Opportunities for ionic liquids in recovery of biofuels
Andrei G. Fadeev*† and Michael M. Meagher
Biological Process Development Facility, The University of Nebraska, Lincoln, NE, 68583, USA.
E-mail: mmeagher@unl.edu; Fax: +1 402-472-1693; Tel: +1 402-472-2342
Received (in Corvallis, OR, USA) 26th July 2000, Accepted 13th December 2000
First published as an Advance Article on the web 23rd January 2001
Room temperature ionic liquids have potential as ex-
tractants in recovery of butyl alcohol from fermentation
broth; water solubility in ionic liquid and ionic liquid
solubility in water are important factors affecting selectivity
of butyl alcohol extraction from aqueous solutions.
The reaction was carried out at 80 °C for 3–4 d. The product was
washed with ethyl acetate and dried in vacuum. OMIM[PF6]
was obtained from 1-octyl-3-methyl-1H-imidazol-3-ium chlo-
ride by adding an equimolar amount of hexafluorophosphoric
acid. The density of [OMIM][PF6] was 1.21 g cm23
.
To determine distribution coefficients and mutual solubil-
ities, the ionic liquid was mixed with water or BuOH–water
solution in an airtight bottle. The mixture was shaken overnight
and then left for 1–3 d to achieve phase separation. Mutual
solubilities were determined by separating BuOH and water
from the IL using a rotary evaporator. The collected BuOH–
water solutions were analyzed by GC. BuOH and water
distribution coefficients, D, and selectivities, a, were estimated
according to eqns. 1 and 2
There has been a renewed interest in processing of biomass into
fuels and chemicals via fermentation during the last decade. In
that respect, ethanol and acetone–butyl alcohol–ethanol (ABE)
fermentations are two of the most attractive bioprocesses.
Recovery of alcohols from the fermentation broth is the most
energy extensive step in fermentative fuel production. The
average energy requirement per litre of EtOH produced is 2446
kcal.1 The amount of energy is about 3 times higher2 for n-
BuOH, since ABE fermentation broth contains only about 2
wt% of BuOH vs. 6 wt% of EtOH in yeast fermentation broth.
EtOH is currently recovered by distillation. Distillative recov-
ery of BuOH from fermentation broth is not economical.
Alternative recovery methods have been considered including:
solvent extraction, stripping, pervaporation, etc. None of the
methods has been economically proved yet.
We investigated the potential of ionic liquids for extraction of
BuOH from aqueous solutions. Room temperature ionic liquids
have extremely low vapor pressure and low solubility in water.
These properties make them interesting solvents for extraction
of organic compounds from aqueous stream.3
Most solvents that have high distribution coefficients for
BuOH are flammable, hazardous and toxic to humans and
microorganisms.4 Tri-n-butyl phosphate, octan-1-ol and diethyl
ether are listed as solvents with high BuOH partition coeffi-
cients, 14.6, 7.6, and 4.1, respectively.5 Tri-n-butyl phosphate
and octan-1-ol were shown to be toxic to bacteria6 and diethyl
ether is extremely flammable.
cIL ◊ r
D =
a =
(1)
(2)
c
H2O
D
BuOH
D
H2O
where cIL and cH O are the concentrations (g per 1000 g of
2
phase) of BuOH and water in ionic liquid phase and aqueous
phase; r is the density of ionic liquid, DBuOH and DH O are the
2
BuOH and water distribution coefficients.
The BuOH distribution coefficient for [BMIM][PF6]–water
was found to be very close to the predicted value, 0.85 (Table 1).
Distribution coefficients of BuOH for both ionic liquids were
similar. Increase in the molecular dimensions of the alkylimida-
zolium cation decreased mutual solubilities of the ionic liquid
and water resulting in higher extractive selectivities for
[OMIM][PF6]. Water solubility of [BMIM][PF6] was 6.5 fold
higher than that of [OMIM][PF6].
Solvent selection criteria for liquid–liquid extractions are
numerous, including distribution coefficient, density, viscosity,
etc. This paper addresses solubility of ionic liquids in water,
water coextraction, density and viscosity of extractant. Using
the correlation of partitioning data between 1-butyl-3-methyl-
1H-imidazol-3-ium hexafluorophosphate ([BMIM][PF6])–
water and octan-1-ol–water,3 it could be found that the BuOH
partition coefficient in the [BMIM][PF6]–water system should
be around 0.8–0.85. Professor K. Seddon advised us that
The larger size of the [OMIM]+ ion also resulted in higher
viscosity of [OMIM][PF6] (Fig. 1). The viscosity had strong
temperature dependence allowing for fast phase separation at
elevated temperature. Higher temperature also improved ex-
tractive selectivity, Table 1. BuOH solubility for the ionic
liquids under investigation was small, 200–400 mol m23. It is,
however, within the range of solubilities in octanol (200 to 2000
mol m23). Huddleston et al.3 noted that even low partition
coefficients could result in an economical recovery process due
to the non-volatility of the extractant.
Pervaporation is often mentioned as the most promising
technology for recovery of organics from aqueous streams, and
polydimethylsiloxane membrane is the most extensively stud-
ied for organophilic pervaporation.
Pervaporative BuOH recovery from 1 wt% aqueous solution
and [OMIM][PF6] was investigated using commercial poly-
dimethylsiloxane membrane MEM-100 (MemPro Co.,
water 97.4–1.0–1.6 wt% was prepared to simulate the composi-
tion of the ionic liquid phase in equilibrium with 1 wt% BuOH–
water solution. The pervaporation set-up is described
1-octyl-3-methyl-1H-imidazol-3-ium
hexafluorophosphate
([OMIM][PF6]) should have lower solubility in water than
[BMIM][PF6].
BMIM[PF6] was prepared according to the procedure
described by Huddleston et al.3 The density of the ionic liquid
was 1.34 g cm23
.
1-Octyl-3-methyl-1H-imidazol-3-ium chloride was prepared
via metathesis reaction of equimolar amounts of 1-methylimid-
azole and 1-chlorooctane. The two components were mixed in
a round-bottom flask equipped with stopcock vacuum adapter,
and the air was removed from the flask using a vacuum pump.
† Current address: Santa Fe Science @ Technology, Santa Fe, NM 87505;
E-mail: fadeev@sfst.net; Fax: +1 505-474-9489; Phone: +1
505-474-3500.
DOI: 10.1039/b006102f
Chem. Commun., 2001, 295–296
This journal is © The Royal Society of Chemistry 2001
295
Fadeev
