Characterization of rice husks and the products of its thermal degradation in air or nitrogen atmosphere

Rice husk is a by-product of rice milling process and are a major waste product of the agricultural industry. They have now become a great source as a raw biomass material for manufacturing value-added silicon composite products, including silicon carbide, silicon nitride, silicon tetrachloride, pure silicon, zeolite, fillers of rubber and plastic composites, adsorbent and support of catalysts. The bulk and true densities of raw rice husk with different moisture and sizes were determined. The rice husk was subjected to pyrolysis in fluidized-bed reactor in air or nitrogen atmosphere. The products obtained were characterized by thermogravimetric and X-ray powder analysis, IR-spectroscopy, scanning electron microscopy and nitrogen adsorption at 77 K. The specific surface area of the products is comparable with this of γ-Al₂O₃. The kinetics of H₂SeO₃ adsorption out of aqueous solutions at 298 K was studied. The adsorption capacity of white rice husks ash was found to be higher than that of black rice husk ash and the adsorption kinetics obeyed the second order kinetic equation.     

Geometrically designed,variable knot regression splines

A new method of Geometrically Designed least squares (LS) splines with variable knots, named GeDS, is proposed. It is based on the property that the spline regression function, viewed as a parametric curve, has a control polygon and, due to the shape preserving and convex hull properties, it closely follows the shape of this control polygon. The latter has vertices whose x-coordinates are certain knot averages and whose y-coordinates are the regression coefficients. Thus, manipulation of the position of the control polygon may be interpreted as estimation of the spline curve knots and coefficients. These geometric ideas are implemented in the two stages of the GeDS estimation method. In stage A, a linear LS spline fit to the data is constructed, and viewed as the initial position of the control polygon of a higher order (\(n>2\)) smooth spline curve. In stage B, the optimal set of knots of this higher order spline curve is found, so that its control polygon is as close to the initial polygon of stage A as possible and finally, the LS estimates of the regression coefficients of this curve are found. The GeDS method produces simultaneously linear, quadratic, cubic (and possibly higher order) spline fits with one and the same number of B-spline coefficients. Numerical examples are provided and further supplemental materials are available online.     

Hydrogen-bonded complexes of nitrous and nitric acids with ethene Ab initio and DFT studies

The complexes formed by ethene with nitric and nitrous (trans and cis) acids have been investigated by ab initio (SCF and MP2) and B3LYP calculations with 6-311++G(d,p) basis set. Full geometry optimisation has been performed for the complexes studied. The most stable structures of the complexes are established. Bearing in mind the corrected values of the dissociation energy the studied hydrogen-bonded complexes can be ordered as follows: C(2)H(4)...HONO(2)>C(2)H(4)...HONO-trans>C(2)H(4)...HONO-cis. In the complexes the acids act as proton donors forming the pi-type of hydrogen bond with ethene. The predicted changes in the vibrational characteristics (vibrational frequencies and infrared intensities) arising from the hydrogen bonding are in good agreement with the experimentally measured. The predicted frequency shift of the stretching OH vibration in the nitric acid is largest (-210 cm(-1)), followed by the shifts in the trans-HONO (-141 cm(-1)) and cis-HONO (-109 cm(-1)). The calculations predict an increase of the IR intensity of the stretching O-H vibration in the complexes from 6 to 10 times.     

Crystallization,crystal-structure refinement,and IR spectroscopy of a synthetic hexahydroborite analog

The crystal structure of the hexahydroborite analog Ca[B(OH)₄]₂ · 2H₂O (a = 7.9941(3) Å, b = 6.6321(2) Å, c = 7.9871(3) Å, β = 104.166(4)°, V = 410.58(3) ų, sp. gr. P2/c, Z = 2, ρ_calc = 1.891 g/cm³; Xcalibur S CCD automated diffractometer, 1196 reflections with I > 2σ(I), λMoK _α), which was synthesized by the hydrothermal method via the recrystallization of calciborite CaB₂O₄ (M) in the M ? B₂O₃ ? H₂O system (t = 250°C and P = 70–80 atm), was refined by the least-squares method with anisotropic displacement parameters (H atoms were located; R ₁ = 0.0260). The structure of synthetic hexahydroborite consists of infinite columns running along the c axis. The columns are formed by Ca polyhedra linked together and to [B(OH)₄] orthotetrahedra by sharing edges. Along the two other axes, the translationally equivalent columns are linked only by hydrogen bonds. The presence of a stronger bond between the discrete (Ca-B-O) columns along the shortest (b = 6.6 Å) axis accounts for the possibility of the shift of the columns by 1/2T _b and the formation of the second modification of Ca[B(OH)₄]₂ · 2H₂O. The crystals of synthetic hexahydroborite were studied by IR spectroscopy. A crystal-chemical analysis was performed for a series of natural metaborates with the general formula CaB₂O₄ · nH₂O (CaO: B₂O₃ = 1: 1, n = 0–6), including calciborite CaB₂O₄ and hexahydroborite CaB₂O₄ · 6H₂O as the end members.     

Crystal structure of Rb-elpasolite Rb2NaAlF6

Single crystals of Rb₂NaAlF₆, the Rb analogue of the mineral elpasolite, are obtained in the NaF-Rb₂CO₃-Al₂O₃-Rb₃PO₄-H₂O system under hydrothermal conditions, and their structure is determined by X-ray diffraction (R = 0.0188): a = 8.3087(1) Å, space group $Fm\bar 3m$ , Z = 4, and ρ_calcd = 3.88 g/cm³. The hypothesis that Rb elpasolite exists in nature in late associations of pegmatites enriched in rubidium is proposed.     

Quantum - Chemical Study of the Relation Between Electronic Structure and IR Spectra of Neutral Molecules,Anion - Radicals,and Dianions of Aromatic Nitriles

The relation between the IR data and the quantum-chemical indices. calculated by the CNDO/2 method has been studied. The linear relationship between the experimentally measured °C≡N and the corresponding Wiberg bond index (W_C≡N) has been found for the series of conjugated nitriles including neutral molecules. anion - radicals and dianions.

The changes in the intensity of the stretching C≡N vibration on transition from a neutral molecule into anion - radical and dianion have been investigated by the CNDO/2 method. The calculated values for δμ/δQ_CN show that in the order: neutral molecule, anion - radical and dianion the IR intensity of the stretching C≡N vibration significantly increases.     

Poly(propylene imine) dendrimer complexes as catalysts for oxidation of alkenes

Complexes of poly(propylene imine) dendrimers D8[DAB-dendr-(NH₂)₈] and D32 [DAB-dendr-(NH₂)₃₂] were prepared by interaction of the dendrimers with transition metal salts such as FeCl₃.6H₂O; CoCl₂.6H₂O; CuCl₂.2H₂O; VOSO₄.5H₂O; Na₂MoO₄.2H₂O and Na₂WO₄.2H₂O at room temperature in aqueous solutions. The content of metal ions in the complexes was found to be from 8.2 to 69.6 mg metal ion/g polymer carrier. The complexes were characterized by using IR, UV-VIS, Moessbauer spectroscopy and EPR. The anticipated co-ordination structure of the compounds was suggested. It was found that the order of the catalytic activity of the complexes of poly(propylene imine) dendrimers D8 and D32 in the reaction of epoxidation of cyclohexene with organic hydroperoxides such as tert-butyl hydroperoxide (t-BHP), ethylbenzene hydroperoxide (EBHP) and cumene hydroperoxide (CHP) was as follows: D32-MoО₂²⁺>D32-VО²⁺>D32-WО₂²⁺ > D32-Co²⁺ > D32-Cu²⁺>D32-Fe³⁺. The order of reactivity of organic hydroperoxides in the reaction studied was: t-BHP > EBHP > CHP.     

The first layer potassium–bismuth-nickel oxophosphate KBi<Subscript>4</Subscript>Ni<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>O<Subscript>4</Subscript>: Synthesis,crystal structure,and expected magnetic properties

The new potassium–bismuth–nickel oxophosphate obtained by hydrothermal synthesis in the Bi(OH)₃–NiCO₃–K₂CO₃–K₃PO₄ system is studied by X-ray diffraction, IR spectroscopy, and Raman spectroscopy. Parameters of the orthorhombic cell are as follows: a = 13.632(1) Å, b = 19.610(2) Å, and c = 5.4377(3) Å; V = 1452.64(2) ų; and space group Pnma. The structure is solved and refined to the final discrepancy factor R ₁ = 5.76% in the anisotropic approximation of atomic displacements using 3606 reflections with I > 2σ(I). The crystal-chemical formula (Z = 4) is KBi₄{Ni₂O₄(PO₄)₃}, where the composition of the layer nickel–phosphate polyanion is enclosed in braces. Theoretical calculations show that all exchange spin interactions between Ni²⁺ ions are antiferromagnetic and very weak (J < 0.1 cm^–1) because of the polyatomic character of bridging Ni–O–P–O–Ni and Ni–O–Bi–O–Ni groups. Thus, this compound is expected to be paramagnetic with very weak antiferromagnetic exchange interactions and appreciable energy of zero-field splitting of the spin levels of Ni²⁺ ions.