Columbia International Publishing
American Journal of Biomass and Bioenergy
(2014) Vol. 3 No. 1 pp. 68-82
doi:10.7726/ajbb.2014.1004
Research Article
Catalytic Depolymerization of Microcrystalline Cellulose
Accomplished in an Ionic Liquid
Matthew P. Foley1, Luke M. Haverhals1, David K. D. Klein1, William B. McIlvain1,
W. Matthew Reichert2, Daniel W. O’Sullivan1, Hugh C. De Long3, and Paul C. Trulove1,*
Received 21 July 2013; published online 17 May 2014
© The author(s) 2014. Published with open access at www.uscip.us
Abstract
Dissolution and depolymerization of cellulose by the action of acid-functionalized ionic liquid catalysts is
reported. Depolymerization of microcrystalline cellulose (MCC) was accomplished by utilizing a binary IL
system composed of an IL solvent, 1-butyl-3-methyl-1H-imidazolium chloride (C4mim-Cl), capable of cellulose
dissolution combined with a homogeneous IL acid catalyst. The sulfonic acid functionalized IL catalysts
employed are capable of hydrolyzing glycosidic bonds to yield cellobiose, glucose and that also enabled
dehydration reactions producing 5-hydroxymethylfurfural. Viscometry was employed to follow the progress
of the depolymerization and follow-on derivatization reactions. Experiments yielded real-time viscosity data
of the reaction system which indicated the type and timing of the derivatization processes that occur. Results
suggest that viscometric analyses can be a useful methodology to rapidly screen solution/catalyst
compositions for desired outcomes.
Keywords: Cellulose; Ionic liquid
1. Introduction
Cellulose is the most abundant biopolymer, by mass, on earth.(Encyclopædia Britannica, 2013) The
conversion of cellulose to high value products such as fuels and synthetic starting materials is vital
to address the ever-growing resource demands of the modern world (Ragauskas et al., 2006).
Unfortunately, cellulose processing methods are often energy intensive and use hazardous
materials (Fort et al., 2007; Lynd et al., 2002). Processing is difficult because cellulose polymers are
held together by extensive hydrogen bonding networks that are difficult to break apart (O’Sullivan,
1997). Harsh conditions (i.e., high Temperature) are, therefore, necessary to overcome this
insolubility issue when utilizing ‘traditional’ solvents (Fort et al., 2007; O’Sullivan, 1997). In 2002,
R. Rodgers and coworkers demonstrated efficient cellulose dissolution at relatively mild conditions
______________________________________________________________________________________________________________________________
*Corresponding e-mail: [email protected] phone: (410) 293-6622
1* United States Naval Academy, Department of Chemistry, Annapolis, MD 21402, USA
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2 University of South Alabama, Department of Chemistry, Mobile, AL 36688, USA
3 Air Force Office of Scientific Research, Arlington, VA 22203, USA
M.P. Foley, L.M. Haverhals, D.K.D. Klein, W.B. McIlvain, W.M. Reichert, D.W. O’Sullivan, H.C. De Long,
and P.C. Trulove / American Journal of Biomass and Bioenergy (2014) Vol. 3 No. 1 pp. 68-82
with alkyl-imidazolium chloride ionic liquids (ILs) (Swatloski et al., 2002). Since the release of this
seminal report, ILs have been vigorously investigated as solvents for dissolving cellulose at
concentrations useful for scalable manufacturing processes (Cao et al., 2009; Fort et al., 2007;
Swatloski et al., 2002; Wang et al., 2012).
In previous work, our group and others have demonstrated the catalytic breakdown of cellulose
that was dissolved in IL solvents (Swatloski et al., 2002). Catalysts utilized have included sulfuric,
hydrochloric, and IL-based acid catalysts (Amarasekara and Wiredu, 2011; Gazit and Katz, 2012;
Gazit and Katz, 2012; Li et al., 2007; Li et al., 2008; McIlvaine et al., 2010; Meine et al., 2012; Rinaldi
et al., 2008; Rinaldi et al., 2008; Rinaldi and Schüth, 2009; Su et al., 2011; Watanabe, 2010; Zhang et
al., 2010). Breakdown products include the monomer, cellobiose, (which is two glucose units),
glucose, 5-hydroxymethylfurfural (HMF) and other components (McIlvaine et al., 2010). The yields
of products are influenced by the reaction conditions and the catalyst employed. In the current
work, the viscometric analyses of the digestion of cellulose in a ‘one pot’ catalytic reaction are
discussed. The one pot reaction was a binary IL system composed of an IL solvent (1-butyl-3methyl-1H-imidazolium chloride) capable of cellulose dissolution combined with a homogeneous IL
acid catalyst capable of cellulose depolymerization. The data provide real-time viscosity changes of
the reaction system that indicate the type and timing of derivatization processes that occur.
2. Experimental
2.1 Materials
Microcrystalline cellulose (MCC), 1-butyl-3-methyl-1H-imidazolium chloride (C4mim-Cl, ≥95%),
glucose, and cellobiose were obtained from Sigma Aldrich (Milwaukee, WI). 5-hydroxymethylfurfural (HMF) was obtained from SAFC (St. Louis, MO). All chemicals were reagent grade
and were used as received. Water used in these experiments was produce by an in-house reverse
osmosis purification system.
The 1-methyl-3-(3-sulfopropyl)-1H-imidazolium 1,1,1-trifluoromethanesulfonate (1:1) ionic liquid
catalyst (C3ILcat) was synthesized using the methods described by Ohno (Yoshozawa et al. 2001).
The 1-methyl-3-(4-sulfobutyl)-1H-imidazolium 1,1,1-trifluoromethanesulfonate (1:1) ionic liquid
catalyst (C4ILcat) was prepared using methods published by Davis and coworkers (Cole et al., 2002).
Both ionic liquid catalysts (ILcat) were clear, yellow, viscous liquids.
Viscosity measurements were made with the aid of an Anton Paar SVM3000 linked with a computer
(data logger). Care was taken to ensure solution viscosities were within the experimental limits of
the viscometer. (See Supplemental Data.) Viscosity data were processed with the aid of MS Excel.
2.2 Methods
Solutions of MCC in C4mim-Cl (~20 g of total solution) were prepared nominally at 5% (wt/wt)
using a FlackTek SpeedMixer. Next, solutions were heated to the working temperature (usually
either 90 or 100 oC) and additional reagents (i.e., water, IL catalysts) were added as appropriate.
After a brief but thorough mechanical mixing (by hand), a 5 mL aliquot of solution was removed
and quickly loaded into the viscometer using a syringe. (Note that the volume of the viscometer is
about 3 mL; excess solution overflowed into a waste beaker to ensure the sample chamber was fully
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M.P. Foley, L.M. Haverhals, D.K.D. Klein, W.B. McIlvain, W.M. Reichert, D.W. O’Sullivan, H.C. De Long,
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charged.) Time zero (t0) was defined as the time when stable viscometer response was obtained to
a particular test mixture. Typically, t0 occurred within 60 to 90 seconds after catalyst introduction
(addition and mixing).
Viscometric data were correlated with HMF concentration measurements by setting aside a portion
(the remainder) of solution not introduced to the viscometer. The reaction was allowed to proceed
in a closed container held at the same temperature as the viscometer set point and with mechanical
mixing. Aliquots of the reaction mixture were collected at various times and subsequently analyzed
by HPLC/UV. A Phenomenex Luna-NH2 column with mobile phase that contained acetonitrile,
methanol, 0.1% formic acid and at a 1 mL/min flow rate were utilized. A gradient pump profile of
80:20 (vol./vol.) ACN:MeOH was used for the first six minutes of each analysis. The gradient was
shifted to 70:30 over the next six minutes and then back to 80:20 for remainder of each analysis.
Results were compared with a calibration curve developed from matrix-matched standards that
contained 50, 100, 240 and 370 mg/L HMF, respectively. The HMF concentration was determined
in units of mass of HMF per unit mass of reaction solution (for each sample). The mass fraction of
HMF was then converted to % conversion (of cellulose) by noting the reaction stoichiometry and
the initial mass fraction of cellulose (i.e., 5%). For example, if the starting ‘concentration’ was 50
mg cellulose per g solution (5% wt/wt), then 100% conversion to HMF would result in ~35 mg
HMF per g solution. (Assuming cellulose to be 100% glucose, the stoichiometry for conversion is
1:1, and taking the molar masses of glucose and HMF to be 180.11 and 126.11 g/mol, respectively.)
Therefore, a measured concentration of 3.5 mg HMF per g reaction solution corresponds with 10%
conversion rate. (10% of available cellulose converted to HMF.)
3. Results and Discussion
3.1 General Reaction Scheme
MCC is almost exclusively cellulose I with a degree of polymerization roughly 100-150 cellobiose
units (Terinte et al., 2011). The acid catalyzed cleavage of glycosidic bonds within the polymer
chain has been studied extensively (Raferty and Rand-Meir, 1968; Swatloski et al., 2002). Fig. 1
details the hydrolysis of the polymer into glucose followed by dehydration (of glucose) to HMF
(Swatloski et al., 2002). This scheme is intentionally simplistic to highlight the fact that a relatively
large polymer, with associated relatively high solution viscosity, is converted to products that
significantly lower the overall observed solution viscosity. The major objective of this work is to
tease out details of the time-dependant depolymerization process as they relate to catalysis and
product formation. To accomplish this goal, we have utilized two different acid-functionalized
(homogeneous) ILs catalysts to perform the hydrolysis reaction, Fig. 2. The cations of both the
C3ILcat and C4ILcat contain terminal sulfonates (alkane sulfonic acid) that are protonated provided
the pKa of the IL anion is sufficiently low (< 2) (Cole et al., 2002). Of course, this also requires the
anion of the IL utilized for cellulose dissolution (Cl‒) to be sufficiently acidic as well. For example,
ILs containing acetate are not suitable to achieve cellulose dissolution for these studies. The cations
of C3ILcat and C4ILcat are the active catalytic species and we are interested in the effect of alkyl chain
length on catalytic activity and product formation (relative yields).
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M.P. Foley, L.M. Haverhals, D.K.D. Klein, W.B. McIlvain, W.M. Reichert, D.W. O’Sullivan, H.C. De Long,
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Fig. 1. Shown is a generalized reaction scheme for the acid-catalyzed degradation of cellulose into
reducing sugars (glucose) and other products (HMF).
Fig. 2. Shown are the generalized structures for IL solvent (C4mim-Cl) and IL catalysts utilized. The
alkyl chain length, n, is equal to three for C3ILcat and four for C4ILcat, respectively, in the
studies discussed.
3.2 Viscosity Relationship to Major Products
The depolymerization of cellulose by the action of an acid-functionalized IL in C4mim-Cl is followed
by measuring the viscosity; Fig. 3. The uppermost data were obtained from analysis of 5% MCC
(wt/wt) in C4mim-Cl at 100 oC. Even without the IL catalyst, the viscosity of the MCC solution
decreases as time progresses. This is likely due to depolymerization of the cellulose by impurities
present in the IL. All of the solutions used in this study were prepared in ambient conditions in the
laboratory. In addition to the ~10 mg water per g IL already present in C4mim-Cl (as received from
the manufacturer), a relatively small amount of water (< 10 mg water per g IL) likely enters
solutions during preparation although care was taken to limit exposure to the atmosphere.
Impurities and adventitious water react with solubilized cellulose and result in degradation of the
polymer chain. We have shown in some unpublished work that weak acids (such as acetic acid) can
catalyze the depolymerization of IL solubilized cellulose at a much slower rate the IL catalysts used
in this study. It is reasonable to assume that the 5% impurities present in the IL, as received from
the manufacturer, could include acidic materials. Those acidic substances would be capable of
catalytic depolymerization of cellulose dissolved in the IL at a much slower rate than that of an ILcat.
The lower data plotted in Fig. 3 are from the analysis of 5% MCC in C4mim-Cl (with similar
adventitious water) and also with 2.5% (wt/wt) C4ILcat at 100 oC. It is obvious that addition of the
acid-functionalized IL catalyst greatly increases the initial rate of change in viscosity. Note that the
viscosity at t0 is recorded as about 375 mPa•sec, but the viscosity drops precipitously immediately
upon addition of C4ILcat. A relatively small portion of this drop is due to extra lubricating effects of
the IL catalyst itself. However, most of the viscosity drop is likely due to catalytic degradation of the
degree of polymerization of cellulose that happens before a stable measurement can be made. The
viscosity is below 100 mPa•sec by ~7.5 minutes and then drops slowly to a value of about 75
mPa•sec after 2.5 hours of reaction time. Curve fitting (not shown) with exponential and
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polynomial equations did not account for the observed behavior.
logarithmically (‘log-log plots’) did not linearize the response.
Even plotting the data
Fig. 3. Comparison plot of viscosity data from analysis of solutions of 5% MCC in C4mim-Cl at 100 °C
with and without added catalyst. The upper curve (solid line) is the analysis of a 5% MCC
solution with no added catalyst. The lower curve (dotted line) is the analysis of a 5% MCC
in C4mim-Cl solution with 2.5% C4ILcat. The inset plots only the first fifteen minutes of the
experiment.
To better understand how (changing) solution composition is related to viscosity, data were
collected for solutions of individual components, glucose and HMF, as well as a reaction mixture at
completion (five days) at 90 oC; Fig. 4. All solutions were 5% (wt/wt) of either glucose, HFM, or
(initially) cellulose in C4mim-Cl and with 2.5% C4ILcat and 2.5% added water. (The addition of
water will be discussed in greater detail below.) The solutions which originally contained cellulose
maintained a steady viscosity of about 33 mPa•sec. The relatively low viscosity is because cellulose
has presumably been converted to glucose, HMF and other products after five days of reaction (at
90 oC). The reaction mixture viscosity falls between the glucose solution (initially: 54 mPa•sec) and
HMF solution (25 mPa•sec). As might be expected, the viscosity of the glucose solution slowly
decreases as, presumably, the catalytic dehydration to HMF and other products proceeds. The HMF
solution, on the other hand is steady with time.
Taken together the viscosity data in Figs. 3 and 4 indicate that the degree of polymerization of
cellulose decreases quickly upon the addition of the acid catalyst. Stochastic scissioning of the
polymer chain (as opposed to the cleavage of only the end glucose units) most likely accounts for
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the rapid drop. Within about 10 minutes of adding the catalyst the polymer chain is degraded to
short cellulose chains (i.e., cellobiose) and glucose. At this point, the viscosity change slows as
dehydration reactions convert glucose into HMF and other products.
Fig. 4. Viscosity data from analysis of 5% (wt/wt) glucose (solid line), 5% HMF (dotted line), and
5% cellulose (dashed line) solutions at 90 oC. Each solution contained 2.5% (wt/wt) C4ILcat
and 2.5% of added water. The cellulose solution was analyzed after reacting for five days to
reach a final solution composition and viscosity.
3.3 Viscosity and Effects of Water Concentration
The reaction scheme in Fig. 1 indicates that either the presence or lack of water may have
important implications for final product distribution. In fact, previous work has demonstrated
fairly complex multistep mechanisms (equilibria) are involved in the conversion of glucose to HMF
(Lanzafame et al., 2012; Rasrendra et al., 2012) . Because small ratios of <10 mg water/g C4mim-Cl
are practically difficult to achieve and maintain, we were curious to document how adding excess
water (water concentration a constant) effects these systems. Data in Figs. 5 and 6 characterize
catalyst activity with the effects of water concentration. One concern was that high concentrations
of water may level the acidity (activity) of IL-based acid catalysts and/or shift equilibria that
influence product composition. That said, there is a practical limit to how much water can be added
to these systems before cellulose precipitates out of solution. Typically, water concentrations must
be well below 10% (wt/wt) to avoid precipitation. At 5% water, the molar ratio IL solvent to water
is ~2:1. Thus, the systems studied do not contain ‘bulk’ water.
Fig. 5 plots the viscosities for reaction mixtures after two days of reaction time at 90 oC. The
viscosities of individual solutions were found to be constant with time after the specified reaction
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time. A solution of 5% MCC in C4mim-Cl solution with 2.5% C4ILcat (similar composition to the
catalyst containing solution in Fig. 3) and with only adventitious water is measured at 53 mPa•sec.
When this solution is diluted with 2.5% (wt/wt) water (after reaction), the viscosity drops to 33
mPa•sec. In comparison, a solution of 5% MCC in C4mim-Cl solution with 2.5% C4ILcat and 2.5%
water (added before reaction, similar composition to the cellulose containing solution in Fig. 4) is
found to be 30 mPa•sec. While there is a 3 mPa•sec difference between the reaction mixtures
(water added before versus after reaction), we do not consider the difference significant given the
reaction mixture (after five days) in Fig. 4 was 33 mPa•sec.
Fig. 5. Comparison of viscosity data for solutions with and without added water. The uppermost
line (solid) is for a solution initially containing 5% MCC in C4mim-Cl solution with 2.5%
C4ILcat after two days at 90 oC. After the reaction, this solution was diluted with 2.5% water
and retested to produce the middle line (dashed line). The bottom line (dotted) is for a
solution 5% MCC in C4mim-Cl solution with 2.5% C4ILcat and 2.5% water (added prior to
heating) after two days at 90 oC.
Fig. 6 contains plots of time dependent viscometeric response for solutions with various amounts of
added water reacted at 100 oC. Data are plotted for solutions containing 5% cellulose in C4mim-Cl,
2.5% C4ILcat, and with either adventitious (solid line), 1% (long dashed line), 2.5% (dashed line), or
5% (dotted line) water. It is apparent that the acid catalyst remains active at all water
concentrations probed. As mentioned earlier, the molar ratio of IL (solvent and catalyst) to water is
sufficiently high such that ‘bulk’ water is not available to level the catalyst.
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Fig. 6. Comparison of viscosity data for solution with variable water concentration reacting at 100
oC. Each solution contained 5% cellulose in C4mim-Cl with 2.5% C4ILcat solutions and either
adventitious (solid), 1% (long dash), 2.5% (dash), or 5% (dotted) water, respectively.
3.4 Viscosity and the Effect of Catalyst Loading
The data in Fig. 7 contrast the viscometric responses for systems containing 5% cellulose in C4mimCl, 2.5% water, and with either 1.5% (solid), 2.5% (dashed), 5% (dotted) C3ILcat. These solutions
correspond to weight ratios of IL catalyst to cellulose of 0.3:1, 0.5:1, and 1:1, respectively. It is clear
that the greater the ratio of IL to cellulose, the faster the system viscosity reaches a near ‘steadystate’ value. Note that the addition of C3ILcat displaces the (%) amount of IL solvent. Structural
similarities between the IL catalyst and C4mim-Cl likely account for the close agreement in the
viscosity at long times (> ~90 minutes). Conversely, the relative disparity in viscosities at t0 is
explained by the fact that depolymerization is rapidly taking place for the ~60 seconds it takes to
mix, load, and record stable data.
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Fig. 7. Comparison of viscosity data for solution with variable water concentration reacting at 90 oC.
Each solution contained 5% MCC in C4mim-Cl and 2.5% water with different amounts of
added C3ILcat: 1.5% (solid), 2.5% (dashed), and 5% (dotted), respectively. The inset plots
only the first fifteen minutes of the experiment.
3.5 Viscosity Correlated with Reaction Composition
Viscosity measurements can be correlated with solution composition by running concurrent
reaction analyses. Data reported in Fig. 8 are for a single reaction mixture broken into two separate
portions. (The reaction mixture was 5% MCC in C4mim-Cl, 2.5% C3ILcat and 2.5% water reacted at
100 oC. This was a similar solution composition to that of the red data curve for 2.5% C3ILcat in Fig.
7.) One portion was, of course, loaded into the viscometer. The remainder of the reaction solution
was held in a sealed container (at 100 oC similar to the viscometer) and had aliquots of solution
periodically removed for analysis. The HMF concentration was determined by HPLC to deduce the
mass of HMF per unit mass of reaction solution as a function of reaction time. As discussed in the
experimental methods section, mass fraction values (of HMF) were converted to % conversion by
noting the starting amount of cellulose, making the assumption that MCC is 100% glucose, and by
noting the reaction stoichiometry and molar masses of glucose and HMF, respectively. As expected,
the amount of cellulose converted to HMF increases with time and also coincides with a slow drop
of the viscosity at times >60 minutes. After 90 minutes, just over 2% of the glucose in cellulose has
been converted to HMF. In separate experiments with similar solutions (5% cellulose in C4mim-Cl,
2.5% C3ILcat and 2.5% water), the HMF conversion was measured at 13.3% and 13.6% for two
different reaction solutions after 48 hours reaction time at 100 oC. This result is supportive of data
in Fig. 4 that suggest glucose dehydrates relatively slowly over time. (Previous work has shown the
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conversion to HMF can be pushed along faster and to higher yields by introduction of additional
catalysts (Binder and Raines, 2009).)
Fig. 8. Viscosity (line, left y-axis) and compositional data (diamonds, right y-axis) as a function of
reaction time for 5% MCC in C4mim-Cl, 2.5% C3ILcat and 2.5% water reacted at 100 oC.
4. Conclusions
Viscometric data are shown to be useful to analyze the acid catalyzed degradation of cellulose to
glucose and HMF. A rapid reduction in the solution viscosity indicates cellulose chains likely
undergo rapid stochastic scissioning as opposed to an orderly rate from the ends of individual
polymers. After about 15 to 20 minutes, the rate of viscosity change slows substantially. At long
times (> ~60 minutes) the conversion of intermediate viscosity sugar (glucose) solutions to lower
viscosity products such as HMF and water are likely primarily responsible for further lowering of
the viscosity. The HMF conversion appears to proceed for hours after the introduction of IL acid
catalysts, although the rates of limiting steps can likely be increased substantially by the
introduction of other catalysts. The role of water concentration on reaction rate and final
composition was not resolved by viscometric measurements alone. However, the IL-based acid
catalysts utilized here do appear to be active at (relatively high) water concentrations up to the
precipitation point of cellulose. Given that the negligible vapor pressure of the IL solvent has been
shown to be advantageous for the recovery/recycling of the solvent, we would expect similar
results in the recovery of IL-based catalysts (Earle et al., 2006). In summary, the results indicate
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viscometric analyses may be a useful methodology to rapidly screen solution/catalyst compositions
for desired outcomes.
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5. Supplemental Data
5.1 Temperature Response of the Viscometer
The viscosity of 5% (wt/wt) MCC dissolved in C4mim-Cl as a function of temperature is shown in
Supplemental Fig. 1. As expected, data indicate that the viscosity of the 5% MCC solution decreases
as temperature increases. Data indicate that within the Anton Paar SVM3000 viscometer, a change
in the set point temperature is rapidly reflected in viscosity. This good thermal response behavior
indicates that test systems quickly reach working temperatures. The scatter in viscosity
measurements at the lowest temperature (70 °C) is due to the high viscosity of the sample that
exceeds the (apparent) measurement maximum. Interestingly, the instrument is capable of
measuring viscosities well above the manufacturer’s specified limit (2000 mPa × sec), as indicated
by the stable measurements obtained at temperatures > 70 oC.
Supplemental Figure 1. Viscosity measurements of a 5% MCC in C4mim-Cl at different
temperatures. The left axis is for viscosity data (open circles, left y-axis) and the right axis displays
system temperature (solid line, right y-axis).
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and P.C. Trulove / American Journal of Biomass and Bioenergy (2014) Vol. 3 No. 1 pp. 68-82
5,2 Viscosity and the Effect of Catalyst Type
Viscosity data that compare the catalytic activity of C3ILcat and C4ILcat are plotted in Supplemental
Fig. 2. Solutions were 5% MCC in C4mim-Cl, 2.5% C3ILcat and 2.5% water and were reacted at 100
oC. The difference between the two catalysts is one methylene in the length of the sulfonic acid
chain tether. Viscosity changes for both reaction systems are similar. Apparently, for the two
catalysts tested, one methylene group does not have a substantial impact upon the reaction rate,
although with the caveat that compositional data for products were not determined. Interestingly,
a recent report by the Reichert et al. indicated the successful synthesis of additional structural
analogs to the catalysts used in this study (Reichert et al., 2012). It is possible that these new
analogs may lead to enhanced reaction rates and controlled product distributions.
Supplemental Figure 2. Plotted are viscosity data from the reaction of 5% MCC in C4mim-Cl, 2.5%
water, and with either 2.5% C3ILcat (diamonds) or 2.5% C4ILcat (squares) at 100 oC.
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