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Vapour-phase heterogeneous catalytic transfer hydrogenation of alkyl methyl ketones on MgO: Prevention of the deactivation of MgO in the presence of carbon tetrachloride
Applied Catalysis A: General 169 (1998) 263±269
Vapour-phase heterogeneous catalytic transfer hydrogenation of alkyl methyl ketones on MgO: Prevention of the deactivation of MgO in the presence of carbon tetrachloride Gy. SzoÈlloÈsi, M. BartoÂk* Department of Organic Chemistry and Organic Catalysis Research Group of the Hungarian Academy of Sciences, JoÂzsef Attila University, DoÂm teÂr 8, H-6720 Szeged, Hungary Received 5 June 1997; received in revised form 19 December 1997; accepted 19 December 1997
Abstract Catalytic transfer hydrogenation (henceforth abbreviated CTH) of alkyl methyl ketones (2-butanone, 2-pentanone and 2-hexanone) by 2-propanol was studied on a commercially available MgO catalyst. Pretreatment of MgO, deactivation of the catalyst, temperature dependence of CTH and the effects of the 2-propanol/ketone ratio and of various catalyst modi®ers were investigated. An important result of these studies is that the deactivation of MgO may be prevented by treatment with carbon tetrachloride. As a consequence, vapor-phase CTH using commercially available MgO may be economically realized. # 1998 Elsevier Science B.V.
1. Introduction Catalytic transfer hydrogenation (CTH) is a widely used reduction procedure in preparative organic chemistry, employed both in liquid and vapour phases [1± 3]. This procedure is conveniently applicable to all of the reducible functional groups. The ®eld most copiously discussed in the special literature is CTH of ketones, since ± in addition to their practical importance ± ketones have been, and are preferred model compounds for studies on the stereochemistry, the mechanism and the catalysts of CTH. Following the discoveries made in the seventies [4±6], as well as recent experimental results, special *Corresponding author. 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(98)00015-5
attention has been focussed on MgO, one of the catalysts of vapour-phase CTH [7±13]. It has been concluded that MgO is an excellent catalyst of vapour-phase CTH, in spite of its being gradually deactivated as conversion progresses. However, the deactivated catalyst may be effectively regenerated. Deactivation has been attributed to various causes [10,13±15] like formation of heavy condensation products on magnesia surface or carbon deposits and the adsorption of substances on their surface or formation of surface carboxylate species. Important conclusions have been drawn regarding the mechanism of CTH. The present report gives an account of our studies which permits the effective utilization of commercially available MgO in CTH of ketones and provides new data on catalyst modi®cations.
264
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2. Experimental 2.1. Materials MgO was a product of Fluka (Fluka 63091 MgO light, purum p.a., >98.0%; chloride<0.05%, sulphate<0.5%, Cu<0.005%, Pb<0.005%, Cd<0.005%, Zn<0.005%, Fe<0.05%) with a speci®c surface area of 64 m2/g (BET measurement was performed in a Gemini 2375 V3.02 apparatus). The organic compounds used were Aldrich and Fluka products; these were puri®ed by distillation prior to use. The purity of helium used in the measurements was 99.996%. Oxygen (99.996%) utilized for pretreatments was dried on an A5 molecular sieve. 2.2. Methods CTH was studied in the vapour phase in a continuous-¯ow system. The catalyst was placed in an 8 mm I.D. vertically disposed glass microreactor. The helium used as carrier gas was saturated with the reactants in separate saturators; the two gas ¯ows were next combined and conducted to the catalyst bed. Samples were withdrawn from the product ¯ow leaving the reactor at regular intervals by means of an automated sampling valve and analyzed by the attached SRI 8610A gas chromatograph equipped with a column ®lled with 15% Carbowax 1000/Chromosorb W (length, 2 m; diameter, 1/8") and a ¯ame ionization detector. Products were identi®ed on the basis of their retention time, and by GC-MS analysis (HP 5890 GC with a 50 m HP-1 capillary columnHP 5970 MSD) of the mixture obtained by freezing the product that is leaving the reactor in liquid nitrogen. The values of hourly liquid space velocity (HLSV) were calculated from data published in the literature [16,17] and were also determined experimentally. The two results were in good agreement.
In a standard experiment, 21 mg of MgO were placed in the reactor and pretreated at 673 K by a 1 : 1 mixture of helium and oxygen for 2 h. The catalyst was then cooled to reaction temperature in helium and the helium ¯ow saturated with the reactants was ®nally conducted into the reactor. Samples were withdrawn from the mixture of products every 30 min and analyzed. In experiments with modi®ed catalysts, modi®cations with acetic anhydride, triethylamine, carbon tetrachloride or 2-chloro-2-methylpropane were effected at the temperature of CTH in helium. Thereafter, ®ve pulses of 5 ml each were injected, then the system was purged with helium for 15 min before the reaction was started. In addition, but only in the case of triethylamine, 5 ml pulses at 90, 150, 215 and 245 min were injected into the feed. Treatment of the catalyst with 2-propanol or 2-butanone was carried out applying 1 h of ¯ow of 2-propanol with HLSV 2.77 ml/h g or of 2-butanone with HLSV 0.93 ml/h g before the CTH. 3. Results The reaction studied is shown in Scheme 1. Ketones used as model compounds were 2-butanone, 2-pentanone and 2-hexanone. Of secondary alcohols, 2-propanol was selected as a hydrogen donor. 2-Propanol, a conventional hydrogen source, has many favourable properties: it is stable, easy to handle, non-toxic, environmentally friendly, inexpensive and is a good solvent of many organic compounds. Acetone, the by-product of the reaction is easy to remove. The main reaction shown above is occasionally accompanied by the dehydration of 2-propanol. Dehydration is minimal at 523 K and does not exceed a selectivity of 3% even at 623 K.
Scheme 1.
Gy. SzoÈlloÈsi, M. BartoÂk / Applied Catalysis A: General 169 (1998) 263±269
Experiments were focussed on the pretreatment of MgO, the deactivation of the catalyst, the temperature dependence of CTH and the effect of the 2-propanol/ ketone ratio and of various catalyst modi®ers. 3.1. Pretreatment of the catalyst The methods listed in Table 1 were selected for the pretreatment of MgO. The pretreatment methods listed in Table 1, differing not just by a change of the temperature but also by modi®cation of the pretreatment time and atmosphere, are convincing with respect to the in¯uence of the pretreatment methods on CTH of methyl ketones on MgO. The results obtained on MgO pretreated in various ways reveal that the activity of the catalyst decreased with the progress of time, independently of the pretreatment (Table 2). The decrease in conversion is Table 1 MgO pretreatment conditions (21 mg MgO, pretreatment: 40 ml gas/min) Method
Pretreatment gas
1 2 3 4 5
HeO2 He HeO2 HeO2 HeO2
1/1 1/1 1/1 1/1
Temperature (K)
Time (h)
673 673 523 773 673
2 2 2 2 1
Table 2 Effect of MgO pretreatment conditions on 2-butanone CTH (21 mg MgO, CTH: 523 K, 20 ml He/min, 2-propanol/2-butanone 1.5, HLSV a 0.3 ml/h g) Minutes 10 40 70 100 130 160 190 220 250 280 310 340 370 a
265
largest when pretreatment was carried out at 773 K and smallest after pretreatment at 523 K, while initial conversion is highest at 673 K. In each case, the selectivity of formation of 2-butanol is close to 100% at the lower pretreatment temperature, while on the sample, pretreated at 773 K, CTH is accompanied by dehydration of 2-propanol. Method 1 was chosen for further measurements, since it was after this pretreatment that the highest initial activities were observed and activity decreased at a relatively slow rate. 3.2. Temperature dependence of CTH Typical data are summarized in Fig. 1. Initial conversion is considerably higher at 523 K than at 423 or 623 K. The relative decrease in conversion is smallest and linear at 523 K and largest at 423 K, while at 623 K conversion remains steady after a large initial relative decrease. 3.3. Deactivation of MgO during CTH Changes in the activity of MgO during CTH are presented in Figs. 2 and 3. The experimental data unambiguously show that the extent of the deactivation of MgO is dependent on the amount of the reactants. It is well demonstrated that, under an identical load, the deactivation of the lower amount of
2-Butanone conversion/mol% 1b
2b
3b
4b
5b
68 65 65 64 63 61 60 59 59 57 56 55 54
67 66 64 63 60 61 57 57 57 56 54 53 51
63 62 62 61 60 59 59 58 57 63 59 61 60
62 56 54 53 49 48 Ð Ð Ð Ð 45 44 42
64 64 62 62 60 62 61 56 62 54 56 Ð Ð
Hourly liquid space velocity. b See Table 1.
Fig. 1. Effect of temperature on CTH of 2-butanone with 2propanol on MgO (pretreatment: 1, reaction conditions: see Table 2).
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Fig. 2. Effect of 2-butanone amount (ml/h g) on CTH with 2propanol (4.8 mg MgO, pretreatment: 1, reaction conditions as in Table 2, 2-propanol/2-butanone molar ratio in parantheses).
Fig. 4. Effect of 2-propanol/2-butanone molar ratio on CTH on MgO (see notes in Fig. 1).
Fig. 3. Effect of treatment of MgO with 2-propanol and 2butanone on CTH of 2-butanone with 2-propanol (5.5 mg MgO, pretreatment: 1, treatment: see Section 2, CTH: 523 K, 20 ml He/ min, 2-propanol/2-butanone 3.0, HLSV 3.70 ml/h g).
Fig. 5. CTH of 2-pentanone and 2-hexanone with 2-propanol on MgO (see notes in Fig. 1, 2-propanol/ketone molar ratio in parantheses, 0.20 ml 2-pentanone2-propanol/h g and 0.16 2hexanone2-propanol/h g).
MgO is faster. From the experimental data, obtained with catalyst treated before the reaction with 2-propanol or 2-butanone, shown in Fig. 3, we can deduce that the deactivation of the catalyst is due to the condensation compounds formed from 2-butanone and acetone. The initial relative decrease in conversion is smaller at lower amounts of 2-butanone. It is clear from these data that MgO available from commercial sources at a relatively low price is indeed applicable to CTH after a suitable pretreatment;
because of its short cycle time due to its relatively fast deactivation, however, its utilization without any modi®cation is not recommendable for practical application. 3.3.1. The effect of the 2-propanol/ketone molar ratio and the chain length of ketones on CTH Experimental data on the three different methyl ketones are shown in Figs. 4 and 5. An increase in the alcohol/ketone ratio enhances conversion, due to a
Gy. SzoÈlloÈsi, M. BartoÂk / Applied Catalysis A: General 169 (1998) 263±269
favourable shift in the equilibrium of the redox reaction. Again, it is worth mentioning that the decrease of the ketone amount led to a much slower deactivation of MgO. At the same time, it is also shown that deactivation is not increased by the length of the alkyl chain. 3.3.2. The effect of the modification of MgO In the case of the conversion of 2-butanone, treatment by acetic anhydride causes immediate deactivation due to poisoning of the active sites of MgO. Treatment by triethylamine has hardly any effect on the initial phase of conversion and the reaction rate is only slightly decreased even by repeated additions during the reaction marked on Fig. 6. Treatment of the catalyst before the reaction by carbon tetrachloride had an unexpected effect on the activity (Fig. 6): both, the deactivation of MgO and the dehydration of the alcohols stopped. This effect was especially spectacular in the case of relatively low amount of catalyst (Fig. 2) and at higher HLSV value. When the catalyst was treated with 2-chloro-2-methylpropane under identical conditions, the initial activity considerably lower than without modi®cation; the activity then increased gradually and remained higher than the value obtained on an untreated catalyst.
Fig. 6. Effect of treatment of MgO with organic modifiers on CTH of 2-butanone with 2-propanol (see notes in Fig. 1; modification, see Section 2; ", NEt3 5 ml pulses).
267
4. Discussion In the course of CTH of ketones, some of the activity of MgO is lost. Deactivation is lowest on MgO pretreated at 523 K (Tables 1 and 2), i.e. at the temperature where the concentration of surface OH groups is maximal; deactivation is very high on the catalyst pretreated at 773 K, indicating that the formation of adsorbates poisoning the surface is inhibited by OH groups. The result obtained on MgO, pretreated only for 1 h, points to the same conclusion. The experiment on MgO pretreated in helium reveals that defects in the crystal lattice do not play an important role under the experimental conditions used. According to our results, the optimal reaction temperature for both, initial conversion and activity is 523 K. At 423 K, the strong adsorption of starting materials and products results in a large decrease in initial conversion. At 623 K, the rate of the formation of poisonous surface products is increased in the initial phase, followed by steady conversion at a lower level (which is due to the decrease in the number of active sites). In addition to the effect to be expected (i.e. conversion is increased parallel with the alcohol/ketone molar ratio), variations in the amount of the reactants identify the determinant role of ketones in the poisoning of the surface. It appears from the experiment on MgO modi®ed by triethylamine that acid centers also play some role in CTH. Treatment by chlorinated hydrocarbons has been widely applied in heterogeneous catalysis in order to increase selectivity [18]. Observations on the modifying effect of Clÿ ions on MgO have also been published. Kasper et al. [10] studied CTH of 4-hexen-3one and reported the effect of Clÿ ions on product selectivity. The speci®c surface area of MgO was signi®cantly decreased by the modi®cation, the basicity and the morphology of the catalyst were altered and, consequently, side reactions were minimized. Burch et al. [18] and Moffat et al. [19] investigated the effect of CH2Cl2 and CCl4 on the activity and selectivity of methane oxidative coupling on MgO. The selectivity of ethylene formation increased by chlorinated compounds. Although the role of the modi®ers as yet is unknown, it may be established with certainty that they modify the structure
268
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of the surface, resulting in the formation of new, catalytically active sites. In the case of CCl4, the chlorinated species on the catalyst was identi®ed as MgCl2 [19]. Our experiments indicate that, in CTH on MgO, CCl4 prevents catalyst poisoning. In our opinion, Clÿ ions are adsorbed on centres responsible for the irreversible adsorption of ketones (probably Lewis acidic centres), thereby inhibiting ketone binding to these sites. In addition to the prevention of deactivation, this assumption is also supported by the total absence of dehydration after CCl4 treatment and by the results obtained on catalysts treated with 2-chloro-2-methylpropane. In the case of the catalyst treated with 2-chloro-2-methylpropane, the low initial conversion is a result of the powerful steric hindrance of the hydrocarbon group. So the participation of basic surface coordinative unsaturated O2ÿ centres is essential for CTH: the deterministic role of basic centres is proven by the result obtained on MgO modi®ed by triethylamine and by acetic anhydride which caused the total inactivity of the catalyst.
CTH is illustrated by the reaction steps shown in Scheme 2, veri®cation of which calls for complex and extensive studies. It would be especially important to identify the nature of the catalytically active sites. It is customary to formulate six-membered transition states (see Fig. 7) formally representing the determinant process, i.e. Hÿ transfer. The following conclusions have been published in the literature on the nature of active sites operating on MgO in CTH: ± ``. . . the substrate is coordinated on a weak acid site while propane-2-ol must be coordinated on an adjacent surface basic site'' [10]. ± ``. . . the basic and one-electron donor sites of MgO surface are responsible for the oxide activity in the CTR. On the other hand, we did not ascertain the necessity of a co-action of acidic surface sites'' [8]. ± ``. . . In the case of a basic oxide, like MgO, coordinative ethanol adsorption is unlikely, the species rather being H-bonded to basic O2ÿ sites, or dissociated on weak Lewis acid±strong base pair site'' [20].
Scheme 2.
Gy. SzoÈlloÈsi, M. BartoÂk / Applied Catalysis A: General 169 (1998) 263±269
269
Fig. 7. Transition states in CTH of ketones.
5. Conclusions
References
The results presented in this paper allowed us to draw the following conclusions: (i) The commercially available MgO suffers a deactivation during CTH of ketones, independent of the pretreatment method used. This deactivation can be mainly attributed to surface species formed from ketones. The deactivation of MgO is not accentuated by the increase of the alkyl chain of methyl alkyl ketones. (ii) Pretreatment of MgO with CCl4 prevented the deactivation of the catalyst during CTH, probably by binding Clÿ to the Lewis acid sites of the surface. As a result, such sites being responsible for the formation of irreversibly bonded surface species, in our experimental conditions, can be blocked. As a consequence, vapour-phase CTH using commercially available MgO may be economically realized. Altogether, the experimental observations have not yet led to the emergence of a uni®ed picture, so our efforts will be focussed in this direction, especially to the modi®cation of magnesia with other organochlorine reagents. These studies will include the titrimetry of MgO with various organochlorine compounds and FTIR, XRD and DSC measurements.
[1] R.W. Johnstone, A.H. Wilby, I.D. Entwistle, Chem. Rev. 85 (1985) 129. [2] C.F. de Graauw, J.A. Peters, H. van Bekkumm, J. Huskens, Synthesis, (1994) 1007. Ï ervenyÂ, Chem. Listy 90 (1996) 93. [3] Z. DobrovolnaÂ, L. C [4] W. Zatorski, S. Malinowski, Rocz. Chem. 45 (1971) 81; Chem. Abstr. 77 (1972) 140850. [5] H. Niyama, E. Echigoya, Bull. Chem. Soc. Jpn. 45 (1972) 938. [6] Y. Okamoto, T. Imanaka, S. Teranishi, Bull. Chem. Soc. Jpn. 45 (1972) 3207. [7] J. KijenÂski, M. GlinÂski, J. Reinhercs, Heterogeneous catalysis and fine chemicals I., Stud. Surf. Sci. Catal. 41 (1989) 231. [8] J. KijenÂski, M. GlinÂski, J. Czarnecki, J. Chem Soc. Perkin Trans. 2 (1991) 1695. [9] J. KijenÂski, M. GlinÂski, R. WisÂniewski, S. Murghani, Heterogeneous catalysis and fine chemicals II., Stud. Surf. Sci. Catal. 59 (1991) 169. [10] J. Kaspar, A. Trovarelli, F. Zamoner, E. Farnetti, M. Graziani, Heterogeneous catalysis and fine chemicals I., Stud. Surf. Sci. Catal. 59 (1991) 253. [11] J. KijenÂski, J. RuszczynÂski, I. Knedler, M. GlinÂski, React. Kinet. Catal. Lett. 49 (1993) 287. [12] M. GlinÂski, J. KijenÂski, J. Gibka, J. GoÂra, React. Kinet. Catal. Lett. 56 (1995) 121. [13] M.A. Aramendia, V. Borau, C. Jimenez, J.M. Marinas, A. Porras, F.J. Urbano, J. Catal. 161 (1996) 829. [14] J. KijenÂski, M. GlinÂski, J. Czarnecki, R. Derlacka, V. Jarzyna, Heterogeneous catalysis and fine chemicals III., Stud. Surf. Sci. Catal. 78 (1993) 631. [15] M. GlinÂski, J. KijenÂski, J. RuszczynÂski, React. Kinet. Catal. Lett. 54 (1995) 1. [16] M.G. Mayberry, J.G. Aston, J. Amer. Chem. Soc. 56 (1932) 2682. [17] C.B. Kretschmer, R. Wiebe, J. Amer. Chem. Soc. 74 (1952) 1276. [18] R. Burch, S. Chalker, S.J. Hibble, Appl. Catal. A 96 (1993) 289. [19] S. Sugijama, K. Satomi, N. Shigemoto, H. Hayashi, J.B. Moffat, Catal. Lett. 25 (1994) 201. [20] V.A. Ivanov, J. Bachelier, F. Audry, J.C. Lavalley, J. Mol. Catal. 91 (1994) 45.
Acknowledgements We acknowledge the ®nancial support provided for this research by the Hungarian National Science Foundation through Grant OTKA 016109. We also  rpaÂd MolnaÂr for valuable discusthank Professor A sions.
Vapour-phase heterogeneous catalytic transfer hydrogenation of alkyl methyl ketones on MgO: Prevention of the deactivation of MgO in the presence of carbon tetrachloride Gy. SzoÈlloÈsi, M. BartoÂk* Department of Organic Chemistry and Organic Catalysis Research Group of the Hungarian Academy of Sciences, JoÂzsef Attila University, DoÂm teÂr 8, H-6720 Szeged, Hungary Received 5 June 1997; received in revised form 19 December 1997; accepted 19 December 1997
Abstract Catalytic transfer hydrogenation (henceforth abbreviated CTH) of alkyl methyl ketones (2-butanone, 2-pentanone and 2-hexanone) by 2-propanol was studied on a commercially available MgO catalyst. Pretreatment of MgO, deactivation of the catalyst, temperature dependence of CTH and the effects of the 2-propanol/ketone ratio and of various catalyst modi®ers were investigated. An important result of these studies is that the deactivation of MgO may be prevented by treatment with carbon tetrachloride. As a consequence, vapor-phase CTH using commercially available MgO may be economically realized. # 1998 Elsevier Science B.V.
1. Introduction Catalytic transfer hydrogenation (CTH) is a widely used reduction procedure in preparative organic chemistry, employed both in liquid and vapour phases [1± 3]. This procedure is conveniently applicable to all of the reducible functional groups. The ®eld most copiously discussed in the special literature is CTH of ketones, since ± in addition to their practical importance ± ketones have been, and are preferred model compounds for studies on the stereochemistry, the mechanism and the catalysts of CTH. Following the discoveries made in the seventies [4±6], as well as recent experimental results, special *Corresponding author. 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(98)00015-5
attention has been focussed on MgO, one of the catalysts of vapour-phase CTH [7±13]. It has been concluded that MgO is an excellent catalyst of vapour-phase CTH, in spite of its being gradually deactivated as conversion progresses. However, the deactivated catalyst may be effectively regenerated. Deactivation has been attributed to various causes [10,13±15] like formation of heavy condensation products on magnesia surface or carbon deposits and the adsorption of substances on their surface or formation of surface carboxylate species. Important conclusions have been drawn regarding the mechanism of CTH. The present report gives an account of our studies which permits the effective utilization of commercially available MgO in CTH of ketones and provides new data on catalyst modi®cations.
264
Gy. SzoÈlloÈsi, M. BartoÂk / Applied Catalysis A: General 169 (1998) 263±269
2. Experimental 2.1. Materials MgO was a product of Fluka (Fluka 63091 MgO light, purum p.a., >98.0%; chloride<0.05%, sulphate<0.5%, Cu<0.005%, Pb<0.005%, Cd<0.005%, Zn<0.005%, Fe<0.05%) with a speci®c surface area of 64 m2/g (BET measurement was performed in a Gemini 2375 V3.02 apparatus). The organic compounds used were Aldrich and Fluka products; these were puri®ed by distillation prior to use. The purity of helium used in the measurements was 99.996%. Oxygen (99.996%) utilized for pretreatments was dried on an A5 molecular sieve. 2.2. Methods CTH was studied in the vapour phase in a continuous-¯ow system. The catalyst was placed in an 8 mm I.D. vertically disposed glass microreactor. The helium used as carrier gas was saturated with the reactants in separate saturators; the two gas ¯ows were next combined and conducted to the catalyst bed. Samples were withdrawn from the product ¯ow leaving the reactor at regular intervals by means of an automated sampling valve and analyzed by the attached SRI 8610A gas chromatograph equipped with a column ®lled with 15% Carbowax 1000/Chromosorb W (length, 2 m; diameter, 1/8") and a ¯ame ionization detector. Products were identi®ed on the basis of their retention time, and by GC-MS analysis (HP 5890 GC with a 50 m HP-1 capillary columnHP 5970 MSD) of the mixture obtained by freezing the product that is leaving the reactor in liquid nitrogen. The values of hourly liquid space velocity (HLSV) were calculated from data published in the literature [16,17] and were also determined experimentally. The two results were in good agreement.
In a standard experiment, 21 mg of MgO were placed in the reactor and pretreated at 673 K by a 1 : 1 mixture of helium and oxygen for 2 h. The catalyst was then cooled to reaction temperature in helium and the helium ¯ow saturated with the reactants was ®nally conducted into the reactor. Samples were withdrawn from the mixture of products every 30 min and analyzed. In experiments with modi®ed catalysts, modi®cations with acetic anhydride, triethylamine, carbon tetrachloride or 2-chloro-2-methylpropane were effected at the temperature of CTH in helium. Thereafter, ®ve pulses of 5 ml each were injected, then the system was purged with helium for 15 min before the reaction was started. In addition, but only in the case of triethylamine, 5 ml pulses at 90, 150, 215 and 245 min were injected into the feed. Treatment of the catalyst with 2-propanol or 2-butanone was carried out applying 1 h of ¯ow of 2-propanol with HLSV 2.77 ml/h g or of 2-butanone with HLSV 0.93 ml/h g before the CTH. 3. Results The reaction studied is shown in Scheme 1. Ketones used as model compounds were 2-butanone, 2-pentanone and 2-hexanone. Of secondary alcohols, 2-propanol was selected as a hydrogen donor. 2-Propanol, a conventional hydrogen source, has many favourable properties: it is stable, easy to handle, non-toxic, environmentally friendly, inexpensive and is a good solvent of many organic compounds. Acetone, the by-product of the reaction is easy to remove. The main reaction shown above is occasionally accompanied by the dehydration of 2-propanol. Dehydration is minimal at 523 K and does not exceed a selectivity of 3% even at 623 K.
Scheme 1.
Gy. SzoÈlloÈsi, M. BartoÂk / Applied Catalysis A: General 169 (1998) 263±269
Experiments were focussed on the pretreatment of MgO, the deactivation of the catalyst, the temperature dependence of CTH and the effect of the 2-propanol/ ketone ratio and of various catalyst modi®ers. 3.1. Pretreatment of the catalyst The methods listed in Table 1 were selected for the pretreatment of MgO. The pretreatment methods listed in Table 1, differing not just by a change of the temperature but also by modi®cation of the pretreatment time and atmosphere, are convincing with respect to the in¯uence of the pretreatment methods on CTH of methyl ketones on MgO. The results obtained on MgO pretreated in various ways reveal that the activity of the catalyst decreased with the progress of time, independently of the pretreatment (Table 2). The decrease in conversion is Table 1 MgO pretreatment conditions (21 mg MgO, pretreatment: 40 ml gas/min) Method
Pretreatment gas
1 2 3 4 5
HeO2 He HeO2 HeO2 HeO2
1/1 1/1 1/1 1/1
Temperature (K)
Time (h)
673 673 523 773 673
2 2 2 2 1
Table 2 Effect of MgO pretreatment conditions on 2-butanone CTH (21 mg MgO, CTH: 523 K, 20 ml He/min, 2-propanol/2-butanone 1.5, HLSV a 0.3 ml/h g) Minutes 10 40 70 100 130 160 190 220 250 280 310 340 370 a
265
largest when pretreatment was carried out at 773 K and smallest after pretreatment at 523 K, while initial conversion is highest at 673 K. In each case, the selectivity of formation of 2-butanol is close to 100% at the lower pretreatment temperature, while on the sample, pretreated at 773 K, CTH is accompanied by dehydration of 2-propanol. Method 1 was chosen for further measurements, since it was after this pretreatment that the highest initial activities were observed and activity decreased at a relatively slow rate. 3.2. Temperature dependence of CTH Typical data are summarized in Fig. 1. Initial conversion is considerably higher at 523 K than at 423 or 623 K. The relative decrease in conversion is smallest and linear at 523 K and largest at 423 K, while at 623 K conversion remains steady after a large initial relative decrease. 3.3. Deactivation of MgO during CTH Changes in the activity of MgO during CTH are presented in Figs. 2 and 3. The experimental data unambiguously show that the extent of the deactivation of MgO is dependent on the amount of the reactants. It is well demonstrated that, under an identical load, the deactivation of the lower amount of
2-Butanone conversion/mol% 1b
2b
3b
4b
5b
68 65 65 64 63 61 60 59 59 57 56 55 54
67 66 64 63 60 61 57 57 57 56 54 53 51
63 62 62 61 60 59 59 58 57 63 59 61 60
62 56 54 53 49 48 Ð Ð Ð Ð 45 44 42
64 64 62 62 60 62 61 56 62 54 56 Ð Ð
Hourly liquid space velocity. b See Table 1.
Fig. 1. Effect of temperature on CTH of 2-butanone with 2propanol on MgO (pretreatment: 1, reaction conditions: see Table 2).
266
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Fig. 2. Effect of 2-butanone amount (ml/h g) on CTH with 2propanol (4.8 mg MgO, pretreatment: 1, reaction conditions as in Table 2, 2-propanol/2-butanone molar ratio in parantheses).
Fig. 4. Effect of 2-propanol/2-butanone molar ratio on CTH on MgO (see notes in Fig. 1).
Fig. 3. Effect of treatment of MgO with 2-propanol and 2butanone on CTH of 2-butanone with 2-propanol (5.5 mg MgO, pretreatment: 1, treatment: see Section 2, CTH: 523 K, 20 ml He/ min, 2-propanol/2-butanone 3.0, HLSV 3.70 ml/h g).
Fig. 5. CTH of 2-pentanone and 2-hexanone with 2-propanol on MgO (see notes in Fig. 1, 2-propanol/ketone molar ratio in parantheses, 0.20 ml 2-pentanone2-propanol/h g and 0.16 2hexanone2-propanol/h g).
MgO is faster. From the experimental data, obtained with catalyst treated before the reaction with 2-propanol or 2-butanone, shown in Fig. 3, we can deduce that the deactivation of the catalyst is due to the condensation compounds formed from 2-butanone and acetone. The initial relative decrease in conversion is smaller at lower amounts of 2-butanone. It is clear from these data that MgO available from commercial sources at a relatively low price is indeed applicable to CTH after a suitable pretreatment;
because of its short cycle time due to its relatively fast deactivation, however, its utilization without any modi®cation is not recommendable for practical application. 3.3.1. The effect of the 2-propanol/ketone molar ratio and the chain length of ketones on CTH Experimental data on the three different methyl ketones are shown in Figs. 4 and 5. An increase in the alcohol/ketone ratio enhances conversion, due to a
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favourable shift in the equilibrium of the redox reaction. Again, it is worth mentioning that the decrease of the ketone amount led to a much slower deactivation of MgO. At the same time, it is also shown that deactivation is not increased by the length of the alkyl chain. 3.3.2. The effect of the modification of MgO In the case of the conversion of 2-butanone, treatment by acetic anhydride causes immediate deactivation due to poisoning of the active sites of MgO. Treatment by triethylamine has hardly any effect on the initial phase of conversion and the reaction rate is only slightly decreased even by repeated additions during the reaction marked on Fig. 6. Treatment of the catalyst before the reaction by carbon tetrachloride had an unexpected effect on the activity (Fig. 6): both, the deactivation of MgO and the dehydration of the alcohols stopped. This effect was especially spectacular in the case of relatively low amount of catalyst (Fig. 2) and at higher HLSV value. When the catalyst was treated with 2-chloro-2-methylpropane under identical conditions, the initial activity considerably lower than without modi®cation; the activity then increased gradually and remained higher than the value obtained on an untreated catalyst.
Fig. 6. Effect of treatment of MgO with organic modifiers on CTH of 2-butanone with 2-propanol (see notes in Fig. 1; modification, see Section 2; ", NEt3 5 ml pulses).
267
4. Discussion In the course of CTH of ketones, some of the activity of MgO is lost. Deactivation is lowest on MgO pretreated at 523 K (Tables 1 and 2), i.e. at the temperature where the concentration of surface OH groups is maximal; deactivation is very high on the catalyst pretreated at 773 K, indicating that the formation of adsorbates poisoning the surface is inhibited by OH groups. The result obtained on MgO, pretreated only for 1 h, points to the same conclusion. The experiment on MgO pretreated in helium reveals that defects in the crystal lattice do not play an important role under the experimental conditions used. According to our results, the optimal reaction temperature for both, initial conversion and activity is 523 K. At 423 K, the strong adsorption of starting materials and products results in a large decrease in initial conversion. At 623 K, the rate of the formation of poisonous surface products is increased in the initial phase, followed by steady conversion at a lower level (which is due to the decrease in the number of active sites). In addition to the effect to be expected (i.e. conversion is increased parallel with the alcohol/ketone molar ratio), variations in the amount of the reactants identify the determinant role of ketones in the poisoning of the surface. It appears from the experiment on MgO modi®ed by triethylamine that acid centers also play some role in CTH. Treatment by chlorinated hydrocarbons has been widely applied in heterogeneous catalysis in order to increase selectivity [18]. Observations on the modifying effect of Clÿ ions on MgO have also been published. Kasper et al. [10] studied CTH of 4-hexen-3one and reported the effect of Clÿ ions on product selectivity. The speci®c surface area of MgO was signi®cantly decreased by the modi®cation, the basicity and the morphology of the catalyst were altered and, consequently, side reactions were minimized. Burch et al. [18] and Moffat et al. [19] investigated the effect of CH2Cl2 and CCl4 on the activity and selectivity of methane oxidative coupling on MgO. The selectivity of ethylene formation increased by chlorinated compounds. Although the role of the modi®ers as yet is unknown, it may be established with certainty that they modify the structure
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of the surface, resulting in the formation of new, catalytically active sites. In the case of CCl4, the chlorinated species on the catalyst was identi®ed as MgCl2 [19]. Our experiments indicate that, in CTH on MgO, CCl4 prevents catalyst poisoning. In our opinion, Clÿ ions are adsorbed on centres responsible for the irreversible adsorption of ketones (probably Lewis acidic centres), thereby inhibiting ketone binding to these sites. In addition to the prevention of deactivation, this assumption is also supported by the total absence of dehydration after CCl4 treatment and by the results obtained on catalysts treated with 2-chloro-2-methylpropane. In the case of the catalyst treated with 2-chloro-2-methylpropane, the low initial conversion is a result of the powerful steric hindrance of the hydrocarbon group. So the participation of basic surface coordinative unsaturated O2ÿ centres is essential for CTH: the deterministic role of basic centres is proven by the result obtained on MgO modi®ed by triethylamine and by acetic anhydride which caused the total inactivity of the catalyst.
CTH is illustrated by the reaction steps shown in Scheme 2, veri®cation of which calls for complex and extensive studies. It would be especially important to identify the nature of the catalytically active sites. It is customary to formulate six-membered transition states (see Fig. 7) formally representing the determinant process, i.e. Hÿ transfer. The following conclusions have been published in the literature on the nature of active sites operating on MgO in CTH: ± ``. . . the substrate is coordinated on a weak acid site while propane-2-ol must be coordinated on an adjacent surface basic site'' [10]. ± ``. . . the basic and one-electron donor sites of MgO surface are responsible for the oxide activity in the CTR. On the other hand, we did not ascertain the necessity of a co-action of acidic surface sites'' [8]. ± ``. . . In the case of a basic oxide, like MgO, coordinative ethanol adsorption is unlikely, the species rather being H-bonded to basic O2ÿ sites, or dissociated on weak Lewis acid±strong base pair site'' [20].
Scheme 2.
Gy. SzoÈlloÈsi, M. BartoÂk / Applied Catalysis A: General 169 (1998) 263±269
269
Fig. 7. Transition states in CTH of ketones.
5. Conclusions
References
The results presented in this paper allowed us to draw the following conclusions: (i) The commercially available MgO suffers a deactivation during CTH of ketones, independent of the pretreatment method used. This deactivation can be mainly attributed to surface species formed from ketones. The deactivation of MgO is not accentuated by the increase of the alkyl chain of methyl alkyl ketones. (ii) Pretreatment of MgO with CCl4 prevented the deactivation of the catalyst during CTH, probably by binding Clÿ to the Lewis acid sites of the surface. As a result, such sites being responsible for the formation of irreversibly bonded surface species, in our experimental conditions, can be blocked. As a consequence, vapour-phase CTH using commercially available MgO may be economically realized. Altogether, the experimental observations have not yet led to the emergence of a uni®ed picture, so our efforts will be focussed in this direction, especially to the modi®cation of magnesia with other organochlorine reagents. These studies will include the titrimetry of MgO with various organochlorine compounds and FTIR, XRD and DSC measurements.
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Acknowledgements We acknowledge the ®nancial support provided for this research by the Hungarian National Science Foundation through Grant OTKA 016109. We also  rpaÂd MolnaÂr for valuable discusthank Professor A sions.