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Activity of Basic Catalysts in the Meerwein–Ponndorf–Verley Reaction of Benzaldehyde with Ethanol
Journal of Colloid and Interface Science 238, 385–389 (2001) doi:10.1006/jcis.2001.7519, available online at http://www.idealibrary.com on
Activity of Basic Catalysts in the Meerwein–Ponndorf–Verley Reaction of Benzaldehyde with Ethanol Mar´ıa A. Aramend´ıa, Victoriano Borau, C´esar Jim´enez, Jos´e M. Marinas, Jos´e R. Ruiz,1 and Francisco J. Urbano Departamento de Qu´ımica Org´anica,Universidad de C´ordoba, Campus de Rabanales,Edificio C-3, Ctra. Nacional IV-A, km 396, E-14014 C´ordoba, Spain E-mail: [email protected] Received December 19, 2000; revised February 27, 2001
EXPERIMENTAL The Meerwein–Ponndorf–Verley (MPV) reaction of benzaldehyde with ethanol in the liquid phase in the presence of basic catalysts consisting of magnesium oxide, calcium oxide, and mixed oxides obtained by calcination of layered double hydroxides, was studied. The catalysts were characterized using various techniques including X-ray diffraction and gas adsorption (viz nitrogen physisorption to determine textural properties and carbon dioxide chemisorption to elucidate surface basic properties). The catalyst consisting of calcium oxide, which was that possessing the highest density of basic sites, was found to be the most active in the process; the MPV reaction was accompanied by two other, competing reactions (viz aldol condensation and the Tishchenko cross-reaction). The MPV reaction of benzaldehyde with other alcohols was also examined, the highest conversion being obtained with secondary alcohols as hydrogen sources. °C 2001 Academic Press
Preparation of Catalysts The magnesium oxide catalyst was prepared by calcining commercially available Mg(OH)2 at 600◦ C in the air for 2 h. The solid thus obtained possessed a very low specific surface area, so it was rehydrated in refluxing water for 6 h. The new resulting solid was air-dried and calcined at 600◦ C to obtain the catalyst named MgO-600 (10). The calcium oxide catalyst was obtained by calcining commercially available calcium hydroxide at 900◦ C in the air for 24 h and was named CaO-900. The mixed solids CaO/Al2 O3 , MgO/Al2 O3 , and MgO/Ga2 O3 were prepared by calcining their corresponding precursors (layered double hydroxides, LDHs) at 500◦ C. The precursors were in turn obtained by using a conventional coprecipitation method previously developed by our group (4, 9).
INTRODUCTION
Aldehydes and ketones have long been known to be effectively reduced by metal alkoxides and alcohols. This process, which essentially involves a hydrogen transfer, is known as the Meerwein–Ponndorf–Verley (MPV) reaction. This reduction reaction is highly selective, so much so that it leaves C=C double bonds untouched. In an MPV reaction, a secondary alcohol acts as a hydrogen donor and the carbonyl group of the substrate as a hydrogen acceptor. Usually, the process is conducted in the presence of a homogeneous catalyst such as a metal alkoxide. Recently, however, heterogeneous catalysts proved to effectively catalyze the MPV reaction under mild conditions. The heterogeneous catalysts used for this purpose include acid solids such as silica (1) and zeolites (2, 3), and basic solids (particularly magnesium oxides (4–7)). These heterogenous systems have the advantage over alkoxides that they avoid the need for separation and reuse. This kind of solid is being used by our Research Group on different base-catalysed reactions such as the reduction of citral (8) or the limonene epoxidation (9). 1
To whom correspondence should be addressed.
Characterization of Catalysts X-ray diffraction (XRD) patterns were recorded on a Siemens D-500 diffractometer using CuKα radiation. Scans were performed over the 2θ range from 5◦ to 80◦ . The textural properties of the solids were established from nitrogen adsorption– desorption isotherms at liquid nitrogen temperature that were recorded on a Micromeritics ASAP 2010 instrument. Specific surface areas were determined using the BET method (11). The amount of CO2 chemisorbed by each solid was measured on a Micromeritics 2900 TPD/TPR analyser. Prior to analysis, samples were heated at 500◦ C in an argon stream for 1 h. Measurements were made at room temperature by alternately passing argon and the same gas containing CO2 over the sample; the amount of chemisorbed CO2 was calculated as the difference between the first adsorption peak (physisorbed plus chemisorbed CO2 ) and the arithmetic mean of the adsorption and desorption peaks. Basicity was assessed under the assumption that one molecule of CO2 was adsorbed at a single basic site.
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0021-9797/01 $35.00
C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
ARAMEND´IA ET AL.
386
TABLE 1 Textural Properties of the Catalysts Studied
Catalytic Transfer Hydrogenation Catalytic hydrogen transfer runs were conducted under refluxing conditions in a two-mouthed flask containing 0.003 mol of benzaldehyde, 0.06 mol of alcohol (the donor-to-acceptor ratio was thus 20), and 1 g of freshly calcined catalyst. One of the flask mouths was fitted with a reflux condenser and the other was used for sampling at regular intervals. The system was shaken throughout the process. Products were identified from their retention times and by GC-MS analysis on an HP 5890 GC instrument furnished with a 30 m ×0.32 mm Supelcowax 10 column and an HP 5971A MSD instrument. RESULTS AND DISCUSSION
Characterization of Catalysts Figure 1 shows the XRD patterns for the catalysts studied (HTCa/Al-500 excluded as it exhibited an amorphous structure). As can be seen, the two solids obtained by calcining Mg/Al and Mg/Ga LDHs (Figs. 1a and 1b) contained periclase MgO as the sole crystalline phase, in addition to an aluminum- or gallium-based amorphous phase (12). Solid MgO-600 (Fig. 1c) also exhibited a periclase phase but was more crystalline than the previous ones as its bands were much sharper and narrower.
Catalyst
SBET (m2 /g)a
Va (ml/g)b
HT-Mg/Al-500 HT-Ga/Al-500 HT-Ca/Al-500 MgO-600 CaO-900
143.5 116.9 23.1 110.8 5.3
0.69 0.61 0.12 0.32 0.04
rp (A)c 107.7 147.0 206.9 75.4 371.6
a
Specific surface area. Pore volume. c Mean pore radius. b
Finally, solid CaO-900 exhibited an XRD pattern consistent with that for calcium oxide. Table 1 summarizes the textural properties of the catalysts. Of the three based on mixed oxides, solid HT-Mg/Al-500 had the highest specific surface area. The solid containing CaO/Al2 O3 possessed a much lower area. On the other hand, solid MgO-600 exhibited an area similar to those of the MgO/M2 O3 systems and CaO-900 possessed a rather low one (only 5.3 m2 /g). Finally, the five solids studied were of the mesoporous type as judged from their mean pore radii. Figure 2 shows the CO2 TPD profiles recorded with a view to determining the surface chemical properties of the solids. In all cases, CO2 was desorbed at a temperature below 300◦ C. This desorption method is frequently used to estimate the number of basic sites in a solid and their strength. Such strength is dependent on the desorption temperature—the higher the temperature is the greater is the strength. Also, the area under the desorption curve provides a measure for the amount of basic sites. The desorption profiles for the five solids studied exhibited three peaks: one at a low temperature (60–130◦ C), another at a medium temperature (150–230◦ C), and the third at a higher level (260–300◦ C). The number, proportion, and density of basic sites thus calculated are shown in Table 2. The profile for MgO600 (Fig. 2d) was quite consistent with previously reported data for this oxide (13), with three desorption bands peaking at 128, TABLE 2 Number, Proportion, and Density of Basic Sites in the Catalysts Studied Number of total Dba Proportion of basic sites(%) basic sites (µmol Catalizador (µmol CO2 /g) 60–130◦ C 150–230◦ C 260–300◦ C CO2 /m2 ) HT-Mg/Al -500 HT-Mg/Ga -500 HT-Ca/Al -500 MgO-600 CaO-900
FIG. 1. XRD patterns for the catalysts (a) HT-Mg/Al-500, (b) HT-Mg/Ga500, (c) MgO-600, and (d) CaO-900.
a
330
30.6
23.9
45.6
2.30
182
19.2
34.1
46.7
1.56
12
58.3
33.3
8.4
0.52
257 89
14.8 16.9
38.5 30.3
46.7 53.1
2.32 17.10
Density of basic sites.
387
THE MEERWEIN–PONNDORF–VERLEY REACTION
SCHEME 1. MPV reaction studied.
FIG. 2. TPD profiles for CO2 adsorption for the catalysts (a) HT-Mg/Al500, (b) HT-Mg/Ga-500, (c) HT-Ca/Al-500, (d) MgO-600, and (e) CaO-900.
171, and 264◦ C (the third, which corresponded to the most basic sites, was the strongest). The results for the mixed oxides containing Mg (Figs. 2a and 2b) were similar to those reported by L´opez-Salinas et al. (14); the strongest peak was also that observed at the highest temperature, so, as in MgO-600, the strongest basic sites predominate over those of medium and low strength. The two calcium-containing solids (HT-Ca/Al-500 and CaO-900) possess much smaller numbers of basic sites than the other catalysts; CaO-900 is similar to Mg-600 in that it abounds with strong basic sites, whereas CaO/Al2 O3 contains weak sites predominantly. As can be seen from Table 2, the density of basic sites of Ca-900 is greater than those of the other catalysts by a factor of least 7. Catalytic Activity Catalytic transfer hydrogenations take place as depicted in Scheme 1. During the reaction, ethanol is oxidized to acetaldehyde and benzaldehyde is reduced to benzyl alcohol. Table 3
shows the catalytic activity of the solids, and the selectivity, as calculated at a total benzaldehyde conversion above 90% in all instances. From the results it follows that CaO-900 is the most active catalyst, followed by MgO-600, HT- Mg/Al-500, and HTMg/Ga-500. The CaO/Al2 O3 -based solid is inactive under the reaction conditions used. A comparison of the catalytic activity results with the basic site densities (Table 2) reveals an identical sequence for all catalysts: the most active one is that possessing the highest density of basic sites. Table 3 shows the turnover frecuency (TOF) obtained by dividing catalytic activity (ra ) by the basic density (Db ). These results indicate that the MgOcontaining catalysts present similar turnover. On the other hand, the different TOF value corresponding to the CaO catalyst could be explained by differences in the structure and composition of the solid in comparison to the MgO-containing catalysts. Based on the conversion results, the reduction reaction competes with two other processes, viz (a) aldol condensation between benzaldehyde and acetaldehyde resulting from the dehydrogenation of ethanol, which yields cinnamaldehyde, and (b) a Tishchenko cross-reaction between benzaldehyde and acetaldehyde that gives ethyl benzoate. Scheme 2 illustrates the overall process. The conversion to benzyl alcohol was close to or even greater than 70% in all cases, aldol condensation prevailing over the other competing reaction, with a conversion of 20–30%. Based on these results, aldol condensation will inevitably occur to some extent under the reaction conditions used. On the other hand, the Tishchenko cross-reaction will only take place
TABLE 3 Catalytic Activity of the Solids Studied in the Reaction of Benzaldehyde with Ethanola Selectivity d (%) Catalyst HT-Mg/Al -500 HT-Mg/Ga -500 HT-Ca/Al -500 MgO-600 CaO-900
ra b (×103 )
TOFc
Ethyl benzoate
Benzyl alcohol
Cinnamaldehyde
9.7
4.22
—
68.1
29.9
7.5
4.81
—
76.9
23.1
—
—
—
—
—
10.0 40.0
4.31 2.38
0.7 6.1
78.2 70.3
21.1 23.6
a Reaction conditions: 0.003 mol of benzaldehyde; 0.06 mol of ethanol; T = 78◦ C; 1 g of catalyst. b mmol benzaldehyde used · g−1 cat · min−1 . c Turnover frecuency. d Conversi´ on of benzaldehyde >90%.
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SCHEME 2. Overall process studied.
with solid CaO-900, which is the most active; this suggests that the cross-reaction requires stronger basic sites than the other processes. These catalytic activity results can be explained in the light of a mechanism similar to that proposed by Ivanov et al. (15) (see Scheme 3), where the hydrogen transfer is a concerted process that takes place via a six-link intermediate formed between ethanol and benzaldehyde. The rate-determining step of the process must be related to the interaction of the alcohol with acid–base sites, which causes its dissociation to the corresponding alkoxide. Carbonyl compounds have been found to interact with acid and basic sites in a solid to give condensation reactions, as have alcohols with metal acid sites to yield olefins. In the proposed scheme, the surface-adsorbed alkoxide formed from the alcohol transfers a hydride ion that attacks the carbonyl group. Once solid CaO-900 was found to be the most active catalyst in the MPV process, the hydrogen transfer reaction was studied by using alternative primary and secondary alcohols with this catalyst. Table 4 shows the total conversion of benzaldehyde after 10 h of reaction, as well as the partial conversions to the different products obtained. The ester column in the table refers to the product of the Tishchenko cross-reaction and the aldehyde column to the product of the aldol condensation between ben-
zaldehyde and the aldehyde resulting from the dehydrogenation of the alcohol used as hydrogen donor. From the data in Table 4 it follows that methanol also gives the MPV reaction. However, it yields no aldol condensation product; rather, it gives benzyl alcohol and a high proportion of methyl benzoate in relation to the other alcohols studied. This is logical as formaldehyde resulting from the dehydrogenation of methanol possesses no α hydrogen with respect to the carbonyl group, so it can never undergo aldol condensation. With other primary alcohols such as 1-propanol and 1-butanol, however, the total conversion with respect to ethanol is similar, whereas that to the reduction product (benzyl alcohol) decreases markedly in favor of that in the Tishchenko cross-reaction, probably as a result of the polar species involved in this reaction being stabilized by the longer chain of the alcohol. With secondary alcohols, the total conversion is virtually 100% and the hydrogen transfer predominates by virtue of the ease with which secondary alcohols can be dehydrogenated under the reaction conditions used.
TABLE 4 Total and Partial Conversions in the MPV Reaction of Benzaldehyde with Various Alcoholsa Selectivity Alcohol Methanol Ethanol 1-Propanol 1-Butanol 2-Propanol 2-Butanol
SCHEME 3.
Mechanism of the MPV reaction studied.
χtb
Ester
Benzyl alcohol
Aldehyde
51.1 78.1 79.9 75.2 100.0 98.7
22.6 6.1 13.0 19.0 — —
77.4 70.3 57.4 58.6 92.7 90.3
— 23.6 29.6 22.4 6.3 9.7
a Reaction conditions: 0.003 mol of benzaldehyde; 0.06 mol of alcohol; refluxing T; 1 g of CaO-900. b χ , total conversion of benzaldehyde (%) at 10 h of reaction. t
THE MEERWEIN–PONNDORF–VERLEY REACTION
ACKNOWLEDGMENTS The authors express their gratitude to Spain’s Direcci´on General de Ense˜nanza Superior e Investigaci´on del Ministerio de Educaci´on y Cultura (Project PB970446) and to the Consejer´ıa de Educaci´on y Ciencia de la Junta de Andaluc´ıa for funding this work.
REFERENCES 1. Quignard, F., Graziani, O., and Choplin, A., Appl. Catal. A: General 182, 29 (1999). 2. van der Waal, J. C., Kunkeler, P. J., Tan, K., and van Bekkum, H., J. Catal. 173, 74 (1998). 3. Jansen, J. C., Creyghton, E. J., Njo, S. L., van Koningsveld, H., and van Bekkum, H., Catal. Today 38, 205 (1997). 4. Aramend´ıa, M. A., Borau, V., Jim´enez, C., Marinas, J. M., Ruiz, J. R., and Urbano, F. J., Mat. Lett. 46, 309 (2000). 5. Sz¨oll¨osi, G., and Bartok, M., J. Mol. Struct. 482, 13 (1999). 6. Sz¨oll¨osi, G., and Bartok, M., J. Mol. Catal. A: Chem. 148, 265 (1999).
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7. Kumbhar, P. S., Sanchez-Valente, J., Lopez, J., and Figueras, F., Chem. Commum. 535 (1998). 8. Aramend´ıa, M. A., Borau, V., Jim´enez, C., Marinas, J. M., Ruiz, J. R., and Urbano, F. J., Appl. Catal. A: General 206, 95 (2001). 9. Aramend´ıa, M. A., Borau, V., Jim´enez, C., Luque, J. M., Marinas, J. M., Ruiz, J. R., and Urbano, F. J., Stud. Surf. Sci. Catal. 130, 1667 (2000). 10. Aramend´ıa, M. A., Ben´ıtez, J. A., Borau, V., Jim´enez, C., Marinas, J. M., Ruiz, J. R., and Urbano, F. J., Langmuir 15, 1192 (1999). 11. Brunauer, E. P., Emmet, P. H., and Teller, E. J., J. Am. Chem. Soc. 60, 73 (1951). 12. Aramend´ıa, M. A., Avil´es, Y., Borau, V., Marinas, J. M., Ruiz, J. R., and Urbano, F. J., J. Mater. Chem. 9, 1603 (1999). 13. Di Cosimo, J. I., Diez, V. K., Xu, M., Iglesia, E., and Apestegu´ıa, C. R., J. Catal. 178, 499 (1998). 14. L´opez-Salinas, E., Garc´ıa-S´anchez, M., Llanos-Serrano, M. E., and Navarrete-Bol´an˜ oz, J., J. Phys. Chem. B 101, 5112 (1997). 15. Ivanov, V. A., Bachelier, J., Audry, F., and Lavalley, J. C., J. Mol. Catal. 91, 45 (1994).
Activity of Basic Catalysts in the Meerwein–Ponndorf–Verley Reaction of Benzaldehyde with Ethanol Mar´ıa A. Aramend´ıa, Victoriano Borau, C´esar Jim´enez, Jos´e M. Marinas, Jos´e R. Ruiz,1 and Francisco J. Urbano Departamento de Qu´ımica Org´anica,Universidad de C´ordoba, Campus de Rabanales,Edificio C-3, Ctra. Nacional IV-A, km 396, E-14014 C´ordoba, Spain E-mail: [email protected] Received December 19, 2000; revised February 27, 2001
EXPERIMENTAL The Meerwein–Ponndorf–Verley (MPV) reaction of benzaldehyde with ethanol in the liquid phase in the presence of basic catalysts consisting of magnesium oxide, calcium oxide, and mixed oxides obtained by calcination of layered double hydroxides, was studied. The catalysts were characterized using various techniques including X-ray diffraction and gas adsorption (viz nitrogen physisorption to determine textural properties and carbon dioxide chemisorption to elucidate surface basic properties). The catalyst consisting of calcium oxide, which was that possessing the highest density of basic sites, was found to be the most active in the process; the MPV reaction was accompanied by two other, competing reactions (viz aldol condensation and the Tishchenko cross-reaction). The MPV reaction of benzaldehyde with other alcohols was also examined, the highest conversion being obtained with secondary alcohols as hydrogen sources. °C 2001 Academic Press
Preparation of Catalysts The magnesium oxide catalyst was prepared by calcining commercially available Mg(OH)2 at 600◦ C in the air for 2 h. The solid thus obtained possessed a very low specific surface area, so it was rehydrated in refluxing water for 6 h. The new resulting solid was air-dried and calcined at 600◦ C to obtain the catalyst named MgO-600 (10). The calcium oxide catalyst was obtained by calcining commercially available calcium hydroxide at 900◦ C in the air for 24 h and was named CaO-900. The mixed solids CaO/Al2 O3 , MgO/Al2 O3 , and MgO/Ga2 O3 were prepared by calcining their corresponding precursors (layered double hydroxides, LDHs) at 500◦ C. The precursors were in turn obtained by using a conventional coprecipitation method previously developed by our group (4, 9).
INTRODUCTION
Aldehydes and ketones have long been known to be effectively reduced by metal alkoxides and alcohols. This process, which essentially involves a hydrogen transfer, is known as the Meerwein–Ponndorf–Verley (MPV) reaction. This reduction reaction is highly selective, so much so that it leaves C=C double bonds untouched. In an MPV reaction, a secondary alcohol acts as a hydrogen donor and the carbonyl group of the substrate as a hydrogen acceptor. Usually, the process is conducted in the presence of a homogeneous catalyst such as a metal alkoxide. Recently, however, heterogeneous catalysts proved to effectively catalyze the MPV reaction under mild conditions. The heterogeneous catalysts used for this purpose include acid solids such as silica (1) and zeolites (2, 3), and basic solids (particularly magnesium oxides (4–7)). These heterogenous systems have the advantage over alkoxides that they avoid the need for separation and reuse. This kind of solid is being used by our Research Group on different base-catalysed reactions such as the reduction of citral (8) or the limonene epoxidation (9). 1
To whom correspondence should be addressed.
Characterization of Catalysts X-ray diffraction (XRD) patterns were recorded on a Siemens D-500 diffractometer using CuKα radiation. Scans were performed over the 2θ range from 5◦ to 80◦ . The textural properties of the solids were established from nitrogen adsorption– desorption isotherms at liquid nitrogen temperature that were recorded on a Micromeritics ASAP 2010 instrument. Specific surface areas were determined using the BET method (11). The amount of CO2 chemisorbed by each solid was measured on a Micromeritics 2900 TPD/TPR analyser. Prior to analysis, samples were heated at 500◦ C in an argon stream for 1 h. Measurements were made at room temperature by alternately passing argon and the same gas containing CO2 over the sample; the amount of chemisorbed CO2 was calculated as the difference between the first adsorption peak (physisorbed plus chemisorbed CO2 ) and the arithmetic mean of the adsorption and desorption peaks. Basicity was assessed under the assumption that one molecule of CO2 was adsorbed at a single basic site.
385
0021-9797/01 $35.00
C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
ARAMEND´IA ET AL.
386
TABLE 1 Textural Properties of the Catalysts Studied
Catalytic Transfer Hydrogenation Catalytic hydrogen transfer runs were conducted under refluxing conditions in a two-mouthed flask containing 0.003 mol of benzaldehyde, 0.06 mol of alcohol (the donor-to-acceptor ratio was thus 20), and 1 g of freshly calcined catalyst. One of the flask mouths was fitted with a reflux condenser and the other was used for sampling at regular intervals. The system was shaken throughout the process. Products were identified from their retention times and by GC-MS analysis on an HP 5890 GC instrument furnished with a 30 m ×0.32 mm Supelcowax 10 column and an HP 5971A MSD instrument. RESULTS AND DISCUSSION
Characterization of Catalysts Figure 1 shows the XRD patterns for the catalysts studied (HTCa/Al-500 excluded as it exhibited an amorphous structure). As can be seen, the two solids obtained by calcining Mg/Al and Mg/Ga LDHs (Figs. 1a and 1b) contained periclase MgO as the sole crystalline phase, in addition to an aluminum- or gallium-based amorphous phase (12). Solid MgO-600 (Fig. 1c) also exhibited a periclase phase but was more crystalline than the previous ones as its bands were much sharper and narrower.
Catalyst
SBET (m2 /g)a
Va (ml/g)b
HT-Mg/Al-500 HT-Ga/Al-500 HT-Ca/Al-500 MgO-600 CaO-900
143.5 116.9 23.1 110.8 5.3
0.69 0.61 0.12 0.32 0.04
rp (A)c 107.7 147.0 206.9 75.4 371.6
a
Specific surface area. Pore volume. c Mean pore radius. b
Finally, solid CaO-900 exhibited an XRD pattern consistent with that for calcium oxide. Table 1 summarizes the textural properties of the catalysts. Of the three based on mixed oxides, solid HT-Mg/Al-500 had the highest specific surface area. The solid containing CaO/Al2 O3 possessed a much lower area. On the other hand, solid MgO-600 exhibited an area similar to those of the MgO/M2 O3 systems and CaO-900 possessed a rather low one (only 5.3 m2 /g). Finally, the five solids studied were of the mesoporous type as judged from their mean pore radii. Figure 2 shows the CO2 TPD profiles recorded with a view to determining the surface chemical properties of the solids. In all cases, CO2 was desorbed at a temperature below 300◦ C. This desorption method is frequently used to estimate the number of basic sites in a solid and their strength. Such strength is dependent on the desorption temperature—the higher the temperature is the greater is the strength. Also, the area under the desorption curve provides a measure for the amount of basic sites. The desorption profiles for the five solids studied exhibited three peaks: one at a low temperature (60–130◦ C), another at a medium temperature (150–230◦ C), and the third at a higher level (260–300◦ C). The number, proportion, and density of basic sites thus calculated are shown in Table 2. The profile for MgO600 (Fig. 2d) was quite consistent with previously reported data for this oxide (13), with three desorption bands peaking at 128, TABLE 2 Number, Proportion, and Density of Basic Sites in the Catalysts Studied Number of total Dba Proportion of basic sites(%) basic sites (µmol Catalizador (µmol CO2 /g) 60–130◦ C 150–230◦ C 260–300◦ C CO2 /m2 ) HT-Mg/Al -500 HT-Mg/Ga -500 HT-Ca/Al -500 MgO-600 CaO-900
FIG. 1. XRD patterns for the catalysts (a) HT-Mg/Al-500, (b) HT-Mg/Ga500, (c) MgO-600, and (d) CaO-900.
a
330
30.6
23.9
45.6
2.30
182
19.2
34.1
46.7
1.56
12
58.3
33.3
8.4
0.52
257 89
14.8 16.9
38.5 30.3
46.7 53.1
2.32 17.10
Density of basic sites.
387
THE MEERWEIN–PONNDORF–VERLEY REACTION
SCHEME 1. MPV reaction studied.
FIG. 2. TPD profiles for CO2 adsorption for the catalysts (a) HT-Mg/Al500, (b) HT-Mg/Ga-500, (c) HT-Ca/Al-500, (d) MgO-600, and (e) CaO-900.
171, and 264◦ C (the third, which corresponded to the most basic sites, was the strongest). The results for the mixed oxides containing Mg (Figs. 2a and 2b) were similar to those reported by L´opez-Salinas et al. (14); the strongest peak was also that observed at the highest temperature, so, as in MgO-600, the strongest basic sites predominate over those of medium and low strength. The two calcium-containing solids (HT-Ca/Al-500 and CaO-900) possess much smaller numbers of basic sites than the other catalysts; CaO-900 is similar to Mg-600 in that it abounds with strong basic sites, whereas CaO/Al2 O3 contains weak sites predominantly. As can be seen from Table 2, the density of basic sites of Ca-900 is greater than those of the other catalysts by a factor of least 7. Catalytic Activity Catalytic transfer hydrogenations take place as depicted in Scheme 1. During the reaction, ethanol is oxidized to acetaldehyde and benzaldehyde is reduced to benzyl alcohol. Table 3
shows the catalytic activity of the solids, and the selectivity, as calculated at a total benzaldehyde conversion above 90% in all instances. From the results it follows that CaO-900 is the most active catalyst, followed by MgO-600, HT- Mg/Al-500, and HTMg/Ga-500. The CaO/Al2 O3 -based solid is inactive under the reaction conditions used. A comparison of the catalytic activity results with the basic site densities (Table 2) reveals an identical sequence for all catalysts: the most active one is that possessing the highest density of basic sites. Table 3 shows the turnover frecuency (TOF) obtained by dividing catalytic activity (ra ) by the basic density (Db ). These results indicate that the MgOcontaining catalysts present similar turnover. On the other hand, the different TOF value corresponding to the CaO catalyst could be explained by differences in the structure and composition of the solid in comparison to the MgO-containing catalysts. Based on the conversion results, the reduction reaction competes with two other processes, viz (a) aldol condensation between benzaldehyde and acetaldehyde resulting from the dehydrogenation of ethanol, which yields cinnamaldehyde, and (b) a Tishchenko cross-reaction between benzaldehyde and acetaldehyde that gives ethyl benzoate. Scheme 2 illustrates the overall process. The conversion to benzyl alcohol was close to or even greater than 70% in all cases, aldol condensation prevailing over the other competing reaction, with a conversion of 20–30%. Based on these results, aldol condensation will inevitably occur to some extent under the reaction conditions used. On the other hand, the Tishchenko cross-reaction will only take place
TABLE 3 Catalytic Activity of the Solids Studied in the Reaction of Benzaldehyde with Ethanola Selectivity d (%) Catalyst HT-Mg/Al -500 HT-Mg/Ga -500 HT-Ca/Al -500 MgO-600 CaO-900
ra b (×103 )
TOFc
Ethyl benzoate
Benzyl alcohol
Cinnamaldehyde
9.7
4.22
—
68.1
29.9
7.5
4.81
—
76.9
23.1
—
—
—
—
—
10.0 40.0
4.31 2.38
0.7 6.1
78.2 70.3
21.1 23.6
a Reaction conditions: 0.003 mol of benzaldehyde; 0.06 mol of ethanol; T = 78◦ C; 1 g of catalyst. b mmol benzaldehyde used · g−1 cat · min−1 . c Turnover frecuency. d Conversi´ on of benzaldehyde >90%.
ARAMEND´IA ET AL.
388
SCHEME 2. Overall process studied.
with solid CaO-900, which is the most active; this suggests that the cross-reaction requires stronger basic sites than the other processes. These catalytic activity results can be explained in the light of a mechanism similar to that proposed by Ivanov et al. (15) (see Scheme 3), where the hydrogen transfer is a concerted process that takes place via a six-link intermediate formed between ethanol and benzaldehyde. The rate-determining step of the process must be related to the interaction of the alcohol with acid–base sites, which causes its dissociation to the corresponding alkoxide. Carbonyl compounds have been found to interact with acid and basic sites in a solid to give condensation reactions, as have alcohols with metal acid sites to yield olefins. In the proposed scheme, the surface-adsorbed alkoxide formed from the alcohol transfers a hydride ion that attacks the carbonyl group. Once solid CaO-900 was found to be the most active catalyst in the MPV process, the hydrogen transfer reaction was studied by using alternative primary and secondary alcohols with this catalyst. Table 4 shows the total conversion of benzaldehyde after 10 h of reaction, as well as the partial conversions to the different products obtained. The ester column in the table refers to the product of the Tishchenko cross-reaction and the aldehyde column to the product of the aldol condensation between ben-
zaldehyde and the aldehyde resulting from the dehydrogenation of the alcohol used as hydrogen donor. From the data in Table 4 it follows that methanol also gives the MPV reaction. However, it yields no aldol condensation product; rather, it gives benzyl alcohol and a high proportion of methyl benzoate in relation to the other alcohols studied. This is logical as formaldehyde resulting from the dehydrogenation of methanol possesses no α hydrogen with respect to the carbonyl group, so it can never undergo aldol condensation. With other primary alcohols such as 1-propanol and 1-butanol, however, the total conversion with respect to ethanol is similar, whereas that to the reduction product (benzyl alcohol) decreases markedly in favor of that in the Tishchenko cross-reaction, probably as a result of the polar species involved in this reaction being stabilized by the longer chain of the alcohol. With secondary alcohols, the total conversion is virtually 100% and the hydrogen transfer predominates by virtue of the ease with which secondary alcohols can be dehydrogenated under the reaction conditions used.
TABLE 4 Total and Partial Conversions in the MPV Reaction of Benzaldehyde with Various Alcoholsa Selectivity Alcohol Methanol Ethanol 1-Propanol 1-Butanol 2-Propanol 2-Butanol
SCHEME 3.
Mechanism of the MPV reaction studied.
χtb
Ester
Benzyl alcohol
Aldehyde
51.1 78.1 79.9 75.2 100.0 98.7
22.6 6.1 13.0 19.0 — —
77.4 70.3 57.4 58.6 92.7 90.3
— 23.6 29.6 22.4 6.3 9.7
a Reaction conditions: 0.003 mol of benzaldehyde; 0.06 mol of alcohol; refluxing T; 1 g of CaO-900. b χ , total conversion of benzaldehyde (%) at 10 h of reaction. t
THE MEERWEIN–PONNDORF–VERLEY REACTION
ACKNOWLEDGMENTS The authors express their gratitude to Spain’s Direcci´on General de Ense˜nanza Superior e Investigaci´on del Ministerio de Educaci´on y Cultura (Project PB970446) and to the Consejer´ıa de Educaci´on y Ciencia de la Junta de Andaluc´ıa for funding this work.
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