Selectively permeable membranes made from inorganic materials, including zeolites, have attracted great interest for a variety of technological applications in areas such as chemical separations, energy conversion, and environmental remediation. The fabrication of new types of functional membranes, as well as the development of their structure-property relationships, has been important challenges in membrane science. A number of different experimental tools have been used with considerable success to study transport phenomena in membranes. However, the increasing structural complexity of the materials used for membrane fabrication leads to increasingly greater ambiguity in the interpretation of experimental data. In particular, previous approaches have not led to a “direct” method for probing transport phenomena in the interior of the membrane, rather preferring to use trans-membrane performance data (obtained from analyzing the properties of the exit streams from the membrane module) in order to infer the transport phenomena within the membrane.

The objective of our work is to develop and validate the application of Step-Scan PhotoAcoustic Spectroscopy (SS-PAS) as a non-destructive, in situ technique for directly acquiring intra-membrane transport information through depth-profiling of the membrane composition and structure with micron-scale resolution. A rigorously developed model is implemented to analyze the PA spectra, allowing the extraction of intra-membrane concentration profiles of diffusing permeants as well as that of membrane structural components. Our comprehensive data analysis methods produce reliable calculation of concentration profile data directly from the PA depth profiling spectra; there is no dependence upon trans-membrane performance data. A membrane/permeate pair of considerable technological significance, MFI/p-Xylene, is considered in this work.

Experimental and data analysis work with this example system shows promising results. The extraction of concentration profiles from SS-PAS data can be accomplished with a measurement model that involves three system-dependent physical parameters. We demonstrate the extraction of these physical parameters through the analysis of thermogravometric and PA spectral data done under permeate-saturated conditions. We subsequently implement these parameters into our model, and demonstrate the direct extraction of intra-membrane concentration profiles in steady-state permeation experiments performed at different concentration gradients across the membrane. A novel and physically significant result of this method, i.e. the direct measurement of the local intra-membrane permeate diffusivity at any point within the membrane, is demonstrated. The depth-resolved PA spectral analysis method established with this work can be utilized in conjuction with in situ steady-state permeation experiments to determine local intra-membrane concentrations and diffusivities for numerous membrane/permeant(s) combinations.