Surfactants silicate treatment

Some synthesis methods can be considered to be modifications of the general EISA synthesis. For example, the direct calcination of wet surfactant-silicate gel and some post-synthesis treatments (phase transformation). 

Post-synthesis hydrothermal treatment in salt solution[202] could be a convenient method for pore expansion and silica-wall thickening for improvement of its stability. The pore size and wall thickness vary with the type of anion in the salt and its concentration. The salt effect follows the well known binding strength of the Hofmeister series of anions for the cationic surfactant, N03 > Br > Cl- > SO 2 FT The anion binds with cationic surfactant molecules in solution to shift the equilibrium of surfactant/silicate binding, leading to less surfactant and water in the pore, and hence less pore expansion. 

In this procedure the synthesis mixture is reduced to three basic components silica (solid), surfactant (halide) and hydroxide as mineralizer (tetramcthylammonium hydroxide, TMA-OH), all supplied as the convenient reagents. The key parameter is the essentially stoichiometric 1 1 ratio of surfactant and hydroxide, which allows treatment of the product (MCM-41) as formally a surfactant silicate resulting from interaction between silica and surfactant hydroxide. The latter reagent is not a conveniently available or cost effective one to use. Consequently the most desirable surfactant source is its halide salt. The presented... 

With chemical treatment, the natural surfactants in crude oil can be activated [1384]. This method has been shown to be effective for highly viscous crude oil from the Orinoco Belt that has been traditionally transported either by heating or diluting. The precursors to the surfactants are preferably the carboxylic acids that occur in the crude oil. The activation occurs by adding an aqueous buffer solution [1382,1383]. The buffer additive is either sodium hydroxide in combination with sodium bicarbonate or sodium silicate. Water-soluble amines also have been found to be suitable [1506]. 

A possible mechanism of the ammonia hydrothermal treatment for the acid-made sample is shown below. The predominant interaction between the silica wall and the surfactant of the acid-made products is the weak hydrogen bond interaction through an intermediate counterion (i.e. N03). Such weaker interaction eases the removals of organic template by hot water or organic solvent [6], Thus, when the acid-made materials are subjected to the ammonia hydrothermal treatment, the interactions between the surfactant and silicate framework would be transformed as ... 

It is interesting that similar low-temperature weight loss peaks on the DTG curves were also observed for lamellar silicates templated by CTMA+, but the overall shape of the DTG curves was somewhat different [22]. It is not fully clear why these lamellar materials exhibited surfactant decomposition at such low temperatures. This behavior may be related to either properties of their silicate surface or the structural changes in their framework upon heat treatment, or both of these factors, which would promote the Hoffmann elimination at lower temperature [22]. The second of these possible factors may be related to the stress in... 

Since the disclosure by Mobil of Micelle-Templated Silicate structures called MCM-41 (hexagonal symmetry) or MCM-48 (cubic symmetry) [1,2] many other structures have been synthesized using different surfactants and different synthesis conditions. All of these Micelle-Templated Silicas (MTS) have attracted much interest in fields as diverse as catalysis, adsorption, waste treatment and nanotechnology. MTS materials possess a high surface area ( 1000 m2/g), high pore volume ( 1 mL/g), tunable pore size (18-150 A), narrow pore size distribution, adjustable wall thickness (5-20 A). The silica walls can be doped with different metals for catalytic applications, like Al orTi, for acidic or oxydation reactions, respectively. 

In 2002, Chiang and coworkers[124] developed a new scheme for the confined synthesis of TPA-silicalite nanocrystals. The surfactant cetyltinmethylammonium bromide (CTAMeBr) (in ethanol solution) was added to the single- and double-heated TPA-silicalite precursor sols (SHPS and DHPS), and the mixture was flocculated at a certain pH value to collect the nano-size silicate species in the precursors, and then dried. The dried precursor/surfactant hybrid was pressed into pellets and then steamed in a stainless steel autoclave at 110 150°C for 7 36 h. Finally, the product was calcined to remove the surfactants and TPA. The particle sizes of silicalite-1 produced in this method are about 30 nm. The study indicates that the nanoparticles collected by surfactants already exhibit the structural features of MFI. They crystallize entirely to form silicalite-1 nanocrystals after steam treatment at 110 150 °C. This new solid-phase approach provides a way to synthesize MFI nanocrystals without the problem of separation and collecting nanocrystals from suspension, and it also avoids the large consumption and cost of special mesoporous templates used in the confined-synthesis methods. 

Even milder conditions for template removal can be achieved using supercritical fluids, such as carbon dioxide. " Treatment with supercritical CO2 alone is effective only in removing triblock copolymer templates from templated silicates, however inclusion of an alcohol improves the extraction efficiency for ionic surfactant templates.Washing with methanol modified supercritical CO2 was shown to remove 76-95 % of the surfactant template from MCM-41 and MCM-48 films, powders and SBA-1, SBA-3 powders at 85 Other groups prefer supercritical... 

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Other studies have shown that a related mesoporous silicate can be formed via hydrothermal treatment of a layered silicate such as kanemite and the same alkyltrimethylammonium cationic surfactants [31, 32]. Variation of the synthesis conditions results in a fairly ordered porous material denoted as FSM-16 that has physical properties and diffraction data similar to that of MCM-41 [32]. 

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