Pd/NbOPO₄ multifunctional catalyst was prepared and used for catalytic hydrotreatment upgrading of bio-oil, which was obtained from low-temperature torrefaction (LTT) of pubescens and contained mainly lignin oligomers. The upgrading temperatures were investigated at 220–280 °C. Pd/NbOPO₄ exhibited good performance for the effective depolymerization of oligomers with increasing temperature. It was revealed that both Brönsted acid sites and Lewis acid sites existed on NbO_x, which contributed to the cleavage of the C–O–C bond. Furthermore, esterification, hydrogenation, and O-alkylation of monomeric derivatives also occurred simultaneously during the depolymerization process. After hydrotreatment upgrading, the average molecular weight of bio-oil decreased from M_w = 320 Da (M_n = 298 Da) to M_w = 273 Da (M_n = 254 Da) and the bio-oil quality was improved dramatically. The oxygen content decreased from 29.53 to 9.78 wt %. The upgraded bio-oil obtained at 280 °C had a heating value of 40.48 MJ kg^–1, which was much higher than that of the original bio-oil (26.96 MJ kg^–1).

Natural microalgae (NA, cyanobacteria) collected from Taihu Lake (Jiangsu, China) were used for biofuel production through pyrolysis. The microalgae were de-ashed via pretreatment with deionized water and hydrochloric acid, and the samples obtained were noted as 0 M, 0.1 M, 1 M, 2 M, 4 M, 6 M, 8 M, respectively, according to the concentration of hydrochloric acid used in the pretreatment. Pyrolysis experiments were carried out at 500 °C for 2 h. The products were examined by various techniques to identify the influence of the ash on the pyrolysis behavior. The results showed that the ash inhibited the thermal transformation of microalgae. The 2 mol/L hydrochloric acid performed the best in removing ash and the liquid yield increased from 34.4% (NA) to 40.5% (2 M). Metal-oxides (mainly CaO, MgO, Al₂O₃) in ash promoted the reaction of hexadecanoic acid and NH₃ to produce more hexadecanamide, which was further dehydrated to hexadecanenitrile. After acid pretreatment, significant improvement in the selectivity of hexadecanoic acid was observed, ranging from 22.4% (NA) to 58.8% (4 M). The hydrocarbon compounds in the liquid product increased from 12.90% (NA) to 26.67% (2 M). Furthermore, the acid pretreatment enhanced the content of C₉–C₁₆ compounds and the HHV values of bio-oil. For natural microalgae, the de-ashing pretreatment before pyrolysis was essential for improving the biocrude yield and quality, as well as the biomass conversion efficiency.