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© Kimia ITS – HKI Jatim 1

Akta Kimindo Vol. 1 No. 1 Oktober 2005: 1 – 10AKTA KIMIA

INDONESIA

Nanomaterials as catalysts in the production of fine chemicals*)

Halimaton Hamdan1)

Ibnu Sina Institute for Fundamental Science Studies

Universiti Teknologi Malaysia

Skudai, Johor, Malaysia

ABSTRACT

Zeolit and mesomorphous materials are porous materials with windows, channels and cavity

architectures of nanometer dimensions. Large pore zeolites, mesomorphous MCM-41, MCM-48 and silica

aerogels have been synthesized from rice husk. The growing interest in these novel systems is due to thebulk behaviour of these nanostructured materials which can be designed and tailored by controlling their

cluster nanostructures which lead to greatly improved performance. Changes in the molecular properties of

materials at the nanoscale level greatly enhance their physical and chemical properties. The zeolite lattice

may also be used as a host for encapsulated complexes or metallic clusters allowing the control of

nuclearity of these active species. MCM-41, for examples and enzymatic species to from molecular wires,

zeozymes and hybrid catalyst.

ABSTRAK

Zeolit dan mesomorfosa adalah bahan berpori dengan arsitektur jendela, terowongan dan rongga

yang berdimensi nanometer. Zeolit berpori besar, mesomorfosa MCM-41, MCM-48 dan silika aerogel telah

disintesis dari sekam padi. Pertumbuhan yang menarik dari sintesis baru ini adalah struktur bahan yang

berukuran nano dapat didesain dan dibuat melalui pengontrolan kluster struktur nano yang memberikan

kinerja yang sangat luas. Perubahan dalam sifat molekuler bahan pada tingkat skala nano meningkatkansifat fisik dan sifat kimianya. Kisi-kisi zeolit dapat juga digunakan sebagai host (sarang) untuk

membungkus komplek atau kluster logam yang memungkinkan untuk mengontrol keterpusatan spesies

aktif ini, MCM-41 misalnya telah digunakan untuk sarang polimer, komplek logam dan spesies enzim untuk

membentuk kawat, zeoenzim dan katalis hibrida.

INTRODUCTION

An exciting new scientific trend emerged

in the 80’s for exploring zeolites and

mesomorphous materials, as advanced solid-

state materials. Zeolites and mesomorphous

materials or commonly referred as molecular

sieves, are porous materials with nanometer

dimension (0.3–10 nm) windows, channels andcavity architectures. They represent a ‘new

frontier’ of solid-state chemistry with great

opportunities for innovative research and

development.

The most recent efforts is to find several

novel applications which include molecular

electronics, “quantum” dots/chains, zeolite

electrodes, batteries, nonlinear optical materials,

enzyme mimics, chemical sensors, molecular

wires and nanodevices (Frost and Sulivan,

2001).

The growing interest in these novel

systems is due to the bulk behaviour of these

nanostructured materials which can be designed

and tailored by controlling their cluster

nanostructures which lead to greatly improved

performance. In addition, the characteristics of

these nanomaterials could be purposely

engineered by the variation of the chemical

composition, structure and size distribution(Frost and Sulivan, 2001; Barrer, 1982; Breck,

1974; Meier and Olson, 1987 ).

In tandem with numerous research

findings which are continuously being reported by

about 20000 zeolite scientists globally, we have

made some important discoveries that contribute

to the development in the science and

nanotechnology of zeolites and mesomorphous

materials. This paper presents a general review

of our contributions and recent advances in the

design and investigation of these fascinating

family of nanostructured materials.*) Makalah kunci yang disajikan pada Seminar

Nasional Kimia VII, di Surabaya 9 Agustus 20051) Corresponding author



Hamdan - Nanomaterials as catalysts in the production of fine chemicals

2 © Kimia ITS – HKI Jatim

ZEOLITES AND MESOMORPHOUS MATERIALS

Zeolites are crystalline, hydrated

aluminosilicates with open three-dimensional

framework structures (Barrer, 1982; Breck, 1974)

built of (SiO4)4– and (AlO4)5– tetrahedra linked by

sharing of an oxygen atom, to form regular

intracrystalline cavities and channels of molecular

dimensions. The first natural zeolite molecular

sieve, stilbite, was discovered by Cronstedt in

1756. He named it ‘ zeolitos’ which means boiling

stone, because the mineral appeared to boil when

heated. Since then about 45 natural zeolites have

been identified.

In 1862, St. Claire Deville attempted,

unsuccessfully, to prepare a synthetic zeolite.

Barrer’s pioneering work in the 1940’s

demonstrated that a wide range of zeolites could

be synthesized from aluminosilicate gels (Frost

and Sulivan, 2001; Barrer, 1982). In 1956, Linde

A, the first commercial zeolite was synthesized byBreck (Breck, 1974). In 1962 Mobil Oil introduced

the use of synthetic zeolite X as a cracking

catalyst, followed by the synthesis of the high

silica zeolites beta and ZSM-5. Today at least 150

synthetic zeolites are known.

Mesoporous MCM-41 and aerogels are

nanostructured materials with great potential as

catalyst and nanocomposites. The interest in new

zeolite-like materials or mesomorphous materials

reflects the importance of improving the

performance of zeolites as molecular sieves or

catalysts. The major problem in the zeolite area is

an apparent restriction of pore size to less than0.8 nm. There have been numerous attempts to

incorporate the selectivity and resilience of zeolite

into a structure which has significantly larger pore

size which is capable of processing large

hydrocarbon molecules. With mesoporous solids

for instance, shape selectivity whose effects on

reactants, products and transition states are well

known in microporous systems may be extended

to larger molecules.

STRUCTURAL CHARACTERISTICS

(i) Zeolites

The various types of zeolites are

characterized by the distinct topology of their

three-dimensional framework, the relative content

of silicon and aluminium, the ordering of the

silicon and aluminium atoms in the tetrahedral

sites of the framework and the type and

distribution of cations. The framework topology

and morphology of zeolites contribute to the

remarkable physical and chemical properties of

these microcrystals. Some of the framework

topologies found in zeolites are shown in Figure 1

zeolite A Zeolit Y

ZSM-5 zeolite beta

ferrierite mordenite

Figure 1 : Framework topologies of various zeolites

ii) Mesomorphous MCM-41

In 1992, a new family of silicate

mesoporous materials, designated as M41S, with

exceptionally large uniform pore structures has

been synthesized. Among these materials, the so-

called MCM-41 family which shows a hexagonal

array of uniform mesopores in the range between

1.6 nm and 10 nm as shown in Figure 2. TheMCM-41 structures were found to be constructed

mainly from amorphous inorganic silica walls of

0.9 to 1.2 nm in thickness around surfactant

molecules. The calcined material have specific

surface areas of about 700 m2 per gram. A so-

called liquid crystal templating mechanism in

which surfactant liquid crystal structures serve as

organic templates has been proposed to explain

the formation of such large pore sizes in the

mesoporous materials. Burning off of the organic

material then leaves back the cylindrical pores.



Akta Kimindo Vol. 1 No. 1 Oktober 2005: 1 – 10


Figure 2: Schematic representation of the

structure of a MCM-41 phase with an

interpore distance of ≈ 3.5 nm,

amorphous wall structure and hexagonal

pores.

(iii) Silica Aerogel

In a recent issue of Science, aerogel was

rated among the top ten scientific and

technological developments (Nur, et al., 2005).

Aerogels are advanced materials yet are literally

next to nothing. They consists of more than 96%

air and the remaining four percent is a matrix of

silica (Figure 3). Aerogels are unique materials

with pores and properties which are smaller than

the wavelength of light. Aerogel is the lightest

solid material known; only three times the density

of air and has tremendous insulating capability.

Aerogel is a good insulator because of its

large internal surface area. It disperses heat

throughout its complex structure and aerogel

makes possible development of extremely

interesting applications in vacuum and heatinsulation of hot water tanks and boiler,

refrigerators and industrial ovens. A double pane

window filled with a one inch layer of aerogel

provides the same insulating value as 15

standard thermopanes. It is just a question of

time as to when technology and markets offer the

benefits of “translucent” building components

that feature full control of heat performance. A

promising material for translucent roofing is silica

aerogel.

Figure 3: Silica matrix in aerogel

Aerogels are inert, non-toxic, environmentallyfriendly insulation materials and, its superior

performance over other foam materials is finally

being recognized by designers, engineers and

architects. Silica aerogel is a potential substitute

for silicon dioxide, the reigning dielectric. Silica

aerogel offers a better way to keep the

interconnecting wires from shorting across the

narrow dividing space between transistors which

avoid propagation delays and excessive crosstalkand subsequently may double computer speeds.

Ultralow density (ULD) silica aerogels

have been taken on NASA space shuttle missions

to capture high velocity cosmic dust particles.

Furthermore, aerogels are also being used by

NASA to insulate the rover vehicle for the Mars

Pathfinder project in 1997.

Maerogel; a silica aerogel which is

directly prepared from rice husk (Figure 4) is a

nanomaterial of a highly divided state and exhibits

unconventional properties which offers more cost

effective methods of production and application.

Maerogel is more superior in quality than the

current commercial TEOS aerogel. Being an inert,

non-toxic and environmentally friendly amorphous

material, Maerogel possesses established

physico-chemical properties al listed in Table 1

which can be modified for specific applications.

Figure 4: SEM micrograph and photograph of Maerogel

Si OO

Si Si

O O

Si

O Si O

Si

O

SiO OO

SiO O

OH

OH

OH

OH

HO

HO





0

0.5

1

1.5

2

2.5

3

without

catalyst

10 20 40 80 sec-

AlMCM-

41Catalyst (SiO2:Al2O3)

A m

o u n t o f d e s i r e d

p r o d u c t ( m

m

o l )

Table 1: Physical Properties of Maerogel

NEW DIRECTIONS

Nanostructured Materials

Large pored zeolites, mesomorphous

MCM-41 and silica aerogels are naturally

nanomaterials due to the existence of pores and

crystalline network of nano dimension. Changes in

the molecular properties of materials at the

nanoscale greatly enhance their physical and

chemical properties. Due to its stable and flexible

framework of variable sizes, the zeolite lattice may

also be used as a host for encapsulated

complexes or metallic clusters allowing the control

of nuclearity of these active species and the steric

contraints imposed on the reactants. MCM-41 and

VPI-5, for example, have been used to host

polymer, metal complexes and enzymetic species

to form molecular wires and zeozymes.

(i) Al-MCM-41 catalysts were prepared with

various SiO2:Al2O3 ratios via direct and

secondary syntheses using sodium aluminate as the aluminium source. Structural studies by 27Al

and 29Si MAS NMR spectroscopy indicated that Al

are in the tetrahedral form and located in the

framework. The presence of distorted framework

aluminium was also observed, more significantly

in the secondary aluminated samples. Maximum

amount of Al was incorporated by direct synthesis

with SiO2:Al2O3 ratio of 10 and a calculated Si/Al

ratio of 15.2. Acidity studies using Pyridine

Desorption Measurement and Temperature

Programmed Desorption of Ammonia

(TPD-NH3) show that the acidity of Al-MCM-41

increases with increase in Al incorporation into theMCM-41 framework. The potential of H-Al-MCM-

41; as a heterogeneous catalyst was studied in

the hydroxyalkylation of benzene with propylene

oxide as a model reaction. Gas chromatography

analysis indicates that H-Al-MCM-41 with

SiO2:Al2O3 ratio of 10 demonstrates the highest

catalytic activity with a conversion of benzene and

selectivity of 92.3% and 87.5% respectively. The

formation of 2-phenyl-1-propanol was optimized at

a temperature of 393 K after 24 hours with

propylene oxide to benzene mol ratio of 0.5 using

nitrobenzene as the solvent. The results indicate

that instead of aluminium content, solvent andreactant mole ratio also play a role to give high

conversion and selectivity of 2-phenyl-1-propanol.

(Mohamed, 2005)

Figure 5 27Al MAS NMR spectra of Zeolite A and Al-

MCM-41 samples with various SiO2:Al2O3

ratios

Figure 6 Amount of 2-phenyl-1-propanol (desired

product) (mmol) with various SiO2:Al2O3 ratio at

constant parameter (Temperature: 363 K; Reactant

Mole Ratio: 0.5; Time: 24 hours; Solvent:Nitrobenzene)

Property Maerogel

Apparent density 0.03 g/cm3

Internal Surface Area 800-900 m2 /g

Mean Pore Diameter 20.8 nm

Thermal Tolerance to 500 C, mp > 1200 C

Thermal Conductivity 0.099 Wm-1 K-1

- 0- ppmAl H2O 6

3+

Dir-Al-MCM-

Dir-Al-MCM-

Dir-Al-MCM-

Dir-Al-MCM-

Sec-Al-MCM-41

Zeolite A

- 56





(ii) The development of heterogeneous

oxidation catalysts which contain metal

complexed Schiff bases such as phthalocyanine,

porphyrin and salen that mimic catalytic activity of

metaloenzyme is of interest. Encapsulated metal

complex, as the guest molecule, into molecular

sieves with suitable pore sizes such as zeolite Y,VPI-5 and MCM-41 as the host, via covalent or

ionic bonding is expected to be as active as those

present in enzyme, structurally and thermally

more stable, remain unchanged during reactions

and give higher conversions (Figure 7).

Figure 7: Metal complex encapsulated molecular sieves

In situ synthesis of Fe(III)-salen and Cu(II)-

salen complexes, Mn(III) complexes based on

diimine and aroylhydrazone ligands in the cavities

of Al-MCM-41, by the flexible ligand method were

attempted with success in our laboratory. The

catalytic activity of the Fe(III)-salen-Al-MCM-41complex was studied in the oxidative

polymerisation of bisphenol-A using aqueous 30%

H2O2 at room temperature.

Cu2+

T=80oC,

air

Table 2. Catalytic tests on polymerization of

bisphenol-Aa

aAll reactions were carried out at room temperature for

3 h: 100 mg catalyst; 5 mmol bisphenol-A; 3.4 mL 30%

H2O2; 10 mL dioxane

X-Ray diffractograms of all samples

demonstrate that the structure of Al-MCM-41 are

still intact after modification process with slightly

decrease in crystallinity. The immobilization of

salen ligand increases the pore diameter and unit

cell parameter of support due to the steric effect.

From DRUV-Vis spectra, it is observed that the

geometry of Table 2. Catalytic tests on

polymerization of bisphenol-Aa

Mn(Salen) and Co(Salen) complexes are

distorted upon encapsulation which can be

evaluated from the splitting of d-d electronic

transition. The main product of benzyl alcohol

oxidation is benzaldehyde and this molecule can

undergo further oxidation to produce benzoic acid.It is observed that the encapsulated Fe(Salen)

catalyst shows the best substrate conversion

followed by Co(Salen) and Mn(Salen).

(iii) Dibenzoylation of benzoyl chloride in the

presence of mesoporous H-Al-MCM-41 forms the

biphenyl 4,4’-dibenzoylbiphenyl (DB) with 100%

selectivity. Catalytic results indicate that samples

with higher Si/Al ratios produced higher yields of

4,4′-dibenzoylbiphenyl. Sample with the highest

Si/Al ratio produced 0.45 µmol 4,4′-dibenzoyl

biphenyl; the highest yield, after 3 hours of reaction. The catalytic test results indicate that

the product yield is influenced and determined by

the presence of both Lewis and Brønsted acid

sites. (Figure 8)

Catalyst

Quantity of

Polybisphenol-

A (g)

Percentage

of reacted

bisphenol-A

Fe(III)-salen(FS)

0.22 19.3

FSAM-40 0.76 66.7

FSAM-60 0.71 62.3

FSAM-120 0.38 33.3

salen

T=140oC,Nitrogen

gas

Cun+

ZEOZIM Cu-salen-Al-MCM-41





Figure 8: 27Al MAS NMR spectra of (a) SO4-AlMCM-

41, (b) H-AlMCM-41, (c) cal-AlMCM-41 and (d)

SO4-AlMCM-41 after treatment with 1.0 M

methanolic HCl solution.

(iv) Sulphated AlMCM-41 (SO4-AlMCM-41)

mesoporous molecular sieves with SiO2 /Al2O3

ratio=15 was prepared via impregnation of

sulphuric acid on the surface of H-AlMCM-41.

Results of this work (Table 3) demonstrate thatSO4-AlMCM-41 is a solid Brönsted acid and active

towards benzoylation and dibenzoylation of

biphenyl. The production of 4, 4’-DBBP is affected

by the amount of acid site, amount of biphenyl

and 4-PBP. The conversion of biphenyl over H-

AlMCM-41 and sulphuric acid, sulphuric acid, SO4-

AlMCM-41 and sulphated amorphous silica are

83.7, 90.6, 75.0, 94.2 and 22.3%, respectively.

The selectivity towards 4-PBP over H-AlMCM-41,

the mixture of H-AlMCM-41 and sulphuric acid,

sulphuric acid, SO4-AlMCM-41 and sulphated

amorphous silica (SO4-silica) are 83.7, 11.1, 19.3,

83.2 and 22.1%, respectively. The SO4-AlMCM-41which contains octahedral aluminium related to

the presence of Bronsted acid (Figure 8 and 9)

was found to be active towards dibenzoylation of

biphenyl reaction, giving 11.0% of 4, 4’-DBBP

whereas sulphuric acid and H-AlMCM-41 catalyst

which contains both Lewis and Bronsted acid sites

only produced 4.1% of 4, 4’-DBBP

(v) Recently, a novel concept of “phase-

boundary catalysis” (PBC) in the catalysis of

immiscible liquid-liquid reaction system was

proposed. In the PBC system, the bifunctional

particles containing both the hydrophilic andhydrophobic regions, which require neither stirring

nor addition of co-solvent, were placed at the

phase boundary in order to catalyze the reaction.

It is of interest to enhance the activity of PBC

system The activity enhancement of the PBC

system by fluorination was explored. Hydrogen

peroxide was chosen as the oxidizing agent

because it produces only water as the by-product.In addition, it is cheaper and more accessible

than other oxidant.

NaY zeolite was used as the host material

for the phase-boundary catalyst. Titanium

tetraisopropoxide was impregnated from

cyclohexanol solution into NaY zeolite powder to

give Ti-NaY. After that, the mixture was suspended

in toluene solution containing

octadecyltrichlorosilane (OTS). The fluorination

was carried out in 1M ammonium

hexafluorosilicate solution [(NH4)2SiF6] to give

F-PB-NaY. In the epoxidation reaction, 1-octene (4

ml), 30% aqueous H2O2 (1 ml) and the catalyst

powder (50 mg) were placed in a glass tube. The

reaction was performed with or without stirring for

24 hours at ambient temperature.

Wavenumber / cm-1

1640 1600 1560 1520 1480 1440 1400

A b s o r b a n c e / a .

u .

(a)

(b)

Figure 9: The pyridine-FTIR spectra of (a) SO4-

AlMCM-41 and (b) H-AlMCM-41 at 250 oC.





Table 3: Benzoylation and dibenzoylation of biphenyl with benzoyl chloride over various types of catalysts at

180oC for 24 h.

Catalyst(s) Conversion of BP

(%)

Selectivity towards 4-

PBP (%)

Selectivity

towards 4, 4’-

DBBP (%)

Selectivity

towards others

(%)

H2SO4 a 75.0 35.3 0.0 39.7

H-AlMCM-41 83.7 83.7 0.0 0.0

H2SO4 + H-AlMCM-41 90.6 13.0 1.65 76.0

SO4-AlMCM-41 94.2 83.2 11.0 0.0

SO4-Silica 22.3 22.1 0.0 0.2

a Homogeneous catalyst.

Activity comparison in Table 4 between

phase-boundary catalysts (PB- and F-PB-) and

hydrophilic catalysts (Ti- and F -Ti-) of all catalysts

(Figure 10) on the epoxidation of 1-octene to give

1,2-epoxyoctane indicated that the amphiphilic

catalysts are more feasible for epoxidation. When

the amphiphilic particles are placed at the phase

boundary with the hydrophobic side facing the 1-

octene phase and the hydrophilic side facing the

H2O2 phase, titanium active sites on the catalysts

are in contact with both the octene substrate and

H2O2. This resulted in a continuous supply of H2O2

and alkene substrate to the active sites on theparticles. Fluorine is the most electronegative

element. Being electron deficient, it has a high

tendency and affinity to attract electrons from its

nearby element. Based on this nature, fluorine

which was introduced to the catalyst by

fluorination would draw the electron from titanium

active sites towards the fluorine sites (Ti4+→F-).

Consequently, titanium as the active site for

epoxidation reaction is further activated by

fluorination. Amphiphilic fluorinated Ti-NaY

catalysts showed a remarkable activity

enhancement as observed in the F-PB-NaY

catalyst which gives the highest TON/BET

vi) Titanium containing silica aerogel was

prepared by the sol-gel method. The tetrahedral

titanium is present in low titanium loading, TSA1

(Si:Ti = 200) is responsible in the catalysis of

cyclohexene to 1,2-cyclohexanediol as a major

product in the presence of hydrogen peroxide.

Higher titanium loaded silica aerogel, (TSA2 (Si: Ti

=33) absorbs at 250 nm in UV DRS, suggesting

the presence of [Ti(SiO)3O-] species. Selectivity of

TSA2 is tuneable by changing the loading of the

catalyst in the reaction mixture. TSA1 catalysed

cyclohexene to produce 1,2-cyclohexanediol at

low loading and 2-cyclohexene-1-one when the

loading was doubled.(Figure11)

Table 4: Epoxidation of 1-octene by various modified Ti-NaY catalysts

All reactions were carried out at room temperature for 24 hours with 1-octene (4 ml), 30% H2O2 (1 ml) and catalyst (50mg) with vigorous stirring. The concentration of Ti and OTS = 500 μmol g -1.

Catalyst Epoxide (μmol) BET (m2/g) TON for Ti

TON/BET

( x 10-3)

None 0.0 - - -

Ti-NaY 74.9 718.46 3.0 4.18

PB-NaY 94.1 118.04 3.8 32.19

F-Ti-NaY 111.2 22.37 4.4 196.69

F-PB--NaY 172.5 14.20 6.9 485.92





Figure 10: Photographs of modified Ti-NaY catalysts: Ti-NaY, OTS-Ti-NaY, F -Ti-NaY, OTS-F -Ti-NaY and TMS-F -Ti- NaY (from

left tto right).

0

10

20

30

40

50

60

70

Alkene Glycol Keton Others

31.3mg

TSA262.5mg

TSA2125mg

TSA2

0

10

20

30

40

50

60

70

Alkene Glycol Keton Others

31.3mgTSA1

62.5mgTSA1

Figure 11: The component percent in the reaction

mixture after reaction using (a) TSA1 and (b) TSA2 as

catalyst

CONCLUDING REMARKS AND FUTURE

OUTLOOK

Several features of the structuralchemistry of zeolites are related to their

importance as sorbents, molecular sieves and

catalysts. Zeolites are potentially very active

catalysts due to the topology of the framework,

shape and size of the pores which can be

modified to accommodate sorbates and impose

shape selective contraints on the products of the

reaction. It is apparent that more applications of

these remarkable zeolite systems will be realised

as our knowledge of the chemistry and structureof the framework continues to grow.

Improvements in the technologies for the

synthesis of zeolites and development of zeolites

with larger pore sizes hold great promise in

better use of the depleting petroleum resources.

Industrial application of aerogel-based

catalyst or catalyst supports have so far been

limited due to the rather expensive method of

preparation and difficulties in reactors operation.

Production of silica aerogel from rice husk does

not only reduce the cost but at the same time

minimise prolonged environmental problem. The

future beneficial use of aerogels in catalysismainly requires tailoring and design of the

surface structure and overcoming the technical

limitations.

REFERENCES

Barrer, R. M. 1982. Hydrothermal Chemistry of

Zeolites. Academic Press, London

Breck, D. W. 1974. Zeolite Molecular Sieves:

Structure, Chemistry and Use. John Wiley

and Sons, London.





Chai, Lee Soon and Hamdan, H., 2004, Proc. of

the 2nd Annual Fundamental Science

Seminar AFSS 2004, ISBN-983-9805-54-

1, 2005, 138-140.

Frost and Sullivan, 2001. Zeolites Industry

Trends and Worldwide Markets in 2010.

Technical Insights, New York.Hamdan, H. 2003. Design and Molecular

Engineering fo Nanostructured Zeolites

and Mesomorphous Materials –

Professorial Inaugural Lecture, UTM, Siri 7,

Penerbit UTM.

Hamdan, H., Navijanti, V., Nur, H., and Mohd

Nazlan Mohd Muhid, J. of Solid State

Sciences, 2005 Vol 7, Issue 2, 239-244.

Halimaton Hamdan, Vivin Navijanti, Hadi Nur,

and Mohd Nazlan Mohd Muhid and, J. of

Solid State Sciences, 2005 Vol 7, Issue

2, 239-244. Nur, . Amir Faizal Naidu Abdul Manan, Lim Kheng

Wei, Mohd Nazlan Mohd Muhid and

Halimaton Hamdan, J. of Hazardous

Materials, 117,2005, 35-40.

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