Synthesis And Charcterization Of Heteroatomic Composite Molecular Sieve Based On The Conversion Of Methanol To Xylene

It is well kown that xylene as the synthesis monomer or solvent is widely used in medicine, dynamite, pesticides and other industries, which is a mixture of p-xylene (PX), o-xylene (OX) and m-xylene (MX). Methanol conversion of xylene (Methanol To Xylene, MTX) process, involving a variety of reaction pathways and routes, such as methanol and toluene alkylation, toluene disproportionation, low-carbon alkane/alkene aromatization reaction and xylene isomerization reaction. Therefore it can be improved through designing and modifing the catalyst to make the reaction easier for xylene generated direction.ZSM-5 zeolite with highly catalytic activity and selectivity in methanol aromatization for its larger specific surface area, shape-selective catalysis performance and unique surface acidity. Simultaneously EU-1 zeolite is a kind of high-silica molecular sieve, contains ten-ring through the hole and vertical with connected twelve-ring side pockets structure, because of the molecular sieve hole dimensions and the characteristics of the collateral bags and the special performance in the catalytic reaction. EU-1 is used for good shape-selective catalyst of xylene isomerization reaction. Therefore taking advantage of EU-1 and ZSM-5 zeolites their respective strengths to form composite crystal, while improving the selectivity of catalytic products to xylene and paraxylene in the process of methanol to xylene.ZSM-5/EU-1 composite zeolite was synthesized by hydrothermal method with silica sol, aluminum sulfate, sodium hydroxide, TPABr, boric acid, iron nitrate and EU-1 as raw material. The synthetic samples were investigated by XRD, SEM, N2 adsorption-desorption and NH3-TPD. The results showed that the ZSM-5/EU-1 had diffraction peaks of both ZSM-5 and EU-1; The average pore size had significantly enlarged and the amount and strength of acid was increased, which was better for aromatization trend of methanol and diffusion of xylene. The performance of catalyst was evaluated on a continuous-flow reactor. The selectivity could reached 84.70% in aromatic oil phase. The contents of xylene were always high in the two kinds of zeolites. The content of Fe-ZSM-5/EU-1 ranged from 33.88% to 39.16% and B-ZSM-5/EU-1 from 41.32% to 45.88%. This was because the B-ZSM-5/EU-1 had the higher strong acid and relatively smaller pore diameter (0.8060 nm), leading to more conducive to the production of xylene. The selectivity of p-xylene in xylene was in the range of 29.75%-47.47% in B-ZSM-5/EU-1 catalytic product; while the selectivity of p-xylene in xylene could reach up to 53.75% in the Fe-ZSM-5/EU-1 catalytic product. This result was due to the larger particle size of Fe-ZSM-5/EU-1, providing longer pore structure for catalytic reaction, which could be easier to convert o-xylene and m-xylene into p-xylene in the diffusion process.The B-EU-1/ZSM-5 composite zeolite with a porous structure was synthesized by a hydrothermal method using hexamethonium bromide (HMBr2) and four with propyl bromide (TPABr) as co-templates in one step hydrothermal synthesis method in Na2O-SiO2-Al2O3-TPABr-HMBr2-H2O synthesis system for the first time. The properties of synthetic samples were characterized by the XRD, FT-IR, SEM, finding that the composite zeolite was dispersed phase material which has diffraction peaks with ZSM-5 and EU-1 and better crystallinity; Exploring the synthesis conditions, found the growth process of composite molecular sieve was in connection with the proportion of two templates. Composite molecular sieve would be suitable for the growth in a range of n(TPABr)/n(HMBr2)=0.2～0.5; The best conditions of synthesis was n(TPABr)/n(HMBr2)=0.4 and reaction time. The selectivity of the catalytic effect of composite molecular sieves synthesized with dual-mode was higher than sample mechanical composite in methanol conversion of xylene catalytic reaction. In the reaction of catalyst in the dual-mode version, aromatic hydrocarbons of the oil phase reached a maximum of 66.72%, and xylene of aromatic hydrocarbon content is 46.15%, and p-xylene content of xylene reached 30.75%.Because the morphology and pore structure of catalyst results have an important effect on the catalytic synthesis, morphology of samples are explored experimentally in this thesis. Different alkali metal cations (Li+, Na+, K+) should be added into the composite of EU-1/ZSM-5 zeolite which was synthesized. In this process, it turned out that Li+ and K+ showed much weaker structure-directing ability than Na+, although Li+ was superior to K+.