KINETICS OF HYDROGENOLYSIS OF METHYLAMINE ON A RHODIUM CATALYST

The kinetics of hydrogenolysis of methylamine to methane and ammonia w ere investigated over a catalyst consisting of small clusters of rhodi um dispersed on silica. Data obtained in the temperature range 353-408 K exhibit a characteristic pattern in which the rate passes through a maximum as the hydrogen partial pressure is increased by two orders o f magnitude from 0.01 to 1.0 atm. At a given temperature, the position of the maximum shifts slightly in the direction of higher hydrogen pa rtial pressure when the methylamine partial pressure increases by one to two orders of magnitude. Of particular interest is the finding that the rate increases with decreasing methylamine partial pressure over a broad range of hydrogen partial pressures covered in the investigati on. As the hydrogen pressure increases, the inverse dependence of the rate on methylamine pressure becomes less pronounced and eventually di sappears at a sufficiently high hydrogen pressure. At hydrogen partial pressures somewhat higher than those at which the rate maxima are obs erved, there is some indication that the inverse dependence changes to a positive dependence, especially at the lowest temperatures investig ated. It seems likely that the rate limiting step of the reaction chan ges when the hydrogen pressure varies over a wide range. At the highes t hydrogen pressures studied, it is suggested that the limiting step i s one in which the scission of the carbon-nitrogen bond occurs in a hy drogen deficient surface intermediate formed in the chemisorption of m ethylamine, with no direct participation of hydrogen as a reactant in the step. On the other hand, at the lowest hydrogen pressures investig ated, it is proposed that the rate is limited by a step in which chemi sorbed hydrogen does participate directly as a reactant.