Treatment with dimethyldichlorosilane

Reversed-phase TLC also uses the common supporting facilities. In the laboratory, the nonpolar stationary phase can be produced by impregnating the layer (kieselguhr G or silica gel G) with long-chain hydrocarbons or liquid paraffin or by treatment with dimethyldichlorosilane (DMCS). Although commercial RP-TLC plates are available, so far it has been experimented with Ci8 plates only. 

Two materials marketed by Applied Science Lab. were employed, Celite 545 and GAS CHROM Q with particle size distribution from 110 to 140 y and a specific area of about 1 m2.g J. Celite 545 is made hydrophobic by treatment with dimethyldichlorosilane. Impregnation of the stationary phase materials by TBP, TOA, HDEHP, and HD(DiBM)P were carried out as follows a mass of material is placed in contact with a solution of extractant in hexane, the solvent is then evaporated under reduced pressure by means of a Buchi Rotavapor rotary evaporator. Impregnation levels in the final mixture are respectively TBP = 27 %, TOA 25 %, HDEHP = 20 % and HD(DiBM)P = 30 %. 

Some researchers have tried to stabilize the MCM wall by a complete hydrofobization of the surface, replacing every silanol group with a trimethy Isi ly 1 group, using e.g. trimethylchlorosilane of hexamethyldisilazane [9], Although this treatment is very effective in se, it yields a surface that is completely unreactive towards subsequent grafting of transition metals. We therefore present a silylation procedure with dimethyldichlorosilane (DMDCS), which allows - upon hydrolysis - a recreation of surface silanols. 

The reaction of pure silica MCM-48 with dimethyldichlorosilane and subsequent hydrolysis results in hydrophobic materials with still a high number of anchoring sites for subsequent deposition of vanadium oxide structures. The Molecular Designed Dispersion of VO(acac)2 on these silylated samples results in a V-loading of 1.2 mmol/g. Spectroscopic studies evidence that all V is present as tetrahedral Vv oxide structures, and that the larger fraction of these species is present as isolated species. These final catalysts are extremely stable in hydrothermal conditions. They can withstand easily hydrothermal treatments at 160°C and 6.1 atm pressure without significant loss in crystallinity or porosity. Also, the leaching of the V in aqueous conditions is reduced with at least a factor 4. 

The coupling of dimethyldichlorosilane by treatment with lithium in tetrahydrofuran in the presence of a catalytic amount of triphenylsilyllithium affords another convenient method for synthesis of dodecamethylcyclohexasilane. The yields are in the range of 60-70% (76). 

Thermal treatment of 2-pyridone derivatives, such as (76), with dimethyldichlorosilane, triethylamine and chloranil in benzene gives 8-hydroxyquinoline (77) in high yield by oxidative intramolecular [4+2] cycloaddition (Scheme 40) <94CC701). These isoquinolines are used in the synthesis of fully functionalized DEF rings of fiederimycin A. 

Surface treatments for Cab-O-Sil influence the viscosity and thixotropic properties of an adhesive as shown in Figure 3.19 where MS-5 is untreated Cab-O-Sil , TS-720 is treated with polydimethylsiloxane, and TS-610 and TS-530 are treated with dimethyldichlorosilanes. The silica filler treated with polydimethylsiloxane (TS-720) produces greater van der Waals forces of attraction and thus higher viscosity compared to TS-610 and TS-530. ... 

The use of dimethyldichlorosilane as a coupling agent for the grafting of VOx structures on the MCM-48 surface, produces a material that is simultaneously hydrophobic (inmiscible with water) and very active (all V-centers are accessible, even for water molecules and the catalytic activity for methanol oxidation has increased). The VOx surface species are grafted by the Molecular Designed Dispersion of VO(acac)2 on the silylated surface, followed by a calcination in air at 450°C. These hydrophobic MCM-48 supported VOx catalysts are stable up to 500°C and show a dramatic reduction in the leaching of the V-centers in aqueous media. Also the structural stability has improved enormously. The crystallinity of the materials does not decrease significantly, even not when the samples are subjected to a hydrothermal treatment at 160°C and 6.1 atm. pressure. 

These three problems will be dealt with in this presentation the MCM-48 support is prepared by a controlled extraction of the cationic gemini surfactant, in such a way that no thermal post-treatment step is required. Secondly, we present an approach of selective, partial hydrofobization of the silica walls, using dimethyldichlorosilane (DMDCS), rendering it essentially hydrophobic to withstand the water attack, but creating simultaneously sufficient active sites for a subsequent grafting of the surface. Finally, VOx surface species are grafted on the silylated MCM-48 surface, in such a way that leaching is almost completely suppressed. 

The production of polydimethylphenylsilazane and polydimethylphen-ylsilazaboroxane varnishes (Fig.78) comprises the following main stages the coammonolysis of dimethyldichlorosilane and phenyltrichlorosilane and the treatment of the reactive mixture with alkali solution the distilla-... 

The following method has been found satisfactory (Schwartz and Zabin 1966) prepare a 1% solution of dimethyldichlorosilane [(CH3)2SiCl2] in benzene in a fume cupboard. Make sure the glass to be treated is clean and completely dry. Rinse it with the solution several times and then dry it at 250°C. Chlorine is given off during these processes. Protect the hands from the solution. The treatment is now complete and the siliconising can be checked as above. If it is not satisfactory the treatment is repeated, possibly with a higher concentration of dimethyldichlorosilane in benzene. 

Fumed silica, or fumed silicon dioxide, is produced by the vapor phase hydrolysis of silicon tetrachloride in an H2/O2 flame. The reactions are shovm in Chapter 19. Hydrophilic fumed silica bearing hydroxyl groups on its surface is produced by this process. Hydrophobic fumed silica is made by processing fumed hydrophilic silica through in-line hydrophobic treatments such as with silanes, siloxanes, silazanes, and so on [1]. Examples of different types of hydrophobic fumed silica coatings include DMDS (dimethyldichlorosilane), TMOS (trimethoxyoctylsilane), HMDS (hexamethyldisilazane). 

---