WO2015095902A1

WO2015095902A1 - Method for producing carbon particles

Info

Publication number: WO2015095902A1
Application number: PCT/AT2014/000224
Authority: WO
Inventors: Gisela Goldhalm; Josef Innerlohinger; Martin Häubl
Original assignee: Lenzing Ag
Priority date: 2013-12-23
Filing date: 2014-12-15
Publication date: 2015-07-02
Also published as: AT515234A1

Abstract

The invention relates to a method for producing carbon particles from precursors with a cellulose II structure, in which non-cylindrical cellulose particles with a cellulose II structure are used as precursors.

Description

Process for the production of carbon particles

The present invention relates to a process for the preparation of

Carbon particles from precursors with cellulose-II structure, in which

Precursor non-cylindrical cellulose particles with cellulose II structure can be used.

State of the art

The production of carbon particles from polymers by carbonization has been known for a long time. In addition to the well-known carbon fibers and carbon particles are formed with another form of appropriately shaped

Polymer precursors, so-called precursors prepared. Such

Carbon particles are used, among other things, for the production of electrodes in modern energy storage media, where high charge and discharge rates are important, for example for batteries and supercapacitors in vehicles with electric drive or start-stop function.

In the meantime, not only polyacrylonitrile is widely used as a suitable precursor polymer. Also cellulose is already often used as a precursor for the production of carbon moldings. In addition to the naturally grown cotton and the pulp produced mainly from wood, cellulosic shaped bodies which were produced by means of viscose or lyocell processes are particularly suitable.

The generic name "lyocell" was assigned by BISFA to cellulose fibers made from solutions in an organic solvent without formation of a derivative.

To date, however, has only one method for large-scale production of fibers of the genus Lyocell prevailed and that the amine oxide method. In this process, the solvent used is a tertiary amine oxide, preferably N-methylmorpholine-N-oxide (NMMO). Tertiary amine oxides have long been known as alternative solvents for cellulose. From US Pat. No. 2,179,181 it is known, for example, that tertiary amine oxides are able to dissolve pulp without derivatization and that cellulosic shaped bodies such as, for example, fibers can be produced from these solutions. In US 3,447,939 cyclic amine oxides as

Solvent for cellulose described.

How this method is carried out, the expert is known in principle for many years from many patents and other publications. For example, EP 356 419 B1 describes the preparation of solutions and EP 584 318 B1 describes the spinning of such solutions of cellulose in

hydrous tertiary amine oxides.

A process for carbonization and activation of lyocell fibers is described in US 2008/0254972 A1 patent, wherein the fibers are impregnated with a phosphorus-containing reagent, thermally carbonized under inert gas and also thermally activated with carbon dioxide or water vapor in the rotary kiln. This patent describes a significantly higher activability of lyocell fibers compared to other cellulosic precursors, resulting in a higher electrical gravimetric capacity when used as electrode material for supercapacitors.

US 2007/0178310 A1 describes the use of fiber fragments having a similar fiber cross-section as electrode material for energy storage.

The use of lyocell fibers as a carbonization precursor is also described in the article "Tencel carbon precursor", Lenzinger Berichte, 90, 2012, 58-63.

Electrodes made of carbon powder or activated carbon powder are manufactured using two principally different methods:

1. The dry method in which the dried carbon or

Activated carbon powder mixed with binder (eg PTFE), then heated and as long is kneaded until a dough is formed, which is sheet-shaped rolled and laminated with aluminum foil.

2. The slurry method in which the dried carbon or activated carbon powder is dispersed in a suitable liquid (e.g., acetone) and added with a binder such as PVDC. One usually pretreated with primer

Aluminum foil is coated with this suspension and dried. The dried layer is often subsequently densified by hot calendering.

For the large-scale production of electrodes made of carbon powders, the majority of the slurry method is used.

According to US 2008/0254972 A1 and US 2007/0178310 A1, lyocell fibers are said to have particular suitability for the production of electrode material. The uniform fiber shape should ensure a high packing density. It has now been found that this is true for the application of the dry method of electrode production in the laboratory, but not for the large-scale applied slurry method. If the slurry method is carried out with fibrous powder, extremely fragile, brittle electrodes are produced whose calendering and densification is possible only when very high binder quantities are used. Although this material has high electrical capacities, it can not be used on an industrial scale.

task

The object of this prior art therefore was to find a carbonized material which, on the one hand, permits a high electrical capacitance during electrode production and, on the other hand, results in mechanically stable electrodes. Description of the invention

The solution to the above-described problem consists in a process for the production of carbon particles from precursors with cellulose II structure, non-cylindrical cellulose particles with cellulose II structure being used as precursor. The carbon particles are often also called

Activated carbon particles called.

The use of non-cylindrical cellulose particles with cellulose II structure as precursor for the production of carbon particles has not yet been described. It was not close either. Namely cylindrical

Cellulose II molded articles can be produced, for example, by comminution of commercially available lyocell fibers, were non-cylindrical

Cellulose particles with cellulose II structure, in particular those which had been prepared by the Lyocellverfahren, so far not readily available. In a particularly preferred embodiment of the present invention, the non-cylindrical cellulose particles are prepared by a special Lyocell method.

The advantages of the invention are, in particular, the combination of high activability by lyocell structure of cellulosic precursors and large-scale processability to electrodes, with high gravimetric electrical capacitances by the inherent lyocell structure, with recovery of stable electrodes by means of slurry method with high electrode density and thus high volumetric capacity.

Furthermore, there are advantages of particles or granules compared to fibers in the carbonization or activation process: particles are easier to promote, since they have no compressible behavior in the promotion and no

Show bridging. Particles are also easier to dose because the bulk density is much more constant.

Particles can also be more easily introduced into a complete atmosphere of inert gas or activating gas. Fibers transport a large amount of air through their high trapped volume must be displaced with nitrogen before the thermal treatment; This costs time and resources. For particles the effort is much lower.

The passage of particles or granules through the rotary kiln is much more constant and can be better controlled than that of fibers, since fibers form by rotation in the rotary kiln differently sized fiber bales, which run at different speeds through the furnace. The activation of particles or granules is also much more homogeneous, since the fiber bales described above in the outer region stronger, inside

be activated weaker. Due to the much higher bulk density of particles or granules, all systems for heat treatment can be designed up to an order of magnitude smaller.

Preparation of precursors:

WO 2009/036480 describes suitable three-dimensional cellulosic shaped bodies and a lyocell method for their production. This method is characterized in that the cellulose solution free-flowing, that is without substantial shearing of the cellulose solution under their

Solidification temperature is cooled, the solidified cellulose solution is crushed, the solvent is washed out and the crushed, washed out particles are dried. The so produced

Cellulose particles have a cellulose-II structure with a particle size in the range of 1pm to 5000μιτι, preferably 5pm to 2000μιη, more preferably 10 to 200μιη and are also characterized in that they have an approximately spherical particle shape with irregular surface and a crystallinity in the range of 15% to 45% according to the Raman method.

Another method for producing three-dimensional cellulosic molded bodies is described below. It includes the following

Manufacturing steps: a. Dissolving a cellulosic raw material, for example pulp, to obtain a solution containing 10 to 15% by weight of cellulose;

b. Extrusion of in step a. obtained cellulose solution without air gap directly into a precipitation bath;

c. Regeneration process, wherein upon entry of the cellulose solution into the precipitation bath, the difference in the NMMO concentrations of cellulose solution and precipitation bath 15-78 wt .-%, preferably 40-70 wt .-%, and the difference in temperature of cellulose solution and precipitation bath 50 - 120 K, preferably 70 to 120 K, particularly preferably 80 to 120 K to amount;

d. Washing process according to the percolation principle with at least one alkaline washing step, preferably at pH 9-13;

e. optionally a drying process by which the outer skin of the molded body is not damaged abruptly; the laundry mentioned in point d.) is preferably multi-stage and in the

Countercurrent takes place and contains at least one alkaline step. The cellulosic shaped bodies produced in this way have an optically detectable core-shell structure.

As dissolution processes come about the viscose, the lyocell or the

Cuprammonium process into consideration; It is also possible to dissolve the cellulose in NaOH or suitable ionic liquids. In general, the method described here is not limited to specific solvents or processes, but by using different methods, the structure of the resulting particles can be additionally influenced. However, preference is given to the lyocell process which is fundamentally known to the person skilled in the art and is described, inter alia, in EP 0356419. When using the lyocell process, care should be taken to ensure that the difference between the NMMO concentrations of cellulose solution and precipitation bath 15 occurs when the cellulose solution enters the precipitation bath - 78 wt .-%, preferably 40 - 70 wt .-%, and the difference in temperature of cellulose solution and precipitation bath 50 - 120 K, preferably 70 - 120 K, particularly preferably 80 - 120 K should be. Based on the cellulose solution, the shaping takes place, in which - especially in the precipitation process - care must be taken to ensure that no fibrous structures are formed. This is not a trivial requirement because

Cellulose, because of its macromolecular structure, seeks to form fibrous domains. This problem is solved by the fact that the

Cellulose solution is first brought into the desired shape without significant shearing and then the regeneration conditions are selected accordingly. It is essential that the cellulose solution directly, d. H. without an air gap, is extruded into a precipitation bath and the comminution of the solution strand takes place in a manner which results in substantially equal particles. Suitable aggregates for this step are, for example, underwater or stranded granulators, which in addition to spherical particles also produce cylinders, angular ellipsoids and ovoids. The aforementioned units also meet the additional

Requirements for the manufacturing process. The particles produced are supposed to have as uniform a size as possible, with the

Control properties via the process parameters. At the same time, the process should have a high throughput.

Granules of different sizes can be made from Lyocell dope, for example, with an EUP50 type underwater granulator from Econ. The beads produced can be separated from the process water via a mechanical centrifugal dryer. In other embodiments, such solid-liquid separations may be e.g. by hydrocyclone, pusher centrifuges or by sieves

be accomplished. Granulators are available in a variety of sizes on the market, and due to the simplicity of the process of granulation of dope, upscaling on an industrial scale is relatively easy. Thus, e.g. with a single pelletizer of the type EUP 3000 about 5000 tons of pellets are produced per year. Furthermore, there are much larger machines available from other manufacturers.

In a further embodiment, cellulose strands can also be produced with special Lyocell nozzles with nozzle hole diameters of 0.5 to 5 mm, which are fed to a strand pelletizer after a washing cycle.

Critical in this regard are the washing out, the feeding and the feeding of the individual strands into the strand granulator, since the strands are very flexible. In this way, cylindrical granules can be obtained.

The viscosity of the cellulose solution also has a great influence on the properties of the particles obtained, since these normally also have the viscosity

Viscosity difference between cellulose solution and precipitation bath dominates. The precipitation bath is preferably aqueous (with a viscosity in the range of 1 Pa * s), but the viscosity of the precipitation bath can be increased significantly by adding thickeners (polymers). A smaller difference in viscosity leads to a thinner coat. According to the invention is a

Viscosity difference between cellulose solution and precipitation bath of at least 600 Pa.s, preferably in the range 750-1200 Pa.s. (based on the zero viscosity).

The thickness of the shell is decisively influenced by the NMMO concentration difference when the cellulose solution enters the precipitation bath. The larger this is, the thicker the coat gets

produced moldings. The difference in concentration becomes maximum when pure water is used as the precipitation bath and the precipitation bath at the point of entry of the cellulose solution is thoroughly mixed so that all outgoing NMMO is immediately transported away.

The thickness of the jacket is also influenced by the temperature difference when the cellulose solution enters the precipitation bath. The larger this is, the thicker the jacket of the granules produced according to the invention becomes.

In addition to the possibility of underwater granulation in a liquid precipitation bath, there is also the possibility of coagulation in a gaseous medium.

The preferred process principle for washing out the moldings on an industrial scale is the countercurrent washing in order to keep the necessary amount of wash water and the recovery costs within limits. In order to achieve the required purity of the moldings, 10 to 12 washing stages are necessary for this. In addition, with low residual NMMO contents, increasing the wash water temperature is advantageous. A washing water temperature of 60 ° C. to 100 ° C. is preferred here. In order to remove even small amounts of decomposition products of the solvent efficiently, an alkaline step is additionally necessary, wherein preferably pH values of 9-13 are used.

The cellulose II molded bodies produced by this second process variant are characterized in that they have an optically detectable core-shell structure, wherein the shell has a higher density and a lower crystallinity than the core and the core has a sponge-like structure. Preferably, the sheath has a relative density of 65% to 85% and the core has a relative density of 20% to 60%, based in each case on compact cellulose. Also preferably, the jacket thickness between 50 μιη and 200 μιη. Among other things, the ratio of shell thickness to total diameter of the molding can be between 1: 5 and 1:50. Due to the production, the ratio of the semiaxes of the ellipsoidal shaped body 3: 1 will not exceed. For the purposes of the present invention, cellulose particles of the invention having such a shape, i. ellipsoidal shaped bodies with a ratio of the semi-axes of less than or equal to 3: 1 are referred to as "spherical granules".

The spherical granules thus obtained can be coarsely or finely ground depending on the desired final size.

For the particles following one of the two above

As a generic term for the purposes of the present invention, the term "non-cylindrical cellulose particles having a cellulose II structure" is used, this being intended in particular to differentiate from the cellulose II particles known in the above cited prior art, which are made of lyocell or viscose fibers by comminution and therefore always have a cylindrical shape (see Figs. 1 and 2). In particular from Fig. 2 shows that even at very fine

Milling of cellulose fibers, the resulting carbon particles continue to have a cylindrical shape. A cylindrical shape should also be understood to mean one with a lobed cross-section, as is the case when viscose fibers are comminuted (see, for example, FIG. 2 in Lenzinger Berichte, 90, 2012, 58-63).

According to the invention, the cellulosic shaped bodies produced by the processes described above can be used to produce carbon particles, at least one heat treatment step being necessary.

In a preferred embodiment of this method, the

Heat treatment in an inert gas atmosphere. In a further likewise preferred embodiment, the heat treatment is carried out in one

activating atmosphere. Particularly preferably, the activating atmosphere consists essentially of water vapor or carbon dioxide.

As a heat treatment is also a combination of several

Heat treatment steps possible and in many cases even particularly advantageous. A preferred embodiment consists in the use of said cellulosic moldings for carbonization and activation, wherein at least the following process steps are necessary:

Fire protection pretreatment, carbonation and activation. Depending on the desired shape of the final product or possibly also for

Optimization in a subsequent second heat treatment step may be added to comminution steps.

A preferred solution to the above-described problem consists, for example, in a process for the production of carbon particles from precursors with cellulose II structure, non-cylindrical cellulose particles having cellulose II structure being used as precursor,

a. the cellulose particles are optionally subjected to a fire protection pretreatment,

b. A first heat treatment in an inert gas atmosphere, c. Optionally, a shredding of the in step b. obtained particles,

d. A further heat treatment of the particles thus obtained takes place in an activating atmosphere,

e. And optionally a shredding of the in step d. obtained particles takes place.

A particularly preferred solution of the above-described object consists in a process for the production of carbon particles from precursors with cellulose II structure, non-cylindrical cellulose particles having cellulose II structure being used as precursor,

a. the cellulose particles are subjected to a fire protection pretreatment,

b. A first heat treatment in an inert gas atmosphere, c. a shredding of the in step b. obtained particles, d. A further heat treatment of the particles thus obtained takes place in an activating atmosphere,

e. And a shredding of the stage d. obtained particles takes place.

A further preferred solution of the above-described object consists in a process for the production of carbon particles from precursors with cellulose II structure, non-cylindrical cellulose particles having cellulose II structure being used as precursor,

b. A heat treatment in an inert gas atmosphere takes place and c. a shredding of the in step b. obtained particles takes place.

A further possible solution to the above-described problem consists in a process for the production of carbon particles from precursors with cellulose II structure, non-cylindrical cellulose particles having cellulose II structure being used as precursor, the cellulose particles being subjected to fire protection pretreatment, preferably by impregnation one aqueous solution of a phosphate, and finally a heat treatment in activating atmosphere takes place. With this variant, relatively large carbon particles are obtained since no comminution steps take place. Of course, a crushing step can also be added to obtain correspondingly smaller carbon particles. For example, Fig. 3 shows an inventive crushed, carbonized and activated ball granules, while Fig. 4 shows an inventive carbonized and activated, not further crushed ball granules, in which the core-shell structure of the original precursor is still clearly visible.

The fire protection pretreatment can either be a thermal oxidation at temperatures of 150 ° C - 400 ° C under an air atmosphere or a

Impregnation with a solution of a fire retardant, e.g.

Phosphates, for example Diammoniumhydrogenphosphat (DAHP) or aqueous potassium hydroxide (KOH). As a result, a protective layer is built up on the particle surface, through which both structural

Improvements as above all also higher yields in the carbonization result.

Preferred is an embodiment of the method according to the invention, wherein the fire retardant is selected from the group containing diammonium hydrogen phosphate (DAHP) and potassium hydroxide (KOH).

The carbonization may be batch or continuous in an inert atmosphere such as e.g. Nitrogen be performed. Essential elements here are the heating rate, the maximum temperature, the

Residence time at maximum temperature and the cooling phase, as well as the volume flow of purge gas. In Lenzinger Berichte, 90, 2012, 58-63, such a suitable batch process is described. In the continuous process, for example, precursor particles are purged with nitrogen in the storage tank and thus rendered inert and then introduced through a cooled screw conveyor into a rotary kiln. The particles then pass through, for example, three temperature zones, for example 400 ° C., 900 ° C., 200 °, and are subsequently cooled under inert gas. By pretreatment and heating rate, the properties of the resulting carbon particles can be influenced, such as the resulting C content, which can be set between 80% and 99% depending on the pre-treatment.

The activation step can be carried out either directly with the precursor or with already carbonized particles. A pretreatment with e.g. Phosphates increase the formation of active surface area (see Example 1). As with the carbonization are essential elements of the

Activation step the heating rate, the maximum temperature, the

Residence time at maximum temperature and especially the type and the

Volume flow of the purge or activation gas. As activating gases, e.g. Carbon dioxide or water vapor can be used. In a preferred form of the method according to the invention, therefore, the activating atmosphere during the heat treatment consists essentially of water vapor or carbon dioxide.

When pretreating the precursor with KOH, the thermal activation will preferably be carried out in an inert gas stream. The activation can be done in the rotary kiln, the rotation should ensure a uniform activation. By pretreatment, maximum temperature and

Dwell time, the formation of the active surface is controlled. A measure of the active surface area may be the BET value in m ² / g. Another

Possibility to characterize the carbon particles is the measurement of the electrical capacitance by means of cyclic voltammetry of two-electrode or three-electrode cells at 1 or 1, 5 V with a scan rate of 20mV / s.

Another aspect of the present invention is the use of cellulosic cellulose non-cylindrical cellulose particles to produce carbon particles by the method described above.

The non-cylindrical carbon particles produced in this way can be used, inter alia, for the production of electrodes in modern energy storage media which require high charging and discharging rates, for example for batteries and supercapacitors in vehicles with electric drive or start-stop function. Also for the so-called "supercabatteries" - hybrid systems between supercapacitors and batteries - the carbon particles produced according to the invention are suitable. Other uses include uses as a filter medium, especially for the filtration of gases, air and liquids as well as storage medium for gases, such as hydrogen.

In the following the invention will be described by way of examples. However, the invention is expressly not limited to these examples, but includes all other embodiments based on the same inventive concept.

Examples

Example 1: Activation of lyocell fibers with and without pretreatment

Precursors used: Commercially available Tencel® fiber 1, 7 dtex / 38 mm cut length

Pretreatment: Impregnation of the lyocell fibers with an aqueous solution of diammonium hydrogen phosphate (2% by weight, based on cellulose), followed by drying at 70 ° C. under reduced pressure.

Heating and cooling rate ° C / min 3

T max ° C 850

Residence time carbonation min. 30

Residence time activation min 60-180

Gas flow argon l / h 36

Gas stream carbon dioxide l / h 24

DAHP on lyocell% 2

The characterization was carried out via yield and active surface by means of BET measurement (measuring device BELsorp mini II). Table 1 :

The experiments of Example 1 show that both the yield and the active surface are increased by the pretreatment. By a longer residence time in the carbonation, although the active surface is increased, but the yield decreases. This finding is unchanged from Lyocell fibers to non-cylindrical lyocell particles as a precursor transferable.

Example 2: Activation of lyocell fibers and lyocell pellets in

comparison

Commercially available lyocell fibers as well as various lyocell pellets produced by the method described above were pretreated, carbonated and activated under the same conditions. The electrical capacitance of the products was determined by cyclic voltammetry at 1.5 V with a scan rate of 20 mV / s in a two-electrode cell with the following

Composition measured: current collector (aluminum foil) - activated carbon electrode - electrolyte (1, 8 mol / l Triethylmethylammoniumtetraborat in propylene carbonate) - separator - activated carbon electrode - current collector (aluminum foil).

Tested Precursors: Commercially available Tencel® fiber 1, 7 dtex / 38 mm cut length, Lyocell granules unmilled 1 mm in diameter, coarse milled Lyocell granules (100 pm average

Particle size), finely ground lyocell granules (8 pm average particle size). The lyocell pellets were prepared as described in the above Description shown method produced. The determination of the particle size was carried out with a laser diffraction meter.

Heating and cooling rate ° C / min 10

T max ° C 850 residence time carbonation min. 30

Residence time activation min. 150

Gas stream carbon dioxide l / h 60

DAHP on lyocell% 2

Table 2:

The experiments of Example 2 show that lyocell pellets show a similar good activability as lyocell fibers under the same conditions.

Claims

claims

1. A process for the preparation of carbon particles from precursors with cellulose-II structure by heat treatment, characterized in that are used as precursor non-cylindrical cellulose particles with cellulose-II structure.

2. The method according to claim 1, wherein the heat treatment is carried out in an inert gas atmosphere.

3. The method according to claim 1, wherein the heat treatment is carried out in an activating atmosphere.

4. The method of claim 3, wherein the activating atmosphere consists essentially of water vapor or carbon dioxide.

5. The process according to claim 1, wherein the cellulose particles are subjected to a fire protection pretreatment before the heat treatment.

6. The method according to claim 5, wherein the fire protection pretreatment is an impregnation of the cellulose particles with a solution of a fire retardant.

7. The method according to claim 6, wherein the fire retardant is selected from the group comprising diammonium hydrogen phosphate (DAHP) and potassium hydroxide (KOH).

8. The method according to claim 5, wherein the fire protection pretreatment is a thermal oxidation of the cellulose particles at temperatures of 150 ° C - 400 ° C under air.

9. The method according to claim 1, wherein after the heat treatment, a comminution of the particles obtained by the heat treatment takes place.

10. The method according to claim 1, wherein

a. the cellulose particles are optionally impregnated with a solution of a fire retardant,

d. A further heat treatment of the particles thus obtained takes place in an activating atmosphere, e. And optionally a shredding of the in step d. obtained particles takes place.

11. Use of non-cylindrical cellulose particles with cellulose-II structure for the production of carbon particles by means of a method according to claim 1.