STUDY of the mechanism of
CO TRANSFORMATION ON a Zn-Cu-Al
CATALYST USING QUANTUM
CHEMICAL CALCULATION
METHODS
Talgatova Aisulu
Dauletovna
Zhetysu University named
after I. Zhansugurov
Email: td.ai@mail.ru
The main approaches to
considering the mechanism of CO adsorption on low-temperature
Zn-Cu-Al catalysts for methanol synthesis are investigated. Using
quantum "chemical methods for the calculation of the analysis of
the synthesis mechanism, taking into account obra" the use of
positively charged chemisorbed complex. The structures of active
centers of catalysts are studied. The binding energies between the
adsorbed CO molecule and the active center of the catalyst are
calculated.
Keywords: Methanol, surface
synthesis mechanism, quantum chemical calculation methods, active
center, adsorption, binding energy.
Methanol is one of the most
widely used chemical products in the world, used in the oil and gas
industry as an inhibitor of hydrate formation in the production and
transportation of oil and gas. In addition, methanol is used to
produce chemical products that are processed into polymer
materials, dyes, solvents, medicines and other substances. Methanol
by its physical and chemical properties makes it possible to use it
not only directly as a fuel [1]. In modern installations, methanol
synthesis is carried out using two types of catalysts: high –
temperature-zinc – chromium and low-temperature-zinc-copper.
Zinc-chromium catalysts are thermally more stable, but have less
activity compared to low-temperature zinc-copper catalysts. When
using highly active Zn-Cu-Al catalytic systems, the selectivity of
methanol synthesis decreases in comparison with Zn-Cr catalysts.
However, the service life and thermal stability of Zn-Cu-Al
catalysts is significantly higher [2]. Therefore, there is a need
to preserve selectivity and create conditions for inhibiting the
side process of hydrocarbon synthesis by the Fischer-Tropsch
reaction on the metal centers of the catalyst formed under the
conditions of thermal decomposition of Zn-Cu clusters. The
description of this process using various physical and chemical
approaches allows us to take into account the mechanism of
formation of the active center on the surface of the Zn-Cu-Al
catalyst and the modes of cluster activation. Using the method of
mathematical modeling also allows us to solve the problem of
testing catalysts of various brands for the synthesis of methanol.
Therefore, the main goal of this work is to substantiate the
mechanism of methanol synthesis on the surface of Zn-Cu-Al
catalysts using quantum chemical calculations.Using the results
obtained, it is planned to create a mathematical model of the
methanol synthesis process, and the obtained thermodynamic values
will be used to find the constants of the reaction rates occurring
on the surface of the catalyst. The mathematical model developed in
this way will solve the main issues of existing industrial methanol
synthesis plants: forecasting the operation of the plant and
testing of catalysts from various manufacturers. When drawing up a
mathematical model of the process, it was decided to rely on the
generally accepted multi-level scheme for constructing the
structure of mathematical models of chemical processes, proposed by
M. G. Slinko [3]. At the first stage, the mechanism of the process
is clarified, on the basis of which a kinetic model of the process
and a model of substance transfer in the catalyst layer are
compiled. The next step is to create a mathematical model of the
reactor and the technological scheme as a whole. Currently, the
scientific literature considers two variants of elementary
mechanisms that occur in the synthesis of methanol on Zn-Cu-Al
catalysts. The first approach is based on the fact that the
synthesis of methanol is preceded by reactions of shock
substitution of water with carbon dioxide [4]. The water molecule
adsorbed at the first stage of the mechanism is subsequently
replaced on the surface of the catalyst by a CO2 molecule coming
from the gas phase, while the water molecule passes into the gas
phase. Then the adsorbed CO2 interacts with hydrogen, resulting in
the final product – methanol. This mechanism is characterized by
the following scheme of transformation of basic substances, where
[Me] is the metal center on the surface of the
catalyst:
The second approach is based
on the fact that the main substance of synthesis is a CO molecule,
which, adsorbed on the active center of a weakly reduced catalyst,
forms a positively charged chemisorbed me–CO complex [5, 6, 10].
The diffusion of hydrogen atoms under the influence of high
pressure increases, so that they "dissolve" in the near-surface
layer of the catalyst and when interacting with its surface, they
acquire an effective negative charge. In the course of successive
stages (Fig. 1) the Me–CO bond is saturated with negatively charged
hydrogen atoms, resulting in loosening of this bond and splitting
off of the final product – methanol. The main side reaction is the
formation of hydrocarbons by the Fischer–Tropsch reaction on the
metal centers of the Zn-Cu-Al catalyst that dynamically arise
during operation, which in the industrial synthesis of methanol are
continuously passivated by CO2 present in the initial synthesis gas
at a concentration of about 8 vol.%.
Appropriate laboratory studies
were conducted to confirm the proposed mechanisms. Experiments with
a labeled oxygen atom [7] have shown that the source of the oxygen
atom in the methanol molecule is carbon dioxide or water. At the
same time, spectroscopic analyses indicate the formation of a
positively charged chemisorbed Me–CO complex [5]. This paper
analyzes the mechanism that takes into account the formation of
surface compounds on the catalyst. This mechanism allows us to take
into account the physical and chemical properties of the surface of
the catalyst and most accurately predict its industrial operation.
Researchers do not have a clear idea about the structure of the
active center of the catalyst. Based on EPR studies, the authors of
[8] indicate that the structure of the catalyst contains fragments
of Cu2+ - O-M–O-Cu2+, which are probably the active centers of
synthesis. Some authors [6] talk about the introduction of copper
atoms into the crystal lattice of zinc oxide. To clarify the
structure of the active center and substantiate the chosen
mechanism, we performed confirming quantum chemical calculations.
The DFT – Density Functional Study method was chosen as the
calculation method for this work [9]. The theoretical approximation
was the B3LYP model, the theory of the Becke density functional
(B3), using the electronic correlation of Li Yang and Pair (LYP).
This model is suitable for calculations of heterogeneous systems,
and allows you to calculate the necessary energy parameters with
sufficient accuracy. The set 6–311G** was chosen as the basis [10].
The main criterion confirming the existence of this surface
structure was chosen as the binding energy between the active
center of the catalyst and the adsorbed carbon monoxide molecule.
This energy was calculated from the difference between the total
energy of the adsorbed structure and the sum of the total energies
of the CO molecule and the active center:
ERU=EClst–CO -
(EClst+ECO),
where ECB is the binding
energy between the cluster and the adsorbed CO molecule; EClst-CO
is the total energy of the cluster with the adsorbed CO molecule;
EClst is the total energy of the cluster; ECO IS the total energy
of the CO molecule. The calculation was performed for a temperature
of 543 K and a pressure of 6 MPa, since these values are the
average for the methanol synthesis reaction. The optimized
structures shown in Fig. 2. Geometric parameters of optimized
structures are shown in table. 1. in the course of optimization, no
imaginary oscillation frequency was obtained, which proves their
stationarity.
In the periodic literature [5,
11], there are opposite opinions about which of the metal atoms
adsorbs the CO molecule, so the cluster energy during CO adsorption
was calculated for both the zinc atom and the copper atom.
Simultaneous adsorption of a CO molecule on each metal is not
possible energetically, and therefore this form was not taken into
account. Also in the literature there is a mention of the structure
of the active center consisting of six atoms [8], i.e. of the
structure Cu2+ - O-M–O-Cu2+. Based on this view, the form of the
active center of the catalyst is proposed, Fig.
3.
An important task was to find
out the possibility of adsorption of the CO molecule on both the
zinc atom and the copper atom. The team of authors [11] speaks
about the adsorption of CO molecules on individual zinc or copper
atoms, but this statement is doubtful, since the probability of
complete destruction of the crystal lattice of the oxide catalyst
is extremely small. To calculate the binding energy of an adsorbed
CO molecule, the energy of a separate CO molecule and a free active
center was calculated at the same pressure and temperature values,
including using the calculation method used for adsorbed forms,
table. 2. as can be seen from table 1, the strongest bond is a
triatomic cluster with a CO molecule adsorbed on copper (Fig. 2, b)
– 308.46 kJ/mol. If the active metal is a zinc atom (Fig. 2, a),
the binding energy decreases by 25.1 kJ/mol – a value sufficient to
state that the active metal is a copper atom. The binding energy of
the adsorbed CO molecule on the surface of hexatomic clusters (Fig.
2, b, d) is more than 2 times less, and therefore it can be argued
that these structures are energetically
unprofitable.
Thus, in the course of this
study, a confirmatory calculation of the adsorption of a CO
molecule on a low-temperature methanol synthesis catalyst was
performed using quantum chemical methods. During the calculation,
it was shown that the most likely active center of the catalyst is
a three-atom cluster containing copper, zinc and oxygen atoms (Fig.
2, b). The most likely active metal in the cluster is a copper
atom, since the bond of the CO molecule adsorbed on copper is
greater than the rest. This does not exclude the possibility of
adsorption of the CO molecule on zinc, since the energy of this
bond is quite large – 283.36 kJ/mol, but still less than in the
case of adsorption on copper. Therefore, a three-atom cluster with
CO molecule adsorption on copper was chosen for the mathematical
model.
Conclusions: the main approaches to
consideration of CO adsorption on low-temperature
Zn-Cu-Al-catalytic systems of methanol synthesis are Analyzed. The
synthesis mechanism based on the formation of a positively charged
chemosorbed complex on the surface of the catalyst was analyzed
using quantum chemical calculation methods. The m ost frequently
encountered structures in the literature, consisting of three and
six atoms, were selected as the active center of the catalyst.
Based on the values of binding energies obtained during the
calculation, it is shown that the most energetically advantageous
is the triatomic active center, whose binding energy is 308.46
kJ/mol.
LIST OF
REFERENCES
1. Sekunova M. V. Methanol-an
important raw material resource for the production of fuels / / Mir
petrochemicals. - 2007. - № 4. - P. 4-8.
2. Karpov S. A., Kunashev L.
H., Mortikov E. S., Kapustin V. M. Production of methanol: the
current state of industry and development trends / / oil Refining
and petrochemistry. - 2009. - № 7. - P.
3-8.
3. Slinko M. G. Modeling of
chemical reactors. Novosibirsk: Nauka, 1968, 96
p.
4. Kravtsov A.V., Novikov A.
A., Koval P. I. Computer analysis of technological processes.
Novosibirsk: Nauka, 1988, 216 p.
5. Kravtsov A.V. on the
dynamic features of the reaction mechanism of carbon monoxide
hydrogenation // Questions of kinetics and catalysis.
Interuniversity collection. Ivanovo: Nauka, 1980, Pp.
33-40.
6. Weigel J., Koeppel R.,
Baiker A. Surface species in CO and CO2 hydrogenation over
copper/zirconia: on the methanol synthesis mechanism / / Langmuir.
- 1996. - no. 12. - P. 5319-5329.
7. Takeuchl A., Katzer J.
Mechanism of methanol formation / / Jour nal of Physical Chemistry.
- 1981. - V. 52. - No. 85. - P. 937-939.
8. Altynnikov A. A.,
Anufrienko V. F., Rozovsky A. Ya. Detection of copper ion clusters
in cu-Zn-Al oxide catalysts for methanol synthesis according to EPR
data / / Kinetics and catalysis. - 1999. - Vol. 40. - No. 1. - Pp.
129-133.
9. Gokhale A. A., Kandoi S.,
Greely J. P., Mavrikakis M., Dumesic J. A. Molecular-level
description of surface chemistry in kinetic models using density
functional theory // Chemical Engineering Science. – 2004. – V. 59.
– № 22-23. – P. 4679-4691.
10. Polishchuk O. Kh., Kizhner
D. M. Chemical research by methods of calculating the electronic
structure of molecules. - Tomsk: Izdvo TPU, 2006. - 146
p.
11. HyeWon Lim, MyungJune
Park, SukHwan Kang, HoJeong Chae, Jong Wook Bae, KiWon Jun.
Modeling of the Kinetics for Methanol Synthesis using
Cu/ZnO/Al2O3/ZrO2 Catalyst: Influence of Carbon Dioxide during
Hydrogenation // Ind. Eng. Chem. Res. – 2009. – V. 23. – № 48. – P.
10448–10455.