Bifunctionality of amine-modified metal-organic frameworks for CO2 capture and selective utilization in cycloaddition

Abstract

In this study, copper- and chromium-based (HKUST-1 and MIL-101(Cr), respectively) metal-organic frameworks (MOF) functionalized with amine groups (HKUST-1‒NH2 and MIL-101(Cr)‒NH2, respectively) were directly synthesized using 2-aminoterephthalic acid as an organic linker via hydrothermal method without adding hydrofluoric acid. They were then investigated for their potential applications in dynamic carbon dioxide (CO2) adsorption and conversion of epoxides with CO2. The functionalized MOF (HKUST-1‒NH2 and MIL-101(Cr)‒NH2) retained their desired textural properties, while gaining a significantly enhanced Lewis basic character for CO2 capture and catalysis application. Both HKUST-1‒NH2 and MIL-101(Cr)‒NH2 not only showed an improved CO2 uptake capability, but also an excellent and stable regenerability over multiple adsorption-desorption cycles. MIL-101(Cr)‒NH2 exhibited a higher performance than the parent MOF and HKUST-1‒NH2 in the transformation of styrene oxide (SO) with CO2 to styrene carbonate (SC) and carbonate oligomers (COL) due to combined effect of its textural properties and basicity. Under solvent-free system, COL from monomeric SC was directly obtained, up to 72.4 % yield, via in situ oligomerization. Optimization of the solvent-free reaction conditions was carried out to control the selective pathway of CO2 utilization between cycloaddition and oligomerization. In the presence of acetonitrile, a > 97 % yield of SC was achieved over MIL-101(Cr)‒NH2 under a mild reaction condition (120 °C and 20 bar of CO2). Reaction mechanisms for the cycloaddition and oligomerization of SO with CO2 are also proposed to comprehend the role of MOF, amine group, and co-catalyst. The combined efficient CO2 adsorption and capability to produce CC and COL makes the synthesized MOF promising materials for CO2 capture and selective utilization.