Lotus root residue cellulose nanofiber and preparation method thereof

Abstract

The application provides a lotus root residue cellulose nanofiber and a preparation method thereof, and comprises the following steps: (1) pretreatment of lotus root residue; (2) swelling of cellulose; (3) preparation of cellulose nanofiber. The application removes lignin and hemicellulose in the lotus root residue by using sodium hydroxide and sodium sulfite, avoids hydrogen peroxide bleaching and other methods for treating and purifying cellulose, improves safety, and increases the yield of cellulose. Then, the cellulose is directly swollen and oxidized in a weak alkali to produce cellulose nanofiber with high aspect ratio in a high-efficiency and low-toxicity manner. The swelling of cellulose in a dilute alkali solution can maximize the avoidance of the hydrolysis of glycosidic bonds in the cellulose, retains the integrity and stability of the cellulose structure, endows the cellulose with rich charges and sufficient reactive sites, reduces energy loss, and forms a uniform and extremely stable suspension in water. The application is a mild, low-toxicity and high-efficiency method for preparing lotus root residue cellulose nanofiber, and has important practical significance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of comprehensive utilization of agricultural product processing by-products and nanomaterials, and particularly relates to a preparation method of lotus root residue cellulose nanofiber and lotus root residue cellulose nanofiber prepared by the method. BACKGROUND

[0002] Natural biomass cellulose resources based on renewable plant resources are one of effective ways to solve the problem of increasingly exhausted fossil resources and environmental problems. Cellulose is a polysaccharide widely existing in nature, accounting for 30% to 50% of the total amount of cell wall material, derived from trees, cotton and other agricultural and sideline products. According to statistics, the annual renewable cellulose on earth is more than 2.16 x 10 11 As the most abundant natural polysaccharide on earth, cellulose has the advantages of non-toxicity, low price, biocompatibility and biodegradability, and is widely used in many fields including biomedical field, food, agriculture, water treatment and cosmetics. However, due to the difficulty of dissolving cellulose and the lack of active sites (such as amino or carboxyl) involved in the reaction, cellulose of nanoscale size obtained by proper treatment is expected to expand its application range. Among them, cellulose nanofiber with excellent mechanical properties and ultra-high aspect ratio, exposing more active sites, has attracted widespread attention, therefore, new cellulose sources and cellulose nanofiber preparation are of great significance.

[0003] Lotus root is the largest area of planting and the highest yield of aquatic vegetables in agricultural products in China, with a yield of nearly 7000 tons, and the industry is included in the top ten key agricultural industry chains. However, during the processing of lotus root, the lotus root residue by-products such as lotus root node and lotus root skin are faced with the problems of low utilization and benefit, serious loss and waste, short industry chain, high post-processing cost and other social problems. In addition, the random disposal of lotus root residue will cause environmental problems such as rotting and deterioration, therefore, the lotus root residue has multiple problems to be solved. The lotus root residue contains rich dietary fiber, which is an ideal cellulose source and can be used to prepare cellulose nanofiber to give it high added value.

[0004] The most common method of cellulose extraction is mainly acid method, alkali method, organic solvent method and the like, wherein the acid treatment method has higher recovery treatment cost, more complex process flow and longer reaction time, the organic solvent method has more extraction procedures, time-consuming, and it is difficult to recover and treat the organic solvent, and it is highly polluting. The separation of cellulose in lignocellulose resources is usually treated and purified by sodium hydroxide, acidic sodium chlorite and hydrogen peroxide bleaching. There are many problems in using hydrogen peroxide, and the World Health Organization lists hydrogen peroxide as a class 3 carcinogen, which is low in safety. Common preparation of nanocellulose is to use a high-pressure homogenizer to shake cellulose in water, or a ball mill to produce cellulose nanofibers, which has high energy consumption and small aspect ratio. In addition, enzyme method is used to produce nanocellulose mildly, but there are problems such as harsh production environment and low yield.

[0005] In summary, it is of great significance to develop a mild, low-toxicity and high-efficiency lotus root residue cellulose nanofiber method. SUMMARY

[0006] In view of the deficiencies of the prior art, the present application first removes lignin and hemicellulose in lotus root residue by using sodium hydroxide and sodium sulfite, and then directly swells and oxidizes in weak alkali to produce high-aspect-ratio cellulose nanofibers with high efficiency and low toxicity, which endows cellulose with rich charge and sufficient reactive active sites, reduces energy loss, and can form a uniform and stable suspension in water for long-term storage.

[0007] In order to achieve the above purpose, the present application provides a preparation method of lotus root residue cellulose nanofiber, comprising the following steps:

[0008] (1) Pretreatment of lotus root residue

[0009] Fresh lotus root is selected, the waste including lotus root skin and lotus root node is crushed to obtain lotus root residue material, pure water is added, amylase is added, heating treatment is performed to remove starch in raw materials, then the enzyme is inactivated after temperature rising, and the upper clear liquid is removed to obtain lotus root residue after preliminary treatment, 1-10wt% sodium hydroxide solution and 1-10wt% sodium sulfite are added to the lotus root residue after preliminary treatment, and stirring reaction is performed at 40-100℃, then filtration and washing are performed until neutralization to obtain lotus root residue cellulose;

[0010] (2) Swelling of cellulose

[0011] The lotus root residue cellulose prepared in step (1) is dispersed in dilute sodium hydroxide solution, and stirring is performed to fully swell the cellulose, so as to obtain a cellulose swelling solution;

[0012] (3) Preparation of cellulose nanofiber

[0013] The 2,2,6,6-tetramethylpiperidine oxide and sodium bromide are dissolved in the cellulose swelling solution prepared in step (2), and the aqueous sodium hypochlorite solution is slowly added dropwise while maintaining the pH at 10-10.5; after the dropwise addition is completed, dilute sodium hydroxide solution is continuously added dropwise until the pH no longer changes, and the mixture is stirred overnight; the mixture is then dispersed, dialyzed against water, and centrifuged to obtain the supernatant, which is the cellulose nanofiber suspension.

[0014] According to a preferred embodiment of the present application, in step (1), pure water is added at a material-to-liquid ratio of 1:8-12, and 0.1-0.3 wt% amylase is added; the concentration of the sodium hydroxide solution is 2-8 wt%.

[0015] According to a preferred embodiment of the present application, in step (1), the heating treatment for removing the raw starch includes water bath treatment at 50-60℃ for 2-6h.

[0016] The heating treatment for removing the raw starch further includes iodine solution testing for non-blue coloration.

[0017] According to a preferred embodiment of the present application, in step (1), the temperature-increasing inactivation of the enzyme includes water bath treatment at 85-95℃ for 1-3h.

[0018] According to a preferred embodiment of the present application, in step (2), the concentration of the dilute sodium hydroxide solution is 0.1-5 wt%, and the concentration of the lotus root residue cellulose dispersed in the dilute sodium hydroxide solution is 0.2-1 wt%; the stirring reaction time is 6-48h.

[0019] According to a preferred embodiment of the present application, in step (3), the mass ratio of 2,2,6,6-tetramethylpiperidine oxide to cellulose is 80-120:1, the mass ratio of sodium bromide to cellulose is 8-12:1, and the mass ratio of the aqueous sodium hypochlorite solution to cellulose is 1:2-10.

[0020] According to a preferred embodiment of the present application, in step (3), the dispersion time is 1-10min.

[0021] According to a preferred embodiment of the present application, in step (3), the dialysis bag used for dialysis against water has a molecular weight cut-off of 8000-14000.

[0022] The present application also provides cellulose nanofibers prepared by the above preparation method.

[0023] According to a preferred embodiment of the present application, the cellulose nanofibers have a length in the range of 1000-10000nm and an aspect ratio of 50-500. The cellulose nanofibers prepared in the examples of the present application have an average length of about 1500nm and an aspect ratio greater than 100.

[0024] The present application can solve the problem that the cellulose in lignocellulosic resources is treated and purified by sodium hydroxide, acidic sodium chlorite and hydrogen peroxide bleaching, etc., and the use of hydrogen peroxide results in low safety, etc. The lignin and hemicellulose in lotus root residues are removed by sodium hydroxide and sodium sulfite, avoiding the treatment and purification of cellulose by hydrogen peroxide bleaching, etc., to improve safety and increase the yield of cellulose.

[0025] The present application can solve the problem that the traditional nanocellulose is prepared by strong mechanical method and enzyme method, which has the problems of high energy consumption, small aspect ratio, low yield and harsh production environment, etc. The present application directly swells and oxidizes in weak alkali to produce high aspect ratio cellulose nanofiber with high efficiency and low toxicity. Swelling cellulose in dilute alkali can maximize the hydrolysis of glycosidic bond in cellulose, retain the integrity and stability of cellulose structure, and endow cellulose with rich charge and sufficient reactive active sites, which not only reduces energy consumption loss, but also forms a uniform and stable suspension in water. In summary, the present application is a mild, low toxicity and high efficiency method for lotus root residue cellulose nanofiber, which has important practical significance.

[0026] Other features and advantages of the present application will be described in detail in the following specific embodiments. BRIEF DESCRIPTION OF DRAWINGS

[0027] The above and other objects, features and advantages of the present application will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:

[0028] Figure 1 Lotus waste and lotus root residue purified cellulose materials are shown.

[0029] Figure 2 a-2c are transmission electron microscope images, diameter statistical graphs and length statistical graphs of lotus root residue cellulose nanofiber prepared by the present application.

[0030] Figure 3 The lotus root residue cellulose nanofiber solution prepared by the present application is shown. DETAILED DESCRIPTION

[0031] The preferred embodiments of the present application will be described in more detail below. Although the preferred embodiments of the present application are described below, it should be understood that the present application can be implemented in various forms and should not be limited by the embodiments described herein.

[0032] Example 1

[0033] (1) Pretreatment of lotus root residues

[0034] Fresh lotus root, lotus root skin and lotus root node waste were crushed and added with pure water at a ratio of 1:10. Then 0.2% amylase was added and the mixture was placed in a water bath at 55°C for 4 hours to remove starch. After the blue color disappeared, the mixture was heated to 90°C for 2 hours to inactivate the enzyme. The supernatant was removed and the lotus root residue was obtained. The lotus root residue was added with 5wt% sodium hydroxide solution (2.5wt% concentration, 2wt% sodium sulfite) and stirred at 80°C for 3 hours. After filtration and washing, the lotus root residue was obtained.

[0035] (2) Swelling of cellulose

[0036] The lotus root residue cellulose was dispersed in 0.5wt% sodium hydroxide solution to form a 0.5wt% cellulose dispersion solution. The solution was stirred for 12 hours to fully swell the cellulose.

[0037] (3) Preparation of high aspect ratio cellulose nanofiber

[0038] 2,2,6,6-tetramethylpiperidine oxide (100:1 by mass ratio with cellulose) and sodium bromide (10:1 by mass ratio with cellulose) were dissolved in the cellulose swelling solution. Sodium hypochlorite aqueous solution (1:5 by mass ratio with cellulose) was slowly added to maintain the pH at 10-10.5. After the addition was completed, dilute sodium hydroxide solution was continuously added until the pH no longer changed. The mixture was stirred overnight, dispersed for 2 minutes, and then placed in a dialysis bag with a molecular weight cutoff of 10000. The dialysis was performed against water, and the supernatant cellulose fiber suspension was obtained by centrifugation.

[0039] The lotus root waste and lotus root residue purified cellulose material is shown in Figure 1 The lotus root waste contains abundant starch, cellulose, hemicellulose and lignin. Through the action of amylase, sodium hydroxide and sodium sulfite under heating conditions, purified cellulose can be efficiently separated. Since the cellulose molecular chain structure is stable and low in toxicity, the yield of cellulose is improved.

[0040] Figure 2 Figure a shows the transmission electron microscope image of the lotus root residue cellulose nanofiber prepared by the above method. The lotus root residue cellulose is uniformly dispersed in water in the form of nanofiber. The diameter histogram is shown in Figure 2 Figure b, with an average diameter of about 15nm, indicating that the method can prepare uniform nanoscale cellulose nanofiber. The length histogram is shown in Figure 2 Figure c, the length of the lotus root residue cellulose nanofiber is uniformly about 1500nm, and the high aspect ratio is more conducive to the application of material mechanical properties enhancement, etc.

[0041] Figure 3The actual figure of the lotus root residue cellulose nanofiber solution prepared by the application is shown, which shows obvious Tyndall effect under the concentration condition of 0.5wt%, indicating that the lotus root residue cellulose nanofiber prepared by the application is uniformly dispersed and stable.

[0042] The above has described various embodiments of the application, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes are obvious to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A method for preparing lotus root residue cellulose nanofiber, comprising the following steps: (1) pretreatment of lotus root residue fresh lotus roots are selected, and waste including lotus root skin and lotus root joints are crushed to obtain lotus root residue materials, pure water is added, amylase is added, and the raw material starch is removed by heating treatment, then the enzyme is inactivated by heating, and the supernatant is removed to obtain the lotus root residue after preliminary treatment, 1-10 wt% sodium hydroxide solution and 1-10 wt% sodium sulfite are added to the lotus root residue after preliminary treatment, and the mixture is stirred at 40-100 ℃ for reaction, filtration and washing until neutral to obtain lotus root residue cellulose; (2) swelling of cellulose the lotus root residue cellulose prepared in step (1) is dispersed in a dilute sodium hydroxide solution, and the cellulose is fully swollen by stirring to obtain a cellulose swelling solution; the concentration of the dilute sodium hydroxide solution is 0.1-0.5 wt%; (3) preparation of cellulose nanofiber 2,2,6,6-tetramethylpiperidine oxide, sodium bromide are dissolved in the cellulose swelling solution prepared in step (2), and sodium hypochlorite aqueous solution is slowly added dropwise, and the pH is maintained at 10-10.5, then dilute sodium hydroxide solution is continuously added dropwise until the pH no longer changes, and the mixture is stirred overnight, dispersed, dialyzed against water, and centrifuged to obtain the supernatant, which is a suspension of cellulose nanofiber.

2. The method for preparing lotus root residue cellulose nanofibers according to claim 1, wherein, In step (1), pure water is added at a solid-liquid ratio of 1:8-12, and 0.1-0.3 wt% amylase is added. The concentration of the sodium hydroxide solution is 2-8 wt%.

3. The method for preparing lotus root residue cellulose nanofibers according to claim 1, wherein, In step (1), the conditions for removing the raw material starch by heating treatment include 50-60 ℃ water bath for 2-6 h. After removing the raw material starch by heating treatment, iodine solution is used for testing whether there is a blue change.

4. The method for preparing lotus root residue cellulose nanofibers according to claim 1, wherein, In step (1), the conditions for inactivating the enzyme by heating include 85-95 ℃ water bath for 1-3 h.

5. The method for preparing lotus root residue cellulose nanofibers according to claim 1, wherein, In step (2), the concentration of the lotus root residue cellulose dispersed in the dilute sodium hydroxide solution is 0.2-1 wt%, and the stirring reaction time is 6-48 h.

6. The method for preparing lotus root residue cellulose nanofibers according to claim 1, wherein, In step (3), the mass ratio of 2,2,6,6-tetramethylpiperidine oxide to cellulose is 80-120:1, the mass ratio of sodium bromide to cellulose is 8-12:1, and the mass ratio of sodium hypochlorite aqueous solution to cellulose is 1:2-10.

7. The method for preparing lotus root residue cellulose nanofibers according to claim 1, wherein, In step (3), the dispersion time is 1-10 min.

8. The method for preparing lotus root residue cellulose nanofibers according to claim 1, wherein, In step (3), the molecular weight cut-off of the dialysis bag used for dialysis against water is 8000-14000. 9.The cellulose nanofiber prepared by the method of any one of claims 1-8.

10. The cellulose nanofiber according to claim 9, wherein, The length of the cellulose nanofiber ranges from 1000 nm to 10000 nm, and the aspect ratio ranges from 50 to 500.

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