Dehydration of silica

It is generally accepted that the only important polar adsoiption sites on the silica surface are the silanol functions, i.e., hydroxyl groups, that are attached to silicon atoms (2). They can interact with the sample molecules by hydrogen bonding and various physical observations ctAi be used to prove this statement. Complete dehydration of silica by beating, i.e., removal of all surface hydroxyl groups, yields a hydrophobia silica which no longer shows adsorption for unsaturated and polar molecules and is no more wetted by water (15). Chemical modification of the surface hydroxyls such as used in the preparation of chemically bonded phases also eliminates the selective adsorption properties of the silica. ... 

Lewis Acid Sites. Many other mechanisms (66, 85) are best described in terms of the more general concept of aprotic, or Lewis acidity which is defined in terms of the capacity to donate or share pairs of electrons. Aprotic acid sites are commonly derived from the coordinatively unsaturated cations at crystal edges or adsorbed on crystal faces, from deydration of hydroxylated surfaces, and from deamination or deamination and dehydration of silica-aluminum catalysts or similarly treated clays having extensive tetrahedral substitution (130, 132). Formation of Lewis acid sites by deamination or deamination-dehydration is dependent on inversion of the basal oxygens of the aluminum-substituted tetrahedron away from the surface, in order to expose the aluminum (131). 

Supporting the Metallocene, then Treating with the Activator. Because of their method of preparation, silicas have a high concentration of surface hydroxyl groups and complexed water. Normally both hydroxyl groups and water will promote decomposition of a metallocene or metallocene/activator complex. Therefore it is necessary to remove the water either by thermal or chemical dehydration. Full dehydration of silica can occur at 150°C. At this point the silica gel will have a surface that is fully hydroxylated. Even these hydroxyl groups can decompose some types of metallocenes. In most cases the silica gel can be further dehydrated at 400°C (see Fig. 19). 

The sensitivity of Eu " photoluminescence properties to structural changes of the host was explored as a function of time and temperature of dehydration of silica-based gels. Similarities of the Eu final local structure with the one known for glasses obtained by classical melting of oxides were put forward [3]. In fact, the SGM became an optional strategy in the preparation of glasses or when low temperatures are required [5]. 

Fig. 5.16 Surface concentration of hydroxyl groups of silica, as a function of the temperature of dehydration. Data are +, from Fripiat and Uytterhoeven A, from Kiselev and Zhuralev O, from Taylor (cf.

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. 

Catalyst acid properties depend on several parameters, including method of preparation, dehydration temperature, silica-to-alumina ratio, and the ratio of Bronsted to Lewis acid sites. 

The preferred choice of a water-selective membrane up to now has been hydrophilic membranes because of their high water affinity. However, recently Kuhn et al. reported an all-silica DDR membrane for dehydration of ethanol and methanol with high fluxes (up to 20kg m h ) and high selectivities (H20/ethanol 1500 and H20/methanol 70 at 373 K) in pervaporation operation. The separation is based on molecular sieving with water fluxes comparable to well-performing hydrophilic membranes [51]. 

New acid catalyst comprising Keggin-type heteropoly acid supported on mesoporous silica for dehydration of acetic acid... 

The first attempt to synthesize and characterize Kegj -type heteropoly acid supported on various mesoporous silicas and its application to add catalysis in the formation of acetic anhydride via dehydration of acetic acid were described in this study. A variety of characterization techniques such as Na adsorption, TEM and XRD were applied... 

The selectivity for acetic anhydride in the catalytic dehydration of acetic acid could be controlled by the pore size of pure mesoporous silica SBA-15. New acid catalyst comprising Keggin-type heteropoly acid supported on SBA-15 enhanced the activity etfectively when tungstophosphoric acid was highly dispersed on the silica substrate. 

The dehydration of 1-hexanol to hexene was conducted over heterogeneous sulfated zirconium oxide catalyst [19, 138]. The zirconia was treated with sulfuric acid and is known as super acid catalyst, having well documented performance for many reactions [19]. The reaction conditions are notably milder as for other acid catalysts, such as silica-alumina. 

Silica gel. Silica gel is a porous amorphous form of silica (Si02) and is manufactured by acid treatment of sodium silicate solution and then dried. The silica gel surface has an affinity for water and organic material. It is primarily used to dehydrate gases and liquids. 

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. 

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%. 

This method has been applied (M5) for modeling the vapor-phase rate of dehydration of secondary butyl alcohol to the olefin over a commercial silica-alumina cracking catalyst. Integral reactor data are available at 400, 450, and 500°F. Two models considered for describing this reaction are the single site... 

Let us consider the data taken by Laible (LI) on the dehydration of normal hexyl alcohol at 450°F over a silica alumina catalyst. The single- and dualsite surface reaction controlled models applying to alcohol dehydration were discussed in Section V,A,2. We now consider, however, the functional forms given, for example, by Eq. (84), as probably being capable of describing the data, but do not restrict the Ct and C2 plots to a linear pressure dependence as before. Rather, we obtain an empirical pressure dependence from the... 

---