Synthesis and Characterizations of Metal Oxide-Sulfonic Acid Functionalized ZSM-5 for Photocatalytic Degradation and Adsorption of

Dimethylarsenic Acid

Alias Mohd Yusof1, a, Nur Nadira Sulaiman 1, b and Hadi Nur 2, c 1Department of Chemistry,Universiti Teknologi Malaysia, Malaysia

2Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, Malaysia aalias@kimia.fs.utm.my, bnadiraslayman@gmail.com, chadi@kimia.fs.utm.my

Keywords: titanium dioxide; zinc oxide; ZSM-5; dimethylarsenic acid; photodegradation; adsorption.

Abstract. TiO2 and ZnO were incorporated into ZSM-5 using titanium tetrachloride and zinc nitrate

as the precursors respectively. The functional group of ZSM-5 was further modified using

mercaptopropyl trimethoxysilane (MPTS). This method was used to produce a two-in-one

composite namely TiO2-ZSM5-SH and ZnO-ZSM5-SH which holds the abilities to degrade organic

pollutants as well as to remove them from the environment. Determination of the composition and

morphology of all synthesized catalyst complexes were characterized using Fourier Transformed

Infrared (FTIR), X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and High

Resolution Transmission Electron Microscopy (HR-TEM). Results obtained indicated that the

average diameters for TiO2-ZSM5-SH particles are 2.398 µm, slightly larger than the diameter of

ZnO-ZSM5-SH particles with an average of 1.8067 µm. Both TiO2-ZSM5-SH and ZnO-ZSM5-SH

appeared as sphere-like shape and amorphous since both metal oxides were incorporated into the

ZSM-5 pores. The compositions of these complexes were proven by the presence of a strong silica

absorption band showing the existence of Si-O stretching vibrations and SiO2 asymmetric vibrations

in the synthesized complexes. For TiO2-ZSM5-SH, the Ti-O stretching vibration can be seen at

3467.29 cm-1

and at 1637.86 cm-1

while Zn-O absorption band appeared at 455.52 cm-1

in ZnO-

ZSM5-SH. The symmetric and asymmetric vibrations of –CH2 and –CH3 indicate the existence of

alkyl groups in the ZSM-5. The ability of catalyst complexes synthesized as photocatalyst were

applied and tested on the photodegradation of dimethylarsenic acid (DMA) under ultra violet (UV)

radiation which showed that these catalyst complexes are proven be able to degrade DMA into less

toxic intermediates and are able to adsorb the remaining arsenic species from the aqueous solutions.

Introduction

In recent years, photocatalytic degradation of various kinds of organic and inorganic pollutants

using semiconductor powders as photocatalysts has been studied extensively using heterogeneous

catalytic processes in aqueous samples for the removal of toxic pollutants. Metal oxides

photocatalysis is based on the use of metal oxides as light activated catalyst in the destruction of

inorganic and organic materials as well as in organic synthesis chemistry. The most promising

photoctalyst that have been reported - able to degrade both inorganic and organic pollutants are

titanium dioxide (TiO2) and zinc oxide (ZnO). Improvement in the photocatalytic functions of TiO2

and ZnO for achieving efficient decomposition of various pollutants has been an active research

field in recent years [1, 2].

Most of the studies on this issue have been focused on the development of visible light-

driven photocatalysts for efficient utilization of sunlight or indoor fluorescent light. Thus, various

approaches have been utilised for harvesting visible light to be used on TiO2 and ZnO [3]. While

improvements can be made in terms of producing catalysts on nano-scale level, attempts have also

been made to complement the function of the photocatalysts during post-operation to address the

fate of the degraded by-products [3, 4]. Hence the introduction of zeolite is believed to be suitable

in the final clean-up stage. This study attempts to look into new approaches and functions of the

composite TiO2 and ZnO with zeolite combined as a two-in-one composite that can address toxic

Applied Mechanics and Materials Vol. 699 (2015) pp 994-999 Submitted: 18.07.2014Online available since 2014/Nov/26 at www.scientific.net Accepted: 09.09.2014© (2015) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMM.699.994

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 161.139.220.14, Universiti Teknologi Malaysia, Skudai, Malaysia-27/11/14,09:35:03)

pollutants clean up especially in aqueous environment. It is vital to ensure its applicability in

environmental remediation because the capability of this new material is targeted to clean for

consumption of water from underground sources.

In this study, the method implemented was incorporating TiO2 and ZnO into the pores of ZSM-5

using impregnation method and later the grafting procedure was applied to modify the functional

groups on ZSM-5 surfaces using MPTS [5,6]. These procedures were merged to produce a two-in-

one composite where it can degrade organoarsenicals compound such as dimethylarsenic acid

(DMA) as well as to remove their inorganic species from aqueous environment.

Experimental

Synthesis of TiO2-ZSM5-SH and ZnO-ZSM5-SH.The TiO2 and ZnO were incorporated into

zeolite pores using impregnation method. The commercial ZSM-5 (3.0 g) was stirred in ethanolic

solutions of TiCl4 to give a load of TiO2 into the ZSM-5 pores. The sample was stirred under

ambient temperature until it was dehydrated. Then, it was calcined at 500 C for 2 h to yield the

desired TiO2-ZSM5 white powder. The same procedure was applied to yield ZnO-ZSM5 with a

replacement of the precursor of ZnO which is Zn (NO3)2 [5].

The grafting procedure was conducted following the proposed method by Hadi Nur et al. [6].The

reaction was carried out by mixing the synthesized TiO2-ZSM5 (2.0 g) with MPTS (2 mL) in

toluene (4 mL) under stirring at 60˚C for 6 hours. The mixture was then filtered and washed with

toluene and double distilled water before drying at 100˚C. The product collected was expected to be

the TiO2-ZSM5-SH. A similar procedure was carried out for the modification of ZnO-ZSM5 into

ZnO-ZSM5-SH.

Characterizations of TiO2-ZSM5-SH and ZnO-ZSM5-SH. The techniques used for

characterization include the use of High Resolution Transmission Electron Microscopy (HR-TEM)

and Scanning Electron Microscopy (SEM) for morphological determination. The use of X-ray

Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR) is to determine the

qualitative measurement of the synthesized catalyst complexes.

Photocatalytic Degradation of Dimethylarsenic acid. Photocatalytic degradation of DMA was

carried out in a simple degradation box fully lined with aluminum foil in the inside for maximum

exposure of UV light supplied. A 100 ml solution of 50ppb DMA was prepared and added with 50

mg of TiO2-ZSM5-SH and ZnO-ZSM5-SH respectively. The DMA sample solutions were left for

24 h for dispersion under dark condition and 2 ml of the sample solutions were injected out and

marked as the initial concentration. The reaction was then carried out for 2 h under UV using a 6

watt UV lamp of wavelength 365 nm. After the reactions stopped, the samples were injected out

and marked as final concentration and examined by UV-Vis Spectroscopy (UV-Vis) and Graphite

Furnace-Atomic Absorption Spectroscopy (GF-AAS).

Results and Discussion

Synthesis of TiO2-ZSM5-SH and ZnO-ZSM5-SH. Based on the reagents and methods proposed,

the desired photocatalyst complexes namely TiO2-ZSM5-SH and ZnO-ZSM5-SH were successfully

prepared.

Characterizations of TiO2-ZSM5-SH and ZnO-ZSM5-SH.The prepared photocatalyst complexes

were characterized to determine their composition and morphology.

Fourier Transform Infrared Spectroscopy (FTIR)

Fig. 1 shows the spectra of both synthesized TiO2-ZSM5-SH and ZnO-ZSM5-SH. Both spectra

have similar patterns as they hold the same composition, only differ in the existence of Ti-O and

Zn-O stretching vibrations.

Applied Mechanics and Materials Vol. 699 995

Fig. 1. FTIR spectra of the synthesized TiO2-ZSM5-SH and ZnO-ZSM5-SH

It is clearly observed that strong silica absorption band exists in both spectra at 1100.00 cm-1

–

1300.00 cm-1

proving the existence of Si-O stretching vibrations and SiO2 asymmetric vibrations in

the synthesized complexes. For TiO2-ZSM5-SH, the Ti-O stretching vibration can be seen at

3467.29 cm-1

and at 1637.86 cm-1

while Zn-O absorption band appeared at 455.52 cm-1

in ZnO-

ZSM5-SH. The symmetric and asymmetric vibrations of –CH2 and –CH3 indicate the existence of

alkyl groups in ZSM-5.

X-Ray Diffraction (XRD).Diffractograms from XRD analysis provide the composition of the

synthesized catalyst complexes. TIO2-ZSM5-SH

00-046-0862 (Q) - Sodium Aluminum Titanium Silicate Tetrabutylammonium Hydrate - ((C4H9)4N)2O-Al2O3-Na2O-TiO2-SiO2-H2O - Y: 98.61 % - d x by: 1. - WL: 1.54056 - I/Ic PDF 1. - S-Q 100.0 % -

Operations: Import

TIO2-ZSM5-SH - File: TIO2-ZSM5-SH.RAW - Type: 2Th/Th locked - Start: 2.000 ° - End: 60.000 ° - Step: 0.050 ° - Step time: 1. s - Temp.: 25 °C (Room) - Time Started: 2 s - 2-Theta: 2.000 ° - Theta: 1.000 ° - Phi: 0.00 ° - Aux1: 0.0

Lin (C

ounts)

0

10

20

30

40

50

60

2-Theta - Scale

2 10 20 30 40 50 60 Fig. 2. Diffractogram of the synthesized TiO2-ZSM5-SH

From Fig. 2, the diffractogram shows some significant peaks appearing at its own respective

degree on 2 theta scale. It can be clearly seen that the peaks have shifted a little to the left compared

to the expected peaks represented by the red peaks due to the existence of ZSM-5 together with

TiO2. The peaks are not as intense as they were expected also because of the introducing of silica

and other elements from the zeolite.

ZNO-ZSM5-SH

00-056-0499 (Q) - Sodium Aluminum Silicate Hydrate - Na2O-Al2O3-SiO2-H2O - Y: 60.00 % - d x by: 1. - WL: 1.54056 - I/Ic PDF 1. - S-Q 33.7 % -

00-036-0795 (Q) - Sodium Aluminum Hydrogen Silicate Sulfide Hydrate - Na5HAl4Si6S4O24·H2O - Y: 72.58 % - d x by: 1. - WL: 1.54056 - I/Ic PDF 1. - S-Q 40.8 % -

00-021-1486 (Q) - Zinc Oxide - ZnO - Y: 29.22 % - d x by: 1. - WL: 1.54056 - I/Ic PDF 1. - S-Q 16.4 % -

01-076-0603 (*) - Zinc Potassium Aluminum Oxide Silicate Hydrate - Zn5K2Al12Si12O48(H2O)3.5 - Y: 92.89 % - d x by: 1. - WL: 1.54056 - Cubic - a 12.07500 - b 12.07500 - c 12.07500 - alpha 90.000 - beta 90.000 - gamma 90.000

Operations: Import

ZNO-ZSM5-SH - File: ZNO-ZSM5-SH.RAW - Type: 2Th/Th locked - Start: 2.000 ° - End: 60.000 ° - Step: 0.050 ° - Step time: 1. s - Temp.: 25 °C (Room) - Time Started: 2 s - 2-Theta: 2.000 ° - Theta: 1.000 ° - Phi: 0.00 ° - Aux1: 0.0

Lin (C

ounts

)

0

10

20

30

40

50

60

70

80

90

100

2-Theta - Scale

2 10 20 30 40 50 60 Fig. 3.Diffractogram of ZnO-ZSM5-SH

996 Sustainable Energy and Development, Advanced Materials

Fig. 3. shows the diffractogram of ZnO-ZSM5-SH. The existence of ZSM-5 has a huge impact

on the intensity of the peaks recorded. Several peaks seem to interfere one another. The Expected

peaks for ZnO appeared but similar to TiO2, it has been shifted due to the effect of ZSM-5

introduced in the complex.

Scanning Electron Microscopy (SEM).The morphology of the complex TiO2-ZSM5-SH is clearly

shown in Fig. 4 while ZnO-ZSM5-SH shown in Fig. 5.

Fig.4. SEM image for TiO2-ZSM5-SH Fig. 5. SEM image of ZnO-ZSM5-SH

The shapes resemble those of non-uniform sphere-like particles. The average diameter for TiO2-

ZSM5-SH is 2.398 µm slightly longer than the diameter of ZnO-ZSM5-SH with an average of

1.8067 µm. Both TiO2-ZSM5-SH and ZnO-ZSM5-SH appeared as amorphous materials as both

metal oxides were incorporated into the ZSM-5 pores.

High Resolution Transmission Electron Microscopy (HR-TEM).The images obtained from HR-

TEM analysis shows the cross-section of both synthesized catalyst complexes.

Fig. 6. HR-TEM images of TiO2-ZSM5-SH

Fig. 6 shows the micrographs of TiO2-ZSM5SH. The appearance of a straight lattice line indicates

the existence of Ti crystal lattice line. The gradient colors appeared from black to grey showing the

difference between the crystalline TiO2 particles as indicated by the dark area and the bright area

which has no specific line structure portraying the degree of amorphous and mesoporosity of ZSM-

5. It can be clearly seen that there are several crystals lattice line appearing in the TiO2-ZSM5SH

complex.

Applied Mechanics and Materials Vol. 699 997

Fig. 7. HR-TEM images of ZnO-ZSM5-SH

Fig. 7 shows the images of the synthesized ZnO-ZSM5-SH. The image proves the mesoporousity of

the catalyst complex. Similar to TiO2-ZSM5-SH, the dark shade shows the existence of crystalline

metal which in this complex is the ZnO while the brighter shade shows the existence of amorphous

silica from the ZSM-5.

Degradation of Dimethylarsenic acid. Photocatalytic degradation of DMA was carried out in a

batch process for 2 h with 50 mg of each catalyst complex added into a 100 mL DMA and left for

24 hours dispersion before the reaction started under UV radiation. Samples were injected out at

one hour, half an hour and two hours and were further analyzed using GF-AAS and UV-Vis.

Fig. 8. Percentage of arsenic adsorption by catalyst complexes.

Fig. 8 shows the percentage of arsenic adsorption by each catalyst complex by analyzed using GF-

AAS technique. Results from GF-AAS prove that TiO2-ZSM5-SH holds the best adsorption

characteristics as compared to ZnO-ZSM5-SH within a 1.5 h of reaction time. The amount of

arsenic species being adsorbed by TiO2-ZSM5-SH are higher compared to ZnO-ZSM5-SH. Both

catalyst complexes gave the same trend. The percentage of adsorbed arsenic increase from 1 h to

1.5 h but as the time of reaction proceeded, the adsorbed arsenic species would be released back

into the medium as evident from the increase in the amount of arsenic species detected after 2 h. It

can be concluded that the optimum time required for efficient arsenic adsorption by both catalyst

complexes is 1.5 h.

998 Sustainable Energy and Development, Advanced Materials

Fig. 9. Percentage of DMA Concentration Reduction

Results from UV-Vis analysis show similarities to the trends observed from GF-AAS results. Fig. 9

portrays the results obtained from UV-Vis. Wavelength from DMA was detected at 197 nm which

interfering with wavelength from the other arsenic species such as As (III) and As (V) and

monomethyl arsenic acid (MMA). From UV-Vis data obtained, it shows that the ability of TiO2-

ZSM5-SH to degrade DMA is better as compared to ZnO-ZSM5-SH. The concentration of DMA

decreasing as time but the optimum time for best reduction of DMA concentration is 1.5 h. As time

is prolong, the percentage of DMA concentration reduction decreased.

Summary

A dual function photocatalyst namely TiO2-ZSM5-SH and ZnO-ZSM5-SH was successfully

synthesized having the abilities to degrade toxic pollutant; DMA and to adsorb the remaining

arsenic species in the degradation by-products. Both photocatalyst complexes were amorphous and

sphere-like shape composing of titanium dioxide, zinc oxide and silica from the zeolite introduced;

ZSM-5. The ability of TiO2-ZSM5-SH was proven to give better performance to degrade and

adsorb toxic pollutant since the percentage of DMA degradation and arsenic adsorption are higher

as compared to ZnO-ZSM5-SH.

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