Hydrogenolysis and related reactions of hydrocarbons (C ₃ to C ₅ ) on silica-supported Rh-Pt bimetallic catalysts

Abstract

The reactions with hydrogen of propane, butane, 2-methylpropane, pentane, 2-methylbutane, 2, 2-dimethylpropane and cyclopentane have been studied in a static reactor using a series of highly dispersed rhodium-platinum catalysts supported on high-area silica. The main reaction over all catalysts was hydrogenolysis involving the breaking of a single carbon-carbon bond. Most of the compounds reacted at similar rates over platinum with activation energies in the range 132-144 kJ mol -1 . With rhodium, rates varied with hydrocarbon structure by factors of more than 10 2 . The selectivities for the formation of the various products showed that the relative rates of breaking different carbon-carbon bonds over rhodium were S-S > P-S > S-T > P-T > P-Q (P = primary, Q = quaternary, S = secondary and T = tertiary). At temperatures above 455 K, there was evidence of a change of rate-determining step over rhodium with the overall reaction becoming controlled by the rate of desorption of methane. Probable mechanisms over platinum and rhodium are discussed. Rhodium was more active than platinum by factors of about 200 for branched hydrocarbons and of 10 3 or more for straight-chain compounds. Patterns of activity with metal composition are interpreted in terms of active sites consisting of ensembles of about six metal atoms. But an alternative description of the active site as a single metal atom with properties influenced by a number of nearest neighbours cannot be excluded.

Kinetics of Hydrogen Adsorption and Desorption on Silica-Supported Pt, Rh, and Ru Catalysts Studied by Solid State 1H NMR

“…This can be qualitatively correlated with the differences in the catalytic activity of these metal catalysts for various reactions. For example, Rh, Ru, and Ir show higher activity for hydrogenolysis whereas Pt is more active for skeletal isomerization. − Clearly, the rates of hydrogen adsorption and desorption described here are much faster than the rates of these catalytic reactions. However, the surface coverage of the intermediates can alter the overall rates as well as the selectivities of the reactions 7 even in the situations where these intermediates form at much faster rates than the slowest step of the overall reaction network.…”

Effect of Macro- and Microstructures on Catalytic Hydrogenolysis of Polyolefins

“…A corresponding decrease in signal from methylene units in 1 H NMR when all other signals increase suggests redistribution of hydrogen at CH 2 to methane and methine units. An inverse isotope effect may account for this redistribution, as a deuterium on the Pt surface is known to increase the rate of hydrogenolysis and isomerization vs hydrogen, and hydrogenolysis rates are fastest for methylene-associated C–C bonds . It should also be noted, however, that other characteristics such as local chain configurations and steric hindrance may play a role in the extent of hydrogenolysis within a given run time.…”

Controlled Pretreatment and Reconstruction of a Bimetallic Pt–Ir/Al2O3/ZSM-5 Catalyst for Increased Stability during Butane Hydrogenolysis

“…A similar trend was observed in a different report of butane hydrogenolysis over 0.5 wt % bimetallic Pt−Ir catalysts at 473 K, demonstrating the robust observation of improved selectivity of Pt−Ir bimetallic particles for butane hydrogenolysis as compared to monometallic Ir catalysts. 20,21 Alkane hydrogenolysis on Ir catalysts is structure-sensitive and requires ensembles of 3−6 Ir atoms 28 for multiple hydrogenolysis to occur, which results in undesirable methane formation. 23,24 Pt alloying has been hypothesized to break up ensembles of surface Ir atoms to minimize multiple hydrogenolysis while also slowing oxidative agglomeration of Ir nanoparticles during calcination or regeneration.…”

“…Alkane hydrogenolysis on Ir catalysts is structure-sensitive and requires ensembles of 3–6 Ir atoms for multiple hydrogenolysis to occur, which results in undesirable methane formation. , Pt alloying has been hypothesized to break up ensembles of surface Ir atoms to minimize multiple hydrogenolysis while also slowing oxidative agglomeration of Ir nanoparticles during calcination or regeneration . Bimetallic Pt–Ir catalysts thus capitalize on the high activity of Ir for cleaving C–C bonds, while Pt surface atoms limit Ir ensemble sizes to selectively promote single hydrogenolysis of butane to ethane. , Furthermore, charge transfer between Pt and Ir has been proposed to increase the inherent reactivity of both elements, suggesting the potential influence of geometric (ensemble) and electronic effects on hydrogenolysis reactivity. , …”