Production of bio-aromatic by catalytic biomass pyrolysis over metal modified biomass-derived biochar-based catalyst

Aromatic hydrocarbons are important bulk chemicals in the petrochemical industry. The renewable biomass-derived bio-oil is widely expected to produce aromatics. Catalytic pyrolysis is one of the most promising thermochemical conversions to prepare high quality bio-oil, thereby realizing energy enrichment against energy crisis. However, many undesired properties have confined the application of bio-oil, such as high oxygen content, low target product, and selectivity. In this study, four Activated Carbons (ACs) were prepared for catalytic fast pyrolysis of Yunnan pine to produce the upgrading bio-oil and improve the yield of aromatics. Pyrolysis-gas chromatography×gas chromatography/mass spectrometry (Py-GC×GC/MS) were carried out to reveal the influences of metal type (Al, Cu, Zn, Ni) and loading amount (1%, 5%, 10%) on the product yield and aromatic selectivity. The porous structures, catalytic activities, crystal structure, micro morphology, functional group structure, and element composition of metal modified AC were characterized by XRD, BET, NH3-TPD, SEM, FTIR, and element analyzer. The experimental results illustrated that the specific surface area and mesoporous pore volume of catalysts were significantly improved by metal modification, from 148.69 m2/g and 0.047 mL/g to 308.55-419.62 m2/g and 0.158 mL/g, respectively. The average diameter of the pore decreased from 7.76 to 4.44-6.17 nm. The metal addition significantly improved the active sites and acidity of catalysts, and further promoted depolymerization of biomass to generate hydrocarbons, due to the electron-transfer or charge-transfer interactions, while catalyzing the demethoxylation, deoxygenation, and aromatization reactions of primary oxygen-containing compounds for the aromatic hydrocarbon. The order of the largest hydrocarbons produced by different metals was ranked: 10% Ni (73.78%) > 5% Zn (55.14%) > 10% Cu (42.53%) > 5% Al (40.75%), while the largest aromatic hydrocarbon produced by metal modified AC catalysts was in order of 10% Ni (67.54%) > 5% Zn (46.87%) > 10% Cu (33.16%) > 10% Al (19.30%) under the optimal conditions (pyrolysis temperature of 450 ℃, catalytic temperature of 500 ℃, pyrolysis time of 30min, biomass to catalyst ratio of 1:2, N2 flow ratio of 150mL/min). The main role of HZSM-5 zeolite was the acid-catalyzed deoxygenation, aromatization of olefins, alkylation of aromatics, and isomerization of xylenes. The metal activity was ranked in order of Ni , Zn, Cu, Al for the gas phase upgrading in catalytic pyrolysis. Al with high Lewis acid content promoted the formation of olefins and phenols, while low acid Cu promoted the increase of coke content. Zn and Ni with high Bronsted acid content promoted the formation of monocyclic aromatic hydrocarbons, thereby enhancing the selectivity of benzene and the derivatives, phenol, and alkylphenols, indicating a remarkable effect of deoxygenation and demethoxylation reaction. When the loading of Ni reached 10%, the highest hydrocarbon content was 73.78%, while the selectivity of aromatic hydrocarbon compounds and benzene reached 67.54% and 52.15%, respectively, significantly higher than those of non-catalytic pyrolysis at the same pyrolytic temperature, together with the lowest methoxy phenols and alkylated phenols content (0% and 0.74%, respectively), indicating an effective improvement of bio-oil quality.