Proton conduction in alkali metal ion-exchanged porous ionic crystals.

Proton conduction in alkali metal ion-exchanged porous ionic crystals A2[Cr3O(OOCH)6(etpy)3]2[α-SiW12O40]·nH2O [I-A+] (A = Li, Na, K, Cs, etpy = 4-ethylpyridine) is investigated. Single crystal and powder X-ray diffraction measurements show that I-A+ possesses analogous one-dimensional channels where alkali metal ions (A+) and water of crystallization exist. Impedance spectroscopy and water diffusion measurements of I-A+ show that proton conductivities are low (10-7-10-6 S cm-1) under low relative humidity (RH), and protons mostly migrate as H3O+ with H2O as vehicles (vehicle mechanism). The proton conductivity of I-A+ increases with the increase in RH and is largely dependent on the types of alkali metal ions. I-Li+ shows a high proton conductivity of 1.9 × 10-3 S cm-1 (323 K) and a low activation energy of 0.23 eV under RH 95%. Under high RH, alkali metal ions with high ionic potentials (e.g., Li+) form a dense and extensive hydrogen-bonding network of water molecules with mobile protons at the periphery, which leads to high proton conductivities and low activation energies via rearrangement of the hydrogen-bonding network (Grotthuss mechanism).

Synergetic effect in heterogeneous acid catalysis by a porous ionic crystal based on Al(iii)-salphen and polyoxometalate.

A porous ionic crystal is synthesized with a cationic Al(iii)-salphen complex (Al(iii)-salphen) and a α-Keggin-type polyoxometalate (POM). The compound possesses stable three dimensional porous structure and shows high activity as a heterogeneous catalyst in pinacol rearrangement, which is a typical acid reaction. Notably, Al(iii)-salphen, POM, and a physical mixture of the two components are much less active, suggesting a synergetic effect of Al(iii)-salphen and POM in a porous framework.

Conductive Hybrid Crystal Composed from Polyoxomolybdate and Deprotonatable Ionic-Liquid Surfactant.

A polyoxomolybdate inorganic-organic hybrid crystal was synthesized with deprotonatable ionic-liquid surfactant. 1-dodecylimidazolium cation was employed for its synthesis. The hybrid crystal contained δ-type octamolybdate (Mo8) isomer, and possessed alternate stacking of Mo8 monolayers and interdigitated surfactant bilayers. The crystal structure was compared with polyoxomolybdate hybrid crystals comprising 1-dodecyl-3-methylimidazolium surfactant, which preferred ß-type Mo8 isomer. The less bulky hydrophilic moiety of the 1-dodecylimidazolium interacted with the δ-Mo8 anion by N-H···O hydrogen bonds, which presumably induced the formation of the δ-Mo8 anion. Anhydrous conductivity of the hybrid crystal was estimated to be 5.5 × 10(-6) S·cm(-1) at 443 K by alternating current (AC) impedance spectroscopy.

Reduction-Induced Highly Selective Uptake of Cesium Ions by an Ionic Crystal Based on Silicododecamolybdate.

Cation adsorption and exchange has been an important topic in both basic and applied chemistry relevant to materials synthesis and chemical conversion, as well as purification and separation. Selective Cs(+) uptake from aqueous solutions is especially important because Cs(+) is expensive and is contained in radioactive wastes. However, the reported adsorbents incorporate Rb(+) as well as Cs(+) , and an adsorbent with high selectivity toward Cs(+) has not yet been reported. Highly selective uptake of Cs(+) by an ionic crystal (etpyH)2 [Cr3O(OOCH)6 (etpy)3]2 [α-SiMo12 O40 ]⋅3 H2O (etpy =4-ethylpyridine) is described. The compound incorporated up to 3.8 mol(Cs(+) ) mol(s)(-1) (where s=solid) by cation-exchange with etpyH(+) and reduction of silicododecamolybdate with ascorbic acid. The amount of Cs(+) uptake was comparable to that of Prussian blue, which is widely recognized as a good Cs(+) adsorbent. Moreover, other alkali-metal and alkaline-earth-metal cations were almost completely excluded (<0.2 mol mol(s)(-1)).

A functional mesoporous ionic crystal based on polyoxometalate.

A mesoporous ionic crystal is synthesized with a polyoxometalate and a macrocation with polar cyano groups. The compound possesses one-dimensional mesopores with an opening of 3.0 × 2.0 nm. The compound shows high proton conductivity and catalytic activity, which are due to the water molecules in the mesopores.

Concerted functions of anions and cations in a molecular ionic crystal with stable three-dimensional micropores.

The molecular ionic crystal [Cr3O(OOCCH═CH2)6(H2O)3]3[α-PW12O40]·15H2O [Ia] with stable three-dimensional micropores and a minimum aperture of 3.3 Å was synthesized with a phosphododecatungstate [α-PW12O40](3-) (polyoxometalate, POM) and a macrocation with acrylate ligands [Cr3O(OOCCH═CH2)6(H2O)3](+). The porous structure of Ia was basically constructed by an arrangement of macrocations forming a six-membered ring: vinyl groups (CH═CH2) of adjacent macrocations were aligned parallel to each other, suggesting a weak dispersion force between them. A guest-free phase [Cr3O(OOCCH═CH2)6(H2O)3]3[α-PW12O40] [Ib] was formed by the treatment of Ia in vacuo at room temperature without any structure change. Compound Ib showed shape-selective sorption of CO2 and C2H2 (molecular size = 3.3 Å) over N2 (3.6 Å) and methane (3.7 Å), and the sorption enthalpy of C2H2 was larger than that of CO2. The high affinity toward C2H2 was further confirmed as follows: the Monte Carlo simulations of the optimized geometries of C2H2 in Ib showed that both hydrogen atoms were in the vicinity of the surface oxygen atoms of POMs. The gas sorption profiles showed a much faster diffusion for C2H2. All these results suggest that the anion and cation mainly play the guest-binding and structure-directing roles, respectively, (i.e., concerted functions) in an ionic crystal with stable three-dimensional micropores.

Highly selective sorption and unique packing geometries of unsaturated hydrocarbons and CO2 in a fluorine-substituted organic-inorganic ionic crystal.

An ionic crystal [Cr3O(OOCCF3)6(H2O)3]3[α-PW12O40]·8CH3COCH3·8H2O·CHCl3 [Ia] was synthesized by the complexation of a fluorine-substituted macrocation with a phosphododecatungstate. Compound Ia possessed a layered structure with an interlayer distance of ca. 3.5 Å, and the inner surface of the layer was decorated with CF3 groups of the macrocations. The solvent molecules (acetone and chloroform) spontaneously desorbed and exchanged with water under an ambient atmosphere and a hydrated phase [Cr3O(OOCCF3)6(H2O)3]3[α-PW12O40]·25H2O [Ib] was formed. The water molecules were partially desorbed by the treatment of Ibin vacuo or under a dry N2 or He flow at 298-303 K, and [Cr3O(OOCCF3)6(H2O)3]3[α-PW12O40]·15H2O [Ic] was formed. The powder XRD pattern of Ic well agreed with that calculated for Ia, showing that the structure was maintained after the exchange and partial desorption of guests. Compound Ic sorbed CO2 and unsaturated hydrocarbons, while saturated hydrocarbons such as ethane and methane were almost excluded despite the similar kinetic diameters. Acetylene/methane, CO2/methane, and ethylene/ethane sorption ratios were 13, 15, and 4.9, respectively, at 198 K and 100 kPa. Typical Monte Carlo-based optimized geometries of acetylene and CO2 showed different alignments in the structure of Ic despite the similar amounts of sorption and molecular sizes. Acetylene and CO2 were aligned mainly parallel and perpendicular, respectively, to the layer, which is probably due to the differences in the electron density distributions of the HOMO orbitals.

Metabolic mechanism of mannan in a ruminal bacterium, Ruminococcus albus, involving two mannoside phosphorylases and cellobiose 2-epimerase: discovery of a new carbohydrate phosphorylase, ß-1,4-mannooligosaccharide phosphorylase.

Ruminococcus albus is a typical ruminal bacterium digesting cellulose and hemicellulose. Cellobiose 2-epimerase (CE; EC 5.1.3.11), which converts cellobiose to 4-O-ß-D-glucosyl-D-mannose, is a particularly unique enzyme in R. albus, but its physiological function is unclear. Recently, a new metabolic pathway of mannan involving CE was postulated for another CE-producing bacterium, Bacteroides fragilis. In this pathway, ß-1,4-mannobiose is epimerized to 4-O-ß-D-mannosyl-D-glucose (Man-Glc) by CE, and Man-Glc is phosphorolyzed to α-D-mannosyl 1-phosphate (Man1P) and D-glucose by Man-Glc phosphorylase (MP; EC 2.4.1.281). Ruminococcus albus NE1 showed intracellular MP activity, and two MP isozymes, RaMP1 and RaMP2, were obtained from the cell-free extract. These enzymes were highly specific for the mannosyl residue at the non-reducing end of the substrate and catalyzed the phosphorolysis and synthesis of Man-Glc through a sequential Bi Bi mechanism. In a synthetic reaction, RaMP1 showed high activity only toward D-glucose and 6-deoxy-D-glucose in the presence of Man1P, whereas RaMP2 showed acceptor specificity significantly different from RaMP1. RaMP2 acted on D-glucose derivatives at the C2- and C3-positions, including deoxy- and deoxyfluoro-analogues and epimers, but not on those substituted at the C6-position. Furthermore, RaMP2 had high synthetic activity toward the following oligosaccharides: ß-linked glucobioses, maltose, N,N'-diacetylchitobiose, and ß-1,4-mannooligosaccharides. Particularly, ß-1,4-mannooligosaccharides served as significantly better acceptor substrates for RaMP2 than D-glucose. In the phosphorolytic reactions, RaMP2 had weak activity toward ß-1,4-mannobiose but efficiently degraded ß-1,4-mannooligosaccharides longer than ß-1,4-mannobiose. Consequently, RaMP2 is thought to catalyze the phosphorolysis of ß-1,4-mannooligosaccharides longer than ß-1,4-mannobiose to produce Man1P and ß-1,4-mannobiose.