Structure Optimization Of Mg-Al Composite Oxides And Theirs Performance Study

For the demands of sustainable development and green chemistry,the use of new non-toxic and efficient heterogeneous catalytic materials instead of conventional homogeneous ones,to achieve a green chemical production pathway at atomic economical level,has received increasing attention.In the field of alkali catalysis,solid bases have extensive application prospects in the fields of chemical,energy and environment,since they are much less corrosive,easier to separate,and can catalyze diverse reactions under mild conditions with minimizing waste production,which makes them become an important way to replace the traditional liquid base catalysts to achieve environmentally friendly catalytic processes.In addition,for a complete and sustainable catalytic reaction,the introduction of mesostructure into solid base catalyst is helpful to improve the diffusion rate of reactive molecules,and further to achieve a more ideal heterogeneous catalytic effect.Therefore,a better understanding of the structural properties with regard to catalytic applications is strongly required to enable the design and synthesis of mesoporous solid base with high performance by a simple and easily reproducible preparation process,which has become a hot topic in the field of materials science and catalysis.Ordered mesoporous alumina?OMA?as a nonpolluting material should be a good carrier for the preparation of solid bases,because the reaction of strongly basic species with the host that occurred in ordered msesoporous silica-supported materials is absent.Therefore,the thoughtful combination of both magnesia with a unique basic characteristic and OMA allows the fabrication of a new type of material with excellent basic properties.Herein,we aimed to design a controllable approach to prepare highly ordered Mg-Al composite oxide solid basic materials with highly uniform channel,large surface area and pore volume,narrow pore size distribution,tunable pore size,and remarkably increasing basicity?basic sites amount and strength?by introducing Mg species into alumina mesoporous framework.By improving the understanding of the interaction between Mg and Al inorganic species and organic template micelle molecuels during the evaporation-induced triconstituent cooperative co-assembly process,the function mechanism of Mg species existing within mesoporous walls with a highly homogenous distribution on mesostructural modification,mesostructural stability,and porous walls surface properties were further explained.Based on various characterization techniques,the effect of introducing way and introducing amount of Mg species on the structure,texture,and wall properties were comprehensively investigated,which remarkably affected the mesostructural stability and the generation of basic sites.The following results could be achieved:?1?At present,it is difficult to achieve the ordered mesoporous Mg-Al composite oxide solid basic materials with high degree of atomic mixing,due to the fast hydrolysis and condensation rates of aluminum precursor and phase transitions accompanied by the mesostructural collapse during the calcination process at high temperature.Consequently,in most reported cases,the resulting mesoporous phases with a disordered "wormhole-like" pore structure were generally the mixture of alumina and periclase or the mixture of hydrotalcite and periclase,and exhibited poor thermal stability and low surface area.Based on the deep understanding the hydrolysis process of aluminum alkoxide and the interaction between Mg,Al inorganic species and the surfactant molecules during the solvent evaporation induced self-assembly process,we proposed a cooperative co-assembly method and successfully prepared a series of highly stable ordered mesoporous Mg-Al composite oxide?OMMA-x?.The results of XRD,TEM,EDX,N₂ adsorption,NMR,DTG and CO₂-TPD confirmed that the addition of magnesium nitrate and citric acid in the synthesis system can remarkably affect the hydrolysis-condensation rate of aluminum alkoxide and the hydrophilic-hydrophobic property of surfactant molecules.As a result,the appropriate introduction of magnesium nitrate and citric acid can effectively increase the Al-OH species amount and enhance their interaction with surfactant molecules by the hydrogen bonds,which plays an important role on promoting the formation of organic-inorganic composite micelles.Compared with OMA material prepared without introducing Mg species during the synthesis solution,the resultant OMMA-x exhibited a more regular 2D hexagonal mesostructure,higher specific surface area and pore volume,narrower pore size distribution and stronger basicity?basic sites amount and basic strength?.More importantly,the highly homogeneous incorporation of Mg species into the mesoporous walls and the formation of framework Mg-O-Al bond can significantly inhibit the formation of crystalline alumina,thereby effectively improving the thermal stability of the resulting materials.Taking sample OMMA-8 for example,after high temperature thermal treatment at 1000 ? for 1 h,the ordered mesostructure of OMMA-8 could be well maintained with a high specific surface area?182 m2/g?and pore volume?0.24 cm3/g?.?2?In order to obtain a highly efficient mesoporous solid base material for biodiesel production by the transesterification of soybean oil with methanol,OMMA-x with excellent structure and surface properties was selected as carrier-loaded potassium species to further improve the alkalinity of obtained materials.The results of transesterification reaction showed that the structure,texture and surface properties of carrier could directly affect the existence and distribution of the supported potassium species,further affecting the reaction performance of the obtained catalyst.Compared with OMA and Mg-Al mixed oxide?Mg/OMA-8?,the carrier OMMA-x can significantly increase the dispersion of potassium species and promote the formation of potassium active components due to its highly regular mesostructure,high specific surface area and pore volume,high structural stability,and rich aluminum hydroxyl content of pore wall surface.The obtained catalyst K/OMMA-x showed higher transesterification reactivity and stability.Take K/OMMA-8 for example,the conversion of soybean oil was more than 94% after 5 h reaction at 80 °C.?3?Based on the deep understanding of the synthesis mechanism for ordered mesoporous Mg-Al composite oxide and the hard template technology,a series of ordered meso-macroporous Mg-Al composite oxide materials?OMMMA-Y?have been successfully prepared using polystyrene microspheres as the hard template for the formation of macroporous structure.The introduction of three-dimensional ordered macropores with uniform pore size into ordered mesoporous Mg-Al composite oxides can favor to improve the connectivity between the mesoporous channels,thus the diffusion rate of reactive molecules in the catalyst can be significantly increased.The results of XRD,SEM,and N2 adsorption showed that the use of hard template in the synthesis system does not affect the formation of ordered mesoporous Mg-Al composite oxide and the uniformly doping of magnesium species into alumina framework,and the macroporous size can be successfully controlled in the range of 100-300 nm by adjusting the particle size of the hard template.The resultant meso-macroporous Mg-Al composite oxide materials exhibit high thermal stability and mechanical stability,which could be due to the important role of Mg species existing within mesoporous walls on mesostructural modification,pore wall modification and skeleton support.Taking sample OMMMA-300 for example,after calcination at 900 ?,it's specific surface area and pore volume can still be retained by 74.8% and 67.1% respectively;while after 17 MPa pressure treatment,the macrostructure of OMMMA-300 could still be well maintained.It can be seen that our dual template method is simple and easy to repeat,which may provide theoretical guidance for the preparation of alumina-based hierarchically porous materials with excellent structure and surface performance.