单一模板线性多胺一步合成多级孔ZSM-5沸石微球

陈 丽 薛 腾 朱树燕 王一萌(华东师范大学化学系,上海市绿色化学与化工过程绿色化重点实验室,上海200062)

单一模板线性多胺一步合成多级孔ZSM-5沸石微球

陈 丽 薛 腾 朱树燕 王一萌*
(华东师范大学化学系,上海市绿色化学与化工过程绿色化重点实验室,上海200062)

线性多胺模板剂(二乙烯三胺(DETA)、三乙烯四胺(TETA)、四乙烯五胺(TEPA))作为单一模板合成了均匀的10-15 μm的多级孔沸石ZSM-5微球.实验发现,在水热过程中,50 nm纳米的沸石晶体粒子自发地堆积成微球,并且这些微球具有较大的介孔体积,在傅-克烷基化反应中具有高的催化活性.同时,我们发现线性多胺起到了模板剂和空隙填充剂的多重功能,这点明显不同于以前我们报道的线性二胺作为模板剂在合成ZSM-5沸石时,主要是起模板剂的作用.

多级孔沸石;聚集体;单一模板;线性多胺

1 Introduction

The hierarchical ZSM-5(MFI)zeolites possessing large surface area,bimodal porosity containing both micropore and mesopore and interconnected mesopores with zeolitic pore walls have been widely investigated in the synthesis and catalytic applications by many researchers.Therefore,significant efforts have been devoted to developing methods to introduce mesoporosity into zeolite materials.1-14Studies show that structure-directing agents/templates could influence the physicochemical properties,catalytic activity,and crystalline morphology,and so on.Generally,the templating approaches make it possible a priori to tailor the pore size of the mesopores by using mesopore template with a special size,which leaves mesopores with essentially the same size and shape as that of the mesopore template after the removal of template.In the multi-template synthesis strategies,crystalline zeolite containing both microporous and mesoporous structures in a single phase is difficult to be obtained,if the aluminosilicate gel is directly crystallized in the presence of ordinary template formesopore and molecular templates for zeolites.Only when both surfactants and molecular templates are not expelled from the aluminosilicate phase during the zeolite crystallization process,it is possible to synthesize mesoporous zeolite without the formation of physical mixture of bulk zeolites and mesoporous materials.1-7Recently,one-step synthesis of hierarchical zeolite aggregates with single template has attracted great attentions comparing with double template methods mentioned above,where the synthesis mechanism and the bimodal pore size can be easily controlled in the synthesis system using a single template.8-12

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Ryoo and co-workers7directly synthesized mesoporous MFI and LTAzeolites using specifically designed amphiphilic organosilanes.Later,the same group reported that stable single-unit-cell nanosheets of zeolite MFI were synthesized with a specially designed bifunctional template that possesses two quaternary ammonium groups spaced by a C6alkyl linkage and a long chain alkyl group(C22)on the end.9In these two cases,the template for mesopore was directly connected with zeolitic template via covalent bond in a single molecule,which effectively prevented the formation of physical mixture of bulk zeolite and mesoporous materials.Fang et al.8firstly reported in-situ assembly of zeolite nanocrystals into mesoporous aggregate with single-crystal-like morphology using small organic molecule as single template but without secondary template.Furthermore,Ryoo and co-workers showed that cyclic diammonium as single template can direct the formation of zeolite structures on both mesoporous and microporous length scales simultaneously,where the formation of zeolites with nanocrystalline morphologies and intercrystalline mesoporosity could attribute to particularly high affinity of the molecular template having an organic cyclic diammonium(CDM) structure with silicate species.10,11Recently,house-of-cards-like ZSM-5 prepared by adding the N-methyl-2-pyrrolidone in zeolite synthetic mixture presented much better catalytic performances in the cracking of cumene and 1,3,5-triisopropylbenzene(TIPB) than conventional porous catalysts(ZSM-5,Y zeolite,and Al-MCM-41).12Chen et al.13reported that mesoporous zeolite aggregates can be synthesized using diamine with linear carbon chain as single template without secondary template by self-assembly of in-situ formed zeolite nanocrystals.Since large hierarchical zeolite aggregates not only have reduced diffusion limitation but also can be easily recovered by conventional filtration, the development of new methods to synthesize large ZSM-5 hierarchical zeolite aggregates with large mesopore volume,large surface area and high catalytic activity are still highly anticipated.

Here,our work has been extended to linear alkyl-polyamines with different oligomerization degrees that were used to control the morphology,textual property,and catalytic property of hierarchical zeolite.Alkyl-polyamines have received important attentions since they were used in the synthesis of open-framework metal phosphates,membranes,and chelating resins and showed excellent properties for their active imino-nitrogen in host-guest charge balance,coordinative activity,and hydrogen bonding capability with the framework O atoms.15-17Meanwhile,they were widely used in the synthesis of ZSM-5 zeolite due that alkylpolyamines exhibited their strong structure-directing role because of the charge balance and hydrogen bonding capability of active imino-nitrogen with the framework O atoms,where the rectangular sheet conglomeration crystals in size of ca 15 μm×2.5 μm× 1 μm were obtained.18As we all know,the reduction of zeolite particle sizes could been employed as a means to reduce the intracrystalline diffusion path length,a hierarchical aggregates assembled by nanocrystallines have large surface area and the realtively large aggregates may also curtail the filtration difficulities in the preparation and catalytic applications to some extent.Adetailed study about synthesis and catalytic performance of the hierarchical microspheres assembled by nanocrystal aluminosilicate with the use of alkyl-polyamines as single template has not reported.

In our work,we successfully synthesize hierarchical ZSM-5 microspheres using linear alkyl-polyamine as single template.The effects of different chain lengths of alkyl-polyamines,SiO2/Al2O3molar ratio,alkalinity,and H2O/SiO2molar ratio on the physical and catalytic properties were investigated.Furthermore,the role of the alkyl-polyamine templates in the synthesis of mesoporous zeolite aggregate was discussed.

2 Experimental

2.1 Reagent

Sodium metasilicate nonahydrate(Na2SiO3·9H2O),hydrochloric acid(HCl),aluminum sulfate(Al2(SO4)3·18H2O)were purchased from Sinopharm Chemical Reagent Co.,Ltd.,China.All the chemicals were used directly without further purification.

2.2 Synthesis of hierarchical ZSM-5 micropheres

The mesoporous ZSM-5 zeolite material was hydrothermally synthesized as follows:the solution of Al2(SO4)3·18H2O dissolved in an aqueous HCl solution was added drop-by-drop to the aqueous solution of Na2SiO3·9H2O,which was kept under stirring until pH value lowered to 8.And then the fresh precipitated alumino-silica was recovered by filtration after being continuously stirred for 2 h.Subsequently the obtained solid was added into H2O and required linear alkyl-polyamines(such as diethylenetriamine (DETA),triethylenetetramine(TETA),tetraethylenepentamine (TEPA)).The molar composition of the synthetic mixtures was 60SiO2:xAl2O3:6Na2O:1440H2O:alkyl-polyamines(N/Si molar ratio is 0.42),where x=1,2,respectively.After being stirred for 2 h,the mixtures were transferred into Teflon-lined stainless steel autoclave,where they were heated under autogenous pressure for prescribed time at 448 K.The products obtained were filtered, washed and then dried overnight at 373 K.The obtained samples were denoted as M-ZSM-5-N,where M is the template;N refers to SiO2/Al2O3molar ratio.The organic component was removed by calcinations in air at 823 K for 6 h.Ion exchanges were carried out four times with 0.6 mol·L-1NH4NO3solution at 353 K each for 1 h,followed by calcination at 823 K in air for 6 h to give H-form products.

2.3 Characterization

The particle sizes and morphology of the mesoporous zeolitemicrosphere were characterized by scanning electron microscopy (SEM,type HITACHI S-4800)with an accelerating voltage of 3 kV.Transmission electron microscopy(TEM)characterization was carried on a JEM-2010 operating at 200 kV.For the TEM images the specimens were dispersed in ethanol and placed on holey copper grids.Powder X-ray diffraction patterns(XRD)of as-synthesized samples on a Bruker D8 Advance powder diffractometer using Cu Kαradiation(λ=0.154184 nm)over a 2θ range from 5°to 35°,the accelerating voltage and the applied current were 40 kVand 40 mA,respectively.Nitrogen adsorptiondesorption measurements were carried at 77 K on a nitrogen adsorption apparatus(Quantachrome Autosorb-3B).The surface area(SBET)was calculated using the Brunauer-Emmett-Teller (BET)method,the total pore volume was estimated from the amount of nitrogen adsorbed at the relative pressure(p/p0)of 0.99, and the micropore volume was calculated from the t-plot.Solidstate magic angle spinning(MAS)NMR spectra were recorded on a Bruker DSX300 spectrometer.And Infrared(IR)spectra were characterized by SpectrumTM GX.

The alkylation of phenol with tert-butanol was carried out in an oil bath at 100°C for 4 h under stirring condition in a 50-mL round-bottomed flask immersed in a thermostatted water bath and equipped with a condenser.The stirring was sufficiently strong to rule out the effect of external diffusion on catalytic performance for all of the batches.In a typical batch,0.15 g of catalyst,7.78 g of cyclohexane as solvent,0.74 g of tert-butanol,0.47 g of phenol were used.The product was analyzed using a Shimadzu GC-2014 gas chromatograph equipped with a 30-m Wax capillary column and a flame ionization detector.

3 Results and discussion

The X-ray powder diffraction patterns of zeolites are shown in Fig.1.Diffraction peaks located at about 8°,9°,23°,24°,25°(2θ) clearly reveal that ZSM-5 zeolite had been synthesized19and no other peak could be observed,indicating that high purity of ZSM-5 products with MFI structure can be synthesized under the SiO2/ Al2O3molar composition of 30.However,when the SiO2/Al2O3molar ratio increased from 30 to 60 in the synthetic mixtures, some impurities ofα-quartz formed along with ZSM-5,which indicated that there was a relatively narrow range of SiO2/Al2O3molar ratio for the crystallization of ZSM-5 zeolite using alkylpolyamines as template.18This is different from our previous report,13where pure ZSM-5/ZSM-11 zeolites can be obtained with the SiO2/Al2O3molar ratios ranging from 30 to 100.Noticeably, TETAas structure-directing agent/template was more hydrophilic than 1,8-diaminoctane(DAOT)and showed a strong interact with hydrophilic framework aluminum,which is unfavorable for the formation of high silica zeolite.Moreover,ZSM-11 with MEL structure can be obtained when the DAOT acted as the structuredirecting agent/template,while the ZSM-5 structure with MFI can be synthesized when the TETAwas used as the structure-directing agent/template.This may be due that the configuration of DAOT is straight and it main located at the straight pore channel in the ZSM-11,while,the configuration of the TETAis a zigzag chain which was used to construct the ZSM-5 three-dimensional framework.18TETAexhibits stronger structure-directing role than DAOT because of its host-guest charge balance and hydrogen bonding capability with the framework O atoms,leading to the formation of highly crystalline ZSM-5 zeolite only after 7 days, while zeolite ZSM-11 well formed after 10 days in the presence of DAOT.

Fig.1 Powder XRD patterns of the as-synthesized ZSM-5 microspheres synthesized by DETA-ZSM-5-60(a),

As shown in the SEM images of Fig.2,the resulting materials were both composed of relatively uniform spherical particles of 10-15 μm in diameter,obviously larger than the spherical aggregates(5-7 μm)obtained in the literature.13The insets were the high magnification SEM images of a single microsphere,showing that the microsphere is built of small particles.Further increasing the magnification reveals that the small particles have regular crystal-like shapes and are in size of only～50 nm(Fig.2(a-c), inset),much smaller than that for ZSM-5 nanocrystals synthesized with the SiO2/Al2O3molar ratio of 60(Fig.2d,inset).These results suggest that the microspheres are aggregates of nanosized ZSM-5 crystals in present study,quite different from the shape of rectangular sheet conglomeration crystals with a size of about 15 μm×2.5 μm×1 μm,18which may attributed to the different nature and concentration of the ingredients and the different molar composition of synthetic mixture result in the different nuclei amount of ZSM-5 and crystalline rate despite same alkyl-polyamine used template.Relatively large size of the aggregates is apositive fact,in favor of easy recovery of the zeolite product by conventional filtration or centrifugation,otherwise it would be very difficult to separate and recover completely isolated or highly dispersed nanocrystals.The TEM image is in good agreement with SEM observation,showing a micrometer-sized particle with spherical morphology made of zeolite nanocrystals rather than amorphous silica,as shown in Fig.2f.

Fig.2 SEM images of zeolite ZSM-5 microspheres DETA-ZSM-5-30(a),TETA-ZSM-5-30(b),TEPA-ZSM-5-30(c),and TEPA-ZSM-5-60(d);TEM image of a single microsphere TETA-ZSM-5-30(e);high-resolution TEM image taken at the edge of the microsphere TETA-ZSM-5-30(f)

The effects of Na2O/SiO2and H2O/SiO2molar ratios in the synthesis mixture are presented in the SEM images of Fig.3. When Na2O/SiO2molar ratios range from 0.08 to 0.12,the size of primary particles in zeolite microspheres becomes non-uniform. However,the aggregates retain microspherical morphology with a similar size.Furthermore,the morphology of the TEPA-ZSM-5-30 samples is affected by the H2O/SiO2molar ratio.In case of n(H2O)/n(SiO2)=19,both the size of primary particle and the aggregate morphology are quite similar to those for TEPA-ZSM-5-30 synthesized under the conditions of n(Na2O)/n(SiO2)=0.08 in Fig.3a.With H2O/SiO2molar ratio increasing up to 50,a decreasing of alkalinity in the synthesis mixture results in slow crystallization rate and some amorphous silica was presented in the sample as shown in Fig.3d.The larger is the molar ratio of H2O/SiO2,the more amorphous silica is present in the product after the same crystallization time,as shown in the Fig.3d.

Fig.3 SEM images of the as-synthesized TEPA-ZSM-5-30 zeolites synthesized at 428 K for 214 h at the molar ratios of

N2adsorption-desorption isotherms measured at 77 K are shown in Fig.4,and the data of surface area,total volume,and micropore volumes of all the samples with different SiO2/Al2O3molar ratios are listed in Table 1.The isotherms show type IV isotherms with hysteresis loops in the relative pressure(p/p0)range of 0.5-0.9, indicating the presence of irregular mesopore.The insert of Fig.4B shows that the amount of physical adsorption increases continuously in the range of 2-50 nm,further proving the mesopore in these zeolites.All the samples have similar micropore volume of 0.10-0.13 cm3·g-1,while the mesopore volume varies from 0.09 to 0.16 cm3·g-1.The samples TEPA-ZSM-5-30 have quite a large mesopore volume of 0.16 cm3·g-1.These results prove that alkyl-polyamines as single template could also lead to hierarchical zeolite aggregates with rich mesopore volume. Compared with that of zeolite synthesized with DAOT,the mesopore volume of zeolite prepared by TETA template was low.13BET surface areas of these zeolite microspheres amount to 270-350 m2·g-1,similar to the hierarchical zeolite microsphere synthesized using diamines template,13slightly higher than the hierarchical zeolite microspheres prepared using F127 (EO106PO70EO106,EO:ethylene glycol,PO:propylene glycol) polymer as a secondary template20and slightly lower than the hierarchical ZSM-5 mciroshperes prepared in the presence of the urea-formaldehyde polymer.21The high BET surface areas and large mesopore volume of zeolites may favor the reaction of bulky molecular,resulting in the higher catalytic activity,22excellent anticoke formation ability,and anti-sulfur poisoning ability in the catalytic reaction.23,24

Thermogravimetric(TG)analysis of the as-prepared zeolite microsphere was conducted in air.The TG plot showed three weight-loss steps with a total weight loss of 8.6%-12.0%ranging from room temperature to 800°C(Fig.5).The first step appearing below 200°C was associated with the desorption of water.And the peak appearing at 300-500°C come from the desorption of alkyl-polyamines in the micropore of zeolite varied with the linear chain length.18The weight loss step between 200 and 300°Cmight correspond to the decomposition of the alkyl-polyamines adsorbed on the surface and interstitial voids of the ZSM-5 microspheres.18This is obviously different from the thermogravimetric analysis of zeolites synthesized using diamines as template,13there are no any weight loss peaks appeared during the range of 200-300°C,indicating that there are no or a little diamines adsorbed on the surface and interstitial voids of the ZSM-5/ZSM-11 zeolite microspheres.And the last step between 500 and 800°C might correspond to the dehydroxylation of the zeolite microsphere.The weight loss of zeolites synthesized with alkyl-polyamines is more than that of the zeolites synthesized using diamine templates,indicating that there are possibly more hydroxyls in the zeolites synthesized using alkyl-polyamines as template.

Fig.4 Nitrogen adsorption-desorption isotherms of the calcined mesoporous DETA-ZSM-5-60(a),TETA-ZSM-5-60(b),TEPAZSM-5-60(c),DETA-ZSM-5-30(d),TETA-ZSM-5-30(e), and TEPA-ZSM-5-30(f)zeolites

Fig.6Ashows the27Al NMR spectra of the zeolites synthesized by the different alkyl-polyamines.In general,the resonance centered at 54 is commonly assigned to four-coordinated framework aluminum,while the peak at 0 is referred to non-framework octahedral aluminum.There is almost no peak at 0 in all the samples,indicating the absence of non-framework aluminum no matter what alkyl-polyamines and SiO2/Al2O3molar ratios were used,this is in agreement with that reported result.25

As shown in Fig.6B,29Si MAS NMR spectra indicate a main peak at a chemical shift(δ)ca-114 from siloxane bridges(Q4) species with a shoulder centered at-106 from single silanol(Q3) entities in the all samples.The Q4/Q3intensity ratios calculated from the deconvoluted peaks(dashed lines)were 2.1,1.9,and 1.7 corresponding to the(a)DETA-ZSM-5-30,(b)TETA-ZSM-5-30, and(c)TEPA-ZSM-5-30 zeolites,and when increasing the SiO2/ Al2O3molar ratio to 60,the Q4/Q3intensity ratio calculated from the deconvoluted peaks(dashed lines)was changed to 3.1 corresponding to(d)TEPA-ZSM-5-60 zeolite,respectively.The higher percentage of Q3in all the samples than that of the samples prepared using diamines as template indicated that mesoporous ZSM-5 aggregates had more hydroxyl groups and thus a more hydrophilic surface,which resulted in an obvious loss peak during the temperature range of 500-800°C in the TG-DTG analysis (Fig.5 and Table 1).Almost no Q2signals are observed in either spectrum,suggesting high degree of condensation of the framework in the zeolites.

Table 1 Textual properties and thermogravimetric analysis of the mesoporous ZSM-5 zeolite samples

The SEM images and IR spectra of the TEPA-ZSM-5-30 prepared for different crystallization time of 1,3,5,and 7 days are shown in Fig.7.As shown in the SEM images,there are some irregularly-shaped gel lumps at early stage of the hydrothermal reaction(1-3 days).Some microspheres randomly dispersed in amorphous gel after 3 days as shown in Fig.7b SEM image. Correspondingly,a small band at 550 cm-1appears in IR spectra, indicating the formation of some 5-membered ring in pentasil zeolites.With the crystallization time increasing up to 5 days, more and more irregularly-shaped gels transform into microspheres with the diameter of～15 μm.Meanwhile,the band at 550 cm-1becomes larger and no well crystallized zeolites show in XRD.After 7 days,almost all amorphous gel disappears and well crystallized zeolite ZSM-5 microspheres in size of 15 μm.And the band at 550 cm-1becomes distinctly.The band at 550 cm-1is the characteristic of pentasil zeolites,including ZSM-5 and ZSM-11,with a five-membered ring.26-29The intensity ratio of the 550 to 450 cm-1bands has been used to assess the formation of ZSM-5 and is named as IR crystallinity.The band at 550 cm-1grows stronger and sharper gradually as the crystallization time is prolonged.As indicating by all SEM images and IR spectra with varied crystallization time,the formation of macroscopic morphology and zeolitic framework seem to go simultaneously.

Fig.5 TG-DTG analyses of the DETA-ZSM-5-60(A),DETA-ZSM-5-30(B),TETA-ZSM-5-60(C),TETA-ZSM-5-30(D), TEPA-ZSM-5-60(E),and TEPA-ZSM-5-30(F)samples

Fig.6 27Al NMR(A)and29Si NMR(B)spectra of the as-synthesized mesoporous(a)DETA-ZSM-5-30,(b)TETA-ZSM-5-30, (c)TEPA-ZSM-5-30,and(d)TEPA-ZSM-5-60 zeolites

The liquid alkylation of phenol with tert-butyl alcohol is a typical Friedel-Crafts alkylation,which can be catalyzed by acid catalysts.The different acid sites and pore structures lead different distributions of products.30-32The catalytic activities of zeolite microspheres synthesized by alkyl-polyamines in the alkylation of phenol and tert-butyl alcohol are listed in Table 2.The zeolite microspheres exhibited high catalytic activity with a phenol conversion that reached 16.4%after a reaction time of 4 h at 373 K.The higher catalytic activity of TEPA-ZSM-5-30 is due to the higher mesoporous volume,indicating that the existence of mesopores resulted from nanoparticle aggregation reduced thediffusion limitation,although the catalytic activity was slightly lower than the early report.13Moreover,the larger size of zeolite aggregates than the zeolite ZSM-5 microshpere synthesized by diamines may further curtail the filtration difficulties during the synthesis and applications to some extent.

Fig.7 SEM images and IR spectra of the TEPA-ZSM-5-30 with the crystallization time of 1(a),3(b),5(c),and 7(d)days

Table 2 Catalytic activity and product distribution for alkylation reaction of phenol with tert-butanol

Here,linear alkyl-polyamines as single template can be used to synthesize larger zeolite ZSM-5 microspheres with larger mesoporosity than the report.13This synthesis method is very simple and convenient,easy compared with those mesoporous ZSM-5 synthesized using a secondary template.20,21Similar to diamines, the alkyl-polyamines as template not only direct the formation of crystalline zeolite structure,but also favor the control of macroscopic morphology and tectonics of zeolite crystals.These results prove that non-amphiphilic organic molecules could also lead to hierarchical zeolite with both micro-and meso-porosity via a single hydrothermal treatment.Furthermore,the zeolites microspheres with mesoporosity could not only reduce diffusion limitation in catalytic reaction,but also curtail the recovery difficulties in the preparation and applications.

4 Conclusions

Alkyl-polyamine as a single template was used to prepare mesoporous zeolite ZSM-5 microspheres.The obtained hierarchical ZSM-5 zeolite microspheres have the bulky aggregate morphology and high catalytic activity in the Friedel-Crafts alkylation,similar to the zeolite synthesized using diamines as template.Furthermore,it is found that the diamine templates only act as the structure directing agent in the synthesis of zeolite, while the alkyl-polyamine templates act as both the structure directing agent and space filling agent in the mesoporous zeolites.

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One-Step Synthesis of Hierarchical ZSM-5 Zeolite Microspheres Using Alkyl-Polyamines as Single Templates

CHEN Li XUE Teng ZHU Shu-Yan WANG Yi-Meng*
(Shanghai Key Laboratory of Green Chemistry and Chemical Processes,Department of Chemistry, East China Normal University,Shanghai 200062,P.R.China)

Diethylenetriamine(DETA),triethylenetetramine(TETA),and tetraethylenepentamine(TEPA)were used as single templates to synthesize mesoporous zeolite ZSM-5 microspheres.The obtained 10-15 μm hierarchical aggregates had a uniform spherical morphology,which was spontaneously assembled by primary zeolite nanocrystals of 50 nm in size during the hydrothermal synthesis.The ZSM-5 aggregates had a tunable textual porosity,large mesopore volume,and high catalytic activity in the Friedel-Crafts alkylation.Diamine templates only acted as structure directing agents in our previous work.The current alkyl-polyamine templates acted as structure directing agents and space fillers in the synthesis of the hierarchical zeolites.

Hierarchical zeolite;Aggregate;Single template;Alkyl-polyamine

O643

10.3866/PKU.WHXB201411174www.whxb.pku.edu.cn

Received:September 29,2014;Revised:November 14,2014;Published on Web:November 17,2014.

∗Corresponding author.Email:ymwang@chem.ecnu.edu.cn;Tel:+86-21-62232251.

The project was supported by the National Natural Science Foundation of China(20890122)and National Key Technology R&D Program of China (2012BAE05B02).

国家自然科学基金(20890122)和国家科技支撑计划(2012BAE05B02)资助