Hydrogen Transfer and Dehydrogenation Reactions

The photolytic dehydrogenation of methanol in the presence of Rh catalysts is sensitized by acetone products are CH2O, H2, CH2(0Me)2 and those derived from CH2OH radicals. Binuclear Rh and Pd dppm complexes are also effective. Addition of acetone to the methanol medium changes the gas-phase composition from pure H2 to methane and CO. The first thermal methanol dehydrogenation uses Ru2(0Ad)i,Cl/PEtPh2 as catalyst precursor.  

P (p-anisyl)3 PMePh2, dppe) whose activities decrease with increasing ligand basicity as shown . The hydrogenolysis of aryl bromides by benzyl alcohol is catalysed by a Pd(II) complex under phase transfer conditions (reaction 2) . 

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The highest pyridine-yd-picoline yields with MFI were found with Si02/Al203 ratios in the 150-400 range [10,11,22]. At such low aluminium concentrations it is likely that Brpnsted acid sites, rather than Lewis sites, assist in the Aldol condensation, cyclization, and hydrogen transfer (or dehydrogenation) reaction steps. Discussion of the mechanism of formation of pyridine bases in the vapor phase and the nature of the acid site dates back to the time of Chichibabin and remains a topic for debate [5,27]. 

Hydrogen transfer, addition and dehydrogenation reactions occur among the produced radical species to yield new compounds. These reactions are summarized in TABLE 2-2. 

As with all pyrolytic reactions, CARBONIZATION is a complex process in which many reactions take place concurrently, such as dehydrogenation, condensation, hydrogen transfer and isomerization. The final pyrolysis temperature applied controls the degree of CARBONIZATION and the residual content of foreign elements (e.g. at 1200K, the carbon content of the residue exceeds a mass fraction 90 wt%, whereas at 1600K, more than 99 wt.% carbon is found). 

Heterogeneous catalytic hydrogenation and dehydrogenation reactions proceed in a pathway of elementary steps that involve mass transfer of reactant molecules to the catalyst surface, adsorption and migration of reactant molecules on the catalyst surface to active sites, reaction conversion to the product on catalyst surface and then desorption of product molecules from the catalyst surface. Microreactors in their various geometries have the potential for detailed studies of reactions for multiple... 

The carbon content in continuous, previously stabilized (-500 K in air) carbon fibers is increased progressively by pyrolysis in an inert atmosphere. The high-temperature carbonization process (1400-1650 K) reduces the molecular weight of the resulting fibers, and includes complex reactions (dehydrogenation, hydrogen transfer and isomerization, etc.) that are required to remove the volatile species that may be present (Hj, Nj, Oj, etc.). The pyrolysis temperature determines the degree of carbonization (e.g., the carbon content can exceed 90 and 99 wt% for processes at 1200 and 1600 K, respectively) (UIPAC, 1997). 

L = P(CH3)3 or CO, oxidatively add arene and alkane carbon—hydrogen bonds (181,182). Catalytic dehydrogenation of alkanes (183) and carbonylation of bensene (184) has also been observed. Iridium compounds have also been shown to catalyse hydrogenation (185) and isomerisation of unsaturated alkanes (186), hydrogen-transfer reactions, and enantioselective hydrogenation of ketones (187) and imines (188). 

Cracking, isomerization, and hydrogen transfer reactions account for the majority of cat cracking reactions. Other reactions play an important role in unit operation. Two prominent reactions are dehydrogenation and coking. 

However, the pattern is complicated by several factors. The sugar molecules to be hydrogenated mutarotate in aqueous solutions thus coexisting as acyclic aldehydes and ketoses and as cyclic pyranoses and furanoses and reaction kinetics are complicated and involve side reactions, such as isomerization, hydrolysis, and oxidative dehydrogenation reactions. Moreover, catalysts deactivate and external and internal mass transfer limitations interfere with the kinetics, particularly under industrial circumstances. 

Ethylene dehydrogenation was poisoned by oxygen, and direct hydrogen transfer reactions between water and oxygen and between methanol and oxygen were observed. 

Sym-octahydrophenanthrene (HgPh) would be expected to follow the same rearrangement-dehydrogenation reactions as Tetralin, except with more isomer and product possibilities. The reactions shown in Figure 1 illustrate the many structures expected from sym-HgPh in the presence of free radical acceptors. Unlike Tetralin, hydrophenanthrenes have multiple structures which each, in turn, form various isomers. The amounts of these isomers are dependent upon the type of hydrogen-transfer reactions and the environment of the system. 

With propene, n-butene, and n-pentene, the alkanes formed are propane, n-butane, and n-pentane (plus isopentane), respectively. The production of considerable amounts of light -alkanes is a disadvantage of this reaction route. Furthermore, the yield of the desired alkylate is reduced relative to isobutane and alkene consumption (8). For example, propene alkylation with HF can give more than 15 vol% yield of propane (21). Aluminum chloride-ether complexes also catalyze self-alkylation. However, when acidity is moderated with metal chlorides, the self-alkylation activity is drastically reduced. Intuitively, the formation of isobutylene via proton transfer from an isobutyl cation should be more pronounced at a weaker acidity, but the opposite has been found (92). Other properties besides acidity may contribute to the self-alkylation activity. Earlier publications concerned with zeolites claimed this mechanism to be a source of hydrogen for saturating cracking products or dimerization products (69,93). However, as shown in reaction (10), only the feed alkene will be saturated, and dehydrogenation does not take place. 

Thus, the role of zinc in the dehydrogenation reaction is to promote deprotonation of the alcohol, thereby enhancing hydride transfer from the zinc alkoxide intermediate. Conversely, in the reverse hydrogenation reaction, its role is to enhance the electrophilicity of the carbonyl carbon atom. Alcohol dehydrogenases are exquisitely stereo specific and by binding their substrate via a three-point attachment site (Figure 12.7), they can distinguish between the two-methylene protons of the prochiral ethanol molecule. 

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