Identification Of New Catalysts Research Articles

Ni- and Ni/Pd-Catalyzed Reductive Coupling of Lignin-Derived Aromatics to Access Biobased Plasticizers.

Lignin-derived aromatic chemicals offer a compelling alternative to petrochemical feedstocks, and new applications are the focus of extensive interest. 4-Hydroxybenzoic acid (H), vanillic acid (G), and syringic acid (S) are readily obtained via oxidative depolymerization of hardwood lignin substrates. Here, we explore the use of these compounds to access biaryl dicarboxylate esters that represent biobased, less toxic alternatives to phthalate plasticizers. Chemical and electrochemical methods are developed for catalytic reductive coupling of sulfonate derivatives of H, G, and S to access all possible homo- and cross-coupling products. A conventional NiCl2/bipyridine catalyst is able to access the H-H and G-G products, but new catalysts are identified to afford the more challenging coupling products, including a NiCl2/bisphosphine catalyst for S-S and a NiCl2/phenanthroline/PdCl2/phosphine cocatalyst system for H-G, H-S, and G-S. High-throughput experimentation methods with a chemical reductant (Zn powder) are shown to provide an efficient screening platform for identification of new catalysts, while electrochemical methods can access improved yields and/or facilitate implementation on larger scale. Plasticizer tests are performed with poly(vinyl chloride), using esters of the 4,4'-biaryl dicarboxylate products. The H-G and G-G derivatives, in particular, exhibit performance advantages relative to an established petroleum-based phthalate ester plasticizer.

One Decade of Computational Studies on Single-Atom Alloys: Is In Silico Design within Reach?

ConspectusSingle-Atom alloys (SAAs) are an emerging class of materials consisting of a coinage metal (Cu, Ag, and Au) doped, at the single-atom limit, with another metal. As catalysts, coinage metals are rarely very active on their own, but when they are, they exhibit high selectivity. On the other hand, transition metals are usually very active but not as selective. Incorporating transition metals (guest elements) into coinage metals (host material) is therefore appealing for combining the activity and selectivity of each constituent in a balanced way. Additionally, first-principles calculations have shown that single atoms embedded in the surface of a coinage metal can exhibit emergent properties. Here, we describe how computational studies based on density functional theory (DFT) and kinetic Monte Carlo (KMC) simulations, often undertaken in close collaboration with experimental research groups, have shaped, over the past decade, the way we understand SAA catalysis.This Account reviews our contributions in elucidating the stability of SAAs, their electronic structure, and the way adsorbates interact and react on SAA catalytic surfaces. By studying in detail the processes that affect the stability of the SAA phase, we have shown that out of several bimetallic combinations of coinage metals with prominent Pt-group metals only PtCu and PdCu are stable surface alloys under vacuum. However, more surface alloy structures are possible in the presence of adsorbates because the latter can stabilize, via strong binding, dopants in the surface of the material. More interestingly, a large number of these surface alloys are resistant to the aggregation of dopant atoms into clusters, thereby favoring the SAA structure. These major results from DFT calculations serve as a guide for experimentalists to explore new SAA catalysts. Further analysis has shown that SAAs have a unique electronic structure with a very sharp d-band feature close to the Fermi level, analogous to the electronic structure of molecular entities. This is one of the reasons that SAAs are particularly sought after: although they are metallic nanoparticles, they have properties akin to those of homogeneous catalysts. In this context, we have contributed extensive screening studies, focusing on molecular fragments of catalytic relevance on a range of SAAs, which have driven the identification of new catalysts. We have also explored the rich chemistry of two-adsorbate systems via kinetic modeling, demonstrating how a spectator species with greater affinity for the dopant can modulate the reactivity of the catalyst via the so-called (punctured) molecular cork effect.Since the first experimental characterization of SAAs about a decade ago, theoretical models have been able to support and explain various experimental observations. These models have served as benchmarks for assessing the predictive capability of the underlying theoretical methods. In turn, the predictions that have been delivered have guided and continue to guide the experimental research efforts in the field. These advancements show that the in silico design of new SAA catalysts is now within reach.

On the Generality of Molecular Volcano Plots

AbstractIn homogeneous catalysis, the structure and electronic configuration of the active catalysts can vary significantly. Changes in ligation, oxidation, and spin states have the potential to influence the catalytic cycle energetics strongly that, to a large degree, dictate the catalytic performance. With the increased use of computational screening strategies aimed towards the identification of new catalysts, ambiguity surrounding structure/electronic configurations can be problematic, as it is unclear which species should be computed to determine a catalyst's properties. Here, we show that a single volcano plot constructed from linear free energy scaling relationships is able to account for variations in ligation, oxidation, and spin state. These linear scaling relationships can also be used to predict the free energies associated with a specific structure and electronic configuration of a catalyst based on a single descriptor. As a result, a single volcano plot can be used to screen prospective new catalysts rapidly.

ADF-STEM Imaging of Nascent Phases and Extended Disorder Within the Mo–V–Nb–Te–O Catalyst System

Materials investigated for their potential use as heterogeneous selective oxidation catalysts may sometimes show highly ordered regions at the nanoscale which are embedded within highly disordered matrices from which they nucleate. These are often referred to as X-ray amorphous due to the lack of higher-order Bragg reflections; however, here we use annular dark-field scanning transmission electron microscopy (ADF-STEM) real space imaging to characterize the local structure of nanometer size domains of compositionally and structurally complex nascent oxide phases present in multi-phase disordered solids. These images have been obtained using aberration-corrected scanning transmission electron microscopy and point to its potential for local structural characterization where the synthesis conditions for a particular phase are not optimized or where self-assembly to extended structures has been locally hindered. Such studies may lead to identification of new catalysts, provided their targeted synthesis can be scaled up.

Electroreduction of CO2 to Synthesis Gas and Hydrocarbons on Doped Carbons

Strategies to close the anthropogenic carbon cycle through a renewable energy-driven efficient chemical conversion of CO2 into usable chemical feeds have become a scientific priority. Conversion of CO2 rich streams from steel mills, concrete- or coal-fired power plants, into syn-gas alcohols or hydrocarbons is of tremendous interest for industry. Of all chemical ways to convert CO2 into hydrocarbons, the electrocatalytic coupling of water splitting and CO2 reduction appears attractive, as it is a one-step process step, occurs at ambient temperature and pressures, involves solely water and CO2 as reactants, and may involve renewable electricity from wind, solar or hydro power plants. CO2 electroreduction is typically carried out on the surface of metal catalysts. Cu catalysts generate various gas products such as H2, CO, methane and ethylene, while Au and Ag are the catalysts of choice for the selective electroreduction of CO2 into CO/H2 synthesis gas mixtures. On all metal catalysts the CO2 electroreduction suffers from prohibitively high kinetic overpotentials and limited product selectivity. This is why the identification of new catalysts or new catalyst formats, such as nanoparticles or core shell architectures, with reduced overpotentials combined with the exploration of new strategies to control and improve product selectivities has become the two key goals for CO2 reduction.In this contribution, we share recent work on the electroreduction of CO2. In particular, we discuss a family of heteroatom-doped carbon catalysts for the generation of synthesis gas. We show evidence that carbon catalysts meet and exceed the activity and selectivity of traditional Au catalysts providing an intriguing cost-effective alternative.

Structure-Activity Relationship Studies of Cyclopropenimines as Enantioselective Brønsted Base Catalysts.

We recently demonstrated that chiral cyclopropenimines are viable Brønsted base catalysts in enantioselective Michael and Mannich reactions. Herein, we describe a series of structure-activity relationship studies that provide an enhanced understanding of the effectiveness of certain cyclopropenimines as enantioselective Brønsted base catalysts. These studies underscore the crucial importance of dicyclohexylamino substituents in mediating both reaction rate and enantioselectivity. In addition, an unusual catalyst CH···O interaction, which provides both ground state and transition state organization, is discussed. Cyclopropenimine stability studies have led to the identification of new catalysts with greatly improved stability. Finally, additional demonstrations of substrate scope and current limitations are provided herein.

Strategies for the Synthesis of Supported Gold Palladium Nanoparticles with Controlled Morphology and Composition

The discovery that supported gold nanoparticles are exceptionally effective catalysts for redox reactions has led to an explosion of interest in gold nanoparticles. In addition, incorporating a second metal as an alloy with gold can enhance the catalyst performance even more. The addition of small amounts of gold to palladium, in particular, and vice versa significantly enhances the activity of supported gold-palladium nanoparticles as redox catalysts through what researchers believe is an electronic effect. In this Account, we describe and discuss methodologies for the synthesis of supported gold-palladium nanoparticles and their use as heterogeneous catalysts. In general, three key challenges need to be addressed in the synthesis of bimetallic nanoparticles: (i) control of the particle morphology, (ii) control of the particle size distribution, and (iii) control of the nanoparticle composition. We describe three methodologies to address these challenges. First, we discuss the relatively simple method of coimpregnation. Impregnation allows control of particle morphology during alloy formation but does not control the particle compositions or the particle size distribution. Even so, we contend that this method is the best preparation method in the catalyst discovery phase of any project, since it permits the investigation of many different catalyst structures in one experiment, which may aid the identification of new catalysts. A second approach, sol-immobilization, allows enhanced control of the particle size distribution and the particle morphology, but control of the composition of individual nanoparticles is not possible. Finally, a modified impregnation method can allow the control of all three of these crucial parameters. We discuss the effect of the different methodologies on three redox reactions: benzyl alcohol oxidation, toluene oxidation, and the direct synthesis of hydrogen peroxide. We show that the coimpregnation method provides the best reaction selectivity for benzyl alcohol oxidation and the direct synthesis of hydrogen peroxide. However, because of the reaction mechanism, the sol-immobilzation method gives very active and selective catalysts for toluene oxidation. We discuss the possible nature of the preferred active structures of the supported nanoparticles for these reactions. This paper is based on the IACS Heinz Heinemann Award Lecture entitled "Catalysis using gold nanoparticles" which was given in Munich in July 2012.

Imaging the electrocatalytic activity of single nanoparticles

The electrocatalytic properties of nanoparticles depend on their size, shape and composition. These properties are typically probed by measuring the total electrocatalytic reaction current of a large number of nanoparticles, but this approach is time-consuming and can only measure the average catalytic activity of the nanoparticles under study. However, the identification of new catalysts requires the ability to rapidly measure the properties of nanoparticles synthesized under various conditions and, ideally, to measure the electrocatalytic activity of individual nanoparticles. Here, we show that a plasmonic-based electrochemical current-imaging technique can simultaneously image and quantify the electrocatalytic reactions of an array of 1.6×10(5) platinum nanoparticles printed on an electrode surface, which could facilitate high-throughput screening of the catalytic activities of nanoparticles. We also show that the approach can be used to image the electrocatalytic reaction current and measure the cyclic voltammograms of single nanoparticles.

Libraries of Bidentate Phosphorus Ligands; Synthesis Strategies and Application in Catalysis (Eur. J. Inorg. Chem. 19/2008)

AbstractThe cover picture shows structures that represent three different ways of making phosphorus‐based ligand libraries: modular synthesis, solid‐phase synthesis and formation of ligands by assembly of smaller building blocks. These ligand libraries are important for the rapid identification of new catalysts for (asymmetric) conversions. In this Microreview on p. 2939 ff J. N. H. Reek et al. show examples that illustrate these strategies.

Heterogeneous catalysis: Enigmas, illusions, challenges, realities, and emergent strategies of design

Predominantly this article deals with the question of how to design new solid catalysts for a variety of industrial and laboratory-orientated purposes. A generally applicable strategy, illustrated by numerous examples, is made possible based on the use of nanoporous materials on to the (high-area) inner surfaces of which well-defined (experimentally and computationally) active centers are placed in a spatially separated fashion. Such single-site catalysts, which have much in common with metal-centered homogenous catalysts and enzymes, enable a wide range of new catalysts to be designed for a variety of selective oxidations, hydrogenations, hydrations and hydrodewaxing, and other reactions that the "greening" of industrial processes demand. Examples are given of new shape-selective, regio-selective, and enantioselective catalysts, many of which operate under mild, environmentally benign conditions. Also considered are some of the reasons why detailed studies of adsorption and stoichiometric reactions at single-crystal surfaces have, disappointingly, not hitherto paved the way to the design and production of many new heterogeneous catalysts. Recent work of a theoretical and high-throughout nature, allied to some experimental studies of well-chosen model systems, holds promise for the identification of new catalysts for simple, but industrially important reactions.

Identification of New Catalysts to Promote Imidazolide Couplings and Optimisation of Reaction Conditions Using Kinetic Modelling

Three catalysts were used to promote an imidazolide coupling. These catalysts were 2-hydroxypyridine (HOPy), endo-N-hydroxy-5-norbornene-2,3-dicarboximide (HONB) (both reported for the first time to catalyse this type of reaction), and 1-hydroxybenzotriazole (HOBt). The thermal safety, cost, and catalytic effectiveness of these three catalysts are compared. In addition, kinetic modelling using the Dynochem software was used to optimise the HOBt- and HOPy-catalysed reactions. By use of this simulation method, the optimal reaction conditions, such as catalyst quantity, reaction time, and reaction temperature were predicted. Subsequent experiments confirmed that the predictions were accurate.

Identification of new catalysts for the asymmetric reduction of imines into chiral amines with polymethylhydrosiloxane using high-throughput screening

The use of high-throughput techniques allowed the rapid identification of new catalysts for the enantioselective reduction of imines using polymethylhydrosiloxane (PMHS) as a reducing agent. By a simple modification of the chiral ligand structure that came out of the screening, the enantioselectivity of the reduction was increased from 40% ee to 60% ee.