Ethylene carbonylation to propionic acid is a powerful route to the synthesis of oxygenates. Mo(CO)(6) is a known homogenous catalyst for this reaction. When supported on HY zeolite, prepared by incipient wetness impregnation from pentane or by vapor deposition, turnover numbers dramatically increase to over 40,000 in 5 h of reaction, particularly for the supports with lower Si/Al ratios. Diffuse reflectance UV-visible spectra and thermogravimetric analysis indicate that lower Si/Al ratios promote more and stronger interactions between Mo(CO)(6) and the support, leading to higher reactivity under liquid-phase reaction conditions. Although some leaching occurs under these conditions, the active catalyst is the supported Mo(CO)(x)/HY, and the recovered catalysts are still stable and active for ethylene carbonylation with turnover numbers exceeding 30,000 mol propionic acid/mol of Mo over 5 hat 190 degrees C. FTIR provides evidence for the formation and stabilization of under-coordinated carbonyl species during heat treatment, and such sub-carbonyls are known to be relevant in previously-established catalytic mechanisms. (C) 2016 Elsevier Inc. All rights reserved.