Raman Time-Resolved Ion Exchange Studies in Natrolite Rachel Hentz and Aaron Celestian, Geography and Geology

ABSTRACT: The mechanics of ion exchange and ion mobility within zeolitic materials and aqueous solutions are not well understood due to the rate of reaction and the difficulty in probing samples in situ. Knowing the reaction process and understanding the behaviors of ion mechanics in the solid state can we tailor materials for specified functions. In this study, we conducted time resolved ion exchange using Raman spectroscopy, on natrolite with a focus on understanding the crystallographic and chemical transformations. Natrolite successfully sequestered ions through its elliptical channels and has previously exhibited high selectivity for large ion radius cations. Our studies had shown that there is a two step exchange process: 1) softening of the 8 member rings as K exchanges directly into the Na site, and 2) after an unmeasured amount of K had exchanged, the 4 member ring columns rapidly distort to open the 8 member rings as K migrates to one side (see figure below).

Background Methods/Experiment Results and Discussion

DISCUSSION: Using a combined set of experimental techniques, our measurements suggest that there are two steps during ion exchange. First, there is a ‘softening’ of the 8MR and 4MR as the polyhedra distort to accommodate the ingoing K cation. Second, after a maximal strain, the 8MR rapidly (>1min.) open to allow K to migrate to the walls of the 8MR. Future work will be focused on detailed structural transitions and on increasing the effectiveness of ion exchanges to use in the industry field for waste water filtration.

METHODS: A single crystal of natrolite was mounted to 0.0125” diameter polyimide tubing that had been cut to a diagonal point. Larger 0.0625” diameter polyimide tubing was cut to create a window for viewing the crystal inside and to allow the solution to escape. The smaller tubing was then inserted into larger 0.0625” diameter polyimide tubing and glued in position so that the crystal appeared flat in the window. The data acquisition routine was set with the following experimental parameters: laser wavelength, 780nm; laser power at sample, 24mW; aperture, 50µm slit; grating, 830 line/mm; estimated spot size, 1.6µm; allowed range, 1871 to 23cm-1

; min range limit, 50cm-1; max range limit

1868cm-1; objective, 50x 0.5 N.A.; collect exposure time 10sec; sample

exposures, 3. A dry Raman spectrograph was taken first and followed by a deionized “wet” graph sampling. Then a macro program was written for 400 loops of the Raman data acquisition routine, automatic smoothing each graph, and saving each 30 sec. complete scan. The program was started with the starting of the Masterflex Console Drive, set at .6 (effectively 1 mL/min.), that connected the ion solution to the crystal. Natrolite crystals were exchanged with 0.1M solutions of KCl (CsCl and LiCl were also examined but not presented here) under constant advection for 4 hours at room temperature. A new crystal was used for each experiment and the finished crystal was taken to the single crystal x-ray diffractometer for structural analysis.

AKNOWLEDGEMENTS: This work was supported by a grant through the NSF-REU program and the Advanced Materials Institute at WKU.

ABOVE: Upon ion exchange the natrolite channel locks in the ion into its nanoporous crystalline framework. This distorts the channel and traps the ion into the structure, thus effectively sequestering the ion from solution.

PICTURE SERIES BELOW: This series displays our method setup in the lab. (1) Natrolite crystal before ion exchange. (2) Natrolite crystal after potassium ion exchange. (3) Single crystal mount on polyimide tubing. (4) Window setup for ion exchange, with the crystal mounted on the smaller tubing that lies inside the larger tubing. (5) Smaller tubing was inserted into the larger tubing. (6) Setup in the Raman DXR machine ready for ion exchange.

LEFT IMAGE: This is the connection we had setup between the Raman DXR to the Masterflex Console Drive and ion solution bottles.

ABOVE: Time resolved data of K exchange into Na-natrolite. Note the two different exchange steps: (1) a decrease in peak intensity and (2) a shift in peak position to lower wavenumbers. Peak assignments from Liu et al. (submitted).

RIGHT: This is the Iterative Targeted Transformation Factor Analysis which helps determine when changes are occurring in the spectra. This analysis is for the K exchange into Na-natrolite and displays a possible two-step exchange process.

RESULTS: For the K exchange with Na-natrolite the Iterative Targeted Transformation Factor Analysis displays one significant slope decrease (Step 1) in intensity between frames 10 and 15 and a second slope decrease (Step 2) between frames 15 and 17. There was a decrease in intensity of the peaks in the Raman spectrum at 443cm-1 and 535cm-1 at 43 min. (onset of Step 1) and at second decrease and peak shift after 48 min. (onset of Step 2).

INTRODUCTION: Particular toxic species of heavy metals such as Cs, Ni, Cu, and Zn, and light metals such as Li, K, and Na contaminate water when they are released from industrial waste, mine tailings, and rocks. The low concentrations of these metals in surface and ground waters make it difficult and expensive for selective removal, thus offering no economic incentive to reprocess those metals, such as into a sellable commodity. Naturally occurring or synthetic nanoporous materials have large channel structures (on the scale of angstroms) and have applications in petroleum refinement, gas separation, fertilizers, and environmental remediation to adsorb/desorb metals from solutions. Time-resolved Raman spectroscopy is an useful way to observe changes in the shift of energy, which gives information about the vibrational modes in the system. This information can give insight to the structure of zeolitic materials as a reaction takes place. Collecting spectrums and surface maps in real-time during the ion exchange experiment allows us to monitor chemical and structural properties as cations diffuse in and out of the crystalline host.

From Liu et al. (submitted to American Mineralogist) Used with permission.