Heterogeneous alcohol dehydration

In this Section, we consider examples of application of semiconductor sensors in investigation of heterogeneous catalytic reactions of dehydration of isopropyl alcohol and dissociation of hydrosine on zinc oxide. 

In conclusion, we note that the appearance of hydrogen atoms in the gas volume in catalytic reaction of dehydration of alcohol at low pressures observed in [25] by the sensor technique confirms that dehydration of alcohol on the surface of the zinc oxide catalyzer yields hydrogen atoms. In other words, this heterogeneous reaction does not result in production of hydrogen molecules through the process... 

The steric effects may be more pronounced in heterogeneous catalysts than in homogeneous reactions in solution. The rigid, solid surface restricts the approach of the reactants to the active centers and interaction between the reactants. The steric requirements are quite stringent when a two-point adsorption is necessary and when, in consequence, the internal motion of the adsorbed molecules is limited. In this way, the stereoselectivity of some heterogeneous catalytic reactions, for example, the hydrogenation of alkenes on metals (5) or the dehydration of alcohols on alumina and thoria (9), have been explained. 

The analogy between homogeneous and heterogeneous eliminations and substitutions has been pursued further. Joint action of an acidic and a basic site, suggested quite early for the heterogeneous dehydration of alcohols [6], has been gradually accepted as a general mode of operation... 

In general, dehydration means loss of water molecules from chemical substances, irrespective of their structure. Even if all cases where water is bonded in hydrate form are excluded, a number of reactions remain which also include formation of nitriles from amides, lactones from hydroxy acids etc. However, the present treatment will concentrate on the heterogeneous catalytic decomposition of alcohols in the vapour phase, which can be either olefin-forming or ether-forming reactions, and on the related dehydration of ethers to olefins. 

Although theoretical and computational advances now afford powerful insights into the mechanisms of heterogeneous catalysis, especially on acidic, zeolitic solids (6a-d), experimental studies (7, 8) still hold sway. This we hope to demonstrate here by reference to the wide range of techniques—spectroscopic, kinetic, and analytical—that we have brought to bear in our studies of the catalytic dehydration of butyl alcohols. 

Typical acid-catalyzed reactions like the dehydration of alcohols and double bond shifts in olefins have been mentioned occasionally as reactions catalyzed by organic heterogeneous catalysts. An extensive kinetic study of the dehydration of tertiary butyl alcohol over pyrolized polyacrylonitrile has been describ-... 

This chapter has focused on heterogeneous catalysis in supercritical media, but the relationship between supercritical fluids and catalysis is much broader. There have been numerous studies of homogeneous catalysis in SCFs. Examples include hydroformylation via cobalt carbonyl complexes in supercritical CO2, oxidation via metal salts dissolved in supercritical water, and acid-catalyzed dehydration of alcohols in supercritical water. 

Many steroid secondary alcohols are resistant to acid-catalysed dehydration, but can be converted into olefins by elimination reactions of their sulphonate esters, usually the toluene 3 -sulphonate ( tosylate ) or methanesulphonate ( mesylate ). These elimination reactions always accompany attempted nucleophilic substitution of the sulphonate (p. 41), and can be made to predominate by appropriate choice of reaction conditions. Activated alkaline alumina has been used to prepare A - and AH-olehns from the sulphonates of 3 - [131] and i2a-alcohols [132]. In the latter case the naphthalene-i-sulphonate was most effective, and the methanesulphonate distinctly less so, suggesting that steric acceleration occurs with the more bulky naphthalenesulphonate [ 132c]. This view is strengthened by the enhanced efficiency of elimination of a i2a-sulphonate in the presence of a 17a-methyl group, which increases the compression at C(i2) [132b]. The mechanism and stereochemical features of these heterogeneous reactions have not been described in detail. 

The mechanism by which proton acids catalyze the dehydration of primary and secondary alcohols in water is not perfectly well understood (1). There is universal agreement that the dehydration of tertiary alcohols can be explained by an El mechanism (1,2) involving either a II complex ( ) or a symmetrically solvated carbonium ion (4) as the key reaction intermediate. Although an occasional text ( ) also describes the dehydration of primary alcohols by an El mechanism, authoritative reviews (1/4) conclude that a concerted E2 type mechanism is more probable. The dehydration behavior of secondary alcohols is presumed to be similar to primary alcohols (4). Discussions of the gas phase dehydration of alcohols by heterogeneous Lewis acid catalysts admit more possibilities. In their authoritative review Kut, et al. (1) consider E1-, E2-, and ElcB-like mechanisms, as well as the possible role of diethyl ether as a reaction intermediate, but they reach no conclusion concerning the relative importance of these mechanisms in the formation of olefins from alcohols. 

R. L. Banks takes up the subject of olefin metathesis previously discussed by J. J. Rooney and A. Stewart in Volume 1 and gives an authorative review of the very substantial literature which has appeared in the last four years. Naturally his account covers both heterogeneous and homogeneous catalysis and he summarizes as well the industrial applications which have been made to date of metathesis reactions. S. Malinowski and J. Kijeriski review the specialist field of very highly basic catalysts largely developed by the work of the Polish school. In their chapter they discuss the evidence for the nature of catalysts such as alkali-treated magnesium and other oxides and the kind of reactions that take place thereon. J. M. Winterbottom in a chapter with emphasis on the literature since 1973 concentrates mainly on the dehydration of alcohols as the fundamental studies on dehydration far exceed those on hydration, which features mainly in the patent literature. His chapter dis-... 

Shell subsequently developed a heterogeneous, silica-supported titania catalyst [11,12] which forms the basis of the commercial process for the epoxidation of propylene with ethylbenzene hydroperoxide. The co-product alcohol is dehydrated, in a separate step, to styrene. Ti(IV)Si02 was the first truly heterogeneous epoxidation catalyst useful for continuous operation in the liquid phase. 

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