EP2989127A1

EP2989127A1 - Method for catalytic oxidation of cellulose

Info

Publication number: EP2989127A1
Application number: EP14726181.2A
Authority: EP
Inventors: Markus Nuopponen; Lauri Kuutti; Heikki Pajari; Timo PÄÄKKÖNEN
Original assignee: UPM Kymmene Oy
Current assignee: UPM Kymmene Oy
Priority date: 2013-04-25
Filing date: 2014-04-24
Publication date: 2016-03-02
Also published as: WO2014174152A1; FI20135423L; FI126386B

Abstract

In a method for catalytic oxidation of cellulose using a heterocyclic nitroxyl compound as catalyst and a main oxidant acting as oxygen source, the heterocyclic nitroxyl compound is activated by electrolysis to oxidized form, whereafter the catalytic oxidation of cellulose is performed.

Description

METHOD FOR CATALYTIC OXIDATION OF CELLULOSE Field of the invention The invention relates to a method for catalytic oxidation of cellulose. Background of the invention

It is known to convert polysaccharides to chemical derivatives by reactions performed on their monomeric units and to impart in this way desired properties to the polymer through altered chemical structure, for example by adding functional groups to the polymer molecule. Cellulose, which is an abundant renewable natural substance, is one example of a polymer that can be converted to many chemical derivatives. The derivatization takes place mostly by chemical reactions of the hydroxyl groups in the β-D- glucopyranose units of the polymer. By chemical derivatization the properties of the cellulose can be altered in comparison to the original chemical form while retaining the polymeric structure. Heterocyclic nitroxyl compounds are known as catalysts that participate in the selective oxidation of C-6 hydroxyl groups of cellulose molecules to aldehydes and carboxylic acids, the corresponding oxoammonium salt being known as the active direct oxidant in the reaction series. One of these chemical oxidation catalysts known for a long time is "TEMPO", i.e. 2,2,6,6- tetramethylpiperidinyl-1 -oxy free radical. Thus, the oxidized forms of the nitroxyl radicals, N-oxoammoniumions, act as direct oxidants in the oxidation of the target cellulose molecule, whereas a main oxidant is used to bring oxygen to the reaction chain and to convert the nitroxyl compound back to the oxidized form.

It is known to oxidize primary alcohols to aldehydes and carboxylic acids through "TEMPO" by using sodium hypochlorite as the main oxidant (for example Anelli, P. L.; Biffi, C; Montanari, F.; Quici, S.; J. Org. Chem. 1987, 52, 2559). To improve the yield in the oxidation of the alcohols to carboxylic acids, a mixture of sodium hypochlorite and sodium chlorite was also used (Zhao, M. M.; Li, J.; Mano, E.; Song, Z. J.; Tschaen, D. M.; Org. Synth. 2005, 81, 195).

It is also known procedure to catalytically oxidize cellulose in native cellulose fibers through "TEMPO" by using sodium hypochlorite as main oxidant (oxygen source) and sodium bromide as activator (Saito, T. et al.; Cellulose Nanofibers Prepared by TEMPO-Mediated Oxidation of Native Cellulose, Biomacromolecules 2007, 8, 2485-2491 ). The primary hydroxyl groups (C6- hydroxyl groups) of the cellulosic β-D-glucopyranose units are selectively oxidized to carboxylic groups. Some aldehyde groups are also formed from the primary hydroxyl groups. When the fibers of oxidized cellulose so obtained are disintegrated in water by applying sufficient mechanical energy , they give stable transparent dispersion of individualized cellulose fibrils of 3- 5 nm in width, that is, so-called nanofibrillar cellulose.

The use of sodium bromide as activator is preferred because it accelerates the reaction. For example WO01/29309 recommends using 3 parts by weight NaBr to 4 parts of NaOCI. In the reaction series, the bromide ion acts as oxygen mediator between the main oxidant and the nitroxyl radical by oxidation to hypobromite and reduction back to bromide.

It has been presumed that high level of cellulose oxidation (such as 1 .5 mmol COOH/ g pulp for example), attainable by the use of sodium bromide at an optimum pH of about 10, is preferable so that the pulp can be easily disintegrated to nanofibrillar cellulose. However, in these oxidation conditions side reactions increase, which results in lowered DP (degree of polymerization) of the cellulose and consequently in lowered strength properties and gel formation ability of the nanofibrillar cellulose. In EP2216345A1 , the lowering of the DP is compensated for by performing the oxidation in acidic or neutral conditions.

The use of bromine compounds in the oxidation reaction is problematic because of environmental concerns. Sodium bromide is usually used in the reaction mixture in relatively large amounts (conventionally 100-125 kg / 1000 kg pulp) and it is difficult to remove bromide residues from the final cellulose product by washing. Bromine compounds also accumulate in process waters. Further, the use of bromine in industrial scale is undesirable. Use of large amounts of sodium bromide cause also corrosion problems in the equipment. Bromine compounds are generally recognized as hazardous to health, for example bromate anion which is formed as a result of side reactions is a suspected carcinogen.

Summary of the invention

It is a purpose of the invention to provide a method for effectively and selectively oxidizing the C-6 hydroxyl groups of cellulose by avoiding the use of bromine compounds.

One purpose of the invention is to provide a method for activating the catalyst for the catalytic oxidation of cellulose.

Still one purpose of the invention is to provide a method for making nano- fibrillar cellulose.

In the method, the heterocyclic nitroxyl compound is activated by electrolysis to oxidized form; whereafter the catalytic oxidation of cellulose is performed. It has been found that heterocyclic nitroxyl radicals such as 2,2,6,6-tetra- methylpiperidinyl-1 -oxy radical (TEMPO) can be activated by electrolysis from the stable radical form to the active oxidized form, whereafter the catalytic oxidation of cellulose can proceed to the desired degree of oxidation by means of the catalyst and a main oxidant (such as hypochlorite), which is the oxygen source.

The heterocyclic nitroxyl radical can be activated by electrolysis without the use of bromide or iodide. The catalytic oxidation of cellulose by means of the catalyst can be performed in optimum conditions in regard to structural integrity (DP value) of the cellulose.

The catalyst is activated by passing a solution containing the heterocyclic nitroxyl compound in the radical through an electrochemical cell, where the compound is oxidized by the anode to the oxidized nitrosonium form, which can be used for starting the catalytic oxidation. It is also possible that in the electrolytic treatment the compound is converted to a mixture of the oxidized form and the reduced hydroxylamine forms, which are different from the radical form and can start the catalytic activity in the reaction conditions in the presence of main oxidant and cellulose.

In the oxidation process, the cellulose is oxidized at C-6 carbons to carboxyl groups through the catalytic activity using a main oxidant, which provides the oxygen for the reaction and whose amount in relation to the amount of cellulose can be used to adjust the degree of conversion of the cellulose. Hypochlorite, such as sodium hypochlorite, may be used as the main oxidant. Residual aldehyde groups may be oxidized to carboxyl groups in a second step to complete the oxidation process and to attain a desired oxidation degree. After the cellulose is subjected to oxidation, it can be processed to a final cellulose product. When the starting material is pulp derived from plants, especially wood, the cellulose exists in fiber form. The fibers that contain the cellulose in oxidized form as a result of the oxidation process are easy to disintegrate by mechanical methods to small-scaled fragments, nanofibrillar cellulose (NFC). The method for forming the cellulose product comprises the first process of catalytic oxidation of the fibrous starting material and the second process of disintegration the oxidized starting material to nanofibrillar cellulose. Brief description of the drawings

In the following, the invention will be described with reference to the appended drawings, where

Figure 1 shows schematically the activation step of the catalyst.

Detailed description of the invention

In the following disclosure, all percent values are by weight, if not indicated otherwise. Further, all numerical ranges given include the upper and lower values of the ranges, if not indicated otherwise. In the present application all results shown and calculations made, whenever they are related to the amount of pulp, are made on the basis of dried pulp.

In the invention, the primary hydroxyl groups of cellulose are oxidized cataly- tically by a heterocyclic nitroxyl compound, for example 2,2,6,6-tetramethyl- piperidinyl-1 -oxy free radical, "TEMPO". Other heterocyclic nitroxyl compounds known to have selectivity in the oxidation of the hydroxyl groups of C- 6 carbon of the glucose units of the cellulose may also be used, and these compounds are widely cited in the literature. Hereinafter, the oxidation of cellulose refers to the oxidation of these hydroxyl groups to aldehydes and/or carboxyl groups. It is preferred that the hydroxyl groups are oxidized to carboxyl groups, that is, the oxidation is complete.

Whenever the catalyst "TEMPO" is mentioned in this disclosure, it is evident that all measures and operations where "TEMPO" is involved apply equally and analogously to any derivative of TEMPO or any heterocyclic nitroxyl radical capable of catalyzing selectively the oxidation of the hydroxyl groups of C-6 carbon in cellulose after it has been activated by electrolysis. Other known members of this group are the TEMPO derivatives 4-methoxy-TEMPO and 4-acetamido-TEMPO.

In the following description, catalytic oxidation refers to nitroxyl-mediated (such as "ΤΕΜΡΟ''-mediated) oxidation of hydroxyl groups. The catalytic oxidation of fibers or fibrous material in turn refers to material which contains cellulose that is oxidized by nitroxyl-mediated (such as "ΤΕΜΡΟ''-mediated) oxidation of hydroxyl groups of the cellulose.

The term "nanofibrillar cellulose" refers to a collection of isolated cellulose microfibrils or microfibril bundles derived from cellulose raw material. Micro- fibrils have typically high aspect ratio: the length might exceed one micrometer while the number-average diameter is typically below 200 nm. The diameter of microfibril bundles may also be larger but generally less than 1 μιτι. The smallest microfibrils are similar to so called elementary fibrils, which are typically 2-12 nm in diameter. The dimensions of the fibrils or fibril bundles are dependent on raw material and disintegration method. Nanofibrillar cellulose may be characterized as a cellulose-based material, in which the median length of particles (fibrils or fibril bundles) is not greater than 50 μιτι, for example in the range of 1-50 μιτι, and the particle diameter is smaller than 1 μιτι, suitably in the range of 2- 500 nm. In case of native fibril cellulose, in one example the average diameter of a fibril is in the range of 5-100 nm, for example in the range of 10-50 nm. Typically, non-ionic grades have such wider fibril diameter while the chemically modified grades are a lot thinner (for example in the range of 5-20 nm). Nanofibrillar cellulose is characterized by a large specific surface area and a strong ability to form hydrogen bonds. In water dispersion, the nanofibrillar cellulose described herein typically appears as either light or turbid gel-like material. The nanofibrillar cellulose may also contain some hemicelluloses; the amount is dependent on the plant source. Mechanical disintegration of the oxidized cellulose raw material is carried out with suitable equipment such as a refiner, grinder, homogenizer, colloider, friction grinder, ultrasound sonicator, fluidizer such as microfluidizer, macrofluidizer or fluidizer-type homogenizer.

The heterocyclic nitroxyl compound used as catalyst in the oxidation process (such as "TEMPO") is stable in its neutral, radical form, and it can be stored in that form. The catalyst is activated electrolytically to oxidized form, which can participate at once in the reaction, and the oxidation process of the cellulose starts quickly. The structural formula of "TEMPO" in its radical form is given below

TEMPO

After the activation, the oxidation reaction can be started and performed to completion to a desired conversion degree in a reaction medium in the presence of the catalyst, cellulose and main oxidant. The reaction medium may be water-based medium where the materials are dissolved and suspended. In the case of pulp raw material, the cellulose exists in fiber form as suspension in water in a suitable consistency, whereas the catalyst and the main oxidant are dissolved in the water. The pH of the reaction medium is controlled during the reaction to keep it in the optimum range. Temperature of the reaction medium may also be controlled.

The catalytic oxidation of the hydroxyl groups at carbon C-6 in the pulp takes place by means of the active TEMPO. Hypochlorous acid (HOCI), which is in equilibrium with the hypochlorite (pKa 7.53), acts as a TEMPO activator, returning the reduced TEMPO back to the oxidized form. The NaCIO chemical is consumed in the reaction, and the catalyst remains. In this reaction, no bromide or other alkaline metal halide, such as iodide, will be needed for activating the radical TEMPO or returning the TEMPO from the reduced form to the oxidized form.

Activation of the catalyst

The catalyst is activated before the oxidation of cellulose by passing the catalyst in an electrolyte solution through an electrolytic cell comprising an anode and a cathode and an electrolysis space between the anode and the cathode. The cell may have several parallel spaces between an anode and a cathode for parallel passage of different electrolytes. The electrolyte undergoes electrolysis when passing through the electrolysis space. In the electrolysis, the anode oxidizes the radical form of the catalyst to the oxidized form. The electrolyte is preferably an aqueous solution. The principle is shown in Figure 1 .

Figure 1 shows the anode, the cathode, and the electrolysis space between the anode and cathode for passing the electrolyte with the catalyst dissolved therein. The space is preferably divided by a membrane to two compartments, the cathode compartment and the anode compartment. The membrane prevents the passage of at least some substances of the electrolyte from one compartment to the other. There are three alternatives for the membrane: a cation exchange membrane, an anion exchange membrane, or a neutral membrane. The electrolyte in contact with the anode contains in addition to the heterocyclic nitroxyl radical some dissolved salt to increase the conductivity. The salt should be preferably inert and not reactive to the anode so that there are no anions or cations producing additional oxidizers and competing with the nitroxyl radical. For example sodium sulfate can be used.

According to Figure 1 , different electrolytes are passed through the anode compartment and the cathode compartment. A salt that is inert towards the anode is used in the anode compartment and alkali metal hydroxide in the cathode compartment. The inert salt is sodium sulfate and the alkali metal hydroxide is sodium hydroxide. The membrane is cation exchange membrane which allows cations to pass from the anode compartment to the cathode compartment. In Figure 1 , the radical, neutral form of the catalyst is denoted with TE^* and the oxidized form with TE⁺. The oxidized form of the catalyst, being a cation, may pass through the membrane, depending on the permeability of the membrane for cations of this molecular size. However, if the concentration of other cations that can pass through the membrane is higher than that of the catalyst (expressed in M) in the anode compartment, the losses of the oxidized catalyst are not significant.

The salt used in the electrolyte passed through the anode compartment need not be an inert salt. For example a chloride salt produces chlorine or hypochlorite, depending on pH, through the reactions at the anode. These oxidative species can be used as oxidant or catalyst activator in the catalytic oxidation step of cellulose, and the electric current can be used more efficiently in the electrolysis. For example sodium chloride may be used in the electrolyte.

In Figure 1 , the anode material is DSA (dimensionally stable anode) and the cathode material is titanium. Other known electrode materials suitable for the electrolysis may also be used.

Another membrane material that may be used is anion exchange membrane which allows the passage of anions from the cathode department but blocks the passage of cations (incl. oxidized catalyst) from the anode department. A neutral membrane which only separates the solutions of the anode compartment and the cathode compartment allows the passage of both ionic species (anions and cations) may also be used. By choice of the permeability, the retention of the oxidized catalyst in the anode compartment can be influenced.

The electrolytic activation can take place without a membrane, but in an undivided cell, part of the oxidized form reduces back to radical form at the cathode. However, it is important that all catalyst in various forms can be recovered from the electrolysis. In this case, the composition of the electrolyte is not in two fractions, and it may be used as such in the subsequent oxidation step. In case of a membrane separating the anode and cathode compartments, only the electrolyte passed through the anode compartment and containing the catalyst in the oxidized form is preferably used.

Oxidation of cellulose using the activated catalyst

The reaction medium for oxidation of the cellulose comprises activated heterocyclic nitroxyl catalyst, cellulose, and main oxidant, which is preferably sodium hypochlorite. The oxidation process is performed in a reactor which is equipped with mixing of the reactions medium and control of reaction conditions. The catalyst and the main oxidant are added preferably to a fibrous suspension of cellulose fibers to achieve a desired starting consis- tency of the reaction medium. The activated catalyst may be added by adding the whole volume of the electrolyte to the reaction medium, without the need to separate the catalyst. The main oxidant may be added portionwise during the reaction. It is advantageous to add the main oxidant, such as hypochlorite, continuously as the oxidation of cellulose proceeds to avoid excess concentrations which may cause unwanted side reactions.

The fibrous starting material which is suspended in the reaction medium may be any of the above mentioned materials, especially fibers of plant origin which form, when suspended in aqueous reaction medium, a pulp of given consistency. The fibers may be especially from wood. Chemical pulp, such as softwood or hardwood pulp, for example bleached birch pulp, may be used.

The oxidation reaction is allowed to proceed till a required conversion degree (oxidation level has been achieved. As expressed in carboxylate groups generated as the result of oxidation, this is normally 0.5-1 .4 mmol COOH/g pulp.

For the purpose of making NFC, it has been found that the oxidation level (conversion degree) of 0.5-1 .1 mmol COOH/g pulp, preferably 0.6-0.95 and most preferably 0.7-0.9 is already sufficient that the cellulose fibers can be easily disintegrated to fibrils by mechanical energy.

The dosage of hypochlorite to cellulose, to reach the above-mentioned conversions, may be 1 .7 to 5 mmol/g pulp, preferably 2.2-2.7 mmol/g pulp.

The consistency of the pulp in the reaction medium where the oxidation is performed is preferably above 3%.

In fact, according to an advantageous embodiment, the reaction is performed at medium consistency of the pulp to increase selectivity. When medium consistency of the pulp is used, the selectivity of the cellulose oxidation can be improved, because the desired reactions take place in the fiber, whereas the unwanted side reactions take place in the solution phase.

The medium consistency is initial consistency of the cellulosic raw material that is higher than normally used. The consistency of the pulp is above 6%, especially above 6% and at the most 12%, more preferably equal to or higher than 8%, and most preferably in the range of 8-12% by weight. Within the last-mentioned range, the optimum consistency is supposed to be in the range of 9-1 1 %. The consistency values are the initial consistency at the beginning of the oxidation. In all above-described embodiments the catalytic oxidation may be performed without the use of bromide. Sodium bromide, which is conventionally used as activator and cocatalyst because of the faster reaction rate and high degree of oxidation, can be avoided in the catalytic oxidation process according to still one embodiment. Conventionally, the optimum pH when sodium bromide is used is 10. However, side reactions occur at this pH which cannot be avoided even at the relatively fast reaction rate. The DP value (degree of polymerization) will decrease considerably, which decreases the strength characteristics and gel forming ability of the NFC. Thus, according to still one embodiment, the catalytic non-bromine oxidation with the heterocyclic nitroxyl catalyst initially activated by electrolysis, and preferably at the above mentioned medium consistency of the pulp, may be performed by using carefully defined conditions with regard to pH and temperature. The reaction is performed in neutral or slightly alkaline pH, at 7- 10, more preferably in the range of 7-9, most preferably in the range of 7.5- 8.5, and at room temperature or slightly elevated temperature, in the range of 15-50°C, preferably in the range of 20-40°C, most preferably in the range of 26-35°C, in the absence of added alkali metal halide. The selectivity (less C- 2 and C-3 reactions) is improved, and bromine compounds are avoided. The slower oxidation reaction rate due to the lower pH is compensated by the temperature, which does not increase the side reactions as much as the higher pH.

Temperature control may be used to keep the temperature within the above ranges during the reaction. Because the oxidation is exothermic, the temperature of the reaction medium will rise if cooling is not provided. The rise is about 10°C between the start and the end point. Thus, in the range of 15-50°C the reaction can start at below 30°C and end at below 40°C, for example start at 24-26°C and end at 34-36°C.

During the catalytic oxidation, part of the hydroxyl groups of the cellulose in C-6 carbon is oxidized incompletely to aldehydes. If aldehydes are not wanted in the oxidation product, the oxidation may be completed by oxidizing the aldehyde groups to carboxylate groups in a second step, using different reaction conditions. When the first step using the electrolytically activated catalyst has proceeded so that a desired conversion degree is reached, the first step is stopped. The partly oxidized cellulose may be washed and the second step for converting the residual aldehydes to carboxylates to reach the final carboxylate content is performed in a reaction medium where the pH is clearly on acidic side, about 1 .5-4, preferably 2-3. Preferably the second step is performed at a pH below 3. The stop point of the first step may be chosen according to the consumption of the main oxidant or any other way. Alternatively, the pH of the reaction medium of the first step may be lowered directly to the pH range of the second step at the stop point of the first step.

When the pH is lowered, chlorite, for example NaCIO2, is added to the reaction medium. In this second step, the remaining aldehyde groups are rapidly oxidized to carboxyl groups with chlorite as the main oxidant. Dimethyl sulfoxide (DMSO) may be used in the reaction medium in the second step to eliminate the formation of hypochlorite from chlorite.

It is also possible that the aldehyde groups are reduced back to hydroxyl groups by using a suitable reducing agent, such NaBH .

Thus, it is also possible to complete the oxidation in a second step by oxidizing the residual aldehydes to carboxyl groups to obtain cellulose with the above-mentioned oxidation levels (COOH/g pulp) that are suitable for making NFC.

After the desired conversion degree has been attained, the reaction medium is taken out from the reactor. The fibers containing the oxidized cellulose are separated from the reaction medium, and the reaction medium is possibly reused. Regeneration measures making it possible to reuse at least some of the constituents of the reaction medium are not described herein in closer detail. The fibers are washed to remove the remnants of the chemicals and processed further, especially to NFC.

The method will be described in the following by way of examples. The following methods were used for characterizing the nanofibrillar cellulose product.

Turbidity

Turbidity may be measured quantitatively using optical turbidity measuring instruments. There are several commercial turbidometers available for measuring quantitatively turbidity. In the present case the method based on nephelometry is used. The units of turbidity from a calibrated nephelometer are called Nephelometric Turbidity Units (NTU). The measuring apparatus (turbidometer) is calibrated and controlled with standard calibration samples, followed by measuring of the turbidity of the diluted NFC sample.

In the method, a nanofibrillar cellulose sample is diluted in water, to a concentration below the gel point of said nanofibrillar cellulose, and turbidity of the diluted sample is measured. Said concentration where the turbidity of the nanofibrillar cellulose samples is measured is 0.1 %. HACH P2100 Turbidometer with a 50 ml measuring vessel was used for turbidity measurements. The dry matter of the nanofibrillar cellulose sample was determined and 0.5 g of the sample, calculated as dry matter, was loaded in the measuring vessel, which was filled with tap water to 500 g and vigorously mixed by shaking for about 30 s. Without delay the aqueous mixture was divided into 5 measuring vessels, which were inserted in the turbidometer. Three measurements on each vessel were carried out. The mean value and standard deviation are calculated from the obtained results, and the final result is given as NTU units.

Rheometer viscosity The NFC was diluted with deionised water to a concentration of 0.5 w% and 200 g of the mixture was homogenised with a Buchi-mixer (B-400, max 2100 W, Buchi Labortechnik AG, Switzerland) for 3 x 10 s.

The viscosity of the NFC dispersions was measured at 22°C with a stress controlled rotational rheometer (AR-G2, TA Instruments, UK) equipped with a narrow gap vane geometry (diameter 28 mm, length 42 mm) in a cylindrical sample cup having a diameter of 30 mm. After loading the samples to the rheometer they were allowed to rest for 5 min before the measurement was started. The steady state viscosity was measured with a gradually increasing shear stress (proportional to applied torque) and the shear rate (proportional to angular velocity) was measured. The reported viscosity (=shear stress/shear rate) at a certain shear stress was recorded after reaching a constant shear rate or after a maximum time of 2 min. The measurement was stopped when a shear rate of 1000 s^"1 was exceeded. The method was used for determining zero-shear viscosity.

Brookfield viscosity

The apparent viscosity of NFC is measured with a Brookfield viscometer (Brookfield viscosity) or another corresponding apparatus. Suitably a vane spindle (number 73) is used. There are several commercial Brookfield viscometers available for measuring apparent viscosity, which all are based on the same principle. Suitably RVDV spring (Brookfield RVDV-III) is used in the apparatus. A sample of the nanofibrillar cellulose was diluted to a concentration of 0.8% by weight in water and mixed for 10 min. The diluted sample mass was added to a 250 ml beaker and the temperature was adjusted to 20°C ± 1 °C, heated if necessary and mixed. A low rotational speed 10 rpm was used.

Examples

Example 1 .

The activation of "TEMPO" molecule by electrolysis was done. The catalyst was initially in its radical form. The used equipment, materials and concentrations are those shown in Fig. 1 . A cation exchange membrane (area 100 cm²) prevented the reduction of TEMPO+ (the oxidized form) at the cathode. The electrolysis volume was 0.5 liter and the current was constant 0.9 A and the voltage was 2.5 V and temperature 20°C. Electrolysis took 4 hour activation time (to ensure total oxidation of TEMPO) and the pH dropped during the electrolysis to pH 2.0 due to the decomposition of water at the anode. The most of the electric current was consumed to oxygen evolution.

To the activated solution 20 g pulp was added as dry matter and the 3.4 mmol NaCIO/ g pulp. The reaction time was three hours at room temperature and pH during the reaction was kept constant by adding NaOH, to neutralize the carboxyl acid groups being formed. The charge density was 869 μιτιοΙ COOH/g pulp. Dry substance of the washed sample was 18.8%. The sample was disintegrated to nanofibrillar cellulose in Masuko Super masscolloider MKZA10-15J using one pass. The sample was diluted with tap water to 2 w% and predispersed for 10 minutes at 700 rpm in Diaf dissolver 100 WH. Characterization was done by measuring turbidity and viscosity by rheometer.

Turbidity was 44 NTU. The zero shear viscosity was 28 000 Pa s and yield stress 1 1 Pa.

Example 2

2.8 kg of bleached birch pulp was dispergated in water in reaction vessel, 9 w%, total weight 31 kg. 1 .8 g of electrolytically activated TEMPO in 0.5 I was added to the reaction vessel. 3.8 I of 15% NaCIO was added by pump during the reaction. 1 .2 I 1 .5 M NaOH was added to keep up reaction pH 8. Reaction temperature was 20-25 °C and reaction time was 225 min. Carboxyl content after washing step was 0.82 mmol COOH / g pulp.

The sample was disintegrated to nanofibrillar cellulose in Atrex dispergator. The sample was diluted to 2.5 w% and run through the dispergator four times. Characterization was done by measuring turbidity and Brookfield viscosity.

Turbidity was 26 NTU and Brookfield viscosity was 24500 mPa. Manufacture and general properties of nanofibrillar cellulose The NFC prepared from cellulose raw material oxidized with the methods above has excellent gelling ability, which means that it forms a gel at a low consistency in aqueous medium. When the oxidized pulp is ground at a consistency of about 1-4% in aqueous medium, a clear gel consisting of microfibrils in water (NFC gel) is obtained.

The fibril cellulose is preferably made of plant material that has been subjected to the oxidation to convert the hydroxyl groups of the cellulose to carboxyl groups with a conversion degree that enhances the disintegration of the material to nanofibrillar cellulose. One preferred alternative is to obtain the microfibrils form non-parenchymal plant material where the fibrils are obtained from secondary cell walls. One abundant source of cellulose fibrils is wood fibers. The nanofibrillar cellulose is manufactured by homogenizing oxidized wood-derived fibrous raw material, which may be chemical pulp. The pulp can be for example softwood pulp or hardwood pulp or a mixture of these. The fibrils originating in secondary cell walls are essentially crystalline with degree of crystallinity of at least 55%.

In any of the preceding oxidation processes, the carboxylate content of 0.5- 1 .1 mmol COOH/ g starting pulp (on dry matter), preferably 0.7-0.9 mmol COOH/ g pulp is desirable so that the gel formation as a result of mechanical disintegration would be easy.

Before the oxidized pulp is disintegrated to make the NFC, the pH of the medium is adjusted to 7-10, preferably 7-9, and most preferably to 7-8.5, which lowers the energy needed.

The obtained NFC gel is characterized by shear thinning behaviour. The mean diameter of the microfibrils is 2-20 nm, preferably 2-6 nm, and the mean length is in the range of 0.3-5 μιτι, preferably 0.5-2 μιτι. The turbidity of the NFC is 3-90, preferably 10-70, more preferably 20-60 NTU (0.1 % concentration, nephelometric measurement). Measured at a 0.5% concentration in water, the gel has zero shear viscosity ("plateau" of constant viscosity at small shearing stresses approaching zero) of 5000-50000 Pa s and yield stress (shear stress where shear thinning begins) of 5-40 Pa, preferably 10-30 Pa.

Claims

Claims:

1 . A method for catalytic oxidation of cellulose using a heterocyclic nitroxyl compound as catalyst and a main oxidant acting as oxygen source, characterized in that the heterocyclic nitroxyl compound is activated by electrolysis to oxidized form, whereafter the catalytic oxidation of cellulose is performed.

2. The method according to claim 1 , characterized in that hypochlorite is used as the main oxidant.

3. The method according to claim 1 or 2, characterized in that the cellulose is oxidized catalytically using the main oxidant and the heterocyclic nitroxyl compound at a pH of 7-10, preferably 7-9, most preferably 7.5-8.5.

4. The method according to any of the preceding claims, characterized in that the cellulose is oxidized catalytically using the main oxidant and the heterocyclic nitroxyl compound at a temperature of 20-50°C.

5. The method according to any of the preceding claims, characterized in that the cellulose is oxidized catalytically using the main oxidant and the heterocyclic nitroxyl compound at pulp consistency of above 4%, preferably above 5.5%.

6. The method according to any claim 5, characterized in that the pulp consistency is at least 8%, for example in the range of 8-12%.

7. The method according to any of the preceding claims, characterized in that the cellulose is oxidized in the absence of bromide.

8. The method according to any of the preceding claims, characterized in that the cellulose is oxidized to the level of 0.5-1 .4 mmol COOH / g pulp, preferably 0.5-1 .1 mmol COOH / g pulp, most preferably 0.7-0.9 mmol COOH / g pulp.

9. The method according to any of the preceding claims, characterized in that the heterocyclic nitroxyl compound is activated in electrolysis by passing a solution comprising the heterocyclic nitroxyl compound through an anode compartment.

10. The method according to claim 9, characterized in that the anode compartment is isolated from the cathode by a membrane, which is anion exchange membrane, cation exchange membrane or neutral membrane.

1 1 . The method according to claim 10, characterized in that the membrane is cation exchange membrane.

12. The method according to claim 9, 10 or 1 1 , characterized in that the solution comprises an electrolyte which is inert towards the anode, such as sodium sulfate.

13. The method according to claim 9, 10 or 1 1 , characterized in that the solution comprises chloride, which is converted to chlorine or hypochlorite at the anode.

14. The method according to any of the above claims, characterized in that the cellulose subjected to catalytic oxidation is the cellulose in fibrous raw material, especially fibers obtained from plant material .

15. A method for making a cellulose product, comprising

- subjecting fibrous starting material, especially fibers obtained from plant material, to catalytic oxidation by any of the methods 1 to 14 to obtain oxidized fibrous material, and

- disintegrating the oxidized fibrous raw material.

16. The method according to claim 15, characterized in that the oxidized fibrous raw material is disintegrated to nanofibrillar cellulose (NFC).

17. The method according to claim 15, characterized in that the nanofibrillar cellulose is essentially crystalline with degree of crystallinity of at least 55%, being preferably made of plant material.

18. The method according to claim 16 or 17, characterized in that the fibrils of the nanofibnilar cellulose have mean diameter of 2-20 nm, preferably 2-6 nm, and mean length of 0.3-5, preferably 0.5-2 μιτι.

19. The method according to any of claims 16 to 18, characterized in that the nanofibrillar cellulose has the turbidity of 3-90, preferably 10-70, more preferably 20-60 NTU (nephelometric turbidity units) as measured at 0.1 % concentration in water.

20. The method according to any of claims 16 to 19, characterized in that the nanofibrillar cellulose has the zero shear viscosity of 5000-50000 Pa-s and yield stress of 5-40 Pa, preferably 10-30 Pa, as measured at 0.5% concentration in water.