Catalytic Conversion of Macroalgae-derived Alginates to Valuable Organic Compounds under Hydrothermal Conditions

Abstract

Recently, marine biomass is gaining attraction as a promising renewable feedstock for production of biofuels and valuable chemicals. This aquatic biomass including microalgae and macroalgae has important advantages such as easy cultivation on non-arable land, a rapid growth rate and its lignin-free composition, in comparison with corn, sugarcane and lignocellulosic biomass. Microalgae is extensively studied for producing biodiesels based on the high lipid content in maicroalgae. On the other hand, macroalgae contains abundant carbohydrate components such as alginate and mannitol instead of lipid, which can be applied in production of bio-oil, volatile fatty acids (VFAs) and bio-alcohol. Alginate is one of the main components of macroalgae, especially brown seaweeds. It is composed of two monomeric subunits, β-D-mannuronic acid and α-L-guluronic acid, connected by β-1,4-glycosidic bonds. This cellulose-like structure of alginate has great potential for production of valuable chemicals via thermochemical or biological conversion processes. Among various conversion processes, hydrothermal process using hot-compressed water is promising, since it is a low-cost and eco-friendly biorefinery technique valorizing biomass-derived carbohydrate materials. The hydrothermal conversion of alginate was performed at reaction temperatures from 150 to 200 °C as a function of pH in order to investigate the effects of acidity and basicity on the production of value-added chemicals. The pH value of aqueous medium was used as a quantitative standard indicating acidity and basicity. A base-catalyzed reaction at pH 13 enahnced the decomposition of alginate, resulting in the production of lactic acid, fumaric acid and malic acid as major species. At pH 1, monomers (mannuronic acid and guluronic acid), furfural and glycolic acid were predominantly produced by the acid-catalyzed hydrothermal decomposition of alginate. Increasing the reaction temperature promoted both the acid- and base-catalyzed reactions, indicating that hot-compressed water can play a role of catalyst itself due to the increasing ion product (Kw). This study demonstrates that optimizing the acidity, basicity and temperature is significantly important for the efficient conversion of alginate to valuable products. To study the effect of catalysts on the production of lactic acid from alginate, metal oxides were employed as solid base catalysts. The CaO catalyst exhibited the highest catalytic performance, yielding 14.66 % lactic acid at 200 °C for 6 h, while other metal oxide catalysts showed little activity. The yield of lactic acid was proportional to the number of Brønsted bases (OH-) in an aqueous medium, measured by titration of metal oxides. The catalytic activity of CaO catalyst was maintained for two subsequent reaction cycles and the deactivated catalyst was successfully regenerated by calcination. The deactivation of the CaO catalyst during subsequent repeated uses arose from decrease in the number of available active sites providing both Lewis acid site and Brønsted basic site, by covering the active sites with various byproducts. Plausible reaction pathway for the catalytic conversion of alginate to lactic acid over CaO was also discussed. The effect of metal oxide catalysts on the depolymerization of alginate was also analyzed based on the molecular weight distribution of products. The catalytic effect of metal cations on the hydrothermal production of furfural from alginic acid was investigated with using various metal ions as catalysts. Among the metal ions, Cu (II) ions demonstrated the highest furfural yield (13.19 %) at 200 °C for 30 min. As the reaction temperature increased from 160 to 220 °C, the production of furfural was enhanced, however, the maximum furfural yield at each reaction temperature decreased after certain reaction times due to the conversion of furfural to humins or organic acids. The yield of furfural was strongly dependent upon metal ion concentration and the optimal concentration of Cu (II) ions was 0.01 M. In addition to furfural, lactic acid, glycolic acid and formic acid were produced in different amounts. The effect of Cu (II) ion on the depolymerization of alginate was studied with gel permeation chromatography (GPC) analysis. A plausible reaction pathway of furfural production catalyzed by Cu (II) ions was proposed.