Benzene alkylation with methanol over phosphate modified hierarchical porous ZSM-5 with tailored acidity☆

Jinghui Lyu ,Hualei Hu ,Carolyn Tait,Jiayao Rui,Caiyi Lou ,Qingtao Wang ,Wenwen Han ,Qunfeng Zhang ,Zhiyan Pan ,Xiaonian Li,*

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

2 Department of Chemical and Biochemical Engineering,University of Western Ontario,1151 Richmond Street,London,ON N6A 5B9,Canada

3 Department of Environmental Engineering,Zhejiang University of Technology,Hangzhou 310032,China

1.Introduction

Xylene is an important chemical material and widely used in producing phthalate plasticizers,polyethylene terephthalate and polybutylene terephthalate.Now instead of the traditional petroleumbased catalytic reformation or naphtha pyrolysis,the alkylation of benzene with methanol over ZSM-5 is a potential way in xylene production[1].However,the products distribution of benzene alkylation with methanol is complicated,the coke and by-product ethylbenzene fro m side reaction methanol to ole fins are still a great challenge due to the acidity of the catalyst[2].Moreover,the selectivity of unwantedmetaxylene is limited by the thermodynamic equilibrium distribution of three xylene isomers because of the isomerization on the external acid sites ofthe zeolite[3–5].In ourprevious work,the alkylation ofbenzene was rationally regulated by decreasing of Br?nsted acid sitesviaconverted Br?nsted acid sites into Lewis acid sites or increasing the Si/Al ratio of hierarchical porous ZSM-5[2,6].However,the effect of the strength of the Br?nsted acid sites on the catalytic performance of hierarchical porous ZSM-5 for catalyzing benzene alkylation with methanol is still unexplored.Literatures reported that the strong Br?nsted acid sites of the ZSM-5 zeolite could be converted into weaker Br?nsted acid sites when ZSM-5 zeolite was treated with phosphoric acid[7–9].Moreover,the introduction of phosphorus passivated the external surface acid sites and narrowed the pore size,which in turn inhibited the isomerization of xylene[10–12].Therefore,we conclude that both the acidity and acid sites distribution of hierarchical porous ZSM-5 can be tailoredviaphosphate modification,the catalytic performance was expected to be improved,and the thermodynamic equilibrium distribution of xylene isomers would be broken down at the same time.

In the present work,the catalytic performance of phosphate modified hierarchical porous ZSM-5 in benzene alkylation with methanol was investigated,and the catalysts were characterized in detail.These results showed that the modification of hierarchical porous ZSM-5 with phosphate could indeed suppress the side reaction and optimize production distribution.

2.Experimental

The hierarchical porous ZSM-5(SiO2/Al2O3ratio 360)was preparedviasolvent evaporation assisted dry-gel route as reported in the reference[2,13].Phosphate modified hierarchical ZSM-5 catalysts(P/ZSM-5)with a nominal phosphorus loading of 0 to 3 wt%were prepared by impregnating method.Detailed experimental,and characterization data of P/ZSM-5 are available in supporting information.

Fig.1.XRD patterns of synthesized and phosphate modified hierarchical porous ZSM-5 catalysts.

3.Results and Discussion

3.1.Physical and chemical properties of catalyst

XRD patterns of synthesized and phosphate modified hierarchical porous ZSM-5 catalysts are presented in Fig.1.The patterns showed distinct broad diffraction peaks in 8°–10°and 20–25°(2θ)ranges,which can be indexed with Mficrystal structure[14,15].In addition,XRD patterns of all phosphate modified samples matched well with unmodified ZSM-5,which suggested that the structural integrity of ZSM-5 after phosphate modification remained.

As shown by SEM(Fig.2),the synthesized hierarchical porous ZSM-5 has an ellipsoidal morphology with the lateral size in the range of 200–300 nm.High-resolution transmission electron microscopy(HRTEM)was used to explore the basic building blocks of hierarchical porous ZSM-5 in more detail.As shown in Fig.2C and D,the aggregates were made of nanoparticles ranging from 10 to 50 nm and these nanoparticles were crystalline in nature(Fig.2D).Fig.3 shows the TEM image and SEM image of phosphate modified hierarchical porous ZSM-5,respectively.No obvious differences were observed between the synthesized and phosphate modified hierarchicalporous,indicating that the morphology of hierarchical porous ZSM-5 was maintain during the modification process.

The N2adsorption–desorption isotherms of synthesized and phosphate modified hierarchical porous ZSM-5 are shown in Fig.4.An obvious hysteresis loop(categorized as a type IV isotherm)associated with capillary condensation in mesopores was observed,suggesting that all the samples were mesopores in nature.The structural properties of synthesized and modified hierarchical porous ZSM-5 catalysts are summarized in Table 1.The reduction of surface area from 408 to 228 m2·g?1after phosphate modification may be attributed to that both external and microporous areas were occupied by the impregnated phosphate groups[13].It should be noted that the decrease of mesopore volume was more remarkable than that of microporous,suggesting that most of the phosphate was distributed in the mesopore areas.In addition,considering thatthe phosphorus mightbe evaporated atthe calcination stage,the realphosphorus contentin each sample was detected by the XRF.As shown in Table 1,the real phosphorus content was slightly lower than the nominal phosphorus loading,indicating that a small amountof phosphorus was lost in the modification process.

The acid properties of phosphate modified hierarchical porous ZSM-5 samples were investigatedviaNH3-TPD.As shown in Fig.5,with the introduction ofphosphate,strong acid sites decreased whereas the total acidity of the catalyst was increased.It is believed that due to the reaction between phosphate salts and acid sites,new kinds of acid sites were generated[12].This phenomenon was consistent with the results reported by Hodala,the reason might be attributed to that the well dispersion of acid sites on ZSM-5 with high Si/Al ratio(＞100)was favored for the interaction of phosphorous with acid site to form two new acid sites(Fig.6)[8,16].The TPD results showed that the high temperature ammonia desorption peak shifted towards lower temperature with the increasing of phosphate owning to the reduction of the acid strength.The amounts of ammonia desorbed from the catalyst surface could be estimatedviaTPD peak areas.For further analysis of the strength change of the acid sites,the NH3-TPD curve was divided into three peaks based on the desorption temperature.The quantities of strong,medium and weak acid sites were measured by the amounts of ammonia desorbed.As shown in Fig.5 and Table S1,the desorption temperature of the strong acid sites of 3 wt%P/ZSM-5(265°C)is lower than that of the moderately strong acid sites of the unmodified sample(328°C).Due to the strength of strong acid sites of 3 wt%P/ZSM-5 is weaker than thatof the moderately strong acid sites of the unmodified catalyst.Besides,the amount of the weak acid sites of the unmodified and phosphate modified hierarchical porous ZSM-5 catalysts has not changed much.As compared with the unmodified ZSM-5,the amount of medium acid sites on phosphate modified ZSM-5 increased at the expense of the strong acid sites.Thus,it is believed that the phosphate modification of the high Si/Al ratio hierarchical porous ZSM-5 converted a strong acid site into two moderately strong acid sites,and the strong acid sites decreased.

Fig.2.SEM image(A)(B)and TEM image(C)(D)of synthesized hierarchical porous ZSM-5.

Fig.3.TEM image and SEM image of phosphate modified hierarchical porous ZSM-5.(A,B,1 wt%P/ZSM-5;C,D,2 wt%P/ZSM-5;E,F,3 wt%P/ZSM-5).

Fig.4.Nitrogen adsorption–desorption isotherms of synthesized and phosphate modified hierarchical porous ZSM-5.

Table 1Chemical and structural property of synthesized and modified hierarchical porous ZSM-5 catalysts

The infrared spectra in the region of pyridine ring vibrations are shown in Figs.7 and 8.The spectra were recorded after removing physicaland weak adsorbed pyridine molecules at200°C and removing the moderate strength adsorbed pyridine molecules at 400°C,respectively.Three main absorption bands were observed at about 1546(Br?nsted acid sites),1490(Br?nsted acid sites+Lewis acid sites)and 1448 cm?1(Lewis acid sites)for all samples.The ratio of Br?nsted to Lewis acid sites(B/Lratio)wascalculated by the formula reported by Emeiset al.and the extinction coefficients of ε(B)and ε(L)were 1.88 and 1.42 cm2·mmol?1,respectively[17].Compared with the unmodified ZSM-5,the peak area corresponding to the Br?nsted acid sites of phosphate modified catalyst increased after desorption at200 °C while decreased after desorption at400 °C,indicating that the amount of moderately strong Br?nsted acid sites increased while the amount of strong Br?nsted acid sites decreased on phosphate modified catalyst.According to the literature,the new formed Br?nsted acid sites(P--OH)on phosphate modified ZSM-5 were not strong enough to retain pyridine at high temperatures(＞350 °C)[8,18].Thus,it is reasonable to conclude that the modification of P converted the strong Br?nsted acid site into the weaker Br?nsted acid sites(Fig.6).This could be attributed to the deposition of phosphate that converted a strong Br?nsted acid site into two moderately strong Br?nsted acid sites(Fig.6)and the new formed Br?nsted acid sites(P--OH)were not strong enough to retain pyridine at high temperatures(＞350 °C)[8,18].While for Lewis acid sites,the corresponding peak area was significantly decreased after desorption at either 200 or 400°C,suggesting that the amount of both strong and moderately strong Lewis acid sites decreased on phosphate modified catalyst.This could be attributed to the phosphate species interacting with the Lewis acid sites but not being able to form the Lewis acid species.

The acid sites located at the external surface of the zeolite are main active sites for the isomerization reaction according to the mechanism of diffusion[10].The large probe molecule 2,6-di-tert-butlyl-pyridine(DTBPy)cannot penetrate into the interior pore of ZSM-5 zeolite and is adequate to characterize it.The evolution ofIRspectra after DTBPy adsorption on phosphate modified ZSM-5 is shown in Fig.9.Weakly adsorbed DTBPy was removedviaevacuating the sample at 200°C for 30 min.The band at 3367 cm?1is characteristic of the N--H vibration,which is proposed to characterize the Br?nsted acid sites on the external surface ofthe zeolite[19].As consistentwith the mechanism mentioned before,the amount of Br?nsted acid sites of the high Si/Al ratio ZSM-5 zeolite increased after the phosphate modification.After adsorption of DTBPy on zeolite,the asymmetric stretching vibration of the--CH3groups in DTBPy was shifted to 2979 cm?1,indicating the existence of a hydrogen bonding between the--CH3groups and the hydroxyl groups of the zeolite[20].After the phosphate modification,the--CH3groups in DTBPy were significantly reduced,indicating thatthe majority of acid sites on the external surface were passivated by phosphate.

Fig.5.TPDofammonia trace and acidity ofphosphate modified hierarchicalporous ZSM-5 catalysts.

Fig.6.Phosphate interaction with zeolite framework as adopted from A and B.

Fig.7.IR spectra of the samples after pyridine desorption at 200°C.

Fig.8.IR spectra of the samples after pyridine desorption at 400°C.

Fig.9.IR spectra after DTBPy adsorption on phosphate modified ZSM-5.

3.2.Catalytic performances

The catalytic performance of synthesized and phosphate modified hierarchical porous ZSM-5 in benzene alkylation with methanol was showed in Fig.10.It is clearly shown that with the increase of P content from0 to 2 wt%,the conversion ofbenzene and the selectivity to toluene and xylene gradually increase from 51.3 to 54.3%,49.0 to 52.5%and 33.9 to 37.2%,respectively.Moreover,the suppression of ethylbenzene formation was observed on a phosphate modified catalyst.Further increasing P content to 3 wt%,the benzene conversion and xylene selectivity decreased to 30.5%and 25.6%respectively.We note that higher concentration of moderately strong Br?nsted acid sites in the 3 wt%P/ZSM-5 was insufficient for catalyzing the alkylation reaction.In addition,2 wt%of phosphorus was considered an optimal amount for the modification of hierarchical porous ZSM-5 catalyst.It was previously reported that alkylation of benzene with methanol and side reaction of methanol to ole fins were simultaneous catalyzed by Br?nsted acid[21].However,the presence of methanolto ole finsreaction was associated with the costof product separation and the stability of catalyst.As reported in our previous work,reducing the Br?nsted acid sites of hierarchical porous ZSM-5 could inhibit the side reaction of methanol to ole fins and thus in turn suppress the formation of ethylbenzene.We noted that the selectivity of ethylbenzene on 2 wt%P/ZSM-5 was only 0.12%,indicating that the formation of ethylbenzene was suppressed effectively.Moreover,the increase of benzene conversion meant that a higher percentage of methanol was alkylated with benzene.Thus,it was reasonable to conclude that the methanol to ole fins reaction was suppressed.It should be noted that with the introduction of phosphate,the amount of strong Br?nsted acid sites decreased with the increase of the amount of moderately strong Br?nsted acid sites,suggesting thatthe alkylation reaction and main side reaction methanol to ole fins were both catalyzed by the strong Br?nsted acid sites.Appropriate amount of strong Br?nsted acid sites was the key method for suppressing the side reactions of methanol to ole fins.

Fig.10.Catalytic performance of phosphate modified hierarchicalporous ZSM-5 in benzene alkylation with methanol.(A)Conversion of benzene,(B)Ethylbenzene selectivity,(C)Xylene selectivity,(D)(toluene+xylene)selectivity.

We further present the stability of phosphate modified hierarchical porous ZSM-5.As shown in Fig.11,as the amount of phosphorus increased from 0 to 2 wt%,the benzene conversion was stable during the investigated time.It also should be noted that the benzene conversion on 3 wt%P/ZSM-5 decreased obviously.As reported in many literatures,the deactivation of ZSM-5 catalyst could be attributed to the formation of coke[22–24].To further analyze the carbon deposition on the used catalyst,TG-DTA was performed to evaluate the coke content(remove within 200 to 600°C)on the catalyst.As seen in Fig.14,with the increase ofphosphorus amountfrom 0 to 1 wt%,the coke contentincreased from 2.2 wt%to 3.3 wt%.The diffusion of carbon precursor would be promoted by introducing mesopores into conventional ZSM-5,which further inhibited the formation of coke[25].Therefore,the increase of coke content of 1 wt%P/ZSM-5 could be attributed to the decrease ofmesopore volume,which limited the diffusion ofcarbon precursor.Furtherincreasing the phosphorus amountto 2 wt%,the coke content decreased to 2.3 wt%due to the effective regulation of acidity and reduction of carbon precursor by suppressing the side reaction of methanol to ole fins.As for the 3 wt%P/ZSM-5catalyst,although the mesopore volume reduced and the activity for alkylation decreased,the coke content decreased to 1.1 wt%.Moreover,both methanol and dimethyl ether were detected in the products,which indicated that the methanol to ole fins reaction was indeed suppressed.In order to further investigate the coke formation,the catalytic performance of 3 wt%P/ZSM-5 and the content of coke were measured after 4-hour reaction.The result showed that at the beginning 4 h,the conversion was decreased from 44.6 to 41.2%and the coke content is 0.9.While after 10-hour reaction,the benzene conversion decreased from 44.6 to 16.1%and the coke content is 1.1%(Fig.S1),suggesting that the rate of coke formation was faster at the begin reaction and this might be attribute to the cokes were mainly produced over the strong acid site.It should be noted that the amount of strong acid site on 3 wt%P/ZSM-5 was lower than that of other catalysts,whereas there were sufficient moderately strong Br?nsted acid sites,the results confirmed that a small amount of strong Br?nsted acid site was necessary for the alkylation[1,6].Hence,with the increasing of reaction time,the strong acid site of 3 wt%P/ZSM-5 was rapidly covered by coke and this in turn lead to the obviously decrease of benzene conversion.

Fig.11.The effect of phosphate modified on the stability of hierarchical porous ZSM-5.

Fig.12.TG-DTA pro files of phosphate modified catalysts after successive reaction time(10 h).

The selectivity ofxylene isomers on phosphate modified sample was listed in Fig.13.For unmodified hierarchicalporous ZSM-5,the selectivity ofp-xylene,m-xylene ando-xylene were 22.9,54.4 and 22.7%respectively,which was close to the thermodynamic equilibrium distribution.With the introduction of 1 wt%P,the composition of xylene isomers changed slightly,indicating that the isomerization of xylene was not prevented.Further increasing the P amount to 2 wt%,the selectivity ofm-xylene was significantly decreased from 54.4%to 32.0%,suggesting that the formation ofm-xylene was suppressed.However,the selectivity ofp-xylene increased slightly while that ofo-xylene increased from 22.8%to 44.5%.Meanwhile,the yield of xylene was increased from 17.4 to 20.2%.In order to verify the accuracy of the data,the phosphate modified catalyst was synthetized and evaluated many times,and the catalytic performance of additional sample(1.5 wt%and 2.5 wt%P/ZSM-5)was provided in Table S1 and S2.The results indicating that the variation tendency of xylene isomers selectivity on phosphate modified sample was veritable and repeatable.According to the literature,although the diffusion coefficient ofpara-xylene in the channel of ZSM-5 zeolite is about 1000 times ofmeta-xylene and about 100 times ofortho-xylene above 250°C[3],the composition of xylenes produced is close to the thermodynamic equilibrium distribution(22.55%p-xylene,54.42%m-xylene,and 23.03%o-xylene at 400°C)due to the isomerization ofp-xylene occurring on external sites of the zeolite[4].In general,suppressing the isomerization ofp-xylene by passivating the active sites on the external surface and limiting the diffusion ofm-xylene ando-xylene by narrowing the pore size of conventional ZSM-5 could achieve higherp-xylene selectivity[10,26].The xylene isomers were fed as reactants separately under the reaction condition in orderto investigate the effectofphosphate modification on the isomerization ofxylene.As shown in Fig.14,the unmodified ZSM-5 catalysthad a high activity ofisomerization and the productselectivity was close to that of the thermodynamic equilibrium distribution of xylene isomers.With the introduction of phosphate,the selectivity of xylene of2 wt%sample changed slightly,while the main productof3 wt%sample was strongly dependent on the reactant.Due to the different diffusion coefficient of the three isomers in the channel,the molecular size ofxylene isomers waspara＜ortho＜metaand the conversion the xylene isomers wasmeta＜ortho＜para.These results indicated that the modification of phosphate could effectively suppress the isomerization of xylene.According to our previous discussion,the majority of acid sites on the external surface of modified catalyst were passivated by phosphate.Moreover,the activity of xylene isomerization was decreased when the higher amount of moderately strong Br?nsted acid sites existed on the external surface.These moderately strong Br?nsted acid sites express very low activity or no activity on the xylene isomerization.Therefore,it is reasonable to deduce that the suppression of xylene isomerization on phosphate modified catalyst could be attributed to the passivation of the active sites on the external surface.

Fig.13.The effect of phosphate modified on the selectivity of xylene isomers.

We further demonstrate a highero-xylene selectivity in the alkylation reaction over the phosphate modified catalyst with the suppressed isomerization of xylene.However,the highero-xylene selectivity did notemerge in the isomerization ofxylene overphosphate modified ZSM-5.According to reference[7],the loaded phosphorus may narrow the pore size,but the effect of the narrowed the pore size will lead to the increasing selectivity ofpara-xylene rather thanorthoxylene.Thus we conclude that the narrowing of pore size was not the main reason forthe change ofselectivity.The highero-xylene selectivity in alkylation reaction might be summarized into two con fluent factors.First,the length of the diffusion pathways:The selectivity ofp-xylene was strongly dependent on the length of the diffusion pathways due to the higher diffusion coefficient ofp-xylene[27].The hierarchical porous ZSM-5 has a large number of mesopores and the catalyst is stacked by small particles(10–50 nm),which would shorten the diffusion pathways of production and the diffusion advantage ofp-xylene is not as prominent compared to conventional ZSM-5.Second,the preference ofo-xylene formation:In addition,it was widely realized that the electrophilic methoxonium ion will preferentially interact with the carbon atoms of toluene,which bears the highest negative partial charge(orthoandparapositions)[28].Voset al.had reported that in absence of steric constraints,the order of activation energies of xylene wasortho＜para＜meta[29].In addition,Bautistaet al.had observed that the Br?nsted acid sites on AlPO4catalysts were hard sites which favoredortho-substitution in toluene methylation[30].With the modification of phosphate,one strong Br?nsted acid site was converted into two weak Br?nsted acid sites(P--OH)which was similar to that of AlPO4.Moreover,Bhattacharyyaet al.had reported that in the alkylation of phenol with methanol,the catalyst with the lower acidity favored theorthoalkylation[31].Therefore,the high selectivity ofo-xylene on phosphate modified catalyst could be attributed to the fact that theo-xylene formation was favored and the isomerization of xylene was suppressed.

Fig.14.The isomerization of xylene on phosphate modified hierarchical porous ZSM-5 catalysts.

4.Conclusions

In summary,owning to the suppression of the side reactions from methanol to ole fins with phosphate modification of hierarchical porous ZSM-5,both conversion and selectivity of xylene in benzene alkylation with methanol improved.The thermodynamic equilibrium distribution of xylene isomers is broken and the selectively ofm-xylene is decreased as a result of phosphate covered external surface acid sites.The moderately strong Br?nsted acid sites are unable to catalyze benzene alkylation reaction,the benzene alkylation was catalyzed by the strong Br?nsted acid sites.Appropriate amount of strong Br?nsted acid sites of hierarchical porous ZSM-5 was the key factor to improve catalytic performance in the benzene alkylation with methanol.

Appendix A.Supplementary Data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.cjche.2016.12.005.

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