Despite the interest in niobia-based catalysts and the importance of biomass valorisation, studies on these catalysts typically utilize model substrates like simple sugars. In this study, a series of niobium oxide-based catalysts was prepared for the application in aqueous phase catalytic conversion of sugars extracted from Chlorella sp. microalga into value-added furans. The solid catalysts were firstly characterized by various techniques including X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), Raman and X-ray photoelectron (XPS) spectroscopy as well as low-temperature N2 physisorption. Moreover, the acidity of the catalysts was assessed by using the temperature-programmed NH3 desorption (NH3-TPD), by titration of water suspended catalyst with NaOH solution, and by P-bearing molecular probes loaded catalysts through 31P and 1H solid-state nuclear magnetic resonance (NMR) techniques. Herein, we focused on the catalytic transformation of Chlorella sp. and glucose solution as model molecule into furans. The best Nb2O5 catalysts for valorizing Chlorella sp. into furans exhibited a larger number of Brønsted acid sites, achieving conversion yields to 5-HMF and furfural of ca. 20–22 % with respect to the extracted sugars from algae. The results showed a discernible dependence of the conversion yields to 5-HMF and furfural on catalyst acidity, specific surface area, and the presence of the Brønsted acid sites. Conversely, when using the glucose solution as substrate is concerning, the highest yield to 5-HMF was reached by using a catalyst that showed also the presence of Lewis acid sites. A systematic investigation of the structure–activity relationships in niobium oxide application for aqueous phase dehydration using real biomass substrates to obtain furanic derivatives has not been documented thus far. Therefore, the current research is significant as it demonstrates the feasibility of transforming the carbohydrate content in microalgal biomass into furans by identifying the best catalyst to use.

IK acknowledges support by Spanish MCINN (PID2020-113558RB-C41) and Gobierno del Principado de Asturias (IDI-2021-000048). SV-R and JIP gratefully acknowledge funding by Spanish Ministerio de Ciencia e Innovación and Agencia Estatal de Investigación (MCIN/AEI/ 10.13039/501100011033) as well as the European Regional Development Fund (ERDF, A way of making Europe) through grant PID2021-125246OB-I00, and by Plan de Ciencia, Tecnología e Innovación (PCTI) 2018-2022 del Principado de Asturias and by ERDF through grant IDI/2021/000037. This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020, UIDP/50011/2020, and LA/P/0006/2020, financed by national funds through the FCT/MEC (PIDDAC). We also acknowledge funding from project PTDC/QUI-QFI/28747/2017 (GAS2MAT-DNPSENS-POCI-01-0145-FEDER-028), financed through FCT/MEC and cofinanced by FEDER under the PT2020 Partnership Agreement. The NMR spectrometers are part of the National NMR Network (PTNMR) and are partially supported by Infrastructure Project 022161 (cofinanced by FEDER through COMPETE 2020, POCI and PORL, and FCT through PIDDAC). SL acknowledges support by European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement 865974) and with the co-funding of European Union, European Social Fund – REACT EU, PON Ricerca e Innovazione 2014- 2020, Azione IV.4 “Dottorati e contratti di ricerca su tematiche dell’innovazione” and Azione IV.6 “Contratti di ricerca su tematiche Green” (DM 1062/2021). MI acknowledges FCT for Researcher Position (CEECIND/00546/2018) and Beatriz Galindo Scholarship (MU-23-BG22/00145).

Peer reviewed