Preparation of modified hierarchical HZSM-5 catalyst and its application on pyrolysis of biomass to enhance aromatics products

Biomass catalytic pyrolysis vapor upgrading is an important way for converting biomass to high-value chemical products. Studies on pyrolysis behavior and products distribution of biomass are of great importance to the research on the interactions and catalytic conversion mechanism between model components of biomass and catalyst. At the same time, aromatic hydrocarbon compounds was the important chemical in nowadays, catalytic pyrolysis process produces a highly oxygenated bio-oil containing over 100 different compounds. The selectivity of fast pyrolysis can be greatly enhanced through the use of a catalyst, in particular with HZSM-5 zeolite which gives the highest selectivity toward mono-cyclic platform aromatics (such as benzene, toluene, xylene and ethyl benzene) essential to the chemical industry. Despite this high selectivity, a significant fraction of the freed-stock’s renewable carbon is lost to coke and char, two undesired byproducts. Coke formation has been attributed to the polymerization of small oxygenates on the external surface of the zeolite and to the formation of polyaromatic hydrocarbons through condensation reactions inside the micropores. So in order to improve selectivity toward desired products of aromatic and lower the formation of coke, the mesoporous HZSM-5 was synthesized by desilication method. In this paper, we used alkali treatment of commercial HZSM-5 zeolites using K2CO3 solutions to introduce mesopores into microporous system, and to investigate the effects of alkali concentration (0.2-0.6 mol/L) on aromatics yield and selectivity, The pyrolysis of experimental sample (pine, cellulose and lignin) was conducted in a fixed bed reactor with the conditions of pyrolysis temperature of 450 ℃, and catalytic temperature of 550 ℃, the ratio of biomass to catalyst of 1:2. Materials and products characteristics were investigated with various testing approaches, such as elements analysis, X-ray diffraction (XRD), surface area and pore size analyzer (BET), scanning electron microscope (SEM), Fourier infrared spectroscopy (FTIR), X-ray photoelectron spectrometer (XPS), temperature programmed desorption (NH3-TPD) and gas chromatography mass spectrograph (GC-MS) to study the effect of desilication on the structure, acidity, pore size and performance of aluminum-rich ZSM-5 as well the product distribution of pyrolysis bio-oil. The results showed that alkali treatment of HZSM-5 can remove the extra-framework Si species to enhance the diffusion property, and at the same time, creation the mesopores, and the pore diameter of mesopores increased with the increase of K2CO3 concentration however, the total acid amount decreased. Detailed characterization of the obtained zeolite catalysts indicated that mild desilication conditions significantly affect the elemental composition, crystallographic structure, microporosity, and distribution of aluminum atoms in framework and extra framework sites. The number of accessible Brønsted acid sites increased as a result of the enhanced mesoporosity. At the same time, the concentration of alkali solution, the corrosion degree, the degree of desilication and desilication, and the mesoporous degree increased, as such the hysteresis phenomenon became more and more obvious. Moreover, the hierarchical HZSM-5 catalyst produced more aromatic hydrocarbons and less coke formation in CFP of lignin and lignocelluloses’ biomass that contained the lignin component than that of the control HZSM-5, which effectively improved the selectivity for naphthalene family products (methylnaphthalene and 2-methylnaphthalene) in the bio-oil and decreased the content of aromatic hydrocarbons larger than C10. The highest aromatic yield (82.81%) and lowest coke yield (28.06%) were obtained in CFP of pine wood with mildly desilicated zeolite treated with 0.5mol/L K2CO3 solution.