Uptake-curves for the determination of diffusion coefficients and sorption equilibria for n-alkanes on zeolites

Sorption capacities and diffusion coefficients were measured for n-alkanes on a 5A-zeolite and ZSM-5 with a gravimetric setup. Significant heat effects were observed during the experimental determination of diffusion coefficients, which complicate the interpretation and the analysis of the experimental Uptake-curves. This effect depends significantly of the sorption rate, heat of adsorption and dissipation of heat. Taking into account these heat effects, reliable transport coefficients can be extracted. These agree well with diffusion coefficients obtained from other techniques, like PFG–NMR or ZLC. With the carbon chain-length the measured diffusivities decrease from 1.6 × 10−12 to 0.5 × 10−12 m2/s for propane to n-hexane for 5A-zeolite at 473 K. For n-butane and n-hexane values about 8 × 10−11 and 4.6 × 10−11 m2/s on MFI-type zeolites at 303 K are measured.From the equilibrium data, heats of adsorption were calculated and compared with literature data. The heat of adsorption for 5A-zeolite changes with the carbon chain-length from 32 to 44 kJ/mol for n-butane to n-octane, respectively. The heats of adsorption of alkanes on ZSM-5 were 60 kJ/mol for n-butane and 57 kJ/mol for hexane. Due to different adsorption sites in MFI-type zeolites two heats of adsorption were calculated for each alkane.