A liquid phase, a functionalized pocket, and small inorganic voids provide an environment that acts in concert with a “binding point” to favor (or disfavor) certain bound transition states (TS) or intermediates. This study addresses how non-polar liquids (alkanes, alkenes) influence reactivity/selectivity in coupling reactions of alkenes (dimerization) and alkanones (aldol condensation). The presence (and transport corruptions) of bulk liquids is avoided here by using an intrapore condensed phase. Intrapore n-heptane liquids in TiO2 powders increase acetone condensation turnover rates by solvating the TS that mediates kinetically-relevant enolate formation steps at Ti-O acid-base pairs; this “entry” step TS occurs early along the reaction coordinate and resides, in part, within the liquid. The TS for “exit” desorption steps resembles the desorbed and benefits from solvation effects more than that for C-C coupling, thus enabling the desorption of primary C6 products before additional C-C bonds form and leading to high C6 selectivities and much slower deactivation. Alkene (C2-C4) dimerization rates on (Ni-OH)+ monomers grafted onto Al-MCM-41 are limited by C-C bond formation. This bimolecular TS is coordinated to acid-base centers in (Ni-OH)+ and is weakly affected by a liquid outer sphere. The intrapore condensation of alkenes (or C2-C3 alkanes) leads to higher selectivity to primary dimers, as late TS for desorption “exit’ steps is solvated by the liquid; this inhibits further C-C coupling and prevents the formation of larger deactivating residues (&lt;1 h to &gt;600 h catalyst lifetimes; 40,000 h-1 turnover rates; 250 K). Intrapore n-alkanes also lead to higher alkene dimerization rates on acidic zeolites via preferential stabilization of the C-C coupling TS over alkoxide precursors. Their presence may enable local perturbations of the void space around active protons and tuning of reactivity and selectivity for a given framework through changes in the number of n-alkane molecules within a zeolite unit cell.

Consequences of non-polar liquids for the stability of transition states and their kinetically-relevant precursors in surface catalysis

ACS Spring 2022

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