Influence of Methanol on the Dissolution of ... - ACS Publications

Article pubs.acs.org/IECR

Influence of Methanol on the Dissolution of Lignocellulose Biopolymers with the Ionic Liquid 1‑Ethyl-3-methylimidazolium Acetate María C. Castro, Alberto Arce, Ana Soto, and Héctor Rodríguez* Departamento de Enxeñería Química, Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, A Coruña, Spain S Supporting Information *

ABSTRACT: Ionic liquids are the basis of a promising technology for the dissolution and fractionation of lignocellulosic materials. The use of a miscible organic cosolvent as additive may improve the physical properties of the dissolving fluid, as compared to those of the ionic liquid alone. Alternatively, this organic substance added in larger amounts can act as antisolvent for the regeneration of the nonvolatile lignocellulose fractions dissolved. In this work, mixtures of methanol and 1-ethyl-3methylimidazolium acetate (an archetypal ionic liquid for dissolution of lignocelluloses) were characterized from a thermal and physical point of view. Their ability to dissolve representative standards of the lignocellulose biopolymers (cellulose, hemicellulose, lignin) was assessed. Very different solubilities of the three biopolymer standards were observed as a function of composition of the solvent, suggesting the possibility of a selective dissolution procedure to separate mixtures of the disengaged polymers. Precipitation tests showed that fractionated recovery of the dissolved biopolymers from the ionic liquid was not possible by direct addition of moderate amounts of methanol, due to formation of gel-like phases.

1. INTRODUCTION Lignocellulosic biomass is a nonedible, renewable resource with the capacity to serve as a feedstock for a new and sustainable technological platform for the production of chemicals and materials and without competing with the food market.1 Traditionally the exploitation of this resource has focused on the production of cellulose, thus neglecting the potential of its other two major biopolymeric components: hemicellulose and lignin. It is currently acknowledged that the development of acceptable biorefinery schemes must take into consideration the valorization of all these biopolymers.1−3 Unfortunately, the technologies available nowadays, which pose a series of safety and environmental concerns, are based on the utilization of hazardous chemicals and on the application of harsh operation conditions to the lignocellulosic feedstock in order to isolate the cellulose by degradation of the other components.4 Over the past decade, a new type of solvents for cellulose has gained increasing popularity: ionic liquids.5 Some ionic liquids (definable as salts that are liquid below 373 K) have the ability to dissolve cellulose at atmospheric pressure and at moderate or even ambient temperatures. In 2007 the first scientific papers appeared reporting ionic liquids that were also able to dissolve different sources of lignocellulosic biomass, including wood, at similarly mild conditions.6,7 This opens a door to the development of more sustainable processes for the full exploitation of all the chemical richness naturally embedded in the lignocellulosic biomass, through its dissolution and the subsequent separated recovery and further transformation of its major structural biopolymers.4 Although ionic liquids often exhibit an appealing set of properties for their use as solvents in sustainable processes,8 one of their most recurrent drawbacks is their relatively high viscosity, as compared to conventional volatile solvents. The use of the latter as cosolvents might be of interest, if they do © 2015 American Chemical Society

not reduce significantly the dissolution capacity of the ionic liquid, since the kinetics of the dissolution process would be facilitated as a result of the diminution of the viscosity.9 This lowering in viscosity would also be a concomitant benefit for the process from an engineering perspective. Moreover, the use of a cosolvent together with the ionic liquid would allow modulation of the solubility capacity by control of the composition of the resulting solvent fluid−with the advantages that a fractionated solubility of the different biopolymers might have for some specific purposes. Obviously, the cosolvent of choice should be miscible with the biomass-dissolving ionic liquid and should also possess reasonably green credentials to fit within the general context of sustainability of the new process to be proposed. Both the ionic liquids and the lignocellulose biopolymers are nonvolatile. If the lignocellulosic fractions are to be regenerated from ionic liquid solution prior to any further transformation, or for the regeneration of any modified solutes but still in a polymeric form, a logical approach is the utilization of an antisolvent to force precipitation. In fact, it would be particularly interesting to find a suitable substance that could act as a convenient cosolvent if added in small amounts to the ionic liquid and also as antisolvent if added in large amounts to the solution medium. Besides the characteristics mentioned in the paragraph above for the cosolvent, the antisolvent should have a negligible ability to dissolve the biopolymers and also an intermediate volatility. This requirement for the volatility is needed to balance two aspects: the energy required for its removal by vaporization from the mixture with the nonvolatile Received: Revised: Accepted: Published: 9605

July 15, 2015 September 16, 2015 September 17, 2015 September 17, 2015 DOI: 10.1021/acs.iecr.5b02604 Ind. Eng. Chem. Res. 2015, 54, 9605−9614

Article

Industrial & Engineering Chemistry Research

Table 1. CAS Numbers, Molar Masses (M), and Experimental and Literature Values for the Density (ρ) and Viscosity (η) of Methanol and the Ionic Liquid [C2mim][OAc] at 298.15 K ρ (g/cm3) compound

CAS no.

methanol

64-17-5

[C2mim][OAc]

143314-17-4

η (mPa·s)

M (g/mol)

exp.

lit.

32.04

0.78674

170.21

1.09903

0.7863713 0.7866514 1.099315 1.0977816

exp. 0.533 139.0

lit. 0.551313 0.55314 143.6115 132.9116

critical influence that residual water can have on properties and performance of ionic liquids,12 the [C2mim][OAc] batch was subjected to high vacuum (absolute pressure lower than 5 Pa) while magnetically stirred at ca. 343 K, to reduce the water content, as well as other volatile impurities potentially present. The preservation of the chemical identity and the absence of major impurities in the purified product were confirmed by 1H and 13C nuclear magnetic resonance spectroscopy (Figures S1 and S2 in the Supporting Information), and a water content of 0.0012 in mass fraction was determined by the Karl Fischer titration method using a Metrohm 737 KF coulometer. Experimentally measured values for selected physical properties of both methanol and [C2mim][OAc], along with values from the literature for pertinent comparison,13−16 are reported in Table 1. The experimental measurements of density and viscosity were carried out as described below. Microcrystalline cellulose (MCC) powder (Aldrich), xylan from beechwood (Sigma), and the kraf t pine lignin Indulin AT (MeadWestvaco) were used as received. 2.2. Thermal and Physical Characterization of the System Methanol + [C2mim]OAc]. Thermogravimetric analysis (TGA) runs were carried out for mixtures of methanol and the ionic liquid in a TA Instruments Q500 thermogravimetric analyzer with a weight precision of 0.01%. Approximately 15−35 mg of liquid sample was placed in an open platinum pan, which was automatically hung to the balance hook, and immediately followed by sealed closure of the furnace chamber and tare of the balance. The thermal program consisted of a simple heating ramp at a rate of 5 K/min from room temperature to 673 K, using flows of N2 (Praxair, 99.999%) of 40 mL/min and 60 mL/min as balance purge gas and sample purge gas, respectively. Analysis of the TGA curves was made by means of the Universal Analysis 2000 software, version 4.5.0.5, by TA Instruments. Differential scanning calorimetry (DSC) experiments for the same mixtures were run in a TA Instruments Q2000 differential scanning calorimeter, equipped with an RCS 90 refrigerated cooling system, and with an estimated uncertainty of 1 K in temperature. The liquid samples were placed in 40 μL aluminum pans (whose weights were recorded to within 0.01 mg by means of a Mettler Toledo AE240 analytical balance), sealed hermetically with aluminum lids, and loaded into the measuring chamber with an automatic autosampler. An empty pan with its lid was used as the reference, and a 50 mL/min flow of N2 was used as sample purge gas. The thermal program consisted of three cycles, each of them comprising a cooling ramp down to 183 K, a 5 min isotherm at this temperature, a heating ramp up to 313 K, and a 5 min isotherm at this temperature. The cooling and heating ramps were carried out at a constant cooling/heating rate of 5 K/min. Due to the observed loss of stability of the thermogram baseline below ca. 200 K, the portions of the thermogram below this temperature were systematically disregarded. It was ensured that the second

ionic liquid for recycling of both and the safety and environmental risks associated with a too volatile compound. In this context, the two most common antisolvent choices in the literature to date have been water and ethanol,5 with which the typical biomass-dissolving ionic liquids are, in most cases, totally miscible. The green credentials of water per se are unbeatable; however, it is a substance with a high specific heat and a relatively high boiling temperature, which lead to an excessive energy penalty at the stage of recycling the ionic liquid by distillation of the water from the solvent + antisolvent (ionic liquid + water) mixture. Regeneration of the mixture of ionic liquid and ethanol (which has lower specific heat and boiling temperature than water) would be less energy intensive and would possibly lead to an improved environmental friendliness of the overall process, in spite of its flammability. Within the family of light alcohols, methanol has a moderately lower boiling temperature than ethanol and could lead to larger energy savings in the regeneration of solvent and antisolvent in the process. Methanol can also be obtained via renewable pathways,2 and globally it has a sustainable character comparable to that of ethanol.10 In addition, taking into account the current and prospected low prices for natural gas,11 methanol is likely to be more economically favored than ethanol. Thus, the utilization of methanol might ease the implementation and development of the suggested ionic liquid based technology in a first instance. In this work, we explore the use of methanol as a potential cosolvent and/or antisolvent for the fractionated solubility and precipitation of lignocellulose biopolymers from the archetypical biomass-dissolving ionic liquid 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]).4 First, the binary system constituted by methanol and [C2mim][OAc] was characterized from a thermal and physical point of view. In particular, thermogravimetric analyses were run for samples covering the entire composition range, and their density and viscosity (two key properties in process design involving fluid systems) were determined and analyzed as a function of temperature and composition. The solubility of standards of the main lignocellulose biopolymers was then measured in some of those mixtures, covering the entire composition range. The selected representative standards of cellulose, hemicellulose, and lignin were microcrystalline cellulose (MCC), beechwood xylan, and Indulin AT, respectively.4 Finally, the feasibility of using methanol as a precipitation agent from biopolymer solutions in [C2mim][OAc] is discussed in a quantitative approach, on the basis of the solubility results and on the output of real precipitation tests.

2. MATERIALS AND METHODS 2.1. Materials. Methanol was purchased from Aldrich with a nominal purity of 99.9% and used as received. The ionic liquid [C2mim][OAc] was purchased from Iolitec with a nominal purity of >95%. Given its hygroscopic nature and the 9606

DOI: 10.1021/acs.iecr.5b02604 Ind. Eng. Chem. Res. 2015, 54, 9605−9614

Article


observation. This procedure was tested with different concentrations until reaching the solubility limit, manifested by undissolved solute at the bottom of the vial, suspended particles, or a turbid liquid mixture. In the case of Indulin AT, occasionally very dark solutions were obtained, which prevented the confirmation of solubilization by simple visual observation. Therefore, the solubility was determined by measuring the absorbance of mixtures with different amounts of added Indulin AT, following a similar procedure to that reported by Lee et al.17 Specifically, the supernatant of these decanted mixtures was taken, it was diluted with 0.1 M aqueous NaOH to avoid precipitation of the lignin dissolved, and its absorbance was measured at a wavelength of 360 nm using an Agilent 8453 UV−vis spectrophotometer. The solubility value was assumed to correspond to the composition for which a plateau was observed in the plot of absorbance versus amount of polymer added. For the specific experiment on selective dissolution of a mixture of the three biopolymer standards by mixtures of [C2mim][OAc] and methanol, details on the procedure and masses involved are supplied directly in the Results and Discussion section. All FT-IR spectra were carried out in a Varian FT-IR 670 spectrometer, using KBr pellets, and recording a total of 32 scans in the wavelength range 500− 4000 cm−1. The precipitation experiments to evaluate the ability of methanol as antisolvent were carried out at room temperature. Mixtures of 3 g of [C2mim][OAc] and 0.15 g of the biopolymer standard (or a combination of biopolymer standards) were placed in capped glass vials and stirred magnetically until total dissolution. Controlled amounts of methanol were gradually added. After each addition the resulting system was stirred for 30−45 min and allowed to settle for several minutes.

and third cycles of each thermogram were essentially coincident, and the signal of the third cycle was used for evaluation with the Universal Analysis 2000 software mentioned in the previous paragraph. Densities (ρ) were performed at atmospheric pressure in an Anton Paar DMA 5000 vibrating U-tube density meter, with an uncertainty of 3 × 10−5 g/cm3. The temperature of the sample was internally controlled and kept constant during the measurements by an integrated system based on the Peltier effect, with a precision of 1 × 10−3 K. The instrument also carries out an automatic correction of the influence of the viscosity on the density determination. At least two measurements were performed, ensuring that they were repetitive within the reported uncertainty. Viscosities were determined with micro-Ubbelohde glass capillary viscometers by Schott. A certified calibration of each tube was provided by the manufacturer, and the calibration constants (K) were checked in our laboratory by measuring the viscosity of pure liquids and comparing the experimental results obtained with reference values from the literature. Efflux times in the capillary tubes were measured by a Lauda PVS1 Processor Viscosity System equipped with a photoelectric cell, with a resolution of 0.01 s. A Lauda D20 KP clear view thermostat, filled with water and coupled with a Lauda DLK 10 through-flow cooler, was used to keep the temperature constant during the measurements, with an estimated uncertainty of 0.05 K. At least three measurements of efflux time were carried out for each sample, with the average efflux time (t) being calculated after discard of possible outliers. The kinematic viscosity (ν) was then calculated by means of the following formula

ν = K · (t − y )

(1)

where y stands for the correction of kinetic energy, applied to the shorter efflux times in accordance with the tables supplied by the viscometer manufacturer. Only in a few cases there was a need to apply this correction. The ν values thus calculated were checked to lie within the range of the specifications of the capillary viscometer used. If otherwise, the measurements were repeated with another viscometer with a capillary tube of different diameter. The dynamic viscosity (η) was finally calculated, with an estimated overall uncertainty of 0.5%, as the product of the density and the kinematic viscosity: η = ρ·ν (2)

3. RESULTS AND DISCUSSION 3.1. Thermal Analysis of the Mixtures of Methanol and [C2mim][OAc]. Either in its role as cosolvent or as antisolvent, it is desirable to have the possibility of easily removing methanol from its mixtures with [C2mim][OAc], to get back the pure ionic liquid as needed. In principle, the vaporization of the alcohol from the nonvolatile ionic liquid at moderate temperatures would be a preferred strategy. To confirm that this was possible for the methanol + [C2mim][OAc] system, TGA experiments were carried out for samples covering the composition range of the binary system. Figure 1 shows the TGA thermograms, from pure [C2mim][OAc] down to a mixture with a 0.10 molar fraction of the ionic liquid, at a step of 0.10 in molar fraction. For pure [C2mim][OAc] a regular one-step decomposition curve was obtained, with a 5% onset decomposition temperature (Td,5%onset) of 427 K. The Td,5%onset corresponds to the temperature of the onset calculated using the tangent line to the thermogram at the point for which a loss of 5% of the original mass of the sample occurs. It is a more conservative value than the regular onset decomposition temperature and also a better estimation of the maximum temperature at which a substance or mixture can be used in practice in an industrial process. For all other samples (containing both methanol and the ionic liquid), a two-step decomposition is observed. The first step takes place quite abruptly from the very beginning of the recording of data in the heating ramp, as a result of the inherent volatility of methanol at room temperature. After an inflection point in the thermogram with a horizontal tangent

All mixtures of methanol and [C2mim][OAc] were prepared by weight using the Mettler Toledo AE240 analytical balance mentioned previously but in the weighing mode with a precision of 0.1 mg. The uncertainty in the composition of the prepared mixtures was estimated to be 0.0001 in molar fraction. 2.3. Solubility Measurements and Precipitation Experiments. The solubilities of MCC, xylan, and Indulin AT in mixtures of methanol and [C2mim][OAc] were determined by direct addition and dissolution of controlled amounts of the polymer to the corresponding liquid solvent (always a freshly prepared mixture, to minimize alteration in the composition due to volatilization of the alcohol) in capped glass vials. After addition of the solute, the content of the vial was magnetically stirred (or, in the case of samples of high viscosity, mechanically stirred using a metal rod coupled to an IKA RW 16 Basic overhead stirrer) overnight. The complete dissolution of the added polymer was then inspected by direct visual 9607


Article


liquid does not seem negatively affected by the presence of methanol. Overall, there is a quite wide temperature window in which the distillation of methanol can be carried out comfortably below the thermal stability limit of the ionic liquid. This behavior is similar to that observed for the system [C2mim][OAc] + ethanol in our previous work.18 The specific heats and enthalpies of vaporization of these alcohols (2.54 J/ (g·K) and 1.17 kJ/g for methanol and 2.44 J/(g·K) and 0.92 kJ/g for ethanol, at 298 K) are largely lower than those of water (4.18 J/(g·K) and 2.44 kJ/g, at 298 K).13 Therefore, the use of methanol or ethanol as antisolvents, as compared to the use of water, is expected to reduce notably the energy cost of recovering the ionic liquid by distillation of the latter. Vapor− liquid equilibrium (VLE) data for the binary mixture [C2mim][OAc] + methanol, and pertinent comparison with analogous binary systems with ethanol and water, would complement the previous statement, analyzing the appropriate design and operation conditions of the distillation unit to be used for removal of methanol from its mixtures with [C2mim][OAc]. In this regard the VLE data reported by Cai et al.19 on the ternary system methyl acetate + methanol + [C2mim][OAc] point out initially to a small effect of the ionic liquid in the boiling temperature of methanol (a variation of less than 5 K) up to ionic liquid molar fractions of 0.30. Since the TGA runs are dynamic experiments, a word of caution must accompany the temperature thresholds of stability reported here. Lower temperature limits could apply when talking about process plants operating in the mid- or long-term. Nevertheless, the values reported in this section can be taken as fair estimations of the actual limiting temperatures at which industrial units in a real process could be operated for the intended purpose of this work. A detailed explanation of the calculation of the inflection points and T′d,5%onset values reported in this section is provided in the Supporting Information (see Figure S3). In a separate piece of thermal analysis, the liquid range of the mixtures [C2mim][OAc] + methanol was characterized by DSC. Pure methanol is known to have a melting temperature of 175.47 K at atmospheric pressure,13 whereas for pure [C2mim][OAc] a glass transition near 200 K was reported by Troshenkova et al.20 The DSC thermograms (see Figure S4 in the Supporting Information) confirmed that the mixtures of both components, of any composition, remained liquid at least down to ca. 200 K (the reliable lower temperature limit of the apparatus used), not showing any thermal events at higher temperatures. 3.2. Effect of Methanol on the Density and Viscosity of [C2mim][OAc]. The thermophysical characterization of fluid systems is critical for the adequate design of engineering processes in which such systems are involved. The mixing of methanol with the denser and more viscous ionic liquid will have a substantial impact on the properties as compared to the pure ionic liquid. In addition, an insightful analysis of the experimental results may provide clues on the chemical interactions occurring between both substances at a molecular level. In this section two key thermophysical properties such as density and viscosity were investigated for the system [C2mim][OAc] + methanol over the entire composition range and in the temperature range from 278.15 to 318.15 K. The density and viscosity of mixtures of [C2mim][OAc] and methanol at different temperatures and atmospheric pressure are reported in Table 3. As expected, a decrease in both properties with increasing concentration of methanol and/or

Figure 1. TGA thermograms for mixtures of [C2mim][OAc] and methanol, from pure [C2mim][OAc] (top) to a 0.10 molar fraction of [C2mim][OAc] (bottom), with a step of 0.10 in molar fraction. Solid and dashed patterns are alternated for better identification of each individual thermogram.

(transition from convex to concave), the second decomposition step occurs at higher temperatures, in the vicinity of the decomposition of pure [C2mim][OAc]. Interestingly, as shown quantitatively in Table 2, the referred inflection points Table 2. Temperatures (Tip) and Weight Percents (%wtip) for the Inflection Points with Horizontal Tangent Line of the TGA Thermograms of Mixtures of [C2mim][OAc] and Methanol with Different Concentrations of [C2mim][OAc] (xIL for Molar Fraction, wIL for Mass Fraction)a xIL

wIL

Tip (K)

%wtip

T′d,5%onset (K)

0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 0.7000 0.8003 0.9003 1.0000

0.3712 0.5705 0.6948 0.7798 0.8416 0.8885 0.9253 0.9551 0.9796 1.0000

419 406 403 413 413 415 419 418 415

46.2 65.6 73.8 79.4 84.2 88.1 92.0 94.7 97.2

448 438 438 445 445 444 444 451 448 427

a

The corresponding pseudo 5% onset decomposition temperatures (T′d,5%onset), or the regular 5% onset decomposition temperature in the case of pure [C2mim][OAc], are also reported.

correspond to weight percents (%wtip) that roughly match the mass fractions of ionic liquid (wIL) in the samples (with exception of the samples with a higher concentration of methanol, likely due to experimentally unavoidable losses of methanol by evaporation from the sample prior to the set of the 100% weight during the taring step). This coincidence is indicative of a practically total vaporization of methanol from its mixture with the ionic liquid at that point. If the mentioned inflection points are used to draw the baseline for calculation of pseudo 5% onset decomposition temperatures (T′d,5%onset) for the second decomposition step,18 values in the range 438−451 K are obtained (see Table 2). These values are just slightly higher than that of pure [C2mim][OAc]; whereas those for the temperature at which the inflection points occur (Tip) are in the range 403−419 K (see Table 2), lower than Td,5%onset of pure [C2mim][OAc]. Therefore, the thermal stability of the ionic 9608


Article


Table 3. Density (ρ) and Viscosity (η) for the System [C2mim][OAc] + Methanol, at Atmospheric Pressure and Different Temperatures, with xIL Representing the Molar Fraction of [C2mim][OAc] T (K) xIL

278.15

288.15

298.15

308.15

318.15

0.78674 0.89702 0.95875 0.99819 1.02558 1.04577 1.06131 1.07380 1.08377 1.09214 1.09903

0.77726 0.88907 0.95143 0.99122 1.01884 1.03920 1.05488 1.06748 1.07754 1.08599 1.09293

0.76768 0.88112 0.94413 0.98431 1.01216 1.03269 1.04849 1.06120 1.07136 1.07989 1.08686

ρ (g·cm−3) 0.0000 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 0.7000 0.8003 0.9003 1.0000

0.80550 0.91300 0.97359 1.01227 1.03922 1.05907 1.07434 1.08655 1.09631 1.10466 1.11157

0.79614 0.90499 0.96612 1.00520 1.03237 1.05239 1.06780 1.08016 1.09002 1.09826 1.10520 η (mPa·s)

0.0000 0.1000 0.2000 0.3000 0.4000 0.5000 0.6999 0.7000 0.8003 0.9003 1.0000

0.713 2.018 4.358 9.022 18.14 35.84 68.41 144.0 245.4 443.5 690.3

0.614 1.693 3.523 6.897 12.97 23.72 41.82 78.55 121.8 198.8 283.8

0.533 1.393 2.911 5.410 9.684 16.60 27.37 46.97 69.71 104.4 139.0

(3)

where T is the absolute temperature, and a0, a1, and a2 are the fit parameters. These parameters, obtained by least-squares regression for each composition series, are summarized in Table S1 in the Supporting Information, along with the corresponding standard deviation values. The quality of the fits can also be visually assessed in Figure 2a. The typical exponential decrease of viscosity with temperature is shown for the studied system in Figure 2b. This evolution was suitably correlated by means of the Vogel− Fulcher−Tammann (VFT) equation, in particular the modified version by Cohen and Turnbull22 ⎛ k ⎞ η = A ·T 0.5·exp⎜ ⎟ ⎝ T − T0 ⎠

0.411 1.050 2.089 3.645 5.972 9.336 13.94 21.14 28.45 38.41 47.37

obtained by the VFT equation.24,25 In particular, this is the case for [C2mim][OAc].18 Therefore, and to uniformly correlate the viscosity dependence with temperature over the entire composition range with a single expression, all series were correlated with the VFT equation. The fit parameters are summarized in Table S2 in the Supporting Information, along with the corresponding relative standard deviations. The obtained fits are plotted together with the experimental data points in Figure 2b. From an application perspective in a chemical process context, it is obvious that low temperatures should be avoided especially with streams rich in the ionic liquid, due to their high viscosity. For an insightful analysis of the density and viscosity as a function of composition, the excess molar volumes (VE) and viscosity deviations (Δη) for the system [C2mim][OAc] + methanol can be calculated from the experimental data, for series at constant temperature, with the following equations

increasing temperature is observed, although the evolution of each property with temperature and with composition is starkly different, as discussed below. Figure 2a shows the approximately linear decrease of the density with temperature, at different constant compositions. However, a detailed analysis with the F-test,21 conducted at a significance level of 0.05, determined that the second-order term in a polynomial expression of the density as a function of temperature was statistically significant for several of the series at constant composition. Therefore, the following equation was preferred over a linear fit to correlate the experimental data ρ = a0 + a1·T + a 2 ·T 2

0.466 1.202 2.444 4.403 7.489 12.22 19.07 30.62 42.85 60.83 77.32

VE =

⎛1

∑ xi·Mi ·⎜⎜

Δη = η −

⎝ρ

∑ xi·ηi

−

1⎞ ⎟⎟ ρi ⎠

(5) (6)

where xi, Mi, ρi, and ηi are respectively the molar fraction, molar mass, density, and viscosity of the i-th compound, and ρ and η are the density and viscosity of the solution. Figure 3 shows their graphical representation as a function of composition for the different isotherms. (The numerical values of VE and Δη are reported in Table S3 in the Supporting Information.) All excess molar volumes and viscosity deviations are negative, over the entire composition range, at all the investigated temperatures. The fact of VE < 0 can be attributed to attractive forces, among which hydrogen bonding and ion-dipole interactions of the

(4)

where T stands for the absolute temperature, and A, k, and T0 are the fit parameters. Although the viscosity dependence of methanol could be nicely described by the well-known and simpler Arrhenius-like equation proposed by Andrade,23 for ionic liquids a significantly better description is typically 9609


Article


Figure 2. Density ρ (top) and viscosity η (bottom) for the binary system [C2mim][OAc] + methanol, at atmospheric pressure and as a function of temperature (T), at different molar fractions of [C2mim][OAc] (xIL): ★, 0.00; Δ, 0.10; ▲, 0.20; ◊, 0.30; ⧫, 0.40; □, 0.50; ■, 0.60; ▽, 0.70; ▼, 0.80; ○, 0.90; ●, 1.00. (For a greater degree of precision of the molar fractions, please refer to Table 3.) Solid lines correspond to the correlation by means of a second-degree polynomial (density) or to the correlation by means of the VFT equation (viscosity).

Figure 3. Excess molar volume VE (top) and viscosity deviation Δη (bottom) for the binary system [C2mim][OAc] + methanol, at atmospheric pressure and as a function of the molar fraction of [C2mim][OAc] (xIL), at different temperatures: ●, 278.15 K; ○, 288.15 K; ▼, 298.15 K; ▽, 308.15 K; ■, 318.15 K. Redlich−Kister polynomial fits are plotted as solid lines. The viscosity deviations predicted by the mixing rule of eq 7 are plotted as dashed lines.

alcohol molecules with the ionic liquid26 may be cited. This type of interaction would be expected to weaken with increasing temperature and consequently diminish the absolute value of VE; however, the opposite is observed (Figure 3a). This is likely due to the contribution of a second factor: a relative improvement in the packing of the chemical moieties at a molecular level with an increment in temperature, as a result of the increment of molar volumes Vi of the pure compounds (from 153.13 cm 3 ·mol −1 to 156.61 cm 3 ·mol −1 for [C2mim][OAc] and from 39.78 cm3·mol−1 to 41.74 cm3· mol−1 for methanol, in the temperature interval 278.15−318.15 K studied in this work). This increment in the Vi values (mathematically obtained as the quotient of Mi over ρi) is speculated to facilitate a more compact spatial arrangement of the species in the liquid mixture. With this effect occurring at a faster pace than the weakening of the ion-dipole interactions mentioned above, the overall effect would be an increase in the absolute value of the excess molar volumes (i.e., VE becoming more negative) with an increase in temperature, as it is experimentally observed. Regarding the viscosity deviation, its evolution with temperature follows the opposite trend: its absolute value decreases (i.e., Δη becomes less negative) as the temperature is increased

(Figure 3b). The decreasing pace is particularly high at the lowest temperatures, coinciding with the sharpest decrease of viscosity with temperature (Figure 2b). Nevertheless, the viscosity deviation remains negative even at the highest temperature tested, indicating that the viscosity of all essayed mixtures is lower than the composition-weighted average of the viscosities of the pure compounds. At low concentrations of ionic liquid in the mixture (ca. xIL < 0.40), it was found that the deviation viscosity (and consequently the viscosity) was well described by the simple and classical mixing rule of viscosity than can be credited to Arrhenius and Kendall27 ln ηpred = x1·ln η1 + x 2·ln η2

(7)

where the subscript pred indicates that the viscosity obtained is interpreted here as a value predicted by the mixing rule. The dashed lines in Figure 3b correspond to the “predicted” viscosity deviations Δηpred calculated by means of eq 6 but using the values of ηpred instead of the experimental η values. Interestingly, at high concentrations of ionic liquid the prediction with the Arrhenius-Kendall mixing rule was worse, in particular at low temperatures. 9610


Article


as compared to the solubility of cellulose in other solvents, or even in other ionic liquids.5 This solubility decreased moderately with a moderate increase in the percentage of methanol in the solvent composition (xIL = 0.60, 0.80). With a further increase in the presence of methanol (xIL = 0.40 or lower), MCC became practically insoluble (less than 0.5 g per 100 g of solvent in any case). Xylan was less soluble than MCC in the pure ionic liquid. Interestingly, with the presence of as little as 20 mol % methanol in the solvent (equivalent to only 4.5 mass% of methanol), the xylan solubility became practically negligible (

Influence of Methanol on the Dissolution of ... - ACS Publications

Recommend Documents