Stainless Steel As a Catalyst for the Total Deoxygenation of Glycerol and Levulinic Acid in Aqueous Acidic Medium

Exposing 316 Stainless Steel pressure reactor bodies to an aqueous Bronstedt acidic solution (trifluoromethane sulfonic acid) at elevated temperatures (100-250 degrees C) under reducing atmosphere (hydrogen gas at 800 psi) leads to the formation of insoluble inorganic precipitates, identified as mixed chromium oxides by scanning electron microscopy X-ray fluorescence (SEM-XRP). A catalytically active metal surface is generated, that is, under these conditions the <100 angstrom thick chromium oxide layer that normally passivates 316 Stainless Steel (316SS) against corrosion is etched away, and the reactor body itself becomes an active hydrogenation catalyst. The effect is specific to aqueous acidic medium and therefore water-soluble substrates as encountered in biomass conversion, for example, sugar alcohols and levulinic acid, which can be deoxygenated to the corresponding alkanes and alkenes using only a Bronstedt acid and the reactor body as the catalyst. Control experiments in several different 316SS reactors built by different manufacturers from different batches of 316SS as well as inductively coupled plasma optical emission spectroscopy (ICP-OES) and mass spectrometry (ICP-MS) analysis of the chromium oxide precipitates formed and steel samples from the reactor body itself indicate that the catalytic activity is not caused by trace amounts of ruthenium or another hydrogenating metal such as Re, Rh, Ir, Pd, or Pt. The observed catalytic activity scales with the concentration of acid and the addition of 316SS added to the reaction mixture as a powder conclusively establishing 316SS as the active catalyst.