WO2013001288A1

WO2013001288A1 - Preparing porous carbon

Info

Publication number: WO2013001288A1
Application number: PCT/GB2012/051495
Authority: WO
Inventors: Peter Branton; An-hui LU; Wen-cui LI
Original assignee: British American Tobacco (Investments) Limited
Priority date: 2011-06-27
Filing date: 2012-06-26
Publication date: 2013-01-03
Also published as: AR088132A1; TW201315762A; CN102849718A

Abstract

The present invention relates to a method of preparing porous carbon having micropores and macropores using a lipid, the method comprising phase separation. The present invention also relates to the porous carbon produced by such methods, as well as filters and smoking articles comprising said porous carbon.

Description

Preparing Porous Carbon Technical Field

The present invention relates to methods for preparing porous carbon and, in particular, to methods for preparing carbon which has a pore structure including both micropores and macropores.

Background

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

Activated carbons are commonly produced from organic 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 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.

An alternative source of microporous carbon is synthetic carbons, such as those formed by a polymerisation reaction, such as resin-based synthetic carbons. Such carbons may, for example, be prepared by polycondensation of an aldehyde and a phenol. These synthetic carbons are attractive because some of their physical properties can be controlled during manufacture, allowing them to be tailored to provide desired filtration characteristics. However, these materials are significantly more expensive than activated coconut carbon and the like.

The performance and suitability of porous 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 distribution of pore sizes, the surface area of the material, and so on. Generally, the larger the surface area of a porous material, the greater its adsorption capacity. However, as the surface area of the material is increased, the density and the structural integrity of the material are reduced. Furthermore, while the surface area of a material may be increased by increasing the number of pores and making the pores smaller. However, 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. In accordance with nomenclature used by those skilled in the art, pores in an adsorbent material that are less than 2 nm in diameter are called "micropores" and pores having diameters of between 2 nm and 50 nm are called "mesopores". Pores are referred to as "macropores" if their diameter exceeds 50 nm. Pores having diameters greater than 500 nm do not usually contribute significantly to the adsorbency of porous materials.

The precise method used to manufacture porous carbon material has a strong influence on its physical properties. Indeed, various parameters, including the shape and size of the particles, the pore size and surface area of the material, may be controlled by manipulating the process and conditions by which the porous carbon is produced. It is therefore possible to manufacture 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 the particles' 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.

For example, 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 of the material (but produces no real change in pore size or distribution).

It is known to incorporate porous carbon materials into smoking articles and smoking article filters in order to reduce the level of certain particulates and/ or vapour phase constituents of tobacco smoke inhaled during smoking, typically by physisorption. It is important that this is achieved without removing significant levels of other components, such as organoleptic components, as this would degrade the quality or taste of the product.

Porous carbon materials that have traditionally been used in smoking articles and smoking article filters include those made from the carbonised form of organic materials, most commonly plant-based materials such as coconut shell.

Alternatively, synthetic carbons are used, such as resins prepared by

polycondensation reactions.

It is believed that carbon material that is rich in both micropores and macropores will exhibit excellent filtration of select substances from the vapour phase of tobacco smoke and will thus be an improvement over conventional activated carbon which includes essentially only micropores.

However, previous attempts to incorporate macropores into conventional activated carbon have been met with difficulties. It has also proven difficult to reliably and reproducibly prepare a porous carbon that has a pore size distribution comprising micropores and macropores. It has previously been difficult or even impossible to tailor the micropore and macropore volumes of a porous carbon, in order to optimise the carbon's adsorption characteristics.

At least some of the embodiments facilitate the provision of a simple and highly reproducible method for producing carbon having both micropores and

macropores.

In particular, at least some of the embodiments facilitate the provision of a method for producing a porous carbon with enhanced adsorption of smoke vapour phase toxicants for use in smoke filtration, for example, in the filter of a smoking article such as a cigarette.

Summary At least some of the embodiments of the invention provide, in a first aspect, a method of preparing porous carbon having micropores and macropores using a lipid, the method comprising a phase separation step.

In an embodiment, the lipid is a fatty acid, preferably linoleic acid or oleic acid. Preferably, the fatty acid is not provided as a metal salt. The fatty acid may be provided as an ammonium salt. In these embodiments, the method may further comprise the use of an amine.

In an embodiment, the porous carbon is a resin-based synthetic carbon, preferably prepared by polycondensation of an aldehyde and a phenolic compound. The aldehyde is preferably formaldehyde and the phenolic compound is preferably phenol or resorcinol. The lipid is preferably added to the aldehyde and the phenolic compound prior to the polycondensation. The molar ratio of the amount of phenolic compound to the amount of lipid used is preferably between 30:1 and 3:1. The lipid, aldehyde and phenolic compound are preferably provided in aqueous solution.

In an embodiment, the phase separation step is carried out at about 70-90°C over a period of about 3-24 hours. Preferably, the phase separation step is a liquid phase separation step resulting in a lipid phase and an aqueous phase. Preferably, the phase separation step provides the porous carbon with its macropores.

In an embodiment, the method further comprises a pyrolysis step that is carried out at a temperature in the range of 700°C to 1000°C. The pyrolysis step preferably provides the porous carbon with micropores.

In an embodiment, the method further comprises activating the porous carbon using nitrogen and steam. In an embodiment, the method further comprises activating the porous carbon using carbon dioxide. At least some of the embodiments of the invention provide, in a second aspect, a porous carbon is provided which is obtained or obtainable by a method according to the first aspect of the invention. At least some of the embodiments of the invention provide, in a third aspect, a filter element for a smoking article is provided, comprising a porous carbon according to the second aspect.

At least some of the embodiments of the invention provide, in a fourth aspect, a smoking article is provided, comprising a porous carbon according to the second aspect.

Description of the Drawings

In order that the invention may be more fully understood, embodiments thereof will be described, by way of example only, with reference to the accompanying drawings.

Figure 1 illustrates the pores of the carbon at the various stages of the method according to an embodiment. Figure 2 illustrates a filter cigarette which has been partially unwrapped comprising a filter according to an embodiment of the invention. The figure is not to scale.

Figures 3 and 4 show the infrared spectrums, sorption isotherms and BJH plots of two porous carbon samples ('sample 1 ' and 'sample 2', respectively) produced as described in Example 1.

Figure 5 shows the infrared spectrum and sorption isotherm of a control porous carbon ('control sample') produced as described in Example 2. Detailed Description

Due to the high flow rates and short contact times involved in the smoking process, it is desirable to have a combination of micropores and larger pores, i.e. mesopores or, more preferably, small macropores, in a porous carbon for use in a smoking article. The larger pores act as 'transporters' to the micropores, the latter being used to trap the smoke toxicant vapour. As macropores offer better transport to the micropores than mesopores, general adsorption under the high flow rates encountered in the smoking process will be more efficient using a micro- macroporous carbon as opposed to a micro-mesoporous material.

Various embodiments relate to a method involving the formation of a porous carbon using a lipid. The method results in the presence of both micropores and macropores in the surface structure of the carbon material. Mesopores may also be present.

Various embodiments relate to a porous carbon that is obtained or obtainable by such a method. The present invention provides a porous carbon having micropores and macropores that is obtained or obtainable by a method using a lipid, the method comprising a phase separation step. The porous carbon may also have mesopores.

Such material is useful for smoke filtration in smoking articles, as the porous structure provides improved adsorption of smoke vapour phase constituents compared to conventional activated carbon.

By "micropore", "mesopore" and "macropore" can be meant the following. In accordance with nomenclature used by those skilled in the art, pores in an adsorbent material that are less than 2 nm in diameter can be called "micropores" and pores having diameters of between 2 nm and 50 nm can be called "mesopores". Pores can be referred to as "macropores" if their diameter exceeds 50 nm. In an embodiment, the macropores formed in the porous carbon materials of the invention have a diameter that is less 500 nm. In one embodiment, the porous carbon is a resin-based synthetic carbon, such as the carbon prepared by polycondensation of an aldehyde and a phenol. If available, commercially available polycondensates may be used. To produce the polycondensate, the starting material may be a phenolic compound such as phenol, resorcinol, catechin, hydrochinon or phloroglucinol, and an aldehyde such as formaldehyde, glyoxal, glutaraldehyde or furfural. A commonly used and preferred reaction mixture comprises resorcinol (1,3-dihydroxybenzol) and formaldehyde, which react with one another under alkaline conditions to form a gellike polycondensate. The polycondensation process will usually be conducted under aqueous conditions.

The rate of the polycondensation reaction, as well as the degree of cross-linking of the resultant gel, can, for example, be influenced by the presence of a catalyst and/ or the relative amounts of the phenol and the catalyst. The skilled person would know how to adjust the amounts of these components to achieve the desired outcome. Suitable catalysts include any base, for example, sodium carbonate, sodium hydroxide or potassium hydroxide. The quantity of the catalyst should be adjusted so that the final pH of the reaction mixture is in the range of 7-10.

The lipid used may be any type of lipid, provided that it is soluble in a solvent; any lipid soluble in any solvent could be used. Suitable lipids include fats, waxes, sterols, fat-soluble vitamins, monoglycerides, diglycerides, triglycerides, glycolipids and phospholipids. The lipid may be a fatty acyl, glycerolipid, glycerophospholipid, sphingolipid, sterol lipid, prenol lipid, saccharolipid or polyketide, for example. Fatty acyls, which include fatty acids, fatty acid conjugates and fatty acid derivatives, are preferred. Examples of suitable fatty acyls are the eicosanoids (including prostaglandins, leukotrienes and thromboxanes), fatty esters (including wax esters, fatty acid thioester coenzyme A derivatives, fatty acid thioester ACP derivatives and fatty acid carnitines) and fatty amides (including N-acyl ethanolamines).

Fatty acids are an example of a lipid which may be used. By "fatty acid" is meant a carboxylic acid with a long aliphatic tail (chain), which may be branched or unbranched, saturated or unsaturated. In the following examples of suitable fatty acids, the lipid numbers C:D are indicated in brackets, where C is the number of carbon atoms in the fatty acid and D is the number of double bonds in the aliphatic tail. Examples of suitable saturated fatty acids are lauric acid (12:0), myristic acid (14:0), palmitic acid (16:0), stearic acid (18:0), arachidic acid (20:0), behenic acid (22:0), lignoceric acid (24:0) and cerotic acid (26:0).

If unsaturated, the two carbon atoms in the fatty acid chain that are bound either side of the (each) double bond may occur in a as or trans configuration. Examples of suitable unsaturated fatty acids are myristoleic acid (14:1), palmitoleic acid (16:1), sapienic acid (16:1), oleic acid (18:1), linoleic acid (18:2), linolenic acid (18:3), arachidonic acid (20:4), eicosapentaenoic acid (20:5), erucic acid (22:1) and docosahexaenoic acid (22:6).

Fatty acids suitable for use in various embodiments may be of any chain length, provided the fatty acid is soluble in the chosen medium. Thus they may be short- chain fatty acids (having fewer than six carbons), medium-chain fatty acids (having six to 12 carbons), long-chain fatty acids (having 12 to 21 carbons) or very long- chain fatty acids (having more than 22 carbons). However, a chain that is from four to 28 carbons in length is preferred. This includes all of the particular saturated and unsaturated fatty acids identified above.

In an embodiment, the fatty acid is linoleic acid (18:2). Oleic acid (18:1) is a preferred agent, due to its good availability.

The fatty acid may be produced industrially by the hydrolysis of triglycerides, with the removal of glycerol. Alternatively, phospholipids may be used as the source or they may be produced synthetically by hydrocarboxylation of alkenes.

The lipid is added during formation of the porous carbon. For example, the lipid may be added to the starting materials used to produce the porous carbon.

Alternatively, it may be added at a later stage of the formation process. If a polycondensation reaction is employed, it is preferably carried out in the presence of the lipid, i.e. the lipid is added prior to the polymerisation step, for example, by mixing it with the component monomers. In preferred embodiments, for example, an aqueous solution (such as water or an aqueous ethanol solution) containing a fatty acid is added to an aqueous solution of the constituent reagents of the polycondensate (such as, for example, resorcinol and formaldehyde, or phenol and formaldehyde), under vigorous stirring at room temperature, to yield a homogeneous solution. A suitable aqueous solution of constituent reagents contains resorcinol and formaldehyde at 37% (w/w).

The use of a salt form of a fatty acid increases its solubility in an aqueous solution. Ammonia may thus be mixed with a fatty acid used, to yield a fatty acid salt. If a polycondensation reaction is employed, the fatty acid may be reacted with ammonia to form an ammonium salt prior to addition of the fatty acid to the reaction mixture containing the constituent reagents of the polycondensate. Other suitable fatty acid salts will be known to the skilled person. Preferably, however, the fatty acid is not provided as a metal salt. The salt will preferably be capable of leaving no toxic residue in the finished product (the porous carbon produced by the processes of the invention).

The amount of lipid or fatty acid used as a molar ratio to the amount of the phenolic compound used may be between 1 :30 and 1 :3. The molar ratio of phenolic compound to water is preferably in the range of between 1 :100 and 1 :3.

An amine, such as lysine or 1 -methylimidazole (MIM), may be used together with a fatty acid in the methods of the invention, to act as a structural directing agent in the final stage prior to carbonisation (the amine increases the nitrogen content of the final carbon). In a preferred embodiment, the amine is included in the aqueous solution of fatty acid, prior to its mixing with the constituent reagents of the polycondensate. A suitable amount of amine to be added is up to 2% by weight, but preferably about 1 % by weight. To produce the polycondensate, the reaction mixture may be warmed (this process is often referred to as 'aging' or, more usually, 'curing'). Usually, the

polycondensation reaction will be carried out at a temperature above room temperature and preferably between 40 and 90°C, for example at about 50°C. The incubation period may be between 5 minutes and 24 hours. The exact conditions will, however, depend upon the system at hand, but will be known to the skilled person. If a lipid, such as a fatty acid, is present in the reaction solution, molecules of the same will become trapped by the cross-links being formed as polymerisation progresses. The resultant resin will thus have molecules of lipid or fatty acid encapsulated within it.

The methods of the invention include a phase separation step. By "phase separation" is meant the process by which a single phase is separated into two or more new phases. A single liquid phase may be separated into two or more new liquid phases; for example, a homogeneous solution of two immiscible liquids may be separated into two phases, the lower phase comprising one of the immiscible liquids and the upper phase comprising the other.

Thus, in a preferred embodiment, the reaction solution emanating from a polycondensation reaction as described above is subjected to a phase separation step.

The conditions for phase separation will depend upon the particular system being separated. However, it may be preferable to carry out the phase separation at about 70-90°C over a period of about 3-24 hours; for example, 3 hours at 90°C or 24 hours at 70°C. Any temperature or temperature range in the preferred range may be used, for example, about 70, about 75, about 80, about 85, about 90, about 70-80, about 80-90, or about 75-85°C. Any period of time in the preferred range may be used with any of these temperatures, for example, about 3-5, about 3-10, about 3- 12, about 5-15, about 10-15, about 10-20, about 15-20 or about 20-24 hours. The precise conditions for any particular system will be known to the skilled person. A solvent can be used to improve solubility, if necessary. In the embodiments where a polycondensate is formed under aqueous conditions in the presence of a lipid, the phase separation step will be a liquid phase separation step resulting in the formation of (at least) a lipid phase and an aqueous phase. At least some of the lipid molecules that became encapsulated by the polymer as it formed will also escape from the polymer during this phase separation step. These molecules become part of the lipid phase that is formed on the surface of the aqueous solvent phase. Micropores are formed during the phase separation step. In an embodiment, it is only the phase separation step that provides the porous carbon with its macropores, i.e. macropores are not formed in the carbon material at any other stage of the formation process.

In the embodiments where a polycondensate is formed in the presence of a lipid and an amine, these agents are thought to act together as a structural directing agent to form mesopores and macropores in the resultant polymer, i.e. using the concept of self-assembly. The amine introduces nitrogen into the structure, which can provide enhanced selectivity for aldehydes and hydrogen cyanide (HCN). The amine is added during the polymerisation process (as above); this does not change the phase separation step.

Following phase separation, the porous carbon material (for example, the polycondensate) may be separated from the other phases by simply drying (the solvent will evaporate). Drying will be quicker if the temperature is increased. The drying time depends on the solvent used. The exact conditions will be known to the skilled person.

In one embodiment, the polycondensate then undergoes pyrolysis. The pyrolysis may also be described as carbonisation or charring. Pyrolysis (or carbonisation) is a chemical process of incomplete combustion of a solid when subjected to high heat. By the action of heat, pyrolysis removes hydrogen and oxygen from the solid, so that the remaining product, the char, is composed primarily of carbon. Pyrolysis also causes micropores and/ or mesopores to form in the material. The pores are formed by the burning away of the carbon. Pyrolysis does not cause macropores to form in the material.

Suitable pyrolysis 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. Pyrolysis should be carried out at an incubation temperature and time that is suitable for the material at hand; the appropriate conditions will be known to the skilled person. The incubation temperature and time may be between 300 °C and 1000 °C, and between 30 minutes and 4 hours, respectively.

In a preferred embodiment, the carbon is heated to a temperature falling in the range of 700°C to 1000°C, 700°C to 900°C or even 700°C to 800°C. Pyrolysis at such temperatures is believed to be advantageous, as it provides the carbon material with micropores and/or mesopores.

In a preferred embodiment, the desired temperature, once reached, is maintained for one hour or more. The temperature may be in the range of 700-900°C. The heating rate should not be too high; it may be in the range of 2-10°C/minute, for example, 5°C/minute or more.

For example, the pyrolysis step may involve heating the 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 pyrolysis step involves heating the carbon to the desired temperature at a heating rate of about 5°C/minute and maintaining the desired temperature for one hour. In another embodiment, the pyrolysis step involves heating the carbon at a rate of 5-10°C/minute to 700°C.

During pyrolysis, the carbon can be heated under a flow of nitrogen. The skilled person will be able to determine a suitable nitrogen flow rate for the furnace used.

In the embodiments where an ammonium salt of a fatty acid is present during polycondensation of carbon precursors, any ammonium cations (NH₄ ⁺) present in the resultant polycondensate will be decomposed by the pyrolysis process. It is an important feature that the porous carbon produced in accordance with embodiments of the invention does not contain any residues that would be toxic to the end user when the carbon is smoked in a smoking article or smoking article filter. Where a fatty acid salt is used, therefore, it is preferably a salt capable of leaving no toxic residue in the finished product. A preferred example is an ammonium salt of a fatty acid. In a preferred embodiment, the fatty acid salt is not a metal salt. After pyrolysis, the carbon is cooled and the carbon surface is preferably

deactivated, for example by exposure to a humid nitrogen flow. This deactivation is necessary because of the high risk of exothermic oxygen adsorption causing red- heat. According to an embodiment, the surface properties of the carbon are changed by treating the polycondensate to 'activate' it. Thus, in other words, the dried gels used may be non-activated or, in some embodiments, activated, for example nitrogen-activated, steam-activated and/ or activated with carbon dioxide.

Activation may be carried out before, during or after pyrolysis.

Where the starting material is a carbon precursor (such as a polycondensate of an aldehyde and a phenol), the carbon precursor is preferably pyrolysed before being activated. Conventional methods of pyrolysis may be used. During activation of the carbon material, micropores are formed by the burning away of the carbon. Existing microscopic cavities, pores and pits in the material that were formed during the pyrolysis process are opened and enlarged to form micropores. Consequently, the microporosity of the material may be regulated by regulating the degree of activation. Activation of pyrolysed carbon thus yields a material with a large surface area. Activation is preferably included in the methods of embodiments of the invention, in order to provide this improved pore structure. Activation may be done by either physical or chemical means, and conventional activation techniques can be used. Preferably the material is activated by physical means, for example, with steam, air, carbon dioxide, oxygen or a mixture of gases, which may be diluted with nitrogen or another inert gas. It is preferred to use a mixture of nitrogen and steam, or alternatively, carbon dioxide. The activation stage thus preferably takes place in a gaseous atmosphere comprising nitrogen, water and/ or carbon dioxide.

In one embodiment, the 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. The appropriate conditions for activation will depend upon the batch size and particular agents used.

For example, it may be appropriate to activate the material using nitrogen and steam at a temperature of between 600°C and 1100°C, optionally between 700°C and 900°C, such as at about 850°C. The length of the activation period will depend upon batch size; for example, small batches may require an activation period of between 30 minutes and 4 hours, such as one hour, whereas larger batches may require a longer period (tonne batches may require over a 24 hour period of activation, for example). The length of the activation period may also depend upon the temperature used. As the temperature is increased, the nitrogen content is decreased.

In an alternative embodiment, the material is activated by reaction with carbon dioxide. In this case, it may be appropriate to activate the material at a temperature of between 400°C and 1000°C, optionally between 600°C and 800°C. The activation process may be carried out for between 30 minutes and 4 hours, but, again, larger batches or lower temperatures may require a longer period of activation. Chemical activation methods may be used. For example, potassium hydroxide or zinc chloride may be used to activate the material. However, chemical activation methods may result in the deposition of chemicals in the carbon material, which may be undesirable. Such chemicals may be removed using an intensive washing procedure.

In any event, the skilled person will be able to determine the appropriate conditions for activation of the material at hand using the available equipment.

Figure 1 illustrates the pores of the carbon at the various stages of the method exemplified in Example 1 and described below.

A resin-based synthetic carbon (Figure 1A) is prepared by polycondensation of resorcinol and formaldehyde in the presence of oleic acid. As polymerisation progresses, molecules of oleic acid become trapped by the cross-links being formed in the polymer. The resultant gel-like polycondensate 1 thus contains oleic acid encapsulated by these cross-links 2. The degree of cross-linking (and hence fatty acid encapsulation) is influenced by the presence of the catalyst, 1-methylimidazole (MIM), and/ or the relative amounts of the resorcinol and the catalyst.

The products of the polycondensation reaction are subjected to a phase separation step (step I in Figure 1). At least some of the encapsulated oleic acid molecules escape from the polycondensate during this step. The resultant polycondensate (Figure IB) contains macropores 3, which are formed during the phase separation step. The polycondensate is dried at atmospheric pressure.

The polycondensate then undergoes pyrolysis (step II in Figure 1). The pyrolysed product (Figure 1C) contains microscopic cavities, pores and pits in its surface 4, which are formed by the burning away of the carbon during the pyrolysis process. The pores formed during pyrolysis are micropores (and possibly mesopores);

however, no macropores are formed during this step. After pyrolysis, the carbon is cooled. The product of this process (Figure ID) is thus a porous carbon material containing both micropores 5 and macropores 6, which offers enhanced adsorbency of smoke vapour phase toxicants compared to conventional porous carbon materials.

Whilst the precise mechanism by which the microporous and macroporous carbon produced in accordance with the present invention offers enhanced adsorbency of smoke vapour phase toxicants is not known, it is believed that the larger pores allow more of the target molecules to access the carbon's surface than otherwise would be the case. This hypothesis is supported by the experimental data provided and discussed below, and in particular by that disclosed in Examples 2 and 3.

The surface areas of porous 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 porous carbon materials produced in accordance with the present invention is important for the adsorption of smoke constituents. In particular, the activation step may be controlled to ensure that the resultant product contains the desired volume of micropores. The porous carbon materials produced according to the present invention preferably have a BET surface area of at least 400, 450, 500, 550, 600, 650, 700, 750, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800 or at least 1900 m²/g. Porous carbon materials with BET surface areas of between 500 m²/g and 1300 m²/g are preferred, and materials with surface areas of between 800 m²/g and 1300 m²/g are most 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 methods of the invention yield a carbon material having a pore structure that includes micropores and macropores. In the preferred carbon materials of the present invention, at least 20%, but desirably no more than 65% of the pore volume (as estimated by nitrogen adsorption), is in macropores. Typical minimum values for the volume of macropores as a percentage of the combined micropore and macropore volumes of the carbon materials of the invention are 25%, 35%, or 45%. Typical maximum values for such volumes are 55%, 60%, or 65%. Preferably the macropore volume of the carbon materials of the invention is in the range of between 25% and 55% of the combined micropore and macropore volume.

Preferably the carbon material contains only micropores and macropores, and no mesopores.

The porous carbon materials of the invention preferably have a pore volume (as estimated by nitrogen adsorption) of at least 0.3 cm³/g, and desirably at least 0.5 cm³/g. Carbon materials with pore volumes of at least 0.5 cm³/g are particularly useful as an adsorbent for tobacco smoke. Carbon materials according to the invention with pore volumes significantly higher than 1 cm³/g, however, 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 pore structure and density of porous carbon material are closely related.

Generally, the greater the pore volume of the material, the lower is the density.

Porous carbon materials produced by the method of the invention preferably have bulk densities greater than 0.25 g/ cm³, and preferably greater than 0.3 g/ cm³. The activated carbon material may have a bulk density of up to 0.7 g/ cm³, 0.6 g/ cm³ or 0.5 g/ cm³. Ideally, the bulk density is between 0.35 g/ cm³ and 0.55 g/ cm³. The porous carbon produced by the methods of the present invention may be provided in monolithic or particulate form.

If a monolith is formed, it is broken into pieces and sieved to the desired fraction.

Particles of a desired size may be produced using conventional mechanical size reduction techniques or grinding. It is preferred that the size reduction step results in the formation of granules with the desired size distribution, whereby the formation of a powder portion is substantially avoided. By reducing the size of the particles in this way, a greater surface area is also accessible for adsorption of target molecules.

Following size reduction, particles having an optimal size are selected. Size selection is by any suitable method, for example, by sieving of the material.

The particles of porous carbon should be small enough to provide a large surface area for smoke filtration. The particles should, however, be large enough that the smoke drawn through a filter comprising the particles is not restricted. It is also important that the particles are large enough that they cannot become entrained in the smoke and drawn through the filter to be inhaled by the smoker. The carbon is not harmful, but inhaling particles thereof would nevertheless be unpleasant for the user. If the size of the particles used is too small, the particles can also interfere with manufacturing processes, especially high speed processes as used to

manufacture cigarette filters.

On the other hand, if the particles are too large, the surface area to volume ratio of the particles will be such that the filtration efficiency would be reduced.

Taking these factors into account, porous carbon produced in accordance with the present invention will preferably have a particle size in the range of between 10 μπι and 1500 μπι. Preferably the mean particle size is between 100 μπι and 1000 μπι, and more preferably between 150 μπι and 800 μπι. Most preferably, the particles of carbon material have a mean size of between 250 μπι and 750 μπι. A porous carbon obtained or obtainable by a method of the invention may be used in a smoking article for smoke filtration. The porous carbon may be provided in the filter of such a smoking article.

As used herein, the term "smoking article" includes smokeable products such as cigarettes, cigars and cigarillos whether based on tobacco, tobacco derivatives, expanded tobacco, reconstituted tobacco or tobacco substitutes. Figure 2 shows a smoking article 7 comprising a filter 8.

The filter 8 is substantially cylindrical and has a mouth end 9 and smoking material end 10. The filter comprises three segments wherein the segments at the mouth end 11 and smoking material end 12 comprise plugs of filter material. The central filter segment comprises a cavity 13 which contains the porous carbon material of the invention.

The filter 8 is wrapped in a plugwrap 14 around its circumferential surface. The smoking article further comprises a cylindrical rod of smokeable material, in this case tobacco 15, aligned with the filter 8 such that the end of the tobacco rod 15 abuts the end of the filter 8. The tobacco rod is joined to the filter 8 by tipping paper 16 in a conventional manner.

Clearly the more porous carbon that is present in the filter, the greater is the capacity to filter smoke. It is important, however, that the filter does not contain too much porous carbon. For example, if the density at which porous carbon is packed into the filter is too great, then this may inhibit the smoke flow path and the smoker will experience an unsatisfactorily high resistance to draw. In addition, if the size of the cavity is increased and the amount of filter plug material is reduced to compensate, then particulate material may not be sufficiently filtered from the smoke. Porous carbon may be incorporated into the filter by a number of methods, in addition to the cavity filter embodiment shown in Figure 2. In some embodiments, the filter may comprise a Dalmatian-type filter, wherein the porous carbon is distributed throughout the filter material. In other embodiments, the filter may comprise a patch-type filter wherein the porous carbon material is attached to the plugwrap or tipping paper. In further embodiments, porous carbon may be incorporated into the filter in a combination or two or more of the above methods.

The amount of porous carbon that can be incorporated into the filter depends upon the type of filter. For example, filters for use with Super Slim cigarettes typically comprise between 12 and 20 mg, and preferably 16 mg of activated carbon. Filters for use in connection with King Size cigarettes on the other hand, typically comprise between 20 and 80 mg, preferably between 30 and 60 mg. In general, the filter may comprise between 5 mg and 120 mg of porous carbon, and preferably comprises between 10 mg and 100 mg of porous carbon. In the embodiment shown in Figure 2, the filter comprises 60 mg of porous carbon material produced according to the method of the invention.

Above is described what are believed to be the preferred embodiments of the invention. However, those skilled in the art will recognise that changes and modifications may be made without departing from the scope of the invention.

Examples

The following examples illustrate some embodiments of the invention. Example 1 - Preparation of Micro-Macroporous Carbon

Two samples of micro-macroporous carbon material were prepared from resorcinol and formaldehyde using a fatty acid. Briefly, approximately 10 g of sample 1 was prepared using 1 -methylimidazole (MIM) as a catalyst and oleic acid as a macropore- forming agent, based upon a polycondensation of resorcinol with formaldehyde at 50°C for 4 hours. The molar ratio of resorcinohMIM was 26:1. After phase separation, the resultant polycondensate was dried at atmospheric pressure, then carbonised at 800°C. Approximately 10 g of sample 2 was prepared in exactly the same way as sample 1, expect the molar ratio of resorcinohMIM was 13:1. The samples contained both micropores and macropores.

Example 2 - Assessment of Texture Parameters

Various physical properties of the micro-macroporous carbon samples prepared in Example 1 were assessed. The parameters measured were the BET surface area (S_BET), total pore volume (V_total), total pore volume present in micropores (V_mic) and the average pore diameter (D_av).

For reference purposes, a control carbon material was used, which had an entirely microporous structure. The control carbon was prepared as follows. Briefly, approximately 10 g of the control carbon was prepared using ethylenediamine (EDA) as a catalyst, based upon a polycondensation of resorcinol with

formaldehyde. The mass percentage of resorcinol, formaldehyde and EDA in solution was about 30 wt%, with thermal curing (4 h at 90°C). After drying, the control was thermally treated at 800°C under nitrogen (N₂). The synthetic conditions and texture parameters of the samples and control are shown in Table 1.

Table 1: Synthetic Conditions and Texture Parameters of Porous Carbons

R = resorcinol; F = formaldehyde; MIM = 1-methylimidazole; OA = oleic acid; EDA = ethylenediamine As Table 1 shows, the micro-macroporous carbon samples had a higher BET surface area, total pore volume, total pore volume present in micropores and average pore diameter than the control (which contained micropores only).

The infrared spectra, sorption isotherms and BJH plots of samples 1 and 2 are given in Figures 3 and 4. The infrared spectrum and sorption isotherm of the control sample are given in Figure 5. The skilled person will understand from the sorption isotherms that samples 1 and 2 comprise macropores (and some mesopores), in addition to micropores. In contrast, the control sample is microporous only. Thus, samples 1 and 2 have a structure that is both microporous and macroporous and is therefore suitable for use in tobacco smoke filtration.

Example 3 - Assessment of Adsorption Capacity

The micro-macroporous carbon samples prepared in Example 1 were assessed for their capacity to adsorb selected smoke vapour phase toxicants. Approximately 60 mg of sample 1 or 2 was used in a cavity filter, similar to that shown in Figure 2. The filters were attached to tobacco rods and the resulting smoking articles were conditioned at 22°C and 60% relative humidity for three weeks prior to smoking. Smoke analysis was performed in accordance with the International Organization for Standardization (ISO) method for smoke collection, which comprises a 35ml puff lasting two seconds taken every 60 seconds. The composition of the smoke drawn through each filter was then assessed. A control smoking article comprising 60mg of the control carbon (prepared in

Example 2) was used. In addition, a smoking article comprising an identical filter having no adsorbent was also studied under the same conditions. The results of these analyses are given in Table 2.

Table 2: Performance in a Cigarette Filter (60 mg carbon, ISO smoking)

NFDPM = nicotine-free, dry particulate matter ("tar")

The data shown in Table 2 were used to calculate percentage reductions in various components of tobacco smoke; these percentage reductions are given in Table 3. The efficacy of the micro-macroporous carbon samples, and the control, in reducing the levels of various organic molecules in tobacco smoke was assessed in comparison to the empty filter (i.e. the identical filter having no adsorbent).

Table 3: Percentage Reductions Compared to Empty Filter

Carbon Sample 1 Sample 2 Control

Acetaldehyde 46 59 15

Acetone 24 59 6

Acrolein 77 93 24

Butyraldehyde 22 48 16

Crotonaldehyde 82 93 42

Formaldehyde 61 66 58

Methyl ethyl ketone 23 61 9

Propionaldehyde 26 59 6 Hydrogen cyanide 87 87 63

1 ,3-butadiene 34 51 21

Acrylonitrile 77 85 45

Benzene 24 49 23

Isoprene 9 34 20

Toluene 30 61 26

The micro-macroporous carbon samples and the control were all good at reducing formaldehyde and hydrogen cyanide (HCN) in the smoke vapour phase because they contain nitrogen in their structures.

However, the micro-macroporous carbon samples were better at reducing the remaining toxicants compared to the control because they have macropores in addition to micropores in their structure. Furthermore, sample 2, which was prepared using a greater proportion of catalyst, had a greater capacity to remove the organic chemicals than sample 1.

In conclusion, it can be seen that the methods generate a porous carbon having a pore structure that is rich in micropores and macropores. This pore structure correlates with the capacity of the material to remove undesirable organic molecules from tobacco smoke since both of the micro-macroporous carbon samples remove these chemicals with greater efficiency than the exclusively microporous control.

Claims

1. A method of preparing porous carbon having micropores and macropores using a lipid, the method comprising a phase separation.

2. A method as claimed in claim 1, wherein the lipid is a fatty acid.

3. A method as claimed in claim 2, wherein the fatty acid is linoleic acid or oleic acid.

4. A method as claimed in claim 2 or claim 3, wherein the fatty acid is not provided as a metal salt.

5. A method as claimed in claim 4, wherein the fatty acid is provided as an ammonium salt.

6. A method as claimed in any one of claims 2-5, the method further comprising the use of an amine.

7. A method as claimed in any one of claims 1 -6, wherein the porous carbon is a resin-based synthetic carbon.

8. A method as claimed in claim 7, wherein the porous carbon is prepared by polycondensation of an aldehyde and a phenolic compound.

9. A method as claimed in claim 8, wherein the aldehyde is formaldehyde and the phenolic compound is phenol or resorcinol.

10. A method as claimed in claim 8 or claim 9, wherein the lipid is added to the aldehyde and the phenolic compound prior to the polycondensation.

11. A method as claimed in any one of claims 8-10, wherein the molar ratio of the amount of phenolic compound to the amount of lipid used is between 30: 1 and 3:1.

12. A method as claimed in any one of claims 8-11 , wherein the lipid, aldehyde and phenolic compound are provided in aqueous solution.

13. A method as claimed in any one of claims 1 -12, wherein the phase separation is carried out at about 70-90°C over a period of about 3-24 hours.

14. A method as claimed in any one of claims 1 -13, wherein the phase separation is a liquid phase separation resulting in a lipid phase and an aqueous phase.

15. A method as claimed in any one of claims 1 -14, wherein the phase separation provides the porous carbon with its macropores.

16. A method as claimed in any one of claims 1 -15, the method further comprising pyrolysis that is carried out at a temperature in the range of 700°C to 1000°C.

17. A method as claimed in claim 16, wherein the pyrolysis provides the porous carbon with micropores.

18. A method as claimed in any one of claims 1 -17, the method further comprising activating the porous carbon using nitrogen and steam.

19. A method as claimed in any one of claims 1 -18, the method further comprising activating the porous carbon using carbon dioxide.

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

21. A filter element for a smoking article comprising a porous carbon as claimed in claim 20.

22. A smoking article comprising a porous carbon as claimed in claim 20.