Fast and selective sugar conversion to alkyl lactate and lactic acid with bifunctional carbon-silica catalysts

A novel catalyst design for the conversion of mono- and disaccharides to lactic acid and its alkyl esters was developed. The design uses a mesoporous silica, here represented by MCM-41, which is filled with a polyaromatic to graphite-like carbon network. The particular structure of the carbon-silica composite allows the accommodation of a broad variety of catalytically active functions, useful to attain cascade reactions, in a readily tunable pore texture. The significance of a joint action of Lewis and weak Br?nsted acid sites was studied here to realize fast and selective sugar conversion. Lewis acidity is provided by grafting the silica component with Sn(IV), while weak Br?nsted acidity originates from oxygen-containing functional groups in the carbon part. The weak Br?nsted acid content was varied by changing the amount of carbon loading, the pyrolysis temperature, and the post-treatment procedure. As both catalytic functions can be tuned independently, their individual role and optimal balance can be searched for. It was thus demonstrated for the first time that the presence of weak Br?nsted acid sites is crucial in accelerating the rate-determining (dehydration) reaction, that is, the first step in the reaction network from triose to lactate. Composite catalysts with well-balanced Lewis/Br?nsted acidity are able to convert the trioses, glyceraldehyde and dihydroxyacetone, quantitatively into ethyl lactate in ethanol with an order of magnitude higher reaction rate when compared to the Sn grafted MCM-41 reference catalyst. Interestingly, the ability to tailor the pore architecture further allows the synthesis of a variety of amphiphilic alkyl lactates from trioses and long chain alcohols in moderate to high yields. Finally, direct lactate formation from hexoses, glucose and fructose, and disaccharides composed thereof, sucrose, was also attempted. For instance, conversion of sucrose with the bifunctional composite catalyst yields 45% methyl lactate in methanol at slightly elevated reaction temperature. The hybrid catalyst proved to be recyclable in various successive runs when used in alcohol solvent.     

Structures and absolute stereochemistry of nipponallene and neonipponallene, new brominated allenes from the red alga Laurencia nipponica

Two new natural brominated allenes named as nipponallene 1 and neonipponallene 2 were isolated from the red alga Laurencia nipponica, collected near the Russian shore of the Sea of Japan (Troitsa Bay). The structures and absolute stereochemistry of 1 and 2 were elucidated using NMR spectroscopy, chemical transformations, modified Mosher’s method and on the basis of biogenetic understanding.     

Chiral three-dimensional microporous nickel aspartate with extended Ni-O-Ni bonding

In the course of our investigation aimed at the preparation of homochiral coordination polymers using readily available in optically pure form ligands and building blocks of condensed metal polyhedra, we recently reported a one-dimensional nickel aspartate compound [Ni(2)O(l-Asp)(H(2)O)(2)].4H(2)O (1) based on helical chains with extended Ni-O-Ni bonding. Here we report a new nickel aspartate [Ni(2.5)(OH)(l-Asp)(2)].6.55H(2)O (2) with a three-dimensional Ni-O-Ni connectivity that forms at a higher pH and is based on the same helices as in 1 which are connected by additional nickel octahedra to generate a chiral open framework with one-dimensional channels with minimum van der Waals dimensions of 8 x 5 A. The crystal structure of 2 was determined by synchrotron single-crystal X-ray diffraction on a 10 x 10 x 240 microm crystal.     

Atoms-in-molecules study of intra- and intermolecular bonding in the pentaerythritol tetranitrate crystal

Chemical bonding in the pentaerythritol tetranitrate crystal based on the experimental electron density obtained from X-ray diffraction data at 100 K and theoretical calculations at the experimental molecular geometry have been analyzed in terms of the Quantum Theory of Atoms in Molecules. Features of the intra- and intermolecular bond critical points and the oxygen atom lone-pair locations are discussed. Numerous intermolecular bonding interactions, including O...H and O...O, have been found and characterized. Atomic charges and atomic energies were integrated and compared with those for similar compounds. The N-O topological bond orders have been calculated for the first time, and the PETN atomic valences have been estimated.     

Application of the Low Frequency Raman Spectroscopy for Studying Ultra-High Molecular Weight Polyethylenes

In the frequency range below ∼150 cm⁻¹, the longitudinal acoustic modes (LAM) localized along straight chain segments (SCS) of macromolecules emerge in the Raman spectra of linear semicrystalline polymers. The LAM frequency is inversely proportional to the SCS length; therefore, the LAM band contour reflects the SCS length distribution in a sample. The opportunities given one by the low-frequency Raman spectroscopy for studying the ordered structures in polyethylene are demonstrated and discussed. The illustrating material consists of both previously published and original data on nucleation and transformation of the ensemble of SCS in powder-like, gelled, and drawn ultra-high molecular weight polyethylene.     

Isoprenoid-chained lipid β-XylOC16+4—A novel molecule for in meso membrane protein crystallization

Here we report a successful use of a recently developed isoprenoid-chained lipid family for in meso crystallization of membrane proteins. The isoprenoid-chained lipid 1-O-(3,7,11,15-tetramethylhexadecyl)-β–d-xyloside (β-XylOC₁₆₊₄) used as a host lipid for in meso crystallization provided high quality bacteriorhodopsin (bR) crystals (P6₃ space group) diffracting to high resolution and characterized by low twinning ratio. β-XylOC₁₆₊₄ has an isoprenoid chain with methyl branches at each 4th position and a xylose group in the water-soluble part. These peculiarities make the lipid clearly distinguishable in the bR crystalline lattice and provides a unique opportunity to study the role of the host lipid in the in meso crystallization. We conclude that β-XylOC₁₆₊₄ may have a general application for in meso crystallization for a wide range of membrane proteins. The cubic phase of β-XylOC₁₆₊₄ is present over a wide range of temperatures and is stable at low temperature (down to about 8 °C). This opens up the possibility of using temperature as a tool for the optimization of in meso crystallization with additional advantages for the crystallization of membrane proteins at lower temperatures where the proteins of interest may be more stable.     

Formation of assemblies comprising Ru-polypyridine complexes and CdSe nanocrystals studied by ATR-FTIR spectroscopy and DFT modeling

The interaction between CdSe nanocrystals (NCs) passivated with trioctylphosphine oxide (TOPO) ligands and a series of Ru-polypyridine complexes-[Ru(bpy)(3)](PF(6))(2) (1), [Ru(bpy)(2)(mcb)](PF(6))(2) (2), [Ru(bpy)(mcb)(2)](BarF)(2) (3), and [Ru(tpby)(2)(dcb)](PF(6))(2) (4) (where bpy = 2,2'-bipyridine, mcb = 4-carboxy-4'-methyl-2,2'-bipyridine, tbpy = 4,4'-di-tert-butyl-2,2'-bipyridine; dcb = 4,4'-dicarboxy-2,2'-bipyridine, and BarF = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate)-was studied by attenuated total reflectance FTIR (ATR-FTIR) and modeled using density functional theory (DFT). ATR-FTIR studies reveal that when the solid film of NCs is exposed to an acetonitrile solution of 2, 3, or 4, the complexes chemically bind to the NC surface through their carboxylic acid groups, replacing TOPO ligands. The corresponding spectral changes are observed on a time scale of minutes. In the case of 2, the FTIR spectral changes clearly show that the complex adsorption is associated with a loss of proton from the carboxylic acid group. In the case of 3 and 4, deprotonation of the anchoring group is also detected, while the second, "spectrator" carboxylic acid group remains protonated. The observed energy difference between the symmetric, ν(s), and asymmetric, ν(as), stretch of the deprotonated carboxylic acid group suggests that the complexes are bound to the NC surface via a bridging mode. The results of DFT modeling are consistent with the experiment, showing that for the deprotonated carboxylic acid group the coupling to two Cd atoms via a bridging mode is the energetically most favorable mode of attachment for all nonequivalent NC surface sites and that the attachment of the protonated carboxylic acid is thermodynamically significantly less favorable.     

Numerical simulations of cessation flows of a Bingham plastic with the augmented Lagrangian method

The augmented Lagrangian/Uzawa method has been used to study benchmark one-dimensional cessation flow problems of a Bingham fluid, such as the plane Couette flow, and the plane, round, and annular Poiseuille flows. The calculated stopping times agree well with available theoretical upper bounds for the whole range of Bingham numbers and with previous numerical results. The applied method allows for easy determination of the yielded and unyielded regions. The evolution of the rigid zones in these unsteady flows is presented. It is demonstrated that the appearance of an unyielded zone near the wall occurs for any non-zero Bingham number not only in the case of a round tube but also in the case of an annular tube of small radii ratio. The advantages of using the present method instead of regularizing the constitutive equation are also discussed.