Surface silylation

Numerous modifications of chromium-based catalysts have been made through the introduction of various additives, the most effective of which are titanium alkoxides (38,39). These additives apparentiy reduce surface silyl chromate moieties to chromium titanates, which are then oxidized to titanyl chromates. These catalysts offer a better control of the resin molecular weight (39). 

Reactions of alkenes with H-Si(l 0 0)-2 x 1 surfaces have been shown to yield films with one-dimensional (ID) molecular lines through Si-C linkages, contrary to formation of the islands observed on H-Si(l 11). The reaction can be initiated from isolated surface silyl radicals created using the tip of the STM. The STM images showed molecular lines running along and across the dimer rows depending on the chemical constituent of R in the CH2 = CH-R molecules. 

Inorganic salts/bases, slaked lime in, 15 65 Inorganic silicate coatings, 7 200 Inorganic surfaces, silylation of, 22 697-698, 701 Inorganic tellurium compounds, 24 417-422... 

Whatever the initial step of formation of surface silyl radicals, the mechanism for the oxidation of silicon surfaces by O2 is expected to be similar to the proposed Scheme 8.10. This proposal is also in agreement with the various spectroscopic measurements that provided evidence for a peroxyl radical species on the surface of silicon [53] during thermal oxidation (see also references cited in [50]). The reaction being a surface radical chain oxidation, it is obvious that temperature, efficiency of radical initiation, surface precursor and oxygen concentration will play important roles in the acceleration of the surface oxidation and outcome of oxidation. 

The reaction is formally a hydrosilylation process analogous to the homogeneous reactions described in Chapter 5. Scheme 8.11 shows the proposed H—Si(lll) surface-propagated radical chain mechanism [48]. The initially formed surface silyl radical reacts with alkene to form a secondary alkyl radical that abstracts hydrogen from a vicinal Si—H bond and creates another surface silyl radical. The best candidate for the radical translocation from the carbon atom of the alkyl chain to a silicon surface is the 1,5 hydrogen shift shown in Scheme 8.11. Hydrogen abstraction from the neat alkene, in particular from the... 

Low-angle powder X-ray diffractometry. In Figure 1 XRD reflection patterns of a non silylated calcined Si-MCM-41 (a), an one which is pre-silylated (b) and an one silylated with APTES (c) are compared. The calcined and the outer surface silylated MCM-41 show the characteristic reflexions. The intensities of the reflections in the functionalized sample are reduced as expected from an increased scattering for filled pores.9 The unessentially changed d spacings confirm the preserved structure.10... 

Figure 2. DRIFT spectra of MCM-41 a) calcined, b) outer surface silylated, c) silylated in dichloromethane and d) azo dye grafted.

Fig. 7.14. IR spectra (a) of an Si-H groups on the silica surface, correlation diagram (b) between EPR constants (tfiSo(29Sia)) for surface silyl radicals and frequencies of Si-H stretching vibrations of the products of their hydrogenation, and (c) the Si-H group absorption band in H2-loaded silica glass (4) [55], and surface groups =Si-H (1), >Si(H)(OH) (2), and >Si(H)(H) (3) [54].

The typical Phillips catalyst comprises chemically anchored chromium species on a silica support. The formation of a surface silyl chromate, and eventually silyl dichromate [scheme (29)], is significant during the catalyst preparation, because at the calcination temperature chromium trioxide would decompose to lower-valent oxides. Chromium trioxide probably binds to the silica as the chromate initially, at least for the ordinary 1% loading. However, some rearrangement to the dichromate at high temperature may occur. It is incorrect to regard only one particular valence state of chromium as the only one capable of catalysing ethylene polymerisation. On the commercial CrOs/silica catalyst the predominant active species after reduction by ethylene or carbon monoxide [scheme (59)] is probably Cr(II), but other species, particularly Cr(III), may also polymerise ethylene under certain conditions ... 

Fumed silica A-200 (Pilot plant at the Institute of Surface Chemistry, Kalush, Ukraine specific surface area Ascorbic acid (vitamin C) and all-rac-a-Tocopheryl acetate (vitamin E acetate) were used as adsorbates. Folin-Ciocalteu s phenol reagent (Merck) was used to measure the total polyphenolic index. Silica samples with different degree of surface silylation were obtained using gas-phase chemical modification of highly disperse silica (A-200) surface by trimethylchlorosilane.6... 

A comprehensive physicochemical study found that wetting of trimethyl-silylated silica samples by water is determined by the amount of grafted groups.7 In particular, for hydrophilic trimethylsilylated silica (with a degree of surface silylation less than 0.55) in an aqueous medium, the increase of modify-cation degree was accompanied by an increase of the structure restoration ability.7... 

C. Zapilko and R. Anwander, Size-selective Surface Silylation of Cagelike Mesoporous Silica SBA-2 with Disilazane Reagents. Chem. Mater., 2006, 18, 1479-1482. 

The surface silylated zeolites were prepared by suspending the H-ZSM-5 or FeH-ZSM-5 zeolite in n-hexane into which a calculated amount of tetraethyl orthosilicate was added to obtain addition of 1.5 wt. % of Si in the final product (abbrev. SiH-ZSM-5 or SiFeH-ZSM-5). n-Hexane was evaporated and the zeolites were dried and calcined in an oxygen stream at 770 K for 5 hours. To add some Fe cations to the surface of silylated zeolites, the SiH-ZSM-5 was introduced into a FeCl solution, filtered and dried (abbrev. FeSiH-ZSM-5). The characteristics of the parent and modified zeolites are given in the Table. The alkylation of toluene with ethylene was carried out in a vapour phase continuous flow microreactor at atmospheric pressure. Nitrogen as a carrier gas was saturated with toluene to 18.5 vol. %, the toluene to ethylene molar ratio was 3.8. The reaction products were analyzed by an "on-line" gas chromatograph (Hewlett-Packard 5890) with MS and FID detection. 

---