甲苯甲醇甲基化

关于延长集团MTA甲醇制芳烃项目的几点建议由清华大学和华电合作的甲醇制芳烃技术1万吨/年工业实验装置已于2013年2月初，由华电煤业集团投资、华电煤业和清华大学共同合作开发的万吨级流化床甲醇制芳烃工业试验项目，在陕西省榆林榆横煤化学工业园区获得成功。

第一次投料原料甲醇转化率高于99．99％，油相产物中甲基苯（主要指甲苯、二甲苯和三甲苯）的含量达到90％以上。

截至1月1 5日，原料甲醇累计进料约100吨，装置平稳运转54小时，工业试验装置实现了一次点火成功，一次投料试车成功，打通了关键流程。

甲醇制芳烃（MTA）是指甲醇在催化剂的作用下，经过一系列反应，最终转化为芳烃的过程，产品以苯、甲苯、二甲苯（BTX）为主，副产品主要是LPG。

MTA的芳烃理论收率为40．6％，但是实践中由于副产物的存在，通常需要3吨以上甲醇才能获得1吨BTX。

在我国甲醇产能过剩已成为现实，进口甲醇具有低成本优势的市场现状下，MTA技术的开发和工业化示范对于开拓有竞争力的甲醇下游衍生物产品具有重要意义，将为我国甲醇行业提供新的产品方向。

中科院山西煤化所技术中科院山西煤化所和赛鼎工程公司合作固定床甲醇制芳烃技术，以甲醇为原料，以改性ZSM－5分子筛为催化剂，在操作压力为0．1～5．0MPa，操作温度为300～460℃，原料液体空速为0．1～6．0h－1条件下催化转化为以芳烃为主的产物；经冷却分离将气相产物低碳烃与液相产物C5＋烃分离；液相产物C5＋烃经萃取分离，得到芳烃和非芳烃。

该发明具有芳烃的总选择性高，工艺操作灵活的优点。

该技术属于大规模甲醇下游转化技术，目标产物是以BTX为主的芳烃。

以MoHZSM－5（离子交换）分子筛为催化剂，以甲醇为原料，在T＝380～420℃、常压、LHSV＝1h－1条件下，甲醇转化率大于99％，液相产物选择性大于33％（甲醇质量基），气相产物选择性小于10％。

液相产物中芳烃含量大于60％。

已完成实验室催化剂筛选评价和反复再生试验，催化剂单程寿命大于20天，总寿命预计大于8000小时。

H-ZSM-5催化甲苯与碳酸二甲酯和甲醇甲基化的反应机理李玲玲1JANIK J.Michael 2,3,*聂小娃1,4宋春山1,2,3郭新闻1,*(1大连理工大学化工学院精细化工国家重点实验室,PSU-DUT 联合能源研究中心,辽宁大连116024;2宾夕法尼亚州立大学能源与矿物工程系能源研究所,PSU-DUT 联合能源研究中心,宾夕法尼亚16802,美国;3宾夕法尼亚州立大学化学工程系,宾夕法尼亚16802,美国;4俄亥俄州立大学化工与生物分子工程系,俄亥俄43210,美国)摘要:对二甲苯是重要的石油化工产品之一,可以通过甲苯甲基化生产.本文采用“our own-N -layeredintegrated molecular orbital+molecular mechanics ”(ONIOM)和密度泛函理论(DFT)结合的方法,计算了H-ZSM-5催化甲苯与碳酸二甲酯(DMC)和甲醇甲基化反应机理.考察了反应物吸附和产物脱附.描述了主要的中间物种和过渡态的结构.用计算的速率常数来估计甲苯甲基化反应的动力学活性.H-ZSM-5催化的甲苯与DMC 和甲醇甲基化的机理不同.甲苯和DMC 甲基化包括DMC 完全解离,接着甲基化生成二甲苯异构体.相比而言,在甲苯甲基化反应中,甲醇作为甲基化试剂的活性比DMC 更好.甲苯和甲醇甲基化的分步反应路径和联合反应路径的本征活化能相似.在773K,分步反应路径的速率常数比联合反应路径更高.在甲苯和这两种试剂甲基化的反应中,生成对二甲苯为动力学优先,而间二甲苯为能量最低产物.我们的计算结果和实验观察到的现象一致.关键词:密度泛函理论;ONIOM;甲苯甲基化;碳酸二甲酯;甲醇;H-ZSM-5中图分类号:O641Reaction Mechanism of Toluene Methylation with Dimethyl Carbonate orMethanol Catalyzed by H-ZSM-5LI Ling-Ling 1JANIK J.Michael 2,3,*NIE Xiao-Wa 1,4SONG Chun-Shan 1,2,3GUO Xin-Wen 1,*(1State Key Laboratory of Fine Chemicals,PSU-DUT Joint Center for Energy Research,School of Chemical Engineering,Dalian University of Technology,Dalian 116024,Liaoning Province,P .R.China ;2EMS Energy Institute,PSU-DUT Joint Center for Energy Research and Department of Energy &Mineral Engineering,Pennsylvania State University,University Park,P A 16802,USA ;3Department of Chemical Engineering,Pennsylvania State University,University Park,P A 16802,USA ;4Department ofChemical &Biomolecular Engineering,The Ohio State University,Columbus,OH 43210,USA )Abstract:Para -xylene is an important petrochemical that can be produced by the methylation of toluene.Here,the mechanism of toluene methylation with dimethyl carbonate (DMC)or methanol catalyzed by H-ZSM-5was studied using the “our own N -layered integrated molecular orbital +molecular mechanics ”(ONIOM)in combination with density functional theory (DFT)methods.The adsorption of reactants and desorption of products are considered,and the structures of important intermediates and transition states are putational rate constants are used to estimate the kinetic activity of toluene methylation reactions.The reaction mechanism of toluene methylation with DMC and that with methanol catalyzed by H-ZSM-5differ.Toluene methylation with DMC involves full decomposition of DMC prior to methylation to form xylene isomers.In contrast,methanol is more active than DMC as the methylation reagent in toluene[Article]doi:10.3866/PKU.WHXB201304262物理化学学报(Wuli Huaxue Xuebao )Acta Phys.-Chim.Sin .2013,29(7),1467-1478July Received:January 29,2013;Revised:April 26,2013;Published on Web:April 26,2013.∗Corresponding authors.GUO Xin-Wen,Email:guoxw@;Tel:+86-411-84986133.JANIK J.Michael,Email:mjanik@;Tel:+814-863-9366.The project was supported by the Program for New Century Excellent Talent in University,China (NCET-04-0268),Plan 111Project of the Ministry of Education of China,and High Performance Computing Department of Network and Information Center,Dalian University of Technology,China.新世纪优秀人才项目(NCET-04-0268)及教育部111计划工程基金和大连理工大学网络与信息化中心高性能计算部基金资助ⒸEditorial office of Acta Physico-Chimica Sinica1467Acta Phys.-Chim.Sin.2013Vol.29methylation.The stepwise and concerted paths of toluene methylation with methanol have similar intrinsic activation energies.At773K,the stepwise path has a higher rate constant than the concerted one.For toluene methylation with both reagents,para-xylene formation is kinetically preferred,whereas meta-xyleneis the lowest-energy product.The results of our calculations agree well with experimental observations.Key Words:Density functional theory;ONIOM;Toluene methylation;Dimethyl carbonate;Methanol;H-ZSM-51IntroductionPara-xylene,an important petrochemical,can be produced from multiple processes including isomerization of xylenes and methylation of toluene.ZSM-5,a solid zeolite catalyst, shows a greater selectivity in synthesis of para-xylene by tolu-ene methylation than other zeolites.1-3High selectivity of para-xylene has been obtained in toluene methylation with methanol as a traditional methylation reagent.3Recently,the use of di-methyl carbonate(DMC),which is an important green chemi-cal intermediate,has gained attention in xylene production.4,5 The methylation of toluene with DMC in a fixed-bed reactor demonstrated positive selectivity to para-xylene over MgO-modified MCM-22.4Toluene methylation with methanol,how-ever,could give higher selectivity to para-xylene than methyla-tion with DMC which has a faster coke deposition rate over Si-P-Mg modified ZSM-5.5Increasing both the selectivity to para-xylene and the stabili-ty of the catalyst remains important issues in catalyst develop-ment for toluene methylation.Mechanism determination will facilitate rational design of selective zeolite catalysts for tolu-ene methylation.Experimental kinetic studies show promising results of toluene alkylation with DMC and methanol using a ZSM-5catalyst,and determination of differences in key steps within the two alkylation processes can aid in interpreting dif-ferences in catalytic activity and optimal catalyst design be-tween the two reagents.Herein,we apply density functional theory(DFT)6calculations,using the embedded quantum me-chanics/molecular mechanics(QM/MM)approach,to eluci-date mechanistic differences between methanol and DMC re-agents in the methylation of toluene.The mechanism of aromatic methylation with DMC over ze-olites is not well established.The DMC molecule has three iso-mer configurations including trans-trans,cis-cis,and trans-cis structures,with the trans-trans being the most stable configura-tion.7DMC can be decomposed to dimethyl ether,methanol, and carbon dioxide over H-ZSM-5zeolite,and the dimethyl ether is speculated to be produced by reaction of methoxide and methanol.8-10The decomposition products can further react with toluene.11-15We can speculate the mechanism of toluene methylation with DMC based on these clues.Aromatic alkylation with methanol as a reagent has received more prior attention.Previous experimental and theoretical re-search suggests that toluene alkylation with methanol to xylene isomers can occur through either stepwise or concerted paths.11-15The stepwise path passes through a surface methox-ide species whereas the concerted path includes a direct alkyla-tion by methanol.The exploration of the dominant reaction path might help guide rational catalyst design to increase the selectivity to para-xylene.V os et al.15examined the concerted mechanism of toluene methylation with methanol using a peri-odic model of MOR zeolite.The difference in pore structure and therefore active site structure in highly active ZSM-5moti-vates our further mechanistic study of this reaction. Theoretical calculation based on quantum chemistry is a helpful tool to investigate molecular level catalytic phenome-na.The“our own-N-layered integrated molecular orbital+mo-lecular mechanics”(ONIOM)16,17approach with two layers was used herein to examine toluene methylation with DMC and methanol.Equilibrium and transition state configurations were located within the H-ZSM-5pore,and relative energies of these states provide elementary step activation barriers and re-action energies.The QM region,including the acid site and re-acting species,provided an accurate description of reaction en-ergetics and was combined with a MM representation of the confining extended framework.Following B3LYP/UFF optimi-zation,single point energies with theωB97X-D functional were used to provide accurate zeolite-hydrocarbon interaction energies including dispersion energies.2Computational model and methodsA128T(128tetrahedra)cluster of H-ZSM-5,including the zigzag and straight channels,was taken from the lattice struc-ture of MFI zeolite and is shown in Fig.1(a,b).18-21A single Al atom is most preferred to substitute for Si at the T12position to introduce a Brønsted acid site of ZSM-5.22-25The cluster was terminated by H atoms bound to Si atoms,with the terminal Si－H bond length fixed in the crystalline lattice direction at 0.147nm.The methods and model used herein are equivalent to that used in our previous study of methylation of2-methyl-naphthalene.18A two-layer ONIOM(ONIOM2)approach was employed for the128T cluster.The inner12T portion,covering a10-membered-ring(10MR)and two additional basal T units near-by the Al atom,was considered to represent the active region of the zeolite and treated at the B3LYP/6-31G(d,p)level.The remaining extended framework was considered with the univer-1468LI Ling-Ling et al .:Reaction Mechanism of Toluene Methylation with DMC or Methanol Catalyzed by H-ZSM-5No.7sal force field (UFF).26During optimization,only the basic 5T of the active region,[(≡(SiO)3Al(OH)Si ≡],and the adsorbates were allowed to relax,whereas the rest of the zeolite model was fixed at the crystallographic coordinates.Structural optimiza-tion of the most bulky adsorbed states within the 128T ONIOM cluster was performed considering different QM-portion sizes to test the validity of the 12T QM model.Similar structural pa-rameters and relevant energies determined from the ONIOM clusters with different QM sizes (12T and 27T)demonstrate a reasonable description of stable states using a 12T QM region.The model in Fig.1is a reasonable balance of accuracy and computational cost considering the bulky reacting species.The quadratic synchronous transit (QST)method within the frame-work of the ONIOM2(DFT:UFF)model was applied to locate transition states,and each transition state was confirmed to have a single imaginary vibrational frequency.The quasi-intrinsic re-action coordinate (IRC)approach followed geometry optimiza-tions were used to obtain initial reactant and product species to-ward reaction tendency.27,28In the ONIOM system,error may be introduced by the coarse combination of the low level and the more accurate DFT method.Based on the definition of Morokuma ʹs group,29a ΔS -value was used to confirm the reliability of energetics ob-tained by the level/model combination.A large ΔS -value indi-cates inaccuracy of the ONIOM2(DFT:UFF)extrapolation.Therefore,after optimization or transition state identification with the ONIOM2model,a zero point vibrationally corrected single-point energy was calculated using the expensive full ωB97X-D method for the whole 128T cluster.The ωB97X-D functional was developed to consider the binding energies of adsorbates covalently and non-covalently interacting with a ze-olite framework.28,30All energetics reported herein refer to the values calculated for the single-point energy using the ωB97X-D on the B3LYP(12T)/UFF(116T)optimized structure.All cal-culations were performed with the Gaussian 0331and Gaussian 0932packages.3Results and discussion3.1Overview of possible reaction mechanismsChemical differences between DMC and methanol lead to different reaction mechanisms of toluene methylation.Metha-nol,as an alcohol,can dehydrate to form a water molecule and a methoxide species which further alkylates toluene,which is generally called the stepwise path.Alternatively,methanol can co-adsorb with toluene and alkylate through a concerted dehy-dration-methylation reaction.Based on the intermediates observed in the DMC decomposi-tion reaction 8-10and the feasible methylation of toluene with methoxide,11-13a stepwise toluene methylation with DMC may proceed through DMC decomposition and toluene methylation with the surface methoxide species (path A in Scheme 1(a)).A concerted mechanism involving DMC,as speculated by experi-mental reports,4,33,34partially decomposes DMC to form a meth-yl group and releases a methyl carbonate (MC)molecule.The methyl group could alkylate toluene in a concerted step,how-ever we found that the decomposed methyl carbenium ion in-teracts with a zeolite basic oxygen atom instead of directly at-tacking toluene.The zeolite basic oxygen atom serves as a tem-porary acceptor.We propose that a “concerted ”mechanism of toluene methylation with DMC starts with DMC partial decom-position to form a methyl carbenium ion which interacts with the zeolite medium releasing a MC molecule,then the medium donates the carbenium ion to alkylate toluene as shown by path B and path C in Scheme 1(a).Though we term this as a “con-certed ”mechanism,the effect of toluene in the initial decompo-sition step can be ignored to decrease the space restraint and the computational cost.The three paths of toluene methylation with DMC differ in the first decomposition step,forming dif-ferent DMC-derived intermediate products.The methylation of toluene with DMC and methanol via the methoxide intermediate is examined in Section 3.2,and the concerted methylation of toluene with methanol is examined in Section 3.3.3.2Toluene methylation with DMC and methanol viamethoxide intermediate 3.2.1DMC decompositionThe methylation reaction of toluene with DMC starts with DMC decomposition to form a methoxide intermediate via pathFig.1ONIOM2model of the 128T H-ZSM-5The inner 12T (balls)was optimized at the B3LYP/6-31G(d ,p )level;the remaining extended zeolite framework (lines)was optimized with UFF.Full 128T single-point calculations were used for the reported energetics.(a)zigzag channel view,(b)straight channel view.green:silicon,red:oxygen,pink:aluminum,white:hydrogen1469Acta Phys.-Chim.Sin.2013Vol.29A,path B or path C.DMC adsorbs strongly over the Br ønsted acid site of H-ZSM-5through either bidentate adsorption or un-identate adsorption.35The adsorption structure is preferred for full decomposition or partial decomposition and further methyl-ation with toluene.The optimized structures of key intermedi-ates and transition states during DMC decomposition are shown in Fig.2.In path A of toluene methylation with DMC,DMC fully decomposes to methoxide,methanol,and carbon di-oxide,and further the methoxide species then alkylates toluene.In path B or C,DMC partially decomposes to form a methyl carbocation and MC,and the methyl carbocation alkylates tolu-ene.The energetics of key states are given relative to the sum of gas phase DMC and the isolated zeolite in Fig.3.Relevant bond parameters of the key structures are given in Table1.Fig.3(a)illustrates the reaction energy profile for path A of toluene methylation with DMC,including the full decomposi-tion reaction of DMC to the methoxide intermediate.In the de-composition step,formation of surface methoxide,methanol,and carbon dioxide occurs in a concerted action.Fig.2(a)shows the initial adsorption structure of bidentate DMC which initiates path A of toluene methylation with DMC.DMC ad-sorbs at the Br ønsted acid site (H1)by formation of a hydrogen bond between H1and O4,labeled as Ads1_DMC.The O1－H1bond is elongated from 0.097nm in the isolated zeolite to 0.099nm with DMC adsorbed.The DMC molecule is activat-ed after adsorbing,reflected by the lengthening of C3－O4and C1－O3bond distances by 0.002and 0.001nm,respectively.Catalyzed by H-ZSM-5,DMC completely dissociates through the transition state,TS1_DMC,shown in Fig.2(b).The cleav-age of the C3－O4bond,motivated by H1attack,promotes the breaking of the C1－O3bond due to instability of the nega-tive CH 3OCO -group.At the transition state,a close-to-planar methyl group suggests a carbenium-like transition state,with the positive charge stabilized by interaction with both the O2(a)(b)Scheme 1Proposed reaction paths for methylation of toluene with DMC and methanol over H-ZSM-5(a)the mechanism of toluene methylation with DMC including the path A,path B,and path C;(b)the concerted path of toluene methylation withmethanol1470LI Ling-Ling et al .:Reaction Mechanism of Toluene Methylation with DMC or Methanol Catalyzed by H-ZSM-5No.7zeolite atom and the O3atom of what will be the CO 2decom-position product.The bond angle of OCO is 172.3°,which shows a tendency towards the generation of a CO 2molecule.In the product structure,Int1(Fig.2(c)),the surface methoxide forms a covalent interaction between C1and O2at the inter-atomic distance of 0.150nm.The formed methanol molecule adsorbs at the framework basic O1atom by forming a hydro-gen bond between H1and O1.The carbon dioxide molecule is co-stabilized by methoxide (C1…O3)and methanol (C3…O4).The adsorption energy of Ads1_DMC is -157.7kJ ·mol -1.This exothermic value is relatively similar to the dimethyl ether adsorption energy (-123.0kJ ·mol -1)19computed with ONIOM2(M06-2X/6-311+G(2df ,2p ):UFF)+SCREEP calcula-tion.The activation energy (E a )of the DMC full decomposition step to methoxide,methanol,and carbon dioxide is 186.6kJ ·mol -1catalyzed by H-ZSM-5,which is 28.4kJ ·mol -1higher than that calculated for the same reaction catalyzed by dicylo-hexylurea in the gas phase.36The produced methanol and car-Table 1Optimized geometric parameters of species involved in decomposition of DMC overH-ZSM-5(i)(j)(k)(l)Fig.2Optimized structures of equilibrium and transition states along the paths of DMC decomposition and methanol dehydration(a)bidentate adsorption structure for DMC,Ads1_DMC;(b)transition state for the DMC decomposition step in path A,TS1_DMC;(c)product for the DMC decomposition step in path A,Int1;(d)bidentate adsorption structure for DMC,Ads2_DMC;(e)transition state for the DMC decomposition step in path B,TS2_DMC;(f)product for the DMC decomposition step in path B,Int2;(g)unidentate adsorption structure for DMC,Ads3_DMC;(h)transition state for the DMC decomposition step in path C,TS3_DMC;(i)product for the DMC decomposition step in path C,Int3;(j)side-on adsorption structure for methanol,Ads1_Met;(k)transition state formethanol dehydration step,TS_Met,(l)product for the methanol dehydration step,Int_Met_H 2O.Only the active region of the H-ZSM-5is shown for clarity.green:silicon,red:oxygen,pink:aluminum,grey:carbon,white:hydrogen(e)(f)(g)(h)(a)(b)(c)(d)1471Acta Phys.-Chim.Sin.2013Vol.29bon dioxide diffuse from the active site by overcoming a col-lective desorption energy of 125.1kJ ·mol -1.The produced methanol can diffuse to another Br ønsted active site to partici-pate in a further methylation reaction with a toluene molecule.Fig.3(b)and 3(c)illustrate the reaction energy profiles for DMC partially decomposing to a methoxide species and a MC molecule in path B and path C of toluene methylation with DMC,respectively.Fig.2(d)displays the initial adsorption structure of bidentate DMC which initiates path B of toluene methylation with DMC.DMC adsorbs at the Br ønsted acid site of H-ZSM-5through the formation of a hydrogen bond be-tween H1and O3.In the adsorption structure,Ads2_DMC,the C1－O3bond of DMC is weakened,as reflected by the expan-sion of the C1－O3bond length from 0.144to 0.146nm.In the transition state shown in Fig.2(e),TS2_DMC,H1transfers from the initial basic O1to O3atom,and the C1－O3bond is completely broken to form a methyl carbocation.A methoxide and a MC molecule (Fig.2(f),Int2)are produced via the transi-tion state.To initiate path C,DMC adsorbs at the Br ønsted acid site of the H-ZSM-5catalyst by the O5atom,forming a uniden-tate adsorption structure (Fig.2(g)).This structure is labeled asAds3_DMC.The bond distance of C1－O3is lengthened by 0.001nm,illustrating a tendency to dissociate.In the transition state (Fig.2(h),TS3_DMC),the H-ZSM-5catalyst donates the H1proton to bond with the O5atom,and the C1－O3bond is motivated to simultaneously rupture.This decomposition step ends by releasing a MC molecule adsorbing in the vicinity of methoxide (Fig.2(i),Int3).The adsorption energies of Ads2_DMC and Ads3_DMC are -166.1and -176.9kJ ·mol -1,respectively.The E a for DMC partial decomposition to methoxide and MC in path B and path C are 220.5and 206.7kJ ·mol -1,respectively.The formed MC intermediates desorb from the active site overcoming 105.0and 113.1kJ ·mol -1desorption energies in the corresponding paths.The MC intermediates in path B and path C are isomers.After desorbing from the active site,the MC in path C could transform to the more stable MC produced in path B.The MC is unstable and may quickly dissociate to methanol and carbon dioxide over another Br ønsted acid site of H-ZSM5.Catalyzed by H-ZSM-5,E a for MC dissociation is 41.8kJ ·mol -1,which is 21.4kJ ·mol -1lower than the calculated barrier reported for gas-phase decomposition and 6.7kJ ·mol -1lower than the barrier reported using a solvent model.33Path C is more preferred for DMC to partially decompose to methoxide and MC than path B due to a lower E a (13.8kJ ·mol -1).However,DMC partial de-composition to form methoxide and MC via path C (E a =206.7kJ ·mol -1)needs to overcome a higher E a than full decomposi-tion to form methoxide,methanol,and carbon dioxide via path A (E a =186.6kJ ·mol -1).3.2.2Methanol dehydrationThe stepwise path of toluene methylation with methanol re-sembles toluene methylation with DMC,with the difference be-ing that the methoxide intermediate forms from methanol dehy-dration.The energy profile for methanol dehydration is given in Fig.4.A methanol molecule adsorbs on the Br ønsted acid site,dehydrates to methoxide,and then the methoxide attacks toluene to form xylene isomers.The methanol is adsorbed on the acid site with two hydrogen bonds between O3and H1and between H methanol and O framework forming a side-on adsorbed struc-ture reported by Ivanova and Corma,11as shown in Fig.2(j).The adsorption energy is -128.4kJ ·mol -1which is within range of the data calculated by Mazar et al .37by PBE+D meth-od (-118kJ ·mol -1in sinusoidal channel and -132kJ ·mol -1in straight channel),and agrees well with the experimental value ((-115±5)kJ ·mol -1)38.To form a methoxide,the C1－O3bond dissociates as H1is transferred to O3,releasing a water mole-cule and forming a methyl-carbenium transition state (Fig.2(k)).The methyl-carbenium ion is stabilized by O3and O2framework oxygen atoms.The E a of methanol dehydration to methoxide is 120.9kJ ·mol -1.The methyl group binds to a framework oxygen atom (O2)to form the surface methoxide intermediate (Fig.2(l)).The desorption energy of the produced water molecule is 47.2kJ ·mol -1.We explicitly define the step-wise path to require the desorption of water,as the lack of sta-Fig.3Reaction energy profile for methylation of toluene with DMC to form para -,ortho -,and meta -xylene(a)path A;(b)path B;(c)path C.The solid line belongs to the para -xylene formation path,the dotted line represents the ortho -xylene formation path,andthe dashed line belongs to the meta -xylene formationpath.1472LI Ling-Ling et al .:Reaction Mechanism of Toluene Methylation with DMC or Methanol Catalyzed by H-ZSM-5No.7bilization of the alkylation transition state by a water molecule differentiates the stepwise and concerted transition states.Key bond distances within the methanol dehydration sequence are shown in Fig.2.3.2.3Methylation stepToluene methylation with DMC and methanol occurs via an identical methylation step.The reaction energy profiles,refer-encing the DMC or methanol gas phase states,are shown in Fig.3and Fig.4,respectively.To initiate the methylation step,methoxide/toluene co-adsorption structures are formed via σ-πinteraction between the methyl group and the electrons of the aromatic ring of toluene.Ortho -xylene and meta -xylene forma-tion occur from the same co-adsorption species due to the suit-able reaction position,whereas para -xylene formation initiates from a different co-adsorbed species.The toluene adsorption energies to a methoxide containing site are -112.9and -105.0kJ ·mol -1,suggesting a substantial attraction between the methoxide and toluene.To alkylate,toluene is attacked by the methyl group to form the xylene isomers.Both methylation and deprotonation of tol-uene occur in a concerted reaction step to both form the xylene product and regenerate the acid site of H-ZSM-5catalyst.The methylation transition states are denoted as TS_P-Xylene,TS_O-Xylene,and TS_M-Xylene for formation of para -,ortho -,and meta -xylene,respectively.The corresponding struc-tures are illustrated in Fig.5.The geometric parameters of the key species are given in Table 2.In these transition state struc-tures,the C1－O2bond of methoxide is completely broken and new C1－C p ,C1－C o ,and C1－C m bonds are partially formed.A C toluene －H toluene bond within the toluene molecule is elongated towards deprotonation and reformation of the acid site.The activation energies for the formation of para -,ortho -,and meta -xylene are 63.2,68.2,and 112.9kJ ·mol -1,respective-ly,when referencing their respective initial co-adsorbed struc-tures.Differences in the rates of the methylation step will dic-tate selectivity,though with the transition state referenced to the lowest energy toluene co-adsorbed state in all cases,selec-tivity to para over ortho -xylene is not clearly predicted.The space restraints of the ZSM-5pore lead to transition state selectivity against formation of the meta -xylene product,as evidenced by interatomic distances.At the transition state,the attacking methyl group is near to planar,suggesting that much of the positive charge of the reacting species is on this fragment.This methyl fragment is stabilized by interaction with the πelectrons of toluene and the negative zeolite frame-work charge through the O2atom.The bond distances of C1－C p ,C1－C o ,and C1－C m are 0.222,0.219,and 0.231nm,re-spectively.C1－C m has the longest interatomic distance.The basic O1site of the zeolite also assists to stabilize the carbeni-um ion.The interatomic distances of C1…O1are both 0.275nm in TS_P-Xylene and TS_O-Xylene,and the C1…O1dis-tance is 0.021nm longer in TS_M-Xylene.This indicates the weakest stabilization effect of the framework of the zeolite on the meta -transition state.Though these distances show little differentiation between forming ortho -or para -xylene,space restraints lead to a greater barrier for meta -xylene production.Thermodynamically,there is less than a 1.2kJ ·mol −1differ-ence calculated between the three gas-phase products energies.However,there are more significant differences in reaction en-ergies for formation of the adsorbed products.Meta -xylene is the most thermodynamically stable adsorbed product,39.4and 47.3kJ ·mol -1more stable than adsorbed ortho -xylene and ad-sorbed para -xylene.The thermodynamic stability is similar be-tween adsorbed para -xylene and ortho -xylene.The desorption energies are 125.9,133.0,172.0kJ ·mol -1for para -,ortho -,and meta -xylene respectively.The desorption energies for xy-lene desorbing from the acid site inside the pore are higher than that for xylene desorbing from the external surface,which may be due to the additive interaction between xylene and the internal zeolite framework atoms.39At low conversions,selec-tivity may be presumed to be dictated purely by the kinetic se-lectivity.At higher conversion,the relative irreversibility of meta -xylene formation could impact selectivity.The desorp-Table 2Optimized geometric parameters of species involved in toluene methylation with methoxide to form para -,ortho -andfor formation of para -,ortho -,or meta -xylene,X:Xylene.L is bond length andA is bond angle.Fig.4Reaction energy profile for stepwise mechanism of methylation of toluene with methanol to form the para -,ortho -,and meta -xylene including methanol dehydration step andmethylation stepThe solid line belongs to the para-xylene formation path,the dotted line represents the ortho -xylene formation path,and the dashed line belongs to themeta -xylene formation path.1473。

甲苯甲醇烷基化技术的分析与优化王宏乐，韦鹏，刘燕 (蒲城清洁能源化工有限责任公司, 陕西 渭南 715500)摘要：文章对甲苯甲醇烷基化制备对二甲苯技术进行了分析和讨论，分析了甲苯转化率对产品收率、运行能耗及投资的影响，分析了甲醇转化率对污水排放系统的影响。

经过分析得出，甲苯甲醇烷基化技术制备对二甲苯，产品的选择性很高，非常适于生产对二甲苯。

甲苯甲醇烷基化反应产物中剧毒物质苯的含量非常少，在环境保护方面优于其他芳烃生产工艺。

关键词：甲苯；甲醇；烷基化；分析中图分类号：TQ02文献标志码：A文章编号：1008-4800(2021)15-0092-02DOI:10.19900/ki.ISSN1008-4800.2021.15.046The Analysis and Optimization about Alkylation Technology of Toluene with MethanolWANG Hong-le, WEI Peng, LIU Yan (Pucheng Clean Energy Chemical Co., Ltd., Weinan 715500, China)Abstract: The process using alkylation of toluene and methanol to produce paraxylene is analyzed. The influence of toluene’s conversionon product yields, unit energy consumption and investment are also analyzed. Otherwise, the conversion of methanol also influences the downstream waste water processing system. Alkylation technology of toluene with methano is especially suitable for paraxylene production due to the high selectivity of paraxylene in the product yields. In the product, the amount of toxic benzene is approaching to zero, so it is more environment-friendly than other technologies. Keywords: toluene; methanol; alkylation; analysis0引言由于我国芳烃资源较少，工业上采用甲苯、C9芳烃的烷基转移和甲苯歧化技术生产苯和二甲苯。

第十一届全国青年催化会议论文集文章编号：ＰＡ－０２５　分子筛催化甲苯甲醇烷基化反应的研究刘晔，姚明恺，刘月明**，王勇，吴鹏（华东师范大学化学系上海市绿色化学与化工过程绿色化重点实验室上海）关键词：甲苯甲醇烷基化，分子筛，硅铝比，硼改性作为合成对苯二甲酸和对苯二甲酸二甲酯的主要原料，对二甲苯的需求量巨大且逐年增加。

出于降低石油依赖程度的考虑，开发甲苯甲醇烷基化直接合成对二甲苯的技术工艺引起了国内外学术和工业界的极大兴趣。

迄今为止，甲氧基碳正离子机理[1]被广泛地认为是该反应的反应机理，后Ann M.V os[2]等通过在丝光沸石上建立模型计算，验证了甲苯和甲醇分子共吸附的协同反应机理。

如何提高产物的对位选择性一直是研究的热点，虽然目前对此还没有一致的看法，但总的看来，分子筛的孔道孔口大小和外表面的酸性分布是影响对位选择性的主要因素。

邹薇等[3]通过镧、镁氧化物对ZSM-5分子筛的复合改性提出孔径效应比酸性分布对催化剂的对位选择性影响更大。

Vu Van Dung等[4]通过在ZSM-5分子筛表面二次生长Silicalite-1分子筛来钝化外表面酸性中心，极大地提高了产物的对位选择性，为研究该反应提供了新的思路。

本文考察了硅铝比和硼改性对ZSM-5分子筛催化性能的影响，发现高硅铝比有利于提高对二甲苯的选择性，硼改性可以显著提高对二甲苯的选择性；同时对比ZSM-5在不同温度和原料空速下的反应结果，发现350℃～400℃，空速大于0.5适合作为该反应的条件。

1．实验部分采用水热法合成了不同铝含量的HZSM-5分子筛，ICP测试表明分子筛的硅铝比(分子比)分别为38、57、80、120、291。

以硅铝比为38的ZSM-5分子筛为基础，采用浸渍法进行硼改性。

反应在固定床微型反应器上进行。

催化剂0.5g, 20～40目；原料甲苯甲醇摩尔比等于2，空速等于2；氢气作载气，流量为200ml/min；反应区温度300～480℃，常压下进行。

甲苯甲醇甲基化制高浓度对二甲苯技术甲苯甲醇烷基化合成对二甲苯是增产对二甲苯的一条新的工艺路线，为甲苯转化和廉价甲醇利用提供了新的途径。

20世纪70年代以来，国内外相继开展以Y分子筛和ZSM-5分子筛催化剂为基础的甲苯选择性烷基化合成研究，特别是对ZSM-5分子筛硅铝比、晶粒大小、Pt，Mg，Sb/碱(土)金属改性及P，Si，B等元素改性和水蒸气处理等对催化剂结构、酸性与催化性能之间的关联进行了大量研究。

以Mobil公司采用分子筛硅铝摩尔比为450、970℃蒸汽处理45min的P/HZSM-5催化剂为例，在反应温度600℃、反应压力0.28MPa、WHSV4h-1、n(甲苯)/n(甲醇)/n(水)/n(氢)=2/1/6/6的工艺条件下进行甲基化反应，甲醇转化率为97.8%，甲苯转化率为28.4%，PX选择性为96.8%。

反应中不生成苯，副产物很少，主要是C5以下烃类，其质量分数不到1%。

该工艺目前尚未有工业化报道，其关键在于稳定性好、寿命长的工业催化剂研究开发及技术经济性是否具有优势两大问题。

最近印度石油化工公司(IPCC)和GTC公司联合报道了所开发的GT-T o-lAlkSM甲苯甲醇烷基化工艺技术的新进展，并对200kt/aPX生产装置的技术经济性进行了评价。

甲苯烷基化采用固定床反应器和专有的高硅沸石催化剂，在反应温度400-450℃、反应压力0.1-0.5MPa、甲苯与甲醇质量比为1.35/1条件下，PX选择性达到85%以上，催化剂操作周期6-12月，该技术的主要特点：可把所有的重整甲苯直接送至甲苯烷基化单元，与低成本的甲醇共同作为原料生产高浓度PX的芳烃，二甲苯馏分可通过低成本的简单结晶单元，有效回收PX，得到高纯度PX，结晶分离单元建设投资比传统吸附分离单元低得多。

此外，副产物苯可忽略不计。

每生产1tPX只需耗用1t甲苯(而甲苯选择性歧化工艺中，生产1t PX需耗约2.5t甲苯，副产苯多，B与PX质量比为1.36-1.60)。