Fluorescent lignin prepared based on tea stems and preparation method thereof

Abstract

The invention provides fluorescent lignin prepared on the basis of tea stems and a preparation method of the fluorescent lignin, the tea stems are used as raw materials, the lignin is extracted by pretreating tea stem waste, namely polyethylene glycol and organic acid are used as deep eutectic solvents, and the lignin is extracted from the tea stem waste by utilizing hydrothermal reaction for pretreatment. After deep eutectic solvent pretreatment, a reverse solvent is used for treatment to obtain regenerated lignin; and mixing the regenerated lignin with a solvent, and reacting in an oil bath at 120 DEG C for 6 hours to obtain the fluorescent lignin, thereby realizing separation and high-value utilization of biomass components.

Description

Technical Field

[0001] The invention relates to the field of agricultural waste renewable biomass wood, and in particular to fluorescent lignin prepared based on tea stalks and a preparation method thereof. Background Art

[0002] Tea stems refer to the leaf stems picked from finished tea leaves, which are a by-product of tea processing. Further processing and utilization of tea stem waste to increase added value will help increase farmers' income while also achieving sustainable development of resources and promoting ecological balance. Tea stems contain about 37% wood lignin, which is mainly composed of cellulose, hemicellulose, and lignin.

[0003] Lignin is a polymer of amorphous high molecular weight aromatic compounds, mainly composed of guaiacylglycerol (G), p-hydroxyphenyl (H) and syringol (S) units, connected by a large number of CO bonds and CC bonds. Its aromatic structure makes lignin considered an important natural source of phenolic chemicals for the preparation of various biofuels, chemicals and materials. Summary of the invention

[0004] The invention provides a fluorescent lignin prepared based on tea stems and a preparation method thereof. The invention uses tea stem waste as raw materials, polyethylene glycol and organic acid as low eutectic solvents, pre-treats the tea stem waste to efficiently separate and extract and prepare the fluorescent lignin, and the prepared fluorescent lignin exhibits strong photoluminescence performance under the excitation of a 365nm ultraviolet lamp.

[0005] The technical means adopted by the present invention are as follows: A method for preparing fluorescent lignin based on tea stems, comprising the following steps:

[0006] S1: Using tea stems as raw materials, pretreatment with a low eutectic solvent and reverse solvent treatment in sequence to obtain regenerated lignin;

[0007] S2: The regenerated lignin obtained in step S1 is mixed with a solvent, and reacted in an oil bath at 120° C. for 4 to 6 hours to obtain fluorescent lignin.

[0008] Furthermore, the method for preparing regenerated lignin in step S1 comprises the following steps:

[0009] The tea stems, the low eutectic solvent and the deionized water are mixed evenly, and heated at 100-150° C. for 1-5 hours. After the reaction is completed, the mixture is washed and filtered until the washing liquid under filtration is transparent. After the solvent is removed from the washing liquid, a reverse solvent is added for precipitation, and the precipitate is freeze-dried to obtain regenerated lignin.

[0010] Furthermore, in step S2, the solvent is selected from hydrogen peroxide.

[0011] Furthermore, in step S2, the mass ratio of the regenerated lignin to the solvent is 5:1.

[0012] Further, in step S1, the deep eutectic solvent comprises polyethylene glycol and an organic weak acid in a molar ratio of (1-4):1;

[0013] The counter solvent is selected from deionized water or ethanol.

[0014] Furthermore, in step S1, the molar ratio of polyethylene glycol to the organic weak acid can be any one of 1:1, 2:1, 3:1, 4:1 or a range between any two of them.

[0015] Furthermore, the organic weak acid is selected from at least one of oxalic acid, formic acid, lactic acid and levulinic acid.

[0016] Furthermore, in step S1, the mass ratio of the tea stems to the deep eutectic solvent is 1:20;

[0017] The amount of deionized water used is no more than 50 wt % of the mass of the deep eutectic solvent.

[0018] Further, in step S1, the amount of deionized water used is any one of 50wt%, 45wt%, 40wt%, 35wt%, 30wt%, 25wt%, 20wt%, 15wt%, 10wt%, 5wt%, 0wt% or a range between any two of the mass of the low eutectic solvent.

[0019] Another object of the present invention is to provide fluorescent lignin prepared by a method for preparing fluorescent lignin based on tea stems.

[0020] Furthermore, the fluorescent lignin is used to detect iron ions.

[0021] Furthermore, the excitation wavelength of the fluorescent lignin is about 340 nm; and / or

[0022] The emission wavelength is 450 nm when excited at 340 nm.

[0023] Compared with the prior art, the fluorescent lignin prepared from tea stems and the preparation method thereof described in the present invention have the following beneficial effects:

[0024] First, the present invention uses polyethylene glycol and organic acid as low eutectic solvents, and uses hydrothermal reaction for pretreatment to extract lignin from tea stem waste. After pretreatment with the low eutectic solvent, it is treated with reverse solvents such as water and ethanol to obtain regenerated lignin, which is reacted with hydrogen peroxide to obtain fluorescent lignin. The fluorescence intensity of the regenerated lignin extracted by this system is better than that of the lignin extracted by the traditional method.

[0025] Second, the fluorescence intensity of the fluorescent lignin prepared by the present invention is significantly stronger than that of the lignin (ground wood lignin) prepared by the traditional method. At the same time, the fluorescence quenching of the fluorescent lignin solution by iron ions is the most obvious, while the change of fluorescence intensity by other metal ions is not obvious, indicating that the prepared fluorescent lignin is particularly effective as a probe for detecting iron ions. The experimental results show that the presence of other metal ions has an effect on the fluorescence quenching of the fluorescent lignin solution. 3+ The quenching response of Fe was not significantly affected. The new fluorescent sensor developed in this study has the potential to detect Fe in water environment. 3+ The fluorescent lignin method for detecting metal ions has the advantages of low cost, high sensitivity, good selectivity, and fast response time, and is particularly effective as a probe for detecting iron ions. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0027] Figure 1 is the yield of levulinic acid from cellulose after pretreatment;

[0028] Figure 2 The fluorescent lignin obtained in Example 1 emits obvious fluorescence under a 365nm ultraviolet lamp;

[0029] Figure 3 Fluorescence emission spectrum of metal ions on the fluorescent lignin obtained in Example 1;

[0030] Figure 4 Competing ions and iron ions for the fluorescence emission spectrum of the fluorescent lignin obtained in Example 1;

[0031] Figure 5 Effect of iron ion concentration on the fluorescence emission spectrum of the fluorescent lignin obtained in Example 1;

[0032] Figure 6 The linear relationship between the fluorescent lignin obtained in Example 1 and the iron ion concentration and the fluorescence intensity;

[0033] Figure 7 Fluorescence emission spectra of the fluorescent lignin obtained in Example 1 and the groundwood lignin obtained in Comparative Example 1. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] Preparation example of regenerated lignin

[0036] The regenerated lignin involved in the present application is obtained by using tea stems as raw materials and pre-treating with a low eutectic solvent.

[0037] Preparation Example 1-4

[0038] The preparation method of regenerated lignin comprises the following steps:

[0039] Tea stem powder (1g), low eutectic solvent (20g, composed of polyethylene glycol and organic acid in a molar ratio of 2:1) and deionized water (0g) were mixed evenly to obtain a mixed solvent; the obtained mixed solvent was added to a thick-walled pressure bottle (100mL), immersed in a preheated 100°C oil bath, and after reacting for 3h, the thick-walled pressure bottle was taken out of the oil bath and immediately immersed in an ice water bath to quench the reaction; ethanol / water (1:1v / v) was used for suction filtration and washing in a Buchner funnel until the filtered washing liquid was transparent, and the filtered residue was dried in an 80°C forced air drying oven to constant weight. The washing liquid was subjected to a rotary evaporator to remove ethanol and water at 80°C to obtain a concentrated solution; 200mL of deionized water was added to the concentrated solution for precipitation, and finally freeze-dried to obtain regenerated lignin.

[0040] The only difference between Preparation Examples 1-4 is the different organic acids, which are: oxalic acid (Preparation Example 1), formic acid (Preparation Example 2), lactic acid (Preparation Example 3), and levulinic acid (Preparation Example 4).

[0041] The cellulose retention rate and hemifiber removal rate of Preparation Examples 1-4 were tested respectively:

[0042] The regenerated lignin was first hydrolyzed with 72% sulfuric acid at 30°C, and then hydrolyzed with 4% sulfuric acid in an autoclave (121°C, 1h), and then filtered with a G3 glass filter. The filtrate was filtered through a 0.22μm water filter and then measured for glucose and xylose by high performance liquid chromatography; at the same time, the filtrate was diluted with an ultraviolet spectrophotometer at a wavelength of 205 to make its absorbance in the range of 0.2-0.8 to measure its acid-soluble lignin, and then more than 50ml of pure water was used to dilute the filtrate. The residue in the pressure bottle is fully washed into the crucible, and then the crucible is dried at 105°C for 4-5h until constant weight is reached, cooled in a dryer, weighed, and recorded as W1, which is the mass of acid-insoluble lignin and ash. After weighing, the crucible is placed in a muffle furnace and calcined at 550°C for 4-5h. After calcination, the crucible is cooled in a dryer and then weighed and recorded as W2. The remaining component of the crucible after ashing is acid-insoluble ash. After deducting the mass of acid-insoluble ash from W1-W2, the content of acid-insoluble lignin (AIL) can be obtained. The pretreatment efficiency is comprehensively determined by the cellulose retention rate, hemicellulose removal rate, and lignin removal rate of the cellulose solid residue.

[0043] Chromatographic column: HPX-87H, 300mm x 7.8mm, mobile phase: 5mM sulfuric acid, column temperature: 50°C, detector: RI detector, flow rate: 0.6mL / min, injection volume: 20μL. See Table 1 for test results.

[0044] Table 1

[0045]

[0046] Based on Table 1, it can be seen that the removal rate of lignin is the highest under the oxalic acid system, so the organic acid is preferably oxalic acid.

[0047] The only difference between Preparation Examples 5-9 and Preparation Example 1 is the preheating temperature of the oil bath, and the samples are immersed in preheated oil baths at 110°C (Preparation Example 5), 120°C (Preparation Example 6), 130°C (Preparation Example 7), 140°C (Preparation Example 8), and 150°C (Preparation Example 9), respectively.

[0048] The cellulose retention rate, hemicellulose removal rate and lignin removal rate of the regenerated lignin prepared in 5-9 were tested by the above method. The test results are shown in Table 2.

[0049] Table 2

[0050]

[0051] Based on Table 2, it can be seen that as the temperature increases, the lignin removal rate also increases. When the temperature reaches 140°C, the lignin removal rate tends to the temperature. When the temperature is too high, other substances will be generated or decomposed. Therefore, the oil bath temperature is preferably 140°C.

[0052] The only difference between Preparation Examples 10-13 and Preparation Example 1 is the oil bath time. Specifically, they are immersed in a preheated oil bath at 140°C and reacted for 1h (Preparation Example 10), 2h (Preparation Example 11), 4h (Preparation Example 12), and 5h (Preparation Example 13), respectively.

[0053] The cellulose retention rate, hemicellulose removal rate and lignin removal rate of the regenerated lignin prepared by the above method were tested. The test results are shown in Table 3.

[0054] Table 3

[0055]

[0056] Based on Table 3, it can be seen that when the pretreatment temperature is 4 h, the lignin removal rate is the highest, so the oil bath treatment time is preferably 4 h.

[0057] The only difference between Preparation Examples 14-16 and Preparation Example 1 is the different molar ratios of polyethylene glycol to oxalic acid, which are 1:1 (Preparation Example 14), 3:1 (Preparation Example 15), and 4:1 (Preparation Example 16), respectively.

[0058] The cellulose retention rate, hemicellulose removal rate and lignin removal rate of the regenerated lignin prepared by the above method were tested. The test results are shown in Table 4.

[0059] Table 4

[0060]

[0061] Based on Table 4, it can be seen that the lignin removal rate is higher when the molar ratio is 1:1 and 2:1, but the system synthesized with a molar ratio of 1:1 is unstable, so the molar ratio of polyethylene glycol to oxalic acid is preferably 2:1.

[0062] The only difference between Preparation Examples 17-21 and Preparation Example 1 is the amount of deionized water added. Specifically, the amount of deionized water added is 2g (Preparation Example 17), 4g (Preparation Example 18), 6g (Preparation Example 19), 8g (Preparation Example 20), and 20g (Preparation Example 21), respectively.

[0063] The cellulose retention rate, hemicellulose removal rate and lignin removal rate of the regenerated lignin prepared by the above method were tested. The test results are shown in Table 5.

[0064] Table 5

[0065]

[0066]

[0067] Based on Table 5, it can be seen that when the water content is 20%, the lignin removal rate is the highest, so the addition amount of deionized water is preferably 20%.

[0068] Based on the above preparation examples 1-21, it can be seen that when the system is polyethylene glycol 200 and oxalic acid, the molar ratio is 2:1, the reaction temperature is 140°C, and the reaction time is 4h, the highest lignin removal rate is achieved, and the optimal preparation example is 18.

[0069] 0.2g tea stem powder, 0.2g regenerated cellulose obtained in Preparation Example 18 above, 6g deionized water and 1.5g 1-sulfonic acid butyl-3-methylimidazole hydrogen sulfate (purchased from Shanghai Chengjie) were mixed in a 35mL pressure-resistant glass container, heated in an oil bath at 180°C for 4h, and continuously stirred by magnetic force. After heating for 4h, the pressure bottle was placed in an ice water bath and quickly cooled to stop the reaction. The conversion rate of levulinic acid was calculated by the standard regression equation. The conversion rate of tea stem powder was 35.69%, the conversion rate of cellulose residue after DES pretreatment for 1h was 65.28%, the conversion rate of cellulose residue after DES pretreatment for 2h was 54.85%, the conversion rate of cellulose residue after DES pretreatment for 3h was 48.78%, the conversion rate of cellulose residue after DES pretreatment for 4h was 51.51%, and the conversion rate of cellulose residue after DES pretreatment for 5h was 59.86%. It was proved that the pretreatment of polyethylene glycol 200 and oxalic acid system can improve the accessibility of cellulose and significantly increase the conversion rate of levulinic acid. Figure 1 shown.

[0070] Examples of fluorescent lignin

[0071] Example 1

[0072] The preparation method of the fluorescent lignin involved in this application is:

[0073] The regenerated lignin obtained in the above Preparation Example 18 was dispersed in a H2O2 solution (10 wt%, 20 mL) to obtain a mixture; the mixture was reacted in an oil bath at 120°C for 6 hours to obtain fluorescent lignin.

[0074] Embodiment 2-5

[0075] The only difference from Example 1 is that the regenerated lignin obtained in the above-mentioned Preparation Example 1 (Example 2), the regenerated lignin obtained in Preparation Example 8 (Example 3), the regenerated lignin obtained in Preparation Example 12 (Example 4), and the regenerated lignin obtained in Preparation Example 14 (Example 5) are dispersed in H2O2 solution (10wt%, 20mL) to obtain a mixture; the mixture is reacted in an oil bath at 120°C for 6 hours to obtain fluorescent lignin.

[0076] Comparative example of fluorescent lignin

[0077] Comparative Example 1

[0078] The preparation method of groundwood lignin is as follows:

[0079] Place wheat straw in a planetary ball mill and mill at 220 rpm for 4 h. Take 5 g of ball-milled wheat straw powder into a 100 mL beaker, add 50 mL of dioxane aqueous solution (v:v=96:4), seal the beaker to avoid light, oscillate for 24 h, centrifuge, and retain the filtrate.

[0080] The above experiment was repeated, and the two filtrates were mixed and placed on a rotary evaporator, heated at 50°C to remove dioxane, and the filtrate was concentrated to 3 mL.

[0081] To 3 mL of concentrated filtrate, add 9 mL of 95% ethanol solution, shake thoroughly and filter by centrifugation, rotary evaporate the filtrate to 3 mL, repeat this step 3 times, add 30 mL of pH = 2 hydrochloric acid solution, filter by centrifugation to obtain a solid substance, dry it, and fully dissolve the above-obtained dry solid substance in a mixture of dichloroethane and ethanol (V:V = 2:1), and add it dropwise to 10 times the volume of anhydrous ether to obtain precipitated groundwood lignin.

[0082] Performance Testing

[0083] 1. The fluorescent lignin solution obtained in the above examples 1-5 was placed under 365nm ultraviolet light. The test results are shown in Figure 2 .

[0084] from Figure 2 It can be seen that the fluorescent lignin solution obtained in Example 1 of the present invention can emit obvious blue light under the excitation of a 365nm ultraviolet lamp, showing a strong photoluminescence performance.

[0085] The luminescence performance of the fluorescent lignin obtained by using Examples 2-5 and the fluorescent lignin of Example 1 under 365nm ultraviolet light is highly similar to that of the fluorescent lignin of Example 1. Figure 2 , so I won’t list them one by one.

[0086] 2. Fluorescent lignin obtained in Examples 1-5 Response of fluorescent lignin to metal ions:

[0087] ①Study the effect of 1 mg / mL fluorescent lignin and Ca 2+ 、Ni 2+ , Pb 2+ 、Zn 2+ Cr 3+ , Mn 2+ 、Cu2+、Al 3+ , Fe 3+ 、Co 2+ Responses of different metal ions.

[0088] Mix 3 ml of 1 mg / mL fluorescent lignin solution and 0.3 mL of 1600 μM different metal ion solutions. Add 0.3 mL of deionized water as the blank control group, and scan the fluorescence spectrum with a fluorescence spectrophotometer to observe the change in fluorescence intensity of lignin before and after adding metal ions. The test results are shown in Figure 3 .

[0089] from Figure 3 It can be seen that Fe 3+ The ions can specifically quench the fluorescence of the fluorescent lignin obtained in Example 1, while the effects of other metal ions on the fluorescence intensity of the fluorescent lignin are weak or even negligible.

[0090] The fluorescence intensity changes of the fluorescent lignin obtained by using Examples 2-5 and the fluorescent lignin of Example 1 before and after adding metal ions are highly similar to those of the attached Figure 3 , so I won’t list them one by one.

[0091] ②Different concentrations (0, 400, 600, 800, 1000 μM) of Fe 3+ Add to the fluorescent lignin solution, and scan the fluorescence spectrum with a fluorescence spectrophotometer to observe the addition of different Fe 3+ The fluorescence intensity of fluorescent lignin before and after ionization is changed. Thus, a standard curve is established to show the Fe 3+ The relationship between the concentration and fluorescence quenching efficiency, the test results are shown in Figure 6 .

[0092] ③ In order to evaluate the effect of fluorescent lignin on Fe 3+ The selectivity of various metal ions (Ca 2+ 、Ni 2 + , Pb 2+ 、Zn 2+ Cr 3+ , Mn 2+ , Cu 2+ 、Al 3+ and Co 2+) as a competitive metal ion. Together with iron ions, they were added to the fluorescent lignin solution. The fluorescence intensity changes of lignin before and after the addition of metal ions were observed by scanning the fluorescence spectrum with a fluorescence spectrophotometer. The test results are shown in Figure 4 .

[0093] like Figure 4 As shown, the presence of these other ions has an important effect on Fe 3+ The quenching response of Fe was not significantly affected. The new fluorescent sensor developed in this study has the potential to detect Fe in water environment. 3+ potential.

[0094] ④ In order to evaluate the effect of fluorescent lignin on Fe 3+ To study the selectivity of the concentration, different concentrations of iron ions (0, 400, 800, 1200, 1600, 2000 umol / L) were prepared and added to the fluorescent lignin solution. The fluorescence intensity change of lignin before and after the addition of iron ions was observed by scanning the fluorescence spectrum with a fluorescence spectrophotometer. The test results are shown in Figure 5 .

[0095] from Figure 4-Figure 6 It can be seen that Fe 3+ The concentration of lignin has an effect on the fluorescence intensity and the concentration is proportional to the fluorescence intensity, which proves that the prepared fluorescent lignin solution can selectively detect Fe 3+ Metal ions.

[0096] The fluorescence intensity changes of the fluorescent lignin obtained by using Examples 2-5 and the fluorescent lignin of Example 1 before and after adding metal ions are highly similar to those of the attached Figure 4 The fluorescence intensity change of lignin before and after the addition of iron ions is highly similar to that of Figure 5 , so I won’t list them one by one.

[0097] 3. The regenerated lignin obtained in Examples 1-5 of the present invention and Comparative Example 1 was dispersed in a H2O2 solution (10 wt%, 20 mL) to obtain a mixture; the mixture was reacted in an oil bath at 120°C for 6 hours, and the fluorescence intensity between the two was observed by scanning the fluorescence spectrum with a fluorescence spectrophotometer. The test results are shown in Figure 7 .

[0098] Depend on Figure 7 It can be seen that the fluorescence intensity of the regenerated lignin obtained in Example 1 of the present invention is significantly higher than that of the groundwood lignin in Comparative Example 1, which proves that the fluorescent lignin prepared by the regenerated lignin extracted under this system is better than the groundwood lignin extracted by the traditional method.

[0099] The fluorescence intensity of the fluorescent lignin obtained by using Examples 2-5 and the fluorescent lignin of Example 1 is highly similar to that of the attached Figure 7 , so I won’t list them one by one.

[0100] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing fluorescent lignin based on tea stems, characterized in that: The steps include: S1: Using tea stems as raw materials, pretreatment with a low eutectic solvent and reverse solvent treatment in sequence to obtain regenerated lignin; S2: The regenerated lignin obtained in step S1 is mixed with a solvent, and reacted in an oil bath at 120° C. for 4 to 6 hours to obtain fluorescent lignin.

2. The method for preparing fluorescent lignin based on tea stems according to claim 1, characterized in that: Step S1: The method for preparing regenerated lignin comprises the following steps: The tea stems, the low eutectic solvent and the deionized water are mixed evenly, and heated at 100-150° C. for 1-5 hours. After the reaction is completed, the mixture is washed and filtered until the washing liquid under filtration is transparent. After the solvent is removed from the washing liquid, a reverse solvent is added for precipitation, and the precipitate is freeze-dried to obtain regenerated lignin.

3. The method for preparing fluorescent lignin based on tea stems according to claim 1, characterized in that: In step S2, the solvent is selected from hydrogen peroxide.

4. The method for preparing fluorescent lignin based on tea stems according to claim 1, characterized in that: In step S2, the mass ratio of the regenerated lignin to the solvent is 5:

1.

5. A method for preparing fluorescent lignin based on tea stems according to claim 1 or 2, characterized in that: In step S1, the deep eutectic solvent comprises polyethylene glycol and an organic weak acid in a molar ratio of (1-4):1; The counter solvent is selected from deionized water or ethanol.

6. The method for preparing fluorescent lignin based on tea stems according to claim 5, characterized in that: The organic weak acid is selected from at least one of oxalic acid, formic acid, lactic acid and levulinic acid.

7. The method for preparing fluorescent lignin based on tea stems according to claim 2, characterized in that: In step S1, the mass ratio of the tea stems to the deep eutectic solvent is 1:20; The amount of deionized water used is no more than 50 wt % of the mass of the deep eutectic solvent.

8. A fluorescent lignin prepared by the method according to any one of claims 1 to 7.

9. The fluorescent lignin according to claim 8, characterized in that The fluorescent lignin is used to detect iron ions.

10. The fluorescent lignin according to claim 8, characterized in that The excitation wavelength of the fluorescent lignin is about 340 nm; and / or The emission wavelength is 450 nm when excited at 340 nm.

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