DOI: 10.1126/science.1090313, 792 (2003);302 Science

Robin D. Rogers and Kenneth R. SeddonIonic Liquids--Solvents of the Future?

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sheets reach some critical latitude, they re-flect so much solar energy back into spacethat the entire planet freezes over. In thefrozen world, weathering stops. Hydro-thermal iron becomes more abundant thanweathering sulfur in the anoxic ocean, gener-ating the first banded iron formations onEarth in 1000 million years (2). Ultimately,Walker et al.s thermostat overcomes the icealbedo, because CO2 degassing from Earthsinterior drives atmospheric pCO2 upward.The ice melts abruptly, transforming Earthinto a hothouse, which the thermostat even-tually ameliorates. In the process, weatheringconsumes large amounts of CO2, generatingthe cap carbonates.

There were two to four snowball glacia-tions during the Neoproterozoic (7). Whydid the thermostat break repeatedly duringthis interval, but not at any other time?Ridgwell et al. have identified a mecha-nism that may help to answer this question.To understand their idea, we need to con-sider a second feedback mechanism in thecarbon cycle: CaCO3 compensation (8).

The balance this time is between weath-ering of CaCO3 and its burial in the ocean.The homeostat switch is the pH of the ocean.CaCO3 is a base and dissolves in acid. If therate of weathering exceeds that of burial, theocean becomes more basic, enhancing burialuntil the two fluxes balance. CaCO3 com-

pensation operates more quickly than Walkeret al.s thermostat; under todays conditionsthe time scale is about 10,000 years (9).Ridgwell et al. have identified a mechanismby which CaCO3 compensation might havegone awry, drawing down enough CO2 to ex-plain the descent into the Snowball state.

The mechanism begins with a drop in sealevel. When continental crust is partiallyflooded, as it is today, the ocean covers a largearea of shallow-water sea floor. If the sea lev-el drops by a few hundred meters, the area ofshallow-water sea floor decreases dramatical-ly, by about a factor of 10 in todays ocean.Today, such a sea-level drop would shift theburden of CaCO3 deposition from the shal-low to the deep ocean. This was not an optionin the Neoproterozoic, when no organismssecreted CaCO3 in the open ocean.

Ridgwell et al. (3) argue that deep-seaburial of CaCO3 is more responsive tochanges in ocean pH than shallow-waterburial would be. CaCO3 solubility increaseswith pressure. Today, the deep ocean strad-dles equilibrium (intermediate waters aresupersaturated, whereas the abyss is under-saturated). Hence, a change in pH alters theboundary between supersaturated and un-dersaturated sea floor, driving a largechange in CaCO3 burial. In contrast, shal-low waters are supersaturated almost every-where. Shallow CaCO3 burial can therefore

only be enhanced by increasing the precipi-tation rate for CaCO3, which requires amuch larger pH change (see the figure).

Because the Neoproterozoic only had re-course to saturation stateinsensitive shal-low-water CaCO3 deposition, a drop in sealevel would have driven a larger excursion inocean pH, and atmospheric CO2, than wouldoccur today. The mechanism identified byRidgwell et al. (3) would still require an ini-tial kickperhaps a glaciationto lower sealevel, but would then amplify that cooling towithin reach of the runaway ice-albedo feed-back. In the aftermath of the Snowball, afterWalker et al.s thermostat melted the ice, theocean would have relieved its CaCO3 burdenin newly flooded shallow seas, providing acomplementary or alternative explanationfor the observed cap carbonates.

References1. A. C. Doyle, Silver Blaze, in Strand Mag. (December 1892).2. P. F. Hoffman, D. P. Schrag, Terra Nova 14, 29 (2002).3. A. Ridgwell, M. J. Kennedy, K. Caldeira, Science 302,

859 (2003).4. H. C. Urey, The Planets, Their Origin and Development

(Yale Univ. Press, New Haven, CT, 1952).5. J. C. G. Walker, P. B. Hays, J. F. Kasting, J. Geophys. Res.

86, 9776 (1981).6. R. A. Berner, Z. Kothavala, Am. J. Sci. 301, 182 (2001).7. A. J. Kaufman, A. H. Knoll, G. M. Narbonne, Proc. Natl.

Acad. Sci. U.S.A. 94, 6600 (1997).8. W. S. Broecker, T. H. Peng, Global Biogeochem. Cycles

1, 15 (1987).9. D. Archer, H. Kheshgi, E. Maier-Reimer, Global

Biogeochem. Cycles 12, 259 (1998).

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Research into ionic liquids is booming.The first industrial process involvingionic liquids was announced in

March 2003, and the potential of ionic liq-uids for new chemical technologies is be-ginning to be recognized. The burgeoninginterest in the field was obvious at the re-cent American Chemical Society (ACS)meeting in New York, where ionic liquidswere the focus of 10 sessions (1).

Ionic liquids are composed entirely ofions. For example, molten sodium chloride isan ionic liquid; in contrast, a solution of sodi-um chloride in water (a molecular solvent) isan ionic solution. The term ionic liquidshas replaced the older phrase molten salts

(or melts), which suggeststhat they are high-tempera-ture, corrosive, viscous me-dia (like molten minerals).The reality is that ionic liq-uids can be liquid at temper-atures as low as 96C.Furthermore, room-tempera-ture ionic liquids are fre-quently colorless, fluid, andeasy to handle. In the patentand academic literature, theterm ionic liquids nowrefers to liquids composedentirely of ions that are fluidaround or below 100C.

One of the primary driv-ing forces behind researchinto ionic liquids is the per-ceived benefit of substitutingtraditional industrial sol-vents, most of which are

volatile organic compounds (VOCs), withnonvolatile ionic liquids. Replacement ofconventional solvents by ionic liquids wouldprevent the emission of VOCs, a majorsource of environmental pollution. Ionic liq-uids are not intrinsically greensome areextremely toxicbut they can be designed to

be environmentally benign,with large potential benefitsfor sustainable chemistry (2).

There are four principalstrategies to avoid usingconventional organic sol-vents: No solvent (heteroge-neous catalysis), water, su-percritical fluids, and ionicliquids. The solventless op-tion is the best established,and is central to the petro-chemical industry, the leastpolluting chemical sector.The use of water can also beadvantageous, but many or-ganic compounds are diffi-cult to dissolve in water, anddisposing of contaminatedaqueous streams is expen-sive. Supercritical fluids,which have both gas- andliquid-like properties, arehighly versatile solvents for

R. D. Rogers is at the Center for Green Manufacturing,The University of Alabama, Tuscaloosa, AL 35487,USA. E-mail: rdrogers@bama.ua.edu K. R. Seddon iswith Queens University Ionic Liquid Laboratories(QUILL), The Queens University of Belfast, BelfastBT9 5AG, UK. E-mail: k.seddon@qub.ac.uk

The BASIL reactor. Upper phase,the solvent-free pure product;lower phase, ionic liquid.

C H E M I S T RY

Ionic LiquidsSolvents of the Future?

Robin D. Rogers and Kenneth R. Seddon

31 OCTOBER 2003 VOL 302 SCIENCE www.sciencemag.org

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chemical synthesis (3). This technologywas recently commercialized by ThomasSwan & Co., Ltd., in a chemical plant de-signed for multipurpose synthesis. Togetherwith ionic liquids (46), these alternativesolvent strategies (sometimes referred to asalternative reaction media or green sol-vents) provide a range of options to indus-trialists looking to minimize the environ-mental impact of their chemical processes.

What are the advantages of using aroom-temperature ionic liquid in an indus-trially relevant catalytic process? As notedabove, ionic liquids have no detectable va-por pressure, and therefore contribute noVOCs to the atmosphere. But this is not theonly reason for using ionic liquids. Anotheris that at least a million binary ionic liquids,and 1018 ternary ionic liquids, are potential-ly possible (7). (For comparison, about 600molecular solvents are in use today.)

This diversity enables the solvent to bedesigned and tuned (2) to optimize yield, se-lectivity, substrate solubility, product separa-tion, and even enantioselectivity. Ionic liq-uids can be highly conducting (8), dissolveenzymes (9), form versatile biphasic systemsfor separations (10), can form both polymersand gels for device applications (8), are me-dia for a wide range of organic and inorgan-ic reactions (46), and are the basis for atleast one industrial process, called theBASIL process (see the figure) (11).

The BASIL process was developed andis operated by BASF. At the meeting,Matthias Maase (BASF) revealed that useof the BASIL process increases the produc-tivity of their alkoxyphenylphosphine for-mation process by a factor of 80,000 com-pared with the conventional process. Othercompanies are also pursuing the use of ion-ic liquids. Bernd Weyershausen (Degussa)presented an ionic liquidbased process forthe synthesis of organosilicon compounds.Use of an ionic liquid solvent enabled thecatalyst to be easily recycled and reusedwithout further treatment after separationfrom the product at the end of the reaction.Christian Mehnert (ExxonMobil) describedbiphasic hydroformylation with rhodiumcatalysts in ionic liquids.

Because research into ionic liquids is atan early stage, many of their properties re-main to be elucidated. Nonetheless, ionicliquids have already provided access tonew chemical processes. Recent papers de-scribe their potential application as em-balming fluids (12), in ion drives for spacetravel (13), for desulfurization of fuels(14), and as lubricants (15).

Ionic liquids have already found manylaboratory applications in synthesis, catal-ysis, batteries, and fuel cells (46, 8, 16),and numerous new combinations of ionicliquid solvent properties are available or

predicted. The next decade should see ion-ic liquids being used in many applicationswhere conventional organic solvents areused today. Furthermore, ionic liquids willenable new applications that are not possi-ble with conventional solvents. In the fu-ture, solvents will be designed to controlchemistry, rather than the chemistry beingdictated by the more limited range of mo-lecular solvents currently used.

References and Notes1. ACS Fall Meeting, 7 to 11 September 2003, New York.

The Ionic Liquids symposium was sponsored by theACS Division of Industrial and Engineering Chemistry,the Green Chemistry and Engineering Subdivision, theSeparation Science and Technology Subdivision, andthe Green Chemistry Institute.

2. M. Freemantle, Chem. Eng. News 76, 32 (30 March1998).

3. M. Poliakoff, J. M. Fitzpatrick, T. R. Farren, P. T. Anastas,Science 297, 807 (2002).

4. R. D. Rogers, K. R. Seddon, Eds., Ionic Liquids as GreenSolvents: Progress and Prospects (ACS Symp. Ser. 856,American Chemical Society, Washington, DC, 2003).

5. R. D. Rogers, K. R. Seddon, Eds., Ionic Liquids: IndustrialApplications for Green Chemistry (ACS Symp. Ser.818, American Chemical Society, Washington, DC,2002).

6. P. Wasserscheid, T. Welton, Eds., Ionic Liquids inSynthesis (Wiley-VCH, Weinheim, Germany, 2003).

7. K. R. Seddon, in The International GeorgePapatheodorou Symposium: Proceedings, S.Boghosian et al., Eds. (Institute of ChemicalEngineering and High Temperature ChemicalProcesses, Patras, Greece, 1999), pp. 131135.

8. H. Ohno, Ed., Ionic Liquids: The Front and Future ofMaterial Developments (CMC, Tokyo, 2003).

9. R. A. Sheldon, R. M. Lau, M. J. Sorgedrager, F. vanRantwijk, K. R. Seddon, Green Chem. 4, 147 (2002).

10. K. E. Gutowski et al., J. Am. Chem. Soc. 125, 6632(2003).

11. K. R. Seddon, Nature Mater. 2, 363 (2003).12. P. Majewski, A. Pernak, M. Grzymislawski, K. Iwanik, J.

Pernak, Acta Histochem. 105, 135 (2003).13. M. Gamero-Castano,V. Hruby, J. Propulsion Power 17,

977 (2001).14. A. Bosmann et al., Chem. Commun., 2494 (2001).15. W. M. Liu, C. F. Ye, Q. Y. Gong, H. Z. Wang, P. Wang,

Tribol. Lett. 13, 81 (2002).16. R. D. Rogers, K. R. Seddon, S. Volkov, Eds., Green

Industrial Applications of Ionic Liquids (Kluwer,Dordrecht, Netherlands, 2002), vol. 92.

17. R.D.R. acknowledges financial support from the U.S.Environmental Protection Agency; NSF; Air ForceOffice of Scientific Research; U.S. Department ofEnergy, Environmental Management Science Pro-gram and Office of Basic Energy Sciences, Office ofEnergy Research; National Renewable Energy Lab-oratory; and the PG Research Foundation. QUILLs in-dustrial sponsors include Avecia, bp, Chevron, C-Tri,Cytec, Eastman, ICI, Merck, Novartis, SASOL, Shell,and UOP.

After replication, mammalian DNAbecomes marked by the addition ofmethyl groups to certain cytosine

bases, almost exclusively those in the se-quence 5CpG. Just how the resulting pat-tern of methylated and nonmethylated cy-tosines is converted into biological out-comes is now starting to become clear.Methyl-CpG has emerged as a gene silenc-ing signal that usually ensures the long-term shutdown of gene expression. Likelymediators of this effect are the methyl-CpGbinding domain (MBD) proteins that re-cruit transcriptional silencing machinery tothe DNA. One of these proteins, MeCP2, isof particular interest because about 80% ofpatients with a profound neurological con-dition called Rett syndrome carry a muta-tion in their MECP2 gene (1). Mice lackingthe Mecp2 gene exhibit several features ofRett syndrome. Furthermore, targeted dele-tion of Mecp2 in mouse brain causes a Rett-like phenotype that is virtually indistin-guishable from the phenotype of mice inwhich every tissue lacks MeCP2. Global

microarray analyses designed to search fortarget genes that are derepressed in thebrains of MeCP2-deficient mice, and thusmight be causally implicated in Rett syn-drome, have not yielded any clear candi-dates (2). The existence of a genuine targetgene is now highlighted on pages 885 (3)and 889 (4) of this issue. Chen et al. (3) andMartinowich et al. (4) describe their dis-covery that normal MeCP2 regulates ex-pression of the gene encoding brain-de-rived neurotrophic factor (BDNF), a secret-ed protein that is essential for neural plas-ticity, learning, and memory. Their newfindings, together with recent work in theamphibian Xenopus laevis (5), confirm thatMeCP2 is a methyl-CpGdependent tran-scriptional repressor and reveal its unex-pected role in the induction of gene expres-sion in the nervous system.

Belonging to a set of proteins synthe-sized in response to neuronal activity, BDNFis thought to be essential for converting tran-sient stimuli into long-term changes in brainactivity (6). Understanding how BDNF isregulated in neurons is important if we are tocomprehend brain development, learning,and memory. There are four BDNF promot-ers, one of which (promoter III) responds

M O L E C U L A R B I O L O G Y

MeCP2 RepressionGoes Nonglobal

Robert Klose and Adrian Bird

The authors are at the Wellcome Trust Centre for CellBiology, University of Edinburgh, The Kings Buildings,Edinburgh EH9 3JR, UK. E-mail: a.bird@ed.ac.uk

www.sciencemag.org SCIENCE VOL 302 31 OCTOBER 2003

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