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RESEARCH NOTES Removal of the Template Molecules from MCM-41 with Supercritical Fluid in a Flow Apparatus Xiao-Bing Lu,*,† Wen-Hua Zhang,‡ Jing-Hai Xiu,† Ren He,† Lu-Guang Chen,† and Xiao Li† State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, P. R. China, and State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China
Supercritical CO2 modified with a methanol/dichloromethane mixture was used to quantitatively remove the template molecules (cetyltrimethylammonium bromide) from the pores of assynthesized pure siliceous MCM-41 in a flow apparatus. The resulting mesoporous MCM-41 materials, characterized by powder X-ray diffraction, nitrogen adsorption, and high-resolution transmission electron microscopy, exhibited higher surface areas, larger pores, and narrower pore size distributions than those obtained by conventional calcination at high temperature. The removed template retained its structures and properties and could be reused in the synthesis of MCM-41 materials. Introduction Since the discovery of the M41S family by Mobil scientists in 1992,1,2 the synthesis and application of ordered mesoporous materials using surfactants as poredirecting agents has attracted wide attention.3 Among this family of materials, MCM-41 is the most extensively studied. It exhibits regularly hexagonal arrays of cylindrical mesopores and variable pore diameters between 1.5 and 20 nm. These properties make MCM-41 materials the best candidates for catalysts or catalyst supports4 and the hosts for many guest materials.5 The preparation of MCM-41 materials usually includes hightemperature calcination to destroy the surfactant template used to form mesopores. This procedure easily causes structural shrinkage.6 It has been reported that the template molecules in porous materials can be removed by extraction with polar solvents or mixtures such as ethanol or its azeotropic mixture with heptane, together with a cation donor such as dissolved HCl.7 The main drawback of this approach lies in its low extraction efficiency or/and its negative effect on the pore structure.8 Ozone treatment has been shown to be effective in eliminating the organic template from as-synthesized MCM-41 materials.9 However, questions remain as to the forms in which nitrogen and bromine are eliminated from the pores. Supercritical fluids have been used as low-viscosity, low-surface-tension reaction media in which kinetic rate enhancements are frequently observed.10,11 In fact, the low viscosity and high diffusivity inherent to supercritical fluids are ideally suited for rapid transport of reagents in to and out of the channels of porous materials. Indeed, Kawi et al. reported that * Corresponding author. Tel.: +86-0411-3631333-3243. Fax: +86-411-3633080. E-mail:
[email protected]. † Dalian University of Technology. ‡ Chinese Academy of Sciences.
up to 90% of the surfactant template (cetyltrimethylammonium hydroxide) inside the pores of MCM-41 materials could be extracted using methanol-modified supercritical CO2 at a high pressure of up to 35 MPa.12 In the present paper, supercritical CO2 modified with a methanol/dichloromethane mixture was used to quantitatively remove the template (cetyltrimethylammonium bromide, designated as CTAB) from the pores of as-synthesized pure siliceous MCM-41 at a relatively low pressure in a flow apparatus. The resulting mesoporous MCM-41 materials exhibit higher surface areas, larger pores, and narrower pore size distributions than those obtained by conventional calcination at 550 °C. Experimental Section The parent MCM-41 was synthesized according to a previously described method with CTAB as the template.13 The obtained white solid was washed with adequate deionized water at ambient temperature and dried for 24 h at 100 °C in vacuo. The experiments for removing the template from assynthesized MCM-41 were carried out using the apparatus shown in Figure 1. Two sample cells, made of stainless steel tubing of 8-mm inner diameter and 300mm length, were used to hold the parent MCM-41 during supercritical fluid treatment. A frit (glass fiber) at the bottom of each tube kept the sample in place. Supercritical fluid was easily made to flow alternately through the sample cells by switch valves. In a typical procedure, liquid CO2 and cosolvent were pumped with syringe pumps (Oriental Scientific Instrument Factory, Beijing, China; model SY-04) and allowed to flow through the system at constant pressure. They met in the mixing area and then passed through the preheater. Then, cosolvent-modified supercritical CO2, after being warmed in the preheater area, was introduced into the
10.1021/ie020422c CCC: $25.00 © 2003 American Chemical Society Published on Web 01/11/2003
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Figure 2. TG profiles of the series MCM-41 materials: (a) MCM41U, (b) MCM-41 in which 51.2% of surfactant template has been removed, and (c) MCM-41E. Table 1. Removal Efficiencies of the Template under Various Conditions in a Flow Apparatus at 95 °C for 6 h
Figure 1. Flow apparatus for removing the template from assynthesized MCM-41 silica with supercritical fluid. P, syringe pump; F, one-way valve; M, mixer; PH, preheater; R, relief valve; TS, switch valve; C, sample tube; S, pressure regulator; T, device for collecting the template; G, gas flow gauging device.
sample cells. The effluent from the sample cells was introduced into the collecting vessel, where the pressure was reduced to atmospheric. Caution: High-pressure equipment such as that required for the experiments described in this paper should be equipped with a relief valve and/or (preferably) a rupture disk to minimize the risk of personal injury. The removal efficiencies of the template under various conditions were determined by comparing the mass loss in air between the as-synthesized sample and the samples after supercritical fluid treatment by means of thermogravimetric analysis (TGA, Mettler-Toledo TGA/ SDTA851e). Three samples were used: MCM-41U (uncalcined as-synthesized mesoporous material), MCM41C (the same sample but calcined at 550 °C), and MCM-41E (the same sample but treated with supercritical CO2 modified with a CH3OH/CH2Cl2 mixture in a flow apparatus at 95 °C and 15 MPa for 6 h). The series of samples were characterized by powder X-ray diffraction (XRD, Rigaku D/Max 3400 instrument, Cu KR radiation). The pore diameters, pore volumes, and surface areas were calculated from adsorption and desorption isotherms of N2 at 77 K according to the Barrett-Joyner-Halenda (BJH) method using a micromeritics instrument from Quantachrome Corporation. Infrared spectra were recorded under ambient conditions with a Nicolet 50X FTIR spectrophotometer. TEM analyses were performed using a JEOL 200cx electron microscope operating at 200 kV. Results and Discussion The removal efficiencies of the template molecules from as-synthesized MCM-41 under various conditions are shown in Table 1. Although supercritical CO2 is a good solvent for most nonpolar and some polar organic compounds with low molecular weights, it barely dissolves the surfactant template. Therefore, no template
entry
cosolvent
1 2 3 4 5 6
CH3OH CH2Cl2 CH3OH/CH2Cl2 CH3OH/CH2Cl2 CH3OH/CH2Cl2
liquid CO2 cosolvent removal pressure flow rate flow rate efficiency (mL/h) (mL/h) (%) (MPa) 15.0 15.0 15.0 15.0 5.0 0.1
45 36 36 36 36 0
9 9 9 9 30
0.0 69.7 74.1 92.4 51.2 9.0
was removed from the mesoporous channels of MCM41 when pure supercritical CO2 was used as the solvent. However, the solvating strength of supercritical CO2 can be improved by adding a small amount of polar solvents such as methanol or dichloromethane as modifiers.14 In fact, methanol-modified supercritical CO2 has been demonstrated to be effective in removing the template from as-synthesized MCM-41 materials at the high pressure of 35 MPa.12 In the present case, a number of cosolvents were tried, including methanol, ethanol, dichloromethane, and their mixtures. Among them, the CH3OH/CH2Cl2 mixture was found to be the most effective, perhaps because of the very high solubility of the surfactant template in the mixture. A removal efficiency of up to 92.4% was achieved at the relatively low pressure of 15 MPa when supercritical CO2 modified with a CH3OH/CH2Cl2 mixture was used as the solvent; this value was higher than those obtained in systems with CH3OH or CH2Cl2 alone as the cosolvent. The results in Table 1 also indicate that reducing the pressure results in lower removal efficiencies. The higher removal efficiency obtained under supercritical conditions can be ascribed to the enhanced desorption and transfer of template molecules by internal and external diffusion in the mesopores of MCM-41, because the low viscosity and high diffusivity inherent to supercritical fluids support the rapid transport of reagents in to and out of the channels of porous materials. The TG profiles of some resulting MCM-41 materials are shown in Figure 2. The small-angle XRD (SAXRD) patterns of the samples obtained in the present study are depicted in Figure 3. All of the samples display three or four reflection peaks that are assigned to the (100), (110), (200), and (210) reflections of hexagonally ordered MCM-type structures with unit cell parameters between 4.5 and 4.9 nm. The d100 value and the positions of all reflection peaks for MCM-41E are the same as those for MCM-41U, indicating that pore contraction did not occur during the removal of the template from the as-synthesized MCM-
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Figure 3. Powder X-ray diffraction patterns of the series of MCM41 materials: (a) MCM-41U, (b) MCM-41U for which the removed template from the as-synthesized MCM-41 material was reused, (c) MCM-41E, and (d) MCM-41C.
Figure 5. HRTEM images of MCM-41E sample taken with the beam direction (a) parallel to the pores and (b) perpendicular to the pores. Table 2. Properties of the Series of MCM-41 Molecular Sieves
Figure 4. Nitrogen adsorption (squares) and desorption (circles) at 77 K of (a) MCM-E and (b) MCM-41C.
41 by supercritical fluid treatment. As with MCM-41C, the intensity of the (100) reflection for MCM-41E increased by about 20%, indicating that MCM-41E is more ordered than MCM-41C. The adsorption-desorption isotherms (Figure 4) of N2 at 77 K are of type IV for the series of MCM-41 samples, which is typical of mesoporous materials. The capillary condensation step in the isotherm of MCM-41E is, in reality, sharper and slightly higher in relative pressure (P/P0 ) 0.40 compared with 0.35) than that of MCM41C. This would seem to indicate that a larger pore diameter and a narrower pore size distribution could be achieved for MCM-41 mesoporous materials using the supercritical fluid technique, which is in agreement with the SAXRD results. The parameters calculated from the nitrogen adsorption data using the BJH method are listed in Table 2.
sample
d100 (nm)
lattice parameter (nm)
pore diameter (BJH) (nm)
SBET (m2 g-1)
pore volume (cm3 g-1)
MCM-41U MCM-41E MCM-41C
42.5 42.5 39.4
4.91 4.91 4.55
3.05 2.76
901 866
0.878 0.829
Further evidence that the highly ordered pore structure has been preserved during supercritical fluid treatment is provided by transmission electron micrographs, as shown in Figure 5. In the micrograph taken with the beam direction parallel to the pores (Figure 5a), the hexagonally ordered pore structure can be observed. On the other hand, the micrograph taken with the beam direction perpendicular to the pores (Figure 5b) clearly shows the alternating structure of mesoporous channels and framework of MCM-41 materials. The FTIR spectrum (Figure 6) of the removed template is similar to that of pure CTAB, indicating that the chemical structure and properties of the template removed from the as-synthesized MCM-41 remain the same as those of fresh CTAB. The removed template
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Literature Cited
Figure 6. FTIR spectra of (a) fresh CTAB and (b) the template removed from the as-synthesized MCM-41 material using supercritical fluid treatment.
could easily be isolated by simple distillation and reused for synthesizing MCM-41 materials, the XRD results for which are shown in Figure 3b. The d100 value and the positions of all reflection peaks for MCM-41U (b) are the same as those of MCM-41U (a), which is made of fresh template, indicating that the template removed from the as-synthesized MCM-41 by supercritical fluid treatment has retained its structure and properties. Conclusion Supercritical CO2 modified with a methanol/dichloromethane mixture is an effective solvent for the removal of the template molecules from as-synthesized MCM41 materials in a flow apparatus. The resulting MCM41 materials exhibit higher surface areas, larger pores, and narrower pore size distributions than those obtained by high-temperature calcination. The removed template is shown to retain its structure and properties, so that it could be reused in the synthesis of MCM-41 materials. Acknowledgment Gratitude is expressed to the National Science Foundation (NSFC) program (Grant 20204002) for financial support.
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Received for review June 6, 2002 Revised manuscript received December 4, 2002 Accepted December 12, 2002 IE020422C
