Ethanol dehydration and dehydrogenation on γ-Al₂O₃: Mechanism of acetaldehyde formation

Steady state kinetics and measured pyridine inhibition of ethanol dehydration and dehydrogenation rates on γ-alumina above 623 K show that ethanol dehydrogenation can be described with an indirect hydrogen transfer mechanism to form acetaldehyde and ethane and that this mechanism proceeds through a shared surface intermediate with ethylene synthesis from ethanol dehydration. Ethane is produced at a rate within experimental error of acetaldehyde production, demonstrating that ethane is a coproduct of acetaldehyde synthesis from ethanol dehydrogenation. Steady state kinetic measurements indicate that acetaldehyde synthesis rates above 623 K are independent of co-fed water partial pressure up to 1.7 kPa and possess an ethanol partial pressure dependence between 0 and 1 (P_ethanol = 1.0-16.2 kPa), consistent with ethanol dehydrogenation rates being inhibited only by ethanol monomer surface species. The surface density of catalytically active sites for ethylene and diethyl ether production were estimated from in situ pyridine titration experiments to be ∼0.2 and ∼1.8 sites nm^-2, respectively, at 623 K. Primary kinetic isotope effects for ethylene and acetaldehyde are measured only when the C-H bonds of ethanol are deuterated, verifying that C-H bond cleavage is kinetically limiting for both products. The proposed indirect hydrogen transfer model for acetaldehyde synthesis is consistent with experimentally observed reaction rate dependences and kinetic isotope effects and highlights the complementary role of hydrogen adatom removal pathways in the formation of aldehydes on Lewis acidic systems. (Chemical Equation Presented).