abstracts - Институт катализа им. Г.К. Борескова

KS-I-2Experiments performed at varied flow rates and initial concentrations of oxygen and CO 2added to the reaction mixture demonstrate the following trends:1. Whereas in the empty reactor propane and oxygen conversions follow S-shape curvesindicative for the development of radical-chain reaction, in the presence of catalyst theyincrease almost linearly at increasing formal residence time. In the latter case a slight decreasein propylene (C 3= ) and ethylene (C 2= ) selectivities is accompanied by a small increase ofmethane selectivity at increasing propane conversion.2. If up to 1 vol.% CO 2 is added to the initial gas mixture, some increase in selectivity to C 3=is accompanied by a corresponding decrease of C 2= ; at higher concentrations of CO 2selectivities to all main products remain almost constant. The overall reaction rate graduallydecreases at increasing CO 2 concentration.3. At 600 o C and low initial oxygen concentration (< 0.01 vol.%) the rates of C 3= , C 2= andmethane are almost equal; at increasing oxygen concentration the C 3= formation rateincreases, while that of C 2= remains almost constant and methane decreases. Simultaneously,the rates of H 2 and CO formation are increasing and their ratio reaches ~1 at 20% O 2 in theinitial mixture.4. If a void volume after the catalyst bed is added, the C 3= formation rate increases much morethan any other; the formation of H 2 and CO significantly drops.The analysis of the above observations together with available gas phase kinetic data andproposed radical-type heterogeneous reactions and their estimated rate constants suggests thefollowing about the reaction mechanism and olefin formation pathways in particular.1. Li/MgO catalyst is active in the formation of initial propyl radicals, but at the same time itactively terminates secondary radicals; as a result, general kinetic features (the developmentof the process in time) change from those typical for chain reactions to "linear".2. Propylene can form via the following radical processes:C 3 H 7 → C 3 H 6 + H (1)C 3 H 7 + O 2 → C 3 H 6 + HO 2 (2)C 3 H 7 + X → C 3 H 6 + XH (3)[O] S + C 3 H 7 → [OH] S + C 3 H 6 (4)Relative importance of each process depends on the reaction conditions:- in the absence of catalyst and at very low concentrations of oxygen (e.g., at high O 2conversions) C 3= forms mainly via mono-molecular dissociation (1);- at higher oxygen concentration the reaction process (2) dominates;26