Reaction Mechanisms and Production of Hydrogen and Acetic Acid from Aqueous Ethanol Using a Rn-Sn/TiO2 Catalyst in a Continuous Flow Reactor

Abstract

The catalytic reforming of bioethanol can produce green hydrogen (H2) and acetic acid (AcOH). In the present study, the conversion of aqueous ethanol (EtOH) over 4 wt%Ru-4 wt%Sn/TiO2 in a flow reactor was investigated at different temperatures at 0.1 MPa or at various pressures at 260 °C. The ethanol conversion was rather slow in liquid water, while the reactivity increased significantly when water was evaporated. Under gas-phase conditions at 0.1 MPa, the conversion rate increased with increasing reaction temperature, but the AcOH yield and H2 purity decreased due to by-production of CH4, CO, and CO2. The CH4 and CO generated by fragmentation of acetaldehyde (AA), an intermediate, were suppressed by increasing reaction pressure, although the formation of CH4 and CO2 generated from AcOH was pressure independent. Thus, the highest-pressure conditions in steam at a given reaction temperature are preferred for the production of pure H2. The initial step, EtOH → AA, was the rate-determining reaction, and the model experiments using AA as a substrate showed that the Cannizzaro reaction of two AA molecules to form EtOH and AcOH occurred preferentially. This oxidation system was confirmed to be effective at EtOH concentrations of up to 500 g/L in water.