HYDROTHERMAL SYNTHESIS AND ION EXCHANGE PROPERTIES OF THE NOVEL FRAMEWORK SODIUM AND POTASSIUM NIOBIUM SILICATES

ABSTRACT

Two novel metastable sodium niobium silicates of the empirical formula: Na_l+x?yH_y(Nb_1?xSi_x)O₃ nH₂O, where x=0.33?0.38, y<l+x, n=0,7-l.l (NbSi-Na, 6.0 Å phase), and Na_3-x H_xNb₃Si₂O₁₃ nH₂O, where x<1.5, n=2.5?3.5 (NbSi-Na, 12.6 Å phase), and two novel potassium niobium silicates: K_4?xH_xNb₄SijO₂₂nH₂O, where x<l, n=3.5-4.0 (NbSi-K., 10.0 Å phase), and K_1?xH_xNbSi₄O₁₁nH₂O, where x<0.2, n=0.4-0.5 (NbSi-K, 6.05 Å phase), were synthesized in the homogeneous alkaline reaction system NbCl₅ - SiO₂ - NaOH (KOH) -H₂O₂ - H₂O under mild hydrothermal conditions. The compounds were characterized by elemental analysis, FTIR, TGA, MAS ²⁹Si NMR and X-ray diffraction. It was found that alkali metal niobium silicates have open framework structures. Their ion exchange affinity towards alkali, alkaline earth and some transition metal ions was studied. All alkali metal niobium silicates are moderately acidic ion exchangers. Both sodium niobium silicates show a distinct affinity for Cs⁺ ion among alkali metal ions, whereas potassium niobium silicate, the NbSi-K, 10.0 Å phase, exhibits affinity for Rb⁺ ion. The affinity of the sodium niobium silicate, NbSi-Na, 6.0 Å, toward strontium ion in neutral solutions is equal or superior to the best Sr-selective inorganic ion exchangers. The sodium niobium silicate (NbSi-Na, 12.6 Å phase) exhibits extremely high affinity for Pb²⁺ ion in acidic and neutral media, and both sodium niobium silicates also show a moderate affinity for Hg²⁺ ion in neutral and highly alkaline media. These exchangers could be promising for the treatment of some specific nuclear waste and contaminated environmental and biological liquors containing lead, mercury and radioactive strontium.     

SiAlON Composites Containing Rare-Earth Melilite and Neighboring Phases

Sintering aids with a formulation of rare-earth (R) melilite solid solution M'(R) have been introduced to several SiAlON composites to obtain high-density bodies by pressureless and gas-pressure sintering. These compositions include ones along the Si₃N₄–R₂O₃:9AlN line, which contain no Al₂O₃ and are known to have excellent strength but very poor sinterability. Phase analysis and property (hardness, toughness, strength, and oxidation resistance) measurements reveal a complex set of structure–property–processing relationships that are composition-dependent. Of the most importance is the Al—O content in various phases, which is dictated by the starting composition as well as Al—O solubility in M'(R). This influences the sinterability, strength, and phase assemblage through the compositions of the intermediate liquid and the final β'-SiAlON. La₂O₃ is further established as an effective sintering aid that does not affect the equilibrium phase assemblage. Overall, M'(R) and La₂O₃ show promise as SiAlON sintering aids that allow high density to be achieved without compromising intrinsic mechanical and thermal properties.     

Modeling of the Crack Tip Plastic Zone by Two Slip Lines and the Order of Stress Singularity

Modeling of initial plastic zone by two slip-lines is considered. It is shown that such a model does not remove stress singularity at the crack tip.     

Solving Linear Interval Systems Is NP-Hard Even If We Exclude Overflow and Underflow

Reliable Computing -     

Polymerization of α-Olefins Using Benzamidinate Zirconium Complexes Supported on Mesoporous Silica Materials

The polymerization of ethylene and propylene were studied using an anchored dimethyl benzamidinate zirconium complex. As supports, mesoporous silicas MCM-41 and HMS were utilized. These inorganic materials were reacted with methylalumoxane to serve as the cocatalyst and to avoid the deactivation of the active cationic complex. The reaction of the octahedral benzamidinate zirconium complex with the supported cocatalysts results in the formation of active catalytic systems for the polymerization of olefins. The polymer properties were found to depend on the nature of the supports. The activity of the heterogeneous catalysts and the polymer characteristics were compared with those obtained with the homogeneous catalyst.     

Atmospheric integrated water vapour measured by IR and MW techniques at the Peterhof site (Saint Petersburg,Russia)

Regular comparison of different systems for monitoring atmospheric integrated water vapour (IWV) is part of their testing and validation protocol. We compared coincident measurements of IWV over Saint Petersburg (Russia) from ground-based Fourier-transform spectrometer Bruker IFS 125 HR (FTIR) and microwave radiometer RPG-HATPRO (MW) at the Peterhof site between March 2013 and June 2015. This study is a contribution towards global efforts to make such inter-comparisons at various ground-based sites. Since FTIR measures solar radiance, the vast majority of coincident pairs correspond to the spring and summer seasons. The numbers of measurements in the dry season (from October to April) and in the wet season (from May to September) are almost identical, comprising 616 and 638 pairs, respectively. MW and FTIR data sets demonstrate a high level of agreement: the mean relative difference between MW and FTIR data is less than 3% (0.3 mm), with standard deviation from the means of about 4% (0.4 mm). Notwithstanding the short distance between both instruments (150 m), they can monitor different air masses: MW is a zenith-viewing instrument whereas FTIR follows the sun. We analysed the FTIR observation fields under different solar zenith and azimuth angles, taking into account the location of the Peterhof site between the Gulf of Finland and rural suburbs of Saint Petersburg. Although in general MW measurements slightly overestimated IWV in comparison with the FTIR data, we detected several episodes when FTIR gave higher values than MW. These episodes relate to the FTIR observations directed at the coastal region with more humid air than that above the measurement site. We may conclude at this stage of our investigations that the spatial inhomogeneity of humidity fields in the atmosphere causes the most significant differences between the two data sets. Detailed analysis of variation in spatial IWV, e.g. using a MW radiometer in angular scanning mode, is an issue for future research.     

Comparisons of satellite (GOSAT) and ground-based spectroscopic measurements of CH4 content near Saint Petersburg: influence of data collocation

We compared atmospheric column average mole fractions of methane measured with ground-based Fourier transform spectroscopy at the Physical Department of Saint Petersburg State University (59.9° N, 29.8° E) in years 2009–2012 with similar data obtained from the Japanese Greenhouse Gases Observing Satellite (GOSAT) satellite. For the GOSAT data version V02.xx, average and median values of biases between satellite and ground-based methane mole fractions are ?(1.7–4.1) ppb and their standard deviations are 10–13 ppb. These values are similar to biases between the GOSAT satellite and the ground-based Total Carbon Column Observation Network and Network for the Detection of Atmospheric Composition Change making Fourier transform spectroscopic observations. Average and median biases for satellite data selected within α = ±1° latitude–longitude vicinity from the ground-based observations site are smaller than those for α = ±3° and α = ±5°.     

Effect of the method of preparation on the surface area and porosity of titanium(IV) phenylphosphonate

By using three synthetic procedures we have prepared layered -titanium phenylphosphonate samples with different degrees of crystallinity and distinct textural parameters. The solids were prepared by reacting phenylphosphonic acid with different titanium compounds used as precursors. The materials obtained have a high thermal stability as shown by TGA measurements.An analysis of nitrogen adsorption-desorption isotherms on the resulting materials allowed determination of the corresponding specific surface area and porous texture. The N₂-isotherms correspond to type IV of the BDDT classification with hysteresis loops of the type H-1. The materials are essentially mesoporous and it was not detected any mensurable microporosity. Crystallinity, BET surface areas and porosity are markedly dependent on the preparation procedure.