Visible-Light Photochemical Reduction of CO2 to CO Coupled to Hydrocarbon Dehydrogenation

Research on the photochemical reduction of CO₂, initiated already 40 years ago, has with few exceptions been performed by using amines as sacrificial reductants. Hydrocarbons are high-volume chemicals whose dehydrogenation is of interest, so the coupling of a CO₂ photoreduction to a hydrocarbon-photodehydrogenation reaction seems a worthwhile concept to explore. A three-component construct was prepared including graphitic carbon nitride (g-CN) as a visible-light photoactive semiconductor, a polyoxometalate (POM) that functions as an electron acceptor to improve hole–electron charge separation, and an electron donor to a rhenium-based CO₂ reduction catalyst. Upon photoactivation of g-CN, a cascade is initiated by dehydrogenation of hydrocarbons coupled to the reduction of the polyoxometalate. Visible-light photoexcitation of the reduced polyoxometalate enables electron transfer to the rhenium-based catalyst active for the selective reduction of CO₂ to CO. The construct was characterized by zeta potential, IR spectroscopy, thermogravimetry, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). An experimental Z-scheme diagram is presented based on electrochemical measurements and UV/Vis spectroscopy. The conceptual advance should promote study into more active systems.     

A weak cation-exchange monolith as stationary phase for the separation of peptide diastereomers by CEC

A CEC weak cation-exchange monolith has been prepared by in situ polymerization of acrylamide, methylenebisacrylamide and 4-acrylamidobutyric acid in a decanol-dimethylsulfoxide mixture as porogen. The columns were evaluated by SEM and characterized with regard to the separation of diastereomers and α/β-isomers of aspartyl peptides. Column preparation was reproducible as evidenced by comparison of the analyte retention times of several columns prepared simultaneously. Analyte separation was achieved using mobile phases consisting of acidic phosphate buffer and ACN. Under these conditions the peptides migrated due to their electrophoretic mobility but the EOF also contributed as driving force as a function of the pH of the mobile phase due to increasing dissociation of the carboxyl groups of the polymer. Raising the pH of the mobile phase also resulted in deprotonation of the peptides reducing analyte mobility. Due to these mechanisms each pair of diastereomeric peptides displayed the highest resolution at a different pH of the buffer component of the mobile phase. Comparing the weak-cation exchange monolith to an RP monolith and a strong cation-exchange monolith different elution order of some peptide diastereomers was observed, clearly illustrating that interactions with the stationary phase contribute to the CEC separations.     

Ready access to 3-amino-2,3-dideoxysugars via regio- and stereo-selective tandem hydroamination-glycosylation of glycals

A highly stereoselective BF(3)·OEt(2)-promoted tandem hydroamination-glycosylation on a glycal scaffold has been developed to form 3-amino-2,3-dideoxysugars in a one-pot procedure. This efficient multicomponent reaction protocol offers simplicity and general applicability to a broad range of variations on each component.     

Protonation of phosphovanadomolybdates H(3+x)PV(x)Mo(12-x)O40: computational insight into reactivity

Protonated phosphovanadomolybdates of the Keggin structure, H(3+x)PV(x)Mo(12-x)O(40) where x = 0, 1, 2, and derivatives with surface defects formed by loss of constitutional water were studied using high-level DFT calculations toward determination of the most stable species and possible active forms in oxidation catalysis in both the gas phase and in polar solutions. The calculations demonstrate that protonation at bridging positions is energetically much more favorable than protonation of terminal oxygen atoms. The preferential protonation site is determined by the stability of the metal-oxygen bond rather than the negative charge on the oxygen atom. In H(3)PMo(12)O(40), maximum distances between protons at bridging oxygen atoms are energetically favored. In contrast, for H(4)PVMo(11)O(40) and H(5)PV(2)Mo(10)O(40) protons prefer nucleophilic sites adjacent to vanadium atoms. Up to three protons are bound to the nucleophilic sites around the same vanadium atom in the stable isomeric forms of H(5)PV(2)Mo(10)O(40) that result in strong destabilization of oxo-vanadium(V) bonding to the Keggin unit. Such behavior arises from the different nature of the Mo-O and V-O bonds that can be traced to the different sizes of the valence d orbitals of the metals. Coordination of two protons at the same site yields water and an oxygen defect as a result of its dissociation. The energetic cost for the formation of surface defects decreases in the order: O(t) ? O(c) ? O(e) and is lower for the sites adjacent to vanadium atoms. Vanadium atoms near defects also have a significant contribution to the LUMO. Thus, vanadium-substituted polyoxometalates with defects near and, especially, between vanadium atoms present a plausible active form of polyoxometalates in oxidation reactions.     

A model for enhanced coal bed methane recovery aimed at carbon dioxide storage

Numerical simulations on the performance of CO₂ storage and enhanced coal bed methane (ECBM) recovery in coal beds are presented. For the calculations, a one-dimensional mathematical model is used consisting of mass balances describing gas flow and sorption, and a geomechanical relationship to account for porosity and permeability changes during injection. Important insights are obtained regarding the gas flow dynamics during displacement and the effects of sorption and swelling on the ECBM operation. In particular, initial faster CH₄ recovery is obtained when N₂ is added to the injected mixture, whereas pure CO₂ allows for a more effective displacement in terms of total CH₄ recovery. Moreover, it is shown that coal swelling dramatically affects the gas injectivity, as the closing of the fractures associated with it strongly reduces coal’s permeability. As a matter of fact, injection of flue gas might represent a useful option to limit this problem.     

Triflimide-catalyzed allyl-allyl cross-coupling: a metal-free allylic alkylation

A highly efficient metal-free intermolecular C(sp3)-C(sp3) allyl-allyl cross-coupling protocol between allyl acetates and allyltrimethylsilanes, which proceeded smoothly in the presence of catalytic triflimide to form 1,5-dienes with good to excellent regioselectivity, has been developed.     

3D Mapping of Gas Physisorption for the Spatial Characterisation of Nanoporous Materials

Nanoporous materials used in industrial applications (e. g., catalysis and separations) draw their functionality from properties at the nanoscale (1–10 Å). When shaped into a technical form these solids reveal spatial variations in the same properties over much larger length scales (1 μm–1 cm). The multiscale characterization of these systems is impaired by the trade-off between sample size and image resolution that is bound to the use of most imaging techniques. We show here the application of X-ray computed tomography for the non-invasive spatial characterization of a zeolite/activated carbon adsorbent bed across three orders of magnitude in scale. Through the unique combination of gas adsorption isotherms measured locally and their interpretation by physisorption analysis, we determine three-dimensional maps of the specific surface area and micropore volume. We further use machine learning to identify and locate the materials within the packed bed. This novel ability to reveal the extent of heterogeneity in technical porous solids will enable a deeper understanding of their function in industrial reactors. Such developments are essential towards bridging the gap between material research and process design.     

In‐situ photopolymerized polyhedral oligomeric silsesquioxane‐derived monolithic capillary columns with quinidine functionality for enantioseparation by nano‐liquid chromatography

The successful fabrication of monolithic capillary columns for enantiomer separations was achieved within vinylized fused silica capillaries via fast “one‐pot” photo‐initiated free radical polymerization reaction. A mixture consisting of polyhedral oligomeric silsesquioxane, O‐[2‐(methacryloyloxy)ethylcarbamoyl]‐10,11‐dihydroquinidine was copolymerized in the presence of n‐butanol, ethylene glycol and photo‐initiator 2,2‐dimethoxy‐2‐phenylacetophenone. The morphology of the resultant polymeric hybrid inorganic‐organic material and its permeability as well as porosity can be controlled by adjusting the composition of the monomers and binary porogenic solvent. The chromatographic characteristics of the columns have been investigated. Separation factors of N‐acetyl‐phenylalanine (Ac‐Phe) and dichlorprop dropped with decrease of chiral functional monomer. Permeability was better when the macroporogen ethyleneglycol was present at higher concentrations during the polymerization. In general, the chiral compounds were well separated (dichlorprop: α = 1.53, R_s up to 4.14; Ac‐Phe: α = 1.36, R_s up to 2.69) by nano‐HPLC with an optimized enantioselective monolithic capillary column which can be prepared within a few minutes.     

Large Scale Oscillations in the Continuous Casting Process

Long-term oscillations of the flow in a continuous casting tundish are investigated numerically and experimentally. The numerical model is based on the Reynolds averaged Navier-Stokes equations (URANS). A comparison between numerical results and experimental data from a water model is presented. Frequencies due to long-term oscillations are resolved in the simulation. These frequencies are in satisfactory agreement with the experimental results of LDA measurements. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)