Methylamine hydrogenolysis on a rhodium catalyst: kinetics at high hydrogen partial pressures

Abstract

The kinetics of hydrogenolysis of methylamine to methane and ammonia on a rhodium catalyst were investigated at hydrogen partial pressures in the range of 2–10 atm at temperatures of 368, 383, and 408 K. At a fixed methylamine partial pressure, the rate decreased with increasing hydrogen partial pressure. When the hydrogen pressure was held constant, the rate increased with increasing methylamine pressure. Results of a previous investigation by our group at lower hydrogen partial pressures (0.01–1 atm) indicated that the hydrogenolysis rate passed through a maximum with increasing hydrogen pressure. Moreover, at the lower hydrogen pressures, there was an inverse rather than positive dependence of the rate on methylamine partial pressure. With the aid of the present results, there is a much clearer definition of the maximum in the experimental data relating the reaction rate to hydrogen partial pressure. The inversion of the effect of methylamine pressure on the rate as the hydrogen pressure is varied over a sufficiently wide range is also firmly established. With regard to the interpretation of the many interesting features of the kinetics, we retain the suggestion from our earlier work that the rate limiting step at the highest hydrogen pressures is the scission of the carbon-nitrogen bond in a partially dehydrogenated methylamine intermediate chemisorbed on the rhodium, with no direct participation of hydrogen as a reactant in this step. At the lowest hydrogen pressures, however, there is a different rate limiting step in which hydrogen does participate directly as a reactant.