Silica thermal treatment

However, it should be said in passing that the pore size and surface area of the silica, which can be critical for certain LC applications, is controlled by the conditions of gelling, the subsequent washing conditions and any ensuing thermal treatment. 

The composition of silica depends on the method of preparation and thermal treatment (21,39,42). Three temperature... 

Alumina is not widely used in modem HPLC [48]. Porous gamma alumina is prepared by dehydration and thermal treatment of crystalline bayerite [8,49]. It is available in several types with pore diameters from 6-lS nm, surface areas 70-250 m /g and pore volumes 0.2-0.3 ml/g. After conditioning with acid or base its apparent surface pH can be adjusted between pH 3-9. The alumina surface is more heterogeneous than silica containing both hydroxyl... 

First attempts to incorporate pre-formed magnetite colloids within alginate/silica nanocomposites via a spray-drying process have been described, but formation of lepidocrocite y-FeOOH and fayalite Fe2Si04 was observed, attributed to Fe2+ release during the aerosol thermal treatment [53],... 

It was also shown that thermal treatment of an ammonium zeolite under steam causes not only framework dealumination, but also a structural rearrangement in the zeolite framework. The defect sites left by dealumination are filled to a large extent by silica, which leads to a very stable, highly silicious framework (11,12) (Figure IB) Defect sites not filled by silica are occupied by "hydroxyl nests" (13). 

In one approach, polymethacrylate-type monoliths have been fabricated by copolymerization of the chiral monomer 0-9-[2-(methacryloyloxy)ethylcarbamoyl]-10,11-dihydroquinidine 1 or 0-9-(tert-butylcarbamoyl)-ll-[2-(methacryloyloxy) ethylthio]-10,ll-dihydroquinine 2 (see Figure 1.34a), the comonomer 2-hydroxyethylmethacrylate (HEMA), the crosslinker ethylenedimethacrylate (EDMA) in presence of the binary porogenic solvent mixture cyclohexanol and 1-dodecanol, directly in a single step within fused-silica capillaries. Initiation of the polymerization by either thermal treatment or UV irradiation yielded microglobular polymer morphologies, such as those well known from their corresponding nonchiral... 

The reaction between Os3(CO),2 and silica and alumina has been the object of different studies. Appropriate thermal treatment under argon of the OssjCOjn initially physisorbed leads to [Os3(CO)io( t-H)( t-OSi)] or [Os3(CO)io(n-H)(p-OAl)] surface species that are similar to those obtained from RusjCOjn [129, 130]. An increase of temperature can produce osmium oxidation by the hydroxyl groups and the breaking of the Os-Os bonds, resulting in a surface carbonyl species of Os(II), Os (CO) ( = 2 or 3), surface anchored fragments that in the case of alumina have been characterized by EXAFS [131, 132]. 

Early studies of the interaction of lr4(CO)i2 with a silica surface indicate that physisorption of the cluster takes place. Although the cluster can sublime during thermal treatments after impregnation [198], the loss of metal carbonyl can be avoided by mild thermal treatments that produce a redispersion of the physisorbed lr4(CO)i2 onto the silica surface [199]. An XPS and FTIR study of the evolution of physisorbed lr4(CO)i2 under different conditions pointed to the formation of metallic particles by mild thermal decomposition under Ar or H2, with the particle size increasing with increasing temperature [200]. 

Physisorbed [Co2(CO)g] is converted into physisorbed [Co4(CO)i2] on the surface of silica by a mild thermal treatment (40 °C) [97], such as in petroleum ether solution [98], but the yield was not determined. 

Immobilization of the resulting Chirasil-Dex stationary phases on the fused silica surface by thermal treatment represents another refinement of the methodology130 and such a phase has been employed for the enantiomer separation of a series of /-lactones (Figure 16). 

Strength. The fracture strength of vitreous silica depends on its surface quality, which can be affected by thermal treatment and handling conditions. Microcracks, surface contamination, and crystallization can reduce the strength from the value of pristine vitreous silica by several orders of magnitude. 

Studies with sulfated zirconia also show similar fast catalyst deactivation in the alkylation of isobutane with butenes. It was found, however, that original activities were easily restored by thermal treatment under air without the loss of selectivity to trimethylpentanes. Promoting metals such as Fe, Mn, and Pt did not have a marked effect on the reaction.362,363 Heteropoly acids supported on various oxides have the same characteristics as sulfated zirconia.364 Wells-Dawson heteropoly acids supported on silica show high selectivity for the formation of trimethylpentanes and can be regenerated with 03 at low temperature (125°C).365... 

The solid so obtained was introduced in two different sealed refractory crucibles and immersed in a coke bed. One of the crucibles was submitted to the following thermal treatment in an electric furnace 20-1000 "C at 5"C/min. heating rate, followed by 3 h soaking time at this temperature, obtaining a porous solid composite called SC-100. The other crucible was treated in equal conditions but varying the final temperature to 1550 °C, obtaining in this case another porous composite called SC-155. The SC-155 showed a little volumetric expansion respect to the SC-100 material. (The names SC-100 and SC-155 means S= silica C= carbon and the number is the treatment temperature in °C/10). 

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