Transalkylation of Multi-secbutylbenzenes with Benzene over Hierarchical Beta Zeolite

Yingxia Li*,Can Luo2,Xuan WangChongpin HuangBiaohua Chen

Transalkylation of Multi-secbutylbenzenes with Benzene over Hierarchical Beta Zeolite

Yingxia Li1,*,Can Luo1,2,Xuan Wang1,Chongpin Huang1,Biaohua Chen1

1State Key Laboratory ofChemicalResource Engineering,Beijing University ofChemicalTechnology,Beijing 100029,China
2Changling Petrochemical engineering Design Co.Ltd.,Yueyang 414000,China

A R T I C L E I N F o

Article history:

Received 2 January 2014

Received in revised form 8 January 2014

Accepted 21 January 2014

Available online 19 June 2014

Hierarchicalzeolite

Multi-secbutylbenzene

Transalkylation

Mesopore

Nanoparticles

Reactivity

A hierarchical beta zeolite synthesized by quasi-solid phase conversion method was characterized by BET, scanning electron microscope(SEM),transmission electron microscope(TEM),X-ray diffraction(XRD), temperature-programmed desorption of ammonia(NH3-TPD),27Al and29Si magic angle spinning nuclear magnetic resonance(27Al and29Si MAS NMR),and its catalytic performance was compared with that of conventional microporous beta zeolite for liquid phase transalkylation of multi-secbutylbenzenes(MSBBs) with benzene.The results indicate that the hierarchical beta zeolite consists of nanosized crystals with a meso/ microporous structure and has stronger acid strength than the microporous beta zeolite.The higher conversion oftri-secbutylbenzene(TSBB)and selectivity of sec-butylbenzene(SBB)are achieved on hierarchical beta zeolite than microporous beta zeolite,while the conversion of di-secbutylbenzene(DSBB)is slightly higher.The improvement of catalytic performance over hierarchical beta zeolite can be ascribed to the presence of mesopores,nanosized crystals and stronger acidity.

?2014 The ChemicalIndustry and Engineering Society ofChina,and ChemicalIndustry Press.Allrights reserved.

1.Introduction

Sec-butylbenzene(SBB)is an important intermediate for the production ofphenoland methylethylketone[1].Currently the synthesis of SBB is carried out through the alkylation ofbenzene with butene over the zeolite catalysts[2,3].However,there is an abundant quantity ofmulti-secbutylbenzene(MSBB)as byproducts in this process[4].In order to improve the SBB yield,it is necessary to convert MSBBs to SBB by transalkylation reaction with benzene.

At present,the study oftransalkylation reactions is mainly focused on the transalkylation ofdi-isopropylbenzene with benzene and the transalkylation of di-ethylbenzene with benzene.Due to their high activity,selectivity,easy regeneration and long lifetime,severalzeolites, such asβ,Y and SAPO-5 zeolite[5-7],and amorphous silica-alumina [8]have been applied in the transalkylation reactions.Among these zeolites,beta zeolite has been proven to be suitable for such reactions[5]. However,the diffusion performance of multi-alkylbenzenes in microporous beta zeolite is relatively poor[9]because multi-alkylbenzenes are bulky molecules and microporous beta zeolite possesses micropore structure only.Thus,it is necessary to introduce a new method to enhance diffusion in beta zeolite and improve its catalytic performance in transalkylation reactions.

The hierarchicalzeolite refers to zeolites possessing atleasttwo levels ofpore size.It also has strong acidity and large surface area.It is reported that hierarchical zeolites are used in transalkylation,alkylation, isomerisation,and cracking reactions[10-15],and they exhibit better catalytic performances in comparison with conventionalzeolites.In our previous work,we synthesized the hierarchical beta zeolite by quasisolid phase conversion method with a meso/microporous structure[16]. In this study,the catalytic behavior of hierarchical beta zeolite synthesized by using inorganic silicon source and aluminum source was tested for liquid phase transalkylation of MSBB with benzene and compared with that of microporous beta zeolite.Crystal size,pore structure and acidity of hierarchical beta zeolite were investigated to clarify their effects on catalytic performance in the transalkylation reaction.

2.Experimental

2.1.Catalyst preparation

The hierarchical beta zeolite was synthesized by quasi-solid phase conversion method reported by Wang etal.[16].The aluminum source is aluminum sulfate and silicon source is fumed silicon dioxide,with the Si/Almolar ratio in the feeding being 20.The microporous beta zeolite was prepared according to Ref.[17],and the initial Si/Almolar ratio in precursor solution is also 20.

2.2.Catalytic reaction

The zeolite catalysts were evaluated for transalkylation of MSBBs with benzene in a fi xed bed reactor at 240°C under 3 MPa.Thebenzene to DSBB molar ratio was 16:1,and the weight hourly space velocity(WHSV)of DSBB was 1.4 h-1.The MSBB composition(disecbutylbenzene(DSBB)to tri-secbutylbenzene(TSBB)molar ratio)of is 3.1:1(provided by Lanzhou Zhuoyue Chemical Material Co.,China,purity 98%).The products were analyzed with a VARIAN GC 3900 gas chromatograph equipped with OV-101 capillary column (60 m×0.25 mm×0.25μm)and a FID detector.

2.3.Catalystcharacterization

The nitrogen adsorption-desorption isotherms were measured by a Micromeritics TriStar II3020.The sample was fi rst degassed at room temperature for 3 h and then heated at 300°C for 5 h under vacuum. The speci fi c surface area of the samples was estimated with the Brunauer Emmett Teller(BET)model.The totalpore volume was calculated at P/P0=0.99;the micropore volume and externalsurface area were calculated by the t-plot method;the mesopore size distribution was estimated by the Barrett Joyner Halenda(BJH)modelusing desorption isotherm.SEMimages were obtained on Hitachi S-4700,and high resolution transmission electron microscopy(HRTEM)was performed on JEOL JEM-3010.XRD patterns were measured on a Bruker D8 Advance series with Cu Kαradiation.NH3-TPD was performed with a Thermo TPD/RO1100 Catalyst Analyzer System.The sample was activated in He flow at 300°C for 2 h,then equilibrated with NH3gas for 1 h atroom temperature and fl ushed with He for1 h at100°Cto remove physicaladsorbed ammonia.The desorption ofammonia was carried out in He flow from 100°C to 630°C at a heating rate of10°C·min-1. Solid-state27Aland29SiNMR experiments were carried outon a Bruker AV300 spectrometer.27Al MAS NMR spectra were recorded at a frequency of 104.218 MHz with a spinning rate of 9 kHz with a 16.7° pulse,and a recycle delay of0.5 s.The29SiNMR spectra were recorded with a spinning rate of 5 kHz at a frequency of 79.45 MHz with a 30° pulse,and a recycle delay of60 s.The27Aland29Sichemicalshifts are reported relative to Al(H2O)3+6and TMS,respectively.

3.Results and Discussion

3.1.Catalytic activity

The catalytic behaviors of the hierarchical beta zeolite and the microporous beta zeolite with a similar SiO2/Al2O3ratio were estimated for transalkylation of MSBBs with benzene.The reaction results are shown in Fig.1.For the microporous beta zeolite,DSBB conversion at 3 h time on stream(TOS)is 87%,but TSBB conversion and SBBselectivity are relatively low,only 37%and 69%respectively.And the decrease of DSBB and TSBB conversions can be observed,indicating the deactivation ofmicroporous beta zeolite catalyst.However,DSBB and TSBB conversions and SBB selectivity over the hierarchical beta zeolite stillremains above 88%,50%and 85%respectively after 10 h TOS.These demonstrate thathierarchicalbeta zeolite shows bettercatalytic activity and selectivity than the microporous beta zeolite for the transalkylation of MSBBs with benzene.We willdiscuss below the effects ofpore structure,crystalsize and acidity on catalytic behavior to fi nd the factors leading to an improved catalytic activity.

3.2.Effects ofpore structure and crystalsize on catalytic performance

Zeolite structure and crystalsize play an important role in most reactions catalyzed by zeolites,as has been reported[18].We characterized the morphology and pore structure of the hierarchical beta zeolite and the microporous beta zeolite to study their effects on catalytic performance.

3.2.1.Characterization ofpore structure

Fig.2 lists the nitrogen adsorption-desorption isotherms and pore size distribution of the two samples.It can be seen from Fig.2(a)that the isotherm of the hierarchical beta zeolite exhibits a type IV isotherm and one declining uptake at P/P0＜0.02 is observed,which indicates the presence ofmicropores.There is a steep in fl ection with type H1 hysteresis loops at P/P0=0.7-1.0,which is usually present in mesoporous materials with approximately uniform particles[19]and indicates the existence of mesopores in hierarchical beta zeolite.It can be seen that the BJH mesopore size distribution of the hierarchical beta zeolite [Fig.2(b)]shows a narrow peak between 6 nm and 15 nm centered at 9.3 nm.In contrast,for the microporous beta zeolite,the isotherm presents the representative characteristics oftype Iisotherms and the mesopore is not found.Table 1 lists the texturalproperties of the two samples.It can be seen that hierarchical beta zeolite has higher mesopore volume(0.45 ml·g-1)and larger external surface area (253 m2·g-1)than that of microporous beta zeolite.These results indicate that hierarchical beta zeolite possesses a meso/microporous structure with high speci fi c surface area and large mesopore volume, while microporous beta zeolite has a microporous structure only.

Fig.1.Catalytic performance of different beta zeolites.T=240°C,P=3.0 MPa,WHSV (DSBB)=1.4 h-1,n(DSBB):n(DSBB)=16.■DSBB conversion over hierarchical beta zeolite;▼DSBB conversion over microporous beta zeolite;TSBB conversion over hierarchical beta zeolite;▲TSBB conversion over microporous beta zeolite;SBB selectivity over hierarchical beta zeolite;SBB selectivity over microporous beta zeolite.

3.2.2.Characterization of zeolite morphology

Fig.3(a)and(b)shows the SEMimages of hierarchical beta zeolite and microporous beta zeolite.We can see uniform globular beta crystals with a diameter about 20 nm for hierarchical beta zeolite.There are some interconnected mesopores formed by the aggregation of the nanosized crystals,increasing mesopore volume and external surface area.However,microporous beta zeolite consists of cubic crystalwith a diameter of around 200 nm,which is much larger than that of hierarchical beta zeolite.The XRD patterns of the two samples are shown in Fig.4.It can be obviously seen that the XRD patterns of hierarchicalbeta zeolite(Fig.4)have two peaks centered at7.8°and 22.4°,which are the characteristic diffraction peaks of beta crystalline phase.It should be noted that hierarchical beta zeolite shows two broadening peaks with decrease in peak height in comparison with microporous beta zeolite. The broadening of the(101)peak may come from the domain size of the complex inter growth[20],while the broadening of the(302)peak is correlated to the smallsize crystal[21].The crystal size calculated by the Scherrer equation with the full width at half maximum of the (302)peak is 12 nm,which corresponds with the result of the SEM image.

Fig.2.N2adsorption-desorption isotherms and BJHpore size distribution of different beta zeolites.■hierarchical beta zeolite;microporous beta zeolite.

Fig.3(c)presents the TEM image of hierarchical beta zeolite,where the parallel lattice fringes can be observed and extend to the entire crystal cluster.In some areas,lattice fringes of some contiguous nanosized crystals show the same direction,indicating the oriented assembly of hierarchical beta zeolite[16].The mesopores may be produced through the inter growth of these nanosized crystals.

From the results of characterization,we can see that hierarchical beta zeolite possesses nanosized crystals and a hierarchicalmeso/ microporous structure with high specific surface area and mesopore volume,while microporous beta zeolite only has a microporous structure and large size crystals.

3.2.3.Effects of pore structure and crystalsize on catalytic performance

It is generally accepted that the accessibility of active sites plays an important role in the diffusion-controlled reactions[12].The presence of mesopores in hierarchical beta zeolite can contribute to the enhanced accessibility of catalytic active sites and lower diffusion barrier of reactants and products,especially the bulky molecule.Nesterenko et al.[22] and Yang etal.[12]investigated transalkylation reactions over hierarchicalzeolites,and the results indicated that the presence of mesopores or macropores can improve the acid sites accessibility,leading to high catalytic activity.

Table 1 N2adsorption-desorption date of hierarchical beta zeolite and microporous beta zeolite

In addition,the nanosized crystals of hierarchical beta zeolite may show a short intracrystalline diffusion path length and make reactants contact with the active sites easier than that in microporous beta zeolite. Derouane et al.[23]and Serrano et al.[24]applied nanosized beta or nanosized ZSM-5 zeolite in acylation of anisol with acetic anhydride. They all observed an improved catalytic performance and attributed it to the lowered average diffusion length.

Fig.3.SEMand TEMimages of different beta zeolites.

We can see from Fig.1 that the conversion of TSBB and the selectivity of SBB over hierarchical beta zeolite are much higher than that ofmicroporous beta zeolite,while the conversion of DSBB is just slightly higher.Since TSBB molecules are bulkier than DSBB molecules,the diffusion barrier of TSBB in the zeolite is larger than that of DSBB and the diffusion of TSBB in the pore may be the limiting step of transalkylation of TSBB with benzene.In addition,some successive reactions of SBB may occur at active sites in the pore,such as is omerization and cracking reactions,which lead to the decline of SBB selectivity.The high accessibility of catalytic active sites and low diffusion barrier of reactants partially break the diffusion limitation on MSBBs in the zeolite, which leads to the improvement of conversion.Small diffusion resistance of SBB reduces the occurrence of side reactions,and could make the selectivity of SBB higher.Therefore,the presence of mesopores and nanosized crystals of hierarchical beta zeolite are beneficial to the improvement of catalytic activity and selectivity.

Fig.4.XRDpatterns of different beta zeolites.1—hierarchical beta zeolite;2—microporous beta zeolite.

3.3.Effects of acidity on catalytic performance

3.3.1.NH3-TPD study

Since the transalkylation reaction is an acid catalyzed reaction,the catalyst acidity plays an important role in the transalkylation of MSBBs.Thus,the acidity properties of the hierarchical beta zeolite and microporous beta zeolite were characterized by NH3-TPD.It can be seen from Fig.5 that the desorption profiles of the two samples are similar and they have two ammonia desorption peaks.The first peak is assigned to the weak acid site,and the second peak corresponds to strong acid sites[25,26].In order to get details about the acidity of weak and strong acid sites,we fit the NH3-TPD profiles of the two samples by Gaussian deconvolution,and the results are shown in Table 2.

From Table 2,it can be seen that the ammonium desorption temperature of the hierarchical beta zeolite in high-temperature region is higher than that of microporous beta zeolite,and the peak area of the hierarchical beta zeolite is larger.But the total area of two peaks of hierarchical beta zeolite is smaller than that of microporous beta zeolite. The results indicate that the acid strength of strong acid of hierarchical beta zeolite is greater than that of microporous beta zeolite and the acidity of strong acid is slightly larger,while the total acidity of hierarchical beta zeolite is less.

Table 2 Peak temperature and peak areas of NH3-TPD of hierarchical beta and conventional beta zeolites

3.3.2.27AlMAS NMR and29Si MAS NMR spectroscopy study

To explain the difference in acidity properties,the two samples were characterized by27Al MAS NMR and29Si MAS NMR.Fig.6 presents the27AlMAS NMR spectra of the two samples.All spectra have a sharp resonance atδ=56,which is assigned to tetrahedral aluminum[20].However,for microporous beta zeolite,there is a weak resonance at0 ppmin the spectrum,which is characteristic of octahedral aluminum[27].On account of the relative intensities of the27AlMAS NMRsignals of the tetrahedrally and octahedrally coordinated Alspecies,it can be seen thata majority of aluminum atoms are incorporated into the framework of the two samples.The results explain that the aluminum distribution is an important reason why the totalacidity of the hierarchical beta zeolite is less than that of the microporous beta zeolite.

Fig.7 presents the experimental results of29Si MAS NMR spectra and deconvoluted spectra of hierarchical beta and microporous beta zeolites using Gaussian deconvolution.In the two samples, two lines centered at around-115 and-111 ppm are assigned to Si(0Al)of two different crystallographic sites[28-30],namely Si(0Al)A and Si(0Al)B.Fig.7 shows that Si(1Al),Si(OH),and Si(2Al) sites are centered at-105--104,-103--102,-99--98,respectively[20,29,31].On account of the relative intensities of the29Si MAS NMR signals of different Sispecies,it can be seen that the concentration of framework Si(1Al)and Si(2Al)sites of hierarchical beta zeolite are slightly larger than that of microporous beta zeolite,while the concentration offramework Si(OH)sites is less.It is reported that acid strength of the bridging OH groups generated from Si(1Al)is stronger than that of bridging OH groups generated from Si(2Al)and that of silanols generated from Si(OH)1[17,32].Thus,it can be inferred that the acid strength of the hierarchical beta zeolite is stronger than that of microporous beta zeolite,which is in accordance with the results of NH3-TPD.

Fig.5.NH3-TPD pro fi les of different beta zeolites.1—hierarchical beta zeolite;2—microporous beta zeolite.

Fig.6.27Al MAS NMR spectra of different beta zeolites.1—hierarchical beta zeolite; 2—microporous beta zeolite.

3.3.3.Effects ofacidity on catalytic performance

From the results ofNH3-TPDand NMRspectroscopy,we can see that the hierarchical beta zeolite has a stronger and larger strong acid thanmicroporous beta zeolite,while the totalacidity is less.The acid strength required for transalkylation reactions is higher than that required for the alkylation.As have been reported and confirmed[12,33,34],the strong acid amount and acid strength have significant effect on the activity of catalyst in transalkylation reactions.Yang etal.[12]found that stronger acidity can improve the catalytic activity in transalkylation of di-isopropyl benzene with benzene.Cheng et al.[34]investigated transalkylation of 1,2,4-trimethyl benzene with benzene over beta zeolite and nanosized ZSM-5,and the results showed that beta zeolite has a higher 1,2,4-trimethylbenzene conversion because beta zeolite has more strong acid sites and low diffusion limitation of reactants.Therefore it can be inferred that,apart from the presence of mesopore and nanosize crystals,the stronger acidity and larger strong acidity of hierarchical beta zeolite can also improve the catalytic activity.

Fig.7.29Si MAS NMR spectra of different beta zeolites.1—hierarchical beta zeolite; 2—microporous beta zeolite.

4.Conclusions

The catalytic performance of the hierarchical beta zeolite prepared via quasi-solid phase conversion method was evaluated for transalkylation of MSBBs with benzene and compared with that of microporous beta zeolite.The reaction results show that the higher conversion of MSBBs and higher SBB selectivity are obtained on hierarchical beta zeolite than conventional microporous beta zeolite.The effects of crystalsize,pore structure and acidity on catalytic activity were investigated.The results indicate that the improvement of catalytic performance can mainly be attributed to the enhanced accessibility of catalytic active sites,lower diffusion barrier and strong acid strength, due to the presence of mesopores,nanosized crystals and strong acidity of hierarchical beta zeolite.In addition,these results imply that hierarchical beta zeolite may offer an opportunity for reactions in which both good mass transfer of bulky reactants and strong acidity are needed.More work is needed to further study the lifetime of hierarchical beta zeolite in transalkylation reaction,and investigate the effects of pore structure and acidity on reaction stability.

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*Corresponding author.

E-mailaddress:liyx@mail.buct.edu.cn(Y.Li).