Activation energy ethylene oxide production

The products of the thermolysis of 3-phenyl-5-(arylamino)-l,2,4-oxadiazoles and thiazoles have been accounted for by a radical mechanism.266 Flash vacuum pyrolysis of 1,3-dithiolane-1-oxides has led to thiocarbonyl compounds, but the transformation is not general.267 hi an ongoing study of silacyclobutane pyrolysis, CASSF(4,4), MR-CI and CASSCF(4,4)+MP2 calculations using the 3-21G and 6-31G basis sets have modelled the reaction between silenes and ethylene, suggesting a cyclic transition state from which silacyclobutane or a trcins-biradical are formed.268 An AMI study of the thermolysis of 1,3,3-trinitroazacyclobutane and its derivatives has identified gem-dinitro C—N bond homolysis as the initial reaction.269 Similar AMI analysis has determined the activation energy of die formation of NCh from methyl nitrate.270 Thermal decomposition of nitromethane in a shock tube (1050-1400 K, 0.2-40 atm) was studied spectrophotometrically, allowing determination of rate constants.271... 

Reaction with ethylene sulfide yields ethylene and, possibly, a silanethione (47). With ethylene oxide, the first step is abstraction of an oxygen atom to yield the silanone (48), which adds to a second ethylene oxide molecule to give the siladioxolane (49). The products were identified mainly by infrared spectroscopy in several cases the reaction kinetics were studied and activation energies determined348. 

The mechanism of formation of the cracking products had not been resolved at the time of publication of Shtern s review [2]. Semenov [3], however, had pointed out that the direct decomposition of prop-l-yl and prop-2-yl radicals at 300 °C was most unlikely, due to the large activation energies involved (27—29 and 40 kcal. mole", respectively). He therefore suggested the following alkylperoxy radical isomerization and decomposition reactions to explain the formation of propene and ethylene in propane oxidation, viz. 

Cluster DFT calculations were also used to identify a transition state for the formation of ethylene oxide. In this transition state, the Ag-0 bonds are elongated relative to the oxametallacycle. The product of this reaction is gaseous EO. The activation energy for this step had previously been determined by Linic and Barteau experimentally.62 The predicted activation barrier from DFT calculations, 16 kcal/mol, is in very good agreement with the experimental result of 17 kcal/mol. 

Fig. 9. Influence of chlorine coverage on the kinetic parameters for selective ethylene oxidation over a Ag(llO) surface. Parameters for both the production of ethylene epoxide (EtO, circles) and the undesired side reaction to full combustion (C02, squares) are presented. Steady-state reaction orders in P02 and Pei and activation energies a versus chlorine coverage near 563 K, Pb = 20 torr, and P02 = 150 torr. From Ref. 118.

Ethylene oxidation over five CuO catalysts prepared with various amounts of KOH was found to have a rate equal to A(C2H4) by Koutsoukos and Nobe 172). They used 0.02-0.1% C2H4 in air at 217-340°, and observed activation energies of 20-27 kcal/mole. In related work, Accomazzo and Nobe 173) used a supported copper catalyst (50 % CuO, 50 % AlgOj) for the oxidation of ethylene, propylene, and other hydrocarbons at low concentrations in air. The rate was expressed as equal to l (olefin) . Activation energies for ethylene and propylene were 18 and 17.5 kcal/mole, respectively. Propylene could be oxidized to an equal extent at about 20° lower temperature than for ethylene. The products were essentially only COg and HgO. Diffusion effects and conversion equations were discussed. 

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