EP4126976A1

EP4126976A1 - Method for producing xylan

Info

Publication number: EP4126976A1
Application number: EP21717488.7A
Authority: EP
Inventors: Lari VÄHÄSALO; Sebastian VON SCHOULTZ; Nicholas LAX
Original assignee: Bln Woods Ltd AB
Current assignee: Bln Woods Ltd AB
Priority date: 2020-03-31
Filing date: 2021-03-30
Publication date: 2023-02-08
Also published as: FI129258B; FI20205330A1; WO2021198561A1

Abstract

The invention relates a method for producing xylan, which comprises - obtaining an aqueous starting solution comprising xylan and optionally at least one non-cellulosic polysaccharide, and adjusting pH of the aqueous solution to a first pH value ≥7, subjecting the aqueous solution to reducing conditions, and heating the aqueous solution to a reaction temperature of ≥50 °C. The aqueous solution is maintained at the reaction temperature, and xylan particles are precipitated from the aqueous solution by cooling the aqueous solution to a precipitation temperature and/or by reducing pH of the aqueous solution to a precipitation pH. Finally, the precipitated xylan particles are separated from the aqueous solution.

Description

METHOD FOR PRODUCING XYLAN

The present invention relates to a method for producing xylan according to preambles of enclosed independent claims.

Polymeric xylans from wood, straw, grains and a lot of other biomass have a backbone formed from monomeric xylose units and side chain groups bound to the backbone. Typical side chain groups are sugar units comprising of arabinose, methylated glucuronic acids and non-methylated glucuronic acids, as well as acetyl groups. The side chain groups depend on the source of the xylan.

High molecular mass hemicelluloses, i.e. non-cellulosic polysaccharides, can be effectively extracted in a polymeric form from biomass, for example by hot-water- extraction disclosed in WO 2014/009604. Hemicellulose extracts usually comprise a variety of different non-cellulosic polysaccharides, one of which is xylan. The xylose content of the total extracted non-cellulosic polysaccharides varies depending on the raw material. For example, in birch wood, xylose content in total extracted polysaccharides is close to 70% of dry matter, methylated glucuronic acid content close to 10% and acetyl groups can vary in the range of 3- 10%. In addition to xylans, the hemicellulose extracts also contain other non-cellulosic polysaccharides. Examples of other non-cellulosic polysaccharides are galactoglucomannans, arabino-galactans/galactans and pectins, which are rhamno- galacturonic acid polysaccharides, that may make up in the range of 20% of total non-cellulosic polysaccharides in birch hemicellulose extracts. In hemicellulose hot- water extracts of wheat straw the amount of xylose is around 80% and is comparable with birch hemicellulose extracts. In wheat bran hemicellulose extracts the amount of xylose can be as low as 30% due to the high starch content. In oat husk hemicellulose extracts the amount of xylose is ca. 55%, also due to a rather high amount of starch.

Traditionally, xylose is produced from a hydrolysate or pre-hydrolysate from biomass with a significant xylan hemicellulose content. After hydrolysis into monomers, the hydrolysate contains a number of sugar monomers in addition to xylose, such as arabinose, galactose, glucose, mannose and a variety of uronic acids, such as glucuronic and galacturonic acids. In addition, many hydrolysates contain low-molar-mass organic acids and aldehydes such as oxalic acid, furfural and hydroxymethylfurfural (HMF). The purification of such a hydrolysate into a stream of xylose with high purity is extremely expensive process where techniques such as chromatographic separation using anion and cation exchange resins are used. More efficient methods involve the use of xylanase enzymes which selectively hydrolyze xylans into xylose monomers. However, the drawback is that xylans contain side groups such as arabinose and 4-O-Me-glucuronic acid units which are not cleaved using xylanase.

In xylan/xylose production, the main problem is to be able to reach high enough purity of the product. Typically, over 98% purity is required, especially for xylitol production. The only solution to date is to monomerize xylan/xylose rich material either by acid, catalytic or enzymatic hydrolysis with subsequent purification. The purification can be done by membrane filtration steps and/or a variety of chromatographic separation techniques, followed by xylan/xylose crystallization, which are both expensive and tedious.

There is no known prior art for production of xylan, where xylan can be precipitated as xylan particles, even in a nano-particle size range.

It is the aim of this invention to reduce or even eliminate the problems related to known art.

The aim of the present invention is to provide a simple and effective method for producing xylan, especially with a high xylose unit content.

A further aim of this invention is to provide a simple method for producing xylan in particle form.

These objects are achieved by the features disclosed in the independent claim and the invention is defined by the features of the enclosed independent claim. Some preferred embodiments of the present invention are presented in the dependent claims.

All the described embodiments and advantages apply to all aspects of the present invention, even if not always explicitly stated so.

A typical method according to present invention for producing xylan comprises

- obtaining an aqueous starting solution comprising xylan and optionally at least one non-cellulosic polysaccharide,

- adjusting pH of the aqueous solution to a first pH value >7, subjecting the aqueous solution to reducing conditions, and heating the aqueous solution to a reaction temperature of >50 °C,

- maintaining the aqueous solution at the reaction temperature, and

- precipitating xylan particles from the aqueous solution by cooling the aqueous solution to a precipitation temperature and/or by reducing pH of the aqueous solution to a precipitation pH, and

- separating the precipitated xylan particles from the aqueous solution.

Typical xylan particles according to the present invention are obtained by a method according to the invention.

Now it has been surprisingly found that subjecting alkaline xylan containing aqueous solution to reducing conditions and heating the aqueous solution to an elevated temperature it is possible to precipitate xylan particles with high purity from the aqueous solution by simply cooling the solution and/or by adjustment of pH of the solution. In the present invention the combination of elevated temperature, alkaline environment and the reducing conditions, i.e. the negative redox potential of the aqueous solution, provides unexpected advantages. For example, it is assumed, without wishing to be bound by a theory, that the reduction of terminal sugar units in xylan prevents a “peeling” reaction which would otherwise degrade xylan into fragments with low molecular mass. This means that the molecular mass of the xylan is not significantly reduced during the process. The combination of the present invention also may also provide removal of the side chain groups from the xylan backbone as well as effective deacetylation of the xylan backbone. It is further assumed, that non-degraded, deacetylated and side chain cleaved high molecular mass xylan molecules are then able to form self-assembled “crystalline” water- insoluble particles, i.e. xylan nanocrystals, which can be easily precipitated after lowering the temperature and/or pH of the aqueous solution. This enables the production of water insoluble polymeric xylan with high purity since the precipitated xylan particles can be mechanically separated from other water-soluble polysaccharides, lignin-containing contaminants and degradation products. Furthermore, the obtained xylan particles show pleasant appearance, as they have high whiteness. The precipitated xylan particles can be used in many applications, especially in those where traditionally cellulose nanocrystals are used. Further, the high purity, i.e. high xylose content, of the precipitated particles enables their efficient hydrolysis into pure xylose monomers and then into xylitol, furfural and other platform and bulk chemicals.

The aqueous starting solution comprises xylan. The aqueous starting solution may optionally also comprise at least one other non-cellulosic polysaccharide. According to one embodiment of the invention the aqueous starting solution may comprise 1 - 70 weight-%, preferably 10 - 60 weight-%, more preferably 20 - 55 weight-%, of xylan, calculated from the total dry solids content of the aqueous solution. The starting solution may be an extract, which contains a mixture of hemicelluloses and which originates from wood, straw, grains, algae, husks, spent grains from brewery and distillery industry, preferably from hardwood. Hemicellulose denotes in this context a non-cellulosic polysaccharide originating from plants.

The aqueous starting solution may further comprise, in addition to xylan, at least one other non-cellulosic polysaccharide selected from mannose, galactose, rhamnose, arabinose and any of their mixtures. The amount of this non-cellulosic polysaccharides in the staring solution is usually 0.1 - 20 weight-%, preferably 1 - 20 weight-%, more preferably 2 - 10 weight-%, calculated from the total dry solids content of the aqueous solution. The present invention is especially suitable for producing xylan from extracts comprising a mixture of high molecular mass hemicelluloses, i.e. non-cellulosic polysaccharides. Such extracts can be obtained, for example, by hot-water- extraction of wood-based or non-wood based biomass. Suitable hot-water- extraction process is disclosed e.g. in WO 2014/009604. Hemicellulose extracts which can be used as aqueous starting solution in the present invention usually comprise a mixture of different non-cellulosic polysaccharides, such as xylan, mannose, galactose, rhamnose, arabinose galactoglucomannans, arabino- galactans/galactans, and/or pectins.

The aqueous starting solution may comprise at least 30 weight-% of water. Preferably the aqueous starting solution is free of any organic solvents.

The pH of the aqueous starting solution is adjusted to a first pH value >7, preferably >8, more preferably >9. The pH of the aqueous starting solution may be adjusted to the first pH preferably before the aqueous solution is subjected to reducing conditions. However, in some embodiments the aqueous solution is first subjected reducing conditions and then the pH of the aqueous starting solution is adjusted to the first pH. The pH of the aqueous solution may be adjusted by addition of an alkali, such as NaOH, KOH, Ca(OH)2 or Mg(OH)2 or any of their mixtures, or by addition of an oxide, such as Na20.

The aqueous solution is subjected to reducing conditions. According to one embodiment the aqueous solution may be subjected to reducing conditions by addition of a reducing agent, which is able to donate electron(s) in a redox reaction, such as NaBH4, Ca(BH4)2 or NaHSC>3, to the aqueous solution. Preferably the reducing agent is selected from a group consisting of NaBH₄, Ca(BH4)2 and NaHSOs, but it is possible to use any suitable reducing agent available. The reducing agent may be added before or after adjustment of the pH of the aqueous solution to the first pH value >7, preferably >8, more preferably >9. The reducing agent may be added in amount of 1 - 100 mg/g, preferably 5 - 50 mg/g, more preferably 13 - 20mg/g. The amount of reducing agent is based on the amount of xylan, as dry, in the starting solution. According to one embodiment of the invention the aqueous solution is subjected to reducing conditions under an inert atmosphere. The inert atmosphere may be a nitrogen atmosphere. It is also possible to remove or reduce oxygen from the aqueous solution by bubbling through it inert gas, such as nitrogen.

According to one embodiment of the invention the pH of the aqueous starting solution may be adjusted under the reducing conditions to a second pH value of >10, preferably >11. The pH of the aqueous solution may be adjusted by addition of alkali, such as NaOH or KOH, Ca(OH)2 or Mg(OH)2 or any of their mixtures, or by addition of an oxide, such as Na₂0. According to one embodiment the alkali concentration in the aqueous solution may be over 0.5 mol/l, preferably over 1 mol/l. The alkali concentration may preferably be less than 4 mol/l. The alkali concentration may be 0.3 - 4 mol/l, preferably 0.5 - 3 mol/l, more preferably 1 - 2.5 mol/l. For example, when NaOH is used for the adjustment of the pH of the aqueous solution, the NaOH concentration in the aqueous solution may be about 0.5 mol/l, preferably about 1 mol/l.

The alkaline aqueous solution, preferably after the addition of the reducing agent, is heated to the reaction temperature of >50 °C. According to one embodiment of the invention the aqueous solution may be heated to the reaction temperature of >80 °C or >120 °C, preferably >130 °C, more preferably >150 °C. The reaction temperature is preferably <250 °C, more preferably <200 °C. The reaction temperature may be, for example, in a range of 50 - 250 °C, preferably 80 - 225 °C, more preferably 120 - 200 °C or 155 - 200 °C. In one embodiment the aqueous solution is first heated to the desired reaction temperature and then subjected to the reducing conditions.

The aqueous solution is maintained a desired time at the reaction temperature. It has been observed that xylan with sufficient purity may be obtained even if the aqueous solution is maintained at the reaction temperature a relatively short time, which makes the process especially suitable for industrial production. However, a longer maintenance at the reaction temperature may improve the purity of the obtained xylan. According to one embodiment of the invention the aqueous solution may be maintained at the reaction temperature for at least 15 min, preferably at least 30 min, more preferably at least 1 h and/or for less than 12 h or less than 8 h, preferably less than 6 h, more preferably less than 5 h.

Xylan particles are precipitated from the aqueous solution by cooling the aqueous solution to a precipitation temperature and/or by reducing pH of the aqueous solution to a precipitation pH. The precipitation pH may be <10, preferably <8, more preferably <7.5 or <7. The suitable precipitation temperature may be <50 °C, preferably <60 °C, more preferably <100 °C, depending on the reaction temperature. The pH may be adjusted to the desired precipitation pH by addition of suitable acid, such as formic acid or the like. The precipitation of xylan particles from the aqueous solution can be performed without addition of organic solvents, such as glycerol, methanol and ethanol. When the temperature and/or pH value of the aqueous solution is lowered, xylan effectively aggregates and precipitates from the aqueous solution. Other non-cellulosic polysaccharides, such as mannose, galactose, rhamnose or arabinose, are not precipitated but remain dissolved in the aqueous solution. This enables the production of water-insoluble polymeric xylan particles with high purity, as the precipitated xylan particles can be easily separated from the aqueous solution as well as from other non-cellulosic polysaccharides, lignin-containing contaminants and possible degradation products.

If desired, the separated xylan particles may be washed by using any suitable washing liquid, such as water, ethanol, (iso)propanol, methanol or any of their mixtures.

The method according to invention provides an effective method, preferably where the steps of pH adjusting, heating in the reducing environment and precipitation are all performed in a single reactor, reaction vessel, tank or the like.

The present invention provides a possibility to produce high molecular mass xylan with a high purity. According to one embodiment of the invention the xylan may have a weight average molecular weight >3000 g/mol, preferably >5000 g/mol, more preferably >10 000 g/mol. It has been observed that the precipitation step is more effective when the xylan has a high molecular mass, which leads a high yield of precipitated xylan.

The precipitated xylan preferably has a high purity. High purity denotes that the precipitated xylan has a xylose content of at least 85 weight-%, calculated from the weight of dry precipitated xylan. According to one preferable embodiment the precipitated xylan may have a xylose content of at least 90 weight-%, more preferably at least 93 weight-%, even more preferably at least 95 weight-%, sometimes even 97 weight-% or 99 weight-%, calculated from the weight of dry precipitated xylan.

The average particle size of the precipitated xylan may be 0.1 - 5 pm, preferably 1 - 3 pm, more preferably 1 - 2 pm.

According to one embodiment of the invention, the particle size of the precipitated xylan particles may be adjusted or controlled with the precipitation pH and the precipitation temperature. For example, if the pH of the aqueous solution is reduced at a precipitation temperature, which is above approximately 80 °C, the particle size of the precipitated xylan is significantly smaller than in a case where the reducing of the pH is performed at 25 °C. The latter precipitation temperature produces xylan particles with large size, which enable a faster sedimentation of the particles. In general, lower precipitation temperature may produce larger particles.

The precipitated xylan particles are may be used in films, paints, medical devices, composites, adhesives, medicines and any other suitable applications, such as 3D- printing and, but not limited to, extrusion molding.

EXAMPLES

Some embodiments of the present invention are described in the following non limiting examples. Example 1

1.5 g dry xylan extract was treated at alkaline conditions either a) as such, or b) under reducing conditions with 0.02 g of NaBhU. The materials (xylan extract, optional NaBhU) were dissolved in 15 mL of 1M NaOH. The test solutions were either purged overnight with N2 or treated as such.

Each test solution was transferred into a mini-reactor and heated at 150°C for different time periods (15 min, 30 min, 1 h and 2 h). After cooling, 0.6 mL of glycerol was added and the solution was neutralized to pH 7 with formic acid. The precipitated xylan was separated by centrifugation at 1500 rpm for 30 min; washed first with MeOH-water (1:1) washing solution, separated from washing solution by centrifugation. Washing was then repeated with MeOH washing solution. Xylan particles were separated by centrifugation and freeze-dried. Test scheme and the production yields are shown in Table 1.

Table 1 Test scheme and production yields for Example 1.

*non-precipitated in water, but precipitated in MeOH:H20 solution, corresponds to low molecular weight xylan fraction

From Table 1 it can be clearly seen that the production yield for precipitated xylan is significantly higher in reducing conditions than in oxidative reaction conditions, which are assumed to produce xylan backbone “peeling” reactions. The addition of a reducing agent doubles the production yields.

The results in Table 2 show that the alkaline treatment of the xylan hemicellulose extract at elevated temperature increases the purity of the produced xylan precipitate.

Table 2 Chemical composition, purity and yield of water insoluble xylan particles. * Starting HMM xylan

Figure 1 shows a microscope image of the water insoluble xylan particles with a uniform particle size distribution in polarized light. The fluorescence from the water insoluble particles in polarized light can be evidence for well-ordered isotropic material. Figure 2 shows a TEM image of the same material as in Fig. 1 after its suspension was mechanically treated in a Nano Flomogenize Machine AFI-100D (ATS Engineering Ltd., China) at 1100 bars during 20 min. The xylan particle size was found in the range of 20-40 nm. The homogenized suspension of xylan particles was stable over 12 months at room temperature.

Example 2 6 g of EtOH-precipitated birch xylan (Mw 7.6 kDa, Mn 5.6 kDa, Mw/Mn 1 .357) was dissolved in 60 mL of 1 M NaOH and 80 mg NaBhU was added. The solution was bubbling with N2 overnight. 15 mL of alkaline solution was heated at 150 °C during different time periods in a hermetically closed reactor. The reactor was cooled down, opened and equipped with a magnetic bar. 0.6 g of glycerol was added in the reactor, the content was mixed and neutralised to pH ca. 7 with 90 % formic acid. The water-precipitated material was centrifuged at 1500 rpm during 90 min in a test tube, resuspended with distilled water and centrifuged again. This was repeated three times; water-washed precipitate was frozen and freeze-dried.

The yield of water insoluble xylan crystals was depended on the reaction time at the reaction temperature. The tested reaction times were 1 , 2, 4 and 6 hours and the corresponding yields were 30 %, 32 %, 36.1 % and 36.8 % resp. The colour of the material was clear white.

Example 3

23.7 g of 25.3 % birch xylan concentrate (corresponds to 6 g of dissolved solids) was mixed with 42.3 mL of 1.4 M NaOH and 80 mg NaBH₄ was added. The experimental protocol for the process was otherwise the same as in Example 2.

The yield of water insoluble xylan particles was depended on the reaction time at the reaction temperature. The tested reaction times were 1 , 2, 4 and 6 hours and the corresponding yields were 13.7 %, 14.5 %, 16 % and 17.4 % resp. The colour of the particles was clear white and the xylose content of the precipitated particles was above 94% in all cases. The lower yield compared to Example 2 was attributed to the larger amount of low molecular mass xylans present in the starting concentrate compared to EtOH-precipitated xylans.

Example 4

700 g of 27% birch xylan concentrate (corresponds to 189 g of dissolved solids) was mixed with 350 mL of 12.5 M NaOH and 0.4 g of NaBH4 was added. The final alkali concentration was 4.16 M NaOH. The experimental protocol for the process was otherwise the same as in Example 2. The yield of water insoluble xylan particles was depended on the reaction time at the reaction temperature. The tested reaction time was 4 hours and the yield was 5% and the color of the water insoluble particles was gray, and the xylose content of the precipitated particles was more than 96%. 4 M NaOH was concluded to be too high for good quality and yield of xylan particles.

Example 5 (Reference)

The same treatment as described in example 1 was performed for spruce galactoglucomannan hemicelluloses. Regardless of the treatment time or the use of NaBhU reducing agent no water insoluble particles could be produced.

It is apparent to a person skilled in the art that the invention is not limited exclusively to the examples described above, but that the invention can vary within the scope of the claims presented below.

Claims

1. Method for producing xylan, which comprises

- obtaining an aqueous starting solution comprising xylan and at least one non- cellulosic polysaccharide,

- maintaining the aqueous solution at the reaction temperature, and - precipitating xylan particles from the aqueous solution by cooling the aqueous solution to a precipitation temperature and/or by reducing pH of the aqueous solution to a precipitation pH, and

- separating the precipitated xylan particles from the aqueous solution.

2. Method according to claim 1 , characterized in that the aqueous solution is heated to the reaction temperature of >80 °C, preferably >120 °C, more preferably >130 °C, even more preferably >150 °C.

3. Method according to claim 1 or 2, characterized in that the reaction temperature is <250 °C, preferably <200 °C.

4. Method according to claim 1, 2 or 3, characterized in that the aqueous solution is maintained at the reaction temperature for less than 12 h, preferably less than 8 h, more preferably less than 6 h.

5. Method according to any of preceding claims 1 - 4, characterized in that the aqueous solution is maintained at the reaction temperature for at least 15 min, preferably at least 30 min, more preferably at least 1 h.

6. Method according to any of preceding claims 1 - 5, characterized in that the aqueous starting solution comprises 1 - 70 weight-%, preferably 10 - 60 weight-%, more 20 - 55 weight-%, of xylan.

7. Method according to any of preceding claims 1 - 6, characterized in that the aqueous solution is free of any organic solvents.

8. Method according to any of preceding claims 1 - 7, characterized in that the aqueous solution is subjected to reducing conditions by addition of a reducing agent, such as NaBhU, Ca(BH4)2 or NaHSC>3, to the aqueous solution.

9. Method according to claim 8, characterized in that the reducing agent is added in amount of 1 - 100 mg/g, preferably 5 - 50 mg/g, more preferably 13 - 20 mg/g, based dry xylan.

10. Method according to any of preceding claims 1 - 9, characterized in that the aqueous starting solution comprises at least one non-cellulosic polysaccharide selected from mannose, galactose, rhamnose, arabinose and any of their mixtures.

11 . Method according to any of preceding claims 1 - 10, characterized in that the precipitated xylan has a xylose content of at least 90 weight-%, more preferably at least 93 weight-%, even more preferably at least 95 weight-%.

12. Method according to any of preceding claims 1 - 11 , characterized in that the pH of the aqueous starting solution is adjusted under the reducing conditions to a second pH value of >10, preferably >11 , preferably by an addition of alkali to the aqueous starting solution.

13. Method according to claim 12, characterized in that the alkali concentration in the aqueous solution is over 0.5 mol/l, preferably over 1 mol/l.

14. Method according to any of preceding claims 1 - 13, characterized in that the particle size of the precipitated xylan is adjusted with the selected precipitation temperature and/or precipitation pH.

15. Xylan particles obtained by a method according to any of claims 1 - 14.