Dissolution of wet wood biomass without heating

Abstract

Lignocellulose biomass including wood is an abundant, natural, and renewable material, and is a promising fossil fuel substitute. However, there are still no powerful solvents to extract polysaccharides from biomass. Here we report a tetra-n-butylphosphonium hydroxide–water mixture as a potential solvent for wood dissolution without heating. Upon gentle stirring at room temperature, this solution containing 40 wt% water extracted 37% of polysaccharides after stirring for only 1 hour. This excellent dissolution ability was maintained in a wide range of water content.

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Introduction

Since plants cannot escape predation by moving, many protect themselves using an armor made of high strength polymers such as cellulose, which has limited solubility in most solvents. The only way to dissolve them is the use of enzymes such as cellulase, as termites do. In spite of its insolubility, polysaccharide, which is a major component of plants, is a useful material. From the viewpoint of energy conversion, lignocellulose can be regarded as accumulated energy from the sun. Hydrolysis of lignocelluloses yields glucose, which is also the hydrolysis product of starch. Glucose is a potential starting material for several chemicals, as well as a source of energy. Since lignocellulose does not have a competing use as food, it could be a suitable fossil fuel substitute.¹ The problem is its insolubility. Because of the lignin network and the strong intra- and inter-hydrogen bonding networks of cellulose,² the effective extraction of polysaccharides has not yet been achieved. A variety of pretreatment processes of wood biomass have been reported that disrupt the cellulose structure and make it possible to use the biomass as an renewable material; these processes may be physical, biological, or chemical.³ However, they still require a large amount of energy, multi-step, long-term treatment, and much equipment.³

Several solvents exist for cellulose, including a NaOH–water mixture with or without additives,^4,5N-methylmorpholine N-oxide monohydrate,⁶ and a LiCl–N,N-dimethylacetamide mixture.^7–9 These solvents have disadvantages such as very limited ranges of conditions, toxicity, inapplicability to lignocellulose, and others. In contrast, ionic liquids are promising novel solvents for pure cellulose and lignocellulose.^10–13 Some ionic liquids have the potential to dissolve cellulose. We recently reported that polar ionic liquids dissolved polysaccharides, and we also successfully extracted them from plant biomass without heating.¹⁴ These cellulose-dissolving ionic liquids suffer from a drastic decrease of cellulose solubility with increasing water content.^15,16 Since polar ionic liquids are highly hygroscopic, like polar molecules, they readily absorb water from the atmosphere.¹⁷ Plant biomass contains a considerable amount of water. It is therefore necessary to include a drying process for both the ionic liquids and plant biomass when using ionic liquids as solvents for lignocellulosic biomass. There is a strong motivation to design a new system without the drying processes.

Recently, hydroxide solutions that have organic cations with or without additives have been suggested as novel solvents for cellulose.^18,19 In particular, tetra-n-butylphosphonium hydroxide (TBPH) has a strong proton accepting ability even in the presence of water.¹⁸ Here we report on wood biomass treatment without heating using TBPH containing water. This solvent successfully extracted polysaccharides without heating or biomass drying.

Experimental

Materials

TBPH containing 60 wt% water was provided by Hokko Chemical Industry Co. Ltd., and TBPH–water mixtures were prepared by the evaporation or addition of water. Wood disks (34 mm in diameter and 1 mm in thickness) and poplar powder (36–200 mesh) were provided by the Forestry and Forest Products Research Institute. Methanol (>99.8%) was purchased from Kanto Chemical Co. Ltd.

Poplar dissolution test

Poplar powder was first dried under vacuum for 6 hours at room temperature before use (the water content was about 1%). The water content of the poplar powder was measured using thermogravimetric analysis (TGA). The TGA measurements were performed using a SEIKO TG/DTA 220 instrument with a heating rate of 10 °C min⁻¹ from 25 to 110 °C under nitrogen gas. After holding the sample for 30 min at 110 °C, the weight loss detected indicated the water content of the sample. TBPH solutions (9.5 g) containing different amounts of water were put into 30 ml vials, and the dried poplar powder (0.5 g) was added (Scheme S1 in ESI†). The mixture of the TBPH solution and poplar powder was gently stirred (300 rpm) at 25 °C. After that, the residue was separated by centrifugation. The collected supernatant solution containing poplar extracts was stirred vigorously, and methanol was added to precipitate out the materials extracted from the poplar. After washing, the dried weight of the precipitated materials was evaluated, and the extraction rate was determined with the following equation:

extraction rate (%) = (9.5 × Y)/{0.5 × (X − Y)} × 100

, where X is the weight of the poplar dissolving TBPH solution (g), Y is the weight of the extracted material (g), and the numbers 9.5 and 0.5 are the initial weights of the TBPH aqueous solution and the poplar powder, respectively.

Results and discussion

Dissolution of wood disks without heating

Pine, cedar, and poplar were used as typical types of wood. Samples were cut into disks with a diameter of 34 mm and a thickness of 1 mm. TBPH containing 40 wt% water was used as a solvent because this water content is the best for dissolving pure cellulose.¹⁸ These disks were put into the TBPH solution (15 g), and changes in their shape were observed visually without stirring at room temperature. Every wood disk became swollen and distorted, and gradually broke down; in particular, the pine disk was fragmented more easily than the other disks (Fig. 1). These results indicate that TBPH solution is powerful for the extraction of constituents in wood biomass by simply soaking without heating. Changes of shape on a smaller scale were tracked by using wood powder and optical microscopy. The results are summarised in the ESI (Fig. S1).†

Fig. 1 Changes in the shape of the wood disks in TBPH containing 40 wt% water. Wood disks (diameter 34 mm, thickness 1 mm, about 0.5 g) were soaked in TBPH containing 40 wt% water (15 g) without heating or stirring.

The effect of water content on the dissolution of wood powder

Dissolution tests of wood samples were conducted in TBPH solutions containing different amounts of water. As poplar is a typical hardwood and is often used as a standard wood sample, poplar wood powder was used as a standard biomass sample. The material extracted from poplar powder is a mixture of polysaccharides. Precise analysis of the ratio of components is set out in a latter section. Table 1 summarises the degree of extraction of wood components after stirring for 1 hour. The degree of extraction increased with increasing water content in the TBPH from 30 to 50 wt%, but that with 70 wt% water could extract only 4.9% of the polysaccharides, since the solubility of cellulose had dropped at that water content. TBPH containing 40–50 wt% water successfully extracted 36–37% of the polysaccharides from the poplar powder. TBPH containing 30 wt% water extracted 24% of the polysaccharides within 1 hour. TBPH containing 40–50 wt% water was concluded to be the best for extracting polysaccharides from wood powder at room temperature.

Table 1 Effect of the water content of TBPH on the degree of polysaccharide extraction after 1 hour stirring

Water content of TBPH (wt%)	Extraction degree (%)^a
a Weight percent relative to added poplar, the poplar powder was added at a final concentration of 5 wt% relative to the TBPH solution.
70	4.9
60	28
50	36
40	37
30	24

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Wet biomass treatment

TBPH solution is more effective in the presence of some water, unlike other cellulose-dissolving ionic liquids. In this section, we evaluate the ability of the TBPH solution to extract polysaccharides from wet wood biomass. In the previous section, the poplar was dried under vacuum in advance, and the water content of the wood was reduced to about 1%. Of course, natural wood contains more water. Consequently, we attempted to extract polysaccharides from air-dried wood. The poplar powder was dried naturally by leaving it at room temperature, and the water content of the wood fell to 7.5% (7.5 g of water per 100 g of dried wood, details are described in the ESI†). Water was added to the poplar powder (20% relative to the weight of the dried poplar) and the mixture was stirred to make it homogeneous. The degree of extraction of polysaccharides from the air-dried poplar using TBPH containing 40 wt% water was then determined. The final concentration of added poplar was 5 wt% (as a dried weight). The TBPH solution extracted 41% of the polysaccharides from the air-dried poplar, even though only 37% was extracted from the vacuum-dried poplar. This difference was considered to be not due to experimental error, because the value of the experimental error in the measured degrees of extraction was about 2–3%. Spinu et al. reported that wet biomass was hydrated more easily than dried biomass in an aqueous salt solution.²⁰ Similar hydration might occur in the case of the TBPH solution with air-dried poplar (not the vacuum-dried specimen). This finding shows that it is not necessary to dry wood to extract polysaccharides. Air-drying is sufficient in the case of treatment with TBPH solutions.

Air-drying is an easy pre-treatment. However, natural plant biomass contains a considerable amount of water, so the air-drying needs time. We therefore evaluated the capability of the TBPH solution to treat “wet” biomass without any pre-treatment, even air-drying. Water was added to the poplar powder (200% relative to the weight of the dried poplar, i.e., 20 g of water was added to 10 g of dried wood powder) and the mixture was stirred to make it homogeneous. The prepared wet wood was then added to TBPH containing 40 wt% water so as to make the final concentration 5 wt% relative to the TBPH solution (as a dried weight). After 1 hour of stirring, TBPH containing 40 wt% water was found to extract 32% of the polysaccharides from the added wet poplar.

In spite of the ion penetration effect in the wet wood biomass, the degree of extraction of the polysaccharide decreased. This might be caused by the decrease of the basicity of the solution.

The water content of the TBPH solution was changed by the addition of wet wood biomass, and this might decrease the hydrogen bonding basicity of the TBPH solution. Since the polysaccharide extraction is dependent on water content, as mentioned above, we calculated the water content of the TBPH solution after addition of the wet wood biomass. In this paper, 1.5 g of wet poplar (a mixture of 0.5 g poplar and 1.0 g water) was added to 9.5 g of TBPH solution (5.7 g TBPH and 3.8 g water). Therefore the water content of the TBPH solution was changed from 40 wt% to 45.7 wt% after addition of the wet poplar. This TBPH solution was expected to have the ability to extract polysaccharides, because the TBPH solutions containing both 40 and 50 wt% water showed potential for extraction, as summarised in Table 1. Accordingly, it should be mentioned here that water-containing TBPH is effective for the extraction of polysaccharides from wood powder unless large excess amounts of wet powder are added.

In this experiment, we evaluated the degrees of extraction of polysaccharides with only 1 hour of stirring. Thus, differences in the dispersibilities may also affect the degrees of extraction. The degrees of extraction will be improved with time.

Auto-recovery of the water content

TBPH containing water dissolves and extracts polysaccharides even from wet wood biomass. However, this wet biomass treatment may lead to increased water content in the TBPH solution. As the ability of the TBPH solution to treat biomass was weakened by increasing the water content above 70 wt%, the TBPH solution should be concentrated to recover its ability for polysaccharide extraction from wet biomass. The process of concentration, by heating or decompression, for example, requires energy and therefore increases the energy cost of the overall process of wood biomass treatment. An energy-saving TBPH solution concentrating process would therefore be helpful.

The fresh TBPH solution used in this paper contains 60 wt% water. This solution was left open to the atmosphere without heating or stirring, and the change in water concentration was traced with time (25 °C, humidity: 45%). The water content was found to fall to about 45 wt% after a week (Fig. S2†). This result suggests that the TBPH solution can easily be concentrated, again automatically without any energy consumption. Although ionic liquids are potential solvents for biomass treatment, their polysaccharide-extracting ability is significantly reduced by water addition and they also easily absorb water from the air,¹⁷ so polysaccharide extraction requires drying of both the ionic liquid and the biomass. In contrast, TBPH solution is automatically concentrated even in the open air.

These findings for wet biomass treatment and auto-recovery suggest an energy saving recycling system for treating biomass as depicted in Scheme 1. The plant biomass will be separated into residues and extracted by adding a poor solvent. After that, the filtrate will be re-concentrated with time when water is used as the poor solvent. However, there are a few more steps required to realize recycling to avoid the effect of impurities and other factors.

Scheme 1 A proposed procedure for plant biomass treatment with TBPH solution.

The effect of time on the degree of extraction

It is empirically demonstrable that the stirring time is another important factor to determine the degree of extraction. Poplar powder containing 1% water was added to TBPH containing 30–70 wt% water, and the mixture was stirred for up to 24 hours at room temperature. The results are summarised in Fig. 2. When the TBPH contained a large amount of water, the degree of extraction of polysaccharide was low (see Table 1). TBPH containing 70 wt% water could extract only 5.7% of the polysaccharides even after 24 hours of stirring. On the other hand, TBPH containing 40 wt% water extracted more than 50 wt% of the polysaccharides after mixing for 24 hours. The extent of polysaccharide extraction with TBPH solutions containing 40 and 50 wt% water and 24 hours stirring reached 59% and 44%, respectively. Furthermore, the value with TBPH containing 30 wt% water reached 62% after 24 hours stirring. This could be due to the high concentration of hydroxide anions. A certain amount of water is known to be needed to suppress the decomposition of TBPH.¹⁸ On the other hand, more than 70% water lowered the extraction ability of the TBPH solution. Considering both the stability of the TBPH aqueous solution and the extraction performance, TBPH containing 40 wt% water is proposed to be the best medium for treating wood biomass.

Fig. 2 The effect of stirring time on polysaccharide extraction. The degree of extraction is in wt%, relative to the amount of added poplar.

Selective extraction of polysaccharides

Here we have analysed the composition of the extracted material. After 1 hour of treatment of the poplar powder with TBPH containing 40 wt% water, the extracted material was analysed by a standard sugar analysis (Fig. 3 and Table S1†).²¹ The extract was found to consist mainly of polysaccharides such as cellulose (glucan) and xylan. Lignin was found mostly in the residue rather than in the extract. It was confirmed that the extract contains a significant amount of cellulose when TBPH solution containing 40 wt% water was used as a solvent.

Fig. 3 The component ratios of intact poplar, and the extract and residue after treatment with TBPH containing 40 wt% water for 1 hour. The value of the glucan ratio essentially indicates the amount of cellulose.

However, the solubility of these components was not the same in TBPH solutions with different amounts of water. Then, selective extraction of polysaccharides was examined by changing the water content of the TBPH.

We found that the power of the TBPH solutions to dissolve xylan was a function of the water content of the TBPH. Commercially available xylan from beechwood was added to several TBPH solutions with different water contents and stirred at 25 °C. The best concentration of TBPH for dissolving xylan was not the same as that for cellulose (Table S2†). TBPH containing 30 wt% water did not dissolve xylan powder. That with 40 wt% water dissolved only a small amount of xylan. On the other hand, xylan was rapidly dissolved in TBPH containing 50 wt% water or more. TBPH containing larger amounts of water should therefore dissolve xylan more efficiently than cellulose. These results suggested that xylan-rich materials should be extracted from lignocellulose simply by controlling the water content of the TBPH solutions.

We then examined the extraction of xylan-rich materials from wood. Poplar was treated with TBPH containing 70 wt% water, and the components of the extracted material were investigated with sugar analysis. When TBPH containing 40 wt% water was used to treat poplar material, the main component of the extract was cellulose (Fig. 4A, based on the same data as shown in Fig. 3; the width of each bar relates to the amount of material extracted). In contrast, TBPH containing 70 wt% water successfully extracted xylan-rich materials from poplar, and the xylan content exceeded 50% (Fig. 4B; again the width of the bar indicates the amount of material extracted (4.24%), see Table S3†). We also carried out FT-IR and ¹³C-NMR measurements to analyse the components of the extracted materials. These results also suggested that the extract was a xylan-rich material (Fig. S3†). The amount of extracted material was 4.24%, which was less than that using TBPH containing 40 wt% water (37.0%), but it clarified that TBPH containing 70 wt% water dissolves and extracts xylan-rich materials easily.

Fig. 4 The component ratios of the materials extracted from poplar that had been treated with TBPH containing 40 wt% water (A) or 70 wt% water (B) with 1 hour of stirring, and the materials precipitated by adding excess water to the TBPH solution containing both 40 wt% water and extracts from poplar (C).

We also propose a simple process for obtaining cellulose-rich materials selectively with the effective use of water. We have studied the precipitation characteristics of cellulose and xylan from TBPH solutions. When water was added to the TBPH solutions containing both cellulose and xylan, cellulose was selectively precipitated from the solution (Table S4†). Based on these results, we have studied the capability of selectively precipitating cellulose from poplar-dissolving TBPH solution. After dissolution of poplar in TBPH containing 40 wt% water, an excess amount of water was added to the solution. The precipitated materials were collected after washing, and the component ratio was analysed using the sugar analysis method (Fig. 4C; the width of the bar indicates the amount of material extracted (14.4%), see Table S3†), as well as FT-IR and ¹³C-NMR measurements (Fig. S4†). The results indicated that the precipitated material comprises cellulose-rich polysaccharides. This tells us that cellulose-rich compounds are collected as precipitates selectively from a TBPH solution which has been used to dissolve poplar, simply by using water as a poor solvent.

The component ratios of extracts from poplar have been found to be easily controlled by the use of water. Cellulose-rich materials and xylan-rich materials can be extracted preferentially from wood by using both a TBPH solution and a suitable poor solvent, such as water. Since these selective collection processes do not require any complex or energy-dependent processes such as heating, cooling, or explosions, they may become important methods to obtain polysaccharides in diverse fields.

Conclusions

We have proposed a novel treatment process for wood using TBPH solution. Using this solvent, much polysaccharide is extracted from poplar within a short period of time without heating or cooling. This solvent extracts a certain quantity of polysaccharides from wood biomass even under wet conditions (e.g., containing 200% water). The component ratio of extracts from poplar is easily controlled by changing the amount of water.

Acknowledgements

This work was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (no. 21225007). M. A. acknowledges the financial support of the Japan Society for the Promotion of Science (Research Fellowship for Young Scientists).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Optical micrographs of poplar in TBPH solution, poplar powder dissolution test, auto-recovery of water content, xylan dissolution test, xylan rich material extraction from wood, polysaccharides precipitation test, and cellulose rich material extraction from wood, including FT-IR and ¹³C-NMR spectra. See DOI: 10.1039/c4ra01038h

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