Studies On Preparation, Structure And Catalytic Performances Of Hierarchical Core-Shell Layered Double Hydroxides-Based Magnetic Composite Microspheres

Recently, magnetic functional nanocomposites with core-shellstructure are widely used in catalysis, biomedicine, selective separationand chemical microsensors due to their versatile properties of bothmagnetic materials and other functional materials. In this paper, a seriesof novel hierarchical core-shell structured magnetic compositemicrospheres composed of superparamagnetic Fe₃O₄core and ternary/quaternary hydrotalcite （LDH） containing transition metal ions asfunctional shell with varied compositions and controllable morphologieshave been first facilely synthesized via one step coprecipitation underambient conditions. The structure, composition, morphology andphysicochemical properties of as-synthesized magnetic composites aresystematically investigated using SEM/EDX, TEM/HRTEM, XRD, TG,BET, VSM and XPS techniques. The catalytic performance andreusability are evaluated using phenol hydroxylation as a probe reaction.The main contents and innovations are as follows: The centered-hollowed Fe3O4microspheres particles are pre-synthesized by modified solvothermal method with mean size of ca.400nm. The ternary LDH-based hierarchical core-shell structured magneticcomposite microspheres Fe₃O₄@CuNiAl-LDH with controlled shell LDHparticle morphology and thickness are facilely synthesized via one stepcoprecipitation on the surface of pre-synthesized Fe₃O₄under ambientconditions by modulating media methanol/water ratio using economicinorganic salt and base. The composites obtained in pure waterpossessing honeycomb-like hierarchical structure with CuNiAl-LDHnanoplatelets staggered perpendicularly oriented grown on the core. Byadjusting the molar ratio of metal cation and Fe₃O₄（nCu/nFe=0.15,0.30,0.60）, the mass content of CuNiAl-LDH phase （54.03%,68.70%,81.02%）and shell thickness （56,105,120nm） of magnetic composites aregradually increased, while the thickness of a single nanoplatelets isdecreased （18.8,14.1,8.8nm）, probably owing to forming more crystalnucleus with increasing the salt concentration. The formation mechanismof hierarchical core-shell magnetic composites is tentatively proposedbased on pre-adjusted reaction medium pH value leading to rapidlyinterface nucleation via electrostatic attraction and followed epitaxialgrowth of LDH hexagonal particles over the core.The catalysis tests show that Fe₃O₄@CuNiAl-LDH catalysts showobviously higher catalytic activity than that of the pure CuNiAl-LDH, among which the maximal conversion, selectivity and TON are30.9%,38.1%and27.1h^-1, respectively. The highest catalytic activity is due tothe homogeneously well-dispersed catalytic activity centre bivalent ionCu²⁺.The quaternary LDH-based hierarchical core-shell magneticcomposite microspheres Fe3O4@CuNiZnAl-LDH with varied Zn/Nimolar ratio have been synthesized in methanol/water media under similarconditions. It is found that the thickness of CuNiZnAl-LDH shell are50,65and30nm with increasing Zn/Ni molar ratio （Zn/Ni=0.3/2.7,0.5/2.5,0.7/2.3）, while the thickness of platelets without obvious change （12,13,17nm）. The normalized catalytic activity of Fe₃O₄@CuNiZnAl-LDH ishigher than the related Fe₃O₄@CuNiAl-LDH with TON of38.6h^-1,probably due to the favorable Cu²⁺-O-Zn²⁺linkage making more stableand uniformly dispersed Cu²⁺species on octahedral layer by doping Zn²⁺.Hierarchical core-shell magnetic composites possess high magnetiz-ation, which enable facile catalyst separation after reaction by using anexternal magnetic field, greatly facilitating the catalysis recyclability.Therefore, the ecofriendly method used in the assembly ofhierarchical core-shell magnetic composites can be used to the design andsynthesis of multifunctional nanomaterials which is very important in theapplication of catalysis and separation.