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In situ FTIR spectroscopy was used to monitor the sorption and desorption kinetics of pyridine into and out of microporous materials such as mordenite, ZSM-5 and silicalite. A previously described experimental technique and method of evaluation, which had been successfully used to study sorption and sorption kinetics of aromatics in H-ZSM-5, was now employed for the investigation of the behavior of pyridine. A modification of the original flow-through IR cell, however, was necessary to provide better resistance to and handling at higher sorption temperatures. Sorption and desorption of pyridine was measured through the intensity changes of the IR bands typical of sorbed pyridine. At sufficiently high temperatures (525–575 K), the uptake of pyridine even into strongly acidic hydrogen mordenite (H-MOR) and H-ZSM-5 could be monitored and formally described by a solution of Fick's second law using uniform diffusion coefficients for the whole sorption processes, i.e. D=1×10−12 and D=6×10−11 cm2 s−1 for H-MOR (at 525 K) and H-ZSM-5 (at 575 K), respectively. The process was, however, not reversible, i.e. no `random walk' of the sorbate molecules occurred. Therefore, it was tentatively assumed that the slow transport of pyridine into the micropores is the rate-determining step, which may indeed be described as a diffusion, but followed by a rapid reaction with the acidic Brönsted and/or Lewis sites. The reaction step is, at the temperatures applied, almost irreversible. By contrast, with less strongly acidic sorbents such as Li-ZSM-5 and Na-ZSM-5, pyridine sorption was found to be reversible, the diffusivities determined at 575 K amounted to 8×10−12 and 2×10−11 cm2 s−1, respectively. In the case of pyridine/silicalite, reversibility was observed as well. Pyridine in silicalite showed a diffusion behavior similar to that of benzene in H-ZSM-5, exhibiting a Darken-corrected diffusivity of about 5×10−10 cm2 s−1 at 395 K. In fact, the isosteric heats of adsorption of pyridine in Li-ZSM-5 and Na-ZSM-5 were determined to 195–155 kJ mol−1 and 120 kJ mol−1 and were thus significantly higher than those previously measured for benzene in H-ZSM-5 (65±5 kJ mol−1).
