Ordering of Intercalated Water and Carbonate Anions in Hydrotalcite. An NMR Study

Intercalation chemistry and thermal characteristics of layered double hydroxides possessing organic phosphonates and sulfonates

The distinct roles of organic sulfonates that enable delamination in water and formation of microporous structures via thermal activation are elucidated.

Understanding the High Longitudinal Relaxivity of Gd(DTPA)-Intercalated (Zn,Al)-Layered Double Hydroxide Nanoparticles

In this study, biocompatible gadolinium diethylenetriaminepentaacetate (Gd(DTPA))-intercalated (Zn,Al)-layered double hydroxide (LDH) nanoparticles were synthesized, characterized for Gd(DTPA) loading percentage and nanostructure, and the spin-lattice relaxation times (T₁) measured to determine their suitability as a potential T₁-weighted contrast agent for magnetic resonance imaging (MRI). Compared to the most commonly used contrast agent in clinical MRI (i.e., molecular Gd(DTPA)), significant increases in longitudinal relaxivity (r₁) were measured for all Gd(DTPA)-intercalated nanoparticles. For a specific Zn₂Al(OH)₆(Cl,0.5CO₃)_0.56Gd(DTPA)_0.086·xH₂O composition, r₁ was found to be 28.38 s^-1 mM^-1, which is over 6 times the r₁ of molecular Gd(DTPA). This dramatic increase in r₁ is attributed to (a) the much longer rotational correlation time (τ_R) of nanoparticles and (b) the inherent water of LDH that forms the second-/outer-sphere in the vicinity of intercalated Gd(DTPA)^2-. The latter, with an extensive hydrogen bonding network and insignificant translational motion, results in a longer mean residence lifetime (τ_M), which makes the contribution of second-/outer-sphere significant. Therefore, when the Gd(DTPA)^2- loading percentage increases from 8.6 to 55%, the diminution of the ratio of inherent water to Gd(DTPA)^2- concomitantly diminishes the contributions by second-/outer-sphere water to r₁. Additionally, the modest increase in r₁ with decreasing particle size (∼315-540 nm) is perhaps due to the shortening of τ_M. Finally, the spin-spin relaxation times (T₂) of ¹⁷O, determined at various temperatures, show a negligible exchange of water molecules at room temperature. Therefore, the very high r₁ of nanoparticles indicate that protons of the bulk water are still accessible to the Gd³⁺ centers, possibly dominated by prototropic exchange through the hydrogen bonding network.

Nb2O5 supported on mixed oxides catalyzed oxidative and photochemical conversion of anilines to azoxybenzenes

Supported niobium oxide as an efficient heterogeneous catalyst for chemoselective preparation of azoxybenzenes from anilines.

Memory Effect of Activated Mg–Al Hydrotalcite: In Situ XRD Studies during Decomposition and Gas-Phase Reconstruction

The thermal decomposition of Mg-Al hydrotalcite and the subsequent reconstruction of the decomposed products in the presence of water vapor (2 vol. % H(2)O in N(2)) have been investigated by in situ XRD. Thermographic analysis and temperature-programmed desorption MS results complemented the diffraction data. Valuable mechanistic and kinetic insights into these processes, which are of prime importance for optimal activation of this type of material for catalytic applications, were obtained. Hydrotalcite decomposition to the mixed oxide proceeds via formation at 423-473 K of an intermediate phase, consisting of a highly disordered, dehydrated, layered structure. The latter evolves by removal of interlayer water on heating, causing a shrinking of the interlayer space (it is up to 45 % smaller than in the as-synthesized hydrotalcite). Above 623 K, Mg(Al)O(x) oxide with the periclase structure is formed. Reversion of the intermediate dehydrated structure to hydrotalcite upon contact with water vapor is complete and very fast at room temperature. Recovery of hydrotalcite from the oxide calcined at 723 K is two orders of magnitude slower than rehydration of the intermediate layered structure and one order of magnitude slower than the typically practiced liquid-phase reconstruction. In contrast to the decomposition, the reconstruction mechanism does not involve an intermediate phase. The gas-phase rehydration and reconstruction was interrupted above 303 K. This is attributed to the poor wetting of the surface of the decomposed materials induced by hampered H(2)O adsorption above room temperature at the water vapor pressure applied. The Avrami-Erofe'ev model describes the reconstruction kinetics well.

Removal of B, Cr, Mo, and Se from wastewater by incorporation into hydrocalumite and ettringite

Boron, chromium, molybdenum, and selenium often occur in high concentrations in fly ash leachates. During the leaching of fly ash in alkaline environments, hydrocalumite (Ca4Al2(OH)12(OH)2 x 6H2O) and ettringite (Ca6Al2(OH)12(SO4)3 x 26H20) form as secondary precipitates. In this study, the removal of B, Cr, Mo, and Se oxyanions from high pH waters by incorporation into hydrocalumite and ettringite was examined. Experiments were performed by precipitating these minerals in solutions containing B, Cr, Mo, and Se oxyanions at conditions relevant to lime-leaching of fly ash as well as to fly ash containing concrete. The uptake of all four anions by hydrocalumite and ettringite was high. Anion uptake by hydrocalumite was larger than that by ettringite, and hydrocalumite was able to reduce anion concentrations to below drinking water standards. Ettringite showed an anion preference in the order of B(OH)4- > SeO4(2-) > CrO4(2-) > MoO4(2-). In contrast, borate was least preferred by hydrocalumite. Coordination, size, and electronegativity are likely the factors that result in the observed differences among the oxyanions.

The thermal decomposition of Mg-Al hydrotalcites: effects of interlayer anions and characteristics of the final structure

The thermal decomposition of hydrotalcites (HTs) with different interlayer anions in the 298-523 K region has been investigated by using transmission electron microscopy (TEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and IR, 27Al MAS-NMR and X-ray absorption near-edge structure (XANES) spectroscopy. The thermal stability of the HT with interlayer oxalate was remarkably higher than that of HTs with interlayer carbonate; the onset temperatures for decomposition were 523 K and 473 K, respectively. It is proposed that the basicity of the interlayer anion is the key factor in the onset of dehydroxylation of the brucite-like layers: the lower the basicity, the more thermally stable the HT compound. After heat treatment at 723 K, small Mg(Al)O domains (approximately 5 nm) within the HT crystallites cause broadening of the XRD reflections. The electron diffraction pattern consists of spots and rings, due to nonrandomly oriented crystalline material. Formation of disordered bonds, caused by nonideal packing between the decomposing adjacent cation layers in the (111) direction, could explain the expanded d value in the resulting MgO-like phase observed with XRD and electron diffraction. The orientation of the Mg(Al)O domains in the heat-treated material may be crucial for the so-called "memory effect" of HTs.