Catalysis Science & Technology: Catalysis in the USA

Catalysis Science & Technology is a multi-disciplinary journal that focuses on all fundamental science and technological aspects of catalysis, including biological, heterogeneous, and homogeneous. This USA themed issue builds upon the journal's focus with 24 contributions from scientists and engineers performing research in all areas of catalysis within the borders of the USA. The impact of catalysis in the United States and the rest of the world can't be understated. Catalysis, in its various forms provides society with a quality of life that we typically take for granted. The economic contributions of catalysis are enormous; and the estimated value of products generated through catalysis was nearly a trillion USD in 2005 (ref. 1) and the global catalyst market is expected to reach nearly $20 billion USD by 2016.² Discoveries and innovations in catalysis and its utilization to produce products we use on a daily basis will continue to drive the economy of the USA. As the United States explores new potential sources of energy and end-products, such as biomass and shale gas, catalysis is positioned to play a key role in ensuring the sustainable processing of these new sources.

Biomass to renewable chemicals is a current research direction of great interest in all sectors of catalysis in the United States. Since most biomass-related conversion reactions will be conducted in the liquid-phase, there is significant interest to develop a fundamental understanding of the influence of solvent on activity and selectivity of heterogeneous catalysts. In his mini-review, Gounder (10.1039/c4cy00712C) demonstrates how hydrophobically-modified zeolites containing Lewis and Brønsted bases can be highly active for carbohydrate reactions in the presence of liquid water. Neumann and colleagues (10.1039/c4cy00569d) examine the influence of lignin structure utilizing model compounds to aromatic products during catalytic fast pyrolysis. Lignin represents the only renewable source of aromatics; the development of this feed stream will invariably rely on catalysts and catalytic processes that can reduce oxygen content in lignin macromolecules while leaving the native aromaticity intact. Deng and co-workers (10.1039/C4CY01285B) demonstrate the influence of catalyst pretreatment of a bimetallic Ir–Re supported catalyst for the hydrogenolysis of glycerol to 1,3-propanediol. Catalysts directly reduced after Ir and Re metal precursor impregnation were significant more active than Ir–Re catalysts that were calcined prior to reduction demonstrating that reduced Re was key for the formation of an Ir–Re alloy, and subsequently high hydrogenolysis activity. One of the most important reactions in the conversion of biomass to chemicals is hydrodeoxyenation (HDO) – a reaction in which hydrogen is used to reduce the oxygen content in biomass derivatives. Behtash et al. (10.1039/c4cy00511b) have utilized density functional theory (DFT) calculations and microkinetic modeling to determine the predominant mechanism of HDO of methyl propionate over Pd(111). The authors' analysis demonstrated dehydrogenation of the α-carbon of methyl propionate and propanoyl–methoxy bond dissociation are rate-controlling steps and possible activity descriptors. Ethanol, a common end-product of biomass fermentation is commonly reformed to produce hydrogen. In this themed issue, Xiong et al. (10.1039/c4cy00914b) utilize bimetallic PdZn catalysts to reform ethanol in aqueous solutions at low temperatures. The authors found that PdZn nanoparticles confined within the pores of carbon nanotubes proved to be an excellent catalyst for aqueous-phase reforming of ethanol capable of producing CO-free H₂. Utilizing DFT, Luo and Asthagiri (10.1039/c4cy00582a) perform an ab initio thermodynamic study to determine the stable phases of cobalt during the steam reforming of ethanol. Based on calculated phase diagrams under ethanol steam reforming conditions, the authors suggested that a reducible support may be important for the stabilization of Co surfaces against reduction to metallic Co.

The discovery of vast quantities of shale gas reserves throughout the United States has revitalized an interest in the selective activation of aliphatic C–H bonds in methane. The development of catalysts for the selective activation/oxidation of methane to oxygenates and fuel intermediates that does not proceed through syn-gas is vital to the development of a domestic source of liquid fuel. C–H bond activation/C–X functionalization of hydrocarbons remains an active area of research in all sub-disciplines of catalysis. A number of publications in this themed issue demonstrate this is an active area of research in both homogeneous and heterogeneous catalysis. Webster-Gardiner and co-workers (10.1039/c4cy00972j) demonstrate a Rh(I) catalyst ligated by bidentate nitrogen chelates are capable of activating C–H bonds in arenes as determined by the extent of H/D exchange using benzene as a model arene. Wu et al (10.1039/c4cy00197d) demonstrate how the oxidation state and local structure of Fe as determined by X-ray absorption spectroscopy influences the ammoxidation of propylene. Weinstein and Stahl (10.1039/c4cy00764f) demonstrate through a unique solvent-assisted mechanism that aryl group can be animated over homogeneous Pd catalysts in the presence of oxygen. Some shale reserves located in the United States contain a considerable amount of “wet” gas (methane containing larger hydrocarbons, such as ethane, propane, etc.). In this themed issue, a number of potential approaches for the conversion of the larger alkanes to useful products are highlighted in select papers. Ethylene formed through the steam cracking of ethane is the world's largest produced chemicals here in the United States. Polyethylene and ethylene oxide represent two significant end products, both of which are produced by a catalytic-based process. Yan et al. (10.1039/c4cy00877d) demonstrate W and Nb-loaded mesoporous silica catalysts that are selective ethylene epoxidation catalysts when hydrogen peroxide is used as the oxidant. Incorporation of W and Nb into the silicate framework leads to high selectivity to epoxidation products, but it was determined that the tetrahedrally-coordinated cations (i.e., Lewis acids) leached into solution negatively influencing catalyst stability and lifetime. Hydrogen is a clean energy carrier for future energy systems, such as fuel cells. A potential route to H₂ is via reforming of the “wet” gas found in shale gas reserves. Krcha and Janik (10.1039/c4cy00619d) examine the reaction mechanism for propane reforming over Zr-doped CeO₂(111) utilizing density functional theory. Calculations by these authors provide evidence for a reduced surface under reforming conditions, and it was further determined that the relevant reaction pathway varied based on the chemical potential of oxygen, since this alters the step in the mechanism where the surface is re-oxidized. In addition to utilizing n-propane as an appropriate model compound for reforming chemistry, it serves the same role in mechanistic studies of catalytic cracking. Yun and Lobo (10.1039/c4c00731j) examine the influence of pretreatment temperature on the monomolecular cracking of propane by H-SSZ-13 zeolites. Depending on the pretreatment temperature, the active sites of H-SSZ-13 evolved to favor dehydrogenation over cracking.

The development of new synthetic methods is critical to understand the inner details of how all types of catalysts operate. In particular, new methods of synthesis employing colloidal approaches to synthesize metallic nanoparticles with well-controlled size and shape have shed light on how these properties of the catalyst influence activity and selectivity. The reactivity and selectivity of Au as a catalyst for the selective oxidation of benzyl alcohol under neat conditions is examined by Li and co-workers (10.1039/c3cy01064c) utilizing a thermally stable Au/alumina aerogel catalyst produced by a synthesis approach in which alkylamines mediated the deposition of aluminum oxide on to Au nanoclusters. Zeolites are the workhorse of the petroleum industry, yet robust methods to control the macroscopic properties which have significant implications for their catalytic efficacy have only recently been explored in a systematic fashion. In their mini-review, Rimer and co-workers (10.1039/c4cy00858h) summarize current efforts in zeolite synthesis in the absence of organic templating agents, and the addition of growth modifiers to tailor macroscopic crystal habit. The former topic seeks to reduce the cost associated with zeolite synthesis, while the latter is critical for the development of zeolites with improved catalytic performance. Ligand design is an essential component of an active and selective organometallic catalyst. The development of new ligands enables the application of orgnanometallic catalysts to harsher environments and chemistries where various forms of selectivity must be met. Gupta and co-workers (10.1039/c4cy00742e) provide a comprehensive study of the synthesis, characterization and reactivity studies of BOROX catalysts with a new biaryl atropisomeric ligand, iso-VAPOL. BOROX catalysts ligated by iso-VAPOL were effective in terms of both yield and high enantio- and diastereo-selectivity for the aziridination of imines with ethyl diazo acetate. Yahya et al. (10.1039/c4cy01394h) demonstrate that the modification of polyoxometallates with oligomeric amine ligands yields a catalyst capable of oxidative desulfurization of dibenzothiophene and the epoxidation of cylcooctene in a biphasic solvent (heptane/water) system using hydrogen peroxide as the oxidant.

Enzymes can be tailored through mutations to catalyze reactions that the wild-type enzyme fails to accomplish. Utilizing a mutated variant of cytochrome P450 from Bacillus megaterium, Renata et al. (10.1039/c4cy00633j) demonstrated that the engineered mutations led to an enzyme with high activity, enantioselectivity and substrate versatility for the cycloproponation of acrylate olefins with ethyl diazo acetate under aerobic conditions. In addition to the development of new catalysts, novel modifications of the process itself can have significant fundamental and applied implications. For example, Upadhye et al. (10.1039/c4cy01183j) demonstrate that localized surface plasmon resonance (LSPR) phenomena can be used to drive a chemical reaction over catalysts containing LSPR-active metals, such as Au. Even with a visible photon-to-chemical energy conversion efficiency of 5%, the authors demonstrated that the activity of the reverse water gas-shift reaction was enhanced by 30–1300% over a supported Au catalyst.

Characterization of catalysts is integral to understanding what physicochemical properties influence reactivity and selectivity. Synchrotron-based techniques have become integral for the characterization of catalysts under in situ conditions. Zhang and co-workers (10.1039/c4cy00414k) utilize synchrotron-based X-ray diffraction to characterize the structure of very small Pt nanoparticles. Childers et al. (10.1039/c4cy00846d) utilized a number of characterization techniques, including X-ray absorption spectroscopy, to understand the influence of Au addition to supported Pd catalysts on the hydrogenolysis of neopentane. Zhang and colleagues (10.1039/c4cy00938j) pursue a surface science approach to understand the mechanism of CO oxidation over Pd oxide on Pd(111) utilizing reflection absorption infrared spectroscopy (RAIRS). Although, this reaction is well-studied on metallic Pd, the current work demonstrates the importance of oxygen vacancies and metallic domains on partially-reduced PdO(101) for the promotion of CO oxidation.

In the last few decades, DFT calculations have greatly improved our understanding of how catalyst structure – both physical and chemical – influences catalytic reactivity and selectivity. The use of DFT has been highlighted many times in this USA themed issue. Many of these calculations are costly in terms of the amount of computer time required for their complete execution. Montemore and Medlin (10.1039/c4cy00335g) review the use of scaling relationships, which allow for the prediction of adsorption energies of several related species on a particular surface based on the adsorption energy of a single species on the same surface. These linear relationships ultimately decrease the computational effort required to compute relevant thermodynamic parameters. Frey and co-workers (10.1039/c4cy00763h) utilize DFT calculation and Monte Carlo simulations of the oxidation of NO to NO₂ over various late transition models to demonstrate the influence of coverage effects on the catalytic activity. Due to the influence of modified energetics due to coverage effects and the statistical availability of reaction sites, rates deviate substantially from the coverage-independent scenario. DFT methods have proven equally as powerful for the prediction of optimal catalysts and evaluation of reaction mechanisms in electrocatalysis. Tsai et al. (10.1039/c4cy01162g) utilize DFT to examine how the doping of transition metal into MoS₂ catalysts influences the catalyst's structure and its activity for the hydrogen evolution reaction (HER). Doping metals on edge sites modifies the strength of sulfur binding which ultimately controls hydrogen binding onto sulfur. A simple descriptor model allowed for the rational design of new MoS₂-type catalysts; in this work, six candidates, in addition to MoS₂ itself were identified as promising HER catalysts.

The 24 papers showcased in this themed collection offer just a glimpse of the infinite promise of catalysis; however, together they are an indication that research in catalysis in the United States is a highly active and diverse research field. This collection is a demonstration that catalysis continues to evolve and has become increasingly multidisciplinary. Research efforts in all branches of catalysis, as well as at the interface of these branches, are being pursued for the development of a complete fundamental understanding of catalytic processes and their potential industrial applications. As with past advances in catalysis, these research efforts are likely to have profound economic and societal benefits.

References

1. Google Book: U.S. Climate Change Technology Program: Technology Options for the Near and Long Term. (A Compendium of Technology Profiles and Ongoing Research and Development at Participating Federal Agencies), p. 56.
2. World Catalyst Market, February, 2013; published by Freedonia.

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