Sihca gel surfaces

The mptt-modified siHca gel surface was prepared as follows the synthesized mptt was allowed to react with 5.0 g of the previously activated silica, in dry xylene at 343 K and the mixture was mechanically stirred for 72 h. Then, the sohd was filtered, washed with xylene and acetone until the filtered solution gave no positive test for thiol groups. The sequence of reactions involved in the synthesis of the modified silica is summarized in Fig. 3.4. 

Dimerization of Acenaphthylene in the Crystalline or Solid State Dimerization on Sihca Gel Surfaces ... 

Alkyl aryl ketone photolysis has been also studied on a silica-gel surface.Molecules in the outermost layer are responsible for the reaction, which has been consequently used for the determination of the solid support surface area. The effect of the alkyl chain length on the type 11 photoreactivity of alkyl phenyl ketones on the surface was found to be insignificant. Based on the quantum yield measurements of valerophenone andp-substituted valerophenone photolysis, a sihca-gel surface was reported to be as strong a polar medium as methanol the surface might cause the inversion of the n,n and n,n triplet states. 

A nonreactive solid surface, such as a sihca-gel surface, provides an ordered two-dimensional environment for effecting and controlling photochemical processes. A sihca-gel surface is composed of an interconnected network of polar silanol (Si-OH) and siloxane (Si-O-Si) groups and therefore provides a strong polar reaction medium. Sihca gel has a Kosower s Z value of 88, intermediate between... 

Hasegawa, T., Imase, Y, Imada, M., Kumakura, K., and Yoshioka, M., Surface photoreactions of 2-benzoylcyclopentanones sihca gel surface as a possible field bringing out the latent reactivity of dispersed molecules, / Phys. Org. Chem., 9, 677,1996. 

The photochemical reactions of carbonyl compounds has been thoroughly investigated and the behavior is well estabhshed. Some ketones show different photochemical behavior on a sihca gel surface from that in solution as reported by Hasegawa et al. Scheffer et al. reported that the irradiation of an... 

Eig. 1. Schematics of (a) acid-cataly2ed and (b) base-cataly2ed siHca gels showing the differences in microstmcture and surface functional groups. 

Desiccants. A soHd desiccant is simply an adsorbent which has a high affinity and capacity for adsorption of moisture so that it can be used for selective adsorption of moisture from a gas (or Hquid) stream. The main requkements for an efficient desiccant are therefore a highly polar surface and a high specific area (small pores). The most widely used desiccants (qv) are siHca gel, activated alumina, and the aluminum rich zeoHtes (4A or 13X). The equiHbrium adsorption isotherms for moisture on these materials have characteristically different shapes (Fig. 3), making them suitable for different appHcations. 

Amorphous sihca exists also ia a variety of forms that are composed of small particles, possibly aggregated. Commonly encountered products iaclude sihca sols, sihca gels, precipitated sihca, and pyrogenic sihca (9,73). These products differ ia their modes of manufacture and the way ia which the primary particles aggregate (Fig. 8). Amorphous sihcas are characterhed by small ultimate particle si2e and high specific surface area. Their surfaces may be substantially anhydrous or may contain silanol, —SiOH, groups. These sihcas are frequentiy viewed as condensation polymers of sihcic acid, Si(OH)4. 

Characterization. When siHca gel is used as an adsorbent, the pore stmcture determines the gel adsorption capacity. Pores are characterized by specific surface area, specific pore volume (total volume of pores per gram of solid), average pore diameter, pore size distribution, and the degree to which entrance to larger pores is restricted by smaller pores. These parameters are derived from measuring vapor adsorption isotherms, mercury intmsion, low angle x-ray scattering, electron microscopy, gas permeabiHty, ion or molecule exclusion, or the volume of imbibed Hquid (1). 

Condensation occurs most readily at a pH value equal to the piC of the participating silanol group. This representation becomes less vaUd at pH values above 10, where the rate constant of the depolymerization reaction k 2 ) becomes significant and at very low pH values where acids exert a catalytic influence on polymerization. The piC of monosilicic acid is 9.91 0.04 (51). The piC value of Si—OH decreases to 6.5 in higher order sihcate polymers (52), which is consistent with piC values of 6.8 0.2 reported for the surface silanol groups of sihca gel (53). Thus, the acidity of silanol functionahties increases as the degree of polymerization of the anion increases. However, the exact relationship between the connectivity of the silanol sihcon and SiOH acidity is not known. 

Silica Polymei Metal Ion Interactions in Solution. The reaction of metal ions with polymeric sihcate species in solution may be viewed as an ion-exchange process. Consequently, it might be expected that sihcate species acting as ligands would exhibit a range of reactivities toward cations in solution (59). Sihca gel forms complexes with multivalent metal ions in a manner that indicates a correlation between the ligand properties of the surface Si-OH groups and metal ion hydrolysis (60,61). For Cu +, Fe +, Cd +, and Pb +,... 

For the synthesis of quinolines and isoquinolines the classical approaches are the Skraup and the Bischler-Napieralski reactions. The reaction of substituted anilines with different carbonyl compounds in acid medium has been reported to be accelerated under microwave irradiation to give differently substituted quinolines and dihydro quinolines [137]. Although the yields are much better and the conditions are milder than under conventional heating, the acidity of the medium may prevent the preparation of acid-sensitive compounds. Thus, alternative approaches have been investigated. Substituted anilines and alkyl vinyl ketones reacted under microwave irradiation on the surface of sihca gel doped with InCU without solvent [137] to furnish good yields of quinohnes 213 (Scheme 77). 

The maximum surface density of the reactive silanol groups in a fuUy hydrated state is 8-9 pmol mT (or 5 SiOH nm ). This can be achieved by a hydrothermal activation of sihca gel [48, 49] or, for planar substrates, a solution with hydrogen... 

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