The formation and cleavage of C-C bonds and the isomer distribution and chain length in products formed from hydrocarbons on acid catalysts depend sensitively on the dynamics of alkene oligomerization and β-scission. H-transfer (HT) reactions terminate chains, thus controlling chain length and incorporating alkanes into alkene oligomerization cycles. Kinetic, isotopic, spectroscopic and theoretical methods show that alkene oligomerization occurs via kinetically-relevant formation of C-C bonds between alkenes and bound alkoxides present at near saturation coverages on Brønsted acid sites. Oligomerization rate constants reflect the stability of ion-pair transition states (TS) that are larger and more highly-charged than their alkoxide precursors; these size and charge differences lead to higher turnover rates on stronger acids and on protons located within confining voids that provide van der Waals contacts without significant structural distortions of organic guests or inorganic hosts. These alkene oligomerization events form equilibrated skeletal and regio isomers of all oligomers; channel undulations locally form larger oligomers that require subsequent β-scission to traverse the interconnecting constrictions. HT from alkanes to bound alkoxides leads to alkoxide desorption as alkanes and forms alkane-derived alkoxides that react with alkanes to incorporate alkanes into larger oligomers. HT and oligomerization are both mediated by ion-pair TS structures with a full positive charge, causing their respective turnover rates to depend similarly on acid strength. The TS structures for HT to a given alkoxide are, however, larger than those forming such alkoxides via alkene dimerization; as a result, voids within large-pore zeolites and mesoporous aluminosilicates favor HT over oligomerization. The stability of oligomerization and HT transition states also depends on the proton affinity (PA) of the alkene that forms the alkoxides involved in both reactions and on the hydride-abstraction energies (HAE) of the alkane co-reactants. HAE and PA depend, in turn, on the size and structure of the alkene and alkane reactants. Rate and selectivities are thus determined by the carbocation stability conferred by these molecular properties and by the fit, in size and shape, between these TS structures and the voids within inorganic hosts, which leads to van der Waals stabilization without significant enthalpic penalties from host-guest structural distortions.

Site and structural requirements for stabilization of transition states for oligomerization and hydrogen transfer reactions on solid acids

255th National Meeting (2018)

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