TEMPO-Appended Metal–Organic Frameworks as Highly Active, Selective, and Reusable Catalysts for Mild Aerobic Oxidation of Alcohols

Abstract

Metal−organic frameworks (MOFs) decorated with stable organic radicals are highly promising materials for redox catalysis. Unfortunately however, the synthesis of chemically robust MOFs typically requires harsh solvothermal conditions, which are not compatible with organic radicals. Here, we describe the synthesis of two isoreticular families of stable, mixed component, zirconium MOFs with UiO-66 and UiO-67 structures and controlled amounts of covalently attached TEMPO radicals. The materials were obtained using a relatively low-temperature, HCl-modulated de novo method developed by Hupp and Farha and shown to contain large amounts of missing cluster defects, forming nanodomains of the reo phase with 8-connected clusters. In the extreme case of homoleptic UiO-67-TEMPO(100%), the material exists as an almost pure reo phase. Large voids due to missing clusters and linkers allowed these materials to accommodate up to 2 times more of bulky TEMPO substituents than theoretically predicted for the idealized structures and proved to be beneficial for catalytic activity. The TEMPO-appended MOFs were shown to be highly active and recyclable catalysts for selective aerobic oxidation of a broad range of primary and secondary alcohols under exceptionally mild conditions (room temperature, atmospheric pressure of air). The influence of various parameters, including the pore size and TEMPO content, on the catalytic activity was also comprehensively investigated.

Surface‐Deactivated Core–Shell Metal–Organic Framework by Simple Ligand Exchange for Enhanced Size Discrimination in Aerobic Oxidation of Alcohols

“…Accordingly, to adopt this catalytic system to heterogeneous catalysts, UiO‐67‐TEMPO with Eu(NO 3 ) 3 was used for aerobic oxidation with a variety of aldehydes with different sizes in the present study (Figure , Tables S1 and S2). To the best of our knowledge, this is the first attempt to utilize a combination system of a metal catalyst and TEMPO species for aerobic oxidation by MOF catalysis, although the previously reported TEMPO only‐functionalized MOFs were successfully demonstrated in the aerobic oxidation of alcohols . The experiments conducted with different catalyst combinations for benzyl alcohol ( 1 a ) oxidation clearly showed that the simultaneous use of UiO‐67‐TEMPO and Eu(NO 3 ) 3 exhibited 97 % conversion to benzaldehyde ( 2 a ), which is the comparable result from homogeneous reaction of Eu(NO 3 ) 3 and TEMPO (entries 3 and 4 in Table S1).…”

Engineering UiO‐66 Metal Organic Framework for Heterogeneous Catalysis

“…[80] The same oxidation reaction was completed in one third of the time using UiO studied. [81] The aerobic oxidation of benzyl alcohol to benzaldehyde resulted in quantitative yield with UiO-67-TEMPO (38 %) at 25°C in acetonitrile, dioxane, and 1,2-dichloroethane. Interestingly, the use of UiO-67-TEMPO (38 %) as catalyst afforded 48 % yield at 0°C.…”

Electroactive Organic Building Blocks for the Chemical Design of Functional Porous Frameworks (MOFs and COFs) in Electronics

“…More recently,T EMPO-decorated UiO-66, UiO-67 and UiO-68 MOFs have been synthesized under mild conditions and have been shown to be efficient catalysts fort he selective aerobic oxidation of alcohols. [228,229] In these studies, the authors prepared the TEMPO-substituted MOFs using an ew HClmodulated and low-temperature method to avoid the decompositiono ft he radicals, obtaining highly defective materials with surfacea reas( up to 1310m 2 g À1 )h igher than the parent frameworks. The radical-based catalysts are highly activea nd recyclable for the aerobic oxidation of alcohols under ambient conditions.…”