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The Mechanism of Pseudomorphic Transformation of Spherical Silica Gel into MCM-41 Studied by PFG NMR Diffusometry
Institute of Chemical Technology, Universität Leipzig, Linnéstr. 3, 04103 Leipzig, Germany
Institute of Experimental Physics I, Universität Leipzig, Linnéstr. 5, 04103 Leipzig, Germany
* Author to whom correspondence should be addressed.
Received: 1 July 2013; in revised form: 16 August 2013 / Accepted: 19 August 2013 / Published: 26 August 2013
Abstract: The pseudomorphic transformation of spherical silica gel (LiChrospher® Si 60) into MCM-41 was achieved by treatment at 383 K for 24 h with an aqueous solution of cetyltrimethylammonium hydroxide (CTAOH) instead of hexadecyltrimethylammonium bromide (CTABr) and NaOH. The degree of transformation was varied via the ratio of CTAOH solution to initial silica gel rather than synthesis duration. The transformed samples were characterized by N2 sorption at 77 K, mercury intrusion porosimetry, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Thus, MCM-41 spheres with diameters of ca. 12 μm, surface areas >1000 m2 g−1, pore volumes >1 cm3 g−1 and a sharp pore width distribution, adjustable between 3.2 and 4.5 nm, were obtained. A thorough pulsed field gradient nuclear magnetic resonance (PFG NMR) study shows that the diffusivity of n-heptane confined in the pores of the solids passes through a minimum with progressing transformation. The final product of pseudomorphic transformation to MCM-41 does not exhibit improved transport properties compared to the initial silica gel. Moreover, the PFG NMR results support that the transformation occurs via formation and subsequent growth of domains of <1 μm containing MCM-41 homogeneously distributed over the volume of the silica spheres.
Keywords: pseudomorphic transformation; silica gel; LiChrospher® Si 60; MCM-41 spheres; PFG NMR; diffusion
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Einicke, W.-D.; Enke, D.; Dvoyashkin, M.; Valiullin, R.; Gläser, R. The Mechanism of Pseudomorphic Transformation of Spherical Silica Gel into MCM-41 Studied by PFG NMR Diffusometry. Materials 2013, 6, 3688-3709.
Einicke W-D, Enke D, Dvoyashkin M, Valiullin R, Gläser R. The Mechanism of Pseudomorphic Transformation of Spherical Silica Gel into MCM-41 Studied by PFG NMR Diffusometry. Materials. 2013; 6(9):3688-3709.
Einicke, Wolf-Dietrich; Enke, Dirk; Dvoyashkin, Muslim; Valiullin, Rustem; Gläser, Roger. 2013. "The Mechanism of Pseudomorphic Transformation of Spherical Silica Gel into MCM-41 Studied by PFG NMR Diffusometry." Materials 6, no. 9: 3688-3709.