Document Sample
              J.M.Valente Nabais1), P.J.M. Carrott1), M.M.L. Ribeiro Carrott1)
                             J.A. Menéndez2), A. Dominguez2)
    Centro de Química de Évora & Departamento de Química, Universidade de Évora,
                   Rua Romão Ramalho 59, 7000-671 Évora, Portugal
         Instituto Nacional del Carbon, CSIC, Apartado 73, 33080 Oviedo, Spain

Corresponding author e-mail address: jvn@uevora.pt


Carbon molecular sieves (CMS) have important applications in the chemical and
petrochemical industry in separation and catalytic processes based on the different
adsorption of components from gaseous or liquid phases. The work to be presented
here focuses on two commercially significant separations: CO2/CH4 and O2/N2. The first
is important for the recovery of CH4 in fermentation processes [1] and in purification of
natural gas [2]. The second is used worldwide for the production of nitrogen and oxygen
from air by pressure swing adsorption (PSA) [3,4].

Activated carbon fibres (ACF) are a comparatively modern form of porous carbon
material with a number of significant advantages over the more traditional powder or
granular forms. These include high surface area and adsorption capacity, as well as very
high rates of adsorption from the gas or liquid phase. The micropore size is in general
uniform and can often be controlled by adjusting the conditions of preparation of the ACF
or by post-preparation modifications which include CVD of suitable organic precursors,
such as benzene [5] and propylene [6], thermal treatments [7] and impregnation with
organic additives with subsequent carbonisation [8]. These characteristics and the fact
that the micropores open directly to the exterior make ACF excellent alternatives in PSA
systems because the time needed for pressurisation/depressurisation cycles can be

In this work we report a novel method of making CMS which involves the control of
porosity of activated carbon fibres using thermal treatment in a microwave furnace under
an inert atmosphere. This methodology leads to the production of CMS that are
indicated for the separations CO2/CH4 and/or O2/N2.

The use of a microwave furnace has several advantages over conventional heating
devices related to the different manner of sample heating. In conventional heating the
heat source is located outside the carbon bed which is heated by conduction and/or
convection. With microwave heating, on the other hand, the microwaves supply energy
to the carbon particles themselves according to the mechanism of the Maxwell-Wagner
effect [9].
In a microwave furnace the heating is very fast (temperatures of 900ºC can be reached
in about 1 minute [10]) and the treatment time is reduced when compared with the
treatment time in conventional furnaces. This leads to important savings in time, energy
and gas consumption.

For carbon materials there are relatively few publications that describe the use of
microwaves for producing [11,12] and regenerating [13,14] activated carbons. Surface
chemistry modification of carbon materials by means of microwave heating was also
studied in previous works [10,15] as was the production of molecular sieving effects

The separations using CMS are based on kinetics and not on equilibrium. For instance
the equilibrium adsorption capacities for O2 and N2 can be nearly identical. These
different adsorption kinetics allow the adsorption of the gas that has the faster diffusion
rate, in general the gas that also has the smaller molecular size, which can then be
separated from other gases with slow diffusion rates. Although simple in principle, in
practice it is a relatively difficult separation when compared to an equilibrium separation.
Regarding the kinetic diameter of oxygen and nitrogen, 0.346 and 0.364nm [18], it is
easy to see that this separation is quite an impressive feat.

The diffusion model more appropriate for the ACF used is the empirical
phenomenological model for mass transfer in gas adsorption because, as can be seen
in Fig. 1, the materials are not homogeneous and the particles are not spherical [19]. In
this case the adsorption rate is controlled by the diffusivity into the micropores rather
than by the superficial resistance to mass transfer. This model, also called the linear
driving force (LDF) mass transfer kinetic model, assumes that the surface concentration
of gas is constant and that the diffusion is controlled by the concentration gradient
through the particle. The kinetics of diffusion can then be expressed by the empirical
diffusion equation:

                                       mt/minf = ktn                            (1)

where mt is the gas uptake at time t, minf the gas uptake at equilibrium, t the time, n the
diffusional exponent and k the constant.

The diffusional exponent can be evaluated from a plot of ln(mt/minf) vs ln t. When n=0.5
the diffusion mechanism obeys the Fick Equation. In our case n is always different from
0.5 which is coherent with our choice for the diffusional model.
The gas uptake into CMS may be considered as a pseudo-first order mass transfer
between the gas phase and the ACF. Then the rates of gas uptake can be compared in
terms of the pseudo-first order rate constant (k) with the gas uptake following equation

                                       mt/minf = 1-ekt                           (2)
The plot ln(1-mt/minf) vs t should be a straight line with gradient equal to k that is
essentially comparable to Dπ2/a2 in the Fickian diffusion model for spherical particles
where mt/minf>0.5.

        a)                                              b)

                     Figure 1: SEM image of a) F2-0 and b) F1-37.

The precursors used for the production of ACF were three acrylic textile fibres (designed
F1, F2 and F1N) provided by Fisipe (Barreiro, Portugal). According to the manufacturer
all of the fibres had been polymerised from acrylonitrile (~90w%) and vinyl acetate
(~10w%) monomers. For the production of the ACF about 12g of fibre and a horizontal
tubular furnace were used. The fibres were carbonised at 800ºC for 1 hour under a
constant N2 flow of 85cm3min-1 (samples F1N-0, F1N-0 and F2-0) and activated at
900ºC in a CO2 flow of 85cm3min-1 for 2 hours, to 37% burn-off (sample F1-37). Details
of the process are given elsewhere [20].

Microwave treatments
Sub-samples (ca. 1-2g) sized to less than 1mm of activated carbon fibres were placed in
a quartz reactor, which in turn was placed inside a multimode resonant microwave
cavity. Microwave treatments consisted in subjecting the samples to microwave action
for 15min. The time of treatment is indicated in the sample designation (XXXm15,
means 15 minutes of microwave treatment). An inert atmosphere was maintained during
treatment and cool-down intervals by passing a N2 flow of 100cm3min-1 through the
sample bed. The input power of the microwave equipment was set at 1000W and the
microwave frequency used was 2450MHz. The temperature of the carbon bed during
microwave treatment was measured using an infrared optical pyrometer. Details of the
microwave device as well as temperature measurements are given elsewhere [21,22].

Gas uptake experiments
The adsorption of O2, N2, CO2 and CH4 at 25ºC was determined using a classical glass
adsorption apparatus. After outgassing the ACF at 300ºC, the probe molecule at an
initial pressure of 955mbar was introduced and the uptake measured as a function of
Results and Discussion

Typical uptake curves obtained are shown in Fig.2 were the plots of volume adsorbed (in
volume NPT obtained by the ideal gas equation) versus contact time are represented.
From this data we can evaluate the ability of the materials for performing the separations
wanted. This evaluation is done by the determination of the volume adsorbed, at a
specific contact time, and the kinetic selectivity obtained by the ratio between the
volumes of gas adsorbed. These data are shown in tables 1 and 2. We chose 100s for
the contact time because the PSA apparatus normally uses similar times.

The adsorption kinetics analysis shows that for all samples the uptake of O2 and CO2 is
faster than the adsorption of N2 and CH4, respectively. The uptake at 100s contact time
is greater for O2 and CO2 than for N2 and CH4. This behaviour is typical for CMS but
opposite to other molecular sieves like zeolites 5A or 13X that have more affinity for N2
than for O2, with the faster adsorption of the first gas [23].

The results show that the microwave treatments lead to important changes in the values
of gas adsorbed and selectivities. It can be seen in tables 1 and 2 that after microwave
treatment the adsorption of the larger molecules N2 and CH4 was, in almost every case,
reduced to zero. On other hand, the adsorption of the smaller molecules CO2 and O2
was less pronounced leading to significant increase of the selectivities.

For samples F1-0 and F1-37 the treatment caused a decrease in the mean width of the
micropores [22] and consequently a decrease of the adsorption capacity. For sample F1-
0m15 this results in null adsorption of CH4, N2 and O2. Despite the reduction of 80% in
the CO2 adsorption the fast adsorption obtained makes this sample a good CMS for the
separation of CO2. The sample F1-37 improved the CMS characteristics after the
treatment with the selectivity values for the separations CO2/CH4 and O2/N2 changing
from 2.3 to 13.9 and from 0.9 to 13.9, respectively, as can be seen in the tables.
For samples F1N-0 and F2-0 the treatment caused an increase in the mean micropore
diameter [22] and also an increase in the adsorption capacity. These samples have
much amorphous carbon which was in part burnt-out during the microwave treatment
with the opening of the micropores. We can say that the micropores still have some
constrictions after the treatment because the uptake curves show a slower
approximation to equilibrium adsorption, which leads to smaller values of k when
compared with F1-0m15 (tables 1 and 2). The sample F1N-0m15 is very interesting
because the null adsorption of methane and nitrogen lead to infinite selectivities for the
separations CO2/CH4 and O2/N2.
                        a)                                           b)

                       c)                                            d)




                   0         200   400            600   800   1000

                       e)                                            f)

      Figure 2: Uptake curves at 25ºC of CO2(!), CH4(O), O2(!) and N2(!).a) F1-0,
                 b)F1-0m15, c)F1-37, d)F1-37m15, e)F1N-0, f)F1N-0m15.
       Table 1. Adsorption kinetics (k/s-1) and uptakes (v/cm3g-1) of O2 and N2 at 25ºC.
Sample           V(O2)             V(N2)               V(O2)/V(N2)     k(O2)              k(N2)
F1-0             2.8               2.4                 1.2             0.020              0.009
F1-0m15          0                 0                   id              0                  0
F2-0             0.1               0                   inf             0.039              0
F2-0m15          0.3               0                   inf             0.035              0
F1N-0            0                 0                   id              0                  0
F1N-0m15         1.5               0                   inf             0.008              0
F1-37            2.2               2.4                 0.9             0.097              0.096
F1-37m15         1.3               0.1                 13.9            0.010              0
Note: Uptakes after 100s of contact time, nd=not determined, id=indeterminate, inf=infinite
   Table 2. Adsorption kinetics (k/s-1) and uptakes (v/cm3g-1) of CO2 and CH4 at 25ºC.
Sample           V(CO2)           V(CH4)         V(CO2)/V(CH4)         k(CO2)            k(CH4)
F1-0             24.9             4.4            5.6                   0.015             0.012
F1-0m15          3.6              0              inf                   0.025             0
F2-0             0.7              0              inf                   0.004             0
F2-0m15          1.4              0              inf                   0.004             0
F1N-0            2.8              0              inf                   0.003             0
F1N-0m15         7.5              0              inf                   0.008             0
F1-37            28.7             12.4           2.3                   0.025             0.030
F1-37m15         5.7              0.5            13.9                  0.011             0.011

Published work using the same type of experimental method that we have used
indicates that for O2/N2 separation a selectivity of about 10 with O2 uptake of about
4cm3g-1 after 100s contact time can be considered reasonable for a laboratory sample
[24,25]. Verma has reported higher selectivities, of about 20, but only at the expense of
much lower O2 uptake [26], while Villar-Rodil et al. have reported higher O2 uptakes but
with slightly lower selectivities [27]. Other authors have reported results for commercial
CMS which are inferior both in relation to selectivity and to O2 uptake [24,25]. The results
obtained by us, and shown in tables 1 and 2, therefore appear to be quite reasonable in
comparison with other published work. Of the microwave treated samples, the best
results were obtained with F1-37 and F1-0, which both had higher selectivities, although
the O2 uptakes were lower than ideal.
For CO2/CH4 separation various authors have reported selectivities greater than 40 with
CO2 uptakes as high as 45cm3g-1 [24,27]. The best result in table 2 was obtained with
F1N-0 and is comparable to the results reported by Vyas et al. [28]. However, it is clear
that a more detailed study is still necessary in order to achieve a higher CO2 uptake
necessary to improve the productivity of the process.

The application of the phenomenological model for adsorption kinetics to the gas uptake
(eq. 2) is shown in Fig. 3. As expected the plots show good linear regions. For samples
F1-0, F1-37m15, F1N-0m15 and F1-37 (in the last sample only for CO2 and CH4 uptake)
it is possible to observe two linear regions for contact times less than and greater than
100s. In these cases we only considered the first region because, as explained before,
the PSA instruments use short cycle times and it is therefore more useful to evaluate the
k value for these conditions.

The pseudo-first order rate constant, k, for the uptake of oxygen (table 1) is in general
comparable with the values reported in previous work with similar materials. In some
cases the values found for our samples are 10 times higher. Hu and Vansant [29]
reported for the best sample studied for O2/N2 separation (selectivity=9.2) a k value of
0.0008s-1. In comparison, for sample F1-37m15 (table 1) we report here selectivity=13.9
and k=0.008s-1.

In the samples that after the microwave treatment have increased the gas uptake as a
consequence of micropore enlarging, F2-0 and F1N-0, the rate constant also increased
because it is easier for the probe molecules to enter the micropores. On the other hand,
for samples F1-0 and F1-37 where the treatment caused a decrease in micropore mean
width the rate constant values also decreased.

The rate constant for the CO2 uptake is also comparable to the values already reported
in previous works. Reid and Thomas reported values between 0.004 and 0.012s-1 for
commercial CMS [30] and Prasetyo and Do reported values in the range 0.0013-0.0030
in modified CMS [31].

The comparison with reported values is sometimes not totally meaningful because the
linear ranges chosen for the application of the model are not the same. In several cases
the authors don't refer the linear range used, which can lead to misleading comparisons.


The results presented here suggest that microwave heating could be a useful alternative
to more commonly used methods for introducing molecular sieve properties into
activated carbon adsorbents. One of the advantages of the microwave method is that,
unlike conventional heating, where there can be temperature gradients from the outside
towards the centre of the carbon bed, in the microwave furnace the carbon bed is
uniformly heated throughout. It should therefore be possible to scale up the process
while still obtaining highly homogeneous products. Furthermore, the heating time is
      considerably shorter which would result in significant energy and process gas cost
      The properties of samples F1N-0m15 and F1-37m15 (reasonably good O2 adsorption
      capacity, very good selectivity and interesting adsorption rate) altogether make these
      samples the best ones for O2/N2 separation.
                 0                                                 a)                                0                                                                      b)

                -1                                                                                  -1

                -2                                                                                  -2

                -3                                                                                  -3

                -4                                                                                  -4

                -5                                                                                  -5

                       0       200    400            600    800          1000                                       0       200         400             600         800           1000
                                      time /s                                                                                            time/s

                 0                                                      c)
                                                                                                               0                                                                 d)






                       0       200    400            600    800          1000
                                                                                                                        0     200        400             600         800              1000

                                                                        e)                                      0                                                                f)






                           0    200    400            600    800             1000                                       0         200         400             600         800           1000

                                            time/s                                                                                             time/s

         Figure 3: Representation by phenomenological model for adsorption kinetics for the
       uptake of CO2(!), CH4(O), O2(!) and N2(!).a)F1-0, b)F1-0m15, c)F1-37, d)F1-37m15,
                                       e)F1N-0, f)F1N-0m15.
  Despite the relatively low CO2 uptake (3.6cm3g-1), the sample F1-0m15 can be a good
  choice for the CO2 separation from CH4, O2 and N2 because the only probe molecule
  taken up was CO2 with a relatively good adsorption rate (0.025s-1). For the CO2/CH4
  separation we must refer also to samples F1N-0m15 and F1-37m15 that can be very
  promising alternatives.


  The authors are grateful to the Fundação para a Ciência e a Tecnologia (Portugal), the
  European Regional Development Fund (FEDER) and the program PRAXIS XXI for
  financial support (project nº PRAXIS/3/3.1/MMA/1781/95).


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