Selective oxidative dehydrogenation kinetics

Heteroatom Oxidation, Dehydrogenation Electrooxidative kinetic resolution of rac alcohols mediated with a catalytic amount of an optically active A-oxyl was performed in an undivided cell at constant current conditions. A high enantiomeric purity for the recovered alcohol was found, which could be increased by electrolysis at lower temperatures. The optically active A-oxyl was recovered and used repeatedly without change in efficiency and selectivity [368]. Cyclovoltammetry with the A-oxyl (GR, 7S, 10/f)-4-oxo-2,2,7-trimethyl-10-isopropyl-l-azaspiro[5.5]undecane-A-oxyl as catalyst showed for rac-1-phenylethanol a highly enhanced catalytic... 

The kinetics of the electrochemical oxidation of ammonia on platinum to dinitrogen in basic electrolytes has been extensively studied. In the widely supported mechanism originally suggested by Gerischer and Mauerer[ll], the active intermediate in the selective oxidation to N2 is a partly dehydrogenated ammonia adsorbate, NH2 ads or NHaatomic nitrogen adsorbate N ag, which is apparently formed at more positive potentials, is inactive toward N2 production at room temperature. Generally, only platinum and iridium electrodes exhibit steady-state N2 production at potentials at which no sur-... 

The oxidative dehydrogenation of methane to ethane and ethene (CH4 + O2 —> C2 s + H2O) shows great potential for methane utilization if the selectivity is controlled t. The use of a catalyst to control the kinetics of the reaction has been examined, with some success. 

Process studies on conversion of m-butenes to butadiene by oxidative dehydrogenation over unspecified catalysts were reported by Kolobikhin and co-workers (133) and by Alkazov and co-workers (134). These studies show high selectivities to butadiene, and present a certain amount of kinetic information. 

There are several examples of investigations of oxidation processes, involving radiotracers. Many substances can be partially oxidized by O2, if selective catalysts are used to control kinetically the oxidation process, otherwise the partial oxidation products will react further resulting in total combustion to CO2 and H2O. It is possible to introduce O2 into hydrocarbons such as olefins and aromatics to synthesize aldehydes (for example, acrolein and benzaldehyde) and acids (for example, acrylic acid, phthalic acid anhydride). In some reactions a selective oxidation can also result in dehydrogenation (butene butadiene) or dealkylation (toluene benzene) processes. 

A detailed kinetic study of oxidative dehydrogenation of propane, isobutane, n-butane (23 runs) and LPG (27 runs) was conducted over a wide range of partial pressures of pure and mixed hydrocarbons (0-0.3 atm), oxygen (0-0.2 atm) and steam (0.2-0.7) atm and temperature 600-670°C. Oxidation of Hj, CjHg, C H, CH and CO was also tested at 600-650°C. A set of reactions was selected based on the distribution of products ... 

This preparation procedure also creates solid-state phases that are key to the performance of the Mo-V-Nb-Te-0 catalyst for propane ammoxidation. High activity and selectivity result when the x-ray powder diffraction pattern shows the presence of specific diffraction lines attributed to two separate phases denoted as Ml and M2 by Mitsubishi Chemical Corp. The diffraction lines assigned to these two phases are given in Table 7 (146). The coexistence of these two phases is viewed as key to the successful functioning of the catalyst. Specifically, the Ml phase is purportedly responsible for the oxidative dehydrogenation of propane to propylene, the key intermediate in the reaction network. This reaction sequence, in which the first step is the formation of a propylene intermediate, is the same as noted previously with other propane ammoxidation catalysts, most notably with the V-Sb-0 catalyst (see above). The M2 phase of the Mo-V-Nb-Te-0 catalyst is reportedly the center for the selective ammoxidation of the propylene intermediate to acrylonitrile. As the first-formed intermediate, propylene is apparently the source of all the observed reaction products. Although a detailed kinetic analysis has not been presented, a cursory report, published in Japan, summarized the kinetic experiments for the conversion of propane and propylene over a... 

Heracleous, E. and Lemonidou, A. (2006). Ni-Nb-O Mixed Oxides as Highly Active and Selective Catalysts for Ethene Production via Ethane Oxidative Dehydrogenation. Part II Mechanistic Aspects and Kinetic Modeling, J. Catal., 237, pp. 175-189. 

It is beyond the scope of the present contribution to provide a detailed report on the complexity of the mechanism and kinetics of short contact time processes. Instead, an effort is made to summarize the advancement of the research in this field, trying to focus on the specific characteristics of monolithic structures that are requested and exploited in the short contact time production of chemicals. The focus of this review is on the selective oxidation (or oxidative dehydrogenation) of small alkanes to olefins. Mention is also made of other short contact time oxidation processes, such as the ammoxidation of methane to HCN. 

There are two ways to produce acetaldehyde from ethanol oxidation and dehydrogenation. Oxidation of ethanol to acetaldehyde is carried out ia the vapor phase over a silver or copper catalyst (305). Conversion is slightly over 80% per pass at reaction temperatures of 450—500°C with air as an oxidant. Chloroplatinic acid selectively cataly2es the Uquid-phase oxidation of ethanol to acetaldehyde giving yields exceeding 95%. The reaction takes place ia the absence of free oxygen at 80°C and at atmospheric pressure (306). The kinetics of the vapor and Uquid-phase oxidation of ethanol have been described ia the Uterature (307,308). 

The lower total activity for Rh electrodes may be partly due to increased CO poisoning and slower CO electro-oxidation kinetics compared with Pt electrodes, as demonstrated by the number of voltammetric cycles required to oxidize a saturated CO adlayer from Rh electrodes (see Section 6.2.2) [Housmans et al., 2004]. In addition, it is argued that the barrier to dehydrogenation is higher on Rh than on Pt, leading to a lower overall reaction rate [de Souza et al., 2002]. These effects may also explain the lower product selectivity towards acetaldehyde and acetic acid, which require the dehydrogenation of weakly adsorbed species. 

Alcohols will serve as hydrogen donors for the reduction of ketones and imi-nium salts, but not imines. Isopropanol is frequently used, and during the process is oxidized into acetone. The reaction is reversible and the products are in equilibrium with the starting materials. To enhance formation of the product, isopropanol is used in large excess and conveniently becomes the solvent. Initially, the reaction is controlled kinetically and the selectivity is high. As the concentration of the product and acetone increase, the rate of the reverse reaction also increases, and the ratio of enantiomers comes under thermodynamic control, with the result that the optical purity of the product falls. The rhodium and iridium CATHy catalysts are more active than the ruthenium arenes not only in the forward transfer hydrogenation but also in the reverse dehydrogenation. As a consequence, the optical purity of the product can fall faster with the... 

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