Dehydrogenation of hydrocarbons

Dehydrogenation of hydrocarbons with a multimetallic catalytic composite Catalyst development Pt-Sn-Li 81... 

Beside their use in equilibrium-restricted reactions, CMRs have been also proposed for very different applications [6], like selective oxidation and oxidative dehydrogenation of hydrocarbons they may also act as active contactor in gas or gas-liquid reactions. 

Kineti cs. To date only addition reactions have been reported. These reactions produce products or adducts that are the result of complete addition, or addition and subsequent elimination. An example of the later reaction is dehydrogenation of hydrocarbons on platinum clusters. These addition reactions are in many ways analogs to the chemisorption process on metal surfaces. 

Searching for a better catalyst than platinum to oxidize methanol to COad ° y be a new direction of the catalyst search. If such a catalyst is combined with a good catalyst for COad oxidation to C02> the overall catalytic activity may exceed that of platinum based catalysts. Platinum has catalytic activities for both reactions to some extent. For each reaction, however, platinum is not necessarily the best. Palladium may be a good catalyst to oxidize methanol to COad because it is known as a good catalysts for dehydrogenation of hydrocarbons. 

In catalytic dehydrogenation of hydrocarbons the catalyst activity decays with use because of carbon deposition on the active surfaces. Let us study this process in a specific system. 

Professors Alex Bell and Enrique Iglesia of the University of California, Berkeley have used UV-Vis DRIFTS and Raman spectroscopy to elucidate the role of many catalytic systems ranging from mixed metal oxides to precious metal formulations for applications ranging from dehydrogenation of hydrocarbons to oxidation of alkanes to the role of exposed species on dispersed surfaces. ... 

Early studies with respect to the dehydrogenation of hydrocarbons to alkenes on oxide catalysts indicated that carbonaceous deposits formed in the early stages of the process on the surface of acidic catalysts act as the real active centers for the oxidative dehydrogenation. The hypothesis was later confirmed377 378 and verified by using carbon molecular sieves. With this catalyst 90% styrene selectivity could be obtained at 80% ethylbenzene conversion.379 Various coals for the synthesis of isobutylene380 and activated carbon in the synthesis of styrene381 were used in further studies. 

In organic chemistry, elimination processes are those decompositions of molecules whereby two fragments are split off and the multiplicity of the bonds between two carbon atoms or a carbon atom and a hetero atom is increased. Such a broad definition also embraces the dehydrogenation of hydrocarbons and alcohols which is dealt with in Chap. 2. Here we shall restrict our review to the olefin-forming eliminations of the type... 

One of the most promising methods for controlling the intensity and selectivity of processes is the introduction of various substances into the reaction mixture. Venuto et al. (58) attained a highly selective dehydrogenation of hydrocarbons over cation exchanged zeolite X by conducting the reaction in the presence of NH3. It is also well known that the addition of small amounts of water increases the activity of zeolites for carbo-nium-ion type reactions cracking (59), alkylation (58), isomerization (56,60), disproportionation (60,61,62) and others (56). 

Spinel oxides with a general formula AB2O4 (i.e. the so-called normal spinels) are important materials in industrial catalysis. They are thermally stable and maintain enhanced and sustained activities for a variety of industrially important reactions including decomposition of nitrous oxide [1], oxidation and dehydrogenation of hydrocarbons [2], low temperature methanol synthesis [3], oxidation of carbon monoxide and hydrocarbon [4], and oxidative dehydrogenation of butanes [5]. A major problem in the applications of this class of compound as catalyst, however, lies in their usually low specific surface area [6]. 

All data on the kinetics of the catalytic dehydrogenation of hydrocarbons, amines, and alcohols obtained in our laboratory are expressed by the equation 12) ... 

Table 1 summarizes the experimental results obtained in our laboratory on the kinetics of the normal dehydrogenation of hydrocarbons (hexahydro-aromatics to aromatics, the open chain compounds butylene to butadiene, and ethylbenzene to styrene), of amines to ketimines, and of alcohols to aldehydes or to ketones, respectively, in the presence of metallic or oxide catalysts. Equation (1) was found to apply in all cases. Ko and h are given by... 

Fig. 20. Volcano-shaped curves for the dehydrogenation of hydrocarbons (I) and alcohols (II) and dehydration of alcohols (III) on CrjOs (85). For E and q, the units are kg. calories.

Experimental Activation Energies 1 e Bond Energies of Atoms in the Reacting Molecules with the Catalyst Qak Adsorption Potentials q and the Heights of the Potential Barriers E kg. cat./mole on Chromias of Different Methods of Preparation-, the Subscripts Designate 1—Dehydrogenation of Hydrocarbons, II—Dehydrogenation of Alcohols and Acids, III—Dehydration of Alcohols ... 

Selective oxidations, e.g., propane to acroleine, butane to maleic anhydride, ethylene to ethylene oxide Oxidative dehydrogenations of hydrocarbons Oxidative coupling of methane Methane oxidation to syngas... 

Let us discuss in general gas-phase processes of oxidative dehydrogenation of hydrocarbons involving as reagents substances that easily induce free radical transformations of substrates. Many such substances are known that dissociate to free radicals or induce free radical reactions. However, the most widespread in investigations are compounds that are able to shift dehydrogenation and cracking product ratios toward the first process. 

Skarchenko, V.K., Dehydrogenation of Hydrocarbons, Naukova Dumka, Kiev, 1981, 328 pp. (in Russian). 

Atomic oxygen formation from H202 at conjugated dehydrogenation of hydrocarbons on the reactor walls is of low probability, but to justify the possibility of oxygen participation in dehydrogenation, nitrous oxide decomposition [3] ... 

As follows from a brief consideration of the role of HO radicals in gas-phase oxidation reactions, they are the key active sites in the high temperature range, and studies of homogeneous oxidative dehydrogenation of hydrocarbons allocate the dominating role in unsaturated compound formation to them. 

In this case, the origin of the dehydrogenating effect displayed in the interaction between hydrogen peroxide and the substrate and conjugated dehydrogenation of hydrocarbons in the gas phase is the same. 

Supercritical C02 has also recently attracted much attention as a reaction medium for C-H bond activation, because C02 is miscible with organic compounds, including organometallic compounds, and potentially stable toward alkane activation conditions. We have successfully applied supercritical C02 to the carbonyla-tion and dehydrogenation of hydrocarbons [34], The technique is effective for conversion of gaseous substrates such as methane and ethane [35]. 

Chromium ions at the surfaces of inorganic oxides are characterized by a wide variability of the oxidation state, coordination number, and local structure. Consequently, Cr-based materials are especially attractive as catalysts. Much is known about the catalytic activity of pure Cr203 for various reactions (469), including polymerization of alkenes (470-472), hydrogenation-dehydrogenation of hydrocarbons (473-481), reduction of NO and decomposition of N2O4 (482), and oxidation of organic compounds (483, 484). 

A large number of intermediate pathways arc possible when catalytic reactions interfere with the polymerization-dehydrogenation steps. A common scenario is the catalytic dehydrogenation of hydrocarbons on nickel surfaces followed by dissolution of the activated carbon atoms and exsolution of graphene layers after exceeding the solubility limit of carbon in nickel. Such processes have been observed experimentally [40] and used to explain the shapes of carbon filaments. In the most recent synthetic routes to nanotubes [41] the catalytic action of in situ-prepared iron metal particles was applied to create a catalyst for the dehydrogenation of cither ethylene or benzene. 

Rozanska X, Sauer J. Oxidative dehydrogenation of hydrocarbons by V307+ compared to other vanadium oxide species. J Phys Chem A. 2009 113(43) 11586—94. 

The discussion to this point has emphasized kinetics of catalytic reactions on a uniform surface where only one type of active site participates in the reaction. Bifunctional catalysts operate by utilizing two different types of catalytic sites on the same solid. For example, hydrocarbon reforming reactions that are used to upgrade motor fuels are catalyzed by platinum particles supported on acidified alumina. Extensive research revealed that the metallic function of Pt/Al203 catalyzes hydrogenation/dehydrogenation of hydrocarbons, whereas the acidic function of the support facilitates skeletal isomerization of alkenes. The isomerization of n-pentane (N) to isopentane (I) is used to illustrate the kinetic sequence associated with a bifunctional Pt/Al203 catalyst ... 

Hydrothermal stability. Steam or water vapor is sometimes present in the reaction mixture not necessarily as a reactant or product. It is a convenient form of heat source for endothermic reactions such as dehydrogenation of hydrocarbons. It can also oxidize carbon deposits that deactivate many catalysts. Steam is regularly used in the dehydrogenation of ethylbenzene to produce styrene for the above reasons. 

Much of the research and development activity on inorganic membrane reactors in recent years has been directed toward hydrogenation or dehydrogenation of hydrocarbons, especially the latter, and other reactions involving production or depletion of hydrogen. This can be attributed to a number of factors. 

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