Introduction
The invention of a new family of mesoporous silica materials by scientists at Mobil Oil Corporation at 1992 has substantially expanded the range of crystallographically defined pore sizes from the micropore (<1.3 nm) to the mesopore (2 to 10 nm) range [1,2]. Their synthesis uses ordered arrays of surfactant molecules as a template for the three-dimensional polymerization of silicates or aluminosilicates. The mesoporous materials obtained are characterized by several remarkable features:
Experimental
The samples of MCM-41 were prepared according to the following procedure. An aqueous solution of sodium aluminate was combined with a mixture of tetraethylammonium hydroxide and colloidal silica. The gel formed was mixed with an aqueous solution of surfactant (hexadecyltrimethylammonium chloride) and left to age 104$^{o}$C, aliquots being removed at specified time intervals. These samples were filtered, washed with water, extracted with ethanol and finally calcined in air at 600$^{o}$C for 22 hours.
Powder X-ray diffraction data were obtained on a Seifert 3000 P diffractometer in the Bragg-Brentano geometry arrangement using CoK$\alpha$ radiation with a graphite monochromator and a scintillation detector.
Results and discussion
The distinctive characteristic of powder X-ray diffraction patterns of both composites and mesoporous materials obtained after the removal of the organic component by subsequent extraction and calcination is that they exhibit reflections at very small angles, from which only the first one is intensive. As transmission electron micrographs had confirmed a hexagonal lattice typical of MCM-41, diffractograms were indexed on this lattice [1,2].
The time evolution of X-ray diffractograms of composites suggests that no changes in the geometry of channel arrangement occur in the course of ageing and that the position of the (100) reflection does not practically depend on the length of the synthesis (Fig.1). While the material synthesized for 1 hour already contains some proportion of the MCM-41 structure a striking increase in the intensity of the (100) reflection has been observed with the lengthening of the synthesis time from 1 to 2-6 hours. The X-ray diffractogram of composite synthesized for 24 hours has the highest intensity of the (100) reflection of all. With the sample synthesized for 72 hours a marked decrease in the intensity of this reflection has been observed. This unusual phenomenon might be perhaps explained by the mutual time changes in the structure and periodicity of the aluminosilicate walls and surfactant micelles.
From the comparison of the X-ray diffractograms of composites with those of calcined products a shrinking of the structure and an increase in the intensity of the (100) reflection have been observed (Fig.2). This effect is probably caused by the decrease in the linear absorption coefficient after the removal of micelles, which increases the penetration of rays into the solid, and by the shrinking of the MCM-41 structure after calcination, which may increase the density of the walls. The samples synthesized for 24 hours or longer have practically attained the integral intensity of this reflection corresponding to the fully developed material.
The study of the time development of the MCM-41 structure has shown that it first appears after the reaction time of 1 hour and is not preceded by any lamellar structure. While the development of practically fully organized surfactant-% aluminosilicate composite takes only about 5-6 hours, that of thermally stable structure much longer.
1. C.T.Kresge, M.E.Leonowicz, W.J.Roth, J.C.Vartuli and
J.S.Beck: Nature 359 (1992) 710
2. J.S.Beck, J.C.Vartuli, W.J.Roth, M.E.Leonowicz,
C.T.Kresge, K.D.Schmitt, C.T.-W. Chu, D.H.Olson,
E.W.Scheppard, S.B.McCullen, J.B.Higgins and J.L.Schlenker:
J.Am.Chem.Soc., 114 (1994) 10834
3. Q.Huo, D.I.Margolese, U.Ciesla, D.G.Demuth, P.Feng,
T.E.Gier, P.Sieger, A.Firouzi, B.F.Chmelka, F.Schžth and
G.D.Stucky: Chem.Mater., 6 (1994) 1176
Legends to figures:
Figure 1: Time development of the (100) reflection of
uncalcined composites.
Figure 2: Time development of the (100) reflection of calcined
molecular sieves.