An efficient and green method for preparing nanocellulose, new modified nanocellulose and its application

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 a solvent, as well as to novel modified nanocellulose and applications thereof.

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 a solvent, as well as to novel modified nanocellulose and applications thereof. Background Art

[0002] Cellulose, a linear polymer of β(1,4)-linked D-glucose units, is the most readily available polymer on Earth and is used in a variety of applications due to its biocompatibility, non-toxicity, and excellent mechanical properties. The isolation of cellulose, particularly from plant fibers, typically involves chemical treatments consisting of alkaline extraction and bleaching.

[0003] Over the past decade, the preparation and novel applications of so-called nanocelluloses have attracted considerable interest. The term nanocellulose is often used for cellulose materials with at least one dimension on the nanoscale. Their unique combination of cellulose properties and the characteristics of nanomaterials has opened up new areas of materials science.

[0004] Today, there are three main types of nanocellulose materials: bacterial nanocellulose (BNC), mechanically layered cellulose nanofibers (CNF), and hydrolysis-extracted cellulose nanocrystals (CNC) (see the following reviews: Klemm et al., Nanocelluloses: A New Family of Nature-Based Materials, Angew. Chem. Int. Ed. 2011, 50, p. 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 the manufacturing cost of BNC is quite high due to the low space-time yield, BNC is usually obtained in such high purity that it can be used in medical applications, even without complicated purification procedures. The most commonly used bacteria are acetic acid bacteria of the genus Gluconacetobacter. During biosynthesis, cellulose chains are produced and aggregated in fibrils that usually have a cross-sectional size of 2 to 20 nm and a degree of polymerization of 4,000 to 10,000 glucose units. Such fibrils usually show a small amount of defects or amorphous domains.

[0006] CNF is most commonly and is produced on a larger scale from delignified and preferably bleached pulp. Mechanical delamination of the fibers is achieved, for example, by using high-pressure homogenizers, microfluidizers, conventional refiners, high-speed blenders, and extruders, or techniques such as ball milling, steam explosion, and ultrasound. These methods are very simple but require high energy input, damage the fibers, and produce CNF 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 more.

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

[0008] Newer methods for isolating CNCs involve oxidation and hydrolysis with acids such as hydrochloric acid, hydrobromic acid, citric acid, or phosphoric acid. The choice of acid directly affects the colloidal and thermal stability, size, and surface charge of the CNCs. For example, phosphoric acid and hydrochloric acid hydrolysates produce CNCs with low or no charge content, and CNCs are often aggregated but have higher thermal stability. Therefore, it is important to optimize the reaction conditions for each separation procedure to ensure the preparation of stable and predictable nanomaterials. The most common starting materials for CNCs are wood pulp and cotton, but also include algae, bacteria, and tunicates, as well as waste materials such as coconut shells, rice husks, and banana pseudostems.

[0009] However, despite the enormous potential of nanocellulose in various applications, the major drawback of its commercial application is the very high energy consumption, especially for CNF and CNC. Their poor long-term stability and storability are also found to be key issues.

[0010] Therefore, various attempts have been made to overcome these problems.

[0011] These attempts include pretreatments such as mechanical cleavage, acid hydrolysis, enzymatic pretreatment, and introduction of charged groups by carboxymethylation or 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO)-mediated oxidation to aid decomposition 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 therein, US 2014 / 0155301A1, US 2015 / 0171679, and CN 102180979B).

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

[0013] Other cutting-edge approaches include pretreatment or preparation methods using ionic liquids or deep eutectic solvents as reaction media (an overview is given 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, 1, 2018, p. 219-227, a new modified nanocellulose containing a guanidine group is disclosed, which is prepared by a two-step procedure, including cationization of dialdehyde cellulose with aminoguanidine hydrochloride and glycerol (a deep eutectic solvent, as a reagent and reaction medium), followed by mechanical decomposition. The starting material dialdehyde cellulose is prepared by oxidizing cellulose (bleached kraft birch pulp) with sodium periodate.

[0015] Oxidation and modification procedures use expensive chemicals and weaken the mechanical integrity of cellulose, preventing commercial applications.

[0016] The use of ammonium formate as a reagent and reaction medium for the conversion of 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.

[0017] Despite the aforementioned progress, there is still a need to provide an efficient and green method to prepare nanocellulose materials starting from readily available compounds without involving toxic or hazardous reagents.

[0018] Another object of the present invention is to provide nanocellulose with increased stability, ie reduced tendency to irreversible agglomeration when applied as a dispersion or colloid. Summary of the Invention

[0019] According to one aspect of the present invention, there is now provided a method for preparing nanocellulose, the method comprising at least the following steps:

[0020] a) providing a mixture comprising i) ammonium formate, ii) at least one acid and iii) at least one cellulose-containing raw material

[0021] b) heating the mixture provided in step a) at a reaction temperature of 100° C. or higher.

[0022] In other aspects, the present invention includes nanocellulose obtained by the above method and uses thereof. DETAILED DESCRIPTION

[0023] The present invention also includes all combinations of preferred embodiments, range parameters as disclosed below with each other or with the broadest disclosed range or parameter.

[0024] Whenever the terms "including", "for example", "eg", "such as" and "etc." are used herein, they mean "including but not limited to" or "for example but not limited to", respectively.

[0025] As used herein, the term nanocellulose refers to polymer particles comprising β(1,4) linked D-glucose units having an average degree of polymerization of at least 50 D-glucose units and at least one dimension less than 1000 nm. Such nanocellulose may or may not be chemically derivatized.

[0026] In one embodiment, the average degree of polymerization is from 100 to 15,000, preferably from 200 to 10,000.

[0027] For the avoidance of doubt, the specification "at least one dimension less than 1000 nm" includes particles having an average cross-section of 3 to 200 nm, preferably in the range of 5 to 100 nm, more preferably in the range of 5 to 30 nm and most preferably in the range of 5 to 20 nm, and an average length of 15 to 5000 nm, preferably in the range of 50 to 1000 nm, more preferably 70 to 800 nm.

[0028] In one embodiment, the aspect ratio, ie the ratio of the length to the cross-section of the nanocellulose, is greater than 1, preferably 2 or greater, more preferably 2 to 100 or 2 to 50.

[0029] In step a) of the process, a mixture comprising i) ammonium formate, ii) at least one acid and iii) at least one cellulose-containing raw material is provided.

[0030] Suitable acids include organic acids, such as organic compounds with one, two, or three carboxylic acid (—COOH) or sulfonic acid groups, and inorganic acids, such as sulfuric acid, hydrohalic acids, perhalic acids, and phosphoric acid.

[0031] 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, with formic acid, propionic acid, glycolic acid, lactic acid, levulinic acid and succinic acid being even more preferred.

[0032] The key discovery of the present invention is that the mixing of ammonium formate and an organic acid results in a mixture with a significantly lower melting point compared to the individual components, allowing the mixture to be used as both a reagent and a solvent without the need to add additional solvents. These so-called eutectic mixtures facilitate processing and increase the solubility of cellulose-containing raw materials.

[0033] In one embodiment, for example, the molar ratio between ammonium formate and the sum of acids is 0.2 to 1000, preferably 0.5 to 10.0, more preferably 1.0 to 5.0, and even more preferably 2.0 to 2.5.

[0034] Higher and lower molar ratios are possible in principle but offer no advantages.

[0035] Therefore, the present invention also includes the use of ammonium formate and a mixture thereof with an organic acid for preparing nanocellulose.

[0036] The mixture provided in step a) also comprises a cellulose-containing raw material.

[0037] As used herein, cellulose-containing feedstock includes any feedstock containing cellulose, whether or not in combination with lignin and / or hemicellulose and / or other structural building blocks.

[0038] Examples include microcrystalline cellulose, microbial cellulose, cellulose derived from marine or other invertebrates, recycled or waste paper (such as office and municipal waste), wood pulp (such as softwood pulp and hardwood pulp, whether bleached or not), chemical (dissolving) pulp, delignified pulp, pulp waste, natural biomass in the form of plant fibers, wood chips, sawdust, straw, leaves, stems or shells, and cellulosic synthetic fibers (such as tire cord) and other cellulose sources (such as mercerized cellulose). Other examples include bagasse, miscanthus and bamboo.

[0039] The cellulose-containing feedstock may or may not be chemically derivatized, for example by carboxymethylation, carboxylation, oxidation, sulfation, or esterification.

[0040] The cellulose-containing feedstock may or may not be mechanically pretreated, for example by cutting, fractionation, high pressure homogenization, sonication, or other known methods, or pretreated by enzymatic hydrolysis.

[0041] However, in one embodiment, the cellulose-containing feedstock has not been chemically derivatized, enzymatically or mechanically pretreated.

[0042] Specific examples of cellulose-containing raw materials include bleached softwood pulp, microcrystalline cellulose (such as Avicel PH-101), and pulp obtained from uncoated delignified paper.

[0043] In one embodiment, for example, the weight ratio of the cellulose-containing feedstock to the sum of ammonium formate and the at least one acid is 0.001 to 1, preferably 0.01 to 0.25, more preferably 0.02 to 0.20, even more preferably 0.03 to 0.10.

[0044] Unless specifically stated otherwise, the amounts of cellulose-containing raw materials are given and calculated based on their dry weight, even though they generally contain varying amounts of (residual) water.

[0045] The reaction was found to be not very sensitive to the presence of water. Consequently, a certain amount of water in the reaction mixture provided in step a) can be tolerated.

[0046] Thus, in one embodiment, the sum of ammonium formate, the at least one acid, the cellulose-containing starting material and water is 80-100 wt.-%, preferably 90-100 wt.-%, in another embodiment 95-100 wt.-%, relative to the total weight of the mixture provided in step a), the remainder being generally impurities from the starting materials used.

[0047] Providing the reaction mixture comprising the above compounds can be carried out in any manner known to a person skilled in the art, in any order of addition and in any vessel known to a person skilled in the art to allow the reaction as defined above.

[0048] In step b), the reaction temperature is 100°C, preferably 140°C or higher, more preferably 155°C or higher.

[0049] In one embodiment, the reaction temperature is in the range of 100°C to 190°C, preferably 140°C to 185°C, more preferably 155°C to 180°C, especially 160°C, 170°C or 180°C.

[0050] Ammonium formate is known to begin to decompose at temperatures above 180°C, so higher temperatures as described above may occur, but will increase the formation of undesirable by-products such as formamide. At temperatures below 100°C, the reaction becomes too slow to effectively obtain the desired nanocellulose.

[0051] The pressure conditions are not particularly limited, and the pressure in step b) may be 500 hPa to 50 MPa, preferably 1000 hPa to 1 MPa. However, due to the potential decomposition of ammonium formate and the formation of low-boiling components (such as water, formic acid or other organic acids), the reaction is carried out at a pressure established after confining the reaction mixture in reaction step a) and heating it to the desired temperature (i.e., under isochoric or near-isochoric conditions).

[0052] The process according to the invention, in particular step b) thereof, can be carried out in any vessel or reactor suitable for the purpose and known to a person skilled in the art.

[0053] Preferably, the reaction is carried out in an autoclave or a reactor that allows the process to be carried out under isochoric or near isochoric conditions.

[0054] For example, the reaction time is at least 30 minutes, preferably at least 90 minutes, more preferably at least 2 hours.

[0055] In one embodiment, the reaction time is from 60 minutes to 48 hours, preferably from 90 minutes to 12 hours, even more preferably from 2 to 4 hours.

[0056] Longer reaction times are possible but do not actually add any advantage over shorter reaction times, although shorter reaction times may reduce the yield of the desired nanocellulose.

[0057] In step b), a reaction mixture containing the desired nanocellulose is obtained. Water, formic acid and other acids as well as volatile by-products present (such as formamide) can be removed by simple washing with water and / or alcohol or by distillation, fractionation or vacuum to isolate the nanocellulose.

[0058] Nanocellulose can be redispersed in water by vortex mixing or sonication to form a colloid, dispersion or suspension, which are also encompassed by the present invention.

[0059] If desired, formic acid and other acids as well as excess ammonium formate can be recycled to step a).

[0060] The nanocellulose obtained by the method according to the invention exhibits a higher zeta potential compared to mechanically prepared nanocellulose, and the nitrogen content indicates that at least the reducing ends of at least some of the cellulose chains within the nanocellulose are chemically modified and are therefore novel and comprised by the present invention.

[0061] Without wishing to be bound by theory, it is assumed that in step b) ammonium formate reacts with the reducing ends of at least some of the cellulose chains to form by reductive amination a cellulose polymer comprising repeating units of formula (I) (typically for cellulose as a polymer of β(1,4) linked D-glucose units) and terminal units of formula (II):

[0062]

[0063]

[0064] Since cellulose-containing raw materials, depending on their origin, generally contain more or less structural defects and oxygen is generally not excluded during the treatment and / or performance of reaction steps a) and b), during reaction step b), further amino groups can be introduced into the cellulose chains of the nanocellulose by reductive amination of aldehyde groups already present in the cellulose chains or generated by partial oxidation, explaining the typical nitrogen content observed for the nanocellulose according to the invention as defined below.

[0065] As a macroscopic effect of amination, the nanocellulose according to the present invention exhibits exceptionally high stability when dispersed in water or as a colloid. Such dispersions and colloids are stable without forming significant gels even after two weeks of storage at room temperature.

[0066] Nanocellulose also exhibits high crystallinity.

[0067] The zeta potential of the nanocellulose according to the present invention is typically in the range of 2.0 to 50.0 mV, preferably 5.0 to 40.0 mV, more preferably 8.0 to 35.0 mV, as measured according to the procedure described in the experimental part below.

[0068] The nitrogen content of the nanocellulose is typically between 0.2 and 2.0 wt.-%, preferably between 0.3 and 1.8 wt.-%, as measured by elemental analysis according to the procedure described in the experimental part below.

[0069] The crystallinity index of the nanocellulose is generally in the range of 70% to 100%, preferably 75% to 100%, as measured by X-ray diffraction according to the procedure described in the experimental part below.

[0070] The degree of polymerization of nanocellulose depends strongly 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.

[0071] Nanocelluloses according to the present invention and colloids, dispersions and suspensions containing them can be used in various applications. This includes their use in food and beverages, for example as additives (such as low-calorie additives, thickeners, stabilizers (e.g., foam stabilizers) and texture modifiers), and as microcapsules or coatings for protecting odor and taste.

[0072] They are more useful in technological applications, such as membranes for fuel cells and supercapacitors, as conductive membranes, speaker diaphragms, in or as packaging materials, in water absorption or purification (such as hydrogel beads for removing water-based dyes), water filtration membranes, nanocomposite heavy metal sensors, aerogels, flocculants and nanocomposite filters for groundwater mediation, and as reinforcing additives for synthetic polymers (such as thermoplastics and elastomers).

[0073] Other technical applications include paper / paperboard coating and reinforcement applications, additives for coatings, adhesives, latex and cement, as stimulation fluids, drilling fluids, completion fluids and spacer fluids, where the new nanocellulose is used as a stabilizer, thickener, shear thinning agent, proppant or reinforcement.

[0074] Other applications include their use in cosmetic or pharmaceutical compositions and biomedical applications, such as for drug delivery, tissue engineering, bone repair materials, biosensors, bioadhesives and microcapsules.

[0075] Therefore, the present invention also includes foods, beverages, membranes, films, packaging materials, water absorption or purification materials, heavy metal sensors, aerogels, flocculants, reinforced synthetic polymers, paper, paperboard, coatings, 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 or its colloids, suspensions or dispersions according to the present invention.

[0076] The main advantage of the present invention is that it provides a very efficient and green method for preparing nanocellulose and novel nanocelluloses that allow the formation of highly stable dispersions and colloids.

[0077] Hereinafter, the present invention is explained by way of examples, however, these examples are not intended to limit the scope of the present invention.

[0078] Experimental part:

[0079] General Information:

[0080] Material:

[0081] Ammonium formate (≥98%) was purchased from Alfa Aesar, glycolic acid (≥98%) was purchased from Alfa Aesar, propionic acid (99.5%) was purchased from Fluca, levulinic acid (98+%) was purchased from Acros Organics, succinic acid (99.5%) was purchased from Roth, and lactic acid (90 wt% aqueous solution) was purchased from Acros Organics.

[0082] If not noted, all chemicals were used as received without further purification.

[0083] Representation.

[0084] Elemental analysis

[0085] 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.

[0086] Electromotive force

[0087] The zeta potential based on electrophoretic light scattering was measured using a Zetasizer NanoZS from Malvern Instruments (Malvern, United Kingdom). The wet sample after centrifugal washing was diluted with distilled water to obtain an approximately 1% (nano-)cellulose suspension. The suspension was placed in a disposable folded capillary cell (DTS1070). The electrophoretic mobility of the (nano-)cellulose suspension was measured using Malvern software and converted to a zeta potential according to the Smoluchowski equation. For the zeta potential measurement, the sample average with 95% confidence was reported for three measurements.

[0088] TEM imaging

[0089] Transmission electron microscopy (TEM) images were recorded on a Zeiss Libra 912 microscope operated at 120 kV. Negative staining was performed with 1% uranyl acetate in distilled water to obtain higher image contrast.

[0090] Crystallinity index

[0091] The crystallinity index of nanocellulose was calculated from the XRD data as the ratio between the maximum intensity of the (002) lattice diffraction (at 22.8°) and the intensity of the amorphous region in the same unit cell (at 18.6°), see also Segal et al., Textile Research Journal, October 1959, p. 786 to 794.

[0092] II. Preparation of Nanocellulose

[0093] Experimental procedures

[0094] A. Preparation of low melting point mixture

[0095] To obtain a low-melting-point mixture for use as both a solvent and a reactant, dry ammonium formate (AF) is mixed with an organic acid in a 2:1 molar ratio. The mixture is ground in a mortar or thoroughly mixed in a glass beaker. As the mixture gradually liquefies during grinding / mixing, the desired low-melting-point mixture is visually formed. To promote this formation, the mixture is maintained at 60°C in a sealed glass bottle with continuous stirring for at least two hours or until the crystals disappear completely.

[0096] B. Cellulose-containing raw materials used in reaction.

[0097] SP: Bleached softwood kraft pulp obtained from Mercer Pulp was disintegrated in deionized water with constant stirring at room temperature overnight.

[0098] DP: 5 g of uncoated high-quality delignified paper purchased from Inapa Germany was cut into approximately 1 cm 2 The resulting pulp was filtered on a glass funnel filter and washed with deionized water and ethanol in that order. The washed pulp was dried at 60°C for 24 hours. MC: Commercially available microcrystalline cellulose (Avicel PH-101, 100% cellulose content) was used.

[0099] C reaction conditions

[0100] Add the cellulose-containing raw material to the corresponding low-melting-point mixture of ammonium formate and acid in a glass beaker. Transfer the resulting reaction mixture to The further reactions were carried out in an autoclave reactor under static conditions (ie without stirring) or with stirring as follows:

[0101] Static: Place the reaction mixture in a Beaker The lid was sealed and placed in a stainless steel Parr reactor (autoclave). The autoclave was kept 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.

[0102] Stirring: Place the reaction mixture in Beaker The mixture was stirred at 200 rpm for 1 h. The reaction mixture was cooled by a water cooling system. After the reactor was cooled to room temperature, the product mixture was transferred to a glass beaker.

[0103] The composition of the reaction mixture for preparing the nanocellulose according to the present invention, the cellulose-containing raw materials used and the reaction conditions are summarized in Table 1:

[0104] Table 1. Composition of the reaction mixture, amount and type of cellulose-containing feedstock used, and reaction conditions.

[0105]

[0106]

[0107] *Lactic acid was used as a 90 wt% aqueous solution

[0108] D washing. The product mixture obtained according to part c) was diluted with a few milliliters of distilled water, mixed with a spatula, and ultrasonicated in a laboratory sonicator for 30 minutes. Subsequently, the colloid was precipitated by centrifugation at 10,000 RPM for 5 minutes on an Avanti JE centrifuge (Beckman Coulter) equipped with a JA-25.50 fixed angle rotor, and the supernatant was removed. The precipitate was washed in the following order: redispersed in water, vortexed for 30 seconds, ultrasonicated for 30 minutes, centrifuged at 10,000 RPM (20,000 RPM for the last three runs) for 5 minutes, and the washing solution was decanted. This procedure was repeated four times with water and twice with ethanol until a clear washing solution was obtained. Ethanol was replaced with water, the sample was centrifuged at 25,000 RPM, the supernatant was decanted, and the final product was freeze-dried to obtain a white to beige powder.

[0109] Table 2 summarizes the characteristics of the nanocellulose obtained in Examples 1 to 26.

[0110] Table 2: Characterization of nanocellulose

[0111]

[0112] Figure 1 and Figure 2 Shown is a TEM image of nanocellulose prepared from microcrystalline cellulose according to the above-described Example 1. Whiskers having a size of 10 to 20 nm in diameter and a length of up to 200 nm are thus obtained.

[0113] Figure 3 and Figure 4 TEM images of nanocellulose prepared from softwood pulp according to Example 6 above are shown.

[0114] Figure 5 and Figure 6 TEM images of nanocellulose prepared from delignified pulp according to Example 11 above are shown.

[0115] All nanocelluloses obtained according to the invention showed a high stability of their aqueous colloids over at least two weeks, clearly indicating an increase in stability by the formation of amino groups at the reducing ends of the cellulose chains, which leads to an increased nitrogen content in the nanocelluloses 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 organic acid; b) adding iii) at least one cellulose-containing raw material to the mixture, and c) heating the mixture provided in step b) at a reaction temperature of 100°C to 190°C, The molar ratio of ammonium formate to the at least one organic acid is from 0.2 to 1000, and the weight ratio of the cellulose-containing raw material to the sum of ammonium formate and the at least one organic acid, calculated based on their dry weight, is from 0.001 to 1.

2. The method according to claim 1, wherein the nanocellulose refers to polymer particles comprising β(1,4) linked D-glucose units, having an average degree of polymerization of at least 50 D-glucose units, at least one dimension less than 1000 nm, and being chemically derivatized or not.

3. The method according to claim 1 or 2, wherein the at least one organic acid comprises an organic compound with one, two or three carboxylic acid or sulfonic acid groups.

4. The method according to claim 1 or 2, wherein the at least one organic acid is selected from monocarboxylic acids and dicarboxylic acids.

5. The process according to claim 1 or 2, wherein the molar ratio between ammonium formate and the at least one organic acid is from 0.5 to 10.

0.

6. The method of claim 1 or 2, wherein the cellulose-containing raw material is selected from microcrystalline cellulose; microbial cellulose; cellulose derived from marine or other invertebrates; recycled or waste paper; wood pulp; chemical pulp; chemical dissolving pulp; delignified pulp; pulp waste; natural biomass in the form of plant fibers; wood chips; sawdust; straw; leaves; stems or shells; and cellulose synthetic fibers; and other cellulose sources.

7. The method of claim 1 or 2, wherein the cellulose-containing feedstock is chemically derivatized by carboxymethylation, carboxylation, oxidation, sulfation, or esterification, or is not chemically derivatized.

8. The method according to claim 1 or 2, wherein the cellulose-containing raw material is mechanically pretreated or not by cutting, layering, high-pressure homogenization, ultrasonic treatment or other known methods, or is pretreated or not by enzymatic hydrolysis.

9. The method according to claim 1 or 2, wherein the cellulose-containing raw material is selected from the group consisting of bleached softwood pulp; microcrystalline cellulose; and pulp obtained from uncoated delignified paper.

10. The process according to claim 1 or 2, wherein the weight ratio between the cellulose-containing raw material and the sum of ammonium formate and the at least one organic acid, calculated based on their dry weight, is from 0.01 to 0.

25.

11. The process according to claim 1 or 2, wherein the sum of ammonium formate, the at least one organic acid, the cellulose-containing feedstock and water is 80 to 100 wt.-%, relative to the total weight of the mixture provided in step b).

12. The process according to claim 1 or 2, wherein in step c), the reaction temperature is in the range of 155°C to 180°C.

13. The process according to claim 1 or 2, wherein the pressure in step c) is 500 hPa to 50 MPa.

14. The process according to claim 1 or 2, wherein the reaction time in step c) is at least 30 minutes.

15. The method according to claim 1 or 2, wherein the reaction time in step c) is 60 minutes to 48 hours.

16. The process according to claim 1 or 2, wherein the nanocellulose is separated from the reaction mixture obtained in step c) by washing with water and / or alcohol or removing volatiles by distillation, fractionation or vacuum.

17. The process according to claim 1 or 2, wherein formic acid and other acids, where present, and excess ammonium formate are recycled to step a).

18. Nanocellulose obtained by the method according to any one of claims 1 to 17. The nanocellulose according to claim 18 , having a zeta potential of 2.0 to 50.0 mV.

20. Nanocellulose according to claim 18, having a nitrogen content of 0.2 and 2.0 wt.-%.

21. The nanocellulose according to claim 18, having a crystallinity index measured by X-ray diffraction in the range of 70% to 100%.

22. The nanocellulose according to claim 18, having a degree of polymerization of 100 to 15,000 glucose units.

23. A suspension, dispersion or colloid comprising the nanocellulose according to any one of claims 18 to 22.

24. Use of the nanocellulose according to any one of claims 18 to 22 or the dispersion or colloid according to claim 23 in food and beverages.

25. Food, beverage, film, packaging material, water absorption or purification material, heavy metal sensor, aerogel, flocculant, reinforced synthetic polymer, paper, paperboard, coating, adhesive, latex, cement, stimulation fluid, drilling fluid, completion fluid, spacer fluid, cosmetic or pharmaceutical composition, tissue and bone repair material, biosensor and bioadhesive comprising the nanocellulose according to any one of claims 18 to 22 or the dispersion or colloid according to claim 23.

26. Use of ammonium formate in combination with an organic acid for preparing the nanocellulose according to any one of claims 18 to 22.

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