Preparation of Nanoporous MgAl 2 O 4 by Combined Utilization of Sol-Gel Process and Combustion of Biorenewable Oil
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Preparation of Nanoporous MgAl2O4 by Combined Utilization of Sol-Gel Process and Combustion of Biorenewable Oil Christian Hörtz1, Danielle M. Ladd2 and Dong-Kyun Seo2 1 Johannes Gutenberg University Mainz, Duesbergweg 10-14, D-55128 Mainz, Germany 2 Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, U.S.A.
ABSTRACT Nanoporous MgAl2O4 particulates with high porosities were successfully prepared from sol-gel reactions, solvent exchange with castor oil and subsequent combustion and calcination at 700 °C. The products were crystalline and semitransparent. Changes in the metal precursor concentrations allowed control of pore volumes from 0.7 to 1.1 cm3/g and average pore sizes from 14 to 19 nm. The specific surface areas are about 200 m2/g regardless of the precursor concentrations. After heating at 1000 °C for 10 hours, the products kept about 70% of their original pore volume and about 60% of the original surface area. Heating at 1100 °C caused a drastic reduction of pore volume and surface area to 40 and 36%, respectively, as the average particle size increased to 23 nm. INTRODUCTION MgAl2O4 is attractive as a catalyst or catalyst support due to its low acidity, good interaction with the supported metals, hydrophobic character and high thermal resistance1 for various petroleum processing and fine chemical productions.2 Furthermore, its high thermal stability is ideal for CO2 reforming, oxidative dehydrogenation of ethane with CO2, and chemical looping combustion/reforming which are particularly relevant to clean energy technologies.3, 4 Nanoporous MgAl2O4 materials have been prepared through several different methods such as coprecipitation,1 supercritical drying,5 hydrothermal synthesis,6 freeze drying,7 surfactant templating,8 and nanocasting with carbon.9 However, nanoporous crystalline MgAl2O4 with a high surface area and high porosity is relatively rare because of the undesired pore collapsing during calcination.9 Recently, we have reported a new synthetic method for production of high surface area, high porosity metal oxides such as γ-alumina, which employs sol-gel process but can be drastically fast by forgoing the supercritical drying step in removing the pore liquid. The new method utilizes biorenewable oil as a pore liquid in the metal oxide gels. Combustion of the gel materials with the oil inside the pores (“olegels”) can result in high surface area metal oxides (“pyrogels”) which exhibit mostly mesopores, without a significant loss of pore volume.10, 11 Herein we show that the same method can be applied to produce nanoporous crystalline MgAl2O4 materials. The thermal stability of the products was examined for the highest pore volume sample up to 1100 °C.
EXPERIMENT Five different samples were prepared with different precursor concentrations in order to examine the effect of the concentrations on pore properties (Table 1). The initial wet gel was prepared by following the procedure described by Baumann et al. using inorganic salt precursors and an epoxide as a gel
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