Room-temperature synthesis of hydrothermally stable aluminum-rich periodic mesoporous organosilicas with wormlike pore channels

Microporous and Mesoporous Materials 85 (2005) 32–38 www.elsevier.com/locate/micromeso

Room-temperature synthesis of hydrothermally stable aluminum-rich periodic mesoporous organosilicas with wormlike pore channels Wanping Guo, X.S. Zhao

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Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore Received 23 August 2004; received in revised form 18 February 2005; accepted 6 June 2005 Available online 27 July 2005

Abstract Hydrothermally stable Al-containing periodic mesoporous organosilicas with various Si/Al ratios as low as 5.6 were easily prepared at room temperature by NH4OH-catalyzed, CTAB-templated hydrolysis and condensation of 1,2-bis(triethoxysilyl)ethane and aluminum isopropoxide. Characterization data with XRD, TEM, SEM, nitrogen adsorption, 29Si and 27Al MAS NMR, pyridine-TPD, and hydrothermal test showed that the mesoporous materials possess wormlike channels with high textural porosity, tetrahedrally and octahedrally coordinated Al atoms in their textural structures, and weak acid sites whose contents are proportional to the amount of Al incorporated. It was also observed that the morphology of PMO samples changed with the incorporation of aluminum into the framework. Both Al-PMO with medium aluminum amount (Si/Al = 29) and Al-PMO with high aluminum amount (Si/Al = 5.6) were stable in boiling water for as long as 12 days owing to the presence of hydrophobic ethane bridging groups as well as the absence of sodium ions in the frameworks. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Periodic mesoporous organosilicas; Al incorporation; Wormlike channels; Hydrothermal stability; Characterization

1. Introduction The discovery of a novel class of organic–inorganic hybrid materials called periodic mesoporous organosilicas (PMOs) has opened up a new area in advanced porous materials research [1–8]. PMOs are synthesized by hydrolysis and condensation of silsesquinoxane (RO)3Si–R 0 –Si(OR)3 in the presence of surfactant template. It has been demonstrated that cationic, anionic, neutral, non-ionic oligomeric and triblock copolymer surfactants can all be used as the templates to prepare PMO materials with various hierarchical structures

*

Corresponding author. Tel.: +65 68744727; fax: +65 67791936. E-mail address: [email protected] (X.S. Zhao).

1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.06.004

and bridging organic groups such as methane, ethane, ethylene, benzene, thiophene, biphenylene, and cyclohexane [9–15]. These PMO materials exhibit unique chemical, physical and mechanical properties suitable for a wide range of applications such as catalysis, adsorption and separation. It is known that pure silicas or organosilicas themselves are catalytically inactive in many reactions. However, when metal atoms such as Al, Ti are incorporated into their frameworks, catalytically active sites can be generated. So far, there have been only a few articles dealing with this topic [16–19]. Inagaki and co-workers [16] first reported the effective incorporation of Ti with different amounts into the pore channel walls of PMO materials. It was observed that the Ti-containing PMO materials are highly hydrophobic and very efficient in the catalytic reactions of epoxidizing a-pinene to

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a-pinene oxide [16], epoxidizing propene to propene oxide using H2 and O2 [17], and ammoximating bulky ketones to oximes [18]. The hydrophobicity of the Ticontaining PMO materials was directly correlated with their catalytic effectiveness. Very recently, Yang et al. [19] described the synthesis of Al-containing PMOs with ordered hexagonal structures by one-pot condensation of aluminum isopropoxide and 1,2-bis(trimethoxysilyl)ethane using octadecyltrimethylammonium chloride as the template. They observed that the Al-containing PMOs are hydrothermally stable in boiling water for 100 h and are suitable catalyst for the alkylation of 2,4-di-tert-butylphenol. However, the reported method involved using relatively expensive template and the Si/Al ratio was restricted to 29. A facile and cost-effective route to the preparation of Al-containing PMOs with high hydrothermal stability and much acidity remains a challenge. Hydrothermal stability and acidity are two key factors in the development of new water-tolerant solid acid catalysts for practical applications [20]. In the case of Al-containing mesoporous materials, the number of acidic sites could be enhanced by increasing the amount of Al atoms incorporated, namely, decreasing the Si/Al ratios in the resultant mesostructures obtained [21]. In general, the hydrothermal stability of mesoporous materials depends on their inherent wall properties, where the adsorbed water interacted with the wall surface. The collapse of an ordered mesoporous structure has been attributed to the hydrolysis of bare Si–O–Si (Al) bonds caused by water adsorbed onto the nearby silanol groups [22,23]. In order to improve the hydrothermal stability of mesoporous aluminosilicates, two main approaches have been employed. One is to provide a protective layer on the pore wall surfaces to prevent the framework from water attack. The other one is to introduce more water-tolerant structural units into the mesopore walls to lessen the hydrolysis of the framework. The former approach is typical of post-synthesis grafting of heteroatoms or hydrophobic groups while the latter one can be illustrated as the use of protozeolitic nanoclusters or hydrophobic organosilicas as framework precursors [19,24]. Recent studies [25–27] have demonstrated the enhanced hydrothermal stability of PMO materials due to the hydrophobicity imparted by the organic components in the framework. It is the purpose of this study to present room-temperature synthesis of Al-containing PMOs (Al-PMOs) with a wide range of Si/Al ratios as low as 5.6 using cheap template cetyltrimethylammonium bromide by NH4OH-catalyzed condensation of 1,2-bis(triethoxysilyl)ethane and aluminum isopropoxide. The effects of incorporating different aluminum amount on the structure, acidity and hydrothermal stability of Al-PMOs are discussed.

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2. Experimental 2.1. Chemicals 1,2-Bis(triethoxysilyl)ethane (BTEE, 96%, Aldrich), tetraethyl orthosilicate (TEOS, 98%, Fisher), aluminum isopropoxide (Al(OiPr)3, 98%, Acros), cetyltrimethylammonium bromide (CTAB, 99%, Merck), ammonia solution (35%, Fisher), and absolute ethanol (99.98%, Merck) were used as received. 2.2. Synthesis of mesoporous materials A typical synthesis procedure is described as follows. 0.67 g of CTAB was dissolved in 23 g of deionized water and 12 g of ammonia solution under stirring at room temperature. To this solution was added 2.66 g of BTEE and a given amount of Al(OiPr)3. The resulting mixture was continuously stirred at ambient temperature for 24 h. The white powder was collected by filtration, washed thoroughly with water and dried at ambient conditions. Template removal from the as-synthesized samples was performed by ethanol extraction at 60 °C for 6 h and repeated for three times. Six samples were involved in this study. Four Al-containing PMO samples were prepared from the gels with molar ratios of Si/ Al = 50, 25, 10, and 5.0, respectively. They were denoted as Al-PMO-x, where x is the Si/Al ratio in the resultant solids. One pure PMO sample denoted as Si-PMO was prepared without the presence of aluminum, and one mesoporous aluminosilicate sample was prepared from the gel with a molar ratio of Si/Al = 25 by substituting a bimolar amount of TEOS for BTEE. 2.3. Characterization Powder X-ray diffraction (XRD) patterns were obtained on a SHIMADZU XRD-6000 diffractometer using Ni-filtered CuKa radiation (k = 0.1542 nm) at 40 kV and 30 mA. Transmission electron microscopy (TEM) images were acquired on a JEOL 2010 electron microscope operated at an accelerating voltage of 200 kV. Prior to TEM measurement, a powder sample was dispersed in ethanol, followed by depositing two drops onto a carbon-coated copper grid. Scanning electron microscope (SEM) images were obtained with a JEOL JSM-5600L microscope operated at an accelerating voltage of 15 kV. N2 adsorption and desorption isotherms were measured at 196 °C on a Quantachrome NOVA 1200 system. Samples were degassed at 200 °C for 3 h before measurements. The specific surface area of a sample was calculated from its adsorption isotherm using the multiple-point BET method in the relative pressure range of 0.05–0.2. The pore volume was calculated from the amount adsorbed at a relative pressure of 0.99. The pore diameter was analyzed from the

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adsorption branch of the isotherm by using the BJH method. The molar ratios Si/Al of the solid samples were determined by using a Perkin–Elmer Optima 3000DV inductively coupled plasma (ICP) instrument. Solid-state MAS NMR spectra were recorded on a Bruker DRX400 FT-NMR spectrometer operated at a resonance frequency of 79.5 MHz for 29Si NMR and 104.3 MHz for 27Al NMR. The chemical shifts were ref29 erenced to tetramethylsilane and AlðH2 OÞ3þ Si and 6 for 27 Al signals, respectively. Temperature-programmed desorption of pyridine (pyridine-TPD) was carried out on a TGA 2050 thermogravimetric analyzer. A template-free Al-PMO sample was first activated at 140 °C for 24 h under vacuum before it was exposed to saturated pyridine vapor overnight at room temperature, followed by thermal treatment at 100 °C for 2 h. Desorption of pyridine was conducted from room temperature to 850 °C with a heating rate of 10 °C/min under N2 flow of 100 cm3/min. The hydrothermal stability of a template-free sample was tested by autoclaving 0.20 g of the sample in 20 g of deionized water at 100 °C for a given period of time.

3. Results and discussion

Intensity (a. u.)

The powder XRD patterns of the solvent-extracted Si-PMO and Al-PMO samples are shown in Fig. 1.

a b

c d e 2

3

4

5

6

2 theta (degree) Fig. 1. Powder XRD patterns of solvent-extracted: (a) Si-PMO, (b) Al-PMO-57, (c) Al-PMO-29, (d) Al-PMO-12, and (e) Al-PMO-5.6.

The corresponding d spacings are listed in Table 1. It can be seen that all PMO samples give only one reflection peak at the low-angle region. The incorporation of Al into the framework of periodic mesoporous organosilicas is evidenced by the decrease of d spacings of Al-PMO samples, suggesting a more condensed framework in Al-PMO samples. Furthermore, the structural order of Al-PMO samples was deteriorated upon incorporating of high amount of Al. Nevertheless, the mesopore structure of Al-PMO-5.6 sample is still reserved although the main peak becomes broader and less intense, which has demonstrated the successful preparation of Al-PMO with a high Al amount (Si/Al = 5.6) under room temperature. The TEM images, represented in Fig. 2, show the existence of wormlike pore channels in the Al-PMO samples regardless of the amount of Al incorporated. It should be noted that the d spacing of the Al-PMO-29 sample is larger than that of an aluminosilicate MCM-41 sample with a Si/Al ratio of 27 (see Table 2). This could be attributed to the weak assembly interaction between positively charged CTAB micelles and negatively charged BTEE precursors by S+I pathway, as well as the low pH of ammonia used herein. The negative charge density of BTEE precursors is lower than that of TEOS precursors in basic media because the silyl groups are separated by ethane fragments in BTEE [1]. The low pH would give rise to the incomplete hydrolysis and condensation of organosilane species [28]. The combination of the above two effects may result in the formation of Al-PMOs with larger d spacings than mesoporous aluminosilicates and the presence of wormlike pore channels. The SEM images of Si-PMO and Al-PMO samples are presented in Fig. 3. Interestingly, the Si-PMO sample appears as aggregates of crystal-like platelets, about 5 lm in diameter. With the incorporation of aluminum, the smooth platelets become loose and some aggregates emerge in ball-like morphology (Fig. 3b). Finally, the Al-PMO-5.6 sample with the highest Al amount incorporated displays ball-like discrete particles with diverse sizes, as shown in Fig. 3c. This indicates that the incorporation of aluminum into the framework has an important influence on the morphology of the obtained mesoporous materials. Fig. 4 shows the nitrogen adsorption and desorption isotherms of solvent-extracted Si-PMO and Al-PMO samples. The surface area, pore volume, and pore

Table 1 Textural properties and acidity of solvent-extracted Si-PMO and Al-PMO samples Sample

Si/Al (gel) Si/Al (solid) d Spacing (nm) Surface area (m2/g) Pore volume (cm3/g) Pore diameter (nm) Acidity (mmol/g)

Si-PMO Al-PMO-57 Al-PMO-29 Al-PMO-12 Al-PMO-5.6

1 50 25 10 5.0

1 57 29 12 5.6

5.51 5.43 5.08 5.05 4.92

871 767 946 896 910

0.71 0.58 0.76 0.65 0.66

3.08 2.82 2.89 2.91 2.13

– 0.16 0.22 0.27 0.31

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Fig. 2. TEM images of: (a) Al-PMO-29 and (b) Al-PMO-5.6.

Table 2 Textural properties of solvent-extracted Al-PMO and Al-MCM-41 samples before and after hydrothermal treatment in boiling water Sample

d Spacing Surface Pore Pore (nm) area volume diameter (m2/g) (cm3/g) (nm)

Al-PMO-29 Al-PMO-29 (4 days) Al-PMO-29 (8 days) Al-PMO-29 (12 days) Al-MCM-41 (Si/Al = 27) Al-MCM-41 (4 days) Al-PMO-5.6 (4 days) Al-PMO-5.6 (8 days) Al-PMO-5.6 (12 days)

5.08 5.07 5.07 5.06 3.89 – 4.89 4.85 4.83

946 968 914 918 940 459 878 861 842

0.76 0.77 0.72 0.73 0.86 0.48 0.59 0.58 0.55

2.89 2.88 2.87 2.87 2.66 – 2.17 2.17 2.18

diameter of the samples are presented in Table 1. It is seen that all samples possess high surface area and large pore volume. The isotherms of all PMO samples except for that of Al-PMO-5.6 sample are of type IV with a pronounced uptake in the relative pressure range of 0.25–0.45 due to capillary condensation of mesopores [29], indicating the presence of relatively uniform mesopores. However, the isotherm of Al-PMO-5.6 sample

Fig. 3. SEM images of: (a) Si-PMO, (b) Al-PMO-29, and (c) Al-PMO5.6.

is between type I and type IV [29], which is evidenced by the presence of small pore diameter (2.13 nm). In addition, the shift of the mesopore filling step to the lower relative pressure with the Al incorporation indicates that the pore diameter of Al-PMO is smaller than that of Si-PMO, as confirmed by the data in Table 1. The 29Si and 27Al MAS NMR spectra of the solventextracted Al-PMO-29 sample are given in Fig. 5. The 29 Si NMR spectrum exhibits two signals at 60.1 and 67.2 ppm corresponding to T2 [RSi(OSi)2OAl] and

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a

2

T

Volume adsorbed (cm3/g STP)

1000

T

3

b

800

c 600

d

400

e

a

0

-30

200

-60

-90

-120

Chemical shift (ppm) Tetrahedron

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0) Octahedron Fig. 4. Nitrogen adsorption (solid line) and desorption (hollow line) isotherms of solvent-extracted: (a) Si-PMO, (b) Al-PMO-57, (c) AlPMO-29, (d) Al-PMO-12, and (e) Al-PMO-5.6. The isotherms are offset by 150 cm3/g for clarity.

T3 [RSi(OSi)3] resonances, respectively. The absence of signals due to Qn [Si(OSi)n(OH)4n] species between 90 and 120 ppm indicated that all carbon–silicon bonds were intact in the course of synthesis and extraction [5]. As expected, the incorporation of Al into the mesoporous framework was demonstrated by the upfield shift from T2 resonance at 57.0 ppm for SiPMO to T2 resonance at 60.1 ppm for Al-PMO [19]. Furthermore, the 27Al MAS NMR spectrum (Fig. 5b) provides direct evidence on the environments of Al species incorporated. The peak at 53.9 ppm is assigned to tetrahedrally coordinated aluminum in the framework of Al-PMO while the peak at 6.8 ppm is due to octahedrally coordinated non-framework Al species. Pyridine-TPD technique [30] was employed to characterize the acidity of Al-PMO samples. Fig. 6 shows the pyridine—TPD curves of four Al-containing PMO samples. Included in Fig. 5 is also the first derivative weight loss (DrTGA) curve of Al-PMO-29 sample before pyridine adsorption. Four thermal desorption peaks can be observed on Al-PMO-29 sample (Fig. 6a) before pyridine adsorption. The peak below 100 °C corresponds to the desorption of physically adsorbed species like water. The very small peak at about 228 °C may be attributed the loss of surface ethoxy groups from incomplete hydrolysis of BTEE and/or formed during ethanol extraction [25]. The peaks from 300 to 800 °C are due to the gradual decomposition of ethane bridging groups in the mesoporous framework [31]. In comparison with the DrTGA curve of solvent-extracted Al-PMO-29, the pyridine— TPD curves of the four Al-PMO samples exhibit one extra peak at about 166 °C, which is related to the

b 150

100

50

0

-50

-100

Chemical shift (ppm) 29

Fig. 5. (a) Si and (b) Al-PMO-29.

27

Al MAS NMR spectra of solvent-extracted

desorption of pyridine adsorbed on weak acid sites. As shown in Table 1, the number of the acidic sites calculated from the pyridine—TPD curves was 0.16, 0.22, 0.27, and 0.31 mmol/g for Al-PMO-57, Al-PMO-29, Al-PMO-12, and Al-PMO-5.6, respectively, suggesting a proportional relationship of the number of acidic sites with the amount of Al incorporated. The powder XRD patterns of the solvent-extracted mesoporous materials before and after hydrothermal treatment in boiling water are shown in Fig. 7. The textural properties, such as d spacing, surface area, pore volume, and pore diameter for these materials are provided in Table 2. Interestingly, the XRD patterns remain essentially unchanged for the Al-PMO-29 sample after treated in boiling water for 4–12 days. Almost no decrease in the d spacing occurred for the Al-PMO sample before and after hydrothermal treatment. The structural properties obtained from nitrogen adsorption such as similar surface area, comparable pore volume, and nearly same pore diameter of the Al-PMO samples before and after hydrothermal treatment further attest the high stability of the Al-PMO materials. By contrast, the aluminosilicate MCM-41 sample with a Si/Al ratio of 27 was unable to withstand the 4-day treatment in

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DrTGA (%/°C)

0.15

0.10

0.05 e d c

166

b a

0.00

100 200 300 400 500 600 700 800

Temperature (°C)

Intensity (a. u.)

Fig. 6. DrTGA curves of (a) Al-PMO-29 before pyridine adsorption and pyridine-adsorbed Al-PMO samples: (b) Al-PMO-57, (c) AlPMO-29, (d) Al-PMO-12, and (e) Al-PMO-5.6.

a b c d e f 2

3

4

5

6

2 theta (degree) Fig. 7. Powder XRD patterns of solvent-extracted samples before and after hydrothermal treatment in boiling water for: (a) Al-PMO-29 (untreated), (b) Al-PMO-29 (4 days), (c) Al-PMO-29 (8 days), (d) AlPMO-29 (12 days), (e) Al-MCM-41 (Si/Al = 27, untreated), and (f) AlMCM-41 (4 days).

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boiling water. After the treatment, the mesostructure of Al-MCM-41 collapsed as revealed by the XRD pattern shown in Fig. 7f. Consequently, a significant decrease occurred in surface area from 940 to 459 m2/g, and in pore volume from 0.86 to 0.48 cm3 /g. Furthermore, the hydrothermal stability of the Al-PMO sample with the highest Al amount incorporated was investigated. Fig. 8 gives the nitrogen adsorption isotherms and the corresponding pore size distribution plots for the Al-PMO-5.6 sample before and after hydrothermal treatment. Similarly, the isotherms and the pore diameters are almost same for the Al-PMO-5.6 sample after treated in boiling water for 4–12 days. As shown in Table 2, the Al-PMO-5.6 sample retained 93% of surface area with a change from 910 m2/g to 842 m2/g via 12-day hydrothermal test. This result reveals that the Al-PMO samples with various Si/Al ratios prepared at room temperature are very stable in boiling water. The high hydrothermal stability of Al-PMO materials can be mainly attributed to the hydrophobic ethane bridging groups in the mesoporous framework [19,25]. A recent study [32] on the adsorption properties of PMO materials for water and n-hexane corroborated hydrophobic surface nature of the framework contributed by the ethane groups. The surface property of lacking affinity for water would protect bare Si–O–Si (Al) bonds against hydrolysis, resulting in an increase of hydrothermal stability. Another reason for the high hydrothermal stability of the Al-PMO materials studied herein may be due to the absence of sodium ions in their frameworks. Pauly et al. [33] observed that entrapped framework sodium ions are able to catalyze the collapse of mesoporous structures upon hydrothermal treatment, showing the deleterious effect of sodium ions on hydrothermal stability. Thus, the absence of sodium ions in our samples in this study not only eliminates the ion exchange procedure for creation of acidic sites, but also enhances the hydrothermal stability of the Al-PMO materials.

Sovent-extracted Al-PMO-5.6 Al-PMO-5.6 (4 days) Al-PMO-5.6 (8 days) Al-PMO-5.6 (12days)

0.03

500 400

dV / dD

Volume ads. (cm3/g STP)

600

300 200 100 0 0.0

Solvent-extractedAl-PMO-5.6 Al-PMO-5.6 (4 days) Al-PMO-5.6 (8 days) Al-PMO-5.6 (12days)

0.2 0.4 0.6 0.8 Relative pressure (P/P0)

1.0

0.02

0.01

0.00 2

4 6 8 Pore diameter (nm)

10

Fig. 8. Nitrogen adsorption isotherms and the corresponding pore size distribution plots for the Al-PMO-5.6 sample before and after hydrothermal treatment in boiling water. The isotherms are offset by 50 cm3/g for clarity.

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4. Conclusions A convenient and cost-effective route to the synthesis of Al-containing periodic mesoporous organosilicas with a wide range of Si/Al ratios was described in this study. Especially, The hydrothermally stable Al-PMO with a Si/Al ratio as low as 5.6 was easily prepared at room temperature. The Al-PMOs samples exhibited wormlike channels with high textural porosity. It was observed that the incorporation of aluminum into the framework has an important influence on the morphology of the PMO materials. The number of the acidic sites of the Al-PMO materials was found to be proportional to the amount of Al incorporated. The Al-PMO materials were hydrothermally very stable that can withstand treatment in boiling water for as long as 12 days without observed structure collapses. The high hydrothermal stability is attributed to the presence of hydrophobic ethane bridging groups as well as the absence of sodium ions in their frameworks. The Al-containing mesoporous materials are expected to find applications as a water-tolerant solid acid catalyst for those reactions requiring weak acidic sites and low temperatures.

Acknowledgement Financial support from ARF of NUS is gratefully acknowledged.

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