CA1057730A - Platinum group metal catalyst and support - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE:
Graphitized carbon support has a basal plane area of greater than 100 m2/g, a ration of BET surface area to basal surface area of less than 2:1 and a ratio of basal plane area to edge surface area of at least 5:1 The support is highly crystalline on the surface and readily accepts catalytic metals.
A platinum group metal is applied to prepare a reforming catalyst.

Description

` 1057730 This invention relatea to a no~el form of carbon, a novel supported platinum group metal càtalyst and to its use. ~he catalyst i3 particularly auitable for use in hydrogen transfer reactions such as the hydrogenation, dehydrogenation and/or dehydrocyclisation of hydrocarbons.
According to one aapect of the present in~ention there ia provided a novel form of graphite-containing carbon having (1) a basal plane surface area of at leaat 100 m2/g, (2) a ratio of B~T
surface area to basal plane surface area of not more than 5:1 and 1Q (3) a ratio of basal plane surface area to edge surface area of at - least 5:1.
The graphite-containing carbon compriseq a crystalline layered structure in ~hich the conatituent atoma form layers attached to each other by relatively weak Van der Waals dispersion force~. The crystalline surface area of the material i3i formed largely of the basal planes of the layers with a smaller contribution from the edges of the layerq. There will usually be some amorphous carbon associated with the crystalline material.
The basal surface srea i3 determined by measuring the heat of adsorption ~f n-dotriacontane from n-heptane. Similarly the edge surface area is determined by the heat of adsorption of n-butanol from n-heptane.
Heats of adsorption can be measured using a flow microcalorimeter a~ described in "Chemistry and Induqtry" for 20th I~arch, 1965 at pages 482-485.
~he BET ~urface area i8 the surface area determined by the nitrogen adsorption method of Brunauer, Emmet and ~eller di~closed in J. Am. Chem.
Soc. 60, 309, (19~8). Thiq corre3ponds to the total surface area, i.e., the cry~talline basc~l plane surfac~ area, tl~e crystalline edge surfnce 3 area and the amorphous ~urface area.

- 2 -This novel form of graphite-containing carbon can be used as a catalyst support for platinum group metals and the supported metal catalysts are effective for hydro-gen transfer reactions.
Thus according to a further feature of the invention there is provided a catalyst compris~g (i) as support, a graphite-containing carbon having (a) a basal plane surface area of at least 100 m /g. (b) a ratio of BET surface area to basal plane surface area of not more than 5:1 and (c) a ratio of basal plane surface area to edge surface area of at least 5:1; and (ii) as active component, 0.01 to 10% by weight, preferably 0.1 to 5%
by weight, of the total weight of catalyst of a platinum group metal disposed thereon.
By "platinum group metal" we mean ruthenium, rhodium, palladium, osmium, iridium and platinum.
The preferred metals are platinum itself and iridium.
Preferably the basal plane surface area of the graphite-containing carbon is in the range 150 m2/g to 1000 m2/g. If the area is greater than 1000 m2/g, the carbon is unlikely to have sufficient strength for a catalyst support.
The closer that the ratio of the BET surface area to the basal plane surface area is to the theoretical minimum of 1, the higher is the quality of the material, i.e., the higher is the proportion of crystalline material and the lower is the pro-portion of amorphous. For practical reasons this ratio is pre-ferably in the range 2 to 1 to 1.01 to 1.
Preferably the ratio of the basal plane surface area to the edge surface area is greater than 10:1, most preferably greater than 300:1.
Preferably the graphite-containing carbon support has a pH in the range from 5 to 9, more preferably from 6 to 8, most preferably

-3-I) about 7, and contain~ less th n l~o by wt. of adsorbed oxygen, more preferably less than 0.5~0 by wt. of a~sorbed oxygen. The lower the proportion of adsorbed oxygen, the closer is the pH to 7.
The graphite-containing carbon should be of high purity~
e.g., having an ash content of less than 0.1~ by wt., preferably less than 0.05~ and morc preferably less than 0.02~.
Preferably the catalyst composition also contains a minor proportion of a modifying metal ion selected from the :~lkali and alkaline earth metnl ions. The modifying metal ion gives a marked 1~ increase in the dehydrocyclisation activity of the platinum group metal. The preferred quantity of modifying metal i~ from lO to 300 atomic per cent of the platinum group metal. Any alkali metal is ~uitable but the preferred one is sodium. The alkaline earth metal can be magnesium, calcium,~trontium or barium.
The particle size of the graphite-containing carbon is not significant and can be controlled in known manner in view of its intended applic~tion, ranging from fine particles for use in slurry processe~ to granules for use in fixed bed processes.
The graphite may be prepared from many different forms of carbon, including (a) activated carbons derived from coconut charcoal, coal, peat, etc. (b) carbon blacks,(c) carbons produced by the coking of petroleum re~idues, and (d) oleophilic graphite, e.g., as prepared according to our British Patent Specification No. 1168785.
The support may be prepared by heating in an inert atmosphere a carbon having a BET surface area in the range 100 to 3000 m /g to a tempe-ature in the range 900 to 3300C, preferab]y 1500 to 2700C
for sufficient time to produce a graphite-containing carbon.
Preferabl~ the carbon emploJed as a starting material is one nhic.~, prior to heat treatment as above, at approximately 1000C, has a B~T ~lrf~ce area of at ler1st 500 M2/g.

lOS7730 The preparation of the graphite-containing carbon varies according to the type of carbon selected and utilises combinations of heat treatment under inert and oxidising conditions chosen so as to optimise the ratios of BET to ba~al p1ane areas and basal plane to edge surface areas.
Heating in an inert atmosphere increases the proportion of Kraphitic material, i.e., decreases the first ratio and increases the second. ~iith most forms of carbon, however, the total surface area is significantly reduced by this treatme~t, but this is not always thc case and some forms of carbon sho~ only a relatively small decrease in surfAce area on heating.
Oxidation under carefully controlled conditions, by contrast, increases the surface area.
The preparation of the carbon support involves typically and generally three steps: (1) an initial heat treatment in an inert atmosphere at temperatures between 900 and 3300C, (2) an oxidation stage at a temperature between 300 and 1200C and (3) a further heat treatment in an inert atmosphere at temperatures between 1000 and 3000C and preferably not higher than the temperature of the initial heat treatment.
In Steps (1) and (3) nitrogen provides a suitable atmosphere for temperatures up to 1000C. Above thi~ an inert gas, e.~ rgon or helium, should preferably be used. In Step (2) suitable oxidising media include air, steam and carbon dioxide. If air is employed, the temperature is preferably in the range 300 to 450C; if steam or carbon dioxide, in the range 800-1200C.
During the heating in the inert atmosphere a portion at least of the carbon ia converted to graphite, and it is believed that ad30rbed organic oxygen-containing group~ ~uch as ketones, hydroxyl, car~oxylic acids and the like are removeA. ~he absence of organic ~o57730 oxygen-containing groups from the treated carbon (less than 1~) is believed to be significant in the context of selectivity of the catalyst employing the carbon as a support, since oxygen-containing groups have been reported to promote side reactions.
The cataly~t is prepared by impregnating the graphite-contàining carbon support with a solution of a reducible platinum group metal compound and reducing the reducible compound to the metal.
~ uitable solutions include a~ueous solutions of tetrammine platinous chloride, platinum tetrammine hydroxide and chloroplatinic ucid. Suitable conditions for the impregnation are temperatures of 20 to 90C, times of 1 to 6 hours and solution concentrations of 10 to 1 molar.
After each impregnation the catalyst may be dried at, e.g., 100-~50C,for 1 - 24 hours.
The quantity of modifying metal ion which can be added to the catalyst is much greater when the latter has been prepared from chloroplatinic acid or similar compounds than when it has been prepared from a tetrammine complex. Maximum activity can be maintained up to about 300 atomic per cent for the former and up to about 150 atomic per cent for the latter.
The platinum group metal is preferably reduced before use, e.g., by heating to 200 to 700C, preferably 300-600C, for 1 to 5 hours in a reducing atmosphere, preferab]y a stream of hydrogen flowing at ~rom 500 to 10,0C0 v/v/hr. The alkali or alkaline earth metal ion is preferably added before the reduction of the platinum group metal.
The catalyst is particularlg suitable for the dehydrocyclisation of acyclic straight chain hydrocnrbons having at least 6 carbon atoms or other hydrocarbons hPvine in their structure a straight chain with at 1east 6 carbon atoms which is capable of cys]isation. The preferred S0 ~Iydrocarbona are paraffins, although olefins may be used. Particularly lOS7'730 suitable feedstocks are C6 10 paraffin hydrocarbons which will give ben%ene and/or lower alkyl aromatics in good yield with a minimum of side reactions. The feedstocks may be pure hydrocarbons, or mixtures of acyclic hydroc~rbons. Such mixtures may also contain naphthenes and aromatics ~md may be, for example, petroleum fractions, particularly those boilin~ in the range 60-250C.
Sulphur compounds are normally undesirable in feedstocks for platinum group metal catalysts and preferably the sulphur content of the feedstock is less than lO ppm (wt), more particulnrly, less than 1 ppm (wt).
Thus according to a further aspect of the present invention there is provided a hydrocarbon hydrogen transfer proces3 which process comprises contacting the hydrocarbons under conversion conditions with a catalyst as hereinbefore defined.
Broad and preferred ranges of process conditions for dehydrogenation and/or dehydrocyclisation are as follows Broad Ran~ePreferred Ran~e Temperature C 200-650 400-600 Pressure bars(ga) l-210 1-70 Space Velocity v/v/hr 0.01-20 0.1-lO
H2: hydrocarbon mole ratio0.01:1-20:10.5:1-10:1 The catalyst is also suitable for use in hydrogenating aromatic compounds such as benzene and other compounds such as olefins and acetylenes.
Broad and preferred ranges oE process conditions for hydrogenation are as follows:
Broad Ran~ePreferred Ran~e Temperature C 0-400 100-300 Pressure bars(ga) l-210 1-70 S~ace 'lelocity v/v/hr 0.01-20 0.1-lO
H2:h/drocarbon mole ratio0.01-20:1 0.5:1-lO:l ~05~73() The invention is illustrated with reference to the following Examples, of which Examples 4, 7 and 8 are not accor-ding to the invention and are included for comparative purposes only.
~ Example 1 Preparation of the carbon 45 gms of carbon black sold by the Cabot Corporation under the Trade Mark "Black Pearls 71" was heated under an atmos-phere of nitrogen to 1000C to remove volatile matter and allowed to cool to room temperature (wt loss 11.4%). The sample was then heated in an atmosphere of argon from room temperature to 2700C which temperature was maintained for about 30 minutes, and then allowed to cool (further wt loss 2.3%). This second heating and cooling treatment lasted about 3 hours. After cool-ing the carbon had a black and grey mottled appearance and BET surface area 180 m /g Basal surface area 150 m /g Edge surface area 0.36 m /g BET/Basal ratio 1.2:1 Basal/Edge ratio 427:1 pH 7 (measured by slurrying with water) graphite 30% by wt. (X-ray diffrac-tion) %amorphous carbon 70% by wt.
total metals content 13 ppm ash content zero ~ adsorbed oxygen zero As a result of the heat treatments the total weight loss was 13.7% wt.
The carbon was in the form of spheres 200-4QOA in dia-meter which are believed to have an outer graphite skin.
Preparation of catalyst 0.7~ wt. of platinum was added to the support by impregnating with a 1/10 molar aqueous solution of Pt(N~13)4(OH)2 ati90C for

4 hours, followed by drying at 120C for 2 hours.
A portion of this impregnated material was then fur-ther impregnated with an aqueous solution of sodium carbonate containing 0.74 gms per litre, at 90C for 2 hours, following by drying at 110C for 2 hours, sieving to 40-100 BSS mesh granule size and then reducing at 500C for 2 hours in a stream of hydro-gen flowing at 4000 v/v/hour. The resulting catalyst contained 600 parts per million of sodium, corresponding to 73 atomic % of the platinum.
During the impregnation of platinum and sodium and the subsequent removal of water, the catalyst was continuously agi-tated to ensure even distribution of both components within and between catalyst pellets.
Use of the carbon prepared as described above as a catalyst support (i) 0.225 gms of the catalyst was used for the dehydro-cyclization of _-hexane at 500C in a microreactor at atmospheric pressure using a molar ratio of hydrogen:n-hexane of 7:1 and a liquid space velocity of 4 v/v/hour. The microreactor had a ca-pacity for 0.45 ml catalyst.
(ii) a portion of the catalyst prepared as describedabove but omitting the sodium addition step was also tested under the same conditions. The results are recorded below.

No added sodium ¦ 600 ppm sodium ¦

% Activity 63 92 % Benzene 38 72 Example 2 A sample of carbon black sold by the Cabot Corporation under the Trade Mark "Black Pearls 2" was heated in two stages a~ described in Example 1 above and allowed to cool (total wt 108S 32~ wt).

lOS7730 After cooling, the carbon had a black and grey mottled appearance and BET surface area 235 m2/g Basal surface area 230 m2/g Edge surface area o,3 m /g BET/Ba~al ratio 1.02:1 3asal/Edge ratio 766:1 pH 7 total metals content 13 ppm ash content zero adqorbed oxygen zero raphite 30~o amorphous carbon 70~
A ~atalyst containing 0.7~0 by wt platinum and 600 ppm sodium was prepared by treating the above support as described in Example 1.
(i) n-hexane was dehydrocyclised as in Example 1.
(ii) a portion of the cntalyst prepared as described but omitting the sodium addition step was also tested under the same conditions. The results are recorded below No addad sodium 600 ppm sodium Activity 75 98 Benzene 46 85 .

Example 3 The sodium containing catalyst described in Example 2 wa~
employed for the dehydrocyclisation of n-hexane to benzene over a period of 42 hour~ on stream under the processing conditions of ~xample 1.
S0 Durin~ the run the results obta;ned we~re i49 set out overleaf.

lOS7730 , . . . . . . . , .
Hours on Strenm .~; Acti~ity ~0 Selectivity to Benzene % Benzene Yield 2 92.5 7~.0 76.0 80.0 76.0 60.0 77.0 ~7.5 52.5 42 72.0 63.0 46.0 amPle 4 A number of different commercially available dehydrocyclisation cataly~t~ were tested under the proces~ing conditions of E~ample 1 and their acLivity and benzene yield compared with those of the sodium -containing catalyst described in Exa~ple 2.
The reqults are sst out below . . '.'- Activity -~0 ~enzane Yield . .. . . _ 0.~5 ~ms (0.45 ml) of 8 commercially available-reforming catalyqt containinfr 0.3~ Pt on A120~ 56.0 13.5 .. . . ...
0.45 g;n~ (0.45 ml) of a co~mercially nvnilable reforming catalyst containing Pt and Re on Al20 66.0 19.5 . . - , .
0.45 g~3 (0.45 ml) of a co~mercially available reforaing catalyst conLaining 0.3 ~ Pt on Al203 and containinf~ chlorine 41.0 12 0 .

Exa~le 5 A sodium doped catalyst similar to that of Example 2 W8~ emFloyed to hydro.~enate benzene at various temp~ratl)r~: ~n~ atmo~heric prq~ ?
in th^ mi._~or.~mctor u~in~ a molar r tio of hydrogen to h7.~rcc_.bon of 6:1 and e liquid ~pace velocity of 1 v/v/hour. Instead of being reduced at 500C for 2 hour~ it was reduced at 250C for h~lf an hour.
The follo~ling conver3ion~ were ac~ieved:

Temperature C ¦ Conversion '~
150 I lO0 34~6 after 3 hours on ¦ stream Example 6 A catalyst s,imilar to t;hat of Exnmple ', was employed to hy(1rogenate benzene under the same conditions and at 75C~ In t~liS e~ample the catalyst was reduced at 225C for ~ hour.
The followin~ conversions were acnieved:
__ , Time (minutes) ¦ Conversion c~o " 5 T -96.
?0 ~ 100 Example 7 Example 5 was repeated using a commercially availzlble reforming cata]yst conta~ning 0~35r~t Pt on alumina which had been reduced at 250C for 1~ hours.
The following conversions were achieved.

Temperature CConversion ~ , 100 11.5 ~ 9 ,' Example ~
~xample 7 was repeated using a commercia]ly available reforming c~tal~/xt contzlinin~ 0~35~ Pt on alumina which had been reduced at '~0 500~C for 1~ hours.

The following conversions were achieved Time (minutes~) Conversion 94.9 82.3 33.5 It will be noted that all hydrocarbon conversions were carried out in a microreactor with a capacity for 0.45 ml cata-lyst. This volume is equivalent to 0.225 g catalyst based on graphite-containing carbon and 0.45 g catalyst based on alumina.
Thus in order to ensure comparable platinum contents within the reactor it is necessary to load the carbon base catalyst with twice as much platinum as the alumina based catalyst.
Example 9 A sample of activated carbon sold by Norit Clydesdale Ltd. under the Trade Mark Norit Extra was heated in argon at 1500C partially to graphitise it. The material partially col-lapsed, i.e., its total surface area was considerably reduced.
2 g. were then oxidized to increase the surface area by heating at 450C in a stream of air flowing at the rate of 50 ml/min for 5 hours.
The following results were obtained:
BET surface area before oxidation 740 m /g BET surface area after oxidation 1350 m /g Basal surface area before oxidation 169 m2/g Basal surface area after oxidation 430 m /g Edge surface area before oxidation 2.5 m2/g Edge surface area after oxidation 35 m2/g Final BET/Basal ratio 3:1 Final Basal/Edge ratio 12:1 As a result of the heat treatments, the total weight lo## was 13.7%. The pH wa# 7.

lOS7730 Example 10 A sample of activated carbon sold by Wimborne Chemicals Ltd. under the Trade Mark SASC 13 was treated as in Example 9 with the difference that the initial heat treatment in the argon atmosphere was at 1900C.
Results set out in the following table were obtained.
Example 11 A sample of activated carbon sold by Ketjen Carbon N.V.
under the Trade Mark Ketjen EC16 was treated as in Example 9.
Results set out in the following table were obtained.
Example 12 A sample of activated carbon sold by the British Ceca Co. Ltd. under the Trade Mark Anti Carbone AC 40 was treated as in Example 9.
Results set out in the following table were obtained.

105';~30 TABLE
.
. Ex. 10Ex. 11E~:. 12 BE~ surfacearea before o~cidationm2/g 525 565 B~ surface area àfter oxidation m2/~ 345 750 1230 Basal surface area before oxidation m2/g401 278 Basal surface area after oxidationm2/g 213 726 721 Edge surface area before oxidationm2/g0.15 1.7 11 Edge surface area after oxidationm2/g . 14 73 Final BE~/Basal ratio 1.62:11.03:1 1.71:1 Final Basal/Edge ratio 24:1 54~1 10~1 ~ totai weight l08~ . 10.672.94 39.88 .

Claims (15)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Graphite-containing carbon having (1) a basal plane surface area of at least 100m2/g (2) a ratio of BET sur-face area to basal plane surface area of not more than 5:1 and (3) a ratio of basal plane surface area to edge surface area of at least 5:1.

2. Graphite-containing carbon according to claim 1, wherein the basal plane surface area is in the range 150 to 1000 m2/g.

3. Graphite-containing carbon according to claim 1, wherein the ratio of the basal plane surface area to the edge surface area is greater than 10:1.

4. Graphite-containing carbon according to claim 3, wherein the ratio of the basal plane surface area to the edge surface area is greater than 300:1.

5. A catalyst comprising (a) as support, a graphite-containing carbon according to claim land (b) as active component, 0.01 to 10% by weight of a platinum group metal disposed thereon.

6. A catalyst according to claim 5, wherein the active component is present in amount 0.1 to 5, by weight of the plati-num group metal.

7. A catalyst according to either of claims 5 or 6, wherein the platinum group metal is platinum or iridium.

8. A catalyst according to claim 5, wherein a modi-fying metal ion is present in amount 10 to 300 atomic per cent of the platinum group metal.

9. A catalyst according to claim 8, wherein the modi-fying metal ion is sodium.

10. A hydrocarbon hydrogen transfer process which process comprises contacting a hydrocarbon feedstock under con-version conditions with a catalyst according to claim 5.

11. A dehydrogenation and/or dehydrocyclisation process according to claim 10, wherein the hydrocarbon feed-stock is a C6-10?-paraffin and the process is carried out under a temperature in the range 200° to 650°C, a pressure in the range 1 to 210 bars gauge, a liquid space velocity in the range 0.01 to 20 vol/vol/hour and a hydrogen to hydrocarbon mole ratio in the range 0.01:1 to 20:1.

12. A dehydrogenation and/or dehydrocyclisation pro-cess according to claim 11, wherein the temperature is in the range 400° to 600°C, the pressure is in the range 1 to 70 bars gauge, the liquid space velocity is in the range 0.1 to 10 vol/
vol/hour and the hydrogen to hydrocarbon mole ratio is in the range 0.01:1 to 20:1.

13. A hydrogenation process according to claim 10, wherein the hydrocarbon feedstock is an unsaturated compound and the process is carried out under a temperature in the range 0 to 400°C, a pressure in the range 1 to 210 bars gauge, a li-quid space velocity in the range 0.01 to 20 vol/vol/hour and a hydrogen to hydrocarbon mole ratio in the range 0.01 to 20:1.

14. A hydrogenation process according to claim 13, wherein the temperature is in the range 100 to 300°C, the pres-sure is in the range 1 to 70 bars gauge, the liquid space velo-city is in the range 0.1 to 10 vol/vol/hour and the hydrogen to hydrocarbon mole ratio in the range 0.5:1 to 10:1.

15. A hydrogenation process according to either of claims 13 or 14, wherein the unsaturate compound is benzene.