Science Concentrates

Elusive CO₂ binding mode snagged

University of California, San Diego, chemists have isolated and structurally characterized a synthetic uranium complex that binds CO₂ in an unusual way: CO₂ is linearly coordinated to the metal via one of its oxygen atoms [Science, 305, 1757 (2004)]. This linear metal-CO₂ coordination mode had previously been observed in the crystal structure of an iron enzyme involved in antibiotic synthesis, but until now scientists had not been able to obtain definitive structural evidence for its existence in synthetic systems, according to Karsten Meyer and Ingrid Castro-Rodriguez. In their new synthetic complex, bulky adamantane groups that surround the metal force U­OCO coordination (shown; purple = uranium, red = oxygen, gray = carbon, and blue = nitrogen). From bond lengths, magnetization data, and electronic and vibrational spectra, Meyer and Castro-Rodriguez conclude that upon CO₂ binding, U(III) is oxidized to U(IV), and CO₂ is reduced by one electron. The study of this and other metal complexes that bind and reduce CO₂ may someday lead to the development of simple compounds that can convert excess CO₂ into useful chemicals, they say.

Nanotubes as long as desired?

Researchers have grown individual single-walled carbon nanotubes (SWNTs) as long as 4 cm and say that their results "suggest the possibility of growing SWNTs continuously without any apparent length limitation" [Nat. Mater., published online Sept. 12, http://dx.doi.org/10.1038/nmat1216]. Yuntian T. Zhu of Los Alamos National Laboratory and his colleagues synthesized the ultralong nanotubes by iron-catalyzed decomposition of ethanol in a quartz tube furnace at 900 ºC. The nanotubes were grown on a 4.8-cm-long silicon substrate that was used to obtain scanning electron microscope images of the nanotubes--hence the 4-cm limit on nanotube growth in these experiments. The SWNTs grew at an estimated rate of 11 µm per second--several times faster than a high growth rate reported last year by a different group. The researchers note that previous reports of long nanotube structures "have been limited to yarns, strands, and fibers formed from much shorter individual nanotubes or nanotube-polymer matrix composite fibers." Ultralong single SWNTs could find use in electronic devices and biosensors.

More evidence for inorganic origin of oil

Crude oil is widely believed to have formed from plant and animal material encased in near-surface sedimentary rocks in Earth's crust. A counterclaim holds that petroleum is formed abiotically from carbonates at high temperatures and pressures deep in Earth's mantle. Now, a research team led by Henry P. Scott of Indiana University, South Bend, reports some of the first experimental evidence to bolster the inorganic theory [Proc. Natl. Acad. Sci. USA, published online, http://www.pnas.org/cgi/doi/10.1073/pnas.0405930101]. Scott and coworkers reacted iron oxide, calcite (CaCO₃), and water in a diamond anvil cell and observed in situ formation of methane at a range of temperatures and pressures. The most favorable conditions for methane formation were 500 °C and 70,000 atm, corresponding to a depth of 100 to 200 km below Earth's surface. In 2002, a different group reported formation of methane as well as heavier hydrocarbons for a similar set of experiments in which the reactions were quenched before the products were characterized.

Self-assembling molecule for zeolite synthesis

Zeolites are porous aluminosilicates that can be used as catalysts and adsorbents for applications such as petroleum refining. They are usually formed by allowing the inorganic material to assemble around a "shape-directing agent" (SDA). One type of zeolite, known as zeolite A, generally does not make a good material for petroleum refining because its low Si/Al ratio (about 1 to 1) leads to poor acid-catalytic activity, poor hydrothermal stability, and low hydrophobicity. Avelino Corma and coworkers at the Polytechnic University of Valencia, in Spain, describe a self-assembling supramolecular organic SDA (shown; red = carbon, green = nitrogen, gray = hydrogen) made from a quinolinium derivative that allows them to synthesize zeolite A structures with high Si/Al ratios [Nature, 431, 287 (2004)]. Using the quinolinium derivative with tetramethylammonium cations, they are even able to synthesize zeolites made of pure silica. The zeolites synthesized using this SDA can be used for separations and petroleum processing.

Toxic iodoacid by-products of clean water

A new class of highly toxic contaminants has shown up among the disinfection by-products of a Texas drinking water treatment plant [Environ. Sci.Technol., 38, 4713 (2004)]. Iodoacids (including iodoacetic acid and some brominated compounds such as 3-bromo-3-iodopropenoic acid) are the likely result of a pair of reactions. A chloramine disinfectant reacts with iodide (which seeps into the Texas water from an ancient seabed) to form HOI, and HOI reacts with natural organic matter to form a variety of iodoacids. Michael J. Plewa of the University of Illinois, Urbana-Champaign, and coworkers tested iodoacetic acid for its biological toxicity. "Iodoacetic acid is the most potent cytotoxic and genotoxic disinfection by-product that I have ever seen in mammalian cells," Plewa reports. He suspects that only treatment plants that rely solely on chloramination disinfection and encounter high levels of iodide and bromide should produce the iodoacids. He adds that his data highlight the dangers of not having a comparative database of disinfection by-products.