Note: Descriptions are shown in the official language in which they were submitted.
CA 02821581 2013-06-13
WO 2012/098405 PCT/GB2012/050117
- 1 -
Method of Preparing Porous Carbon
Field of the Invention
The present invention relates to methods for preparing porous carbon material,
and
in particular to methods designed to produce porous carbon which exhibits
selectivity for low molecular weight aldehydes, such as formaldehyde, and for
hydrogen cyanide. The selective porous carbon is particularly useful for smoke
filtration in smoking articles, as the porous structure provides improved
adsorption
of these smoke vapour phase constituents which are generally poorly adsorbed
by
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, most commonly
plant-based materials such as coconut shell. Alternatively, synthetic carbons
are
used, such as resins prepared by polycondensation reactions.
Activated carbon materials have become widely used as versatile adsorbents
owing
to their large surface area, microporous structure, and high degree of surface
reactivity. In particular, these materials are especially effective in the
adsorption of
organic and inorganic pollutants due to the high capacity of organic molecules
to
bind to carbon.
Activated carbons are commonly produced from materials including coconut
shell,
wood powder, peat, bone, coal tar, resins and related polymers. Coconut shell
is
particularly attractive as a raw material for the production of activated
carbon
because it is cheap and readily available, and is also environmentally
sustainable.
CA 02821581 2013-06-13
WO 2012/098405
PCT/GB2012/050117
- 2 -
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 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
CA 02821581 2013-06-13
WO 2012/098405
PCT/GB2012/050117
- 3 -
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 Adsotption (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 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.
Whilst it is well established that activated carbon material exhibits
excellent general
filtration of unwanted substances from the vapour phase of tobacco smoke,
there
are some smoke vapour constituents that are poorly adsorbed and these include
low
molecular weight aldehydes (such as formaldehyde) and hydrogen cyanide (HCN).
The presence of free groups on the surface of the porous carbon material has
been
found to also affect the carbon's adsorption properties. It is known that the
presence of free nitrogen groups may enhance the selective adsorption of
constituents including low molecular weight aldehydes and HCN.
The present invention seeks to provide a method for preparing porous carbon
materials which have nitrogen-containing groups on the surface of the carbon,
to
enhance selective adsorption of low molecular weight aldehydes and HCN.
The present invention seeks to provide porous carbon materials having nitrogen-
containing groups on their surfaces.
CA 02821581 2013-06-13
WO 2012/098405 PCT/GB2012/050117
- 4 -
Summary of the Invention
Accordingly, in a first aspect of the invention there is provided a method of
preparing porous carbon with adsorbent properties for use in smoke filtration,
the
method comprising preparing the porous carbon in the presence of a nitrogen-
donating agent.
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.
Detailed Description of the Invention
The present invention relates to a method involving the addition of nitrogen-
containing groups to the surface of porous carbon by preparing the carbon in
the
presence of a nitrogen-donating agent. Preferably the porous surface structure
of
the carbon is formed in the presence of the nitrogen-donating agent, so that
the
nitrogen groups are present within the porous structure
In one 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-dihydroxyben2ol)
and
formaldehyde, which react with one another under alkaline conditions to form a
gel-
CA 02821581 2013-06-13
WO 2012/098405 PCT/GB2012/050117
- 5 -
like polycondensate. The polycondensation process will usually be conducted
under
aqueous conditions.
To produce the polycondensate, the reaction mixture may be warmed. Usually,
the
polycondensation reaction will be carried out at a temperature above room
temperature and preferably between 40 and 90 C. According to the present
invention, the polycondensation reaction is carried out in the presence of a
nitrogen-donating agent so that the resultant resin has an increased nitrogen
content
and increased presence of nitrogen-containing groups on its surface. In
preferred
embodiments, for example, an aqueous solution containing the nitrogen-donating
agent is added to an aqueous solution of resorcinol and formaldehyde under
vigorous stirring to yield a homogeneous solution. This solution is then
incubated
to provide the polycondensate. The incubation period may be between 5 minutes
and 24 hours.
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.
In one embodiment of the present invention, the polycondensate (which has
optionally been reduced in particle size) then undergoes pyrolysis. The
pyrolysis
may also be described as carbonisation.
According to an embodiment of the invention, the surface properties of the
resultant carbon are changed by treating the polycondensate before, during or
after
CA 02821581 2013-06-13
WO 2012/098405
PCT/GB2012/050117
- 6 -
pyrolysis with a nitrogen-donating agent, optionally as well as with other
more
conventional means, such as steam, air, COõ oxygen or a mixture of gases,
which
may be diluted with nitrogen or another inert gas. It is particularly
preferred to use
a mixture of nitrogen and steam.
The activation stage preferably takes place in a gaseous atmosphere comprising
nitrogen, water and/or carbon dioxide. In other words, the dried gels used in
the
present invention may be non-activated or, in some embodiments, activated, for
example steam activated or activated with carbon dioxide. Activation is
preferred in
order to provide an improved pore structure.
Where the starting material is a carbon precursor, the carbon precursor is
preferably
pyrolysed before then being activated. Conventional methods of pyrolysis may
be
used. The nitrogen-donating agent may be added to the material before or after
the
pyrolysis step, but it is preferably added before any pyrolysis step and
before the
activation step.
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. 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. 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 N,
flowing at a rate of 10-200 cm3/min. In one embodiment, the pyrolysis step is
carried out at a temperature of no more than 600 C, more preferably at a
temperature of no more than 550 C, or of about 500 C. Pyrolysis at these
temperatures is preferred as they provide a high nitrogen content and good
surface
CA 02821581 2013-06-13
WO 2012/098405
PCT/GB2012/050117
- 7 -
area. Pyrolysis at higher temperatures may lead to a reduction in nitrogen
content
and can result in a lower surface area due to structural shrinkage.
After pyrolysis, the carbon is cooled and the carbon surface is preferably
deactivated, for example by exposure to a humid N, flow. This deactivation is
necessary because of the high risk of exothermic 02 adsorption causing red-
heat.
Subsequently, to increase the surface area, the pyrolysed carbon is activated.
This
may be done by either physical or chemical means, and conventional activation
techniques can be used. Preferably the material is activated by physical
means, and
most preferably the material is activated using nitrogen and steam, or
alternatively,
CO,.
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.
Activation of the material using nitrogen and steam may be performed at a
temperature of between 600 C and 1100 C, and preferably activation is
performed
at a temperature of between 700 C and 900 C. Most preferably, the material is
activated at about 850 C. The activation process is preferably carried out for
between 30 minutes and 4 hours. Most preferably, the material is activated for
1
hour. 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, activation of the material may be performed at a
temperature
of between 400 C and 1000 C, and preferably activation is performed at a
temperature of between 600 C and 800 C. The activation process is preferably
carried out for between 30 minutes and 4 hours.
CA 02821581 2013-06-13
WO 2012/098405
PCT/GB2012/050117
- 8 -
Whilst the precise mechanism by which the nitrogen-containing groups are
transferred to the surface of the carbon is not known, it is believed that the
donating agent is able to add nitrogen-containing groups to the carbon's
surface and
these groups act as preferential sites for aldehyde and HCN adsorption. This
hypothesis is supported by the experimental data provided and discussed below,
and
in particular by the XPS (X-ray photoelectron spectroscopy) results.
It has also been demonstrated that the porous carbon produced according to the
methods of the present invention exhibit significantly increased selectivity
for
formaldehyde and HCN.
According to the present invention, the nitrogen-donating agent may be an
amino
acid or amino acid derivative, an amine, including an aromatic amine, or an
imidazole, an imidazole derivative or a compound including the pyridine-like
nitrogen of imidazole, such as 1-methylimidazole. In a preferred embodiment,
the
nitrogen-donating agent is lysine, L-hydroxylysine, L-arginine, L-histidine, L-
aspartic acid, 1-methylimida2ole (MIM), ethylenediamine (EDA), propylamine,
dimethyamine, 2-propylamine, trimethylamine or aniline. Particularly preferred
agents are lysine, MIM and EDA.
The nitrogen-donating agent is preferably added to the constituent reagents of
the
polycondensate in the form of an aqueous solution. This solution may be added
to
the mixture of phenolic compound and aldehyde prior to polymerisation. The
amount of nitrogen-donating agent used as a molar ratio to the amount of the
phenolic compound may be between 30:1 and 3:1 (phenolic compound: nitrogen
donating agent). The molar ratio of phenolic compound to water is preferably
in the
range of between 1:3 and 1:50 (phenolic compound: water).
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.
CA 02821581 2013-06-13
WO 2012/098405
PCT/GB2012/050117
- 9 -
The BET surface area of the activated carbon materials produced by the method
is
still important for the adsorption of smoke constituents other than low
molecular
weight aldehydes and HCN. 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 material with surface areas of between 600 m2/g and 1200 m2/g
are
most preferred.
The porous carbon materials of the invention preferably have a pore volume (as
estimated by nitrogen adsorption) of at least 0.3 cm9g, and desirably at least
0.5 cm9g. Carbon materials with pore volumes of at least 0.5 cm9g are
particularly
useful as an adsorbent for tobacco smoke. Carbon materials according to the
invention with pore volumes significantly higher than 1 cm9g 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 activated carbon produced by the methods of the present invention may be
provided in monolithic or particulate form. Particles will preferably have a
particle
size in the range of between 10 p.m and 1500 ,m. Preferably the mean particle
size
is between 100 ,m and 1000 p.m, and more preferably between 150 p.m and 800
p.m.
Most preferably, the particles of activated carbon material have a mean size
of
between 250 ,m and 750 p.m. The smaller the particles are, the larger is the
combined surface area, however, if the size of the particles used is too
small, the
particles can interfere with manufacturing processes, especially high speed
processes as used to manufacture cigarette filters.
Experiments
Lysine catalyzed carbon monolith
CA 02821581 2013-06-13
WO 2012/098405 PCT/GB2012/050117
- 10 -
Two carbon samples, 08-12-05 and 09-05-04 (in the form of carbon monoliths)
were prepared by polycondensation of resorcinol and formaldehyde in the
presence
of lysine.
Sample 08-12-05
The mass percentage of (resorcinol+formaldehyde) and lysine in solution was
approximately 30 weight A and molar ratio of resorcinol:lysine was 6.6 with
thermal
curing (1 day at 50 C and 1 day at 90 C). The obtained bulk polymer was dried
at
50 C for 1 day and pyrolysed at 700 C for 2 hours under nitrogen atmosphere.
The resultant carbon had a BET surface area of 460 m2/g, a total pore volume
of
0.23 cm3/g, and a micropore volume of 0.22 cm3/g. The surface nitrogen was
detected using IR but the surface groups were not identified.
Sample 09-05-04
The process used to synthesise sample 08-12-05 was repeated, except that the
surface area and porosity were slightly increased, so that the sample had a
BET
surface area of 580 m2/g, a total pore volume of 0.27 cm3/g and a micropore
volume of 0.24 cm3/g.
Performance of both of these carbon samples in a cigarette was measured by
placing
60 mg into the cavity filter of a reference cigarette. The filter construction
was a
triple filter in the form of cellulose acetate ¨ carbon granules ¨ cellulose
acetate.
Filters having an empty cavity, or an equal weight of sorbite (coal based
carbon)
were used as controls. Sorbite was chosen as a control carbon because it has
similar
physical properties to sample 08-12-05 (460 m2/g surface area, 0.26 cm3/g
total
pore volume and 0.25 cm3/g micropore volume).
Once prepared, cigarettes were aged at 22 C and 60% Relative Humidity for
approximately 3 weeks prior to smoking.
All cigarettes were smoked under ISO conditions, i.e. a 35m1 volume puff of 2
second duration was taken every minute. The smoke chemistry results are shown
CA 02821581 2013-06-13
WO 2012/098405
PCT/GB2012/050117
- 11 -
below in Table 1, which provides the percentage reductions of various smoke
constituents.
Table 1
Carbon Sorbite 08-12-05 09-05-04
Acetaldehyde 10 14 16
Acetone 25 9 10
Acrolein 22 16 26
Butyraldehyde 30 13 20
Crotonaldehyde 42 24 48
Formaldehyde 16 32 67
Methyl ethyl ketone 36 6 15
Propionaldehyde 21 9 12
HCN 14 47 51
1,3-butadiene 12 12 11
Acrylonitrile 22 28 8
Benzene 19 5 39
Isoprene 24 10 20
Toluene 13 1 28
A 67% reduction in formaldehyde (as observed using sample 09-05-04) by carbon
is
outstanding. The reduction in HCN is also significantly improved by using both
of
the samples produced according to the present invention in comparison to
conventional activated carbon.
Further samples of materials produced by polycondensation of resorcinol and
formaldehyde in the presence of lysine were pyrolysed at different
temperatures
(400, 500, 600, 700 and 800 C). The samples were not subsequently activated.
The
nitrogen content was measured by element analysis and XPS measurements (Axis
Ultra, Kratos Analytical) and the results are set out in Table 2 below.
CA 02821581 2013-06-13
WO 2012/098405
PCT/GB2012/050117
- 12 -
Table 2 - Element analysis of the carbon monoliths
Sample Pyrolysis N 0
temperature ( C) (wt.%) (wt.%) (wt.%) (wt.%)
1 400 1.71 24.67 3.07 70.55
2 500 1.92 10.15 1.68 86.25
3 600 1.84 9.11 1.66 87.40
4 700 1.43 4.03 0.61 93.93
800 1.28 3.44 0.34 94.89
XPS measurements (Axis Ultra, Kratos Analytical) on N ls signal are enabled to
reveal the changes occurring in nitrogen species present on carbon surfaces
after
5 pyrolysis. The fitting of the N Is peaks gave the following binding
energies: 399.8
0.3 eV (amide and/or pyrrolic nitrogen), 401.4 0.3 eV (quaternary nitrogen),
and
402.8 eV (pyridine-N-oxide). Upon pyrolysis, the nitrogen species within the
material change significantly with the temperature applied. The nitrogen
content of
samples are accordingly of 1.3, 1.9, 1.3, 0.8 and 0.5 wt%.
The XPS results clearly show that nitrogen has been incorporated into the
carbon
structure. They also show that the pyrolysis temperature is significant and
that the
nitrogen content begins to drop significantly when the carbon is pyrolysed at
a
temperature above 600 C. The optimal temperature for nitrogen incorporation
appears to be around 500 C.
Alternative nitrogen-donating agents
Three carbon samples were evaluated against a conventional reference cigarette
for
filter additive studies. The samples were based on a resorcinol-formaldehyde
resin
with increased nitrogen content via synthesis with either 1-methylimida2ole
(MIM)
or ethylenediamine (EDA). This work was conducted to establish whether other
nitrogen-containing molecules could be used to increase the activated carbon
nitrogen content which would then also show similar reductions in formaldehyde
and HCN.
CA 02821581 2013-06-13
WO 2012/098405
PCT/GB2012/050117
- 13 -
Table 3 sets out the properties of the carbon samples that were prepared. RF
polymer means resorcinol formaldehyde polymer, and the catalyst used was
either 1-
methylimida2ole (MIM) or ethylenediamine (EDA). The polycondensation of
resorcinol with formaldehyde in the presence of MIM was conducted at 50 C for
4
hours and an ambient pressure drying followed by incubation at 800 C. For the
polycondensation in the presence of EDA, thermal curing was carried out at 90
C
for 4 hours and, after drying, the sample was thermally treated at 800 C under
N2.
Table 3
Sample Carbon Type Surface area Micropore
Total pore
code (m2/g)
volume volume
(cm3/g) (cm3/g)
09-12-01 RF polymer + MIM 670 0.27 0.37
(R:MIM=26:1)
09-12-02 RF polymer + MIM 580 0.23 0.33
(R:MIM=13:1)
09-12-03 RF polymer + EDA 520 0.22 0.25
The micropore volumes of the samples were lower than that of coconut carbon
currently typically used in cigarette filters (which is generally 0.4-0.5
cm3/g). This
was expected to have an effect on the adsorption characteristics of the
samples
compared to the reference cigarette.
Infra red analysis showed that a number of different nitrogen-containing
groups
were present on the surfaces of the samples, including NH and CN groups.
60 mg of the carbon samples was weighed into the cavity filter of a reference
cigarette. The filter construction was a triple filter in the form of
cellulose acetate ¨
carbon granules ¨ cellulose acetate. A filter having an empty cavity of
similar
dimensions was used as a control.
Once prepared, the cigarettes were aged at 22 C and 60% Relative Humidity for
approximately 3 weeks prior to smoking.
CA 02821581 2013-06-13
WO 2012/098405
PCT/GB2012/050117
- 14 -
All cigarettes were smoked under ISO conditions, i.e. a 35m1 volume puff of 2
second duration was taken every minute. The smoke chemistry results are shown
below in Table 4.
Table 4 - Smoke analysis results
Smoke Yields ( /0 Reductions)
Carbon Empty 09-12-01 09-12-02 09-12-
03
Puff No 6.8 6.9 6.7 6.6
NFDPM (mg/cig) 11.2 9.7 8.4 7.1
Nicotine 0.93 0.83 0.72 0.59
Water 2.5 1.8 1.5 0.8
CO 11 11.2 10.3 10.6
Acetaldehyde
550.1 299.7 (46) 223 (59) 470 (15)
( g/cig)
Acetone 285.1 217.6 (24) 116.9 (59) 267.5
(6)
Acrolein 64.3 14.7 (77) 4.7 (93) 48.9
(24)
Butyraldehyde 37.4 29.3 (22) 19.3 (48) 31.6
(16)
Crotonaldehyde 21.4 3.8 (82) 1.5 (93) 12.4
(42)
Formaldehyde 34.6 13.5 (61) 11.7 (66) 14.5
(58)
Methyl ethyl ketone 68.3 52.5 (23) 26.9 (61) 62.3
(9)
Propionaldehyde 48.3 35.6 (26) 19.6 (59) 45.6
(6)
HCN 122.1 15.7 (87) 16 (87) 45 (63)
1,3-butadiene 72.8 47.7 (34) 35.5 (51) 57.6
(21)
Acrylonitrile 15.2 3.5 (77) 2.3 (85) 8.3
(45)
Benzene 53.5 40.9 (24) 27.3 (49) 41.4
(23)
Isoprene 644 588 (9) 428 (34) 515
(20)
Toluene 73.4 51.5 (30) 28.6 (61) 54.4
(26)
All of the samples show much higher reductions in formaldehyde and HCN than is
usually observed with activated carbon (compare to the reductions observed
using
Sorbite provided in Table 1).
CA 02821581 2013-06-13
WO 2012/098405
PCT/GB2012/050117
- 15 -
The granule strength of the samples was weak and it is likely that some 'dust'
was
generated causing an increase in pressure drop and thus resulting in the lower
yields
of NFDPM and nicotine observed (i.e. the smoke particulate phase).
Differences in pressure drop will not affect vapour phase analyte yields and,
although some caution should be exercised, percentage reductions are shown
without normalising to tar.
It is clear that all of the agents used to incorporate nitrogen into the
activated
carbon structure have resulted in enhanced adsorption towards formaldehyde and
HCN. The greater the nitrogen addition, the greater the reductions, as shown
using
MIM, where despite a lower surface area and pore volume, sample 09-12-02
(having
R:MIM of 13:1) outperformed sample 09-12-01 (in which R:MIM was 26:1).
Sample 09-12-03 gave small reductions in the majority of smoke analytes
measured
(as expected using a physisorption mechanism with a surface area of only about
500m2/g). However enhanced selectivity (via a chemisorption mechanism) was
shown towards formaldehyde and HCN.
From the results of the experiments described above, it appears that the
precise
nitrogen source is not critical. The experimental data show that the use of
activated
carbons produced using lysine, MIM, or EDA all provide excellent reductions in
smoke formaldehyde and HCN.
