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Catalysis Looks to the Future 1 Introduction The following are some benchmark discoveries made over the years in the science and technology of catalysis. 100 years ago: Paul Sabatier (Nobel Prize 1912) at the University of Toulouse started work on his method of hydrogenating organic molecules in the presence of metallic powders. 70 years ago: Irving Langmuir (Nobel Prize 1932) at General Electric laid down the scientific foundations for the oxidation of carbon monoxide on palladium. 50 years ago: Vladimir Ipatieff and Herman Pines at UOP developed a process to make high-octane gasoline that was shipped just in time to secure the victory of the Royal Air Force in the Battle of Britain. 30 years ago: Karl Ziegler and Giulio Natta (Nobel Prize 1963) invented processes to make new plastic and fiber materials. 17 years ago: W. S. Knowles at Monsanto Company obtained a patent for a better way to make the drug L-Dopa to treat Parkinson's disease. 16 years ago: General Motors Corporation and Ford Motor Company introduced new devices in cars to clean automotive exhaust. These devices found worldwide acceptance. 10 years ago: Tennessee Eastman Corporation started a new process for converting coal into chemicals used for the production of photographic film. Yesterday: Procter and Gamble Company manufactured a new environmentally safe bleach mixed with laundry soap. Today: Thomas Cech (Nobel Prize 1989) at the University of Colorado received U.S. patent 4,987,071 to make ribozymes, a genetic material that might, one day, be used to deactivate deadly viruses.
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Catalysis Looks to the Future Tomorrow: A new, homogeneous catalyst to make methanol may be commercialized following preliminary work at Brookhaven National Laboratory. The above examples deal with materials for health, clothing, consumer products, fuels, and protection of the environment, but all have a common feature: they rely on chemical or biochemical catalysts. What are catalysts? What is catalysis, this growing field of science and technology that holds the keys to better products and processes, and continues to have such a strong impact on our economy and quality of life? WHAT ARE CATALYSTS? The word ''catalyst'' is often used in everyday conversation: it is said, for instance, that a person is a catalyst, meaning a go-between who facilitates a transaction but withdraws when the transaction is ended. Similarly, a catalyst is, in principle, found intact at the end of a chemical reaction, ready to be engaged in the same reaction again and again. What the catalyst THE BATTLE OF BRITAIN: CATALYSTS FOR VICTORY Fifty years ago, between July 10 and October 31, 1940, Royal Air Force fighter pilots defeated the Luftwaffe in a heroic air battle over Britain. The British lost 915 planes versus 1733 for the Germans. The impact of the British victory was immortalized by Winston Churchill in the House of Commons when he said, "Never in the field of human conflict was so much owed by so many to so few." In the Chicago Tribune Magazine of July 15, 1990, Herman Pines reminds us of the critical role played by 100-octane fuel that provided British planes with 50% faster bursts of acceleration than were available to them during the May 1940 French campaign fought with 87-octane fuel. With the same planes but new fuel, British pilots were able to outclimb and outmaneuver the enemy. The new fuels that contributed to victory came just in time from the United States, as a result of discovery and development by Universal Oil Products (now UOP Inc.) of sulfuric acid-catalyzed gasoline alkylation. Vladimir Ipatieff, Herman Pines, and Herman S. Bloch played key roles in this work. Since 1940, hydrofluoric acid has, in part, replaced sulfuric acid as the catalyst for gasoline alkylation. Today, in the battle for the environment, efforts are under way to replace hydrofluoric acid. Eventual success will be another achievement of researchers in catalysis.
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Catalysis Looks to the Future does is to provide a path for the reaction to proceed swiftly and selectively to the desired products. Yet what about an operational definition of a catalyst? A catalyst is a substance that transforms reactants into products, through an uninterrupted and repeated cycle of elementary steps, until the last step in the cycle regenerates the catalyst in its original form. Many types of materials can serve as catalysts. These include metals, compounds (e.g., metal oxides, sulfides, nitrides), organometallic complexes, and enzymes. Because not all portions of a catalyst participate in the transformation of reactants to products, those portions that do are referred to as active sites. Most industrial catalysts are used in the form of porous pellets, each of which contains typically 1018 catalytic sites. The total amount of catalyst is small compared to the amount of reactants and products made during the life of the catalyst. The turnover frequency of the cycle is the quantity that defines the activity of a catalyst. Strictly speaking, the turnover frequency is the number of molecules of a given product made per catalytic site per unit time. In heterogeneous catalysis, the turnover frequency is typically of the order of one per second. Who develops these catalysts? The development of catalysts is carried out by chemists and chemical engineers, often in large multidisciplinary teams that bring together expertise in the areas of physical, organic, and inorganic chemistry, as well as materials science and chemical reaction engineering. Such teams work on determining the optimal composition and physical structure of the catalyst, its activity and selectivity over the desired range of operating conditions, and its deactivation rate over time. Attention is also paid to developing methods for catalyst reactivation and recovery. The generalities cited above can be illustrated by examples borrowed from the chemical, oil, and pharmaceutical industries, as well as environmental protection. The first triumph of large-scale catalytic technology goes back to 1913 when the first industrial plant to synthesize ammonia from its constituents, nitrogen and hydrogen, was inaugurated in Germany. From the outset, and until the present, the catalyst in such plants has consisted essentially of iron. The mechanism of the reaction is now well understood. Small groups of iron atoms at the surface of the catalyst are capable of dissociating first a molecule of nitrogen and then a molecule of hydrogen, and finally of recombining the fragments to ultimately form a molecule of ammonia. The catalyst operates at high temperature to increase the speed of the catalytic cycle and at high pressure to increase the thermodynamic yield of ammonia. Under these severe conditions, the catalytic cycle turns over more than a billion times at each catalytic site before the catalyst has to be replaced. This high productivity of the catalyst explains its low cost: the
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Catalysis Looks to the Future catalyst results in products worth 2000 times its own value during its useful life. The refining of petroleum to produce fuels for heating and transportation involves a large number of catalytic processes. One of these is the catalytic reforming of naphtha, a component derived from petroleum, used to produce high-octane gasoline. In modern catalytic reforming, many different catalytic reactions proceed on small particles made of platinum and a second metal such as rhenium or iridium. These bimetallic clusters are expensive but chemically robust. They can be reactivated after long-term use, thus making possible the use of precious metals to produce an affordable consumer commodity. The metallic clusters are so small that practically all metal atoms are exposed to the reactants and take part in the catalytic cycle. These metal clusters are supported within the pores of an acidic metal oxide that also takes part in the reforming process. The next illustration of catalysis shows that industrial catalysts can be biomimetic, in the sense that they imitate the ability of enzymes to produce optically active molecules (i.e., molecules whose structures are such that the reflection of the molecule in a mirror does not superimpose on the original molecule). Many pharmaceuticals are known to be active in only one form, let us say the left-handed form. It is therefore critical to obtain the left-handed form with high purity. This is particularly important when the drug is toxic, even if only slightly so, and must be administered over many years. It is true of a molecule called L-Dopa used in the treatment of Parkinson's disease. Here, the right-handed molecule is inactive. In ordinary synthesis, both forms (right and left) are produced in equal amounts. Their separation is costly. Is it possible to produce only the left-handed form by means of a synthetic catalyst? The first success of an industrial synthesis of this kind was achieved at Monsanto, and a patent for the selective synthesis of L-Dopa was granted in 1974. The catalytic process used to make L-Dopa today may be regarded as an important achievement in industrial catalysis. Finally, more recent developments in catalytic technology are targeted at the protection of the environment. The best-known example deals with catalytic converters that remove pollutants from the exhaust gases of automobiles. Catalytic converters for automobiles were first installed in the United States in the fall of 1974. These devices were subsequently introduced in Japan and are currently spreading through Europe. The most advanced catalyst now contains three metals of the platinum group and controls the emissions of carbon monoxide, nitrogen oxides, and unburned hydrocarbon molecules by use of a complex network of catalytic reactions. This application has contributed more than any other to public awareness of catalysis and of its many applications for the benefit of mankind.
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Catalysis Looks to the Future AN IMMOBILIZED ENZYME AS AN INDUSTRIAL CATALYST While the oil crisis of the 1970s was front-page news, the soft drink industry was experiencing a less-heralded shock of its own. High sugar cane prices sparked a scramble to other sweeteners—not artificial sweeteners, mind you, but other forms of sugar. Table sugar—sucrose—is just one member of a family of several dozen closely related natural sugars. Other members include fructose, found in fruits; glucose, found in honey and grapes; lactose, found in milk; and maltose, found in malted grain. Fructose became the sweetener of choice, thanks to the adaptation of a catalyst to its large-scale industrial production. The catalyst, glucose isomerase, was derived from Streptomyces, a common, soil-dwelling bacterium best known as the source of many antibiotics, including streptomycin. Glucose isomerase is an enzyme—one of nature's own catalysts. All living things use enzymes, each one tailored to carry out one of the multitude of reactions essential for life itself. An enzyme is essentially a protein molecule, although it may have other atoms or molecules attached that help it do its job. A protein molecule is a long chain made up of hundreds or thousands of smaller units called amino acids assembled in a very specific order. When dissolved in water, this chain naturally kinks and knots up. The sequence of amino acids making up the protein determines the shape that the protein knots itself into, and it is this shape that allows the protein to catalyze its reaction. The molecules that participate in the reaction fit into a crevice in the protein, like a key in its lock. Once inside the crevice, called the "active site," the molecules are held in just the right relative orientation for the reaction between them to proceed. Adapting an enzyme to a continuous-flow industrial process requires that the soluble protein be immobilized somehow. (If the protein were left in its soluble form, it would be well-nigh impossible to separate it from the process stream, and it would all wash away in the flow.) To keep doing its job, the immobilized enzyme must retain its dissolved shape, yet it must also be firmly anchored to its solid support. In addition, the catalyst-support combination must be stable at the processing temperature and strong enough not to break up under processing conditions. Resin and polymer supports were tried first, because these molecules are chemically very similar to enzymes, which makes it easy to attach enzymes to them. Unfortunately, the resin and polymer beads were crushed into a gummy mass under the processing conditions and clogged the works. One way around the problem is to attach the enzyme molecules to a ceramic material. Tiny ceramic particles have a high surface area, allowing a lot of catalyst to be attached to them and increasing the reaction's efficiency. Ceramics are also incompressible, and so the
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Catalysis Looks to the Future system can be run at high pressure without crushing the catalyst or clogging the works. Because proteins do not stick naturally to ceramics, an intermediary is needed. The ceramic particles are coated with a special polymer that adheres well to both the ceramic and the enzyme while allowing the enzyme to retain its shape. The enzyme does tend to decompose slowly under process conditions, but the extra stiffness imparted to it by the ceramic backbone makes it more stable than the natural enzyme, so that each batch of ceramic-supported enzyme lasts longer. The fructose production process has proved to be remarkably efficient. One pound of catalyst-coated ceramic will produce an average of 14 1/4; tons, and sometimes as much as 18 tons, of fructose (measured as a dry solid) before the enzyme loses its activity. The process converts a watery, honey-colored syrup containing 95% glucose—a by-product of the wet-milling process used to make starch from corn—into a 42-45% fructose syrup—the "corn sweeteners" on a soft drink ingredient label. (Both Coca-Cola and Pepsi-Cola allow their bottlers to replace up to 100% of the sugar in their soft drinks with corn sweeteners.) Although glucose, fructose, and sucrose are all sugars, they are not equally sweet. Glucose is not picked up by the taste buds as quickly as fructose or sucrose, nor does its sweetness linger as long on the palate. If sucrose scores 100 on a sweetness scale, fructose rates a supersweet 173 and glucose an unsatisfying 74—the main reason glucose itself is not sold as a sweetener. A complicated separation process keeps recycling unreacted glucose back through the system, while drawing off fructose as it forms. Pure fructose comes out as a 90% solution, which is diluted to 55%—equivalent in sweetness to pure sucrose—before the syrup is sold. Thus fructose is as convenient to use as sucrose—the bottler does not have to install any extra tanks or plumbing to dilute the fructose, or alter the recipes to allow for its greater sweetness. Fructose has the added advantage of being safe for diabetics. THE FUNCTION OF RESEARCH Research plays a vital role in advancing the frontiers of scientific understanding of catalysis and in assisting the development of catalysts for industrial application. Because of the complexity of catalysts and catalytic phenomena, information must be drawn from a large number of supporting disciplines, in particular, organometallic chemistry, surface science, solid-state chemistry and materials science,
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Catalysis Looks to the Future biochemistry and biomimetic chemistry, chemical reaction engineering, and chemical kinetics and dynamics. Although research in catalysis is still dominated largely by experimental studies, theoretical efforts are becoming increasingly important. Theory provides a framework for understanding the relationships among catalyst composition, structure, and performance. The advent of supercomputers has made it possible to model a still larger body of catalytic phenomena and even, in some cases, to predict catalyst properties a priori. The availability of high-resolution computer graphics has proved particularly useful in visualizing the results of complex calculations and understanding the spatial relationships between catalysts and reactants on a molecular scale. The industrial development of catalysts is an expensive and labor-intensive activity because catalysts currently cannot be designed from first principles. Rather, they must be developed via a sequence of steps involving formulation, testing, and analysis. An important aim of research in catalysis is to accelerate this process by providing critically needed knowledge and techniques. Another important function of research is to provide a reservoir of new information and materials that may contribute to the identification of new catalytic materials or processes. Thus, not only does research provide the tools and knowledge needed for direct facilitation of catalyst development, but it also increases opportunities for the discovery of new materials and new techniques. One example suffices to illustrate the impact of research on catalytic science and technology. Because catalysis is a kinetic phenomenon based on the turning over of the catalytic cycle, the example deals with the prediction of overall kinetics for a catalyzed reaction based on a knowledge of elementary steps in the cycle. This information cannot be obtained theoretically at present, but it can be determined from experimental investigations. In the case of solid catalysts, some of these measurements are carried out on large single crystals, exposing one defined facet about 1 cm in size. These facets are nearly perfect in structure and are extremely pure. The chemistry of elementary processes occurring on such surfaces can be studied in great detail, to determine not only the rate of the process but also what intermediate species are formed. By studying different crystal facets, the effects of catalyst surface structure on reaction dynamics can be established. Information on the rates of elementary reactions can be assembled to describe the kinetics of a multistep process. This approach has been used to understand ammonia synthesis, over iron, and to establish which facet of iron is most effective in promoting this reaction. Such knowledge can be used to guide the preparation of industrial catalysts so as to expose the desired facets of iron preferentially. Recent studies have demonstrated that the best
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Catalysis Looks to the Future commercially available ammonia synthesis catalysts operate at a rate that is almost equal to that observed on the preferred facet of iron. SUMMARY AND PERSPECTIVE In summary, catalysts play a vital role in providing society with fuels, commodity and fine chemicals, pharmaceuticals and means for protecting the environment. To be useful, a good catalyst must have a high turnover frequency (activity), produce the right kind of product (selectivity), and have a long life (durability), all at an acceptable cost. Research in the field of catalysis provides the tools and understanding required to facilitate and accelerate the development of improved catalysts and to open opportunities for the discovery of new catalytic processes. The aim of this report is to identify the research opportunities and challenges for catalysis in the coming decades and to detail the resources necessary to ensure steady progress. Chapter 2 discusses opportunities for developing new catalysts to meet the demands of the chemical and fuel industries, and the increasing role of catalysis in environmental protection. The intellectual challenges for advancing the frontiers of catalytic science are outlined in Chapter 3. The human and institutional resources available in the United States for carrying out research on catalysis are summarized in Chapter 4. The findings and recommendations of the panel for industry, academe, the national laboratories, and the federal government are presented in Chapter 5.
Representative terms from entire chapter: