Mechanisms of hydrocarbon conversion reactions on heterogeneous ...

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Topics in Catalysis Vol. 34, Nos. 1–4, May 2005 (Ó 2005) DOI: 10.1007/s11244-005-3806-4

Mechanisms of hydrocarbon conversion reactions on heterogeneous catalysts: analogies with organometallic chemistryq Francisco Zaera* Department of Chemistry, University of California, Riverside, California 92521, USA

Catalysis is central to most industrial processes for chemical manufacturing. As catalytic processes have become more complex and more demanding, selectivity has become the central issue in their design. Selectivity is defined by the relative rates of competing reaction pathways available to crucial intermediates, and can be controlled by subtle changes in the nature of the catalyst, the reactants, and/or the reaction conditions. In order to be able to do this in a systematic manner, a good understanding of the catalytic reaction mechanisms is needed. Here a connection is drawn between the key elementary steps comprising hydrocarbon conversion reactions on surfaces and those known to occur on discrete organometallic complexes. This way, the hydrogenation, dehydrogenation, hydrogenolysis, chain growth, and isomerization reactions typical in heterogeneous catalysis are redefined in terms of hydride elimination, oxidative addition, reductive elimination, migratory insertion, and. 1, 2-shift elementary steps, among others. It is suggested that the knowledge already available from organometallic chemistry can be used to further advance the understanding of the surface science involved in heterogeneous catalysis. Thanks to the commonality of the chemistry involved, a better synergy could also be established between homogeneous and heterogeneous catalytic development. These ideas are discussed in this article in a critical and personal way. KEY WORDS: heterogeneous catalysis; surface science; hydrocarbon conversion; transition metals; reaction mechanisms.

1. Introduction Catalysis is at the heart of most modern industrial processes [1,2]. Catalytic processes can be carried out either homogeneously or heterogeneously, depending on if the reactants and the catalysts are present in the same or different phases. In practice, the majority of the catalytic processes used nowadays are heterogeneous in nature, typically involving a solid catalyst and gas- or liquid-phase reactants. Compared to homogeneous catalysts, heterogeneous catalysts are easier to prepare, handle, separate from the reaction mixture, recover and reuse, and are often more stable, cheaper, and less toxic. On the other hand, homogeneous catalysts, typically organometallic compounds, are better understood and easier to manipulate for the design of selective processes. In fact, many catalytic reactions can be carried out both homogeneously and heterogeneously, as is the case with, for instance, the hydrogenation of olefins, which can be easily promoted by either organometallic complex such as Wilkinson’s catalyst (a rhodium (I) complex) or the surfaces of most late transition metals [3–5]. The ability to promote reactions by using both organometallic complexes and solid metals points to a q

Invited contribution to the special volume entitled ‘‘The Interface between Heterogeneous and Homogeneous Catalysis,’’ stemming from contributions at the recent International Symposium on Relations between Heterogeneous and Homogeneous Catalysis, and dedicated to the memory of Robert L. Burwell. *To whom correspondence should be addressed. E-mail: [email protected]

similar chemistry in both cases. This organometallicsurface analogy has been already advanced by several research groups [6–9]. Specifically, the nature of the chemical bond between solid surfaces and adsorbed molecules has been shown to be quite localized, and to share the same qualitative molecular orbital picture that helps explain the binding of ligands to metal complexes [10]. For instance, the model developed by Blyholder to describe the interaction of carbon monoxide to metal surfaces [11] closely follows the p-bonding ideas so well accepted in inorganic chemistry [12,13]. Similarly, the determination of much of the structural information .available on adsorbates on metal surfaces [14] has been aided by crystallographic information from organometallic analogs. The resolution of the controversy surrounding the nature of the species that forms on Pt(1 1 1) upon adsorption of ethylene at room temperature, for one, required a comparison with data from an ethylidyne tricobalt complex [15,16]. Unfortunately, studies on the reactivity of adsorbed species have benefit to a much lesser extent from this organometallic-surface science analogy. There have been some clear correlations identified between surface reactions and those seen with organometallic ligands [8,9,17,18], but much more remains unanswered in terms of reaction mechanisms and reaction kinetics on solid substrates. It has been one of the main aims of our research group to use the ideas of organometallic chemistry to better understand the thermal chemistry of adsorbates, of hydrocarbon moieties in particular, on transition metal surfaces [19–23]. Below we provide a 1022-5528/05/0500–0129/0 Ó 2005 Springer Science+Business Media, Inc.

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brief summary of the results from that work so far. Specifically, a number of surface elementary steps are described and grouped in terms of the corresponding reactions that take place in discrete molecules, and the similarities and differences between the two types of systems and the relevance to catalysis are discussed.

2. Oxidative additions Many catalytic hydrocarbon conversions of industrial relevance such as the activation of methane for the production of more useful chemicals and oil reforming start with mixtures of one or more alkanes as feedstocks [24]. Since the CAH bonds in those molecules are strong, saturated hydrocarbons are quite stable and difficult to activate. In spite of some interesting recent advances [25,26], practical applications of alkane activation by organometallic compounds are still in the future. On the other hand, the conversion of alkanes with heterogeneous catalysts is widespread [27]. The starting point for these reactions in both homogeneous and heterogeneous systems is believed to be an agostic interaction between a given CAH bond and a metal center, a step that is followed by the oxidative addition of that bond [8]. Unfortunately, the isolation and full characterization of such a reaction has proven elusive. In contrast, the oxidative addition of weaker bonds involving carbon atoms can be easily induced. For instance, the activation of carbon–halogen bonds, of carbon–iodine bonds in particular, have been reported repeatedly both in organometallic compounds and on metal surfaces. We have over the years taken advantage

of this reaction to prepare alkyls and other hydrocarbon moieties on many transition-metal single-crystal surfaces [21,28–31]. Figure 1 presents typical data for the thermal conversion of adsorbed alkyl iodides to alkyls on metal surfaces. Shown are reflection-absorption infrared spectra (RAIRS) from normal and deuteriumlabeled neopentyl iodides adsorbed on Pt(1 1 1) at 115 and 170 K to point to the subtle differences in vibrational modes that occurs upon the scission of the CAI bond [32]. For instance, the CH3 symmetric stretching (around 2965–2963 cm)1) and deformation (1366 and 1390–1395 cm)1) modes in normal and neopentyl-a-d2 become narrower and split upon scission of the CAI bond, and the CD2 deformation in both a-d2 and perdeuterio isotopomers shifts down (from 927–917 to 918–902 cm)1). Typically, most CAI or CABr bonds in halohydrocarbons on late transition metals cleave below 200 K, and display activation barriers on the order of 5 kcal mol)1 [33,34]. The preparation of surface species, alkyls in particular, via oxidative addition of appropriate halohydrocarbon precursors has allowed us to characterize the surface chemistry of hydrocarbon conversion catalysis in great detail [19,21,23]. Dehalogenation using supported metal catalysts is also critical in removal of CFCs (chlorofluorohydrocarbons) and other halogenated solvents from the environment [35].

3. b-Hydride eliminations The dehydrogenation of hydrocarbon fragments coordinated to metals often occurs via hydride elimination steps. In addition, depending on the skeletal

Figure 1. Reflection–absorption infrared spectra (RAIRS) for neo-pentyl iodide adsorbed on Pt(111) single-crystal surfaces at 115 and 170 K. The four panels correspond to four different isotopomers obtained by selective deuterium labeling: normal neopentyl iodide (d0, left panel), neopentyl iodide fully deuterium-labeled at the alpha (a-d2 , second panel from left) and the gamma (c-d9, third panel from left) positions, and perdeuterio neopentyl iodide (d11, right panel). The subtle changes seen in each cases between the 115 and 170 K data attest to the oxidative addition of the CAI bond and the corresponding formation of neo-pentyl surface groups [32].


structure of the hydrocarbon moieties, there may be a specific regioselectivity to this hydrogen removal. It has long been established that the loss of hydrogen atoms in organometallic compounds occurs preferentially at the beta position, that is, at the second carbon away from the metal center [36]. Since our initial report of selective hydrogen abstraction from the methyl moiety in ethyl groups on Pt(1 1 1) surfaces [37], b-H elimination has also been widely recognized in adsorbed systems [34,38– 41]. In fact, a number of clever experiments have provided some insight on the microscopic details of this elementary step. For one, studies on the effect of substituents based on the inductive effect of fluorine atoms [42] have been used to establish the hydride character of the leaving hydrogen atom [43]. Additional work using alkyl groups with constrained structures, cyclic compounds in particular, have been employed to point to the planar nature of the transition state [44]. Selectivity towards the abstraction of hydrogens from inner carbons has also been reported [34,41]. All these characteristics parallel the behavior reported on discrete organometallic compounds. The initial surface-science work on b-H elimination on metals mainly involved chemisorbed alkyl species. Figure 2 displays data indicating the occurrence of that step for the case of 1-propyl groups adsorbed on Pt(1 1 1) single crystals. The temperature programmed desorption (TPD) data on the left correspond to the detection of the resulting propene in the gas phase, while the infrared spectra on the right prove the formation of di-r adsorbed propene on the surface (note in particular

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the new weak features at 1023, 1042, 1092, 1436, 2827 and 2880 cm)1, corresponding to the CH3 rocking, the CH2 twist, the CAC stretch, the CH2 scissoring, and the CH2 and CH3 symmetric stretches, respectively) [45]. More recently, similar b-hydride elimination reactions have been indicated for other surface moieties. For instance, metallacyclobutane intermediates, prepared via thermal activation of adsorbed 1,3-diiodopropane, undergo b-hydride elimination on Pt(1 1 1) around 220 K to yield fairly stable g3–allyl surface moieties [46]; indirect evidence suggest that a similar reaction is also available on Ni(1 0 0) [47]. This dehydrogenation pathway is available for hydrocarbon fragments with heteroatoms as well. For instance, extensive work has confirmed the thermal conversion of alkoxide surface species to aldehydes or ketones, a reaction that requires the removal of a hydrogen atom from the carbon adjacent to the oxygen atom [17,18,20]. In this case b-hydride elimination is significantly slower than with alkyl groups, but still generally preferred over H removal from other positions in the hydrocarbon chain. For instance, on Ni(1 0 0), while 2-propyl and sec-butyl surface groups are converted to propene and isobutene at temperatures around 170 and 135 K, respectively [40,48], 2-propanol activation on Ni(1 0 0) produces acetone only about 300 K [49]. b-Hydride elimination may be easier from adsorbed terminal alkoxide groups, but the resulting aldehydes are also more reactive, and therefore more difficult to isolate [50]. b-Hydride elimination directly accounts for the formation of olefins from alkanes. Equilibria are indeed

Figure 2. Propene temperature programmed desorption (TPD, left panel) and RAIRS (right) data for 1-iodopropane adsorbed on Pt(1 1 1). After formation of 1-propyl groups on the surface via CAI bond activation, a b-hydride elimination step leads to the formation of propene. This is evidenced both by the detection of the propene in the gas phase around 255 K and by the significant changes in the vibrational spectrum of the surface species around 200 K [45].

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rapidly established between alkanes and alkenes in reforming, Fischer-Tropsch, and other hydrocarbonconversion processes [24,51]. Under the reducing conditions used in most of those processes, however, the extent of the dehydrogenation is small. This step is also directly connected to the academic H–D exchange reaction in alkanes and alkenes [52–54], and is key in the mechanism for double bond migration and cis-trans isomerization [22,55,56]. 4. a- and c-hydride eliminations Hydride eliminations at other position within the hydrocarbon chains are more difficult, but still possible. Indeed, many examples are already available for a-hydride eliminations to alkylidenes in the organometallic literature, most involving early transition metals [36]. In some cases, a-hydride elimination may even compete favorably with b-hydride removal steps [57]. On surfaces, hydride elimination from adsorbed methyl, a fragment with alpha hydrogens only, has been characterized on a number of transition metals [58–61]. In general, it has been found that a-hydride

elimination on surfaces is quite demanding, occurring at temperatures somewhere between 50 and 200 K higher than those needed for b-hydride elimination [21]. On the other hand, hydrogen removal at the alpha position also displays a higher activation energy, so the difference in rates between the two hydride elimination steps is reduced at the higher temperatures typical of catalytic reforming processes [23]. A couple of experiments in our laboratory have indicated that a- and b-H eliminations may in fact display similar rates about 800–900 K [53,62]. In particular, NMR and mass spectrometry analysis of a reaction mixture produced by catalytic isotope exchange of normal ethane with deuterium gas promoted by a platinum foil pointed to the formation of comparable amounts of 1,1- and 1,2-didueriated ethanes, implying close competition between eliminations of hydrogen atoms from adsorbed ethyl groups at the alpha versus beta positions (respectively) [53]. The data in figure 3 corresponds to tandem mass spectrometry studies on a sample collected after 30% conversion [63]. The composition of that mixture is quite complex, containing significant amounts of all ten possible deuterium isotopologues of ethane, and only 1% of

Figure 3. Mass spectrum of the reaction mixture collected after 30% conversion of a mixture of 500 Torr of D2 and 50 Torr of normal ethane on a platinum foil at 590 K. The data correspond to ions produced in the second stage of a tandem mass spectrometry/collision-induced decomposition mass spectrometry (MS/CID-MS) instrument from the 32 amu ion, and focus on the CHxDy+ fragments. The detection of signals at 15 (CH3+) and 17 (CHD2+) amu indicates the formation of some 1,1-dideuterioethane, CH3CHD2, which is proposed to involve an a-hydride elimination step. The large signal at 16 amu corresponds mainly to CD2+ ions from perdeuterioethane [53,63].


dideuterioethane. Nevertheless, it is clear from the 15 (CH3+ ) and 17 (CHD2+ ) amu peaks obtained in the second stage of the mass spectrometry analysis of the 32 amu fragment that some ethane-d2,d1 is indeed made in this reaction. A complementary study using 13C NMR indicated a 1,1-to-1,2 substitution ratio for the dideuteriated isotopologue of approximately 1:2 [64]. Evidence for a-hydride elimination has also been reported for the thermal conversion of neopentyl groups on Ni(1 0 0) [65] and Pt(1 1 1) [32] surfaces. In catalysis, a-hydride elimination has been indirectly associated with hydrogenolysis reactions [66], an idea directly supported by our detection of isobutene formation from neopentyl groups on Ni(1 0 0) [65]. Dehydrogenation steps at hydrocarbon chain positions farther away from the surface are also possible, and in fact favored over a-H eliminations on lowvalence late transition metal complexes [67,68]. These reactions can be considered a particular example of intramolecular CAH oxidative additions, and typically lead to the formation of dicoordinated cyclic compounds. Only a few examples are available for c-H eliminations from alkyl groups adsorbed on surfaces. In one study, a close competition between a- and chydride elimination from neopentyl moieties was observed on Pt(1 1 1) [69–71], in contrast with the exclusive elimination at the a-carbon reported on Ni(1 0 0) [62,65]. It is quite possible that c-hydide elimination and metallacycle formation are central to the mechanism of isomerizations and other skeletal rearrangement catalytic reactions [72]. More recently, inhibition of the b-hydride elimination route in 2propoxide groups adsorbed on Pt(1 1 1) by fluorine substitution at one of the terminal methyl moieties allowed for the detection of a competing pathway to propene. This propene is presumably produced via chydride elimination to form an oxametallacycle surface intermediate [73]. Figure 4 reports the key temperatureprogrammed desorption data indicating this behavior. Oxametallacycle formation via this type of c-hydride elimination from alcohols may be responsible for the industrial production of epoxides [74].

5. Other dehydrogenation steps A number of other CAH bond-scission steps are also possible on hydrocarbon moieties coordinated to metal centers [8]. Indeed, good precedents are available in the organometallic literature for the activation of CAH bonds in allyl [75], vinyl [76], alkylidene [77], vinylidene [78] and acetylene [79] ligands, among others. In heterogeneous catalysis these dehydrogenation steps may lead to the production of specific chemicals. On surfaces, we and others have over the years reported a few examples of dehydrogenation steps from fragments other than alkyl groups. Allylic hydrogens in

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Figure 4. Arrhenius plot of reaction rate constants for the b-and c-hydride elimination steps from 2-propoxide groups adsorbed on Ni(100). These data were acquired by temperature-programmed desorption (TPD) experiments starting with saturation coverages of the corresponding alcohols. The figure compares the data obtained for normal against partially fluorinated 2-propoxides to highlight the retarding effect exerted on the b-hydride elimination pathway to ketones by fluorine substitution at the gamma position. In turn, the slow down of the b-H elimination step leads to a competition with the more difficult c-hydride elimination reaction step to the alkene [73].

particular are quite labile and easy to remove, in particular because of the stability of the resulting allyl intermediate. Experiments carried out with allyl halides have indicated the formation of both r–g1 and p–g3 coordinated allyl moieties on Pt(1 1 1) [80,81], Cu(1 0 0) [82] and Ag(1 1 0) [83], an interesting observation in light of the fact that many p–g3 allyl organometallic complexes are prepared via expulsion of a ligand from a r–g1 allyl complex [84]. Additional work with 1, 3-diiodopropane, a precursor to surface platinacyclobutane, have shown the ease with which such intermediate can undergo b-hydride elimination to a g3-allyl moiety [46]. More direct evidence for allylic-H activation comes from studies with cyclohexene, which has been shown to convert to a stable C6H9 allylic intermediate on Pt(1 1 1) at fairly low temperatures [85,86]. This C6H9 allylic moiety has also been isolated and characterized on other metal surfaces [87,88]. Finally, H–D exchange at the allylic position has been inferred from the observation of an isomerization of methylene cyclopentane to 1-methyl cyclopentene on Ni(1 0 0) [89]. Activation of allylic hydrogens is key in many selective oxidation catalytic processes [90].

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Vinyl hydrogens are more strongly held to the hydrocarbon chain than their allylic counterparts, and are therefore less reactive. Nevertheless, their activation has been clearly seen in ethylene on Ni(1 0 0) surfaces [91–93]. Figure 5 displays kinetic results from a laserinduced desorption (LID) study of that reaction. Interestingly, a large isotope effect was observed, a difference of a factor of ~60 in reaction rate between normal versus perdeuteriated ethylene [91], indicative of severe secondary effects including a potential rehybridization of the CAC bond. The resulting vinyl surface moiety was identify by vibrational spectroscopy [93], and has also been isolated on other metals [94–96]. More extensive dehydrogenation of hydrocarbon fragments is common in surface-science experiments under vacuum, but often involves a sequence of fast steps which ultimately yields surface carbon, and has been seldom properly characterized. Evidence is available for a-H elimination from methylene to methylidyne [97,98] and from ethylidene to ethylidyne [99], for the conversion of vinyl to vinylidene [95] and/or acetylene [92,95], and for the dehydrogenation of acetylene to acetylide [92,100–102]. Also, the scission of CAH bonds from adsorbed aromatic species tends to be regioselective. For instance, benzylic hydrogens are typically removed first in substituted benzenes

[103,104]. In the case of heterocyclic aromatic molecules such as thiophene [105–107] and furan [108], the alpha positions (the carbon adjacent to the heteroatom) are typically the most reactive. This latter regioselectivity may explain the origin of some of the main products obtained during the hydrodesulfurization (HDS) and hydrotreating of crude oil [109].

6. Reductive eliminations Reductive eliminaztions can be viewed as the reverse of oxidative addition steps [36]. Therefore, the preference for a particular reaction taking place in either the addition or elimination direction is usually defined by the relative stability of the reactants versus the products [8]. In the case of alkyl halides, for instance, oxidative addition is usually facile. On the other hand, the reductive elimination of alkyl groups with halide ligands or adsorbates is rare. More relevant to catalysis, the reductive elimination of either alkyl–hydride or alkyl– alkyl pairs is typically quite exothermic, and therefore easy to promote; most hydrogenation reactions can in fact be viewed as the result from reductive elimination of alkyl-hydride pairs.

Figure 5. Arrhenius plot of reaction rate constants for the first hydrogen elimination from ethylene adsorbed on Ni(1 0 0). The rates were measured isothermally by following the surface coverage of ethylene using a laser-induced desorption (LID) technique. Data are provided from experiments with different types of deuterium substitutions to illustrate the significant kinetic isotope effect seen for the reaction, an indication that rehybridization of the resulting vinyl hydrocarbon surface moiety may accompany this dehydrogenation step [91].


Ample examples are available for alkyl + hydride eliminations from organometallic compounds, from mononuclear complexes [36,68] as well as from clusters [110], and the same can be said for reactions on metal surfaces. Examples of this class of reactions from our laboratory include a large collection of experiments showing the formation of alkanes from thermal activation of alkyl groups coadsorbed with hydrogen on nickel [40,87,111–113], platinum [45,70,114,115], and copper [41,116] surfaces. Combined with b-hydride eliminations and the reverse hydrogenation of adsorbed olefins (olefin insertions into metal-hydrogen bonds, see below), these reactions account for the alkane–alkene equilibria often seen in reforming and other catalytic hydrocarbon conversion processes [21,23,52,117], and can also be used to explain H–D exchange, double-bond migrations, and cis-trans isomerization [53,54,118] 167. Hydrogen addition to other surface fragments, including aryls [87], metallacycles [46,47,71,119], alkylidenes [71,99,120], vinyls [95], and allyls [81] have been reported as well.

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Coupling between two alkyl fragments is less favorable but also possible, and more common with late transition metals [8,36]. On surfaces, extensive work on alkyl coupling has been reported by Bent et al. on copper [9,121–123] and gold [124,125] surfaces, and by White et al. on silver [126]. Other metals often favor dehydrogenation steps instead, b-hydride elimination in particular, but can sometimes be passivated with coadsorbed atoms such as oxygen to favor the coupling route. Figure 6 shows a recent example of this from our group in the form of temperature programmed desorption data for the production of ethylene by coupling of two methylene groups on an oxygen-predosed Ni(1 1 0) surface [127]. The determination in this case of the 180 K peak as originating from a coupling step was assessed by deuterium labeling. Similar ethane production from methyl coupling has been observed as well [128]. Finally, coupling between ethyl and ethoxide groups to yield diethyl ether has been reported on Ag(1 1 0) [129]. Coupling reactions of use to industrial

Figure 6. Ethylene TPD data for perduteriomethylene adsorbed on an oxygen-passivated Ni(110) surface. The surface was pretreated with 0.3 L of oxygen at 300 K to deposit a submonolayer coverage of atomic oxygen, and the perdeuteriomethylene moieties were prepared via thermal activation of adsorbed perdeuteriodiiodomethane. It is seen in this figure that ethylene is produced in two stages, around 180 and 245 K. The isotope-labeling strongly suggests that the low-temperature product is the result of a direct coupling of two (perdeuterio) methylene surface groups, a reductive elimination step. The high-temperature desorption, on the other hand, shows some normal hydrogen incorporation, and is therefore likely to result from (perdeuterio) methylene insertion into surface methyl groups (from hydrogenation of other methylene surface moieties with background hydrogen) followed by b-hydride elimination [127].

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applications include the Ullman, Heck and Suzuki reactions [130,131]. One interesting subset of alkyl–alkyl oxidative additions is that seen intramolecularly within metallacycle intermediates adsorbed on metal surfaces. For instance, thermal activation of nickellacyclobutane moieties on Ni(1 0 0) results in the formation of cyclopropane [47]. Interestingly, the same reaction could not be observed on platinum [46], even though that metal is less active towards dehydrogenation and therefore more likely to favor carbon–carbon bond formation. Metallacycles with five [132] and six [119] carbon atoms can also form cyclic compounds on nickel, but that reaction appears to be preceded by extensive dehydrogenation steps. Oxametallacyclic intermediates are believed to couple intramolecularly to form epoxides as well, in particular on silver surfaces, which are used commercially as catalysts for the manufacturing of ethylene oxide from ethylene [74,133]. However, none of the surface-science experiment carried out on this system to date have been successful in identifying such a step [134–137]; only indirect evidence has been reported for the production of isobutylene oxide from a chloro-tertbutoxy intermediate on Ag(1 1 0) [138]. 7. Insertions Another common family of reactions leading to bond formation in catalysis involves migratory insertions into metal–hydrocarbon bonds. In organometallic complexes, the insertions of alkenes (as in polymerization processes) [139] and carbonyls [140] are quite common, and insertions of other hydrocarbon moieties such as carbenes (alkylidenes), alkynes, allenes, arenes, dienes and polyenes, as well as groups with heteroatoms like isonitriles, nitrosyls, thiocarbonyls, oxygen and sulfur [36,68] are also possible. In heterogeneous catalysis carbene insertion is perhaps the most common, being responsible for processes such as Fischer-Tropsch synthesis [51]. This group of reactions also includes insertions into metal–hydrogen bonds, which can be viewed as hydrogenation steps. For instance, the insertion of olefins into MAH bonds is the microscopic reverse of b-hydride elimination, and is required in hydrogenation, H–D exchange, and double-bond migration processes. Methylene insertions on metal surfaces were first identified on copper [141], but has also been seen by us on nickel [120]. Interestingly, this reaction seems to exhibit some structure sensitivity, since no chain growth products are detected upon thermal activation of either methyl or methylene groups on Ni(1 0 0) or Ni(1 1 1) single crystals, but up to C4 hydrocarbons are produced with the same surface species on Ni(1 1 0). As an example of the latter chemistry, figure 6 reports temperature-programmed desorption data highlighting the formation of ethylene via methylene insertion on

oxygen-passivated Ni(1 1 0) [127]. The desorption of ethylene about 245 K in that figure is associated with methylene insertion on methyl surface groups produced by hydrogenation of other methylene adsorbed moieties followed by b-hydride elimination of the resulting ethyl groups, hence the large amount of monohydroethylene (CD2@CDH) produced [127]. Methylene insertion may also take place on platinum, where methyl adsorbates can be converted to ethylidyne (Pt3@CACH3), but the mechanistic details of that reaction are still unknown [142]. Insertion of heteroatoms into metal-carbon bonds on surfaces is possible as well. The best document example of this in surface science studies is the insertion of adsorbed oxygen atoms into alkyl or carbene surface moieties. As an example, figure 7 displays infrared data pointing to the formation of a CAO bond via the insertion of an oxygen atom into the Ni(1 0 0) 2-propyl bond [143]. The infrared spectrum obtained at 300 K can be clearly assigned to the expected 2-propoxide groups, as indicated in particular by the appearance of a new CAO vibration around 1100 cm)1. Analogous reactions are presumed with terminal alkyls, except that the resulting alkoxides are unstable and undergo facile b-hydride elimination to the aldehyde, and later to carbon, hydrogen, and CO [144]. More cleanly, oxygen insertion into methylene–nickel bonds leads directly to the production of formaldehyde [127]. Oxygenated products have also been detected in studies on the chemistry of alkyl or alkylidene groups with adsorbed oxygen on rhodium [17], palladium [145], molybdenum [17], and nickel [146]. This type of oxygen insertion may be responsible for the formation of oxygenates in hydrocarbon oxidation catalysis.

8. Other elementary steps It could be said that the chemistry of carbon– hydrogen bonds dominates hydrocarbon conversion catalysis. Many key mechanistic steps in those processes involve the scission of CAH bonds via oxidative additions or hydride eliminations, and/or the formation of new CA H bonds by reverse reductive eliminations and insertions into metal–hydride bonds. In fact, evidence from our group suggests that the selectivity in many catalytic processes such as reforming and selective oxidations is controlled by the regioselectivity of dehydrogenation (hydride elimination) steps [20,23]. Nevertheless, the formation and breaking of other types of bonds such as CAC and CAO are also crucial in the manufacturing of many chemicals. As mentioned already, reductive eliminations and migratory insertions are two common mechanisms for the formation of CAC (and other) bonds. A different and quite interesting, example is that of the formation of benzene via acetylene trimerization, a reaction originally


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Figure 7. RAIRS for 2-propyl iodide adsorbed on hydroxo-covered Ni(1 0 0). The hydroxide groups were prepared by sequentially dosing 0.5 L of O2 at 300 K and 2.0 L of water at 200 K, and the 2-propyl groups were obtained via thermal activation of adsorbed 2-propyl iodide. Oxygen insertion into the 2-propyl–nickel bond is clearly made evident by the appearance of a CAO stretching around 1100 cm)1 associated with the resulting 2-propoxide surface moiety [143].

observed on palladium surfaces [147–149]. There is abundant evidence pointing to the fact that this reaction occurs sequentially via the formation of a C4 intermediate [150]. In fact, in the presence of heteroatoms such as oxygen or sulfur, it is possible to produce thiophene or furan instead [151]. Another important process is olefin metathesis, by which alkenes swap their alkylidene moieties around their double bond [152]. The most accepted mechanism for this reaction involves the formation of carbenes followed by their subsequent reversible reaction with coordinated alkenes to yield metallacyclobutane intermediates. Tysoe et al. have performed extensive surface-science studies on this reaction over molybdenum substrates, but have been only partially able to elucidate the reaction mechanism [153]. Carbon–carbon bond-breaking reactions are believed to be prevalent in the thermal chemistry of hydrocarbons adsorbed on metal surfaces. Unfortunately, little is known about the elementary steps involved there. What is clear is that they are often preceded by extensive dehydrogenation of the carbonaceous adsorbed moieties, and that the resulting surface carbon may reassociate at higher temperatures to form either graphitic or carbidic deposits depending on the nature of the metal. Regarding the former point, we have shown that neopentyl moieties can indeed undergo CaACb bond scission on nickel after a-hydride elimination to neopentylidene to ultimately yield isobutene [65]. Nevertheless, that appears to be a complex reaction involving simultaneous CAH activations, and displays significant kinetic isotope effects upon deuterium substitutions at either alpha or gamma position. CAC bond breaking

steps are responsible for the often undesirable hydrogenolysis products in reforming and other hydrocarbon conversion catalysis [24,154]. Another bond scission reaction of significance in catalysis is that associated with decarbonylation. The reverse of carbonyl insertion, this step is common in organometallic complexes [155]. We have reported clear vibrational and thermal desorption evidence for the decarbonylation of acrolein and crotonaldehyde adsorbed on Pt(1 1 1) [156]. Interestingly, a second channel is available for these systems leading to CAC bond breaking and ketene and olefin formation [157]. Additional decarbonylations have been reported for acetaldehyde [158], acrolein and allyl alcohol on rhodium [159], and for regular alcohols [160] and for furan on palladium [161], among others. Finally, isomerization reactions need to be considered. There is some indication that tautomerization of coordinated species is possible via a 1,2 hydrogen shift [8]. For one, this may be the mechanism by which acetylene isomerizes to vinylidene, as seen on Pd(1 1 1) [162]. A more complex case not fully understood yet is that of the conversion of alkenes to alkylidynes. This is a fairly common reaction on transition metal surfaces, and appears to require an initial 1,2-H shift in the adsorbed olefin to produce an unstable alkylidene intermediate [163]. In a third example, adsorbed butadiene sometimes isomerizes to a species resembling 2-butene, perhaps an unsaturated metallacyclopentene [164]. However, it is not clear if in that case the double bond migration occurs via a dehydrogenation/rehydrogenation sequence or directly by hydrogen transfer.

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CAC skeletal rearrangements have been particularly difficult to isolate in surface-science studies, and, to the best of our knowledge, have only been reported in association with ring expansions or contractions, such as in the case of the production of cycloheptatriene, norbornadiene and/or benzene from cyclooctene, 1,3-cyclooctadiene, 1,5-cyclooctadiene, or cyclooctatetraene on Pt(1 1 1) [165,166], of benzene from bicyclo[2.2.2]octene and norbornene, also on Pt(1 1 1), and of benzene again, but from 1-methylcyclopentene, on Ni(1 0 0) [89]. Clearly, these reactions are complex, and involve multiple surface elementary steps.

9. Concluding remarks In the paragraphs above some of the key elementary steps involved in heterogeneous catalysis processes have been identified and discussed in terms of the similar chemistry known on discrete organometallic complexes. This approach can be justified by at least two rationales, one fundamental, by which the knowledge acquired from research in organometallic chemistry may aid in the understanding of more difficult-to-study surface systems, and another more practical associated with the search of extensions of selective homogeneous catalysis to heterogeneous systems. Catalysis relies on kinetics, and selectivity in catalysis is defined by relative reaction rates among competitive steps. Therefore, it is highly desirable to understand the mechanism of catalytic processes in order to optimize their performance. Critically, selectivity may often depend on elementary steps different to those that control overall activity [19,23]. We have already pointed out above that this may very well be the case in hydrocarbon reforming, where the rate limiting step often is the activation of the initial alkanes in the feedstock, but where selectivity may be defined by the regioselectivity of the subsequent dehydrogenation of the resulting alkyl surface intermediates [21,23]. Similar arguments can be made for the conversion of alcohols and for the oxidation of hydrocarbons [20]. Figure 8 displays the energy diagram of a third example, that of the migration and cis–trans isomerization of double bonds in olefins [167]. It can be seen there that the relative yields for 1-, cis-2-, and trans-2-butenes in a given catalytic process may rely on the relative rates for b-hydride elimination from different positions within the common 2-butyl surface intermediate. The similarities among those three steps highlight the subtleties involved in controlling selectivity in many catalytic processes. Here is where the analogy between surface science and organometallic chemistry may help. The identification of elementary surface steps in terms of coordination chemistry allows for the definition of selectivity problems in heterogeneous catalysis in terms of the betterestablished reactivity of ligands in discrete metal-con-

Figure 8. Energy diagram for the interconversion of the different butene isomers on metal surfaces. Shown are the elementary steps responsible for double bond migration between 1- and 2-butene as well as for cis–trans isomerization within 2-butene. Of particular importance here is the role that b-hydride elimination from the common 2-butyl surface intermediate plays in the interconversion. This diagram highlights the fact that selectivity in these reactions is defined by subtle relative differences among the energy barriers of competing elementary steps.

taining molecules. In this review we have argued that hydrogenation, dehydrogenation, hydrogenolysis, chain growth, and isomerization processes on solid metal catalysts can be understood in terms of hydride eliminations, oxidative additions, reductive eliminations, migratory insertions, and 1,2-shifts, among others. With this in mind, the thinking involved in designing better catalysts may be guided by first identifying the surface elementary step(s) to be promoted, and by then recalling the knowledge available from organometallic chemistry in terms of reactivity trends and applying it to the heterogeneous catalytic system. This organometallic thinking approach to surface reactivity is still in its infancy, but may have already shown some promise, For instance, the viability of imparting chirality to solid catalysts via the adsorption of chiral modifiers has been advanced in recent studies [168–170], in an analogous way as chiral ligands are used to impart chirality to homogeneous catalysts [171]. At a more fundamental level, it has become clear that b-hydride elimination is prevalent on surfaces, the same as it dominates much of the chemistry of alkyl groups in organometallic compounds. The challenge now is to extend those analogies to more complex reactions in order to answer pressing unanswered questions such as what the optimum conditions should be for improving specific bond-forming and skeletal-rearrangement reaction steps. As the bridge between organometallic and surface chemistry is built, however, some attention must be placed on identifying and understanding the limitations


of this analogy. It is interesting to note, for instance, that much of the chemistry seen on late transition metal surfaces is equivalent to that of early transition metal complexes. This may be at least in part connected to the fact that organometallic compounds can sustain different and well-defined oxidation states at their metal centers. In fact, many catalytic processes using organometallic compounds involve interconversions between so called 18 and 16 electron complexes [172]. The electronic structure of solid metal catalysts can be tuned somewhat by the use of additives or alloys or by altering the coordination number of some surface atoms via changes in surface structure, but in general this variability is much more limited and much harder to control than in discrete molecules. It is interesting to note, though, that by using small particles with radically different electronic properties it has recently been possible to make quite active catalysts based on gold, a metal usually considered inert [173,174]. On the other hand, solid surfaces are more likely to support unique atomic ensembles with complex geometries than simple metal complexes. Unique catalytic sites may promote specific reactions with great selectivities, as in enzymatic catalysis. For instance, nitrogen activation is believed to require so-called C7 sites, in which the active metal center is coordinated to seven other metal surrounding atoms [175–177]. Then there is the influence of ligands on the chemistry of the metal center. Again, these may modify the electronic properties of the complex in significant ways. It is common on surfaces for coadsorbates to be present near the active catalytic site, and to modify the kinetics of the reaction in complex ways [178]. However, the coverage and nature of those ‘‘spectator’’ surface species is typically hard to control (other than by the use of additives or modifiers, or by tuning the reaction conditions). In this respect, the formation of complex surface carbonaceous deposits is of particular relevance to the passivation of the activity of metal catalysts for the promotion of facile hydrocarbon conversion catalysis [22,23,179,180]. Nevertheless, none of these differences preclude us from taking advantage of the knowledge available from organometallic chemistry and homogeneous catalysis to advance our understanding of surface chemistry and our ability to design better heterogeneous catalysts. Acknowledgements Financial support for this research was provided by the U.S. National Science Foundation and by the U.S. Department of Energy. References [1] A.M. Thayer, Chem. Eng. News (March 9) (1992) 27. [2] Z. Ma, F. Zaera, in: Encyclopedia of Inorganic Chemistry, R.B. King (ed.), 2nd ed. (John Wiley & Sons, New York, 2004), in press.

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