Preparation Of Mesoporous γ-Alumina And Supported Ni-Mg Oxides And Their Catalytic Applications

γ-aluminas have been shown to be of crucial importance as industrial adsorbents andcatalyst components in a wide variety of chemical processes However, conventionalγ-aluminas formed through the thermal transformation of aluminum hydroxides andoxyhydroxides at a temperature above450oC typically present only textural porosityfeatured by low surface areas and broad pore size distributions （PSDs）, which limit theirpotential applications. Therefore, following the M41S mesoporous silica, mesoporousalumina materials become an important research topic in the field of catalysis.The sol-gel approaches have been widely applied for the synthesis of porous alumina. Inmost cases mesoporous aluminas with disordered or ordered channels prepared via thesurfactant-assisted method give only amorphous framework walls at temperatures below800oC, which lack the structure stability and surface characteristics of crystalline aluminasfor use as absorbents and catalyst supports. While higher temperature calcinations fortransition alumina phase inevitably destroy the pore structure and cause drasticdeterioration of texture properties. On the other hand, most of these procedures use organicsolvents and aluminum alkoxides besides expensive structure-directing agents. Therefore,it is important to develope a novel, economic and environmentally benign approach tofabricate stable mesoporous γ-aluminas with large surface areas, pore volumes and narrowPSDs.In this paper, high-surface-area mesoprous γ-alumina and alumina supported Ni-Mgoxides are successfully prepared using inorganic Al salts as Al source and their catalyticproperties are also studied. The main research contents and results are summarized asfollows:1. Preparation of mesoporous γ-alumina and CO₂adsorption capacityThe facile synthesis of mesoporous γ-alumina is developed through partial hydrolysis ofAl（NO₃）3aqueous solution with （NH₄）2CO₃without organic surfactants. In this synthesis,stable NH₄NO₃/Al₁₃species （AN/Al） hybrid containing Keggin-Al₁₃polycations is firstprepared, which is the key for the successful formation of mesoporous γ-alumina. XRD, 27Al MAS NMR, TEM, and N₂adsorption and desorption results demonstrate that theas-prepared AN/Al hybrid can be transformed to γ-alumina by treatment at200oC andexhibit a wormhole-like mesoporous structure with large surface area up to～450m2g-1,pore volume of～0.3cm3g-1and narrow pore size distribution peaked at～3.9nm aftercompletely removing NH₄NO₃at300oC. The obtained mesoporous γ-aluminas have highthermal stability up to900oC and excellent hydrothermal stability.The effects of different inorganic aluminum salts on the formation and pore structure ofmesoporous γ-aluminas are investigated; the decomposition of NH₄NO₃in the hybrid areanalyzed; and the phase transformation of Al species in the AN/Al hybrid are detailedlystudied during the calcination process. The results imply that the synergetic effect betweenNH₄NO₃and Al₁₃species promotes crystallization of Al species to γ-alumina, which mayhave a unique mechanism distinct from the mesoporous aluminas reported previously.CO₂adsorption measurements indicate that these mesoporous γ-aluminas have muchhigher CO₂adsorption capacity than ordered mesoporous alumina synthesized by thesurfactant-templating method and conventional γ-alumina derived from aluminumoxyhydroxides.2. Preparation of mesoporous Ni-Al oxides and catalytic reaction performanceThe mesoporous γ-alumina supported nickel oxides （NixAl） with various Ni/Al molarratios （x=0.05,0.10,0.125,0.25,0.50） are prepared by a simple co-hydrolysis of aqueoussolution of Al（NO₃）3and Ni（NO₃）2with （NH₄）2CO₃without surfactants and characterizedby XRD, SEM, TEM, and N₂adsorption and desorption. The prepared NixAl samplesexhibit wormhole-like mesoporous structures with large specific surface areas and narrowpore size distributions are similar to mesoporous γ-alumina. XRD and TPR resultsdemonstrate that ammonium nitrate is completely decomposed and γ-alumina formed as thecalcination temperature of over300oC. The nickel oxide species are highly dispersed in thenickel-aluminum composite oxide without free NiO crystal phase. As the temperatureincreases to800oC, most of Ni species are transfermed into NiAl₂O4. CO₂and H₂methanation reaction are used as a model reaction. The catalytic reactionperformances of the Ni-Al composite oxide with different Ni/Al molar ratio calcined at400oC are investigated. The results show that the Ni-Al composite oxides prepared by partialhydrolysis method have better catalytic activity and100%methane selectivity. When theNi/Al ratio is0.25, Ni-Al catalyst exhibits the highest activity.The steam reforming of benzene is used as a model reaction. The reforming reactionperformance of the mesoporous Ni0.1Al oxide prepared by partial hydrolysis and sol-geltemplate method calcined at800oC are compared. The results show that mesoporous Ni-Aloxide prepared by partial hydrolysis have a higher catalytic activity and better resistance tocarbon deposition. This may be due to the three-dimensional pore structureis are moreconducive to the diffusion of the product, thereby reducing the polymerization of theproduct in the pore.3. Preparation of mesoporous Ni-Mg-Al composite oxide and its catalytic reactionperformanceKeeping the Ni/Mg molar ratio equivalent, a series of different Ni/Al molar ratio of themesoporous Ni-Mg-Al composite oxides （NixMgxAl x=0.125,0.17,0.25,0.5） areprepared and characterized by means of XRD, TPR and N₂adsorption-desorption.Theprepared mesoporous NixMgxAl oxides also have high specific surface area, the larger poresize and a narrow pore size distribution.The NixMgxAl oxides are employed for the liquefied petroleum gas （LPG） steamreforming reaction. The effects of Ni/Al molar ratio and calcination temperature ofNixMgxAl catalyst on the performance of LPG reforming show that Ni0.25Mg0.25Al-500catalyst calcined at500oC has a high resistance to carbon deposition and high catalyticactivity.