Lignins Isolated via Catalyst-Free Organosolv Pulping from Miscanthus x giganteus, M. sinensis, M. robustus and M. nagara: A Comparative Study

Abstract

As a low-input crop, Miscanthus offers numerous advantages that, in addition to agricultural applications, permits its exploitation for energy, fuel, and material production. Depending on the Miscanthus genotype, season, and harvest time as well as plant component (leaf versus stem), correlations between structure and properties of the corresponding isolated lignins differ. Here, a comparative study is presented between lignins isolated from M. x giganteus, M. sinensis, M. robustus and M. nagara using a catalyst-free organosolv pulping process. The lignins from different plant constituents are also compared regarding their similarities and differences regarding monolignol ratio and important linkages. Results showed that the plant genotype has the weakest influence on monolignol content and interunit linkages. In contrast, structural differences are more significant among lignins of different harvest time and/or season. Analyses were performed using fast and simple methods such as nuclear magnetic resonance (NMR) spectroscopy. Data was assigned to four different linkages (A: β-O-4 linkage, B: phenylcoumaran, C: resinol, D: β-unsaturated ester). In conclusion, A content is particularly high in leaf-derived lignins at just under 70% and significantly lower in stem and mixture lignins at around 60% and almost 65%. The second most common linkage pattern is D in all isolated lignins, the proportion of which is also strongly dependent on the crop portion. Both stem and mixture lignins, have a relatively high share of approximately 20% or more (maximum is M. sinensis Sin2 with over 30%). In the leaf-derived lignins, the proportions are significantly lower on average. Stem samples should be chosen if the highest possible lignin content is desired, specifically from the M. x giganteus genotype, which revealed lignin contents up to 27%. Due to the better frost resistance and higher stem stability, M. nagara offers some advantages compared to M. x giganteus. Miscanthus crops are shown to be very attractive lignocellulose feedstock (LCF) for second generation biorefineries and lignin generation in Europe.

Keywords: Miscanthus nagara; Miscanthus robustus; Miscanthus sinensis; Miscanthus x giganteus; lignin; low-input crops; monolignol ratio.

Figure 1

Monolignol structures: p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol forming the specific residues p-hydroxylphenyl (H), guaiacyl (G) and syringyl (S). Reprinted from [30] under open access license.

Figure 2

Ether linkages: β-aryl-ether (β-O-4′) (a), α-aryl-ether (α-O-4′) (b), biphenyl ether (4-O-5′) (c); R = CH₂OH, lignin. Reprinted from [31] under open access license.

Figure 3

Carbon-carbon linkages: 1,2-diarylpropane (β-1′) (a), biphenyl (5-5′) (b), resinol (β-β’) (c); R=CH₂OH, lignin. Reprinted from [31] under open access license.

Figure 4

More complex linkages: dibenzodioxocin (α-O-4′/β-O-4′/5-5′) (a), phenylcoumaran (α-O-4′/β-5′) (b), spirodienone (β-1′/β-O-4′) (c). Reprinted from [31] under open access license.

Figure 5

Leaf versus stem content (weight ratio of dry matter) of different Miscanthus genotypes: M. x giganteus (Gig17, Gig34, Gig35), M. nagara (NagG10), M. sinensis (Sin2), and M. robustus (Rob4) harvested in September (09/15), December (12/14), and April (04/15), respectively, arranged to follow the seasonal order from autumn to spring. Reprinted from [45] under open access license. Error bars are standard deviation of triplicates.

Figure 6

Dry matter of the leaves (a) and stems (b) of all six genotypes harvested in September (09/15), December (12/14) and April (04/15), respectively.

Figure 7

Ash content of the leaves (a) and stems (b) of the six M. genotypes harvested in September (09/15), December (12/14) and April (04/15), respectively.

Figure 8

Monolignol units (H, G, S) and corresponding linkages (A: β-O-4 linkage, B: phenylcoumaran, C: resinol, D: β-unsaturated ester) of lignins according to HSQC NMR. Reprinted from [44] under open access license.

Figure 9

Monolignol ratios (H, G, S in %) of the crop mixtures (stem and leaves) harvested in 2013 (a) and 2015 (b) according to HSQC NMR. Precision of HSQC integration is about 5%.

Figure 10

Monolignol ratios (H, G, S in %) of the leaf-derived lignins, harvested in September (09/15) (a), in December (12/14) (b) and in April (04/15) (c), due to HSQC NMR. NagG10-1 and NagG10-2 (Figure 10, above/right) are duplicates. Precision of HSQC integration is about 5%.

Figure 11

Monolignol ratio (H, G, S in %) of stem-derived lignins harvested in September (09/15) (a), in December (12/14) (b) and in April (04/15) (c), analyzed via HSQC NMR. Precision of HSQC integration is about 5%.

Figure 12

Monolignol linkages (A, B, C, D in %) of the stem/leaf mixture harvested in 2013 (a) and 2015 (b) analyzed via HSQC NMR. Precision of HSQC integration is about 5%. The two linking patterns, phenylcoumaran (B) and resinol (C), were the smallest proportion of the lignin structure, sometimes well below 10%. There are fluctuations for all genotypes and for both harvests, with NagG10, Sin2, and Rob4 showing the smallest deviations.

Figure 13

Monolignol ratio (H, G, S in %) of leaf-derived lignins harvested in September (09/15) (a), in December (12/14) (b) and in April (04/15) (c), analyzed via HSQC NMR. NagG10-1 and NagG10-2 are duplicates. Precision of HSQC integration is about 5%.

Figure 14

Monolignol linkages (A, B, C, D in %) of stem-derived lignins harvested in September (09/15) (a), in December (12/14) (b) and in April (04/15) (c), due to HSQC NMR. Precision of HSQC integration is about 5%.