A Study Of Hydrogenolysis Of Glycerol To Glycols Over Ni-based Catalysts

The present thesis focuses on the development of key catalyst in the production of valuable commodity chemicals from bio-glycerol through hydrogenolysis.Works are put on the correlation of catalyst preparation,structure and performance.Herein the crystallized Ni-Co-B and Ni-Fe-B catalysts were prepared by chemical reduction in aqueous solution and then by H₂-reduction in gas-phase at elevated temperatures.The effect of bimetal atomic ratio and reduction temperature on the catalytic performance of Ni-Co-B and Ni-Fe-B catalysts was investigated by controlled experimental conditions.The samples were characterized by N₂-adsorption/desorption,X-ray diffraction,NH₃-TPD,H₂-TPD and XPS so as to correlate between structure and performance.The reaction mechanism was also disussed on the basis of characterization results.The crystallized Ni-Co-B and Ni-Fe-B catalysts exhibited excellent activities and good selectivities to 1,2-propanediol(1,2-PDO) and ethylene glycol(EG) in the glycerol hydrogenolysis under relatively mild conditions.The bimetal atomic ratio and reduction temperature showed a considerable influence on the glycerol conversion.As for Ni-Co-B catalyst,the hydrogenolysis of a 20wt%glycerol aqueous solution with an optimized Ni/Co atomic ratio at 1/7 could give a glycerol conversion higher than 95%with combined selectivity to 1,2-PDO and EG above 70%when the reaction was run at 473 K and 2.0 MPa for 12 h.The conversion tended to decrease when the Co amount was increased in the crystallized Ni-Co-B catalyst with almost retention of the selectivities to 1,2-PDO and EG.As for Ni-Fe-B catalyst,the glycerol conversion reached a maximum of 82.9%with an optimized Ni/Fe atomic ratio at 4/1. The conversion tended to decrease at higher content of Fe in the crystallized Ni-Fe-B catalyst,but the selectivity to 1,2-PDO kept increasing gradually.In addition,the glycerol conversion tended to decrease when both catalysts were reduced at higher temperatures,but the selectivities to 1,2-PDO and EG kept unchanged essentially. Besides,it was worthy noting that acetol was detected with different degrees in all products of glycerol hydrogenolysis over the Ni-Co-B and Ni-Fe-B catalysts. The XRD results indicated that the phase transition occurred to the amorphous Ni-Co-B and Ni-Fe-B precursors by reduction at 523 K in 5%H₂-N₂.The diffraction lines became more intensive after the glycerol hydrogenolysis.As for Ni-Co-B catalyst,the incorporation of Ni into Co-B could bring about changes in the BET surface area.The sample with Ni/Co molar ratio at 1/7 gave a maximal surface area of 59.4 m² g^-1.As for Ni-Fe-B catalyst,the sample with Ni/Fe molar ratio at 1/1 gave a maximal surface area by 87.1 m² g^-1.Moreover,there was an obvious drop in the surface area of both Ni-Co-B and Ni-Fe-B catalysts reduced at higher temperatures. The NH₃-TPD and H₂-TPD results indicated that there existed both acid-sites and hydrogenation-sites on the crystallized Ni-Co-B and Ni-Fe-B surfaces.Moreover,the desorption amount of hydrogen increased with increasing the Ni amount in the crystallized Ni-Co-B and Ni-Fe-B samples.Therefore,the concurrence of acid and hydrogenation sites in the catalysts might be beneficial for the reactivity towards glycerol hydrogenolysis.Glycerol was believed to be dehydrated to form an acetol intermediate-acetol over the acid sites,which was then hydrogenated to produce 1,2-PDO.On the other hand,glycerol could also be dehydrogenated to form glyceraldehydes,followed by dehydration to 2-hydroxylacrolein,and then by hydrogenation to 1,2-PDO.Nevertheless,the glyceraldehyde molecule could also undergo C-C bond cleavage through a reverse aldolization reaction to give 2-hydroxyacetaldehyde,subsequently giving EG by catalytic hydrogenation.