Development of a novel experimental technique for studying zeolites : combining zeolite coated ATR elements and FTIR spectroscopy

Sammanfattning: Thin zeolite films have great potential in several novel application areas such as structured catalysts, membranes and sensors. To fully exploit the advantages of these films it is of great importance to determine the adsorption properties of the films. A powerful technique for studies of phenomena at surfaces or in thin films is FTIR/ATR-spectroscopy (Fourier Transform Infra Red / Attenuated Total Reflection). In this work, MFI zeolite films were prepared on ATR elements using two methods. One method produced 200 nm thick films with small crystals (<200nm). The other method was used for preparing b-oriented ZSM-5 films. These films were discontinuous and ca 420 nm thick and consisted of well intergrown, and substantially larger crystals, ca 1.1 ìm in diameter. Silicalite-1 coated elements were evaluated as gas sensors and the sensitivity for a hydrocarbon for zeolite-coated elements was compared with a standard 10 cm gas cell. The sensitivity was approximately 85 times higher for the coated elements at low hydrocarbon concentration. The response time was investigated by exposing the coated element to a step increase of an analyte and recording the response as a function of time. The response was relatively fast, equilibrium was achieved after approximately 250 s, but already after a few seconds, a strong signal could be detected. The coated elements were also used for determining single gas adsorption isotherms. The studied systems were n-hexane/silicalite-1, p-xylene/silicalite-1 and p-xylene/ZSM-5. The observed isotherms for temperatures between 323 and 423 K were typical for microporous materials with a fast increase of the amount adsorbed at low partial pressures. p-Xylene isotherms of type I were observed as opposed to the type IV isotherms reported in literature for powders. This difference was assigned to strain in the films and/or to reduced flexibility of the MFI framework when attached to a support. Further, some capillary condensation occurred at higher pressures in the films consisting of smaller crystals, which was assigned to condensation in open grain boundaries. Henry's constants and heats of adsorption determined from low-pressure data agreed well with previous reports. Measurements with polarized radiation revealed that p- xylene molecules are mainly oriented with their long axis in the b- direction of the crystals and it was also observed that some kind of rearrangement of n-hexane occurs with increasing loading, both of these findings were in agreement with previously reported results.