Predicting a generalized mechanism of branched alkane hydrogenolysis on Ru, Ir, and Pt surfaces relevant to polymer upcycling applications

Abstract

Here, we present a fundamental study investigating the mechanisms governing C–C hydrogenolysis of branch points within small alkanes on Ir, Pt, and Ru surfaces using Density Functional Theory (DFT). Previous work has shown that activation of unsubstituted C–C bonds occurs through the dehydrogenation of the C–C bond to form a bound alkyne, followed by a kinetically relevant C–C activation and the hydrogenation of the cleaved intermediates to form smaller alkane products. Substituted bonds, in contrast, involve the dehydrogenation of the C–C bond being cleaved, as well as other C atoms near the reacting center. This leads to the counterintuitive observation, that reactions of unsubstituted C–C bonds (having more H to lose) are less inhibited by H₂ than reactions of substituted C–C bonds (having less H atoms to lose). These prior studies of branched alkane activation, however, focused on Ir catalysts and on methyl-substituted alkanes and cycloalkanes, such that the impact of catalyst identity or of long branches (i.e., like those found in some polymers) on substituted C–C hydrogenolysis mechanisms is largely unexplored. Here, we consider isobutane activation mechanisms on Ir, Ru, and Pt catalysts, and use these results to predict how a larger branched alkane, 3-ethylpentane, would react, as that molecule is more reminiscent of the branches in polyethylene. DFT-estimated free energy barriers and turnover rates indicate that hydrogenolysis activity and rate inhibition from hydrogen pressure follow a general trend with catalysts following a reactivity trend of Ru > Ir > Pt, where Ru is the most active and most inhibited by H₂, with Pt being the least reactive and least inhibited by H₂ pressure. By categorizing the isobutane-derived transition states based on whether they are ‘extendable’ to larger compounds, we predict how the size of branches and the alkane backbone influence substituted C–C bond activation (e.g., comparing isobutane to 3-methylpentane or 3-ethylpentane). These data demonstrate why, on Ru, isobutane hydrogenolysis measurements are unlikely to be informative about the mechanisms that activate branches present in polyethylene or polypropylene molecules. This study lays a foundation for a better mechanistic understanding of how branch points activate in alkanes and relates those changes to polymer upcycling via hydrogenolysis.