The effect of rhenium on the conversion of glycerol to mono-alcohols over nickel catalysts under continuous flow conditions

Abstract

Ni/Al₂O₃ and Ni/SiO₂ catalysts with nominal loadings of 30 wt% Ni and promoted with 1 wt% Re were synthesized via wet impregnation. The catalysts were characterized using XRD, TPR, TPD, TEM, SEM and BET surface area. TPR results showed that promotion with rhenium decreases the reduction temperature of the catalysts and TPD showed that it increases the acidity of the catalysts. SEM showed discernible changes in the distribution of metal oxide on the support after the incorporation of rhenium. These catalysts were evaluated in the hydrogenolysis of glycerol in a continuous flow fixed bed reactor in a temperature range of 250–325 °C and at a H₂ pressure of 60 bar. All catalysts were active, with activity increasing by 3 or 4 times when rhenium was added due to an increase in acidity of the catalyst, thus aiding the dehydration step to form ethylene glycol and 1,2-propanediol. At the highest temperature (325 °C), ethylene glycol and 1,2-propanediol had the lowest yield and, consequently, the yield of lower alcohols such as methanol, ethanol and 1-propanol increased with temperature as the ethylene glycol and 1,2-propanediol reacted further to give these products due to C–C bond cleavage. The Re-promoted catalysts showed a higher total yield of lower alcohols compared to the unpromoted catalysts. Among all the catalysts Re–NiAl₂O₃ showed the highest TOF which is also in agreement with the acidic site concentration, metallic surface area and metal dispersion of the catalyst.