Study of the Constitutive Superficial Water of Precipitated Amorphous Silicas using ¹H NMR:  Broad-Line at 4 K and HR MAS at 300 K

Abstract

Two silicas obtained by precipitation have been compared using two 1H NMR techniques: broad-line at 4 K with simulation of the spectra and MAS at room temperature. Previous 29Si high resolution NMR results have been used to interpret the spectra. Geminal silanols, single silanols, water molecules relatively strongly hydrogen-bonded to silanols, and water not interacting with OH groups have been characterized and quantified. Differences are found between the two precipitated silicas. Silanols, 70% of which can hydrogen-bond a water molecule, occupy all the surface of one of them. The other sample surface is not so crowded with silanols, and only 37% of them hydrogen-bond water molecules. Both geminal and single silanols hydrogen-bond water. Physisorbed water molecules not interacting with silanols can be equilibrated on both sample surfaces. Room temperature dehydration of the samples in vacuum is studied.

Multinuclear Solid-state NMR Investigation of Nanoporous Silica Prepared by Sol-gel Polymerization Using Sodium Silicate

“…9,17 The signal at 3.1 ppm was interpreted as the hydroxyl groups hydrogen-bonded with water while the broad signal at 4.7 ppm was assigned to the hydroxyl groups hydrogen-bonded with the hydroxyl groups. 15,16,18 Thus isolated hydroxyl groups, geminal hydroxyl groups, hydroxyl groups hydrogen-bonded with water, and hydroxyl groups hydrogenbonded with hydroxyl groups were present on the surface of the as-synthesized silica. Figure 3(b) presents a representative 1 H MAS NMR spectrum of the hydrothermally treated silica and the deconvoluted spectrum was used to generate the peaks of isolated hydroxyl groups at 1.8 ppm, geminal hydroxyl groups at 2.1 ppm, and hydroxyl groups hydrogen- bonded with water at 3.1 ppm.…”

Adsorption of biomolecules and polymers on silicates, glasses, and oxides: mechanisms, predictions, and opportunities by molecular simulation

“…[6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Measurements of surface areas with the Brunauer-Emmett-Teller (BET) method, thermal gravimetric analysis (TGA), surface adsorption of gases and liquids, spectroscopy, as well as of ionization of silanol surface groups have been carried out by numerous research teams. [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] Glasses are typically produced from oxide melts of various composition, often the Stober method based on hydrolysis of liquid precursors is applied to synthesize nanostructured and microstructured silica. 23 Recent studies also explored the use of biomimetic synthesis approaches of various oxides using soluble precursors in combination with amines and peptide-based ligands.…”

Synthesis of heteropolyacid (H3PW12O40)/SiO2 nanoparticles and their catalytic properties

“…The peak at 4.0 ppm could be assigned to the hydroxyl proton which was related to the germinal (Q 2 ) silanol of silica [35,37]. The peak of physically adsorbed water was sharper and higher than that of bulk powder.…”

“…In the HPA/SiO 2 bulk powder, the broad line centered at 5.0 ppm is assigned mainly to hydrogen bonded silanol groups with overlapping of signals of physically adsorbed water [34]. And the small peak around 1.5 ppm is isolated silanol (i.e., non-hydrogen bonded) [35,36]. The amorphous silica always contains hydrogen atoms belonging to different chemical species, such as hydroxyls and adsorbed water molecules.…”