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UDC 547.133;547.518;543.422.27
PARAMAGNETIC PROPERTIES OF COBALT-MOLYBDENUM CATALYSTS OF LIQUID PHASE OXIDATION OF C6-C9 CYCLIC OLEFINS
Kh.M.Alimardanov, M.A.Najafova, N.I.Garibov, N.R.Dadashova, E.S.Musayeva, M.E.Huseynova, A.D.Guliyev
Yu.Mamedaliyev Institute of Petrochemical Processes, NAS of Azerbaijan
maisa. [email protected]
Received 20.12.2016
The paramagnetic nature of cobalt-molybdenum catalysts, synthesized on the basis of ammonium hepta-molybdate or a mixture of oxobromide of molybdenum and cobalt(II) bromide has been studied by the EPR method. Hyperfine structure lines were observed in EPR spectra of these catalysts, indicating formation of stable radical complex. The conditions for preparing the catalysts notable for stable catalytic action were found. It is shown that the increase of Co content relative to Mo from 1:1 to 3:1 leads to increased conversion of the substrate to 10-15% in oxidation reactions of saturated and unsaturated hydrocarbons.
Keywords: liquid-phase oxidation, electron paramagnetic resonance, synthesis, steady radical complex,
saturated, unsaturated hydrocarbons.
Introduction
The development of highly efficient catalyst systems for the selective functionalization of saturated and unsaturated hydrocarbons is considered an actual problem of modern petrochemical industry [1]. In this aspect, one of the promising areas is the using of an oxide as precursors - heteropoly compounds (HPC), representing the complex inorganic compounds formed by atoms of V, Mo or W in the heteroligand environment, serving as the complexing agent [1]. In the studies [2-4] it has been shown that in the oxygen functionalization of unsaturated hydrocarbons with aqueous H2O2, the oxidation with HPC effectively proceeds using ammonium compounds, containing C4-C18 alkyl groups. However, the using of the latter, complicates the selection process and reuse of these catalyst systems. The solution of this issue may be found, either by preparing catalyst systems soluble in both phases, or their heterogenization in the form of microstruc-tured particles providing holding pseudo-homogeneous phase process.
According to [5, 6], Mo6+ complexes promoted by cobalt compounds exhibit the high activity in the oxidation and the epoxidation of olefins in the liquid phase systems.
Using different methods (inhibitors, EPR) have shown that these catalyst systems act as radical initiators. The latter transfer from the catalyst surface to the volume of the liquid
phase, where the oxidation products are formed by radical chain mechanism [7]. The role of the surface of catalysts in the liquid phase oxidation of hydrocarbons is, mainly to generate radicals, the initiators of chain process occurring in the volume. In this regard, the EPR, more than the other methods of investigation, helps to find out the regularities of formation of intermediate forms (complexes, radicals) and the final reaction products. EPR spectroscopy can also be used for studying ion pairs, if one of the ions is paramagnetic, and the other contains an atom whose nucleus has a spin different from zero (electron-nuclear interaction and hyperfine structure of the EPR spectrum). The use of EPR is due to the fact that the main parameters of the spectrum: the anisotropy of the g-factor, the parameters of the fine structure, the relaxation times of the ions, radicals and other paramagnetic compounds with unpaired electrons are more sensitive to both the symmetry of the environment of paramagnetic centers in the crystal lattice, and the nature of relation of these centers with the environment. The formation of co-valent bond of paramagnetic ions with ligands, often revealing itself as the additional hyperfine structure (HFS), makes it possible to explore the nature of relation of ions in inorganic systems, and especially in complex compounds. EPR data are important also for the understanding of the electronic structure of transition metal
compounds. For example, molybdenum EPR signals are characterized by definite values of g-factor, width, intensity, hyperfine splitting, saturation parameters, of UHF power that distinguish them from the iron-containing systems signals. Sixcomponent HFS, characteristic of EPR spectra of molybdenum, is determined by the interaction of 95Mo and 97Mo (I = 5/2), the content of which in the natural isotopes are 15.78 and 9.60%, respectively. And the intensive central line is due to even isotopes of 92Mo, 94Mo, 96Mo, 98Mo, 100Mo (I = 0) [8, 9]. One of the distinguishing features of the complex molybdenum catalysts is the lability of some of their components, which are likely to provide the high efficiency of these catalysts in the oxidation of hydrocarbons.
This work aimed to investigate the possibility of using synthesized cobalt-molybdenic samples and their heterogenized catalysts on microstructured carbon material in the reaction of liquid phase oxidation of C6-C9 cyclic olefins by EPR and XRD methods.
Experimental
Synthesis of the samples of polyoxomet-alate catalysts on the basis of molybdenum compounds with the introduction of cobalt ions was carried out by the known method [10, 11]. For the partial substitution of ammonium groups for Co, (NH4)6Mo7O24, CoBr2 is taken in equimolar ratio, and in the case of a full substitution in a molar ratio of 1:3. A definite amount of ammonium heptamolybdate and cobalt(II) bromide are placed in a round bottom flask with a stirrer and for completing homogenization 100 ml of 50% ethanol solution is added. The experiment is carried out under intensive stirring (T = 80-900C) for 12 h. At the initial stage, the solution tinges a kind of green colour indicating the presence of dissolved cobalt(II)bro-mide. Further the color of the solution changes to blue, indicating the beginning of the ex-
+ 9+
change reaction of (NH4) groups to the Co . The blue color of the solution can also indicate the presence of Mo5+ in the solution (molybdenum blue). The reaction was carried out until complete decolouration of the solution. The
light brown precipitate was dried at 100-1200C. The samples of these systems heterogenized on the carbon material have also been prepared.
For investigation of prepared samples the EPR method has been used as the main physical method. Methods for testing and recording of EPR spectra are refered in [12, 13]. EPR spectra were recorded on a "Bruker BioSpin". Setting of the internal field standard was defined by the UDD (ultradisperse diamond) model in the X-range (operating frequency of 9.85 Hz). The amplitude of the HF modulation (100 Hz) was varied in the range of 150-550 mTl. The determination of the g-factor values was performed using a UDD with a g-factor equal to 2.0036.
X-ray diffraction (XRD) was conducted on the X-ray diffractometer "PANalytical Empyrean".
The crystal structure of the prepared catalysts was determined using an S-3400 N scanning electron microscope equipped with an Oxford Instruments Nano Analysis (OINA) micro-analysis system.
The experiment of cyclic olefins oxidation by air oxygen was carried out in the absence of solvent in a laboratory unit with a bubble type glass reactor provided with a Schott filter, a thermocouple, refrigeration system and traps. The feed rate of purified air was 30 l/h per 100 g of raw materials. The quantity of formed hydroperoxide was determined by io-dometric titration.
The composition of oxidate after its reduction by aqueous Na2SO3 was analyzed using a "^eT-500" chromatograph with a flame-ioni-zation detector: a 2000*2 mm column, phase of polyethylene glycol succinate (5 wt. %) on chromosorb, helium as carrier gas, Tcolumn -1600C, Tevap. - 2800C.
Results and discussion
The main objective of this work was to investigate the EPR spectra of the synthesized catalysts in order to identify their the paramagnetic centers, formation and revealing the nature of the radicals, the effect of the ratio of components on their paramagnetic and oxidizing properties. We have suggested that the full sub-
stitution of (NH4) group for Co occurs at the triple excess of CoBr2:
(NH4)6Mo7O24+3CoBr3^Co3Mo7O24+6NH4Br (1)
and a partial replacement of group of (NH4)+ with Co2+ at an equimolar ratio of the starting compounds:
(NH)6Mo7O24+CoBr2^(NH4)4CoMo7O24+2NH4Br. (2)
However, according to XRD data, obtained solid mass consists of several phases. Analysis and quantity of the interplanar spacings, calculated on the values for 29 by diffractogram (Figure 2) showed the presence of mainly 2 phases: CogMo36O80 (4Co2Mo9O2o, d 4.97, 2.88, 2.43, 1.67 A, 29 17.85, 31.0, 37.02, 55.03); Moi6O44[4(Mo2O5-2MoO3], d 2.26, 1.93, 1.91, 1.79, 1.72, 1.62 A, 29 39.82, 46.92, 47.48,
50.91, 53.35, 56.91), as well as admixtures of Co2Mo2O8(2CoMoO4, d 2.15, 1.74 A, 29 41.90, 52.08).
The EPR spectra of the studied catalysts are shown in Figure 1 (A and B), from which it is evident that the total width of spectra reaches 285 mTl. It has been established that there are two signals, due to the presence of HFS from Co2+ ions (AHtotal = 25.0 mTl, g = 4.25) [14] and a broad band (AHtotal = 80.0 mTl, g = 2.00398), representing 6 lines characteristic of Mo5+ ions (Figure 1 A). Against the background of these six lines, a singlet (AHtotal =5.5 mTl, g=1.937) is seen. According to the authors [15, 16], this singlet is typical for short "molybdenyl bond" Mo=O.
60
/ 103 40 20 0 -20 -40 -60 -80
1500 2000 2500 3000 3500 4000 4500 5000 5500
1 B
( —
14.0695 7.0348 4.6898 3.5174 2.8139 2.3449 2.0099 1.7587 5500
g-factor
Fig. 1. The EPR spectra of cobalt-molybdenum catalysts prepared in ratios of CoBr2:(NH4)6Mo7O2 - 1:1 (A) and 3:1 (B).
0 - <D 1 3600 -1600 - 400 -
I 1 I 1 I 1 I 1 I 1 I 1 I 1 I 1 1 1 I 1 I 1 I 1 I 10 15 20 25 30 35 40 45 50 55 60 65 2Theta (°)
Fig. 2. The diffractogram of catalytic system obtained on the basis of MoOBr3 and CoBr2.
The appearance of the EPR spectra of Mo5+ ions is one of the stages of the formation of MoO3 with tetrahedral coordination of ions. Such coordination is carried out at the displacement of Mo5+ ions from the equatorial plane in octahedral structures towards one of the axial oxygen atoms, whereby the second axial oxygen atom is removed, resulting in a C4 symmetry.
Some of the EPR lines are not identified until the end, although we assume that they are due to electron-hole centers, formed mainly in mono- and polycrystals of cobalt-molybdenum system. As seen from the EPR spectra (Figure 1 A and B), the concentration of ions Mo5+ signif-
icantly reduces at a ratio of Mon+ to Co2+ (1:3) (Figure 1, B). As can be viewed, HFS appears in the spectrum from the cobalt radical, that is, increasing the amount of Co relative to the molybdenum-base, EPR spectrum will consist of only Co radicals. The catalyst systems obtained in this way exhibit high activity in the reactions of liquid phase oxidation of C6-C9 mono- and bicyclic cyclic olefins, and their epoxidation by the forming hydroperoxides (Table). As shown in Table, at a degree of conversion of the cyclic olefin to 30-45% with the above catalyst systems, the main products of reaction are oxygen-containing monofunctional compounds (epox-ides, alcohols, ketones).
Results of the liquid-phase oxidation of Cg-C9 cyclic olefins with air oxygen in the presence of a catalytic system obtained from (NH4)6[Mo7O24] and CoBr2 (7=800C, Vair=30 l/h, x=4 h, Co2+:Mon+=3:1 molar ratio, Colef.=1 mol, Cch3cooh=0.3 mol)
Cyclic olefins Conver version, % Selectivity, % on converted cyclic olefin
By epoxides By unsatu-rated alcohols By unsatu-rated ketones By epoxy alcohols Unidentified compounds
cyclohexene 47.0 28.3 47.2 12.3 9.2 3.0
1 -methylcyclopentene 44.0 31.6 54.2 6.4 3.6 4.2
1 -methylcyclohexene 51.0 33.0 49.3 9.1 4.5 4.1
4-vinylcyclohexene 41.0 36.0 44.4 7.6 6.0 6.0
tetrahydroindene 54.5 40.5 39.7 5.1 8.4 6.3
5-vinylbicyclo[2.2.1]hept-2-ene 26.4 51.7 33.1 3.8 - 11.4
*4 -vinylcyclohexene 36.4 32.0 47.3 9.1 3.2 8.4
* tetrahydroindene 49.0 48.0 32.8 7.6 - 11.6
* Note: As a catalyst was used a sample on a microstructured carbon material supporter.
С0 1-MCHP10-2 mol/l 9 -I
8 -
7
6 -
5 -
4 -
3 -
2
1
0
0
5
10
1 x, h
15
Fig. 3. The kinetic curves of the accumulation of 1-MCHP hydroperoxide (according to iodometric titration) depending on the concentra-
tion of the cation Со2+ mol/l: 1 - 13710-3,
2 - 2.74-10-3, 3 - 3.410-3
4 - 4.1110-3 (1-MCHP - 0.15 mol/l, Fair - 30 l/h, T=50°C.
Under the conditions that ensure a higher degree of conversion of hydrocarbons (T>800C), the share of multifunctional and oligomeric oxygen-containing compounds increases (Figure 3). From the kinetic curves of the accumulating of 1-methylcyclohexen hydroperoxide (1-MCHP) depending on the number of CoBr2 in catalytic complex, it is seen that for 4-8 h duration of the experiment, the proportion of monofunctional products of degradation of hydroperoxide (un-saturated alcohols, saturated and unsaturated ke-tones) accumulates in the oxidate. With increasing time of experiment (>8 h) the content of the products secondary conversion of monofunc-tional oxygen-containing compounds increases in oxidate (epoxy alcohols, glycols, dicarboxylic acids, oligo esters).
Obtained data confirm that the increase of molar quantity of CoBr2 relative to (NH4)6Mo7O24 from 1:1 to 3:1 raises the selectivity of conversion of the substrate for 10-15% in the liquid phase oxidation reactions of un-saturated C6-C9 hydrocarbons.
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C6-C9 tsíkloolefínlorIn maye fazada oksídlo§mo reaksíyasina kobaltmolíbden katalízatorlarinin paramaqnít xassolorInín tosírí
H.M.0limardanov, M.A.Nac3fova, N.LQaribov, N.R.Dada$eva, E.S.Musayeva, M.O.Hüseynova, A.D.Quliyev
Ammoniumheptamolibdat va ya molibdenla kobalt(II) oksobromidlar qan§igi asasinda sintez olunmu§ kobaltmolibden katalizatorlannin EPR metodu ila paramagnit xassalari öynanilmiijdir. EPR spektrlarindan müayyan edilmi§dirki, bu katalizatorlarda dayaniqli radikal komplekslar amala galir. Katalizatorlarin stabil katalitik tasirli xassalarla alinma üsüllari tapilmi§dir. Co:Mo münasibatinin 1:1-dan 5:1-a kimi artirilmasi tikloolifinlarin oksidla§ma reaksiyasinda konversiyasinin 10-15% artirilmasi ila naticalanir.
Agar sözlar: maye fazada oksidh§m3, elektron paramaqnit rezonans, sintez, dayaniqli radikal kompleks, doymu§ v3 doymami§ karbohidrogenbr.
ПАРАМАГНИТНЫЕ СВОЙСТВА КОБАЛЬТМОЛИБДЕНОВЫХ КАТАЛИЗАТОРОВ ЖИДКОФАЗНОГО ОКИСЛЕНИЯ С6-С9-ЦИКЛООЛЕФИНОВ
Х.М.Алимарданов, М.А.Наджафова, Н.И.Гарибов, Н.Р.Дадашева, Э.С.Мусаева,
М.Э.Гусейнова, А.Д.Кулиев
Методом ЭПР изучена парамагнитная природа кобальтмолибденовых катализаторов, синтезированных на основе гептамолибдата аммония или смеси оксобромидов молибдена и бромида кобальта(11).Установлено, что в спектрах ЭПР наблюдается сверхтонкая структура линий, свидетельствующая об образовании в этих катализаторах устойчивого радикального комплекса. Найдены условия получения катализаторов, отличающихся стабильностью каталитического действия. Показано, что увеличение содержания Со по отношению к Мо от 1:1 до 5:1 приводит к увеличению конверсии субстрата на 10-15% в реакциях окисления насыщенных и ненасыщенных углеводородов.
Ключевые слова: жидкофазное окисление, электронно-парамагнитный резонанс, синтез, устойчивый радикальный комплекс, насыщенные и ненасыщенные углеводороды.
