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.