Preparation And Properties Studies Of Al-based Oxides With Super-micropores

Alumina has been widely used in catalysis area for its broad applications as industrialcatalysts and catalyst supports employed in petroleum refinement, organic synthesis, finechemistry and others. However, most traditional alumina with mesopores or macroporespossess only textural porosity featured by low surface area and broad pore size distributionand almost has no micropore structure, which limits their practical applications to meet thecatalytic process with special requirements on the selectivity, stability, contact area of thereaction. Therefore, preparation of the alumina with higher surface areas, smaller pore sizesand narrower pore size distribution for enhancing the catalytic performance has attracted wideresearch attention.There are numerous routes of synthesis to achieve porous alumina materials, for instance,sol-gel, hydrotheral treatment, precipitation, microemulsion, cation-anion double hydrolysisand nanocasting, then alumina can be prepared through calcination or solvent extraction toremove all or part of the template. However, these are obviously laborious and industrialunfavorable method with multi-step, high-cost and time-consuming procedure. Furthermore,to the best of our knowledge, up to now, there are few reports for the synthesis of microporousalumina via the soft template method. Herein, we describe the synthesis of super-microporous(pore size in the range of1-2nm) alumina by using readily available fatty alcoholpolyoxyethylene ether as the template via an evaporation-induced self-assembly (EISA)method. Super-microporous alumina materials break through the limitations of traditionalpore size, which can effectively control the state of the precursor aggregates to generate the microporous structure by changing the limited reaction conditions, thus effectively changingthe pore size and performance of alumina nanomaterials in order to expand its practicalapplication.The as-synthesized alumina materials will offer great potential applying for theshape-selective sensing, adsorption, and catalysis to overcome the limitation of traditionalmesoporous molecular sieves and microporous zeolites. Systematic studies on this subject indomestic and other countries are still in stage of starting up to now, there are still manyproblems needing to be solved, for example, the use of toxic or expensive surfactants ortemplating agents. Synthesis of microporous alumina materials with good thermal stabilityand high catalyst performance is a great challenge in the coming years. Therefore, a simplebut cost-effective synthetic methodology for preparation of a high surface area microporousalumina with a narrow pore size distribution were developed, commercial development wouldbe likely to proceed.In this paper, super-microporous alumina (pore size between1and2nm) withpolycrystalline walls and high surface area (more than650m2/g) was synthesized successfullyvia an evaporation induced self-assembly (EISA) pathway using readily available andinexpensive nonionic surfactant fatty alcohol polyoxyethylene ether (AEO-7) as the template.FTIR pyridine adsorption and NH3-TPD measurement suggested the presence of strong Lewisacid sites. Transmission electron microscopy (TEM) measurements indicated that the aluminapossesses the disordered “wormhole-like” super-microporous structure with polycrystallinewalls. The method has advantages of simple operation, low cost, readily available andenvironmentally templates. Meanwhile, the influence of the amount of template and inorganicacid (nitric acid), the different types of carboxylic acids and synthesis temperature on theproperties and structural parameters of samples was investigated also.On the basis of the synthetic strategy for the preparation of super-microporous alumina, aseries of super-microporous alumina-zirconia composites have been successfully designedand synthesized for the first time through adding different amount of ZrOCl2·8H2O usingone-pot synthesis strategy. The results demonstrated that after the introduction of ZrO2, thecrystalline growth and aggregation, and the crystal phase transfornation fromamorphous-to-γ-Al2O3was strongly retarded. When calcined at600℃, super-microporous aluminas transform to the mesoporous aluminas. However, for alumina-zirconiananomaterials, even if treated at900℃, microporous pore size (about1-2nm) was notchanged, indicating a high thermal stability of the microporous structure.Then, porous alumina (PA) which enables control of pore structure was prepared using amixture of fatty alcohol polyoxyethylene ether (AEO-7) and poly (alkylene oxide) triblockcopolymers (P123) as the template via the EISA pathway associated with thermal treatment.The effect of the P123/AEO-7ratio on the pore structures of our obtained porous aluminaswas studied. Results show that the mixed template method can remarkably increase thethermal stability. When transformed to γ-alumina after calcination at900℃, the surface areaof the sample is still188m2/g, which is much higher than the sample synthesized with theonly agent AEO-7due to the presence of microporous structure. More importantly, porestructures of these synthesized alumina range from super-micropore to mesopore by changingthe ratio of the mixed template.Super-microporous copper-alumina catalysts were successfully synthesized by usingin-situ and impregnation method, which were tested for the selective catalytic reduction ofNO with methane (CH4), and the relationship between copper percent and catalyticperformance was studied. N2adsorption-desorption results indicated the as-synthesizedcatalysts are microporous structure, and possess stable high activity. When the copper contentis around15mol%by in-situ method or5mol%by impregnation method, the conversion ofNO increases with temperature, and reaches almost100%around600℃in the absence ofoxygen. Interestingly, they exhibit high activity for direct NO decomposition, which is barelyreported about copper-alumina catalyst.