J. Phys. Chem. B 2000, 104, 4835-4839
4835
Highly Ordered MCM-41 Silica Prepared in the Presence of Decyltrimethylammonium Bromide Abdelhamid Sayari*,† and Yong Yang Department of Chemical Engineering and CERPIC, UniVersite´ LaVal, Ste-Foy, Quebec, Canada G1K 7P4 ReceiVed: January 13, 2000; In Final Form: March 3, 2000
Literature data show that for MCM-41 silica made in the presence of alkyltrimethylammonium bromide (ATAB) surfactants, the best quality materials in terms of framework order are those obtained with surfactants having 12-16 carbon atom alkyl chains. Lower molecular weight surfactants seem to be more difficult to selforganize, thus leading to less ordered materials with broader pore size distributions. Here, we provide reliable recipes which afford excellent quality materials using decyltrimethylammonium bromide. This method can also be applied to ATABs with 12 and 14 carbon chains.
Introduction The discovery of M41S silicas in the early 1990s generated very strong interest in periodic mesoporous materials,1 and more recently in ordered macroporous materials.2 Not only were a number of new synthesis strategies developed for the preparation of MCM-41 (p6mm) and MCM-48 (Ia3d) mesophases, but mesoporous materials with other structures such as the MSUn,3 MSU-G,4 SBA-2 (p63/mmc),5 SBA-1 (Pm3n),6 SBA-8 (cmm),7 KIT-1,8 and a variety of non-silica materials9,10 were also synthesized. In addition, extensive work was devoted to modifications of periodic mesoporous silicas. This included framework11 and surface12,13 modifications as well as encapsulation of organic and inorganic species for various applications.14 One important reason for this remarkable growth is the high flexibility of synthesis conditions. Indeed, ordered mesoporous silica may be prepared from subambient temperature to 160 °C under the whole range of pH in the presence of a large variety of surfactants and polymers.1 Long-chain alkyltrimethylammonium bromides (ATABs) were by far the most used surfactants. Under otherwise similar conditions, the length of the ATAB alkyl chain was found to affect the pore size.1,15-18 However, the materials with the highest quality in terms of framework periodicity and pore size distribution were usually those made in the presence of ATABs with 12-16 carbon chains. Heavier surfactants were rarely used because they are difficult to dissolve. Lower molecular weight ATAB surfactants seem to be more difficult to self-organize, thus giving rise to less ordered materials with broader pore size distributions.16-19 The aim of the current work was to provide simple recipes for the preparation of high quality MCM-41 silica using decyltrimethylammonium bromide. Experimental Section Materials. Samples were prepared as follows. 3.85 g of TMAOH (25%) was diluted with 37.1 g of water before adding 4.2 g of decyltrimethylammonium bromide (DTAB) under vigorous stirring. After 15 min, 2 g of Cab-O-Sil silica was added. The overall mixture composition was 1.0 SiO2:0.317 † E-mail:
[email protected]. Phone: (418) 656 3563. Fax: (418) 656 5993.
Figure 1. X-ray diffractograms for samples prepared in the presence of DTAB at the indicated temperatures.
TMAOH:0.45 DTAB:67 H2O. In all cases, the pH whether before or after addition of silica was between 12.6 and 12.8. The gel obtained after stirring for an additional 30 min was transferred into a Teflon-lined autoclave, and heated statically under autogenous pressure for 40 h at different temperatures in the range 100-150 °C. The obtained materials were filtered, washed extensively, dried, and calcined at 540 °C, first in flowing nitrogen and then in air. Two additional series of samples were made using the same composition, but in the presence of either do- or tetradecyltrimethylammonium bromide. Samples will be referred to as MCM-41-x-y, where x and y stand for the number of carbon atom in the alkyl chain of the surfactant and the synthesis temperature in degree Celsius, respectively. Measurements. X-ray diffraction (XRD) spectra were obtained on a Siemens D5000 diffractometer using Cu KR radiation (λ ) 0.154 18 nm). Scanning electron microscopy (SEM) images were recorded on a JEOL 840A microscope operated at an accelerating voltage of 10 kV. Transmission electron micrographs (TEM) were obtained using a Philips 430 instru-
10.1021/jp0001900 CCC: $19.00 © 2000 American Chemical Society Published on Web 05/02/2000
4836 J. Phys. Chem. B, Vol. 104, No. 20, 2000
Sayari and Yang
Figure 2. Nitrogen adsorption-desorption isotherms for samples prepared in the presence of DTAB at the indicated temperatures. The isotherms were shifted upward (in 200 unit steps) for clarity.
Figure 4. Typical scanning electron micrograph for samples prepared at 100 (upper frame) and 130 °C (lower frame) in the presence of DTAB. Figure 3. Pore size distributions for samples prepared in the presence of DTAB at the indicated temperatures.
TABLE 1: Structural Parameters of the MCM-41-10-y Samplesa sample
d100 (nm)
SBET (m2/g)
Vp (cm3/g)
Vt (cm3/g)
wKJS (nm)
wd (nm)
MCM-41-10-100 MCM-41-10-120 MCM-41-10-130 MCM-41-10-140 MCM-41-10-150
2.95 3.22 3.18 3.06 3.66
1360 1236 1204 1199 978
0.70 0.72 0.73 0.73 0.70
0.72 0.74 0.75 0.75 0.71
2.70 2.75 2.88 2.94 3.42
2.79 3.06 3.03 2.91 3.46
ad 100 ) XRD (100) interplanar spacing; Vp ) primary mesopore volume; Vt ) total pore volume; wKJS ) primary mesopore size calculated using Kruk-Jeroniec-Sayari method; wd ) primary mesopore size calculated using: wd ) cd100(FVp/(1 + FVp))1/2 with c ) 1.213 and F ) 2.2 cm3/g.
ment operated at 100 kV. The specimen embedded in an epoxy resin were cut in ultrathin sections (ca. 60 nm) and examined. Nitrogen adsorption measurements were performed using a Coulter Omnisorp 100 gas analyzer. Before analysis, the samples were degassed under vacuum (ca. 10-5 Torr) at 300 °C for 1 h. The specific surface area, SBET, was calculated using the BET (Brunauer-Emmett-Teller) method.20 The range of relative pressure used was limited to 0.05-0.12 to avoid overestimation of the surface area due to nitrogen capillary condensation in
TABLE 2: Structural Parameters of MCM-41-x-y with x ) 12 and 14a sample
d100 (nm)
SBET (m2/g)
Vp (cm3/g)
Vt (cm3/g)
wKJS (nm)
wd (nm)
MCM-41-12-100 MCM-41-12-120 MCM-41-12-140 MCM-41-12-150 MCM-41-14-100 MCM-41-14-120 MCM-41-14-130 MCM-41-14-140 MCM-41-14-150
3.03 3.07 3.21 3.94 2.99 3.08 3.31 3.43 4.22
1427 1489 1070 1289 1408 1425 1203 1467 1131
0.72 0.78 0.74 1.09 0.77 0.82 0.81 1.01 0.99
0.73 0.80 0.76 1.11 0.78 0.83 0.82 1.02 1.01
2.76 2.85 3.18 4.06 2.87 2.97 3.26 3.37 4.40
2.87 2.96 3.06 4.01 2.88 3.00 3.21 3.46 4.24
ad 100 ) XRD (100) interplanar spacing; Vp ) primary mesopore volume; Vt ) total pore volume; wKJS ) primary mesopore size calculated using Kruk-Jeroniec-Sayari method; wd ) primary mesopore size calculated using: wd ) cd100(FVp/(1 + FVp))1/2 with c ) 1.213 and F ) 2.2 cm3/g.
primary mesopores, i.e., ordered mesopores within the particles. The onset of such condensation takes place at a relative pressure of ca. 0.15. Pore size distributions (PSDs) were calculated using the recently developed KJS (Kruk, Jaroniec, Sayari) approach.21 This method uses the BJH (Barrett-Joyner-Halenda) procedure22 based on adsorption data and calibrated specifically for ordered mesoporous silica. The pore diameter corresponding to the maximum of PSD will be denoted as wKJS. The size of primary mesopores, wd, was also calculated using a simple
Highly Ordered MCM-41 Silica
J. Phys. Chem. B, Vol. 104, No. 20, 2000 4837
Figure 5. Transmission electron microscopy image for a sample prepared in the presence of DTAB at 130 °C.
Figure 6. Nitrogen adsorption-desorption isotherms for samples prepared in the presence of dodecyltrimethylammonium bromide at the indicated temperatures.
geometric model which consists of an infinite array of hexagonally packed cylindrical pores.16 The relationship between wd, the primary mesopore volume Vp, and the d100 distance obtained from XRD data is as follows:
(
)
FVp wd ) cd 1 + FVp
1/2
where c ) (8/(31/2π))1/2 is a constant and F is the density of the silica walls taken as equal to 2.2 cm3/g.
Figure 7. Nitrogen adsorption-desorption isotherms for samples prepared in the presence of tetradecyltrimethylammonium bromide at the indicated temperatures. The isotherms were shifted upward (in 200 unit steps) for clarity.
Results and Discussion Figure 1 shows the XRD spectra for a series of MCM-4110-y samples prepared in the presence of DTAB at different temperatures. All spectra exhibited three well-resolved peaks indicative of good quality materials. For synthesis temperatures
4838 J. Phys. Chem. B, Vol. 104, No. 20, 2000
Figure 8. Pore size distributions for samples prepared in the presence of dodecyltrimethylammonium bromide at the indicated temperatures.
Figure 9. Pore size distributions for samples prepared in the presence of tetradecyltrimethylammonium bromide at the indicated temperatures.
Figure 10. Pore size distributions for samples prepared in the presence of alkyltrimethylammonium bromide at 100 °C. The alkyl chain carbon number is indicated.
below 150 ˚C, the main (100) peak hardly shifted. Provided that no major changes in pore wall thickness occurred, this indicates that the pore size remained constant. At 150 °C, the d100 value shifted from 31 ( 1.5 to 36.6 nm indicative of a significant pore size expansion. Figure 2 shows a series of nitrogen adsorption desorption isotherms for materials prepared at different temperatures. In all cases, the process was fully reversible. The isotherms exhibited a sharp nitrogen condensa-
Sayari and Yang tion-evaporation step, indicating the occurrence of mesopores with narrow size distributions. In addition, for synthesis temperature below 150 °C, the condensation step took place almost at the same relative pressure (ca. 0.2), consistent with the inference from XRD data that the pore size remained constant. In agreement with this observation, as shown in Figure 3, the PSDs for samples made in the temperature range 100140 °C, exhibited maxima at 2.8 ( 0.15 nm. Moreover, as inferred from XRD data and from the sharpness of the N2 condensation steps, the PSDs were quite narrow. At 150 °C, in addition to the shift to larger sizes, the pore system lost some uniformity as indicated by the broadening of both the PSD (Figure 3) and the (100) XRD peak (Figure 1). Additional structural properties given in Table 1 show that all samples exhibited high surface areas and pore volumes. Comparison between the total and the primary mesopore volumes indicates that there is little porosity other than that corresponding to the honeycomb structure. This is consistent with the fact that wKJS and wd were very close for all samples. As demonstrated in earlier publications regarding cetyltrimethylammonium bromide derived MCM-41 silica either prepared or postsynthesis treated at high temperature,23-25 the increase in pore size is associated with the partial decomposition of the surfactant and the formation of neutral N,N-dimethylhexadecylamine which plays the role of micelle expander within the silica channels. It is therefore inferred that a similar mechanism applies for the current materials. Scanning electron microscopy showed that all samples were composed of spherroidal particles whose size increased steadily from ca. 0.1 µm for MCM-41-10-100 to about 0.6 µm for MCM-41-10-140. Typical SEM images for samples prepared in the presence of DTAB at 100 and 130 °C are shown in Figure 4. The latter was also studied using TEM. As shown in Figure 5, it exhibited a very well ordered honeycomb structure of mesopores. The proposed synthesis for highly ordered materials in the presence of DTAB, also works for ATABs with 12 and 14 carbon atom chains. Figures 6 and 7 shows the nitrogen adsorption-desorption isotherms for a series of materials prepared at different temperatures in the presence of do- and tetradecyltrimethylammonium bromide, respectively. Here also, the nitrogen condensation step occurs at an approximately constant relative pressure for samples synthesized in the temperature range 100-140 °C. A significant shift to higher relative pressure occurs for samples prepared at 150 °C indicative of an increase in the pore size. Table 2 shows that all samples exhibited high surface areas and pore volumes. The PSDs for samples prepared in the presence of do- and tetradecyltrimethylammonium at different temperatures are shown in Figures 8 and 9, respectively. Consistent with nitrogen adsorption and XRD (not shown) data, the pore size, whether calculated using the KJS method or the geometric model, did not change significantly for materials synthesized at 100-140 °C. Increasing the synthesis temperature to 150 °C gave rise to more than 1 nm jump in the pore size without as much loss in uniformity as in the case of DTAB-derived silicas. As proposed above, this expansion may also be attributed to the formation of neutral N,N-dimethylalkylamines. For comparison, Figure 10 shows the PSDs for samples prepared at 100 °C in the presence of surfactants with different chain lengths. It is interesting to notice that contrary to methods reported previously,16-18 DTAB gave rise to the material with the narrowest PSD. This makes the current method particularly suitable for the preparation of highly ordered MCM-41 silica
Highly Ordered MCM-41 Silica in the presence of DTAB. Owing to the great variety of synthesis recipes available in the literature, it is difficult to pinpoint a single reason as to why the current method works well for DTAB as surfactant. The absence of sodium cations, the pH, the use of fumed silica, and its addition at the end of the preparation procedure may all play a role in the occurrence of improved long-range order in the current materials. It is important to notice that applied in the presence of cetyltrimethylammonium using the same gel molar composition, the proposed method gave rise to different phases depending on the synthesis temperature. Detailed information on this system will be the subject of a separate paper. References and Notes (1) Sayari, A. Stud. Surf. Sci. Catal. 1996, 102, 1. (2) Go¨ltner, C. G. Angew. Chem., Int. Ed. Engl. 1999, 38, 3155, and references therein. (3) Tanev, P. T.; Pinnavaia, T. J. Science 1996, 271, 1267. (4) Kim, S. S.; Zhang, W.; Pinnavaia, T. J. Science 1998, 282, 1302. (5) Huo, Q.; Leon, R.; Petroff, P. M.; Stucky, G. D. Science 1995, 269, 1324. (6) Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147. (7) Zhao, D. Huo, Q.; Feng, J.; Kim, J.; Han, Y.; Stucky, G. D. Chem. Mater. 1999, 11, 2668. (8) Ryoo, R.; Kim, J. M.; Shim, C. H.; Lee, J. Y. Stud. Surf. Sci. Catal. 1997, 105, 45.
J. Phys. Chem. B, Vol. 104, No. 20, 2000 4839 (9) Sayari, A.; Liu, P. Microporous Mater. 1997, 12, 149. (10) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. Engl. 1999, 38, 56. (11) Sayari, A. Chem. Mater. 1996, 8, 1840. (12) Maschmeyer, Th. Curr. Opin. Solid State Mater. Sci. 1998, 3, 71. (13) Brunel, D. Microporous Mesoporous Mater. 1999, 27, 329. (14) Moller, K.; Bein, T. Chem. Mater. 1998, 10, 2950. (15) 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. (16) Kruk, M.; Jaroniec, M.; Sayari, A. J. Phys. Chem. B. 1997, 101, 583. (17) Beck, J. S.; Vartuli, J. C.; Kennedy, G. J.; Kresge, C. T.; Roth, W. J.; Schramm, S. E. Chem. Mater. 1994, 6, 1816. (18) Ravikovitch, P. I.; Wei, D.; Chueh, W. T.; Haller, G. L.; Neimark, A. V. J. Phys. Chem. B. 1997, 101, 3671. (19) Eswaramoorthy, M.; Neeraj, S.; Rao, C. N. R. Microporous Mesoporous Mater. 1999, 28, 205. (20) Brunauer, S.; Emmett, P. H.; Teller, E. J. J. Am. Chem. Soc. 1938, 60, 309. (21) Kruk, M.; Jaroniec, M.; Sayari, A. Langmuir 1997, 13, 6267. (22) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373. (23) Sayari, A.; Kruk, M.; Jaroniek, M.; Moudrakovski, I. L. AdV. Mater. 1998, 10, 1376. (24) Sayari, A.; Yang, Y.; Kruk, M.; Jaroniek, M. J. Phys. Chem. B 1999, 103, 3651. (25) Kruk, M.; Jaroniek, M.; Sayari, A. J. Phys. Chem. B 1999, 103, 4590.
