Monitoring the Brönsted acidity of zeolites by means of in situ FT-IR and catalytic testing using chloromethane as probe molecule

The Brönsted acidity of a series of protonated zeolites with different structures and Si/Al ratios such as HY (Si/Al = 2.4), H(Na)EMT (3.6), HMOR (9.5), HZSM-5 (21.7) and HBeta (12.5) have been studied at room temperature and at various temperatures by means of in situ FT-IR spectroscopy using chloromethane as probe molecule. CH₃Cl was found to interact by its negatively charged chlorine atom with the polarised hydrogen atoms of hydroxyls of zeolites through hydrogen bonding, causing the red-shift of the hydroxyls vibration. The extent of the red-shift supplies important information on the strength of Brönsted acidity of zeolites which ranks in the order: HBeta > HMOR ≈ HZSM-5 > H(Na)EMT ≥ HY. The study made at various temperatures revealed that the interaction of chloromethane with hydroxyls of zeolites was dependent on the temperature, reflecting the strength and the heterogeneity of acidic hydroxyls of zeolites. This temperature-dependent study confirms also the acid strength ranking obtained on the basis of the extent of the red-shift. The direct conversion of CH₃Cl was carried out in the temperature range of 200-500 °C. The intrinsic activity of the five investigated zeolites has been compared with and was found to vary in the sense as acid strength. The selectivity of products of chloromethane conversion was also correlated with the Brönsted acid strength of the zeolites.