Adsorption of n-Hexane and p-Xylene in Thin Silicalite-1 Films Studied by FTIR/ATR Spectroscopy

Adsorption isotherms for p -xylene and n -hexane in silicalite-1 films with a thickness of 200 nm were determined at 323, 343, 368, 393, and 423 K using Fourier transform infrared/attenuated total reflection (FTIR/ATR) spectroscopy. For both adsorbates, the low-pressure data agreed with literature data for MFI powder and the estimated Henry’s constant and adsorption enthalpy were close to previously reported results. The upper region of the n -hexane isotherm ( p > 2 kPa at 323 K) was likely influenced by micropores in open grain boundaries, as expected for a polycrystalline film of small (<200 nm) crystals. As for n -hexane, the first part (0 ≤ p ≤ 65 Pa at 323 K) of the p -xylene isotherm agreed with data for powder. However, the saturation capacity was only about half of that previously reported for powders, which indicates that p -xylene molecules do not adsorb in the sinusoidal channels in the film. We speculate that the small crystals used in the present work may behave differently from the larger crystals in previous works. Another explanation for the lower saturation capacity may be the bonding of crystals to the supports, which are known to induce strain in the attached crystals.