Research Article pubs.acs.org/journal/ascecg
The Fate of 4‑O‑Methyl Glucuronic Acid in Hardwood Xylan during Alkaline Extraction Christian Hutterer,† Karin Fackler,‡ and Antje Potthast*,§ †
Kompetenzzentrum Holz GmbH, Altenberger Strasse 69, A-4040 Linz, Austria Lenzing AG, Werkstraße 1, A-4860 Lenzing, Austria § Division of Chemistry of Renewable Resources, Department of Chemistry, University of Natural Resources and Life Sciences Vienna, Konrad-Lorenz-Straße 24, A-3430 Tulln, Austria Downloaded via NAGOYA UNIV on June 26, 2018 at 21:16:52 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: Highly pure wood cellulose pulps are a prerequisite to the production of regenerated fibers and cellulose derivatives. Harsh treatments for their preparation promote the digestion of wood-derived impurities, but also cause substantial yield losses on cellulose. Kraft pulping technology offers the possibility to almost completely preserve cellulose integrity, but this also applies to hemicelluloses, which must be quantitatively removed to ensure stable rayon fiber production. Alkaline postextractions of paper pulps are able to selectively remove hemicellulose impurities without degrading cellulose. The xylan can be recovered as a polymer in neat form. Hardwood kraft pulps in particular show good applicability, as xylan is easily extractable by alkaline treatments. The present study was designed to profile xylan distribution in alkali-treated pulps using 4-O-methylglucuronic acid as a marker for xylan. Labeling the uronic acid with the fluorescence marker FDAM (9H-fluoren-2-yl-diazomethane) and subsequent size exclusion chromatography (SEC) allowed xylan profiling relative to the molar mass distribution of cellulose. The method revealed an enrichment of uronic acids in the cellulosic fraction with increasing alkalinity during pulp treatment. Additionally, side reactions could be visualized, as the labeling technique is very sensitive. KEYWORDS: Caustic hemicellulose extraction, FDAM carboxyl group determination, Size exclusion chromatography, Methanolysis GC, Xylan substitution pattern, Cellulose purification
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INTRODUCTION For several years, the pulp and paper industry has faced increasing prices of wood raw material. In parallel, the market for cellulose fibers and derivatives is still growing due to an increasing world demand for fiber products. In order to produce high-quality yarns from cellulose, highly pure dissolving pulps are indispensable. Hence, processes for their manufacture with simultaneous utilization of accompanying wood components are desired. These high-yield separations embody the cost- and resource-effectiveness of biorefining. The wood-derived impurities in pulp, mainly residual hemicelluloses and lignins, are mostly removed during pulp harvesting techniques conventionally used for dissolving pulp production, such as the sulfite and prehydrolysis kraft (PHK) process. The preparation of pulps by these cooking technologies is accompanied by substantial yield losses on cellulose through acid-catalyzed reactions at high temperatures, affecting the profitability of the whole pulp manufacture. The kraft process, the worldwide dominant technology for producing high-yield paper pulps, offers a possibility to bypass this problem. Carbohydrate polymers, being alkali-stable, are mostly preserved in alkaline cooks, enabling high-yield pulp © 2016 American Chemical Society
production. As hemicelluloses remain in these pulps to large extent, they have to be removed afterwards almost quantitatively. A promising technology being simply integrable into pulp production lines is alkaline postextraction of shortchain noncellulosic carbohydrate polymers. In addition, the harvest of this carbohydrate fraction from process effluents allows their subsequent utilization as a coproduct.1−4 The dissolution and the molecular transport of xylan in the alkaline heterogeneous pulp dispersion were found to be merely physical processes influenced by diffusion. Hence, the temperature, alkali ion concentration, pulp consistency, and residence time parameters are crucial factors triggering the extraction efficiency.5 The close-to-complete removal of the noncellulosic glycosides is imperative, as their presence in spinning dopes reduces the quality of the regenerated fibers.6 Additionally, the reactivity of pulps and therefore the accessibility of derivatization chemicals must be guaranteed in order to permit a stable rayon process. The reactivity of pulp Received: October 24, 2016 Revised: December 7, 2016 Published: December 12, 2016 1818
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ACS Sustainable Chemistry & Engineering was found to decrease during alkaline treatments of pulps due to the partial conversion of the original cellulose I allomorph to cellulose II.7 This conversion may result in the formation of molecular aggregates in pulps, which is generally defined as pulp hornification. Such pulps contain a reduced surface area and pore volume, showing decreased accessibility toward xanthogenation agents.8 The hardwood hemicelluloses extracted by alkaline treatments of kraft paper pulps contain mainly xylan with a high average molar mass, as it remains almost unaffected during alkaline cooks. Naturally, hardwood xylan is substituted by 4-Omethyl-glucuronic acid at carbon C-2 and carries in addition 4O-α-D-galactopyranosyluronic acid units.9 The degree of substitution differs among hardwood species. For native birch xylan, a substituent is present at roughly every tenth monomer unit.10 Alkaline degradation of hemicelluloses includes peeling reactions lowering the chain lengths of the dissolved macromolecules from the reducing end11,12 and alkaline depolymerization reactions.13,14 Both types of degradation occur only to a minor extent, as temperatures for alkaline extractions are moderate and below 80 °C. The peeling reactions are additionally hindered by xylan substitution. Physical effects during alkaline treatments controlling the extraction efficiency of hemicelluloses, including alkali ion diffusion and polymer dissolution, are well-known. Consequently, the precise adjustment of process variables leads to pulps with predictable properties. The interrelation of extraction efficiency and xylan structures is yet not fully understood and requires further investigation. The present study focused on the role of xylan substitution patterns on xylan extractability during alkaline treatments. Therefore, birch and eucalyptus kraft paper pulps were treated with white liquor or pure sodium hydroxide, applying different treatment conditions and varying the crucial variables temperature and effective alkalinity. The carboxyl groups of the xylan-bound uronic acids in extracted pulps were quantitatively profiled by FDAM fluorescence labeling followed by SEC. In addition, the uronic acid contents in extracted xylan samples were measured by methanolysis. This experimental strategy helped to clarify the role of the substitution pattern of hardwood xylan on the selectivity of simple alkaline postextraction for purifying cellulose. As a result, a more precise prediction of pulp product would become possible, enabling a quality by design in addition to further process understanding.
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Table 1. Pulp Data of Birch and Eucalyptus Pulps Used for Alkaline Extraction Experiments kappa number (−) brightness (%ISO) intrinsic viscosity (CUEN viscosity) (η) (mL g−1 odp) water retention value (%) total carboxyl croup content (μmol g−1) glucan (%) xylan (%) mannan (%) arabinan (%) rhamnan (%) galactan (%) weight-average molar mass (kg mol−1) cellulose crystallinity (%) cellulose II content (%)
birch
eucalyptus
7.6 66.4 925 103 109.6 76.0 23.8 0.2 0.0 0.0 0.0 420 57 10
7.4 67.7 880 98 126.7 77.5 22.1 0.0 0.0 0.0 0.2 514 56 10
Preparation of Xylan Samples. The xylan of the applied hardwood species used in this study was precipitated from alkaline liquors by acidification with 1 M sulfuric acid until a pH of 2.0 was reached. After centrifugation for 10 min at 4500 rpm, the hemicelluloses were washed three times with deionized water. The dispersed hemicelluloses were then recovered from deionized water by filtration using a 0.45 μm glass fiber filter. The resulting wet hemicelluloses were subsequently freeze-dried. Analytical Methods. The kappa number was determined according to TAPPI T 236 cm-85 (1993a), brightness according to ISO 2470−1 (2009), and the intrinsic viscosity [η] according to SCAN-CM 15:99 (1999). The total carboxyl group content (COOH) was determined according to Philipp et al.16 Water retention value (WRV) was measured according to Zellcheming Merkblatt IV/33/57 (1957). The degree of crystallinity (CrI) and the cellulose II content (Cell II) were determined with Fourier transform Raman spectroscopy (FT-Raman) according to Röder et al.17 The carbohydrate content of pulps and hemicelluloses was determined as described by Schelosky et al.18 For determining the content of 4-O-methylglucuronic acid in extracted hemicelluloses, methanolysis-GC was applied according to Sundberg et al.19 All samples were analyzed in duplicate. The uronic acid contents in rising pulps were profiled and quantified during SEC after labeling with FDAM according to the method described by Bohrn et al.20 Pulp samples for FDAM labeling were in a never-dried state. Method for FDAM-Labeling in Brief. A total of 20 mg of dry pulp was preconditioned in 0.1 M HCl and agitated for 20 s in a mixer. Afterward, samples were washed with 0.1 M HCl, 96% ethanol, and dimethylacetamide (DMAc), filtered, and transferred into a 4 mL vial followed by derivatization. Pulps were suspended in 3 mL of DMAc and 1 mL of 9H-fluoren-2-yl-diazomethane (FDAM) solution (0.125 mol L−1 in DMAc). Pulp suspensions were agitated at 40 °C for 7 days, followed by filtration, washing in DMAc, and transfer into a dry vial. For dissolution, 1.6 mL of DMAc/LiCl 9% (m/v) was added. Finally, the samples were diluted and filtered through 0.45 μm filters. SEC was done with DMAc/LiCl (0.9% m/v) after filtration through a 0.02 μm filter. Samples were injected automatically into the chromatographic system (four serial SEC columns), monitored by fluorescence, multiple-angle laser light scattering (MALLS), and refractive index (RI) detection. Molar mass distributions and related polymer-relevant parameters were calculated based on a refractive index increment of 0.136 mL g−1 for cellulose in DMAc/LiCl (0.9% m/v). Parameters used: flow, 1.00 mL min−1; columns, four PL gel mixed A LS, 20 μm, 7.5 × 300 mm; detectors, MALLS-fluorescenceRI; fluorescence detection: excitation, 252 nm; emission, 323 nm; injection volume, 100 μL; run time, 45 min.
MATERIALS AND METHODS
Starting Pulps. Never-dried oxygen-delignified eucalyptus (Eucalyptus globulus) kraft pulps were kindly supplied by ENCE. Neverdried oxygen-delignified birch (Betula papyrifera) kraft pulp was also used. The kraft pulps used for alkaline extractions show the parameters illustrated in Table 1. Alkaline Treatments of Pulps. Hardwood hemicelluloses were extracted by alkaline treatments of oxygen-delignified kraft paper pulps according to the method of Wallis and Wearne15 for 30 min. White liquor was used as an alkali source for all extractions, as it is the dominating alkali source in kraft pulp production sites. The liquor in our experiments consists of sodium hydroxide, sodium sulfide, and sodium carbonate with a sulfidity of 30% and a causticizing degree of 90%. Similar extractions were done with pure sodium hydroxide. Extraction temperatures varied between 20 and 80 °C and alkalinities between 40 and 160 g L−1. Pulps were separated from the hemicellulose-containing lyes by filtration through a glass frit. After a subsequent washing step with a lower-concentrated caustic solution, the pulp was washed several times with hot water until it reached a pH value below 10. 1819
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Figure 1. Molar mass distributions and corresponding profiles of uronic acids (inserts) of alkali-treated pulps compared to the untreated paper pulp. The figure illustrates extractions performed at 60 and 80 °C, up to 160 g L−1 effective alkalinity.
Figure 2. Uronic acid profiles of sodium hydroxide-treated pulps compared to the untreated paper pulp at molar masses above 250 kg mol−1. The figure illustrates extractions performed at 20, 40, 60, and 80 °C, up to 160 g L−1 effective alkalinity.
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RESULTS AND DISCUSSION The selective labeling of carboxyl groups by FDAM prior to SEC represents a well-suited method to visualize uronic acidsubstituted hardwood xylan. Figures 1 and S1 illustrate molar mass distributions of birch paper pulps treated with white liquor compared to the untreated paper pulp (thick lines). The latter reveals a bimodal distribution of polysaccharides, both a high-molar-mass cellulose peak with a maximum at 500 kg mol−1 and a low-molar-mass xylan peak at about 25 kg mol−1. Xylan quantification by total hydrolysis and subsequent HPLC affords 23.8% for this pulp, directly reflecting the SEC xylan peak, which corresponds to about 26% based on the molar mass distribution. A decrease in xylan content is accomplished by alkali extraction. At 20 °C and 120 g L−1 alkali, the xylan
concentration levels off at around 5% and remains visible by a small hemicellulose peak. Lower hemicellulose concentrations can be reached by applying treatments at 40 °C, leading to 4.3% xylan. For further purification, the temperature must be raised to 60 or 80 °C. The area of the xylan peak decreases for both temperatures and levels off at 3.2%. Further increasing the lye alkalinity from 120 to 160 g L−1 does not lower the xylan concentration any further. The cellulose peaks shift to lower molar masses after alkaline treatments; hence, some cellulose degradation is also induced, although the temperature is very moderate compared to kraft cooking conditions. The FDAM labeling method is selective for uronic acids, as the gluconic acids do not yield a stable product with FDAM. Hence, FDAM labeling can be used as a selective marker for 41820
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ACS Sustainable Chemistry & Engineering O-methylglucuronic acid bound to hardwood xylan. The suitability of the method for xylan profiling has already been demonstrated21 and is also displayed in Figure 1. The uronic acid content directly relates to the amount of xylan in the pulp. The effect of alkaline treatment can also be visualized by massnormalized fluorescence signals in the xylan region of the MWD (Figure 3). The quantity of uronic acids increases with increasing alkalinity in the high-molar-mass region over 750 kg mol−1 (Figure 1). These may originate from xylan or from autoxidized cellulose. It is likely that those uronic acids mostly come from alkali-induced cellulose autoxidation,22 as they are present in the untreated cellulose to a significantly lesser extent. Those present in the high-molar-mass region of the untreated paper pulps result from high alkalinity during kraft cooking, as well as from oxidative conditions during oxygen delignification. In order to exclude effects induced by the white liquor treatment, which not only contains sodium hydroxide but also sodium sulfide and sodium carbonate, all pulp treatments were done in parallel with pure sodium hydroxide. Figure 2 shows the uronic acid contents of corresponding pulps at molar masses above 250 kg mol−1. Data are given for treatments at 20, 40, 60, and 80 °C and for alkalinities ranging from 40 to 160 g L−1. The sodium hydroxide treatment yields similar results compared to white liquor, as given in Figures 1 and S1. Hence, the effect observed for the uronic acid contents is independent from the lye type used. As shown above, uronic acids increase with rising alkali concentration in the region above 750 kg mol−1, but interestingly decrease in the region between 250 and 750 kg mol−1. Table 2 illustrates the total uronic acid content in the alkalitreated pulps. The data indicate a leveling-off of overall uronic
limited extraction of xylan from hardwood paper pulps, depending on the swelling behavior of pulps at temperatures below 40 °C and a lowest possible residual xylan concentration in pulps of approx. 3% at elevated temperatures.5 A shift in the molar mass of the residual xylan in dependence on the extraction conditions is also clearly visible. The concentration has a rather small effect compared to the temperature, which is the driving force in this process. The molar mass of the residual xylan increases steadily from 20 to 60 °C (Figure 3A−C). This is in agreement with the statements in Hutterer et al.5 At 20 °C, pulp swelling leads to the liberation of xylan macromolecules with a similar degree of polymerization (DP) not depending on the alkalinity used. Xylan degradation is limited at this temperature. Increasing the temperature to 60 °C leads to the dissolution and extraction of low-molar-mass xylan, in addition to starting alkali-induced degradations. At elevated temperatures, gaps between lye densities and viscosities are small among the alkalinities used that allow a broader molar mass fraction of xylan to be dissolved. The enhanced extraction performance at this temperature leads to 3% residual xylan in pulps. Hence, molar masses of xylan in pulps are rising. A further increase in temperature to 80 °C causes a smaller xylan DP in pulps as degradation reactions become more dominant, and degraded cellulose fractions may also influence the molar mass in this region (Figure 3D). This behavior can be concluded from broad molar masses of extracted xylan at this temperature, not depending on the alkalinity used. To confirm the xylan extraction values from Table 2, similar data from pulps treated with pure sodium hydroxide are obtained and given in Table S1. To investigate whether substituted xylan is better or worse when extracted by alkali, the total uronic acid contents of white liquor-treated pulps were determined for the xylan peak of the molar mass distributions. The resulting concentrations of uronic acids are shown in Figure 4, with higher concentrations of uronic acids in the residual xylan with increasing alkalinity of the treatment liquid. The effect is similar for all temperatures applied between 20 and 80 °C. From these data, we can conclude that, with higher alkalinity, the extractable xylan has fewer uronic acid side chains, represented by linear unbranched molecules, which leads to an enrichment of uronic acids in the residual xylan (Figure 4). Hence, the C-2 substitution hinders not only the peeling from the reducing end but also the overall extractability of the polymeric xylan. The same is true if pure sodium hydroxide is applied (Figure S2). Complementary to the determination of residual uronic acid contents in pulps, the 4-O-methylglucuronic acid contents of the extracted xylan samples were determined by methanolysis. This method is able to quantify uronic acids in pulps with high reproducibility. Table 3 illustrates the contents of 4-Omethylglucuronic acid for four birch xylans and two eucalyptus xylans. The samples were extracted with pure sodium hydroxide before applying the treatment temperatures and alkalinities shown in the table. The content of uronic acids in the extracted birch xylan reached around 61−64 μmol g−1 for low alkalinity and decreased to 55−56 μmol g−1 at high alkali concentrations. A similar trend is obtained for extracted eucalyptus xylan, which contains overall about three times as much uronic acid as extracted birch xylan. Overall, this nicely confirms the SEC data for residual xylan in the pulp.
Table 2. Xylan Content and Total Uronic Acid Contents in Alkali-Treated Birch Pulps Determined by FDAM Fluorescence Labelinga sample number untreated pulp 1 2 3 4 5 6 7 8 9 10 11 12 13 14 a
temperature (°C)
20 20 20 40 40 40 60 60 60 60 80 80 80 80
alkalinity (g L−1)
40 80 120 40 80 120 40 80 120 160 40 80 120 160
xylan in pulp (%)
total COOH (μmol g−1)
23.8
46.1
10.1 4.7 5.2 11.2 4.6 4.3 12.4 5.6 3.2 3.7 12.9 6.5 3.2 3.4
22.2 12.2 14.9 23.1 12.4 13.1 23.6 13.4 11.0 13.0 23.9 13.9 10.6 16.3
Pulps were prepared by treatment with white liquor.
acid at alkalinities above 80 g L−1 for treatments at 20 and 40 °C. This stagnation occurs for pulps treated at 60 and 80 °C above an alkalinity of 120 g L−1. Additional data illustrate the percentage of residual xylan in the pulps determined by total hydrolysis of pulps with subsequent HPLC that show a similar leveling-off of xylan concentrations. Both data sets indicate a 1821
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Figure 3. Mass-normalized fluorescence signals of alkali-treated pulps after FDAM/SEC with varying extraction conditions between 20 and 80 °C, as well as 40 and 160 g L−1 compared to the untreated paper pulp. Extractions were performed using white liquor as the alkali source.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02552. Molar mass distributions of alkali-treated kraft pulps with the corresponding uronic acid profiles applying the treatment temperatures 20 and 40 °C, xylan and total uronic acid contents in alkali-treated kraft pulps using pure sodium hydroxide as extraction medium, percentages of uronic acids in the residual xylan depending on temperature and effective alkalinity with pure sodium hydroxide as the extraction medium (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
Figure 4. Contents of uronic acids in the residual (xylan peak of the molar mass distributions). The figure illustrates extractions performed at 20, 40, 60, and 80 °C up to 160 g L−1 effective alkalinity with white liquor use.
ORCID
Antje Potthast: 0000-0003-1981-2271 Notes
The authors declare no competing financial interest.
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Table 3. 4-O-Methyl-Glucuronic Acid Contents of Extracted and Precipitated Xylans Using Pure Sodium Hydroxide as the Extraction Medium wood species
temperature (°C)
alkalinity (g L−1)
4-O-methyl-glucuronic acid (μmol g−1)
birch birch birch birch eucalyptus eucalyptus
40.0 40.0 80.0 80.0 80.0 80.0
40.0 120.0 40.0 120.0 40.0 120.0
61.1 56.0 64.4 55.2 170.8 142.9
ACKNOWLEDGMENTS Financial support to WOOD Kplus was provided by the Austrian government; the provinces of lower Austria, upper Austria, and Carinthia, and by Lenzing AG. We also express our gratitude to the Johannes Kepler University, Linz; the University of Natural Resources and Life Sciences, Vienna, and Lenzing AG for their in-kind contributions. We thank Dr. S. Böhmdorfer for fruitful discussions.
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