Impact of the Spatial Organization of Bifunctional Metal–Zeolite Catalysts on the Hydroisomerization of Light Alkanes

Improving product selectivity by controlling the spatial organization of functional sites at the nanoscale is a critical challenge in bifunctional catalysis. We present a series of composite bifunctional catalysts consisting of one-dimensional zeolites (ZSM-22 and mordenite) and a γ-alumina binder, with platinum particles controllably deposited either on the alumina binder or inside the zeolite crystals. The hydroisomerization of n-heptane demonstrates that the catalysts with platinum particles on the binder, which separates platinum and acid sites at the nanoscale, leads to a higher yield of desired isomers than catalysts with platinum particles inside the zeolite crystals. Platinum particles within the zeolite crystals impose pronounced diffusion limitations on reaction intermediates, which leads to secondary cracking reactions, especially for catalysts with narrow micropores or large zeolite crystals. These findings extend the understanding of the “intimacy criterion” for the rational design of bifunctional catalysts for the conversion of low-molecular-weight reactants.     

The direct formation of ketones by reaction of methyl- and aryl-(carbonyl)(iodo)pentamethylcyclopentadienylrhodium complexes with organic iodides

[C₅Me₅Rh(aryl)(CO)I] reacts with methyl iodide to give [(C₅Me₅RhI₂)₂] and arylCOMe; similar reactions occur between [C₅Me₅Rh(Me)(CO)I] and RI to give the ketones RCOMe (R = Ph, Me, Et, or Pr).     

Selective methylative homologation: an alternate route to alkane upgrading

InI3 catalyzes the reaction of branched alkanes with methanol to produce heavier and more highly branched alkanes, which are more valuable fuels. The reaction of 2,3-dimethylbutane with methanol in the presence of InI3 at 180-200 degrees C affords the maximally branched C7 alkane, 2,2,3-trimethylbutane (triptane). With the addition of catalytic amounts of adamantane the selectivity of this transformation can be increased up to 60%. The lighter branched alkanes isobutane and isopentane also react with methanol to generate triptane, while 2-methylpentane is converted into 2,3-dimethylpentane and other more highly branched species. Observations implicate a chain mechanism in which InI3 activates branched alkanes to produce tertiary carbocations which are in equilibrium with olefins. The latter react with a methylating species generated from methanol and InI3 to give the next-higher carbocation, which accepts a hydride from the starting alkane to form the homologated alkane and regenerate the original carbocation. Adamantane functions as a hydride transfer agent and thus helps to minimize competing side reactions, such as isomerization and cracking, that are detrimental to selectivity.     

Specificity of sites within eight-membered ring zeolite channels for carbonylation of methyls to acetyls

The acid-catalyzed formation of carbon-carbon bonds from C1 precursors via CO insertion into chemisorbed methyl groups occurs selectively within eight-membered ring (8-MR) zeolite channels. This elementary step controls catalytic carbonylation rates of dimethyl ether (DME) to methyl acetate. The number of O-H groups within 8-MR channels was measured by rigorous deconvolution of the infrared bands for O-H groups in cation-exchanged and acid forms of mordenite (M,H-MOR) and ferrierite (H-FER) after adsorption of basic probe molecules of varying size. DME carbonylation rates are proportional to the number of O-H groups within 8-MR channels. Na+ cations selectively replaced protons within 8-MR channels and led to a disproportionate decrease in carbonylation turnover rates (per total H+). These conclusions are consistent with the low or undetectable rates of carbonylation on zeolites without 8-MR channels (H-BEA, H-FAU, H-MFI). Such specificity of methyl reactivity upon confinement within small channels appears to be unprecedented in catalysis by microporous solids, which typically select reactions by size exclusion of bulkier transition states.     

Conversion of methanol to 2,2,3-trimethylbutane (triptane) over indium(III) iodide

InI3 is able to catalyze the conversion of methanol to a mixture of hydrocarbons at 200 degrees C with one highly branched alkane, 2,2,3-trimethylbutane (triptane), being obtained in high selectivity. The mechanism for InI3-catalyzed reactions appears to be basically the same as that proposed for the previously studied ZnI2-catalyzed system in which sequential methylation of olefins is followed by competing reactions of the resulting carbocation: proton loss to give the next olefin vs hydride transfer to give the corresponding alkane. Although the reaction conditions and typical triptane yields achievable with ZnI2 and InI3 are quite similar, the two systems behave rather differently in a number of important particulars, including significant differences between the detailed product distributions. Most of the differences in behavior can be ascribed to the stronger Lewis acidity of InI3, including the ability to activate some alkanes, the higher activity for methylation of arenes, and the fact that methanol conversion can be observed at somewhat lower temperatures with InI3 than with ZnI2.