Reference LPI-ARTICLE-1999-017View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

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Metal Oxide Surfaces and Their Interactions with Aqueous Solutions and Microbial Organisms.

Thisreviewdiscusses preparationanduseofthesyntheticlayerednanoparticlesinpolymermatrices, i.e., in the polymeric nanocomposites (PNCs). Several types of synthetic or semi-synthetic layered materials are considered, namely the phyllosilicates (clays), silicic acid (magadiite), layered double hydroxides (LDHs), zirconium phosphates (ZrPs), and di-chalcogenides. The main advantage of synthetic clays is their chemical purity (e.g. absence of amorphous and gritty contaminants, as well as arsenic, iron, and other heavy metals), white to transparent color that assures reproducibly of brightly colored products, as well as a wide range of aspect ratios, p ¼ 20 to £6000. Several large scale production facilities have been established. The synthetic clay and LDH industries are oriented toward big volume markets: catalysis, foodstuff, cosmetics, pharmaceuticals, toiletry, etc. The use of these materials in PNCs is limited to synthetic clays and LDHs, mainly for reinforcement, permeability control, reduction of flammability, and stabilization, e.g. during dehydrohalogenation of chlorinated macromolecules. The use of lamellar ZrPs and di-chalcogenides is at the laboratory stage of functional polymeric systems development, e.g. for electrically conductive materials, catalysts or support for catalysts, in photochemistry, molecular and chiral recognition, or in fuel cell technologies, etc. Copyright # 2007 John Wiley & Sons, Ltd.

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Synthetic, layered nanoparticles for polymeric nanocomposites (PNCs)

This paper reviews the synthesis of smectites and porous pillared clay catalysts. Synthetic as well as natural smectites serve as precursors for the synthesis of Al, Zr, Ti, Fe, Cr, Ga, V, Si, and other pillared clays as well as mixed Fe/Al, Ga/Al, Si/Al, Zr/Al and other mixed metal/Al pillared clays. The use of these pillared clays in some catalytic reactions is also briefly reviewed.

Synthesis of Smectites and Porous Pillared Clay Catalysts: A Review

Smectites are one of the most important groups of phyllositicates found in soils and sediments, and certainly one of the most difficult to study. New information about the formation mechanisms, impact of structural features on surface properties, and long-term stability of smectites can best be gained from the systematic study of single-phase specimens. In most instances, these specimens can only be obtained through synthesis under controlled conditions. Syntheses of smectites have been attempted (1) at ambient pressure and low-temperature ( 1000~ or pressures >10 kbars), and (4) in the presence of fluoride. Of these approaches, syntheses performed under moderate hydrother- mal conditions are the most numerous and the most successful in terms of smectite yield and phase- purity. Using hydrothermal techniques, high phase-purity can be obtained for beidellites and several transition-metal smectites. However, synthesis of montmorillonite in high purity remains difficult. Starting materials for hydrothermal syntheses include gels, glasses, and other aluminosilicate minerals. The pres- ence of Mg 2+ seems to be essential for the formation of smectites, even for phases such as montmorillonite which contain low amounts of Mg. Highly crystalline smectites can be obtained when extreme temper- atures or pressures are used, but other crystalline impurities are always present. Although the correlation between synthesis stability fields and thermodynamic stability fields is good in many instances, metastable phases are often formed. Few studies, however, include the additional experiments (approach from under- and over-saturation, reversal experiments) needed to ascertain the conditions for formation of thermody- namically stable phases. Thorough characterization of synthetic products by modern instrumental and molecular-scale techniques is also needed to better understand the processes leading to smectite formation.

Synthesis of smectite clay minerals; a critical review

Synthesis of clay minerals

Ammonium-saponite is hydrothermally grown at temperatures below 300°C from a gel with an overall composition corresponding to (NH4)0.6Mg3Si34Al0.6O10(OH)2. The synthetic saponite and coexisting fluid have been characterized by means of X-ray powder diffraction, X-ray fluorescence, Induced Coupled Plasma-Atomic Emission Spectroscopy, thermogravimetric analysis, transmission electron microscopy, CEC determination using an ammonia selective electrode, and pH measurement. In the crystallization model developed, crystallization started with the growth of individual tetrahedral layers with an aluminum substitution not controlled by the A1IV/A1VI ratio in the gel and hydrothermal fluid, on which the octahedral Mg layers can grow. During the synthesis, individual sheets stacked to form thicker flakes while lateral growth also took place. The remaining A1VI partly replaced ammonium as the interlayer cation.

Development of ammonium-saponites from gels with variable ammonium concentration and water content at low temperatures*

Saponites were hydrothermally grown in the presence of amounts of NH4+, Na+, K+, Rb+, Ca2+, Ba2+, and Ce4+ equivalent with the CEC of the saponite (155 meq/100 g), with or without F−, at a temperature of 200°C for 72 hr. XRD and CEC data revealed the formation of a two-water-layer saponite with mainly Mg2+ as interlayer cation. Dehydration occurred between 25° and 450°C and dehydroxylation occurred in two steps between 450° and 790°C and between 790° and 890°C. The relatively small length of the b-axis between 9.151 and 9.180 A is explained by considerable octahedral Al substitution (between 0.28 and 0.70 per three sites) and minor tetrahedral Al substitution (between 0.28 and 0.58 per four sites). Under the synthesis conditions applied in this study, less than 13% of the interlayer sites are occupied by Na+, K+, and Rb+; between 13.3% and 21% by Ca2+ and Ba2+; while NH4+ gives the highest value at 34%. The remaining sites are mainly filled by Mg2+. Ce4+ is not found in the saponite structure due to the formation of cerianite, CeO2. The presence of F− had little influence on the saponite composition. The formation of Mg-saponites is explained by a model in which an increased bayerite formation resulting in a higher octahedral Al3+ substitution and more Mg2+ in solution. Mg2+ is preferentially incorporated compared with the other interlayer cations due to its smallest ionic radius in combination with its 2 + charge.

Characterization of Mg-saponites synthesized from gels containing amounts of Na+, K+, Rb+, Ca2+, Ba2+, or Ce4+ equivalent to the CEC of the saponite

In the chemical system Na2O-Al2O3-SiO2-H2O, the stability field of Na-beidellite is presented as a function of pressure, temperature, and Na- and Si-activity. Na0.7-beidellite was hydrothermally synthesized using a stoichiometric gel composition in the temperature range from 275° to 475°C and at pressures from 0.2 to 5 kbar. Below 275°C kaolinite was the only crystalline phase, and above about 500°C paragonite and quartz developed instead of beidellite. An optimum yield of 95% of the Na0.7- beidellite was obtained at 400°C and 1 kbar after 20 days. Gels with a Na-content equivalent to a layer charge lower than 0.3 per O20(OH)4 did not produce beidellite. They yielded kaolinite below 325°C and pyrophyllite above 325°C. With gels of a Na-content equivalent to a layer charge of 1.5, the Na-beidellite field shifted to a minimum between temperatures of 275° and 200°C. This procedure offers the potential to synthesize beidellite at low temperatures. Beidellite synthesized from Na1.0-gel approach a Na1.35 composition and those from Na1.5- and Na2.0-gels a Na1.8 composition.

https://link.springer.com/content/pdf/10.1346/CCMN.1993.0410403.pdf

Synthesis and paragenesis of Na-beidellite as a function of temperature, water pressure, and sodium activity

The Al concentration and forced hydrolysis influence the polymerization of aqueous Al(III) and thus the 27Al NMR spectra An increase of the Al concentration results in an increase of the fraction present as monomer, in the formation of an oligomer of an OH/Al molar ratio of 24, and in the disappearance of the tridecamer above an Al concentration of 015M Increasing the OH/Al molar ratio at a constant Al concentration leads to a linear drop of the fraction present as monomer over the entire range between 1 and 25 and a maximum in oligomer fraction between 15 and 25 At low Al concentrations, the fraction of tridecamer exhibits an optimum yield between OH/Al ratios of 22 and 24 At ratios higher than 24, NMR unobservable polymers are formed The chemical shift and the linewidth of the monomer resonance are lowered by increasing spin-spin relaxation, T2, and a consequently decreasing quadrupole coupling constant upon increasing Al concentration With increasing OH/Al molar ratio, [Al(H2O)6]3+ is mainly replaced by [Al(H2O)5(OH)]2+ and [Al(H2O)4(OH)2]+ and subsequently the chemical shift and linewidth of the monomer resonance increase The Al concentration or OH/Al molar ratio hardly affect the resonance of the central fourfold coordinated Al of the tridecamer, due to its very strong shielding

The effects of concentration and hydrolysis on the oligomerization and polymerization of Al(III) as evident from the 27Al NMR chemical shifts and linewidths

The 27 Al monomer resonance of 0.05M aluminum nitrate solutions gradually broadens from 7.82 to 89.40 Hz upon forced hydrolysis from an OH/Al molar ratio of 0.0 to 2.2. The broadening is due to increasing amounts of [AlOH] 2+ and [Al(OH) 2 ] + up to 2.24% and 0.11%, respectively. Additional measurements on the hydrolysis with ammonium hydroxide and the dissolution of aluminum sec-butoxide in dilute HCl give similar results. To calculate the linewidth of the monomer resonance, it is assumed that [AlOH] 2+ and [Al(OH) 2 ] + have identical linewidths as the isoelectronic compounds [AlF] 2+ and [AlF 2 ] + , respectively. The calculation is based on pK values of 4.99 for [AlOH] 2+ and 10.13 for [Al(OH) 2 ] + . The results of the theoretical calculations agree well with observed linewidth data.

Aluminum monomer line-broadening as evidence for the existence of [AlOH]2+ and [Al(OH)2]+ during forced hydrolysis: a 27Al nuclear magnetic resonance study

J. Theo Kloprogge

Papers

Development of ammonium-saponites from gels with variable ammonium concentration and water content at low temperatures*

Characterization of Mg-saponites synthesized from gels containing amounts of Na+, K+, Rb+, Ca2+, Ba2+, or Ce4+ equivalent to the CEC of the saponite

Synthesis and paragenesis of Na-beidellite as a function of temperature, water pressure, and sodium activity

The effects of concentration and hydrolysis on the oligomerization and polymerization of Al(III) as evident from the 27Al NMR chemical shifts and linewidths

Aluminum monomer line-broadening as evidence for the existence of [AlOH]2+ and [Al(OH)2]+ during forced hydrolysis: a 27Al nuclear magnetic resonance study