Fabrication of Keggin heteropolyacid modified macroporous covalent triazine frameworks with cyano group-rich defects for high-value utilization of carbohydrates

Abstract

Designing an efficient, stable and recyclable solid acid for catalytic conversion of biomass and its derived compounds to synthesize high value-added organic chemicals could effectively alleviate the environmental problems caused by the energy crisis and carbon dioxide emissions. We prepared a Keggin heteropolyacid modified macroporous covalent triazine frameworks (PW12-MCTF) catalyst using a simple template method and post-impregnation technology for efficient conversion of carbohydrates to produce 5-hydroxymethylfurfural (HMF). Abundant structural defects of MCTF can be constructed in situ through incomplete polymerization, which can firmly bind H3PW12O40 through physical confinement effect and bonding interactions. Under optimized conditions, PW12-MCTF exhibited high HMF yield (79.4 % or 44.4 %) and the highest turnover frequency (4.39 min−1 or 1.86 min−1) in microwave-assisted catalytic conversion of fructose or inulin, respectively. Its catalytic activity surpassed that of bulk PW12-CTF and commercial acid zeolite and resin catalysts, due to its appropriate acid properties, well-dispersed nature in DMSO, inter-connected pore structure and outstanding nucleophilic effect of N-rich units. Density Functional Theory (DFT) study provides a plausible mechanism in PW12-CTF-catalyzed fructose dehydration. The catalytic performance of PW12-MCTF remained unchanged after three cycles due to strong bonding interaction between the Keggin units and framework of MCTF. This study will provide a new strategy for the fabrication of heteropolyacid modified covalent organic frameworks with structural defects to achieve the high-value utilization of carbohydrates.