The Mechanism of Pseudomorphic Transformation of Spherical Silica Gel into MCM-41 Studied by PFG NMR Diffusometry

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.

Structural Changes of Hierarchically Nanoporous Organosilica/Silica Hybrid Materials by Pseudomorphic Transformation

“…Chemical structure and the structural stabilityo fo rganic bridge and siloxane linkageb efore and after pseudomorphic transformation are examined by 13 Ca nd 29 Si CP MAS NMR spectroscopy. Figure 8a comparest he 13 CCPMAS NMR spectra of 50BTEVB, 50BTEVB_6 h_C 16 TAOH and 50BTEVB_4 d_C 16 TAOH with the liquid-state NMRo ft he precursorm olecule BTEVB in CDCl 3 .T he spectra are in good agreement with each other,i ndicatingt he integrity of the organic bridge after the treatments despite the harsh alkaline conditions. Signals between 150-120ppm result from the divinyl-benzene bridge.…”

“…[44] Hysteresis occurs which can be due to either interparticle cavitation or desorptionf rom some of the large initial pores through the newly formed mesopores.S imilarc avitation effects have also been observed in pseudomorphic transformedC PGs before. [16,17] The shape of the isotherms indicates the presence of small mesopores and micropores, as can be seen in the pore diameter distribution with two maximaat0 .6 nm and 2.9 nm (Figure 5c). In addition, p-XRD patterns indicate formation of smaller mesoporesw hen C 10 TABs urfactant is used in comparison to C 16 TAOH ( Figure 6).…”

“…Under optimal conditions these competing reactions occur in spatialp roximityg enerating mesoporous network, while the initial morphology of the silica phase can be preserved. [14][15][16] Enke et al and Grandjean et al adapted the process of the pseudomorphic transformation to CPGs with different initial pore diameters ranging from meso to macroscales. [17][18][19] In the case of mesoporousC PGs, the initialp ores ystem collapses during the transformation,b ut a hierarchically porous silica materialc an be generated from macroporous CPGs.…”

Pseudomorphic Transformation of Porous Glasses into Micelle‐Templated Silica

“…Einicke et al used a modified approach for the pseudomorphic transformation of pre‐shaped particles of a commercial zeolite Y , a spherical silica gel (LiChrospher®) , and biogenic silica from rice husk ash into MCM‐41 containing materials. An aqueous alkyltrimethylammonium hydroxide solution was used instead of the mostly applied transformation solution on the basis of alkyltrimethylammonium bromide and NaOH.…”

Charakterisierung von Kieselgelen mit bimodaler Porenstruktur mittels Positronenlebensdauer‐Spektroskopie

“…In der zweiten Phase werden auch die Wände der eingekapselten Mesoporen durch weitere Auflösungs-und Umlagerungsprozesse zu MCM-41 umgewandelt, wodurch eine reine MCM-41-Struktur entsteht. Ä hnlich wird die Strukturentwicklung auch von Einicke et al [9] beschrieben. Bei den Untersuchungen an partiell und vollständig transformierten LiChrospher 60-Partikeln mittels PFG-NMR wurde festgestellt, dass sogenannte MCM-41-Domänen gebildet werden.…”