Catalytic reactions dehydrogenation

The main by-products ia the dehydrogenation reactor are toluene and benzene. The formation of toluene accounts for the biggest yield loss, ie, approximately 2% of the styrene produced when a high selectivity catalyst is used. Toluene is formed mostly from styrene by catalytic reactions such as the foUowiag ... 

Hydride species were also formed in the dehydrogenative coupling of hydrosilanes with DMF [45]. The catalytic system is applicable to tertiary silanes, which are known to be difficult to be converted into disiloxanes (Fig. 17). The catalytic reaction pathway involves the intermediacy of a hydrido(disilyl)iron complex... 

Second, catalytic reactions do not necessarily proceed via the most stable adsorbates. In the ethylene case, hydrogenation of the weakly bound Jt-C2H4 proceeds much faster than that of the more stable di-cr bonded C2H4. In fact, on many metals, ethylene dehydrogenates to the highly stable ethylidyne species, =C-CH3, bound to three metal atoms. This species dominates at low coverages, but is not reactive in hydrogenation. It is therefore sometimes referred to as a spectator species. Hence, weakly bound adsorbates may dominate in catalytic reactions, and to observe them experimentally in situ spectroscopy is necessary. 

The presence of solution at a metal surface, as has been discussed, can significantly influence the pathways and energetics of a variety of catalytic reactions, especially electrocatalytic reactions that have the additional complexity of electrode potential. We describe here how the presence of a solution and an electrochemical potential influence the reaction pathways and the reaction mechanism for methanol dehydrogenation over ideal single-crystal surfaces. 

Catalytic dehydrogenation of alcohol is an important process for the production of aldehyde and ketone (1). The majority of these dehydrogenation processes occur at the hquid-metal interface. The liquid phase catalytic reaction presents a challenge for identifying reaction intermediates and reaction pathways due to the strong overlapping infrared absorption of the solvent molecules. The objective of this study is to explore the feasibility of photocatalytic alcohol dehydrogenation. 

Thaller, L.H., and G. Thodos, "The Dual Nature of a Catalytic-Reaction The Dehydrogenation of sec-Buty Alcohol to Methyl Ethyl Ketone at Elevated Pressures", AIChE Journal, 6(3), 369-373, 1960. 

Theoretical studies of catalytic alkane-dehydrogenation reactions by [(PCP )IrH2], PCP rf-C6H3(CH2P112)2-l, 3 and [cpIr(PH3)(H)]+, suggest that they proceed through similar steps in both cases namely (i) alkane oxidation, (ii) dihydride reductive elimination, (iii) /3-II transfer from alkyl ligand to metal, (iv) elimination of olefin.402 The calculated barriers to steps (i), (ii), and (iv) are more balanced for the PCP system than for cp(PH3). 

Ziman et al. [Kinetika i Kataliz, 9 (117), 1968] have studied the kinetics of the catalytic oxidative dehydrogenation of various butene isomers to form 1,3-butadiene. Over a Bi-Mo catalyst the following reactions are important. 

In contrast, a catalytic reaction pair of tetralin dehydrogenation/naphthalene hydrogenation (Equation 13.3) is another choice for stationary applications. Although the storage densities of tetralin are relatively low (3.0 wt%, 28.2 kg-H2/m3), rates of absorbing and desorbing hydrogen... 

At the beginning stage of dehydrogenation, the substrate organic hydride is adsorbed onto the catalyst surface from the liquid phase directly and easily. Catalytic reaction processes will succeed it, until the surface sites are filled with the adsorbed reactant and products. Once product desorption starts to form and grow a bubble, product readsorption becomes unfavorable due to the increment of translational entropy of the product molecule in the bubble, if compared with that in the solution, shifting the adsorption equilibrium for the product and suppressing its effect of rate retardation. 

Remarkably, the same Shvo complex can be used for the catalytic transfer dehydrogenation of aromatic amines to give imines (Scheme 7.14) [80]. This reaction produces high yields when carried out for 2-6 h in refluxing toluene with 2 mol.% catalyst. A quinone is used as the hydrogen acceptor, giving the corresponding hydroquinone. 

Oxidative Dehydrogenation of Ethane. The dehydrogenation of alkanes also occurs, but in a catalytic manner, over molybdenum supported on silica (22,23). In addition to the stoichiometric reactions, the role of the 0 ion in this catalytic reaction is further suggested by the observation that N2O is an effective oxidant at temperatures as low as 280°C, but no reaction is observed at these temperatures with O2 as the oxidant (22). It should be noted that at moderate temperatues N2O gives rise to 0 , whereas O2 yields O2 over Mo/Si02. Under steady-state conditions the rates of formation of C2Hi were in the ratio of 7 1 at 375°C and 3.7 1 at 450°C when N2O and O2 were used as the oxidants, respectively (23). ... 

As discussed in the previous section, metal oxides have both acidic and basic properties. The acid-base properties of metal oxides have led to many interesting catalytic reactions. Catalytic reactions such as H2-D2 exchange, hydrogenation, isomerization, dehydrogenation, dehydrohalo-genation, and benzylation can be considered as examples of acid-base catalysis reactions.31-36 These reactions will be briefly discussed in the following section. The remarkable properties of MgO as a catalyst have been well documented in the literature and we shall discuss some of these unique catalytic properties. 

In this chapter, we have discussed the application of metal oxides as catalysts. Metal oxides display a wide range of properties, from metallic to semiconductor to insulator. Because of the compositional variability and more localized electronic structures than metals, the presence of defects (such as comers, kinks, steps, and coordinatively unsaturated sites) play a very important role in oxide surface chemistry and hence in catalysis. As described, the catalytic reactions also depend on the surface crystallographic structure. The catalytic properties of the oxide surfaces can be explained in terms of Lewis acidity and basicity. The electronegative oxygen atoms accumulate electrons and act as Lewis bases while the metal cations act as Lewis acids. The important applications of metal oxides as catalysts are in processes such as selective oxidation, hydrogenation, oxidative dehydrogenation, and dehydrochlorination and destructive adsorption of chlorocarbons. 

The diagram doesn t show the ethylene and propylene made by metathesis of methanol or the propylene made by catalytic reaction of ethylene and butylene or by dehydrogenation of propane. The volumes are small, and besides, it would make the diagram too messy. 

In the case of iridium, complex [IrH2(PPh3)2(acetone)2] BF4 (11) was the first to carry out catalytically the dehydrogenation of cycloalkanes [13, 14]. However, it was later realized that the halocarbons used as solvents reacted with 11 to produce the stable species [HL2lr(p-Cl)2(. i-X)IrL2H]BF4 (X = Cl (14) or H (15)) [16] (Scheme 13.8), and that elimination of the solvent by running the reactions in neat alkane not only improved yields but also permitted the activation of other previously unreactive cycloalkanes, such as methyl- and ethyl-cyclopentane. However, it was also noted that the system in some cases was not catalytic, due mainly to decomposition of the catalyst at the temperatures employed [16]. 

Recent advances in the convenient synthesis of spinels has witnessed its application for a wide variety of catalytic reactions, such as oxidation, dehydrogenation, acylation, alkylation... 

Other catalytic reactions involving a transition-metal allenylidene complex, as catalyst precursor or intermediate, include (1) the dehydrogenative dimerization of tributyltin hydride [116], (2) the controlled atom-transfer radical polymerization of vinyl monomers [144], (3) the selective transetherification of linear and cyclic vinyl ethers under non acidic conditions [353], (4) the cycloisomerization of (V2V-dia-llyltosylamide into 3-methyl-4-methylene-(V-tosylpyrrolidine [354, 355], and (5) the reduction of protons from HBF4 into dihydrogen [238]. 

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