Dynamic Copper Site Redispersion through Atom Trapping in Zeolite Defects.

Single-site copper-based catalysts have shown remarkable activity and selectivity for a variety of reactions. However, deactivation by sintering in high-temperature reducing environments remains a challenge and often limits their use due to irreversible structural changes to the catalyst. Here, we report zeolite-based copper catalysts in which copper oxide agglomerates formed after reaction can be repeatedly redispersed back to single sites using an oxidative treatment in air at 550 °C. Under different environments, single-site copper in Cu-Zn-Y/deAlBeta undergoes dynamic changes in structure and oxidation state that can be tuned to promote the formation of key active sites while minimizing deactivation through Cu sintering. For example, single-site Cu2+ reduces to Cu1+ after catalyst pretreatment (270 °C, 101 kPa H2) and further to Cu0 nanoparticles under reaction conditions (270-350 °C, 7 kPa EtOH, 94 kPa H2) or accelerated aging (400-450 °C, 101 kPa H2). After regeneration at 550 °C in air, agglomerated CuO was dispersed back to single sites in the presence and absence of Zn and Y, which was verified by imaging, in situ spectroscopy, and catalytic rate measurements. Ab initio molecular dynamics simulations show that solvation of CuO monomers by water facilitates their transport through the zeolite pore, and condensation of the CuO monomer with a fully protonated silanol nest entraps copper and reforms the single-site structure. The capability of silanol nests to trap and stabilize copper single sites under oxidizing conditions could extend the use of single-site copper catalysts to a wider variety of reactions and allows for a simple regeneration strategy for copper single-site catalysts.

Oxidation and hydration of U3O8 materials following controlled exposure to temperature and humidity.

Chemical signatures correlated with uranium oxide processing are of interest to forensic science for inferring sample provenance. Identification of temporal changes in chemical structures of process uranium materials as a function of controlled temperatures and relative humidities may provide additional information regarding sample history. In this study, a high-purity α-U3O8 sample and three other uranium oxide samples synthesized from reaction routes used in nuclear conversion processes were stored under controlled conditions over 2-3.5 years, and powder X-ray diffraction analysis and X-ray absorption spectroscopy were employed to characterize chemical speciation. Signatures measured from the α-U3O8 sample indicated that the material oxidized and hydrated after storage under high humidity conditions over time. Impurities, such as uranyl fluoride or schoepites, were initially detectable in the other uranium oxide samples. After storage under controlled conditions, the analyses of the samples revealed oxidation over time, although the signature of the uranyl fluoride impurity diminished. The presence of schoepite phases in older uranium oxide material is likely indicative of storage under high humidity and should be taken into account for assessing sample history. The absence of a signature from a chemical impurity, such as uranyl fluoride hydrate, in an older material may not preclude its presence at the initial time of production. LA-UR-15-21495.

Aerobic oxidation of pinacol by vanadium(V) dipicolinate complexes: evidence for reduction to vanadium(III).

The reactivity of vanadium complexes bearing the ligand dipicolinic acid (H(2)dipic) with alcohols has been explored. Dipic vanadium complexes are able to catalyze the aerobic oxidative C-C bond cleavage of pinacol. Reaction under anaerobic conditions allowed for isolation of a V(III) mu-oxo dimer, supporting the involvement of V(III) in aerobic oxidation reactions. Stoichiometric oxidation of unactivated aliphatic alcohols has also been observed, with oxidation of cyclobutanol producing cyclobutanone in 93% yield. The absence of ring-opening products in this reaction provides further support for the involvement of V(III) intermediates.

The Conversion of Starch and Sugars into Branched C10 and C11 Hydrocarbons.

Oligosaccharides, such as starch, cellulose, and hemicelluloses, are abundant and easily obtainable bio-derived materials that can potentially be used as precursors for fuels and chemical feedstocks. To access the pertinent molecular building blocks (i.e., 5- or 6-carbon containing sugar units) located within these biopolymers and transform them into useful fuel precursors, oligosaccharide depolymerization followed by chain extension is required. This chain extension can readily be performed via a Garcia-Gonzalez-like approach using ß-diketones under mild conditions to provide fuel precursors containing an increased carbon atom content that meets fuel requirements. In a subsequent step, ring opening and hydrodeoxygenation chemistry of these species allows for the preparation of branched alkanes under relatively mild conditions. This approach can be applied to monomeric sugars (glucose and xylose), oligosaccharides (starch), and potentially to hydrolyzed dedicated energy crops to allow the conversion of real biomass into fuel type molecules.

The Effect of Functional Groups in Bio-Derived Fuel Candidates.

Interest in developing renewable fuels is continuing to grow and biomass represents a viable source of renewable carbon with which to replace fossil-based components in transportation fuels. During our own work, we noticed that chemists think in terms of functional groups whereas fuel engineers think in terms of physical fuel properties. In this Concept article, we discuss the effect of carbon and oxygen functional groups on potential fuel properties. This serves as a way of informing our own thinking and provides us with a basis with which to design and synthesize molecules from biomass that could provide useful transportation fuels.

Synthesis of Acetone-Derived C6 , C9 , and C12 Carbon Scaffolds for Chemical and Fuel Applications.

A simple, inexpensive catalyst system (Amberlyst 15 and Ni/SiO2 -Al2 O3 ) is described for the upgrading of acetone to a range of chemicals and potential fuels. Stepwise hydrodeoxygenation of the produced ketones can yield branched alcohols, alkenes, and alkanes. An analysis of these products is provided, which demonstrates that this approach can provide a product profile of valuable bioproducts and potential biofuels.

Coordination of pertechnetate [TcO4]- to actinides.

The ability of [TcO(4)](-) to coordinate directly to tetra- and hexa-valent actinides in the presence of organic P[double bond, length as m-dash]O ligands is confirmed in the crystallographically characterised complexes [UO(2)(TcO(4))(2)(Ph(3)PO)(3)] and [Th(TcO(4))(4)((n)Bu(3)PO)(4)].

The hydrodeoxygenation of bioderived furans into alkanes.

The conversion of biomass into fuels and chemical feedstocks is one part of a drive to reduce the world's dependence on crude oil. For transportation fuels in particular, wholesale replacement of a fuel is logistically problematic, not least because of the infrastructure that is already in place. Here, we describe the catalytic defunctionalization of a series of biomass-derived molecules to provide linear alkanes suitable for use as transportation fuels. These biomass-derived molecules contain a variety of functional groups, including olefins, furan rings and carbonyl groups. We describe the removal of these in either a stepwise process or a one-pot process using common reagents and catalysts under mild reaction conditions to provide n-alkanes in good yields and with high selectivities. Our general synthetic approach is applicable to a range of precursors with different carbon content (chain length). This allows the selective generation of linear alkanes with carbon chain lengths between eight and sixteen carbons.

Regeneration of ammonia borane spent fuel by direct reaction with hydrazine and liquid ammonia.

Ammonia borane (H(3)N-BH(3), AB) is a lightweight material containing a high density of hydrogen (H(2)) that can be readily liberated for use in fuel cell-powered applications. However, in the absence of a straightforward, efficient method for regenerating AB from dehydrogenated polymeric spent fuel, its full potential as a viable H(2) storage material will not be realized. We demonstrate that the spent fuel type derived from the removal of greater than two equivalents of H(2) per molecule of AB (i.e., polyborazylene, PB) can be converted back to AB nearly quantitatively by 24-hour treatment with hydrazine (N(2)H(4)) in liquid ammonia (NH(3)) at 40°C in a sealed pressure vessel.

Synthesis and thermolytic behavior of tin(IV) formates: in search of recyclable metal-hydride systems.

The synthesis and characterization of the series of organotin formates together with their thermolytic behavior are described. The diformate Bu(n)(2)Sn{OC(O)H}(2) (1) was synthesized by the reaction of Bu(n)(2)SnH(2) with formic acid. The triorganotin monoformate compounds R(3)SnOC(O)H (R = Cy (cyclohexyl)) 3, Mes, (mesityl, 2,4,6-trimethylphenyl) 4, and Dmp (2,6-dimethylphenyl 5) were obtained by the reaction of R(3)SnOH with formic acid. Their X-ray crystal structures along with that of the previously reported formate (PhCH(2))(3)SnOC(O)H (2) were determined. The diformate 1 exhibits an extended two-dimensional polymeric structure in which six-coordinate tin centers are linked by formate bridges. The tribenzyltin formate (2) possesses a chain structure in which the five-coordinate Sn(CH(2)Ph)(3) units are bridged by formate ions. The cyclohexyl derivative 3 was observed to have a similar structure. In contrast, the Mes and Dmp derivatives 4 and 5 support monomeric structures in which the four-coordinate tin atom is bound to an oxygen of the formate ligand. Heating the compounds in various high boiling solvents produced no decomposition up to 120 °C in the case of 1 and refluxing a solution of 2 or 3 in mesitylene or diglyme left the starting material mostly unchanged, although 3 decomposed to an insoluble orange solid in refluxing decalin. In contrast, the heating of 4 and 5 in refluxing mesitylene led to elimination of CO to give the tin hydroxides. The results are in contrast to the known thermolytic behavior of R(3)SnOC(O)H (R = Pr(n) or Bu(n)) complexes, which eliminate CO(2) to generate R(3)SnH. Compounds 3-5 are rare examples of structurally characterized tin formates.

Recycle of tin thiolate compounds relevant to ammonia-borane regeneration.

The use of benzenedithiol as a digestant for ammonia-borane spent fuel has been shown to result in tin thiolate compounds which we demonstrate can be recycled, yielding Bu(3)SnH and ortho-benzenedithiol for reintroduction to the ammonia-borane regeneration scheme.

Synthesis and characterization of low-coordinate divalent aryl transition-metal halide analogues of grignard reagents: precursors for reduction to metal-metal-bonded complexes.

The synthesis and characterization of the series of divalent first-row aryl transition-metal(II) halide compounds [Cr(mu-Cl)Ar']2 (1) and (Li(OEt2)Ar'MI2]2 (M=Mn (2), Fe (3), and Co (4); Ar'=C6H3-2,6-(C6H3-2,6-iPr2)2) are described. 1-4 were prepared by the addition of one equiv of Ar'Li to the respective transition-metal dihalides. They were characterized by UV-vis spectroscopy, magnetic measurements, and by X-ray crystallography. In dimeric 1, each chromium center has quasi-four-coordinate, square-planar geometry, in which the metal is terminally bound to a terphenyl ligand through the ipso carbon of the central ring and to two bridging chloride ligands. There is a further interaction between chromium and an ipso carbon from one of the flanking -C6H3-2,6iPr2 rings. In contrast, for the iodo derivatives 2-4, LiI is not eliminated upon addition of LiAr' to MI2. Instead, the diethyl ether solvated adducts, [Li(OEt2)Ar'MI2]2 (M=Mn (2), Fe (3), or Co (4)) were isolated. These possess a distorted cubane Li2M2I4 core, in which the lithiums are bound to an ether and the transition metals are bound to a terphenyl group. Magnetic measurements between 2 and 300 K reveal the expected weak antiferromagnetic exchange coupling in each of the complexes.

Synthesis and reactivity of metal complexes supported by the tetradentate monoanionic ligand bis(2-picolyl)(2-hydroxy-3,5-di-tert-butylbenzyl)amide (BPPA).

Metal-halide complexes of Ti, V, Y, Zr, Al, Ga, and U supported by the tetradentate monoanionic (TDMA) ligand bis(2-picolyl)(2-hydroxy-3,5-di-tert-butylbenzyl)amine, H(BPPA), were synthesized and spectroscopically characterized. In addition, the complexes (BPPA)TiCl2, (BPPA)VBr2, [(BPPA)YCl2]2, (BPPA)AlCl2, (BPPA)GaCl2, and (BPPA)UI3 were characterized by single-crystal X-ray crystallography. In all cases the ligand is bound kappa4 to the metal center. All structurally characterized compounds are monomeric in the solid-state with the exception of [(BPPA)YCl2]2, which exists as a dimer in the solid-state. The metal-alkyl complexes (BPPA)AlMe2 and (BPPA)Zr(CH2Ph)3 were also synthesized and characterized, and an X-ray structure of (BPPA)Zr(CH2Ph)3 was obtained. The transformation of BPPA from a monoanionic to a dianionic ligand via proton abstraction was observed and monitored by NMR spectroscopy.

Substituent effects in formally quintuple-bonded ArCrCrAr compounds (Ar = terphenyl) and related species.

The effects of different terphenyl ligand substituents on the quintuple Cr-Cr bonding in arylchromium(I) dimers stabilized by bulky terphenyl ligands (Ar) were investigated. A series of complexes, ArCrCrAr (1-4; Ar = C6H2-2,6-(C6H3-2,6-iPr2)2-4-X, where X = H, SiMe3, OMe, and F), was synthesized and structurally characterized. Their X-ray crystal structures display similar trans-bent C(ipso)CrCrC(ipso) cores with short Cr-Cr distances that range from 1.8077(7) to 1.8351(4) A. There also weaker Cr-C interactions [2.294(1)-2.322(2) A] involving an C(ipso) of one of the flanking aryl rings. The data show that the changes induced in the Cr-Cr bond length by the different substituents X in the para positions of the central aryl ring of the terphenyl ligand are probably a result of packing rather than electronic effects. This is in agreement with density functional theory (DFT) calculations, which predict that the model compounds (4-XC6H4)CrCr(C6H4-4-X) (X = H, SiMe3, OMe, and F) have similar geometries in the gas phase. Magnetic measurements in the temperature range of 2-300 K revealed temperature-independent paramagnetism in 1-4. UV-visible and NMR spectroscopic data indicated that the metal-metal-bonded solid-state structures of 1-4 are retained in solution. Reduction of (4-F3CAr')CrCl (4-F3CAr' = C6H2-2,6-(C6H3-2,6-iPr2)2-4-CF3) with KC8 gave non-Cr-Cr-bonded fluorine-bridged dimer {(4-F3CAr')Cr(mu-F)(THF)}2 (5) as a result of activation of the CF3 moiety. The monomeric, two-coordinate complexes [(3,5-iPr2Ar*)Cr(L)] (6, L = THF; 7, L = PMe3; 3,5-iPr2Ar* = C6H1-2,6-(C6H-2,4,6-iPr3)2-3,5-iPr2) were obtained with use of the larger 3,5-Pri2-Ar* ligand, which prevents Cr-Cr bond formation. Their structures contain almost linearly coordinated CrI atoms, with high-spin 3d5 configurations. The addition of toluene to a mixture of (3,5-iPr2Ar*)CrCl and KC8 gave the unusual dinuclear benzyl complex [(3,5-iPr2Ar*)Cr(eta3:eta6-CH2Ph)Cr(Ar*-1-H-3,5-iPr2)] (8), in which a C-H bond from a toluene methyl group was activated. The electronic structures of 5-8 have been analyzed with the aid of DFT calculations.

The coordination of perrhenate and pertechnetate to thorium(IV) in the presence of phosphine oxide or phosphate ligands.

A series of thorium(IV) perrhenato- and pertechnetato-complexes with P[double bond, length as m-dash]O donor ligands have been prepared and characterised both in the solid state and in solution. Isostructural complexes of general formula [Th(MO(4))(4)(L)(4)], where M = Re or Tc and L = triethylphosphate (TEP) (2 and 7), tri-iso-butylphosphate (TiBP) (3 and 8) and tri-n-butylphosphine oxide (TBPO) (4 and 9) have been prepared from the novel starting materials [Th(ReO(4))(4)] x 4H(2)O (1) and [Th(TcO(4))(4)] x 4H(2)O (6). The reaction of or with triphenylphosphine oxide (TPPO) in MeOH has also led to the synthesis of [Th(MO(4))(3)(TPPO)(3)(OCH(3))(HOCH(3))] (M = Re (5) or Tc (10)). While the structural characterisation of 4 and 9 has been previously described, we report for the first time the structural characterisation of 2 and 5, with a partial structural refinement of 3. Vibrational spectroscopic analysis confirms that the Tc complexes not characterised by single crystal X-ray diffraction are indeed isostructural with the perrhenate complexes with the same P[double bond, length as m-dash]O donor ligand. In all cases, monodentate coordination of the Group 7 tetraoxo anion is observed. (31)P NMR spectroscopy indicates that in all the phosphine oxide-based complexes there is one dominant solution species. For the phosphate based systems, the presence of pertechnetate appears to inhibit P[double bond, length as m-dash]O donor ligand complexation in solution, whereas a significant proportion of each phosphate remains coordinated to Th(IV) when perrhenate is present as the counter ligand. These results give some indication as to the mechanism of pertechnetate co-extraction with tetravalent cations in the presence of tri-n-butyl phosphate in the Plutonium and Uranium Recovery by Extraction (PUREX) process.

Temperature-resolved study of three [M(M'O4)4(TBPO)4] complexes (MM' = URe, ThRe, ThTc).

The crystal structures of the title complexes were measured at several temperatures between room temperature and 100 K. Each sample shows reversible crystal-to-crystal phase transitions as the temperature is varied. The behaviour of [U(ReO4)4(TBPO)4] (I) and [Th(ReO4)4(TBPO)4] (II) (TBPO = tri-n-butylphosphine oxide) is very similar; at room temperature, crystals of (I) and (II) are isostructural, with space group I42m, and reducing the temperature to 100 K causes a lowering of the space-group symmetry to C-centred cells, space groups Cc for (I) and Cmc2(1) for (II). The variation of lattice symmetry of [Th(TcO4)4(TBPO)4] (III) was found to be somewhat different, with the body-centred cubic space group, I43m, occurring at 293 K, a reduction of symmetry at 230 K to the C-centred orthorhombic space group, Cmc2(1), and a further transition to the primitive orthorhombic space group, Pbc2(1), below 215 K. Elucidation of the correct space-group symmetry and the subsequent refinement was complicated in some cases by the twinning by pseudo-merohedry that arises from the lowering of the space-group symmetry, occurring as the temperature is reduced. All three of the crystal structures determined at room temperature have high atomic displacement parameters, particularly of the (n)Bu groups, and (III) shows disorder of some of the O atoms. The structures in the space group Cmc2(1), show some disorder of nBu groups, but are otherwise reasonably well ordered; the structures of (I) in Cc and (III) in Pbc2(1) are ordered, even to the ends of the alkyl chains. Inter-comparison of the structures measured below 293 K, using the program OFIT from the SHELXTL package, showed that generally, they are remarkably alike, with weighted r.m.s. deviations of the M, M' and P atoms of less than 0.1 A, as are the 293 K structures of (I) and (II) with their low-temperature counterparts. However, the structure of (III) measured in the space group Cmc2(1) is significantly different from both the structure of (III) at 293 K and that found below 215 K, with weighted r.m.s. deviations of the Th, Tc and P atoms of 0.40 and 0.37 A, respectively. An extensive network of weak intra- and intermolecular C-H...O hydrogen bonds found between the atoms of the nBu and [M'O4] groups probably influences the packing and the overall geometry of the molecules.

Synthesis of a stable compound with fivefold bonding between two chromium(I) centers.

Although in principle transition metals can form bonds with six shared electron pairs, only quadruply bonded compounds can be isolated as stable species at room temperature. Here we show that the reduction of {Cr(mu-Cl)Ar'}2 [where Ar' indicates C6H3-2,6(C6H3-2,6-Pri2)2 and Pr indicates isopropyl] with a slight excess of potassium graphite has produced a stable compound with fivefold chromium-chromium (Cr-Cr) bonding. The very air- and moisture-sensitive dark red crystals of Ar'CrCrAr' were isolated with greater than 40% yield. X-ray diffraction revealed a Cr-Cr bond length of 1.8351(4) angstroms (where the number in parentheses indicates the standard deviation) and a planar transbent core geometry. These data, the structure's temperature-independent paramagnetism, and computational studies support the sharing of five electron pairs in five bonding molecular orbitals between two 3d5 chromium(I) ions.

The synthesis, structural, and spectroscopic characterization of uranium(IV) perrhenate complexes.

We report the synthesis, structural, and spectroscopic characterization of a series of uranium(IV)-perrhenato complexes. Three isostructural complexes with general formula [U(ReO4)4(L)4] (where L = tri-n-butylphosphine oxide/TBPO (2), triethyl phosphate/TEP (3), or tri-iso-butyl phosphate/TiBP (4)), have been synthesized, both through the photoreduction of ethanolic {UO2}2+ solutions and also via a novel U(IV) starting material, U(ReO4)4.5H2O (1). Compound 1 has also been used in the preparation of [U(ReO4)4(TPPO)3(CH3CN)].2CH3CN (5) and [U(ReO4)(DPPMO2)3(OH)][ReO4]2.2CH3CN (6), where TPPO represents triphenylphosphine oxide and DPPMO2 represents bis(diphenylphosphino)methane dioxide. All six complexes have been spectroscopically characterized using NMR, UV-vis-NIR, and IR techniques, with 2, 3, 5, and 6 also fully structurally characterized. The U atoms in compounds 2-6 all exhibit eight-coordinate geometry with up to four perrhenate groups in addition to three (DPPMO2 and TPPO) or four (TEP, TiBP, TBPO) coordinated organic ligands. In the case of compounds 5 and 6, the coordination of eight ligands to the U(IV) center is completed by the binding of a solvent molecule (CH3CN) and OH-, respectively. Solid-state physical analysis (elemental and thermogravimetric) and infrared spectroscopy are in agreement with the structural studies. The crystallographic data suggest that the strength of the U(IV)-O-donor ligand bonds decreases across the series R3PO > [ReO4]- > (RO)3PO. Solution-state IR and 31P NMR spectroscopy appear to be in agreement with these solid-state results.

Raman spectroscopy of silver pertechnetate.

The title compound, AgTcO4, contains close Ag-O contacts, and Raman spectroscopy shows a reduction in the Tc-O stretching frequencies on changing the pertechnetate counter-cation from K+ to Ag+.