Production of carbon molecular sieves by plasma treated

activated carbon fibersq

T. Orfanoudakia,b, G. Skodrasb,c, I. Doliosb, G.P. Sakellaropoulosa,b,*

aChemical Process Engineering Laboratory, Department of Chemical Engineering, Aristotle University of Thessaloniki,

P.O. Box 1520, Thessaloniki 540 06, GreecebLaboratory of Solid Fuels and Environment, Chemical Process Engineering Research Institute, Thessaloniki, Greece

cCentre for Solid Fuels Technology and Applications, Ptolemais, Greece

Received 6 November 2002; accepted 27 February 2003; available online 11 June 2003

Abstract

Carbon molecular sieves (CMS) are valuable materials for the separation and purification of gas mixtures. In this work, plasma deposition

was used aiming to the formation of pore constrictions, by narrowing the surface pore system of commercial activated carbon fibers (ACF).

For this reason propylene/nitrogen or ethylene/nitrogen discharges of 80 and 120 W were used. The molecular sieving properties of the

plasma treated ACF were evaluated by measuring the adsorption of CO2 and CH4. The CO2/CH4 selectivity was significantly improved and

depended on plasma treatment conditions (discharge gas and power). The optimum CO2/CH4 selectivity (26) was observed for C2H4/N2

plasma treated ACF at 80 W. Sample scanning electron microscopy (SEM) analysis after plasma treatment revealed an external film

formation and X-ray photoelectron spectroscopy (XPS) analysis showed the incorporation of nitrogen functional groups in the film, which

probably interact with CO2, thereby altering CO2/CH4 selectivity.

q 2003 Elsevier Ltd. All rights reserved.

Keywords: Plasma deposition; Activated carbon fibers; Carbon molecular sieves

1. Introduction

Carbon molecular sieves (CMS) are high added value

materials used in gas separation processes. Their unique

ability to separate gases based on the different size and shape

of molecules has been exploited in commercial applications

such as pressure swing adsorption (PSA) [1–5].

The most important feature of CMS is their narrow pore

size distribution accomplished either by controlled acti-

vation [6] or by employment of pore narrowing techniques

on an inherent pore structure. This latter technique has been

used on various carbon materials for the production of CMS

suitable for air separation. Hu et al. [6] used 3-methylpen-

tane as a source for carbon deposition on walnut shells,

chemically activated by KOH. The best oxygen–nitrogen

separation selectivity reported in this study was 9.2. Freitas

et al. [7] reported the modification of two activated carbons

of different texture by the pyrolysis of benzene in an attempt

to obtain CMS for O2/N2 separation. Their results showed

that this objective can be attained when the carbon precursor

has been activated only to a limited extent and when carbon

deposition is carried out in the proper kinetic regime. Vyas

et al. [8] obtained CMS by methane cracking on bituminous

coal and coconut shells. The O2/N2 uptake ratio of the best

CMS produced was 2.667. Cabrera et al. [9] described the

preparation of CMS for air separation by a two-step

hydrocarbon deposition with a single hydrocarbon. They

found that the concentration of the carbon containing

compound used in the first step should be larger than that of

the second step, so that the pore openings of the micropores

of the support narrowed gradually, avoiding pore plugging.

CMS for CO2/CH4 separation has also been produced by

the same method. Praseyto et al. [10] tailored the pore

structure of activated carbon by benzene deposition, and

improved the CO2/CH4 kinetic selectivity from 7 to 26.

However, cobalt catalyst was used to enhance benzene

0016-2361/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0016-2361(03)00172-8

Fuel 82 (2003) 2045–2049

www.fuelfirst.com

q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com

* Corresponding author. Address: Chemical Process Engineering

Laboratory, Department of Chemical Engineering, Aristotle University of

Thessaloniki, P.O. Box 1520, Thessaloniki 540 06, Greece. Tel.: þ30-

2310-996271; fax: þ30-2310-996168.

E-mail address: sakel@vergina.eng.auth.gr (G.P. Sakellaropoulos).

cracking. Kawabuchi et al. [11] modified the pore size of

several types of carbon adsorbents, suitable for CO2/CH2

separation, by chemical vapor deposition of benzene. They

showed that pyrolytic carbon should be deposited only on

the pore mouth in order to avoid pore plugging and to retain

the CO2 adsorption capacity.

Surface modification techniques, such as plasma, could

be an alternative for CMS production. The unique ability of

plasma to modify the surface of a material without changing

the bulk properties is attractive for the modification of a

carbon material to CMS by a one step treatment.

Hydrocarbon plasma has been widely used to improve

polymeric membrane efficiency [12,13]. However, studies

concerning plasma deposition of a thin film on carbon-

aceous material in order to improve its molecular sieve

properties are limited [14].

In this work commercial activated carbon fibers (ACF)

based on phenol resin were modified by N2/propylene or

N2/ethylene RF plasma discharges aiming to enhance the

molecular sieve properties of the material. Plasma treated

fibers were examined for possible surface modifications and

for their selectivity towards adsorption of CO2 and CH4

gases.

2. Experimental

The deposition apparatus used in this study consisted of a

quartz reactor, an RF generator with an impedance matching

network and a mechanical pump. The reactor was a quartz

cylinder 1 m long, 65 mm diameter placed coaxially

through a working coil. The coil was made of 9 turns of

1=4 in diameter copper tube. RF power was supplied from a

1 KW, 13.56 MHz generator. The system was also equipped

with a water circulation unit, necessary for cooling the

various RF plasma components.

About 35–40 mg of ACF (FR-10, Kuraray Chemical

Co.) were introduced in the reactor in the middle of the coil

and sealed therein by vacuum flanges. After system

evacuation, a gas mixture, of 20% hydrocarbon and 80%

nitrogen was introduced in the reactor, and plasma was

ignited. The sample was always treated for 15 min at an 80

or 120 W plasma power. Since no additional heating was

employed, the temperature of the ACF rose only by

inductive heating and energy transfer from the plasma.

The raw and treated ACF were characterized by N2

adsorption at 77 K, from which their BET surface area was

estimated using the BET multiple point equation. Raman

spectroscopy was employed to characterize the type of

carbon–carbon bonds before and after plasma treatment.

The 514 nm line spectra of an Arþ laser was used for

excitation. XPS analysis was also employed to characterize

the functional groups on the ACF surface before and after

plasma treatment. The ionizing radiation, Mg Ka, was

provided by a non-monochromatic X-ray source with

characteristic energy 1253.6 eV. The range of kinetic

energies of the analyzer was calibrated according to the

ASTM-E 902-88 standard method. Scanning electron

microscopy (SEM) was also used for surface examination

of the initial and the plasma treated ACF. The molecular

sieving properties of samples were evaluated by measuring

the adsorption of CO2 and CH4, volumetrically under

ambient conditions.

3. Results and discussion

3.1. BET surface area measurements

The N2 adsorption isotherm of untreated carbon fibers,

Fig. 1, is of type I, according to BDDT classification and

corresponds to a microporous material. Untreated samples

have a BET surface area of 650 m2/g. Plasma treated carbon

fibers gave negligible N2 adsorption. This indicates that the

film deposited on ACF surface, due to plasma treatment,

reduced significantly the surface pore entrance, possibly in

the range of molecular dimensions. Hence, N2 is probably

kinetically restricted to enter such narrow pores, and to

diffuse in the interior pore structure of the ACF. Such a

behavior is not unusual in CMS [6,15].

3.2. Raman spectra

The Raman spectrum of the untreated carbon fibers is

shown in Fig. 2(a). It consists of two peaks, characteristic of

a graphite structure [16]. The presence of the Raman peak at

1350 cm21, in addition to the main one at 1580 cm21,

suggests that small crystals are present [16]. In the Raman

spectra of the plasma treated fibers the two peaks were

replaced by a continuous line, Fig. 2(b). A similar result was

also obtained by Hayashi and his co-workers, although the

reason for such a behavior is still unclear [17].

Fig. 1. Nitrogen adsorption isotherm of commercial activated carbon fibers

(FR-10) at 77 K. No nitrogen adsorption for plasma treated ACFs.

T. Orfanoudaki et al. / Fuel 82 (2003) 2045–20492046

3.3. XPS analysis

A typical XPS spectrum of raw ACF, before plasma

treatment, is shown in Fig. 3. Similar spectra were obtained

with plasma treated ACF. The main C1s peak of all samples

can be deconvoluted to three components at around 284.6,

286 and 288 eV, Fig. 3, which probably correspond to C–C

(sp2 or sp3), C–OH or bridged –CyO–H–OyC–, and

COOH or COOR [18–20]. A fourth peak, at the highest

binding energy, is attributed to the filter used as substrate for

the XPS analysis. However, C–N bonds show quite similar

binding energies, 286–288 eV [19]; hence, the assignment of

peaks at 286 and 288 eV to C–O bonds, based on C1s spectra,

is ambiguous. For this reason, N1s spectra of all samples

were also obtained. Raw fibers before plasma treatment

showed no N1s spectra, therefore, the peaks observed from

286 to 288 eV in the C1s spectra, Fig. 3, can be assigned to

C–O groups as discussed earlier. The N1s XPS spectra of

plasma treated samples are shown in Figs. 4 and 5. All

N1s spectra after deconvolution show characteristic peaks

around 399 and 400 eV, which correspond to pyridine and

pyrole nitrogen groups, respectively, [21–24]. The presence

of NH2 groups cannot be excluded, whose characteristic

binding energy is 399.3 eN [18,24]. In addition, to these two

peaks, the N1 s spectrum of ethylene–nitrogen plasma

treated ACF shows one additional peak at 402 eV, Fig. 5.

The binding energy of this latter peak corresponds to

oxidized nitrogen forms [24]. These results suggest that

nitrogen present in the feed stream reacts and remains on the

sample surface after plasma treatment.

3.4. SEM surface examination

Fig. 6, shows the surface of the ACF, as observed by

SEM. In Fig. 6(a) the pore structure of ACF surface can be

identified, while Fig. 6(b) demonstrates that the fiber surface

is rough and contains defects. From Fig. 6(c), it is evident

that a thin film was deposited on the activated carbon

surface after propylene–nitrogen plasma treatment. The

film thickness along the fiber varies between 150 and

300 nm.

Fig. 2. Raman spectrum of commercial activated carbon fibers (FR-10) (a)

before plasma treatment (b) after plasma treatment.

Fig. 3. XPS C1s spectrum of raw (untreated) commercial activated carbon

fibers (FR-10).

Fig. 4. XPS N1s spectrum of commercial activated carbon fibers treated by

propylene–nitrogen plasma at 80 W.

Fig. 5. XPS N1s spectrum of commercial activated carbon fibers treated by

ethylene–nitrogen plasma at 80 W.

T. Orfanoudaki et al. / Fuel 82 (2003) 2045–2049 2047

3.5. Molecular sieving properties

The capacity of carbon fibers to act as molecular sieves

for CO2 and CH4 separation was studied before and after

plasma treatment. Fig. 7 shows the uptake curves of CO2

and CH4 for raw and plasma treated samples in a nitrogen–

propylene and nitrogen–ethylene discharge at 80 W. In all

cases the adsorption rate of both gases decreases by the

plasma treatment, especially during the first minute of

adsorption. This is probably due to film formation on

the fiber surface, which causes restrictions to gas diffusion.

Although the adsorption of both CO2 and CH4 is suppressed

by film formation, this is stronger in the case of CH4. The

linear CO2 molecules probably diffuse easier through the

film than the spherical CH4 molecules. Basic pyridine or

amino surface groups present on plasma treated ACF may

interact with acidic CO2 to enhance and improve its

adsorption capacity. Pyridinic or amino groups were

detected by N1 s XPS on the fiber surface after plasma

treatment, as discussed previously. Therefore, an acid–base

interaction between surface nitrogen groups and CO2 is very

probable. This leads to a significant enhancement of CO2

diffusion and adsorption on the plasma treated ACF as

compared to CH4. The difference in the adsorption rates of

the two gases leads to a significant improvement of ideal

selectivity (expressed as the ratio of the amount of CO2

adsorbed to that of CH4), as shown in Table 1. The CO2/CH4

ideal selectivity, measured at 60 s of adsorption, improves

from 2.4, for the raw ACF, to 26 for C2H4/N2 and 18.5 for

C3H6/N2 plasma treated ACF at 80 W. The selectivity

decrease, observed at longer adsorption times, 120 s, is in

agreement with the kinetic separation rules. The molecular

sieving ability of the deposited film, Table 1, varies with the

hydrocarbon used for the deposition process, due to the

different deposition rates for different hydrocarbon sourcesFig. 6. SEM characterisation of ACF (a,b) before plasma treatment (c) after

propylene–nitrogen plasma treatment.

Fig. 7. CO2 and CH4 uptake curves of commercial ACF treated by

ethylene–nitrogen and propylene–nitrogen plasma at 80 W. closed

symbols: CO2 open symbols: CH4 (VS FR-10, BA C2H4/N2, XW

C3H6/N2).

Table 1

Ideal selectivity (CO2/CH4) of raw and plasma treated ACF (FR-10) at 60

and 120 s of adsorption

Gas feed Plasma power

(W)

Ideal selectivity

(CO2/CH4)-60 s

Ideal selectivity

(CO2/CH4)-120 s

– – 2.4 2.4

C2H4/N2 80 26.0 20.0

C3H6/N2 80 18.5 12.5

C2H4/N2 120 19.5 10.0

C3H6/N2 120 15.6 9.2

T. Orfanoudaki et al. / Fuel 82 (2003) 2045–20492048

[25,26]. The rate of deposition depends on the energy input

per gram of hydrocarbon used, which in turn depends on the

molecular weight of the hydrocarbon employed (C2H4 and

C3H6). Therefore, for the same energy supplied to C2H4 and

C3H6 by plasma at a specific plasma power, e.g. 80 W, the

energy input per gram of C2H4 is higher than that of C3H6,

for the same flow rate. Thus, for the same treatment time

(15 min) the film thickness is not the same when different

hydrocarbon sources are used. Considering that film

structure is related to the film thickness [27], it is reasonable

to attribute the differences in CO2 and CH4 adsorption to the

different film thickness obtained with ethylene or propylene.

The capacity of ACF for CO2 and CH4 uptake increases

with increasing plasma power, as shown in Fig. 8. At 120 W

plasma power, CO2 gas uptake increases in the first 2 min by

15–20%, compared with that of the 80 W treated samples.

In order to explain this, one should consider not only the

film formation but also possible film ablation [25,26]. The

film ablation could be stronger in the case of 120 W plasma

treatment [28] resulting in lower net deposition rates (net

deposition rate ¼ deposition rate 2 ablation rate). Thus, for

samples modified at 120 W plasma power and at the same

treatment time, a lower net deposition rate has probably

resulted in the formation of a thinner film than that of 80 W

and, therefore, in lower restrictions to CO2 and CH4

adsorption. This is in good agreement with the reduced

CO2/CH4 selectivity for ACF treated at 120 W, compared to

those treated at 80 W.

4. Conclusions

Carbon films were deposited on ACF by propylene–

nitrogen and ethylene–nitrogen RF discharges. XPS

analysis revealed that nitrogen reacted and remained on

ACF surface during plasma treatment, but the nitrogen form

in the deposited material did not depend on hydrocarbon

used. Ideal selectivity of ACF for CO2 and CH4 gas

adsorption improved significantly after plasma treatment

due to a film formation on the ACF surface. Although

diffusion through the film of both CO2 and CH4 was

hindered, compared to raw ACF, CO2 transport and

adsorption was favored probably because of acid–base

interaction between CO2 and basic pyridine or amino groups

detected by XPS. Ideal selectivity differences at 80 and

120 W plasma power can be attributed to different film

thicknesses obtained in the two cases.

Acknowledgements

We thank the European Coal and Steel Community for

financial support of this work. We also thank the Physics

Division, School of Engineering of Aristotle University of

Thessaloniki for Raman measurements, and the Institute

of Chemical Engineering and High Temperature Processes

of Patra for XPS measurements.

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Fig. 8. CO2 and CH4 uptake curves of commercial activated carbon fibers

treated by ethylene–nitrogen and propylene–nitrogen plasma at 120 W.

closed symbols: CO2 open symbols: CH4 (VS FR-10, BA C2H4/N2, XW

C3H6/N2).

T. Orfanoudaki et al. / Fuel 82 (2003) 2045–2049 2049