EP2566812A1

EP2566812A1 - Method of preparing porous carbon

Info

Publication number: EP2566812A1
Application number: EP11754714A
Authority: EP
Inventors: Peter Branton; Elizabeth Dawson; Gareth Parkes
Original assignee: British American Tobacco Investments Ltd; British American Tobacco Co Ltd
Current assignee: British American Tobacco Investments Ltd
Priority date: 2010-05-07
Filing date: 2011-05-06
Publication date: 2013-03-13
Also published as: RU2562285C2; ZA201208231B; KR20130062291A; CA2797212A1; RU2012152543A; GB201007667D0; KR20160044051A; AR081904A1; KR101756223B1; JP2013531596A; WO2011148156A1; CL2012003096A1

Abstract

The invention provides a method of preparing porous carbon with adsorbent properties for use in smoke filtration, the method comprising: pre-treating a starting material with an alkali solution; removing the alkali solution from the pre-treated material; and then activating the pre-treated material, wherein the starting material is a carbon precursor or a microporous carbon material. Preferably, the alkah solution is removed before the activation step. The invention also provides a porous carbon with micropores and mesopores, and uses thereof.

Description

Method of Preparing Porous Carbon

Field of the Invention

The present invention relates to methods for preparing porous carbon material, and in particular to methods designed to produce porous carbon with micropores and mesopores. The resultant porous carbon is particularly useful for smoke filtration in smoking articles, as the porous structure can provide improved adsorption of smoke vapour phase toxicants compared to conventional activated carbon.

Background to the Invention

Filtration is used to reduce certain particulates and/ or vapour phase constituents of tobacco smoke inhaled during smoking. It is important that this is achieved without removing significant levels of other components, such as certain organoleptic components, thereby degrading the quality or taste of the product.

Smoking article filters may include porous carbon materials (dispersed throughout filter material or in a cavity in the filter) to adsorb certain smoke constituents, typically by physisorption. Such porous carbon materials can be made from the carbonized form of many different organic materials, most commonly plant-based materials such as coconut shell.

Activated carbon materials have become widely used as versatile adsorbents owing to their large surface area, microporous structure, and high degree of surface reactivity. In particular, these materials are especially effective in the adsorption of organic and inorganic pollutants due to the high capacity of organic molecules to bind to carbon.

Activated carbons are commonly produced from materials including coconut shell, wood powder, peat, bone, coal tar, resins and related polymers. Coconut shell is particularly attractive as a raw material for the production of activated carbon because it is cheap and readily available, and is also environmentally sustainable. Furthermore, it is possible to produce from coconut shell activated carbon material which is highly pure and has a high surface area. The performance and suitability of activated carbon material as an adsorbent in different environments is determined by various physical properties of the material, including the shape and size of the particles, the pore size, the surface area of the material, and so on. These various parameters may be controlled by manipulating the process and conditions by which the activated carbon is produced.

Generally, the larger the surface area of a porous material, the greater is the adsorption capacity of the material. However, as the surface area of the material is increased, the density and the structural integrity are reduced. Furthermore, while the surface area of a material may be increased by increasing the number of pores and making the pores smaller, as the size of the pores approaches the size of the target molecule, it is less likely that the target molecules will enter the pores and adsorb to the material. This is particularly true if the material being filtered has a high flow rate relative to the activated carbon material, as is the case in a smoking article.

The precise method used to manufacture porous carbon material has a strong influence on its properties. It is therefore possible to produce carbon particles having a wide range of shapes, sizes, size distributions, pore sizes, pore volumes, pore size distributions and surface areas, each of which influences their

effectiveness as adsorbents. The attrition rate is also an important variable; low attrition rates are desirable to avoid the generation of dust during high speed filter manufacturing.

As explained in Adsorption (2008) 14: 335-341, conventional coconut carbon is essentially microporous, and increasing the carbon activation time results in an increase in the number of micropores and surface area but produces no real change in pore size or distribution.

In accordance with nomenclature used by those skilled in the art, pores in an adsorbent material that are less than 2nm in diameter are called "micropores", and pores having diameters of between 2nm and 50nm are called "mesopores". Pores are referred to as "macropores" if their diameter exceeds 50nm. Pores having diameters greater than 500nm do not usually contribute significantly to the adsorbency of porous materials. The distribution of pore sizes in a porous carbon material affects the adsorption characteristics, and it has been found that activated carbon material that is rich in micropores and mesopores exhibits excellent filtration of unwanted substances from the vapour phase of tobacco smoke, and an improvement over carbon which includes essentially only micropores.

Previous attempts to incorporate mesopores into activated carbon have met with difficulties and it has been found that it is difficult to reliably and reproducibly prepare mesoporous activated carbon. It has certainly not previously been possible to tailor the mesopore and micropore volumes of an activated carbon, in order to optimise the carbon's adsorption characteristics. Synthetic mesoporous carbons have been made, but these are relatively expensive.

In view of the foregoing, it is an aim of the present invention to provide a cheap and simple method for producing porous carbon materials for smoke filtration which have both micropores and mesopores.

Summary of the Invention

Accordingly, in a first aspect of the invention there is provided a method of preparing porous carbon having mesopores with adsorbent properties suitable for use in smoke filtration, the method comprising: pre-treating a starting material with an alkali solution; and then activating the pre-treated material, wherein the starting material is a carbon precursor or a microporous carbon material. Where the starting material is a carbon precursor, the alkali solution is preferably removed before the activation step and the pre-treated material is preferably charred prior to activation.

According to a second aspect of the invention, a porous carbon is provided which is obtained or obtainable by a method according to the first aspect of the invention. According to a third aspect of the invention, a filter element is provided, which may be a filter for a smoking article, comprising a porous carbon according to the second aspect of the invention. According to a fourth aspect of the invention, a smoking article is provided, comprising a porous carbon according to the second aspect of the invention.

Brief Description of the Figures

Figure 1A shows the N₂ adsorption isotherms of various samples of activated carbon.

Figure IB shows a bar graph indicating the adsorption properties of the activated carbon samples of Figure 1A.

Figure 2 shows a table indicating the physical properties of the activated carbon samples of Figures 1A and IB.

Figure 3 shows a table indicating the physical properties of various activated carbon samples.

Figure 4 shows the N₂ adsorption isotherms of the activated carbon samples of Figure 3.

Figure 5 shows a bar graph indicating the adsorption properties of the activated carbon samples of Figure 3.

Figure 6 shows the N₂ adsorption isotherms of various samples of activated carbon. Figure 7 shows a bar graph indicating the adsorption properties of various samples of activated carbon.

Figure 8 shows the effect of NaOH molarity on the textural properties of various samples of activated carbon.

Detailed Description of the Invention

The present invention relates to a method involving pre-treating a starting material with an alkali solution prior to an activation step. The activation of this pre-treated or doped material encourages the formation of mesopores during the activation step. Without the pre-treatment, the same activation step would result in the formation of micropores only. The starting material for the methods of the present invention may be any carbon precursor material. This carbon precursor material may be any lignocellulosic material, including, for example, coconut shell, or other naturally-occurring materials including pistachio nut shells, wood chips and bamboo.

Where the starting material is a carbon precursor material, the alkali solution is preferably removed from the pre-treated material, preferably by repeated washing steps, prior to the activation step. In an alternative embodiment, the starting material for the method of the present invention is a microporous carbon. In this embodiment, the method introduces mesopores to the already microporous carbon structure. The microporous carbon starting material may be conventionally activated carbon, such as microporous activated coconut carbon, or it can be a synthetic microporous carbon.

Where the starting material is a carbon precursor, the carbon precursor is pre- treated with an alkali solution and the resultant pre-treated material is preferably charred before then being activated. Conventional charring methods may be used. No charring step is required where the starting material is a microporous carbon material.

Whilst the precise nature of the effect of the alkali solution is not entirely understood, it is believed that the alkali may be involved in the dissolution of some of the lignin, hemicellulose and/or other components of the cell structure of a lignocellulosic carbon precursor such as coconut shell. This hypothesis is supported by the experimental data provided below. Residual sodium ions which remain bound to the precursor may catalyse the carbon gasification. It has been surprisingly found that the production of mesopores in the activated carbon is independent of the alkali concentration over the range stated. Therefore, it is possible to use an alkali solution in the pre-treatment step having a

concentration from as low as 0.1M up to 4M or higher. It does appear that the mesopores are at a maximum when using a solution having a concentration of 1M, as indicated by the graph shown in Figure 8, suggesting that relatively weak alkali solutions may be used. Figure 8 shows the effect of NaOH molarity on the textural properties of unwashed carbon, activated at 700°C for 4 hours in steam. The graph shows the molarities of NaOH used to pre-treat coconut shell before carbonisation and subsequent activation and the curves indicate the effect on the surface area/porosity of the activated product.

In a preferred embodiment of the invention, the alkali solution is an aqueous sodium hydroxide (NaOH) solution. It has been found that the use of NaOH is particularly effective in creating mesopores in the method according to the present invention. NaOH solutions may have a concentration of from about 0.1M, 0.2M, 0.5M, or 0.7M to about 1M, 2M, 3M, 4M, or greater. Once again, relatively weak solutions, for example 1M solutions may be preferred.

Alternatively, the alkali solutions used may be, for example, Na₂C0₃, OH, K₂C0₃, KHC0₃ and NH₄OH. NaOH and Na₂C0₃ solutions are preferred.

In further embodiments of the methods of the present invention, the pre-treatment with an alkali solution is followed by treatment with solutions of salts that can catalyse carbon gasification, such as iron and copper. Thus, for example, the carbon precursor may be treated using an iron sulphate solution. As the metal will precipitate if the metal salt solution is mixed with an alkali solution such as NaOH, the treatment with a salt solution is preferably in the form of a separate step to the pre-treatment step. The treatment with a salt solution may be carried out before or after the alkali solution pre-treatment step.

The pre-treatment step may involve contacting the starting material with an alkali solution at room temperatures. In yet further embodiments, the alkali solution used in the pre-treatment step of the present invention has a temperature above room temperature. In a particularly preferred embodiment, the alkali solution used to pre-treat the carbon has a temperature of between 30-60°C, which seems to enhance the formation of mesopores. Once the starting material has been doped or pre-treated with the alkali solution, if the alkali solution is to be removed, this may be achieved by repeated washing until the liquor is pH neutral which indicates that all of the alkali has been removed. Washing with an acid may speed up the removal of the alkali.

When the starting material is a carbon precursor, the next step in the methods of the present invention is preferably charring of the pre-treated material. Charring (or carbonisation) is a chemical process of incomplete combustion of a solid when subjected to high heat. By the action of heat, charring removes hydrogen and oxygen from the solid, so that the remaining product, the char, is composed primarily of carbon.

Suitable charring or carbonisation methods that may be used include those that will be familiar to the skilled person, such as the pit method, the drum method, and destructive distillation.

For example, the charring step may involve heating the pre-treated carbon to a temperature of at least 500°C and maintaining the carbon at that temperature for a number of hours. In one embodiment, the charring step involves heating the pre- treated carbon at a rate of 10°C/minute to 600°C under N₂ flowing at a rate of 100 cmVmin.

After charring, the carbon is cooled and the carbon surface is preferably

deactivated, for example by exposure to a humid N₂ flow. This deactivation is necessary because of the high risk of exothermic 0₂ adsorption causing red-heat. Subsequently, the charred material is activated.

Activation in the methods of the present invention may be by either physical or chemical means, and conventional activation techniques can be used. Preferably the material is activated by physical means, and most preferably the material is activated using nitrogen and steam, or alternatively, C0₂. In one embodiment of the invention, the pre-treated material is activated by reaction with steam under controlled nitrogen atmosphere in a kiln such as a rotary kiln. The temperature is important during the activation process. If the temperature is too low, the reaction becomes slow and is uneconomical. On the other hand, if the temperature is too high, the reaction becomes diffusion controlled and results in loss of the material.

Activation of the material using nitrogen and steam may be performed at a temperature of between 700°C and 1100°C. The activation process is preferably carried out for between 30 minutes and 6 hours. Most preferably, the material is activated using nitrogen and steam at about 700°C for 4 hours.

In an alternative embodiment, the material is activated by reaction with carbon dioxide. In this case, activation of the material may be performed at a temperature of between 700°C and 1100°C, and preferably activation is performed at a temperature of between 800°C and 1000°C. The activation process is preferably carried out for between 1 and 6 hours. Most preferably, the material is activated by reaction with carbon dioxide at about 800°C for 2-4 hours. The surface areas of activated carbon materials are estimated by measuring the variation of the volume of nitrogen adsorbed by the material in relation to the partial pressure of nitrogen at a constant temperature. Analysis of the results by mathematical models originated by Brunauer, Emmett and Teller results in a value known as the BET surface area.

The BET surface area of the activated carbon materials produced by the method of the invention is at least 500, 550, 600, 650, 700, 750, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800 or at least 1900 m²/g. Typical values for BET surface area of carbon materials produced by the method of the invention are 850 n ²/g. Porous carbon materials with BET surface areas between 700 m²/g and 1300 m²/ g are preferred. The relative volumes of micropores, mesopores and macropores in an activated carbon material can be estimated using well-known nitrogen adsorption and mercury porosimetry techniques. Mercury porosimetry can be used to estimate the volume of mesopores and macropores. Nitrogen adsorption can be used to estimate the volumes of micropores and mesopores, using the so-called BJH mathematical model. However, since the theoretical bases for the estimations are different, the values obtained by the two methods cannot be compared directly with each other.

The method of the invention yields an activated carbon material having a pore structure that includes mesopores and micropores. In the preferred carbon materials of the present invention, at least 20% but desirably no more than 95% of the pore volume (as estimated by nitrogen adsorption) is in mesopores. Typical minimum values for the volume of mesopores as a percentage of the combined micropore and mesopore volumes of the carbon materials of the invention are 25%, 35%, or 45%. Typical maximum values for such volumes are 95%, 90%, or 85%. Preferably the mesopore volume of the carbon materials of the invention is in the range of between 55% and 70% of the total pore volume. The ratio of mesopores to micropores is preferably between about 1:1 and about 3:1, and is most preferably in the region of about 2:1.

The porous carbon materials produced by the methods of the present invention preferably have a pore volume (as estimated by nitrogen adsorption) of at least 0.4cm³/g, and desirably at least 0.5 cm³/g. Carbon materials with pore volumes of at least 0.5cm³/g are particularly useful as an adsorbent for tobacco smoke. Carbon materials according to the invention with pore volumes significantly higher than 2cm³/g are low in density and are therefore less easy to handle in cigarette production equipment. Such carbon materials are less favourable for use in cigarettes or smoke filters for that reason. The activation step is preferably controlled to ensure that the resultant product contains the desired volume of micropores. In one embodiment of the invention, the product of the present invention has a ratio of at least 1 :2 of micropores to mesopores, which is desirable for good smoke adsorption characteristics. The activated carbon produced by the methods of the present invention may be provided in monolithic or particulate form. Particles will preferably have a particle size in the range of between ΙΟμηι and ΙΟΟΟμηι. Preferably the mean particle size is between 50μηι and 500μηι, and more preferably between ΙΟΟμπι and 400μιη. Most preferably, the particles of activated carbon material have a mean size of between

Experiments using cellulosic carbon precursors as starting material

Solution treatment of the raw shell

Most of the inorganic compounds studied as solutes were chosen because they have been used in conventional chemical activation of cellulosic carbon precursors, where they are dry-mixed with the precursor but in much higher concentrations (at least 1 :1) than used in this project. The accepted reaction route is by

dehydration/ decomposition and this tends to yield more mesoporous activated carbons than those formed by physical activation of a char (using steam or C0₂). However, the inorganic component should generally be washed out at the end of the activation process. The coconut shell (100 g) was stirred for 4 hours at 50°C in 300 cm³ aqueous solutions (usually 1-2M) of: NaOH (4M, 2M, 1M), Na₂C0₃, KOH, K₂C0₃, and KHC0₃. The shell was then washed until the liquor was neutral and dried at 100°C overnight. Charring

Coconut shell was heated at 10°C/min to 600°C in flowing N₂ (100 cm³/niin). The coconut shell was then held at 600°C for 4 hours. When cool, the charred shell was exposed to a humid N₂ flow (bubbler) to de-activate the carbon surface (necessary because of the high risk of exothermic 0₂ adsorption causing red-heat). 100 g of the shell/impregnated coconut shell yielded 28-29 g char. Measurement of the surface area of one of the chars derived from NaOH-treated shell, using krypton adsorption, gave a value of 0.65 m²/g. This indicates that the NaOH treatment alone does not generate significant surface area in the char.

Activation

The standard activation method used involved water (20 cm³ at 5 cm³/hour) introduced using a syringe pump, for 4 hours at 700°C, in N₂ flow of 20 cm³/min, with 5 g char.

Textural characterisation of the activated products

The link between NaOH treatment and ultimate mesoporosity in the steam- activated carbon appears very strong.

It appears that the shell is degraded by hot NaOH solution, leaving it highly susceptible to the formation of mesopores on subsequent activation in steam. This is possibly due to the dissolution of some of the hgnin/hemicellulose or other components (see Table 1) of the cell structure. The deep red-brown colour of the liquor when the NaOH-treated shell was washed supports this proposal. The effect does not appear to be dependent on the molarity of the alkali solution above 1M, althotigh more dilute concentrations have not yet been investigated. However, the figures in Table 2 indicate that only 6-7% material is removed by treatment with the alkali solution.

Table 1— Analysis of coconut shell

Table 2 - Mass loss after treatments

Treatment Mass lost (%) Dried 9

Washed with H₂0 and dried 10

Washed with NaOH solution and dried 16

Although the shell was washed thoroughly after NaOH treatment, the presence of a small amount of residual Na⁺ cannot be ruled out and this may have a catalytic or other effect in the steam activation process.

Samples for smoke testing

Scale-up (5x) activations produced enough activated sample, after sieving to give the correct particle sizes, for smoke testing. 100 g raw shell yields approx 28-29 g char, which in turn yields 16-20 g activated carbon. A control sample was also prepared from untreated shell, to give the same microporosity, but no mesopores. The sample data and isotherms are shown in the graph of Figure 1A. The adsorption properties of the samples are shown in the graph of Figure IB. Properties of the samples are set out in the table of Figure 2. There was also good agreement between the scaled-up and normal 5 g scale products with respect to their porosity characteristics.

The smoke analysis data shows that the carbon samples with increased mesopore volumes (samples 1, 2, and 5) exhibited much improved adsorption, with

significantly better reductions in all toxicants tested than the control (sample 6). All of the carbons used had similar micropore volumes, but whilst the control had a very low mesopore volume of just 0.04 cm³/g, the other samples, which were all pre-treated in methods according to the invention, had much greater mesopore volumes (see the table of Figure 2).

Further samples were provided for testing (samples 20, 21, and 22). These were prepared from NaOH-doped shell, steam-activated at 700 °C. One of these samples (sample 22) was washed and re-activated in C0₂ to increase the surface area and microporosity. A purely microporous carbon (sample 24) was also prepared for comparison. All these samples had significantly higher surface areas, micro- and nie sop of o sides than the first samples prepared for testing (see the data in Figure 1A, IB and 2). The sample details are given in the table of Figure 3 and the isotherms are shown in Figure 4.

Smoke analysis data for these re-activated samples is shown in Figure 5. The results indicate that the reduction in toxicants was further increased compared to the first set of samples. In addition, the washed sample (sample 22) showed significantly improved adsorption characteristics than the other sample across all of the toxicants tested.

Post-Treatment Washing

Despite attempts to remove the NaOH completely from the shell before

carbonisation by washing, it appears that some NaOH is retained in the final activated carbon which is therefore alkaline. Experiments have been conducted in an attempt to remove this residual NaOH by washing and, in doing so, a further increase in surface area and/ or porosity is achieved.

In most of the experiments conducted, washing the activated carbon in water or dilute acid removes the alkalinity and gives an increase in surface area and/ or porosity, as shown by the data in Tables 3 and 4 below. There appears to be no significant difference in the mass lost using water or acid for post-activation washing.

Table 3— Mass lost on post-activation washing of NaOH doped coconut shell

Table 4 - Effect of post-activation washing on textural characteristics

The data in Tables 3 and 4 indicate that there could be approximately 20% water- soluble ash in the final activated carbon. At least some of this could be due to alkaline Na compounds, e.g. the oxide (which will react with the wash water to produce NaOH). This material is likely to be situated within the pore network and could therefore be responsible for blocking pores. When this ash is dissolved and removed by a post-activation washing step, the whole pore network is opened up to gas phase adsorption and the measured surface area and porosity increase.

From these arguments, it can be concluded that despite attempts to remove all the caustic from the coconut shell by thorough washing, some Na is retained and it is fundamental to the development of mesoporosity when the carbon is activated, as it is entrained in the pre-cursor matrix and catalyses the activation.

Solution treatment of the raw shell

Treatment of raw coconut shell with NH₄OH (0.880) removed a comparable amount of material to the NaOH treatment (16%) but the standard activation conditions gave a relatively low burn off (11%) and a product with micropores only, supporting the theory that although the lignin in the shell may be extracted by alkali, entrained Na⁺ ions are necessary for catalysis of the activation process to give mesopores.

Experiments using microporous carbon as starting material

Since the treatment of coconut shell with NaOH solutions has been shown to give mesoporous activated carbons, the procedure was extended to the commercially available activated carbon sold as Aquasorb® by Jacobi. It was hypothesised that the presence of adsorbed sodium ions in the carbon pores of the activated starting material might catalyse further activation and selectively widen these pores.

The as-received activated carbon (designated JAC) was stirred at 50°C with 2M NaOH solution, and then filtered before drying overnight at 100°C. Samples were then re-activated in steam at 700°C or in C0₂ at 800°C. The large difference in mesopore volume between the steam and C0₂ activated samples shown in Table 5 below may be due to steam washing out the impregnated NaOH during activation.

Table 5 - Jacobi Aquasorb re-activation after NaOH doping

The N₂ adsorption isotherms shown in Figure 6 demonstrate that the effect is significant, but that in the case of the C0₂ re-activation, nearly all the microporosity has been widened. In this respect the product is very different from the activated carbon from NaOH-doped shell.

It would appear that there is no chemical dissolution of material when activated carbon is treated with the alkali solution, unlike that observed with raw coconut shell. Instead, it would seem that the solution will merely fill the existing micropore network and is easily removed. Therefore, if the alkali solution is washed out of the activated carbon before re-activation, essentially the starting material is unchanged and further activation will only produce micropores. For that reason, when the starting material is microporous carbon, the alkali solution is preferably not removed prior to the activation step. It is assumed that the mesoporosity develops around the Na deposits provided by doping using an NaOH solution, by a catalysed activation process.

Experiments on alternative carbon precursors

The NaOH treatment was also shown to result in mesoporous activated carbons prepared from pistachio nut shells, oak woodchips and bamboo. The carbons formed were generally less dense than the coconut carbons. The results for pistachio (sample 4, 0.28 cm³g^_1 micropore volume, 0.35 cm³g^_1 mesopore volume) suggest that this would be worth further investigation. The treatment was also used on activated carbon. For results, see the table in Figure 7.

Claims

1. A method of preparing mesoporous carbon, the method comprising:

pre-treating a starting material with an alkali solution; and

activating the pre-treated material,

wherein the starting material is a carbon precursor or a microporous carbon material.

2. A method as claimed in claim 1, wherein the carbon precursor is a lignocellulosic material and wherein the pre-treated material is not activated in the presence of the alkali.

3. A method as claimed in claim 2, wherein the pre-treated material is washed in order to remove the alkali solution prior to the activation step.

4. A method as claimed in either of claims 2 or 3, wherein the lignocellulosic material is coconut shell, pistachio shell, wood chips or bamboo.

5. A method as claimed in any one of claims 2 to 4, wherein the method further includes charring the pre-treated material prior to activation.

6. A method as claimed in claim 1, wherein the microporous carbon material is conventionally activated microporous carbon or a synthetic microporous carbon.

7. A method as claimed in any one of the preceding claims, wherein the alkali solution is an aqueous NaOH solution.

8. A method as claimed in claim 7, wherein the NaOH solution has a concentration of from about 0.1M to about 4M.

9. A method as claimed in any one of the preceding claims, wherein the method further includes treating the starting material or pre-treated material with a salt solution, such as an iron sulphate solution.

10. A method as claimed in any one of the preceding claims, wherein the pre- treatment step involves washing the starting material with an alkali solution having a temperature between ambient temperature and 60°C.

11. A method as claimed in any one of the preceding claims, wherein the activation of the pre-treated material involves treatment with steam at a temperature of 700°C.

12. Porous carbon obtained or obtainable by a method as claimed in any one of the preceding claims.

13. Porous carbon as claimed in claim 12, wherein the carbon has a ratio of at least 1:2 of micropores to mesopores.

14. A filter element comprising a porous carbon as claimed in claim 12 or 13.

15. A filter element as claimed in claim 14, wherein the filter element is a filter for a smoking article.

16. A smoking article comprising a porous carbon as claimed in claim 12 or 13.