A Hierarchical Metal-Organic Framework Composite Aerogel Catalyst Containing Integrated Acid, Base, and Metal Sites for the One-Pot Catalytic Synthesis of Cyclic Carbonates.

Abstract

Catalysis has played a decisive role in the development of unique chemical reactions to produce important chemicals. However, conventional stepwise synthetic routes that rely on individual catalysts to promote each step often suffer from ponderous processes for the isolation of intermediates that result in massive material losses and large economic expenditures. In addition, traditional powder forms of these catalysts suffer from poor processability and recoverability. Herein, we designed and prepared a hierarchical metal-organic framework (MOF) composite monolithic catalyst IL-Au@UiO-66-NH2/CMC that contains integrated acid (Zr4+), base (ionic liquid (IL)), and metal sites (Au nanoparticles (NPs)) to promote the one-pot preparation of cyclic carbonates from styrene derivatives and CO2. Highly dispersed Au NPs, IL 1-aminoethyl-3-methylimidazolium bromide ([C2NH2 MIM] [Br]), and MOF-positioned Lewis acid sites within this composite aerogel are separately responsible for catalyzing selective epoxidation of the styrene derivatives and the subsequent cycloaddition reaction of CO2 with intermediate styrene oxides. Importantly, inclusion of the imidazolium-based IL effectively modulates the size and chemical microenvironment of the Au NPs via electrostatic protection, leading to catalyst stability and its selective oxidation of styrene. Benefiting from the rapid mass transfer and high exposure of active sites within the pore-rich hierarchical nanostructure, IL-Au@UiO-66-NH2/CMC promotes high conversion (90.5%) of the styrene and selectivity (80.5%) for styrene carbonate (SC) formation in the one-pot process, a performance level that far exceeds those of related catalysts containing only Au NPs or IL (the selectivity of SC < 42%). Furthermore, the composite aerogel catalyst can be readily separated and recycled at least five times without a remarkable loss of activity and selectivity. The controllable integration of various active components in the hierarchical MOF composite aerogel herein should serve as the foundation for the design of multifunctional monolithic catalysts for other valuable tandem processes.