Transition-State Analysis Reveals Unexpected Coordination-Specific Reactivity That Drives Alkene Dimerization by Sulfated Metal–Organic Frameworks

Metal–organic frameworks (MOFs) with metal nodes and organic linkers have significant potential as reaction catalysts. However, one major factor impeding the development and use of MOFs as organic reaction catalysts is the detailed understanding of catalytically active sites and their molecular reaction mechanism and selectivity. Experimentally, sulfated and Brønsted acidic MOF-808 catalyzes isobutylene dimerization. Here, we report a density functional theory (DFT)-based multi-chemical model (cluster and periodic) transition-state study that reveals the likely active catalytic sites/species and the resulting reaction mechanism and selectivity for isobutylene dimerization catalyzed by sulfated MOF-808. Surprisingly, while there are dozens of acidic sites within a single MOF crystal, catalysis is likely the result of only a few specific active sites. Of the two-dozen acidic sites we extensively examined, only one aqua group from the tri-sulfated MOF-808 Zr6 node exhibits strong enough acidity to produce a dimerization barrier consistent with the experiment. Reaction coordinate and transition state models indicate a concerted dimerization process that couples C–C bond formation, C–H bond cleavage, and proton shuttling. The selectivity between terminal and internal alkene dimerization products is likely kinetically controlled with negligible influence from the steric effects of the MOF framework.