Atomic Description of the Interface between Silica and Alumina in Aluminosilicates through Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy and First-Principles Calculations

Abstract

Despite the widespread use of amorphous aluminosilicates (ASA) in various industrial catalysts, the nature of the interface between silica and alumina and the atomic structure of the catalytically active sites are still subject to debate. Here, by the use of dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS) and density functional theory (DFT) calculations, we show that on silica and alumina surfaces, molecular aluminum and silicon precursors are, respectively, preferentially grafted on sites that enable the formation of Al(IV) and Si(IV) interfacial sites. We also link the genesis of Brønsted acidity to the surface coverage of aluminum and silicon on silica and alumina, respectively.

Brønsted acid sites based on penta-coordinated aluminum species

“…Similarly, Al IV species incorporated into the amorphous silica network are able to generate BASs on ASA92021. The proximity between Al IV and silanol sites in ASA has recently been observed by nuclear magnetic resonance (NMR) correlation experiments between 29 Si and 27 Al nuclei, the sensitivity of which was enhanced by dynamic nuclear polarization22. However, the strength of these BASs is generally lower than that on crystalline zeolites7 and thus the presence of bridging OH groups (Fig.…”

“…However, the strength of these BASs is generally lower than that on crystalline zeolites7 and thus the presence of bridging OH groups (Fig. 1a) in ASA is still strongly under debate2223.…”

“…1b)9212425 or a pseudobridging silanol (PBS) with a nearby Al atom (Fig. 1c)2627 have been proposed22, to account for the longer Al-O distances (2.94–4.43 Å) in ASA26, with respect to those in the crystalline zeolite framework (1.88–2.0 Å)28. So far, most efforts focus on tuning the concentration of Al IV as the main route to increase the Brønsted acidity on zeolites or silica-alumina2930313233.…”

Pairwise hydrogen addition in the selective semihydrogenation of alkynes on silica-supported Cu catalysts

“…12 Dynamic nuclear polarization (DNP) is a technique that exploits the transfer of polarization from electrons to nuclei. 13,14 DNP was successfully applied to study surfaces of solid materials, 10,15,16 including catalysts. 17,18 Another hyperpolarization technique is parahydrogen-induced polarization (PHIP), which creates non-equilibrium magnetization using a high spin order of parahydrogen (p-H 2 ).…”

17O‐NMR‐Spektroskopie mit Kernpolaritätsverstärkung zur Bestimmung der Säurestärke

“…b) Strukturvorschlagf ürdie saure Brønsted-Silanolgruppe (Al-O(H)-Si)i mkürzlich untersuchten Siliciumdioxid-Aluminiumoxid [14]. Die Strukturen von terminalem (Al-OH) und verbrückendem Aluminol (Al-O(H)-Al) stammena us Bçhmit.…”