Al molar ratios and different silicon sources

432

Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) © 2004 Elsevier B.V. All rights reserved.

SYNTHESIS AND CHARACTERIZATION OF MCM-41 WITH DIFFERENT Si/Al MOLAR RATIOS AND DIFFERENT SILICON SOURCES Blanco, C , Pesquera, C. and Gonzalez, F. Department of Chemical Engineering and Inorganic Chemistry. University of Cantabria. Avda de los Castros, s/n 39005-Santander. Spain. E-mail: [email protected]

ABSTRACT A series of MCM-41 samples have been synthesized with different silicon sources (silica filmed/ sodium silicate) and Si/Al molar ratios. The MCM-41 solids were synthesized hydrothermally at 423K in an autoclave or under reflux for 48 hours. The calcined products were characterized by means of XR diffraction, nitrogen adsorption and EDXRA. Keywords: Mesoporous molecular sieves, MCM-41, Si/Al molar ratios, nanoporous materials, catalysts, adsorbents. INTRODUCTION Since its discovery in 1992 [1-2], MCM-41 has become the most popular member of the M41S family of mesoporous silicates. This attracted considerable interest because of the potential of these materials for use as solid-acid catalysts. The member of this family designated as MCM-41 shows a hexagonal array of uniform mesopores with pore diameter ranging between 15 and lOOA, depending on the template and synthesis conditions. The presence of these very large uniform pores combined with acidic properties opens new possibilities in the catalytic conversion of large molecules [3-7]. Since then, much attention has been placed on the substitution of heteroatoms, particularly aluminum [8-13], into the silicon network to modify the composition of the material. Aluminum incorporation is of special interest in the application of mesoporous materials in various processes because of the beneficial effects which influence the properties of the MCM-41. It is known that substitution of aluminum atoms into a silicate framework requires the accompanying introduction of extra-frame work charge-compensating cations, and that these cationic species can impart desirable adsorption and catalytic behaviors to the porous aluminosilicates. During calcination, the surfactant and other cationic molecules decompose, leaving charge compensating alkyl ammonium or proton species as Lewis or Bronsted acid sites in the calcined mesoporous material. These species compensate the negative framework charge associated with tetrahedral coordinated aluminum in the polymerized and calcined framework. The aim of the current study is to synthesize a series of MCM-41 molecular sieves by using different Si/Al molar ratios and two sources of silicon and characterize the resulting structures using various techniques. The influence of different synthesis methods in the final properties of the mesoporous aluminosilicate MCM-41 structure is also investigated.

EXPERIMENTAL SECTION A I - M C M - 4 1 molecular sieves were synthesized starting from an aqueous solution of silica fumed or silica fumed and sodium silicate solution as the silicon source and an aqueous suspension of sodium aluminate as the aluminium source and tetramethylammonium hydroxide (TMAOH) in water as the cationic agent. This gel was vigorously stirred for 1 h and then an aqueous solution of cetyltrimethylammonium bromide (CATBr) aged for 1 h was dropped under stirring at room temperature and the stirring was continued for 5 min. The resultant gel was transferred to a static teflon-lined stainless steel autoclave under autogeneous pressure and heated to 423 K or alternatively into a glass vessel and heated under reflux with stirring for 48 hours. The final solid reaction product was extracted from autoclave or from the reflux beaker, filtered with distilled water and dried at 333K overnight. The molar composition of the gels subjected to hydrothermal

433 synthesis was as follows: Si/Al=25; 50 and 100; ISiOs: 0.25 CTABr: 0.1 TMAOH: 100 H2O. When the silicon used in the synthesis process was from two sources the molar ratio of Si02/Na2Si03 was 1/0.25 (Si/Al molar ratio will be denoted in the samples as follows: AlM41S(x-y) where x indicated the molar ratio and y can be 1 or 2 depending the number of the silicon sources used). All samples were calcined in air at 823 K for 6 hours. The samples synthesized in autoclave will denoted as AlM41S(x-y)P, while the ones which are made under reflux will be denoted as AlM41S(x-y)R. XRD, EDXRA and BET analysis were applied to characterize all the samples. X-ray powder diffraction patterns of the samples were collected in air at room temperature in a Bruker diffractometer using CuKa (A.=1.5418A) radiation, O.OT step size and a 6 s step time from 1.5°(26) to 10.0°(26). Energy-dispersive X-ray analysis (EDXRA) was carried out with a Jeol electron microscopic (Model JSTM-T 330 A) with a Link Analytical AN 10.000 microanalyzer. The adsorption isotherm of N2 at 77K was determined in a Micromeritics ASAP 2010 with a micropore system, and prior to measurement, the samples were outgassed at 413K for at least 16 h. Specific surface area was determined by applying the BET equation to the isotherm [14]. Mesopore size distribution was calculated using the adsorption branch of the nitrogen adsorption isotherm and the Barrett-Joyner-Halenda (BJH) formula [15]. The pore diameter medium (DBJH) was calculated and the cumulative pore volume, VBJH, of the mesopores was obtained from the mesopore size distribution curves. RESULTS EDXRA data In Table 1 the results of the real Si/Al obtained by EDXR analysis are shown. It can be seen that in all the samples, the Si/Al molar ratio obtained is lower that the value that we have tested, thus indicating that the proportion of aluminum incorporated in the samples is higher than the proportion of silicon tested in all the samples, independently of the hydrothermal method and silicon source used. Table 1. Structural an d textural parameters of the synthesized mesoporous materia s. SAMPLE A1M41S(25-1)P AiM41S(50-l)P AIM41S(100-1)P A1M41S(25-2)P A1M41S(50-2)P AIM41S(100-2)P A1M41S(25-2)R A1M41S(50-2)R A1M41S(100-2)R A1M41S(100-1)R

Si/Al real 20.5 30.6 56.2 18.9 35.2 56.0 20.2 38.4 65.8 46.1

d(100)(nm) 3.82 3.64 3.98 3.86 3.59 3.56 3.81 3.71 4.13 4.21

SBET (mVg)

1126 1010 640 1396 1229 1307 1280 1249 1004 873

DBJH (nm)

2.88 3.14 3.12 2.98 3.20 3.06 2.92 3.06 2.98 3.12

VBJH (cc/g)

0.91 0.94 0.51 0.87 0.68 1.00 1.03 1.01 0.72 0.72

X-ray diffraction The small-angle X-ray diffraction pattern for a typical MCM-41 sample shows four well-resolved peaks that can be indexed as (100), (110), (200) and (210) reflections associated with the hexagonal symmetry. Figure 1 shows the XRD results of the materials with different silicon to aluminum molar ratios prepared with one silicon source and synthesized in an autoclave. These show a strong diffraction at 26 smaller than 2.5'' along with the presence of small peaks which are not very clear; thus, we confirmed the formation of the mesoporous materials. However, the most intense peak corresponds to A1MS41(25-1)P sample, but the smallest angle is for the A1MS41(100-1)P sample. For the aluminosilicate MS41 with two sources of silicon and also synthesized in an autoclave (Figure 2) one very intense peak (100) appeared in all the samples. For the A1M41S(100-2)P sample this peak along with three others peaks were observed. This suggests that this sample has the highest degree of long range ordering of the structure and well-formed hexagonal pore array than the other samples. It can be seen that as the amount of aluminum increases, the interplanar spacing dioo of samples shifts gradually to lower angle. On the contrary, as the amount of aluminum decreases in the sample, the quality of the samples, as estimated from the width of the main peak and the appearance of the other diffraction peaks, increases gradually.

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435 Figure 3 shows the XRD results of the materials with different silicon to aluminum molar ratios synthesized with two silicon sources and under reflux conditions and for comparison, the sample synthesized with one silicon source and a Si/Al ratio of 100, [A1M41S(100-1)R], under the same hydrothermal conditions has been included. It can be observed that the interplanar spacing dlOO of the samples shifts gradually to a lower angle but the peak becomes wider as the amount of aluminum decreases. The A1M41S(100-1)R sample shows a XRD pattern similar to the A1M41S(100-2)R. Table 1 shows d-spacing sizes of the crystal plane (100) based on X-ray diffraction results. As can be seen, the values are very similar for all the samples. Despite that, in general, the interplanar spacing dlOO of the samples synthesized with two sources of silicon are higher than the other samples.

Adsorption isotherms The isotherm of nitrogen adsorption on these materials was measured at 77K. Figures 4, 5 and 6 show the isotherms of all the samples. These isotherms showed a typical IV-type adsorption profile consisting of a step condensation behavior due to the formation of mesopores. luuu -

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436 The adsorption of nitrogen in the samples at low relative pressure (P/Po<0.3) is from the monolayer adsorption of nitrogen on the wall of the mesopores and does not imply the presence of any micropores. As the relative pressure increases (P/Po>0.3), the isotherms exhibit sharp inflection, which is characteristic of capillary condensation within uniform mesopores, where the P/Po position of the inflection point is related to the diameter of the mesopores. Moreover, the sharpness of the isotherm in the range 0.28

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Pressure Relative (P/Po) Figure 6. Nitrogen adsorption isotherms of AlM41S(x-y)R samples Figure 6 shows the isotherms of the samples synthesized with two silicon sources under reflux. The tendency is paralell to the other series of samples. The sample with the lowest amount of aluminum [A1M41 S(100-2)R], has a similar isotherm to the same sample but synthesized with one source of silicon. Table 1 reflects the specific surface area for all the synthesized samples. These data clearly illustrates that the use of one or two silicon sources have an influence in the specific surface area. This parameter increases due to the use of two silicon sources instead of one, while there is not a great difference when the sample is synthesized under the two different hydrothermal methods. However, the values of the specific surface area are a little greater when the synthesis was in autoclave than under reflux. Moreover, the highest value is when the Si/Al molar ratio was the smallest tried in the two hydroyhermal method used, while the lowest value correspond to the samples synthesized with one silicon source and with the highest Si/Al molar ratio used. Pore size distribution Though the BJH method may systematically underestimate the pore size up to 1.0 nm [16], and although many papers have developed more accurate methods for pore size calculation [17, 18], we think that the data obtained by applying the same method are comparable, and therefore the quantitative resuhs based on these data are credible.

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438 In Figures 7, 8 and 9 the pore size distribution for all the synthesized samples is shown. The sample which has the highest pore size is the A1M41S(50-2)P, although the A1M41S(100-2)P sample is the one which has the highest incremental volume. The pore size distribution is narrow for most of the synthesized samples except for A1M41S(25-2)P sample. Table 1 reflects the data of pore diameter medium of all the samples synthesized. As can be seen, all the samples have similar values. However, when the samples were synthesized with two silicon sources and with the same Si/Al molar ratios in autoclave, the samples presented a higher pore size than when the samples were synthesized under reflux. The cumulative pore volume of the samples are illustrated in Table 1. As can be noted, the highest value (1.03 cc/g) was obtained for the A1M41S(25-2)R sample and the lowest (0.51 cc/g) was for the A1M41S( 100-1)? sample. CONCLUSIONS A series of mesoporous materials have been synthesized with different silicon sources (silica fumed/ sodium silicate) and Si/Al molar ratios. All the samples have a lower Si/Al molar ratio than that tested in the synthesized process, independently of the hydrothermal method used. The obtained materials, in general, exhibited high specific surface area, but these data were higher when two sources of silicon was used and the synthesis was in autoclave than under reflux. Most of the samples showed a narrow pore size distribution.

ACKNOWLEDGMENTS Our acknowledgment to D.G. I. of M.C. y T. for financial support of this work under projects: MAT2002-03808 and MAT2002-02158. REFERENCES 1. Kresge, C.T., Leonowicz, M.E., Roth, W.J., Vartuli, J.C, Beck, J.S., Nature, 359 (1992), 710-712. 2. 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, 114 (1992), 10834-10843. 3. Corma, A., Fomes, V., Navarro, M.T., Perez-Pariente, J., J. Catal., 148 (1994), 569-574. 4. Zhao, D., Feng, J., Huo, Q., Melosh, N., Fredickson, G.H., Chmelka, B.F., Stucky, G.D., Science, 279 (1998), 548-552. 5. Kim, S.S., Zhang, W., Pinnavaia, T.J., Science, 282 (1998), 1302-1305. 6. Ryoo, R., Kim, J.M., Ko, C.H., Shin, C.H., J. Phys. Chem., 100 (1996), 17718-17721. 7. Mokaya, R., Angew. Chem. Int. Ed., 38 N° 19 (1999), 2930-2934. 8. Trong On, D., Zaidi, S.M.J., Kaliaguine, S., Microp. Mesop. Mater., 22 (1998), 211-224. 9. Wu, C.-G., Bein, T., J.Chem. Soc, Chem. Commun., 8 (1996), 925-926. 10. Kim, J.-H., Tanabe, M., Niwa, M., Microp. Mater., 10 (1997), 85-93. 11. Kosslick, H., Lischke, G., Parlitz, B., Storek, W., Fricke, R., Appl. Catal. A: General, 184 (1999), 49-60. 12. Reddy, K.M., Song, C , Catal. Lett., 36 (1996), 103-109. 13. Kolodziejski, W., Corma, A., Navarro, M.-T., Perez-Pariente, J., Solid State Nuc. Magn. Reson., 2 (1993), 253-259. 14. Gregg, S.J., Sing, K.S.W., Adsorption, Surface Area and Porosity, Academic Press, New York, 1982. 15. Barrett, E.P., Joyner, L.G., Halenda, P. P., J. Am. Chem. Soc. 73 (1951), 373-380. 16. Broekhoff, J.C.P., de Boer, J.H., J. Catal., 9 (1967), 8-14. 17. Kruk, M., Jaroniec, M., Sakamoto, Y., Terasaki, O., Ryoo, R., Ko, C.H., J. Phys. Chem. B, 104 (2000), 292-301. 18. Kruk, M., Jaroniec, M., Chem. Mater., 12 (2000), 222-230.