Enhanced Catalytic Activity for Levulinic Acid Esterification Using Covalent Organic Framework Heterogenized Heteropolyacids

Alkyl levulinates act as crucial additives in gasoline and biodiesel and are largely produced through esterification of levulinic acid (LA) with alcohols using homogeneous acids suffering from equipment corrosion and low reusability issues. Here, in this work, a homogeneous phosphotungstic acid (HPW) catalyst was heterogenized via incorporation on a 2D imine-based covalent organic framework material (LZU1) to obtain a heterogeneous HPW-LZU1 catalyst. The material synthesis process was comprehensively monitored through 1H NMR, solid 13C MAS NMR, ATR-FTIR, etc. When the HPW/LZU1 mass ratio was 8.6, the HPW-LZU1 catalyst exhibited an excellent catalytic activity, with >90% conversion of LA and 100% selectivity of alkyl levulinates obtained, comparable to the HPW catalyst. Catalytic performance without obvious decrease after five cycles demonstrated excellent stability and reusability of the as-prepared catalyst. In situ FTIR study shows that LA was dominantly adsorbed on the Brönsted acid sites, while methanol was adsorbed on the Brönsted and Lewis acid sites. The HPW-π interaction-induced electron delocalization effect over the catalyst effectively enhanced the adsorption of LA and alcohols and the attack of alcohols to LA through AAc1 mechanism, attributing to the high catalytic performance of the catalyst.