The Role of Surfactant Micelles in the Synthesis of the mesoporous

The Role of Surfactant Micelles in the Synthesis of the mesoporous

(Parte 1 de 3)

Langmuir 1995,1, 2815-2819 2815

The Role of Surfactant Micelles in the Synthesis of the Mesoporous Molecular Sieve MCM-41

Chi-Feng Cheng, Zhaohua Luan, and Jacek Klinowski"

Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 IEW, U.K.

Received March 27, 1995@

The role of the surfactant cetyltrimethylammonium chloridehydroxide in the synthesis ofthe mesoporous molecular sieve MCM-41 was studied by powder X-ray diffraction and 29Si magic-angle-spinning NMR. The aim was to test the hypothesis that the surfactant micelles template the formation of the solid and/or catalyze the hydrolysis of organic silicate and its subsequent polymerization. MCM-41 can be synthesized with surfactant concentrations as low as, but not below, the critical micelle concentration, which provides the first direct proof that surfactant micelles indeed template the synthesis. In the absence ofthe surfactant the product is invariably amorphous, and the rate of silicate polymerization increases by a factor of more than 2000 when the surfactant is added. The micelle catalysis mechanism relies on electrostatic interaction at the micelle-silicate interface and the higher silicate concentration near the interface than in the bulk.

Introduction

Considerable efforts have been devoted to synthesizing wide-pore molecular sieves for chemical processing of large molecules. The microporous materials with the largest intracrystalline space prepared until 1992 had been the aluminophosphates AlP04-8,1 VPI-52 and ~l$verite,~ all of which have pore diameters in the 8-13 A range. The widest aperture in conventional aluminosilicate molecular sieves (zeolites) is circumscribed by a ring of 12 tetrahedral atoms (Si or Al) and is ca. 7.4 A in diameter. A new family of highly uniform silicat? mesoporous materials with pore diameter in the 15- 100 Arange,4,5 has therefore attracted much attention. These solids allow faster diffusion of large organic molecules than the zeolitic and aluminium phosphate-based microporous sieves. Their high thermal and hydrothermal stability and the uniform size and shape of the pores over micrometer length scales, as well as the prospect of "tuning" the pore aperture by selecting a suitable template, make these materials potentially useful as catalysts for fluidized catalytic cracking (FCC) and for the manufacture of fine chemicals.

MCM-41, a member of this family of solids, lacks strict crystallographic order on the atomic level, and its powder X-ray diffraction (XRD) pattern consists of a 3-4 low- index peaks reflecting the quasi-regular arrangement of the mesopores. Several studies concerning the synthesis and characterization of MCM-41 have been publi~hed.~-l~

Pore morphologyll and adsorption properties have been Abstract published in Advance ACS Abstracts, June 1, 1995. (1) Dessau, R. M.; Schlenker, J. L.; Higgins, J. B. Zeolites 1990,10,

(2) Davis, M. E.; Saldarriaga, C.; Montes, C.; Garces, J.; Crowder, (3) Estermann, M.; McCusker, L. B.; Baerlocher, C.; Merrouche, A.;

C. Nature 1988, 331, 698.

Kessler. H. Nature 1991. 352. 320. (4) Kksge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck,

J. S. Nature 1992, 359, 710. (5) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge,

C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. SOC. 1992, 114, 10834. (6) Reiss-Husson, F.; Luzzati, V. J. Phys. Chem. 1964, 68, 3505 (7) Chen, C.-Y.; Li, H.-X.; Davis, M. E. Microporous Mater. 1993,2, 17. (8) Monnier, A.; Schuth, F.; Huo, Q.; Kumar, D.; Margolese, D.;

Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299. (9) Coustel,N.;Renzo, F. D.;Fajula, F. J. Chem. Soc.,Chem. Commun. 1994,967. (10) Steel, A.; Carr, S. W.; Anderson, M. W. J. Chem. SOC., Chem. Commun. 1994, 1571.

studied,12J3 and elements such as Al,14-18 Ti,19 and Pa were incorporated into the framework to generate catalytic activity. Conducting filaments of polyanilineZ1 and carbon (Iwires~m have been encapsulated in the channels.

MCM-41 is formed hydrothermally in the presence of a cationic surfactant cetyltrimethylammonium chloride/ hydroxide. Different formation mechanisms were pro- po~ed,~,*,~~ but the experimental support for each has been insufficient. It is clear that the surfactant plays an vital role for the formation of MCM-41, but the precise mechanism of the process is unclear. Dubois et al. found that the growth of silica polymers in a lamellar mesophase in the didodecyldimethylammonium bromidelwater sys- tem proceeds very fast in the acidic media,24 while Monnier et al. have shown that surfactant-silicate interface region favors silicate polymerization.8 However, the mechanism of the catalytic action of the surfactant is unknown. We report a detailed study of the role of the surfactant in the synthesis ofMCM-41. Numerous syntheses with different surfactant concentrations and in the absence of the surfactant show that the surfactant micelle is a template and catalyzes the hydrolysis of the organic silicate and its subsequent polymerization.

Experimental Section

Synthesis. The source of silica was tetraethylorthosilicate (TEOS, 98%). A solution of cetyltrimethylammonium (CTA)

(1) Alfredsson, V.; Keung, M.; Monnier, A.; Stucky, G. D.; Unger, (12) Rathousky, J.; Zukal, A.; Franke, 0.; Schulz-Ekloff, G. J. Chem. (13) Branton, P. J.; Hall, P. G.; Sing, K. S. W.; Reichert, H.; Schuth, (14) Luan, Z.; Cheng, C.-F.; Zhou, W.; Klinowski, J. J. Phys Chem.

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1994,148, 569. Solid State NMR 1993, 2, 253. Chem. Commun. 1994, 147. Commun. 1994, 1059.

(18) Kolodziejski, W.; Corma, A.; Navarro, M.-T.; Perez-Pariente, (19) Corma, A, Navarro, M. T.; Perez-Pariente, J. J. Chem. SOC., (20) Reddy, K. M.; Moudrakovski, I.; Sayari, A. J. Chem. SOC., Chem.

(21) Wu, C.-G.; Bein, T. Science 1994,264, 1757. (2) Wu, C.-G.; Bein, T. Science 1994,266, 1013. (23) Chen, C.-Y.; Burkett, S. L.; Li, H.-X.; Davis, M. E. Microporous

(24) Dubois, M.; Gulik-Krzywicki, Th.; Cabane, B. Langmuir 1993, Mater. 1993,2, 27.

9, 673.

0743-746319512411-2815$09.0/0 0 1995 American Chemical Society

2816 Langmuir, Vol. 1, No. 7, 1995 chloridehydroxide was prepared by batch ion exchange of a 25 wt % aqueous solution of (CTA)Cl with the IRA-420 (OH) ion- exchange resin (both from Aldrich) to achieve different degrees of exchange determined by acidhase titration. We refer to the products as (CTA)CL/OH. A 25 wt % aqueous solution of tetramethylammonium hydroxide ((TMAIOH) was obtained from Aldrich.

A typical procedure used for the synthesis of purely siliceous

MCM-41 was as follows. (TMA)OH andNaOH were added to an aqueous solution of (CTA)Cl or (CTA)Cl/OH of appropriate concentrations and stirred for 10 min. TEOS was combined with the resulting solution at room temperature under stirring. Unless specified otherwise, the gel was left to react for 24 h at room temperature. The gel turns cloudy after 2-5 min, and the product instantly begins to precipitate. The initial precipitation time was so clear-cut and reproducible that it could be used directly to provide relative kinetic data. The molar composition of final gel mixtures (pH = 1.0-1.3) was 0.5 TEOS:O.12 NaOH:0.06 TMAOH:5.6 H@:(0-0.5) (CTA)Cl or (CTA)CVOH. The solid product was filtered, washed with distilled water, dried in air at 120 "C and finally calcined at 550 "C for 16 h.

Sample Characterization. X-ray Diffraction. XRD pat- terns were recorded using a Philips 1710 powder diffractometer with Cu Ka radiation (40 kV, 40 mA), 0.025" step size and 1 s step time. Well-resolvedXRD patterns were recorded with 0.01" step size and 10 s step time.

Solid-state NMR. 29Si magic-angle-spinning (MAS) NMR spectra were recorded at 79.4 MHz using a Chemagnetics CMX- 400 spectrometer and a Doty probehead with 7 m cylindrical MAS rotors spun at 3 kHz. The 30" radiofrequency pulses and 90 s recycle delays were used, and 400-600 scans were acquired for the solid products. 29Si spectra ofgels were obtained without sample spinning~sing45~pulses, 10 s recycle delays, and 13 500-

27 0 scans. Chemical shifts are given in ppm from external tetramethylsilane (TMS).

Elemental Analysis. The content of carbon, hydrogen, and nitrogen was determined using a Carlo Erba Elemental Analyzer (gas chromatograph with a thermal conductivity detector).

Results and Discussion

Surfactant Micelle as a Template. XRD patterns of products synthesized using different concentrations of the surfactant are shown in Figure 1. Diffraction peaks at 28 below 2" could not be measured accurately for instru- mental reasons. The relatively well-defined pattern in Figure If is typical of MCM-41 as described by Kresge et al.4 All four XRD peaks can be indexFd on a hexagonal lattice with repeat distance a, 41 A (for a hexagonal latticea, = 2dlod31'z). Figures l(b)-(g) show at least three reflections, (loo), (110), and (210), which can be indexed on a hexagonal lattice. It is clear that MCM-41 is formed only at [(CTA)Cl] 1 0.0013 M. The relative crystallinity of the MCM-41 product, evaluated from the relative intensity and full width at half-maximum of the XRD peaks, increases with increasing [(CTA)ClI between 0.0013 and 0.12 M, but decreases again above 0.12 M, at which concentration the best quality MCM-41 is formed. Since the critical micelle concentration of (CTA)Cl in water is

0.0013 M,25 it is clear that the presence of surfactant micelles is essential for the synthesis of MCM-41.

Figure 2 shows the percentages of Si02 and CTA+ which are recovered in the solid product when different con- centrations of (CTA)Cl concentrations are used, assuming that after overnight calcination at 550 "C all silicate is in the form of SiOz. At [(CTA)Cl] > 0.12 M, virtually all silica is recovered, but some CTA+ remains in the mother liquor. Conversely, at [(CTA)ClI < 0.12 M all CTA+ is found in the solid, but some surplus silica is left over in the solution. At [(CTA)Cl] = 0.12 M virtually all of the Si02 and CTA+ are recovered. The XRD pattern shown in Figure If demonstrates that the highest quality MCM-

1.0-

Cheng et al.

(25) Ralston, A. W.; Eggengerger, D. N., Harwood, H. J.; Du Brow, P. L. J. Am. Chem. SOC. 1947, 69, 883.

XRD patterns

100 different gel composition (e' n molar 01 CTACl

- x20.5
200

0.12 (e)

'..,
0.025 x2 .....
0.013
0.0013

o.o.5

(Parte 1 de 3)

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