Mechanistic insights and consequences of intrapore liquids in ethene, propene, and butene dimerization on isolated Ni2+ sites grafted within aluminosilicate mesopores

Abstract

The stability, selectivity, and reactivity conferred by intrapore liquids onto Ni2+ species grafted within mesoporous aluminosilicates for the dimerization of C2-C4 alkenes are reported and interpreted by the preferential stabilization of dimer desorption transition states, which inhibit further growth during the surface sojourn that forms these dimers. The marked decrease in deactivation rate constants leads to catalyst half-lives > 600 h for all alkenes and to turnover rates (per Ni) at sub-ambient temperatures (240–260 K) that exceed those at the higher temperatures previously used for Ni-based solid catalysts (350–500 K). These effects appear at the relative pressures required for alkene capillary condensation within Ni-Al-MCM-41 catalysts with different mesopore diameters and are also evident when the intrapore liquids consist of unreactive alkanes, indicative of solvation effects conferred by non-covalent interactions mediated by dispersion forces. For all alkenes, turnover rates (per Ni) are unaffected by Ni content until each acidic proton in the Al-MCM-41 is replaced by one Ni atom, consistent with the involvement of grafted (Ni-OH)+ monomers as active centers and excess Ni leading to inactive NiO species. The stable nature of these catalysts allows accurate mechanistic assessments and a demonstration of the kinetic relevance of the CC coupling elementary steps on essentially bare (Ni-OH)+ centers, leading to rates described by second-order dimerization constants (αn′) and adsorption constants (βn′) for each alkene (Cn); these parameters are influenced only slightly by intrapore liquids, because they reflect free energies of formation of surface-bound species and transition states from gaseous precursors. These αn′ and βn′ parameters increase with alkene size, with the enthalpic component of αn′ increasing from 21 to 46 kJ mol−1 and the activation entropy becoming more negative (−103 to −187 J (mol K)−1), consistent with CC coupling transition states that occur late along the reaction coordinate and resemble its bound dimer product. The low C3n/C2n ratios observed in the presence of intrapore liquids (<0.04, for n = 2–4) reflect the endothermic nature of dimer desorption events, for which product-like transition states benefit from the solvation of desorbing products by a non-polar liquid phase, thus allowing detachment from Ni centers before subsequent growth. The inhibition of such growth events accounts not only for the unique dimer selectivity observed, but also for the unprecedented stability conferred by intrapore liquids by inhibiting the formation of oligomers that bind more strongly with increasing chain size, as evident from the βn′ values reported here for alkene reactants.