KR20130062291A - Method of preparing porous carbon - Google Patents

Abstract

The present invention is a method of preparing porous carbon having adsorptive properties for use in smoke filtration, wherein the starting material is pretreated with an alkaline solution, the alkaline solution is removed from the pre-treated material and thereafter activated the pre-treated material. System, wherein the starting material is a carbon precursor or a microporous carbon material. Preferably, the alkaline solution is removed before the activation step. The present invention also provides a porous carbon having micropores and mesopores, and uses thereof.

Description

Method for producing porous carbon {METHOD OF PREPARING POROUS CARBON}

The present invention relates to a method for producing a porous carbon material and in particular a method designed for producing porous carbon having micropores and mesopores. The obtained porous carbon is particularly useful for smoke filtration of smoking articles, because the porous structure can improve the adsorption of vaporous smoke toxicants as compared to conventional activated carbon.

Filtration is used to reduce the vapor phase components of certain particulates and / or tobacco smoke that are inhaled during smoking. It is important that this is achieved without significantly degrading the level of other ingredients such as certain organoleptic components and thereby degrading the quality or taste of the product.

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

Activated carbon materials are widely used as multipurpose sorbents because of their large surface area, microporous structure, and high degree of surface reactivity. In particular, these materials are particularly effective in the adsorption of organic and inorganic contaminants due to the high capacity of organic molecules binding to carbon.

Activated carbon is typically formed from materials including coconut shells, wood powder, peat, bone, coal tar, resins and related polymers. Coconut husk is particularly attractive as a raw material for the production of activated carbon because it is inexpensive, readily available and environmentally friendly. It is also possible to produce activated carbon materials which are very pure and have a high surface area from coconut shells.

The performance and suitability of activated carbon materials as adsorbents in different environments is determined by the various physical properties of the material, including the appearance and size of the particles, pore size, surface area of the material, and the like. These various parameters can be controlled by manipulating the processes and conditions under which activated carbon is produced.

In general, the larger the surface area of the porous material, the greater the adsorption capacity of the material. However, because the surface area of the material increases, the density and structural integrity are poor. In addition, the surface area of the material can be increased by increasing the number of pores and making the pores smaller, but as the pore size approaches the size of the target molecule, the target molecule is less likely to enter the pores and adsorb to the material. Lose. This is especially true when the material to be filtered has a high flow rate with respect to the activated carbon material, such as in smoking articles.

The precise method used to fabricate porous carbon materials greatly affects their properties. Thus, it is possible to form carbon particles having a wide range of shapes, sizes, size distributions, pore sizes, pore volumes, pore size distributions and surface areas, each of which affects its effectiveness as an adsorbent. Attrition rate is also an important variable, where low loss rate is desired to prevent the generation of dust during high speed filter fabrication.

As described in Adsorption (2008) 14: 335-341, conventional coconut carbon is essentially microporous and an increase in carbon activation time results in an increase in the number of micropores and the surface area. However, it does not form any practical change in pore size or distribution.

According to the nomenclature used by those skilled in the art, the pores in an adsorbent material less than 2 nm in diameter are referred to as "micropore" and the pores having a diameter of 2 nm to 50 nm are referred to as "mesopore". . Pores are referred to as "macropores" when their diameter exceeds 50 nm. Pores with diameters larger than 500 nm usually do not contribute significantly to the adsorptivity of the porous material.

The pore size distribution in porous carbon materials affects adsorption characteristics, and activated carbon materials rich in micropores and mesopores provide improved filtration of unwanted materials from vaporous tobacco smoke, and improvements over carbon, including essentially micropores only. It was found to indicate.

Previous attempts to introduce mesopores into activated carbon have encountered difficulties, and it has been found to be difficult and reliable to produce mesoporous activated carbon. In order to optimize the adsorption properties of carbon, it is certainly not possible to conventionally adjust the mesopores and micropore volumes of activated carbon. Synthetic mesoporous carbon has been produced, but these are relatively expensive.

In view of the above technique, it is an object of the present invention to provide an inexpensive and simple method for producing porous carbon materials for smoke filtration with both micropores and mesopores.

Accordingly, a first aspect of the invention includes pretreating the starting material with an alkaline solution and then activating the pre-treated material, wherein the starting material is a carbon precursor or a microporous carbon material. Provided is a method of making porous carbon with mesopores having adsorptive properties suitable for use. If the starting material is a carbon precursor, the alkaline solution is preferably removed before the activation step, and the pre-treated material is preferably carbonized before activation.

According to a second aspect of the invention, there is provided a porous carbon obtained or obtainable by the process according to the first aspect of the invention.

According to a third aspect of the invention, there is provided a filter element, 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, there is provided a smoking article comprising the porous carbon according to the second aspect of the invention.

1A shows N ₂ adsorption isotherms of various activated carbon samples.
FIG. 1B shows a bar graph showing the adsorption properties of the activated carbon sample of FIG. 1A.
FIG. 2 shows a table showing the physical properties of the activated carbon samples of FIGS. 1A and 1B.
3 shows a table showing the physical properties of various activated carbon samples.
4 shows an N ₂ adsorption isotherm of the activated carbon sample of FIG. 3.
FIG. 5 shows a bar graph showing the adsorption properties of the activated carbon sample of FIG. 3.
6 shows N ₂ adsorption isotherms of various activated carbon samples.
7 shows a bar graph showing the adsorption properties of various activated carbon samples.
8 shows the effect of NaOH molar concentration on the textural properties of various activated carbon samples.

The invention relates to a process comprising pre-treating the starting material with an alkaline solution prior to the activation step. Activation of this pre-treated or doped material encourages the formation of mesopores during the activation step. If not pre-treated, the same activation step will only result in the formation of micropores.

The starting material for the process of the invention can be any carbon precursor material. Such carbon precursor material may be any lignocellulosic material, including, for example, coconut shells or other natural materials including pistachio nut shells, wood chips and bamboo.

If the starting material is a carbon precursor material, the alkaline solution is preferably removed from the pre-treated material, preferably by a repeated washing step, prior to the activation step.

In an alternative embodiment, the starting material for the process of the invention is microporous carbon. In this embodiment, the method introduces mesopores into a carbon structure that is already microporous. The microporous carbon starting material can typically be activated carbon, for example microporous activated coconut carbon, or it can be synthetic microporous carbon.

If the starting material is a carbon precursor, the carbon precursor is pre-treated with an alkaline solution, and the obtained pre-treated material is preferably carbonized and subsequently activated. Conventional charring methods can be used.

No carbonization step is required if the starting material is a microporous carbon material.

Although the precise nature of the alkaline solution effect is not entirely understood, it is believed that alkali may participate in the dissolution of some of the other components of the cell structure of lignocellulosic carbon precursors such as lignin, hemicellulose and / or coconut husks. Is considered. This hypothesis is supported by the experimental data provided below. Residual sodium ions that remain bound to the precursor can catalyze carbon gasification.

Surprisingly, it has been found that the production of mesopores in activated carbon is independent of alkali concentrations over the described ranges. Thus, it is possible to use alkaline solutions with concentrations as low as 0.1M to 4M or higher in the pre-treatment step. As indicated by the graph shown in FIG. 8, mesopores appear to be maximum when using a solution with a concentration of 1 M, suggesting that a relatively weak alkaline solution can be used. FIG. 8 shows the effect of NaOH molar concentration on the structural properties of unwashed carbon, activated at 700 ° C. for 4 hours in steam. This graph shows the molar concentration of NaOH used to pre-treat the coconut shell before carbonization and after activation, and this curve shows the effect on the surface area / porosity of the activated product.

In a preferred embodiment of the invention, the alkaline solution is an aqueous sodium hydroxide (NaOH) solution. The use of NaOH has been found to be particularly effective in producing mesopores in the process according to the invention. The NaOH solution may have a concentration of about 0.1M, 0.2M, 0.5M, or 0.7M to about 1M, 2M, 3M, 4M, or greater. Again, relatively weak solutions may be preferred, for example 1M solutions.

Alternatively, the alkaline solution used may be Na ₂ CO ₃ , KOH, K ₂ CO ₃ , KHC0 ₃ and NH ₄ OH, for example. NaOH and Na ₂ CO ₃ solutions are preferred.

In another embodiment of the process of the invention, after pre-treatment with alkaline solution, treatment with a solution of salts capable of catalyzing carbon gasification, such as iron and copper, is carried out. Thus, for example, the carbon precursor can be treated using ferrous sulfate solution. Since the metal precipitates when the metal salt solution is mixed with an alkaline solution such as NaOH, treatment with the salt solution is preferably in the form of separate steps for the pre-treatment step. Treatment with the salt solution may be carried out before or after the alkaline solution pre-treatment step.

The pre-treatment step may comprise contacting the starting material with an alkaline solution at room temperature. In another embodiment, the alkaline solution used in the pre-treatment step of the present invention has a temperature higher than room temperature. In a particularly preferred embodiment, the alkaline solution used to pre-treat the carbon has a temperature of 30 to 60 ° C., which will improve the formation of mesopores.

If the alkaline solution is to be removed immediately after the starting material is doped or pre-treated with the alkaline solution, this can be achieved by repeated washing until the liquid is pH neutral indicating that all alkali is removed. Washing with acid can accelerate the removal of alkali.

When the starting material is a carbon precursor, the next step in the process of the invention is preferably charring of the pre-treated material. Carbonization (or carbonisation) is a chemical process of incomplete combustion of solids when treated with high heat. By the action of heat, carbonization removes hydrogen and oxygen from the solids such that the remaining product, char, consists mainly of carbon.

Suitable carbonization or carbonization methods that can be used include methods well known to those skilled in the art, such as the pit method, drum method, and destructive distillation.

For example, the charring step may include heating the pre-treated carbon to a temperature of at least 500 ° C. and maintaining the carbon at this temperature for several hours. In one embodiment, the carbonization step comprises heating the pre-treated carbon at a rate of 10 ° C./min to 600 ° C., under N ₂ flowing at a rate of 100 cm 3 / min.

After carbonization, the carbon is cooled and the carbon surface is preferably deactivated, for example by exposure to a humid N ₂ stream. Due to the high risk of exothermic O ₂ adsorption causing red-heat, this deactivation is essential. Thereafter, the charred material is activated.

Activation in the method of the present invention can be accomplished by either physical or chemical means, and conventional activation techniques can be used. Preferably, the substance is activated by physical means, most preferably the substance is activated using nitrogen and steam, or alternatively CO ₂ .

In one embodiment of the invention, the pre-treated material is activated by reaction with steam under a controlled nitrogen atmosphere in a kiln, such as a rotary kiln. Temperature is important during the activation process. If the temperature is too low, the reaction is slow and uneconomical. On the other hand, if the temperature is too high, the reaction is diffusion controlled and leads to loss of material.

Activation of the material with nitrogen and steam can be carried out at temperatures of 700 ° C to 1100 ° C. The activation process is preferably carried out for 30 minutes to 6 hours. Most preferably, the material is activated using nitrogen and steam at about 700 ° C. for 4 hours.

In another embodiment, the substance is activated by reaction with carbon dioxide. In this case, the activation of the material can be carried out at a temperature of 700 ° C. to 1100 ° C., preferably, the activation is carried out at a temperature of 800 ° C. to 1000 ° C. The activation process is preferably carried out for 1 to 6 hours. Most preferably, the material is activated by reaction with carbon dioxide at about 800 ° C. for 2-4 hours.

The surface area of activated carbon material is estimated by measuring the change in the volume of nitrogen adsorbed by the material to the partial pressure of nitrogen at a constant temperature. Analysis of the results by mathematical models derived from Brunauer, Emmmett and Teller leads to a figure known as the BET surface area.

The BET surface area of the activated carbon material formed by the method of the present 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 M 2 / g. Typical values for the BET surface area of the carbon materials formed by the process of the invention are 850 m 2 / g. Preference is given to porous carbon materials having a BET surface area of 700 m 2 / g to 1300 m 2 / g.

The relative volumes of micropores, mesopores and macropores in activated carbon materials can be estimated using well known nitrogen adsorption and mercury porosimetry techniques. Mercury porosity measurements can be used to estimate the volume of mesopores and macropores. Nitrogen adsorption can be used to estimate the volume of micropores and mesopores using a so-called BJH mathematical model. However, because the theoretical bases for this estimation are different, the values obtained by the two methods cannot be directly compared with each other.

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

The porous carbon material produced by the process of the invention preferably has a pore volume (estimated by nitrogen adsorption) of at least 0.4 cm 3 / g, and preferably at least 0.5 cm 3 / g. Carbon materials having a pore volume of at least 0.5 cm 3 / g are particularly useful as adsorbents for tobacco smoke. Carbon materials according to the invention having a pore volume significantly greater than 2 cm 3 / g have a low density and are therefore less easy to operate in cigarette production equipment. Such carbon materials may for this reason be less desirable for use in cigarettes or smoke filters.

The activation step is preferably adjusted so that the product obtained contains the desired volume of micropores. In one embodiment of the invention, the product of the invention has a ratio of micropores to mesopores of at least 1: 2, which is desired for good smoke adsorption characteristics.

Activated carbon formed by the method of the present invention may be provided in monolithic or particulate form. The particles will preferably have a particle size in the range of 10 μm to 1000 μm. Preferably, the average particle size is between 50 μm and 500 μm, more preferably between 100 μm and 400 μm. Most preferably, the particles of activated carbon material have an average size of 150 μm to 250 μm.

Experiment using cellulose carbon precursor as starting material

Raw shell ( raw shell Solution treatment

Most inorganic compounds studied as solutes were selected because these are used in the conventional chemical activation of cellulose carbon precursors, which in general chemical activation are dry mixed with the precursors, but are much larger than those used in these projects. Mix in concentration (at least 1: 1). Acceptable reaction pathways are by dehydration / decomposition, which tends to produce activated carbon that is more mesoporous than that formed by physical activation of char (using steam or CO ₂ ). However, the inorganic component generally has to be washed at the end of the activation process.

Coconut shell (100 g) was stirred for 4 h at 50 ° C. in a 300 cm 3 aqueous solution of NaOH (4M, 2M, 1M), Na ₂ CO ₃ , KOH, K ₂ CO ₃ , and KHC0 ₃ (usually 1 to 2M). . The shell was then washed and dried at 100 ° C. overnight until the liquid became neutral.

carbonization( charring )

The coconut shell was heated to 10 ° C./min to 600 ° C. in flowing N ₂ (100 cm 3 / min). Thereafter, the coconut shells were kept at 600 ° C. for 4 hours. Upon cooling, the carbonized shell was exposed to a wet N ₂ stream (bubble) to inactivate the carbon surface (essential due to the high risk of exothermic O ₂ adsorption causing redness). 100 g shells / impregnated coconut shells yielded a 28 to 29 g difference.

Using krypton adsorption, the surface area measurement of one of the teas derived from the NaOH-treated husk provided a value of 0.65 m 2 / g. This indicates that treatment with NaOH alone does not result in significant surface area in the car.

Activation

The standard activation method used included a 20 g 3 / min N ₂ flow, water introduced using a syringe pump for 4 hours at 700 ° C. (20 cm 3/5 cm 3 / hour) with a 5 g difference.

Structural Characterization of Activated Products textural characterisation )

The association between NaOH treatment and final mesoporosity in steam-activated carbon is very high.

The shell appears to decompose by hot NaOH solution, which greatly facilitates the formation of mesopores in subsequent activation in steam. This is probably due to the dissolution of some of the lignin / hemicellulose or other components of the cell structure (see Table 1). Dark reddish brown liquid supports this proposal when NaOH-treated shells are washed. No dilute concentrations were studied, but this effect did not appear to be dependent on the molar concentration of the alkaline solution above 1M. However, the numbers in Table 2 indicate that only 6-7% of the material was removed by treatment with alkaline solution.

Table 1-Analysis of Coconut Shells

Table 2-Mass Loss After Treatment

The shells were thoroughly washed after NaOH treatment, but the presence of small amounts of residual Na ⁺ could not be ruled out, which could have a catalytic or other effect in the steam activation process.

Sample for smoke test

For smoke testing, scale-up (5 times) activation formed a sufficiently activated sample after sieving to provide the correct particle size. 100 g raw shell yields approximately 28 to 29 g difference, which also yields 16 to 20 g of activated carbon. A control sample was also prepared from the untreated shell that provided the same microporosity but not mesopores. Sample data and isotherms are shown in the graph of FIG. 1A. Adsorption properties of the samples are shown in the graph of FIG. 1B. The properties of the samples are described in the table of FIG.

In addition, for their porous characteristics, there was a good agreement between the scale-up and the typical 5 g scale product.

Smoke analysis data showed that the carbon samples with increased mesopore volume (samples 1, 2 and 5) showed very improved adsorption and markedly better reductions compared to the control (sample 6) in all toxicants tested. It was. All carbons used had similar micropore volumes, but the control had a very low mesopore volume of only 0.04 cm 3 / g, while in the method according to the invention all other pre-treated samples had very large mesopore volumes. (See table in FIG. 2).

Additional samples were provided for testing (Samples 20, 21 and 22). These were prepared from NaOH-doped shells steam-activated at 700 ° C. One of these samples (Sample 22) was washed and re-activated in CO ₂ to increase surface area and microporosity. In addition, purely microporous carbon (Sample 24) was prepared for comparison. All these samples have much greater surface area, micro- and mesoporosity than the first sample prepared for testing (see data in FIGS. 1A, 1B and 2). These sample details are provided in the table of FIG. 3, with the isotherm shown in FIG. 4.

Smoke analysis data for this re-activated sample is shown in FIG. 5. This result indicates that the reduction of toxins was further increased compared to the first sample set. In addition, the washed sample (Sample 22) showed very improved adsorption characteristics compared to the other samples for all toxicants tested.

After processing  wash( post - treatment washing )

Despite attempts to completely remove NaOH from the shell prior to carbonization by washing, some NaOH remains in the final activated carbon, indicating alkalinity. Experiments were conducted in an attempt to remove residual NaOH by washing, through which further increases in surface area and / or porosity are achieved.

In most of the experiments performed, washing activated carbon in water or dilute acid removes alkalinity and provides an increase in surface area and / or porosity, as can be seen by the data in Tables 3 and 4 below. The mass lost using water or acid for post-activation washing did not appear to differ significantly.

Table 3-Lost mass for post-activation washing of NaOH doped coconut husks

Table 4-Effect of post-activation wash on structural features

The data in Tables 3 and 4 show that approximately 20% water soluble ash may be present in the final activated carbon. At least some of these may be due to alkaline Na compounds, for example oxides, which may react with wash water to form NaOH. Such materials are likely to be located in the pore network, and thus may cause blocking of the pores. When this ash is dissolved and removed by the post-activation washing step, the entire pore network is released to gaseous adsorption, and the measured surface area and porosity increase.

From this point, despite attempts to remove all caustic from coconut shells by thorough washing, some Na is retained, which is incorporated into the precursor matrix and promotes activation, which is mesoporous when carbon is activated. It can be concluded that it is essential to the development of.

Solution treatment of raw shell

Treatment of the raw coconut shell with NH ₄ OH (0.880) removes similar amounts of material as in NaOH treatment (16%), but standard activation conditions have relatively low burn off (11%) and only micropores. It provides a product, which supports the theory that although lignin in the shell can be extracted by alkali, the incorporated Na ⁺ ions are necessary for the catalysis of the activation process to provide mesopores.

Experiment using microporous carbon as starting material

Although treating coconut shells with NaOH solution has been shown to provide mesoporous activated carbon, this procedure extends to commercially available activated carbon sold by Jacobi as Aquasorb®. It is assumed that the presence of sodium ions adsorbed to the carbon pores of the activated starting material promotes further activation and selectively widens these pores.

As-received activated carbon (named JAC) was stirred at 50 ° C with 2M NaOH solution, filtered and dried at 100 ° C overnight. The sample was then re-activated at 700 ° C., steam or at 800 ° C. in CO ₂ . The large difference in mesopore volume between the steam activated sample and the CO ₂ activated sample described in Table 5 may be due to steam washing NaOH impregnated during activation.

Table 5-Jacobi Aquasorb Re-Activation After NaOH Doping

The N ₂ adsorption isotherm shown in FIG. 6 indicates that, although this effect is significant, in the case of CO ₂ re-activation, almost all microporosity is widened. In this regard, this product is very different from activated carbon from NaOH-doped shells.

Contrary to what was observed in raw coconut shells, it was found that the material did not chemically dissolve when activated carbon was treated with alkaline solution. Instead, the solution will only fill the existing microporous network and will be easily removed. Thus, if the alkaline solution is washed from the activated carbon prior to re-activation, the starting material will not necessarily change and further activation will only form micropores. For this reason, when the starting material is microporous carbon, the alkaline solution is preferably not removed before the activation step. Mesoporosity is estimated to develop around the Na deposit provided by doping with NaOH solution by a catalyzed activation process.

Experiment with other carbon precursors

NaOH treatment has also been shown to form mesoporous activated carbon made from pistachio nut shells, oak wood chips and bamboo. The carbon formed was generally less dense than coconut carbon. The results for pistachios (sample 4, 0.28 cm 3g ⁻¹ micropore volume, 0.35 cm 3g ⁻¹ mesopore volume) suggest that this is worth further investigation.

This treatment was also used on activated carbon. See the table in FIG. 7 for the conclusion.

Claims (16)

Pre-treat the starting material with alkaline solution,
Activating a pre-treated material,
A process for producing mesoporous carbon, wherein the starting material is a carbon precursor or a microporous carbon material. The method of claim 1 wherein the carbon precursor is a lignocellulosic material and the pre-treated material is not activated in the presence of alkali. The method of claim 2, wherein the pre-treated material is washed to remove the alkaline solution prior to the activation step. The method according to claim 2 or 3, wherein the lignocellulosic material is coconut shell, pistachio shell, wood chips or bamboo. 5. The method of claim 2, further comprising charring the pre-treated material prior to activation. 6. The method of claim 1 wherein the microporous carbon material is typically activated microporous carbon or synthetic microporous carbon. The method according to any one of claims 1 to 6, wherein the alkaline solution is an aqueous NaOH solution. The method of claim 7 wherein the NaOH solution has a concentration of about 0.1M to about 4M. 9. The process according to claim 1, further comprising treating the starting material or the pre-treated material with a salt solution, eg iron sulphate solution. The method of claim 1, wherein the pre-treatment step comprises washing the starting material with an alkaline solution having a temperature from ambient temperature to 60 ° C. 11. The method of claim 1, wherein the activation of the pre-treated material comprises treating with steam at a temperature of 700 ° C. 12. Porous carbon obtained or obtainable by the process according to any one of the preceding claims. 13. The porous carbon of claim 12, wherein the carbon has a ratio of micropores to mesopores of at least 1: 2. A filter element comprising the porous carbon of claim 12. The filter element according to claim 14, wherein the filter element is a filter for a smoking article. A smoking article comprising the porous carbon of claim 12.

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