Catalyst Libraries Research Articles

An Efficient and Stable Hydrophobic Molecular Cobalt Catalyst for Water Electro-oxidation at Neutral pH

The synthesis of a library of molecular water oxidation catalysts based on the Co complex of tris(2-benzimidazolylmethyl)amine is described. Hydrophobicity was identified as the key variable in mediating the catalytic competence of the complexes. The change in this parameter correlates with both the conformational mobility of the ligand core and the structural changes in the local solvent environment around the metal site. The optimal Co complex identified is hydrophobic, because of three semifluorinated side chains. It catalyzes water electro-oxidation efficiently at neutral pH, with an overpotential of 390 mV and a turnover frequency (TOF) of 1.83 s–1 in the absence of soluble Co salts. The catalyst can be immobilized through physisorption, and it remains stable in prolonged electrolysis experiments.

Quantitative screening of an extended oxidative coupling of methane catalyst library

A comprehensive microkinetic model, including catalyst descriptors, that accounts for the homogeneous as well as heterogeneously catalyzed reaction steps in Oxidative Coupling of Methane (OCM) was used in the assessment of large kinetic datasets acquired on five different catalytic materials. The applicability of the model was extended from alkali magnesia catalysts represented by Li/MgO and Sn-Li/MgO and alkaline earth lanthana catalysts represented by Sr/La2O3 to rare earth-promoted alkaline earth calcium oxide catalysts, represented by LaSr/CaO, and to a Na-Mn-W/SiO2 catalyst. The model succeeded in adequately simulating the performance of all five investigated catalysts in terms of reactant conversion and product selectivities in the entire range of experimental conditions. It was found that the activity of Sr/La2O3, in terms of methane conversion, is approximately 2, 5, 30 and 33 times higher than over the La-Sr/CaO, Sn-Li/MgO, Na-Mn-W/SiO2 and Li/MgO catalysts, respectively, under identical operating conditions. This was attributed mainly to the high stability of adsorbed hydroxyls, the high stability of adsorbed oxygen and the high concentration of active sites of Sr/La2O3. The selectivity towards C2 products was found to depend on the methyl radical sticking coefficient and the stability of the adsorbed oxygen and was the highest on the Na-W-Mn/SiO2 catalyst, that is 75% at about 1% methane conversion and 1023K, 190kPa and inlet molar CH4/O2 ratio of 4.

Et3N catalyzed cascade reaction of Meldrum’s acid with ortho-hydroxyaryl aldehydes for the synthesis of coumarin-3-carboxylic acids under solvent-less condition

The synthesis of coumarin-3-carboxylic acids in good yields is realized through a triethylamine catalyzed Knoevenagel-intramolecular cyclization tandem reaction of Meldrum’s acid with various ortho-hydroxyaryl aldehydes. This method expands the catalyst library about the synthesis of coumarin-3-carboxylic acids and also has advantages of using much less water as solvent, a cheap and eco-friendly catalyst, clean reaction conditions, simple workup procedure and easy isolation.

Site-Selective Acylations with Tailor-Made Catalysts.

The acylation of alcohols catalyzed by N,N‐dimethylamino pyridine (DMAP) is, despite its widespread use, sometimes confronted with substrate‐specific problems: For example, target compounds with multiple hydroxy groups may show insufficient selectivity for one hydroxyl, and the resulting product mixtures are hardly separable. Here we describe a concept that aims at tailor‐made catalysts for the site‐specific acylation. To this end, we introduce a catalyst library where each entry is constructed by connecting a variable and readily tuned peptide scaffold with a catalytically active unit based on DMAP. For selected examples, we demonstrate how library screening leads to the identification of optimized catalysts, and the substrates of interest can be converted with a markedly enhanced site‐selectivity compared with only DMAP. Furthermore, substrate‐optimized catalysts of this type can be used to selectively convert “their” substrate in the presence of structurally similar compounds, an important requisite for reactions with mixtures of substances.

A Concise Access to C2-Symmetric Chiral 4-Pyrrolidinopyridine Catalysts with Dual Functional Side Chains.

A practical method was developed for the preparation of a diastereomeric library of C2-symmetric chiral 4-pyrrolidinopyridine catalysts with dual amide side chains. Use of a racemic precursor is the key to the concise production of catalysts with diverse stereochemisty.

Fluorescence imaging technology (FI) for high-throughput screening of selenide-modified nano-TiO2 catalysts.

A high-throughput screening (HTS) method based on fluorescence imaging (FI) was implemented to evaluate the catalytic performance of selenide-modified nano-TiO2. Chemical ink-jet printing (IJP) technology was reformed to fabricate a catalyst library comprising 1405 (Ni(a)Cu(b)Cd(c)Ce(d)In(e)Y(f))Se(x)/TiO2 (M6Se/Ti) composite photocatalysts. Nineteen M6Se/Tis were screened out from the 1405 candidates efficiently.

A Novel High‐Throughput Screening of Multicomponent Photocatalysts for Decomposition of Organic Pollutants Based on Fluorescence Imaging

AbstractA novel high‐throughput screening (HTS) method based on fluorescence imaging was developed for the exploration of photocatalysts. In our HTS method, UV lithography technique was adopted to prepare a chip with 1262 independent microreactors. Aiming to fabricate a large group of catalysts, we selected the sulfides of six metals, Cd, Ni, Zn, Sr, Ce, Y to modify nano‐MoS2. In practice, a color ink‐jet printing technology was adapted to obtain a catalyst library. The structural feature and chemical composition of the individual catalysts were typically characterized by scanning electron microscopy and X‐ray photoelectron spectroscopy. The catalytic performance of the library was evaluated with fluorescence imaging. The selected catalysts with the highest catalytic activity are the following three combinations of (Cd0.03Ni0.77Zn0.04Sr0.02Ce0.12Y0.01)Sx/MoS2, (Cd0.23Ni0.06Zn0.18Sr0.00Ce0.26Y0.27) Sx/MoS2, and (Cd0.04Ni0.32Zn0.31Sr0.00Ce0.18Y0.15)Sx/MoS2, which were verified in conventional batch reactions.

Correlation of Electronic Structure with Catalytic Activity: H2–D2Exchange across CuxPd1–xComposition Space

The relationship between alloy catalyst activity and valence band electronic structure has been investigated experimentally across a broad, continuous span of CuxPd1–x composition space. CuxPd1–x composition spread alloy films (CSAFs) were used as catalyst libraries with a 100 channel microreactor to measure the H2–D2 exchange kinetics over a temperature range of 333–593 K at 100 discrete CuxPd1–x compositions spanning the range x = 0.30–0.97. The H2–D2 exchange activity exhibits a monotonic decrease over the composition range x = 0.30–0.97. A steady state, microkinetic model was used to estimate the energy barriers to dissociative H2 adsorption, ΔEads‡, and recombinative H2 desorption, ΔEdes‡, as functions of alloy composition, x. Their values fall in the ranges ΔEads‡(x) = 0.15 to 0.45 eV and ΔEdes‡(x) = 0.55–0.65 eV. Spatially resolved UV photoemission spectra were obtained from the CuxPd1–x CSAF and used to estimate the average energy of the filled states of the valence band as a function of alloy com...

1D Models for Enzymatic Biological Fuel Cells

Enzyme electrocatalysts offer a unique approach to fuel cell system design by broadening the range of fuel and oxidants available expanding the library of low cost organic and transition metal catalysts. Increased understanding of redox enzymes will further our ability to design catalyst active sites and possibly lead to breakthroughs in enzyme electrocatalytic technology. The design and analysis of bioelectrocatalytic systems, not only for fuel cells but also for biosensor and bioconversion applications, requires detailed quantitative understanding of the physicochemical phenomena controlling such systems (kinetics, mass transport etc.). Similarly, building real-world systems with bioelectrocatalysts requires engineering approaches to predict the effect of length scales on device performance. This article describes some modeling approaches for enzymatic fuel cells that are applicable to many other systems, including biosensors and bioreactors, with some emphasis on recent advances. We discuss basic thin film 1D models and their analytical and numerical solutions, and then introduce the treatment of porous bioelectrodes, which covers the vast majority of practical devices. Finally we describe one type of complete cell model.

On the mechanism of the palladium bis(NHC) complex catalyzed CH functionalization of propane: experiment and DFT calculations.

We report a detailed mechanistic study on the CH functionalization of alkanes by palladium complexes with chelating bis(N-heterocyclic carbene) (NHC) complexes. The experimental results are complemented by detailed DFT calculations, which allow us to rationalize the regioselectivity and the catalytic activity. The study includes a library of catalysts with different electronic and steric properties, kinetic data, and isotope effects. The combined experimental and computational results favor a mechanism involving organometallic palladium(IV) intermediates. Furthermore, it is shown that at high halide loadings a different mechanism is operative.

Stereoselectivity in Asymmetric Catalysis: The Case of Ruthenium-Catalyzed Ketone Hydrogenation.

The ruthenium-catalyzed asymmetric hydrogenation of simple ketones to generate enantiopure alcohols is an important process widely used in the fine chemical, pharmaceutical, fragrance, and flavor industries. Chiral diphosphine-RuCl2-1,2-diamine complexes are effective catalysts for the reaction giving high chemo- and enantioselectivity. However, no diphosphine-RuCl2-1,2-diamine complex has yet been discovered that is universal for all kinds of ketone substrates, and the ligands must be carefully chosen for each substrate. The procedure of finding the best ligands for a specific substrate can be facilitated by using virtual screening as a complement to the traditional experimental screening of catalyst libraries. We have generated a transition state force field (TSFF) for the ruthenium-catalyzed asymmetric hydrogenation of simple ketones using an improved Q2MM method. The developed TSFF can predict the enantioselectivity for 13 catalytic systems taken from the literature, with a mean unsigned error of 2.7 kJ/mol.

The effect of the metal precursor-reduction with hydrogen on a library of bimetallic Pd-Au and Pd-Pt catalysts for the direct synthesis of H2O2

Two sets of bimetallic Pd-Pt (Pd: 1.0; Pt: 0.25-1.0%, w/w) and Pd-Au (Pd: 1.0; Au: 0.25-1.0%, w/w) catalysts have been used, with no added promoter, in the catalytic direct synthesis (CDS) of hydrogen peroxide from its elements at 2°C with a CO2/O2/H2 mixture (72/25.5/2.5%, respectively). The catalysts were supported on the commercial macroreticular ion-exchange resin Lewatit K2621 and were obtained from the reduction with H2 of ion-exchanged cationic precursors at 5bar and at 60°C. The addition of Pt or Au to Pd produced an increase of the initial overall catalytic activity in comparison with monometallic Pd with both the second metals, but with Pt the increase was much higher than with Au. Moreover, the addition of 0.25% (w/w) Pt, or more, invariably made all the Pd-Pt catalysts less selective with respect to Pd alone. In the case of Au, by contrast, the addition of 0.25% w/w produced an increase, albeit small, of the selectivity. As the result, the most active and productive Pd-Pt catalyst was 1Pd025PtK2621 with 1891 mol(H2) mol(Pd+Pt)−1 h−1 initially consumed, 1875 mol(H2O2) mol(Pd+Pt)−1 h−1 initially produced, a 45% selectivity towards H2O2 at 50% conversion of H2. In the case of the Pd-Au bimetallic catalysts, 1Pd025AuK2621 was the best one, with 1184 mol(H2) mol(Pd+Pt)−1 h−1 initially consumed, 739 mol(H2O2) mol(Pd+Pt)−1 h−1 initially produced, a 55% selectivity towards H2O2 at 50% conversion of H2. Although the characterization of the Pd-Pt and Pd-Au catalysts with TEM showed that the morphology of the nanostructured metal phases in the Pd-Pt and Pd-Au catalysts was very different from each family to the other, no clear correlation between the size of the nanoparticles and their distribution and the catalytic performance was apparent. These catalysts were also generally different, especially the Pd-Au ones, from previously reported related materials obtained from the same support and the same precursor, but with a different reducing agent (formaldehyde).

Pd-Au and Pd-Pt catalysts for the direct synthesis of hydrogen peroxide in absence of selectivity enhancers

Organic polymers are suitable candidates as catalytic materials for the direct synthesis of hydrogen peroxide (DSR) from dihydrogen and dioxygen, in view of the low temperature applied. In particular, strongly acidic ion-exchange resins have been already proposed for this reaction, either as carriers of PdII ions or Pd0 nanoparticles, but not for bimetallic Pd/Pt and Pd/Au catalysts, which are also active in this reaction. The introduction of PdII, PtII and AuIII precursors into the commercial ion-exchangers Lewatit K2621 and their subsequent reduction with an aqueous formaldehyde solution produced a library of bimetallic catalysts with 1% Pd (w/w) and a variable content of either Pt (0.1–1%, w/w) or Au (0.25–1%, w/w). The catalysts were tested with no selectivity enhancer in the DSR under batchwise condition at 2°C in MeOH at 2MPa, with a CO2/O2/H2 (72/25.5/2.5) mixture. In the Pd-Pt catalysts the addition of the smallest amount of Pt (0.1%, w/w) gave the best results in terms of the initial selectivity (43%) and the top concentration of H2O2 achieved in solution increased to 14.3mM from 10.8mM with the monometallic catalyst. With higher platinum amount the initial selectivity was lower, but in all these catalysts the addition of platinum limited the decrease of selectivity with time in comparison with the monometallic one. The addition of platinum in increasing amount seems also to favour poisoning of the catalysts by the products (most likely by H2O2). The presence of small amount of gold is also effective in achieving a comparatively high initial selectivity. With the addition of only 0.25% Au (w/w), the initial selectivity towards H2O2 and the top concentration of the product were boosted up to 61% and 17mM, respectively. Gold in relatively little amount is also useful to limit the decrease of selectivity with time. In particular, it seems to limit the combustion of hydrogen and to be much more effective than platinum in this respect. Preliminary TEM analysis of the size and size distribution of the metal nanoparticles in the Au-Pd bimetallic catalysts shows that relatively large nanoparticles are preferable to achieve higher and stable selectivity, but also that size is likely not the most important parameter in controlling the catalyst performance.

Combined Catalysis and Optical Screening for High Throughput Discovery of Solar Fuels Catalysts

Considerable research and development efforts are being devoted to the efficient generation of solar fuels. A solar fuels device couples a solar photoabsorber with catalysts to convert solar energy to chemical energy via reactions such as oxygen evolution (water splitting). Widespread deployment of this technology hinges upon discovery of new materials through efforts such as the high throughput screening of oxygen evolution catalysts, as discussed in this manuscript. We derive an expression for the efficiency of the oxygen evolution catalyst that combines catalytic and optical properties. Using this hybrid efficiency, we screen 5456 samples in a (Fe-Co-Ni-Ti)Ox pseudo-quaternary catalyst library using automated, high throughput electrochemistry and optical experiments. The observed compositional trends in this catalyst efficiency lead to the discovery of a new high performance composition region.

An albumin-derived peptide scaffold capable of binding and catalysis.

We have identified a 101-amino-acid polypeptide derived from the sequence of the IIA binding site of human albumin. The polypeptide contains residues that make contact with IIA ligands in the parent protein, and eight cysteine residues to form disulfide bridges, that stabilize the polypeptide structure. Seventy-four amino acids are located in six α-helical regions, while the remaining thirty-seven amino acids form six connecting coil/loop regions. A soluble GST fusion protein was expressed in E. coli in yields as high as 4 mg/l. This protein retains the IIA fragment’s capacity to bind typical ligands such as warfarin and efavirenz and other albumin’s functional properties such as aldolase activity and the ability to direct the stereochemical outcome of a diketone reduction. This newly cloned polypeptide thus represents a valuable starting point for the construction of libraries of binders and catalysts with improved proficiency.

Double-asymmetric hydrogenation strategy for the reduction of 1,1-diaryl olefins applied to an improved synthesis of CuIPhEt, a C2-symmetric N-heterocyclic carbenoid.

A library of iridium and rhodium phosphine catalysts have been screened for the double-asymmetric hydrogenation of 2,6-di-(1-phenylethenyl)-4-methylaniline to produce the C2-symmetric aniline precursor of the N-heterocyclic carbenoid CuIPhEt. The best catalyst produced the desired enantiomer in 98.6% selectivity. This rare example of a highly selective hydrogenation of a 1,1-diaryl olefin enables a four-step asymmetric synthesis of the C2-symmetric phenylethyl imidazolium ion (IPhEt) from p-toluidine and phenylacetylene and its conversion to the hydrosilylation catalyst CuIPhEt.

A moth pheromone brewery: production of (Z)-11-hexadecenol by heterologous co-expression of two biosynthetic genes from a noctuid moth in a yeast cell factory

BackgroundMoths (Lepidoptera) are highly dependent on chemical communication to find a mate. Compared to conventional unselective insecticides, synthetic pheromones have successfully served to lure male moths as a specific and environmentally friendly way to control important pest species. However, the chemical synthesis and purification of the sex pheromone components in large amounts is a difficult and costly task. The repertoire of enzymes involved in moth pheromone biosynthesis in insecta can be seen as a library of specific catalysts that can be used to facilitate the synthesis of a particular chemical component. In this study, we present a novel approach to effectively aid in the preparation of semi-synthetic pheromone components using an engineered vector co-expressing two key biosynthetic enzymes in a simple yeast cell factory.ResultsWe first identified and functionally characterized a ∆11 Fatty-Acyl Desaturase and a Fatty-Acyl Reductase from the Turnip moth, Agrotis segetum. The ∆11-desaturase produced predominantly Z11-16:acyl, a common pheromone component precursor, from the abundant yeast palmitic acid and the FAR transformed a series of saturated and unsaturated fatty acids into their corresponding alcohols which may serve as pheromone components in many moth species. Secondly, when we co-expressed the genes in the Brewer’s yeast Saccharomyces cerevisiae, a set of long-chain fatty acids and alcohols that are not naturally occurring in yeast were produced from inherent yeast fatty acids, and the presence of (Z)-11-hexadecenol (Z11-16:OH), demonstrated that both heterologous enzymes were active in concert. A 100 ml batch yeast culture produced on average 19.5 μg Z11-16:OH. Finally, we demonstrated that oxidized extracts from the yeast cells containing (Z)-11-hexadecenal and other aldehyde pheromone compounds elicited specific electrophysiological activity from male antennae of the Tobacco budworm, Heliothis virescens, supporting the idea that genes from different species can be used as a molecular toolbox to produce pheromone components or pheromone component precursors of potential use for control of a variety of moths.ConclusionsThis study is a first proof-of-principle that it is possible to “brew” biologically active moth pheromone components through in vitro co-expression of pheromone biosynthetic enzymes, without having to provide supplementary precursors. Substrates present in the yeast alone appear to be sufficient.

Nanocatalysts for hydrogen generation from hydrazine

Hydrogen is an key future fuel of interest because it is considered as an efficient energy carrier, like electricity, releasing only water when combining with oxygen (e.g. in a fuel cell) and therefore has no negative impact on the environment. Unfortunately we are not yet able to clear the economical and engineering hurdles to store hydrogen safely and efficiently. To overcome this, onboard hydrogen generation by hydrogen storage materials comes out to be an attractive and effective approach. Therefore, the concept of onboard hydrogen generation and use based on our requirements are gaining interest, reflected by a huge number of hydrogen storage materials with high hydrogen contents. Among them, hydrous hydrazine, which is a liquid and has a hydrogen content for hydrogen release as high as 8.0 wt%, proves to be a strong candidate for onboard hydrogen generation at ambient conditions. Nanoparticle catalysts (nanocatalysts) play a significant role to control the selective generation of hydrogen from catalytic decomposition of hydrous hydrazine. Nanocatalysts based on monometallic or two-component alloy catalysts involving noble (Ru, Rh, Pt, Pd, Ir) and non-noble metals (Fe, Co, Ni, Cu) and various non-metals have been extensively studied. Screening of a vast range of nanocatalysts thus provides a library of active and selective catalysts. The structure and activity of nanocatalysts are discussed focusing on the structure–activity relationship for selective hydrogen generation from hydrous hydrazine.

Efficient approach for the design of effective chiral quaternary phosphonium salts in asymmetric conjugate additions

An efficient approach for the design of chiral quaternary phosphonium bromides as chiral phase-transfer catalysts was demonstrated. A catalyst library of phosphonium salts with various structures was readily constructed using commercially available chiral phosphines as catalyst precursors, and an optimized catalyst was successfully applied to highly enantioselective conjugate additions under base-free phase-transfer conditions with low catalyst loading.

Combined Catalysis and Optical Screening for High Throughput Discovery of Solar Fuels Catalysts

Considerable research and development efforts are being devoted to the efficient generation of solar fuels. A solar fuels device couples a solar photoabsorber with catalysts to convert solar energy to chemical energy via reactions such as oxygen evolution (water splitting). Widespread deployment of this technology hinges upon discovery of new materials through efforts such as the high throughput screening of oxygen evolution catalysts, as discussed in this manuscript. We derive an expression for the efficiency of the oxygen evolution catalyst that combines catalytic and optical properties. Using this hybrid efficiency, we screen 5456 samples in a (Fe-Co-Ni-Ti)Ox pseudo-quaternary catalyst library using automated, high throughput electrochemical and optical experiments. The observed compositional trends in this catalyst efficiency lead to the discovery of a new high performance composition region.