An efficient green process for the preparation of nanocellulose, a novel modified nanocellulose and its applications

Abstract

The present invention relates to an efficient method for preparing nanocellulose using a mixture of ammonium formate and at least one acid as a reactant and solvent, a novel modified nanocellulose and its applications.

Description

[Technical Field]

[0001] The present invention relates to an efficient method for preparing nanocellulose using a mixture of ammonium formate and at least one acid as a reactant and solvent, a novel modified nanocellulose and its applications. [Background technology]

[0002] Cellulose, a linear polymer of β(1,4)-linked D-glucose units, is the most available polymer on earth and is used in numerous applications due to its biocompatibility, non-toxicity, and exceptional mechanical properties. Isolation of cellulose, especially from plant fibers, typically involves chemical processing consisting of alkaline extraction and bleaching.

[0003] In the past few decades, the preparation and novel applications of so-called nanocellulose have attracted great interest. The term "nanocellulose" is frequently used to describe a cellulose material with at least one dimension at the nanoscale. This unique combination of the properties of cellulose and the characteristics of nanomaterials has opened up new areas in materials science.

[0004] Today, there are three main types of nanocellulose materials: bacterial nanocellulose (BNC), mechanically exfoliated cellulose nanofibers (CNF), and hydrolytically extracted cellulose nanocrystals (CNC) (see the following overviews: Klemm et al., "Nanocelluloses: A New Family of Nature-Based Materials", Angew. Chem. Int. Ed. 2011, 50, pp. 5438 to 5466; A. Dufresne, "Nanocellulose: a new ageless bionanomaterial", Materials Today, Vol. 16, No. 6, 2013 and Klemm et al., "Nanocellulose as a natural source for groundbreaking applications in materials science: Today's state", Materials Today, Vol. 21, Number 7, 2018).

[0005] Although their production is very costly due to low space-time yields, BNCs are typically obtained with very high purity, i.e., they are applicable to pharmaceutical applications without laborious purification steps. The most commonly used bacteria are acetic acid bacteria of the genus Gluconacetobacter. During biosynthesis, cellulose chains are produced and aggregate into fibrils with cross-sectional dimensions typically ranging from 2 to 20 nm and a degree of polymerization of 4,000 to 10,000 glucose units. These fibrils usually exhibit a small number of defects or amorphous domains.

[0006] CNFs are most commonly and on a large scale produced from delignified and preferably bleached pulp. Mechanical delamination of the fibers is carried out using techniques such as high-pressure homogenizers, microfluidizers, conventional refiners, high-speed blenders and extruders, or ball mills, steam explosion, and ultrasonication. These methods are very simple but require high energy input, damage the fibers, and produce CNFs with a wide distribution of fibril diameters and lengths. Typically, CNFs exhibit diameters of 5 to 60 nm and lengths of 100 nm to 10 mm, with a degree of polymerization of 500 or greater.

[0007] The isolation of CNCs from wood pulp and cotton via acid hydrolysis using sulfuric acid was first reported in the 1940s. It is well understood that the acid decomposes the more accessible and / or irregular cellulose domains, leaving the highly crystalline domains intact. CNCs typically have dimensions of 100 to 250 nm in length and 5 to 70 nm in diameter, with a degree of polymerization of 500 to 15,000.

[0008] Novel methods for isolating CNCs include oxidation and hydrolysis using acids such as hydrochloric, hydrobromic, citric, and phosphoric acid. The choice of acid directly affects the colloidal and thermal stability, size, and surface charge of CNCs. For example, phosphoric and hydrochloric acid hydrolysis produces CNCs with low or no charge content, and the CNCs typically aggregate but have high thermal stability. Therefore, it is important to optimize the reaction conditions for each isolation step to ensure stable and predictable nanomaterials are prepared. The most common starting materials for CNCs are wood pulp and cotton, algae, bacteria, and urochordates, as well as waste materials such as coconut shells, rice husks, and banana pseudostem.

[0009] However, despite their enormous potential in various applications, the major drawback of the commercial implementation of nanocellulose is its very high energy consumption, and their poor long-term stability and shelf life, especially for CNFs and CNCs, have also been found to present significant problems.

[0010] As a result, various attempts have been made to address these problems.

[0011] These attempts include mechanical cleavage, acid hydrolysis, enzymatic pretreatment, and pretreatments such as the introduction of charged groups through carboxymethylation or 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidation, which aid in degradation through electrostatic repulsion. (See Klemm et al., Nanocellulose as a natural source for groundbreaking applications in materials science: Today's state, Materials Today, Vol. 21, Number 7, 2018, and references cited in U.S. Patent Application No. 2014 / 0155301, U.S. Patent Application No. 2015 / 0171679, and Chinese Patent No. 102180979.)

[0012] A similar approach was disclosed by K. Watanabe et al., Cytotechnology 13 (1993) 107-114, in which cellulose was chemically modified by introducing cationic surface charges such as trimethylammonium hydroxypropyl groups, diethylaminoethyl groups, aminoethyl groups, and carboxymethyl groups.

[0013] More advanced approaches include pretreatment or preparation methods using ionic liquids or deep eutectic solvents as reaction media (an overview is provided in H. Tadesse and R. Luque, "Advances on biomass pretreatment using ionic liquids", Energy Environ. Sci., 2011, 4, 3913).

[0014] In Li et al., "Recyclable deep eutectic solvent for the production of cationic nanocelluloses," Carbohydrate Polymers, Vol. 199, No. 1, 2018, pp. 219-227, a novel modified nanocellulose bearing guanidinium groups is disclosed. It is prepared by a two-step procedure involving the cationization of dialdehyde cellulose using aminoguanidine hydrochloride and glycerol, a deep eutectic solvent acting as reagent and reaction medium, followed by mechanical degradation. The starting material, dialdehyde cellulose, is prepared by oxidation of cellulose (bleached kraft birch pulp) with sodium periodate.

[0015] The oxidation and modification procedures are carried out using expensive chemicals and weaken the mechanical strength of cellulose, thus hindering its commercial application.

[0016] The use of ammonium formate as both a reagent and a reaction medium to convert carbohydrates into valuable fine chemicals is known from S. Filonenko, A. Voelkel and M. Antonietti, “Valorization of monosaccharides towards fructopyrazines in a new sustainable and efficient eutectic medium,” Green Chem., 2019, 21, 5256. [Prior art documents] [Chartered documents]

[0017]

Patent Document 1

Patent document 2

Patent document 3

Non-licensed literature

[0018] [Non-licensed document 1] Klemm et al., Nanocelluloses: A New Family of Nature-Based Materials", Angew. Chem. Int. Ed. 2011, 50, p. 5438 to 5466 [Non-licensed document 2] A. Dufresne, Nanocellulose: a new ageless bionanomaterial“, Materials Today, Vol. 16, No. 6, 2013 [Non-licensed document 3] Klemm et al., Nanocellulose as a natural source for groundbreaking applications in materials science: Today's state", Materials Today, Vol 21, Number 7, 2018

Non-licensed Document 4

Non-licensed Document 5

[0019] Despite the above-mentioned advances, there remains a need to provide an efficient, green process for preparing nanocellulose materials that does not require the use of toxic or hazardous reagents and that starts from readily available compounds.

[0020] A further object of the present invention was to provide nanocellulose with improved stability, i.e. with reduced tendency to irreversibly aggregate when applied as a dispersion or colloid. [Means for solving the problem]

[0021] In one aspect of the invention, there is provided a method for preparing nanocellulose, comprising at least the following steps: a) providing a mixture comprising i) ammonium formate, ii) at least one acid, and iii) at least one cellulose-containing feedstock; b) heating the mixture provided in step a) at a reaction temperature of 100°C or higher; A method is provided, comprising:

[0022] Further aspects of the present invention include nanocellulose obtained by the above-mentioned method and uses thereof. [Brief explanation of the drawings]

[0023] [Figure 1] FIG. 1 shows a TEM image of nanocellulose prepared from microcrystalline cellulose according to Example 1 above. [Figure 2] FIG. 2 shows a TEM image of nanocellulose prepared from microcrystalline cellulose according to Example 1 above. [Figure 3] FIG. 3 shows a TEM image of nanocellulose prepared from softwood pulp according to Example 6 above. [Figure 4] FIG. 4 shows a TEM image of nanocellulose prepared from softwood pulp according to Example 6 above. [Figure 5] FIG. 5 shows a TEM image of nanocellulose prepared from delignified pulp according to Example 11 above. [Figure 6] FIG. 6 shows a TEM image of nanocellulose prepared from delignified pulp according to Example 11 above. DETAILED DESCRIPTION OF THE INVENTION

[0024] The present invention includes all combinations of preferred embodiments, range parameters disclosed herein below, with each other or with the broadest ranges or parameters disclosed.

[0025] The terms "including," "for example," "eg," "such as," and "like" as used herein are meant to mean "including but not limited to" or "for example, without limitation," respectively.

[0026] The term "nanocellulose," as used herein, refers to polymeric particles comprising β(1,4) linked D-glucose units having at least one dimension less than 1000 nm and an average degree of polymerization of at least 50 D-glucose units. These nanocelluloses may or may not be chemically derivatized.

[0027] In one embodiment, the average degree of polymerization is from 100 to 15,000, preferably from 200 to 10,000.

[0028] For the avoidance of doubt in the specification, "having at least one dimension less than 1000 nm" includes particles having an average cross section in the range of from 3 to 200 nm, preferably in the range of from 5 to 100 nm, more preferably in the range of from 5 to 30 nm, and most preferably in the range of from 5 to 20 nm, and an average length in the range of from 15 to 5000 nm, preferably in the range of from 50 to 1000 nm, more preferably in the range of from 70 to 800 nm.

[0029] In one embodiment, the aspect ratio, i.e. the ratio between the length and cross section of the nanocellulose, is greater than 1, preferably greater than or equal to 2, more preferably between 2 and 100 or between 2 and 50.

[0030] In step a) of the method, a mixture is provided that includes i) ammonium formate, ii) at least one acid, and iii) at least one cellulose-containing feedstock.

[0031] Suitable acids include organic acids, such as organic compounds having one, two, or three carboxylic acid (-COOH) or sulfonic acid groups, and inorganic acids, such as sulfuric acid, hydrohalic acids, perhalogen acids, and phosphoric acid.

[0032] Preferred acids are mono- and dicarboxylic acids such as formic acid, acetic acid, propionic acid, glycolic acid, lactic acid, oxalic acid, levulinic acid, malonic acid, succinic acid, malic acid, maleic acid and adipic acid, of which formic acid, propionic acid, glycolic acid, lactic acid, levulinic acid and succinic acid are even more preferred.

[0033] A key finding of the present invention is that by mixing ammonium formate and an organic acid, the melting point of the mixture is significantly lowered compared to the single components, allowing the mixture to function simultaneously as both a reagent and a solvent without the need for the addition of additional solvents. These so-called deep eutectic mixtures facilitate the handling and increase the solubility of cellulose-containing raw materials.

[0034] In one embodiment, the molar ratio of ammonium formate to the total acid is, for example, from 0.2 to 1000, preferably from 0.5 to 10.0, more preferably from 1.0 to 5.0, and even more preferably from 2.0 to 2.5.

[0035] Higher and lower molar ratios are possible in principle, but do not offer any advantage.

[0036] Thus, the present invention encompasses the use of ammonium formate and mixtures thereof with organic acids to prepare nanocellulose.

[0037] The mixture provided in step a) further comprises a cellulose-containing raw material.

[0038] As used herein, cellulose-containing feedstock includes any feedstock that contains cellulose, whether or not bound to lignin and / or hemicellulose and / or other structural building blocks.

[0039] Examples include: Included are microcrystalline cellulose, microbial cellulose, marine or other invertebrate, recycled or waste paper such as office waste and municipal waste, bleached or unbleached softwood and hardwood pulp, chemical (dissolving) pulp, delignified pulp, wood pulp such as pulp rejects, native biomass in the form of plant fibers, cellulose derived from wood chips, sawdust, straw, leaves, stems or husks, cellulosic synthetic fibers such as tire cord, and other sources of cellulose such as mercerized cellulose. Further examples include bagasse, miscanthus and bamboo.

[0040] The cellulose-containing raw materials may or may not be chemically derivatized by, for example, carboxymethylation, carboxylation, oxidation, sulfation, or esterification.

[0041] The cellulose-containing feedstock may or may not have been mechanically pretreated, for example, by cutting, peeling, high-pressure homogenization, sonication, or other known methods, or may or may not have been pretreated by enzymatic hydrolysis.

[0042] However, in one embodiment, the cellulose-containing feedstock is not chemically derivatized, enzymatically or mechanically pretreated.

[0043] Specific examples of cellulose-containing feedstocks include bleached softwood pulp, microcrystalline cellulose such as Avicel PH-101, and pulp obtained from uncoated delignified paper.

[0044] In one embodiment, the mass ratio of the cellulose-containing feedstock to the sum of ammonium formate and the at least one acid is, for example, from 0.001 to 1, preferably from 0.01 to 0.25, more preferably from 0.02 to 0.20, and even more preferably from 0.03 to 0.10.

[0045] Unless otherwise stated, amounts of cellulose-containing raw materials are calculated and given based on their dry weight, even though they typically contain varying amounts of (residual) water.

[0046] It has been found that the reaction is not very sensitive to the presence of water, and as a result, a certain amount of water in the reaction mixture provided in step a) is tolerable.

[0047] Thus, in one embodiment, the sum of ammonium formate, at least one acid, cellulose-containing raw material and water is from 80 to 100% by weight, preferably from 90 to 100% by weight and in another embodiment from 95 to 100% by weight, based on the total weight of the mixture provided in step a), the remainder being impurities typically from the applied starting materials.

[0048] Providing the reaction mixture containing the above-mentioned compounds can be carried out in any manner, in any order of addition, and in any container known to those skilled in the art that allows for the reaction defined above.

[0049] In step b), the reaction temperature is 100°C, preferably 140°C or higher, more preferably 155°C or higher.

[0050] In one embodiment, the reaction temperature ranges from 100°C to 190°C, preferably from 140°C to 185°C, more preferably from 155°C to 180°C, in particular 160°C, 170°C or 180°C.

[0051] Ammonium formate is known to begin decomposing at temperatures above 180°C, and although higher temperatures are possible, they may increase the formation of undesirable by-products such as formamide. At temperatures below 100°C, the reaction is too slow to efficiently yield the desired nanocellulose.

[0052] The pressure conditions are not particularly limited, and the pressure in step b) can be 500 hPa to 50 MPa, preferably 1000 hPa to 1 MPa. Due to the possibility of decomposition of ammonium formate and the formation of low-boiling components such as water, formic acid or other organic acids, however, the reaction in step a) is carried out under pressure, i.e., under constant volume or near constant volume conditions, by confining the reaction mixture to build up and heating to the desired temperature.

[0053] The process of the present invention, in particular step b) thereof, can be carried out in any vessel or reactor suitable for the purpose and known to those skilled in the art.

[0054] Preferably, the reaction is carried out in an autoclave or in a reactor capable of carrying out the process under constant volume or near constant volume conditions.

[0055] The reaction time is, for example, at least 30 minutes, preferably at least 90 minutes, more preferably at least 2 hours.

[0056] In one embodiment, the reaction time is from 60 minutes to 48 hours, preferably from 90 minutes to 12 hours, more preferably from 2 to 4 hours.

[0057] Longer reaction times are possible but do not add any substantial benefit, and shorter reaction times may decrease the desired nanocellulose yield.

[0058] In step b), a reaction mixture containing the desired nanocellulose is obtained. To isolate the nanocellulose, water, formic acid and other acids, and volatile by-products such as formamide, if present, can be removed by simple washing with water and / or alcohol, or by distillation, fractionation, or in vacuo.

[0059] Nanocellulose can be redispersed in water by vortex mixing or sonication to form a colloid, dispersion or suspension encompassed by the present invention.

[0060] If desired, formic acid and other acids and excess ammonium formate can be recycled to step a).

[0061] The nanocellulose obtained by the method of the present invention exhibits a higher zeta potential compared to mechanically prepared nanocellulose and a higher nitrogen content indicating chemical modification of at least the reducing ends of at least some of the cellulose chains within the nanocellulose, and is therefore novel and encompassed by the present invention.

[0062] Without wishing to be bound by theory, it is believed that in step b) ammonium formate reacts with at least some of the reducing ends of the cellulose chains to form repeating units of formula (I) typical of cellulose as a polymer of β(1,4) linked D-glucose units. [ka] and a terminal unit of formula (II) [ka] It is presumed that the cellulose polymer containing

[0063] Due to the fact that cellulose-containing raw materials, depending on their origin, typically contain more or less structural defects, and due to the fact that oxygen is typically not removed during the treatment and / or reaction steps a and b), further amino groups may be introduced into the cellulose chains of the nanocellulose, via reductive amination of aldehyde groups already present or generated by partial oxidation, during reaction step b), or into cellulose chains that exhibit the typical nitrogen content observed for the nanocellulose of the invention as defined below.

[0064] As a macro effect resulting from the amination, the nanocellulose of the present invention generally exhibits high stability when dispersed in water or as a colloid: these dispersions and colloids are stable even after two weeks of storage at room temperature without forming significant amounts of gel.

[0065] Nanocellulose further exhibits a high degree of crystallinity.

[0066] The zeta potential of the nanocellulose of the present invention, as measured by the method described in the Examples below, typically ranges from 2.0 to 50.0 mV, preferably from 5.0 to 40.0 mV, and more preferably from 8.0 to 35.0 mV.

[0067] The nitrogen content of the nanocellulose, as determined by elemental analysis according to the method described in the Examples below, is typically 0.2 to 2.0% by weight, preferably 0.3 to 1.8% by weight.

[0068] The crystallinity of the nanocellulose, as measured by X-ray diffraction according to the method described in the Examples below, typically ranges from 70% to 100%, preferably from 75 to 100%.

[0069] The degree of polymerization of nanocellulose strongly depends on the cellulose-containing raw material, but is typically between 100 and 15,000 glucose units, and in another embodiment, between 500 and 5,000.

[0070] The nanocellulose of the present invention, and colloids, dispersions and suspensions comprising same, are useful in a wide range of applications, including their use in foods and beverages as additives such as low-calorie additives, thickeners, stabilizers such as foam stabilizers and texture modifiers, and as microcapsules or coatings to protect aroma and flavor.

[0071] They are further useful in technical applications such as membranes for fuel cells and supercapacitors, as conductive membranes, vibration films in loudspeakers, in or as packaging materials, in water absorption or purification, as hydrogel beads for removing aqueous dyes, water filtration membranes, nanocomposite heavy metal sensors, aerogels, flocculants and nanocomposite fillers for groundwater remediation, as reinforcing additives for synthetic polymers such as thermoplastics and elastomers.

[0072] Further technical applications include paper / board coating and reinforcing applications, additives for paints, adhesives, latex and cement, stimulating, drilling, finishing and spacer fluids, where the novel nanocellulose acts as a stabilizer, thickener, shear thinning agent, proppant or reinforcing agent.

[0073] Other applications include their use in cosmetic or pharmaceutical compositions and in biomedical applications, such as for drug delivery, tissue engineering, bone recovery materials, biosensors, bioadhesives and microcapsules.

[0074] The present invention encompasses foods, beverages, membranes, films, packaging materials, water absorption or purification materials, heavy metal sensors, aerogels, flocculants, reinforced synthetic polymers, paper, board, paints, adhesives, latex, cement, stimulation fluids, drilling fluids, completion fluids, spacer fluids, cosmetic or pharmaceutical compositions, tissue and bone repair materials, biosensors and bioadhesives comprising nanocellulose according to the present invention or colloids, suspensions or dispersions thereof.

[0075] A major advantage of the present invention is that it provides a highly efficient and green process for preparing nanocellulose and novel nanocellulose that is capable of forming highly stable dispersions and colloids.

[0076] The present invention will be described below with reference to examples, but the scope of the present invention is not intended to be limited thereto. [Example]

[0077] General information: material: Ammonium formate (≥98%) was purchased from Alfa Aesar, glycolic acid (≥98%) from Alfa Aesar, propionic acid (99.5%) from Fluca, levulinic acid (98+%) from Acros Organics, succinic acid (99.5%) from Roth, and lactic acid (90% by weight in water) from Acros Organics.

[0078] Unless otherwise stated, all chemicals were used as received without further purification.

[0079] Characterization elemental analysis Elemental analysis (EA) was performed using a vario MICRO cube CHNOS Elemental Analyzer (Elementar Analysensysteme GmbH, Langenselbold). Elements were detected using a thermal conductivity detector (TCD) for C, H, N, and O, and an infrared detector (IR) for sulfur. Each sample was measured twice, and the average value was calculated.

[0080] Zeta potential Electrophoretic light scattering-based zeta potential was measured using a Malvern Instruments Zetasizer Nano ZS (Malvern, United Kingdom). After centrifugation, the wet sample was diluted with distilled water to obtain a 1% (nano-)cellulose suspension. The suspension was placed in a disposable collapsible capillary cell (DTS1070). The electrophoretic mobility of the (nano-)cellulose suspension was measured and converted to zeta potential according to the Smoluchowski equation using Malvern software. Zeta potential measurements were reported as sample averages from triplicates with 95% confidence.

[0081] TEM images Transmission electron microscopy (TEM) images were recorded on a Zeiss Libra 912 microscope operated at 120 kV. To enhance image contrast, negative staining with 1% uranyl acetate dissolved in distilled water was applied.

[0082] Crystallinity The crystallinity of nanocellulose was calculated from the XRD data as the ratio of the maximum intensity of the (002) lattice diffraction (22.8°) to the intensity of the amorphous region at the same unit (18.6°), see Segal et al., Textile Research Journal, October 1959, pp. 786 to 794.

[0083] II. Preparation of nanocellulose Experimental Method A. Preparation of low melting point mixture To obtain a low-melting-point mixture for use as a solvent and reactant, dry ammonium formate (AF) was mixed with an organic acid in a 2:1 molar ratio. The mixture was either ground in a mortar or thoroughly mixed in a glass beaker. Visual formation of the desired low-melting-point mixture was confirmed when the mixture gradually liquefied under grinding / mixing. To promote its formation, the mixture was kept at 60°C in a sealed glass bottle under constant stirring for at least 2 hours or until the crystals had completely disappeared.

[0084] B. Cellulose-containing raw material used in the reaction SP: bleached softwood kraft pulp obtained from Mercer Pulp digested in deionized water overnight under constant agitation at room temperature. DP: 5g of uncoated premium delignified paper purchased from Inapa Deutschland was cut into approximately 1cm pieces with scissors. 2 The mixture was cut into square pieces and placed in a 1 L glass bottle. 1 L of deionized water was added, and the contents were stirred overnight. The resulting pulp was filtered through a glass funnel filter and washed successively with deionized water and ethanol on the filter. The washed pulp was dried at 60°C for 24 hours. MC: commercially obtained microcrystalline cellulose (Avicel PH-101, 100% cellulose content) was used.

[0085] C. Reaction conditions The cellulose-containing raw material was added to a corresponding low-melting mixture of ammonium formate and acid in a glass beaker. The resulting reaction mixture was transferred to a Teflon beaker. Further reactions were carried out in autoclaved reactors under static conditions (i.e., without stirring) or under stirring, as described below. Static: The Teflon® beaker containing the reaction mixture was sealed with a Teflon® cap and placed in a stainless steel Parr reactor (autoclave). The autoclave was held at 180°C for 4 hours. The reaction was stopped by cooling the autoclave in an ice bath and the resulting product mixture was transferred to a glass beaker. Stirring: The Teflon® beaker containing the reaction mixture was sealed with a Teflon® gasket. The beaker was placed in a stainless steel high-pressure benchtop reactor with an internal stirring system. The reactor was heated to 180°C and held at that temperature for 4 hours unless otherwise specified in Table 1). The reaction mixture was stirred at 200 rpm. The reaction was stopped by cooling the reactor with a water cooling system. After the reactor was cooled to room temperature, the product mixture was transferred to a glass beaker.

[0086] The composition of the reaction mixture, the cellulose-containing raw materials used, as well as the reaction conditions for preparing nanocellulose of the present invention are summarized in Table 1: [Table 1]

[0087] D. Cleaning The product mixture obtained in section c) was diluted with a few mL of distilled water, mixed with a spatula, and sonicated for 30 minutes in a laboratory sonicator. This was followed by centrifugation at 10,000 RPM for 5 minutes in an Avanti JE centrifuge (Beckman Coulter) equipped with a JA-25.50 fixed-angle rotor to precipitate the colloids, and the supernatant was removed. The precipitate was washed in the following order: redispersion in water, vortexing for 30 seconds, sonication for 30 minutes, centrifugation at 10,000 RPM for 5 minutes (the last three runs were at 20,000 RPM), and decantation of the wash fluid. This process was repeated four times with water and twice with ethanol until a clear wash solution was obtained. The ethanol was replaced with water, the sample was centrifuged at 25,000 RPM, the supernatant liquid was decanted, and the final product was freeze-dried to obtain a white to beige powder.

[0088] The properties of the nanocellulose obtained in Examples 1 to 26 are summarized in Table 2. [Table 2]

[0089] Figures 1 and 2 show TEM images of nanocellulose prepared from microcrystalline cellulose according to Example 1 above, resulting in whiskers with dimensions of 10 to 20 nm in diameter and up to 200 nm in length.

[0090] Figures 3 and 4 show TEM images of nanocellulose prepared from softwood pulp according to Example 6 above.

[0091] Figures 5 and 6 show TEM images of nanocellulose prepared from delignified pulp according to Example 11 above.

[0092] All nanocelluloses obtained according to the invention showed high stability of their aqueous colloids for at least two weeks, clearly indicating further stabilization through the formation of amino groups at the reducing ends of the cellulose chains, which leads to an increase in the nitrogen level in the nanocellulose according to the invention.

Claims

1. A method for preparing nanocellulose, comprising at least the following steps: a) providing a mixture comprising i) ammonium formate, ii) at least one acid, and iii) at least one cellulose-containing feedstock; b) heating the mixture provided in step a) at a reaction temperature of 100°C or higher; A method comprising:

2. 2. The method of claim 1, wherein the nanocellulose represents polymeric particles having at least one dimension less than 1000 nm and comprising β(1,4) linked D-glucose units with an average degree of polymerization of at least 50 D-glucose units.

3. 3. The method of claim 1 or 2, wherein the at least one acid comprises an organic acid as well as an inorganic acid.

4. 4. The method according to any one of claims 1 to 3, wherein the at least one acid is selected from mono- and dicarboxylic acids.

5. 5. The method of claim 1, wherein the molar ratio of ammonium formate to the sum of the acids is from 0.2 to 1000.

6. 6. The method of any one of claims 1 to 5, wherein the cellulose-containing raw material is selected from microcrystalline cellulose, microbial cellulose, invertebrates, recycled or waste paper, wood pulp, chemical (dissolving) pulp, delignified pulp, pulp rejects, native biomass in the form of plant fibers, wood chips, sawdust, straw, cellulose derived from leaves, stems or husks, cellulosic synthetic fibers, mercerized cellulose, bagasse, miscanthus and bamboo.

7. 7. The method of any one of claims 1 to 6, wherein the cellulose-containing feedstock is or is not chemically derivatized by carboxymethylation, carboxylation, oxidation, sulfation, or esterification.

8. 8. The method of claim 1, wherein the cellulose-containing raw material is or is not mechanically pretreated or pretreated by enzymatic hydrolysis.

9. 9. The method of any one of claims 1 to 8, wherein the cellulose-containing raw material is selected from bleached softwood pulp, microcrystalline cellulose, and pulp obtained from uncoated delignified paper.

10. 10. The method according to claim 1, wherein the mass ratio of the cellulose-containing feedstock to the sum of the ammonium formate and the at least one acid, calculated on a dry mass basis, is from 0.001 to 1.

11. 11. The method according to any one of claims 1 to 10, wherein the sum of the ammonium formate, the at least one acid, the cellulose-containing feedstock and water is from 80 to 100% by weight, and in another embodiment from 95 to 100% by weight, based on the total weight of the mixture provided in step a).

12. 12. The method according to claim 1, wherein in step b) the reaction temperature is from 100°C to 190°C.

13. 13. The method according to any one of claims 1 to 12, wherein the pressure in step b) is from 500 hPa to 50 MPa.

14. 14. The method according to any one of claims 1 to 13, wherein the reaction time in step b) is at least 30 minutes.

15. 14. The method according to claim 1, wherein the reaction time in step b) is from 60 minutes to 48 hours.

16. 16. The method according to any one of claims 1 to 15, wherein the nanocellulose is isolated from the reaction mixture obtained in step b) by washing with water and / or alcohol or by removal of volatile substances.

17. 16. The method according to claim 1, wherein, if formic acid and other acids and excess ammonium formate are present, formic acid and other acids and excess ammonium formate are recycled into step a).

18. 1. Nanocellulose comprising at least some cellulose polymers, the cellulose polymers comprising repeating units of formula (I): 【Chemical 1】 and a terminal unit of formula (II) 【Chemistry 2】 Nanocellulose, including:

19. 19. The nanocellulose of claim 18, further comprising amino groups obtained via reductive amination of aldehyde groups already present or generated by partial oxidation of the cellulose polymer.

20. 20. Nanocellulose according to claim 18 or 19, having a zeta potential of 2.0 to 50.0 mV.

21. 21. Nanocellulose according to any one of claims 18 to 20, having a nitrogen content of 0.2 and 2.0% by mass.

22. 22. Nanocellulose according to any one of claims 18 to 21, having a crystallinity in the range of 70% to 100%, as measured by X-ray diffraction.

23. 23. Nanocellulose according to any one of claims 18 to 22, having a degree of polymerization of between 100 and 15,000 glucose units or between 500 and 5,000 glucose units.

24. 24. A suspension, dispersion or colloid comprising nanocellulose according to any one of claims 18 to 23.

25. 26. Use of the nanocellulose according to any one of claims 18 to 23 or the dispersion or colloid according to claim 24 in food and beverages, as membranes for fuel cells and supercapacitors, as conductive membranes, vibration films in loudspeakers, in or as packaging materials, in water absorption or purification, as a strengthening additive for synthetic polymers, for paper / board coating and strengthening applications, as an additive for paints, adhesives, latex and cement, as a stimulating fluid, drilling fluid, completion fluid and spacer fluid, in cosmetic or pharmaceutical compositions and in biomedical applications.

26. 25. Foods, beverages, membranes, films, packaging materials, water absorption or purification materials, heavy metal sensors, aerogels, flocculants, reinforced synthetic polymers, paper, board, paints, adhesives, latex, cement, stimulation fluids, drilling fluids, completion fluids, spacer fluids, cosmetic or pharmaceutical compositions, tissue and bone repair materials, biosensors and bioadhesives comprising the nanocellulose of any one of claims 18 to 23 or the dispersion or colloid of claim 24.

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