Novel valorization method of green tides macroalga Ulva rigida using heterogeneous catalysts and mechanochemistry for the production of furan-derived platform molecules

Ulva rigida is a green macoalga that has recently raised concerns among the research community due to its accumulation in intertidal environments and coastal areas after green tides. In this work, Ulva rigida retrieved from affected local intertidal areas was valorized through a two-step hydrothermal process, involving an autohydrolysis in aqueous medium and a heterogeneous catalytic dehydration in a water/methyl isobutyl ketone medium. This process was enhanced by a pretreatment that consisted in a wet mechanochemical milling of the macroalga. Mechanochemical treatment reduced significantly the degree of polymerization calculated via viscosimetry analysis, from 1160 to 600 and promoted the release of glucose-, xylose- and rhamonose-rich species in the autohydrolysis step. For the catalytic dehydration, two mesoporous acid silica-based catalysts were assayed. Their different textural and acidic properties led to different catalytic performances and selectivity. HMS-Zr promoted preferentially the production of furfural at 160 °C, while SBA Nb, enhanced the transformation of glucose to 5-hydroxymethylfurfural (5-HMF) at 180 °C. The highest efficiencies, defined as the molar moiety of furans to molar moiety of potential pentoses or hexoses, obtained for furfural and 5-HMF were 78.2% and 57.7% respectively, which translated into 30 and 51 g∙kg −1 of dry Ulva rigida . Whereas, HMS-Zr retained a constant catalytic performance and could be reused up to five cycles, SBA-Nb exhibited a slight deactivation after the third cycle and its performance dropped consequently as carbonaceous deposits appeared over the surface of the catalyst.