Geometric hydration shells for anionic platinum group metal chloro complexes

Selective Pd recovery from acidic leachates by 3-mercaptopropylphosphonic acid grafted TiO2: does surface coverage correlate to performance?

Modification of metal oxides with organophosphonic acids (PAs) provides the ability to control and tailor the surface properties. The metal oxide phosphonic acid bond (M-O-P) is known to be stable under harsh conditions, making PAs a promising candidate for the recovery of metals from complex acidic leachates. The thiol functional group is an excellent regenerable scavenging group for these applications. However, the research on organophosphonic acid grafting with thiol groups is very limited. In this study, four different metal sorbent materials were designed with different thiol surface coverages. An aqueous-based grafting of 3-mercaptopropylphosphonic acid (3MPPA) on mesoporous TiO₂ was employed. Surface grafted thiol groups could be obtained in the range from 0.9 to 1.9 groups per nm². The different obtained surface properties were studied and correlated to the Pd adsorption performance. High Pd/S adsorption efficiencies were achieved, indicating the presence of readily available sorption sites. A large difference in their selectivity towards Pd removal from a spend automotive catalyst leachate was observed due to the co-adsorption of Fe on the titania support. The highest surface coverage showed the highest selectivity (K _d: 530 mL g^-1) and adsorption capacity (Q _max: 0.32 mmol g^-1) towards Pd, while strongly reducing the co-adsorption of Fe on remaining TiO₂ sites.

Origins of Clustering of Metalate-Extractant Complexes in Liquid-Liquid Extraction

Effective and energy-efficient separation of precious and rare metals is very important for a variety of advanced technologies. Liquid-liquid extraction (LLE) is a relatively less energy intensive separation technique, widely used in separation of lanthanides, actinides, and platinum group metals (PGMs). In LLE, the distribution of an ion between an aqueous phase and an organic phase is determined by enthalpic (coordination interactions) and entropic (fluid reorganization) contributions. The molecular scale details of these contributions are not well understood. Preferential extraction of an ion from the aqueous phase is usually correlated with the resulting fluid organization in the organic phase, as the longer-range organization increases with metal loading. However, it is difficult to determine the extent to which organic phase fluid organization causes, or is caused by, metal loading. In this study, we demonstrate that two systems with the same metal loading may impart very different organic phase organizations and investigate the underlying molecular scale mechanism. Small-angle X-ray scattering shows that the structure of a quaternary ammonium extractant solution in toluene is affected differently by the extraction of two metalates (octahedral PtCl₆^2- and square-planar PdCl₄^2-), although both are completely transferred into the organic phase. The aggregates formed by the metalate-extractant complexes (approximated as reverse micelles) exhibit a more long-range order (clustering) with PtCl₆^2- compared to that with PdCl₄^2-. Vibrational sum frequency generation spectroscopy and complementary atomistic molecular dynamics simulations on model Langmuir monolayers indicate that the two metalates affect the interfacial hydration structures differently. Furthermore, the interfacial hydration is correlated with water extraction into the organic phase. These results support a strong relationship between the organic phase organizational structure and the different local hydration present within the aggregates of metalate-extractant complexes, which is independent of metalate concentration.

Selective and Preferential Separation of Rhodium (III) from Palladium (II) and Platinum (IV) Using a m-Phenylene Diamine-Containing Precipitant

Although Rh is an industrially important and the most expensive platinum group metal (PGM), the selective and preferential separation of Rh from PGM mixtures still remains as a big challenge. In this work, the separation of Rh (III) from Pd (II) and Pt (IV) in a hydrochloric acid (HCl) solution was studied using a m-phenylene diamine-containing precipitant (m-PDA). At high HCl concentrations (6.0–8.0 M), most of the Rh (III) (>90%) was precipitated, and Pd (II) and Pt (IV) were hardly precipitated (<5%). On the other hand, over 85% of Pd (II) and Pt (IV) precipitated along with small amount of Rh (III) (<25%) at low HCl concentrations (1.0–2.0 M). As a consequence, m-PDA enabled selective and preferential precipitation of Rh (III) at high HCl concentrations. XPS and TG analyses revealed that the Rh-containing precipitate is an ion-pair complex composed of one [RhCl₆]³⁻ anion and three m-PDA cations. The Rh desorption from the precipitate as well as the recovery of m-PDA was successfully achieved using an NH₄OH solution. This method is a promising practical approach to Rh recovery.

Preferential Precipitation and Selective Separation of Rh(III) from Pd(II) and Pt(IV) Using 4-Alkylanilines as Precipitants

Rhodium (Rh) is the most expensive platinum group metal (PGM) and is of great industrial importance. Although the recycling of PGMs from secondary sources is in high demand, the preferential and selective separation of Rh from PGM mixtures remains a great challenge. Here, a selective Rh separation method involving the precipitation of Rh from an HCl solution containing palladium (Pd), platinum (Pt), and Rh is reported. 4-Butylaniline and 4-hexylaniline were used as precipitants for Rh, and selective Rh precipitation was achieved at high HCl concentrations. We revealed that Rh in HCl selectively forms a unique and highly stable ion-pair complex comprising [RhCl₆]^3-/anilinium/chloride ions in a 1:6:3 ratio. The formation and high stability of the Rh complex were found to play important roles in the selective recovery of Rh from PGM mixtures.

195Pt NMR and Molecular Dynamics Simulation Study of the Solvation of [PtCl6]2- in Water-Methanol and Water-Dimethoxyethane Binary Mixtures

The experimental ¹⁹⁵Pt NMR chemical shift, δ(¹⁹⁵Pt), of the [PtCl₆]^2- anion dissolved in binary mixtures of water and a fully miscible organic solvent is extremely sensitive to the composition of the mixture at room temperature. Significantly nonlinear δ(¹⁹⁵Pt) trends as a function of solvent composition are observed in mixtures of water-methanol, or ethylene glycol, 2-methoxyethanol, and 1,2-dimethoxyethane (DME). The extent of the deviation from linearity of the δ(¹⁹⁵Pt) trend depends strongly on the nature of the organic component in these solutions, which broadly suggests preferential solvation of the [PtCl₆]^2- anion by the organic molecule. This simplistic interpretation is based on an accepted view pertaining to monovalent cations in similar binary solvent mixtures. To elucidate these phenomena in detail, classical molecular dynamics computer simulations were performed for [PtCl₆]^2- in water-methanol and water-DME mixtures using the anionic charge scaling approach to account for the effect of electronic dielectric screening. Our simulations suggest that the simplistic model of preferential solvation of [PtCl₆]^2- by the organic component as inferred from nonlinear δ(¹⁹⁵Pt) trends is not entirely accurate, particularly for water-DME mixtures. The δ(¹⁹⁵Pt) trend in these mixtures levels off for high DME mole fractions, which results from apparent preferential location of [PtCl₆]^2- anions at the borders of water-rich regions or clusters within these inherently micro-heterogeneous mixtures. By contrast in water-methanol mixtures, apparently less pronounced mixed solvent micro-heterogeneity is found, suggesting the experimental δ(¹⁹⁵Pt) trend is consistent with a more moderate preferential solvation of [PtCl₆]^2- anions. This finding underlines the important role of solvent-solvent interactions and micro-heterogeneity in determining the solvation environment of [PtCl₆]^2- anions in binary solvent mixtures, probed by highly sensitive ¹⁹⁵Pt NMR. The notion that preferential solvation of [PtCl₆]^2- results primarily from competing ion-solvent interactions as generally assumed for monatomic ions, may not be appropriate in general.

Intrinsic 37/35Cl and 18/16O isotope shifts in 195Pt and 103Rh NMR of purely inorganic Pt and Rh complexes as unique spectroscopic fingerprints for unambiguous assignment of structure

Well-resolved intrinsic ¹ΔM(^37/35Cl) and ¹ΔM(^18/16O) isotope shifts (where M = ¹⁹⁵Pt or ¹⁰³Rh) are visible in the ¹⁹⁵Pt NMR peak profiles of relatively kinetically inert [PtCl_n(H₂O)_6-n]^4-n (n = 1-6) complexes, their corresponding hydroxido [PtCl_6-n(OH)_n]^2- (n = 1-5/6) anions, and [RhCl_n(H₂O)_6-n]^3-n (n = 3-6) complexes in aqueous solutions at ca. 293 K. Although some such isotope effects have been previously reported, there are very limited published data in the open literature, and the first systematic studies of such intrinsic ¹ΔM(^37/35Cl) and ¹ΔM(^18/16O) isotope effects are reviewed in this perspective. In high magnetic-field NMR spectrometers, the ¹⁹⁵Pt and ¹⁰³Rh NMR peak profiles acquired within a relatively narrow temperature range (288-300 K) constitute unique 'spectroscopic fingerprints', which allow unambiguous structural assignment in solution. Available data for Pt(iv) and Rh(iii) complexes give rise to intrinsic isotope ¹Δδ¹⁹⁵Pt/¹⁰³Rh(^37/35Cl) profiles, which are extraordinarily sensitive to the structure of a particular complex or its geometric isomer. The profiles of aquated Pt(iv) and Rh(iii) complexes in acidic solutions may be resolved at either an isotopologue level only or at both an isotopologue and an isotopomer level depending on the structure. By contrast, in the series of [PtCl_6-n(OH)_n]^2- (n = 1-6) anions, ¹Δδ¹⁹⁵Pt(^37/35Cl) isotope shifts are resolved only at an isotopologue level. Relatively larger ¹Δ¹⁹⁵Pt(^18/16O) isotope shifts obtained by the partial ¹⁸O enrichment of both the [PtCl_n(H₂O)_6-n]^4-n (n = 1-6) and [PtCl_6-n(OH)_n]^2- (n = 1-6) series give rise to remarkable ¹⁹⁵Pt NMR peak profiles showing both ^37/35Cl and ^18/16O shifts. In the [PtCl_6-n(OH)_n]^2- (n = 1-5/6) anions a typical NMR peak profile spanning ∼2 ppm only may be resolved at both the isotopologue and isotopomer levels, depending on whether ^18/16OH^- ions are coordinated trans to chloride ions or not. The potential utility of such ¹Δ¹⁹⁵Pt(^37/35Cl) and ¹Δ¹⁹⁵Pt(^18/16O) isotope shifts in selected practical applications involving such complexes is briefly illustrated.

Molecular Scale Description of Anion Competition on Amine-Functionalized Surfaces

Many industrial and biological processes involve the competitive adsorption of ions with different valencies and sizes at charged surfaces; heavy and precious metal ions are separated on the basis of their propensity to adsorb onto interfaces, often as anionic ion clusters (e.g., [MCl_x]^n-). However, very little is known, both theoretically and experimentally, about the competition of factors that drive preferential adsorption, such as charge density or valence, at interfaces in technologically relevant systems. There are even contradictory pictures described by interfacial studies and real life applications, such as chlorometalate extractions, in which charge diffuse chlorometalate ions are extracted efficiently even though charge dense chloride ions present in the background are expected to occupy the interface. We studied the competition between divalent chlorometalate anions (PtCl₆^2- and PdCl₄^2-) and monovalent chloride anions on positively charged amine-functionalized surfaces using in situ specular X-ray reflectivity. Chloride anions were present in vast excess to simulate the conditions used in the commercial separation of heavy and precious metal ions. Our results suggest that divalent chlorometalate adsorption is a two-step process and that the divalent anions preferentially adsorb at the interface despite having a charge/volume ratio lower than that of chloride. These results provide fundamental insight into the structural mechanisms that underpin transport in phases that are relevant to heavy and precious metal ion separations, explaining the high efficiency of low charge density ion transport processes in the presence of charge dense anions.

Molecular structures and solvation of free monomeric and dimeric ferriheme in aqueous solution: insights from molecular dynamics simulations and extended X-ray absorption fine structure spectroscopy

CHARMM force field parameters have been developed to model nonprotein bound five-coordinate ferriheme (ferriprotoporphyrin IX) species in aqueous solution. Structures and solvation were determined from molecular dynamics (MD) simulations at 298 K of monomeric [HO-ferriheme](2-), [H2O-ferriheme](-), and [H2O-ferriheme](0); π-π dimeric [(HO-ferriheme)2](4-), [(H2O-ferriheme)(HO-ferriheme)](3-), [(H2O-ferriheme)2](2-), and [(H2O-ferriheme)2](0); and μ-oxo dimeric [μ-(ferriheme)2O](4-). Solvation of monomeric species predominated around the axial ligand, meso-hydrogen atoms of the porphyrin ring (Hmeso), and the unligated face. Existence of π-π ferriheme dimers in aqueous solution was supported by MD calculations where such dimers remained associated over the course of the simulation. Porphyrin rings were essentially coplanar. In these dimers major and minor solvation was observed around the axial ligand and Hmeso positions, respectively. In μ-oxo ferriheme, strong solvation of the unligated face and bridging oxide ligand was observed. The solution structure of the μ-oxo dimer was investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy. The EXAFS spectrum obtained from frozen solution was markedly different from that recorded on dried μ-oxo ferriheme solid. Inclusion of five solvent molecules obtained from spatial distribution functions in the structure generated from MD simulation was required to produce acceptable fits to the EXAFS spectra of the dimer in solution, while the solid was suitably fitted using the crystal structure of μ-oxo ferriheme dimethyl ester which included no solvent molecules.

The genesis of a heterogeneous catalyst: in situ observation of a transition metal complex adsorbing onto an oxide surface in solution

Transition metal complex adsorption onto an oxide surface was observed in situ using a quartz crystal microbalance at the solid–liquid interface.

195Pt NMR Study of the Speciation and Preferential Extraction of Pt(IV)−Mixed Halide Complexes by Diethylenetriamine-Modified Silica-Based Anion Exchangers

A detailed 195Pt NMR study of the distribution of Pt(IV) complex species resulting from the aquation of H2PtCl6, H2PtBr6, and mixtures of H2PtCl6/H2PtBr6 in water/dilute HClO4 has been carried out to obtain an understanding of the speciation in these solutions as relevant to the recovery of Pt(IV) complexes from process solutions. A species distribution plot of the [PtCl6]2-, [PtCl5(H2O)]-, and [PtCl4(H2O)2] shows that in equilibrated, relatively concentrated H2PtCl6 solutions ([Pt]t > 0.12 M), the [PtCl4(H2O)2] species is below the 195Pt NMR detection limit; for [Pt]t concentrations < 0.1 M, the relative concentrations of the [PtCl5(H2O)]- and [PtCl4(H2O)2] species increase significantly, as a result of relatively rapid aquation of the [PtCl6]2- and [PtCl5(H2O)]- complexes under these conditions. From this (195)Pt NMR data the aquation constants of [PtCl6]2- and [PtBr6]2- of log K6 approximately 1.75 +/- 0.05 and log K6 approximately 2.71 +/- 0.15, respectively, have been determined at 30 degrees C. In mixtures of H2PtCl6/H2PtBr6 in water, a number of previously unidentified aquated complexes of the general formula [PtCl(5-n)Br(n)(H2O)]- (n = 0-5) could be identified, including the possible geometrical isomers of these complexes. These 195Pt NMR assignments were confirmed by remarkably systematic, linear relationships between the 195Pt chemical shift increments induced by substitution of Cl- ions by n Br- ions in [PtCl(6-n)Br(n)]2- and [PtCl(5-n)Br(n)(H2O)]- complexes. Preferential extraction of the [PtX6]2- (X = Cl, Br, or a mixture of the two halides) species over their corresponding aquated [PtX5(H2O)]- counterparts by silica-based diethylenetriamine anion exchangers could be demonstrated by means of 195Pt NMR spectroscopy.

Monte Carlo Simulation of Cisplatin Molecule in Aqueous Solution

The Lennard-Jones (12-6) parameters were obtained for all atoms of cisplatin molecule using the ab initio quantum mechanical potential energy surface for the water-cisplatin interaction as reference data. The parameters found were (epsilon/kcal.mol(-1) and sigma/angstroms) 1.0550, 3.6590 (Pt); 0.0381, 4.6272 (Cl); 0.0455, 3.3783 (N); and 0.0185, 0.0936 (H) and provided very good results for the description of the aqueous solution of cisplatin through Monte Carlo simulation. From statistical analysis of solute-solvent interactions, we observed that the NH3 groups are involved in 53% of the calculated hydrogen bonds with a significant contribution from chlorides (41%) and only 6% involving the Pt center. This is in agreement with the expected behavior for such molecules. Two hydration shells with 22 and 81 water molecules, respectively, centered around 4.6 and 7.3 angstroms were found from the center of mass pair correlation function analysis. The cisplatin atomic Lennard-Jones parameters are reported for the first time, and they might be useful for studying the structure, properties, and processes of cisplatin-like molecules in aqueous solution, including explicitly the solvent effect.

195Pt NMR and DFT computational methods as tools towards the understanding of speciation and hydration/solvation of [PtX6]2−(X = Cl−, Br−) anions in solution

195Pt NMR together with DFT calculations and MD simulations, offer a powerful toolkit with which to probe the hydration shells of the [PtCl6]2- anions, which may lead to a more profound understanding of the solute-solvent interactions of such complexes.

Contact Ion Pair between Na+ and PtCl62- Favored in Methanol

Ion-pair formation between a Na+ cation and the [PtCl62-] anion in methanol is observed from195Pt NMR chemical shift trends as well as from molecular dynamics computer simulations. Free energy of association calculations reveal that contact ion pairs (CIPs) are the most favored configuration in methanol, followed by solvent shared ion pairs (SSHIPs). By contrast, such ion-pair formation is not observed for comparable solutions in water.