Junhua Gao,Keming Ji,Hao Zhou,Jiayao Xun,Zenghou Liu,Kan Zhang,Ping Liu

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

Keywords:ZSM-5 Catalysis Methanol Hydrocarbons

ABSTRACT Five kinds of BZSM-5 molecular sieve with different Si/B ratio and a SiZSM-5 molecular sieve were prepared by hydrothermal synthesis method followed by acid exchange and pelletization.The samples were characterized by XRD,SEM,FT-IR,ICP,low temperature N2 physical adsorption and desorption,NH3-TPD and Py-IR.The catalytic performance in the reaction of methanol to hydrocarbons was evaluated in the fixed bed reactor.Compared with SiZSM-5,the amount and strength of Br?nsted(B)acid were enhanced by introducing skeleton boron and the activity of the catalyst was greatly improved.The characterization and evaluation results indicated that the BZSM-5 catalyst synthesized from the gel of SiO2/B2O3 20 with Si/B ratio 74.48 had modest acidity strength,acid amount of 0.18 mmol NH3·g-1 and large mesopore volume of 0.23 cm3·g-1.The B acid ratio was higher and the acid strength of BZSM-5 was weaker than that of AlZSM-5,which could inhibit the deep coke formation and increase the activity stability.B-2 had the best lifetime which could reach 672 h under the same evaluation reaction conditions,due to the best matching of moderate acidity and good diffusion properties.

1.Introduction

Methanol to hydrocarbon (MTH) is an important route for coal from syngas to oil and bulk chemicals.The mature technology of methanol production from coal through syngas leads to over capacity of methanol,as a result,the MTH process becomes a hot research direction in the last twenty years.The rapid development of this process is accord with the energy situation of rich coal,poor oil and lack gas in China.Methanol could be converted into the mixture of alkane,olefin and aromatic by solid acid catalysts,and it can be categorized into methanol to gasoline (MTG) methanol to olefin (MTO) and methanol to aromatics (MTA) according to the product selectivity [1,2].ZSM-5 molecular sieve is a common catalyst used in MTH in fixed-bed process.It is a kind of inorganic aluminosilicate materials with three-dimensional crossing pore structure,which has unique shape selectivity,good hydrothermal stability and strong resistance to carbon deposition[3].The serious deep carbon deposition in MTH reaction process could cover active centers and block pores,which is the main reason for the one-way deactivation of the catalyst[4,5].Therefore,the activity stability of ZSM-5 catalysts is one of the key factors restricting its industrial application.With the further research on the ‘‘hydrocarbon pool”mechanism of MTH reaction [6,7],the hydrocarbon pool species are realized as active intermediates,as well as the precursors of carbon deposition.Reducing acid strength and density of acid site,especially strong Br?nsted(B)acid center,could inhibit the formation of carbon deposition [8,9].On the other hand,it is also an effective method to construct hierarchical pore structure through reducing crystal size and improve the diffusion performance of the catalyst,so that the macromolecular aromatics can diffuse out in time without deep reaction [10–13].

In order to obtain the catalyst with high activity and stability,the researchers adjusted the acidity and pore structure of the catalyst by various means,such as controlling silicon aluminum ratio of framework,modification by other element,high temperature steam treatment,acid or alkali solution treatment and so on [14–18].Modification ZSM-5 by other elements is one of the most commonly used methods.As a kind of Lewis(L)acid or Lewis base,the modified elements cover the surface and pore of ZSM-5 molecular sieve and combine with aluminum oxide tetrahedron or surface hydroxyl groups,which can change the acidity and pore structure of the catalyst,thus affecting the catalytic performance.By introducing other elements to reduce the acid strength,acid center density and pore distribution,the product distribution can be adjusted and the deep carbon deposition can be inhibited.It has a significant effect on improving the activity stability of the catalyst and improving the selectivity of a product [19,20].

The basic structural unit of conventional AlZSM-5 molecular sieve is composed of aluminum oxide tetrahedron and silicon oxygen tetrahedron,and the active center is produced by aluminum oxygen tetrahedron on the framework of ZSM-5 molecular sieve.MTH is an acid catalyzed reaction process,therefore,modifying the composition of skeleton elements is a direct and effective method to modify acidity.The element B and Al are main group elements of IIIA,thus the heteroatom BZSM-5 molecular sieve is easy to synthesis.As mentioned above,the acid produced by aluminum oxide tetrahedron is the source of acid active center,the introduction of framework heteroatoms can essentially adjust the acidity of the catalyst,and the introduction of boron can significantly reduce the acid strength and change acid distribution of the catalyst,so it is necessary to characterize and study it systematically.The previous researches mainly focused on the differences between GaZSM-5,BZSM-5,FeZSM-5 and AlZSM-5 [21–25].However,there are few researches on the MTH reaction performance by using BZSM-5 and few discussions about the effect of pore structural features of BZSM-5 on the activity stability in MTH reaction[26–30].In this research,by adjusting the amount of boron source using common and cheap synthetic materials,five kinds of BZSM-5 molecular sieves with different Si/B ratio and a SiZSM-5 molecular sieve have been synthesized,and the MTH catalysts have been prepared by further acid exchange and compression method.It is proposed that the acidity of ZSM-5 can be adjusted by introducing skeleton boron,the effect of Si/B ratio on the physical and chemical properties and catalytic performance of BZSM-5 catalyst will be investigated.

2.Experimental

2.1.Catalysts preparation

BZSM-5 molecular sieves are synthesized by hydrothermal method by using water glass (with modulus of 3.3,Qingdao Dongyue Sodium silicate Co.,Ltd) as silicon source,boric acid (H3BO3,Analytical purity,Tianjin Fuchen Chemical Reagent Factory) as boron source and tetrapropylammonium bromide (TPABr,Chemical purity,Zhejiang Kente Chemical Co.,Ltd) as template R.The synthesis gel ratio is SiO2:nB2O3:0.1R:0.33Na2O:30H2O,the SiO2/B2O3ratio of gel is low to high,and the value of n is 0.1,0.05,0.025,0.0125,0.00625 and 0,respectively.The gel are placed in a 2 L stainless steel reactor,then stirred,aged in low temperature,crystallized in high temperature,controlled of the synthesis temperature and crystallization time,and then five kinds of BZS09M-5 and a SiZSM-5 molecular sieve crystals are finally produced.The crystals are washed by water to neutral,dried at 120C in 12 h,calcined at 540C in 4 h to remove the template.Each gram of molecular sieve raw powder is exchanged in 50 ml ammonium nitrate (NH4NO3,chemical purity,Sinopharm Chemical Reagent Co.,Ltd) solution of 0.5 mol·L-1three times,and then the hydrogen type BZSM-5 and SiZSM-5 (HBZSM-5 and HSiZSM-5)raw powder is prepared.The ZSM-5 molecular sieve raw powder is formed by pressing,and broken into 20–40 mesh,which named as B-1,B-2,B-3,B-4,B-5 and SiZSM-5 for their different Si/B ratio from low to high.

2.2.Catalysts characterization

Phase analysis by X-ray diffraction(XRD)was carried out on D8 Advance X-ray powder diffractometer made in Germany,using Cu target,K α ray(λ=0.15 nm),tube voltage of 40 kV,tube current of 30 mA,scanning area of 5to 50,and the step size is 0.02.

The catalysts morphology was measured on JSM-7001F thermal field emission scanning electron microscope (SEM) with a voltage of 10 kV.

Fourier transform infrared (FT-IR) spectra characterization was carried out on Bruker Vector 22 infrared spectrometer made in Germany.The samples and potassium bromide powder were mixed and grinded as the mass ratio of 1:(200–400),and then the spectra were scanning at room temperature after forming by pressing.

The content of elements was determined by Thermo ICAP 6300 inductively coupled plasma emission spectrometer (ICP).

The low temperature nitrogen physical adsorption and desorption characterization was carried out on the ASAP2020 physical adsorption instrument of Micromeritics instrument company made in the United States.The surface area,pore distribution and micropore volume were calculated by Brunauer,Emmett and Teller methods(BET method),Barret,Joyner and Halenda methods(BJH method) andt-plot method,respectively.

Ammonia temperature programmed desorption(NH3-TPD)was characterized on TP-5080,a micro-automatic multi-function adsorption instrument of Tianjin Xianquan company.The catalyst was first purged with N2at 500C for 60 min,cooled to 100C to adsorb NH3,and then heat to 700C in 10C·min-1after the baseline steadied.

Pyridine infrared(Py-IR)spectrum was characterized on Bruker Vector 22 infrared spectrometer.The catalyst was first treated at 400C and 0.05 Pa for 30 min to remove the impurities adsorbed.After falling to room temperature,pyridine was adsorbed,and the scanning spectrum was gotten after treated at 300C and 0.05 Pa.The characteristic peaks of B acid and L acid were integrated.The ratio of the peak area of the two kinds of acids represents the distribution of B acid and L acid.

2.3.Catalysts evaluation

The evaluation of the catalyst was carried out on a continuous flow fixed bed reactor,which is a stainless steel pipe with 100 cm long and 1 cm in diameter.The reactor was filled with 3 g catalyst at constant temperature area and quartz sand at both ends of the bed.The performance of MTH reaction was evaluated at atmospheric pressure,380C and WHSV=2 h-1.The gas and liquid products were separated by cold trap,the gas phase and water phase were analyzed by SP-2000 gas chromatograph of Beijing Beifen Reili Analytical Instrument (Group) Co.,Ltd.,the chromatographic column is TDX-01 packed column with thermal conductivity detector(TCD),and Al2O3capillary column with flame ionization detector (FID).The water phase was analyzed by Porapak-Q packed column with TCD.The oil phase was analyzed by SP-3420 gas chromatograph of Beijing Beifen Reili Analytical Instrument (Group) Co.Ltd.,the chromatographic column is HPINNOWax capillary column with FID.The methanol conversion and distribution of products are defined as follows.

Conversion of methanolx=（Methanol mass of income-Methanol mass in aqueous phase）/Methanol mass of income×100%

Product distributions=The number of moles of carbon in a product/Total carbon molar number of product×100%

3.Results and Discussi on

3.1.Crystal structure and morphology of the catalysts

The XRD patterns of HBZSM-5 with different Si/B ratios and SiZSM-5 catalysts are shown in Fig.1.It can be seen that all samples have MFI characteristic structure diffraction peaks at 7.96,8.86,23.18,24.00and 24.50.There is no diffraction peak of other impurity crystal phase in the samples,which shows that the synthesized crystals are ZSM-5 molecular sieves with good crystallinity.The intensity of HBZSM-5 diffraction peaks increases with the increase of Si/B ratio,it may be due to the fact that the crystal size of HBZSM-5 increase with the increasing of Si/B ratio,which can lead to the broadening of XRD characteristic peak.

Fig.1.XRD patterns of the HBZSM-5 with different Si/B ratios and SiZSM-5 catalysts.

The scanning electron micrographs of the HBZSM-5 with different Si/B ratios and SiZSM-5 catalysts are shown in Fig.2.It can be seen that the B-1 is nano-sized crystals with the size less than 100 nm.The crystal size distribution is relatively uniform,and the nano crystals agglomerate into slightly larger crystals.With the increasing of the SiO2/B2O3in the gel,the crystal size increase gradually.The crystal size of B-5 is nonuniform,the smallest crystal is about 100 nm and the largest crystal is about 1 μm.If the SiO2/B2O3ratio in the gel is low,more crystal nucleus will be formed,and nano crystals are easier to grow,otherwise,the size of the crystals increase gradually.Under the same synthesis conditions,the morphology of SiZSM-5 is similar to B-5.The crystal distribution of SiZSM-5 is uneven,most of the SiZSM-5 crystals have larger size,about 1 μm,and a small number of the SiZSM-5 crystals are about 200 nm.

Fig.2.Scanning electron micrographs of the HBZSM-5 with different Si/B ratios and SiZSM-5 catalysts.

Fig.3.FT-IR spectra of the HBZSM-5 with different Si/B ratios and SiZSM-5 catalysts.

The FT-IR spectra of the HBZSM-5 with different Si/B ratios and SiZSM-5 catalysts are shown in Fig.3.It can be seen that the HBZSM-5 prepared by this method all have the characteristic peaks at about 450,550,790,1093 and 1225 cm-1of the molecular sieve framework.In the FT-IR spectra,the band near 450 cm-1reflects TO-T flexural vibration,the band near 550 cm-1reflects flexural vibration of five-membered ring in the structural of ZSM-5,the band near 790 cm-1reflects T-O-T out-of-plane symmetric telescopic vibration,the band at 1093 cm-1reflects T-O-T antisymmetric stretching vibration of tetrahedron,and the band at 1225 cm-1reflects antisymmetric stretching vibration of inner tetrahedron.The peak positions of HBZSM-5 are similar to that of SiZSM-5,the position of the vibration peaks of 450,550 and 1225 cm-1have not shift obviously,the position of the vibration peaks of 790 and 1093 cm-1have shift to low wave number.The bond length of B-O bond is longer than that of Si-O bond,indicating that boron enters into the framework of molecular sieve.All the FTIR characteristic peaks appeared,which shows that the synthesized molecular sieves have MFI structure [31].

3.2.Acidity and composition of BZSM-5 catalysts

Fig.4.NH3-TPD spectra of the HBZSM-5 with different Si/B ratios and SiZSM-5 catalysts.

The NH3-TPD spectra of the HBZSM-5 with different Si/B ratios and SiZSM-5 catalysts are shown in Fig.4.It can be seen that all the catalysts have two desorption peaks.The low-temperature peaks represent weak acids and high temperature peaks represent strong acid.The temperature of weak acid peak of B-1 is the lowest about 223C,and its weak acid is the weakest.The temperature of weak acid peak of B-2,B-3 and B-4 is about 230–235C,the weak acid strength of these catalysts is in the middle.The temperature of weak acid peak of B-5 is about 248C,the weak acid of B-5 is the strongest.The peaks temperature of strong acid in all HBZSM-5 samples are close to 428C,which means the acid strength of different HBZSM-5 catalysts is close.With the increase of Si/B ratio,the area of weak acid peak decreases gradually,while the area of strong acid peak does not change obviously,which indicates that the amount of weak acid decreases gradually and the amount of strong acid changes little.Compared with BZSM-5,the strong acid of SiZSM-5 is weaker and less.The acidity of SiZSM-5 mainly comes from Si-OH on the surface,while the acidity of BZSM-5 comes from the negative charge of boron oxygen tetrahedron,which indicates that the substitution of boron for skeleton Si can increase the amount of strong acid and enhance the acid strength partly.

The Py-IR spectra of the HBZSM-5 with different Si/B ratios and SiZSM-5 catalysts are shown in Fig.5.It can be seen that the peaks near 1545 cm-1are formed by pyridine adsorbing on B acid,the peaks near 1490 cm-1are formed by pyridine adsorbing both on B acid and L acid,the peaks near 1454 cm-1are formed by pyridine adsorbing on L acid.The peak of B acid was larger than that of L acid,which means that the HBZSM-5 catalysts mainly exists in the form of B acid.Especially for B-5,the characteristic peaks of L acid almost disappeared and almost all existed in the form of B acid.The B acid peak and L acid characteristic peak of SiZSM-5 almost disappeared,but a larger B acid peak appears after the introduction of boron,which indicates that boron enters the framework of molecular sieve and produces B acid center with a certain acid strength.

The ICP and NH3-TPD results of the HBZSM-5 with different Si/B ratios and SiZSM-5 catalysts are shown in Table 1.It can be seen that the Si/B ratio of BZSM-5 is higher than that of its gel.With the increases of SiO2/B2O3ratio in the gel,the boron content of the catalyst decreases and the Si/B ratio increases gradually from 48.11 to 256.80.The acid analysis by NH3-TPD shows that the acid amount of BZSM-5 does not exceed 0.21 mmol NH3·g-1.When the skeleton Si and Al is replaced by boron,acidity is produced primarily by the boric oxygen tetrahedron,the acidity of HBZSM-5 is stronger than that of SiZSM-5,while weaker than that of AlZSM-5.With the decreases of boron content,the acid amount of catalysts also decreases.The acid amount of B-1 is slightly higher,the acid content of B-2 and B-3 is close,and the acid amount of B-4 and B-5 is close.The acid amount of SiZSM-5 is the same as that of B-5,there are mainly weak acid sites on SiZSM-5 and the strong acid amount is merely 0.03 mmol NH3·g-1.However,according to the NH3-TPD and Py-IR spectra,the strong acid strength of BZSM-5 is obviously stronger than that of SiZSM-5,and it is mainly B acid center.With the decrease of Si/B ratio,the amount of weak acid decreased gradually.The strong acid amount of all HBZSM-5 was very low,not more than 0.10 mmol NH3·g-1.The strong acid amount of B-1,B-2 and B-3 are the same,which is 0.08 mmol NH3·g-1.The strong acid content of B-4 is close to B-5,B-5 was only 0.01 mmol NH3·g-1higher than that of B-4,the reason is not clear,the difference may be caused by the surface silicon hydroxyl groups of the BZSM-5 catalysts.

Fig.5.Py-IR spectra of the HBZSM-5 with different Si/B ratios and SiZSM-5 catalysts.

Table 1ICP and NH3-TPD results of the HBZSM-5 with different Si/B ratios and SiZSM-5 catalysts

3.3.Pore structure of BZSM-5 catalyst

The isotherms of low temperature N2physical adsorption and desorption of the HBZSM-5 with different Si/B ratios and SiZSM-5 catalysts are shown in Fig.6.It can be seen that the adsorption and desorption isotherms of the HBZSM-5 catalysts do not completely coincide,and hysteresis loops with different size are formed.The size of hysteresis loops formed by the adsorption and desorption isotherms decrease with the increases of Si/B ratio,indicating that different intercrystal mesoporous have been piled up between HBZSM-5 crystals.This is mainly due to the smaller crystal size and more intercrystal pores in HBZSM-5 with low Si/B ratio,thus forming a larger hysteresis ring.The largest hysteresis loop of B-1 indicates that there are more intercrystal mesoporous.There is a small hysteresis loop in the N2physical adsorption and desorption isotherms of SiZSM-5,which indicates that a small amount of intercrystal mesoporous are formed.

Fig.6.The isotherms of low temperature N2 adsorption and desorption of the HBZSM-5 with different Si/B ratios and SiZSM-5 catalysts.

The pore distribution of the HBZSM-5 with different Si/B ratios and SiZSM-5 catalysts are shown in Fig.7.It can be seen that the catalysts have a certain amount of mesoporous in addition to its micropores.With the increases of Si/B ratio,the number of pores distributed around 2.3 nm increases,and the number of pores between 20 to 80 nm decreases gradually.The HBZSM-5 catalysts with low Si/B ratio have more mesoporous and a certain number of large pores,whose hierarchical pore structure is more obvious.The intercrystal mesoporous depend on the properties of crystal and the interaction between crystals.The previous studies have shown that the common ZSM-5 with high aluminum content has larger intercrystal mesoporous,and BZSM-5 has the same rule[32].The pore diameter distribution curve of SiZSM-5 shows that the number of mesoporous is small.

The Low temperature N2physical adsorption and desorption results of the HBZSM-5 catalysts with different Si/B ratios are shown in Table 2.It can be seen that all of the catalysts have higher BET surface area than 400 m2·g-1.The HBZSM-5 with low Si/B ratio has a larger micropore surface area and a smaller external surface area,while the HBZSM-5 with high Si/B ratio is opposite.The micropore surface area of B-1 is the highest,reaching 289 m2·g-1,which is 106 m2·g-1higher than that of B-5.The external surface area increases with the increase of Si/B ratio.The external surface area of B-5 is the highest,reaching 231 m2·g-1,which is 99 m2·g-1higher than that of B-1.In theory the crystal size of HBZSM-5 becomes larger with the increase of Si/B ratio,the external surface area decreases with the increase of Si/B ratio,but HBZSM-5 is not so,which may be determined by the property of the BZSM-5 itself,the bond length of the Si-O bond is longer than the bond length of the B-O bond.The total pore volume,micropore volume and mesoporous volume decrease with the increase of Si/B ratio.B-1 has the largest total pore volume of 0.45 cm3·g-1,while the B-5 catalyst has the lowest total pore volume of 0.24 cm3·g-1.From B-1 to B-5,the micropore volume decreases from 0.11 cm3·g-1to 0.07 cm3-·g-1and the mesoporous volume decreases from 0.34 cm3·g-1to 0.17 cm3·g-1.The average pore diameter decreases with the increase of Si/B ratio too,and the pore volume agree well with the pore size distribution.The difference of pore volume between catalysts is mainly determined by the intercrystal mesoporous volume,which is mainly determined by the special crystal properties and the crystal size.The BET surface area of SiZSM-5 is much lower than that of HBZSM-5,which is only 374 m2·g-1.The microporous surface area is between B-2 and B-3,and the external surface area is a little higher than B-1.The micropore volume of SiZSM-5 is larger than B-4 and lower than B-3.The mesoporous volume of SiZSM-5 is the smallest,which is 0.09 cm3·g-1.

Table 2Low temperature N2 adsorption and desorption results of the HBZSM-5 with different Si/B ratios and SiZSM-5 catalysts

3.4.Evaluation of the catalysts

Fig.7.Pore distribution of the HBZSM-5 with different Si/B ratios and SiZSM-5 catalysts.

The methanol conversion of the HBZSM-5 with different Si/B ratios and SiZSM-5 catalysts are shown in Fig.8.It can be seen that the order of activity stability is B-2>B-3>B-4>B-5=B-1>SiZSM-5.In general,HBZSM-5 shows higher activity and stability than SiZSM-5 in MTH reaction.B-2 has the longest lifetime,reaching 672 h;B-3 and B-4 is next,reaching 500 h;B-5 and B-1 have the same lifetime,less than 500 h.Compare to previous research results of hierarchical AlZSM-5,the activity and stability of HBZSM-5 are further improved [32].The activity of SiZSM-5 is poor,the conversion of methanol is less than 96%,the activity stability is less than 24 h,and there is mainly gaseous hydrocarbon products.Because the acidity of SiZSM-5 is weak and there are almost no B acid centers,it cannot form active hydrocarbon pool species,so it cannot carry out autocatalytic reaction.

By acid characterization of NH3-TPD and Py-IR comparison,the ratio of B acid on HBZSM-5 is higher than that of AlZSM-5,however,its weak and medium strength acid amount are larger,and its strong acid is weaker than that of AlZSM-5,which can slow down the coke deposition rate.Therefore under the same evaluation conditions the stability of BZSM-5 catalyst is longer than that of AlZSM-5 [33,34].On the view of pore structure,the intercrystal mesoporous volume of B-1 reaches 0.34 cm3·g-1,which is favorable for the diffusion of carbon deposit precursor species.Even though the pore volume of B-2 is not the largest,the weak adsorption is also beneficial to the diffusion of precursor species of carbon deposition because of the relative weak acidity and low acid site density of B-2.It can be seen that for molecular sieve catalysts with MFI structure,the influence of the nature of acid active center on the activity and stability of catalyst is the main reason,which is more important than the diffusion factor.The acidity and pore structure of B-1 and B-5 are quite different,but their activity and stability are the same.This result shows that acidity and pore structure could affect the catalytic performance together.B-1 has a wide pore size distribution and well-developed intercrystal mesoporous structure,but its acid center density is high.As a result,the hydrocarbon pool species are easy to further react to form macromolecule carbon deposit species,which can reduce the catalyst lifetime.B-5 has low acid site density and large external surface area,which is favorable for product diffusion.The acid strength,acid site density,B/L ratio,pore volume and external surface area of B-2 are in the middle,whose acid active center and pore structure matches the best,thus its activity and stability is the highest.

Fig.8.Methanol conversion of the HBZSM-5 with different Si/B ratios and SiZSM-5 catalysts.

The hydrocarbon product distribution of the HBZSM-5 catalysts with different Si/B ratios are shown in Fig.9.It can be seen that the content ofof B-1 is the highest,which is about 13%higher than that of other catalysts,the corresponding alkane and olefin content of B-1 is obviously lower than that of other catalysts.The high acid site density of the HBZSM-5 with low Si/B ratio is conducive to the polymerization reaction of carbon chain growth,the large intercrystal mesoporous is conducive to the diffusion of macromolecular products,which improves the selectivity oin the products.The HBZSM-5 with high Si/B ratio has a high ratio of B acid,which promotes cracking reaction,and the low hydrocarbon product content is higher than that of other catalysts.The olefin content in the average product distribution of all the HBZSM-5 is higher than that of alkanes,which indicated that the hydrogen transfer ability of the HBZSM-5 is low.This is because HBZSM-5 has a high proportion of B acid,however,its acidity is weak,so its hydrogen transfer capacity is insufficient.

The alkane content in the product distribution decreases,the olefin content increases,and thecontent decreases with prolong of reaction time.The conversion of methanol to hydrocarbons involves a series of reactions,firstly,methanol dehydrates to dimethyl ether,and dimethyl ether further reacts to produce light olefins,light olefins polymerize to form long chain hydrocarbons,and then a series of reactions,such as cyclization,hydrogen transfer,cracking,alkylation,and so on.Finally,olefins,alkanes and aromatics are produced.According to the reaction mechanism of‘‘hydrocarbon pool”,methanol forms hydrocarbon pool species rapidly on the catalyst,and methanol reacts with hydrocarbon pool species to generate various hydrocarbons.The acidity,pore structure and reaction conditions all affect the reaction behavior of hydrocarbon pool species with methanol.The acidity of BZSM-5 is weaker than that of AlZSM-5 and stronger than that of SiZSM-5,so the interaction between hydrocarbon pool species and acid active center has the same law,which can inhibit the deep carbon deposition reaction,so stability of BZSM-5 is higher than AlZSM-5 and the activity of BZSM-5 is better than SiZSM-5 in MTH reaction.With the prolong of reaction time,the active sites of catalyst are gradually covered by carbon deposition,resulting in the decrease of acid amount and acid strength,the olefin polymerization,cracking,hydrogen transfer,cyclization and aromatization ability of the catalyst reduces.At the same time,the diffusion performance is reduced,therefore,the low carbon number products increase and the selectivity of high carbon number product ofdecreases with the increase of TOS.So the above results appear.

Fig.9.Hydrocarbon product distribution of the HBZSM-5 catalysts with different Si/B ratios.

Table 3 shows the average distribution of C5+products of HBZSM-5 during the period of 100% methanol conversion.It can be seen that the content of non-aromatics (non-Ar) in C5+products gradually decreases with the decrease of Si/B ratio of HBZSM-5 catalysts,while the aromatics content increases gradually.The content of benzene,toluene and ethylbenzene in aromatic products increased first and then decreased,xylene content increased gradually,C9aromatics (C9-Ar) and C10aromatics (C10-Ar) decreased first and then increased,and the content of hydrocarbons >C10was less than 1%.The acidity of HBZSM-5 is stronger than that of SiZSM-5,but weaker than that of AlZSM-5.Therefore,SiZSM-5 has almost no oil phase products and the content of non-Ar in C5+products of HBZSM-5 is higher than that of AlZSM-5.Combined with the characterization results of HBZSM-5 catalyst,the difference of C5+products may also be result in the common interaction of acidity and pore structure.B-1,B-2 and B-3 have the same amount of strong acid,but B-1 has nano crystal size,large number of mesoporous and wide pore size distribution,therefore,B-1 hasthe best diffusibility,and the formation of macromolecular aromatics and small molecular hydrocarbons is easy to diffuse out,which weakens the cracking reaction.So in the products the content of non-Ar is high,the content of high carbon number aromatics is high and the content of low carbon number aromatics is low.Similarly,the strong acid amount of B-4 and B-5 is close and the product distribution is different,which can be attributed to their different diffusion properties.

Table 3The averageproduct distribution of the HBZSM-5 catalysts with different Si/B ratios

Table 3The averageproduct distribution of the HBZSM-5 catalysts with different Si/B ratios

4.Conclusions

(1) Five kinds of BZSM-5 molecular sieves and a SiZSM-5 samples were synthesized by hydrothermal method with common and cheap raw materials.With the increase of Si/B ratios,the amount of acid decreases gradually,the weak acid becomes stronger,the strength of strong acid changes little,the main acid type of HBZSM-5 is weak and moderate B acid center,the pore volume of intercrystal mesoporous decreases and the external surface area increases in the meantime.

(2) Compared with SiZSM-5,the B acid of BZSM-5 molecular sieve is increased and the strong acid acidity is enhanced by introducing skeleton boron.The BZSM-5 catalyst prepared by initial gel of SiO2/B2O3of 20,which has moderate acid center density,acid type distribution,acid strength distribution,external specific surface area and intercrystal mesoporous.The best matching of acidity and pore structure makes the highest activity stability,reaching 672 h.

(3) Compared with AlZSM-5 and SiZSM-5,the BZSM-5 catalyst has weaker strong acid strength and the higher proportion of weak and moderate B acid center,which is conducive to improving the activity and stability of MTH reaction.The distribution ofand the content of non-Ar inproducts of BZSM-5 is higher which is due to the common interaction of acidity and pore structure.BZSM-5 has low hydrogen transfer ability,so the olefin content is relative high and the alkane content is low in the product distribution.

Acknowledgements

This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (CAS) (XDA21020500).