Substantial Turnover Frequency Enhancement of MOF Catalysts by Crystallite Downsizing Combined with Surface Anchoring

We report on the preparation of surface-anchored nanoparticles of the metal-organic framework (MOF) UiO-66 (Universitet i Oslo; Zr6O4(OH)(4)(bdc)(6); bdc(2-) = 1,4-benzene dicarboxylate). The surface -anchored nano-MOFs (SA-NMOFs) were prepared by covalent anchoring of a presynthesized, functionalized UiO-66 nano-MOF (NMOF) on surface-modified poly(dimethylsiloxane). The SA-NMOFs exhibit discrete NMOFs (<30 nm) which do not aggregate. The SA-NMOFs retain a high surface area, rendering them interesting catalysts. We compared the catalytic activities of SA-NMOFs in the cyanosilylation of benzaldehyde with those of the bulk UiO-66 and colloidal-dispersed UiO-66 NMOFs (size: 22 +/- 3 nm). The SA-NMOFs exhibit a boost in activity by a factor of 100,000-1,000,000 owing to (a) the generally larger surface area of NMOFs and (b) the suppressed aggregation of the nanoparticles by surface immobilization. In contrast, colloidal NMOFs rapidly aggregate, as shown by dynamic light scattering. The general applicability of our approach for other Lewis acid-catalyzed reactions is demonstrated by comparing the activities of the three catalyst systems for the cycloaddition of CO2 and propylene oxide to propylene carbonate, where SA-NMOFs by far outperform the bulk MOFs and defect-engineered MOFs, respectively. This discovery paves the way for application of SA-NMOFs as efficient catalyst materials.