Macroporous silicas

Polymerization of triphenylmethyl methacrylate in the presence of a chiral anion catalyst results in a polymer with a helical structure that can be coated onto macroporous silica [742,804). Enantioselectivity in this case results from insertion and fitting of the analyte into the helical cavity. Aromatic compounds and molecules with a rigid nonplanar structure are often well resolved on this phase. The triphenylmethyl methacrylate polymers are normally used with eluents containing methanol or mixtures of hexane and 2-propanol. The polymers are soluble in aromatic hydrocarbons, chlorinated hydrocarbons and tetrahydrofuran which, therefore, are not suitable eluents. 

Morphosynthesis of macroporous silica frameworks in bicontinuous microemulsions. Advanced Materials, 10, 151—154. 

The chiral recognition ability of the insoluble (+)-l was estimated by HPLC using a column packed with small particles of l.25 However, this column showed a poor efficiency because of a low theoretical plate number. This defect was overcome by coating soluble poly(TrMA) with a DP of 50 on macroporous silica gel.26 The 1-coated silica gel had higher resistance against compression and longer lifetime than the CSP of insoluble 1. Moreover, the two 1-based CSPs show quite different chiral recognition for several race-mates, which may be attributed to the different orientation of 1 in bulk and on the surface of the silica gel.27... 

In contrast, the use, in chromatography, of poly(trityl methacrylate) appears much more promising. Both the insoluble polymer and macroporous silica gel coated with a soluble polymer have been used. The latter system gives better results, especially with regard to elution time. The columns have proved quite efficient in resolution of a great variety of chiral organic compounds (365, 388). Other examples of usefiil chiral polymer supports are the substituted polyacrylamides (389). Earlier used adsorbents obtained by reacting optically active amines with polyacryloyl chloride have been superseded by new chiral phases prepared by direct polymerization of optically active acrylamides. 

This results in almost the same specific surface area as found in capillary packed with macroporous silica beads. Next, the heparin was covalently immobilized on the surface of an etched capillary through a spacer using silane chemistry. 

In section 6.3.1 it was described, that macroporous silica particles are veiy effective carriers for lipase inunobilization, provided that the pore diameter is larger than about 500A. For industrial use the particle size catmot be less than 200-300 m. 

The BET specific surface area [28] was calculated in the relative pressure range between 0.04 and 0.2. The total pore volume was determined from the amount adsorbed at a relative pressure of 0.99 [28], The primary mesopore volume and external surface area were evaluated using the as-plot method [24, 28, 29] with the reference adsorption isotherm for macroporous silica [29], The pore size distributions were determined using the Kruk-Jaroniec-Sayari (KJS) equation [30] and the calculation procedure proposed by Barrett, Joyner and Halenda (BJH) [31]. 

A different semisynthetic method involves the acylation of an amino alcohol with a peptide ester and the resulting amino alcohol is subsequently oxidized to the aldehyde 40 The acylation of H-Phe[CH2OH] with the peptide ester Z-Ala-Ala-Leu-OMe is carried out in 5% DMF/MeCN with the subtilisin distributed on the surface of macroporous silica gel. The resulting peptide alcohol is oxidized under mild conditions using anhydrous dimethyl sulfoxide and 20-fold excess of acetic anhydride with purification via flash chromatography 40] Z-Phe[CH2OH] has been oxidized under these conditions and the optical rotation indicates little epimerization as compared to literature values 11 40 ... 

Zhang proposed the chromatographic partitioning of cesium from HN03 (4 M) to implement the calixarene MC9 with TBP immobilized on a macroporous silica-based polymeric composite.104 The authors do not explain the choice of the calixarene MC9, more difficult to synthesize than classical dipropoxy or octyloxy calix[4]... 

D Ru on mesoporous silica E Pd on microporous alumina F Macroporous silica/alumina G Pt on mesoporous polystyrene... 

Okamoto, Y., Kawashima, M., Yamamoto, K., and Hatada, K. (1984) Useful chiral packing materials for high-performance liquid chromatographic resolution. Cellulose triacetate and tribenzoate coated on macroporous silica gel, Chem. Lett., 739-742. 

Structure Same structure as cellulose trobenzoate (coated on macroporous silica gel)... 

Somewhat surprisingly, the Pd-bppfa complex 24 tethered to a macroporous silica was shown to be moderately active for the enantioselective hydrogenation of ethyl nicotinate (25) (a rare example of a homogeneous Pd-catalyzed hydrogenation), but ee s were very low (Scheme 15.1).10a... 

Figure 1.6. Zero coverage energy of adsorption of n-alkanes versus carbon number Nc, on three microporous solids (and two others, for comparison). Molecular sieve carbon, after Carrott and Sing (1987) Silicalite after Canott and Sing (1986) NaX and macroporous silica, after Kiselev (1967) non-porous carbon, after Carrott and Sing (1987) and Avgul and Kiselev (1965).

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