Aerogels catalysis

Keywords Aerogels, Catalysis, Polysaccharides, Porous materials. Remediation, Shaping, Textural properties... 

Catalysis. Kistler explored the catalytic appHcations of aerogels ia the 1930s because of the unique pore characteristics of aerogels (24), but this area of research stayed dormant for about three decades until less tedious procedures to produce the materials were introduced (25,26). Three recent review articles summarize the flurry of research activities since then (63—65). Table 3 is a much abbreviated Hst of what has been cited in these three articles to demonstrate simply the wide range of catalytic materials and reactions that have been studied. 

Some work has also been achieved with heterogeneous catalysis. These catalysts include Amberlyst-15, Nafion-H, montmorillonite KSF clay, ferrihydrite silica gel aerogels containing 11-13% iron, silica sulfuric acid, and zeolites. ... 

Transition-metal mixed oxides active in combustion catalysis have been prepared by two main procedures i) classical coprecipitation / calcination procedures starting from metal nitrates and/ or alkoxides ii) preparation based on the supercritical drying of gels prepared from organic complexes (alkoxides, acetylacetonates or acetates), producing aerogels . Details on the second preparation can be found in Ref. 13. 

Maury, S., Buisson, P., Perrard, A. and Pierre, A.C. (2004) Influence of the sol-gel chemistry on the activity of a lipase encapsulated in a silica aerogel. Journal of Molecular Catalysis B-Enzymatic, 29, 133-148. 

Oxidative catalysis over metal oxides yields mainly HC1 and C02. Catalysts such as V203 and Cr203 have been used with some success.49 50 In recent years, nanoscale MgO and CaO prepared by a modified aerogel/hypercritical drying procedure (abbreviated as AP-CaO) and AP-MgO, were found to be superior to conventionally prepared (henceforth denoted as CP) CP-CaO, CP-MgO, and commercial CaO/MgO catalysts for the dehydrochlorination of several toxic chlorinated substances.51 52 The interaction of 1-chlorobutane with nanocrystalline MgO at 200 to 350°C results in both stoichiometric and catalytic dehydrochlorination of 1-chlorobutane to isomers of butene and simultaneous topochemical conversion of MgO to MgCl2.53-55 The crystallite sizes in these nanoscale materials are of the order of nanometers ( 4 nm). These oxides are efficient due to the presence of high concentration of low coordinated sites, structural defects on their surface, and high-specific-surface area. 

Microporosity is a feature observed in many different materials (e g., activated carbons, aerogels, and xerogels). However, with regard to heterogeneous catalysis, zeolites are practically the only microporous catalysts used at present. The following chapter thus only addresses zeolites and their use in catalysis. 

Since 1905, when Coblentz obtained the first IR spectrum, vibrational spectroscopy has become an important analytical research tool. This technique was then applied to the analysis of adsorbates on well-defined surfaces, subsequently moving towards heterogeneous reaction studies. Terenin and Kasparov (1940) made the first attempt to employ IR in adsorption studies using ammonia adsorbed on a silica aerogel containing dispersed iron. This led to a prediction by Eischens et al. from Beacon Laboratories in 1956 that the IR technique would prove to be extremely important in the study of adsorption and catalysis. For an excellent review article in IR spectroscopy, see Ryczkowski and references therein and for a more recent review with applications, see Topsoe. ... 

These porous solids obviously have very high surface areas, and they lend themselves naturally to service in catalysis. Palladium catalysts supported on alumina aerogels have been used successfully to remove CO and NO from automobile exhausts,13 and a V205/Ti02 aerogel is itself a catalyst for the selective reduction of NO to N2 and water by gaseous ammonia.14... 

Aerogels are used in diermal insulation, catalysis, detection of high energy particles, piezoceramic, ultrasound transducers, integrated circuits, and as dehydrating agents. 

A series of Chromia-Alumina aerogel catalysts containing different contents of chromium was prepared by autoclave method. The specific areas of the catalysis were measured with Ng at 77°K according to the BET method. Their structural properties were determined from the X ray diffraction patterns recorded on a philips diffractometer PW 1050/70. EPR measurements were performed with a 8ruker ZOO TT spectrometer at 77°K operating in X band. DPPH was used as the g value standard. Kinetic data were obtained in dynamic pyrex microreactor operating at atmospheric pressure as described elsewhere (ref. 3). 

H. Zarrouk, A. Ghorbel, G. M. Pajonk and S. J. Teichner, EPR Investigation of Chromia Alumina Aerogel Catalysts for the Transformation of Isobutane by NO into Methacrylonitrile, Proc. 9 th Ibero Americain Symposium on Catalysis, Lisbon, July 16-21, 1984, pp 339 - 348. 

The non-ideality of catalyst surfaces has ever been one the major difficulties in understanding the detailed mechanisms of contact catalysis. The Advances in Catalysis were opened in 1948 by an article of Taylor on the heterogeneity of catalyst surfaces for chemisorption [4] that the matter was not easy to model is understood by observing that 41 years later the role of particle size on the catalytic activity of supported metals was the subject of of another review in the same series [5] moreover, the family of solids of catalytic interest has since Taylor s review been increased by the availability of new techniques for the preparation of highly dispersed solids, like crystalline zeolites and amorphous aerogels. 

Temperature resistant oxide supports for catalytic active phases are necessary for the catalytic combustion of hydrocarbon pollutants in the field of environment as well for energy production through hydrocarbon clean combustion. Four commonly used pure support oxides in catalysis silica, alumina, titania, and zirconia were synthesised by the sol-gel and aerogel methods and, in parallel, the same oxides were doped with yttria. Pure yttria aerogel was also made and characterised. Heat treatments were performed from ambient up to 1200°C and BET surface areas and XRD patterns were recorded after heat treatments at 300, 600, 900 and 1200 °C for pure and doped oxides, respectively. 

Considering the hydrophilic properties of the support, the effectiveness of polysaccharide aerogel microspheres as catalyst support was evidenced in the so-called Supported Aqueous Phase Catalysis [131]. The stability of the catalyst obtained was investigated in terms of textural stability and catalytic activity in the reaction of substitution of an allyl carbonate with morpholine catalyzed by the hydrosoluble Pd (TPPTS)3 complex [132]. 

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