US20130160354A1

US20130160354A1 - Organic nitrates as ignition enhancers

Info

Publication number: US20130160354A1
Application number: US13/721,499
Authority: US
Inventors: Widyai Prenish BANSIE; Matthias EGGENSTEIN; Renate Uitz; Andrew David Horton
Original assignee: Shell Oil Co
Current assignee: Shell USA Inc
Priority date: 2011-12-22
Filing date: 2012-12-20
Publication date: 2013-06-27
Also published as: WO2013093100A1

Abstract

A diesel fuel composition comprising an organic nitrate is described. The organic nitrate may be a terpene nitrate. Methods of using an organic nitrate for achieving a desired cetane number, and uses of organic nitrates for the purpose of reducing the ignition delay of the fuel and/or for increasing its cetane number to a defined level are also described, as are methods of operating a compression ignition engine.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of European Patent Application No. 11195433.5, filed on Dec. 22, 2011, the disclosure of which is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

Embodiments of the present invention relate to a method of improving diesel fuels, and in particular to the use of organic nitrates as additives in a diesel fuel composition to give improvements in fuel combustion and cetane number.

BACKGROUND OF THE INVENTION

This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of any prior art.
The cetane number of a fuel composition is a measure of its ease of ignition and combustion. With a lower cetane number fuel a compression ignition (diesel) engine tends to be more difficult to start and may run more noisily when cold; conversely a fuel of higher cetane number tends to impart easier cold starting, to lower engine noise, to alleviate white smoke (“cold smoke”) caused by incomplete combustion after.
There is a general preference, therefore, for a diesel fuel composition to have a high cetane number, a preference which has become stronger as emissions legislation grows increasingly stringent, and as such automotive diesel specifications generally stipulate a minimum cetane number. To this end, many diesel fuel compositions contain ignition improvers, also known as cetane boost additives or cetane (number) improvers/enhancers, to ensure compliance with such specifications and generally to improve the combustion characteristics of the fuel.
Organic nitrates have been known for some time as ignition accelerants in fuels, and some are also known to increase the cetane number of diesel fuels. Such organic nitrates generally include short- and medium-chain linear and branched alkanols and nitrates of cycloalkanols, such as those described in U.S. Pat. No. 4,479,905.
A commonly used diesel fuel ignition improver is 2-ethylhexyl nitrate (2-EHN), which operates by shortening the ignition delay of a fuel to which it is added. However, 2-EHN is also a radical initiator, and can potentially have an adverse effect on the thermal stability of a fuel. Poor thermal stability in turn results in an increase in the products of instability reactions, such as gums, lacquers and other insoluble species. These products can block engine filters and foul fuel injectors and valves, and consequently can result in loss of engine efficiency or emissions control.
The organic nitrates described in the prior art as combustion improvers and/or cetane number improvers have a series of disadvantages, especially lack of thermal stability, excessively high volatility and insufficient efficacy. However, it may be expected that by decreasing the volatility of a cetane enhancer, e.g. by using a molecule of higher molecular weight, its efficacy as a combustion improver and/or cetane number improver may then decline.
There are also health and safety concerns regarding the use of 2-EHN, which is a strong oxidising agent and is also readily combustible in its pure form. It can also be difficult to store in concentrated form as it tends to decompose, and so is prone to forming potentially explosive mixtures. Furthermore, it has been noted that 2-EHN functions most effectively under mild engine conditions.
These disadvantages, taken together with the often significant cost of incorporating 2-EHN as an additive into a fuel composition, mean that it would be generally desirable to reduce or eliminate the need for 2-EHN and other known cetane number improvers in diesel fuel compositions, whilst at the same time maintaining acceptable combustion properties.
WO2008/000778 describes one such approach to reducing the amount of an ignition enhance required in a diesel fuel by using a Fischer-Tropsch derived fuel component, in a fuel composition containing an ignition improver, which acts to enhance the effect of the ignition improver and thus reduce the amount required to achieve the same desired cetane number.
WO2006/067234 relates to the use of fatty acid alkyl esters (FAAEs) in diesel fuels to increase the cetane number.
Thus, it is desirable to overcome or alleviate at least one of the problems associated with the prior art.

SUMMARY OF THE INVENTION

Embodiments of the invention provide alternative organic nitrates which are effective as combustion improvers or cetane number improvers. Embodiments of the invention also provide alternative organic nitrates which have similar or lower volatility than known cetane number improvers, or which meet acceptable safety levels for use in commercial diesel fuels. In addition, embodiments of the invention provide alternative organic nitrates for use as ignition/combustion improvers and that are most cost-effective and/or more convenient to manufacture than known organic nitrate cetane number improvers. Also, embodiments of the invention provide alternative organic nitrates for use as cetane number enhancers that work well under harsh engine conditions (for example, some known cetane enhancers may undesirably over-advance combustion. Embodiments of the invention further provide cetane enhancers derived from renewable (or waste) feedstocks or by-products. Additionally, embodiments of the invention provide methods for producing organic nitrates useful as cetane number improvers by nitration of corresponding organic alcohols.
Surprisingly, it has been found that certain long chain linear organic nitrates, certain cyclic terpene organic nitrates, and certain nitrates derived from fatty alcohols and fatty acid alkyl esters can serve to reduce the ignition delay and/or as effective cetane number improvers in diesel fuels.
Accordingly, in a first aspect of the invention, there is provided a diesel fuel composition for use in a compression ignition engine, which comprises an organic nitrate selected from the group consisting of:
a cyclic nitrate of Formula (4):
wherein each of R₁to R₉is independently selected from H or C₁-C₆alkyl, or nitrate(—ONO₂), wherein optionally one of R₄and R₅forms an optionally substituted alkylene bridge with one of R₈and R₉, which may be substituted by one or more C₁-C₆alkyl, and/or nitrate(—ONO₂); wherein at least one of R₁to R₉is not H, and provided that no more than one R₂to R₉comprises a nitrate group.
In one embodiment, the organic nitrate has the effect of increasing the cetane number of fuel, such as to a desired or target cetane number.
One or more additional organic nitrates may be used in the diesel fuel composition. Embodiments of the present invention also define the addition organic nitrates.
In one embodiment, the diesel fuel composition has a cetane number of 40 or more, 50 or more, 60 or more, or 70 or more.
In another aspect of the invention, there is provided a method for reducing the ignition delay and/or increasing the cetane number of a diesel fuel composition, which method comprises adding to the composition an amount of an organic nitrate according to the invention.
The method may involve increasing the cetane number of the diesel fuel composition to achieve a target cetane number. In some embodiments, the method may involve adding one or more additional organic nitrate to the fuel composition.
In one embodiment, the method may further be for reducing the amount of 2-ethylhexyl nitrate (2-EHN) or any other known cetane enhancer in the diesel fuel composition to achieve the target cetane number.
A further aspect of the invention is directed to the use of an organic nitrate in a diesel fuel composition for the purpose of reducing the ignition delay (ID) of the diesel fuel composition, wherein the organic nitrate is as defined herein.
The diesel fuel composition of this or any other aspect may comprise a biofuel, and optionally may comprise FAAEs, such as FAMEs.
The organic nitrate may be present in the diesel fuel composition at a concentration of: (a) between 0.025% and 2.0% w/w; (b) between 0.05% and 1.0% w/w; or (c) one of 0.05% w/w, 0.1% w/w, 0.5% w/w or 1.0% w/w; based on the total weight of the fuel composition.
In a preferred embodiment, the organic nitrate is selected from the group consisting of bornyl nitrate, fenchyl nitrate, menthly nitrate, and any combination thereof.
The embodiments of the present invention may additionally or alternatively be used to adjust any property of the fuel composition which is equivalent to or associated with cetane number, for example, to improve the combustion performance of the fuel composition, e.g. to shorten ignition delays (i.e. the time being fuel injection and ignition in a combustion chamber during use of the fuel), to facilitate cold starting or to reduce incomplete combustion and/or associated emissions in a fuel-consuming system running on the fuel composition) and/or to improve fuel economy or exhaust emissions generally.
Accordingly, in further aspects of the invention there is provided a method or use of an organic nitrate in a diesel fuel composition for improving the fuel economy of an engine into which the fuel composition is or is intended to be introduced, or of a vehicle powered by such an engine, wherein the organic nitrate is defined herein.
In yet another aspect of the invention there is provided a method for the preparation of a diesel fuel composition having a target cetane number for use in a compression ignition engine. The method comprises adding an organic nitrate, as defined elsewhere herein, to the diesel fuel composition; and blending the organic nitrate with the diesel fuel composition to provide a diesel fuel composition having the target cetane number.
Still yet another aspect of the invention relates to a method of operating a compression ignition engine and/or a vehicle which is powered by such an engine, which method involves introducing into a combustion chamber of the engine a diesel fuel composition as defined elsewhere herein, or as obtained by the uses and methods of the invention.
Other features of embodiments of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further illustrated by the accompanying drawings in which:

FIG. 1 depicts the structures of exemplary embodiments of organic nitrates according to aspects of the invention;

FIG. 2 illustrates the reduction in ignition delay (ID) as a percentage for a diesel base fuel comprising certain embodiments of the organic nitrates according to aspects of the invention;

FIG. 3 illustrates the results of derived ignition quality (DIQ) studies for a diesel base fuel comprising an exemplary embodiment of organic nitrates according to aspects of the invention under certain combustion conditions;

FIG. 4 is a graph illustrating correlations between the measured ignition delay at the various combustion conditions (a01 to a11—see Key) against the organic nitrate used as a cetane enhancer in a diesel fuel composition according to the invention. The results are shown against molecular weight of the cetane enhancer: bornyl nitrate (1), menthyl nitrate (2), 1,10-decyl dinitrate (3), oleyl nitrate (4), hexadecyl nitrate (5), nitro-substituted methyl oleate (6), and nitro-substituted ethyl abietate (7); and

FIG. 5 is a differential scanning calorimetry (DSC)/thermogravimetric analysis (TGA) plot illustrating the thermal decomposition of 1,10-decyl dinitrate.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In order to assist with the understanding of the invention several terms are defined herein.
The terms “cetane (number) improver” and “cetane (number) enhancer” are used interchangeably to encompass any component that, when added to a fuel composition at a suitable concentration, has the effect of increasing the cetane number of the fuel composition relative to its previous cetane number under one or more engine conditions within the operating conditions of the respective fuel or engine. The term cetane number improvers/enhancers of the invention are organic nitrates as described herein. As used herein, a cetane number improver or enhancer may also be referred to as a cetane number increasing additive/agent or the like.
In one embodiment, the cetane number of a fuel composition may be determined in any known manner, for instance using the standard test procedure ASTM D613 (ISO 5165, IP 41) which provides a so-called “measured” cetane number obtained under engine running conditions. In a preferred embodiment, the cetane number may be determined using the more recent and accurate “ignition quality test” (IQT; ASTM D6890, IP 498), which provides a “derived” cetane number based on the time delay between injection and combustion of a fuel sample introduced into a constant volume combustion chamber. This relatively rapid technique can be used on laboratory scale (ca 100 ml) samples of a range of different fuels. Alternatively, cetane number may be measured by near infrared spectroscopy (NIR), as for example described in U.S. Pat. No. 5,349,188. This method may be preferred in a refinery environment as it can be less cumbersome than for instance ASTM D613. NIR measurements make use of a correlation between the measured spectrum and the actual cetane number of a sample. An underlying model is prepared by correlating the known cetane numbers of a variety of fuel samples with their near infrared spectral data.
In some embodiments, the methods/uses encompass adding a cetane enhancer according to aspects of the present invention to a fuel composition so as to adjust the cetane number or to achieve or reach a desired target cetane number. In the context of the embodiments of this invention, to “reach” a target cetane number can also embrace exceeding that number. Thus, the target cetane number may be a target minimum cetane number.
In one embodiment, the present invention results in a fuel composition which has a derived cetane number (IP 498) of 50 or greater, more preferably of 51, 52, 53, 54 or 55 or greater. For example, in some embodiments, the resultant fuel composition may have a cetane number of 60 or greater, 65 or greater or even 70 or greater.
Embodiments of the present invention may additionally or alternatively be used to adjust any property of the fuel composition which is equivalent to or associated with cetane number, for example, to improve the combustion performance of the fuel composition, e.g. to shorten ignition delays (i.e. the time between fuel injection and ignition in a combustion chamber during use of the fuel), to facilitate cold starting or to reduce incomplete combustion and/or associated emissions in a fuel-consuming system running on the fuel composition) and/or to improve fuel economy or exhaust emissions generally.
In accordance with embodiments of the invention, therefore, cetane number improvers also encompass additives that increase the combustability of the fuel to which it is added and, as such, decrease the ignition delay. Therefore, as used herein, an organic nitrate that increases the combustability (i.e. a “combustion enhancer/improver”) and/or decreases the ignition delay (i.e. an “ignition enhancer/improver”) is also considered to be a cetane number improver or enhancer.
Cetane number improvers of the invention may be used to increase the cetane number of a fuel composition. As used herein, an “increase” in the context of cetane number embraces any degree of increase compared to a previously measured cetane number under the same or equivalent conditions. Thus, in a preferred embodiment, the increase is compared to the cetane number of the same fuel composition prior to incorporation of the cetane number increasing (or improving) component or additive. Alternatively, the cetane number increase may be measured in comparison to an otherwise analogous fuel composition (or batch or the same fuel composition) that does not include the cetane number enhancer of the invention. Alternatively, an increase in cetane number of a fuel relative to a comparative fuel may be inferred by a measured increase in combustability or a measured decrease in ignition delay for the comparative fuels.
The increase in cetane number (or the decrease in ignition delay, for example) may be measured and/or reported in any suitable manner, such as in terms of a percentage increase or decrease. By way of example, the percentage increase or decrease may be at least 1%, such as at least 2%. In one embodiment, the percentage increase in cetane number or decrease in ignition delay is at least 5%, at least 10%, at least 15% or at least 20%. In some embodiments, the increase in cetane number or decrease in ignition delay may be at least 25%, at least 30%. However, it should be appreciated that any measurable improvement in cetane number or ignition delay may provide a worthwhile advantage, depending on what other factors are considered important, e.g. availability, cost, safety and so on.
The engine in which the fuel composition of the invention is used may be any appropriate engine. Thus, where the fuel is a diesel or biodiesel fuel composition, the engine is a diesel or compression ignition engine Likewise, any type of diesel engine may be used, such as a turbo charged diesel engine, provided the same or equivalent engine is used to measure fuel economy with and without the cetane number increasing component. Similarly, the invention is applicable to an engine in any vehicle. Generally, the cetane number improvers of the invention are suitable for use over a wide range of engine working conditions. However, some organic nitrates of the invention may provide optimal effects under a particular narrow range of engine working conditions, such as under mild conditions and more suitably under harsh conditions.
Cetane Number Enhancers/Ignition Improvers
Cetane number enhancers are known and commercially available, and may also be known (in the context of diesel fuels) as “cetane (number) improvers”, “combustion improvers” and “ignition improvers” etc. as previously described.
Cetane enhancers are often added to diesel fuels, at additive levels (typically 0.1 to 2.0% w/w), to improve the combustion properties of the fuel. They function to reduce the ignition delay, i.e. the period between the time of injection of the fuel and the start of combustion (ignition). This, in turn, leads to better engine performance, for example, in terms of higher fuel efficiency, lower emissions, reduced combustion noise and improved cold starting. Addition of a cetane enhancer to a diesel fuel allows the point in the diesel cycle at which heat is released to be advanced, which results in improved thermodynamic efficiency (maximum efficiency at about 10° after top dead centre.
Although there are various explanations of the working of cetane enhancers, such as their effect in increasing the heating rate of the fuel, it is generally accepted that they act as sources of chain-initiating radicals.
The cetane number (CN) of a fuel is defined by reference to the ignition properties of standard mixtures of n-hexadecane (cetane, CN=100) and 2,2,4,4,6,8,8-hepta-methylnonane (CN=15). A fuel with a high CN has a short ignition delay. Typically, molecules with high octane numbers, which confer a resistance to spontaneous ignition in gasoline spark ignition engines, have low cetane numbers. The addition of small amounts of cetane enhancers to a diesel fuel may, therefore, result in improved fuel properties based on the shorter ignition delay.
Known cetane number enhancers include: a) certain organic nitrates (e.g. isopropyl nitrate, 2-ethylhexyl nitrate (2-EHN), cyclohexyl nitrate, and methoxyethyl nitrate); b) organic peroxides and hydroperoxides (e.g. di-tert-butyl peroxide); and c) organic peracids and peresters. The most commonly used cetane enhances are dialkylperoxides (ROOR, di-t-butylperoxide) and organic nitrates (R—ONO₂), of which the most important is 2-ethylhexylnitrate (2-EHN). European consumption of 2-EHN grew from 75 kt/a to 101 kt/a from 2000 to 2008, and an average annual growth of approximately 3.5% has been predicted from 2008 to 2013.
The consumption of 2-EHN in North America (USA: 7.2 kt/a in 2008) is much lower than in Europe.
2-EHN is produced industrially by the nitration of 2-ethylhexanol, and in Europe this consumes almost a quarter of the production of this alcohol. The nitration of the alcohol involves reaction with a 1/1 mixture of undiluted nitric and sulfuric acids (using stoichiometric amounts of alcohol and nitric acid).
There are some safety concerns surrounding the production, transport and use of 2-EHN. For example, it is conceivable that 2-EHN drums exposed to high temperatures during transport could be subject to runaway decomposition reaction. The low flash point of 2-EHN (76° C.) is also a concern. Furthermore, the auto-ignition temperature of 2-EHN of 130° C. is lower than normal hydrocarbons. There have been very many studies of the thermal stability of 2-EHN (e.g. Pritchard (1989), Combustion and Flame, 75, 415; Bornemann & Scheidt (2000), F. Int. J. Chem. Kinetics, 34, 34; and Zeng et al. (2008), J. Thermal Analysis & calorimetry, 91, 359).
Accordingly, it was desired to identify further alternative cetane number improvers, which may provide benefits over the known cetane enhancer. Any useful benefit may be achieved, such as in relation to their synthesis, storage, transportation; or in use, e.g. under certain operating conditions or in certain diesel fuels. Particular benefits of the invention are directed to one or more of the following: increased stability under storage and transport conditions; at least equal and more suitably greater effectiveness as a cetane enhancer; organic nitrates derivable from renewable feedstocks or waste streams; effectiveness under harsh engine conditions; and effectiveness at component or more suitably at additive concentration levels.
Thus, embodiments of the present invention provide alternative organic nitrates for use as cetane number improvers in diesel fuels and optionally for achieving one or more associated benefit.
The cetane number improver according to aspects of the invention may be selected from nitrated:
(a) terpene alcohols, particularly monoterpene alcohols and most preferably monocyclic and bicyclic molecules including pinenes, such as borneol, fenchol and menthol;
(b) fatty alcohols, particularly obtained by hydrogenation of fatty acid esters (e.g. the synthetic alcohol mixture, Neodol 23, derived from the SHF process)
(c) unsaturated fatty esters, particularly FAMEs, such as methyl oleate;
(d) tall oil derived resin esters, particularly abietate esters such as ethyl abietate; and
(e) long-chain linear alkanols, particularly linear C₁₀-C₁₈alkanols and diols, such as 1-octanol, 1,10,-decanediol, 1-dodecenol, 1-tridecanol, 1-tetradecanol, 1-hexadecanol, 1-octadecanol; and any combination thereof.
Accordingly, the cetane number improver has one or two nitrate (NO₃) groups in place of the hydroxyl groups in the above-mentioned compounds.
With the exception of 1,10,-decanediol, generally the alcohol feedstocks are commercially available at scales of at least 10 kt/a, which would potentially be compatible with production of new useful cetane enhancers for worldwide consumption. 1,10-decanediol may be obtained by the hydrogenation of sebacic acid (1,8-octanedicarboxylic acid), which itself is produced via the alkali fusion of ricinoleic acid, the major constituent of castor oil.
Thus, in accordance with a first embodiment of the invention, the cetane number improver of the invention has the Formula (1): R—ONO₂, wherein R is a terpene or an oxygenated (saturated) terpene. Optionally, the terpene may be natural or substituted by up to three (e.g. 1, 2 or 3) C₁-C₆alkyl groups or a further nitrate (—ONO₂) group.
Terpenes are classified according to the number of units of the basic structure methylbuta-1,3-diene or isoprene, which make up the terpene. Monoterpenes contain two isoprene units and are generally considered to have the chemical formula C₁₀H₁₆. However, monoterpenes are particularly sensitive to oxygenation at the carbon-carbon double bond and so monoterpenes are typically saturated hydrocarbons lacking the carbon-carbon double bond. These oxygenated, saturated molecules are sometimes also referred to as monoterpenes, and are encompassed by the term “monoterpene” as used in the context of this invention. Monoterpenoid is another term that is understood to include the monoterpenes and other related compounds having the monoterpene skeleton, and such monoterpenoid structures are also encompassed within the definition of monoterpene used herein.
Monoterpenes may be acyclic such as myrcene and ocimene or cyclic such as limonene and pinene. In one embodiment, the terpene is a monocyclic or a bridged-monocyclic (i.e. bicyclic) alkyl.
In one embodiment, the cetane enhancer of the invention has the Formula (2): C₁₀H₁₆X—ONO₂, wherein X is selected from H, C₁-C₆alkyl and ONO₂. In a preferred embodiment, X is selected from H, methyl, ethyl and ONO2; and still more preferably, X is H. Most preferably, R of Formula (1) is a monoterpene selected from menthyl, fenchyl and bornyl, which may optionally be substituted by X as defined above.
According to embodiments of the invention, therefore, the cetane number improver may be a compound of Formula (3):
wherein each of R₁to R₉is independently selected from H or C₁-C₆alkyl, or nitrate(—ONO₂), wherein optionally two of R₁to R₉may be connected together to form a bridge, which may be substituted by one or more C₁-C₆alkyl, and/or nitrate(—ONO₂); provided that no more than 1 R₁to R₉comprises a nitrate group. In one embodiment, R₁to R₉are independently selected from H or C₁-C₆alkyl, wherein optionally one of R₄and R₅forms an optionally substituted alkylene bridge with one of R₈and R₉. Preferably at least one of R₁to R₉is not H; more preferably 1, 2, 3, 4 or 5 of R₁to R₉is not H.
The C₁-C₆alkyl may be straight chain (i.e. linear) or branched chain, wherein the number 1 to 6 refers to the total number of carbon atoms in the group. In one embodiment, C₁-C₆alkyl radicals include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, sec-butyl, n-pentyl, isopentyl, tert-pentyl, sec-pentyl, n-hexyl, 2-ethylbutyl, and 2,3-dimethylbutyl.
In accordance with a preferred embodiment, R₁to R₉are independently selected from H, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl and tert-butyl, wherein optionally one of R₄and R₅forms an optionally substituted alkylene bridge with one of R₈and R₉. In one embodiment, the alkylene bridge has the formula —(CR_aR_b)_n—, wherein R_aand R_bare independently selected from H, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl and tert-butyl; and n is 1 or 2. In a preferred embodiment, R_aand R_bare independently selected from H, methyl and ethyl, and n is 1; and yet more preferably selected from H and methyl, and n is 1.
In a preferred embodiment, R₁, R₆and R₇are H. In another preferred embodiment R₁, R₆and R₇are H, one of R₄and R₅is H, one of R₈and R₉is H, and the other of R₄and R₅, and R₈and R₉is selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl and tert-butyl or they are connected together to form an alkylene bridge of the formula —CR_aR_b—, wherein R_aand R_bare defined above. In one particularly preferred embodiment, one of R₄and R₅is H, and the other is methyl; and one of R₈and R₉is H, and the other is isopropyl; wherein R₁, R₂, R₃, R₆and R₇are as defined in any of the above embodiments; preferably wherein R₁, R₆and R₇are H, and R₂and R₃are as defined in any of the above embodiments; and most preferably wherein R₁, R₂, R₃, R₆and R₇are H.
In yet another particularly preferred embodiment of Formula (3) or (3A), one of R₄and R₅is H, one of R₈and R₉is H or methyl, and the other of R₄and R₅and of R₈and R₉are connected together to form an alkylene bridge of the formula —CR_aR_b—, wherein R_aand R_bare H or methyl. Beneficially in this preferred embodiment, one of R₈and R₉is methyl, R₁, R₆and R₇are H, R_aand R_bare H or methyl, and R₂and R₃are as defined in any of the above embodiments. In one preferred group of compounds of this embodiment, at least two of R_a, R_b, R₂and R₃are methyl. For example, in one embodiment R_aand R_bare methyl, and in another embodiment R₂and R₃are methyl.
Accordingly, in another preferred embodiment of the invention, the cetane number improver is defined by Formula (4):
wherein each of R₁to R₉is as defined in connection with Formula (3). In one embodiment, at least one of R₁to R₉is not H, and more preferably, 1, 2, 3, 4 or 5 of R₁to R₉is not H. In a preferred structure of Formula (4) or Formula (4A), R₈and R₄are connected together to form an alkylene bridge of the formula —(CR_aR_b)_n—, wherein R_aand R_bare independently selected from H, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl and tert-butyl; and n is 1 or 2. More preferably the alkylene bridge has the formula —CR_aR_b—, wherein R_aand R_bare H or methyl. Also preferred are compounds of Formula (4) and Formula (4A) in which R₁, R₆and R₇are H. Still more preferred are compounds of Formula (4) and Formula (4A) wherein R₁, R₆and R₇are H and at least two of R_a, R_b, R₂and R₃are methyl. In one particular embodiment R_aand R_bare methyl, and in another particular embodiment R₂and R₃are methyl.
In yet another preferred embodiment, the cetane number improver is defined by Formula (5):

or Formula (5A):

wherein each of R₁to R₃, R₅to R₇, R₉, R_aand R_bare as defined in connection with any embodiment of Formulas (3) or (3A), or as defined in connection with any embodiment of Formulas (4) or (4A). Preferably, in the compound of Formulas (5) and (5A), R₁, R₆and R₇are H; and R₂, R₃, R₅, R₉, R_aand R_bare independently selected from H, methyl and ethyl. Beneficially at least one of R₂, R₃, R₅, R₉, R_aand R_bis not H, and preferably 1, 2 or 3 of R₂, R₃, R₅, R₉, R_aand R_bis not H. Most preferred are compounds wherein three of R₂, R₃, R₅, R₉, R_aand R_bare methyl.
In the cetane number improver compounds of embodiments of the invention, the nitrate may be arranged in an axial or equatorial position relative to the alkyl ring. In a preferred embodiment, however, the nitrate is attached in an equatorial position on the ring.
Most preferred cetane improvers of this embodiment are bornyl nitrate, fenchyl nitrate and menthly nitrate.
In accordance with another embodiment of the invention, the cetane number improver is a compound of the Formula (6): R_x—ONO₂, wherein R_xis a linear (straight-chain) aliphatic group having 8 to 24 carbon atoms. In one embodiment, the aliphatic group has between 10 and 20 carbon atoms, and preferably between 10 and 18 carbon atoms. For example, in one embodiment, R_xgroups have 8, 10, 12, 13, 14, 16 or 18 carbon atoms. In a preferred embodiment, R_xgroup has 18 carbon atoms. The aliphatic group may be saturated or unsaturated. When unsaturated it is preferably mono-unsaturated. The R_xgroup may optionally be substituted with a nitrate (—ONO₂) group to form a dinitrate compound. Preferably the one to two nitrate groups are attached to the terminal carbon atoms of the aliphatic group.
Embodiments of cetane enhancers of the invention of Formula (6) include 1-octyl nitrate, 1,10,-decyl dinitrate, 1-dodecyl nitrate, 1-tridecyl nitrate, 1-tetradecyl nitrate, 1-hexadecyl nitrate, and 1-octadecyl nitrate.
In accordance with one embodiment, the R_xgroup a fatty acid derivative, particularly a fatty alcohol derivative. A preferred fatty alcohol derivative is monounsaturated, and is more preferably a cis-monounsaturated fatty alcohol derivative (i.e. wherein adjacent hydrogen atoms are on the same side of the double bond). Thus, in some embodiments, the cetane number improver of the present invention has the Formula (7):
CH₃(CH₂)_xCH═CH(CH₂)_yONO₂
wherein X is between 3 and 8 and Y is between 4 and 9. In one embodiment, X is between 5 and 7 and Y is between 6 and 8. In a preferred embodiment, X is 5 or 7 and Y is 6 or 8. In another preferred embodiment, the carbon-carbon double bond is in the cis formation.
A preferred monounsaturated aliphatic nitrate of the invention is oleyl nitrate.
In accordance with a third embodiment of the invention, the cetane number improver of the invention is a nitro-substituted fatty acid ester of the Formula (7): R_y-[O_sNO_t]_u, wherein R_yis a alkyl ester of a fatty acid, i.e. a fatty acid alkyl ester (FAAE) moiety, S is 0 or 1, t is 1 or 2, and u is 1 to 4 or 1 to 3.
In one embodiment, the FAAE group is a linear (straight-chain) aliphatic group having 8 to 24 carbon atoms, preferably between 10 and 22 carbon atoms, preferably between 12 and 20 carbon atoms, and most preferably between 14 and 18 carbon atoms. For example, particularly suitable R_ygroups have 14, 16 or 18 carbon atoms; and advantageously the R_ygroup has 18 carbon atoms. The aliphatic group may be saturated or unsaturated. Preferably R_yis mono-unsaturated, and more preferably the R_ygroup has a cis double-bond. The R_ygroup may optionally be substituted with one or two additional nitro-substituted groups of formula O_sNO_t.
A preferred nitro-substituted fatty acid ester of the invention is a methyl oleate substituted with 1, 2 or 3 O_sNO_tgroups.
In accordance with yet another embodiment of the invention, the cetane number improver of the invention is a nitro-substituted diterpene of the Formula (8): R_z—[O_sNO_t]_u, wherein R₁is a diterpene or an alkyl ester of a diterpene, and wherein [O_sNO_t]_uis as defined above. In one embodiment, the compound of Formula (8) is an alkyl ester of abietic acid. In a preferred embodiment, the ester is a methyl or ethyl ester. The R_zgroup may optionally be substituted with one or two additional nitro-substituted groups of formula O_sNO_t. In another preferred embodiment, the nitro-substituted diterpene is nitro-substituted ethyl abietate substituted with 1, 2 or 3 O_sNO_tgroups.
In accordance with another embodiment of the invention, the cetane number improver is a nitro-substituted steroid containing from 1 to 3 nitrate groups, such as cholesterol nitrate.
In one embodiment, in use, the cetane number improving additive of the invention may be pre-dissolved in a suitable solvent, for example an oil such as a mineral oil or Fischer-Tropsch derived hydrocarbon mixture; a fuel component (which again may be either mineral or Fischer-Tropsch derived) compatible with the diesel fuel composition in which the additive is to be used (for example a middle distillate fuel component such as a gas oil or kerosene); a poly alpha olefin; a so-called biofuel such as a fatty acid alkyl ester (FAAB), a Fischer-Tropsch derived biomass-to-liquid synthesis product, a hydrogenated vegetable oil, a waste or algae oil or an alcohol such as ethanol; an aromatic solvent; any other hydrocarbon or organic solvent; or a mixture thereof. Preferred solvents for use in this context are mineral oil based diesel fuel components and solvents, and Fischer-Tropsch derived components such as the “XtL” components referred to below. Biofuel solvents may also be preferred in certain cases. In one embodiment, the cetane enhancer will be part of an additive (performance) package additionally containing other additives such as detergents, anti-foaming agents, corrosion inhibitors, dehazers etc. Alternatively, the cetane enhancing agent of the invention may be blended directly with the base fuel.
The concentration of the cetane number enhancing additive used may depend on desirable fuel characteristics/properties, such as: the desired combustability of the overall fuel composition; the combustability of the composition prior to incorporation of the additive; the combustability and/or stability of the additive itself; and/or the properties of any solvent in which the additive is used. By way of example, the concentration of the cetane number improving additive in the fuel composition may be up to 2% w/w and preferably up to 1.0% w/w. Thus, the concentration of the cetane number improver may be from 0.025% w/w to 2% w/w, or from 0.05% w/w to 1% w/w. In some cases, the concentration of the cetane number improver is from 0.05% w/w to 1.0 w/w, such as 0.05% w/w, 0.1% w/w, 0.25% w/w, 0.5% w/w, 0.75% w/w or 1.0% w/w based on the total weight of the fuel composition.
Where a combination of two or more cetane number improving additives is used in the fuel composition, the same concentration ranges may apply to the total combination of cetane number improving additives. It will be appreciated that amounts/concentrations may also be expressed as ppm, in which case 1% w/w corresponds to 10,000 ppm w/w.
The remainder of the composition will typically consist of one or more automotive base fuels optionally together with one or more fuel additives, for instance as described in more detail below.
The relative proportions of the cetane number enhancer, fuel components and any other components or additives present in a diesel fuel composition prepared according to the invention may also depend on other desired properties such as density, emissions performance and viscosity.
The synthesis of embodiments of the cetane enhancers of the invention are described further below.

Synthesis of Cetane Number Enhancers

A range of alcohols and olefins were evaluated as precursors of cetane enhancers. The conversion of alcohols to nitrates is generally well-known in the art (Olah et al. Nitration: Methods and mechanisms, Chapter 4, Aliphatic nitration in Organic nitro chemistry series, VCH, New York, 1989, p. 219; Boschan et al. (1955) Chem. Rev., 55, 485), and any appropriate procedure can be used to produce the organic nitrates of the invention.
Although olefinic substrates may afford nitrates (i.e. R—ONO₂) on reaction with typical nitrating agents (HNO₃, N₂O₄, N₂O₅etc.), more typically they react to give products with nitro (R—NO₂) and other substituents. Since nitroalkanes are inherently unstable (decomposing via both non-radical, HONO elimination; and radical mechanisms, C—NO₂bond homolysis); like nitrates, they are potential sources of radicals and may, therefore, also be useful as cetane enhancers.
It was anticipated that, in most cases, the alcohols could be easily converted to the corresponding nitrates by reaction with nitric acid. Alcohol nitration involves attack of the —OH functionality on NO₂ ⁺, and may be generated using the following reagents: (i) nitric acid for secondary alcohols; (ii) a mixture of nitric acid, sulfuric acid and urea for primary or secondary alcohols; and (iii) a mixture of nitric acid and acetic anhydride (precursor of reactive acetyl nitrate) in acetic acid for unsaturated alcohols.
The choice of reagent used to synthesise the nitrate or nitro-compounds of the invention may depend on the reactivity of the alcohol starting material and the desirability to avoid side reactions, such as oxidation of the alcohol to a ketone, or sulfation of the olefin. Reactive olefins, such as pinenes, can be cleanly converted to nitrates, for example, by (ring-opening) reaction with nitric acid (e.g. Canoira et al. (2007) Fuel, 86, 965; Bakhvalov et al. (2000) J. Organic Chem., 36, 1601; Bakhvalov et al. (2002) J. Organic Chem., 38, 507). However, olefins containing ester functionalities (such as unsaturated constituents of FAME) may undergo partial hydrolysis of the ester under these conditions, affording free acid, which is not desirable in a diesel fuel component. In these cases, N₂O₄may suitably be used instead of nitric acid. Reaction with olefins normally, therefore, results in a mixture of products, containing nitro (R—NO₂), as well as nitrate (R—ONO₂) and hydroxyl functionalities.
Synthesis of embodiments of the cetane enhancing agents of the invention were initially performed at small scale (ca. 2 g) to investigate optimum conditions (e.g. yield, purity), and for safety reasons. The reactions were then scaled up to afford 20 to 40 g of the cetane enhancers, which (following thermal stability tests) were sent for assays in a combustion research unit (CRU).
Following the synthesis of the organic nitrates, a differential scanning calorimetry/thermal gravimetric analysis (DSC/TGA measurement) was performed to determine the thermal decomposition behaviour of the molecules, before shipping for further analysis.

Conversion of Primary Alcohols

A common method of converting primary/secondary alcohols to nitrates involves the use of mixed nitric and sulfuric acids (molar ratio substrate: nitric acid: sulfuric acid generally 1:3:8), together with urea (0.25 equivalent based on substrate). The urea is used to remove any HNO₂(nitrous acid) formed. The mixed acid is a more powerful nitrating agent than nitric acid alone, due to the higher concentration of NO₂ ⁺, which is the reactive species. A general method is shown in Scheme 1 below.
The primary alcohols were nitrated at 0° C. for 2 hours. Following neutralisation, extraction with dichloromethane and drying, the primary nitrates were obtained in a yield of 80-98%. Where a di-nitrate is to be synthesised from a di-hydroxy starting material the relative proportions of reaction components are selected to achieve two equivalents of HNO₃to ensure complete conversion of hydroxyl groups. In some cases, lower than expected yields of nitrated alkyls were obtained, which may be due to dissolution of part of the nitrated product in the water layer.
Exemplary nitrated alkyls (compounds 1 to 6) of the invention are illustrated in FIG. 1.

Conversion of Secondary Alcohols

In some cases secondary alcohols may be nitrated using the same reagent mixture and conditions as used for primary alcohols. Alternatively, nitric acid may be used, which is a less powerful oxidising agent than the mixed acid.
One exemplary secondary alcohol is endo-borneol, which may be converted into a bornyl nitrate.
By way of example, the secondary alcohols may be nitrated by reacting with nitric acid at room temperature (RT) for approximately 24 hours. In another example, exo-borneol was reacted with nitric acid at RT to obtain the exo-nitrate of borneol (see reaction Scheme 2 below and FIG. 1, compound 7; yield 87%).
Nitrated products from exo-fenchol and endo-fenchol may be obtained in a similar manner to the borneol compounds (see reaction Scheme 3 below). Unlike endo-fenchol, exo-fenchol is not commercially available, but may be prepared by the reduction of L-fenchone via a Meerwein-Ponndorf-Verley reduction (MPV; see e.g. Hückel & Rohrer (1960) Chem. Ber., 93, 1053; Mojtahedi et al. (2007) Org. Lett., 9, 2791). This was found to give a mixture of the endo- and exo-fenchol in the ratio 1:3. The MPV reaction is an aluminium-catalyzed hydride shift (of RCHOH), from the alcohol (isopropanol) to the carbonyl carbon (L-fenchone). Isopropanol may be used as a hydride donor, because the acetone formed can be easily removed by distillation. The reaction was refluxed at 95° C. for 7 days followed by an extraction. This mixture of secondary alcohols was allowed to react with nitric acid for 24 hours at RT, to give a mixture of two nitrated compounds (see FIG. 1, compounds 8a, 8b) as the main products, together with unreacted endo-fenchol. The sterically crowded endo-isomer appears not to react with the milder nitrating reagent, whereas the exo-fenchol affords the two isomeric nitrates, as confirmed by ¹H and ¹³C NMR spectroscopy.
The secondary alcohol, menthol (e.g. L-isomer), may be nitrated using the same conditions as for primary alcohols in general (i.e. mixed acid; see Scheme 4). After extraction with diethyl ether, the nitrated product (see FIG. 1, compound 9) was obtained in a yield of 95%.

Conversion of α-Terpineol

β-pinene may be reacted with nitric acid, e.g. at −15° C. in dichloromethane, according to methods known to the person of skill in the art (see reaction Scheme 5 below). After neutralisation, the product may be isolated by extraction with dichloromethane. NMR may be used to confirm the identity of the product.
As illustrated, this reaction proceeds via protonation of the double bond in β-pinene, followed by rearrangement (β-fragmentation) to relieve ring strain and attack of NO₃ ⁻ on the tertiary carbonium ion.
Tertiary nitrates are generally known to be less thermally stable than primary and secondary nitrates. Therefore, it may be necessary to store and ship tertiary nitrates, such as α-terpineol nitrate, with particular care.

Conversion of Unsaturated Alcohols

In general, unsaturated alcohols can be nitrated selectively at the alcohol position to leave the double bond intact, provided that the mode of introducing the reactants and the amount of nitrating agent is suitably controlled, and the use of sulfuric acid is avoided (as this may react with the double bond to give a sulfate). Thus, any suitable method can be used.
By way of example, oleyl alcohol (e.g. cis-9-octadecen-1-ol) was nitrated slowly adding nitric acid to a mixture of the alcohol and acetic anhydride in acetic acid solvent at 15° C. (see Scheme 6 below). Advantageously, this embodiment avoids build-up of significant concentrations of acetyl nitrate (which may undergo runaway decomposition above 60° C.). Acetyl nitrate is a powerful nitrating agent, being a good source of NO₂ ⁺, which reacts with the alcohol. The product (FIG. 1, compound 10) can then be extracted with a suitable solvent, such as diethyl ether. 1H-NMR confirmed the clean formation of the desired single product without significant by-products.
Nitration of unsaturated secondary alcohols, such as cholesterol, may be performed similarly so as to form cholesterol nitrate (compound 11) shown in FIG. 1.

Conversion of Olefinic Esters

The formation of nitrate or nitro derivatives of olefins may be carried out using any appropriate reaction scheme. Typically, conditions used will be different to those described above for non-olefinic esters. For example, it is desirable to avoid sulfuric acid (to reduce risk of olefin sulfation). Exemplary conditions include: (i) HNO₃(70%) and acetic anhydride (precursor of acetyl nitrate); (ii) fuming HNO₃, acetic acid and NaNO₂; (iii) N₂O₄in chloroform or hexane.
When used in the nitration of FAMEs, processes (i) and (ii) have been reported to result in the partial hydrolysis of the ester. For example, scheme (i) has been found to form nitro/nitrate and nitro/acetate products as well as nitro-substitution of the allylic position. Scheme (ii) has been found to result in low conversion rates of unsaturated FAMEs, with nitro-olefins formed in low yields. Therefore, reaction scheme (iii) may be preferred, since it is expected to avoid ester hydrolysis. N₂O₄may conveniently be used as a dilute solution (boiling point 30-100° C.), and excess reagent removed under low pressure at the end of the procedure. Chloroform and hexane were both used as solvents.
Three unsaturated FAMEs and a resin ester were nitrated using this procedure. In a typical reaction, methyl oleate was added to a stock solution of N₂O₄in chloroform at 0 C and stirred for 48 hours. After, excess N₂O₄was removed in vacuo and the product quenched in an ice bath. The products can be extracted in a suitable solvent, such as ether, and dried. A mixture of products was identified by ¹H and ¹³C NMR analysis.
Analysis indicated that the main products of the nitration reaction were the 1,2-dinitro compound, (O₂N)C(R)H—C(R′)H(NO₂) and nitro alcohols (e.g. NO₂. . . OH). It is likely that addition of a nitro radical to the double bond is followed by reaction of the resulting radical with a second nitro radical to give the 1,2-dinitroalkane and 1-nitro-2-nitritoalkane. The nitrite group (R—ONO) in the 1-nitro-2-nitrito product tends to undergo hydrolysis during work-up to give the nitro-alcohol. Only small amounts of nitroalkene (RCH═C(R′)NO₂) and the allylic nitro products were observed. The reaction and the resultant mixture of products is illustrated in Scheme 7 below.
Similar reactions were carried out to convert other FAMEs, e.g. methyl linoleate (methyl cis,cis-9,12-octadecadienoate) and methyl linolenate (methyl cis,cis,cis-9,12,15-octadecatrienoate) in chloroform and hexane as solvents.
Nitration products of abietic acid—the most prevalent of the organic acids that form the largest constituent of rosin—such as ethyl abietate were also synthesised in similar fashion. Reactions were carried out using N₂O₄in either hexane or chloroform and a similar mixture of nitrogen-containing products was formed. Hexane is a preferred solvent for this reaction. The NMR data was consistent with the formation of dinitro and nitro-alcohol products containing a single double bond (e.g. 1,4-dinitro-2-alkene).
DSC/TGA analysis showed that the product mixture underwent an exothermic decomposition reaction in the temperature range 150-250° C., making a full analysis of the products difficult.
Since the relative thermal stability of the organic nitrates of the invention is one of the key characteristics for determine (and explain) how rapidly they combust and, hence, how effective they may be as combustion improvers (cetane number enhancers) when used in a fuel within a diesel engine, thermal stability assays (e.g. differential scanning calorimetry/thermogravimetric analysis (DSC/TGA)) of the products were also conducted, as described in the Examples. Mass spectrometry (MS) may also be used to provide information on the decomposition mechanism.
DSC measures heat flow resulting from evaporation (endotherm) or thermal decomposition of the compound (exotherm). TGA measures the weight loss on evaporation or decomposition. MS of the gas space allows the identity of the volatile decomposition products to be determined. As well as the decomposition temperature, the associated exotherm may, in principle, be determined. These measurements on embodiments of the organic nitrates according to the invention were intended to provide an initial assessment of their relative thermal decomposition behaviour and not limit the scope of the invention.

Diesel Fuel Compositions

In one aspect of the invention, there is provided a diesel fuel composition, which comprises an embodiment of a cetane number improver of the invention. In particular, the cetane number improver is present at a concentration sufficient and appropriate for achieving a desired cetane number in the resultant fuel composition.
A diesel fuel composition prepared in accordance with aspects of the present invention may in general be any type of diesel fuel composition suitable for use in a compression ignition (diesel) engine; and it may itself comprise a mixture of diesel fuel components.
Thus, in addition to the cetane enhancer, a diesel fuel composition prepared according to aspects of the present invention may comprise one or more diesel fuel components of conventional type. It may, for example, include a major proportion of a diesel base fuel, for instance of the type described below. In this context, a “major proportion” means at least 50% w/w, and typically at least 85% w/w based on the overall composition. In a preferred embodiment, a “major proportion” also includes at least 90% w/w or at least 95% w/w, and in some cases, at least 98% w/w or at least 99% w/w of the fuel composition consists of the diesel base fuel. Accordingly, in some embodiments, the base fuel may itself comprise a mixture of two or more diesel fuel components of the types described below.
Typical diesel fuel components comprise liquid hydrocarbon middle distillate fuel oils, for instance petroleum derived gas oils. Such base fuel components may be organically or synthetically derived, and are obtained by distillation of a desired range of fractions from a crude oil. They will typically have boiling points within the usual diesel range of 150 to 410° C. or 170 to 370° C., depending on grade and use. They will typically have densities from 0.75 to 0.9 g/cm³, such as from 0.8 to 0.86 g/cm³, at 15° C. (IP 365) and measured cetane numbers (ASTM D613) of from 35 to 80, more preferably from 40 to 75. Their initial boiling points will be in the range 150 to 230° C. and their final boiling points in the range 290 to 400° C. Their kinematic viscosity at 40° C. (ASTM D445) might suitably be from 1.5 to 4.5 centistokes. Such fuels are generally suitable for use in compression ignition (diesel) internal combustion engines, of either the indirect or direct injection type.
An automotive diesel fuel composition which results from carrying out aspects of the present invention also falls within these general specifications or standards. Accordingly, it will generally comply with applicable current standard specification(s) such as for example EN 590 (for Europe) or ASTM D975 (for the USA). By way of example, the fuel composition may have a density from 0.82 to 0.845 g/cm³at 15° C.; a T₉₅boiling point (ASTM D86) of 360° C. or less; a cetane number (ASTM D613) of 45 or greater; a kinematic viscosity (ASTM D445) from 2 to 4.5 mm²/s at 40° C.; a sulphur content (ASTM D2622) of 50 mg/kg or less; and/or a polycyclic aromatic hydrocarbons (PAH) content (IP391 (mod)) of less than 11% w/w. Relevant specifications may, however, differ from country to country and from year to year and may depend on the intended use of the fuel composition. In particular, its measured cetane number will preferably be from 45 to 70, to 75 or to 80, more preferably from 50 to 65, or at least greater than 50, greater than 55, greater than 60, or greater than 65.
A petroleum derived gas oil, e.g., obtained from refining and optionally (hydro)processing a crude petroleum source, may be incorporated into a diesel fuel composition. It may be a single gas oil stream obtained from such a refinery process or a blend of several gas oil fractions obtained in the refinery process via different processing routes. Examples of such gas oil fractions are straight run gas oil, vacuum gas oil, gas oil as obtained in a thermal cracking process, light and heavy cycle oils as obtained in a fluid catalytic cracking unit, and gas oil as obtained from a hydrocracker unit. Optionally, a petroleum derived gas oil may comprise some petroleum derived kerosene fraction. Such gas oils may be processed in a hydro-desulphurisation (HDS) unit so as to reduce their sulphur content to a level suitable for inclusion in a diesel fuel composition. This also tends to reduce the content of other polar species such as oxygen- or nitrogen-containing species. In some cases, the fuel composition will include one or more cracked products obtained by splitting heavy hydrocarbons.
In some embodiments of the present invention, the base fuel may be or contain another so-called “biodiesel” fuel component, such as a vegetable oil, hydrogenated vegetable oil or vegetable oil derivative (e.g. a fatty acid ester (FAE), in particular a fatty acid methyl ester (FAME)), or another oxygenate such as an acid, ketone or ester. Such components need not necessarily be bio-derived. Where the fuel composition contains a biodiesel component, the biodiesel component may be present in quantities up to 100%, such as between 1% and 99% w/w, between 2% and 80% w/w, between 2% and 50% w/w, between 3% and 40% w/w, between 4% and 30% w/w, or between 5% and 20% w/w. In one embodiment, the biodiesel component may be FAME.
A diesel base fuel may consist of or comprise a Fischer-Tropsch derived diesel fuel component, typically a Fischer-Tropsch derived gas oil. As used herein, the term “Fischer-Tropsch derived” means that a material is, or is obtained from, a synthesis product of a Fischer-Tropsch condensation process. A Fischer-Tropsch derived fuel or fuel component will therefore be a hydrocarbon stream in which a substantial portion, except for added hydrogen, is derived directly or indirectly from a Fischer-Tropsch condensation process.
Fischer-Tropsch fuels may be derived by converting gas, biomass or coal to liquid (XtL), specifically by gas to liquid conversion (GtL), or from biomass to liquid conversion (BtL). Any form of Fischer-Tropsch derived fuel component may be used as a base fuel in accordance with aspects of the invention.
In one embodiment, the base fuel has a low sulphur content, for example at most 1000 mg/kg (1000 parts per million by weight/ppmw). In a preferred embodiment, it will have a low or ultra low sulphur content, for instance at most 500 mg/kg (500 ppmw), such as no more than 350 mg/kg (350 ppmw), and still more preferably no more than 100 or 50 or 10 or even 5 mg/kg (5 ppmw) of sulphur. It may be a so-called “zero-sulphur” fuel; although in some cases it may be desired that the base fuel is not a sulphur free (“zero sulphur”) fuel. In a preferred embodiment, a fuel composition which results from carrying out aspects of the present invention will also have a sulphur content falling within these limits.
The diesel fuel composition according to aspects of the present invention may, if desired, contain no, or only low levels of additional cetane improving (ignition improving) additives such as 2-ethylhexyl nitrate (2-EHN). In other words, embodiments of the present invention embrace the use of certain organic nitrates in a diesel fuel composition for the purpose of reducing the level of a second (or further) cetane improving additive in the composition.
Furthermore, a fuel composition prepared according to aspects of the present invention, or a base fuel used in such a composition may contain one or more fuel additives, or may be additive-free. If additives are included (e.g. added to the fuel at the refinery), the composition may contain minor amounts of one or more additives. Selected examples or suitable additives include (but are not limited to): anti-static agents; pipeline drag reducers; flow improvers (e.g. ethylene/vinyl acetate copolymers or acrylate/maleic anhydride copolymers); lubricity enhancing additives (e.g. ester- and acid-based additives); viscosity improving additives or viscosity modifiers (e.g. styrene-based copolymers, zeolites, and high viscosity fuel or oil derivatives); dehazers (e.g. alkoxylated phenol formaldehyde polymers); anti-foaming agents (e.g. polyether-modified polysiloxanes); anti-rust agents (e.g. a propane-1,2-diol semi-ester of tetrapropenyl succinic acid, or polyhydric alcohol esters of a succinic acid derivative); corrosion inhibitors; reodorants; anti-wear additives; antioxidants (e.g. phenolics such as 2,6-di-tert-butylphenol); metal deactivators; combustion improvers; static dissipator additives; cold flow improvers (e.g. glycerol monooleate, di-isodecyl adipate); antioxidants; and wax anti-settling agents. The composition may for example contain a detergent. Detergent-containing diesel fuel additives are known and commercially available. Such additives may be added to diesel fuels at levels intended to reduce, remove or slow the build up of engine deposits. In some embodiments, it may be advantageous for the fuel composition to contain an anti-foaming agent, more preferably in combination with an anti-rust agent and/or a corrosion inhibitor and/or a lubricity enhancing additive.
Where the composition contains such additives (other than the cetane number increasing components of the invention), it preferably contains a minor proportion (such as 1% w/w or less, 0.5% w/w or less, 0.2% w/w or less), of the one or more fuel additives, in addition to the cetane number increasing component(s). Unless otherwise stated, the (active matter) concentration of each such additive component in the fuel composition may be up to 10000 ppmw, such as in the range of 0.1 to 1000 ppmw; and advantageously from 0.1 to 300 ppmw, such as from 0.1 to 150 ppmw.
If desired, one or more additive components, such as those listed above, may be co-mixed (e.g. together with suitable diluent) in an additive concentrate, and the additive concentrate may then be dispersed into a base fuel or fuel composition. In some cases, it may be possible and convenient to incorporate the cetane number increasing component of the invention into such an additive formulation. Thus, the cetane number improving additive may be pre-diluted in one or more such fuel components, prior to its incorporation into the final automotive fuel composition. Such a fuel additive mixture may typically contains a detergent, optionally together with other components as described above, and a diesel fuel-compatible diluent, which may be a mineral oil, a solvent such as those sold by Shell companies under the trade mark “SHELLSOL”, a polar solvent such as an ester and, in particular, an alcohol (e.g. hexanol, 2-ethylhexanol, decanol, isotridecanol and alcohol mixtures such as those sold by Shell companies under the trade mark “LINEVOL”, especially LINEVOL 79 alcohol which is a mixture of C_7-9primary alcohols, or a C_12-14alcohol mixture which is commercially available).
In one embodiment, the total content of the additives in the fuel composition may be between 0 and 10000 ppmw and preferably below 5000 ppmw.
As used herein, amounts (e.g. concentrations, ppmw and % w/w) of components are of active matter, i.e., exclusive of volatile solvents/diluent materials.
In one embodiment, the present invention involves adjusting the cetane number of the fuel composition, using the cetane number enhancing component, in order to achieve a desired target cetane number.
The maximum cetane number of an automotive fuel composition may often be limited by relevant legal and/or commercial specifications, such as the European diesel fuel specification EN 590 that stipulates a cetane number of 51. Thus, typical commercial automotive diesel fuels for use in Europe are currently manufactured to have cetane numbers of around 51. Thus, embodiments of the present invention may involve manipulation of an otherwise standard specification diesel fuel composition, using a cetane number enhancing additive, to increase its cetane number so as to improve the combustability of the fuel, and hence reduce engine emissions and even fuel economy of an engine into which it is, or is intended to be, introduced.
In one embodiment, the cetane number improver increases the cetane number of the fuel composition by at least 3 cetane numbers. In some particular embodiments, the cetane number increase may be up to approximately 9, or any value in between these ranges. Accordingly, in other embodiments, the cetane number of the resultant fuel is between 51 and 60.
In a preferred embodiment, an automotive diesel fuel composition prepared according to aspects of the present invention will comply with applicable current standard specification(s) such as, for example, EN 590 (for Europe) or ASTM D-975 (for the USA). By way of example, the overall fuel composition may have a density from 820 to 845 kg/m³at 15° C. (ASTM D-4052 or EN ISO 3675); a T95 boiling point (ASTM D-86 or EN ISO 3405) of 360° C. or less; a measured cetane number (ASTM D-613) of 51 or greater; a VK 40 (ASTM D-445 or EN ISO 3104) from 2 to 4.5 mm²/s; a sulphur content (ASTM D-2622 or EN ISO 20846) of 50 mg/kg or less; and/or a polycyclic aromatic hydrocarbons (PAH) content (IP 391 (mod)) of less than 11% w/w. Relevant specifications may, however, differ from country to country and from year to year, and may depend on the intended use of the fuel composition.
It will be appreciated, however, that diesel fuel composition prepared according to aspects of the present invention may contain fuel components with properties outside of these ranges, since the properties of an overall blend may differ, often significantly, from those of its individual constituents.

Uses and Methods

In accordance with one aspect of the invention, there is provided the use of an embodiment of the cetane number improver of the invention to achieve a desired cetane number of the resultant fuel composition. In some embodiments, the desired cetane number is achieved or intended to be achieved under a specified set or range of engine working conditions, as described elsewhere herein. Accordingly, an advantage of embodiments of the present invention is that cetane number enhancers of the invention may be suitable for reducing the combustion delay of a fuel composition under all engine running conditions, or under mild, or under harsh engine conditions. Embodiments of the cetane number enhancer of the invention may serve to improve combustion and, hence, improve associated engine factors, such as exhaust emissions and/or engine deposits under a range of engine operating conditions—particularly under harsh engine conditions when fuel emissions might otherwise be expected to increase.
In the context of the present invention, “use” of a cetane number improver in a fuel composition means incorporating the component into the composition, typically as a blend (i.e. a physical mixture) with one or more fuel components (typically diesel base fuels) and optionally with one or more fuel additives.
The cetane number improver is preferably incorporated into the fuel composition before the composition is introduced into an engine which is to be run on the composition. Accordingly, the viscosity increasing component may be dosed directly into (e.g. blended with) one or more components of the fuel composition or the base fuel at the refinery. For instance, it may be pre-diluted in a suitable fuel component, which subsequently forms part of the overall automotive fuel composition. Alternatively, it may be added to a diesel fuel composition downstream of the refinery. For example, it may be added as part of an additive package containing one or more other fuel additives. This can be particularly advantageous because in some circumstances it can be inconvenient or undesirable to modify the fuel composition at the refinery. For example, the blending of base fuel components may not be feasible at all locations, whereas the introduction of fuel additives, at relatively low concentrations, can more readily be achieved at fuel depots or at other filling points such as road tanker, barge or train filling points, dispensers, customer tanks and vehicles.
Accordingly, the “use” of embodiments of the invention may also encompass the supply of a cetane number improver together with instructions for its use in a diesel fuel composition to achieve one of the benefits of the present invention. The cetane number increasing component may therefore be supplied as a component of a formulation which is suitable for and/or intended for use as a fuel additive, in particular a diesel fuel additive. By way of example, the cetane number improver may be incorporated into an additive formulation or package along with one or more other fuel additives. As described above, the one or more fuel additives may be selected from any useful additive, such as detergents, anti-corrosion additives, esters, poly-alpha olefins, long chain organic acids, components containing amine or amide active centres, and any combination thereof, as is known to the person of skill in the art.
According to another aspect of the invention, there is provided a process for the preparation of an automotive fuel composition, which process involves blending a diesel base fuel (or base fuel mixture) with an embodiment of the cetane number improver of the invention. The blending may be carried out for one or more of the purposes described herein.
In some cases the cetane number improver of the invention may not be suitable for pre-mixing with other fuel additives and may, therefore, be dosed directly into the fuel composition from a concentrated (100%) or pre-diluted stock.
In accordance with one embodiment of the present invention, two or more cetane number increasing additives may be used in a diesel fuel composition to provide one or more of the effects of the invention described herein.
For example, embodiments of the present invention can provide an effective way of improving fuel combustion/combustability in an internal combustion engine.
It has surprisingly been found that certain organic nitrate molecules of the invention can, at relatively low concentrations, increase the cetane number of a diesel fuel composition by an amount greater than known organic nitrate cetane enhancers under some engine operating conditions. In particular, embodiments of the cetance number enhancing agents of the invention may be capable of providing greater benefits than some prior art cetane number improvers, particularly under harsh engine working conditions (e.g. high engine speeds and powers).
While the amount of the cetane number increasing component for use in accordance with aspects of the invention may vary depending of fuel type and/or engine working conditions to be used; a further benefit of the invention is that under some engine conditions the amount of cetane number improver needed to observe the benefit of the invention may be surprisingly low, such as at the level of typical fuel additives.
This in turn can reduce the cost and complexity of the fuel preparation process. For example, it can allow a fuel composition to be altered in order to improve fuel combustability, by the incorporation of additives downstream of the refinery, rather than by altering the content of the base fuel at its point of initial preparation. The blending of base fuel components may not be feasible at all locations, whereas the introduction of fuel additives, at relatively low concentrations, can more readily be achieved at fuel depots or at other filling points such as road tanker, barge or train filling points, dispensers, customer tanks and vehicles. This in particular may be achievable where the cetane number improver is sufficiently stable to allow it to be transported under suitable conditions without taking unnecessary safety risks. Of course, in some case it may not be appropriate due to safety factors to transport the cetane number improver.
Moreover, an additive which is to be used at a relatively low concentration can naturally be transported, stored and introduced into a fuel composition more cost effectively than can a fuel component which needs to be used at concentrations of the order of tens of percent by weight.
Another aspect of the invention provides a method of operating an internal combustion engine and/or a vehicle powered by such an engine, which comprises introducing into a combustion chamber of the engine a fuel composition prepared in accordance with aspects of the invention. The fuel composition is advantageously introduced for one or more of the purposes described in connection with aspects of this invention. Thus, the engine is preferably operated with the fuel composition for the purpose of improving ease of fuel ignition during use of the engine (by increasing fuel combustability) and, for example, associated benefits such as reduced engine emissions, engine noise, etc. The engine is in particular a diesel engine, and may be a turbo charged diesel engine. The diesel engine may be of the direct injection type, for example of the rotary pump, in-line pump, unit pump, electronic unit injector or common rail type, or of the indirect injection type. It may be a heavy or a light duty diesel engine. For example, it may be an electronic unit direct injection (EUDI) engine.
Where relevant to a particular assessment, emission levels may be measured using standard testing procedures such as the European R49, ESC, OICA or ETC (for heavy-duty engines) or ECE+EUDC or MVEG (for light-duty engines) test cycles. In a preferred embodiment, emissions performance is measured on a diesel engine built to comply with the Euro II standard emissions limits (1996) or with the Euro III (2000), IV (2005) or even V (2008) standard limits.
Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.
Thus features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the present invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. Thus, features of the “uses” of the invention are directly applicable to the “methods” of the invention. Moreover, unless stated otherwise, any feature disclosed herein may be replaced by an alternative feature serving the same or a similar purpose.
Aspects of the invention will now be further illustrated by way of the following non-limiting examples.

EXAMPLES

Introduction

Various organic nitrates were synthesised and assayed for their potential to act as cetane enhancers in diesel fuels. Cetane enhancers of the invention may provide various benefits, in use, that are associated with increased combustability and a reduction in ignition delay, such as reducing engine noise, reducing build-up of engine deposits, reducing engine emissions, and may even improve fuel economy in some cases.
Organic nitrates of the invention were also tested for thermal stability to obtain useful information on necessary storage and transportation conditions.
Organic nitrate synthesis processes may also help in determining suitability of the organic nitrates for use as cetane enhancers in diesel fuel at a commercial level (e.g. with respect to ease of synthesis and cost of production).

Reagents and Chemicals

In general, standard reagents, chemicals and solvents were purchased from Sigma-Aldrich. Ethyl abietate (70%) was obtained from TCI and ABCR. Methyl linoleate was purchased from TCI. Neodol 23 (C12-C13 alcohol mixture) was obtained from Shell Chemicals.
To form the solution of N₂O₄, gaseous yellow/brown NO₂/N₂O₄(Sigma-Aldrich) was slowly bubbled into the solvent (chloroform or hexane) in a round-bottomed-flask in an ice bath. The dew point of the N₂O₄is around 20° C., so the heavy gas condenses into the cold solvent (at ca. 4° C.). The amount of N₂O₄added was determined by weighing the flask before and after addition of N₂O₄.

General

The following general synthesis methods were used for the preparation of nitrate and nitro compounds of the invention:
Nitric acid, for secondary alcohols
Mixture of nitric acid, sulfuric acid and urea, for primary or secondary alcohols
Mixture of nitric acid and acetic anhydride (precursor of reactive acetyl nitrate) in acetic acid, for unsaturated alcohols
N₂O₄, for olefins
A 500 ml three-necked round-bottomed flask, equipped with a ‘bubbler’, thermometer, a dropping funnel and a magnetic stirring bar was used in all the experiments. Reactions were performed under nitrogen.

Synthesis of Organic Nitrates

Nitration Product of 1-Octanol

A mixture of concentrated nitric acid (45 ml, 0.96 mol), concentrated sulfuric acid (134 ml, 2.35 mol) and urea (5 g, 0.08 mol) were placed in a 500 m three-necked flask and cooled to 0° C. 1-Octanol (48 ml, 0.30 mol) in dichloromethane (30 ml) was added from a dropping funnel for 60 min. The temperature was maintained at 0 to 5° C. with an ice-salt bath. After 2 hours the reaction mixture was poured in an ice bath (quenched) and dichloromethane (100 ml) was added. The organic phase was washed for three times with water, followed by sodium bicarbonate. The organic phase was dried over magnesium sulfate and the solvent was evaporated by a rotorvap (40° C. bath temperature, 5 mbar vacuum). The yield was 43 g (81%).
¹H NMR (CDCl₃, 300 MHz): 4.4 (t, 2H), 1.8-1.6 (m, 2H), 1.4-1.1 (m, 10H), 0.9-0.8 (m, 3H)
¹³C NMR (CDCl₃): 73.7, 32.0, 29.5, 29.4, 27.1, 26.0, 23.0, 14.4

Nitration Product of 1,10-Decanediol

A mixture of concentrated nitric acid (67 ml, 1.44 mol), concentrated sulfuric acid (200 ml, 3.50 mol) and urea (7 g, 0.12 mol) were placed in a 500 ml three-necked flask and cooled to 0° C. 1,10-octanediol (48 ml, 0.30 mol) in dichloromethane (30 ml) was added from a dropping funnel for 60 min. The temperature was maintained at 0 to 5° C. with an ice-salt bath. After 2 hours the reaction mixture was poured in an ice bath (quenched) and dichloromethane (100 ml) was added. The organic phase was washed for three times with water, followed by sodium bicarbonate. The organic phase was dried over magnesium sulfate and the solvent was evaporated by a rotorvap (40° C. bath temperature, 5 mbar vacuum). The yield was 49 g (98%).
¹H NMR (CDCl₃, 300 MHz): 4.4 (t, 4H), 1.8-1.6 (m, 4H), 1.4-1.2 (m, 12H)
¹³C NMR (CDCl₃): 73.7, 29.5, 29.3, 27.0, 25.9

Nitration Product of 1-Tetradecanol

A mixture of concentrated nitric acid (27 ml, 0.58 mol), concentrated sulfuric acid (81 ml, 1.42 mol) and urea (3 g, 0.05 mol) were placed in a 500 ml three-necked flask and cooled to 0° C. 1-Tetradecanol (48 ml, 0.19 mol) in dichloromethane (30 ml) was added from a dropping funnel for 45 min. The temperature was maintained at 0 to 5° C. with an ice-salt bath After 2 hours the reaction mixture was poured in an ice bath (quenched) and dichloromethane (100 ml) was added. The organic phase was washed for three times with water, followed by sodium bicarbonate. The organic phase was dried over magnesium sulfate and the solvent was evaporated by a rotorvap (40° C. bath temperature, 5 mbar vacuum). The yield was 38 g (79%).
¹H NMR (CDCl₃, 300 MHz): 4.4 (t, 2H), 1.8-1.6 (m, 2H), 1.5-1.2 (m, 22H), 0.9-0.8 (m, 3H)
¹³C NMR (CDCl₃): 73.7, 32.3, 30.0, 29.9, 29.8, 29.7, 29.5, 26.0, 23.1, 14.5

Nitration Product of Hexadecyl Alcohol

A mixture of concentrated nitric acid (24 ml, 0.52 mol), concentrated sulfuric acid (72 ml, 1.26 mol) and urea (2 g, 0.04 mol) were placed in a 500 ml three-necked flask and cooled to 0° C. Hexadecyl alcohol (40 g, 0.16 mol) in dichloromethane (30 ml) was added from a dropping funnel for 60 min. The temperature was maintained at 0 to 5° C. with an ice-salt bath. After 2 hours the reaction mixture was poured in an ice bath (quenched) and dichloromethane (100 ml) was added. The organic phase was washed for three times with water, followed by sodium bicarbonate. The organic phase was dried over magnesium sulfate and the solvent was evaporated by a rotorvap (40° C. bath temperature, 5 mbar vacuum). The yield was 39 g (80%).
¹H NMR (CDCl₃, 300 MHz): 4.5 (t, 2H), 1.8-1.6 (m, 2H), 1.4-1.2 (m, 26H), 0.9-0.8 (m, 3H)
¹³C NMR (CDCl₃): 73.6, 32.3, 30.1, 29.8, 27.0, 26.0, 23.1, 14.5

Nitration Product of 1-Octadecanol

A mixture of concentrated nitric acid (16 ml, 0.35 mol), concentrated sulfuric acid (48 ml, 0.85 mol) and urea (2 g, 0.03 mol) were placed in a 500 ml three-necked flask and cooled to 0° C. 1-Octadecanol (36 g, 0.11 mol) in dichloromethane (30 ml) was added from a dropping funnel for 60 min. The temperature was maintained at 0 to 5° C. with an ice-salt bath. After 2 hours the reaction mixture was poured in an ice bath (quenched) and dichloromethane (100 ml) was added. The organic phase was washed for three times with water, followed by sodium bicarbonate. The organic phase was dried over magnesium sulfate and the solvent was evaporated by a rotorvap (40° C. bath temperature, 5 mbar vacuum). The yield was 28 g (80%).
¹H NMR (CDCl₃, 300 MHz): 4.5 (t, 2H), 1.8-1.6 (m, 2H), 1.4-1.2 (m, 30H), 0.9-0.8 (m, 3H)
¹³C NMR (CDCl₃): 73.7, 32.3, 30.1, 30.0, 29.9, 29.7, 29.5, 27.1, 26.0, 23.1, 14.5

Nitration Product of Neodol 23

A mixture of concentrated nitric acid (31 ml, 0.66 mol), concentrated sulfuric acid (92 ml, 1.61 mol) and urea (3 g, 0.05 mol) were placed in a 500 ml three-necked flask and cooled to 0° C. Neodol 23 (48 ml, 0.21 mol) in dichloromethane (30 ml) was added from a dropping funnel for 60 min. The temperature was maintained at 0 to 5° C. with an ice-salt bath. After 2 hours the reaction mixture was poured in an ice bath (quenched) and dichloromethane (100 ml) was added. The organic phase was washed for three times with water, followed by sodium bicarbonate. The organic phase was dried over magnesium sulfate and the solvent was evaporated by a rotorvap (40° C. bath temperature, 5 mbar vacuum). A mixture of dodecyl and tridecyl nitrates was obtained with a yield of 45 g (93%).
¹H NMR (CDCl₃, 300 MHz): 4.5 (t, 2H), 1.8-1.6 (m, 2H), 1.4-1.2 (m, 16H), 0.9-0.8 (m, 3H)
¹³C NMR (CDCl₃): 73.7, 32.3, 30.0, 29.9, 29.7, 29.5, 27.1, 26.0, 23.1, 14.5

Nitration Product of Exo-Borneol

In a 500 ml three neck-flask filled with nitric acid (150.21 ml, 3.24 mol) was added exo-borneol (50 g, 0.32 mol) slowly (3.5 hours) at room temperature. After 2 hours the reaction mixture was poured in an ice bath (quenched) and diethyl ether (200 ml) was added. The organic phase was washed for three times with water, followed by sodium bicarbonate. The organic phase was dried over magnesium sulfate and was the solvent was evaporated by a rotorvap, 40° C. bath temperature and ±5 mbar vacuum. The yield was 57 g (87%).
¹H NMR (CDCl₃, 300 MHz): 4.8 (t, 1H), 2.0-1.9 (m, 2H), 1.8-1.6 (m, 3H), 1.3-1.1 (m, 2H), 1.0-0.9 (m, 6H), 0.8 (s, 3H)
¹³C NMR (CDCl₃): 90.5, 49.9, 47.4, 45.1 38.3, 34.5, 27.2, 20.2, 11.5

Nitration Product of Exo-Fenchol

In a 500 ml three neck-flask was filled L-Fenchone (40 g, 0.26 mol), aluminium isopropoxide (2 g) and isopropyl alcohol (300 ml). This mixture was refluxed (during reflux the acetone was evaporated and filled with isopropyl alcohol) for 168 hours, extracted with diethyl ether, dried and evaporated. 25 g exolendo-fenchol in ratio 3:1 was isolated.
In a 500 ml three neck-flask filled with nitric acid (75.10 ml, 1.62 mol) was added the exo/endofenchol mixture (25 g, 0.16 mol) slowly (2 hours) at room temperature. After 2 hours the reaction mixture was poured in an ice bath (quenched) and diethyl ether (100 ml) was added. The organic phase was washed for three times with water, followed by sodium bicarbonate. The organic phase was dried over magnesium sulfate and the solvent was evaporated using a rotorvap, 40° C. bath temperature and ±5 mbar vacuum. The yield was 26 g (82%).
¹H NMR (CDCl₃, 300 MHz): 4.8-4.6 (m, 1H), 4.2-4.2 (m, 1H), ratio 1:0.47
¹³C NMR (CDCl₃): 97.2, 88.2

Nitration Product of L-Menthol

A mixture of concentrated nitric acid (15.79 ml, 0.34 mol), concentrated sulphuric acid (47.37 ml, 0.83 mol) and urea (1.62 g, 0.03 mol) were placed in a 500 ml three necked flask and cooled to 0° C. L-menthol (17 g, 0.11 mol) in diethyl ether (20 ml) was added from a dropping funnel for 60 min. The temperature was maintained at 0 to 5° C. with an ice-salt bath. After 2 hours the reaction mixture was poured in an ice bath (quenched) and diethyl ether (100 ml) was added. The organic phase was washed for three times with water, followed by sodium bicarbonate. The organic phase was dried over magnesium sulfate and was the solvent was evaporated by a rotorvap, 40° C. bath temperature and ±5 mbar vacuum. The yield was 41 g (95%).
¹H NMR (CDCl₃, 300 MHz): 4.9-4.8 (m, 1H), 2.2-1.9 (m, 2H), 1.8-1.7 (m, 2H), 1.6-1.4 (m, 2H), 1.2-1.0
(m, 2H), 1.0-0.9 (m, 6H), 0.9-0.8 (m, 3H)
¹³C NMR (CDCl₃): 84.4, 45.8, 39.7, 34.2, 31.8, 26.5, 24.2, 22.2, 20.8, 16.8

Nitration Product of Oleyl Alcohol

To a mixture of acetic anhydride (80 mL), acetic acid (80 ml) and oleyl alcohol (40 g) at 15° C., was added nitric acid (10 ml) drop wise (60 min). After 30 min the reaction mixture was poured in an ice bath (quenched) and diethyl ether (100 ml) was added. The organic phase was washed for three times with water, followed by sodium bicarbonate. The organic phase was dried over magnesium sulfate and was the solvent was evaporated by a rotorvap, 40° C. bath temperature and ±5 mbar vacuum. The yield was 34 g (74%).
¹H NMR (CDCl₃, 300 MHz): 5.4-5.2 (m, 2H), 4.4 (t, 2H), 2.1-1.9 (m, 4H), 1.4-1.2 (24H), 1.0-0.8 (m, 3H)
¹³C NMR (CDCl₃): 130.2, 129.8, 73.7, 32.3, 30.1, 30.0, 29.9, 29.7, 29.6, 29.5, 29.4, 27.6, 27.5, 27.1, 26.0, 23.1, 14.5

Nitration Product of Cholesterol

To a mixture of cholesterol (35 g) in chloroform (10 ml) and acetic anhydride (90 ml) in chloroform (10 ml) at 15° C., was added drop wise (60 min) a mixture of nitric acid (12.6 ml) in acetic acid (45 ml). After 60 min the reaction mixture was poured in an ice bath (quenched) and diethyl ether (100 ml) was added. The organic phase was washed for three times with water, followed by sodium bicarbonate. The organic phase was dried over magnesium sulfate and was the solvent was evaporated by a rotorvap, 40° C. bath temperature and ±5 mbar vacuum. The yield was 24 g (63%). Cholesterol nitrate is a solid at RT.
¹H NMR (CDCl₃, 300 MHz): 5.5-5.4 (m, 1H), 4.9-4.7 (m, 1H), 2.6-2.3 (m, 2H), 2.1-1.1 (m, 25H), 1.1-0.9 (m, 15H), 0.8-0.6 (m, 3H)
¹³C NMR (CDCl₃): 138.3, 122.3, 83.5, 57.6, 56.5, 50.2, 42.5, 40.2, 40.1, 37.8, 37.5, 31.8, 29.5, 26.8, 23.4, 18.5, 17.5, 11.2

Nitration Product of Methyl Oleate

To a solution of N₂O₄(13.96 g, 0.15 mol) in chloroform (20 ml) at 0° C. was added drop wise (3 hours) methyl oleate (30 g, 34.48 ml). The reaction mixture was stirred for 48 hours with a low nitrogen flow. The reaction mixture was poured in an ice bath (quenched) and diethyl ether (50 ml) was added. The organic phase was washed for three times with water. The organic phase was dried over magnesium sulfate and was the solvent was evaporated by a rotorvap, 40° C. bath temperature and ±5 mbar vacuum. The yield was 31.2 g.

Nitration Product of Ethyl Abietate

To a solution of N₂O₄(12.75 g, 0.14 mol) in hexane (20 ml) at 0° C. was added drop wise (2 hours) ethyl abietate (30 g, 34.48 ml). The reaction mixture was stirred for 48 hours with a low nitrogen flow. The reaction mixture was poured in an ice bath (quenched) and diethyl ether (50 ml) was added. The organic phase was washed for three times with water. The organic phase was dried over magnesium sulfate and was the solvent was evaporated by a rotorvap, 40° C. bath temperature and ±5 mbar vacuum. The yield was 33.2 g. The products of the reaction are solid at RT.

Analytical Methods

¹H and ¹³C NMR spectra were recorded on a Varian Mercury 300 MHz or a Varian Inova 400 MHz system. All NMR samples were measured in CDCl₃.
Infrared spectra were measured on a Nicolet 6700 FT-IR Spectrometer (Fourier transform infrared spectroscopy) from Thermo Scientific.
Gas chromatography-mass spectrometry (GC-MS) analyses were performed on a Trace GC Ultra chromatograph from Interscience equipped with a 50 m×0.2 mm×0.5 μm RTX-1 PONA column and an DSQII mass-selective EI detector. The following temperature profile was used in the GC-MS for measuring the components. The oven started at 35° C. for the first 5 min, then increased with 10° C./min to 300° C. followed by a hold time of 10 min at 300° C. DSC/TGA was measured on a STA 409 PC (Simultaneous Thermal Analysis) with QMS 403 C (quadrupole mass spectrometers) from NETZSCH.

Testing of Blended Fuels

The organic nitrate (or nitro) cetane number enhancing agents were performance tested in diesel fuel blends. The assessment of their ability to increase the cetane number of the fuel was carried out indirectly by measuring changes in ignition delay (ID) of the blended fuels using a combustion research unit (CRU). The CRU is operated to determine ignition quality in a similar fashion to the ignition quality tester (IQT; International Standard EN 15195:2007:E) known to the person of skill in the art, i.e. using a heated, pressurised constant volume chamber. Typically, the cetane number is given as a dimensionless number, which describes the ignition behaviour of a fuel in comparison to primary reference fuels (PRFs) with a defined cetane number. PRFs are bimodal mixtures of n-hexadecane (or “cetane”; CN=100) and heptamethylnonane (CN=15).

Fuel Blending

All test compounds were blended into an EN590 compliant diesel (European specification), zero-sulfur fuel (i.e. ZSD base-fuel from Stanlow refinery; density 837.4 kg/m³at 15° C.; viscosity 2.89 mm²/s at 40° C.) at concentrations of 0.1% w/w and 1.0% w/w. Some test compounds were also blended at concentrations of 0.05% w/w and 0.5% w/w. The mixtures were stirred at RT for 1 hour, after which all solid compounds were dissolved in the base fuel.
For purposes of comparison, similar fuel blends containing 2-EHN were prepared.

Combustion Research Unit (CRU) Testing

In CRU tests, diesel-like fuel is injected into a high temperature, high pressure chamber where it mixes with the hot air and ignites, thus mimicking combustion in a compression-ignition engine. The combustion process is monitored via a pressure sensor inside the chamber.
The CRU delivers p-t-charts of the ignition process from which the ignition delay, the burn rate and the maximum pressure increase (MPI) can be determined. A comparison of blended fuels with standard fuels demonstrates changes in ignition delay/combustability of the fuels, and may also allow determination of cetane numbers under different operating conditions.
The engine conditions selected for CRU tests are designed to simulate a wide range of engine operating conditions so as to assess the cetane enhancers under mild, intermediate and harsh conditions; e.g. temperature and pressure are both varied from low to high. This allows the temperature and pressure dependence of ignition delay to be demonstrated. Thus, the slope of the isothermal or isobaric charts plotted from the data obtained from these tests provide direct information about how the response of the fuel blend changes from mild to harsh conditions.
Each of the fuel blends (at each concentration of cetane enhancer used) and the base fuels were tested for ignition quality on the CRU under the 11 different sets of parameters (engine operating conditions) illustrated in Table 1.

TABLE 1

Conditions 01 to 11 used for CRU measurements of fuel blends.

						Delay
						Pre-
		p_Chamb/				Main
		bar		Pre-Inj.	Main Inj.	(usec)
Working	T_Wall/° C.	range	p_Fuel/bar	Period/	Period/usec	range	EGR/%
point	range 350-590	10-75	range 200-1600	usec 0-1400	range 100-1500	100-3000	range 0-100	No. Injections

01	590	30	900	0	900	100	0	10
02	590	50	900	0	900	100	0	10
03	590	75	900	0	900	100	0	10
04	560	30	900	0	900	100	0	10
05	560	50	900	0	900	100	0	10
06	560	75	900	0	900	100	0	10
07	530	30	900	0	900	100	0	10
08	570	21.4	200	0	1500	100	0	10
09	530	75	900	0	900	100	0	10
10	530	50	900	0	900	100	0	10
11	590	65	1600	0	1500	100	0	10

Ignition Delay

The primary data obtained from CRU measurements are pressure-time traces, from which the ignition delay (ID), burn period (BP) and maximum pressure increase (MPI) can be determined.
There are different definitions for ignition delay. Typically, ID^5%or ID^0.2measurements are used, which are defined as the time taken for the pressure in the combustion chamber to rise to its initial value plus 5% of the MPI, or to 0.2 bar above its initial value, respectively. In these studies both values are used, although ID^0.2is preferred due to the lower standard deviations observed.
A derived ignition quality (DIQ) is obtained from the ignition delay via comparison with primary reference fuels (PRFs), which have a known cetane number. Since ignition delay correlates with the cetane number of the fuel, a calibration model can be obtained for all conditions. With such calibrations, DIQ can be determined for any unknown fuel and derived as a pseudo-cetane number.
The PRFs used in this study are shown in Table 2.

TABLE 2

PRFs used to determine the ID-DIQ calibration models.

PRF	Cetane	Heptamethylnonane	CN
number	(Vol %)	(Vol %)	(by definition)

1	40.0	60.0	49
2	100.0	0.0	100
3	64.7	35.3	70
4	88.2	11.8	90
5	52.9	47.1	60
6	76.5	23.5	80
7	35.3	64.7	45

The effect of the different organic nitrates in fuel blends was compared by measuring ignition delay and calculating the percentage ID-reduction relative to the respective base fuel. The results for exemplary combustion conditions 03, 05, 07 and 08 are illustrated in FIG. 2.
The graphs show the data obtained at organic nitrate concentrations of 1.0% w/w (left, graphs A, C, E and G) and 0.1% w/w (right, graphs B, D, F and H) at representative combustion conditions (first row, condition a03, graphs A and B; second row, condition a05, graphs C and D; third row, condition a07, graphs E and F; and fourth row, condition a08, graphs G and H). Data for organic nitrates is as follows: exo bornyl nitrate (column 1); menthly nitrate (column 2); oleyl nitrate (column 3); 1,10-decyl dinitrate (column 4); 1-octadecyl nitrate (column 5); nitro-substituted methyl oleate (column 6); cholesterol nitrate (column 7); 1-octyl nitrate (column 8); 1-tetradecyl nitrate (column 9); 2-ethylhexyl nitrate (positive control, column 10); 1-hexadecyl nitrate (column 11); dodecyl/tridecyl nitrate mixture (column 12); nitro-substituted ethyl abietate (column 13); and exo fenchyl nitrate (column 14)
These data show that all of the organic nitrates provided a benefit compared to the base fuel by achieving a shorter ignition delay. In general, menthyl nitrate achieved a greater reduction in ignition delay than the known cetane enhancer, 2-EHN, under all test conditions, and was the most effective combustion enhancer tested. Under some test conditions exo-fenchyl nitrate or 1,10-decyl dinitrate gave the greatest reductions in ignition delay. In general, 1,10-decyl dinitrate, neodo123 nitrate (mixture of dodecyl and tridecyl nitrates), and 1-octyl nitrate achieved similar reductions in ignition delay to 2-EHN under all test conditions. Some organic nitrates displayed a concentration-dependent effect. For example, exo-fenchyl nitrate was generally the most effective cetane enhancer at low concentrations (e.g. 0.1% w/), while it was generally slightly less effective than menthyl nitrate at higher concentrations (e.g. 1.0% w/w). Furthermore, the effect was most pronounced under the milder test conditions. Interestingly, the fatty acid derived nitrates displayed slight reductions in effectiveness as chain length increased. Of the saturated and unsaturated nitrates of the same carbon-chain length, it appears that the saturated nitrates provide a stronger cetane boost than unsaturated molecules. The results also suggest that the nitrated molecules are more effective cetane enhancers than the nitro molecules.

Dose Rate Response

The effectiveness of the terpene nitrates, exo-bornyl nitrate and L-menthyl nitrate, were tested at different dose levels (i.e. 0.05% w/w, 0.1% w/w, 0.5% w/w and 1.0% w/w). The results (not shown) illustrate that there is a saturation effect for cetane enhancement. Thus, at the low concentrations tested (0.05% w/w and 0.1% w/w) there was a clear concentration-dependent reduction in ignition delay. However, a relatively small additional reduction in ignition delay was observed in the concentration range of 0.1% w/w to 0.5% w/w and 1.0% w/w. This saturation effect was found under all combustion conditions tested.
Importantly, therefore, the effective additives can be used to good effect at low concentrations, which is useful in many respects, such as production level, storage, cost, handling and safety.

Derived Ignition Quality (DIQ)

The derived ignition quality (DIQ) was determined for each of the organic nitrates at concentrations of 0.1% w/w and 1.0% w/w.
By way of example, the DIQ for the diesel base fuel comprising exo-fenchyl nitrate at different concentrations (i.e. 0.1% w/w and 1.0% w/w) was determined relative to the non-modified base fuel composition and relative to blends with the known cetane enhancer, 2-EHN. Studies were carried out under all 11 sets of combustion conditions and the results are shown in FIG. 3. The left graph shows the results of a base fuel containing exo-fenchol nitrate at concentration of 0.1% w/w, and the right graph shows the results for concentration of 1.0% w/w, measured under all test combustion conditions, a01 to a08 (columns 1 to 8, respectively) and a09 to all (columns 10 to 12, respectively). In each graph, for comparative purposes the DIQ of the diesel base fuel under reaction condition a08 is illustrated (column 9);
By way of example, these data show that at IQT-conditions 08, as specified in cetane number measurement according to EN15195 (Oxley et al. (2000) Energy Fuels, 14, 1252-1264), the 0.1% w/w exo-fenchyl nitrate fuel blend achieved a DIQ of 52.2, while the 1.0% w/w exo-fenchyl nitrate fuel blend had a DIQ of 63.7. This represents a significant increase on the DIQ of 44.1 for the base fuel composition alone. These results thus show a cetane number enhancement of 7.1 at 1000 ppm, which is similar to that expected for 2-EHN.

Isothermal and Isobaric Response

The isothermal and isobaric responses of the fuel blends were measured and compared to see the effect of temperature and pressure on ignition delay for each organic nitrate. In particular, the experimental design that was chosen for CRU measurements allowed for isobaric and isothermal comparison of the additives, which provided an opportunity to follow the response of additives as combustion conditions changed from mild to harsh.
The data, not illustrated, demonstrated that in general all additives showed the same response pattern irrespective of their specific performance as ignition enhancers. Higher temperature and higher pressure (harsh conditions) resulted in reduced ignition delays (i.e. each additive demonstrated a parallel down-shift compared to the base fuel as pressure and/or temperature increased. Thus, under combustion condition 11 (maximum power), typically, the ignition delay was the shortest. This may be due to faster injection rate and mixing. However, it was found that the relative difference in ignition delay in comparison to the base fuel increased with reducing temperature. In other words, relative to base fuel the reduction in ignition delay is greater at mild conditions.

Thermal Stability

Differential scanning calorimetry/thermogravimetric analysis (DSC/TGA) was used to evaluate the thermal stabilities of the nitrates and other compounds prepared in the study; and also to provide information on the decomposition mechanism from mass spectrometry (MS) of the volatile products.
The experiments were carried out under argon flow, with a temperature profile of 5° C./min up to 200-350° C. Assays were carried out under atmospheric pressure and, hence, the thermal degradation of products that evaporate prior to thermal decomposition could not be studied (e.g. 1-octyl nitrate).
An exemplary DSC/TGA assay trace for 1,10-decyl dinitrate is illustrated in FIG. 5. As illustrated, the product begins to decompose at a temperature of approximately 170° C. The line beginning in the top left of the graph illustrates the weight loss of the product on decomposition. The two traces showing sharp peaks towards the right hand side of the graph represent the exotherm (right-hand peak: maximum at about 200° C.) and the concentration of the decomposition product (left-hand peak).
Typically several ions are observed in MS studies of the organic nitrates under thermal decomposition (e.g. M (or M/2)=12, 14, 18, 28, 30, 44, 46 and 56).
Without wishing to be bound by theory, the rate-determining step in the decomposition of primary and secondary nitrates is thought to be the homolysis of the RO—NO₂bond to give the alkoxyl radical and NO₂. This may be followed by one or more competing reactions of the alkoxyl radical, e.g.: 13-cleavage of the alkoxyl radical to liberate formaldehyde (CH₂O) and form an alkyl radical, which may undergo β-cleavage or other reaction; loss of a hydrogen radical (via the alternative-cleavage) to give an anhydride; and hydrogen abstraction from a suitable hydrogen donor (e.g. PhCH₂R) to give the alcohol.
The degradation mechanism and product distribution for a particular nitrate depends on whether it occurs in the liquid or gas phase and the presence of other components. Without being bound by theory, it is expected that the compound 1,10-decyl dinitrate would decompose in a similar fashion to the mononitrates, except perhaps, more quickly as the two nitrate functionalities are effectively isolated.
The DSC/TGA method was initially used to confirm that product samples were sufficiently stable to allow safe transportation and storage. However, the methods also provide detailed information on the thermal degradation of potential cetane enhancers, including on the mechanism of decomposition, at least for those products that do not (partially) evaporate below the degradation temperature (linear organic nitrates having less than 14 carbon atoms may require a modified method).
The DSC method provides some qualitative information on the relative stability of different cetane enhancers, but, as it involves a temperature scan, it does not directly reflect the relative thermal stability at any particular temperature. Hence, the stability order derived from the temperature of exotherm maximum may differ from that determined at a particular temperature in kinetic experiments if the activation energy of the compounds differs. Components exhibiting a broad exotherm would be expected to have a lower activation energy than those with a narrow exotherm.
Methods are known for determining kinetic parameters from single scan DSC heat flow data, from the dependence of the temperature of maximum exotherm on the heating rate, and using isothermal DSC methods. For example, the activation energy and pre-exponential factor for the decomposition of di-t-butylperoxide have been determined from DSC. It should, in principle, be possible (e.g. using adiabatic measurements) to extend these methods for the determination of Arrhenius kinetic constants to cetane enhancers based on organic nitrates and related compounds.
The identification of more stable cetane enhancers is an attractive prospect as it may then be possible to transport and supply such cetane enhancers in more concentrated stocks than is possible for the current commonly used cetane enhancers, such as 2-EHN. This may then provide a number of advantages over existing formulations, such as by reducing the volume of chemicals that need to be added to fuels and, thus, transported and stored. It may also reduce or eliminate the need for some specialist storage, transportation and/or handling equipment to avoid combustion hazards. While the organic nitrates of the invention in general are more stable than some known cetane enhancers (e.g. Di-tert-butylperoxide, DTBP), tetradecyl nitrate is a particularly attractive cetane enhancer of the invention that appears to be more stable than 2-EHN.

Adiabatic Thermal Stability Measurements

To obtain a quantitative measurement of the thermal stability of some cetane enhancers adiabatic heat-wait search (HWS) experiments were carried out using a Phi-Tech calorimeter, which gives an accurate determination of kinetic and thermodynamic parameters of the molecules decomposition reaction.
The Phi-Tec equipment is usually operated in a heat-wait-search mode, which means that the temperature of the reactor is increased stepwise until the decomposition onset temperature (DOT) of the reaction is detected. Onset temperatures are graphically derived by searching for the temperature at which deviation from linear temperature rise takes place due to self-heating of the sample. The apparatus than automatically tracks the runaway exotherm to 490° C. maximum (at short exposure time; otherwise 400° C.). The decomposition onset is detected when the self heating rate exceeds 0.02° C./min.
Typical Phi-Tec reaction conditions:
Intake sample: ca. 66 g;
Test cell material: SS-316 (STRCA type, 1*⅛″ tube connection);
Void cell volume: ca. 110 ml;
Start temperature: ca 20° C. below T_onset(ex thermal screening/TSu);
Detection limit: 0.02° C./min;

Step: 5 to 20° C.;

Maximum search temperature: 270° C.;
Maximum track temperature: 300° C.;
Maximum pressure: 80 bara.
The organic nitrates tetradecyl nitrate (TDN), menthyl nitrate (MN)— both of the invention, 2-Ethylhexylnitrate (2-EHN; Aldrich), and di-tert-butylperoxide (DTBP; Merck)—known in the prior art were assessed in consecutive experiments. Cetane enhancers were diluted to approx. 15% w/w in toluene for thermal measurements. Tetradecyl nitrate was also blended at approx. 27% w/w for PhiTec measurements due to its high molecular weight. Experiments with DTBP were conducted in an inert atmosphere (under N₂), which is expected to increase the onset temperature compared to an oxygen-containing atmosphere. Other experiments were carried out under air to mimic typical conditions in use. The data obtained from Phi-Tec measurements are summarised in Table 3.
The actual start temperature at which the Phi-Tec switched to adiabatic tracking mode (i.e. first detection of decomposition onset temperature, DOT) is the temperature of the sample at which the self-heating rate exceeds 0.02° C./min. From the final tracking temperature, the adiabatic temperature rise can be calculated by multiplication with Phi, the heat distribution factor for the reaction (as is known by the skilled person in the art). The variable heat capacity (Cp) for the novel nitrates is assumed to be 2.54 J/gK.
As indicated in Table 3, in these experiments MN was found to be slightly less stable than 2-EHN, which correlates with its effectiveness as a cetane enhancer described above. Overall, TDN was found to be the most stable molecule, such that the stability of the compounds was in the order DTBP<<MN<EHN<<TDN.
Since the maximum temperature rise is within the Phi-Tec operating window (i.e. below 400° C.), 1^Storder kinetic data can be obtained from the exothermic decomposition reaction by plotting in an Arrhenius plot ln((dT/dt)/60)/(T−T_max)) against 1/T. The frequency factor k (intercept) and activation energy Ea (slope) can thus be derived. It is reasonable to assume the decomposition reaction as 1^storder, as it is solely dependent on the concentration of the nitrate or peroxide respectively, which approach has been documented by other investigators (Oxley et al. (2000) Energy & Fuels, 14, 1252).

TABLE 3

Kinetic and thermodynamic data derived from Phi-Tec HWS experiments

					Phi-tec
	Conc.			Ea	Onset
Compound	(w %)	Atmos.	Phi	(kJ/mol)	Temp. (° C.)

DTBP	15.0	N₂	1.10	160.2	111
DTBP	14.6	N₂	1.10	162.0	110
DTBP	14.9	N₂	1.10	157.6	110
DTBP	14.9	N₂	1.10	158.4	112
2-EHN	15.2	air	1.10	178.2	132
2-EHN	14.9	air	1.10	177.5	135
TDN	15.1	air	1.10	190.3	142
TDN	27.2	air	1.10	179.1	136
TDN	27.3	air	1.11	179.5	137
MN	14.9	air	1.10	171.1	123
MN	14.9	air	1.10	171.1	127

As illustrated in the data, the ranking described for thermal stability of the compounds is also reflected in the activation energies (Ea), which indicates that compounds with lower reaction onset temperature also exhibit lower activation energy. The reaction enthalpies (not shown) for the respective compounds correlate with the adiabatic temperature rise. DTBP demonstrated the lowest reaction enthalpy and the lowest temperature rise, whereas the organic nitrates compared well, with both the reaction enthalpy and the adiabatic temperature rise are in the same order of magnitude.

CONCLUSIONS

The synthesis of possible alternative embodiments of cetane enhancers to the commonly used 2-ethylhexylnitrate (2-EHN) was investigated, with the focus on the use of renewable feedstocks. There are some safety concerns surrounding the production, transport and use of this current compound. Furthermore, 2-EHN functions most effectively under mild engine conditions and a cetane enhancer that also works well under harsher engine conditions is desirable.
Potential cetane enhancers were prepared via the nitration of various bio-feedstocks: terpene alcohols—borneol, fenchol and menthol (and pinene); fatty alcohols; unsaturated FAME's; 1,10-Decandiol, ultimately derived from ricinoleic acid (castor oil); and ethyl abietate, a resin ester derived from tall oil.
Following determination of the optimum experimental conditions for nitration in small-scale experiments, different nitrated products were prepared in 25-50 g scale. The majority of the samples were well-defined nitrates (R—ONO₂), prepared from alcohol precursor by reaction with nitric acid, optionally in combination with other reagents. Treatment of the two olefin feedstocks with dinitrogen tetroxide (N₂O₄) led to mixtures of compounds. All samples were characterised by NMR and IR spectroscopy. DSC/TGA analyses was used to provide information on the exothermic decomposition of the products. As expected, most nitrates underwent exothermic decomposition in a narrow temperature range, around 210° C., whereas products derived from olefins decomposed over a much broader temperature range.
Following confirmation (from DSC/TGA and NMR monitoring of storage stability) that the samples were sufficiently stable to transport, products were shipped for evaluation of their effectiveness as cetane enhancers in fuel compositions at a Combustion Research Unit.
A thermal screening and calorimetric assessment of the organic nitrates for use as cetane enhancers has been conducted for comparison with cetane enhancers known in the art (e.g. 2-EHN and DTBP).
Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims.

Claims

1. A diesel fuel composition for use in a compression ignition engine, said diesel fuel composition comprises an organic nitrate selected from the group consisting of:

a cyclic nitrate of Formula (4):

and Formula (4A):

wherein each of R₁to R₉is independently selected from H or C₁-C₆alkyl, or nitrate(—ONO₂), wherein optionally one of R₄and R₅forms an optionally substituted alkylene bridge with one of R₈and R₉, which may be substituted by one or more C₁-C₆alkyl, and/or nitrate(—ONO₂); wherein at least one of R₁to R₉is not H, and provided that no more than one R₂to R₉comprises a nitrate group.

2. The diesel fuel composition of claim 1, wherein at least one of R₁to R₉is selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, and tert-butyl.

3. The diesel fuel composition of claim 1, wherein one of R₄and R₅is methyl and one of R₈and R₉is isopropyl.

4. The diesel fuel composition of claim 1, wherein one of R₄and R₅forms an optionally substituted alkylene bridge with one of R₈and R₉; and wherein the alkylene bridge has the formula —(CR_aR_b)_n—, wherein R_aand R_bare independently selected from H, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl and tert-butyl; and n is 1 or 2.

5. The diesel fuel composition of claim 4, wherein the cetane number improver is defined by Formula (5):

or Formula (5A):

wherein (i) at least two of R_a, R_b, R₂, R₃and R₉are methyl; (ii) R_aand R_bare methyl; (iii) R₂and R₃are methyl; (iv) R_a, R_band R₉are methyl; or (iv) R₂, R₃and R₉are methyl.

6. The diesel fuel composition of claim 1, wherein the cetane number of the diesel fuel composition containing the organic nitrate is higher than the cetane number of the diesel fuel composition lacking the organic nitrate.

7. The diesel fuel composition of claim 1, wherein said diesel fuel composition has a cetane number of between 52 and 58.

8. The diesel fuel composition of claim 1, wherein the diesel fuel composition comprises one or more additional organic nitrate.

9. A method for reducing the ignition delay and/or increasing the cetane number of a diesel fuel composition, said method comprises adding to the composition an amount of an organic nitrate, wherein the organic nitrate is selected from the group consisting of:

a cyclic nitrate of Formula (4):

and Formula (4A):

10. The method of claim 9, wherein at least one of R₁to R₉is selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, and tert-butyl.

11. The method of claim 9, wherein one of R₄and R₅is methyl and one of R₈and R₉is isopropyl.

12. The method of claim 9, wherein the cetane number improver is defined by Formula (5):

or Formula (5A):

wherein (i) at least two of R_a, R_b, R₂, R₃and R₉are methyl; (ii) R_aand R_bare methyl; (iii) R₂and R₃are methyl; (iv) R_a, R_band R₉are methyl; (iii) R₂, R₃and R₉are methyl.

13. The method of claim 9, wherein said method is configured to increase the cetane number of the diesel fuel composition to achieve a target cetane number.

14. The method of claim 9 further comprising adding one or more additional organic nitrate to the fuel composition.

15. The method of claim 9, wherein said method is configured to reduce the amount of 2-ethylhexyl nitrate (2-EHN) in the diesel fuel composition to achieve the target cetane number.

16. The diesel fuel composition of claim 1, wherein the organic nitrate is present in the diesel fuel composition at a concentration of:

(a) between 0.025% and 2.0% w/w;

(b) between 0.05% and 1.0% w/w; or

(c) 0.05% w/w, 0.1% w/w, 0.5% w/w or 1.0% w/w;

based on the total weight of the fuel composition.

17. The diesel fuel composition of claim 1, wherein the organic nitrate is further defined as follows:

(i) R₁, R₆and R₇are H;

(ii) R₁, R₆and R₇are H; R₂and R₃are methyl;

(iii) R₁, R₆and R₇are H; R₅is H and R₉is methyl; or

(iv) R₁, R₆and R₇are H; R₂and R₃are methyl; R₅is H and R₉is methyl.

18. The diesel fuel composition of claim 1, wherein the organic nitrate is selected from the group consisting of: bornyl nitrate, fenchyl nitrate, and menthly nitrate.