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Oligomerization of higher olefins and oxidation in a series of isobutanol — isobutyric aldehyde — isobutyric acid in the presence of some heteropoly compounds
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
1703
O l i g o m e r i z a t i o n o f higher olefins and oxidation in a series o f i s o b u t a n o l isobutyric aldehydeisobutyric acid in the presence o f some heteropoly compounds S.M. Zulfugarova, M.K. Munshieva, D.B. Tagiev, P.S. Mamedova Institute of inorganic and physical chemistry Azerbaijan Academy of sciences, Baku, 370143, H.Javid avenue, 29, Azerbaijan Republic
The results of oligomerization of C6-olefins and oxidation in a series: isobutanol isobutyric aldehyde - isobutyric acid. in the presence of some 12-Heteropoly acids (HPA) and their compounds are given. It is shown that the reaction may takes place on the surface of these catalysts as well as in their bulk. By modifying the studied heteropoly acids and their compounds or supporting them on the surface of some substrates can increase its efficiency.
1. INTRODUCTION Heteropoly compounds have recently attracted considerable attention as catalysts for various industrial processes. The wide diapason of catalytic possibilities allows their use as homogeneous and heterogeneous catalysts for acidic and oxidation reactions [ 1]. In the present paper we wish to report the results of oligomerization of C6-olefins and oxidation in a series: isobutanol - isobutyric aldehyde - isobutyric acid. in the presence of some 12-Heteropoly acids and their compounds.
2. EXPERIMENTAL HPA and their compounds were synthesized, according to [1,2]. For identification of obtained compounds the method of IR-spectroscopy was used: the bands of absorption at 1060, 970, 870 and 790 cm l, belonging to the Keggin's cell were observed. HPA was investigated in free form and supported on the various substrates, such as CaCO3, silica with the specific surface 120-300 m2/g and zeolite HZSM with SIO/A1203=33. The oligomerization process was taking place at the boiling point of C6-olefins (6572~ and atmospheric pressure. The reaction of the oxidative conversion in a series: isobutanol - isobutyric aldehyde isobutyric acid, was studied in the flow reactor at the temperature 250-460~ molar correlation - raw material: oxygen between 1:1-1:4 and at the volume velocity 2-3 hours 1. The products of these reactions after separating from the catalyst were analyzed by method of GLC.
1704
3. RESULTS AND DISCUSSIONS
Free and supported HPA were researched in oligomerization process. For correct comparison of activity of these catalysts, the amount of active mass in both cases was equal. (Table 1). Table 1 Conversion of C6-01efins in the presence of some heteropoly acids The Conversion The yield of products, % mass Catalyst amount of C6-01edimers tri- and aromatics of fins, % oligomers HPA, mass g 0.8 55.0 100 H3PWI204o 1.0 61.2 100 0.5 56.0 25% H3PW1204o/SiO2 68.6 5.7 25.7 1.0 630 30% H3PWI204o/SiO2 72.2 5.6 22.2 0.15 24.0 83.3 16.7 0.50 470 87.0 13.0 HsPWloV204o 1.0 650 88.5 11.5 0.15 24.0 83.3 16.7 8% HsPWloV2040/SiO2 0.46 47.0 86.8 13.2 30% HsPWloV204o/SiO2 0.92 650 88.5 11.5 05 6O 5 93.0 6.1 0.9 H4SiWl204o 10 69 0 94.0 4.5 1.5 0.5 6O 5 93.0 6.2 0.8 25% H4SiWl204o/SiO2 94.2 4.3 1.5 10 69.0 30% H4SiWl204o/SiO2 100 O8 26 0 H3PMo1204o 100 10 29.0 45 55 O7 8.0 H4PMOllVO4o 50 50 10 14.0 23 77 4.0 1.0 HsPMoloV204o 0.5 95.7 4.3 57.5 25% H3PW1204o/SiO2 95.4 4.6 77 1.0 30% H3PW1204o/SiO2 95.0 5.0 0.5 62 25% H4SiWl204o/SiO2 94.9 4.4 0.7 68.5 1.0 30% H4SiWl2040/SiO2 100 10 0.5 25% HsPWloV2Ono/CaCO3 90 10 41 30% H3PW12Oao/HZSM 1.0 90 10 3 30% H3PWI204o/HZSM 1.0 100 53 2.0 NH4HEPW1204o The samples 2,4,6 were prepared by use of silica with the specific surface - 120 m2/g as a support. In the samples 10.11 as a support was used the silica with the specific surface 300 m2/g. It is shown that the activity and selectivity of HPA are dependent on their nature. So PW and SiW HPA allow to conduct the dimerization process of C6-olefins with 100% selectivity [3]. The olefin conversion is within the limits 60-70%. As opposed to the
1705 aforementioned catalysts PMo I-IPA has lower activity (approximately 30%) and there the aromatizasion process is taking place. The substitution of Mo- and W-ions for V-ion influences on the nature of HPA's acidic centres. It is established that when the dimerization process is going on in the presence of PW HPA then both dimerization and oligomerization are taking place on the PWV HPA. But the replacement of Mo-ions by V-ones decreases the catalytic activity of HPA in oligomerization of olefins The free HPA are not thermal stable and their specific surface is 1-5 m2/g. Therefore we supported these catalysts on the different substrates having surface hydroxyl groups and minimum surface area of about 80 m2/g. Our investigations showed that the strong interaction of HPA with CaCO3 having the basic properties decreases their catalytic activity in oligomerization process. HPA supported on the HZSM interact with the surface hydrogen atoms of zeolite. Therefore it is supposed the acidic properties ofHPA's surface become stronger. However, these catalysts exhibit a lower activity in the studied reaction. Optimal support among the researched ones was silica-SiO2, the surface of which has undergone the physical adsorption of HPA in distinction from chemisorption in the presence of other supports. The substantial distinction of supported catalysts from free ones in oligomerization of olefins is the decrease of activity of free I-IPA after 4-6 experiments and maintaining of high activity of suppoIled HPA even after numerous (20-25) experiments. Thus, the support of HPA on the surface of silica stabilizes the activity of HPA [4,5]. According to the experimental data, the conversion of olefins on the free and silica supported HPA is almost identical. Therefore, it is assumed that the process of oligomerization takes place on the outer surface of both catalysts. The hydrated protons included in the structure of the surface acidic centres, probably, have located on the oxygen atoms of angle of bridges O which have linked the metal atoms in triads M3013. M/\M Monosubstituted NH4 salts of PW HPA (NH4H2PW12040) also exhibit high catalytic activity in oligomerization of C6-olefins. The conversion of olefins reaches 55%, after 10-12 experiments it decreases to 12-14%. The investigation of the structure and adsorptive capabilities of salts of monovalent cations of HPA, recorded by method BET shows the dependence of surface on size of cation has maximum for NH4 and equals 100-200 mZ/g, according to [6]. Therefore, ifHPA is characterized by virtual absence of evident porosity the reaction mainly takes place on their surface or on boundary layers. The substitution of H + for other cations leads to expansion of pores and bulk acidic centres become more accessible to reacting molecules, which explains the high activity NH4H2PW12040 in oligomerization process. The study of HPA acidic properties was carried out by IR-spectroscopy of adsorbed pyridine. The samples of silica supported H~PW10V2040 before and after oligomerization of C6-olefins in their presence were chosen for this purpose. It is noted that these samples hadn't been exposed to high temperature treatment. Before the pyridine adsorption the catalysts were only pumped out at the room temperature. In this case the sharp decrease of the band at 3500 cm 1, which belongs to the OH-groups, and the appearance of the strongest, band at 1540 cm x, characteristic of the bond of pyridinium-ion with HPA surface and the strength of the Bronsted acidic centres are observed.
1706 Atter the preliminary thermal treatment of catalysts at 140~ and subsequent adsorption of pyridine the band at 1450 cm 1 , belonging to the Lewis acidic centres appears in IR spectra. This band is more intensive than the aforementioned one at 1540 cm -1. After pyridine pumping out the intensity of band at 1540 cm ~ decreases, but the band at 1450 cm -~ doesn't change. This fact characterizes the firmness of the link of pyridine with the Bronsted acidic centres. The essential role of these centres of HPA in oligomerization of olefins is confirmed by the experiments in which the catalyst samples heated at 150-250~ appeared to be less active than those which hadn't been exposed to any thermal treatment. The investigation of PMo, PW, PMoV and PWV acids and some of their salts in oxidation in a series of isobutanol- isobutyric aldehyde- isobutyric acid allowed to establish undermentioned regularities: PMo acid does not practically show any activity in oxidation of isobutanol in a isobutyric aldehyde. The deep oxidation process and the formation of acetone, methylketone and acetate acid take place, in particular, in the presence of this catalyst. The substitution of atoms of Mo for V-ones in the initial PMo acid leads to decrease of the total conversion of isobutanol and the insignificant increase of the yield of isobutyric aldehyde. (Figure 1). 6f
......
80
4 6o
~
40
3L
o
r..)
20 /J V Mo
I-
I
!
1 11
2 10
3 9
[-
4 8
~r Mo
1 11
2 10
3 9
4 8
Figure 1. The oxidation of isobutanol to isobutyric aldehyde over H3PMo12-nVnO40/SiO2 (n--0, 1, 2, 3, 4). The content of H P A - 30% wt. PW acid exhibits a higher activity in oxidation of isobutyric aldehyde into a isobutyric acid in the comparison with PMo-one; the yield of isobutyric acid reaches 14% mass in the presence of H3PW12040/SiO2. The substitution of two W-atoms for V-ones leads to the increase of the yield of isobutyric acid to about 38% mass, but in the case of replacement of W-atoms by Mo- or Si-ones the yield of isobutyric acid decreases to about 2% mass (Table 2). In oxidation of isobutyric acid PMo-, PMoV-, PWV HPA have not exhibited the notable activity. The investigation of Cs salts of these HPA showed their higher activity in oxidation of isobutyric acid to methacrilic one. This is a result of expansion of the channels between the different heteropolyanions and the formation of stable structures while modifying
1707 Table 2 Oxidative conversion of isobutyric aldehyde Catalyst: HPA/SiO2 The amount of HPA, %wt H3PW12040 20 30 H3PWloV204 10
H3Px,V6Mo604o HaSiMol204o
20 30 30 30
The yield of isobutyric acid, % mass 10 14 25 30 38 5 2
I-[PA with cations with a large radius. Therefore, in the presence of Cs2H2PMol1VO40 the yield of the methacrylic acid totals 60% with a selectivity of about 80% (Table 3). Table 3 The catalytic activity of some heteropoly compounds in the oxidative conversion of isobutyric acid to methacrylic one Catalyst Temperature, Conversion of The yield of meSelectivity T~ isobutyric acid, thacrylic acid, % % mass % mass CsH2PW12040 460 45 22 49 CsH4PW10V2040 460 36 11 30 Cs3PWI2040 460 40 CsH2PMol2040 400 88 C szH 2PMo I 1VO40 400 75 60 80 CszH3PMoloV204o 400 80 55 68 380 32 10 31 Mol0VPCu0.2 440 32 16 50 460 46 22 48 CuH2PMo I 1VO40 400 73 27 38 460 69 31 41 The investigation of the complex substance catalyst prepared by including the different elements in its composition in one stage showed the small activity (23%) in formation of the methacrylic acid. The modification of this catalyst with CsNO3 almost tripled its initial activity. Perhaps, the inclusion of Cs + ions in the composition of catalyst changes not only its main acidic capabilities but also the structure. As a result of this, the active V centres O=Mo 6+ become accessible for the reagents. 2 M o 6+ + O2+ 2 V 5+ ---~2Mo5+ +2V 5+ / > 2Mo6++2V 4+ isobutyric acid ~ methacrylic acid In the reactions of selective oxidation the important role belongs to the absorbed oxygen stability of its linkage with catalyst and the amount of reaction capable oxygen. To
1708 make clear this problem the thermodesorption of oxygen from the surface of various HPA was investigated. For this purpose, the adsorption of oxygen was carried out at 300-450~ for 20-30 min. Aiter this, the system was cooled until the room temperature and then it was heated with the linear velocity of 15~ until 500~ On the spectra of thermodesorption of oxygen from the catalysts HsPWIoV2040/SiO2 three peaks with the maximum of temperature 120-160, 200-290 and 440-450~ are observed (Table 4). The displacement of W-atoms by V-ones leads to a slight removal of the maximum of temperature to the side of low temperature, which can testify the weak linkage of oxygen with lhe catalyst. The amounts of adsorbed oxygen corresponding to these forms also differ. According to the data presented in the Table 4, it follows that under the substitution of W-atoms for V-ones the amount of low temperature oxygen decreases but, on contrary, the volume of high temperature oxygen increases. Table 4 Thermodesorption of oxygen from HPA Sample
H3PWlzO40/SiO2
HsPW10V2040/SiO2
The temperature of maximum, T~ 160 290 440 120 200 440
The amount of desorbed oxygen, mmol/g 1.04 0.630 0.554 0.545 0.668 0.682
Therefore, the catalytic activity of HPA in selective oxidation of isobutyric aldehyde into a isobutyric acid also increases. REFERENCES
1. J.V.Kojevnikov, Russian Chemical Review, No. 9 (1987) 1417. 2. E.A.Nikytina, Heteropoly Compounds, Mir, Moscow, 1961. 3. S.M.Zulfugarova, M.K.Munshieva, D.B.Tagiev, P.S.Mamedova, The Method of the Obtaining of C6-, C8-olefins Dimers, Author Certificate (USSR) No. 1723792 (1991). 4. D.B.Tagiev, S.M.Zulfugarova, M.K.Munshieva, P.S.Mamedova, The Catalyst for Oligomerization of C6-olefins, Author Certificate (USSR) No. 1743035 (1992). 5. S.M.Zulfugarcwa, D.B.Tagiev, M.K.Munshieva, P.S.Mamedova, The Method of Oligomerization of C6-olefins, Author Certificate (USSR) No. 1823415 (1992). 6. J.B.Mollat, G.B.Garvey, NATO Adv. Res. Workshop, Chattily, London, (1990) 193.
1703
O l i g o m e r i z a t i o n o f higher olefins and oxidation in a series o f i s o b u t a n o l isobutyric aldehydeisobutyric acid in the presence o f some heteropoly compounds S.M. Zulfugarova, M.K. Munshieva, D.B. Tagiev, P.S. Mamedova Institute of inorganic and physical chemistry Azerbaijan Academy of sciences, Baku, 370143, H.Javid avenue, 29, Azerbaijan Republic
The results of oligomerization of C6-olefins and oxidation in a series: isobutanol isobutyric aldehyde - isobutyric acid. in the presence of some 12-Heteropoly acids (HPA) and their compounds are given. It is shown that the reaction may takes place on the surface of these catalysts as well as in their bulk. By modifying the studied heteropoly acids and their compounds or supporting them on the surface of some substrates can increase its efficiency.
1. INTRODUCTION Heteropoly compounds have recently attracted considerable attention as catalysts for various industrial processes. The wide diapason of catalytic possibilities allows their use as homogeneous and heterogeneous catalysts for acidic and oxidation reactions [ 1]. In the present paper we wish to report the results of oligomerization of C6-olefins and oxidation in a series: isobutanol - isobutyric aldehyde - isobutyric acid. in the presence of some 12-Heteropoly acids and their compounds.
2. EXPERIMENTAL HPA and their compounds were synthesized, according to [1,2]. For identification of obtained compounds the method of IR-spectroscopy was used: the bands of absorption at 1060, 970, 870 and 790 cm l, belonging to the Keggin's cell were observed. HPA was investigated in free form and supported on the various substrates, such as CaCO3, silica with the specific surface 120-300 m2/g and zeolite HZSM with SIO/A1203=33. The oligomerization process was taking place at the boiling point of C6-olefins (6572~ and atmospheric pressure. The reaction of the oxidative conversion in a series: isobutanol - isobutyric aldehyde isobutyric acid, was studied in the flow reactor at the temperature 250-460~ molar correlation - raw material: oxygen between 1:1-1:4 and at the volume velocity 2-3 hours 1. The products of these reactions after separating from the catalyst were analyzed by method of GLC.
1704
3. RESULTS AND DISCUSSIONS
Free and supported HPA were researched in oligomerization process. For correct comparison of activity of these catalysts, the amount of active mass in both cases was equal. (Table 1). Table 1 Conversion of C6-01efins in the presence of some heteropoly acids The Conversion The yield of products, % mass Catalyst amount of C6-01edimers tri- and aromatics of fins, % oligomers HPA, mass g 0.8 55.0 100 H3PWI204o 1.0 61.2 100 0.5 56.0 25% H3PW1204o/SiO2 68.6 5.7 25.7 1.0 630 30% H3PWI204o/SiO2 72.2 5.6 22.2 0.15 24.0 83.3 16.7 0.50 470 87.0 13.0 HsPWloV204o 1.0 650 88.5 11.5 0.15 24.0 83.3 16.7 8% HsPWloV2040/SiO2 0.46 47.0 86.8 13.2 30% HsPWloV204o/SiO2 0.92 650 88.5 11.5 05 6O 5 93.0 6.1 0.9 H4SiWl204o 10 69 0 94.0 4.5 1.5 0.5 6O 5 93.0 6.2 0.8 25% H4SiWl204o/SiO2 94.2 4.3 1.5 10 69.0 30% H4SiWl204o/SiO2 100 O8 26 0 H3PMo1204o 100 10 29.0 45 55 O7 8.0 H4PMOllVO4o 50 50 10 14.0 23 77 4.0 1.0 HsPMoloV204o 0.5 95.7 4.3 57.5 25% H3PW1204o/SiO2 95.4 4.6 77 1.0 30% H3PW1204o/SiO2 95.0 5.0 0.5 62 25% H4SiWl204o/SiO2 94.9 4.4 0.7 68.5 1.0 30% H4SiWl2040/SiO2 100 10 0.5 25% HsPWloV2Ono/CaCO3 90 10 41 30% H3PW12Oao/HZSM 1.0 90 10 3 30% H3PWI204o/HZSM 1.0 100 53 2.0 NH4HEPW1204o The samples 2,4,6 were prepared by use of silica with the specific surface - 120 m2/g as a support. In the samples 10.11 as a support was used the silica with the specific surface 300 m2/g. It is shown that the activity and selectivity of HPA are dependent on their nature. So PW and SiW HPA allow to conduct the dimerization process of C6-olefins with 100% selectivity [3]. The olefin conversion is within the limits 60-70%. As opposed to the
1705 aforementioned catalysts PMo I-IPA has lower activity (approximately 30%) and there the aromatizasion process is taking place. The substitution of Mo- and W-ions for V-ion influences on the nature of HPA's acidic centres. It is established that when the dimerization process is going on in the presence of PW HPA then both dimerization and oligomerization are taking place on the PWV HPA. But the replacement of Mo-ions by V-ones decreases the catalytic activity of HPA in oligomerization of olefins The free HPA are not thermal stable and their specific surface is 1-5 m2/g. Therefore we supported these catalysts on the different substrates having surface hydroxyl groups and minimum surface area of about 80 m2/g. Our investigations showed that the strong interaction of HPA with CaCO3 having the basic properties decreases their catalytic activity in oligomerization process. HPA supported on the HZSM interact with the surface hydrogen atoms of zeolite. Therefore it is supposed the acidic properties ofHPA's surface become stronger. However, these catalysts exhibit a lower activity in the studied reaction. Optimal support among the researched ones was silica-SiO2, the surface of which has undergone the physical adsorption of HPA in distinction from chemisorption in the presence of other supports. The substantial distinction of supported catalysts from free ones in oligomerization of olefins is the decrease of activity of free I-IPA after 4-6 experiments and maintaining of high activity of suppoIled HPA even after numerous (20-25) experiments. Thus, the support of HPA on the surface of silica stabilizes the activity of HPA [4,5]. According to the experimental data, the conversion of olefins on the free and silica supported HPA is almost identical. Therefore, it is assumed that the process of oligomerization takes place on the outer surface of both catalysts. The hydrated protons included in the structure of the surface acidic centres, probably, have located on the oxygen atoms of angle of bridges O which have linked the metal atoms in triads M3013. M/\M Monosubstituted NH4 salts of PW HPA (NH4H2PW12040) also exhibit high catalytic activity in oligomerization of C6-olefins. The conversion of olefins reaches 55%, after 10-12 experiments it decreases to 12-14%. The investigation of the structure and adsorptive capabilities of salts of monovalent cations of HPA, recorded by method BET shows the dependence of surface on size of cation has maximum for NH4 and equals 100-200 mZ/g, according to [6]. Therefore, ifHPA is characterized by virtual absence of evident porosity the reaction mainly takes place on their surface or on boundary layers. The substitution of H + for other cations leads to expansion of pores and bulk acidic centres become more accessible to reacting molecules, which explains the high activity NH4H2PW12040 in oligomerization process. The study of HPA acidic properties was carried out by IR-spectroscopy of adsorbed pyridine. The samples of silica supported H~PW10V2040 before and after oligomerization of C6-olefins in their presence were chosen for this purpose. It is noted that these samples hadn't been exposed to high temperature treatment. Before the pyridine adsorption the catalysts were only pumped out at the room temperature. In this case the sharp decrease of the band at 3500 cm 1, which belongs to the OH-groups, and the appearance of the strongest, band at 1540 cm x, characteristic of the bond of pyridinium-ion with HPA surface and the strength of the Bronsted acidic centres are observed.
1706 Atter the preliminary thermal treatment of catalysts at 140~ and subsequent adsorption of pyridine the band at 1450 cm 1 , belonging to the Lewis acidic centres appears in IR spectra. This band is more intensive than the aforementioned one at 1540 cm -1. After pyridine pumping out the intensity of band at 1540 cm ~ decreases, but the band at 1450 cm -~ doesn't change. This fact characterizes the firmness of the link of pyridine with the Bronsted acidic centres. The essential role of these centres of HPA in oligomerization of olefins is confirmed by the experiments in which the catalyst samples heated at 150-250~ appeared to be less active than those which hadn't been exposed to any thermal treatment. The investigation of PMo, PW, PMoV and PWV acids and some of their salts in oxidation in a series of isobutanol- isobutyric aldehyde- isobutyric acid allowed to establish undermentioned regularities: PMo acid does not practically show any activity in oxidation of isobutanol in a isobutyric aldehyde. The deep oxidation process and the formation of acetone, methylketone and acetate acid take place, in particular, in the presence of this catalyst. The substitution of atoms of Mo for V-ones in the initial PMo acid leads to decrease of the total conversion of isobutanol and the insignificant increase of the yield of isobutyric aldehyde. (Figure 1). 6f
......
80
4 6o
~
40
3L
o
r..)
20 /J V Mo
I-
I
!
1 11
2 10
3 9
[-
4 8
~r Mo
1 11
2 10
3 9
4 8
Figure 1. The oxidation of isobutanol to isobutyric aldehyde over H3PMo12-nVnO40/SiO2 (n--0, 1, 2, 3, 4). The content of H P A - 30% wt. PW acid exhibits a higher activity in oxidation of isobutyric aldehyde into a isobutyric acid in the comparison with PMo-one; the yield of isobutyric acid reaches 14% mass in the presence of H3PW12040/SiO2. The substitution of two W-atoms for V-ones leads to the increase of the yield of isobutyric acid to about 38% mass, but in the case of replacement of W-atoms by Mo- or Si-ones the yield of isobutyric acid decreases to about 2% mass (Table 2). In oxidation of isobutyric acid PMo-, PMoV-, PWV HPA have not exhibited the notable activity. The investigation of Cs salts of these HPA showed their higher activity in oxidation of isobutyric acid to methacrilic one. This is a result of expansion of the channels between the different heteropolyanions and the formation of stable structures while modifying
1707 Table 2 Oxidative conversion of isobutyric aldehyde Catalyst: HPA/SiO2 The amount of HPA, %wt H3PW12040 20 30 H3PWloV204 10
H3Px,V6Mo604o HaSiMol204o
20 30 30 30
The yield of isobutyric acid, % mass 10 14 25 30 38 5 2
I-[PA with cations with a large radius. Therefore, in the presence of Cs2H2PMol1VO40 the yield of the methacrylic acid totals 60% with a selectivity of about 80% (Table 3). Table 3 The catalytic activity of some heteropoly compounds in the oxidative conversion of isobutyric acid to methacrylic one Catalyst Temperature, Conversion of The yield of meSelectivity T~ isobutyric acid, thacrylic acid, % % mass % mass CsH2PW12040 460 45 22 49 CsH4PW10V2040 460 36 11 30 Cs3PWI2040 460 40 CsH2PMol2040 400 88 C szH 2PMo I 1VO40 400 75 60 80 CszH3PMoloV204o 400 80 55 68 380 32 10 31 Mol0VPCu0.2 440 32 16 50 460 46 22 48 CuH2PMo I 1VO40 400 73 27 38 460 69 31 41 The investigation of the complex substance catalyst prepared by including the different elements in its composition in one stage showed the small activity (23%) in formation of the methacrylic acid. The modification of this catalyst with CsNO3 almost tripled its initial activity. Perhaps, the inclusion of Cs + ions in the composition of catalyst changes not only its main acidic capabilities but also the structure. As a result of this, the active V centres O=Mo 6+ become accessible for the reagents. 2 M o 6+ + O2+ 2 V 5+ ---~2Mo5+ +2V 5+ / > 2Mo6++2V 4+ isobutyric acid ~ methacrylic acid In the reactions of selective oxidation the important role belongs to the absorbed oxygen stability of its linkage with catalyst and the amount of reaction capable oxygen. To
1708 make clear this problem the thermodesorption of oxygen from the surface of various HPA was investigated. For this purpose, the adsorption of oxygen was carried out at 300-450~ for 20-30 min. Aiter this, the system was cooled until the room temperature and then it was heated with the linear velocity of 15~ until 500~ On the spectra of thermodesorption of oxygen from the catalysts HsPWIoV2040/SiO2 three peaks with the maximum of temperature 120-160, 200-290 and 440-450~ are observed (Table 4). The displacement of W-atoms by V-ones leads to a slight removal of the maximum of temperature to the side of low temperature, which can testify the weak linkage of oxygen with lhe catalyst. The amounts of adsorbed oxygen corresponding to these forms also differ. According to the data presented in the Table 4, it follows that under the substitution of W-atoms for V-ones the amount of low temperature oxygen decreases but, on contrary, the volume of high temperature oxygen increases. Table 4 Thermodesorption of oxygen from HPA Sample
H3PWlzO40/SiO2
HsPW10V2040/SiO2
The temperature of maximum, T~ 160 290 440 120 200 440
The amount of desorbed oxygen, mmol/g 1.04 0.630 0.554 0.545 0.668 0.682
Therefore, the catalytic activity of HPA in selective oxidation of isobutyric aldehyde into a isobutyric acid also increases. REFERENCES
1. J.V.Kojevnikov, Russian Chemical Review, No. 9 (1987) 1417. 2. E.A.Nikytina, Heteropoly Compounds, Mir, Moscow, 1961. 3. S.M.Zulfugarova, M.K.Munshieva, D.B.Tagiev, P.S.Mamedova, The Method of the Obtaining of C6-, C8-olefins Dimers, Author Certificate (USSR) No. 1723792 (1991). 4. D.B.Tagiev, S.M.Zulfugarova, M.K.Munshieva, P.S.Mamedova, The Catalyst for Oligomerization of C6-olefins, Author Certificate (USSR) No. 1743035 (1992). 5. S.M.Zulfugarcwa, D.B.Tagiev, M.K.Munshieva, P.S.Mamedova, The Method of Oligomerization of C6-olefins, Author Certificate (USSR) No. 1823415 (1992). 6. J.B.Mollat, G.B.Garvey, NATO Adv. Res. Workshop, Chattily, London, (1990) 193.