Heterogenization of [Ti(η-C5HMe4)Cl3] on to MCM-41 and organomodified MCM-41 to form epoxidation catalyst

This paper describes the heterogenization of a tetramethylmonocyclopentadienyl titanium (IV) trichloride complex, [Ti(η⁵-C₅HMe₄)Cl₃] onto mesoporous MCM-41. Its immobilization has been performed via a straightforward grafting process of the organometallic precursor in the pores of an MCM-41 host material and by reaction with previously organomodified MCM-41 material with a hydroxyl triazine based compound. Applying all-silica MCM-41 hosts, stable and heterogeneous liquid-phase epoxidation catalysts are obtained. Powder X-ray diffraction and nitrogen adsorption-desorption analysis indicated that the structural integrity of the support has been preserved during the titanium complex immobilization. These materials have been also extensively characterized using diffuse reflectance UV-vis, ¹³C and ²⁸Si MAS NMR and FT-IR spectroscopy. With these techniques the strong adsorption of the intact catalytic complex within an all-silica MCM-41 host is demonstrated. These materials have been tested as catalyst for the epoxidation of aliphatic and aromatic alkenes with TBHP as oxidant exhibiting a significant selectivity toward the epoxide with negligible leaching of titanium species. The conversion values are moderated, being the olefin trend reactivity 1-octene > cyclohexene > styrene.     

Depth profile characterization of Zn-TiO2 nanocomposite films by pulsed radiofrequency glow discharge-optical emission spectrometry

In recent years particular effort is being devoted towards the development of radiofrequency (rf) pulsed glow discharges (GDs) coupled to optical emission spectrometry (OES) for depth profile analysis of materials with technological interest. In this work, pulsed rf-GD-OES is investigated for the fast and sensitive depth characterization of Zn-TiO₂ nanocomposite films deposited on conductive substrates (Ti and steel). The first part of this work focuses on assessing the advantages of pulsed GDs, in comparison with the continuous GD, in terms of analytical emission intensities and emission yields. Next, the capability of pulsed rf-GD-OES for determination of thickness and compositional depth profiles is demonstrated by resorting to a simple multi-matrix calibration procedure. A rf forward power of 75 W, a pressure of 600 Pa, 10 kHz pulse frequency and 50% duty cycle were selected as GD operation parameters.Quantitative depth profiles obtained with the GD proposed methodology for Zn-TiO₂ nanocomposite films, prepared by the occlusion electrodeposition method using pulsed reverse current electrolysis, have proved to be in good agreement with results achieved by complementary techniques, including scanning electron microscopy and inductively coupled plasma-mass spectrometry. The work carried out demonstrates that pulsed rf-GD-OES is a promising tool for the fast analytical characterization of nanocomposite films.     

Cold vapor-solid phase microextraction using amalgamation in different Pd-based substrates combined with direct thermal desorption in a modified absorption cell for the determination of Hg by atomic absorption spectrometry

In this work, different Pd-based substrates (i.e. Pd wire, Pd-coated stainless steel wire and Pd-coated SiO₂) are tried for microextraction of Hg prior to its release into a modified quartz T-cell so as to develop a cost-effective, sensitive and easy-to-handle coupling between solid-phase microextraction (SPME) and atomic absorption spectrometry. The new design allows a direct sample injection from the SPME device into a quartz T-cell thus avoiding analyte dilution. Mercury amalgamation onto a Pd wire provided the best performance in respect to sensitivity and fiber lifetime, but Pd wires could not be implemented in the SPME device due to their poor mechanical characteristics. On the contrary, Pd-coated SiO₂ fibers could be easily adapted to the typical sampling device used for SPME. Narrow time-dependent absorption signal profiles that could be integrated within 25 s were obtained. The detection limit was 90 pg mL⁻¹ of Hg, and the repeatability expressed as relative standard deviation was 4.3%.     

Carbon materials as electrodes for electrosorption of NaCl in aqueous solutions

Different porous carbons (MWCNT, a carbon aerogel, an activated carbon cloth and a chemically activated carbon) were evaluated as electrode material for the electrosorption of NaCl. The results obtained from the chronoamperometric experiments were correlated to the surface area and the size of the pores present in each carbon. These results indicate that all the surfaces are equivalent for the electrosorption process, demonstrating that both, mesopores and micropores, are equally effective. Nevertheless, the kinetics of the process is influenced by the pore size distribution of the carbon, although it is rather fast for all the carbons studied. The chemically activated carbon seems to be the most suitable carbon material for electrosorption of NaCl due to the combination of a high surface area and an appropriate pore size distribution.     

Harmonic mappings and conformal minimal immersions of Riemann surfaces into {\mathbb {R}^{\rm N}}

We prove that for any open Riemann surface ${\mathcal{N}}$ , natural number N ≥ 3, non-constant harmonic map ${h:\mathcal{N} \to \mathbb{R}}$ ^N?2 and holomorphic 2-form ${\mathfrak{H}}$ on ${\mathcal{N}}$ , there exists a weakly complete harmonic map ${X=(X_j)_{j=1,\ldots,{\sc N}}:\mathcal{N} \to \mathbb{R}^{\sc N}}$ with Hopf differential ${\mathfrak{H}}$ and ${(X_j)_{j=3,\ldots,{\sc N}}=h.}$ In particular, there exists a complete conformal minimal immersion ${Y=(Y_j)_{j=1,\ldots,{\sc N}}:\mathcal{N} \to \mathbb{R}^{\sc N}}$ such that ${(Y_j)_{j=3,\ldots,{\sc N}}=h}$ . As some consequences of these results (1) there exist complete full non-decomposable minimal surfaces with arbitrary conformal structure and whose generalized Gauss map is non-degenerate and fails to intersect N hyperplanes of ${\mathbb{CP}^{{\sc N}-1}}$ in general position. (2) There exist complete non-proper embedded minimal surfaces in ${\mathbb{R}^{\sc N},}$ ${\forall\,{\sc N} >3 .}$      

A Binomial Approximation Method for the Ising Model

A large portion of the computation required for the partition function of the Ising model can be captured with a simple formula. In this work, we support this claim by defining an approximation to the partition function and other thermodynamic quantities of the Ising model that requires no algorithm at all. This approximation, which uses the high temperature expansion, is solely based on the binomial distribution, and performs very well at low temperatures. At high temperatures, we provide an alternative approximation, which also serves as a lower bound on the partition function and is trivial to compute. We provide theoretical evidence and the results of numerical experiments to support the strength of these approximations.     

Symmetries of Equivelar 4-Toroids

We derive some general results on the symmetries of equivelar toroids and provide detailed analysis of the subgroup lattice structure of the dihedral group D ₄ and of the octahedral group to complete classification by symmetry type of those in ranks 3 and 4.     

Development of a Method for the Determination of Polychlorinated Biphenyls in Microplastics Present in Marine Samples

Journal of Analytical Chemistry - In this work, a method for the determination of nine polychlorinated biphenyls (PCBs) in three polymer types (polyethylene terephthalate, PET; unplasticized...     

Experimental and Theoretical Study of Two Pyridinium-Based Ionic Liquids

Thermophysical properties of two pyridinium-based ionic liquids, 1-ethyl-2-methylpyridinium bis(trifluoromethylsulfonyl)imide and 1-propyl-2-methylpyridinium bis(trifluoromethylsulfonyl)imide, have been measured from 278.15 to 323.15?K, with a temperature step of 2.5?K. The properties measured were: densities, speeds of sound, refractive indices, surface tensions, isobaric molar heat capacities, electrical conductivities and viscosities. Thermal properties were also recorded in the temperature range from 100 to 320?K. From the experimental results coefficients of thermal expansion, isentropic compressibilities, molar refraction and surface enthalpies and entropies have been determined. Moreover, a theoretical study has been performed using ab initio calculations, level of theory HF/6-31G(d). From this study, we have obtained qualitative information about the magnitude and the directionality of the cation?Canion interactions which allows a better understanding of the physico-chemical properties of these ionic liquids.