Dynamic studies of catalyst poisoning: Effect on adsorption and surface reaction rates for hydrogenation of α-methyl styrene by Pd/Al2O3



Pulse-response data for the hydrogenation of α-methyl styrene were obtained as a function of poisoning of the Pd/Al2O3 catalyst. The reaction is a first-order, reversible adsorption of hydrogen followed by a first-order, irreversible surface reaction. The experiments, at 298 to 323 K and atmospheric pressure, were carried out in a three-phase slurry reactor for which mass transport effects could be accurately accounted.

Independent adsorption equilibrium data in cumene slurries showed that adsorption occurred to the same extent on both poisoned and unpoisoned sites with a heat of adsorption, ΔHc = −16.3 kJ/mol. Converted to styrene as the slurry liquid, ΔHs = −20.9 kJ/mol.

The zero and first moments of the response curves indicated that the rate constant for the surface reaction was independent of the extent of poisoning. The analysis required developing a new theory for interpreting dynamic data where reactant adsorption occurred on both poisoned and unpoisoned sites. The rate constant (per unit mass of catalyst) for adsorption decreased continuously to zero as the extent of poisoning increased.

The results are consistent with the concept that poisoning reduced the number of sites active for reaction, but that the activity per site for the surface reaction was constant. However, the activity per site for adsorption, while constant at poison levels up to 40%, appeared to decrease sharply at higher levels.