Catalytic isomerization over metal, acid and hybridsites

T.S.R. Prasada Rao and G. Murali Dhar (Editors) Recent Advances in Basic and Applied Aspects of Industrial Catalysis Studies in Surface Science and Catalysis, Vol. 113 9 1998 Elsevier Science B.V. All rights reserved

CATALYTIC SITES

ISOMERIZATION

OVER METAL,

41

ACID AND HYBRID

Wolfgang M.H. Sachtler

V.N. Ipatieff Laboratory, Center for Catalysis and Surface Science Department of Chemistry, Northwestern University Evanston, I1, 60208, USA ABSTRACT Recent results are presented illustrating principal mechanistic differences between alkane isomerization in liquid acids and over solid acids, including bifunctional catalysts. Isotopic labeling shows that butane isomerization over solid acids proceeds preferentially as a bimolecular process, i.e. via a C8 intermediate, which subsequently decomposes, preferentially into two iso-C4 structures. Bronsted acid sites in zeolites form chemical bonds with metal clusters. The resulting metal-proton adducts function as "collapsed bifunctional sites". Key Words: Acid Catalysts, Bifunctional Catalysts, Sulfated Zirconia, Metal-Proton Adducts, Carbenium Ions, Protonated Cyclopropane, Butane Isomerization, Collapsed Bifunctional Sites, Electron Deficiency. 1.

INTRODUCTION

In 1951 J. W. Otvos et al. published some remarkable observations they had made when dissolving /-butane in deutero-sulfuric acid, D2SO4 [1]. They found that rapid exchange of H atoms and D atoms took place, as expected, but two observations puzzled them: (1) exchange started only after an induction period (2); in an excess of D2SO4, nine of the ten H atoms in C4H10 were exchanged, but the tenth H atom, bonded to the tertiary carbon atom, was not exchanged. They also found that adding a small amount of butene eliminated the induction period. They concluded that butene acts as a base and an equilibrium of the following type is established"

CH3 H2C=C

I i

CH3 + I-~

CH3 Scheme 1

-.

I 4

-."- HsC ~ C O

CH3

42 Nowadays, the positive ion on the right of this equation is called a carbenium ion, or more specifically, the trimethylcarbenium ion. Carbenium ions are a large, important subgroup of carbocations. Rapid equilibration in an excess of D + ions will lead to the perdeutero form of the trimethylcarbenium ion, in which all nine H atoms have been replaced by a D atom. This carbenium ion then reacts with another/-butane molecule: ~D3 D3C--C@

fH3 + H--

CH

~D3

C---CH3.~---~ D3C-- ~ -

I

I

CD 3

CH 3

CD 3

i 3

H

@ C ~ CH CH 3

Scheme 2

The second reaction is appropriately called a hydride ion transfer. Carbenium ions are very reactive, isomerizing readily. They can also react with olefins; large carbenium ions undergo gjfission, which is the basis of catalytic cracking, currently the largest scale industrial operation of the world petroleum industry. By virtue of the hydride transfer reaction an infinite number of alkane molecules can temporarily be in the active state of the carbenium ion and, for instance, isomerize, as illustrated in scheme 3.

RI-H

RI +Rz-H K'h | -. Kr -R2 K'r

+ R2 Q

Kh

RQl R1-H

Kov K'ov

R2-H

Scheme 3

Different carbenium ions have different energies; among the alkyl ions, the tertiary carbenium ion is the most stable. Isomerization in the liquid phase can make use of secondary carbenium ions or protonated cyclopropane intermediates, as shown by Brouwer and Hoogeveen [2]. Primary carbenium ions have a very high energy and therefore do not play an important role as reaction intermediates in alkane conversions in the liquid phase. Isomerizations which lead to a change in the degree of branching make use of the protonated cyclopropane intermediates, as shown in scheme 4 for a C5 carbenium ion.

43

/Ell2 CH3--CH~ ~ ~CH2--CH3 l I-I-transfer

-

CH2

/ I ~c~-cH~ CH3--CH2 C3 ring formation CH,

/ CH3-- CH t ~ - \ C H - - C H 3

l ring opening

~M~!H-shi~ CH3--C--CH2--CH3 cH~, H-

transfer

CN3--CH--CH2--CH3 Scheme

4

Pentane is the smallest alkane which can be isomerized in this way. With butane, however, a new difficulty arises. Although a methylcyclopropane ring can be formed, its opening toward an isobutyl skeleton would result in a primary carbenium ion. Because of the high energy of this species, isomerization of butane in liquid acids is estimated to be roughly 101~ times slower than that of pentane or higher alkanes. The methyl-cyclopropane ring can, however, be opened in a different way, such that no primary carbenium ion is formed. In this way a new cation of n-C4 is formed, it differs from the original ion only by the fact that two carbon atoms have changed their places. Scheme 5 illustrates this using a C4 ion with one labeled C atom.

c89*

C--C'~

~/C~c,

c

Scheme

- -

5

c\

c\

c--~ vI

Q

~...

*

~C--C--

C

~|

C --C--C*

--C*

Y ----~C- - C - - C *

44 On solid acids, such as zeolites in their H-form or sulfated zirconia, Bronsted sites of high acid strength exist. These are O-H + groups, adjacent to an O= ion. Although the polarity of the O-H dipole is very small, they can react with weak bases such as alkenes. The adsorption complex is best described by an alkoxy group, i.e. a new C-H bond and a C-O bond is formed with the Bronsted acid/Lewis base pair at the surface I3, 4]. This is illustrated in scheme 6"

+0.021e

H,~,, +0.056e 1.45 H.... ,C ~ C .... H 1.16,,'" "'H ,'

1.32 . . . . . . H

H~...

2.22,/ ' ,' 9

H,,,

+0.384e

H.... C ~ L ~

_ H ~;~H

/ i .09

/.55

:2.94

,'

1.55l

H /0.98

o

1

AI 1~.72\ ~O ~

"'"O--

pi-complex of ethylene

_

2.07

AI 1.77

O/ """ b

the transition state of the ethyxylation reaction

/ AI , --O

O--

the final ethoxide

structure

Scheme 6

Although this looks very different from an "adsorbed carbenium ion", reactions at the surface of solid acids somewhat resemble those in liquid acids. Kazansky proposes that the cause for this similarity is that the transition state leading to the alkoxy group is very similar to a carbenium ion. The present paper's objective is to identify some fundamental differences in the catalytic chemistry between liquid and solid acid catalysts. The obvious advantages of solid over liquid acid catalysts include the easy separation of products from the catalyst and the easy applicability of higher temperatures. However, allylic carbenium ions can also be formed at elevated temperature; they are precursors of carbonaceous deposits that tend to deactivate the catalyst. To minimize formation of such "coke", transition metals are often deposited at the catalyst's surface and hydrogen is applied at a high pressure. Over these "bifunctional catalysts" coke precursors are hydrogenated in situ so that the catalyst surface is kept clean. Therefore, when comparing liquid and solid acids, it appears appropriate to include bifunctional systems in the group of heterogeneous acid catalysts. 2.

CHARACTERIZATION OF METAL AND ACID SITES

As competent textbooks [5] and review papers [6] exist on the characterization of metal sites, only a brief overview will be given here. Hydrogen chemisorption is often used to obtain a rough estimate of the metal dispersion. It will, however, be shown below that this method is unreliable when small clusters of a transition metal coexist with strong Bronsted

45 sites on a solid surface. The formation of metal-proton adducts leads to strong suppression of the propensity of the metal to chemisorb hydrogen [7, 8]. Extended X-ray Absorption Fine Structure (EXAFS) is a more reliable technique for estimating the average size of small metal particles and clusters [9-11]. Differential X-ray scattering [12], Nuclear Magnetic Resonance (NMR) [13, 14] and Xe-NMR have also been used successfully [15]. Transmission electron microscopy shows metal particles above a critical size, depending on the metal and the instrument. For "invisible" particles Energy Dispersive Spectroscopy (EDS) using X-rays (EDX) identifies the presence or absence of the metal element in a surface region of the order of 10 x 10 nm 2 [ 16]. Whereas the heat of adsorption of an adsorptive can in principle be determined by calorimetry, it is often easier to derive the activation energy for desorption from the kinetics of temperature programmed desorption (TPD), sometimes also called TDS for "thermal desorption spectroscopy. Acid sites on catalyst surfaces have often been characterized by adsorption of indicators, as originally proposed by Benesi [17]. However, some of these large organic molecules tend to react with a diversity of sites on a complex catalyst surface. IR spectroscopy of ammonia adsorbed on Bronsted sites shows bands typical for ammonium ions; on Lewis sites ammonia forms a Lewis acid/base complex. Other strong bases, such as pyridine, are used in a similar manner. A combination of IR spectroscopy with TPD provides information on the enthalpy of formation of these complexes and thus an indication of the relative acid strength. Recently, Dumesic et al. used microcalorimetry to characterize acid sites on sulfated zirconia [ 18, 19]. They showed that only Bronsted sites are instrumental in the isomerization of butane. A very small fraction of sites with strength in the range conventionally ascribed to superacids is also present, but these sites deactivate very rapidly and are of negligible effect in the steady state of the isomerization process. For Lewis acid sites, a reliable method to estimate the acid strength was proposed by Zaki and Kn6zinger [20], who use carbon monoxide as the probe. The C-O bond in this molecule is destabilized by electrons in an antibonding orbital, but when this electron pair interacts with a Lewis acid, the C=O bond is stabilized and the vibrational frequency shifts to higher values. This shift of the IR band is a direct measure for the strength of the Lewis acid. Adeeva et al. applied this method to characterize the Lewis acid sites on sulfated zirconia [21]. They found that these sites are weaker acids than exposed A1 ions in properly dehydrated y-A1203 catalysts. The same group interrogated the Bronsted sites in sulfated zirconia by adsorbing a very weak basis such as acetonitrile. The adsorption of such molecules causes a shift of the IR band characteristic of the O-H vibration; likewise the chemical shift of the proton-NMR signal characterizing the surface H atoms is changed. Both the IR and the NMR shifts were compared for different acid catalysts. The results show that the Bronsted sites in sulfated zirconia are strong acids, but are weaker than the Bronsted sites in acid zeolites such as HY or HZSM-5. This laid to rest speculations that the particular catalytic activity of sulfated zirconia might be caused by the presence of some "superacid" sites.

46 3.

CLASSICAL R E A C T I O N MECHANISMS

Platinum was originally introduced into reforming catalysts such as silica-alumina for the purpose of providing hydrogenation sites. It appeared, however, that transition metals also catalyze skeletal isomerizations and hydrogenolysis of hydrocarbons. Research on the isotopic exchange of, alkanes by Kemball [22, 23], Burwell [24, 25] and others showed that these molecules are chemisorbed by rupture of one C-H bond and the formation of a metalhydrogen and a metal-alkyl bond. This initial step is often followed by ones in which a second C-H bond is broken. The authors assumed that ix,ix- or ix,[3 diadsorbed molecules are formed, i.e that the molecule is assumed to be bonded to the surface by either a M - C double bond or two M-C single bonds. In the presence of an excess of deuterium the initial product contains significant concentrations of molecules with more than one D atoms. A well known example is the formation of cyclopentane with five D atoms. This CsHsD5 molecule is assumed to be formed by successive step-overs of an tx,13 diadsorbed complex, leading to complete exchange of the five H atoms at one side of the C5 ring. Kemball observed a correlation between the propensity of a transition metal to form M=C double bonds and its hydrogenolysis activity [22]. Among the Group VIII metals, Ru, Ni, Co and Ir have the highest propensity to form double bonds: under a fixed set of conditions they catalyzed hydrogenolysis near 100~ in contrast, the metals Pd and Pt form very little M=C double bonds: they require roughly 300~ to achieve a similar extent of C-C fission. Metal catalyzed isomerizations of alkanes and other hydrocarbons were extensively studied by F. Gault et al. [26]. Using 13C labeled molecules, these authors showed that skeletal isomerization makes use of two major routes. In both routes one can assume that the molecule is diadsorbed on the surface. For isomerization, the prevalent types seem to be either c~,,/ or ot,~ bonded molecules, i.e the reactions can be described schematically by assuming that a new C-C bond is formed, resulting in either a cyclopropane or a cyclopentane "pseudo-intermediate"[27]. Subsequent opening of this C3 or C5 ring leads to the isomerization product. For instance, Gault et al. describe the isomerization of 2methylpentane, labeled at position 2, to n-hexane and 3-methylpentane by assuming a methylcyclopentane intermediate, as shown in scheme 7:

Scheme 7

Methylcyclopentane (MCP) is a convenient probe molecule for interrogating the metal and acid sites of a bifunctional catalysts. For instance, metal clusters are formed in the cavities of zeolite Y by ion exchange, followed by calcination and reduction with hydrogen. Protons which act as Bronsted acid sites are formed during reduction of the metal ions. A monofunctional catalyst can be obtained by neutralizing these protons with NH3 or by secondary exchange with Na + ions. With this acid-free form of such catalysts the ring-

47

opening step, proposed by Gault as the second step in isomerization, is the only observed reaction in the presence of hydrogen, i.e. MCP is exclusively converted into the three ringopening products n-hexane, 2-methylpentane and 3-methylpentane:

Scheme 8 However, a totally different catalysis is observed when the protons are not neutralized, so that transition metal clusters and Bronsted sites co-exist in the same catalyst. For such bifunctional catalysts, for instance Pd/HNaY or Pd/HY, ring opening is a minor side reaction, but ring-enlargement becomes the major reaction pathway, with benzene and cyclohexane as the predominant reaction products [28.29]. Apparently, a carbenium ion has been formed from MCP, it is isomerized via the fused cyclopropane ring to the cyclohexylcarbocation, as depicted in scheme 9:

H2C--CH2

H2 H2C---CH2

H2C'--CH2

H

H2C......CH2 i~.t2

Scheme 9 Subsequent hydride transfer and metal catalyzed dehydrogenation steps lead to benzene. Qualitatively, the hydrocarbon conversion over bifunctional catalysts can thus be described as a reaction network using two types of sites. The reactions taking place on the Bronsted sites are similar to those in liquid acids, as described in the first part of this paper; a second group of reactions takes place on the metal sites; these steps are identical with those observed on the same metal in the absence of acid sites. Mills et al. devised a simple model

48 on this basis: they assumed that reaction intermediates shuttle between metal and acid sites [30]. For the isomerization of an alkane they assumed that the alkane molecule is first dehydrogenated over Pt; the resulting olefin then reacts with a proton, forming a carbenium ion. This ion will isomerize, subsequently splitting into a proton and the isomerized olefin, which will ultimately be hydrogenated to the alkane isomer. 4.

REVISED MECHANISMS

The classical model of solid acid and bifunctional heterogeneous catalysis basically assumed that the acid catalyzed reactions followed the laws known for liquid acids. It also assumed that catalysis over bifunctional catalysts, exposing both metal sites and acid sites, could be described by assuming simple additivity, reaction intermediates were thought to shuttle frequently between metal sites and acid sites. More recent research has, however, shown that both assumptions are inadequate. Some reactions which are not catalyzed by liquid acids are fast over solid acids, moreover bifunctional catalysts display substantial deviations from the additivity model.

4.1

Bifunctional Catalysts

We shall start with bifunctional systems. Using zeolite supported catalysts, recent findings are at variance with the classical model: Metal clusters and protons react with each other, forming "metal-proton adduets" [6, 29]. Their catalytic propensities are distinctly different from those of a metal cluster of the same size. The metal in the adduct is "electron-deficient", as indicated by XPS data [3133]. Consequently, the rates of some metal catalyzed reactions are much higher than on the neutral metals. The isomerization and hydrogenolysis of neopentane is an example for this [34]. Metal-proton adducts are also able to act as "collapsed bifunetional sites", because they act simultaneously as acid and as metal sites. This is advantageous for catalysis because no shuttling of intermediates is required: all reaction steps required for isomerization can be carried out by an adsorbed molecule during one residence at this bifunctional site. Whether the molecule in this state behaves as an adsorbed olefin or as an adsorbed carbenium ion depends on the relative energies. Usually, the carbenium ion is more stable. This is why adding acid sites to a given metal catalyst not only opens a new "bifunctional" avenue for reaction, but also lowers the rate of the monofunctional, i.e. the metal catalyzed reaction. This was observed for the conversion of MCP over zeolite supported Pd [29, 35], and Rh catalysts [36]. These metals can catalyze typical metal catalyzed processes such as ring opening of MCP at low temperatures. However, when they are present as metal-proton adducts, carbenium-like adsorbates are formed which block the surface of these clusters for the monofunctional, i.e. metal-catalyzed processes. In the case of rhodium this is manifest at low temperature where the carbenium ions do not react further. As the proton in a zeolite is fixed near the negative charge of an A1 containing tetrahedron, the formation of a metal-proton adduct imposes considerable immobilization on the metal cluster. This "chemical anchoring" inhibits migration and coalescence. Therefore, the formation of this adduct helps to achieve extremely high metal dispersion. Even monoatomic platinum has been stabilized in H-mordenite by exploiting the anchoring action of zeolite protons [37].

49 4.2

Butane Isomerization Over SolidAcids

Some isomerizations that are undetectable in liquid acids are fast over solid acids at rather low temperature and in the absence of a transition metal. A case in point is the isomerization of n-butane to /-butane. As mentioned above, the formation of a protonated cyclopropane intermediate is possible with butane, but its opening to yield a n iso-C4 carbon skeleton would lead to the energetically prohibitive formation of a primary carbenium ion. Therefore, the isomerization of butane in liquid acids is many orders of magnitude slower than the isomerization of pentane. Over some solid acid catalysts, however, butane isomerization has been reported even below 300K [38]. Some catalysts of ttiis category are based on sulfated zirconia, usual!y in the tetragonal modification. The most active members contain additional oxides, for instance of manganese and iron. As with these materials, a reaction which is not catalyzed by even the strongest liquid acids under comparable cofiditions proceeds rapidly; the problem arises of whether or not there are principal differences between liquid and solid acids. It was already mentioned that carbenium ions are well identified by NMR in liquid acids but not on solid surfaces. The alkoxy groups identified by Kazansky might suggest that it is more difficult to form carbenium ions on solids, but this assumption obviously fails to explain the superior activity of some solid acid catalysts in butane isomerization. The mechanism of butane isomerization over sulfated zirconia and some industrial catalysts has been unraveled by Adeeva et al. [39-41]. In the following Table the proprietary catalyst samples provided by industry are simply denoted A, B, C, and D. The authors used double labeled n-butane, 13CH3-12CH2-12CH2-'3CH3 and studied the distribution of 13C atoms in the initial reaction product, /-butane. Isotope Distribution in Isomerization Product of n- butane over sulfated zirconia (SZ) and four industrial catalysts on the basis of aluminum chloride

13C1

13C0

39.31

25.86

6.35

14.4

64.6

14.0

5.2

4.6

18.9

47.5

23.0

6.0

C

1.6

24.3

49.0

21.8

3.2

D

2.1

19.3

51.9

19.7

6.9

(Binomial)

6.25

25.0

37.5

25.0

6.25

Catalyst

13C4

SZ

5.78

23.70

A

1.7

13C3

13C2

....

.......

50 The results show that considerable isotopic scrambling takes place over all five catalysts. At low conversion, the unconverted n-butane retained its isotopic identity, nbutane molecule contains two 13C and two 12C atoms. However, the fragmentation pattern in the mass spectrum showed that considerable internal rearrangement of 1 3 C H 3 - 1 2 C H z - l Z c H 2 13CH3 to '2CH3-13CH2-12CH2-13CH3takes place. In this respect solid and liquid acid catalysis are identical. A crucial difference between solid and liquid acids, however, is that the isomerization of n-butane is a bimolecular process only over solid acid catalysts. A Cs intermediate is formed which isomerizes easily. Fission of this intermediate results in two C4 fragments, one or both of which have a n iso-C4 structure. If the adsorbed C4 intermediate is assumed to be essentially a carbenium ion, the formation of the C8 intermediate requires reaction of this carbenium ion with a C4 olefin: (C4H9) +

+ C4H8 = (C8H17) +

Scheme

10a

Most feeds contain some olefin as an impurity; moreover many sulfated zirconia catalysts contain traces of iron or other transition metal ions that are able to dehydrogenate butane. In the presence of such sites, the olefin concentration is limited by thermodynamics, i.e a high pressure of H2 leads to a low olefin concentration. That aspect of the reaction mechanism has been proven in independent experiments. The isomerization rate over sulfated zirconia was dramatically lowered by H2. This effect is most pronounced when a small amount of platinum is deposited on the catalyst, so that thermodynamic equilibrium between butane, hydrogen and butene was established. In this way it was found that the isomerization reaction has a reaction order of +1.3 in n-butane, but -1.2 in hydrogen [40, 41]. The byproducts, propane and pentane, are additional evidence that a C8 intermediate is formed in this process. As expected, this kinetics is typical for butane isomerization only; in contrast pentane isomerization is mainly a monomolecular process, because for this molecule the protonated cyclopropane ring can be opened without forming a primary carbenium ion [42]. While intermolecular processes are evidently required to obtain isotopic scrambling between /-butane molecules, not every bimolecular route will lead to scrambling. Remarkably, scrambling reaches the extent predicted by random statistics and expressed by the binomial law only over sulfated zirconia. It is therefore useful to inspect the formation of the C8 intermediate and its [3-fission more carefully. One can assume that the C8 carbocation is initially formed from a but-2-ene molecule and a secondary C4 ion, following Markovnikov's rule:

13CH-'~"cH#C(~I-I'-..I(~H3 + lac~...~CH....q3 CH

CH3

1H3 13 "

C1"43"C H2/CH'cH/ ~ q 3 CH3 13(~H3

Scheme 10b

In this intermediate all four terminal groups contain a 13C atom, all internal groups a lzC atom. Obviously, this distinction will not disappear if subsequent isomerization is limited to facile internal shifts of hydride ions and methyl groups. For isomers of this category, B-fission will lead to a secondary C4 carbenium ion and an isobutene molecule.

51 Each of these units will still contain two ~3C atoms; no isotopic scrambling takesplace. This statement is rigorously correct only if no internal rearrangement in the Ca monomers occurs prior to the formation of the C8 dimer. In reality , internal rearrangement is well documented. Its rate has, however, been found to be lower than the rate of isomerization for all catalysts. Randomization of the 13C label, as observed over sulfated zirconia would require an extremely fast internal atom rearrangement prior to the formation of the C8 intermediate, if only simple methyl shifts inside the dimer took place before J3-fission. This model can thus be discarded. It follows that substantial carbon scrambling in the C8 carbocation' is required to achieve randomization of the carbon atoms in the ultimate C4 entities. Even isomerizations of the C8 carbocation via protonated cyclopropane intermediates can lead to isomers which, after 13-fission, each have two 13C atoms. Scrambling, leading to binomial distribution, requires that an additional condition be fulfilled: an isomer which could undergo 13-fission must isomerize further, before breaking into two Ca entities. This is illustrated in reaction scheme 11: 13 CH3 13 C..H.H3 ~../ 1

]

CH2 CH ,o -..., / "--.~,, L; L;I-12 L;I-13 0

|

Qc

, C /~ closure

H3

131 ............

C

L,;I-13

CH3

|

13

I

H2\C H CH / "-43.

c C/c' CH H2

I

CH 13 H/ "J'~ H3

13 13 OH3 (~H3 QH3 / 13 [ | / C H 13 CH3_,..__CHy --.:CH3 13(~H3

13CH3

9- 13C H 3 ~ C - - - C H /~ opening 13 CH3 OH3 I CH3 shift H- shift

"13 CH3~ C H

130H3

'

/CH13 CH3

1 3(rH3

CH 13[ CH3

(~H3

s "~ ~-CH3

13 H3

I I~.,,~CH~cH~C-.~H~ ~n3

13

CH;shift H- shift

1~ / C ~ CH3

_ _ I~CH ~ CH2

H3 13

jC~

CH3

Scheme 11

3 1 OH3

CH3

13

CH3

l, 131 CH3

I

~ CH3-----C~ C H 2

CH2-~C~CH3

B-fission

13CH3 +('~-)C-CH3 13~1H3

52 It is a special feature of sulfated zirconia that over this catalyst even the 2,2,4trimethylpentyl carbocation undergoes numerous isomerizations before its B-fission into two iso-C4 fragments. 5.

CONCLUSIONS

Although true carbenium ions have never been detected on solid surfaces, catalysis over solid and liquid acids displays many similarities, but also some characteristic differences. A crucial difference between solid and liquid acids is the ability of certain solids with strong Bronsted sites to isomerize n-butane to/-butane via a bimolecular mechanism. A C8 carbocation is formed which isomerizes and undergoes 13-fission. In this fission, the formation of two iso-C4 units is apparently preferred. Only if fission is preceded by extensive isomerization of the C8 carbocation can isotopic scrambling reach the randomization predicted by the binomial law. Catalysts which contain reduced transition metal clusters besides acid sites are able to catalyze reactions that are not observed on catalysts exposing one type of site only. The reaction network is inadequately described by models which assume only additivity of catalytic functions and shuttling of intermediates between sites. There is strong evidence that metal clusters and Bronsted sites form metal-proton adducts. These act as "collapsed bifunctional sites"; all alkane isomerization steps can take place on such sites during one single residence of the adsorbed molecule. At low temperature, adsorption in a mode reminiscent of a carbenium ion can suppress pure metal catalysis. ACKNOWLEDGMENTS Financial support for this research by the US National Science Foundation and the Director for Basic Energy Sciences, US Department of Energy, and Shell Development Company is gratefully acknowledged. We thank Magnesium Electron Inc, Cytec Industries Inc., Engelhard Corp. and UOP for kindly providing us with catalyst samples. REFERENCES

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