Ultrafine Palladium/covalent Organic Frameworks For Hydroxycarbonylation Of Aryl Iodides

Ultrafine metal NPs invariably tend to aggregate during catalytic processes owing to high surface energy,leading to the gradual decrease of the recyclabilities and catalytic activities.To solve this problem,various supporting substrates,have been applied to avoid the serious aggregation.As a versatile class of porous materials,covalent organic frameworks(COFs)have also been practised as supporting materials to stabilize the metal NPs for catalysis owing to their high surface areas and excellent chemical stabilities.Most previous studies in which metal NPs-loaded COFs were virtually synthesized via the solution infiltration method by applying additional capping agents,surfactants and reductants.Spontaneous electroless deposition of metals was exploited for the loading of metal NPs owing to its simple and straightforward process without any additives and capability for facilitating a large-scale production of supported metal NPs.Palladium(Pd)NPs decorated on various substrates have been hotly reported as heterogeneous catalysts with high catalytic activities,but the uncontrollable sizes,obstructive additives,and weak interactions between substrates and NPs frequently bring restrictions to their use and reuse.In this work,we used poly(phenylene butadiynylene)s(PPBs)as the support material.On the one hand,the material can be easily synthesized under mild conditions,and the synthesis method is simple and quick;on the other hand,because of SP~2-and SP-hybridized carbon atoms,and a lower oxidation-reduction potential,it is very suitable for spontaneous electroless deposition of palladium nanoparticles on the support material.In the process of loading Pd nanoparticles,we found that the spontaneous electroless deposition of Pd nanoparticles was carried out.Therefore,we use this spontaneous electroless deposition to load ultrafine sub-3 nm palladium nanoparticles with highly dispersed nanoparticles on covalent organic frameworks.In the loading process,the PPBs material can not only be used as support material for fixed palladium nanoparticles but also can play the role of the reducing agents,so there are no additional additives in the preparation process.The as-formed Pd/PPBs heterogeneous catalyst exhibits excellent recyclability and superior catalytic activity towards hydroxycarbonylation of aryl iodides in the presence of carbon monoxide and water.In addition,the as-prepared Pd/PPBs,due to carbon-carbon triple bonds in PPBs can play a certain coordination role,have very good chemical stability.Because of the strong immobilization of Pd nanoparticles on PPBs materials,the designed composite catalysts have very good cycle efficiency.