* Corresponding author. Tel.: #015-278-6725; fax: #015-278-4452. E-mail address: t.a.nijhuis@stm.tudelft.nl (T.A. Nijhuis) Chemical Engineering Science 54 (1999) 4423} 4436 Measurement and modeling of the transient adsorption, desorption and di!usion processes in microporous materials T.A. Nijhuis*, L.J.P. van den Broeke, M.J.G. Linders, J.M. van de Graaf, F. Kapteijn, M. Makkee, J.A. Moulijn Delft University of Technology, Department of Chemical Process Technology, Section Industrial Catalysis, Julianalaan 136, 2628 BL Delft, Netherlands Abstract Using pulse-response experiments the adsorption and di!usion behavior of a number of gases in microporous materials was measured in the temperature range 300}773 K. By modeling these pulse responses it is possible to simultaneously determine the equilibrium constant for adsorption, the absolute rate constants for adsorption and desorption, as well as the di!usivity inside micropores. The pulse-response measurements are very reproducible and the calculated parameters are in good agreement with values reported in literature. A unique characteristic of this method is the possibility to accurately determine all adsorption and di!usion parameters from one single measurement. Furthermore, this macroscopic method is the "rst technique that yields di!usivity values which are in good agreement with those determined using microscopic methods, such as Pulse-Field-Gradient NMR. The technique is demonstrated for the adsorption and di!usion in MFI-type zeolites and microporous carbon.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Adsorption; Di!usion; Microporous materials; Zeolites; TAP; Transient 1. Introduction The usual procedure to determine gas-phase adsorp- tion parameters of a sorbent material is by studying the rate of adsorption using gravimetric, volumetric or baro- metric techniques (Ka K rger and Ruthven, 1992). By a gravimetric technique the mass changes of a sorbent sample are measured as a function of the sorbent gas partial pressure. Volumetric and barometric techniques measure the adsorbed amount of gas from the change in gas volume or pressure, respectively. The disadvantage of these methods is that they are time consuming, since it is necessary to reach adsorption equilibrium with the sur- rounding gas phase, which is often reached only after a prolonged time. The di!usion coe$cient in microporous materials can be determined by a number of di!erent techniques. These methods are for example NMR relaxation, tracer desorp- tion, chromatographic methods, measurement of di!u- sional #uxes through a membrane, and determination of the uptake (or desorption) rate in adsorption (or desorp- tion) measurements. A disadvantage of these methods is that they are elaborate. Furthermore, these methods produce di!usion coe$cients, which can di!er over two orders in magnitude (Ka K rger and Ruthven, 1992; Kapteijn et al., 1994). Especially the NMR techniques yields values for the di!usion coe$cient that are usually up to an order of two higher than the values determined by other techniques, although the NMR technique is the most direct method for determining a di!usion coe$- cient inside a microporous material. A satisfactory ex- planation for these di!erences is, however, not yet available. A new method for determining both the di!usion coef- "cient inside a porous particle and the adsorption behav- ior on that sorbent particle, is the application of a transient pulse-response technique (Nijhuis et al., 1997). This is carried out by pulsing a small amount of gas over the sorbent material and measuring the pulse response of the gas. By modeling these pulse responses, the rate constants of adsorption and desorption can be deter- mined. The advantage of this transient method is that it is experimentally very fast and reproducible. Multitrack is 0009-2509/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 9 9 ) 0 0 1 3 1 - 1