Hexagonal Layer Manganese Metal–Organic Framework for Photocatalytic CO₂ Cycloaddition Reaction

Abstract

A novel manganese metal–organic framework (Mn-MOF) termed UAEU-50 assembled from a benzenedicarboxylate linker (BDC) and trinuclear manganese clusters was synthesized and fully characterized using different spectroscopic and analytic techniques (e.g., X-ray powder diffraction, UV–vis diffuse reflectance spectroscopy, thermogravimetric analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy). UAEU-50 adopted a hexagonal layer structure and exhibited superior thermal stability and robust chemical stability. Photocatalytic activities of UAEU-50 were investigated using the cycloaddition of CO 2 to different epoxides, forming cyclic carbonates. Impressively, UAEU-50 can transform up to 90% photocatalytic CO 2 conversion to cyclic carbonates in the visible-light region at ambient conditions.

S-Scheme Bi-oxide/Ti-oxide Molecular Hybrid for Photocatalytic Cycloaddition of Carbon Dioxide to Epoxides

“…Finally, we found a combination of TiO 2 P25 and Bi­(OAc) 3 afforded higher yields of 1a than TiO 2 P25 alone, that is, 57% versus 38% (entry 9). After the reaction, TiO 2 P25 turned dark, presumably due to the formation of Bi(0) on its surfaces, indicating that the TiO 2 P25/Bi­(OAc) 3 combination was not chemically stable and that Bi(0) could be responsible for the increased photocatalytic activity . This contrasts with the current Ti 18 Bi 4 , which remained stable throughout the photocatalytic reactions, as demonstrated later.…”

“…Next, in the absence of TBAB, no conversion occurred, indicating that it could not be avoided (entry 5). To the best of our knowledge, ammonium or other nitrogen-containing halide salts were always essential for activation of epoxides in CO 2 insertion in previous reports using Lewis acid catalysts or photocatalysts. − We also studied the activities of the undoped Ti-oxide carboxylates including Ti 6 O 6 Bz 6 (O i Pr) 6 and Ti 8 O 8 Bz 16 and the commercial TiO 2 P25. However, their activities were much lower than that of Ti 18 Bi 4 (entries 6–8).…”

“…While this conversion could be effectively catalyzed with Lewis acidic and basic reagents at high temperatures, there have been unremitting endeavors to develop new catalytic systems that use sunlight as the energy source and operate under mild conditions (e.g., ambient temperature and 1 atm CO 2 ). In this regard, photocatalysis, which uses photogenerated electrons and holes to trigger CO 2 cycloadditions via new reaction pathways distinct from traditional Lewis acid/base thermocatalysis, represents a promising new direction. − Due to their high charge-separation efficiency, semiconductor heterojunctions composed of two transition metal-oxide components are a class of highly efficient photocatalysts. Among them, the S-scheme systems (also known as the direct Z-scheme) are known for their high redox power due to their unique charge carrier-transfer mode, − which has been shown to benefit challenging reactions like CO 2 reduction. − Given the cycloaddition reactions of CO 2 and epoxides, it is assumed that S-scheme systems may benefit from a synergistic effect of the very strong driving force and the surface coordination-unsaturated transition metal ions that could serve as Lewis acidic sites.…”

Cr–Ti Mixed Oxide Molecular Cages: Synthesis, Structure, Photoresponse, and Photocatalytic Properties

“…Hence, both are molecular catalysts. Tetrabutylammonium bromide (TBAB) was used as the cocatalyst, which is critical for promoting the ring-opening step of the cycloaddition reaction, as has been demonstrated in numerous previous studies using Lewis acid catalysts or photocatalysts. − …”

“…Note that the reaction conditions used herein are very mild, indicating that the two clusters’ Lewis acidic thermocatalytic activities are good. ,,− Meanwhile, the reaction conditions used herein such as solvent-free, room temperature, and atmospheric pressure are all highly desirable when considering the principle of green chemistry. Because both clusters are photoresponsive, their photocatalytic activities in the model reaction were investigated.…”