Identification of the strong Brønsted acid site in a metal–organic framework solid acid catalyst

It remains difficult to understand the surface of solid acid catalysts at the molecular level, despite their importance for industrial catalytic applications. A sulfated zirconium-based metal–organic framework, MOF-808-SO4, was previously shown to be a strong solid Brønsted acid material. In this report, we probe the origin of its acidity through an array of spectroscopic, crystallographic and computational characterization techniques. The strongest Brønsted acid site is shown to consist of a specific arrangement of adsorbed water and sulfate moieties on the zirconium clusters. When a water molecule adsorbs to one zirconium atom, it participates in a hydrogen bond with a sulfate moiety that is chelated to a neighbouring zirconium atom; this motif, in turn, results in the presence of a strongly acidic proton. On dehydration, the material loses its acidity. The hydrated sulfated MOF exhibits a good catalytic performance for the dimerization of isobutene (2-methyl-1-propene), and achieves a 100% selectivity for C8 products with a good conversion efficiency. Solid acid heterogeneous catalysts are widely used in industrial chemical processes, but understanding the exact molecular structures responsible for catalytic activity has proved difficult. Now, the structure of the strong Brønsted acid site for a sulfated zirconium-based metal–organic framework has been shown to consist of a specific arrangement of adsorbed water and sulfate moieties on the zirconium clusters.