Abstract
The promoting effect of Pd on a Cu/ZnO/Al2O3 catalyst for the aqueous glycerol hydrogenolysis process to produce 1,2-propanediol was studied. At a lower hydrogen pressure (2.07 MPa), using the Cu/ZnO/Al2O3 catalyst with 2 wt% Pd doped, could significantly improve the glycerol conversion (97.2%) and 1,2-propanediol selectivity (93.3%) compared with the unpromoted catalyst (69.4% and 89.7%, respectively). A power-law kinetic model, which took into account all the elementary reactions including glycerol dehydration and its reverse reaction, acetol hydrogenation, side reactions and ethylene glycol formation, was developed to comprehensively investigate the effect of Pd. Though the rate of glycerol dehydration using the Pd-promoted catalyst was found to be slightly lower, mainly due to the reduced number of acidic sites after adding Pd, the glycerol conversion rate was notably higher compared with using the unpromoted catalyst, mainly attributed to the enhanced activity of acetol hydrogenation by Pd. The rapid hydrogenation of acetol can inhibit the reverse reaction of glycerol dehydration, resulting in a higher glycerol conversion rate, so that glycerol dehydration is considered as the rate-determining step. In contrast, when the unpromoted catalyst was used, the rate of reverse glycerol dehydration was drastically increased due to the elevated acetol concentration, especially at a lower hydrogen pressure, resulting in a slower glycerol conversion rate; thus, acetol hydrogenation became the rate determining step. In addition, Pd can improve the reducibility of the catalyst, allowing the CuO to be reduced in situ during the reaction. Therefore, catalyst deactivation due to any potential oxidation of metallic copper during the reaction can be prevented.
Highlights
The promoting effect of Pd on a Cu/ZnO/Al2 O3 catalyst for the aqueous glycerol hydrogenolysis process to produce 1,2-propanediol was studied
The results showed that the spillover effect induced by Pd on the Cu/ZnO/Al2 O3 catalyst can significantly enhance the acetol hydrogenation activity, resulting in a higher glycerol conversion and 1,2-PD selectivity at relatively lower hydrogen pressures, compared with using an unpromoted catalyst
The experimental results showed that adding Pd onto the CZA catalyst was essential in order to obtain a high glycerol conversion and 1,2-PD selectivity when the glycerol hydrogenolysis reactions were carried out at a hydrogen pressure lower than 2.76 MPa
Summary
The promoting effect of Pd on a Cu/ZnO/Al2 O3 catalyst for the aqueous glycerol hydrogenolysis process to produce 1,2-propanediol was studied. At a lower hydrogen pressure (2.07 MPa), using the Cu/ZnO/Al2 O3 catalyst with 2 wt% Pd doped, could significantly improve the glycerol conversion (97.2%) and 1,2-propanediol selectivity (93.3%) compared with the unpromoted catalyst (69.4% and 89.7%, respectively). Producing 1,2-PD from biomass-derived glycerol has been extensively studied, since it can increase the profits of biodiesel manufacturers and further reduce the fossil resource dependence of 1,2-PD production processes, in the spirit of green chemistry and sustainability. When its concentration is high it can undergo a series of side reactions, such as dehydration, oxidation, condensation and polymerization, to name a few, forming numerous types of byproducts [3,5,6]. In order to minimize the yield of the undesired byproducts, a high rate of acetol hydrogenation becomes crucial to obtain a good selectivity of 1,2-propanediol
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