Silica gels surface-modified

Molecular weight distribution (MWD) is most important factor to determine the mechanical property and processability of polyethylene (PE). The control of MWD is one of the main targets for development of new generation polymerisation catalyst. In this work, a novel Si02-supported Ziegler-Natta catalyst designed in a molecular level on the silica gel surface modified with linear long alkene chains was developed for ethylene polymerisation. The coordinative nature of alkyl-Al co-catalyst should be expected to vary with its structure. The catalysts modified by various alkyl-Al co-catalysts were studied by Al MAS method, and the relationship between the coordinative states of A1 species and the MW and MWD of PE produced was clarified. ... 

Nevertheless, silica gel is the material of choice for the production of the vast majority of LC stationary phases. Due to the reactive character of the hydroxyl groups on the surface of silica gel, various organic groups can be bonded to the surface using standard silicon chemistry. Consequently, the silica gel surface can be modified to encompass the complete range of interactive properties necessary for LC ranging from the highly polar to almost completely dispersive. 

Attachment of the Fe(n) complex of tetraphenylporphyrin to a modified silica gel support also resulted in an efficient dioxygen carrier (Basolo, Hoffman Ibers, 1975). In this system the silica gel was modified by direct attachment of imidazoyl-propyl groups to its surface. 

This contribution will deal mainly with recent photophysical studies of the behavior of aromatic hydrocarbons on silica gel and modified silica gel surfaces. 

Hydrated silica gel was modified with APTS (sample 1) and studied by DRIFT (Diffuse Reflectance Infrared Fourier Transform) and CP MAS NMR. The IR spectrum of the modified silica (figure 9.4) shows silane NH, CH and Si-O-Si bands along with silica lattice and surface vibrations. Assignments of IR bands of APTS modified silica are given in table 9.2. 

The CSC precursor build-up has been studied after modification of the silica gel surface from the gas phase. This gas phase modification involves the deposition of one molecular layer at the time. For thicker coatings, a cyclic procedure is needed. Liquid phase modification of the silica surface may also yield valuable ceramic precursors. The precursor molecular structure and layer thickness is controlled by other parameters compared to gas phase procedures. Parameters such as reaction solvent, silane concentrations and presence of water are of primal importance. Those have been discussed in detail in chapter 9. In this chapter, the application of silica modified with aminosilanes, will be discussed. The aminopropylsilica is used as a prototype compound for the production of ceramics by liquid phase chemical surface coating. 

Highly structured mesoporous silicas using a surfactant template provide a well characterized, ultra-high surface area material.5 In this volume Jaroniec provides an excellent summary of various novel materials based on this chemistry for Hg2+ adsorption. In this report we discuss results from a recent study utilizing two types of silica gels and a nanoparticulate fumed silica, all surface modified with complexing functional groups, for the adsorption of Cu2+ and Pb2+. 

The use of silica gel surfaces to modify photoprocesses continues to be of interest. The present report deals with photoaddition of alkenes to the steroidal enone (44).It is presumed that, in the adsorbed state on silica gel, the steroid will present the normally hindered side, i.e., the g-face is attacked by the alkene and the a-face is adsorbed on to the gel. The photoadditions were compared with those carried out in methanol. Thus with allene the two photoproducts (45) and (46) were obtained in 90 and 10% yields, respectively, at -78 °C in methanol, while on silica gel, both at -78 °C and at ambient temperature, the same two products were obtained in 46 and 53% yield. Similar results were obtained using ethene and cyclopentene. The photocycloaddition of ethyne and propyne to progesterone and testosterone in solution yields the cycloadducts (47) and (48), with a preference for the former mode of addition being observed. These results are in conflict with those of de Mayo and co-workers. ... 

Figure 8. Dependence of In Vn, and — AH values on the number of carbon atoms of the modifying group bonded to the silica gel surface for cymarin from water-ethanol (7 3) eluent.

In this chapter investigations using excireer formation with pyrene (Py) and a number of its derivatives adsorbed on dry and modified silica gel surfaces are discussed. These studies were carried out to obtain information on the properties of the silicas from the surface behaviour of the probe molecules. The preferential use of pyrenes for investigations of silica gel (1-3) and other oxide materials (3-9), finds its origin in the fact that Py possesses a number of distinct advantages as a fluorescent probe (10-15) ... 

The adsorptive selectivity of the gels may be influenced to a certain extent by the preparative conditions. One may, for instance, produce silica gels which adsorb a specific dye of characteristic molecular shape and charge distribution. This is done by dispersing this dye in the silicate solution and the precipitating the gel in the presence of the dye [2, 4, 9]. Similar experiments have been carried out with optically active compounds ([3], see also [1]). The silica gel surface may also be modified in a specific way by adsorbing on it appropriate substances this yields preparations with completely new adsorptive properties [14, 19]. 

Such modified surface was then used as a substrate for the adsorption of the luminescent complex Eu(fbd)3.2H20 (fod is the anion of the 2,2-dimethyl-6,6,7,7,8,8,8-heptafluoro-3,5-octanedione). Eu(fod)3.2H20 and the sUicon hybrid were mixed in stoichiometric amormts to produce samples with 1%, 5%, and 10% (m/m) of adsorbed complex. The adsorbed complex was then studied from the spectroscopic point of view. The measured lifetimes for the emission process are 476, 542, and 566 )Is for the 1%, 5%, and 10% samples, respectively. Furthermore, comparison with experimental data for -[3(trimethoxysilyl) propyl]-ethylenediamine-modified silica gel surfaces [8] emphasizes the prominent role of the chemical composition of the silica-modified surface in the spectroscopic properties of the adsorbed complex. 

ROCHA, R. R ROSATTO, S. S. BRUNS, R. E. and KUBOTA, L. T. Factorial design optimization of redox properties of methylene hlue adsorbed on a modified silica gel surface. J. Electroanal. Chem. 433, 1997, pp. 73-76. 

This reaction has been modified extensively to proceed under different conditions. For example, it has been performed in liquid crystalline matrixes, sg in zeolite,in supercritical C02, on a silica gel surface, or in the presence of cyclo-dextrin. In addition, this reaction has been catalyzed by photoacid. Moreover, this reaction has been extended to various substrates other than the regular phenyl (or aryl) esters as described earlier. 

The inorganic materials involve the silica, montmorillonite, allapulgite, FCjO, ZnCl, CuCl, etc. The as-prepared composites can absorb versatile organic and inorganic pollutants from aqueous solutions. Different silica crystalline forms were utilized to fabricate various PANI/silica composites for water purification. PANI were coated on silica gel to modify its surface morphology and to improve its adsorption capacity for phenol [35] and acid green 25 [36] adsorption from aqueous solutions. 

Physical adsorption and chemical immobilization techniques were used for the modification of silica gel surface by 1-aminoanthraquinone to produce three modified silica gel phases. These modified phases were studied for their metal sorption and extraction properties. The chemically modified silica phases were applied for selective extraction and preconcentration of Cu(II) and Cr(III) from seawater samples. ... 

Here the surface of silica gel is modified by the introduction of quaternary amine groups. These groups are permanently positively charged thus, they attract negatively charged analytes (anions). 

Scheme 22.11 Asymmetric aldol reaction in the presence of proline immobilized in the ionic layer covering a modified silica-gel surface.

Silica gel surfaces are modified for reagents and catalysts with silane coupling agents as in Scheme Each silane could be bound to the SG surface with one, two, or three Si-O bonds,... 

Asymmetric catalysis is an important technique for the synthesis of chiral compounds. The introduction of supported IL catalyst into the field of asymmetric catalysis might offer new approaches to improve the catalytic performance and also the reusabiUty of chiral catalysts. The first example of a supported IL asymmetric catalyst is the proUne-catalyzed aldol reaction [116]. In this work, the IL molecule covalently attached to modified silica gel was used as the support for IL-phase containing L-proUne. The modification of the silica gel surface by the IL molecule is crucial to gain high enantioselectivity. In the model reaction of acetone and benzaldehyde, the yield to 4-hydroxy-4-phenylbutan-2-one was 51% with 64% ee. Otherwise, the yield was only 38% with 12% ee without the silica gel modification. 

The stationary phase in LC is a fine granular solid such as silica gel. It can be used as such (mainly for nonpolar compounds), or the granules can be modified by a surface-bonded coating that changes (reverses) the polarity of the gel. A very small selection of stationary phases is listed in Table 35.2. 

---