Langmuir 1995,11, 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 29Simagic-angle-spinningNMR. 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 electrostaticinteraction at the micelle-silicate interface and the higher silicate concentration near the interface than in the bulk.
Introduction
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 polyanilineZ1and carbon (Iwires~mhave been encapsulated in the channels. MCM-41 is formed hydrothermally in the presence of a cationic surfactant cetyltrimethylammonium chloride/ hydroxide. Different formation mechanisms were prop o ~ e d , ~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 system proceeds very fast in the acidic media,24while 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.
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 AlP04-8,1VPI-52and ~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- 100Arange,4,5has 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 lowExperimental Section index peaks reflecting the quasi-regular arrangement of Synthesis. The source of silica was tetraethylorthosilicate the mesopores. Several studies concerning the synthesis (TEOS, 98%). A solution of cetyltrimethylammonium (CTA) and characterization of MCM-41 have been p u b l i ~ h e d . ~ - l ~ (11)Alfredsson, V.; Keung, M.; Monnier, A.; Stucky, G. D.; Unger, Pore morphologyll and adsorption properties have been Abstract published in Advance ACS Abstracts, J u n e 1, 1995. (1)Dessau, R. M.; Schlenker, J. L.; Higgins, J. B. Zeolites 1990,10, 522. (2) Davis, M. E.; Saldarriaga, C.; Montes, C.; Garces, J.; Crowder, C. Nature 1988,331,698. (3)Estermann, M.; McCusker, L. B.; Baerlocher, C.; Merrouche, A.; 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.
0743-746319512411-2815$09.00/0
K. K.; Schiith, F. J. Chem. SOC.,Chem. Commun. 1994,921. (12) Rathousky, J.;Zukal, A.; Franke, 0.;Schulz-Ekloff, G. J. Chem. SOC.,Faraday Trans. 1994,90,2821. (13) Branton, P. J.;Hall, P. G . ;Sing, K. S. W.; Reichert, H.; Schuth, F.;Unger, K. K. J. Chem. SOC.,Faraday Trans. 1994,90,2965. (14)Luan, Z.; Cheng, C.-F.; Zhou, W.; Klinowski, J. J. Phys Chem. 1995,99,1018. (15)Luan, Z.; Cheng, C.-F.; He, H.; Klinowski, J. J. Phys Chem., in Dress. (16) Schmidt, R.; Akporiaye, D.; Stocker, M.; Ellestad, 0.H. J. Chem. SOC.,Chem. Commun. 1994,1493. (17) Corma,A,;Fornes,V.; Navarro, M. T.; PBrez-Pariente, J.J. Catal. 1994,148,569. (18)Kolodziejski, W.; Corma, A.; Navarro, M.-T.; Perez-Pariente, Solid State NMR 1993,2,253. (19) Corma, A, Navarro, M. T.; Perez-Pariente, J. J. Chem. SOC., Chem. Commun. 1994,147. (20) Reddy, K. M.; Moudrakovski, I.;Sayari,A. J . Chem. SOC.,Chem. Commun. 1994,1059. (21) Wu, C.-G.; Bein, T. Science 1994,264,1757. (22) Wu, C.-G.; Bein, T.Science 1994,266,1013. (23) Chen, C.-Y.; Burkett, S. L.; Li, H.-X.;Davis, M. E. Microporous Mater. 1993,2,27. (24) Dubois, M.; Gulik-Krzywicki, Th.; Cabane, B. Langmuir 1993, 9,673.
0 1995 American Chemical Society
2816 Langmuir, Vol. 11, No. 7, 1995 chloridehydroxide was prepared by batch ion exchange of a 25 wt % aqueous solution of (CTA)Cl with the IRA-420 (OH) ionexchange 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 w t % aqueous solution of tetramethylammonium hydroxide ((TMAIOH)was obtainedfrom Aldrich. A typical procedure used for the synthesis of purely siliceous MCM-41was as follows. (TMA)OHandNaOH were added to an aqueous solution of (CTA)Cl or (CTA)Cl/OH of appropriate concentrations and stirred for 10min. 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 = 11.0-11.3) was 0.5 TEOS:O.12 NaOH:0.06 TMAOH:55.6 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 patterns 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. 29Simagic-angle-spinning (MAS) NMR spectra were recorded at 79.4 MHz using a Chemagnetics CMX400 spectrometer and a Doty probehead with 7 mm 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. 29Sispectra ofgels were obtained without 10 s recycle delays,and 13 500samplespinning~sing45~pulses, 27 000 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 instrumental reasons. The relatively well-defined pattern in Figure I f is typical of MCM-41 as described by Kresge e t 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)ClIbetween 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 concentrations of (CTA)Clconcentrations are used, assuming that after overnight calcination at 550 "Call 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( 2 5 ) Ralston, A. W.; Eggengerger, D. N., Harwood, H. J.;Du Brow, P. L. J.Am. Chem. SOC.1947, 69, 883.
Cheng et al. XRD patterns
n 100
(e'
different
gel composition
molar concantration 01
-
(1)
................. .....x 2
.......
(e)
CTACl
0.5
200 0.12
.................. ..'.., ................ x 2 .....
0.025
.......... ....................
0.013
X?
.................
0.005
......... ...... .................
0.0013
o.ooo.5 2
4
6
&i
10
2 e (degrees)
Figure 1. XRD patterns of purely siliceous MCMdl prepared from reaction mixtures of composition 0.5 TEOS:0.12 NaOH: 0.06 TMAOH:55.6 H20:(0.0005-0.5) (CTA)Cl.The concentration of (CTAIC1was (a)0.0005 M, (b) 0.0013 M, (c) 0.005 M, (d) 0.013 M, (e) 0.025 M, (0 0.12 M, and (g) 0.5 M. Dotted lines represent vertical expansion of sections of the XRD patterns.
100
CTACI
-
0
2
82
10-
3
1.0-
0.0001
0.001
0.01
0.1
1 .o
molar concentration of CTACI Figure 2. Yields of CTA+and Si02 in as-synthesized MCM-41 vs the concentration of (CTA)Cl in the reaction mixture with composition 0.5 TEOS:0.12 NaOH:0.06 TMAOH55.6 HzO: (0.0005-0.5) (CTA)Cl.
41 is formed at this concentration of the surfactant. A surplus of silica or of the surfactant degrades the crystallinity of the product. The peaks at ca. -100 ppm and -110 ppm in the 29Si MAS NMR spectra of MCM-41 synthesized using different surfactant concentrations (Figure3) correspond to Q3 and
Role o f Surfactant Micelles
Langmuir, Vol. 11, No. 7, 1995 2817
2 9 ~ MAS i NMR
- 100
*
e
5.
.
$
4-
*
r'
0
21 0.0001
- 80
- 1bo
-li0
0.01
1.o
0.1
molar concentration of CTACI
1 \ - 60
0.001
Figure 4. SiOdCTA+mole ratio in the as-synthesized MCM41 vs the (CTA)Cl concentration in the reaction mixture. 0.005 M
29si NMR
- lio
- 160
Q4
ppm from TMS
Figure 3. 29SiMAS NMR spectra of the as-synthesizedMCM41 prepared with the following respective concentrations of (CTA)Cl: (a)0.005 M; (b) 0.12 M; (c) 0.5 M. Asterisks denote spinning sidebands.
Q4 silicons in Si(OSi)30H and Si(0Si)l structural units, respectively. Using the Q4/Q3ratio as a measure of the degree of polymerization, we obtain values of 1.30,1.02, and 0.87 for 0.005,0.12, and 0.5 M (CTA)Cl,respectively. Thus lower surfactant concentrations result in increased polymerization. The SiOdCTA+molar ratio of the product decreases with increasing concentration of (CTA)Cl (Figure 4).
Surfactant Micelle as a Hydrolysis and Silicate Polymerization Catalyst. We compared the reaction rates with and without the surfactant by carrying out.the reactions under otherwise the same conditions. TEOS and the aqueous solution are initially immiscible. In the absence of the surfactant, the organic silicate is hydrolyzed within 1h, but after 24 h the solution is still quite clear and there is no solid product. The solution becomes cloudy after 72 h and a large amount of amorphous Si02 suspension is found after 14 days. The amount of amorphous Si02 increases with time. By contrast, TEOS hydrolyzed and began to precipitate in the solution containing 0.12 M (CTA)Cl after only 4 min. Over 92% of the silicate was recovered as the mesoporous product after only 10 min. It is clear t h a t the surfactant catalyzes the hydrolysis of the organic silicate. Figure 5 shows the 29SiNMR spectra of the (CTA)Cl-free gel after 72 h and 14 days and of a solid product made with (CTA)Cl after only 10 min. The &" silicate species are defined as Si(OSi),(OHk-,. The value of n is, of course, a measure of the degree of condensation of the silicate. We see that the product made using a surfactant is more highly polymerized after 10 min than the amorphous
Q2
I\ without CTACI
3 days
- 40
I -
- 60
-i o
-100
-120
-140
ppm from TMS
Figure 5. 29SiMAS NMR spectra of (a) gel without (CTAICl after 3 days, (b) gel without (CTA)Clafter 14 days, or (c) MCM41 prepared with 0.12 M (CTA)Clfor 10 min. Asterisks denote spinning sidebands.
material from a surfactant-free gel after 14 days. In other words, in the presence ofthe surfactant the rate of silicate polymerization increases by a factor of more than 2000.
Cheng et al.
2818 Langmuir, Vol. 11, No. 7, 1995 1000
100 h
m
I
c.
a
-g U
10
0
.-h 1.o
20
0
: 0
80
100
-
Figure 7. Initial precipitation time of solid products vs the degree of C1- OH- ion exchange in (CTA)Cl. XRD patterns
100
Two polymerization reactions proceed in the gel. The amorphous product forms in the bulk, while MCM-41 forms on the surface of the micelles. The catalytic action of the surfactant drives the reaction. Consequently, a t [(CTA)Cl]< 0.12MtheyieldofMCM-41 isalinearfunction of the concentration of (CTA)Clin the gel, and is virtually constant a t [(CTA)Cl] > 0.12 M when the entire supply of the silicate has been consumed (Figure 6). We propose the following mechanism for the surfactant micelle catalysis. The organic silicate accumulates within the Stern layerz6 formed by the charged micelle head groups. The concentrated OH- ions attracted to the head groups of the micelle hydrolyze the organic silicate more readily than that in bulk of the liquid phase, especially since TEOS is insoluble in water. Figure 7 shows that increased C1OH- exchange in (CTA)Cl accelerates the hydrolysis and condensation of the organosilicate, even though the total OH- concentration (from the surfactant and the sodium hydroxide) is constant. Most of the OHand C1- counterions are outside the Stern layerz6 and balance of cationic micelle head groups. The increased C1- OH- exchange increases the basicity of the micelle surface, which promotes the hydrolysis and condensation of the organosilicate. This is the most likely reason why the Mobil group used CTAOWCl instead of (CTA)Cl for the synthesis of MCM-41. The hydrolyzed organic silicate forms monomeric and oligomeric silicate anions which are stabilized by the charged micelle head groups. On the other hand, the silicate anions in the bulk repel one another, which does not favor condensation. Assuming that the micelles are spherical, the calculated aggregation numberz7 of (CTA)Cl is 169. If the silicate anions are one;to-one paired with CTA+ cations within a sphere 2520 A in radius,z8the concentration of the silicate in the interfacial region is 18 times larger than in the bulk. Thus the basis of the action of the surfactant as a catalyst for
60
Degree of Cl-* OH- ion exchange in CTACI (“10)
molar concentration of CTACl
Figure 6. Plots of the weight for as-made MCM-41 vs the (CTA)Clconcentrationin the reaction mixture.A 0.5 mol sample of (CTA)Cl was used in this synthesis.
40
A
I
(f’
different synthesis times
synthesis time
-
-
(26) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: London, 1975; pp 31 and 119. (27) Debye, P. Ann. N. Y.Acad. Sci. 1949, 51, 575. (%)Feuston, B. P.; Higgins, J. B. J. Phys. Chem. 1994, 98, 4459.
10 minutes
5 minutes
2
4
6
8
10
2 e (degrees )
Figure 8. XRD patterns of MCM-41 synthesized from gels of composition 0.5 TEOS:0.12 NaOH:0.06 TMAOH55.6 HzO: (0.0005-0.5) (CTAIC1 and reaction times of (a) 5 min, (b) 10 min, (c) 20 min, (c) 30 min, (d) 2 h, and (f) 24 h.
hydrolysis and polymerization is the electrostatic interaction a t the interface and the higher concentration of the silicate in the interfacial region than in the bulk.
Role of Surfactant Micelles According to the liquid crystal mechanism for the formation of MCM-41 proposed by Beck et al.,4 the template are the aggregates of the surfactant, rather than single organic molecules as in the case of synthesis of conventional microporous materials. Two possible pathways for the formation of the silicate have been proposed. According to pathway 1,the liquid crystal surfactant forms micelles and then micellar rods, which arrange themselves in a hexagonal array. The silicate species accumulate in the continuous water region between the rods and polymerize, thus creating the inorganic walls of MCM-41. According to pathway 2, the silicate is involved a t an earlier stage: it is the interaction of the silicate with the surfactant micelle which enables the formation of the micellar rods and the self-assembly of the silicate/micelle aggregates to form the hexagonal phase. Monnier et a l a and Steel et al.1° have suggestsed that the hexagonal phase is formed via an intermediate lamellar phase. However, the XRD patterns of products taken a t different reaction times (Figure 8) show that only hexagonal MCM-41 is ever present under the experimental conditions described above (Le. when TEOS
Langmuir, Vol. 11, No. 7, 1995 2819 is used a s the source of silica a t ambient temperature). The liquid crystal phase in the (CTA)Cl/water system forms6 when the (CTA)Cl concentration is >40 wt %. In the conventional synthesis of MCM-41 the concentration of (CTAICl is lower, and only micelles can exist. Our results show that MCM-41 can be synthesized with surfactant concentrations as low as, but not below, the critical micelle concentration. Therefore, the model according to which the hexagonal liquid crystal forms first is unlikely to be correct. Our work supports the model in which the individual micelles are the first to form, and then undergo silication and self-assembly to the hexagonal phase. However, one cannot rule out the possibility that the mechanism may be different when a different source of silica or different synthesis conditions (particularly as concerns the temperature) are employed.
Acknowledgment. We are grateful to the Royal Society and the K. C. Wong Foundation for a Research Fellowship to Z.L. LA9502406
