Synthesis,modification,and Catalytic Cracking Performance Of Multi-stage Pore ZSM-35

Low carbon olefins are important organic chemical raw materials,and with the heavy and poor quality of cracking raw materials,catalytic cracking has become an effective way to produce low carbon olefins.ZSM-35 zeolite has a special acid distribution and unique two-dimensional pore structure,which has good application prospects in the field of catalytic cracking of large molecule hydrocarbons to produce low carbon olefins.Based on the bottleneck problems of ZSM-35 zeolites,such as acidity is not easy to be controlled and coking is easy in the process of hydrocarbon catalytic cracking,this paper synthesized a series of micron-scale multi-stage pore ZSM-35 zeolites by dynamic hydrothermal synthesis process parameter control,and the acidity of ZSM-35 zeolites was controlled by organic acid treatment and doping with alkaline earth metals,and the crystalline phase,pore structure,acidity and morphology of zeolite samples before and after modification were characterized and analyzed.The crystalline phase,pore structure,acidity and morphology of the zeolite samples before and after modification were characterized,and the performance evaluation of ZSM-35 for diesel catalytic cracking to olefins was carried out before and after modification.The main findings of this paper are as follows:（1）ZSM-35 zeolite was synthesized by dynamic hydrothermal synthesis,and the optimized crystallization parameters were 135°C and 72 h.The suitable molar ratios for synthesis were n（Si O₂）:n（Al₂O₃）:n（pyrrolidine（PYR））:n（OH^-）:n（H₂O）=1:0.0333:0.5:0.31:26,at which time the synthesized The crystallinity of the zeolite is 99.24%,and the morphology is mainly a micron-scale spherical structure formed by the accumulation of flakes,with a specific surface area of 310 m²/g and a pore size of 3 nm.（2）The ZSM-35 zeolite was modified by organic acid treatment,and when the citric acid concentration was 1 mol/L and the temperature was 80℃for 4 h,it could remove part of the non-skeletal aluminum in the pore channel of ZSM-35,and the acid amount was reduced from236.14μmol/g to 115.96μmol/g,and the regulation of the pore structure of ZSM-35 was realized at the same time.Its yield of gas-phase products in the evaluation of diesel catalytic cracking performance was increased from 19.1%to 24.5%,the selectivity of low carbon olefins was increased from 51.3%to 56.7%,and the corresponding carbon accumulation rate was reduced from 9.83%to 2.35%,which effectively suppressed coking and helped to extend the lifetime of ZSM-35.（3）ZSM-35 zeolite is modified with alkaline earth metal magnesium by in situ doping method and impregnation method,when n（Mg）/n（Al）=0.15 in in situ doping method and the loading of metal magnesium is 0.81%in impregnation method,the acid amount is reduced from236.14μmol/g to 134.19μmol/g and 166.12μmol/g respectively.meanwhile,in situ doping can make the coordination environment of silicon atoms in ZSM-35 change,and magnesium elements enter into ZSM-35 zeolite skeleton;while impregnation method does not change the coordination mode of silicon atoms,and magnesium elements are distributed on the surface of zeolite grains or subsurface region.Its performance evaluation of diesel catalytic cracking performance of gas products from 19.1%to 26.5%and 22.2%,low carbon olefin selectivity from 51.3%to 58.0%and 56.6%,carbon accumulation rate from 9.83%to 6.52%and 6.89%,reducing the harm caused by coking on the acidic sites of zeolite.