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Redox potentials of molybdovanadophosphoric heteropoly acids in aqueous solutions
Journal of Molecular Catalysis A: Chemical 158 Ž2000. 453–456 www.elsevier.comrlocatermolcata
Redox potentials of molybdovanadophosphoric heteropoly acids in aqueous solutions V.F. Odyakov ) , E.G. Zhizhina, K.I. Matveev BoreskoÕ Institute of Catalysis, Pr. Akad. LaÕrentieÕa 5, 630090, NoÕosibirsk, Russia
Abstract Redox potentials E and pH values were measured in 0.2 and 0.01 M solutions of partly reduced heteropoly acids V H 3q x q m PVmIV Vxym Mo 12yxO40 ŽH m HPA-x, 1 F x F 4, 0 F m F x .. The potentials were compared to the redox potential of q a couple VO 2 rVO 2q which is in equilibrium with various HPA-x m anions. It was found that all HPA-x m anions as VOq 2 and VO 2q cations are one-electron oxidantsrreductants. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Heteropoly acids; Molybdovanadophosphates; Redox potentials; Solutions
1. Introduction Heteropoly acids H 3qx PVxV Mo 12yxO40 ŽHPA-x, 1 F x F 4. are reversible oxidants that are widely used as homogeneous catalysts for various substrates ŽSu. oxidation w1x Ž Su s olefins, CO, H 2 S, phenols, etc.. via the overall reaction Ž3. : m 2Su q HPA-xqm 2H 2 O
™m 2SuO q H HPA-x H HPA-xqm 4O ™ HPA-xqm 2H O 2Su q O ™ 2SuO m
m
2
2
HPA-x
2
Ž1. Ž2. Ž3.
Their reduced forms H m HPA-xs H 3qx q m PVmIV VxVy m Mo 12y x O40 Ž where m s wV IV xr wHPA-x x. are easily regenerated by dioxygen w2x. Rate and selectivity of oxidation of various )
Corresponding author. Fax: q7-3832-34-30-56. E-mail address: [email protected] ŽV.F. Odyakov..
Su are significantly affected by redox potentials of the H m HPA-x in solution w3x. However, these potentials were often measured in strong acidic media w4x when equilibrated composition of H m HPA-x solutions is very complicated due to 2q Ž the presence of cations VOq see 2 and VO . later . In the present work, the redox potentials of H m HPA-x solutions at various m are measured in 0.2 and 0.01 M H m HPA-x solutions without the addition of strong acids, with 0.2 M H m HPA-x solutions mostly used as catalysts w1x. Nevertheless, 0.01 M H m HPA-x solutions 2q are little affected by cations VOq , 2 and VO and therefore they are more preferable for potentiometric study.
2. Experimental 0.2 M aqueous solutions of H 3qx PVxVMo 12yxO40 were synthesized using stoichiomet-
1381-1169r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 1 - 1 1 6 9 Ž 0 0 . 0 0 1 2 3 - 0
V.F. OdyakoÕ et al.r Journal of Molecular Catalysis A: Chemical 158 (2000) 453–456
454
ric quantities of V2 O5 , H 2 O 2 , H 3 PO4 , and MoO 3 w5x and diluted with water if it was necessary. Their compositions were determined by 31 P and 51 V NMR spectroscopy w5x. Redox potentials E and pH values of H m HPA-x solutions Ž0 F m F x . were measured at 293 K using a digital ionomerer I-130. The concentration of V IV was determined by titration of H m HPA-x solution by KMnO4 in H 3 PO4 w6x.
In aqueous solutions, HPA-x are always in equilibrium with cations VOq 2 due to their acidity w7x:
Ž 13 y x . H 3qx PVx Mo 12yxO40 q 12Hq
° Ž12 y x .H k ox
q 2qx PVxy1 Mo 13yx O40 q 12VO 2
q H 3 PO4 q 12H 2 O 3. Discussion The relationships E s f Ž m. and pH s w Ž m. for 0.01 M H m HPA-x solutions are pictured in Fig. 1. Similar 0.2 M solutions Ž not shown. have greater E values Žby ; 0.1 V. and smaller pH ones Ž by ; 0.5 unit. than the 0.01 M solutions.
V 4y H xqmy1 PVmIV Vxym Mo 12yxO40 q eyq Hq
Ž HPA-x m .
Ž4.
The reduced forms H m HPA-x are in similar equilibrium with cations VO 2q w8x. Therefore, the more concentrated are H m HPA-x solutions Žwith high Hq concentration., the higher are the q VOq 2 and VO 2 concentrations in them. Each H m HPA-x solution contains Ž x q 1. individual anions Žfrom HPA-x o to HPA-x x ., which are in equilibria Ž 5. .
l
IV V 4y H xqm PVmq1 Vxymy1 Mo 12yxO40
Ž HPA-x mq1 .
Ž5.
For each m Ži. e., each HPA-x mrHPA-x mq1 pair., the redox potential Em is given by formula Ž6.: Em ™ mq1 s Emo ™ mq1 y 0.0591pH q 0.0591log
HPA-x m HPA-x mq1
Ž6.
For HPA-3 and HPA-4 solutions, calculated fractions of individual HPA-x m anions are given in Table 1. Redox potentials of various anion pairs HPAx mrHPA-x mq1 Ž Em ™ mq1 . are equal to the re2q Ž dox potential of cation pair VOq EV . 2 rVO due to equilibria Ž4. , Ž5., and Ž7.:
l VO
q y VOq 2 P aq q 2H q e
2q
P aq q H 2 O
Ž7.
Fig. 1. E and pH measured for 0.01 M H m HPA-x solutions as a function of the reduction degree m. ŽDigits 2, 3 and 4 are x values..
V.F. OdyakoÕ et al.r Journal of Molecular Catalysis A: Chemical 158 (2000) 453–456
455
Table 1 Fractions of individual partly reduced anions HPA-x m , a m , in 0.01 M solutions of H m HPA-3 and H m HPA-4 Calculation according to formula Ž6. following Fig. 1. Solution
m
H m HPA-3
0 0.5 1 1.5 2 2.5 3 0 0.5 1 1.5 2 2.5 3 3.5 4
™ mq1a ŽV.
Eq m
a m Ž%. HPA-x o
H m HPA-4
a
100
HPA-x 2
–
–
8.9
78.7
0.2
HPA-x 3
HPA-x 4
–
–
12.1
0.3
10.6
74.2
15.0
– – –
–
–
0.817 0.713 0.622 – 100
100
0.834 8.3
82.7
8.3
0.6
0.1
0.1
10.8
59.7
25.3
4.1
1.6
21.4
54.6
22.3
–
–
0.716 0.650 ; 0.02 0.603 –
–
100
The middle-point potential for half-reduction of anion HPA-x m to HPA-x mq1.
Let b V be a fraction of cation VOq 2 in all forms of V V, and b IV be a fraction of cation VO 2q in all forms of V IV:
For the last pair E V s E Vo y 0.1182 pH q 0.0591log
VOq 2 VO 2q
Ž8. E Vo s 1.00
where V w9x. The ratio of equilibrium concentrations 2q x x w wVOq 2 eqr VO eq in the H m HPA-x solution can be found from comparison between the measured potential E for this solution and the for cation pair middle-point potential Eq V 2q VOq rVO at the same pH. 2 2q x x w Ž . If wVOq 2 eq s VO eq , then Eq. 8 transforms to Eq. Ž9.: o Eq Vs E V y 0.1182 P pH
Ž9.
2q x wVOq x w 2 eq / VO eq ,
If then Eq. Ž8. transŽ . forms to Eq. 10 considering Eq. Ž9. and the equality E V s Em ™ mq1 s E: log
HPA-x 1
VOq 2 VO
eq
2q
s
E y E Vo q 0.1182 P pH
s
E y Eq V
b V s VOq 2
eqr
b IV s VO 2q
VSV
eqr
Ž 11.
VSIV
Ž 12 .
If a m is a fraction of individual anion HPA-x m in H m HPA-x solution, then VSV s b V VOq 2
eq q
Ž1 y b V . S a m Ž n y m .
= HPA-x m fS a m Ž n y m . HPA-x m Ž 13. VSIV s b IV VO 2q
eq q
Ž 1 y b IV .
=S a m m HPA-x m fS a m m HPA-x m Ž 14.
0.0591
eq
0.0591
Ž 10.
The values E and pH measured for H m HPA-x solutions allow us to determine only the b IVrb V ratio, not b IV and b V separately. Taking into
V.F. OdyakoÕ et al.r Journal of Molecular Catalysis A: Chemical 158 (2000) 453–456
456
H m HPA-x solutions. In all cases, b IV ) b V , i.e. wVO 2q xeq ) wVOq x 2 eq . Analogous conclusion was earlier made only for HPA-2 solutions using the very complicated computer program LAKE w10x. These results confirm that HPA-x m and HPA2q x mq1 anions as VOq cations in the 2 and VO H m HPA-x solutions are one-electron oxidantsr reductants.
References V Ž Ž .. and Fig. 2. Ratio between fractions of VOq 2 in VS Eq. 11 VO 2q in VSIV ŽEq. Ž12.. as a function of the ratio of total concentrations of VSV and VSIV for 0.2 M and 0.01 M H m HPA-x solutions. ŽDigits 2, 3, and 4 are x values for 0.01 M solutions. X X X Ones 2 , 3 , and 4 are x values for 0.2 M solutions..
account Eqs. Ž11. and Ž12., Eq. Ž10. transforms to Eq. Ž15.: logG s log
b IV bV
s log
VSV VSIV
y log
VOq 2 VO 2q
eq eq
Ž 15. Fig. 2 shows that the ratio b IVrb V depends on both x and m. It decreases if x increases, and goes through maximum if m increases. These maxima are especially clear for 0.2 M
w1x I.V. Kozhevnikov, Chem. Rev. 98 Ž1998. 171. w2x K.I. Matveev, in: Study of Properties and Application of Heteropolyacids in Catalysis, 1978, p. 3, Novosibirsk Žin Russian.. w3x K.I. Matveev, V.F. Odyakov, E.G. Zhizhina, J. Mol. Catal. A: Chem. 114 Ž1996. 151. w4x E.G. Zhizhina, L.I. Kuznetsova, E.N. Yurchenko, K.I. Matveev, Koord. Khim. 6 Ž1980. 1846. w5x V.F. Odyakov, E.G. Zhizhina, R.I. Maksimovskaya, K.I. Matveev, Kinet. Catal. 36 Ž1995. 733. w6x L.S.A. Dikshitulu, G. Gopala Rao, Fresenius’ Z. Anal. Chem. 189 Ž1962. 421. w7x P. Souchay, F. Chauveau, P. Courtin, Bull. Soc. Chim. Fr. Ž1968. 2384. w8x L.I. Kuznetsova, E.N. Yurchenko, R.I. Maksimovskaya, N.P. Kirik, K.I. Matveev, Koord. Khim. 3 Ž1977. 51. w9x J. Israel, L. Meites, in: Standard Potentials in Aqueous Solution ŽIUPAC., Marcer Dekker, New York, 1985, p. 520. w10x L. Pettersson, in: Polyoxometalates. From Platonic Solids to Anti-Retroviral Activity, Kluwer Academic Publishing, Dordrecht, 1994, p. 26.
Redox potentials of molybdovanadophosphoric heteropoly acids in aqueous solutions V.F. Odyakov ) , E.G. Zhizhina, K.I. Matveev BoreskoÕ Institute of Catalysis, Pr. Akad. LaÕrentieÕa 5, 630090, NoÕosibirsk, Russia
Abstract Redox potentials E and pH values were measured in 0.2 and 0.01 M solutions of partly reduced heteropoly acids V H 3q x q m PVmIV Vxym Mo 12yxO40 ŽH m HPA-x, 1 F x F 4, 0 F m F x .. The potentials were compared to the redox potential of q a couple VO 2 rVO 2q which is in equilibrium with various HPA-x m anions. It was found that all HPA-x m anions as VOq 2 and VO 2q cations are one-electron oxidantsrreductants. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Heteropoly acids; Molybdovanadophosphates; Redox potentials; Solutions
1. Introduction Heteropoly acids H 3qx PVxV Mo 12yxO40 ŽHPA-x, 1 F x F 4. are reversible oxidants that are widely used as homogeneous catalysts for various substrates ŽSu. oxidation w1x Ž Su s olefins, CO, H 2 S, phenols, etc.. via the overall reaction Ž3. : m 2Su q HPA-xqm 2H 2 O
™m 2SuO q H HPA-x H HPA-xqm 4O ™ HPA-xqm 2H O 2Su q O ™ 2SuO m
m
2
2
HPA-x
2
Ž1. Ž2. Ž3.
Their reduced forms H m HPA-xs H 3qx q m PVmIV VxVy m Mo 12y x O40 Ž where m s wV IV xr wHPA-x x. are easily regenerated by dioxygen w2x. Rate and selectivity of oxidation of various )
Corresponding author. Fax: q7-3832-34-30-56. E-mail address: [email protected] ŽV.F. Odyakov..
Su are significantly affected by redox potentials of the H m HPA-x in solution w3x. However, these potentials were often measured in strong acidic media w4x when equilibrated composition of H m HPA-x solutions is very complicated due to 2q Ž the presence of cations VOq see 2 and VO . later . In the present work, the redox potentials of H m HPA-x solutions at various m are measured in 0.2 and 0.01 M H m HPA-x solutions without the addition of strong acids, with 0.2 M H m HPA-x solutions mostly used as catalysts w1x. Nevertheless, 0.01 M H m HPA-x solutions 2q are little affected by cations VOq , 2 and VO and therefore they are more preferable for potentiometric study.
2. Experimental 0.2 M aqueous solutions of H 3qx PVxVMo 12yxO40 were synthesized using stoichiomet-
1381-1169r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 1 - 1 1 6 9 Ž 0 0 . 0 0 1 2 3 - 0
V.F. OdyakoÕ et al.r Journal of Molecular Catalysis A: Chemical 158 (2000) 453–456
454
ric quantities of V2 O5 , H 2 O 2 , H 3 PO4 , and MoO 3 w5x and diluted with water if it was necessary. Their compositions were determined by 31 P and 51 V NMR spectroscopy w5x. Redox potentials E and pH values of H m HPA-x solutions Ž0 F m F x . were measured at 293 K using a digital ionomerer I-130. The concentration of V IV was determined by titration of H m HPA-x solution by KMnO4 in H 3 PO4 w6x.
In aqueous solutions, HPA-x are always in equilibrium with cations VOq 2 due to their acidity w7x:
Ž 13 y x . H 3qx PVx Mo 12yxO40 q 12Hq
° Ž12 y x .H k ox
q 2qx PVxy1 Mo 13yx O40 q 12VO 2
q H 3 PO4 q 12H 2 O 3. Discussion The relationships E s f Ž m. and pH s w Ž m. for 0.01 M H m HPA-x solutions are pictured in Fig. 1. Similar 0.2 M solutions Ž not shown. have greater E values Žby ; 0.1 V. and smaller pH ones Ž by ; 0.5 unit. than the 0.01 M solutions.
V 4y H xqmy1 PVmIV Vxym Mo 12yxO40 q eyq Hq
Ž HPA-x m .
Ž4.
The reduced forms H m HPA-x are in similar equilibrium with cations VO 2q w8x. Therefore, the more concentrated are H m HPA-x solutions Žwith high Hq concentration., the higher are the q VOq 2 and VO 2 concentrations in them. Each H m HPA-x solution contains Ž x q 1. individual anions Žfrom HPA-x o to HPA-x x ., which are in equilibria Ž 5. .
l
IV V 4y H xqm PVmq1 Vxymy1 Mo 12yxO40
Ž HPA-x mq1 .
Ž5.
For each m Ži. e., each HPA-x mrHPA-x mq1 pair., the redox potential Em is given by formula Ž6.: Em ™ mq1 s Emo ™ mq1 y 0.0591pH q 0.0591log
HPA-x m HPA-x mq1
Ž6.
For HPA-3 and HPA-4 solutions, calculated fractions of individual HPA-x m anions are given in Table 1. Redox potentials of various anion pairs HPAx mrHPA-x mq1 Ž Em ™ mq1 . are equal to the re2q Ž dox potential of cation pair VOq EV . 2 rVO due to equilibria Ž4. , Ž5., and Ž7.:
l VO
q y VOq 2 P aq q 2H q e
2q
P aq q H 2 O
Ž7.
Fig. 1. E and pH measured for 0.01 M H m HPA-x solutions as a function of the reduction degree m. ŽDigits 2, 3 and 4 are x values..
V.F. OdyakoÕ et al.r Journal of Molecular Catalysis A: Chemical 158 (2000) 453–456
455
Table 1 Fractions of individual partly reduced anions HPA-x m , a m , in 0.01 M solutions of H m HPA-3 and H m HPA-4 Calculation according to formula Ž6. following Fig. 1. Solution
m
H m HPA-3
0 0.5 1 1.5 2 2.5 3 0 0.5 1 1.5 2 2.5 3 3.5 4
™ mq1a ŽV.
Eq m
a m Ž%. HPA-x o
H m HPA-4
a
100
HPA-x 2
–
–
8.9
78.7
0.2
HPA-x 3
HPA-x 4
–
–
12.1
0.3
10.6
74.2
15.0
– – –
–
–
0.817 0.713 0.622 – 100
100
0.834 8.3
82.7
8.3
0.6
0.1
0.1
10.8
59.7
25.3
4.1
1.6
21.4
54.6
22.3
–
–
0.716 0.650 ; 0.02 0.603 –
–
100
The middle-point potential for half-reduction of anion HPA-x m to HPA-x mq1.
Let b V be a fraction of cation VOq 2 in all forms of V V, and b IV be a fraction of cation VO 2q in all forms of V IV:
For the last pair E V s E Vo y 0.1182 pH q 0.0591log
VOq 2 VO 2q
Ž8. E Vo s 1.00
where V w9x. The ratio of equilibrium concentrations 2q x x w wVOq 2 eqr VO eq in the H m HPA-x solution can be found from comparison between the measured potential E for this solution and the for cation pair middle-point potential Eq V 2q VOq rVO at the same pH. 2 2q x x w Ž . If wVOq 2 eq s VO eq , then Eq. 8 transforms to Eq. Ž9.: o Eq Vs E V y 0.1182 P pH
Ž9.
2q x wVOq x w 2 eq / VO eq ,
If then Eq. Ž8. transŽ . forms to Eq. 10 considering Eq. Ž9. and the equality E V s Em ™ mq1 s E: log
HPA-x 1
VOq 2 VO
eq
2q
s
E y E Vo q 0.1182 P pH
s
E y Eq V
b V s VOq 2
eqr
b IV s VO 2q
VSV
eqr
Ž 11.
VSIV
Ž 12 .
If a m is a fraction of individual anion HPA-x m in H m HPA-x solution, then VSV s b V VOq 2
eq q
Ž1 y b V . S a m Ž n y m .
= HPA-x m fS a m Ž n y m . HPA-x m Ž 13. VSIV s b IV VO 2q
eq q
Ž 1 y b IV .
=S a m m HPA-x m fS a m m HPA-x m Ž 14.
0.0591
eq
0.0591
Ž 10.
The values E and pH measured for H m HPA-x solutions allow us to determine only the b IVrb V ratio, not b IV and b V separately. Taking into
V.F. OdyakoÕ et al.r Journal of Molecular Catalysis A: Chemical 158 (2000) 453–456
456
H m HPA-x solutions. In all cases, b IV ) b V , i.e. wVO 2q xeq ) wVOq x 2 eq . Analogous conclusion was earlier made only for HPA-2 solutions using the very complicated computer program LAKE w10x. These results confirm that HPA-x m and HPA2q x mq1 anions as VOq cations in the 2 and VO H m HPA-x solutions are one-electron oxidantsr reductants.
References V Ž Ž .. and Fig. 2. Ratio between fractions of VOq 2 in VS Eq. 11 VO 2q in VSIV ŽEq. Ž12.. as a function of the ratio of total concentrations of VSV and VSIV for 0.2 M and 0.01 M H m HPA-x solutions. ŽDigits 2, 3, and 4 are x values for 0.01 M solutions. X X X Ones 2 , 3 , and 4 are x values for 0.2 M solutions..
account Eqs. Ž11. and Ž12., Eq. Ž10. transforms to Eq. Ž15.: logG s log
b IV bV
s log
VSV VSIV
y log
VOq 2 VO 2q
eq eq
Ž 15. Fig. 2 shows that the ratio b IVrb V depends on both x and m. It decreases if x increases, and goes through maximum if m increases. These maxima are especially clear for 0.2 M
w1x I.V. Kozhevnikov, Chem. Rev. 98 Ž1998. 171. w2x K.I. Matveev, in: Study of Properties and Application of Heteropolyacids in Catalysis, 1978, p. 3, Novosibirsk Žin Russian.. w3x K.I. Matveev, V.F. Odyakov, E.G. Zhizhina, J. Mol. Catal. A: Chem. 114 Ž1996. 151. w4x E.G. Zhizhina, L.I. Kuznetsova, E.N. Yurchenko, K.I. Matveev, Koord. Khim. 6 Ž1980. 1846. w5x V.F. Odyakov, E.G. Zhizhina, R.I. Maksimovskaya, K.I. Matveev, Kinet. Catal. 36 Ž1995. 733. w6x L.S.A. Dikshitulu, G. Gopala Rao, Fresenius’ Z. Anal. Chem. 189 Ž1962. 421. w7x P. Souchay, F. Chauveau, P. Courtin, Bull. Soc. Chim. Fr. Ž1968. 2384. w8x L.I. Kuznetsova, E.N. Yurchenko, R.I. Maksimovskaya, N.P. Kirik, K.I. Matveev, Koord. Khim. 3 Ž1977. 51. w9x J. Israel, L. Meites, in: Standard Potentials in Aqueous Solution ŽIUPAC., Marcer Dekker, New York, 1985, p. 520. w10x L. Pettersson, in: Polyoxometalates. From Platonic Solids to Anti-Retroviral Activity, Kluwer Academic Publishing, Dordrecht, 1994, p. 26.