Promoting Xylene Production in Benzene Methylation using Hierarchically Porous ZSM-5 Derived from a Modified Dry-gel Route☆

WeiDeng,Xuan He,Chao Zhang,YunyiGao,Xuedong Zhu,Kake Zhu*,,Qisheng Huo,Zhijie Zhou3College of chemical engineering,East China University of Science and Technology,Shanghai0037,China

2State Key Laboratory of Inorganic Synthesis and Preparative Chemistry,College of Chemistry,Jilin University,Changchun 130012,China

3Key Laboratory of Coal Gasification,Ministry of Education,East China University of Science and Technology,Shanghai200237,China

Promoting Xylene Production in Benzene Methylation using Hierarchically Porous ZSM-5 Derived from a Modified Dry-gel Route☆

WeiDeng1,Xuan He1,Chao Zhang1,YunyiGao1,Xuedong Zhu1,Kake Zhu*,1,Qisheng Huo2,Zhijie Zhou31College of chemical engineering,East China University of Science and Technology,Shanghai200237,China

2State Key Laboratory of Inorganic Synthesis and Preparative Chemistry,College of Chemistry,Jilin University,Changchun 130012,China

3Key Laboratory of Coal Gasification,Ministry of Education,East China University of Science and Technology,Shanghai200237,China

A R T I C L E I N F o

Article history:

Received 24 December 2013

Received in revised form 21 February 2014

Accepted 13 April2014

Available on line 18 June 2014

Hierarchicalzeolite

Benzene

Methanol

Methylation

ZSM-5

Process intensification

Methylation of benzene is an alternative low-costroute to produce xylenes,but selectivity to xylene remains low over conventional zeolitic catalysts.In this work,a combined dry-gel-conversion and steam-assisted crystallization method is used to synthesize hierarchically porous zeolite ZSM-5 with varied Si/Almalar ratios. X-ray diffraction(XRD),N2physisorption,NH3-temperature programmed desorption(TPD),scanning electronic microscopic(SEM)measurement and Fourier transform infrared(FT-IR)are employed to characterize the structure and acidity of both hierarchically porous zeolites and their conventional counterparts.The method is found to be applicable to ZSM-5 with molar ratios of Si/Al from 20 to 180.The ZSM-5 zeolites are used as catalysts for benzene methylation at460°C to investigate the effect of additional porosity and Si/Al ratios.At low Si/Al ratios, the benzene conversions over conventional and hierarchical ZSM-5 are close,and selectivity to toluene is high over hierarchical ZSM-5.It is found that hierarchical porosity markedly enhances the utility of zeolite and the selectivity towards xylenes via improved mass transport at higher Si/Al ratios.Under an optimized hierarchical ZSM-5 catalyst,xylene selectivity reaches 34.9%at a Si/Al ratio of 180.

?2014 The Chemical Industry and Engineering Society of China,and Chemical Industry Press.Allrights reserved.

1.Introduction

Xylenes are important feedstock for the industrial production of terephthalic acid,isophthalic acid,dimethyl terephthalate,polyester resins,and the synthesis of vitamins or other pharmaceutical products. Nowadays,mostxylenes are mass produced in industrialscale via either naphtha catalytic reforming or toluene disproportionation[1].Separation of xylene isomers and the ethylbenzene mixture co-product is expensive due to their close boiling points.In the established toluene disproportionation process,a large amount of benzene is produced simultaneously,which was previously added to gasoline as co-fuels. Nonetheless,governments are imposing stringent guidelines to reduce benzene content in gasoline,with the awareness of its toxicity to public health and our ecological system.An alternative route to produce xylenes is through the methylation of toluene or benzene,and zeolites are the preferred solid acidic catalyst for their shape-selectivity[1-9]. For toluene methylation,since the pioneering work by Yashima et al. [2]over various cation modified Y-zeolite(FAU)catalysts in the 1970s, Kaeding and co-workers have optimized the modification methods of ZSM-5(MFIstructure)with a wide range of modifiers(Mg,P,Si,etc.)to maximize the p-xylene yields[3,4,11-16].Later on,other zeolites,such as ZSM-11(MEL),MCM-22(MWW)[17],ITQ-2(MWW)[17],Mordenite (MOR)[18,19],β-(BEA)[20],SAPO-34(CHA)[18],SAPO-11/MnAPO-11 (AEL)[6,18],and SAPO-5/MnAPO-5(AFI)[8,18,21]have been extensively explored for catalyzing methylation of toluene.In mostcases,valuable p-xylene is the major product because p-xylene diffusivity in ZSM-5 channels is 3 orders of magnitude faster than m-or o-xylene,which endows desirable product selectivity[3,22].On the other hand,there are only sporadic reports on benzene methylation using methanol compared with that of toluene;a complicated product spectrum with high selectivity to toluene and low selectivity towards xylenes is normally observed[5-10,23,24].It is demanding to design a single intensified process to convert low-cost benzene and methanol through consecutive methylation to value added xylenes,for which the current selectivity to xylene remains low(≤20%)[5,23].

As the crystalsize of mostzeolites is in the range of micrometers,and molecular transport within microporous channels is solely governed by configurational diffusion[25].One side effect of using zeolitic catalysts is the restriction of diffusion that limits the utility of zeolites or accessibility to and from the internalactive centers.In the pastdecade or so,it has been recognized that creating additional porosity to zeolitic crystals ordecreasing crystalsize can alleviate the diffusion limitations and thereby promote the catalytic performance of zeolitic catalysts[25-34].Thezeolite with atleast two types of porosity is called a hierarchically porous zeolite(HPZ),which normally contains both meso-(2-50 nm)and micro-porosity in one crystalline material.The merits of using HPZs with respectto their conventional analogues include high activity,high catalytic lifetime,higher activity towards bulky molecules,as well as improved stability,that has recently been highlighted in a recent review by Egeblad etal.[35].The product distribution can also be altered due to the different shape or size of their controlled mass transportinside zeolitic architectures[26,35].In the case of toluene methylation,however, there are controversial conclusions.For instance,Jahn et al.reported no size effect of ZSM-5 to production distribution for toluene methylation [36],whereas Wang and co-workers[13]showed the merits of using nanosized ZSM-5 by promoting activity(conv.4%)in both toluene disproportionation and methylation.Ding and co-workers[37]have observed a jump of10%in conversion when ZSM-5 after steaming treatment was used to catalyze the methylation of toluene;however,the zeolitic size effect is not clearly shown as steaming affects both porosity and acid strength.From early studies by Christensen et al.[26],it has been observed that over HPZ ZSM-5,benzene alkylation with ethylene to ethylbenzene is enhanced in terms of benzene conversion and ethyl benzene selectivity.The rate limiting process for the reaction over conventional ZSM-5 is intracrystalline diffusion of ethylbenzene,because the ethylbenzene molecule is larger in size than other reactants.On the other hand,the rate limiting process on HPZ ZSM-5 is shifted to the intrinsic reaction controlled process.These findings inspire us to find the effect of product distribution by HPZ,but no report on crystalsize effect for benzene methylation has been known thus far.

From the point of view of mass transport,it was once believed that the shape-selectivity stems from the intracrystal diffusion of products and thereby a larger crystal size enhances selectivity[38].However, recent transport studies show that benzene,toluene and xylene(BTX) transport depends on crystal sizes.Lercher and co-workers[39-41] found that the rate-controlling step in mass transport is switched from intracrystal diffusion to the surface process as the primary sizes of ZSM-5 are reduced down to below 100 nm,as a result of small values(≤10-6)for the BTX sticking probability.As small molecules (i.e.,CH3OH)access pore apertures via both sinusoidal/straight channels and BTXenters mainly via straight channels,traffic-controlcan also play a role in mass transport[42].Keeping in mind that the majority of the conventionalZSM-5 crystalis covered by sinusoidal channels as a result of subunit inter growth,only traces of straight channels are exposed to external surfaces[43].Moreover,the pore mismatch in boundaries of inter growth subunits can incur a more intricate effect to mass transport in conventional ZSM-5[43].From a kinetic angle of recent studies by Svelle et al.[5],it is evident that as the apparent reaction rate is first order with respect to benzene partialpressure,improving BTXtransport is expected to influence the concentration of BTXin the vicinity of active sites,and thereby the kinetic process.Although the reaction rate is zeroth order with respect to CH3OH pressure,excessive CH3OH adsorbed strongly on acidic sites can block the aperture to channels and lead to slow mass transport[5].Changing the Si/Al ratio will adjust the concentration of pre-adsorbed CH3OH.Inspired with the above knowledge,one can expect that both additional porosity and Si/Al ratio can influence the catalytic performance to benzene methylation[38].

Herein,we adopt H-ZSM-5,a widely employed archetypalzeolitic catalyst to explore the effect of hierarchical porosity and Si/Alratio on the catalytic performance for the methylation of benzene.As the methylation of benzene is also an important step to understand methanol to hydrocarbon(MTH)[5]or methane methylation with benzene[7,8],these results can also be fundamentally important to these relevant reactions.We demonstrate here that HPZ ZSM-5 enhances both benzene methylation to toluene and its consecutive methylation to xylene at high Si/Al ratios.HPZ ZSM-5 is thereby a catalyst that intensifies the benzene methylation selectivity to xylene, which constitutes another merit of using hierarchical zeolites to consecutive reactions.

2.Experimental

2.1.Materials and syntheses

2.1.1.Materials

Tetraethyl orthosilicate(TEOS,99%)and tetrapropylammonium hydroxide[TPAOH,25%(by mass)aqueous solution]were obtained from Meyer Chemical,China.Ethanol(90%),aluminum tritertbutoxide(ATTB,99%)and ammonium hydroxide(28%-30%)solutions were purchased from Shanghai Lingfeng Chemicals,while hexadecyltrimethoxysilane(HTS)was obtained from Gelest,USA. Distilled H2O was used throughout this work.All commercial chemicals were used directly without further purifications.

2.1.2.Synthesis

HierarchicalZSM-5(denoted as ZSM-5-HTS-x,x is the molar ratio of Si/Al)was prepared via a modified approach developed by the author [44].In a typicalsynthesis of ZSM-5-HTS,3.12 g TEOS,0.21 g HTS and 0.035 g ATTB were dissolved in 20.0 ml ethanol under stirring.This mixture was further stirred for 30 min to form a solution,followed by the addition of another solution of2.0 g TPAOH in 5.0 g ethanol under vigorous stirring;hydrolysis occurred at this stage as evidenced by heat release,although a stable so-called“clear solution”is produced(the transparent“clear solution”is stable up to one month in a closed vial). The“clear solution”was stirred for 1 h before being transferred to a petri dish where the solvent was allowed to evaporate overnight.A transparent dry gel resulting from solvent evaporation was ground, then moved to a teflon cup.The cup was placed in a teflon-lined autoclave of150 ml,and 10.0 g of water is added outside the cup to create steam for the hydrothermal synthesis conditions.The autoclave was moved into an oven set at 180°C and kept there for 3 days for a steam-assisted crystallization.The final solid,retaining the shape of the original dry gel particles,was filtrated and washed with water, and then calcined at 550°C for 20 h by a ramp of 3°C·min-1in air.

To prepare a conventional ZSM-5 with the same Si/Alratio,3.24 g TEOS,0.07 g ATTB,2.0 g TPAOH and 7.7 ml ammonium hydroxide were mixed in 30.0 g DI-H2O and stirred for 4 h,then transferred to an autoclave and allowed to crystallize for 72 h at180°C[44].Combustion to remove the template was conducted using the same procedure as for ZSM-5-HTS.For both samples,the Si/Almolarratio was controlled to be 20,40,70 and 180.

2.2.Characterizations

X-ray diffraction patterns of powders were carried out on a Rigaku D/max 2550 VB/PC(40 kV,100 mA)X-ray diffractometer operated with a monochromatized Cu Kαradiation(λ=0.15406 nm)enabled by nickel filtration of Cu Kβ.The scan rate was 8(°)·min-1,ranging from 5-50°.N2physisorption was employed to analyze the surface area and pore architecture of both powders.The specific surface areas of both conventional ZSM-5 and HPZ ZSM-5-HTS were deduced from the Langmuir method and the Brunauer-Emmett-Teller(BET)method, respectively,for the purpose of comparison by using a Micromeritics ASAP 2020 Accelerated Surface Area and Porosimetry System.A micropore volume is calculated from the standard t-plot approach,while the totalvolume pores were based on the adsorption capacity at a relative pressure of 0.99.The macro-plus meso-pore volume was referred to as the subtraction of the micropore volume from the totalpore volume. Mesopore size distribution was calculated from a Barrett-Joyner-Halenda(BJH)model using the adsorption branch of an isotherm. Scanning electronic micrographic(SEM)images were taken from a Hitachi S-4800(II)field-emission SEM apparatus(FESEM).Ammonia temperature programmed desorption(NH3-TPD)was measured using a PX200A type TPD/TPR instrument from Tianjin Pengxiang Tech. Corp.,with 0.10 g conventional ZSM-5 and 0.25 g ZSM-5-HTS sampled respectively.Before measurement,each sample was heated underultrapure He atmosphere at a flow rate of30 ml·min-1at500°C for 1 h to remove surface adsorbate,cooled to 150°C and saturated with 5% NH3-He before flushing with He to remove physisorbed NH3.The effluent of NH3-TPD was quantified by a flame ionization detector (FID)under a heating rate of10°C·min-1.Fourier transform infrared (FT-IR)spectra of all samples were recorded on a Nicolet 6700 FTIR spectrometer in KBr pellets.

2.3.Catalytic performance assessment

Catalytic reaction was carried out at0.1 MPa of pressure with a 2.5 g catalyst,which was squeezed into strips with sizes of 1-2 mm,in a stainless steel reaction tube of 10 mm ID and 600 mm length.The flow rate of N2which was used as carrier gas was 50 ml·min-1.Premixed solutions of methanol and benzene with a mole ratio 1:1 were introduced directly into the reactor at the rate of about 0.20 ml·min-1without being preheated.All the reactions were carried out at 460°C. Reaction products were analyzed by gas chromatography(Agilent 6820).The selectivity for reaction products S,the yield of reaction products Y,and the conversion of benzene X are defined as

where M is the mole number of benzene in the raw material,N is the mole number of the benzene series in the products,P is the mole number of toluene in the products and Q is the mole number of xylene in the products.B,T,and Xstand for benzene,toluene,xylene respectively,and TX is the sum of benzene and toluene.

3.Results and Discussions

3.1.SEMimages of dry geland XRD patterns for ZSM-5

Details of preparation to a ZSM-5 with a Si/Al ratio of 70 has been reported in a preceding report by one of the present authors,the structural characterizations of this specific sample were also included[44].It is noteworthy that the method merits an improved pore connectivity as evidenced by129Xe NMR measurements,and is thereby superior to alternative routes.Herein,we show the synthesis and characterization results for the prepared ZSM-5 with different Si/Alratios to validate the wide applicability of the modified method.Representative SEM images of the dry gel after ethanolevaporation for a Si/Alratio of 180 are disclosed in Fig.1.Atlow magnification,silica gelmade up of aggregated particles in the sample can be observed unanimously,showing that the dry gelis uniform in morphology[Fig.1(a)].When we zoom in,a more detailed structure is exhibited in Fig.1(b);the gelis found to consist of large numbers of tiny particulates ranging around 20 to 40 nmin diameter.The uniformity of these particles is visually observed and demonstrates that all particles are wellde fined in size by the presence of HTS.As the condensable silane head is hydrophilic and the alkyl tails are hydrophobic,the HTS acts as a surfactant during the solventevaporation process for the solid-gas interface.After hydrothermal treatment under steam-assisted crystallization(air itself being hydrophobic)and combustive removals of organic templates,ZSM-5 white powders are collected.The X-ray diffraction patterns of both conventional ZSM-5 and hierarchically porous ZSM-5-HTS are displayed in Fig.2.It is evident that the diffraction peaks are characteristic of reported MFI-structured materials of the same type(JCPDS No.43-0784),and no additional crystalline phase or amorphous silica is found.ZSM-5-HTS shows some broadening of the diffraction peaks and relatively weak intensity as compared to the conventionally prepared ZSM-5 catalyst, similar to observations also reported in nanozeolites or HPZ syntheses. This result demonstrates a decrease in the crystalline size of the primary zeolitic particles.Besides,when the Si/Alratio is 20,a perceivable broadening of peaks can also be identified.At lower Si/Alratios,the charge density of an organic structure-directing-agent(SDA)does not match so well in the absence of inorganic cations such as Na+.Such a mismatch can lead to the formation of defective sites or a decrease of particle size that causes the broadening of XRD peaks.

3.2.N2-physisorption isotherms

Fig.1.SEMimage ofa dry gelafter evaporation induced self-assembly with Si/Al molar ratio of180.

N2adsorption-desorption isotherms of ZSM-5 and ZSM-5-HTS are displayed in Fig.3 and Table 1.For conventionalZSM-5,the microporesare filled at a relative pressure of 0.1,as evidenced by the uptake of N2. For conventional ZSM-5 with Si/Al ratios of 20 and 40,no obvious hysteresis loop can be discerned from the adsorption-desorption isotherms,showing that there is no additional porosity on the conventionalZSM-5.However,for conventionalZSM-5 with Si/Alratios of70 and 180,a clear hysteresis loop at relative pressures between 0.1 and 0.2 are observed.This artifact hysteresis loop stems from a fluid-to-crystalline transition of liquid N2inside MFIstructured micropores,which is a unique phenomenon for MFItopology with high Si/Alratios[45,46].

Fig.2.XRD patterns ofconventional ZSM-5 and hierarchically porous ZSM-5-HTS with varied Si/Almolar ratios(20,40,70 and 180).

A t-plotanalysis of the adsorption atmicropore range gives pore volumes of ca.0.10 ml·g-1for both conventional and HPZ ZSM-5,evidencing the presence of micropores in these catalysts.For ZSM-5-HTS-20, the isothermbelongs to a transition between type II and type IV,according to IUPAC classifications.The large slope at a relative pressure between 0.01 and 0.6 indicates that multilayer adsorption dominates for ZSM-5-HTS-20,which is pertinent to the BET model.The large slope is consistent with the extraordinarily large BET surface area (567 m2·g-1)observed for this sample than the rest.As the micropores cannot accommodate multilayers,itis inferred that addition of HTS expands the external surface areas in dry-gel conversion.At a relative pressure of0.7 to 0.9 a small uptake of xliquid nitrogen can be observed, indicating the presence of a small number of mesopores with a broad pore size distribution.When the Si/Al ratio is increased above 40, ZSM-5-HTS,on the other hand,exhibits a typical isotherm of type IV, and an obvious jump of the N2uptake at a relative pressure of0.6 can be observed.The jump in both adsorption and desorption branches at a relative pressure＞0.6 is caused by the capillary condensation ofliquid N2inside mesopores,showing a large number of mesopores present in these ZSM-5-HTS materials.The possibility of a tensile-strength-effect (TSE)due to bottle-neck shaped pores can be excluded as the loop closes at a relative pressure above 0.6,while the TSE caused the forced closure of isotherms which happens at～0.45[46].As TSE is a result of confined mesopores by smallersized pores,the absence of TSE indicates that the mesopores are connected to the externalsurfaces.This is one important feature of the MFI structure derived from our preparation method,and the result is also corroborated by previous129Xe NMR measurements[44].The hysteresis loop can be categorized to a H3 type,implying that the additionalpores belong to slit-shaped ones.To estimate the pore size distribution in ZSM-5-HTS,the BJH model is employed from the desorption branch,which is shown in the insets of Fig.3.Rather broad pore-size-distributions are observed for ZSM-5-HTS with a Si/Alratio above 40.The sum of microporous and mesoporous volumes is estimated by subtracting the micropore volume derived from thet-plotfromthe over all adsorption volume at a relative pressure of0.99.

Data shown in Table 1 also demonstrate that the surface area and pore volume are almost independent of the Si/Al ratio for samples with a Si/Alratio above 40.As the BET surface area is more pertinent to the mesopores that can accommodate multi-layers of N2,a prominent increase in BET surface is detected for HPZ samples.These measurements clearly manifest that with the inclusion of HTS to the dry gel system,MFI structured zeolite with varied Si/Al ratios above 20 can be produced with hierarchical porosity.

3.3.SEM characterization

The microscopic images measured by SEMare unraveled in Fig.4. For Si/Al molar ratio of20,conventional ZSM-5 are made up of uniform aggregates of3-5μm that are seen allover the sample.The surface of conventional ZSM-5 appears corrugated,and the particles appear tobe aggregates of tiny primary particulates ranging from 50 to 300 nm. Those primary particulates appear to be coffin-like in shape,which is typical for the MFI structure,but the sizes are atypically small.These observations are in line with XRD results showing that peaks for Si/Al (molar ratio)=20 are broadened substantially.The HPZ ZSM-5 with a Si/Al ratio of 20 possesses even smaller aggregates ranging from 200 to 400 nm.The aggregates are also made up of even smaller primary particulates of 80 to 150 nm.The primary particulates in HPZ ZSM-5 [Si/Al(molar ratio)=20]are irregular in shape with respect to those for the HPZ counterpart.For Si/Al(molarratio)samples,the charge density of inorganic gel and SDA matches well and a typical coffin-like morphology is observed for conventional ZSM-5.These particles are 2 to 3μm wide and 5 to 8μm long,with clear edges and corners that indicates perfect crystallinity.The HPZ ZSM-5 with a Si/Al ratio of 180 looks close to the previously reported ZSM-5 with a Si/Al ratio of 70 [44],and aggregates of200 to 400 nm are unanimously seen in the samples.Close investigation of the surface shows that they are rough and made of very fine primary particles of20 to 40 nm.Between the primary particles,voids or inter particular spaces are found,which constitute the macro-or meso-pores that are measurable by N2physisorptions.

Fig.3.N2physisorption isotherms for ZSM-5 and ZSM-5-HTS with varied Si/Alratios,the corresponding pore-size-distribution patterns for ZSM-5-HTS samples are shown in the insets.

Table 1 Surface area and pore structure analyses from N2physisorption

3.4.Infrared spectra

FT-IR spectroscopy was also employed to identify the crystal phase, as depicted in Fig.5.Contributions from ubiquitous H2O can be found from the stretching bands of OH from 760 to 1660 cm-1.All samples display diagnostic ZSM-5 absorption bands at 1225,1093(strong), 550(media)and 450 cm-1(strong)[47].The featured vibration band at 550 cm-1had been assigned to the five-membered rings of T-O-T (T=Sior Al)in MFI-type zeolites,strongly indicating that all the frameworks are made up of pentasil walls,in line with XRD designations[48, 49].The OH stretch vibrations normally appear in the range of3000-3800 cm-1.The broad and shallow ones at～3500 cm-1are generally ascribed to silanol nests that consist of a number of silanol groups interacting through hydrogen bonding[50].The bands appear at 3675 cm-1,corresponding to an overall contribution from external free silanols,internal silanols and bridging hydroxyls[50].More silanol groups as a result of the existence of silanol groups in different environments are found for conventional ZSM-5 with Si/Alratios of20 and 40 [50].The evidences that there are more defective sites of surface silanolsat lower Si/Al ratios are also confirmed by XRD measurements or SEMobservations.These defective sites are actually healed by the addition of HTS as lacking of such bands for HPZ ZSM-5 with similar compositions.

Fig.4.SEMimage of conventional ZSM-5(a,c,d)and hierarchically porous(b,e,f)ZSM-5-HTS with Si/Alratios of20(a,b)and 180(c-f).

Fig.5.FT-IR spectra ofconventionaland HPZ ZSM-5 with varied Si/Almolar ratios.

3.5.NH3-TPD measurements

NH3-TPDtechnique is employed to identify the strength of acidity in ZSM-5 and ZSM-5-HTS,and the profiles are displayed in Fig.6.NH3,as a typical probe molecule to measure the strength of acidic sites,can penetrate both micropores and larger mesopores,and therefore is used to give an overview of the acidic strength.NH3-TPD patterns can be categorized into high temperature(＞350°C)and low temperature (＜350°C)ranges.It has been corroborated that weak acidity sites from＜350°C are from physisorbed ammonia on surface silanol groups, that are unimportantto catalysis.Peaks that desorb at＞350°C are from NH4+cations on the sites,and can be attributed to strong Br?nsted acidic sites,which are catalytically active sites[51,52].For conventional ZSM-5-20,a relatively large contribution from desorption of NH4+fromsilanols is detected.The number of weak silanols decreases progressively with respect to a Si/Alratio increase.The results show that there are more silanols for low Si/Alratios,which is also observed in IR measurements.The high temperature desorption of NH4+from Br?nsted acidic sites are also found in all conventional ZSM-5 samples,with desorption temperatures ranging from 400 to 500°C.The small shift of desorption temperature for NH3is tentatively ascribed to the existence of defective silanols,as also observed via XRD,IR or SEM characterizations.For ZSM-5-180,the high temperature desorption is hardly perceivable for the low Alcontent and related low desorbed NH3content.For ZSM-5-HTS, two distinct peaks can be discerned at 209 to 250 and 350 to 450°C as well.The low temperature desorption patterns are similar to the conventionalZSM-5.The profiles athigh temperature ranges are almostsuperimposable,indicating very close acid strength for these ZSM-5-HTS samples.In the synthetic protocol,we have employed TPAOH as structure directing agents,which can cause Al zoning in the MFIcrystal. This can complex the general trends in acidity with respect to Si/Al ratios.In the case of HPZ MFIcrystal,the presence of HTS can inhibit the migration of Al in the crystal and heal the defective sites(in accordance with IR observations),and hence generates an obviously almost identicalacidity when the Si/Alratio is above 10.

Fig.6.NH3-TPD patterns of conventional ZSM-5(a)and HPZ ZSM-5-HTS(b).

3.6.Catalytic performance

The catalytic test results of benzene methylation using ZSM-5 with varied Si/Al ratios are presented in Fig.7.Toluene(T)and xylene(X)are found to be the predominantproducts after methylation, other byproducts such as ethyl benzene,trimethylbenzene or methylethylbenzene are also detected in small amounts(sel.%=7%-12%). According to a previously reported reaction mechanism and the fact that ZSM-5 is also a catalyst for methanol to olefin(MTO)conversion [5,23,24],a possible reaction pathway is illustrated in Schemes 1 and 2.The reaction is initiated on Br?nsted acid sites where protons on zeolites(Z?H⊕)attack methanol to generate surface Z?CH3⊕and H2O. Z?CH3⊕then attacks benzene to form protonated toluene.Deprotonation produces toluene as the primary product and regenerate the Br?nsted site to its originalstate(Fig.8).Toluene can be methylated to xylene or trimethylbenzenes via a similar mechanism after sequential methylations,as suggested by the reaction network in Fig.9.At levated temperatures,it is proven that contribution from methanol to hydrocarbons will occur to afford ethylene and some methylated benzenes[7]. The presence of ethylene incurs benzene alkylation to ethylbenzene, which can further be methylated to methylethylbenzenes.

Fig.7.The catalytic performance ofconventional(a)and ZSM-5-HTS(b)ZSM-5 with varied Si/Almolar ratios in benzene methylation.Space velocity=2.0 h-1,temperature=460°C.

Fig.8.Reaction mechanism of benzene methylation over ZSM-5.

Fig.7 shows benzene conversion on ZSM-5 zeolites with different Si/ Alratios.For conventional ZSM-5,the conversion reduces from 41.0% (molar ratio of Si/Alis 20)to 38.3%(molar ratio of Si/Alis 40),before increasing to 43.1%(molarratio of Si/Alis 70),and the conversion of35.9%is reached at a Si/Alratio of180.A slow and continuous increase in toluene selectivity from 60.1%to 69.2%is observed with the increase of the Si/Al ratio for conventionalZSM-5,butthe selectivity towards xylene decreases from 28.3%(molar ratio of Si/Alis 20)to 23.2%at a Si/Al ratio of180.The above results are close to previously reported values[5,23].For HPZ ZSM-5,on the other hand,conversion of benzene increases from 41.4%(molar ratio of Si/Alis 20)to 49.9%(molar ratio of Si/Alis 180)with a Si/Al ratio increase.Simultaneously,selectivity to toluene exhibits a monotonical declination from 72.9%to 52.9%.At the same time,selectivity to secondary methylation xylene increases from 38.5%to 43.8%.

For both HPZ and conventional ZSM-5,the total selectivity of TX (toluene+xylene)is around 90%.The increase in conversion and TX selectivity for HPZ ZSM-5 suggests that trimethylbenzene selectivity is still restricted as a result of shape selectivity imposed by the micropores of MFI structure.The optimum TXyield of xylene is 34.9%for HPZ with a Si/Al ratio of180,whereas the maximum TX yield is 25.6%for conventional ZSM-5 with a Si/Al ratio of 70.The effect of acidic strength can be negligible because the acid strength for all HPZ ZSM-5-HTS samples is close or slightly weaker than conventionalZSM-5(NH3-TPD),whereas they over perform in the catalytic tests.The overall carbon balance is estimated to be above 96%.It is important to note that the catalyst is currently running in an industrialside-track experiment,and remains stable for up to a test of 2000 h.The high conversion of benzene and the high selectivity to xylene over HPZ ZSM-5 can be explained by the positive effects of additional porosity.

To understand the effect of additional porosity to the product distribution,it is important to take into consideration the role of mass transport of related molecules.For conventional ZSM-5,according to the Thiele theorem[25,53],for a reaction of an n th order,the Thiele modulusφcan be expressed usingφ=with R being the typical particle size,k the intrinsic reaction rate constant,D being the effective diffusivity,and C the reactant concentration at the particle surface.The relationship betweenφand effectiveness factorηis given by

from which one knows that whenφ＜1,η=1;and whenφ＞10,

The effectiveness factor denotes the utility of active sites in a porous catalytic materialsuch as zeolite.At Si/Al=20,the primary particulates that build up the aggregates of conventional or HPZ ZSM-5 crystals are close in size,i.e.,with similar R values.The effect of zeolite utility in terms ofηis notso obvious,and the benzene conversions for conventional and HPZ ZSM-5 are close.Besides,the low Si/Al ratio also increases the Al content and associated acid density in micropores,and more CH3OHis adsorbed on acidic sites which can aggravate mass transport even in HPZ ZSM-5.Both factors contribute to the close benzene conversion when Si/Al(molar ratio)=20.As the Si/Alratio increases, R for HPZ remains small and for conventionalZSM-5 it appears larger, and the effectiveness factor governs the conversion and selectivity of benzene in conventional ZSM-5.From the above speculations,one can expect that it is the length of the diffusion path in zeolitic micropores that determines the conversion/selectivity.For HPZ ZSM-5,the intracrystal diffusion is no longer the rate-limiting process of the mass transport process.According to recent mass transport studies,benzene, toluene or xylene transport in HPZ or nanosized ZSM-5 is dictated by a surface barrier that is associated with sticking probability,whereas in conventional micron-sized ZSM-5 it is restricted by intra crystalline diffusion [39,40].As sticking probability of benzene,toluene,and xylene follows the order:B＞T＞X;itis likely that reactants such as benzene and toluene are more kinetically favorable to enter HPZ ZSM-5 than xylenes[39,40]. This surface barrier controlled mass transport,in turn,means that more benzene/toluene can be converted on HPZ than xylene to trimethylbenzenes.Interestingly,this effectiveness factor also reflects an increase in primary producttoluene methylation to xylene,and the corresponding increase in xylene selectivity is detected simultaneously.A quantified description of the observed selectivity is still lacking thus far, due to the unknown basic parameters to describe the reaction kinetic data.But the trend is obviously manifested that using HPZ zeolite not only enhances the methylation of benzene to toluene,but also the toluene to xylene methylation.The acceleration of two consecutive reactions constitutes a merit to use HPZ zeolite for process intensification purposes.

Fig.9.Reaction network of benzene methylation and related products.

4.Conclusions

A series of MFI structured ZSM-5 with varied Si/Alratios have been successfully prepared by extrapolating a combined dry-gelconversion and steam-assisted crystallization method.When employed as catalyst for the methylation of benzene,ata Si/Al ratio of20,the primary particulates of conventionalZSM-5 and HPZ ZSM-5 are both small after drygel conversion,and only toluene selectivity is improved.As Si/Al ratio increases,it is found that selectivity towards toluene increases with increasing Si/Al ratios in conventional ZSM-5,and xylene selectivity does not exhibit remarkable change.On HPZ ZSM-5,selectivity to toluene decreases with increasing Si/Alratios,and a simultaneous increase in benzene conversion and xylene selectivity is observed.The presence of additionalporosity not only accelerates benzene methylation,but also contributes to the faster sequentialmethylation ofprimary product toluene to xylenes.The selectivity to multi-methylated benzene or methylated ethylbenzene is notnotably affected by the presence ofhierarchical pores.A high selectivity of 34.9%to xylene by benzene methylation is achieved by a HPZ with a Si/Alratio of180,in contrastto 25.6%for conventional ZSM-5.

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☆Supported by the National Natural Science Foundation of China(21006024),the CNPC Innovation Foundation(2011D-5006-0507),the Shanghai Pujiang Program(11PJ1402600), the New Century Excellent Talents in University(NCET-11-0644),and the Fundamental Research Funds for the Central Universities(WB1213004-1).

*Corresponding author.

E-mailaddress:kakezhu@ecust.edu.cn(K.Zhu).