NEWS 0*THEWEEK
NATURAL GAS FORMATION Catalysis perhaps made geologic deposits ew evidence suggests that natural gas in geologic deposits may have been produced not by thermal decomposition of organic matter, as tradi tionally believed, but rather as the result of catalytic influences of sedimentary rock on kerogen—the complex fossilized or ganic material present in such rocks. Laboratory simulation of the geolog ic processes that would be involved has produced a gas mixture with com position typical of natural gas sources. The catalysts evidently are transition metals and their compounds, and the dominant reaction appears to be con densation of hydrogen and rc-alkenes. The work was carried out in Hous ton by Frank D. Mango, professor of
N
Catalytic hydrogenolysis scheme leads to methane CHo CH, M
M
v ^ C E
-VVNM—L:
M—CH 3
M—H CH4
H2
Reprinted with permission from Nature Copyright 1994 Macmillan Magazines Ltd.
Catalytic scheme proposed for hy drogenolysis of a long-chain n-alkene to methane shows wavy-line compounds representing long carbon chains of any length, with those trun cated by double bonds representing α-alkenes. Metals (M) are weakly co ordinated to the double bonds. The metal maintains a σ-bond in each in termediate, indicated by solid lines, and is free to move up and down the carbon chain by sequential additionelimination steps. The scheme would thus lead to generation of ethane and higher alkanes if the metal moves to the number four and higher carbon atoms. 4
APRIL 11,1994 C&EN
geology and geophysics, and Joe W. Hightower, professor of chemical engi neering, both at Rice University, along with Alan T. James, a petroleum geolo gist with Exxon Production Research Co. Funded by the Department of En ergy, the research was published last week in Nature [368, 536 (1994)]. If the finding of a catalytic pathway to natural gas is validated, it could influ ence how and where fossil fuel explora tions are carried out. It also could pro vide insight into other geologic phenom ena, such as the hydrothermal processes involved at oceanic rifts, where not only temperatures but pressures are high. In recent years, increased attention has been paid to the origins of petro leum and natural gas, with some theo rists suggesting that natural gas may even be abiogenic in origin. One theo ry, for example, attributes gas forma tion to processes deep within the Earth's crust where there have never been kerogen deposits. The traditional view, however, is that biomass was de posited, covered, and pressurized over geologic time. During this time, it was converted, principally by thermolysis, to form petroleum and gas. It is also sometimes suggested that coal may have been formed by similar processes. A problem with traditional theory, however, is that pyrolysis of organic matter in the laboratory does not pro duce methane in the concentrations typically observed in natural gas. Higher yields of methane are obtained in catalytic crackers, but Mango and his coworkers doubt that the conditions encountered in a catalytic cracker are ever encountered in geologic processes. Independent theoretical analysis in dicates that the thermodynamics of the C-H-O reaction system permit methane formation but that such formation is kinetically inhibited. Thus, catalysis would be necessary to increase the rates to appreciable values. Hence, an alternative theory of natu ral gas formation points to the selective
Mango: reaction of hydrogen, n-alkenes catalytic conversion of materials from the decomposition of kerogen. In par ticular, Mango suggests that natural gas may be formed via the catalytic re action of hydrogen with n-alkenes. The catalysts in question could be transition metals or their compounds occurring naturally in the sedimentary rocks of the geologic formation. Decomposition of the kerogen may produce α-olefins, which may react readily with metal hy drides to produce metal alkyls—key in termediates in the reaction sequence. The hydrogen necessary for the reac tions may come either from kerogen decomposition or from the reaction of water with metal-oxide minerals. To test this idea, the Houston re searchers devised an experiment that simulated conditions underground. For periods of several days, n-octadecene-1 and hydrogen were maintained at 190 °C in contact with a preheated sam ple of sedimentary source rock. The product was a C r to-C 4 mixture that was about 80% by weight (90 mol %) CH4. The CH4 was generated at a rate on the order of 10"7 g per day per g of rock. This rate was observed to remain con stant for periods up to a year, the nomi nal length of the experiment.
To demonstrate that true catalysis had occurred, Mango and coworkers conducted additional experiments un der identical conditions with alkene and hydrogen (with no rock), with hy drogen and rock (no alkene), and with alkene and rock (no hydrogen). In each case, only trace amounts of hydrocar bons were detected. The addition of small amounts of water increased cata lytic activity and selectivity to methane. Above about 2.5% by weight of water, catalytic activity remained unaffected, but selectivity to methane dropped in favor of increased conversion to C5 and higher hydrocarbons. Since the source rock does not con tribute material to the reaction, but is essential for it to proceed, the research ers reason, the reaction must be catalyt ic. As additional evidence, the reaction rate is fairly constant over time— consistent with catalytic reactions and inconsistent with thermal cracking. Scientists may question, however,
whether the experimental conditions correspond to conditions in geologic formations. Mango responds by noting that the experimental temperature range is well within that suggested for gas generation in sedimentary basins, and below some temperatures mea sured in actual reservoirs and deep gas deposits. Similarly, hydrogen condi tions in the experiments are well with in the limits encountered in nature. The researchers also tried the reaction with a sample of commercial cracking catalyst (Si0 2 /Al 2 0 3 ). At the reaction temperature, the catalyst was active with π-octadecene-l, but the products were more typical of acid catalysis and totally unlike those produced with the source rock catalyst. Mango has not identified the specific catalysts in the source rock, but the principal candidates are nickel and va nadium compounds. He is now work ing on more precise identification. Joseph Haggin
North Korea suspected of hiding plutonium The International Atomic Energy Agen cy (IAEA) says several types of isotopic tests lead it to believe North Korea may have produced far more plutonium than it has declared. However, evidence obtained so far is somewhat sketchy and circumstantial. North Korea has blocked IAEA from conducting further tests and inspections needed to prove that that nation is un derstating its plutonium production. The U.S. Central Intelligence Agency and the Department of Defense fear that North Korea is extracting plutoni um from spent fuel to build nuclear weapons, and that it is developing nu clear facilities to enable it to sell nucle ar technologies to nations like Iraq. North Korea now operates a 5-MW nuclear reactor, and is rapidly complet ing a 50-MW reactor. Construction of a 200-MW reactor also is well under way. All are Calder Hall reactors, used in the U.K. in the 1950s and ideally suited for producing weapons-grade plutonium (mostly Pu) from natural uranium ore. North Korea has one reprocessing line for extracting plutonium from spent fuel and is completing a second line, which will double its capacity for plutonium production. It also has a plant that manufactures fuel for its re actors. All the nuclear facilities are lo
cated at Yongbyong, 60 miles north of the capital, Pyongyang. North Korea claims it has produced less than 100 g of plutonium in one re processing run. But IAEA says it may be hiding as much as 10 kg. About 5 kg are needed to make a crude bomb. When IAEA tested three samples of plutonium provided by North Korea, it found the samples had different isoto pic compositions of americium-240 and -241, says David R. Kyd, director of public information at the agency's headquarters in Vienna. When plutoni um is extracted from spent fuel, americium is also extracted. The longer the fuel stays in the reactor, the more americium develops. So if one sample of plutonium has a different level of americium than another, it was probably extracted at a different time. Another way to test whether North Korea is hiding plutonium is to deter mine whether the isotopic ratios of 239Pu to 240Pu in the extracted plutonium sam ples are the same as the ratios in the waste from reprocessing this plutonium. The 239Pu/240Pu ratios in the samples do not match the ratios in the waste IAEA inspectors had access to, says Kyd. IAEA tests indicate that the 239Pu/240Pu ratios in the waste differ from those in the samples by about 0.5 to 1%.
This leads IAEA to suspect North Korea is hiding waste. But that nation has barred IAEA from conducting fur ther tests to confirm this. "We have reason to believe there is a great deal of concealed waste material at their facilities because we have satellite photos of facilities that were camou flaged before our inspections began," Kyd tells C&EN. "We believe waste is hidden below ground at the site. If we could get into that, we would be able to see whether the volume of waste is such as to make us believe a good deal more plutonium has been produced than North Korea claims. They have refused to let us into those sites on the grounds that they are militarily related." A third way to test North Korea's ve racity is to check the fuel rods in July when the core is removed from the 5-MW reactor for refueling. IAEA is demanding it be allowed to test these rods to see if they are the original ones installed in 1986, or whether more rods were removed in 1989 for plutonium reprocessing than North Korea ac knowledges. North Korea claims it withdrew only a few damaged rods at one time in 1989, and experimentally reprocessed a rod to produce a few ounces of plutonium. North Korea denies it is building nu clear reactors and setting up reprocess ing facilities to build nuclear weapons. It
Yongbyon is site of North Korean nuclear facilities China North Korea YQNGBYOM Korea Bay
Sea of Japan P'yongyang
* Seoul
Yellow Sea
South Korea
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