Znd. Eng. Chem. Res. 1991, 30, 232-240
232
Evidence for the Heterogeneity of Glycol Lignin Ronald W. Thring, Esteban Chornet,* Jean Bouchard, and Pierre F. Vidal Department of Chemical Engineering, University of Sherbrooke, Sherbrooke, Quebec, Canada J1K 2Rl
Ralph P . Overend National Research Council of Canada, Ottawa, Ontario, Canada K I A OR6
Glycol and kraft lignins have been fractionated by solvent extraction using a sequence of organic solvents of increasing hydrogen-bonding capability. A comparison between the fractions from both lignins has been made in terms of yield, elemental analysis, Fourier transform infrared spectroscopy, and molecular weight distribution. In addition, the glycol lignin and its fractions were further characterized by methoxyl and total hydroxyl group determination, as well as by I3C NMR spectroscopy. The heterogeneity of glycol lignin is particularly demonstrated. More importantly, 26% of this lignin has been found to consist of a low molecular weight fraction soluble in ethyl acetate, which is richer in guaiacyl units and hydroxyl groups than the other fractions. This fraction is not present in Indulin AT kraft lignin.
Introduction The isolation of lignin by chemical pulping and by organosolvolysis is known to produce a "lignin" that is heterogeneous in terms of both chemical (associated with functionalities and interlinking units) and physical (associated with different degrees of polymerization of the lignin aggregates) composition. The origin of this heterogeneity may stem from differences in the types of lignin synthesized in the different parts of the cell wall during growth of the cell, for example, middle lamella versus S1-S2-S3 lignins (Fergus and Goring, 1970; Musha and Goring, 1975; Fengel and Wegener, 1983). They may also be caused by differences in cellular metabolism due to the influence of external variables, i.e., temperature, during the growth process (Wenzl, 1970; Wardrop, 1981). At any rate, the lignin isolated from macroscale (whole wood) samples is heterogeneous and has been demonstrated by fractional precipitation obtained by varying the pH of aqueous solutions of sulfonated kraft lignins (Wada et al., 1962), by successive precipitation with water from a dioxane solution (Hatakeyama et al., 1975), or by fractional solubilization in a range of solvents (Morck et al., 1986, 1988). Lignin heterogeneity in terms of molecular weight has also been demonstrated by means of ultrafiltration (Lin and Detroit, 1981) and gel permeation chromatography (Kringstad et al., 1981). The majority of reported studies on lignin fractionation have used commercially produced kraft lignins as substrate. As part of an overall fractionation strategy for the upgrading of lignocellulosics, our group has developed an organosolv procedure for selectively isolating lignin from a prototype hardwood Populus deltoides (Chornet et al., 1986). The technique involves the preferential dissolution of lignin and hemicelluloses by a thermomechanical treatment using ethylene glycol as solvent, leaving the cellulosic residue virtually undegraded. The process, conducted in a process development unit capable of treating up to 4 kg of dry wood per hour, involves two stages: pretreatment of the slurry at 200 OC for 1 h by continuous recirculation through a homogenizing valve maintained at a high-pressure gradient; liquefaction by passage through a tubular reactor at 220 "C for a residence time of 5 min. The lignin, referred to as "glycol lignin", is recovered as a precipitate from the spent glycol liquor
* Author to whom correspondence should be addressed.
by dilute acidulation with an aqueous solution of hydrochloric acid (Thring et al., 1990b). The amount of lignin recovered by this method represents approximately 72% of the Klason lignin found to be initially present in the untreated wood. The present paper describes the fractionation of a solvolytic glycol lignin and of a commercially obtained pine kraft lignin by successive solvent extraction. The fractions are compared in terms of yield, elemental composition, molecular weight distribution, functional group analysis, FTIR, and 13CNMR spectroscopy. A preliminary report on this work has been presented (Thring et al., 1989b).
Experimental Section (a) Materials. Glycol lignin was prepared from airdried debarked aspen woodmeal by solvolysis delignification using ethylene glycol at high temperature, followed by dilute acidification of the spent black liquor from the process to precipitate the lignin (Thring et al., 1989a). Kraft Indulin AT lignin from Westvaco Company (Charleston,NC) was initially extracted with water at room temperature by stirring a dilute suspension for 1 h. The remaining water-insoluble fraction was then air-dried at room temperature and used thereafter. Ethyl acetate (ACS Reagent Grade) was purchased from Mallinckrodt Inc., Paris, KY. Methanol (ACS Reagent Grade) was obtained from Anachemia, Montreal, QC, Canada. (b) Lignin Characterization. Elemental analyses (C, H, and N) were carried out by using a Perkin-Elmer 240 C instrument. Oxygen was calculated by difference (100% - (C + H + N)%). Methoxyl content was determined by using a modified microscale Zeisel method (Haluk and Metche, 1986). Reproducibility was estimated to be within 4% by duplicate analysis of the same sample. Total hydroxyl content was determined by initial acetylation (see method below) of the lignins followed by determination of the 0-acetyl groups by gas chromatography (Solar et al., 1987). High-Performance Size-Exclusion Chromatography (HPSEC). Analysis by HPSEC was performed by using a Varian liquid chromatograph connected to an online computer via a Waters Maxima 820 data system. Two stainless steel columns connected in series were used (length 30 cm; i.d. 7.5 mm) packed with PLgel spherical particles (Polymer Laboratories) of particle size 5 pm and pore size 50 A (column 1)and 500 A (column 2). Detection
0000-5005/91/2630-0232$02.50/00 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991 233 LIGNIN
FRACTDN 1
FRACTION 2
I FRACTION4 I Figure 1. Scheme for the fractionation of glycol and kraft lignins.
was made with a UV monitor at 254 nm. A constant flow rate through the system was maintained by a single piston, reciprocating pump. The following operating conditions were employed: eluent, degassed tetrahydrofuran (purged with N2);flow rate, 1 mL/min; pressure, 7.0 MPa; temperature, 25 " C ; injection volume, 10 pL; sample concentration, 1 mg/mL. Calibration was achieved by using monodisperse polystyrene standards (Polymer Laboratories, England, UK). A third-order polynomial of the form y = ax3 + bx2 + cx + d best describes the relation between the log of the actual molecular weights of polystyrene standards (y) and their retention times (x). The flow rate accuracy of the instrument was 0.370, and the following coefficients were calculated: a = 4.0071, b = 0.325, c = -5.15, and d = 30.7. These gave a fit with a multiple regression coefficient of r2 = 0.998. Fourier Transform Infrared Spectroscopy (FTIR). Lignin samples were prepared in a KBr matrix at a concentration of 1-2% (w/w) by grinding in a stainless steel ball mill for 30 s at room temperature. Spectra were obtained from a 5DXB Nicolet FTIR spectrometer in diffuse reflectance mode using the Collector cell from SpectraTech. Conditions of analysis were as follows: resolution, 4 cm-'; co-adding, 400 scans; frequency range, 400-4600 cm-' (DTGS detector); Happ-Genzel apodization. '3c NMR Spectroscopy. The spectra of the acetylated samples of glycol lignin and its fractions were obtained by using a Bruker 250 WM Fourier Transform spectrometer. Samples were prepared by dissolving ca. 800 mg of lignin in 2 mL of DMSO-d6and placing the solution in 10-mmi.d. glass tubes. These were run at a temperature of 50 "C. The instrument was set in proton inverse gated decoupled mode, and the delay between scans was 2 s. (c) Fractionation Scheme. Both lignins were fractionated into four fractions at ambient conditions. An ultrasonic water bath was used for agitation to enhance solvent penetration into the lignin residue and also cause rapid dissolution of the soluble portion of lignin. The method of fractionation, illustrated in Figure 1, may be described as follows: Ethyl acetate (50 or 150 mL) was added to an evaporating flask containing (3 or 30 g, respectively) lignin. The suspension was agitated by ultrasound for 10 min and then stood to allow particles to settle before decanting the liquid into a 500-mL volumetric flask. The same procedure was repeated with clear ethyl acetate until the extract was virtually colorless. The remaining solvent in the residue after decantation was evaporated under reduced pressure after addition of about 10 mL of methanol to facilitate the process. Addition of this aliquot of methanol, followed by agitation, increased the methanol solubility of the residue and enhanced the removal of ethyl acetate.
Methanol was used next in the sequence. The residue remaining, after the repetitive extraction procedure with methanol, was seen as a highly viscous but flowing black fluid, particularly in the case of glycol lignin. To obtain a third fraction, a 50/50 mixture of ethyl acetate and methanol was employed. The soluble material was deemed effectively removed by observing an almost colorless supernatant after the last extraction. The remaining residue in the extraction flask constituted the fourth fraction. Each fraction was evaporated under reduced pressure by rotavapor and the final remaining residue dried to constant weight to within f10 mg. For this, it was necessary to increase the water bath temperature to 40 "C. A higher temperature was suspected to adversely effect the properties of the residues and was therefore not used. To ensure complete removal of solvent, all four residues were then freeze-dried at 0.1 mmHg and -60 "C for 2 days. Each fraction was powdered and stored in opaque colored bottles under nitrogen at -10 "C until further use. In this manner, a fraction for each solvent extraction sequence was obtained. An identical procedure was used for fractionating water-washed kraft lignin. An interesting observation was the difference in the texture of the fractions from each lignin. Glycol lignin fractions existed as aggregates of black particles with the first fraction a viscous, sticky polymer with no visible granular particles. Kraft lignin fractions, on the other hand, were all free running powders and light brown in color. Acetylation of Lignins. Both the starting lignins and their fractions were acetylated by dissolving approximately 2 g of sample in 30 mL of a 1:l mixture of acetic anhydride-pyridine in a 100-mL flask. The contents of the flask were agitated for 2 days at room temperature by using a rotary shaker. The acetylated product was recovered by precipitation in an ice-water mixture. The filtered product was washed a few times with 0.1 N HC1, rinsed thoroughly with distilled water, dried under vacuum at room temperature, and stored in the dark under a nitrogen blanket. The somewhat darker coloration of the filtrate, particularly from Fr-1, strongly suggested that this acetylation technique solubilizes a little lignin. Hence, all subsequent data reported in this work pertaining to acetylated samples refer to the water-insoluble fraction only. Results and Discussion The basis for fractionating lignin according to the solubility by extraction with organic solvents is the work of Schuerch (1952). Schuerch found that (a) organic solvents of weak or moderate hydrogen-bondingcapacity solubilize low molecular weight lignin fractions and (b) a mixture of a hydroxylated solvent (e.g., methanol) and a solvent of lower hydrogen-bonding capacity, having a Hildebrand solubility parameter (a) greater than or equal to 18.4 MPa'/*, will have a higher solvating power for lignin than either solvent. A 1:l (v/v) mixture of ethyl acetate (moderate hydrogen-bonding capability, u = 18.6 MPa112 (Barton, 1983)) and methanol (strong hydrogen-bonding capability, u = 29.5 MPa'I2) was thus selected and used as the third elution solvent in our fractionation sequence. The fractionation method described was rather time consuming but very reproducible, within experimental error, with a high solvent recovery. Repetition of the procedure on 3-g and 30-gquantities of lignin gave a deviation of f4% around the average values of the yields of the fractions reported. For kraft lignin, the water-soluble fraction, accounting for approximately 7% of the initial lignin, was discarded
234 Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991 Table I. Elemental Analysis" of Original Lignins and Fractions kraft glycol fraction C % H % N % 0 %b C % H % N % 1 61.1 7.3 0.1 31.5 64.6 6.1 0.4 2 61.9 7.3 0.2 30.6 64.2 6.1 0.4 3 62.2 6.0 0.4 31.4 61.0 5.5 0.4 4 60.9 6.1 0.4 32.6 61.2 5.6 0.4 original 61.3 6.0 0.2 32.5 63.2 5.5 0.3 lignin
Maf basis.
O%b 29.0 29.1 33.2 33.0 31.0
Determined by difference.
Table 11. Methoxyl and Total Hydroxyl Contents"of Glycol Lignin and Fractions content fraction methoxyl total hydroxyl 1 21.5 10.6 18.1 7.1 2 23.6 7.1 3 18.1 8.4 4 original lignin 20.8 7.4
" Maf basis. and not analyzed in this study. It has been previously reported (Morck et al. 1986) that this fraction consists of inorganic salts, organic acids, carbohydrates, and watersoluble phenols. Elemental Composition. The elemental compositions shown in Table I are average values from duplicate analyses. The instrument used gave errors within 0.4% (C and N) and 2% (H)from triplicate analyses of a test sample. For glycol lignin, the C content increases from Fr-1 to Fr-3 but diminishes to a low of 60.7% in Fr-4. The H content is highest at 7.3% for Fr-1 and Fr-2 but is lowest for Fr-3 and Fr-4 at 6.0%. The 0 content is lowest in Fr-2 a t 30.690 and highest at 32.6% for Fr-4. In kraft lignin, there is a steady decrease in the C content from Fr-1 (64.6%) to Fr-4 (61.2%). A similar trend on the C content in fractions from the sequential solvent extraction of a softwood kraft lignin has also been reported (Morck et al., 1986). The H content is highest for Fr-1 and Fr-2 a t 6.1% but reduces to 5.6% for Fr-3 and Fr-4. Oxygen is lowest in Fr-1 and Fr-2 at 29.0% but increases to 33.0% for Fr-3 and Fr-4. The variations in elemental composition are indicative of the differences between both lignins and the corresponding fractions due to their source and method of isolation from the wood. The above interpretation does not take into account the presence of residual carbohydrates in these lignin samples. Total carbohydrate content has been previously reported to be below 1% in both the water-washed kraft lignin (Vidal et al., 1989) and glycol lignin (Thring et al. 1989a). Analytical determinations of the content of methoxyl and total hydroxyl groups were carried out only for glycol lignin and its fractions. The results are listed in Table 11.
As seen, the content of methoxyl groups is highest in Fr-3, indicating that this fraction is the richest in syringyl units. Indeed, this finding is supported by results from I3C NMR analysis (see below). The content of total hydroxyl groups is highest in Fr-1. I3C NMR (see below) shows the aromatic hydroxyl groups to be more than the aliphatic hydroxyl groups in this fraction. This suggests that Fr-1 constitutes the most cleaved material from the original lignin. Liberation of the phenolic hydroxyl groups most likely occurs during delignification involving splitting of the P-0-4 ether bonds. The results show a general increase in total hydroxyl groups with increasing molecular weight from Fr-2 to Fr-4. As it has been suggested (Morck et al., 1988) from work on the fractionation of kraft lignin from birchwood, this trend probably reflects an increase in the amount of etherified and/or non-etherified syringyl units with increasing molecular weight. This is further substantiated by the steady increase in the oxygen content with increasing molecular weight, in spite of the similarity in methoxyl content (except Fr-3). Yields. As shown in the first row of Table 111, the extraction procedure clearly demonstrates the inhomogeneity of both hardwood and softwood lignins irrespective of their differences in origin and method of isolation from the plant: 94% of the glycol lignin is solubilized by this procedure while the solubilization yield for kraft lignin is 72%; fraction 1 represents 26% of the glycol lignin but is almost nonexistent in kraft lignin (3%); the methanolsoluble fraction (Fr-2) is the major fraction in both lignins but is more important in kraft lignin (63% versus 40% respectively); the solvent mixture allows isolation of a large fraction (Fr-3) from glycol lignin (28%) while the effect of this high hydrogen-bonding solution is poor on kraft lignin (5%). HPSEC. Following fractionation, both glycol and kraft lignins yield product fractions of increasing molecular weight, as shown in Table 111. It is important to mention here that since the molecular weights discussed throughout this work were obtained by using monodisperse polystyrene standards for calibration, these values should only be considered as relative, not absolute. Thus, molecular weights throughout this work are "apparent molecular weights". In each case, Fr-4 was found to have the highest apparent molecular weight with some material excluded (MW > 20000) in the column set used. Other differences noted in the molecular weight (polystyrene-equivalent) distribution patterns shown in Figures 2 and 3 of the fractions from the two lignins may be stated as follows. (1) Both starting lignins exhibit a bimodal distribution pattern with kraft lignin and fractions having higher apparent molecular weights than glycol lignin and corresponding fractions. (2) Ethyl acetate extracts a significant amount of low molecular weight material from glycol lignin, which has the lowest apparent weight-average = 1155 g/mol and polydispersity molecular weight of M,,,
Table 111. Fractionation Yields, Molecular Weight Averages (Polystyrene Standards), and Polydispersity of Original Lignins and Fractions" glycol kraft fraction yieldb M, M. d yieldb M, Mm d 1 960 2.6 1.6 3 2 527 26 1155 742 2 1807 846 2.1 40 2 788 1368 2.0 63 3 314 1488 2.3 3 28 10 959 2038 5.4 5 15954 2239 7.1 4 6 17 429 2406 7.2 29 4612 1092 4.2 100 3 874 1210 3.2 100 original lignin
" M, = apparent weight-average molecular weight; an= apparent number-average molecular weight; d = polydispersity (MW/Mn).Yields were determined on 3-g samples; losses, calculated by difference, amounted to 0.8% and 0.9% (initial sample) for glycol and kraft lignins, respectively.
Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991 235 MOLECULAR WEIGHT (Polystyrene Standards)
*
2 10
3 IO
4
I
Table IV. Assignments of Absorption Bands i n the I n f r a r e d S p e c t r a of Lignins a n d T h e i r Fractions' wavelength, band cm-' assignment a 3450-3420 0-H stretching (H-bonded) b 3000-2840 C-H stretching in methyl and methylene groups C 1720-1710 carbonyl stretching-unconjugated ketone and carbonyl groups d 1675-1660 carbonyl stretching-aryl ketone 1595 e aromatic skeletal vibrations; intensity proportional to aromatic C-0 stretching mode f 1515 aromatic skeletal vibrations coupled with C-H in-plane deformations C-H deformations (asymmetric) in methyl, 1460 g methylene, and methoxyl groups 1425 aromatic skeletal vibrations coupled with C-H h in-plane deformations; affected by nature of ring substituents i 1370 (sh) C-H deformation (symmetric) syringyl ring breathing with C-0 stretching 1330 j k guaiacyl ring breathing with C-0 stretching 1270 syringyl and guaiacyl ring breathing with C-0 1 1220 stretching m 1150 aromatic C-H in-plane deformation, guaiacyl type n aromatic C-H in-plane deformation, syringyl 1120 type aromatic C-H in-plane deformation, guaiacyl 1030 0 type, and C-H deformation, primary alcohol 970 =C-H out-of-plane deformation (trans.) P aromatic C-H out-of-plane deformation 920 q r same 880 (sh) same S 860-820 ~
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RETENTION TIME ( M I N )
Figure 2. HPSEC chromatograms of acetylated glycol lignin and fractions (calibration based on polystyrene standards). UV absorbance scale is not normalized. It is expanded in the high and low molecular weight regions to facilitate viewing of the separation.
4 10
MOLECULAR WEIGHT (Polystyrene Standards) 3
10
2
10
~~
~
"Based on the works of Sarkanen et al. (1967), Hergert (1971), and Winston (1987).
10
12
14
16
RETENTION TIME (MIN)
Figure 3. HPSEC chromatograms of acetylated water-extracted kraft lignin and fractions (calibration based on polystyrene standards). UV absorbance scale has not been normalized. It is expanded for Fr-3 to enhance its monodisperse character.
of 1.6 when compared to the other fractions. This is not the case for kraft lignin where Fr-1 accounts for only 3% of the initial lignin and in fact could be assimilated to Fr-2 since their molecular weight distribution patterns and infrared spectra are similar. (3) Fr-3 from kraft lignin shows a surprisingly very narrow molecular weight distribution profile. However, the yield of this fraction only accounts for 5 % of the initial lignin extracted. (4) Fr-3 and Fr-4 for glycol lignin have similar MWD profiles, but the solvent mixture used to extract the former exhibits a greater solvating power for high molecular weight material than either solvent alone (see yields above). In the case of kraft lignin, this mixture is not as selective for fractionating the high molecular weight component recovered as Fr-4. Also, polydispersity (it?,,./it?,,) increases with molecular weight in the fractions from glycol lignin from a low of 1.6 (Fr-1) to 7.2 (Fr-4). A similar trend is observed for kraft lignin fractions (except Fr-1). Fr-1 and Fr-2 in both lignins can be considered as being dominated by low molecular weight material, whilst Fr-3 and Fr-4 consist essentially of lignin from the high molecular weight part of the starting material.
FTIR. (i) Glycol Lignin and Fractions. The spectra of glycol lignin and fractions are shown in Figure 4. Assignments of the absorption bands, listed in Table IV, are based on information obtained from other works (Sarkanen et ai., 1967; Hergert, 1971; Winston, 1987) on characterization of lignins by infrared spectroscopy. Fr-1 appears to be chemically different from the original lignin and the other three fractions, which are almost identical. The maximum absorbance for 0-H stretching shifts from 3310 cm-' for Fr-1 to 3410 cm-' for fraction 4. This may be interpreted that Fr-1 is richer in phenolic hydroxyl groups while the aliphatic 0-H group stretching is the major vibration in the other fractions. The band at 2755 cm-', attributed to C-H stretching of the aldehyde group, is almost absent in Fr-1. Moreover, the absence of bands in the 820-cm-' region, due to C-H deformation of CHO, confirms the low aldehyde content in this fraction. The 1330-cm-' and 1120-cm-' bands, both associated with the syringyl unit (ring breathing with C-0 stretching and C-H in-plane deformation), are smaller in Fr-1 than in the other fractions. Consequently, this fraction can be described as rich in guaiacyl units. (ii) Kraft Lignin and Fractions. As observed in Figure 5, there are no profound differences between the spectra of the original lignin sample and extracted fractions. This suggests the absence of significant differences in functional group composition between kraft lignin and its fractions. In particular, the similarity between the spectra of Fr-1 and Fr-2 indicates these are chemically identical. The similarity in molecular weight distribution patterns between these two fractions (see below) reinforces this assumption. One difference is the 1120-cm-' band associated with aromatic C-H in-plane deformation of the syringyl type
236 Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991 n
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4000. 0
9400. 0
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2200. 0
1000. 0
WAVENUMBERS
1800. 0
1300. 0
1000. 0
eon.
00
CM-'
Figure 4. Infrared spectra of glycol lignin and fractions.
is well resolved from the 1150-cm-' band related to the guaiacyl-type deformation in Fr-1 and Fr-2 but disappears in Fr-3 and Fr-4. This observation suggests a richer composition in syringyl units in the first two fractions. Small variations in the relative intensities of the 860-cm-' and 820-cm-l bands related to the C-H out-of-plane deformation of substituted aromatics may be explained in the same manner. '3c NMR Spectroscopy. Comparison by '3c NMR was only carried out for glycol lignin and its fractions. Assignments of important signals for glycol lignin were previously presented (Thring et al., 1990a). In this work, we extend the assignment of signals to include others that exhibit noticeable differences in their intensity when comparing the spectra of glycol lignin and its fractions, as shown in Figure 6. The assignments, listed in Table V, are again based on previous works by other workers (Lapierre et al., 1982; Kringstad and Morck, 1983; Morck and Kringstad, 1985; Robert, 1982; Marchessault et al., 1982). OH Region. Signals 1,2, and 3, attributed to primary, secondary, and phenolic acetyl carbonyl carbons respectively, yield information on the frequency of the different hydroxyl groups in lignin. A coarse estimation of the relative abundance of these groups is possible in routine NMR analysis from the intensities of the signals because of the similarity in spin-lattice relaxation times of the
various acetyl carbonyl carbon atoms (Morck and Kringstad, 1985). As observed in Figure 6, the content of aliphatic and phenolic hydroxyl groups thus appears to be similar in the original glycol lignin and Fr-2, with the primary hydroxyl groups slightly dominating in both cases. In the spectrum of Fr-1, the high intensity of signal 3 relative to signal 1 indicates that the high total hydroxyl group content found analytically in this fraction and shown in Table I1 is mainly due to aromatic hydroxyl groups. As seen in the spectra for Fr-3 and Fr-4, aliphatic hydroxyl groups are dominant in both of these fractions. All the spectra indicate that the aliphatic hydroxyls consist of essentially primary hydroxyl goups, with much less secondary hydroxyl groups. In this region, signals from carbons other than those attributed to acetyl carbons from phenolic groups are observed in the lower molecular weight fractions, especially in Fr-2. Assignments for these signals, along with signal 13, wer not found in the open literature or even guessed a t in this work due to our uncertainty in their source. Aromatic Region. The monomeric composition of glycol lignin and its fractions may be estimated by determining the syringyl/guaiacyl ratio. According to Lapierre et al. (1985), the quantitative use of routine 13C NMR spectra may be a suitable method to determine the monomeric composition of a hardwood lignin sample, as
Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991 237 a
fI
WAVE NUM B E RS Figure 5. Infrared spectra of water-extracted kraft lignin and fractions.
long as the signals used are those of homologous syringyl and guaiacyl carbons. Integration of the areas under the signals at 103.8 ppm (syringyl C2-C6) and 111.8and 119.8 ppm (guaiacyl C2, C6 respectively) thus gave the results shown in Table VI. As seen, these results are generally in concordance with the variation in the methoxyl group content determined analytically, if methoxyl group content is taken to be an indicator of syringyl content in lignin, and the reproducibility of the data is taken into account. The values of both methoxyl content and S/G ratio in Fr-1 reaffirm that indeed this fraction is guaiacyl rich, a conclusion previously reached from analysis of the FTIR data above. Furthermore, the intensity of signal 5 (C-4 in etherified guaiacyl) is highest in Fr-1, demonstrating the high guaiacyl content in this fraction yet again. Aliphatic Region. (i) At 82-49 ppm. Signals in this region are attributed to oxygenated and non-oxygenated intermonomeric linkages in lignin. The most striking difference is the low intensity of these signals (except signal 25) in the spectrum for Fr-1 as compared to the other spectra. This lack of interunit bonds indicates that Fr-1 consists of essentially low molecular weight material. The high intensity of signal 25, attribute to a-and 6-carbons in dilignol, suggests these low molecular weight structures (most likely dimers, trimers, and tetramers) are mostly linked by C-C linkages.
k
C M"
The intensity of signal 24 due 7-C in 0-0-4 with a-CO, cinnamyl alcohols, is lowest in Fr-1. This may mean that Fr-1 contains structures that lack terminal hydroxyl groups in the propane side chain. These hydroxyl groups have probably been removed by a water-elimination reaction and/or cleavage between 0- and a-carbons (Sudo et al., 1985) during our solvolysis process. Indeed, the low intensity of signal 1 (aliphatic hydroxyl groups) as well as signals 20 and 23 (0 and a interunits carbons) confirms this hypothesis. Signals 19 and 22, assigned to a-C and 0-C in 6-0-4 units, respectively, are of comparably low intensity in all the spectra, suggesting this type of interlinkage is present in both glycol lignin and its fractions. (ii) At 34-14 ppm. In this region, the strong signal at 20.6 ppm, corresponding to C atoms of the CH3 group in acetyl, is similar in intensity in all the spectra. A distinct difference between the spectra is the high abundance and intensity of signals in this region in Fr-1 and their virtual absence in the higher molecular weight fractions, particularly in Fr-3 and Fr-4. These signals are present but much weaker in the spectrum of the original lignin. The strong signal at 29.2 ppm (signal 30),attributed to C atoms of the C-4 and C-5 CH3 groups in n-alkyl moieties or to the C-2 CH2 groups of a CH3CH2CH(CH3)Cgroup, is highest in intensity in Fr-1 and becomes progressively weaker with increase in number of fraction. The weaker
238 Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991
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Table V. Assignments of Signals in the NMR Spectra of Acetylated Glycol Lignin and Fractions' chem shift, signal ppm assignments 1 170.2 primary alcoholic acetate (CO) 2 169.6 secondary alcoholic acetate (CO) 3 168.3 phenolic acetate (CO) 4 152.6 C-3/C-5: syringyl, etherified 5 151.8 C-4: guaiacyl; etherified 6 149.9 C-3: guaiacyl; etherified 7 137.4 C-1: syringyl; 0-0structures 8 136.3 C-4: syringyl; etherified 9 134.5 (2-1: guaiacyl, syringyl; etherified 10 130.9 C-2/C-6 p-coumaric acid ester 11 129.1 n-C/@-C: ArCH=CHCH,OH 12 128.4 C-2/C-6: p-hydrophenyl. CH: olefinic and aromatic 13 125.5 14 124.1 C-1: p-coumaric acid ester 15 122.8 C-6: guaiacyl, ferulic acid ether 16 119.3 C-6: guaiacyl; etherified 17 111.9 C-2: guaiacyl: etherified 18 103.9 C-2/C-6 syringyl; etherified or not 19 85.2 8-C in 8-0-4 units; syringyl 20 81.2 8-C in 8-0-4 units; guaiacyl 21 74.3 C-3 in xylan 22 71.5 n-C in 6-0-4 units; syringyl and guaiacyl 23 67.4 y-C in 0-C-5 24 63.3 y-C in 0-0-4 with a-CO;cinnamyl alcohols 25 62.1 y-C and 0-C in 0-1 dilignol; y-C in phenylcoumaran 26 55.9 OCHs syringyl and guaiacyl 27 49.3 C-1: 0-C in 0-5 28 33.5 CH2, CH3 groups 29 31.5 same 30 29.2 same 31 24.6 same 32 20.6 CH3 (acetyl) 33 18.7 CH2 34 14.1 CH2
'Based on the works of Lapierre e t al. (1982), Kringstad and Morck (1983), Morck and Kringstad (1985), Robert (1982), and Marchessault et al. (1982). Signal 13 was not assigned. Table VI. Estimates of Syringyl/Guaiacyl Ratio in Glycol Lignin and Its Fractions from lsC NMR Spectroscopy fraction svringvl/euaiacvl ratio Fr-1 1.0 Fr-2 1.6 Fr-3 1.9 Fr-4 1.7 original lignin 1.5
signals a t 33.5-31.5 and 24.6 ppm, corresponding to C-2 CH2groups in n-alkyl moieties, as well as signals at 18.7-14 ppm (CH,in n-alkyl chains), also follow a similar trend. All these signals are present in the spectrum of the original glycol lignin but are much weaker in intensity. I t may be concluded then that glycol lignin contains a substantial amount (26% ) of material containing saturated aliphatic residues, due in some part to contamination with fatty acids from extractives, which may be selectively removed by extraction with ethyl acetate. The importance of the fatty acids in Fr-1 was estimated by an ether extraction in a Soxhlet overnight. The procedure, which removed approximately 115% material, may have dissolved and fractionated some of the lignin components. It should be noted that the initial aspen wood used to isolate the glycol lignin was not free of extractives. Conclusions Fractionation of medium quantities of a solvolytic glycol lignin and water-washed kraft Indulin A T into four frac-
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I 40
I
20
PPM
Figure 6. 13C NMR spectra of acetylated glycol lignin and fractions: solvent, DMSO-d6.
tions by successive extraction with organic solvents of increasing hydrogen-bonding capacity has been achieved. In general, the yields of fractions from glycol lignin were found to be of the same order of magnitude, whereas there is a vast difference in the yields of fractions from kraft lignin. For the same solvent, the magnitude of the yields is very different for each lignin, indicative of the differences not only in the source but in the method of isolation between glycol and kraft lignin. A comparison of the first and last fractions from each lignin shows that glycol lignin has more low molecular weight material soluble in ethyl acetate. This is not the case from kraft lignin, which is found to contain a higher proportion of high molecular weight material. The variations in elemental composition are indicative of the differences existing between the two lignins due to their source and method of isolation from the wood. In addition, both lignins and their fractions were characterized by functional group analysis (FTIR)and mo-
Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991 239 lecular weight distribution (HPSEC). Fourier transform infrared spectroscopy gives a first indication that Fr-1 of glycol lignin is chemically different from Fr-2 to Fr-4. In particular, the spectra suggest that fraction 1 is richer in phenolic hydroxyl groups and guaiacyl groups but poorer in the aldehyde function than the other fractions. However, the same conclusions cannot be reached for the corresponding kraft lignin fractions. High-performance size-exclusion chromatography shows that the fractionating sequence with solvents gives fractions of increasing molecular weight. The molecular weight distribution patterns show that the solvents used are more selective for fractionating glycol lignin than kraft lignin. Fr-3 from kraft lignin has a narrow molecular weight distribution pattern but accounts for only 5 % of the starting material. Further characterization of glycol lignin and its fractions was made by determination of methoxyl and total hydroxyl group content as well as by 13CNMR spectroscopy. The results not only complemented each other but confirmed the chemical differences existing between Fr-1 and the other fractions. The data suggested that the low molecular weight fraction (Fr-1) is the richest in guaiacyl units, whilst Fr-3 has the highest syringyl character. Residues containing saturated aliphatic side-chain structures partially from extractives were selectively extracted with ethyl acetate from glycol lignin to give the low molecular weight fraction. The presence of the xylan peak due to carbohydrate contamination appears in all the NMR spectra of the high molecular weight fractions. The results demonstrate the heterogeneity of both softwood and hardwood lignins in terms of functional groups, chemical structure, and molecular weight distribution. Fractionation by successive solvent extraction revealed that glycol lignin contains a high portion of low molecular weight material rich in guaiacyl units and aromatic hydroxyl groups and a medium molecular weight fraction rich in syringyl units. Finally, we may conclude that in light of the data presented lignins are heterogeneous whether isolated on a semitechnical or technical basis by kraft or solvolytic procedures.
Acknowledgment We thank T. S. Nguyen, M. Trottier, Jean-Pierre Lemmonier, and Jacques Bureau for their technical assistance. Thanks are also expressed to Dr. John Ripmeester (National Research Council of Canada, Ottawa, Canada) for recording the 13CNMR spectra. Financial assistance by FCAR, NSERC, and EMR (Canada) is deeply appreciated. Registry No. Glycol lignin, 129832-89-9; Kraft Indulin AT, 8068-05-1.
Literature Cited Barton, A. F. M. Practical Liquid Solubility Scales. Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press Inc.: Boca Raton, FL, 1983;Chapter 8,pp 139-190. Chornet, E.; Vanasse, C.; Overend, R. P. Liquefaction Fractionge de la Biomasse en Milieu Solvolytique par le Procedi5 UdeS-S. Entropic 1986,No. 130/131,89-97. Fengel, D.; Wegener, G. Lignin. Wood: Chemistry, Ultrastructure, Reactiona; Walter de Gruyter and Co.: Berlin, West Germany, 1983;Chapter 6, pp 132-181. Fergus, B. J.; Goring, D. A. I. The Location of Guaiacyl and Syringyl Lignins in Birch Xylem Tissue. Holzforschung 1970,24,113-117. Haluk, J. P.; Metche, M. Charact6risation Chimique et Spectrographique de la Lignine par Acidolyse et Ozonolyse. Cell. Chem. Technol. 1986,20,31-50. Hatakeyama, H.; Iwashita, K.; Meshitsuka, G.; Nakano, J. Effect of Molecular Weight on Glass Transition Temperature of Lignin.
Mokuzai Gakkaishi 1975,21 (ll),618-623. Hergert, H. L. Infrared Spectra. In Lignins, Occurrence, Formation, Structure, and Reactions; Sarkanen, K. V., Ludwig, C. H., as.; Wiley-Interscience: New York, 1971;Chapter 7,pp 267-293. Kringstad, K. P.; Morck, R. laC-NMR Spectra of Kraft Lignins. Holzforschung 1983,37 (5), 237-244. Kringstad, K.P.; Mansson, P.; Morck, R. Changes in the Molecular Weight Distribution of Kraft Lignins Resulting from Various Chemical Treatments. International Symposium on Wood and Pulping Chemistry-the Eckman Days, 1981;Vol. V, pp 91-93. Lapierre, C.; Lallemand, J. Y.; Monties, B. Evidence of Poplar Lignin Heterogeneity by Combination of 13C NMR and 'H NMR Spectroscopy. Holzforschung 1982,36, 275-282. Lapierre, C.; Monties, B.; Guittet, E.; Lallemand, J. Y. The Quantitative Measurements in Hardwood Lignin 13C NMR Spectra. Holzforschung 1985,39 (6),367-368. Lin, S. Y.; Detroit, W. J. Chemical Heterogeneity of Technical Lignins-its Significance in Lignin Utilization. International Symposium on Wood and Pulping Chemistry-the Eckman Days, 1981;Vol. IV, pp 44-50. Marchessault, R. H.; Coulombe, S.; Morikawa, H. Characterization of Aspen Exploded Wood Lignin. Can. J. Chem. 1982, 60, 2372-2382. Morck, R.; Kringstad, K. P. 13C-NMRSpectra of Kraft Lignins. 11. Kraft Lignin Acetates. Holzforschung 1985,39 (2),109-119. Morck, R.;Yoshida, H.; Kringstad, K. P. Fractionation of Kraft Lignin by Successive Extraction with Organic Solvents. 1. Functional Groups, 13C NMR and Molecular Weight Distributions. Holzforschung 1986,40 (Suppl.), 51-60. Morck, R.; Reimann, A.; Kringstad, K. P. Fractionation of Kraft Lignin by Successive Extraction with Organic Solvents. 111. Fractionation of Kraft Lignin from Birch. Holzforschung 1988, 42 (21,111-116. Musha, Y.; Goring, D. A. I. Distribution of Syringyl and Guaiacyl Moieties in Hardwoods as Indicated by Ultraviolet Microscopy. Wood Sci. Technol. 1975,9, 45-58. Robert, D. Quantitative Structural Analysis of Lignins by lacNMR Analysis. Canadian Wood Chemistry Symposium (CIC) Proceedings, Niagara Falls, Ontario, Canada, 1982;pp 63-66. Sarkanen, K.; Chang, H. M.; Allan, G. Species Variation in Lignins. 111. Hardwood Lignins. TAPPI 1967,50 (12),587-590. Schuerch, C. The Solvent Properties of Liquids and Their Relation to Solubility, Swelling, Isolation and Fractionation of Lignin. J. Am. Chem. SOC. 1952,74,5061-5067. Solar, R.; Kacik, F.; Melcer, I. Simple Semimicro Method for the Determination of 0-acetyl Groups in Wood and Related Materials. Nordic Pulp Paper Res. J. 1987,No. 4, 139-141. Sudo, K.; Shimizu, K.; Sakurai, K. Characterization of Steamed Wood Lignin from Beech Wood. Holzforschung 1985,39 (9, 281-288. Thring, R. W.; Chornet, E.; Overend, R. P.; Heitz, M. Production and Hydrolytic Depolymerization of Ethylene Glycol Lignin. In Lignin: Properties and Materials; Glasser, W. G., Sarkanen, K. V., Eds.; ACS Symposium Series 397;American Chemical Society: Washington, DC, 1989a;pp 228-244. Thring, R. W.; Chornet, E.; Vidal, P. F.; Bouchard, J.; Overend, R. P. Solvent Fractionation of Ethylene Glycol Lignin. Proceedings of the International Symposium on Wood and Pulping Chemistry (TAPPI), Raleigh, NC, May 22-24, 1989b;pp 343-345. Thring, R. W.; Chornet, E.; Bouchard, J.; Vidal, P. F.; Overend, R. P. Characterization of Lignin Residues Derived from the Alkaline Hydrolysis of Glycol Lignin. Can. J. Chem. 1990a,68,82-89. Thring, R. W.; Chornet, E.; Overend, R. P. Recovery of a Solvolytic Lignin: Effects of Spent Liquor/Acid Volume Ratio, Acid Concentration, and Temperature. BIOMASS 1990b,in press. Vidal, P. F.; Bouchard, J.; Chornet, E.; Overend, R. P.; Giroux, H.; Lamy, F. The Degradation of Indulin ATR by Streptomyces: Chemical Characterization of the Water Soluble Acid Precipitable Products. Can. J. Chem. 1989,67, 350-355. Wada, S.; Iwamida, T.; Iizima, R.; Yabe, K. The Bonds between Thiolignin and Synthetic Rubber VII. Fractional Precipitation of Thiolignin from Kraft Pulping Waste Liquor with the Variation of pH and Reinforcement of SBR with Each Fraction. Chem. High Polym. Jpn. 1962, 19 699-703.
240
Ind. Eng. Chem. Res. 1991,30, 240-248
Wardrop, A. B. Anatomical Aspects of Lignin Formation in Plants. The Eckman Days 1981. Int. Symp. Wood Pulp. Chem., Stockholm, Sweden, 1981; Vol. 1, pp 44-51. Wend, H. F. J. Anatomy and Physical Properties of Wood. The Chemical Technology of Wood;Academic Press, Inc.: New York, 1970; Chapter 2, pp 32-88. Winston, M. H. Characterizationof the Lignin Residue from Hy-
drolysis of Sweetgum Wood with Superconcentrated Hydrochloric Acid. Ph.D. Thesis, North Carolina State University, Raleigh, 1987, 113 pages.
Received for review January 5, 1990 Revised manuscript received June 30, 1990 Accepted July 17, 1990
An Equation of State for Electrolyte Solutions. 3. Aqueous Solutions Containing Multiple Salts Gang Jint and Marc D. Donohue* Department of Chemical Engineering, The Johns Hopkins Unioersity, Baltimore, Maryland 21218
An equation of state for mixtures containing electrolytes was derived by taking moleculemolecule interactions, charge-charge interactions, and charge-molecule interactions into account by using perturbation expansions based on their potentials. Previous calculations for a large number of aqueous solutions containing single strong or volatile weak electrolytes demonstrated the potential of this equation of state. In this paper, we give a new physical description of this model and extend the work to aqueous systems containing multiple salts. The only adjustable parameter in the model is related to the ionic size. In our previous papers, we had one parameter for each electrolyte (i.e., each ion pair). In this work, we have used a single adjustable parameter for each ion. These parameters were determined from mean ionic activity coefficient data for a large number of single-salt aqueous systems. The values then are used in the calculations for multisalt systems without any additional adjustable binary or ternary parameters. Preliminary calculations are presented for several double-salt aqueous solutions and one triple-salt system, CaS04-MgC12-NaC1-H20. Our calculations are compared with experimental data and with calculations made with other models, which contain a number of adjustable parameters. This equation of state shows good agreement with experimental data over wide ranges of temperature and composition.
I. Introduction There is widespread interest in the solubility of calcium sulfate and its hydrates (CaS04,CaS04-H20,CaS04.2H20, and CaS04.4H20)in solutions of sodium chloride (NaCl) and/or magnesium chloride (MgClJ. Geologists and geochemists are interested because of gypsum and anhydrite conversion that occurs in nature (Bock, 1961; Zen, 1965). Engineers involved in the production of petroleum and in water desalination are interested because of deposition of calcium sulfate scales. These scales also are a problem in the operation of both boilers and cooling towers (Denman, 1961). Much experimentalwork has been required in order to understand the complex solubility behavior of calcium sulfate in water and in electrolyte solutions (Marshall et al., 1964; Power et al., 1964; Ostroff and Metler, 1966; Yeatts and Marshall, 1969). However, these experiments are costly and time consuming. It is desirable to have a model that could give reliable predictions for such systems (Gering and Lee, 1989). In contrast to the extensive amount of modeling given in the literature for aqueous solutions containing single electrolytes, only limited reports have been made on multisalt systems. Generally, models used for multisalt systems are built upon the calculation of activity coefficients of single salt electrolyte solutions together with the application of certain "mixing rules", All these equations are empirical or semiempirical and contain a number of adjustable binary and ternary parameters. For example, the models developed by Guggenheim (1967), Bromley (19731, and Meissner and Kusik (1972) can be applied to *Author to whom correspondence should be addressed. Current address: Benger Laboratory, Dupont Company, Waynesboro, VA 22980.
multisalt systems with one adjustable parameter per ion pair. Pitzer's model (1973,1974, 1979) uses three to four parameters per ion pair, one parameter for each like charged ion pair and one ternary interaction parameter. Chen (1980) developed a model that contains two parameters per water-salt pair and two parameters for each salt-salt pair. In all these models, the user must regress experimental data to determine the values of the parameters in the equations. Recently, a model for electrolyte solutions has been developed by using perturbation theory (Jin and Donohue, 1988a,b; Jin, 1989). The only adjustable parameters used in the model are for the sizes of the ions. By use of an equation of state from this model, predictions of thermodynamic properties for a large number of aqueous solutions containing single strong or weak electrolytes were made. In previous calculations, one parameter was used for each electrolyte (ion pair). This introduced an inconsistency when using the model for multisalt calculations because, for example, a chloride ion would have two different sizes in a mixture containing sodium chloride and potassium chloride. Here, we refit experimental data to obtain radii for individual ions and then apply the equation of state to aqueous solutions containing multiple salts. In the following sections, we will give a new physical description of this model and then discuss the use of multiple sets of single salt data to determine the ionic radii. Finally, we present calculations of thermodynamic properties of multiple-salt aqueous solutions. 11. Physical Description of the Model While there are hundreds of different models for nonelectrolyte systems, there are relatively few for mixtures containing electrolytes because little is known about the structure of electrolyte solutions. Some of the questions 0 1991 American Chemical Society
