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1 ~ Opportunities for Catalysis Research in Energy and Transportation R. Thomas Baker, Los Alamos National Laboratory INTRODUCTION Catalytic processes, key to a variety of efficient transformations, are of enormous importance and are responsible in many ways for the high standard of living we enjoy today. Approximately one-third of the Gross Domestic Product of the United States today can be traced to the success of one or more catalytic processes. In the chemical industry approximately 80 percent of the processes use catalysts, and the percentage is expected to grow as concern over waste dis- posal grows. By definition, catalysis saves energy and plays a key role in diverse areas in energy and transportation. Future opportunities for catalysis in energy and transportation will rely on the development of more efficient catalytic processes that take place under milder conditions (lower temperature and pres- sure) along with the discovery and development of selective new catalysts for transformations that either are not known or are too inefficient today. A catalyst simply provides a reaction pathway that is not possible in its absence. A perfect catalyst would catalyze a reaction forever with no activation energy and 100 percent selectivity. Although many catalysts have turnover num- bers in the millions, most catalyst lifetimes are limited by side reactions that lead to catalyst deactivation. Modern study of chemical catalysis is an interdisciplinary enterprise that involves scientists from every chemical discipline collaborating with each other and with material scientists, theoreticians, and chemical engineers. The largest area of catalysis in industry today is heterogeneous catalysis. Heterogeneous catalysts have the advantage of being insoluble and often rela- tively robust. Therefore, gaseous or liquid reactants and products readily pass through or over a heterogeneous catalyst, and the product can be separated from 70
OPPORTUNITIES FOR CATALYSIS RESEARCH IN ENERGY AND TRANSPORTATION 71 the catalyst readily. Disadvantages of heterogeneous catalysts include the inability to control activity by systematically altering the surface structure, the difficulty of identifying precisely the type of site that promotes a given reaction, and competing side reactions at other catalytic sites on the surface. It is also difficult to guarantee that a high percentage of a metal in a heterogeneous catalyst is participating in the catalytic reaction, thereby requiring either higher tempera- tures or high metal loading for practical operation. Homogeneous catalysts, reactants, and products are all in the same phase, usually liquid. Homogeneous catalysts often employ metal complexes with well- defined ligand coordination environments and reactivities, which lead to a high percentage of active catalyst sites, and predictable and reproducible activity that depends on a 'single site' being present. The main disadvantage of homogeneous catalysts is separation of the product from the catalyst, a problem that has been solved in some cases using phase transfer catalysis, or by 'tethering' a homoge- neous catalyst to an insoluble support. A second disadvantage is the widespread use of hydrocarbon solvents, which has led to an exploration of the possible use of other solvents (water, supercritical carbon dioxide, and ionic liquids) in many reactions. In the future new metal catalysts and catalytic reactions will be perfected primarily as a consequence of fundamental studies of reactions at a metal center. One must be able to control the timing of a sequence of steps in the primary coordination sphere of a metal with exquisite delicacy and thereby direct the metal through a maze of possible outcomes, including no reaction at all. The challenge is to control the outcome of a desired catalytic reaction, either through ligand design in homogeneous catalysts or surface design in heterogeneous catalysts. OPPORTUNITIES FOR CATALYSIS RESEARCH Below are some examples of catalysts and areas of catalyst research that could contribute significantly in the future in terms of energy savings and im- proved transportation. De-Nitrogen Oxide Catalysts The transportation sector accounts for 68 percent of all of the petroleum used and one-third of the anthropogenic carbon dioxide emissions in the United States. In addition, the utilization of internal combustion engines for transportation results in a significant amount of nitrogen oxides and particulate emissions. Technology exists today to manufacture much more thermally and fuel effi- cient 'lean burn' engines (compression ignition, direct injection engines such as diesels or homogeneous charge compression ignition engines) that operate under lean conditions. Existing noble metal-based catalytic converter technology for
72 ENERGY AND TRANSPORTATION stoichiometric gasoline engines is not suitable for the oxygen-rich exhaust of a lean-burn engine, so there is no suitable existing technology for emissions control for the lean-burn engine. The scientific challenge is to catalytically reduce nitrogen oxides under oxidizing conditions over a wide operating temperature range, from below 200 °C up to 500 °C. To achieve this goal, new catalysts are being sought that could affect the selective catalytic reduction of nitrogen oxide in the presence of oxygen by exhaust hydrocarbons or by introduction of ammonia from the hydrothermal decomposition of urea. Molecular sieve zeolites and other oxide supports, ion- exchanged with transition metals and other ions, are the best of the catalysts known to affect this chemistry. However, the mechanism of the selective cata- lytic reduction is still not understood. In addition to discovering catalysts of high activity, the challenge is to maintain the activity of the catalyst under conditions of high temperature, steam, sulfur, and the long operating times necessary to make these materials viable catalysts for use in lean-burn engines. The past decade saw additional major advances in automotive catalysts. The amount of costly precious metals such as palladium, platinum, and rhodium in catalytic converters has been reduced by more than 20 percent through the use of precision coating onto increasingly sophisticated mixed metal oxide supports. To reduce existing "smog" levels, Engelhard's PremAir technology uses a platinum- based coating on the radiator and air-conditioner condenser to convert ground- level ozone into oxygen and carbon monoxide into carbon dioxide. Clean Coal Technologies The fact that a massive supply of coal still exists in this country argues for technologies that allow for more efficient use of coal and that burn coal with fewer emissions and less carbon dioxide. One of the technologies presently being pursued is to combine anaerobic hydrolysis of coal with the capture of carbon dioxide by calcium oxide. The thermodynamics of these combined processes indicate that coupling these reactions leads to a thermodynamically neutral over- all reaction. The benefit of this type of process is that all potential emissions could be handled at once. However, burning coal releases not just small molecule emis- sions but sulfur and heavy metals in addition to carbon formation and radioactive emissions. Therefore, it is necessary to perform in situ diagnostics to receive feedback on transient concentrations in order to maintain the most efficient coal burning. New Catalysts for Refining Applications Over the past 60 years catalysis has transformed the refinery from a distilla- tion plant to a sophisticated chemical-processing plant. One major challenge
OPPORTUNITIES FOR CATALYSIS RESEARCH IN ENERGY AND TRANSPORTATION 73 facing the refining industry is the cost-effective manufacture of ultra low sulfur transportation fuels to enable vehicles to meet stringent reduction in emissions. To enable the production of less than 10 ppm of sulfur gasoline, a number of new catalysts and processes have been developed to selectively desulfurize part or all of the FCC (Fluid Catalytic Cracking) naphtha and preserve the olefins that pro- vide octane value. There are a variety of selective desulfurization options avail- able that preserve octane value. These include IFP's Prime G+, BP's OATS process (Olefin Alkylation of Thyophenic Sulfur), Phillips' S-Zorb process, and ExxonMobil's SCANfining process. A breakthrough distillate hydrotreating catalyst, jointly developed by ExxonMobil and AkzoNobel, was recently commercialized and deployed in three refineries. This catalyst has three to ten times the activity of the most active cur- rent HDS/HDN catalysts, depending on pressure. This catalyst also exhibits novel and unique pressure sensitivity from 400 psi Hydrogen-2 pressure up to 2,000 psi Hydrogen-2. It represents a major advance in catalyst performance, composition, structure, and morphology. The product is stripped not only of sulfur but also nitrogen and achieves substantial aromatic saturation. Short contact time FCC has significantly improved the yield and quality of naphtha. A major improvement in cracking catalyst design came about from the recognition of bifunctionality of FCC catalysts. An example of a new catalyst is one where detrital alumina deposited on the active zeolite doubled activity and increased selectivity. The ADA catalyst family has been developed in a joint research program by Grace Davison and ExxonMobil. Much higher product value has been generated by significant improvements in the ZSM-5 catalyst system to produce olefins in FCC units, specifically ethylene and propylene. Major im- provements in lube hydroprocessing have come about through the use of the new MSDW-2 isodewaxing catalyst. Selective Reactions of Methane and Other Alkanes Currently a great deal of energy is expended to obtain starting materials, usually alkenes, for the chemical industry from petroleum or coal. These alkenes are then converted into polymers, alcohols, and other downstream chemical products. In view of the large reserves of methane (natural gas), it would be highly desirable to use it as a source of products for the chemical industry. Although methane can be reformed with steam and oxygen to produce a mixture of carbon oxide and hydrogen ('synthesis gas'), which is then reacted over a catalyst to produce a variety of hydrocarbons, this 'Fischer-Tropsch' chemistry involves high pressure and temperatures. It would be highly desirable to be able to convert methane directly into value- added products, most importantly into methanol by direct oxidation with oxygen at low temperatures and pressures. Selective direct oxidation of methane to methanol would allow plants to be built at the sources of methane, thereby elimi-
74 ENERGY AND TRANSPORTATION eating problems associated with transportation of methane. Methanol at present is the starting point for the synthesis of multicarbon commodity chemicals and can be converted into alkalies using zeolite catalysts. Significant progress has been made in selectively oxidizing methane to methanol with oxygen and plati- num catalysts, although an efficient catalytic process has yet to be perfected. It also would be desirable to directly functionalize higher alkalies, for example to, alcohols, or to dehydrogenate them selectively to alkenes.i Catalysts for Photocatalytic Water Splitting One of the grand challenges is to develop catalysts that will split water into hydrogen and oxygen using sunlight as the source of energy for this uphill reaction. This would be the artificial counterpart of photosynthesis in green plants. Hydrogen and oxygen could then be recombined in fuel cells, thereby effectively utilizing the energy of the sun to provide electricity with an efficiency that is not inherently limited theoretically to a small yield, as in silicon-based solar cells. Photocatalytic splitting of water is an ambitious project with a huge and important payoff. Complex new transition metal complexes must be designed that operate in water and allow the production and separation of hydrogen and oxygen using photochemistry. These catalysts are likely to be either homoge- neous or tethered to a support that doubles as a hydrogen-permeable membrane, thereby allowing hydrogen to be separated from oxygen. The most ambitious approach would be to link photocatalytic splitting of water with a fuel cell in order to effectively convert sunlight into electricity in a single device. Although progress has been made in driving unfavorable reactions photochemically, we are not close to a solution to the problem. The solution will require long term sup- port, research effort, and patience. Linking Catalysis Theory with Experiment With advancements in computer power and the advantage of reasonable scalability to larger systems, density functional theory has made computational chemistry widely accessible in the chemical sciences, permitting direct compari- sons to be made between theory and a wide variety of experiments. Examples of the application of density functional theory for prediction, understanding, and interpretation in surface science and heterogenous catalysis include: · understanding active site structures and structures of key intermediates in acid catalysis of hydrocarbon conversion by zeolites, iAs hydrocarbons will most likely be used for the foreseeable future, the development of hydro- carbon anode catalysis is another area of research that could potentially provide significant break- throughs.
OPPORTUNITIES FOR CATALYSIS RESEARCH IN ENERGY AND TRANSPORTATION 75 . understanding the importance of dynamic changes of the catalyst surface in ammonia synthesis, identification of new catalytic intermediates in selective oxidation; prediction of reaction energetics and the kinetics of selective and non- selective catalytic pathways, · predictions leading to the discovery of a new Diels-Alder reaction for organic functionalization of semiconductor surfaces, and · prediction leading to invention of new steam reforming catalyst. Although computational chemistry is far from predicting the outcome of a chemical reaction through full-scale modeling of the catalytic reaction in question, it can be used in conjunction with experiments to guide the experimentalist in potentially fruitful directions. Efforts should be made to formalize collaborations between experimentalists and theoreticians as much as possible in the future.
