A Study Of Carbon Nanotubes-Promoted Cu-Zn Catalyst For Hydrogenolysis Of Glycerol To Glycols

Glycerol is a potentially important renewable resource and feedstock,available from animal oil and fat,vegetable oil or sugars by simple processes.Often glycerol exists as byproduct in many chemical and biorefinery processes.For every 9 kg of biodiesel produced,about 1 kg of a crude glycerol byproduct is formed.Therefore,the production of high-added value chemicals from glycerol is particularly crucial for the biodiesel enterprise economically viable.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.Specifically,a class of carbon nanotubes-promoted Cu-Zn catalysts(Cu-Zn-CNTs) was prepared using co-precipitation method.Their catalytic performance in glycerol hydrogenolysis to produce glycols,namely 1,2-propanediol(1,2-PDO) and ethylene glycol(EG),was investigated,and the nature of promotion effect of CNTs and the reaction mechanism were discussed on the bass of the characterization results by means of N₂O titration,XPS,H₂-TPR,CO₂/CO-TPD,Raman,XRD,HRTEM and SEM.The glycerol conversion is significantly enhanced over Cu-Zn-CNTs catalyst under mild conditions,keeping the similar selectivities to 1,2-PDO and EG in comparison to the CNTs-free Cu-Zn one.The experimental results indicate that the Cu-Zn-CNTs catalyst performance is strongly influenced by several parameters such as the pretreatment time for as-grown CNTs,the CNTs content in the catalyst,the atomic ratio of Cu/Zn,the reduction temperature,the reaction conditions(temperature, H₂-pressure,time) and the glycerol concentration.The highest performance can be obtained when using the following optimization conditions:i.e.,the as-grown CNTs was pretreated in concentrated HNO₃ for 4-12 h,the content of purified CNTs in the catalyst was ranging 2.5-12.5 wt%,the atomic ratio of Cu/Zn was set at 3/1,and the catalyst was reduced at 523 K.The hydrogenolysis of a 20 wt%glycerol aqueous solution over the Cu₃-Zn₁-CNTs catalyst may give a glycerol conversion of 74.9% with a selectivity of 75.3%to 1,2-PDO when the reaction was run at 473 K and 2.5 MPa for 18 h.Such conversion is 67.9%higher than that over CNTs-free Cu₃-Zn₁ catalyst.On the other hand,when the hydrogenolysis of concentrated glycerol of 99 wt%over the Cu₃-Zn₁-CNTs catalyst was carried out at 453 K and 2.5 MPa for 6 h,a conversion of 73.8%and a selectivity of 92.2%to 1,2-PDO can be achieved.The performances over the Cu₃-Zn₁-CNTs in the present study are higher than those over the Cu-based catalysts reported in literature so far.The CNTs before and after pretreatment were characterized using XRD,XPS, Raman,CO₂/CO-TPD,SEM and HRTEM.The results show that the residual catalyst and the amorphous carbons at the CNTs can be removed and the functional groups, mainly as carboxyl groups,are created on the surfaces of CNTs after the as-grown CNTs were pretreated in a concentrated HNO₃ at 363 K for several hours.The CNTs are structurally stable during the pretreatment.The structure features of the catalysts with and without CNTs were studied by means of XRD,XPS,H₂-TPR,N₂O titration and SEM/HRTEM.The entities of Cu⁰ and ZnO are the major species besides the CNTs in the working Cu-Zn-CNTs catalyst. No differences in the valences of Cu and Zn species are observed in the samples with and without CNTs.The incorporation of a minor amount of CNTs into the Cu₃-Zn₁ leads to a lowering in CuO_x reduction-temperature and an enhancement in the Cu-dispersion.The reaction TOF(by mol of glycerol converted over per mol of copper on the catalyst surface per hour) over the Cu₃-Zn₁-CNTs catalyst is higher than that over the CNTs-free one.In addition,the apparent active energies in the glycerol hydrogenolysis over the catalysts with and without CNTs are quite similar to each other(81 kJ/mol over Cu₃-Zn₁-CNTs and 86 kJ/mol over the Cu₃-Zn₁),suggesting that the glycerol hydrogenolysis over the two catalysts is essentially underwent the same reaction pathway.Therefore,the CNTs may do not take part in the construction of the active sites and the promoting action is more probably indirect,including enhancing the adsorption/desorption and promoting the spillover/activation of hydrogen species.Nevertheless,to understand and clarify the promotional nature of CNTs in the Cu-Zn-CNTs catalyst,much detail work is highly demanded.