Hydrothermal liquefaction (HTL) of microalgae for biofuel production is suffering from low bio-oil yield and high heteroatomic compositions owing to their low efficiency and selectivity to hydrolysis of cellular compounds. Hereby we report Keggin-type (MoÀ VÀ P) heteropolyacids (HPAs)-catalyzed HTL of microalgae for efficient low-nitrogen biocrude production. The increases of reaction temperature, reaction time, and vanadium substitution degrees of HPAs are favorable to biocrude yield initially, whereas a significant decrease of biocrude yield is observed owing to the enhanced oxidation of carbohydrates above the optimum reaction conditions. The maximum biocrude yield of HPAs-catalyzed HTL of microalgae is 29.95 % at reaction temperature of 300°C, reaction time of 2 h, and 5 wt% of HPA-4, which is about 19.66 % higher than that of control with 71.17 % less N-containing compounds, including 1,3-propanediamine, 1-pentanamine, and 2, 2'heptamethylene-di-2-imidazoline than that of control. This work reveals that HPAs with Brønsted acidity and reversible redox properties are capable of both enhancing biocrude production via catalyzing the hydrolysis of cellular compounds and reducing their nitrogen content through avoiding the Maillard reactions between the intermediates of hydrolysis of carbohydrates and proteins. HPAs-catalyzed HTL is an efficient strategy to produce low N-containing biofuels, possibly paving the way of their direct use in modern motors.

Brønsted acid ionic liquids (BAILs) are effective to biodiesel production of in situ transesterification of wet algae due to their dual role as both solvents of cellulose and catalysts of transesterification. The cellulose solubilities of BAILs are depending on H-bonding of BAILs-cellulose in varied solvent conditions, which subsequently affect the biodiesel productions. For this reason, the effects of H-bonding between BAILs includingand cellulose under different methanol and water conditions on cellulose extractions are experimentally and theoretically investigated. The cellulose extractions in BAILs are decreasing as [Bmim][H 2 PO 4 ] > [Bmim] 2 [HPO 4 ] > [Bmim][HSO 4 ], contrary to the variation of their thermal stabilities and crystallinities caused by varied Hbonding of BAILs-cellulose. Increasing methanol is positive to cellulose extractions in [Bmim][HSO 4 ], while negative to those in [Bmim][H 2 PO 4 ] and [Bmim] 2 [HPO 4 ], due to the solvent effect of methanol confirmed by cyclic voltammetry (CV) measurements. The effect of water on H-bonding of BAILs-cellobiose is contrary to the effect of methanol. It is confirmed via the bonding interaction of BAILs-cellobiose through the density functional theory (DFT) computational method. However, the biodiesel yields are varied differently due to the competition of H-bonding of BAILs-cellobiose and deprotonation of BAILs under different methanol and water conditions. This study might pave the way to neutralize the negative effect of water on in situ transesterification via enhancing the cellulose extractions of BAILs.

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High oxygen content of microalgae-derived bio-oil limits their direct use in modern motors. In this study, instead of noble metal-catalyzed two-step hydrodeoxygenation, MgAl layered double hydroxides/oxides (MgAl-LDH/LDO) with tunable acidic and basic properties are developed for catalyzing the hydrothermal liquefaction (HTL) of microalgae to obtain bio-oil with a low oxygen content. The results show that both MgAl-LDH 3 and MgAl-LDO 3 enhance low O/C bio-oil production through catalyzing both the hydrolysis of cellular compounds and decarboxylation and decarbonylation of biocrude during HTL of microalgae. MgAl-LDH 3 with more acidic sites is more effective at catalyzing the hydrolysis of cellular compounds than MgAl-LDO 3 according to the relative increases of 12.98% and 9.72% of biocrude yields, respectively. However, MgAl-LDO 3 with more basic sites is more efficient in catalyzing the decarboxylation and decarbonylation and amidation of fatty acids to form hydrocarbons, esters, alcohols, and amides, which contributes to a 22.6% decrease of O/C and 28.4% increase of N/C in the bio-oil product, respectively. This work reveals that MgAl-LDH x and MgAl-LDO x are efficient at catalyzing not only the hydrolysis of cellular compounds but also the deoxidation of the reaction intermediates to produce low Ocontaining bio-oil, which might pave the way for its direct use in modern motors.

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