Hydrocarbon Reforming

This project focuses on the reactivity of hydrocarbon moieties on transition-metal surfaces as it relates to catalytic hydrogenation-dehydrogenation and reforming processes. One of our key contributions to this field has been the development of a method for preparing alkyl and other relevant hydrocarbon fragments cleanly on surfaces by using alkyl halides, a procedure that is now widely employed in other laboratories and that has allowed us to characterize the reactivity of those species in some detail. The basic ideas extracted from this research have been summarized in a number of review articles [F. Zaera Acc. Chem. Res. 25 (1992) 260; F. Zaera, J. Mol. Catal. 86 (1994) 221; F. Zaera, Chem. Rev. 95 (1995) 2651]. 

Evidence for the formation of alkyl groups on metal surfaces upon thermal activation of chemisorbed alkyl iodides.  Left:  Reflection absorption infrared spectra (RAIRS) from methyl, ethyl, 1-propyl, and 2-propyl moieties on Cu(110) prepared by thermal activation of the corresponding adsorbed alkyl iodide precursors.  Right:  Static secondary ion mass spectra (SSIMS) from perdeutero methyl and ethyl fragments chemisorbed on Ni(100).

One of the key early advances made in this project was the establishment of beta-hydride and reductive eliminations as the main reactions available to most adsorbed alkyl fragments [F. Zaera, J. Am. Chem. Soc. 111 (1989) 8744; F. Zaera, Surf. Sci. 219 (1989) 453; S. Tjandra and F. Zaera, Langmuir 10 (1994) 2640; S. Tjandra and F. Zaera, J. Am. Chem. Soc. 117 (1995) 9749].  More recently, new reaction pathways have been identified using other moieties.  For instance, it was determined that on Ni(100) single-crystal surfaces thermal activation of neopentyl groups leads to the selective alpha-hydride elimination to neopentylidene [F. Zaera and S. Tjandra J. Am. Chem. Soc. 118 (1996) 12738].  Interestingly, the major carbon-containing desorbing product in that system is isobutene, which isotope labeling experiments demonstrated comes from the scission of the C(alpha)-C(beta) bond.  On Pt(111), on the other hand, it was found that hydrogen elimination from the alpha and gamma positions display comparable rates [T. V. W. Janssens, G. Jin and F. Zaera J. Am. Chem. Soc. 119 (1997) 1169; F. Zaera, S. Tjandra and T. V. W. Janssens,  Langmuir 14 (1998) 1320], and that, because of a strong primary kinetic isotope effect, regiospecific deuterium substitutions can lead to a complete switch in dehydrogenation selectivity [T. V. W. Janssens and F. Zaera, Surf. Sci., 501, 1-15 and 16-30 (2002)].

Schematic representation of the elementary surface reaction steps proposed for the hydrogenolysis and isomerization of hydrocarbons during catalytic reforming.  The example provided here corresponds to neopentyl intermediates.  The arguments is made that while alpha-hydride elimination eventually leads to C–C bond scission and yields hydrogenolysis products, hydride elimination at the gamma position results in isomerization and cyclization instead.  Explicit reference is also made in this figure to the steps already identified on Pt and Ni single-crystal surfaces.

The interesting observation in terms of selectivity in reforming is that the hydrogenolysis of alkyl intermediates may be rate limited by the C–C bond breaking step, but is decided by a much earlier and facile a dehydrogenation reaction.  In the same way, the gamma-hydride elimination step likely responsible for the isomerization products appears to be viable on platinum but not on nickel.  This observation explains the unique ability of platinum for catalyzing reforming processes, as opposed to nickel, which leads to exclusive cracking instead [Zaera, 1998 #809].  The central conclusion from this work is that it is the relative rates among the different available paths on a given surface what matters in terms of defining selectivity.  All dehydrogenation steps follow the same qualitative trends, that is, they become faster on early transition metals or with heavier hydrocarbon chains, but the relative rates for alpha, beta, and gamma dehydrogenations within the same metal also change across the periodic table.  It is the relative changes in dehydrogenation rates that explains the different performance of different metals in hydrocarbon reforming in terms of selectivity [F. Zaera, Appl. Catal., 229 (2002) 75; F. Zaera, Catal. Lett. 91 (2003), 1; F. Zaera, J. Mol. Catal. A 228 (2005), 21; F. Zaera, Top. Catal. 34 (2005), 129; F. Zaera, Chem. Rec. 5 (2005), 133].

Prototypical reaction mechanism for the catalytic reforming of hydrocarbons on transition metal surfaces, for isobutane in this example.  The rate-limiting step in most reforming processes is the initial C–H bond activation to the corresponding alkyl surface intermediates (sec-butyl in this case), but selectivity is defined by the regioselectivity of the subsequent dehydrogenation reactions.  Beta-hydride elimination dominates the chemistry of adsorbed alkyl groups, but only leads to fast alkane-alkene equilibria; what determines selectivity between the undesirable hydrogenolysis reactions and the desirable cyclization and isomerization processes is the relative rates between the alpha- and gamma-H elimination steps.

In reference to the surface chemistry of metallacycles, a number of diiodoalkane precursors have been used to prepare and characterize the appropriate intermediates [S. Tjandra and F. Zaera, J. Phys. Chem. 101 (1997) 1006; 151. D. Chrysostomou, A. Chou and F. Zaera, J. Phys. Chem. B, 105, 5968-5978 (2001)].  For instance, thermal activation of dihalopropanes on Ni(100) lead to the production of propene, propane, cyclopropane and halopropanes, some via the formation of propenyl surface moieties.  A somewhat similar chemistry is seen on Pt(111), as summarizes below. The thermal chemistry of six-membered hydrocarbon cyclic moieties is more complex, and has only been partially ellucidated [S. Tjandra and F. Zaera, J. Phys. Chem. A 103 (1999) 2312].

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Comparison of alpha- vs. beta-hydride elimination reaction temperatures across the periodic table.  The numbers in each cell correspond to the temperature maxima for the associated methane (left) and ethylene (right) production reactions during temperature-programmed desorption with methyl and ethyl surface species, respectively.  In general, higher reactivities are seen for all reactions with early transition metals and shorter hydrocarbon chains.  More significant are the variations in relative rates between the alpha- and beta-hydride eliminations: the former appears to be particularly difficult with coinage metals.

Last modified June 27, 2005