Method for the fluorescence spectroscopic determination of the lignin content in cellulose

Abstract

The invention relates to methods for the fluorescence spectroscopic determination of the lignin content of cellulose samples, in which either the lignin content is determined by comparing the fluorescence emission spectrum of the sample with a reference sample or via the fluorescence decay spectrum. The invention further relates to methods for the delignification of lignocellulose, in particular in the form of wood, in which such methods are used to monitor the progress of the delignification, in particular such methods in which cellulose and lignin from the lignocellulose are separated by an extraction process using an ionic liquid.

Description

[0001] METHOD FOR THE FLUORESCENCE SPECTROSCOPIC DETERMINATION OF LIGNING CONTENT IN CELLULOSE

[0002] Description

[0003] The present invention relates to methods for the fluorescence spectroscopic determination of the lignin content of cellulose samples, in which the lignin content is determined either by comparing the fluorescence emission spectrum of the sample with a reference sample or via the fluorescence decay spectrum. The present invention further relates to methods for the delignification of lignocellulose, in which such methods are used to monitor the progress of delignification, in particular to methods in which cellulose and lignin from the lignocellulose are separated from each other by extraction with an ionic liquid.

[0004] State of the art

[0005] Cellulose is technically used in a wide variety of products, of which paper is perhaps the most obvious, and has become practically indispensable in everyday life. But cellulose also occurs in a number of other everyday products, such as in derivatized form as cellulose acetate in cigarette filters, as "carboxymethylcellulose," methylcellulose, or in the form of hydroxyalkyl derivatives of cellulose such as HMPC or HPC. The latter usually aims to make cellulose more soluble in water or polar organic solvents in order to utilize its properties.

[0006] Cellulose is technically obtained from plants, where the cellulose is formed from CO2 through photosynthesis and used as a structural substance in the plants. Cellulose does not occur in its pure form in plants, but is usually found as a mixture with lignin and hemicelluloses. These additional substances are undesirable for the use of cellulose (for example, if wood, as a lignocellulosic material, is to be used not as such but as a cellulose source) and must therefore be separated from the cellulose in more or less complex processes. Such processes include, for example, sulfite digestion with sodium sulfite, in which lignin is converted into water-soluble ligninsulfonic acids, which can then be separated from the insoluble cellulose.A newer process used for the separation of cellulose, lignin, and hemicelluloses involves the reaction of non-cellulose biomass (e.g., in the form of wood) with ionic liquids, in which the lignin is dissolved without prior chemical modification of the lignin. Such a process is described, for example, in DE 10 2015 006 926 A1.

[0007] For cellulose required for further use, such as in consumer products or pharmaceuticals (e.g., as microcrystalline cellulose or in derivatized form), there are different purity requirements regarding the permissible lignin content. For example, residual lignin in cellulose generally leads to a slight discoloration compared to pure white cellulose, and residual lignin can also have a negative impact on further processing, for example, when it is used as a raw material for the production of fibers (as regenerated cellulose).

[0008] The problem here is that the purity of the cellulose in such a delignification process depends on the raw material used to obtain the cellulose and the process conditions. For example, to reproducibly produce cellulose with a residual lignin content of "less than 5%," it would be necessary to control the purity of the cellulose in the extraction process. However, according to the available state of the art, this requires the cellulose isolated from the process to be subjected to a wet-chemical separation process that corresponds to the separation process used to produce the cellulose. In this case, the separation must be carried out with a higher proportion of "extractant" for lignin or for a longer period of time. Such a test is therefore clearly only possible as "quality control" (i.e., after the extraction reaction has already ended).

[0009] This results in the extraction reaction either having to be carried out longer than necessary to achieve a desired maximum residual lignin content, or additional control and processing effort being required if quality control reveals that the desired lignin content has not (yet) been achieved for a particular cellulose. Both scenarios clearly result in significant economic disadvantages.

[0010] Against this background, there is a need for methods that can determine the lignin content of a cellulose sample with minimal technical effort and at a speed that allows monitoring the progress of an ongoing delignification reaction. Furthermore, there is a need for methods for determining the lignin content of a cellulose sample that allow non-destructive analysis of the cellulose sample.

[0011] The present invention addresses this need.

[0012] Description of the invention

[0013] In the investigations underlying this invention, it was surprisingly discovered that the lignin content of a cellulose sample can be determined using the fluorescence emission of lignin. The reason for this is that the fluorescence spectrum in a specific wavelength range shows a dependence of the fluorescence intensity on the lignin content if the spectrum has previously been normalized to a specific wavelength in which the fluorescence occurs. It was further surprisingly discovered that the fluorescence decay time of cellulose samples containing lignin is influenced by the lignin content, so that this content can be determined by evaluating the fluorescence decay spectrum. In this way, a complex chemical separation of lignin and cellulose, which would be necessary using conventional procedures, can be avoided.Furthermore, since the samples for fluorescence measurements require minimal preparation, the methods can also be used to monitor the progress of common wood delignification reactions.

[0014] According to a first aspect, the present invention accordingly relates to a method for the fluorescence spectroscopic determination of the lignin content in a cellulose sample, which comprises recording a fluorescence emission spectrum of the cellulose sample in the range > 500 nm, normalizing the fluorescence emission spectrum with a fluorescence emission spectrum of a cellulose sample with known lignin content, preferably at a wavelength in the range of 500 to 700 nm and in particular at about 520 nm, and comparing the fluorescence intensity of the cellulose sample with the fluorescence intensity of the reference cellulose sample.

[0015] In this inventive method, "normalization" refers to the fact that the emission of the measured spectrum at the normalization wavelength (which is identical for both spectra) is adjusted by multiplying it by a factor, and this factor is subsequently also used to multiply the entire measured spectrum. In this way, the two spectra assume the same value at the reference wavelength and differ from each other only at higher and lower wavelengths.

[0016] In the context of the invention, "comparison of the fluorescence intensity determined in this way with the fluorescence intensity of the reference cellulose sample" refers to the fact that the deviations in the fluorescence intensity at wavelengths higher or lower than the standardization wavelength are characteristic of the lignin content and thus correlate with the fluorescence intensity of the reference cellulose sample. According to the invention, this deviation can be used to determine the lignin content of the sample.

[0017] The specified method can be advantageously further developed in that the fluorescence intensity measured in the range from 520 nm to 630 nm, and preferably in the range from 540 nm to 580 nm, is compared with a reference cellulose sample with a known lignin content in such a way that the spectrum of the reference cellulose sample is subtracted from the spectrum of the cellulose sample from which the lignin content is to be determined, wherein a positive difference indicates a lignin content that is higher than the lignin content of the reference cellulose sample and a negative difference indicates a lignin content that is lower than the lignin content of the reference cellulose sample.

[0018] Furthermore, within the scope of the specified invention, it is advantageous if the cellulose sample whose lignin content is to be determined is irradiated with light having a wavelength of < 500 nm, preferably in the range of 390 to 500 nm and more preferably 395 to 440 nm, to generate fluorescence. As mentioned above, a lignin content-dependent fluorescence decay was also surprisingly observed, with the lignin content of the sample having an influence in particular on the amplitudes Ai and A2 of the fluorescence decay spectra determined by biexponential curve fitting, and the resulting quotient AI / A2.

[0019] A further aspect of the present invention therefore relates to a method for the fluorescence spectroscopic determination of the lignin content in a cellulose sample, wherein the fluorescence is determined as a fluorescence decay spectrum generated by excitation with a short laser pulse. Preferably, the fluorescence is generated by excitation with laser light having a wavelength of <500 nm, in particular in the range from 390 nm to 500 nm and even more preferably in the range from 450 to 490 nm. It is further preferred for this method if the fluorescence is generated with a pulse length of the irradiated light of <100 ps, ​​in particular <50 ps and more preferably in the range from 2 to 10 ps.

[0020] Since, as mentioned above, an influence of the lignin content of the sample was observed, especially on the amplitudes Ai and A2 of the fluorescence decay spectra determined by a biexponential curve fitting, and the resulting ratio AI / A2, it is further advantageous for the method if the lignin content of the cellulose sample is determined based on the amplitudes Ai and A2 determined from the fluorescence intensity and preferably from the ratio AI / A2. These amplitudes result from a biexponential curve fitting according to I(t) = Ai e -t / T1 + A2e -t / T2 with the fluorescence lifetimes TI and T2.

[0021] For the method, it is further preferred if the fluorescence emission is recorded at a wavelength of > 510 nm.

[0022] The measurement of the fluorescence and fluorescence decay spectra can be carried out in the context of the invention specified here, for example, in the manner specified in the examples section of this application.

[0023] Regardless of whether the first or second of the methods described above is used to determine the lignin content of the cellulose sample, it is preferred that the fluorescence spectrum for determining the lignin content be determined from a sample area measuring less than 1 x 1 mm. The thickness of this sample can also be very thin (so that only a very small amount of sample is required for the measurement), with a sample thickness in the range of 10 pm to 1 mm being suitable.

[0024] During the experiments conducted for this invention, it was observed that the determination of lignin content is not as reliable for lignin contents of less than 2% in the sample as for lignin contents of more than 2%. Accordingly, the determination of lignin content is preferably carried out for samples for which a lignin content of > 2% is assumed. It is further preferred if the determination of lignin content is carried out for samples for which a lignin content of < 15% and especially < 10% is assumed, since a nearly linear dependence of the fluorescence intensity or lifetime on the lignin content has been observed for this range.

[0025] As is clear from the discussion in the introduction and the state of the art, the specified methods are particularly applicable in the context of a process for the delignification of lignocellulose, preferably in the form of wood, to monitor the progress of delignification. This can be done, for example, by taking a small sample from the delignification process, separating the cellulose from the lignin solution, and subsequently recording a fluorescence spectrum of the cellulose sample. From this spectrum, the residual lignin content in the sample can then be determined.

[0026] Accordingly, a further aspect of the present invention relates to a process for delignifying lignocellulose, preferably in the form of wood, wherein the lignocellulose is treated with a solvent which dissolves lignin and substantially does not dissolve cellulose, and the extent of delignification is determined by a process as set out above.

[0027] Such a process is preferably designed in such a way that, after different treatment times of the lignocellulose with the solvent, cellulose samples are taken, the solvent is washed out of these samples with the dissolved lignin, and the lignin content of the respective sample is subsequently determined according to a method as described above. For the process, particularly if it is a process according to the first aspect described here, it is further advantageous if the lignin content of the respective sample is compared with an expected value for the lignin content, and the treatment of the lignocellulose with the solvent is terminated when the expected value for the lignin content is exceeded. The "expected value" for the lignin content here corresponds to a desired maximum lignin content for the target product to be produced via delignification.A suitable value for such an "expected value" can be, for example, 10%, preferably 8%, and more preferably in the range of 2% to 6%. % refers to proportions in weight percent.

[0028] In addition to wood (which is usually preferred for delignification in the context of the process described here, especially in the form of hardwood or softwood), the following materials in particular can be considered as lignocellulose: bamboo, plant residues such as corn stalks, cereal straw, rice straw, sugar cane, hay, etc. as well as waste materials such as residues from biogas plants, residues from fermentation and fermentation processes.

[0029] It is further advantageous for the process if the solvent which dissolves lignin and essentially does not dissolve cellulose is selected from ionic liquids, and preferably acidic ionic liquids, which are in particular selected from compounds of formulas 1 to 9 (see below)

[0030] where the residues R 1 to R 5 may be the same or different from one another and represent branched and unbranched alkyl, substituted and unsubstituted alicyclyl, aryl, aralkyl, alkylaryl, heteroaryl radicals, each of which may be substituted by hydroxy, alkoxy, alkoxycarbonyl and amino or dialkylaminocarbonyl groups, and where the radicals R x -R 2 , R x -R 3 , and R 3 -R 4can be bonded together to form cyclic structures, and wherein X- is selected from the group comprising chloride, bromide, iodide, methanesulfonate, methanedisulfonate, methanetrisulfonate, methanetetrasulfonate, ethanesulfonate, 1,1- and 1,2-ethanedisulfonate, 1,3-propanedisulfonate, sulfate, hydrogensulfate, tetrafluoroborate, hydroxytrifluoroborate, trifluoroacetate, trichloroacetate, oxalate, hydrogenoxalate, trifluoromethanesulfonate, nonafluorobutanesulfonate, bis(trifluoromethylsulfonyl)imide, methylsulfate, ethylsulfate, propylsulfate, and butylsulfate. It is very particularly preferred if the ionic liquid is an ionic liquid according to the formula 1 shown above, where in particular R 1 , R 2 and R 3 H or a C1-4 alkyl group, which may be the same or different. In a specifically preferred embodiment, the ionic liquid is the adduct of sulfuric acid and dimethylformamide.

[0031] For further preferred embodiments of the delignification process in which the specified methods for determining the lignin content in the cellulose sample can be used, reference is made to the disclosure of DE 10 2015 006 926 A1, the relevant disclosure of which is hereby incorporated by reference into this application.

[0032] In the following, the present invention is further illustrated by means of some embodiments, which, however, are not to be regarded in any way as limiting the scope of protection of this application.

[0033] Examples

[0034] Preparation of cellulose samples with different lignin contents

[0035] For the following investigations, cellulose samples with lignin contents ranging from 1.4% to 9% were used. The corresponding starting materials (beech or oak shavings, 50 g) were extracted with a mixture of DMF / H2SO4 (3-12 g) and a solvent (THFA, 250 ml) at 140°C for 3 h. The purities and compositions of the resulting cellulose samples are given in Table 1 below:

[0036] Table 1

[0037] 1 = extracted with ethylene glycol instead of THFA as solvent; 2 = extracted with butyl glycol instead of THFA as solvent; 3 = extracted with glycerol instead of THFA as solvent

[0038] The samples prepared in this way were examined with a confocal laser scanning microscope (Axiovert 200M with scanning unit Pascal 5, Carl Zeiss Microimaging GmbH, Jena, Germany) with an excitation wavelength X ex= 488 nm and fluorescence detection at X = 505-530 nm and at X<560 nm. The use of a 10x / 0.30 objective lens allowed the acquisition of sample areas of 920 pm x 920 pm and layers of 5-10 pm thickness. For microspectral analysis, a high-pressure mercury lamp with a bandpass filter for 395-440 nm for excitation and a longpass filter for X>470 nm for detection was used. A custom-built polychromator (resolution: AX<10 nm) was mounted on the microscope (Axioplan 1, Carl Zeiss Microimaging GmbH, Jena, Germany) and combined with an image intensifier system (IMD D4562, Hamamatsu Photonics, Ichino-Cho, Japan).

[0039] For analysis, the spectra were normalized at 520 ± 5 nm; the resulting superimposed spectra for 2.4%, 3.0%, 5.0%, 6.0%, and 9.0% lignin are shown in Fig. 1. As can be seen from this figure, the spectra show a fluorescence intensity dependent on the lignin content, particularly in the wavelength range from 540 to 630 nm. The following table shows the fluorescence intensities (mean counts per channel) of the individual cellulose samples with the specified lignin concentrations in the spectral range 540-580 nm, with reference to Sample No. 1 (2.4% lignin):

[0040] Table 2

[0041] Figure 1 and the table show that in the technically interesting range of lignin contents between 2% and approximately 10%, it is possible to correlate fluorescence intensity with lignin content. The samples require minimal preparation for the measurement, and the measurement can be performed within a short time.

[0042] Determination of the fluorescence decay spectra of the cellulose samples

[0043] To measure the fluorescence decay kinetics, a supercontinuum fiber laser operating at 450–490 nm with a 5 ps pulse duration and a 78 MHz repetition rate (NKT Photonics, Birkeröd, Denmark) was used in combination with an image intensifier camera system (Picostar HR12 image intensifier coupled with a cooled ICCD camera; LaVision, Göttingen, Germany). This system allowed the fluorescence decay kinetics to be recorded with a resolution of 200 ps, ​​and the fluorescence lifetime could be determined from this. Figure 2 shows the fluorescence decay kinetics and a bi-exponential curve fit.

[0044] From the determined bi-exponential decay behavior and the dependence relationship for the fluorescence intensity IF = Ai e -t / T1 + A2e -t / T2 The fluorescence lifetimes n and z2, the corresponding amplitudes Ai and A2, and the ratio AI / A2 were determined. The values ​​thus determined are shown in Table 3 below:

[0045] Table 3

[0046] As can be seen from the table, higher amplitudes and shorter fluorescence lifetimes result for higher lignin contents. The AI / A2 ratio shows an approximately linear increase from low lignin contents up to lignin contents in the range of approximately 10%.

Claims

Claims 1. Method for the fluorescence spectroscopic determination of the lignin content in a cellulose sample, comprising recording a fluorescence emission spectrum of the cellulose sample in the range > 500 nm, normalizing the fluorescence emission spectrum with a fluorescence emission spectrum of a cellulose sample with known lignin content, and comparing the fluorescence intensity of the cellulose sample with the fluorescence intensity of a reference cellulose sample.

2. The method according to claim 1, wherein the fluorescence intensity measured in the range from 520 nm to 630 nm, preferably in the range from 540 nm to 580 nm, is compared with a reference cellulose sample with known lignin content in such a way that the spectrum of the reference cellulose sample is subtracted from the spectrum of the cellulose sample from which the lignin content is to be determined, a positive difference indicating a lignin content that is higher than the lignin content of the reference sample and a negative difference indicating a lignin content that is lower than the lignin content of the reference cellulose sample.

3. The method according to claim 1 or 2, wherein the cellulose sample is irradiated with light having a wavelength < 500 nm, preferably in the range of 390 to 500 nm and more preferably 395 to 440 nm, to generate the fluorescence.

4. A method for the fluorescence spectroscopic determination of the lignin content in a cellulose sample, wherein the fluorescence is determined as a fluorescence decay spectrum generated by excitation with a short laser pulse, preferably with a wavelength < 500 nm, more preferably with a pulse length < 100 ps.

5. The method according to claim 4, wherein the fluorescence emission is recorded at a wavelength of > 510 nm.

6. Method according to claim 4 or 5, wherein the lignin content of the cellulose sample is determined from the amplitudes Ai and A2 determined from the fluorescence intensity, preferably from the ratio AI / A2 of a biexponential curve fitting according to I(t) = Ai e -t / T1 + A2e -t / T2 with the fluorescence lifetimes n and T2.

7. Method according to at least one of the preceding claims, wherein a fluorescence spectrum of a sample from an area of ​​less than 1 x 1 mm is determined.

8. Method according to one of the preceding claims, wherein the determination of lignin contents is carried out in the range > 2%.

9. A process for the delignification of lignocellulose, in particular for the delignification of wood, wherein the lignocellulose is treated with a solvent which dissolves lignin and substantially does not dissolve cellulose, and the extent of delignification is determined by a process according to any one of claims 1 to 8.

10. The method according to claim 9, wherein cellulose samples are taken after different treatment times of the lignocellulose with the solvent, the solvent is washed out of these samples with the dissolved lignin and then the lignin content of the respective sample is determined according to a method according to one of claims 1 to 8.

11. The method according to claim 10, wherein the lignin content of the respective sample is compared with an expected value for the lignin content, and the treatment of the lignocellulose with the solvent is terminated when the expected value for the lignin content is exceeded.

12. The process according to claim 11, wherein the expected value of the lignin content is 10%, preferably 8% and more preferably in the range of 2% to 6%.

13. A process according to any one of the preceding claims 9 to 12, wherein the solvent which dissolves lignin and substantially does not dissolve cellulose is selected from ionic liquids, in particular acidic ionic liquids selected from compounds of formula where the residues R 1 to R 5may be the same or different from one another and represent branched and unbranched alkyl, substituted and unsubstituted alicyclyl, aryl, aralkyl, alkylaryl, heteroaryl radicals, each of which may be substituted by hydroxy, alkoxy, alkoxycarbonyl and amino or dialkylaminocarbonyl groups, and where the radicals R x -R 2 , R x -R 3 , and R 3 -R 4 can be linked to form cyclic structures, and where X- is selected from the group comprising chloride, bromide, iodide, methanesulfonate, methanedisulfonate, Methane trisulfonate, methane tetrasulfonate, ethanesulfonate, 1,1- and 1,2- ethane disulfonate, 1,3-propanedisulfonate, sulfate, hydrogen sulfate, tetrafluoroborate, hydroxytrifluoroborate, trifluoroacetate, trichloroacetate, oxalate, hydrogen oxalate, trifluoromethane sulfonate, nonafluorobutane sulfonate, Bis(trifluoromethylsulfonyl)imide, methyl sulfate, ethyl sulfate, propyl sulfate, and Butyl sulfate.

14. The method according to claim 13, wherein the ionic liquid is an ionic Liquid according to formula 1, wherein preferably R 1 , R 2 and R 3 H or a Cl-4 alkyl group, which may be the same or different, whereby the ionic liquid is most preferably the adduct of sulfuric acid and dimethylformamide.

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