Tailoring selective de-hydroxylation/hydrogenation reactions of bio-based aldaric acids towards adipic acid derivatives by Re catalyst metal–support interactions

Rhenium-based catalysts are highly active in the catalytic deoxydehydration (DODH) reaction, which results in selective dehydroxylation of vicinal diols to unsaturated products. Hereby, DODH combined with subsequent hydrogenation, is applied to upgrade biomass-derived mucic acid towards renewable adipates using supported Re catalysts. The influence of different support materials, as well as the effect of catalyst pretreatment conditions, were examined in our study. The results strongly suggest that only C and TiO2 serve as suitable supports for Re to achieve noticeable DODH activity. Furthermore, reductive pre-treatment protocol crucially affects DODH activity. This pre-treatment is essential for achieving yields of up to 88 % for fully dehydroxylated products at low reaction temperature of 120 °C with methanol as reducing agent. Most notably, DODH activity depends on the co-existence of metallic and high-valent Re sites, as catalyst characterization indicates. Particle growth during (reductive) thermal treatment probably increases the formation of bifunctional sites. A corresponding bifunctional reaction mechanism of the DODH on monometallic Re catalysts is proposed, which can explain the experimental results.