Design,Preparetion And Liquid Hydrogenation Performance Of Core-shell Catalysts With ZIF-8 Membrane Coated On Pd Nanoparticles-supported Particles

Core-shell structured catalysts with porous materials as the shells have demonstrated outstanding activity,selectivity and stability in many industrially important reactions covering selective hydrogenation,catalytic oxidation,C-C coupling and so on,which largely relies on the selection of shell materials.ZIF-8,one of the most widely studied MOFs,not only possesses the superior properties of ultrahigh porosity,enormous specific surface area,tunable structure and tailorable properties,but also performs well in terms of thermal and chemical stability,resulting in an excellent candidate for shell materials when fabricating core-shell catalysts.In this work,we focus on the design and synthesis of core-shell catalysts with Pd nanoparticles-supported catalysts as the cores and ZIF-8 membrane as the shell.According to the surface characters of different cores,several unique strategies were developed to obtain well-defined core-shell particles.Then,we also conducted liquid hydrogenations of alkenes as the model reactions to explore the influence of ZIF-8 shell on the performance of core-shell catalysts in terms of selectivity,anti-poisoning and anti-leaching.The main content is as follow:?1?We fabricated a new type of Pd/SiO₂@ZIF-8 core-shell catalyst composed of monodispersed Pd/SiO₂?300 nm in diameter?microsphere as the core and ZIF-8 membrane as the shell.It is crucial that the modification of the Pd/SiO₂ core with PDDA and PSS was employed to improve adsorption capacity of the core surface for zinc ions and assist the formation of a continuous ZIF-8 shell.Neither in-situ method nor second growth approach can archive intact core-shell structure.The compactness and thickness of the shell are optimized by adopting temperature-controlled crystallization and increasing the cycles of ZIF-8 growth.The catalytic tests show that the Pd/SiO₂@ZIF-8 catalysts can provide excellent selectivity?The conversion of 1-hexene and cyclooctene is 100%and 0%,respectively,while for Pd/SiO₂,it is 100%for 1-hexene and 91.7%for cyclooctene.?,anti-poisoning and anti-leaching properties for the hydrogenations of alkenes.?2?Metal oxide is reported to be an alternative metal source for the formation of MOFs.Based on this principle,Pd/ZnO@ZIF-8 core-shell catalyst was efficiently fabricated with ZnO not only as the support for Pd nanoparticles?NPs?but also the zinc source to induce the growth of ZIF-8 shell.Further insight into the synthesis process of Pd/ZnO@ZIF-8 gives rise to a formation mechanism coined as "dissolution-recrystallization",according to which,ethanol was used as the solvent for the 2-methylimidazole?2-MIM?solution to facilitate the encapsulation of Pd NPs with ZIF-8 by equilibrating ZnO dissolution and ZIF-8 growth.Compared with the ZIF-8 synthesis strategy of PDDA/PSS pre-modification,this strategy is more efficient and at the same time can produce ZIF-8 shell with higher quality.Therefore,the resulting Pd/ZnO@ZIF-8 core-shell catalyst performs very well in terms of selectivity,anti-leaching and sulfur poison-resistance properties in the hydrogenations of alkenes:the conversion of cyclohexene is confined below 14%,while it is 29.2%for Pd/SiO₂@ZIF-8;the activity of 1-hexene can be retained to above 70%with the present of smaller poisons T-Dodecyl mercaptan;the activity and selectivity still kept unchanged after recycled for four times.?3?Generally,the fabrication of MOFs-coated core-shell particles needs to undergo tedious steps as well as time-consuming and energy intensive operations of separation and purification.In order to solve these issues,we effectively integrated the procedures of Pd NPs formation and loading on ZnO supports and the subsequent growth of ZIF-8 shells over them into one-pot synthetic route and obtained a novel Pd/F-ZnO@ZIF-8 core-shell catalysts.Notably,the utilization of 2D nanosheet-composed ZnO particle as the support is favorable for the formation of the well-defined core-shell catalyst due to its enhanced functions in anchoring Pd NPs and inducing ZIF-8 formation.Further insight indicates that the as-prepared catalyst deliveries improved performance in terms of selectivity?The conversion of 1-hexene and cyclooctene is 100%and 0%,respectively,while for Pd/SiO₂,it is 100%for 1-hexene and 56.4%for cyclooctene.?,anti-poisoning and anti-leaching?Pd loading only decreased by 3.3%after being reused for four times?.?4?A unique Pd/ZnO@hollow ZIF-8 yolk-shel-1 catalyst was prepared with ZIF-8 hollow shell to enclose Pd/ZnO core so as to improve the activity of ZIF-8 membrane-coated catalysts.Of prime importance is that ZnO was selected as self-template since it can not only be etched in the solution of ligands but also assist the formation of a compact ZIF-8 shell via linker activation.It is also demonstrated that the formation process of the yolk-shell catalysts is composed of ZIF-8 membrane growth and ZnO dissolution and a ZIF-8 growth solution with the concentration of 0.08mol/L or higher should accelerate the outward growth of ZIF-8 layer as well as dissolution of ZnO,resulting in well-defined yolk-shell materials.When evaluated as catalysts,Pd/ZnO@hollow ZIF-8 particles demonstrate excellent behaviors in terms of size-selectivity,anti-poisoning and leaching-resistance.Moreover,a better activity?100%conversion of 1-hexene in 6 h?beyond their core-shell counterparts?no more than 35%in 6 h?was detected since the presented interior hollow space between core and shell is likely to expose more active surfaces to reactants.