Competition of Dual Roles of Ionic Liquids during In Situ Transesterification of Wet Algae

Ionic liquid (IL)-catalyzed in situ transesterification\r\n(IST) of wet algae is a promising strategy for energy-efficient\r\nbiodiesel production owing to the dual roles of ILs as both solvents\r\nof the cell wall and catalysts of transesterification, while their\r\nvariations and interactions within different water contents, which\r\nsubsequently affect biodiesel production, are still out of knowledge.\r\nAccordingly, the variations of fatty acid methyl ester (FAME)\r\nyields and cellulose solubilities of [Bmim][HSO4], [Bmim]-\r\n[H2PO4], and [Bmim]2[HPO4] under different water conditions\r\nare experimentally and theoretically studied. Results indicate that\r\n[Bmim][HSO4], [Bmim][H2PO4], and [Bmim]2[HPO4] are acid,\r\nweak alkaline, and strong alkaline catalysts of transesterification of\r\nlipid with methanol, respectively. FAME yields of both [Bmim]-\r\n[HSO4]- and [Bmim][H2PO4]-catalyzed reactions increase initially\r\nwith the addition of 1.5 g of water, while they decrease with the increase of water content because of the consequent decline of the\r\nnucleophilic index and electrophilic index of the OH groups of [Bmim][HSO4] and [Bmim][H2PO4], respectively. Analogously,\r\nslight decreases of FAME yields of [Bmim]2[HPO4]-catalyzed transesterification are observed within 0.0−3.0 g of water, while a\r\nnotable decrease is presented when 6.0 g of water is added. Contrary to the variation of catalytic properties, the cellulose solubilities\r\nof [Bmim][HSO4], [Bmim][H2PO4], and [Bmim]2[HPO4] all decrease with the addition of 1.5 g of water, as the water molecules\r\noccupy the H-bonding sites of [Bmim][HSO4]−cellobiose (clb), while the H-bonding interactions of water−clb and water−\r\n[Bmim][H2PO4] formed by shared oxygen atoms are enhanced with the increase of water content, subsequently enhancing the Hbonding interactions of ILs−clb and relative cellulose solubilities. The variations of cellulose solubilities of ILs with the addition of\r\n0.0−6.0 g of water are opposite to those of their catalytic properties. This study indicates that the cellulose solubilities and catalytic\r\nproperties of [Bmim][HSO4], [Bmim][H2PO4], and [Bmim]2[HPO4] compete with each other under different water conditions,\r\nproviding a potential strategy to offset the negative effect of water content on IST of wet algae.