Isolation and characterization of herbaceous lignins for applications in biomaterials
Highlights
► Lignin was extracted from triticale, wheat, corn, flax, and hemp residues. ► Similar microwave-assisted organosolv conditions were applied for all extractions. ► Extracted lignins were characterized by 31P NMR, GPC, TGA, elemental, sugar analyses. ► Measured physico-chemical properties allowed proposing an application for each lignin.
Introduction
With the current rapid depletion of non-renewable fossil fuel based resources, there is a strong interest worldwide in using renewable biomass to replace petroleum-based chemicals in products manufacturing. In this context, lignin, which is the second most abundant polymer in nature after cellulose, appears as a promising candidate. Lignin is an amorphous, highly branched polyphenolic macromolecule of complex structure. It consists primarily of three phenylpropane units, namely syringylpropane (3,5-dimethoxy-4-hydroxyphenylpropane) (S), guaiacylpropane (4-hydroxy-3-methoxyphenylpropane) (G), and 4-hydroxyphenylpropane (H) groups resulting from the enzymatic polymerization of sinapyl, coniferyl, and p-coumaryl alcohols, respectively. Lignin exhibits a great variability of functional groups including aliphatic hydroxyl groups, phenolic groups, and carboxylic groups, which makes it an attractive macromonomer for the synthesis of polymers (Lora, 2008, Gandini and Belgacem, 2008, Stewart, 2008, Doherty et al., 2011). However, the physico-chemical properties of lignin, and therefore its suitability for specific polymer applications, are largely dependent on the plant species and the isolation processes (Gandini and Belgacem, 2008).
Although until recently, all commercial lignins were byproducts of the pulp and paper industry, lignins extracted from herbaceous plants are currently receiving increasing attention (Buranov and Mazza, 2008) as proven by the recent industrial and pilot scale production of non-wood lignins by GreenValue SA and CIMV, respectively (Lora, 2008, Delmas, 2008). The reasons justifying this recent interest stand in the annual renewability of herbaceous plants, their high tonnage per year, the increased conversion of cellulosic substrate into biofuel when lignin is first extracted from the lignocellulosic feedstock, and the improved economics of cellulosic processes when developing value-added lignin coproducts. In contrast to lignins of softwood (gymnosperm) that contain mainly G units, and lignins of hardwood (dicotyledonous angiosperm) that contain both G and S units, the lignins of grass or annual plants (monocotyledonous angiosperm) contain the three types of phenylpropane units, i.e. G, S, and H units. The resulting increased heterogeneity and complexity of herbaceous lignins compared to wood lignins and the much more limited knowledge of their chemistry requires that additional efforts be dedicated to their characterization. A significant number of data has been reported on the physico-chemical characteristic of wheat lignins (Billa et al., 1996, Crestini and Argyropoulos, 1997, Sun et al., 1997, Sun et al., 2002, Sun et al., 2005, Sahoo et al., 2011, Delmas et al., 2011) but literature is much more scarce on the properties of lignins extracted from other agricultural feedstocks (Sun et al., 2002, Gosselink et al., 2004, Xiao et al., 2001, Sahoo et al., 2011, del Rio et al., 2011). In addition, published data often provide properties of one lignin extracted from one biomass under specific conditions that cannot be compared to lignins isolated using different processes and conditions.
The purpose of this work was to compare the physicochemical parameters of lignins extracted from various agricultural feedstocks under identical conditions to determine the most suitable application for each one. Recently, we showed that lignin could be efficiently extracted from triticale straw using microwave irradiation (Monteil-Rivera et al., 2012). Microwave energy was applied here with a unique set of conditions to extract lignins from various agricultural residues and the extracted lignins were characterized in terms of functional groups, molecular weights, and thermal stability. Residues from crops grown in Canada were investigated, which include wheat straw, corn residues (cobs + stalks), flax shives, and hemp hurds. Straw from triticale, a hybrid of rye and wheat presently considered as a potential new major crop in Canada (http://www.ctbi.ca), was also included in the comparison. Although the lignins extracted herein certainly differ from lignins extracted industrially using different processes, the present study provides insight on the intrinsic differences occurring between lignins originating from various annual plants.
Section snippets
Materials
Triticale straw and wheat straw were provided by Agriculture and Agri-Food Canada, Lethbridge, AB. Corn residues (mixture of stalks and stovers remaining after grain harvest) were provided by a local producer from Quebec. Flax shives were provided by Schweitzer-Mauduit, Winkler, MB. Hemp straw was received from Lanaupôle Fibres, Berthierville, QC, and decorticated mechanically into fibers and hurds. The hurds were used for lignin extraction. All air-dried agricultural residues were ground with
Composition of biomass
The composition of the agricultural residues used in this study was determined in terms of ash, carbohydrates, and lignin (Table 1). Wheat and triticale straw exhibited comparable compositions, with, however, a significantly higher content of ash for triticale than for wheat. Corn residue (stovers + stalks) contained slightly more lignin than wheat and triticale straws, roughly the same content of carbohydrates and as much ash as in wheat straw. Flax and hemp residues contained significantly more
Conclusion
Organosolv lignins were extracted from triticale, wheat, corn, flax, and hemp residues using microwave irradiation under similar conditions to assess their applicability in polymer chemistry. All the extracted lignins were of high purity with very low content of ash, carbohydrates, and sulfur, and 0–5% proteins. Corn lignin, with a high proportion of H phenolic units, is a promising candidate for phenolic resins but the reactivity of p-coumaric esters under downstream processing conditions
Acknowledgements
This work was funded by the National Bioproducts Programs of National Research Council Canada and the Canadian Triticale Biorefinery Initiative of the Agricultural Bioproducts Innovation Program (ABIP-CTBI) of Agriculture and Agri-Food Canada. The technical assistance of Louise Paquet is gratefully acknowledged. Dr. Francois Eudes from Agriculture and Agri-Food Canada, Lethbridge, AB, Canada, and Dr. Denis Rho from NRC-BRI are thanked for providing the raw materials.
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