Keming Ji,Jiayao Xun,Ping Liu,Qingwen Song,Junhua Gao,Kan Zhang*,Jingyuan Li

State Key Laboratory of Coal Conversion,Institute of Coal Chemistry,Chinese Academy of Sciences,Taiyuan 030001,China

Keywords:Aromatization ZSM-5 Metal modified

A B S T R A C T In this article,transition metals of Cu,La and Zn were used as adjuvant to prepare modified HZSM-5 by impregnation method.The catalysts were characterized by XRD,BET,NH3-TPD and Py-IR to reveal the microstructure and acid property.The catalysis performances of methanol aromatization of catalysts were investigated in a fixed-bed reactor.The results show that the strength and distribution of acid center of these catalysts are significantly influenced by the species of transition metal. There are more mediate strong Lewis acid center in Zn modified HZSM-5 catalyst and therefore exhibits higher selectivity to aromatic,benzene,toluene and xylenes in the MTA reaction..

1.Introduction

Benzene(B),toluene(T),xylenes(X)and other aromatics are widely used in fields of energy,transportation,materials,daily chemical and pesticide.The industrial production of aromatic is mainly depending on aromatics complex plant and petroleum is used as raw material.Developing the technology of methanol to aromatic (MTA)which uses coal as raw material can effective reduce the dependence on crude oil resource and has good economic value,which is becoming highly attractive in the field of coal chemical industry.

The MTA reaction employs molecular sieve as catalyst.Among these catalysts,ZSM-5 shows unique shape selectivity,good hydrothermal stability and coke resistance and is widely used in this field[1,2].To improve the selectivity of aromatic products in the MTA process,especially the selectivity of BTX,various methods have been used to modify the HZSM-5 catalyst,such as water vapor treatment,impregnation,chemical deposition,ion exchange,and direct synthesis[3–7].Among these methods,metal modified HZSM-5 can change the selectivity of aromatic products markedly and has been widely studied in recent years[8,9].

Bi et al. have researched the effect of a variety of Zn salts on the properties of HZSM-5 molecular sieves and the catalytic performances in methanol aromatization(MTA)process.The result shows that the molecular sieves modified by zinc nitrate have a weaker acid center and less strong acid center, the B/L ratio declined, and the aromatics selectivity improves 14.6%[10].Niu et al.have researched properties and catalytic performances in MTA of Zn modified HZSM-5 molecular sieves prepared by various methods.The result shows that the Zn modified molecular sieves prepared by impregnation method have ZnOH+and ZnOn anoparticles,the strong acid center decreases and mediumstrong acid increases,the B/L ratio declined,the dehydrogenation of low carbon hydrocarbons to aromatics is promoted, and the aromatics selectivity is improved by7.5%[11].Yang et al.have researched the influence of addition of La to the structure and performance of HZSM-5 molecular sieves by using density functional methods.The result shows that when a small amount of La ions is introduced, they could strongly interact with molecular sieves, the L acid will increase as reaction center, and the thermal stability of molecular sieves could be improved[12].Ni et al.have researched the catalytic performances of MTA of La modified HZSM-5 molecular sieves.The result shows that the selectivity of benzene,toluene and xylene(BTX)of catalysts La modified is increased 1%[13].Zaidi et al.have researched the influence of addition of Cu in HZSM-5 molecular sieves to the catalytic performances of MTA.The result shows that after the Cu modification,the conversion of methanol increases to 59%and selectivity of aromatics increases to 55.6%[14].It can be seen from the above literature that the addition of transition metals,such as Zn,La,and Cu,could modulate the properties of ZSM-5 remarkably,and then the catalytic performances of MTA would be improved.Nevertheless,the effect of metallic modification by the different metals has not been compared directly above,and the influence of the species of the transition metal on the properties of catalysis and performances ofMTA has not been given. Conte et al. have used HZSM-5modified by Ag,Cu,Ni,Pd,Ir and Ru into the MTA process.The result shows that the different metals could influence the microstructure,surface properties and electronic properties of HZSM-5 catalysts,and then influence the selectivity of products[15].However,the relationship between species of metals and acidity of molecular sieves has not been researched in-depth,and the major research object is noble metals,yet the comparison between non-noble metals is absent.

In this paper,modified HZSM-5 catalysts are prepared by using impregnation method,the influences of metal additives on the structure and acidity are researched,and the effect of reaction conditions,such as temperature,space velocity and length of time,on catalytic performances of aromatization is discussed.

2.Experimental

2.1.Catalyst preparation and reaction performance evaluation

Reagents applied in the experiment are as follows:zinc nitrate(Beichen Fangzheng Reagent Company,AR),lanthanum nitrate(Sinopharm Group,AR),copper nitrate(Beichen Fangzheng Reagent Company,AR),and HZSM-5 zeolite(Shanxi Institute of Coal Chemistry,Chinese Academy of Science,GY-1).

The HZSM-5 zeolite was crushed to 20–40 mesh(0.42–0.90 mm)and then added into Zn(NO3)2solutions by using an isometric impregnation method for16 h,then it was dried at120°C for 3 handcalcinated at550°Cfor5 h.This sample with 3 wt.%of Zn was denoted as Zn/HZ-5.Other samples prepared by the same method modified by La and Cu are denoted as La/HZ-5 and Cu/HZ-5,respectively.The unmodified catalyst was also treated by wetting out,drying and calcining,denoted as HZSM-5.

The catalytic performances of catalysts were investigated by using a laboratory-scale fixed-bed reactor.In each run,the catalyst(3 g)was loaded into the middle part of the reactor with an inner diameter of 10 mm,and then heated at 10 °C·min-1to the reaction temperature.The methanol feedstock was fed into the reactor by a liquid pump(Dalian Jiangshen LC-05P),and the product was cooled by a congealer which was full of 2°C ethylene glycol solution.The quantity of tail gas was recorded by the wet type gas flow meter and the reaction product was analyzed in a gas chromatograph.A separating funnel was used to separate the liquid product into oil product and aqueous product.The oil product was analyzed by a gas chromatograph(Beifen SP-2100A,HP-INNOWAX column,FID detector)to detect aromatic components and other oil phase products.The aqueous product distribution was obtained by a gas chromatograph(Beifen SP-2100A,Porapak-Q column,TCD detector)to detect methanol and water component.The gaseous products were analyzed by using a gas chromatograph(Beifen SP-2100A,Al2O3column,FID detector)to detect hydrogen and gas phase hydrocarbon.

2.2.Catalyst characterization

X-Ray diffraction(XRD)was performed with a D8 Advance diffractometer(Cu Kαradiation)at 40 kV and 40 mA.The samples were scanned with Bragg's angles between 5°and85°atarate of 4(°)·min-1.

Nitrogen adsorption–desorption isotherms were obtained on a Micrometrics ASAP 2020 automatic device.

Temperature-programmed desorption of NH3(NH3-TPD)experiments were carried out with a self-made adsorption instrument.200 mg of a sample was pretreated in Ar(50ml·min-1)at 500 °C for 120min.Then,the sample was cooled to 100°C,adsorbed NH3for 60 min,and heated in Ar(50ml·min-1)to 600 °C at the heating rate of 10 °C·min-1.

Pyridine adsorption Fourier-transform infrared(Py-IR)experiments were carried out with a Bruker Vector 22 infrared spectrometer.A sample was polished to 100–200 mesh(0.075–0.147 mm),and a very small part of the sample was compressed as a semitransparent flake.The flake was handled in high vacuum at 300°C for 30 min,and then cooled to room temperature and the infrared spectra were recorded.

3.Results and Discussion

3.1.Physical and chemical properties of catalysts

3.1.1.Microstructure of catalysts

XRD characterization results of catalysts are shown in Fig.1.It can be seen that the catalysts before and after modification have MFI topology and exhibit characteristic diffraction peaks of ZSM-5,which indicate that these catalysts have a ZSM-5 zeolite structure.Compared with the spectrum of HZSM-5 zeolite,the spectrum of catalysts after modification has no characteristic diffraction peaks of Cu,La or Zn species,which indicate that the metal atoms at the loading dispersed well on the catalysts.

Fig.1.XRD patterns of HZSM-5 zeolite catalysts.

Taking the four relatively stronger diffraction peaks of unmodified HZSM-5 at 8.0°,8.8°,23.1°and 24.0°as comparison standard,the relative degrees of crystallization of catalysts are calculated and shown in Table 1.It can be seen that the relative degree of crystallization of all the modified catalysts decreases.Among them,the relative degree of crystallization of Cu/HZSM-5 catalyst is less decreased to 88.4%,however,that of Zn/HZSM-5 catalyst is more decreased to 48.6%.The decrease of the relative degree of crystallization indicates that the modification with transition metal leads to decrease of structure orderliness.

Table 1Relative degree of crystallization and N2adsorption–desorption result of zeolite catalysts

N2adsorption–desorption results of catalysts before and after modification are shown in Table 1.It can be seen that the specific surface area of HZSM-5 catalyst is as large as 342.9 m2·g-1,and the pore volume and average pore diameter are 0.457 cm3·g-1and 5.9 nm,respectively.The specific surface area of catalysts after modification reduces slightly to 320–330 m2·g-1,and the pore volume also reduces slightly,however,the average pore diameter does not change.It indicates that the additives cover a part of zeolite surface and block a part of pore,but the structure of zeolite has not change.

3.1.2.Acidity of catalysts

NH3-TPD spectra of HZSM-5 before and after modified are shown in Fig.2.It can be seen that theHZSM-5 catalyst has NH3desorption peaks in100to600°C.According to the difference of temperature, the desorption peaks are divided into α-peak at low temperature,β-peak at moderate temperature and γ-peak at high temperature,which correspond to weak acid center,dium-strong acid center and strong acid center.In accordance with the temperature of the desorption peak,the spectra are Gaussian fitted,and the results are shown in Table 2.The total area of the desorption peaks of unmodified HZSM-5 catalyst is 282.2,among them,the α-peak and γ-peak have bigger areas of 109.4 and 100.6.The total area of the desorption peaks of each catalyst modified by transition metals increases,which indicates that the transition metals could induce the increase of total acid amount.The total desorption peak area of the Cu/HZ-5 catalyst is 290.2,and the peak area of β peak reaches to161.2,however,the peak are a of γ-peak significantly reduces to 13.8,which indicates that the addition of Cu results in the reduction of a strong acid center,and the formation of a more diumstrong acid center.The total area of the desorption peaks of La/HZ-5 is 364.0,and the peak area of α,β and γ all increased compared to that of HZSM-5.The total area of the desorption peaks of Zn/HZ-5 catalyst is obviously higher than that of unmodified zeolite catalyst and other catalysts modified by transition metals and reaches 382.0,and the peak areas of α,β and γ reach to 145.5,100.8 and 135.7.

Fig.2.NH3-TPD profiles of HZSM-5 zeolite catalysts.

Table 2Gaussian fitting analysis of NH3-TPD profiles of ZSM-5 catalysts

To further reveal the distribution of surface acidity,the catalysts are characterized by Py-IR,and the results are shown in Fig.3.In this figure,the adsorption peaks near 1454 and 1546 cm-1are corresponding to characteristic vibration peaks of pyridine molecule adsorbed at Lewis acid sites and Br?nsted acid sites;the adsorption peaks near 1490 and 1636 cm-1are applied to characterize the synergistic effect between Lewis acid and Br?nsted acid;the strong adsorption peaks near 1615 cm-1are corresponding to ZnOH+formed by Zn species and Br?nsted acid center[16].The Py-IR spectra of HZSM-5 zeolite catalysts and transition metal modified catalyst are fitted by Gaussian function,and the peak areas of Br?nsted acid and Lewis acid are shown in Table 3.

Fig.3.Py-IR spectra of catalysts.

Table 3Peak areas of acid sites and Br?nsted acid/Lewis acid in Py-IR spectra

At the desorption temperature of 350°C,the total acid peak area of unmodified HZSM-5 zeolite is 1.65 cm2,the peak areas of Lewis acid(1454 cm-1)and Br?nsted acid(1546 cm-1)are 0.6 cm2and 1.05 cm2,the ratio of Br?nsted acid and Lewis acid is 1.74,which shows that the catalyst has less Lewis acid.Compared with HZSM-5 catalyst,the characteristic peak area of Cu/HZ-5 increases a little,however,the peak area of Br?nsted acid decreases obviously to 0.40 cm2.Meanwhile,the adsorption peak area representing the synergistic effect between Br?nsted acid and Lewis acid decreases obviously,which results from the decrease of Br?nsted acid.The total acid amount of La/HZ-5 catalyst decreases a little,the characteristic peak area of Lewis acid decreases obviously to 0.46cm2,and the ratio of Br?nsted acid and Lewis acid is 2.06. The total acid amount of Zn/HZ-5 is more than others,the sum area of all characteristic peaks is 2.32cm2,and the Br?nsted acid decreases a little,while the Lewis acid increases notably.This is probably because the Znspecies loaded at catalyst interact with Br?nsted acid centers and transform to Lewis acid centers;on the other hand,the Zn species might interact with framework aluminum and generate new Lewis acid centers.

3.2.Aromatization performance of catalyst

3.2.1.Effect of reaction temperature

The aromatization performances of catalyst before and after modification at different temperatures are investigated and the product distributions of different phases are shown in Table 4.It can be seen that less gas-phase hydrocarbon products and more oil phase products are formed for any catalyst at the reaction temperature of 430°C.The aqueous phaseproducts at each reaction temperature are all near theoretical yield.Comparing the product composition of different catalysts at 430°C,the gasphase hydrocarbon products and oil phase products of HZSM-5 catalyst are 17.3%and 22.6%;the yield of gas-phase hydrocarbon products of Cu/HZ-5 reaches to 19.2%,while the yield of oil phase products is as low as 20.0%.The yield of gas-phase hydrocarbon products of La/HZ-5 and Zn/HZ-5 is near that of HZSM-5;however the oil phase of La/HZ-5 and Zn/HZ-5 is higher than that of HZSM-5,among them that of Zn/HZ-5 is the highest one,which reaches to 25.4%.

Table 4Phase distribution of product

In recent,based on one of the most popular reaction mechanisms of methanol aromatization,methanol is firstly catalytic dehydrated and polymerizated to light olefins(1),then forms cycloalkane by cyclization reaction(2),a part of cycloalkane forms aromatics and H2by dehydrogenation reaction(3),another part of cycloalkane takes hydrogen transfer reaction with olefin and forms aromatics and alkanes byproduct(4),all of the process is acid-catalyzed reactions.Combined with NH3-TPD characterization,it can be seen that the zeolites modified by transition metals increases the total acid amount,which is beneficial to aromatization reaction,as a result the yield of oil phase products increased.

The result of different catalysts has different yields of products which could also be further explained by the result of NH3-TPD.Cu/HZ-5catalyst has more weak acid and medium strong acid center,while short of strong acid center.The insufficiency of strong acid restricts the formation and growth of C--C bonds,more hydrocarbons with shorter carbon chain and less hydrocarbons with longer carbon chain are in the product,which is one of the reasons for the higher yield of gas phase and lower yield of oil phase.The weak acid,medium strong acid and strong acid increase in La/HZ-5 and Zn/HZ-5.Catalysts with sufficient medium strong acid and strong acid could promote the formation and growth of C--C bonds,which causes higher yield of gas phase and lower yield of oil phase.

3.2.2.Effect of additive type

The oil phase products of the ZSM-5 catalysts before and after modification contain different hydrocarbon types,and the results are shown in Fig.4.It can be seen that the aromatization products of HZSM-5 catalyst contain 39.5%of non-aromatic hydrocarbons;benzene,toluene and xylene increase in turn as 1.9%,13.0%and 20.8%;aromatic of C9,C10and C10+decreases in turn as 15.9%,5.3%and 1.5%.Compared to the unmodified catalyst,non-aromatic hydrocarbons in products of Cu/HZ-5 are as high as 40.8%,toluene is as low as 12.1%,contents of other components are similar to the unmodified catalyst.Non-aromatic hydrocarbons in products of La/HZ-5 catalyst are 45.8%,and the aromatic heavier than C9is less than that of HZSM-5,the other components are similar to the unmodified catalyst.Non-aromatic hydrocarbons in products of Zn/HZ-5 catalyst are28.3%,which is lower than that of HZSM-5;while the components of benzene, toluene and xylene are as high as 3.1%,20.5%and24.0%,and the aromatic heavier than C9is slightly less than that of HZSM-5.

According to the result of NH3-TPD and Py-IR,it can be summarized that a large amount of Br?nsted acid sites exists on the surface of unmodified HZSM-5 catalyst.Considering the product distribution result,it can be considered that Br?nsted acid could effectively catalyze methanol conversion reaction(1)and olefin cyclizing reaction(2),then prefer to take hydrogen transfer reaction(4)subsequently,thereby the product has more non-aromatics.Cu/HZ-5 catalyst has more Lewis acid sites but is short of strong acid.In the reaction process, reactions (1) and (2) are successfully carried out, and then the catalytic hydrogen transfer reaction(4)is also in a dominant position,non-aromatic content in product is relatively high[17].The Br?nsted acid sites of La/HZ-5 catalyst are more than that of HZSM-5,and the yield of non-aromatic product is obviously higher than HZSM-5,the result confirms Br?nsted acid sites have relatively high catalytic performance for hydrogen transfer reaction. Py-IR results show that Zn/HZ-5 catalyst has more Lewis acid sites,meanwhile,NH3-TPD results indicate the catalyst has more strong acid and diumstrong acid center, the stronger Lewis acid center is in favor of catalytic dehydrogenation reaction(3),the yield of non-aromatic product obviously decreases,while the yield of aromatic product increases.

Fig.4.Product distribution in oil phase.Reaction conditions:430°C and 1 h-1.

Fig.5.Yield of oil,aromatics and BTX.(a)HZSM-5;(b)Zn/HZ-5.■,●and▲are yields of oil,aromatics and BTX at1 h-1;□,○and△are yields of oil,aromatics and BTX at2 h-1.Reaction conditions:430°C,1 or 2 h-1.

3.2.3.Effect of reaction space velocity

The process of aromatization on HZSM-5 and Zn/HZ-5 at space velocity 1h-1and 2 h-1was tested,the yields of oil phase,total aromatic and BTX are shown in Fig.5.It is obvious in Fig.5 that the yield of oil phase,total aromatic and BTX decreased when the space velocity increased,levels of yield dropped by an average of 4%,7%and 4%,respectively.When the space velocity increased the yield of oil phase products decreased to 4%averagely,similar to that of HZSM-5.The yield of total aromatic and BTX decreased to 5% and 2%, and the degree of reduction is less than that of HZSM-5.Owing to the Zn-modified catalyst exist more acid sites and more active centers;it exhibits excellent selectivity at higher space velocity and longer reaction time.

3.2.4.Effects of reaction time

An 11 day long evaluation of Zn/HZ-5 has been taken to record the methanol conversion rate and selectivity of each component.The results are shown in Fig.6.In full reaction time,methanol conversion rate remained at 100%,while the selectivity of oil phase product,aromatic and BTX appears to decline.The selectivity of aromatic falls 6%,followed by BTX whose decline is only less than 2.5%.

Fig.6.Methanol conversion and product selectivity of Zn/HZSM-5 catalyst.▲is conversion of methanol;●,■,□,○ and △ are selectivity of oil,aromatics,benzene,toluene and xylene.Reaction conditions:430°C,1.5 h-1.

4.Conclusions

The HZSM-5 zeolite catalyst is modified by transition metal Cu,La or Zn,the strength and distribution of the acid center on the surface of catalyst change significantly.The increase of dium-strong Lewis acid could promote the intermediate reaction of cycloalkane dehydrogenation and enhance the selectivity of aromatics in product.Catalyst modified by Zn has more dium-strong Lewis acid center,as a result the yields of oil phase,aromatics and BTX are higher than that of unmodified HZSM-5 zeolite.The reaction conditions are investigated,and the result shows that the product has the highest yield of oil phase at a reaction temperature of 430°C.The yields of oil phase,aromatics and BTX decrease as the reaction space velocity increases; catalyst modified by Zn has relatively excellent stability,the methanol conversion remains at 100%for 11 days,and the selectivity of aromatics has not changed much.