| The framework of molecular sieve ZSM-23 is MTT,containing one-dimensional straight channel defined by teardrop-shaped ten-membered rings with diameter of 0.52×0.45 nm.Compared with ZSM-5,ZSM-23 has smaller pore diameters,due to its unique pore structure,it has good selectivity in the catalytic reaction.Therefore,on the ZSM-23 molecular sieve,methylation-alkylation cleavage,skeletal isomerization of octadecane,platinum-supported hydroisomerization,and selective catalysis of propylene cracking to propylene can be carried out.The synthesis of ZSM-23 molecular sieves reported so far usually uses Pyrrolidine?PYR?as template by static hydrothermal method,whose steps are adding Na OH and there adjusting the p H with concentrated sulfuric acid.This method has some shortcomings: 1)Adjusting the p H with concentrated sulfuric acid is cumbersome and the titration cannot guarantee the solution to reach the optimum p H value.2)In the static hydrothermal synthesis method,the product is often agglomerated and the post process is hard to perform.3)The synthesis range is narrow and heterogeneous crystals are prone to occur.To solve above problems,two structure directing agents,pyrrolidine and N,N-dimethylformamide?DMF?,were selected to investigate the synthesis of ZSM-23 molecular sieves.When using pyrrolidine as a template without alkali,it was found that the way of crystallization had a great influence on the synthesis of ZSM-23 molecular sieve;we studied the effect of series parameters such as the ratios of PYR/SiO2,H2O/SiO2 and SiO2/Al2O3,the crystallization time and the seeds.When DMF was used as the template,the ratio of SiO2/Al2O3,Na2O/SiO2,DMF/SiO2,H2O/SiO2,the crystallization time and the seed crystals on the target product were studied;then the synthesis was carried out in a 2 L reactor.In the end,we have also explored the template-free synthesis of ZSM-23 using seeds.The main results are as follows:1)Using Pyrrolidine as a template,mordenite,ZSM-5 molecular sieve,ZSM-35 molecular sieve and cristobalite are four heterocrystalline phases competing with ZSM-23 molecular sieve during crystallization,and the main factor causing the above results is Si/Al,water content and template agent content.When the ratio of SiO2/Al2O3 is less than 100,it is easy to synthesize mordenite and ZSM-5 molecular sieve.The ratio of SiO2/Al2O3 is more than100,and it is easy to form MOR and FER structure;when the PYR/SiO2 is relatively high or low,ZSM-5 molecular sieve is easily formed;When the H2O/SiO2 is too low,ZSM-5 molecular sieve and the quartz phase are easily formed.When the ratio of SiO2/Al2O3 is 100,the PYR/SiO2 is 19-27,and the ratio of H2O/SiO2 is 19-23,the pure molecular phase target molecular sieve can be synthesized.2)The ZSM-23 molecular sieve obtained by static crystallization and dynamic crystallization has a large difference in morphology.The former has a single dumbbell shape and the latter has two shapes of dumbbell-shaped and clustered needle-like crystals;in addition,both dynamic crystallization and seeding can effectively shorten the induced nucleation period of the molecular sieve.3)When DMF was used as the template,the pure phase ZSM-23 molecular sieve with SiO2/Al2O3 of 100-120 was successfully synthesized,the morphology of the needle-like monomer or needle-like aggregate can be observed from the SEM.When the ratio is equal to 100,the other influencing parameters are investigated.It is found that the most suitable ratio of synthetic molecular sieve is: n?Na2O?/n?SiO2?= 0.34-0.37,n?H2O?/n?SiO2?=35-48,n?DMF?/n?SiO2?= 0.65-0.70,and the crystallization time was 50 h.After seeding,the crystallization time was shortened to 37 h,and the shape was still needle-shaped,but the needle tip was more dispersed than the seed crystal.4)Using DMF as a template,ZSM-23 with higher crystallinity and tidy morphology was successfully synthesized in a 2 L reactor.However,when pyrrolidine is used as a template,the crystallinity of the synthesized ZSM-23 molecular sieve is very low due to the presence of the system gel and theamplification effect,and further research is needed.The method of rapidly synthesizing MTT zeolite without organic template was investigated,and the crystallization time was shortened to 10 h.However,compared with the conventional method,the combined ZSM-23 molecular sieve has a crystallinity of 40% lower. |
PDF Full Text Request