PRODUCTION

TECHNICAL FIELD

The present disclosure relates in general to aerogels and xerogels containing cellulose as well as processes for their production.

BACKGROUND

Aerogels and xerogels are solids that feature very low density, high specific surface area and high porosity. They are derived from gels by replacement of the liquid component with a gas, in most cases air. Aerogel can be produced from numerous compounds, the most frequently commercially used aerogel probably being silica aerogel.

The replacement of the liquid component in the gel with air can be performed through different processes. The key issue is avoiding collapse of the solid matrix in the gel resulting from capillary action when removing the liquid component. Therefore, the drying step is often performed by supercritical drying or freeze-drying.

Recently, the development of aerogels comprising various kinds of cellulose has attained great interest since cellulose is an abundant, bio-derived and renewable source. For example, aerogels of nanocellulose or microcrystalline cellulose have been developed.

Ganesan et al., "Design of aerogels, cryogels, and xerogels of cellulose with hierarchical porous structures", Materials and Design, 92 (2016), p. 345-355, discloses for example a method for producing a cellulose aerogel comprising preparing a solution of microcrystalline cellulose in calcium thiocyanate hydrate, washing with ethanol to remove the calcium thiocyanate, and subjecting the alcogel to supercritical drying to obtain the aerogel.

CN 105420108 A discloses a preparation method for cellulose aerogel. The method comprises cooling a NaOH/urea solvent system to -8 to -12 °C, adding and dissolving cellulose in the NaOH/urea solvent system, adding a precipitator to obtain cellulose aquagel, washing, freezing to form a frozen gel and freeze-drying to obtain the cellulose aerogel.

Pour et al., "Xerocellulose: lightweight, porous and hydrophobic cellulose prepared via ambient drying", J Material Sci (2015) 50:4526-4535, discloses preparation of a low density, highly porous and hydrophobic cellulose-based material which they called "Xerocellulose". Tritylcellulose with different degrees of substitution was synthesised and subsequently dissolved in DMF, coagulated in methanol, washed in ethanol and dried at ambient conditions. Pour et al. found that the obtained Xerocellulose had a drastically decreased water vapour uptake compared to cellulose aerogel prepared by dissolving microcrystalline cellulose followed by coagulation and washing in ethanol and finally dried with supercritical carbon dioxide.

CN 103536954 B discloses cross-linked alginate-bacterial cellulose sponge, wherein the alginate biomaterial accounts for 80-95 % by mass of the sponge. The sponge is intended for adsorption of blood and is produced by cross-linking and freeze-drying.

Raman et al., "Hybrid alginate based aerogels by carbon dioxide induced gelation: Novel technigue for multiple applications”, J. of Supercritical Fluids, 106 (2015), p. 23-33, discloses a technique for manufacturing alginate-based hybrid aerogels by carbon dioxide induced gelation. The hybrid aerogels may comprise for example carboxymethyl cellulose or methyl cellulose. Raman et al. also discloses chitosan and cellulose hydrogels produced by CO2 induced gelation.

Previously derived aerogels comprising different kinds of cellulose may however suffer from drawbacks such as insufficient mechanical stability, poor adsorption properties and/or can only be produced through complex and costly manufacturing processes. In case a cellulose aerogel can be obtained having good adsorption properties as well as sufficient mechanical stability for being able to carry the adsorbed matter, it would be possible to use the obtained aerogel in numerous commercial applications. From a cost perspective, it would also be advantageous if costly drying processes could be avoided such that drying in ambient atmosphere would be possible, thereby obtaining a cellulose xerogel. SUMMARY

The object of the present disclosure is to provide an aerogel or xerogel with excellent adsorption properties and sufficient mechanical stability for carrying the adsorbed matter.

The object is achieved by the process according to independent claim 1.

The process for preparing a hybrid aerogel comprising cellulose or hybrid xerogel comprising cellulose according to the present invention comprises the following steps:

a) providing an aqueous sodium hydroxide solution;

b) dissolving a cellulose pulp in the aqueous sodium hydroxide solution at a temperature of 10 °C or lower, preferably a temperature of 5 °C or lower;

c) adding a chargeable carbohydrate in an amount of 0.1-100 weight-% based on dry weight of the cellulose pulp to the aqueous sodium hydroxide solution either before, during or after step b);

thereby obtaining a hybrid gel solution comprising the aqueous sodium hydroxide solution, dissolved cellulose pulp and charged carbohydrate;

d) adjusting the temperature of the hybrid gel solution to a temperature sufficient to cause gelation of the solution, thereby obtaining a hybrid gel;

e) regenerating the hybrid gel in an organic protic solvent, preferably wherein the organic protic solvent is ethanol, isopropanol, or mixtures thereof; and

f) after regenerating, drying the hybrid gel by supercritical drying to thereby obtain a hybrid aerogel, or drying the hybrid gel in ambient air to thereby obtain a hybrid xerogel.

In view of the fact that both a cellulose pulp and a chargeable carbohydrate are added and dissolved in the aqueous sodium hydroxide solution, the resulting aerogel or xerogel will constitute a hybrid aerogel or hybrid xerogel, respectively. The chargeable carbohydrate may be a charged

carbohydrate, such as carboxymethyl cellulose, or alternatively, a carbohydrate which will be charged when present in the aqueous sodium hydroxide solution, such as chitosan or alginate.

By means of the process described above, a hybrid aerogel or hybrid xerogel is obtained which demonstrate excellent adsorption properties and moreover has excellent mechanical stability such that it is able to carry the adsorbed matter. The chargeable carbohydrate may be carboxymethyl cellulose (CMC). In such a case, the chargeable carbohydrate in the form of CMC may suitably be added in an amount of 10-100 weight-% based on dry weight of the cellulose pulp.

Alternatively, the chargeable carbohydrate may be chitosan. In such a case, the chargeable carbohydrate in the form of chitosan may suitably be added in an amount of 10-100 weight-% based on dry weight of the cellulose pulp.

Alternatively, the chargeable carbohydrate may be alginate. In such a case, the chargeable carbohydrate in the form of alginate may suitably be added in an amount of 0.1-30 weight-% based on dry weight of the cellulose pulp. Preferably, alginate may be added in an amount of up to 20 weight-% of the dry weight of the cellulose pulp, more preferably in an amount of up to 10 weight-% of the dry weight of the cellulose pulp.

The aqueous sodium hydroxide solution may suitably comprise 5-15 % by weight of sodium hydroxide. Such an amount facilitates dissolving the cellulose pulp in the aqueous sodium hydroxide solution. Preferably, the aqueous sodium hydroxide solution comprises 6-10 % by weight of sodium hydroxide, more preferably 7-9 % by weight of sodium hydroxide.

The process may further comprise adding one or more metal oxides to the aqueous sodium hydroxide solution. The purpose of such an addition of one or more metal oxides is primarily to further facilitate the dissolution of the cellulose pulp. Preferably, at least zinc oxide is added to the aqueous sodium solution since this provides the best results. It has further been found that the addition of zinc oxide to the aqueous sodium hydroxide solution further improves the adsorption properties of the resulting hybrid aerogel or hybrid xerogel. When added, zinc oxide may suitably be added in an amount of up to 1.5 weight-% based on the weight of the aqueous sodium hydroxide solution (before comprising dissolved cellulose pulp and/or chargeable carbohydrate). Preferably, 0.5-1.0 weight-% of zinc oxide is added, more preferably 0.6-0.9 weight-%.

The dissolution of the cellulose pulp in the aqueous sodium hydroxide solution may preferably be performed at a temperature of from -15 °C to 0 °C. Such a temperature facilitates the dissolution and ensures that as much as possible, preferably essentially all, of the cellulose pulp is dissolved. The step of adjusting the temperature of the hybrid gel solution to a temperature sufficient to cause gelation of the solution to thereby obtain a hybrid gel may suitably comprise adjusting the temperature to between 30 °C and 70 °C (including the end values of the range), preferably between 40 °C and 60 ° (including the end values of the range). The hybrid gel solution is held at said temperature for a period of time sufficient to cause the desired gelation. The holding time is dependent of the temperature but is at least 1 hour. In general, the holding time is in most cases at least 10 hours.

The process may further comprise a step of physically forming particles of the hybrid gel prior to regenerating the hybrid gel in the organic protic solvent. Such particles facilitate the handling of the hybrid gel as well as the subsequent drying thereof.

The process may further comprise a step of neutralising the hybrid gel prior to drying the hybrid gel by supercritical drying or drying in ambient air. The neutralisation is performed after regenerating the hybrid gel in the organic protic solvent. The neutralisation is preferably performed by subjecting the hybrid gel to acetic acid. It has been found that the adsorption properties are significantly further improved by such a neutralisation step.

The cellulose pulp may suitably be prepared by a process comprising the following steps:

I. providing a fiber source material;

II. subjecting the fiber source material to pre-hydrolysis;

III. subjecting the pre-hydrolysed fiber source material to alkaline chemical pulping process, preferably kraft pulping, to obtain an alkaline pulp;

IV. optionally adjusting the pH of the obtained pulp to above pH 9; and

V. subjecting the alkaline pulp to a bleaching sequence comprising a step of contacting the pulp with ozone (Z) in alkaline conditions.

Such a process for preparing a cellulose pulp results in a cellulose pulp, which may easily be dissolved in an aqueous sodium hydroxide solution.

Preferably, the bleaching sequence further comprises at least a step of contacting the pulp with oxygen prior to contacting the pulp with ozone, and/or at least a step of contacting the pulp with chlorine dioxide after the step of contacting the pulp with ozone. This further improves the dissolution of the cellulose pulp in an aqueous sodium hydroxide solution.

The present disclosure also relates to hybrid aerogel containing cellulose or hybrid xerogel containing cellulose obtained by the process as disclosed above. Such a hybrid aerogel or hybrid xerogel may be considered to be cellulose based and comprising charged carbohydrate polymer.

The present disclosure also relates to a cellulose based hybrid aerogel or a cellulosed based hybrid xerogel comprising charged carbohydrates with carboxylic acid groups as sodium salts and zinc salts, respectively. Moreover, the sodium cations and the zinc cations in the hybrid aerogel or hybrid xerogel are attached to cellulose hydroxyl groups in the form of complexes. Such a cellulose based hybrid aerogel or hybrid xerogel is obtainable by the process as disclosed above in case zinc oxide has been added to the aqueous sodium hydroxide solution. Such a cellulose based hybrid aerogel or cellulose based hybrid xerogel exhibits superior adsorption properties since the sodium cations and zinc cations participate actively in the adsorption.

The present disclosure further relates to the use of a hybrid aerogel or xerogel, produced according to the process disclosed above, for adsorption of gas or moisture, or as carrier of at least one cationic or anionic active substance.

The present disclosure further relates to the use of a hybrid aerogel or hybrid xerogel containing cellulose, as described above, for adsorption of gas or moisture, or as carrier of at least one cationic or anionic active substance.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 illustrates a flow chart of the process of the present invention.

Fig. 2 illustrates a DVS isothermal plot for a hybrid xerogel obtained from cellulose and CMC, regenerated in isopropanol.

Fig. 3 illustrates a DVS isothermal plot for a hybrid xerogel obtained from cellulose and CMC,

regenerated in ethanol, the hybrid gel subjected to acetic acid. Fig. 4a illustrates a DVS isothermal plot for a hybrid xerogel obtained from cellulose and CMC, ratio 1:1, regenerated in 95% ethanol and 5% isopropanol, the hybrid gel subjected to acetic acid.

Fig. 4b illustrates a DVS isothermal plot for a hybrid xerogel obtained from cellulose and CMC, ratio 3:1, regenerated in 95% ethanol and 5% isopropanol, the hybrid gel subjected to acetic acid.

Fig. 5 illustrates a DVS isothermal plot for a hybrid xerogel obtained from cellulose and alginate, ratio 4:0.1, regenerated in ethanol.

Fig. 6 illustrates a DVS isothermal plot for a hybrid xerogel obtained from cellulose and alginate, ratio 1:0.1, regenerated in 90% ethanol and 10% isopropanol, the hybrid gel subjected to acetic acid.

Fig. 7 illustrates a DVS isothermal plot for a xerogel obtained from cellulose, regenerated in

ethanol.

DETAILED DESCRIPTION

In the following, the present invention will be described in more detail with reference to certain embodiments and experimental results. These embodiments and experimental results shall however not be construed as limiting the scope of the present invention and are to be considered for illustrative purposes only. The invention may be varied within the scope of the appended claims.

The process of the present invention enables a cellulose based hybrid aerogel or hybrid xerogel. The difference between aerogel and xerogel resides in the step of the replacement of the liquid component in the gel with air when drying the gel. An aerogel may be obtained when using methods such as supercritical drying or freeze-drying, whereas a xerogel is obtained when drying using ambient air.

For reasons of process economy and efficiency, drying in ambient air is desirous in comparison with supercritical drying and freeze-drying. Flowever, in many cases, drying of a gel in ambient air may cause a collapse due to the capillary forces or result in a shrinkage of the structural network of particles. Therefore, supercritical drying or freeze-drying is generally used.

It has however been found that the process according to the present invention may utilise drying in ambient air and still obtain a structurally stable xerogel. This is believed to be a result of the usage of a chargeable carbohydrate in addition to the cellulose pulp. The chargeable carbohydrate may be a charged carbohydrate or alternatively a carbohydrate which becomes charged when present in an aqueous sodium hydroxide solution. The hybrid xerogel does not hornify, not even during cyclic adsorption conditions. Furthermore, even though drying in ambient air is desired, it is also possible to perform drying by means of supercritical drying and thereby obtain an aerogel.

Figure 1 illustrates a flow chart of the process for preparing a hybrid aerogel or hybrid xerogel according to the present invention. The process comprises the following steps:

a) providing an aqueous sodium hydroxide solution; S100

b) dissolving a cellulose pulp in the aqueous sodium hydroxide solution at a temperature of 10 °C or lower, preferably a temperature of 5 °C or lower; S200

c) adding a chargeable carbohydrate in an amount of 0.1-100 wt-% based on dry weight of the cellulose pulp to the aqueous sodium hydroxide solution either before, during or after step b); S300;

thereby obtaining a hybrid gel solution comprising the aqueous sodium hydroxide solution, dissolved cellulose pulp and charged carbohydrate;

d) adjusting the temperature of the hybrid gel solution to a temperature sufficient to cause gelation of the solution, thereby obtaining a hybrid gel; S400;

e) regenerating the hybrid gel in an organic protic solvent, preferably wherein the organic protic solvent is ethanol, isopropanol, or mixtures thereof; S500; and

f) after regenerating, drying the hybrid gel by supercritical drying to thereby obtain a hybrid aerogel, or drying the hybrid gel in ambient air to thereby obtain a hybrid xerogel; S600.

The chargeable carbohydrate may be a charged carbohydrate, such as carboxymethyl cellulose. Alternatively, the chargeable carbohydrate may be a carbohydrate which will be charged when present in the aqueous sodium hydroxide solution, such as chitosan or alginate.

In the following, the different steps of the process are disclosed in more detail. Process for preparing and obtaining dissolvable cellulose pulp

The cellulose pulp used in the process according to the present invention is a dissolvable cellulose pulp. The dissolvable cellulose pulp is preferably prepared by a process comprising the following steps:

Firstly, a fiber source material is provided. The fiber source material can be based on hardwood, softwood, non-wood, such as annual plants, or mixtures thereof. According to one embodiment, the fiber source material comprises softwood, more preferably consists of softwood, whereby a high yield with good quality can be obtained in an alkaline chemical pulping process. Examples of suitable softwood species include, but are not limited to, spruce, pine, fir, larch, cedar, and hemlock.

Examples of suitable hardwood species include, but are not limited to, birch, oak, poplar, beech, eucalyptus, acacia, maple, alder, aspen, gum trees and gmelina. Examples of annular plants include, but are not limited to, bamboo and bagasse.

The fiber source material is suitably debarked, chipped into desired size and screened to remove pin chips, oversized and over-thick chips before being used in the subsequent steps of the process for preparing a dissolvable cellulose pulp.

The fiber source material is thereafter subjected to a pre-hydrolysis step. The purpose of the pre hydrolysis step is to reduce hemicellulose of the fiber source material by degrading hemicellulose, thereby enabling it to be removed in subsequent steps of the process. Pre-hydrolysis may for example be made by treating the fiber source material with steam or water in liquid form at elevated temperatures during a prolonged time period, i.e. autohydrolysis or a dilute mineral acid can be added to the fiber source material. The resulting pre-hydrolysate, i.e. the liquid into which hemicelluloses have been transferred during the pre-hydrolysis step, may suitably be withdrawn after the pre-hydrolysis step.

According to one aspect, pre-hydrolysis can be performed by treating the fiber source material at an elevated temperature of about 120-180 °C during at least 20 minutes under acidic conditions. The pre-hydrolysed fiber source material is thereafter subjected to an alkaline chemical pulping process to obtain an alkaline pulp. The alkaline chemical pulping process is preferably kraft pulping, but soda pulping process is also plausible. In the alkaline chemical pulping process, the fiber source material is delignified into a pulp which has high cellulose content and in which the lignin is degraded and solubilized leading to defibration of the fibers of the fiber source material. Kraft pulping is well known and will therefore not be described in more detail herein. Optionally, the pre-hydrolysed fiber source material may be impregnated with white liquor at a temperature which is lower, such as about 20-70 °C lower, than the temperature of the alkaline chemical pulping process temperature, prior to the alkaline chemical pulping process. Such an impregnation enables a better distribution of the cooking chemicals into the fiber source material and thus a more homogenous cook can be obtained.

The process may optionally comprise adjusting the pH of the obtained alkaline pulp, if necessary for the subsequent bleaching sequence. The pH may for example be adjusted to above 9, such as pH 10-

13.

Thereafter, the alkaline pulp is subjected to a bleaching sequence to ultimately obtain the dissolvable cellulose pulp. By bleaching is meant a chemical processing of pulp by means of a bleaching agent to increase the brightness and/or whiteness of the pulp. Bleaching also involves delignification of the pulp, i.e. removing residual lignin from the pulp. A bleaching sequence comprises in general a plurality of bleaching steps. Bleaching agents that are commonly used in steps of bleaching sequences include oxidants such as oxygen (O), chlorine dioxide (D), ozone (Z), or any oxidant containing a peroxide group such as hydrogen peroxide (P, Pa, Px). Further bleaching agents generally used are chlorine (C), sodium hypochlorite (H), sodium hydroxide extraction (E, Eo, Ep,

Eop), chelation agents to remove metals (Q), enzymes (X), peracetic acid (T, Paa) and sodium hydrosulphite (Y) or any combination thereof.

The bleaching sequence used in the present process for preparing a dissolvable cellulose pulp comprises a step of contacting the pulp with ozone under alkaline conditions, i.e. a Z-step. This Z-step is preferably performed at a pH of 10-13, however at least at a pH at or above 9. In case the pH of the alkaline pulp resulting from the alkaline chemical pulping process is below 9, the pH is adjusted to the desired pH before the ozone step of the bleaching sequence. It has been found that a process as described above results in a cellulose pulp with significantly higher dissolving yield, when compared to a pulp of similar chemical composition and intrinsic viscosity that has not been subjected to the above described process for preparing a cellulose pulp.

The bleaching sequence suitably also comprises a step of contacting the pulp with oxygen, i.e. an O step, prior to contacting the pulp with ozone under alkaline conditions. By means of such a step, a more gentle way of reducing the kappa number (residual lignin) in the pulp than with a prolonged cook can be achieved. The step of contacting the pulp with oxygen may be performed in several steps following each other, such as (OO). Several steps may further improve the delignification effect. The different O steps may be performed in the same way or differently. For example, the first O step can be made shorter that the second O step to further increase the gentleness of the process and so that minimal amount of cellulose is deteriorated in the delignification process.

The bleaching sequence may further comprise a step of contacting the pulp with chlorine dioxide, i.e. a D step after the step of contacting the pulp with ozone under alkaline conditions. Thereby, further improved delignification can be achieved. The bleaching sequence may also comprise more than one D step.

Examples of especially preferred bleaching sequences in the process for preparing a dissolvable cellulose pulp include OOZDD and OOZDED. Thereby, a cellulose pulp with high dissolve yield is achieved.

Examples of processes for producing a dissolvable cellulose pulp are for example disclosed in WO 2016/080895 Al.

Preparation of hybrid gel solution

In accordance with the present invention, a hybrid gel solution is prepared for the purpose of preparing a hybrid aerogel or hybrid xerogel. In the present disclosure, a hybrid gel solution shall be interpreted as a solution comprising the dissolved constituent components, i.e. dissolved cellulose pulp as well as dissolved charged carbohydrate. Thus, even though denominated as a "gel solution", the hybrid gel solution is not a gel but a solution provided for the purpose of forming a gel in subsequent steps. The hybrid gel solution is prepared by dissolving a cellulose pulp, preferably the cellulose pulp described above, and a chargeable carbohydrate in an aqueous sodium hydroxide solution. When the chargeable carbohydrate is added and dissolved in the aqueous sodium hydroxide solution, it will be present in a charged state. Thus, the resulting hybrid gel solution comprises dissolved cellulose pulp as well as a charged carbohydrate.

According to one option, the chargeable carbohydrate may be carboxymethyl cellulose.

Carboxymethyl cellulose is in itself charged and remains charged when present in the aqueous sodium hydroxide solution. According to another option, the chargeable carbohydrate is chitosan. Chitosan will become charged when present in the aqueous sodium hydroxide solution. According to yet an option, the chargeable carbohydrate is alginate. Alginate will also become charged when present in the aqueous sodium hydroxide solution. Other plausible chargeable carbohydrates include starch, hemicelluloses and pectin.

Even though the cellulose pulp used in accordance with the present invention preferably has been prepared so as to increase its solubility in an aqueous sodium hydroxide solution, it is necessary to reduce the temperature to 10 °C, preferably 5°C, or lower in order to be able to dissolve the cellulose pulp to a sufficient degree. Preferably, the temperature during dissolution of the cellulose pulp is equal to or below 0 °C, such as -15 °C to 0°C. In contrast, chargeable carbohydrates, such as carboxymethyl cellulose, chitosan and alginate, can in general dissolve at any temperature in an aqueous sodium hydroxide solution. Therefore, the addition of the chargeable carbohydrate to the aqueous sodium hydroxide solution can be made either before the dissolution of the cellulose pulp, simultaneously with the dissolution of the cellulose pulp or after the dissolution of the cellulose pulp. If the addition of the chargeable carbohydrate is not performed simultaneously with the dissolution of the cellulose pulp, the aqueous sodium hydroxide solution may have a temperature of 10 °C or below, but may alternatively have a higher temperature, such as room temperature.

Even though the process for preparing a dissolvable cellulose pulp as disclosed above is preferred, it may be possible to dissolve other cellulose pulps. However, in such cases the yield is lower and the process has to be more closely controlled, such as in terms of concentration windows and/or temperature. The aqueous sodium hydroxide solution may suitably comprise 5-15 % by weight of NaOH, preferably 6-10 % by weight of NaOH, most preferably 7-9 % by weight of NaOH, based on total weight of the aqueous sodium hydroxide solution. In this context, the total weight of the aqueous sodium hydroxide solution is considered to be the total weight of the aqueous sodium hydroxide solution without comprising the cellulose pulp and chargeable carbohydrate.

The aqueous sodium hydroxide solution is preferably stirred during the dissolution of the constituent components in order to facilitate the dissolution and mixing. Stirring can be performed by any conventional method used for such purposes.

A metal oxide may suitably be added to the aqueous sodium hydroxide solution. If desired, more than one metal oxide can be added. The metal oxide may suitably be added in the form of a salt to the aqueous sodium hydroxide solution, but the invention is not limited thereto. The purpose of the metal oxide is primarily to facilitate the dissolution of the cellulose pulp. Any metal oxide, except for sodium oxide and potassium oxide, may be used for this purpose. It has however been found that zinc oxide provides significantly better results compared to other metal oxides. Zinc oxide transfers to zinc hydroxide or zincate anion in the aqueous sodium hydroxide solution. It is believed that the zinc ions ensure that the glucan chains of the cellulose pulp are sufficiently separated from each other by avoiding hydrogen bonds or London forces between the cellulose fibrils or microfibrils.

Thus, the zinc oxide is preferably added to the aqueous sodium hydroxide solution. The addition of a metal oxide may suitably be performed before adding the cellulose pulp, but may also be performed simultaneously.

The metal oxide(s) may be added in a total amount of up to and including 1.5 % by weight based on the total weight of the aqueous sodium hydroxide solution. In this context, the total weight of the aqueous sodium hydroxide solution is considered to be the total weight of the aqueous sodium hydroxide solution without comprising the cellulose pulp and chargeable carbohydrate. In accordance with a preferred embodiment, up to 1.5 % by weight of ZnO is added to the aqueous sodium hydroxide solution, more preferably 0.5-1.0 % by weight of ZnO, most preferably 0.6-0.9 % by weight of ZnO, based on the total weight of the aqueous sodium hydroxide solution excluding any dissolved cellulose pulp and/or chargeable carbohydrate.

In accordance an embodiment, the aqueous sodium hydroxide solution consists of: 5-15 % by weight of NaOH, preferably 6-10% by weight NaOH, more preferably 7-9 % by weight of NaOH;

up to 1.5 % by weight in total of one or more metal oxides, preferably wherein said one or more metal oxides constitute ZnO and is present in an amount of 0.5-1.0 % by weight, more preferably in an amount of 0.6-0.9 % by weight; and

the remainder being water.

The cellulose pulp is preferably allowed to swell in an aqueous sodium hydroxide solution before being dissolved in the aqueous sodium hydroxide solution as described above, which may also be considered to constitute a second aqueous sodium hydroxide solution. In practice this may be conducted by allowing the cellulose pulp to swell in a first aqueous sodium hydroxide solution for a suitable time and at a temperature above 10 °C (preferably about 20-25 °C), and thereafter adjusting the composition of the first aqueous sodium hydroxide solution to obtain the above described aqueous sodium hydroxide solution having a composition suitable for dissolving the cellulose pulp and reducing the temperature to 10 °C or lower. Swelling can for example suitably be conducted during a couple of minutes at room temperature. By way of example only, the cellulose pulp may have a dry content of 15-30 % and the first aqueous sodium hydroxide solution may comprise about 5 % NaOH.

When the cellulose pulp is to be dissolved, the cellulose pulp may suitably be added to the aqueous sodium hydroxide solution in an amount corresponding to 1-5 % by weight of dry cellulose pulp based on the total weight of the resulting hybrid gel solution. Preferably, the cellulose pulp is added to the aqueous sodium solution in an amount corresponding to 2-4 % by weight of dry cellulose pulp based on the total weight of the resulting hybrid gel solution. This ensures that the cellulose is sufficiently dissolved in the aqueous sodium hydroxide solution.

In the embodiment wherein carboxymethyl cellulose is used, carboxymethyl cellulose may suitably be added to the aqueous sodium hydroxide solution in an amount of 10-100 % of the dry weight of the cellulose pulp. In other words, carboxymethyl cellulose may be added such that the ratio of carboxymethyl cellulose to cellulose pulp is from 0.1:1 to 1:1 based on dry weight. Preferably, carboxymethyl cellulose is added to the aqueous sodium hydroxide solution in an amount of 20-55 % by dry weight of cellulose pulp. Below an amount of 10 % by weight of carboxymethyl cellulose based on the dry weight of the cellulose pulp, there will not be a sufficient degree of carboxylic acid groups and therefore the adsorption properties will not be sufficient. The charged groups are essential for obtaining the desired adsorption properties. Furthermore, there is no reason for adding more than 100 % by weight of carboxymethyl cellulose of the dry weight of the cellulose pulp and is therefore not desirous for reasons of cost.

In the embodiment wherein chitosan is used, chitosan is suitably added to the aqueous sodium hydroxide solution in an amount of 10-100 % of the dry weight of the cellulose pulp, preferably 20-50 % of the dry weight of the cellulose pulp. This ensures that there is a sufficient amount of charged groups available and therefore that the desired adsorption properties are achieved.

In the embodiment wherein alginate is used, alginate may suitably be added in an amount of up to 30 % of the dry weight of the cellulose pulp. Even when added in such a small amount as 0.1 % of the weight of the cellulose pulp, a sufficient amount of charged groups will be available in the resulting hybrid aerogel or xerogel to significantly improve the adsorption properties compared to a pure cellulose aerogel. An addition of alginate above 30 % of the dry weight of cellulose pulp will still enable an aerogel or xerogel to be formed. However, the amount of cellulose in such an aerogel or xerogel will be too low and thereby result in an inferior mechanical stability. Preferably, alginate is added in an amount of 0.1-10 %, more preferably 0.1-1%, by dry weight of the cellulose pulp.

For reasons of economy, the chargeable carbohydrate is preferably alginate.

Gelation of hybrid gel solution

After the hybrid gel solution has been prepared, the temperature of the hybrid gel solution is adjusted to a temperature sufficient to cause gelation of the hybrid gel solution and held at said temperature for a period of time sufficient to allow for the gelation. Thereby, a hybrid gel is obtained.

Gelation of the hybrid gel solution occurs spontaneously when the temperature of the hybrid gel solution is increased to a temperature of about 30 °C or higher. However, there is no beneficial effect of increasing the temperature too high. Furthermore, there is a risk that gelation proceeds too quickly or in an uncontrolled manner if the temperature is too high, which in turn may affect the viscosity such that formation of particles may be difficult. Thus, the temperature is suitably adjusted to from 30 °C to 70 °C. Preferably, the temperature of the hybrid gel solution is adjusted to from 40 °C to 60 °C (the end values of the range included).

A lower temperature during the gelation in general requires a longer holding time whereas a higher temperature enables a shorter holding time. However, the holding time is preferably at least 1 hour, more preferably at least 10 hours. When the hybrid gel solution has gelled to a hybrid gel, there is no reason to hold the hybrid gel at said temperature and thus the temperature can be lowered.

However, a longer holding time at said temperature does not lead to any deteriorated properties and it is thus not critical that the temperature is lowered after the formation of the hybrid gel has been completed.

According to a preferred aspect, the temperature of the hybrid gel solution is 40-60 °C during gelation and the holding time at least 24 hours, such as about 24-48 hours.

The holding time is also dependent of the amount of hybrid gel solution, wherein a larger amount requires longer holding times. The holding time dependent on the amount of hybrid solution can be readily adjusted by the skilled person by trial and error experiments.

Formation of particles

The process according to the present invention preferably comprises a step of physically forming of particles of the hybrid gel. The purpose of forming particles in the process according to the present invention is primarily to facilitate the subsequent drying step but also facilitates the handling of the resulting hybrid aerogel or xerogel. The particles may suitably have an average size of about 200 pm to about 1 mm, preferably about 300 pm to about 700 pm. In this context, a size of a particle is considered to mean the diameter of the particle which is equivalent to the diameter for a spherical particle. In other words, the size of a particle should be interpreted to mean the diameter of the particle in case it would have been a spherical particle having the same volume unless the particle is in fact a spherical particle, in which case the size constitutes the actual diameter. It should be noted that in general, the particles will be substantially spherical.

The particles may suitably be formed by means of JetCutter technology (for example described by RGϋbq et al. "Production of Spherical Beads byJetCutting”, Chem. Eng. Technol. 23 (2000) 12, p. 1105- 1110). In accordance with the JetCutter technology, a viscous material is pressed with a high velocity out of a nozzle as a solid jet. Directly underneath the nozzle the jet is cut into cylindrical segments by a rotating cutting tool. The cutting tool may be composed of small wires fixed in a holder which rotates. As a result of the surface tension, the cut cylindrical segments form spherical particles while falling further down by means of gravity to an area or receptacle where the spherical particles finally can be gathered. The JetCutter technology is capable of processing fluids with viscosities up to several thousands mPa-s, and particles with sizes ranging from about 200 pm to several millimetres are obtainable.

Alternatively, the particles may be formed by other processes known in the art such as spray forming or by means of emulsification in oil. The emulsification method is however less preferred in accordance with the present invention as it requires that the oil is removed before the drying step, something which may be difficult to achieve and require numerous repeating exchange steps.

Regeneration of the hybrid gel

After the hybrid gel has been formed, the hybrid gel is regenerated. Regeneration is sometimes also referred to as coagulation within the technical field. The purpose of the regeneration is to facilitate the subsequent drying step by replacing the solvent in the gel with a liquid which is easier to remove. It is an important step for maintaining the three-dimensional structure in the hybrid gel.

The regeneration is performed by contacting the hybrid gel with an organic protic solvent, preferably ethanol, isopropanol or mixtures thereof. Most preferably, the organic protic solvent is ethanol. The reason for using ethanol is that regeneration is somewhat faster and it may be easier to obtain spherical particles, thereby obtaining a more homogenous structure of the aerogel or xerogel.

Regeneration is preferably performed after formation of particles to ensure that the organic protic solvent is sufficiently distributed within the hybrid gel.

The regeneration can easily be performed under ambient conditions, such as at room temperature and without any particular atmosphere. If desired, regeneration can be performed in multiple sub-steps, each sub-step comprising subjecting the hybrid gel to the organic protic solvent.

Subjecting the to acetic acid

It has been found that the adsorption capacity of the hybrid aerogel or xerogel according to the present invention can be significantly improved further in case the regenerated hybrid gel is subjected to acetic acid before drying. As will be demonstrated in the experimental results below, the adsorption capacity of a xerogel at a target relative humidity of 90% can be improved in the order of about 30% compared to a xerogel obtained by the present process but without subjecting the regenerated hybrid gel to acetic acid.

Subjecting the hybrid gel to acetic acid may suitably be performed by washing the hybrid gel with acetic acid in one or more steps. Subjecting the hybrid gel to acetic acid neutralises the sodium hydroxide comprised in the hybrid gel. Said neutralisation may also be advantageous for the reasons that it enables the resulting aerogel or xerogel to be used in further applications, in particular applications having high environmental demands or where there may be an issue with regard to corrosion of components coming into contact with the aerogel or xerogel or liquids desorbed from the aerogel or xerogel during cyclic conditions.

It is thus preferable to subject the hybrid gel to acetic acid before the drying step, even though good adsorption properties can be obtained even without such a step.

Drying of the hybrid gel

Drying of the hybrid gel can be performed by supercritical drying, in which case the result will be a hybrid aerogel. Alternatively, drying can be performed under ambient conditions, in which case the result will be a hybrid xerogel. In this context, drying under ambient conditions shall be considered to encompass for example drying in ambient air, drying by forced air or drying by nitrogen gas.

Supercritical drying per se is previously known for example for synthesis of aerogels, and any previously known supercritical drying process for this purpose may be used in the process according to the present invention. During supercritical drying, the liquid of the gel is transformed into gas in the absence of surface tension and capillary stresses, which ensures that the network (out of which the resulting aerogel is composed) does not collapse during drying.

In contrast to supercritical drying, drying under ambient conditions is not possible in most cases where the gel comprises cellulose since the pores generally collapse during such drying due to capillary forces. However, for the hybrid gel according to the present invention, drying under ambient conditions is possible.

The hybrid aerogel or hybrid xerogel

The process described above results in a cellulose containing, preferably cellulose based, hybrid aerogel or hybrid xerogel. The hybrid aerogel or hybrid xerogel comprises charged carbohydrate polymers.

In case the process comprises a step of adding zinc oxide to the aqueous sodium hydroxide solution, the resulting hybrid aerogel or xerogel will comprise carboxylic acid groups attached to

carbohydrates as sodium salts and zinc salts. Furthermore, the sodium cations and the zinc cations, respectively, are attached to cellulose hydroxyl groups in the form of complexes. These sodium cations and zinc cations are able to actively participate in the adsorption of gas or moisture and therefore improve the adsorption properties of the hybrid aerogel or hybrid xerogel. Sodium and zinc originate from the aqueous sodium hydroxide solution.

The hybrid aerogel and the hybrid xerogel as described above have excellent adsorption properties, even under cyclic environmental conditions, as well as high mechanical stability. Furthermore, the cellulose based hybrid aerogel and the cellulose based hybrid xerogel have good insulating properties as well as low thermal conductivity.

The hybrid aerogel and the hybrid xerogel can be used for various applications, in particular for gas or moisture adsorption or as carrier of cationic or anionic active substances. Some examples of more specific applications include (but are not limited to):

in cosmetics and hygiene products, for example for the replacement of micro plastics conventionally used,

in cleaning products, for example for large oil spill clean-up, in biocatalysis, such as for immobilization of enzymes, anti-bodies etc.,

in nutrition encapsulation, for example encapsulation of minerals etc.,

for controlled release, for example for specific controlled release of drugs, or

construction materials, in particular where mechanical stability, insulation properties and low thermal conductivity is required.

Experimental results

Experimental result 1

Cellulose pulp was obtained by pre-hydrolysis of wood chips, sulphate cook, followed by oxygen delignification and bleaching using an (OODED) sequence.

The bleached cellulose pulp was dissolved in an aqueous mixture of NaOH (8 weight-%) and ZnO (0.8 weight-%) at a temperature of between -8 °C and -12 °C together. Furthermore, carboxymethyl cellulose (CMC) was added simultaneously with the cellulose pulp. The ratio of cellulose pulp to CMC was 2:1 based on dry weight.

300 ml of the aqueous sodium hydroxide solution comprising the dissolved matter was allowed to gel at about 50 °C for a few hours.

Particles with a diameter of about 500 pm were prepared by means of JetCutter. Regeneration was performed by means of isopropanol. Thereafter, drying was performed by air drying at room temperature whereby a hybrid xerogel was obtained.

Moisture adsorption was investigated by Dynamic Vapour Sorption (DVP) technique, whereby the weight of an amount of sample of approximately lOg dry was recorded on a balance in an enclosed environment. The environment was successively changed between different RH, preferably 0, 80 and 90 % RH by mixing dry and saturated air at the specified temperature in accurate proportions for the desired RH +/- 0.1%. The result is shown in Figure 2. As can be seen from the figure, a mass change of about 60 % was obtained in the first cycle at a relative humidity of 80 % and about 50 % in the second cycle at a relative humidity of 80%. At 90 % relative humidity, the mass change is about 100 % and about 90 % for the first and second cycle, respectively. The reason for considering a relative humidity of 80% and 90% is that it in many cases constitutes a practical target relative humidity since at higher relative humidity condensation of moisture may occur.

These figures may for example be compared with commercially available silica aerogels which often demonstrates less than 20% mass change at a relative humidity of 80%.

At a relative humidity of 100 %, a mass change of about 100 % was obtained.

Experimental result 2

Experimental result 1 was repeated except that regeneration was performed using ethanol, and that prior to the drying step, the hybrid gel was subjected to acetic acid.

Moisture adsorption was investigated as described above in Experimental result 1. The result is shown in Figure 3. As can be seen from the figure, a mass change of about 90 % was obtained at a relative humidity of 80 % both in the first and in the second cycle. At a relative humidity of 90 %, a mass change of about 130 % was obtained.

It should be noted that the organic protic solvent used for regeneration does not significantly affect the adsorption properties of the hybrid xerogel. Therefore, when comparing the result with the result from Experimental result 1, it can be seen that subjecting the hybrid gel to acetic acid significantly improves the adsorption capacity of the hybrid xerogel.

Experimental result 3

Hybrid xerogels were prepared as described above with regard to Experimental result 1 subject to alteration of the relative amounts of cellulose pulp and CMC, that regeneration was performed using a mixture of 95% ethanol and 5% isopropanol, and that the hybrid gels were subjected to acetic acid.

In the first sample, the ratio cellulose pulp to CMC was 1:1 based on dry weight. In the second sample, the ratio cellulose pulp to CMC was 3:1 based on dry weight. Moisture adsorption was investigated as described in Experimental result 1. Figure 4a illustrates the result for the first sample and Figure 4b illustrates the result for the second sample. As can be seen from the figures, both samples have good adsorption properties with a mass change at 90 % relative humidity of about 100 % and 120 %, respectively. The second example comprising a higher relative amount of cellulose pulp demonstrates a higher mass change than the first sample. The adsorption remains the same for both samples up to the 5 cycles investigated.

Experimental result 4

The bleached cellulose pulp prepared as described in Experimental result 1 was dissolved in an aqueous mixture of NaOFI (8 weight-%) and ZnO (0.8 weight-%) at a temperature of between -8 °C and -12 °C together with alginate. The ratio between cellulose pulp and alginate was 4:0.1 based on dry weight.

300 ml of the aqueous sodium hydroxide solution comprising the dissolved matter was allowed to gel at about 50 °C for a few hours.

Particles with a diameter of about 500 pm were prepared by means of JetCutter. Regeneration was performed by means of ethanol. Thereafter, drying was performed by air drying at room

temperature whereby a hybrid xerogel was obtained.

Moisture adsorption was investigated as described above in Experimental result 1. The result is shown in Figure 5. As can be seen from the figure, a mass change of about 60 % was achieved at a relative humidity of 90 % and a mass change of above 30 % at a relative humidity of 80%. The adsorption capacity is similar for all five cycles investigated.

Experimental result 5

Experimental result 4 was repeated except that the ratio between cellulose pulp and alginate was 1:0.1 based on dry weight, regeneration was performed in a mixture of 90% ethanol and 10% isopropanol, and that the hybrid gel was subjected to acetic acid. Moisture adsorption was investigated as described in Experimental result 1. The result is shown in Figure 6. As can be seen from the figure, a mass change of 70% was obtained at a relative humidity of 90% and a mass change of about 40 % was obtained at a relative humidity of 80%. The adsorption capacity is similar for all five cycles investigated.

Experimental result 6 - Comparative

A xerogel was prepared from the same cellulose pulp used in Experimental result 1. The xerogel was produced by dissolving the cellulose pulp in an aqueous sodium hydroxide solution having the same composition as given in Experimental result 1. A chargeable carbohydrate was not added to the solution.

300 ml of the solution comprising the dissolved cellulose pulp was allowed to gel at a temperature of about 50 °C for a few hours. Particles with a diameter of about 500 pm were formed using JetCutter. Regeneration was performed using ethanol. Drying was performed in ambient air at room temperature.

Moisture adsorption was investigated by Dynamic Vapour Sorption (DVP) technique in the same way as described above with regard to Experimental result 1. Figure 7 illustrates the result. As can be seen in the figure, a mass change of between 20% and 25% is obtained at a relative humidity of 80 % and a mass change of about 30 % at a relative humidity of 90%.