Adsorbents high-silica

The effect from the top is behind the differences in IR spectra of CO adsorbed on various Na-zeolites (Fig. 1). The IR spectrum of CO adsorbed on the high-silica Na-FER shows only one band (centred at 2175 cm 1) that is due to the carbonyl complexes formed on isolated Na+ sites. When the content of Na+ in the sample increases (Na-FER with Si/Al=8), in addition to the band at 2175 cm 1 a new band at 2158 cm"1 appears due to the formation of linearly bridged carbonyl complexes on dual cation sites. The IR spectrum of CO adsorbed on Na-A,which has a large concentration of Na+ cations, shows bands centred at 2163, 2145, and 2129 cm 1 the band at 2163 cm"1 is due to the carbonyl species formed on dual cation sites, while bands at 2145 and 2129 cm"1 are due to carbonyls formed on multiple cation sites (Table 1), i.e., on adsorption sites involving more than two cations. 

A. K. Chakraborti and R. Gulhane, Perchloric acid adsorbed on silica gel as a new, highly efficient, and versatile catalyst for acetylation of phenols, thiols, alcohols, and amines, Chem. Commun., 15 (2003) 1896-1897. 

Ag exchanged zeolite is used to remove iodine compounds. More recently Ag-LZ-210 , an Ag-exchanged zeoUte-Y adsorbent developed by UOP and having a high silica/alumina ratio (Si/Al > 5), has been used commercially to remove iodide from acetic acid streams [250-252]. 

We define zeolites with Si02/Al203 molar ratio > 5 as high silica zeolites. These, typically exemplified by ZSM-5, play an important role as industrial catalysts. Studies to apply them as hydrophobic adsorbents have also been conducted. 

Cyclohexadienones. The perturbation of the electronic absorption spectra by adsorption on silica gel can have a significant effect on subsequent photochemical reactions. For example, Hart has shown that 2,4-cyclo-hexadienones photochemically degrade cleanly in nonpolar solvents to a ketene (1) but in highly polar solvents or adsorbed on silica gel bicyclic ketone (2) is the predominant product.33 The absorption spectra indicate that in nonpolar solvents the lowest singlet state is the n,n state, from which the formation of the ketene proceeds. This n -n band is obscured by the band in polar media, inversion of the energy levels of the n,n, and the first... 

Activities of tri-n-butylammonium and tri-n-butylphosphonium ions with two different spacer chain lengths are compared in Table 8 1I8). The greater activity of the phosphonium ions is opposite to what has been reported for analogous soluble phase transfer catalysts119). Activities of the catalysts bound to silica gel were as high as activities of soluble catalysts adsorbed to silica gel118). Without some independent determination of the role of intraparticle diffusion it is not possible to determine whether the reduced activity of the adsorbed catalysts is due to lower intrinsic activity at the silica gel surface or to diffusional limitations. The size selectivity for alkyl bromides suggests that intraparticle diffusion was not a problem. 

Interesting variations in product distributions could be observed when ozonolysis was carried out with alkenes adsorbed on silica gel.628 The low-temperature ozonolysis on dry silica gel of cyclopentene led to the formation of the normal, monomeric ozonide in high yield [Eq. (9.114)]. This corresponds to the product formed in aprotic and nonparticipating solvents. In contrast, reaction of cyclopentene on wet silica gel resulted in the formation of the corresponding oxo acid... 

On the face of it, the structural sensitivity of high-silica zeolites to the adsorbed species is not altogether surprising. The fact that physical adsorption can cause structural changes in an adsorbent is well known. Bangham et... 

The high silica version of 7.SM-5, also known as silicalite. is a hydrophobic adsorbent capable of adsorbing, e.g.. ethanol from an aqueous solution. 

Ogawa, H. Amano, M. Chihara, T. Facile and highly selective monoacylation of symmetric diols adsorbed on silica gel with MeCOCl. Chem. Commun. 1998, 495—496. 

To gain insight into the effect of physical state and/or molecular organization on lipid oxidation, a variety of model systems have been used. These include dispersions, liposomes or vesicles (37,38), monolayers adsorbed on silica (39,40,41), and red blood cell ghosts (42). In most of these studies, oxidation was conducted at relatively low temperatures, i.e., 20 - 40°C. Very little information is available on the effects of physical state on high temperature oxidative reactions or interactions of lipids. 

The amount of water, which is sorbed on zeolites, can be very significant, up to 25 wt% for Na and H-FAU samples with low Si/Al ratios. This amount depends on the partial pressure of water and on various characteristics of the zeolite, such as the pore system, which determines the micropore volume accessible to water, the framework Si/Al ratio, the nature of the cations and the crystallite size. Thus, a linear decrease was found in the amount of water adsorbed at low pressure (that is, strongly adsorbed) over various protonic high silica zeolite materials MOR,[28] bea[29] or MFI,[30] with a decrease in the framework A1 content. This can be attributed to the decrease in the number of partially ionic, hydrophilic centres associated with the tetrahedrally coordinated A1 atoms at the profit of nearly homopolar (hydrophobic) =Si-0-Si= bonds. A stoichiometry of four water molecules per H(A1) was found, suggesting the formation of H9O4 species. 

A feature of the neutron reflectivity study on polyDMDAAC and surfactant adsorption by Penfold et al. [74] was that the adsorbed layer of polyDMDAAC was remarkably robust and unaffected by the subsequent surfactant adsorption. This is not always the case, and Fielden et al. [76] reported a large increase in the thickness of the surface layer of AM-MAPTC on mica due to complex formation with SDS. Thickness increases with electrolyte and pH were reported for high molecular weight polyacrylamide adsorbed onto silica, measured by null ellip-sometry by Samoshina et al. [82] in the absence of surfactant. Complex formation at the interface, resulting in layer thickening, was also reported by Dedinaite et al. [83] for PCMA/SDS mixtures on mica from AFM measurements. 

A cyclic adsorption process for citrus oil processing in supercritical carbon dioxide (SC-C02) was studied with silica gel adsorbent. Based on the adsorption equilibrium properties, where adsorbed amounts decreased with the increase in the solvent density and oxygenated compounds were selectively adsorbed on silica gel, a continuous cyclic operation between the adsorption step at 8.8 MPa and 313 K, and the desorption step at 19.4 MPa and 313 K was demonstrated Highly concentrated fraction of oxygenated com pounds was continuously obtained for the desorption and blowdown step. The proposed system showed the feasibility of the continuous operation for citrus oil processing. 

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