Qingtao Wang,Wenwen Han,Hualei Hu,Jinghui Lyu,Xiaolong Xu,Qunfeng Zhang,Hongjing Wang,Xiaonian Li*

Institute of Industrial Catalysis of Zhejiang University of Technology,State Key Laboratory Breeding Base of Green Chemistry Synthesis Technology,Hangzhou 310032,China

1.Introduction

Toluene and xylene(TX)are important raw materials for commodity production in chemical industry[1].Traditionally,they are produced from catalytic reforming or naphtha pyrolysis;however,the process is now limited by the shortage of petroleum resources[2].Compared with toluene and xylene,benzene has been produced in excess[3].Therefore,the conversion of the surplus benzene to the more valuable toluene and xylene is a potential way to balance the demand for aromatic production.Currently,alkylation of benzene is receiving increased attentions in petrochemical industry duo to its possible nature as the promising alternative for the synthesis of toluene and xylene from natural gas and coal industry[4].

Alkylation of benzene with methanol,as a well-known acid catalyzed reaction,can produces a mixture of aromatics and the selectivity for productions highly depends on the characteristics of the catalyst.ZSM-5 zeolite catalysts were believed to be optimal catalysts for alkylation reaction because of their high thermal stability,tunable acidity and remarkable shape-selectivity within the uniform micropores[5,6].However,the intrinsic micropores structures in conventional ZSM-5 inhibit the diffusion of bulky substrates,which resulted in low conversion of benzene and low selectively of xylene in the alkylation of benzene[7].Hierarchical ZSM-5 zeolite,which possess the porosity and channelstructure with the coexistence ofmicropore and mesopore,can observably improve the diffusion property and exhibit excellent catalytic performance in different reactions[8,9].Recently,Lu et al.[10]synthesized hierarchical ZSM-5 catalyst using solid-phase hydrothermal method and found that the hierarchical ZSM-5 showed an enhanced performance compared with conventional ZSM-5:the conversion ofbenzene increased by 8%,the selectivity oftoluene and xylene increased by 5%and 10%respectively.Hu et al.[11]pointed out that the excellent catalytic activity and high xylene selectivity was mainly due to the presence of mesopores,which promoted the diffusion of the reactants and products and provided easier access to the active sites in micropores.Catalytic tests in the alkylation of benzene with methanol show that selectivity of TX is improved over hierarchical ZSM-5,which is potentially important for the selective formation of TX chemicals in the future.However,the traditional template method for synthesizing hierarchical ZSM-5 zeolite is considered to be expensive and environmental unfriendly which would limit its commercial application on a large scale[12,13].Fortunately,it has been demonstrated that post-treatment of zeolites is a facile and cost-effective route to obtain hierarchical materials[14,15].

Recently,a wide variety of post-treatment ways for hierarchically porous structures have been reported,exemplified by the desilication and dealumination of MCM-22,ZSM-5,beta,mordenite,etc.in relevant basic and acidic conditions[16,17].Ogura etal.[18]pointed outthatalkalitreatmentby sodium hydroxide solution created mesopores withoutchanging the microporous structures of zeolite,and resulted in an enhanced catalytic activity for cumene cracking.Groen et al.[19]reported the detailed investigation on the formation of mesopores in ZSM-5 crystals by NaOH treatment,they found that mesoporosity was originated from selective removalofframework Siand the framework aluminum played an important role in regulating mesopore formation.According to the pioneering work on controlled desilication of ZSM-5 zeolites,a specific threshold of the framework Si/Al ratio has been proven available,being in the range of25-50,in which the desilicated ZSM-5 zeolites exhibited optimalstructure properties,especially for the hierarchical pore development[20,21].The desilication has been proven to be an effective method to generate mesoporosity.However,some extra-framework aluminum species are often observed under the base treatment due to the realumination[17].Therefore,an additionalacid treatmentstep is needed to remove the species for opening the micropores and mesopores.On the other hand,dealumination is also a well-known process to modify pore structure which has been widely applied in industry[22,23].Many researchers[24,25]declared that mesoporosity was generated from the loss of aluminum from the zeolite framework.Steaming and acid leaching are facile and the most common methods for dealumination.Steaming is generally performed at high temperature above 500°C in the presence of steam,mesopores established under these circumstances are mostly connected to the surface of the crystals by bottle-necks[26].During the treatment extra-framework aluminum species(EFAl)are generally formed,which may cause a negative effect,so acid-leaching is always employed to remove the undesired amorphous species,and secondary mesoporosity might be created[27].

The above findings inspired us to investigate the effect of posttreatment method on the catalytic performance of ZSM-5 zeolite for the alkylation of benzene with methanol.Compared with conventional ones,this study focused on post-synthesis modification with treatmentin alkaline and/oracid media to the zeolite crystals.The structural changes and the alteration in the catalytic performance have been thoroughly examined.The catalysts were characterized by various techniques(including ICP-AES,XRD,nitrogen sorption isotherms,SEM,NH3-TPD,Py-IR and TG).We observed that the hierarchical structure was well formed in ZSM-5 zeolite by means of post-treatment and the modified ZSM-5 zeolite showed an excellent catalytic performance in the alkylation of benzene with methanol.

2.Experimental

2.1.Catalyst preparation

A commercial ZSM-5 with Si/Al ratio of 42(supplied by Nankai Chemical Plant,NH4-form)was used in this study.HZSM-5 catalyst was prepared as follows:the zeolite powder was tableted,crushed and screened to 0.45-0.90 mm successively,then calcined in air at 550°C for 7 h to obtain HZSM-5,denoted as ZSM-5-P.

2.1.1.Alkaline treatment

Alkali-treatment of the ZSM-5 zeolite was carried out based on method described in literatures[19,28].In brief,appropriate amount of parent NH4-ZSM-5 was suspended in 0.5 mol·L-1NaOH solution at 353 K for 2 h under stirring and re fluxing to produce a mixture.Then the mixture was cooled down immediately,followed by washing with deionized water.Finally,the as-prepared zeolite samples were dried at 383 K overnight,followed by three consecutive exchanges in 0.5 mol·L-1NH4Cl solution and calcined in air at 823 K for 7 h to produce ZSM-5-AT.

2.1.2.Acid treatment

Acid treatment of the zeolites was performed in a re flux-attached round bottom flask,containing 10 ml of oxalic acid solution and 1 g of the zeolite solid,stirring at 363 K for 2 h.Then,the solid was filtered and washed with distilled water repeatedly until complete elimination of the acid.Lastly,the sample was dried at 383 K and calcined at 823 K for 7 h to obtain the pretreated zeolite,labeled as ZSM-5-OT.Zeolites treated by successive alkaline and acid was denoted as ZSM-5-AT-OT.

2.2.Catalyst characterization

The morphology of the catalyst was observed by using a scanning electron microscopy(SEM)(Hitachi S-4700(II)).The crystallography of the catalysts was characterized by X-ray diffraction(XRD)(SCINTAG X″TRA),using CuKα(154.2 nm)at 30 mA and 40 kV with scanning angle(2θ)from 5°to 80°.The nitrogen sorption isotherms were measured by using a Micromeritics ASAP-2020 system.Chemical Al and Si analyses of the materials were obtained by inductively coupled plasma atomic emission spectroscopy(ICP-AES)using a Winlab Optima 3300 DV ICP spectrometer from Perkin-Elmer.NH3temperatureprogrammed desorption(TPD)of the catalysts was carried out using a Tianjin XQ TP-5076 chemisorption instrument equipped with a thermal conductivity detector(TCD).Fourier transform infrared(FT-IR)spectra were measured on a VERTEX 70 FT-IR spectrometer.The concentrations of Br?nsted acid(B)and Lewis acid(L)were calculated from the peak areas of adsorbed pyridine at around 1545-1and 1454 cm-1,and the extinction coefficients of ε(B)and ε(L)were 1.88 and 1.42 cm·mmol-1,respectively[29].

2.3.Catalytic performances evaluation

All the catalytic performance evaluations were carried out in a continuous- flow fixed-bed reactor with a stainless steel tube(8-mm i.d.)at atmospheric pressure.0.5 g catalyst diluted with 5.0 g inert quartz sand was loaded in the reactor.The catalystwas in-situ heated from room temperature to 400 °C at 10 °C·min-1and maintained for 1 h in N2flow with a space velocity of 2400 h-1.Benzene and methanol mixture(B/M molar ratio=1:1)was then fed into the reactor(WHSV=2.0 h-1)with a co-feed N2flow of 120 ml·min-1.The product was directly analyzed online by the gas chromatography(Fuli GC9790)equipped with a DB-1 capillary column(30 m×0.25 mm×1.00 μm)and a flame ionization detector.

Conversion of benzene and selectivity to toluene,xylene and ethyl benzene were defined as follows respectively:

3.Results and Discussion

3.1.Catalyst characterization

Fig.1.SEM images of parent and treated zeolites:a.ZSM-5-P b.ZSM-5-OT c.ZSM-5-AT d.ZSM-5-OT-AT.

Representative SEM micrographs for the parent and the treated ZSM-5 zeolites are shown in Fig.1.As observed,the ZSM-5-P sample is typical twin hexahedrons with smooth morphological surface and highly ordered structures,demonstrating the high crystallinity of the parentzeolites(Fig.1a).The ZSM-5-OT maintains the cubic morphology after acid treatment except sporadic defects(Fig.1b).In contrast,the morphology of the zeolite was obviously changed after alkali treatment.Many faults and voids on the surface of the zeolite can be found.A portion of the framework has collapsed,suggesting that desilication happened under the alkaline conditions(Fig.1c).As for ZSM-5-AT-OT(Fig.1d),irregular solid particles and amorphous debris which deposited on external surface almost disappeared upon the sequential acid leaching treatment.V.Mavrodinova et al.[30]suggest that extra-lattice can be expelled from the zeolite structure upon the acid re flux which could improve the accessibility to the intrinsic micropores in the zeolite.

Fig.2.XRD patterns of HZSM-5 zeolites before and after treatment.

XRDcharacterization was performed to determine the possible changes in the relative crystallinity and the framework structure of ZSM-5 with the modification(Fig.2).As indicated by the intensity of the peaks in the XRD patterns,all samples exhibit the same peaks,which are exclusively corresponded to the structure of MFI topology[31].It can be concluded that the alkali treatment or/and the followed oxalic acid leaching did not destroy the structure of the ZSM-5 zeolite.However,the crystallizations of the samples changed.Due to the formation of the amorphous phase,the crystallinity of ZSM-5-AT decreased[32].This indicates the desilication happened under the alkali treatment.While upon acid washing the crystallinity increased slightly(ZSM-5-OT),suggests that dealuminization in the zeolite framework is negligible.According to Camblor et al.[33],the enhance intensity of these re flections can be due to the removalofEFAlfrom lattice positions.Particularly,owing to the removalofthe amorphous phase,the crystallizations ofZSM-5-AT-OT were recovered which is consistent with the observation from SEM images.

The elemental analysis results decided by ICP-AES are listed in Table 1.The Si/Al ratio of ZSM-5(P)zeolite is 42.5(the nominal Si/Al ratio is 42),which increased to 44.9 after a moderate acid treatment(ZSM-5-OT).The similarity in the Si/Al ratio suggests that dealuminization in the zeolite framework is negligible during the acid leaching post-treatment,which is in agreementwith the XRDcharacterization and scanning electron microscopy measurement.While for the alkali-treated samples(ZSM-5-AT),a distinct decline of the Si/Al ratio is presented.Such variation demonstrates that the process of alkali treatment affects the environment of the framework and leads to the dissolution of the siliceous species,which agrees with prior studies[8,9].However,the acid treatment performed upon desilication sample(ZSM-5-AT-OT)leads to a more significantly change of the Si/Al ratio(from 29.1 to 87.4),indicating that a part of aluminum species has been removed.On the basis of previous study[17,24],some extraframe work aluminum species are generally produced simultaneouslywith the removal of Si atoms(Si-OH-Si)from the ZSM-5 lattice.Combined with these results,we can attribute the increase in the Si/Al ratio to be consequences of the removal of the amorphous phase,which formed during the alkali treatment.The result will be discussed in detail together with the following physicochemical analysis.

Table 1 The different treatment methods and the characteristics of the resulting zeolites

Fig.3 displays the N2adsorption isotherms and pore size distributions of parent and treated ZSM-5 zeolites.As observed,the parent zeolites gave a typical Langmuir type I nitrogen adsorption isotherm,which represented a microporous material.According to isotherms of ZSM-5-OT,the adsorption isotherm did not change dramatically which indicates negligible leaching of framework Al at the conditions applied here[34].Comparatively,the isothermal curves of ZSM-5-AT and ZSM-5-AT-OT zeolites exhibited the behavior of type IV with a type-H3 hysteresis loop,which was typically associated with the filling and emptying of mesopores[35],suggesting these two samples possessed mesopore structures.Comparing the isothermal curves in Fig.3,the sequent treatments(alkaline plus acid leaching)over the ZSM-5 samples lead to a progressive increase of the N2adsorbed amount at high relative pressures,which was indicative of the additional mesoporosity.The results are consistent with SEM micrographs that more defects emerged upon the sequent acid treatments.The textural parameters derived from N2adsorption were listed in Table 2.After the alkali treatment,the total pore volume of the sample increased from 0.19 to 0.33 cm3·g-1,while the micropore volume slightly decreased from 0.11 to 0.09 cm3·g-1.It indicates that the alkali treatment has created mesopore system in ZSM-5-P sample,while the micropore systems did not dramatically affected.As to the sample of ZSM-5-AT-OT,the specific surface area and pore volumes increased further,which as consequence of the removal of the extra-framework species formed during the desilication treatment.In brief,the results presented above show that hierarchical zeolites have formed during post-treatments.

Fig.3.Nitrogen adsorption-desorption isotherms of ZSM-5 zeolites.

Table 2The porosity properties for ZSM-5 zeolites before and after treatment

As mentioned above,the zeolites structure has changed during the post-treatment.The acidity of zeolites is an important factor in determining the catalytic activity and product selectivity for alkylation reaction.The acid site properties were studied using NH3-TPD and the pro files are displayed in Fig.4.As well known the amounts of ammonia desorbed from the catalyst surface could be estimated via TPD peak areas and the acid strength attributes to the peak temperature.It is observed that different degrees of change in the number and strength of acid sites occurred for the treated samples when compared to the untreated ZSM-5(ZSM-5-P).Table 3 summarizes the acidic properties of these zeolites.As compared with the parent ZSM-5,the peak of desorption temperatures and the corresponding peak area of the ZSM-5-OT sample changed very little in Peak I and Peak II,indicating that only small changes in the acid properties occurred after the oxalic acid treatment steps.On the other hand,a noticeable decrease of the amount of acid sites is produced by the alkali treatment that attributes to desilicification.According to previous study[36],these phenomena are owing to the selectively leaching of siliceous species during alkalitreatment,the leaching of the framework resulting in a loss of acid sites.Furthermore,the spectrum of sample ZSM-5-AT-OT,prepared by alkali treatment subsequent with acid washing,exhibits minimal peak area and the lowest temperature ammonia desorption peak with respect to the other samples which indicative of minimal amount of acid sites and the weakest acid strength.According to literatures,the function of acid leaching for the zeolite is the removal of extraframework species[23,27].Therefore,this result demonstrates that the extraframework species which generated by alkali treatment is removed by the subsequent oxalic acid washing procedure.This result is in agreement with the proposed mechanism for alkali desilicification[19,20],in which the destruction of silanol groups(Si-O-Si bonds)could lead to the presence of extraframework species.From the research above,we conclude that alkali and/or acid treatment does not only improve the pore accessibility(according to the nitrogen adsorption analysis)but also further adjust the acid properties of ZSM-5 zeolites.

Fig.4.NH3-TPD pro files of parent and modified ZSM-5.

Table 3 The deconvolution results of NH3-TPD pro files of the parent and modified ZSM-5

Fig.5.FT-IR spectra of pyridine adsorption on parent and modified ZSM-5 catalysts.

Further acidity characterization was done by FTIR spectroscopy of adsorbed pyridine.As shown in Fig.5,the absorption peak at 1454 cm-1is assigned to the C-C stretching of coordinately bonded pyridine on the Lewis acid sites while the peak at 1545 cm-1is attributed to the C-C stretching vibration of pyridinium ion by interaction with Br?nsted acid sites.The amount of both Br?nsted and Lewis acid sites calculated with respect to the integrated area of the absorption bands is given in Table 4.It is obvious that the quantity of Br?nsted acid sites in the NaOH treated zeolites ZSM-5-AT is much lower than parent samples,which owing to the removal of a few Al atoms(Si-OH-Al)that occurs simultaneously with the preferential removal of Si atoms(Si-OH-Si)from the HZSM-5 framework[36,37].Based on the previous research[38],the alkali treatment inevitably produced partially extra-framework Al species,which was associated with the Lewis acidity.Hence,the presence of EFAl species that resulted from alkali treatment should be responsible of the enhancement of Lewis acid sites.As for ZSM-5-AT-OT samples,sequential acid leaching treatment has no remarkable effect on the amount of Br?nsted acid sites over ZSM-5-AT[26].However,the intensity of the Lewis acid decreased because of the removal of the amorphous phase(SiO2and Al2O3)in the acid washing step[24].This is consistent with the observation from N2physical adsorption and SEM images.

Table 4 Br?nsted acid sites and Lewis acid sites of zeolites by IR spectra of absorbed pyridine

3.2.Catalytic performance

The parent and treated ZSM-5 samples were evaluated in alkylation of benzene with methanol,the catalytic performance and product distribution are listed in Table 5.As data indicated,methanol was not observed in the products,which could lead us to the conclusion that all catalysts showed efficient performance of the methanol conversion.Meanwhile,alkene and alkane were observed in the reaction products which indicated that the side reaction of methanol to ole fins had occurred,which was attributed to the effect of strong acid sites on ZSM-5[35].In the MTO process,methanol is first dehydrated to dimethyl ether(DME)and then converted to light ole fins,which can further react to form paraffin,aromatics,and oligomer through hydrogen transfer,alkylation,or polycondensation(Fig.6)[39].The key step in theconversion of methanol is to control the reaction during ole fin formation,where the acidity of the catalyst plays an essential role.According to the literatures,although the alkylation of benzene with methanol and the side reaction of methanol to ole fins were both catalyzed by the Br?nsted acid,acid strength was different.MTO process prefers stronger Br?nsted acid than alkylation reaction[17].Therefore,the Br?nsted acidity of ZSM-5 catalyst was closely related to the product distribution.Compared the performance of the three catalysts,the percentage composition of alkene and alkane was in the order ZSM-5-P＞ZSM-5-AT-OT,which implied that the amount of strong acid sites on the ZSM-5 was also in the order ZSM-5-P＞ZSM-5-AT-OT.The decreasing trend of alkene and alkane indicates the reduction of strong Br?nsted acid sites.The experiment alresults are in good agreement with Py-IRresults(Table 4 and Fig.5).

Table 5 Products content of benzene alkylation with methanol over different HZSM-5

Fig.6.Reaction path of methanol during the MTO process.

As shown in Fig.7,the treated ZSM-5 presented a much higher benzene conversion compared with the parent ZSM-5 catalyst,which was due to the channels in treated ZSM-5 catalyst that provide larger reaction volume for reactant to access active site.Furthermore,the ZSM-5-AT-OT showed a better activity than ZSM-5-AT,which was attributed to the effect of acid leaching resulting in more active sites exposed and the accessibility to the zeolite internal porosity improved.This is supported by the N2physical adsorption,both the pore volume and surface area of the samples increased after treatment.On the other hand,the selectivity of xylene is also enhanced,suggesting that the existence of mesopores could promote the diffusion of bulky aromatics in the reactions[7].In conclusion,the hierarchical pores ZSM-5 modified by alkali and the sequential acid treatment improved the catalytic activity and product selectivity for the alkylation of benzene with methanol.

Fig.7.Catalytic performance of different zeolites in benzene alkylation with methanol.

According to the literatures[40],alkylation of benzene with methanol and methanol to ole fins were both catalyzed by the Br?nsted acid,while ethyl benzene was formed by benzene alkylation with ethylene that was converted from methanol to ole fins over ZSM-5 catalyst.Moreover,several excellent reviews have proven that suppressing the side reaction of methanol would help to reduce the formation of ethyl benzene.As seen in Fig.7,the ethyl benzene selectivity of the alkylation of benzene with methanol over different ZSM-5 zeolites was compared.It was found that the selectivity to ethyl benzene was decreased over treated ZSM-5 catalysts(8.8%for ZSM-5-P and 0.9%for ZSM-5-AT-OT).This result suggested that the modified ZSM-5 zeolite could inhibit the side reaction of methanol to ole fins,thus improved the utilization rate of methanol in the alkylation reactions.This result was in accordance with the Py-IR results and previous researches[17,41].

Fig.8.Stability of different zeolite in benzene alkylation with methanol.

Fig.9.TG-DTA pro files of the catalysts after successive reaction time(24 h).

The long-term stability of the catalyst is vital for industrial application.Thus,it is necessary to investigate the effect of the modified catalysts on the stability for the alkylation of benzene with methanol,as shown in Fig.8.Compared to parent ZSM-5,the ZSM-5-AT-OT catalyst exhibited remarkably high resistance to deactivation with over 250 h times on stream.As pointed out by many researchers,accumulation of coke deposits blocks the micropores of the catalyst and physically restricts the access of reactants towards the catalytic centers causing rapid deactivation[42].Furthermore,alkenes are primary precursors of the mechanism of coke formation[43,44].Hence,the decrease of alkenes could reduce the coke deposit rate and prolong the lifetime of the catalyst.TG-DTA was performed to evaluate the coke content of the catalysts after 24 h usage(Fig.9).Depending on the analysis,a less amount of coke was observed over ZSM-5-AT-OT.As mentioned above,the modified ZSM-5 zeolite(ZSM-5-AT-OT)could decrease the acidity of the HZSM-5 zeolite and inhibit the side reaction of methanol to ole fins,thus decreased precursors ofthe mechanismofcoke formation.In addition,the ZSM-5-AT-OT catalyst possesses a larger pore volume and surface area to bear the coke than conventional microporous zeolite.Consequently,hierarchically structured zeolites synthesized by posttreatment are not only bene ficial in enhancing the catalytic performance,but also prolonged the catalytic lifetime.

4.Conclusions

In this work,we successfully prepared hierarchical ZSM-5 zeolite by combining alkali treatment with acid leaching under mild conditions.After the suitable treatment,not only the mesopores introduced into the structure of ZSM-5 zeolite by selective extraction of silicon,but also amorphous particles and crystalsplinters within the ZSM-5 zeolites were cleared.The hierarchical ZSM-5 zeolite presented appropriate properties in terms of acidity and accessibility for promoting the conversion of benzene,a higher selectivity to toluene and xylene as well as a higher anti-coking performance than conventional microporous zeolites in the alkylation of benzene with methanol was achieved.Most importantly,as compared to the template method to synthesize hierarchical ZSM-5 zeolite,the method adopted here was technically feasible and economically reasonable which would help to improve the commercial viability for the alkylation of benzene with methanol in industrial production.

[1]W.Vermeiren,J.P.Gilson,Impact of zeolites on the petroleum and petrochemical industry,Top.Catal.52(2009)1131-1161.

[2]C.D.Chang,C.Song,J.M.Garcés,Y.Sugi(Eds.),Shape Selective Catalysis,Am.Chem.Society,Washington,DC 2000,p.96.

[3]X.M.Wang,J.Xu,G.D.Qi,B.J.Li,C.Wang,F.Deng,Alkylation ofbenzene with methane over ZnZSM-5 zeolites studied with solid-state NMR spectroscopy,J.Phys.Chem.C 117(2013)4018-4023.

[4]H.L.Hu,J.H.Lv,J.Y.Rui,J.Cen,Q.F.Zhang,Q.T.Wang,W.W.Han,X.N.Li,The effect of Si/Al ratio on the catalytic performance of hierarchical porous ZSM-5 for catalyzing benzene alkylation with methanol,Catal.Sci.Technol.6(2016)2647-2652.

[5]M.O.Adebajo,M.A.Long,The contribution of the methanol-to-aromatics reaction to benzene methylation over ZSM-5 catalysts,Catal.Commun.4(2003)71-76.

[6]N.M.Tukur,S.Al-Khattaf,Comparison studies of xylene isomerization and disproportionation reactions between SSZ-33,TNU-9,mordenite and ZSM-5 zeolite catalysts,Chem.Eng.J.166(2011)348-357.

[7]C.H.Christensen,K.Johannsen,I.Schmidt,C.H.Christensen,Catalytic benzene alkylation over mesoporous zeolite single crystals:Improving activity and selectivity with a new family of porous materials,J.Am.Chem.Soc.125(2003)13370-13371.

[8]W.Deng,X.He,C.Zhang,Y.Y.Gao,X.D.Zhu,K.K.Zhu,Q.S.Huo,Z.J.Zhou,Promoting xylene production in benzene methylation using hierarchically porous ZSM-5 derived from a modified dry-gel route,Chin.J.Chem.Eng.22(2014)921-929.

[9]I.I.Ivanova,E.E.Knyazeva,ChemInform abstract:Micro-mesoporous materials obtained by zeolite recrystallization:Synthesis,characterization and catalytic applications,Chem.Soc.Rev.42(2013)3671-3688.

[10]L.Lu,H.Z.Zhang,X.D.Zhu,Synthesis of hierarchical ZSM-5 and its application in benzene alkylation with methanol,Pet.Process.Sect.1(2012)111-116.

[11]H.L.Hu,Q.F.Zhang,J.Cen,X.N.Li,High suppression ofthe formation of ethylbenzene in benzene alkylation with methanol over ZSM-5 catalyst modified by platinum,Catal.Commun.57(2014)129-133.

[12]Y.Wei,T.E.Parmentier,K.P.Jong,J.Ze?evi?,Tailoring and visualizing the pore architecture of hierarchical zeolites,Chem.Soc.Rev.44(2015)7234-7261.

[13]D.P.Serrano,J.Aguado,J.M.Escola,J.M.R.,ángel Peral,Hierarchical zeolites with enhanced textural and catalytic properties synthesized from organofunctionalized seeds,Chem.Mater.18(2006)2462-2464.

[14]K.Gao,S.Z.Li,W.Y.Wang,Study of the alkylation of benzene with methanol for the selective formation of toluene and xylene over Co3O4-La2O3/ZSM-5,RSC Adv.5(2015)45098-45105.

[15]J.Pérez-Ramírez,C.H.Christensen,K.Egeblad,C.H.Christensen,J.C.Groen,Hierarchical zeolites:Enhanced utilisation of microporous crystals in catalysis by advances in materials design,Chem.Soc.Rev.37(2008)2530-2542.

[16]W.Ding,Y.Cui,J.Li,Y.Yang,W.Fang,Promoting effect of dual modification of H-ZSM-5 catalyst by alkali treating and Mg doping on catalytic performances for alkylation of benzene with ethanol to ethylbenzene,RSC Adv.4(2014)50123-50129.

[17]M.D.González,Y.Cesteros,P.Salagre,Comparison of dealumination of zeolites beta,mordenite and ZSM-5 by treatment with acid under microwave irradiation,Microporous Mesoporous Mater.144(2011)162-170.

[18]M.Ogura,S.Shinomiya,J.Tateno,Y.Nara,M.Nomura,E.Kikuchi,M.Matsukata,Developments and structures of mesopores in alkaline-treated ZSM-5 zeolites,Appl.Catal.A 219(2001)33-43.

[19]J.C.Groen,J.A.Moulijn,J.Perez-Ramirez,Alkaline posttreatmentof MFI zeolites from accelerated screening to scale-up,Ind.Eng.Chem.Res.46(2007)4193-4201.

[20]J.C.Groen,J.A.Moulijn,J.Perez-Ramírez,Desilication:On the controlled generation of mesoporosity in MFI zeolites,J.Mater.Chem.16(2006)2121-2131.

[21]J.H.Ahn,R.Kolvenbach,C.Neudeck,S.S.Al-Khattaf,A.Jentys,J.A.Lercher,Tailoring mesoscopically structured H-ZSM-5 zeolites for toluene methylation,J.Catal.311(2014)271-280.

[22]S.Shu,S.Husain,J.Koros,Sonication-assisted dealumination of zeolite a with thionyl chloride,Ind.Eng.Chem.Res.46(2007)767-772.

[23]N.Viswanadham,M.Kumar,Effect of dealumination severity on the pore size distribution of mordenite,Microporous Mesoporous Mater.9(2006)231.

[24]S.M.Almutairi,B.Mezari,G.A.Filonenko,P.C.Magusin,M.S.Rigutto,E.A.Pidko,E.J.Hensen,In fluence of extraframework aluminum on the Br?nsted acidity and catalytic reactivity of Faujasite zeolite,ChemCatChem 5(2013)452-466.

[25]W.Chen,Y.Geng,K.H.Chung,Achievement of thick mesoporous TiO2crystalline films by one-step dip-coating approach,Microporous Mesoporous Mater.111(2008)544-550.

[26]R.Astala,S.M.Auerbach,The properties of methylene-and amine-substituted zeolites from first principles,J.Am.Chem.Soc.126(2004)1843-1848.

[27]Fan Yu,Xiaojun Bao,Xiuying Lin,Gang Shi,Haiyan Liu,Acidity adjustmentofHZSM-5 zeolites by dealumination and realumination with steaming and citric acid treatments,J.Phys.Chem.B 110(2006)15411-15416.

[28]B.M.Lin,Q.H.Zhang,Y.Wang,Fluoride-treated H-ZSM-5 as a highly selective and stable catalyst for the production of propylene from methyl halides,Ind.Eng.Chem.Res.48(2009)10788-10794.

[29]C.A.Emeis,Determination of integrated molar extinction coefficients for infrared absorption bands of pyridine adsorbed on solid acid catalysts,J.Catal.141(1993)347-354.

[30]R.M.Mihályi,M.Kollár,P.Király,Z.Karoly,V.Mavrodinova,Effect of extraframework Al formed by successive steaming and acid leaching of zeolite MCM-22 on its structure and catalytic performance,Appl.Catal.A 417-418(2012)76-86.

[31]Jinghui Lyu,Zhiyan Pan,Xiaonian Li,Benzene alkylation with methanol over phosphate modified hierarchical porous ZSM-5 with tailored acidity,Chin.J.Chem.Eng.25(9)(2017)1187-1194.

[32]L.L.Su,L.Liu,X.H.Bao,Creating mesopores in ZSM-5 zeolite by alkali treatment:Anew way to enhance the catalytic performance of methane dehydroaromatization on Mo/HZSM-5 catalysts,Catal.Lett.91(2003)3-4.

[33]M.A.Camblor,A.Corma,M.J.Dias-Cabanas,Synthesis and structural characterization of MWW type zeolite ITQ-1,the pure silica analog of MCM-22 and SSZ-25,J.Phys.Chem.B 102(1998)44-51.

[34]G.Ana,Gayubo,Ainhoa Alonso,Javier Bilbao,Selective production of ole fins from bioethanol on HZSM-5 zeolite catalysts treated with NaOH,Appl.Catal.B 97(2010)299-306.

[35]H.Teng,J.Wang,X.Q.Ren,D.M.Chen,Disproportionation of toluene by modified ZSM-5 zeolite catalysts with high shape-selectivity prepared using chemical liquid deposition with tetraethyl orthosilicate,Chin.J.Chem.Eng.19(2011)292-298.

[36]Y.Ni,A.Sun,X.Wu,Reparation of hierarchical mesoporous Zn/HZSM-5 catalyst and its application in MTG reaction,J.Nat.Gas Chem.20(2011)237-242.

[37]Y.Wei,E.Petra,L.M.Bonati,J.David,J.Glenn,Enhanced catalytic performance of zeolite ZSM-5 for conversion of methanol to dimethyl ether by combining alkaline treatment and partial activation,Appl.Catal.A 504(2015)211-219.

[38]U.Fleischer,W.Kutzelnigg,A.Bleiber,J.Sauer,Proton NMR chemical shift and intrinsic acidity of hydroxyl groups Ab initio calculations on catalytically active sites and gas-phase molecules,J.Am.Chem.Soc.50(1992)27-30.

[39]D.Chen,K.Moljord,A.Holmen,A methanol to ole fins review:Diffusion,coke formation and deactivation on SAPO type catalysts,Microporous Mesoporous Mater.164(2012)239-250.

[40]J.R.Anderson,T.Mole,V.Christov,Mechanism of some conversions over ZSM-5 catalyst,J.Catal.61(1980)477-484.

[41]H.L.Hu,J.H.Lyu,Q.T.Wang,Q.F.Zhang,J.Cen,X.N.Li,Alkylation of benzene with methanol over hierarchical porous ZSM-5:Synergy effects of hydrogen atmosphere and zinc modification,RSC Adv.5(2015)32679-32684.

[42]J.C.Zhang,H.B.Zhang,X.Y.Yang,Z.Huang,W.L.Cao,Study on the deactivation and regeneration of the ZSM-5 catalyst used in methanol to ole fins,J.Nat.Gas Chem.20(2011)266-270.

[43]F.Schmidt,C.Hoffmann,F.Giordanino,S.Bordiga,P.Simon,W.Carrillo-Cabrera,S.Kaskel,Coke location in microporous and hierarchical ZSM-5 and the impact on the MTH reaction,J.Catal.307(2013)238-245.

[44]Q.Qian,J.Ruiz-Martínez,M.Mokhtar,A.M.Asiri,H.Schulz,Single-catalyst particle spectroscopy of alcohol-to-ole fins conversions:Comparison between SAPO-34 and SSZ-13,Catal.Today 154(2010)183-194.