Analysis of catalytic biomass pyrolysis by a novel solid phosphoric acid

Pyrolysis can convert the biomass into pyrolysis gas, coke, and bio-oil. With the aid of pyrolysis, the biofuels can be utilized to produce the high-value value-added chemicals. Recently, the phosphorous catalysts have been introduced into biomass pyrolysis to produce the chemicals, such as levoglucosan, furans, and high-surface-area activated carbon. However, most research has focused mostly on the directed preparation of bio-oil. It is still lacking on the effects of the solid phosphoric acid on the biomass catalytic pyrolysis. This study aims to explore the catalytic performance of solid phosphoric acid in biomass pyrolysis. A quartz sand-based solid phosphoric acid catalyst (PS) was also prepared using impregnation. A series of experiments were conducted on the catalytic pyrolysis using poplar wood. A diatomaceous earth-based solid phosphoric acid catalyst (PD) was also prepared for comparison. Firstly, the catalysts were characterized using BET, SEM, and XRD techniques. A comparison was then made on the composition and morphology of the catalysts before and after the reaction. The catalytic mechanisms were then determined during pyrolysis. Finally, the pyrolysis experiments were performed to investigate the effect of the PS catalyst and its addition ratio on the yield and composition of pyrolysis products. The results showed that the PS was reduced the yield of pyrolysis gas by 32.2% to 41.2%, compared with non-catalytic pyrolysis, while the yield of coke increased by 18.8% to 28.2%. This trend was attributed to the cross-linking reactions between the phosphoric acid and the macromolecular organic structures in the poplar wood. In terms of pyrolytic gas, the proportion of CO increased by 27.1% to 32.4%, whereas the proportions of CH₄, CO₂, and C₂-C₃ decreased, leading to a reduction of 5.6% to 16.2% in the heating value of the gas. Furthermore, the PS was has significantly enhanced the selectivity of furan compounds in the bio-oil. The relative abundance of furan compounds reached 88.6%, when the PS-to-poplar wood ratio was 1:1. Although the PD was also enhanced the selectivity of furan compounds in the bio-oil, its effect was significantly lower than that of PS. In terms of the char, the carbon retention rate increased by 40.6% to 72.3% in the catalytic pyrolysis, compared with the non-catalytic pyrolysis. The active species in the PS were formed C-O-P, C-PO₃, and C₂-PO₂ chemical bonds on the char surface after the cross-linking reactions with the oxygen-containing functional groups of the biomass, indicating the inhibited cracking of side-chain structures. In contrast, the PD was improved only the carbon retention rate by 6.8%, indicating the a much lower retention of solid carbon, compared with the PS. The products were obtained from the solid phosphoric acid-catalyzed pyrolysis of poplar wood. It was found that the PS exhibited the a strong selectivity for the furan compounds in bio-oil, indicating the great potential to for the production of furans from biomass pyrolysis. Additionally, the PS significantly was improved the carbon retention rate in char after cross-linking reactions with oxygen-containing functional groups. Therefore, the PS shared the significant potential in the preparation of furan chemicals for the high carbon retention rate during biomass pyrolysis. These findings can provide valuable insights into the new solid phosphoric acid catalysts and the high-value utilization of lignocellulosic biomass.