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5CHEMICAL SCIENCES
MICROWAVE-ASSISTED HYDROTHERMAL SYNTHESIS OF NANOSIZED Co-Mn-OXIDE CATALYSTS AND THEIR INITIATING ACTIVITY IN LIQUID-PHASE OXIDATION OF ALKYL
AROMATIC HYDROCARBONS
Hasanguliyeva N.,
ORCIDID: orcid.org/0000-0003-4709-238X
Shakunova N.,
ORCID ID: orcid.org/0000-0002-9218-6839
Litvishkov Y.
ORCID ID: orcid.org/0000-0003-0662-1257
Institute of Catalysis and Inorganic Chemistry named after academician M. Nagiyev of Azerbaijan National
Academy of Sciences, Baku
DOI: 10.5281/zenodo.7014299
ABSTRACT
It has been carried out the synthesis of highly dispersed, nano-sized Co-Mn-oxides as prospective catalysts for the liquid-phase oxidation of alkyl aromatic hydrocarbons by the method of hydrothermal treatment of co-precipitated cobalt and manganese hydroxides and subsequent exposure to microwave radiation. The possibility of varying the phase composition and dimensioning specifications of the synthesized Co- and Mn- double oxides by changing the conditions of their main preparation stage - thermal treatment in a microwave field has been shown.
The activity of the Co-Mn-oxide samples, which combine manganese cobaltite and cobalt manganite in their composition at the stage of initiating the model reaction of liquid-phase oxidation of m-xylene has been studied. The correlation between the initiating activity of the samples and the content of the manganese cobaltite phase has been established.
Keywords: Cobalt and manganese oxides, hydrothermal synthesis, microware exposure, liquid-phase oxidation, nano-sized catalysts.
Introduction
Liquid-phase oxidative conversion of alkylar-omatic hydrocarbons is based on the promising methods for obtaining various oxygen-containing organic compounds - acids, aldehydes, ketones, alcohols, which are used in the synthesis of a wide range of products and materials with complex valuable properties [1-4, p.240, p. 3958-3997, p. 2743, p. 99-101].
Homogeneous systems (salts, or organic complexes of metals of variable valence) are used as catalysts in implementing most of the existing methods for liquid-phase oxidation of hydrocarbons; their use, as is known, makes certain technological difficulties associated with the separation of final products and residual amounts of catalysts, solvent, bromine compounds, etc. contained in their mass [5,6; p. 9, P. 303].
Carrying out the processes in the presence of heterogeneous contacts, among which mixed oxides of Co and Mn are most often mentioned, does not eliminate the difficulties of controlling the total reaction rate and selectivity by targeted products [7, p. 203-204], although it makes it possible to significantly simplify the separation technology and improve the degree of purity of the isolated products.
This situation is mostly due to the uneven distribution of the dispersed catalyst in the volume of the liquid phase and leads to the occurrence of undesirable, side, radical chain conversions in the non-catalytic volume of the reaction space.
The above difficulties are aggravated due to the uneven heating of the peripheral and deep margins of the reaction medium, as well as due to the occurrence of a gradient during the distribution of the temperature field from the oxidizable hydrocarbon to the surface of heterogeneous catalyst particles.
Previously, we found that carrying out the liquid-phase oxidation reaction of meta- and para-xy-lene to aromatic monocarboxylic acids in the presence of bulk structure heterogeneous catalysts capturing the entire reaction space makes it possible to minimize the undesirable non-catalytic conversion of xylenes and significantly increase the selectivity to monocarboxylic acids [8, p. 21-23].
However, the complexity of the preparation of catalysts of this type caused to organize studies in direction of obtaining ultradisperse (nano-sized) binary combinations of manganese and cobalt oxides as potential catalysts for the liquid-phase oxidation of alkylaromatic hydrocarbons with their expected uniform distribution in the reaction volume.
In recent years, the hydrothermal method has been mostly used to obtain nano-crystalline oxide materials due to its relative simplicity and low cost, which makes it possible to control the morphology of a dispersed product by varying the process parameters [9, 10; p. 388-393, p.103].
It should be noted that, as a result of subsequent thermal treatment of hydrothermal synthesis products under conventional long-term heating, the
formation of oxide phases is accompanied by undesirable agglomeration of fine particles leading to a decrease in their catalytically active surface.
In this work, it has been made an attempt at combination of the stage of hydrothermal synthesis and thermal treatment of precursors (drying and calcination) under microwave radiation in order to improve the method of obtaining nano-dispersed Co-Mn - oxide systems. A comparative evaluation of the initiating activity of the synthesized catalysts has been carried out in the reaction of liquid-phase oxidation of m-xylene.
Experimental part
Cobalt and manganese nitrates of chemically pure label have been used as precursors in the synthesis of binary Co-Mn-oxide systems, coprecipita-tion of which has been carried out before hydrothermal treatment with an aqueous solution of ammonium hydroxide at pH = 7.5-8.0 in the form of amorphous hydroxides.
The stage of hydrothermal (autoclave) treatment of Co and Mn hydrogels has been carried out in a hermetically-sealed autoclave heated by an electric spiral furnace made of stainless steel of 12X18H10T mark designed for an overpressure of 5 MPa and equipped with a standard pressure gauge and also a vapor locking system for inlet and outlet.
The sample processing have been accomplished in a temperature regime of 200-2500C; a water vapor pressure was 15-30 atm.; variation of the reaction time - in the range of 3.0-5.0 hours.
Thermal treatment of the samples after auto-claving has been carried out on a setup designed on the basis of the EM-G5593V microwave oven (Panasonic) with a resonator volume of 25 L by varying the power of the magnetron from 200-800 W with an operating frequency of 2450 MHz. The temperature of the charge has been monitored with a CEMDT-8858 infrared pyrometer with an interval of the temperature measurement (50-1300°C).
Variation of microwave treatment mode of the samples (drying 110-1150C, calcination 400-
5500C) has been carried out by changing both the power of microwave radiation (250-800 W) and the exposure time of the samples in the oven resonator 5.0-10.0 min.
Liquid-phase oxidation of m-xylene has been achieved in a bubbling-type column reactor in a unit that combined the reaction unit with an external feeding system for oxygen gas flow. The absorption rate of oxygen has been estimated from the readings of a liquid manometer. X-ray phase analysis of the obtained binary Co-Mn - oxide systems has been carried out on an automatic diffractometer D2Phazer of Bruker Company. The interpretation of the diffraction patterns have been accomplished using the ICDD, PDF-2 database. The phase ratio of manganese cobaltite and cobalt manganite has been determined according to the integral intensity of the main reflexes of the phases using the internal standard method [11]. The sizes and shapes of mi-crocrystallines of the synthesized samples have been studied on a JSM-6460LV high-resolution scanning electron microscope with auto emission cathode 7500 F of JEOL Company (Japan). The images have been recorded in the mode of low-energy secondary electrons.
Results and discussion
Methods that combine the traditional alkaline precipitation of basic precursors - Co and Mn nitrates with hydrothermal treatment of the obtained precipitates of mixed hydroxides and their subsequent thermal treatment by exposure to microwave radiation have been used to achieve this goal.
It has been established that there are mixed spinel phases - CoMnO3 and MnCo2O4 along with the monometallic oxides Mn3O4 and Co3O4in the freshly prepared samples.
According to the XRF data of the samples, the mentioned sequence of the operations allows the formation of mixed manganese cobaltites and cobalt manganites with the spinel structure both under the conditions of traditional heat treatment (Fig. 1A) and microwave thermal treatment (Fig. 1B).
Figure 1.X-ray diffraction patterns of binary Co-Mn oxide systems, the heat treatment carrying out by traditional heating (A) and microwave radiation (B).
However, the ratio of the content of the manganese cobaltite phase in the samples depends on the parameters of the hydrothermal treatment and the nature of the final heat treatment.
As is seen from the presented data, the phase composition of the binary Co-Mn oxide samples formed under the conditions of conventional and microwave thermal treatment is qualitatively identical.
It has been also established that the relative content of the targeted phase of manganese cobal-tite (MnCo2Ü4) increases with an increase in the atomic ratio Co:Mn under comparable conditions of hydrothermal and microwave treatment of samples, the maximum amount of which corresponds to the samples with an atomic ratio Co:Mn = 2:1 (Table 1).
Table 1.
Phase composition of binary Co-Mn- oxide catalysts in variation of atomic ratio of metals
Mn : Co = 2:1 Mn : Co = 1:1 Mn : Co = 1:2
d/n, A I Line belonging d/n, A I Line belonging d/n, A I Line belonging
4,87 20 Mn3Ü4 4,87 15 Mn3Ü4 4,87 10 Mn3Ü4
2,87 10 Co3Ü4 2,87 10 Co3Ü4 2,87 20 Co3Ü4
2,69 60 CoMnÜ3 2,69 30 CoMnÜ3 2,69 20 CoMnÜ3
2,60 50 MnCo2Ü4 2,60 60 MnCo2Ü4 2,60 70 MnCo2Ü4
2,37 10 MnCo2Ü4 2,37 15 MnCo2Ü4 2,37 25 MnCo2Ü4
2,23 25 MnCo2Ü4 2,23 30 MnCo2Ü4 2,23 45 MnCo2Ü4
1,77 20 Mn3Ü4 1,77 20 Mn3Ü4 1,77 10 Mn3Ü4
1,70 15 CoMnÜ3 1,70 15 CoMnÜ3 1,70 10 CoMnÜ3
1,53 30 Mn3Ü4 1,53 25 Mn3Ü4 1,53 20 Mn3Ü4
1,27 10 Co3Ü4 1,27 15 Co3Ü4 1,27 20 Co3Ü4
1,01 10 MnCo2Ü4 1,01 15 MnCo2Ü4 1,01 25 MnCo2Ü4
The closeness of the qualitative composition of the also results from a comparison of the spectra of temperoxide forms of metals that including in the composition ature-programmed reduction (TPR) of the samples pre-of the active surface of the samples of Co-Mn catalysts pared by the electric heating - the traditional method of
heat treatment and under the field action of the microwave (Fig. 2).
Figure 2. Curves of temperature-programmed reduction of the Co-Mn - catalysts samples obtained using the method of heat treat-ment by electric heating (A) and heat treatment in the microwave field (B).
Comparison of the TPR results for samples of Co-Mn catalysts with data on the intensity of the phase reflexes (MnCo2Û4) presented in Table 1 provides basis to suppose that the maximum absorption of hydrogen in the temperature range of 240-3200C corresponds to its interaction with crystallites manganese cobaltite.
The larger value of the area of the above-mentioned hydrogen absorption maximum in the spectrum (B) and its shift to the low-temperature region with respect to the spectrum (A) demonstrates a higher content and greater dispersion of the manganese cobaltite phase
in the samples, the thermal treatment of which has been carried out under microwave exposureat the final stage of phase formation.
Therefore, under the conditions of traditional heat treatment, micro-crystallites agglomerate reaching a size of 10-30 |imat the final stage of sample preparation due to the increased time for structuring phase composition (4-5 hours) in the relatively low content of the active phase of manganese cobaltite (MnCo2Û4) ( Fig. 3A).
Figure 3. Electron micrographs of samples of Co-Mn-catalysts; A— thermal treatment by traditional heat-ing, B - thermal treatment in the microwave field.
The thermal treatment of the samples in the microwave field at the stage of formation of the phase composition occurs in a significantly short time (6-10 min). At the same time, crystal particles do not have time to agglomerate and their average statistical size is 90-200 nanometer (Fig. 3B). According to XRF, the content of the active phase of manganese cobaltite (MnCo2Û4) reaches 60-70% (w/w).
Thus, the particles of Co-Mn catalysts formed as a result of hydrothermal-microwave treatment are characterized by relatively greater uniformity in size and have a sufficiently high active surface (15-20 m2/g).
Assuming that the formation process of the free radicals under the conditions of an undeveloped chain reaction is determined by the stage of heterogeneous catalytic initiation of chains, it seems possible to estimate its rate from the absorption rate of oxygen.
Fig. 4-6 shows the dependence of the initial rate of heterogeneous catalytic initiation of chains in the oxidation of m-xylene, under conditions of its conversion not exceeding 3-5%, when the change in hydrocarbon concentration and the process of homogeneous generation of free radicals can be neglected.
As is seen from the data presented in Fig. 4, the greatest decrease in the induction period of oxygen absorption is observed at the stoichiometric ratio of man-
T,min
10,0-
s.o -
6.0-
4,0-
ganese and cobalt oxides 1:2 in precursors, which corresponds to the maximum content of the manganese co-baltite phase (see Table 1).
£>2, mlmin
2,5
2,0
'1,5
1,0
_L
_L
_L
_L
50 100 150 200 S/V,(sm ') Figure 4. Dependence of the induction period (t) and the oxygen absorption rate (WO2 ) on the ratio of the surface of samples of nanostructures manganese cobaltite catalysts to the volume of oxidized xylene
(V, sm3);
A noticeable decrease in the induction period of oxygen absorption and an increase in the initial rate of oxygen absorption are observed (Fig. 5) with an increase in the reaction temperature, the upper limit of T,min
10.0-
which is limited by the boiling point of m-xylene under the experimental conditions.
^02, mlmin
-2,0
MnO;
2:1
1:1
1:2
=-0,5
Co;Oj
Figure 5. Dependence of the induction period (t) and the initial absorption rate of oxygen (WOJ on the
composition of Co-Mn-catalysts in the liquid-phase oxidation of m-xylene; 1-3 — me-chanical mixture of manganese and cobaltoxides, 2-4 — nanostructured catalyst.
Figure 6. Dependence of the induction period (t) and the initial oxygen absorption rate (WOJ on the temperature of the liquid-phase oxidation reaction of m-xylene in the presence of the nano-structured catalyst sample with the content of manganese cobaltite phas.e
It follows from the data in Fig. 6 that at fixed values of the established mass-average temperature in the reaction apparatus, the induction period of oxygen absorption and its absorption rate depend on the amount of feed catalyst presented as the ratio of the geometric surface to the volume of oxidized xylene. The nature of the above dependence indicates the presence of a heterogeneous catalytic pathway for the initiation of free m-xylene radicals under conditions of liquid-phase oxidation of m-xylene.
The observed proportionality of the oxygen absorption rate w°2 to the amount of feed catalyst can be explained by the following scheme of free radical formation during the interaction of m-xylene with active centers on the surface of heterogeneous catalyst (stage 2 is assumed to be slow):
ki >
RH +Z(oxid.) R- + Z(reduc.) (1)
k2slow v
Z(reduc.) + O2 Z(oxid.) + H2O
here, Z(oxid.) - oxidized center, Z(reduc.) - reduced center on the catalyst surface.
According to the above scheme, the generation rate of the free radicals with a heterogeneous surface in
its quasi-stationary state can be equated to the formation rate of reoxidized centers Z(oxidized):
dR
W — = k1[RH]-©0
'R-= dx
oxid.
dZ
= k2 "@reduc. [O2 ]-ki[RH]-©oxid.
dx
(2)
here, [RH ] - the initial concentration of m - xylene (mol/L) [02] - the concentration of dissolved oxygen (10-3 mol/L) ; ki and k2 - the rate constants of reduction and reoxidation of the active centers; ®oxH- and
®reduc. - the concentrations of the oxidized and reduced centers on the surface of the catalyst.
Comparison of values of the oxygen absorption rate in the liquid-phase oxidation of m-xylene in the presence of our previously proposed bulk structure Co-Mn/A12O3/A1-frame-work [12, p. 22-26] and nano-sized Co-Mn-catalyst samples synthesized by the hy-drothermal-microwave method showed the closeness of the obtained values with some prevalence of the latter parameters (Fig. 7)
Figure 7. Comparative results of the values of the oxygen absorption rate during the liquid-phase oxidation ofm-xylene in the presence of bulkstructure Co-Mn/AhOi/Al-framework (1) and nano-sized Co-Mn catalyst sam-ples
(2); Reaction temperature 1250C.
Thus, the obtained results give reason to suppose that the hydrothermal-microwave method proposed by us for obtaining nano-structured Co-Mn-samples can be recommended for the synthesis of catalysts exhibiting a high initiating activity in the liquid-phase oxidation reactions of alkylaromatic hydrocarbons.
Conclusions
1. A method for obtaining nano-dispersed Co-Mn - complex oxide systems as potential catalysts for the liquid-phase oxidation of alkylaromatic hydrocarbons that combines the stage of hydrothermal treatment of amorphous Co and Mnhydrogels and subsequent heat treatment of metal hydroxides by exposure to microwave radiationhas been proposed.
2. It has been shown that the degree ofdispersi-tyof the samples obtainedunder the conditions of microwave thermal treatmentsharply increases, and the average statistical size of crystallites is 90-200 nanometers, in contrast to the samples obtained under the conditions of traditional heating.
3. It has been established that under the realized conditions of hydrothermal and microwave treatment of samples, the relative content of the manganese co-baltite phase (MnCo2O4) increaseswith an increase in the atomic ratio Co : Mn, the maximum content of which corresponds to the samples with an atomic ratio Co: Mn = 2:1.
4. The activity assessment of samples of Co-Mn - complex oxide systems in the limiting stage of initiation of free radicals has been carried out using the model reaction of liquid-phase oxidation of m-xylene, and a correlation has been revealed between their initiating ability and the quantitative content of the manganese cobaltite phase with the spinel structure.
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