The Synthesis and Modification of HZSM-5 Zeolite and Its Catalytic Performance on Toluene Alkylation with CH3Br

Author:Xie Jun

Supervisor:yin shuang feng

Database:Doctor

Degree Year:2018

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Pages:106

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p-Xylene is one kind of important organic materials in chemical industry,and its demand is increasing in recent years.Catalytic synthesis of p-xylene is an important production technology in petrochemical field.Conventional production technologies of p-xylene have meet great challenges since the decline of oil resources.A new concept is proposed by our group to prepare p-xylene by catalytic methylation of toluene by CH3Br intermediate which is synthesized from bromination or oxybromination of natural gas.Some research results has been achieved.It is necessary to research highly efficient catalysts for reaction of toluene alkylation.HZSM-5 zeolite with adjustable framework structure,high surface area,controllable surface acidity and good thermal stability is often used in toluene alkylation.However,the catalytic performance of HZSM-5 zeolite is closely related to the pore structure,particle size,surface acidity and other properties of the material.Therefore,the properties of HZSM-5 zeolite can be controlled by different synthesis conditions and modification of the material to obtain better performance.In this dissertation,the pore structure,particle size and surface acidity of HZSM-5 zeolite is controlled by different synthesis conditions or post-processing modification,thus improve the catalytic performance towards the alkylation of toluene with CH3Br.The physical and chemical properties of the as-synthesized zeolites were characterized and analyzed by XRD,SEM,NH3-TPD,XRF and so on.The relationship between the surface properties of catalysts and catalytic performances was also discussed.The reaction thermodynamics of toluene alkylation with CH3Br was also investigated.In addition,the industrial production of p-xylene by toluene alkylation with CH3Br intermediate which is synthesized from bromination or oxybromination of natural gas was designed.The main innovative results and conclusions were summarized as follows.(1)The reaction thermodynamics of toluene alkylation with CH3Br was detailed studied according to thermodynamic laws.The self transformation of CH3Br and toluene disproportionation are endothermic reactions,while other alkylation reactions are exothermic reactions.The self transformation of CH3Br and the methylation of benzene(or toluene and xylene)are spontaneous,while toluene disproportionation is a reversible reaction and can not be carried out spontaneously.High temperature is beneficial to inhibit the reaction of benzylation,propylation of benzene and toluene ethylation.While other reactions are irreversible due to the high Kp value.Furthermore,compared with o-xylene and p-xylene,m-xylene can be more easily generated in the toluene methylation.Therefore,synthesis of p-xylene by toluene methylation with CH3Br is feasible.(2)The HZSM-5 zeolite was modified by alkali treatment,and tested in the toluene methylation with CH3Br.Alkali treatment can remove the Si species in the frameworks of zeolite and obtain the mesoporous structure,and it also affects the distribution of acid sites of the zeolite.The zeolite modified by alkali treatment shows a higher toluene conversion,which may be related to more medium strong and strong acid sites.In addition,compared with the unmodified catalyst,the modified zeolite has higher stability due to better diffusion properties of the material.(3)HZSM-5 zeolites of different crystal sizes were directly synthesized by a single-step method using TPAOH and/or TEA as templates.The as-synthesized zeolites show excellent catalytic activity towards the methylation of toluene with CH3Br.The one denoted as HZSM-5(30,10)shows the highest p-xylene yield,giving toluene conversion of 44.8%and p-xylene selectivity of 48.5%.It was found that the increase of mid-strong and strong acid sites enhances the toluene conversion.And the selectivity to p-xylene is determined by the amount of strong acid sites as well as the crystal size of the catalysts.(4)Series of HZSM-5 zeolites were synthesized by hydrothermal method using TPAOH and/or MeEt3N+I–as templates.The use of MeEt3N+I–affects the crystal growth and the distribution of acid sites on the external surface of the zeolites.The catalysts show excellent catalytic activity towards toluene alkylation with CH3Br.With toluene conversion of 36.3%and p-xylene selectivity of 66.2%,HZ-40/0 is the most effective among the prepared catalysts,showing a p-xylene yield of 21.2%.It is deduced that the ample presence of acid sites could be beneficial for the improvement of toluene conversion,and the improved para-selectivity is mainly due to the reduction of acid sites on the external surface of the catalysts.(5)The process design of producing p-xylene from methane is strictly complying with relevant laws and regulations,policy requirements and standard specifications of the state and industry.The process flowchart was drawn,and the material balance is calculated for the reaction.The process has the advantages of simple operation and product purification,less investment and high utilization of raw materials.A highly purity p-xylene product can be obtained by this process.In addition,a more practical control measures of“three wastes”and noise were put forward.