An accelerated route of glycerol carbonate formation from glycerol using waste boiler ash as catalyst

Article abstract of DOI:10.1039/c4ra02910k

Waste boiler ash was successfully utilised as catalyst for the direct synthesis of glycerol carbonate from glycerol and urea. A series of catalysts were prepared using various calcination temperatures. The physico-chemical properties of the catalysts have been investigated by using XRD, BET, TGA, FESEM-EDX, ICP-MS, Hammett test and CO₂-TPD. From the study it was found that boiler ash had significant catalytic activity towards conversion of glycerol into glycerol carbonate. It is believed that the potassium metal ion which detaches from potassium silicate had a major impact on the catalytic data where the potassium ion being a weak Lewis acid causes selective catalytic transformation of glycerol into glycerol carbonate. The mechanistic pathway through glycerol carbamate intermediate was confirmed through time online analysis study using ¹³C-NMR and ATR-FTIR, respectively. However, the selective transformation of glycerol carbamate into glycerol carbonate is reported to be different where it is formed in an accelerated manner. The highest catalytic activity resulted in an average percentage of 93.6 ± 0.4% glycerol conversion, 90.1 ± 1.0% glycerol carbonate selectivity and 84.3 ± 1.1% glycerol carbonate yield. Besides, for the first time the novel idea of using waste material, specifically boiler ash, is proposed as a catalyst for synthesis of glycerol carbonate from glycerol and urea. The current research employed suggests an alternative route for proper disposal of waste boiler ash.

Full text of DOI:10.1039/c4ra02910k

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V. Paroo Indran, N. A. S. Zuhaimi, M. A. Deraman, G. Pragas Maniam, M. M. Yusoff and Y. H. Taufiq-Yap,
RSC Adv., 2014, DOI: 10.1039/C4RA02910K.
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PAPER
An Accelerated Route of Glycerol Carbonate
Formation from Glycerol using Waste Boiler Ash as
Catalyst
Cite this: DOI:
a
a
Received
Accepted
Vidhyaa Paroo Indran , Nor Ain Syuhada Zuhaimi , Mohd Asyrak
a
a,b
a
Deraman , Gaanty Pragas Maniam , Mashitah Mohd. Yusoff , TaufiqꢀYap
Yun Hin and Mohd Hasbi Ab. Rahim*
c
a,d
DOI:
www.rsc.org/
Waste boiler ash was successfully utilised as catalyst for the direct synthesis of glycerol carbonate from
glycerol and urea. A series of catalysts were prepared using various calcination temperatures. The
physicoꢀchemical properties of catalyst have been investigated by using XRD, BET, TGA, FESEMꢀ
EDX, ICPꢀMS, Hammett Test and CO ꢀTPD. From the study it was found that boiler ash had significant
2
catalytic activity towards conversion of glycerol into glycerol carbonate. It is believed that the potassium
metal ion which detaches from potassium silicate had the major impact on the catalytic data where the
potassium ion being a weak Lewis acid causes selective catalytic transformation of glycerol into glycerol
carbonate. The mechanistic pathway through glycerol carbamate intermediate was confirmed through
13
time online analysis study using CꢀNMR and ATRꢀFTIR, respectively. However, the selective
transformation of glycerol carbamate into glycerol carbonate is reported to be different where it is
formed in an accelerated manner. The highest catalytic activity obtained in an average percentage of 93.6
±
0.4% glycerol conversion, 90.1 ± 1.0% glycerol carbonate selectivity and 84.3 ± 1.1% glycerol
carbonate yield. Besides, for the first time the novel idea of using waste material specifically boiler ash is
proposed as catalyst for synthesis of glycerol carbonate from glycerol and urea. The current research
employed suggests an alternative route for proper disposal of waste boiler ash.
Glycerol carbonate is one of the value added product of glycerol
Introduction
which is important to many industries. Glycerol carbonate has low
3
toxicity, noble biodegradability and high boiling point. There are
also further investigations that supports glycerol carbonate to be
used in cosmetic and medical institutes as they have low toxicity,
Over the years production of glycerol as a crude waste from
biodiesel making industries has been largely increasing. For every
1
00 kg of biodiesel produced, 10 kg of glycerol is produced as a byꢀ
4
volatility, combustibility and good moisturizing ability. Besides,
product which corresponds to 10 weight percentage of biodiesel. It is
estimated that by the year 2016, the production of crude glycerol will
reach 4 billion gallons. Thus, in order to make this waste a useful
substance for future use, the conversion of glycerol to other value
added product is suggested.
they also play a major role as a component of surfactants, paints,
3
1
coatings and gasꢀseparation membranes. Additionally, there is a
general consensus that the potential of glycerol carbonate carbonate
as an antiꢀexplosive additive for gasoline and diesel will result in an
2
5,6,7
unprecedented growth in the coming years.
_
______________________________________________________
a
Faculty of Industrial Sciences & Technology, Universiti Malaysia Pahang,
Lebuhraya Tun Razak, 26300 Kuantan, Pahang, Malaysia.
E-mail: mohdhasbi@ump.edu.my
There are various routes for producing glycerol carbonate from
glycerol. One of the methods employed traditionally was synthesis
of glycerol carbonate from glycerol by utilising phosgene.
8
b
Central Laboratory, Universiti Malaysia Pahang, Lebuhraya Tun Razak,
However, due to the toxicity and hazard issues, other synthesis
routes were suggested such as from propylene and ethylene
2
6300 Kuantan, Pahang, Malaysia.
c
Catalysis Science and Technology Research Centre, Faculty of Science,
Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia.
9
carbonate. Ethylene carbonated was later found to be nonꢀcost
effective chemical. Thus, the direct carbonylation using carbon
d
Centre for Earth Resources Research & Management, Universiti Malaysia
dioxide (CO ) was suggested. Nevertheless, the synthesis route also
2
Pahang, Lebuhraya Tun Razak, 26300 Kuantan, Pahang, Malaysia.
suffered from optimisation under high pressure reactors and had
some thermodynamic limitations which also affected the cost
†
Electronic supplementary information (ESI) available.
10,11,12
effectiveness of the reaction.
Glycerol carbonate was also
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proposed to be produced with dimethyl carbonate (DMC). The catalyst at various temperatures did not significantly alter the
reaction required higher ratio of DMC to glycerol besides requiring conversion of glycerol. All catalyst used reported had almost
13
expensive catalyst as well as shift in chemical equilibrium. Another relatively comparable glycerol conversion which was ≥ 90% at 4 h
route is by reacting glycerol and urea. Glycerol and urea are reaction time (table 1). BA 900 showed the highest selectivity and
3
,14
relatively cheaper and readily available.
yield of about 90.1 ± 1.0% and 84.3 ± 1.1% respectively which was
significantly higher than reaction conducted without catalyst. It is
worth to note that glycerol carbonate yield obtained from blank
Malaysia was testified to be the world’s second leading palm oil
producer in the year of 2010 by Foreign Agricultural Service,
Production, Supply and Distribution Online, United States
Department of Agriculture (USDA) with over 18.6 million tonnes of
production. Besides, about 4 million tonnes of boiler ash is
produced per year from incineration of palm fruits, palm kernel,
2
3
reaction is comparable with previously reported literature. The
yield of glycerol carbonate obtained follows the order of BA 900 >
BA 700 > BA 1100 > BA 110 > BA 500. This could have resulted
from the influence of basic metal oxides present in the catalyst with
almost similar strength even after different heat preꢀtreatment was
carried out. It is also justified that potassium as the most major metal
in the catalyst could have been the target metal that has actively
played the role in the catalytic activity of boiler ash. The catalytic
data was also comparably higher with recently reported study where
15
1
6,17
palm shells, and palm fiber.
Therefore, there were rising costs on
proper disposal of these ashes that resulted in research to make use
1
8
of these ashes as catalysts. To add on, boiler ash over the years has
been vastly used in many area of application. It was found that boiler
ash could act as a good absorbent due to its spongy and porous
characteristic. Thus, it was employed in a wastewater treatment
research where it was capable removing 97% zinc ions from
wastewater with capacity of 0.01 mg/ g absorbance. Besides, this
research affirmed that boiler ash could act as good absorbent of
moisture. Recent research also proposed the use of boiler ash as
additional cementꢀlike material where it improved the strength of the
Furthermore, boiler ash was also utilised to convert
waste cooking oil into biodiesel through transesterification.
7
5% yield of glycerol carbonate was obtained using metal
2
4
monoglycerolates as catalyst under same temperature conditions.
Besides, the turn over frequency, TOF (mmol/g.h ) of boiler ash
was relatively higher than most of the previously reported study
ꢀ
1
(Supplementary data, Table S1). It can be said that the current study
19
promises good yield and selectivity with utilisation of boiler ash as
catalyst from waste. Therefore, the research could critically ensure
as an alternative method for proper disposal of waste boiler ash.
20,21
cements.
22
Effect of Metal on Catalytic Activity
The current study, for the first time focuses on the use of waste
material as catalyst for the synthesis of glycerol carbonate from
glycerol and urea. For that reason, boiler ash from palm oil mill in
Lepar Hilir, Malaysia is introduces as catalyst herein. The reaction is
carried out as previously reported by Hasbi and coꢀresearchers in
In order to clearly identify the metal which has played the role in
enhancing the catalytic activity, a series of reaction comprising
various metals from most dominant metal content in boiler ash were
carried out. The metals for analysis were chosen based on the
elemental analysis carried out using ICPꢀMS. Reaction involving
sylvite (KCl), potassium hydroxide (KOH), calcium oxide (CaO),
23
2
012. However, the catalysts used in the previous study were of
gold and metal alloys supported catalysts meanwhile in the current
study, it focuses in the use of boiler ash (BA) waste which contains
variety of metals of those being major are K, Ca, and Mg. This is a
new, cheap and abundant catalyst which is introduced in this
research for the transformation of glycerol into glycerol carbonate.
Significantly, development of new catalyst from waste is addressed
in the study following the synthesis pathway as in figure 1.
magnesium oxide (MgO) and silicon oxide (SiO ) were carried out
2
and are illustrated in figure 2. From figure 2, it is evident that the
potassium regardless in form of hydroxide (KOH) or sylvite (KCl)
has the highest catalytic effect towards glycerol carbonate yield and
selectivity compared to other metal oxides such as MgO and CaO.
Besides, the catalytic reaction of SiO also yielded lower glycerol
2
carbonate. This phenomenon is expected since SiO is a covalently
2
O
bonded nonꢀpolar molecule which does not affect the catalytic
behavior of the reaction. Pottasium predominantly exist as KCl and
potassium silicate (K SiO ) in boiler ash, but for this reaction, KOH
was also used for the reason that potassium does not only leach
while dissolving into reaction solution as potassium ion and activate
the transformation of glycerol into the target product but also
absorbs moisture from the environment. Thus, it is supposed that the
preform of potassium metal does not play a significant role in the
reaction but the catalytic reaction is subjected to the presence of
potassium ion.
O
HO
OH
Boiler Ash Catalyst
2
3
2
NH3
+
O
O
+
OH
Ammonia
H
2
N
NH
2
Glycerol
Urea
HO
Glycerol Carbonate
Figure 1: Reaction network for glycerol carbonate synthesis using
Additionally, study on other minor metal elements presence in
boiler ash such as aluminium (Al) and iron (Fe) was not carried out
as it was previously reported with poor yield and selectivity of
glycerol carbonate in form of mixture of Al/Ca (metal oxide) and
Fe/Mg hydrotalcites. The yield obtained was only 10% and 39%
glycerol and urea as feedstock.
Results and discussion
Effect of calcination temperature on boiler ash towards glycerol respectively. An increase in aluminium and iron content decreases
the basic site density which decreases active sites to catalyse the
reaction. Besides, other metal such as zirconium in form of ZrO2
conversion, glycerol carbonate selectivity and yield
25
was also reported to produce lower yield of glycerol carbonate
26
Table 1 illustrates the effect of calcination heat preꢀtreatment of (31%). Zr was detected to be present in boiler ash in low levels
boiler ash towards glycerol conversion, glycerol carbonate through FESEMꢀEDX analysis (table 2). Therefore, it can be
selectivity and glycerol carbonate yield. From the reaction inferred that potassium ion is the responsible metal that acts as a
conducted, it is evident that boiler ash is capable of selectively weak Lewis acid to catalyse the reaction.
converting glycerol into glycerol carbonate. However, calcination of
2
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Table 1: Effect of boiler ash calcination temperature on glycerol conversion, glycerol carbonate selectivity and yield
No.
Catalyst
Temp. heat
treatment (°C)
Gly.
conv. %
Selectivity %
Comp. Comp.
3)
GC Yield %
TOF (mmol/g
ꢀ
1
GC
Comp.
(6)
cat. h )
(
(5)
43.1
11.1
10.0
4.8
1
2
3
4
5
6
Blank
BA 110
BA 500
BA 700
BA 900
BA 1100
ꢀ
78.7
91.1
91.1
94.1
93.6
89.8
32.8
83.5
82.7
88.6
90.1
85.6
24.0
1.9
4.3
3.5
4.1
5.5
ꢀ
25.8
76.2
75.3
83.4
84.3
77.0
ꢀ
110
500
700
900
1100
3.5
2.9
2.9
1.9
2.4
136.7
136.7
141.2
140.4
134.7
3.9
6.4
Reaction conditions: Temperature, 150 ˚C; Gas, N ; Glycerol: Urea, 1: 1.5 (Molar ratio); Catalyst mass, 0.25 g; Time, 4 h; Standard stirring
2
rate, 340 rpm. TOF: calculated based on the mmol of glycerol converted per gram catalyst per total reaction time (h). Relative Standard
Deviation (RSD): < 5%. Note: Gly. is Glycerol; GC is Glycerol carbonate; Comp. (3) is 2, 3ꢀdihydroxypropyl carbamate; Comp. (5) is 4ꢀ
(hydroxymethyl) oxazolinꢀ2ꢀone; Comp. (6) is (2ꢀoxoꢀ1,3ꢀdioxolanꢀ4ꢀyl)methyl carbamate.
suitable time of reaction with BA 900 to assist production of
maximum yield. On the other hand, figure 4 illustrates the selectivity
percentage of glycerol carbonate, compound (3), compound (5) and
compound (6). It is evident that by prolonging the reaction time,
decomposition of glycerol carbonate into (6) increases and at the
same time formation of compound (5) originated from carbamate
intermediate also increased. The figure also depicts that the glycerol
carbamate, compound (3) formed was decomposed into glycerol
carbonate in an accelerated manner. It is important to note that in
previous studies, compound (3) was observed to be present in large
amount in the initial stages of reaction and then gradually converted
25, 23, 27
into glycerol carbonate at prolonged reaction time.
Figure 2: Effect of metals towards glycerol conversion (%),
glycerol carbonate selectivity (%) and glycerol carbonate yield
(
%). Reaction conditions: Temperature, 150 ˚C; Gas, N2;
Glycerol: Urea, 1: 1.5 (Molar ratio); Catalyst mass, 0.25 g;
Time, 4 h; Standard stirring rate, 340 rpm.
Time Online Study
Figure 3 illustrates the time online study for boiler ash in the reaction
with glycerol and urea from 0 minute to 10 h reaction time. From the
figure it is evident that glycerol conversion using boiler ash
specifically BA 900 shows a consistent increase in conversion from
0
h to 6 h. At 6 h, 8 h and 10 h, the conversion hits a maximum level
Figure 3: Time online analysis (TOL) of glycerol conversion (%),
glycerol carbonate selectivity (%) and glycerol carbonate yield (%)
of BA 900. Reaction conditions: Temperature, 150 ˚C; Gas, N2;
Glycerol: Urea, 1: 1.5 (Molar ratio); Catalyst mass, 0.25 g; Standard
stirring rate, 340 rpm.
and remains unchanged which is about 97% conversion. In this
study, we report that the maximum yield of glycerol carbonate is at 4
h reaction time reaching 84.3 ± 1.1% of yield from the triplicate
study of BA 900. Moreover, the maximum yield stipulated the most
optimum time for the reaction to be conducted. On the contrary,
selectivity percentage and yield percentage of glycerol carbonate
decreased gradually from 5 h to 10 h. Low selectivity and yield at
prolonged reaction time was due to the decomposition of glycerol
carbonate into carbamate of glycerol carbonate. This is due to the
availability of primary hydroxyl group (–OH) in glycerol carbonate
molecule which has higher possibility to react with urea to form (2ꢀ
7
oxoꢀ1,3,ꢀdioxolanꢀ4ꢀyl)methyl carbamate. The availability of urea
at prolonged reaction time was confirmed by the time online analysis
1
3
using ATRꢀFTIR and C NMR, respectively. Therefore, the time
online analysis (TOL) depicts that 4 h reaction time is the most
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Figure 4: Time online analysis (TOL) of glycerol carbonate Figure 5: ATRꢀFTIR spectra of a) Standard glycerol, b) Standard
synthesis in presence of BA 900 catalyst. Key: Selectivity of glycerol carbonate, c) Product of reaction catalysed using BA 900 at
glycerol carbonate,
compound (5) and
selectivity of compound (3), selectivity of 4 h reaction time and d) Product of reaction catalysed by BA 900 at
selectivity of compound (6). Reaction 10 h reaction time.
conditions: Temperature, 150 ˚C; Gas, N ; Glycerol: Urea, 1: 1.5
2
(
Molar ratio); Catalyst mass, 0.25 g; Standard stirring rate, 340 rpm.
Mechanistic Study
Figure 6(a) and 6(b) illustrates the C NMR analysis for time online
13
Analysis of Reaction Products using ATR-FTIR
13
analysis (TOL) of BA 900. From the C NMR pattern, it is evident
To corroborate results from analysis of product using GCꢀFID, that glycerol carbonate (4) was selectively formed through
3
0
ATRꢀFTIR analysis (figure 5) was carried out on glycerol standard, carbonylation of intermediate 2, 3ꢀdihydroxypropyl carbamate (3).
glycerol carbonate standard, product of reaction at 4 h and 10 h. At 4 h of reaction time, 4ꢀ(hydroxymethyl) oxazolinꢀ2ꢀone (5) and
From figure 5, it is evident that glycerol is detectable through ATRꢀ (2ꢀoxoꢀ1,3ꢀdioxolanꢀ4ꢀyl)methyl carbamate (6) was seen to be
ꢀ
1
FTIR where transmittance of peak at range (1500ꢀ1200) cm
present. As the reaction time was prolonged up to 10 h, gradual
indicates glycerol moiety as in pattern (a) and (c). These peaks are increase in the peak pattern of (5) and (6) was observed which
assigned to overlapping of CꢀH in planes and OꢀH bending in the clearly indicated that selectivity towards glycerol carbonate
glycerol molecule. The OꢀH bending is further supported by the – formation decreased from 4 h onwards due to formation of (5) and
ꢀ
1 28
OH stretching at peak range of (3500 ꢀ 3000) cm . On the other (6). This is due to the further reaction of glycerol carbonate with
hand, glycerol carbonate is evident to be present in pattern (b) and excess of urea forms (6) and the intermediate 2, 3ꢀdihydroxypropyl
3
1
(
c). The occurrence of OꢀH vibration from 2ꢀhydroxyethyl chain at carbamate (3) forms compound (5) through a parallel reaction.
ꢀ
1
ꢀ1
peaks 3432 cm and 3343.57 cm indicates presence of –OH
functional group. Besides, peak observation from range (2950 –
2
From figure 6(a), glycerol (1) is detected through peak at 74.0
ꢀ
1
850) cm further specifies presence of glycerol carbonate. The two ppm and 64.5 ppm where the peak is observed from the presence of
peaks observed are due to CH and CH vibration of the Oꢀmethylene –CH–O– and –CH –O– bonds respectively. Compound (3) was
and Oꢀmethylindyne groups of cyclic carbonate. Moreover, the peak detected at 71.8 ppm which may attribute to –C–N– bond.
present at range (1790ꢀ1760) cm signifies C=O functional group Compound 5 on the other hand attributed to peaks at 43.0 ppm, 63.5
which is a result of C=O stretching of the 5 membered cyclic ppm and 78.9 ppm which are resultant of –CH–C– (C tertiary), –
2
2
ꢀ
1
carbonate.
CH–N–as well as –C–O– bonds respectively. Compound 4 exhibits
peaks at 62.3 ppm, 67.8 ppm and 79 ppm which correspond to –
ꢀ
1
Finally, peaks at range (1200ꢀ1000) cm also support presence CH –O–, –CH–O– as well as –C–O–. Besides, compound 6 showed
2
of glycerol carbonate where these peaks indicate CꢀC and CꢀO presence of peaks at 55.3 ppm, 64.1 ppm and 68.9 ppm which is
2
9
stretching of 2ꢀhydroxyethyl chain. In pattern (c) the two peaks attributed by –CH –O–, –C–N– as well as –CH–O–. On the other
2
ꢀ
1
ranging from (1610ꢀ1670) cm indicate presence of urea as hand, figure 6(b) is labelled with urea (2) at 165 ppm. This peak
previously reported. The urea peak is still visible in pattern (d) for directly corresponds to the C=O group that is attached to amide
7
the product of reaction catalysed by BA 900 at 10 h reaction time group in urea. The pattern of the decomposition of urea is also clear
signifying urea is not completely used up in the prolonged reaction as the C=O peak of urea gradually reduces from 30 min to 10 h until
as urea is employed excessively. However, the decrease of glycerol only a minimal amount of urea is left. The result also corresponds to
moiety peaks were noticed in pattern (d) indicating loss of glycerol spectra (d) in figure (5) for analysis of product using ATRꢀFTIR at
where it suggest almost 97% conversion of glycerol in prolonged 10 h reaction time where presence of urea is still detected at minimal
reaction time. Besides, the C=O stretch was also shifted in pattern amount. On the other hand, peak at 157.8 corresponds to C=O of
ꢀ
1
ꢀ1
(
d) to the right from 1790 cm into 1730 cm due to decrease in glycerol carbonate adjacent to –O which belongs to glycerol
selectivity of formation of glycerol carbonate and increase in carbonate. However, in this study we report that the quantity of (3)
formation of 4ꢀ(hydroxymethyl) oxazolidinꢀ2ꢀone (compound 5) at did not show direct correlation to glycerol carbonate formation as (3)
prolonged reaction time at 10 h. The shifted peak was observed to was not clearly observed at 0.5 h of reaction time. Nonetheless,
occur due to the (C=O) being adjacent to NꢀH group in compound presence of intermediate (3) may have attributed to the formation of
(5). As for glycerol carbonate, the C=O is partly adjacent in between glycerol carbonate but in an acceleration manner as the intermediate
two –O groups which indicates the peak in spectra (b) and (c) to be (3) formed quickly transformed into glycerol carbonate. These
inꢀline and sharper compared to spectra (d). Thus, C=O group is still results also correlated to the previous analysis of TOL using GCꢀFID
expected to persist at 10 h due to the increase formation in (figure 3) which resulted in the highest glycerol carbonate yield at 4
compound (5).
h reaction time and the yield decreased with prolonged reaction time
as the selectivity towards glycerol carbonate decreased due to
formation of (5) and (6). From the study we believe that the
4
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ꢀ
1
synthesis route of glycerol carbonate still follow the mechanistic peaks were observed in range of 2250 cm indicating the presence
pathway reported by Hasbi and coꢀresearchers in 2012 as well as of NꢀCꢀO stretching of isocyanic acid which was also similar to the
Kim and coꢀresearchers in 2014 but the selective transformation of study previously reported by Aresta and in his coꢀresearchers in
3
,31
glycerol carbamate intermediate decomposed in an accelerated 2009.
manner to form glycerol carbonate.
Therefore the mechanistic pathway for the synthesis of
2
3,31
glycerol carbonate using BA 900 is suggested as in scheme 1.
13
Figure 6(a): C NMR overlay of standard glycerol (STD GLY),
standard glycerol carbonate (STD GC) and TOL of BA 900. Key: (1)
Glycerol; (3) 2, 3ꢀdihydroxypropyl carbamate, (4) Glycerol
carbonate; (5) 4ꢀ(hydroxymethyl) oxazolinꢀ2ꢀone; (6) (2ꢀoxoꢀ1,3ꢀ
dioxolanꢀ4ꢀyl) methyl carbamate.
Figure 7: ATRꢀFTIR spectra for time online analysis of BA 900
reaction from 0.5 h to 10 h. Key: (1) Glycerol; (2) Urea; (3) 2, 3ꢀ
dihydroxypropyl carbamate; (4) Glycerol carbonate; (5) 4ꢀ
(hydroxymethyl) oxazolinꢀ2ꢀone.
O
O
HO
OH
+
HO
O
NH
2
H
2
N
NH
2
OH
OH
(
1)
(2)
(
3)
O
O
O
O
O
O
O
O
NH
13
Figure 6(b): C NMR overlay of standard glycerol (STD GLY),
standard glycerol carbonate (STD GC) and TOL of BA 900. Key: (2)
Urea and (4) Glycerol carbonate.
O
NH
2
HO
HO
(6)
(4)
(5)
O
13
To validate the C NMR results on time online analysis of BA 900,
analysis using ATRꢀFTIR was carried out for the reaction from 0.5 h
to 10 h (figure 7). From the spectra it is observed that glycerol (1)
Scheme 1: Mechanistic pathways for synthesis of glycerol carbonate
using glycerol, urea and boiler ash (BA 900) as catalyst. Key: (1)
Glycerol; (2) Urea; (3) 2, 3ꢀdihydroxypropyl carbamate, (4) Glycerol
carbonate; (5) 4ꢀ(hydroxymethyl) oxazolinꢀ2ꢀone; (6) (2ꢀoxoꢀ1,3ꢀ
dioxolanꢀ4ꢀyl) methyl carbamate.
ꢀ
1
ꢀ1
peak gradually shifted from 1450 cm closer to 1400 cm as the
time for reaction was prolonged. Peak of urea (2) also showed
ꢀ
1
similar behavior of where the two peaks (1665 and 1620) cm was
observed to gradually shift. This phenomenon is expected to persist
as most glycerol quickly converted into glycerol carbonate and other
products. Intermediate (3) was detected at 2 h and 4 h reaction time
_
______________________________________________________
ꢀ
1
in range of (1710ꢀ1715) cm . However, from 6 h to 10 h the peak
ꢀ
1
shifted to 1730 cm indicating the conversion of carbamate (3) into
31
ꢀ1
product (5). Presence of peak at 1790 cm indicated C=O stretch
of glycerol carbonate (4) at 2 h onwards. However, glycerol
carbamate peak (3) was not clearly observed at 0.5 h to 4 h due to
reaction mechanism occurring in an accelerated manner to directly
decompose intermediate (3) into (4). This result was also in
1
3
agreement with the previously discussed C NMR. Besides, no
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R
/
S
C
C
4R
A
A
d
02
v
9
a
1
n
0
c
K
es
, CaO, KOH and
K SiO , where there is sharp peak indicating presence of oxide at
Catalyst Characterisation
and other forms of metals such as MgO, Al O
2 3
2
3
ꢀ
1 33
(
1010ꢀ2010) cm
and broad range of –OH functional group at
325.47 cm , respectively. From figure 9, it is also evident that both
Thermogravitmetric (TGA) Analysis
ꢀ
1
3
BA 700 and BA 900 exhibits similar FTIR pattern which may
attribute to findings in this study that the catalytic behaviour is
relatively similar.
Based on figure 8, the TGA thermogram (figure 8, a) displays
significant weight loss in boiler ash where the weight decreased from
1
00% to 60% from temperature 0 ˚C to 1100 ˚C. There is about 40%
loss in weight percentage which is due to presence of many
components in the boiler ash such as moisture and other organic
matter such as carbon that can be lost through heating. Besides,
halogenated metal compound containing chlorine could have
resulted in evaporation under high thermal treatment. For example,
sylvite (KCl) could have been transformed into potassium silicate
(
K SiO ) under high thermal treatment at 900 °C.
2 3
Figure 9: FTIR spectra of a) BA 110, b) BA 700 and BA 900.
X-Ray Diffraction Analysis
It can be seen from figure 10 that BA 110 showed the characteristics
peak of sylvite (KCl) at 2θ= 28.60˚, 40.69˚, 66.71˚ and 73.85˚,
respectively. The presence of calcite (CaCO ) could be examined at
3
Figure 8: Thermoꢀgravimetric analysis (TGA) of boiler ash. (a)
Weight Percentage (%) Loss and (b) Derivative Thermal
Gravimetric (DTG).
diffraction peak of 2θ= 30.61˚ whereas potassium silicate (K SiO )
2
3
was detected at 2θ= 30.93˚. For BA 700, similar phase composition
as BA 110 containing KCl, CaCO and K SiO were observed to
3
2
3
presence. However, for BA 900, in addition to CaCO and K SiO ,
3
2
3
These phenomena can be also compared and observed from the XRD
diffractogram. Moreover the DTG thermogram (figure 8, b)
illustrates the possibility of the boiler ash catalyst to exist in various
phases at different calcination temperature. At temperature 100 ˚C to
calcium oxide (CaO) was identified to be present at 2θ= 26.46˚ and
2.83˚ respectively. The occurrence of CaO in BA 900 is expected
3
since CaCO was known to decompose into CaO as temperature for
3
o
calcination treatment is higher than 800 C.
2
00 ˚C there is variation in peaks drop that could be indicating
moisture lost through heating. The transformation of peak from
temperature 400 ˚C to 1100 ˚C could indicate loss of elements such
as organic matters and phase transformation of the catalyst. This
analysis was done to identify changes in phase transformation at
various level of temperature. Thus, the boiler ash was dried at 110 ˚C
and calcined from 350 ˚C to 1100 ˚C in order to obtain suitable
catalyst.
The change in the XRD pattern could be mainly influenced by
calcination temperature where at different calcination temperatures,
phase transformation occurred as evident from TGA data in figure 8
and FTIR analysis in figure 9. In general, potassium from sylvite
form in BA 110 transformed into potassium silicate with higher
thermal heat treatment as observed in BA 700 and BA 900. Even
though, KCl was not detected in BA 900 in the XRD data we believe
that KCl is still present in BA 900 as referred to the elemental
analysis using FESEMꢀEDX. Moreover, the presence of potassium
as the predominant peak is detected in all three catalysts. Therefore,
this factor contributed to the insignificant difference in catalytic
results for all three catalysts. However, BA 900 and BA 700 exhibits
better catalytic results than BA 110 due to the dominance presence
of potassium silicate phase at higher calcination temperature.
However, it is important to note that the present reported study on
boiler ash characteristic is not relatively similar of those reported
Fourier Transform Infrared (FTIR) Analysis of Catalysts
Results from FTIR analysis (figure 9) on the boiler ash (BA)
suggests that major peak for BA 700 and BA 900 catalyst was
ꢀ
1
observed at 3325.47 cm which is attributed to the presence of –OH
functional group. Similarly the –OH band was observed in BA 110
ꢀ
1
at 3245.63 cm . The presence of –OH group was confirmed with
ꢀ
1
peaks observed in both catalyst at 680ꢀ620 cm where it assigns –
OH out of plane bending. The presence of the band indicates water
33
earlier. The difference in the particular characteristic could have
32
adsorbed on the surface of the catalyst from the atmosphere. Boiler
ash is found to have spongy and porous characteristic that can easily
been due to difference in calcination method, where in previous
study calcination in closed system was employed and in the current
study the calcination process was carried out under static air in tube
furnance. Even though, magnesium was not detected in the major
peaks visible from the XRD diffractogram, it is believed that
magnesium metals are present in minor amount in boiler ash which
is supported by the elemental analysis using EDX and ICPꢀMS.
Since magnesium and calcium are generally present in lower amount
ꢀ
1
absorb moisture. The band at range of (1300ꢀ1400) cm attributes
2
2
the deformation of –OH. Other than that, the presence of band at
ꢀ
1
range (1010 ꢀ1020) cm was attributed to alternating –SiO and –AlO
3
3
bands in BA 700 and BA 900. It is visible from the FTIR spectrum
that metal ions present in the boiler ash BA 700 as well as BA 900
have gone transformation to form metal oxides, metal hydroxides
6
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in oil palm boiler ash as reported in elemental analysis study by Morphology and Elemental Analysis of Boiler Ash using
3
3
Boey and coꢀresearchers in 2011, using Xꢀray Fluorescence (XRF), FESEM-EDX and ICP-MS
it plainly supports the detection level of these specific elements to be
very minimal in this characterisation technique by XRD. These From table 2 and table 3, it is apparent that potassium is the most
element were particularly looked through because magnesium and dominant metal present in boiler ash which supports the catalytic
calcium supported catalyst have shown good yield of glycerol activity of the boiler ash to be influenced by potassium. Besides, the
7
carbonate in previous studies. Besides, these metals are reported to results are also inꢀline with the XRD data where due to minimal
be in the form of oxides in palm oil boiler ash by Boey and coꢀ amount, peaks containing magnesium element were not visible.
researches in 2011. Thus, it is likely evident that metal present in
boiler ash exist in solid metal oxides phase.
Table 2: Elemental analysis of BA 900 using FESEMꢀEDX
Element
Weight (%)
9.97
C
O
Mg
Al
Si
53.24
3.39
0.70
6,01
S
1.42
Cl
1.10
K
Zr
18.53
5.65
Total
100.00
These elemental results also correlated to previous results reported
32
by Yin and coꢀresearchers in 2008. Figure 11 on the other hand,
simply depicts the nature of boiler ash to be spongy and porous
1
9,34
where it could act as a good absorbent of moisture.
This
characteristic is important because waste glycerol contains moisture
and catalyst which can absorb moisture will favour the forward
reaction in case of if direct utilisation of waste glycerol with urea is
proposed.
Table 3: Metal content in BA 900 (ICPꢀMS)*
Metals
Potassium (K)
Calcium (Ca)
Percentage (%)
86.8
9.8
2.0
1.1
0.4
Magnesium (Mg)
Aluminium (Al)
Others (Fe, Ag, Cu, and Na)
Note: * analysis carried out for environmental metals only
.
Figure 10: XRD Diffractogram of (a) BA 110, (b) BA 700 and (c)
BA 900 catalysts.
(a)
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Temperature Programmed Desorption (CO –TPD)
2
From figure 12, BA 110 is classified as a basic material with total
gas CO desorbed about 7066.9 µmol/ g. A total of four distinct
2
peaks were observed at different temperatures with different amount
of CO gas desorbed ranging from weak basic property to high basic
2
property. This is regard to the influence of homogeneously
distributed weak alkaline metals and strong alkaline metals in BA
1
10. BA 700 (figure 12) on the other hand showed basic property
with total amount of CO gas desorbed about 780.3 µmol/ g while
2
BA 900 showed 1249.3 µmol/ g of CO gas desorbed. A total of
2
three peaks were observed with different amount of gas desorbed.
According to carbon dioxide temperature programmed desorption
technique (CO ꢀTPD), it can be roughly classified that peaks
2
(
b)
absorbed at temperature (< 200 °C) contains weak basic sites, at
temperature (200 °C – 400 °C) contains medium basic sites and at
temperature (> 400 °C) contains strong basic sites respectively.
3
6
Figure 11: FESEM of (a) BA 110 and (b) BA 900.
For the case of BA 110, strong basic sites corresponds to the
peak desorbed at temperature 446 °C (b), 635 °C (c) and 744 °C (d)
respectively. On the other hand for BA 900 strong basic site is
present at peak 647 °C (g) while BA 700 exhibited strong basic sites
BET Surface Area Analysis
From the BET analysis it was found that BA 110 had surface area of at peak 667 °C (h) and 834 °C (i). Even though, all three BA 110,
2
2
about 8.05 m / g and BA 900 had surface area about 2.00 m / g. The BA 700 and BA 900 contain strong basic sites, the Hammett test
surface area of the dried boiler ash at 110 °C was relatively larger indicated these three catalysts to show mild basic property. Thus,
than the boiler ash calcined at 900 °C. This is because at higher this could be an influence of mixture of weak and strong metal
temperature agglomeration of particles occur causing particles to oxides that results in mild basic property of boiler ash in the
bind closely to each other and exist as cluster where this Hammett Test as depicted in table 5. Even though, BA 110 exhibits
dramatically causes surface area to decrease. However, surface area almost 6 times higher basic property from the CO ꢀTPD analysis,
2
did not particularly influence the catalytic results in this study BA 700 and BA 900 still depicted higher yield of glycerol carbonate
because no distinct difference in catalytic data was observed.
compared to BA 110. Thus, it is expected that the potassium metal
ion which acts as a weak Lewis acid had played the major role in the
catalytic activity. This is because it is reported that Lewis acid sites
activates the carbonyl group of urea while the conjugated base site
activates hydroxyl group on glycerol to form glycerol carbonate. It
is also understood that the preform of potassium does not show a
Acidity and Basicity Analysis
3
0
Hammett Test
Based on table 4, it can be summarized that the Hammett test major effect on the catalytic data obtained. In addition to that,
showed that all catalyst BA 110, BA 500, BA 700, BA 900 and BA presence of K SiO3 is more dominant in BA 700 and BA 900
2
1
100 changed the colour of phenolphthalein (H_=8.2) from compared to BA 110 which is evident from the XRD as well as
ꢀ
2
colourless to pink. However, for all three indicator 2, 4 – FESEMꢀEDX data. Therefore, the availability of O species could
Dinitroaniline (H_=15), 4 – Nitroaniline (H_=18.4) and methyl red assist in superior catalytic activity of BA 700 and BA 900 to form
(H_=4.8) remained unchanged. Therefore, it can be concluded that glycerol carbonate.
the basicity of the catalyst lie in the range of 8.2 < H_< 15 and the
catalyst is not acidic in nature. In concise, the basic strength of the
catalysts tested is of mildly basic in nature. The low basicity of the
ꢀ
catalyst is essentially influenced by OH groups, medium basicity by
ꢀ
2
35
metalꢀO groups and strong basicity by O groups.
Table 4: Hammett Test
Catalyst
Methyl
Red
Phenolphthalein
(H_=8.2)
2, 4 –
4 –
Dinitroaniline Nitroaniline
(
H_=4.8)
(H_=15)
(H_=18.4)
BA 110
BA 500
BA 700
BA 900
BA
No
changes
No
changes
No
changes
No
changes
No
changes
Colourless to
pink solution
Colourless to
pink solution
Colourless to
pink solution
Colourless to
pink solution
Colourless to
pink solution
No changes
No changes
No changes
No changes
No changes
No changes
No changes
No changes
No changes
No changes
Figure 12: CO ꢀTPD of BA 110, BA 700 and BA 900.
2
1
100
8
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Table 5: Basic sites distribution calculated on the basis of CO ꢀTPD molar ratio of glycerol to urea used was (1:1.5). Sampling was done
2
profile
from 0 h to 4 h (typical or otherwise mentioned) with the time
interval of 1 h. 50 µl of sample was transferred in 1450 µl of
deionised water. The results were analysed using Gas
Chromatography ꢀ Flame Ionised Detector (GCꢀFID) Agilent
Technologies 7890A equipped with Varian Capillary Column, CPꢀ
ParaBOND Q (25m, 0.53mm, 10µm).
Catalyst
Basic sites ( µmol/ g)
Medium Strong
Contribution
of strong
basic site
Weak
Total
(%)
BA 110
BA 700
BA 900
265.1
ꢀ
44.9
ꢀ
ꢀ
6801.7
780.3
1149.6
7066.9
780.3
1249.3
96
100
92
Product Analysis
54.8
Gas Chromatography ꢀ Flame Ionised Detector (GCꢀFID) Agilent
Technologies 7890A equipped with Varian Capillary Column, CPꢀ
ParaBOND Q (25m, 0.53mm, 10µm) was used to analyse the
product of reaction. The temperature ramping was set at 10 ˚C per
minute increased up to 300 ˚C. Sampling was done during reaction
for analysis from 0 h to 4 h with time interval of 1 h (typical or
otherwise mentioned) where 50 µl of sample was added to 1450 µl
of deionized water. Standard calibration for glycerol and glycerol
carbonate was done using GCꢀFID before samples from catalytic
reaction were analysed. Tetraethylene Glycol (TEG) was used as
external standard. Helium gas was used as the carrier gas with a flow
rate of 35 ml/ min. The temperature of the injector and the detector
were 225 °C and 250 °C, respectively. The temperature of the
column was programmed to have a 2 min initial hold at 80 °C, then
Conclusions
In concise, it can be concluded that the use of waste boiler ash as
catalyst for the reaction of producing glycerol carbonate from bioꢀ
renewable feedstock glycerol and urea is an economic synthesis
route. Other than that, the reaction is also environmentally friendly
where it ensures proper disposal of waste besides providing an
alternative route for a modest and cheap synthesis of glycerol
carbonate at larger scale due the amount of waste boiler ash and
glycerol generated yearly. Besides, the percentage of yield of
glycerol carbonate and percentage of conversion of glycerol which
are 84.3% as well as 93.6% respectively also shows a promising
approach for the synthesis of glycerol carbonate utilising waste
boiler ash. From the overall study, it was found that the potassium
metal ion acts as a weak Lewis acid played a major role as the active
metal ion for the catalytic activity of boiler ash while O groups in
boiler ash could have also acted as a strong base to activate glycerol
to form glycerol carbonate. The reaction pathway was confirmed to
follow glycerol carbamate route. However, the decomposition of
glycerol carbamate was found to occur at an accelerated manner
from initial stages of the catalytic reaction.
1
2
0 °C/ min ramp from 80 °C to 250 °C and 15 °C/ min ramp from
50 C to 300 C with 3 min hold time. The split ratio was 1:10 and
°
°
injection volume was 1 µl.
ꢀ
2
Time online analysis of products was also analysed using
Attenuated Total ReflectanceꢀFourier Transform infrared
spectroscopy (ATRꢀFTIR) Perkin Elmer, USA with resolution of
ꢀ1
4
cm per second and Nuclear Magnetic Resonance (NMR)
BRUKER Ultra Shield Plus 500 MHz. For the case of analysis of
liquid product using ATRꢀFTIR, 50 µl of sample was immediately
sampled and one drop was placed onto the sample holder. The drop
was then immediately analysed. On the other hand, the sample for
Experimental
13
Materials
C NMR analysis was prepared by sampling 150 µl into the NMR
tubes and then diluting them with 500 µl deuterated Methanolꢀd4.
Glycerol (99.5%) and Urea (AR Grade) were purchased from
Friendemann Schmidt Chemical. Waste boiler ash used as catalyst
was collected from palm oil mill located in Lepar Hilir, Pahang,
Catalyst characterisation
Malaysia. The boiler ash used is the ash obtained from incineration XꢀRay Diffraction RigakuMini Flex II was set to analyse the XRD
of palm fruits, palm kernel, palm shells and palm fiber. Potassium
hydroxide ≥85.0%, potassium chloride ≥99.0%, magnesium oxide
pattern of the catalysts. Diffraction patterns were recorded with Cu
Kα radiation over the range of 3 degree to 80 degree at 2 Theta (θ)
≥99.0% and calcium oxide ≥99.99% used were purchased from for crystalline phase determination.
SigmaꢀAldrich.
Thermoꢀgravimetric Analyser Mettler Toledo TGA/DSCꢀ
HT/1600 used was analysed in the range of temperature between
ambient to 1200 ˚C under static air.
Catalyst preparation
Raw boiler ash obtained from palm oil mill in Lepar Hilir, Pahang,
Malaysia was dried at 110 ˚C overnight and then powdered using
mortar and pestle (denoted as BA 110). Then the ash was sieved
The Hammett test were carried out on BA 110, BA 500, BA
7
00, BA 900 and BA 1100 where phenolphthalein, 2, 4ꢀ
using 200µm sized sieve. Later, 2 g of the sieved ash were loaded dinitroaniline, 4ꢀnitroaniline and methyl red were used as indicators
on the combustion boat and calcined under static air at temperature to determine the qualitative acidic as well as basic properties of the
(
500 ˚C, 700 ˚C, 900 ˚C and 1100 ˚C respectively) for 4 h. The
catalysts were later denoted as BA 500, BA 700, BA 900 and BA
100, respectively.
catalyst. 25 mg of catalyst were weighed and prepared in three
batches. 5 ml of methanol was added to the catalyst. Then 1 mL of
the indicator was added to 4 ml of methanol. Final volume of 5 ml
indicators were added separately to the catalysts weighed in batches.
The mixture was then left to equilibrate for 2 h. The colour changes
were observed and noted.
1
Catalytic Testing
The reaction was carried out using a threeꢀneck round bottom flask.
Typically, 13.8 g of glycerol was allowed to heat up to 150 ˚C for
under the flow of nitrogen gas. When the temperature reached 150
The quantitative basicity strength was analysed using CO2ꢀ
Temperature Programmed Desorption, (Thermo Finnigan TPD/R/O
˚C, 13.5 g urea and 0.25 g catalyst was added to the reaction and
1
100) Thermo electron for BA 110, BA 700 and BA 900. The TPD
stirred using magnetic stirrer under 340 rpm of stirring rate. The
was equipped with a thermal conductivity detector (TCD). 50 mg of
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sample was treated at 200 °C for 1 h by passing helium at a flow rate 9.
of 20 ml/min. Then, the sample was saturated with carbon dioxide at
a flow rate of 30 ml/min and at temperature ramping of 10 °C/ min 10.
up to 200 °C for 1 h. Subsequently, the system was flushed with
helium (20 ml/ min) at 200 °C for 1 h to eliminate physisorbed CO2. 11.
Desorption was carried out from ambient temperature to 900 °C at a
Z. Mouloungui, J.ꢀW. Yoo, C.ꢀA. Gachen, A. Gaset, and G.
Vermeersch, 1996, European Patent, EP0739888 A1, 1–13.
C. Vieville, J. W. Yoo, S. Pelet, and Z. Mouloungui, Catal.
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M. Aresta, A. Dibenedetto, F. Nocito, and C. Pastore, J.
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heating ramp of 5 °C/ min.
12.
J. George, Y. Patel, S. M. Pillai, and P. Munshi, J. Mol.
Catal. A Chem., 2009, 304, 1–7.
Other than that, Field Emission Scanning Electron Microscope 13.
FESEM) and Energy Dispersive XꢀRay (EDX), JEOL (JSMꢀ7800f)
S. C. Kim, Y. H. Kim, H. Lee, D. Y. Yoon, and B. K. Song,
J. Mol. Catal. B Enzym., 2007, 49, 75–78.
(
with spatial resolution up to 1 nm was used to study the surface 14.
morphology and elemental analysis of the catalyst.
S. Fujita, Y. Yamanishi, and M. Arai, J. Catal., 2013, 297,
137–141.
1
5.
Foreign Agric. Serv. Prod. Supply Distrib. Online, United
States Dep. Agric. (USDA). Available <http//www.
fas.usda.gov/psdonline/psdHome.aspx>.
Inductive Coupled Plasma ꢀ Mass Spectrometer (ICPꢀMS)
Agilent 7500c was used to study the environment metal content in
the catalyst using inꢀhouse method CHEMITEL/WI/CHEMꢀTM/001 16.
based on AOAC999.10.
K. T. Lee, N. F. Zainudin, and A. R. Mohamed, in The
Seventh Asia-Pacific International Symposium on
Combustion and Energy Utilization, 2004, pp. 1–8.
N. Zainudin, K. Lee, A. Kamaruddin, S. Bhatia, and A.
Mohamed, Sep. Purif. Technol., 2005, 45, 50–60.
S. H. Shuit, K. T. Tan, K. T. Lee, and A. H. Kamaruddin,
Energy, 2009, 34, 1225–1235.
K. H. Chu and M. A. Hashim, J. Chem. Technol.
Biotechnol., 2002, 77, 685–693.
W. Tangchirapat, J. Tangpakasit, S. Waewꢀkum, and C.
Jaturapitakkul, KMUTT Res. Dev. J., 2003, 26, 459–473.
V. Sata, C. Jaturapitakkul, and K. Kiattikomol, J. Mater.
Civ. Eng., 2004, 16, 623–628.
L. H. Chin, B. H. Hameed, and A. L. Ahmad, Energy &
Fuels, 2009, 23, 1040–1044.
M. Hasbi Ab Rahim, Q. He, J. A. LopezꢀSanchez, C.
Hammond, N. Dimitratos, M. Sankar, A. F. Carley, C. J.
Kiely, D. W. Knight, and G. J. Hutchings, Catal. Sci.
Technol., 2012, 2, 1914.
T. W. Turney, A. Patti, W. Gates, U. Shaheen, and S.
Kulasegaram, Green Chem., 2013, 15, 1925–1931.
M. J. Climent, A. Corma, P. De Frutos, S. Iborra, M. Noy,
A. Velty, and P. Concepción, J. Catal., 2010, 269, 140–149.
M. Lelovský and A. Kaszonyi, in Glycerol carbonate from
bioglycerol. 44th International Petroleum Conference,
Bratislava, Slovak Republic., 2009, pp. 1–6.
Fourier Transform infrared spectroscopy (FTIR) Perkin Elmer, 17.
USA was used to study the functional group present in the catalyst
that could also attribute to depict the phase of metal present in the 18.
catalyst. Fourier transform infrared spectroscopy (FTIR)
transmission data were collected for pressed catalyst disk made with 19.
KBr with ratio of (1:10) catalyst to KBr in the range of 4000–400
−
1
cm .
20.
The BET (BrunauerꢀEmmettꢀTeller) method was used to 21.
measure the total specific surface areas of the boiler ash catalysts
using a Thermo Finnigan Sorptomatic 1990 nitrogen adsorptionꢀ 22.
desorption analyzer. Sample was oven dried at 100 °C for 12 h
before analysis. Degassing of sample was conducted for 24 hours 23.
with 8 hours slow degassing followed by 16 h fast degassing before
analysis.
Acknowledgements
24.
The authors thank Universiti Malaysia Pahang, Universiti Putra
Malaysia and the Ministry of Education for Research Acculturation
Collaborative Effort (RACE) and Universiti Malaysia Pahang
Internal Grants Scheme (RDU121301 as well as 120363). We also
would like to thank the Ministry of Education and Universiti
Malaysia Pahang for funding MyBrain15 as well as Graduate
Research Scheme (GRS) scholarship, respectively.
2
2
5.
6.
2
7.
K. Jagadeeswaraiah, C. R. Kumar, P. S. S. Prasad, S.
Loridant, and N. Lingaiah, Appl. Catal. A Gen., 2014, 469,
1
65–172.
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