EP0051237A1 - Process for preparing water-soluble saccharides from cellulosic material - Google Patents

Abstract

1. A process for obtaining water-soluble saccharides from cellulose-containing material by treating the latter with gaseous hydrogen fluoride, optionally diluted with an inert gas at temperatures between about 20 and 120 degrees C, preferably between about 40 and 80 degrees C, which comprises subjecting cellolignin, a material composed largely of cellulose and lignin and being essentially free of pentosans and hexosans which has been obtained by pre-hydrolysis of natural cellulose-containing material with dilute mineral acid at an elevated temperature and under an elevated pressure, to treatment with hydrogen fluoride.

Description

Cellulose-containing materials are found in nature in large numbers and diversity. A known such natural cellulosic material is e.g. the wood. It consists essentially of cellulose (a material mainly composed of glucose), hemicellulose (a substance mainly composed of pentoses and hexoses) and lignin (a polymeric substance with aromatic rings substituted by methoxy groups). Wood is recycled in a variety of ways, e.g. for heat generation (firing), as a building material in the furniture and building materials sector, etc .; A purely chemical recycling of the wood is also possible.

Chemical digestion processes that not only separate wood into its components, hemicellulose, cellulose and lignin, but also break it down and transform it, have long been known. The chemical processes generally provide aqueous solutions of mono-, di- and oligomeric saccharides, which may be subjected to post-hydrolysis to glucose or directly fermented to ethanol, concentrated or evaporated to dryness. Possible areas of application for products obtained in this way are e.g. in the field of animal feed additives or preferably that of fermentation raw materials.

In the past, two principles have been applied on an industrial scale among the chemical processes for wood saccharification: the wood pulping with concentrated aqueous hydrochloric acid (Bergius-Rheinau-Udic) and digestion with dilute sulfuric acid (Scholler-Tornesch-Madison); see, for example, Ullmann's Encyclopedia of Industrial Chemistry, 3rd Edition, Vol. 8 (1957), p. 591 ff.

The digestion of raw materials containing cellulose with anhydrous hydrofluoric acid has also been repeatedly investigated. Here, however, all the processes that have become known have not yet led to a technically satisfactory solution. DE-PS 560 535 describes the digestion of wood with liquid or vaporous pure HF at low temperatures, the HF being recirculated by evaporation or blowing off and subsequent condensation. In continuation of this work, a process for the digestion of wood with gaseous hydrogen fluoride is described in DE-PS 585 318, which involves three stages via an absorption of HF on wood at 10 ° to 20 ° C, the digestion at 20 ° to 50 ° C and desorption works at 100 ° - 150 ° C, the HF can be diluted with an inert gas stream. The cooling effort for the condensation of HF has a disadvantage here, as does the fact that when condensation occurs, only a very uneven distribution of the hydrogen fluoride on the reaction material occurs, a circumstance which can only be counteracted by very long dwell times or a sharp increase in the use of hydrogen fluoride otherwise the yields can be severely impaired.

In DE-PS 606 009 an extraction with liquid HF is described, which, however, requires large quantities of HF and has the disadvantage that large amounts of heat are added to the evaporation of the hydrogen fluoride from the extract and extraction residue (lignin), and at subsequent condensation must be removed again.

More detailed information on yields in processes of this type can be found in Angew. Chem. 46 (1933). 113/7, with the absorption of the HF from the gas phase in a vessel with external cooling from 0 ° C. with a loading of 50% by weight HF. 32% sugar on wood based on existing carbohydrates and at 100% by weight loading 86% sugar. on carbohydrates. No further details are given there about the repatriation of the HF.

All of these processes have the disadvantage that they consume large amounts of the expensive hydrofluoric acid, the recovery of HF from the reaction products being very expensive and, in practice, large HF losses occurring.

A further process is described in AT-PS 147 494, where the previous state of the art is described as follows: "If you work with highly concentrated or anhydrous hydrofluoric acid in a liquid or gaseous state at low temperatures, the wood can only be broken down very uneven and therefore imperfect to begin with. At such low temperatures, the distribution form of the hydrogen fluoride, which is present as a fine mist in the air, is very uneven, all the more so since the air present complicates the uniformity of the reaction that when saccharifying wood with concentrated hydrogen fluoride, both in the liquid and in the gaseous state, the wood particles react quickly on the surface with the concentrated hydrogen fluoride, form a hard, rather impenetrable skin and shrink, which inhibits the further penetration of the gas into the interior Furthermore, d It is difficult for the wood particles to penetrate the air in the cells. It An outer crust forms very quickly, which includes unsugared material and prevents further saccharification. To eliminate these bad conditions, it has also been proposed to carry out the digestion with concentrated liquid hydrofluoric acid by means of an extraction process or to avoid the formation of crusts by adding inert gases to the hydrofluoric acid in order to achieve a more uniform and complete digestion. However, the extraction process works with a disproportionately high excess of hydrofluoric acid, and the reactant retains large amounts of hydrofluoric acid without preventing the crust formation with all of its disadvantages. The dilution with inert gases can reduce the formation of crusts somewhat, but can never cancel them out and also does not lead to the gas penetrating evenly into the interior of the wood, since the wood is filled with air. As is well known, wood consists only of the smallest part of wood mass itself and for the most part of air, which is located between and in the wooden cells. For example, practically water-free wood consists of approximately 15% wood mass and approximately 85% air. Since the wooden cells are extremely small in relation to the size of the wood, no matter how small it is, the air content plays a major role even with sawdust.

Hardening of the surface of wood particles also seems to have been found in the saccharification of wood with aqueous mineral acids such as aqueous hydrochloric or sulfuric acid, because Z. Angew. Chem. 37 (1924) 221 the substances present in the wood such as lignin, mannan, galactan etc. are referred to as "incrustations", which should also be removed as possible before the actual wood saccharification due to disruptive degradation products (furfural, acetic acid, formic acid etc.). For the distance one would have - since that Hydrolyzability of these "incrustations" was known - even in the case of wood saccharification using hydrogen fluoride, one could think of a kind of "pre-hydrolysis" with dilute mineral acid at elevated temperature and possibly increased pressure. However, such pre-hydrolysis had not been considered; rather, in order to avoid the disadvantages of Hoch and Bohunek described above, it was proposed to apply a vacuum of approx. 30 torr to the saccharification of wood with hydrogen fluoride [AT-PS 147 494 + addition 151 241; the wood saccharification process with hydrogen fluoride according to Hoch and Bohunek is also described in the journal "Holz Roh- und Werkstoff" 1, pp. 342-344 (193817.

Disadvantages of this method are the difficulties of the technical inevitably occurring when working in a vacuum. Realization, as well as the relatively complicated reaction process. A deficiency inherent in all processes is the formation of mixtures of pentoses and hexoses by simultaneous hydrolysis of the hemicelluloses and the cellulose of the wood.

Another problem is the separation of the acetic acid formed in the hydrolysis of hemicellulose, which makes the loss of HF in the circle as difficult as possible, as well as the easy decomposition of the pentoses to furfural.

Surprisingly, it has now been found that these disadvantages of the prior art described can be avoided and a slight saccharification of cellulose is possible if the plant materials are not in their native form, but after a pretreatment in the form of "cellolignin" with water-free, gaseous HF unlocks.

“Cellolignin” here means vegetable materials such as wood, straw, bagasse and similar raw materials which have been subjected to a pre-hydrolysis which is known per se.

This pre-hydrolysis of the wood, which is known per se, consists of a relatively short-term treatment with highly dilute mineral acid at higher temperatures and pressures, with essentially the pentosans and hexosans contained in the hemicelluloses down to the monomer units, e.g. Xylose or mannose, split. Depending on the reaction conditions, these can then be isolated as such or undergo further changes, e.g. Dehydration to furfural or hydroxymethylfurfural (see Ullmann, loc. Cit., Vol. 7 (1957), p.711). Apart from fermentation, the reduction of xylose to xylitol is another example of a technical use of hemicellulose degradation products. It is therefore possible to obtain valuable products from wood by pre-hydrolysis before the digestion process according to the invention is used.

Furthermore, cellolignin is also understood to mean paper material (e.g. waste paper) which is low in hemicelluloses. During the pre-hydrolysis of wood, its structure is largely preserved, but the cellolignin that can be obtained in this way has a much more wear-resistant and porous quality than the native state, so that HF, even when mixed with air or another inert carrier gas, can easily penetrate without one Encrustation of the surface occurs. Working in a vacuum is not necessary.

Another advantage of using cellolignin instead of native wood is that the reaction material is significantly easier to handle in terms of process technology. On the one hand, this is due to the fact that cellolignin has a bulk volume of only about half that of wood of the same grain size and therefore has a significantly lower degree of shrinkage when digested with hydrogen fluoride gas, which e.g. a great relief for the dimensioning of reactors. On the other hand, reaction material made of cellolignin remains pourable and free-flowing even when it is loaded with hydrogen fluoride, whereas that made from native wood tends to stick together due to resinous accompanying substances and fission products of the hemicelluloses and is difficult to promote.

Naturally, such a tendency to stick also complicates the desorption of hydrogen fluoride, especially if this is to proceed quickly and as quantitatively as possible. However, this is readily possible when using cellolignin as the substrate.

Furthermore, in this process it is no longer necessary to separate the sugar mixtures formed during the hydrolysis of hemicellulose from the oligomeric glucose units or glucose formed during the hydrolysis of cellulose, which enables these different sugars to be used more easily by fermentation.

It is also advantageous that when cellolignin is digested, no more acetic acid and furfural are formed, so that the HF can be circulated without having to condense these components. This avoids separation difficulties and RF losses.

Another advantage is the possibility
the absorption of HF on cellolignin above the boiling point of HF, so that external cooling is no longer necessary. It was also surprising that in the process according to the invention yields of> 90% glucose or oligomeric glucose based on the cellulose used in cellolignin can be achieved in a simple manner, the resulting sugars being of high quality, ie almost colorless.

The subject of the invention is therefore a process for obtaining water-soluble saccharides (glucose or oligomeric glucose) from cellulose-containing material by treating the same with gaseous hydrogen fluoride, optionally diluted with an inert gas, at temperatures between about 20 and 120 ° C. between about 40 and 80 ° C; the process is characterized in that cellolignin is subjected to a treatment with hydrogen fluoride.

As previously defined, cellolignin is understood to mean a material largely consisting of cellulose and lignin.

In view of the state of the art, the most recent of which the author (Hoch and Bohunek, loc.cit.) Tries to remedy the disadvantages associated with the uneven digestion and crust formation by using the complex vacuum method - although the easy hydrolyzability of hemicelluloses is known (Oesterr. Cheme-Zeitg. 40, 5 ff (1937), the use of prehydrolyzed material was by no means obvious. It was therefore extremely surprising that this measure, from which the state of the art just led away, a smooth and problem-free saccharification of Wood and wood-like materials allowed.

The cellolignin which is particularly suitable according to the invention for degradation to water-soluble sugars is obtained by pre-hydrolysis of natural cellulose-containing material (wood, straw, bagasse, etc.) with dilute aqueous mineral acid, preferably dilute hydrochloric or sulfuric acid. Prehydrolysis is - as already indicated in the description of the prior art - known in wood saccharification and can also be found in the more recent literature such as Ullmann's Encyclopedia of Industrial Chemistry, 3rd Edition, Volume 8 (-1957), pp. 591-595 as well as in the book by W. Sandermann, "Chemische Holzverwertung", Bayrischer Landwirtschaftverlag, Munich 1963, p. 253.

It consists in a relatively short-term treatment of the natural starting material with a highly diluted mineral acid at elevated temperature (preferably between about 100 and 160 ° C.) and elevated pressure (preferably up to about 10 atm), essentially the pentosans and hexosans contained in the hemicelluloses up to the monomer units (xylose, arabinose, mannose etc.)

be split. Depending on the reaction conditions, these can then be isolated as such or undergo further changes, e.g. through dehydration to furfural, etc.

They are preferably used as fermentation raw materials or for the extraction of xylitol.

Waste paper low in hemicellulose is also well suited for use.

The digestion according to the invention can be accomplished, for example, in such a way that the pre-digested material (cellolignin or, for example, paper shredder material) dried to a moisture content of 0 - about 20%, advantageously about 2 - 5% and crushed if necessary, either discontinuously in one suitable mixing vessel made of hydrogen fluoride - resistant material in contact with HF gas, possibly in a mixture with air or another inert carrier gas, or that an HF-containing gas mixture is advantageously countered in a conveyor system to a continuous flow of the substrate to be digested.

The temperature rises due to the spontaneously released heat of reaction and can be Dilution with inert gases in the desired range between about 20-120 ° C., preferably between 40 and 80 ° C.

The contact of the substrate with hydrogen fluoride gas is maintained until one part by weight of the material has absorbed about 0.2 to 3.0, preferably about 0.4 to 0.8, part by weight of hydrogen fluoride.

The reaction is now advantageously carried out in such a way that, depending on the type of substrate and the conditions of HFw absorption, a residence time is selected which Achieving the high yield is sufficient. Longer dwell times are not disadvantageous, but they are also of no advantage. They can be between about 15 minutes and several hours. Reaction conditions are preferred in which the residence time does not exceed about one hour.

The subsequent HF desorption can, according to the prior art, be carried out by heating the reaction material and / or by evacuation or by treatment with an inert gas stream (for example nitrogen, air, CO ₂ or noble gas) of suitable strength, in turn with or without simultaneous heating and / or evacuation , respectively. The hydrogen fluoride thus recovered can be isolated by condensation or reacted directly with fresh substrate, so that a cycle of gaseous hydrogen fluoride is formed. The further processing of the now digested ("saccharified") material can also be carried out in a manner known per se, as described, for example, by K. Fredenhagen and G. Cadenbach, Angewandte Chemie 46 (1933), pp. 113 to 117 Hot water, filtered from the insoluble lignin, neutralized the small amount of hydrogen fluoride carried in the filtrate by means of calcium carbonate or hydroxide and concentrated.

The amount of "wood sugar" (or "straw sugar" etc.) obtained after drying the evaporation residue in the procedure according to the invention is consistently over about 90% of the cellulose contained in the substrate (calculated on dry substance).

Because of the high "sugar" yield, the extremely simple and smooth process implementation (increasing the porosity of the substrate and thereby facilitating the penetration of HF!). as well as low-cost hydrogen fluoride absorption (no cooling necessary, no vacuum), the invention represents a not inconsiderable advance in this area.

The oligomeric glucose building blocks can be subjected to further utilization in the form obtained (fermentation to ethanol, concentration or evaporation and use as animal feed additives or as fermentation raw materials etc.) or also in a manner known per se for post-hydrolysis to monomeric glucose.

The invention is now explained in more detail by the following examples:

example 1

500 g of spruce wood cellolignin (59% cellulose + 41% lignin) with a grain size of approx. 2 mm were placed in a round 2 1 container made of transparent polyethylene with a stirrer, thermometer and gas inlet and with a mixture of air and fluorine gas which was obtained prepared by passing air over liquid hydrogen fluoride at 20 ° C (water bath) treated. The material was slowly stirred and turned dark brown. The air flow and HF evaporation were regulated so that the internal temperature did not exceed 70 ° C.

After 300 g of hydrogen fluoride had been taken in, an internal temperature of 50 ° C. was maintained for 30 minutes. With further stirring, the hydrogen fluoride was then expelled by introducing warm air. Part of the necessary desorption heat was also applied by external heating. Desorption was continued with a steadily increasing temperature up to a hydrogen fluoride content of about 5% in the substrate. Then the material was transferred to a fluid bed dryer and the hydrogen fluoride was blown off to a residual amount of about 0.5%. The resulting HF-air mixtures could be used immediately for further approaches.

The contents of the reactor were then digested with about 2 liters of hot water for 15 minutes, filtered off with suction and washed with a little water. The dark brown filter residue weighed about 250 g after drying and thus consisted of 82% lignin and 18% undigested cellulose. The filtrate was made alkaline while still hot with technical calcium hydroxide, the excess hydroxyl ion was neutralized with carbon dioxide, and the calcium fluoride and carbonate were filtered off, possibly with the aid of a filtration aid. The clear, pale yellow, neutral solution was brought to dryness in a vacuum. This gave about 250 g of slightly yellowish wood sugar, corresponding to a yield of 85% of theory The product was clearly water-soluble and contained between 2 and 10% monomeric glucose, the rest consisted of oligomeric glucose.

Examples 2-13

A jacketed, hydrogen fluoride-resistant tube 30 cm long and 4 cm wide was filled in the horizontal position with 30 g cellolignin with a grain size of 1-2 mm approximately half-way and closed at both ends with pierced rubber stoppers. In the cellolignin layer as well as in the free space above was a thin steel tube perforated over the entire length. These pipes led on both sides through bores of the sealing plugs to the outside and were used for supplying and discharging the HF-air mixture. In this way it was possible to gas the cellolignin perpendicular to the surface of the bed. The material was allowed to absorb hydrogen fluoride and, during the subsequent dwell time, an internal temperature of 50 ° C. was ensured by appropriate heating. Subsequently, instead of the HF Air mixture was blown hot air through the bed for 15 minutes and the reaction product thus obtained and freed from the majority of the hydrogen fluoride was worked up as described in Example 1.

The following table shows the yields as a function of the amount of HF absorbed and the residence time.

Example 14

In a horizontally arranged, long tube made of hydrogen fluoride-resistant material, in which a free-flowing solid can be moved continuously by means of a screw conveyor, a cellulose lignin filling was entrained with a hydrogen fluoride / carrier gas mixture demonstrated that the material at the HF inlet end of the tube had a content of approx. 60% HF, based on cellolignin, but only pure carrier gas flowed out at the cellolignin inlet end. The reaction material was continuously discharged at the HF inlet end, while fresh cellolignin was supplied on the opposite side. The material discharged was freed of hydrogen fluoride by blowing off after passing through a half-hour residence time and the HF-rich gas mixture thus obtained was returned to the reaction tube. The digested cellolignin is worked up in the manner already described in Example 1. The yield of wood sugar was approx. 85% of theory

Example 15

150 g of shredder material made from newsprint was gassed with a mixture of hydrogen fluoride and air in the manner described in more detail in Example 1. After the reaction mixture had been left to stand at 50 ° C. for one hour, the hydrogen fluoride was removed to a residual content of 2% by introducing a stream of warm air and the dark-colored residue was digested with hot water. After filtering and drying, 50 g of insoluble material, mainly consisting of lignin, were obtained. The filtrate was neutralized with hydrated lime and suctioned off from the calcium fluoride. The evaporation residue of the filtrate weighed 80 g and contained approximately 10% monomeric glucose (remainder: oligomeric glucose).

Claims (3)

1. A process for obtaining water-soluble saccharides from cellulose-containing material by treating the same with gaseous - optionally diluted with an inert gas - hydrogen fluoride at temperatures between about 20 and 120 ° C, preferably between about 40 and 80 ° C, characterized in that cellolignin Subjects treatment with hydrogen fluoride. 2. The method according to claim 1, characterized in that the cellolignin used is a material obtained by pre-hydrolysis of natural cellulose-containing material with dilute mineral acid at elevated temperature and pressure. 3. The method according to claim 1, characterized in that used as cellolignin hemicellulose-poor waste papers.