Organic base silicates

Since silica dissolves above about pH 10.7-11.0, silicates can be prepared with organic bases having dissociation constants greater than about 10 , although some silica will dissolve in aqueous solutions of weaker bases with constants as low as lO or 10 (123-125). Merrill and Spencer (124) reported the preparation of a number of water-soluble quaternary ammonium silicates by grinding silica gel with a solution of the free base. However, the compounds all appear to have a ratio of 2 1 when expressed as SiOj (NR )jO by analogy with the alkali metal system. 

In the case of the interaction of tetramethylammonium hydroxide with colloidal silica, there is evidence that, unlike NaOH, this quaternary ammonium base does not allow the colloidal and ionic species of silica to equilibrate readily. This is probably because the (CHj)4N ions are strongly adsorbed on the surface of the colloidal particles and retard dissolution of Si(OH)4. This was described by Beard (39), who examined mixtures of colloidal silicas with (CHj)4N OH (TMA) or NaOH with equivalent molar SiOj NajO ratios from 0.5 to 3.25 by infrared spectroscopy. This method does not require diluting the sample or otherwisc disturb-ing the equilibrium. Silica concentrations were 13.3, 15. and 20 wt. %. Mixtures were aged for up to 6 days to reach equilibrium. 

Polymeric or colloidal species could be detected in sodium silicate solutions by an absorption band at about 1120 cm . At a ratio of 2.0 a peak at about 1025 cm is assumed to be due to dimer, but is more likely HSiO, . At a ratio of 0.67, where 

However. (CHj)4N- - silicate solutions differ from the sodium silicate solutions in that even at 1.0 ratio, colloid still persists along with HSiOj . Likewise, in 3.3 ratio solutions, there is much more colloid in the (CH3)4N based solutions than with sodium base. The authors conclude that TMA cation shifts the equilibrium to a mixture of low and high species. It is likely that the colloid phase is stabilized by an adsorbed monolayer of (CH3)4N . 

It will be noted that in these experiments the solutions were made from TMA hydroxide and colloidal silica. It is possible that if TMA had been added to a dilute solution of polysilicic acid of low molecular weight at a ratio of 3.3 and then vacuum evaporated to 10% silica, solubility equilibrium would become established and the distribution of silica species would be like that in sodium silicate solutions. 

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In the field of pharmaceuticals, an unusual type of organic base silicate is methionine mcthylsulfonium silicate, which has the advantage of being nonhygro-... 

Enichem made one of the most important steps forward in the development of general heterogeneous oxidation catalysts in the early 1990s with the commercialization of titanium silicate (TS-1) catalysts. TS-1 has a structure similar to ZSM-5 in which the aluminium has been replaced by titanium it is prepared by reaction of tetraethylorthosilicate and tetra-ethylorthotitanate in the presence of an organic base such as tetrapropy-lammonium hydroxide. This catalyst is especially useful for oxidation reactions using hydrogen peroxide (Scheme 4.11), from which the only byproduct is water, clean production of hydroquinone being one of the possibilities. 

Method 3052. Microwave Assisted Acid Digestion of Siliceous and Organically Based Matrices, http //www.epa.gov/epawaste/hazard/testmethods/sw846. 2012. Available at http //www.epa.gov/epawaste/hazard/testmethods/sw846. Accessed June 3, 2013. 

PEBBLEs are water-soluble nanoparticles based on biologically inert matrices of cross-linked polymers, typically poly(acrylamide), poly(decylmethacrylate), silica, or organically modified silicates (ORMOSILs), which encapsulate a fluorescent chemo-sensor and, often, a reference dye. These matrices have been used to make sensors for pH, metal ions, as well as for some nonionic species. The small size of the PEBBLE sensors (from 20 to 600 nm) enables their noninvasive insertion into a living cell, minimizing physical interference. The semipermeable and transparent nature of the matrix allows the analyte to interact with the indicator dye that reports the interaction via a change in the emitted fluorescence. Moreover, when compared to naked chemosensors, nanoparticles can protect the indicator from chemical interferences and minimize its toxicity. Another important feature of PEBBLEs, particularly valuable in intracellular sensing applications, is that the polymer matrix creates a separate... 

Basic silicate solutions were prepared by using silicic acid (ex Baker, dried at 350 °C) and a solution of the organic bases [25 wt % tetramethylammonium hydroxide (TMAOH), 40 wt % tetra-ethylammonium hydroxide (TEAOH), 20 wt % tetrapropylammonium hydroxide (TPAOH) ex Fluka] and, optionally, dimethyl sulfoxide (DMSO). A solution of hexamethonium hydroxide [ (MejNCgH NM ) (OH) 2] was prepared from the bromide salt (ex Sigma) and A O. 

First, although the use of bulky organic bases clearly shifts the silicate equilibrium to the DnR species, there may be a large amount (up to more than 90%) of polymeric species present in silicate solutions. This is true especially at low OH/Si ratios (<0.5) or high Si concentrations (>2), i.e., normal values for a zeolite synthesis composition. This range of polymeric silicates cannot at present be characterized satisfactorily, and the presence of zeolite precursor species other than DnR silicates in this range cannot be excluded. 

Organic base (ethyl silicate) lough hard excellent bond Good Poor Poor Good Excellent Excellent Easy... 

In contrast to the chemistry of living organisms based on systems of linked C atoms there is the chemistry of silicates based on Si atoms linked via O atoms. We shall note some of the more important differences between C and Si at the beginning of Chapter 23. Here we note only one other feature of carbon chemistry which differentiates this element from silicon, namely, the presence of multiple bonds in many simple molecules, a characteristic also of nitrogen and oxygen. 

Figure 10.10. Schematic diagram of the enhanced protonation of an organic base (prometryne) at pH 8 in the presence of layer silicate clays.

In 1982, workers from Union Carbide (ref. 4) announced the synthesis of a new microporous oxide system based on the aluminophosphates. The synthesis of this system resulted from the application of an organic base to an aluminophosphate gel followed by hydrothermal treatment. The resulting aluminopRosphate structures have indeed been shewn capable of isomorphous substitution (refs 5-8) and the range of structures and substituting elements continues to grew at a nuch faster rate than that for the conventional silicate systems. The substituted aluminophosphates have been shown to be catalytic and could provide a complementary series of shape selective catalysts. 

Figure 5.9 A typical procedure used for the microwave digestion of siliceous and organically based matrices [10].

SW-846 EPA Method 3052 (1998) Microwave assisted acid digestion of siliceous, and organically based matrices, in Test Methods for Evaluating Solid Waste, Rev. 0, Environmental Protection Agency, Washington, DC. 

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