Selectively Regulating Lewis Acid-Base Sites in Metal-Organic Frameworks for Achieving Turn-On/Off of the Catalytic Activity in Different CO2 Reactions.

Abstract

Regulating Lewis acid-base sites in catalysts to investigate their influence in the chemical fixation of CO2 is significant but challenging. A metal-organic framework (MOF) with open metal Co sites, {(NH2 Me2 )[Co3 (μ3 -OH)(BTB)2 (H2 O)]⋅9 H2 O⋅5 DMF}n (1), was obtained and the results of the catalytic investigation show that 1 can catalyze cycloaddition of CO2 and aziridines to give 99 % yield. The efficiency of the cyclization of CO2 with propargyl amines is only 32 %. To improve the catalytic ability of 1, ligand XN with Lewis base sites was introduced into 1 and coordinated with the open Co sites, resulting in a decrease of the Lewis acid sites and an increase in the Lewis base sites in a related MOF 2 ({(NH2 Me2 )[Co3 (μ3 -OH)(NHMe2 )(BTB)2 (XN)]⋅8 H2 O⋅4 DMF}n ). Selective regulation of the type of active centers causes the yield of oxazolidinones to be enhanced by about 2.4 times, suggesting that this strategy can turn on/off the catalytic activity for different reactions. The catalytic results from 2 treated with acid solution support this conclusion. This work illuminates a MOF-construction strategy that produces efficient catalysts for CO2 conversion.