Preparation、Characterization And Catalytic Performance Of Meso-microporous HZSM-5 Zeolite

Recently, the strict environmental laws put forward higher and higher request on quality of fuel oil. At the same time, faced with a fact that the crude oil is becoming heavier and inferior, the technology of clean fuel production by a deep process to heavy oil has become a hot subject in domestic and foreign petroleum refining industry. The ZSM-5 zeolite that can transfer gasoline olefins and straight-chain component from first cracking reaction to high octane components by isomerization and hydrogen transfer reaction, optimizing the property of oil product,so it was used as a octan enhancer in fluid catalytic cracking widely. However, the light hydrocarbon molecular were easy to cause second reaction because of restricted diffusion and strong acidity, which can product coke and lead to cover acidity center or reduce the reaction efficiency of zeolite.Therefore, adjusting the pore structure and acidity reasonablely becomes a effective way to improve the catalytic efficiency and prolong the service life of ZSM-5 zeolite. Currently, many scientific research institutions at home or abroad have made a lot of exploration to product a meso-microporous ZSM-5 zeolite, and some significant results have been achieved.In this paper, HZSM-5 zeolites with larger average pore diameter, increased external surface area and mesopore volume, also broad pore size distribution were prepared using HZSM-5 zeolite(Si/Al=37) as the raw material by different temperature steam treatment, different concentration of alkali treatment and different kinds of acid solution treatment methods. The HZSM-5 zeolite before and after modified were characterized by means of scanning electron microscope, X-ray diffraction, X-ray fluorescence, NH3 temperature-programmed desorption, N2 adsorption-desorption and MAT- Ⅱ catalytic cracking assessment, also a further analysis and scientific demonstration about the mechanism or effect on the structure and property of HZSM-5 zeolite modified by different means were drawed. The result of experiment indicated that: the framework aluminum of ZSM-5 zeolite can be removed by high temperature steam, and the crystal structure destroyed, so as to reduce the acid strength and acid amount on the surface of zeolite. Besides, it was useful to expand average pore diameter and increase the pore volume. In which, the catalytic activity of HZSM-5 treated by steam at 500℃ got the maximum improvement. Low concentration of NaOH solution could only remove the framework silicon, but the framework aluminum species also be removed by higher concentration of NaOH solution, which made a significant influence to expand diameter, but it also caused a serious damage to the crytallinity and acid amount. Furthermore, the higher of the concentration, the stronger of the effect to remove framework silicon and aluminum species, and the HZSM-5 treated by NaOH solution with the concentration of 0.5 mol/L obtained the specific surface area increasing from 335.0 m2/g to 447.1 m2/g. The framework silicon of HZSM-5 zeolite can be removed by NaAlO2 solution and to produce some second mesopore structure; Compared with NaOH solution, the HZSM-5 zeolite treated by NaAlO2 solution would maintain a better crystallinity and surface acidity when it got the same effect in expanding pore diameter. The amorphous silicon and aluminum particles could be cleared by C2H2O4 and C6H8O7 solution, so it dredged the channel of zeolite and caused crystallinity improved, average diameter and specific surface area increased slightly. Because of its strong acidity and oxidability, HNO3 solution could cause a serious damage to the skeleton structure of HZSM-5 zeolite, removing a large number of framework aluminum species and lead the strong acid center suffered serious losses; At the same time, a lot of second mesopore structure produced, which caused the average pore size and pore volume of HZSM-5 increased, but the specific surface area and micro-reaction activity decreased.By analyzing and comparing the effect and mechanism of each method to enlarge pore diameter, this paper provides a experimental reference to design a HZSM-5 zeolite with excellent catalytic performance and suitable acidity, which also possesses a large specific surface, pore volume and broad pore size distribution.