Vapor phase synthesis of methylpyrazine using aqueous glycerol and ethylenediamine over ZnCr2O4 catalyst: Elucidation of reaction mechanism

Article abstract of DOI:10.1016/j.catcom.2011.03.021

A novel method has been developed for the synthesis of methylpyrazine (MP) by using aqueous glycerol and ethylenediamine (EDA) over Zn-Cr catalyst derived from hydrotalcite precursors. The X-ray diffraction analysis of the oven-dried Zn-Cr samples synthesized at various pH ranging from 7 to 11 showed hydrotalcite phase whereas the calcined catalysts displayed ZnO and ZnCr₂O ₄ phases. The cyclisation activity of Zn-Cr catalyst prepared at pH ～ 9 demonstrated 99.4% conversion of EDA and 94% of glycerol with ～ 72% selectivity to MP at a reaction temperature of 400 °C. This process demonstrates direct utilization of bio-glycerol for the synthesis of MP.

Full text of DOI:10.1016/j.catcom.2011.03.021

Catalysis Communications 12 (2011) 1067–1070
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Vapor phase synthesis of methylpyrazine using aqueous glycerol and
ethylenediamine over ZnCr O catalyst: Elucidation of reaction mechanism
2
4
Reema Sarkari, Chatla Anjaneyulu, Vankudoth Krishna, Ramineni Kishore,
Medak Sudhakar, Akula Venugopal ⁎
Catalysis and Physical Chemistry, Indian Institute of Chemical Technology, Tarnaka – 500 607, Hyderabad, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel method has been developed for the synthesis of methylpyrazine (MP) by using aqueous glycerol and
ethylenediamine (EDA) over Zn–Cr catalyst derived from hydrotalcite precursors. The X-ray diffraction
analysis of the oven-dried Zn–Cr samples synthesized at various pH ranging from 7 to 11 showed hydrotalcite
Received 15 December 2010
Received in revised form 21 February 2011
Accepted 14 March 2011
2 4
phase whereas the calcined catalysts displayed ZnO and ZnCr O phases. The cyclisation activity of Zn–Cr
Available online 22 March 2011
catalyst prepared at pH ~9 demonstrated 99.4% conversion of EDA and 94% of glycerol with ~72% selectivity to
MP at a reaction temperature of 400 °C. This process demonstrates direct utilization of bio-glycerol for the
synthesis of MP.
Keywords:
Hydrotalcite
Methylpyrazine
Glycerol
© 2011 Elsevier B.V. All rights reserved.
Ethylenediamine
ZnCr O
2 4
1
. Introduction
Fuel and energy crisis surge R&D, firstly on alternate fuels for
on the vapor phase synthesis of methylpyrazine, directly using aqueous
glycerol and EDA (instead of the normal procedure that requires
1,2-propylene glycol and EDA) avoiding a process step i.e. the
production of 1,2-propylene glycol by hydrogenolysis of bio-glycerol.
Hence our methodology is exceptionally beneficial from the economical
perspective in the transformation of bio-glycerol into useful com-
pounds. Hydrotalcite like anionic clays are found to be new class of
catalyst precursors for commodity and fine chemical synthesis. An
excellent review recently by Centi et al. reports on the usage of layered
materials for the synthesis of various chemical entities in domestic and
industrial applications [12]. In the present investigation the Zn–Cr
hydrotalcite precursors have been synthesized, characterized and the
calcined form of catalysts is explored for the cyclisation of EDA and
aqueous glycerol. Here we report for the first time, the synthesis of
methylpyrazine using aqueous glycerol and ethylenediamine and a
probable reaction mechanism has been proposed.
future and secondly on clean fuels in the context of pollution
abatement. Recent advancements in the development of bio-diesel
production from non-edible oils seem a promising approach [1–3].
Bio-glycerol is a by-product that is inevitably formed in large amounts
during bio-diesel production by transesterification of oils. On one
hand generation of alternate fuels is a prime objective; on the other
hand utilization or safe disposal of the by-products obtained in the
process is equally important. Conversion of bio-glycerol into value
added compounds and fine chemicals is extensively studied by several
authors [4–6]. One of such processes is the production of 1,2-
propylene glycol (PG) by hydrogenolysis of bio-glycerol over
supported noble metal catalysts at high pressures [7,8].
Methyl pyrazine is an intermediate compound for the synthesis of
2
-amido pyrazine, a well-known bacteriostatic and antitubercular
drug. Conventionally methyl pyrazine is synthesized by cyclisation of
ethylenediamine (EDA) and 1,2-propylene glycol (Scheme 1). Forni
et al. have studied Pd-promoted zinc chromite catalyst for the synthesis
of methyl pyrazine using EDA and 1,2-propylene glycol [9]. Studies
pertaining to the same have been reported by others [10,11].
2. Experimental
The Zn–Cr catalysts employed in this investigation were pre-
pared by a simple coprecipitation method using Zn(NO
3 2 2
) ·6H O and
Cr(NO ·9H O (Sigma–Aldrich, AR grade) with Zn:Cr=2:1 (mole
3
)
3
2
Synthesis of methylpyrazine using aqueous glycerol and EDA is not
reported in the literature. This paper reports our exploratory research
ratio), in order to get the hydrotalcite structure. The various samples
were prepared at pH ranging from 7 to 11 using a mixture of 2 M
2 3
NaOH+1 M Na CO as precipitating agent. The gels were washed
thoroughly, filtered and oven-dried for 12 h at 120 °C, subsequently
calcined in static air at 450 °C for 5 h. The surface properties of the
⁎
Corresponding author. Tel.: +91 40 27193510; fax: +91 4027160921.
E-mail address: akula@iict.res.in (A. Venugopal).
2
fresh and the used samples were measured by N adsorption at
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.03.021

1068
R. Sarkari et al. / Catalysis Communications 12 (2011) 1067–1070
The cyclisation activities over calcined Zn–Cr (HT) catalysts were
performed at 300 to 400 °C at atmospheric pressure in a fixed-bed
vertical quartz reactor (I.D=0.8 cm, length=46 cm) placed in a two-
zone furnace operated in a down flow mode. The first one was pre-
heating zone, which was maintained at 300 °C and the second one
was catalyst bed temperature, both monitored by temperature
controller cum programmers using K-type thermocouple. Glycerol
(
(
Fluka) and EDA supplied by SDFCL, India were used. Nitrogen
IOLAR-I grade, BOC, India) was used as a carrier gas. The cyclisation
activities were carried out using −18/+23 sieved (BSS) catalyst
particles. The carbon mass balance was done based on the inlet and
outlet concentration of the organic moiety. Prior to the reaction about
0
.2 g of calcined catalyst (sieved particles −18/+25 BSS) was reduced
in 5% H balance Ar at 400 °C for 5 h. A 20 wt.% aqueous glycerol
solution was used and the glycerol to EDA mole ratio of 1:1 and flow
2
−
1
rate of the reaction mixture of 2 mL h along with N
2
as carrier gas at
was maintained. The reaction mixture
O:N =1:1:20.5:20.4 (mole ratio) and
the gas hourly space velocity (GHSV)=18909 cc g
−
1
a flow rate of 1800 cc h
contained a glycerol:EDA:H
2
2
−
1
− 1
h
. The
samples were collected after 3 h of continuous operation at each
temperature and analyzed by gas chromatograph (Shimadzu, GC-17A)
equipped with flame ionization detector (FID) using ZB-5 capillary
−
1
column at a ramping rate of 10 °C min
from 60 to 280 °C. The
samples were analyzed by GC-MS (QP5050A Shimadzu) using a ZB-5
capillary column with EI mode. The mass spectra confirmed the product
distribution and the corresponding m/z values for methylpyrazine:
Scheme 1. Proposed reaction mechanism for the formation of methylpyrazine during
dehydrocyclization of glycerol and ethylenediamine over Zn–Cr catalyst.
+
.
+.
+.
+.
+
M
3 3 4
m/z: 94, (M-HCN) m/z: 67, (M-CH CN) m/z: 53, (M-C H N)
+.
+.
−
196 °C in a Autosorb 3000 physical adsorption apparatus. The
specific surface areas were calculated applying the BET method. The
temperature programmed desorption of NH of Zn–Cr samples
m/z: 40; pyrazine: M m/z: 80, (M-HCN)
m/z: 59, (M-NH
{M-(CH
(M-CH
(M-C
m/z: 108; (M-H) m/z: 107; (M-CO) m/z: 80; (M-C
2 2
{M-(H,CO,C H )} m/z: 53; and 2,3-dimethylpyrazine: M m/z: 108;
(M-CH
isolated and analyzed by H NMR spectra which revealed H NMR
(CDCl , 200 MHz): δ=8.32–8.5(m, 3 H); 2.56 (s, 3 H), attributed to
m/z: 53; EDA: (M-H)
+
.
+.
3
)
m/z: 43; glycerol: (M-CH
O)} m/z: 43; 2,5-dimethylpyrazine: M m/z: 108;
2
OH)
m/z: 61;
+
.
+.
+.
OH, H
)
2
3
2
+
.
+.
prepared by varying pH from 7 to 11 were measured using an Auto
Chem 2910 (Micromeritics, USA). In a typical method about 0.1 g of
calcined Zn–Cr sample was reduced at 400 °C 5 h in hydrogen at a
3
m/z: 93; (M-HCN)
m/z: 81; (M-CH
3
CN)
m/z: 67;
+.
3
H
6
N)+. m/z: 52; (M-C
H
4 4
N) +. m/z: 42; pyrazinealdehyde: M
+.
+.
+.
N
2 2
)
m/z: 56;
−
1
+.
+.
flow rate of 30 mL min . After the reductive pretreatment the
sample was saturated with 10% NH balance He at 60 °C at a flow rate
+
.
+.
3 4 6
CN) m/z: 67; (M-C H N) m/z: 40. The methylpyrazine was
3
−
1
1
1
of 50 mL min subsequently flushed with He gas at 60 °C for 1 h. The
TPD was carried out from 60 °C to 600 °C at a ramping rate of
3
−
1
1
0 °C min . The amount of desorbed NH
3
was calculated using
methylpyrazine.
GRAMS/32 software. The carbon contents of the used Zn–Cr samples
were measured using VARIO EL, CHNS analyser. The oven-dried and
calcined forms of Zn–Cr catalysts were characterized by powder X-ray
diffraction (XRD) analysis using a Rigaku miniflex X-ray diffractom-
3. Results and discussion
The XRD patterns of all the Zn–Cr samples prepared by varying pH
displayed similar hydrotalcite patterns in oven-dried form and ZnO–
ZnCr O phases over calcined samples. Fig. 1 shows the XRD patterns
eter using Ni filtered Cu K
0°, at a scan rate of 2°min
current of 30 kV and 15 mA respectively.
α
radiation (λ=0.15406 nm) from 2θ=2 to
−
1
8
with the beam voltage and a beam
2
4
of the oven-dried (Zn–Cr prepared at pH=9) sample revealed the
Fig. 1. XRD patterns of the Zn–Cr oven-dried, calcined in air at 450 °C/5 h and the used Zn–Cr catalyst prepared at pH=9.

R. Sarkari et al. / Catalysis Communications 12 (2011) 1067–1070
1069
Table 1
Cyclisation activity of ethylenediamine and aqueous glycerol at a reaction temperature of 350 °C over Zn–Cr catalysts prepared at pH=7 to 11 and calcined in air at 450 °C/5 h.
Catalyst wt.=0.2 g, feed rate=2 mL h− ; glycerol:EDA:H
1
O:N
=1:1:20.5:20.4 (mole ratio); and GHSV=18909 cc g
−1 −1
h
.
2
2
apH
BET–SA (m2 g−1
bCarbon
(wt.%)
cNH
dSpecific rate×108
)
%Conv.
EDA
%Conv.
3
uptake
%Selectivity
MP
eCalcined
fReduced
μmol g−1
gOthers
mol m2 s−1
Glycerol
Pyrazine
7
8
9
1
1
.0
.0
.0
0.0
1.0
31.7
38.1
45.3
56.4
63.4
35.2
40.8
37.6
56.2
60.0
57.5
69.4
74.0
56.2
44.5
43.7
55.6
71.3
59.4
28.3
2.83
2.68
1.91
2.76
2.75
360
284
101
302
466
65.2
64.0
69.2
66.2
55.7
18.8
18.5
16.5
18.3
21.5
15.9
17.4
14.2
15.4
22.8
6.8
7.4
10.4
5.7
2.6
a
b
c
d
e
f
pH maintained during the preparation Zn–Cr LDH catalysts.
Carbon contents of the used catalysts measured by CHN analysis.
NH
Specific rate is measured with respect to glycerol conversion and surface area of the reduced catalyst.
BET–surface areas of the fresh calcined ZnO–ZnCr catalysts.
BET–surface areas of the calcined and reduced (in 5%H balance Ar at 400 °C/5 h) ZnO–ZnCr
Other compounds such as pyrazinealdehyde, 2,3-dimethyl pyrazine, and 2,5-dimethyl pyrazine.
3 3
uptakes measured by TPD of NH of the fresh calcined and reduced catalysts.
2 4
O
2
2 4
O catalysts.
g
presence of HT phase layered double hydroxide (LDH) which is
the acidities of the catalysts (Table 1) measured by temperature
programmed desorption (TPD) of NH . The TPD of NH studies indicated
decomposed to form ZnO and ZnCr
O
2 4
phases [13] upon calcination
3
3
(
Fig. 1) in air at 450 °C for 5 h. The lattice parameters corresponding to
that the sample prepared at pH~9 is found to have lower acidity
compared to other catalysts. It appears that strong acid sites are
undesirable for the EDA and glycerol cyclisation reaction. The carbon
content on the used catalysts (Table 1) showed ca. 1.91% over pH=9
sample, which is lower compared to other samples.
the HT structure are found to be a=3.10 and c=22.5 for Zn–Cr LDH.
The basal spacing is calculated from the average of (00l) peaks
(
position of the (110) peaks (a~0.3106 nm). This is in good agreement
with the literature value [14,15]. The XRD patterns of the calcined
catalyst indicated the presence of reflections at 2θ=36.2, 31.7, 34.4,
d~0.775 nm), while the "a" dimension is calculated as twice the
To gain an insight into the methylpyrazine yields, we performed
experiments to check the influence of reaction temperature on the
5
0
6.6, and 62.8° and their corresponding ‘d’ values of 0.247, 0.281,
.260, 0.162, and 0.147 nm are attributed to ZnO [ICDD # 89–0510]
2 4
activity over ZnO–ZnCr O catalyst prepared at pH~9 and the data is
represented in Fig. 2. It is interesting to note that there is not much
change in selectivity towards MP with increase in reaction temper-
ature. The extent of conversion of EDA is slightly higher than that of
glycerol at all temperatures. The cyclisation activity of the ZnO–
phase and diffraction lines at 2θ=35.7, 30.3, 63.1, 57.4, and 43.4° with
the ‘d’ values of 0.251, 0.294, 0.147, 0.160, and 0.208 nm are ascribed
to the ZnCr O [ICDD # 22–1107] phase. The BET–surface areas of the
2 4
calcined and reduced Zn–Cr catalysts are reported in Table 1. It is
observed that there is not much variation in the BET–surface areas of
the calcined and reduced samples.
2 4
ZnCr O catalyst showed (Fig. 2) almost 99.4% conversion of EDA and
94% glycerol with ~72% selectivity towards methylpyrazine at 400 °C.
A number of experiments on the cyclisation activity over Zn–Cr
catalysts are carried out under standard conditions using 20 wt.%
aqueous glycerol and EDA. Based on the experimental data a plausible
reaction mechanism (Scheme 1) is proposed in conjunction with
earlier postulates for the formation of methylpyrazine [16].
First, we examined the EDA and aqueous glycerol cyclisation
activity over several ZnO–ZnCr
ranging from 7 to 11) at a reaction temperature of 350 °C and the
results are reported in Table 1. The ZnO–ZnCr samples prepared at
2 4
O catalysts (prepared at various pH
2 4
O
pH~9 shows relatively high EDA and glycerol conversion as well as
selectivity towards methylpyrazine compared to other catalysts.
Formation of about 18–20% of pyrazine along with by-products such
as 2,3-dimethylpyrazine, 2,5-dimethylpyrazine, and pyrazinealdehyde,
A well-known mechanism on methylpyrazine formation is via
dehydrocyclisation of EDA and 1,2-propylene glycol (Scheme 2) over
mixed oxide catalysts is reported [10,11,17]. We anticipated that if
EDA and glycerol undergo similar transformation, the end product
could be pyrazylmethanol (Scheme 1, 2nd step or Scheme 3). On the
contrary a large amount of methylpyrazine is obtained in this study
along with some by-products. This is explained based on the
conversion of pyrazylmethanol into methylpyrazine (see reaction
mechanism Scheme 1) and pyrazinaldehyde [16].
(
~14–18%) were observed over all catalysts and slightly higher ~22% on
the catalyst prepared at pH=11. The better activity and selectivity of
the ZnO–ZnCr (sample prepared at pH~9) are explained based on
2 4
O
Several reaction parameters are evaluated during the cyclisation
of EDA and aqueous glycerol. We envisioned that the methylpyrazine
could be formed by condensation followed by cyclisation of glycerol
and EDA that proceeds through dehydrogenation, which undergoes
homo-coupling (Scheme 1, 3rd step) reaction subsequently forming
methylpyrazine and pyrazinealdehyde [18]. The high ratio of
methylpyrazine compared to pyrazinealdehyde is probably due to the
hydrogenation of pyrazinealdehyde [19] to form pyrazylmethanol
(Scheme 1, 5th step) during the course of reaction, as one would expect
equal proportions of pyrazinealdehyde and methylpyrazine for
reaction Scheme 1. Pyrazine the main by-product obtained by
Fig. 2. Influence of reaction temperature over calcined Zn–Cr catalyst prepared at
−
1
pH=9, calcined in air at 450 °C/5 h. Catalyst wt.=0.2 g, feed rate=2 mL h ; glycerol:
=1:1:20.5:20.4 (mole ratio); and GHSV=18909 cc g−
1
h
−1
. Methylpyr-
Scheme 2. Dehydrocyclization of EDA and 1,2-propylene glycol for the formation of
EDA:H
2
O:N
2
azine yields were calculated with respect to glycerol conversions.
methylpyrazine.

1070
R. Sarkari et al. / Catalysis Communications 12 (2011) 1067–1070
cyclisation activity. Further characterization of the catalyst studies are
in progress.
Acknowledgements
Scheme 3. Anticipated reaction mechanism for the formation of pyrazylmethanol.
Authors RS, CA, VK, RK and MS thank CSIR New Delhi for the award
of Junior Research Fellowship.
hydrogenolysis of methylpyrazine is observed in the product mixture.
Formation of the by-products such as 2,3-dimethylpyrazine and 2,5-
dimethylpyrazine is probably due to disproportionation of methylpyr-
Appendix A. Supplementary data
azine in presence of ZnO–Cr
2
O
3
catalysts. However, phases due to any of
Supplementary data to this article can be found online at
doi:10.1016/j.catcom.2011.03.021.
the chromium oxides are not observed in the XRD analysis of calcined
catalysts. They are either present in small amounts or below the X-ray
detection limit. Formation of the second major product i.e. pyrazine
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2
O
4
[
9
1
reaction temperature of 400 °C.
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