Zeolite-catalyzed one-pot sucrose-to-HMF transformation: Ge outperforms Sn, Zr, and Al sites

Abstract

The conversion of renewable compounds to versatile platform molecules over environmentally friendly heterogeneous catalysts is a major challenge. Zeolites stand as active, selective, and reusable solid catalysts for various acid-catalyzed reactions involved in the one-pot cascade transformation of polysaccharides to 5-hydroxymethylfurfural (HMF), a platform molecule opening the way to various valuable chemicals. However, the acidity-performance relationships of zeolite catalysts in HMF synthesis have not been fully elucidated. Here, we have addressed the effect of acid site nature in zeolite catalysts for sucrose-to-HMF transformation by comparing the performance of conventional Al-substituted IWW zeolite with that of Sn-, Zr-, and Ge-containing zeolite catalysts of the same structure. Ge-associated acid sites were found to exhibit superior HMF selectivity compared to Sn, Zr, and Al acid centers, while experiencing evolution into Bronsted acid centers during the catalytic run. The conversion of sucrose over germanosilicate zeolites enhances with increasing catalyst pore diameter or decreasing crystal size. Specifically, the extra-large pore Ge-UTL catalyst, featuring intersecting 14- and 12-ring pores (crystal size 10 x 20 x <1 μm), and large-pore Ge-IWW with 12-, 10- and 8-ring pores (crystal sizes of 1 μm) showed a yield of targeted HMF comparable to or even exceeding the values previously reported for homogeneous or heterogeneous catalysis (54 % after 3 h at 120 °C). Ge-IWW catalyst demonstrated reusability across a minimum of 3 catalytic runs, while in situ structural transformation precluded stable performance of Ge-UTL catalyst in the repetitive catalytic cycles. The results of this study highlight zeolites with uncharacteristic chemical compositions as active, selective, and reusable catalysts for highly demanding applications in biomass valorization.