Dual function catalysts and oxides

IV. Cyclization over Dual Function Catalysts and Oxides.311... 

Historically, the isomerization catalysts have included amorphous siUca-aluminas, zeoHtes, and metal-loaded oxides. AH of the catalysts contain acidity, which isomerizes the xylenes and if strong enough can also crack the EB and xylenes to benzene and toluene. Dual functional catalysts additionally contain a metal that is capable of converting EB to xylenes. 

In contrast to this mechanism, the one proposed in our work operates direct from the oxidation state of the alkane feedstock. The same alkyl cation intermediate can lead to both alkane isomerization (an alkyl cation is widely accepted as the reactive intermediate in these reactions) and we have shown in this paper that a mechanistically viable dehydrocyclization route is feasible starting with the identical cation. Furthermore, the relative calculated barrier for each of the above processes is in accord with the experimental finding of Davis, i.e. that isomerization of a pure alkane feedstock, n-octane, with a dual function catalyst (carbocation intermediate) leads to an equilibration with isooctanes at a faster rate than the dehydrocyclization reaction of these octane isomers (8). 

This article will deal with metal-catalyzed cyclization reactions, with reference to oxide and dual-function catalysts. Product cycles may contain five or six carbon atoms. The respective prefixes C5 and will point to the resulting structure (5). The term dehydrocyclization will be applied to reactions that end up with aromatic products the formation of saturated (cycloalkane) rings will henceforth be called cyclization. ... 

The oxides and sulfides formerly used in fluidized beds have been abandoned in favor of the following categories of dual-function catalysts ... 

A major factor in the rapid commercial utilization of catalytic reforming processing for upgrading low octane naphthas, and the production of aromatics from petroleum sources, has been the development of more active and selective dual-functional catalysts. These catalysts contain a very active hydrogenation-dehydrogenation agent such as platinum, in combination with an acidic oxide support such as alumina or silica-alumina. 

Gallucci, F., Kuipers, J.A.M., and van Sint Annaland, M. (2012) Integrated autothermal oxidative coupling and steam reforming of methane. Part 1. Design of a dual-function catalyst particle. Chem. Eng. ScL, 82, 200-214. 

The dual function of the oxidizing tower sets a practical limit to the air flow operating range because the flotation process requires a relatively constant air rate. Therefore, it is not practical to control arsenate formation solely by the intensity of the solution aeration, and a small amount of catalyst is added to promote and control arsenate formation. 

Bis-hydroxylation. Molecular oxygen or air was used as the terminal oxidant in the osmium-catalyzed oxidation of alkenes to form cis-diols with high conversions at low catalyst amount.1298 In a triple catalytic system using H2O2 as the terminal oxidant, a cinchona alkaloid ligand has a dual function—it provides stereocontrol and acts as reoxidant via its IV-oxide. The formation of the latter is catalyzed by a biomimetic flavin component.1299... 

In studies of dual-functional catalysis the mixed catalyst technique has many advantages, two of which are mentioned. (1) It allows separate and independent preparation of each component for example, a platinum preparation can be made in any manner desired in order to obtain a certain platinum activity without regard to what such procedures might do to the acidic properties of the oxide base, this interdependence always being a matter of concern in conventional direct impregnation techniques. (2) A component s relative activity contribution can be flexibly varied in a perfectly known and controllable manner by simply varying its bulk amount in admixture with the other. 

These reactions take place normally in a trickle bed downflow reactor at high hydrogen pressures (1500 to 4000 psi) and temperatures in excess of 600°F. The catalyst is frequently referred to as dual function since it promotes both cracking and hydrogenation. The catalyst base material is usually silica-alumina with metal oxides from groups VI and VIII (nickel, cobalt, molybdenum, tungsten). The active forms are the metal sulfides. Fractionation provides the waxy lubes cuts which are subsequently dewaxed and hydrofinished. 

Often multicomponent catalyst systems are utilized to carry out reactions consisting of two or more active metal components or both oxide and metal constituents. For example, a Pt-Rh catalyst facilitates the removal of pollutants from car exhausts. Platinum is very effective for oxidizing unburned hydrocarbons and CO to H2O and CO2, and rhodium is very efficient in reducing NO to N2, even in the same oxidizing environment. Dual functional or multifunctional catalysts are frequently used to carry out complex chemical reactions. In this circumstance the various catalyst components should not be thought of as additives, since they are independently responsible for different catalytic activity. Often there are synergistic effects, however, whereby the various components beneficially influence each other s catalytic activity to provide a combined additive and multifunctional catalytic effects. 

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