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ACS Catalysis

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https://www.readbyqxmd.com/read/28824823/influence-of-the-reaction-temperature-on-the-nature-of-the-active-and-deactivating-species-during-methanol-to-olefins-conversion-over-h-sapo-34
#1
E Borodina, H Sharbini Harun Kamaluddin, F Meirer, M Mokhtar, A M Asiri, S A Al-Thabaiti, S N Basahel, J Ruiz-Martinez, B M Weckhuysen
The selectivity toward lower olefins during the methanol-to-olefins conversion over H-SAPO-34 at reaction temperatures between 573 and 773 K has been studied with a combination of operando UV-vis diffuse reflectance spectroscopy and online gas chromatography. It was found that the selectivity toward propylene increases in the temperature range of 573-623 K, while it decreases in the temperature range of 623-773 K. The high degree of incorporation of olefins, mainly propylene, into the hydrocarbon pool affects the product selectivity at lower reaction temperatures...
August 4, 2017: ACS Catalysis
https://www.readbyqxmd.com/read/28824822/rationalizing-acid-zeolite-performance-on-the-nanoscale-by-correlative-fluorescence-and-electron-microscopy
#2
Jordi Van Loon, Kris P F Janssen, Thomas Franklin, Alexey V Kubarev, Julian A Steele, Elke Debroye, Eric Breynaert, Johan A Martens, Maarten B J Roeffaers
The performance of zeolites as solid acid catalysts is strongly influenced by the accessibility of active sites. However, synthetic zeolites typically grow as complex aggregates of small nanocrystallites rather than perfect single crystals. The structural complexity must therefore play a decisive role in zeolite catalyst applicability. Traditional tools for the characterization of heterogeneous catalysts are unable to directly relate nanometer-scale structural properties to the corresponding catalytic performance...
August 4, 2017: ACS Catalysis
https://www.readbyqxmd.com/read/28824821/mild-and-regioselective-pd-oac-2-catalyzed-c-h-arylation-of-tryptophans-by-arn2-x-promoted-by-tosic-acid
#3
Alan J Reay, L Anders Hammarback, Joshua T W Bray, Thomas Sheridan, David Turnbull, Adrian C Whitwood, Ian J S Fairlamb
A regioselective Pd-mediated C-H bond arylation methodology for tryptophans, utilizing stable aryldiazonium salts, affords C2-arylated tryptophan derivatives, in several cases quantitatively. The reactions proceed in air, without base, and at room temperature in EtOAc. The synthetic methodology has been evaluated and compared against other tryptophan derivative arylation methods using the CHEM21 green chemistry toolkit. The behavior of the Pd catalyst species has been probed in preliminary mechanistic studies, which indicate that the reaction is operating homogeneously, although Pd nanoparticles are formed during substrate turnover...
August 4, 2017: ACS Catalysis
https://www.readbyqxmd.com/read/28824820/promoted-iron-nanocrystals-obtained-via-ligand-exchange-as-active-and-selective-catalysts-for-synthesis-gas-conversion
#4
Marianna Casavola, Jingxiu Xie, Johannes D Meeldijk, Nynke A Krans, Andrey Goryachev, Jan P Hofmann, A Iulian Dugulan, Krijn P de Jong
Colloidal synthesis routes have been recently used to fabricate heterogeneous catalysts with more controllable and homogeneous properties. Herein a method was developed to modify the surface composition of colloidal nanocrystal catalysts and to purposely introduce specific atoms via ligands and change the catalyst reactivity. Organic ligands adsorbed on the surface of iron oxide catalysts were exchanged with inorganic species such as Na2S, not only to provide an active surface but also to introduce controlled amounts of Na and S acting as promoters for the catalytic process...
August 4, 2017: ACS Catalysis
https://www.readbyqxmd.com/read/28804677/haloselective-cross-coupling-via-ni-photoredox-dual-catalysis
#5
Kingson Lin, Rebecca J Wiles, Christopher B Kelly, Geraint H M Davies, Gary A Molander
The chemoselective functionalization of polyfunctional aryl linchpins is crucial for rapid diversification. Although well-explored for Csp(2) and Csp nucleophiles, the chemoselective introduction of Csp(3) groups remains notoriously difficult and is virtually undocumented using Ni catalysts. To fill this methodological gap, a "haloselective" cross-coupling process of arenes bearing two halogens, I and Br, using ammonium alkylbis(catecholato)silicates, has been developed. Utilizing Ni/photoredox dual catalysis, Csp(3) -Csp(2) bonds can be forged selectively at the iodine-bearing carbon of bromo(iodo)arenes...
August 4, 2017: ACS Catalysis
https://www.readbyqxmd.com/read/28890842/catalytic-mechanisms-for-cofactor-free-oxidase-catalyzed-reactions-reaction-pathways-of-uricase-catalyzed-oxidation-and-hydration-of-uric-acid
#6
Donghui Wei, Xiaoqin Huang, Yan Qiao, Jingjing Rao, Lu Wang, Fei Liao, Chang-Guo Zhan
First-principles quantum mechanical/molecular mechanical (QM/MM)-free energy calculations have been performed to uncover how uricase catalyzes metabolic reactions of uric acid (UA), demonstrating that the entire reaction process of UA in uricase consists of two stages-oxidation followed by hydration. The oxidation consists of four steps: (1) chemical transformation from 8-hydroxyxythine to an anionic radical via a proton transfer along with an electron transfer, which is different from the previously proposed electron-transfer mechanism that involves a dianion intermediate (UA(2-)) during the catalytic reaction process; (2) proton transfer to the O2(-) anion (radical); (3) diradical recombination to form a peroxo intermediate; (4) dissociation of H2O2 to generate the dehydrourate...
July 7, 2017: ACS Catalysis
https://www.readbyqxmd.com/read/28713644/kinetic-modeling-of-the-nickel-catalyzed-esterification-of-amides
#7
Nicholas A Weires, Daniel D Caspi, Neil K Garg
Nickel-catalyzed coupling reactions provide exciting tools in chemical synthesis. However, most methodologies in this area require high catalyst loadings, which commonly range from 10-20 mol % nickel. Through an academic-industrial collaboration, we demonstrate that kinetic modeling can be used strategically to overcome this problem, specifically within the context of the Ni-catalyzed conversion of amides to esters. The successful application of this methodology to a multigram-scale coupling, using only 0...
July 7, 2017: ACS Catalysis
https://www.readbyqxmd.com/read/28713643/solvent-polarity-induced-pore-selectivity-in-h-zsm-5-catalysis
#8
Alexey V Kubarev, Eric Breynaert, Jordi Van Loon, Arunasish Layek, Guillaume Fleury, Sambhu Radhakrishnan, Johan Martens, Maarten B J Roeffaers
Molecular-sized micropores of ZSM-5 zeolite catalysts provide spatial restrictions around catalytic sites that allow for shape-selective catalysis. However, the fact that ZSM-5 has two main pore systems with different geometries is relatively unexploited as a potential source of additional shape selectivity. Here, we use confocal laser-scanning microscopy to show that by changing the polarity of the solvent, the acid-catalyzed furfuryl alcohol oligomerization can be directed to selectively occur within either of two locations in the microporous network...
July 7, 2017: ACS Catalysis
https://www.readbyqxmd.com/read/28824819/predicting-product-distribution-of-propene-dimerization-in-nanoporous-materials
#9
Yifei Michelle Liu, Berend Smit
In this work, a theoretical framework is developed to explain and predict changes in the product distribution of the propene dimerization reaction, which yields a mixture of C6 olefin isomers, resulting from the use of different porous materials as catalysts. The MOF-74 class of materials has shown promise in catalyzing the dimerization of propene with high selectivity for valuable linear olefin products. We show that experimentally observed changes in the product distribution can be explained in terms of the contribution of the pores to the free energy of formation, which are directly computed using molecular simulation...
June 2, 2017: ACS Catalysis
https://www.readbyqxmd.com/read/28798888/identical-active-sites-in-hydroxynitrile-lyases-show-opposite-enantioselectivity-and-reveal-possible-ancestral-mechanism
#10
Bryan J Jones, Zsófia Bata, Romas J Kazlauskas
Evolutionarily related hydroxynitrile lyases from rubber tree (HbHNL) and from Arabidopsis thaliana (AtHNL) follow different catalytic mechanisms with opposite enantioselectivity toward mandelonitrile. We hypothesized that the HbHNL-like mechanism evolved from an enzyme with an AtHNL-like mechanism. We created ancestor-like composite active-sites in each scaffold to elucidate how this transition may have occurred. Surprisingly, a composite active site in HbHNL maintained (S)-selectivity, while the identical set of active site residues in AtHNL maintained (R)-selectivity...
June 2, 2017: ACS Catalysis
https://www.readbyqxmd.com/read/28603658/insights-into-the-activity-and-deactivation-of-the-methanol-to-olefins-process-over-different-small-pore-zeolites-as-studied-with-operando-uv-vis-spectroscopy
#11
Joris Goetze, Florian Meirer, Irina Yarulina, Jorge Gascon, Freek Kapteijn, Javier Ruiz-Martínez, Bert M Weckhuysen
The nature and evolution of the hydrocarbon pool (HP) species during the Methanol-to-Olefins (MTO) process for three small-pore zeolite catalysts, with a different framework consisting of large cages interconnected by small eight-ring windows (CHA, DDR, and LEV) was studied at reaction temperatures between 350 and 450 °C using a combination of operando UV-vis spectroscopy and online gas chromatography. It was found that small differences in cage size, shape, and pore structure of the zeolite frameworks result in the generation of different hydrocarbon pool species...
June 2, 2017: ACS Catalysis
https://www.readbyqxmd.com/read/28603657/o-benzyl-xanthate-esters-under-ni-photoredox-dual-catalysis-selective-radical-generation-and-csp-3-csp-2-cross-coupling
#12
Brandon A Vara, Niki R Patel, Gary A Molander
Alkyl xanthate esters are perhaps best known for their use in deoxygenation chemistry. However, their use in cross-coupling chemistry has not been productive, which is due, in part, to inadequate xanthate activation strategies. Herein, we report the use of O-benzyl xanthate esters, readily derived from alcohols, as radical pronucleophiles in Csp(3)-Csp(2) cross-couplings under Ni/photoredox dual catalysis. Xanthate (C-O) cleavage is found to be reliant on photogenerated (sec-butyl) radical activators to form new carbon-centered radicals primed for nickel-catalyzed cross-couplings...
June 2, 2017: ACS Catalysis
https://www.readbyqxmd.com/read/28603656/simultaneous-no-x-and-particulate-matter-removal-from-diesel-exhaust-by-hierarchical-fe-doped-ce-zr-oxide
#13
Ying Cheng, Weiyu Song, Jian Liu, Huiling Zheng, Zhen Zhao, Chunming Xu, Yuechang Wei, Emiel J M Hensen
Particulate matter and NO x emissions from diesel exhaust remains one of the most pressing environmental problems. We explore the use of hierarchically ordered mixed Fe-Ce-Zr oxides for the simultaneous capture and oxidation of soot and reduction of NO x by ammonia in a single step. The optimized material can effectively trap the model soot particles in its open macroporous structure and oxidize the soot below 400 °C while completely removing NO in the 285-420 °C range. Surface characterization and DFT calculations emphasize the defective nature of Fe-doped ceria...
June 2, 2017: ACS Catalysis
https://www.readbyqxmd.com/read/28503355/stereoselective-cis-vinylcyclopropanation-via-a-gold-i-catalyzed-retro-buchner-reaction-under-mild-conditions
#14
Bart Herlé, Philipp M Holstein, Antonio M Echavarren
A highly stereoselective gold(I)-catalyzed cis-vinylcyclopropanation of alkenes has been developed. Allylic gold carbenes, generated via a retro-Buchner reaction of 7-alkenyl-1,3,5-cycloheptatrienes, react with alkenes to form vinylcyclopropanes. The gold(I)-catalyzed retro-Buchner reaction of these substrates proceeds by simple heating at a temperature much lower than that required for the reaction of 7-aryl-1,3,5-cycloheptatrienes (75 °C vs 120 °C). A newly developed Julia-Kocienski reagent enables the synthesis of the required cycloheptatriene derivatives in one step from readily available aldehydes or ketones...
May 5, 2017: ACS Catalysis
https://www.readbyqxmd.com/read/28593082/control-of-selectivity-through-synergy-between-catalysts-silanes-and-reaction-conditions-in-cobalt-catalyzed-hydrosilylation-of-dienes-and-terminal-alkenes
#15
Balaram Raya, Stanley Jing, T V RajanBabu
Readily accessible ( (i)(-Pr)PDI)CoCl2 [ (i)(-Pr) PDI = 2,6-bis(2,6-diisopropylphenyliminoethyl)pyridine] reacts with 2 equivalents of NaEt3BH at -78 °C in toluene to generate a catalyst that effects highly selective anti-Markovnikov hydrosilylation of the terminal double bond in 1,3- and 1,4-dienes. Primary and secondary silanes such as PhSiH3, Ph2SiH2 and PhSi(Me)H2 react with a broad spectrum of terminal dienes without affecting the configuration of the other double bond. When dienes conjugated to an aromatic ring are involved, both Markovnikov and anti-Markovnikov products are formed...
April 7, 2017: ACS Catalysis
https://www.readbyqxmd.com/read/28413693/stable-fe-zsm-5-nanosheet-zeolite-catalysts-for-the-oxidation-of-benzene-to-phenol
#16
Lingqian Meng, Xiaochun Zhu, Emiel J M Hensen
Fe/ZSM-5 nanosheet zeolites of varying thickness were synthesized with di- and tetraquaternary ammonium structure directing agents and extensively characterized for their textural, structural, and catalytic properties. Introduction of Fe(3+) ions in the framework of nanosheet zeolites was slightly less effective than in bulk ZSM-5 zeolite. Steaming was necessary to activate all catalysts for N2O decomposition and benzene oxidation. The higher the Fe content, the higher the degree of Fe aggregation was after catalyst activation...
April 7, 2017: ACS Catalysis
https://www.readbyqxmd.com/read/28413692/photoredox-mediated-routes-to-radicals-the-value-of-catalytic-radical-generation-in-synthetic-methods-development
#17
REVIEW
Jennifer K Matsui, Simon B Lang, Drew R Heitz, Gary A Molander
Photoredox catalysis has experienced a revitalized interest from the synthesis community during the past decade. For example, photoredox/Ni dual catalysis protocols have been developed to overcome several inherent limitations of palladium-catalyzed cross-couplings by invoking a single-electron transmetalation pathway. This Perspective highlights advances made by our laboratory since the inception of the photoredox/Ni cross-coupling of benzyltrifluoroborates with aryl bromides. In addition to broadening the scope of trifluoroborate coupling partners, research using readily oxidized hypervalent silicates as radical precursors that demonstrate functional group compatibility is highlighted...
April 7, 2017: ACS Catalysis
https://www.readbyqxmd.com/read/28413691/proton-transfer-in-the-catalytic-cycle-of-nife-hydrogenases-insight-from-vibrational-spectroscopy
#18
REVIEW
Philip A Ash, Ricardo Hidalgo, Kylie A Vincent
Catalysis of H2 production and oxidation reactions is critical in renewable energy systems based around H2 as a clean fuel, but the present reliance on platinum-based catalysts is not sustainable. In nature, H2 is oxidized at minimal overpotential and high turnover frequencies at [NiFe] catalytic sites in hydrogenase enzymes. Although an outline mechanism has been established for the [NiFe] hydrogenases involving heterolytic cleavage of H2 followed by a first and then second transfer of a proton and electron away from the active site, details remain vague concerning how the proton transfers are facilitated by the protein environment close to the active site...
April 7, 2017: ACS Catalysis
https://www.readbyqxmd.com/read/28405491/transient-behavior-of-ni-nio-x-functionalized-srtio3-in-overall-water-splitting
#19
Kai Han, Tomas Kreuger, Bastian Mei, Guido Mul
Transients in the composition of Ni@NiO x core-shell co-catalysts deposited on SrTiO3 are discussed on the basis of state-of-the-art continuous analysis of photocatalytic water splitting, and post-XPS and TEM analyses. The formation of excessive hydrogen (H2:O2 ≫ 2) in the initial stages of illumination demonstrates oxidation of Ni(OH)2 to NiOOH (nickel oxyhydroxide), with the latter catalyzing water oxidation. A disproportionation reaction of Ni and NiOOH, yielding Ni(OH)2 with residual embedded Ni, occurs when illumination is discontinued, which explains repetitive transients in (excess) hydrogen and oxygen formation when illumination is reinitiated...
March 3, 2017: ACS Catalysis
https://www.readbyqxmd.com/read/28367354/mild-redox-neutral-alkylation-of-imines-enabled-by-an-organic-photocatalyst
#20
Niki R Patel, Christopher B Kelly, Allison P Siegenfeld, Gary A Molander
An operationally simple, mild, redox-neutral method for the photoredox alkylation of imines is reported. Utilizing an inexpensive organic photoredox catalyst, alkyl radicals are readily generated from the single-electron oxidation of ammonium alkyl bis(catecholato)silicates and are subsequently engaged in a C-C bond-forming reaction with imines. The process is highly selective, metal-free, and does not require a large excess of the alkylating reagent or the use of acidic additives.
March 3, 2017: ACS Catalysis
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