Boosting the Long-Term Stability of Hydrotalcite-Derived Catalysts in Hydrogenolysis of Glycerol by Incorporation of Ca(II)

Abstract

Selective hydrogenolysis of biomass-derived glycerol into 1,2-propanediol (1,2-PDO) is of practical importance to upgrade the huge amounts of surplus glycerol derived from renewable biomass. Herewith, we report that hydrotalcite-derived catalysts have been employed for selective hydrogenolysis of glycerol into 1,2-PDO under aqueous phase and base-free conditions. In particular, the catalyst of Co₂-Ca₄-Al₃ was recorded as a good catalyst with an optimum glycerol conversion of 100% and 1,2-PDO selectivity of 90.5% in glycerol hydrogenolysis. Meanwhile, it exhibited high stability in consecutive catalytic recycles under batch-wise operation conditions. Furthermore, the catalyst also demonstrated high glycerol conversion (ca. 95.0%) and selectivity to 1,2-PDO (ca. 90.0%) with a long-term catalytic stability (280 h) by a continuous-flow fixed-bed reactor. Additionally, the catalyst can be further extended to selective hydrogenolysis of some other sugar polyols to 1,2-PDO (selectivity up to 50.5%–68.5%). The activity tests and characterizations demonstrated that Co⁰ existing in the catalysts acted as active sites in glycerol hydrogenolysis. Also, the further characterization by XRD, HRTEM, and EDS element maps revealed that the distribution of Co was basically consistent with that of Al, while Ca species were adjacent to Co and Al species. The mixed metal oxides CaO-CoAl₂O₄ were generated arising from a strong interaction between Co(II) and Ca(II) oxides species, which endowed more electron-deficient Co(II) sites and also led to large amounts of moderate basic sites, being favorable for enhancing catalytic activity and selectivity. Notably, the forming CaCO₃ phase played a crucial role in preventing the leaching of moderate basic sites and metal species by covering CaO-CoAl₂O₄ mixed metal oxides, which was the crucial reason that the present Co₂-Ca₄-Al₃ catalyst showed the remarkable catalytic stability under aqueous phase reaction conditions. On the basis of the results above, the reaction pathway has been discussed as well.