Sustainable acid catalyst from the hydrothermal carbonization of carrageenan: use in glycerol conversion to solketal

The reaction of acetone with glycerol to yield 2,2-dimethyl-1,3-dioxolane-4-methanol (solketal) was successfully catalyzed by a novel, sustainable solid acid catalyst formed by the hydrothermal carbonization (HTC) of carrageenan, a sulfated natural polysaccharide obtained from red macroalgae. All evaluated carrageenan HTC material incorporated some sulfur atoms on their structure, leading to remarkable strong acidity, in the range of 1.26 to 1.29 mmol g(-1), depending on the time of HTC. The glycerol conversion was around 80-85% within 1 h and 95-98% within 6 h, similar to Amberlyst-15 and better than many commercial solid acids. These results were interpreted in terms of the formation of strong acid sites, probably of sulfonic acid nature present in low concentration, as shown by FTIR. The carrageenan HTC materials showed excellent activity and stability, being truly heterogeneous catalysts that can be reused without significant deactivation. These results indicate the potential of these materials as sustainable acid catalysts, especially for the conversion of biomass.