Project 5: Studying Zeolitic Diffusion by Interference and IR Microscopy

Transcription

1 Application for Financial Support of a Proposal within a Package of Proposals Submitted for Joint Evaluation by CNRS, DFG and EPSRC Project 5: Studying Zeolitic Diffusion by Interference and IR Microscopy Prof. Dr. Jörg Kärger Institut für Experimentalphysik I der Universität Leipzig Financial Support Requested from: DFG

2 1. General Information 1.1. Applicant Prof. Dr. Jörg Kärger Lehrstuhlinhaber (C4) Grenzflächenphysik Born 3 October 1943 Institut für Experimentelle Physik I Universität Leipzig Linnéstr. 5 D Leipzig phone: private address: Ahornweg 10 D Markranstädt phone: Topic Studying Zeolitic Diffusion by Interference and IR Microscopy 1.3. Code Word Diffusion by Microscopy 1.4. Research Areas Material Sciences Chemical Engineering Physical Chemistry Interface Sciences 1.5. Scheduled Total Duration of Support six years 1.6. Period of Support Applied for three years 1.7. Begin of Support April Summary Interference and IR microscopy are so far the only two techniques, which allow the direct observation of the evolution of concentration profiles in the interior of zeolite crystals. In the present project they are applied for the investigation of intracrystalline transport diffusion in a series of zeolitic adsorbate-adsorbent systems. The observed transport properties are compared with the results of the diffusion measurements of the identical systems by the other techniques of the consortium. These investigations shall be accompanied by methodological developments of the techniques. They focus on a further enhancement of space and time resolution and sensitivity, as well as on an extension of the temperature range of the measurements. Project 5, page 2

3 2. State of the Art, Own Activities in the Field 2.1. State of the Art As an ingenious alternative to the gravimetric method, H. Karge for the first time applied IR spectroscopy for the observation of sorption phenomena on zeolites [1-4]. This approach offers the important advantage that, by a judicious choice of the IR wavelength, the sorption of any individual component may be followed separately. Another advantage of this approach is that IR measurements can be applied to individual crystallites, although this technique has not yet been widely exploited [5-7]. It suffers, however, from limited resolution (~ 10µm) because of the relatively long wavelength. This limits the application of this technique to relatively large crystals. This deficiency may be overcome by the use of optical interference microscopy. After preliminary attempts to use this approach to measure the rate of moisture uptake from the surrounding atmosphere [8], interference microscopy has now been used to determine the evolution of intracrystalline diffusion profiles during the uptake of methanol in zeolite NaCaA [9,10], of isobutane in silicalite-1 [11,12] and of methanol and water in AFI-type zeolites [13] Own Activities in the Field The only measurements of zeolitic diffusion by interference microscopy so far performed have been carried out in this laboratory [8-14]. They have for the first time provided information of transient intracrystalline concentration profiles during molecular uptake and release [9,10] and led to the establishment of a novel technology for the structural analysis of nanoporous host materials by tracing the distribution of the guest molecules [11,14]. Following refs. [1-7], in this laboratory meanwhile also the facilities for the measurement of zeolitic diffusion by IR microscopy are given. First diffusion studies have been performed with water and methanol in CrAPO-5 [14,15] and with ethanol in silicalite-1 [16]. The application of interference microscopy to the identical systems yielded satisfactory agreement in the observed concentration profiles [14]. Comparison of this type highly benefits from the complementarity of the two techniques, viz. the high sensitivity of IR microscopy with respect to the individual molecular species and the high spatial resolution of interference microscopy References 1 W. Niessen, H. G. Karge, Studies in Surf. Sci. Catal. 60 (1991) W. Niessen, H. G. Karge, Microporous Mater. 1 (1993) 1 3 H. G. Karge, W. Niessen, H. Bludau, Appl. Cat. A 146 (1996) R. Schumacher, H. G. Karge, J. Phys. Chem. B 103 (1999) M. Hermann, W. Niessen, H. G. Karge, Studies in Surf. Sci. Catal. 94 (1995) H. Karge, in Proc. Third Polish German Zeolite Coll, Torun, 1998, p.11 7 Y.S.Lin, N. Yamamoto, Y. Choi, T. Yamaguchi, T. Okulo, S.-I. Nakao, Micropor. Mesopor. Mater. 38 (2000) J. Kärger, R. Danz, J. Caro, Feingerätetechnik 27 (1978) U. Schemmert, J. Kärger, C. Krause, R. A. Rakoczy, J. Weitkamp, Europhys. Lett. 46 (1999) U. Schemmert, J. Kärger, J. Weitkamp, Micropor. Mesopor. Mater. 32 (1999) O. Geier, S. Vasenkov, E. Lehmann, J. Kärger, U. Schemmert, R. A. Rakoczy, J.Weitkamp, J. Phys. Chem.B. 105 (2001) S. Vasenkov,, O. Geier, U. Schemmert, J. Kärger, R. A. Rakoczy, J. Weitkamp, in Fundamentals of Adsorption 7 (K. Kaneko et al., Edts.), IK Internat. Publ., Shinjuku 2002, O. Geier, S. Vasenkov, E. Lehmann, J. Kärger, U. Schemmert, R. A. Rakoczy, J. Weitkamp, Stud. Surf. Sci. Catal. 13 (2001) E. Lehmann, C. Chmelik, H. Scheidt, S. Vasenkov, B. Staudte, J. Kärger, F. Kremer, G. Zadrozna, J. Kornatowski, J. Amer. Chem. Soc. 124 (2002) 8690 Project 5, page 3

4 15 H. Scheidt, Diplomarbeit, Leipzig, C. Chmelik, Diplomarbeit, Leipzig, Issues and Working Programme 3.1. Issues With the application of IR and interference microscopy, microscopic techniques for the measurement of transport diffusion are applied. In this way direct evidence about the evolution of the intracrystalline concentration profiles during transient sorption measurements is obtained, which has to be compared with the findings of the other techniques of diffusion measurement, in particular of the macroscopic transient measurements. In order to ensure the largest possible overlap with the ranges of measurement of the other techniques, the investigations shall be accompanied by further improvements in the experimental procedure, including the enhancement of time and space resolution and of detection sensitivity, as well as the possibility of temperature variation Working Programme The microscopic transient diffusion measurements start in the sequence as predetermined by the accessibility of the zeolite host materials indicated in tab.1 of the synthesis project. In a following, second period of time, the measurements are repeated with identical or modified materials taking account of the findings by the other groups of the consortium. In comparison with all other techniques, these microscopic methods are the only ones, which deduce their information about intracrystalline diffusion from the directly observable evolution of the (partially averaged) internal concentrations. This information is attainable without any additional assumption. Checks of consistency of the attained data are therefore of less relevance than in the other projects. On the other hand, operation with single crystals is much more subjected to the risk of sample deterioration. Such possibilities have to be excluded by ensuring data reproducibility by operation with different crystallites NaCaA On investigating the n-alkanes, the IR measurements will suffer from the small crystal sizes (probably with diameters around or less 50 µm) as a consequence of their pourer spatial resolution, while in the interference measurements one has to check, whether the changes in the refractive index are sufficient. The latter problems shall not exist with methanol Cation-free LTA Though such experiments are probably most interesting, it is unlikely that syntheses products of sufficient size shall become available Silicalite-1/ZSM-5 In previous microscopic studies, zeolite crytallites of this type were generally found to contain an extended system of internal structural defects affecting the transport properties. The information provided in this project for the material synthesized and applied by the consortium is therefore of particular relevance. Special attention has to be given to the influence of sample pretreatment/calcinations. Project 5, page 4

5 NaX/NaY Both types of measurements attain their highest accuracy when applied to the observation of intracrystalline molecular concentration distributions between parallel surface faces. Therefore, the octahedric shape of the crystallites makes diffusion measurements with X/Y type zeolites by IR and interference microscopy much more complicated than with the other host systems considered by the consortium. Measurements of this type will be performed, therefore, only if in the course of the experiments they might become of particular interest. This might in particular be the case with IR microscopy, if it is sufficient to exclusively apply as a mesoscopic technique (cf. tab.1 of the Introduction to this package), i.e., if one restricts oneself to information about the mean intracrystalline concentration rather than to intracrystalline concentration differences AlPO 4-5 Owing to their hexagonal shape, crystals of type AlPO 4-5 offer excellent conditions for observation in the direction of and parallel to the channels. In addition to the potentials of interference microscopy, which (though with a higher spatial resolution) can only be applied to the measurement of transport diffusion, IR microscopy by simply observing the counterdiffusion of two isotopes - can also be applied to study self-diffusion processes. Thus, the comparative experiments suggested in the respective projects for transient sorption and PFG NMR, could be performed, using IR microscopy, by one and the same technique, simply using two different ways of application. Thus, with isobutane (expected to be a single-file system) the differences in the uptake and tracer-exchange rates should be much larger than with methane, where a mutual passage of the molecules is more likely Ferrierite Again, as in the case of AlPO 4-5, methane, isobutane and methanol are used as diffusants. While methane should be accessible to both channel systems, isobutane will probably only penetrate into the system with the larger channel diameters Methodological Developments a) So far, the observation rate of molecular uptake by interference microscopy is limited by the necessity of a series of subsequent measurements for any individual data point for unequivocally discriminating the relevant phase from the observed light intensities. By developing and implementing the appropriate software for data analysis, this limitation has to be overcome, so that time constants of fractions of seconds may be attained. In conventional (i.e.) macroscopic transient diffusion experiments, processes with such small time constants are generally assumed to provide little information about intracrystalline diffusion. This is due to the fact that such fast processes are most likely controlled by external effects (such as heat release and external transport resistances). With the option of measuring intracrystalline concentration profiles, however, such a limitation does not exist for the microscopic measurements (provided that there is still a notable contribution of intracrystalline transport, which prohibits a totally homogeneous sorbate concentration over the intracrystalline space) b) With the design of appropriate observation cells both temperature variation and temperature control and an enhancement of the frequency range of the IR measurements shall become possible. As a consequence of the rather critical conditions, the observation cell has to be subjected due to the necessity of sample activation, the present observation Project 5, page 5

6 cells allow only measurements at room temperature and between 2700 and 8000 cm -1. Extending the frequency range to the finger-print region ( cm -1 ) would allow to pinpoint many adsorbates, which have similar structure, by their characteristic IR bands. It would also increase the precision of the concentration measurements due to the wellknown fact that the shape of the IR adsorption bands in this region does not depend significantly on the intracrystalline concentration, thus leading to a direct correlation between the band intensity and the intracrystalline concentration. c) Up to now, spatial resolution in IR microscopy could only be achieved by application of a motorized (sample-) stage. With the recent advent of focal plane array detectors the situation has completely changed. Allowing a spatial resolution of µm 2 with typical time steps of 0.36 s, the so far inevitable compromise between space and time resolution has become unnecessary. IR microscopes equipped with focal plane array detectors have therefore all potentials for a further decisive step ahead in monitoring transient intracrystalline concentration profiles with unprecedented spatial and temporal resolution. We intend (involving partners from this University and the University of Halle) to establish a user network, which will justify the installation of such an equipment as an HBFG (Hochschulbau-Förderungsgesetz) project. 4. Requested Financial Support 4.1. Personnel The complexity of the task involving two different microscopic techniques and their instrumental development requires the employment of a post-doctoral fellow (Research Associate, BAT IIa). For performing a part of the measurements, funding of a research student (studentische Hilfskraft) is applied for Durable Equipments Turbo-molecular pump (Pfeiffer Vacuum TSU 071 E) 5,300, is to be applied to achieve high vacuum (< mbar) in the vacuum system of the interference microscope. The high-performance characteristics of this pump will ensure that the vacuum system will be free of tracer amounts of vacuum oil. Small amounts of vacuum oil can be expected to occur in a vacuum system if a lower-cost mechanical or diffusional pump is used. Both, high vacuum and vacuum oil-free conditions are essential when performing the activation and the diffusion measurements with single crystals in the apparatus. As explained under item c), we are going to establish a local network of users of an IR microscope, equipped with a focal plane array detector (DIGILAB), at the Universities of Leipzig and Halle, which should give us the possibility of applying for such an instrument as an HBFG (Hochschulbau-Förderungs-Gesetz) project Consumables Pure and isotope-labelled substances 5,000 Vacuum-tight glass cuvettes for both interference and IR microscopy 5,000 Project 5, page 6

7 4.4. Travelling The work within the package of projects depends on the quality of mutual exchange between the partners. This has to include the possibility of mutual visits in addition to the regular meetings (1000 per year). Therefore, a total amount of 2000 per year, corresponding to 6000 for the total period of time is requested for, to support these visits and meetings. 4.5 Other Costs For the support of conference participation (Annual German Zeolite Conferences, Conferences of the International Zeolite Association and Adsorption Association) an amount of 3000 is requested for. 5. Requirements for the Success of the Application 5.1. Team Members a) Prof. Dr. Jörg Kärger (applicant) b) PD Dr. Brigitte Staudte, c) Dr. Sergey Vasenkov, d) Dipl.-Phys. Christian Chmelik b) e) Dipl.-Chem. Enrico Lehmann, f) Dipl.-Phys. Cordula Krause g) Lutz Moschkowitz 5.2. Cooperation with Other Scientists The primary partners of cooperation are the members of the consortium. Following a long cooperation, the investigations shall be carried out in close contact with Prof. Dr. D. M. Ruthven, Univ. of Maine, USA.. As an 2002 Humboldt Awardee he is going to stay in our laboratory over a longer period of time. With respect to IR microscopy, continuous contact with Dr. H. G. Karge shall be maintained Available Equipments Vacuum devices for out-of-system sample activation, Interference microscope Jenamap p dyn IR microscope FTS 6000/UMA 500, programmable sample holder 5.4. Support from the Own Budget From the budget of the institute an amount of about 1000 may be used for ensuring the operating conditions of the equipments Further Prerequisites The institute offers the working places for the co-workers, applied for, as well as the necessary laboratories. The infrastructure of the institute allows the use of data-networks and computer systems free of charge. Project 5, page 7

8 6. Statements 6.1. Collaborative Research Centres (Sonderforschungsbereiche) The present application cannot be part of any of the existing collaborative research centres of the University Application for Support by Different Sources I have never applied for financial support of this or a similar project. If I shall do so, I will immediately inform the German Science Foundation (DFG) Information to the University Representative of DFG (Vertrauensmann) The university representative of the DFG, Prof. Dr. F. Jacobs has been informed about this proposal. 7. Signature Leipzig, November 1, 2002 Prof. Dr. Jörg Kärger Project 5, page 8