Glycerol Hydrogenolysis With Or Without External Added Hydrogen Over Ni-Containing Bimetallic Catalysts

With the large-scale production and application of biodiesel, abundant surplus glycerol byproduct emerged as feedstock of biomass conversion and building block platform. It is an important value-added route to convert biodiesel-derived glycerol into 1,2-propanediol and ethylene glycol by hydrogenolysis pathway. Therefore, it has been a hot topic for industrial and academic research. Current research emphasis about glycerol hydrogenolysis is on the catalyst development and characterization. Among all the catalysts, Ni catalysts are widely applied in industrial production due to their many advantages such as low cost, flexible preparation and easy regulation of activity and selectivity. In this work, aiming at Ni-containing bimetallic catalyst, we explored the effect of the second metal (Pd, Co) introduction on the catalytic performance for glycerol hydrogenolysis. This study focused on the synergistic effect between metals, metals and supports or metals and promoters. Secondly, glycerol aqueous-phase reforming (APR) for hydrogen generation and catalytic transfer hydrogenation of glycerol 2-propanol solution (CTH) were incorporated into the reaction system of glycerol hydrogenolysis to produce glycols respectively. And the in-situ hydrogenation of glycerol in the absence of added hydrogen was investigated. Based on the several series of prepared Ni-containing bimetallic catalysts, the effect of operation conditions on the glycerol hydrogenolysis was also studied. Besides, the effects of metal types, structure of supports and promoters on the structural properties and catalytic performance were investigated by modern physicochemical technologies. In addition, the different hydrogenation processes and catalytic performance over different catalysts were compared to establish the structure-performance relationship and the corresponding mechanisms were also discussed(1) Pd-M series catalysts were prepared and used for glycerol hydrogenolysis. The activity was shown in the order of Pd-Co>Pd-Fe>Pd-Zn>Pd-Ni>Pd-Cu. Among these, Pd-Ni exhibited the highest selectivity of ethylene glycol. For example,89% of glycerol conversion and 22% of EG selectivity were achieved at 220℃ under 6.0MPa of H2 with 24h. After four times of recycling, the glycerol conversion was decreased by no more than 10% and the selectivities of 1,2-PDO and EG remained unchanged. The results indicated that Pd-Ni catalyst has excellent catalytic stability for glycerol hydrogenolysis. H2-TPR revealed that the reduction temperatures of metal oxides were significantly decreased by the introduction of Pd component. XRD patterns and TEM images showed that the Pd nanoparticles with an average size of～4nm promoted the reduction of oxide supports and resulted in the formation of Pd-Ni species (possibly alloy). XPS curves indicated that unique performance of the Pd-Ni bimetallic catalysts might be attributed to the formation of Pd-Ni phase and the promotion of C-C bond cleavage. In the end, hydrogenolysis reactions for the main products of glycerol conversion such as 1,2-PDO, EG and methanol were carried out over Pd-Ni catalyst to disclose the synergistic effect between Pd and Ni and verify the reaction mechanism of retro-aldol reaction leading to C-C bond cleavage.(2) Ni-Co bimetallic catalysts supported on γ-Al2O3 with different Ni/Co mass ratio were prepared by incipient wetness impregnation method for glycerol hydrogenolysis. The catalyst with a Ni/Co mass ratio of 1:3 (denoted as Ni1Co3) exhibited the highest conversion at 220℃ under 6.0MPa of H2 with 10h. The catalytic performance was compared with that of catalysts promoted by Ce. The addition of Ce showed a remarkable promoting effect on the catalytic performance when the cerium content was 2.5 wt% and the conversion and glycols selectivity reached 71.3% and 81.1%, respectively. H2-TPR profiles revealed that the coexistence of Ni and Co components on support changed the respective reduction behavior of Ni or Co alone, showing the synergistic effect between Ni and Co species. Compared with the TPR profiles of Ni1Co3, it was clearly observed that the reduction peak of nickel oxide and/or cobalt oxide shifted down to the lower temperature zone gradually with the addition of Ce. It was most probable that the addition of Ce favored the formation of the strong interaction between metal species and ceria. The TEM images showed that the addition of Ce component could improve the dispersion of Ni-Co species on support and inhibited the agglomeration of metal particles during the reaction process, which might be responsible for the enhanced stability. Besides, the introduction of ceria was responsible for the enhancement of interaction between metals and support, inhibiting the formation of C-C bond cleavage and promoting hydrogenation of acetol which increased the selectivity of 1,2-PDO.(3) The thermodynamic features of glycerol aqueous solution hydrogenolysis in the absence of added hydrogen was studied and the calculation results indicated that the in-situ hydrogenation system was feasible based on the coupling endothermal with exothermal and coupling hydrogen generation with hydrogenation. Raney Ni catalyst was prepared and tested for hydrogenolysis of glycerol without external hydrogen supply.55.2% of glycerol conversion and 40.5% of 1.2-PDO selectivity were achieved at 220℃ under 0.1 MPa of N2 with 12h. Considering the disadvantages of Ni monometallic catalyst such as excessive cracking and low utilization efficiency. Pt-M series of catalysts were prepared and used for in-situ glycerol hydrogenation process. Through catalyst screening,71.4% of glycerol conversion and 52.4% of 1,2-PDO selectivity were achieved over 2.5Pt-2.5Ni/γ-Al2O3 catalyst. XRD and H2-TPR indicated that Pt and Ni metals were dispersed evenly over γ-Al2O3 support. The strong interaction between active metals and support can influence the reduction temperature remarkably and the subsequent catalytic performance. SEM and EDX results illustrated that the damage of structure and carbon deposition were responsible for the catalyst deactivation.(4) A series of Ni-Cu bimetallic catalysts were developed and used for glycerol hydrogenolysis to produce 1,2-PDO in the absence of added hydrogen along with the 2-propanol as hydrogen donor. At 220℃ under 1.0MPa of N2 with 8h, when Ni/Cu mass ratio was 1:1, the conversion and 1,2-PDO selectivity reached 76.1% and 82.8%, respectively. XRD indicated that the monometallic Cu catalyst can catalyze the dehydrogenation of 2-propanol effectively, but the Cu species would be oxidized to Cu2O which led to the catalyst deactivation. Ni/Al2O3 showed better stability but the excessive cracking would result in lower glycols selectivity. Ni incorporation can change the reduction behavior of Cu catalyst and the formation of Ni-Cu alloy might promote the hydrogenation of acetol and inhibited the oxidation of Cu component. Furthermore, the effect of ZnO introduction on the catalytic performance of Ni-Cu/Al2O3 bimetallic catalyst for glycerol hydrogenolysis in the absence of added hydrogen was investigated. The addition of ZnO enhanced the acidity of catalyst surface, resulting in higher C-O bond cleavage activity and decreased the reduction temperature remarkably. XRD and SEM images revealed that the addition of ZnO inhibited the catalyst sintering and favored the formation of the strong interaction between Ni-Cu species and Al2O3 by the formation of ZnAl2O4 phase. In the end, a glycerol dehydrogenation-dehydration-hydrogenation mechanism was proposed and discussed in detail.