Technology
Progress in d chemistry aired Researchers gather in Belgium to exchange results on catalysts, syngas sources, mechanisms, and various methanol processes Joseph Haggin C&EN, Chicago
Diverse views of the role of singlecarbon molecules in the future synthetic chemicals industry characterized the recent International Symposium on Catalytic Reactions of One Carbon Molecules held in Bruges, Belgium. The symposium was the first of three meetings on the subject scheduled for the next few years. Probably the strongest and most fervently voiced of these views is that presented by the West German contingent at the meeting, namely, that soon there will be a rapid changeover in the raw material base for organic chemistry at the commercial level from oil to coal. Jurgen Falbe, executive vice president of Ruhrchemie, says he thinks this change is inevitable and, barring political problems, it may be well along within the next 10 years. He predicts the demise of the present petrochemical industry during that period and its replacement by some new arrangement based on Ci chemistry. Key elements in this scenario include a new wave of development of industrial catalysts and the development of cheaper methods to produce synthesis gas. Also important will be the direct liquefaction of coal, which Falbe expects will make a large contribution to the commercial organic chemistry of the future. Much of the immediate progress of Ci chemistry will depend on the availability and the cost of syngas, a general term for the mixtures of carbon monoxide and hydrogen produced by gasifiers. No two gasifiers produce exactly the same mix of these gases, a situation that is at once a curse and a blessing. It means that there are a certain number of competing choices in gasifiers, but it also means that a lot of intermediate ad-
justments in gas composition frequently are required before a syngas can be used as the feedstock for a particular process. The ideal would be a gasifier that could be adjusted within wide limits during operation to produce whatever syngas composition was desired. However, no such gasifier exists. Among the present gasifiers, says Falbe, the Lurgi and Winkler types aren't really very good for chemical production. Instead, their product is quite well suited for production of methane or substitute natural gas. The best of the present gasifiers for chemical production, at least in West Germany, he says, is the KoppersTotzek gasifier. Second-generation gasifiers now under development appear to offer more flexibility in product composition and lower production costs. Assuming that syngas will be available in the required amounts, Falbe has mapped out three general paths for the conversion of Ci molecules to commercial products. The first is direct conversion of syngas to substitute natural gas, FischerTropsch chemicals, and methanol. The second route involves indirect conversion through a methanol intermediate. Such a route would permit the addition of functional groups to the basic hydrocarbon structure of the product molecules. The third route is further reaction of syngas with hydrocarbons and alcohols. Falbe believes that direct hydrogenation of carbon monoxide to methane and methanol already proceeds about as far as selectivity and
Mobil's gasoline process may go like this -H20 CH3OCH3
CH3OH -H20 C
nH2n
Paraffins
Naphthenes
Aromatics
conversion will permit. The real choice between existing processes is not which is the most selective or has the highest conversion efficiency, but which process will optimize the commercial production of fuels and chemicals, he says. After 50 years of development, Fischer-Tropsch catalysts still are not well understood. All of them have problems with selectivity, and the choice of catalysts available is still rather limited. Current catalysts also have a problem with cost. Most of the better Fischer-Tropsch catalysts are noble metals that are very expensive. Any new wave of interest in Ci catalysis should have as one aim the replacement of these catalysts with less expensive ones that have good stability with at least as great selectivity as present catalysts. Falbe assumes that coal will be the chief future source of carbon monoxide in West Germany, so it will be particularly important to develop more efficient methods of coal gasification to lower the cost of syngas from this source. Some profitable syngas-to-chemicals processes already are operating, he points out, and the outlook is for more processes to be developed and for petroleum gradually to be displaced as a feedstock for synthetic chemicals. About the only place where one can speak with authority about current commercial production of chemicals on a large scale from Ci feedstocks is South Africa. Mark E. Dry described production at South Africa's Sasol facility. Dry has been at Sasol since it began in 1955 and was directly involved in both the 1980 expansion and the further expansion of the facility that was completed last year. All three Sasol units presently use one of two types of reactors: either entrained-bed Synthol reactors or the older fixed beds. However, a new section using ebulating beds will go on stream in 1983, Dry says, and Sasol is doing a pilot study of a slurry reactor for future use. In general, the fixedbed reactors give maximum yields of heavier hydrocarbons (up to 35 carbons per molecule). The entrained beds yield more of the light olefins and gasoline. Iron catalysts are used exclusively at Sasol, Dry says, because they are June 28, 1982 C&EN
31
Technology Interest in C^ chemistry may be short-lived Although they put together the International Symposium on Catalytic Reactions of One Carbon Molecules, the organizers themselves question whether Ci chemistry is in fact a separate area of study that deserves all the attention it is receiving. The organizers are Robert Schoonheydt of Catholic University of Louvain, Philippe J. Teyssie and A. Hubert of the University of Liege, and Peter A. Jacobs, also of Catholic University of Louvain. The symposium was organized under the auspices of the Belgian Inter-University Consortium for Research In Catalysis, the Blaamse Chemische Vereniging, and the Societe Chimique Beige. Sponsors included about a dozen industrial firms. In the organizers' opinion, the current interest in C-i chemistry per se may be short-lived. To Jacobs and Teyssie, for example, Ci chemistry is more properly considered one facet of a growing new interest in industrial catalysis. They suggest that emphasis should be placed on the catalytic aspects of the work now taking place rather than on maintaining the integrity of the "unitary carbon" atom. In fact, they point out, all of the work now under way in Ci chemistry is, in some respect, aimed at producing chemicals that have more than one carbon atom in their molecules. In this sense, the subject of Ci chemistry may be a bit contrived.
cheap and because they tend to form more olefins. The variety of products that are produced at Sasol is the result of the way the plants are operated, he explains, rather than any big changes in the catalysts used. There are considerable variations in the way the limited number of catalysts are prepared, however. The Sasol catalysts permit some manipulation of selectivity and there is considerable refinement of primary products. Hydrocracking of waxes, for example, can yield up to 80% diesel fuel, which is one of the products in demand in South Africa. The mechanism of FischerTropsch synthesis still is being debated, Dry points out, but in general it involves stepwise carbon chain growth. The main point of debate, he observes, is whether the carbon monoxide dissociates before it inserts into the growing chain. Over the years, debate has gone full circle on this point. Now, Dry says, the prevailing view seems to be that the for32
C&EN June 28, 1982
Symposium's organizers {from left): Schoonheydt, Teyssie, Hubert, Jacobs It is perhaps not surprising that these researchers should see Ci chemistry in terms of its place in the broader field of industrial catalysis since each of them is an expert in some branch of catalysis or a related field, and most of them have considerable experience in areas far afield from Ci chemistry. Thus, their concern with catalysis rather than with the single carbon atom reflects, in part, their own natural interests. Nevertheless, Ci chemistry has developed something of a life of its own in the past couple of years based on a history that goes back at least 50 years in chemical research and includes some very large commercial applications in Europe and in South Africa as well as smaller operations elsewhere. One measure of its present independent status is that the field now has its own journal, C-\ Molecule Chemistry, edited
mation of surface-active carbides is the key step in the process. Dry doesn't believe that resolution of the debate is really all that important, at least for commercial purposes. Mechanisms, he says, don't make much difference in operating a plant. It is more important to know the operating characteristics of a particular plant and how it runs best. Mechanism is important to Peter A. Jacobs of Catholic University of Louvain, Belgium. He points out that most of the work on Fischer-Tropsch chemistry involves a spectrum of products t h a t corresponds to a Shultz-Flory molecular weight distribution, a characterization derived from polymer-forming sequences. If such a distribution is indicative of the mechanism of the reaction, it places some unfortunate constraints on the chemistry, Jacobs says. For one thing it means that chain growth occurs one carbon atom at a time and that termination of the growth occurs through chain desorption from the
by Igor B. Tkatchenko of the French Institute for Catalytic Research. Teyssie and Jacobs also depart from mainstream thinking in their view of the future potential of coal as a chemical feedstock. Here they are less optimistic than some others. They expect the influence of petrochemistry to remain strong for a long time. Even though oil is more expensive than it once was and reserves are decreasing, the assumption that mixtures of carbon monoxide and hydrogen will displace petroleum as a key chemical feedstock in the near future doesn't seem likely to them. It is possible to make synthesis gas from many sources, including coal, but such gas is very expensive, particularly in Europe. This high cost is a big impediment to the immediate success of C-i chemistry on a commercial scale, they say.
catalyst surface. Jacobs suggests that such a description may be good for organizing data but it is not very helpful in elucidating mechanistic details. Furthermore, it implies that chain growth is independent of chain length. Some recent work indicates that the Shultz-Flory distribution may not always be operative. Deviations from the classical Fischer-Tropsch pattern usually are associated with a high proportion of methylene units and a dip in C2 yields. Some of the causes may be mass transfer limitations, but others may be mechanistic. One possibility is that the mildness of some reactions may permit the insertion of C2 units into the growing chain. One problem common to all the Fischer-Tropsch catalyst studies is determining whether the activity of the catalyst is limited by intrinsic catalyst activity or by the number of active sites. P. Biloen of Shell Laboratory in Amsterdam believes that he has shown that the number of sites is
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the limiting factor. In isotope labeling studies, he finds that reaction intermediates cover only one tenth to one hundredth of the surface under conditions in which the total coverage with carbonaceous species exceeds a monolayer. Chain growth is known to be a much faster reaction than can be explained by the surface concentration of intermediates. Thus he concludes that there are not enough active sites available. Another possible limitation in Fischer-Tropsch reactions was mentioned by J. B^gild Hansen of Haldor T0psoe, Denmark. The limitation is intrinsic in the catalyst and is illustrated in the simple case of methane formation from carbon monoxide and hydrogen. As methane is formed, a side reaction (the water gas shift reaction) also produces some carbon dioxide. Hansen believes that this carbon dioxide reduces the catalytic carbides at the active sites to an inactive form and thereby reduces overall catalytic activity. Besides Fischer-Tropsch synthesis, the biggest commercial interest in Ci chemistry centers on the Mobil methanol-to-gasoline process. Mobil's Werner 0. Haag described two versions of the process. One uses a fluid bed and is under development by a Mobil-West German consortium. The other is a fixed-bed process which Haag says is ready for commercialization. The New Zealand government will make the first installation of the Mobil process in connection with an integrated refinery and gas field optimization effort. This installation is now beginning and should be operational in a couple of years. The fixed-bed reactor typically is operated at about 400 °C and 20 bar. Recycle ratio is 9 to 1. Haag reports a direct gasoline yield of 80% with a research octane number of 93. All the reactions in Mobil's methanol-to-gasoline process are acid catalyzed. They include olefin polymerization, cyclization, aromatization via hydrogen transfer, and alkylation by methanol of olefins and aromatics. The alkylation of olefins by methanol is the step that actually produces the higher olefins. There is also some simultaneous cracking of these higher olefins. The product distribution of olefins is nominally a Shultz-Flory distribution, but the catalyst also has a strong influence. Shape-selective control comes through the use of specific zeolites. Increased severity yields more aromatics, but at the higher temperatures the aromatics have lower molecular weights.
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Technology Although the methanol-to-gasoline process has been promoted primarily as a producer of gasoline, it also may be developing into a producer of olefins and aromatics. Haag notes that control of the silicon-to-aluminum ratio in the zeolite catalyst permits great control of the kinetics, and possibly of the products, of the reaction. Some studies indicate that C2-C4 olefins are produced in low conversion rates under some conditions. Ethylene (62%) is the dominant olefin. At low temperatures there appears to be a dominance of polymerization, and at higher temperatures cracking is common. There also seems to be a pronounced tendency toward oligomerization in the sequence C3, C6, C9, etc. The conclusion, says Haag, is that the process, although primarily a gasoline producer, also yields some olefins and aromatics. How much control can be obtained with catalyst manipulations and process conditions remains to be seen. What may be the formative stages of a new process to make ethylene and the higher aldehydes from formaldehyde were discussed by David L.
Thorn of Du Pont. Thorn described the possibility of making a slate of C2 compounds from carbon monoxide and formaldehyde. He also is looking at the possibility of producing C3 compounds by using iridium complexes as catalysts. The compounds most sought after would be propylene and cyclopropane. Most of the emphasis in Ci chemistry has been on carbon monoxide, but there is a growing interest in carbon dioxide as well. For thermodynamic reasons, carbon dioxide is not considered a desirable feed for very many processes. One investigator who holds out some hope for this molecule, however, is R. P. A. Sneeden of the French Institute of Catalysis Research. To generate a viable technology based on carbon dioxide, it is first necessary to determine which of the transformations of carbon dioxide do not proceed through a carbon monoxide intermediate. In Sneeden's laboratory recent work has given a partial answer to this question. There are some syntheses—for example, the production of ethylene—in which both oxygen atoms in the carbon dioxide are retained. Furthermore,
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C&EN June 28, 1982
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carbon dioxide may replace carbon monoxide in other syntheses. Homologation to higher hydrocarbons, however, apparently requires transformations of the products of carbon dioxide reactions. It appears that carbon dioxide generates Ci compounds exclusively but that higher homologs can be generated from the chemical fragments. One general conclusion that appeared from the Bruge symposium is that methanol is becoming the Ci compound of principal interest to most people, and not merely as a precursor to high-octane gasoline. Methanol can be generated from synthesis gas via well-established technology. It can, among other things, be converted to higher alcohols, presumably by homologation. E. Freund of the French Petroleum Institute described some features of a new process for higher alcohols based on the general reactions n(CO + 2H 2 ) -
C n H 2 n + iOH + (n - 1)H 2 0
n ( C 0 2 + 3H 2 ) -
C n H 2 n + 1 OH + nH20
The main products are ethanol and propanols. The usual catalysts are cobalt-copper-based metal oxides with sodium and potassium promoters. A serious drawback is that 35% of the product mix is water and that must be removed before the alcohols can be used as products. One of the more novel processes presented at Bruges came from Argonne National Laboratory. Michael J. Chen described some chemistry for the selective homologation of methanol catalyzed by transition metal complexes in basic media. In particular the process can be used to produce "dry" ethanol without the coproduction of water. Water always has been a problem in most Ci chemistry of commercial interest and a great economic impediment to most processes. The Argonne synthesis is based on the reaction CH3OH + H 2 + 2CO Fe/NR 3
*CH 3 CH 2 OH + C 0 2 Producing carbon dioxide as a side product may not be much better from the commercial viewpoint than producing water. However, the reaction represents a potentially valuable new approach to controlling reaction chemistry. Chen says that in addition to producing dry ethanol, these systems were highly selective in converting methanol to ethanol. •
