Tetrahedral Coordination State Research Articles

Mössbauer spectroscopy and gamma-ray shielding performance for iron phosphate glasses containing various heavy metals oxides

Mössbauer spectroscopy analysis and gamma (γ)-ray shielding performance for some iron phosphate glasses with composition 50 %P2O5 + 20 %Na2O + 20 %Fe2O3 + 10 %X, whereX = As2O3, SrO, BaO, CdO, and Sb2O3 w.t.% were investigated. The melt quenching technique was used for glass sample preparation, where the XRD patterns investigated the amorphous nature of the prepared glasses. Moreover, SEM and EDS mapping were used to provide information on the element distribution of the added heavy metals. Mössbauer spectroscopy showed that, most of the iron is present in the glass as Fe3+oxidation state, which is composed of tetrahedral and octahedral coordination states. The remaining iron ions have the form Fe2+ which occupied the tetrahedral coordination state. The γ-ray attenuation parameters like, mass attenuation coefficient (MAC), half value layer (HVL) and the effective atomic number (Zeff) were experimentally measured at different γ-ray energies using 133Ba, 137Cs, and 60Co radioactive sources. Results indicated that the sample containing Sb2O3 is the best-modified glass for gamma radiation protection.

Effect of Zr Incorporation on Mordenite Catalyzed Dimethyl Ether Carbonylation

Zr-mordenite catalysts were prepared by post-treatment method and H-mordenite catalyst was synthesized by hydrothermal method for the dimethyl ether carbonylation reaction. XRD, N2 adsorption/desorption, ICP-OES, SEM, TEM, UV–Vis, XPS, Raman, CO-TPD, NH3-TPD, 27Al MAS NMR, FTIR and Py-IR were employed to characterize the catalysts. The results demonstrated that part of Zr atoms existed in a tetrahedral coordination state after incorporating into the mordenite framework. Zirconium Lewis acid sites with the ability to absorb CO molecules were formed during this process although the amount of Bronsted acid sites in 8-member ring channels decreased upon the incorporation of zirconium. During the concentration of zirconium nitrate solution increased from 0.02 to 0.08 mol/L, the Zr-mordenite catalyst through post-treatment with 0.04 mol/L zirconium nitrate solution exhibited the best catalytic performance at 473 K and 1 MPa. Dimethyl ether conversion and methyl acetate selectivity increased to 78% and 96% respectively compared with H-mordenite catalyst. Schematic of Zr incorporating MOR framework

Bi1.9Mn0.1Te3 nanocrystal functionalized diamagnetic glasses: optical, luminescence, magnetic and Faraday rotation

Mn doped Bi2Te3 nanocrystal, both as topological insulator and diluted magnetic semiconductor, has gained significant interest due to its interesting magnetic performance roused by the sp-d exchange interaction and unique time-reversal symmetry protected surface states. In this paper, for the first time, Bi1.9Mn0.1Te3 nanocrystal doped diamagnetic TeO2–PbO–B2O3 glass was studied. Pure 50nm hexagonal Bi1.9Mn0.1Te3 nanocrystals were synthesized with a specific surface area of 64.28 m2/g using the optimized PEG as surfactant. Due to the complex spin-orbit interactions between sp band of Bi2Te3 and d band of Mn, the prepared Bi1.9Mn0.1Te3 displayed characteristic luminescence, A11g, E2g, A21g Raman modes, good photocatalytic (70.2%) and ferromagnetic/paramagnetic co-existence behavior.Bi1.9Mn0.1Te3 doped diamagnetic glasses were prepared by melting-quenching technique and the influence of different doping amount to glass structure and properties was evaluated through X-ray diffraction, Fourier transforms infrared, Raman, Ultraviolet-visible, photoluminescence spectra and Verdet constant measurement. The glasses structure and properties were doping amount dependent. X-ray diffraction, Fourier transforms infrared and Raman spectra revealed the evolution of structure units of TeO4, TeO3, BO3 etc under the modification of Bi1.9Mn0.1Te3. SEM revealed the homogeneity and growth of nanocrystals in matrix. X-ray photoelectron spectroscopy displayed the chemical states changes of Mn2p, Bi4f, Te3d and O1s under different doping amounts. Both the optical absorption band at 540nm and cutoff wavelength increased with the dopant amount. The luminescence in visible range revealed the tetrahedral coordination state of paramagnetic Mn ions which also influenced the magnetization and Faraday rotation accordingly. Experimental results showed that glass with 2%Bi1.9Mn0.1Te3 exhibited good optical, luminescence, magnetic, significant high Verdet constant (0.2522min/G.cm) and good thermal stability as well. Such good quality diamagnetic glass is attractive for magneto-optical devices applications in isolator and current transducer etc.

Facile photoinduced pH-adjusting synthesis of high amount framework Ti-containing meso-silica catalyst for cyclohexane oxidation

Here, we report a facile photoinduced pH-adjusting method to prepare highly-active Ti-containing mesoporous silica catalysts (PIMTS) for cyclohexane oxidation, featuring high amounts of framework titanium species (Ti ca. 2.92 mol%) with uniform tetrahedral coordination state. Ordered meso-structure and framework substitutions of titanium are evidenced by a set of mutually complementary characterizations, catalytic tests and item-by-item comparisons with the hydrothermal crystallized counterparts (Ti-SBA-15). PIMTS acts as an efficient catalyst for cyclohexane oxidation, leading to the maximum KA oil TOF up to 393 min−1 when cyclohexane conversion is 49.9%.

The synthesis of hierarchical Sn-Beta zeolite via aerosol-assisted hydrothermal method combined with a mild base treatment

Abstract The Sn-Beta zeolite is an efficient Lewis acid catalyst for a wide range of chemical reactions. Here a seed-free aerosol-assisted hydrothermal strategy is used to synthesize plate-like Sn-Beta zeolite with higher tin content. A majority of Sn species were transformed into the framework of Beta zeolite as the desired tetrahedral coordination state, which creates strong Lewis acid sites. Moreover, the diffusion property of the Sn-Beta zeolite can be further improved by a mild base-surfactant (NH3˙H2O-CTAB) treatment. The addition of CTAB can not only narrow the mesopore distribution but also prevent the excessive loss of Sn species. The obtained hierarchical Sn-Beta zeolite exhibits an enhanced catalytic performance in the Baeyer–Villiger oxidation of bulky ketone.

Highly Active Layered Titanosilicate Catalyst with High Surface Density of Isolated Titanium on the Accessible Interlayer Surface

AbstractMetallosilicate catalysts with tetrahedrally coordinated transition metals are attractive because of their high selectivity in the oxidation of hydrocarbons. Here, we report highly active titanosilicate catalysts featuring an isolated titanium species with a uniform tetrahedral coordination state, unusually high surface density, and high accessibility of the Ti active sites. The layered titanosilicate prepared by grafting of TiIV acetylacetonate onto the surfaces of the crystalline layered silicate, HUS‐7, showed excellent catalytic performance in the epoxidation of cyclohexene with tert‐butyl hydroperoxide as an oxidant (turnover frequency of 230 h−1 and reaction rate of 314 mmol g−1 h−1).

Ultrafast synthesis of TS-1 without extraframework titanium species in a continuous flow system

When preparing porous crystalline materials, low energy utilization and long crystallization times (∼48 h) as well as the instability of the products makes it difficult for traditional hydrothermal technology using batch reactors to meet the ever increasing environmental requirements of the chemical industry. However, rapid crystallization can be realized when using a continuous flow system such as a tubular reactor due to large heat transfer coefficient. Herein, we demonstrate the rapid continuous hydrothermal synthesis of nanoparticles of TS-1 with a tubular reactor. The particles have MFI structures and a very narrow size distribution of 100–150 nm after an 8 min synthetic step in tubes with an inner diameter of 2 mm and an external diameter of 3 mm. The results show that a faster nucleation rate improved the proportion of titanium entering into the silicon skeleton, with almost all Ti species existing in the tetrahedral coordination state at a grain size of 42 nm. This simple and intrinsically safe route allows good quality control and high productivity with very little equipment, and allows for easy scale-up. This first case of fast synthesis of TS-1 may provide some valuable references for the development of green synthesis of much more crystalline materials.

Crystallization of Ti-Rich *BEA Zeolites by the Combined Strategy of Using Ti–Si Mixed Oxide Composites and Intentional Aluminum Addition/Post-Synthesis Dealumination

Titanosilicate zeolites are well-known catalysts for selective oxidation using hydrogen peroxide, an environmentally friendly oxidant. To effectively synthesize these materials with high Ti contents, we have focused on using a Ti–Si mixed oxide composite as the ingredient along with intentional addition of an aluminum source to promote crystallization. Ti-beta, a *BEA-type zeolite containing titanium at the framework sites, was chosen as a model zeolite. First, (Ti, Al)-beta, a *BEA-type zeolite containing both Ti and Al, was prepared; the occluded aluminum inside the product was subsequently removed by an acid treatment. This treatment not only lead to the reduction of the aluminum content to trace levels but also improved the states of the titanium species to the desired tetrahedral coordination state. Thus, Ti-beta zeolites with little extra-framework Ti were successfully obtained with molar compositions up to Ti/(Ti + Si) = 4.0 mol %. As a titanosilicate zeolite catalyst, high functionality was demons...

Association between Field Liming and the Epidemic of Type 1 Diabetes

Zinc is an essential nutrient for mammals. Zinc has only one oxidation state Zn2+ but it has many coordination states, which can alter without demanding energy. Coordination states depend on prevailing pH. In soils the coordination state changes from octahedral (nonphysiologic) form to tetrahedral (physiologic) form when pH rises to about 6.5. Weathering processes of common soil mineral mica are also pH dependent. Large scale argricultural field liming began from 1950's onward in the Western Countries and since that time the incidence of T1D began to increase in the Western Countries. Liming elevates soil pH often near 6.5 and favours mosaic mica-vermiculite nanoparticle formation in which vermiculite corner binds zinc in tetrahedral (physiological) coordination state. In this pH mica corner remains in native form and offers the plane for soluble pMHC molecule (binded with antigenic beta-cell specific self peptide) to adhere on mica and as a consequence to trigger the activation of autoreactive CD4 and/or CD8 T cells. Beta-cell specific autoantigens are released because abundant zinc derived from endocytosed zinc-mica-vermiculite particles in lysosomes leads to incresed beta-cell apoptosis, also the physiological neonatal beta-cell mass remodeling enhance beta-cell specific auto-antigen release. Furthermore enterovirusinfections during first years of life are common and can release beta-cell specific auto-antigens. The probable disease mechanism is dealt with this review article.

Synergetic photo-epoxidation of propylene over V[sbnd]Ti/MCM-41 mesoporous photocatalysts

The photo-epoxidation of propylene has gained preference among many alternative processes because the current processes for propylene oxide (PO) production are neither economical nor environmentally sustainable. We synthesized various single and binary metal oxides (Ti and V) constructed within the MCM-41 framework and demonstrated their favorable efficiency for the photo-epoxidation of propylene. The results indicated that highly dispersed tetrahedral coordination states played a role in photo-epoxidation. The maximum PO formation rate was 634±17 (μmolg−1h−1) on V0.2Ti0.3/MCM-41 under UV-light irradiation of 0.3mWcm−2 at 323K. The Delplot method revealed two pathways to produce PO. One is indirectly from propionaldehyde via either V or Ti oxides, and the other is directly from a C3H6 reactant via Ti oxides. This paper clearly illustrates how V and Ti oxides affect the photo-epoxidation pathways. Furthermore, a possible photo-epoxidation mechanism via the metal oxide active site is proposed to elucidate the synergetic photoactivity.

Structural and Optical Correlation of Gamma-Irradiated 3d Transition Metals-Doped Lithium Disilicate Glasses

Undoped lithium disilicate glasses together with glasses of the same nominal composition containing 0.2 % of one of the 3d transition metal oxides (TMO) were prepared. Combined optical and FT infrared absorption spectral analysis were carried out for the prepared glasses before and after gamma irradiation with a dose of 8 Mrad. The optical spectrum of undoped glass reveals strong UV absorption which is related to the presence of unavoidable trace iron impurities (mainly Fe3+ ions). UV/Visible absorption spectra of 3d TM ions show additional characteristic absorption bands due to each respective TM ion. The TM ions mostly exhibit the higher valance or tetrahedral coordination states. Gamma irradiation of the undoped glass produces some variations in the optical spectra. The UV spectra reveal a slight increase in intensity while the visible spectra exhibit an induced defect at about 420 nm due to a silicon oxygen hole center or a nonbridging oxygen hole center. Some TM ions are assumed to share or compete for generated electrons and positive holes during the irradiation process. Infrared absorption spectra of the glasses reveal repetitive characteristic vibrational bands due to bending, symmetric stretching and antisymmetric stretching of the Si-O-Si or O-Si-O modes in the silicate network. The introduction of TM ions within the doping level (0.2 %) causes no effect on the IR spectra. The minor effects of irradiation on the IR spectra are related to some changes in the bond angles and/or bond lengths of some structural building units.

Identification of the hydrate gel phases present in phosphate-modified calcium aluminate binders

The conversion of hexagonal calcium aluminate hydrates to cubic phases in hydrated calcium aluminate cements (CAC) can involve undesirable porosity changes and loss of strength. Modification of CAC by phosphate addition avoids conversion, by altering the nature of the reaction products, yielding a stable amorphous gel instead of the usual crystalline hydrate products. Here, details of the environments of aluminium and phosphorus in this gel were elucidated using solid-state NMR and complementary techniques. Aluminium is identified in both octahedral and tetrahedral coordination states, and phosphorus is present in hydrous environments with varying, but mostly low, degrees of crosslinking. A 31P/27Al rotational echo adiabatic passage double resonance (REAPDOR) experiment showed the existence of aluminium–phosphorus interactions, confirming the formation of a hydrated calcium aluminophosphate gel as a key component of the binding phase. This resolves previous disagreements in the literature regarding the nature of the disordered products forming in this system.

Effect of 3d-transition metal doping on the shielding behavior of barium borate glasses: A spectroscopic study

UV–visible and FT infrared spectra were measured for prepared samples before and after gamma irradiation. Base undoped barium borate glass of the basic composition (BaO 40%–B2O3 60mol.%) reveals strong charge transfer UV absorption bands which are related to unavoidable trace iron impurities (Fe3+) within the chemical raw materials. 3d transition metal (TM)-doped glasses exhibit extra characteristic absorption bands due to each TM in its specific valence or coordinate state. The optical spectra show that TM ions favor generally the presence in the high valence or tetrahedral coordination state in barium borate host glass.Infrared absorption bands of all prepared glasses reveal the appearance of both triangular BO3 units and tetrahedral BO4 units within their characteristic vibrational modes and the TM-ions cause minor effects because of the low doping level introduced (0.2%).Gamma irradiation of the undoped barium borate glass increases the intensity of the UV absorption together with the generation of an induced broad visible band at about 580nm. These changes are correlated with suggested photochemical reactions of trace iron impurities together with the generation of positive hole center (BHC or OHC) within the visible region through generated electrons and positive holes during the irradiation process.

Structural and electric-dielectric properties of some bismuth -phosphate glasses

In the present study, glasses with composition XBi 2O 3 (60− X)P 2O 5 20Fe 3O 4 20Li 2O (0≤ X≤15) mol% have been prepared by the conventional melt quenching technique. The IR studies show the presence of FeO 4, FeO 6, BiO 6, PO 3, PO and PO 4 structural groups. The hyperfine structure of these glasses is investigated using Mössbauer spectroscopy. ME spectroscopy indicated the presence of two different oxidation states of iron (Fe 2+ and Fe 3+). The ferrous ions Fe 2+ occupied tetrahedral coordination states, while the ferric ions Fe 3+ occupied both tetrahedral and octahedral coordination states. The effect of partial replacement of P 2O 5 by Bi 2O 3 on the electric-dielectric properties is studied in more details. It is found that, unmonotonic variation in the σ dc, ( θ D /2), σ ac( ω), ε 1( ω), and ε 2( ω), as a function of Bi 2O 3 contents. Also it is found that, the power law exponent, s, is temperature dependent and the CBH model is the most applicable conduction mechanism in all glass samples. Pseudo Cole-Cole diagram of the investigated glassy samples exhibit similar behavior where all plots show a single semicircle indicating a single relaxation process.

Coordination and valence state of transition metal ions in alkali-borate glasses

Borate glasses of the 20R 2O·80B 2O 3 type, where R = Li, Na and K, were colored by doping with transition metal ions (Co, Ni, Cr and Mn). The glasses were obtained by melting at the temperature of 1150 °C. For these glasses optical absorption in UV–VIS–NIR range were recorded. Analysis of the spectra allows to be determined the coordination and oxidation states of the doping transition metal ions. Changes of their coordination or oxidation are presented as a function of the optical basicity Λ after Duffy. Cobalt and nickel are present in examined borate glasses as divalent ions (Co 2+, Ni 2+) in octahedral coordination mainly, but the tetrahedral coordination state of cobalt is also possible. Chromium and manganese are present in the borate glasses in various oxidation state, though Cr 3+ and Mn 3+ ions in the octahedral coordination are probably dominant. A decrease of the electronegativity of the modifiers (Li → Na → K) and an increase of the glass matrix basicity cause a shift of the oxidation/reduction equilibrium towards higher valences of the transition metals (Cr 6+, Mn 3+).

Identification of Modular Chiral Bisphosphines Effective for Cu(I)-Catalyzed Asymmetric Allylation and Propargylation of Ketones

New modular chiral phosphines effective for two distinct Cu(I)-catalyzed asymmetric tetrasubstituted-carbon-forming reactions, namely, allylation and propargylation of ketones, were identified. The optimized phosphine 8 was readily synthesized on a gram scale in high yield via three facile transformations (O-alkylation, bisaminal formation, and phosphination) from commercially available materials. In both reactions, excellent enantioselectivity (up to 98% ee) was produced from a range of substrates, including aromatic and aliphatic ketones, using 0.1-5 mol % catalyst loading. Specifically, catalytic enantioselective propargylation was the first example, affording synthetically useful chiral building blocks that have not been easily accessed to date. In addition to the enantioselectivity, the high catalytic activity of the CuOAc-8 complex is noteworthy. Preliminary studies to elucidate the structure-catalyst activity relationship suggested that the high catalytic activity of the Cu-8 complex is due to the extraordinarily wide bite angle ( angleP-Cu-P = 137.8 degrees ), leading to the stabilization of the active monomeric catalytically active species. Furthermore, mechanistically intriguing nonconventional hydrogen bonds existed between the acetate ligand of Cu and the bisaminal hydrogen atoms, stabilizing the distorted tetrahedral coordination state of the Cu atom.

Organotin compounds immobilized on mesoporous silicas as heterogeneous catalysts for direct synthesis of dimethyl carbonate from methanol and carbon dioxide

Organotin compounds of (MeO) 2ClSi(CH 2) 3SnCl 3 have been grafted on a series of mesosporous silicas, as proved by XRD, N 2 adsorption/desorption experiments and FTIR, diffuse reflectance UV–vis and 13C CP/MAS NMR spectroscopies. A further substitution of Cl − bonded to Sn 4+ by CH 3O − led to the formation of a series of active heterogeneous catalysts for the direct synthesis of dimethyl carbonate from methanol and CO 2. The catalytic properties of these catalysts depended on the structure, crystal size and surface properties of mesoporous silicas. SBA-15 as a host was superior to SBA-16 and large-pore Ia3d. In addition, the methods used for removing surfactants occluded in mesoporous silicas had strong influences on the catalytic performance as a result of significantly affecting the surface areas and the concentrations of surface hydroxyl groups of the mesoporous silicas. The extraction of surfactants by an ethanol–HCl solution should be available. An increase in the reaction time, reaction temperature, methanol amount and CO 2 pressure significantly increased the dimethyl carbonate yield before reaching thermodynamic equilibrium. The dimethyl carbonate yield could also be increased by adding dehydration agents to the reaction system. 2,2-Dimethoxypropane gave a much higher dimethyl carbonate yield than 3A molecular sieve and tetramethyl orthorsilicate. The prepared organotin/mesoporous silica catalysts were highly stable, as corroborated by the findings that the dimethyl carbonate yield obtained over the SBA-15-immoilized organotin compound did not decrease within six repeated runs with regeneration under the same reaction conditions and that the tetrahedral coordination state of Sn species was maintained after reaction.

Structure, acidity and redox properties of MCM-22 ferrisilicate

Aluminum-free MCM-22 type ferrisilicate, [Fe]MCM-22 (Si/Fe = 19.2) and, for comparison, MCM-22 aluminosilicate, [Al]MCM-22 (Si/Al = 13.1), were synthesized. The materials and their precursors were characterized by X-ray diffractometry (XRD), thermogravimetry (TG), transmission and diffuse reflectance Fourier-transform infrared spectroscopy (FT-IR and DRIFTS), temperature-programmed ammonia evolution (TPAE), in situ Mössbauer spectroscopy and temperature-programmed reduction with hydrogen (H 2-TPR). The presence of Fe 3+ T atoms (Fe FW) were indicated by the appearance of a ν OH IR-band at 3634 cm −1. The red shift of the ν OH band, induced by the H-bonding of bridged hydroxyls and N 2, was smaller for the [Fe]MCM-22 (64 cm −1) than for the [Al]MCM-22 (86 cm −1). The smaller shift evidences sites of weaker intrinsic acid strength. The rate and temperature of the TG-release of hexamethyleneimine template showed relation with the acidity of the sample. The amount of Fe 3+ species, present in highly distorted tetrahedral coordination state, was in good correspondence with the concentration of the Brønsted acid sites. After reduction with H 2 or CO, Fe 2+ appeared in a novel coordination state. Results suggest that these Fe 2+ species are bound to the framework via oxygen atoms. Reducibility of the framework iron is related to the high concentration of hydroxyl nests generated in both the wide (12-MR) and the narrow (10-MR) channels of [Fe]MCM-22 during the synthesis and/or template removal.

Hydrogarnet: A Host Phase for Cr(VI) in Chromite Ore Processing Residue (COPR) and Other High pH Wastes

For understanding both the environmental behavior and developing remediation treatments for chromium ore processing residue (COPR) it is important to identify all the potentially soluble sources of Cr(VI). Hydrogarnet has been identified as a major phase in COPR and it has been previously speculated that it has a capacity to host Cr(VI). Here we provide direct evidence of this capacity by demonstrating the incorporation of Cr(VI) into laboratory synthesized hydrogarnet. Electron microscopy and energy dispersive X-ray microanalysis show incorporation of approximately 17000-22000 mg Cr(VI) kg(-1) hydrogarnet. X-ray powder diffraction data show that peak intensities are altered by chromium substitution and that chromium substituted hydrogarnets have a smaller unit cell than the pure Ca-Al end member. This is consistent with substitution of hydroxyl tetrahedra by smaller chromate tetrahedra. Electron energy loss spectroscopy confirms the tetrahedral coordination and hexavalent oxidation state of chromium in the hydrogarnets. The maximum amount of hexavalent chromium that can be introduced synthetically corresponds to a replacement of about one out of every eight hydroxyl tetrahedral per unit cell by a CrO4(2-) tetrahedra and tallies closely with the amount of chromium measured in hydrogarnets from COPR. Chromium-bearing hydrogarnet is the most abundant crystalline phase in millions of tons of COPR contaminating land around Glasgow, Scotland, and was recently identified in COPR from sites in North America. Calculations based on its abundance and its Cr(VI) content indicate that hydrogarnet can host as much as 50% of the Cr(VI) found in some COPR samples.

Unusual Coordination Behavior of Cr3+ in Microporous Aluminophosphates

A CrAPO-5 molecular sieve has been investigated with X-ray absorption spectroscopy (EXAFS-XANES) as dehydrated material and after loading with water and ammonia to unravel the coordination geometries of Cr3+ in the framework of a microporous crystalline aluminophosphate, more particularly of the AFI-type. A comparison of the XANES data, a preedge analysis with crystal field multiplet calculations and EXAFS data, pointed toward the presence of framework Cr3+ which, on dehydration, takes on a distorted tetrahedral coordination state. Due to the 3d3 configuration of Cr3+, this unusual tetrahedral coordination environment strongly tends to transform into the more stable 6-fold coordination geometry by binding two extraframework water molecules during hydration. In the presence of ammonia, tetrahedral Cr3+ readily transforms into a 5-fold coordination geometry by binding one ammonia molecule. Therefore, depending on the environmental conditions, the Cr3+ ions can occur in a 4-, 5-, or 6-fold coordination. This observation underlines the flexibility of transition metal ions, such as Cr3+, to cope with unusual coordination geometries in inorganic hosts, making them interesting as potential active sites in heterogeneous catalysis.