Mechanism of Substitution in Carbonated Apatites

Single-phase AB-type carbonate apatites were prepared by sintering appropriate mixtures of CaHPO₄ and CaCO₃ at 870°C in a CO₂ atmosphere with a partial water vapor pressure of 5 mm Hg. Chemical and physical analyses indicate that at a constant CO₃^2?/OH^? ratio in the hydroxyl sublattice, carbonate substitutes for phosphate on a 1:1 mole basis. For every three PO₄^3? ions substituted, two vacancies in the Ca²⁺ sublattice and one in the OH^? sublattice are created. The same substitution mechanism seems to apply in pure B-type carbonate apatite.     

Infrared study of trapped carbon dioxide in thermally treated apatites

The formation of molecular CO₂ in synthetic apatites (prepared in aqueous systems), dental enamel, dentine, and various apatitic rock phosphates after heating in the range 120–900°C has been investigated by infrared spectroscopy. The CO₂ band at 2340 cm^?1 was observed in the synthetic samples, enamel, and some of the rock phosphates, but not in dentine or bone. It is suggested that the absence of this band in dentine and bone is caused by the small crystal size of their apatites. The CO₂ band at 667 cm^?1 was never observed. The polarized infrared spectrum of heated dental enamel showed that the linear CO₂ molecules were either randomly oriented or oriented so that the length of the molecules made an angle of about 56° with the c axis. It is suggested that, if the latter is correct, the CO₂ originates from a CO^2?₃ ion which occupied the sloping face of the phosphate ion site.     

Monitoring of the evolved gases in apatite-ammonium sulfate thermal reactions

Summary Thermal reactions in natural fluorapatite or fluorcarbonate apatite and ammonium sulfate mixtures with mole ratio 1:4 at calcination up to 500°C were studied by simultaneous thermogravimetry and FTIR analysis of the evolved gases. The composition of natural apatite has little impact on the release of NH₃. Upon the evolution of NH₃ nitrous oxides were found in minor amounts. The release of SO₂ at temperatures above 400°C is more intensive and occurs at lower temperatures in the case of fluorapatite than of carbonate containing apatites. Evolution of CO₂ starts at 250°C with maximum at 350-360°C.     

Etude de la decomposition thermique de fluorapatites carbonatees

Using a precipitation method, variably carbonated samples of fluorapatite-like francolite were prepared from solutions containing diammonium phosphate, ammonium fluoride and ammonium carbonate. Thermal analysis, gas chromatography and IR spectroscopy were performed. The results show that cyanate ions (CNO^?) appear between 400 and 500 °C, probably as a result of the thermal evolution of ammonium ions. Above 500°C, nitrogen gas was detected with CO₂. N₂ arises presumably from cyanate ion decomposition. Decarbonation of these apatites occurs in three steps, the first of these is attributed to carbamate ions.     

Fourier transform infrared determination of CO2 evolved from carbonate in carbonated apatites

A quantitative method based on FTIR has been developed to determine carbonate in synthetic apatites. The method measures the evolved CO2 after reaction of 50 mg apatite with 2 mL of hydrochloric acid (0.5 M) in a reaction vessel, heated to 40 degrees C. The CO2 evolved was swept by a carrier of nitrogen to a laboratory-made infrared gas cell of 39 mm pathlength and 490 microL volume. The signals were recorded as a function of time and the areas of the chemigram peaks obtained from the measurements in the wavenumber range of 2,500-2,150 cm(-1), were interpolated using a calibration curve. The method can be used to study apatites with carbonate contents below 0.2% with a sampling frequency of 8 h(-1).     

Thermal analysis of apatite structure

Two types of thermal effects, caused by substitutions (Ca²⁺↔ Na⁺, ↔ CO₃ ^2-, SO₄ ^2-, OH^-↔ F^-) in synthetic precipitated apatites as well as by sorption of Cd²⁺, Zn²⁺, and Cr³⁺ ions from the solutions were studied by TG/DTA, XRD and FTIR analysis. The temperatures of exothermic effect at 330-340°C and of decomposition of carbonate and sulfate apatites at 650-950°C were shown to depend on the substitutions in the apatite structure. This revised version was published online in July 2006 with corrections to the Cover Date.     

Abiotic C?C bond formation under environmental conditions: Kinetics of the aldol condensation of acetaldehyde in water catalyzed by carbonate ions (CO32−)

The role of C? C bond‐forming reactions such as aldol condensation in the degradation of organic matter in natural environments is receiving a renewed interest because naturally occurring ions, ammonium ions, NH⁺₄, and carbonate ions, CO₃^2?, have recently been reported to catalyze these reactions. While the catalysis of aldol condensation by OH^? has been widely studied, the catalytic properties of carbonate ions, CO₃^2?, have been little studied, especially under environmental conditions. This work presents a study of the catalysis of the aldol condensation of acetaldehyde in aqueous solutions of sodium carbonate (0.1–50 mM) at T = 295 ± 2 K. By monitoring the absorbance of the main product, crotonaldehyde, instead of that of acetaldehyde, interferences from other reaction products and from side reactions, in particular a known Cannizzaro reaction, were avoided. The rate constant was found to be first order in acetaldehyde in the presence of both CO₃^2? and OH^?, suggesting that previous studies reporting a second order for this base‐catalyzed reaction were flawed. Comparisons between the rate constants in carbonate solutions and in sodium hydroxide solutions ([NaOH] = 0.3–50 mM) showed that, among the three bases present in carbonate solutions, CO₃^2?, HCO₃^?, and OH^?, OH^? was the main catalyst for pH ≤ 11. CO₃^2? became the main catalyst at higher pH, whereas the catalytic contribution of HCO₃^? was negligible over the range of conditions studied (pH 10.3–11.3). Carbonate‐catalyzed condensation reactions could contribute significantly to the degradation of organic matter in hyperalkaline natural environments (pH ≥ 11) and be at the origin of the macromolecular matter found in these environments. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 676–686, 2010     

Thermal decomposition of copper(II) and zinc carbonate hydroxides by means of TG-MS

Summary For the quantitative analyses of evolved CO₂and H₂O during the thermal decomposition of solids, calibration curves, i.e. the amounts of evolved gases vs. the corresponding peak areas of mass chromatograms measured by TG-MS, were plotted as referenced by the reaction stoichiometry of the thermal decomposition of sodium hydrogencarbonate NaHCO₃. The accuracy and reliability of the quantitative analyses of the evolved CO₂and H₂O based on the calibration curves were evaluated by applying the calibration curves to the mass chromatograms for the thermal decompositions of copper(II) and zinc carbonate hydroxides. It was indicated from the observed ratio of evolved CO₂and H₂O that the compositions of copper(II) and zinc carbonate hydroxides examined in this study correspond to mineral malachite, Cu₂CO₃(OH)₂, and hydrozincate, Zn₅(CO₃)₂(OH)₆, respectively. Reliability of the present analytical procedure was confirmed by the fairly good agreement of the mass fraction of the evolved gases calculated from the analytical values with the total mass-loss during the thermal decompositions measured by TG.     

Sequence Control in Asymmetric Terpolymerizations of meso-Epoxides,CO2, and Phthalic Anhydride: Unprecedented Statistical Ester–Carbonate Distributions

The statistical terpolymerization of epoxides, CO₂ and cyclic anhydrides remains challenging, mainly because epoxide/CO₂ and epoxide/anhydride copolymerizations typically proceed at considerably different rates. Herein, we report the syntheses of novel chiral terpolymers with unprecedented statistical distributions of carbonate and ester units (up to 50 % junction units) via the one-pot reaction of cyclohexene oxide, phthalic anhydride, and CO₂ under mild conditions using enantiopure bimetallic aluminum-complex-based catalyst systems. Notably, all resulting terpolymers exhibited excellent enantioselectivities (≥96 % ee) that were independent of the carbonate–ester distribution. The statistical compositions of the carbonate and ester units in the resulting terpolymers were determined via ¹H and ¹³C NMR spectroscopies. Furthermore, thermal properties were tuned by altering the ester content of the chiral terpolymer without influencing the enantioselective ring-opening step involving the meso-epoxide. This asymmetric terpolymerization methodology is also compatible with a variety of meso-epoxides to afford the corresponding terpolymers with 17 %–25 % junction units and excellent enantioselectivities (94 %–99 % ee). The present study is expected to provide new guidelines for preparing a broad range of biodegradable polymers with excellent enantioselectivities and adjustable properties.     

Non-3d metal modulated zinc imidazolate frameworks for CO2 cycloaddition in simulated flue gas under ambient condition

Cycloaddition of CO₂ and epoxide into cyclic carbonate is one of the most efficient ways for CO₂ conversion with 100% atom-utilization. Metal–organic frameworks are a kind of potential heterogeneous catalysts, however, high temperature, high pressure, and high-purity CO₂ are still required for the reaction. Here, we report two new Zn(II) imidazolate frameworks incoporating MoO₄^2– or WO₄^2– units, which can catalyse cycloaddition of CO₂ and epichlorohydrin at room temperature and atomospheric pressure, giving 95% yield after 24 h in pure CO₂ and 98% yield after 48 h in simulated flue gas (15% CO₂ + 85% N₂), respectively. For comparison, the analogic Zn(II) imidazolate framework MAF-6 without non-3d metal oxide units showed 71% and 33% yields under the same conditions, respectively. The insightful modulation mechanisms of the MoO₄^2– unit in optimizing the electronic structure of Zn(II) centre, facilitating the rate-determined ring opening process, and minimizing the reaction activation energy, were revealed by X-ray photoelectron spectroscopy, temperature programmed desorption and computational calculations.     

Standard enthalpies of formation of “A” type carbonate phosphobaryum hydroxyapatites

Phosphobaryum carbonate hydroxyapatites Ba₁₀(PO₄)₆(OH)_(2?2x)(CO₃) _x with 0????x????1, were synthesized in this study by solid gas reaction at a high temperature. Carbonate content was determined by thermogravimetric analysis and X-ray diffraction. The heat of the solution in a 3 wt% nitric acid solution was measured at 298?K using an isoperibol calorimeter. The combination of the enthalpies of solution with the enthalpies of formation of the reactants allowed us to determine the standard enthalpies of formation of the studied apatites. The result showed a decrease in formation enthalpy with the carbonate amount introduced in the lattice, suggesting an increase in stability of these compounds as the ratio of the substitution increases.     

Influences of evolved gases on the thermal decomposition of zinc carbonate hydroxide evaluated by controlled rate evolved gas analysis coupled With TG

The influences of atmospheric CO₂ and H₂O on the kinetics of the thermal decomposition of zinc carbonate hydroxide, Zn₅(CO₃)₂(OH)₆, were investigated by means of controlled rate evolved gas analysis (CREGA) coupled with TG. Although CO₂ and H₂O were evolved simultaneously in a single mass-loss step of the thermal decomposition, different effects of those evolved gases on the kinetic rate behavior were observed. No distinguished effect of atmospheric CO₂ was detected within the possible range of self-generated CO₂ concentration. On the other hand, apparent acceleration effect by the increase in the concentration of atmospheric H₂O was observed as the reduction of reaction temperature during the course of constant rate thermal decomposition. The catalytic effect was characterized by the decrease in the apparent activation energy for the established reaction with increasing the concentration of atmospheric H₂O, accompanied by the partially compensating decrease in the pre-exponential factor.     

Synthesis of cyclic carbonates from epoxides or olefins and CO2 catalyzed by metal-organic frameworks and quaternary ammonium salts

Catalytic properties of the metal-organic framework Cr-MIL-101 in solvent-free cycloaddition of CO₂ to epoxides to produce cyclic carbonates using tetrabutylammonium bromide as co-catalyst have been explored under mild reaction conditions (8 bar CO₂, 25 ⁰C). Styrene and propylene carbonates were formed with high yields (95% and 82%, respectively). Catalytic performance of Cr-MIL-101 was compared with other MOFs: Fe-MIL-101, Zn-MOF-5 and HKUST-1. The catalytic properties of different quaternary ammonium bromides, Cr-MIL-101 as well as PW₁₂/Cr-MIL-101 composite material have been assessed in oxidative carboxylation of styrene in the presence of both tert-butyl hydroperoxide and H₂O₂ as oxidants at 8–100 bar CO₂ and 25–80 ⁰C with selectivity to styrene carbonate up to 44% at 57% substrate conversion.     

碳酸钙晶须合成过程中可溶性磷酸盐的作用机理研究

以可溶性磷酸盐为控制剂,一步碳化法制备了文石相碳酸钙晶须。借助于XRD和FTIR,分析了可溶性磷酸盐在碳酸钙晶须合成过程中的作用机理。研究结果表明：通入CO₂进行碳化反应前,可溶性磷酸盐与Ca(OH)₂反应生成了热力学上最稳定的磷酸钙化合物——羟基磷灰石;在通入CO₂初期,[CO₃^2-(OH)]进入到羟基磷灰石的晶格,部分替代[PO₄^3-],生成碳酸羟基磷灰石,然后以此为结晶中心诱导文石相的异相成核,Ca²⁺、CO₃^2-不断叠加,进而生长为碳酸钙晶须。     

The structure and solubility of carbonated hydroxyl and chloro lead apatites

The properties of carbonated hydroxyl and chloro lead apatites, Pb₁₀(PO₄)₆(OH)₂ and Pb₁₀(PO₄)₆Cl₂, serve as models for the incorporation of carbonate into their medically important calcium analogs, and there is likely incorporation of carbonate in an insoluble lead phosphate phase during lead remediation. We have synthesized a series of carbonated lead hydroxyl- and lead chloro-apatites at 60–80 °C. The incorporation of carbonate into the apatite structure was documented by X-ray powder diffraction, IR and Raman spectroscopy, ²⁰⁷Pb solid state NMR spectroscopy, and elemental analysis. The carbonate content was determined by combustion analysis and confirmed by Raman spectroscopic analysis. As carbonate content increases in hydroxyl lead apatite, Raman spectra show changes in the phosphate stretching modes at 925 and 950 cm⁻¹, an increase in intensity and downshift of a new peak at 1050 cm⁻¹, and changes in the spectral features of the O–H stretch at about 3560 cm⁻¹. The variation in unit cell parameters for the chloro lead apatite as a function of carbonate content is similar to that documented for B-type substitution in calcium apatites. The ²⁰⁷Pb NMR spectra corroborate B-type substitution. For the hydroxyl lead apatite, the changes in cell parameters suggest a combination of A- and B-type substitution. Solubilities of the carbonated lead apatites, determined by ICP-MS, increase slightly at low to moderate carbonate content, but more strongly at ca. 5.0 wt.% carbonate content. K_sp values extrapolated to zero carbonate content reveal that the chloro lead apatite is indeed less soluble than the hydroxyl analog.     

Adsorption d'acides aminés sur des phosphates de calcium carbonatés analogues aux tissus calcifiés

The adsorption of alanine and phenylalanine onto apatites, containing simultaneously HPO₄²⁻ and CO₃²⁻ ions in various amount, was studied. These apatites are similar to the mineral part of calcified tissues at different stages of its maturation. The adsorption isotherms are Langmuirian in all cases whatever the amounts of HPO₄² or CO₃²⁻. However, these amounts play a role in the adsorption properties of these apatites. The largest amounts of aminoacids were adsorbed by the compounds with the highest HPO₄²⁻ content. The influence of the presence of PO₄³⁻ ions in the incubation media was studied. These ions hinder the adsorption process by competing with the aminoacid carboxylate groups for the calcium ions at the surface of the solid. The nature of the aminoacid lateral chain, especially its size, plays a role in the adsorption. The amount of aminoacid bound at saturation decreases as the size of the side chain increases.     

Activation of carbon dioxide and carbon monoxide at aluminium surfaces

Dynamic photoelectron spectroscopy has shown that the adsorption of carbon dioxide at aluminium surfaces is followed by a dissociative reaction leading to the formation of a metastable surface carbonate in the temperature range 80-120 K. The carbonate is subsequently reduced (120–475 K) (deoxygenated) to generate two different forms of surface carbon, one carbidic C^δ- (a) and the other less ionic C⁰(a) possibly graphitic. Quantification of the C(ls) and O(ls) spectra enable each of the species O^2-(a), CO₃^2-(a), C^δ-(a) and C⁰ (a) to be distinguished and their surface concentrations calculated over a wide temperature range. The temperature and pressure dependences of CO₂ reduction suggest the participation of a precursor dimer state (CO₂^---CO₂)(a) which then disproportionates. Furthermore studies of the coadsorption of ammonia and carbon dioxide in analogous systems indicate that a discrete and specifically reactive species, O^- (s), is formed during carbonate formation. The results are discussed in the context of recent theoretical studies of FREUND and MESSMER and also comparisons made with metal-CO₂ complexes.The facile surface reduction of CO₂ via a surface carbonate suggested that a possible route to carbon-oxygen bond cleavage in carbon monoxide interaction with an sp-metal surface (aluminium) was a step-wise oxidation to CO₂ leading to surface carbonate which was then readily deoxygenated. Studies of carbon monoxide: dioxygen mixtures (100: I) confirmed that this indeed occurred. A modified ELEY-RIDEAL type mechanism involving a hopping "non-adsorbed" CO molecule and a short-lived surface O^- (s) species is suggested.     

Mechanism of the Cycloaddition of Carbon Dioxide and Epoxides Catalyzed by Cobalt‐Substituted 12‐Tungstenphosphate

Co^II‐substituted α‐Keggin‐type 12‐tungstenphosphate [(n‐ C₄H₉)₄N]₄H[PW₁₁Co(H₂O)O₃₉]‐ (PW₁₁Co) is synthesized and used as a single‐component, solvent‐free catalyst in the cycloaddition reaction of CO₂ and epoxides to form cyclic carbonates. The mechanism of the cycloaddition reaction is investigated using DFT calculations, which provides the first computational study of the catalytic cycle of polyoxometalate‐catalyzed CO₂ coupling reactions. The reaction occurs through a single‐electron transfer from the doublet Co^II catalyst to the epoxide and forms a doublet Co^III–carbon radical intermediate. Subsequent CO₂ addition forms the cyclic carbonate product. The existence of radical intermediates is supported by free‐radical termination experiments. Finally, it is exhilarating to observe that the calculated overall reaction barrier (30.5 kcal mol^?1) is in good agreement with the real reaction rate (83 h^?1) determined in the present experiments (at 150 °C).     

On the Carbonate Formation in Thermally Stressed Hydroxysodalite – Some Facts to Notice for SOD Application in Separation Processes

Basic sodalite (hydroxysodalite) was synthesized from two Si-Al sources: (1) kaolin to obtain |Na_7.5(OH)_1.5(H₂O)_3.5|[AlSiO₄]₆ sodalites (SOD) with small crystals (< 0.5 μm) and (2) a mixture of cristobalite and corundum (CC) to obtain larger microcrystals (1–5 μm) with ideal composition |Na₈(OH)₂(H₂O)₂|[AlSiO₄]₆. Both SOD were exposed to thermal stress by long-time heating at 773 K under open conditions, in N₂ and CO₂ atmosphere and in presence of a NaOH-Na₂CO₃ melt. The crystals obtained from kaolin were dehydrated and developed remarkable degrees of carbonate cage fillings already under open conditions. The large microcrystals obtained from CC exhibit this effect only at very low scale even after long-time heating in CO₂ atmosphere. Whereas heating in presence of the melt showed no effect, investigations in CO₂ clearly indicate an intra-cage reaction between CO₂ and the enclathrated [Na₄OH]³⁺ ions as the carbonate generating mechanism instead of destruction of hydroxysodalite followed by recrystallization. A model is proposed, in which cage fillings [Na₃□]³⁺ with a vacancy in the Na tetrahedron known from dehydrated hydrosodalites |Na₆|[AlSiO₄]₆ are required to induce the intra-cage reaction of hydroxysodalite. As those fillings only occur in a noticeable number in dehydrated hydroxysodalite obtained from kaolin, the large extent of the carbonate formation inside this sample becomes obvious. The results are significant for future improvement of hydroxysodalite membranes.     

N‐Heterocyclic Olefin–Carbon Dioxide and –Sulfur Dioxide Adducts: Structures and Interesting Reactivity Patterns

Depending on the amount of methanol present in solution, CO₂ adducts of N‐heterocyclic carbenes (NHCs) and N‐heterocyclic olefins (NHOs) have been found to be in fully reversible equilibrium with the corresponding methyl carbonate salts [EMIm][OCO₂Me] and [EMMIm][OCO₂Me]. The reactivity pattern of representative 1‐ethyl‐3‐methyl‐NHO–CO₂ adduct 4 has been investigated and compared with the corresponding NHC–CO₂ zwitterion: The protonation of 4 with HX led to the imidazolium salts [NHO–CO₂H][X], which underwent decarboxylation to [EMMIm][X] in the presence of nucleophilic catalysts. NHO–CO₂ zwitterion 4 can act as an efficient carboxylating agent towards CH acids such as acetonitrile. The [EMMIm] cyanoacetate and [EMMIm]₂ cyanomalonate salts formed exemplify the first C?C bond‐forming carboxylation reactions with NHO‐activated CO₂. The reaction of the free NHO with dimethyl carbonate selectively led to methoxycarbonylated NHO, which is a perfect precursor for the synthesis of functionalized ILs [NHO–CO₂Me][X]. The first NHO‐SO₂ adduct was synthesized and structurally characterized; it showed a similar reactivity pattern, which allowed the synthesis of imidazolium methyl sulfites upon reaction with methanol.