Functional silica gels

Instead, the aim is to provide a unified picture of the chemistry of functional silica gels. Hence, in place of a complete coverage of what has been done with these immensely versatile materials, an attempt is made to provide readers with an understanding of the principles behind the applications. 

The formation of a sol-gel functional silica gel takes place by hydrolytic polycondensation of suitable precursors in the presence of a dopant in solution. An example is given in the following (unbalanced) equation ... 

SiliCycle manufactures a number of functionalized silica gels to meet the specific requirements of pharmaceutical professionals, and has... 

We tried a different approach to create a supported chloroaluminate IL on silica to be used for the toluene carbonylation reaction. A functionalized silica (2-(2-pyridyl)ethyl-functionalized silica gel (Sigma Aldrich 53,798-5) was reacted with one equivalent of n-propyl chloride in toluene (0.2 M) for 12 h at 140 °C to form the quaternary ammonium chloride." The pyridyl concentration on the silica was 1.3 mmol/g silica per the vendor. Prior to reaction with the -propyl chloride, the solid was evacuated at 150 °C for 1 h. The supernatant liquid was decanted, then the solid was evacuated at room temperature overnight. A small portion of this solid was examined by C-MAS-NMR to determine how well the quaternization reaction had occurred. The spectra of the sample before and after treatment with the -propyl chloride were compared to show the same peaks in both samples 1.42, 12.22, 29.87, 47.81, 110.5, 120.8, 134.9, 147.4, and 163.5 ppm however, the treated sample showed the peak at 110.5 ppm grew after the treatment confirming the success of the quaternization reaction. This... 

Scheme 3.—Immobilization on Functionalized Silica Gel by Means of Glutaraldehyde.

C8- or Ci8-functionalized silica gel, for the extraction of nonpolar products (reversed-phase technique) ... 

The incorporation of organofunctional groups on the silica surface may be effectuated during the synthesis of the silica material. The addition of organofunctional alkoxysilanes to the TEOS solution in the sol-gel process, produces functionalized silica gels. This procedure does not allow a careful control of the obtained surface morphology. Since the relative amounts of silane and TEOS is the only variable parameter, neither layer thickness, nor modification density can be precisely tuned. This results in an irreproducible functionalization of the surface. 

Just as the interiors of micelles in water provide a hydrophobic environment to solubilize chlorobiphenyls and then photodegrade them by reductive dechlorination, so do octadecyl-functionalized silica gel [89], normally used as reverse phase packing in HPLC and for solid-phase extraction, and the hquid-semisohd polydimethylsiloxane [-OSi(CH3)2 -] [90,91], also used for solid-phase extraction. In both media, reductive dechlorination is the primary photochemical pathway. 

Figure 10.5.38 Schematic Representation of the Sapphyrin-Functionalized Silica Gel 10.55 Used to Generate Oligonucleotide-separating HPLC Chromatography Columns...

The solid support was the silyl-capped, sapphyrin-functionalized silica gel shown schematically as structure 10.55 in Figure 10.5.38. Conditions Isochratic buffer consisting of 1.0 M ammonium phosphate dibasic, at 1.5 mL min , pH 7.0, monitored at 260 nm. This figure was redrawn with permission from reference 323... 

HydrosUylation of Ethynylborazines and Their Use for the Formation of a Highly Functionalized Silica Gel... 

Sessler and coworkers have recently developed a high-performance liquid chromatography (HPLC) support with sapphyrin-functionalized silica gel [67]. Oxyanions were found to interact with the sapphyrin unit, and thus they were retained longer compared to an appropriate control support. The separations of adenosine, adenosine 5 -phosphate (AMP), adenosine 5 -diphosphate (ADP) and ATP, and dimers to nonamers of polydeoxyadenylic acids at neutral pH were also achieved. 

Having elucidated the optimum conditions for the Knoevenagel reaction in a flow reactor, a range of other reactions using different activated methylene derivatives and aldehydes (Table 14.3) was conducted. In all cases excellent product purities and yields were obtained. The reaction of benzaldehyde and ethyl cyanoacetate was also performed using 3-(dimethylamino)propyl-functionalized silica gel, 3-aminopropyl-functionalised silica gel, 3-(l,3,4,6,7,8-hexahydro-2H-pyrimidojl, 2-l]pyrimidino)propyl-functionalized silica gel and polymer-supported diazabicyclo[2.2.2]octane, whereby excellent conversions were obtained (> 99.0%) in all cases [37]. 

Alley and Lu (1995) have described a method that combines the extraction of PCBs and their separation from large quantities of fat in biological matrixes by combined supercritical fluid extraction and supercritical fluid cleanup. PCBs were separated from lipids on silica gel, alumina, fiorisil and various functionalized silica gel when eluted with supercritical CO2. 

A novel chiral stationary phase was prepared by immobilization of heptakis(6-azido-6-deoxy-2,3-di-0-phenylcarbamoylated)-jS-CyD onto the surface of amino-functionalized silica gel via multiple urea linkages [65]. 

A wide variety of functionalized silica gels is available, as are many other supports such as alumina, clays and graphite. Many of these other supports are not covered in this volume, but will be in a broader volume on supported reagents (22). 

A rhodium sulfide oxidation catalyst on silica gel has highest activity when the rhodium sites are isolated (112). Mercaptopropyl-functionalized silica gel gives varied amounts of the monomeric and dimeric species in Equation 26. The lower the DF, the higher the activity per rhodium atom for the autoxidation of 1-hexene to 2-hexanone with Cu(II) as a cocatalyst... 

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