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Study on the structure and properties of xylan extracted from eucalyptus, sugarcane bagasse and sugarcane straw
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Wood Chemistry and Pulp Technology.
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Lignocellulosic biomasses are an important source of chemical components such as cellulose, lignin and hemicelluloses, and can be used for a variety of purposes in both the pulp and paper and chemical conversion industries. Xylan, the main hemicellulose found in hardwood and grass plants, plays an important role during the pulping/pretreatment process reactions, including those used in 2nd generation bioethanol production. It may also play an important role in the production of certain novel materials.

This thesis evaluates the composition of eucalyptus (Eucalyptus urophylla x Eucalyptus grandis), sugarcane bagasse and sugarcane straw, with a specific focus on the structure and properties of xylan. The chemical characterization of biomasses showed that sugarcane bagasse and straw contain larger amounts of extractives, ash and silica than eucalyptus. The large amount of silica leads to an overestimation of the Klason lignin content, if not corrected. By using a complete mass balance approach, sugarcane bagasse and straw were shown to contain smaller amounts of lignin (18.0% and 13.9%, respectively) than previously reported for these raw materials, and certainly a much smaller amount of lignin than was found in eucalyptus (27.4%). The hemicellulose content in sugarcane bagasse (28.7%) and straw (29.8%) was much higher than that in eucalyptus (20.3%).

In order to investigate the structure of the xylan in greater detail, it was extracted with dimethyl sulfoxide from holocellulose, obtained by either peracetic acid or sodium chlorite delignification. The structure of the isolated xylans was confirmed by FTIR and 1H NMR analysis. In eucalyptus, the O-acetyl-(4-O-methylglucurono)xylan (MGX) was identified. This had a molar ratio of xylose units to branches of 4-O-methylglucuronic acid of 10:1.1 and a degree of acetylation of 0.39. All 4-O- methylglucuronic acid groups were attached to position O-2 of the xylose units, which had an acetyl group in position O-3. The acetyl groups were distributed in positions O-3 (64%), O-2 (26%) and O-2,3 (10%). The MGX had a molecular weight (Mw) of about 42 kDa.

In bagasse and straw, arabinoxylan (AX) was identified. This had a molar ratio of xylose units to arabinosyl substitutions of 10:0.5 for bagasse and 10:0.6 for straw. A degree of acetylation was 0.29 and 0.08 for bagasse and straw, respectively. The arabinose units were attached preferentially to position O-3 in AX. In the xylan from bagasse, the acetyl groups were found in positions O-3 (60%), O-2 (13%) and O-2,3 (27%), while in the xylan from straw, the acetyl groups were distributed between positions O-3 (67%) and O-2 (33%). The AX had a molecular weight (Mw) of about 38 kDa and 30 kDa for bagasse and straw, respectively.

The differences in the structure of xylan present in the various biomasses played an important role during hydrothermal pretreatment, which is often used as the first step in 2nd generation ethanol production. The varying amounts of uronic acid and acetyl groups resulted in different starting pH levels of liquor and, thus, affected the chemical transformation in the biomasses in different ways. The hydrothermal pretreatment resulted mostly in the removal and/or transformation of hemicelluloses, but also in the formation of a significant number of pseudo-lignin structures. In addition, in eucalyptus, pseudo-extractives structures were generated. The sugarcane straw showed the highest mass loss during the investigated pretreatment.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , 53 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2015:59
National Category
Polymer Technologies
Identifiers
URN: urn:nbn:se:kth:diva-175774ISBN: 978-91-7595-718-0 (print)OAI: oai:DiVA.org:kth-175774DiVA: diva2:862284
Presentation
2015-11-13, Rånbyrummet, Teknikringen 56, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20151023

Available from: 2015-10-23 Created: 2015-10-21 Last updated: 2015-10-23Bibliographically approved
List of papers
1. Assessment of chemical transformations in eucalyptus, sugarcane bagasse and straw during hydrothermal, dilute acid, and alkaline pretreatments
Open this publication in new window or tab >>Assessment of chemical transformations in eucalyptus, sugarcane bagasse and straw during hydrothermal, dilute acid, and alkaline pretreatments
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2015 (English)In: Industrial crops and products (Print), ISSN 0926-6690, E-ISSN 1872-633X, Vol. 73, 118-126 p.Article in journal (Refereed) Published
Abstract [en]

The impact of hydrothermal, dilute acid, and alkaline pretreatments on the chemical structure of eucalyptus, sugarcane bagasse, and straw were compared with a view to their subsequent bioconversion into ethanol. Sugarcane bagasse and straw contain high amounts of extractives (15.0% and 12.2%, respectively), ash (2.3% and 7.9%, respectively), and silica (1.4% and 5.8%, respectively). If not properly corrected, the presence of silica would lead to the overestimation of the lignin, while high amounts of extractives would cause the overestimation of the content of sugars in biomass. Applying a novel approach through the use of complete mass balance, bagasse and straw were proven to contain lower amounts of lignin (18.0% and 13.9%, respectively) than previously reported for these raw materials, and certainly a much lower amount of lignin than eucalyptus (27.4%). The syringyl to guaiacyl units ratio (SIG) for lignin in bagasse and straw (1.1 and 0.5, respectively) was lower than that for eucalyptus (2.7), indicating a different reactivity during chemical pretreatments. The xylan content in sugarcane bagasse and straw was much higher than that in eucalyptus, with a significantly lower degree of substitution for uronic acids and acetyl groups. The sugarcane straw showed the highest mass loss during the investigated pretreatments, especially under alkaline conditions, with a total biomass yield of only 37.3%. During the hydrothermal and dilute acid treatments, mostly hemicelluloses were removed, followed by the formation a significant amount of pseudo-lignin structures, while the alkaline pretreatment affected the lignin content. With eucalyptus, the formation of structures similar in their behavior to extractives (i.e., soluble in toluene and ethanol, subsequently referred to as "pseudo-extractives") was observed during all three pretreatments, with 12.4% for hydrothermal, 18.9% for dilute acid, and 8.7% for alkaline pretreatment. This information, combined with actual yields, should be taken into account when assessing the impact of pretreatments on the chemical composition and structure of biomass.

Keyword
Silica content, Complete mass balance, Sugarcane, Pretreatments, Pseudo-lignin, Pseudo-extractives
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-171871 (URN)10.1016/j.indcrop.2015.04.021 (DOI)000357839300014 ()2-s2.0-84929453022 (Scopus ID)
Funder
Knut and Alice Wallenberg Foundation
Note

QC 20150817

Available from: 2015-08-17 Created: 2015-08-10 Last updated: 2017-12-04Bibliographically approved
2. Comparative characterization of acetylated heteroxylan from eucalyptus, sugarcane bagasse and sugarcane straw
Open this publication in new window or tab >>Comparative characterization of acetylated heteroxylan from eucalyptus, sugarcane bagasse and sugarcane straw
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(English)Manuscript (preprint) (Other academic)
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-175832 (URN)
Note

QS 2015

Available from: 2015-10-23 Created: 2015-10-23 Last updated: 2015-10-23Bibliographically approved

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