Advanced Materials Research
ISSN: 1662-8985, Vol. 699, pp 155-160
doi:10.4028/www.scientific.net/AMR.699.155
© 2013 Trans Tech Publications, Switzerland
Online: 2013-05-27
Ionic Solid Nanomaterials: Synthesis, Characterization and Catalytic
Properties Investigation
Nur Aainaa Syahirah Ramli1,a, Nor Aishah Saidina Amin2,b
1,2
Chemical Reaction Engineering Group (CREG), Energy Research Alliance, Faculty of Chemical
Engineering, Universiti Teknologi Malaysia, 81310 UTM, Skudai, Johor, Malaysia
a
[email protected], [email protected]
Keywords: Ionic Solid, Synthesis, Characterization, Catalytic Properties
Abstract. A series of ionic solid nanomaterials denoted as IS1, IS2 and IS3 have been prepared using
butylmethylimidazolium bromide ([BMIM][Br]) ionic liquid as cation, and three types of
heteropolyacid; phosphotungstic acid (H3PW12O40), phosphomolybdic acid (H3PMo12O40), and
silicotungstic acid (H4SiW12O40) as anion. The nanomaterials were characterized by FTIR, XRD,
SEM, TGA, NH3-TPD and BET. Its catalytic performance was investigated by catalyzing glucose
conversion to levulinic acid and hydroxymethylfurfural. It was observed that the ionic solids have
higher acidity with semi amorphous structure, higher thermal stability and insignificant water content
compared to the parent compound. Among the three prepared ionic solids, phosphomolybdic based
ionic solid (IS2) exhibited the best catalytic performance due to its highest total acidity.
Introduction
Ionic solid (IS) is a nanosize material that has several advantages as heterogeneous catalyst. Its
preparation method is simple and displayed good catalytic performance [1-3]. Ionic solid is prepared
by combining heteropolyacid (HPA) as an anion with ionic liquid (IL) as a cation. Ionic liquid is an
organic compound with variety attractive properties such as high stability, negligible vapour pressure,
and non-flammable [4]. HPA is known as active catalyst in homogeneous and heterogeneous
reactions[5]. Bronsted acidity of HPA is stronger than conventional solid acids such as SiO2-Al2O3
and zeolites [6]. In this paper, a series of IS were prepared, characterized, and their catalytic
properties were investigated in the conversion of glucose to levulinic acid (LA) and
hydroxymethylfurfural (HMF).
Experimental
Synthesis, Characterization and Catalytic Properties Investigation of Ionic Solids
Ionic solids were prepared by mixing [BMIM][Br] with respective aqueous HPA at room
temperature with constant stirring for 1 hour. In order to prepare one mole of [BMIM]3[PW12O40] and
one mole of [BMIM]3[PMo12O40], molar ratio of 3:1 were taken for IL:HPA. Meanwhile, IL and
HPA were used at molar ratio 4:1 for the synthesis of [BMIM]4[SiW12O40]. The precipitate formed
was filtered, washed, and dried overnight at 85°C. The prepared ionic solids are denoted as IS1, IS2
and IS3 for [BMIM]3[PW12O40], [BMIM]3[PMo12O40], and [BMIM]4[SiW12O40], respectively.
All IS were characterized by XRD employing High Resolution X-Ray Diffractometer using
CuKα radiation with wavelength of 1.54Ǻ. The FTIR specta were recorded in the range of
400-4000cm-1. Elementary (CHN) analysis was performed using ThermoFinnigan EA1112 elemental
analyzer. TGA of the samples were conducted under N2 gas flow; samples were heated from ambient
to 800°C at a heating rate of 20°C/min on a Mettler Toledo TGA/SDTA851e. SEM images were
taken using JEOL JSM-6390LV, operated at 15kV. The acidity properties evaluation was done using
ammonia (NH3) TPD method using a TCD detector. Surface area of IS was measured using
Micromeritics ASAP2020 analyzer.
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Materials Science and Chemical Engineering
The catalytic performances were tested for glucose conversion in a 100ml stirred autoclave.
Initially, 1g of glucose was mixed with 50 ml water before 1g of catalyst was charged. The
experiments were conducted at 170°C for 180min with stirring rate of 250 rpm. HPLC system was
used with Aminex HPX-87H column for monosaccharide and organic acid analysis. UV 210nm
detector was used to detect LA and HMF while refractive index detector was used to detect glucose.
Glucose conversion and products yield were calculated based on the glucose weight.
Results and Discussion
Characterization of Ionic Solid
The IR spectra of IS and its parent compounds are illustrated in Fig. 1. The IL, [BMIM][Br]
spectrum exhibited characteristic peaks of imidazolium ring C-H stretch (3088–3147cm-1), aliphatic
C-H stretch (2874–2962cm-1), imidazolium ring stretch (1573cm-1), imidazolium H-C-C and H-C-N
bending (1168cm-1), in plane imidazolium ring bending (847cm-1), out of plane C-H imidazole ring
bending (754cm-1), and imidazole C2-N1-C5 bending (623cm-1). All solid IS have insignificant water
content, while the parent HPA compounds contained large amount of water. Water content presence
was identified at 3450cm-1 and 1630cm-1 vibration for O-H stretch and bending, respectively. The IR
band for imidazole C-H stretch (3000–3170cm-1) signified a strong interaction between [BMIM] and
HPA. The interaction resulted in splitting of the imidazole C-H stretch peaks of [BMIM][Br] into
several peaks for IS. The parent compound of H3PW12O40 displayed vibration modes of P-O stretch
(1080cm-1), W=Oter stretch (984cm-1), W-Oc-W stretch (891cm-1), and W-Oe-W stretch (802cm-1).
H3PMo12O40 revealed bands of P-O stretch (1065cm-1), Mo=Oter stretch (950cm-1), Mo-Oc-Mo stretch
(847cm-1), and Mo-Oe-Mo stretch (755cm-1), while H4SiW12O40 expressed Si-O stretch (1018,
925cm-1), W=Oter stretch (980cm-1), W-Oc-W stretch (881cm-1), and W-Oe-W stretch (784cm-1).
During IS preparation, HPA go through structural alteration where most of the water
molecules have been replaced by [BMIM]. The XRD pattern (Fig. 2) exhibit there is one intense peak
at 8.5o, 9.0o and 9.0o for IS1, IS2 and IS3, respectively. These peaks specified the semi amorphous
structure of the nanomaterial [7]. The average particle size of IS was calculated using Scherrer’s
formula, with size of 11.7nm, 6.6nm and 9.3nm for IS1, IS2 and IS3, respectively. The morphology
of powdered IS, examined using SEM, clarified small bunch of irregular forms and nanosize adhere
thread-like particles (Fig. 3). The porosity of the IS has been analyzed by nitrogen sorption analysis.
The isotherm adsorption exhibited Type II isotherm; where the absence of hysteresis indicated
adsorption and desorption of a non-porous surface. The surface area of IS was calculated from BET
method to be 2.2m2/g, 2.0m2/g, and 1.7m2/g for IS1, IS2 and IS3, respectively. The low surface area
of the compounds is attributed to the low surface area of HPA, ~2m2/g [6].
The thermal stability of IS and its HPA parent compound are examined using TGA (Fig. 4).
The TGA curves for HPA demonstrated two regions of weight loss: due to evaporation of water and
decomposition of HPA. [BMIM][Br] IL parent compound totally decomposed at 800°C [8]. The
thermal stability of IS were improved compared to the parent compounds [2]. As discussed in FTIR
study, IS have insignificant water content and is proven from the insignificant weight loss below
300°C. The bonding between [BMIM] and HPA have removed the water content. Significant weight
loss in IS was scrutinized above 300°C and 500°C, suggesting the decomposition of IL and HPA
respectively. The thermal stability of IS3 was lower compared to the other two IS. The differences
might be related to the number of [BMIM] present in each IS.
Advanced Materials Research Vol. 699
157
(c)
Transmittance (a.u.)
Transmittance (a.u.)
(c)
(b)
(a)
500
1000
1500
2000
2500
-13000
Wavenumber (cm )
3500
(b)
(a)
500
4000
1000
1500
2000
2500 -1 3000
Wavenumber (cm )
3500
4000
(i) IS1 or [BMIM]3[PW12O40] and parent compounds (ii) IS2 or [BMIM]3[PMo12O40] and parent
compounds
Transmittance (a.u.)
(c)
(b)
(a)
500
1000
1500
2000
2500
-13000
Wavenumber (cm )
3500
4000
(iii) IS3 or [BMIM]4[SiW12O40] and parent compounds
Fig. 1 FTIR spectra of (a) HPA, (b) [BMIM][Br], (c) ionic solid
20
40
2θ (degree)
60
Intensity (a.u)
Intensity (a.u)
0
[BMIM]4[SiW12O40]
[BMIM]3[PMo12O40]
[BMIM]3[PW12O40]
80 0
20
40
2θ (degree)
60
80 0
20
40
2θ (degree)
60
80
Fig. 2 XRD pattern of IS1, IS2 and IS3
The thermal stability of IS and its HPA parent compound are examined using TGA (Fig. 4).
The TGA curves for HPA demonstrated two regions of weight loss; due to evaporation of water and
decomposition of HPA. [BMIM][Br] IL parent compound totally decomposed at 800°C [8]. It can be
seen that thermal stability of IS were improved compared to the parent compounds [2]. As discussed
in FTIR study, IS have insignificant water content and is proven from the insignificant weight loss
below 300°C. The bonding between [BMIM] and HPA have removed the water content. Significant
weight loss in IS was scrutinized above 300°C and 500°C, suggesting the decomposition of IL and
HPA respectively. By comparison, thermal stability of IS3 was lower compared to the other two IS.
The differences might be related to the number of [BMIM] present in each IS.
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Materials Science and Chemical Engineering
(a)
(b)
Fig. 3 SEM images of (a) IS1, (b) IS2 and (c) IS3
100
95
95
90
H3PW 12O40
85
H4SiW 12O40
80
H3PMo12O40
[BMIM]3[PW 12O40]
90
Weight (%)
Weight (%)
100
(c)
85
[BMIM]3[PMo12O40]
80
[BMIM]4[SiW 12O40]
75
75
0
100
200
300
400
500
o
Temperature ( C)
600
700
0
100
200
300
400
500
o
Temperature ( C)
600
700
Fig. 4 TGA curves of IS and HPA parent compounds
According to desorption temperature, the acidity sites are classified into three: weak
(150-300°C), medium (300-500°C), and strong (500-650°C) acid sites. Two peaks were found at
medium and strong acid sites for IS1 (447, 600°C) and IS3 (427, 582°C), while there were three peaks
for IS2; two for medium acid site (302,418°C), and one for strong acid site (523°C). The total
acidities of the prepared IS were in an order of IS2 > IS3 >IS1 with values of 7.13mmol/g, 4.3mmol/g,
3.43mmol/g, respectively. The high acidity of IS should be beneficial for heterogeneous catalytic
systems such as hydration, dehydration, isomerization, and hydrolysis since HPA and IL have been
used for those reactions [9-11].
Catalytic properties of ionic solid
The distinction of glucose conversion and products yield emphasize that catalyst properties
have influenced its activity (Fig. 5). The general reaction pathway of glucose conversion is glucose
was first isomerized to produce fructose. Fructose then goes through three steps of dehydration to be
converted to HMF. With the effect of acidic solution, HMF combined with water and rehydrated to
produce LA. From the overall results, glucose conversion was higher over IS compared to HPA. The
catalytic activity of IS came from both parent compounds. Among all the ionic solids, IS3 gave the
highest glucose conversion since it contained the highest amount of IL cation, [BMIM]+. In addition,
glucose conversion for IS2 was higher compared to IS1. It can be seen that [BMIM]+ part in IS was
more effective towards glucose conversion, since the performance using [BMIM][Br] was
comparable to IS3 as catalyst. The acid concentrations also played major roles for enhancing glucose
conversion.
The anion in IL acted as nucleophile and promoted fructose dehydration to HMF [12]; proven
by the highest HMF yield over [BMIM][Br] as catalyst. IS1 which gave highest HMF yield inferred
that higher [BMIM]+ amount also promoted fructose dehydration to HMF. The same amount of
Advanced Materials Research Vol. 699
159
[BMIM]+ for IS1 and IS2 resulted in a similar HMF yield. IS3 gave the highest glucose conversion,
IS2 with the highest acidic content produced the highest LA yield. The catalytic testing results
indicate that catalyst acidity plays a major role in glucose conversion to LA [13-15]. Another factor
that influences glucose conversion is the surface area of catalyst [14-16] . Albeit the surface area of IS
was small and may retard glucose conversion, the strong Bronsted acidity of IS seemed to favor the
reaction. Large surface area materials like zeolite, and highly acidic materials like metal chloride
have been used as catalyst for LA production with good yield [14-16]. More recently, the performance
of HPA for LA formation has been attributed to the solubility of HPA in water creating homogeneous
system with Bronsted acidic properties [17].
Fig 5 Catalytic performance of different catalyst on (a) glucose conversion and (b) product yield.
(IS1=[BMIM]3[PW12O40], IS2=[BMIM]3[PMo12O40], IS3=[BMIM]4[SiW12O40], HPA1:H3PW12O40,
HPA2:H3PMo12O40, HPA3:H4SiW12O40 , IL:[BMIM][Br], 1g catalyst, 1g glucose, 170C, 180min)
Conclusions
Ionic solid nanomaterials formed by reaction between IL and HPA have better characteristic and
catalytic performance for glucose conversion to HMF and LA compared to the parent compound. The
catalytic process was demonstrated to be efficient using IS2, where the glucose conversion, LA and
HMF yield reached 60.5wt%, 22.4wt%, and 5.8wt% respectively. The IS performance was attributed
to the strong acidity of HPA and small size of alkyl substituent from the ionic liquid.
Acknowledgement
The authors would like to expresse their sincere gratitude to the Ministry of Higher Education
(MOHE), Malaysia for supporting the project under Research University Grant (vote 02H75) and for
sponsoring one of the authors, N.A.S.Ramli under MyBrain15-MyPhD programme.
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Materials Science and Chemical Engineering
10.4028/www.scientific.net/AMR.699
Ionic Solid Nanomaterials: Synthesis, Characterization and Catalytic Properties Investigation
10.4028/www.scientific.net/AMR.699.155