Dissolution of Cello with Ionic Liquids
Richard P.Swatloski,Scott K.Spear,John D.Holbrey,and Robin D.Rogers*
克雷登斯
Center for Green Manufacturing and Department of Chemistry,The Uni V ersity of Alabama,
Tuscaloosa,Alabama 35487
Received February 1,2002
Cellulo is the most abundant biorenewable material,with a long and well-established technological ba.1Derivitized products have many important applications in the fiber,paper,membrane,polymer,and paints industries.
Cellulo consists of polydisper linear gluco polymer chains (Figure 1)which form hydrogen-bonded supramolecular structures;2cellulo is insoluble in water and most common organic liquids.The growing willingness to develop new cellulosic materials results from the fact that cellulo is a renewable resource,although many of the technologies currently ud in cellulo processing are decidedly nongreen.3For example,visco rayon is prepared from cellulo xanthate (production over 3,000,000tons per year)utilizing carbon disulfide as both reagent and solvent.Most r
ecently,process using more environmentally acceptable nonderivitizing solvents (N -methylmorpholine-N -oxide (NMNO)and phosphoric acid)have been commercialized.Solvents are needed for dissolution that enable homogeneous pha reactions without prior derivitiza-tion.4
Graenacher 5first suggested in 1934that molten N -ethylpyri-dinium chloride,in the prence of nitrogen-containing bas,could be ud to dissolve cellulo;however,this ems to have been treated as a novelty of little practical value since the molten salt system was,at the time,somewhat esoteric and has a relatively high melting point (118°C).We were interested in examining whether other solvents that would now be described as ionic liquids (ILs)6would dissolve cellulo and,especially,whether the availability of a wide and varied range of ILs,coupled with the current understanding of their solvent properties,7would allow flexibility and control in the processing methodology,with incread solution efficiency and reduction or elimination of undesirable solvents.
Ionic liquids,containing 1-butyl-3-methylimidazolium cations ([C 4mim]+)were screened with a range of anions,from small,hydrogen-bond acceptors (Cl -)to large,noncoordinating anions ([PF 6]-)also including Br -,SCN -,and [BF 4]-.In addition,variations in cation alkyl-substituent from butyl through octyl were investigated for the chloride salts.Dissolution experiments were carried out using cellulo-dissolving pulps (from cellulo acetate,lyocell,and rayon production lines),fibrous cellulo
(Aldrich),and Whatman cellulo filter papers.The cellulo samples were added to the ionic liquids without pretreatment,in glass vials,and heated without agitation on a heating plate or in a domestic microwave oven.
Table 1summarizes the results obtained using high MW dissolving pulp (DP ≈1000).Stirring cellulo in the ILs under ambient conditions did not lead to dissolution,although the cellulo fibers were wetted by the ILs.However,on heating to 100-110
°C,cellulo slowly dissolved in the Cl --,Br --,and SCN --containing ILs to yield increasingly viscous solutions.
Dissolution rates could be significantly improved by heating in a microwave oven.In a typical procedure to prepare a 10wt %solution,0.5-1.0g of fibrous cellulo was placed in a glass vial and [C 4mim]Cl ionic liquid (10g)was added as a liquid at 70°C (i.e.,above the melting point).The vial was then looly capped,placed in a microwave oven,and heated with 3-5s puls at full power.Between puls,the vial was removed,shaken or vortexed,and replaced in the oven.A clear,colorless,viscous solution was obtained.ILs are heated with exceptional efficiency by microwaves,8and care must be taken to avoid excessive heating that induces cellulo pyrolysis.The decomposition appears to be more rapid in contact with the ILs than for isolated cellulo under equivalent
conditions.
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arminSolutions containing up to 25wt %cellulo can be formed as viscous pastes in the chloride-containing ILs,although compositions between 5and 10wt %cellulo are more readily prepared.The
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greatest solubility was obtained using [C 4mim]Cl as the solvent.When high concentrations of cellulo (>10wt %)were dissolved in [C 4mim]Cl,the viscous solutions obtained were optically anisotropic between crosd polarizing filters and displayed bire-fringence.The formation of liquid crystalline solutions of cellulo may have uful applications for the generation of new,advanced materials.9High-strength materials that conrve anisotropy in the solid pha are especially desirable,yielding enhanced mechanical properties.
Nonderivitizing solvents for cellulo effect dissolution by disrupting and breaking the intramolecular hydrogen-bonding network.For dimethylacetamide (DMAC)/LiCl solvents,complex-ation of lithium ions by DMAC mobilizes chloride ions which
*To whom correspondence should be addresd.E-mail:rdrogers@
bama.ua.edu.
Figure 1.A cellulo polymer chain,n is typically 400-1000.Table 1.Solubility of Dissolving Pulp Cellulo in Ionic Liquids
ionic liquid
method
flirtsolubility (wt %)
[C 4mim]Cl heat (100°C)10%(70°C)
3%[C 4mim]Cl heat (80°C)+sonication 5%
[C 4mim]Cl microwave heating 25%,clear
propofol
3-5-s puls viscous solution [C 4mim]Br microwave 5-7%[C 4mim]SCN microwave 5-7%[C 4mim][BF 4]microwave insoluble [C 4mim][PF 6]microwave insoluble [C 6mim]Cl heat (100°C)5%
[C 8mim]Cl heat (100°C)slightly soluble
Published on Web 04/17/2002
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interact with the cellulo hydroxyl groups.In a typical 10wt %LiCl/DMAC solution,free chloride ion concentration is about 6.7mol %.In contrast,[C 4mim]Cl has a chloride concentration almost 3times as high,(approximately 20mol %).In all the cas,the
chloride ions are nonhydrated.
We speculate that the high chloride concentration and activity in [C 4mim]Cl is highly effective in breaking the extensive hydrogen-bonding network prent,thus allowing for much
quicker dissolution times,and the ability to dissolve higher concentrations of cellulo than the traditional solvent systems.The longer-chain substituted ionic liquids ([C 6mim]Cl and [C 8mim]Cl)appear to be less efficient at dissolving cellulo.This may be due to the reduced effective chloride concentration within the liquids;however,the choice of IL can also be a compromi between solubilizing power and rheological properties;when cooled to room temperature,[C 8mim]-Cl is a (viscous)liquid,whereas [C 4mim]Cl forms a crystalline solid.
The prence of water in the ionic liquid was shown to significantly decrea the solubility of cellulo,presumably through competitive hydrogen-bonding to the cellulo microfibrils which inhibits solubilization.When water was added to the IL at concentrations greater than ca.1wt %(approximately 0.5mole fraction H 2O)the solvent properties were significantly impaired,and cellulo was no longer soluble.Hydrogen-bonding properties are important in solvents for the dissolution of cellulo.For example,anhydrous and monohydrated NMNO are good solvents;however,when completely hydrated by two or more waters,NMNO is no longer a good solvent for cellulo.10Similarly,the ability of molten inorganic salt hydrates 11to dissolve cellulo depends on the degree of ion hydration and is thought to occur via coordinating the cellulo hydroxyl groups to the metal cation (when deficient in water).
Cellulo could be precipitated from the IL solution by the addition of water,or other precipitating solutions including ethanol and acetone.The macroscopic morphology of the regenerated cellulo varied depending on how the contacting of the IL solution and the regeneration liquid is achieved.To explore the scope for processing of cellulo from ILs,we prepared monoliths,fibers,and films by forming into an aqueous pha.Rapid mixing of the IL solution with an aqueous stream results in precipitation of cellulo as a powdery floc.By extrusion of the IL/cellulo solution into water,thin fibers and rods were prepared.橡皮擦用英语怎么说
The regenerated materials were characterized by differential scanning calorimetry (DSC),thermogravimetric analysis (TGA),and size exclusion chromatography (SEC).Scanning Electron Microscopy (SEM)was ud to obrve the bulk structure,scanning electron micrographs of dissolving pulp and regenerated cellulo
are shown in Figure 2.The dissolving pulp shows fibers at 300×magnification in the SEM.After regeneration,the morphology of the material was significantly changed,displaying a rough,but conglomerate texture in which the fibers are fud into a relatively homogeneous macrostructure.SEC indicated that the dissolving pulp could be solubilized in [C 4mim]Cl and regenerated from water without significant change in the degree of polymerization or polydispersity.
TGA curves are shown in Figure 3for dissolving pulp regener-ated from [C 4mim]Cl.Rapid decomposition in a narrow temperature range from 350to 360°C is obrved for the initial cellulo sample.The regenerated sample exhibits a lower ont temperature for decomposition,but gives a higher char yield (nonvolatile carbonaceous material)on pyrolysis,indicated by the high residual mass after the decomposition step.
This rearch has shown that ILs can be ud as nonderivatizing solvents for cellulo.ILs incorporating anions which are strong hydrogen bond acceptors were most effective,especially when combined with microwave heating,whereas ILs containing ‘non coordinating’anions,including [BF 4]-and [PF 6]-were nonsolvents.Chloride containing ILs appear to be the most effective solvents,presumably solubilizing cellulo through hydrogen-bonding from hydroxyl functions to the anions of the solvent.
Acknowledgment.We thank PG Rearch Foundation Inc.for financial support,International Paper (Natchez,MS)for the gift of dissolving pulp samples,and Dr.M.-A.Rouslle (USDA,Cotton Textile Chemistry Rearch Unit)for performing SEC measure-ments.
References
(1)Kirk-Othmer Encyclopedia of Chemical Technology ,4th ed.;Wiley:New
York,1993;Vol.5,p 476.
(2)Finkenstadt,V.L.;Millane,R.P.Macromolecules 1998,31,7776-7783.(3)Johnson,D.C.In Cellulo Chemistry and its Application ;Nevell,T.P.,
Zeronian,S.H.,Eds.;E.Horwood:Chichester,1985;p181.
(4)(a)Augustine,A.V.;Hudson,S.M.;Cuculo,J.A.In Cellulo Sources
and Exploitation ;Kennedy,J.F.,Philipps,G.O.,Williams,P.A.,Eds.;E.Horwood:New York,1990;p59.(b)Dawy,T.R.In Cellulosic Polymers ,Blends and Composites ;Gilbert,R.D.,Ed.;Carl Hanr Verlag:New York,1994;p157.
(5)Graenacher,C.Cellulo Solution.U.S.Patent 1,943,176,1934.
(6)See,for example:(a)Huddleston,J.G.;Visr,A.E.;Reichert,W.M.;
Willauer,H.D.;Broker,G.A.;Rogers,R.D.Green Chem.2001,3,156-164.(b)Holbrey,J.D.;Seddon,K.R.Cle
从现在开始英文an Prod.Proc.1999,1,223-236.(c)Gordon,C.M.Appl.Catal.A 2001,222,101-117.(d)Sheldon,R.A.Chem.Commun.2001,2399-2407.
(7)Holbrey,J.D.;Visr,A.E.;Rogers,R.D.In Ionic Liquids in Synthesis ;
domain controller>时光网首页
Wasrscheid,P.,Welton,T.,Eds.;VCH -Wiley,2002.In press.(8)Varma,R.S.;Namboodiri,V.V.Chem.Commun.2001,643-644.(9)Boerstoel,H.;Maatman,H.;Westerink,J.B.;Koenders,B.M.Polymer
2001,42,7371-7379.
(10)Maia,E.;Peguy,A.;Perez,S.Acta Crystallogr.B 1981,37,1858.(11)Leipner,H.;Fischer,S.;Brendler,E.;Voigt,W.Macromol.Chem.Phys.
2000,201,2041-2049.
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Figure 2.SEM micrographs of the initial dissolving pulp (left)and after dissolution in [C 4mim]Cl and regeneration into water (right).
Figure 3.Thermal decomposition profiles of (i)regenerated cellulo and (ii)original dissolving pulp.Samples were heated in platinum sample containers under a nitrogen atmosphere at 10°C min -1.
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