Fast adsorption-desorption kinetics of hydrocarbons in silicalite-1 by the single- step frequency response method N. Van-Den-Begin and L.V.C. Rees Physical Chemistry Laboratories, Imperial College of Science and Technology, London, UK and J. Carp and M. Billow Zentralinstitut fi~r physikal£whe Chemie der Akademie der Wissenschaften der DDR, Berlin, GDR Fast adsorption-desorption kinetics with dead times of 25-35 ms have been followed using a new technique. At sorption equilibrium, a zeolite sample is subjected to small (less than +1%) pressure changes brought about by very rapid, square-wave volume perturbations. By recording the gas pressure response on compression/expansion of the system, sorption uptake/desorption processes, which are governed by intracrystalline diffusion, can be followed. By fitting appropriate solutions of Fick's diffusion law to the adsorption/desorption curves, and correcting the as-measured diffusivities by the Darken factor, intracrystalline diffusion coefficients of short-chain hydrocarbons, n-hexane and benzene, have been determined. These corrected diffusion coefficients for methane, ethane, and propane, are ~< 5 smaller than the self-diffusivities measured directly by the n.m.r, pulsed-field- gradient technique. The corresponding coefficients for n-hexane and benzene agree well with those determined by piezometric measurements. Keywords: Adsorption-desorption kinetics; intracrystalline diffusion coefficients; single-step frequency response method INTRODUCTION The pore diameters of 10-ring zeolites, such as ZSM-5, are of a size similar to many important aromatic and branched hydrocarbon molecules. Thus, ZSM-5, including its 'pure' silica varient silicalite-1, are important molecular sieves for sorp- tive separation and shape-selective catalysis processes. Such guest molecules can have a low mobility within the ZSM-5/silicalite channel network; for example, the mobility of sorbed benzene molecules in silicalite, as determined by 2H- and 13C-n.m.r. investigations, have been found to be solid state-like.l'2 Because of their structure-dependent transport properties, the measurement of intracrystalline diffu- sion coefficients in ZSM-5-type zeolites has become of increasing interest. For this purpose, new ex- perimental techniques have been developed and classical ones considerably improved. For example, sorption u ptake/desorption, "~-7 several n.m.r. methods,S-! U frequency-response, Xl-14 permeation This paper is dedicated to Professor Harry Pfeifer on the occasion of his 60th birthday. Address reprint requests to Professor L.V.C. Rees, Physical Chemistry Laboratories, Imperial College of Science and Tech- nology, London SW7 2AY, UK. Received 12 July 1988 methods, tS'l° neutron scattering, 17 zero-length col- umn' chromatog pyra h ,is and the 'gas-concentration jump' technique, i.o However, the diffusion coefficients determined by these different methods and investigators for the same sorption system have been fotmd to vary over five orders of magnitude. 1.~.20.'_, l Despite these discre- pancies, however, only a few studies have been carried out to ascertain the interference on intracrys- talline diffusion by intercrystalline mass transport, by sorption heat transfer, and by surface barriers.: 7.14:22--24 It is the aim of this paper to address these problems by reporting diffusion coefficients measured using a newly developed single-step frequency response method that is really a fast adsorption-desorption. technique. The results obtained will be compared with those obtained by n.m.r, self-diffnsion and piezometric sorption-uptake measurenlents. EXPERIMENTAL Fast adsorption/desorption kinetics have been mea- sured using the frequency-response system recently developed at the Imperial College, London. This improved system will be fully described elsewhere. '-'~ In the single-step modification the zeolite sample © 1989 Butterv~orth Publishers ZEOLITES, 1989, Vol 9, July 287