Packing effects in the liquid-phase adsorption of C5-C22 n-alkanes on ZSM-5

The liquid-phase adsorption of C-5-C-22 linear alkanes on ZSM-5 was studied using a batch adsorption technique. Saturation capacities of the alkanes depend strongly on the chain length. The number of -CHx groups adsorbed per unit cell drops steeply between C-7 and C-8 and then increases steadily to reach a plateau of 53-54 for C-14-C-22. In this plateau region, the pores of ZSM-5 are densely packed with alkane molecules. The mechanisms of packing in liquid-phase adsorption were studied using configurational-bias grand canonical Monte Carlo simulations. These simulations revealed that the probability of bending and crossing in the intersections increases with chain length, explaining the very good packing of long-chain alkanes. Short alkanes tend to stay away from the intersections, resulting in a lower packing density.