Direct spectroscopic evidence for isolated silanols in SiO_x/Al₂O₃ and their formation mechanism

The preparation and unambiguous characterization of isolated Brønsted-acidic silanol species on silica−alumina catalysts presents a key challenge in the rational design of solid acid catalysts. In this report, atomic layer deposition (ALD) and liquid-phase preparation (chemical liquid deposition, CLD) are used to install the SiO_x sites on Al₂O₃ catalysts using the same Si source (tetraethylorthosilicate, TEOS). The ALD-derived and CLD-derived SiO_x sites are probed with dynamic nuclear polarization (DNP)enhanced²⁹Si−²⁹Si double-quantum/single-quantum (DQ/SQ) correlation NMR spectroscopy. The investigation reveals conclusively that the SiO_x/Al₂O₃ material prepared by ALD and CLD, followed by calcination under an O₂ stream, contains fully spatially isolated Si species, in contrast with those resulting from the calcination under static air, which is widely accepted as a postgrafting treatment for CLD. Insight into the formation mechanism of these sites is obtained via in situ monitoring of the TEOS + γ-Al₂O₃ reaction in an environmental diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) cell. Upon calcination, the DRIFTS spectra of SiO_x/Al₂O₃ reveal a signature unambiguously assignable to isolated Brønsted-acidic silanol species. Surprisingly, the results of this study indicate that the method of preparing SiO_x/Al₂O₃ catalysts is less important to the final structure of the silanol sites than the post-treatment conditions. This finding should greatly simplify the methods for synthesizing site-isolated, Brønsted-acidic SiO_x/Al₂O₃ catalysts.