Kinetic isotope effect for H-2 and D-2 quantum molecular sieving in adsorption/desorption on porous carbon materials

Adsorption and desorption of H-2 and D-2 from porous carbon materials, such as activated carbon at 77 K, are usually fully reversible with very rapid adsorption/desorption kinetics. The adsorption and desorption of H-2 and D-2 at 77 K on a carbon molecular sieve (Takeda 3A), where the kinetic selectivity was incorporated by carbon deposition, and a carbon, where the pore structure was modified by thermal annealing to give similar pore structure characteristics to the carbon molecular sieve substrate, were studied. The D-2 adsorption and desorption kinetics were significantly faster ( up to x 1.9) than the corresponding H-2 kinetics for specific pressure increments/decrements. This represents the first experimental observation of kinetic isotope quantum molecular sieving in porous materials due to the larger zero-point energy for the lighter H-2, resulting in slower adsorption/desorption kinetics compared with the heavier D-2. The results are discussed in terms of the adsorption mechanism.