Highly Dispersed Zirconium–Oxo Species Anchored on Ultrathin g-C3N4 Nanosheets for Efficient Catalytic Transfer Hydrogenation of Furfural

Abstract

The development of efficient heterogeneous catalysts for the sustainable upgrading of biomass-derived platform molecules remains a pivotal challenge. Herein, we report a facile strategy for constructing active Lewis acid–base pairs by anchoring well-dispersed zirconium–oxo species onto thermally exfoliated ultrathin g-C₃N₄ nanosheets. By simply extending the calcination time of melamine to 10 hours, g-C₃N₄ is simultaneously synthesized and exfoliated into nanosheets with a thickness of ~1.2 nm (approximately 3 layers). These nanosheets offer abundant pyridinic and pyrrolic N atoms, which effectively immobilize the zirconium–oxo species through robust Zr–N coordination, yielding the g-C₃N₄ /Zr(IV) composite catalyst. In the transfer hydrogenation of furfural to furfuryl alcohol using 2-propanol as the hydrogen donor, this catalyst achieves >90% conversion and >99% selectivity under mild conditions (120 °C). Structural and catalytic characterizations indicate that prolonged calcination increases the specific surface area and the density of edge N–H functional groups, facilitating the generation of more Zr–N active pairs. Poisoning experiments confirm the indispensable role of Lewis acid sites in achieving high conversion and highlight the essential function of basic sites in maintaining exceptional selectivity toward furfuryl alcohol. Furthermore, isotopic labeling studies verify that the hydroxyl hydrogen in furfuryl alcohol originates exclusively from the hydroxyl group of 2-propanol, while its α-hydrogen is supplied from the α-position of 2-propanol. The catalyst demonstrates robust structural and crystalline stability over five consecutive reaction cycles. Notably, partial activity could be restored by methanol washing, and the remaining deactivation is attributed to minimal Zr leaching. This work highlights the potential of nitrogen-rich two-dimensional supports in anchoring zirconium–oxo species for efficient and sustainable biomass conversion.