Hydrogen adsorption on M-ZSM-12 zeolite clusters (M = K,Na and Li): a density functional theory study

The molecular adsorption of hydrogen has been studied theoretically via DFT on additional framework with alkali metal atoms (K, Na and Li) in ZSM-12 zeolite. A 14T channel zeolite cluster model was used. Lewis acidity of alkali metals decreases with increasing atomic radius of alkali metal and H₂ adsorption. Adsorption enthalpy values were computed to be ?7.4 and ?5.1 kJ/mol on Li- and Na-ZSM-12 clusters, respectively. Hydrogen adsorption enthalpy values for Li- and Na-cases are meaningfully larger than the liquefaction enthalpy of hydrogen molecule. This designates that Li- and Na-ZSM-12 zeolites are potential cryoadsorbent materials for hydrogen storage.     

Alkali-metal promoted rhodium-on-alumina catalysts for nitrous oxide decomposition

Catalytic activity of γ-alumina supported rhodium catalysts in nitrous oxide decomposition into dinitrogen and dioxygen has been determined, the total conversion being reached at 450 °C. Rhodium is present at alumina surface in three forms: metal particles, Rh₂O₃ and rhodium zero valent atoms interacting with oxygen ions at the metal/support interface. Linear dependence of the catalytic activity on rhodium dispersion has been found. Deposition of alkali metal cations: Li, Na, K and Cs as promoters at the surface of alumina results in a considerable increase of rhodium dispersion and hence catalytic activity. The effect of promoters depends strongly on the speciation of alkali metals and rhodium used in the preparation of the catalyst. Both alkali metal cations and rhodium compete for the same OH groups at the alumina surface. The electronegativity of alkali metal oxides is much greater than that of alumina and their deposition increases the negative charge of surface oxide ions hindering the diffusion of rhodium and preventing the growth of its larger particles.     

Characterization of γ-alumina doped with Li and K by infrared studies of CO adsorption and27Al-NMR

The influence of alkali metals on the Lewis acid sites of gamma-alumina has been studied with NMR and IR of adsorbed CO. The physical properties of the alumina are slightly changed by the addition of low alkali metal concentrations. Li and K are adsorbed selectively on tetrahedral Lewis Al³⁺ sites of the alumina surface. These effects on the tetrahedral cations are more marked with increasing molar concentration and ionic radius of the alkali metal added to alumina. Results demonstrate that the influence of the alkali metals is not only due to steric factors but also to possible electronic modifications.This work was supported by a grant from CONICET, Argentina.     

Graft copolymerization of styrene with carbon black-alkali metal complex

It was found that an ion radical complex is formed by the reaction of carbon black with an alkali metal (Li, Na, K) in tetrahydrofuran. The graft copolymerization of styrene with carbon black was carried out using the ion radical complexes as initiator, and free polystyrene and carbon black-polystyrene graft copolymer were obtained. The identification of the graft copolymer with carbon black was carried out by fractionation and thermal degradation. The initiation activity of the alkali metals increased in the order Li < Na < K. The effect of the particle size of the carbon black on the graft copolymerization was studied.     

碱金属与碱土金属对电光源玻璃性能的影响

采用传统高温熔融法制备得到了多组无铅电光源玻璃,利用热膨胀仪对各组分玻璃的热膨胀性能进行测试.分析了不同氧化物替代Na2O后对热膨胀系数和膨胀软化温度的影响.结果显示:当三元碱金属氧化物(Li2O,Na2O与K2O)共存时,热膨胀性能出现复杂的混合碱效应;当碱土金属部分取代碱金属时,碱土金属氧化物对性能的影响并非按照离子的大小呈现一种简单的线性关系.玻璃的电阻率和介电性能不仅与碱金属的含量有关,还与其相对含量有关.     

(MMgBO3)n (n = 1, M = Li,Na, K,Rb and n = 4, M = Cs): An investigation on the structure transition and optical properties

In crystal engineering, modification of the crystal structure by cation substitution is one of the most important and efficient methods. This paper reveals the origin of the structure transition of a series of (MMgBO₃)_n (n = 1, M = Li, Na, K, Rb and n = 4, M = Cs) compounds. A force-equilibrium model was established with the comparison of different structures of these compounds. After substituted by other alkali metal cations with different atomic radii, the changed bond between oxygen and cations leads to structural transition. The electronic and optical properties of NaMgBO₃, KMgBO₃ and RbMgBO₃ were also investigated by using DFT methods to give more comprehension of these compounds.     

Synthesis of higher alcohols with cobalt and copper based model catalysts: effect of the alkaline metals

The synthesis of higher alcohols, obtained from CO hydrogenation over transition metals, is an alternative for the production of chemical commodities and less pollutant renewable fuels nowadays. In this work, the influence of different alkali metals (Li, Na, K, Cs) over the formation of active phases of supported catalysts, based on cobalt and copper, as well as the performance of these solids in the reaction of CO hydrogenation under a 50 bar pressure were studied. The solids were prepared by continuous coprecipitation of sodium carbonate with metal nitrate solutions (Al–Co, Al–Cu or Al–Co–Cu) promoted with the alkali metals with dry impregnation. Characterization studies by atomic absorption spectrophotometry, BET, XPS and also the results of catalytic tests show that physico-chemical properties of the obtained catalyst are similar to those of the industrial catalyst used as reference (I).     

Study of the poisoning of γ-Al2O3 by alkali metals addition

The effect of alkali metal addition to alumina on the acid characteristics and on the isopropanol dehydration capacity was analyzed. It was found that the incidence on the catalytic and acid properties of the alkali metal doped alumina depended on the ionic radii of the added element. Thus, K produces a higher poisoning of the acid function than Na and Li. Catalytic results can be interpreted in terms of a modification of the acid sites distribution by the alkali metal addition, the stronger acid sites being mainly affected.     

Catalytic combustion of soot. Effects of added alkali metals on CaO–MgO physical mixtures

The effect of adding alkali metals (Li, Na, K) to a CaO–MgO mixture, on the catalytic combustion of carbon black (CB), a model compound for soot, was studied. Catalysts were prepared by the Sol–Gel method and characterized by surface (BET surface area, XPS, DRIFTS) and bulk (AAS, XRD and TPR) techniques. Samples with a 4:1 catalyst-CB ratio were subjected to catalytic oxidation in a thermo-gravimetric apparatus and the temperature T_m, at which combustion occurs at its maximum rate, was recorded for comparison of catalytic activity. The addition of alkali metals (Li, Na, K) over the CaO–MgO mixture significantly increased the catalytic activity, due to the formation of surface oxygenated species that enhanced the oxidizing properties of the catalyst surface. That activity for CB combustion increases with the atomic number of the alkali metal contained in the catalyst. The presence of alkali metals also diminished the amount and stability of carbonates formed on the catalyst. The K-containing catalyst showed the largest activity for the catalytic CB combustion, because it shows the largest capacity to enrich its surface with α-oxygen type and promotes best the surface dissociation of that oxygen. Furthermore, surface-adsorbed OH and carbonate groups that disable the active sites and prevent the oxygen adsorption and dissociation, were less abundant and desorbed at lower temperatures, showing to be less stable on this K-containing catalyst.     

The possible use of alkali metal modified NbMCM-41 in the synthesis of 1,4-dihydropyridine intermediates

In this work we have prepared the catalysts containing all alkali metals (from Li to Cs) supported on mesoporous niobosilicate molecular sieve, NbMCM-41, and for comparison also loaded on silicate MCM-41. The materials were characterised by N₂ adsorption, XRD, and test reactions (acetonylacetone cyclisation and Knoevenagel condensation). It has been found that niobium located in the mesoporous material plays a role of structural promoter which prevents the solid from the disordering caused by the treatment with alkali metal solution. Moreover, it changes the catalytic properties of basic centres generated by alkali metal loading in relation to those observed for alkali metals supported on silicate MCM-41. Rb/NbMCM-41 has been proposed as active and selective catalyst in the condensation of benzaldehyde and different substituted benzaldehydes with ethyl acetoacetate towards the synthesis of intermediates in the preparation of some dihydropyridines.     

碱金属对CO加氢制备低碳醇Cu-Fe-Co基催化剂的影响

考察了碱金属(Li、Na、K、Cs)对CO加氢制备低碳醇Cu-Fe-Co基催化剂反应性能的影响.研究发现:碱金属的添加能明显降低甲烷化反应,提高催化剂活性,使醇分布向碳链增长方向转移;碱金属Na的添加对催化剂活性的提高影响最大,与未添加碱金属的催化剂相比,液体产物中总醇质量分数提高45.98%,其中C2+醇和C5+醇在总醇中的摩尔分数分别提高了21.88%和22.89%,尾气中CH4摩尔分数下降42.83%.XRD结果表明,碱金属的加入使Cu、Fe、Co三种组分在载体上分散性加强,增加了CO与活性组分的接触位点从而使碳链增长;FE-SEM结果表明,碱金属Na的添加提高了催化剂孔隙结构和活性组分的均匀性和稳定性,同时减轻了催化剂积炭现象,提高了醇收率.     

Synthesis of alkali-metal-doped C60 nanotubes

C₆₀ nanotubes have been synthesized by a solution-solution method. After degassing in a dynamic vacuum, the C₆₀ nanotubes doped with alkali metals by means of vapor evaporation method. Different temperatures have been studied to evaporate the alkali metals for the doping experiments. Raman spectrum was further employed to analyze the doping concentration of the obtained samples. It was found that all three alkali metals (Li, Na and K) used can be efficiently doped into the C₆₀ nanotubes, forming A_xC₆₀ nanotubes. The doping concentration of Li, Na changed from low to high level, depending on the experiment temperatures, while K doping always gave saturated doping. The melt points, the ionic sizes and vapor pressures of alkali metals were thought to affect the final doping results.     

Electrodeposition of the alkali metals from propylene carbonate

Journal of Applied Electrochemistry - The alkali metals were electrodeposited from various solutions in propylene carbonate (PC). Li, Na, K, Rb and Cs were electrodeposited from solutions of their...     

Evidence of Nitrate Formation from the Thermal Decay of Alkali Metal Dinitramides

The thermal decomposition products of five alkali metal dinitra-mides were examined by mass spectral and X-ray diffraction analysis. All five salt residues (Li, Na, K, Rb and Cs) showed no evidence of the parent dinitramide by mass spectrometry and showed only metal nitrate adducts to be present. X-ray diffraction patterns of the decay residues matched JCPDS reference patterns and confirmed that the dinitramide salts thermally decomposed to their respective alkali metal nitrate.     

High-Temperature Instability of Li- and Ta-Modified (K,Na)NbO3 Piezoceramics

This paper addresses the high-temperature instability of Li- and Ta-modified (K,Na)NbO₃ piezoceramics. The grains with abnormal size evolve out of the fine matrix grains during high-temperature annealing. They are found to be precipitates with a tetragonal tungsten bronze structure, which result from the volatilization and segregation of the alkali metal elements. With the growth of the abnormal grains the composition of the perovskite matrix phase also changes remarkably, as has been suggested by EDX analysis (for Na) and electric measurements (for Li). These variations lead to a large increase in the tetragonal/orthorhombic phase transition temperature and appreciable variations in the dielectric, ferroelectric, and piezoelectric properties of the ceramic samples. Control of the volatilization of the alkali metal elements can efficiently depress the abnormal grain growth and the compositional segregation.     

Ligands for selective metal ion extraction: A molecular modeling approach

We present a density functional theory (DFT) based study on interaction of alkali metal cations (Li⁺ and Na⁺) with macrocyclic crown ethers of different ring sizes. The minimum energy structures, binding energies, and binding enthalpies of crown ether–cation complexes have been determined with a correlated hybrid density functional, namely Becke’s three-parameter functional, B3LYP using a split valence basis function, 6-311++G(d, p). Geometry optimizations for all the crown ether–cation complexes were carried out with several initial guess structures based on semi-empirical PM3 optimized results. For both metal ions, the calculated values of binding energy and binding enthalpy increase with the increase in size of the crown ether ring, i.e. with the increase in the number of donor oxygen atoms in crown ether. The calculated values of gas phase binding energy for lithium ions are always higher than those for sodium ions in the case of all macrocyclic crown ethers studied at present. The calculated values of binding enthalpy are in good agreement with the reported experimental data.     

碱金属离子对丙烷脱氢催化剂中活性组分与载体间作用的影响

用程序升温还原（TPR）研究了添加Li^ 、Na^ 、K^ 等碱金属离子对丙烷脱氢催化剂中活性组分与载体间作用的影响。结果表明，Li^ Na^ 的引入对Sn-Al2O3之间的作用影响很小，而K^ 引入很大程度地削弱了PtSn/Al2O3催化剂中Sn组分与载体之间的相互作用，使得锡离子容易还原成金属态的Sn，易形成PtSn合金，使催化剂中毒。     

Temperature-Induced Structural Modifications Between Alkali Borate Glasses and Melts

High-temperature neutron diffraction and Raman spectra have been obtained on M₂O–2B₂O₃ (M=Li, Na, K) glasses and melts. Both techniques indicate a coordination change of boron atoms: the tetrahedral boron sites present in the glasses are converted into triangular boron sites. These changes of the borate network yield modifications of the alkali environment, as assessed for Li using the isotopic substitution technique. We observe that Li atoms are in a charge-compensating position in the glass and in a modifying position in the liquid. These structural modifications have important implications toward understanding the physical properties of borate melts.     

Mechanistic investigation of the reaction of thiourea with dialkyl acetylenedicarboxylates: a theoretical study

In this work, mechanistic investigation of the addition–cyclization reaction between thiourea (TU) and dialkyl acetylenedicarboxylate (DAAD) was made theoretically with the B3LYP method using a 6-311++G(2d,2p) basis set. In order to investigate the effect of substituted alkyl groups, the reaction was evaluated in the presence of dimethyl acetylenedicarboxylate, diethyl acetylenedicarboxylate and di-tert-butyl acetylenedicarboxylate. To evaluate the effect of structure and dielectric constant of the solvent, all structures were optimized in different solvent phases such as dichloromethane and acetone at the B3LYP/6-311++G(2d,2p) level of theory. Different orientations of alkyl groups make different kinetic paths. Results indicated that the reaction mechanism did not change with different orientations of COOMe, COOEt or COOtBu groups, but caused significant changes in the potential energy surfaces of the reaction coordinate. The reaction product (alkyl E-2-(2-imino-4-oxo-1,3-thiazolane-5-yliden) acetates) can be converted to different tautomeric forms in two proton transfer processes. So, in order to investigate the effect of different substituted alkyl groups, intermolecular proton transfer in the product was investigated both in gas phase and in the presence of methanol, ethanol and tert-butanol molecules as a bridge to facilitate the proton transfer process.     

Low Temperature Water–Gas Shift Reaction Over Alkali Metal Promoted Cobalt Carbide Catalysts

Low temperature water–gas shift (LT-WGS) was performed over various group I alkali metal (Li, Na, K, Rb, Cs) promoted cobalt carbide (Co₂C) catalysts at temperatures ranging from 453 to 573 K and atmospheric pressure. Cobalt carbide (Co₂C) was found to be active for the WGS reaction. The stability of the catalyst is related to the stability of the cobalt carbide phases under reaction conditions. Potassium promoted cobalt carbide catalysts exhibited higher activity and stability compared to the other alkali promoted catalysts for LT-WGS. X-ray diffraction analyses of fresh and used catalysts suggest that the origin of deactivation of the catalysts is primarily due to the chemical transition of cobalt from carbide to metal during WGS.