Fang Jin *,Xianqiao Wang #,Tieliang Liu Linbo Xiao 2,Ming Yuan Yangchun Fan

1 Key Laboratory for Green Chemical Process of Ministry of Education,Hubei Novel Reactor&Green Chemical Technology Key Laboratory,School of Chemical Engineering and Pharmacy,Wuhan Institute of Technology,Wuhan 430073,China

2 Hubei SanNing Chemical Industry Joint Stock Co.,Ltd,China

1.Introduction

Phosphate fertilizer industries have produced mass of gaseous silicon tetra fluoride(SiF4)and it was absorbed by water to form hexa fluorosilicic acid[1,2].Hexa fluorosilicic acids are naturally hazardous for the human and creature living in the water,soil and even the air[3].However,the by-product hexa fluorosilicic acid of phosphate fertilizer industry is regarded as a major fluorine and silicon-containing source[4].Hexa fluorosilicic acid has many applications by different industrial processes[5–7].Hexa fluorosilicic acid has been reported to produce fluorine–silicate,such as sodium fluorosilicate,potassium fluorosilicate,magnesium fluorosilicate and zinc fluorosilicate for many years[8].However,the market share of these salts is very small,that is why the applications of hexa fluorosilicic acid are greatly limited.Another product from hexa fluorosilicic acid is siliceous reinforcing agent,white carbon black[9–11].However,the economic value-added of this product is very low.

ZSM-5 zeolite with the characteristics of three-dimensional straight channel and acidity modification has been widely used in petrochemical industry as catalysts,adsorbents and others[12,13].Zeolite has been widely synthesized with silicon source ofwaterglass,silica sol,tetraethyl orthosilicate,fumed silicaetc.[14].The high cost of the silica source blocks the further industrialapplication of ZSM-5.If the hexa fluorosilicic acid can be used as silicon source for zeolite synthesis,this new method can greatly improve the economic value-added and solve the environmental problem of the phosphorus chemical industry by-product hexa fluorosilicic acid and reduce the cost of ZSM-5 production.

Industrial C9 components from oil reforming or pyrolysis process contain trimethylbenzene,ethyl benzene,propylbenzene,indan and other aromatic components.The majority of content in C9 aromatic is trimethylbenzene.Ifthe trimethylbenzene can be efficiently transferred to benzene,toluene and xylene(BTX),this method can achieve the rational utilization of resources and bene fit maximization of the resource.The hydrodealkylation and transalkylation of trimethylbenzene to produce high value-added products BTX have aroused the attention in the domestic and foreign researchers[15].

Here,a systemic method for ZSM-5 synthesis with the crude industrial hexa fluorosilicic acid as silicon and fluorine resources is reported.The ZSM-5 zeolite with a different SiO2/Al2O3ratio can be synthesized successfully under the optimal synthesis conditions.The obtained Mfizeolite from the process is applied as catalyst for toluene and 1,2,4-trimethylbenzene transalkylation and disproportionation.

2.Experimental

2.1.Preparation of materials

Industrial hexa fluorosilicic acid(10%–25%)was provided by a phosphate fertilizer factory from the phosphoric acid of wet process.Tetrapropyl ammoniumbromide(98%)comes from Aladdin Industrial Corporation;aluminum hydroxide brought from Sinopharm Chemical Reagent Co.,Ltd.;nano fumed silica(99.8%)comes from Aladdin Industrial Corporation;ammonium hydroxide(25%–28%),ammonium nitrate(99%)and toluene(99.5%)are commercial products from Xilong Chemical Co.,Ltd.;1,2,4-trimethylbenzene(98%)comes from Aladdin Industrial Corporation.Commercial Products ZSM-5 zeolite(SiO2/Al2O3molar ratio=38)comes from Sinopec Jinling Petrochemical Corporation South Re fined Institute.

2.2.Procedure and methods

Two-step ammoniation method was applied to recover high-quality silica and produce ammonium fluoride from hexa fluorosilicic acid reported in[1].The recovered SiO2and NH4F are applied assilicon and fluorine source for hydrothermalsynthesis ZSM-5 zeolites.The compositions were the following molar ratio:SiO2:NH4F:Al(OH)3:TPABr:H2O=10:x:y:z:330(x=0 or 9;y=0.83,0.33,0.16 or 0.08;z=2.5,1.25,0.625 or 0.5).The reagents were mixed in the following order:NH4F solid was added to distilled water,then tetrapropyl ammonium bromide(TPABr),Al(OH)3and SiO2hydrogel from the second step of the two-step ammoniation method are added sequentially with the constant stirring in 4 h.Then the stirred mixture was transferred to a hydrothermal crystallization kettle and crystallization for 1–7 d at 443 K.Then the crystallized products are filtered,washed with 1000 mlD.I.waterand dried at333 K overnightto obtain ZSM-5 precursor.The precursor is calcined at 823 K for 7 h in the Muf fle furnace to obtain ZSM-5 zeolite that indicated asx-ZSM-5(xstand for molar ratio of SiO2/Al2O3).To comparison with thex-ZSM-5 synthesized from recovered SiO2,the commercial fumed pure silica was used to synthesize ZSM-5 with the same procedure under the condition ofNH4F or not,the synthesized samples are indicated asx-ZSM-5(f).The parentx-ZSM-5 zeolite was exchanged with 1.0 mol·L?1NH4NO3solution at re flux for 3 times(8 h),fully washed,then dried at 393 K for 12 h and calcinated at 823 K in air,at last the H-formx-HZSM-5 zeolites were obtained.After calcinations,the commercial ZSM-5 zeolite is indicated as38-ZSM-5(S).

2.3.Catalysts characterization

Powder X-ray diffraction(XRD)was performed using a Panalytical X'Pert automatic diffractometer.The light source is Cu targetKαradiation(X-ray wavelength 0.1541874 nm),tube current is 40 mA,tube voltage is 45 kV,the diffraction pattern is recorded in 2θ of 5°–50°.Phase and relative crystallinities of zeolite samples were judged by JADE5 software.Apparent morphology and crystal size of the zeolite were observed with S-4800 type cold field emission scanning electron microscope(SEM).FALCON type Energy Dispersive Spectrometer(EDS)produced by EDAX company of America is applied to measure the elemental composition of the sample.The machine is rated at 4 kW,with cathode maximum current 160 mA.The thermogravimetric(TG)analysis is carried out at 473 K with 30 mg sample of particle sizes 0.18–0.28 mm at atmospheric pressure.The system was quenched at 823 K for 2 h in a flow of He at 20 ml·min?1(STP),then it was performed under the same He flow with a ramp of 10 K·min?1to 1073 K.N2adsorption–desorption measurements were carried out on Micromeritics VII2390 apparatus at 77 K.Prior to the analysis,the samples were degassed at 373 K under nitrogen purging for 1 h,then at 573 K under nitrogen purging for 1 h.The specific surface area was calculated by the Brunauer–Emmett–Teller(BET)method in the 0.05–0.25P/P0range.The pore volumes and their size distributions were derived from the desorption branch of the N2isotherms using the Barrett–Joyner–Halenda(BJH)method.

2.4.Catalytic reaction experiments

Toluene and 1,2,4-trimethylbenzene(3:1 ratio mixed by weight)transalkylation was carried out in a fixed-bed flow reactor under 3 MPa pressure with N2as carrier gas.The catalyst was activated in a gas flow at 773 K for 1 h and cooled down to the reaction temperature of 723 K.The WHSV was 3 h?1.Reaction products were analyzed by GC using capillary column FL-INNOWAX capillary column.The toluene and 1,2,4-teimethylbenzene conversion and product selectivities are calculated by the following equations.

3.Results and Discussion

3.1.Hydrothermal synthesis of ZSM-5 zeolite

3.1.1.The two-step ammoniation method for high purity SiO2recover from hexafluorosilicic acid

The results of the two-step ammoniation with different allocation proportion of ammonia usage in the first and second ammoniation steps are shown in Table 1.According to the report of Yuet al.[1],the high-grade amorphous silica can be obtained in the second step with the two-step hexa fluorosilicic acid ammoniation method.With the increasing amount of NH3·H2O in the first step,most of the impurities,such as Fe,Mg,Ca,and Na,in the fluorosilicic acid can be deposited in the first step.Although the amount of rest produced SiO2in the second step is decreased,it is getting purer.The obtained silica of the second step is more suitable for the preparation of the zeolite.According to the yield and purity of SiO2generated in the second step,the most important condition for preparing silica hydrogel is that the molar ratio of ammonia and fluorosilicic acid in the first step and the second step is 3 and 4.2,respectively.Under this condition,the yield of amorphous silica is 42.6%and 52.5%in the first and second ammoniation steps,respectively.Moreover,the produced SiO2purity in the second step is 97.5%according to EDS analysis results.It can be seen fromAppendix A(Table A1).The element composition of this SiO2is Al 0.71 wt%,Si 49.12 wt%,Mg 0.56 wt%,F 1.81 wt%and does not detect other impurities.

Table 1Effect of ammonia addition on the two-step ammoniation

3.1.2.Influenceofsilicatoaluminamolarratio(SiO2/Al2O3)onthesynthesis of ZSM-5

To investigate the in fluence of SiO2/Al2O3molar ratio on the zeolite synthesis,the ZSM-5 zeolite is hydrothermally synthesized with the SiO2/Al2O3ratio from 12 to 125 under the same reaction condition,such as reaction time stirring 4 d;SiO2/TPABr=8;SiO2/NH4F=1.11;H2O/SiO2=33.The EDS analysis results of the produced ZSM-5 are illustrated in Table 2,in which the relative crystallinity is calculated based on the XRD patterns as shown in Fig.1.

The XRD pattern of the zeolite samples with different SiO2/Al2O3ratio is shown in Fig.1,which only contains the characterized(1 0 1),(2 0 0),(0 2 0),(3 0 1)and(5 0 3)diffraction peaks of ZSM-5.The peaks at 2θ =23.2°(5 0 1)and 2θ =24.2°(3 1 3)indicate that the ZSM-5 is orthogonal phase[16].The results in Fig.1 show that with the increase of SiO2/Al2O3ratio,the peaks in the region of 2θ=7.5°–9.5°and 2θ =22.5°–24.5°changed obviously.In order to describe the change of these two regions more clearly,the partial enlarged detail of these two diffraction regions are shown in Fig.1(B)&(C).From Fig.1(B),it can be seen that the single diffraction peak at the(2 0 0)of 2θ=8.5°has become bimodal as the growth of SiO2/Al2O3ratio.At the same time,the two diffraction peaks at the(3 3 2)and(5 0 1)crystal planes merged to form one peak at the 2θ=23.08°in Fig.1(C).These results show that the crystal phase of ZSM-5 transfers from the orthogonal crystal phase to monoclinic phase[17].

The EDS analysis of this sample indicates that the element compositions ofthe ZSM-5 are Al1.62 wt%,Si35.17 wt%,O 63.21 wt%and do not detect other impurities.Although the silica gel from the second ammoniation step of the industrial hexa fluorosilicic acid contains the impurities of Mg ions,the metal ions are actually hard to be incorporated into the structure skeleton of ZSM-5 to replace silicon atoms.This result indicates that the impurity of Mg ions in the silica gel is hard to in fluent the synthesized zeolite.The elemental analysis results show that with the presence of the Al elements in the ZSM-5,the actual SiO2/Al2O3ratio in the zeolite is a little smaller than the theoretical ratio of hydrothermal solution as shown in Table 2.It can be deduced that the impurity Al of the silica gel can also be incorporated into the zeolite.Finally,the success ofthe synthesis ofthe ZSM-5 with different SiO2/Al2O3ratio is con firmed.

Table 2In fluence of SiO2/Al2O3 molar ratio and crystallization time on the synthesis of ZSM-5 zeolite

3.1.3.Influence of crystallization time on the synthesis of ZSM-5

Under the same molar ratio of SiO2:NH4F:TPABr:Al(OH)3:H2O=10:9:1.25:0.16:330,the XRD patterns of synthesized ZSM-5 zeolites with different crystallization time are shown in Table 2.The XRD patterns in Fig.2 show the in fluence ofcrystallization time on the synthesis of ZSM-5 zeolite.They have only the characteristic structure peaks of ZSM-5 zeolite as(1 0 1),(2 0 0),(0 2 0),(3 0 1)and(5 0 3).These results indicate that only Mfizeolite crystals are formed during the hydrothermal synthesis.There are separate(5 0 1)and(3 1 3)peaks in the 2θ=23.2°and 24.2°,which are corresponding to the orthogonal phase.With the increase of hydrothermal time,the diffraction peak in the 2θ=22.5°–24.5°has great change.From the enlargement of partial diffraction of Fig.2(A)&(B),it can be seen that the intensity of the separate diffraction peak at the(5 1 1)crystal plane in 2θ=23.8°and(3 1 3)crystal plane in 2θ =24.2°decreased and merged with the increase of crystallization time.This phenomenon is corresponding to the transfer of orthogonal crystal system to monoclinic system[17].

Fig.1.XRD patterns ofsynthesized ZSM-5 zeolite with different SiO2/Al2O3 molar ratio(a)23-ZSM-5,(b)26-ZSM-5,(c)32-ZSM-5 and(d)45-ZSM-5.(SiO2/NH4F=1.11;SiO2/TPABr=8;reaction time(stirring 4 d);H2O/SiO2=33).

Fig.2.XRD patterns of synthesized 32-ZSM-5 zeolite under dynamic condition with different hydrothermal time(a)dynamic 1d,(b)dynamic 2 d,(c)dynamic 3 d,(d)dynamic 4 d,(e)dynamic 7 d.(SiO2/Al2O3=32;SiO2/TPABr=8;SiO2/NH4F=1.11;H2O/SiO2=33).

3.1.4.The influence of dynamic and static hydrothermal condition on the ZSM-5 zeolite synthesis

Fig.3.XRD patterns of synthesized ZSM-5 zeolite under different hydrothermal condition(a)dynamic 1 d and(b)static 1 d.(SiO2/Al2O3=32;SiO2/TPABr=8;SiO2/NH4F=1.11;H2O/SiO2=33).

The XRD patterns of the ZSM-5 zeolite with dynamic or static reaction are shown in Fig.3.The zeolites synthesized both in the dynamic and static methods exhibit the ZSM-5 characteristic diffraction peaks.The intensity of the peak indicates that the ZSM-5 synthesized with dynamic method under 1 d hydrothermal treatment possesses higher crystallinity than that synthesized with static method under the same time.The detailed topography information shown in Fig.3(B)indicates that the ZSM-5 synthesized in the dynamic condition has two diffraction peaks at around 2θ =23.2°and 24.2°for the(5 0 1)and(3 1 3)crystal planes,which is corresponding to a typical orthogonal crystal system.However,these two peaks merge to a single peak at 2θ =23.5°–24°for ZSM-5 synthesized under the static reaction,which is the characteristic of the monoclinic phase[17].

3.1.5.Influence of the molar ratio of silica to TPABr(SiO2/TPABr)on the synthesis of ZSM-5

The synthesized ZSM-5 with the composition of SiO2:NH4F:TPABr:Al(OH)3:H2O=10:9:x:0.16:330 and different ratio of SiO2/TPABr is characterized by the thermogravimetric analysis(DTG/TG)and N2de/adsorption and shown in Table 3.The DTG/TG curves of ZSM-5 zeolite precursor with different SiO2/TPABr ratio are shown in Fig.4.There is no mass loss with the temperature lower than 373 K,which suggests that there is no adsorption water retained in the sample,because the ZSM-5 zeolite samples have been dried in the 333 K of oven for 12 h before test.The total mass loss is about 12%from 333 K to 1073 K,the organic templates are burned out between 673 and 773 K[18].The samples with the SiO2/TPABr ratio lower than 8 have almost the same mass loss for the removing surfactant during TG test.These phenomena show that the surfactant incorporated to the samples does not increase with the increase of TPABr contents in the hydrothermal solution,when the SiO2/TPABr ratio is up to 8.The maximum amount of TPABr incorporated in the sample is about 12%in the synthesized ZSM-5 precursor[19].The nitrogen sorption isotherm in Appendix Fig.A1 is typical type I isotherm and depicts that the adsorption curve rapidly rising before the relative pressure is 0.1,which is attributed to typical of microporous nature.

Table 3The effect of the SiO2/TPABr molar ratio on the synthesis of 32-ZSM-5 zeolite

3.1.6.Influence of the molar ratio of silica to NH4F(SiO2/NH4F)on the synthesis of ZSM-5

The XRD patterns of synthesized ZSM-5 by the silica source from industrial hexa fluorosilicic acid are shown in Fig.5.All the32-ZSM-5 samples with NH4F and without NH4F as mining agent during hydrothermal process only contain the ZSM-5 character peaks.The detailed topography information shown in Fig.5(C)indicates that the ZSM-5 synthesized with NH4F and without NH4F has two diffraction peaks at 2θ=23.2°and 24.2°for the(5 0 1)and(3 1 3)crystal planes,which are corresponding to a typical orthogonal crystal system.However,the relative intensity of the peaks(1 0 1),(2 0 0)and(0 2 0)between 2θ=7.5°and 9.5°is different for the samples32-ZSM-5 with NH4F and without NH4F.The increased relative intensity of peak(2 0 0)and(0 2 0)to(1 0 1)for32-ZSM-5 with NH4F compared with the32-ZSM-5 without NH4F indicates that the32-ZSM-5 with NH4F has longer order structure alongboraaxis direction.These phenomena are further con firmed by SEM characterization as shown in Fig.6.The crystal structure of synthesized32-ZSM-5 with or without fluorine as mining agent during hydrothermal process is almost the same[13].It can be seen that the crystal size of the32-ZSM-5 with fluorine ions in the hydrothermal process is about 50 μm × 10 μm × 10 μm.While the crystal size of the ZSM-5 without fluorine ions in the hydrothermal process is about 10 μm × 1 μm × 0.2 μm.Therefore,the32-ZSM-5 with NH4F is thicker than that without NH4F along thebaxis direction.

To investigate the in fluence of fluorine ions on the synthesis of the ZSM-5,the synthesized samples of32-ZSM-5(f)with NH4F and without NH4F from fumed pure SiO2are also characterized as the XRD pattern in Fig.5 and SEMpictures in Fig.6.The crystalsize ofZSM-5(f)with fluorine ions is about 50 μm × 10 μm × 10 μm.It can be seen from Figs.5&6 that under the fluorine ion hydrothermal condition the32-ZSM-5(f)synthesized fromfumed silica hasalmostthe same SEMmorphology and XRD pattern with32-ZSM-5 synthesized from recovered silica.While the XRDpattern ofsample(e)in Fig.5 shows thatwithout fluorine ion condition during the hydrothermal process,the ZSM-5 crystal structure cannotbe synthesized with the pure fumed SiO2.Therefore,itcan be deduced that the fluorine content of the recovered SiO2from H2SiF6has great in fluence on the hydrothermal process for ZSM-5 crystal structure formation,while the small amount of fluorine ion content in the recovered SiO2causes the formation of the thin disk crystal along thebaxis direction compared with32-ZSM-5 synthesis with NH4F addition.Moreover,the additional amount of fluorine ion content in the hydrothermal process can control the crystal morphology and size of synthesized ZSM-5.

Fig.4.The thermal gravimetric analysis results of the ZSM-5 with SiO2/TPABr=(8,16,20 or 40).(SiO2/Al2O3=32;SiO2/NH4F=1.11;reaction time(stirring 4 d);H2O/SiO2=33).

For further comparison,the commercial38-ZSM-5(S)with small particle size synthesized under the hydrothermal condition without fluorine ion content is also applied to XRD and SEM characterizations.Ithas smallcrystalparticle size and uniform morphology in 3Ddirection compared with the sample32-ZSM-5(f)hydrothermal synthesized under NH4F addition.These results are consistent with the report of Aielloet al.[20]that the addition of fluorine ion can increase the growth of zeolite crystal,because the addition of fluoride ion can promote the solution of SiO2and Al(OH)3to form the complex fluorides and speed up the crystallization rate and shorten the reaction time[21].

Fig.5.XRD patterns of different zeolite(a)38-ZSM-5(S),(b)32-ZSM-5 with NH4F and(c)32-ZSM-5 without NH4F.(d)32-ZSM-5(f)with NH4F,(e)32-ZSM-5(f)without NH4F.(SiO2/TPABr=8;SiO2/Al2O3=32;reaction time(stirring 4 d);H2O/SiO2=33).

3.2.Catalytic properties of different SiO2/Al2O3 molar ratio HZSM-5

The average results of the toluene and 1,2,4-teimethylbenzene conversion and product selectivities after 8 h reactions are shown in Table 4.According to the reaction products,the possible reaction elementary steps in the transalkylation process are as Fig.7.Benzene is product from toluene disproportionation or dealkylation reaction[22].Xylene isomers can be produced from the toluene disproportionation or transalkylation of toluene and trimethylbenzene or disproportionation of trimethylbenzene.Compared with the purposed product of xylene isomers from transalkylation reaction,trimethylbenzene isomerization product selectivities are less than 10%in total product selectivity.For the synthesizedx-HZSM-5 with the increase of SiO2/Al2O3ratio,the amount of acid sites should decrease and the acid strength raised[23,24].At the same time,the toluene and 1,2,4-trimethylbenzene conversion have a clear decreasing trend with the decreased acid amount.While the sum of benzene and xylene selectivities decreases with the increase of the SiO2/Al2O3ratio,the selectivities of trimethylbenzene isomerization reactions are increased,because the strength acid sites are more favorable for trimethylbenzene isomerization,which is consistent with the phenomena reported in the literature[25].For the23-ZSM-5,toluene and 1,2,4-trimethylbenzene conversion can reach 53%and 54%,respectively.The maximum of averagep-xylene selectivity reaches 15%as shown in the Figs.8&9.

3.3.Catalytic propertiesofdifferentparticle size and morphology ofHZSM-5

Catalytic properties for toluene and 1,2,4-trimethylbenzene conversion of38-HZSM-5(S)and the32-HZSM-5 with or without NH4F addition during hydrothermal process are compared and shown in Table 5.For the sample32-HZSM-5 without NH4F,the average toluene and 1,2,4-trimethylbenzene conversion reach 55%and 63%,while the total xylene selectivity and benzene selectivity is 62.5%and 28%,respectively.On the other hand,the average toluene conversion and the 1,2,4-trimethylbenzene conversion are 56%and 60%,respectively,and the total xylene selectivity and benzene selectivity are 61.5%and 27.5%for38-HZSM-5(S),respectively.According to Figs.10&11,the32-HZSM-5 without NH4F has a similarcatalytic properties with38-HZSM-5(S)for the 1,2,4-trimethylbenzene and toluene conversion.

Fig.6.SEM photographs of the different ZSM-5 zeolite:32-ZSM-5 with NH4F(A&B),32-ZSM-5 without NH4F(C&D),commercial 38-ZSM-5(S)(E&F);32-ZSM-5(f)with NH4F(G&H).(SiO2/TPABr=8,SiO2/Al2O3=32,reaction time(stirring 4 d);H2O/SiO2=33).

Table 4The catalytic performance of different SiO2/Al2O3 molar ratio of HZSM-5 samples for transalkylation of toluene and trimethylbenzene

Fig.7.Reaction elementary steps in transalkylation and disproportionation process of toluene and 1,2,4-trimethylbenzene.

Fig.8.Conversion as a function of the time on stream over HZSM-5 catalysts with various SiO2/Al2O3 molar ratio(a)23-HZSM-5,(b)26-HZSM-5,(c)32-HZSM-5 and(d)45-HZSM-5 at 723 K with WHSV of 3 h?1.

ZSM-5 zeolite possesses a uniform pore size distribution without cage,which inhibits the generation of macromolecular coke precursor deposited in the zeolite[24].Moreover,the commercial38-HZSM-5(S)has smaller particle size but with a homogeneous 3D orientation of Mfizeolite crystals.Therefore,the38-HZSM-5(S)processes a good catalytic stability.The synthesized32-ZSM-5 without NH4F is a thin circular disk alongbaxial direction with a bigger(0 1 0)crystal plane and a smaller section(1 0 0)crystal plane[26],as shown in Appendix Fig.A2.Therefore,it has relatively short straight channel along thebdirection,which is more unblocked,particularly conducive to molecule diffusion.The pore distance alongbaxis of ZSM-5 is shortened,and the diffusion of the reaction molecules in it is easier during catalytic reaction.Although,the32-ZSM-5 without NH4F has a larger size than38-HZSM-5(S),the short distance alongbaxial direction makes it a good service life as the38-HZSM-5(S).

Fig.9.The average product selectivity during 8 h reaction over HZSM-5 catalysts with various SiO2/Al2O3 molar ratio(a)23-HZSM-5,(b)26-HZSM-5,(c)32-HZSM-5 and(d)45-HZSM-5 at 723 K with WHSV of 3 h?1.

Table 5The average activity data of the samples 38-HZSM-5(S),32-HZSM-5 with NH4F or without NH4F

Fig.10.Conversion as a function of the time on stream over(a)commercial 38-HZSM-5(S),(b)32-HZSM-5 without NH4F and(c)32-HZSM-5 with NH4F at 723 K with WHSV of 3 h?1.

Although the crystal structure of synthesized ZSM-5 with or without NH4F as mining agent during hydrothermal process has similar morphology ofacplan preferred orientation growth crystal.The crystal size of ZSM-5 with NH4F is much bigger than the32-HZSM-5 without NH4F and commercial38-HZSM-5(S)[27].The larger size crystal particles own the smaller external surface area and less active center for large size molecule,i.e.,1,2,4-trimethylbenzene reaction.The diameter of the corresponding aromatic molecules is shown in Appendix Table A2.Therefore,the32-HZSM-5 with NH4F has relatively lower catalytic properties than the other two samples[28].But the32-HZSM-5 with NH4F has the highest ratio for theSxylene/Sbenzene,it is more conducive to transalkylation of toluene and 1,2,4-trimethylbenzene than other side reaction,such as disproportionation of toluene or 1,2,4-trimethylbenzene.

Fig.11.Product selectivity as a function of the time on stream over(a)commercial 38-HZSM-5(S),(b)32-HZSM-5 with NH4F and(c)32-HZSM-5without NH4F at 723 K with WHSV of 3 h?1.

Appendix

Table A1The element content in high purity SiO2 recovered from hexa fluorosilicic acid

4.Conclusions

The two-steps ammoniation method recovered silica hydrogel from hexa fluorosilicic acid can be directly used for ZSM-5 zeolite synthesis.The optimal hydrothermal conditions of ZSM-5 synthesis are investigated.It was found that with the increase of SiO2/Al2O3ratio and reaction time,the crystal type of ZSM-5 transforms from the orthorhombic to the monoclinic phase.The EDS analysis ofthese samples indicatesthatthe elementcomposition ofZSM-5 only contains Al,Si,and Owithoutotherimpurities.The ZSM-5 synthesized with recovered SiO2has the morphology ofacplane preferred orientation growth crystal.It can be deduced that without the impurity fluorine content of the recovered SiO2from H2SiF6the ZSM-5 crystal structure cannot be produced on the hydrothermal process for our synthesis procedure.The increased fluorine ion content by adding NH4F during hydrothermal process can improve the growth of ZSM-5 crystal alongbaxis and get large size crystal,because the fluorine ions can promote the dissolution of the silica and aluminum source and shorten the hydrothermal reaction time.The catalytic performance ofZSM-5 in the transalkylation oftoluene and 1,2,4-trimethylbenzene reaction indicates that with the increase of SiO2/Al2O3ratio of ZSM-5,the toluene and 1,2,4-trimethylbenzene conversions decrease,the sum of benzene and xylene selectivities decreases,and the 1,2,3-trimethylbenzene selectivity is on the rise.The morphology character of ZSM-5 produced from recovered SiO2has relatively short straight channel along thebdirection,which shows similar catalytic activity and service life as the performance of small particle size commercial ZSM-5.The32-HZSM-5 hydrothermal synthesized with NH4F as mining agent has larger particle size and highest ratio for theSxylene/Sbenzene,which is more conducive to transalkylation oftoluene and 1,2,4-trimethylbenzene than other side reaction,such as disproportionation of toluene or 1,2,4-trimethylbenzene.

Table A2The diameter of the aromatic molecules

Fig.A1.N2 ad/desorption spectra of 32-ZSM-5 zeolite.(SiO2/TPABr=8;SiO2/Al2O3=32;reaction time(stirring 4d);H2O/SiO2=33).

Fig.A2.Channel structure of ZSM-5 zeolite[24].

[1]H.S.Yu,K.I.Rhee,C.K.Lee,D.H.Yang,Two-step ammoniation of by-product fluosilicic acid to produce high quality amorphous silica,Korean J.Chem.Eng.17(2000)401–408.

[2]J.X.Sun,R.X.Lin,et al.,Ammonification technology of silica production by using waste fluorosilicic acid as by product of phosphate fertilizer production,Inorg.Chem.Ind.41(2009)48–50.

[3]Y.N.Wu,J.W.Tang,Effect of template type and template silica mole ratio on the crystallinity of synthesized nanosized ZSM-5,Chem.Ind.Eng.Prog.30(2011)332–335.

[4]H.Xu,T.An,Y.Liu,G.Li,New process of preparing anhydrous hydrogen fluoride ammonium by fluorosilicic acid,Chem.Eng.8(2014)76–78.

[5]A.Krysztafkiewicz,B.Rager,M.Maik,Silica recovery from waste obtained in hydro fluoric acid and aluminum fluoride production from fluosilicic acid,J.Hazard.Mater.3(1996)31–49.

[6]L.S.Du,Y.P.Li,Comprehensive utilization status and development thinking of the byproducts fluorine and silicon of phosphate fertilizer production,Phos.Comp.28(2013)66–69.

[7]A.J.Lu,H.L.Xu,F.Y.Lu,Preparation ofhigh purity silica and ammonium fluoride from fluosilicic acid,He Nan.Chem.Ind.12(2002)17–19(in Chinese).

[8]X.Liu,A.Lu,Y.Liu,X.Liu,A new process for producing high purity SiO2 from lf uosilicic acid and annonium hydrocarbonate,Appl.Chem.Ind.6(2002)23–25.

[9]S.Y.Jeong,J.K.Suh,J.M.Lee,O.Y.Kwon,Preparation of silica-based mesoporous materials from fluorosilicon compounds:gelation of H2SiF6in ammonia surfactant solution,Colloid Interface Sci.192(1997)156–161.

[10]S.R.Ying,Z.Jiang,Technology and process of high purity quartz sand produced by fluosilicic acid,Chem.Prod.Tech.20(2013)27–30.

[11]P.B.Sarawade,J.Kim,A.Hilonga,H.T.Kim,Preparation of hydrophobic mesoporous silica powder with a high specific surface area by surface modification of a wet-get slurry and spray-drying,J.Hazard.Mater.173(2010)576–580.

[12]J.L.Guth,L.Delmotte,M.Soulard,Synthesis of Al,Si,MFI-type zeolites in the presence of fanions:structural and physicochemical characteristics,Zeolites12(1992)929–935.

[13]H.Jon,Y.Oumi,K.Itabashi,T.Sano,Synthesis and characterization of large beta zeolite crystals using ammonium fluoride,J.Cryst.Growth307(2007)177–184.

[14]R.M.Mohamed,H.M.Aly,M.F.El-Shahat,I.A.Ibrahim,Effect of the silica sources on the crystallinity of nanosized ZSM-5 zeolite,Microporous Mesoporous Mater.79(2005)7–12.

[15]O.Xu,H.Su,J.Ji,X.Jin,J.Chu,Kinetic model and simulation analysis for toluene disproportionnation and C9-aromatics transalkylation,Chin.J.Chem.Eng.15(2007)326–322.

[16]I.Schmidt,C.Madsen,C.J.Jacobsen,Con fined space synthesis:a novel route to nanosized zeolites,Inorg.Chem.39(2000)2279–2283.

[17]G.T.Kokotailo,L.Riekert,A.Tissler,Phase transformations and changes in lattice parameters of ZSM-5 as a function of Al content and temperature,Stud.Surf.Sci.Catal.46(1989)821–826.

[18]O.A.Fouad,R.M.Mohamed,M.S.Hassan,I.A.Ibrahim,Effect of template type and template/silica mole ratio on the crystallinity of synthesized nanosized ZSM-5,Catal.Today116(2006)82–87.

[19]Y.Cheng,L.Wang,J.Li,Y.Yang,X.Sun,Preparation and characterization of nanosized ZSM-5 zeolites in the absence of organic template,Mater.Lett.59(2005)3427–3430.

[20]R.Aiello,F.Crea,E.Nigro,F.Testa,R.Mostowicz,A.Fonseca,J.B.Nagy,The in fluence of alkali cation on the synthesis of ZSM-5 in fluoride medium,Microporous Mesoporous Mater.28(1999)241–259.

[21]B.Louis,L.Kiwi-Minsker,ZSM-5 coating on stainless steel grids in one-step benzene hydroxylation to phenol by N2O:reaction kinetics study,Microporous Mesoporous Mater.74(2004)171–178.

[22]A.Baduraig,T.Odedairo,S.Al-Khattaf,Disproportionation and methylation of toluene with methanol over zeolite catalysts,Top.Catal.53(2010)1446–1456.

[23]L.Shirazi,E.Jamshidi,M.R.Ghasemi,The effect of Si/Al ratio of ZSM-5 zeolite on its morphology,acidity and crystal size,Cryst.Res.Technol.12(2008)1300–1306.

[24]J.M.Berak,B.Kanik,P.Eysymontt,J.Mejsner,Deactivation of high silica zeolite by steam treatment,React.Kinet.Catal.Lett.25(1984)3–4.

[25]Z.K.Xie,Q.L.Chen,Disproportionation of toluene and transalkylation of C9 aromatics over H-βzeolite,East China Univ.Sci.Technol.26(2000)260–264(in Chinese).

[26]Y.Liu,X.Zhou,X.Pang,Y.Jin,X.Meng,X.Zheng,X.Gao,F.Xiao,The in fluence of alkali cations on the synthesis of ZSM-5 in fluoride medium,Chem.Cat.Chem.5(2013)1517–1523.

[27]J.Shi,G.L.Zhao,J.W.Teng,etal.,Advances in the research of Mfizeolite morphology,Prog.Chem.26(2014)545–552.

[28]Q.Dai,S.Bai,X.Wang,G.Lu,Facile synthesis of HZSM-5 with controlled crystalmorphology and size as efficient catalysts for chlorinated hydrocarbons oxidation and xylene isomerization,J.Porous.Mater.21(2014)1041–1049.