The challenge of silanol species characterization in zeolites

Abstract

The chemistry of silica-based materials, including zeolites, is strongly influenced by the nature and amount of their silanols. In zeolites, they are either isolated (non H-bonded) silanol species, or silanol...

Elucidating the Nature of the External Acid Sites of ZSM‐5 Zeolites Using NMR Probe Molecules

“…[21] In contrast, the nature of the TMPOinteracting species giving the 31 P NMR resonances below 60 ppm is contentious. They have been assigned to the interaction of TMPO with terminal silanol groups, [22] formation of (TMPO) 2 H + dimers, [21,23] interaction with Lewis acid sites, [24] weak Brønsted acid sites, [25] and physisorbed (~44 pm), crystalline (~41 ppm), confined (~37 ppm), and mobile TMPO (3 1 ppm). These conflicting assignments do not elucidate the reason for the higher relative spectral intensity below 60 ppm…”

“…The absence of Al in silicalite-1 and, thus, of Brønsted and Lewis acid sites, confirms that this correlation peak is due to the interaction of TMPO with Si-species, such as SiOH, as suggested in a recent study. [22] Because all samples described in this work contain particles of smaller size than the samples studied in previous NMR studies, and no treatment was performed to change the external surface of the materials, the correlation peak at δ H ~5 to 12 ppm is assigned to TMPO interaction with sites at the external surface of the crystals. Although it is often assumed that the external surface of zeolite crystals is coated with SiOH groups, other external surface species have been proposed for silicalite-1 and ZSM-5.…”

Facile and fast synthesis of highly active Lewis acid MWW zeolite from pure silica ITQ-1

“…Specifically, the appearance of the 3510 cm −1 band suggests the formation of hydroxyl nests in the deboronated B-MWW zeolite. 36,42,43 After the hydrothermal regrowth, the substantial decrease in the intensity of the bands at 3510 cm −1 indicates that the Si-OH nests have healed significantly (Fig. 5b).…”

Recent advances in solid-state NMR of zeolite catalysts

“…Taking these experiments together, the 1 H signals at 3.3 and 4.0 ppm were assigned to hydrogen-bonded SiOH groups and the signals at 4 and 6 ppm were assigned to unperturbed and hydrogen-bonded BASs on high-silica H-ZSM-5, respectively [ 76 ]. FTIR is often used complementarily to NMR spectroscopy in the characterization of OH groups in zeolites [ 77 ]. The work by Chizallet et al theoretically predicted the signals of possible hydroxyls on the external of H-ZSM-5.…”