A comparative study on the catalytic activity of ZnAl, NiAl, and CoAl mixed oxides derived from LDH obtained by mechanochemical method in the synthesis of 2-methylpyrazine

Article abstract of DOI:10.1016/j.catcom.2019.105829

Mixed oxides (mo)-ZnAl; NiAl; CoAl (M²⁺/Al = 3 M ratio) were obtained from layered double hydroxides (LDH) precursors synthesized by mechanochemical method. The solids were characterized by XRD, DRIFT, H₂-TPR, NH₃/CO₂-TPD and determination of textural properties by N₂ adsorption-desorption isotherms. The catalytic activity was tested in the dehydration-cyclization of ethylenediamine with propylene glycol, followed by the dehydrogenation of the obtained piperazine, yielding the 2-methylpyrazine (2-MP) as the main reaction product. At 400 °C, the conversion of both reactants was quasi-total, but the yield to 2-MP was close to 90% on the solids with more active sites favoring dehydration-cyclization (mo-ZnAl and mo-CoAl).

Full text of DOI:10.1016/j.catcom.2019.105829

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A comparative study on the catalytic activity of ZnAl, NiAl,
and CoAl mixed oxides derived from LDH obtained by
mechanochemical method in the synthesis of 2-methylpyrazine
F. Teodorescu, A.I. Slabu, O.D. Pavel, R. Zăvoianu
PII:
S1566-7367(19)30291-2
DOI:
https://doi.org/10.1016/j.catcom.2019.105829
CATCOM 105829
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
12 July 2019
10 September 2019
20 September 2019
Please cite this article as: F. Teodorescu, A.I. Slabu, O.D. Pavel, et al., A comparative
study on the catalytic activity of ZnAl, NiAl, and CoAl mixed oxides derived from LDH
obtained by mechanochemical method in the synthesis of 2-methylpyrazine, Catalysis
Communications (2018), https://doi.org/10.1016/j.catcom.2019.105829
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A comparative study on the catalytic activity of ZnAl, NiAl, and CoAl mixed oxides
derived from LDH obtained by mechanochemical method in the synthesis of 2-
methylpyrazine
F. Teodorescu1*, A.I. Slabu1, O.D. Pavel2, R. Zăvoianu2
1”C.D. Nenitzescu” Center for Organic Chemistry, 202 B Spl. Independenței, S6, 060023
Bucharest, Romania
2University of Bucharest, Faculty of Chemistry, Department of Organic Chemistry,
Biochemistry and Catalysis, 4-12 Regina Elisabeta Av., S3, 030018 Bucharest, Romania
*Tel:+40 213167900, e-mail: florina.teodorescu@ccocdn.ro
Abstract
Mixed oxides (mo)-ZnAl; NiAl; CoAl (M2+/Al=3 molar ratio) were obtained from layered
double hydroxides (LDH) precursors synthesized by mechanochemical method. The solids
were characterized by XRD, DRIFT, H2-TPR, NH3/CO2-TPD and determination of textural
properties by N2 adsorption-desorption isotherms. The catalytic activity was tested in the
dehydration-cyclization of ethylenediamine with propylene glycol, followed by the
dehydrogenation of the obtained piperazine, yielding the 2-methylpyrazine (2-MP) as the
main reaction product. At 400°C, the conversion of both reactants was quasi-total, but the
yield to 2-MP was close to 90% on the solids with more active sites favoring
dehydration-cyclization (mo-ZnAl and mo-CoAl).
Keywords: mechanochemical, LDH, bifunctional catalyst, 2-methylpyrazine

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1. Introduction
Alkylpyrazines are a class of N-heterocyclic compounds often found in the composition of
natural and synthetic compounds. Consequently, there are numerous applications of
alkylpyrazines in the industrial area (e.g. food, agriculture and pharmaceutics). Due to the
wide variety of applications, the synthesis of alkylpyrazines is a matter of high interest in
organic chemistry. The most often utilized synthesis route is a coupling reaction assisted by a
catalyst. 2-methylpyrazine (2-MP) is an important intermediate for the synthesis of the front-
line antitubercular drug pyrazinamide [1]. As most alkyl pyrazines, 2-MP has flavor
properties, which makes it suitable for usage in the food and beverage industry.
One of the most effective synthesis methods applied for the obtaining of 2-MP is the
dehydration-cyclization of ethylenediamine with propylene glycol, followed by the
dehydrogenation of the obtained piperazine (Supplementary Information: Scheme 1) [1].
The first catalysts patented for this process contained mixtures of oxides with redox properties
such as Cu-Cr [2], Cu-Zn-Cr [3]; or Ag-La-Al2O3 [4]. Other patented catalytic systems were
based on the combined action of zinc phosphates with manganese or silver acting as the
element with redox properties [5].
The catalysts containing ZnO and Cr2O3 promoted with Pd salts were intensively studied by
Forni and coworkers [1] and showed a maximum yield to 2-MP of 54%. The structural
characterization of the catalyst precursor prepared by co-precipitation contains a mixture of
crystallographic phases, namely two different hydrotalcite-like phases, zincite and ZnCr2O4.
Meanwhile, the crystallographic structure of the catalyst obtained after the addition of Pd
promoter followed by calcinations and reduction revealed a mixture of zincite and ZnCr2O4
phases [6]. The mechanistic studies performed on this type of catalyst revealed that the
formation of 2-MP involves an Eley-Rideal mechanism on bifunctional active sites and the
fact that Pd plays the role in decreasing the selectivity to acetone by-product [6].

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Other bifunctional catalysts, with dehydration and dehydrogenation properties such as: zinc-
modified ferrierite [7], zinc modified zeolites (ZSM-5 or BETA) [8], and ZnO/SiO2 based
catalysts [9] were also studied, but the yield to 2-MP did not exceed 49%. Among the most
promising catalysts developed for this reaction are a Cr-promoted Cu-Zn/Al2O3 catalyst which
allowed reaching 84.5% yield to 2-MP at 380 °C [10] and Mn2Fe2O4 that yielded 80% 2-MP
at 450 °C [11].
A catalyst obtained by mechanical mixing of ZnO and Cr2O3 allowed the achieving of a high
selectivity to 2-MP (85%), but the yield was only 20.4% [12].
More promising results were obtained using catalytic systems obtained from layered double
hydroxide precursors such as Zn-Cr or CuCoAl [13,14] which gave high yields to 2-MP at
400 °C (e.g. 71.5% and 56% respectively).
Layered
double
hydroxides
(LDH)
are
anionic
clays
having
3+
[M2+
M
1-x
x(OH)2]x+[An−x/n]·mH2O general formula, where M2+and M3+ represent divalent
and trivalent cations in the brucite-type layers, A is the interlayer anion with a n charge
balancing the exceeding charge occurring by isomorphic substitution of M2+ by M3+, x is the
fraction of the trivalent cation and m is the crystallization water [15]. These materials are
obtained by numerous methods of preparation such as co-precipitation [16] of the salts
aqueous solutions containing the target cations with an alkaline solution, hydrothermal
synthesis of physically mixed oxides under temperature and pressure conditions [17], ion
exchange [18], etc. Among the above mentioned methods, constant pH at low supersaturation
co-precipitation is the most commonly used one. However, all these methods present the
disadvantage of a multistep process and the high energy consumption. A new unconventional
preparation method for LDH, but not new to getting other materials, is the so called
mechanochemical method, where only a milling of the reactants followed by washing and
drying is involved [19]. In the literature there are several publications on the use of this

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method for LDH obtaining, mostly on the obtaining of MgAl systems with different
-
2-
2-
compensation anions such as Cl-, NO3 , SO4 , CO3 , OH- [20]. In addition to the synthesis of
MgAl type LDH using mechanochemical route, other layered materials have been synthesized
in which other cations than magnesium and aluminum are involved such as: MgFeAl and
MgNiAl [21], MgFe and ZnAl [22], CoFe [23], NiFe [24], LiAl [25], CaAl [26], ZnCaAl
[27], CuAl [28] etc. It has also been attempted to prepare an MgAlLa-LDH modified with
lanthanum in order to increase the basic character [29] with no success due to the too large
ionic radius of the modifying cation.
Considering the large interest for the obtaining of 2-MP, finding new materials able to
catalyze this reaction is still a challenge. One desiderate would be to avoid the use of Cr in the
composition of the catalyst due to the environmental issues caused by Cr species. Another
desiderate would be to apply an economical synthesis route for the obtaining of the catalyst
which could be achieved using the mechanochemical route. To this aim it was considered
interesting to investigate the possibility to obtain binary catalytic systems ZnAl, NiAl and
CoAl from the corresponding LDH precursors prepared by mechanochemical method. Among
these three systems, only the synthesis of ZnAl by mechanochemical method was already
reported. Based on these assumptions, this work is intended to perform a comparative analysis
of the physico-chemical properties as well as the catalytic activity for 2-MP synthesis
exhibited by NiAl, ZnAl and CoAl catalytic materials prepared by calcination of
mechanochemically-synthesized LDH.
2. Experimental
2.1. Preparation of the catalysts
ZnAl-, NiAl-, and CoAl-LDH with molar ratio of 3 were prepared by the mechanochemical
route, where the corresponding quantities of nitrates as well as Na2CO3 and NaOH were

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directly milled in the mortar grinder apparatus with pestle speed 100 rpm for 1 h, at 25 °C.
The resulted paste was then washed with bi-distilled water until pH of 7 of the washing water
was reached and dried at 90 °C for 24 h in air flow. The solids obtained were annotated with
ZnAl; NiAl and CoAl. The calcination of the dried samples at 460 °C for 18 h in air flow led
to corresponding mixed oxides annotated mo-ZnAl; mo-NiAl and mo-CoAl.
2.2. Characterization of the catalysts
Powder X-ray diffraction patterns were recorded with a Shimadzu XRD 7000 diffractometer
using Cu Kα radiation (λ = 1.5418 Å , 40 kV, 40 mA) at a scanning speed of 0.10°·min-1 in
the 5-70° 2θ range. DRIFTS spectra obtained from accumulation of 400 scans in the domain
400-4000 cm-1 were recorded with a NICOLET 4700 spectrometer. N2 adsorption-desorption
isotherms were determined using a Micromeritics ASAP 2010 instrument. Prior to nitrogen
adsorption, samples were out gassed under vacuum for 24 h. The measurements of acidity and
basicity were carried out by CO2-TPD and NH3-TPD using a micro-reactor assembly coupled
to a TCD detector. The mixed oxides samples were pre-treated in nitrogen at 460 °C for 2 h
and then cooled at 100 °C prior to the adsorption of CO2/NH3 at this temperature. After the
adsorption of CO2/NH3 (30 mL∙min-1) for 1 h, the sample was flushed with N2 for 1 h in order
to remove the physisorbed CO2/NH3 from the surface. The desorption was performed under
nitrogen flow (1 mL·s-1) at a heating rate of 10 °C∙min-1 from 100 to 500 °C in the case of
NH3-TPD and up to 650 °C in the case of CO2-TPD and the signal of the TCD detector was
recorded. The acidity and basicity of the supports were quantified by integration of the
desorption curves of NH3 and CO2, respectively. The values of the areas are proportional to
the number of moles of NH3 and CO2, respectively, adsorbed on the catalyst surface. H2
temperature programmed reduction (H2-TPR) was measured on a Porotec TPDRO 1100
device (Thermo Fisher Scientific Inc., Waltham, MA, USA). A 100 mg sample was placed in

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the microreactor and the surface was cleaned in He flow at 460 °C for 1 h and cooled to room
temperature under He (40 mL·min-1). The sample was heated under a mixture of 5 vol. % H2
in He from room temperature to 800 °C with a heating rate of 5 °C·min-1 and the amount of
H2 consumption during the reduction was measured using the thermal conductivity detector of
the apparatus. The morphology of the samples was investigated using a Hitachi Scanning
Electron Microscope (SU8010) at landing voltage 2kV, and 500 magnification. The
microphotographs of the solids are presented in the Supplementary Information.
2.3. Catalytic tests
The chemicals used were both purchased from Sigma Aldrich: propylene glycol 99% purity
and ethylenediamine for synthesis, which was purified by distillation prior to the reactions.
The reactions for 2-methylpyrazine synthesis using ZnAl; NiAl; CoAl mixed oxides as
catalytic materials were performed in a fixed bed reactor made of heat resistant glass (230 mm
height – 15 mm diameter). The middle part of the reactor, containing the catalyst bed (11 mm
height, dp=60 mesh), was heated at the desired temperature. Prior to the reactions, the catalyst
was activated at 400 °C for 2h in a flow of gas mixture (H2/Ar = 1/1, vol.). The liquid
reactants were introduced with a controlled feed rate (1 mL·h-1) from the top side of the
reactor tube, using a peristaltic pump. The aqueous reaction mixture was prepared by diluting
propylene glycol and ethylenediamine (mole ratio = 1/1) in distilled water (50% vol.). The
water was used for reducing the viscosity of the mixture. The inert carrier gas used was pure
argon, at a flow rate of 15 mL∙min-1. The reaction products were captured in an ice-cooled
trap and further analyzed by Agilent GC-MS 6890N equipped with Agilent HP-5ms capillary
column (0.25 mm x 30 m x 0.25 μm). A standard calibration method with synthetic mixtures
was used for quantification of data. The reaction products were also evidenced by HPLC
studies with an Agilent 1200 series chromatograph at 40 °C using H2O/ACN 30/70 (vol.)

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mobile phase on a Zorbax SB-C18 column (250 x 4.6 mm) at a flow rate of 0.6 mL∙min-1. The
products were identified by comparison with a standard sample.
3. Results and discussions
3.1. Catalysts characterization
The XRD patterns show narrow, symmetrical and high intensity lines at low values of 2 theta
and low intensity and non-symmetrical lines at high values of 2 theta, structure characteristic
for solid layered materials (Supplementary Information: Fig. SI1). Only CoAl and NiAl
present typical patterns of a pure layered double hydroxide structure while ZnAl shows some
impurities from ZnO phase presence (JCPDS 36-1451). Assuming packing layers of 3R, from
the first and second harmonic of the reflection planes 003 and 006, a lattice parameter
c = (3/2)(d003+2d006) (related to the interlayer distance depending on the hydrated anion size
and on the electrostatic forces between the layers and interlayer anions) can be calculated, as
well as cation-cation distance from the diffraction line position derived from the crystalline
plane (110), a = 2d110 [30]. The values are presented in Supplementary Information: Table
SI1. However, the ZnAl present the larger interlayer (c=23.744 Å compared to 23.562 Å and
23.586 Å for CoAl and NiAl, respectively) which can be explained by ZnO presence as well
as the ionic radius of Zn2+ (0.74 Å compared to 0.6 Å and 0.69 Å for Co and Ni respectively).
The large ionic radius has major influences even on the network parameter a. These patterns
certify that the mechanochemical method affords the obtaining of layered materials by simply
mixing the precursors.
The calcination at 460 °C for 18 h of the layered double hydroxides leads to mixed oxides
type materials, Fig. 1. However, for mo-NiAl there is only a line for NiO with no others
diffraction lines corresponding to alumina or spinel-type phases. The presence of aluminum in
layers shifts the reflections of NiO from mixed oxides towards 2 theta higher values than in

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pure nickel oxide. mo-CoAl presents the diffraction lines corresponding to Co3O4(JCPDS 42-
1467) which is the spinel phase of cobalt (II) and cobalt (III) due to the easy oxidation of Co2+
to Co3+ in contact with the air (calcination atmosphere used). However, from the
thermodynamic point of view the stability of Co3O4 is highest compared to CoO [31]. The
reflections in XRD pattern of calcined ZnAl (mo-ZnAl) are characteristic of the ZnO.
Fig. 1.
DRIFT spectra of dried samples, Supplementary Information: Fig. SI2, shows the typically
absorption bands of LDH with 3500-3600 cm-1 attributed to the H-bonding stretching of the
OH group in the layer, at 3000 cm-1 related to hydrogen bonding between water and carbonate
in the interlayer of the LDH, 1720 cm-1 is due to H2O bending vibration of interlayer water in
the LDH structure, 1450 cm-1 and 674 cm-1 are assigned to CO32- group vibration bands in the
LDH. In addition to these, the spectra show bands in the region 1200-700 cm-1 characteristic
2-
to carbonates. During calcination, Fig. 2, part of OH- and CO3 are removed from the
structure and the intensity of the absorption bands at 3500-3600 cm-1, 3000 cm-1 and
1720 cm-1 decreased, except for the sample mo-ZnAl. For mo-CoAl the bands at 760 and
660 cm-1 correspond to ν1 and ν2 vibrations of spinel phase [32].
Fig. 2.
The specific surface areas and the results of basicity and acidity determinations by CO2-TPD
and NH3-TPD are displayed in Table 1. The thermo desorption profiles are presented in the
Supplementary Information (Fig. SI3 and SI4).

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The specific surface areas of the mixed oxides samples vary in the range of 128-151 m2∙g-1
Table 1. The mo-CoAl presents the highest surface area, as well as the highest acid character
as revealed by the ratio between the total number of acid and base sites (e.g. 1.81). The
variation of the acid character follows the order: mo-CoAl (1.81) > mo-NiAl (1.5) > mo-ZnAl
(0.84) which does not respect the variation of electronegativity (in Pauling scale) presented by
divalent cations, namely: Ni (1.91) > Co (1.88) > Zn (1.65). A possible explanation for this
behavior could be the presence of Co3O4 impurities in mo-CoAl. The number of strong Lewis
acid sites was associated to the area of the NH3 desorption peak at the highest temperature, as
it was suggested by Anand et al. [7] and Roessner et al. [33] who established a correlation
between the amount of these types of sites determined by pyridine adsorption and the NH3-
TPD profiles. Based on the aforementioned considerations it may be concluded that the
distribution of acid sites strength revealed that the strong Lewis acid sites prevailed on mo-
CoAl and mo-ZnAl, while on mo-NiAl medium strength acid sites were prevailing.
Table 1.
The microphotographs of the solids presented in the Supplementary Information (Fig. SI5)
revealed that mo-ZnAl has the most regular morphology of with small oval shaped particles
(5-12 m size). The mo-CoAl sample showed a mixture of some triangular shaped particles
with large dimensions (50-60 m) and smaller oval shaped particles (3-10 m). The mo-NiAl
sample had the most heterogeneous morphology, presenting a mixture of small oval particles
along with larger oval and prismatic shapes.
The characterization of the samples using H2-TPR, Fig. 3, showed for mo-CoAl sample two
different thermoreduction peaks one at lower temperatures (370 °C) (Co2+→Co1+) and the
second one at high temperature (694 °C) (Co1+→Co0). For mo-NiAl both peaks appear at

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lower temperatures than in the previous case, while for mo-ZnAl the intensity of the TPR
signal is significantly lower than that for the previous samples and the maximum intensity is
noticed at 715 °C indicating a better stability of the mixed oxide under reduction conditions.
Fig. 3.
3.2. Catalytic activity
3.2.1. Influence of reaction temperature
The experiments carried out in the temperature range of 340-400 °C showed that temperature
has a significant effect on the course of the reaction, both the reduction temperature of the
catalyst and the reaction temperature playing an important role in what concerns the
selectivity to 2-MP. In all cases the total conversion of ethylenediamine and propylene glycol
was reached. For instance, in the case of mo-ZnAl material, Supplementary Information:
Table SI2, the resultant products distribution changes according to the temperature.
As the temperature was increased, there was an increase in selectivity of 10% for 2-MP from
79.2% at 340 °C to 89.4% at 400 °C while the selectivity for both pyrazine and alkylpyrazine
type compounds halved. At lower temperature, there was an important quantity of side
products obtained through the cracking of propylene glycol [34] and formation of alkyl
carbocations necessary for alkylation to the detriment of the cyclo-dehydration reaction which
is energy demanding [35]. In order to support our assumption that the distribution of products
changes with the temperature, the same experiments were performed in the case of the other
materials, under the same reaction conditions. The effect of the temperature on the
distribution of the resultant products was almost the same for all the materials. Taking these
findings into account, 400 °C was chosen as suitable reduction and reaction temperature for
further experiments.

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3.2.2. Influence of the catalyst quantity
Catalytic tests have been performed on different quantities of catalyst, at the same reactant
flow rate of 1 mL∙h-1. It has been observed that the selectivity for 2-MP decreases with the
increase in catalyst quantity, while the selectivity for secondary products, especially alkyl
pyrazines is increasing. The same tendency has been observed for all three catalysts. The
results are presented in Fig. 4 for mo-ZnAl and in Supplementary Information: Fig. SI6 and
SI7 for mo-CoAl and mo-NiAl respectively.
Fig. 4.
It appears that a higher number of acid catalytic sites leads to a higher probability of side
reactions, mainly alkylation of pyrazine and 2-MP with propylene glycol cracking products.
The increase in catalytic bed height could also contribute to the decrease in 2-MP selectivity,
due to the increase of contact time, which also increases the probability of side reactions.
3.2.3. Influence of the reactants flow rate
Reactions have been performed at different reactant flow rates (1-3 mL·h-1), using the same
amount of catalyst (0.5 g). Out of the catalysts tested, the mo-ZnAl and the mo-CoAl catalysts
presented similar activities, with the mo-ZnAl catalyst exhibiting a slightly higher selectivity
for 2-MP. The mo-NiAl catalyst presents a different activity, with the conversion as high as
for the other two, but with a much lower selectivity for 2-MP, most probably due to the
activity of Ni for competing hydrocracking reactions. This result can also be correlated to the
distribution of the acid sites presented in Table 1, showing an increased amount of strong acid
sites responsible for dehydration-cyclization for both mo-CoAl and mo-ZnAl, while the
amount of this type of sites is much lower for mo-NiAl. On the other hand considering the

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TPD profiles presented in the Supplementary Information (Figs. SI3, SI4) it may be inferred
that in the case of mo-NiAl, due to the higher amount of strong base sites and lower amount
of strong acid sites, the adsorption of propylene glycol which has a higher pKa value than
ethylendiamine (14.9 vs. 9.69; see ChemAxon.com) would be lower compared to mo-ZnAl
and mo-CoAl. Meanwhile, the adsorption of propylene glycol and its activation would be
favored on mo-CoAl and mo-ZnAl which have lower strong basicity and higher strong
acidity. Therefore it could be inferred that an Eley-Rideal reaction mechanism involving the
reaction between the adsorbed propylene glycol species and ethylenediamine from the
gaseous phase could be responsible for the transformation, as it was suggested by several
other authors [6, 11]. Another explanation for the lower selectivity to 2-MP exhibited by mo-
NiAl could be the heterogeneous morphology of its particles which was revealed by SEM
(See Supplementary Information Fig. SI5). The lower selectivity noticed for mo-CoAl
compared to mo-ZnAl may be also a consequence of the lower homogeneity of the
morphology of the former sample (see Fig. SI5), as well as of the presence of Co3O4 phase.
As it can be observed in Fig. 5, for all the 3 catalysts, the selectivity for 2-MP decreases with
the increase of the reactants flow rate for a low catalyst quantity.
Fig. 5.
For instance, a reaction performed over 0.5 g of mo-CoAl catalyst, at a reactant flow rate of
1 mL∙h-1, gave 88% selectivity for 2-MP and 5.5% selectivity for alkyl pyrazines. Increasing
the reactant flow rate to 3 mL∙h-1, for the same catalyst quantity, leads to a slight increase in
alkyl pyrazines selectivity (9.6%), at the expense of 2-MP (85.4%). However, if the catalyst
quantity is increased, the selectivity for 2-MP also increases, at higher flow rates as shown in
Fig. 6 in case of mo-CoAl material. Even if the increase in catalyst quantity leads to an

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increase in catalytic sites, a higher flow rate leads to a decrease in contact time, which lowers
the probability of secondary products formation. For a maximum selectivity to 2-MP the
increase of reactant flow rate can be compensated by the increase of catalyst quantity. An
optimal contact time is very important for obtaining the desired selectivity.
Fig. 6.
3.3. Catalyst reusability
To establish the stability over time of the layered catalytic materials the catalyst which
showed the most suitable activity (mo-ZnAl) has been tested for 5 consecutive reaction cycles
under the same conditions (0.5 g of catalyst, reactant flow rate 1 mL∙h-1) and the results are
presented in Supplementary Information: Fig. SI8. After the fifth reaction cycle the results
show that there are no sensible changes of the conversion compared to the initial value, while
the selectivity for 2-MP decreased slightly, from 89 to 86%. It may be concluded that the mo-
ZnAl catalyst is thermally stable, and the catalytic sites can be regenerated after they were
submitted to the reaction conditions, since as it is proven by the TPR analysis, the reduction
of ZnO species does not begin until 500 °C (see Supplementary Information, Fig. SI9). The
characterization of the recycled catalyst by XRD ((see Supplementary Information, Fig. SI10)
revealed that its structure was not altered.
4. Conclusions
This research presented the mechanochemical method as a facile and economic route to
produce hydrotalcite-type materials where the M2+ are Co, Zn and Ni and M3+ is Al. The
layered structure has been demonstrated by X-ray diffraction and DRIFT. We have proved
that the mixed oxides obtained by calcination of the corresponding layered double hydroxides

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present the ditopic character having acid and base sites (bifunctional catalyst) necessary for
2-methylpyrazine synthesis by dehydration-cyclization of ethylene diamine with propylene
glycol, followed by the dehydrogenation of the obtained piperazine. Even if in terms of
conversion the three materials presented similar activities, regarding the selectivity to
2-methylpyrazine, the mo-ZnAl and mo-CoAl presented better results as a consequence of the
increased concentration of strong Lewis-type sites that favored the dehydration-cyclization. In
these binary systems, the redox capacity of the bivalent cation was not a key factor to the
transformation.
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgment
This work was supported by the Executive Agency for Higher Education, Research,
Development and Innovation (UEFISCDI): PN-III-P3-3.5-EUK-2016-0040.
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Figure captions
Fig. 1. The XRD patterns of mixed oxides obtained by calcination of LDH.
Fig. 2. DRIFT spectra of mixed oxides obtained by calcination of LDH.
Fig. 3. Comparative H2-TPR traces obtained for mixed oxides samples.
Fig. 4. Influence of mo-ZnAl catalyst loading on pyrazine compounds selectivity at a reaction
temperature of 400 °C. Flow rate of reaction mixture 1 mL∙h-1.
Fig. 5. Influence of the reactant flow rate on pyrazine compounds selectivity, for all three materials,
at a reaction temperature of 400 °C, over 0.5 g of catalyst.
Fig. 6. Influence of the reactant flow rate on pyrazine compounds selectivity, over different CoAl
catalyst loadings.

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Table 1. The textural properties of the mixed oxides obtained by calcination of LDH.
Total
acidity
(mmol
NH3∙g-1)
2.9
Acidity
Surface
area
Basicity
(mmol
Catalysts
Weak
Medium
Strong
(m2∙g-1)
CO2∙g-1)
160-200 °C 201-350 °C 351-500 °C
mo-CoAl
mo-NiAl
mo-ZnAl
151
131
128
0.3
0.6
0.2
1
1.6
0.3
0.8
1.6
1.4
1.3
2.1
1.2
0.1
1.1

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 ZnAl, CoAl and NiAl LDH materials have been mechanochemically synthesized
 NiAl, CoAl mixed oxides were obtained for the first time from the above precursors
 mo-CoAl and mo-ZnAl catalysts yielded cca. 90% to 2-MP at 400 °C
 the catalytic activity was influenced mostly by the strong Lewis acid sites
 mo-ZnAl showed exceptional good recyclability

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6