KR20160044051A - Method of preparing porous carbon - Google Patents

Abstract

The present invention relates to a process for preparing porous carbon having adsorptive properties for use in smoke filtration, comprising pretreatment of the starting material with an alkali solution, removal of the alkali solution from the pre-treated material and subsequent activation of the pre- Wherein the starting material is a carbon precursor or a microporous carbon material. Preferably, the alkali solution is removed prior to the activation step. The present invention also provides porous carbon having micropores and mesopores, and uses thereof.

Description

METHOD OF PREPARING POROUS CARBON < RTI ID = 0.0 >

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

Filtration is used to reduce the vapor phase constituents of the particulate and / or cigarette smoke inhaled during smoking. It is important that this is achieved without significantly degrading the levels of other components such as certain organoleptic components and thereby without degrading the quality or taste of the product.

The smoking article filter may typically comprise a porous carbon material (dispersed throughout the filter material or dispersed in the cavities of the filter) to adsorb certain smoke constituents by physical adsorption. Such porous carbon materials can be prepared from a variety of different organic materials in the form of carbonates, most commonly plant-based materials, such as coconut shells.

Activated carbon materials are widely used as multipurpose sorbents due to 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 bound to carbon.

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

The performance and suitability of the activated carbon material as adsorbent in different environments is determined by the various physical properties of the material including the particle size and shape, 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.

Generally, the larger the surface area of the porous material, the larger the adsorption capacity of the material. However, as the surface area of the material increases, the density and structural integrity deteriorate. 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 size of the pores is closer to the size of the target molecule, the target molecules enter the pores and are less likely to adsorb to the material Loses. This is especially so when the material to be filtered has a high flow rate with respect to the activated carbon material, as in the case of smoking articles.

The precise method used to make the porous carbon material greatly influences its properties. Accordingly, it is possible to form carbon particles having a wide range of geometry, size, size distribution, pore size, pore volume, pore size distribution and surface area, each of which influences its effectiveness as a sorbent material. The attrition rate is also an important variable, which 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 the increase in carbon activation time results in an increase in the number of micropores and surface area , Does not form any practical change in pore size or distribution.

According to the nomenclature used by those skilled in the art, pores in adsorbent materials having a diameter of less than 2 nm are referred to as "micropores " and pores with diameters between 2 nm and 50 nm are referred to as" mesopores " . Pores are referred to as "macropores" when their diameter exceeds 50 nm. Pores with diameters larger than 500 nm do not largely contribute to the adsorbability of the porous material.

The distribution of pore size in the porous carbon material affects the adsorption characteristics and the activated carbon material rich in micropores and mesopores is superior to the carbon containing only the micropores and the excellent filtration of the unwanted material from the vapor phase cigarette smoke .

Previous attempts to introduce mesopores into activated carbon have been found to be difficult to fabricate reliably and reproducibly in mesoporous activated carbon, as it has stuck to the fallopian tubes. In order to optimize the adsorption characteristics of carbon, conventionally it is not possible to adjust the mesopores and micropore volumes of activated carbon reliably. Synthetic mesoporous carbon was prepared, but these are relatively expensive.

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

Accordingly, a first aspect of the present invention relates to a process for the preparation of a compound of formula I, which comprises pre-treating the starting material with an alkali solution and then activating the pre-treated material, wherein the starting material is a carbon precursor or a microporous carbon material, There is provided a method for producing a porous carbon having mesopores having adsorption properties suitable for use. If the starting material is a carbon precursor, the alkali solution is preferably removed prior to the activation step, and the pre-treated material is preferably carbonized prior to activation.

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

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

According to a fourth aspect of the present invention, there is provided a smoking article comprising porous carbon according to the second aspect of the present invention.

Figure 1a shows the N ₂ adsorption isotherms for various activated carbon samples.
Figure 1b shows a bar graph showing the adsorption properties of the activated carbon sample of Figure 1a.
Figure 2 shows a table showing the physical properties of the activated carbon samples of Figures 1a and 1b.
Figure 3 shows a table showing the physical properties of various activated carbon samples.
Figure 4 shows the N ₂ adsorption isotherm of the activated carbon sample of Figure 3;
Figure 5 shows a bar graph showing the adsorption properties of the activated carbon sample of Figure 3;
Figure 6 shows the N ₂ adsorption isotherms for various activated carbon samples.
Figure 7 shows a bar graph showing the adsorption properties of various activated carbon samples.
Figure 8 shows the effect of NaOH molarity on the textural properties of various activated carbon samples.

The present invention relates to a method comprising pre-treating the starting material with an alkaline solution prior to the activation step. Activation of this pre-treated or doped material promotes 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 present invention may be any carbon precursor material. Such carbon precursor materials 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 alkali solution is preferably removed from the pre-treated material prior to the activation step, preferably by a repeated washing step.

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

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

No carbonation step is required when the starting material is a microporous carbon material.

Although the precise nature of the alkali solution effect is not entirely understood, it is believed that the alkali may be involved in dissolving some of the other components of the cell structure of the lignocellulosic carbon precursor, such as lignin, hemicellulose and / or coconut shell It 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 the alkali concentration over the range described. Accordingly, it is possible to use an alkaline solution having a concentration as low as 0.1M to 4M or higher in the pre-treatment step. As indicated by the graph shown in FIG. 8, when using a solution with a concentration of 1 M, the meso pore appears to be the maximum, suggesting that a relatively weak alkaline solution can be used. Figure 8 shows the effect of NaOH molarity on the structural properties of uncleaned carbon activated in steam at 700 ° C for 4 hours. These graphs show the molar concentration of NaOH used to pre-treat the coconut shell before and after carbonization, and this curve shows the effect on the surface area / porosity of the activated product.

In a preferred embodiment of the present invention, the alkali solution is an aqueous solution of sodium hydroxide (NaOH). 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 from 0.7M to about 1M, 2M, 3M, 4M, or greater. Again, a relatively weak solution, for example a 1 M solution, may be preferred.

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

In another embodiment of the process of the present invention, a pre-treatment with an alkali solution is followed by treatment with a solution of a salt capable of catalyzing carbon gasification, such as iron and copper. Thus, for example, the carbon precursor can be treated using ferrous sulfate solution. Since the metal is precipitated when the metal salt solution is mixed with an alkaline solution such as NaOH, the treatment with the salt solution is preferably a separate stepwise step for the pre-treatment step. Treatment with a salt solution can be performed before or after the alkaline solution pre-treatment step.

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

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

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

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

For example, a charring step may include heating the pre-treated carbon to a temperature of at least 500 ° C and holding the carbon at this temperature for several hours. In one embodiment, the carbonization step is pre-included that under N ₂ flow to a processing carbon at a rate of 100 ㎤ / min, heated to 10 ℃ / min up to 600 ℃.

After carbonization, the carbon is cooled and the carbon surface is preferably deactivated by exposure to, for example, a wet N ₂ flow. This deactivation is essential because of the high risk of exothermic O ₂ adsorption which causes red-heat. Thereafter, the charred material is activated.

Activation in the method of the invention can be by either physical or chemical means, and conventional activation techniques can be used. Preferably, the material is activated by physical means, most preferably, the material 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. The 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 results in loss of material.

The activation of the material using nitrogen and steam can be carried out at a temperature 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 < RTI ID = 0.0 > 700 C < / RTI > for 4 hours.

In another embodiment, the material is activated by reaction with carbon dioxide. In this case, the activation of the material may be carried out at a temperature of 700 ° C to 1100 ° C, and 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 for 2 to 4 hours at about < RTI ID = 0.0 > 800 C. < / RTI >

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

The BET surface area of the activated carbon material formed by the process 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 Lt; 2 > / g. A typical value for the BET surface area of the carbon material formed by the process of the present invention is 850 m < 2 > / g. Porous carbon materials having a BET surface area of from 700 m 2 / g to 1300 m 2 / g are preferred.

The relative volume of micropores, mesopores and macropores in the activated carbon material 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, since the theoretical bases for this estimation are different, the values obtained by the two methods can not 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 the preferred carbon material of the present invention, the pore volume (as estimated by nitrogen adsorption) of at least 20% but preferably less than 95% relates to mesopore. Typical minimum values for the volume of mesopore 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 such volumes are 95%, 90%, or 85%. Preferably, the mesopore volume of the carbon material of the present invention is in the range of 55% to 70% of the total pore volume. The ratio of mesopores to micropores is about 1: 1 to about 3: 1, and most preferably about 2: 1.

The porous carbon material produced by the process of the present invention preferably has a pore volume (estimated by nitrogen adsorption) of at least 0.4 cm3 / g, and preferably at least 0.5 cm3 / g. Carbon materials having a pore volume of at least 0.5 cm3 / g are particularly useful as adsorbents for tobacco fumes. The carbon material according to the present invention having a pore volume significantly greater than 2 cm3 / g has a low density and thus is less easy to operate in a cigarette production apparatus. Such a carbon material may for this reason be less desirable for use in a cigarette or smoke filter.

The activation step is preferably controlled so that the resulting product contains micropores of the desired volume. In one embodiment of the invention, the product of the present invention has a ratio of micropores to mesopores of at least 1: 2, which is desirable 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 [mu] m to 1000 [mu] m. Preferably, the average particle size is from 50 μm to 500 μm, more preferably from 100 μm to 400 μm. Most preferably, the particles of activated carbon material have an average size of from 150 [mu] m to 250 [mu] m.

Experiments using a cellulose carbon precursor as a starting material

Solution treatment of raw shell

Most inorganic compounds investigated as solutes have been chosen because they are used in the usual chemical activation of cellulose carbon precursors and in conventional chemical activation these compounds are dry-blended with precursors, but they are much larger Concentration (at least 1: 1). The acceptable reaction pathway is by dehydration / decomposition, which tends to produce more mesoporous activated carbon than that formed by the physical activation of the char (using steam or CO ₂ ). However, the inorganic components generally have to be washed at the end of the activation process.

A coconut shell (100 g) at 50 ℃ in NaOH (4M, 2M, 1M) , Na 2 C0 3, KOH, K 2 C0 3, and KHC0 ₃ 300 ㎤ solution (usually 1 to 2M) in the mixture was stirred for 4 hours . Thereafter, the skin was washed and dried at 100 < 0 > C overnight until the liquid became neutral.

Charring

The coconut shell was heated to 600 ° C in flowing N ₂ (100 cm 3 / min) at 10 ° C / min. Thereafter, the coconut shell was kept at 600 DEG C for 4 hours. Upon cooling, the charred shell was exposed to a wet N ₂ flow (bubbler) to deactivate the carbon surface (essential due to the high risk of exothermic O ₂ adsorption causing glow). 100 g of shell / impregnated coconut shell yielded 28 to 29 g of tea.

Using krypton adsorption, a surface area measurement of one of the cars derived from the NaOH-treated shell gave a value of 0.65 m 2 / g. This indicates that NaOH alone treatment does not produce significant surface area in the car.

Activation

The standard activation method used included 5 liters of water, 20 cm 3 / min N ₂ flow, water introduced at 700 캜 for 4 hours using a syringe pump (20 cm 3 at 5 cm 3 / hr).

* Textural characterization of the activated product (textural characterization)

The relationship between NaOH treatment and final mesoporosity in steam-activated carbon is very large.

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). When the NaOH-treated skin is washed, a dark reddish brown liquid supports this proposal. Even more dilute concentrations have not been studied, but these effects did not appear to be dependent on the molarity of the alkali solution above 1M. However, the values in Table 2 indicate that only 6 to 7% of the material was removed by treatment with an alkaline solution.

Table 1 - Analysis of coconut shells

Table 2 - Mass loss after treatment

Though the shell was washed thoroughly after NaOH treatment, the presence of a small amount of residual Na ⁺ can not be excluded, which may have catalytic or other effects in the steam activation process.

Sample for smoke test

For smoke testing, scale-up (5 times) activation was performed to sieving to provide an accurate particle size, followed by the formation of fully activated samples. 100 g of raw material shell yields approximately 28 to 29 g difference, which also yields 16 to 20 g of activated carbon. Control samples providing the same microporosity but not providing mesopores were also prepared from untreated shells. The sample data and isotherms are shown in the graph of FIG. 1A. The adsorption properties of the sample are shown in the graph of FIG. The properties of the samples are described in the table of FIG.

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

The smoke analysis data showed that the carbon sample with increased meso pore volume (samples 1, 2 and 5) exhibited greatly improved adsorption and showed a significantly better reduction in all tested toxicants than the control (sample 6) . All the carbon used had a similar micropore volume, while the control had a very low meso pore volume of only 0.04 cm3 / g, whereas in the method according to the invention, all the pre-treated other samples had very large mesopore volumes (See the table of FIG. 2).

Additional samples were provided for testing (Samples 20, 21 and 22). These were prepared from a steam-activated, NaOH-doped shell at 700 ° C. One of these samples (Sample 22) was washed and re-activated at CO ₂ to increase surface area and microporosity. Also, for comparison, purely microporous carbon (Sample 24) was prepared. All these samples have a much larger surface area, micro-and meso porosity than the first sample prepared for the test (see data in Figures la, lb and 2). These sample details are provided in the table of FIG. 3, and the isotherm is shown in FIG.

The smoke analysis data for this re-activated sample is shown in FIG. These results indicate that the decrease in toxicity was further increased compared to the first set of samples. In addition, the washed sample (Sample 22) exhibited greatly improved adsorption characteristics relative to other samples for all toxic materials tested.

Post-treatment washing

Despite attempts to completely remove NaOH from the pre-carbonization shell by washing, some NaOH remains in the final activated carbon, indicating alkalinity. Experiments have been conducted in an attempt to remove residual NaOH by washing, through which an additional increase in surface area and / or porosity is achieved.

In most of the experiments carried out, washing of activated carbon in water or dilute acid removes alkalinity and provides increased surface area and / or porosity, as can be seen by the data in Tables 3 and 4 below. The mass loss using water or acid for post-activation washing was not significantly different.

Table 3 - Weight lost for washing after activation of NaOH-doped coconut shell

Table 4 - Effect of cleaning after activation on structural features

The data in Tables 3 and 4 indicate that approximately 20% water soluble ash (ash) may be present in the final activated carbon. At least some of which may be due to an alkaline Na compound, such as an oxide (which may react with wash water to form NaOH). These materials are likely to be located within the pore network and thus may cause them to block the pores. When such ash is dissolved and removed by a post-activation washing step, the entire pore network is exposed to gaseous adsorption, and the measured surface area and porosity are increased.

From this point, in spite of attempts to remove all the caustic from the coconut shell by intensive washing, some Na is retained, which is incorporated into the precursor matrix and promotes activation, Can be concluded to be crucial to the development of.

Solution treatment of raw material shell

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

Experiments using microporous carbon as a starting material

Although treatment of the coconut shell with NaOH solution has been shown to provide mesoporous activated carbon, this procedure extends to commercially available activated carbon sold by Jacobi as Aquasorb (R). It is believed that the presence of sodium ions adsorbed on the carbon pores of the activated starting material promotes further activation and selectively widenes these pores.

Activated carbon (named JAC) as-received was stirred with 2M NaOH solution at 50 < 0 > C, filtered and then dried overnight at 100 < 0 > C. Subsequently, the sample was re-activated at 700 캜, in steam or at 800 캜 in CO ₂ . The large difference in mesophase volume between the steam activated sample and the CO ₂ activated sample described in Table 5 below 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 this effect is significant, but in the case of CO ₂ re-activation, almost all micropores are widened. In this regard, these products are very different from activated carbon from NaOH-doped shells.

Unlike the one observed in the raw coconut shell, when the activated carbon was treated with an alkaline solution, the material was found not to be chemically dissolved. Instead, the solution will only fill the existing micro pore network and will be easily removed. Thus, when the alkali solution is washed from 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 a microporous carbon, the alkali solution is preferably not removed prior to the activation step. The mesoporosity is believed to develop around the provided Na deposits by doping using a NaOH solution, by a catalyzed activation process.

Experiments on 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 the pistachio (Sample 4, 0.28 cm ^{3 -1} min pore volume, 0.35 cm ³ g ^-1 meso pore volume) suggest that this is worth further investigation.

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

Claims (9)

The starting material is pre-treated with an alkaline solution,
≪ / RTI > activating the pre-treated material,
Wherein the starting material is a carbon precursor which is a lignocellulosic material and the pre-treated material is not activated in the presence of an alkali. The method of claim 1, wherein the pre-treated material is washed to remove the alkaline solution prior to the activation step. The method according to any one of claims 1 to 3, wherein the lignocellulosic material is a coconut shell, a pistachio shell, a wood chip or bamboo. 4. The method according to any one of claims 1 to 3, further comprising charring the pre-treated material prior to activation. 5. The process according to any one of claims 1 to 4, wherein the alkali solution is a NaOH aqueous solution. 6. The method of claim 5, wherein the aqueous NaOH solution has a concentration of from about 0.1M to about 4M. 7. The method according to any one of claims 1 to 6, further comprising treating the starting material or the pre-treated material with a salt solution, for example a solution of iron sulphate. 8. The method according to any one of claims 1 to 7, wherein the pre-treatment step comprises washing the starting material with an alkaline solution having a temperature of from ambient to 60 < 0 > C. 9. The process according to any one of claims 1 to 8, wherein the activation of the pre-treated material comprises treating with steam at a temperature of 700 < 0 > C.

Images