Silica gel in alkali metal hydroxides

Figure 2. Normalized rate of dissolution of amorphous silica gel in alkali metal hydroxides as determined from the initial, integrated peak area of dissolved species (5 wt % silica suspensions M20 3SiC>2 I8OH2O). Other experimental details are given in the caption of Figure 1.

In 1992, R.M. Laine (University of Michigan, Ann Arbor) announced the development of a process that transforms sand and other forms of silica into reactive silicates that can be used to synthesize unusual silicon-based chemicals, polymers, glasses, and ceramics. The Lame procedure produces pentacoordinate silicates directly from low-cost raw materials—silicon dioxide,ethylene glycol, and an alkali base. The mixture is approximately a 60 1 ratio of silica gel, fused silica (or sand) to metal hydroxide and ethylene... 

Because the only variable changed in this dissolution study was the type of alkali metal hydroxide, differences in dissolution rate must be attributed to differences in adsorption behavior of the alkali metal cations. The affinity for alkali metal cations to adsorb on silica is reported (8) to increase in a continuous way from Cs+ to Li+, so the discontinuous behavior of dissolution rate cannot simply be related to the adsorption behavior of the alkali metal cations. We ascribe the differences in dissolution rate to a promoting effect of the cations in the transport of hydroxyl anions toward the surface of the silica gel. Because differences in hydration properties of the cations contribute to differences in water bonding to the alkali metal cations, differences in local transport phenomena and water structure can be expected, especially when the silica surface is largely covered by cations. Lithium and sodium cations are known as water structure formers and thus have a large tendency to construct a coherent network of water molecules in which water molecules closest to the central cation are very strongly bonded slow exchange (compared to normal water diffusion) will... 

This phenomenon was confirmed by the introduction of symmetric tetraalkylammonium hydroxides in the dissolution of silica gel. In TMAOH the observed rate of dissolution was slow compared to the rate observed for cesium hydroxide dispersions, and cesium hydroxide has the lowest rate for the different alkali metal hydroxides. Results in Figure 3 clearly reveal an inhibition time between mixing of the silica gel with the aqueous TMAOH and the onset of dissolution. This observation is attributed to the strong interaction of the rather apolar TMA cation with the negatively charged silica gel surface. Because in this case no hydration shell is present, dissolution only occurs very slowly. The observed inhibition period of the dissolution reaction can be related to specific interactions of TMA cations with relatively large oligomeric species of the monomeric... 

Yamazaki and Kawai reported a study on the reaction of HCHO with acetonitrile or propionitrile using silica-supported metal salts or hydroxides as catalysts. Formalin is used as the source of HCHO. The performances are summarized in Table 15. It is concluded that silica-supported alkali metal hydroxide catalysts show the best performances. The optimum loading of alkali metals is in the range of 0.01 to 0.1 mol/60 g of silica gel. The optimum reaction conditions are nitrile/HCHO molar ratio of 5, temperature of 500 °C, and contact time of 2.5 x 10 s-g-cat/mol. The single-pass yields of acrylonitrile and methacrylonitrile are 75 and 65 mol%, respectively, based on the charged HCHO (25 and 22 mol% based on the charged nitrile) with a nitrile/HCHO molar ratio of 3. The reaction rate is first order with respect to the concentrations of both nitrile and HCHO. 

In a study [6] of the dissolution of amorphous silica gels in aqueous alkali metal hydroxides, the rate of dissolution was found to depend on the cation used in the dissolution reaction. A maximum in dissolution rate was found for potassium hydroxide solutions, whereas both intrinsically smaller and larger cations (lithium-sodium and rubidium-cesium) showed slower dissolution rates, as can be concluded from the concentration of dissolved silicate species (normalized peak areas) as a function of alkali metal cation (Figure 45.2). This result is contradictory to the expectation that a monotonic increase or decrease in dissolution rate is to be observed for the different cations used. One major effect that occurs at the high pH values of this study is that the majority of silanol... 

Reaction of acetone with formaldehyde in the gas phase passing over lead zeolite or alkali metal hydroxide-impregnated silica gel at 200 to 300 °C [280] ... 

Catalysts. - The catalysts and sources of HCHO that appeared in patents are listed in Table 6. Solid bases such as hydroxides of alkali and alkaline earth metal supported on a support such as silica gel or aluminosilicate have mainly been claimed in patents to be effective as the catalysts. In addition, another type of compounds, which possess acidic property as well as basic property, are also claimed in patents, for example, PbO, Mn02, AI2O3, metal phosphates, metal borates, multicomponent oxides containing V, Nb, W, and Mo. 

As catalysts for the condensation reactions, two types of compounds with opposite properties show the best performances. One is hydroxides of alkali and alkaline earth metal supported on silica gel, aluminosilicate, or the like. They are generally considered to be typical bases. However, it should be noted that these hydroxides are much less active when they are not supported on a support, and that there exists an optimum value in the activity with the variation in the loading of these hydroxides. These findings led... 

Another bond used in basic castables and for producing acid resistant alumino-silicate castables involves the use of alkali silicates, either sodium silicate or potassium silicate. Alkali silicates will react with acids, salts, and metal hydroxides and stiffen or set by formation of a silica hydrogel. This gel will dewater continuously as temperature increases with complete dehydration at 350°C. Setting agents used to set alkali silicate bonded castables include sodium silicofluoride, aluminum polychloride, sodium phosphate, aluminum polyphosphate, magnesium polyphosphate, and calcium and magnesium hydroxides (13). 

For practical (real) catalyst systems, precipitation, ion exchange, impregnation and sol-gel processing procedures are used. In precipitation methods, a hydroxide or a carbonate of a metal may be precipitated from a solution of a metal salt onto the support material held in the solution. Thus, a copper-silica catalyst may be prepared using a Cu-nitrate solution in which silica is suspended. Additives of any alkali cause the precipitation of copper hydroxide onto the silica support. This is then dried and normally reduced in hydrogen at moderate temperatures ( 400-500 °C) to form the catalyst. In co-precipitation techniques , the support is precipitated simultaneously with the active catalyst. In the ion-exchange method, for example, highly dispersed Pt on... 

---