WO2012160354A1

WO2012160354A1 - Method of preparing enhanced porous carbon

Info

Publication number: WO2012160354A1
Application number: PCT/GB2012/051112
Authority: WO
Inventors: Peter Branton; An-hui LU; Wen-cui LI
Original assignee: British American Tobacco (Investments) Limited
Priority date: 2011-05-20
Filing date: 2012-05-17
Publication date: 2012-11-29
Also published as: TW201311558A; AR086494A1; CN102784618A

Abstract

The present invention relates to a method of preparing porous carbon. In particular, the carbon exhibits enhanced adsorption selectivity for low molecular weight aldehydes, such a formaldehyde, and for hydrogen cyanide. The method comprises applying magnesium carbonate to carbon prior to polycondensation and/or carbonisation, so that the porous carbon becomes impregnated with magnesium carbonate. The carbon is suitable for use in smoke filtration.

Description

Method of Preparing Enhanced Porous Carbon Field of the Invention

The present invention relates to a methods of preparing porous carbon. In particular, the carbon exhibits enhanced adsorption selectivity for low molecular weight aldehydes, such as formaldehyde, and for hydrogen cyanide (HCN). The porous carbon of the invention is particularly useful for smoke filtration in smoking articles, as it provides improved adsorption of the aforementioned smoke vapour phase constituents when 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 organoleptic components, thereby degrading the quality or taste of the product.

Smoking article filters may include porous carbon materials to adsorb certain smoke constituents, typically by physisorption. Such porous carbon materials can be made from the carbonized form of many different organic materials. Alternatively, synthetic carbons are used, such as resins prepared by polycondensation reactions.

Porous carbon materials have become widely used as versatile adsorbents owing to their large surface area, microporous structure, and high degree of surface reactivity. Porous carbons are commonly produced from materials including coconut shell, plant-based materials, 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.

As mentioned above, 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 manufacturing, 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 surface area of the material, and so on. These various parameters may be controlled by manipulating the process and conditions by which the porous 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.

Whilst it is well established that porous carbon material exhibits excellent general filtration of unwanted substances from the vapour phase of tobacco smoke, there are some smoke vapour constituents that exhibit relatively lower levels of adsorption and these include low molecular weight aldehydes (such as

formaldehyde) and hydrogen cyanide (HCN).

The presence of different compounds on the surface of the porous carbon material has been found to also affect the carbon's adsorption properties.

The present invention seeks to provide a method for preparing porous carbon having enhanced selective adsorption of low molecular weight aldehydes and HCN. Summary of the Invention

Accordingly, in a first aspect of the invention there is provided a method of preparing porous carbon for use in smoke filtration, the method comprising applying magnesium carbonate to carbon prior to polycondensation and/or carbonisation so that the carbon becomes impregnated with magnesium carbonate.

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 for a smoking article is provided, 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.

Description of the Drawings

For a fuller understanding of the invention, reference is made to Figure 1, which illustrates the percentage reduction in certain components of the smoke for activated carbon with magnesium carbonate applied (+ MgC0₃) versus activated carbon without magnesium carbonate applied (sorbite).

Detailed Description

The present invention relates to a method involving the application of magnesium carbonate to porous carbon so that the porous carbon becomes impregnated with magnesium carbonate and, as a result, has enhanced adsorbent properties.

In one embodiment of the invention, the porous carbon is a carbonated form of an organic material, such as coconut shell.

In another embodiment of the invention, 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 and 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 crosslinking of the resultant gel can, for example, be influenced by the relative amounts of the alcohol and catalyst. The skilled person would know how to adjust the amounts of these components used to achieve the desired outcome.

In order to produce particles of a desired size, it has been shown to be

advantageous to reduce the size of the polycondensate before further processing. The size reduction of the polycondensate may be carried out 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.

The organic material or polycondensate (which has optionally been reduced in particle size) then undergoes pyrolysis. The pyrolysis may also be described as carbonisation.

Carbonisation describes the procedure in which the material is pyrolysed in the absence of air to remove most of the elements other than carbon as volatile compounds.

Carbonisation may be achieved using any suitable method, and such methods will be familiar to the skilled person. Suitable methods include the pit method, the drum method, and destructive distillation. Carbonisation methods which utilise H₃P0₄ or ZnCl₂ increase the proportion of micropores in the material, and also increase the proportion of carbon in the material. However, this process may also increase the proportion of undesirable mineral compounds in the carbon material. Such mineral compounds may be removed by an intensive washing procedure.

Conditions may vary depending upon scale. However, the incubation temperature and time may be between 300°C and 1000°C, and between 30 minutes and 4 hours, respectively. For example, the pyrolysis 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 pyrolysis step involves heating the pre-treated carbon at a rate of 5-10°C/minute to 600°C under N₂ flowing at a rate of 10-200cm³/niin.

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

deactivated, for example by exposure to a humid N₂ flow. This deactivation is preferable due to the high risk of exothermic 0₂ adsorption causing red-heat.

Subsequently, to increase the surface area, the pyrolysed carbon is activated. In this process, existing pores and pits in the material are opened and enlarged to yield a material with a large surface area that is rich in micropores and mesopores.

The material may be activated by any means, and the skilled person would be aware that either physical or chemical means may be suitable. 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 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 activation process is preferably carried out for between 30 minutes and 4 hours. Most preferably, the material is activated for 1 hour.

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. Most preferably, the material is activated at about 900°C. Chemical activation methods may be used. For example, KOH or 2nCl₂ 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.

Following activation, the particle size of the material is reduced by between 10% and 40%, and preferably the particle size of the material is reduced by between 20% and 30%. Material produced according to the method of the invention will have particles that are small enough to provide a large surface area for smoke filtration. The particles of activated carbon material should, however, be large enough that the smoke drawn through the filter is not restricted. It is also important that the particles are large enough that they can't become entrained in the smoke and drawn through the filter to be inhaled by the smoker. The carbon is not harmful, but inhaling fragments would nevertheless be unpleasant for the user.

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

Taking these factors into account, activated carbon produced by the claimed method should preferably have a particle size in the range of between ΙΟμηι and 1500μηι. Preferably the mean particle size is between ΙΟΟμηι and ΙΟΟΟμηι, and more preferably between 150μηι and 800μηι. Most preferably, the particles of activated carbon material have a mean size of between 250μηι and 750μη .

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 used in the present invention is ideally at least 800m²/g, preferably at least 900m²/g, and desirably at least 1000, 1100, 1150, 1200, 1250, 1300, or 1350m²/g. Typical values for BET surface area of carbon materials produced by the method of the invention are up to about 1000, 1050, 1100, 1150, 1200, 1250, 1300, 1400, 1500, 1600, 1700, 1800, or 1900m²/g. Porous carbon materials with BET surface areas of between 1000m²/g and 1500m²/g are preferred, and material with surface areas of between 1200m²/g and 1400m²/g are most preferred. However, MgC0₃ impregnation works even for activated carbon with a low surface area down to 450m²/g as shown in the example below.

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 can use an activated carbon material having any pore structure that is generally suitable for smoke filtration, i.e. it may include micropores, mesopores and possibly macropores. In some suitable 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 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 55%, 60%, or 65%. Preferably the mesopore volume of the carbon materials of the invention is in the range of between 25% and 55% of the combined mesopore and micropore volume.

The porous carbon materials used in 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, 0.6, 0.7, 0.8, or 0.9cm³/g. Carbon materials with pore volumes of at least 0.5cm³/g are particularly useful as an adsorbent for tobacco smoke. Carbon materials with pore volumes significantly higher than the preferred values may be 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 activated carbon material are closely related.

Generally, the greater the pore volume of the material, the lower is the density. Activated carbon materials used in the invention preferably have bulk densities greater than 0.25g/cm³, and preferably greater than 0.3g/cm³. The activated carbon material may have a bulk density of up to 0.7g/cm³, 0.6g/cm³, or 0.5g/cm³.

A saturated basic solution of magnesium carbonate may be used in the present invention. A saturated solution in excess is used and 1% of saturated solution by carbon weight is added. However, other solvents may also be used in conjunction with the magnesium salt. This solution can then be applied to the carbon by soaking or spraying, where it acts as a catalyst. The porous carbon is not washed after application of the magnesium carbonate to remove excess carbonate.

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

A sample was made, referred to as "+ Mg0₃". This was synthesized using resorcinol and formaldehyde (40:60 ratio). Polycondensation was induced using magnesium carbonate as a catalyst, using a saturated basic solution of magnesium carbonate with thermal curing (3 gays at 40°C or 6 hours at 90°C). A gel is formed which is dried (2 days at room temperature or overnight at 90°C). The dried gel was pyrolysed at 600-900°C, preferably 800°C, for 2-8 hours, preferably 3 to 4 hours, in nitrogen. In this particular example, activation was not employed, but could have been used in order to achieve a higher surface area and porosity.

Sorbite, which is a mineral carbon, was used as a control. This material has a similar surface area and porosity to the sample referred to in the preceding paragraph. Both carbons are predominantly microporous with surface areas of 450m²/ g and pore volumes of 0.24 cm³/ g.

As can be seen in Figure 1, with the magnesium carbonate impregnated carbon there are significant reductions in the levels of acrolein, formaldehyde and HCN, as well as reductions in other constituents.

In summary, the Example indicates that applying magnesium carbonate to porous carbon according to the method of the invention provides porous carbon having enhanced adsorption of smoke vapour phase constituents.

Claims

1. A method of preparing porous carbon for use in smoke filtration, the method comprising applying magnesium carbonate to carbon prior to

polycondensation and/ or carbonisation, so that the porous carbon becomes impregnated with magnesium carbonate.

2. A method as claimed in claim 1, wherein the carbon is a form of an organic material.

3. A method as claimed in claim 2, wherein the organic material is a plant based material.

4. A method as claimed in claim 3, wherein the organic material is coconut shell.

5. A method as claimed in claim 1, wherein the carbon is a resin-based synthetic carbon.

6. A method as claimed in claim 5, wherein the resin-based synthetic carbon is prepared by polycondensation of an aldehyde and a phenolic compound.

7. A method as claimed in claim 6, wherein the phenolic compound is resorcinol.

8. A method as claimed in claims 6 or 7, wherein the aldehyde is

formaldehyde.

9. A method as claimed in any of the preceding claims, wherein the magnesium carbonate is applied to the porous carbon by soaking or, spraying.

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

11. A filter element for a smoking article comprising a porous carbon as claimed in claim 10. smoking article comprising a porous carbon as claimed in claim 10.