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Acid-Functionalized Nanoparticles for Pretreatment of Wheat Straw

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DOI: 10.4236/jbnb.2012.33032    4,931 Downloads   8,274 Views   Citations


Perfluoroalkylsufonic (PFS) and alkylsufonic (AS) acid-functionalized magnetic nanoparticles were synthesized and characterized, then evaluated for their ability to hydrolyze hemicelluloses. The magnetic core was made of cobalt spinel ferrite and was coated with silica to protect it from oxidation. The silanol groups allowed surface chemical modification of the nanoparticles with the PFS and AS acid functionalities. Thermogravimetric analysis gave a total organic load of 12.6% and 32.5% (w/w) for AS and PFS nanoparticles, respectively. The surface sulfur content was calculated from XPS analysis as 1.37% and 1.93% for PFS and AS nanoparticles, respectively. Wheat straw samples were treated with the acid-functionalized nanoparticles at two different conditions: 80℃ for 24 h and 160℃ for 2 h. These experiments aimed to hydrolyze wheat straw hemicelluloses to soluble oligosaccharides. PFS nanoparticles solubilized significantly higher amounts of hemicelluloses (24.0% ± 1.1%) than their alkyl-sulfonic counterparts (9.1% ± 1.7%) at 80℃, whereas the hydrothermolysis control solubilized 7.7% ± 0.8% of the original hemicelluloses in the sample. At 160℃, PFS and AS nanoparticles gave significantly higher amounts of oligosaccharides (46.3% ± 0.4% and 45 ± 1.2%, respectively) than the control (35.0% ± 1.8%). The hemicelluloses conversion at 160?C reached 66.3% ± 0.9% using PFS nanoparticles and 61.0% ± 1.2% using AS nanoparticles compared with the control experiment, which solubilized 50.9% ± 1.7% of hemicelluloses in the biomass.

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D. Wang, M. Ikenberry, L. Peña and K. Hohn, "Acid-Functionalized Nanoparticles for Pretreatment of Wheat Straw," Journal of Biomaterials and Nanobiotechnology, Vol. 3 No. 3, 2012, pp. 342-352. doi: 10.4236/jbnb.2012.33032.


[1] Renewable Fuels Association, “2010 Ethanol Industry Outlook,” 2010.
[2] Senate and House of Representatives of the United States of America in Congress, “Energy Indepence and Security Act,” 2007.
[3] Office of the Biomass Program, “Biomass: Multi-Year Program Plan, U.S. DOE,” 2010.
[4] S. Iborra, A. Corma and G. Huber, “Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts, and Engineering,” Chemical Reviews, Vol. 106, No. 9, 2006, pp. 4044-4098. doi:10.1021/cr068360d
[5] C. E. Wyman and B. J. Goodman, “Biotechnology for Production of Fuels, Chemicals, and Materials from Biomass,” Applied Bio-chemistry and Biotechnology, Vol. 39-40, No. 1, 1993, pp. 41-59. doi:10.1007/BF02918976
[6] R. Preston, “Fi-brillar Units in the Structure of Native Cellulose,” Discussions of the Faraday Society, Vol. 11, No. 11, 1951, pp. 165-170. doi:10.1039/df9511100165
[7] O. Bobleter, “Hydrothermal Degradation of Polymers Derived from Plants,” Progress in Polymer Science, Vol. 19, No. 5, 1994, pp. 797-841. doi:10.1016/0079-6700(94)90033-7
[8] J. D. McMillan, “Pretreatment of Lignocellulosic Bio- mass,” In: M. E. Himmel, J. O. Baker and R. P. Overend, Eds., Enzymatic Conversion of Biomass for Fuels Production, American Chemical Society, 1994, pp. 292-294. doi:10.1021/bk-1994-0566.ch015
[9] N. Rodrussamee, N. Lertwattanasakul, K. Hirata, S. Limtong and T. Kosaka, “Growth and Ethanol Fermentation Ability on Hexose and Pentose Sugars and Glucose Effect Under various Conditions in Thermotolerant Yeast Kluyveromyces Marxianus,” Applied Microbiology and Biotechnology, Vol. 90, No. 4, 2011, pp. 1573-1586. doi:10.1007/s00253-011-3218-2
[10] F. M. Girio, C. Fonseca, F. Carvalheiro, L. C. Duarte and S. Marques, “Hemicelluloses for Fuel Ethanol: A Review,” Biore-source Technology, Vol. 101, No. 13, 2010, pp. 4775-4800. doi:10.1016/j.biortech.2010.01.088
[11] L. Olsson, H. R. Soerensen, B. P. Dam, H. Christensen and K. M. Krogh, “Separate and Simultaneous Enzymatic Hydrolysis and Fermentation of Wheat Hemicellulose with Recombinant Xylose Utilizing Saccharomyces Cerevisiae,” Applied Biochemistry and Biotechnology, Vol. 129, No. 1-3, 2006, pp. 117-129. doi:10.1385/ABAB:129:1:117
[12] A. Bacic and B. Stone, “A (1-]3)-Linked and (1-]4)- Linked Beta-D-Glucan in the Endosperm Cell-Walls of Wheat,” Carbohydrate Research, Vol. 82, No. 2, 1980, pp. 372-377. doi:10.1016/S0008-6215(00)85713-4
[13] V. Menon, G. Prakash and M. Rao, “Enzymatic Hydrolysis and Ethanol Production using Xyloglucanase and Debaromyces Hansenii from Tamarind Kernel Powder: Galac-toxyloglucan Predominant Hemicellulose,” Journal of Biotechnology, Vol. 148, No. 4, 2010, pp. 233-239. doi:10.1016/j.jbiotec.2010.06.004
[14] R. de Vries and J. Visser, “Aspergillus Enzymes Involved in Degradation of Plant Cell Wall Polysaccharides,” Microbiology and Mo-lecular Biology reviews, Vol. 65, No. 4, 2001, pp. 497-522. doi:10.1128/MMBR.65.4.497-522.2001
[15] J. Vincken, W. York, G. Beldman and A. Voragen, “Two General Branching Patterns of Xyloglucan, XXXG and XXGG,” Plant Physiology, Vol. 114, No. 1, 1997, pp. 9-13. doi:10.1104/pp.114.1.9
[16] A. Matsushika, H. Inoue, K. Murakami, O. Takimura and S. Sawayama, “Bioethanol Production Performance of Five Recombinant Strains of Laboratory and Industrial Xylose-Fermenting Saccharo-myces Cerevisiae,” Bioresource Technology, Vol. 100, No. 8, 2009, pp. 2392-2398. doi:10.1016/j.biortech.2008.11.047
[17] J. Song and D. Wei, “Production and Characterization of Cellulases and Xylanases of Cellulosimicrobium Cellulans Grown in Pretreated and Extracted Bagasse and Minimal Nutrient Medium M9,” Biomass Bioenergy, Vol. 34, No. 12, 2010, pp. 1930-1934. doi:10.1016/j.biombioe.2010.08.010
[18] C. Q. Zhang, W. Qi, F. Wang, Q. Li and R. X. Su, “Ethanol from Corn Stover using SSF: An Economic Assessment,” Energy Sources Part B: Economics, Planning and Policy, Vol. 6, No. 2, 2011, pp. 136-144.
[19] D. Y. Corredor, X. S. Sun, J. M. Salazar, K. L. Hohn and D. Wang, “Enzymatic Hy-drolysis of Soybean Hulls using Dilute Acid and Modified Steam-Explosion Pretreatments,” Journal of Biobased Materials and Bioenergy, Vol. 2, No. 1, 2008, pp. 43-50. doi:10.1166/jbmb.2008.201
[20] E. Viola, F. Nanna, E. Larocca, M. Cardinale, D. Barisano and F. Zimbardi, “Acid Impregnation and Steam Explosion of Corn Stover in Batch Processes,” Industrial Crops and Products, Vol. 26, No. 2, 2007, pp. 195-206. doi:10.1016/j.indcrop.2007.03.005
[21] U.S. Department of Energy, “Concentrated Acid Hydro- lysis,” 2006.
[22] Y. Lee and S. Kim, “Diffusion of Sulfuric Acid within Lignocellulosic Biomass Particles and its Impact on Dilute-Acid Pretreatment,” Bioresource Technology, Vol. 83, No. 2, 2002, pp. 165-171. doi:10.1016/S0960-8524(01)00197-3
[23] N. S. Mosier, C. M. Ladisch and M. R. Ladisch, “Cha- racterization of Acid Catalytic Domains for Cellulose Hydrolysis and Glucose Degradation,” Biotechnology and Bioengineering, Vol. 79, No. 6, 2002, pp. 610-618. doi:10.1002/bit.10316
[24] K. Cejpek, J. Velisek and O. Novotny, “Formation of Carboxylic Acids during Degra-dation of Monosaccharides,” Czech Journal of Food Science, Vol. 26, No. 2, 2008, pp. 113-131.
[25] J. Del-genes, “Effects of Lignocellulose Degradation Pro- ducts on Ethanol Fermentations of Glucose and Xylose by Saccharomyces Cerevisiae, Zymomonas Mobilis, Pichia Stipitis, and Candida Shehatae,” Enzyme and Microbial Technology, Vol. 19, No. 3, 1996, pp. 220-225. doi:10.1016/0141-0229(95)00237-5
[26] J. M. Oliva, M. J. Negro and F. Saez, “Effects of Acetic Acid, Furfural and Catechol Combinations on Ethanol Fermentation of Kluyveromyces Marxianus,” Process Biochemistry, Vol. 41, No. 5, 2006, pp. 1223-1228. doi:10.1016/j.procbio.2005.12.003
[27] A. Corma and H. Garcia, “Silica-Bound Homogeneous Catalysts as Reco-verable and Reusable Catalysts in Organic Synthesis.” Advanced Synthesis & Catalysis, Vol. 348, No. 12-13, 2006, pp. 1391-1412. doi:10.1002/adsc.200606192
[28] M. A. Harmer, Q. Sun, A. J. Vega, W. E. Farneth, A. Heidekum and W. F. Hoel-derich, “Nafion Resin-Silica Nanocomposite Solid Acid Catalysts. Microstructure-Processing-Property Correlations,” Green Chemistry, Vol. 2, No. 1, 2000, pp. 7-14. doi:10.1039/a907892d
[29] M. Yurdakoc, M. Akcay, Y. Tonbul and K. Yurdakoc, “Acidity of Silica-Alumina Catalysts by Amine Titration using Hammett Indicators and FT-IR Study of Pyridine Adsorption,” Turkish Jour-nal of Chemistry, Vol. 23, No. 3, 1999, pp. 319-327.
[30] J. A. Bootsma and B. H. Shanks, “Cello-biose Hydrolysis Using Organic-Inorganic Hybrid Me-soporous Silica Catalysts,” Applied Catalysis A-General, Vol. 327, No. 1, 2007, pp. 44-51.doi:10.1016/j.apcata.2007.03.039
[31] P. L. Dhepe, M. Ohashi, S. Inagaki, M. Ichikawa and A. Fukuoka, “Hydrolysis of Sugars Catalyzed by Water- Tolerant Sul-fonated Mesoporous Silicas,” Catalysis Letters, Vol. 102, No. 3-4, 2005, pp. 163-169. doi:10.1007/s10562-005-5850-x
[32] A. Onda, T. Ochi and K. Yanagisawa, “Selective Hydrolysis of Cellulose into Glucose Over Solid Acid Catalysts,” Green Chemistry, Vol. 10, No. 10, 2008, pp. 1033-1037. doi:10.1039/b808471h
[33] K. Shimizu, H. Furukawa, N. Kobayashi, Y. Itaya and A. Satsuma, “Effects of Bronsted and Lewis Acidities on Activity and Selectivity of Hete-ropolyacid-Based Catalysts for Hydrolysis of Cellobiose and Cellulose,” Green Chemistry, Vol. 11, No. 10, 2009, pp. 1627-1632. doi:10.1039/b913737h
[34] P. Dhepe and R. Sahu, “A Solid-Acid-Based Process for the Conversion of Hemi-cellulose,” Green Chemistry, Vol. 12, No. 12, 2010, pp. 2153-2156. doi:10.1039/c004128a
[35] A. Bell, “The Impact of Na-noscience on Heterogeneous Catalysis,” Science, Vol. 299, No. 5613, 2003, pp. 1688-1691. doi:10.1126/science.1083671
[36] V. Polshettiwar and R. Varma, “Green Chemistry by Nano-Catalysis,” Green Chemistry, Vol. 12, No. 5, 2010, pp. 743-754. doi:10.1039/b921171c
[37] M. Zhang, B. L. Cushing and C. J. O’Connor, “Synthesis and Characterization of Mo-nodisperse Ultra-Thin Silica- Coated Magnetic Nanopar-ticles,” Nanotechnology, Vol. 19, No. 8, 2008, pp. 1-5. doi:10.1088/0957-4484/19/8/085601
[38] C. S. Gill, B. A. Price and C. W. Jones, “Sulfonic Acid- Functionalized Silica-Coated Magnetic Nanoparticle Catalysts.” Journal of Catalysis, Vol. 251, No. 1, 2007, pp. 145-152. doi:10.1016/j.jcat.2007.07.007
[39] P. D. Stevens, J. Fan, H. M. R. Gardimalla, M. Yen and Y. Gao, “Superpara-magnetic Nanoparticle-Supported Catalysis of Suzuki Cross-Coupling Reactions.” Organic Letters, Vol. 7, No. 11, 2005, pp. 2085-2088. doi:10.1021/ol050218w
[40] N. T. S. Phan and C. W. Jones, “Highly Accessible Catalytic Sites on Recyclable Organosilane-Functionalized Magnetic Nanoparticles: An Alternative to Functionalized Porous Silica Catalysts,” Journal of Molecular Catalysis A: Chemical, Vol. 253, No. 1-2, 2006, pp. 123-131. doi:10.1016/j.molcata.2006.03.019
[41] A. J. Rondinone, A. C. S. Samia and Z. J. Zhang, “Superparamagnetic Relaxation and Magnetic Anisotropy Energy Distribution in CoFe2O4 Spinel Ferrite Nanocrystallites,” Journal of Physical Chemistry B, Vol. 103, No. 33, 1999, pp. 6876-6880.doi:10.1021/jp9912307
[42] A. Sluiter, B. Hames, R. Ruiz, et al., “Determination of Structural Carbohydrates and Lignin in Biomass,” 2008.
[43] A. Sluiter, B. Hames, R. Ruiz, C. Scarlata, J. Sluiter and D. Templeton, “Determination of Sugars, Byproducts, and Degradation Products in Liquid Fraction Process Samples,” 2008.
[44] J. Silva, W. de Brito and N. Mohallem, “Influence of Heat Treatment on Cobalt Ferrite Ceramic Powders,” Materials Science Engineering B: Solid-State Materials for Advanced Technology, Vol. 112, No. 2-3, 2004, pp. 182-187.
[45] M. Naseri, E. Saion, H. Ahangar, A. Shaari and M. Hashim, “Simple Synthesis and Characterization of Cobalt Ferrite Nanoparticles by a Thermal Treatment Method,” Journal of Nanomaterials, Vol. 2010, 2010, pp. 1-8. doi:10.1155/2010/907686
[46] X. S. Zhao, G. Q. Lu and X. Hu, “Characterization of the Structural and Surface Properties of Chemically Modified MCM-41 Material,” Microporous and Mesoporous Materials, Vol. 41, 2000, pp. 37-47.
[47] M. Colilla, I. Izquierdo-Barba, S. Sanchez-Salcedo, J. Fierro, J. Hueso and M. Vallet-Regi, “Synthesis and Characterization of Zwitterionic SBA-15 Nanostructured Materials,” Chemistry of Materials, Vol. 22, No. 23, 2010, pp. 6459-6466. doi:10.1021/cm102827y
[48] M. Alvaro, A. Corma, D. Das, V. Fornes and H. Garcia, “Nafion-Functionalized Mesoporous MCM-41 Silica Shows High Activity and Selectivity for Carboxylic Acid Esterification and Frie-del-Crafts Acylation Reactions,” Journal of Catalysis, Vol. 231, No. 1, 2005, pp. 48-55. doi:10.1016/j.jcat.2005.01.007
[49] S. Suganuma, K. Nakajima, M. Kitano, D. Yamaguchi, H. Kato, S. Hayashi and M. Hara, “Hydrolysis of Cellulose by Amorphous Carbon Bearing SO3H, COOH, and OH Groups,” Journal of the American Chemical Society, Vol. 130, No. 38, 2008, pp. 12787-12793. doi:10.1021/ja803983h
[50] R. Buzzoni, S. Bordiga, G. Ricchiardi, G. Spoto and A. Zecchina, “Interaction of H2O, CH3OH, (CH3)2O, CH3CN, and Pyridine with the Superacid Perfluorosulfonic Membrane Nafion: An IR and Raman Study,” Journal of Physical Chemistry, Vol. 99, No. 31, 1995, pp. 11937- 11951. doi:10.1021/j100031a023
[51] G. Blanco Brieva, J. Campos Martin, M. de Frutos and J. Fierro, “Preparation, Characterization, and Acidity Evaluation of Perfluoro-sulfonic Acid-Functionalized Silica Catalysts,” Industrial Engineering Chemistry Research, Vol. 47, No. 21, 2008, pp. 8005-8010.doi:10.1021/ie800221f
[52] T. H. Kim, Y. H. Im and Y. B. Hahn, “Plasma Enhanced Chemical Vapor Deposition of Low Dielectric Constant SiCFO Thin Films,” Chemical Physics Letters, Vol. 368, No. 1-2, 2003, pp. 36-40. doi:10.1016/S0009-2614(02)01715-3
[53] J. Scaranto, A. P. Charmet and S. Giorgianni, “IR Spectroscopy and Quantum-Mechanical Studies of the Adsorption of CH2CClF on TiO2,” Journal of Physical Chemistry C, Vol. 112, No. 25, 2008, pp. 9443-9447. doi:10.1021/jp801075n
[54] C. Biloiu, I. A. Biloiu, Y. Sakai, Y. Suda and A. Ohta, “Amorphous Fluorocarbon Polymer (a-C: F) Films Obtained by Plasma Enhanced Chemical Vapor Deposition from Perfluoro-Octane (C8F18) Vapor I: Deposition, Morphology, Structural and Chemical Properties,” Journal of Vacuum Science & Technology A, Vol. 22, No. 4, 2004, pp. 1158-1165. doi:10.1116/1.1759354
[55] K. Zavadil, N. Armstrong and C. Peden, “Reactions at the Interface between Multi-Component Glasses and Metallic Lithium Films,” Journal of Materials Research, Vol. 4, No. 4, 1989, pp. 978-989. doi:10.1557/JMR.1989.0978
[56] K. Senapati, C. Borgohain and P. Phukan, “Synthesis of Highly Stable CoFe2O4 Nanoparticles and Their Use as Magnetically Separable Catalyst for Knoevenagel Reaction in Aqueous Medium,” Journal of Molecular Catalysis A, Vol. 339, No. 1-2, 2011, pp. 24-31. doi:10.1016/j.molcata.2011.02.007
[57] S. Hamoudi, S. Royer and S. Kaliaguine, “Propyl- and Arene-Sulfonic Acid Functionalized Periodic Mesoporous Organosili-cas,” Microporous and Mesoporous Materials, Vol. 71, No. 1-3, 2004, pp. 17-25. doi:10.1016/j.micromeso.2004.03.009
[58] F. Carvalheiro, L. Duarte, R. Medeiros and F. Girio, “Optimization of Brewery’s Spent Grain Dilute-Acid Hydrolysis for the Production of Pentose-Rich Culture Media,” Applied Bi-ochemistry and Biotechnology, Vol. 113, 2004, pp. 1059-1072. doi:10.1385/ABAB:115:1-3:1059
[59] S. E. Jacobsen and C. E. Wyman, “Xylose Monomer and Oli-gomer Yields for Uncatalyzed Hydrolysis of Sugar- cane Bagasse Hemicellulose at Varying Solids Concentration,” Industrial Engineering Chemistry Research, Vol. 41, No. 6, 2002, pp. 1454-1461. doi:10.1021/ie001025+
[60] C. E. Wyman, B. E. Dale, R. T. Elander, M. Holtzapple, M. R. Ladisch and Y. Y. Lee, “Coordinated Development of Leading Biomass Pretreatment Technologies,” Bioresource Technology, Vol. 96, No. 18, 2005, pp. 1959-1966. doi:10.1016/j.biortech.2005.01.010
[61] S. Allen, D. Schulman, J. Lichwa, M. Antal and E. Jennings, “A Comparison of Aqueous and Dilute-Acid Sin-gle-Temperature Pretreatment of Yellow Poplar Sawdust,” Industrial Engineering Chemistry Research, Vol. 40, No. 10, 2001, pp. 2352-2361. doi:10.1021/ie000579+
[62] T. Marzialetti, C. Sievers and P. Agrawal, “Dilute Acid Hydrolysis of Loblolly Pine: A Comprehensive Approach,” Industrial Engineering Chemistry Research, Vol. 47, No. 19, 2008, pp. 7131-7140. doi:10.1021/ie800455f
[63] A. Corma, S. Iborra and A. Velty, “Chemical Routes for the Transformation of Biomass into Chemicals,” Chemical Reviews, Vol. 107, No. 6, 2007, pp. 2411-2502. doi:10.1021/cr050989d
[64] B. Girisuta, L. Janssen and H. Heeres, “A Kinetic Study on the Conversion of Glucose to Levulinic Acid,” Chemical Engineering Research and Design, Vol. 84, No. 5, 2006, pp. 339-349. doi:10.1205/cherd05038
[65] G. Bonn and O. Bobleter, “Determination of the Hydrothermal Degradation Products of D-(U-14C) Glucose and D-(U-14C) Fructose by TLC,” Journal of Radioanalytical Chemistry, Vol. 79, No. 2, 1983, pp. 171-177. doi:10.1007/BF02518929
[66] A. P. Dunlop, “Furfural Formation and Behavior,” Industrial Engineering Chemistry, Vol. 40, No. 2, 1948, pp. 204-209. doi:10.1021/ie50458a006
[67] M. J. Antal, T. Leesom-boon, W. S. Mok and G. N. Richards, “Mechanism of Formation of 2-Furaldehyde from D-Xylose,” Carbohy-drate Research, Vol. 217, 1991, pp. 71-85. doi:10.1016/0008-6215(91)84118-X
[68] F. Carrasco and C. Roy, “Kinetic-Study of Dilute-Acid Prehydrolysis of Xylan-Containing Biomass,” Wood Science and Technology, Vol. 26, No. 3, 1992, pp. 189-208. doi:10.1007/BF00224292
[69] R. Torget and T. Hsu, “Two-Temperature Dilute-Acid Prehydrolysis of Hard-wood Xylan Using a Percolation Process,” Applied Bio-chemistry and Biotechnology, Vol. 45-46, 1994, pp. 5-21. doi:10.1007/BF02941784
[70] S. B. Kim, Y. Y. Lee and R. Torget, “Kinetics in Acid-Catalyzed Hydrolysis of Hardwood Hemicellulose,” Biotechnology & Bioengi-neering Symposium, Vol. 17, 1987, pp. 71-84.
[71] K. Grohmann, R. Torget and M. Himmel, “Optimization of Dilute Acid Pretreatment of Biomass,” Biotechnology & Bioengineering Symposium, Vol. 15, 1986, pp. 59-80.
[72] C. Liu and C. E. Wyman, “Effect of the Flow Rate of a Very Dilute Sulfuric Acid on Xylan, Lignin, and Total Mass Removal from Corn Stover,” Industrial & Engineering Chemistry Research, Vol. 43, No. 11, 2004, pp. 2781-2788. doi:10.1021/ie030754x
[73] J. Grondin, R. Sagnes and A. Commeyras, “Perfluorosulfonic Acids-3. Hammett Acidity Functions of Perfluoroalkanesulfonic Acids and of their Mixtures with SbF5,” Bulletin de la Société Chimique de France, No. 11, 1976, pp. 1779-1783.
[74] A. Corma, D. Das, V. Fornes, H. Garcia and M. Alvaro, “Single-Step Preparation and Catalytic Activity of Mesoporous MCM-41 and SBA-15 Silicas Functionalized with Perfluoroalkylsulfonic Acid Groups Analogous to Nafion (R),” Chemical Communications, No. 8, 2004, pp. 956-957.
[75] W. M. Van Rhijn, D. E. De Vos, B. F. Sels, W. D. Bossaert and P. A. Jacobs, “Sulfonic Acid Functionalized Ordered Mesoporous Materials as Catalysts for Condensation and Esterification Reactions,” Chemical Communications Cambridge, Vol. 1998, No. 3, 1998, pp. 317- 318. doi:10.1039/a707462j

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