US20150122429A1

US20150122429A1 - Method for producing pulp having low lignin content from lignocellulosic material

Info

Publication number: US20150122429A1
Application number: US14/397,919
Authority: US
Inventors: Alexander Dybov; Florian Boeck
Original assignee: Annikki GmbH
Current assignee: Annikki GmbH
Priority date: 2012-05-03
Filing date: 2013-04-24
Publication date: 2015-05-07
Also published as: WO2013164234A1; EP2844796B1; AR091998A1; EP2844796A1

Abstract

A method for the production of cellulose having a low lignin content characterized by a combination of the measures that

a) a lignocellulosic material is treated with an aqueous solution comprising a C_1-6-alcohol and having a pH from 10 to 14 at a temperature of below 100° C., whereupon the aqueous solution, in which lignin split off from the lignocellulose is present in dissolved form, is separated from the solid which represents a material enriched with cellulose and hemicellulose,
b) hemicellulose is removed from the material of a) enriched with cellulose and hemicellulose,
c) the material obtained in b), which is hemicellulose depleted, is treated with an alkali sulfite, an alkaline earth sulfite or ammonium sulfite, in particular Na₂SO₃and/or with oxygen in aqueous alkaline solution, whereby lignin dissolves and cellulose having a low lignin content is obtained;
as well as cellulose which can be produced by the method.

Description

The present invention relates to a method for producing pulp having a low lignin content from lignocellulosic material.
The conversion of lignocellulose may occur in two fundamentally different ways:

1) with the aid of the so-called “Thermochemical Platform”, wherein the lignocellulose is at first gasified and the synthesis gases are converted to desired products, and
2) with the aid of the so-called “Sugar Platform”, wherein the main interest lies in utilizing the sugars bound in the polymers cellulose and hemicellulose or, respectively, in utilizing the polymers themselves.

The present invention follows the second path: The polymeric components (cellulose and lignin) are to be separated from lignocellulose, with changes as few as possible appearing in said components after the separation process.
In classical pulpings for the production of pulp, nonselective degradation methods are generally applied, wherein lignin and xylan are simultaneously removed from the lignocellulose material at high temperatures in the presence of sulphur-containing reagents. The removed lignin is thereby changed significantly in its structure by anellation and an attack by the sulphur-containing reagents and normally can be used only as a combustible. Moreover, high-temperature methods may lead to the formation of heterocyclic substances (e.g., furfural) from sugars which necessitates sophisticated recycling methods.
A technological improvement in said field would be the development of low-temperature methods (i.e., at a temperature of less than 100° C. and below), with all substances accumulating in the highest possible purity and the material yield being maximized. This would mean a decisive progress, since lignocellulosic material could be sepatated in consecutive degradations at low temperatures (<100° C.) into the individual pure substance classes lignin, xylan and cellulose which subsequently could be converted into higher-quality products. In addition, such methods may constitute an energetically economical process due to the low temperatures.
Various methods of producing delignified cellulose are known from literature. In the following, methods are described which have found industrial application, namely “Soda pulping processes” (soda process), “Kraft pulping processes” (kraft process), and “Sulfite pulping processes” (sulfite process). Many examples and detailed descriptions of the techniques can be found in the “Handbook for Pulp & Paper Technologies”, 2nd Edition, G. A. Smook, Angus Wilde Publications (1992)“. Therein, the prior art is described based on publications which, in each case, depict the most current improvements and modifications of said processes.

The “Soda Process”

The soda method was developed in 1851 by Burgess and Watts and refrains from the use of sulphur compounds which are harmful to the environment. Only sodium hydroxide serves as a degradation chemical, for which reason the process must be carried out at high temperatures for an adequate delignification. In comparison to other processes, a high decomposition of carbohydrates takes place so that a lower degree of polymerization and less tear strength of the cellulose are obtained. In addition, the method is suitable only for materials which are easy to bleach. In some cases, anthraquinone is added for stabilizing the carbohydrates and for a better delignification (soda-anthraquinone process).
In U.S. Pat. No. 6,946,057 B2, an environmentally friendly process is described, wherein a lignocellulosic material is treated in several stages with an alkaline buffer solution containing, as main components, metaborates and sodium carbonate. The process is performed at a temperature of 160° C. A main aspect of the method described in the patent is an effective recovery of the degradation chemicals.
From U.S. Pat. No. 3,954,553, a process is known wherein hardwood is treated for one hour with an alkaline solution containing sodium hydroxide and sodium carbonate at a temperature ranging from 340° F. to 376° F. (from 171° C. to 191° C.). Via the combination of the two chemicals, efficiencies and selectivities are described which are comparable to those of kraft methods.
As compared to the kraft process, the soda process provides a possibility of obtaining lignin with a very small or without a sulphur content. However, in order to achieve the same delignification effect as with the kraft process, the removal of lignin must be performed at even higher temperatures than in the kraft process, whereby lignin is obtained which is condensed to a higher extent. In addition, during the heating of carbohydrates at soda-process temperatures, heterocyclic substances such as furfural emerge which must be separated in a sophisticated fashion.

the “Sulfite Process”

The first patent for the sulfite process was obtained by Benjamin Tilghman in 1867. The first industrial method utilizing said process started in Sweden in 1874. In said method, mixtures of different salts of a sulfurous acid are added in most cases as active reagents. Calcium and magnesium salts are most widely used. By appropriate choice of the counterions, the process may be operated from a highly acidic to an alkaline environment. Also, a two-stage combination of both has proved to be useful in some cases. Classical methods operate in a highly acidic (calcium, pH=1-2) or slightly acidic (magnesium, pH=3-4) medium. The sulfite thereby chemically modifies the lignin and renders it more readily soluble in water. Due to its harmfulness to the environment, the process is today of secondary importance, however, it is frequently used for the production of chemical cellulose, since, in this way, a cellulose is produced which is easy to bleach.
In U.S. Pat. No. 8,038,842, a method of fractionating a lignocellulosic material into cellulose, hemicellulose and lignin is described. Therein, a treatment with a vapour-shaped mixture of aliphatic alcohol, SO₂, ammonia and water occurs in a continuous or gradual procedure, the concentration of SO₂ranging between 10% by weight and 50% by weight in the solution. Upon the direct contact of said vapour-shaped mixture with a lignocellulosic material for a duration of between 5 minutes and 3 hours at a temperature of between 115° C. and 150° C., a mixture of cellulose, hemicellulose and lignin is formed, from which cellulose can be removed.
From U.S. Pat. No. 3,630,832, a controlled process for the production of sulfite pulp is known, wherein the use of sodium sulfide is avoided. The process is performed at a temperature of more than 145° C. and a pH-value of between 8 and 11. Using online analytics, the pH-value is thereby monitored and adjusted appropriately.
From EP 1 375 734, a method for the production of cellulose is known, wherein a lignocellulosic material is treated with magnesium bisulfite, optionally with a surplus of SO₂, at a temperature of between 130° C. and 165° C. in order to remove lignin. The method also involves a combination with a bleaching sequence after the removal of the lignin, which is performed exclusively with reagents which are free from chlorine.
In DE 103 23 376, a method of delignifying lignocellulosic raw materials is described, wherein the lignin is removed by the use of sulfites in the presence of an alkaline component, in particular sodium hydroxide or sodium carbonate, or a mixture thereof, in an aqueous solution, applying increased temperature and elevated pressure. Thereby an aqueous sulfite solution is used at the beginning of degradation, and the temperature is raised as far as to the maximum reaction temperature. The alkaline component is added only after the start of the degradation.
Although, in those sulfite processes, the molecular mass of cellulose is largely maintained and a decomposition of lignin is avoided, they have the major drawback that the lignin obtained may contain a sulphur content of approx. 5% (S. Brudin, P. Schoenmaker, J. Sep. Sci. 2010, 33, 439-452), which greatly limits any possible applications thereof.

The “Kraft Process”

The kraft process was developed in 1884 in Danzig by Carl Ferdinand Dahl (see, e.g., U.S. Pat. No. 296,935) and involves a method in which lignin is dissolved out of the pulp at an excess pressure of from 5 to 14 atmospheres, using a mixture of sodium sulfate, sodium carbonate, sodium hydroxide and sodium sulfide. The lignin is thereby obtained in the black liquor as a soluble alkali lignin and has a high sulphur content. Lignin according to the kraft process contains between 1.5 and 3.0% of sulphur (Marton J. “Lignins; Occurrence, formation, structure and reactions”, 1971, Wiley-Interscience, Eds. Sarkanen K. V. and Ludwig C. H., USA, p 666). In most cases, it is combusted as an energy source for generating the heat requirement for the process. Currently, the kraft method constitutes the most common method for the production of paper pulp.
From US 2011/0073264 A1, a method of increasing the yield of kraft pulp and of increasing the viscosity of the pulp obtained is known. This is achieved by pretreating a lignocellulose material with hot water or water vapour at a temperature of 160° C. Remaining lignin is then removed by treatment with Na₂S and NaOH under kraft conditions.
In US 2011/0067829 A1, an optimization of the kraft process is described, wherein the sulphur concentration is kept constant during the digestion of pulp by an optimized method of recycling chemicals.
From U.S. Pat. No. 6,613,192 B1, a modification of the kraft process is known, wherein eucalyptus chips are fermented by inoculation with white-rot fungi, whereby lignin is modified and, as a result, becomes more readily degradable in the kraft process.
From U.S. Pat. No. 7,824,521 B2, an optimization of the kraft process is known, wherein a lignocellulosic material is treated with an aqueous solution containing a further base in order to extract acidic material from the lignocellulosic material. In this way, a portion of the hemicellulose is extracted, whereby the actual kraft process proceeds more effectively.
From U.S. Pat. No. 7,520,958 B2, a method of producing a modified pulp is known, wherein wood chips are treated with peroxides in an acidic solution in order to remove 5-20% of the hemicellulose. With this pretreated wood, a kraft pulp containing only between 5% and 9% of hemicellulose in the dry matter can be produced.
In U.S. Pat. No. 6,153,052 A and U.S. Pat. No. 7,828,930 B2, improved pulp processes are described, wherein polysulfides are used, by which use the yield of pulp can be increased. The polysulfides thereby act primarily as reducing agents inhibiting undesired “peeling” reactions.
In U.S. Pat. No. 7,828,930 B2 and U.S. Pat. No. 6,143,130, a modified method of the kraft process is described, wherein polysulfides are used as active delignification components. Said procedure gives higher yield of pulp and allows to remove lignin at temperatures which are lower (100° C. to 160° C.) than in the classical kraft process.
From EP 0 468 016, a process for the production of kraft pulp is known, wherein a lignocellulosic material is pre-treated with a charged cooking liquor at a temperature between 20° C. and 100° C. The lignocellulosic material pretreated is this way is heated at a temperature of between 135° C. and 155° C. and delignified.
From U.S. Pat. No. 6,576,084 B1, a method of increasing the yield as well as the viscosity of pulp is known. The technique is based on the pre-treatment of a lignocellulosic material with anthraquinone or polysulfides at a temperature from 80° C. to 130° C. By that measure, a decomposition of cellulose is avoided. Subsequently, the pre-treated material is reacted with cooking liquor at a temperature of more than 130° C.
An advantage of the above-described kraft process is the relatively simple possibility of producing a delignified cellulose which can easily be subjected to further bleaching. A major drawback associated therewith is that the potentially high-quality lignin is suitable for a further application only to a limited extent, since, after being removed from the other raw material components, it is significantly changed in its structure by high-temperature condensation reactions. Moreover, the lignin is strongly modified with sulphur via the reaction with sulfide.

Further Delignification Methods

As relatively new concepts, organosolv processes have recently come back into use. They were described for the first time in the 1970ies. Strategies for an “extended cooking” were then developed, which essentially was based on increasing the degree of delignification and lowering the bleaching effort. As solvents, alcohols such as ethanol or methanol were used predominantly, and they were mainly supposed to increase the solubility of the lignin, whereas acids, alkali, sulfite or sulfide or oxidative reagents continued to act as actual degradation chemicals (H. Hergert, 1998, Developments in organosolv pulping; In: R. A. Young and M. Akhtar, Environmentally friendly technologies for the pulp and paper industry; John Wiley & Sons Inc., New York, 5-68).
Organosolv processes can basically be divided into acidic and alkaline variants. An acidic process is, for example, the Allcell process, which was taken up and developed further by the company LIGNOL (C. Arato, E. K. Pye, G. Gjennestad, 2005, The Lignol approach to biorefining of woody biomass to produce ethanol and chemicals; Appl. Biochem. Biotechnol., Vol. 121-124, p. 871-882). As substrates, wood, straw or bagasse are processed. The underlying chemical reaction is the autohydrolytic cleavage of hemicelluose at a pH from 2.0 to 3.8, which results from the acetic acid separated from the xylan (conditions: 180° C. to 195° C., ethanol concentration: 35% by weight to 70% by weight, liquid-to-solid ratio: from 4:1 to 10:1, reaction time: 30 to 90 minutes). In this way, cellulose is split off partly in the form of insoluble oligosaccharides, and a large part of the hemicellulose is separated into soluble oligo- and monosaccharides. A partthe pentoses is oxidized into furfural under the reaction conditions. Lignin is likewise hydrolyzed partly and accumulates together with the other decomposition products in the cooking liquor, from which the decomposition products are then extracted. The other non-hydrolyzed portion remains in the solid and may be hydrolyzed, for example enzymatically, into sugars and fermented into ethanol. The lignin remaining in the solid (20% to 25% of the original one) accumulates as a fermentation residue and can only be combusted.
It may be said that, in acidic processes, the amount of lignin obtained is relatively small on the one hand, and the decomposition of lignin cannot be decoupled from the decomposition of hemicellulose. Because of the relatively poor decomposition of lignin a fibre material emerges with a residual content of lignin which, in the event of being used as a chemical raw material, would require a substantial bleaching effort and is unsuitable for said use. Therefore, primarily endeavoured is the use as a raw material for the production of bio-alcohol, although reports exist which confirm that the accessibility of the residual lignin in the pulp is relatively high (E. K. Pye, J. H. Lora, 1991, The Allcell Process—A Proven Alternative to Kraft Pulping, TAPPI Journal March 1991, 113-117).
In the past, basic organosolv processes were investigated far less often than acidic ones, since high technical requirements are imposed on the recovery of sodium hydroxide, if high amounts of sodium hydroxide are used, in particular if straw is used as a substrate (Marton & Granzow 1982, Use of ethanol in alkaline pulping; WO 82/01568).
In Germany in the 1990ies, the organocell process for pulp cooking was developed for industrial application (N. Zier, 1996, Strukturelle Merkmale eines Organosolv-Lignins bei Variation der Parameter; Dissertation, Technische Universität Dresden). The process proceeds in two stages, starting with an alcohol-water impregnation (ratio: 3 parts of alcohol: 7 parts of water) at 110° C. to 140° C. and subsequent cooking at 165° C. to 170° C., with 30% sodium hydroxide and 0.1% anthraquinone based on the dry weight of the substrate. The process is suitable for the degradation of hardwood and softwood as well as for annual plants. The pulp grade is comparable to that of kraft pulp and could be bleached with oxygen in a chlorine-free way. According to various information, the facility was closed soon after start-up due to technical problems, which, partly, were associated with the recovery of the high amount of sodium hydroxide (El-Sakhawy et al., 1996: Organosolv pulping, (3), ethanol pulping of wheat straw; Cellul. Chem. Technol. 30, 281-296).
For a cost-efficient biorefinery process which is not aimed for the production of bio-alcohol, but at utilizing all the main components of lignocellulose as a chemical or raw material, it is necessary to recover a portion as large as possible of the available lignin. This should occur in a homogeneous product stream with minor contaminations from decomposition products from other components.
From WO 2011/014894 and WO 2012/027767, low-temperature methods for separating said lignin from a lignocellulosic material such as straw, bagasse, energy grasses and/or husks are known. The degradation process aims for a low-temperature process for delignification at below 100° C., whereupon the material formed, which has been enriched with cellulose and hemicellulose can be treated with at least one carbohydrate-cleaving enzyme. One variant is the use of xylanases, whereby xylan is decomposed selectively and a material strongly enriched with cellulose is formed. A disadvantage of such a low-temperature method is the lignin content in the cellulose which is high in comparison to pulp from kraft or soda processes.

Separation of Xylan

In WO 2010/124312, a method for the separation of hemicellulose from a material enriched with hemicellulose and cellulose is described, which material is produced according to WO 2011/014894. Thereby the solid mixture obtained after the degradation is suspended in an acetate buffer and treated with xylanases in order to selectively depolymerize and solubilize hemicellose. The solid obtained after the separation, which is strongly enriched with cellulose, subsequently may be used for the production of glucose, for example, by a conversion with cellulases.
A chemical method of separating xylan from kraft pulp for the production of chemical cellulose is described in WO 2005/118923 A1. Thereby, the pulp obtained is treated with an at least 5%, preferably 9%, sodium hydroxide solution at a temperature of less than 25° C. The xylan-containing solution which is obtained as in US 2010/0021975 A1 may be concentrated by nanofiltration and precipitated by adding a mineral acid. As described in AT 503 624, the addition of a mono- or multivalent alcohol may boost said precipitation.

Removal of (Residual) Lignin

For the removal of lignin and, respectively, for the cleavage of lignin, for example, the following methods are known from literature:
From EP 1 025 305, a chemical method for lignin depolymerization is known. It is based on the catalytic effect of complexed copper in combination with hydrogen peroxide or organic hydroperoxides and is capable of oxidatively cleaving lignin at temperatures of below 100° C. The complexing agents used thereby are pyridine derivatives. Using synthetic lignin models, it has been possible to demonstrate that, if H₂O₂is used as an oxidant, a cleavage occurs in the propyl side chain of the lignin molecule, as a result of which the lignin polymer disintegrates into oligomeric subunits. If the copper system is used with hydroperoxides, it is possible to delignify wood. The system based on H₂O₂appears to be more readily technically feasible. It has been tested as a bleaching additive during the peroxide bleaching of kraft pulp and has led to an improved degree of delignification. A disadvantage of such a system is the large consumption of expensive reagents such as, e.g., H₂O₂or pyridine derivatives, and the structural change in the lignin.
In U.S. Pat. No. 4,560,437, it is described that, prior to the actual bleaching process, wood pulp may be subjected to a second degradation under addition of sodium sulfite, whereby the effectiveness of the following bleaching steps, especially of oxygen bleaching, is supposed to be increased and, hence, costs for bleaching chemicals are supposed to be diminished. The best results could be achieved by using 25 to 30 kg of SO₂(in the form of sodium sulfite) per ton of unbleached pulp at a pH of 8.
In addition, it is described in “Oxidation of Lignin Using Aqueous Polyoxometalates in the Presence of Alcohols” (ChemSusChem 2008, 1, 763-769) that the delignifying effect of polyoxometalates can be increased substantially under an oxygen atmosphere in the presence of aliphatic alcohols. It is concluded that, in the catalytic cycle, alkoxy radicals are formed which initiate the cleavage of lignin.
In EP 0 524 127, the effect of ethanol as an additive in the oxygen bleaching of pulp is addressed in detail. It is shown that, by adding catalytic amounts of ethanol, the efficiency of delignification and the selectivity can be increased equally.
Also in U.S. Pat. No. 5,609,723, it is described that a higher efficiency of oxygen bleaching can be achieved at the same pulp viscosity, if multivalent alcohols are added to the bleaching solution. Moreover, it is known from U.S. Pat. No. 6,923,887 that the efficiency of peroxide bleaching can be increased by adding an organic solvent, preferably a C_1-2-alcohol.
In U.S. Pat. No. 4,004,967, an increased selectivity of the bleaching of kraft pulp by adding polyols or monovalent alcohols is likewise reported. However, optimum effects were achieved by adding formaldehyde.
In all the methods described, the quality of the cellulose is considered to be the only criterion. Thereby, it is neglected that lignin also constitutes a valuable raw material component.
In contrast to the processes described, it is an object of the present invention to provide a method wherein, in addition to the isolated lignin, a material strongly enriched with cellulose and having a low lignin content (Kappa ≦10) can be obtained without any substantial depolymerization of the cellulose.
In one aspect, the present invention provides a method for the production of cellulose having a low lignin content which is characterized by a combination of the measures that

a) a lignocellulosic material is treated with an aqueous solution comprising a C_1-6-alcohol and having a pH from 10 to 14 at a temperature of below 100° C., whereupon the aqueous solution, in which lignin split off from the lignocellulose is present in dissolved form, is separated from the solid which represents a material enriched with cellulose and hemicellulose,
b) hemicellulose is removed from the material of a) enriched with cellulose and hemicellulose,
c) the material obtained in b), which is hemicellulose depleted, is treated with an alkali sulfite, an alkaline earth sulfite or ammonium sulfite, such as Na₂SO₃and/or with oxygen in aqueous alkaline solution, whereby lignin dissolves and cellulose having a low lignin content is obtained.

A method which is provided according to the present invention is herein designated also as a “method of (according to) the present invention”.
In a method according to the present invention, an organic material containing lignin, for example, annual plants such as (dry) grasses, or parts of grasses, preferably grasses, straw, energy grasses such as, e.g., switch grass, elephant grass or abaca, sisal, bagasse, or atypical lignocellulose substrates such as husks, e.g., lemmas such as rice husks, particularly preferably straw, energy grasses, bagasse or husks, even more preferably straw or bagasse, e.g., straw such as wheat straw, is used as a lignocellulosic material.
In a method according to the present invention, the content of solids at the beginning of the delignification process according to measure a) preferably amounts to 3 to 30% by weight of the lignocellulosic material in the aqueous solution and preferably is provided at a consistency from 3 to 30% by weight, in particular from 5 to 20% by weight.
In a method according to the present invention, an aliphatic alcohol, such as a C_1-6-alcohol, particularly preferably a C_1-4-alcohol such as ethanol or isopropanol, is preferably used as an alcohol in order to separate, according to measure a), lignin from a lignocellulosic material.
In a method according to the present invention, the pH of the solution may be adjusted according to measure a) with a base, preferably an inorganic base, for example, a hydroxide such as sodium hydroxide, potassium hydroxide.
The lignin degradation according to measure a) in a method according to the present invention is performed at a temperature of below 100° C., preferably from 40° C. to 90° C., particularly preferably from 50° C. to 70° C. It has turned out that, if temperatures of more than 100° C. are applied, condensation products of the lignin will emerge to an increasing extent, for example, condensed phenolic fragments of the formulae
which are formed by ring condensation reactions. It has been found by 2D NMR analysis that the content of such fragments in lignin isolated according to the present invention is surprisingly significantly lower than in comparative lignins which have been produced by high-temperature extraction, as is shown in Table 1 below:

	TABLE 1

	Condensed phenolic fragments
	in lignin, mmol/g

	Diphenyl			in
Lignin	methane	4-0-5	5-5	total

Lignin obtained according to the	0.02	0.06	0.10	0.18
method of the present invention
Soda lignin from wheat straw	0.04	0.09	0.12	0.25
Soda lignin from sarkanda grass	0.36	0.21	0.28	0.84
Soda lignin from agricultural fibres	0.40	0.25	0.31	0.96
Organosolv lignin from hardwood	0.45	0.25	0.27	0.97

Moreover, sulphur containing reagents do not have to be used in the degradation process according to measure a) so that the lignin isolated according to the method of the present invention is free from sulphur from sulphur-containing reagents.
In one aspect, the present invention is based on the finding that from a lignocellulosic material treated with an aqueous basic solution comprising an alcohol, in particular a C_1-6-alcohol, and having a pH-value of from 10 to 14 lignin may be obtained which is less condensed. In addition, the treated material which is enriched with cellulose and hemicellulose proves to be a material more readily applicable for an enzymatic degradation into carbohydrate cleavage products.
An advantage of the method according to the present invention is that a selective isolation of pure lignin is possible as a result of the low solubility of hemicellulose at higher alcohol concentrations. In a subsequent step, hemicellulose may be isolated according to measure b) as a pure product from the lignocellulose material enriched with cellulose and hemicellulose.
In a method according to the present invention, approximately 59% to 92% of the total lignin may be removed. This enables a further enzymatic conversion in order to separate hemicellulose, preferably xylan, from cellulose. Alternatively and in one aspect according to the present invention, hemicellulose may be separated by a combination of chemical and enzymatic treatments in order to obtain dexylanized cellulose which is as pure as possible.
In contrast, a preceding enzymatic degradation of the xylan has proved to be less effective since the sugars of the lignocellulose are present in tightly crosslinked, polymeric, crystalline structures of the cellulose and hemicellulose, which, in addition, are enveloped by a lignin coat, whereby an extremely dense complex is created. Direct access to the individual classes of polymers is thereby exacerbated. Due to their high molecular weights, the enzymes are incapable of penetrating into the lignocellulose through the narrow pores. This means that a first step must be taken which increases the porosity of the lignocellulose and thereby enables a further enzymatic conversion.
The depletion and optionally isolation of the hemicellulose in a method according to the present invention according to measure b) may occur chemically or enzymatically.
In a further aspect, the present invention provides a method according to the present invention in which, according to measure b), either
hemicellulose is solubilized from the material of a) enriched with cellulose and
hemicellulose by treatment with at least one carbohydrate-cleaving enzyme, or
hemicellulose is solubilized from the material of a) enriched with cellulose and
hemicellulose by treatment with an aqueous alkaline solution at a temperature from 20° C. to 50° C., preferably from 25° C. to 35° C., or
hemicellulose is solubilized from the material of a) enriched with cellulose and
hemicellulose both by treatment with an aqueous alkaline solution at a temperature from 20° C. to 50° C., preferably from 25° C. to 35° C., and by treatment with at least one carbohydrate-cleaving enzyme.
The dissolved hemicellulose may be isolated from the solution by separating the solution from the solid.
The combined measures a) and b) in a method according to the present invention have proved to be particularly suitable for the depletion of cellulose from a lignocellulosic material.
In comparison to the methods of the prior art, the method according to the present invention for the production of delignified pulp from a lignocellulosic material is a process by which the separate isolation of all partial streams is rendered possible, whereby a high-quality sulphur-free lignin is obtained. A significant advantage of the method according to the present invention is a high delignification along with high selectivity.
By the delignification according to measure a), which is performed during the degradation, the porosity of the cell walls of the lignocellulosic material is increased so that carbohydrate-cleaving enzymes can penetrate into the straw and hydrolyze the xylan contained therein. In addition, activity losses of the enzymes can be reduced by nonspecific bonds to the lignin. Pure endo-1,4-β-xylanases, e.g., enzyme preparations with pure endoxylanase activity, are suitable for the removal of hemicellulose, in particular xylan, from the lignocellulose material, wherein the cellulose is preserved according to measure b) in a method of the present invention. Mixed enzymes with β-xylosidase or α-L-arabinofuranosidase activity convert also cellulose in too large proportions and therefore must be excluded. For example, Pentopan BG™ as well as Pulpzyme HC™ are suitable as enzyme preparations with pure endoxylanase activity. Depending on the enzyme, the enzymatic reactions are performed at optimum conditions and pH-values.
Besides the enzymatic hydrolysis, xylan can also be extracted chemically according to one aspect of measure b). Thereby the functionalization with acidic residues which is high in comparison to cellulose, is exploited. By use of a strong basic solution, xylan may be extracted selectively from a material enriched with cellulose and hemicellulose. In the method according to the present invention, the pH is adjusted according to measure b) by an inorganic base, e.g., a hydroxide, preferably sodium hydroxide, with an amount of substance from 1 to 20% by weight, based on the amount of liquid. The extraction is preferably carried out at a temperature from 20° C. to 50° C., particularly preferably from 25° C. to 35° C.
A material strongly enriched with cellulose such as, for example, a material produced by delignification and dexylanization according to the method described above is referred to as a pulp, unless otherwise indicated.
In a method according to the present invention, the material obtained according to measure b), which is hemicellulose depleted, lignin, namely residual lignin, is removed from the pulp according to measure c). For this purpose, either a sulfite, in particular an alkali, alkaline earth or ammonium sulfite, preferably sodium sulfite and/or oxygen, is used in order to obtain a pulp with a low content of lignin. It has turned out that the use of a combination of sulfite and oxygen leads to a substantially increased efficiency of delignification.
Furthermore, the solution obtained after the removal of the (residual) lignin provides possibilities of isolating and utilizing the removed lignin.
The efficiency of delignification is defined as follows:
$Efficiency of delignification = \frac{(initial value Kappa - final value Kappa) \times 100}{initial value Kappa}$
The Kappa value is a measure of the residual content of lignin in the pulp.
The removal of the (residual) lignin in a variant according to measure c) in a method of the present invention using sulfite, e.g., sodium sulfite, is performed at a temperature from 90° C. to 150° C., preferably from 120° C. to 140° C.
Surprisingly, it has been ascertained that better delignification occurs under alkaline conditions. Sulfite such as sodium sulfite is preferably used in an amount from 10 to 100% by weight, particularly preferably of 20% by weight, based on the dry matter. An inorganic base, preferably a hydroxide, e.g., sodium hydroxide, at a concentration of from 0.1 to 3.0% by weight, based on the dry matter, is used as a base. In this way, an optimum pH from 9 to 13 may be adjusted. In addition, in one embodiment of the method, an anthraquinone derivative, preferably sodium anthraquinone-2-sulfonate, may be added as a mediator in an amount preferably from 0.05 to 5% by weight, particularly preferably in an amount from 0.1 to 1% by weight, most preferably in an amount of 0.5% by weight, based on the dry matter, in order to increase the efficiency of delignification.
In a different variant of the method according to the present invention according to measure c), the (residual) lignin can be removed by oxygen treatment at a temperature ranging from 60 to 130° C., preferably from 80° C. to 100° C., particularly preferably at 90° C. Thereby an inorganic base, e.g., a hydroxide such as sodium hydroxide, is used in an amount from 1.0 to 10.0% by weight, preferably in an amount of 6% by weight, based on the dry matter. With an appropriate oxygen bleaching, an oxygen pressure, preferably from 1 to 20 bar, particularly preferably of 10 bar, is applied. In addition, in one embodiment of said method, an anthraquinone derivative, preferably sodium anthraquinone-2-sulfonate, is added as a mediator in an amount from 0.05 to 5% by weight, preferably in an amount of 0.1 and 1% by weight, particularly preferably in an amount of 0.5% by weight, based on the dry matter. Surprisingly, it has been found that an embodiment in which an aqueous solution with 0.1 to 60% by volume of alcohol, preferably from 1 to 5% by volume, particularly preferably with 3% by volume, is used will yield an improved efficiency of delignification.
In order to achieve an intended efficiency of delignification during the removal of residual lignin of the pulp obtained after the separation of hemicellulose and lignin, the respective compositions of all delignification components on a percentage basis as well as the reaction parameters temperature, reaction time and solids concentration can be adjusted.
Celluose produced according to the present invention and having, for example, the Kappa number and the viscosity which are achievable by the method according to the present invention is novel and likewise is a subject matter of the present invention.
In a different aspect, the present invention provides cellulose which can be produced, and in particular is produced, according to a method of the present invention.
By means of the following examples, the impact of the contents of ethanol and mediator as well as of the reaction time on the efficiency of delignification is illustrated.
In the following examples, all temperatures are in ° C.

EXAMPLE 1

a) Pulping

Wheat straw (21.0% by weight of lignin content, 20.9% by weight of xylan content, 36.8% by weight of cellulose content, based on the dry matter) is crushed with an ultra-centrifugal mill to a particle size of 2 mm. The crushed straw is suspended in a mixture of water, ethanol (alcohol content of 60% by volume) and sodium hydroxide (8% by weight, based on the dry matter). The mixture is mechanically stirred for 18 hours at a constant temperature of 70° C. Subsequently, the obtained solid is squeezed and washed with a sufficient amount of water, whereby a material enriched with cellulose and hemicellulose is obtained. The yield and important parameters of the obtained material are illustrated below in Table 2.

	TABLE 2

	Digestion parameters

	Solid yield	60.1 ± 3.1%
	Delignification	78 ± 6%
	Lignin content in the solid	8.2 ± 1.1%
	Delignification	75 ± 5%
	Extracted lignin	15.8 ± 1.18 g
	per 100 g of wheat straw
	Extracted sugar	1.4 ± 0.4 g
	per 100 g of wheat straw

b) Dexylanization (Chemically and Enzymatically)

The material of a) enriched with cellulose and hemicellulose (approx. 58.2% by weight of cellulose content, approx. 36.9% by weight of xylan content, approx. 8.2% by weight of lignin content) is dexylanized in a two-stage sequence. For this purpose, the material is initially suspended in 10% sodium hydroxide, the temperature of which has been adjusted to 30° C., to a dry matter content of 10% by weight and is stirred at this temperature for 30 minutes. After such time, the homogeneous mixture is filtered off and washed neutrally with diluted hydrochloric acid and water. The filter cake obtained is suspended to a solids content of 10%, based on the dry matter, in a 50 mm acetate buffer solution, whereupon the pH is adjusted to 5. 150 U of Pentopan BG™ per g of cellulose is added to said mixture, whereupon the suspension is heated to 50° C. for 72 hours. Thereupon, the aqueous phase is squeezed and washed with twice the amount of water, whereby the pulp is obtained. A solid yield of 75% of the dry matter used was obtained. Important parameters of the pulp are compiled below in Table 3.

	TABLE 3

	Parameters	composition

	Cellulose	80.9%
	(Residual) xylan	2.2%
	Ash	1.0%
	(Residual) lignin	10.4%
	(Residual) arabinan	0.7%
	Other sugars and uronic acids	1.8%
	MW cellulose	2200 kDa
	Crystal form	Cellulose I + II
	Kappa number	57.6
	CuEn viscosity	694 ml/g
	Degree of polymerization	1850
	Degree of whiteness	32.6% ISO

c) Delignification

The pulp of b) was suspended in the water to a dry matter content of 4% by weight. 7.6% by weight of NaOH and 0.5% by weight of Mg²⁺, based on the dry matter of the pulp, were added to the reaction mixture. Delignification was carried out for 4 hours under an O₂pressure of 10 bar and at a temperature of 90° C. Thereupon, the pulp was washed with a sufficient amount of water and dried. Subsequently, the Kappa number and the viscosity were determined

EXAMPLE 2

Pulping and dexylanization were performed as in Example 1. The implementation and the batch of the delignification correspond to Example 1a), except that, instead of water, a 3% aqueous ethanol solution was used as a reaction medium.

EXAMPLE 3

Pulping and dexylanization were performed as in Example 1. The implementation and the batch of the delignification correspond to Example 1, wherein, however, 6.3% by weight of anthraquinone-2-sulfonic acid sodium salt monohydrate, based on the entire dried mass, was additionally added as a mediator during the delignification.

EXAMPLE 4

Pulping and dexylanization were performed as in Example 1. The implementation and the batch of the delignification correspond to Example 3, except that, instead of water, a 3% aqueous ethanol solution was used as a reaction medium.

EXAMPLE 5

Pulping and dexylanization were performed as in Example 1. The implementation and the batch of the (residual) delignification correspond to Example 3, except that the reaction time was extended to 16 hours.

EXAMPLE 6

Pulping and dexylanization were performed as in Example 1. The implementation and the batch of the (residual) delignification correspond to Example 5, except that, instead of water, a 3% ethanol-water mixture was used as a reaction medium.

EXAMPLE 7

Pulping and dexylanization were performed as in Example 1. For delignification, the pulp was suspended in the water to a dry matter content of 4% by weight and reacted with 64% by weight of sodium sulfite and 1.5% by weight of sodium hydroxide in the presence of 2.0% by weight of anthraquinone-2-sulfonic acid sodium salt monohydrate, based on the dry matter of the pulp. The reaction time was 1 hour at a temperature of 130° C. After delignification, the pulp was washed with a sufficient amount of water and dried. Subsequently, the Kappa number and the viscosity were determined

EXAMPLE 8

The pulp produced according to Example 7 was bleached with oxygen and alkali. The implementation of the reaction and the reaction batch were as described in Example 4 regarding the delignification.

EXAMPLE 9

Pulping and dexylanization were performed as in Example 1. The implementation and the batch of the delignification correspond to Example 7, except that the reaction was carried out for 3 hours at a temperature of 140° C.

EXAMPLE 10

The pulp produced according to Example 9 was bleached with oxygen and alkali. The implementation of the reaction and the reaction batch corresponded to the data specified in Example 4 with regard to delignification.
The Kappa numbers and viscosity results of the individual delignification methods are illustrated below in Table 4.
The Kappa value is determined according to DIN 54357 and is a measure of the (residual) content of lignin in the pulp. The viscosity determination was performed in 0.5 m of an aqueous copper ethylene diamine solution (CuEn) with an Ubbelohde capillary viscometer according to a simplified test set-up, with the same capillary being used for the sample and the blank value (SCAN-C 15:62, SCAN-C 16:62 and SCAN-CM 15:88).
As can be seen in Table 4 below, a considerably stronger reduction in the Kappa number could be achieved in those trials in which EtOH was added right from the beginning. Therein, the efficiency of delignification is surprisingly significantly higher than in comparable bleaching processes of kraft pulp.

TABLE 4

Kappa,	efficiency of	viscosity,
mL	delignification, %	mL/L

Example 1	31.8	44.5	699
Example 2	27.8	51.5	740
Example 3	28.3	50.6	717
Example 4	24.7	56.9	758
Example 5	18.1	68.4	715
Example 6	14.6	74.5	736
Example 7	29.7	48.2	915
Example 8	9.7	83.1	809
Example 9	19.4	66.3	971
Example 10	7.7	86.6	843

When the pulp was delignified additionally with Na₂SO₃prior to the oxygen bleaching, a substantially lower Kappa number of 7.7 ml could surprisingly be achieved after two bleaching steps, which, in total, corresponds to a delignification of 86.6%. In comparison to pulp bleached only with oxygen (Example 6), a Kappa number difference of approx. 6.9 ml exists. Said difference is significant in view of an industrial application, considering that, in general, the removal of lignin is more difficult with a low Kappa number and expensive selective delignification reagents are required. Moreover, the viscosity of the pulp delignified in a two-stage method is higher than that of a pulp bleached only with oxygen. For example, the pulp produced according to Example 10 was furthermore bleached with peroxyacetic acid. Peroxyacetic acid was used in an amount of 10% by weight, based on the dry matter. The consistency amounted to 4%, the temperature was 70° C. After a reaction time of 3 hours, the pulp was washed with a sufficient amount of water and dried. Subsequently, a Kappa number of 1.2 ml was measured.

Claims

1. A method for the production of cellulose having a low lignin content, comprising:

a) treating a lignocellulosic material with an aqueous solution comprising a C_1-6-alcohol and having a pH from 10 to 14 at a temperature of below 100° C., whereupon the aqueous solution, in which lignin split off from the lignocellulose is present in dissolved form, is separated from a solid which represents a material enriched with cellulose and hemicellulose,

b) removing hemicellulose from the material of a) enriched with cellulose and hemicellulose to yield a hemicellulose depleted material,

c) treating the material obtained in b) with an alkali sulfite, an alkaline earth sulphite, or ammonium sulfite and/or with oxygen in aqueous alkaline solution, whereby lignin dissolves and cellulose having a low lignin content is obtained.

2. Method according to claim 1, wherein in b) hemicellulose is solubilized from the material of a) enriched with cellulose and hemicellulose by treatment with at least one carbohydrate-cleaving enzyme.

3. Method according to claim 1, wherein in b) hemicellulose is solubilized from the material of a) enriched with cellulose and hemicellulose by treatment with an aqueous alkaline solution at a temperature from 20° C. to 50° C.

4. Method according to claim 1, wherein in b), hemicellulose is solubilized from the material of a) enriched with cellulose and hemicellulose both by treatment with an aqueous alkaline solution at a temperature from 20° C. to 50° C. and by treatment with at least one carbohydrate-cleaving enzyme.

5. Method according to claim 1, wherein in c) a temperature from 90° C. to 150° C. is applied if alkali sulfite, an alkaline earth sulphite, or ammonium sulfite is used.

6. Method according to claim 1, wherein in c) a temperature from 60° C. to 130° C. is applied if oxygen is used.

7. Method according to claim 1 wherein straw, bagasse, energy grasses are used as a lignocellulosic material.

8. Method according to claim 1 wherein the lignocellulosic material is provided in the aqueous solution at a consistency of from 3 to 30% by weight, based on the dry matter.

9. Method according to claim 1 wherein in c) an anthraquinone derivative is added as a mediator in an amount from 0.05 to 5% by weight based on the dry matter.

10. Cellulose produced according to claim 1.

11. Method according to claim 1, wherein the material obtained in b) is treated with Na₂SO₃.

12. Method according to claim 3, wherein the temperature is from 25° C. to 35° C.

13. Method according to claim 4, wherein the temperature is from 25° C. to 35° C.

14. Method according to claim 5, wherein the temperature is from 120° C. to 140° C.

15. Method according to claim 6, wherein the temperature is from 80° C. to 100° C.

16. Method according to claim 7, wherein elephant grass, switch grass, and/or husks are used as the lignocellulosic material.

17. Method according to claim 16, wherein lemmas are used as the lignocellulosic material.

18. Method according to claim 9, wherein the anthraquinone derivative is sodium anthraquinone-2-sulfonate.

19. Method according to claim 9, wherein the anthraquinone derivative is added in an amount from 0.1 to 1% by weight based on the dry matter.

20. Method according to claim 9, wherein the anthraquinone derivative is added in an amount of 0.5% by weight based on the dry matter.