The CENTC currently supports ten collaborative research projects.  Each project involves collaborators from multiple institutions, including faculty, students, and postdocs.  These researchers, while physically separated around the U.S., communicate regularly via email and cyber-videoconferencing.  Summaries of these projects are given below. 

1. Alkane Metathesis

Watch the video - "Today's Chemistry, Tomorrow's Fuels" presented by the Morehead Planetarium and Science Center

Collaborators: Prof. Alan Goldman (Rutgers), Prof. Maurice Brookhart (UNC-CH), Prof. Susannah Scott (UCSB), Prof. Richard Schrock (MIT).

Metathesis of alkanes is a potentially highly valuable transformation for processing petroleum.  It allows for upgrading low carbon number alkanes to higher carbon number alkanes useful for transportation fuels and for degrading waxes (alkanes which are solid at ambient temperatures) to lower molecular weight alkanes in the liquid fuel range.  A method for achieving alkane metathesis has been conceived and developed by this CENTC team.  As illustrated below for hexane, a homogeneous catalyst system is employed to redistribute the carbon and hydrogen chains.  Hexane is redistributed give a distribution of linear alkanes from C2 up to C20.  With one of the catalyst systems employing decane as the initial substrate, an equilibrium distribution of alkanes was obtained under the reaction conditions.  Details of the catalyst system used and the unique mechanism for this incredible interconversion will be published shortly. 

2. Anti-Markovnikov Hydroamination

Prof. John Hartwig (UI-UC), Prof. Karen Goldberg (U. Wash.).

Listed among the ten great challenges in catalysis in the early 1990s, the hydroamination of alkenes remains, despite tremendous research efforts, an unsolved synthetic problem. Such a transformation would provide an atom-economic, environmentally benign synthesis of alkyl amines directly from relatively inexpensive and abundant alkene feedstock. A process favoring the formation of anti-Markovnikov products would be particularly valuable.

  The discovery of viable catalysts for large-scale processes could fundamentally change the bulk production of surfactants, detergents and plasticizers, as well as the synthesis of fine chemicals and pharmaceutical intermediates. Looking toward the future, terminal amines could potentially be available directly from alkanes by coupling the transformation with selective alkane dehydrogenation. The Hartwig research group at UIUC and Goldberg research group at UW are collaborating in the exploration of various strategies to accomplish the metal-catalyzed anti-Markovnikov hydroamination of alkenes.

3. Harnessing and Making Oxygen

Prof. Karen Goldberg (U. Wash.), Prof. Jim Mayer (U. Wash.).

Molecular oxygen, O₂, is the greenest, most abundant and inexpensive oxidant imaginable. Yet, it is notoriously difficult to carry out selective partial oxidation reactions using O₂.  Through the examination of the reactivity between a variety of transition metal complexes and molecular oxygen, we are attempting to develop a general means to activate O₂ for selective oxidation of substrates, particularly alkanes.

Conversely, efficient oxidation of water to O₂ is a logical, green source of electrons needed for production of fuels (such as H₂).  We hope to gain fundamental mechanistic insight into this process by studying O-O bond formation in bimolecular complexes. The ultimate goal is to develop an efficient electrocatalytic system for the oxidation of water to O₂.  

(A⁺ is a one-electron acceptor, B is a base)

4. A New Generation of Electrophilic Oxidation Catalysts

Prof. Karen Goldberg (U. Wash.), Prof. Mike Heinekey (U. Wash.), Prof. William Jones (U. Rochester), Prof. Melanie Sanford (U. Mich.), Prof. Wes Borden (U. North Texas).

The ability to use alkanes directly as a feedstock for the chemical production of alcohols and other organics would significantly impact the chemical industry.  Some thirty years after Shilov’s initial discovery of platinum-catalyzed alkane oxidation, platinum-based systems continue to represent some of the most promising leads for the direct conversion of alkanes to alcohols on industrially viable scales.  Surprisingly, other late metals that are also known to activate alkanes via a two-electron couple (e.g. iridium and rhodium) and could in principle perform similar Shilov-type chemistry, perhaps even more successfully than platinum, have not been extensively explored for this purpose.  Using the large knowledge base gained from detailed mechanistic studies on the well-defined platinum systems, related electrophilic catalysts of other metals are under investigation. Efforts are focused on developing and applying fundamental mechanistic understanding of C-H bond activation to the development of new catalytic systems for alkane oxidation.

5. Development of a Methane/Methanol Fuel Cell

Prof. William Jones (U. Rochester), Prof. Jim Mayer (U. Wash.).

Recent developments in the field of alkane activation indicate that it should be possible to deprotonate methane at a cationic metal complex.  Combination of this step with nucleophilic attack by water and further electrochemical oxidation could lead to a process for controlled chemical methane oxidation by a transition metal complex.  Support of this complex on a conducting PEM membrane could lead to a direct process for a methane fuel cell.  Use of these same catalysts for methanol oxidation is also possible.  Existing industry currently works with hydrogen based fuel cells, although working examples of methanol fuel cells are also appearing.  Extension to methane would therefore represent a technological leap forward for this industry.  This project would initiate ground-breaking research for the next generation of fuel-cell technology. 

6. Oxidative Oligomerization of Methane

Prof. Melanie Sanford (U. Mich.), Prof. Jim Mayer (U. Wash.).

Methane (the primary component of natural gas) is highly abundant on earth, and will be an increasingly important source of global energy as petroleum supplies are depleted. However, the widespread use of methane remains limited by the difficulty and expense associated with transporting this flammable gas over long distances. As a result, efficient chemical reactions that directly convert methane into more readily transportable higher alkanes would be of great significance for helping to meet growing global energy demands. The goal of this project is to develop transition metal catalysts for the direct oxidative dimerization and/or oligomerization of methane to form higher alkanes using dioxygen as a terminal oxidant. This approach would be highly advantageous, as the current route from methane to higher alkanes requires an energy-intensive sequence involving initial steam reforming to generate syngas followed by Fisher Tropsch synthesis to form the desired hydrocarbon products. 

In addition to applications in methane chemistry, we anticipate that catalysts developed in this CENTC project will prove useful for direct oxidative C–C bond-forming reactions of more complex molecules. The construction of aryl–aryl, alkyl–aryl, and alkyl–alkyl bonds is of critical importance in the synthesis of polymeric materials, pharmaceuticals, and specialty chemicals. A route to achieve such C–C bond-forming reactions selectively starting directly from C–H substrates would be particularly efficient and environmentally beneficial.

7.  Directed Evolution of Metalloenzymes for Organic Synthesis

Prof. John Hartwig (UI-UC), Prof. Huimin Zhao (UI-UC).

Directed evolution has become a proven method for the development of enzymes that display non-natural functions. Most directed evolution experiments have been conducted on enzymes such as lipases and nitrile hydrolases that lack transition metal containing active sites. However, a few studies have been reported on the development of heme-iron systems, including one that operates by a peroxide shunt pathway without cofactors, and these studies demonstrate that directed evolution can be conducted on metalloenzymes. By combining the expertise of the Hartwig group in the identification and development of transformations useful for synthetic organic chemistry with the expertise of the Zhao group in developing and enacting methods for directed enzyme evolution, we are: 1) developing selective oxidations for reactions useful in organic synthesis; 2) developing selective halogenations that provide building blocks for medicinal chemistry; and 3) developing metalloenzymes that catalyze new classes of enzyme-catalyzed reactions. The capability of metalloenzymes to catalyze amination reactions is limited, but nitrogen-containing molecules serve as a cornerstone of medicinal chemistry. Thus, a portion of our studies on new reactivity will attack the ambitious goal of developing metalloenzymes that catalyze oxidative amination chemistry.

8.  Computational Studies of Catalytic Amination Reactions

Prof. Wes Borden (U. North Texas), Prof. John Hartwig (UI-UC).

Direct amination of hydrocarbons remains an elusive, but highly sought, catalytic reaction for the synthesis of both aliphatic and aromatic amines. Wes Borden and his UNT collaborator, Tom Cundari, will work with John Hartwig to (a) develop catalysts for amination of benzene, to produce aniline, (b) investigate new synthetic methods for metal-catalyzed, intermolecular, nitrene-transfer, and (c) utilize calculations to elucidate the mechanisms of these and other catalytic reactions.

Scheme 1 shows a model catalytic cycle for benzene amination. The goal of the proposed research will be to identify a metal system for which all the steps in Scheme 1 are kinetically and thermodynamically viable. Particular attention will be paid to chemical systems and pathways that best leverage existing CENTC experimental synthetic expertise.

9.  Selective Transformations of Carbohydrates

Prof. James Dumesic (U. Wisc.), Prof. Susannah Scott (UCSB), Prof. Huimin Zhao (UI-UC).

In the US, the availability of non-food dry biomass is estimated at 1.3 billion tons annually, representing the energy equivalent of 3 billion barrels of liquid transportation fuel. Unlocking this natural resource could have major ramifications for both national security and global warming. One of the major biomass components suitable for feedstock use is carbohydrates. Unfortunately, the chemical catalysts developed for processing of petroleum-based feedstocks usually do not work well with carbohydrates. Thus, we are using a combined chemical, biochemical, and reaction engineering approach to the design of new catalytic transformations of carbohydrates for production of renewable chemicals and fuels. Specifically, we are investigating catalytic processes for selective transformations of carbohydrates to produce furfural, 5-hydroxymethylfurfural (HMF), L-xylulose, and xylitol. We focus on mechanistic understanding of various chemical catalysts and biological catalysts with a goal of identifying the synergies between these different catalytic approaches and ultimately establishing a common framework for their applications to selective transformations of carbohydrates.

10.  Towards Selective Disassembly of Lignocellulose: C-C (vs. C-O) Bond Cleavage via Bimetallic Oxidation Catalysis using Dioxygen and Metal-Substituted Hydrotalcites for Catalyzed Lignin Hydrolysis

Prof. Peter Ford (UCSB), Prof. Susannah Scott (UCSB), Prof. Wes Borden (U. North Texas), Dr. David Thorn (LANL), Dr. Tom Baker (LANL).  

Bio-derived chemical feedstocks and fuels should preferably be obtained from high productivity crops that do not require synthetic fertilizers and biocides.[1]  The lignocellulosic biomass of wood, for example, offers higher area-based energy yield per year compared to carbohydrates from agricultural crops.[2]  While progress has been made toward low cost enzymatic hydrolysis of cellulose’s b-1,4 glucose C-O linkages, selective C-C bond cleavage approaches could yield valuable new chemical intermediates.[3]  We propose to build on some promising preliminary results to develop a bimetallic catalytic oxidation process using dioxygen.

Using tridentate iminodioxy ligands, we have shown that oxovanadium complexes catalyze pinacol oxidation to acetone using dioxygen (Scheme 1).  The redox properties of the ligands balance the electron-withdrawing capacity of V^V for the C-C activation with the need to reoxidize V^IV to V^V using dioxygen.  Catalysis is inhibited by substrate and water due to formation of chelate complexes and V-O-V bridges, respectively.  We are using computational chemistry to guide the design of new ligands with increased steric demands that maintain the desired redox properties.  Dinucleating ligands that control the intervanadium spacing are also being investigated both for soluble catalysts and metal oxide-tethered variants. These materials are being evaluated for selective catalytic C-C bond cleavage using a number of increasingly complex model substrates in a variety of reaction media including water/polar solvent mixtures and ionic liquids.

Scheme 1. Bimetallic Pinacol Oxidation Catalysis using Dioxygen

After cellulose, lignin is the most common biopolymer on the planet. It is extremely difficult to dissociate, however, due to heavy cross-linking and a somewhat random structure (Figure 1). Lignin is typically acidified to lignosulfate or pyrolyzed to syngas. In Nature, white rot fungi use enzymes that rely on metal centers capable of reversible electron transfer for efficient degradation of lignin using hydrogen peroxide as oxidant.

Figure 1. Model lignin structure

We are investigating metal-substituted hydrotalcites (Figure 2) as robust catalysts for low temperature lignin hydrolysis to afford phenolic building blocks that could be used to make biorenewable polymers.

Figure 2. Hydrotalcite structure

References

[1] E. Chornet, S. Czernik, Nature 2002, 418, 928.

[2] J. O. Metzger, Angew. Chem. Int. Ed. 2006, 45, 696.

[3] D. J. Miller, J. E. Jackson, Catalysis for Biorenewables Conversion, 2004 NSF Workshop report, www.egr.msu.edu/apps/nsfworkshop.

The Center for Enabling New Technologies through Catalysis is a Center for Chemical Innovation funded by the National Science Foundation