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CHEMICAL PROBLEMS 2022 no. 4 (20) ISSN 2221-8688
UDC 541.128.542.547
PHASE COMPOSITION AND OXIDATION DEGREE OF VANADIUM IN CATALYSTS FOR OXIDATIVE DEHYDROGENATION OF PROPANE
A.M. Sardarly
Acad. M. Naghiyev Institute of Catalysis and Inorganic Chemistry, H.Javid ave., 113, AZ1143, Baku, Azerbaijan, e-mail:[email protected]
Received 19.07.2022 Accepted 19.10.2022
Abstract: The phase composition of vanadium and vanadium-antimony-containing samples of y-Al2O3 and the degree of oxidation of vanadium in them were examined by XRD and EPR methods, respectively. The data of XRD and EPR spectroscopy show the formation of highly dispersed vanadium-oxygen and vanadium-antimony-oxygen structures on the surface of the support. It is shown that the EPR spectra of the studied samples are due to "isolated" vanadium ions with an oxidation state of +4 in non-stoichiometric vanadium-oxygen and vanadium-antimony-oxygen formations with a square pyramid local environment structure with a characteristic V=O double bond. The EPR spectra of two types of paramagnetic centers of vanadium, which differ in the distortion of the square-pyramidal structure of the local environment of the vanadium ion, were identified. It was established that the number of ions with an oxidation state of+4 in V,Sb-containing samples depends on the V/Sb ratio in them.
Keywords: oxidative dehydrogenation of propane, V,Sb-containing oxide catalysts, phase composition, EPR spectra.
DOI: 10.32737/2221-8688-2022-3-358-365
Introduction
Supported vanadium oxide systems draw attention of researchers as catalysts for various redox reactions, including oxidative dehydrogenation (OD) of light alkanes [1-3]. The catalytic properties of such systems are associated with the presence of VOx vanadium structures on the support surface, the composition, structure, and distribution of which are depend upon the amount of the supported component, the nature of the support, and the nature of the vanadium precursor used to prepare the samples [4-10]. A fairly wide range of OD catalysts is known [11-18], however, the search for an effective OD catalyst for C2-C4 alkane is still ongoing. Mixed oxides of vanadium and antimony are well known as active and selective catalysts for selective partial oxidation processes [19, 20]. V-Sb-O catalysts gave a good account of themselves in the selective oxidation of isobutene to
methacrolein [21], methane to formaldehyde [22], propane to acrylic acid [23-25], and selective ammoxidation to acrylonitrile [26-29]. In mixed V-Sb-O oxide catalysts, during their formation, the SbOx and VOx structures react with the formation of the VSbO4 rutile phase with cationic vacancies [30]. This mixed oxide phase can have different stoichiometry compositions [31, 32]. In V-Sb-O oxide catalysts, in addition to mixed V-Sb oxide phases, phases of amorphous antimony oxide (Sb2O3, a -Sb2O4) [33] and/or V2O5 may also be present. Studies into catalysts based on mixed oxide of vanadium and antimony by ion scattering spectroscopy showed that their surfaces were enriched with the surface structures of vanadium oxide [34]. Although there is a fairly wide range of studies of the physicochemical properties of mixed oxides of vanadium and antimony, the role of vanadium-.
CHEMICAL PROBLEMS 2022 no. 4 (20)
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antimony-containing structures in reactions of selective oxidative catalysis is still the subject of research.
This paper presents the results of research into samples of vanadium-, antimony-containing aluminum oxide by the method of electron
paramagnetic resonance (EPR) in combination with the method of X-ray analysis in order to establish the phase composition and the effect of antimony on the degree of oxidation of vanadium in them.
Experimental part
Vanadium-containing samples were samples of y-Al2O3 impregnated with a mixture of aqueous solutions of ammonium metavanadate and tartaric acid with a content of 15 wt.% vanadium in terms of ammonium metavanadate. Vanadium-antimony-containing samples were samples of y-Al2O3 containing 15 wt.% vanadium oxide with different content of antimony trichloride SbCl3 (1; 2; 2.7; 3.5 and 5 wt.%) dissolved in tartaric acid. Catalysts were prepared using y-Al2O3 produced by the Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences (Russia) with a specific surface area of 85 m2/g, ammonium metavanadate NH4VO3 (Qualikems, CAS No 7803-55-6, India) and antimony chloride SbCl3 (Aldrich, Germany) as supports), dried at a temperature of 110°C and then subjected to stepwise calcination at 2000, 4000, 6000C for 2 hours at each temperature. X-ray diffraction patterns and EPR spectra were recorded at room temperature using an X-ray diffractometer Phaser D2 X-ray diffractometer and an EMR spectrometer EMRmicro, Bruker, Germany, respectively.
The catalytic properties of the samples in
the oxidative propane dehydrogenation (ODP) reaction were studied in a flow reactor with a fixed catalyst bed at 550°C and atmospheric pressure. Analysis of the products before and after the reactor was carried out in on-line mode on an LXM-80 chromatograph. Two chromatographic columns filled with Porapak QS and NaX molecular sieves were used for analysis. The reactor was a quartz tube with an inner diameter of 10 mm, into which ~0.2 g of a sample of a fraction of 0.25-0.5 mm was placed and diluted with quartz chips of a similar fraction. Before testing, the catalyst was calcined in a stream of air at 600°C for 1 hour. The propane/air reaction mixture contained 10 vol. % C3H8. Conditional contact time was changed by varying the volume of the catalyst and/or the space velocity of the reaction mixture. The propane conversion, as well as the selectivity for the formation of COx and C3H6, was calculated according to [11]. Note that, the y-Al2O3 support samples used are not active in the oxidative dehydrogenation of propane and acquire this activity after the introduction of vanadium and antimony into them.
Results and discussion
Table 1 shows the results of testing reaction of oxidative dehydrogenation of samples of V-Sb-O/y-Al2O3 containing 15% propane. (wt.) vanadium and 1-5 wt.% antimony in the
Table 1. Test results for V-Sb-O/yAl2O3 samples containing 15% (wt.) vanadium and 1-5 wt.% antimony in the reaction of oxidative dehydrogenation of propane (reaction conditions: T=5500C, contact time - 4 s.)
Catalyst Conversion Selectivity,% Exit C3H6,
* C3H8, % C3H6 C1-C3 COx %
1 11.5 11.6 6.6 81.8 1.33
2 12.6 11.7 5.6 82.7 1.47
3 14.3 12.0 7.2 81.8 1.72
4 12.9 12.2 5.4 82.6 1.57
5 12.7 13.0 6.1 82.9 1.65
* Samples of catalysts based on y-Al2O3 containing (in wt.% in terms of ammonium metavanadate and antimony trichloride): 1 - 15% V and 1% Sb, 2 - 15% V and 2% Sb, 3 - 15% V and 2.7%Sb, 4 - 15%V and 3.5%Sb, 5 - 15% Vand 5%Sb, respectively.
The data presented in Table 1 show that with the introduction of antimony into the composition of the vanadium-containing catalyst, the selectivity of the ODP reaction with respect to propylene and the yield of propylene increase. In this case, the maximum yield of propylene is found for a catalyst containing 2.7% antimony.
Figures 1 and 2 show X-ray diffraction
patterns and EPR spectra, respectively, recorded at room temperature, of the following samples: a) 15% V/Al2O3 with vanadium deposited from a mixture of aqueous solutions of ammonium metavanadate and tartaric acid; b) on (a) a solution of 2.7% SbCl3 in tartaric acid was added, c) on (a) solution of 5.0% SbCl3 in tartaric acid was added, followed by drying at 110 °C and calcining in air at 600°C for 4 hours.
i jv
HV*V
so
rt) « «I Ml Ml
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Two tWi/tW^Wît w«
c)
Fig. 1. X-ray diffraction patterns of samples calcined at 600 °C: a) 15% V/Al2O3 with vanadium deposited from a mixture of aqueous solutions of ammonium metavanadate and tartaric acid, b) on (a) a solution of 2.7% SbCl3 in tartaric acid was added; c) on (a) a solution of 5.0% SbCl3 in tartaric acid was added.
340 mi 380 b)
Fig. 2. EPR spectra recorded at room temperature of samples calcined at 600 °C: a) V-15% / y-Al2 O3 and b, c) y-AhO3 -containing: b) 15% V-2.7% Sb, c) 15 %V, 5.0%SbCl3
The X-ray diffraction patterns of these samples show phases characteristic of y-Al2O3 (PDF 411426) and VOX (V2O5 - PDF#10-0425; VO2 -PDF#44-0252; V6O13 - PDF #71-2235) structures. Depending on the oxygen partial pressure and temperature, VO2, V6O13 and V2O5 can be obtained as pure or mixed phases. Reduction of V2O5 in a hydrogen atmosphere at 400°C for 10 min. led to its reduction to V2O3.
During the subsequent admission of air into the system at the same temperature, the V2O3 phase was oxidized to VO2. Studies demonstrated that through sequential redox procedures, a controlled redox cycle V2O3 ^ VO2 can be achieved. The XRD results show that VOx is present either as an amorphous phase or in a highly dispersed form on the support surface.
The results of EPR studies suggest that the changes in the content of vanadium+4 ions in the VSbO/Al2O3 catalysts as compared to the VOx/Al2O3 catalyst were due to the interaction between the particles of SbOx and VOx oxides. The introduction of antimony reduces the redox properties of VOx structures while the introduction of ~3% antimony into the composition of VOx/Al2O3 is useful not only for improving catalytic activity, but also for obtaining catalysts that work stably in a continuous mode for 10 hours. The stable and increased activity of VOx/Al2O3 catalysts with the introduction of antimony is mainly due to the formation of (-V - Sb - O-)n structures and easy transfer of oxygen from the volume of antimony oxide. In the diffraction patterns of the VSbO/Al2O3 catalysts, in addition to the y-Al2O3 carrier phase, weak reflections were found, most likely belonging to the SbOx (Sb2O3, a -Sb2O4) structures [33] and the nonstoichiometric, defective V1-xSbxO4 phase [30], which exhibits a characteristic EPR spectrum from V4+ ions.
In the process of calcination at 600°C in an air flow of samples, which were ammonium metavanadate deposited from an aqueous solution on aluminum oxide by impregnation at room temperature, the ammonium metavanadate decomposes to form vanadium(V) oxide, releasing ammonia and water (2NH4VO3 ^ 2NH3 + V2O5 + H2O). Calcination at 600°C of samples obtained by impregnation of aluminum oxide at room temperature with a mixture of aqueous solutions of ammonium metavanadate NH4VO3 and tartaric acid HOOC-CH(OH)-CH(OH)-COOH and then dried at 110°C, was accompanied by the formation of vanadium(V) oxide, evolution of ammonia , water, oxides of nitrogen and carbon.
The EPR spectra of these samples contain signals characteristic of vanadyl ions VO2+. The data of EPR measurements unambiguously indicate the presence of "isolated" vanadium ions with an oxidation state of +4 in the samples. The formation of vanadium ions with an oxidation state of +4 during calcination in this case is, most probably,
due to the formation of nonstoichiometric vanadium structures with anion defects on the aluminum oxide surface: V5+2.xV4+xO5.x/ y-Al2O3, where x< 1. For samples of V,Sb-containing samples obtained by impregnation of aluminum oxide samples at room temperature with a mixture of aqueous-acid solutions of ammonium metavanadate and antimony trichloride, followed by drying at 110°C and further calcination at 600°C in air, the X-ray diffraction patterns mainly revealed the y-Al2O3 phase. The lines characteristic of vanadium-oxygen V6O13 and vanadium-antimony-oxygen Sb(VO3)3 structures were rather weakly manifested.
The calcination of these samples at 600°C leads to the decomposition of these structures with the formation of two phases - a-Sb2O4 and VSbO4. In the X-ray diffraction patterns of V, Sb-containing alumina (Fig. 1,b,c) against the background of the y-Al2O3 phase, reflections from vanadium and antimony and V/Sb phases appear very weakly, due to the low content of antimony and the highly dispersed state vanadium structures. For catalysts with a vanadium content of <5%, only peaks characteristic of the gamma modification of alumina appeared. For catalysts with a vanadium content of >5%, X-ray diffraction patterns showed weak, broadened peaks in the range of 20-35°, which were due to vanadate-like structures dispersed on the aluminum oxide surface [4-6]. The EPR spectra of vanadium-containing samples contained signals characteristic of VO2+ ions, which differed only in the intensity of these signals. For these samples, EPR spectra were found, characterized by a hyperfine structure due to the interaction of the unpaired 3d1 electron of VO2+ ions with the spin of the 51V nucleus (I = 7/2) and belonging to "isolated" ions (VO)2+ with distorted octahedral coordination and a "strong" bond V =O. The values of the magnetic resonance parameters of the EPR spectra of these samples are given in Table. 2.
The overall intensity of the EPR spectrum was measured and compared with the intensity of the standard - polycrystalline VOSO4.
Table 2. Values of the magnetic resonance parameters of the EPR spectra of the studied samples
*Samples g-factor Hyper-fine structure constant, in mT **Ration V4+/V5+
g|| g"1 All A"
V-15%/Al2Ü3 1.934 1.998 1.963 19.4 10.2 0.092
V-15%-Sb-2.7% /AI2O3 1.933 1.998 1.964 19.8 9.9 0.0821
V-15%-Sb-5%/Al2Ü3 1935 1.995 1.958 19.9 10.3 0.0793
*The weights of the studied samples were ~30 mg. To estimate the number ofparamagnetic centers in the samples, polycrystalline VOSbO4 •5H2O was used as a standard. ** - the ratio of the number of VO2+ ions recorded by the EPR spectra to the total number of vanadium ions in the sample.
This calculation showed that approximately 10% of the vanadium ions in these samples were in the +4 oxidation state. The XRD results show that the VOx particles were present either as an amorphous or highly dispersed phase on the carrier surface. In general, the changes observed in the EPR spectra and X-ray diffraction patterns of the studied samples with an increase in the antimony content are due to the transformations of the surface vanadium oxide phases VOx. It ought to be noted that the intensity of the EPR signals from V4+ ions in the V2O5/Al2O3 sample was higher than in the V-Sb containing samples,
which was due to a decrease in the ability of vanadium to reduce when vanadium samples were modified with antimony.
Note that changes in the phase composition of the surface of supported VOx/y-Al2O3 catalysts by the introduction of antimony and a decrease in the amount of V4+ ions affected their catalytic properties. In general, the results of EPR studies indicate that changes in the first and second coordination spheres of vanadium in surface vanadium oxide structures are due to the formation of -O -V-O-Sb-O-V-O-structures with the introduction of antimony into the samples.
Conclusion
The phase composition of vanadium and vanadium-antimony-containing samples of y-Al2O3 and the degree of oxidation of vanadium in them were studied by XRD and EPR methods, respectively. X-ray diffraction patterns showed that the synthesized samples were gamma alumina with highly dispersed vanadium-oxygen and vanadium-antimony-oxygen structures on the carrier surface. It revealed that the EPR spectra of the studied samples were due to "isolated" vanadium ions with an oxidation state of +4 in nonstoichiometric vanadium-oxygen and
vanadium-antimony-oxygen structures with a square-pyramidal structure of the local environment of vanadyl (VO)2+ ions with a characteristic "silt" bond V=O. It is assumed that the increased activity and selectivity of the supported V, Sb-containing catalyst in the target reaction is due to the Sb-V-O structures. These structures are formed as a result of the interaction of VOx and SbOx particles, although it should be noted that SbOx structures are not active in the oxidative dehydrogenation of alkane.
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ФАЗОВЫЙ СОСТАВ И СТЕПЕНЬ ОКИСЛЕНИЯ ВАНАДИЯ В КАТАЛИЗАТОРАХ ОКИСЛИТЕЛЬНОГО ДЕГИДРИРОВАНИЯ ПРОПАНА
А.М. Сардарлы
Институт Kaтализа и Hеорганической химии им. акад. М. Нагиева AZ1143 Баку, пр. Г. Джавида, 113; e-mail: afet. sardarly@gmail. com
Аннотация: Методами РФА и ЭПР исследованы, соответственно, фазовый состав ванадий, ванадий-сурьма-содержащих образцов у-Л1203 и степень окисления ванадия в них. Данные РФА и ЭПР спектроскопии указывают на формирование высокодиспергированных ванадий-кислород и ванадий- сурьма-кислородных структур на поверхности носителя катализаторов. Показано, что спектры ЭПР исследованных образцов обусловлены «изолированными» ионами ванадия со степенью окисления +4 в нестехиометрических ванадий-кислород и ванадий- сурьма-кислородных образованиях с квадратно-пирамидальной структурой локального окружения с характерной двойной связью V=0. Идентифицированы спектры ЭПР двух типов парамагнитных центров ванадия, различающихся искажением квадратно-пирамидального строения локального окружения иона ванадия. Установлено, что количество ионов со степенью окисления +4 в V,Sb - содержащих образцах зависит от соотношения V/Sb в них.
Ключевые слова: окислительное дегидрирование пропана, V,Sb -содержашие оксидные катализаторы, фазовый состав, ЭПР спектры
PROPANIN OKSÎDLaÇDÎRÎCi DEHÎDROGENL9ÇM9 KATALlZATORLARININ FAZA T9RKlBi Va ONLARDA VANADiUMUN OKSiDL9§M9 DOROCaSl
A.M. Sardarli
Akad. M. Nagiyev adina Kataliz vd Qeyri-üzvi Kimya institutu, H. Cavidpr., 113, AZ1143, Baki, Azdrbaycan e-mail:[email protected]
Xülasa: Tarkibinda vanadium va vanadium-sürma olan y-Al2O3 nümunalarinin faza tarkibi va onlarda vanadiumun oksidlaçma daracasi müvafiq olaraq RFA va EPR metodlarinin istifadasi ila tadqiq edilmi§dir. Müayyan edilmi§dir ki, qeyd olunan metodla sintez olunmu§ katalizatorlarin daçiciyisi sathinda yüksak dispersli vanadium-oksigen, vanadium-sürma-oksigen quruluçlari amala galir. Göstarilmi§dir ki, tadqiq olunan nümunalarin EPR spektrlari qeyri-stexiometrik vanadium-oksigen va vanadium-sürma-oksigen birlaçmalarinda ikiqat V=O rabitali, lokal sahasi kvadrat piramida quruluçlu va oksidlaçma daracasi +4 olan "izola olunmu§" vanadium ionlarina xasdir. EPR spektrlar asasinda lokal sahasinin kvadrat-piramidal quruluçunun pozulma daracasina göra farqlanan iki növ paramaqnit vanadium markazlari müayyan edilmiçdir. Müayyan edilmiçdir ki, tarkibinda V, Sb olan nümunalarda oksidlaçma daracasi +4 olan ionlarin miqdari V/Sb nisbatindan asilidir.
Açar sözlar: propanin oksidlaçdirici dehidrogenlaçmasi, V, Sb tarkibli oksid katalizatorlari, faza tarkibi, EPR spektrlari.
