Frederico M. Relvas†
a,
Ana Rita C. Morais†ab and
Rafal Bogel-Lukasik*a
aUnidade de Bioenergia, Laboratório Nacional de Energia e Geologia, I.P., Estrada do Paço do Lumiar 22, 1649-038, Lisboa, Portugal. E-mail: rafal.lukasik@lneg.pt; Fax: +351 217163636; Tel: +351 210924600 ext. 4224
bLAQV/REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal
First published on 24th August 2015
The processing of wheat straw using high-pressure CO2–H2O technology was studied with the objective to evaluate the effect of CO2 as catalyst on the hydrothermal production of hemicellulose-derived sugars either as oligomers or as monomers. Also, the reduction of the crystallinity of the cellulose-rich fraction was assessed. Over a range of reaction conditions (0 to 50 bar of initial CO2 pressure and 0 to 45 minutes of holding time, at T = 180 °C), the addition of CO2 to water-based processes led to the in situ formation of carbonic acid, which allowed us to obtain a higher dissolution of wheat straw hemicellulose. Furthermore, this approach led to a xylo-oligosaccharide (XOS) rich fraction, yielding 79.6 g of XOS per 100 g of the initial xylan content (at 50 bar of initial CO2 pressure and 12 min of residence time) while the water-only process gave only 70.8 g of XOS per 100 g of initial xylan content. Furthermore, for higher pressures of CO2, a decrease in oligosaccharide content was found and was counterbalanced by production of monomer sugars, achieving a maximum of 5.7 g L−1 at the severest condition.
Lignocellulosic biomass has a very heterogeneous composition as it is generally composed of three main fractions: cellulose, hemicelluloses and lignin.11 Cellulose and hemicelluloses are constituted by polymers of hexosans and pentosans representing 35–50% and 20–40% of biomass, respectively. Lignin is a complex polymer matrix of aromatic alcohols constituting between 10 and 25% of the weight of entire biomass. The aforementioned complex composition and recalcitrant structure of lignocellulosic biomass creates a great challenge for its valorization in the biorefinery framework. In an effort to obtain all benefits of each biomass component, specific technologies are needed to deconstruct them and to make biomass available for further conversion to value-added products.12 Various physical, chemical, physico-chemical and biological pretreatment technologies have demonstrated to be efficient in deconstruction of recalcitrant structure of biomass increasing its susceptibility to enzymatic-based processes.13 On the other hand, most of these pretreatments are characterized by low selectivity influencing negatively the production of diverse value commodities at competitive costs. Thus, beyond the need to find alternative sources of energy, the development of novel and more environmentally benign technologies for lignocellulosic biomass processing is still strongly required.
Recently, green technologies such as high-pressure CO2–H2O approach have been used in the valorization of lignocellulosic and starch-based biomass to produce a wide-range of chemicals and others value-added products.14–20 Recently, Morais et al. published a review where the applicability and effectiveness of high-pressure CO2 and CO2–H2O technology for biomass pretreatment and its potential as alternative to conventional methods such as acid-catalyzed and water-only reactions were demonstrated.19 The presence of CO2 in hydrothermal processes allows to the in situ formation of acidic environment (CO2 + H2O ↔ (H2CO3), 2H2CO3 ↔ H3O+ + HCO3−, HCO3− ↔ H3O+ + CO32−), which promotes acid-catalyzed hydrolysis of biomass-derived hemicellulose21 and simultaneously decreases cellulose crystallinity,22 without the typical disadvantages of acid-catalyzed reactions. In this respect, van Walsum et al. observed that the addition of CO2 to water-only reactions allowed to hydrolyze pure xylan to produce xylose oligomers at lower temperatures and at shorter holding times in comparison to those obtained with autohydrolysis (water-only) technology.23 Miyazawa and Funazukuri explored the effect of compressed CO2 in the hydrolysis of carbohydrates to monosaccharides under hydrothermal conditions.24 In water-only process, the final xylose yield was less than 5% while in CO2-assisted process a great improvement in the yield was achieved with lower production of degradation products in comparison to acid-catalyzed processes.
In this work, high-pressure CO2–H2O technology was selected for the pretreatment and hydrolysis of wheat straw. Previous results demonstrated the potential of this technology in hydrolysis of hemicellulose fraction into both oligosaccharides and monosaccharides25–27 concurrently with reduction of crystallinity of the processed materials. The kinetics of the wheat straw hemicellulose hydrolysis using high-pressure CO2–H2O is also reported in literature.27 The objective of this work was to evaluate the effect of holding time and initial CO2 pressures on the conversion of hemicellulose present in wheat straw to C5 sugars (either in oligomeric or in monomeric form) and its simultaneous effect on other constituents of biomass such as cellulose and lignin.
The chemical composition of wheat straw was presented elsewhere25 and is as follows (w/w): 38.5 ± 0.1 cellulose (as glucan), 19.1 ± 0.1 xylan, 3.0 ± 0.1 arabinan, 2.7 ± 0.2 acetyl groups, 17.7 ± 0.1 Klason lignin, 4.7 ± 0.1 protein, 10.7 ± 0.1 ash.
In order to study the effect of CO2 concentration on the severity of reaction (R0), the CO2 density was calculated according to Peng–Robinson equation of state29 using both initial temperature and CO2 pressure employed in each experiment. For the same calculations, the Henry's constant (H) was determined according to the literature23 using the empirical equation H(T) = −0.017037T2 + 6.1553T + 78.227. The CO2 solubility in water was taken from literature30 and modelled using PE software31 for required temperature.
a Calculated according to literature.23 | |||||||||||||||||||
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t (min) | 0 | 4 | 6 | 12 | 18 | 20 | 25 | 30 | 35 | 45 | 0 | 4 | 8 | 12 | 16 | 20 | 25 | 30 | 40 |
pinitial (bar) | 50 | 35 | |||||||||||||||||
CSpCO2a | −1.16 | −0.64 | −0.49 | −0.25 | −0.09 | −0.05 | 0.07 | 0.14 | 0.19 | 0.30 | −1.25 | −0.70 | −0.45 | −0.30 | −0.19 | −0.10 | 0.00 | 0.08 | 0.19 |
Estimated pH | 3.72 | 3.78 | |||||||||||||||||
Final pH | 4.46 | 4.38 | 4.37 | 4.11 | 3.94 | 3.99 | 3.92 | 3.62 | 3.73 | 3.64 | 4.5 | 4.33 | 4.23 | 4.22 | 3.92 | 3.9 | 3.65 | 3.66 | 3.58 |
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Yield (g per 100 g of initial amount present in raw material) | |||||||||||||||||||
XOS | 38.9 | 55.3 | 60.1 | 79.6 | 73.7 | 66.1 | 64.2 | 40.7 | 27.5 | 18.6 | 36.8 | 60.0 | 67.9 | 73.0 | 72.4 | 57.7 | 50.4 | 33.5 | 31.9 |
AOS | 53.0 | 62.3 | 54.6 | 39.9 | 39.3 | 15.3 | 29.7 | 8.0 | 9.7 | 7.3 | 49.1 | 57.0 | 47.7 | 35.5 | 44.4 | 29.4 | 19.9 | 10.1 | 15.3 |
GlcOS | 11.5 | 9.5 | 9.2 | 10.4 | 11.3 | 9.5 | 9.9 | 9.3 | 6.7 | 7.5 | 11.6 | 9.9 | 11.3 | 12.3 | 12.0 | 11.0 | 10.7 | 7.2 | 8.7 |
Xylose | 6.1 | 7.1 | 6.4 | 9.0 | 12.6 | 12.0 | 13.9 | 23.9 | 21.4 | 26.9 | 5.8 | 6.5 | 7.9 | 9.8 | 9.9 | 14.5 | 15.6 | 21.1 | 21.8 |
Arabinose | 30.2 | 32.5 | 27.4 | 34.8 | 34.0 | 35.9 | 27.6 | 41.0 | 18.6 | 20.4 | 20.6 | 28.9 | 34.8 | 41.4 | 29.8 | 34.5 | 23.0 | 23.2 | 20.3 |
Glucose | 1.9 | 1.9 | 1.3 | 1.5 | 1.5 | 1.5 | 1.2 | 3.0 | 1.4 | 2.1 | 0.9 | 1.5 | 1.5 | 1.3 | 0.6 | 1.4 | 0.8 | 1.6 | 1.5 |
5-HMF | 0.0 | 0.1 | 0.1 | 0.3 | 0.4 | 0.4 | 0.4 | 0.8 | 0.7 | 1.4 | 0.1 | 0.1 | 0.1 | 0.2 | 0.3 | 0.4 | 0.5 | 0.6 | 0.9 |
Furfural | 0.0 | 0.0 | 0.7 | 3.3 | 5.3 | 5.1 | 7.7 | 16.4 | 17.8 | 25.2 | 0.0 | 0.7 | 0.7 | 1.6 | 4.7 | 6.4 | 6.4 | 9.7 | 16.4 |
a Calculated according to literature.23b log![]() |
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t (min) | 0 | 4 | 8 | 12 | 16 | 20 | 25 | 30 | 40 | 0 | 4 | 8 | 12 | 16 | 20 | 25 | 30 | 40 |
pinitial (bar) | 20 | 0 | ||||||||||||||||
CSpCO2a/log![]() |
−1.28 | −0.70 | −0.45 | −0.30 | −0.19 | −0.09 | 0.00 | 0.07 | 0.20 | 2.74b | 3.20b | 3.39b | 3.52b | 3.63b | 3.71b | 3.80b | 3.87b | 3.99b |
Estimated pH | 3.78 | — | ||||||||||||||||
Final pH | 4.35 | 4.04 | 3.95 | 3.85 | 3.8 | 3.6 | 3.6 | 3.6 | 3.53 | 4.48 | 4.4 | 4.15 | 4.02 | 4.07 | 3.93 | 3.78 | 3.8 | 3.73 |
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Yield (g per 100 g of initial amount present in raw material) | ||||||||||||||||||
XOS | 34.4 | 55.0 | 65.6 | 67.5 | 63.6 | 60.3 | 52.1 | 40.7 | 22.7 | 28.9 | 46.3 | 60.5 | 70.8 | 73.1 | 72.5 | 70.5 | 66.6 | 50.3 |
AOS | 49.8 | 56.6 | 41.0 | 36.4 | 35.2 | 26.8 | 23.2 | 20.7 | 14.9 | 53.2 | 55.3 | 56.9 | 55.7 | 47.6 | 40.7 | 30.6 | 28.7 | 20.1 |
GlcOS | 10.0 | 11.3 | 11.0 | 9.5 | 12.3 | 9.1 | 9.3 | 9.3 | 8.0 | 11.9 | 11.6 | 12.5 | 11.8 | 11.0 | 12.1 | 10.7 | 10.7 | 10.0 |
Xylose | 5.7 | 6.4 | 7.9 | 10.9 | 16.2 | 15.7 | 18.0 | 20.5 | 25.1 | 5.8 | 6.0 | 6.3 | 6.2 | 8.0 | 10.9 | 11.6 | 13.9 | 19.9 |
Arabinose | 21.6 | 30.8 | 39.7 | 32.4 | 29.7 | 28.3 | 25.1 | 20.7 | 18.9 | 18.0 | 28.9 | 32.0 | 34.1 | 32.0 | 33.7 | 31.2 | 31.7 | 27.4 |
Glucose | 1.6 | 1.2 | 1.0 | 1.1 | 1.7 | 1.5 | 1.5 | 1.3 | 1.8 | 0.9 | 1.2 | 1.5 | 0.8 | 0.8 | 0.8 | 0.7 | 0.9 | 1.16 |
5-HMF | 0.0 | 0.1 | 0.2 | 0.3 | 1.7 | 0.4 | 0.5 | 0.7 | 1.0 | 0.0 | 0.1 | 0.1 | 0.1 | 0.2 | 0.3 | 0.3 | 0.4 | 0.6 |
Furfural | 0.1 | 0.6 | 1.3 | 4.1 | 4.4 | 8.8 | 7.7 | 11.8 | 23.0 | 0.0 | 0.2 | 0.7 | 1.1 | 2.7 | 4.3 | 5.9 | 8.8 | 14.0 |
Xylo-oligosaccharides were found to be the main products present in produced liquors. The processing of wheat straw using high-pressure CO2–H2O, performed with 50 bar of initial CO2 pressure for 12 min of holding time (CSpCO2 = −0.25), yielded a 79.6% xylan conversion to XOS with corresponding concentration of XOS as high as 14.8 g L−1. As increase of reaction severity (namely holding time), a decline in XOS content was observed, achieving its minimum (3.6 g L−1 or 18.6% of xylan conversion to XOS) for 45 min of reaction time (CSpCO2 = 0.30). Under this condition, an extended xylan hydrolysis to xylose and furfural (26.9% and 25.2%, respectively) coupled with loss of 77% of XOS yield were observed. Additionally, interesting is that comparing the reactions with different initial CO2 pressures, the XOS concentration was 18% higher for reactions performed with 50 bar of initial pressure of CO2 than this obtained at 20 bar of initial CO2 pressure. On the other hand, higher CO2 pressures favored quick decay of XOS yield along the reaction time than in the case of lower CO2 pressures.
Considering the effect of CO2 presence, it can be stated that, the addition of CO2 (initial pressure of 50 bar) to water-only reaction improved the XOS concentration by almost 10% and at the same time the highest XOS concentration was observed at shorter holding reaction time (shift from 16 to 12 min) as presented in Tables 1 and 2.
Xylose was the main monosaccharide present in liquors as depicted in Fig. 2. Under the best condition for XOS production (CSpCO2 = −0.25), the concentration of released xylose corresponded to 9% of the initial xylan content. The concentration of xylose increased with the progress of reaction severity achieving a maximum concentration of 5.7 g L−1 (26.9% xylan yield) at severest condition. Evaluating the influence of CO2 presence, the xylose concentration increased 71% with an initial CO2 pressure of 50 bar than in water-only reactions for the same holding reaction time (30 min).
Other hemicellulose-derived products such as arabino-oligosaccharides and arabinose exhibited similar profiles to those found for XOS and xylose. Under, the best condition for XOS production, the yield of released AOS and arabinose corresponded to 39.9% and 34.8% of initial arabinan content, respectively. Due to low content of arabinan in the raw material, the maximum concentration of AOS was only 1.9 g L−1 and it was achieved at the shortest examined holding time (4 min at 50 bar of initial CO2 pressure). For longer holding times, the concentrations of AOS and arabinose decreased rapidly and reached a minimum of 0.2 g L−1 and 0.7 g L−1, respectively.
The obtained liquors also contained free acetic acid and acetyl groups linked to oligosaccharides. As expected, the concentration of acetic acid increased along the reaction progress but demonstrated a tendency to stabilize (2.7 g L−1) for prolonged reactions.
The major C5-sugar degradation product, furfural, was detected in almost all experiments. The formation of furfural is highly influenced by either initial CO2 pressure or holding time. The increase of holding time from 12 min (CSpCO2 = −0.25) to 45 min (CSpCO2 = 0.30), increased furfural concentration almost 7.5-fold reaching even 25.2% xylan conversion yield. Furthermore, an increase of 85% of furfural concentration was observed in case of high-pressure CO2–H2O with 50 bar of initial CO2 pressure for 30 min in comparison to water-only reaction at the same holding time.
Among C6-derived products, gluco-oligosaccharides and glucose were found in the liquors. The formation of glucose in either oligomeric or monomeric form may have its origin in hydrolysis of amorphous cellulose, which is highly prone to hydrolysis even at very mild conditions. Analyzing the produced data it is clear that both GlcOS and glucose generally followed patterns of XOS and xylose. Even more, scrutinizing the effect of CO2, it can be concluded that in high-pressure CO2–H2O reactions performed at 50 bar of initial CO2 pressure for 30 min (CSpCO2 = 0.14) and in water-only process, the obtained GlcOS concentrations were very similar (3.4 g L−1 and 4.0 g L−1, respectively). On the other hand, the concentration of glucose was relatively different and in the case of high-pressure CO2–H2O was two times higher than in water-only reaction.
a Calculated according to literature.23 | |||||||||||||||||||
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t (min) | 0 | 4 | 6 | 12 | 18 | 20 | 25 | 30 | 35 | 45 | 0 | 4 | 8 | 12 | 16 | 20 | 25 | 30 | 40 |
pinitial (bar) | 50 | 35 | |||||||||||||||||
CSpCO2a | −1.16 | −0.64 | −0.49 | −0.25 | −0.09 | −0.05 | 0.07 | 0.14 | 0.19 | 0.30 | −1.25 | −0.70 | −0.45 | −0.30 | −0.19 | −0.10 | 0.00 | 0.08 | 0.19 |
Solid yield | 90.0 | 92.8 | 91.0 | 67.2 | 73.8 | 66.9 | 72.3 | 60.4 | 60.1 | 65.5 | 90.7 | 89.2 | 87.0 | 63.9 | 68.8 | 69.7 | 67.4 | 67.2 | 66.9 |
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Composition | |||||||||||||||||||
Glucan | 36.8 | 41.3 | 41.5 | 33.9 | 37.6 | 33.0 | 36.4 | 31.0 | 32.3 | 31.6 | 37.5 | 41.7 | 43.1 | 31.4 | 35.1 | 34.4 | 35.0 | 35.3 | 32.2 |
Xylan | 14.2 | 13.9 | 12.8 | 6.4 | 5.8 | 5.4 | 4.8 | 2.5 | 2.4 | 1.8 | 16.0 | 11.3 | 8.9 | 5.2 | 4.9 | 4.7 | 3.2 | 2.5 | 2.3 |
Arabinan | 1.2 | 1.0 | 1.0 | 0.3 | 0.4 | 0.3 | 0.4 | 0.1 | 0.3 | 0.0 | 1.2 | 0.7 | 0.3 | 0.1 | 0.1 | 0.1 | 0.0 | 0.0 | 0.0 |
Acetyl groups | 2.4 | 2.4 | 2.3 | 1.2 | 1.2 | 1.1 | 1.0 | 0.7 | 0.7 | 0.7 | 2.7 | 2.1 | 1.9 | 1.2 | 1.2 | 1.1 | 0.9 | 0.9 | 0.9 |
Klason lignin | 17.7 | 19.1 | 20.2 | 17.6 | 20.3 | 18.5 | 20.3 | 19.4 | 19.1 | 22.8 | 18.5 | 20.7 | 21.7 | 17.3 | 19.6 | 20.8 | 20.9 | 21.6 | 22.1 |
a Calculated according to literature.23b log![]() |
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t (min) | 0 | 4 | 8 | 12 | 16 | 20 | 25 | 30 | 40 | 0 | 4 | 8 | 12 | 16 | 20 | 25 | 30 | 40 |
pinitial (bar) | 20 | 0 | ||||||||||||||||
CSpCO2a/log![]() |
−1.28 | −0.70 | −0.45 | −0.30 | −0.19 | −0.09 | 0.00 | 0.07 | 0.20 | 2.74b | 3.20b | 3.39b | 3.52b | 3.63b | 3.71b | 3.80b | 3.87b | 3.99b |
Solid yield | 92.9 | 70.9 | 72.5 | 67.0 | 64.6 | 68.7 | 67.7 | 65.8 | 67.6 | 92.8 | 83.7 | 75.4 | 74.4 | 71.4 | 65.5 | 67.0 | 66.9 | 69.0 |
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Composition | ||||||||||||||||||
Glucan | 37.5 | 31.6 | 34.4 | 32.0 | 31.4 | 35.1 | 33.4 | 33.7 | 35.1 | 35.9 | 35.6 | 34.2 | 37.2 | 31.1 | 32.6 | 34.4 | 33.4 | 35.9 |
Xylan | 15.5 | 9.1 | 8.0 | 5.3 | 4.6 | 4.2 | 3.2 | 2.6 | 2.5 | 17.8 | 13.3 | 10.6 | 9.3 | 7.3 | 6.4 | 5.2 | 5.0 | 4.0 |
Arabinan | 1.3 | 0.4 | 0.2 | 0.1 | 0.1 | 0.0 | 0.0 | 0.0 | 0.0 | 1.6 | 0.7 | 0.4 | 0.2 | 0.2 | 0.2 | 0.1 | 0.0 | 0.0 |
Acetyl groups | 2.6 | 1.6 | 1.4 | 1.1 | 1.0 | 1.0 | 0.8 | 0.7 | 0.7 | 2.5 | 1.8 | 1.3 | 1.0 | 0.9 | 0.7 | 0.5 | 0.5 | 0.4 |
Klason lignin | 19.4 | 16.0 | 17.1 | 17.6 | 17.8 | 19.5 | 21.0 | 20.7 | 22.1 | 19.5 | 18.3 | 17.9 | 18.8 | 18.4 | 18.4 | 19.0 | 20.1 | 21.2 |
For all experiments, the lowest observed solid recovery yield was 60.4%. The water-only reactions demonstrated lower biomass dissolution resulting in solid recovery yield of 66.9% (for 30 min of reaction). This 11% of difference is mainly caused by more extensive CO2-assisted hydrolysis of hemicellulose, in particularly xylan, arabinan and acetyl groups. The presence of CO2 led to an efficient decrease of hemicellulose content in the processed materials and consequently in lower solid recovery yield. Considering the holding time effect, it is clear that it played a great role on biomass recovery yield either for CO2-assisted or for water-only reaction. As increase of the reaction time, the solid recovery yield decreased by 1/3 in comparison to the initial solid recovery yield found for 0 min holding time.
The performed pretreatments (high-pressure CO2–H2O and water-only reaction) also resulted in noticeable changes in chemical composition of processed solids. For the most severe CO2-assisted reaction (CSpCO2 = 0.30), up to 90.2% of hemicelluloses were removed. Despite the extensive hemicelluloses removal, an incomplete hydrolysis of xylan and minor amounts of acetyl groups present in processed solids were observed. For example, the content of xylan in processed solids gradually decreased with an increase of reaction severity reaching only 2.5% for the severest condition (CSpCO2 = 0.30). For water-only process at similar holding reaction time, the xylan content was 2-fold higher.
Similarly to the composition of the raw material, glucan is the major constituent of all processed solids, and for two the highest pressures examined its concentration decreased by less than 10% along the reaction time. For 20 bar of initial CO2 pressure and for water-only reaction, the glucan content was kept constant and varied within the experimental error. Another component of lignocellulosic biomass remaining in the processed materials is lignin. The lignin recovery was found to be between 16.0 and 22.8%. The reactions performed at two the highest initial CO2 pressures led to an increase of the lignin content in the processed materials to values above the lignin content in raw material. This fact could be explained by the formation of solid carbonaceous species (i.e. humins) due to lignin condensation reactions.36–39
Even at the highest initial CO2 pressure (50 bar) conditions, the solubility of CO2 in water phase is very low (0.01 mol fraction of CO2).30 Nevertheless, this limited solubility is high enough to contribute to lower pH value of the medium promoting the hydrolysis of hemicellulose. In addition, the obtained results clearly show that even lower pressure of CO2 (e.g. 35 bar) is sufficient to play an important role in hemicellulose hydrolysis. For instances, at maximal XOS concentration (CSpCO2 = −0.45), the solubility of CO2 in aqueous phase is as low as xCO2 = 0.007.
For the lowest initial CO2 pressure conditions, XOS concentration remains lower because the solubility of CO2 in water is null creating a system with three immiscible phases constituted by gaseous CO2, aqueous liquid phase and solid biomass. This explains why the three phase system formed by 20 bar of initial CO2 pressure allowed to obtain a pH of liquor and the concentration of XOS very similar to those obtained in the water-only reaction. This result also demonstrates the beneficial catalytic effect of CO2, which can only be achieved when CO2 is added at determined pressures.
Contrary to hemicellulose, cellulose is a very resistant polymer to hydrolysis, since it is mainly composed of a crystalline structure with just some amorphous regions.44,45 Hydrothermal technologies proved the ability to hydrolyze hydrogen-bond-linked structure of cellulose and its glycosidic bonds into glucose monomers. However, due to harsher conditions required for the cellulose hydrolysis, both GlcOS and glucose undergo quick conversion to degradation products such as 5-HMF. The conditions employed in this work are relatively mild to perform the hydrolysis of crystalline cellulose, thus it can be expected that the presence of GlcOS and glucose observed in all experiments was rather originated from the hydrolysis of amorphous cellulose than crystalline as it was also already reported in the literature.5,26
Various literature reports show that extensive hydrolysis of hemicellulosic acetyl groups after autohydrolysis experiments guided to lower pH.5,48,49 van Walsum et al. discovered very similar behavior by founding that the pH of liquors from corn stover and aspen wood treatment were quite different (3.68 and 4.95, respectively). This difference in final pH can be explained by the autocatalytic hydrolysis effect of acetyl groups of aspen wood in comparison to those of corn stover.43 McWilliams et al. did not report any beneficial effect of CO2 addition to water-only reaction on hydrolysis of aspen wood at 180–220 °C since the formation of carbonic acid improved neither xylose nor furfural compounds yield.50 Although this work does not show any benefits in the use of CO2 it is important to understand that aspen wood contains highly acetylated hemicelluloses and these compounds are highly susceptible to autohydrolysis at temperatures above 170 °C, thus no additional of CO2 was required51 as the formed acetic acid catalyzes the hydrolysis of the hemicellulose.
It is known that arabinan is one of the easiest hydrolysable fractions of hemicellulose5 and arabinan content in the processed solids decreased with the increase of the reaction severity. Similarly, the content of acetyl group in produced solids decreased accordingly to the increase of acetic acid concentration in the liquor reaching the content as low as 0.7% at CSpCO2 = 0.30 similarly to literature reports where comparable range of acetyl groups content was found in processed solids.5,25,26
At the severest condition examined (CSpCO2 = 0.30), the processed solids presented high cellulose and Klason lignin contents. Among all polysaccharides present in lignocellulosic biomass, cellulose is the least prone fraction for hydrolysis and this was also observed in liquor composition in which the concentration of GlcOS, glucose and 5-HMF were lower than products derived from hemicellulose. This cellulose characteristic is even more visible considering the relative amount of glucan in processed solids that was strongly enriched in comparison to the untreated biomass. Nevertheless, there are other parameters, such as both biomass and cellulose crystallinity that constitute a hurdle to achieve high enzymatic hydrolysis yields. To evaluate the effect of high-pressure CO2–H2O on cellulose crystallinity, native biomass and two processed solids samples (produced from high-pressure CO2–H2O performed at CSpCO2 = 0.14 and water-only reaction carried out with logR0 = 3.87), underwent the FTIR analysis. Two absorption bands were selected for analysis of cellulose-rich fraction crystallinity. A band at 1437 cm−1 is characteristic to the scissoring vibration assigned to CH2 in the crystalline cellulose and the band at 898 cm−1, assigned to C–O–C bonds of β-1,4-glycosidic bonds is typical for amorphous fractions i.e. amorphous cellulose and hemicellulose.52 To compare the cellulose-rich fraction crystallinities, the LOI index, which is the ratio between absorption bands at 1437 cm−1 and 898 cm−1, was calculated.35 The LOI results for native wheat straw and processed solids by water-only reaction and high-pressure CO2–H2O are given in Table 5.
A1437 | A898 | LOI (A1437/A898) | |
---|---|---|---|
Native wheat straw | 0.239 | 0.104 | 2.30 |
Water-only reaction (log![]() |
0.217 | 0.061 | 3.56 |
High-pressure CO2–H2O (CSpCO2 = 0.14) | 0.183 | 0.044 | 4.16 |
The analysis of produced data shows that LOI for the untreated biomass has lower value (LOI = 2.30) than processed biomasses from water-only reaction (3.56) and high-pressure CO2–H2O processed solid (4.16). However, close inspection of obtained data shows that water-only process removed amorphous fractions (either cellulose or hemicellulose) as the absorption of the band at 898 cm−1 was reduced by 41% in comparison to untreated wheat straw while at the same time crystalline cellulose was affected insignificantly. In the case of high-pressure CO2–H2O, both “amorphous” and “crystalline” bands were affected because both were reduced significantly. Although the LOI data does not reflect directly the reduction of crystallinity but the understanding of the vibrations resulting in creation of both bands allows to state that the water-only reaction in comparison to high-pressure CO2–H2O is less severe, reduces the crystallinity less and is more selective for hemicellulose hydrolysis.
Klason lignin is the second major component of processed solids and its content increased with the reaction progress and with the increase of exerted CO2 pressure. Analogously to cellulose, the Klason lignin content in processed solids increased due to enhancement of a xylan removal. The Klason lignin content in processed materials is typical either for autohydrolysis process or for high pressure CO2-assisted autohydrolysis as reported in literature.5,25,26,49
Footnote |
† The authors wish it to be known that the first two authors should be regarded as joint first authors. |
This journal is © The Royal Society of Chemistry 2015 |