Study On Preparation And Performance Of Methanol And Toluene Alkylation Catalyst

In recent years,with the development of the polyester industry,as an important chemical raw material paraxylene market gap is huge,import dependence is high,the urgent need to increase the production capacity of refined paraxylene.Coal resources account for about 94%of the fossil energy reserves in China,and they are a cornerstone in ensuring national energy security and economic development.Developing green,efficient,and sustainable utilization of coal is a strategic choice that aligns with the current national conditions and has significant implications.Methanol with excess capacity in coal chemical industry and low value-added toluene were selectively catalyzed to produce high additional refined p-xylene.The development of methanol-toluene alkylation technology（MTPX）is not only conducive to the clean and efficient utilization of coal and the safety of polyester industry chain,but also helps to form a new pattern of complementary and coordinated development of coal chemical industry and petrochemical technology.The current main catalyst for the methylation of toluene with methanol is HZSM-5 zeolite molecular sieve,which has a wide range of applications in catalytic reforming,catalytic cracking,and petroleum processing due to its unique MFI three-dimensional pore structure and abundant acidic sites.However,the abundant yet narrow micropores of HZSM-5 limit the diffusion of molecules and the utilization of the molecular sieve,and the internal acidity is difficult to exploit,resulting in poor overall catalyst performance.Therefore,precise control of the pore structure and pore size distribution of the molecular sieve and determining the most suitable pore structure are crucial to improving the overall performance of the molecular sieve in methylation of toluene with methanol.This article adopts appropriate regulation for the pore structure,element distribution,and acidity distribution of HZSM-5 molecular sieve and studies the performance of each regulation in the reaction.The main research content and results are as follows:1.Three approaches were employed to improve the pore structure of zeolites,and the effects of pore structure and pore size on the methylation of toluene with methanol were investigated.Firstly,a mixed alkali treatment（tetrapropylammonium hydroxide and sodium hydroxide）was used to introduce mesopores into commercial ZSM-5 zeolite.The mixed alkali treatment effectively etched away Si elements in the ZSM-5framework,destroying the original micropores and introducing new mesopores.The crystallinity of the zeolite decreased,while the specific surface area and pore volume increased significantly,especially in the mesopores.The results showed that the better diffusion ability and accessible internal acidic sites increased the toluene conversion rate from 8.4%to 11.3%,but the selectivity for xylene was low,both below 30.5%.Secondly,a controllable silicon-rich region was used to truncate the zeolite pores,shortening the pore length.The inert silicon-rich region provided abundant mesoporous structures,allowing the zeolite to have a balanced specific surface area between micropores and mesopores,providing a good diffusion environment while retaining some selective micropores.Therefore,compared to the alkali-treated zeolite,both the conversion rate and selectivity were improved,with the highest toluene conversion rate reaching 13.3%,and the optimal Si@HZSM-5-S having a toluene conversion rate of 10.8%and a selectivity for paraxylene of42.8%.Thirdly,twin-crystal zeolite HZSM-5-T was synthesized by hydrothermal method.The twin-crystal zeolite had a richer microporous structure,and the inert twin-crystal covered the straight pore channel（b-axis）of the zeolite,so that the molecules could only diffuse out through the tortuous sinusoidal channel,maximizing the use of selective micropores.Therefore,the selectivity for paraxylene was as high as 80.6%,but the diffusion of toluene was restricted by more micropores,resulting in a toluene conversion rate of only7.2%.2.By using in situ Ru modification of the twin crystal zeolite,the performance of Ru in anti-coking was studied.The twin crystal HZSM-5-T has a more abundant and densely distributed microporous structure,providing a good environment for the selective alkylation of toluene,thus it has higher selectivity for p-xylene than the hierarchical pore molecular sieves HZSM-5-B and Si@HZSM-5.However,the micropores also limit its diffusion and lead to severe coking.Ru metal was successfully encapsulated in the twin crystal HZSM-5-T by in situ synthesis,and its introduction successfully covered most of the strong acid sites,reducing the toluene conversion rate（7.2%→3.7%）,but also reducing the isomerization of p-xylene,further improving the selectivity for p-xylene（～80%→～96%）.Combined with the distribution of organic products in the gas phase,the introduction of Ru changed the distribution of by-product coking products,generating more methane instead of easily coked olefins.The methane proportion increased from 5%to～90%,while the olefin proportion decreased from～50%to～5%.Thermogravimetric analysis showed that the coking rate of Ru@HZSM-5 after reaction was only 64%of that of HZSM-5.3.Based on DFT theory,the oxygen ion mechanism in MTH and the reaction mechanism of methane as a byproduct from methanol were calculated for the HZSM-5 and Ru@HZSM-5 models.The computational results showed that the dissociation energy of C₂H₄ on the Ru site was as high as 257.62 k J·mol^-1,while on the Br（?）nsted acid site of HZSM-5 it was only 59.82 k J·mol^-1.Moreover,the reaction barriers for the formation of dimethyl ether and trimethyl oxonium ion on the Ru site were much higher than those on the=Br（?）nsted acid site of HZSM-5,making it more difficult for the first C-C bond to form through methanol dehydration at the Ru site compared to the Br（?）nsted acid site.In addition,by comparing the reaction pathways and intermediate energy distributions of the formation of olefins and methane as byproducts from methanol at the Ru active site,it was found that there were five intermediate states with reaction barriers greater than150 k J·mol^-1 in the process of the first C-C bond formation,while in the process of methane formation,only two reaction barriers were greater than 100 k J·mol^-1,with the highest occurring in the desorption of methane at 110.48 k J·mol^-1.This suggests that at the Ru active site,methane formation is the dominant pathway for methanol as a byproduct.The reaction path of methanol is changed over Ru@HZSM-5 catalyst and showed the high resistance to carbon deposition.