Assessing Mass Transport of Isooctane in Zeolites by In Situ FTIR Spectroscopy

Abstract In situ FTIR spectroscopy was established as a standalone method for quantifying mass transport in zeolites. By monitoring two independent IR markers, global isooctane uptake (δ(CH) bands) and Brønsted acid site accessibility (3610 cm –1 band), the method reveals a 5 to 11 times difference between overall uptake and true micropore diffusion, demonstrating its ability to distinguish surface adsorption from intracrystalline transport. While overall diffusion is slowest for ZSM-11, its micropore diffusion is comparable to that of ZSM-5 samples. This suggests that the straight intersecting channel system of the MEL framework facilitates intracrystalline transport, although this advantage is masked by slower surface adsorption in the overall uptake. A slight difference between TS-1 and ZSM-5 is attributed to a combination of pore geometry and weaker Lewis acidity of titanium-containing frameworks. In situ FTIR offers a direct, chemically specific means to assess diffusion contributions, providing a valuable tool for optimizing mass transport in zeolite-based catalysts.