NWBC 2017, Stockholm, March 28-30
γ-valerolactone/water Fractionation of Softwood
Sherif Elsayed 1, Huy Quang Lê1, Marc Borrega 1, Herbert Sixta 1
1Aalto University, Department of Forest Products Technology, Espoo, Finland
Abstract
The feasibility of fractionating Scots pine (Pinus sylvestris L.) wood employing γ-valerolactone (GVL)/water mixtures was here studied. Pine sawdust exhibited high recalcitrance to GVL/water fractionation, with only about 50% of lignin being extracted without compromising the cellulose yield. Kinetic parameters for delignification and carbohydrate degradation during cooking were determined. In addition, GVL/water fractionation of pine wood chips yielded high amount of rejects, even when prehydrolysis was employed. Alternative pretreatment methods are being investigated.
Introduction
The conversion of lignocellulosic biomass into bio-based materials and chemicals requires the development of selective fractionation processes that are in line with biorefinery concepts. Recently, we have shown that Eucalyptus (Eucalyptus globulus) wood can be successfully fractionated into its main components in a binary mixture of γ-valerolactone (GVL) and water at elevated temperature [1]. In this work, we have investigated the feasibility of employing GVL/water mixtures to fractionate Scots pine.
Materials and methods
Scots pine (Pinus sylvestris L.) sawdust was fractionated in 30mL vials heated in a monowave reactor (Anton Paar Monowave 300) for cooking optimization and kinetics study. The sawdust was treated in GVL solution containing 0 – 98 wt% GVL (obtained from Sigma Aldrich) at 180 – 210°C for 5 – 180 minutes with liquor-to-wood ratio (L:W) of 10 L/kg. Pine chips were employed in upscaled fractionation trials with 50 wt% GVL, at 180°C, for 60 – 180 minutes with L:W=10 L/kg in 225mL bombs heated in a silicon oil-bath reactor (Haato 43427). Prehydrolysis of the chips with P-factor of 50 – 200 was performed in 2.5L bombs heated in an air-bath reactor (Haato 16140-538). The pulps were analyzed for carbohydrate and lignin content. The spent liquors were analyzed for carbohydrates, dissolved lignin, furanic, and organic acids content.
A model was constructed for the removal of lignin, cellulose and hemicelluloses based on the components residual values in pulp. The reactions were assumed to follow a first order reaction kinetics model. Wolfram Mathematica 10.3 non-linear curve fitting functions were employed for modelling and determining the reaction rate constants.
Results and discussion
The behavior of the wood main components in GVL/H2O fractionation of sawdust is illustrated in Figure 1. The results from the fractionation of sawdust indicate that the cellulose fraction was well-preserved at any GVL/H2O content, while the delignification reached a maximum when fractionation liquor contained about 50 – 60 wt% GVL. Hemicellulose removal increased with increasing water content due to the enhanced hydrolytic degradation. These findings agree with previous research on hardwood [1, 2] with the remark that delignification proceeded in a significantly lesser extent due to the recalcitrance of softwood biomass. Prolongation of reaction time beyond 120 minutes did not improve the delignification, while the increase of reaction temperature to 210°C facilitated the lignin and hemicellulose removal at the expense of cellulose yield.
Figure 1. Effect of GVL content in fractionation liquor on the separation of pine sawdust main components at 180°C. (odw: oven-dried wood).
The hydrolysis of the cellulose was significantly slower compared to that of hemicelluloses at any given temperature, and the hydrolysis of xylan was found to be faster than that of glucomannan. Models explaining the degradation of lignin and polysaccharides were fitted to the experimental data, and the kinetic parameters were determined and summarized in Table 1.
Table 1.Activation energies and rate constants of the delignification and the hydrolysis of cellulose and hemicelluloses at 180, 195 and 210°C
Component / Ea[kJ/mol] / K180°C [1/min] / K195°C [1/min] / K210°C [1/min]
Lignin / 140.1 / 0.0035 / 0.0225 / 0.0378
Cellulose / 134.2 / 0.0006 / 0.0039 / 0.0054
Galactoglucomannan / 90.3 / 0.0102 / 0.0342 / 0.0448
Xylan / 84.1 / 0.0222 / 0.0622 / 0.0882
Selectivity / - / 5.83 / 5.77 / 7.00
The potential of GVL/water mixtures to fractionate softwoods was also tested on pine wood chips. Fractionation of the chips at 180°C and 50% wt GVL yielded a considerable amount of rejects (about 25% after 180 minutes of cooking). When mild autohydrolysis was employed as pretreatment, the amount of rejects increased with increasing prehydrolysis intensity.
Conclusion
GVL/water cooking has been proven a potential sustainable method for fractionation of hardwood biomass. However, the adaption of such method for softwood biomass is still challenging. Alternative pretreatment methods arebeing investigated to improve the delignification and defibrillation of pine chips in GVL/water process.
References
1.Lê H Q, Ma Y, Borrega M, Sixta H (2016): Wood biorefinery based on γ-valerolactone/water fractionation. Green Chem., 18:20, p. 5466.
2.Fang W, Sixta H (2015): Advanced biorefinery based on the fractionation of biomass in γ-valerolactone and water. ChemSusChem, 8:1, p. 73.