No 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.

The aim of the present solid-state NMR study was to characterize the surface species of γ-aminopropyltriethoxysilane (APTS), γ-aminopropyltrimethoxysilane (APTMS), and γ-aminopropyldiethoxymethylsilane (APDMS) on porous silica when the deposition was performed via the gas phase. The reaction temperature used, that is, 150−300 °C, in an atomic layer deposition reactor at a pressure of 20−50 mbar, was observed to distinctly affect the surface species of aminopropylalkoxysilanes on silica. The gas−solid reactions of the precursors with the silica surface were observed to be surface-limiting at the deposition temperatures of ≤150 °C. On the basis of 29Si CP/MAS NMR, the amino ends of APTS and APTMS molecules were observed to react both with alkoxy groups of other precursor molecules and silanols of silica at deposition temperatures of ≥150 °C forming Si−N linkages. The amino groups of APDMS molecules were observed to react at 150 °C only on silica heat-treated at 200 °C in a similar way. The reaction of amino groups affected also the chemical shifts of the carbon atoms in the propyl chain causing splitting of the peaks in the 13C CP/MAS NMR spectra. At still higher reaction temperatures, especially at 300 °C, decomposition of the surface structures was observed to occur. The bonding modes of trifunctional APTS and bifunctional APDMS on silica heat-treated at 200−800 °C were systematically studied by 29Si and 13C NMR when the deposition was performed at 150 °C. Bi- and tridentate species of APTS were observed on silica pretreated at 200 °C, and mono- and bidentately bound surface structures were observed when silica was heat-treated at 450−800 °C. APTMS was also observed to attach onto silica pretreated at 600 °C in a similar way. APDMS was bound both mono- and bidentately on silica pretreated at 200 °C and at 600−800 °C but solely bidentately on silica pretreated at 450 °C.

A novel gas-phase procedure for the control of amino group density on porous silica through consecutive reactions of aminopropylalkoxysilanes and water vapor was developed. First heat-treated silica was saturated with trifunctional γ-aminopropyltrimethoxysilane (APTMS) or γ-aminopropyltriethoxysilane (APTS) in an atomic layer deposition reactor. During this step, precursor molecules were bound onto the surface both mono-and bidentately forming siloxane bridges with the silanol groups of silica. Then surface densities of 1.8 APTMS or 2.0 APTS molecules/nm 2 were achieved. Next the aminosilylated surface was treated with water vapor in order to hydroxylate the free alkoxy groups of chemisorbed aminosilane molecules. At the same time, the silanol groups on the silica surface, which had remained unreacted during the first step, were revealed below the hydrolyzed alkoxy groups. These silanol groups of silica and hydrolyzed alkoxy groups were able to react further with the next feed of aminosilane molecules. The above-mentioned aminosilane/water vapor cycles, that is, two consecutive steps, could be repeated several times, and the amino group content on silica could be controlled through the number of aminosilane/water cycles. After four cycles, the surface was observed to be saturated and maximum amino group density was achieved. Then, by performing four or five cycles, surface densities of up to 3.0 APTS or APTMS molecules/nm 2 were obtained. With this procedure, a high-density aminopropylsiloxane network is grown through horizontal polymerization of aminosilane molecules on the surface. With bifunctional γ-aminopropyldiethoxymethylsilane (APDMS), the repetition of aminosilane/water cycles did not increase the amino group content because of a lack of free and reactive ethoxy groups on the aminosilylated silica surface due to the bidentate bonding of APDMS molecules on silica.