Sol-gel synthesis routes

Recently, Dai et al. [86] investigated 4 wt.% Au catalysts supported on Ti02-Si02 mixed oxides prepared by nonhydrol)4 ic sol-gel synthesis route for direct propylene epoxidation. High-resolution TEM studies showed that the Au particle size was around 2-A nm. The Ti loading was varied systematically, and the best performance was observed over a catalyst containing 10 mol% Ti 61.3 gpo kg J j h... 

The stoichiometric compound of gallium and oxygen, Ga203, exists in a wide variety of polymorphic phases, including a, p, y, b, e, and 0 phases that so far have heen identified using powder X-ray diffraction (XRD) [72]. The polymorphs a and p are usually prepared at ambient pressure from thermal decomposition of the nitrate (Ga(N03)3), or via sol-gel synthesis routes followed hy dehydration and recrystal-lization of the gel (Figure 3.3 adapted from [72]). 

Clapsaddle BJ, Sprehn DW, Gash AE, Satcher JH, Simpson RL (2004) A versatile sol-gel synthesis route to metal-silicon mixed oxide nanocomposites that contain metal oxides as the major phase. J Non-Cryst Sohds 350 173-181. 

There are a number of reasons for the particular value of, and high interest in, sol-gel synthesis routes [1,12-15] ... 

Amjoud M, Rhouta B, Alimoussa A, Hajji L, Mezzane D, Ahamdane H (2005) Effect of pH adjustment in sol-gel synthesis route on grain size of tin dioxide intended for gas sensors application. Phys Chem News 22 120-124 Arthur JA (2002) Molecular beam epitaxy. Surf Sci 500 189-217... 

Although nonaqueous sol-gel synthesis routes were mainly designed for metal oxide nanomaterials, the underlying chemical principles can be extended to many materials beyond metal oxides. In the following, we will separately discuss composites, organic-inorganic hybrids, metal sulfides, and metals, which all also have important technological applications [114],... 

The classical sol-gel synthesis certainly is one of the most powerful synthesis routes in terms of the wide variety of synthesis approaches and techitical applications. For a long time, especially the aqueous (hydrolytic) sol-gel synthesis route, mainly forced by the development of sitica, has been the main focus of thousands of chemists and materials scientists worldwide. Thus, it is no wonder that many nanoscaled metal oxide-based catalysts have been intensively investigated and reviewed for many different catalytic reactions. 

Although this was in fact the first sol-gel synthesis route leading to the formation of nanoscale metal fluorides, several drawbacks are connected with this route (1) not all metals form stable metal fluoroacetates, (2) not all metal fluoroacetates decompose into metal fluorides but may also form more stable metal oxides, and (3) even the formation of the respective metal oxide fluorides or mixtures of fluorides and oxides cannot be ruled out. Since Equation (6.2) is an oversimplification of the thermal decomposition reaction, the formation of highly corrosive hydrogen fluoride and other harmful decomposition products has to be taken into account. 

In conclusion, the new access towards nanoscopic metal fluorides via this recently developed fluorolytic sol-gel synthesis route opens a wide range of applications for metal fluorides due to the distinctive different properties of these nano materials. 

One of the most promising applications of enzyme-immobilized mesoporous materials is as microscopic reactors. Galameau et al. investigated the effect of mesoporous silica structures and their surface natures on the activity of immobilized lipases [199]. Too hydrophilic (pure silica) or too hydrophobic (butyl-grafted silica) supports are not appropriate for the development of high activity for lipases. An adequate hydrophobic/hydrophilic balance of the support, such as a supported-micelle, provides the best route to enhance lipase activity. They also encapsulated the lipases in sponge mesoporous silicates, a new procedure based on the addition of a mixture of lecithin and amines to a sol-gel synthesis to provide pore-size control. 

Nakanishi, K., Minakuchi, H., Ishizuka, N., Soga, N., and Tanaka, N. (1998). Monolithic columns via sol-gel route in sol-gel synthesis and processing. Ceramic Transactions, 95, American Ceramic Society, Westerville, 139. 

It was also reported by PruP et al. [31] that in situ formed cobalt(III) complexes of pyridine-4-ylmethyl-propyl-amine (PYPA) on preformed organomodified HMS are active as catalysts in the aerobic oxidation of styrene and also 1-decene (Figure 3). Incorporation of PYPA may be achieved by following several routes viz. sol-gel synthesis, post modification of sol-gel AMP-HMS, and grafting. The authors proposed that all materials are able to act as... 

The earliest routes for forming ceramics from sol-gel solutions involved the precipitation of metal oxide particles from solutions. These form a true colloidal suspension a sol. Upon destabilization of this sol, aggregation takes place and a rigid network is formed a gel. A gel is intermediate between a solid and a liquid. The term sol—gel has since been used by the materials science community to describe, albeit erroneously, virtually all chemical processing of ceramics from solutions (e.g., metal oxide particle precipitation or metalorganic decomposition). This discussion focuses on the gel aspects of sol—gel synthesis and not on the sol aspects, which are treated separately in this book. 

All samples were prepared by sol-gel synthesis in alkali-free medium via a polymeric gel route involving alkoxide hydrolysis and condensation, catalysed by base or acid. Similar reactant mixtures were used in all preparations and only the type of gelling agent was changed. 

In this work a sol-gel synthesis of sulfated zirconia (SZ) in basic medium is presented. The new synthesis route, claimed in patents [9, 10], permits to obtain in a single step SZ materials that, after calcination in the range 500-600°C, show a pure tetragonal phase stabilised by the nano dimension of crystallites. 

Sol-gel synthesis of LiA102 involves an alcohol-alkoxide route. Different alcohols and alkoxide combinations can be used. The alkoxide and alcohols are mixed and hydrolyzed by addition of pure water. The mixture is then gelled by heating to 60°C and is subjected to hydrothermal treatment in an autoclave. The morphology of the crystalline phases is dependent on the length of the alkoxy groups used in the alkoxide-alcohol mixing step. For example, rod-like crystals are produced with butoxide and propoxide mixtures. 

The last approach (route c) [34] involves the direct sol-gel synthesis of MTS materials containing chloropropyl groups 84 by reaction between (3-chloropropyl)trimethoxysilane 66 and letraethoxysilane (TEGS) 83. This was converted into a TMG-containing material 85 by the method described above. The T.MG loading values of some of the so-prepared catalysts are summarized in Table 9. 

N. L. Wu, S. Y. Wang, and I. A. Rusakova. Inhibition of crystallite growth in the sol-gel synthesis of nanocrystalline metal oxides. Science, 285 1375-1377, 1999 A. Vioux. Nonhydrolytic sol-gel routes to oxides. Chem. Mater., 9 2292-2299, 1997 J. Rockenberger, E. C. Scher, and A. P. Alivisatos. A new nonhydrolytic single-precursor approach to surfactant-capped nanocrystals of transition metal oxides. J. Am. Chem. Soc., 121 11595-11596, 1999... 

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