- Email: [email protected]
Oxidation of cyclohexane and cyclohexene on sol-gel prepared birnessites
Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) 9 2004 Elsevier B.V. All fights reserved.
2639
O X I D A T I O N OF C Y C L O H E X A N E AND C Y C L O H E X E N E ON SOL-GEL P R E P A R E D B I R N E S S I T E S Rives, V. l, Prieto, O. 1, Del Arco, M. l, Fetcu, A. 2 and P~rvulescu, V.I. 2 1Departamento de Quimica Inorgfinica, Universidad de Salamanca, Salamanca 37008, Spain. E-mail: [email protected] 2University of Bucharest, Faculty of Chemistry, Department of Chemical Technology and Catalysis, B-dul Regina Elisabeta 4-12, Bucharest 030018, Romania. Fax: 4021-3320588. E-mail: [email protected]
ABSTRACT Layered manganese oxides prepared by sol-gel in both the Na and K form were ionic exchanged with Cu and characterized by chemical analysis, DTA, adsorption-desorption isotherms of N2 a t - 1 9 6 ~ XRD, FTIR, and thermal analysis. They were tested in liquid phase oxidation of cyclohexane and cyclohexene with hydrogen peroxide and air showing high conversion of hydrocarbon (over 70%) and rather good selectivitity of H202. INTRODUCTION Molecular sieves have been widely investigated for the selective oxidation of various organic substrates [ 1]. All these studies indicated that the cooperation between a redox cation in framework positions and the shape selectivity generated by the specific texture of these materials provide both active and selective catalysts. Among the different oxidation reactions, epoxidation received a special interest [2]. This an oxygen-transfer reaction for which hydrogen peroxide and its derivatives are particularly well suited. Metallic complex species which contain W, Mo, V, Mn, Re, Ti, Zr, etc. are currently used as active catalysts, these elements being able to generate peroxometal intermediates, which facilitate the oxygen transfer to alkene. Oxidation of cyclohexane is also an oxygen-transfer reaction and various redox molecular sieves containing as active elements Ti, V, Cr, Mn, Fe, Co, Mo, etc. were used as well [3]. The common drawback of these materials is the leaching of the active metals under reaction conditions. Hutchings et al. [4], using a 2.4 wt% Ti TS-1, reported that in several reaction media, even Ti has leached out. Another disadvantage of Ti-containing molecular sieves is the relative complicate synthesis routes to avoid precipitation of TiO2, as separate phase which, often acts as a catalyst poison in the subsequent oxidation reactions by hydrogen peroxide [5]. In the presence of the most molecular sieves, strongly coordinating solvents, like water or alcohols, severely retard the reaction by competing for coordination sites. Therefore another option is to use selective gas-phase oxidation of the organic substrates with oxygen [6]. Birnessites are layered manganese oxides, formed by two-dimensional layers of edge-sharing MnO6 octahedra, with metal cations (usually alkaline or alkaline-earth cations) in the interlayer together with water molecules [7]. The properties of these materials depend on the synthetic route, and in a previous paper we reported about the efficiency of the use of the sol-gel method in preparing such structures [8]. These solids undergo ionic exchange of the interlayer cations, such a process usually leading to small changes in the interlayer spacing. This represents also a possibility to modify the catalytic properties of these layered oxides by introducing redox cations in the interlayer. The aim of this contribution is to report about the sol-gel synthesis of MnNa and MnK birnessites, the ionic exchange of these solids with copper, and the catalytic behavior and stability of these materials in the liquid phase oxidation of cyclohexane and cyclohexene with hydrogen peroxide. Table 1 compiles the investigated structures. EXPERIMENTAL
SECTION
MnNa and MnK birnessites were prepared by a sol-gel method, starting .from glucose and sodium or potassium permanganate using the reported procedure [8, 9]. The molar ratio glucose/cation was in the range 1 to 1.5. After drying, the samples were gently calcined at 400~ to remove unreacted glucose. MnCuK1 and
2640 MnCuK2 samples correspond to ion exchange of calcined sample MnK with aqueous solutions of copper nitrate at 80 ~ for 3 hours and 8 hours, respectively. As it is presented in Table 1, the exchange was almost complete, but a small fraction of potassium (ca. 10% of the originally existing) still existed. The resulted catalysts were characterized using several techniques: chemical analysis, DTA, adsorption-desorption isotherms a t - 1 9 6 ~ (Gemini apparatus, from Micromeritics), XRD (Siemens D500, Cu K~ radiation), FTIR (Perkin Elmer 1710, KBr pellets), thermal analysis (TG and DTA in TG7 and DTA7 instruments, from Micromeritics). Table 1. Chemical composition and surface area of the investigated bimessites. Catalyst
Formula
Mn
(%) MnNa MnK MnCuK1 MnCuK2
Na0.33MnO2"0.70H20 K019MNO2"0.3 5H20 K0021Cu00941MnO2"0.64H20 Ko.o173Cuo.oq96MnO2"0.63H20
51.8 55.4 55.0 54.3
Na (%) 7.2
K (%) 7.5 0.8 0.7
Cu (%)
Specific surface area (m2 g-l)
6.0 6.2
33 16 25 26
The catalytic tests were carried out in liquid phase in a 50 mL flat bottom flask with an attached condenser. In each experiment 9 mmoles hydrocarbon (cylochexene or cyclohexane) were mixed with 50 mg catalyst. A solution of H202 3 5% (Fluka) in dioxane was slowly added with alkene/H202 molar ratio between 1:3. The reaction was carried out under a vigorous stirring at temperatures in the range 4-100 ~ under N2 atmosphere. In some experiments, the catalyst was separated from the hot solution after 4h and the filtrate was kept under reaction conditions for another 2h as a test for leaching of reactive species. Some of the recovered catalysts were reused in repeated reactions. Air oxidation of cyclohexene and cyclohexane was carried out in a stirred teflon-lined stainless steel autoclave at an air pressure of 5 atm and in the same range of temperatures. These experiments used the same amount of hydrocarbon and catalysts as those used in the liquid phase oxidation. The reaction products were analysed using a Hewlett Packard MS-5988 gas-chromatograph fitted with a DBI silicon column and a FID detector. The reaction products were identified by GC/MS and IH and ~3C NMR. RESULTS
Catalyst characterization XRD clearly indicate the layered structure (Fig. 1). The reflexions close to 2theta 38, 44, and 6 5 ~ correspond to the AI sample holder. The ion exchange with copper did not alter this structure, suggesting it is a topotactic process. The FT-IR spectra were typical of the birnessite structure [7-9], with only a strong band in the 3400-3200 cm -~ range, due to OH stretching modes of interlayer water molecules, and a structured absorption in the 600-400 cm -~ range, due to Mn-O stretching modes. On heating, interlayer water evolution is observed below 200~ and above 400~ some effects correspond to collapsing of the layered structure, forming tunnel-structure mixed Mn-metal oxides [9]. The nitrogen adsorption-desorption isotherms correspond to type II in the IUPAC classification characteristic of non-porous or macroporous adsorbents, indicating that, under the experimental conditions used, nitrogen molecules are unable to enter the interlayer space; the absence of microporosity is concluded from the t-plots. In addition, the isotherms display a H3 hysteresis loop closing at p/p~ usually associated to adsorption on aggregates of particles with a layered morphology, forming slit-like pores [10]. The Na containing sample shows a specific surface area ca. twice that of the K precursor; as the samples are not microporous, this difference cannot be attributed to different accessibility to the interlayer region, depending on the size of the interlayer cation, but is probably due to different properties of the gel obtained during the synthesis procedure, as the sodium gel is more consistent and thicker than the potassium one. The weak peaks recorded in the XRD diagram of sample MnK are due to a small impurity of Mn203, which can be probably blocking interparticle pores, thus decreasing the surface area of this sample.
2641
500
6 v
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nNa
MnK
" I
I
I0
I
20
I
30
I
40
20
(~
50
I
I
60
70
80
K0~)
Figure 1. XRD pattern of samples MnK and MnNa (* lines due to the A1 sample holder). Catalytic activity
Liquid-phase oxidation of cyclohexene with HeOe Figure 2 shows the variation in the yield to cyclohexene-epoxide and H202 efficiency with temperature on the parent Na- and K-birnessites. On both the catalysts, the increase of the temperature till 70~ led to a decrease of the yield in epoxide. A further increase of the temperature to 100~ was consistent with an increase of the yield. H 2 0 2 efficiency paralleled the variation of the yield to cyclohexene-epoxide. However, the values determined at 70~ and merely at 100~ are completely unusual as compared with values at 4~ Typically, the increase of the temperature determines a decrease of the H202 efficiency. In all these experiments the selectivity to cyclohexene-epoxide was higher than 90%, the secondary product being the diol. 100
i
'
~
9
i
9
i
'
i
'
i
60
/k
80
"--------~.
40
30 ~-
20-
20 10
0
0
Temperature, "C Figure 2. The variation in the yield to cyclohexene-epoxide and H202 efficiency with temperature on the parent Na- and K-birnessites MnK : II - yield to cyclohexene-epoxide; A - H202 Efficiency; MnNa : 9 - yield to cyclohexene-epoxide; 0 - H202 Efficiency. Figure 3 shows the variation in the yield to cyclohexene-epoxide and H202 efficiency with temperature on the exchanged Cu-K-birnessites. In accordance with the chemical composition of the two exchanged catalysts the differences in the yield to cyclohexene-epoxide are rather small. The replacement of K with Cu
2642
has as an effect an increase of the yield both at 70 and 100~ As for the case of the un-exchanged birnessites, the increase of the temperature led to an unusual increase in the H202 efficiency. But, contrarily to the yield, the H202 efficiency indicated differences between the catalysts. The values determined for CuMnK2 were larger those for CuMnK1 in the whole temperature range. The selectivity to cyclohexen-epoxide indicated almost no differences comparatively to un-exchanged catalysts being higher than 90%. As in that case, the secondary product was the diol. 80
30
,
25 602o
4o
C~ ~..~.
.-..t g)
10
"
20
0
,
20
40
60
I
|
I
80
100
Ternp er at-ure,~ Figure 3. The variation in the yield to cyclohexene-epoxide and H202 efficiency with temperature on the exchanged Cu- K-birnessites. CuMnKI 9 I - yield to cyclohexene-epoxide; A - H202 Efficiency; CuMnK2 9 9 - yield to cyclohexene-epoxide; 0- H202 Efficiency.
Liquid-phase oxidation of cyclohexane with H202 Figure 4 presents data on cyclohexane oxidation on MnK and exchanged Cu-K-bimessites. Only cyclohexanol and cyclohexanone were resulted from this reaction in the investigated temperature range. The yield in cyclohexanol reached rather small values and decreased from MnK to CuMnK2.
25--
- 27 - 26
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- 25
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~Yield [ , H202 Efficiency
o
22 ~ - 21 ~ii~ii:i~i~i'?i~i~i~~ii~?i~~;ii~i~ii~i~i~izi .......................................
I
MnI(.
20
I
C uMnV,_1
C uMnI(2
Catalyst Figure 4. The variation in the yield to cyclohexanol and H202 efficiency on MnK and exchanged Cu- K-bimessites. (15 mg catalyst, 70~
2643 Cyclohexanol was the main reaction product, the selectivity being almost independent on the catalyst composition and always around 80%. H20~ efficiency was very appropriate to that determined in the case of oxidation of cyclohexene and its decrease paralleled the decrease of the yield. However, as for cyclohexene, the H20~ efficiency increased with temperature, showing the higher values at 100~
Oxidation of eyclohexene with air The increase of the H202 efficiency in liquid phase oxidation of cyclohexene and cyclohexane with temperature suggested that on these catalysts the released oxygen may direct participate in the oxidation of hydrocarbon. Therefore, separate catalytic tests using dried oxygen as oxidant have been also carried out. The oxidation of cyclohexene occurred with the formation of the products indicated in Scheme 1, indicating that, indeed, these catalysts are able to activate oxygen at low temperatures. The epoxide was the major product in all these reactions, which means that these systems exhibit not only activity, but also selectivity. Figure 5 presents the variation of the yield in cyclohexene-epoxide on the investigated catalysts when cyclohexene was used as substrate. In the investigated conditions CuMnK1 led to the higher yields in epoxide. It is also worth to notice that the selectivity in epoxide was higher than 60%.
O
~
c at, T o~ 1
"C 2
3
Scheme 1. Oxidation of cyclohexene with air on birnessites.
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I
............
Cu_MnK2
Catalyst Figure 5. The variation in the yield to cyclohexene-epoxide on K- and Na-Mn, and exchanged Cu- K-birnessites. (15 mg catalyst, 70~
Oxidation of cyclohexane with air Oxidation of cyclohexane with air occurred with the formation of the products indicated in Scheme 2. Except these, but in very small amounts, cyclohexene and advanced oxidation products were also identified. Figure 6 shows the yield in cyclohexanol on these catalysts. The exchange with copper appeared to be detrimental for this reaction, CuMnK2 exhibiting an extremely low activity. A similar tendency was also observed for the selectivity. While on K-Mn the selectivity to cyclohexanol was 81%, on CuMnK1 it decreased to 56% and to 41% for CuMnK2.
2644 OH
O
O
O2
atl,
+
+
o.
Scheme 2. Oxidation of cyclohexane with air on birnessites.
40353025-
2o1510_
I
i,,
,,~
,,,
....,;,
, ,!
CuMnK 1
I
liiill
....
I
C ~ 2
Catalyst Figure 6. The variation in the yield to cyclohexanol on K-Mn, and exchanged Cu- K-birnessites. (15 mg catalyst, 70 oC).
Catalysts recycling The recycle of the catalysts in all the investigated reaction led to very similar results. No leaching of manganese or copper species, and no deactivation as a result of deposition of heavy products have been identified during these experiments. Each catalyst was recycled five times.
DISCUSSION Birnessites are mixed oxides in which part of manganese is reduced to Mn 3+, the neutrality of the network being compensated, like in the case of zeolites, by alkaline cations. In the investigated samples the amount of incorporated alkaline species is different, according to the nature of the cation. Sodium is almost 30%, potassium 20% and the ionic exchange of potassium by copper reduces this content to about 10%. Very small amounts of Mn203 also remain from the synthesis process. The synthesis process also controls the surface area of the resulted materials. In conclusion the framework of these materials contains Mn 4~ and M 3t cations and, by ionic exchange some other redox species could be also introduced. Mn 4~ is a known non-selective H202 decomposition species [11], but Mn 3+ was reported recently as an active species in oxidation of cyclohexane, cyclohexanone and cyclohexanol to adipic acid [12]. Therefore, it results that such a structure fits very well with an oxidation reaction. Liquid phase epoxidation of cyclohexene occurred on these catalysts as a very sensitive reaction to temperature. Very good yields to cyclohexene-expoxide and H202 efficiency resulted at 4 and, in a very surprising way for a liquid-phase oxidation, at 100~ The results at 100~ suggested that the released oxygen may participate direct in this reaction and, indeed, experiments carried out in air at 70~ confirmed this behavior. The yields to cyclohexene-expoxide obtained on CuKMnK1 under air oxidation were almost
2645 similar with those obtained in oxidation with hydrogen peroxide. Actually, CuKMnK1 was the best catalyst in cyclohexene epoxidation in the investigated series. The selectivity to cyclohexene-epoxide was higher than 90% in all the liquid-phase catalytic experiments. Such very high values could be related with the supposed low acidity of these materials. It is well known for such reactions that the presence of the acid sites catalyzes the hydrolysis of epoxides to the corresponding diols [13, 14]. The same acidic sites catalyze the non-selective decomposition of hydrogen peroxide in water and oxygen. In titaniumsilicalites it has been shown that sodium ions [15] and even surface silanols [16] are detrimental to the selective use of H202. In this particular case the catalytic tests were the only one possibility to appreciate the acidity of these materials. FTIR and DRIFT experiments failed because these compounds are black and NH3-TPD measurements gave false information due to their oxidative properties. The same catalytic tests indicated that sodium, indeed, exhibits a negative influence on the selective use of H202 even in these reactions. However, the experiments with Cu-K-birnessites indicated that all the alkaline cations decompose non-selectively H202 and for an advanced ionic exchange (CuKMn2) the H202 efficiency was higher. The replacement of H202 with air led to a decrease of the selectivity to cyclohexene-epoxide, and therefore the very high selectivity using H202 at 100 ~ is rather intrigued. We may just speculate that the released oxygen from the non-selective decomposition of hydrogen peroxide is more easily to be handled. However, it is worth to notice that even under these conditions the selectivity to cyclohexene-epoxide exceeded 60%. These data come to indicate that these materials are able to activate oxygen at very low temperature and to use it in very selective reactions. This was also confirmed by the experiments carried out in oxidation of cyclohexane. Liquid-phase oxidation led to smaller yields than in the case of cyclohexene and the comparative results on the investigated catalysts suggest that the role of the exchanged cation is smaller. Under these conditions the role of Mn-framework seems to be more important and K-birnessite led to the higher yield in cyclohexanol (which was the main reaction product) and to the higher H202 selectivity. K-birnessite was also the most active catalyst in cyclohexane oxidation with air. CONCLUSIONS In conclusion birnessites are able to carry out selectively oxidation of hydrocarbons at low temperatures. These reactions may use as oxidant both hydrogen peroxide and oxygen from air. In the first case the selectivity in the desired product is higher. Both the manganese framework and the exchanged species may participate in this process. ACKNOWLEDGMENTS V. Rives, O. Prieto and M. Del Arco thank financial support from DGES (grant PB96-1307-C03-01). REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Vayssilov, G. N., Catal. Rev.-Sci. Eng., 39 (1997) 209-244. Sheldon, R. A., van Vliet, M. C. A., in R. A. Sheldon, H. van Bekkum, Fine Chemicals through Heterogeneous Catalysis, Wiley-VCH, Weinheim, 2001, p. 473 Schuchardt, U., Cardoso, D., Sercheli, R., Pereira, R., da Cruz, R. S., Guerreiro, M. C., Mandelli, D., Spinace, E. V., Pires, E. L., Appl. Catal., 211 (2001) 1-17. Davies, L., McMorn, P., Bethell, D., Bulman Page, Ph. C., King, F., Hancook, F. E., Hutchings, G. J., Chem. Corn., (2000) 1807-1808. Trong On, D., Kapoor, M. P., Thibeault, E., Gallot, J. E., Lemay, G., Kaliaguine, S., Microporous Mater., 20 (1998) 107-131. Dusi, M., Mallat, T., Baiker, A., Catal. Rev., 42 (2000) 213-262. Feng, K., Kanoh, H., Mijay, Y., Ooi, K., Chem. Mater., 7 (1995) 1226-1241. Prieto, O., Rives, V., Bol. Soc. Esp. Ceram. Vid., 39 (2000) 233-238. Prieto, O., del Arco, M., Rives, V., Thermochimica Acta, 41 (2003) 95-105. Sing, K. S. W., Everett, D. H., Haul, R. A. W., Moscou, L., Pierotti, R. A., Rouquerol, J., Sieminiewska, T., Pure Appl. Chem., 57 (1985) 603-615. Pirault-Roy, L., Kappenstein, Ch., Guerin, M., Eloirdi, R., Pillet, N., J. Propulsion Power, 18 (2002) 1235-1241.
2646 12. Chavan, S. A.' Srinavas, D., Ratnasamy, P., J. Catal., 212 (2002) 39-45. 13. Gelbard, G., Gauducheau, T., Vidal, E., Faure, Y., Parvulescu, V. I., Pop, V., Crosman, A., J. Mol. Cat. A: Chem. 182/183 (2002) 251-266. 14. Gelbard, G., Breton, F., Sherrington, D. C., Quenard, M., J. Mol. Cat., 153 (2000) 7-18. 15. Gallot, J. E., and Kaliaguine, S., Can. J. Chem. Eng., 76 (1998) 833-844. 16. Trong On, D., Kapoor, M. P., Thibeault, E., Gallot, J. E., Lemay, G., and Kaliaguine, S., Microporous Mater. 20 (1998) 107 -131.
2639
O X I D A T I O N OF C Y C L O H E X A N E AND C Y C L O H E X E N E ON SOL-GEL P R E P A R E D B I R N E S S I T E S Rives, V. l, Prieto, O. 1, Del Arco, M. l, Fetcu, A. 2 and P~rvulescu, V.I. 2 1Departamento de Quimica Inorgfinica, Universidad de Salamanca, Salamanca 37008, Spain. E-mail: [email protected] 2University of Bucharest, Faculty of Chemistry, Department of Chemical Technology and Catalysis, B-dul Regina Elisabeta 4-12, Bucharest 030018, Romania. Fax: 4021-3320588. E-mail: [email protected]
ABSTRACT Layered manganese oxides prepared by sol-gel in both the Na and K form were ionic exchanged with Cu and characterized by chemical analysis, DTA, adsorption-desorption isotherms of N2 a t - 1 9 6 ~ XRD, FTIR, and thermal analysis. They were tested in liquid phase oxidation of cyclohexane and cyclohexene with hydrogen peroxide and air showing high conversion of hydrocarbon (over 70%) and rather good selectivitity of H202. INTRODUCTION Molecular sieves have been widely investigated for the selective oxidation of various organic substrates [ 1]. All these studies indicated that the cooperation between a redox cation in framework positions and the shape selectivity generated by the specific texture of these materials provide both active and selective catalysts. Among the different oxidation reactions, epoxidation received a special interest [2]. This an oxygen-transfer reaction for which hydrogen peroxide and its derivatives are particularly well suited. Metallic complex species which contain W, Mo, V, Mn, Re, Ti, Zr, etc. are currently used as active catalysts, these elements being able to generate peroxometal intermediates, which facilitate the oxygen transfer to alkene. Oxidation of cyclohexane is also an oxygen-transfer reaction and various redox molecular sieves containing as active elements Ti, V, Cr, Mn, Fe, Co, Mo, etc. were used as well [3]. The common drawback of these materials is the leaching of the active metals under reaction conditions. Hutchings et al. [4], using a 2.4 wt% Ti TS-1, reported that in several reaction media, even Ti has leached out. Another disadvantage of Ti-containing molecular sieves is the relative complicate synthesis routes to avoid precipitation of TiO2, as separate phase which, often acts as a catalyst poison in the subsequent oxidation reactions by hydrogen peroxide [5]. In the presence of the most molecular sieves, strongly coordinating solvents, like water or alcohols, severely retard the reaction by competing for coordination sites. Therefore another option is to use selective gas-phase oxidation of the organic substrates with oxygen [6]. Birnessites are layered manganese oxides, formed by two-dimensional layers of edge-sharing MnO6 octahedra, with metal cations (usually alkaline or alkaline-earth cations) in the interlayer together with water molecules [7]. The properties of these materials depend on the synthetic route, and in a previous paper we reported about the efficiency of the use of the sol-gel method in preparing such structures [8]. These solids undergo ionic exchange of the interlayer cations, such a process usually leading to small changes in the interlayer spacing. This represents also a possibility to modify the catalytic properties of these layered oxides by introducing redox cations in the interlayer. The aim of this contribution is to report about the sol-gel synthesis of MnNa and MnK birnessites, the ionic exchange of these solids with copper, and the catalytic behavior and stability of these materials in the liquid phase oxidation of cyclohexane and cyclohexene with hydrogen peroxide. Table 1 compiles the investigated structures. EXPERIMENTAL
SECTION
MnNa and MnK birnessites were prepared by a sol-gel method, starting .from glucose and sodium or potassium permanganate using the reported procedure [8, 9]. The molar ratio glucose/cation was in the range 1 to 1.5. After drying, the samples were gently calcined at 400~ to remove unreacted glucose. MnCuK1 and
2640 MnCuK2 samples correspond to ion exchange of calcined sample MnK with aqueous solutions of copper nitrate at 80 ~ for 3 hours and 8 hours, respectively. As it is presented in Table 1, the exchange was almost complete, but a small fraction of potassium (ca. 10% of the originally existing) still existed. The resulted catalysts were characterized using several techniques: chemical analysis, DTA, adsorption-desorption isotherms a t - 1 9 6 ~ (Gemini apparatus, from Micromeritics), XRD (Siemens D500, Cu K~ radiation), FTIR (Perkin Elmer 1710, KBr pellets), thermal analysis (TG and DTA in TG7 and DTA7 instruments, from Micromeritics). Table 1. Chemical composition and surface area of the investigated bimessites. Catalyst
Formula
Mn
(%) MnNa MnK MnCuK1 MnCuK2
Na0.33MnO2"0.70H20 K019MNO2"0.3 5H20 K0021Cu00941MnO2"0.64H20 Ko.o173Cuo.oq96MnO2"0.63H20
51.8 55.4 55.0 54.3
Na (%) 7.2
K (%) 7.5 0.8 0.7
Cu (%)
Specific surface area (m2 g-l)
6.0 6.2
33 16 25 26
The catalytic tests were carried out in liquid phase in a 50 mL flat bottom flask with an attached condenser. In each experiment 9 mmoles hydrocarbon (cylochexene or cyclohexane) were mixed with 50 mg catalyst. A solution of H202 3 5% (Fluka) in dioxane was slowly added with alkene/H202 molar ratio between 1:3. The reaction was carried out under a vigorous stirring at temperatures in the range 4-100 ~ under N2 atmosphere. In some experiments, the catalyst was separated from the hot solution after 4h and the filtrate was kept under reaction conditions for another 2h as a test for leaching of reactive species. Some of the recovered catalysts were reused in repeated reactions. Air oxidation of cyclohexene and cyclohexane was carried out in a stirred teflon-lined stainless steel autoclave at an air pressure of 5 atm and in the same range of temperatures. These experiments used the same amount of hydrocarbon and catalysts as those used in the liquid phase oxidation. The reaction products were analysed using a Hewlett Packard MS-5988 gas-chromatograph fitted with a DBI silicon column and a FID detector. The reaction products were identified by GC/MS and IH and ~3C NMR. RESULTS
Catalyst characterization XRD clearly indicate the layered structure (Fig. 1). The reflexions close to 2theta 38, 44, and 6 5 ~ correspond to the AI sample holder. The ion exchange with copper did not alter this structure, suggesting it is a topotactic process. The FT-IR spectra were typical of the birnessite structure [7-9], with only a strong band in the 3400-3200 cm -~ range, due to OH stretching modes of interlayer water molecules, and a structured absorption in the 600-400 cm -~ range, due to Mn-O stretching modes. On heating, interlayer water evolution is observed below 200~ and above 400~ some effects correspond to collapsing of the layered structure, forming tunnel-structure mixed Mn-metal oxides [9]. The nitrogen adsorption-desorption isotherms correspond to type II in the IUPAC classification characteristic of non-porous or macroporous adsorbents, indicating that, under the experimental conditions used, nitrogen molecules are unable to enter the interlayer space; the absence of microporosity is concluded from the t-plots. In addition, the isotherms display a H3 hysteresis loop closing at p/p~ usually associated to adsorption on aggregates of particles with a layered morphology, forming slit-like pores [10]. The Na containing sample shows a specific surface area ca. twice that of the K precursor; as the samples are not microporous, this difference cannot be attributed to different accessibility to the interlayer region, depending on the size of the interlayer cation, but is probably due to different properties of the gel obtained during the synthesis procedure, as the sodium gel is more consistent and thicker than the potassium one. The weak peaks recorded in the XRD diagram of sample MnK are due to a small impurity of Mn203, which can be probably blocking interparticle pores, thus decreasing the surface area of this sample.
2641
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MnK
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30
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40
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70
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Figure 1. XRD pattern of samples MnK and MnNa (* lines due to the A1 sample holder). Catalytic activity
Liquid-phase oxidation of cyclohexene with HeOe Figure 2 shows the variation in the yield to cyclohexene-epoxide and H202 efficiency with temperature on the parent Na- and K-birnessites. On both the catalysts, the increase of the temperature till 70~ led to a decrease of the yield in epoxide. A further increase of the temperature to 100~ was consistent with an increase of the yield. H 2 0 2 efficiency paralleled the variation of the yield to cyclohexene-epoxide. However, the values determined at 70~ and merely at 100~ are completely unusual as compared with values at 4~ Typically, the increase of the temperature determines a decrease of the H202 efficiency. In all these experiments the selectivity to cyclohexene-epoxide was higher than 90%, the secondary product being the diol. 100
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Temperature, "C Figure 2. The variation in the yield to cyclohexene-epoxide and H202 efficiency with temperature on the parent Na- and K-birnessites MnK : II - yield to cyclohexene-epoxide; A - H202 Efficiency; MnNa : 9 - yield to cyclohexene-epoxide; 0 - H202 Efficiency. Figure 3 shows the variation in the yield to cyclohexene-epoxide and H202 efficiency with temperature on the exchanged Cu-K-birnessites. In accordance with the chemical composition of the two exchanged catalysts the differences in the yield to cyclohexene-epoxide are rather small. The replacement of K with Cu
2642
has as an effect an increase of the yield both at 70 and 100~ As for the case of the un-exchanged birnessites, the increase of the temperature led to an unusual increase in the H202 efficiency. But, contrarily to the yield, the H202 efficiency indicated differences between the catalysts. The values determined for CuMnK2 were larger those for CuMnK1 in the whole temperature range. The selectivity to cyclohexen-epoxide indicated almost no differences comparatively to un-exchanged catalysts being higher than 90%. As in that case, the secondary product was the diol. 80
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Ternp er at-ure,~ Figure 3. The variation in the yield to cyclohexene-epoxide and H202 efficiency with temperature on the exchanged Cu- K-birnessites. CuMnKI 9 I - yield to cyclohexene-epoxide; A - H202 Efficiency; CuMnK2 9 9 - yield to cyclohexene-epoxide; 0- H202 Efficiency.
Liquid-phase oxidation of cyclohexane with H202 Figure 4 presents data on cyclohexane oxidation on MnK and exchanged Cu-K-bimessites. Only cyclohexanol and cyclohexanone were resulted from this reaction in the investigated temperature range. The yield in cyclohexanol reached rather small values and decreased from MnK to CuMnK2.
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2643 Cyclohexanol was the main reaction product, the selectivity being almost independent on the catalyst composition and always around 80%. H20~ efficiency was very appropriate to that determined in the case of oxidation of cyclohexene and its decrease paralleled the decrease of the yield. However, as for cyclohexene, the H20~ efficiency increased with temperature, showing the higher values at 100~
Oxidation of eyclohexene with air The increase of the H202 efficiency in liquid phase oxidation of cyclohexene and cyclohexane with temperature suggested that on these catalysts the released oxygen may direct participate in the oxidation of hydrocarbon. Therefore, separate catalytic tests using dried oxygen as oxidant have been also carried out. The oxidation of cyclohexene occurred with the formation of the products indicated in Scheme 1, indicating that, indeed, these catalysts are able to activate oxygen at low temperatures. The epoxide was the major product in all these reactions, which means that these systems exhibit not only activity, but also selectivity. Figure 5 presents the variation of the yield in cyclohexene-epoxide on the investigated catalysts when cyclohexene was used as substrate. In the investigated conditions CuMnK1 led to the higher yields in epoxide. It is also worth to notice that the selectivity in epoxide was higher than 60%.
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Scheme 1. Oxidation of cyclohexene with air on birnessites.
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Oxidation of cyclohexane with air Oxidation of cyclohexane with air occurred with the formation of the products indicated in Scheme 2. Except these, but in very small amounts, cyclohexene and advanced oxidation products were also identified. Figure 6 shows the yield in cyclohexanol on these catalysts. The exchange with copper appeared to be detrimental for this reaction, CuMnK2 exhibiting an extremely low activity. A similar tendency was also observed for the selectivity. While on K-Mn the selectivity to cyclohexanol was 81%, on CuMnK1 it decreased to 56% and to 41% for CuMnK2.
2644 OH
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Scheme 2. Oxidation of cyclohexane with air on birnessites.
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Catalysts recycling The recycle of the catalysts in all the investigated reaction led to very similar results. No leaching of manganese or copper species, and no deactivation as a result of deposition of heavy products have been identified during these experiments. Each catalyst was recycled five times.
DISCUSSION Birnessites are mixed oxides in which part of manganese is reduced to Mn 3+, the neutrality of the network being compensated, like in the case of zeolites, by alkaline cations. In the investigated samples the amount of incorporated alkaline species is different, according to the nature of the cation. Sodium is almost 30%, potassium 20% and the ionic exchange of potassium by copper reduces this content to about 10%. Very small amounts of Mn203 also remain from the synthesis process. The synthesis process also controls the surface area of the resulted materials. In conclusion the framework of these materials contains Mn 4~ and M 3t cations and, by ionic exchange some other redox species could be also introduced. Mn 4~ is a known non-selective H202 decomposition species [11], but Mn 3+ was reported recently as an active species in oxidation of cyclohexane, cyclohexanone and cyclohexanol to adipic acid [12]. Therefore, it results that such a structure fits very well with an oxidation reaction. Liquid phase epoxidation of cyclohexene occurred on these catalysts as a very sensitive reaction to temperature. Very good yields to cyclohexene-expoxide and H202 efficiency resulted at 4 and, in a very surprising way for a liquid-phase oxidation, at 100~ The results at 100~ suggested that the released oxygen may participate direct in this reaction and, indeed, experiments carried out in air at 70~ confirmed this behavior. The yields to cyclohexene-expoxide obtained on CuKMnK1 under air oxidation were almost
2645 similar with those obtained in oxidation with hydrogen peroxide. Actually, CuKMnK1 was the best catalyst in cyclohexene epoxidation in the investigated series. The selectivity to cyclohexene-epoxide was higher than 90% in all the liquid-phase catalytic experiments. Such very high values could be related with the supposed low acidity of these materials. It is well known for such reactions that the presence of the acid sites catalyzes the hydrolysis of epoxides to the corresponding diols [13, 14]. The same acidic sites catalyze the non-selective decomposition of hydrogen peroxide in water and oxygen. In titaniumsilicalites it has been shown that sodium ions [15] and even surface silanols [16] are detrimental to the selective use of H202. In this particular case the catalytic tests were the only one possibility to appreciate the acidity of these materials. FTIR and DRIFT experiments failed because these compounds are black and NH3-TPD measurements gave false information due to their oxidative properties. The same catalytic tests indicated that sodium, indeed, exhibits a negative influence on the selective use of H202 even in these reactions. However, the experiments with Cu-K-birnessites indicated that all the alkaline cations decompose non-selectively H202 and for an advanced ionic exchange (CuKMn2) the H202 efficiency was higher. The replacement of H202 with air led to a decrease of the selectivity to cyclohexene-epoxide, and therefore the very high selectivity using H202 at 100 ~ is rather intrigued. We may just speculate that the released oxygen from the non-selective decomposition of hydrogen peroxide is more easily to be handled. However, it is worth to notice that even under these conditions the selectivity to cyclohexene-epoxide exceeded 60%. These data come to indicate that these materials are able to activate oxygen at very low temperature and to use it in very selective reactions. This was also confirmed by the experiments carried out in oxidation of cyclohexane. Liquid-phase oxidation led to smaller yields than in the case of cyclohexene and the comparative results on the investigated catalysts suggest that the role of the exchanged cation is smaller. Under these conditions the role of Mn-framework seems to be more important and K-birnessite led to the higher yield in cyclohexanol (which was the main reaction product) and to the higher H202 selectivity. K-birnessite was also the most active catalyst in cyclohexane oxidation with air. CONCLUSIONS In conclusion birnessites are able to carry out selectively oxidation of hydrocarbons at low temperatures. These reactions may use as oxidant both hydrogen peroxide and oxygen from air. In the first case the selectivity in the desired product is higher. Both the manganese framework and the exchanged species may participate in this process. ACKNOWLEDGMENTS V. Rives, O. Prieto and M. Del Arco thank financial support from DGES (grant PB96-1307-C03-01). REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Vayssilov, G. N., Catal. Rev.-Sci. Eng., 39 (1997) 209-244. Sheldon, R. A., van Vliet, M. C. A., in R. A. Sheldon, H. van Bekkum, Fine Chemicals through Heterogeneous Catalysis, Wiley-VCH, Weinheim, 2001, p. 473 Schuchardt, U., Cardoso, D., Sercheli, R., Pereira, R., da Cruz, R. S., Guerreiro, M. C., Mandelli, D., Spinace, E. V., Pires, E. L., Appl. Catal., 211 (2001) 1-17. Davies, L., McMorn, P., Bethell, D., Bulman Page, Ph. C., King, F., Hancook, F. E., Hutchings, G. J., Chem. Corn., (2000) 1807-1808. Trong On, D., Kapoor, M. P., Thibeault, E., Gallot, J. E., Lemay, G., Kaliaguine, S., Microporous Mater., 20 (1998) 107-131. Dusi, M., Mallat, T., Baiker, A., Catal. Rev., 42 (2000) 213-262. Feng, K., Kanoh, H., Mijay, Y., Ooi, K., Chem. Mater., 7 (1995) 1226-1241. Prieto, O., Rives, V., Bol. Soc. Esp. Ceram. Vid., 39 (2000) 233-238. Prieto, O., del Arco, M., Rives, V., Thermochimica Acta, 41 (2003) 95-105. Sing, K. S. W., Everett, D. H., Haul, R. A. W., Moscou, L., Pierotti, R. A., Rouquerol, J., Sieminiewska, T., Pure Appl. Chem., 57 (1985) 603-615. Pirault-Roy, L., Kappenstein, Ch., Guerin, M., Eloirdi, R., Pillet, N., J. Propulsion Power, 18 (2002) 1235-1241.
2646 12. Chavan, S. A.' Srinavas, D., Ratnasamy, P., J. Catal., 212 (2002) 39-45. 13. Gelbard, G., Gauducheau, T., Vidal, E., Faure, Y., Parvulescu, V. I., Pop, V., Crosman, A., J. Mol. Cat. A: Chem. 182/183 (2002) 251-266. 14. Gelbard, G., Breton, F., Sherrington, D. C., Quenard, M., J. Mol. Cat., 153 (2000) 7-18. 15. Gallot, J. E., and Kaliaguine, S., Can. J. Chem. Eng., 76 (1998) 833-844. 16. Trong On, D., Kapoor, M. P., Thibeault, E., Gallot, J. E., Lemay, G., and Kaliaguine, S., Microporous Mater. 20 (1998) 107 -131.