Bayer Poisons: Degradation of Klason Lignin in Sodium Hydroxide at

Although plant components are modified during extraction, they can be useful in demonstrating the role of different types of structural bonds during d...
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Ind. Eng. Chem. Res. 2002, 41, 6493-6502

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Bayer Poisons: Degradation of Klason Lignin in Sodium Hydroxide at 145 °C Amanda V. Ellis,† Michael A. Wilson,*,‡ and Peter Forster§ Department of Chemistry, Materials and Forensic Science, University of Technology, Sydney, P.O. Box 123, Broadway NSW 2007, Australia, Deans Unit, College of Science, Technology and Environment, University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW 1797, Australia, and Technology Delivery Group, Alcoa World Alumina, Kwinana Alumina Refinery, P.O Box 161 Kwinana, Western Australia 6167, Australia

During bauxite refining, roots are dissolved in sodium hydroxide with the bauxite. This material is largely responsible for the poisoning activity of organic matter in the Bayer process. Although plant components are modified during extraction, they can be useful in demonstrating the role of different types of structural bonds during dissolution under bauxite refining. In this work, three Klason lignins have been prepared from Callitris rhomboidea plants (a gymnosperm) and the roots of Corymbia calophylla and Eucalyptus marginata (both angiosperms), and the dissolution of these materials in 3.5 M sodium hydroxide at 145 °C has been studied to understand β-O-4 aromatic carbon cleavage from syringyl and guaiacyl groups. The dissolution rates fall in the order Corymbia calophylla > Eucalyptus marginata > Callitris rhomboidea, which is the same order as their syringyl contents. By means of carbon balances and solid-state NMR spectroscopy, it has been established that, for Callitris rhomboidea, aromatic carbon is rapidly hydroxylated on initial dissolution and converted to other carbon types. This differs from the behavior of angiosperm lignins, where reactions of aromatic carbons are much slower. Introduction The precipitation of aluminum trihydroxide in the Bayer process for the industrial-scale production of alumina from bauxite by extraction and precipitation from sodium hydroxide is adversely affected by soluble organic contaminants from the bauxite.1 In the process, insoluble organic matter is removed in the red mud, while soluble organic degradation products accumulate on recycling of the sodium hydroxide.2 Established roles of these degradation products include suppression of precipitation yields, incorporation of sodium ions, excessive liquor foaming, evolution of odors, increased liquor viscosity and density, and effects on alumina crystallization and agglomeration. Although much is known about the lower-molecular-weight degradation products,1,3 the structure of the larger (>2000 Da) species in solution is not well understood.4,5 A logical approach to the understanding of the composition of organics in the Bayer process is to study the decomposition of plant remains present in bauxite so that the role of each plant component can be elucidated and the pathways of decomposition determined. This is the approach used here. One important component is the residual root systems of macromolecular vascular plants (0.02-0.03% wt/wt) that have passed through the crushing and sieving pretreatment with the mined bauxite. This woody material consists of up to 33% lignin.6 In elucidating the role of different organic components in the wood, it is desirable to separate lignin from * Corresponding author. Address: Deans Unit, College of Science, Technology and Environment, University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW 1797, Australia. Phone: 61-(0)2-4570-1210. Fax: 61-(0)2-4570-1403. E-mail: [email protected]. † University of Technology. ‡ University of Western Sydney. § Alcoa World Alumina.

carbohydrate.7-12 The isolation of lignin from carbohydrate is not trivial. All extraction processes modify lignin chemically13-23 because it forms cross-links with hemicellulose through ether and ester linkages,9 and these cross-links must be broken. Hemicellulose is then bound via hydrogen bonding to cellulose. One method of removing cellulose and hemicellulose is to pretreat the wood with sulfuric acid, with subsequent washing. This leaves behind material termed Klason lignin. Klason lignin is found to be the least contaminated in terms of cellulose and hemicellulose23,24 and provides the most accurate quantitative estimate of the total lignin content of the wood as compared to any other isolation process.23-25 There is also a good correlation between Klason lignin measured as yield and lignin content determined by NMR spectroscopy in wood.17,26 In removing cellulose,17,23,26,27 some cross aryl reactions occur mainly at the 6-position in the aromatic ring but with some at the 5-position.23 Klason lignin retains the relevant β-O-4 linkages and other linkages between monomeric species17,29 and methoxy carbon26-28 found in in situ lignin, and as such, it is of interest to understand the breakdown of these units and linkages under Bayer conditions. This is also the material that would be left in any pretreatment sulfuric acid process for removing poisonous carbohydrates prior to dissolution. Such a process is expensive but has been given serious consideration if implemented in a fractionation process. In the other possible lignin isolation processes, namely, ball milling, dioxane acidolysis, and enzymatic treatment, the removal of carbohydrates is not effective,23,34 and periodate treatment causes extensive oxidative.23 Hence, in this work, we have chosen Klason lignin both because it has potentially interesting structures and because studies of its dissolution might have a practical value in real situations.

10.1021/ie010896w CCC: $22.00 © 2002 American Chemical Society Published on Web 11/15/2002

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Table 1. Elemental, Ash, and Yield Data for Klason Lignins % wt/wt genus and species

yield

stda

Callitris rhomboidea Eucalyptus marginata Corymbia calophylla

23.40 29.40 22.60

(2.60 (1.60 (1.00

a

C

H

N

58.79 58.01 60.83

6.05 5.00 5.20

0.94 0.41 0.40

mole ratios ash

Ob

O/C

H/C

N/C

0.88 0.42 0.10

33.34 36.16 33.47

0.43 0.47 0.41

1.24 1.03 1.03

0.014 0.006 0.006

Standard deviation of preparations. b By difference.

In this study, we follow the structure and decomposition of Klason lignin isolated from Callitris rhomboidea (gymnosperm) and Corymbia calophylla and Eucalyptus marginata (angiosperms) under Bayer process conditions, using solid-state 13C cross polarization magicangle spinning (CP/MAS) nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared spectroscopy (FTIRS), and pyrolysis gas chromatography/ mass spectrometry (py-GC/MS). These techniques have proved valuable in understanding the structure of lignin13-21 and, therefore, should be powerful tools in understanding its dissolution. This work complements a study on the carbohydrates present.22 Experimental Section Sample Preparation. Klason lignin was isolated from Callitris rhomboidea (leaves and stems) and Eucalyptus marginata and Corymbia calophylla (roots). The plant material was soaked overnight in distilled water and then washed thoroughly to removed soil. The Callitris rhomboidea material was finely chopped, blended, and freeze-dried at -55 °C and 26.7 Pa. The Eucalyptus marginata and Corymbia calophylla roots were air-dried. Batches (10 g) of each dried plant sample (200 g in total) were then ring-milled in a tungsten carbide ring-mill carried out on an N. V. Tema ringmill shaker type T.100 at 50 Hz. Each batch was run from 1 to 2 min and then the contents sieved to pass a 150-µm2 grid. Removal of Organic and Water Solubles. A sample of 40 g of each ground plant sample was Soxhlet extracted with a sequence of absolute ethanol/toluene mixture (1:2 v/v) for 48 h, absolute ethanol for 48 h, and distilled water for 10 days; in the latter sequence, the water was changed every 3 days. The residue material was then washed with acetone and dried in an oven at 65 °C for 12 h. Isolation of Klason Lignin. A subsample (10.00 g) of each dried extractive-free plant material was ground in a mortar and pestle and digested in 600 mL of 5% w/v sodium hydroxide under nitrogen (to avoid oxygen uptake) with constant stirring for 5 h at 50 °C in a silicon oil bath. While still hot, 43.0 mL of glacial acetic acid was added, and the solution was filtered. The residue was washed with water until neutral.9,35 It was then dried in an oven at 65 °C. The dried residue (2-5 g) was treated with sulfuric acid (72% v/v H2SO4, 40.0 mL) for 12 h at room temperature, after which time 1.5 L of distilled water was added, and the solution was refluxed for 4 h. The Klason lignin was isolated as residue after filtering and washing with distilled water until neutral. It was dried at room temperature under vacuum and weighed. The yields are given in Table 1. Ash and Elemental Analysis. Ash analyses were performed on a TA Instruments SDT (simultaneous differential techniques) 2960 TGA/DTA instrument. Samples (8-10 mg weighed accurately) were heated in an open platinum crucible under an air atmosphere with

a flow rate of 130 mL/min. The furnace was programmed to heat rapidly to 600 °C, then ramp at 30 °C/min to 1000 °C, and hold for 15 min. The furnace was cooled with nitrogen to 30 °C. The carbon, hydrogen, and nitrogen compositions of the samples were determined using a Carlo Erba EA1108 CHNS-O elemental analyzer after being vacuum-dried for 4 h in an oven at 60 °C. The ash and elemental compositions are given in Table 1. Alkaline Digestion of Klason Lignin. The alkaline digestions were carried out in a 25-mL Parr reactor (Parr Instrument Company, Moline, IL), with a flexible graphite (4740) gasket. Klason lignin, 30.0 g/L total organic carbon (TOC), was added to 20 mL of 3.5 M sodium hydroxide, and the bomb was sealed under 5.2 × 105 Pa of nitrogen. The heating was carried out in a silicon oil bath at 145 °C ((3 °C) for different extraction time periods of 0, 1, 5, 24, 50, and 96 h. The time 0 h is designated as the time after which the bomb was added to the bath and the temperature dropped to 120 °C (15 min) plus the time to reheat to 145 °C at 2 °C/min (15 min). After each digestion time the bomb was cooled for 5 min under cold tap water, and the charge was emptied into 300 mL of distilled water. This solution was filtered through GF/D 15.0-cm glass filter paper. The residue was washed with distilled water until the outlet was neutral, dried, and weighed. The filtrate was protonated on a cation-exchange column (Amberlite 120, 60 cm × 2 cm), and the eluant was freeze-dried at -55 °C and 26.7 Pa and then weighed. Both the residue and the protonated alkali soluble fraction were subsequently used for 13C NMR analysis. Fourier Transform Infrared Spectroscopy (FTIRS). Klason lignin (10 mg) was ground with oven-dried potassium bromide (IR grade, Aldrich) and made into a pellet. Infrared spectra were recorded on the pellet under nitrogen on a Nicolet Magna-IR 760 Fourier transform spectrometer using 256 scans with 4 cm-1 resolution. 13C Cross Polarization Magic-Angle Spinning Nuclear Magnetic Resonance (13C CP/MAS NMR) Spectroscopy. Solid-state nuclear magnetic resonance (NMR) spectra were obtained on a Bruker DPX200W Avance 200-MHz instrument operating at 50.3 MHz. Approximately 200 mg of each ground Klason lignin, residue, and alkali soluble fraction was analyzed by using the cross-polarization technique with magic-angle spinning (CP/MAS). A spinning speed of 8 kHz was used. Pulse widths of 4 µs were used, with a 2-s recycle time and a 1-ms contact time, except in T1FH determinations, when the contact time was varied from 1 µs to 20 ms. Spectra were collected at 16K points. Free induction decays (5000) were signal averaged and Fourier transformed with line broadening of 20 Hz to obtain the frequency-domain spectra. The chemical shifts were measured relative to adamantane but are reported relative to tetramethylsilane. For quantitation, the signal intensity, It, was corrected for different T1FH

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Figure 1. Partial lignin structure showing linkage units relevant here. Linkage A is the bond to the 4-carbon in the aromatic ring. Linkage B is the bond between oxygen and the side-chain β-carbon. Linkage C is the bond cross-linking hydroxylated alkyl chains at the R- and γ-positions through C-C bonds at the β-position.

relaxation of different structural groups by extrapolating all exponentials back to contact time of zero from It ) I0 exp(t/T1FH), where t is the contact time. I0 values are used to calculate44 the percentage of structural group i as I0i/ΣI0i. Pyrolysis Gas Chromatography/Mass Spectrometry (py-GC/MS). Py-GC/MS was performed on a Hewlett-Packard 5890 gas chromatograph with a Hewlett-Packard 5970 mass-selective detector. Chromatography was carried out on a fused silica column (30 m × 0.25 mm i.d., 0.25 µm film thickness) coated with DB5MS (modified 5% phenyl, 95% methyl silicone). Accurately weighed samples (0.7-1.0 mg) were introduced as solids by using an injector to the SGE pyrojector and were pyrolyzed at 450 °C. It should be noted that the pyroinjector performance is different from that of Curie point pyrolysers in that the residence time in the pyrolysis chamber zone is different and higher temperatures are inappropriate.36 The temperature of the column was kept at 5 °C for 2 min, then raised at 5 °C/min to 290 °C, and kept at that temperature for 15 min. Helium was used as the carrier gas at 100 kPa. The injection-split ratio was approximately 1:5. Mass spectra were recorded under electron impact at 70 eV. The assignments of compounds from Klason lignin released from the pyrolysis were based on a search of a mass spectral library. Peak areas were used to distinguish differences between lignins. Results and Discussion Lignins. Although there has been some suggestion of order,37,38 lignin is generally believed to be a random C3 aryl polymer. Three linkages are the most important and are labeled A, B, and C in structure 1, Figure 1. Linkage A is the oxygen to 4-carbon of the aromatic ring. This is usually linked to the side-chain β-carbon on another monomer unit (linkage B) and can make up to 80% of the linkages.39 Cross-linking of hydroxylated alkyl chains at the R- and γ-positions through oxygen and β through C-C bonds is also important (Figure 1, linkage C). However, there are other ways these structures can join, and the amounts of these linkages vary

Figure 2. para-Hydroxyphenyl, syringyl, and guaiacyl structures in lignin.

Figure 3. FTIR spectra of Klason lignins (KBr disks): (a) Callitris rhomboidea, (b) Corymbia calophylla, (c) Eucalyptus marginata.

between species. The most important of these other linkages is the one that occurs through the 5-carbon atom on the aromatic ring, either to the β-position or to another 5-carbon.40,41 A dibenzodioxocin linkage has also recently been observed.42 Depending on the species, syringyl, para-hydroxyphenyl, or guaiacyl units (Figure 2) can be the aromatic entities in the aromatic structures in Figure 1 at various degrees of concentration. Figure 3 shows Fourier transform infrared spectra of the lignins. Assignments have been reported elsewhere9,20,29 and are not repeated. Absorptions at 1313,

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Figure 4. 13C CP/MAS NMR spectra of Klason lignins: (a) Callitris rhomboidea, (b) Corymbia calophylla, (c) Eucalyptus marginata.

1217, and 1113 cm-1 are associated with syringyl. The spectra show that Corymbia calophylla (angiosperm) has the greatest intensity at these wavenumbers (Figure 3b), followed by Eucalyptus marginata (angiosperm) and then Callitris rhomboidea (gymnosperm).9,29 In addition, absorptions at around 1505 cm-1 are observed for angiosperms but are shifted toward a higher wavenumber (>1510 cm-1) for gymnosperms. There are also differences due to aliphatic components. Callitris rhomboidea has a larger contribution from aliphatic hydrocarbon, as indicated by the C-H asymmetric stretching at 2932 cm-1 associated with CH2. This can be differentiated from CH3, which absorbs closer to 2960 ( 10 cm-1.43 The Callitris rhomboidea spectrum also has many fine overlapping bands between 1345 and 1180 cm-1 due to the various CH2 waggings. Solid-state NMR spectroscopy is also useful for distinguishing between lignins.13,16,18,20,21,44 Solid-state highresolution 13C NMR spectra of the lignins are shown in Figure 4. Resonances from aromatic carbons are observed at approximately 115, 135, and 150 ppm. All samples show a large peak at 147 ppm, assigned to mainly C-3 and C-4 carbons of guaiacyl units. There is a shoulder at 153 ppm, assigned to C-3 and C-5 carbons of syringyl that have β-O-4 linkages, but some guaiacyl derivatives also resonate in this region. The ratio of the intensities of the 147/153 resonances falls in the order Corymbia calophylla > Eucalyptus marginata > Callitris rhomboidea in agreement with the infrared data, but the difference is small and unconvincing. However, protonated aromatics on syringyl groups concentrate at >115 ppm in the aromatic region, and it is clear from this region of the spectrum that the order for concentration of syringyl groups is Corymbia calophylla > Eucalyptus marginata > Callitris rhomboidea. There are differences in the hydroxy region of the spectra. At 100-50 ppm, the specific resonances are attributed to Cβ (84 ppm), CR-OH (73 ppm), Cγ-OH (60 ppm), and OCH3 (56 ppm).45 Excluding the methoxy signal, the total carbon corresponding to this region was 10.9, 16.8, and 21.3% for Callitris rhomboidea, Eucalyptus marginata, and Corymbia calophylla, respectively. These values are rather low because, for a 1011-carbon monomer, they would be expected to be around 27-30%. Although quantitative accuracy is not claimed, this shows that there are other different bonding structures of the side chains in the different lignins. In Callitris rhomboidea, more of the side chains are linked possibly in C structures (Figure 1, structure 1) and/or R groups in B structures.

Integration of the methoxy peak area against the total aromatic carbon area of each Klason lignin gives an estimate of the relative contributions of methoxy substitution on the aromatic rings as 13.5, 20.8, and 26.3% for Callitris rhomboidea, Corymbia calophylla, and Eucalyptus marginata, respectively. Another difference between the angiosperm and gymnosperm lignins is in the intensity of the peak at 33 ppm in the gymnosperm. This is due to alkyl carbon, especially CH2 in long chains.16,27 This appears to be strongly bound although not normally regarded as a lignin component.16 Callitris rhomboidea also shows a larger contribution from carboxyl carbon at 175 ppm compared with the other lignins. Pyrolysis gas chromatography/mass spectrometry was also instructive. Spectra are displayed in Figure 5, and the percentages of the important compounds as a fraction of total pyrolysate are reported in Table 2. These included phenol, guaiacyl, syringyl and their para, methyl, ethyl, and propenyl derivatives, which can be ascribed to the thermal decomposition of lignin. As expected, phenol (6), 2-methylphenol (7), 3-methylphenol (8), and 1-(2-hydroxy-5-methylphenyl)-ethanone (15), characteristic of phenyl structures, were observed. Likewise, 2-methoxyphenol (9), 2-methoxy-4-methylphenol (11), 4-ethyl-2-methoxyphenol (13) and 4-hydroxy-3-methoxybenzaldehyde (18), characteristic of guaiacol structures were also observed. Peaks 19 and 20 were initially matched to trimethoxybenzenes by the mass spectra library; however, on closer analysis of the fragmentation pattern, trimethoxybenzenes were found to be impossible, as both peaks showed no fragment at m/z 110. Instead, the fragment m/z 107 was seen as being associated with the tropylium ion of benzyl alcohol. Further evidence of this was the loss of H2O as a prominent M-18 peak most noticeably found in primary alcohols.46 Peaks 19 and 20 were then matched to 2,5-dimethoxybenzyl alcohol and 3,5-dimethoxy-2methylbenzyl alcohol, respectively. These species are lignin-derived, specifically from syringyl units in lignin. The compounds 2,6-dimethoxyphenol (16), 1-(2,5-dimethoxyphenyl)-ethanone (22), and 2,6-dimethoxy-4-(2-propenyl)-phenol (23) also derive from syringyl units in the lignin. The ratios of peaks (16 + 19 + 20 + 21 + 22 + 23) to peaks (9 + 11 + 13 + 17 + 18) are indicative of the syringyl-to-guaiacyl ratio content and are 0.286, 0.691, and 2.27 for Callitris rhomboidea, Eucalyptus marginata, and Corymbia calophylla, respectively. These data are in agreement with the infrared and NMR data. The presence of 2,6-dimethoxy-4-(2-propenyl)-phenol (23) represents the remnants of the β-O-4 linkage attached to a syringyl unit after the thermal cleavage of the C-O bond. Corymbia calophylla (angiosperm) shows the largest presence of this linkage (Table 2), whereas it is absent in the Callitris rhomboidea (gymnosperm). The formation of the aldehyde, and ethyl ketone derivatives is also entirely consistent with the cleavage of the β-O-4 bond, followed by elimination involving side-chain units.47 All samples show the presence of minor amounts of furans. These could be derived from C unit linkages (Figure 1). In summary, there are clear differences between the lignins in their syringyl-to-guaiacyl ratio and C3 β-0-4 linkage structures (B linkages, Figure 1). The trend is Corymbia calophylla > Eucalyptus marginata > Callitris rhomboidea. For other linkage types, the trend is Callitris rhomboidea > Eucalyptus marginata > Corym-

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Figure 5. Pyrolysis gas chromatography/mass spectra of lignins at 450 °C using an SGE pyrojector: (top trace) Callitris rhomboidea, (middle trace) Corymbia calophylla, (bottom trace) Eucalyptus marginata.

bia calophylla. This information is useful in the extraction study described below. Extraction Kinetics. Figure 6 shows the lignin remaining from the extraction at 145 °C in 3.5 M NaOH plotted against time of extraction. Cursory examination of Figure 6 shows that Callitris rhomboidea (gymnosperm) dissolves at the slowest rate and Corymbia calophylla at the fastest. For all three lignins, dissolution occurs rapidly over the first 5 h, then levels off after 50 h, but does not fit a simple exponential decay. The data are best fitted to three empirical equations of the form

cA ) a + b exp(-t/x) + c exp(-t/y) where cA is the concentration of residual lignin. For Callitris rhomboidea, the line of best fit is

cA ) 19.54 + 11.68e-t/0.760 + 23.19e-t/19.68 For Corymbia calophylla, the line of best fit is

cA ) 5.817 + 51.01e-t/1.131 + 23.68e-t/8.629

Finally, for Eucalyptus marginata, the line of best fit is

cA ) 4.504 + 23.92e-t/0.710 + 48.57e-t/16.06 The fact that the kinetics are not single-order in lignin and need to be expressed effectively by a series of exponentials suggests that more than one bond is broken during dissolution and that these break at different rates. It also suggests that there are different concentrations of these bonds and/or different bonds are involved for different lignins. Carbon Yields. Table 3 shows elemental data and ash data for the extracts and residues from dissolution of the lignin of Callitris rhomboidea in sodium hydroxide, which was used for carbon yield calculations. Data were also obtained for the other lignins, but only final yields are given for brevity. The ash contents are higher than those in the original lignins because sodium ions are exchanged onto the residues and do not exchange completely on the cation-exchange resins. For all three lignins, the carbon content of the residues decreases with extraction time. Appropriately, the carbon content of the extracts increases. The nitrogen contents decrease in the extracts with time, but this is not reflected as an appropriate concentration in residues. This suggests that ammonium salts are formed during hydrolysis.

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Table 2. Pyrolysis Products from Py-GC/MS Analyses of Callitris rhomboidea (CR), Eucalyptus marginata (EM), and Corymbia calophylla (CC) Klason Lignins peak area (%)b and weight analyzed no.

compound

retention time (min)

m/ea

CR (1.2 mg)

EM (0.7 mg)

CC (0.7 mg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

2-methylfuran 2,5-dimethylfuran toluene 2-furancarboxyaldehyde 5-methyl-2-furancarboxyaldehyde phenol 2-methylphenol 3-methylphenol 2-methoxyphenol 2,4-dimethylphenol 2-methoxy-4-methylphenol 3-methoxy-1,2-benzenediol 4-ethyl-2-methoxyphenol 3-methyl-1,2-benzenediol 1-(2-hydroxy-5-methylphenyl)-ethanone 2,6-dimethoxyphenol 3,4-dimethoxyphenol 4-hydroxy-3-methoxybenzaldehyde 3,5-dimethoxybenzyl alcohol 3,5-dimethoxy-2-methylbenzyl alcohol 3,4-dimethoxy-5-hydroxybenzaldehyde 1-(3,4-dimethoxyphenyl)-ethanone 2,6-dimethoxy-4-(2-propenyl)-phenol hexadecanoic acid oleic acid octadecanoic acid 1-eicosene squalene

4.22 6.08 7.46 9.51 14.24 15.02 17.39 18.26 18.57 20.53 22.23 24.44 25.02 25.43 26.08 27.27 27.48 28.56 30.04 31.27 32.01 33.08 36.32 42.16 45.50 46.14 46.59 58.43

82 96 92 96 110 94 108 108 124 122 138 140 152 124 150 154 154 152 168 182 182 180 194 284 282 284 280 410

0.62 1.14 0.87 0.75 1.52 1.00 2.37 5.95 1.46 8.87 3.18 2.08 4.89 2.90 0.90 0.84 0.75 1.94 1.60 0.84 2.31 0.64 0.61 0.34 -

1.17 1.17 1.48 0.84 1.39 1.02 1.42 10.60 1.02 12.78 7.01 3.61 1.14 2.78 8.01 2.50 1.39 7.69 1.02 2.50 1.37 0.74 -

0.88 0.55 0.65 0.59 0.67 0.91 0.59 1.30 5.00 0.70 3.34 6.77 1.64 1.19 1.23 13.44 2.07 1.32 9.68 1.90 2.05 1.60 1.70 2.26 0.77 0.51 1.12 0.59

a

Molecular ion mass-to-charge ratio. b Peak areas are percentages total peak area.

Figure 6. Amount of Klason lignin remaining (% wt/wt) as a function of time (h) during extraction of lignin at 145 °C with 3.5 M sodium hydroxide.

Such salts would be lost in the soluble fractions upon ion exchange. When weighted back to the original lignin analytical data, the carbon and ash data for the residues and solution are also inconsistent. This inconsistency can be analyzed by examining the yield data. In Table 3, there is almost quantitative weight recovery on initial dissolution (0 h) for Corymbia calophylla; this is also true for Eucalyptus marginata lignins on a weight basis, but the latter result is deceiving. The amount of carbon recovered is much lower. For example, for Eucalyptus marginata lignin, after initial dissolution (0 h), the recovered carbon is 51.43 g from 58.04 g of lignin carbon, which corresponds to 88.6% recovery. Thus, hydroxide ions initially convert some carbon to volatiles.

Structural group contents, aromatic, methoxy, Oalkyl, etc., can be obtained from the NMR data. Ideally, to be quantitative, NMR spectra should be obtained by single-pulse methods, but 13C relaxation times are prohibitive for signal averaging to obtain adequate signal. The CP/MAS technique allows relaxation at the faster proton rate but is semiquantitative. Nevertheless, data can be improved by correcting for spin lattice relaxation in the rotating frame (T1FH). The T1FH values for the original lignins are much higher than those for the residues or extracts. The values for the recovered extractables are particularly low, showing, as expected, that the material is not very rigid.44 The fact that the T1FH values of the residues are considerably less than those of the original lignins (e.g., 12 versus 4 ms) is testament to the fact that the removal of extractables breaks up the rigidity of the lignin structure. Table 4 show the integration NMR data for the residues and extract fractions for Callitris rhomboidea after correction for spin lattice relaxation in the rotating frame. Data were also obtained for the other extracts. In themselves, these data are not particularly useful, except that they show trends in dissolution. However, by combining yield data, it is possible to determine changes in structural group content as the original lignin dissolves. The amount of carbon in any functional group (ΣfC) is given in units of grams per 100 g of original lignin by

ΣfC ) [(%CresiduefC residue%yieldresidue)/100] + [(%CextractfC extract%yieldextract)/100] (1) Here, %Cresidue and %Cextract are the percentages of carbon in each Klason lignin in the residue and extract, respectively. The parameter fC is the fraction of a

Ind. Eng. Chem. Res., Vol. 41, No. 25, 2002 6499 Table 3. Elemental, Ash, and Yield Data (% wt/wt) and Mole Ratios for Callitris rhomboidea Klason Lignin Products as a Function of Extraction Time (h) % wt/wt

extraction time (h)

a

yield

C

H

0 1 5 24 50 96

28.43 42.25 41.15 55.74 56.86 55.20

54.81 56.16 55.75 58.19 59.55 59.23

5.49 5.42 5.31 5.52 5.63 5.73

0 1 5 24 50 96

54.41 44.71 37.65 25.88 22.26 19.22

55.59 56.01 55.61 53.99 55.88 53.73

5.17 4.98 5.00 5.08 5.07 5.02

N

alkali extract 1.52 3.96 0.61 2.02 0.41 N/Db 0.58 0.00 0.46 0.00 0.48 0.00 residue 0.58 Corymbia calophylla. For Corymbia calophylla, there is no loss. These results are confirmed by the change in methoxy content. There is an initial rapid loss for Callitris rhomboidea due to aryl ring opening. For all three lignins, this continues but at a much slower rate, if at all, and aromatic ring decomposition stops. The most interesting feature in Table 5 is the decrease in alkoxy carbon. Callitris rhomboidea, which has the lowest apparent content (6.4%), has a value of only 2.0% after 96 h. Eucalyptus marginata has 9.7% carbon of this type but shows a loss to 4.3% and a faster rate of dissolution. On the other hand, Corymbia calophylla has 13.0% O-alkyl carbon, and this decreases to 6.3%. Most of this loss occurs during the initial dissolution; for Corymbia calophylla, O-alkyl cleavage is almost over after the

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Figure 7. Breakage of β-O-4 linkages.

initial reaction, but for the other lignins, the reaction continues at a slower rate. Mechanism. The breakage of O-alkyl groups, e.g., β-O-4 linkages, Figure 7, is believed to be the ratedetermining step through base attack and removal of protons on the oxygen at the R-carbon.10,49-51 The polymeric and cross-linked nature of lignin induces physical constraints that are supposed to make it less reactive toward attacking species after the initial removal of the β-O-4 linkages. Although our results show that the rate of dissolution is clearly dependent on the syringyl content, the electrondonating ability of a second methoxy group on the aryl ring would destabilize the β-O-4 transition state through the 4-position, decreasing the rate. However, it is the gymnosperm that dissolves slowest, not the angiosperms. A possible explanation is that, because the methoxy group in gymnosperms can be replaced by a di-aryl linkage, the lignin is held together, preventing dissolution. However, it is difficult to see how hydroxide slowly breaks this linkage, and it appears that dissolution requires the β-O linkage to hydrolyze. Perhaps, in the early loss of aromatics, there are internal rearrangements from β-O to R-O, which then hydrolyze more slowly. Such reactions have been documented for model lignin dimers.49 We do not see any accumulation of polyhydroxy compounds, as would be expected by the scheme shown in Figure 7. Rather, further hydrolysis occurs. The production of formaldehyde52 might occur with methylation of the C5-position of phenolic units, eventually creating new diphenyl methane structures.53 This and any carbonate and other volatiles formed by further oxidation accounts for the loss of carbon on workup acidification and evaporation. Other functional groups such as methoxy and carboxyl also decrease. Runge and Ragauskas11 have suggested that the carboxylic acids formed increase on dissolution in 18.8% alkali at 172 °C as a result of the hydrolysis of lignin end-chain muconic acid esters, which can form lactones. They found no loss of methoxy at this temperature until later chlorination. The time of reaction was not given. It appears that our conditions are harsher and any free carboxylic acids formed are decarboxylated, even though our conditions were under nitrogen and not oxygen. Relevance to the Bayer Process. Although the lignin used in this work differs from that in the original wood, there are clear indications of the rates of cleavage

of important bond types. Moreover, the results demonstrate the changes possible on sulfuric acid treated wood. One possible technology is to sieve root-rich bauxite fractions by cyclone techniques and acid treat them specifically. During Bayer processing, it is of importance to note that any insoluble lignin (residue) will be removed with the red mud (insoluble iron, titanium, and silicon oxides) and, hence, dissolution rates are important. Given a residence time of approximately 90 min, dissolution of gymnosperm of the type here is much slower than that of the angiosperms. These results suggest that selective vegetation removal might be useful. It is well-established that many polyhydroxyphenols and oxidation products containing carboxylic acid functionalities can interfere with the precipitation of both alumina and the byproduct sodium oxalate.2,54 The results show that the degree of β-O-4 linkages, and hence the production of these phenols, is plant-specific, indeed Klason lignin type specific. Hence, any technology that can remove specific plants might be of value. Polyalcohol structures are also known poisons.55-57 The aliphatic chain structures could form alcoholic species that are undesirable. It does seem that the conditions are harsh enough to make these not a problem that arises from the lignin component of plant tissue. Conclusions The structures of Klason lignins from three different species have been shown to differ. The syringyl/guaiacyl ratios fall in the order Corymbia calophylla > Eucalyptus marginata > Callitris rhomboidea. Similarly, differences are present in the O-alkyl carbon content, which was 10.9, 16.8, and 21.3% for Callitris rhomboidea, Eucalyptus marginata, and Corymbia calophylla, respectively. The dissolution rates at 145 °C in 3.5 M sodium hydroxide for the three lignins is best described by double-exponential equations. For Callitris rhomboidea, the line of best fit is cA ) 19.54 + 11.68e-t/0.760 + 23.19e-t/19.68, where cA is the concentration of residual lignin. The rate of dissolution falls in the order Corymbia calophylla > Eucalyptus marginata > Callitris rhomboidea and is directly proportional to the syringyl contents of the lignins. The electron-donating ability of a second methoxy group on the aryl ring cannot be responsible for destabilizing the transition state result-

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ing in oxyanion intermediates, and the lignins must dissolve through different mechanisms. By means of carbon balances and solid-state NMR spectroscopy, it was established that, for the gymnosperm lignin, aromatic carbon is rapidly hydroxylated upon initial dissolution and transformed into other carbon types. This differs from the angiosperm lignins, where reactions of aromatic carbons are much slower. For all of the lignins, O-alkyl cleavage of β-O-4 carbon continues more slowly. Small amounts of volatile small molecules are formed through further decomposition of products, probably polyhydroxy compounds. Literature Cited (1) Atkins, P.; Grocott, S. C. Impact of organic impurities on the product of refined alumina. In Proceedings of the Science, Technology and Utilization of Humic Acids; CSIRO Division of Coal and Energy Technology: Sydney, Australia, 1988; p 85. (2) Grocott, S. C.; Rosenberg, S. P. Soda in Alumina. Possible mechanisms for soda incorporation. Proceedings of the First International Alumina Quality Workshop; Queensland, Australia, 1988; p 271. (3) Lever, G. Identification of organics in Bayer liquor. Light Met. 1978, 71. (4) Smeulders, D. E.; Wilson, M. A.; Patney, H.; Armstrong, L. Structure of molecular weight fractions of Bayer humic substances. 2. Pyrolysis behaviour of high-temperature products. Ind. Eng. Chem. Res. 2000, 39, 3631. (5) Wilson, M. A.; Ellis, A. V.; Lee, G. S. H.; Rose, H. R.; Lu, X.; Young, B. R. Structure of molecular weight fractions of Bayer humic substances. 1. Low-temperature products. Ind. Eng. Chem. Res. 1999, 38, 4663. (6) Higuchi, T. Lignin structure and morphological distribution in plant cells walls. In Lignin Biodegradation: Microbiology, Chemistry and potential Applications, 2nd ed.; Kirk, T., Higuchi, T., Chang, H., Eds.; CRC Press: Boca Raton, FL, 1981. (7) De Groot, B.; Van Dam, J. E.; Van’t Riet, K. Alkaline pulping of hemp woody core: Kinetic modelling of lignin, xylan and cellulose extraction and degradation. Holzforschung 1995, 49, 332. (8) Lawther, J. M.; Sun, R.; Banks, W. B. Fractional characterization of alkali-labile lignin and alkali-insoluble lignin from wheat straw. Ind. Crops Prod. 1996, 5, 291. (9) Sun, R. C.; Fang, J. M.; Goodwin, A.; Lawther, J. M.; Bolton, A. J. Physico-chemical and structural characterization of alkali lignins from Abaca fibre. J. Wood Chem. Technol. 1998, 18, 313. (10) McKague, B.; Reeve, D. W. Reaction of a lignin model dimer sequentially with chlorine and sodium hydroxide. Env. Sci. Technol. 1994, 28, 573. (11) Runge, T. M.; Ragauskas, A. J. NMR analysis of oxidative alkaline extraction stage lignins. Holzforschung 1999, 53, 623. (12) Bentivenga, G.; D′Auria, M.; De Bona, A.; Mauriello, G. Singlet oxygen mediated degradation of Klason lignin. Chemosphere 1999, 39, 2409. (13) Bartuska, V. J.; Maciel, G. E. Structural studies of lignin isolation procedures by 13C NMR. Holzforschung 1980, 34, 214. (14) Bates, A. L.; Hatcher, P. G. Quantitative solid-state 13C nuclear magnetic resonance spectrometric analyses of wood xylem: Effect of increasing carbohydrate content. Org. Geochem. 1992, 18, 407. (15) Cody, G. D.; Saghi-Szabo, G. Calculation of the 13C NMR chemical shift of ether linkages in lignin derived geopolymers: Constraints on the preservation of lignin primary structure with diagenesis. Geochim. Cosmochim. Acta 1999, 63, 193. (16) Hatcher, P. G. Chemical structural studies of natural lignin by dipolar dephasing solid-state 13C nuclear magnetic resonance. Org. Geochem. 1987, 11, 31. (17) Haw, J. F.; Maciel, G. E.; Schroeder, H. A. Carbon-13 nuclear magnetic resonance spectrometric study of wood and wood pulping with cross polarization and magic-angle spinning. Anal. Chem. 1984, 56, 1323. (18) Hawkes, G. E.; Smith, C. Z.; Utley, J. P.; Vargas, R. R.; Viertler, H. A comparison of solution and solid state 13C NMR spectra of lignins and lignin model compounds. Holzforschung 1993, 47, 302. (19) Kolodziejski, W.; Frye, J. S.; Maciel, G. E. Carbon-13 nuclear magnetic resonance spectroscopy with cross polarization

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Received for review November 7, 2001 Revised manuscript received July 28, 2002 Accepted July 28, 2002 IE010896W