Hollow mesoporous aluminosilicate spheres with acidic shell

Materials Chemistry and Physics 125 (2011) 286–292

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Hollow mesoporous aluminosilicate spheres with acidic shell Junhua Li a,b , Dan Zhang b , Qiang Gao a,d , Yao Xu a,∗ , Dong Wu a , Yuhan Sun a , Jun Xu c , Feng Deng c a

State Key Laboratory of Coal conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China School of Chemistry and Environmental Engineering, Liaoning University of Technology, Jinzhou 121001, China c State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China d Graduate University of the Chinese Academy of Sciences, Beijing 10049, China b

a r t i c l e

i n f o

Article history: Received 1 June 2010 Received in revised form 5 September 2010 Accepted 19 September 2010 Keywords: Mesoporous silica Zeolite precursors Catalytic cracking

a b s t r a c t Hollow mesoporous aluminosilicate spheres (HMAS) with middle strong acidity have been successfully synthesized by simply hydrothermally treating mesoporous silica spheres (MSS) in zeolite precursor solution. Based on a set of time-dependent experiments, it was found that the hollowing process was associated with a progressive mass redistribution and changes of pores structures. The surfactant cetyltrimethylammonium bromide (CTAB) located in the pores of MSS protected the mesoporous silica spheres from dissolving into strongly basic zeolite precursor solution and acted as the template for mesoporous shell. Under the templating function of CTAB, primary zeolite units were introduced into the mesopore walls of HMAS. The acidity of the resultant samples HMAS was measured by NH3 -TPD techniques. Catalytic tests showed that the HMAS catalysts exhibited high catalytic activity compared with the MSS and H␤ zeolite for catalytic cracking of 1,3,5-triisopropylbenzene. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Hollow mesoporous spherical materials with a large inner cavum and penetrating mesopores have attracted much attention due to their properties of low density, high specific surface area and well-defined mesoporous wall structure [1–20]. Up to now, hollow mesoporous silica spheres have been extensively studied because of their wide applications in drug-delivery [7], catalysis [8], separation [9], and large biomolecular-release systems [10]. A variety of chemical and physicochemical methods have been developed to synthesize hollow mesoporous silica spheres, including sol–gel process [11,12], emulsion/interfacial polymerization [13,14], self-assembly techniques [15–17] and layer-by-layer method [18–20]. Among these methods, two were particularly interesting and usually used: soft-template self-assembly approach [15–17] and sacrificial-core method [18–21]. In soft-template selfassembly method, surfactant and co-template formed spherical micelles under appropriate solution condition. After silica source was added, the Si-containing species self-assembled on the spherical micelles and then a template-core/mesoporous-shell structure was obtained. Hollow mesoporous silica spheres were finally produced by extraction or calcination to remove the template. In sacrificial-core method, solid spherical beads were coated with the surfactant micelles at first, then the hydrolyzed siloxane species condensed on the sphere surface to form silica-coated

∗ Corresponding author. Tel.: +86 351 4049859; fax: +86 351 4041153. E-mail address: [email protected] (Y. Xu). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.09.036

core/shell composites, finally hollow mesoporous silica spheres were obtained after removing the solid core and the template. Although the above-mentioned two methods were very interesting, the preparation processes were laborious and time-consuming because a strict control must be made on the synthesis condition [15–17], such as preparation of template, coating of the template on solid cores and removing of the template. From the viewpoint of material synthesis, it is still a challenge to simplify the synthetic process of these interesting hollow mesoporous spheres. In addition, many efforts have been devoted to modify the shell of hollow mesoporous spheres [22,23], which were expected to further alternate the chemical properties and thus enable them to meet more needs of technological applications. In the potential applications, acid catalysis is highly attractive and has been paid much attention by many researchers [24–26]. Among solid acid catalysts, hydrothermally stable mesoporous aluminosilicates with strong acidity are of special importance in industrial application. This kind of mesoporous aluminosilicate was firstly synthesized from zeolite precursors by Pinnavaia et al. [27] and Xiao et al. [28]. Furthermore, catalytically cracking large molecule needs strong acid catalysts with bigger pores than that of zeolites, if taking the size and shape of molecule into account. Undoubtedly, hollow spherical mesoporous silica is a promising material if strong acidity can be introduced. So far, hollow mesoporous aluminosilicate spheres (HMAS) with strong acidity has not been reported. In this paper, we report a simple hydrothermal method to synthesize hollow mesoporous aluminosilicate spheres using mesoporous silica spheres (MSS) and zeolites precursor solution as starting materials. Middle strong acidity was successfully intro-

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287

Table 1 The surface areas and pore volumes of samples. Samples

Si/Al ratio

BET surface area (m2 g−1 )

Pore size (nm)

Pore volume (cm3 g−1 )

MSS MSS-B HMAS-H0.5 HMAS-H2 HMAS-H6 HMAS-H12

– 141 87 72 61 48

945.6 942.3 336.5 589.7 755.1 930.5

2.8 2.7 2.5 2.4 2.7 2.9

0.53 0.52 0.21 0.46 0.69 0.97

duced by constructing the zeolite structural units into mesoporous walls. Based on the HMAS, excellent activity was obtained for catalytically cracking 1,3,5-triisopropylbenzene.

2. Experimental 2.1. Materials

2.4. Catalytic test Catalytically cracking 1,3,5-triisopropylbenzene (TIPB) was carried out by pulse method to evaluate the catalytic performance of the resultant materials. In each run, 0.2 ␮l of TIPB was pulsed and cracked over 60 mg of catalyst, and nitrogen was used as carrier gas at a flowing rate of 50 ml min−1 . The reaction products were analyzed by a GC-920 gas chromatograph equipped with a FID detector. The reaction temperature was set to three points: 250 ◦ C, 300 ◦ C, and 350 ◦ C.

Hexadecyltrimethylammonium bromide (CTAB), tetraethoxysilane (TEOS, 99%,), absolute ethanol, tetraethylammonium hydroxide solution (TEAOH, 20 wt% in water) and silica aerogel were used as received from Beijing Reagent Co.

3. Results and discussion

2.2. Synthesis of hollow mesoporous aluminosilicate spheres

3.1.1. Effect of time Time-dependent synthetic experiments were carried out to explore the formation process of HMAS, and the TEM images taken at different hydrothermal treating time are shown in Fig. 1. It was obvious that MSS particles were smooth and solid spheres. At the very early stage of synthesis (just stirring for 4 h at room temperature), sample MSS-B was still solid, as shown in Fig. 1b. The influence of hydrothermal time on the particle morphology was studied at 140 ◦ C from 0.5 h to 12 h. Sample HMAS-H0.5 was still solid (Fig. 1c), but the density of a particle seemed much lower than that of MSS and MSS-B. Extending the reaction time to 2 h, sample HMAS-H2 (shown in Fig. 1d) was of a semi-hollow structure, but the interior vacuum was very small. When the reaction time was prolonged to 6 h, the vacuum diameter of HMAS-H6 was obviously bigger than HMAS-H2, as shown in Fig. 1e. Finally, with the reaction time increased to 12 h, the TEM image of HMAS-H12 (Fig. 1f) clearly showed a big vacuum at the center of these monodispersed HMAS particles. Based on the above TEM results, it was revealed that the hollow vacuum was generated gradually under the hydrothermal treating. Fig. 2 shows the low-angle XRD patterns of the calcined samples including MSS, MSS-B and those product obtain at different hydrothermal time. MSS exhibited a broad diffraction peak (Fig. 2a) corresponding to worm-like mesoporous phase [33]. The peak intensity of MSS-B (see Fig. 2b) decreased a little compared with that of MSS. This may due to the partial dissolution of MSS in strongly basic zeolite precursor solution [34]. For HMAS-H0.5 (see Fig. 2c), the intensity of diffraction peak decreased greatly and almost disappeared indicating that the original mesoporous structure of MSS has been largely destroyed. However, the intensity of diffraction peak increased again for HMAS-H2 (Fig. 2d), HMAS-H6 (Fig. 2e) HMAS-H12 (Fig. 2f). For HMAS-H12, the diffraction peak was stronger than all the other samples, showing that the perfect worm-like mesopores has been reconstructed after 12 h hydrothermal treatment [33]. The HRTEM images of MSS and HMAS-H12 are shown in Fig. 3. The mesoporous morphologies of two samples are similarly worm-like. Fig. 4 shows the overall morphologies of samples observed on SEM. It could be clearly seen that HMAS-H12 was overall uniform spherical morphology with rough surface (see Fig. 4a). Based on the results of the above time-dependent experiments, it was believed that the hollowing process was associated with a progressive mass redistribution from the interior to the exterior of silica spheres probably through Ostwald ripening [35,36]. Under the mass redistribution,

MSS particles were prepared following the method provided by the Ref. [29]. In a typical synthesis, 1.0 g hexadecyltrimethylammonium bromide (CTAB) was dissolved in a ethanol–water (50 v/50 v) mixture solvent followed by an addition of 45 ml aqueous ammonia (25 wt%). After the solution became homogeneous, 10 ml TEOS was dropwise added under vigorous stirring. After 2 h, the solid product was filtered, washed with deionized water and dried at 60 ◦ C. Beta zeolite precursors (Si/Al ratio is 25) were prepared according to the literature [30]. Typically, 0.26 g sodium aluminates, 0.16 g sodium hydroxide and 4.8 g silica aerogel were added into 25 ml TEAOH solution under stirring. The mixture was transferred into a Teflon-lined stainless steel autoclave and hydrothermally treated at 140 ◦ C for 4 h to yield a clear beta zeolite precursor solution. To synthesis HMAS, 2.0 g MSS powder was dispersed into the above-obtained beta zeolite precursor solution (25 ml) under stirring at room temperature and stirred for 4 h. As a reference, the sample at this state was set as MSS-B to differ from the final products. Finally, the mixture was subjected to a hydrothermal transformation at 140 ◦ C [27,28] for different time. The obtained product was filtered, washed with deionized water, dried at 80 ◦ C, and then calcined at 550 ◦ C for 6 h. The final product was designated as HMAS-HX, in which X represented the hydrothermal treatment time. To compare with HMAS samples, beta-zeolite with an Si/Al ratio of 25 was synthesized following the Refs. [31,32], and the as-synthesized samples were converted into the H-form by three consecutive ion exchanges in 0.1 M NH4 NO3 solution and subsequent calcination in air at 550 ◦ C for 5 h.

2.3. Characterization The powder X-ray diffraction (XRD) patterns were recorded on a Bruker diffractometer with Cu K␣ irradiation (at 40 kV and 30 mA). Counting step was accumulated every 0.02◦ and the preset time was 0.2 s. N2 adsorption–desorption isotherms were obtained at −196 ◦ C on a Tristar 3000 sorptometer. Samples were degassed at 150 ◦ C and 10−6 Torr for 12 h prior to analysis. BET surface area was calculated from the linear part of the BET plot according to IUPAC recommendations. Pore size distribution was calculated using the Barrett–Joyner–Halenda (BJH) model. Transmission electronic microscopy (TEM) images were taken on Hitachi H-600. Scanning electronic microscopy (SEM) images were obtained on LEO 1530VP. 27 Al MAS NMR spectra were performed on a Bruker MSL-400 NMR spectrometer equipped with a 7 mm zirconia rotor. The detailed operating parameter were 27 Al resonance frequency, 130.245 MHz; pulse width, 1 ms; recycle delay time, 1 ␮s; spinning frequency, 4 kHz; and Al(OH)6 3+ was used to assign 0 chemical shift. Temperature-programmed desorption of ammonia (NH3 -TPD) was performed in a quartz micro-reactor. At first, 0.20 g of HAMS sample was heated in Ar gas at 600 ◦ C for 0.5 h. Then NH3 was introduced into micro-reactor after the local temperature around sample was cooled down to 100 ◦ C. To remove the weakly adsorbed NH3 , the sample was swept using argon at 100 ◦ C for 2 h. The TPD experiment was then carried out in Ar with a carrier-gas flowing rate of 40 ml min−1 from 100 to 600 ◦ C using a heating rate of 10 ◦ C min−1 . The NH3 desorption was detected by Shimadzu GC-9A equipped with a TCD detector. The final Si/Al ratio in sample was determined by inductively coupled plasmaatomic emission spectrometry (TJA AtomScan16).

3.1. Formation of HMAS

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Fig. 1. TEM images of (a) MSS, (b) MSS-B, (c) HMAS-H0.5, (d) HMAS-H2, (e) HMAS-H6, and (f) HMAS-H12.

mesoporous shell was gradually obtained with the template of CTAB. Table 1 summarizes the pore structure parameters of MSS and different HMAS samples. The pore size distribution was calcu-

lated from the adsorption branch of isotherm by BJH method and shown in Fig. 5. The mean pore size of the samples was similar. Seen from Fig. 5, the BJH pore size distribution for sample HMAS-H0.5 was relatively broad. With the increasing of

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the Al atoms have been gradually incorporated into the frameworks of HMAS particles in the hydrothermal treating process.

Intensity (a.u.)

f

3.1.2. Effect of temperature The hydrothermal temperature also played an important role in the hollowing process of MSS [35,36]. Fig. 7 reveals the similar morphologies of three products with similar pore size distributions that are shown in Fig. 8. The three samples were obtained under different hydrothermal temperature and different time. These three products were chosen to make sure that the hollowing process was at approximately same status. From Fig. 7, it can be seen that the vacuum diameters were very close to each other. Accordingly, the time that was consumed to reach the status shown by Fig. 7 can be used to evaluate the speed of hollowing process. At the highest temperature (180 ◦ C), the hollowing process was fastest. Meanwhile, at the lowest temperature (100 ◦ C), the longest treating time was needed in the complete transformation from MSS particles to HMAS. It was very clear that the hollowing process was highly dependent on the hydrothermal temperature. This confirmed that the mass redistribution was accelerated at high temperature, thus the hollowing process was fastened.

e a b d

c 2

289

4

6

8

10

2 theta/degree Fig. 2. Low-angle XRD patterns of (a) MSS, (b) MSS-B, (c) HMAS-H0.5, (d) HMAS-H2, (e) HMAS-H6, and (f) HMAS-H12.

hydrothermal treating time, the BJH pore size distribution became gradually narrower, indicating that mesopores were gradually formed. The sample HMAS-H12 had pore diameter of ∼2.9 nm with the most narrow pore size distribution. Upon hydrothermal synthesis at 140 ◦ C for only 0.5 h (HMAS-H0.5), the BET surface area decreased to 336.5 m2 g−1 , resulted from the collapse of mesopores in MSS [36]. It is obvious that the pore volume and surface area gradually increased with the hydrothermal treating time because of the regeneration of the mesopores during the hollowing process. The 27 Al MAS NMR spectrums of calcined samples HMAS were shown in Fig. 6. The 27 Al chemical shift at ı = 50 ppm was characteristic of tetrahedral aluminum atoms. This strongly suggested that

3.1.3. Synthesis mechanism of HMAS Under a high alkalinity of the zeolite precursor solution, the outer surface of MSS particles was eroded gradually [34,37] and the CTAB molecules located in the mesopores of MSS was released. Subsequently the zeolite precursors self-assembled around the micelles of CTAB and deposited onto the retained surface of MSS particles at once [34]. From the SEM images of the samples MSSB (Fig. 4b) and MSS (Fig. 4c), it can be clearly observed that there existed a thin layer on the outer surface of the MSS-B particles. The protozeolitic nanoclusters located on the outermost surface would serve as starting points to attract other nanoclusters from the solution to proceed the subsequent self-transformation process during which new depositions grew on the renewed edge of particle and resulted to the formation of rough surface of HMAS particles. According to a reported lecture [38], hollow silica nanoparticles can be spontaneously produced without a template on the basis of the porous nature of silica and its high surface energy at the nanometer dimension. Small pores can be developed inside the solid nanoparticles synthesized by the Stöber methods under basic conditions. With further reaction, those small pores merge into a single big vacuum to reduce the surface energy of small pores, and thus generating well-defined hollow nanoparticles. In our work, MSS spheres were synthesized by a modified Stöber method. Due to the existence of lots of mesopores, the surface energy at the

Fig. 3. HRTEM images of (a) MSS and (b) HMAS-H12.

J. Li et al. / Materials Chemistry and Physics 125 (2011) 286–292

HMAS-H12 MSS MSS-B HMAS-H6 HMAS-H2 HMAS-H0.5

3

Pore volume (cm /g )

290

1

10

100

D (nm) Fig. 5. Pore size distributions of MSS, MSS-B, HMAS-H0.5, HMAS-H2, HMAS-H6, and HMAS-H12.

3.2. Acidity An important application of Al incorporated material is used as solid acid catalyst. NH3 -TPD technology has been widely used to determine the total acidity of solid catalyst [39,40]. The corresponding NH3 desorption temperature is taken to measure the acid strength and the desorption amount of ammonia at characteristic temperature is taken as the measurement of acid content [41]. Fig. 9 shows the NH3 -TPD profiles of MSS and HMAS-HX samples. As shown by Fig. 9a three NH3 -TPD peaks for H␤ were distinguished at 230 ◦ C, 300 ◦ C and 430 ◦ C. As shown in Fig. 9b, the NH3 amount desorbed from MSS was very low, showing nearly no acidity. For HMAS-H0.5 (Fig. 9c), the amount of desorbed NH3 obviously increased, because one peak was observed at ca. 226 ◦ C corresponding to a weak acidity. More interestingly, the acid strength of sample increased with hydrothermal treating time. For samples HMAS-H2 (Fig. 9d) and HMAS-H6 (Fig. 9e), the shoulder peak appeared at ca. 310 ◦ C corresponding to a middle-acidity. Two NH3 -TPD peaks were observed at ca. 220 ◦ C and 340 ◦ C for HMAS-H12, as shown by Fig. 9f, indicating that HMAS-H12 had moderate acid strength [41]. The increased amount of NH3 adsorbed on HMAS-H12 showed that much more acid sites were created. The gradually increased acid content should be attributed to that more zeolite precursor units

Fig. 4. SEM images of (a) HMAS-H12, (b) MSS-B and (c) MSS.

d c b

particle core should be higher than that at the exterior surface of spheres and the reactive small molecules can diffuse relatively easily through the abundant mesoporous. With a similar mechanism [38], the higher surface energy of the interior causes the small pores to collapse into larger voids and finally these voids merged into a vacuum. This is the driving force for hollowing process and the reason responsible for the progressive mass redistribution from the interior to the exterior of the silica spheres.

a 200

150

100

50

0

-50

-100

-150

-200

ppm Fig. 6. 27 Al MAS NMR spectra of samples (a) HMAS-H0.5, (b) HMAS-H2, (c) HMASH6, and (d) HMAS-H12.

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Fig. 7. TEM images of samples obtained at (a) 100 ◦ C for 24 h, (b) 140 ◦ C for 12 h, and (c) 180 ◦ C for 4 h.

5

a

5

b

4

4

4

3

3

3

2

2

2

1

1

1

c

a

0

0 1

e

10

100 1

10

f

c d b

0

100 1

10

D (nm)

TCD signal (a.u.)

3

Pore volume (cm /g )

5

100

D (nm)

D (nm)

100

Fig. 8. Pore size distribution of the samples hydrothermal treating at (a) 100 ◦ C, (b), 140 ◦ C, and (c) 180 ◦ C.

200

300

400 o

500

600

Temperature ( C)

were introduced into the shell of HMAS with prolonged hydrothermal treatment, also indicated by the Si/Al ratio of HMAS in Table 1.

Fig. 9. NH3 -TPD curves of (a) H␤, (b) MSS, (c) HMAS-H0.5, (d) HMAS-H2, (e) HMASH6, and (f) HMAS-H12.

3.3. Catalytic activity

ously, MSS did not perform any catalytic activity. The H␤ with the strongest acidity showed very low conversion of 2.7% at 250 ◦ C, and the product was only m-diisopropylbenzene. This result suggested that H␤ was basically inactive for catalytically cracking large

Table 2 presents the result of catalytically cracking 1,3,5triisopropylbenzene (TIPB) over MSS and HMAS samples. ObviTable 2 Catalytic activities in the cracking of 1,3,5-triisopropylbenzene. Sample

Temperature (◦ C)

Conversion (mol%)

Selectivity (wt.%) Propylene

MSS HMAS-H0.5

HMAS-H2

HMAS-H6

HMAS-H12

H␤

a b c

300 250 300 350 250 300 350 250 300 350 250 300 350 250 300 350

Standing for m-diisopropylbenzene. Standing for p-diisopropylbenzene. The product was undetectable.

0 27.2 39.5 44.3 54.8 58.4 60.7 70.5 74.9 80.4 88.27 97.23 100 2.7 7.5 10.2

– 20.1 22.5 23.9 25.2 27.7 29.4 31.5 36.9 37.1 38.1 43.1 46.8 –c –c –c

Benzene – 12.1 12.8 13.2 14.8 15.3 15.9 16.7 17.9 17.8 8.4 8.9 10.7 –c –c 5.2

Isopropylbenzene

m-Da

p-Db

– 45.2 43.1 43.0 42.7 40.1 39.2 35.8 34.2 35.3 38.0 39.8 40.9 –c 11.7 13.5

– 13.9 13.0 11.9 11.6 10.0 9.2 10.1 5.7 6.2 10.9 4.9 1.6 100 75.0 69.6

– 8.6 9.6 8.0 5.7 6.9 6.3 5.9 4.3 3.6 4.6 3.3 –c –c 13.3 11.7

292

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molecule of TIPB, because H␤ had 12-ring micropores with diam˚ while TIPB had the dynamic diameter of eter no larger than 6.7 A, 9.4 A˚ and cannot diffuse through the inner micropores of H␤ [25]. In contrast, HMAS samples gave very high conversion. HMAS-H0.5, with much weaker acidity than H␤, showed much higher conversion than H␤. Because HMAS-H0.5 was of mesoporous structure with acidic sites, TIPB can easily diffuse through the mesopores and be cracked on the acidic sites. With the increasing of acid strength and acid amount of HMAS (as shown in Fig. 9), the conversion also gradually increased. HMAS-H12 had the highest conversion that was reasonably attributed to its strongest acidity among the four HMAS catalysts. At 350 ◦ C, TIPB was almost completely cracked. 4. Conclusions Hollow mesoporous aluminosilicate spheres with middle strong acidity can be synthesized using zeolite precursor solution and mesoporous silica spheres as starting materials through a simple hydrothermal method. The hollowing process of silica spheres was associated with a progressive mass redistribution from the interior to the exterior of the microspheres. The zeolite primary building units were introduced into the shell of HMAS. High activity for catalytically cracking 1,3,5-triisopropylbenzene has been obtained over HMAS. Acknowledgment The financial support from the National Nature Science Foundation (No. 20573128) was gratefully acknowledged. References [1] S. Liu, H. Rao, X. Sui, P. Cool, E.F. Vansant, G. Van Tendeloo, X. Cheng, J. Non-Cryst. Solids 354 (2008) 826. [2] H. Zou, S.S. Wu, Q.P. Ran, J. Shen, J. Phys. Chem. C 112 (2008) 11623. [3] Y.Q. Yeh, B.C. Chen, H.P. Lin, C.Y. Tang, Langmuir 22 (2006) 6. [4] Y. Wan, S.H. Yu, J. Phys. Chem. C 112 (2008) 3641. [5] S. Fujita, H. Nakano, M. Ishii, H. Nakamura, S. Inagaki, Micropor. Mesopor. Mater. 96 (2006) 205. [6] X. Cheng, S.Q. Liu, L.C. Lu, X.Y. Sui, V. Meynen, P. Cool, E.F. Vansant, J.H. Jiang, Micropor. Mesopor. Mater. 98 (2007) 41.

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