Catalytic Conversion of Biomass for Aromatics Over HZSM-5 Modified by Dawson-Type Phosphotungstic Acid

The multi-functional zeolites modified with different phosphotungstic acid (PW) ratios were prepared and characterized. Thermogravimetric tests coupled with kinetics analysis were performed to evaluate catalytic pyrolysis behavior of biomass; The presence of HZSM-5 inhibited the product diffusion, causing a slight delay of pyrolysis; The PW modification made the reaction more complex; reaction order increased from 1.96 to 2.12. The activation energy decreased from 53.32 to 31.15 kJ/mol with the increase of PW. Then, fixed-bed catalytic experiments were further conducted to investigate aromatics production; 10%PW modification gave the appropriate acidic distribution and pore structure, resulting in oxygen being more likely to be removed in the form of COx. Although the organic yield was only 10.73%, HHV reached 38.01 MJ/kg. The organic phase catalyzed by 10%PW/HZSM-5 exhibited higher aromatization degree, and the structures of benzene ring were mainly single-rings. The oxygenates (especially for phenols from 16.88% to undetected) reduced obviously with increasing PW loading, and the 10%PW modification gave the highest content (peak area, %) of desirable mono-aromatic hydrocarbons (52.29%). The GC/MS analysis results were basically consistent with the 1H/13C NMRs. Besides, the 10%PW/HZSM-5 had the highest catalytic stability and the spent catalyst could recover high activity after regeneration. Therefore, catalytic pyrolysis of biomass using Dawson-structured PW-modified HZSM-5 is a promising approach for production of light aromatic hydrocarbons.