(219d) Frequency Response Methods for the Investigation of Diffusion within Microporous Zeolite Structures

The Frequency Response method measures the pressure response of a closed sorption system to a
periodic volume perturbation over a spectrum of frequencies [1] spanning several orders of
magnitude, which is then fit to a theoretical transport model. Frequency response (FR) techniques
have been used to measure thermal transport properties[2], adsorption-desorption[3-5], mass
diffusion[6-9] and a combination of several transport processes[10, 11]. Recently, Teixeira and Qi
[12] have integrated asymmetric surface barrier dynamics (i.e., differing uptake and release kinetics at
particle surface) with the Yasuda model[4, 7, 10, 13] and evaluated the relative contributions of
separate transport processes to mass transfer of cyclohexane in MFI (mordenite framework
inverted). The underlying assumptions (Fickian diffusion, kinetic parameters demonstrating only
temperature dependence, spatial homogeneity) are yet to be justified to be appropriate for a broad
range of particle characteristics and applied frequency spectrum. Previous frequency response
devices[14-17] have demonstrated better accuracy, faster response time, exact reproducibility
and have eliminated a lot of sources of possible discrepancy in obtaining data over Yasuda’s first
apparatus in 1976[4, 5]. However, they still lack the superior accuracy needed during measurement
at high frequency and smaller particle size regime. Moreover, even though the latest devices are
capable of measuring a frequency range spanning over three orders of magnitude[12, 17] for
mesoporous particles of size ranging from 200 nm to 3 μm, accuracy in data acquisition and noise-free
operation is not guaranteed. Additionally, all previous models assumed Fickian diffusion inside the
pores, which often is invalid to hierarchical materials and very small particles sizes, where anomalous
and nonlinear diffusion are prominent[18].
We attempt to make use of FR method in a novel way for a theoretical investigation of
diffusion through zeolite and other complex porous structures. Alongside with in-phase and out-of-phase
information, the classical concept of Bode plots was implemented in order to extract more
information from the time-series data analysis. A new frequency response device was designed and
constructed using state-of-the-art technology to operate over the frequency range spanning several
orders of magnitude. A detailed analysis of the blank system before introducing the particles in the
gas chamber elucidated the dynamics of the blank system and sources of additional delay in the
pressure response of the system. Zeolite particles of a broad range of sizes were subjected to
periodic adsorption-desorption and diffusion steps using sinusoidal volume perturbation and time-series
data of pressure response were collected for analysis. Comprehensive mathematical models
based on the principles of transport processes inside and on the surface of a porous particle were
derived. Finally, the raw data were subjected to parameter identification to evaluate the transport
parameters of microporous particles. This study provides us with a better knowledge of the
processes involved and paves the way to further investigation of nonlinear/anomalous transport
processes in hierarchical particles of microscopic scale.
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