Synthesis of Zn/Al mixed-oxide catalyst for carbonylation of glycerol with urea

Article abstract of DOI:10.1007/s11164-015-2336-3

Zn/Al mixed oxide was prepared by the coprecipitation or the hydrothermal method under different conditions and used as catalyst for synthesis of glycerol carbonate by carbonylation of glycerol with urea. The physical properties of the prepared Zn/Al mixe

Full text of DOI:10.1007/s11164-015-2336-3

Res Chem Intermed
DOI 10.1007/s11164-015-2336-3
Synthesis of Zn/Al mixed-oxide catalyst
for carbonylation of glycerol with urea
Young Bok Ryu1,2 Ji Sun Kim1 Kyeong Ho Kim1
•
•
•
Yangdo Kim2 Man Sig Lee1
•
Received: 10 July 2015 / Accepted: 22 October 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract Zn/Al mixed oxide was prepared by the coprecipitation or the
hydrothermal method under different conditions and used as catalyst for synthesis of
glycerol carbonate by carbonylation of glycerol with urea. The physical properties
of the prepared Zn/Al mixed oxide particles were investigated, as well as their
activity as catalyst in the mentioned synthesis. The dried Zn/Al mixed-oxide par-
ticles prepared by the coprecipitation method showed higher activity in synthesis of
glycerol carbonate than those prepared by the hydrothermal method. The Zn/Al
mixed oxide prepared by the coprecipitation method without NaNO3 showed the
highest catalytic activity in synthesis of glycerol carbonate.
Keywords Glycerol Á Glycerol carbonate Á Zn/Al mixed oxide Á Carbonylation
Introduction
Biodiesel as defined by ASTM D6751-12 is fuel comprising monoalkyl esters of
long-chain fatty acids derived from vegetable oils or animal fats [1]. Use of biodiesel
is increasing and becoming more common worldwide, because it is renewable,
nontoxic, biodegradable, and essentially free of sulfur and aromatics. Also, addition
of biodiesel to diesel can improve its lubricity, ensure complete combustion, and
increase engine performance [2]. However, one major drawback of biodiesel is
&
&
Yangdo Kim
yangdo@pusan.ac.kr
Man Sig Lee
lms5440@kitech.re.kr
1
2
Korea Institute of Industrial Technology (KITECH), Ulsan 681-802, Republic of Korea
Department of Materials Science and Engineering, Pusan National University, Busan 609-735,
Republic of Korea
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Y. B. Ryu et al.
byproduction of large amounts of undesired glycerol during the transesterification
process, resulting in a large surplus in the current glycerol market [3]. Glycerol is
normally generated at the rate of 1 mol glycerol for every 3 mol methyl ester
synthesized, approximately 10 wt% of the total product. Since production of biofuels
from biomass is increasing, such surplus production of glycerol is expected to
continue for years to come. Utilization of glycerol as a platform chemical may
partially compensate for the production costs of biodiesel, making it economically
feasible as well as overcoming the problem of disposal of the surplus crude glycerol
from biodiesel industries [4]. Glycerol is widely available and rich in functionalities.
Recent progress in catalysis has enabled conversion of glycerol to commodity
chemicals such as acrolein, 1,3-propanediol [5], docosahexaenoic acid, polyglyc-
erols, and glycerol carbonate (GC). One important glycerol derivative is GC, which
is widely used as a protic solvent (in resins and plastics), additive, and chemical
intermediate [6]. Due to its low toxicity, low evaporation rate, low flammability, and
moisturizing ability, GC is also used as a wetting agent in cosmetics [7] and carrier
solvent for medical preparations. The main methods for preparation of GC are based
on reaction of glycerol with a carbonate source (phosgene, a dialkyl carbonate [8] or
an alkylene carbonate [9]), urea [10], carbon monoxide [11], carbon dioxide [12],
and oxygen [13, 14]. Concerning preparation of GC from urea and glycerol, the same
authors have patented a process for preparing GC using metallic or organometallic
salts, or supported metallic compounds [14, 15]. As described above, the reaction
between urea and glycerol has been performed using zinc sulfate (ZnSO4) and zinc
oxide (ZnO) catalysts. High yields are reported when using ZnSO4 as catalyst [16,
17], but this salt is soluble in glycerol and the reaction takes place under
homogeneous conditions with the catalyst being partially recovered after the
reaction. Several catalysts have been used in carbonylation of glycerol with urea,
mainly Zn-based and other metal catalysts [18–21]; For example, a high yield of
glycerol carbonate (91 %) was obtained using La2O3 as catalyst, and the conversion
of glycerol reached 98 % when using metal monoglycerolates as catalyst [18].
However, these methods suffer from the serious environmental problems of use of
heavy metals and the nonrenewability and exhaustion of mineral resources.
In this study, we prepared Zn/Al mixed-oxide particles by the coprecipitation and
the hydrothermal method. The physical properties of the prepared Zn/Al mixed-oxide
particles were investigated. We also investigated the effect of synthesis conditions on
the physical properties of the Zn/Al mixed oxide, and examined the activity of the Zn/
Al mixed-oxide particles as catalyst in synthesis of glycerol carbonate.
Experimental
Materials
Glycerol (99.5 %), urea (99 %), glycerol carbonate (99 %), zinc nitrate hexahydrate
[Zn(NO3)2Á6H2O, 98 %], aluminum nitrate nonahydrate [Al(NO3)3Á9H2O, 98 %],
sodium nitrate (NaNO3, 99 %), and tetraethylene glycol (TEG, 99 %) were
purchased from Sigma-Aldrich. Sodium hydroxide (NaOH, 97 %) was purchased
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Synthesis of Zn/Al mixed-oxide catalyst for…
from Daejung Chemicals and Metals. All chemicals were used directly without
further purification or pretreatment.
Preparation of catalysts
Zn/Al mixed oxides were prepared by the coprecipitation and the hydrothermal
method. Specifically, 25.5 g Zn(NO3)2Á6H2O and 13.78 g Al(NO3)3Á9H2O (nominal
Zn:Al molar ratio 7:3) were dissolved in 100 ml deionized water at room
temperature under stirring (solution A). NaOH (11.38 g) and 4.74 g NaNO3 (or not
included) were dissolved in 100 ml deionized water (solution B). Solution B was
added dropwise to a beaker containing 100 ml solution A, resulting in a pH of 13 of
the mixture. The dropping rate of solution B was under 1.0 ml/min for 2 h. Then,
the mixture was aged at 50 °C for 18 h or was transferred to a 250-ml Teflon
container held in a stainless-steel vessel for hydrothermal treatment at 200 °C
during 3 h. After that, the precipitate was filtered and washed to eliminate alkali
metals and nitrate ions until the pH of the washing water was 6–8. The particles
were dried at 105 °C for 24 h and then calcined at 700 °C for 3 h.
Characterization of catalysts
Dried fine powder of synthesized Zn/Al mixed oxide was subjected to thermo-
gravimetric–differential thermal analysis (TG–DTA, TA Instruments SDT Q600) to
determine the temperature of possible decomposition and phase changes. The
samples were heated at a rate of 10 °C/min from 30 to 1400 °C.
The major phase of the obtained particles was analyzed by X-ray diffraction
(XRD, Philips X’Pert) using Cu Ka radiation.
The surface acidity and basicity of the catalysts were measured by temperature
programmed desorption (TPD) of NH3 and CO2, and carred out on TPD flow system
(Micromeritics AutoChem II 2920) equipped with a thermal conductivity detector
(TCD). Each sample (100 mg) was preheated in He flow at 500 °C. Adsorption of NH3
(or CO2) was performed at 50 °C, followed by He purge to remove physisorbed NH3
(or CO2). The desorption process was performed ata heating rate of 10 °C/min from 50
to 800 °C.
The particle size and external morphology of the prepared particles were
observed using a field-emission scanning electron microscope (FE-SEM, Hitachi,
SU-8020) at accelerating voltage of 30.0 kV.
Activity test
The catalytic performance of the Zn/Al mixed oxide was evaluated in a 100-ml
round-bottomed flask equipped with a vacuum pump. In a typical experiment,
0.2 mol glycerol and 0.2 mol urea were first fed to the reactor and stirred at a
constant rate. After attainment of the desired temperature (140 °C), a known
quantity of catalyst (5 wt% of glycerol) was added to the reactor. Then, the reaction
temperature was ramped to 160 °C under vacuum to remove ammonia as a
byproduct. The reaction was maintained for 5 h, then the reactants and products
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Y. B. Ryu et al.
were analyzed using a gas chromatograph (Agilent 7890A) equipped with a flame
ionization detector (FID) and
a
capillary column (HP-INNOWax,
30 m 9 0.32 mm 9 0.25 lm). Tetraethylene glycol (TEG) was used as internal
standard. The conversion and selectivity were calculated based on the assumption
that glycerol was the limiting reactant.
Results and discussion
Characterization of catalysts
TG–DTA
Figure 1 shows typical DTA and TGA thermodiagrams for the Zn/Al mixed-oxide
particles prepared under different conditions. There are three obvious mass loss
stages in the TG curve. The initial stage under 100 °C can be associated with release
of superficial absorbed water. Zn/Al mixed-oxide particles showed an endothermic
peak at about 100 °C [22]. It is thought that the endothermic peak is due to free
adsorbed water. The minor endothermic peak at 210 °C is accompanied by weight
loss of 10 %, being attributed to release of hydrate water from the Zn/Al mixed-oxide
particles prepared by coprecipitation. In the temperature range of 200–500 °C, the
exothermic peak at 300 °C is due to decomposition of residual nitrate in all particles.
XRD
Figure 2 shows the XRD patterns of the Zn/Al mixed-oxide particles prepared under
different conditions. These particles were only dried at 105 °C without any
calcination. The patterns of all the Zn/Al mixed-oxide particles show strong peak
reflections from hexagonal ZnO [Joint Committee on Powder Diffraction Standards
(JCPDS) file no. 79-0205] as well as weak peak reflections from cubic ZnAl2O4
(JCPDS file no. 73-1961). In addition, no peak corresponding to alumina crystal phase
was observed for any particles. This means that Al2O3 was well mixed with ZnO.
The Zn/Al mixed-oxide particles prepared by the hydrothermal method exhibited
higher crystallinity than those prepared by coprecipitation. It was confirmed that the
Zn/Al mixed-oxide particles prepared by the hydrothermal method could reach high
crystallinity without calcination. The peak at 2h of 29.5° is probably caused by
residual NaNO3 in the particles [23].
TPD
Temperature-programmed desorption (TPD) is one of the most widely used and
flexible techniques for characterizing oxide surfaces. Analysis of the quantity and
strength of active (acidic and basic) sites on particles is crucial to understand and
predict catalyst performance. According to previous reports [18, 24], desorption
peaks can be roughly divided into three regions, corresponding to weak
(50–150 °C), moderate (150–400 °C), and strong ([400 °C) sites.
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Synthesis of Zn/Al mixed-oxide catalyst for…
Fig. 1 TGA (a) and DTA (b) curves of Zn/Al mixed-oxide particles prepared under different conditions
Figure 3 shows the NH3 (Fig. 3a) and CO2 (Fig. 3b) TPD profiles for the Zn/Al
mixed-oxide particles prepared under different conditions. All the profiles in Fig. 3a
show weakly and moderately acidic sites. The Zn/Al mixed-oxide particles prepared
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Y. B. Ryu et al.
Fig. 2 XRD pattern of Zn/Al mixed-oxide particles prepared under different conditions
by coprecipitation using NaNO3 showed a strong desorption peak at 600 °C
(strongly acidic site).
The CO2 desorption profiles in Fig. 3b show moderately and strongly basic sites.
The total basicities of the samples follow the sequence: coprecipitation
(NaNO3) [ coprecipitation [ hydrothermal [ hydrothermal (NaNO3). It should
also be noted that the basicity of the moderately and strongly (but not too strongly)
basic sites of the samples follows the order: coprecipitation [ coprecipitation
(NaNO3) [ hydrothermal [ hydrothermal (NaNO3). The Zn/Al mixed-oxide par-
ticles prepared by coprecipitation using NaNO3 showed a too strong desorption peak
at 600 °C (too strongly basic site).
The peak area can be considered to represent the amount of active sites. The
acidity (acidic site density) and basicity (basic site density) of the Zn/Al mixed-
oxide particles are listed in Table 1.
SEM
The surface morphology of Zn/Al mixed-oxide particles prepared under different
conditions was studied by scanning electron microscopy (SEM); the images
obtained are shown in Fig. 4. The Zn/Al mixed-oxide particles exhibited irregular
particle shape such as rod, dumbbell, etc., thus being constructed from a net-like,
porous structure. In addition, the surface of the Zn/Al mixed-oxide particles
prepared using NaNO3 comprised smaller aggregates with variable morphologies.
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Synthesis of Zn/Al mixed-oxide catalyst for…
Fig. 3 a NH3–TPD and b CO2–TPD profiles of Zn/Al mixed-oxide particles prepared under different
conditions
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Synthesis of Zn/Al mixed-oxide catalyst for…
Fig. 4 SEM images of Zn/Al mixed-oxide particles prepared under different conditions:
a coprecipitation, b hydrothermal, c coprecipitation (NaNO3), and d hydrothermal (NaNO3)
Activity of catalysts
Figure 5 shows the catalytic activity of the Zn/Al mixed-oxide particles in terms of
glycerol conversion, glycerol carbonate selectivity, and yield. The yield of glycerol
carbonate obtained through the catalyzed reaction was significantly higher
compared with the uncatalyzed homogeneous reaction. In the blank test in the
absence of catalyst, a GC yield of 22.2 % was obtained with glycerol conversion of
22.2 %. It is apparent that the important role of the catalyst increased the conversion
of glycerol under our reaction conditions.
The conversion of glycerol increased with increasing basicity of the catalyst
(Table 2). A possible reason for this variation in catalytic activity may derive from
the various surface acid–base properties, as measured by CO2–TPD. The catalytic
activity can be roughly correlated with the order of basicity, suggesting that the
basicity (expect for too strongly basic) plays an important role in the superior
activity. A possible reason for the lower activity achieved after coprecipitation with
NaNO3 than coprecipitation is the too strong basicity, thus resulting in strong
interaction with the reactants and blocking of active sites [25].
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Conversion
Selectivity
Yield
100
80
60
40
20
0
Blank
Coprecipitation Coprecipitation Hydrothermal
(NaNO3)
Hydrothermal
(NaNO3)
Fig. 5 Activities of different catalysts for synthesis of GC
Table 2 Catalytic activity and physical properties of Zn/Al mixed-oxide particles prepared under dif-
ferent conditions
Conditions
Conversion
(%)
Selectivity
(%)
Yield
(%)
Basic site density
(mmol/g)
Intensity
(XRD)
Blank
22.2
87.2
82.0
100
22.2
82.4
78.6
–
–
Coprecipitation
94.4
95.8
2.76
559
381
Coprecipitation
(NaNO3)
4.44 (1.92, excluding too
strongly basic site)
Hydrothermal
73.7
58.9
99.6
94.2
73.3
55.5
1.56
1.44
818
925
Hydrothermal
(NaNO3)
The selectivity for GC increased with increasing diffraction peak intensity. The
Zn/Al mixed-oxide particles prepared using NaNO3 exhibited lower conversion of
glycerol than those prepared without NaNO3. This suggests that the effect of the
synthesis temperature in the preparation of Zn/Al mixed oxides in these systems by
the hydrothermal method is worthy of study in future work.
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Synthesis of Zn/Al mixed-oxide catalyst for…
Conclusions
We prepared Zn/Al mixed oxides by the coprecipitation and the hydrothermal
method. The physical properties of the prepared Zn/Al mixed oxides were
investigated, as well as their activity as catalyst for synthesis of glycerol carbonate
by carbonylation of glycerol with urea. The Zn/Al mixed oxides prepared by the
coprecipitation method without NaNO3 showed the highest catalytic activity in
synthesis of glycerol carbonate. The conversion of glycerol increased with
increasing catalyst basicity, and the selectivity for GC increased with increasing
diffraction peak intensity.
Acknowledgments This research was financially supported by the Technology Innovation Develop-
ment Program (Code No. SE140001) of the Small & Business Administration (SMBA) and also by the
Project of Research and Development for Regional Industry (Code No. R0003028) through the Ministry
of Trade, Industry, & Energy (MOTIE) and Korea Institute for Advancement of Technology (KIAT).
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