345 / 2018-07-31 15:29:14

Preparation of nano-sized hierarchical ZSM-5 with different mesostructure and catalytic performance of co-cracking phenolic bio-oil model compounds and ethanol to aromatics

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For the utilization of lignocellulosic biomass resources, one of the major difficulties is the bio-oil derived by fast pyrolysis has a high content of phenolic compounds, which decreases the quality of bio-oil and is easy to generate severe coke in the further upgrading process, especially catalytic cracking. Hierarchical zeolites are often considered as ideal catalysts for cracking because of its improved accessibility of actives sites and diffusion of reactants. However to design specific hierarchical zeolite structure under certain reaction condition still remains to be studied.
In this work, mesosructured ZSM-5 with different particle size were synthesized by a hydrothermal treatment in the ammonia solution with or without surfactant CTAB, and the nano-sized ZSM-5 were prepared by a seed-induced method. The synthesized meso-MFI were noted as K-NH3 (without CTAB) and K-IC (with CTAB), respectively. Through the results of BET, SEM and TEM, it showed that both two kinds of samples have obvious mesoporous structure, while K-NH3 samples only have a broad distribution of about 10nm and K-IC have a ordered distribution of 3nm. It can be inferred that K-IC has experienced a recrystallization process because of the addition of surfactant.
Six model compounds (phenol, guaiacol ,eugenol, acetic acid, hydroxyacetone and methylfuran) were used to represent the phenolic pyrolysis bio-oil, and then mixed with ethanol. Under a typical reaction condition (450℃, WHSV=2h-1), the nano ZSM-5 showed much higher selectivity on aromatics than that of conventional ZSM-5(Si/Al=50), which revealed a remarkable size effect. By introducing mesopores into ZSM-5, the catalytic performance was further improved. Of all samples with three different sizes (250 nm, 700 nm and 1 μm), the selectivity over BTX and conversion over guaiacol, eugenol and small reactants had increased significantly. The nano-sized sample K-IC-250nm had a selectivity over aromatics up to 25.8% (GC-MS peak area %) and only 47.9% over phenolic products, with guaiacol and eugenol completely converted. Compared with K-NH3, K-IC had higher ratio of MAH to PAH products, which indicated that hierarchical ZSM-5 with ordered mesostructure can suppress the formation of unwanted PAH and the consequent quick deactivation. This improved catalytic performance of nano-sized mesoZSM-5 catalysts can offer a new route for designing novel cracking catalysts with better resistance to coke formation and higher hydrocarbon selectivity.