Tetraalkylammonium silicate

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. 

The largest silicate species in the extracts of the concentrated TEA system after 45 min, detected with GPC as a shoulder on the main peak, has a retention volume characteristic for dimer of tetracyclic undecamer (Fig.2). This indicates that the first steps of the TEOS polycondensation processes till the formation of (4) and (4 ) occur with TEA as well. This is in agreement with the proposed model in which TEOS hydrolyses at the TEOS-aqueous interfaces in the vicinity of the alkyl groups of the tetraalkylammonium cations (Fig.l). These alkyl groups favor the formation of the hydrophobic silica surfaces encountered already in the smallest species (l)-(3). The absence of trimers... 

Quaternary amines, such as tetraalkylammonium bromides and hydroxides (the alkyl group being Q to C4) are the typical zeolite templates. Quaternary amines fulfill the above-mentioned requirements of stability, specific interaction with the precursor (electrostatic interaction between quaternary amines and silicate), and easy removal (by calcination). 

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. 

One can quote exceptions to these generalizations. The tetraalkylammonium salts as a class are liquid at temperatures below 300 K. There are liquid electrolytes— produced from dissolving AICI3 into some complex organics—which are liquid at room temperature (Tables 5.3 and 5.4). Above the normal range of 300-1300 K is another set of molten electrolytes, the molten silicates, borates, and phosphates, for which the characteristic temperature range is 1300-2300 K (Tables 5.5 and 5.6). 

The question of solubility is coupled to the charge and the polarity of the SDA. Anionic molecules cannot act as SDAs because the silicate ions that aggregate around the SDA during crystallization are also negatively charged. Neutral molecules can act as SDAs, but must possess a certain minimum polarity. Primary, secondary, and tertiary amines are usually good SDAs, as are cationic species, e.g., tetraalkylammonium ions. For cationic SDAs, the solubility can be influenced by means of the counteranion (e.g., OH, F, CF, Br", PF 6). 

Of particular practical interest are methods based on the conversion of halides into the corresponding tetraalkylammonium salts, thermal decomposition of these salts to give alkyl halides, and GC analysis of the latter [144,145]. Reaction chromatographic methods of determining other anions, such as phosphates [146—148], silicates [149— 151], cyanides and thiocyanides [153] have also been described. 

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. 

Figure 18-2 presents the distribution of silicate species according to the Q-resonance frequency, as a function of dissolution time, for a silica to base ratio (Si02/Cs20) equal to 3. Variations in silicate anion distribution are also induced by differences in alkali metal hydroxide (Wijnen et al. 1990, McCormick 1988), temperature (Groenen et al. 1986) and the presence of organic tetraalkylammonium cations (Hoebbel et al. 1984 Hoebbel et al. 1985). 

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. 

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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