SCIENCE/TECHNOLOGY
What works and what doesn' work in industrial catalysis John W. Geus, a professor at the Debije Institute of the University of Utrecht in the Netherlands, gave a keynote address at Europacat-1 that was, in effect, a tour-de-force sketch of much of industrial catalysis today. The gist of his remarks constitutes a noteworthy statement covering a lot of practical catalysis. • Metal catalysts may make or break C—C and C—H bonds. Those metals that are not active in breaking G—C bonds may be used to synthesize methanol and dehydrogenate alcohol to aldehydes and/or ketones. Similarly, carbonyl groups can be hydrogenated to hydroxyl groups. • Noble metals that do not form stable oxides are suitable catalysts for oxidation. Precious metals may be used for nonselective oxidation of hydrocarbons to carbon dioxide and water. For example, platinum may be used for oxidation of ammonia to nitrogen oxide in the manufacture of nitric acid, as well as for the oxidation of methane in the presence of ammonia to yield hydrogen cyanide. Silver is a good catalyst for the selective oxidation of ethylene to ethylene oxide. • At temperatures of industrial interest, metals usually sinter rapidly. Consequently, most metals used as catalysts are supported with metallic oxides. There are problems, however. Small particles of the catalyst precursors in highly dispersed form over a support surface may react with the support to form compounds that are catalytically inactive or that have diminished activity. Examples include the formation of aluminates following reaction with alumina, the formation of mixed oxides after reaction with
nature and dynamics of the elementary processes. For example, basic understanding of catalytic surfaces has improved but remains insufficient for many purposes. Some insights can be gained from welldefined surfaces, but results using these surfaces are changed by the interaction of the surfaces and the gas phase. Among other points made by Ertl in a similar vein are the following: • In-situ experiments are not necessarily limited to low pressures, as usually thought, but can be carried out at higher pressures. • Partial oxidation reactions on single silver crystals can sometimes be followed with Raman spectroscopy. 30
OCTOBER 18,1993 C&EN
magnesia, and the reactions with silica to form hydrosilicates. • In methane-steam reforming with nickel catalysts, only alumina and alumina-based supports are stable enough to avoid sintering or volatilization at the usual reaction conditions. However, at these conditions, deposition of carbon within the catalyst particles and the formation of carbon filaments can be crucial. Suppression of carbon formation is not favored by the optimum reaction conditions, and attempts are being made to suppress carbon by depositing sulfur on the nickel surface. Carbon deposition is also a problem in therich-gasprocess, in which naphthas react with steam to form methane and carbon dioxide. • In classic Fischer-Tropsch chemistry, carbon monoxide and hydrogen react to form higher hydrocarbons of many varieties. The synthesis gas (CO + H2) is usually produced by methane-steam reforming or from oil fractionation. In the Fischer-Tropsch technology, there may be carbon deposition which deactivates the catalysts by, first, encapsulating the metal catalyst particles with carbon, then the growth of carbon filaments within the catalyst bed, and, eventually, plugging of the reactor. Another consequence is the formation of stable metal carbides on catalyst surfaces. In rich-gas processes, the carbon may also directly deactivate the nickel catalysts. • Because nickel can be easily reduced, it is usually the preferred catalyst in nonselective hydrogénations. However, the nickel may also attack C—C bonds leading to methane hydrogenolysis. This is the case in hightemperature benzene hydrogénation.
• For oscillatory reactions, the overall kinetics can sometimes be described with simple models—indeed, in most cases, simple models are the best approximations to fact. All of the topics mentioned by the keynote speakers were treated in depth by some 700 papers delivered at Europacat-1. It seems likely that the conference will provide governments and industry with a concentrated and sustaining picture of the importance of catalysis, which was the original intention of EFCATS. Europacat-2 will be held in September 1995 in Maastricht, the Netherlands. John W. Geus of the University of Utrecht will be the general chairman.
• In gas-phase reactions, larger catalyst particles several millimeters in diameter can be easily reduced, but transport limitations on a liquid phase call for smaller particles. Often, Raney nickel catalysts are specified, since they do not require reduction. The alternative is to use supported precious metals, which can be reduced at low temperatures with hydrogen. • Breaking of C—C bonds with metal catalysts in general is invariably associated with a demonstrated tendency toward formation of carbides. Copper is a possible exception, though, because it forms carbides with great difficulty and displays a pronounced activity for hydrogenating oxygencontaining molecules to alcohols. The principal fault with copper catalysts is that they have a low activity for the dissociation of hydrogen, requiring high pressure and temperature for practical hydrogénation rates. • In the case of catalytic reforming, the acidity of the support is a critical parameter. The same is true for hydrogénation of nitriles over nickel. For selective transformation to primary or secondary amines, selectivity is completely determined by the acidity of the support. • Precious metals are used to catalyze the oxidation of hydrocarbons and carbon monoxide. This oxidative activity often displays oscillatory behavior, which is of fundamental significance. The oxidation of carbohydrates in liquid media can be achieved with palladium or platinum catalysts, but an excess of oxygen will deactivate the catalysts and, sometimes, cause their disintegration over long periods of exposure.
Unit cuts polysaccharide sequencing time The first automated instrument able to determine sequences of simple sugars in polysaccharides in as little as 48 hours has been introduced by Oxford GlycoSystems of Abingdon, England. Previously, polysaccharides have taken weeks or months to sequence with more costly equipment, such as mass and nuclear magnetic resonance spectrometers. Sequencing is important because the order of sugars in polysaccharides from glycoproteins (proteins bonded to
Glycoprotein sugars have branched structures Galpl — • 4 G l c N H A c p l — • 2Mancxl ^Manpi
4GlcNHAc(il — • 4GlcNHAc
Galpl — • 4 G l c N H A c | 5 1 —*• 2 M a n a l «"^ Gal = Galactose GlcNHAc : A/-Acetylglucosamine Man = Mannose
Greek letters (blue) denote configuration at C-l. Numbers (red) denote bond positions of linkages between sugars. Arrows (green) show bonds between sugars and point toward the oligosaccharide reducing end
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Researcher prepares samples for automated sequencing
carbohydrates) dictates how cells inter act with hormones, antibodies, and oth er cells; how hormones and other mes senger substances are turned on and off; how glycoproteins themselves are made; and how quickly the body degrades them. Moreover, glycoprotein polysaccha ride sequences are as varied as those of amino acids in any protein, and they often have complex, branched structures. Polysaccharides are chains of simple sugars whose reducing (aldehyde) ends are plugged into hydroxyl groups of the preceding sugars by acetal links. Thus, polysaccharide chains have a re ducing "front end" and a nonreducing "rear end." The GlycoSystems tech nique depends on tactical use of exoglycosidases, which are enzymes that chop off sugars one at a time from the nonre ducing end. A sequencing operation using the GlycoSystems instrument begins by treating a polysaccharide with tritiated sodium cyanoborohydride (NaB3H3CN), which reduces the aldehyde end to a radioactively labeled carbinol. The sample solution is then divided into nine aliquots. One aliquot is treated with no
enzymes and serves as a blank. A second aliquot is treated with a mix ture of all seven enzymes to de grade the polysaccharide complete ly and verify that the seven en zymes are sufficient to accomplish the degradation. Each of the remaining seven aliquots is treated with a mixture of six of the seven enzymes. Each enzyme is specific for the sugar cleaved and for attack of either a- or β-links. In an early version of the method [Proc. Nat Acad. Sri., 89, 6338 (1992)], for example, one enzyme mixture lacked only the β-galactosidase; a second lacked the β-N-acetylhexosaminidase; and so on. Each mix of enzymes breaks down the polysaccha ride until a stop point is reached. The stop point is the sugar in the chain for which there is no cleaving enzyme. After an incubation period, all nine aliquots are recombined and the result ing single solution is chromatographed using a radioactivity detector. This step allows determination of amounts and effective "sizes" of all tritium-labeled strands. A GlycoSystems computer pro gram uses these data to suggest possi ble sequences and to assign a statistical confidence value to each. GlycoSystem spokesmen liken the process to a per son seeing an endgame on a chess board and trying to divine the pattern of moves that led from the opening to that result. The new instrument is priced at $65,000. GlycoSystems has previously marketed a $42,500 preparative unit to cleave polysaccharides from glycopro teins and a $45,000 mapping unit to sep arate cleaved polysaccharides to print out both a "fingerprint" of the glycopro tein and to provide purified polysaccha rides for sequencing.
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