Catalytic synthesis of 2-methylpyrazine over Cr-promoted copper based catalyst via a cyclo-dehydrogenation reaction route

Article abstract of DOI:10.1007/s12039-010-0097-5

The cyclo-dehydrogenation of ethylene diamine and propylene glycol to 2-methylpyrazine was performed under the atmospheric conditions at 380°C. The Cr-promoted Cu-Zn/Al ₂O ₃ catalysts were prepared by impregnation method and characterized by ICP-AES, N ₂ adsorption/desorption, XRD, XPS, N ₂O chemisorption, TPR and NH3-TPD techniques. The amorphous chromium species existing in Cu-Zn-Cr/Al ₂O ₃ catalyst enhanced the dispersion of active component Cu, promoted the reduction of catalyst. Furthermore, the catalytic performance was significantly improved. The acidity of the catalyst played an important role in increasing the 2-MP selectivity. To optimize the reaction parameters, influences of different chromium content, reaction temperature, liquid hourly space velocity (LHSV), reactants molar ratio and time on stream on the product pattern were studied. The results demonstrated that addition of chromium promoter revealed satisfying catalytic activity, stability and selectivity of 2-methylpyrazine. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-010-0097-5

J. Chem. Sci., Vol. 122, No. 4, July 2010, pp. 621–630. © Indian Academy of Sciences.
Catalytic synthesis of 2-methylpyrazine over Cr-promoted copper
based catalyst via a cyclo-dehydrogenation reaction route
FANGLI JING, YUANYUAN ZHANG, SHIZHONG LUO*, WEI CHU*, HUI ZHANG
and XINYU SHI
Department of Chemical Engineering, Sichuan University, NO. 24 South Section 1, Yihuan Road,
Chengdu 610065, China
e-mail: luoszscu@gmail.com, chuwei65@yahoo.com.cn
MS received 14 May 2009; revised 4 October 2009; accepted 15 October 2009
Abstract. The cyclo-dehydrogenation of ethylene diamine and propylene glycol to 2-methylpyrazine
was performed under the atmospheric conditions at 380°C. The Cr-promoted Cu-Zn/Al O catalysts
were prepared by impregnation method and characterized by ICP-AES, N adsorption/desorption, XRD,
2
2
3
XPS, N O chemisorption, TPR and NH -TPD techniques. The amorphous chromium species existing in
2 3
Cu-Zn-Cr/Al O catalyst enhanced the dispersion of active component Cu, promoted the reduction of
2 3
catalyst. Furthermore, the catalytic performance was significantly improved. The acidity of the catalyst
played an important role in increasing the 2-MP selectivity. To optimize the reaction parameters, influ-
ences of different chromium content, reaction temperature, liquid hourly space velocity (LHSV), reac-
tants molar ratio and time on stream on the product pattern were studied. The results demonstrated that
addition of chromium promoter revealed satisfying catalytic activity, stability and selectivity of
2-methylpyrazine.
Keywords. 2-Methylprazine; copper-based catalyst; chromium-promoter; ethylene diamine; propylene
glycol; characterization.
10 11
copper-chromium and Ag were patented. Other
1. Introduction
12
catalysts such as ZnO-MoO3, ZnO-V2O5, Zn-WO3,
13
ZnO-MnSO4–H3PO4, Ag-La-Zn/Al2O3, Ag-Pt/Al2O3,
Pyrazine and its derivatives are valuable compounds
because of their application in the synthesis of per-
14
and Ag-Mg/Al2O3,
Pd-promoted ZnO/Zn-Cr-O
15
16
fumery, pharmaceutical and agricultural chemical
catalysts,
zinc-modified
ferrierite
(FER),
1–3
industries.
As an example, 2-methylpyrazine
H-ZSM-5, zinc-modified zeolite catalysts (including
17
ZSM-5 and Beta) were also used for synthesis of
(2-MP), an important lower alkyl-substituted
pyrazine, is widely used as a key intermediate for
2-methylpyrazine.
4,5
pyrazineamide, an effective anti-tubercular drug.
The catalysts containing chromium were exten-
Currently, 2-methylpyrazine is prepared by dehydra-
sively used for dehydrogenation of piperazine to
18,19
tion-cyclization and/or partial dehydrogenation of
pyrazine,
which indicated that catalysts contain-
6
ethylene diamine (ED) and propylene glycol (PG).
ing chromium promoted effectively on the dehydro-
7
Forni and Pollesel studied the mechanism of the
20,21
genation reaction. Park et al
reported that metal
cyclization of ethylene diamine and propylene gly-
col to 2-methylpyrazine by means of the TPD-TPR-
MS technique. They pointed out that the reaction in-
volves a Rideal–Eley mechanism leading to a fully
hydrogenated intermediate 2-methylpiperazine be-
tween adsorbed propylene glycol and gaseous ethyl-
ene diamine. The mechanism is shown in scheme 1.
(copper, nickel, cobalt) oxide-modified ZnO/SiO2
for cyclo-dehydrogenation of ethylene diamine with
propylene glycol to 2-methylpyrazine, demonstrated
that the metallic copper and zinc oxide were
assigned to dehydrogenation and dehydration,
respectively. Since the catalytic synthesis of 2-
methylpyrazine is related with cyclization (dehydra-
tion) and dehydrogenation of piperazine intermedi-
ate, the catalysts which should possess dual
functional active sites, they have been designed,
8
Catalytic systems based on zinc, zinc-chromium,
9
*For correspondence
621

622
Fangli Jing et al
Scheme 1. Reaction pathway for the synthesis of 2-methylpyrazine from ethyl-
ene diamine and propylene glycol.
prepared and measured for 2-methylpyrazine syn- apparatus at liquid nitrogen temperature. Samples
thesis. were degassed at 300°C for 3 h prior to analysis.
In this work, a novel type bifunctional Cr- X-ray diffraction (XRD) tests were performed on
promoted Cu-Zn/Al2O3 oxide catalyst was proposed, DX-100 diffractometer with CuKα radiation, scan-
characterized using inductively coupled plasma- ning 2θ angles in the range of 20–80°.
atomic emission spectrometry (ICP-AES), X-ray dif- X-ray photoelectron spectra (XPS) were recorded
fraction (XRD), X-ray photoelectron spectra (XPS), with XSAM800 spectrometer with an AlKα
N2O chemisorption, etc. methods, to understand the (1486⋅6 eV) radiation.
chemical composition, the structure, the chemical Temperature-programmed reduction (TPR) meas-
state of elements and Cu surface area and dispersion urements were carried out at atmospheric pressure in
of the catalyst and correlated these physicochemical a fixed-bed. 50 mg sample was loaded in a quartz
properties with its performance.
reactor in N2 flow at 50°C for 30 min. Then the
nitrogen was replaced by the reductive gas (5%
H2/N2) at a flow rate of 30 mL/min. The temperature
of the reactor was increased linearly from 100 to
550°C at a rate of 10°C/min by a temperature-
programmed controller. The effluent stream was
analysed by a thermal conductivity detector (TCD).
Cu surface area, particles size and the metal dis-
persion were measured by N2O passivation method
including a two-step analysis: N2O oxidization sur-
2. Experimental
2.1 Catalyst preparation
The Cu–Zn–Cr /Al2O3 catalysts were prepared by a
x
3-step-impregnation method. Appropriate amount of
Cu (NO3)2 ·3H2O, Zn (NO3)2⋅6H2O and Cr (NO3)3
9H2O were dissolved in 9 mL deionized water as
impregnating solution. The one third solution was
uniformly added drop-wise on 3 g alumina support
(40–60 mesh, pre-treated at 450°C for 4 h), for
10 min, and then dried at 110°C for 20 min. Repeat-
ing above operations 2 more times, then it was dried
at 110°C for 4 h and calcined at 450°C for 5 h. For
the comparison, the Cu/Al2O3 and Cu-Zn/Al2O3
catalysts were prepared in the same way.
face Cu to Cu2O and H2 temperature-programmed
22,23
reduction of the formed Cu2O surface species.
A
fresh 50 mg catalyst was pre-reduced at 350°C (ris-
ing the temperature at a rate of 10°C/min) for 1 h in
H2/N2 mixture gas flow (5 vol.% at 30 mL/min). The
temperature was in situ cooled to the adsorption
temperature 60°C in pure N2 flow (40 mL/min) and
then the reduced-catalyst was exposed to the pure
N2O (30 mL/min) for 1 h to oxidize the metallic Cu
to Cu2O. After N2O passivation, the catalyst was
purged with N2 (40 mL/min) to remove the residual
oxidant and subsequently the second TPR was per-
2.2 Catalyst characterization
The ICP-AES IRIS Advantage (TJA Solution, USA) formed to reduce Cu2O to Cu. The H2 consumption
was employed to determine the chemical composi- in the two procedures was monitored by TCD and
tions of the different catalyst samples by inductively calculated quantitatively from the TPR peak area,
coupled plasma atomic emission spectroscopy tech- calibrated with standard CuO samples.
nique.
The acidity of the calcined catalyst was character-
The surface area, total pore volume and average ized by temperature-programmed desorption of am-
pore diameter were measured by the N2 adsorp- monia (NH3-TPD) techniques. 50 mg catalyst
tion/desorption, using a Quantachrome Nova 1000e sample, loading in the middle of quartz tube, was

Catalytic synthesis of 2-methylpyrazine over Cr-promoted copper based catalyst
623
Figure 1. The reaction apparatus for synthesis of 2-methylpyrazine.
used for NH3–TPD. Prior to analysis the sample was 112A) using a capillary column (cross-linked SE-30
pre-treated in situ in an Ar flow (30 mL/min) at gum, 0⋅33 mm × 30 m) and flame ionization detector
380°C for 2 h, then cooled to the NH3 adsorption (FID). The identification of the liquid products was
temperature 100°C. NH3 was absorbed by a pulsed done by GC-MS (Agligent Technol. 6890N/Agligent
injection in an Ar flow until saturation which was Technol. 5973Network Mass Selective Detector).
determined by the TCD signal. The TPD profiles
were monitored by a thermal conductivity detector
3. Results and discussion
and recorded from 100 to 700°C at a heating rate of
10°C/min.
3.1 Chemical composition and the texture property
The chemical compositions and BET data of
2.3 Catalytic performance in synthesis of
Cu/Al2O3, Cu–Zn/Al2O3 and Cu–Zn–Cr3/Al2O3 cata-
2-methylpyrazine
lysts were listed in table 1. From ICP–AES results,
the copper content was 11⋅2 wt.% for Cu/Al2O3 and
Cu–Zn/Al2O3, 12⋅4 wt.% for Cu–Zn–Cr3/Al2O3 sam-
ple. The zinc content in Cu–Zn/Al2O3 catalyst was
16⋅5 wt.%, and for Cu–Zn–Cr3/Al2O3 was 16⋅2 wt.%.
The chromium content in Cu–Zn–Cr3/Al2O3 catalyst
was 2⋅74 wt.%. The results of metal contents were
close to those of initially calculated ones. The BET
surface areas of Cu/Al2O3, Cu–Zn/Al2O3 and
Cu–Zn–Cr3/Al2O3 catalysts were 187⋅8, 126⋅6 and
The catalytic activity measurements of the calcined
catalysts were performed at atmospheric pressure in
a continuous fixed-bed reactor (stainless steel, 8 mm
i.d., 300 mm length). The reaction apparatus was
showed in figure 1. The catalyst was loaded at the
middle of the reaction tube with a packing height of
60 mm, reduced under the H2 and N2 gas mixture
flow (H2/N2 = 1, molar ratio) at 380°C for 2 h and
then reacted at the same temperature. The aqueous
liquid reactants were prepared by mixing ethylene
diamine and propylene glycol in the mole ratio of
1 : 1, and diluted with deionized water (50 wt.%).
Dilution with water was helpful for reducing the
viscosity and smooth feeding of the reactants. The
liquid reactants were injected into the top-side of
reactor by a pump at 3 mL/h and the pure nitrogen
(25 mL/min) was introduced as dilute gas. The
liquid products were collected in an ice-water con-
denser and analysed by gas chromatography (GC-
2
124⋅7 m /g, respectively. A large amounts of doped
Zn in Cu/Al2O3 resulted in the surface area decrease
significantly, while only slight decrease in surface
area could be found as few chromium were intro-
duced into Cu–Zn/Al2O3 catalyst. The decrease in
surface area, total pore volume and pore size indi-
cated that not only the metallic oxide particulates
were dispersed on the alumina support surface but
also they could be accreted to inner wall of the pore
channel.

624
Fangli Jing et al
Table 1. Compositions and textural properties of Cu/Al O , Cu–Zn/Al O and Cu–Zn–
2 3 2 3
Cr/Al O catalysts.
2 3
b
Composition/wt. %
2
/m g
–1
–1
Catalyst
Cu
Zn
Cr
Al
S
V/cc g
D/nm
BET
Cu/Al O
2
11⋅2
11⋅2
12⋅4
–
–
–
47⋅0
34⋅7
31⋅9
187⋅8
126⋅6
124⋅7
0⋅3558
0⋅2312
0⋅1984
7⋅576
7⋅416
6⋅269
3
Cu–Zn/Al O
2
16⋅5
16⋅2
3
a
Cu–Zn–Cr /Al O
3 2 3
2⋅74
a
b
Chromium initial calculated content is 3 wt.%. calculated from ICP-AES
highly characteristic reflection of ZnO and CuO.
While the chromium species, generally existing as
24–26
spinel in Cr rich Cu-Zn-Cr catalytic system,
had
very weak intensity, which probably could explain
that the amorphous chromium species existed in the
catalyst with low chromium content.
Figure 2b showed XRD patterns of used
Cu–Zn/Al2O3 and Cu–Zn–Cr3/Al2O3 catalysts. For
Cu–Zn/Al2O3 catalyst, the diffraction peaks of
metallic copper phase were observed as the effect of
pre-reduction with H2 and ZnO could not be
reduced. As to be expected, there was no obvious
diffraction peak of chromium species because of its
possible amorphous state in Cu–Zn–Cr3/Al2O3 cata-
lyst, which may be enhanced by H2 pre-treatment.
3.3 Chemical state of the surface elements by XPS
The surface analysis of the catalysts and the chemi-
cal state of the elements were evaluated by XPS
technique, the catalyst samples were pre-treated by
using hydrogen gas at 380°C for 2 h prior to analy-
sis. The XPS spectra of the Cu2p3/2 of Cu–Zn/Al2O3
and Cu–Zn–Cr3/Al2O3 catalysts were shown in fig-
ure 3. The copper oxide displayed strong shake-up
peak near 935⋅0 eV, which is the characteristic of
2+
Cu species. Another means of identifying Cu
2+
cation was the satellite peak, about 8 eV on the high
binding energy side, due to the shake-up transition
by ligand → metal 3d charge transfer, which cannot
+ 0
occur in Cu and Cu because of their fully filled 3d
Figure 2. a, XRD patterns of fresh Cu–Zn/Al O and
2 3
27–29
Cu–Zn–Cr /Al O catalysts. b, XRD patterns of used Cu–
3 2 3
shell.
The peaks around 932⋅4 eV were assigned
30
to the cuprous cations and/or metallic copper. In
Zn/Al O and Cu–Zn–Cr /Al O catalysts.
2 3 3 2 3
the XPS spectrum of Cu–Zn–Cr3/Al2O3 catalyst, the
2+
Cu species with considerable peak intensity were
3.2 Phase composition of the catalyst
detected, because the reduced sample was exposed
to air before XPS examination, the detected amounts
of copper species in the sample did not characterize
the surface composition of the catalyst under the
reaction conditions. Instead, these data illustrated
partial reoxidation of the catalyst when the sample
XRD analysis was employed to investigate the crys-
tal of the elements of the catalyst samples, and the
XRD patterns of the fresh catalysts and used cata-
lysts were obtained (see figures 2a and b). Both
patterns of two fresh catalyst samples revealed

Catalytic synthesis of 2-methylpyrazine over Cr-promoted copper based catalyst
625
2+
conditioned in reaction environment was exposed to lower temperature side also indicated that the Cu
31
air at room temperature. This low-temperature species were easily reduced to lower valency spe-
33–36
reoxidation apparently affected only a thin surface cies by promotion of chromium.
As a result, lar-
layer, because XRD analysis did not detect copper ger number of metallic Cu could be reduced,
oxides in the reduced sample. The XPS analysis corresponding to larger peak intensity and peak area
results suggested that the metallic copper content proportion of metallic Cu in XPS spectra.
increased significantly on the catalyst surface and
the introduction of chromium enhanced the reduc-
3.5 Cu surface area, dispersion and Cu/CuO
tion compared to the catalyst without chromium.
crystallite size
Cu surface area and dispersion, are important
3.4 The reducibility for the catalyst
parameters for Cu-based catalyst, are generally
37,38
The TPR profiles of Cu/Al2O3, Cu–Zn/Al2O3 and obtained by N2O chemisorption.
Copper disper-
Cu–Zn–Cr3/Al2O3 catalysts were presented in figure sion (DCu), defined as the ratio of Cu exposed at the
4. It was shown that the Cu/Al2O3 catalyst had two surface to total, was calculated from the amount of
reduction peaks detected at 251 and 297°C, which H2 consumed in the TPR process of Cu2O → Cu.
2+
+
+
0 32
could be assigned to Cu → Cu and Cu → Cu .
Two similar TPR patterns, different from that of (MSA) was calculated.
Cu/Al2O3 catalyst, were observed in Cu–Zn/Al2O3
Starting from the DCu value, Cu metal surface area
39
and Cu–Zn–Cr3/Al2O3 catalysts with only one reduc-
Mol ⋅SF⋅ N
H
A
2
MSA (m g ) =
−1
Cu
2
2+
tion peak assigned the reduction of Cu direct to
,
4
10 ⋅C ⋅ W
0
Cu under different reduction temperatures.
M
Cu
The reduction peak area was in proportion to the
reducible copper content for the copper-based cata-
lyst. The variation of reduction peak area indicated
that the catalyst reducibility was modified by the
addition of promoter. From the results of the quanti-
tative calculation, the Cu–Zn–Cr3/Al2O3 catalyst
sample displayed the higher peak area than that of
Cu–Zn/Al2O3 catalyst, corresponding to larger
amounts of hydrogen consumption, which suggested
that more copper oxides were enriched on the cata-
lyst surface. The reduction temperature shifted to
where MolH2, SF, NA, CM and WCu are moles of
hydrogen experimentally consumed per unit mass of
–1
catalyst (μmolH2g cat), stoichiometric factor (2),
23 –1
Avogadro’s number (6⋅022 × 10 mol ), number of
19
surface Cu atoms per unit surface area (1⋅47 × 10
–2
atoms m ), and Cu content (wt.%), respectively.
The O/Cu ratio is assumed to be 1/2 (SF = 2) on the
39
basis of UPS results, which proved that after oxi-
dation with N2O at temperatures up to 100–120°C,
Figure 4. TPR profiles of Cu/Al O , Cu–Zn/Al O and
2 3 2 3
Cu–Zn–Cr /Al O catalysts. The chromium initial calcu-
3 2 3
Figure 3. XPS spectra of Cu–Zn/Al O and Cu–Zn–
2 3
lated content was 3 wt.% in Cu–Zn–Cr /Al O catalyst.
3 2 3
Cr /Al O catalysts.
3 2 3

626
Fangli Jing et al
Table 2. Surface characteristics of dispersed copper catalysts.
TPR–N O passivation method
2
2
Mol (µmol/g ) MSA (m /g )
Catalyst
Surface Cu content (µmol/g )
cat
D
(%)
Cu
H2
cat
Cu
Cu–Zn/Al O
2
8⋅0
12⋅2
4⋅0
21⋅9
183⋅1
358⋅8
28⋅4
55⋅7
3
Cu–Zn–Cr /Al O
3 2 3
Table 3. Crystallite size variation via H treatment for
2
tion peaks by means of the Scherrer equation
L = 0⋅89λ/β(θ)cosθ, where L is the crystallite size, λ
is the wavelength of the radiation used, θ is the
Cu–Zn/Al O and Cu–Zn–Cr /Al O samples.
2 3 3 2 3
Catalyst
CuO (202) [nm] Cu (200) [nm]
40
Bragg diffraction angle, and β(θ) is the FWHM.
Cu–Zn/Al O
2
31⋅0
31⋅2
–
–
3
The results are given in table 3. For the fresh sam-
ples, the crystallite size value of CuO of Cu–Zn–
Cr3/Al2O3 catalyst was slightly bigger than that of
Cu–Zn/Al2O3 catalyst, which may be responsible for
the decrease to a small extent in surface area as de-
scribed in table 1. The CuO phases were reduced to
metallic Cu in H2 gas flow under the reaction condi-
tions, the crystallite size values decreased by 7⋅10%
and 13⋅46% for Cu–Zn/Al2O3-used and Cu–Zn–
Cr3/Al2O3-used samples, respectively. It was sug-
gested that the reducibility of Cu–Zn/Al2O3 catalyst
was improved significantly by adding chromium
promoter, the smaller Cu particles generated larger
metallic Cu surface area and dispersion, resulting in
the Cu particles enriched on the catalyst surface but
Cu–Zn–Cr /Al O
3 2 3
–
Cu–Zn/Al O used
2 3
28⋅8
27⋅0
Cu–Zn–Cr /Al O used
3 2 3
–
31
no sintering.
3.6 The amount of acid sites and acid strength
distribution of the catalyst
NH3–TPD analysis was carried out in order to inves-
tigate the amount of acid sites and acid strength dis-
tribution, the TPD curves were depicted in figure 5
and the details were reported in table 4. In the both
TPD curves, four desorption regions of 100–300°C,
300–400°C, 400–590°C and 590–700°C, were
observed. The desorption peak below 300°C was not
ascribed to the ammonia directly adsorbed on the
acid sites but for possible ammonia molecules inter-
action with the terminal OH groups on the catalyst
Figure 5. NH -TPD curves for Cu–Zn/Al O and
3 2 3
Cu–Zn–Cr /Al O samples. The TPD curves were decon-
3 2 3
volved using a Gaussian shaped function. The solid lines
are the TPD curves and the dashed lines are the fitting
curves.
+
the surface copper is primarily in the Cu oxidation
state. The results from N2O chemisorption were
summarized in table 2. It was shown that the
Cr-promoted Cu–Zn/Al2O3 displayed larger value of
surface Cu content, corresponding to larger amount
of H2 consumption and Cu surface area (MSA)
which increased by almost 1 time. So did the Cu
dispersion.
surface, which were desorbed easily on the TPD test
41,42
(this process was denoted as P1 in figure 5).
The
high-temperature peaks (400–590°C) were assigned
43
to desorption of NH3 from the strong acid sites.
The difference between the TPD curves for
Cu–Zn/Al2O3 and Cu–Zn–Cr3/Al2O3 catalysts was
found, it was single peak for that of Cu–Zn/Al2O3
while it was double peaks for that of Cu–Zn–
The crystallite size of CuO and Cu may be esti-
mated from the values of the full-width at half-
maximum (FWHM) of the (202) and (200) diffrac-

Catalytic synthesis of 2-methylpyrazine over Cr-promoted copper based catalyst
627
Table 4. Acid strength distribution for Cu–Zn/Al O and Cu–Zn–Cr /Al O catalysts and their catalytic
2 3 3 2 3
performances for 2-MP synthesis.
Acidity (NH uptake, mmol/g)
3
Catalyst
100–300 300–400 400–590 590–700°C Total acidity 2-MP selectivity (%)
Cu–Zn/Al O
2
0⋅78
1⋅08
0⋅21
0⋅12
2⋅37
2⋅41
0⋅48
0⋅42
3⋅84
4⋅03
68⋅0
84⋅8
3
Cu–Zn–Cr /Al O
3 2 3
Table 5. Effect of chromium content on synthesis of 2-methylpyrazine.
Selectivity (%)
2-MP 2-MPIP Alkyl pyrazines Others
Catalyst
ED conv. PG conv. Lb
Py
Cu/Al O
2
76⋅7
88⋅1
93⋅6
97⋅9
95⋅7
95⋅4
80⋅2
90⋅5
96⋅1
100
11⋅0 14⋅79 53⋅35
2⋅82 21⋅47 68⋅02
3⋅65 10⋅58 81⋅46
5⋅02
4⋅26
2⋅23
2⋅42
3⋅04
2⋅86
14⋅01
2⋅06
0⋅82
1⋅01
1⋅04
1⋅37
1⋅83
1⋅38
1⋅25
1⋅40
1⋅37
1⋅90
3
Cu–Zn/Al O
2
3
Cu–Zn–Cr /Al O
1 2
3
3
3
3
Cu–Zn–Cr /Al O
3 2
3⋅43
3⋅32
3⋅08
6⋅99 84⋅75
6⋅59 84⋅64
8⋅17 82⋅62
Cu–Zn–Cr /Al O
5 2
98⋅7
98⋅2
Cu–Zn–Cr /Al O
7 2
ED = ethylene diamine, PG = propylene glycol, Lb = lower boiling point components, Py = pyrazine,
2-MP = 2-methylpyrazine, 2-MPIP = 2-methylpiperazine. Reaction conditions: atmosphere, tempera-
–1
ture = 380°C, time on stream = 2 h, LHSV = 1 h , ED : PG = 1 : 1 (mol) in 50 wt.% aq. solution
Cr3/Al2O3 catalyst. The overlapped peaks indicated catalysts containing chromium produced less
that there might be stronger acid sites (denoted as amount of pyrazine, alkyl pyrazine and intermediate
P2) in Cu–Zn–Cr3/Al2O3 sample, the deconvolution product, 2-methylpiperazine as compared with those
indeed showed that there were two different strength of the catalysts without chromium. The selectivity of
acid sites with different desorption temperature. The 2-methylpyrazine increased to 84⋅75% over Cu–Zn–
each TPD curve displayed a broadening shoulder Cr3/Al2O3 catalyst compared to 68⋅02% on Cu–
peak (denoted as Ps) around 600°C, corresponding to Zn/Al2O3 catalyst. Keeping the copper and zinc con-
the formation of strongly Lewis acid sites generated centration constant, 15 wt.% and 20 wt.% respec-
2+
by Zn , which was identified that the low-tempe- tively, the effects of different chromium concentration
rature peak (100–300°C) was widened and shifted to were studied at the same conditions, and the results
17,44,45
high-temperature side.
Cu-Zn-Cr3/Al2O3 showed showed that the optimum concentration of chro-
an increase of NH3 desorption from 0⋅78 to mium was 3 wt.%. The results of BET revealed that
1⋅08 mmol/g compared to Cu-Zn/Al2O3 (table 4). chromium promoted the dispersion of active copper
The total acidity also increased by 4⋅9%. It was species and acted as a fence to separate active
shown that the increased acidity displayed a consid- species from congregating, addition of chromium
erable effect on the catalytic performance, the selec- promoter was helpful for dehydration-cyclization
tivity of 2-MP increased by 24⋅7%.
and dehydrogenation of ethylene diamine and
propylene glycol to 2-methylpyrazine. Further stud-
ies were carried out over Cu–Zn–Cr3/Al2O3 catalyst,
as it showed good conversion of ethylene diamine
and propylene glycol and selectivity of 2-methyl-
pyrazine.
3.7 Catalytic synthesis of 2-methylpyrazine from
ED and PG
3.7a Effect of chromium content on catalytic per-
formance: The effects of Cu–Zn/Al2O3 catalyst 3.7b Influence of reaction temperature: Figure 6
promoted by chromium of different concentration on showed that the effect of reaction temperature on
the conversions of reactants and the products selec- reactants conversions and products distribution, the
tivities were studied and the results were summa- reaction was preformed over Cu–Zn–Cr3/Al2O3 cata-
–1
rized in table 5. It is clear from the table that the lyst for 2 h time on stream at a LHSV of 1 h . The

628
Fangli Jing et al
ethylene diamine conversion increased with the increased. The concentration of pyrazine increased
reaction temperature from 87⋅5% at 340°C to 98⋅1% from 6⋅69 to 13⋅6% with the mole ratio from 1 to
at 420°C. The selectivity of 2-methylpyrazine was 1⋅5, which was probably contributed to the self-
up to maximum 83⋅12% at 380°C. Only a small cyclocondensation of ethylene diamine. The selec-
amount of lower boiling point components was tivity of 2-methylpyrazine decreased as ethylene
detected in products, whose concentration was diamine concentration increasing in the feed.
higher at lower reaction temperature (340°C) com- It could also be noted that the formation of
pared with higher reaction temperature. At higher 2-methylpiperazine, which was the primary inter-
reaction temperature, the lower boiling point com- mediate product, which had the same trend with
ponents like methanol and acetaldehyde were read- 2-methylpyrazine.
ily reacting with piperazine and pyrazine, giving
alkyl pyrazine, which resulted in decreasing slightly 3.7d Role of LHSV in synthesis of 2-methyl-
in concentration of lower boiling point components pyrazine: It was demonstrated that the effect of
rather than higher to be expected. Besides, the liquid hourly space velocity (LHSV) on the forma-
piperazine ring structure could be easily cracked at tion of 2-methylpyrazine in figure 7. It is clear that
higher reaction temperature with the concurrent the conversions of the reactants as well as selectivity
18
formation of side reaction products and ammonia,
for 2-methylpyrazine decreases with increase in
which also reacted with pyrazine and 2-methyl- LHSV. However, the increase in selectivity of
pyrazine to alkyl pyrazine. As a result, the selecti- 2-methylpiperazine, the main intermediate product,
vities of pyrazine, 2-methylpyrazine and 2-methyl- indicated that dehydrogenation had not been well
piperazine decreased, in contrast to increased of performed due to the less contact time. At low
alkyl pyrazine.
LHSV, 2-methylpyrazine and pyrazine were the
–1
major products. As LHSV increased from 1 h to
–1
3.7c Product distribution versus molar ratio of 2⋅4 h , the selectivity of 2-methylpyrazine de-
reactants: A series of experiments were performed creased from 83⋅1 to 65⋅0%, that of alkyl pyrazines
to determine the optimum feed ratio at 380°C with increased resulting from dehydrogenation of pyrazine,
different mole ratio of ethylene diamine and propyl- piperazine and their methyl derivative and alkylation
ene glycol over Cu–Zn–Cr3/Al2O3 catalyst and the of that with the lower boiling point components like
results were presented in table 6. It could be found acetone, lower alcohols and acetaldehyde, which
from the table that the ethylene diamine conversion were easily formed at a high LHSV value.
decreased when the mole ratio of ethylene diamine
Figure 7. Influence of LHSV on synthesis of 2-
Figure 6. Influence of temperature on synthesis of 2-
methylpyrazine over Cu–Zn–Cr /Al O catalyst. Reaction
methylpyrazine over Cu–Zn–Cr /Al O catalyst. Reaction
3 2 3
conditions: atmosphere, temperature = 380°C, time on
stream = 2 h, ED : PG = 1 : 1 (mol) in 50 wt.% aq. solu-
tion.
3
2
3
conditions: atmospheric, time on stream = 2 h, LHSV =
–1
1 h , ED: PG = 1 : 1 (mol) in 50 wt.% aq. solution.

Catalytic synthesis of 2-methylpyrazine over Cr-promoted copper based catalyst
629
Table 6. Effect of molar ratio of ethylene diamine and propylene glycol on synthesis of 2-methylpyrazine over Cu–
Zn–Cr /Al O catalyst.
3
2
3
Selectivity (%)
2-MPIP Alkyl pyrazines Others
Molar ratio ED : PG
ED conv. PG conv.
Lb
Py
2-MP
1 : 1⋅5
1 : 1.2
1 : 1
100⋅0
98⋅5
97⋅9
92⋅4
79⋅8
95⋅2
98⋅8
100⋅0
97⋅5
97⋅1
5⋅27
2⋅96
3⋅47
3⋅59
3⋅99
2⋅54
5⋅21
6⋅87
8⋅19
13⋅64
85⋅88
86⋅26
85⋅09
82⋅24
75⋅71
2⋅81
2⋅73
2⋅18
2⋅26
1⋅84
1⋅76
1⋅66
1⋅89
3⋅09
4⋅05
1⋅76
1⋅20
0⋅49
0⋅64
0⋅78
1⋅2 : 1
1⋅5 : 1
ED = ethylene diamine, PG = propylene glycol, Lb = lower boiling point components, Py = pyrazine, 2-MP =
2-methylpyrazine, 2-MPIP = 2-methylpiperazine. Reaction conditions: atmosphere, temperature = 380°C, time on
stream=2 h, LHSV = 1 h , water = 50 wt.% in the reactants.
–1
Conclusions
The novel Cr-modified Cu–Zn/Al2O3 catalysts were
prepared by a 3-step impregnation method. The N2
adsorption/desorption tests showed that the addition
of chromium had little effect on the surface area and
pore diameter, only evident changes were observed
in pore volume. Considering the influences of chro-
mium on the catalytic performance, the better cata-
lytic activity (selectivity of 2-MP > 80%) and reaction
stability could be obtained compared with those of
the samples without chromium. It suggested that
chromium acted the role of structure promoter. XRD
results suggested that chromium species might exist
in catalyst as amorphous state. The dispersion of ac-
tive site Cu increased by almost 1 time and the total
Figure 8. Influence of time on synthesis of 2-methyl-
acidity was significantly improved by 4⋅7% after of
pyrazine over Cu–Zn–Cr /Al O catalyst. Reaction condi-
3
2
3
–1
tions: atmosphere, temperature = 380°C, LHSV = 1 h ,
ED : PG = 1 : 1 (mol) in 50 wt.% aq. solution.
addition chromium promoter; furthermore, catalytic
activity was improved. The chromium promoted cata-
lyst displayed better reducibility compared to the
catalyst without chromium according to the results
of XPS, N2O chemisorption and TPR measurements.
The catalytic activity for 2-methylpyrazine syn-
thesis by cyclo-dehydrogenation of ethylene diamine
and propylene glycol was carried out at 380°C over
the chromium-promoted Cu–Zn/Al2O3 catalysts. The
Cu–Zn–Cr3/Al2O3 catalyst has shown good conver-
sions of reactants, high selectivity and excellent
stability of 2-methylpyrazine after reacting for 10 h.
The optimum chromium content was 3 wt.%.
3.7e Relationship between catalytic activity and
time on stream: The stability was investigated over
Cu–Zn–Cr3/Al2O3 catalyst for 10 h (see figure 8).
Initially, the deactivation was faster, the conversion
of ethylene diamine dropped sharply from 97⋅9 to
90% in 4 h and kept constant till the end (10 h). The
selectivity of 2-methylpyrazine decreased from 86⋅2
to 80⋅3%, that of 2-methylpyrazine and pyrazine was
over 90%. At the end of the period of this run, the
selectivity of 2-methylpiperazine increased to a little
extent, which indicated that dehydrogenation was
weakened. The results showed the high activity of
composite catalyst in conversion of ethylene
diamine and high selectivity and stability of 2-
methylpyrazine.
Acknowledgements
The authors wish to thank the Analysis and Test
Centre of Sichuan University for ICP-AES test.
Thanks are also due to Fang Guo, Fenfen Qu and
Jinyan Hu for useful discussion.

630
Fangli Jing et al
25. Fujiwara M, Kieffer R, Ando H and Souma Y 1995
References
Appl. Catal. A: Gen. 121 113
26. Fujiwara M, Ando H, Tanaka M and Souma Y 1995
Appl. Catal. A: Gen. 130 105
1. Cenker M, Trenton and Baxter G E 1961 US 3005820
2. Su W Y and Knifton J F 1988 US 4788289
3. Subrahmanyam, M, Kulkarni S J and Rao A V R
1995 Indian J. Chem. Technol. 2 237
4. Seetharamanjaneya S P, Raghavan K V, Rao P K and
Kulkarni S J 2002 US 143180 A1
27. Murcia-Mascaros S, Navarro R M, Gomez-Sainero L
and Fierro J L G 2001 J. Catal. 198 338
28. Diaz G, Perez-Hernandez R, Gomez-Cortes A,
Benaissa M, Mariscal R and Fierro J L G 1999
J. Catal. 187 1
5. Forni L, Stern G and Gatti M 1987 Appl. Catal. 29
161
29. Campos-Martin J M, Guerrero-Ruiz A and Fierro
J L G 1995 J. Catal. 156 208
6. Subrahmanyam M, Muralidhar G and Kulkarni S J
2000 IN 185171
30. Mathew T, Rao B S and Gopinath C S 2004 J. Catal.
222 107
7. Forni L and Pollesel P 1991 J. Catal. 130 403
8. Sato K 1978 US 4097478
31. Ilinich O, Ruettinger W, Liu X S and Farrauto R
2007 J. Catal. 247 112
9. Subrahmanyam M, Muralidhar G and Verma P K
2001 IN 185482
32. Zhai X, Shamoto J, Xie H and Han Y 2008 Fuel 87
430
10. Lee Y K, Park S E and Kwon Y S 1990 US 4966970
11. Shoji T, Nakaishi T and Mikata M 1996 JP
08225543A
33. Tan Y, Xie H, Cui H and Han Y 2005 Catal. Today
104 25
34. Chen C S, Cheng W H and Lin S S 2003 Appl. Catal.
A: Gen. 238 55
12. Balpanov D S, Krichevskii L A and Kagarlitskii A D
2001 Russ. J. Appl. Chem. 74 2125
35. Huang L H, Chu W, Long Y, Ci Z M and Luo S Z
2006 Catal. Lett. 108 113
13. Fedolyak G T, Morozov A V, Kagarlitskii A D,
Krichevskii L A and Akad I 1990 Nauk Kaz. SSR,
Ser. Khim. 6 85
36. Chu W, Zhang T, He C H and Wu Y T 2002 Catal.
Lett. 79 129
14. Shoji T and Nakaishi T 1997 US 5693806
15. Forni L and Nestori S 1998 Stud. Surf. Sci. Catal. 41
291
37. Guerreiro E D, Gorriz O F, Larsen G and Arrua L A
2000 Appl. Catal. A: Gen 204 33
38. Turco M, Bagnasco G, Cammarano C, Senese P,
Costantino U and Sisani M 2007 Appl. Catal. B:
Environ. 77 46
16. Anand R and Rao B S 2002 Catal. Commun. 3
29
17. Anand R, Hegde S G, Rao B S and Gopinath C S
2002 Catal. Lett. 84 265
39. Gervasini A and Bennici S 2005 Appl. Catal. A: Gen
281 199
18. Dixon J K 1946 US 2400398
40. Zhao Y, Li F, Zhang R, Evans D G and Duan X 2002
Chem. Mater. 14 4286
19. Pfann H F, Greenwich and Dixon J K 1947 US
2414552
41. Mathew T, Vijayaraj M, Pal S, Tope B B, Hegde S G,
Rao B S and Gopinath C S 2004 J. Catal. 227 175
42. Mathew T, Tope B B, Shiju N R, Hegde S G, Rao
B S and Gopinath C S 2002 PCCP 4 4260
43. Barthos R, Lonyi F, Onyestyak G and Valyon J 2001
Solid State Ionics 141 253
20. Park I, Lee J, Rhee Y, Han Y and Kim H 2003 Appl.
Catal. A: Gen 253 249
21. Park I, Rhee Y, Lee J, Han Y H, Jeon J and Kim H
2003 Res. Chem. Intermed. 29 575
22. Cesar D V, Perez C A, Salim V M M and Schmal M
1999 Appl. Catal. A: Gen 176 205
44. Katada N, Endo J, Notsu K, Yasunobu N, Naito N
and Niwa M 2000 J. Phys. Chem. B104 10321
45. Roessner F, Hagen A, Mroczek U, Karge H G and
Steinberg K H 1993 Stud. Surf. Sci. Catal. B75 1707
23. Velu S, Suzuki K and Osaki T 1999 Catal. Lett. 62
159
24. Lindstrom B, Pettersson L J and Menon P G 2002
Appl. Catal. A: Gen. 234 111