Effect of HZSM-5 catalyst with different Si/Al ratios on characterization of pyrolysis bio-oil

Abstract: In order to investigate the effect on biomass liquefaction of HZSM-5 zeolite under different silica to alumina ratio and adding amounts. The HZSM-5 (hydrogen zeolite) catalysts obtained by ion-exchange method were characterized using laser particle size analyzer, specific surface area and pore size analyzer with Brunauer-Emmett-Teller (BET) method and X-ray diffraction (XRD), and applied for the catalytic pyrolysis of wood at the optimum temperature of best oil yield. The bio-oils obtained from direct liquefaction with and without catalysts were characterized by GC-MS (Gas Chromatograph-Mass Spectrometer) analyses. The results indicated that: Compared with ZSM-5 zeolite, the crystal grain size of HZSM-5 zeolite prepared through ion exchange increased, and the particle size distribution was more concentrated, andthe catalyst had a preferable crystalline tacticity and a increased crystallinity, meanwhile, the surface area and the pore volume of HZSM-5 catalyst decreased, while pore size did not change significantly. When the HZSM-5 zeolite of different silica to alumina ratio were applied to pyrolysis, the yield of bio-oil decreased significantly (the maximum was up to 8%), the moisture content of the bio-oil increased. However, the composition of bio-oil and the relative content of the product changed a lot, it indicated that there was obvious catalytic effects during the direct liquefaction of biomass under HZSM-5 zeolite. Compared with non-catalyst pyrolysis, the contents of aldehydes, esters, ketones, furans and acid (oxygenated compounds) were reduced. It showed that HZSM-5 zeolite promoted a series of decarboxylation, decarbonylation reaction during the pyrolysis process, resulted to the reduction of the oxygen content of bio-oil and helped to improve the stability of bio-oil. With the increasing of silica to alumina ratio, the catalytic activity weakened a little. When the adding amounts of HZSM-5 zeolite were increased, a small variation of the yield of bio-oil occured, the moisture content of the bio-oil continued to go up, while the content of oxygen-containing compounds and acid aldehydes, esters, ketones, furans further reduced, promoting the formation of hydrocarbons, phenols, alcohols, the maximum percentage of the peak area could be up to 12.57% and 39.36%. Because HZSM-5 zeolite help to provide rich aperture on the surface, which was conducive to the formation of positive carbon ions, then the products of the first thermal decomposition carried on isomerization, cracking formation to convert as small molecules alkanes and alkenes, then go through polymerization reaction, cyclization reaction and a series of reactions to generate aromatic substances and so on. Owing to the above reaction procedure, there was a greater overall increasing in hydrocarbon substances. After the addition ratio was greater than 10%, the changing rate of various components decreased. Comprehensive analysis results of the type and amount of added catalyst showed that HZSM-5 (25) zeolite with the addition ratio of 10% played a more significant role on the catalytic liquefaction control of wood dust.