Fabrication And Size-selective Hydrogenation Of Silicalite-1 Core-shell Materials

Under harsh reaction condition,on one hand,active noble nanoparticles of conventional supported catalysts tend to form conglomerates due to Ostwald effect.It is because its active nanoparticles expose to the reaction system directly,thus affecting catalytic activities.On the other hand,supported catalysts are poor in size-selectivity.Therefore,core-shell catalysts which active nanoparticles are encapsulated within are devised in order to overcome the drawbacks of supported catalysts.Zeolites already appeared to be the best candidates for the encapsulation of particles in isolated cavities or channels with nanometric dimensions.And what matters is how to construct the core-shell catalysts.Taking Silicalite-1 zeolite as shell materials,we devised core-shell Pd@Silicalite-1 catalysts where active palladium nanoparticles were encapsulated as core in the first part.In order to testify the catalytic activity,hydrogenations of olefins were employed.In the second part,we attempted to synthesize core-shell GO/Silicalite-1 materials using two-dimensional graphene oxide as generalized core structure.The main details are as follows:?1?Given that amino-functionalized Silicalite-1 seeds are inclined to anchor Pd²⁺ions,they were obtained to synthesize core-shell Pd@Silicalite-1 catalysts.We used3-aminopropyltriethoxysilane?APTES?to modify the Silicalite-1 seeds of around 190 nm in size,which were marked as amino-functionalized Silicalite-1 seeds.Then Pd²⁺ions were anchored onto the seeds,which were added into the synthesis solution of Silicalite-1 later,successfully attaining core-shell Pd@Silicalite-1 catalysts.These catalysts were uniform and monodisperse prism-like crystals,meanwhile twinned.Palladium nanoparticles were well-encapsulated in Silicalite-1 characterized by XRD,SEM and TEM.Compared with conventional supported palladium catalysts,Pd nanoparticles have narrower size distribution of around 3 nm in Pd@Silicalie-1 catalysts.When studying the synthesis time,we found that the morphology mentioned above can be formed within short synthesis time and well-crystallized as well.Finally,1-hexene,cyclohexene and triphenyl ethylene were employed as model reactions to investigate size selectivity and activity between supported and core-shell catalysts.The results showed that core-shell catalysts were endowed excellent size selectivity while supported catalysts were more active.Conversions of 1-hexene and cyclohexene catalyzed by Pd@Silicalite-1 were lower than supported Pd/Silicate-1 catalysts owing to zeolitic shell.In the meanwhile,triphenyl ethylene cannot access to active sites of Pd@Silicalite-1 resulting in relative alkane due to its larger size than the pore size of Silicalite-1.Moreover,with the size selectivity of core-shell catalysts,competition reaction on the active sites could be weakened in two-component hydrogenations of 1-hexene and cyclohexene,and it could lead to the hydrogenation of cyclohexene barely influenced by 1-hexene.?2?Taking thin-layered graphene oxide as support,we preliminarily explore the core-shell structure GO/Silicalite-1,where pre-crystallization method,APTES inducing method in synthesis solution,GO-NH₂ inducing method and GO-SiO₂ inducing method were employed.In the first three methods,GO could not interact with Silicalite-1 and induce the growth of Silicalite-1 along its surface.The resultants were simply stacked by GO and Silicalite-1,not by interaction.However,when GO-SiO₂ was employed,coral-like or cubic crystal could be obtained with different water proportion in the synthesis solution.