Deuterium enrichment by cw CO2 laser-induced reaction of methane

Abstract

A cw CO2 laser is used to vibrationally excite CH2D2. Intentional VT deactivation competes with interisotope VV transfer and prepares a sample of gas in which the CH2D2 is excited while the CH4 remains unexcited on a steady-state basis. Subsequent reaction of this mixture with chlorine atoms and molecules forms a stable product, deuterated methyl chloride, enriched in deuterium by up to 72%. Extension to the CH3D system will be discussed. A conservative estimate of the energy requirement for a commercial facility (natural abundance feed) based on the latter system gives a value within a factor of 4 of that required by the currently used H2O/H2S commercial process.

Relating State-Dependent Cross Sections to Non-Arrhenius Behavior for the Cl + CH4 Reaction

“…Overall case 4 yields a better fit to the thermal rate constant and KIE data than the other cases, but the agreement is only marginally better than for case 5 for which the contribution to the rate constant from the bending mode is neglected. Given that several studies have suggested that low-frequency bending modes enhance the reactivity, ,− we assume case 4, which includes such contributions, provides the most realistic representation of this system.…”

“…These latter studies indicate that CH 4 with energy in the umbrella and torsional modes may be more reactive than CH 4 with energy in the stretch mode. Earlier work by Hsu and Manuccia − suggested that exciting the CH 2 rocking mode (υ 7 ) of CH 2 D 2 significantly enhances the reaction rate, whereas experiments by Vijin et al and Chesnokov et al indicated that neither pumping the umbrella mode of CD 4 nor exciting the umbrella or torsional modes of CH 4 enhances the reaction rate significantly. Vibrational enhancement of the reaction probability is indicative of a late barrier along the reaction coordinate on the PES, for which the transition state resembles the products more than the reactants.…”

The Reaction of Cl with CH4: A Connection between Kinetics and Dynamics

“…Earlier work by Hsu and Manuccia − indicated that pumping the CH 2 rocking mode (ν 7 ) of CH 2 D 2 enhances the reaction rate with Cl. In contrast, Vijin et al noted no enhancement in reactivity when directly pumping the umbrella bending mode of CD 4 in a mixture of CH 4 and CD 4 and measuring the isotopic fractionation of products.…”

“…Assuming that both the stretching and bending modes of CH 4 enhance the reaction probability for Cl + CH 4 , − ,− the description of the rate constant by eq 2 must contain at least three terms, each of which is characterized by a distinct P rxn ( E coll , ν i ). Figure shows a fit of eq 2 to the absolute rate constants measured for Cl + CH 4 , with one term representing the nearly isoenergetic symmetric and asymmetric stretching modes, one term representing the umbrella bend mode, and one term representing the ground-state and torsional mode, which was assumed not to couple strongly to the reaction coordinate .…”

Effects of Bending Excitation on the Reaction of Chlorine Atoms with Methane

“…Experimental verification has been lacking in part because selective preparation of low-frequency modes has proven difficult. The few experimental measurements [12][13][14][15][16] that probe the effects of bend excitation on the Cl + CH 4 reaction have been performed using indirect methods, leading to disparate results. Early studies [12] showed no enhancement from the bending mode, whereas more recent measurements [14,15] estimate the enhancement factor to be approximately 200 or approximately 3, respectively, based on studies of residual bend-excited methane present in their molecular beam.…”