Silica hydration-dehydration

Figure 2. Schematic of hydration-dehydration on a silica surface...

Zeolites have been used as (acid) catalysts in hydration/dehydration reactions. A pertinent example is the Asahi process for the hydration of cyclohexene to cyclo-hexanol over a high silica (Si/Al>20), H-ZSM-5 type catalyst [57]. This process has been operated successfully on a 60000 tpa scale since 1990, although many problems still remain [57] mainly due to catalyst deactivation. The hydration of cyclohexanene is a key step in an alternative route to cyclohexanone (and phenol) from benzene (see Fig. 2.19). The conventional route involves hydrogenation to cyclohexane followed by autoxidation to a mixture of cyclohexanol and... 

Metal oxide catalysts can be classified as oxides of transition elements or as oxides of other typical metals. Typical transition elements include Cr, Fe, Co, Mo, and V, whose oxides catalyze oxidation and reduction reactions by changing the oxidation state of the metal ion. For selective oxidation of hydrocarbons, mixed oxides containing Mo and V are widely used. Oxides of other metals (acidic oxides such as silica and silica-alumina, basic oxides such as CaO and MgO, and amphoteric oxides such as alumina) catalyze acid or base reactions such as alkylation, isomerization, and hydration-dehydration. 

Hydration and Dehydration Reactions. Hydration and dehydration catalysts have a strong affinity for water. One such catalyst is AI2O3, which is used in the dehydration of alcohols to form olefins. In addition to aliunina, silica-alumina gels, clays, phosphoric acid, and phosphoric acid salts on inert carriers have also been used for hydration-dehydration reactions. An example of cm industrial catalytic hydration reaction is the synthesis of ethanol from ethylene ... 

A third classifying quantity relates to the surface structure of silicas, which is characterized by the coordination of surface silicon atoms the resulting functional groups their density, topology, and distribution the degree of hydroxylation the hydration-dehydration behavior the acidic... 

All of the silica spectra exhibit certain common features (a) hydroxyl groups on the surface exist in several different states that absorb at distinct frequencies, (b) the population of these states is dependent on the temperature and gas-phase environment of the pretreatment, (c) hydration-dehydration phenomena are reversible, and (d) physisorption effects depend on hydrogen-bonding ability of the adsorbate. 

The hydration-dehydration behavior of the hydroxyl groups on alumina (28) and titania (33) is similar in many respects to that on silica. However, there are some important differences. Peri and Hannan (28) found three absorption bands instead of one for isolated OH groups after a drastic calcination treatment. 

A third classifying quantity relates to the surface structure of silicas, which is characterized by the coordination of surface silicon atoms the resulting functional groups their density, topology, and distribution the degree of hydroxylation the hydration-dehydration behavior the acidic and basic properties of surface functional groups and their adsorption behavior and chemical reactivity. The pattern of the surface structure in terms of these properties is discussed in the section Current View of the Silica Surface. The following section reviews the methods by which reliable information on these properties is obtained. 

FIGURE 34.10 360 MHz CRAMPS spectra of Cab-O-Sil fumed silica as a function of hydration/dehydration state (weight loss due to drying shown in parentheses). Sharp peak at 0.05 ppm due to a PDMS reference. Taken from reference 5q. With permission. 

As silica gel is heated to progressively higher temperatures water is lost from pores and then by dehydration of surface silanol groups. The hydration-dehydration of the surface is considered in Chapter 6. Here only the changes in the physical structure are considered.. ... 

Triruthenium and triosmium complexes obtained from functionalized alkynes Complexes containing ligands with methoxycarbonyl (or acetato) groups Complexes obtained upon CO insertion into M-C bonds Hydration-dehydration reactions of alkynols promofed by silica or alumina Osmium derivafives obfained from oxygenafed ligands... 

To our knowledge, there are no recent examples for hydration-dehydration processes involving osmium. However, the reactions of Os3(CO)io(NCMe)2 with HSi(OR)3 (R = Me, Et) lead to the trinuclear clusters ( -H)Os3(CO)9 -(RO)2SiOR) and (/r-H)Os3(CO)9 /r-(RO)2Si(OR)2, which could represent models for the interaction of metal clusters with silica. Similar reactions were performed by Ugo and co-workers during their work on surface organometallic chemistry . ... 

Protein recovery via disruption has also been achieved by adsorbing water from the w/o-ME solution, which causes protein to precipitate out of solution. Methods of water removal include adsorption using silica gel [73,151], molecular sieves [152], or salt crystals [58,163], or formation of clanthrate hydrates [154]. In most of the cases reported, the released protein appeared as a solid phase that, importantly, was virtually surfactant-free. In contrast to the dilution technique, it appears that dehydration more successfully released biomolecules that are hydrophilic rather than hydrophobic. 

A common industrial method of a-terpineol synthesis consists of the hydration of a-pinene or turpentine oil with aqueous mineral acids to give crystalline cis-terpin hydrate mp 117°C), followed by partial dehydration to a-terpineol. Suitable catalysts are weak acids or acid-activated silica gel [83]. 

Fig. 23a. Hydration and dehydration reactions of the silica surface and the IR absorption frequencies of the surface species. Chemisorption of water produces surface silanols, which serve as adsorption sites for water, b Reaction of octadecyltrichlorosilane (OTS). Hydrolysis of the chloride group by trace amounts of water in solution to silanol is followed by condensation with surface silanols, resulting in covalent bond formation between the monolayer and the substrate. OTS molecules can also cross-link to form polymeric species during film curing [201]...

It is beneficial in terms of yield to convert an a,/3-unsaturated ketone into the cyclopropyl ketone using dimethyloxosulfonium methylide. The epoxide, formed as above, is rearranged during chromatography on silica gel. The corresponding pent-2-ene-l,5-diols are also formed, presumably through hydration of the cation, and these may be dehydrated to the pyran with p-toluenesulfonic acid (74JCS(P1)1674). 

Ordinarily the chromium binds to the silica by reacting with hydroxyls on a fully hydrated surface, because chromium is impregnated aqueously onto the silica and then calcined. However, a different catalyst results if the chromium attaches instead to a surface already dehydrated by calcining. A large promotional effect, particularly on the termination rate, is obtained (76). To do this the silica is first dehydrated at 900°C, for example, then impregnated with chromium anhydrously so that the surface is not rehydrated. A secondary calcining step at some lower temperature such as 300-600°C then fixes the chromium to the silica. The method is especially effective if the support also contains titania. 

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. 

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