Time-Dependent Permeance of Gas Mixtures through Zeolite Membranes Kevin H. Bennett and Kelsey D. Cook* Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600 John L. Falconer and Richard D. Noble Department of Chemical Engineering, University of Colorado, Boulder, Colorado 80309-0424 The time-dependent permeation behavior of binary gas mixtures through a ZSM-5 zeolite membrane was studied. Although steady-state permeation rates were indistin- guishable for CO 2 and N 2 or for cis- and trans-2-butene in binary mixtures, differences in the rate of approach to steady state allowed component distinction. In “normal” systems, one component is initially enriched in the permeate following application of a pulse of analyte gas to the membrane, and then disappears more quickly upon termination of the pulse. Mixtures of cis- and trans-2- butene exhibit qualitatively different behavior; the perme- ate is enriched in cis-2-butene during both the leading and trailing edges of a sample pulse (though not at steady state). These differences in permeation behavior reflect different balances among multiple transport mechanisms through the zeolite membrane, thought to reflect a com- bination of selective component sorption and intra- crystalline diffusion; in the case of cis- and trans-2- butene, these two factors oppose one another. It is known that this mechanistic complexity can engender synergistic effects, wherein the presence of one component can affect the permeation of another. These may limit applicability to true “unknowns”, but resulting complications should be less problematic in well-defined process applications. Membrane inlet mass spectrometry (MIMS) 1 has been used to enhance selectivity and sensitivity of on-line process mass spectrometry. 2,3 Response time (e.g., for following changes in stream composition) is important for monitoring and control applications; in general, MIMS response times are limited by analyte permeation times through the membrane. Permeation times, in turn, are related inter alia to membrane thickness; thinner membranes give faster response (within the limits of membrane robustness). Silicone rubber membranes are used most often, providing selectivity for analysis of volatile organics in aqueous streams; applications include fermentation monitoring, waste stream analysis, and ambient air monitoring. 2 Discrimination against water and hydrophilic solutes (including salts) is effective, providing very low backgrounds and excellent limits of detection for dissolved organic compounds. 4 Imbedding powdered zeolite crystals within a silicone membrane can increase the total flux of organics from organic/water mixtures. 5 Additional adsorptive selectivity can also result. Several other membrane types have been used to provide different selectivities. 6 For example, mi- croporous Teflon membranes derive selectivity primarily from steric (size exclusion) effects. 7 Even in cases where mixture components have identical steady- state membrane permeances, differences in the rates at which different components approach steady state can sometimes be exploited to help resolve mixtures. 8 In such “dynamic MIMS” applications, a sample is pulsed to the inlet side of a membrane and the species-dependent permeation delay in the mass spec- trometric ion signal derived from gas sampled from the other side of the membrane induces a phase shift in signals for different components that can be related to sample composition. 8 Recently, 9,10 we assessed the potential MIMS utility of a new and unusual class of zeolite membranes. 11 In contrast to the silicone membrane-doping experiments, 5 the zeolite crystals in these experiments are grown directly as a thin film on a porous ceramic support, resulting in greatly enhanced physical robustness. 12 The thin-film geometry can also allow for fast response to changes in stream composition, making these membranes good candidates for process-monitoring applications. Permeation of molecules through these membranes has been described as a five-step process involving molecular adsorption, transport to the pores, intracrystalline transport, transport out of the pores, and desorp- (1) Srinivasan, N.; Johnson, R. C.; Kasthurikrishnan, N.; Wong, P.; Cooks, R. G. Anal. Chim. Acta 1997, 350, 257-271. (2) Cook, K. D.; Bennett, K. H.; Haddix M. L. Ind. Eng. Chem. Res., in press. (3) Blaser, W. W.; Bredeweg, R. A.; Harner, R. S.; LaPack, M. A.; Leugers, A.; Martin, D. P.; Pell, R. J.; Workman, J.; Wright, L. G. Anal. Chem. 1995, 67, 47R-70R. (4) Soni, M.; Sauer, S.; Amy, J. W.; Wong, P.; Cooks, R. G. Anal. Chem. 1995, 67, 1409-1412. (5) Hennepe, H. J. C.; Boswerger, W. B. F.; Bargeman, D.; Mulder, M. H. V., Smolders, C. A. J. Membr. Sci. 1994, 89, 185-196. (6) Maden, A. J.; Hayward, M. J. Anal. Chem. 1996, 68, 1805-1811. (7) Kasthurikrishnan, N.; Cooks, R. G. Talanta 1995, 42, 1325-1334. (8) Overney, F. L.; Enke, C. G. J. Am. Soc. Mass Spectrom. 1996, 7, 93-100. (9) Cook, K. D.; Bennett, K. H.; Haddix, M. L.; Keator, E. A.; Seebach, G. L.; Falconer J. L. J. Process Anal. Chem. 1998, 3 (3-4), 115-124. (10) Bennett, K. H.; Cook, K. D. 46th ASMS Conference on Mass Spectrometry and Allied Topics; Orlando, FL, 1998; p 1465. (11) Coronas, J.; Noble, R. D.; Falconer, J. L. Ind. Eng. Chem. Res. 1998, 37, 166-176. (12) Funke, H. H.; Argo, A. M.; Falconer, J. L.; Noble, R. D. Ind. Eng. Chem. Res. 1997, 36, 137-143. Anal. Chem. 1999, 71, 1016-1020 1016 Analytical Chemistry, Vol. 71, No. 5, March 1, 1999 10.1021/ac980991n CCC: $18.00 © 1999 American Chemical Society Published on Web 01/21/1999