EP1585772A1

EP1585772A1 - Method for the production of a porous cellulose body

Info

Publication number: EP1585772A1
Application number: EP04703120A
Authority: EP
Inventors: Heinrich Firgo; Hartmut Rüf; Karl Michael Hainbucher; Hedda Weber
Original assignee: Lenzing AG; Chemiefaser Lenzing AG
Current assignee: Lenzing AG
Priority date: 2003-01-20
Filing date: 2004-01-19
Publication date: 2005-10-19
Also published as: TW200418558A; ATA712003A; AT412404B; WO2004065424A1

Abstract

The invention relates to a method for the production of a porous cellulose body, consisting of the following steps: a cellulose solution is produced in a tertiary amino oxide, particularly N-methylmorpholine-N-oxide; the cellulose used has an average polymerization degree of 150 - 2000 and the solution has a 0.1 - 5 wt. % concentration of cellulose; a body is produced from said cellulose solution; the body is precipitated in a precipating agent, thereby enabling a precipitated cellulose body to be obtained; the precipitating agent is an aqueous medium or a cellulose-precipitating non-aqueous medium which cannot be mixed with water, said non-aqueous medium being able to dissolve the tertiary amino oxide and being able to be mixed with a supercritical solvent. The inventive method is characterised in that if the aqueous medium is used as a precipitating agent, the tertiary amino oxide contained in the precipitated cellulose body is washed out with the aqueous medium; the aqueous medium is replaced by a liquid medium which can be mixed with a supercritical solvent; the cellulose body which is wetted with the replacement medium is treated with the supercritical solvent, enabling a porous cellulose body to be obtained; or if a non-aqueous medium is used as a precipitating medium, the tertiary amino oxide contained in the cellulose body is washed out with the non-aqueous medium; the cellulose body which is wetted with the non-aqueous medium is treated with the supercritical solvent in order to obtain a porous cellulose body.

Description

Process for the production of a porous cellulosic body

The present invention relates to a method for producing a porous cellulosic body, a porous cellulosic body and its use and a porous carbon body which can be produced from the cellulosic body by pyrolysis and its use.

The porous bodies according to the invention are so-called aerogels.

According to Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 2002 Electronic Release, aerogels are highly porous solids with very low density. In a narrower definition, aerogels are materials in which the pore and network structure is completely or largely preserved when the pore fluid of a gel is replaced by air.

A gel consists of a sponge-like, three-dimensional network, the pores of which are filled with a liquid. In contrast to aerogels, in which the network structure remains essentially unchanged during drying, conventionally drying (increasing the temperature and / or reducing the pressure) gives strongly shrunk bodies, which are called xerogels. The shrinkage of the gel is caused by the capillary forces at the liquid / gaseous interface of the evaporating pore liquid.

The type of drying of the gel is therefore the decisive step for the production of highly porous aerogels. The technologies are applied:

- supercritical drying

- in organic solvents

- in carbon dioxide (hereinafter the term "supercritical solvent" is used both for supercritical organic solvents and for supercritical carbon dioxide)

- Freeze drying

- Drying at normal pressure - this is only possible with particularly stable networks.

The unique optical, thermal, acoustic and mechanical properties of aerogels are caused by the combination of a solid matrix with air-filled pores in the nanometer range. The majority of industrially produced aerogels are inorganic in nature and consist of silica and metal oxides. However, organic aerogels are also known (for example US Department of Energy, US 4,873,218).

Highly porous moldings are known for cellulose, but with few exceptions the patent literature only describes the production in the wet state, in these cases it is not aerogels.

A cellulose xanthate solution or cellulose acetate, which is hydrolyzed, is generally assumed.

In the manufacture according to the viscose process according to US 4,055,510

Viscose (solution of cellulose xanthate in dilute sodium hydroxide solution) dispersed in a water-immiscible organic solvent, such as chlorobenzene,

solidified the dispersed viscose by means of a thermally induced sol-gel transition (heating the mixture to 90 ° C.),

- cellulose beads are separated,

- Regenerated with acid in an aqueous medium and

- washed.

The cellulosic bodies obtained in this way have porosities (volume fraction water) of 90%. The water can (according to: J. Stamberg et al., Acta Polymerica 30 (1979) Issue 12, 734-739) be replaced by various organic solvents, and the degree of porosity remains almost unchanged.

After drying, products with porosity values of 0% to 70% are obtained, depending on which solvent was last in contact with cellulose before drying. However, it can be seen from this that even under the gentlest drying conditions there is a considerable loss of porosity.

The highest porosity value of a dried cellulose body known to the applicant is a value of 83.6%, which is described in Peska et al., Cell. Chem. Techn. 21 (1978) pp. 419-428.

Further examples for the production of cellulosic bodies are described in: Stamberg, J. "Bead Cellulose", Separation and Purification Methods 17 (2) (1988)

155-183

US 2,543,928

DE 1, 792,230 corresponds to US 3,597,350

DD 1 18,887 corresponds to US 4,055,510

GB 1, 575,700

US 4,312,980

US 4,946,953

WO 91/00142

WO 91/09878 corresponds to US 5,527,902

US 5,328,603

EP 850,979

DD 118,887 contains the statement that only cellulose balls with a low pore volume of less than 30% are dimensionally stable and can be dried without special measures to maintain the porosity.

In GB 1, 575,700 a hydrolyzable cellulose derivative such as e.g. Cellulose acetate dissolved in a water-miscible organic solvent, the solution divided into drops and introduced into a precipitation bath (e.g. water), the beads separated, washed, the cellulose derivative hydrolyzed to cellulose and washed again. A wet product is thus claimed; the drying with supercritical carbon dioxide mentioned in Example 1 is used exclusively for the preparation of a pattern suitable for characterization by means of electron microscopy.

A disadvantage of many processes known from the prior art are ecological problems, such as the use of chlorinated organic solvents, the emission of toxic sulfur compounds in the viscose process or the use of copper-containing solvents for the cellulose. Processes that use alternative cellulose solvents, such as the combination of dimethylacetamide / lithium chloride, have so far failed to provide evidence of an industrially feasible recovery option for the solvent.

Processes based on viscose or sodium hydroxide are restricted to the dissolution of cellulose with a low degree of polymerization. In addition to an overview of processes previously used for the production of cellulose beads, WO 99/31 141 contains the description of a process with the following steps:

- Preparation of a cellulose solution (degree of polymerization 150 to 2000, cellulose concentration 0.5 to 25% by weight)

- Fine division of the cellulose solution and dispersion with a dispersant that is not miscible with the solution

- Solidification of the solution particles by a) cooling the dispersion below the melting temperature of the cellulose solution and separating the solidified particles from the dispersant or b) by precipitating the particles with a precipitant which is miscible with the dispersant

- separation of the pearl particles from the liquid.

To produce particles with a particle size range from 50 μm to 1000 μm, the solution becomes a jet with a diameter in the range from 40 μm to

Deformed 1000 μm and the beam split into segments using rotating cutting beams.

Particles with a pore volume of 5% to 95% are obtained in the wet state. The possibility of drying the cellulose beads is mentioned, but without further information in WO 99/31 141 it can be assumed on the basis of the above-mentioned literature that only a product with low porosity is produced. N-Methylmoφholin-N-oxide (NMMO) is used as a solvent for cellulose in the examples.

Another method for producing porous cellulose beads is in the

WO 02/057319 described. This document also does not contain any details about the possible drying of the cellulose beads produced.

In one example (example 2) beads with an average diameter of 2.2 mm are obtained. After pyrolysis of the beads, which also contain a proportion of Al ₂ O, and subsequent sintering, the beads have a diameter of 1.1 mm. In another example (example 4), the undried beads have an average diameter of 3.13 mm. After drying (not described in detail), the diameter is only 0.8 mm. It can therefore be assumed that the porosity of these pearls has drastically reduced during drying.

The method according to the invention for the production of porous cellulosic bodies comprises the steps Production of a cellulose solution in a tertiary amine oxide, in particular N-methylmoφholin-N-oxide, the cellulose used having an average degree of polymerization of 150 to 2000 and the solution having a cellulose concentration of 0.1 to 5% by weight, production of a body the cellulose solution

Precipitation of the body in a precipitant, whereby a precipitated cellulosic body is obtained, the precipitant being an aqueous medium or a cellulose-precipitating, water-miscible, non-aqueous medium which is non-aqueous

Medium can dissolve the tertiary amine oxide and with a supercritical

Solvent is miscible and is characterized in that if an aqueous medium is used as the precipitant in the precipitated cellulosic body, tertiary amine oxide contained in the precipitated cellulose is washed out with the aqueous medium, the aqueous medium is exchanged for a liquid exchange medium which is miscible with a supercritical solvent and which is moist with the exchange medium cellulosic body is treated with the supercritical solvent, whereby the porous cellulosic body is obtained or that, if a non-aqueous medium is used as a precipitant, tertiary amine oxide contained in the precipitated cellulosic body is washed out with the non-aqueous medium which is washed with the non-aqueous medium. aqueous medium moist cellulosic body is treated with the supercritical solvent, whereby the porous cellulosic body is obtained.

For the purposes of the present invention, the term “cellulosic” means cellulose, cellulose derivatives soluble in tertiary amine oxides and mixtures of cellulose and / or soluble cellulose derivatives and other polymers soluble in NMMO, such as, for example, polyamides.

The dissolving of cellulose in tertiary amine oxides, such as NMMO, is known from the prior art, usually from a suspension of cellulose in one aqueous tertiary amine oxide, preferably N-methylmoφholin-N-oxide (NMMO), a solution of cellulose is formed by evaporating excess water.

Carbon dioxide is preferably used as the supercritical solvent in the process according to the invention.

The treatment of cellulose with supercritical carbon dioxide is described in:

- EP-A 1, 205.598 (extraction of resin from cellulose to improve absorption capacity)

- PCT WO 2000/015668 (disintegration of cellulosic materials by means of steam explosion).

Both patents are based on dry cellulose, so it is not a matter of preserving a highly porous wet gel network.

Organic solvents which are suitable as precipitants for the purposes of the present invention are water-miscible, act as precipitants in relation to cellulose in amine oxide solutions, are able to dissolve NMMO, should cause the primary gel swelling of the cellulose to be as high as possible and should be good with supercritical carbon dioxide be miscible. Alcohols with 1 to 4 carbon atoms, such as Ethanol. Acetone can also be used as the non-aqueous medium or as the liquid exchange medium.

The aqueous medium, which can be used as a precipitant, can consist of water or of mixtures of water with other solvents (provided the cellulose-precipitating effect is retained).

The precipitant (aqueous medium or non-aqueous medium) can additionally contain further components, such as e.g. Parts of solvent (amine oxide) included.

So-called supercritical drying takes place during the treatment with the supercritical solvent. During supercritical drying, the liquid / gaseous phase interface disappears in the pores.

In the preferred case of supercritical drying with carbon dioxide, for example, an alcohol or acetone-moist gel is covered in an autoclave with excess alcohol / acetone (to prevent premature drying), then the temperature and the CO ₂ pressure are slowly increased and for some time at values above the critical point of C0 ₂ (31, 1 ° C / 73.8 bar) held until the solvent in the cellulose body is removed quantitatively. The pressure and temperature are then adjusted so that the CO _{2 is} slowly removed in the gaseous state.

General information on the technique of supercritical drying with C0 ₂ see PH Tewari, AJ Hunt, KD Lofftus in: J. Fricke (ed.): Aerogels (Springer Proc. Phys. 6), Springer, Berlin 1986, p. 31st

In a preferred embodiment of the method according to the invention, the cellulose solution is processed in a manner known per se, e.g. by extrusion through a molding tool or by casting into a corresponding mold, formed into a shaped body.

The molded body can be, for example, a fiber, a film, a block or a plate.

However, spherical or pearl-shaped particles can also be produced from the cellulose solution by dropping the solution or, for example, using the technology described in WO 99/31 141. Furthermore, particles can be produced from the solution by means of the jet cutter technology (from Genialab) and by means of vibration nozzles.

With the method according to the invention, a powder can also be produced from a large number of porous cellulose particles.

A further preferred embodiment of the method according to the invention is characterized in that the cellulosic body is treated with a crosslinking agent, such as e.g. described in WO 91/09878.

The porous cellulosic body produced by the process according to the invention can also be subjected to pyrolysis.

With the method according to the invention it is possible for the first time to provide a porous cellulosic body with a porosity in the dry state of 85% and more. The body obtained by treatment with the supercritical solvent is thus a highly porous airgel. The cellulosic body according to the invention preferably has a porosity of 90% and more, particularly preferably of 95% and more. The porosity is defined as follows for the purposes of the present invention: The cellulosic body obtained is measured in a microscope and the volume is calculated therefrom. The cellulosic body is weighed, which gives the density of the body (mass / volume). From the measured density of the cellulosic body, the formula (1 - density of the body / 1, 6) * 100 gives the porosity of the body, ie the percentage of voids in the body. In the case of a large number of bodies (eg pearls), the average is taken from several measurements.

The invention further relates to a porous cellulosic body which can be obtained by the process according to the invention. This body differs from known porous cellulosic bodies produced by the amine oxide process, e.g. described in WO 99/31 141 and WO 02/057319, characterized in that the porosity is significantly higher in the dry state.

Due to their low density, cellulosic bodies according to the invention can be used in particular as insulation material, for example in thermal or acoustic insulation. Furthermore, the cellulosic bodies according to the invention can be used as a dielectric in the electrical and electronics industry, for impedance matching in acoustic applications, as a means for absorbing and cleaning gases, as a carrier for catalysts and / or as a means for storing energy sources, such as e.g. Hydrogen.

The invention further relates to a porous carbon body which can be obtained from the cellulosic body according to the invention by means of pyrolysis.

Particularly interesting areas of application open up for these carbon bodies. The carbon airgel resulting after pyrolysis of the cellulose component is suitable as an electrode material for batteries, accumulators, capacitors and fuel cells and for high-temperature insulation. At the same time, such carbon aerogels, in analogy to carbon nanotubes, represent efficient hydrogen stores.

Example:

A cellulose solution with 1% by weight> cellulose (cellulose Solucell with a viscosity according to SCAN CM 15:88 500) / 82% by weight NMMO / 17% by weight H ₂ O was prepared in a manner known per se. This solution was dropped in water as a precipitant Washed out solvent with water and then exchanged the water for ethanol.

The ethanol-moist bodies (spheres with a diameter of approximately 3 mm) were placed in an autoclave, covered with additional ethanol, heated to 50 ° C. and 130 bar of CO _{2 were added} for 2.5 hours. The CO ₂ was kept in circulation, which was reduced in the cycle of pressure in two stages, each with a liquid at 60 and 40 bar, then the C0 ₂ has been re-compressed to 130 bar. After the two and a half hours mentioned, the CO _{2 was then} discharged at 50.degree.

Cellulose balls with the same diameter (approx. 3 mm) as the moist starting product were obtained. The density of the cellulose balls was 0.042 g / cm ³ ; according to the above calculation formula, this means that the pore volume is 97.4%.

Claims

claims:

1. A method for producing a porous cellulosic body, comprising the steps

Production of a cellulose solution in a tertiary amine oxide, in particular N-methylmoφholin-N-oxide, the cellulose used having an average degree of polymerization of 150 to 2000 and the solution having a cellulose concentration of 0.1 to 5% by weight, production of a body the cellulose solution

Medium can dissolve the tertiary amine oxide and with a supercritical

Solvent miscible is characterized in that if an aqueous medium is used as the precipitant in the precipitated cellulosic body, tertiary amine oxide contained in the precipitated cellulose is washed out with the aqueous medium, the aqueous medium is exchanged for a liquid exchange medium which is miscible with a supercritical solvent and which is cellulosic body moist with the exchange medium is treated with the supercritical solvent, whereby the porous cellulosic body is obtained or that, in the case of using a non-aqueous medium as a precipitant, the tertiary amine oxide contained in the precipitated cellulosic body is washed out with the non-aqueous medium or with the non-aqueous medium moist cellulosic body is treated with the supercritical solvent, whereby the porous cellulosic body is obtained.

2. The method according to claim 1, characterized in that carbon dioxide is used as the supercritical solvent.

3. The method according to claim 1 or 2, characterized in that an alcohol having 1 to 4 carbon atoms, in particular ethanol, or acetone is used as the non-aqueous medium or as the liquid exchange medium.

4. The method according to any one of the preceding claims, characterized in that the cellulose solution is formed into a shaped body.

5. The method according to claim 4, characterized in that the molded body is a fiber, a film, a block or a plate.

6. The method according to any one of claims 1 to 3, characterized in that spherical or pearl-shaped particles are produced from the cellulose solution.

7. The method according to any one of claims 1 to 3, characterized in that a powder is produced from a large number of cellulose particles.

8. The method according to any one of the preceding claims, characterized in that the cellulosic body is subjected to a treatment with a crosslinking agent before treatment with the supercritical solvent.

9. The method according to any one of the preceding claims, characterized in that the porous cellulosic body is subjected to pyrolysis.

10. Porous cellulosic body with a porosity in the dry state of 85% and more, preferably 90% and more, particularly preferably 95% and more.

1 1. Porous cellulosic body, obtainable by a process according to one of claims 1 to 9.

12. Use of a cellulosic body according to one of claims 10 and 1 1 as insulation material, such as in thermal or acoustic insulation, as a dielectric in the electrical and electronics industry, for impedance matching in acoustic applications, as a means for absorbing and cleaning gases, as a carrier for catalysts and / or as a means for storing energy sources such as Hydrogen.

13. Porous carbon body, obtainable by a process according to claim 9.

4. Use of a carbon body according to claim 13 as an electrode material for batteries, accumulators, capacitors and fuel cells, for high-temperature insulation and as a hydrogen storage.