RU2850430C1 - Method for obtaining powdered cellulose material from cellulose-containing raw materials (variants) - Google Patents

Abstract

FIELD: cellulose material.

SUBSTANCE: invention relates to a method for obtaining powdered cellulose material, namely powdered cellulose (PC) and microcrystalline cellulose (MCC) from cellulose-containing raw materials (CCM): wheat straw, hemp straw, flax shives, barley straw, miscanthus straw, wood. The method for obtaining powdered cellulose material from cellulose-containing raw materials includes delignification treatment of lignocellulosic raw materials with an aqueous solution containing hydrogen peroxide, acetic acid and a catalyst, at a mixture boiling temperature of 98-100°C, at atmospheric pressure for 2.5-3.5 hours with intensive stirring, followed by separation of the cellulose product.

EFFECT: separation of the catalyst from the cellulose product, improvement of the quality of the cellulose product due to the absence of catalyst impurities in it, simplification of the method of obtaining powdered cellulose material due to the repeated use of the catalyst, as well as a reduction in catalyst costs due to the lower cost of MgSO₄ compared to TiO₂.

6 cl, 15 dwg, 9 tbl, 36 ex

Description

The invention relates to a method for producing powdered cellulose material, namely powdered cellulose (PC) and microcrystalline cellulose (MCC) from cellulose-containing raw materials (CCR): wheat straw, hemp straw, flax shives, barley straw, miscanthus straw, wood.

Structure and application of powdered cellulose material

Powdered cellulose materials (PCM) include powdered cellulose (PC), microcrystalline cellulose (MCC), nanocrystalline cellulose (NCC), microfibrillated cellulose (MFC), and nanofibrillated cellulose (NFC). These types of powdered cellulose materials differ in the degree and nature of destruction of the cellulose they contain, as well as in the content of non-cellulose components [3-7].

Powdered cellulose has a low degree of crystallinity (DC) and contains significant amounts of hemicelluloses and lignin. MCC and NCC have a high degree of crystallinity and the highest chemical purity, but differ in the size of the crystallites themselves. Fibrillated cellulose retains its fibrous morphology.

According to the definition of Commission Regulation (EU) No. 231/2012, powdered cellulose is purified, mechanically disintegrated cellulose obtained by processing α-cellulose pulp from fibrous plant material. The degree of polymerization of PC is predominantly ≥ 1000, and the molecular weight is > 160,000 [1]. PC is a white, odorless powder that is insoluble in water, ethanol, ether, and dilute mineral acids and slightly soluble in sodium hydroxide solution. Powdered cellulose is capable of swelling in water, dilute acids, and most solvents. Alkaline solutions lead to swelling and dissolution of the hemicellulose present in it.

The difference between PC and MCC is the presence of particles with a wide range of sizes, a wider distribution of molecular weight, an increased degree of polymerization and a reduced degree of crystallinity [2].

Microcrystalline cellulose is a product of the chemical degradation of cellulose, with a high degree of purity and crystallinity. MCC is a powdered cellulose material consisting of particles that are aggregates of cellulose microcrystallites containing hundreds and thousands of macromolecules. These aggregates have a high length-to-thickness ratio [9-10].

The degree of polymerization of MCC ranges from 30 to 350 and depends on the type of original cellulose, for example, cotton 200 - 300, wood 120 - 180, viscose fiber cellulose 30 - 50. Depending on the original cellulose material and the production conditions, the average particle sizes of different types of MCC range from 1 to 400 µm [8].

There are also morphological differences between the microcrystalline forms of cellulose from different origins. The microcrystallites of cotton MCC are somewhat longer and thicker than those of wood MCC, and the latter are larger than those of cellulose from annual plants.

Due to its highly developed hydrophilic surface containing active hydroxyl groups, microcrystalline cellulose is capable of forming thixotropic gel-like dispersions in water, which distinguishes MCC from fibrillated and powdered cellulose with a low degree of crystallinity [10].

MCC is used to manufacture adsorbent materials [11, 12]. Its high thermal conductivity, thermal stability, light weight, and rigidity allow it to be used as a reinforcing agent in polymer composites, plastics, ceramics, porcelain, and filter materials. MCC also serves as an additive in biopolymers of various compositions to impart biodegradability [13-15]. MCC is not absorbed by the human and animal body, which is why it is used in the food, pharmaceutical, and biomedicine industries [16-18]. Here, MCC is used in powder form, for example, as a binder and filler in food products and medicinal tablets. In colloidal form, MCC is used as a suspension stabilizer, viscosity regulator, and emulsifier in various pastes and creams.

Existing methods for obtaining powdered cellulose material (powdered and microcrystalline cellulose)

Powdered cellulose materials are typically produced through mechanical, chemical, or combined processing of fibrous cellulose-containing semi-finished products. Chemical or mechanical degradation destroys the fibrous structure of the semi-finished products, producing a fine powder containing destructured cellulose fibers and non-cellulose components.

MCC is typically produced by hydrolysis of purified alpha-cellulose or multi-stage processing of lignocellulosic raw materials. The technology for producing MCC from lignocellulosic wood and non-wood raw materials includes the following main stages: raw material preparation (washing, drying, grinding, screening), delignification, bleaching, and hydrolysis [19, 20].

The sulphate pulping process is primarily used for delignification of wood raw materials, while soda pulping is used for delignification of non-wood raw materials (straw, etc.). Bleaching of sulphate and soda pulps is carried out using either chlorine-containing compounds in an acidic medium, such as calcium hypochlorite (CaClO ₂ ), sodium hypochlorite (NaClO _{2 ), or sodium chlorite (NaClO 2} ), or hydrogen peroxide (H ₂ O ₂ ) in an alkaline medium.

To increase the efficiency of the CSS processing processes in the MCC, various CSS treatments are used: chemical (acids, alkalis, ionic liquids, TEMPO oxidation), physical (mechanical, ultrasonic, steam explosion, gamma irradiation, electron beam and microwave radiation), biological, each of which has its own advantages and disadvantages.

Pre-grinding of the CSS reduces the fiber size and increases their surface area in contact with water, which improves the efficiency of subsequent processing. The fibers also acquire the ability to swell in water, allowing for the removal of water-soluble impurities. If necessary, the water-washed fibers are extracted with benzene/ethanol and toluene/ethanol mixtures to remove waxes, pigments, and soluble sugars. The chopped and extracted fibers are then dried, re-grinded, and sieved to remove surface contaminants and soluble impurities.

Pre-treatment of crushed CSS involves the use of acids and bases to remove non-cellulose components. A widely used method is acid treatment of CSS with sulfuric or hydrochloric acid, and less commonly, nitric and phosphoric acid, under mild conditions (low temperature and acid concentration <4% by weight). Mild acid treatment effectively removes hemicelluloses while preserving the cellulose structure.

In addition to acid treatment, alkaline treatment of lignocellulosic raw materials with solutions of sodium, potassium, calcium, ammonium, and other hydroxides is used. These solutions effectively remove hemicellulose and partially depolymerize lignin. Alkaline treatment promotes the rupture of intermolecular ester bonds that crosslink hemicellulose and lignin, followed by their removal. This also removes residual wax, pectin, and natural fats. However, alkaline treatment at alkali concentrations exceeding 5% by weight disrupts the crystalline structure of cellulose, reducing its degree of crystallinity [21].

Despite their simplicity and low energy consumption, acid and alkaline methods of pre-treatment of CSS have a harmful impact on the environment due to the formation of large quantities of wastewater and lead to corrosion of equipment.

After pre-treatment of the CSS, lignin remains in the pulp product, which must be removed to ensure high-quality MCC. Traditional delignification methods, such as sulfite, sulfate, and neutral sulfite, use environmentally hazardous sulfur- and chlorine-containing delignifying agents.

Oxidative and organosolvent delignification methods for producing MCC are highly environmentally friendly. Oxidative delignification processes use hydrogen peroxide, ozone, oxygen, or air to depolymerize lignin and some hemicelluloses, forming carboxylic acids while preserving the cellulose.

To produce high-quality MCC, residual lignin must be removed from the cellulose product. Bleaching processes are used for this purpose. These processes utilize chlorine-containing compounds in an acidic medium, such as calcium hypochlorite, sodium hypochlorite, sodium chlorite, or hydrogen peroxide in an alkaline medium [22]. The advantage of using hydrogen peroxide is that it _is less toxic than sulfur- and chlorine _- containing compounds. Mixtures of NaOH- _H2O2 and _CH3COOH - _H2O2 are capable of selectively dissolving lignin with minimal damage to the cellulose [23, 24]. Peroxide delignification is carried out at low temperatures, which contributes to low energy consumption, and is applicable to both wood and non-wood plant materials.

After delignification and bleaching, purified cellulose is subjected to hydrolysis, which destroys the amorphous portion of the cellulose. Acid hydrolysis with mineral acids such as sulfuric, hydrochloric, nitric, phosphoric, hydrobromic, and others is the most common method for increasing crystallinity and improving the mechanical properties of cellulose materials [25-27]. The properties of the resulting products depend on the nature and concentration of the acid, the temperature, and the duration of the hydrolysis process.

A significant disadvantage of hydrolysis with mineral acids is their toxicity and corrosive activity, as well as a reduced yield of MCC due to the partial destruction of the crystalline part of cellulose [28].

Traditional methods for producing high-quality microcrystalline cellulose rely on treating low-lignin raw materials (cotton, linters, cotton wool, cellulose, and cotton and textile waste) with mineral acids (hydrochloric, sulfuric, nitric, etc.). Low-grade raw materials (wood and agricultural waste) are primarily used to produce powdered cellulose, which is of low quality due to its high degree of polymerization and significant content of residual lignin and hemicelluloses.

The choice of the optimal method for processing CSS into MCC is determined by the available raw material base, the planned area of application of MCC, its characteristics, as well as considerations of environmental friendliness and production profitability.

A known method for producing microcrystalline and powdered cellulose involves crushing lignocellulosic material (wood chips) by explosive autohydrolysis followed by repeated extraction of lignin with water and solvent (0.41% NaOH, acetone, dioxane, ethyl alcohol, etc.). Next, the isolated cellulose is bleached with a solution of complex composition followed by acid hydrolysis [29]. The disadvantages of this method are its complexity and multi-stage nature, which is due to the requirement for special equipment for explosive autohydrolysis (the operation is used to crush and activate the lignocellulosic material), multiple and prolonged (24-hour) extraction, and the use of toxic solvents (acetone, dioxane). In addition, the disadvantage of the method is the use of a bleaching solution of complex composition, as well as the low quality of the resulting product.

A known method for producing microcrystalline cellulose involves grinding oat straw in a vibratory mill to a powder state, sieve fractionation to particles of 5-0.25 mm, followed by hydrolysis and bleaching of the resulting mass in a solution of sulfuric and peroxymonosulfuric acids at the boiling temperature of the mixture for 1-3 hours [30]. The resulting product is filtered, washed with water until a neutral reaction is achieved, and dried at a temperature of 60°C. A disadvantage of this method is the low quality of microcrystalline cellulose, which contains a significant amount of lignin (20.3-26.1%).

A known method for obtaining microcrystalline cellulose from the straw of herbaceous plants of the cereal family (oats, wheat, etc.) is by delignification with an alkaline solution of NaOH and subsequent acid hydrolysis [31]. In this method, cellulose is isolated by treating the straw powder three to four times with a mixture of nitric acid and ethanol (nitrogen-alcohol mixture) while boiling for 1 hour, filtering, and subsequent washing with a fresh nitrogen-alcohol mixture, then with hot water. The disadvantage of this method is the technological complexity and duration of the process. Thus, the isolation of cellulose by delignification requires multiple (3-4 times) treatment with a nitrogen-alcohol mixture at boiling temperature. In addition, this method uses a high hydromodulus during acid hydrolysis (50) and delignification (25), which requires significant water consumption.

In [32], a method for obtaining a cellulose product based on peroxide delignification of wheat straw was proposed. In the first stage, the straw was treated with an aqueous solution of hydrogen peroxide and acetic acid under the following conditions: acetic acid concentration of 36%, hydrogen peroxide - 12%, sulfuric acid - 0.45%; hydromodulus of 3; temperature of 85°C; duration of 2 hours. Then, the cellulose product obtained with a yield of 85.8% was treated with an aqueous solution of sodium hydroxide to dissolve the residual lignin. The disadvantages of the method are due to the use of a high concentration of hydrogen peroxide (12 wt.%) and a toxic, corrosive sulfuric acid catalyst.

A method for producing microcrystalline cellulose by treating birch, fir, and larch sawdust with a mixture of 35% hydrogen peroxide and 30% acetic acid, taken in a molar ratio of 0.3-0.5, in the presence of sulfuric acid in an amount of 1.5-2.0% of the weight of absolutely dry wood at a water modulus of 10-15, a temperature of 120-140°C, and a duration of 1-3 hours, followed by bleaching of the fibrous product [33]. Bleaching is carried out in a solution of similar composition (a mixture of hydrogen peroxide and acetic acid at a molar ratio of 0.3-0.5), but without a sulfuric acid catalyst, at a temperature of 110-130°C, a water modulus of 10-15, and a duration of 1-2 hours. The integration of delignification and bleaching processes produces high-quality microcrystalline cellulose: a degree of crystallinity of 0.66-0.74 and a degree of polymerization of 100-248. The disadvantages of this method include the use of elevated temperatures during the delignification and bleaching stages, the consequent need for pressure reactors, the use of a toxic and corrosive sulfuric acid catalyst, and lengthy processing times.

A similar method has been proposed for obtaining microcrystalline cellulose from crushed wheat straw, in which the raw material is first treated with a solution containing a mixture of 30% acetic acid and 35% hydrogen peroxide, taken in a molar ratio of 0.2-0.6 in the presence of a catalyst - sulfuric acid in an amount of 1-3% of the weight of absolutely dry straw at a hydromodulus of 5 to 10, a temperature of 110-140 °C and a duration of 2-4 hours, and then with a solution containing a mixture of acetic acid and hydrogen peroxide at a molar ratio of 0.1-0.3 and a temperature of 100-120 °C for 1-3 hours at a hydromodulus of 5 to 10 [34]. The method makes it possible to obtain high-quality microcrystalline cellulose (MCC) with a degree of polymerization of 102-137. The disadvantages of this method are due to the low yield of MCC (about 30% by weight), the need to use elevated temperatures, pressure and long processing times.

A known method for producing cellulose is by treating wood chips with a mixture containing 3.9-5.6% by weight of hydrogen peroxide, 25.2-26.7% by weight of acetic acid in the presence of 2.0% (by weight of chips) of a sulfuric acid catalyst. The treatment is carried out with vigorous stirring at a temperature of 90-98°C, atmospheric pressure, a duration of 2.5-3.5 hours and a hydromodulus of 5-10 [35]. A disadvantage of this method is the use of sulfuric acid as a catalyst, which is a corrosive and environmentally hazardous reagent, as well as the difficulty of regenerating sulfuric acid from the spent cooking solution.

In a known method for producing microcrystalline cellulose, autohydrolysis of crushed wood raw material is first carried out with water vapor at 180-240°C for 2-3 minutes. Then, the autohydrolyzed wood is extracted with boiling water at a hydromodulus of 50 for 3 hours and treated with a solution containing 20-30 wt. % CH ₃ COOH, 4.2-10.2 wt. % H ₂ O ₂ , 1.5-3 wt. % H ₂ SO ₄ , at a temperature of 100-110°C for 2.5-3 hours [36]. This invention makes it possible to obtain good quality MCC. Its degree of polymerization is 83-187, which corresponds to the degree of polymerization of industrial MCC samples (DP < 250). The crystallinity index is 0.68-0.71, which is comparable to that of MCC obtained from wood sulfite cellulose (0.67). The disadvantages of this method include the numerous process steps (autohydrolysis, treatment with boiling water, delignification, hydrolysis), elevated temperature and pressure, and the use of a toxic and corrosive sulfuric acid catalyst.

A single-stage catalytic method for obtaining MCC from coniferous and deciduous wood is proposed, based on peroxide delignification of wood in an acetic acid - water medium under mild conditions (100 °C, atmospheric pressure) in the presence of a solid catalyst TiO ₂ [37]. In this case, MCC was obtained with a yield of 36.3-42.0 wt. % and a residual lignin content of ≤1.0 wt. % under the following optimal process conditions: for aspen - 5 wt. % H ₂ O ₂ , 25 wt. % CH ₃ COOH, hydromodulus ₁₀ ; for birch - 5 wt. % H ₂ O ₂ , 25 wt. % CH 3 COOH, GM 10; for fir - 6 wt. % H ₂ O ₂ , 30 wt. % CH ₃ COOH, hydromodulus 15; for larch - 6 wt. % _H2O2 , 30 wt.% _CH3COOH , hydromodulus 15. The disadvantages of this method are due to the high degree of polymerization of the resulting MCC and the difficulty _of separating the solid catalyst from the MCC.

The use of soluble catalysts in the processing of solid cellulose-containing raw materials allows for more efficient contact between the catalyst and the raw material and simplifies the separation of the catalyst from the resulting cellulose. For example, in [38], the catalytic properties of soluble (CuSO ₄ , ZnSO ₄ , MnSO ₄ , FeSO ₄ , (NH ₄ ) ₆ Mo ₇ O ₂₄ ) and solid (TiO ₂ , zeolites KN-30 and C-FeZSM-5) catalysts were compared in the process of peroxide delignification of larch wood in an acetic acid-water medium (temperature 90°C, H ₂ O ₂ concentration 6 wt.%, CH ₃ COOH 30 wt.%, hydromodulus 15, 4 hours). It was found that the delignifying capacity of the catalysts decreases in the following sequence (NH ₄ ) ₆ Mo ₇ O ₂₄ > MnSO ₄ > ZnSO ₄ > TiO ₂ >> C-FeZSM-5 ~ KN-30 ~ FeSO ₄ ~ CuSO ₄ . In the presence of the most active catalyst (NH ₄ ) ₆ Mo ₇ O _24, MCC was obtained with a yield of 35.0 wt. % of the wood weight. However, the molybdenum catalyst is expensive.

The cost of the MnSO ₄ catalyst is significantly lower compared to molybdenum-containing catalysts. By peroxide delignification of larch wood in the presence of the MnSO ₄ catalyst (temperature 100°C, H ₂ O ₂ content 6 wt.%, CH ₃ COOH 25 wt.%, GM 15, duration 3 h), MCC with a yield of 44.3 wt.% with a low content of residual lignin (≤ 1 wt.%) and a degree of crystallinity of 0.8 was obtained [39].

The conducted analysis of scientific, technical and patent literature showed that the known methods for obtaining PC and MCC from lignocellulosic wood and non-wood raw materials are multi-stage and energy-intensive, causing damage to the environment as a result of the use of sulfur-containing and chlorine-containing delignifying and bleaching agents, toxic and corrosive mineral acids.

To improve the efficiency and environmental safety of the developed MCC production methods, it is advisable to combine peroxide delignification, bleaching, and hydrolysis of lignocellulosic feedstock in a single process cycle, using inexpensive and non-toxic dissolved catalysts that are easily separated from the cellulose product, and a low-toxicity solvent—a mixture of acetic acid and water. Energy consumption can be reduced by conducting the process under mild conditions: at a temperature of 98-100°C and atmospheric pressure.

The closest in technological essence and purpose to the proposed method is a method for obtaining a cellulose product, corresponding in its characteristics to powdered cellulose, based on the treatment of crushed CSS with an aqueous solution containing hydrogen peroxide, acetic acid and a solid catalyst TiO ₂ at a temperature of 98-100 °C, atmospheric pressure, at a hydraulic module of 5-15 for 2.5-3.5 hours, followed by the isolation of the cellulose product. [40]. The delignifying aqueous solution contains 3.0-5.6 wt. % hydrogen peroxide, 15-25 wt. % acetic acid and 0.5-1.0 % catalyst TiO ₂ from the weight of wood. However, the patent does not indicate how the separation of the solid catalyst TiO ₂ from the cellulose product is carried out. The presence of catalyst impurities in the cellulose product degrades its quality. There are also no data on the degree of polymerization of the obtained cellulose products.

The objective of the present invention is to simplify the method for producing and improving the quality of powdered cellulose material by using an easily separable soluble catalyst MgSO ₄ while maintaining a high yield of the cellulose product.

The technical result of the invention is:

- simple separation of the catalyst from the cellulose product;

- improving the quality of the cellulose product due to the absence of catalyst impurities;

- simplification of the method for obtaining powdered cellulose material due to the repeated use of the catalyst.

And also a reduction in catalyst costs due to the lower cost of MgSO ₄ compared to TiO _2.

The technical result is achieved by a method for obtaining a powdered cellulose material from cellulose-containing raw materials, including delignifying treatment of lignocellulose raw materials with an aqueous solution containing hydrogen peroxide, acetic acid and a catalyst, at the boiling temperature of the mixture (98-100°C), atmospheric pressure for 2.5-3.5 hours with intensive stirring, followed by isolation of the cellulose product.

The first variant is new in that magnesium sulfate is used as a catalyst in an amount of 0.5-1.0% of the absolute dry raw material weight, and the treatment is carried out in an aqueous solution containing 4-6 wt.% hydrogen peroxide, 40-60 wt.% acetic acid, with a hydromodulus of 10.

The second variant is new in that the lignocellulosic raw material is first treated with boiling water for 1 hour at a water-to-solid ratio of 30, followed by delignifying treatment of the washed lignocellulosic raw material with an aqueous solution containing 4-6 wt.% hydrogen peroxide, 40-60 wt.% acetic acid, magnesium sulfate in an amount of 0.5-1.0% of the absolute dry raw material weight, at a water-to-solid ratio of 10.

According to the third variant, what is new is that the delignifying treatment of lignocellulosic raw materials is carried out with an aqueous solution containing 4-6 wt.% hydrogen peroxide, 40-60 wt.% acetic acid, magnesium sulfate in an amount of 0.5-1.0% of the absolute dry raw material weight, with a hydromodulus of 10, and hydrolysis of the resulting cellulose product is carried out with a boiling solution of 2 _N sulfuric acid for 45 minutes with a hydromodulus of 20.

According to the fourth variant, what is new is that the lignocellulosic raw material is first treated with boiling water for 1 hour at a hydromodulus of 30, the washed lignocellulosic raw material is delignified with an aqueous solution containing 4-6 wt. % hydrogen peroxide, 40-60 wt. % acetic acid, magnesium sulfate in an amount of 0.5-1.0% of the absolute dry raw material weight, and the resulting cellulose product is hydrolyzed with a boiling solution of 2 _N sulfuric acid for 45 minutes at a hydromodulus of 20.

According to the fifth variant, what is new is that the delignifying treatment of lignocellulosic raw materials is carried out with an aqueous solution containing 4-6 wt. % hydrogen peroxide, 40-60 wt. % acetic acid, magnesium sulfate in an amount of 0.5-1.0% of the weight of absolute dry raw materials, with a hydromodulus of 10, then the resulting cellulose product is bleached in an alkaline medium with 4% NaOH at 25°C for 3 hours with a hydromodulus of 25 and hydrolysis of the bleached cellulose product is carried out with a boiling solution of 2 _H H ₂ SO ₄ for 45 minutes with a hydromodulus of 20.

According to the sixth variant, what is new is that the lignocellulosic raw material is first treated with boiling water for 1 hour at a hydromodulus of 30, then delignifying treatment of the washed lignocellulosic raw material is carried out with an aqueous solution containing 4-6 wt. % hydrogen peroxide, 40-60 wt. % acetic acid, magnesium sulfate in an amount of 0.5-1.0% of the weight of the absolute dry raw material, at a hydromodulus of 10, then the resulting cellulose product is bleached in an alkaline medium with 4% NaOH at 25°C for 3 hours at a hydromodulus of 25 and hydrolysis of the bleached cellulose product is carried out with a boiling solution of 2 _H H ₂ SO ₄ for 45 minutes at a hydromodulus of 20.

Unlike the prototype, the proposed invention utilizes an environmentally safe, readily available, and inexpensive dissolved magnesium sulfate catalyst. Using dissolved magnesium sulfate catalyst simplifies catalyst separation from the cellulose product, improves cellulose quality due to the absence of catalyst impurities, and reduces catalyst costs while maintaining a high cellulose product yield. Depending on the processing conditions and the nature of the cellulose-containing feedstock, cellulose products with varying cellulose, residual lignin, and hemicellulose contents, degrees of polymerization, crystallinity indices, and colors ranging from white to gray are obtained.

Fig. 1 shows the crystallinity index spectra of powdered celluloses obtained from wheat straw (black line) and hemp straw (red line).

Fig. 2 shows a micrograph of a sample of powdered cellulose material from wheat straw.

Fig. 3 shows a micrograph of a sample of powdered cellulose material from hemp straw.

Fig. 4 shows the DTG curves of samples of powdered cellulose material obtained from wheat straw and hemp straw.

Fig. 5 shows the volume distribution curve of particles of powdered cellulose obtained from wheat straw.

Fig. 6 shows the volume distribution curve of particles of powdered cellulose obtained from hemp straw.

Fig. 7 shows the crystallinity indices and spectra of MCC obtained from different types of raw materials.

Fig. 8 shows a diffraction pattern of a sample of hemp straw MCC with the Miller indices for the reflections of the cellulose I crystal lattice, the amorphous cellulose region and the baseline marked.

Fig. 9 shows a diffraction pattern of a MCC wood sample with the Miller indices for the reflections of the cellulose I crystal lattice, the amorphous cellulose region and the baseline marked.

Fig. 10 shows a micrograph of MCC of wheat straw.

Fig. 11 shows a micrograph of hemp straw MCC.

Fig. 12 shows a micrograph of MCC wood.

Fig. 13 shows a micrograph of the MCC of miscanthus straw.

Fig. 14 shows a micrograph of barley straw MCC.

Fig. 15 shows a micrograph of the flax shive MCC.

Methods for determining the characteristics of cellulose products

To determine the characteristics of powdered cellulose material (PC and MCC) obtained from different types of CSS, chemical and physicochemical methods of analysis were used.

The determination of the Klasson lignin content was carried out using the Komarov method by treating PC and MCC with 72% sulfuric acid [41].

The cellulose content was determined using the Kürschner method by treating the obtained PC and MCC with an ethanol solution of nitric acid (4:1) [42].

The content of hemicelluloses in PC and MCC was determined as the amount of easily hydrolyzable polysaccharides formed during their hydrolysis with a 2% HCl solution [42].

The degree of polymerization of PC and MCC was determined by the characteristic viscosity of samples dissolved in ZHVNK (iron-sodium complex) using a capillary viscometer of the VPZh-3 type with a constant of 0.03 ^mm2 / ^s2 [42].

To assess the degree of crystallinity of PC and MCC, the X-ray diffraction method was used. The diffraction patterns of the samples were recorded on a DRON-3 diffractometer using Cu _Ka radiation (wavelength 0.15418 nm) with a graphite monochromator on the secondary beam, voltage of 32 kV, current of 25 mA, scanning step of 0.02° 2θ, accumulation time at a point of 1 s. Measurements were carried out in the range of Bragg angles from 5.00° to 70.00°2θ. From the diffraction data, the crystallinity index (CI) was calculated using the Segal method [43]. For this, the background line was subtracted, the intensity of I ₀₀₂ and I _{AM was determined.} CI was determined by the formula:

AND

where I ₀₀₂ is the intensity of the peak in the region 2θ = 22°, corresponding to the crystalline region of cellulose,

I _AM minimum in the region 2θ = 18° corresponding to the amorphous part of cellulose.

The IR spectra were recorded on an IRTracer-100 FTIR spectrometer (Shimadzu, Japan) using the ATR method. The background spectrum, the absorption spectrum of a pure ZnSe crystal, was recorded. Then, a sample was placed on the crystal (the sample should completely cover the crystal surface), and its IR spectrum was recorded. The IR spectrum recording range was 4000 - 630 cm ^-1 , the shooting resolution was 4 cm ^-1 , the number of scans was 32. The resulting spectrum is the difference between the sample and background spectra. Spectra processing was performed using the OPUS 7.5 software package (Bruker).

The morphology of cellulose samples (PC and MCC) was characterized by scanning electron microscopy (SEM) on a TM4000Plus microscope (Hitachi, Japan) with an accelerating voltage of 5-20 kV and a resolution of up to 20 μm. The study was performed in backscattered electron (BSE) mode. Since cellulose is a nonconductive material, to overcome artifacts in SEM imaging, the samples were sputter-coated with a platinum layer. The thickness was approximately 0.8 nm.

Thermogravimetric analysis of celluloses (PC and MCC) was performed in a corundum crucible using an STA 449 F1 Jupiter instrument (NETZSCH, Germany) in the temperature range from 30 to 900°C under argon flow (the flow rates of the protective and purge gases were 20 and 50 ml/min, respectively). The heating rate was 10°C/min. The measurement results were processed using the NETZSCH Proteus Thermal Analysis 5.1.0 software package supplied with the instrument.

Particle size distribution was determined by laser diffraction using a Bettersizer S3 Plus instrument (Bettersize Instruments LTD, China). Approximately 1-2 mg of dry sample powder (PC or MCC) was added to the cuvette compartment of the instrument, containing approximately 500 ml of distilled water. The medium circulation rate in the cell was 200 ml/s, and the ultrasound power was 30 W. Laser scattering spectra of the samples were recorded and processed using Mie light scattering theory with the equivalent-volume sphere model. Refractive indices calculated from the diffraction patterns of the studied sample were used in the processing.

Specific examples of obtaining powdered celluloses from various types of cellulose-containing raw materials are given below and summarized in Table 1.

Example 1

Air-dried wheat straw is first crushed in a Molot-800 hammer mill, then in a RM 120 rotary mill, after which it is sifted in an RG-300 sifter to obtain straw flour with a size class of no more than 2 mm.

10 g of chopped wheat straw (lignin content 19.7%) are placed in a 0.5 l glass reactor (flask) equipped with a stirrer and reflux condenser. 100 ml of a solution (GM 10) containing 6 wt. % H ₂ O ₂ , 40 wt. % CH ₃ COOH and 0.5 wt. % MgSO ₄ catalyst are added. The delignification process is carried out at a temperature of 100 °C and constant stirring (700 rpm) for 3 hours. The resulting product is separated from the solution by filtration, washed with water to a neutral medium and dried at a temperature of 103 ± 2 °C. The dried PC is ground in a MUL-M laboratory mill. The separated solution is used for regeneration.

Indicators: PC yield - 51.4% of the mass of absolutely dry wheat straw, cellulose content - 72.0%, content of Klasson lignin in PC - 3.6%, hemicelluloses - 18.9%, ash - 2.0%, degree of polymerization - 855, crystallinity index - 0.80 (see Table 1, sample No. 1).

Example 2

This method differs from Example 1 in that, to activate the feedstock and remove low-molecular-weight contaminants after milling, the crushed wheat straw is treated with hot water under the following conditions: temperature 50°C, duration 1 hour, liquid module 1:40 g/ml. For this purpose, a sample of air-dried crushed wheat straw weighing approximately 11.0 g is placed in a heat-resistant 1.0 L beaker, 440 ml of distilled water is added, a magnetic anchor is immersed in the beaker, and the beaker is transferred to a heated magnetic stirrer. The stirring speed is 300 rpm. After 1 hour, stirring and heating are stopped, and the treated wheat straw is separated from the aqueous solution by vacuum filtration on a Buchner funnel. The treated wheat straw is pressed on a filter to a moisture content of 84.4 wt%. This water content in the treated wheat straw eliminates the need to add water when preparing the delignification solution. The wet treated wheat straw is quantitatively transferred to a glass reactor, and the calculated amount of 40% by weight glacial acetic acid (40 ml), 6% by weight hydrogen peroxide (14 ml), and 0.05% by weight _MgSO4 catalyst (0.05 g) are added. Delignification and subsequent operations are carried out as in Example 1.

Indicators: PC yield - 43.7% of the mass of absolutely dry wheat straw, cellulose content - 75.2%, content of Klasson lignin in PC - 2.4%, hemicelluloses - 19.1%, ash - 0.7%, degree of polymerization - 896 (see Table 1, sample No. 2).

Example 3

The grinding of the initial raw material - hemp straw - and the selection of the fraction were carried out similarly to Example 1.

10 g of chopped hemp straw (lignin content 11.7%) are placed in a 0.5 l glass reactor (flask) equipped with a stirrer and reflux condenser. 100 ml of a solution (GM 10) containing 4 wt. % H ₂ O ₂ , 40 wt. % CH ₃ COOH and 0.5 wt. % MgSO ₄ catalyst are added. The delignification process is carried out at a temperature of 100 °C and constant stirring (700 rpm) for 3 hours. The resulting product is separated from the solution by filtration, washed with water to a neutral medium and dried at a temperature of 103 ± 2 °C. The dried PC is ground in a MUL-M laboratory mill. The separated solution is used for regeneration.

Indicators: PC yield - 61.9% of the mass of absolutely dry hemp straw, cellulose content - 67.9%, Klasson lignin content in PC - 1.6%, hemicelluloses - 16.6%, ash - 0.3%, degree of polymerization - 918, crystallinity index - 0.79 (see Table 1, sample No. 3).

Example 4

This differs from example 2 in that hemp straw is used as the starting material. For water treatment, a 12.0 g sample is used (adjusted for the content of water-soluble substances to obtain approximately 10 g of processed hemp straw at the peroxide delignification stage) and 480 ml of distilled water.

Wet processed hemp straw is quantitatively transferred into a glass reactor, and the calculated amount of 40 wt.% glacial acetic acid (40 ml), 4 wt.% hydrogen peroxide (10 ml), and 0.05 wt.% _MgSO4 catalyst (0.05 g) are added. Delignification and subsequent operations are carried out as in Example 1.

Indicators: PC yield - 51.8% of the mass of absolutely dry hemp straw, cellulose content - 74.8%, content of Klasson lignin in PC - 0.6%, hemicelluloses - 10.4%, ash - 0.2%, degree of polymerization - 904 (see Table 1, sample No. 4).

Example 5

The grinding of the initial raw material - miscanthus straw - and the selection of the fraction were carried out similarly to Example 1.

10 g of chopped miscanthus straw (lignin content 20.8%) are placed in a 0.5 l glass reactor (flask) equipped with a stirrer and reflux condenser. 100 ml of a solution (GM 10) containing 6 wt. % H ₂ O ₂ , 50 wt. % CH ₃ COOH and 0.5 wt. % MgSO ₄ catalyst are added. The delignification process is carried out at a temperature of 100 °C and constant stirring (700 rpm) for 3 hours. The resulting product is separated from the solution by filtration, washed with water to a neutral medium and dried at a temperature of 103 ± 2 °C. The dried PC is ground in a MUL-M laboratory mill. The separated solution is used for regeneration.

Indicators: PC yield - 47.5% of the mass of absolutely dry miscanthus straw, cellulose content - 61.0%, Klasson lignin content in PC - 6.9%, hemicelluloses - 16.8%, ash - 3.6%, degree of polymerization - 641, crystallinity index - 0.68 (see Table 1, sample No. 5).

Example 6

This differs from example 2 in that miscanthus straw is used as the starting material. For water treatment, a 12.5 g sample is used (adjusted for the content of water-soluble substances to obtain approximately 10 g of treated miscanthus straw at the peroxide delignification stage) and 500 ml of distilled water.

Wet treated miscanthus straw is quantitatively transferred into a glass reactor, and the calculated amount of 50 wt.% glacial acetic acid (50 ml), 6 wt.% hydrogen peroxide (14 ml), and 0.05 wt.% _MgSO4 catalyst (0.05 g) are added. Delignification and subsequent operations are carried out as in Example 1.

Indicators: PC yield - 54.4% of the mass of absolutely dry miscanthus straw, cellulose content - 53.8%, Klasson lignin content in PC - 6.5%, hemicelluloses - 16.8%, ash - 2.4%, degree of polymerization - 790 (see Table 1, sample No. 6).

Example 7

The grinding of the initial raw material - barley straw - and the selection of the fraction were carried out similarly to Example 1.

10 g of crushed barley straw (lignin content 23.1%) are placed in a 0.5 l glass reactor (flask) equipped with a stirrer and reflux condenser. 100 ml of a solution (GM 10) containing 5 wt. % H ₂ O ₂ , 60 wt. % CH ₃ COOH and 0.5 wt. % MgSO ₄ catalyst are added. The delignification process is carried out at a temperature of 100 °C and constant stirring (700 rpm) for 3 hours. The resulting product is separated from the solution by filtration, washed with water to a neutral medium and dried at a temperature of 103 ± 2 °C. The dried PC is ground in a MUL-M laboratory mill. The separated solution is used for regeneration.

Indicators: PC yield - 51.6% of the mass of absolutely dry barley straw, cellulose content - 58.1%, Klasson lignin content in PC - 6.6%, hemicelluloses - 17.7%, ash - 5.3%, degree of polymerization - 718, crystallinity index - 0.74 (see Table 1, sample No. 7).

Example 8

This differs from example 2 in that barley straw is used as the starting material. For water treatment, a sample weighing 11.5 g is used (adjusted for the content of water-soluble substances to obtain approximately 10 g of treated miscanthus straw for the peroxide delignification stage) and 460 ml of distilled water.

Wet treated barley straw is quantitatively transferred into a glass reactor, and the calculated amount of 60 wt.% glacial acetic acid (60 ml), 5 wt.% hydrogen peroxide (12 ml), and 0.05 wt.% _MgSO4 catalyst (0.05 g) are added. Delignification and subsequent operations are carried out as in Example 1.

Indicators: PC yield - 48.0% of the mass of absolutely dry barley straw, cellulose content - 61.5%, Klasson lignin content in PC - 4.3%, hemicelluloses - 19.6, ash - 2.4%, degree of polymerization - 616 (see Table 1, sample No. 8).

Example 9

The grinding of the initial raw material - wood and the selection of the fraction were carried out similarly to Example 1.

10 g of crushed wood (lignin content 28.5%) are placed in a 0.5 l glass reactor (flask) equipped with a stirrer and reflux condenser. 100 ml of a solution (GM 10) containing 6 wt. % H ₂ O ₂ , 50 wt. % CH ₃ COOH and 0.5 wt. % MgSO ₄ catalyst are added. The delignification process is carried out at a temperature of 100 °C and constant stirring (700 rpm) for 3 hours. The resulting product is separated from the solution by filtration, washed with water to a neutral medium and dried at a temperature of 103 ± 2 °C. The dried PC is ground in a MUL-M laboratory mill. The separated solution is used for regeneration.

Indicators: PC yield - 55.6% of the mass of absolutely dry wood, cellulose content - 78.1%, Klasson lignin content in PC - 3.2%, hemicelluloses - 9.9%, ash - 0.3%, degree of polymerization - 472, crystallinity index - 0.86 (see Table 1, sample No. 9).

Example 10

This differs from example 2 in that wood is used as the feedstock. For water treatment, a 10.6 g sample is used (adjusted for the content of water-soluble substances to obtain approximately 10 g of treated miscanthus straw for the peroxide delignification stage) and 424 ml of distilled water are added.

The wet treated wood is quantitatively transferred into a glass reactor, and the calculated amount of 50 wt.% glacial acetic acid (50 ml), 6 wt.% hydrogen peroxide (14 ml), and 0.05 wt.% _MgSO4 catalyst (0.05 g) are added. Delignification and subsequent operations are carried out as in Example 1.

Indicators: PC yield - 54.9% of the mass of absolutely dry wood, cellulose content - 80.3%, content of Klasson lignin in PC - 1.4%, hemicelluloses - 9.2%, ash - 0.1%, degree of polymerization - 488 (see Table 1, sample No. 10).

Example 11

The grinding of the initial raw material - flax shives - and the selection of the fraction were carried out similarly to Example 1.

10 g of crushed flax shives (lignin content 30.4%) are placed in a 0.5 l glass reactor (flask) equipped with a stirrer and reflux condenser. 100 ml of a solution (GM 10) containing 6 wt. % H ₂ O ₂ , 50 wt. % CH ₃ COOH and 0.5 wt. % MgSO ₄ catalyst are added. The delignification process is carried out at a temperature of 100 °C and constant stirring (700 rpm) for 3 hours. The resulting product is separated from the solution by filtration, washed with water to a neutral medium and dried at a temperature of 103 ± 2 °C. The dried flax shives are ground in a MUL-M laboratory mill. The separated solution is used for regeneration.

Indicators: PC yield - 61.9% of the mass of absolute flax shives, cellulose content - 63.6%, content of Klasson lignin in PC - 2.5%, hemicelluloses - 9.0%, ash - 2.3%, degree of polymerization - 829, crystallinity index - 0.77 (see Table 1, sample No. 11).

Example 12

This differs from example 2 in that flax shive is used as the starting material. For water treatment, a 10.9 g sample is used (adjusted for the content of water-soluble substances to obtain approximately 10 g of treated miscanthus straw for the peroxide delignification stage) and 436 ml of distilled water.

The wet processed flax shives are quantitatively transferred into a glass reactor, and the calculated amount of 50 wt.% glacial acetic acid (50 ml), 6 wt.% hydrogen peroxide (14 ml), and 0.05 wt.% _MgSO4 catalyst (0.05 g) are added. Delignification and subsequent operations are carried out as in Example 1.

Indicators: PC yield - 48.7% of the mass of absolutely dry wood, cellulose content - 69.3%, content of Klasson lignin in PC - 1.8%, hemicelluloses - 7.6%, ash - 0.8%, degree of polymerization - 882 (see Table 1, sample No. 12).

Table 1. Yield and composition of powdered cellulose (PC) samples obtained by catalytic peroxide delignification of raw materials (treatment duration 3 hours at the boiling temperature of the mixture, catalyst 0.5 wt.% MgSO ₄ )

Sample No. Powdered cellulose Exit**,% Cellulose Lignin Hemicelluloses Ash SP IR 1 PC Wheat Straw outg.
6-40-10* 51.4 72.0 3.6 18.9 2.0 855 0.80 2 PC Wheat Straw, sample _H2O
6-40-10* 43.7 75.2 2.4 19.1 0.7 896 3 PC Hemp Straw origin.
4-40-10* 61.9 67.9 1.6 16.6 0.3 918 0.79 4 PC Hemp Straw, sample _H2O
4-40-10* 51.8 74.8 0.6 10.4 0.2 904 5 PC Miscanthus straw, outg.
6-50-10* 47.5 61.0 6.9 19.7 3.6 641 0.68 6 PC Miscanthus straw, sample H ₂ O
6-50-10* 54.4 53.8 6.5 16.8 2.4 790 7 PC Straw barley iskh.
5-60-10* 51.6 58.1 6.6 17.7 5.3 718 0.74 8 PC Straw barley sample H ₂ O
5-60-10* 48.0 61.5 4.3 19.6 2.4 616 9 PC Wood source
6-50-10* 55.6 78.1 3.2 9.9 0.3 472 0.86 10 PC Wood sample H ₂ O
6-50-10* 54.9 80.3 1.4 9.2 0.1 488 11 PC Flax Fire, outgoing
6-50-10* 61.9 63.6 2.5 9.0 2.3 829 0.77 12 PC Flax fire sample H ₂ O
6-50-10* 48.7 69.3 1.8 7.6 0.8 882

*delignification conditions: concentration _{of H2O2 -} concentration of _CH3COOH - Hydromodulus;

**yield to dry weight of original raw material / yield to dry weight of water-treated raw materialAccording to their characteristics, cellulose products obtained from wheat straw, hemp, flax shives, wood, barley straw and miscanthus by a single-stage method, combining the processes of delignification, bleaching and hydrolysis in a single technological cycle, belong to the class of powdered celluloses (Table 1).

For samples of powdered cellulose obtained from wheat and hemp straw, additional studies were carried out using IR, SEM, DTA, and laser diffraction methods.

Fig. 1 shows the IR spectra of powdered celluloses obtained from wheat straw and hemp straw.

All characteristic peaks in the IR spectrum (Fig. 1) correspond to cellulose: 3340, 2897, 1430, 1162, 1106, 1055, 897 cm ^-1 . Hemicelluloses are present in both samples, a band at 1726 cm ^-1 . Lignin is present in trace amounts or is absent, absorption band: ~1600 cm ^-1 (overlap with adsorbed water), 1505 cm ^-1 is absent, 1465 cm ^-1 (overlap with C-H vibrations).

According to SEM data (Figs. 2, 3), powdered cellulose obtained from wheat straw is a variety of particles, both fibers and loose particles and "leaflets". The fibers are 100-400 μm long; diameter 7-12 μm. Aggregates of fibers 500-700 μm long are found (Fig. 2). Particles of powdered cellulose obtained from hemp straw are fibers 180-400 μm long; diameter predominantly 10-11 μm, with the presence of a fraction with a larger diameter (25-30) μm (Fig. 3)

According to thermal analysis data (Fig. 4), the studied PC samples exhibit different thermal stability, characterized by different temperatures at which primary structural decomposition begins. The wheat straw PC sample is more thermally stable than the hemp straw PC sample. At the same time, the rate of mass loss associated with the release of thermal decomposition products is significantly greater for the wheat straw PC sample than for the hemp straw PC sample. This may indicate that thermal degradation of the wheat straw PC sample results in fragments with lower molecular weight and, consequently, lower thermal stability. This is indirectly confirmed by the lower amount of carbon residue formed at the end of pyrolysis.

Based on the volume distribution curves of the particles of the obtained powdered celluloses (Fig. 5, 6), the data presented in Table 2 were obtained.

Table 2. Particle size distribution of powdered celluloses

Percentile Particle size, µm Hemp straw Wheat straw D03 14.1 12.7 D10 30.7 23.2 D50 175.5 90.5 D75 350.9 177.4 D97 753.6 570.1

Powdered cellulose obtained from wheat straw consists of 97% particles of 570 µm in size, powdered cellulose from hemp straw consists of particles of 753 µm in size.

To obtain cellulose products with characteristics corresponding to microcrystalline cellulose from lignocellulose raw materials, additional stages of processing of the CSS and the resulting powdered cellulose were used.

The two-stage process for producing MCC involves catalytic peroxide delignification and acid hydrolysis of the resulting PC. The integration of catalytic peroxide delignification of CSS and acid hydrolysis of the resulting cellulose powder allows for a reduction in the content of hemicelluloses and amorphous cellulose, a lower degree of polymerization, and an increased crystallinity index of the resulting MCC.

The use of boiling water treatment of the initial cellulose-containing raw material, preceded by peroxide delignification and acid hydrolysis, helps to reduce the content of mineral impurities (ash) and residual lignin in the resulting cellulose products.

Specific examples of obtaining microcrystalline cellulose from various types of CSS using two-stage and three-stage methods are given below and summarized in Tables 3-6.

Example 13

This method for producing MCC includes the stages of catalytic peroxide delignification of CSS and sulfuric acid hydrolysis of the cellulose product.

The wheat straw cellulose product obtained in Example 1 is subjected to acid hydrolysis with a 2 N (10%) sulfuric acid solution. For this purpose, 10.0 g of the wheat straw cellulose product is placed in a heat-resistant 1.0 L flat-bottomed flask, and 200 mL of a 10% H ₂ SO ₄ solution (liquid ratio 1:20 g/mL) is added. The flask is transferred to an electric hotplate, a reflux condenser is attached to the flask, and the acid hydrolysis process is carried out with the reaction mixture boiling for 45 minutes from the moment the solution begins to boil. The resulting microcrystalline cellulose is separated from the acid solution by vacuum filtration on a Buchner funnel and washed with distilled water until the washings are neutral (tested with litmus paper). The MCC from the Buchner funnel is transferred to a Petri dish and air-dried for 12 hours. It is then placed in a drying oven, where it is dried to a constant weight at a temperature of 103°C (+2°C). The dried MCC is ground in a MUL-M laboratory mill.

Indicators: MCC yield - 41.2% of the mass of absolutely dry wheat straw, cellulose content - 78.0%, Klasson lignin content in MCC - 4.2%, hemicelluloses - 14.7%, ash - 2.9%, degree of polymerization - 455 (see Table 3, sample No. 1).

Example 14

It differs from example 13 in that the starting material used is a cellulose product obtained from hemp straw, according to the method described in example 3. The conditions for acid hydrolysis are the same.

Indicators: MCC yield - 44.3% of the mass of absolutely dry hemp straw, cellulose content - 86.1%, Klasson lignin content in MCC - 0.3%, hemicelluloses - 10.4%, ash - 0.08%, degree of polymerization - 245 (see Table 3, sample No. 2).

Example 15

It differs from example 13 in that the starting material used is a cellulose product obtained from miscanthus straw, according to the method described in example 5. The conditions for acid hydrolysis are the same.

Indicators: MCC yield - 22.6% of the mass of absolutely dry miscanthus straw, cellulose content - 76.4%, Klasson lignin content in MCC - 7.6%, hemicelluloses - 10.7%, ash - 4.4%, degree of polymerization - 930 (see Table 3, sample No. 3).

Example 16

It differs from example 13 in that the starting material used is a cellulose product obtained from barley straw, according to the method described in example 7. The conditions for acid hydrolysis are the same.

Indicators: MCC yield - 39.0% of the mass of absolutely dry barley straw, cellulose content - 68.9%, Klasson lignin content in MCC - 9.8%, hemicelluloses - 12.3%, ash - 5.7%, degree of polymerization - 408 (see Table 3, sample No. 4).

Example 17

It differs from example 13 in that the starting material used is a cellulose product obtained from flax shives, according to the method described in example 11. The conditions for acid hydrolysis are the same.

Indicators: MCC yield - 39.0% of the mass of absolutely dry barley straw, cellulose content - 78.7%, Klasson lignin content in MCC - 3.0%, hemicelluloses - 10.2%, ash - 0.1%, degree of polymerization - 755 (see Table 3, sample No. 5).

Example 18

It differs from example 13 in that the starting material used is a cellulose product obtained from wood, according to the method described in example 9. The conditions of acid hydrolysis are the same.

Indicators: MCC yield - 38.4% of the mass of absolutely dry wood, cellulose content - 87.9%, Klasson lignin content in MCC - 1.6%, hemicelluloses - 8.1%, ash - 0.0%, degree of polymerization - 300 (see Table 3, sample No. 6).

Example 19

This method for producing MCC includes the stages of treating the CCC with hot water, catalytic peroxide delignification of the washed raw material and sulfuric acid hydrolysis of the cellulose product.

The cellulose product obtained from water-treated wheat straw, according to the method described in Example 2, is subjected to acid hydrolysis with a 2 N (10%) sulfuric acid solution. For this, 10.0 g of the wheat straw cellulose product is placed in a heat-resistant, flat-bottomed, 1.0 L flask, and 200 ml of a 10% H ₂ SO ₄ solution (liquid ratio 1:20 g/ml) are added. The flask is transferred to an electric hotplate, a reflux condenser is attached to the flask, and the acid hydrolysis process is carried out with the reaction mixture boiling for 45 minutes from the moment the solution begins to boil. The resulting microcrystalline cellulose is separated from the acid solution by vacuum filtration on a Buchner funnel and washed with distilled water until the washings are neutral (tested with litmus paper). The MCC from the Buchner funnel is transferred to a Petri dish and air-dried for 12 hours. It is then placed in a drying oven, where it is dried to a constant weight at a temperature of 103°C (+2°C). The dried MCC is ground in a MUL-M laboratory mill.

Indicators: MCC yield - 39.9% of the mass of absolutely dry wheat straw, cellulose content - 84.2%, Klasson lignin content in MCC - 2.2%, hemicelluloses - 12.4%, ash - 1.2%, degree of polymerization - 293 (see Table 4, sample No. 1).

Example 20

It differs from example 19 in that the starting material used is a cellulose product obtained from hemp straw treated with water, according to the method described in example 4. The conditions of acid hydrolysis are the same.

Indicators: MCC yield - 43.0% of the mass of absolutely dry hemp straw, cellulose content - 86.2%, Klasson lignin content in MCC - 0.2%, hemicelluloses - 10.7%, ash - 0.06%, degree of polymerization - 251 (see Table 4, sample No. 2).

Example 21

It differs from example 19 in that the starting material used is a cellulose product obtained from water-treated miscanthus straw, according to the method described in example 6. The conditions of acid hydrolysis are the same.

Indicators: MCC yield - 35.4% of the mass of absolutely dry miscanthus straw, cellulose content - 63.7%, Klasson lignin content in MCC - 5.0%, hemicelluloses - 9.5%, ash - 2.0%, degree of polymerization - 565 (see Table 4, sample No. 3).

Example 22

It differs from example 19 in that the starting material used is a cellulose product obtained from barley straw treated with water, according to the method described in example 8. The conditions of acid hydrolysis are the same.

Indicators: MCC yield - 35.4% of the mass of absolutely dry barley straw, cellulose content - 78.3%, Klasson lignin content in MCC - 5.4%, hemicelluloses - 12.2%, ash - 2.2%, degree of polymerization - 380 (see Table 4, sample No. 4).

Example 23

It differs from example 19 in that the starting material used is a cellulose product obtained from flax shives treated with water, according to the method described in example 12. The conditions for acid hydrolysis are the same.

Indicators: MCC yield - 35.2% of the weight of absolutely dry flax shives, cellulose content - 84.6%, Klasson lignin content in MCC - 0.2%, hemicelluloses - 9.1%, ash - 0.08%, degree of polymerization - 553 (see Table 4, sample No. 5).

Example 24

It differs from example 19 in that the starting material used is a cellulose product obtained from water-treated wood, according to the method described in example 10. The conditions of acid hydrolysis are the same.

Indicators: MCC yield - 38.2% of the mass of absolutely dry wood, cellulose content - 88.7%, Klasson lignin content in MCC - 1.4%, hemicelluloses - 10.3%, ash - 0.0%, degree of polymerization - 222 (see Table 4, sample No. 6).

Example 25

This method for producing MCC includes the stages of catalytic peroxide delignification, alkaline treatment and sulfuric acid hydrolysis of the cellulose product.

A 10-g portion of the cellulose product obtained from wheat straw, according to the method described in Example 1, is transferred to a 1-liter glass beaker, and 250 ml of a 4% NaOH solution (liquid ratio 1:25 g/ml) is added. The beaker is placed on a magnetic stirrer platform, and an alkaline treatment is performed at room temperature for 3 hours at a stirring speed of 350 rpm. The cellulose is separated from the alkaline solution by vacuum filtration on a Buchner funnel, washed with distilled water until the wash water is neutral (tested with litmus paper), thoroughly squeezed to remove residual water, and sent to the acid hydrolysis stage. Acid hydrolysis and MCC recovery are carried out as described in Example 13.

Indicators: MCC yield - 35.0% of the mass of absolutely dry wheat straw, cellulose content - 83.1%, Klasson lignin content in MCC - 1.8%, hemicelluloses - 14.0%, ash - 1.1%, degree of polymerization - 304 (see Table 5, sample No. 1).

Example 26

It differs from example 25 in that the starting material used is a cellulose product obtained from hemp straw, according to the method described in example 3. The conditions for alkaline treatment and acid hydrolysis are the same (example 13, example 25).

Indicators: MCC yield - 41.5% of the mass of absolutely dry hemp straw, cellulose content - 87.7%, Klasson lignin content in MCC - 0.3%, hemicelluloses - 8.7%, ash - 0.05%, degree of polymerization - 308 (see Table 5, sample No. 2).

Example 27

It differs from example 25 in that the starting material used is a cellulose product obtained from miscanthus straw, according to the method described in example 5. The conditions for alkaline treatment and acid hydrolysis are the same (example 13, example 25).

Indicators: MCC yield is 36.2% of the mass of absolutely dry miscanthus straw, cellulose content is 77.8%, Klasson lignin content in MCC is 5.4%, hemicelluloses are 12.8%, ash is 2.1%, the degree of polymerization is 375 (see Table 5, sample No. 3).

Example 28

It differs from example 25 in that the starting material used is a cellulose product obtained from barley straw, according to the method described in example 7. The conditions for alkaline treatment and acid hydrolysis are the same (example 13, example 25).

Indicators: MCC yield - 34.4% of the mass of absolutely dry barley straw, cellulose content - 76.7%, Klasson lignin content in MCC - 6.1%, hemicelluloses - 11.7%, ash - 2.2%, degree of polymerization - 367 (see Table 5, sample No. 4).

Example 29

It differs from example 25 in that the starting material used is a cellulose product obtained from flax shives, according to the method described in example 11. The conditions for alkaline treatment and acid hydrolysis are the same (example 13, example 25).

Indicators: MCC yield - 34.3% of the mass of absolutely dry flax shives, cellulose content - 85.2%, Klasson lignin content in MCC - 0.2%, hemicelluloses - 8.5%, ash - 0.06%, degree of polymerization - 635 (see Table 5, sample No. 5).

Example 30

It differs from example 25 in that the starting material used is a cellulose product obtained from wood, according to the method described in example 9. The conditions for alkaline treatment and acid hydrolysis are the same (example 13, example 25).

Indicators: MCC yield - 36.5% of the mass of absolutely dry flax shives, cellulose content - 90.4%, Klasson lignin content in MCC - 0.3%, hemicelluloses - 6.2%, ash - 0.0%, degree of polymerization - 284 (see Table 5, sample No. 6).

Example 31

This method for producing MCC includes the stages of treating the CCC with hot water, catalytic peroxide delignification, alkaline treatment and sulfuric acid hydrolysis of the cellulose product.

The cellulose product obtained from water-treated wheat straw, according to the method described in Example 2, is first subjected to alkaline treatment, according to the method described in Example 25, and then acid hydrolysis of the cellulose is carried out according to the method described in Example 19.

Indicators: MCC yield - 30.0% of the mass of absolutely dry wheat straw, cellulose content - 91.0%, Klasson lignin content in MCC - 0.7%, hemicelluloses - 6.9%, ash - 0.7%, degree of polymerization - 241 (see Table 6, sample No. 1).

Example 32

The cellulose product obtained from water-treated hemp straw, according to the method described in Example 4, is first subjected to alkaline treatment, according to the method described in Example 25, and then acid hydrolysis of the cellulose is carried out according to the method described in Example 19.

Indicators: MCC yield - 41.8% of the mass of absolutely dry hemp straw, cellulose content - 92.3%, Klasson lignin content in MCC - 0.0%, hemicelluloses - 7.3%, ash - 0.0%, degree of polymerization - 386 (see Table 6, sample No. 2).

Example 33

The cellulose product obtained from water-treated miscatus straw according to the method described in Example 6 is first subjected to alkaline treatment according to the method described in Example 25, and then acid hydrolysis of the cellulose is carried out according to the method described in Example 19.

Indicators: MCC yield - 29.7% of the mass of absolutely dry miscanthus straw, cellulose content - 82.2%, Klasson lignin content in MCC - 4.0%, hemicelluloses - 11.8%, ash - 1.8%, degree of polymerization - 416 (see Table 6, sample No. 3).

Example 34

The cellulose product obtained from water-treated barley straw, according to the method described in Example 8, is first subjected to alkaline treatment, according to the method described in Example 25, and then acid hydrolysis of the cellulose is carried out according to the method described in Example 19.

Indicators: MCC yield - 31.8% of the mass of absolutely dry barley straw, cellulose content - 83.4%, Klasson lignin content in MCC - 2.9%, hemicelluloses - 9.6%, ash - 2.1%, degree of polymerization - 312 (see Table 6, sample No. 4).

Example 35

The cellulose product obtained from water-treated flax shives according to the method described in Example 12 is first subjected to alkaline treatment according to the method described in Example 25, and then acid hydrolysis of the cellulose is carried out according to the method described in Example 19.

Indicators: MCC yield - 32.4% of the mass of absolutely dry flax shives, cellulose content - 86.3%, Klasson lignin content in MCC - 0.12%, hemicelluloses - 8.3%, ash - 0.06%, degree of polymerization - 533 (see Table 6, sample No. 5).

Example 36

The cellulose product obtained from water-treated wood according to the method described in Example 10 is first subjected to alkaline treatment according to the method described in Example 25, and then acid hydrolysis of the cellulose is carried out according to the method described in Example 19.

Indicators: MCC yield - 35.8% of the mass of absolutely dry wood, cellulose content - 91.9%, Klasson lignin content in MCC - 0.04%, hemicelluloses - 6.9%, ash - 0.0%, degree of polymerization - 293 (see Table 6, sample No. 6).

Table 3. Yield and characteristics of microcrystalline cellulose (MCC) samples obtained from various types of cellulose-containing raw materials using the stage of catalytic peroxide delignification of CSS and sulfuric acid hydrolysis of the cellulose product

No. Raw materials Concentration
hydrogen peroxide, wt. % Concentration
acetic acid, wt. % Yield, wt. %
a.s. source raw materials Composition of MCC, wt. % SP Cellulose Lignin Hemicelluloses Ash 1 Wheat straw 6 40 41.2 78.0 4.2 14.7 2.9 455 2 Hemp straw 4 40 44.3 86.1 0.3 10.4 0.08 245 3 Miscanthus 6 50 22.6 76.4 7.6 10.7 4.4 930 4 Barley 5 60 39.0 68.9 9.8 12.3 5.7 408 5 Flax fire 6 50 39.0 78.7 3.0 10.2 0.1 755 6 Wood 6 50 38.4 87.9 1.6 8.1 0.0 300

Delignification conditions: temperature 103°C, GM -10, duration 3 h, catalyst 0.05 wt.% MgSO ₄

Acid hydrolysis conditions: _H2SO4 concentration - 10 _wt .%, GM - 20, duration 45 min at boiling temperature

Table 4. Yield and characteristics of microcrystalline cellulose (MCC) samples obtained from various types of cellulose-containing raw materials using the stages of treating lignocellulosic raw materials with hot water, catalytic peroxide delignification of washed raw materials and sulfuric acid hydrolysis of the cellulose product

No. Raw materials Concentration
hydrogen peroxide, wt. % Concentration
acetic acid, wt. % Yield, wt. %
a.s. source raw materials Composition of MCC, wt. % SP Cellulose Lignin Hemicelluloses Ash 1 Wheat straw 6 40 39.9 84.2 2.2 12.4 1.2 293 2 Hemp straw 4 40 43.0 86.2 0.2 10.7 0.06 251 3 Miscanthus 6 50 35.4 63.7 5.0 9.5 2.0 565 4 Barley 5 60 35.4 78.3 5.4 12.2 2.2 380 5 Flax fire 6 50 35.2 84.6 0.23 9.1 0.08 553 6 Wood 6 50 38.2 88.7 1.4 10.3 0.0 222

Water treatment conditions: temperature 50°C, GM -40, duration 1 hour.

Delignification conditions: temperature 103°C, GM -10, duration 3 h, catalyst 0.05 wt.% MgSO ₄

Acid hydrolysis conditions: _H2SO4 concentration - 10 _wt .%, GM - 20, duration 45 min at boiling temperature

Table 5. Yield and characteristics of microcrystalline cellulose (MCC) samples obtained from various types of cellulose-containing raw materials using the stages of catalytic peroxide delignification, alkaline treatment and sulfuric acid hydrolysis of the cellulose product

No. Raw materials Concentration
hydrogen peroxide, wt. % Concentration
acetic acid, wt. % Yield, wt. %
a.s. original raw materials Composition of MCC, wt. % SP Cellulose Lignin Hemicelluloses Ash 1 Wheat straw 6 40 35.0 83.1 1.8 14.0 1.1 304 2 Hemp straw 4 40 41.5 87.7 0.3 8.7 0.05 308 3 Miscanthus 6 50 36.2 77.8 5.4 12.8 2.1 375 4 Barley 5 60 34.4 76.7 6.1 11.7 2.2 367 5 Flax fire 6 50 34.3 85.2 0.19 8.5 0.06 635 6 Wood 6 50 36.5 90.4 0.3 6.2 0.0 284

Delignification conditions: temperature 103°C, GM -10, duration 3 h, catalyst 0.05 wt.% MgSO ₄

Alkaline treatment conditions: temperature 20°C, GM - 25, duration 3 hours, 4% NaOH solution

Acid hydrolysis conditions: _H2SO4 concentration - 10 _wt .%, GM - 20, duration 45 min at boiling temperature

Table 6. Yield and characteristics of microcrystalline cellulose (MCC) samples obtained from various types of cellulose-containing raw materials using the stages of treating lignocellulosic raw materials with hot water, catalytic peroxide delignification, alkaline treatment and sulfuric acid hydrolysis of the cellulose product

No. Raw materials Concentration
hydrogen peroxide, wt. % Concentration
acetic acid, wt. % Yield, wt. %
a.s. source raw materials Composition of MCC, wt. % SP Cellulose Lignin Hemicelluloses Ash 1 Wheat straw 6 40 30.0 91.0 0.7 6.9 0.7 241 2 Hemp straw 4 40 41.8 92.3 0.0 7.3 0.0 386 3 Miscanthus 6 50 29.7 82.2 4.0 11.8 1.8 416 4 Barley 5 60 31.8 83.4 2.9 9.6 2.1 312 5 Flax fire 6 50 32.4 86.3 0.12 8.3 0.06 533 6 Wood 6 50 35.8 91.9 0.04 6.9 0.0 293

Water treatment conditions: temperature 50°C, GM -40, duration 1 hour.

Delignification conditions: temperature 103°C, GM -10, duration 3 h, catalyst 0.05 wt.% MgSO ₄

Alkaline treatment conditions: temperature 20°C, GM - 25, duration 3 hours, 4% NaOH solution

Acid hydrolysis conditions: _H2SO4 concentration - 10 _wt .%, GM - 20, duration 45 min at boiling temperature

For MCC samples obtained from various types of cellulose-containing raw materials using the stages of processing lignocellulosic raw materials with hot water, catalytic peroxide delignification of washed raw materials and sulfuric acid hydrolysis of the cellulose product, according to examples 19-24 (Table 4), additional studies were carried out using IR, X-ray diffraction, laser diffraction, and SEM methods.

Fig. 7 shows the IR spectra of MCC obtained from different types of raw materials.

All IR spectra contain absorption bands (AB) characteristic of cellulose: 3400, 2900, 1430, 1160, 1105, 1055, 898 cm ^-1 , adsorbed water: 1633 cm ^-1 . Absorption bands characteristic of lignin: ~1600 cm ^-1 , 1506 cm ^-1 , 1465 cm ^-1 and hemicelluloses: 1730 cm ^-1 - are absent. Thus, the obtained MCC has a high degree of purification from lignin and hemicelluloses.

The crystallinity index calculated from diffraction patterns using the Segal method [43] has fairly high values (Table 7).

Table 7. Crystallinity indices of MCC obtained from different types of cellulose-containing raw materials

No. Sample Crystallinity index 1 MCC from wheat straw 0.70 2 MCC from hemp straw 0.86 3 MCC from miscanthus straw 0.77 4 MCC from barley straw 0.78 5 MCC from flax shives 0.83 6 MCC made of wood 0.86

The maximum value of the crystallinity index among the obtained MCCs is observed in MCCs made from hemp straw and wood - 0.86. Figs. 8 and 9 show the diffraction patterns of these samples.

Using the laser diffraction method, data were obtained on the volume distribution of particles by size in MCC samples obtained from various types of cellulose-containing raw materials (Table 8).

Table 8. Particle size distribution of MCC samples

No. Sample Proportion of particles with a size of no more than by percentiles Particle size, µm 1 MCC of wheat straw D03 8.4 D10 15.3 D50 54.7 D75 104.6 D97 173.1 2 MCC hemp straw D03 4.0 D10 7.5 D50 24.2 D75 45.5 D97 146.9 3 MCC wood D03 12.8 D10 24.1 D50 90.0 D75 178.3 D97 485.1 4 MCC of miscanthus straw D03 6.3 D10 10.3 D50 28.3 D75 51.2 D97 163.9 5 MCC of barley straw D03 8.0 D10 14.6 D50 55.3 D75 106.9 D97 319.5 6 MCC flax fires D03 10.6 D10 18.0 D50 76.08 D75 163.7 D97 645.5

The average particle size (D50) of MCC obtained from wheat straw is ~55 µm, MCC from hemp straw ~24 µm, MCC from wood ~90 µm, MCC from miscanthus straw ~28 µm, MCC from barley straw ~55 µm, MCC from flax shives ~76 µm.

Using SEM, some morphological characteristics of the obtained MCCs were established.

Table 9. Morphological characteristics of MCC

No. Sample Parameter Established characteristic 1 MCC of wheat straw (Fig. 10) Length 50-250 microns Diameter 20-100 microns Particle shape long fibers, both aggregated and separated 2 MCC of hemp straw (Fig. 11) Length 25-100 microns Diameter 5-20 microns Particle shape shortened separated fibers 3 MCC wood (Fig. 12) Length 100-600 microns Diameter 20-50 microns Particle shape both aggregated and separated fibers 4 MCC of miscanthus straw (Fig. 13) Length 50-150 microns Diameter 15-50 microns Particle shape 2 factions:
separated fibers, irregularly shaped particles 5 MCC of barley straw (Fig. 14) Length 100-250 microns Diameter 20-100 microns Particle shape long fibers, both aggregated and separated 6 MCC flax fires (Fig. 15) Length 500-800 microns Diameter 100-300 microns Particle shape long aggregated fibers

Based on their characteristics, cellulose products obtained from wheat straw, hemp, oats, miscanthus and wood using multi-stage methods belong to the class of microcrystalline celluloses.

Thus, a simple and environmentally friendly method for producing powdered cellulose material (powdered cellulose and microcrystalline cellulose) from cellulose-containing raw materials (wheat straw, hemp, oats, miscanthus, flax shives, and wood) is proposed. Compared to the prior art, the proposed invention facilitates easier separation of the catalyst from the cellulose product, improves the quality of the cellulose product due to the absence of catalyst impurities, and reduces catalyst costs while maintaining a high cellulose product yield.

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Claims (6)

1. A method for producing powdered cellulose material from cellulose-containing raw materials, including delignifying treatment of lignocellulosic raw materials with an aqueous solution containing hydrogen peroxide, acetic acid and a catalyst, at the boiling temperature of the mixture, atmospheric pressure for 2.5-3.5 hours with vigorous stirring, followed by isolation of the cellulose product, characterized in that magnesium sulfate is used as a catalyst in an amount of 0.5-1.0% of the weight of the absolute dry raw material, while the treatment is carried out in an aqueous solution containing 4-6 wt. % hydrogen peroxide, 40-60 wt. % acetic acid, with a hydromodulus of 10. 2. A method for producing powdered cellulose material from cellulose-containing raw materials, including delignifying treatment of lignocellulosic raw materials with an aqueous solution containing hydrogen peroxide, acetic acid and a catalyst, at the boiling temperature of the mixture, atmospheric pressure for 2.5-3.5 hours, with intensive stirring, followed by isolation of the cellulose product, characterized in that the lignocellulosic raw materials are preliminarily treated with boiling water for 1 hour at a hydromodulus of 30, then delignifying treatment of the washed lignocellulosic raw materials is carried out with an aqueous solution containing 4-6 wt. % hydrogen peroxide, 40-60 wt. % acetic acid, magnesium sulfate in an amount of 0.5-1.0% of the weight of the absolute dry raw materials, at a hydromodulus of 10. 3. A method for producing a powdered cellulose material from a cellulose-containing raw material, which includes delignifying treatment of the lignocellulosic raw material with an aqueous solution containing hydrogen peroxide, acetic acid and a catalyst, at the boiling temperature of the mixture, atmospheric pressure for 2.5-3.5 hours, with intensive stirring, followed by isolation of the cellulose product, characterized in that the delignifying treatment of the lignocellulosic raw material is carried out with an aqueous solution containing 4-6 wt. % hydrogen peroxide, 40-60 wt. % acetic acid, magnesium sulfate in an amount of 0.5-1.0% of the absolute dry raw material weight, at a hydromodulus of 10, and hydrolysis of the resulting cellulose product is carried out with a boiling solution of 2 _N sulfuric acid for 45 minutes at a hydromodulus of 20. 4. A method for producing a powdered cellulose material from a cellulose-containing raw material, which includes delignifying treatment of the lignocellulosic raw material with an aqueous solution containing hydrogen peroxide, acetic acid and a catalyst, at the boiling temperature of the mixture, atmospheric pressure for 2.5-3.5 hours, with intensive stirring, followed by isolation of the cellulose product, characterized in that the lignocellulosic raw material is preliminarily treated with boiling water for 1 hour at a hydromodulus of 30, delignifying treatment of the washed lignocellulosic raw material is carried out with an aqueous solution containing 4-6 wt. % hydrogen peroxide, 40-60 wt. % acetic acid, magnesium sulfate in an amount of 0.5-1.0% of the weight of the absolute dry raw material, and hydrolysis of the obtained cellulose product is carried out with a boiling solution of 2 _N sulfuric acid for 45 minutes at a hydromodulus of 20. 5. A method for producing powdered cellulose material from cellulose-containing raw materials, including delignifying treatment of lignocellulosic raw materials with an aqueous solution containing hydrogen peroxide, acetic acid and a catalyst, at the boiling temperature of the mixture, atmospheric pressure for 2.5-3.5 hours, with intensive stirring, followed by isolation of the cellulose product, characterized in that the delignifying treatment of lignocellulosic raw materials is carried out with an aqueous solution containing 4-6 wt. % hydrogen peroxide, 40-60 wt. % acetic acid, magnesium sulfate in the amount of 0.5-1.0% of the absolute dry raw material weight, with a hydromodulus of 10, then the resulting cellulose product is bleached in an alkaline medium with 4% NaOH at 25 °C for 3 hours with a hydromodulus of 25 and hydrolysis of the bleached cellulose product is carried out with a boiling solution of 2 _H H ₂ SO ₄ for 45 minutes with a hydromodulus of 20. 6. A method for producing powdered cellulose material from cellulose-containing raw materials, including delignifying treatment of lignocellulosic raw materials with an aqueous solution containing hydrogen peroxide, acetic acid and a catalyst, at the boiling temperature of the mixture, atmospheric pressure for 2.5-3.5 hours, with intensive stirring, followed by isolation of the cellulose product, characterized in that the lignocellulosic raw materials are preliminarily treated with boiling water for 1 hour at a hydromodulus of 30, then delignifying treatment of the washed lignocellulosic raw materials is carried out with an aqueous solution containing 4-6 wt. % hydrogen peroxide, 40-60 wt. % acetic acid, magnesium sulfate in an amount of 0.5-1.0% of the absolute dry raw material weight, with a hydromodulus of 10, then the resulting cellulose product is bleached in an alkaline medium with 4% NaOH at 25 °C for 3 hours with a hydromodulus of 25 and hydrolysis of the bleached cellulose product is carried out with a boiling solution of 2 _H H ₂ SO ₄ for 45 minutes with a hydromodulus of 20.