TY - JOUR

T1 - A FTIR study of silanol nests over mesoporous silicate materials

AU - Paukshtis, Eugenii A.

AU - Yaranova, Mariya A.

AU - Batueva, Irina S.

AU - Bal'zhinimaev, Bair S.

N1 - Publisher Copyright: © 2019 Elsevier Inc. Copyright: Copyright 2019 Elsevier B.V., All rights reserved.

PY - 2019/11/1

Y1 - 2019/11/1

N2 - IR spectroscopy and DFT calculation were used to investigate the adsorption of water and toluene on aerosil calcined at a temperature of 250 °C as well as on silica produced by high-power electron-beam evaporation. It was shown that isolated SiOH groups (3746-3750 cm−1) on the surface of silicate materials weakly retain water and are easily displaced by toluene. An amount of adsorbed water raises slightly with vapor pressure. On the surface of aerosil there exist silanol nests that are characterized by two pairs of absorption bands: 3540, 3730 cm−1 and 3657, 3740 cm−1, which correspond to stretching vibrations of isolated and H-bonded hydroxyl groups, respectively. Vicinal OH groups in the nests adsorb molecular water according to the stoichiometry 2H2O: 1SiOH even at low water pressures. Owing to a higher acidity of the silanol nests, water is adsorbed much stronger, so that toluene cannot displace them. As a result, it was explained why MFI zeolites with a low silicate modulus are more hydrophobic than similar zeolites with a high modulus. Finally, a reliable spectral criterion of silanol nests existence in silicate materials was established; it consists in the presence of a pair of bands in IR spectra: a broad band in the region below 3600 cm−1 and a narrow band in the range of 3700–3740 cm−1.

AB - IR spectroscopy and DFT calculation were used to investigate the adsorption of water and toluene on aerosil calcined at a temperature of 250 °C as well as on silica produced by high-power electron-beam evaporation. It was shown that isolated SiOH groups (3746-3750 cm−1) on the surface of silicate materials weakly retain water and are easily displaced by toluene. An amount of adsorbed water raises slightly with vapor pressure. On the surface of aerosil there exist silanol nests that are characterized by two pairs of absorption bands: 3540, 3730 cm−1 and 3657, 3740 cm−1, which correspond to stretching vibrations of isolated and H-bonded hydroxyl groups, respectively. Vicinal OH groups in the nests adsorb molecular water according to the stoichiometry 2H2O: 1SiOH even at low water pressures. Owing to a higher acidity of the silanol nests, water is adsorbed much stronger, so that toluene cannot displace them. As a result, it was explained why MFI zeolites with a low silicate modulus are more hydrophobic than similar zeolites with a high modulus. Finally, a reliable spectral criterion of silanol nests existence in silicate materials was established; it consists in the presence of a pair of bands in IR spectra: a broad band in the region below 3600 cm−1 and a narrow band in the range of 3700–3740 cm−1.

KW - FTIR

KW - Mesoporous silicates

KW - Silanol nests

KW - Water and toluene adsorption

KW - GARNETS

KW - ACTIVE-SITES

KW - SORPTION

KW - SUBSTANCES

KW - ADSORPTION PROPERTIES

KW - SILICALITE-1 ZEOLITE

KW - CATALYTIC-ACTIVITY

KW - BECKMANN REARRANGEMENT

KW - WATER

KW - RANGE

UR - http://www.scopus.com/inward/record.url?scp=85068250108&partnerID=8YFLogxK

U2 - 10.1016/j.micromeso.2019.109582

DO - 10.1016/j.micromeso.2019.109582

M3 - Article

AN - SCOPUS:85068250108

VL - 288

JO - Microporous and Mesoporous Materials

JF - Microporous and Mesoporous Materials

SN - 1387-1811

M1 - 109582

ER -