AT508330A1 - METHOD FOR PRODUCING SUGAR FROM A LIGNOCELLULOSIC MATERIAL - Google Patents

Description

• · · · ··· «A12659

Process for the preparation of sugars from a lienocellulosic material

The invention relates to a process for the preparation of sugars, in particular pentoses and hexoses, from a lignocellulosic material. The invention further relates to a process for the production of alcohol from the sugars.

In connection with the shortage of crude oil and the discussion about grain as an energy supplier, the renewable raw material lignocellulose (straw, wood, paper waste, etc.) is gaining in importance as starting material for fuels or chemical products. The conversion of lignocellulose can be accomplished in two fundamentally different ways: 1) the "thermochemical platform", in which the lignocellulose is first gasified and the syngas is synthesized into desired products, and 2) the "sugar platform", where the main interest is in the Use of the bound in the polymers cellulose and hemicelluloses sugar, while lignin is still mainly used for energy. The present invention is to be associated with the second route.

In contrast to starch, the sugars of lignocellulose are present in tightly networked, polymeric, crystalline structures, cellulose and hemicelluloses, which are additionally surrounded by a lignin coat, resulting in an extremely dense complex. The most obvious way to extract sugar from lignocellulose would be the direct use of cellulases and hemicellulases. However, this is made more difficult on the raw material straw or wood by the density of the above-mentioned complex. Due to their high molecular weight enzymes are unable to penetrate through the narrow pores in the lignocellulose. This means that a first step must be taken which increases the porosity of the lignocellulose and thereby allows further enzymatic saccharification.

This first step is called "pretreatment". It is very complicated throughout, so that e.g. In the production of "second generation biofuels" up to 1/3 of the production costs have to be spent, which negatively influences the profitability. The methods used are aimed either at primarily liquefying the hemicelluloses (e.g., steam explosion, dilute acid-pretreatment) or at increasing the porosity by liquefying lignin (e.g., lime, ammonia-pretreatment).

The digested lignocellulose substrate can be treated enzymatically for the recovery of sugars or their oligomers, wherein the type of pretreatment can have a strong influence on the enzyme activity and the yield. At high level ···· ···· • • • • A12659 • ····················································································· ·· ·· · · · ·

Reaction temperatures often give rise to toxic degradation products (e.g., furfural) which, in the case of direct ethanol fermentation, can inhibit yeasts (Chandra et al., 2007, Mansfield et al., 1999).

All of these methods have a serious drawback that they are either energy-intensive and run predominantly at temperatures just below 200 ° C, or that they require a costly recovery of the digestion chemicals.

A technological improvement in this area, e.g. The development of near-neutral low-temperature processes would mean a significant advance in any material use of the raw material lignocellulose. This is the object of the present invention.

EP 1 025 305 B1 discloses a chemical process for lignin depolymerization. It relies on the catalytic action of complexed copper in conjunction with hydrogen peroxide or organic hydroperoxides and is able to oxidatively cleave lignin at temperatures below 100 ° C. The complexing agents used are pyridine derivatives. Synthetic lignin models have been shown to cleave in the propyl side chain of the lignin molecule when H2O2 is used as the oxidant, causing the lignin polymer to disintegrate into oligomeric subunits. When using the Cu system with an excess of organic hydroperoxides, it is possible to delignify wood. The H2O2-based system appears to be technically more feasible and has been tested as a bleach additive in kraft pulp peroxide bleaching, resulting in improved delignification rate and higher whiteness.

Further, from Chupka et al. (1993), that the efficiency of alkaline catalysis of the oxidation of wood and lignin increases considerably when the aqueous reaction medium contains an organic solvent, e.g. DMSO, acetone, ethanol is added. Furthermore, the authors claim that at pH values above 11 there is a sharp increase in the oxidation of wood and lignin.

From WO 01/059204 a process for the production of pulp is known in which the starting material is subjected to a pretreatment, wherein the material is treated with a buffer solution and a delignification catalyst (transition metal). The delignification is carried out in the presence of oxygen, hydrogen peroxide or ozone. • ·······························································

In contrast, the process of the present invention for the production of sugars is characterized in that lignocellulosic material is treated with an aqueous solution containing hydrogen peroxide, a CM alcohol and a base to oxidatively cleave lignocellulose and separate fission products from the material, one with cellulose and hemicellulose-enriched material is obtained, and the resulting cellulosic and hemicellulose-enriched material is treated with a carbohydrate-splitting enzyme to recover the sugars.

The invention is based on the finding that a lignocellulosic material which has been treated with an aqueous, basic hydrogen peroxide solution which contains a C 1-4 -alcohol can be enzymatically processed into sugars in a higher yield than a material which has been delignified in another way, in particular without the additive from alcohol. The sugars are mainly pentoses and hexoses.

As lignocellulosic material straw is preferably used. Straw has a highly hydrophobic surface, so wetting with aqueous solutions is a problem. It has been found that it is possible by the use of alcohol, even without pressure to introduce the reaction solution into the pores of the substrate and to replace the existing air by reaction solution. If metal ions are introduced with the straw, which partially destroy the hydrogen peroxide, then a complexing agent for the metal ions should be added.

It has also been shown that alcohol accelerates the extraction of the cracking products from straw and helps to keep the lignin cleavage products in solution. Ethanol is particularly suitable as ethanol, propanol and butanol, including their isomers.

By evaporation of the alcohol after the digestion process, the amount of liquid is reduced or the substrate concentration is increased, so that smaller amounts of enzyme for enzymatic hydrolysis, or in other enzymatic processing are required. In alcohol production, enzyme costs are a crucial cost factor.

Alcohol causes in the alkaline region during the reaction in addition to the lignin ffeigesetzten hemicellulose cleavage products adsorb to the cellulose. The advantages for the process are: ······························· ···················· * ·· 4 · · · · · · · · · ·······························

High selectivity of lignin degradation. In the case of separation of the extraction solution from the solid:

Very low concentration of hemicellulose cleavage products in the extraction solution: the hemicellulose remains in the solid content and is thus preserved for enzymatic hydrolysis and sugar extraction.

The last two points, hardly any hemicelluose cleavage products in the extract and the increased solubility of the lignin, increase the throughput rates in case of a possible separation of the solid by filtration.

The alcoholic solution of the lignin extract offers improved possibilities in the further processing of the lignin and the production of lignin products:

It also acts as a scavenger and solvent for fission products from an enzymatic, biomimetic or chemical depolymerization of higher molecular weight lignin cleavage products to low molecular weight.

According to a preferred embodiment of the method according to the invention, straw is used as the lignocellulosic material. Straw has a strongly hydrophobic surface, so that wetting with aqueous solutions is generally a problem. In the known steamer explosion process, the reaction solution is introduced by the pressure. However, it has been found that it is possible by the inventive use of alcohol, even without pressure to introduce the reaction solution in the pores of the substrate and to replace the existing air by reaction solution. This is confirmed by the sinking of the substrate in the aqueous reaction medium.

A preferred variant of the method according to the invention is that the pH of the aqueous solution prior to the treatment of the lignocellulosic material is less than 11.0 and greater than 10.0 and that no base is added during the treatment. This is particularly advantageous for the enzymatic further processing of the sugars to alcohol, since it has been shown that the pH drops during the treatment, so that only a few chemicals to set the optimum pH for the subsequent enzymatic cleavage of carbohydrates and fermentation of Sugar to alcohol needed. In the process according to the invention, the lignocellulosic material is preferably used in the aqueous solution in a consistency of 5-40% by weight. ··· ········

The following example and comparative example document the effect of pretreatment in the presence of alcohol on the yield of reducing sugars after enzymatic hydrolysis.

example

Wheat straw is crushed to a particle size of about 2 cm. 2.5 g of crushed wheat straw is suspended in a 500 ml reaction vessel in 200 ml of a solution consisting of 49.5% water, 50% ethanol and 0.5% hydrogen peroxide. The suspension is heated to 50 ° C. in a water bath, thermostated and the pH of the suspension is adjusted to a starting pH of 12 using aqueous NaOH solution. The mixture is stirred continuously at 200 rpm, 60 ° C., 24 h. Thereafter, the solid content is filtered off and washed with 1 L of distilled water.

For enzymatic hydrolysis, 100 mg pretreated substrate of each parallel experiment was adjusted to pH 4.8 with 9.8 ml of 50 mM Na acetate buffer and admixed with 200 μl of Accellerase 1000 Suspension (www.genencor.com). Accellerase is an enzyme mixture of cellulases and hemicellulases. The enzymatic hydrolysis was carried out at 50 ° C in a shaking water bath. The soluble monomers of hexoses and pentoses released after 48 h were determined in the form of reducing sugars by the DNA method in 1 ml of liquid supernatant, based on the amount of pre-treated substrate weighed and expressed in percent of the maximum theoretical yield.

The theoretical maximum yield of reducing sugars was determined separately and is 705 mg +/- 5% per g of untreated straw.

In each case 5 parallel experiments were carried out per test batch. The yield of reducing sugars was 99% +/- 4%.

Comparative example

The above example was repeated, but without the addition of alcohol. The yield of reducing sugars was only 64% +/- 3%.

Claims (5)

• · · ··· A12659 • ······················· Claims 1. A process for the preparation of sugars, characterized by the combination of the steps of treating lignocellulosic material with an aqueous solution containing hydrogen peroxide, a Cm alcohol and a base to oxidatively cleave lignocellulose and fission products from the material to separate a cellulosic and hemicellulose-enriched material, and the resulting cellulosic and hemicellulose-enriched material is treated with a carbohydrate-splitting enzyme to recover the sugars. 2. The method according to claim 1, characterized in that the pH of the aqueous solution before the treatment of the lignocellulosic material is less than 11.0 and greater than 10.0 and that no base is added during the treatment. 3. The method according to any one of claims 1 or 2, characterized in that straw is used as lignocellulosic material. 4. The method according to any one of claims 1 to 3, characterized in that the lignocellulosic material in the aqueous solution in a fabric density of 5-40 wt .-% is present. 5. The method according to any one of claims 1 to 4, characterized in that the resulting sugars are fermented to alcohol, which is separated and recovered.