WO2017013199A1

WO2017013199A1 - A carbon foam manufacturing process

Info

Publication number: WO2017013199A1
Application number: PCT/EP2016/067381
Authority: WO
Inventors: Bernd Bruchmann; Folke Johannes Toelle; Rolf Muelhaupt
Original assignee: Basf Se
Priority date: 2015-07-23
Filing date: 2016-07-21
Publication date: 2017-01-26

Abstract

The present invention relates to a process for preparing a carbon foam, wherein - a microorganism biomass or a hydrochar obtained by hydrothermal carbonisation of a microorganism biomass is provided as a carbon foam precursor; - the carbon foam precursor is subjected to a blowing and a carbonization by a pyrolytic treatment at a temperature of at least 250°C.

Description

A CARBON FOAM MANUFACTURING PROCESS

Solid foams are a type of cellular material which is generally classified by material type (e.g. metal, ceramic, carbon, polymer) and general structure (open cell, closed cell or combinations thereof). Typically, a solid foam is a porous material made of a rigid, dimensionally stable skeleton.

Carbon foam is a solid foam which has low weight and density, high chemical resistance, high temperature tolerance, and a low coefficient of thermal expansion; and is therefore of high interest for a number of different applications such as thermal insulation, filtration medium (e.g. of molten metals), adsorbent material, components in supercapacitors and batteries, and catalyst supports. Applications of carbon foams have been reviewed e.g. by M. Inagaki et al., Carbon, 87 (2015), pp. 128-152. At present, there are three general routes available for the synthesis of carbon foams, which are (i) carbonization of a carbonaceous foam template, e.g. polymer foams; (ii) blowing and carbonization of carbonaceous starting materials without using a template, and (iii) expansion of graphite derivatives. The first approach was disclosed e.g. in US 3,121 ,050. It yields carbon foams by carbonization of polymeric template foams like phenolic resins or urethane foams.

The second route typically comprises annealing of pitch or coal, which is converted into a solid foam by evaporation of an internal or external blowing agent during the pyrolysis.

The term ..carbonization" refers to a process by which solid residues with increasing content of the element carbon are formed from organic material, usually by pyrolysis in an inert atmosphere. In the third preparation route, expandable graphite or graphite oxide is used as a precursor. These materials show a significant volume expansion when heated, which can be used for generating a solid foam.

The processes mentioned above are based on fossil resources. However, previously there have been described some approaches towards "green" carbon foams from renewable resources such as sugars (H. Ji et al., J. Mater. Res., 2014 (29), pp. 1018-1025) and lignin derivatives (J. Seo et al., Carbon, 2014 (76), pp. 357-367).

For obtaining a solid foam, it is typically necessary that the carbon foam precursor material shows high fluidity when the decomposition rate of the blowing agent reaches its maximum. On the other hand, once the cells have been generated by the gaseous blowing agent (or its gaseous decomposition products), the carbon material needs to form a continuous skeleton which is sufficiently rigid so as to form a dimensionally stable solid foam. Preferably, such a solid foam should have a high mechanical strength (e.g. high compressive strength).

Furthermore, it would be of interest that the final properties of the carbon foam (e.g. porosity, specific surface area, electric conductivity) can be fine-tuned by varying the manufacturing conditions.

Just a limited number of carbonaceous starting materials have so far been described for the preparation of carbon foams. So, it remains a challenge to provide appropriate foam precursor materials, in particular from renewable resources, which allow the preparation of dimensionally stable carbon foams.

It is known that a microorganism biomass (e.g. algae biomass, wood biomass, etc.) can be converted to fuel (e.g. biodiesel) and char by a pyrolytic treatment. Both wet and dry pyrolysis can be applied. A wet pyrolytic treatment is e.g. hydrothermal carbonization ("HTC"). Generally, hydrothermal treatment refers to a thermochemical process used to convert an organic feedstock into a carbonaceous product ("hydrochar") in the presence of water under moderate temperature (such as 160 C-250 C) and pressure (such as 2-10 MPa). The char obtained by hydrothermal carbonization is commonly referred to as "hydrochar". Hydrothermal carbonization of biomass is described e.g. by J. A. Libra, Biofuels, 201 1 , 2(1 ), pp. 89-124.

S.M. Heilmann et al., Biomass Bioenerg., 2010, 34, pp. 875-882 describe a hydrothermal carbonization of microalgae.

It is an object of the present invention to provide a carbon foam from a renewable carbon source. Preferably, the carbon foam should have high mechanical strength and/or other relevant properties such as electric conductivity and porosity can be fine-tuned over a broad range.

The object is solved by a process for preparing a carbon foam, wherein

a microorganism biomass or a hydrochar obtained by hydrothermal carbonisation of a microorganism biomass is provided as a carbon foam precursor;

the carbon foam precursor is subjected to a blowing and a carbonization by a pyrolytic treatment at a temperature of at least 250 C.

As commonly known, biomass is a biological material from living or non-living organisms. In the present invention, it has been realized that microorganism biomass can be used as a starting material for preparing a carbon foam. The microorganism biomass comprises microorganism cells, which can be either living or non-living cells.

As will be described in further detail below, the microorganism biomass can be directly used as a carbon foam precursor material which is subjected to a blowing and carbonization treatment, or can be converted to a hydrochar in a first step and said hydrochar originating from the microorganism biomass is then converted to the carbon foam by a blowing and carbonization treatment.

The microorganism can be a phototrophic microorganism, in particular a photosynthetic microorganism. Phototrophic microorganisms are microorganisms that carry out photon capture to acquire energy. They use the energy from light to carry out cellular metabolic processes.

Preferably, the microorganism is selected from algae (e.g. green algae, microalgae) or bacteria (in particular cyanobacteria).

Compared to biomass from terrestrial plants, algae and cyanobacteria show a range of significant advantages: Their cultivation does not need arable land. Provided with a suitable container, they can grow on unproductive land or even on the facade of buildings. By using areas that would otherwise lie idle, they do not compete with current production of food crops. Furthermore, they do not necessarily depend on fresh water, since many species can even grow in salt- or wastewater and they can double their mass during growth phase within several hours.

Depending on the type of microorganism, the protein content (based on dry weight, of the microorganism biomass) may vary over a broad range. The microorganism biomass can have a protein content, based on dry weight, of e.g. at least 40 wt%, more preferably at least 50 wt% or even at least 55 wt%. The protein content can be determined by standard analytical methods, such as the Kjeldahl method. Preferably, the algae is a microalgae. Typically, microalgae are unicellular species.

In a preferred embodiment, the algae is selected from an algae of the genus chlorella.

Preferably, the bacteria are cyanobacteria. In a preferred embodiment, the cyanobacteria are selected from the genus spirulina.

Microorganism biomass can be produced in facilities which are commonly known to the skilled person, such as photobioreactors. Microorganism biomass is also commercially available, e.g. as a dry powder.

If the microorganism biomass is directly used as the carbon foam precursor, it can be a dry microorganism biomass or a wet microorganism biomass. Preferably, a dry microorganism biomass (e.g. in the form of a powder) is used as the carbon foam precursor. As mentioned above, it is also possible that the microorganism biomass is not directly subjected to a blowing treatment, but is subjected to a hydrothermal carbonization so as to provide a hydrochar which is then subjected to a blowing and carbonization treatment. As known to the skilled person, hydrothermal carbonization (HTC) is a process wherein an organic material is subjected to a hydrothermal treatment at elevated pressure (e.g. in a closed reactor such as an autoclave), thereby obtaining a material which is typically referred to as "hydrochar" and has higher carbon content than the organic starting material. Appropriate conditions for hydrothermal carbonization are generally known to the skilled person.

The temperature of the hydrothermal carbonization can vary over a broad range. Preferably, the hydrochar is obtained by a hydrothermal carbonization at a temperature of from 120 °C to 300 °C, more preferably a temperature of from 150 °C to 250 °C, even more preferably of from 180 C to 210 C.

Typically, the hydrothermal carbonization is carried out in a closed reactor, such as an autoclave.

The microorganism biomass which is subjected to a hydrothermal carbonization can be a dry microorganism biomass or a wet microorganism biomass. However, for improving the energy balance, it might be preferred to use a wet microorganism biomass for hydrothermal carbonization. The wet microorganism biomass may comprise living cells.

During hydrothermal carbonization, nitrogen- and/or phosphorus-containing compounds are typically extracted from the biomass into the surrounding aqueous medium. These extracted compounds are a potential nutrient source for growth of microorganism biomass.

So, if the process of the present invention comprises a hydrothermal carbonization, it is preferred that the aqueous medium in which the hydrothermal treatment was carried out is used as a nutrient source for further growth of a microorganism biomass (e.g. further growth of algae or cyanobacteria in a photobioreactor). This can be accomplished e.g. by isolating nitrogen- and/or phosphorus-containing compounds from the aqueous medium.

As will be described below in further detail, organic material being present in the carbon foam precursor can decompose and release gaseous decomposition products upon pyrolytic treatment, thereby providing a blowing effect ("internal" blowing agent). If the process of the present invention comprises a hydrothermal carbonization, it can be preferred that the HTC hydrochar to be used as a carbon foam precursor still includes decomposable compounds, in particular decomposable organic compounds. However, if the amount of decomposable compounds in the hydrochar is too low for providing a sufficiently strong blowing effect, an external organic or inorganic blowing agent can be added to the hydrochar. The hydrochar to be used as a carbon foam precursor material can be separated from the aqueous medium of the hydrothermal carbonization treatment by commonly known methods, such as filtration. The material separated from the aqueous medium of the hydrothermal carbonization treatment may comprise fatty acids (due to hydrolysis of triglyceride compounds of the biomass). So, the hydrochar which had been separated from the aqueous medium of the hydrothermal carbonization treatment may optionally be subjected to an extraction treatment (e.g. with an organic liquid) for at least partially removing the fatty acids. Alternatively, it is also possible that the hydrochar is not subjected to an extraction treatment. If the hydrochar is not subjected to an extraction treatment, it typically still includes decomposable organic compounds such as fatty acids, which may then act as an internal blowing agent.

Optionally, the hydrochar can be dried prior to the blowing and carbonization treatment.

As indicated above, the carbon foam precursor is subjected to a blowing and a carbonization by a pyrolytic treatment at a temperature of at least 250 C.

The carbon foam precursor can be in a dry or a wet state. However, in a preferred embodiment, the carbon foam precursor is in a dry state.

Preferably, the carbon foam precursor is in the form of a powder. However, other forms can be used as well for the pyrolytic treatment. Preferably, the carbon foam precursor (e.g. in the form of a powder) is pressed to a shaped body (e.g. in the form of a pellet, a tablet, a cylindrical body, etc.), which is then subjected to the pyrolysis at T≥250 C, so as to effect blowing and carbonisation.

The blowing during the pyrolytic treatment can be achieved by thermally decomposing the microorganism biomass, thereby generating gaseous decomposition products which provide a blowing effect. In other words, organic compounds which are inherently present in the biomass and are decomposed by the pyrolytic treatment act as an "internal" blowing agent. If the amount of decomposable compounds which are still present in the hydrochar after the hydrothermal carbonization is too low for providing a sufficiently strong blowing effect, it is also possible that an "external" blowing agent is added to the microorganism biomass or the hydrochar. The external blowing agent can be a chemical blowing agent (i.e. a blowing agent which provides one or more gaseous decomposition products during the pyrolytic treatment) or a physical blowing agent. If an external chemical blowing agent is added, its decomposition temperature (e.g. determined as the onset temperature of decomposition by thermogravimetry) is preferably at least 250 C. Appropriate blowing agents are commercially available and are known to or can easily be established by the skilled person. Exemplary external blowing agents that can be mentioned are isocyanurates (e.g. tris(2-hydroxyethyl)isocyanurate) and nitrogen-containing heterocyclic compounds (such as 5-phenyltetrazole or a salt (e.g. Barium salt) thereof.

However, in the present invention, it is also possible that no external blowing agent is added to the microorganism biomass or the hydrochar during the pyrolytic treatment.

Preferably, the temperature of the pyrolytic treatment is at least 280 C, more preferably at least 500 C or at least 700 C or at least 900 C. Different temperature programs can be used for carrying out the pyrolytic treatment. Just as an example, the temperature can be continuously increased to the desired final pyrolysis temperature, and then kept constant for a sufficient period of time so as to accomplish blowing and carbonization. It is also possible that a first pyrolysis temperature is set and kept constant for a while, followed by further increasing the pyrolysis temperature (either continuously or with at least one intermediate annealing step) to a maximum temperature which may then again be kept constant for a while so as to finalize the blowing and carbonization treatment.

In the process of the present invention, it is also possible that a first pyrolysis temperature is set for blowing the carbon foam precursor material and starting the carbonization, followed by further increasing the pyrolysis temperature for finalizing the carbonization.

Appropriate pyrolysis reactors for carrying out a blowing and carbonization treatment are generally known to the skilled person. The pyrolytic treatment is carried out for a period of time which is sufficient for providing a dimensionally stable carbon foam skeleton. Appropriate time periods may depend e.g. on the temperature of the pyrolytic treatment, the mass of the carbon foam precursor to be blown and carbonized, etc., and can easily be determined by routine experimentation. In principle, the microorganism biomass or the hydrochar acting as a carbon foam precursor can be mixed with other carbon foam precursors from renewable resources. However, in a preferred embodiment, it is only the microorganism biomass and/or the hydrochar obtained from the microorganism biomass which is/are used as a carbon foam precursor in the process of the present invention. In other words, it is preferred that, apart from the microorganism biomass and/or the hydrochar obtained from the microorganism biomass, no further carbon foam precursor is present.

Preferably, an inert gas such as nitrogen is fed into the pyrolysis reactor during the pyrolytic treatment.

Optionally, the microorganism biomass can be brought into contact with a chemical activation agent for carbon prior to and/or during the pyrolysis. Chemical activation agents for carbon are well-known to the skilled person and are commonly used for generating micro- and mesopores by etching the carbon material at elevated temperature.

Exemplary activation agents include an alkali metal hydroxide (such as KOH or NaOH), a mineral acid (such as sulfuric acid, phosphoric acid), an alkaline earth metal carbonate (e.g. magnesium carbonate or calcium carbonate), an alkaline earth metal chloride (e.g. magnesium chloride or calcium chloride), a transition metal chloride (e.g. zinc chloride or iron chloride), a transition metal carbonate (e.g. iron carbonate), or a mixture of at least two of these activation agents.

The microorganism biomass may also be subjected to a physical activation prior to and/or during the pyrolysis, e.g. by treatment with water vapour or other reactive gases.

If an external blowing agent and/or a chemical activation agent for carbon is/are added, the external blowing agent and/or chemical activation agent is/are preferably pressed together with the carbon foam precursor to the shaped body. After blowing and carbonization by pyrolytic treatment, a dimensionally stable carbon foam is obtained.

Optionally, the carbon foam can be subjected to a machining post-treatment so as to provide the foam in a desired shape adapted to its intended use.

According to a further aspect, the present invention relates to a carbon foam which is obtainable by the process of the present invention.

As will be demonstrated below in further detail, it is possible with the process of the present invention to fine-tune the properties (such as electric conductivity, type of porosity and BET surface area) of the carbon foam over a broad range.

Just as an example, the carbon foam can be an electric isolator (i.e. very low bulk conductivity and high surface resistance) but may also be an electrically conductive material.

Furthermore, the carbon foam can have a significant amount of micro- and/or mesopores (in addition to macropores), but it is also possible that the porosity of the carbon foam mainly results from macropores with a very low amount of or even with no detectable micro- and/or mesopores. Pores having a diameter of 2 nm or less are commonly referred to as micropores, whereas pores having a diameter between 2 nm and 50 nm are commonly referred to as mesopores. Macropores have a diameter exceeding 50 nm. It is also possible to fine-tune the BET surface area over a broad range.

Typically, the carbon foam available by the process of the present invention can have a carbon content of at least 60 wt%. The carbon content can be in the range of e.g. from 60 to 90 wt%. One of the carbon foams which is obtainable by the process of the present invention has a high bulk density of from 0.70 to 1 .30 g/cm³, more preferably 0.80 to 1 .20 g/cm³, even more preferably 0.85 to 1.10 g/cm³.

Preferably, the carbon foam of high bulk density has a cumulative pore area, determined with nitrogen at 77 K and using the Horvat-Kawazoe method, of less than 90 m²/g, more preferably less than 60 m²/g or even less than 40 m²/g. In a preferred embodiment, the cumulative pore area is in the range of from 1 to 90 m²/g, or from 5 to 60 m²/g, or from 5 to 40 m²/g.

Preferably, the carbon foam of high bulk density has a cumulative pore volume, determined with nitrogen at 77 K and using the Horvat-Kawazoe method, of less than 0.15 cm³/g, more preferably less than 0.10 cm³/g or even less than 0.04 cm³/g.

Preferably, the carbon foam of high bulk density has a cumulative pore area, determined with nitrogen at 77 K and using the Barrett-Joyner-Halenda (BJH) method, of less than 90 m²/g, more preferably less than 60 m²/g or even less than 50 m²/g. In a preferred embodiment, the cumulative pore area is in the range of from 1 to 90 m²/g, or from 5 to 60 m²/g, or from 5 to 50 m²/g.

Preferably, the carbon foam of high bulk density has a cumulative pore volume, determined with nitrogen at 77 K and using the Barrett-Joyner-Halenda (BJH) method, of less than 0.25 cm³/g, more preferably less than 0.15 cm³/g or even less than 0.10 cm³/g.

By the Horvat-Kawazoe method, micropores are mainly detected, whereas the BJH method is mainly detecting the mesopores. So, it is preferred that the carbon foam of high bulk density has a low amount of micropores (or even no detectable micropores) and a low amount of

mesopores (or even no detectable mesopores). It is preferred that the porosity of the carbon foam of high bulk density mainly originates from macropores.

The carbon foam of high bulk density can have a BET surface area, determined with nitrogen at 77 K, of less than 80 m²/g, more preferably less than 70 m²/g. The BET surface area can be e.g. in the range of from 10 to 80 m²/g, or from 25 to 70 m²/g. The carbon foam of high bulk density can have a porosity of at least 35%, more preferably at least 45%. Preferably, the porosity of the carbon foam is within the range of from 35% to 80%, more preferably of from 40% to 70%, or from 45% to 65%. The carbon foam can have an open cell content of at least 55%, more preferably at least 65%.

The carbon foam can have a carbon content of at least 60 wt%. The carbon content can be in the range of e.g. from 60 to 90 wt%. With the carbon foam of high bulk density which is obtainable by the process of the present invention, very high compressive strength values can be achieved, e.g. at least 25 MPa, more preferably at least 70 MPa, even more preferably at least 100 MPa or even at least 1 10 MPa.

Furthermore, depending on the temperature of the pyrolytic treatment, both an isolating carbon foam and an electrically conductive carbon foam of high compressive strength are available.

With the process of the present invention, it is also possible to provide a carbon foam of lower bulk density, i.e. less than 0.70 g/cm³. The carbon foam of lower bulk density may have a cumulative pore area, determined with nitrogen at 77 K and using the Horvat-Kawazoe method, of at least 95 m²/g, more preferably at least 150 m²/g or at least 200 m²/g; and/or a BET surface area, determined with nitrogen at 77 K, of at least 80 m²/g, more preferably at least 140 m²/g or at least 180 m²/g.

According to a further aspect, the present invention relates to the use of a microorganism biomass for preparing a carbon foam.

With regard to preferred properties of the microorganism biomass and preferred properties of the carbon foam, reference is made to the statements provided above. As already described above, the microorganism biomass can be directly converted to a carbon foam by blowing and carbonization, or can be converted to a hydrochar by hydrothermal carbonization in a first step and the hydrochar is then converted to a carbon foam by blowing and carbonization.

The invention will now be described in further detail by the following Examples. Examples

Measuring methods

If not indicated otherwise, the parameters of the present invention have been determined according to the following methods:

Bulk density

For density measurements the skin covering the carbon foam pellets was removed and they were machined into geometrical cylinders with a diameter of 14 mm and a height of 4 mm by a conventional turning machine. From these samples the bulk density was determined

geometrically.

Porosity and open cell content

Foam porosity and open cell content were determined by using the following equations (1 ) and

(2):

Porosity [%] = (1 - (p_b/p_P)) x 100 (1 )

Open cell [%] = (p_t/p_P) x 100 (2) wherein

pt_>: bulk density of foam; p_p: true density of powder; p_t: true density of foam

Bulk density was determined as described above.

True density of the foam was determined by helium pycnometry ( PY-2, QUANTACHROME GmbH&Co.KG, Germany) on the sample already used for the bulk density measurement.

True density of powder was determined by milling the sample used for the true density of foam measurement in a vibrating mill (CryoMili, Retsch GmbH, Germany) at room temperature, drying the powder overnight in vacuum at 60 C, and measuring the density of the powder by helium pycnometry. Compressive strength

Compressive strength measurements were performed on an Instron Device with a 100 kN power sensor and an extensiometer. The measurements were made on the samples also used for the density measurements (i.e. geometrical cylinders with a diameter of 14 mm and a height of 4 mm). Apart from the sample size, the measurements were carried out in accordance with DIN EN ISO 844.

Bulk conductivity and surface resistance

Electrical conductivity was measured using a four point probe. The surface resistance R_s was calculated according to eq. (3) where U is the measured voltage, I is the current and K is a correction factor (-3.6 for samples with 14 mm diameter):

Bulk conductivity σ was calculated from surface resistance R_s according to eq. (4), where w is the thickness of the sample.

Thermogravimetric analysis (TGA)

TGA measurements were performed on a STA 409 device (Netzsch GmbH, Selb, Germany) from 50 °C to 650 C with a heating rate of 10 K/min in air or nitrogen.

Elemental analysis

For elemental analysis a VarioEL device (Elementar Analysensysteme GmbH, Hanau, Germany) was used and WO3 was added to improve combustion.

Scanning electron microscopy

SEM images were taken by ElectroScan 2020 ESEM. Nitrogen adsorption/desorption measurements

N2 adsorption/desorption measurements were made at 77 K. Prior to the measurement, samples were dried in vacuum overnight. Device: Sorptomatic 1990. BET surface area, cumulative pore area and cumulative pore volume according to the Horvat-Kawazoe method, and cumulative pore area and cumulative pore volume according to the Barrett-Joyner-Halenda (BJH) method were obtained by using a standard evaluation software.

The BET surface area was determined by BET theory using the multi-point method. Cumulative pore area and cumulative pore volume of micropores were determined using the Horvat-Kawazoe method which is described in J. Chem. Eng. Japan 1983, 16, pp. 470-475. Cumulative pore area and cumulative pore volume of mesopores were determined using the Barrett-Joyner-Halenda (BJH) method.

Example 1 The microorganism biomass used in Example 1 was an algae of the genus chlorella (chlorella vulgaris, origin: Korea). It was purchased as a dry powder from Dr. Behr GmbH, Bonn,

Germany.

Thermogravimetric analysis of the chlorella biomass showed an onset temperature for thermal decomposition of about 245 'C, and a maximum rate for weight loss at a temperature of about 327 C. Above the onset temperature for thermal decomposition, gaseous or volatile

decomposition products are generated, and these decomposition products may then effect blowing ("internal" blowing agent). 4 g of the chlorella powder was pressed for about 1 minute with a load of about 10 tons to a cylindrical pellet having a diameter of about 25 mm and a thickness of about 7 mm. The pellets were placed inside a tube furnace (CTF 12/75/700, Carbolite Ltd., Hope, UK), heated to a final temperature at a rate of 5 C/min in a nitrogen atmosphere and kept at this temperature for 8 hours.

Different final temperatures were tested in Example 1 : 300 C; 600 C; 800 C; and 1000 C.

As these temperatures are above the onset temperature for the thermal decomposition of the microorganism biomass, gaseous/volatile decomposition products are generated and blowing of the carbon foam precursor material takes place. With the passing of time, carbonization of the blown material takes place.

Figure 1 shows pellets of the chlorella biomass before pyrolytic treatment (Figure 1 a), and after pyrolytic treatment at 300 C (Figure 1 b) and 1000 C (Figure 1 c).

As apparent from Figure 1 , pyrolytic treatment provides dimensionally stable pellets made of carbon foam still showing the cylindrical shape of the untreated starting pellet.

Surprisingly, the pellet was shrinking, which increases with increasing pyrolytic treatment temperature. Typically, conventional foaming of coal results in a volume expansion. Figure 2 shows scanning electron microscopy (SEM) images of the pressed but untreated chlorella starting material (Figure 2a) and the carbon foam obtained by a pyrolytic treatment at 300 C (Figure 2b). Figure 3 shows a SEM image of the carbon foam obtained by a pyrolytic treatment at 1000 C.

As a result of the blowing and carbonization during the pyrolytic treatment, a continuous and rigid carbon skeleton was formed (see Figures 2b and 3), thereby providing a dimensionally stable carbon foam.

Properties of the carbon foam were determined and are listed in Tables 1 and 2.

Table 1

pyrolysis mass loss bulk dens. true dens, porosity open compr.

temp. during 8h [g/cm³] foam [%] cell strength

[°C] annealing [g/cm³] [%] [MPa]

[%]

300 50 0.83 1 .3 51 79 30

600 71 0.89 1 .8 56 87 83

800 74 0.91 1 .8 52 94 120

1000 77 0.87 1 .4 55 74 130

Table 2

Elemental analysis pyrolysis C H N BET surface bulk

resistance conduct.

[ C] [wt.%] [wt.%] [wt.%] [m²/g] [Ω] [S/cm]

300 65.9 5.6 1 1 .4 15 isolator isolator

600 66.9 2.0 10.0 32 350 0.007

800 70.5 1 .2 8.5 45 0.3 9.2

1000 83.4 0.4 3.0 56 0.04 63

Compared to conventional foams from coal or pitch, the bulk density values shown in Table 1 are rather high. Furthermore, the carbon foams of high bulk density show extremely high compressive strength values. Even at a relatively low pyrolytic treatment temperature of 300 C, a very high compressive strength of 30 MPa was achieved. At higher pyrolysis temperature, a further increase of compressive strength of the carbon foams was detected. This is illustrated in Figure 4. As shown in Table 2, electric conductivity of the carbon foam can be fine-tuned over a very broad range. At low pyrolysis temperature, the carbon foam is an isolator, whereas an electrically conducting carbon foam is obtained at higher pyrolysis temperature. However, as shown by Table 1 , both the isolating carbon foam and the electrically conductive carbon foam have a very high compressive strength.

The nitrogen adsorption/desorption isotherms of the carbon foam prepared by a pyrolytic treatment at 1000 C are shown in Figure 5. The shape of the isotherms and the absence of a hysteresis between adsorption and desorption isotherms at a relative pressure p/p°≤ 0.85 indicate that the carbon foam contains only minor amounts, if any, of micro- and mesopores.

The carbon foam prepared by the process of the present invention are of high thermal stability in air. The carbon foam prepared by a pyrolytic treatment at 1000 C was subjected to a thermogravimetric analysis in air and no weight loss was detected up to a temperature of about 440 C.

Example 2

In Example 2, the effect of a chemical activation agent for carbon on the properties of the carbon foam was studied.

KOH is a well-known chemical activation agent for carbon materials. It is commonly used for generating micro- and mesopores by etching the carbon material at elevated temperature.

The same microorganism biomass as in Example 1 was used in Example 2.

The chlorella powder was mixed with 5 wt%, 8 wt%, and 10 wt%, respectively, of KOH, based on the mass of the chlorella powder. The mixture was homogenized in a vibrating mill.

Subsequently the powder was pressed to a pellet as described above in Example 1. The pellets were placed inside a tube furnace (CTF 12/75/700, Carbolite Ltd., Hope, UK), heated to a final temperature of 1000 C at a rate of 5 C/min in a nitrogen atmosphere and kept at this temperature for 8 hours.

After pyrolytic treatment, a dimensionally stable carbon foam was obtained. Different from Example 1 , the pressed pellets showed a volume expansion during the pyrolytic treatment.

The dimensionally stable carbon foam body obtained after the pyrolytic treatment is shown in Figure 6. Macropores are already visible with the naked eye. A SEM image of the carbon foam prepared in the presence of 5 wt% KOH is shown in Figure 7 and confirms the presence of very large macropores. However, as confirmed by nitrogen adsorption analysis, the presence of a chemical activation agent also generated micro- and mesopores in the rigid carbon foam skeleton.

The nitrogen adsorption/desorption isotherms of the sample with 10 wt% KOH are shown in Figure 8. A hysteresis loop can be detected and indicates the presence of mesopores. From the isotherms, micropores were analyzed using the Horvat-Kawazoe method. The presence of micropores was confirmed.

The results of the nitrogen adsorption measurements are summarized in Table 3.

Table 3

KOH Surface area Cumulative pore Cumulative Cumulative Cumulative [wt.%] (BET) [m²/g] volume, Horvat- pore area, pore volume, pore area,

Kawazoe [cm³/g] Horvat- BJH [cm³/g] BJH [m²/g]

Kawazoe [m²/g]

0 56 0.02 20 0.05 41

5 84 0.04 108 0.05 34

8 202 0.10 275 0.07 50

10 158 0.08 222 0.08 42

So, Example 2 demonstrates that

a dimensionally stable carbon foam is still obtainable if a chemical activation agent for carbon (such as KOH) is added prior to or during the pyrolytic treatment, and

- by adding the chemical activation agent, microporosity and BET surface area can be increased.

The carbon foam body of Example 2 was also subjected to preliminary tests of compression strength. The preliminary results indicate that the samples of Example 1 (i.e. no chemical activation agent) have a compression strength which is clearly higher than the compression strength of the samples of Example 2. Example 3

The microorganism biomass used in Example 3 was a cyanobacterium of the genus spirulina (arthrospira platensis, origin: China). It was purchased as a dry powder from Dr. Behr GmbH, Bonn, Germany.

Thermogravimetric analysis of the spirulina biomass showed an onset temperature for thermal decomposition of about 274 C, and a maximum rate for weight loss at a temperature of about 319 C. Above the onset temperature for thermal decomposition, gaseous or volatile

decomposition products are generated, and these decomposition products may then effect blowing ("internal" blowing agent).

4 g of the spirulina powder was pressed for about 1 minute with a load of about 10 tons to a cylindrical pellet having a diameter of about 25 mm and a thickness of about 7 mm. The pellets were placed inside a tube furnace (CTF 12/75/700, Carbolite Ltd., Hope, UK), heated to a final temperature of 1000 C at a rate of 5 C/min in a nitrogen atmosphere and kept at this temperature for 8 hours. As these temperatures are above the onset temperature for the thermal decomposition of the microorganism biomass, gaseous/volatile decomposition products are generated and blowing of the carbon foam precursor material takes place. With the passing of time, carbonization of the blown material takes place. After pyrolytic treatment at 1000 C, a carbon foam is obtained. Different from Example 1 , the pellets showed a volume expansion during the pyrolytic treatment. A SEM image of the carbon foam is shown in Figure 9. The foam structure comprises large "windows" which are several hundred micrometers in diameter and are partially covered by thin films. A BET surface area, determined by nitrogen adsorption, of 93 m²/g was measured. The carbon foam body of Example 3 was also subjected to preliminary tests of compression strength. The preliminary results indicate that the samples of Example 1 (algae-based biomass) have a compression strength which is clearly higher than the compression strength of the samples of Example 3.

The foam of Example 3 had a carbon content of about 83 wt%.

The carbon foam of Example 3 can be of interest for applications which require a higher surface area but do not require high mechanical strength. Example 4

In Example 4, a hydrochar was prepared by hydrothermal carbonization, and said hydrochar was then used as a carbon foam precursor which was subjected to a blowing and carbonization by a pyrolytic treatment.

The microorganism biomass used in Example 4 was an algae of the genus chlorella. The chlorella biomass was subjected to a hydrothermal carbonization in an autoclave for 2 hours at 200 C (pressure inside the autoclave: about 15 bar), thereby obtaining a hydrochar. During the hydrothermal treatment in the autoclave, organic compounds were extracted from the biomass into the surrounding aqueous phase. These organic compounds are a potential nutrient source for growth of further microorganism biomass.

The hydrochar obtained by the hydrothermal carbonization still includes decomposable organic compounds such as fatty acids. The hydrochar was not subjected to an extraction treatment so that the fatty acids can act as an internal blowing agent in a subsequent pyrolytic treatment. For blowing and carbonization, the hydrochar was placed inside a tube furnace and heated to a final temperature of 1000 C at a rate of 5 C/min in a nitrogen atmosphere. An open cell carbon foam was obtained. Figure 10 shows said open cell carbon foam. According to elemental analysis, the carbon foam had a carbon content of 77%, a hydrogen content of 0.6%, and a nitrogen content of 4%.

Comparative Example 1

In Comparative Example 1 , starch was used as the carbon foam precursor material. 4 g of starch was pressed for about 1 minute with a load of about 10 tons to a cylindrical pellet having a diameter of about 25 mm and a thickness of about 7 mm. The pellets were placed inside a tube furnace (CTF 12/75/700, Carbolite Ltd., Hope, UK), heated to a final temperature of 1000 C at a rate of 5 C/min in a nitrogen atmosphere and kept at this temperature for 8 hours. Blowing was achieved by the gaseous decomposition products of the starch material.

During the pyrolytic treatment, the pellets showed a volume expansion. Finally, a carbon foam was obtained. However, the shape of the final carbon body is significantly deviating from the shape of the pressed starting pellet. The irregular, twisted foam body is shown in Figure 1 1 . Figure 12 shows a SEM image of the carbon foam. The cell walls of the carbon foam matrix are quite thin. The sample of Comparative Example 1 had a surface resistance of 700 Ω, which is significantly higher than the value obtained in Example 1 (0.04 Ω) at the same pyrolysis temperature

The carbon foam of Comparative Example 1 had a carbon content of about 92 wt%. However, although its carbon content is higher than the carbon content value obtained in Example 1 (83 wt%) at the same pyrolysis temperature, it has a much lower compressive strength. The carbon foam of Comparative Example 1 can be crushed just by pressing it between the bare fingertips.

Claims

C L A I M S

A process for preparing a carbon foam, wherein

The process according to claim 1 , wherein the microorganism is selected from algae or bacteria, more preferably from microalgae or cyanobacteria.

The process according to claim 1 or 2, wherein the hydrochar is obtained by a

hydrothermal carbonization at a temperature of from 120 C to 300 C.

The process according to one of the preceding claims, wherein the aqueous medium in which the hydrothermal treatment was carried out is used as a nutrient source for a microorganism biomass.

The process according to one of the preceding claims, wherein the carbon foam precursor is pressed to a shaped body and then subjected to the pyrolytic treatment.

The process according to one of the preceding claims, wherein an external blowing agent is added to the microorganism biomass prior to or during the pyrolytic treatment.

The process according to one of the claims 1 to 5, wherein no external blowing agent is added to the microorganism biomass prior to or during the pyrolytic treatment

The process according to one of the preceding claims, wherein the temperature of the pyrolytic treatment is at least 280 C, more preferably at least 500 C.

The process according to one of the preceding claims, wherein the microorganism biomass is brought into contact with a chemical activation agent prior to and/or during the pyrolysis, the chemical activation agent preferably being selected from an alkali metal hydroxide, a mineral acid, an alkaline earth metal carbonate, an alkaline earth metal chloride, a transition metal chloride, a transition metal carbonate, or a mixture of at least two of these activation agents.

A carbon foam, which is obtainable by the process according to one of the claims 1 to 9. A carbon foam having a bulk density of from 0.70 to 1 .30 g/cm³. The carbon foam according to claim 1 1 , which has a cumulative pore area, determined with nitrogen at 77 K and using the Horvat-Kawazoe method, of less than 90 m²/g, more preferably less than 60 m²/g; and/or has a cumulative pore area, determined with nitrogen at 77 K and using the Barrett-Joyner-Halenda (BJH) method, of less than 90 m²/g, more preferably less than 60 m²/g.

13. The carbon foam according to claim 1 1 or claim 12, which has a cumulative pore volume, determined with nitrogen at 77 K and using the Horvat-Kawazoe method, of less than 0.15 cm³/g, more preferably less than 0.10 cm³/g; and/or has a cumulative pore volume, determined with nitrogen at 77 K and using the Barrett-Joyner-Halenda (BJH) method, of less than 0.25 cm³/g, more preferably less than 0.15 cm³/g.

14. The carbon foam according to one of the claims 1 1 to 13, having a BET surface area, determined with nitrogen at 77 K, of less than 80 m²/g; and/or the carbon foam having a porosity of at least 35%.

15. The carbon foam according to one of the claims 1 1 to 14, having a compressive strength of at least 25 MPa.

16. Use of a microorganism biomass for preparing a carbon foam.