Strongly Lewis Acidic Metal–Organic Frameworks for Continuous Flow Catalysis

Abstract

The synthesis of highly acidic metal−organic frameworks (MOFs) has attracted significant research interest in recent years. We report here the design of a strongly Lewis acidic MOF, ZrOTf-BTC, through two-step transformation of MOF-808 (Zr-BTC) secondary building units (SBUs). Zr-BTC was first treated with 1 M hydrochloric acid solution to afford ZrOH-BTC by replacing each bridging formate group with a pair of hydroxide and water groups. The resultant ZrOH-BTC was further treated with trimethylsilyl triflate (Me 3 SiOTf) to afford ZrOTf-BTC by taking advantage of the oxophilicity of the Me 3 Si group. Electron paramagnetic resonance spectra of Zr-bound superoxide and fluorescence spectra of Zr-bound Nmethylacridone provided a quantitative measurement of Lewis acidity of ZrOTf-BTC with an energy splitting (ΔE) of 0.99 eV between the π x * and π y * orbitals, which is competitive to the homogeneous benchmark Sc(OTf) 3 . ZrOTf-BTC was shown to be a highly active solid Lewis acid catalyst for a broad range of important organic transformations under mild conditions, including Diels−Alder reaction, epoxide ring-opening reaction, Friedel−Crafts acylation, and alkene hydroalkoxylation reaction. The MOF catalyst outperformed Sc(OTf) 3 in terms of both catalytic activity and catalyst lifetime. Moreover, we developed a ZrOTf-BTC@SiO 2 composite as an efficient solid Lewis acid catalyst for continuous flow catalysis. The Zr centers in ZrOTf-BTC@SiO 2 feature identical coordination environment to ZrOTf-BTC based on spectroscopic evidence. ZrOTf-BTC@SiO 2 displayed exceptionally high turnover numbers (TONs) of 1700 for Diels−Alder reaction, 2700 for epoxide ring-opening reaction, and 326 for Friedel−Crafts acylation under flow conditions. We have thus created strongly Lewis acidic sites in MOFs via triflation and constructed the MOF@SiO 2 composite for continuous flow catalysis of important organic transformations.

Reticular Materials for Artificial Photoreduction of CO2

“…The high Lewis acidity of metal-oxo clusters in MOFs has been largely explored in a variety of applications, e.g., from gas storage to catalysis and sensing. [156][157][158][159] Although the exceptional stability of zeolites or metal oxides highlights their industrially applicable level for years, their Figure 15. Structure of TTCOF-Zn and the mechanism of the CO 2 -to-CO photoreduction in water.…”

Functional Metal Organic Framework/SiO2 Nanocomposites: From Versatile Synthesis to Advanced Applications

“…As a kind of crystalline material containing metal or metal cluster junctions, metal–organic skeleton materials are expected to become heterogeneous catalysts or catalyst carriers [113,114,115,116,117,118,119,120]. MOF materials provide a highly tunable platform for structure and performance, integrating all the properties required for catalysis, including high surface area, high porosity, active metal sites, recyclability, and high crystallinity [55,121,122,123,124].…”

Testing Metal–Organic Framework Catalysts in a Microreactor for Ethyl Paraoxon Hydrolysis

“…Although MOF or MOF-based composite catalysts have been proven to be effective for the degradation of CWAs, most examples were tested in batch conditions in basic solutions. In general, there are few examples of catalytic testing of MOF-based catalysts in flow reactors as shown in Scheme 1 [20,21]. Catalyst testing in flow microreactors has many advantages over traditional solid catalyst testing in batch reactors [20].…”

“…In general, there are few examples of catalytic testing of MOF-based catalysts in flow reactors as shown in Scheme 1 [20,21]. Catalyst testing in flow microreactors has many advantages over traditional solid catalyst testing in batch reactors [20]. First, many operating parameters such as temperature, pressure, and feed concentrations can be easily and quickly varied in flow microreactors, which provide an insight into the reaction mechanism and kinetics; second, consumption of chemicals and waste production is significantly reduced; and third, it enables easy testing of catalyst reusability, and catalyst stability under reaction conditions via long-term testing on a stream and changing the feed purity.…”