Zinc hydroxystannate: A promising solid acid-base bifunctional catalyst

Article abstract of DOI:10.1039/c3ra44370a

Zinc hydroxystannate is shown to be a highly active and selective solid bifunctional catalyst for carbonylation of glycerol with urea. It has strong basicity with Lewis acidic Zn, low hygroscopicity and good thermal stability which makes it a potential catalyst for heterogeneous acid-base catalysed reactions. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3ra44370a

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Zinc hydroxystannate: a promising solid acid–base
bifunctional catalyst†
Cite this: RSC Adv., 2014, 4, 974
*
Swetha Sandesh, Ganapati V. Shanbhag and A. B. Halgeri
Received 14th August 2013
Accepted 7th November 2013
DOI: 10.1039/c3ra44370a
www.rsc.org/advances
Zinc hydroxystannate is shown to be a highly active and selective solid Synthesis of zinc hydroxystannate material was rst reported by
bifunctional catalyst for carbonylation of glycerol with urea. It has Basciano et al.,7 and it was also synthesized as controlled
strong basicity with Lewis acidic Zn, low hygroscopicity and good nanocubes for nanodevice applications.8 Metal hydrox-
thermal stability which makes it a potential catalyst for heterogeneous ystannates, in general, have been applied as gas sensors,9
acid–base catalysed reactions.
thermal decomposition material10 and also studied as photo-
catalyst by many researchers,11,12 but have not been used so far
as heterogeneous catalyst for organic transformations.
Glycerol carbonate (GlyC) is an industrially important product
having wide applications in various elds like the pharmaceu-
tical, polymer and paint industries. Synthesis of GlyC from
glycerol, a byproduct of biodiesel synthesis, and urea, a cheap
and abundant chemical has made this reaction more attractive.
Many research groups are working on designing solid base
catalyst for the synthesis of GlyC from carbonylation of glycerol
with urea or alkyl carbonates. The heterogeneous catalysts, such
as lanthanum oxide,1 zinc hydrotalcite2 and recently Fujita
et al.,3 reported homogeneous Zn species as catalysts for this
reaction. However, these catalysts showed activity with
moderate yields under energy intensive vacuum conditions.
Lopez and his co workers4 reported an expensive Au–Pd/MgO
catalyst, where ow of nitrogen was used to remove byproduct,
ammonia. Due to a highly viscous nature of glycerol, various
solvents were used to get high GlyC yield with metallic or
organometallic sulphates as catalysts.5
ZnSn(OH)6 (hereaer, ZHS) was prepared by chemical
co-precipitation method using zinc chloride (ZnCl2) and tin
chloride (SnCl4$5H2O) with NaOH as a co-precipitant. The
precipitate was stirred for 2 h at room temperature, ltered,
washed repeatedly with distilled water till free from chloride
and dried in an oven at 120 ꢀC for 6 h and then activated at
ꢀ
170 C for 1 h under static air. Same method was followed to
synthesize other MSn(OH)6 (M ¼ Mg, Ca) using different MCl2.6
To get more insight on its properties, ZHS was converted into its
ꢀ
oxide by calcining at 600 C for 5 h (named as ZHS-calc). The
XRD patterns conrmed the formation of the above compounds
(Fig. 2). MgO, CaO, ZnO, SnO2 were procured commercially and
ꢀ
activated at 800 C. Mg/Al hydrotalcite (HTc Mg/Al) and Zn/Al
hydrotalcite (HTc Zn/Al) were synthesized as per literature
method.13
In the present work, zinc hydroxystannate (ZnSn(OH)6) has
been reported for the rst time as a solid bifunctional catalyst
and has been applied for the synthesis of GlyC from glycerol and
urea. Crystal Structure of ZHS along ‘c’ direction in the unit cell
is represented in Fig. 1. It has a perovskite type crystal structure
with metal atoms octahedrally coordinated with corner sharing
hydroxyl groups to form Sn(OH)6 and Zn(OH)6 octahedra.6
Materials science division, Poornaprajna Institute of Scientic Research, (PPISR),
Bidalur Post, Devanahalli, Bangalore-562110, India. E-mail: shanbhag@
poornaprajna.org; Fax: +91 2361 1836; Tel: +91 2740 8552
† Electronic supplementary information (ESI) available: Fig. S1. Basicity
measurement of all catalyst by Hammett indicator method. Fig. S2. FT-IR
spectra of ZHS catalyst. Fig. S3. XRD pattern of recycled catalyst. See DOI:
10.1039/c3ra44370a
Fig. 1 Perovskite structure of ZHS.
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Fig. 4 SEM images of ZHS catalyst.
SEM and BET surface area by N2 sorption measurement were
performed to determine the morphology of the ZHS catalyst. It
exhibited uniform cuboid shape with 2 mm particle size as
shown in Fig. 4 and the N2 adsorption measurement showed
Fig. 2 XRD patterns of synthesized catalysts.
very low BET surface area of 7 m2 gꢂ1
.
ZHS was characterised by using various techniques like XRD,
FTIR, TGA, SEM, N2 sorption and basicity measurement by
Hammett indicators. All the diffraction peaks in the XRD
pattern could be indexed to cubic ZHS with a lattice parameter
a ¼ 7.80 Aꢀ. It is also conrmed that ZHS crystallizes with space
The qualitative estimation of basicity in synthesized catalysts
were performed using Hammett indicators. The procedure was
followed as described by J. M. Fraile et al.15 using neutral red
(pKBH+ 6.8–8.0), phenolphthalein (pKBH+ 8.0–9.6), nile blue
(pKBH+ 10.1–11.1) and tropaeolin-0 (pKBH+ 11.1–12.7) indica-
tors. The change in colour from nile blue to pink in ZHS and
other MSn(OH)6 indicated the strength of basicity in the range
pKBH+ ¼ 10.1–11.1 (ESI Table S1†). The amount of basicity
present in all the catalysts was measured by titration method.
ZHS contained basicity of 33 mmol gꢂ1, higher than other solid
base catalysts such as CaO, MgO, HTc MgAl and HTc ZnAl
(Table 1). No leachable basicity was observed for all the above
solid catalysts.
ꢀ
group Pn3 according to the standard XRD data le source
(ICDS ¼ #27767). For ZHS catalyst, the hkl values were indexed
and the major diffraction peaks at 2q values of 19.6, 22.6, 32.5,
36.2, 38.4, 40.1, 46.7, 52.4, 57.8 are ascribed to (011), (111),
(112), (122), (103), (113), (213), (313), and (233) crystal planes of
cubic ZHS respectively (Fig. 2). XRD pattern of all other
MSn(OH)6 catalysts were matched with ICSD patterns.
The thermal stability of the ZHS catalyst was determined by
TGA studies (Fig. 3). Interestingly, there was only 0.5% weight
loss up to 250 ꢀC which indicates the presence of very low
amount of water of hydration in ZHS. This property of low
hygroscopicity is very useful for any base catalysts, where it is
not necessary to maintain moisture free conditions for carrying
out reactions. Hydrophobic environment on the catalyst surface
of bifunctional catalyst plays an important role to achieve high
activity for acid–base catalysed reactions.14 Also the material has
The results of GlyC synthesis from glycerol and urea through
carbonylation reaction using various catalysts are represented
in Table 1. ZHS catalyst showed the maximum glycerol
Table 1 Catalytic activity of different catalystsa
Basicity (indicator method)
ꢀ
reasonably good thermal stability of ꢁ250 C and can be used
Total (mmol gꢂ1
)
Glycerol
conversion
(mol%)
GlyC
selectivity
(mol%)
for the reaction bꢀelow this temperature. The weight loss in ZHS
started aer 250 C with a rapid loss of hydroxyl groups which
led to the formation of oxide.
a ¼ phenolphthalein/
Catalyst
b ¼ nile blue
Blank
ZHS
ZHS-calc
CaSn(OH)6
MgSn(OH)6
ZnO
SnO2
Sn(OH)4
MgO
—
0.1
98
64
90.5
86
30.6
30.2
46.8
12.6
20.2
10
99.9
99.6
99.2
92
98.8
99.2
99.4
99.4
99
99.6
99
99.5
99.6
99.1
99.5
33b
20a/3b
7a/22b
23a/11b
10a
No colour change
3a
13a
18a
26a
18a
—
CaO
HTc (Mg/Al)
HTc (Zn/Al)
Recycle 1
Recycle 2
Recycle 3
65
97.8
96.4
95.2
—
—
a
Reaction conditions: glycerol : urea; 1 : 1; catalyst weight ¼ 5 wt%
temperature ¼ 165 ꢀC, time ¼ 5 h. Basicity error limit ¼ ꢃ1 mmol gꢂ1
.
Recycle catalyst: ZHS.
Fig. 3 Thermal analysis of ZHS catalyst.
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conversion of 98% with almost 100% GlyC selectivity. Similarly,
CaSn(OH)6 and MgSn(OH)6 showed high glycerol conversion of
90.5 and 86% respectively. The calcination of ZHS to 600 ꢀC
decreased the basicity from 33 to 23 mmol gꢂ1 and hence
conversion decreased from 98 to 64%. Mild acidic ZnO and
SnO2, and basic oxides like MgO and CaO showed low activity
for glycerol carbonylation. The higher activity of ZHS (98%)
compared with ZnAl HTc (65%) could be due to difference in
their basicities. The results show that in addition to the strong
basicity, Lewis acidic Zn in the catalyst may also act as a
promoter for carbonylation of glycerol with urea. This is in
agreement with other reports that Lewis acidic Zn helps in
improving the yield of GlyC.‡,2,3
Effect of reaction time was studied for carbonylation reaction
of glycerol with urea under the reaction condition shown in
Table 1. The conversion was 17% for 1 h and increased with the
increase in reaction time and reached 98% for 5 h and remained
constant thereaer (Fig. 5).
Scheme 1 Reaction mechanism of ZHS catalyst with glycerol and
urea.
For leaching test, the reaction was carried out with ZHS for
two hour under reaction conditions shown in Table 1. The
reaction was then stopped and the reaction mixture was
centrifuged to separate the catalyst (glycerol conversion ¼ 23%).
The reaction was then continued without the catalyst for
another 3 hours.
There was no increase in glycerol conversion aer removing
the catalyst which indicated the absence of leached active sites
and the reaction was truely heterogeneous. It is further
conrmed by atomic absorption spectrometer analysis of reac-
tion mixture for the presence of Zn aer separating the catalyst.
The measurement showed the absence of Zn in the reaction
mixture (detection limit ¼ 0.01 ppm). Hence it was conrmed
that there was no leaching of active sites during the carbonyl-
ation reaction with ZHS.
Reusability of ZHS catalyst was carried out for 3 successive
runs by separating catalyst from the reaction media followed by
washing with acetone and dried at 120 ꢀC for 2 h. Only marginal
decrease in conversion by 2.8% aer 3 catalyst recycles was
observed as shown in Table 1. XRD patterns of both fresh and
recycled catalysts indicated that there was no change in phase
purity aer three recycles (ESI Fig. S2†).
Based on the above results, a plausible reaction mechanism
is proposed for ZHS catalyzed carbonylation of glycerol with
urea. Scheme 1 shows that basic site in the ZHS activates the OH
group of glycerol by removing proton whereas Lewis acid Zn
activates the carbonyl group (C]O) of urea.
The carbonylation product is formed by the removal of NH3
in the 2nd step. This intermediate product undergoes cyclization
by losing another molecule of NH3 to form GlyC. Since the
reaction is reversible, the extraction of NH3 is essential to get
high conversion of glycerol to GlyC and hence nitrogen was
bubbled to remove NH3 during the reaction.
Further work on the general applicability of metal hydrox-
ystannate as bifunctional or basic catalyst for other organic
transformations is under progress. The initial screening of the
catalyst showed good activity and selectivity for the trans-
esterication of glycerol with dimethyl carbonate (ESI Table S2†).
In conclusion, ZHS is a novel, bifunctional solid catalyst for
the carbonylation of glycerol with urea. It has strong basicity
and good thermal stability. Lewis acidic zinc present in the
catalyst facilitates the reaction to give a high product yield.
Although it is a double metal hydroxide, it possesses low
hygroscopicity which makes it a potential catalyst for other base
catalyzed organic transformations. It showed excellent glycerol
conversion of 98% with almost 100% glycerol carbonate selec-
tivity and also showed good reusability.
Swetha S acknowledges CSIR, New Delhi for providing Senior
Research Fellowship and also thankful to Manipal University
for permitting this research as a part of the Ph.D programme.
Notes and references
‡ Experimental procedure: carbonylation reaction of glycerol and urea was per-
formed using 2 necked round bottom ask with glycerol to urea mol ratio of 1 : 1,
5 wt% catalyst with respect to total reactant weight. N2 was bubbled to remove NH3
formed during the reaction. The mixture was stirred vigorously at 165 ꢀC under
atmospheric pressure. Obtained product was analysed using GC-FID with Sta-
bilwax column and conrmed through GCMS analysis.
Basicity measurements: the basicity was determined quantitatively by titration with
0.01 M benzoic acid. In a typical procedure, the catalyst was suspended (0.15 g) in
a toluene solution of nile blue/phenolphthalein (2 ml, 0.1 mg mlꢂ1) and stirred for
30 min, and then titrated with a toluene solution of benzoic acid (0.01 M) to
determine the total basicity. The indicator changed colour from colourless to pink
with respect to phenolphthalein and blue to pink in case of nile blue. The colour
Fig. 5 Effect of reaction time using ZHS catalyst.
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returned to colourless (for phenolphthalein) or blue (for nile blue) when the
surface was neutralized with benzoic acid. For leachable basicity measurement,
catalyst (0.5 g) was shaken in water (50 ml) for 1 h at room temperature, ltered
and a methanol solution of nile blue/phenolphthalein (5 ml, 0.1 mg mlꢂ1) was
added to the ltrate. It was titrated with methanolic solution of benzoic acid (0.01
M) to determine the leachable basicity.
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