CATALYSIS
MOLECULAR MACHINES
▸ New catalyst puts CO2 to good use
Material’s rotors spin freely and quickly
A Columbia University research team has prepared a magnesium catalyst that can efficiently convert carbon dioxide to precursors for commodity chemicals such as formaldehyde. Carbon dioxide is usually thought of as an essential molecule for photosynthesis and as a greenhouse gas that helps control Earth’s climate. But because CO2 is abundant and cheap, it also offers much potential as a renewable carbon source for synthesizing chemicals and fuels. The catch is that the gas is not very reactive until it overreacts, so chemists need to come up with catalysts that selectively functionalize CO2 to produce targeted products. One approach is hydrosilylation,
Researchers have designed a crystalline material with tethered but freely spinning molecular groups that rotate incredibly quickly for a solid-state material. The material is the best example yet of an X-ray crystal “amphidynamic material,” a crystalline solid with structure of both relatively static and rapidly moving molecular amphidynamic material with fastunits. Miguel García-Garibay of the University of California, Los Angeles, and coworkers synthesized spinning rotors (blue arrow, 50 the material by combining a Zn4O-based metal-orbillion rotations per ganic framework with spinning bicyclo[2.2.2]ocsecond). tane-1,4-dicarboxylate units that have nearly zero spatial or electronic barriers to rotation (Proc. Natl. Acad. Sci. USA 2017, DOI: 10.1073/pnas.1708817115). The bicyclooctane units spin at up to 50 billion rotations per second, only one order of magnitude slower than the maximum possible rotation rate for unhindered gas-phase molecular rotors in vacuum. At the same time, the Zn4O framework undergoes normal low-frequency vibrations. In previous amphidynamic crystals, rotors had larger internal steric or electronic barriers that caused them to rotate in discrete steps. The new material’s low barriers enable the bicyclooctane groups to rotate continuously instead, and the rotors also switch direction many times a second. Amphidynamic materials like this new one have potential implications for the design of smart materials and molecular machines, the researchers say.—STU BORMAN
2-D MATERIALS but control of the product distribution by high-priced precious-metal catalysts at the hot temperatures they require is challenging and may result in complete reduction of CO2 to methane. The new catalysts prepared by Columbia’s Michael Rauch and Gerard Parkin instead use less costly zinc or magnesium and work selectively at room temperature. In particular, the researchers found that a magnesium benzimidazole hydride treated with the Lewis acid B(C6F5)3 produces a reactive cation-anion pair that coordinates CO2 and transfers the hydride to carbon, efficiently creating a formate intermediate (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b10776). The formate group can be removed from the catalyst by reaction with different hydrosilanes in the presence of B(C6F5)3 to make various products. For example, triphenylsilane (Ph3SiH) generates a bis(silyl)acetal that is a protected form of formaldehyde, whereas phenylsilane (PhSiH3) generates methane.—STEVE RITTER
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C&EN | CEN.ACS.ORG | JANUARY 1, 2018
▸ Ironing out graphene’s wrinkles With a carefully engineered substrate, researchers can grow high-quality graphene free of troublesome wrinkles that often form during manufacture. The supersmooth graphene has improved electrical properties over material grown by the
Scanning electron microscope (left) and atomic force microscope (right) images show that graphene grown on Cu(111) (top) is wrinkle-free; not so for graphene grown on Cu(100) (bottom).
usual methods (ACS Nano 2017, DOI: 10.1021/acsnano.7b06196). Cu(100)—the crystal facet, or face, of copper that is usually used as a growth substrate—expands at a different rate than graphene at a given temperature. Hailin Peng of Peking University says this mismatch leads to mechanical strain that causes wrinkling as graphene grows on that face during chemical vapor deposition. So he and his colleagues looked for a copper crystal structure that’s a better match. They report that graphene grown on Cu(111), a crystal face in which the atom positions differ from those of Cu(100), is perfectly smooth and has an electron mobility of 11,000 cm2/V-second, which ranks among the highest ever measured for graphene grown over large areas using practical methods. The team made the Cu(111) substrates by growing copper films on 10-cm-diameter sapphire wafers. The method is compatible with semiconductor industry technology, Peng says. “We are trying to realize the mass production of this wrinkle-free, single-crystal graphene.”—KATHERINE
BOURZAC, special to C&EN
C R E D I T: COU RTESY O F M I GU E L GA RC Í A-GA R I BAY ( ROTO RS ) ; CO U RT ESY O F G E RAR D PAR K I N (CATALYST ); ACS N A N O (G RAP H E N E )
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