A study of polyaniline (PANI) doping with various cobalt compounds, that is, cobalt(II) chloride, cobalt(II) acetate, and cobalt(II) salen, is presented. The catalysts were prepared by depositing cobalt compounds onto the polymer surface. PANI powders containing cobalt ions were obtained by one- or two-step method suspending PANI in the following acetonitrile/acetic acid solution or acetonitrile and then acetic acid solution. Moreover different ratios of Co(II) : PANI were studied. Catalysts obtained with both methods and at all ratios were investigated using various techniques including AAS and XPS spectroscopy. The optimum conditions for preparation of PANI/Co catalysts were established. Catalytic activity of polyaniline cobalt(II) supported catalysts was tested in dec-1-ene epoxidation with molecular oxygen at room temperature. The relationship between the amount of cobalt species, measured with both AAS and XPS techniques, and the activity of PANI-Co catalysts has been established.
Heterogeneous polymer supported catalysts, in particular on conjugated polymers, have been extensively studied in the last few years [
Many oxidation processes are characterized by low selectivity, which makes them much more difficult in application [
In this study, various Co(II) salts, that is, cobalt(II) chloride, cobalt(II) acetate, and cobalt(II) salen complex, have been selected. Various doping methods and Co(II) : PANI ratios were used to obtain PANI-Co powders. Physicochemical properties determined by AAS and XPS spectroscopy have made it possible to draw conclusions on the structure of cobalt ion binding. These catalysts have been investigated and tested in dec-1-ene epoxidation. The catalytic activity data of the obtained catalysts were correlated with the doping method, Co(II) compound used in PANI-Co system, and finally cobalt amount which was determined by using atomic absorption spectroscopy (AAS) and X-ray photoelectron spectroscopy (XPS). The work continues our research on synthesis, characterization, and testing of oxidation reactions in polyaniline supported cobalt(II) catalysts [
Polyaniline was obtained via an oxidative polymerization method [
Representation of synthesized polyaniline supported catalysts.
Co(II) source | Doping method | Catalyst/PANI (g/g) | |
---|---|---|---|
159A | Co(II) salen | II | 1 : 1 |
|
|||
167A | CoCl2 | I | 1 : 2 |
167B | I | 2 : 1 | |
168A | II | 1 : 2 | |
168B | II | 2 : 1 | |
|
|||
169A | Co(CH3COO)2·4H2O | I | 1 : 2 |
169B | I | 2 : 1 | |
170A | II | 1 : 2 | |
170B | II | 2 : 1 |
A mixture of polyaniline (500 mg) and cobalt acetate (500 mg) was stirred in an MeCN (25 mL) and HOAc (25 mL) mixture for 72 h at r.t. Then the reaction mixture was filtered and the solid catalyst was washed with MeCN (5 × 5 mL). The catalyst was dried at 110°C for 24 h.
A mixture of polyaniline (500 mg) and cobalt acetate (500 mg) was stirred in an MeCN (50 mL) mixture for 72 h at r.t. Then the reaction mixture was filtered and dried. Then catalyst was stirred in an AcOH (50 mL) for 1 h at r.t. The solid catalyst was washed with MeCN (5 × 5 mL). The catalyst was dried at 110°C for 24 h.
The amount of cobalt introduced into PANI was determined by atomic absorption spectrometry (AAS) in Perkin Elmer AAnalyst 300 spectrometer after dissolution of the PANI-Co samples in HNO3.
Surface analysis of the catalyst was made with the XPS method in a VSW 100 spectrometer using Mg
Catalytic activity of our catalysts was controlled on epoxidation of dec-1-ene according to the method described previously [
Taking into consideration content of the cobalt atoms on the surface of the tested catalysts and the total amount based on AAS, in relation to the number of nitrogen atoms, it is clearly evident that the concentration at the surface is at least equal to or in some cases 7.13 times higher than the concentration of cobalt measured in the whole volume of the sample (Table
Comparing results obtained from AAS analysis and semiquantitative chemical analysis obtained from X-ray photoelectron spectroscopy.
Sample | XPS (surface) | AAS (bulk) Co/N |
|
||||
---|---|---|---|---|---|---|---|
C | N | Co | Cl | O | |||
154 | 8.68 | 1 | — | 0.73 | 0.0000 | — | |
159A | 13.32 | 1 | — | 1.51 | 0.0039 | — | |
167A | 7.01 | 1 |
|
0.24 | 0.45 | 0.0175 |
|
167B | 8.35 | 1 |
|
0.25 | 0.86 | 0.0162 |
|
168A | 9.93 | 1 |
|
1.02 | 1.18 | 0.0889 |
|
168B | 9.74 | 1 |
|
1.48 | 0.92 | 0.1065 |
|
169A | 8.54 | 1 |
|
— | 0.87 | 0.0105 |
|
169B | 9.28 | 1 | — | 0.97 | 0.0153 | — | |
170A | 10.09 | 1 | — | 1.03 | 0.0146 | — | |
170B | 11.13 | 1 |
|
— | 1.51 | 0.0097 | — |
Furthermore, the ratio of Co : N atoms for catalysts based on CoCl2 (168A and 168B) is much higher than that for the other catalysts, indicating that in this case more nitrogen atoms are involved in the cobalt-nitrogen bond formation. In the case of catalysts based on cobalt acetate(II) (169A, 170B) saturating the nitrogen atoms with cobalt ones is several times lower and ranges from 0.018 to 0.027 Co atoms per nitrogen atom. In the case of catalysts 169B and 170A, surface concentration of Co was too low to give detectable spectral lines for cobalt. The catalyst based on Co(II) salen (159A) does not show the presence of cobalt spectral lines.
Stoichiometric variation of carbon atoms was observed in some samples from the value expected for the ideal PANI structure. This phenomenon is probably due to a significant amount of oxygen adsorbed in the form of water, which is very difficult to remove from the polyaniline surface. As it was presented in literature water molecules are present even in the dried samples [
Detailed analysis of the XPS spectra allows for a more precise determination of nature of the chemical bonds on the surface. According to literature reports [
Representative N1
Figures
Binding energies of nitrogen N1
Sample | Binding energy (eV) | Peak contribution (%) | ||||||
---|---|---|---|---|---|---|---|---|
–N= | –NH– | – |
–NH+= | –N= | –NH– | – |
–NH+= | |
PANI | 397.9 | 399.3 | 400.7 | 402.4 | 22.9 | 54.3 | 16.8 | 6.0 |
159A | 398.0 | 399.4 | 400.8 | 402.5 | 14.4 | 33.5 | 35.3 | 16.7 |
167A | 398.1 | 399.3 | 400.7 | 402.4 | 51.9 | 20.5 | 21.4 | 6.2 |
167B | 398.2 | 399.4 | 400.9 | 402.5 | 50.3 | 24.3 | 19.4 | 6.0 |
168A | 398.4 | 399.6 | 401.0 | 402.7 | 53.7 | 15.1 | 24.0 | 7.2 |
168B | 398.3 | 399.5 | 401.1 | 402.6 | 61.0 | 10.9 | 21.7 | 6.4 |
169A | 398.0 | 399.2 | 400.6 | 402.4 | 25.5 | 44.1 | 23.3 | 7.1 |
169B | 398.0 | 399.2 | 400.6 | 402.3 | 28.6 | 43.6 | 26.0 | 1.8 |
170A | 398.3 | 399.5 | 400.9 | 402.6 | 19.5 | 36.2 | 36.1 | 8.2 |
170B | 397.9 | 399.1 | 400.5 | 402.2 | 20.1 | 39.9 | 30.6 | 9.3 |
XPS N1
167A
167B
168A
168B
XPS N 1
169A
169B
170A
170B
The decline in the oxidation state of the polymer, as a result of doping with cobalt salts, suggests that Co atoms interact more strongly with the nitrogen atom of the quinonoid units than with amine ones included in the benzenoid units. Theoretical considerations also point to the fact that the imine groups of PANI are much more reactive [
There were no significant changes in protonated units content for CoCl2 based catalysts (167A-168B) with increasing of cobalt amount in the catalyst, with the protonated units content remaining at a relatively low level. The situation is completely different for the catalysts in which the polyaniline is doped with cobalt(II) acetate (169A-170B). Protonation degree for these catalysts is much higher when compare with PANI. Lower protonation degree was observed for catalyst 169A only, for which the ratio of Co/N was 0.0258. Thus, for catalysts which contain measurable amounts of cobalt, protonation is negligible due to blocking the nitrogen atoms with cobalt ones. This is another proof of the existence of the chemical nature of the interaction between nitrogen and cobalt atoms immobilized on polyaniline.
Cobalt 2
XPS Co 2
167A
167B
168A
168B
169A
Based on the 2
Characteristic values in Co 2
Sample | Co 2 |
Satellite/eV |
|
Co 2 |
Satellite/eV |
|
|
---|---|---|---|---|---|---|---|
167A | 781.1 | 786.9 | 0.33 | — | — | — | — |
167B | 783.0 | 788.1 | 0.29 | — | — | — | — |
168A | 781.5 | 786.7 | 1.41 | 797.5 | 803.2 | 1.38 | 16.0 |
168B | 781.4 | 786.4 | 1.57 | 797.4 | 802.9 | 1.56 | 16.0 |
169A | 783.5 | 791.4 | 0.095 | — | — | — | — |
The catalytic activity of the obtained catalysts was tested in the epoxidation of dec-1-ene (Figure
Dec-1-ene epoxidation on PANI Co(II) supported catalysts (reaction time 48 h).
Co (AAS)/(molCo/molN) | Co (XPS)*/(molCo/molN) | Yield/% | |
---|---|---|---|
159A | 0.0039 | — | 38.0 |
167A | 0.0175 | 0.0172 | 39.3 |
167B | 0.0162 | 0.031 | 57.3 |
168A | 0.0889 | 0.222 | 22.8 |
168B | 0.1065 | 0.269 | 26.1 |
169A | 0.0105 | 0.0258 | 44.7 |
169B | 0.0153 | — | 41.8 |
170A | 0.0146 | — | 26.0 |
170B | 0.0097 | 0.018 | 26.8 |
Reaction of dec-1-en epoxidation.
Comparing catalysts synthesized in the same conditions and with the same substrates but with a different content of Co (taking into account the content of Co on the surface), it could be observed that the reaction yield is increasing with an increase of Co amount. Comparing Co content determined by AAS and XPS, it was observed that for catalyst 167A the cobalt amount determined using AAS is higher than in 167B and is equal to 0.0175 and 0.0162 mol of Co/mol of N, respectively. While XPS analysis for catalysts 167A and 167B was given completely different results—0.0172 and 0.031 mol of Co/mol of N. It follows that part of the cobalt was trapped inside polymer clusters. Moreover, taking into account the efficiency of the epoxidation reaction, which increases with increasing of surface concentration of Co (for the same conditions of catalyst synthesis), it can be concluded that in epoxidation reaction only cobalt compounds located on the catalyst surface were involved, while the part of the Co trapped inside polymeric clusters was inactive in the epoxidation reaction.
It was also observed that the epoxidation with use of the catalysts synthesized by a two-step method occurs with higher yields than the corresponding catalysts synthesized by one-step method. The presence of a relatively strong acid in reaction media during immobilization results in the protonation of polyaniline and the blocking of the free electron pair, which may act as a potential electron donor to the cobalt atom. Such a phenomenon is observed in the case of doping with cobalt(II) acetate, where the protonation degree was at the level of 40–45% for the one-step method and 28–30% for the two-step method (Table
A series of novel conductive polymer supported cobalt catalysts based on polyaniline and cobalt(II) compounds (cobalt(II) chloride, cobalt(II) acetate, and cobalt(II) salen) have been developed. Investigations of incorporation of Co(II) ions into polyaniline together with the studies of physicochemical properties of PANI-Co systems have shown the following. Properties of catalysts strongly depend on method of cobalt(II) immobilization on the polymer matrix. Comparing results from AAS and XPS analysis, it may be concluded that immobilized cobalt based molecules are located mainly on the polymer surface. Some steric hindrance is observed when large molecules were used as doping agents. The largest effective immobilization was when CoCl2 was used. Doping reactions occur mainly on unprotonated polyaniline units. Some charge transfer from the nitrogen atom of PANI to the cobalt atom was observed for catalysts 167A-168B.
The authors declare that there is no conflict of interests regarding the publication of this paper.