Stable rates of alkene dimerization are conferred upon Ni-Al-MCM-41 mesoporous catalysts at conditions that lead to the formation of an intrapore liquid phase of alkene reactants or inert alkanes. The remarkable mitigation of the fast deactivation enables detailed mechanistic inquiries into the relevant intermediates, transition states, and elementary steps through a combination of theoretical and experimental evidence. DFT-based assessments of active centers and reaction pathways using a model of an Al-exchanged MCM-41 structure confirmed that active centers consist of (Ni-OH)⁺ moieties grafted at Al-exchange sites, a finding in agreement with observed effects of Ni/Al ratios on reactivity. Alternate Ni⁺ and Ni²⁺-H structures contradict experimental observations of steady-state ethene coverages on the catalyst active sites. These (Ni-OH)⁺ species are much weaker Brønsted acids than the O-H groups that they replace and even the later lead to undetectable dimerization rates at the subambient temperature that leads to high turnover rates, thus precluding C-C bond formation via carbenium-ion pathways. (Ni-OH)⁺ centers act instead as acid-base site pairs that mediate the kinetically relevant C-C coupling steps via a concerted interaction of an alkene on each O and H in (Ni-OH)⁺. This bimolecular transition state interacts predominantly with the site pair and is weakly affected by an outer sphere that differs in solvating properties depending on the presence or absence of a non-polar liquid phase. DFT-derived barriers agree well with experiments and the binding energy of alkene-derived species with (Ni-OH)⁺ sites is weak enough to account for the nearly uncovered nature of such sites, which leads to the observed second-order kinetics for reactions of ethene, propene, and butenes. These results offer a mechanistic interpretation for heterogeneous alkene dimerization on Ni sites that differs from the classical metallacycle or Cossee-Arlman mechanisms on Ni⁺ and Ni²⁺-H centers, which would generate strongly-bound alkene-derived species on Ni/Al-MCM-41.

Shuai Wang

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