Production of light olefins such as ethylene, propylene and isobutylene from acetone was examined over ZSM-5 zeolites. These light olefins are produced from acetone over the acid sites of the zeolite via a series of consecutive reactions where olefins such as ethylene and propylene are obtained by cracking of isobutylene produced from aldol condensation products of acetone. Macro-and nano-sized ZSM-5 zeolites were prepared by conventional hydrothermal and emulsion methods, respectively, and the ZSM-5 zeolites with nearly the same acidity and BET surface area were obtained regardless of the crystal sizes. From SEM observations, the crystal sizes of the zeolites were approximately 2000 nm and 30-40 nm.These zeolites with different crystal sizes were applied to light olefins synthesis from acetone, and the effect of crystal size on catalytic activity and stability was investigated. As compared with the macro-seized zeolite, the nano-sized zeolite exhibited a high activity over a long lifetime. However, because the nano-sized zeolite possesses a large external surface area, undesirable reactions to form aromatics from the produced light olefins occurred on the acid sites located near the external surface. To inhibit aromatics formation, selective deactivation of the acid sites located near the outer surface of the zeolite was achieved via the catalytic cracking of silane (CCS) method using diphenyl silane (DP-silane). The CCS method was effective in deactivating the acid sites located near the external surface of the ZSM-5 zeolite.Moreover, the nano-size zeolite after the CCS treatment using DP-silane exhibited high olefins and low aromatics yields under high acetone conversion conditions.

Selective de-activation of acid sites located near the outer surface of zeolite was examined using the catalytic cracking of silane (CCS) method. From FT-IR analysis, adsorption species of silane compounds on the acid sites of zeolite during calcination were investigated. Moreover, the effects of types of silane compounds on the changes in zeolite properties, including the amounts of adsorbed ammonia and benzene within zeolite, prior to and after CCS treatment, were examined. The CCS method using diphenylmethylsilane (DPM-silane) was effective in de-activating the acid sites located near the outer surface of ZSM-5 zeolite. Olefin synthesis from acetone was carried out using ZSM-5 zeolite catalysts. The ZSM-5 zeolite after the CCS treatment using DPMsilane exhibited high olefin selectivity as well as low aromatic selectivity.

Abstract

A series of nickel(II) complexes comprising N,N,N′,N′-tetramethylethylenediamine (tmen), benzoylacetonate (bzac), and a halide anion (X), Ni(tmen)(bzac)X·n(H2O)(n = 1–4, X = Cl, Br, I), have been synthesized. A crystallographic study revealed that the chloride anion coordinates to the central metal, while bromide and iodide anions are located in the outer coordination sphere as the counter anion; the metal has a slightly distorted octahedral geometry, and the two monodentate ligands (aqua and chloro) are in the cis-position. In bromide and iodide complexes, having two water molecules of crystallization, a ladder-like hydrogen bond network is formed among halide anions, aqua ligands, and water of crystallization. In a chloride complex having no water of crystallization, a dimer is formed by strong intermolecular hydrogen bonding between the aqua ligand and the chloro and the benzoylacetonate ligands of the neighboring molecule. The behaviors of the complexes in solution vary, depending on the nature of the solvents, including deaquation and disproportionation, causing contrasting color changes of the solutions. The presence of a proposed intermediate in the disproportionation reaction, namely a 5-coordinated complex, was confirmed. Thermal gravimetric analysis suggests that these complexes lose water of crystallization, undergo deaquation–anation, and then lose the coordinated water before disproportionation into two ternary complexes.

The narrow-gap magnesium silicide semiconductor Mg 2 Si is a promising mid-temperature (600-900 K) thermoelectric material. It intrinsically possesses n-type conductivity, and n-type dopants are generally used for improving its thermoelectric performance; however, the synthesis of p-type Mg 2 Si is relatively difficult. In this work, the hole doping of Mg 2 Si with various impurity atoms is investigated by performing first principles calculations. It is found that the Ag-doped systems exhibit comparable formation energies ΔE calculated for different impurity sites (Mg, Si, and interstitial 4b ones), which may explain the experimental instability of their p-type conductivity. A similar phenomenon is observed for the systems incorporating alkali metals (Li, Na, and K) since their ΔE values determined for Mg (p-type) and 4b (n-type) sites are very close. Among boron group elements (Ga and B), Ga is found to be favorable for hole doping because it exhibits relatively small ΔE values for Si (p-type) sites. Furthermore, the interstitial insertion of Cl and F atoms into the crystal lattice leads to hole doping because of their high electronegativity.

No abstract

We theoretically investigate the impurity doping effects on the structural parameters such as lattice constant, atomic positions, and site preferences of impurity dopants for Al-doped magnesium silicide (Mg 2 Si) crystal using the first-principles calculation methods. We present comparison between several codes: ABCAP, Quantum Espresso, and Machikaneyama2002 (Akai KKR), which are based on the full-potential linearized augmented plane-wave method, the pseudopotential method, and KKR/GGA Green's function method, respectively. As a result, any codes used in the present study exhibit qualitative consistency both in the dependence of the lattice constants on the doping concentration and the energetic preference of the Al atom for the following sites; substitutional Si and Mg sites, and interstitial 4b site; in particular, ABCAP, which is based on the all-electron full-potential method, and Quantum Espresso, which is a code of the pseudopotential method, produce closely-resemble calculation results. We also discuss the effects of local atomic displacement owing to the presence of impurities to the structural parameters of a bulk. Using the analytical method considering the local atomic displacement, moreover, we evaluate the formation energy of Na-and B-doped systems as examples of p-type doping in order to examine the possilbility of realizing p-type Mg 2 Si.