Showing papers on "Selectivity published in 2015"

High Selectivity for Ethylene from Carbon Dioxide Reduction over Copper Nanocube Electrocatalysts

Abstract: Nanostructured surfaces have been shown to greatly enhance the activity and selectivity of many different catalysts. Here we report a nanostructured copper surface that gives high selectivity for ethylene formation from electrocatalytic CO2 reduction. The nanostructured copper is easily formed in situ during the CO2 reduction reaction, and scanning electron microscopy (SEM) shows the surface to be dominated by cubic structures. Using online electrochemical mass spectrometry (OLEMS), the onset potentials and relative selectivity toward the volatile products (ethylene and methane) were measured for several different copper surfaces and single crystals, relating the cubic shape of the copper surface to the greatly enhanced ethylene selectivity. The ability of the cubic nanostructure to so strongly favor multicarbon product formation from CO2 reduction, and in particular ethylene over methane, is unique to this surface and is an important step toward developing a catalyst that has exclusive selectivity for multicarbon products.

Mesostructure-Induced Selectivity in CO2 Reduction Catalysis.

Abstract: Gold inverse opal (Au-IO) thin films are active for CO2 reduction to CO with high efficiency at modest overpotentials and high selectivity relative to hydrogen evolution. The specific activity for hydrogen evolution diminishes by 10-fold with increasing porous film thickness, while CO evolution activity is largely unchanged. We demonstrate that the origin of hydrogen suppression in Au-IO films stems from the generation of diffusional gradients within the pores of the mesostructured electrode rather than changes in surface faceting or Au grain size. For electrodes with optimal mesoporosity, 99% selectivity for CO evolution can be obtained at overpotentials as low as 0.4 V. These results establish electrode mesostructuring as a complementary method for tuning selectivity in CO2-to-fuels catalysis.

Efficient CO2 Capture by Functionalized Graphene Oxide Nanosheets as Fillers To Fabricate Multi-Permselective Mixed Matrix Membranes

Abstract: A novel multi-permselective mixed matrix membrane (MP-MMM) is developed by incorporating versatile fillers functionalized with ethylene oxide (EO) groups and an amine carrier into a polymer matrix. The as-prepared MP-MMMs can separate CO2 efficiently because of the simultaneous enhancement of diffusivity selectivity, solubility selectivity, and reactivity selectivity. To be specific, MP-MMMs were fabricated by incorporating polyethylene glycol- and polyethylenimine-functionalized graphene oxide nanosheets (PEG–PEI–GO) into a commercial low-cost Pebax matrix. The PEG–PEI–GO plays multiple roles in enhancing membrane performance. First, the high-aspect ratio GO nanosheets in a polymer matrix increase the length of the tortuous path of gas diffusion and generate a rigidified interface between the polymer matrix and fillers, enhancing the diffusivity selectivity. Second, PEG consisting of EO groups has excellent affinity for CO2 to enhance the solubility selectivity. Third, PEI with abundant primary, secondar...

Chemical Environment Control and Enhanced Catalytic Performance of Platinum Nanoparticles Embedded in Nanocrystalline Metal-Organic Frameworks.

Abstract: Chemical environment control of the metal nanoparticles (NPs) embedded in nanocrystalline metal–organic frameworks (nMOFs) is useful in controlling the activity and selectivity of catalytic reactions. In this report, organic linkers with two functional groups, sulfonic acid (−SO3H, S) and ammonium (−NH3+, N), are chosen as strong and weak acidic functionalities, respectively, and then incorporated into a MOF [Zr6O4(OH)4(BDC)6 (BDC = 1,4-benzenedicarboxylate), termed UiO-66] separately or together in the presence of 2.5 nm Pt NPs to build a series of Pt NPs-embedded in UiO-66 (Pt⊂nUiO-66). We find that these chemical functionalities play a critical role in product selectivity and activity in the gas-phase conversion of methylcyclopentane (MCP) to acyclic isomer, olefins, cyclohexane, and benzene. Pt⊂nUiO-66-S gives the highest selectivity to C6-cyclic products (62.4% and 28.6% for cyclohexane and benzene, respectively) without acyclic isomers products. Moreover, its catalytic activity was doubled relative ...

Fabrication of MMMs with improved gas separation properties using externally-functionalized MOF particles

Abstract: Mixed matrix membranes (MMM) have the potential to overcome the limitations of traditional polymeric membranes for gas separation by improving both the permeability and selectivity. The most difficult challenge is accessing defect free and optimized MMM membranes. Defects are generally due to incompatible interfaces between the polymer and the filler particle. Herein, we present a new approach to modify and optimize the surface of UiO-66-NH2 based MOF particles to improve its interaction with Matrimid® polymer. A series of surface modified UiO-66-NH2 particles were synthesized and characterized using 1H NMR spectroscopy, mass spectrometry, XPS, and powder X-ray diffraction. MMMs containing surface optimized MOF particles exhibit improved thermal and mechanical properties. Most importantly, the MMMs show significantly enhanced gas separation properties; CO2 permeability was increased by ∼200% and CO2/N2 ideal selectivity was increased by ∼25%. These results confirm the success of the proposed technique to mitigate defective MOF/Matrimid® interfaces.

Increasing para-Xylene Selectivity in Making Aromatics from Methanol with a Surface-Modified Zn/P/ZSM-5 Catalyst

Abstract: We report a ZSM-5 based catalyst with surface modification of SiO2 to increase the selectivity of para-xylene (PX) in xylene (X) in the methanol-to-aromatics process. The effect of acid strength and acid amount in HZSM-5, Zn/P/ZSM-5, and Zn/P/Si/ZSM-5 on the catalytic performance, including methanol conversion, aromatic yield, and PX selectivity, were studied. The total acid strength and acid amount of the catalyst were crucial for high methanol conversion (around 100%) and high yield of aromatics (>60%), whereas weak external acid sites present in a small amount played an important role in increasing the PX selectivity (in the X isomers) from the usual 23–24% to 89.6%. The results validated the use of a catalyst having a core with strong acid sites in a large amount and an external shell with weak acid sites in a small amount. The contribution of the external surface reaction, including alkylation, isomerization, and dealkylation, to the PX selectivity was evaluated by using PX or ortho-X separately as f...

Impact of Hydrogenolysis on the Selectivity of the Fischer–Tropsch Synthesis: Diesel Fuel Production over Mesoporous Zeolite-Y-Supported Cobalt Nanoparticles

Abstract: Selectivity control is a challenging goal in Fischer–Tropsch (FT) synthesis. Hydrogenolysis is known to occur during FT synthesis, but its impact on product selectivity has been overlooked. Demonstrated herein is that effective control of hydrogenolysis by using mesoporous zeolite Y-supported cobalt nanoparticles can enhance the diesel fuel selectivity while keeping methane selectivity low. The sizes of the cobalt particles and mesopores are key factors which determine the selectivity both in FT synthesis and in hydrogenolysis of n-hexadecane, a model compound of heavier hydrocarbons. The diesel fuel selectivity in FT synthesis can reach 60 % with a CH4 selectivity of 5 % over a Na-type mesoporous Y-supported cobalt catalyst with medium mean sizes of 8.4 nm (Co particles) and 15 nm (mesopores). These findings offer a new strategy to tune the product selectivity and possible interpretations of the effect of cobalt particle size and the effect of support pore size in FT synthesis.

Role of Keto Intermediates in the Hydrodeoxygenation of Phenol over Pd on Oxophilic Supports

Abstract: The performance of Pd catalysts supported on SiO2, Al2O3 and ZrO2 for the hydrodeoxygenation (HDO) of phenol has been compared in the gas phase, at 300 °C and 1 atm using a fixed bed reactor. While Pd supported on SiO2 and Al2O3 exhibits high selectivity to cyclohexanone, when supported on an oxophilic support such as ZrO2, it favors the selectivity toward benzene, reducing the formation of ring-hydrogenated products, cyclohexanone and cyclohexanol. Diffuse reflectance infrared Fourier transform spectroscopy experiments support the participation of a keto-tautomer intermediate (2,4-cyclohexadienone) in the reaction. This intermediate can be hydrogenated in two different pathways. If the ring is hydrogenated, cyclohexanone and cyclohexanol are dominant products, as in the case of Pd/SiO2 and Pd/Al2O3 catalysts. By contrast, if the carbonyl group of the keto-intermediate tautomer is hydrogenated, benzene is directly formed via rapid dehydration of the unstable cyclohexadienol intermediate. This is observed ...

One-step Synthesis of Core-Gold/Shell-Ceria Nanomaterial and Its Catalysis for Highly Selective Semihydrogenation of Alkynes.

Abstract: We report a facile synthesis of new core-Au/shell-CeO2 nanoparticles (Au@CeO2) using a redox-coprecipitation method, where the Au nanoparticles and the nanoporous shell of CeO2 are simultaneously formed in one step. The Au@CeO2 catalyst enables the highly selective semihydrogenation of various alkynes at ambient temperature under additive-free conditions. The core–shell structure plays a crucial role in providing the excellent selectivity for alkenes through the selective dissociation of H2 in a heterolytic manner by maximizing interfacial sites between the core-Au and the shell-CeO2.

Recent advances in selective acetylene hydrogenation using palladium containing catalysts

Abstract: Recent advances with Pd containing catalysts for the selective hydrogenation of acetylene are described. The overview classifies enhancement of catalytic properties for monometallic and bimetallic Pd catalysts. Activity/selectivity of Pd catalysts can be modified by controlling particle shape/morphology or immobilisation on a support which interacts strongly with Pd particles. In both cases enhanced ethylene selectivity is generally associated with modifying ethylene adsorption strength and/or changes to hydride formation. Inorganic and organic selectivity modifiers (i.e., species adsorbed onto Pd particle surface) have also been shown to enhance ethylene selectivity. Inorganic modifiers such as TiO2 change Pd ensemble size and modify ethylene adsorption strength whereas organic modifiers such as diphenylsulfide are thought to create a surface template effect which favours acetylene adsorption with respect to ethylene. A number of metals and synthetic approaches have been explored to prepare Pd bimetallic catalysts. Examples where enhanced selectivity is observed are generally associated with decreased Pd ensemble size and/or hindering of the ease with which an unselective hydride phase is formed for Pd. A final class of bimetallic catalysts are discussed where Pd is not thought to be the primary reaction site but merely acts as a site where hydrogen dissociation and spillover occurs onto a second metal (Cu or Au) where the reaction takes place more selectively.

Selective Heterogeneous CO2 Electroreduction to Methanol

Abstract: Catalytic electroreduction of carbon dioxide to useful chemical feedstocks is an environmentally and technologically important process, yet the low energy efficiency and difficulty in controlling product selectivity are great challenges. The reason for part of the latter is that there are presently no catalyst design principles to selectively control CO2 electroreduction toward a desired product. In this work, as a first attempt, we suggest combining a few criteria (CO binding energy, OH binding energy, and H binding energy) that can be collectively used as activity- and selectivity-determining descriptors to preferentially produce methanol over methane from CO2 electroreduction. We then apply these concepts to near-surface alloys (NSAs) to propose efficient and selective CO2 electrochemical reduction catalysts to produce methanol. The W/Au alloy is identified as a promising candidate to have increased catalyst efficiency (decreased CO2 reduction overpotential and increased overpotential for unwanted hydr...

A colorimetric and ratiometric fluorescent probe for ClO- targeting in mitochondria and its application in vivo.

Abstract: A colorimetric and ratiometric fluorescent probe PMN–TPP for imaging mitochondrial ClO− was prepared. The selectivity of PMN–TPP was excellent and the detection would not be influenced by other ROS. The limit of detection (LOD = 3σ/slope) for ClO− was evaluated to be 0.43 μM, suggesting the probe's high sensitivity to ClO−. For the biological applications, PMN–TPP performed well in detecting endogenous HClO in living RAW264.7 macrophage cells. A co-localization study employing Mito Tracker green revealed that PMN–TPP was specifically located in the mitochondria of living RAW264.7 macrophage cells. In the in vivo experiment, a nude mouse with acute inflammation stimulated by lipopolysaccharide (LPS) was employed. After injection of PMN–TPP, the fluorescence signal changed gradually in 30 min and then remained unchanged in the injection region, demonstrating that PMN–TPP could detect the endogenous HClO in living animals.

Enhancing the CO2 separation performance of composite membranes by the incorporation of amino acid-functionalized graphene oxide

Abstract: Composite membranes are fabricated by incorporating amino acid-functionalized graphene oxide (GO-DA-Cys) nanosheets into a sulfonated poly(ether ether ketone) (SPEEK) polymer matrix. Graphene oxide (GO) nanosheets are functionalized with amino acids through a facile two-step method using dopamine (DA) and cysteine (Cys) in succession. The CO2 separation performance of the as-prepared membranes is evaluated for CO2/CH4 and CO2/N2 systems. GO nanosheets increase more tortuous paths for larger molecules, enhancing the diffusivity selectivity. Amino acids with carboxylic acid and primary amine groups simultaneously enhance the solubility selectivity and reactivity selectivity. Accordingly, CO2 molecules can transport quickly due to the enhanced selectivity. The optimum separation performance is achieved at the GO-DA-Cys content of 8 wt% with selectivities of 82 and 115 for CO2/CH4 and CO2/N2, respectively, and a CO2 permeability of 1247 Barrer, significantly surpassing the Robeson upper bound reported in 2008. Besides, the mechanical and thermal stabilities of the composite membranes are also improved compared with the pristine SPEEK membrane.

Catalytic Conversion of Glucose into 5-Hydroxymethylfurfural by Hf(OTf)4 Lewis Acid in Water

Abstract: A series of Lewis acidic metal salts were used for glucose dehydration to 5-hydroymethylfurfural (HMF) in water. Effect of valence state, ionic radii of Lewis acidic cation, and the type of anions on the catalytic performance have been studied systematically. The experimental results showed that the valence state played an important role in determining catalytic activity and selectivity. It was found that a higher glucose conversion rate and HMF selectivity could be obtained over high valent Lewis acid salts, where the ionic radii of these Lewis acidic metal salts are usually relatively small. Analysis on the effect of the anions of Lewis acid salts on the catalytic activity and the selectivity suggested that a higher glucose conversion and HMF selectivity could be readily obtained with Cl−. Furthermore, the recyclability of high valence state Lewis acid salt was also studied, however, inferior catalytic performance was observed. The deactivation mechanism was speculated to be the fact that high valence state Lewis acid salt was comparatively easier to undergo hydrolysis to yield complicated metal aqua ions with less catalytic activity. The Lewis acidic activity could be recovered by introducing a stoichiometric amount of hydrochloric acid (HCl) to the catalytic before the reaction.

Metal Phosphides Derived from Hydrotalcite Precursors toward the Selective Hydrogenation of Phenylacetylene

Abstract: We report a new synthetic strategy for the fabrication of several supported nickel phosphides (Ni12P5, Ni2P, and NiP2) with particle size ranging from 5 to 15 nm via a two-step procedure: preparation of supported Ni particles from layered double hydroxide precursors, followed by a further reaction with a certain amount of red phosphorus. The selective hydrogenation of phenylacetylene over these metal phosphides was evaluated, and the as-prepared Ni2P/Al2O3 catalyst shows a much higher selectivity to styrene (up to 88.2%) than Ni12P5/Al2O3 (48.0%), NiP2/Al2O3 (65.9%), and Ni/Al2O3 (0.7%) catalysts. EXAFS and in situ IR measurements reveal that the incorporation of P increases the bond length of Ni–Ni, which imposes a key influence on the adsorption state of alkene intermediates: as the Ni–Ni bond length extends to 0.264 nm, the alkene intermediate undergoes di-π(C═C) adsorption, facilitating its desorption and the resulting enhanced selectivity. Moreover, electron transfer occurs from Ni to P, as confirmed...

Iron achieves noble metal reactivity and selectivity: highly reactive and enantioselective iron complexes as catalysts in the hydrosilylation of ketones.

Abstract: Chiral iron alkyl and iron alkoxide complexes bearing boxmi pincers as stereodirecting ligands have been employed as catalysts for enantioselective hydrosilylation reactions with unprecedented activity and selectivity (TOF = 240 h–1 at −40 °C, ee up to 99% for alkyl aryl ketones), which match the performance of previously established noble-metal-based catalysts. This shows the potential of earth-abundant metals such as iron for replacing platinum—metals without any drawbacks for the reaction design.

Combined CO2-philicity and Ordered Mesoporosity for Highly Selective CO2 Capture at High Temperatures

Abstract: Various dry sorbents have been lately introduced as promising media to capture carbon dioxide (CO2). However, it is still desirable to further improve their performance in diverse aspects, and high temperature selectivity of CO2 over other gases is clearly one of them. Here, we report a co-assembly approach to turn nonporous melamine resin to a highly ordered mesoporous polymeric network (space group: Im3m) containing high nitrogen content (∼18 at%). This mesoporous network shows anomalously increasing CO2/N2 selectivity with temperature rise, with the selectivity at 323 K reaching 117 (Henry method). This selectivity behavior is attributed to a combined effect of the high nitrogen content allowing for high binding affinity with CO2 and well-defined mesopores (2.5–2.9 nm) accelerating release of N2 with temperature rise. The given orthogonal approach suggests a new direction in designing dry sorbents with excellent selectivities at high temperatures.

Insights into Hägg Iron-Carbide-Catalyzed Fischer–Tropsch Synthesis: Suppression of CH4 Formation and Enhancement of C–C Coupling on χ-Fe5C2 (510)

Abstract: Probing the product selectivity of Fischer–Tropsch catalysts is of prime scientific and industrial importance—with the aim to upgrade products and meet various end-use applications. In this work, the mechanisms for CH4 formation and C1–C1 coupling on a thermodynamically stable, terraced-like χ-Fe5C2 (510) surface were studied by DFT calculations. It was found that this surface exhibits high effective barriers of CH4 formation for the three cases (i.e., 3.66, 2.81, and 2.39 eV), indicating the unfavorable occurrence of CH4 formation under FTS conditions. The C + CH and CH + CH are the most likely coupling pathways, which follow the carbide mechanism. Subsequently, the effective barrier difference between CH4 formation and C1–C1 coupling was used as a descriptor to quantify FTS selectivity. A comparison of the selectivity between this surface and the reported FTS catalysts’ surfaces was discussed in detail. More interestingly, this surface shows unexpectedly high C2+ selectivity. This indicates that manipul...

Tuning catalytic selectivity of liquid-phase hydrogenation of furfural via synergistic effects of supported bimetallic catalysts

Abstract: Bimetallic catalysts supported over TiO2-ZrO2 binary oxides were prepared by co-impregnation methods and used for catalyzing liquid-phase hydrogenation of furfural Highly selective hydrogenation catalysts can be developed based on bimetallic synergistic effect The coexistence of small proportion of palladium with supported nickel species greatly improves the catalytic performance and transfer the reaction selectivity from partial hydrogenation to total hydrogenation The catalyst with Ni-Pd mole ratio of 5:1 shows the best performance The yield of tetrahydrofurfuryl alcohol (THFA) reaches 934% Ni-Pd synergistic effect is interpreted through XPS measurement and a hydrogen-transfer mechanism is proposed Pt-Re bimetallic catalyst is an excellent partial hydrogenation catalyst for furfural conversion Furfural can be totally converted and the selectivity of partial hydrogenation product (FA) reaches 957% When rhenium oxide species are located on the Pt surface, the hydrogen species on Pt are transferred to adsorbed C = O bond to achieve selective hydrogenation (C) 2015 Elsevier BV All rights reserved