New Catalysts Derived from Inorganic Complexes

B.Imelik e t al. (Editors),Metal-Support and Metal-Additiue Effects in Catalysis

285

0 1982 Elsevier Scientific Publishing Company,Amsterdam -Printed in The Netherlands

NEW CATALYSTS DERIVED FROM INORGANIC COMPLEXES

G. HAIRE, L. HILAIRE, 0. ZAHRAA Laboratoire de Catalyse et Chimie des Surfaces. E.R.A. 385 du C.N.R.S. Universit6 Louis Pasteur - 67000 Strasbourg - France RESUbIE L'isomErisation du mEthyl-2 pentane et l'hyarog6nolyse de plusieurs polym6thylcycloalcanes ont Gt6 GtudiEes sur des catalyseurs au platine pr6parGs a partir de complexes de CBATT dEpos6s sur alumine. Ces catalyseurs montrent une grande s6lectivit6 pour la dGmEthylation, pour l'isom6risation par mecanisme de deplacement de liaison et pour l'ouverture de cycle. Ces rcsultats sont trss diffsrents de ce qu'on obtient sur les catalyseurs classiques. Des 6tudes par EXAFS ont montr6 que la prgparation et l'activation du catalyseur ne modifient pas considgrablement la structure du cluster de dEpart : les liaisons Pt-P sont toujours prgsentes et on ne dgtecte pas de liaisons Pt-Pt en premiers voisins. Des mscanismes mettant en jeu une espsce mgtallacyclobutane intermgdiaire sont proposes pour rendre compte de n o s rEsultats. ABSTRACT The isomerization of 2-methylpentane and the hydrogenolysis of several polymethylcycloalkanes were studied on platinum catalysts prepared from CHATT clusters deposited on alumina. These catalysts showed a great selectivity for demethylation, for Bond-Shift isomerization and for ring opening, which is quite different from the behaviour of classical catalysts. EXAFS studies showed that the preparation and activation of the catalyst

did not dramatically affect the

structure of the cluster deposited on alumina : Pt-P bonds were still present but no Pt-Pt bonds (first neighbours) could be detected. Mechanisms an

involving

intermediairy metallacyclobutane are proposed to account for all these re-

sults.

INTRODUCTION Correlations between the size of metallic aggregates and some of their catalytic properties are now well established ( 1 - 3 ) . For example the skeletal isomerization of hexanes is a reaction very sensitive to the size of the metallic particles. Two mechanisms have be.en well characterized:

bond shift and cyclic

mechanisms, the latter being favoured on highly dispersed catalysts (d < 10 A) ( 4 ) . Several methods have been developed in the industry to increase the disper-

sion: the lowering of the total metal loading and the dilution with another

286 metal catalytically inactive arethe best known. Recently, catalysts prepared by decomposition on alumina of

PARSHALL and WILKINSON inorganiccomplexes were

shown to exhibit the same properties (activity, selectivity) as industrial PtSn catalysts (5, 6). Highly dispersed catalysts (d < 10 A)were prepared by 2large amounts of cyclic complexes and led to ICHIKAWA (7) from [Pt3 (CO)6]n products in the

isomerization of n-hexane. In this laboratory catalysts obtai2were shown to behave like clas(CO)3]

ned from CHINI complexes [Pt3 (p2 CO)3

sical Pt/Al 0 catalysts of low particle size (d 2 3

%

10-20 A) (4, 6, 8).

It is remarkable that these model catalysts, prepared by the controlled con-

densation of a cluster on a support, are directly comparable to industrial catalysts. However they can also show

a specific behaviour. For example a selec-

tive demethylation was found on neutral complexes of the type Co-Pt-Co in the hydrogenolysis of methylcyclopentane (6, 8). The highly specific activity for the hydrogenolysis of C-C bonds using metal cluster compounds as catalyst precursors (Ru3 (CO)12 /SiO2) has been applied to the selective cleavage of the alkyl group in ethylbenzene, leading to toluene and methane.(9) Various elements incorporated during the preparation of the catalysts can act as promotors, leading to specific properties : P , from the phosphine ligands (6), Na or K, counter-ions of CHINI complexes (4, 8) for example. In this paper we report on the properties of catalysts derived from metal complexes of the CHATT type deposited on alumina. Specific properties, such as selective demethylation, are evidenced with a series of cyclic hydrocarbons; although the particle size (d

%

20 A) was similar to classical dispersed cata-

lysts, differences in the catalytic properties were found and correlated to the presence of phosphorus (from the phosphine ligand). CATALYST PREPARATION AND CHARACTERIZATION Following the recipe of BOOTH and CHATT (10) starting with potassium tetrachloroplatinate (K2 Pt C1 ) we formed a platinum carbonyl polymer Pt(C@).

4

.

After addition of triphenyl phosphine or triethylphosphine under CO atmosphere in acetone and evaporation under vacuum we obtained the cluster well crystallized and characterized by I.R..

The predegassed alumina (WOELM Y) was impregna-

ted with the suspension of cluster in acetone at 25°C for 48 hours. The catalyst was activated with hydrogen at 250°C for 24 hours. To precise the role of the ligand we studied CHATT complexes stabilized by attendant ligands like PMe3, PEt3 or PPh and deposited on alumina ( 1 1 ) . The So-called 21 CH, 27 CH, 3 28 CH catalysts had (PEt3) ligands while the 29 CH catalyst had (PPh3) ligands. The So-called 50 CH catalyst was derived from a platinum tris-triphenyl phosphine cluster. For characterization of the activated catalysts greatest emphasis has been placed on chemisorption measurements and on the use of transmission electron

287

microscopy(Phi1ips EM 300 G)for the measurement of metal particle size distributions (6, 8, 11).

From the measurements it is clear that the use of cluster

compounds as catalyst precursors leads to formation of particles of crystallite size as low as 5 A . It is equally clear that significant aggregation of the initial Pt3 (u CO) L cluster units has occured during the overall supporting 2 3 4 /activation process. From XPS measurements by using a V.G. Scientific ESCA 3 spectrometer the oxidation states o f Pt have been systematically determined. Some E.X.A.F.S. measurements using the synchrotron radiation facilities of L.U.R.E. have been made on the 50 CH catalyst both before and after catalyst testing (12). RESULTS 1) Isomerization of 2-methylpentane

The catalytic properties o f cluster derived catalysts were studied using the isomerization of 2-methylpentane as a test reaction. In table 1 the results obtained with a series of "CHATT" catalysts, 21 CH, 27 CH and 2 8 Cil, are given and compared with a "CHINI" catalyst and a classical Pt/A1 0 of comparable me2 3 tal loading 2 % . On CHATT catalysts the conversion in isomerization was remarkably low : the selectivity S in isomers varied from 4% (21 CH) to 14% (27 CH). Conversely the cracking conversion was very high and from the cracking distribution, i t is clear that the demethylation was largely predominant. The mechanisms of isomerization were studied using 13C labelled hexanes. In table 2 we give the distribution of the isomers and cracking products obtained with the 29 CH catalyst. It is clear that the demethylation mechanism involves the tertiary carbon atom. No successive reactions were detected. The Bond Shift mechanism was largely predominant : 92, 96 and 85% for the isomerizat ion of 2-methylpentane-2-I 3C, 2-methylpentane-4-I3C and 3-methylpentane-33 ~ , respectively. 2) Hydrogenolysis of methylcycloalkanes We give in table 3 the distributions obtained in the hydrogenolysis of methylcyclopentane, 1,2- and 1,3-dimethylcyclopentanes, ethylcyclopentane, methylcyclohexane and toluene. Thc first striking result is the importance of cyclopentane (around 65% o f the products) formed by the rupture of a tertiary-primary C-C bond in the case of methylcyclopentane. Another interesting result is the very low value of the ratio r2

=

3-methylpentane/n-hexane

characteristic of a non selective hydroge-

nolysis. The percentages of methylcyclopentane obtained in the hydrogenolysis of 1,2- and 1,3-dimethylcyclopentanes

(43 and 33%) are a l s o a good indication

of selective demethylation. In the case of ethylcyclopentane, there is no rup-

ture of the C-C bonds of the ethyl group as shown by the small amounts of

288

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289

TA8LE 2 I s o m e r i z a t i o n and c r a c k i n g d i s t r i b u t i o n s f o r l a b e l l e d I3C hexanes on Ig of

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290

methylcyclopentane and cyclopentane formed, but there is a non selective nydrogenolysis of the ring. The latter result points out the interest of studying the hydrogenolysis of methylcyclohexane and toluene. On this type of catalyst the same selectivity for demethylation was observed with C6 and C5 rings. If we suppose that the ring opening is casier with a cyclopentanic hydrocarbon than with a cyclohexanic one, the hydrogenolysis of cyclohexanes might proceed via a cyclopentanic intermediate. But it is shown in table 3 tnat ihe ethylcyclopentane does not lead to the formation of 2-methylhexane; moreover a 1,3-dimethylcyclopentanic intermediate would not lead to the distributions given in table 3 and a 1 , Z dimethylcyclopentanic iiitermediate should give more 3-methylhexane than n-heptane, wich is not the case. Lastly the hydrogenolysis of methylcyclohexane gives twice more n-heptane than 3-methylhexane, wich is a further proof to discard the hypothesis of a cyclopentanic intermediate. 3) EXAFS studies EMFS measurements were performed on the cluster Pt (PPh ) /A1 0 before 3 3 2 3 and after activation by d 2 at 300°C. These experiments were done using the syn-

chrotron radiation at L.U.R.E,Orsay. It does not seem that the impregnation on alumina drastically changed the structure of the cluster. Three Pt-P bonds at 2.195 A , instead of 2.26 A in the starting material, were measured for the cluster deposited on A1203 ; these distances were the same after activation but Pt atoms were then surrounded by two P atoms only. One Pt-0 distance at 2.03 A and two Pt-0 bonds at 2.05 A were determined before and after activation, respectively. In both cases no Pt-Pt bonds (nearest neighbours) were detected but after activation eacn Pt atom was surrounded by four Pt atoms (second neigh0

bours) at 3.45 and 3.65 A

.

These

EXAFS determinationsenabled us to trace the

modifications of the structure of the cluster during each step of the preparation of the catalyst and tentative structures of the cluster supported on alumina, before and after activation, can be drawn:

before activation

291

PPh3

\

after activation 4 ) Mechanisms

The results obtained on catalysts prepared from CHATT complexes showed a great selectivity for the demethylation reaction, for the Bond Shiftisomerization and for ring opening. Until now, no classical platinum catalyst had led to such a big amount of selective demethylation (around 60%). These trends were confirmed by the hydrogenolysis of polymethylcycloalkanes and toluene. Our EXAFS results clearly show that the deposition of the cluster on

alu-

mina and the subsequent reduction by hydrogen modify the structure of the cluster but not in a very dramatic way. In particular even after reduction Pt-P bonds are still observed. Another important point is that no Pt-Pt bonds are present as first neighbours. We are very far from the structure of Pt crystallite which prevails in a classical catalyst. T t is therefore tempting to try to explain our results by mechanisms analo-

gous to those invoked in homogeneous catalysis. We believe that the formation of a metallacyclobutane species as a precursor leading either to isomerization

or to selective denethylation ( 6 , 8) may explain our results. In particular this intermediate is in good agreement with our EXAFS results since it involves the participation of a single platinum atom, and not several contiguous atoms like in the mechanisms proposed to explain the results obtained on classical catalysts (6).

292

These t y p e s of experiments on c l u s t e r d e r i v e d c a t a l y s t s a r e i n t e r e s t i n g s i n c e t h e y may p r o v i d e a t o o l t o fill t h e gap between c u r r e n t i d e a s i n homogeneous anti heterogeneous c a t a l y s i s .

REFERENCES

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