Catalyst Libraries Research Articles

ChemInform Abstract: Libraries of Transition-Metal Catalysts: High-Throughput Screening of Catalysts for Synthetic Organic Methodology

ChemInform Abstract: Libraries of Transition-Metal Catalysts: High-Throughput Screening of Catalysts for Synthetic Organic Methodology

Real-Time Monitoring of Millimeter-Tall Vertically Aligned Single-Walled Carbon Nanotube Growth on Combinatorial Catalyst Library

The rapid growth dynamics of millimeter-tall, vertically aligned single-walled carbon nanotubes (VA-SWCNTs) was studied using a simple real-time monitoring method. By using combinatorial catalyst libraries, VA-SWCNT growth curves under various catalyst conditions were obtained in a single chemical vapor deposition (CVD) run. VA-SWCNTs grew at constant or gradually decreasing rates for several minutes and then abruptly ceased growth. This unusual behavior of the growth occurred under wide ranges of catalyst and CVD conditions.

High Throughput Discovery of Families of High Activity WGS Catalysts: Part I - History and Methodology

State-of-art water gas shift catalysts (FeCr for high temperature shift and CuZn for low temperature shift) are not active enough to be used in fuel processors for the production of hydrogen from hydrocarbon fuels for fuel cells. The need for drastically lower catalyst volumes has triggered a search for novel WGS catalysts that are an order of magnitude more active than current systems. Novel catalytic materials for the high, medium and low temperature water gas shift reactions have been discovered by application of combinatorial methodologies. Catalyst libraries were synthesized on 4 inch wafers in 16 x 16 arrays and screened in a high throughput scanning mass spectrometer in the temperature range 200 degrees C to 400 degrees C. More than 200 wafers were screened under various conditions and more than 250,000 experiments were conducted to comprehensively examine catalyst performance for various binary, ternary and higher-order compositions.

Gas-Phase Synthesis of Gradient Catalyst Libraries Consisting of Nanoparticles Supported on High Surface Area Porous Substrates

Gas-Phase Synthesis of Gradient Catalyst Libraries Consisting of Nanoparticles Supported on High Surface Area Porous Substrates

Millimeter-tall single-walled carbon nanotube forests grown from ethanol

Millimeter-tall vertically-aligned carbon nanotubes (VA-CNTs) were grown from ethanol under ambient pressure by Co-catalyzed chemical vapor deposition (CVD), with systematic optimization of the CVD temperature and catalytic conditions using combinatorial catalyst libraries. We investigated the use of both aluminum oxide and silicon oxide as underlayers for the Co catalyst and found that VA-CNTs grew to millimeter heights in 15–30min when the pyrolysis of ethanol was carried out at high temperatures (⩾850°C) and long residence times (⩾10s). Thick Co catalytic layers (⩾1.3nm) produced (sub)millimeter-tall multi-walled VA-CNTs on both the aluminum oxide and silicon oxide underlayers. However, thin Co catalytic layers (0.62–1.0nm) produced (sub)millimeter-tall VA-CNTs, which consisted mainly of single-walled CNTs, only on the aluminum oxide underlayers. Stripe patterns were found in the VA-CNTs near the substrate on both aluminum oxide and silicon oxide, indicating some instability prior to growth termination. The possible roles of aluminum oxide in growing millimeter-tall single-walled VA-CNTs were discussed.

A Simple Combinatorial Method Aiding Research on Single-Walled Carbon Nanotube Growth on Substrates

Establishing fabrication methods of carbon nanotubes (CNTs) is essential to realize many applications expected for CNTs. Catalytic growth of CNTs on substrates by chemical vapor deposition (CVD) is promising for direct fabrication of CNT devices, and catalyst nanoparticles play a crucial role in such growth. We have developed a simple method called “combinatorial masked deposition (CMD)”, in which catalyst particles of a given series of sizes and compositions are formed on a single substrate by annealing gradient catalyst layers formed by sputtering through a mask. CMD enables preparation of hundreds of catalysts on a wafer, growth of single-walled CNTs (SWCNTs), and evaluation of SWCNT diameter distributions by automated Raman mapping in a single day. CMD helps determinations of the CVD and catalyst windows realizing millimeter-tall SWCNT forest growth in 10 min, and of growth curves for a series of catalysts in a single measurement when combined with real-time monitoring. A catalyst library prepared using CMD yields various CNTs, ranging from individuals, networks, spikes, and to forests of both SWCNTs and multi-walled CNTs, and thus can be used to efficiently evaluate self-organized CNT field emitters, for example. The CMD method is simple yet effective for research of CNT growth methods.

Acidity Characterization of Catalyst Libraries by High-Throughput Testing

The development of acid and bi-functional catalysts with controlled properties is a matter of high interest for refinery applications. The issues of characterization and quantification of catalyst acidity in the frame of high throughput (HT) screening are outlined. The present study deals with the development of a HT method for the characterization of the Bronsted acidity of 80 bifunctional catalysts library by means of isomerization of 3,3-dimethylbutene testing. Catalytic results are validated with literature data. The choice of probe molecules, the operating conditions and deactivation issues upon testing are addressed. This method allows quantifying very accurately the acid strength. Catalyst ranking from large library can be achieved an acidity scale in conditions similar to the practical applications. The pros and cons of the use of model reactions for HT quantitative characterization are discussed.

Efficient in situ three-component formation of chiral oxazoline-Schiff base copper(ii) complexes: towards combinatorial library of chiral catalysts for asymmetric Henry reaction

A combinatorial in situ three-component chiral oxazoline-Schiff base copper(II) complex catalyst formation method was developed. This simple combinatorial chiral catalyst approach provided a modular library of chiral oxazoline-Schiff base copper(II) complex catalysts. The catalytic activity of these in situ generated catalysts can be rapidly and conveniently evaluated in the asymmetric Henry reaction. Moderate to good yields and enantioselectivities (up to 92% ee) were obtained under the optimized condition. The combination of modular three-component catalyst formation and in situ asymmetric reaction provides a new technology in asymmetric catalysis.

Predictive modeling in homogeneous catalysis: a tutorial

Predictive modeling has become a practical research tool in homogeneous catalysis. It can help to pinpoint 'good regions' in the catalyst space, narrowing the search for the optimal catalyst for a given reaction. Just like any other new idea, in silico catalyst optimization is accepted by some researchers and met with skepticism by others. The basic requirements for good predictive models are a reliable set of initial experimental data, a method for generating and testing virtual catalyst libraries, and robust validation protocols. Once you have these, the key task is translating the catalysis problems into something that a computer can understand. In this tutorial review we explain in simple terms what predictive modeling actually is, why and when should one use it, and how it can be implemented.

Combinatorial Design of Al2O3 Supported Au Catalysts For Preferential CO Oxidation

Multi-component Au/Al2O3 catalysts were designed and tested for PROX reaction using holographic research strategy. On the bases of our previous study Pb has been selected as the main modifier of the Au. In addition to Au and Pb the catalysts library contained V, Ba, Ce, Sm, Ag and Cu resulting in multi component catalysts tailored for PROX reaction. After preparation and testing of 173 catalysts within five generations new catalyst compositions with excellent performance have been obtained. Upon using the best catalyst CO could be removed almost completely and the selectivity of oxygen towards the CO oxidation was around 75%. In the course of catalyst library design it has been revealed that the selection of the objective function (OF) has high impact both on the rate of optimization and the performance of catalysts designed. The complex OF was created from two single desirability functions related to CO conversion and oxygen selectivity towards CO oxidation. In order to maintain high optimization rate there was a need to change the weights of single desirability functions in the course of catalyst library design. The results show that Pb, Sm, Cu and Ag are the key modifiers for PROX reaction under experimental conditions applied.

Combinatorial Science and High-Throughput Experiments for Catalysis

Inaugural First International Combinatorial Catalysis Symposium (ICCS) was held at KAIST in Daejeon from July 20 to 22, 2008 as a post-conference of the 14th International Congress on Catalysis. Combinatorial technology and high-throughput experimentation (HTE) are becoming a revolutionary research paradigm to discover novel materials more efficiently, rapidly and economically. Conventional one-by-one experiment must be very time-consuming and expensive to perform tremendously large number of experiments which is the case for the development of novel catalysts. During the past decade, automation tools and robotics to prepare catalyst libraries, high-throughput characterization methods and software for informatics and data mining for obtaining the knowledge and optimizing the experimental design have been developed quite successfully. One plenary lecture, two keynote lectures and five invited talks as well as 22 oral and 15 poster presentations reported these recent progresses by world-renowned experts. This symposium provided participants with an open place for exchanging new information and constructive ideas to combinatorial catalysis. Through this symposium our friendship and networking was more intensified for the future. Among these presentations, 16 papers were selected after extensive reviews and revisions for this issue. The recent advances in the optimum library work-up, experimental design, data mining and high-throughput experiments for catalysis research was reported in this issue. We sincerely hope that these papers will be an important reference for combinatorial catalysis research.

Improved Fuel Cell Oxidation Catalysis in Pt1−xTax

Sputter codeposition of platinum and tantalum is used to generate catalyst libraries of Pt1−xTax with 0.05< x <0.9. Extensive characterization of the libraries by high-energy X-ray diffraction reve...

High-throughput spatial resolved tests over planar model catalyst libraries: A novel reactor approach

The development of two different in-flow scanning reactors that could be successfully used for testing the activity of planar catalysts are presented. The first one, which allows stepwise scans, is composed by a reactor probe which is sealed over the planar system using a Teflon or Viton mask that also defines the reaction chamber volume. The presence of the sealing mask causes various problems that are solved with the second prototype. In this reactor a controlled leakage system is used instead of the Viton seal, which allows continuous scanning test on a reduced circular area (12.5 mm 2). In this paper the results of preliminary tests over palladium based catalysts are presented in order to discuss the applicability of both reactors.

High‐Throughput Reaction Optimisation and Activity Screening of Ferrocene‐Based Lewis Acid–Catalyst Complexes by Using Continuous‐Flow Reaction Detection Mass Spectrometry

Optimising synthetic conversions and assessing catalyst performance is a tedious and laborious endeavour. Herein, we present an automated alternative to the commonly applied sequential approaches that are used to increase catalyst discovery process efficiencies by increasing the number of entities that can be tested. This new approach combines conversion of the reactants and determination of product formation into a single comprehensive reaction detection system that can be operated with minimal catalyst and reactant consumption. With this approach, rudimentary reaction conditions can be quickly optimised and the same system can then be used to screen for the optimal homogenous catalyst in a selected solution-phase synthetic conversion. The system, which is composed of standard HPLC components, can be used to screen catalyst libraries at a repetition rate of five minutes and can be run unsupervised. The sensitive mass spectrometric detection that is implemented in the reaction detection methodology can be used for the simultaneous monitoring of reactants, catalysts and product ions. In the experiments, the three-component reaction that gives a substituted 2-imidazoline was optimised. Afterwards, the same method was used to assess a library of ferrocene-based Lewis acid catalysts for performance in the aforementioned conversion in six different solvents. We demonstrate the feasibility of using this methodology to directly compare the performance results obtained in different solvents by calibrating the solvent-specific MS responses.

Design of a Genetic Algorithm for the Simulated Evolution of a Library of Asymmetric Transfer Hydrogenation Catalysts

A library of catalysts was designed for asymmetric-hydrogen transfer to acetophenone. At first, the whole library was submitted to evaluation using high-throughput experiments (HTE). The catalysts were listed in ascending order, with respect to their performance, and best catalysts were identified. In the second step, various simulated evolution experiments, based on a genetic algorithm, were applied to this library. A small part of the library, called the mother generation (G0), thus evolved from generation to generation. The goal was to use our collection of HTE data to adjust the parameters of the genetic algorithm, in order to obtain a maximum of the best catalysts within a minimal number of generations. It was namely found that simulated evolution's results depended on the selection of G0 and that a random G0 should be preferred. We also demonstrated that it was possible to get 5 to 6 of the ten best catalysts while investigating only 10 % of the library. Moreover, we developed a double algorithm making this result still achievable if the evolution started with one of the worst G0.

Backbone Diversity Analysis in Catalyst Design

Abstractmagnified imageWe present a computer‐based heuristic framework for designing libraries of homogeneous catalysts. In this approach, a set of given bidentate ligand‐metal complexes is disassembled into key substructures (“building blocks”). These include metal atoms, ligating groups, backbone groups, and residue groups. The computer then rearranges these building blocks into a new library of virtual catalysts. We then tackle the practical problem of choosing a diverse subset of catalysts from this library for actual synthesis and testing. This is not trivial, since ‘catalyst diversity’ itself is a vague concept. Thus, we first define and quantify this diversity as the difference between key structural parameters (descriptors) of the catalysts, for the specific reaction at hand. Subsequently, we propose a method for choosing diverse sets of catalysts based on catalyst backbone selection, using weighted D‐optimal design. The computer selects catalysts with different backbones, where the difference is measured as a distance in the descriptors space. We show that choosing such a D‐optimal subset of backbones gives more diversity than a simple random sampling. The results are demonstrated experimentally in the nickel‐catalysed hydrocyanation of 3‐pentenenitrile to adiponitrile. Finally, the connection between backbone diversity and catalyst diversity, and the implications towards in silico catalysis design are discussed.

Mass spectrometric screening of chiral catalysts and catalyst mixtures

The use of quasi-enantiomeric substrates and ESI-MS as an analytical tool has made it possible to determine the intrinsic enantioselectivity of chiral catalysts by monitoring catalytic intermediates. In this way, potential problems of methods based on product analysis, which may be caused by catalytically active impurities, partial dissociation of a chiral ligand-metal complex, or a non-catalytic background reaction can be avoided. ESI-MS-based screening is fast, reliable, and operationally simple, as it does not require work-up or purification steps. Moreover, mixtures of catalysts with different molecular masses can be screened simultaneously, which is not possible with methods relying on product analysis. In this way catalyst libraries prepared in one batch by combinatorial methods can be screened without the need to synthesize and purify the catalysts individually. This screening method was successfully applied to Pd-catalyzed allylic substitutions and metal-catalyzed and organocatalytic Diels-Alder reactions.

Multiple “optimum” conditions for Co–Mo catalyzed growth of vertically aligned single-walled carbon nanotube forests

Carbon nanotubes (CNTs) were grown directly on substrates by alcohol catalytic chemical vapor deposition using a Co–Mo binary catalyst. Optimum catalytic and reaction conditions were investigated using a combinatorial catalyst library. High catalytic activity areas on the substrate were identified by mapping the CNT yield against the orthogonal gradient thickness profiles of Co and Mo. The location of these areas shifted with changes in reaction temperature, ethanol pressure and ethanol flow rate. Vertically aligned single-walled CNT (SWCNT) forests grew in several areas to a maximum height of ca. 30 μm in 10 min. A pure Co catalyst yielded a vertically aligned SWCNT forest with a bimodal diameter distribution. The effects of Mo on the formation of catalyst nanoparticles and on the diameter distribution of SWCNTs are discussed and Mo as thin as a monolayer or thinner was found to suppress the broadening of SWCNT diameter distributions.

Optically Active (Peptido‐carbene)palladium Complexes: Towards True Solid‐Phase Combinatorial Libraries of Transition Metal Catalysts

AbstractThis work explores the potential of forming transition metal catalysts by combinatorial solid‐phase peptide chemistry to produce catalysts with enzyme‐like properties of stereoselectivity, regioselectivity and even substrate selectivity. A series of new functionalised carbene precursors/donors – imidazolium salts – each containing both amino acid and carboxylic acid functionality was synthesised in solution. The readily accessible carbene precursors were incorporated within the backbones of peptides attached to PEGA resin, by standard solid‐phase peptide coupling techniques. The synthetic strategy gave easy access to both mono‐ and didentate ligand systems, providing folded structures around the central transition metal atoms, with different degrees of steric congestion and bite angles. Changing the number of incorporated amino acids between the two carbene donors facilitated variation of the properties of the complexes. Both ligand systems were complexed to palladium(II) by standard base treatment, and the Pd complexes were studied by mass spectrometry and NMR spectroscopy. The monodentate (carbene)palladium complexes were each the product of enolisation of a neighbouring carbonyl group and loss of a proton, followed by coordination of the oxy anion to the palladium atom. The established methods are suitable for the combinatorial synthesis of palladium catalysts on solid support. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2008)

Insight into the Mechanism of the Asymmetric Ring-Opening Aminolysis of 4,4-Dimethyl-3,5,8-trioxabicyclo[5.1.0]octane Catalyzed by Titanium/BINOLate/Water System: Evidence for the Ti(BINOLate)2-Bearing Active Catalyst Entities and the Role of Water

The mechanism of the enantioselective ring-opening aminolysis of 4,4-dimethyl-3,5,8-trioxabicyclo[5.1.0]octane with benzylamine, catalyzed by the titanium-BINOLate species generated in situ from a mixture of enantiopure BINOL (1,1'-bi-2-naphthol), Ti(OiPr)4, and H2O in the presence of benzylamine in toluene, was investigated in detail using a combination of reaction profile measurements, nonlinear effect (NLE) studies, solution (1)H NMR analysis, electrospray ionization mass spectrometry (ESI-MS), as well as the results obtained from screening of dynamic catalyst library of complexes L(a)/Ti/L(b) (L(a) or L(b) = chiral diol ligands). The BINOL-to-titanium ratio and the presence or absence of water in the catalytic system were found to exert profound influences on both reactivity and enantioselectivity of the reaction. The NLE studies revealed that the titanium species involved in the catalysis should contain more than one BINOL unit, either within or at the periphery of the catalytic cycle. ESI-MS analysis of the catalytic systems provided strong support in favor of the mechanistic proposal that titanium complexes bearing the Ti(BINOLate)2 moiety should be the active species responsible for the catalysis, which was further confirmed by the observation of synergistic effect of the heteroligand combinations during screening of the dynamic catalyst library. ESI-MS analysis of the reaction system indicated that water does not take part in the catalyst generation, which is an unprecedented finding in contrast to the previous mechanistic understandings in the titanium catalytic chemistry involving the participation of water. Most probably, water functions as a proton shuttle in the catalysis, facilitating the proton transfer between the reactants. Furthermore, the origin of (+)-NLE observed in the present catalytic system is rationalized on the basis of the ESI-MS analysis of the catalyst system prepared from a 1:1 pseudoracemic mixture of (S)-BINOL and (R)-3,3',6,6'-D4-BINOL. Finally, the reactivity differences between several couples of epoxide/amine combinations were tentatively rationalized on the basis of the arguments on their relative coordination preferences, and several other aliphatic amines were also found to efficiently ring-open the titled epoxide in excellent enantioselectivities. The results from this study are expected to shed some light on the often elusive chemistry of Ti(IV)-based catalytic systems where water or molecular sieves (or alcohols, etc.) are found to play an important yet inexplicable role and may help the search for effective asymmetric Ti(IV) catalysts for other types of reactions.