Gas-Phase Deposition of Aminopropylalkoxysilanes on Porous Silica

Abstract

Modification of porous silica with aminopropylalkoxysilanes was studied under an inert atmosphere by a gas-phase technique, atomic layer deposition. Trifunctional γ-aminopropyltrimethoxysilane and γ-aminopropyltriethoxysilane (APTS), bifunctional γ-aminopropyldiethoxymethylsilane (APDMS), and monofunctional γ-aminopropyldimethylethoxysilane were used as precursors to deposit surface-saturated molecular layers onto dehydroxylated silica surfaces. A silica bed was saturated with each of the vaporized aminosilanes studied using a reaction temperature of 150 °C and a pressure of 20−50 mbar. At higher reaction temperatures, viz., 280−300 °C, decomposition of aminosilanes was observed on the surface. Aminosilanes were observed to interact with the silica surface through a site adsorption mechanism. Elemental analyses and DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) were used for the characterization of aminosilylated silica samples. The pretreatment temperature of silica, 200−800 °C, and the precursor used were observed to affect the surface density and the bonding mode of aminosilanes on the surface. The surface densities of amino groups on silica decreased when the pretreatment temperature of silica was increased. Tri- and bifunctional aminosilanes were bound onto the silica surface through fewer alkoxy groups when the heat-treatment temperature of silica was increased. The most dense molecular layers were achieved with APDMS and APTS (2.0−2.1 molecules/nm2 on silica heat-treated at 200 °C) even though the differences between the precursors were not large. A linear correlation between the surface densities of amino groups and the number of isolated silanol groups on silica was observed.

A comparative study of the functionalization of mesoporous silica MCM-41 by deposition of 3-aminopropyltrimethoxysilane from toluene and from the vapor phase

“…Under these conditions, the reaction of methanol, which is formed upon grafting of APTMS, with surface silanol groups is unlikely [11,26]. No secondary reactions involving the formation of Si-N bonds occurred upon deposition of APTMS from toluene under reflux.…”

“…Even when working in dry solvents, trace water is efficiently adsorbed on the silica surface such that hydrolysis becomes possible [10]. It has been proposed that the employment of vapor phase deposition techniques might facilitate the elimination of silane cross-linking, thus minimizing clustering and multilayer formation [11]. In the case of mesoporous silica, this should ultimately lead to a more uniform distribution of the grafted moieties, resulting in a well-defined pore size of the functionalized material.…”

“…In order to assess the validity of this hypothesis, we have in the present contribution investigated the postsynthetic functionalization of mesoporous silica MCM-41 with 3-aminopropyltrimethoxysilane (APTMS) by deposition from toluene and from the vapor phase. In the latter case, the atomic layer deposition (ALD) technique based on self-limiting surface reactions was employed, which is known to enable the deposition of vaporized precursor molecules in a highly reproducible and homogeneous manner [11].…”

Chemical modification/grafting of mesoporous alumina with polydimethylsiloxane (PDMS)

“…Consequently, the morphology and structure of the aminosilane-modified surface strongly depends on (1) reaction conditions [16][17][18][19][20][21][22][23][24][25][26] (e.g. concentration, reaction temperature, type of solvent, reaction time, phase of reaction, surface water or humidity) (2) silane nature (number of reactive groups, size) [23,24,27], (3) curing conditions [16,28] and (4) the properties of the inorganic support (pore size, [OH]). Surface-saturated layers of aminosilanes can be deposited under dry conditions, meaning in a dried organic solvent, or in the gas phase.…”

Formation and catalytic activity of Pd nanoparticles on silica in supercritical CO2

“…The same observation has been made during the heat treatment of silica supported catalysts. 43 The intensity of the CLO band at 1640 cm 21 decreased after the first run and disappeared after the third run. This provides additional confirmation of ligand leaching during the different runs, in addition to 1 H NMR data.…”