A ²⁹Si and ¹³C CP/MAS NMR Study on the Surface Species of Gas-Phase-Deposited γ-Aminopropylalkoxysilanes on Heat-Treated Silica

Abstract

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.