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.