CN115012052A - A method for preparing micro-nano cellulose fibers from corn stalks - Google Patents

Abstract

A method for preparing micro-nano cellulose fibers by taking corn straws as a raw material belongs to the technical field of green biomass refining, and adopts an in-situ induction multiphase Fenton advanced oxidation technology to prepare the micro-nano cellulose fibers. The preparation method comprises the steps of adding NaOH into water, adjusting the pH value to 10.5-11.5, uniformly stirring, and then mixing NaOH and H according to the mass ratio ₂ O, adding corn straws in a ratio of 1:8: 1; heating at 90 deg.C for 3h, and performing solid-liquid separation; soaking the solid in Fe with pH of 2.5-4.0 ²⁺ Adding into the solution for 2H, filtering, and adding the solid into 0.4% H ₂ O ₂ Stirring the solution at room temperature for 24 hours to complete oxidation; and (3) performing solid-liquid separation on the oxidized product, washing the solid with distilled water until the pH value is 5-7, dispersing the solid in water to prepare 2 wt% of suspended matters, and performing ultrasonic treatment at the frequency of 20KHz for 10min to obtain the micro-nano cellulose fiber product. According to the invention, final production of the micro-nano cellulose fiber can be completed only by short-time ultrasonic treatment, so that the energy consumption of equipment is greatly reduced.

Description

A method for preparing micro-nano cellulose fibers from corn stalks

technical field

The invention belongs to the technical field of green biomass refining, and in particular relates to a method for preparing micro-nano cellulose fibers by using corn stover as a raw material.

Background technique

The corn stalk accounts for more than 50% of the biological yield of the whole corn plant, and the cellulose content in the corn stalk can reach 35.0% to 39.6%. It is second only to barley straw (36.0-43.0%) and comparable to wheat straw (32.0-40.0%) and sorghum straw (35.0-39.0%).

Due to its large specific surface area, good biocompatibility and green degradability, micro- and nanocellulose fibers have broad application prospects in the fields of material science, nanotechnology, biomedicine and food science.

According to the previous research results, the preparation methods of micro-nano cellulose fibers mainly include mechanical methods, biological enzymes and chemical methods. Due to the high energy consumption, the mechanical method is difficult to promote industrialization. Although the enzymatic hydrolysis technology can greatly reduce the energy consumption during the production of micro/nanocellulose fibers, this method cannot effectively modify the fiber surface structure. In contrast, chemical methods can effectively introduce various new functional groups on the fiber surface, resulting in novel micro- and nanocellulose-based materials with novel properties. Therefore, this method has better development prospects.

At present, there are no less than ten chemical methods for preparing micro-nanocellulose fibers, such as 2,2,6,6-tetramethylpiperidine radical-1-oxyl radical (TEMPO) oxidation, cationization method, phosphorylation method, methylolation method, sulfoethylation method, etc. Among them, the TEMPO oxidation method has received extensive attention.

However, these reported methods have complex reaction processes and require environmentally unfriendly co-catalysts and solvents, such as hypochlorite and dichloromethane. This makes these reported methods totally inconsistent with the principles of all-green biomass refining.

Due to the advantages of low cost and no additional environmental pollutants, the Fenton reaction system has received extensive attention in the field of biomass refining. For example, the Fenton reaction system was used to simulate the metabolic process of white rot fungi to generate hydroxyl radicals to oxidize lignin in lignocellulose. In another example, the Fenton reaction system was used to de-pretreat lignin to produce phenolic oils.

The biggest disadvantage of introducing the traditional Fenton reaction system into the field of biomass refining mainly comes from the fact that the hydroxyl radical active species cannot effectively act on the organic macromolecules of the target biomass, and most of them are inactivated in the water body, resulting in the Fenton reaction system to treat the biomass system. invalid during the process. The development of in situ induced heterogeneous Fenton advanced oxidation technology is one of the effective methods to overcome the above shortcomings.

Patent CN112227102B discloses mixing cane pith, sodium hydroxide and water, cooking in an oxygen atmosphere, and ultrasonic gradient centrifugation to obtain micro- and nano-cellulose fibers of different sizes; Patent CN106279444B discloses a nano-cellulose fibril The preparation method of the emulsion is as follows: adding natural cellulose fibers to a NaOH solution, stirring at 60-98 °C and synchronous ultrasonic vibration for 0.2-2 hours, and then immersing in a mixed solution containing NaOH and NaClO, at a temperature of 60-98 °C. After stirring and synchronous ultrasonic vibration for 1 to 20 hours, a mixed solution containing nanocellulose fibrils is obtained; the obtained mixed solution containing nanocellulose fibrils is centrifuged, and then dialyzed for 0.5 to 3 days to obtain nanocellulose fibrils. fiber lotion. Patent CN101851801A discloses the preparation of nanocellulose fibers by extracting biomass fibers with benzene alcohol solution, treating with acidified sodium chlorite, lye gradient treatment, ultrasonic cell pulverizer treatment, high-pressure homogenization treatment and drying. There is also some work on the preparation of cellulose micro-nanofibers by using a two-chamber high-pressure homogenizer after the oxidation of softwood bleached kraft pulp with hydrogen peroxide. In summary, the micro-nanocellulose fibers prepared by these reported methods have the disadvantages of uncontrollable size, unfriendly solvent and co-catalyst environment, and high energy consumption for high-pressure homogenization.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is: to provide a method for preparing micro-nano cellulose fibers with corn stalks as raw materials, overcoming the high cost of the micro-nano cellulose fibers manufacturing process, the unfriendly environment of the solvent used and the excessive dependence on wood or wood bleaching The shortcomings of kraft pulp as raw material, using in-situ induced heterogeneous Fenton advanced oxidation technology, using corn stover with abundant sources, low price and low utilization rate as raw material, after pretreatment of corn stover to obtain a product with high cellulose content, further Cellulose fibers with nanoscale scale were obtained by in situ induced heterogeneous Fenton oxidation and short-time sonication.

A method for preparing micro-nano cellulose fibers using corn stalks as a raw material is characterized by comprising the following steps, and the following steps are performed in sequence,

Step 1. Take the NaOH aqueous solution with a pH of 10.5-11.5, put the corn stalks in the mass ratio NaOH:H ₂ O:corn stalk=1:8:1; heat at 90°C for 3h, filter and separate solid-liquid , to obtain solid fibers;

Step 2: Immerse the solid fibers obtained in the first step in Fe ²⁺ solution with a pH of 2.5 to 4 to complete the diffusion and pre-adsorption of Fe ²⁺ between fibers, and remove excess Fe through decompression, filtration and washing. ²⁺ ;

Step 3: Place the solid fiber after adsorption of Fe ²⁺ in the second step in a 0.4% H ₂ O ₂ solution, and stir at room temperature for 24 hours for oxidation; the oxidized product is separated from solid and liquid, and the liquid after separation can supplement Fe ²⁺ After repeated use, the separated solid was washed to pH=5-7, dispersed in water to form a 2wt% suspension, and ultrasonically treated at a frequency of 20KHz for 10min to obtain micro-nano cellulose fibers with a moisture content of 99%.

The diameter of the micro-nano cellulose fibers obtained in the third step is 50-70 nm, the carboxyl group content is 0.59±0.04 mmol/g, and the degree of polymerization is 649±6.

The corn stalks used in the first step are grass-ball-shaped straws made by a grass-ball press.

The Fe ²⁺ used in the second step is an industrial-grade FeSO ₄ ˙7H ₂ O solid.

Through the above-mentioned design scheme, the present invention can bring the following beneficial effects: a method for preparing micro-nano cellulose fibers using corn stalks as raw materials, in the mild environment of normal temperature and pressure, using the in-situ induced heterogeneous Fenton oxidation method to produce The strong oxidizing hydroxyl radicals oxidize the alkali-pretreated corn stalks, and only need a short-time ultrasonic treatment to complete the final production of micro-nano cellulose fibers, which greatly reduces the energy consumption of the equipment.

Further, the filtrate of the solid-liquid separation in step 1 of the present invention can be reused at least three times after replenishing water, and the black liquor is rich in lignin, which can be used for secondary use;

Both step 2 and step trioxidation water of the present invention can be recycled after supplementing Fe ²⁺ , and will not produce discharge and waste of water resources;

In the present invention, all processes from Fe ²⁺ adsorption to finally obtaining micro-nano cellulose fiber products will not produce waste water discharge and water resources waste;

The micro-nano cellulose fibers obtained by the invention have an average length of ≤2 μm, a diameter of 50-70 nm, a distinct nano-scale structure, a carboxyl group content of 0.59±0.04 mmol/g, a DP of 649±6, and a moisture content of 99%. A translucent film can be obtained after further vacuum filtration and hot pressing.

The present invention will be further described below in conjunction with specific embodiments:

Detailed ways

Example 1: Take 20g of NaOH solid and put it into a 2L stainless steel reaction kettle, pour 1.6L of water into it and stir to completely dissolve the NaOH solid, 200g of corn stalks (40 mesh powder) are poured into the reaction kettle and stirred at 90°C for 3 hours. Mix 20g FeSO ₄ ˙ 7H ₂ O with 800ml of water in advance and dissolve it completely, adjust the pH with dilute sulfuric acid in the range of 2.5 to 4, weigh 100g dry weight of lye pretreated straw, put it into the solution, and stir at room temperature for 1 hour After completing the adsorption process of Fe ²⁺ on the fiber, vacuum filtration to remove the excess Fe ²⁺ suspended in the liquid, and washing to remove the excess Fe ²⁺ adhering to the surface of the fiber. After the pre-adsorption was completed, 20ml of H ₂ O ₂ with a concentration of 30% was dispersed in 780ml of water, the solution was mixed with the pre-adsorbed fibers in a 2L stainless steel reactor, and stirred at room temperature for 3h to make it fully oxidized. The oxidized solid product was mixed with water to 2wt%, 800ml of the suspension was taken in a 1L plastic container, and ultrasonically treated at a frequency of 20KHz for 10min to finally obtain a micro-nano cellulose fiber product.

Example 2: Take 200g of NaOH solid and put it into a 50L glass-jacketed reactor, take 16L of water and pour it in and stir to dissolve the NaOH solid completely, and pour 2kg of corn stalk (grass ball-shaped solid) into the reactor and stir it for 3 hours at 90°C , solid-liquid separation after refining with a refiner. Mix 200g FeSO ₄ ˙ 7H ₂ O with 10 water in advance, and dissolve it completely, adjust the pH with dilute sulfuric acid in the range of 2.5 to 4, put the straw pretreated with lye into the solution, and stir at room temperature for 1 hour to complete the effect of fiber on Fe In the adsorption process of ²⁺ , the excess Fe ²⁺ suspended in the liquid is removed by vacuum filtration, and the excess Fe ²⁺ adhering to the surface of the fiber is removed by washing. After the pre-adsorption is completed, 200 ml of H ₂ O ₂ with a concentration of 30% is dispersed in 10 ml of water, and the solution is mixed with the pre-adsorbed fibers in a 50 L glass-jacketed reactor, and stirred at room temperature for 3 h to make it fully oxidized. The oxidized solid product is mixed with water to 2wt%, and ultrasonically treated at a frequency of 20KHz to obtain a micro-nano cellulose fiber product.

Example 3: get 200g of NaOH solid and put it into a 50L glass jacketed reactor, get the black liquor of the lye pretreatment filtered out in example 2, add water and make up to 16L, the pH at this time is 10.5～11 and drops slightly, 2kg corn The straw (grass ball-like solid) was poured into the reaction kettle and stirred at 90°C for 3 hours, and the solid-liquid separation was performed after simple refining with a refiner. In Example 2, an appropriate amount of FeSO ₄ ˙7H ₂ O was added to the pre-adsorption water to make up the Fe ²⁺ concentration, and it was completely dissolved. The pH was adjusted with dilute sulfuric acid in the range of 2.5 to 4, and the lye pretreated straw was put into the solution. The adsorption process of Fe ²⁺ on the fiber was completed by stirring at room temperature for 1 h, the excess Fe ²⁺ suspended in the liquid was removed by vacuum filtration, and the excess Fe ²⁺ adhering to the surface of the fiber was removed by washing. After the pre-adsorption is completed, the oxidation water is reused after supplementing the H ₂ O ₂ concentration. In a 50L glass-jacketed reactor, the solution is mixed with the pre-adsorbed fibers, and stirred at room temperature for 3 hours to fully oxidize. The oxidized solid product is mixed with water to 2wt%, and ultrasonically treated at a frequency of 20KHz to obtain a micro-nano cellulose fiber product.

Table 1. Composition and degree of polymerization of raw materials and products under different treatments

Table 1 explains:

(1) Determination of cellulose and lignin content, using the current national standards of GB/T 2677.10-1995 and GB/T 35818-2018;

(2) The carboxyl group content adopts the current national standard of GB/T 10338-2008, the degree of polymerization DP is determined by the intrinsic viscosity method combined with the current national standard of GB 5888-1986, and the samples are all dried before measurement;

(3) All data are averaged after three or more measurements to avoid measurement errors caused by one measurement and reduce data errors.

Claims (4)

1. A method for preparing micro-nano cellulose fibers by taking corn straws as a raw material is characterized by comprising the following steps: comprises the following steps which are sequentially carried out,

step one, taking a NaOH aqueous solution with the PH of 10.5-11.5, and mixing NaOH and H according to the mass ratio ₂ O, putting corn stalks into the corn stalk powder at a ratio of 1:8: 1; heating at 90 deg.C for 3 hr, filtering, and performing solid-liquid separation to obtain solid fiber;

step two, soaking the solid fiber obtained in the step one in Fe with the pH value of 2.5-4 ²⁺ In solution, Fe is completed ²⁺ Diffusion and pre-adsorption among fibers, decompression, filtration and washing to remove excessive Fe ²⁺ ；

Step three, adsorbing Fe in the step two ²⁺ The solid fiber is placed in 0.4% of H ₂ O ₂ Stirring the solution at room temperature for 24 hours for oxidation; solid-liquid separation is carried out on the oxidized product, and the liquid after separation can complement Fe ²⁺ Repeatedly using the mixture for multiple times, washing the separated solid until the pH value is 5-7, dispersing the solid in water to prepare 2 wt% suspended substance, and performing ultrasonic treatment at the frequency of 20KHzAnd (5) obtaining the micro-nano cellulose fiber with the water content of 99% after 10 min.

2. The method for preparing the micro-nano cellulose fibers by using the corn stalks as the raw materials according to claim 1, which is characterized by comprising the following steps of: the diameter of the micro-nano cellulose fiber obtained in the third step is 50-70 nm, the carboxyl content is 0.59 +/-0.04 mmol/g, and the polymerization degree is 649 +/-6.

3. The method for preparing the micro-nano cellulose fiber by using the corn stalks as the raw materials according to claim 1, which is characterized in that: the corn straw adopted in the first step is the straw ball-shaped straw manufactured by a straw ball press.

4. The method for preparing the micro-nano cellulose fiber by using the corn stalks as the raw materials according to claim 1, which is characterized in that: fe adopted in the second step ²⁺ Is industrial grade FeSO ₄ ˙7H ₂ And (4) O solid.