Study On Synthesis And Selective Hydrocracking And Isomerization Of Zeolite ZSM-22

ZSM-22is a high-silica zeolite molecular sieve material with topological framework TON structure. The framework includes five-membered rings, six-membered rings and ten-membered rings. The one-dimensional core channels of ZSM-22are made of ten-membered rings without cross-channel. Because of the unique pore structure and appropriate surface acidity, ZSM-22zeolite is an ideal catalyst for long chain alkane selective hydrocracking and isomerization reactions. Recently, the synthesis of ZSM-22is usually hydrothermally synthesized with aluminum sulfate as aluminum source, but the problems are the rigorous synthesis conditions and the existence of mixed crystal.The different SiO2/Al2O3ratio and size of H-ZSM-22zeolites were prepared by hydrothermal method with boehmite as aluminum source. The H-ZSM-22samples with different SiO2/Al2O3ratio and size can be used to study in selective hydrocracking and isomerization of n-dodecane. The water vapor passivation, phosphate modified and loading Pt method have been proved effective in improving the catalytic activity and stability by the modulation to the acid content and strength on the zeolites surface. The results showed that:(1) The ZSM-22zeolite with high crystallinity was prepared under the condition that the crystallization temperature and time is about150～160℃and48h, OH7SiO2,(1,6-hexanediamine) DAH/SiO2, H2O/SiO2, SiO2/Al2O3ratio is0-0.12,0.3,35-50,70-130respectively, without stirring. The crystallization time can be reduced to36h with stirring. The ZSM-22samples were characterized by SEM, NH3-TPD, which reveals that the smaller crystal size can be obtained by the lower amount of water and the higher temperature or the condition with stirring. With the increase of SiO2/Al2O3ratio, the amount of surface acidity decreases and the acid strength gradually increases.(2) It was found that using the SiO2/Al2O3mole ratio is90and the smaller crystal size of H-ZSM-22zeolite in the selective Hydrocracking of n-dodecane can obtain the results that the conversion of n-dodecane was88.00%, target product (C6-C11) selectivity was41%, the yield was36.06%under the optimal reaction conditions with reaction temperature300℃, reaction pressure3MPa, WHSV of0.75h-1, Hydrogen/Oil ratio of900.(3) For modification of H-ZSM-22zeolite with water vapor passivation and phosphate modified, the former optimal time and temperature were6h and500℃. The optimal amount of NH4H2PO4solution was1%. Using this treated H-ZSM-22in selective hydrocracking of n-dodecane, the conversion of n-dodecane could reach62.28%and the selectivity of the desired product was54.34%, the yield was33.84%. The conversion of n-dodecane could increase to88.37%and the selectivity of isomerization was77.01%under the further modification of supported noble metal Pt. It indicates that modification of H-ZSM-22has good reaction stability.(4) Using Pt/ZSM-22in selective hydrocracking of n-hexadecane, the conversion of n-hexadecane could reach98.97%and the selectivity of the desired product (C9～C15) was49.29%, the yield was48.78%under the optimal reaction conditions with reaction temperature310℃, reaction pressure3MPa, WHSV of1.12h-1, Hydrogen/Oil ratio of900.