Effect of lignin content on changes occurring in poplar cellulose ultrastructure during dilute acid pretreatment

Abstract

BackgroundObtaining a better understanding of the complex mechanisms occurring during lignocellulosic deconstruction is critical to the continued growth of renewable biofuel production. A key step in bioethanol production is thermochemical pretreatment to reduce plant cell wall recalcitrance for downstream processes. Previous studies of dilute acid pretreatment (DAP) have shown significant changes in cellulose ultrastructure that occur during pretreatment, but there is still a substantial knowledge gap with respect to the influence of lignin on these cellulose ultrastructural changes. This study was designed to assess how the presence of lignin influences DAP-induced changes in cellulose ultrastructure, which might ultimately have large implications with respect to enzymatic deconstruction efforts.ResultsNative, untreated hybrid poplar (Populus trichocarpa x Populus deltoids) samples and a partially delignified poplar sample (facilitated by acidic sodium chlorite pulping) were separately pretreated with dilute sulfuric acid (0.10 M) at 160°C for 15 minutes and 35 minutes, respectively . Following extensive characterization, the partially delignified biomass displayed more significant changes in cellulose ultrastructure following DAP than the native untreated biomass. With respect to the native untreated poplar, delignified poplar after DAP (in which approximately 40% lignin removal occurred) experienced: increased cellulose accessibility indicated by increased Simons’ stain (orange dye) adsorption from 21.8 to 72.5 mg/g, decreased cellulose weight-average degree of polymerization (DPw) from 3087 to 294 units, and increased cellulose crystallite size from 2.9 to 4.2 nm. These changes following DAP ultimately increased enzymatic sugar yield from 10 to 80%.ConclusionsOverall, the results indicate a strong influence of lignin content on cellulose ultrastructural changes occurring during DAP. With the reduction of lignin content during DAP, the enlargement of cellulose microfibril dimensions and crystallite size becomes more apparent. Further, this enlargement of cellulose microfibril dimensions is attributed to specific processes, including the co-crystallization of crystalline cellulose driven by irreversible inter-chain hydrogen bonding (similar to hornification) and/or cellulose annealing that converts amorphous cellulose to paracrystalline and crystalline cellulose. Essentially, lignin acts as a barrier to prevent cellulose crystallinity increase and cellulose fibril coalescence during DAP.

Microstructure Properties and Cellulase Hydrolysis Efficiency of Hybrid Pennisetum with [Amim]Cl Pretreatment

“…It can be inferred that there is a direct relationship between the cellulase hydrolysis efficiency and hemicellulose/lignin removal. Similar results have been reported for a dilute sulfuric acid pretreatment of hybrid poplar (Sun et al 2014). The high glucose recovery may be primarily a result of the decrease of inter-molecular Hbonds, resulting in the loosening of the cellulose chains, and the weaker hydrophobic interaction by transforming cellulose I to cellulose II (Wada et al 2010).…”

Lactic Acid and Biosurfactants Production from Residual Cellulose Films

“…The difference in yields found in this work can be explained by one or a combination of two facts: (1) In raw cellulose from higher plants, cellulose chains present crystalline microfibril, with an amorphous part at the surface [29], and (2) when applying a dilute acid pretreatment to raw or to a reduced lignin content cellulose, crystalline cellulose increases and the degree of polymerization decreases [30] which makes residual cellulose films more susceptible for degradation. For this, it is reasonable to propose the hypothesis that the rearrangement on the structure of the pure cellulose after the viscose treatment [1], due to an increase of the crystalline, and the lowering of the polymerization degree after delignification process [30], can favor the acid hydrolysis process herein proposed. Furthermore, in comparison to enzymatic processes, it has been stated in literature that the amount of hydrolyzate generated during enzymatic treatments is less than the amount required for subsequent studies on fermentation for raw cellulosic substrates [28] or the concentration obtained is too low [4].…”

“…Others processes dealing with raw cellulose reached 22.7 g/L of glucose with the consequent formation of inhibitors [28]. The difference in yields found in this work can be explained by one or a combination of two facts: (1) In raw cellulose from higher plants, cellulose chains present crystalline microfibril, with an amorphous part at the surface [29], and (2) when applying a dilute acid pretreatment to raw or to a reduced lignin content cellulose, crystalline cellulose increases and the degree of polymerization decreases [30] which makes residual cellulose films more susceptible for degradation. For this, it is reasonable to propose the hypothesis that the rearrangement on the structure of the pure cellulose after the viscose treatment [1], due to an increase of the crystalline, and the lowering of the polymerization degree after delignification process [30], can favor the acid hydrolysis process herein proposed.…”

Hydroxyl Availability in Aspen Wood After Dilute Acid Pretreatment and Enzymatic Saccharification

“…One possible cause of crystallinity disruption is the dilute acid pretreatment. However, the literature shows that there are no crystallinity changes detected for isolated cellulose from poplar wood treated with dilute acid, based on the results of nuclear magnetic resonance spectroscopy (Sun et al 2014). The other possible cause is the penetrating or disrupting effects of pyridine, as discussed earlier, during the hydroxyl number determination.…”