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Complex formation during the liquid phase catalytic oxidation of hydrocarbons
COMPLEX FORMATION DURING THE LIQUID PHASE CATALYTIC OXIDATION OF HYDROCARBONS* N. G. DIGUROV, V. I. ZAXHAROVX and A. I. KAMlVEVA D. I. Mendeleyev Chemico-technological Institute, Moscow
(Received 26 June 1965)
THE liquid phase oxidations of hydrocarbons in which transition metals are used as catalysts have wide industrial application in the production of O-containing compounds. The mechanism of the reaction between the transition m e t a l ions and the molecules of oxidized matter has not been fully clarified. Phenomena such as the inactivation of the catalyst and its sedimentation, colour changes of the oxidate during oxidation, have not yet been sufficiently clarified. This paper is a report of the results of studying the composition of a cobalt catalyst used to oxidize o-xylene and tetralin to ascertain the nature of t h e above mentioned phenomena. The reaction of metal ions in liquid media was regarded by a number of authors [1-3] as a reaction of complexes. The introduction of the transition metal salt into the solution produced a solvation film around the ion (ionic pair). This was explained as due to the existence of unfilled d-orbitals on such ~netal ions which then could yield chemical bonds (g-bonds, donor-accepter bonds etc.) with the solvent molecules [4]. The transition metal ion will therefore have an energy of interaction with the ligands composed of the electrostatic energy and that of exchange, i.e. E:E~E~, E~being the energy of electrostatic dipole-dipole reaction of the ionic pair (the catalyst) with the ligands, E~ the exchange energy between catalyst and ligands. The main part in the liquid phase oxidation of hydrocarbons and complexing will be played by the exchange energy between the transition metal ions and the intermediate oxidation products. The oxidizing medium is nonpolar, but the oxidate tends towards formation of a donor-accepter bond with the catalyst. The ligands situated in the first sphere of coordination cause the splitting of the d-terms of the metal ions which can be observed in the visible-range of the spectrum [4]. The colour change, e.g. of cobalt catalysts during oxidation is evidence of the change occurring in the solvation film around the ion, i.e. of the composition of the complex. * Neftekhimiya 6, No. 4, 593-597, 1966. 189
190
N.G.
D m u R o v et al.
The initiation of new chains in the above catalytic processes is largely due to the reaction of the metal ion of a higher valency state with any ligand, e.g. [5]: H
I
R--C~-Co.+ -> R--C.~-H+-~Co~+
IE
0
[I
0
The composition of the complex, i.e. the nature of the ligands in the first co-ordination sphere, will affect the progress and course of the whole oxidation process. To get an idea of the participation of the oxygen-containing ligands in complex formation, we studied the reaction of Co ~+ and Co 3+ in hydrocarbon solutions with different oxidation products of o-xylene and tetralin. EXPERIMENTAL
The study of the liquid-phase oxidation of o-xylene in the presence of different cobalt salts showed a colour change of the oxidate from green to reddish brown. This could have been caused by an accumulation of o-toluic acid because the oxidate turned green again after filtration. A colour change was also observed during tetralin oxidation. The oxidation of tetratin with molecular oxygen in a bubbler type column with cobalt-o-toluate present (10 -3 mole/k; 120°C) showed a gradual paling of the green oxidate during the first 10 rain. The moment of colour disappearance coincided with the moment of appearance of carbonyl compounds in the samples, i.e. ~-tetralone and 3,4-dihydro-l,2-naphthoquinone. It was natural to assume that the disappearance of the green colour was connected with the complex formation between the catalyst and the tetralin oxidation intermediates, the carbonyl compounds. All the compositional changes of the complex were studied by following the shift of the light transmission minimum in the spectral range 400-750 m/a. This work was carried out with spectrophotometer SF-10. The first task was to get the spectra of bi- and tri-valent cobalt in o-xylene and tetralin. Solutions of 10 -3 to 2 x 10 -3 mole/1, concentration ofCo-o toluato were prepared in these hydrocarbons. The Co 3+ transition to Co s+ was produced by adding Hz02 to the Co"+-o-toluate solution, followed by boiling to remove water. The transmission spectra were taken at 22_+ l°C for which the SF-I0 instrument was particularly sensitive. The spectrum showed a characteristic minimum transmission for Co 2+ at A----530 and 570 m/~ (Fig. la, curve 1); that for Co 3+ at 2-----607 m/~ (Fig. 2a, curve 1). An addition of ~-totralone, 3,4-dihydro-l,2-naphthoquinone or o-toluic acid to a bivalent cobalt o-toluate solution in o-xylene caused the disappearance of the lows of transmission at A 530 and 570 m/~ (Figs. la-c). The same amount of oxygen containing ligand was always add~l also to the control cell.
191
Catalytic oxidation of hydrocarbons
The studied ligands showed a varying degree of "reactivity" in the coordination with bivalent cobalt. It was found that coml~lexing with the compounds m e n t i o n e d earlier required different quantities o f ]igand additions. T h e following quantities ( % w/w) h a d t o be a d d e d to m a k e t h e characteristic 5B-~
SO- %
1
lZ
3~
80,~06,0
56
~oo
a
I 5o0
to~ -%
t 60O % ml~
3O 20 ff 0
i
no[%
b
b !
"
6o
"~ 80
500
I
tO0-%
bOO ~.m#
0
40 30 20
YO dO0
10
500 FIG. 1
600~ , m #
.,,eo
4oo
FIG. 2
FIG. 1. Effect of adding different oxygen containing ligands on the transmission spectra of Col+-o-toluate solution in o.xylene, a-- 3,4-Dihydro-l,2-naphthoquinone as ligand:/--without addition; 2--with 0"01% w/w; 3--0-1°//o w/w. b--g-Tetralone as ligand: /--without addition; 2--with 0-1~/ow/w; 3--0.5~oW/W; 4--1~/oW/W. c--o-Toluic acid as ligand:/--without addition; 2--with 1~o w/w; 3--10~o w/w. of ligand. FIG. 2. Effect of carbonyl compound additions on the transmission spectra of Co03+-o-toluatesolution in o-xylene and tetralin, a--3,4-Dihydro-l,2-naphthoqulnone as ligand:/--without addition; 2--with 0.01~ w/w; 3--0.1~/o w/w. b--~-Tetralone as ligand (disappearance of peak at 607 m~): 1--No addition; 2--with 0.03% w/w; 3--with 0-3o//0w/w of ligand, c--~-Tetralone as ligand (appearance of peak at 380 m~; spectra taken on SF-4): / - - W i t h 0"15~/ow/w; 2--0.3o//o w/w of ligand.
192
N.G. DIGUROVet al.
p e a k s of bivalent cobalt disappear: ca. 0.1 of 3,4-dihydro-l,2-naphthoquinone,
ca. 1 of a-tetralone and ca. 10 of o-toluic acid. The larger "reactivity" of the first named compound compared with a-tetralone was explained as due to the presence of two carbonyl groups in o-position and the resultant chelating effect. A similar series of tests was carried out with a Coa+-o-toluate (Figs. 2a-c). I t was found that the characteristic absorption peak (2 607 m/x) disappeared much later than from the Co s+ solutions, often b y 8-10 hr. The colour change was practically instantaneous in this case. The results obtained are in agreement with those of Taube [6] with respect to the kinetic inertness of the octahedral Co 3+ complexes in substitutions. It should be noted that a relatively small concentration of 0.1-1% w/w is sufficient for the complexing of cobalt with oxygen containing compounds of ketone, diketone or acid t y p e present in the hydrocarbon. A 3,4-dihydro-l,2-naphthoquinone addition (0.1% w/w of oxidate) to the o-xylene oxidation at 134°C inhibited the reaction (Fig. 3) and caused a colour change of the oxidate from green to reddish brown. In this case, and during tetralin oxidation, there was a coordination of 3,4-dihydro-l,2-naphthoquinone with cobalt and a "passive" form catalyst was produced. The compound apparently reacted with Co 3÷ because the o-xylene oxidate showed an absorption peak at ;t 422 m/~ after its addition which was characteristic for Co 3+ coordinated with 3,4-dihydro-l,2-naphthoquinone. The cobalt coordinated with the oxidation products can sometimes fall out as a sediment in the form of a complex. A bi- and tri-valent Co solution (as stearate or toluate) in o-xylene, tetralin or cyclohexane at 22°C will fall out as sediment when 3,4-dihydro-l,2-naphthoquinone or a-te~ralone is added
.~5 I "xS
2
4®
5
10
15
T
L 21
20
25 30 V, rain
FxG. 3. Effect of 3,4-dihydro-l,2-naphthoquinone additions on the kinetics of catalytic oxidation of o-xylene: 1 - o-Toluic acid accumulation in experiments without additions; 2--e-toluic aldehyde accumulation in the same test; 3 - - a s 1 with 0.1 ~o w / w of compound added; 4--as 2 with 0.1% w / w of compound added.
Catalytic oxidation of hydrocarbons
193
(catalyst concentration equal to 10 -3 mole/1, in o-xylene or tetralin, 5 × 10-*mole/ /1. in cyclohexane, 2-3~/o w/w of additives). The following complexes were produced and isolated: Co s+- and Co s+- with dihydronaphthoquinone and Co2+-a-tetralone. The complex was washed with cyclohexane to remove a-tetralone traces (to a negative reaction for carbonyl with 2,4-dinitrophenyl hydrazine) or treated with 2,4-dinitrophenyl hydrazine when present in an ethanol solution. Thin layer chromatography was used to show that a-tetralone enters into a complex with Co s+. RESULTS
The above results are evidence for comp]exing taking place in the catalytic, liquid phase oxidation of hydrocarbons. The oxygen containing compounds thus produced appear to form coordination compounds around the transition metal ions in all cases and to a larger extent than the original hydrocarbon. The presence in the liquid phase of different quality ligands will give rise to the formation of a reactive complex of a certain composition which will depend on the structure as well as concentration of the ligands, i.e. a competition of the ligands in their reaction with the catalyst. It must be expected that mainly the oxygen containing compounds will be the branching agents in a catalytic oxidation of hydrocarbon. A smooth progress of the above reaction requires that the yield of oxidized ligands from the first coordination sphere is sufficiently large, otherwise the catalyst will become "passivated" and the reaction inhibited. This was observed when 3,4-dihydro-l,2-naphthoquinone was added to o-xylene during oxidation (Fig. 3). It is well known that the formation of chemical bonds in a complex consisting of the coordination of transition metal ions with various ketones is due to the intact electron pair of the oxygen atom, i.e. that a donor-accepter bond is produced. The coordination of TiCl~ with ketones in a benzene solution was studied by other authors [7]. One must expect in a liquid phase oxidation that a coordination of the catalyst with the oxidized O-containing ligand will take place by means of the oxygen atom. The electron transfer from the ligand to the transition metal ion also appears to take place via the oxygen atom. SUMMARY
l. Oxygen containing ligands will coordinate around the catalyst, the cobalt ion, more easily during liquid phase oxidation than the hydrocarbon molecules. 2. A 0 . 1 ~ w/w addition of 3,4-dihydro-l,2-naphthoquinone inhibited the catalytic oxidation of o-xylene.
194
N . G . DIOUROV et al.
3. Complexes of Co s÷ a n d Co S+ with a-tetralone a n d with d i h y d r o n a p h t h o quinone were isolated from the o-xylene solution. Translated by K. A. fl~LLEN
REFERENCES
1. 2. 3. 4. 5. 6.
N. A. IZMAILOV and Yu. A. KRUGLYAK, Dokl. Akad. Nauk SSSR, 134, 1390, 1960 Ya. K. SYRKIN, Uspekhi Khim. 28, 903, 1959 V. A. ~ H A I L O V and S. I. DRAKIN, Izv. Sibir. Otdel. Akad. Nauk SSSR 6, 44, 1960 L. ORGEL, Introduction to the Chemistry of Transition Metals. "Mir", Moscow 1964 C. BAWN, Disc. Faraday Soc. 14, 181, 1953 N. TAUBE, Progress in Inorganic and Radio Chemistry. Academic Press, New York 1, 1, 1959 7. O. A. OSIPOV and V. I. GAIVORONSKH, Zhur. obsh. khim. 33, No. 4, 1346, 1963
(Received 26 June 1965)
THE liquid phase oxidations of hydrocarbons in which transition metals are used as catalysts have wide industrial application in the production of O-containing compounds. The mechanism of the reaction between the transition m e t a l ions and the molecules of oxidized matter has not been fully clarified. Phenomena such as the inactivation of the catalyst and its sedimentation, colour changes of the oxidate during oxidation, have not yet been sufficiently clarified. This paper is a report of the results of studying the composition of a cobalt catalyst used to oxidize o-xylene and tetralin to ascertain the nature of t h e above mentioned phenomena. The reaction of metal ions in liquid media was regarded by a number of authors [1-3] as a reaction of complexes. The introduction of the transition metal salt into the solution produced a solvation film around the ion (ionic pair). This was explained as due to the existence of unfilled d-orbitals on such ~netal ions which then could yield chemical bonds (g-bonds, donor-accepter bonds etc.) with the solvent molecules [4]. The transition metal ion will therefore have an energy of interaction with the ligands composed of the electrostatic energy and that of exchange, i.e. E:E~E~, E~being the energy of electrostatic dipole-dipole reaction of the ionic pair (the catalyst) with the ligands, E~ the exchange energy between catalyst and ligands. The main part in the liquid phase oxidation of hydrocarbons and complexing will be played by the exchange energy between the transition metal ions and the intermediate oxidation products. The oxidizing medium is nonpolar, but the oxidate tends towards formation of a donor-accepter bond with the catalyst. The ligands situated in the first sphere of coordination cause the splitting of the d-terms of the metal ions which can be observed in the visible-range of the spectrum [4]. The colour change, e.g. of cobalt catalysts during oxidation is evidence of the change occurring in the solvation film around the ion, i.e. of the composition of the complex. * Neftekhimiya 6, No. 4, 593-597, 1966. 189
190
N.G.
D m u R o v et al.
The initiation of new chains in the above catalytic processes is largely due to the reaction of the metal ion of a higher valency state with any ligand, e.g. [5]: H
I
R--C~-Co.+ -> R--C.~-H+-~Co~+
IE
0
[I
0
The composition of the complex, i.e. the nature of the ligands in the first co-ordination sphere, will affect the progress and course of the whole oxidation process. To get an idea of the participation of the oxygen-containing ligands in complex formation, we studied the reaction of Co ~+ and Co 3+ in hydrocarbon solutions with different oxidation products of o-xylene and tetralin. EXPERIMENTAL
The study of the liquid-phase oxidation of o-xylene in the presence of different cobalt salts showed a colour change of the oxidate from green to reddish brown. This could have been caused by an accumulation of o-toluic acid because the oxidate turned green again after filtration. A colour change was also observed during tetralin oxidation. The oxidation of tetratin with molecular oxygen in a bubbler type column with cobalt-o-toluate present (10 -3 mole/k; 120°C) showed a gradual paling of the green oxidate during the first 10 rain. The moment of colour disappearance coincided with the moment of appearance of carbonyl compounds in the samples, i.e. ~-tetralone and 3,4-dihydro-l,2-naphthoquinone. It was natural to assume that the disappearance of the green colour was connected with the complex formation between the catalyst and the tetralin oxidation intermediates, the carbonyl compounds. All the compositional changes of the complex were studied by following the shift of the light transmission minimum in the spectral range 400-750 m/a. This work was carried out with spectrophotometer SF-10. The first task was to get the spectra of bi- and tri-valent cobalt in o-xylene and tetralin. Solutions of 10 -3 to 2 x 10 -3 mole/1, concentration ofCo-o toluato were prepared in these hydrocarbons. The Co 3+ transition to Co s+ was produced by adding Hz02 to the Co"+-o-toluate solution, followed by boiling to remove water. The transmission spectra were taken at 22_+ l°C for which the SF-I0 instrument was particularly sensitive. The spectrum showed a characteristic minimum transmission for Co 2+ at A----530 and 570 m/~ (Fig. la, curve 1); that for Co 3+ at 2-----607 m/~ (Fig. 2a, curve 1). An addition of ~-totralone, 3,4-dihydro-l,2-naphthoquinone or o-toluic acid to a bivalent cobalt o-toluate solution in o-xylene caused the disappearance of the lows of transmission at A 530 and 570 m/~ (Figs. la-c). The same amount of oxygen containing ligand was always add~l also to the control cell.
191
Catalytic oxidation of hydrocarbons
The studied ligands showed a varying degree of "reactivity" in the coordination with bivalent cobalt. It was found that coml~lexing with the compounds m e n t i o n e d earlier required different quantities o f ]igand additions. T h e following quantities ( % w/w) h a d t o be a d d e d to m a k e t h e characteristic 5B-~
SO- %
1
lZ
3~
80,~06,0
56
~oo
a
I 5o0
to~ -%
t 60O % ml~
3O 20 ff 0
i
no[%
b
b !
"
6o
"~ 80
500
I
tO0-%
bOO ~.m#
0
40 30 20
YO dO0
10
500 FIG. 1
600~ , m #
.,,eo
4oo
FIG. 2
FIG. 1. Effect of adding different oxygen containing ligands on the transmission spectra of Col+-o-toluate solution in o.xylene, a-- 3,4-Dihydro-l,2-naphthoquinone as ligand:/--without addition; 2--with 0"01% w/w; 3--0-1°//o w/w. b--g-Tetralone as ligand: /--without addition; 2--with 0-1~/ow/w; 3--0.5~oW/W; 4--1~/oW/W. c--o-Toluic acid as ligand:/--without addition; 2--with 1~o w/w; 3--10~o w/w. of ligand. FIG. 2. Effect of carbonyl compound additions on the transmission spectra of Co03+-o-toluatesolution in o-xylene and tetralin, a--3,4-Dihydro-l,2-naphthoqulnone as ligand:/--without addition; 2--with 0.01~ w/w; 3--0.1~/o w/w. b--~-Tetralone as ligand (disappearance of peak at 607 m~): 1--No addition; 2--with 0.03% w/w; 3--with 0-3o//0w/w of ligand, c--~-Tetralone as ligand (appearance of peak at 380 m~; spectra taken on SF-4): / - - W i t h 0"15~/ow/w; 2--0.3o//o w/w of ligand.
192
N.G. DIGUROVet al.
p e a k s of bivalent cobalt disappear: ca. 0.1 of 3,4-dihydro-l,2-naphthoquinone,
ca. 1 of a-tetralone and ca. 10 of o-toluic acid. The larger "reactivity" of the first named compound compared with a-tetralone was explained as due to the presence of two carbonyl groups in o-position and the resultant chelating effect. A similar series of tests was carried out with a Coa+-o-toluate (Figs. 2a-c). I t was found that the characteristic absorption peak (2 607 m/x) disappeared much later than from the Co s+ solutions, often b y 8-10 hr. The colour change was practically instantaneous in this case. The results obtained are in agreement with those of Taube [6] with respect to the kinetic inertness of the octahedral Co 3+ complexes in substitutions. It should be noted that a relatively small concentration of 0.1-1% w/w is sufficient for the complexing of cobalt with oxygen containing compounds of ketone, diketone or acid t y p e present in the hydrocarbon. A 3,4-dihydro-l,2-naphthoquinone addition (0.1% w/w of oxidate) to the o-xylene oxidation at 134°C inhibited the reaction (Fig. 3) and caused a colour change of the oxidate from green to reddish brown. In this case, and during tetralin oxidation, there was a coordination of 3,4-dihydro-l,2-naphthoquinone with cobalt and a "passive" form catalyst was produced. The compound apparently reacted with Co 3÷ because the o-xylene oxidate showed an absorption peak at ;t 422 m/~ after its addition which was characteristic for Co 3+ coordinated with 3,4-dihydro-l,2-naphthoquinone. The cobalt coordinated with the oxidation products can sometimes fall out as a sediment in the form of a complex. A bi- and tri-valent Co solution (as stearate or toluate) in o-xylene, tetralin or cyclohexane at 22°C will fall out as sediment when 3,4-dihydro-l,2-naphthoquinone or a-te~ralone is added
.~5 I "xS
2
4®
5
10
15
T
L 21
20
25 30 V, rain
FxG. 3. Effect of 3,4-dihydro-l,2-naphthoquinone additions on the kinetics of catalytic oxidation of o-xylene: 1 - o-Toluic acid accumulation in experiments without additions; 2--e-toluic aldehyde accumulation in the same test; 3 - - a s 1 with 0.1 ~o w / w of compound added; 4--as 2 with 0.1% w / w of compound added.
Catalytic oxidation of hydrocarbons
193
(catalyst concentration equal to 10 -3 mole/1, in o-xylene or tetralin, 5 × 10-*mole/ /1. in cyclohexane, 2-3~/o w/w of additives). The following complexes were produced and isolated: Co s+- and Co s+- with dihydronaphthoquinone and Co2+-a-tetralone. The complex was washed with cyclohexane to remove a-tetralone traces (to a negative reaction for carbonyl with 2,4-dinitrophenyl hydrazine) or treated with 2,4-dinitrophenyl hydrazine when present in an ethanol solution. Thin layer chromatography was used to show that a-tetralone enters into a complex with Co s+. RESULTS
The above results are evidence for comp]exing taking place in the catalytic, liquid phase oxidation of hydrocarbons. The oxygen containing compounds thus produced appear to form coordination compounds around the transition metal ions in all cases and to a larger extent than the original hydrocarbon. The presence in the liquid phase of different quality ligands will give rise to the formation of a reactive complex of a certain composition which will depend on the structure as well as concentration of the ligands, i.e. a competition of the ligands in their reaction with the catalyst. It must be expected that mainly the oxygen containing compounds will be the branching agents in a catalytic oxidation of hydrocarbon. A smooth progress of the above reaction requires that the yield of oxidized ligands from the first coordination sphere is sufficiently large, otherwise the catalyst will become "passivated" and the reaction inhibited. This was observed when 3,4-dihydro-l,2-naphthoquinone was added to o-xylene during oxidation (Fig. 3). It is well known that the formation of chemical bonds in a complex consisting of the coordination of transition metal ions with various ketones is due to the intact electron pair of the oxygen atom, i.e. that a donor-accepter bond is produced. The coordination of TiCl~ with ketones in a benzene solution was studied by other authors [7]. One must expect in a liquid phase oxidation that a coordination of the catalyst with the oxidized O-containing ligand will take place by means of the oxygen atom. The electron transfer from the ligand to the transition metal ion also appears to take place via the oxygen atom. SUMMARY
l. Oxygen containing ligands will coordinate around the catalyst, the cobalt ion, more easily during liquid phase oxidation than the hydrocarbon molecules. 2. A 0 . 1 ~ w/w addition of 3,4-dihydro-l,2-naphthoquinone inhibited the catalytic oxidation of o-xylene.
194
N . G . DIOUROV et al.
3. Complexes of Co s÷ a n d Co S+ with a-tetralone a n d with d i h y d r o n a p h t h o quinone were isolated from the o-xylene solution. Translated by K. A. fl~LLEN
REFERENCES
1. 2. 3. 4. 5. 6.
N. A. IZMAILOV and Yu. A. KRUGLYAK, Dokl. Akad. Nauk SSSR, 134, 1390, 1960 Ya. K. SYRKIN, Uspekhi Khim. 28, 903, 1959 V. A. ~ H A I L O V and S. I. DRAKIN, Izv. Sibir. Otdel. Akad. Nauk SSSR 6, 44, 1960 L. ORGEL, Introduction to the Chemistry of Transition Metals. "Mir", Moscow 1964 C. BAWN, Disc. Faraday Soc. 14, 181, 1953 N. TAUBE, Progress in Inorganic and Radio Chemistry. Academic Press, New York 1, 1, 1959 7. O. A. OSIPOV and V. I. GAIVORONSKH, Zhur. obsh. khim. 33, No. 4, 1346, 1963