Acidic Structure Of ZSM-5 And Reaction Process Optimization For Step Conversion Of Methanol To P-xylene

P-xylene is an important raw material for resin,fiber,and film production,but its production depends on petroleum route,and the market gap is large.Based on Chinese energy system of"coal-rich,oil-little and gas-short"and the energy strategy of"carbon neutral and emission peak",the production of p-xylene from coal-based methanol to aromatics can not only promote the clean utilization of coal resources,but also enrich the source of raw materials and make up for the market gap of p-xylene.ZSM-5 is an important MTA catalyst.However,the catalytic process has two main problems:the low aromatics selectivity and poor catalytic stability.It is difficult to achieve synchronous improvement by modifying a single catalyst.Based on the stepwise conversion of methanol to aromatics,MTA reaction was decomposed into two processes of methanol to light olefins and light olefins aromatization.The catalysts were configured for the two processes to achieve high stability of MTA.It was not difficult to understand that the process of methanol to light olefins catalyzed by high Si/Al ratio ZSM-5has excellent catalytic stability.The improvement of the stability of the light olefins’aromatization was an important reason for the improvement of the catalytic life of this process.In order to analyze the intrinsic mechanism,propene aromatization,was used as the model reaction to analyze the effects of acidity on aromatization of light olefins,and the stability difference between methanol aromatization and propene aromatization was analyzed.In order to achieve the high selectivity of p-xylene,the external surface acid of aromatization catalyst in the stepwise conversion of MTA was modified.According to the characteristic that toluene accounts for a large proportion of aromatic,part of methanol was further introduced into the lower bed to control the reaction depth of aromatization process,which leaded part of toluene was converted into xylene..A series of ZSM-5 catalysts with different Si O₂/Al₂O₃ ratios and Zn modified were prepared.Combined catalysts evaluation and propene aromatization performance evaluation found that the increased acid density was in favor of the hydrogen transfer process,resulting in an increase of aromatics selectivity from 31%to 34.4%with the Si O₂/Al₂O₃ ratio decreased from 150 to 75.Meanwhile,an increasing in acid density caused more propene to directly participate in the hydrogen transfer reaction to produce propane which was enhanced from 26.4%to 36%.Further introducing Zn could transform some Br?nsted acid sites into Zn-Lewis acid sites enhancing the dehydrogenation aromatization process.Based on the efficient synergistic between appropriate B acids and Zn-L acids,the aromatics selectivity raised from 34.4%to62.4%.The main product of propene aromatization were toluene and xylene which was formed by reaction of butene and propylene,bimolecular butene respectively.Compared with direct methanol aromatization,propene aromatization had less deep alkylation process,and the addition of toluene further improved aromatics selectivity.What’s more,propene aromatization retarded the formation of coke,and most of the coke assigned to soluble carbons.After 46 h,the conversion and liquid hydrocarbon yield of propene aromatization still remained at 98%and 40%,while which decreased to 69%and 3.9%respectively in methanol aromatization process.These results suggested that the pre-conversion of methanol was an important reason for slow carbon deposition rate and high catalytic stability of two-step methanol conversions,which inhibited the deep alkylation of aromatics in the subsequent aromatization process.The external surface acid of ZSM-5 zeolite was modified by liquid deposition method,and the passivation degree of external surface acid was controlled by changing the amount of TEOS.The total acid density and the external surface acid density of ZSM-5 decreased,resulting in the liquid hydrocarbon yield and aromatic selectivity decreased from 15.6%and 34.0%to 10.8%and 20.8%,respectively.At the same time,Si O₂ deposition blocked the pore channels of the zeolites to a certain extent,accelerated the generation of coke deposition and thus reduced the catalytic life.With the development of the reaction,the coke deposition further modified the surface acid sites.The decrease in acid density reduced the aromatic selectivity and inhibited side reactions such as dealkylation and isomerization,leading to a decrease in toluene selectivity and an increase in xylene selectivity,with PX/X gradually increasing from 54.6%at5 h to 77.7%at 52 h.In addition,part of the methanol was introduced into the lower catalyst bed to control the reaction depth,resulting in the proportion of xylene on 3Si/Zn E75 increased from 40.0%to 55.3%,and PX/X increased from 51.1%to 55.5.The alkylation degree of aromatics was significantly affected by the surface acid density of the aromatization catalyst.The decrease of the surface acid density of 28Si/Zn E75 zeolites inhibited the alkylation reaction of aromatics,contributed to the xylene and PX/X increased from 39.9%to 41.8%and 60.0%to61.7%,respectively.