Sorption and ordering of dibranched alkanes on medium-pore zeolites ferrierite and TON

The sorption of (methyl-substituted) pyridines and 2,2-dimethylalkanes on t he medium-pore zeolites FER and TON has been studied by in situ IR spectros copy, calorimetry, and gravimetry in order to describe sorption at the oute r surface and the pore mouths of the zeolite crystals. Six percent and 3% o f the Bronsted acid sites (determined by adsorption of 2,4,6-trimethylpyrid ine) were found to be on the outer surface and the pore mouth of TON and FE R, respectively. This result agrees fairly well with the fraction of sites that are available for the 2,2-dimethylalkanes, which are unable to fully e nter the pores of the zeolites studied. Remarkably, at low coverage, all th ree dimethylalkanes adsorb in parallel to the outer surface. As the coverag e increases, additional 2,2-dimethylpentane and 2,2-dimethylhexane molecule s sorb in such a manner that the propyl and butyl groups point into the por e. Also, approximately 80% of the 2,2-dimethylpentane and 2,2-dimethylhexan e molecules that initially sorbed parallel to the outer surface rearrange t o this sorption structure. This ordering is well documented by the marked i ncrease in the heat of adsorption as the equilibrium pressure increases and by the IR spectra of hydroxyl groups in interaction with the alkane during this process.