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Liquid phase catalytic transfer hydrogenation of aromatic nitro compounds on perovskites prepared by microwave irradiation
Applied Catalysis A: General 252 (2003) 225–230
Liquid phase catalytic transfer hydrogenation of aromatic nitro compounds on perovskites prepared by microwave irradiation Amrita S. Kulkarni, Radha V. Jayaram∗ Physical Chemistry Research Laboratory, Institute of Chemical Technology, University of Mumbai, N.M. Parikh Marg, Matunga, Mumbai 400019, India Received 30 October 2002; received in revised form 14 March 2003; accepted 8 May 2003
Abstract Preparation of specific tailor-made mixed oxides is one of the major topics of research in the field of heterogeneous catalysis. Perovskites are a class of mixed oxides, which have interesting catalytic and physicochemical properties. We have successfully developed a method using microwave irradiation to synthesize perovskites of the type LaMO3 (M = Mn, Fe, Co, Cr, Al) in just 15 min. The LaMO3 perovskites were used as catalysts for reduction of aromatic nitro compounds with propan-2-ol as hydrogen donor and KOH as promoter. Kinetics of the nitrobenzene reduction has also been studied. © 2003 Elsevier B.V. All rights reserved. Keywords: Perovskites; Catalytic transfer hydrogenation; Microwave irradiation; Nitroarenes
1. Introduction Hydrogenation of nitro aromatics with heterogeneous catalysts is often the method of choice for the production of the corresponding anilines [1,2]. Aromatic amines are important starting materials and intermediates for the manufacture of a variety of chemicals such as dyestuffs, pharmaceutical products, agricultural chemicals, surfactants and polymers. The oldest method and the industrially practiced one is the Bechamp reduction, which involves use of stoichiometric amounts of finely divided iron metal and water in the presence of small amounts of acid. This method has a distinct disadvantage of formation of iron sludges that are difficult to filter and that always contain adsorbed reaction products and hence lead to dis∗ Corresponding author. Tel.: +91-22-414-5616; fax: +91-22-414-5614. E-mail address: [email protected] (R.V. Jayaram).
posal problems [3]. The sulfide reduction has a broader selectivity than the Bechamp reduction and enables chemoselective reduction of nitro compounds in the presence of C=C, azo and other nitro compounds. The major disadvantages are toxicity, odor of most reducing agents and sulfur containing organic side products, and formation of elemental sulfur [4]. Hydrogenation using molecular hydrogen is non-polluting, but presents considerable hazards, since hydrogen is a gas of low molecular weight and gets easily ignited. In contrast to these conventional methods, catalytic transfer hydrogenation is superior in terms of saving resources, reducing waste disposal and being unharmful to the environment [5,6]. Catalytic transfer hydrogenation is a simple and safe operation in which a catalyst and hydrogen gas is replaced with a catalyst and hydrogen donor such as cyclohexene, propan-2-ol and ethanol. In this paper, we report the transfer hydrogenation of aromatic nitro compounds using propan-2-ol
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00417-4
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A.S. Kulkarni, R.V. Jayaram / Applied Catalysis A: General 252 (2003) 225–230
as hydrogen donor and KOH as promoter. The use of propan-2-ol in the presence of transition metal catalysts for transfer hydrogenation has been extensively studied [7]. Although perovskite-type mixed oxides have been used for a gamut of reactions like NOx decomposition [8] and oxidation of CO [9] and NH3 [10], their applications for catalytic transfer hydrogenation are still unexplored. Perovskite-type mixed oxides (ABO3 ) have a welldefined bulk structure and the composition of cations at both A and B sites can be widely changed. Several studies have confirmed that the catalytic activity of LaMO3 type perovskites can be attributed to valency control and thermal stability. The perovskite oxides most frequently employed in heterogeneous catalysis are those containing a lanthanide at A site and a transition element at B site. However, the formation of a perovskite structure can further influence the catalytic activity by changes in the electronic and chemical state of B cations. The conventional ceramic method for synthesis of perovskites requires conductive heating for extended periods (12–24 h) at elevated temperatures with intermittent grindings such that perovskite is obtained as a result of a solid-state diffusion process. In order to circumvent this limitation, there is increasing demand for new preparation methods. Microwave-assisted synthesis of inorganic compounds is one of the attractive fields of research in recent years [11,12]. If one or several of the components in a chemical reaction system strongly absorb microwaves, the resultant heat generated can be used to drive a reaction with other components. In the present investigation, we have achieved reduction in time required for the preparation of perovskites of LaMO3 (M = Mn, Fe, Co, Cr, Al) using this methodology and have also tested the activity of these catalysts for transfer hydrogenation reaction.
This assembly was mounted in a domestic microwave oven operating at a frequency of 2.45 GHz and irradiated for 5 min at a full power setting. The microwave source was then switched off and the sample was left to cool inside the oven. The sample was ground again and irradiated for 10 min in two cycles of 5 min. Decomposition of nitrates results in formation of reactive oxides which can easily combine to form the desired product. The perovskite phase was formed only when the nitrates were microwave irradiated. The oxides themselves did not absorb effectively even in the presence of an external absorber (graphite). The perovskite phase was not formed when the nitrates were irradiated in the absence of graphite. 2.2. Preparation of perovskites by conventional method For comparative purposes, perovskites were also prepared by the conventional ceramic method. LaMO3 (M = Mn, Fe, Co, Cr, Al) perovskites were prepared from oxalate precursors. Stoichiometric amounts of the respective oxalates were ground in an agate mortar and calcined in air at 1173 K for 12–24 h. The samples were analyzed by X-ray diffraction and reground and recalcined if necessary. 2.3. Catalyst characterization The catalysts were characterized by X-ray diffraction recorded on a JEOL JDX-8030 X-ray diffractometer using Cu K␣ radiation. The BET area of LaFeO3 prepared by both conventional and microwave irradiation methods were measured using a Model—Quantachrome Autosorb 1 instrument. 2.4. Catalytic activity
2. Experimental 2.1. Preparation of perovskites by microwave irradiation Stoichiometric quantities of the respective nitrates were thoroughly ground and mixed. The mixture was placed in an open quartz tube, on a bed of graphite.
All reactions were carried out under continuous stirring in a two-necked 100 ml round bottom flask fitted with a reflux condenser. Analysis was done by withdrawing definite aliquots of the sample at regular intervals. In a typical run, 150 mg of the catalyst was dispersed in a solution containing 20 mmol of nitrobenzene, 20 mmol of KOH pellets and 20 ml of propan-2-ol. The mixture was stirred and heated under reflux for 2–6 h in an oil bath. The products were
A.S. Kulkarni, R.V. Jayaram / Applied Catalysis A: General 252 (2003) 225–230
analyzed on the basis of their retention times using a Gas Chromatograph fitted with an OV-1 column (Eshita, Model—Eshika).
3. Results and discussion 3.1. XRD analysis Fig. 1 shows the XRD patterns of the perovskites prepared by microwave irradiation. All the five LaMO3 samples synthesized are essentially pure phases, as evidenced by XRD. No unreacted precursor material was detectable. The unit cell parameters are reported in Table 1. 3.2. Catalytic activity Once the materials were characterized as regards structure, they were tested as catalysts for the reduction of nitrobenzene to aniline with propan-2-ol. Scheme 1 depicts a typical catalytic transfer hydrogenation. During the reaction, the alcohol is oxidized to ketone and nitrobenzene is reduced to aniline. Aniline was the only product obtained under these conditions. The catalytic activities of the perovskites prepared by each method were found to be comparable (Table 2). However, the slight increase in the conversion of nitrobenzene using the microwave-prepared Table 1 Unit cell parameters of perovskite-samples prepared by microwave Catalyst
LaAlO3 LaFeO3 LaCrO3 LaCoO3 LaMnO3
Cell parameters a
b
c
5.365 5.557 5.475 5.441 5.520
– 5.563 5.510 – –
13.12 7.862 7.757 3.085 13.320
Scheme 1.
227
catalyst may be due to the increase in the surface area of catalyst prepared by microwave irradiation method. The surface area of LaFeO3 prepared by microwave irradiation method is ∼15 m2 /g and that prepared by conventional method is ∼10 m2 /g. Based on the results, the catalytic activity in the reaction decreases in the following sequence: LaFeO3 > LaAlO3 > LaCrO3 > LaCoO3 > LaMnO3 . Catalytic transfer hydrogenation reaction is similar to Meerwein–Ponndorf–Verley (MPV) reduction, which involves transfer of protons from propan-2-ol to keto group. Usually this reaction is carried out using Al metal. The isopropoxide anion so formed is stabilized at a Lewis Al site. This reaction is expected to follow a similar mechanism. The high activity of LaFeO3 and LaAlO3 is inherent, as the Lewis acidity of Fe and Al are comparable. The perovskites LaMO3 , where M = Cr, Co and Mn, are weakly acidic and are hence expected to show low activity. As LaFeO3 was found to be the most active catalyst, the reaction was also studied on a stoichiometric mixture of La2 O3 , Fe2 O3 and a mixture of La2 O3 -Fe2 O3 . The results are depicted in Table 2. The reaction did not proceed in the absence of either catalyst or KOH. The effect of KOH concentration on the yield of aniline is illustrated in Table 3. The effect of various promoters is illustrated in Table 4. From the data it follows that KOH is the best promoter. NaOH and K2 CO3 show reasonable activity, whereas the activity of Na2 CO3 was very poor. The dissociation of KOH in propan-2-ol is higher than that of the other three bases (NaOH, Na2 CO3 , K2 CO3 ); Table 2 Reduction of nitrobenzene to aniline over various catalysts prepared by both methods Catalysts
La2 O3 Fe2 O3 La2 O3 -Fe2 O3 LaAlO3 LaFeO3 LaCrO3 LaCoO3 LaMnO3
Method of preparation Microwave (%)
Conventional (%)
– – – 94 98 84 82 79
63 69 79 90 95 82 81 75
Nitrobenzene (20 mmol), KOH pellets (20 mmol), catalyst (150 mg) and propan-2-ol (20 ml).
228
A.S. Kulkarni, R.V. Jayaram / Applied Catalysis A: General 252 (2003) 225–230
Fig. 1. The XRD patterns of perovskites: (a) LaAlO3 , (b) LaFeO3 , (c) LaMnO3 , (d) LaCrO3 , (e) LaCoO3 .
A.S. Kulkarni, R.V. Jayaram / Applied Catalysis A: General 252 (2003) 225–230
229
Table 3 Effect of KOH concentration on the reduction of nitrobenzene to aniline
Table 6 Reduction of various nitro aromatics to anilines on LaFeO3 catalyst.
KOH (mmol)
Yield (%) of aniline
Substrate
Product
0 5 10 15 20 25
0 23 41 69 98 100
Yield (%)
Time (h)
Nitrobenzene 2-Nitrotoluene 4-Nitrotoluene 2-Chloronitrobenzene 4-Chloronitrobenzene 2-Nitroanisole 4-Nitroanisole 2-Nitroaniline 1,3-Dinitrobenzene
Aniline 2-Toluidine 4-Toluidine 2-Chloroaniline 4-Chloroaniline 2-Methoxyaniline 4-Methoxyaniline 1,2-Phenylenediamine 3-Nitroaniline
98 89 94 78 92 78 84 92 78
2 4 4 6 6 6 5 4 6
Nitrobenzene (20 mmol), LaFeO3 (150 mg) and propan-2-ol (20 ml). Table 4 Effect of various promoters on the reduction of nitrobenzene to aniline Promoter
Yield (%) of aniline
KOH NaOH Na2 CO3 K2 CO3
98 75 1.6 42
Nitrobenzene (20 mmol), LaFeO3 (150 mg), propan-2-ol (20 ml) and promoter (20 mmol).
hence, it acts as a strong base. Thus, the maximum yield of aniline is obtained when KOH is used as promoter. The reaction was also studied using alternative primary and secondary alcohols with LaFeO3 as catalyst (Table 5). The yields obtained with primary alcohol are less, because primary alcohols dehydrogenate to give aldehydes, which may act as catalyst poisons [7]. Since the reduction of nitrobenzene to aniline was maximum on LaFeO3 , reduction of several other nitroaromatics was carried out on this catalyst (Table 6). It can be seen that ortho-substituted compounds give comparatively lower yields as compared to para-substituted Table 5 Reduction of nitrobenzene to aniline with various alcohols Alcohol
Yield (%) of aniline
Methanol 1-Propanol 1-Butanol 2-Propanol 2-Butanol
11 53 46 98 62
Nitrobenzene (20 mmol), LaFeO3 (150 mg), KOH (20 mmol) and alcohol (20 ml).
Substrate (20 mmol), KOH pellets (20 mmol), LaFeO3 (150 mg) and propan-2-ol (20 ml).
compounds, which may be attributed to steric hindrance. In the case of 1,3-dinitrobenzene, one nitro group was reduced selectively, leading to formation of 3-nitroaniline. 3.3. Catalyst reusability For the study of reusability, LaFeO3 was chosen. The catalyst was filtered and activated at 120 ◦ C before use for the next cycle. The catalyst could be reused up to five times without any loss in the activity. The XRD of the spent catalyst revealed no structural changes. 3.4. Reaction kinetics Kinetic studies were carried out for the reduction of nitrobenzene on all LaMO3 catalysts. The reduction of nitrobenzene was found to follow pseudo-first-order kinetics. The rate constants were used for the calculation of Ea -values. The Arrhenius Table 7 Activation energy for reduction of nitrobenzene to aniline on various LaMO3 catalysts Catalyst
Ea (kJ/mol)
LaAlO3 LaFeO3 LaCrO3 LaCoO3 LaMnO3
77.5 73.4 83.3 83.3 148.1
230
A.S. Kulkarni, R.V. Jayaram / Applied Catalysis A: General 252 (2003) 225–230
Fig. 2. Arrhenius plots for the estimation of the activation energy.
plot for the estimation of the activation energy is shown in Fig. 2. The values of activation energy are included in Table 7, which was found to follow the order: LaMnO3 > LaCrO3 > LaCoO3 > LaAlO3 > LaFeO3 . The catalytic properties of perovskites for nitrobenzene reduction show that the lower the activation energy, the higher is the catalytic activity.
Acknowledgements We dedicate this paper to (Late) Dr. Bhushan M. Khadilkar who inspired us to carry out this work. We are grateful to T.I.F.R., Mumbai for providing the XRD facility and to Dr. N.M. Gupta and Dr. V.S. Kamble— BARC, for the surface area measurements. References
4. Conclusions The results obtained in this work allow us to conclude that perovskites prepared by microwave irradiation method have activity values similar to those of the ones prepared by conventional methods. The method is simple and does not involve intermittent grindings and calcinations at elevated temperatures. Thus, we have achieved a considerable reduction in the time required for preparation of perovskites. These perovskites have excellent catalytic activity for catalytic transfer hydrogenation in liquid phase. The activity of LaFeO3 was particularly significant giving quantitative conversion of nitrobenzene to aniline. The catalyst was recyclable without any significant loss in activity. To our knowledge, this is the first report of liquid phase catalytic transfer hydrogenation reaction on perovskite-type mixed oxides.
[1] A.M. Stratz, Catalysis of organic reactions, Chem. Ind. (Dekker) 18 (1984) 335. [2] R.S. Downing, P.J. Kunkeler, H. van Bekkum, Catal. Today 37 (1997) 121. [3] A.J. Bechamp, Anal. Chim. Phys. 42 (1854) 140. [4] N. Zinin, J. Prakt. Chem. 27 (1842) 140. [5] W.G. Young, W.H. Hartung, F.S. Crossley, J. Am. Chem. Soc. 58 (1936) 100. [6] S. Sciopni, V. Orsetto, Grazz. Chim. Ital. 81 (1951) 654. [7] R.A.W. Johnstone, A.H. Wilby, I.D. Entwistle, Chem. Rev. 85 (1985) 129. [8] L.G. Tejuca, J.L.G. Fierro, J.M.D. Tascon, Adv. Catal. 36 (1989) 237. [9] G. Kremenic, J.M.L. Nieto, J.M.D. Tascon, L.G. Tejuca, J. Chem. Soc., Faraday Trans. I 81 (1985) 939. [10] Y. Wu, T. Yu, B. Dou, C. Wang, X. Xie, Z. Yu, S. Fan, Z. Fan, L. Wang, J. Catal. 88 (1989) 120. [11] B. Vaidyanathan, P. Raizada, K.J. Rao, J. Mater. Sci. Lett. 16 (1997) 2022. [12] K.E. Gibbsons, M.O. Jones, S.J. Blundell, A.l. Mihut, I. Gameson, P.P. Edwards, Y. Miyazaki, N.C. Hyatt, A. Porch, Chem. Commun. (2000) 159.
Liquid phase catalytic transfer hydrogenation of aromatic nitro compounds on perovskites prepared by microwave irradiation Amrita S. Kulkarni, Radha V. Jayaram∗ Physical Chemistry Research Laboratory, Institute of Chemical Technology, University of Mumbai, N.M. Parikh Marg, Matunga, Mumbai 400019, India Received 30 October 2002; received in revised form 14 March 2003; accepted 8 May 2003
Abstract Preparation of specific tailor-made mixed oxides is one of the major topics of research in the field of heterogeneous catalysis. Perovskites are a class of mixed oxides, which have interesting catalytic and physicochemical properties. We have successfully developed a method using microwave irradiation to synthesize perovskites of the type LaMO3 (M = Mn, Fe, Co, Cr, Al) in just 15 min. The LaMO3 perovskites were used as catalysts for reduction of aromatic nitro compounds with propan-2-ol as hydrogen donor and KOH as promoter. Kinetics of the nitrobenzene reduction has also been studied. © 2003 Elsevier B.V. All rights reserved. Keywords: Perovskites; Catalytic transfer hydrogenation; Microwave irradiation; Nitroarenes
1. Introduction Hydrogenation of nitro aromatics with heterogeneous catalysts is often the method of choice for the production of the corresponding anilines [1,2]. Aromatic amines are important starting materials and intermediates for the manufacture of a variety of chemicals such as dyestuffs, pharmaceutical products, agricultural chemicals, surfactants and polymers. The oldest method and the industrially practiced one is the Bechamp reduction, which involves use of stoichiometric amounts of finely divided iron metal and water in the presence of small amounts of acid. This method has a distinct disadvantage of formation of iron sludges that are difficult to filter and that always contain adsorbed reaction products and hence lead to dis∗ Corresponding author. Tel.: +91-22-414-5616; fax: +91-22-414-5614. E-mail address: [email protected] (R.V. Jayaram).
posal problems [3]. The sulfide reduction has a broader selectivity than the Bechamp reduction and enables chemoselective reduction of nitro compounds in the presence of C=C, azo and other nitro compounds. The major disadvantages are toxicity, odor of most reducing agents and sulfur containing organic side products, and formation of elemental sulfur [4]. Hydrogenation using molecular hydrogen is non-polluting, but presents considerable hazards, since hydrogen is a gas of low molecular weight and gets easily ignited. In contrast to these conventional methods, catalytic transfer hydrogenation is superior in terms of saving resources, reducing waste disposal and being unharmful to the environment [5,6]. Catalytic transfer hydrogenation is a simple and safe operation in which a catalyst and hydrogen gas is replaced with a catalyst and hydrogen donor such as cyclohexene, propan-2-ol and ethanol. In this paper, we report the transfer hydrogenation of aromatic nitro compounds using propan-2-ol
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00417-4
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A.S. Kulkarni, R.V. Jayaram / Applied Catalysis A: General 252 (2003) 225–230
as hydrogen donor and KOH as promoter. The use of propan-2-ol in the presence of transition metal catalysts for transfer hydrogenation has been extensively studied [7]. Although perovskite-type mixed oxides have been used for a gamut of reactions like NOx decomposition [8] and oxidation of CO [9] and NH3 [10], their applications for catalytic transfer hydrogenation are still unexplored. Perovskite-type mixed oxides (ABO3 ) have a welldefined bulk structure and the composition of cations at both A and B sites can be widely changed. Several studies have confirmed that the catalytic activity of LaMO3 type perovskites can be attributed to valency control and thermal stability. The perovskite oxides most frequently employed in heterogeneous catalysis are those containing a lanthanide at A site and a transition element at B site. However, the formation of a perovskite structure can further influence the catalytic activity by changes in the electronic and chemical state of B cations. The conventional ceramic method for synthesis of perovskites requires conductive heating for extended periods (12–24 h) at elevated temperatures with intermittent grindings such that perovskite is obtained as a result of a solid-state diffusion process. In order to circumvent this limitation, there is increasing demand for new preparation methods. Microwave-assisted synthesis of inorganic compounds is one of the attractive fields of research in recent years [11,12]. If one or several of the components in a chemical reaction system strongly absorb microwaves, the resultant heat generated can be used to drive a reaction with other components. In the present investigation, we have achieved reduction in time required for the preparation of perovskites of LaMO3 (M = Mn, Fe, Co, Cr, Al) using this methodology and have also tested the activity of these catalysts for transfer hydrogenation reaction.
This assembly was mounted in a domestic microwave oven operating at a frequency of 2.45 GHz and irradiated for 5 min at a full power setting. The microwave source was then switched off and the sample was left to cool inside the oven. The sample was ground again and irradiated for 10 min in two cycles of 5 min. Decomposition of nitrates results in formation of reactive oxides which can easily combine to form the desired product. The perovskite phase was formed only when the nitrates were microwave irradiated. The oxides themselves did not absorb effectively even in the presence of an external absorber (graphite). The perovskite phase was not formed when the nitrates were irradiated in the absence of graphite. 2.2. Preparation of perovskites by conventional method For comparative purposes, perovskites were also prepared by the conventional ceramic method. LaMO3 (M = Mn, Fe, Co, Cr, Al) perovskites were prepared from oxalate precursors. Stoichiometric amounts of the respective oxalates were ground in an agate mortar and calcined in air at 1173 K for 12–24 h. The samples were analyzed by X-ray diffraction and reground and recalcined if necessary. 2.3. Catalyst characterization The catalysts were characterized by X-ray diffraction recorded on a JEOL JDX-8030 X-ray diffractometer using Cu K␣ radiation. The BET area of LaFeO3 prepared by both conventional and microwave irradiation methods were measured using a Model—Quantachrome Autosorb 1 instrument. 2.4. Catalytic activity
2. Experimental 2.1. Preparation of perovskites by microwave irradiation Stoichiometric quantities of the respective nitrates were thoroughly ground and mixed. The mixture was placed in an open quartz tube, on a bed of graphite.
All reactions were carried out under continuous stirring in a two-necked 100 ml round bottom flask fitted with a reflux condenser. Analysis was done by withdrawing definite aliquots of the sample at regular intervals. In a typical run, 150 mg of the catalyst was dispersed in a solution containing 20 mmol of nitrobenzene, 20 mmol of KOH pellets and 20 ml of propan-2-ol. The mixture was stirred and heated under reflux for 2–6 h in an oil bath. The products were
A.S. Kulkarni, R.V. Jayaram / Applied Catalysis A: General 252 (2003) 225–230
analyzed on the basis of their retention times using a Gas Chromatograph fitted with an OV-1 column (Eshita, Model—Eshika).
3. Results and discussion 3.1. XRD analysis Fig. 1 shows the XRD patterns of the perovskites prepared by microwave irradiation. All the five LaMO3 samples synthesized are essentially pure phases, as evidenced by XRD. No unreacted precursor material was detectable. The unit cell parameters are reported in Table 1. 3.2. Catalytic activity Once the materials were characterized as regards structure, they were tested as catalysts for the reduction of nitrobenzene to aniline with propan-2-ol. Scheme 1 depicts a typical catalytic transfer hydrogenation. During the reaction, the alcohol is oxidized to ketone and nitrobenzene is reduced to aniline. Aniline was the only product obtained under these conditions. The catalytic activities of the perovskites prepared by each method were found to be comparable (Table 2). However, the slight increase in the conversion of nitrobenzene using the microwave-prepared Table 1 Unit cell parameters of perovskite-samples prepared by microwave Catalyst
LaAlO3 LaFeO3 LaCrO3 LaCoO3 LaMnO3
Cell parameters a
b
c
5.365 5.557 5.475 5.441 5.520
– 5.563 5.510 – –
13.12 7.862 7.757 3.085 13.320
Scheme 1.
227
catalyst may be due to the increase in the surface area of catalyst prepared by microwave irradiation method. The surface area of LaFeO3 prepared by microwave irradiation method is ∼15 m2 /g and that prepared by conventional method is ∼10 m2 /g. Based on the results, the catalytic activity in the reaction decreases in the following sequence: LaFeO3 > LaAlO3 > LaCrO3 > LaCoO3 > LaMnO3 . Catalytic transfer hydrogenation reaction is similar to Meerwein–Ponndorf–Verley (MPV) reduction, which involves transfer of protons from propan-2-ol to keto group. Usually this reaction is carried out using Al metal. The isopropoxide anion so formed is stabilized at a Lewis Al site. This reaction is expected to follow a similar mechanism. The high activity of LaFeO3 and LaAlO3 is inherent, as the Lewis acidity of Fe and Al are comparable. The perovskites LaMO3 , where M = Cr, Co and Mn, are weakly acidic and are hence expected to show low activity. As LaFeO3 was found to be the most active catalyst, the reaction was also studied on a stoichiometric mixture of La2 O3 , Fe2 O3 and a mixture of La2 O3 -Fe2 O3 . The results are depicted in Table 2. The reaction did not proceed in the absence of either catalyst or KOH. The effect of KOH concentration on the yield of aniline is illustrated in Table 3. The effect of various promoters is illustrated in Table 4. From the data it follows that KOH is the best promoter. NaOH and K2 CO3 show reasonable activity, whereas the activity of Na2 CO3 was very poor. The dissociation of KOH in propan-2-ol is higher than that of the other three bases (NaOH, Na2 CO3 , K2 CO3 ); Table 2 Reduction of nitrobenzene to aniline over various catalysts prepared by both methods Catalysts
La2 O3 Fe2 O3 La2 O3 -Fe2 O3 LaAlO3 LaFeO3 LaCrO3 LaCoO3 LaMnO3
Method of preparation Microwave (%)
Conventional (%)
– – – 94 98 84 82 79
63 69 79 90 95 82 81 75
Nitrobenzene (20 mmol), KOH pellets (20 mmol), catalyst (150 mg) and propan-2-ol (20 ml).
228
A.S. Kulkarni, R.V. Jayaram / Applied Catalysis A: General 252 (2003) 225–230
Fig. 1. The XRD patterns of perovskites: (a) LaAlO3 , (b) LaFeO3 , (c) LaMnO3 , (d) LaCrO3 , (e) LaCoO3 .
A.S. Kulkarni, R.V. Jayaram / Applied Catalysis A: General 252 (2003) 225–230
229
Table 3 Effect of KOH concentration on the reduction of nitrobenzene to aniline
Table 6 Reduction of various nitro aromatics to anilines on LaFeO3 catalyst.
KOH (mmol)
Yield (%) of aniline
Substrate
Product
0 5 10 15 20 25
0 23 41 69 98 100
Yield (%)
Time (h)
Nitrobenzene 2-Nitrotoluene 4-Nitrotoluene 2-Chloronitrobenzene 4-Chloronitrobenzene 2-Nitroanisole 4-Nitroanisole 2-Nitroaniline 1,3-Dinitrobenzene
Aniline 2-Toluidine 4-Toluidine 2-Chloroaniline 4-Chloroaniline 2-Methoxyaniline 4-Methoxyaniline 1,2-Phenylenediamine 3-Nitroaniline
98 89 94 78 92 78 84 92 78
2 4 4 6 6 6 5 4 6
Nitrobenzene (20 mmol), LaFeO3 (150 mg) and propan-2-ol (20 ml). Table 4 Effect of various promoters on the reduction of nitrobenzene to aniline Promoter
Yield (%) of aniline
KOH NaOH Na2 CO3 K2 CO3
98 75 1.6 42
Nitrobenzene (20 mmol), LaFeO3 (150 mg), propan-2-ol (20 ml) and promoter (20 mmol).
hence, it acts as a strong base. Thus, the maximum yield of aniline is obtained when KOH is used as promoter. The reaction was also studied using alternative primary and secondary alcohols with LaFeO3 as catalyst (Table 5). The yields obtained with primary alcohol are less, because primary alcohols dehydrogenate to give aldehydes, which may act as catalyst poisons [7]. Since the reduction of nitrobenzene to aniline was maximum on LaFeO3 , reduction of several other nitroaromatics was carried out on this catalyst (Table 6). It can be seen that ortho-substituted compounds give comparatively lower yields as compared to para-substituted Table 5 Reduction of nitrobenzene to aniline with various alcohols Alcohol
Yield (%) of aniline
Methanol 1-Propanol 1-Butanol 2-Propanol 2-Butanol
11 53 46 98 62
Nitrobenzene (20 mmol), LaFeO3 (150 mg), KOH (20 mmol) and alcohol (20 ml).
Substrate (20 mmol), KOH pellets (20 mmol), LaFeO3 (150 mg) and propan-2-ol (20 ml).
compounds, which may be attributed to steric hindrance. In the case of 1,3-dinitrobenzene, one nitro group was reduced selectively, leading to formation of 3-nitroaniline. 3.3. Catalyst reusability For the study of reusability, LaFeO3 was chosen. The catalyst was filtered and activated at 120 ◦ C before use for the next cycle. The catalyst could be reused up to five times without any loss in the activity. The XRD of the spent catalyst revealed no structural changes. 3.4. Reaction kinetics Kinetic studies were carried out for the reduction of nitrobenzene on all LaMO3 catalysts. The reduction of nitrobenzene was found to follow pseudo-first-order kinetics. The rate constants were used for the calculation of Ea -values. The Arrhenius Table 7 Activation energy for reduction of nitrobenzene to aniline on various LaMO3 catalysts Catalyst
Ea (kJ/mol)
LaAlO3 LaFeO3 LaCrO3 LaCoO3 LaMnO3
77.5 73.4 83.3 83.3 148.1
230
A.S. Kulkarni, R.V. Jayaram / Applied Catalysis A: General 252 (2003) 225–230
Fig. 2. Arrhenius plots for the estimation of the activation energy.
plot for the estimation of the activation energy is shown in Fig. 2. The values of activation energy are included in Table 7, which was found to follow the order: LaMnO3 > LaCrO3 > LaCoO3 > LaAlO3 > LaFeO3 . The catalytic properties of perovskites for nitrobenzene reduction show that the lower the activation energy, the higher is the catalytic activity.
Acknowledgements We dedicate this paper to (Late) Dr. Bhushan M. Khadilkar who inspired us to carry out this work. We are grateful to T.I.F.R., Mumbai for providing the XRD facility and to Dr. N.M. Gupta and Dr. V.S. Kamble— BARC, for the surface area measurements. References
4. Conclusions The results obtained in this work allow us to conclude that perovskites prepared by microwave irradiation method have activity values similar to those of the ones prepared by conventional methods. The method is simple and does not involve intermittent grindings and calcinations at elevated temperatures. Thus, we have achieved a considerable reduction in the time required for preparation of perovskites. These perovskites have excellent catalytic activity for catalytic transfer hydrogenation in liquid phase. The activity of LaFeO3 was particularly significant giving quantitative conversion of nitrobenzene to aniline. The catalyst was recyclable without any significant loss in activity. To our knowledge, this is the first report of liquid phase catalytic transfer hydrogenation reaction on perovskite-type mixed oxides.
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