Anion Distributions in Sodium Silicate Solutions. Characterization by²⁹SI NMR and Infrared Spectroscopies, and Vapor Phase Osmometry

Abstract

Soluble silicates are complex mixtures of silicate anions. The larger anions are two- or three-dimensional condensation products of silicate monomer, SiO4 -4. In more siliceous silicate solutions, i.e., SiO2:Na2O ratio >2.0, some of the silicate condenses to polymeric (colloidal) silica. Negative charges on the anions are balanced by protons or by cations, typically alkali metal or quaternary alkylammonium cations. The distribution of these anions varies with the concentration of dissolved silicate solids, the relative molar concentrations of cations and silica, and to a lesser extent, trace impurities. 29Silicon NMR spectroscopy of silicate solutions has been a powerful method for studying the connectivity of silicon and oxygen atoms in silicate solutions. In this paper we will show how variations in silicate band shape and peak location in the mid-infrared spectrum from 700 to 1300 cm-1 depend on concentration and silica:alkali ratio. We will interpret the infrared data in consonance with NMR results. We will also relate these variations in anion distribution to the average molecular weight of silicate solutions.

A study of sodium silicate in aqueous solution and sorbed by synthetic magnetite using in situ ATR-FTIR spectroscopy

“…However, when the alkaline condition changed by adding NaOH to the latter system (0.2 mol L −1 Na 2 SiO 3 , 0.4 mol L −1 NaOH), the two bands at 1020 and 890 cm −1 shifted to 1001 and 934 cm −1 caused by undissociated Na-silicate species, although a shoulder still appeared at 890 cm −1 . In another study, Bass and Turner also observed a similar shift of infrared bands from 1020 and 890 cm −1 to 1000 and 930 cm −1 , respectively (0.4 mol L −1 sodium silicate solution), when the ratio of silica to alkali decreased from 3.3 to 0.2 [17]. Considering the two main factors determining the properties of sodium silicate solutions, namely silica to alkali ratio and silicate concentration, a solution with lower ratio of silica to alkali should imply a higher pH value.…”

“…From these studies, it became clear that aqueous silicate solutions is a mixture of anions and that the distribution of various silicate species mainly depended on the SiO 2 :Na 2 O ratio, SiO 2 concentration, pH, temperature and aging. Combining results from 29 Si NMR and infrared spectroscopy, Bass et al [17] confirmed that more of polymeric silicates were present in the silicate solutions at higher SiO 2 :Na 2 O ratio and higher SiO 2 concentration. In alkaline and diluted solutions (SiO 2 :Na 2 O < 1.65, 0.4 mol L −1 ), monomer, dimer, and cyclic trimer silicate species are dominant.…”

“…Although there are a number of studies on the assignments of vibration bands of silica solids, glasses and gels [31,32], the assignments related to the distribution of anionic species in sodium silicate solution, especially very dilute solutions, are still unclear and some even incompatible [16][17][18]. To obtain the distribution of silicate species with pH and concentration, a silicate speciation model was set up using WinSGW program [33].…”

“…These efforts were based on potentiometric titrations as well as infrared, Raman and 29 Si NMR spectroscopy [11][12][13][14][15][16][17]. From these studies, it became clear that aqueous silicate solutions is a mixture of anions and that the distribution of various silicate species mainly depended on the SiO 2 :Na 2 O ratio, SiO 2 concentration, pH, temperature and aging.…”

Conversion of South African Coal Fly Ash into High-Purity ZSM-5 Zeolite without Additional Source of Silica or Alumina and Its Application as a Methanol-to-Olefins Catalyst

“…The appearance of the double five-ring lattice vibration at 553 cm −1 [16] in H-FFAE1-TPABr, H-FFAE1-HDA and H-FFAE1-PA spectra is further confirmation of the transformation of the amorphous FFAE1 into ZSM-5. Moreover, the bands at 946 and 1060 cm −1 in the FFAE1 spectrum characterize the degree of condensation of SiO − species and correspond to the monomeric or dimeric SiO − species and rings formed by condensation of SiO − species, while the band that appears at 1123 cm −1 in H-FFAE1-TPABr, H-FFAE1-HDA and H-FFAE1-PA spectra correspond to the polymers of SiO − species [44]. The disappearance the monomeric or dimeric SiO − species and appearance of polymerized SiO − species in H-FFAE1-TPABr, H-FFAE1-HDA and H-FFAE1-PA is another indication of the zeolitization of FFAE1.…”