A frequency-response study of the diffusion and sorption dynamics of ammonia in zeolites

The dynamics of diffusion and sorption of NH3 was studied in zeolites Na-A, K-A, Na-X, H-Y, H-mordenite, and H-ZSM-5. The batch frequency-response (FR) technique was applied. The NH3 pressure was 133 Pa and the temperature was in the 373-873 K range. The FR spectrawere interpreted according to the model of the isotherm, batch-type FR system containing uniform, isotropic, and spherical sorbent particles. Results suggest that any one of the consecutive process steps of the ammonia transport between the gas and the sorbent can control the rate of the process under the near-equilibrium conditions of the measurement. The nature of the rate-controlling step depends on the structure, composition, and size of the sorbent particles. Diffusion in the macropores was found to determine the rate in commercial zeolite pellets and in large particles built of aggregated zeolite crystallites. When this rate-controlling process was eliminated by grinding the particles, either micropore diffusion or sorption became the slowest rate-controlling step. The dimensions of NH3 molecules and the free diameter of the pores of zeolite K-A are comparable. As a result, the diffusion resistance of the zeolitic micropores controls the NH3 mass transport. The other zeolite samples have wider channels and bind ammonia more strongly than K-A. Owing to the high NH3 coverage and activation energy of desorption, the adsorption-desorption process was much slower than the rate of diffusion. For H-zeolites, different parallel transports could be distinguished. A fast process, dominating at high near-equilibrium ammonia coverage, was assigned to sorption on Lewis acid sites, such as NH4+ ions. The slower processes, appearing mainly at lower coverage, can represent the direct interactions between the NH3 and the Bronsted acid sites. The FR method can be effectively used for most of the zeolites to characterize the dynamics of NH3 sorption.