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.
Highlights
The discovery of the M41-S family of ordered mesoporous silicas [1] led to a large number of studies on potential applications of these materials in the fields of sorptive separation and heterogeneous catalysis
The pseudomorphic transformation of the amorphous silica gel LiChrospher® Si 60 to the ordered mesoporous material MCM-41 was performed using an aqueous solution of cetyltrimethylammonium hydroxide (CTAOH)
The transformation stopped when the CTAOH supplied for dissolution of the initial silica gel and
Summary
The discovery of the M41-S family of ordered mesoporous silicas [1] led to a large number of studies on potential applications of these materials in the fields of sorptive separation and heterogeneous catalysis. In 2002, the pseudomorphic synthesis was introduced by Galarneau and co-workers [9,10,11,12,13] The advantage of this method is that preformed, spherical silica gel particles with a non-ordered pore system can be used and transformed, in the presence of surfactant as a structure-directing agent, into ordered mesoporous materials like MCM-41, MCM-48 and MCM-50 by a dissolution-reconstruction process under preservation of the particle morphology. This so-called “pseudomorphic transformation” allows obtaining MCM-41 materials with pore sizes from 6 to 9 nm by addition of trimethylbenzene to the synthesis mixture containing. We present complementary data from mercury intrusion to support the results obtained from PFG NMR spectroscopy measurements
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