Predicting Solubilities of
Cellulose in Ionic Liquids using COSMO-RS
Kai Masuch, Kai Leonhard, Jens Kahlen
• Tree: lignin, hemicellulose, cellulose, other stuff we don’t want (e.g. water)
• cellulose: hydrogen-bonding hinders conversion to liquid..
• ...which can be overcome when dissolved in appropriate solvents...
Why predict cell. solubilities?
• ...e.g. ionic liquids (IL’s)...
• ...which are trendy and referred as ‘green’ by marketing strategists (negligible vapor pressure).
• Cations and anion can be combined freely: numerous (!!!) candidates.
• solubility (in general) can be predicted by COSMO-RS
• fast (contrary to MD and full scale ab-initio methods for liquids)
• huge database for screening purposes
• fully predictive due to previous ab-inito calculations for each molecule (contrary to group contribution methods e.g. UNIFAC)
Why COSMO-RS?• nice colours (simplifies
understanding of interactions)
• (Luckily) many ‘weak points’ of COSMO-RS are touched:
• Combinatorial entropy for polymers
• Electrolytes in general (no long range electrostatics)
• Hydrogen bonds
• Atomic ions
COSMOOctane
Cl-
H2O
• COSMO: COnductor-like Screening MOdel
• QM-computation of surface-charge-density in a conductor
• Only electrostatic interaction energy with dielectric continuum
• Screening charge probability distribution: ‘sigma-profile’
• Chemical potential is derived from sigma-profiles with COSMO-RS
Realistic Solvation COSMO (-RS)• Chemical potential:
• Partition function:
• Residual partition function:
*
COSMO-filesBMIM+
• COSMO-database: 5000 neutral components
• Modelling Ionic Liquids:
• One COSMO-file for each IL, (conformers!)
• Combination of COSMO-files of independent ions
• IL-database: 70 cations x 30 anions = 2100 IL’s
Cl-
OAc-PF6-
Sigma-Profiles of Conformers
• Conformers might have:
• very different sigma-profiles (intramolecular h-bonds e.g. cellulose)
• very similar sigma-profiles (Alkanes/Alkenes)
Cellulose in COSMO-RS
pics on top: http://en.wikipedia.org/wiki/Cellulose
Dissolution
Structure for COSMO-file
(size and shape effects on entropy neglected)
(memory likewise)
Weighting of atoms• Only screening
surface charges of mid-unit should be used to calculate chemical potential
• screening surface charges associated to atoms of end-units are excluded
Generation of conformers
• Generation of cellotriose-conformers in gas phase
• Geometry optimization in COSMO-cavity
• Conformer clustering:
• COSMO energies (conformer distribution by Boltzmann weighting)
• Similarities in sigma-profiles
Match!
(30 clusters)
Solubility-map
Cations
Ani
ons
good
bad
Gua
nidi
um+
BM
IM+
Cl-EMIM+/OAc-
Map for 0,1% Water-Content (similar to1% and 5%)
Cations
Ani
ons
Gua
nidi
um+
BM
IM+
Anions
BMIM:+H2O:
1000ppm
exp. Data: T, DP, chryst., water % = const.(!)
Data: Vitz, et.al. 09exp.: 12% 20% 3% 2% no sol. no sol. no sol.
Anions
Hexa-methyl-guanidinium+
Acetate-
H-MeGu+H20
1000 ppm
PF6-(bad)
Cl-(good)
OAc-(very good)
‘Typical’ cationBMIM+
• Solubility mostly cation independent, probably due to homogenous charge distribution over large surface
• Solubility explanation: IL-self-associativity when sigma-profiles of + mirrors -
‘Atypical’ cation
Cl-(bad)
Guani-dinium
OAc-(bad)
G+/OAc- have strong symmetryto mirror plane: bad solubility
PF6-(good)
G+/PF6- not:interaction withcellulose morelikely
Summary/Outlook• Application of COSMO-RS as
preselection tool for IL’s dissolving cellulose
• IL’s found which are expected to solve cellulose better than commonly used IL’s...
• ...which seem to be highly intolerant to water (design has to include water effects)
• No kinetics included
• Comparison with exp. Data is difficult
• Forthcoming transfer of the model in finite dilution might improve comparability
• Still degree of polymerisation and crystallinity have to be known
• State of aggregation for IL’s should be included in the model
The End
Atomic Ions in COSMO-RS
COSMO-RSno hb
COSMO-RShb
measured(BioVT)
H2O mol/mol
activ
ity H
2O
activity of water in 1-Allyl-3-Methyl-Imidazolium-chloride
COSMO• ‘Continuum Solvation Model’
• COSMO: COnductor-like Screening MOdel (infinite permittivity)
• electron density and screening charge: iteration to self-consistency
• Dielectric solvent: scaling of screening charges by empiric function (function of permittivity)
• Only electrostatic interaction energy with dielectric continuum
O
H
H
+1.5 e/nm²
-1.5 e/nm²
H-Bonds + Dispersion
*http://www.lsbu.ac.uk/water/
(iterative)
Solubility from COSMO-RS (constant for
cellulose with given DP and crystallinity)
Basic COSMO-RSSize/shape (entropy):e.g. Flory-Huggins
COSMO-RS-comb. not suitable for polymers (Klamt)
Contact statistics• best ‘typical’: hexamethylguanidinium-acetate
• 6/30 conformers used best: weight of 83% (No.23 dE?)
• best ‘atypical’: guanidinium-tris(pentafluoroethyl) trifluorophosphate
• 23/30 conformers used best: weight of 34%
Lignin
http://de.wikipedia.org/wiki/Lignin
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