Atmospheric Environment 37 (2003) 5433–5437

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Technical note

Confined catalytic oxidation of volatile organic compoundsby transition metal containing zeolites and ionizer

Teresa S.C. Lawb, Christopher Chaoa,*, George Y.W. Chanb, Anthony K.Y. Lawb

aDepartment of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kongb Acron International Technology Ltd., 4B The Annex Building, Entrepreneurship Centre, The Hong Kong University of Science and

Technology, Clear Water Bay, Kowloon, Hong Kong

Received 30 March 2003; accepted 4 September 2003

Abstract

Ion exchange of zeolite, NaX, at different concentrations of cobalt (II) solutions can play an important role in

gaseous contaminant destruction. The performance of the resulting zeolites on acetone removal when working together

with an ionizer was evaluated. It was found that the cobalt containing zeolites worked better than the original NaX in

our experiments. Catalytic oxidation mechanism of the volatile organic compounds (VOCs) inside the pores of zeolite

was proposed. It is believed that the octahedral coordinated cobalt (II) complexes shift to tetrahedral coordination

upon entering into the pores of NaX, which then work with the reactive oxygen species released from the ionizer and

catalyze the oxidation reactions of the adsorbed VOCs. The results have shown potential applications in odor removal

and indoor air quality control.

r 2003 Elsevier Ltd. All rights reserved.

Keywords: Catalytic oxidation; Zeolite; Transition metals; Volatile Organic Compounds

1. Introduction

Due to ubiquity in the environment and risk to human

health, volatile organic compounds (VOCs) have re-

ceived great attention in the indoor environment. The

removal of VOCs on activated carbon has been used

extensively (Navarri et al., 2002). However, the hetero-

geneously porous structure of activated carbon would

lead to competitive adsorption between water vapor and

organic compounds, which would eventually reduce its

removal effectiveness (Alcaniz-Monge et al., 2002).

Moreover, heat generated from the adsorption process

would cause further polymerization of the structure.

Consequently, the degradation of the activated carbon

structure further decreases the VOCs adsorption perfor-

mance (Daguerre et al., 2000).

ing author. Tel.: +852-2358-7210; fax: +852-

ess: meyhchao@ust.hk (C. Chao).

e front matter r 2003 Elsevier Ltd. All rights reserve

mosenv.2003.09.016

Zeolite can be a good candidate in place of activated

carbon in VOCs adsorption. A preliminary work has

been conducted by the same research group recently to

compare the performance of removal of VOCs and

formaldehyde using both synthetic zeolite, NaP1 and

activated carbon (Law and Chao, 2001). It was noted

that the VOC removal efficiency of using synthetic

zeolite was better than activated carbon by 15–20%

under laboratory testing environment and a much faster

removal rate was also seen under a similar pressure

drop. More recently, an innovative way of using zeolite

filter together with oxidant-generating devices for air-

borne pollutant removal was investigated. In the

combined system, the zeolite secures the pollutants in

its pore. This allows the reactive oxygen species (ROS)

from the oxidant-generating device to interact with the

pollutants and catalytically oxidize them into simpler

components, such as CO2 and H2O, which are then

released from the zeolite (Law et al., 2003).

On the other hand, transition metal oxides have been

receiving much attention scientifically in the last two

d.

ARTICLE IN PRESST.S.C. Law et al. / Atmospheric Environment 37 (2003) 5433–54375434

decades for their applications as heterogenesis oxidation

catalysts (Hagrman et al., 1999). However, the applica-

tions of them for airborne pollutants removal are limited

as the oxidation reactions are usually restricted to

happen on the surfaces of the oxides only (Zaera, 2002).

Prompted by such considerations, attempts to incor-

porate transition metals species with variable oxidation

states into the zeolite structures and to explore their

performance on pollutants removal when working

together with an oxidant-generating device become the

main aims of this research. The results in this paper offer

a prospect of building a combined system of the zeolite

complementary materials with oxidant-generating de-

vices for more efficient air cleaning and VOCs removal,

particularly in the indoor environment.

2. Experimental procedure

2.1. Materials

Cobalt (II) acetate tetrahydrate (Aldrich Chemical

Company) was used as received without further

purification. Zeolite NaX obtained commercially was

used in our experiments. The NaX was identified by

powdered X-ray diffraction (PXRD) (Philip, Model:

PW1830) and also by energy dispersive analysis using X-

ray spectrometry (EDAX) (JEOL. Model: JSM 6300).

The databases in the joint committee on powder

diffraction standards (JCPDS) were used to verify the

crystal structure and phase purity. The chemical formula

of NaX is Na86(Al86Si106O384) � 264H2O. Pore volume of

it was determined by using a Brunauer Emmett Teller

(BET) Surface Area Analyzer (Coulter, Model: SA3100)

and nitrogen gas was used as adsorbent. The surface

area of the zeolite was measured to be 510m2/g. The

NaX was pretreated at 400�C in an oven for removing

the organic contaminants prior to the experiments for

Centrifufan

Ionizer

Chemical

VOC sensor

Fig. 1. Schematic diagram of

3 h. The crystal structure of NaX remained unchanged

after the calcination process as verified from the PXRD

pattern.

2.2. Preparation of the cobalt–zeolite X

Co(II) solution (0.004 M) was prepared by dissolving

1 g of cobalt (II) acetate tetrahydrate (m.w.=249) into a

1000 cm3 of distilled water. Ion exchange between NaX

and Co(II) solution was allowed to take place by putting

100 g of NaX (equivalent to 0.0055 mol) into the Co(II)

solution. It was gently stirred for 48 h. The ion-

exchanged zeolite was then obtained by filtration and

washed with distilled water. The resulting zeolite, named

CoX-1 was pretreated at 400�C in an oven for 3 h in

order to remove the moisture and other contaminants

prior to the experiments.

Ion exchange between 0.008M of Co(II) solution and

100 g of NaX was also performed. The resulting zeolite,

named CoX-2, was prepared by repeating the above

procedures with 2 g of cobalt (II) acetate tetrahydrate.

2.3. Experimental setup

A schematic diagram of the experimental setup is

shown in Fig. 1. The air-cleaning device was placed in an

enclosed acrylic chamber with a volume of approxi-

mately 1 m3. The temperature and RH inside the

chamber were approximately 22�C and 55%, respec-

tively. The zeolite filter had a cylindrical shape with a

diameter and thickness of 10 and 1 cm, respectively,

holding 100 g of zeolites. The zeolites used were in

pelletized form with a diameter of 3–5 mm. The zeolite

filter was placed at the upstream position of the airflow

induced by a centrifugal fan. This arrangement allowed

the polluted air to be drawn through the zeolite filter

without bypass. The airflow rate and pressure drop

across both the upstream and downstream positions of

Zeolite filter

gal

Direction of air flow

Aircleaningdevice

1m3 Chamber

the experimental setup.

ARTICLE IN PRESST.S.C. Law et al. / Atmospheric Environment 37 (2003) 5433–5437 5435

the air-cleaning device were measured with a sensor and

pitot tube (TSI VelociCalcs Plus, Model: 8386). The

pressure drop across the filter was 38Pa and the airflow

rate was 2.59� 10�3 m3/s. The residence time of the

airflow inside the zeolite was about 0.0033 s. An ionizer

(Ionair, model: Stream 5002) operating at 230V/50 Hz

and 10 W was placed at the upstream position of the

zeolite filter for oxidant generation. A calibrated VOC

sensor (ppbRAE, Model: PGM-7240) using the photo-

ionization detection method was employed for the VOC

level monitoring. As the experiment was aimed to

compare the VOC concentrations at different treatment

and conditions qualitatively, the PID sensor which gives

economic, fast response and easy operation was used. It

was a dual channel detector equipped with a 10.6 eV UV

lamp with a detection limit up to one part per billion

(ppb). Isobutylene was used as the reference gas.

2.4. Performance test for volatile organic compounds

removal in a small chamber

A 0.3ml volume of acetone was loaded into a 5 ml

beaker which was placed inside the enclosed arcylic

chamber. Acetone is typically used as a solvent for

paints and adhesives, and it contains both the alkyl and

ketone functional groups. For these reasons, it was

chosen as a representative chemical of VOCs in this

experiment. The initial concentration of the VOC was

thus controlled to about 8000 ppb. The experiment was

conducted under five different conditions:

(1)

Control: The zeolite filter was removed and the

oxidant generator was switched off. The decay of

the VOCs concentration may be due to the leakage

and adsorption of VOCs by the acrylic chamber.

(2)

NaX treatment: The zeolite filter was filled with

NaX sample while the ionizer was switch off.

(3)

NaX treatment combined with an oxidant treat-

ment: The zeolite filter was filled with NaX while the

ionizer was switched on.

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

1 3 5 7 9 11 13 15

Time (m

Con

cent

ratio

n of

Ace

tone

(pp

b)

Fig. 2. Acetone concentration vs. tim

(4)

17

inut

e a

CoX-1 treatment: The zeolite filter was filled with

CoX-1 sample while the ionizer was switched on.

(5)

CoX-2 treatment: The zeolite filter was filled with

CoX-2 sample while the ionizer was switched on.

During each set of experiment, the amount of zeolite

and organic solvent used were kept constant and the

levels of VOC were monitored. In order to verify the

results of the experiment, the above procedures were

repeated three times.

3. Result

The color of the Co2+ solution changed from deep

blue to colorless after ion-exchange occurred with the

NaX for 48 h. The ion-exchanged zeolite, which was

originally pale yellow in color, became pale blue after

the ion-exchange process. It was also observed that the

color of CoX-2 was more intense when compared to that

of CoX-1. EDAX analysis revealed that the elemental

compositions of both CoX-1 and CoX-2 contained

cobalt.

Fig. 2 shows the result when acetone was used as the

representative VOC. Comparing the system when zeolite

was employed to the control experiment, the ability of

zeolite to capture pollutants was obviously observed.

When the ionizer was combined with the zeolite, the

final concentrations were roughly the same as when the

NaX was used alone. However, the combination of the

zeolite with the ionizer produced a faster rate of

pollutant depletion.

The results also revealed that a much faster rate of

pollutant removal rate was observed when the zeolite

ion exchanged with cobalt was used. It was observed

that CoX-2 gave the fastest pollutant removal rate

among all systems. In all cases, the final concentrations

were roughly the same as when the zeolite was used

alone. Similar observations were found when the

experiment was repeated for verification.

19 21 23 25 27 29 31

e)

No TreatmentTreatment with NaXTreatment with NaX and IonizerTreatment with CoX-1 and IonizerTreatment with CoX-2 and Ionizer

t five different operations.

ARTICLE IN PRESST.S.C. Law et al. / Atmospheric Environment 37 (2003) 5433–54375436

4. Discussion

In aqueous solution of Co(II), six water molecules

acting as ligands to coordinate with the Co2+ ions. The

pink color of the solution was due to the formation of

the hexa-ligand complex, Co(H2O)62+. The size of each

Co(H2O)62+ is about 0.14 nm3 and is in the shape of

octahedron. It is small enough to get into the pore of

NaX, whose diameter is about 1.3 nm.

The cation exchange capacity (CEC) in NaX is

4.73meq/g. There are 86mol of Na+ ions in each mole

of NaX. As 0.0055mol of NaX was used, there was

0.473mol of ion-exchangeable cations. For the reason of

charge balance, the ion-exchange process involves

exchanging each two singly charged atoms (Na+) in

the zeolite by one doubly charged (Co(II)) atom

from the solution. After ion exchanging the NaX with

0.004 and 0.008M of Co(II) solutions, the resulting

chemical formulae of the CoX-1 and CoX-2 became

Na85Co(Al86Si106O384) � 264H2O and Na84Co2(Al86

Si106O384) � 264H2O, respectively.

There was no direct evidence showing whether the

removal effect of VOC is due to the catalytic oxidation

process or the adsorption effect by the zeolite, at least it

could not be verified directly from our current experi-

mental setting. However, the immediate adsorption of

the oxidant into the zeolite when the air-cleaning device

was operated and the slower VOC removal rate when

oxidant was used alone implied that there were reactions

happening between the pollutants and ozone inside the

pores of the zeolite (Law et al., 2003). These reactions

caused a sharp decrease of the VOC compared to the

case where either zeolite or ozone was used alone.

The catalytic functions of zeolite with transition metal

complexes for organic compound transformations have

been reviewed by other researchers (De Vos et al., 2002).

It is not necessary for the Co(II) species to be present in

the form of hexa-ligand complex after entering into the

pores of the zeolite. In our experiment, the ion-

exchanged zeolite became blue in color, which implied

that the complexes, octahedral coordinated Co(H2O)62+,

no longer existed inside the pore of the zeolite. Instead,

Co(II) adopted a tetrahedral coordination, which is blue

in color.

In the presence of excessive ROS, which was

generated from the ionizer, the oxidation conversion of

changing Co(II) to Co(III) would move forward

(Collins, 1994). In such case, the ROS would be

consumed. Owing to this reason, the ionizer should be

designed in such a way that the number of ROSs

generated needs to be more than the amount of Co(II)

species incorporated inside the zeolite filter for an

effective utilization of the Co(II) species. In such case,

the tetrahedral Co(II) with d7 configuration will be

excited and change to a less stable tetrahedral d6

configuration. The ligand field stabilization energies

(LFSE) of tetrahedral d6 and d7 are 0.6DT and 1.2DT ;respectively (Shriver et al., 1989).

The existing Co(III) species are, indeed, favorable to

the adsorption of VOCs. The Co(III) has a particular

affinity to electron rich atoms, such as nitrogen donors

and oxygen donors. In some cases, the Co–C bond may

even be established (Davies, 1993). Owing to the charge

imbalance due to the zeolite framework provided by

Co(III), the Co(III) will be reduced back to Co(II). It is

done by transferring one electron from the nearby VOC

molecule to the metal center. Subsequently, the VOCs

molecules are oxidized with subtraction of the hydrogen

atoms and insertion of the oxygen atoms. Indeed, this is

the major pathway observed for most catalytic oxida-

tions in gas phase and is known as Mars–van Krevelen

mechanism.

In fact, VOC oxidation plants employing transition

metals as catalysts were found very common (Strots

et al., 2003). More innovatively, the cobalt ions in the

current system enhance the adsorption of VOCs. ROS

generated from the ionizer, which should be equipped at

the upstream position of a zeolite filter, will be adsorbed

into the pores of zeolite together with the VOC

molecules. The Co(II) inside the zeolite pores will be

oxidized to the less stable Co(III), which will then be

reduced back to Co(II) by capturing an electron from

the nearby VOC molecules. The VOC is then being

oxidized and broken down into simpler form inside the

confined nano-pore of zeolites. Upon complete oxida-

tion reaction, the non-harmful H2O and CO2 will be

produced. These small molecules will then be too small

to be retained in the nano-pore and will be dismissed.

The nano-pores inside the zeolite will then become

vacant and available for the up-coming reactions.

The mechanism implies that larger amount of Co(II)

species existing inside the zeolite framework will offer

more catalytic sites and thus enhances the rates of

oxidation of VOCs. As a result, highest pollution

removal rate was observed in the system of CoX-2,

where double amount of Co(II) was dosed.

5. Conclusion

The system worked best when the ionizer was used

together with the zeolite containing transition metal

ions. The existing of cobalt species inside the pores of

zeolite promoted not only the rate of adsorption of

pollutant but also the rates of oxidation of VOCs. As the

reaction was carried out inside the pores until comple-

tion, the intermediate species such as carbonyls and

carboxylic acid would possibly only exist inside the pore

of zeolite temporarily. A much more complicated

experimental setup and instrumentation such as electron

spin resonance (ESR) will be required for analyzing

these intermediate species. Moreover, future work

ARTICLE IN PRESST.S.C. Law et al. / Atmospheric Environment 37 (2003) 5433–5437 5437

should also be geared towards system fine-tuning as well

as more detailed understanding of the mass-diffusion

mechanism of the pollutant into the zeolite pores.

Acknowledgements

This research was supported financially by Acron

International Technology Ltd. via Project Number

AITL01/02.EG01.

References

Alcaniz-Monge, J., Linares-Solano, A., Rand, B., 2002.

Mechanism of adsorption of water in carbon micropores

as revealed by a study of activated carbon fibers. Journal of

Physical Chemistry B 106, 3209–3216.

Collins, T.J., 1994. Designing ligands for oxidizing complexes.

Accounts of Chemical Research 27, 279–285.

Daguerre, E., Guillot, A., Py, X., 2000. Zeolite for fine

chemicals production. Carbon 38, 59–64.

Davies, M.B., 1993. Cobalt. Coordination Chemistry Reviews

124, 107–181.

De Vos, D.E., Dams, M., Sels, B.F., Jacobs, P.A., 2002.

Ordered mesoporous and microporous molecular sieves

functionalized with transition metal complexes as catalysts

for selective organic transformations. Chemical Reviews

102, 3615–3640.

Hagrman, P.J., Hagrman, D., Zubieta, J., 1999. Organic–

inorganic hybrid materials: from simple coordination

polymers to organodiamine-templated molybdenum oxides.

Angewandte Chemie-International Edition 38, 2639–2684.

Law, T.S.C., Chao, C.Y.H., 2001. The use of synthetic zeolite in

indoor air quality control. Symposium on Indoor Air

Quality and Energy Efficient Technology, Hong Kong,

pp. 51–59.

Law, T.S.C., Chao, C.Y.H., Chan, G.Y.W., Law, A.K.Y.,

2003. The use of zeolite and oxidant generating devices in

air cleaning. Journal of Indoor and Built Environment, in

press.

Navarri, P., Marchal, D., Ginestet, A., 2002. Activated carbon

fibre materials for VOC removal. Filtration and Separation

38, 34–40.

Shriver, D.F., Atkins, P.W., Langford, C.H., 1989. Inorganic

Chemistry. ELBS, Oxford University Press, Great Britain,

p. 208.

Strots, V.O., Bunimovich, G.A., Matro, Y.S., 2003. Base

metal catalysts for VOC oxidation. Pollution Engineering

34, 14–17.

Zaera, F., 2002. Outstanding mechanistic questions in hetero-

geneous catalysis. Journal of Physical Chemistry B 106,

4043–4052.