WO1996017683A1

WO1996017683A1 - Hydrogenation catalyst and process

Info

Publication number: WO1996017683A1
Application number: PCT/GB1995/002837
Authority: WO
Inventors: John David Scott; Gary Goodyear; John Charles Mccarthy
Original assignee: Imperial Chemical Industries Plc
Priority date: 1994-12-09
Filing date: 1995-12-06
Publication date: 1996-06-13
Also published as: JPH10510206A; CA2206337A1; EP0796146A1

Abstract

A hydrogenation catalyst of improved activity and selectivity comprising palladium and platinum in a ratio by weight of 2:1 to 500:1 carried on a support such as carbon and a process for the production of hydrofluorocarbon such as difluoromethane which comprises contacting a (hydro)halofluorocarbon such as chlorodifluoromethane or dichlorodifluoromethane, preferably in the vapour phase, with hydrogen at elevated temperature in the presence of the catalysts.

Description

HYDROGENATION CATALYST AND PROCESS.

This invention relates to a catalyst, in particular a hydrogenolysis (the replacement of halogen by hydrogen in saturated molecules) and/or hydrogenation (addition of hydrogen across ethylenic double-bonds) catalyst, (hereafter referred to collectively as "hydrogenation") and to hydrogenation processes employing the catalyst, in particular the hydrogenation of halofluorocarbons and hydrohalofluorocarbons to produce hydrofluoroalkanes, for example the hydrogenation of dichlorodifluoromethane and chlorodifluoromethane to produce difluoromethane, the hydrogenation of chloropentafluoroethane to produce pentafluoroethane and the hydrogenation of dichlorotetrafluoroethane and chlorotetrafluoroethane to produce tetrafluoroethane and especially 1, 1 , 1 ,2-tetrafluoroethane.

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 propellants 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, pentafluoroethane, also known as HFA 125 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 or hydrohalocarbons 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, for example, UK Patent No. 1 ,578,933, the hydrogenation of chloropentafluoroethane to pentafluoroethane is described in, for example Japanese Laid-Open Patent Application No. J4-29941 and the hydrogenation of chlorodifluoromethane to difluoromethane is described in, for example, European Patent Application No. 0 508 660.

Much attention has focused on improving the catalyst which is employed in the hydrogenation process, for example to increase the activity of the catalyst and/or to increase the selectivity with which the desired HFA product is produced. Thus, improved catalysts for use in the hydrogenation or hydrogenolysis of chlorofluorocarbons and hydrochlorofluorocarbons have been disclosed, inter alia, in International Patent Application Nos. WO90/08748 WO92/121 13, WO94/1 1328, US Patent No. 5, 136, 1 13, European Patent Application No. 0 347 830 and Japanese Kokai No. J4-179322.

The present invention is based on the discovery that the activity and particularly the selectivity of palladium catalysts is improved by incorporating a controlled amount of platinum in the catalysts.

According to the present invention there is provided a hydrogenation catalyst which comprises palladium and platinum in a ratio by weight of 2: 1 to 500: 1 carried on a support.

The palladium and platinum components may be used alone or in combination with other metals, for example other Group VIII metals such as nickel, Group I B metals such as Ag and Au or even other metals.

The metals are carried on a suitable support, for example alumina, fluorinated alumina, silica, silicon carbide or carbon but in particular alumina or carbon. We particularly prefer to employ a carbon carrier where the carbon has a high surface area, eg greater than 200 m²/g. Activated carbon supports with low inorganic impurity levels are particularly suitable.

The loading of the metal on the support material may be dependent at least to some extent on the particular metal catalyst/support combination being used and the difficulty of the hydrogenation reaction. However the % w/w of combined palladium and platinum to support is typically from about 0.1% w/w to about 40% w/w, and preferably from about 0.5% w/w to about 20% w/w, more preferably from about 2.0% w/w to about 20% w/w and especially from about 5% w/w to about 15% w/w.

The proportions of palladium and platinum present may vary within a wide range, although we prefer a catalyst in which there is at least twice as much palladium as platinum. We particularly prefer to employ palladium and platinum in the ratio by weight from about 2: 1 to about 500: 1 , and more preferably from about 3 : 1 to about 100: 1. A preferred catalyst comprises from about 0.5% to 20%, in particular from about 5 to 15% w/w palladium and from about 0.05% to about 5%, in particular from about 0.1% to about 4% by weight platinum, especially when supported on an active carbon. Overall the amount of platinum is usually in the range from about 0.01 % w/w to about 10% w/w of the catalyst. Many methods of preparation of supported metal catalysts are known to those skilled in the art, for example as described in the prior art documents referred to on page 2 of this specification and the process of the invention may be utilised with a supported noble metal catalyst prepared by any such method. A typical method involves impregnating the support with an aqueous solution of a soluble salt of the metal, for example a halide, and thereafter drying the catalyst.

The improved catalyst of the invention is particularly useful in the hydrogenation or hydrogenolysis of a halofluorocarbon or hydrohalofluorocarbon with hydrogen at elevated temperature and according to a second aspect of the invention there is provided a process for the production of a hydrofluorocarbon which comprises contacting a halofluorocarbon or hydrohalofluorocarbon with hydrogen at elevated temperature in the presence of the improved hydrogenation catalyst according to the invention 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 J -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 CF₂XY 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.

An important feature of successful hydrogenation catalysts is a high hydrogenolysis activity for carbon - chlorine and/or carbon - bromine bonds but a low hydrogenolysis rate for carbon-fluorine bonds, thus avoiding the loss of product by removal of fluorine atoms.

The catalyst of the invention is particularly advantageously employed in the production of difluoromethane where it has a marked effect on improving the selectivity with which difuoromethane is produced by reducing the level of over reaction.

According to a preferred embodiment of the second aspect of the invention there is provided a process for the production of difluoromethane which comprises reacting a compound of formula XYCF₂ wherein X and Y are each H, Cl or Br but at least one of X and Y is an atom other than hydrogen, with hydrogen at elevated temperature in the presence of a hydrogenation catalyst according to the invention.

The process may be conveniently effected by feeding a stream comprising the compound of formula XYCF₂ and hydrogen, as a combined or as separate streams through a vessel containing the hydrogenation catalyst.

The starting compounds of formula XYCF₂ are dichlorodifluoromethane, dibromodifluoromethane, chlorobromodifluoromethane, chlorodifluoromethane and bromodifluoromethane. Mixtures of the above compounds may be employed. Usually the compound of formula XYCF₂ will be a chlorinated difluoromethane and chlorodifluoromethane is the preferred starting compound.

The proportion of hydrogen to starting compound of formula XYCF₂ may be varied considerably. Usually at least the stoichiometric amount of hydrogen is employed to replace the chlorine and/or bromine atom(s), and considerably greater than stoichiometric amounts, for example 4 or more moles of hydrogen per mole of starting compound may be employed. Where X and Y are each chlorine or bromine, it is preferred to employ at least two moles of hydrogen (the stoichiometric amount) per mole of starting compound. Where the starting compound of formula XYCF₂ is chlorodifluoromethane it is preferred to employ between 1 and 2 moles of hydrogen per mole of chlorodifluoromethane.

Atmospheric or superatmospheric pressures, for example up to about 60 barg may be employed. We have found that operation of the process of the invention at superatmospheric pressure substantially increases the selectivity of the process towards the production of difluoromethane. The process is preferably operated at a pressure in the range from about 2 bar to about 60 bar and more preferably from about 2 bar to about 30 bar, especially 5 bar to 30 bar.

The reaction is suitably carried out in the vapour phase at a temperature which is at least about 150°C and not greater than about 500^CC, usually from about 225°C to about 400°C, and preferably from about 240°C to about 360°C. The most preferred temperature is dependent upon the pressure at which the process is operated; at atmospheric pressure, we prefer to operate the process at a temperature in the range from about 220°C to about 320°C whereas at a pressure of about 7.5 barg, we prefer to employ temperatures in the range from about 260°C to about 380°C.

Contact times are usually in the range 1 to 60 seconds, especially 5 to 30 seconds when the reaction is carried out in the vapour phase.

In the process according to this preferred embodiment of the second aspect of the invention any unreacted hydrogen and other starting material, together with any organic by-products, may be recycled.

The invention is illustrated but not limited by the following examples.

A. CATALYST PREPARATION.

A sample of carbon support (supplied by Norrit) with an approximate surface area area of 800 sq.m/g was crushed and sieved to generate particles in the size range 1.0- 1.2 mm. 50 cm³ - 60 cm³ of the crushed carbon was then washed in distilled water and the water drained through a no.4 sinter funnel. The washed carbon was then transferred to a Buchner flask . The target weight of palladium chloride or mixed metal chlorides were then dissolved in the minimum volume of warmed concentrated hydrochloric acid and the solution was added to the carbon particles in the Buchner flask. A further 200 cm³ of distilled water was then added to the flask and the slurry was evaporated to dryness on a rotary evaporator using an oil bath temperature of 120°C. The catalyst was finished by heating the granules in a vacuum oven at 150°C for approximately 16 hours.

Alternatively, the above catalyst preparation was repeated at a larger scale. In this scaled-up preparation, the same method was employed with 300 cm³ of carbon support and the particle diameter was increased to 3 mm.

B . CATALYSTS TESTING IN HYDROGENOLYSIS PROCESSES.

50 cm³ of the l Jmm or 3 mm catalyst to be tested was charged to 1.25cm Inconel reactor and heated to 300°C in a nitrogen stream of 300 cm³/min. After the catalyst had been dried for 2 hours, the nitrogen flow was replaced with a stream of mixed reactant gases. In the following comparitive set of examples a mixed reactant flow rate of 180 cm³/min was employed with an hydrogen:HCFC 22 molar ratio of 2: 1. The reactor vent gases were passed into a water scrubber to remove the acid products and then analysed using conventional gas chromatographic analysis. The catalyst performance was determined for reactor temperatures of 300 - 380°C. EXAMPLE 1.

Eight catalysts were prepared (on small or large scale as indicated in the Tables) and tested, and the results of analysis for % conversion of chlorodifluoromethane and % selectivity to difluoromethane are shown in Tables 1 , 2 and 3. The platinum promoted palladium hydrogenation catalyst* was found to have the highest reaction selectivity and to demonstrate significant advantage over a range of other palladium catalyts containing Cu, Ni, Ru, or Rh.

TABLE 1.

TABLE 2.

TABLE 3.

EXAMPLE 2.

Two catalysts were prepared for a series of comparative studies. Catalyst "A" comprised a 10%w/w Pd metal on carbon catalyst and catalyst "B" comprised a 10% Pd plus 1.8%Pt catalyst on the same Norrit carbon support. The catalysts were each prepared by dissolving the metal halides in the minimum volume of hydrochloric acid and impregnating the 1-1.2mm granules of the carbon with the resultant solution. The slurry was agitated and evaporated to dryness. The catalyst was then degassed under a flow of nitrogen at 120°C for 16 hours to produce the finished catalyst.

50 cm³ of catalyst "A" was then charged to a ^lΛⁿ Inconel reactor and heated to 300°C in a 300 ml/min feed of nitrogen for a 2 hour period prior to testing. The test feed mixture was a 2.J molar feed ratio of hydrogen:CFC 115, which was fed at a flow rate of 180 ml/min to the conditioned catalyst. The catalyst was stabilised under these feed conditions for approximately two hours and then cooled under reaction conditions to approximately 200°C. Conversion of CFC 115 and selectivity to HFC 125 were measured and are shown in Table 4. At measured temperatures of 199°C, 213°C and 236°C, the 1 15 conversion was found to increase from approximately 66% to 84% and finally 97%, see examples Al, A2 and A3. The initial reaction selectivity was found to be 99.77 but fell to 99.59 with increasing feed conversion.

The above catalyst testing procedure was repeated with the Pt promoted catalyst "B". The results are presented in examples B l, B2 and B3 in Table 4. The Pt promoted catalyst was found to be more active (operating at lower temperatures for a given feed conversion) and more selective (producing lower levels of by-products) than the base palladium catalyst.

TABLE 4

°C Temp % Conv % Sel 115 125 134a 143a c2H6

10% Pd on Carbon

Example

Al 199 66.49 99.77 33.51 66.34 0.03 0J2 0

A2 213 84.01 99.65 15.99 83.71 0.05 0.24 0

A3 236 97.24 99.59 2.76 96.85 0.07 0.33 0

10% Pd + 1.8% Pt on Carbon

Bl 192 65.27 99.81 34.73 65.15 0.04 0.08 0

B2 209 83.25 99.76 16.75 83.04 0.05 0J6 0

B3 207 98.8 99.71 1.2 98.51 0.05 0.22 0.02 EXAMPLE 3.

The test procedure described in Example 2 using the two catalysts "A" and "B" and the stated flow rates was repeated but employing a mixture of 1 14 and 1 14a rather than 1 15. The results are shown in Table 5. Using catalyst "A", 1 14 conversions of 81.6-88.7% were achieved at 194-204°C, compared with 173°C for the platinum promoted catalyst. The 1 14a conversions were 100% for both catalysts, however the level of over-hydrogenolysis products were reduced for the Pt containing catalysts. Thus the platinum promoted catalyst was more active and more selectie than the base palladium catalyst.

TABLE 5

Tαnp CF2C1-CF2C1 CF2C1-CHF2 CHF2-CHF2 CF3-CFC12 CF3-CHFC1 CF3-CH2F CF3-CH3 CH3-CH3 Deg C 114 124a 134 114a 124 134* 143* 170

10% Pd on Carbon

Example

Ex. A4 19₄ 18.36 34.51 2.55 0 2.5 37.27 4.82 0

A5 204 11.32 40.39 4.7 0 1.94 36.16 5.44 0.02

10% Pd + 1.8% Pt on Carbon

EX.B4 173 14.55 37.76 2.83 0 0.86 39.51 4.46 0

B5 173 13.02 37.44 2.77 0 0.87 39.46 4.41 0

Claims

CLAIMS:

1. A hydrogenation catalyst comprising palladium and platinum in a ratio by weight of from 2: 1 to 500: 1 carried on a support.

2. A catalyst as claimed in claim 1 wherein the support is carbon.

3. A catalyst is claimed in claim 1 or claim 2 wherein the amount of combined palladium and platinum is from 0.1% to 40% based on the support.

4. A catalyst as claimed in any one of claims 1 , 2 and 3 wherein the ratio by weight of palladium to platinum is from 3 : 1 to 100: 1.

5. A catalyst as claimed in any one ofclaims 1 to 4 wherein the support is carbon and comprising from 0.5% to 20% by weight of palladium and from 0.05% to 5% by weight of platinum.

6. A catalyst as claimed in any one of the preceding claims wherein the amount of platinum is from 0.01% to 10% by weight of the catalyst.

7. A process for the production of a hydrofluorocarbon which comprises contacting a halofluorocarbon or hydrohalofluorocarbon with hydrogen at elevated temperature in the presence of a catalyst as claimed in any one of claims 1 to 6.

8. A process as claimed in claim 7 for the production of difluoromethane wherein the halofluorocarbon or hydrohalofluorocarbon has the formula CF₂XY where X and Y is either H, Cl or Br provided that X and Y are not both H.

9. A process as claimed in claim 7 or claim 8 wherein the halofluorocarbon or hydrohalofluorocarbon is in the vapour phase and the elevated temperature is from 150°C to 500°C.

10. A process as claimed in any one of claims 7, 8 and 9 wherein the hydrogen is present in stoichiometric excess.