Langmuir 1999, 15, 2809-2816
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Structural Characterization of MCM-41 over a Wide Range of Length Scales C. G. Sonwane and S. K. Bhatia* Department of Chemical Engineering, The University of Queensland, St. Lucia, Brisbane QLD 4072, Australia Received June 25, 1998. In Final Form: October 29, 1998 In the present work the meso- and macrostructural characteristics of the recently developed model mesoporous adsorbent MCM-41, of different pore diameters, prepared in our laboratory, have been estimated with the help of various techniques. The structure is found to comprise four different length scalessthose of the mesopores, the crystallites, the grains, and the particles. It was also found that the surface area estimated by the use of small angle scattering techniques is higher, while that estimated by mercury porosimetry is much lower, than that obtained from gas adsorption methods. On the basis of the macropore characterization by mercury porosimetry, and the considerable macropore area determined, it is seen that the actual mesopore area of MCM-41 may be significantly lower than the BET area. TEM studies indicated that even though MCM-41 does not have an ideal mesopore structure (because of the presence of curved pore channels), it may still be treated as a model mesoporous material for gas adsorption studies because of the large radius of curvature of the channels.
1. Introduction The discovery of the family of model mesoporous adsorbents1 MCM-41 has resulted in intensive research into their syntheses and applications in the fields of adsorption, separation, catalysis, and guest-host type materials, as well as into the refinement of existing adsorption models.2-8 These materials are gaining importance as a result of the reported presence of an ideal pore structure (i.e. uniform size pores in nonintersecting straight channels) and the ease of tuning the pore diameter between 1.5 and 10 nm with very high surface area of the order of 1000 m2/g. In addition, the surface properties of MCM-41 can be modified by incorporating metal atoms such as boron, titanium, and vanadium, and their mesopores can be used as hosts for chemical bonding of organic or inorganic complexes/groups.9-11 Most of the literature on MCM-41 since its discovery in 1992 deals with synthesis or modification of the synthesis procedure (pre- and postsynthetic treatments) for variation of surface properties or mesopore size, characterization of mesopores by X-ray diffraction, gas adsorption or transmission electron microscopy, and characterization of the surface properties by NMR and FTIR techniques. Although * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +61 7 3365 4199. Telephone: +61 7 3365 4263. (1) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olsen, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10835. (2) Ravikovitch, P. I.; O’Domhnaill, S. C.; Neimark, A. V.; Schuth, F.; Unger, K. K. Langmuir 1995, 11, 4765. (3) Maddox, M. W.; Olivier, J. P.; Gubbins, K. E. Langmuir 1997, 13, 1737. (4) Kruk, M.; Jaroniec, M.; Sayari, A. Langmuir 1997, 13, 6267. (5) Bhatia, S. K.; Sonwane, C. G. Langmuir 1998, 37, 2271. (6) Inoue, S.; Hanzawa, Y.; Kaneko, K. Langmuir 1998, 14, 3079. (7) Morishige, K.; Shikimi, M. J. Chem. Phys. 1998, 108, 7821. (8) Sonwane C. G.; Bhatia, S. K. Chem. Eng. Sci. 1998, 53, 3143. (9) Sayari, A. Chem. Mater. 1996, 8, 1840. (10) Casci, J. L. In Advanced Zeolite Science and Applications, Studies in Surface Science and Catalysis; Jenson, J. C., Stocker, M., Karge, H. G., Weitkamp, J., Eds.; Elsevier: Amsterdam, 1994; Vol. 85. (11) Zhao, X. S.; Lu, G. Q. M.; Miller, G. Ind. Eng. Chem. Res. 1996, 35, 2075.
macrostructure characterization is important for interpreting adsorption dynamics and also effectiveness in catalysis, it has received very limited attention for MCM41, though some electron microscopic observations of uncalcined material have been reported.12 This is possibly because of the other novel features associated with these materials, such as tunability of the mesopore diameter, very high surface area, narrow pore size distribution of the mesopores, and the absence of hysteresis for gas adsorption below a certain pore size, which have attracted the bulk of the attention. The total surface area estimated from gas adsorption is reported to be very high, but neither are the results verified with any other techniques nor has the actual contribution of the mesopore area been established. Among the objectives of the current work is establishing an overall structure of MCM-41 over the various length scales starting from mesopores up to particles. For estimating the sizes of the features at each length scale, such as mesopore size, crystallite size, grain size, and particle size, various characterization techniques were used, as discussed below. Also, the surface area of MCM-41 has been estimated here by several techniques. To the best of our knowledge such a comprehensive structural characterization has hitherto not been published. 2. Experimental Section MCM-41 materials of various pore sizes were prepared in our laboratory by hydrothermal synthesis using alkyltrimethylammonium halides as templates (with alkyl chain lengths of n ) 8, 10, 12, 14, 16, 18) following the established procedure.1,13 In a typical synthesis, sodium silicate, water, and an appropriate quantity of surfactant template were mixed with stirring. The resulting mixture was allowed to stir for 30 min at room temperature. The gel was then heated in a sealed stainless steel reactor at 373 K for 7 days. The product was then heated, washed, filtered, and dried in ambient conditions for 2 days. It was then heated at 823 K (with an initial heating rate of 5 K/min) for 20 h in the presence of air. These calcined MCM-41 samples were (12) Khushalani, D.; Kuperman, A.; Ozin, G. A.; Tanaka, K.; Garces, J.; Olken, J. J.; Coombs, N. Adv. Mater. 1995, 7, 842. (13) Sonwane, C. G.; Bhatia, S. K.; Calos, N. Ind. Eng. Chem. Res. 1998, 37, 2271.
10.1021/la9807614 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/24/1999
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Figure 1. X-ray powder diffraction pattern of C14-MCM-41. then characterized by various techniques such as scanning and transmission electron microscopy (SEM and TEM. respectively), X-ray diffraction (XRD), mercury porosimetry, small-angle X-ray as well as neutron scattering (SAXS and SANS, respectively), gas adsorption, laser particle size analysis, optical microscopy, and helium pycnometry. The uncalcined samples were not characterized by any of the techniques mentioned above. The adsorption isotherms on calcined MCM-41 samples were measured on a Micromeritics ASAP 2010 analyzer using standard volumetric techniques. Although we utilize only the nitrogen data here, isotherms of oxygen, argon, and carbon dioxide are provided elsewhere.13 Before the analysis, the samples were degassed at 250 °C for a minimum of 12 h at about 6 × 10-6 Torr. The measured mesopore volume and XRD d spacing allowed us to calculate the mesopore diameter of each MCM-41 sample from geometrical considerations.13-15 Mercury porosimetry was carried out on a Micromeritics Autopore II 9220 porosimeter over the pressure range 0-412 MPa. Small angle neutron scattering (SANS) patterns of the sample were obtained using the LOQ facility of ISIS, at the Rutherford Appleton Laboratory in Oxfordshire, U.K. Liquid-hydrogen-moderated neutrons in the wavelength range 2-10 Å were used to investigate structures in the size range 20-1000 Å from the data collected over 0.005 e s e 0.22 Å-1. Optical microscopy was carried out on a Leitz Metallux 3 (Cambridge Instruments Quantimet 570 image analyzer) microscope. SAXS data were collected from samples packed in Lindemann glass capillaries, using a custom-built Kratky camera with a position-sensitive detector, operating with Quartz monochromated Cu KR radiation. The data sets were background subtracted and desmeared using Glatter’s16,17 procedure.
3. Results and Discussion 3.1. Surface Areas Estimated by Various Techniques. All the samples of MCM-41 synthesized in our laboratory were initially characterized by X-ray diffraction and nitrogen gas adsorption. Typical characterization plots for the C14 sample are shown in Figures 1 and 2, respectively. Three or four well-defined peaks were obtained in XRD for all the samples studied, which indicates a typical honeycomb arrangement of pores. The (14) Kruk, M.; Jaroniec, M.; Sayari, A. J. Phys. Chem. B. 1997, 101, 583. (15) Dabadie, T.; Ayral, A.; Guizard, C.; Cot, L.; Lacan, P. J. Mater. Chem. 1996, 6, 1789. (16) Glatter, O. In Small-Angle X-ray scattering; Glatter, O., Kratky, O., Eds.; Academic Press: London, 1990; p 165. (17) Wachtel, E. Personnel communication, Department of Chemical Physics, Weizmann Institute of Science, Israel, 1997.
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Figure 2. Adsorption isotherm of nitrogen at 77.4 K on C14MCM-41. Table 1. Estimates of d Spacing, Pore Size, Crystallite Size, Grain Size, and Particle Size for Various MCM-41 Samplesa d spacing sample (nm) C8 C10 C12 C14 C16 C18
2.89 2.92 3.12 3.47 3.79 4.33
mesopore crystallite grain particle diameter diameter diameter diameter (nm) (µm) (µm) (µm) 2.30 2.73 2.96 3.38 3.78 4.36
0.02 0.05 0.05 0.06 0.05 0.04
0.94 0.84 0.78 0.55 0.33 0.49
6.52 6.28 5.85 5.76 6.23 6.18
a The mesopore size was estimated by using gas adsorption and XRD d spacing, the crystallite size was estimated from line broadening of X-ray diffraction, the grain size was estimated by mercury porosimetry, and the particle size was estimated from optical microscopy, as discussed in the text.
transmission electron micrographs of the samples, as shown in Figure 3a and b for samples C10 and C16, also indicated a typical honeycomb pattern of the mesopores. Estimates of the XRD d spacing (100) and pore size for the various samples studied in the present work have been given in Table 1. It can be seen that the d spacing as well as the pore size increases with an increase in the pore size, consistent with the previous studies.1,4 The estimates of the surface area of MCM-41 by various techniques are given in Table 2. The surface area from gas adsorption (nitrogen adsorption at 77.4 K) was obtained by using the BET equation. Although the surface areas for all the samples using different adsorptives are in the range 900-1300 m2/g, there is no trend observed. The BET surface area is the total surface area rather than the mesopore area. The choice of the range of relative pressures for estimating the BET surface area is critical in MCM-41 because, for most of the samples studied in the present work, the phenomenon of capillary condensation was found to occur in the range of relative pressures 0.05-0.35, which is a recommended range for the BET equation. Further details of this discussion can be found elsewhere.13,14 The surface areas were also obtained using4
R)
2vP Sm
(1)
in which vP is the specific pore volume, R is the pore radius,
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Table 2. Surface Area (m2/g) Estimated by Various Techniques adsorption (BET)
C8 C10 C12 C14 C16 C18
N2 (77 K)
Ar (77 K)
Ar (87 K)
O2 (77 K)
CO2 (195 K)
Hg porosimetry
SAXS
SANS
CWT
VWT
937 1318 1280 1162 1240 1123
1100 973 1027 1015 895 920
873 965 963 954 1125 942
1063 1150 1163 888 909 1043
946 868 1123 1018 1013 1338
19 143 194 299 365 295
939 1665 1835 2579 2599 2745
1333 2616
645 648 656 606 615 607
661 1024 1016 933 986 902
2588 3941
and Sm is the surface area. Under the condition that surface area and pore radius are calculated from geometrical considerations, this leads to
Sm )
4πR x FS[ 3(2R + W)2 - 2πR2]
(2)
in which FS is the skeletal density and W is the wall thickness. This equation has been derived on the basis of the assumption that MCM-41 is made of infinitely long cylindrical pores arranged on a two-dimensional hexagonal lattice, similar to that considered13-15 for estimating pore diameter. The value of W in eq 2 was taken as 1 nm for the constant wall thickness (CWT) approach, while, for the variable wall thickness (VWT) approach,14 it was estimated using
W)
x
2 dXRD - 1.2125dXRD x3
FS v P 1 + FSvP
(3)
where vP is the primary pore volume and dXRD is the d spacing obtained from the X-ray diffraction pattern. Equation 3 has been obtained from the expressions of pore diameter and the XRD d spacing using the structural assumption indicated above, validated by our microscopic observations (cf. Figure 3). This approach has been adopted in preference to the alternative based on conventional adsorption models (Dubinin-Astakov, Saito-Foley, and BJH), in view of their observed13 inaccuracies in describing adsorption in MCM-41. The surface areas obtained by the CWT approach are lower than those estimated by the BET technique, while those from the VWT approach with the wall thickness estimated from the primary mesopore volume are closer. It should be noted that the BET area includes that of the mesopores as well as that of the macropores (assuming that the micropores are negligible). Recently, studies on estimation of the surface area of MCM-41 by various methods in gas adsorption (such as the BET method, eq 2, and the BJH method) have indicated that the BET method overestimates the surface area of MCM-41.4 This was attributed to the possibility of considerable overlap of monolayer and multilayer adsorption in the recommended range of pressures for BET analysis. Surface areas of MCM-41 have also been estimated by SAXS and SANS, using the Debye equation18
area )
(4 × 104)φsφp Rdap
(4)
but these are much higher than those from gas adsorption methods (cf. Table 2). In eq 4 φs and φp represent the solid and pore volume fraction, respectively, while R and dap represent the correlation length and apparent density, respectively. The first two parameters were estimated from helium pycnometry13 and from the t-plot based on the nitrogen adsorption isotherm, while the latter two were
Figure 3. High-resolution transmission electron micrograph of calcined (a) C10-MCM-41 and (b) C16-MCM-41.
estimated from the SAS data. In the estimation of correlation length, which is usually obtained from the slope of a plot of I(s)-0.5 versus s2, it was observed that the plot was not linear, as shown in Figure 4 for the C16 sample, for almost all the samples over the entire range of s2. As we are interested in the mesopore surface area, the correlation length was obtained by linear regression over the range sh . 1, where the constant h was estimated as described in the literature.18 The Debye equation resembles that for the surface area of a random space-filling tesselation. This assumption may not be valid for MCM41, which has regularly arranged mesopores. Nevertheless, estimates of the surface area of C8 by the Debye equation are close to those from the gas adsorption method and indicate its randomness, consistent with our previous findings by HRTEM.13 For the C10-C18 samples, how(18) Debye, P.; Anderson, H. R.; Brumburger, H. J. J. Appl. Phys. 1957, 28, 679.
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Figure 4. Estimation of the surface area of C14-MCM-41 from SAXS data. Figure 6. Comparison of (a) pore diameter, (b) crystallite diameter, (c) grain diameter, and (d) particle diameter for various MCM-41 samples.
Figure 5. Mercury porosimetry curves and fraction of original mesopore volume left with an increase in mercury pressure, for C14-MCM-41.
ever, the earlier suggestions19 of a geometrically heterogeneous bilayered surface have been modified20 to indicate a smooth surface. This is consistent with our own fractal characterization21 using molecular tiling. Consequently, further studies overcoming the structural limitations of the Debye equation are necessary to obtain an unequivocal model of the microstructure for the C10-C18 sample using SAS data. The surface area of MCM-41 obtained by mercury porosimetry, calculated using the Rootare-Prenzlow equation,22 is lower than that found by the adsorption method, as shown in Table 2. In general for MCM-41, the surface areas obtained were in the following order: mercury porosimetry < gas adsorption < SAXS < SANS. The mesopore diameters of the MCM-41 studied in the current work are in the range 2.3-4.4 nm, and the lowest diameter of the cylindrical pores in which mercury can (19) Edler, K. J.; Reynolds, P. A.; White, J. W.; Cookson, D. J. Chem. Soc., Faraday Trans. 1997, 93, 199. (20) Edler, K. J.; Reynolds, P. A.; White, J. W. J. Phys. Chem. B 1998, 102, 3676. (21) Sonwane, C. G.; Bhatia, S. K. Langmuir, submitted 1998.
Figure 7. Transmission electron micrographs of (a) mesopores and (b) crystallites in C16-MCM-41, at two different resolutions.
penetrate (at the highest pressure studied in this work) is about 3.2 nm. Therefore, if the walls of the pores are rigid, for the samples C14, C16, and C18 a higher surface area is expected from mercury porosimetry than that reported in Table 2. Recent studies23 involving monitoring the effect of mechanical pressure on MCM-41 with the
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Figure 9. Pore size distribution of MCM-41 estimated from mercury porosimetry data.
Figure 8. Transmission electron micrographs of (a) crystallite aggregates and (b) individual crystallites, in C16-MCM-41.
help of the adsorption isotherm of nitrogen at 77.4 K and SAXS have indicated that its structure could be affected considerably by mechanical compression at pressures as low as 86 MPa and is essentially destroyed at 224 MPa. A plot of the effect of mechanical compression pressure on the percentage of the original mesopore volume left, as estimated from the literature,23 along with the mercury intrusion curve with increasing mercury pressure for our C14 sample is shown in Figure 5. The marked region (above 18 MPa) indicates the pressure range in which there is a possibility of collapse of the mesopores. In the current work, C14, C16, and C18 were found to have estimated wall thicknesses varying in the range 0.6-1 nm and they could have different rigidities than that of the sample reported in the literature, indicating the possibility of collapse of the structure before entry of mercury in the mesopores. Therefore the reliability of the surface area of MCM-41 obtained by mercury porosimetry is doubtful. Recent studies involving alumino-silicate MCM-41 have indicated that its wall thickness is of order of 0.5 nm at the thinnest point, which corresponds to only two silica monolayers.24 Such a thin curtain can be easily pierced by local hydrolysis of Si-O-Al or Si-O-Si bonds in these materials and by dealumination caused during calcination. The existence of a small hole between parallel pores would create a highly unstable situation according to the laws (22) Rootare, H. M.; Prenzlow, C. F. J. Phys. Chem. 1967, 71, 2733. (23) Gusev, V. Y.; Feng, X.; Bu, Z.; Haller, G. L.; O’Brien, J. A. J. Phys. Chem. 1996, 100, 1985. (24) Coustel, N.; Renzo, F. D.; Fajula, F. J. Chem. Soc., Chem. Commun. 1994, 967.
of sintering. In such a process, there could be a combination of two or more neighboring mesopores to give a macropore, which actually should give a lower surface area than the theoretical estimates. This is consistent with recent findings24 for MCM-41 that the experimental value of the mesopore volume is considerably lower than the theoretical predictions. Additionally, with such thin walls, the possibility of the mesopore structure getting collapsed at higher pressures of mercury cannot be ruled out. 3.2. Mesopores. Details of the calculation procedure for estimating the mesopore size of MCM-41 by geometrical considerations using the mesopore volume and the XRD d spacing have been reported by various researchers recently.13-15 Estimates obtained using the nitrogen adsorption isotherm at 77.4 K indicate a consistent increasing trend in pore size with increasing alkyl chain length, as shown in Figure 6a and Table 1. Similar results have been observed13 when argon (at 77.4 and 87.6 K), oxygen (at 77.4 K), and carbon dioxide (at 195 K) were used. These results are consistent with the literature1,13,14 and have been found to be consistent also with theoretical prediction.13 The wall thickness estimated from these results is in the range 0.6-1.1 nm for the C8-C18 samples. This is consistent with the recently reported wall thickness14,25 based on geometrical considerations. As seen from Figure 7a and b, the pores of MCM-41, which are generally thought to be straight channels, are actually curved with small constrictions along the length of the pore, confirming the recent findings.26 The radius of curvature can be seen to be much higher than the diameter of the pore, as well as the length scale of molecular interactions. Although it is difficult to estimate the constrictions quantitatively at such magnifications, the TEM pictures clearly indicate their presence in all the mesopores. In adsorption, it is known that the fluid-solid and fluid-fluid interaction forces are not significant beyond a few molecular diameters. Therefore, the curvature of the pore channels may not have any significant effect on adsorption. While we have not attempted to quantify the curvature, it should be possible to do so on the basis of statistical image analysis of digitized TEM micrographs. Thus, even though MCM(25) Kruk, M.; Jaroniec, M.; Ryoo, R.; Kim, J. M. Microporous Mater. 1997, 12, 93. (26) Chenite, A.; Page, Y. L.; Sayari, A. 1995, 7, 1015.
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Figure 10. Scanning electron micrographs of particles of C18-MCM-41, depicting (a-c) the highly heterogeneous external surface of calcined uniform size particles at different magnifications and (d) the uncalcined sample.
41 does not have an ideal mesopore structure, it may be treated as a model adsorbent. It may be noted that these observations are based on our findings from the C16 sample. Similar studies with the others are yet to be conducted, but the results are not expected to be different. In analyzing the SAXS data by Kratky and Porod’s analysis27 for estimating the radius of a cylindrical pore, the plots of ln(I(s)s) versus s2 were not linear over the entire range of s2. The values of pore diameter obtained by this analysis for C8-C18 were in the range 3.5-5 nm; however, for a few samples they were significantly different from those estimated from gas adsorption data. Further investigation related to this method of estimation is needed. 3.3. Crystallites. MCM-41 is considered as crystalline on a macroscopic level because of the regular arrangement of the mesopores in a honeycomb fashion; therefore, it is possible to estimate crystallite size perpendicular to the basal plane with the help of the X-ray diffraction pattern using the Scherrer equation,28
d)
0.9λ B cos θ
(5)
where d is the crystallite size (equivalent diameter), λ is the wavelength, B is the peak width at half-maximum, (27) Kratki, O.; Porod, G. J. J. Colloid Sci. 1949, 4, 35. (28) Klug, H. P.; Alexander, L. E. X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials; Wiley: New York, 1974.
and θ is the Bragg angle. The Scherrer equation has been widely used29-31 to estimate the crystallite diameter for various materials. However, this technique must be used with caution, since faults or other defects can also result in line broadening. The crystallite sizes obtained for C8, C10, C12, C14, C16, and C18 are 16, 54, 51, 65, 51, and 36 nm, respectively, as shown in Figure 6b and Table 1. These values are consistent with those from TEM, as shown in Figure 8a and b. TEM of the same crystallite at different magnifications is reported (Figure 8a and b) to stress the fact that some of the crystallites are hexagonal prism in shape with pores opening on the surface of the crystallite. As can be seen from Figure 8, the size of the crystallites shown in the micrograph is of the order of 65 nm, which is close to the value of crystallite size obtained by X-ray line broadening. Also from Figures 7a and b and 8a it can be seen that the mesopores are running parallel to each other and are curved with constrictions along the length of the pore. 3.4. Macropores, Grains, and Particles. The macropore size distribution of C8, C10, C12, C14, C16, and C18 obtained from mercury porosimetry data had two peaks; however, the peak observed at very high pressures of mercury is not considered because of the possible collapse (29) Rao, V. U. S. Energy Fuels 1994, 8, 44. (30) Masson, O.; Rieux, V.; Guinebretiere, R.; Dauger, A. Nanostructured Materials 1996, 7, 725. (31) Ishimori, T.; Yamashita, M.; Senna, M. Part. Part. Syst. Charact. 1994, 11, 398.
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Figure 11. Particle size distribution of MCM-41, determined by laser diffraction.
Figure 13. Hysteresis in secondary mesopores of MCM-41.
Figure 12. Meso- and macrostructure of MCM-41.
of the structure at high pressures, as already discussed in section 3.1. The macropore diameter decreases in going from C8 to C16, with the exception of C12, whose pore diameter is higher than that of C8. The pore diameter of C18 is also found to be higher than that of others except C12. For the purpose of comparison, the pore size distribution of the calcined and the uncalcined C16 samples is shown in Figure 9. Mercury porosimetry data can also be used to estimate grain size/particle size, assuming the shape of the particles to be spherical, using
rg )
1.5RP(1 - M) M
(6)
where rg is the grain radius, M is the macropore porosity, and RP is the average macropore radius. In the above calculations, RP was taken from the peak of the pore size distribution curve (Figure 9). M was estimated from the macropore volume obtained from mercury porosimetry. The diameters of the grains obtained by this method for the samples C8, C10, C12, C14, C16, and C18 are 0.94, 0.84, 0.78, 0.55, 0.33, and 0.49 µm, respectively, as shown in Figure 6c and Table 1. These estimates of grain size are roughly 10 times higher than those obtained for crystallites by X-ray diffraction line broadening, as discussed in section 3.3. Micrographs from SEM of C18 are as shown in Figure 10a-d and indicate that the average particle size is about 8 µm, close to the findings by laser particle size analysis, as shown in Figure 11. It can also be seen from Figure 10a-d (by comparison of the SEM spectra of the calcined and the uncalcined C18 samples) that the macropores are formed during calcination. It is probable that they are created by densification of silica during calcination and
Figure 14. Comparison of macro- and mesoporosity of MCM41.
by escape of H2O. These results are also consistent with our estimates by high-resolution optical microscopy (the average diameter was calculated using image analysis software), as shown in Figure 6d and Table 1. It can also be seen from Figure 10 that the external surface texture of calcined MCM-41 particles is rough/heterogeneous while that of uncalcined MCM-41 is smooth. The above analysis gives a clear picture of the mesoand macrostructure of MCM-41 materials synthesized in our laboratory. It is clear that these MCM-41 samples after calcination consist of particles, which are made up of grains. These grains again consist of crystallites which could be hexagonal prism shaped, consisting of mesopores running parallel to the axis of the prism, as shown in Figure 12. The adsorption isotherms of C8 and C18 samples focusing on the hysteresis in secondary mesopores are shown in Figure 13. Such a hysteresis loop has been observed only for a few MCM-41 samples, as it is located at higher pressures in most of the cases.13,14,32 Estimates of the size of the space between crystallites obtained from (32) Ravikovitch, P. I.; Wei, D.; Chueh, W. T.; Haller, G. L.; Neimark, A. V. Langmuir 1995, 11, 4765.
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the application of the Kelvin equation to the point of closure of secondary hysteresis (as shown in Figure 13) are of the order of 4-5 nm diameter, much lower than the range of crystallite size, indicating that the crystallites are closely packed. Estimates of meso- and macroporosity indicated that both porosities decrease with an increase in mesopore size, as shown in Figure 14. 4. Conclusions On the basis of the results of various characterization techniques, it was found that MCM-41 consists of four levels of structure: mesopores, crystallites, grains, and particles. All these levels have been successfully characterized. Estimates of surface area by SAXS and SANS are higher while those from mercury porosimetry are much lower, than those estimates by BET methods, while the estimates obtained from geometrical consideration using
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variable wall thicknesses are close to the BET results. While analyzing the mercury porosimetry data for MCM41, care should be taken because of the possibility of breaking the structure at significant mercury pressures. It was confirmed that mesopores in MCM-41 are curved rather than straight channels and that, even though they do not have an ideal mesopore structure, they can be considered as model mesoporous materials for gas adsorption studies. Acknowledgment. This research has been supported by a grant from the Australian Research Council. The authors are grateful to Dr. Nick Calos for assistance with SAXS and SANS as well as TEM, and to Dr. Nick Kinaev and Ben Schulz for their help with scanning electron and optical microscopy, respectively. LA9807614
