WO1993024224A1

WO1993024224A1 - Treatment of hydrogenation catalysts

Info

Publication number: WO1993024224A1
Application number: PCT/GB1993/001020
Authority: WO
Inventors: Samuel David Jackson; Jenny O'kell; Peter Paul Clayton
Original assignee: Imperial Chemical Industries Plc
Priority date: 1992-05-28
Filing date: 1993-05-19
Publication date: 1993-12-09

Abstract

A process for the treatment of a hydrogenation catalyst, in particular a carbon supported metal catalyst which comprises contacting the catalyst with an atmosphere comprising an oxygen-containing oxidising agent, in particular oxygen or nitrous oxide and a process for the production of hydrohalofluorocarbons and/or hydrofluorocarbons which comprises contacting a halofluorocarbon or hydrohalofluorocarbon with hydrogen at elevated temperature in the presence of a hydrogenation catalyst which has been treated with an atmosphere comprising an oxygen containing oxidising agent. The catalyst may be a fresh catalyst or one which has been deactivated, particularly by use in the hydrogenolysis of a halofluorocarbon or hydrohalofluorocarbon.

Description

TREATMENT OF HYDROGENATION CATALYSTS.

This invention relates to a process for the treatment of hydrogenation catalysts, in particular carbon supported hydrogenation catalysts which are useful, for example, in the hydrogenolysis (the replacement of halogen by hydrogen in saturated molecules) of chlorofluorocarbons and hydrochlorofluorocarbons and in the hydrogenation (addition of hydrogen across ethylenic double-bonds ) of halogenated hydrocarbons, (hereafter referred to collectively as "hydrogenation") . Hydrogenation catalysts are used, for example, in the hydrogenation of dichlorodifluoromethane and chlorodifluoromethane and in the hydrogenation of dichlorotetrafluoroethane and chlorotetrafluoroethane .

In recent years there has been increasing international concern that chlorofluorocarbons , which are used on a large scale around the world, may be damaging the earth's protective ozone layer and there is now in place international agreement to ensure that their manufacture and use is restricted and eventually completely phased out. Chlorofluorocarbons are used, for example, as refrigerants, as foam blowing agents, as cleaning solvents and as propellents for aerosol sprays in which the variety of applications is virtually unlimited. Consequently, much effort is being devoted to finding suitable replacements for chlorofluorocarbons which will perform satisfactorily in the many applications in which chlorofluorocarbons _are used but which will not have the aforementioned damaging effect on the ozone layer. One approach in the search for suitable replacements has centred on fluorocarbons which do not contain chlorine but which contain hydrogen. Hydrofluorocarbons such as difluoromethane , also known as HFA 32, and 1 , 1 , 1 , 2-tetrafluoroethane , also known as HFA 134a, are of interest as replacements, in particular as replacements in refrigeration, air-conditioning and other applications.

Many processes have been proposed for the production of hydrofluorocarbons and hydrochlorofluorocarbons , including the catalysed gas phase hydrofluorination of halocarbons and the catalysed hydrogenation of halocarbons or hydrohalocarbons which contain fluorine. The catalysed hydrogenation of chlorofluorocarbons and hydrohalofluorocarbons by reaction with hydrogen in the presence of a hydrogenation catalyst is considered to be a potentially commercially attractive process. Hydrogenation of dichlorotetrafluoroethane and chlorotetrafluoroethane to 1 , 1, 1 , 2-tetrafluoroethane is described in UK Patent No. 1,578,933 and the hydrogenation of chlorodifluoromethane to difluoromethane is described in European Patent Application No. 0 508 660.

However, one problem with the known processes is that the catalyst is deactivated over a period of time, and it is necessary to periodically replace the catalyst with fresh catalyst. It is clearly desirable to prolong or restore the activity of the catalyst thereby reducing the frequency with which the catalyst must be replaced.

We have now found that the activity of hydrogenation catalysts may be restored, and furthermore the activity of fresh catalysts may be increased, by treating the catalyst with an oxygen-containing oxidising agent.

According to the present invention there is provided a process for the treatment of a hydrogenation catalyst which comprises contacting the catalyst with an atmosphere comprising an oxygen-containing oxidising agent at elevated temperature .

The temperature should be at least 100°C, usually at least 150°C.

Also according to the present invention there is provided a hydrogenation catalyst obtainable by a process comprising contacting the catalyst with an atmosphere comprising an oxidising agent at elevated temperature .

The catalyst may be one which has been previously employed and consequently deactivated in a hydrogenation reaction, usually a vapour phase hydrogenation reaction, and particularly the hydrogenation of a halo (hydro ) fluorocarbon.

Hydrogenation catalysts are well known, and a variety have been proposed. Such catalysts typically comprise a Group Villa metal, for example nickel, rhodium, iridium, ruthenium, osmium, cobalt, palladium and platinum. Other metals have also been proposed, for example rhenium. The preferred catalyst depends upon the particular hydrogenation reaction in which it is being employed but in general a particularly preferred catalyst comprises palladium. Such preferred catalysts may comprise only palladium or may comprise palladium and other metals which may be present as alloys with the palladium or as mixtures of metals.

The metal catalyst is typically carried on a suitable support, for example alumina, fluorinated alumina, silica, silicon carbide or carbon, in particular an active carbon. Any and all of these hydrogenation catalysts, particularly supported metal catalysts may be beneficially treated according to the present invention. The process of the invention has particular utility for the treatment of carbon supported metal catalysts, the ( re )activation of which has hitherto :.aused particular difficulty due to the need to balance ( re )activation of the catalyst with avoiding destruction of the carbon support. We have surprisingly found that such carbon supported metal catalysts may be ( re )activated with an oxygen-containing oxidising agent, without destruction of the carbon support, even at elevated temperatures at which burning of the carbon support would be expected .

A commonly employed and widely available carbon supported metal hydrogenation catalyst comprises palladium carried on an active carbon support.

Many methods of preparation of metal supported noble metal catalysts are known to those skilled in the art, and the process of the invention may be utilised with a supported noble metal catalyst prepared by any method. A typical method involves impregnating the support with an aqueous solution of the salt of the metal, for example a halide, and thereafter drying the catalyst.

By an "atmosphere comprising an oxidising agent" , there is meant a gaseous composition which comprises an oxygen-containing oxidising agent, for example oxygen itself or a gaseous oxide, for example a gaseous oxide of nitrogen, especially nitrous oxide, N2O. Thus the atmosphere may also comprise other gaseous matter, for example the atmosphere may also comprise an inert diluent or other species which may in fact improve the performance of the treatment process. Thus, for example, the atmosphere may also comprise nitrogen, hydrogen and/or hydrogen chloride. we particularly prefer that the atmosphere comprises nitrogen in addition to the oxygen containing oxidising agent. It may also be beneficial to employ hydrogen chloride in combination with the oxygen-containing oxidising agent.

The composition of the atmosphere may vary over a wide range, the optimum composition in any particular application being dependent upon the particular catalyst being treated, it's degree of deactivation and the particular oxygen-containing oxidising agent and temperature employed. In general, the atmosphere will usually comprise at least 12 by volume, preferably at least 52 by volume, and more preferably at least 10Z by volume of the oxygen-containing oxidising agent. Where the catalyst comprises palladium supported on an active carbon and the oxygen-containing oxidising agent comprises oxygen i ^*elf, we especially prefer that the atmosphere comprises at least 15Z by volume of oxygen, whereas where the oxygen-containing oxidising agent comprises nitrous oxide, we prefer that the atmosphere comprises at least 202 by volume of nitrous oxide, more preferably at least 302 by volume and especially at least 502 by volume. Particularly preferred oxygen containing oxidising agents are oxygen and nitrous oxide, especially oxygen, and a particularly preferred atmosphere is air which may be diluted with, for example, nitrogen so as to dilute the oxygen.

We also prefer, because of the corrosive effect of water, that the atmosphere is substantially anhydrous .

The choice of oxygen-containing oxidising agent, in particular oxygen or nitrous oxide depends upon the particular catalyst being treated, and in particular the specific metal and support being employed. Whilst the efficiency of the treatment process with any particular oxygen-containing oxidising agent may vary from one catalyst combination of metal and support to another, the skilled man may determine by routine trial and error the optimum oxygen containing oxidising agent and conditions to employ with any particular catalyst.

In carrying out the process of the invention, the catalyst may be purged with nitrogen both prior to and after the treatment with the atmosphere comprising an oxygen-containing oxidising agent in order to avoid the formation of any explosive mixtures of hydrogen and oxygen-containing oxidising agent. The conditions under which the catalyst is purged with nitrogen are conveniently the same as those under which the catalyst treatment process is carried out.

The temperature at which the process is effected is also dependent upon the particular catalyst being treated and it's degree of deactivation although in general we prefer that the temperature is at least 250°C. There is generally no benefit in using a temperature greater than about 400°C. Preferred temperatures depend upon the choice of oxygen- containing oxidising agent and the amount thereof present in the atmosphere. Where the oxygen-containing oxidising agent is oxygen itself or nitrous oxide, the temperature is preferably in the range from about 250°C to about 350°C. Where the catalyst comprises a carbon supported metal, in particular palladium supported upon an active carbon and the oxygen-containing oxidising agent is oxygen or nitrous oxide, the temperature is more preferably in the range from about 275°C to about 325°C.

The process is conveniently carried out at about atmospheric pressure, although superatmospheric or subatmospheric pressures may be employed if desired. The flow rate of the atmosphere comprising an oxidising agent may be within a wide range depending to some extent upon the volume of the catalyst being treated. Typically, the flow rate will be in the range from about 0.001 to about 100 times, preferably from about 0.1 to about 10 times, the bed volume of the catalyst being treated per minute.

The preferred (re )activation time between the catalyst and the atmosphere comprising the oxidising agent, that is the time for which the catalyst is exposed to the atmosphere, is dependent upon the composition of the atmosphere, the degree of any deactivation of the catalyst and the temperature and pressure at which the process is carried out. Generally the ( re )activation time will be in the range from a few minutes, say 1 to 5 minutes to a few days, say 2 to 3 days, and is typically in the range from about 10 minutes to about 2 days. Where the catalyst comprises palladium carried on an active carbon support and the oxygen-containing oxidising agent is oxygen, the ( re )activation time is preferably in the range from about 5 minutes to about 24 hours, whereas where the oxygen containing oxidising agent is nitrous oxide, the ( re )activation time is preferably in the range from about 10 minutes to about 2 hours.

The catalyst which has been treated according to the invention is particularly useful in the hydrogenolysis /hydrogenation of halofluorocarbons and halohydrofluorocarbons . Thus, according to a further feature of the invention there is provided a process for the production of hydrohalofluorocarbons and/or hydrofluorocarbons which comprises contacting a halofluorocarbon or hydrohalofluorocarbon with hydrogen at elevated temperature in the presence of a hydrogenation catalyst which has been treated with an atmosphere comprising an oxygen-containing oxidising agent at elevated temperature.

Usually the halofluorocarbons and hydrohalofluorocarbons used as starting materials comprise at least one atom of chlorine or bromine and generally will be chlorofluorocarbons or hydrochlorofluorocarbons . The chlorofluorocarbon or hydrochlorofluorocarbon will typically comprise 1, 2 or 3 carbon atoms although it may comprise more than 3, say up to 6, carbon atoms. The (hydro )halofluorocarbon may be unsaturated or saturated, cyclic or acyclic and straight chain or branched chain, although the (hydro )halofluorocarbon will usually be a straight chain saturated acyclic compound, that is a linear (hydro )halofluoroalkane .

Particularly useful hydrogenation reactions in which a hydrogenation catalyst according to the invention may be employed include (a) the hydrogenation of a haloethane, in particular a chloroethane , having 4 fluorine atoms, for example 1,1-dichlorotetrafluoroethane, 1,2-dichlorotetrafluoroethane and chlorotetrafluoroethane to chlorotetrafluoroethane and/or tetrafluoroethane , in particular 1 ,1 , 1 , 2-tetrafluoroethane ; and (b) the hydrogenation of a compound of formula CF2 where X and Y are independently Cl, Br or H (but not both H) , to difluoromethane.

Conditions for effecting these hydrogenation reactions are described, for example in UK Patent Specification No. 1.578,933 and European Patent Application No. 0 508 660 respectively, the disclosures of which are incorporated herein by reference . The invention is illustrated but not limited by the following examples. In examples 1 to 18, the oxygen-containing oxidising agent was oxygen.

EXAMPLE 1.

60 ml of a catalyst comprising 102 palladium supported on Norit RX3 extrudate active carbon which had been deactivated by operation as a catalyst in the hydrogenation of chlorodifluoromethane to difluoromethane was charged to a 1" diameter Inconel reactor enclosed in a furnace. The activity of this deactivated catalyst was measured by passing 60ml/minute hydrogen and 30 ml/minute chlorodifluoromethane over the catalyst at a temperature of 280°C and atmospheric pressure (standard activity test conditions). The reactor off-gases were sampled and the samples analysed by gas chromatography .

The hydrogen and chlorodifluoromethane feeds were switched off and air was passed over the catalyst at a flow rate of 25ml /minute and a temperature of 300°C for 16 hours. After this time the air was switched off and the activity of the catalyst monitored under standard activity test conditions. The results are shown in Table 1.

EXAMPLES 2 to 7.

300 ml of the deactivated catalyst (as example 1) was divided into 6 x 50 ml batches. In each of examples 2 to 7 the procedure described below was carried out with a 50ml batch of the catalyst except that the temperature of regeneration in each example was as stated in Table 2. The 50ml batch was charged to a 1" diameter

Inconel reactor enclosed in a furnace. The activity of this deactivated catalyst was measured by passing 120ml /minute hydrogen and 60 ml/minute chlorodifluoromethane over the catalyst at a temperature of 300°C and atmospheric pressure

(standard activity test conditions). The reactor off-gases were sampled and the samples analysed by gas chromatography .

The hydrogen and chlorodifluoromethane feeds were switched off and air was passed over the catalyst at a flow rate of 100ml /minute and the temperature stated in Table 2 for 16 hours. After this time the air was switched off and the activity of the catalyst monitored under standard activity test conditions. The results are shown in Table 2. TAB LE 2 .

Conversion Selectivity (2) (Z CHF₂C1) CH₂F₂ CH

BEFORE AIR <1 82 TREATMENT

AFTER AIR TREATMENT:

EXAMPLE: REGEN TEMP (°C) .

260 5.7 84.7 11.4

280 11.05 86.9 11.5

300 28.02 83.8 15.2

320 24.6 84.5 14.3

340 28.2 86.4 12.6

360 17.4 86.7 12.3

EXAMPLE 8.

15 ml of the deactivated catalyst (as example 1) was charged to a 1/2" diameter Inconel pressure microreactor enclosed in a furnace. The activity of this deactivated catalyst was measured by passing 230ml /minute hydrogen and 115 ml/minute chlorodifluoromethane over the catalyst at a temperature of 300°C and 7.5 barg pressure (standard "tivity test conditions) . The reactor off-gases were npled and the samples analysed by gas i. romatography .

The hydrogen and chlorodifluoromethane feeds were switched off and air was passed over the catalyst at a flow rate of 25ml/minute and a temperature of 300°C for 16 hours. After this time the air was switched off and the activity of the catalyst monitored under standard activity test conditions. The results are shown in Table 3.

EXAMPLES 9 and 10.

30ml of a fresh catalyst comprising 102 palladium supported on Norit RX3 extrudate active carbon was divided into two 15ml samples. A first sample (example 9) was treated with air under the conditions of example 8. The activities of the air treated fresh catalyst sample (example 9) and the untreated fresh catalyst sample (example 10) were then measured under the standard activity test conditions of example 8. The results are shown in Table 4, and demonstrate the beneficial effect of air treatment on the activity of the fresh catalyst.

EXAMPLES 11 to 16.

90 ml of the fresh catalyst (as examples 9 and 10) was divided into 6 x 15 ml batches. In each of examples 11 to 16 the procedure described below was carried out with a 15ml batch of the catalyst except that the temperature of activation in each example was as stated in Table 5.

The 15ml batch was charged to a 1/2" diameter Inconel reactor enclosed in a furnace. Air was passed over the catalyst at a flow rate of 25ml/minute, atmospheric pressure and the temperature stated in Table 5 for 16 hours. After this time the air was switched off and the activity of the catalyst monitored under the standard activity test conditions of example 8 ( 230ml /minute hydrogen, 115ml/minute chlorodifluoromethane , 300°C and 7.5 barg pressure. The results are shown in Table 5.

EXAMPLE 17.

40 ml of a deactivated catalyst (as example 1) was charged to a 1" diameter Inconel reactor tube enclosed in a furnace and the activity of the catalyst was measured by passing 90ml/minute hydrogen and 30ml/minute of a mixture of 1,1,1,2-tetrachlorodifluoroethane and

1 , 1 , 2 , 2-tetrachlorodifluoroethane over the catalyst at atmospheric pressure and 185°C (standard activity test conditions). The reactor off-gases were sampled and the sample analysed by Gas Chromatograhy .

The hydrogen and organic feeds were then switched off and air passed over the catalyst at a flow rate of lOOml/minute and 300°C for 16 hours. After this time the air was switched off and the catalyst activity monitored under standard activity test conditions. The results are shown in Table 6.

EXAMPLE 18.

The procedure of example 17 was repeated except that the catalyst was a fresh catalyst comprising 52 Pd carried on Grade 208c active carbon (supplied by Sutcliffe Speakman Ltd). The results are shown in Table 7.

EXAMPLE 19.

40ml of a catalyst comprising 52 palladium carried on Grade 208c active carbon (active carbon supplied by Sutcliffe Speakman Ltd) was aged by passing 90ml/minute hydrogen and 30ml/minute of a mixture of 1 , 1 , 1 , 2-tetrachlorodifluoroethane and

1 , 1 , 2 , 2-tetrachlorodifluoroethane over the catalyst at atmospheric pressure and 350°C for 120 hours. After this time the temperature was lowered to 185^{oC anα"} the activity of the catalyst was monitored (standard activity test conditions) by sampling the off-gases and analysing the sample by Gas Chromatography .

The hydrogen and organic feeds were then switched off and air passed over the catalyst at a flow rate of lOOml/minute and 300°C for 16 hours. After this time the air was switched off and the catalyst activity monitored under standard activity test conditions. The results are shown in Table 8.

In the following examples 20-25, the catalyst comprised 112 by weight palladium carried on a Grade 208c active carbon support (active carbon supplied by Sutcliffe Speakman Ltd), reactions were carried out in a 1 inch internal diameter Inconel tubular reactor and the oxygen-containing oxidising agent was Nitrous oxide .

ACTIVITY OF FRESH AND AGED CATALYST

The activity of a 30 ml sample of fresh catalyst in the hydrogenation of chlorodifluoromethane to difluoromethane was determined by charging the sample of catalyst to the reactor and feeding a gaseous mixture of 50ml/min hydrogen and 25ml/minute chlorodifluoromethane over the catalyst at 292°C. The composition of the off-gas was determined by Gas Chromatograhy and the results are shown in Table 8.

The catalyst was then aged at about 300°C by prolonged exposure to the hydrogen and chlorodifluoromethane gaseous mixture described above until the activity of the catalyst fell below 152 conversion of chlorodifluoromethane . The activity of e aged catalyst is also given in Table 9.

10

EXAMPLE 20.

15

The aged catalyst was treated by passing nitrous oxide at 30ml/minute over the catalyst at 305°C for increasing time increments and its activity was determined after each time increment by passing a

20 gaseous mixture of 50ml /minute hydrogen and 25ml/minute chlorodifluoromethane over the catalyst at the same temperature and analysing the off gas. The results are shown in Table 10.

^~~"^~

-- EXAMPLE 21

30ml of a fresh catalyst was aged and the activity of the aged catalyst was determined as described as previously described. The results are shown in Table 11. The aged catalyst was then treated by passing nitrous oxide at 30ml/minute and at 250°C over the aged catalyst for increasing time increments and its activity was determined after each increment as described in example 20. The results are shown in Table 11.

EXAMPLE 22.

30ml of a fresh catalyst was aged and the activity of the aged catalyst was determined as previously described. The results are shown in Table 12. The aged catalyst was then treated by passing nitrous oxide at 30ml/minute and at 275°C over the aged catalyst for increasing time increments and its activity determined after each increment as described in example 20. The results are shown in Table 12.

EXAMPLE 23.

30ml of a fresh catalyst was aged and the activity of the aged catalyst was determined as previously described. The results are shown in Table 13. The aged catalyst was then treated by passing a gaseous mixture of nitrous oxide at lOml/minute and nitrogen at 20ml/minute at 275°C over the aged catalyst for increasing time increments and its activity was determined after each time increment as described in Example 20. The results are shown in Table 13.

EXAMPLE 24.

30ml of a fresh catalyst was aged and the activity of the aged catalyst was determined as previously described. The results are shown in Table 14. The aged catalyst was then treated by passing a gaseous mixture of nitrous oxide at 10ml /minute and nitrogen at 20ml/minute at 300°C over the catalyst for 2 hours followed by 2 hours at 320°C and its activity was determined after each 2 hours as described in Example 20. The results are shown in Table 14.

EXAMPLE 25.

60ml of a fresh catalyst was aged and the activity of the aged catalyst was determined as previously described. The results are shown in Table 15. The aged catalyst was then treated by passing a gaseous mixture of 4:1 nitrogen to nitrous oxide at 60ml/minute and 300°C over the catalyst for increasing time increments and then 30 minutes at 326°C and its activity was determined after each time / temperature increment as described in Example 20. The results are shown in Table 15.

Claims

C LAIMS .

1. A process for the treatment of a hydrogenation catalyst which comprises contacting the catalyst with an atmosphere comprising an oxygen-containing oxidising agent at elevated temperature.

2. A process as claimed in claim 1 in which the catalyst comprises a supported Group Villa metal.

3. A process as claimed in claim 2 in which the support comprises an active carbon.

4. A process as claimed in claim 3 in which the catalyst comprises palladium supported on active carbon.

5. A process as claimed in claim 1 in which the catalyst is a fresh catalyst.

6. A process as claimed in claim 1 in which the catalyst has been previously deactivated in a vapour phase hydrogenation reaction.

7. A process as claimed in claim 6 in which the reaction was the hydrogenation of a halo(hydroJfluorocarbon.

8. A process as claimed in claim 1 in which the oxygen containing oxidising agent is oxygen or nitrous oxide.

9. A process as claimed in claim 8 in which the oxygen containing oxidising agent is oxygen.

10. A process as claimed in claim 9 in which the atmosphere comprises air.

11. A process as claimed in claim 1 in which the zz> atmosphere comprises at least 12 by volume of the oxygen containing oxidising agent.

12. A process as claimed in claim 1 in which the temperature is in the range from about 200°C to about 400°C.

13. A catalyst obtainable by a process as claimed in any one of claims 1 to 12.

14. A process for the production of hydrohalofluorocarbons and/or hydrofluorocarbons which comprises contacting a halofluorocarbon or hydrohalofluorocarbon with hydrogen at elevated temperature and in the presence of a hydrogenation catalyst which has been treated with an atmosphere comprising an oxygen containing oxidising agent at elevated temperature.

15. A process as claimed in claim 14 in which the starting material is a compound of formula CF₂XY where X and Y are independently Cl, Br or H (but not both H).