Photo-switchable phosphotungstic acid active sites in metal-organic frameworks for a tailorable deacetalization reaction

Solid acids can catalyze various reactions and play an important role in modern chemical processes. Albeit highly desirable, the remote regulation of the activity of solid acid catalysts through external stimuli has remained a great challenge. Here, for the first time, we constructed a smart solid acid catalyst system by introducing photo-switchable azobenzene motifs and a phosphotungstic acid (PTA) center into a robust amino-decorated metal-organic framework, Cr-MIL-101-NH₂. By placing this smart acid catalyst under UV- and vis-light irradiation, the catalytic activity in the deacetalization reaction can be regulated remotely. When azobenzene motifs are in trans configuration, the active sites are sheltered, which is unfavorable to the catalytic reaction. While the azobenzene derivatives are transformed from trans to cis isomers under UV-light irradiation, the active sites are exposed, and thus the catalytic performance is enhanced. Density functional theory (DFT) calculations show that when azobenzene is in cis configuration, the surface electrostatic potential of PTA is close to the initial value, which is conducive to catalyzing the reaction. In contrast, when azobenzene is in trans configuration, the surface electrostatic potential of PTA decreases, which inhibits the catalytic reaction. The present smart catalysts shed light on effective control of catalytic activity remotely through a photo-switchable approach.