Homogeneous gas‐phase pyrolyses with a wall‐less reactor. III. The oxygen–ethane reaction. A double reversal in oxygen and surface effects

Abstract

The impact of surface and oxygen on the oxidative pyrolysis of ethane at temperatures above 590°C was studied using a wall-less reactor. At very low conversions under homogeneous conditions, ethene formation begins at the same temperature regardless of whether oxygen is present or absent. Between 0.00 and 0.13% conversion (592-632"C), the rate with oxygen is actually less than the rate in the absence of oxygen. A reversal occurs at about 633°C above which oxygen has a promoting effect. It is concluded that under homogeneous conditions the initiation step in the oxygen-promoted pyrolysis is the same as in the oxygen-free pyrolysis; therefore, initiation by direct attack of oxygen on ethane does not make an important contribution. The decrease in rate observed upon addition of oxygen implies the formation of the relatively unreactive HO2. radical. AS conversion of the HO2. radical to the more reactive H O . radical becomes significant, the reaction is highly accelerated. If a stainless steel surface is added, the reaction is inhibited at higher conversions in the presence of oxygen. Again at low conversions, a second reversal occurs, and the stainless steel surface acts as a promoter below 649°C. The rate of surfacecatalyzed ethene formation at 590°C equals the rate of homogeneous ethene formation at 630°C.

A Multistep Surface Mechanism for Ethane Oxidative Dehydrogenation on Pt- and Pt/Sn-Coated Monoliths

“…Instead, a fraction of it speeds up homogeneous reactions. As argued previously, ,, oxygen is necessary in the gas phase to speed radical propagation reactions 50 and achieve quick ethylene formation, even though the main paths leading to ethylene are nonoxidative. Consistently, experiments showed that at the exit of the 1 cm long catalytic monolith, O 2 conversion and ethylene formation are not complete .…”

A comparative study of the pyrolyses of the tetramethyl derivatives of silicon, germanium, and tin using a wall-less reactor

“…The validity of this procedure has been repeatedly demonstrated. [8][9][10][11][12] In this manner a point sample taken at center stream at a specified distance from the injection tube is used to determine the concentrations of all reactants and products in that plane. The system is computerized such that the raw chromatographic data are converted directly to rate constants, activation energies, graphs etc.…”

Modeling ethane oxy‐dehydrogenation over monolithic combustion catalysts

“…As pointed out in the work of Taylor and Kulich (1973), O 2 is not involved in the initial stages of the process, until a pool of H radicals is formed (mainly via reaction 13). For this reason, the reaction of consumption of oxygen (step 14) is delayed with respect to ethane, and the formation of dehydrogenation products (that is, ethylene) occurs before the oxygenated ones (CO x ) The reactions of propagation occur with a significant rate via C 2 H 6 consumption, involving radical species, such as H · and OH according to step 15 and 16 As concerns the reactivity of the radicals in the consumption of C 2 H 6 to C 2 H 5 , it can be roughly ordered as follows: H > OH ≫ CH 3 > O.…”