Octahedral-based Molecular Sieve PKU-n: Isomorphous Metal-substitution,Catalytic Property,and Structure-activity Relationship

In recent decades,molecular sieve has been widely used in industry.Generally speaking,molecular sieves are divided into aluminum silicate molecular sieve,aluminophosphate molecular sieve,and hetero-atomic molecular sieve.They can be used in some special reactions due to the different pore sizes.Molecular sieves can show the acid catalytic properties and work efficiently in a lot of organic synthesis reactions such as esterification.Modified with ion exchange and doping heteroatoms,the physico-chemical properties of molecular sieve will be various.A lot of efforts have been putted into doping heteroatoms such as transition metals into molecular sieves for obtaining the multifunctional catalyst.However,it is quite difficult to incorporate heteroatom ions into the lattice of traditional tetrahedral molecular sieve because of the structural properties,such as ionic radius,ionic charge,and coordination preference.An alternative strategy for incorporating metal ions is to use the octahedral molecular sieves?OMS?.A number of porous frameworks consisting of mixed octahedra/tetrahedra were reported in recent years,porous materials based exclusively on octahedral are rare.Recently,another series of octahedral molecular sieves of aluminoborates?denoted as PKU-n?was identified,which framework are mainly consist of edge-sharing Al-octahedra and BO_x groups.In this work,some representative materials in PKU-n will be modified,used as the solid acid or redox molecular sieve,and applied into small molecule organic reactions.Various characterizations,such as X-ray diffraction and XPS spectrum,confirmed that transition metals were embedded successfully inside the PKU-n?PKU-1 or PKU-8?framework with a high level?more than 10 atom%?and presented the valence states of doping atoms.Furthermore,qualitative and quantitative analyzes were performed to illustrate the result of the catalytic reactions.Some in-situ experiments and DFT calculation were utilized to investigate the reaction mechanism in the reaction system under the catalytic effect of modified PKU-n.The main conclusions are summarized here:?1?An aluminoborate?PKU-1?with octahedron-based framework was found to be a highly efficient solid acid catalyst for the rapid and convenient synthesis of?-aminonitriles from imines and TMSCN under mild reaction conditions.The catalytic active sites are related to the borate hydroxyl groups in the 18-ring structural channels.The borate hydroxyl groups play a critical role as Br?nsted acid to protonate imine and activate TMSCN,thus providing the imine intermediate and producing CN anions,respectively.?2?Octahedral-based redox molecular sieves Cr-PKU-1 were synthesized by isomorphous metalsubstitution and used as catalysts for catalytic dehydrogenation of sec-alcohols using H₂O₂ as oxidant.Molecular probe analyzes suggested that Cr sites catalyzed the quick formation of active·OH radicals,however strongly suppressed the generation of superoxide·O₂^-ions.Further more,Cr-PKU-1 was also found to be a dual-centered solid acid with a large density of Br?nsted acid?borate hydroxyl groups,B-OH?and Lewis acid sites(Cr³⁺),and therefore showed much higher catalytic activity than monocentral solid acid in the formation of?-aminonitriles.Although commonly considered as Lewis acid centers,electron-deficient boron atoms?tri-coordinated BO₃?in Cr-PKU-1 failed to exhibit the desired catalytic activity in the present investigations probably due to steric hindrance against the large organic substrates.?3?Al_xGa_1-x-PKU-1?x=0,0.20,0.50,0.80,1.0?,a complete solid-solutions composed of two octahedron-based molecular sieves,were prepared and used as a solid acid with continuously adjustable acidity to heterogeneously catalyze Strecker reaction under the mild conditions.These obtained porous materials have exhibited a higher density and stronger strength of Br?nsted acid sites with the successive incorporation of Al atoms,thus resulted into the superior catalytic activity in the formation of?-aminonitriles.Tri-coordinated boron atoms in the framework of Al_xGa_1-x-PKU-1,despite of electron-deficient configuration,seemed unable to behave as the Lewis acid centers probably due to steric hindrance against the large organic substrates.Futher more,Ga₄B₂O₉ could be synthesized by thermal treatment of Ga-PKU-1 and behave as a durable solid base catalyst for the synthesis of?-aminonitriles by Strecker reaction.?4?The synthesis,physicochemical properties of a new nano-phase aluminum borate chloride by trandition elements doping,called M-PKU-8.This cationic framework material also proved to be quite efficient to catalyze the oxidation of sec-alcohols into the corresponding ketone compounds with a high selectivity?99%?,which is applied in the green solvent water and 70%wt tert-butyl hydroperoxide?TBHP?works as the oxidant.Simultaneously,t-BuOO^·groups are generated in a large quantity and seem to function as the most critical active species to enhance the dehydrogenation of sec-alcohols when using TBHP as oxidant.DFT calculations give an energetically favorable pathway for TBHP activation,agreeing well with the obtained catalytic results.A reaction scheme was finally proposed to interpret the probable mechanistic process.