Study On The Conversion Of Methanol To Light Aromatic Hydrocarbons In Zn Modified HZSM - 5 Molecular Sieve Catalyst

Light aromatics, such as, benzene, toluene and xylene (BTX), which are widely used as feedstores in the industry of medicine, perfumery, dyestuff, etc., show considerably increasing market prospect. In recent years, the production of BTX from the traditional route cannot meet its market demand in China, due to the shortage of petroleum and the high demand of BTX with the fast growth rate. Thus, the conversion of methanol to light aromatics (MTA) has attracted great attention because MTA is considered an effective route for using methanol and increasing BTX production. Although some research have been developed in the aspects of catalyst and the process of MTA, there are still many issues such as the improvement of the catalyst, the relationship of properties and catalytic performance of the catalyst, and the surface reaction mechanism to be studied. In this thesis, Me-modified nano-HZSM-5catalysts were prepared, and used in MTA reaction. In addition, the relationship of physiochemical properties with the catalytic pferformace of these catalysts was discussed, and the results as followed.1) Thermodynamics was calculated based on the reaction system of MTA reaction, which was proposed by the possible transformation pathway and "hydrocarbon pool" reaction mechanism. Nano-HZSM-5zeolites as catalysts were prepared by two-template hydrothermal synthesis method. The catalytic reactivity of these catalysts for the conversion of methanol to light aromatics was tested under the reaction conditions of400℃,0.1MPa, PCH3OH=20kPa, and the total flow rate=50ml/min. The41.5%BTX yield was obtained on HS catalyst, and more than40%BTX yield still retained after40h on stream. In addition, the properties of these catalysts were characterized by powder X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), N2isothermal adsorption-desorption, scanning electron microscopy (SEM), temperature programmed desorption of ammonia (NH3-TPD), and thermal gravity (TG) analysis. These results indicate that, the introduction of little CTAB in the hydrothermal synthesis process significantly decreased the crystal size and total acid sites of HS. Furthermore, the reduction of total acidic amount and crystal size of HS enahcnced the accessibility of active sites and the diffusion of macromolecule, which improved the catalytic stability. 2) Me-modified HS catalysts were prepared and used in methanol aromatization. The effects of the promoter, incorporation method, Zn-loading and reaction conditions (reaction temperature, methanol pressure, and contact time) on the catalytic performance of these catalysts in MTA were investigated. In addition, the properties of these catalysts were characterized by XRD, X-ray photoelectron spectroscopy (XPS), N2isothermal adsorption-desorption, SEM, NH3-TPD, in situ pyridine-Infrared (Py-IR), and TG. These results suggest that, BTX yield was strongly dependent on the reaction conditions and the loading of Zn. Under the optimal reaction conditions of0.1MPa,450℃, PCH3OH=20kPa, and total flow rate=25mL/min, the highest BTX yield of67.7%and good catalytic stability was obtained on0.5%Zn modified HS. The addition of Zn into HS caused the interaction of Zn species with hydroxyl groups (OH) on HS surface. Such interaction decreased the concentration of Bronsted (B) acid sites on the surface of Zn/HS catalyst and generated new Lewis (L) acid sites of ZnOH+, which caused the reduction of the ratio of B/L acid sites. In addition, the reduction of Zn/HZSM-5crystal size improved the interaction of Zn with HZSM-5framework and the resistance of coke deposition, which significantly enhanced the catalytic reactivity and stability.0.5%Zn/HS catalyst exhibited the highest BTX yield and good catalytic stability, mainly due to that the catalyst had the moderate density and distribution (B/L ratio=0.29) of acid sites as well as small crystal size.3) The coking behavior and reaction-regeneration on0.5%Zn/HS catalyst in the conversion of methanol to aromatics were investigated. The physical and chemical properties of these catalysts were characterized XRD, N2adsorption-desorption, SEM, NH3-TPD, Py-IR, XPS, and TG. With the increase of reaction-regeneration cycle, the catalytic reactivity in MTA reaction of0.5%Zn/HS decreased, whereas the catalytic stability significantly improved. The coke deposition was the main reason for the deactivation of0.5%Zn/HS, which covered the acid sites of the catalyst. The reactivity of the catalyst was only partly recovered after oxidative treatment. Some carbon remained in the micropore of catalyst after regeneration, leading to the decrease of strength and concentration of acid sites. With the increase of reaction-regeneration cycle, the ratio of B/L acid sites decreased, which suppressed the synergistic effect of B and L acid sites. The concentration and distribution of acid sites were closely relative to the catalytic reactivity of0.5%Zn/HS catalyst. For achieving the high BTX yield and catalytic stability in methanol conversion, the catalyst should have the high B/L ratio and the low total concentration of acid sites. The results from coking kinetics indicate that, the low reaction temperature and methanol pressure favored to decrease coking rate; in addition, the reduction of catalyst size significantly enhanced the coking activation energy which improved its carbon resistance.4) The reaction kinetic of methanol conversion to light aromatics over Zn/HZSM-5was tested and the kinetic parameters were calculated. Based on the results, the possible transformation pathway and reaction mechanism of MTA on Zn/HZSM-5was proposed. Compared with0.5%Zn/HZ,0.5%Zn/HS exhibited the higher rate constant and lower activation energy, and represented the better catalytic reactivity. The results from kinetic experiment suggest that the high temperature was conductive to methanol aromatization. The MTA reaction over0.5%Zn/HS could follow the "hydrocarbon pool" reaction mechanism with polymethylbenzene as reaction intermediate.