A Study On Zeolite Modified By Selective Cracking Of Silanes And Its Catalytic Applications In C₂⁼/C₃⁼-Oriented Generation

C₂⁼ and C₃⁼ are important base platform chemicals.Developing production technology capable of more productive C₂⁼+C₃⁼ and flexibly adjusting the C₃⁼/C₂⁼ratio?P/E ratio?according to market changes is the focus of academia and industry.For China,the abundant coal resources contribute to the rapid development of the methanol to hydrocarbons?MTH?.And the demand of high value utilization of low value C4=/C5=/C6= components enhances the the urgent need for olefin catalytic cracking?OCP?technology.Thus,the development of MTH and OCP technology capable of obtaining high C₂⁼+C₃⁼ yield and flexible controllable P/E ratio is particularly important.And its core is to develop high performance zeolite catalysts.The acid strength of the zeolite catalyst has different influence on the activation energy of different reaction pathways in the catalytic process.The change of acid strength can often change the proportion of different pathways,so as to achieve the purpose of regulating the product selectivity.In view of the fact that the modification method of zeolitic acidity urgently require an upgrade from the availability to the controllability,therefore,we firstly explored the chemical basis and technical method of controlling zeolitic acidity.Then based on the understanding of the chemical basis of C₂⁼/C₃⁼ generation in OCP and MTH process,the optimized product-oriented high performance zeolite catalysts were studied.The main contents include the following four aspects of work:?1?Based on the general principle of the acid-catalyzed reaction of silane molecules with the surface hydroxyl groups of the zeolite and the general rule of the activation energy of the acid-catalyzed reaction can decreases with the increase of the acid strength of the zeolite,a new idea was proposed to control the acidity of zeolites that by controlling the selective cracking of the silane molecules on the acid sites of zeolites with different acid strength.A method named selective cracking of silanes?SCS?for controlling zeolitic acidity was established.At low temperature,the velocity difference??k?T??of the reaction between the silane molecules and the strong acid and weak acid sites of the zeolites is large.The strong acid sites of zeolites are mainly modified,and the ratio of the strong acid sites to weak acid sites is significantly reduced.At high temperature,Ak?T?is small.Strong acid sites and weak acid sites are shielded almost at the same velocity,and the acid strength distribution is basically unchanged.ZSM-5 with controllable regulation of acid strength while keeping the same amount of total acid sites and ZSM-5 with different acid densities without changing acid strength distribution can easily be prepared by using the SCS method.The establishment of SCS technology provides a new way to control zeolitic acidity.?2?Based on the SCS method,the acidity of ZSM-5 was controlled,and the effects of acid strength of ZSM-5 zeolite on the different pathways of generating C₂⁼and C₃⁼ in the 1-C6= catalytic cracking were studied.Both the monomolecular cracking pathway and the dimerization-cracking pathway exist in the catalytic cracking of 1-C6=.Pathway I-1 of the monomolecular cracking of 1-C6=?C6??C₂⁼+C4=?involves unstable primary carbenium ions,but pathway I-2?C6??2C₃⁼?proceeds thhrough a more stable secondary carbenium ion.That is why the Arrhenius activation energy of the former is higher.Therefore,pathway I-1 needs strong acid sites to reduce the relatively high activation energy and produce more C₂⁼?up to 36.1%?,while weak acid sites of ZSM-5 prefer pathway I-2 and generate more C₃⁼?up to 55.3%?.In addition,the high temperature is favorable for the monomolecular cracking of 1-C6= to produce C₂⁼/C₃⁼,while the low temperature is conducive to the dimerization-cracking of 1-C6= to generate C4=/C5=.When the temperature exceeds 500?,Ce= cacking is dominated by the monomolecular cracking pathway.The optimal design of the acid strength distribution of ZSM-5 by SCS method can effectively adjust the weight relation of the pathways for generating C₂⁼ and C₃⁼ in OCP cracking process?C4=/C5=/C6=?,and realize the orientation generation of C₂⁼/C₃⁼.?3?A series of ZSM-5 zeolites with only one acidity variable?acid density,strength and type?at a time were prepared by SCS technique and SCS combined with pickling,high temperature roasting and A1 exchange.And the effect of the acidity modulation of ZSM-5 on the reaction pathways of MTH was studied.The results show that ZSM-5 catalysts with a weaker Br???nsted acid site contribute to the alkene-based cycle of MTH for C₃⁼ production.The selectivity of C₃⁼ is up to 59.0%and the P/E ratio is 7.56.While ZSM-5 catalysts with a stronger Br???nsted acid site promote the conversion reactions from olefins to aromatics.The amount of species entering the arene-based cycle increases and thus yields more C₂⁼.In addition to the Br???nsted acid density and acid strength of ZSM-5,the Lewis acid center within the confined channel is more favorable for promoting the arene-based cycle to produce C₂⁼.The selectivity of C₂⁼ is up to 39.6%and the P/E ratio reaches 0.72.By controlling the acidity of ZSM-5,the reaction pathway of generating C₂⁼ or C₃⁼ in MTH process is strengthened,and the orientation generation of C₂⁼/C₃⁼ is realized.The P/E ratio is widened from 0.72 to 7.56.?4?Based on the understanding of the promotion of C₂⁼ selectivity in MTH reaction by Lewis acid center of ZSM-5 in the third part,Al-Lewis acid center in different confinement environment?microporous or outer surface or mesoporous?and Zn-Lewis acid center were further investigated in the field of Lewis acid center on the effect of the arene-based cycle pathway in the MTH process.The results show that only the Al-Lewis acid center in the microporous domain has the ability to promote the arene-based cycle and thus produce C₂⁼ in the MTH process.