Size Of Alkali Cation Research Articles

Alkali catalyzes methanethiol synthesis from CO and H2S

Alkali metals are commonly used to promote heterogeneous catalysts. For instance, K can enhance the MoS2-catalyzed synthesis of methanethiol, an important industrial chemical, from CO/H2/H2S. Herein, we not only demonstrate that the synergy increases with alkali cation size (Cs > Rb > K > Na) but also that alkali sulfides themselves are the active sites. The alkali-normalized methanethiol formation rate of Cs sulfide is nearly the same as that of Cs-promoted MoS2 with a much lower yield of unwanted methane. Kinetic measurements show that methanethiol on alkali sulfides is formed via the reaction 3 CO + 2 H2S → COS + CO2 + CH3SH. This represents a first example of H2S as a hydrogen donor for the formation of the thiol counterpart of methanol. The role of MoS2 is that of a promoter that disperses alkali and provides spillover H from H2 dissociation.

Synthesis, characterization and morphotropic transitions in a family of M[(UO2)(CH3COO)3](H2O) n (M=Na, K, Rb, Cs; n=0–1.0) compounds

Abstract Experimental investigations of crystallization in a family of uranyl triacetate compounds with Na, K, Rb and Cs were performed. The crystal structures of two novel Cs- and Rb-bearing tri(acetato)uranylates were solved, and the content of H2O molecules in the crystal structure of K-bearing uranyl triacetate was refined. Synthesized compounds were analyzed using IR spectroscopy and single-crystal X-ray diffraction. Crystal chemical analysis of the M[(UO2)(CH3COO)3](H2O) n family (M = Na, K, Rb, Cs; n = 0–1.0) reveals the sequence of structural transformations depending on the size of alkali cation resulting in the symmetry reduction from cubic P 213 (for Na), through tetragonal I 41/a (for K and Rb) to triclinic P 1̅ space groups (for Cs), which is in accordance with the principle of morphotropism, suggested by Paul von Groth, founder of the Zeitschrift für Krystallographie journal, in 1870.

Low-temperature excess heat capacity and boson peak of mixed alkali effect in borate glass

When two dissimilar kinds of alkaline ions are used to co-doping oxide glass, a maximum in the electric resistivity or the expansion coefficient appears, that is called the mixed alkali effect. The heat capacity Cp of several rapidly-quenched mixed alkali borate glass was measured using a relaxation calorimeter to investigate the mixed alkali effect. A broad peak of Cp/T3 was observed with the maximum Tmax in the range 5–15 K, which is related to the excess vibrational density of states (VDoS) of a non-Debye nature. Such a peak is called the boson peak and it is also observed as the low-energy peak of inelastic neutron and light scattering experiments. The negative deviation from the linear relation of single alkali borate glasses between the alkali cation size and Tmax was clearly observed in a mixed lithium cesium borate glass. The mechanism of this deviation was discussed on the basis of the changes in the BO4 units and the nonbridging oxygens by co-doping. The universal nature of the master plot of Cp/T3 also shows the deviation from that of pure borate glass by the mixed alkali effect.

Low-temperature heat capacity of alkali metal borate glass

As a universal feature of glass, its low-energy inelastic scattering spectra show a broad response known as the boson peak (BP). Since the BP is the peak of g(E)/E2, in which g(E) is the vibrational density of states (VDoS), it originates from the low-energy excess of VDoS over the Debye prediction. As another feature of the BP, namely, the excess part of the heat capacity, Cp, at low temperatures shows a broad peak in the Cp/T3 versus T plot, which is higher than the Debye level, and is also related to the excess VDoS. The heat capacity of rapidly quenched alkali metal borate glass, i.e., xM2O (1 − x)B2O3 of x = 0.22 and M = Na, K, Rb, and Cs, was measured between 2 and 150 K. Upon increasing the alkali cation size, the peak temperature of Cp/T3 decreases, while the peak value of Cp/T3 increases. The peak temperature of Cp/T3 shows a good correlation with the shear modulus. This correlation can be related to the variation of the contraction of boron–oxygen network as the alkali charge density increases with the decrease in the alkali cation size. The composition dependence of Cp/T3 was also studied for a cesium borate glass. The universal nature of Cp/T3 curves among alkali borate glass indicates that the distribution of the low-energy excess VDoS does not depend on the alkali metal and its content below x = 0.22.

Hydrogen sulfide oxidation on R E(R E = Sm, Y, La)–V–Sb catalysts: Effects of R E size and electronegativity

Three R E–V–Sb (R E was Sm, Y, and La) mixed-oxide catalysts were prepared for catalyzing the selective oxidation of hydrogen sulfide to sulfur. The multiphase R E–V–Sb catalysts were characterized by TPR, XRD, SEM and BET techniques. Synergistic phenomena in sulfur yield were observed with the solid-state reaction between R EVO 4 and α-Sb 2O 4 (antimony oxide/R EVO 4 weight ratio = 1/3), which was ascribed to the formation of SbVO 4 species. Under identical reaction conditions, areal reaction rate decreased in the following order: Sm–V–Sb > La–V–Sb ∼ LaVO 4 > Y–V–Sb > SmVO 4 > YVO 4. All R E–V–Sb catalysts exhibited 100% sulfur yield in a certain temperature range. The temperature window width for 100% sulfur yield decreased in the following order: La > Sm > Y, which was the same as the order of decreasing ionic radius (La > Sm > Y) and the order of increasing R E electronegativity (La < Sm < Y). The selectivity difference was explained in terms of active site isolation and product desorption effects. The rare earth cation size effect observed here for H 2S oxidation to elemental sulfur was similar to the alkali cation size effect reported for the hydrocarbon oxidation.

Poly(ethylene glycol) cationization with alkali metals in matrix-assisted laser desorption ionization investigated with the solvent-free method

Matrix-assisted laser desorption ionization (MALDI) was applied to determine the relative efficiencies for attachment of alkali cations to poly(ethylene glycol) polymers. The solvent-free sample preparation method was employed in order to discriminate the gas-phase polymer-cation complex formation mechanism from solubility and co-crystallization effects. We show that the cohesion energy of the cation precursor has a strong influence on the outcome of the MALDI measurements. This potential drawback is circumvented by the simultaneous use of alkali salts of similar lattice energy. The experiments include chain polymers of different length (PEG600 and PEG1000) and alkali cations of different size (Na+, K+, Cs+). We found that the attachment efficiencies decrease with growing cation size. This decrease is significantly more rapid for the shorter PEG600 polymers. These results corroborate the general trends observed in earlier studies, but correct the dependence on alkali cation size assessed in previous MALDI experiments with the conventional dried-droplet sample preparation method.

Phase formation, crystal structure, and electrical conductivity of triple phosphates of alkali metals and titanium

For triple phosphates of composition A′0.5A0.5 Ti2(PO4)3 (A−A′=Li−Na, Na−K, K−Rb), phase formation is studied, the crystal structure is refined, and the electrical conductivity is measured. The compounds are classified with the NaZr2(PO4)3 structure type (NZP, space group R\(\bar 3\)c). The phosphate frameworks are built of TiO6 octahedra and PO4 tetrahedra. Extraframework positions M1 are fully occupied by randomly distributed alkali cations. Positions M2 are vacant. Correlations are found between the structural distortion and electrical conductivity of the phosphates, on one hand, and the alkali cation size, on the other.

Computer-assisted structure simulation, synthesis, and phase formation of molybdophosphates A1−x Zr2(PO4 3−x (MoO4)x (A is an alkali metal)

Structure simulation is performed for molybdophates of variable composition A1−x Zr2(PO4)3−x (MoO4)x, where A is Na (0≤x≤0.6), K (0≤x≤0.6), K (0≤x≤0.3), Rb (0≤x≤0.2), or Cs (0≤x≤0.1), using the minimization of the interatomic interaction energy; these molybdophosphates crystallize in the NaZr2(PO4)3 (NZP) structure type. The results of the computer-assisted structure simulation are verified by the synthesis of the molybdophosphates and their characterization by X-ray powder diffraction and IR spectroscopy. The crystallization field of the NZP molybdophosphate shrinks as the alkali cation size increases. The key factors that govern the stability of the NZP structure in alkali zirconium molybdophosphates are determined.

Crystal-chemical and energetic systematics of wadeite-type phases A2BSi3O9 (A = K, Cs; B = Si, Ti, Zr)

Wadeite K2ZrSi3O9 and its analogues K2TiSi3O9 and Cs2ZrSi3O9, synthesized by high-temperature solid-state sintering, have been investigated using powder X-ray diffraction coupled with Rietveld analysis and high-temperature oxide melt solution calorimetry. The crystal chemistry and energetics of these phases, together with K2SiVISi3IVO9, a high-pressure wadeite analogue containing both tetrahedral and octahedral Si, are discussed in term of ionic substitutions. As the size of the octahedral framework cation increases, Si4+ → Ti4+ → Zr4+, the cell parameter c increases at a much higher rate than a. In contrast, increasing the interstitial alkali cation size (K+ → Cs+) results in a higher rate of increase in a compared with c. This behavior can be attributed to framework distortion around the interstitial cation. The enthalpies of formation from the constituent oxides (ΔHf,ox0) and from the elements (ΔHf,el0) have been determined from drop-solution calorimetry into 2PbO·B2O3 solvent at 975 K. The obtained values (in kJ/mol) are as follows: ΔHf,ox0 (K2TiSi3O9) = −355.8 ± 3.0, ΔHf,el0 (K2TiSi3O9) = −4395.1 ± 4.8, ΔHf,ox0 (K2ZrSi3O9) = −374.3 ± 3.3, ΔHf,el0 (K2ZrSi3O9) = −4569.9 ± 5.0, ΔHf,ox0 (Cs2ZrSi3O9) = −396.6 ± 4.4, and ΔHf,el0 (Cs2ZrSi3O9) = −4575.0 ± 5.5. The enthalpies of formation for K2SiVISi3IVO9 were calculated from its drop-solution enthalpy of an earlier study (Akaogi et al. 2004), and the obtained ΔHf,ox0 (K2SiSi3O9) = −319.7 ± 3.4 and ΔHf,el0 (K2SiSi3O9) = −4288.7 ± 5.1 kJ/mol. With increasing the size of the octahedral framework cation or of the interstitial alkali cation, the formation enthalpies become more exothermic. This trend is consistent with the general behavior of increasing energetic stability with decreasing ionic potential (z/r) seen in many oxide and silicate systems. Further, increasing the size of the octahedral framework cation appears to induce more rapid increase in stability than increasing the interstitial alkali cation size, suggesting that framework cations play a more dominant role in wadeite stability.

Thermochemistry of Framework Titanosilicate A2TiSi6O15 (A=K, Rb, Cs)

A new family of framework titanosilicates, A2TiSi6O15 (A=K, Rb, Cs) (space group Cc), has recently been synthesized using the hydrothermal method. This group of phases can potentially be utilized for storage of radioactive elements, particularly 137Cs, due to its high stability under electron radiation and chemical leaching. Here, we report the syntheses and structures of two intermediate members in the series: KRbTiSi6O15 and RbCsTiSi6O15. Rietveld analysis of powder synchrotron X‐ray diffraction data reveals that they adopt the same framework topology as the end‐members, with no apparent Rb/K or Rb/Cs ordering. To study energetics of the solid solution series, high‐temperature drop‐solution calorimetry using molten 2PbO·B2O3 as the solvent at 975 K has been performed for the end‐members and intermediate phases. As the size of the alkali cation increases, the measured enthalpies of formation from the constituent oxides () and from the elements () become more exothermic, suggesting that this framework structure favors the cation in the sequence Cs+, Rb+, and K+. This trend is consistent with the higher melting temperatures of A2TiSi6O15 phases with increase in the alkali cation size.

The structure of alkali germanophosphate glasses by Raman spectroscopy

A series of alkali germanophosphate glasses ((R 2O) x (GeO 2:P 2O 5) 1− x where R=Na, K and Rb) with variable GeO 2:P 2O 5 ratios (8:1, 6:1, 4:1 and 2:1) have been investigated using Raman spectroscopy. The glass network may be treated as being made up of separate germanate and phosphate components. Addition of alkali cations indicates that the alkalis preferentially modify the phosphate part of the network. Network depolymerization occurs by formation of Q 2 and Q 1 PO 4 tetrahedra. At high Ge:P ratios the glasses exhibit a density anomaly. The anomaly is attributed to the formation of small three-membered GeO 4 rings. However, alkali cation size and mass are contributing factors to the shape of the density anomaly in the K 2O and Rb 2O containing glasses. Depolymerization of the 2:1 alkali germanophosphate glasses is predominantly by formation of Q 2 PO 4 tetrahedra. At low Ge:P ratios (2:1) density trends are linear. No ring transition is observed in these glasses. Furthermore, the presence of [5]Ge cannot be ruled out, but if present, does not play a major role in generating a ‘germanate anomaly’.

The effect of alkali metal promotion on vanadium-containing catalysts in the vapour phase oxidation of methyl aromatics to the corresponding aldehydes

The comparison of vanadium phosphate catalysed ammoxidation with the oxidation of methyl aromatics to the corresponding nitriles and aldehydes showed significant differences in catalytic performance. The ammoxidation proceeds with nearly complete conversion and high nitrile selectivities, whereas the partial oxidation of the same feed to aldehydes under similar conditions leads to poor activity and selectivity only. This result is supposed to be due to the lack of a sufficient basicity in the reaction system; catalyst surface acidity causes strong reactant–solid interaction that is the reason for consecutive overoxidation. Therefore, potassium-containing vanadium phosphates and more basic alkali cation-containing vanadia solids (M/V 2O 5, M=Li, Na, K, Rb, Cs) were synthesised and used as catalysts in the vapour phase oxidation of toluene (TO), p-methoxytoluene (MTO) and p-chlorotoluene (CTO) to the corresponding aldehydes. Moreover, the effect of the addition of a non-oxidisable base (e.g. pyridine) to the reaction mixture was studied. Parent catalyst samples and used specimens were characterised by X-ray diffractometry and infrared spectroscopy. Vanadates and bronze phases of the more bulky alkali metal cations are generated during catalyst synthesis and catalytic reaction that may lead to new surface arrangements with vanadia. The catalytic results showed that alkali metal-containing vanadia catalysts revealed significant higher performance compared to vanadium phosphate solids. The increasing alkali cation size and basicity in the order from Li to Cs leads to an increase in the aldehyde selectivity. The effect of acid–base properties of catalyst bulk and surface in connection with reactant nucleophilicity and the impact of electronic effects of the substituents on the catalytic behaviour are discussed.

Dielectric and structural investigation of alkali triborate glasses

Dielectric data are provided for M 2O·3B 2O 3 glasses (M=Li, Na, K, Rb, Cs) over a broad frequency (5 Hz–2 MHz) and temperature ranges (400–600 K). Activation energies for ionic transport and the parameters of a KWW analysis are presented. The Kohlrausch–Williams–Watts (KWW) exponent, β, increases with alkali cation size. The mid- and far-infrared glass spectra suggest that from Li to Cs both the B–O and M–O bonding changes. The relative intensity of the high frequency far-infrared cation motion band was found to scale linearly with β. Also, β depends linearly on the activation energy, E σ , the microscopic barrier, E α , the interionic distance, d M–M, and the decoupling index at T g. Data available on triborate and trisilicate glasses show that a simple master correlation exists between β and T/ T g over the 0.45 < β < 0.75 and 0.35 < T/ T g < 0.9 ranges.

Na4C60: An Alkali Intercalated Two-Dimensional Polymer

The long-missing stoichiometric phase of ${\mathrm{Na}}_{4}{\mathrm{C}}_{60}$ has been identified by Rietveld analysis of synchrotron powder diffraction data. Its monoclinic structure is based on polymer planes of ${\mathrm{C}}_{60}$ where each molecule forms four ``single'' bonds within the plane. This compound is not only the first fulleride polymer with such bonds, but also the first two-dimensional polymer which is naturally intercalated with alkali ions and can be synthesized at ambient pressure. ${\mathrm{Na}}_{4}{\mathrm{C}}_{60}$ is a metal and is expected to be a prototype structure where electronic overlap between the planes can be tuned by the alkali cation size.

"Prepeaks" and "first sharp diffraction peaks" in computer simulations of strong and fragile ionic liquids.

Computer simulations with polarizable anions are shown to reproduce the distinctive prepeak observed in diffraction studies of molton ${\mathrm{ZnCl}}_{2}$ and its mixtures with alkali chlorides. Two distinct origins for the peak in the microscopic structure of the melts are identified. The influence of alkali cation size on the observed behavior is explained. The relationship between the observation of prepeaks and the existence of network structures in strong ionic liquids is discussed.

Defect-induced smoothing of the superionic phase transition in Cs 1− xM xHSO 4 protonic conductors. III. Rubidium substitution

The preparation of Cs 1− x Rb x HSO 4 crystals allows study of the influence of lattice defects on the superionic conductivity. superionic phase transition disappears above x=0.33 and a major structural change takes place at x=0.5. Above x=0.9 ferroelectric ordering is observed below 250 K. Glassy phase containing S 2O 2− 7 defects is easily obtained by quenching in the 0.3< x<0.8 composition range. The influence of alkali cation size on properties is discussed.

New insights into the structure of alkali borate glasses

The Raman and far-infrared spectra of alkali borate glasses up to the metaborate composition have been measured and analysed to elucidate the role of the alkali metal cation on the glass structure. It is demonstrated that the smaller cations favour borate units containing boron tetrahedra, while non-bridging oxygens are systematically formed upon increasing alkali cation size.

Stabilities of fast atom bombardment desorbed alkali metal adducts of pyrimidine molecules

A pyrimidine derivative has been used as a model compound to evaluate the relative stabilities of protonated and alkali cationized molecules formed via fast atom bombardment (liquid secondary ion mass spectrometry); stabilities are shown to increase with increasing alkali cation size. Relative abundances of daughter ions formed by competitive decompositions (losses of NH 3, CH 4) have been analyzed with respect to the initial internal energy content of protonated and cationized pyrimidine molecules. Comparisons are made of metastable and collisionally-activated decompositions, and of unimolecular decompositions over the course of liquid matrix depeletion. Relative activation energies and frequency factors for ammonia and methane eliminations have been qualitatively assessed. The mechanism for ammonia loss is discussed in the light of results obtained from deuterium labeling of mobile protons.

Crystal structure of LiH2PO4, structural topology and hydrogen bonding in the alkaline dihydrogen orthophosphates

Abstract LiH2PO4 crystallizes in space group Pna21; a = 6.253(4), b = 7.656(4), c = 6.881(4) Å, Z = 4. With 1268 observed reflections measured on a single-crystal diffractometer (MoKα radiation), the structure was solved (Patterson methods) and refined anisotropically (least-squares) to R = 0.022. The hydrogen atoms had been located from difference maps. LiO4 coordination polyhedra are linked by vertices and form [100] isolated chains. The two independent hydrogen bridges, 2.684 and 2.564 Å long, connect the PO4 polyhedra building up a three-dimensional framework; [100] channels containing H atoms are observed in the structure. Similarities with the structure of NaH2PO4 · H2O are discussed. The effect of increasing the alkali cation size in the series MH2PO4 causes a progressive condensation of the coordination polyhedra by sharing vertices and edges, from single chains (LiH2PO4) to very compact frameworks (CsH2PO4) of polyhedra. Shorter hydrogen bonds are observed when larger alkali cations are present; this effect is discussed in the light of bond-strength balance concepts.

Further studies on the action of alkali metal hydroxides on cotton

AbstractX‐Ray diagrams revealed that the conversion of the lattice structure from cellulose I to cellulose II is substantially complete in cotton treated at 0°C with LiOH, NaOH, or KOH of approximately 5N concentration. With concentrations less than 5N, there were marked differences in the ability of these reagents to cause lattice conversions in cotton. Below a minimum concentration which is dependent on both the alkali and the treatment temperature, conversion from cellulose I to cellulose II cannot take place in cotton irrespective of the swelling caused by the reagent. We suggest that extensive swelling by one of these alkalis is not sufficient by itself to cause conversion from cellulose 1 to cellulose II and that the concentration of the alkali is of prime importance. There was no direct relation between the swelling induced in cotton by treatment with LiCH, NaOH, or KOH at 0°C and the sorption ratio of the resulting product. Also, although treatment with 5N KOH caused less swelling than that obtained with either 5N NaOH or LiOH, KOH reduced the level‐off degree of polymerization (LODP) of cotton most, indicating the importance of the size of alkali cation in reducing crystallite length. In additional experiments, cotton was treated at 21°C with a solution of 4.7N NaOH to which boric acid had been added. Although the swelling of the cotton was little affected, the addition of the boric acid caused the sorption ratio of the product to decrease, whereas the LODP increased. Also conversion of the lattice structure from cellulose I to cellulose II was inhibited.