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WHAT'S HOT IN... CHEMISTRY, Jan./Feb. 2008
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Chemists Pioneer a Golden Road to
Oxidation
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by John Emsley
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Chemistry Top Ten
Papers
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Rank
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Papers
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Cites Jul-Aug
07
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Rank
May-Jun 07
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1
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Z.L. Wang, J. Song, "Piezoelectric nanogenerators
based on zinc oxide nanowire arrays,"
Science, 312(5771): 242-6, 14 April 2006. [Georgia
Inst. Tech., Atlanta; Peking U., Beijing, China] *032HK
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29
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†
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2
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G. Ferey, et al., "A chromium
terephthalate-based solid with unusually large pore volumes
and surface area," Science, 309(5743):
2040-2, 23 September 2005. (U. Versailles, France; Inst. U.
de France, Paris; Royal Inst., London, U.K.; European
Radiat. Synchrotron Facility, Grenoble, France] *968SF
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19
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6
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3
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D.I. Enache, et al., "Solvent-free
oxidation of primary alcohols to aldehydes using
Au-Pd/TiO2 catalysts,"
Science, 311(5759): 362-5, 20 January 2006.
[Cardiff U., U.K.; Lehigh U., Bethlehem, PA] *007AE
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19
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†
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4
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A. Abad, et al., "A collaborative effect
between gold and a support induces the selective oxidation
of alcohols," Angew. Chem.-Int. Ed.,
44(26): 4066-9, 27 June 2005. [U. Polytec. Valencia, Spain]
*941KX
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18
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†
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5
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Y. Huang, et al., "Enantioselective
organo-cascade catalysis," J. Am. Chem.
Soc., 127(43): 15051-3, 2 November 2005. [Caltech,
Pasadena] *979HV
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16
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†
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6
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J.K. Edwards, et al., "Direct synthesis of
hydrogen peroxide from H2 and O2
using TiO2-supported Au-Pd catalysts,"
J. Catalysis, 236(1): 69-79, 15 November 2005.
[Cardiff U., U.K.; Lehigh U., Bethlehem, PA] *984DE
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16
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†
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7
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S. Hoffmann, A.M. Seayad, B. List, "A powerful
Brønsted acid catalyst for the organocatalytic
asymmetric transfer hydrogenation of imines,"
Angew. Chem.-Int. Ed., 44(45): 7424-7, 18 November
2005. [Max Planck Inst. Bioinorganic Chem., Mulheim an der
Ruhr, Germany] *987KB
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14
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†
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8
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D. Enders, et al., "Control of four
stereocentres in a triple cascade organocatalytic
reaction," Nature, 441(7095): 861-3, 15
June 2006. [Aachen U., Germany] *052SL
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14
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†
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9
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R.P. Herrera, et al., "Catalytic
enantioselective Friedel-Crafts alkylation of indoles with
nitroalkenes by using a simple thiourea
organocatalyst," Angew. Chem.-Int. Ed.,
44(40): 6576-79, 14 October 2005. [U. Bologna, Italy; U.
Alicante, Spain] *976VR
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14
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10
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X. Wang, et al., "A general strategy for
nanocrystal synthesis," Nature,
437(7055): 121-4, 1 September 2005. [Tsinghua University,
Beijing, China; Natl. Ctr. Nanosci. & Nanotech.,
Beijing] *960AC
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13
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9
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SOURCE: Thomson Scientific's
Hot Papers
Database. Read the
Legend.
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A major contribution to a sustainable future would be for chemists to find
catalysts that enabled key processes to give 100% yields at low
temperatures and without the need for solvents. Gold, once thought of as
the noblest of metals on account of its lack of chemical reactivity, might
well have a key role to play in achieving this gold standard of green
chemistry.
Alcohols are potentially useful resource materials. Converting them to
other products such as aldehydes and ketones is generally a very non-green
process. Ideally it should be done using the oxygen of the air as the
oxidizing agent, with the process carried out at room temperature,
producing little in the way of waste by-products.
Thanks to the catalysts reported in papers #3 and #4, that may be
eventually be possible. The group involved in paper #3 also has a second
paper in the list at #6 where they report a cleaner and greener synthesis
of another key chemical, hydrogen peroxide.
Neither paper #3 or #4 quite achieves the goal of green-chemistry
perfection, but #4, from Avelino Corma and colleagues at the Polytechnic
University of Valencia, Spain, comes close. The team stirred 14 alcohols
under an atmosphere of oxygen with their catalyst of nano-crystalline
CeO2 doped with gold. Reaction times were from 2 to 10 hours and
yields ranged from 66 to 100%.
For example, 2-hydroxybenzyl alcohol needed only 2 hours to achieve the
maximum possible yield of 2-hydroxybenzaldehyde. Corma detected
Au+ and Ce3+ as intermediate reactive species and
deduced that Ce-H and Au-H bonds played a part in the process.
Paper #3 comes from lead author Graham Hutchings of Cardiff University,
Wales, and reports work done by his group in collaboration with researchers
at Lehigh University, Bethlehem, Pennsylvania. Their catalyst was gold
(2.5%) and palladium (2.5%) on a TiO2 substrate and focussed on
primary alcohols and their conversion to aldehydes by reaction with
O2. Reactions were carried out in the absence of a solvent, at
100° C or 160° C, and gave excellent yields after about 8 hours.
They also demonstrated that air could be used as the source of oxygen.
Hutchings concentrated on the catalyst itself, which was made by heating
the components at 400° C. As such it is stable and does not lose Au or
Pd when operating, and he claims that it is 25 times more efficient than
the CeO2-based catalyst. Alternative substrates,
Al2O3 and Fe2O3, were tried but
were not as good. The Au-Pd/ TiO2 catalyst was characterized
using XPS and STEM, which showed its surface was particularly enriched with
Pd atoms.
It was the discovery reported in paper #6 which led Hutchings to the
findings in paper #3. Paper #6 showed that Au-Pd/ TiO2 would
catalyze the formation of hydrogen peroxide from a mixture of O2
and H2, thereby offering a possible commercial route to this key
oxidizing agent with advantages over existing processes.
The reaction was carried out at 2° C in an autoclave using
methanol-water as a solvent and under a pressure of 3.7 MPa. After 30
minutes, yields of 25% of H2O2 were obtained.
Hutchings has recently published on the web further details of the
synthesis of H2O2 (see J.K. Edwards, et al.,
Faraday Disc., 2008, DOI: 10.1039/b705915a) in a report revealing
that carbon-supported catalysts are better than the
TiO2-supported ones.
As Hutchings tells ScienceWatch.com, "The direct synthesis of
hydrogen peroxide from its elements represents one of the grand
challenges in catalysis. Paper #6 showed that catalysts could be designed
that operate under intrinsically safe conditions. The work on oxidizing
alcohols was inspired by this discovery because we reasoned that if the
catalyst could make hydrogen peroxide it probably operated via a
hydro-peroxy intermediate, and this should oxidize alcohols. And we were
right."
Hutchings’ group is now working on perfecting the gold palladium
catalyst for other selective oxidation reactions and in particular for
oxidizing hydrocarbons. "We hope that our work plays a role in changing the
image of gold," he says. "Our immediate aim is to design supported gold
palladium catalysts for the direct synthesis of hydrogen peroxide, as this
is one of the remaining grand challenges of catalysis today."
Dr. John Emsley is based at the Department of Chemistry, Cambridge
University, U.K.
What's Hot In... : What's Hot In Chemistry Menu : Chemists Pioneer a Golden Road to Oxidation - Chemistry - Jan/Feb 2008
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