Use of FTIR spectroscopy of pyridine adsorption at incipient moisture conditions in the interpretation of the hydrophobicity, acidity, and activity on HMOR and HY zeolites

The fatty acids (FFA) adsorption on structurally different (HMOR and HY) zeolites with different Si/Al ratios, were studied by FTIR pyridine spectroscopy on incipient moisture conditions and other complementary characterization techniques to improve the understanding of the relationship between hydrophobicity, acidity, and activity. In HMOR with a low density of Si-OH–Al groups, a significant fraction of the FFA molecules are adsorbed onto the less polar sites with its carboxyl groups oriented away from the surface. In HY, the higher density of bridging Si-OH–Al groups, favors the adsorption of FFA molecules with their carboxyl group hydrogen-bonded to these groups.Highlights 1. The FTIR/pyridine spectroscopy on incipient moisture conditions was employed. 2. Steam dealumination was applied on HMOR and HY (Si/Al ≈ 6 and 3) at 800 and 1000 K. 3. Hydrophobicity increases with the dealumination, but the acidity diminishes. 4. In the HMOR series, hydrophobicity drives the adsorption by the London forces. 5. In HY series, acidity drives the adsorption of the fatty acids through H-bonding