Selectivity hydrocarbon reactions

C2.7.6.7 SHAPE-SELECTIVE HYDROCARBON REACTIONS CATALYSED BY ZEOLITES... 

Composite aluminophosphate-type compositions are synthesized with interfaces that are contiguous with a common crystal framework structure with compositional heterogeneity between layers in the crystal. Applications are claimed for selective hydrocarbon reactions. ... 

Organic hydroperoxides can be prepared by Hquid-phase oxidation of selected hydrocarbons in relatively high yield. Several cycHc processes for hydrogen peroxide manufacture from hydroperoxides have been patented (84,85), and others (86—88) describe the reaction of tert-huty hydroperoxide with sulfuric acid to obtain hydrogen peroxide and coproduct tert-huty alcohol or tert-huty peroxide. 

Polyeyelie aromatic hydrocarbons, however, might undergo selective addition reactions if the change in resonance energy is small. 

Post-riser hydrocarbon residence time leads to thermal cracking and non-selective catalytic reactions. These reactions lead to degradation of valuable products, producing dry-gas and coke at the expense of... 

Catalysis zeolites possess acid sites that are catalytically active in many hydrocarbon reactions, as we shall discuss in Chapter 9. The pore system only allo vs molecules that are small enough to enter, hence it affects the selectivity of reactions by excluding both the participation and formation of molecules that are too large for the pores. 

Data accunnilated in the last years on the Ft/Cu alloys, in particular on the 1) surface composition, 2) electronic structure, 3) adsorption properties, 4) catalytic behaviour and 5) various side effects, make a detailed discussion possible of the catalytic selectivity and mechanism of hydrocarbon reactions. 

Greater durability of the colloidal Pd/C catalysts was also observed in this case. The catalytic activity was found to have declined much less than a conventionally manufactured Pd/C catalyst after recycling both catalysts 25 times under similar conditions. Obviously, the lipophilic (Oct)4NCl surfactant layer prevents the colloid particles from coagulating and being poisoned in the alkaline aqueous reaction medium. Shape-selective hydrocarbon oxidation catalysts have been described, where active Pt colloid particles are present exclusively in the pores of ultramicroscopic tungsten heteropoly compounds [162], Phosphine-free Suzuki and Heck reactions involving iodo-, bromo-or activated chloroatoms were performed catalytically with ammonium salt- or poly(vinylpyrroli-done)-stabilized palladium or palladium nickel colloids (Equation 3.9) [162, 163],... 

Another, highly selective oligomerisation reaction of ethene should be mentioned here, namely the trimerisation of ethene to give 1-hexene. Worldwide it is produced in a 0.5 Mt/y quantity and used as a comonomer for ethene polymerisation. The largest producer is BP with 40 % market share utilizing the Amoco process, formerly the Albemarle (Ethyl Corporation) process. About 25 % is made by Sasol in South Africa where it is distilled from the broad mixture of hydrocarbons obtained via the Fischer-Tropsch process, the conversion of syn-gas to fuel. The third important process has been developed by Phillips. 

The activity of elemental carbon as a metal-free catalyst is well established for a couple of reactions, however, most literature still deals with the support properties of this material. The discovery of nanostructured carbons in most cases led to an increased performance for the abovementioned reasons, thus these systems attracted remarkable research interest within the last years. The most prominent reaction is the oxidative dehydrogenation (ODH) of ethylbenzene and other hydrocarbons in the gas phase, which will be introduced in a separate chapter. The conversion of alcohols as well as the catalytic properties of graphene oxide for liquid phase selective oxidations will also be discussed in more detail. The third section reviews individually reported catalytic effects of nanocarbons in organic reactions, as well as selected inorganic reactions. 

One Interesting and industrially important process developed recently,"the cyclar process" is the catalytic aromatization of light (C3-C5) hydrocarbons over pentasil based catalysts. These new classes of solids have been widely studied (1-9). These Investigations led to the conclusion that the catalysts consisting of gallium, zinc, Pt, and modified H-ZSM-5 were more active and more selective towards aromatics than the parent H-ZSM-5 zeolite. The formation of aromatics from light alkanes comprised several main hydrocarbon reactions alkane... 

Table 2. Activity and selectivity of Ga203 in hydrocarbon reactions at 773 K (the pressure in torr indicated in parentheses)...

Kinetics There have been few comprehensive studies of the kinetics of selective oxidation reactions (31,32). Kinetic expressions are usually of the power-rate law type and are applicable within limited experimental ranges. Often at high temperature the rate expression is nearly first order in the hydrocarbon reactant, close to zero order in oxygen, and of low positive order in water vapor. Many times a Mars-van Krevelen redox type of mechanism is assumed to operate. 

The desired products are hydrocarbons in the C5 to C10 range that can be used in gasoline production. Iron-, cobalt- and nickel-based catalysts plus the proper selection of reaction temperatures and pressures are used to control product output. Increasing residence time in the reactor yields more paraffinic products and reduces the formation of alcohol and acid. 

Oxides are widely exploited as catalysts for the selective oxidation of hydrocarbons. They provide lattice oxygen in selective oxidation reactions and exchange it with oxygen gas (e.g. from air in the reactant stream). The periodic lattice oxygen loss for the hydrocarbon oxidation occurs because of reducing gases, despite the presence of gas phase oxygen in the reactant stream. This results in the formation of anion vacancies, local non-stoichiometry and defect structures as discussed in chapter 1. 

The release and uptake of oxygen without essentially changing the basic bulk structure of the oxides are desirable in catalysis (Gai 1993) and efforts are in progress to use these oxides as catalysts in selective oxidation reactions of hydrocarbons. 

Recent data on other alloys confirm the overall classification presented above, but at the same time lead to some refinement of the picture. For example, the most diluted Pt-Au alloys revealed isomerization, identified as running via 3C intermediates. This evidence was obtained (248) by establishing the fact that pentane isomerizes on most diluted Pt-Au alloys with 100% selectivity, whereas this molecule can only isomerize via the 3C complexes. This conclusion has been confirmed by the isotopic labeling method (269). It is therefore reasonable to assume that this isomerization can also proceed on isolated Pt sites, as can a part of dehydrocyclization and the dehydrogenation. We must conclude on the basis of this information that on metals like Pt, the fast multisite and the slow one-site mechanisms of hydrocarbon reactions may operate in parallel with each other. 

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