© Copyright 2000 American Chemical Society
NOVEMBER 28, 2000 VOLUME 16, NUMBER 24
Letters Stabilization of MCM-41 by Pyrolytic Carbon Deposition M. M. L. Ribeiro Carrott,*,† A. J. Esteva˜o Candeias,† P. J. M. Carrott,† K. S. W. Sing,‡ and K. K. Unger# Department of Chemistry, University of E Ä vora, Cole´ gio Luı´s Anto´ nio Verney, E Ä vora, Portugal, School of Chemistry, Bristol University, U.K., and Institut fu¨ r Anorganische Chemie und Analytische Chemie, Johannes Gutenberg Universita¨ t, Mainz, Germany Received May 15, 2000. In Final Form: September 19, 2000 A new method of surface modification which is effective in stabilizing silica grades of MCM-41 in the presence of water vapor is presented. It is shown by means of XRD, low-temperature nitrogen adsorption, and water vapor adsorption measurements that deposition of carbon from benzene vapor pyrolysis considerably improves the stability and, furthermore, that this is achieved with only a small decrease of the pore volume and pore width in relation to those of the unmodified material.
Previous work1-4 has revealed that silica grades of MCM-41 undergo structural changes when exposed to water vapor. The most significant effects are a loss of pore shape uniformity and a large decrease in surface area, pore size, and pore volume.4 Silylation, which is a widely used method for modifying the surface of silicas,5 has also been used on MCM-41 materials1,4,6-11 and has proven to University of E Ä vora. Bristol University. # Johannes Gutenberg Universita ¨ t. † ‡
(1) Koyano, K. A.; Tatsumi, T.; Tanaka, Y.; Nakata, S. J. Phys. Chem. B 1997, 101, 9436. (2) Ribeiro Carrott, M. M. L.; Carrott, P. J. M.; Candeias, A. J. E.; Unger, K. K.; Sing, K. S. W. In Fundamentals of Adsorption 6; Meunier, F., Ed.; Elsevier: Amsterdam, 1998; p 69. (3) Zhao, X. S.; Audsley, F.; Lu, G. Q. J. Phys. Chem. B 1998, 102, 4143. (4) Ribeiro Carrott, M. M. L.; Esteˆva˜o Candeias, A. J.; Carrott, P. J. M.; Unger, K. K. Langmuir 1999, 15, 8895. (5) Packings and Stationary Phases in Chromatographic Techniques; Unger, K. K., Ed.; Marcel Dekker: New York, 1990. (6) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. T.; 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. (7) Tatsumi, T.; Koyano, K. A.; Igarashi, N. Chem. Commun. 1998, 325. (8) Zhao, X. S.; Lu, G. Q. J. Phys. Chem. B 1998, 102, 1556. (9) Jaroniec, C. P.; Kruk, M.; Jaroniec, M.; Sayari, A. J. Phys. Chem. B 1998, 102, 5503.
be effective in stabilizing the structure1,4 but only at the expense of a considerable reduction in pore volume and pore width.4,8 In this Letter we describe an alternative route to stabilization, which has a number of advantages: the deposition of pyrolytic carbon provides excellent protection with considerably less effect on the pore structure. The sample used in the present work was taken from a pure silica form of MCM-41, designated C16A, which had been previously prepared and characterized4 and then stored for a year. This material was synthesized by a roomtemperature procedure12 using tetraethoxysilane, ammonia, and n-hexadecyltrimethylammonium bromide, and calcined in air at 823 K. The surface modification was carried out as follows. The sample was placed in a vertical silica reactor and heated to 873 K in a dry nitrogen flow of 130 cm3 min-1 with a heating rate of 5 K min-1. When the sample had reached the final temperature, the gas flow was changed to nitrogen saturated at 298 K with benzene, which was continued for 2.5 h. The coated (10) Kruk, M.; Antochshuk, V.; Jaroniec, M.; Sayari, A. J. Phys. Chem. B 1999, 103, 10670. (11) Antochshuk, V.; Jaroniec, M. Chem. Commun. 1999, 2373. (12) Gru¨n, M.; Unger, K. K.; Matsumoto, A.; Tsutsumi, K. In Characterisation of Porous Solids IV; McEnaney, B., Mays, T. J., Rouque´rol, J., Rodriguez-Reinoso, F., Sing, K. S. W., Unger, K. K., Eds.; The Royal Society of Chemistry: Cambridge, U.K., 1997; p 81.
10.1021/la000671a CCC: $19.00 © 2000 American Chemical Society Published on Web 10/26/2000
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Figure 1. X-ray powder diffraction patterns (using Cu KR radiation) of the MCM-41 sample original (C16A) and after carbon deposition (C16A-C). Upper/lower pattern: before/after determination of water vapor adsorption isotherms.
material was then allowed to cool to room temperature under the dry nitrogen flow. It is known that under similar conditions pyrolytic carbon can be deposited on the surface of silicas13,14 and other porous solids.15 Carbon analysis of the modified sample, C16A-C, carried out using a Eurovector EuroEA elemental analyzer, showed the presence of 1.46% carbon. The sample was characterized by means of XRD and nitrogen adsorption at 77 K, both before and after benzene pyrolysis. To evaluate the stability, two complete adsorption-desorption cycles of water vapor adsorption, with outgassing of the samples between runs at 423 K, were carried out over a period of about 6 weeks, on both the unmodified and modified samples. At the end of this prolonged contact with water vapor, the samples were again characterized by means of XRD and nitrogen adsorption. It can be seen from the XRD diffraction patterns shown in Figure 1 that the original C16A gave the three peaks typical of MCM-416 and that the modified sample, C16AC, shows the same three peaks with almost the same intensity and position, which indicates that long range order was retained after modification. Furthermore, evidence for the retention of the regularity of the pores is provided by the results of low-temperature nitrogen adsorption shown in Figure 2, where it is evident that the corresponding isotherms both exhibit a steep reversible pore filling step over a narrow range of p/p°. It can also be seen from Figure 2 that after modification the limiting uptake of nitrogen was reduced and the step shifted to slightly lower p/p°, indicating that there was a decrease in pore volume and mean pore width. This is confirmed by the values of pore volume and hydraulic pore radius given in Table 1 and which were obtained by applying the Rs method16,17 to the isotherms in the manner previously described.4 The decrease in pore width indicates (13) Carrott, P. J. M.; Sing, K. S. W.; Raistrick, J. H. Colloids Surf. 1986, 21, 9. (14) Leboda, R.; Turov, V. V.; Charmas, B.; Skubiszewska-Zieba, J.; Gun’ko, V. M. J. Colloid Interface Sci. 2000, 223, 112. (15) Pan, Z. J.; Chen, S. G.; Tang, J.; Yang, R. T. Adsorption Sci. Technol. 1993, 10, 193. (16) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: New York, 1982. (17) Rouquerol, F.; Rouquerol, J.; Sing, K. S. W. Adsorption by Powders and Porous Solids; Academic Press: London, 1998.
Letters
Figure 2. Nitrogen adsorption-desorption isotherms determined at 77 K on samples: C16A original (solid line); after carbon deposition, C16A-C (circles); C16A-C after determination of water vapor isotherms (squares). Open symbols, adsorption; closed symbols, desorption.
Figure 3. Water vapor adsorption-desorption isotherms determined at 298 K on sample C16A-C (sample C16A after carbon deposition): circles, first run; squares, second run; open symbols, adsorption; closed symbols, desorption.
that during the benzene pyrolysis carbon was in fact deposited in the pores and not just on the external surface. However, the combined results of elemental analysis and nitrogen adsorption give a value for the mean surface coverage of about 0.9 carbon atoms/nm2, indicating that the carbon does not form a complete uniform layer on the surface. In addition, the decrease in pore volume, although small, is still greater than would be expected on the basis of the small amount of carbon deposited, which suggests that part of the porosity was blocked by the carbon deposit. The two complete water vapor adsorption-desorption isotherms determined on the modified sample are shown in Figure 3. The first adsorption-desorption isotherm exhibits extensive hysteresis which extends to very low pressures and is associated with a small residual uptake after re-outgassing after the desorption run. However, the second adsorption branch coincides with the desorption branches at lower values of relative pressure. These features are similar to those obtained on the unmodified sample, and as previously explained,4 the overall changes indicate that some hydroxylation of the surface occurred during the course of the isotherm determination.
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Table 1. Results of the Characterization of Samples C16A (Unmodified) and C16A-C (Modified) before and after Water Vapor Adsorption, by Nitrogen Adsorption at 77 K and XRD nitrogen adsorption
XRD
sample
Atot (m2 g-1)
Aext (m2 g-1)
Vp (cm3 g-1)
rp(H) (nm)
d100 (nm)
C16A (1 year old) C16A-C C16A-C (after water adsorption)
905 810 810
24 17 12
0.71 0.61 0.58
1.61 1.54 1.45
3.98 3.94 3.91
Some of the differences between the modified and unmodified samples are that the uptake on the first adsorption run prior to the pore filling step was reduced by carbon deposition and the pore filling step occurs at higher relative pressure than that for the unmodified material, indicating that the surface was made more hydrophobic, probably due to carbon deposition on some surface silanol groups. However, in the present context of structural stability, the two most notable features of the water vapor isotherms are that the limiting uptake is almost constant (0.58 cm3 g-1 at 0.95p°) and that the adsorption-desorption pore filling-emptying steps of the two runs coincide. This is in marked contrast to the behavior found with the unmodified sample4 and can be taken as good evidence for an overall stabilization of the pore structure as a result of the carbon deposition. This enhancement in stability is confirmed first by the XRD measurements. As can be seen from Figure 1, exposure to water vapor resulted in a considerable change in the XRD pattern of the unmodified sample C16A, indicating a drastic reduction in structural ordering. On the other hand, the XRD pattern of the modified sample C16A-C was not greatly altered by the prolonged exposure to water vapor. Further evidence for the considerable improvement in stability comes from the nitrogen adsorption results in Figure 2, from where it is evident that after determination of the water vapor isotherms the uniformity of the pores was retained and little change occurred in the pore structuresthe initial parts of the isotherms are practically coincident before and after water exposure, and there was only a slight shift of the position of the step and a small decrease in the limiting uptake of nitrogen. From Table 1 it can be seen that the pore volume and hydraulic pore radius decrease by only about 5% while the surface area remains constant. This is also in marked contrast to the results previously obtained with the unmodified sample C16A, for which the pore volume, the pore radius, and the total surface area were reduced by about 43%, 25%, and 23%, respectively, after determination of the water vapor isotherms. It is interesting to note that the modified sample C16A-C was found to be much more stable in the presence of water vapor than the unmodified one, even though the amount
of carbon deposited was insufficient to form a complete layer on the silica surface and some surface hydroxylation still occurred. It appears, therefore, that the instability cannot be due just to hydrolysis of surface siloxanes but that other phenomena must also be involved and that the increase in stability may be associated with the deposition of carbon on silanol groups of the material, as mentioned above. In our previous work4 we suggested that the water-vapor-induced structural alterations could be due to an expansion of the walls and possibly, in some cases, to a collapse involving rupture of some of the walls. This was explained by taking into account that the small water molecules could penetrate inside the walls, which may be defective with some incompletely polimerized regions,18 and could participate in hydrolysis reactions within the walls, leading to their disruption. One possible explanation for the considerable improvement in stability found here is that at least some of the silanol groups of the original material were located at entrances of defect sites and the carbon deposited there restricted access of the water molecules to the interior of the walls and therefore prevented the occurrence of those phenomena found with the unmodified sample. In summary, it is clear from the work presented here that stabilization of the MCM-41 sample was considerably improved by pyrolytic carbon deposition and that this was achieved with a decrease of the pore volume and the pore width by only 14% and 4%, respectively, in relation to those of the unmodified material. In this sense, this procedure seems to be more promising than the previously studied silylation which, although it led to an improvement in the stability toward water vapor, provoked a much greater reduction of the pore size and pore volume (by 64%). Acknowledgment. The authors are grateful to the Fundac¸ a˜o para a Cieˆncia e a Tecnologia for financial support (Contract No. PBIC/C/CTM/1929/95) and to Dr. M. Grun and Mr. K. Schumacher for the XRD measurements. LA000671A (18) Corma, A. Chem. Rev. 1997, 97, 2373.
