Methane pyrolysis in low-cost, alkali-halide molten salts at high temperatures

Abstract

Molten salts have received renewed attention as potential reaction media for methane pyrolysis, in which CO₂-free hydrogen gas can be produced and the solid carbon can be continuously removed and then stored or utilized. Compared to solid catalysts, there is a lack of fundamental understanding of the methane transformation processes in liquid environments which could inhibit progress to industrializing a low-cost commercial process. In this work, we have investigated methane pyrolysis in alkali-halide molten salts (i.e., NaCl, NaBr, KCl, and KBr) that are industrially attractive due to their low cost, high temperature stability, and non-toxicity. We measured first-order effective activation energies in differential bubble column reactors of ∼300 kJ mol⁻¹ for methane pyrolysis in all molten salts, which is lower than the reported value for the homogeneous uncatalyzed reaction of ∼420 kJ mol⁻¹. We also measured an effective activation energy of ∼230 kJ mol⁻¹ for single CH₄-D₂ exchange, which is consistent with some solid metal oxide catalysts. Calculations using combined molecular dynamics and density functional theory support the experimental findings with a calculated energy barrier of ∼3.29 eV (or ∼317 kJ mol⁻¹) on a molten KCl surface. The initial reaction rates may be sufficient for an industrial reactor; however, at high conversion, the hydrogen dependent reverse reaction limits single-pass conversion. We also find that the accumulated solid carbon is ‘wetted’ into the liquid salt medium and does not have significant contact with the gas phase or affect methane decomposition rates. The solid phase products are composed of both amorphous and graphitic carbon domains with no long-range ordering. We find that significant salt residue is retained on the carbon removed from the reactor which is inadequately removed by water washing. Only high temperature heat treatments were effective at cleaning the residual salt from the carbon, presumably due to selective evaporation of the salt.