Catalytic conversion of alginic acid to furfural using 12-tungstophosphoric acid catalyst in tetrahydrofuran/water co-solvent

Abstract

Furfural is one of platform chemicals from biomass conversion processes listed by the US Department of Energy (DOE). The world wide demand for furfural is expected to steadily increase, because furfural can be used as a starting compound in production of furan-derivates such as a polymer unit, organic solvent and various liquid fuel additives. Until now, furfural has been exclusively obtained from hemicelluloses despite increasing demand for furfural, suggesting that diversification of feedstock is necessary for stable supply of furfural. In view of this, we attempted to produce furfural using alginic acid derived from brown seaweeds, and considered a possibility whether alginic acid could be used as an alternative feedstock for furfural production. Alginic acid, main constituent of brown seaweed, is a bio-polymer comprised of hexuronic acid monomers (mannuronic acid and guluronic acid) via β-1,4-glycosidic bond. Notably, the direct conversion of alginic acid to furfural is a somewhat different approach for utilization of the marine algae biomass. In the present work, the decomposition of alginic acid was performed in tetrahydrofuran (THF)/water co-solvent over Brønsted acid catalysts, such as H2SO4, Amberlyst15 and 12-tunstophosphoric acid (H3PW12O40) in order to investigate the effect of the different acid catalysts on the furfural production. Among the catalysts, H3PW12O40 exhibited a higher catalytic performance than other acid catalysts by suppressing the formation of by-product such as an insoluble black precipitate (humin). The maximum furfural yield from alginic acid was 33.8% under the reaction conditions of 180 oC for 30 min in THF/water co-solvent containing 5% (v/v) water over H3PW12O40. To our knowledge, this is the highest furfural yield among the studies about the alginic acid decomposition. In this reaction, THF was a more effective reaction medium than water for furfural production. In particular, addition of small amounts of water to THF drastically enhanced the furfural yield. Furthermore, products distribution with time-stream was investigated to elucidate the reaction pathway. This result showed that alginic acid was initially hydrolyzed to hexronic acid, and then it converted to furfural by removing carbon dioxide and water molecules from the hexuronic acid in presence of acid catalyst. In the reaction pathway, subsequent reaction comprised of decarboxylation and dehydration occurred slower than hydrolysis, which led us to determine rate-limiting steps in the proposed reaction pathway.