Benzene hydrogenation over iron: II. Reaction model over unsupported and supported catalysts

Abstract

The kinetic behavior of unsupported iron and iron dispersed on η-Al 2O 3, SiO 2, carbon, and B-doped carbon in the benzene hydrogenation reaction was studied over a range of 390–515 °K, 300–700 Torr H 2, and 15–100 Torr benzene in a differential, plug-flow reactor. Iron surface areas were measured by CO chemisorption, and turnover frequencies (TOF) over iron were found to be at least 10-fold lower than TOF values on the other Group VIII metals. These iron catalysts showed a maximum in activity vs temperature with all catalysts exhibiting this maximum near 473 °K, except for unsupported iron for which it occurred at 493 °K. Pressure dependencies on H 2 were consistently close to 3rd order whereas benzene pressure dependencies were temperature-dependent and varied between 0 and −1. A Langmuir-Hinshelwood rate expression of the form r = k 1P H 2 3 B (1 + K B P B ) 2 was found to provide a surprisingly satisfactory fit of all the data. In addition, this rate equation predicted the observed activity maximum and provided an explanation of this behavior which has also been observed in benzene hydrogenation over other metals. Values of enthalpy and entropy of adsorption for benzene which were obtained from the optimized rate equation were very reasonable and consistent. This study provided evidence that this reaction is structure insensitive over iron. No major support effect was found on kinetic behavior; however, iron dispersed on graphitized carbon and B-doped graphitized carbon exhibited superior activity maintenance and the maximum TOF values achievable were 2–5 times higher on the B-doped catalysts than on any other catalyst.