Synthesis,characteristics of hierarchical EU-1 zeolite for xylene isomerization probe reaction☆

Xiaofeng Li*,Pengchao Ren Yanting Zhang Xiaozhen Liu Xiaotao Sun Meng Gao Miaojuan Jia Zhiping LüTao Dou 2,*

1 Research Institute of Special Chemicals,Taiyuan University of Technology,Taiyuan 030024,China

2 CNPC Key Laboratory of Catalysis,College of Chemical Engineering,China University of Petroleum,Beijing 102249,China

1.Introduction

The EU-1 zeolite is high-silicon zeolite with EUO topological structure,which possesses one-dimensional channel system with 10-membered-ring(MR)openings(0.58×0.41 nm)running along the direction connected to 12-MR side pockets in the(001)direction[1–5].Due to its peculiar structure,the EU-1 zeolite exhibits the excellent activity and high selectivity in the xylene isomerization reaction[6–8].Meanwhile,the diffusion of macro-molecule in EU-1 catalyst is restricted because of its one-dimensional channel system[9].So,reducing the limitation of diffusion in catalytic reaction(promoting mass transfer efficiency)is an important way to improve the catalytic performance of EU-1 zeolites[10–12].

The restriction of diffusion in the catalytic reaction course can be overcome by introducing mesopores or big pores to EU-1 zeolite catalyst structure[13–16].The mesopore channel of the hierarchic alzeolites allows macro-molecule to enter the inside of the catalyst,which reduces residence time of reactants and products[17–18].At the same time,more activity sites are exposed,and the material use ratio is improved as the surface area increases[14,19].The micro pores of hierarchical zeolites can provide active sites and shape selective catalysis[20].Thus the EU-1 zeolites can possess excellent catalytic performance so as to hierarchical structure[10].

There are several methods currently of synthesizing hierarchical EU-1 zeolite:hard template synthesis[21],soft template synthesis[22–23]and post-processing method[24].Mohamed H.M.Ahmeda et al.[10]fabricated the hierarchical EU-1 zeolite by sequential treatment with alkaline treatment followed by acid treatment.In recent years,synthesizing hierarchical zeolite with organosilane as a template is concerned widely[12,25–26].However,synthesizing hierarchical EU-1 zeolite with organosilane directly is not reported.

In this paper,the hierarchical EU-1 zeolites were synthesized with organosilanes as softtemplate,through aging processes of the zeolite synthesis gel and functioning the zeolite seeds,and the hierarchical EU-1 zeolites were fabricated by hydrothermal crystallization.Moreover,this paper also explores the influence of the following three organosilanes on synthesis of the EU-1:γ-glycidoxy propyl trimethoxy silane(GPTMS),N-β-(aminoethyl)-γ-aminopropyl methyl dimethoxyl silane(APAEDMS),and N-(β-aminoethyl)-γ-aminopropyl dimethoxyl(ethyoxyl)silane(TMPED).At the same time,the catalytic activities of hierarchical EU-1 zeolites were evaluated in the xylene isomerization reaction.

2.Experience

2.1.Materials

Deionized water(self-made);sodium meta-aluminate(Al2O3:41 wt%,Sinopharm Chemical Reagent Co.,Ltd);sodium hydroxide(99 wt%,AR,Tianjin Kermel Chemical Reagent Co.,Ltd.);silica sol(SiO2:25 wt%,Qingdao Haiyang Chemical Co.,Ltd.);hexamethonium bromide(99.5%,self-made);GPTMS(≥99%,Dongying Hengyi Chemical Co.,Ltd.);APAEDMS(≥99%,Dongying Hengyi Chemical Co.,Ltd);TMPED(≥99%,Dongying Hengyi Chemical Co.,Ltd);m-xylene(AR,Tianjin Guangfu Fine Chemical Research Institute);ethylbenzene(AR,Sinopharm Chemical Reagent Co.,Ltd)(see Fig.1).

2.2.Preparation of EU-1 zeolites

The hierarchical EU-1 zeolite was prepared by the hydrothermal method with organosilanes as additive.Its initial gel was mixed with sodium hydroxide,deionized water,hexamethonium bromide,silica sol and sodium metaaluminate,and the initial gel molar ratio was SiO2:0.022Al2O3:0.11Na2O:0.3HMBr2:20H2O.The synthesis procedures were as follows:0.45 g sodium hydroxide was dissolved in 18 g deionized water under string atroomtemperature,then 0.28 g sodium metaaluminate was added as a source of aluminum followed by adding 5.5 g hexamethonium bromide into caustic soda solution as an organic structure directing agent.After the solution became limpid,12 g ludox was dropped into the solution as a source of silica.The mixture was stirred for 2 h.The solution was aged at 90°C for 24 h in a rotational dryer.Then,the zeolite precursor was functionalized by reacting with GPTMS,APAEDMS and TMPED(3 mol%in regard to the silica content in the gel)at 90°C for 6 h,then the zeolite precursor-sat crystallized at180°Cfor48 h in a rotationaloven underthe self-generated pressure;the product was separated by filtering,and then washed with deionized water,dried at 120°C in air overnight,the samples obtained by adding different organosilanes were named EU-G,EU-A and EU-T respectively.The sample synthesized in terms of the above steps but without organosilanes was named as EU-N.

The productwas calcinated at550°Cfor 6 h to remove template,and then by band extrusion,ammonia exchange,platinum dipping and calcination,the catalyst samples were obtained to evaluate the catalytic performance in xylene isomerization.

Fig.1.The structure formulas of organosilanes including GPTMS,APAEDMS and TMPED.

2.3.Characterization

X-ray powder diffraction(XRD):RigakuD/max-2500 X-ray diffractometer;radiation source:CuKα;tube voltage:40 kV;tube current:30 mA;scan rate:8(°)·min?1.Scanning electron microscope(SEM):JEOL/JSM-6700F,made in Japan.Brunauer Emmett Teller(BET):Using Micromeritics/ASAP2000 autophysical absorption meter to measure nitrogen adsorption,desorption isothermalline,specific surface area,pore volume and pore size distribution.FT-IR(Nicolet 360 Fourier infrared spectrometer)analysis was used to characterize the framework vibration of EU-1 zeolites,at a resolution of 2 cm?1and a cumulative frequency of 16 times.The fixed sample was mixed with KBr by a mass ratio of 1:160,then ground and sheeted.Ammonia-temperature programmed desorption(NH3-TPD):TP-5076 adsorption instrument made by Xianquan Industrial and Trading Co.Ltd.;carrier gas:N2;adsorber:ammonia gas;sample granular size:40–60 meshes;heating rate:20 °C·min?1.

2.4.Catalytic test

The xylene isomerization reaction was conducted on a fixed-bed reactor(inner diameter was 8 mm,external diameter was 20 mm)to evaluate the catalytic performance of EU-1 zeolites.The amount of catalyst filling was 1 g.Ethyl benzene and m-xylene(molar ratio was 3:17)were fed by H2(P=0.5 MPa)at a weight hourly space velocity(WHSV)of 4.5 h?1.The carrier H2gas flow rate was 70 ml·min?1.Temperature of the column was maintained at 400°C(activation temperature)for 2 h,and then decreased to 360°C(reaction temperature).The reactant's charging rate was 0.09 ml·min?1.Sampling period was 3 h,and the product was analyzed by a liquid chromatograph(HXSP GC-950)equipped with a flame ionization detector(FID),Agilent capillary-column(60 m × 0.320 mm × 0.25 μm).Abandon the first sample and average the latter three samples as catalytic results.

3.Results and Discussion

3.1.Structure characteristics of hierarchical porous EU-1 zeolites

3.1.1.XRD representation

Fig.2 shows the XRD patterns of EU-1 zeolite samples synthesized with different organosilanes or absent of organosilanes.The diffraction lines of all samples exhibit the characteristic diffraction peaks that occurred at 2θ of 7.9°,8.8°,19.0°,20.5°,22.1°,23.2°,23.9°,25.9°,26.5°and 27.3°,which correspond to the literature report[27].No diffraction peaks of the impurity phase were observed in Fig.2.The crystallinity of samples was analyzed using XRD software.The sharp diffraction peaks indicate that EU-1 zeolites synthesized with different organosilanes possess good crystallinity.

Table 1 shows the relative crystallinity of EU-1 zeolites synthesized with differentorganosilanes.Itis known from Table 1 that EU-1 zeolites by adding silane coupling agents have good crystallinity,and their relative crystallinity is greater than that of the zeolites without organosilanes added.The order of the crystallinity is as follows:EUA＞EU-G＞EU-T＞EU-N.This demonstrates that the proper dosage of organosilane has no adverse impact on the crystallization of EU-1 zeolites.

Fig.2.XRD patterns of EU-1 zeolites synthesized with different organosilanes.

3.1.2.FT-IR characterization

The FT-IR spectra were used to characterize the framework structure and silanol groups of EU-1 zeolites.It is known from Fig.3 that the absorption bands are observed at 471,571,790,976,1058–1138,1215,1492,1640,2359,2936,3427 and 3740 cm?1.The EU-1 zeolites synthesized with different organosilanes have the same infrared framework vibration peak,which is in accordance with the case reported in literatures[3,28].The absorption bands at 471 cm?1,790 cm?1,1058–1138 cm?1and 1215 cm?1are due to T–O bond bending.It is the same with the internal symmetrical stretching vibration at790 cm?1,internal asymmetric stretch vibration at～1058 cm?1,and external asymmetric stretch vibration at 1215 cm?1,which correspond to the siliceous materials.The characteristic band of isolated silanol groups is at 3427 cm?1.The band at 3453 cm?1corresponds to Al–OH framework(Br?nsted acid sites)[29].The characteristic band at 976 cm?1is due to silanol groups,and the band at 1640 cm?1is due to the bending vibration of water molecules that the zeolite samples adsorb from the air.The framework vibration band at571 cm?1presented the double five-ring EUO-type zeolites.The peak intensity at 571 cm?1can be used to determine the crystallinity extent of the EU-1 zeolite.It is observed that the intensity of characteristic vibration peaks of EU-G,EUA and EU-T is greater than that of EU-N con firmed by the XRD patterns shown in Fig.3.

Table 1 Relative crystallinity of different EU-1 zeolites

Fig.3.FT-IR spectra of different EU-1 zeolites.

3.1.3.SEM representation

The crystal morphology of EU-1 zeolites synthesized with different organosilanes is presented in Fig.4.It is seen from Fig.4(a,b)that the EU-N zeolite sample synthesized by the conventional approach mainly consists of relatively regular ellipsoidal crystal(which contains many of nanocrystals)at the size of 0.5–2 μm,presenting a wide range of particle size distribution.However,when the organosilanes were added into synthesis system,the crystal morphology of EU-1 zeolite changed greatly as shown in Fig.4(c–h),the nanocrystals were not stacking as tightly as EU-Nsample.As presented in Fig.4(c–f),the axiolitic particles were notobserved in EU-Gand EU-Azeolites,the zeolite crystalconsists of a large amount of nanocrystals at the size of 50–100 nm,the nanocrystals pile up together and fabricate new crystal morphology.As shown in Fig.4(g,h),the EU-Nzeolite stillhas partofellipsoidalcrystals,but smaller particle size contrast with conventional EU-1 zeolite.The nanocrystals at the size of 100–200 nm may be attributed to the crystallization of EU-1 zeolites which are disturbed by organosilanes.The hydrophobic layer was formed by organosilanes on the EU-1 crystal surface and affected the grain growth of EU-1 zeolite as well as the aggregation of nanocrystals.In all of the samples synthesized with organosilanes added,nanocrystals tend to aggregate in disorder.Obviously,compared with the conventional EU-1 zeolite synthesized without organosilanes as additive,the EU-1 zeolites fabricated by nanocrystals possess more holes and larger specific surface area by loosely stacking in disorder.The hierarchical EU-1 zeolites can be successfully synthesized in this way.This practice is also supported by the N2adsorption–desorption data in Table 2.

It is also noted that part of the pores in EU-1 zeolites are blocked and active sites are covered in some extent,due to the unordered aggregation of nanocrystals and fusion of adjacent grains in EU-G,EUA and EU-T zeolites,but the hierarchical EU-1 zeolites still have great advantages compared with EU-N zeolites[30].

Fig.4.SEM images of different EU-1 zeolites.(a,b):EU-N;(c,d):EU-G;(e,f):EU-A;(g,h):EU-T.

3.1.4.N2adsorption–desorption analysis

Fig.5 shows the N2adsorption–desorption isotherms and pore size distribution of EU-1 zeolites.As shown in Fig.5(a),all of the EU-1 zeolite samples presentthe IV-type isotherm with the hysteresis loop,when relative pressure p/p0≥0.9,adsorption curve rise markedly,and when p/p0≤0.5,the desorption curve declines rapidly and overlaps with adsorption curve ultimately.This indicating that the mesopores exist in the zeolite samples of EU-N,EU-G,EU-A and EU-T.Compared with conventional zeolite sample of EU-N,the other samples show that the hysteresis loop shifts towards a high pressure area,and the sample EUA presents the most distinctive skew.This demonstrates that pore diameter of zeolite becomes larger when organosilanes were added.Fig.5(b)shows the pore size distribution of all samples,compared with the reference sample EU-N,the mesopore volume of EU-G,EU-A and EU-T becomes larger,pore size distribution becomes more extensive.This phenomenon can be explained by that the organosilanes make the grain size smaller significantly and incompact aggregation,the mesopore emerges because of the formation of piled pore.Thus,micropores and mesopores coexist in EU-1 zeolites,rather than micropores dominating the pore size distribution of conventional EU-1 zeolite.

Table 2 Specific surface areas and pore volumes of different EU-1 zeolites

Fig.5.N2 adsorption–desorption isotherms and pore size distribution of different EU-1 zeolites.

Table 2 shows the N2adsorption–desorption detailed data of EU-1 zeolites.It is found from Table 2 that the total specific surface area,total pore volume and mesopore volume of EU-T,EU-A and EU-G increase significantly compared with conventional EU-1 zeolites.The order of sample's total pore volume and mesopore volume is EUA＞EU-G＞EU-T＞EU-N.The EU-G external surface area increases by 53.6%,and the mesopore volume by 69.1%;EU-A external surface area increased by 62.1%and the mesopore volume by 129.1%;EU-T external surface area increased by 38.7%,and the mesopore volume by 54.5%.Both micropore specific surface area and volume revealed a trend of decrease.This is by reason of the formation of mesopore sacrifice a part of micropore when organosilanes were added into the synthetic system.The increasing of surface area can make more active sites exposed,and the mesopore can reduce the diffusion restrictions in reaction[14].So,it can promote catalytic performance of xylene isomerization.

Through analyzing the SEM images and nitrogen adsorption data,it is observed that organosilanes of APAEDMS have advantages in synthesizing hierarchical EU-1,as the sample of EU-A possesses more mesopore volume and larger external surface area.Compared with GPTMS and TMPED,APAEDMS lacks one methoxyl,methoxyl can generate silanol when hydrolysis in water.The condensation reaction among organosilanes becomes weaker,so that the pores will not be blocked severely,and more mesopores are available.

3.1.5.NH3-TPD analysis

The corresponding TPD pro files of the EU-1 zeolites are displayed in Fig.6.As shown in this figure,two desorption peaks could be observed around at 215 °C and 400 °C.The desorption peak at low temperature corresponds to the weak acid center,while the desorption peak(near 400°C)at the high temperature area corresponds to the strong acid center.The NH3-TPD calculation is shown in Table 3.In comparison to the EU-Nzeolite,the amount of weak acid of(low-temperature peak)of EU-G,EU-A and EU-T all increased,and the amount of weak acid of EU-G,EU-A and EU-T tallies better with their BET surface area.The increasing of the amount of weak acid is because organosilanes can reduce the grain size of EU-1 zeolite,therefore improve external surface area and expose more acid sites.We also found that the amount of strong acid(high-temperature area)reduced when using organosilanes as additive.

Fig.6.NH3-TPD patterns of different EU-1 zeolites.

3.2.Formation mechanism of hierarchical EU-1 zeolites

The formation mechanism of hierarchical EU-1 zeolites is shown in Fig 7.When organosilanes are added into the synthetic system of EU-1 zeolites,the morphologies of EU-1 zeolites are changed remarkably and hierarchical EU-1 zeolites are obtained.Conventional EU-1 zeolites consist of irregular ellipsoida crystals at a size of 0.5–2 μm,and the hierarchical EU-1 zeolites contain a large amount of nanocrystals.The nanocrystal is only～100 nm in size.The organosilanes play a crucial role in the crystallization of hierarchical EU-1 zeolites.How do the organosilanes affect the morphology of EU-1 zeolites in crystallization process?After an emulsion mixture of EU-1 was first aged at 90°C for 24 h,the organosilanes were added into zeolite precursors.When the organosilanes are added into gel system,the–SiOCH3of organosilanes hydrolyzes and produces silanol,and every organosilane molecule has more than one–SiOCH3.The agents become amphipathic molecules,where its long chain presents hydrophobicity and its silanol presents hydrophillia.Silanol with Si–O(on the surface of zeolite precursor)is condensated into Si–O–Si covalent bonds and the organosilanes are anchored on surfaces of zeolite precursor[12].The silanolof organosilanes on zeolite surface is further reacted with each other to form Si–O–Si covalent bond,then forming reticulation cover eolite surface.This hydrophobic net structure inhibits the crystal nucleus growing.Adjacent zeolite precursors can also be connected by the Si–O–Si linkages as every organosilane has more than one silanol,and changes the state of grain agglomeration[15].

Table 3 The acidity of different EU-1 zeolites

Fig.7.Formation mechanism of hierarchical EU-1 zeolites.

The organosilanes exist not only on the crystal surface of EU-1 zeolites but also located at the inner of EU-1 zeolites,forming another type of intercrystalline mesopores.After calcination,the EU-1 zeolites with hierarchical pores come into being.

Figs.4 and 5 show that different organosilanes have different influences on crystallinity and texturalproperties ofEU-1 zeolites.Compared with GPTMS and TMPED,APAEDMS lacks one methoxyl,which means lack of one silanol after hydrolysis.The ability of connecting adjacentzeolite precursors by the Si–O–Silinkages is weakened.Therefore the fusion of EU-A zeolite nanocrystals decreases,and the EU-A zeolites have greater external surface areas and more mesopore volume.

3.3.Catalytic performance of hierarchical EU-1 zeolites

Table 4 shows the catalytic performance data of hierarchical EU-1 zeolites.It is found from the table that the yield coefficient of p-xylene(PX)for EU-N zeolites is 16.30%,isomerization activity is 23.83%and the C8aromatic hydrocarbon selectivity is 75.16%.After the organosilanes are added,the PX yield coefficient of EU-G,EU-A and EU-T increases to 18.11%,18.41%and 18.08%respectively,and their isomerization activity all approaches to 24%(the theoretical thermodynamic equilibrium value);their C8aromatic hydrocarbon selectivities are 83.81%,84.87%and 83.05%respectively.Compared with EU-N zeolite,the isomerization activity and C8aromatic hydrocarbonselectivity of hierarchicalEU-1 zeolite are improved remarkably.The excellent catalytic performance of EU-G,EU-A and EU-T due to the hierarchical structure of EU-1 zeolites exhibits a larger external surface area and a bigger mesopore volume.Existence of mesopore promotes the mass transfer efficiency,and shortens the diffusion route in reaction.At the same time,the aggregation of nanocrystals can expose more active sites and possess excellent shape selective catalysis capacity.The catalytic performance of EU-1 zeolites is improved ultimately.

Table 4 Catalytic performance of different EU-1 zeolites

As the strong acid amount decreases as well as mesopores exist,the side reaction is reduced remarkably for the xylene generating heavy aromatics by dimolecular disproportionation and alkyl shift,and the C8aromatic hydrocarbon selectivity increases obviously.

4.Conclusions

Different organosilanes are employed to synthesize hierarchical EU-1 zeolites,and the samples are characterized by XRD,XRF,FT-IR,N2adsorption,HR-SEM and NH3-TPD.Compared with the conventional EU-1 zeolites synthesized without organosilanes,the EU-1 zeolites synthesized with organosilanes added possess smaller grain size,larger specific surface area and mesopore volume,forming a hierarchical pore structure consisting of micropores and mesopores simultaneously.

Different organosilanes have different influences on the synthesis of EU-1 zeolite.The EU-A with APAEDMS added is the best one,with a particle size of 50–100 nm and a uniform size distribution.The external surface area of EU-A increased by 62.1%,and the mesopore volume increases by 129.1%.The diffusion limitation decreases greatly in isomerization reaction.

The hierarchical EU-1 zeolites are used for the xylene isomerization reaction.The catalytic data show that the isomerization activity(PX/X)of hierarchicalEU-1 zeolites comes to around 24.09%in theoretical thermodynamic equilibrium from 23.83%,and the selectivity of C8aromatic hydrocarbon is up from 75.16%to 84.87%.Both activity and selectivity are improved.The organosilanes can promote the catalytic performance of EU-1 zeolites.

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