Study On Dynamic Synthesis Of MCM-22 Zeolite

MCM-22,a kind of layered zeolite,possesses two independent pore systems.One consists of two-dimensional sinusoidal channels composed of 10-MR,and the other is characteristic of 12-MR supercages accessible through 10-MR windows.MCM-22 zeolite has also proven to be high microporosity and pore volume,exellent thermal stability and acidity.The unusual porous network and physical chemistry properties of MCM-22 make it an attractive catalyst for reaction such as FCC catalytic cracking,benzene alkylation,trans-alkylation of toluene to p-xylene and skeletal isomerization of n-alkanes to branched alkanes,etc..In most publications,MCM-22 zeolite are obtained by using a large amount of hexamethyleneimin?HMI?as organic structure directing agent,which not only results in the high production cost,but also causes water and atmospheric pollution.Moreover,the crystallization time of MCM-22 usually takes 7²0 days,which is too long for commercial process.In this thesis,the effect of silica sources,alkalinity,templating agent content,crystallization time,and seed addition amount on the synthesis of MCM-22 are studied systematically.In scale-up experiment of 1L autoclave,both the type of the impeller blade and the stirring speed on the synthesis of MCM-22 are investigated,the main results are as follows:1.Pure MCM-22 zeolite can be obtained by using either silicic acid or silica gel as silica source at low Si/Al ratio.The content of the templating agent,NaOH and the crystallization time affect greatly on the product phase.the optimal alkalinity is n?OH^-?/n?SiO₂?=0.034⁰.054 at Si/Al=50,Quartz phase and laumontite appear with increasing alkalinity;The lowest dosage of templating agent was n?HMI?/n(SiO₂)=0.307.Mordenite are produced as the content of templating agent is too low.The crystallinity of the MCM-22 improves with crystallization time,and MCM-22 with the highest crystallinity is obtained at addition of 5¹0 wt%seeds at crystallization time of 5 days.2.Comparing to Si/Al ratio of the initial precursor gel being 50,pure MCM-22 is obtained at lower NaOH content when Si/Al ratio being 100.MCM-22 with the highest crystallinity is obtained at crystallization time of 5 days when 5 wt%seed crystals are added.On scale-up experiment of 1L autoclave,both the type of the impeller blade and the stirring speed have greatly effect on the synthesis of MCM-22.Pure MCM-22 with uniform morphology is produced when using turbo-type stirring blade at stirring speed of300 rpm.3.The catalytic properties of MCM-22 molecular sieve produced from scale-up experimentis are tested on the reaction of tripolyformaldehyde to polyoxymethylene dimethylether.The results show that the conversion of tripolyformaldehyde is 93.8%and the selectivity to DMM_2-8-8 is 96.3%.