Study On Hydrogenolysis Performance Of Glycerol Based On Transition Metal Catalyst

With the influence of energy crisis and for the demand of environmental protection, H2has been attracted so much attention as a clean, efficient, safe and renewable energy source and chemical material.Besides hydrogen, the reforming gas usually contains10%of CO. The water-gas shift reaction (WGS reaction, H2O+CO→CO2+H2) is a key step to generate high-grade hydrogen for fuel cell and other uses. The CO content can be reduced from10%to around1%after the reaction. The copper-based catalysts, with high activity and good steady-state stability, have been widely used in industrial process, but are less stable to oxidizing gases. WGS catalysts based on noble metals have been investigated in recent years as alternatives to copper-based catalysts. They are nonpyrophoric and can be used without activation. However, the deactivation of these catalysts are much more serious, especially in the shutdown/startup operations. The catalytic performance must be improved to fulfill the stringent requirements of fuel cell systems.On the other hand, with the increased output of biodiesel, glycerol, as the by-product of biodiesel production, is excessive in amount in recent years. As one of the platform chemicals assigned by department of energy (DOE) of the USA, glycerol is a very important feed rock in chemical synthesis. Through hydrogenolysis reaction, glycerol can be selectively converted to1,2-propylene (1,2-PDO) or1,3-propylene (1,3-PDO).1,2-PDO is usually used to synthesize unsaturated polyester, epoxy resin, and polyurethane. Similarly,1,3-PDO is copolymerized with terephthalic acid to produce the polyester known as SORONAfrom DuPont and CORTERRA from Shell. They are used in the manufacture of carpet and textile fibers that exhibit unique properties in terms of chemical resistance, light stability, elastic recovery, and dyeability.In this paper, the catalytic performance of Au/TiO2catalysts with TiO2doped with various elements was investigated first. Then we systematically studied the deactivation mechanisms of the Au/CeO2catalyst in steady-state and shutdown/startup WGS reactions. By understanding the deactivation mechanism, the Au/Ce0.4Zr0.6O2catalyst, which has the better activity and stability than the Au/CeO2catalyst, was designed and prepared.Moreover, we optimized the catalytic systems for the hydrogenolysis of glycerol. RQ Cu catalyst showed excellent glycerol conversion to liquid (CTL) and1,2-PDO selectivity, while ReOx/Pt/ZrO2catalyst is beneficial to1,3-PDO synthesis. Combined with the characterization results, some discussion about the reaction pathway and mechanism was made.1. The reaction performance of Au/TiO2-based catalysts for WGS reactionTiO2doped with various elements were synthesized by sol-gel method. And Au was deposited on the supports by urea-DP method. The results showed that the Mn-doped catalyst exhibited the best activity. XRD and BET characterization proved that the highest activity was not simply attributed to the higher surface area or smaller particle sizes.Optimization of the Mn-doped catalyst was carried out. It indicated that when the Mn content was5mol.%and the calcination temperature of the support was500℃, the catalyst showed the best catalytic performance. The CO conversion over this catalyst was65.2%,whichwas6times higher than that over the Au/TiO2catalyst.The stability test showed that the Au/Mn5TiO2catalyst suffered continuous deactivation in the steady-state operation. After50h of steady-state operation, the catalyst lost about50%of its initial activity. However, in the shutdown/startup operation, the catalyst exhibited good stability. After two cycles of shutdown/startup operation, the CO conversion just decreased from62.4%to60.2%. It was suggested that TiO2can restrain the deposition of carbonate species on the catalyst surface, resulting in the excellent stability of the catalyst.2. The deactivation mechanisms of the Au/CeO2catalyst for the WGS reactionThe deactivation mechanisms of the Au/CeO2catalyst in steady-state and shutdown/startup WGS reactions were investigated in realistic reformate at250℃. Catalyst deactivation due to sintering was excluded. After steady-state operation, the original activity was not recovered by removing the deposited carbonate species. The influences of the component gases of realistic reformate on the activity suggest that loss of the Au-CeO2interaction caused by the reducing H2and CO is the main reason for catalyst deactivation. Under the shutdown/startup condition, the catalyst suffered more drastic deactivation, although it underwent a lesser degree of reduction. A good correlation between the extent of deactivation and the amount of the carbonate species indicates that catalyst deactivation is mai nly caused by enhanced formation of the carbonate species, especially through a combined effect of CO2and H2O, during the low-temperature shutdown and start-up steps. The implications of these findings for improved applications of the Au/CeO2catalyst in WGS are indicated.3. The catalytic performance of Au/CexZr1-xO2catalysts for the WGS reactionAfter knowing the deactivation mechanisms of the Au/CeO2catalyst, we further studied the influence of Zr02-modification of CeO2on the catalytic performance. A series of Au/CexZr1-xO2(x=0,0.2,0.4,0.6,0.8,1) catalysts were synthesized, and their activities and stabilities in the WGS reaction were evaluated. Raising the ZrO2content increased the activities and the shutdown/startup stabilities either, and the maximum values were obtained on the Au/Ceo.4Zro6O2catalyst. The H2-TPR study reveals that higher activities of the Au/CexZri-xO2(x=0.4,0.6,0.8) catalysts can be related to the higher content of O-vacancy. In addition, the FTIR and CO2-TPD were used to explain the usual stabilities of the ZrO2-modifucated catalysts. The results indicated that adding ZrO2to the Au/CeO2catalyst seems to restrain the formation of surface carbonates, which block the active sites and then decrease the WGS reaction activities.4. The catalytic performance of Cu-based catalysts for the hydrogenolysis of glycerol to1,2-PDORQ Cu is a new kind of skeletal copper catalyst prepared by incorporation of rapid quenching technique in the preparation of Raney Cu. That is, first solidify the melt of Cu-Al alloy with a cooling speed of～106°Cs-1,then the aluminum in the alloy was extracted by alkaline leaching, resulting in the porous Cu. In this paper, we optimized the conditions of the leaching process and tried to enhance the catalytic performance. Optimization of the reaction conditions was also carried out. The best CTL (conversion to liquid) of glycerol reached>99%with the1,2-PDO selectivity of96.7%(200℃,8MPa H2, and reaction time of12h). Moreover,1,2-PDO,1,3-PDO and acetol were used as the reactant under the same reaction condition. At200℃, the conversion of1,2-PDO was less than1%. It is said that the1,2-PDO formed in the hydrogenolysis of glycerol would not further convert to n-propanol and then result in the good selectivity.5. The Catalytic performance of noble metal catalysts for the hydrogenolysis of glycerol to1,3-PDOWe systematically studied the influence of supports, noble metals, promoters, and reaction conditions on the hydrogenolysis of glycerol to1,3-PDO. The results indicated that when ZrO2was used as the support, Pt as the active metal and ReOx as the promoter, the catalyst showed the best catalytic performance. The CTL of glycerol and the selectivity of1,3-PDO were91.2and33.2%, respectively (130℃,8MPa H2,400rpm, reaction time of24h).In addition, we optimized the reaction conditions of this reaction. Raising the reaction temperature decreased the selectivities of1,2-PDO and1,3-PDO and promoted the further hydrogenolysis of1,2-PDO and1,3-PDO to propanols. Higher H2pressure is beneficial to both the CTL of glycerol and the selectivity of1,3-PDO. By screening the solvent and the acidity of the solution, we found that the neutral water solution was the best for this reaction.