(CH3CH[NH3]CH2NH3)1/2·ZnPO4, a zincophosphate analogue of aluminosilicate zeolite thomsonite

The synthesis and structure of (CH₃CH[NH₃]CH₂NH₃)_1/2·ZnPO₄, an organically templated zincophosphate (ZnPO) analogue of aluminosilicate zeolite thomsonite (THO), are described. The ZnPO framework is built up from an alternating, vertex-sharing, network of ZnO₄ and PO₄ groups (d_av(Zn–O)=1.944 (8) Å, d_av(P–O)=1.535 (9) Å, θ_av(Zn–O–P)=130.5°) involving distinctive 4=1 secondary building units. The 1,2-diammonium propane cations are highly disordered in the [0 0 1] 8-ring channels. Crystal data: (CH₃CH[NH₃]CH₂NH₃)_1/2·ZnPO₄, M_r=198.42, orthorhombic, space group Pncn (no. 52), a=14.119 (6) Å, b=14.136 (5) Å, c=12.985 (5) Å, V=2591 (3) ų, Z=10, R(F)=0.057, R_w(F)=0.061 (for a twinned crystal).     

Synthesis, structure and thermal properties of a novel 3D aluminophosphate UiO-26

The synthesis of a novel 3D aluminophosphate is described. The thermal properties of the material were investigated, and the existence of three high-temperature variants was revealed. The crystal structures of the as-synthesized material (UiO-26-as) and the material existing around 250°C (UiO-26-250) were solved from powder X-ray diffraction data. UiO-26-as with the composition [Al₄O(PO₄)₄(H₂O)]²⁻[NH₃(CH₂)₃NH₃]²⁺ crystallizes in the monoclinic space group P2₁/c (no. 14) with a=19.1912(5), b=9.3470(2), c=9.6375(2) Å and β=92.709(2)°. It exhibits a 3D open framework consisting of connections by PO₄ tetrahedra with AlO₄ tetrahedra, AlO₅ trigonal bipyramids and AlO₅(H₂O) octahedra forming two types of layers stacked along [1 0 0] and connected by Al–O–P bondings. The structure possesses a 1D 10-ring channel system running along [0 0 1], in which doubly protonated 1,3-diaminopropane molecules are located. UiO-26-250 with the composition [Al₄O(PO₄)₄]²⁻[NH₃(CH₂)₃NH₃]²⁺ crystallizes in the monoclinic space group P2₁/c with a=19.2491(4), b=9.27497(20), c=9.70189(20) Å and β=93.7929(17)°. The transformation to UiO-26-250 involves removal of the water molecule which originally is coordinated to aluminum. The rest of the structure remains virtually unchanged. The crystal structures of the two other variants existing around 400 (UiO-26-400) and 600°C (UiO-26-600) remain unknown.     

Synthesis and single crystal structure of an AFX-type magnesium aluminophosphate

SAPO-56 (framework type: AFX) has a framework topology slightly different from that of zeolite chabazite (framework type: CHA). While metal substituted aluminophosphate chabazite analogues can be prepared under a variety of experimental conditions with dozens of different amines, the synthesis of SAPO-56 type materials has been more difficult, particularly in non-SAPO compositions. Prior to this work, the growth of large crystals of the AFX-type materials suitable for single crystal diffraction has not been possible in any composition. Here we report the synthesis and single crystal structure of a magnesium aluminophosphate denoted as MAPO-AFX. This represents the first time that the AFX-type topology is made in a metal aluminophosphate composition. The synthesis was accomplished with a novel polyether diamine as the structure-directing agent. Crystal data for MAPO-AFX, (RH₂)_0.10(NH₄)_0.45[Mg_0.65Al_1.35(PO₄)₂](H₂O) where R=O[CH₂CH₂O(CH₂)₃NH₂]₂, space group P-31c (#163), Z=12, MoK radiation, 2θ_max=50°, a=13.8425(6) Å, c=20.204(1) Å, V=3352.7(3) ų, refinement on F², R(F)=7.94% for 131 parameters and 1218 unique reflections with I>2.0σ(I).     

Structural phase transition and high temperature phase structure of Friedels salt, 3CaO · Al2O3 · CaCl2 · 10H2O

Friedels salt, the chlorinated compound 3CaO · Al₂O₃ · CaCl₂ · 10H₂O (AFm phase), presents a structural phase transition at about 30°C from a monoclinic to a rhombohedral phase. It has been studied by X-ray powder diffraction and optical microscopy in transmitted light with crossed polarisers on single crystals prepared by hydrothermal synthesis. The high temperature phase was determined at 37°C from X-ray single crystal diffraction data. The compound crystallises in the space group R c with lattice parameters of a = 5.7358(6)Å and c = 46.849(9)Å (Z = 3 and D_x = 2.111 g/cm³). The refinement of 498 independent reflections with I > 2σ(I) led to a residual factor of 7.1%. The Friedels salt can be described as a layered structure with positively charged main layers of composition [Ca₂Al(OH)₆]⁺ and negatively charged layers of composition [Cl⁻,2H₂O]. The chloride anions are surrounded by 10 hydrogen atoms, of which six belong to hydroxyl groups and four to water molecules. The structural phase transition may be related to the size of the chloride anions, which are not adapted to the octahedral cavity formed by bonded water molecules.     

Microwave dielectric properties of rutile (Zn1/3Nb2/3)0.40(Ti1−xSnx)0.60O2 (0.15 ≤ x ≤ 0.30) ceramics

Microwave dielectric properties of (Zn_1/3Nb_2/3)_0.40(Ti_1−xSn_x)_0.60O₂ ceramics were investigated as a function of SnO₂ content (0.15 ≤ x ≤ 0.30). A single phase with tetragonal rutile structure was obtained through the entire composition. The unit-cell volume of the specimens was increased with SnO₂ content, due to the larger ionic radius of Sn⁴⁺ (0.69 Å) than that of Ti⁴⁺ (0.605 Å) for octahedral site. Dielectric constant (K) of the sintered specimens was affected by the dielectric polarizability. Quality factor (Qf) was dependent on the degree of reduction of Ti⁴⁺ ion. With an increase of SnO₂ content, the temperature coefficient of resonant frequency (TCF) of the specimens decreased due to the decrease of the octahedral distortion of rutile structure.     

Crystal structure of zeolite X nickel(II) exchanged at pH 4.3 and partially dehydrated, Ni2(NiOH)35(Ni4AlO4)2(H3O)46Si101Al91O384

Complete Ni²⁺ exchange of a single crystal of zeolite X of composition Na₉₂Si₁₀₀Al₉₂O₃₈₄ per unit cell was attempted at 73°C with flowing aqueous 0.05 M NiCl₂ (pH=4.3 at 23°C). After partial dehydration at 23°C and ≈10⁻³ Torr for two days, its structure, now of composition Ni₂(NiOH)₃₅(Ni₄AlO₄)₂(H₃O)₄₆Si₁₀₁Al₉₁O₃₈₄ per unit cell, was determined by X-ray diffraction techniques at 23°C (space group Fd , a₀=24.788(5) Å). It was refined using all intensities; R₁=0.080 for the 236 reflections for which F_o>4σ(F_o), and wR₂=0.187 using all 1138 unique reflections measured. At four crystallographic sites, 45 Ni²⁺ ions were found per unit cell. Thirty of these are at two different site III′ positions. Twenty of those are close to the sides of 12-rings near O–Si–O sequences, where each coordinates octahedrally to two framework oxygens, to three water molecules which hydrogen bond to the zeolite framework, and to an OH⁻ ion. The remaining 10 are near O–Al–O sequences; only three members of a likely octahedral coordination sphere could be found. In addition, two Ni²⁺ ions are at site I, eight are at site I′, and five are at site II. Forty six H₃O⁺ ions per unit cell, 24 at site II′ and 22 at site II, each hydrogen bond triply to six rings of the zeolite framework. Each of the 22 H₃O⁺ ions also hydrogen bonds to a H₂O molecule that coordinates to a site III′ Ni²⁺ ion. Six of the eight sodalite cages each contain four H₃O⁺ ions at site II′; the remaining two each contains a tetrahedral orthoaluminate anion at its center. Each tetrahedral face of each orthoaluminate ion is centered by a site I′ Ni²⁺ ion to give two Ni₄AlO₄ clusters. The five site II Ni²⁺ ions each coordinate to a OH⁻ ion. With 46 H₃O⁺ ions per unit cell, the great tendency of hydrated Ni²⁺ to hydrolyze within zeolite X is demonstrated. With a relatively weak single-crystal diffraction pattern, with dealumination of the zeolite framework, and with an apparent decrease in long-range Si/Al ordering likely due to the formation of antidomains, this crystal like others treated with hydrolyzing cations appears to have been damaged by Ni²⁺ exchange and partial dehydration.     

Influence of the SiO2/Al2O3 ratio on the synthesis and physicochemical characteristics of levyne-type zeolite

Levyne-type zeolites were synthesized from gels of initial compositions 4.5Na₂O-6MeQI-xAl₂O₃ 30SiO₂-500H₂O, with MeQ = methylquinuclidinium and 0.6 ≤ x ≥ 3 at 150 ≤ t ≥ 190 °C. The ²⁹Si NMR spectra show the presence of two crystallographically different sites in the structure. The ²⁷Al NMR spectra also suggest the presence of two different tetrahedral Al atoms incorporated in the structure. A rather high amount of defect groups SiOM and Si(OM)₂ with M = MeQ, Na and/or H are present in the precursor samples. The Si(OM)₂ groups are eliminated during calcination, and a certain amount of SiOM still persists after calcination. The combined ¹³C NMR and thermal analysis data allowed one to interpret the nature of the two different types of MeQ⁺ ions occluded in the levyne channels.     

Photoelectrochemical behavior of thin, thermally grown polycrystalline Fe2O3 films

The photoelectrochemical behavior of a number of photoelectrodes prepared in polycrystalline form from 99.9% iron (Armco) under controlled thermal oxidation conditions is reported. The steady-state polarization curves show a complicated aspect corresponding to changes in the charge transfer kinetics. A strong influence of the surface oxide thickness (d) on both the electrochemical characteristics and the photocurrent efficiency is found. Maximum photocurrents are measured on oxide covered electrodes with d < 10⁴ Å. The long term stability of the electrodes covered with oxide layers of thickness less than 10³ Å is unsatisfactory.     

Zeolite synthesis in the presence of flexible diquaternary alkylammonium ions (C2H5)3N(CH2)nN(C2H5)3 with n=3–10 as structure-directing agents

The use of flexible diquaternary alkylammonium ions (C₂H₅)₃N⁺(CH₂)_nN⁺(C₂H₅)₃ (Et₆-diquat-n with n=3–10) as structure-directing agents for zeolite synthesis in the presence of alkali metal cation is described. Among the organic structure-directing agents studied here, a considerable diversity in the phase selectivity was observed only for the Et₆-diquat-5 ion: this cation can produce five different zeolite structures (i.e., P1, SSZ-16, SUZ-4, ZSM-57, and mordenite), depending on the oxide composition of synthesis mixtures. Analysis of the variable-temperature ¹H CRAMPS NMR spectra obtained from the Et₆-diquat-5 molecules in these five zeolites reveals that the host–guest interactions occurring within the respective materials maintain in a manner different from one another even at 160 °C at which the zeolite hosts crystallize.     

Porous structures constructed from [SiMo12O40] and [SiW12O40] Keggin units

Three compounds, K₂(H₂O)₄H₂SiMo₁₂O₄₀ · 7H₂O (1), K₂Na₂(H₂O)₄SiW₁₂O₄₀ · 4H₂O (2), and Na₄(H₂O)₈SiMo₁₂O₄₀ · 6H₂O (3) have been synthesized and structurally characterized by single-crystal X-ray analysis, IR, and thermogravimetry. Compounds 1 and 2 both show the high symmetry trigonal space group P3₂21 and a novel 3D network structure. The Keggin anions [SiM₁₂O₄₀]⁴⁻(M = Mo, W) are linked by potassium or sodium cations to generate hexagon-shaped channels along the c-axis, in which water molecules are accommodated. Compound 3 is tetragonal, space group P4/mnc constructed from [SiMo₁₂O₄₀]⁴⁻ anions and Na ions.     

Kinetics of propylene epoxidation using H2 and O2 over a gold/mesoporous titanosilicate catalyst

The oxidation of propylene to propylene oxide (PO) with hydrogen–oxygen mixtures was studied on gold supported on the mesoporous titanium silicate, Ti-TUD. The catalyst gave stable activity at low conversions of propylene (<6%) and high selectivity to PO (>95%). Kinetic data were fit to a power-rate law and gave the following expression: r_PO = k(H₂)^0.54(O₂)^0.24(C₃H₆)^0.36. The fractional orders in hydrogen, oxygen, and propylene indicated that these reactants interacted with the catalyst to form species that led to the final PO product. The catalyst likely operated by the commonly accepted mechanism of hydrogen peroxide production on gold sites, and epoxidation on titanium centers. Carbon dioxide was formed primarily from further oxidation of PO rather than the oxidation of propylene, while water was produced from the reaction of hydrogen and oxygen.     

Synthesis and crystal structure of carbonate cancrinite Na8[AlSiO4]6CO3(H2O)3.4, grown under low-temperature hydrothermal conditions

The synthesis of cancrinite in the system Na₂O–SiO₂–Al₂O₃–Na₂CO₃–H₂O was studied under low-temperature hydrothermal conditions in the 353 K<T<473 K interval. The aim was to reveal the suitable range for the crystallization of pure-phase carbonate cancrinite with the ideal composition Na₈[AlSiO₄]₆CO₃(H₂O)₂ without cocrystallization of sodalite or intermediate disordered phases between cancrinite and sodalite. It was found that cancrinite formation reacts very sensitive on the temperature within the autoclaves whereas the concentration of reactants and the alkalinity of the hydrothermal solution have a much lower influence on the phase formation. Thus the temperature of crystallization of carbonate cancrinite without any by-products should not remain below 473 K. At the lower reaction temperature of 353 K the formation of a disordered intermediate phase between the cancrinite and the sodalite structure has been obtained in every case, independent of the template concentrations and the base. Some problems to detect this in a typical powder product mixture are discussed. Besides the ²⁹Si and ²⁷Al MAS NMR characterization of the products, the crystal structure refinement of pure carbonate cancrinite of ideal composition Na₈[AlSiO₄]₆CO₃(H₂O)_3.4, has been carried out from X-ray powder data using the Rietveld method: P6₃, a=1271.3(1) pm, c=518.6(1) pm, R_WP=0.073, R_F=0.016 for 347 structure factors and 45 variable positional parameters.     

In situ IR and pulse reaction studies on the active oxygen species over SrF2/Nd2O3 catalyst for oxidative coupling of methane

Pulse reaction method and in situ IR spectroscopy were used to characterize the active oxygen species for oxidative coupling of methane (OCM) over SrF₂/Nd₂O₃ catalyst. It was found that OCM activity of the catalyst was very low in the absence of gas phase oxygen, which indicated that lattice oxygen species contributed little to the yield of C₂ hydrocarbons. IR band of superoxide species (O₂⁻) was detected on the O₂-preadsorbed SrF₂/Nd₂O₃. The substitution of ¹⁸O₂ isotope for ¹⁶O₂ caused the IR band of O₂⁻ at 1128 cm⁻¹ to shift to lower wavenumbers (1094 and 1062 cm⁻¹), consistent with the assignment of the spectra to the O₂⁻ species. A good correlation between the rate of disappearance of surface O₂⁻ and the rate of formation of gas phase C₂H₄ was observed upon interaction of CH₄ with O₂-preadsorbed catalyst at 700 °C. The O₂⁻ species was also observed on the catalyst under working condition. These results suggest that O₂⁻ species is the active oxygen species for OCM reaction on SrF₂/Nd₂O₃ catalyst.     

Photocatalytic degradation of aqueous organocholorine pesticide on the layered double hydroxide pillared by Paratungstate A ion, Mg12Al6(OH)36(W7O24)·4H2O

Layered double hydroxide pillared by Paratungstate A ion, Mg₁₂Al₆(OH)₃₆(W₇O₂₄)·4H₂O, was prepared via anion exchange reaction of the synthetic precursor, Mg₄Al₂(OH)₁₂TA·xH₂O (TA²⁻=terephthalate), and [W₇O₂₄]⁶⁻ ion. Some physico-chemical properties were measured and the preparation conditions were studied. Trace aqueous organocholorine pesticide, hexachlorocyclohexane (HCH), was totally degraded and mineralized into CO₂ and HCl by irradiating a Mg₁₂Al₆(OH)₃₆(W₇O₂₄)·4H₂O suspension in the near UV area. Disappearance of trace HCH follows Langmuir–Hinshelwood first-order kinetics. The model and mechanism for the photocatalytic degradation of HCH on the Mg₁₂Al₆(OH)₃₆(W₇O₂₄)·4H₂O were proposed, indicating that the interlayer space is the reaction field, and that photogeneration of OH√ radicals are responsible for the degradation pathway.     

Spectroscopic analysis of carbon dioxide and nitrogen mixed gas hydrates in silica gel for CO2 separation

In this study solid-state NMR spectroscopy was used to identify structure and guest distribution of the mixed N₂ + CO₂ hydrates. These results show that it is possible to recover CO₂ from flue gas by forming a mixed hydrate that removes CO₂ preferentially from CO₂/N₂ gas mixture. Hydrate phase equilibria for the ternary CO₂–N₂–water system in silica gel pores were measured, which show that the three-phase H–L_w–V equilibrium curves were shifted to higher pressures at a specific temperature when the concentration of CO₂ in the vapor phase decreased. ¹³C cross-polarization (CP) NMR spectra of the mixed hydrates at gas compositions of more than 10 mol% CO₂ with the balance N₂ identified that the crystal structure of mixed hydrates as structure I, and that the CO₂ molecules occupy mainly the abundant 5¹²6² cages. This makes it possible to achieve concentrations of more than 96 mol% CO₂ gas in the product after three cycles of hydrate formation and dissociation.     

Microstructure and Properties of Co2+: ZnAl2O4/SiO2 Nanocomposite Glasses Prepared by Sol–Gel Method

Transparent bulk Co²⁺: ZnAl₂O₄/SiO₂ nanocomposites containing nanocrystalline Co²⁺: ZnAl₂O₄ dispersed in silica glass matrix were obtained by the sol–gel method. The gels of composition 89SiO₂–6Al₂O₃–5ZnO− x CoO ( x =0.2, 0.4, 0.6, 0.8, 1.0) (mol%) were prepared at room temperature by using two different aluminum salts, aluminum nitrate and aluminum alkoxide (aluminum-iso-propoxide, Al(OPrⁱ)₃), as starting materials. The transparent gels were converted to the crystalline phase of gahnite by heating above 900°C. The microstructural evolution of gels was characterized. The effect of Co²⁺ concentration on spectroscopic properties was also discussed. Co²⁺: ZnAl₂O₄ nanocrystals dispersed in the SiO₂-based glass are formed at lower heat-treatment temperature and shorter heating time by using Al(OPrⁱ)₃ as raw material.     

Formation of Sm2+ lons in Sol-Gel-Derived Glasses of the System Na2 O-Al2O3-SiO2

Samarium ions (Sm²⁺) incorporated into aluminosilicate glasses by a sol-gel process showed persistent spectral hole burning at room temperature. Gels of the system Na₂O-Al₂O₃SiO₂ synthesized by the hydrolysis of Si(OC₂H₅)₄, Al(OC₄H₉)₃, CH₃ COONa, and SmCl₃·6H₂O were heated in air at 500°C, then reacted with H₂ gas to form Sm²⁺ ions. Whereas Al³⁺ ions effectively dispersed the Sm³⁺ ions in the glass structure, Na⁺ ions were not effective. The Al₂O₃-SiO₂ glasses proved appropriate for reacting the Sm³⁺ ions with H₂ gas and exhibited the intense photoluminescence of Sm²⁺ ions. The reaction of Sm³⁺ ions with H₂ in the Al₂O₂-SiO₂ glasses was determined by first-order kinetics, and the activation energy equaled 95 kJ/mol. At 800°C, the maximum photoluminescence of the Sm²⁺ ions was achieved within 20 min.     

Methane combustion and CO oxidation on LaAl1−xMnxO3 perovskite-type oxide solid solutions

Structural, redox and catalytic deep oxidation properties of LaAl_1−xMn_xO₃ (x=0.0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) solid solutions prepared by the citrate method and calcined at 1073 K were investigated. XRD analysis showed that all the LaAl_1−xMn_xO₃ samples are single phase perovskite-type solid solutions. Particle sizes and surface areas (SA) are in the 280–1180 Å and 4–33 m² g⁻¹ ranges, respectively. Redox properties and the content of Mn⁴⁺ were derived from temperature programmed reduction (TPR) with H₂. Two reduction steps are observed by TPR for pure LaMnO₃, the first attributed to the reduction of Mn⁴⁺ to Mn³⁺ and the second due to complete reduction of Mn³⁺ to Mn²⁺. The presence of Al in the LaAl_1−xMn_xO₃ solid solutions produces a strong promoting effect on the Mn⁴⁺→Mn³⁺ reducibility and inhibits the further reduction to Mn²⁺. Both for methane combustion and CO oxidation all Mn-containing perovskites are much more active than LaAlO₃, so pointing to the essential role of the transition metal ion in developing highly active catalysts. Partial dilution with Al appears to enhance the specific activity of Mn sites for methane combustion.     

Thermoelectric characterization of NaxMx/2Ti1−x/2O2 (M=Co, Ni and Fe) polycrystalline materials

Layered -titanate materials, Na_xM_x/2Ti_1−x/2O₂ (M=Co, Ni and Fe, x=0.2–0.4), were synthesized by flux reactions, and electrical properties of polycrystalline products were measured at 300–800 °C. After sintering at 1250 °C in Ar, all products show n-type thermoelectric behavior. The values of both d.c. conductivity and Seebeck coefficient of polycrystalline Na_0.4Ni_0.2Ti_0.8O₂ were ca. 7×10³ S/m and ca. −193 μV/K around 700 °C, respectively. The measured thermal conductivity of layered -titanate materials has lower value than conductive oxide materials. It was ca. 1.5 Wm⁻¹ K⁻¹ at 800 °C. The estimated thermoelectric figure-of-merit, Z, of Na_0.4Ni_0.2Ti_0.8O₂ and Na_0.4Co_0.2Ti_0.8O₂ was about 1.9×10⁻⁴ and 1.2×10⁻⁴ K⁻¹ around 700 °C, respectively.     

Metal cation–acidic proton bifunctional catalyst for methane activation: conversion of CH4 in the presence of ethylene over metal cations-loaded H-ZSM-5

It has been investigated that ¹³CH₄ reacts with ethylene over metal cations such as indium cations-loaded H-ZSM-5 to form singly ¹³C-labeled propylene (¹³C¹²C₂H₆) and hydrogen at 673 K. The heterolytic dissociation of C---H bond in methane presumably proceeds by the reaction of methane with metal cations for the formation of CH₃^δ+ and metal hydride species, thus allowing the reaction of CH₃^δ+ with ethylene to form propylene and acidic protons. Metal cations are regenerated by the reaction of silver-hydride species with acidic protons, and hydrogen is simultaneously formed. Thus, bifunctionality of metal cations and acidic protons is essential for the activation of methane.