Valorisation of Chlorella sp. biomass in 5-HMF through a two-step conversion in the presence of Nb2O5 and NbOPO4 and optimisation through reactive extraction

Abstract

The valorisation of recalcitrant Chlorella sp. microalgal biomass by the extraction of the sugar content and its conversion into 5-hydroxymethyl furfural (5-HMF) has been carried out in two-steps. In the first step, a pretreatment of the biomass was optimized to obtain, by sonication and hydrothermal treatments also in the presence of acetic acid and SiO2 pellets, the maximum release of carbohydrate and their hydrolysis to monosaccharides. The second step, carried out under hydrothermal conditions, was devoted to the heterogeneous catalytic isomerisation/dehydration of the monosaccharides (essentially glucose and fructose) released from algae in the first reaction step to yield 5-HMF in presence of two commercial samples of niobium-based catalysts that were characterized with several techniques like specific surface area and superficial acidity measurements, SEM, HRTEM, EDX, FTIR and Raman. Other important features of the two catalysts are largely present in the literature due to the fact that they are commercial materials. The isomerisation/dehydration reactions of the monosaccharides proceed through a tandem pathway involving the Lewis and Brønsted acid sites provided by the surface of the acidic solids giving rise to the isomerization of glucose to fructose, followed by dehydration of fructose to 5-HMF. The optimisation of the reaction conditions was carried out in the presence of Nb2O5 by using a Design of Experiment (DoE) approach. The optimised parameters were identified as temperature 210 ◦C and 4.45 h of reaction time, providing a yield to 5-HMF of about 18% on the total sugars contained in the algae. A further optimisation has been achieved in the presence of NbOPO4 instead of Nb2O5, which still led to 21% yield to 5-HMF on the total sugars. Moreover, using an H2O/MIBK system, a reactive extraction was performed, obtaining the very good value of yield to 5-HMF of 29% on total sugars. In this way, a multi-approach process optimisation was performed, in line with the principles of process intensification, with the aim of improving the energy-efficiency of the entire process.