High-temperature applications dehydrogenation

The excellent separation properties of silica membranes prepared at temperatures as high as 825°C enables their use for high temperature applications, such as the dehydrogenation of H2S (chapter 8). Unfortunately no hydrothermal stability of the prepared layers could be tested because the mesoporous intermediate layer was not hydrothermally stable, but an indication of the hydrothermal stability of the unsupported material could be obtained from the specific surface area and XRD measurements. These measurements did not show any structural change in the material during SASRA treatment, which is a very hopeful result for the operation of real, supported, membranes at high temperatures and high pressures. 

One of the most studied applications of Catalytic Membrane Reactors (CMRs) is the dehydrogenation of alkanes. For this reaction, in conventional reactors and under classical conditions, the conversion is controlled by thermodynamics and high temperatures are required leading to a rapid catalyst deactivation and expensive operative costs In a CMR, the selective removal of hydrogen from the reaction zone through a permselective membrane will favour the conversion and then allow higher olefin yields when compared to conventional (nonmembrane) reactors [1-3]... 

The vapor-phase dehydrogenations just mentioned are applicable only to the preparation of aldehydes that tolerate such high temperatures. A catalytic oxidation of alcohols carried out by passing a current of air or oxygen through solutions of alcohols in solvents such as heptane [56] or ethyl acetate [55] in the presence of platinum [55, 56], or, better still, platinum dioxide [56], or active cobalt oxide [1136 is applicable even to alcohols that cannot be vaporized and that contain double bonds (equation 205) [56]. 

Reactions of 1,2-diaminoalkenes with alcohols, aldehydes, or carboxylic acids and derivatives at high temperatures in the presence of a dehydrogenating agent are common approaches to the synthesis of 2-substituted imidazoles <89KFZ1246>. In the absence of the dehydrogenating agent the products are imidazolines. The method is particularly applicable to the synthesis of purines from... 

The catalytic dehydrogenation of light alkanes is, potentially, an important process for the production of alkenes, which are valuable starting chemical materials for a variety of applications. This reaction is endothermic and is, therefore, performed at relatively high temperatures, to improve the yield to alkenes, which is limited, at lower temperatures, by the thermodynamic equilibrium. Operation at high temperatures, however, results in catalyst deactivation (thus, requiring frequent reactivation), and in the production of undesired by-products. For these reasons, this reaction has been from the beginning of the membrane reactor field the most obvious choice for the application of the catalytic membrane reactor concept, and one of the most commonly studied reaction systems. 

Oxides and sulfides such as Cr203 and MoS2 also catalyse dehydrogenation, although only at rather high temperatures 123 they also catalyse cyclization of saturated and unsaturated aliphatic to aromatic compounds.124 Chemical methods of dehydrogenation are applicable in a large number of... 

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