Synthesis Of Xerogels Research Articles

Synthesis of transition metal-doped carbon xerogels by cogelation

The cogelation process, i.e. the co-polymerization of a metal complex with the gel precursors, was used for the synthesis of transition metal-doped resorcinol–formaldehyde gels. The aim of this process is to anchor the metal to the polymer so that the former does not sinter during the pyrolysis step leading to porous carbon. Cu-, Ni-, Pd- or Pt-loaded gels were prepared by this technique. After drying and pyrolysis, Pd and Pt were obtained as metal nanoparticles (2–5 nm in diameter) inserted in the carbon nodules, when the complexing agent and the synthesis conditions were well chosen. These small metal particles were inaccessible to reactive gases, probably due to carbon deposit at the metal surface during pyrolysis: CO almost did not chemisorb. Oxidation of the support or pyrolysis under reductive atmosphere was applied to the metal-doped gels and carbons in order to make the surface of the metal particles accessible, but these treatments develop the macropores only. The cogelation process is then suitable to prepare metal nanoparticles protected from the outside by encapsulation in the carbon matrix.

Synthesis of silica xerogels with high surface area using acetic acid as catalyst

The influence of acetic acid on the pore structure and surface area of silica prepared by the sol-gel method was investigated. Experimental conditions of synthesis, such as gelation temperature and solvents, were also studied. N2 adsorption isotherms of the samples were type 1, typical of microporous materials, explaining the high surface area values (BET) observed. The simultaneous addition of acetic and hydrochloric acids as catalysts and of acetone as solvent, together with the use of a gelation temperature of 20 ºC, made it possible to prepare amorphous silica materials with surface area values up to 850 m² g-1. The high surface area value of these samples could be explained by the microporosity and the nanometric size of the particles.

Second-Harmonic Generation by Ferroelectric Microparticles in Aerogels

The synthesis and preparation of aerogels and xerogels loaded with micrometer-sized ferroelectric particles of BaTiO3 and KNbO3 is described. We show that these composite aerogels and xerogels exhibit nonlinear optical properties, especially second-harmonic generation at an incident wavelength of 1064 nm.

Comparative characteristics of texture and properties of hybrid organic–inorganic adsorbents functionalized by amine and thiol groups

Abstract The structure and texture characteristics of the hybrid organic–inorganic adsorbents, which were obtained by using of two-component systems of “structure-forming agent/trifunctional silane”, are compared as follows: the first component is Si(OC 2 H 5 ) 4 or (C 2 H 5 O) 3 Si–A–Si(OC 2 H 5 ) 3 , where A = –(CH 2 ) 2 – or –C 6 H 4 –; the second one is alkoxysilane with amine (–NH 2 , NH, –NH(CH 2 ) 2 NH 2 ) and thiol (–SH) groups. The adsorbents, derived from TEOS, have more accessible functional groups (2.6–4.2 mmol/g) than xerogels, which are based on bis(triethoxysilanes) (1.0–2.6 mmol/g). On another hand xerogels derived from bis(triethoxysilanes) have a more extended porous structure ( S sp =516–968 m 2 /g, V s = 0.418–1.490 cm 3 /g, d = 2.5–15.0 nm) than those that are based on TEOS ( S sp = 4–631 m 2 /g, V s = 0.005–1.382 cm 3 /g, d = 2.3–17.7 nm). The geometric dimensions of functional groups have a more essential effect on the parameters of porous structure in the case of TEOS-derived xerogels. Using solid-state NMR spectroscopy, it has been shown that in synthesis of xerogels with the use of TEOS, the molecular frame of globules is formed by structural units Q n ( n = 2,3,4), and the functional groups exist as structural units of T n ( n = 2,3). The xerogels obtained with using bis(triethoxysilanes) consist only of structural units of T n -type ( n = 1,2,3).

Synthesis of transition metal-doped carbon xerogels by solubilization of metal salts in resorcinol–formaldehyde aqueous solution

The pore texture of carbon materials obtained from evaporative drying and pyrolysis of resorcinol–formaldehyde aqueous gels is controlled by the initial pH of the precursors solution. In order to produce transition metal-containing carbons with tailored texture, various metallic salts were dissolved in the precursors solution. When necessary, a complexing agent (HEDTA or DTPA) was added to render the metal ions soluble. Ni, Fe and Pd loaded carbon xerogels were synthesized and their pore texture was studied after evaporative drying and after pyrolysis. The carbon texture was also studied with regard to the nature of the metal and the amount of complexing agent. The solubilization of transition metal salts in the resorcinol–formaldehyde aqueous solution does not prevent the texture regulation, even though this texture control is influenced: the limits of the pH interval leading to micro–mesoporous carbon materials can slightly differ when a metal salt and/or a complexing agent are added. The pH range shift depends mainly on the amount and nature of the complexing agent, but also slightly on the nature of the metal ion. Nevertheless, the metal particles obtained are rather big (diameter > 15 nm). For catalytic applications, the metal dispersion must be enhanced, especially in the case of expensive metals.

Synthesis and chemical reactivity of semicarbazide-supported silicas

The synthesis of xerogels possessing a Fmoc-protected semicarbazide group covalently linked to a silica matrix is described. BET and fluorescence analyses showed the homogeneity of the xerogels. The xerogels were deprotected and further reacted with aldehydes to give novel materials.

Formation of Nanostructured Energetic Materials via Modified Sol-Gel Synthesis

ABSTRACTThis study is concerned with the development of a modified sol-gel synthesis of Fe2O3 xerogels that would allow the design and control of the interfacial area between the oxidant iron oxide matrix and the metal reducing agent, thus optimizing the energetic yield of these highly energetic reactions. The modification consisted in the addition of a new class of di-functional template molecules, such as diamines or di-acids, as gelation agents. pH profile measurements indicated that the mechanism of reaction of propylene oxide and of succinic acid as the gelation agents was fundamentally different. Propylene oxide acts as a proton scavenger, reducing the hydrated iron species to Fe2O3, thus reducing the concentration of protons in the reaction mixture leading to an increase in pH. When succinic acid is used as the gelation agent, a decrease in pH versus time during the reaction indicates the formation of carboxylate ions, thus creating reactive molecules that are capable of stabilizing the Fe2O3 clusters during the growth process. Infrared spectra of the products in both reactions support presence of carboxylate groups in the Fe2O3 xerogels. X-ray diffraction analyses revealed low levels of crystallinity in both products, and the presence of different phases of Fe2O3.

New sol–gel synthetic route to transition and main-group metal oxide aerogels using inorganic salt precursors

We have developed a new sol–gel route to synthesize several different transition and main-group metal oxide aerogels. The approach is straightforward, inexpensive, versatile, and it produces monolithic microporous materials with high surface areas. Specifically, we report the use of epoxides as gelation agents for the sol–gel synthesis of chromia aerogels and xerogels from simple Cr(III) inorganic salts. The dependence of both gel formation and its rate was studied by varying the solvent used, the Cr(III) precursor salt, the epoxide/Cr(III) ratio, as well as the type of epoxide employed. All of these variables were shown to affect the rate of gel formation and provide a convenient control of this parameter. Dried chromia aerogels were characterized by high-resolution transmission electron microscopy (HRTEM) and nitrogen adsorption/desorption analyses, results of which will be presented. The results presented here show that rigid monolithic metal oxide aerogels can be prepared from solutions of their respective metal ion salts (Fe 3+, Al 3+, In 3+, Ga 3+, Zr 4+, Hf 4+, Ta 5+, Nb 5+, and W 6+), provided the formal oxidation state of the metal ion is greater than or equal to +3. Conversely, when di-valent transition metal salts are used precipitated solids are the products.

Synthesis of Hydrophilic and Hydrophobic High Surface Area Xerogels at pHs below Silica Iso-electric Point

The synthesis of hydrophilic and hydrophobic high surface area (up to 1087 m2/cc) silica xerogels at pHs below 2 is demonstrated in this work. High surface area xerogels exhibit extremely fine microstructural features and fast gelation times (less than one hour). Contrary to conventional generalization that xerogels synthesized at low pH are polymeric, it is demonstrated that particulate xerogels can also be synthesized at pHs below 2. These xerogels are synthesized by the addition of large amounts of NH4OH and have relatively lower surface areas. The particulate nature of these xerogels is indicated by their opacity and from their coarse microstructural features as observed by TEM.

Nanocomposites containing nanoclusters of Ag, Cu, Pt, Os, Co 3C, Fe 2P, Ni 2P, Ge, or [formula omitted

Neutral, low-valent complexes of transition metals or an organometallic compound of Ge are prepared containing ancillary ligands which bear silicate ester or (alkoxy)silicon functional groups. Inclusion of these molecules as dopant species in a conventional sol-gel synthesis of silica xerogels affords silica xerogels in which dopant molecules have presumably been covalently incorporated into the xerogel matrix with uniform and high dispersion. Subsequent thermal treatment of these molecularly doped xerogels under reducing or oxidizing/reducing conditions gives nanocomposites containing nanoclusters of metals or semiconductor substances. Nanocomposites containing nanoclusters of Ag, Cu, Pt, Os, Co 3C, Fe 2P, Ni 2P, or Ge have been prepared. Preliminary evidence for the formation of a nanocomposite containing Pt-Sn nanoclusters derived from a bimetallic molecular precursor is also presented. These results support the hypothesis that covalent incorporation of molecular precursors containing low-valent metals into a silica xerogel can afford nanocluster phases with high dispersion, relatively small particle size, and unusual elemental composition.

Synthesis of SiO2-TiO2 xerogels by sol-gel process

A new method to synthesize SiO2-TiO2 gels by sol-gel process has been developed. This technique uses tetraisopropylorthotitanate [Ti(OiPr)4] and tetraethylorthosilicate [TEOS]: they are mixed in the same solvent and then directly hydrolysed. This one-step reaction is possible because of the use of 2-methoxyethanol, a protic polar solvent. This alcohol plays two different specific roles: it acts as a solvent as well as a stabilizer of titanium alkoxide towards the hydrolysis-precipitation reaction. So, by an accurate adjustment of the quantity of methoxyethanol in the mixture, we can control the reactivity of the titanium precursor.

Nanocomposites Containing Nanoclusters of Metals or Semiconductors

ABSTRACTNeutral, low-valent complexes of transition metals or an organometallic compound of Ge are prepared containing ancillary ligands which bear silicate ester or (alkoxy)silicon functional groups. Inclusion of these molecules as dopant species in a conventional sol-gel synthesis of silica xerogels affords silica xerogels in which dopant molecules have presumably been covalently incorporated into the xerogel matrix with uniform and high dispersion. Subsequent thermal treatment of these molecularly doped xerogels under reducing or oxidizing/reducing conditions gives nanocomposites containing nanoclusters of metals or semiconductor substances. By this procedure, nanocomposites containing nanoclusters of Ag, Cu, Pt, Os, CCo3, Fe2P, Ni2P, or Ge have been prepared. Preliminary evidence for the formation of a nanocomposite containing Pt-Sn nanoclusters derived from a bimetallic molecular precursor is also presented. Characterization data for the nanocomposite materials include TEM, electron diffraction, EDS, XRD, and selected use of micro-Raman spectroscopy. These results support the hypothesis that covalent incorporation of molecular precursors containing low-valent metals into a silica xerogel can afford nanocluster phases with high dispersion, relatively small particle size, and unusual elemental composition.

ChemInform Abstract: Synthesis and Texture of Xerogels Based on Ultradisperse Powders of Aluminum Oxide and Monohydroxide.

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Synthesis and investigation of xerogels of polysiloxane hydride with various porous structures

1. A method of regulation of the porous structure of polysiloxane hydride xerogels was described. Samples of polysiloxane hydride xerogels with various porous structures were obtained. 2. The porous structure of polysiloxane hydride xerogels is determined chiefly by the surface tension of the intermicellar liquid and by the nature of its interaction with the surface of the lyogel. 3. An analysis of the porous structure of typical samples of polysiloxane hydride xerogels was given on the basis of the sorption isotherms of benzene. 4. The porous structure of polysiloxane hydride xerogels is formed by globules in contact, and its parameters are determined by the average diameter of the globules and the average coordination number. The volumes of the different varieties of pores, the specific surface of the transitional pores, and the constants of the equation of the theory of volume filling of micropores, which are parameters of the microporous structure, characterize the porous structure of polysiloxane hydride xerogels.