Construction Of Palladium Catalysts Encapsulated In Zirconium-based Metal Organic Frameworks And Their Catalytic Performance In Upgrading Ethanol To Butanol

As a gasoline additive,biobutanol is better than bioethanol in terms of energy density and water solubility.Therefore,upgrading bioethanol to butanol has become a key research direction in the energy field.It is generally accepted that butanol can be produced from ethanol by the Guerbet pathway,which consists of four tandem steps:ethanol dehydrogenation,aldol condensation of acetaldehyde,dehydration of the aldol,and hydrogenation of crotonaldehyde.It is challenge to design multiple sites for synergistic catalysis in this tandem reaction within one catalyst.Although the reported metal homogeneous catalysts and metal-supported heterogeneous catalysts have made some progress in the research of ethanol conversion to butanol,there are still some scientific problems to be solved:1)metal-supported catalysts suffered from metal sintering and then the decay of activities;2)the active sites were quickly deactivated due to acetaldehyde polycondensation on strong Lewis base sites of the catalyst.In this paper,Pd@UiO-66 catalysts were prepared by a simple and efficient impregnation reaction method and used for upgrading ethanol to butanol under relatively mild reaction conditions?523 K,2 MPa?N₂??and LHSV=4 h·g_cat·mL^-1,N₂/ethanol=250:1?volume ratio?.As a result,the Pd@UiO-66 catalysts showed up to49.8%ethanol conversion and 24.2%butanol yield,also an excellent stability performance during a 200 h long-term evaluation.To the best of our knowledge,it is the first time to present that Pd@UiO-66 catalyst have been applied in the continuous conversion of ethanol to butanol and showed the best catalytic performance among other heterogeneous catalysts.Subsequently,this paper investigatied the structure-activity relationship between catalysts and their catalytic behaviors by means of techniques inclueding N₂ physical adsorption,XRD,TEM,XPS,In situ FT-IR,TG-MS,NH₃-TPD,CO₂-TPD etc.The results confirmed that Pd nanoparticles were favorable for the ethanol dehydrogenation and crotonaldehyde hydrogenation,while coordinatively unsaturated Zr sites acting as Lewis acids were very active toward the aldol condensation of acetaldehyde.Therefore,the high catalytic activity and selectivity of Pd@UiO-66catalyst are primarily ascribed to the close synergy of highly dispersed Pd nanoparticles and coordinatively unsaturated Zr sites on Zr₆ nodes of UiO-66.The high stability of the catalyst is mainly attributed to the high hydrothermal stability of the support,the electrostatic attraction of Pd nanoparticles with Zr₆ nodes and the confinement effect of the cavities of UiO-66.Therefore,it can be used to disperse and stabilize highly active Pd-NPs to prevent sintering and deactivation during the reaction.Finally,in order to shed light on the catalytic reaction mechanism of upgrading ethanol to butanol on Pd@UiO-66 catalyst,the macro-kinetics expirments of ethanol dehydrogenation,acetaldehyde condensation and crotonaldehyde hydrogenation were studied.Kinetic analysis over the 2wt%Pd@UiO-66 catalyst revealed the apparent activation energy for ethanol dehydrogenation,aldol condensation of acetaldehyde and crotonaldehyde hydrogenation was 37.7,55.8,and 4.2 kJ·mol^-1,respectively,indicating that the aldol condensation is still the decisive step in these tandem steps.However,it is notable that the apparent activation energy of aldol condensation was much lower than our reported state-of-the-art Cu-CeO₂/AC catalysts(70.1 kJ·mol^-1).Kinetic analysis further corroborated that Lewis acid sites?Zr-CUSs?were very active toward the aldol condensation of acetaldehyde.Hence,the high n-butanol yield is ascribed to the cooperation of Pd-NPs and Zr-CUSs on the Zr₆ nodes of UiO-66support.According to the results of characterization and kinetics,the reaction mechanism for this synergy in the conversion of ethanol to n-butanol on Pd@UiO-66catalyst was developed.More over,the present study expands the reaction scope of MOFs-based materials in the field of heterogeneous catalysis,especially in complicated tandem reactions or reactions with high reaction temperatures or pressures?temperature:523 K,pressure:2 MPa?,and thus has a general reference significance for the application of MOFs-based catalysts.