Acetic Acid Reduction to Acetaldehyde over Iron Catalysts: I. Kinetic Behavior

The vapor-phase reduction of acetic acid by H2 over both supported and unsupported iron was studied. Typical Fe catalysts increased in activity over a 4- to 5-h period to reach steady state whereas the Fe/carbon catalyst deactivated completely during this initial time on stream. The best catalysts gave selectivities to acetaldehyde between 95 and 100% at low conversions (<12%), but selectivity dropped to ∼80% as conversions approached 40%. This parameter was enhanced as the H2 pressure increased, but was almost independent of the acetic acid partial pressure. The turnover frequency for acetic acid disappearance to reduction products increased markedly from 0.003 to 0.058 s−1 and the apparent activation energy decreased from about 27 to 16 kcal/mol as the Fe crystallite size increased from 10 to 4000 nm. Highly dispersed 1-nm crystallites on SiO2 exhibited no hydrogenation activity and gave only decomposition products. Acetic acid reduction over Fe is described well by a Langmuir–Hinshelwood model invoking two types of sites, i.e., one set on metallic Fe atoms that adsorbs and activates H2, and the other on an Fe oxide surface that adsorbs acetic acid to form an acetate species. The addition of the second H atom to form acetaldehyde appears to be the rate-determining step.