Tetraalkylammonium silicate solutions

Silicon. The dynamics of silicate exchange in highly alkaline silicate solutions have been investigated using Si NMR spectroscopy. Si NMR spectroscopy has been used to study tetraalkylammonium silicate solutions. The composition of water glass solution has been investigated using Si NMR spectroscopy. 

Synthesis of MCM-41 with Additives. The hydrothermal crystallization procedure as described earlier [10] was modified by adding additional salts like tetraalkylammonium (TAA+) bromide or alkali bromides to the synthesis gel [11]. Sodium silicate solution ( 14% NaOH, 27% Si02) was used as the silicon source. Cetyltrimethylammonium (CTA) bromide was used as the surfactant (Cl6). Other surfactants like octadecylltrimethylammonium (ODA) bromide (C,8), myristyltrimethylammonium (MTA) bromide (C,4) were also used to get MCM-41 structures with different pore diameter. Different tetralkylammonium or alkali halide salts were dissolved in little water and added to the gel before addition of the silica source. The final gel mixture was stirred for 2 h at room temperature and then transferred into polypropylene bottles and statically heated at 100°C for 4 days under autogeneous pressure. The final solid material obtained was washed with plenty of water, dried and calcined (heating rate l°C/min) at 560°C for 6 h. 

Harris, R.K, and Knight, C.T.G. (1982) Silicon-29 NMR Studies of Aqueous Silicate Solutions. Part, IV. Tetraalkylammonium hydroxide solutions, J. Mol. Struct, 78,273-278. 

The structure of species in solution depends on numerous parameters (pH, temperature, nature of the cation of the ba.se, Al/Si ratio, etc.) and remains uncertain. The interpretation of AI and Si NMR spectra is complicated and uncertain because of the high lability of Al-O-Si bonds, 2-3 orders of magnitude higher than that of Si-O-Si bonds [96,97]. In the presence of alkaline cations (Na, Cs ), the complexes [(H0)3A10Si(0H)20Si03H3 J - - " and [(HOlaAlOSiOjHj-j are detected in. solution. With tetraalkylammonium cations, cages similar to those observed in silicate solution were found with different substitution ratios of silicon by aluminum. 

A shift in the silicate equilibrium towards DnR species upon substitution of large organic cations such as tetraalkylammonium (TAA) for the alkali. These silicates are still present in solution at 90 °C, i.e., close to zeolite formation temperatures. 

The effect of tetraalkylammonium ions has not been clearly determined yet. It could be due to the particular structuring role of this type of ions on the solvent, and/or to a polarization effect on the anions. In the presence of [N(CH3)4]", 80% of silicates exist in solution as the double cycles of four silicon atoms ([Sig02o] )-The addition of Na swiftly replaces the cubic units with simple [Si4Qi2]" cycles. Transverse bonds between cycles are broken, probably because of the strong affinity of the solvated Na for the basic oxygen in the silicate. 

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