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Applied Clay Science, Volume 87, 2014, Pages 61-65.
Ingrid Wilke, Vidya Ramanathan, Julienne LaChance, Anthony Tamalonis, Michael Aldersley, Prakash C. Joshi, James Ferris.
Department of Chemistry & Chemical Biology, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180, USA.
Abstract
The real and imaginary parts of the complex index of refraction of montmorillonite between 0.2 THz and 1.4 THz at room temperature are reported. The optical properties of montmorillonite in this frequency range were experimentally determined using time-domain THz spectroscopy. The index of refraction of montmorillonite is n = 2.19 ± 0.02, and the extinction coefficient is k = 0.10 ± 0.01. Distinct frequency dependencies of the index of refraction and the extinction coefficient are not observed within the uncertainties of the measurements. The index of refraction of montmorillonite is modeled using an effective medium theory. The characterization of the optical constants of montmorillonite in the THz-frequency band suggests the potential of time-domain THz spectroscopy for the in-situ investigation of catalytic properties of montmorillonite at the molecular level.
Additional Information
Exploration of Interlayer Chemistry in Clay Minerals by Terahertz Spectroscopy
Clay minerals are a highly important commodity. Large quantities of clay are sold as consumer products or used in mining, processing and manufacturing. In technology, clays are valued for both their mechanical and chemical properties. Clay minerals such as montmorillonites are known to catalyze numerous chemical reactions relevant to the chemical and pharmaceutical industries. Since montmorillonites are a natural resource abundantly present on Earth with no reported toxicity to humans, animals and plants, they are regarded as a promising candidate for the development of chemical processes that reduce or eliminate the generation of hazardous waste (green chemistry).
Synthesis of organic molecules using montmorillonite as catalysts is empirically tested but not understood at the molecular level. This is a major problem for the application of montmorillonite as an efficient and green catalyst for the cost-effective industrial production of organic chemicals.
Catalysis by montmorillonite is the result of the layered structure of the material. In montmorillonite, two tetrahedral sheets of SiO4 sandwich a central octrahedral sheet of Al2O3. The spacing between the sheets is a few nanometers. Naturally, the space is filled with exchangeable cations (e.g. Na+) and water molecules. For catalysis, the molecules of interest are “inserted” into the space between the layers. This is the location where the chemical reaction occurs and the reaction products are formed. The insertion occurs via diffusion processes.
Currently, THz spectroscopy is being evaluated as an experimental method to study the interlayer chemistry of clay minerals. For montmorillonite the penetration depth of electromagnetic radiation in the infrared is calculated to be in the order of 1um. Our measurements demonstrate that the penetration depth of the electric field of THz-radiation in the THz-requency band is in order of ~ 1mm for the material. The much longer penetration depth in the THz-frequency band suggests using time-domain THz spectroscopy for in-situ investigations of the catalytic reactions at the interlayers of montmorillonite. The THz frequency band provides a literal window of opportunity for reactions catalyzed by clays to be studied directly for relevant organic molecules with characteristic absorption bands in this frequency range. In this way, information currently not available from any other source can be obtained providing details on the alignment of adsorbed molecules, number of species present or even a snapshot of the complete catalytic system.
