starved for nutrients. The enzyme catalyzes a one-electron oxidation of lignin to an aryl cation radical, which then undergoes cleavage and further oxidation to produce benzylic alcohols and aldehydes. Hammel and his coworkers find that the isolated enzyme oxidizes polycyclic aromatic compounds to quinones. For example, anthracene is converted to anthroquinone and pyrene is converted to a mixture of pyrene-1,6- and -1,8-quinones. (In whole culture, the fungus produces quinones transiently, but they are degraded further by other unidentified metabolic processes.) An experiment with water labeled with oxygen-18 showed that the oxygen atoms in the pyrene quinones come
from water. "We think the enzyme undertakes a one-electron oxidation of the aromatic compounds to give a radical cation/' Hammel explains. "Presumably the radical cation is further oxidized to the cation, which combines w i t h water to give a quinone." Another well-known peroxidase enzyme, horseradish peroxidase, also oxidizes polycyclic aromatics through a one-electron mechanism, Hammel points out. However, that enzyme will oxidize only compounds with ionization potentials lower than 7.35 eV. In contrast, ligninase will affect aromatic substrates with ionization potentials as high as 7.6 eV, making it significantly more oxidizing than the classical horseradish enzyme. •
Direct catalytic conversion of methane to fuels advances N E W OGUCJgAftS Joseph Haggin, C&EN Chicago
The high cost of producing synthesis gas has prompted a widespread search for processes employing direct catalytic conversion of methane to fuels and chemicals. Progress being made in this search was described to the Division of Petroleum Chemistry at a symposium on hydrocarbon oxidation and to the Division of Fuel Chemistry at a symposium on methane activation. Much encouragement toward direct conversion has been provided by Mobil's d e v e l o p m e n t of the methanol-to-gasoline (MTG) process now being used in New Zealand. However, the MTG process still requires synthesis gas to produce the requisite methanol. Methane is very stable, and, therefore, difficult to convert to higher value materials. However, recent work at Arco Chemicals, a subsidiary of Atlantic Richfield, has resulted in development of the noncatalytic formation of ethane, ethylene, and higher hydrocarbons. According to John A. Sofranko of
Arco's research staff, reducible metal oxides, both unpromoted and alkali-promoted, were highly effective redox agents for stoichiometrically coupling methane. The oxides were subsequently regenerated. Other groups also have reported similar results. The latest results demonstrate that the redox agents manganese and sodium pyrophosphate on silica, and sodium permanganate on magnesia are also effective catalysts when used in a methane-air cofeed process. The oxidative coupling of methane to C2 products can be carried out with Arco's systems with C2 yields of 15 to 16% and high productivity at temperatures in the range of 900 to 925 °C. Similar results are obtained with continuous cofeeding of methane and air and cyclic alternate feeding of the two reactants. The suggested mechanism for the reaction consists of the oxidation of methane to methyl radicals in the gas phase, and carbon oxide formation, principally from encounters of C2 and larger products with the oxidized surface. There is a minor contribution to carbon oxides from the oxidation of methyl radicals. Of particular interest to developers of the MTG process is the
possibility of direct conversion of methane to methanol. At the University of Manitoba, a group headed by chemistry professor Hyman D. Gesser has patented such a process, although the process appears to function better as a noncatalytic h o m o g e n e o u s reaction b e t w e e n methane and oxygen. The only catalyst that Gesser's group seems interested in pursuing may be stannic oxide. The patented process depends on controlled oxidation at 300 to 500 °C and 10 to 100 atm. Oxygen concentrations are preferably between 2 and 10%. Conversions are believed to be low, probably around 4%. The direct conversion of methane also has been under study at the University of Pittsburgh. George Marcelin of the department of chemical and petroleum e n g i n e e r i n g there noted again the thermodynamic prohibitions for direct conversion at commercially desirable temperatures and pressures. However, the addition of an oxidizing agent permits either partial oxidation or direct coupling. This is an extension of some of the pioneering work of Madan M. Bhasin and his coworkers at Union Carbide. The coupling proceeds via the abstraction of lattice oxygen and the formation of water. Oxidation also can lead to the formation of carbon oxides and water in addition to the homologs of methane. The work described by Marcelin involved the oxidation of methane with potassium-modified antimony oxides as a model catalyst. The object was to gain an understanding of the pathways of oxidation reactions. In general, bulk antimony oxide was selective to coupling reactions, but there was also a rapid deactivation of the catalyst. Small additions of potassium stabilized the catalyst but decreased the C2 selec-
Cofeed reaction uses Mars-Van Krevelen-type mechanism
September 14, 1987 C&EN
19
Technology Two-stage reactor could convert methane to gasoline Oxyhydrochlorination
Oligomerization
CH4
+
09
Gasoline (C 4 and higher)
tivity. If the potassium additions were increased to about 33%, stability was improved with higher selectivity. Further increases in potassium were not beneficial. The role of potassium in stabilizing the catalysts and fixing the selectivity is not clear. The work is continuing in efforts to determine the surface species of the catalysts. One of the industrial groups working on direct methane oxidation is that at Standard Oil. Joseph P. Bartek, a research associate in Standard Oil's corporate research department, noted that at moderate temperatures methane is active enough to donate hydrogen to metal surfaces in exchange reactions, but no higher hydrocarbons result. At higher temperatures, methane reacts with ammonia and air over platinum to produce hydrogen cyanide in a single step. Acetylene can be made from methane in a single step at high temperatures, but much of the methane must be used to provide the requisite heat for the reaction. Bartek's goal was to convert methane to higher hydrocarbons oxidatively. The work involves the performance of multicomponent oxide catalysts containing lead and magnesium. The lead catalysts were 20% lead oxide on alumina. The magne20
September 14, 1987 C&EN
sium-containing catalysts were made by adding lead nitrate solution to fresh magnesium hydroxide or hydroxy carbonates. According to Bartek, selectivity to higher hydrocarbons increases with temperature over all catalysts tried. The alumina-supported catalyst i$ most active because of the higher surface area. Selectivity is maximum at about 35%. Selectivities for mixed metal catalysts may reach 60% between 720 and 750 ° C Oxygen is the limiting reagent, meaning that methane conversion is selectivity controlled once the oxygen is consumed. Total methane conversion reached 16%, with about 6% being converted to higher hydrocarbons. Some individual catalyst samples were slightly better. The lead oxide catalysts are much more effective at coupling when oxygen is used. Methane added to a nitrogen stream over mixed metal catalysts displays very low yields. The increase in selectivity with temperature is not clearly understood, and much depends on the eventual deduction of the mechanisms involved in these systems. At Illinois Institute of Technology, a new, patented process converts methane to acetylene and ethylene by chlorine-catalyzed oxidative pyrolysis. According to Selim M. Senkan of IIT's department of chemical engineering, the methane is first chlorinated. In a second stage, the chlorinated methanes are oxidatively pyrolyzed to hydrocarbons and hydrogen chloride. Because of the inhibiting effect of chlorine and the chlorine compounds, there is no flame. Furthermore, the evidence indicates that oxidative pyrolysis of methyl chloride is thermoneutral. Senkan and his coworkers are now considering a detailed mechanism. Evidently, although oxygen interrupts processes that eventually lead to formation of solid deposits, it does not interfere with reactions leading to the formation of acetylene and ethylene. The oxygen appears to convert a transient ethylene radical to carbon monoxide, thereby limiting the extent of the polymerization by the C2 species. The combined yields of ethylene and acetylene have been measured
as high as 60% with 30% conversion of the methyl chloride feed. Senkan reports that economic analyses by the Gas Research Institute and Kinetics Technology Inc. have been favorable. IIT is now exploring the possibilities of licensing the process commercially. In Australia, direct methane conversion is also being investigated at Commonwealth Scientific & Industrial Research Organization. CSIRO's Christopher A. Lukey described his work using lithium/magnesium oxide catalysts with methane/oxygen mixtures in fixed catalyst beds. At the longest contact time, conversion reached a maximum of 3%, with selectivity to hydrocarbons about 94%. As contact time increased, the conversion to ethylene and propylene was favored. With increased oxygen content of the feed, there appears to be an increased effect from secondary reactions and a corresponding decrease in olefin production. Small amounts of other hydrocarbons—for example, butenes, butadiene, benzene, toluene, and acetylene—have been detected. Lukey says the data suggest that, at least in the early stages of the reaction, ethane and carbon oxides are formed in parallel reactions, and there is no obvious indication of a direct route to ethylene. Product selectivity is very dependent on contact time, oxygen concentration, and temperature. Very high selectivities to hydrocarbons (over 90%) can be achieved with very high methane/ oxygen ratios (more than 50:1). Considering the great variety of processes under investigation, it may have been inevitable that somebody would attempt to assemble the best features of all-into a composite process. This has been done at the Department of Energy's Pittsburgh Energy Research Center. Research chemist Charles E. Taylor described a combination of processes that is capable of producing higher hydrocarbons from methane in high yield and with high selectivity. In the combination process, methane, oxygen, and hydrogen chloride react over an oxyhydrochlorination catalyst made by sequential layering of cuprous chloride, potassium chloride, and lanthanum chloride on a fumed silica support in
nonaqueous solvents. The products were predominantly chloromethanes and water. In a second stage, the chloromethane is catalytically converted to higher hydrocarbons over forms of Mobil's ZSM-5 zeolites. The maximum practical temperature used in the first stage is limited by the melting point of the layered catalyst. This is generally around 400 °C. The catalyst is stable in air, but it is also hygroscopic and surface moisture can lower yields. / Two forms of ZSM-5 were used. One was promoted with iron and the other was a sample of H-ZSM-5 from the manufacturer. Both produced similar products. By-products in the first stage were the dimer and trimer of chloromethane. Some chloroform also was produced. The oligomerization of chlo-
romethane in the second stage occurred at conditions comparable to those for the conversion of methanol in the MTG process. Hydrogen chloride was produced as a recyclable by-product. It is evident that considerable work remains to be done to develop a consistent approach to the direct conversion of methane. Thermodynamics are against a direct attack on the very stable methane molecule. The sometimes puzzling operations of metal oxides have yet to be deciphered. Indeed, it is not yet universally agreed that they are legitimately termed catalysts. There are also some indications that uncatalyzed but controlled pyrolysis may offer processes of interest. Homogeneous catalysis is just starting to be heard from in this activity. In almost every case, there is some reason for optimism. •
Oil group assesses joint environmental effort
NEW GBWEAN! Last year 22 leading petroleum companies formed an organization for cooperative environmental research called the Petroleum Environmental Research Forum (PERF). The group's goal is to help solve some of the more difficult problems common to the refining industry, operating under the protection of the National Cooperative Research Act of 1984 (NCRA). PERF chairman Donald M. Fenton of Unocal's science and technology division in Brea, Calif., gave the Division of Petroleum Chemistry a review of PERF's first full year of operation. He noted that PERF was a response to a real need to conduct environmental R&D at a time when resources were stretched to the limit. The founders of PERF felt that the answer to this problem was cooperative environmental research that would augment research done by individual companies and yet not demand industry consensus, which might delay implementation of the research results. A major aid that PERF is counting
on is NCRA, which protects joint R&D ventures from triple-damage, private antitrust suits as long as the Department of Justice is notified of the venture. If such suits eventually surface, the maximum penalty would be limited to the actual damages. Also, a plaintiff bringing what the courts consider an unreasonable suit might have to pay the defendant's costs. NCRA also states that government antitrust agencies cannot declare R&D ventures illegal by definition; they must show that the ventures restrain competition in practice. Fenton says that, in securing the protection of NCRA, PERF appears to be the first R&D venture to function as a formal, chartered organization for the development of cooperative research projects and the exchange of general information about industrial research. The act has been used before but for specific projects. The organizers of PERF elected to develop an organization of company representatives rather than one with a paid staff and permanent facilities. The company representatives elect officers and committee members. Meetings are hosted by member companies and research is
conducted by contractors chosen by the proposal groups. Fenton cites five main advantages of the PERF approach. First is the obvious economic advantage of "leveraging" the funds of individual firms. Second is the ability to assemble quickly the necessary "critical mass" of expertise from among the participants. Third is the ability to conduct larger projects that might be undertaken by individual firms. Fourth is enough money is usually available to simultaneously treat possible alternative solutions to problems. And fifth is the development usually of a more mature and broadly applicable understanding of pollution problems. Proposals for research are brought to PERF sponsored by member firms. However, outside agencies, such as universities or institutes, may also submit proposals when sponsored by a member. Members may opt to keep the proposals in-house or agree to sponsor independent proposals. In any case, the proposal team is generally independent of PERF. If and when a proposal is accepted, the proposal team generally becomes the project team. Once a contract is agreed on, notice is given in case others may wish to participate in the project, always with PERF concurrence. The results of a research project belong to the project members. Other PERF members do not share in
Fenton: response to a real need September 14, 1987 C&EN
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Technology the expenses of the research. However, the project team may elect to share results with the public. The 22 petroleum companies in PERF hold quarterly meetings. In the first year, 17 proposals were received. By early 1987, three joint projects were under way, and others are currently in preparation. One deals with the b i o t r e a t m e n t of spilled hydrocarbons. A second deals with evaluation of hazardous waste solidification processes. And a third is concerned with microbiological processing of oily wastes, particularly in the detoxification of oily sludges. PERF also hosts information transfer sessions dealing with incineration of research wastes, improved sampling devices for analysis of groundwater contamination, and transforming of decommissioned offshore oil rigs to artificial reefs. Joseph Haggin, Chicago
New piperidine process developed at Mitsubishi
NEW Q&OEANS Research at Japan's Mitsubishi Petrochemical Co. has produced a new process for the production of piperidine, an important intermediate in the manufacture of agricultural chemicals, pharmaceuticals, anticorrosive agents, fiber additives, and rubber additives. Tadashi Ayusawa, a research specialist at Mitsubishi's fine chemicals laboratory, told the Division of Petroleum Chemistry that the key to the process is catalytic hydrogenolysis of tetrahydrofurfurylamine with cobalt. The reaction is carried out in a suspended catalytic bed. Although many alternative processes have been proposed over the years, most of the piperidine used today is made from the catalytic hydrogenation of pyridine. In the new process, furfural reacts with furfurylamine or tetrahydrofurfurylamine and the product undergoes hydrogenolysis in the presence of a slight excess of am22
September 14, 1987 C&EN
monia. The main product is piperidine. Because the catalyst is not 100% selective, some by-products are formed, which are minimized by adroit operation of the reactor. The piperidine is formed low in the reaction bed, and it is necessary to remove it rapidly, presumably to avoid additional reaction in the highly catalytic environment. Thus the piperidine is removed in an excess hydrogen stream and subse-
quently condensed. Ayusawa claims that the process can be operated continuously and that any decrease in catalyst activity can be compensated for by raising the reaction temperature. An interesting aspect of the process is that by-products apparently reach a maximum concentration in the reactor effluent and then stabilize at that concentration for prolonged periods. Joseph Haggin, Chicago
Fluid catalytic cracking enhanced
NEW QRUEAN I m p r o v e m e n t s in fluid catalytic cracking (FCC) have resulted from the introduction of new catalytic materials. Some materials provide the improvements without any great penalty in performance or product slate, according to developments described at a Division of Petroleum Chemistry symposium on advances in FCC. Most FCC reactions occur at operating temperatures of about 800 K, where the formation of olefins and aromatics is favored but isomerization to more branched products is not. Major problems with conventional catalysts have arisen from coking and catalytic poisoning by heavy metals, particularly nickel and vanadium. Production of lead-free gasoline with higher inherent octane numbers and more gasoline from a barrel of more expensive crude oil has heightened the interest in improving the materials and techniques of FCC. Enhancement of catalytic effect in FCC may be obtained by the use of nonzeolitic additions to the catalyst beds in the form of porous silica/alumina; the addition of smallcell, ultrastable natural zeolites (faujasite); a n d the a d d i t i o n of ZSM-5, Mobil's synthetic zeolite that is best known for its ability to convert methanol to gasoline. Of these means, one of the more discussed is the addition of ZSM-5. According to Susan P. Donnelly, associate engineer in Mobil Research & Development Co.'s engineering
department, ZSM-5 has been used to enhance FCC since 1983. In general, it has permitted upgrading gasoline octane numbers from 87 to 92.7. ZSM-5 increases the octane rating in FCC by upgrading low-octane components in the gasoline boiling range to higher octane components, with simultaneous production of propylene and butylene. This is done without the addition of coke or any increased production of light gases, both problems with conventional catalysts. The addition of ZSM-5 to conventional cracking catalysts in service produces relatively fast changes in the octane boost, and subsequent halting of ZSM-5 additions allows corresponding increases in the C3 and C4 olefin production. This allows considerable flexibility in operating an FCC unit that can take advantage of short-term price fluctuations in the market. The primary reaction pathway of ZSM-5 is to crack and isomerize C7 to C13 low-octane components to branched C 3 to C 5 olefins. The olefins produced are more branched than those produced with the base catalyst alone, suggesting that isomerization is an integral part of the reaction mechanism. There is no evidence of aromatics formation. However, aromatics become slightly concentrated as unbranched paraffins/ olefins are cracked. ZSM-5 also has demonstrated a pronounced tolerance for the presence of metals in the feedstocks. Commercial tests have shown tolerances for up to 3000 ppm of nickel, 2500 ppm of vanadium, and 6000 ppm of sodium. Joseph Haggin, Chicago
