Preparation And Functionalization Of Hierarchical Porous Gamma Alumina Materials

Porous nanomaterials have critical applications in many fields such as chemical catalysis,photocatalysis,energy,sensing,and detection due to their small size and high specific surface area.In recent years,the hierarchical porous structures have attracted much attention,especially for those with microporous,mesoporous,and macroporous structures.Their micro and mesoporous structure provide a high specific surface area,i.e.,a large number of reactive sites.Their macroporous structure could act as high-efficiency mass transfer channels promoting the transport and transfer of reactants and products.These characteristics could significantly improve the activity of catalysts.Gamma alumina has a unique crystal structure combining tetrahedron and octahedron,it has excellent thermal stability and chemical stability,and its higher hardness also makes a better mechanical strength.The hierarchical porous structure of gamma alumina is of great significance for its application as a catalytic carrier.How to control the porous structure of gamma alumina and explore its application as a functional carrier is still a hot topic and a severe problem in the field of materials science.In this work,the hierarchical porous structure of gamma alumina was prepared by the combination of polymer assisted sol-gel and vacuum freeze-drying technology,and its application in catalyst fixation was explored.The specific research is as follows:1.Preparation of gamma alumina hierarchical porous structure and its adsorption propertiesThe polyvinyl alcohol(PVA)/AlOOH hybrid material was obtained by vacuum freeze-drying with isopropanol aluminum,acetic acid,and PVA as the precursor solution,and further calcined at high temperature to prepare a hierarchical porous gamma alumina(HPA for short).By changing the experimental conditions,the effects of PVA content,water content,and calcination temperature on the morphology and structure of the material were systematically studied.Experimental research results show that:gamma alumina prepared by the above process has a self-supporting macrostructure.The channels are interconnected to form a hierarchical porous structure.Its specific surface area is as high as 402 m2g-1.Polymer PVA was introduced into the precursor solution to protect the interconnected macroscopic pore structure formed during the sublimation of ice crystals.During the calcining process,the decomposition of PVA increased the micropores and mesoporous structures.Larger water ratio in precursor weakened the blocking effect of PVA on ice-growth,resulting in decreased transverse macropores,mesoporous,and microporous structures.Increasing calcination temperature decreased the total micropores,although mesopores also collapsed into micropores in the hierarchical porous gamma alumina.The adsorption properties of hierarchical porous gamma alumina on dye Congo Red(C.R.)were further studied.After 60 minutes,the adsorption amount of C.R.was 92.7%,92%,and 94.5%for the solutions with concentrations of 100,200,and 300 ppm.The adsorption rate of the sample HPA is fast in the first 15 minutes,and the adsorption process could nearly be completed when the C.R.solution concentration is low.For the test with 300 ppm C.R.solution,the first-order and second-order kinetic rates of sample HPA were 4.45 and 3.59 times higher than that of alumina nanoparticle ANps.The sample HPA prepared by vacuum freeze-drying technology has an interconnected pore structure,which significantly improves the material transport rate.The mesoporous and microporous formed by the sublimation of ice crystals increase its specific surface area and active sites,which also enhances its adsorption performance.The macroscopic self-supporting structure of sample HPA enables the excellent separation and recovery performance.2.Study on the catalyst immobilization and the related properties of hierarchical porous gamma aluminaThe HPA has an interconnected pore structure and macroscopic self-supporting structure.The mesopores and micropores provide more reaction sites for the immobilization of catalytic materials,and the macropore structure provides the mass transport channel and growth space for the immobilization reaction process.These characteristics make the hierarchical porous gamma alumina as the catalyst carrier has a better application prospect.Silver nanoparticles were immobilized on HPA,forming Ag/HPA composites by in-situ reduction.It was found that the silver nanoparticles were evenly dispersed on the surface of HPA.The size of the silver nanoparticles could be effectively regulated by adjusting the concentration of added ammonia.The catalytic activity on 4-nitrophenol was also affected.Ag/HPA has excellent catalytic performance for the reduction of 4-nitrophenol.in 180s.The rate constant of Ag/HPA is 2.48 times that of Ag/ANps due to its high specific surface area.The mesoporous and macroporous of Ag/HPA can improve the mass transfer rate.The silver nanoparticles are well dispersed on the surface of Ag/HPA for the uniform reaction process.The macroscopic self-supporting structure of Ag/HPA shows excellent recycling property.g-C3N4 was further immobilized on HPA,forming hierarchical porous C3N4/HPA by the gas-solid reaction.The study found that the high-quality g-C3N4 was uniformly supported on the surface of hierarchical porous gamma alumina.The immobilized capacity of g-C3N4 can be regulated by adjusting the quality of urea.The photocatalytic properties of the composites are also affected.Under visible light irradiation,the degradation rate of Rhodamine B(RhB)by C3N4/HPA reached 92%after 40 minutes.Compared to the composite C3N4/ANps,which using alumina nanoparticles as the carrier,the first-order rate of C3N4/HPA is about 2.7 times that of C3N4/ANps.The C3N4/HPA with a higher specific surface area has a higher fixation capacity.Furthermore,the mesoporous and microporous structures provide much more active sites,the interconnected pore structure provides space for reaction and increases light absorption,and the macroscopic self-supporting structure of C3N4/HPA is beneficial to the separation and sedimentation characteristics of materials.MoSe2/HPA,ZnO/HPA,and CuTNPc/HPA were also prepared successfully by hydrothermal and impregnation reaction process using HPA as functional materials carriers.