Dehydration of gels

Fig. 2.19 Adsorption isotherm of benzene on (I) hydrated, and (II) dehydrated siliea gel. (After Kiselev .)...

The immersion of glass electrodes in strongly dehydrating media should be avoided. If the electrode is used in solvents of low water activity, frequent conditioning in water is advisable, as dehydration of the gel layer of the surface causes a progressive alteration in the electrode potential with a consequent drift of the measured pH. Slow dissolution of the pH-sensitive membrane is unavoidable, and it eventually leads to mechanical failure. Standardization of the electrode with two buffer solutions is the best means of early detection of incipient electrode failure. 

Many chlorine compounds, including methyl chlorosilanes, such as ClSi(CH2)3, Cl2Si(CH3)2, Cl3Si(CH3) tetrachlorosilane [10026-04-7] SiCl chlorine, CI2 and carbon tetrachloride, CCl, can completely react with molecular surface hydroxyl groups to form hydrochloric acid (40), which then desorbs from the gel body in a temperature range of 400—800°C, where the pores are still interconnected. Carbon tetrachloride can yield complete dehydration of ultrapure gel—siUca optical components (3,23). 

The vapor-phase esterification of ethanol has also been studied extensively (363,364), but it is not used commercially. The reaction can be catalyzed by siUca gel (365,366), thoria on siUca or alumina (367), zirconium dioxide (368), and by xerogels and aerogels (369). Above 300°C the dehydration of ethanol becomes appreciable. Ethyl acetate can also be produced from acetaldehyde by the Tischenko reaction (370—372) using an aluminum alkoxide catalyst and, with some difficulty, by the boron trifluoride-catalyzed direct esterification of ethylene with organic acids (373). 

Dehydration of TSK-GEL columns can result from improper use or during long-term storage. This condition can be remedied by the following procedure. 

An unusual method for the preparation of 3-(trifluoromethyl)-4-aryl-furazans 49a,b in 47-77% yield has been reported (99H627) (Scheme 19). Thus, dehydration of l,l,l-trifluoroalkane-2,3-dione dioximes 48a,b was accomplished on heating with silica gel. If, as in 48b, Ar was an electron-withdrawing moiety, the conversion proceeded more smoothly. The dehydration of the same dioximes using traditional methods failed. 

A new synthetic route for functionalized polyhydroxyalkyl-pyrimidines starting from unprotected aldoses and based on montmorillonite K-10 catalysis and solvent-free microwave irradiation conditions, has been reported by Yadav et al,m Thus, reaction of D-glucose and D-xylose with semicarbazide or thiosemicarbazide (186) in the presence of montmorillonite K-10, under microwave irradiation, proceeded via domino cycloisomerization, dehydrazination, and dehydration of the intermediate semi- or thiosemicarbazones (187) to afford l,3-oxazin-2-ones or l,3-oxazine-2-thiones (188) in one single step and in yields between 79% and 85% (Scheme 34). Other mineral catalysts tested, such as silica gel and basic alumina, were far less effective for this transformation and only silica gel was active at all, giving low yields (15-28%) of compounds 188a-d. The l,3-oxazin-2-ones(thiones) thus synthesized were subsequently converted into the target pyrimidines by reaction with aromatic... 

In the context of preparing analogues of chiral l,2-dimethyl-3-(2-naphthyl)-3-hy-droxy-pyrrolidines, which are known non-peptide antinociceptive agents, Collina and coworkers have reported the solvent-free dehydration of hydroxypyrrolidines to pyrrolines under microwave conditions (Scheme 6.141) [278]. In a typical experiment, the substrate was adsorbed onto a large excess of anhydrous ferric(III) chloride on silica gel and then irradiated as a powder under microwave conditions for 30 min at 150 °C. The microwave method leads to dehydration without racemiza-tion and provides higher yields in considerably shorter times than the conventionally heated process. 

As the gel is very dilute, the temperature sensitivity in this region is likely related to the increased Brownian motion which breaks the weakly bonded structure into discrete floes. However, whether this occurs as a result of changes of particle charge and potential and/or dehydration of the dispersed phase is unknown at this time (19). 

The crude residue is applied to the column head using a minimum of dichloromethane. The submitters use flash-grade (230-400 mesh) silica gel purchased from E. Merck and a column 10 cm in diameter. TLC values for 2 are Rf = 0.18 (hexane acetone = 3 1) and for 3 (see Note 10) are Rf = 0.08 (hexane acetone = 3 1), employing Whatman K6F silica gel TLC plates 60 A. In a typical purification, collecting 200-mL fractions, 2 would elute in fractions 9-18 and 3 in fractions 20-30. The checkers found that immediate purification of the crude residue was necessary. Yields decreased dramatically with time between isolation and purification. Furthermore, the activity (related to the degree of dehydration) of the silica gel greatly affected yields. Only half the amount of silica mentioned above was used by the checkers to get the reported yields. When the full amount was used, the yield decreased to 40-50%. 

Sindorf, D.W. and Maciel, G.E., Silicon-29 nuclear magnetic resonance study of hydroxyl sites on dehydrated silica gel surfaces, using silylation as a probe, J. Phys. Chem.,%1, 5516, 1983. 

On the progressive dehydration of a silica gel the gel rapidly contracts to a certain point. Van Bemmelen s first inversion point after which but little contraction takes place. On continued dehydration the clear gel suddenly becomes cloudy, then opaque, and finally loses its opacity again—Van Bemmelen s second point of inversion, when the water content has sunk to a ratio of one mol. of water to one of silica. 

The volume change occurring on dehydration of a typical silica gel is shown in the following curve ... 

The fundamental property at work is the interaction of water and polymer, hi the Kuhn experiments, the acrylic acid functioned as the contractile unit. A change in pH or ionic strength will either hydrate or dehydrate a gel. This affects the size of the molecule, which in turn causes the molecule to contract or expand. This phenomenon is most pronounced in ionic polymers. 

New high-surface-area zirconia-carbon composites were prepared by a sol-gel method followed by a high-temperature treatment in inert gas.420 The samples proved to be very active and selective in the aromatization of Cg+ alkanes (n-hexane and n-octane). From n-octane, as main products, only ethylbenzene and o-xylene were formed. The catalysts have low acidity due to significant dehydration of the zirconia surface, and high surface area since the carbon matrix prevents sintering of the Zr02 particles. 

Water Regain and Bed Volume Gel chromatography media are often supplied in dehydrated form and must be swollen in a solvent, usually water, before use. The weight of water taken up by 1 g of dry gel is known as the water regain. For G-50, this value is 5.0 0.3 g. This value does not include the water surrounding the gel particles, so it cannot be used as an estimate of the final volume of a packed gel column. Most commercial suppliers of gel materials provide, in addition to water regain, a bed volume value. This is the final volume taken up by 1 g of dry gel when swollen in water. For G-50, bed volume is 9 to 11 mL/g dry gel. 

Water concentration is important in catalyst performance. The catalyst bases are gel structures, and some water is needed to avoid dehydration of the gels. On the other hand, excessive water vapor will... 

With regard to a solubility equilibrium, the fact that vitreous silica behaves like a precipitate of polymeric silicic acid must be caused by the similarity between polymeric silicic acid and the hydrated surface of vitreous silica. Both forms can release silicic acid by hydrolysis and desorption, and likewise both forms are able to adsorb and condense silicic acid by means of silanol groups randomly distributed on their surfaces. Thus, in order to explain equal final states, the only assumption necessary is that the condensates will not attain the degree of dehydration of the bulk of the vitreous silica. The resulting equilibrium then relates to the two-phase system silicic acid—polymeric precipitate, and strictly speaking, this system is in a supersaturated state with respect to vitreous silica, which can be considered as an aged form of silica gel. 

Over the past five years, we and our colleagues have undertaken an extensive study of the acid-catalyzed dehydration of the four isomeric butyl alcohols. In so doing, we compared the performance of crystalline, molecular-sieve acid catalysts (HZSM-5) in a range of crystal sizes (so as to vary diffusion path and active-site concentration) with that of amorphous aluminosilicate (AAS) gels in which the pore size is significantly larger. Our results, which permit the... 

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