In-situ self-assembly of two ultrastable bifunctional Keggin-type polyoxometalates-based zeolite-like frameworks for efficient cascade biomass chemicals conversion

Abstract

The cascade conversion of 5-hydroxymethylfurfural (HMF) using a single-component catalyst to produce valuable biomass chemicals represents a challenging and intriguing area for investigation. In this study, we present the remarkable potential of two ultrastable polyoxometalates-based zeolite-like frameworks (PZFs), {[La(H2O)3Cl0.5(pdc)][La(H2O)4(pdc)]}2[PMo12O40]·2H2O (PZF-1) and [La(H2O)4(pdc)]4[SiW12O40]·2H2O (PZF-2) (H2pdc = pyridine-2,6-dicarboxylate) that can be self-assembled from the commercialized raw materials by the facile hydrothermal reaction under the controllable pH environments and their cascade conversion of biomass chemicals for the first time. The structural analyses have unequivocally demonstrated the in-situ formation of Keggin-type polyoxometalates, which are encapsulated within the cationic Lewis-acid Lanthanide-organic frameworks. The syntheses yield highly stable and tolerant crystalline bifunctional PZFs catalysts, capable of withstanding thermal conditions (up to 425 °C) and a wide pH range (pH = 1–11) far better than those reported cases. Most notably, PZF-1 demonstrated outstanding catalytic performance in the cascade reactions of HMF oxidation and 2,5-diformylfuran (DFF) Knoevenagel condensation, achieving remarkable yields of up to 99 %. The structure of PZF-1 remained well-preserved even after undergoing five cycles of catalytic reaction. These encouraging findings provide a solid foundation for the further development of PZFs catalysts for the advancement of biomass conversion processes.