Highly Dispersed SiO_x/Al₂O₃ Catalysts Illuminate the Reactivity of Isolated Silanol Sites

Abstract

The reaction of γ-alumina with tetraethylorthosilicate (TEOS) vapor at low temperatures selectively yields monomeric SiO(x) species on the alumina surface. These isolated (-AlO)3Si(OH) sites are characterized by PXRD, XPS, DRIFTS of adsorbed NH3, CO, and pyridine, and (29)Si and (27)Al DNP-enhanced solid-state NMR spectroscopy. The formation of isolated sites suggests that TEOS reacts preferentially at strong Lewis acid sites on the γ-Al2O3 surface, functionalizing the surface with "mild" Brønsted acid sites. For liquid-phase catalytic cyclohexanol dehydration, these SiO(x) sites exhibit up to 3.5-fold higher specific activity than the parent alumina with identical selectivity.

Silica deposition as an approach for improving the hydrothermal stability of an alumina support during glycerol aqueous phase reforming

“…The maxima located at ∼280°C can be assigned to weak and medium strength acid sites [43]. Acid site concentration (Table 1) decreased with increasing modification time, similar to previous observations of selective consumption of the strong Al Lewis acid sites upon exposure to TEOS [44]. Pyridine adsorption followed by IR spectroscopy indicated that only Lewis acid sites (LAS) were present in the parent Pt/Al 2 O 3 catalyst (Fig.…”

“…Given the change in catalyst acidity before and after modification, it can be inferred that Al-O-Si linkages were formed successfully at the expense of specific surface Lewis acid sites, which serve as original hydration sites for the undesired conversion to boehmite. Similarly, Mouat et al [44] suggested that TEOS modification of an γ-alumina surface preferentially occurs on strong Lewis acid sites, also observing the formation of "mild" Brønsted acid sites. As stated above, silica deposition was hypothesized to inhibit water adsorption of Al 2 O 3 by Al coordinative saturation of unsaturated Al species and the formation of AleOeSi bonds, with the characterization data now suggesting that improved stability under hydrothermal conditions can indeed be anticipated.…”

“…The broad peak at 3590 cm −1 has been attributed to hydrogen-bonded OH groups [47,48]. Upon modication with TEOS, a sharp peak centered at 3742 cm −1 appeared, attributed to isolated silanols, considered to be Brønsted acid sites [44,49]. The IR spectra of Pt/Al 2 O 3 -8 and 12 showed a decrease in intensity of the absorption bands at 3672 and 3725 cm −1 compared to pristine Pt/Al 2 O 3 , suggesting that surface unsaturated Al atoms were consumed in the overcoating process.…”

Natural Abundance 17O DNP NMR Provides Precise O−H Distances and Insights into the Brønsted Acidity of Heterogeneous Catalysts

“…[3] TheB rønsted acid sites are,h owever, recognized as being surface hydroxyls, which challenges their characterization by conventional SSNMR. [3] TheB rønsted acid sites are,h owever, recognized as being surface hydroxyls, which challenges their characterization by conventional SSNMR.…”

Facile growth of well-dispersed and ultra-small MoS2nanodots in ordered mesoporous silica nanoparticles

“…17 And, the only presence of silica tetrahedron framework peak and surface silanols peak at 103.1 eV and 532.6 eV, respectively revealed no obvious bonding between MSN and the MoS2 nanoparticles. 18,19 SAXS pattern of UsMSND@MSN exhibited two resolved diffraction peaks with reciprocal spacing, 1/dhk, ratios of approximate 1:√3 ( Figure 2D). Together with the TEM results of MSN precursor ( Figure S3), this could be indexed as the typical (100) and (110) reflections of an ordered hexagonal P6mm mesostructure with unit cell parameter (a0) of 3.73 nm.…”