Catalytic Conversion Of Methanol To Fuel Oil Over Hierarchical HZSM-5 Catalysts

At present, the shortage of petroleum supplies has seriously affected national strategic security and the rapid development of national economy. Making full use of rich coal resources and developing project of CTL as a substituendum for petroleum resource will be one of the most effective solutions for petroleum shortage. The capacity of methanol production has been surplus with the development of high growth and high demand of several years, so methanol to fuel oil to make up the shortage of energy resource will be a favorable way according to our country sustainable development in the future.In this paper, we prepared hierarchically porous HZSM-5 catalysts by three different synthesis methods, characterized them by the modern analysis technologies and investigated their catalytic performance in catalytic conversion of methanol to fuel oil. Main content can be listed as following points:(1) A HZSM-5-AT zeolite was prepared by treating HZSM-5 (SiO2/Al2O3=36) at 60℃for 6 h in 0.1-1mol/L NaOH solution. XRD patterns showed that ZSM-5-AT still maintained its original MFI crystal structure upon post-treating in NaOH solution, but the crystallinity of ZSM-5-AT zeolites decreased compared with its original precursor and the decreasing degree increased with the increase of the employed NaOH solution concenration, indicating that the structure of ZSM-5-AT zeolites were partly damaged. SEM and N2 adsorption-desorption results showed that mesopores were created in the ZSM-5-AT zeolites upon alkali-treating and the amount of the mesopore increased with the increase of the alkalinity. Infrared spectroscopy of adsorbed pyridine and NH3-TPD showed that the HZSM-5-AT catalysts possessed fewer Bronsted strong acid sites and more Lewis weak acidic sites compared with its original precursor, indicating that a part of aluminum species were extracted from the framework of HZSM-5 and no-framework aluminum species were formed during the alkali treatment process.(2) A HZSM-5-Z zeolite was synthesized by adding organic silane during the ZSM-5 synthesis process. XRD patterns showed that the MFI characteristic diffraction peaks of the as-synthesized were widened compared with those of ZSM-5 synthesized in the absent of organic silane. The result suggested that the organic silane inhibited the growth of ZSM-5 crystals, which resulted in the formation of small crystal particles, this suggestion could be further confirmed by the results of SEM. HRTEM and N2 adsorption-desorption showed that ZSM-5-Z zeolites had a hierarchically porous structure. Infrared spectroscopy of adsorbed large size 2,6-dimethyl pyridine indicated that the acidity accessibilities of HZSM-5-Z was significantly improved due to the hierarchical pores.(3) A HZSM-5-F composite zeolite was prepared by a two-step synthesis. XRD and SEM showed the as-synthesized samples have the same characteristic diffraction peaks and the crystalline morphology as that synthesized by a one-step synthesis. N2 adsorption-desorption result showed that HZSM-5-F had the hierarchically porous structure. The mesoporous pores could be attributed to the dissolution of the framework of MFI by the synthesis solution in the second step or cumulate crystal particles.(4) Catalytic reaction of methanol to fuel oil indicated the hierarchically porous HZSM-5-AT, HZSM-5-Z and HZSM-5-F catalysts showed higher the conversion of methanol and the yield of fuel oil and enhanced resistance towards deactivation as compared to the HZSM-5 catalysts without hierarchically porous structure. The better catalytic performance for the prepared hierarchically porous HZSM-5 catalyts can be attributed to the following reasons:1) hierarchical pores in the catalysts improved diffusion of reactant and product; 2) hierarchical pores in the catalysts improved acid accessibility, which is favourable for the conversion of methanol; 3) hierarchical pores in the catalysts could inbibited secondary cracking process and the deposition of coke, which is favourable for improving the stability of the catalysts and increasing the yield of fuel oil.