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Nanoscale motor goes full circle
RESEARCH NEWS
Nanoscale motor goes full circle
Porous material comes with catalytic sites built in POROUS MATERIALS
NANOTECHNOLOGY Generation of controlled, rotational motion is central to the fabrication of molecular machines. However, creation of nanoscale rotary motors is a considerable challenge. Now, chemists from the Stratingh Institute at the University of Groningen in The Netherlands have devised a way to power unidirectional molecular rotation by chemistry alone [Fletcher et al., Science (2005) 310, 80]. The novel system makes its full-circle rotation in four separate stages. Each 360° cycle involves two directionally selective bond-breaking steps alternated with two directionally selective bond-making chemical reactions. Completion of each stage moves the molecule another 90° clockwise about a fixed axis. Prior reports of synthetic molecular motors have used light or electricity to power full-circle motion, says Ben L. Feringa, coauthor of the study. However, natural processes generally do not require an external energy source. They instead generate their own power from the conversion of adenosine triphosphate to the diphosphate form. Researchers looking to nature for inspiration when designing their nanomolecular machines are consequently eager to synthesize a 100% chemically driven molecular motor. Although the system cannot yet propel anything, it demonstrates that such a motor could be made in principle, Feringa says. The next steps will be to improve its operational speed and simplicity. “Of course, this is an extremely primitive system and not at all useful because we need several chemical steps. Also, to build something that can really function as a motor, you need to have fast rotation.”
Paula Gould
Structure of the cubic phthalocyanine clathrate showing the large >8 nm3 void. (© 2005 Wiley-VCH.)
A novel clathrate, Zn octa(2,6-diisopropylphenoxy) phthalocyanine, with interconnected voids >8 nm3 in volume has been synthesized by researchers at Cardiff University, the University of Manchester, and CCLRC Daresbury Laboratory in the UK, as well as the University of Kuwait [McKeown et al., Angew. Chem. Int. Ed. (2005) doi: 10.1002/anie.200502668]. Organic-based porous materials need to possess builtin sites for selective heterogeneous catalysis, adsorption, and separations if they are to have advantages over zeolites and other conventional porous materials. With this aim, Neil B. McKeown and
colleagues investigated nanoporous materials based on highly functional phthalocyanine molecular units with transition metal ions at their center. However, the planar, square-shaped unit tends to form closely packed structures. To prevent close packing and form open nanoporous structures, McKeown’s group attached 2,6-diisophenoxy groups at the peripheral positions of the phthalocyanine ring. These caused steric crowding next to the phthalocyanine core, twisting the phthalocyanine unit out of planarity. “Simple recrystallization of a readily prepared phthalocyanine derivative gives a beautiful cubic packing arrangement in which six of the square-shaped phthalocyanine molecules self-assemble to define the nanovoid, each of which contains over 80 molecules of solvent,” explains McKeown. “The enclosed solvent can be exchanged (even for water), therefore the voids are interconnected, and the clathrate is able to act as a nanoporous material.” Chemical functionalities present in the phthalocyanine unit could provide catalytic sites embedded in the nanovoid wall that are accessible only through the narrow size-selective channels created by the void space. To investigate this possibility, the researchers are studying similar clathrates containing catalytically active transition metal ions such as Co2+ and Fe3+. They are also trying to stabilize the clathrate structure by placing polar and hydrogen-bonding substituents on the 2,6-diisopropylphenoxyl units. John K. Borchardt
Novel catalysts for direct methanol fuel cells CATALYSIS Direct methanol fuel cells (DMFCs) are a promising alternative energy technology because they offer high energy conversion efficiency, low emissions of pollutants, and operate at a low temperature while providing the convenient handling and processing of a liquid fuel. However, electrocatalysts with higher activity in room-temperature methanol oxidation are needed for DMFCs to reach commercialization. Pt is the only single-component catalyst that provides good activity for methanol oxidation, but the coproduct CO poisons the catalyst. Therefore Pt-based catalysts with improved methanol oxidation activity and greater resistance to CO poisoning are needed. Yuehe Lin and coworkers from Pacific Northwest National Laboratory and the University of Idaho have deposited Pt/Ru nanoparticles on multiwalled carbon nanotubes (CNTs) dispersed in supercritical carbon dioxide (scCO2). The resulting
bimetallic alloy catalyst, PtRu/CNT, provides better electrochemical performance for methanol oxidation than Pt nanoparticles alone on CNTs [Lin et al., Langmuir (2005), doi: 10.1021/la051272o]. The PtRu particles have a face-centered cubic crystal structure and are 5-10 nm in size. The Pt:Ru ratio in the particles is 45:55 and the loading on the CNTs is 4.1% Pt and 2.3% Ru. The researchers attribute the higher catalytic activity of PtRu/CNT to the large CNT surface area, the nanostructure of Pt/Ru, and the decrease in the overpotential for methanol oxidation. PtRu/CNT is thought to provide a higher catalytic activity than a Pt/CNT system because oxygen adsorption onto Ru provides a source for the oxidation of CO molecules that are strongly bound to the Pt.
John K. Borchardt
DECEMBER 2005 | VOLUME 8 | NUMBER 12
19
Nanoscale motor goes full circle
Porous material comes with catalytic sites built in POROUS MATERIALS
NANOTECHNOLOGY Generation of controlled, rotational motion is central to the fabrication of molecular machines. However, creation of nanoscale rotary motors is a considerable challenge. Now, chemists from the Stratingh Institute at the University of Groningen in The Netherlands have devised a way to power unidirectional molecular rotation by chemistry alone [Fletcher et al., Science (2005) 310, 80]. The novel system makes its full-circle rotation in four separate stages. Each 360° cycle involves two directionally selective bond-breaking steps alternated with two directionally selective bond-making chemical reactions. Completion of each stage moves the molecule another 90° clockwise about a fixed axis. Prior reports of synthetic molecular motors have used light or electricity to power full-circle motion, says Ben L. Feringa, coauthor of the study. However, natural processes generally do not require an external energy source. They instead generate their own power from the conversion of adenosine triphosphate to the diphosphate form. Researchers looking to nature for inspiration when designing their nanomolecular machines are consequently eager to synthesize a 100% chemically driven molecular motor. Although the system cannot yet propel anything, it demonstrates that such a motor could be made in principle, Feringa says. The next steps will be to improve its operational speed and simplicity. “Of course, this is an extremely primitive system and not at all useful because we need several chemical steps. Also, to build something that can really function as a motor, you need to have fast rotation.”
Paula Gould
Structure of the cubic phthalocyanine clathrate showing the large >8 nm3 void. (© 2005 Wiley-VCH.)
A novel clathrate, Zn octa(2,6-diisopropylphenoxy) phthalocyanine, with interconnected voids >8 nm3 in volume has been synthesized by researchers at Cardiff University, the University of Manchester, and CCLRC Daresbury Laboratory in the UK, as well as the University of Kuwait [McKeown et al., Angew. Chem. Int. Ed. (2005) doi: 10.1002/anie.200502668]. Organic-based porous materials need to possess builtin sites for selective heterogeneous catalysis, adsorption, and separations if they are to have advantages over zeolites and other conventional porous materials. With this aim, Neil B. McKeown and
colleagues investigated nanoporous materials based on highly functional phthalocyanine molecular units with transition metal ions at their center. However, the planar, square-shaped unit tends to form closely packed structures. To prevent close packing and form open nanoporous structures, McKeown’s group attached 2,6-diisophenoxy groups at the peripheral positions of the phthalocyanine ring. These caused steric crowding next to the phthalocyanine core, twisting the phthalocyanine unit out of planarity. “Simple recrystallization of a readily prepared phthalocyanine derivative gives a beautiful cubic packing arrangement in which six of the square-shaped phthalocyanine molecules self-assemble to define the nanovoid, each of which contains over 80 molecules of solvent,” explains McKeown. “The enclosed solvent can be exchanged (even for water), therefore the voids are interconnected, and the clathrate is able to act as a nanoporous material.” Chemical functionalities present in the phthalocyanine unit could provide catalytic sites embedded in the nanovoid wall that are accessible only through the narrow size-selective channels created by the void space. To investigate this possibility, the researchers are studying similar clathrates containing catalytically active transition metal ions such as Co2+ and Fe3+. They are also trying to stabilize the clathrate structure by placing polar and hydrogen-bonding substituents on the 2,6-diisopropylphenoxyl units. John K. Borchardt
Novel catalysts for direct methanol fuel cells CATALYSIS Direct methanol fuel cells (DMFCs) are a promising alternative energy technology because they offer high energy conversion efficiency, low emissions of pollutants, and operate at a low temperature while providing the convenient handling and processing of a liquid fuel. However, electrocatalysts with higher activity in room-temperature methanol oxidation are needed for DMFCs to reach commercialization. Pt is the only single-component catalyst that provides good activity for methanol oxidation, but the coproduct CO poisons the catalyst. Therefore Pt-based catalysts with improved methanol oxidation activity and greater resistance to CO poisoning are needed. Yuehe Lin and coworkers from Pacific Northwest National Laboratory and the University of Idaho have deposited Pt/Ru nanoparticles on multiwalled carbon nanotubes (CNTs) dispersed in supercritical carbon dioxide (scCO2). The resulting
bimetallic alloy catalyst, PtRu/CNT, provides better electrochemical performance for methanol oxidation than Pt nanoparticles alone on CNTs [Lin et al., Langmuir (2005), doi: 10.1021/la051272o]. The PtRu particles have a face-centered cubic crystal structure and are 5-10 nm in size. The Pt:Ru ratio in the particles is 45:55 and the loading on the CNTs is 4.1% Pt and 2.3% Ru. The researchers attribute the higher catalytic activity of PtRu/CNT to the large CNT surface area, the nanostructure of Pt/Ru, and the decrease in the overpotential for methanol oxidation. PtRu/CNT is thought to provide a higher catalytic activity than a Pt/CNT system because oxygen adsorption onto Ru provides a source for the oxidation of CO molecules that are strongly bound to the Pt.
John K. Borchardt
DECEMBER 2005 | VOLUME 8 | NUMBER 12
19