Heterogeneous catalysis alcohol dehydration

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. 

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. 

Hydrogen gas is adsorbed onto the surface of these metal catalysts, and the catalyst weakens the H—H bond. In fact, if H2 and D2 are mixed in the presence of a platinum catalyst, the two isotopes quickly scramble to produce a random mixture of HD, H2, and D2. (No scrambling occurs in the absence of the catalyst) Hydrogenation is an example of heterogeneous catalysis, because the (solid) catalyst is in a diffo-ent phase from the reactant solution. In contrast, homogeneous catalysis involves reactants and catalyst in the same phase, as in the acid-catalyzed dehydration of an alcohol. 

Another example includes the use of a heated microreactor containing heterogeneous catalysis for the dehydration of alcohols to the associated alkene [22]. The dehydration of 1-hexanol to hexene at 155-160 °C has been carried out using an injection pump (Scheme 5.10). A conversion of 85-95% can be achieved without byproducts. This compares favorably with the 30% conversion achieved by conventional reactors. 

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-... 

The epoxide is used as a monomer for polymer production. The byproduct ethylbenzene alcohol can be dehydrated to styrene, also a monomer for the production of polymers. If isobutane is used, iso-butylhydroperoxide replaces ethylbenzene-hydroperoxide as the oxidant. The byproduct tert-butanol can be converted with methanol to an ether that is an important additive in new environmental friendly gasolines. Complexes of Mo, V, or Ti are used in homogeneous epoxidation catalysis, while heterogeneous Ti02/Si02 catalysts can be used also. The active sites consist of a titanium ion with a fourfold coordination of oxygen in a tetrahedral geometry. Titanium acts essentially as a Lewis acid to activate the 0-0 bond in the hydroperoxide. 

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