Alumina-Based Aerogel

C. Hoang-Van, B. Pommier, R. Harivololna, and P. Pichat, Alumina-based aerogels as carriers for automotive Pd catalysts. J. Non-Cryst. Solids 145, 250-254 (1992) [this issue is devoted to aerogels]. 

In a previous study, we found that Pd (lwt%) catalysts supported on finely divided aliunina aerogels were more active, in the oxidation of CO by NO and O2, than both Pd and Pt-Rh supported on commercial aluminas [2], More recently, a study of propane oxidation over Pd supported on alumina-based aerogel catalysts led to the conclusion that favorable ensembles of active sites contituted by Pd° atoms and Pda" " ions might exist for this reaction [3],... 

In this paper, we describe methods for the preparation of single or binary inorganic oxide aerogels that could be used as carriers for high performance supported Pd automotive catalysts. Indeed we have shown that alumina - based aerogel supported palladium catalysts were very active in propane oxidation [2]. Here we present the catalytic performances, in the reaction CO + NO -1- O2, of palladium catalysts supported on some of the prepared aerogels and compare their performances with those of Pd or Pt-Rh catalysts supported on a commercial alumina. 

Silica and, to a less extent, alumina aerogels are detailed below because most of the catalytic aerogels are silica- or alumina-based composites. As mentioned in the Abstract section, the synthesis of an aerogel results from a two-step process First, a solution-to-sol-to-gel is prepared, and second, this gel is supercritically dried in an autoclave. 

Figure 8.7. MonoUthic aerogels prepared via the epoxide addition method A. alumina, B. chromia, C. p-FeOOH (akaganeite), D. Ni(ll)-based aerogel, E. tungsten oxide, and F. tin oxide.

Binary alumina-based oxide-oxide aerogels were synthesized either by a co-hydrolysis or by a two - step hydrolysis procedure. 

Aluminum oxide (alumina), AI2O3, has lugh technological value. Sol-gel processing of alumina has created novel applications and improved some of its properties. Products such as catalyst earners, abrasives, fibers, films for electronic applications, aerogels, and membranes for molecular filtration have been developed based on sol-gel processing. See also Alumina Adsorption (Process) and Bauxite Aluminum and Aluminum Alloys and Engineered Materials. 

For comparison with the carbon-based catalysts, several MgO catalysts were prepared on silica and alumina supports, and a very high-sur-face MgO aerogel was also prepared. Testing these under the same conditions gave very poor yields of the desired alcohols. It is clear that the carbon structures promote the activity, very likely by catalyzing the dehydrogenation and hydrogenation steps. 

The pore structure of two types of catalyst support material were studied 7-alumina and silica aerogel. The alumina samples were commercial catalyst supports made in 1/8 inch diameter pellet form by Harshaw Chemical. Silica aerogels were prepared from silica gels synthesized by a two step acid/base catalyzed procedure employing TE0S with a water to silicon ratio equal to 3.7 [17] and ammonium hydroxide concentration of 0.005 M (sample A) or 0.01 M... 

It is also interesting to compare various types of solid catalysts in the epoxidation of a-isophorone. Unfortunately, a real comparison is rather difficult, as the reaction conditions (temperature, oxidant, concentrations) are different for each catalyst. Due to the lack of information, the comparison shown in Table 2 is based only on the productivity, i.e. the amount of isophorone oxide produced in unit time using unit amount of catalyst. Two set of data were chosen for the 20 wt% titania - 80 wt% silica aerogel, and the best published data were chosen for the hydrotalcite [16, 17] and the alumina-supported KF [17, 18]. We assumed that the... 

Very recently, hierarchically arranged aerogel monoliths [209], silica-based monoliths from swollen liquid crystals [210] or heptane/water/ethanol/sodium dodecylsulfate microemulsion [211], monolith-based hybrid components such as the PMOs [212], and silicon oxycarbide monoliths with better thermal properties than the pure (organo) silica monoliths [213] have been reported. These approaches have been extended also to alumina or aluminumsilicates [214, 215], titania [216, 217], zirconia [218], or ceramic-based [219] systems. 

Pietron J J, Rolison D R (2004) Improving the efficiency of titania aerogel-based photovoltaic electrodes by electrochemically grafting isopropyl moieties on the titania surface. J Non-Cryst Solids 350 107-112 Tursiloadi S, Imai H, Hirashima H (2004) Preparation and characterization of mesoporous titania-alumina ceramic by modified sol-gel method. J Non-Cryst Solids 350 271-276... 

Chandradass J, Kang S, Bae D-S (2008) Synthesis of silica aerogel blanket by ambient drying method using waterglass based precursor and glass wool modified alumina sol, J Non-Cryst Solids 354 4115-4119... 

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