Amorphous NH₂-MIL-68 as an efficient electro- and photo-catalyst for CO₂ conversion reactions

To produce metal-organic framework (MOF) catalysts with both high activity and durability is interesting but challenging. We report an amorphous MOF (NH₂-MIL-68), which combines the advantages of (1) a large number of open metal sites, (2) the basic building blocks and connectivity of crystalline NH₂-MIL-68, and (3) hierarchically meso- and microporous structure. It exhibits high performances in electrocatalytic reduction of CO₂ and photochemical cycloaddition of CO₂ under mild conditions. For the former reaction, the maximum Faradaic efficiency for product formic acid (FE_HCOOH) reaches 93.3% with a current density of 34.2 mA·cm⁻² at −2.05 V vs. Ag/Ag⁺ catalyzed by amorphous NH₂-MIL-68, while the crystalline NH₂-MIL-68 shows FE_HCOOH of 67.7% with considerable productions of CO and H₂ at the same experimental conditions. For the photochemical cycloaddition of CO₂ with styrene oxide, the yield by amorphous NH₂-MIL-68 can reach 94.1% at 12 h, which is higher than the reported value under similar conditions. The structure–efficiency relationship of the catalyst for the two reactions was investigated. This work opens up new possibility for designing high-performance MOF and MOF-based catalysts.