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

Abstract

• HKUST-1‒NH 2 and MIL-101(Cr)‒NH 2 were directly synthesized without adding HF. • Both materials showed a high CO 2 uptake and stability over ten adsorption cycles. • MIL-101(Cr)‒NH 2 had a high activity in cycloaddition and in situ oligomerization. • Using acetonitrile solvent is crucial for controlling cyclic carbonate selectivity. • The styrene carbonate yield of > 97 % was obtained over MIL-101(Cr)‒NH 2 . In this study, copper- and chromium-based (HKUST-1 and MIL-101(Cr), respectively) metal-organic frameworks (MOF) functionalized with amine groups (HKUST-1‒NH 2 and MIL-101(Cr)‒NH 2 , 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 (CO 2 ) adsorption and conversion of epoxides with CO 2 . The functionalized MOF (HKUST-1‒NH 2 and MIL-101(Cr)‒NH 2 ) retained their desired textural properties, while gaining a significantly enhanced Lewis basic character for CO 2 capture and catalysis application. Both HKUST-1‒NH 2 and MIL-101(Cr)‒NH 2 not only showed an improved CO 2 uptake capability, but also an excellent and stable regenerability over multiple adsorption-desorption cycles. MIL-101(Cr)‒NH 2 exhibited a higher performance than the parent MOF and HKUST-1‒NH 2 in the transformation of styrene oxide (SO) with CO 2 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 CO 2 utilization between cycloaddition and oligomerization. In the presence of acetonitrile, a > 97 % yield of SC was achieved over MIL-101(Cr)‒NH 2 under a mild reaction condition (120 °C and 20 bar of CO 2 ). Reaction mechanisms for the cycloaddition and oligomerization of SO with CO 2 are also proposed to comprehend the role of MOF, amine group, and co-catalyst. The combined efficient CO 2 adsorption and capability to produce CC and COL makes the synthesized MOF promising materials for CO 2 capture and selective utilization.