Concentration profiling of a molecular sieve membrane by step-scan photoacoustic spectroscopy

We report the quantitative, nondestructive determination of the concentration profile of an organic molecule in a nanoporous polycrystalline zeolite molecular sieve membrane by step-scan IR photoacoustic experiments and analysis. A heterogeneous zeolite membrane model system was constructed by growing a zeolite MFI layer on a macroporous alpha-alumina substrate, followed by calcination to remove the organic tetrapropylammonium (TPA) structure-directing agent, and finally the growth of a second TPA-containing layer over the first. Step-scan photoacoustic spectroscopy is then used with a large range (10-500 Hz) of incident signal modulation frequencies to obtain a series of depth-dependent IR spectra. Deconvolution of these spectra and analysis of band intensities by the theory of photoacoustic signal generation allows for the determination of the TPA concentration profile. We briefly discuss the implications of this technique for understanding structure-property relationships in materials deposited as membranes for molecular sieving applications.