Preparation of Mesoporous ZSM-5 Zeolite under Double Ester Base Long Carbon Chains Organosilane Quaternary Ammonium Salt Template Agent

QIAO Wenjuan()， LIU Dongliang()

College of Chemistry，Chemical Engineering&Biotechnology，Donghua University，Shanghai201620，China

Abstract：The pore sizes of traditional zeolites are in the range of 0.3-1.5 nm，which strongly hinder the diffusion of large reactant and product molecules within the zeolite pores.To compensate for it，we tried to create mesopores in traditional microporous zeolites and retain all advantages of microporous zeolites.Mesoporous Zeolite Socony Mobile-Five(ZSM-5)zeolite was synthesized by a new double ester base long carbon chains organosilane quaternary ammonium salt as the soft template agent in hydrothermal method.The structure of the acquired zeolite crystals was confirmed by field-emission scanning electron microscopy(FE-SEM)，transmission electron microscopy(TEM)，nitrogen adsorption-desorption measurements and X-ray diffraction(XRD)，which indicated that their structure had the same characteristics as traditional ZSM-5 zeolites.Compared with traditional ZSM-5 zeolite，there were 4 nm and 15 nm mesopores in the crystal.The prepared hierarchical porous ZSM-5 zeolite was expected to be effective catalytic materials for chemical reactions involving large molecules.

Key words：mesoporous Zeolite Socony Mobile-Five(ZSM-5)zeolite；double long carbon chains；organosilane quaternary ammonium salt

Introduction

Zeolite molecular sieve is the crystalline silicon aluminum material with microporous channels[1]. The traditional microporous zeolite was widely used in catalysis， purification， adsorption and separation field， especially in the refining and(petro) chemical industries due to its high surface area， intrinsic acidity and high(hydro) thermal and chemical stability[2-4]. However， the sole presence of micropores in zeolites strongly hinder the diffusion of reactants and products， resulting in the low catalytic efficiency and the fast deactivation of the zeolites by the frequent blocking of the diffusion path[5-8]. The appearance of mesoporous materials partially compensates for the poor performance of the microporous zeolite in the catalytic process of large molecules. However， the hole wall of mesoporous materials is amorphous， and its hydrothermal stability and thermal stability are far worse than the zeolite molecular sieve crystals. This limits the application of mesoporous materials in the field of actual industrial catalysis[9-15]. Hierarchical porous zeolites， combining both the advantages of mesoporous materials and microporous zeolites， have been considered as novel catalytic materials. In recent years， the synthesis of hierarchical porous zeolites have drawn a great deal of attention in the field of chemistry and materials. Many researchers tried to introduce mesopores in traditional zeolites under different synthetic conditions or template agents[16-18]. Among them， the Zeolite Socony Mobile-Five(ZSM-5) zeolite molecular sieve， as the most classic also the most important zeolite molecular sieve， attracted people’s study enthusiasm naturally[19-21].

Ref.[22] creatively used amphiphilic organosilane to synthesize hierarchical porous zeolites， which showed that the amphiphilic organosilane surfactants were effective templates for hierarchical porous zeolites preparation[22-34]. And herein， we designed and synthesized a new double ester base long carbon chains organosilane quaternary ammonium salt template，[2， 3-bis(dodecanoyloxy)-propyl](3-(trimethoxysilyl)propyl)diethylammonium iodide(BDTPI)， to prepare mesoporous zeolite crystals， and its structure is shown in Fig. 1.

Fig. 1 Molecular structure of the synthesized organosilane template BDTPI

Given that the hydrophobic alkyl chainin template can direct the formation of mesopore structure when zeolite preparation， there are double long carbon chains in BDTPI， which is different from the single carbon chain in Ryoo’s template， so it is expected to have a better application performance.

X-ray diffraction(XRD)，field-emission scanning electron microscopy(FE-SEM)， transmission electron microscopy(TEM) and nitrogen adsorption-desorption tests proved that the zeolites prepared with this new template had the same crystal form as ZSM-5， and contained 4 nm and 15 nm mesopores in the crystals.

1 Experimental

1.1 Materials

Dodecanoic acid， 3-diethylamino-1， 2-propanediol， p-toluenesulfonic acid， methylbenzene， 3-iodopropyltrime-thoxysilane， 1-diethylamino-2， 3-bis-dodecanoyloxy-propane， dimethylformamide， sodium bicarbonate， acetone， petroleum ether， aluminum isopropoxide(AIP)， tetrapropylammonium hydroxide(TPAOH， 25% by weight aqueous solution) and tetraethyl orthosilicate(TEOS) were purchased from the Sinopharm Chemical Reagent Co.， Ltd., China.

1.2 Synthesis and purification of double-acyloxy compound

Dodecanoic acid(15.64 g， 78.0 mmol)， 3-diethylamino-1， 2-propanediol(4.79 g， 32.5 mmol) and p-toluenesulfonic acid(0.76 g， 4.0 mmol) were dissolved in 150 mL methylbenzene. The mixture was heated to 150 ℃， stirred and removed water with a Dean-Stark trap at this temperature for 24 h. Then， it was cooled to room temperature， and the yellowish liquid product was obtained after rotary evaporation of solvent， alkali washed(three times with saturated sodium bicarbonate solution)， washed with deionized water three times， dried by anhydrous calcium sulfate， column chromatography with the eluent acetone/petroleum ether(1/15 in volume ratio).

1.3 Characterization of double-acyloxy compound

The structure of the intermediate was confirmed by electrospray ionization mass spectrametry(ESI-MS)，1H nuclear magnetic resnoance spectroscopy(NMR) and13C NMR. Its spectral data are as follows.

Yield： 75%. ESI-MS m/z for C31H62NO4([M+H]+) found： 512.4； calculated： 512.5.

1H NMR(400 MHz， CDCl3)：[δ=5.13(s， 1H， COOCH)]，[δ=4.38-4.08(m， 2H， COOCH2)]，[δ=2.67-2.49(m， 6H， N(CH2)3)]，[δ=2.32-2.24(m， 4H， 2×COCH2)]，[δ=1.70-1.59(m， 4H， 2×COCH2CH2)]，[δ=1.26(s， 32H， 2×(CH2)8]，[δ=1.03-0.97(m， 6H， 2×NCH2CH3)]，[δ=0.88(s， 6H， 2×CH3)].

1.4 Synthesis and purification of organosilane template BDTPI

3-iodopropyltrimethoxysilane(0.83 g， 2.8 mmol) was added to 1.03 g(2.0 mmol) of 1-diethylamino-2， 3-bis-dodecanoyloxy-propane at room temperature， 5 mL of dimethyl formanide(DMF) as solvent. The mixture was heated to 100 ℃ and stirred at this temperature for 24 h under nitrogen environment. Then， it was cooled to room temperature， and the yellowish solid product was obtained after rotary evaporation of solvent， washed with hexane several times and dried under high vacuum at 50 ℃.

1.5 Characterization of organosilane template BDTPI

The structure of BDTPI was confirmed by Fourier transform mass spectrometry(FT-MS)，1H NMR and13C NMR. Its spectral data are as follows.

Yield： 50%.FT-MS m/z for C37H76NO7Si([M-I]+) found： 674.5； calculated： 674.5.

1H NMR(400 MHz， CDCl3)：[δ=5.63-5.61(m， 1H， COOCH)]，[δ=4.54-4.48(m， 2H， OCH2)]，[δ=4.20-4.16(m， 2H， NCH2CH)]，[δ=3.84-3.75(m， 2H， NCH2CH2)]，[δ=3.73-3.52(m， 9H， 3×OCH3)]，[δ=3.33-3.31(m， 4H， 2×NCH2CH3)]，[δ=2.40-2.33(m， 4H， 2×COCH2)]，[δ=1.63- 1.56(m， 2H， NCH2CH2)]，[δ=1.49-1.46(m， 4H， 2×COCH2CH2)]，[δ=1.29-1.27(m， 38H， 2×(CH2)8， N(CH2CH3)2)]，[δ=0.86-0.90(m， 8H， 2×CH3， SiCH2)].

1.6 Synthesis of mesoporous zeolite

AIP (0.08 g) and 4.08 g of microporous template tetrapropylammonium hydroxide(TPAOH， 25% by weight aqueous solution) were dissolved in 29.34 mL of deionized water under mechanical stirring(350 r/min) at room temperature. Ten minutes later， 4.17 g of TEOS was added to the above solution under vigorous stirring. Then， 0.40 g of BDTPI(dissolved in 3 mL ethanol) was added slowly under stirring. The mole composition of the obtained mixture wasn(Al2O3)∶n(SiO2)∶n(TPAOH)∶n(H2O)∶n(BDTPI)=0.01∶1∶0.25∶90∶0.25. The mixture was stirred continuously to gain clear solution. Then， it was transferred into a Teflon-lined stainless steel autoclave. The autoclave was heated at the temperature of 150 ℃ for 3 d in an oven. After cooling to room temperature， the mixture was filtered and washed with deionized water to neutral and then dried at 100 ℃ for 6-8 h. Subsequently， the sample was calcined at 600 ℃ for 6 h with a temperature ramping rate of 1 K/min in air. The obtained mesoporous zeolite was called ZSM-5-Q.

2 Results and Discussion

Figure 2 showed the XRD pattern of ZSM-5-Q. Compared with the database provided by the international center for diffraction data， the ZSM-5-Q XRD pattern matched well with the “standard” ZSM-5 structure(ICSD201183). In addition， the XRD pattern of ZSM-5-Q showed well-resolved peaks of the ZSM-5 structure(2θin the range of 5°- 40°) with no evidence of other crystalline phase， which illustrated the uniform formation of ZSM-5[35].

Fig. 2 XRD patterns of ZSM-5-Q(up) and ICSD201183(lower)

In order to observe the surface of ZSM-5-Q， FE-SEM and TEM images were obtained， as shown in Figs. 3-4.

Fig. 3 SEM images of the ZSM-5：(a)-(c) SEM images of the ZSM-5-Q sample；(d) conventional ZSM-5 zeolite synthesized in the absence of BDTPI

Figure 3(a) shows that the ZSM-5-Q sample contained many platelet-like crystals with relatively uniform size， and they were closely stacked. Moreover， large mesopores could be observed on the froth-like surface of the primary crystals(Fig. 3(c)). For comparison， the SEM image of conventional ZSM-5 zeolite synthesized in the absence of BDTPI surfactant was also obtained(Fig. 3(d))， whose surface was much smoother. It was evident that the organosilane template agent had the ability to create mesopores in the zeolite crystals.

Fig. 4 TEM images of ZSM-5-Q： (a) and (b) low-magnification；(c) high-magnification

The TEM images also illustrated the presence of mesopores in the obtained sample ZSM-5-Q. Figures 4(a)-4(b) show the platelet-like morphology and sponge-like surface that have been observed in the SEM images(Figs. 3(a)-3(b)). As shown in Fig. 4(c)， the lattice fringes revealed the high crystallinity of ZSM-5-Q sample， indicating that this sample had higher thermal stability than conventional amorphous mesoporous materials.

The presence of mesopores in ZSM-5-Q crystals was further confirmed by nitrogen adsorption-desorption measurement and the Barrett-Joyner-Halenda(BJH) pore size distribution isotherms， which were shown in Fig. 5. As shown in Fig. 5(a)， a type IV adsorption-desorption isotherm with a hysteresis loop appeared in the relative pressure(P/P0) range of 0.45-0.95， which indicated the presence of mesoporous structure in ZSM-5-Q. The mesoporous volume， the Brunauer-Emmett-Teller(BET) surface area and total porous volume of ZSM-5-Q were 0.24 cm3/g， 318 m2/g and 0.31 cm3/g， respectively. The BJH pore size distribution isotherm showed mesopores in the size range of around 4-15 nm， with two distribution peaks centered at 4 and 15 nm respectively(Fig. 5(b)). These data also indicated that ZSM-5-Q possessed mesoporous structure， and the size of mesopores in ZSM-5-Q crystals had even been enlarged to 15 nm， which may be helpful for mass transfer and macromolecule catalysis.

Fig. 5 Nitrogen adsorption-desorption results of ZSM-5-Q：(a) adsorption-desorption isotherm；(b) BJH pore size distribution

3 Conclusions

In summary， XRD， FE-SEM， TEM and nitrogen adsorption-desorption measurements suggested that the template BDTPI contributed to introduce mesopores in ZSM-5 zeolite. And the prepared hierarchical porous ZSM-5 zeolites were expected to be effective catalytic materials for chemical reactions revolving large molecules.