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Al2O3 catalysts for the reduction of benzaldehyde with ethanol
Applied Catalysis A: General 325 (2007) 322–327 www.elsevier.com/locate/apcata
Influence of composition and preparation method on the activity of MnOx/Al2O3 catalysts for the reduction of benzaldehyde with ethanol N. Stamatis a,b, K. Goundani b, J. Vakros b, K. Bourikas b, Ch. Kordulis b,c,* a
Department of Aquaculture and Fisheries, Faculty of Agricultural Technology, Technological Education Institute of Messolonghi, Messolonghi, Greece b Department of Chemistry, University of Patras, GR-265 00 Patras, Greece c Institute of Chemical Engineering & High Temperature Chemical Processes (FORTH/ICE-HT), P.O. Box 1414, GR-265 00 Patras, Greece Received 25 July 2006; accepted 7 February 2007 Available online 3 March 2007
Abstract Two series of MnOx/Al2O3 catalysts with varying Mn loading (0–1.2%, w/w Mn) prepared by equilibrium deposition filtration (EDF) and wet impregnation (WI) methodology were used for studying the influence of the composition and the preparation method on their activity for the reduction of benzaldehyde with ethanol. The prepared catalysts were characterized by BET measurements, X-ray photoelectron spectroscopy and diffuse reflectance UV–vis spectroscopy. It was found that following EDF methodology, the deposition of MnOx species on the alumina takes place via adsorption of [Mn(H2O)6]2+ and [Mn–Ac]+ species on the negatively charged surface sites. On the contrary, following wet impregnation the deposition takes place mainly via precipitation in the step of the solvent evaporation. The extent of the interactions exerted between the support and supported phase is higher in the EDF samples. These interactions created during the impregnation step were detected by DRS after drying and calcination. Higher dispersion of the MnOx phase is achieved when it is deposited on the g-Al2O3 surface by the EDF than the WI method. The catalysts of the first series exhibited higher activity related well with the dispersion of MnOx supported nano-particles. # 2007 Elsevier B.V. All rights reserved. Keywords: MnOx/Al2O3 catalysts; Supported manganese oxide catalysts; Equilibrium deposition filtration; MPVO reaction; Active sites
1. Introduction The oxidation of alcohols into the corresponding carbonyl compounds is undoubtedly one of the indispensable transformations in organic synthesis, and numerous studies have been devoted to the development of a more efficient and milder oxidation method [1]. Oppenauer (OPP) oxidation, the reverse process of the Meerwein–Ponndorf–Verley (MPV) reduction is a classical, yet useful, method and is widely employed for the synthesis of steroids, terpenoids and other important intermediates in the pharmaceutical, fragrance, flavor, and agrochemical industry. The Meerwein–Ponndorf–Verley–Oppenauer (MPVO) process permits the reduction of carbonylic group by alcohols via a hydrogen transfer reaction, usually catalyzed by metal alkoxides such as Al(O-iPR)3 in the homogeneous phase [2].
* Corresponding author at: Department of Chemistry, University of Patras, GR-265 00 Patras, Greece. Fax: +30 2610 994 796. E-mail address: [email protected] (Ch. Kordulis). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.02.044
In spite of the simplicity of the above reaction the alkoxides needed as catalysts must be used in stoichiometric amounts and cannot be regenerated. This leads to large waste streams (corrosive streams in many cases), and causes undesirable side reactions, i.e., dehydration of alcohols and aldol condensations. The use of a solid catalyst can overcome these disadvantages [3]. These catalysts can be separated from the reaction mixture and reused. Moreover, these ‘‘green’’ materials are not harmful to the environment, because they are not corrosive and do not produce corrosive side-products. Recently, various heterogeneous catalysts have been demonstrated to efficiently catalyze MPVO reactions under mild conditions. Acid solids, such as silica [4] and zeolites [5,6], and basic solids such as magnesium oxides [7–9], calcium oxides [10], and double-layered hydroxides [11–13] have been used as catalysts for these reactions. The MPVO reactions on the solid catalyst surfaces seems to proceed via a hydrogen transfer mechanism from an alcohol to carbonyl compound adsorbed in the vicinity of the alcohol. If the hydrogen transfer is assumed to be the reaction
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rate-determining step, then pairs of adjacent acid–base sites will be required to adsorb both reactants [14]. Taking into account the above we studied, in the present work, the catalytic performance for the MPVO reaction of solid catalysts consisted by two oxides, one exposing weak basic surface sites and another one exposing weak acidic sites. Gamma alumina has been selected as a weak base and manganese oxide as a weak acid. Alumina is known as high surface area catalytic support and the deposition of manganese oxide on its surface is expected to result in an increase of population of acid sides decorated by basic sites. Impregnation is the simplest way to deposit a catalytically active phase on a high surface area support. In the last decades, our group has studied and developed an impregnation methodology called ‘‘equilibrium-deposition-filtration’’ (EDF), alternatively called equilibrium adsorption, for depositing such phases on oxidic supports [15–24]. This methodology favors monolayer structures of the deposited phases. In the frame of this work, we prepared two series of manganese oxide supported on alumina catalysts with varying Mn loading using the above-mentioned methodology (EDF) and the conventional wet impregnation (WI) one. Thus, we studied the influence of the preparation method and the composition of the prepared catalysts on their catalytic activity in the MPVO reaction of benzaldehyde with ethanol trying to clarify the role of basic and acidic centers in the activity and the selectivity of these catalysts.
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Table 1 The catalysts prepared, the initial concentration of the impregnating solution used to prepare the EDF samples, the specific surface area (SSA) of the samples and the Mn/(Mn + Al + O) surface atomic ratio determined by XPS Catalyst EDF-0.5 EDF-1.0 EDF-1.2 WI-0.5 WI-1.0 WI-1.2 g-Al2O3 MnOx
C0 (mol dm3) 2
1.75 10 3.5 102 5.5 102 1 102 2 102 2.5 102 – –
SSA (m2 g1)
Mn/(Mn + Al + O)
252 233 213 220 232 219 260 24
– – 1.52 – – 0.94 – –
pressure 40–50 mbar. The resulting solid was then dried and calcined as described above. A sample of unsupported manganese oxide was also prepared by precipitation from an aqueous solution, which had been prepared by mixing two aqueous solutions containing (CH3COO)2Mn and NH4OH at stoichiometric amounts. The precipitate was aged for 24 h in contact with the mother liquids. Then it was filtered and underwent the same thermal treatment with the supported samples. The supported catalysts prepared are symbolized by the abbreviation of the preparation method used (EDF or WI) followed by a number indicating the Mn loading (%, w/w Mn). All the prepared samples are illustrated in Table 1.
2. Experimental
2.2. Characterization of the catalysts
2.1. Preparation of catalysts
2.2.1. Specific surface area measurements Specific surface area (SSA) measurements of the prepared samples have been carried out in a laboratory constructed apparatus by the three-point dynamic BET method. Pure nitrogen and helium (Air Liquide) were used as adsorption and carrier gas, respectively. A thermal conductivity detector of a gas chromatograph (Shimadzu GC 8A) was used to detect the adsorbed amount of nitrogen at a given partial pressure.
Six samples of manganese oxide supported on g-Al2O3 catalysts were prepared using the EDF and the WI method. Following EDF, 5 g of g-Al2O3 (AKZO, Alumina Extrudates, HDS-000-1.5 mm E) crushed and sieved into particles of 90– 150 mm were placed in a 10 L spherical flask immersed in a thermostated bath at 25 1 8C. 4.5 L of dilute manganese acetate ((CH3COO)2Mn4H2O, Alfa, ref. 87336) impregnating solution of suitable concentration was added. NH4NO3 was used for the regulation of ionic strength of this solution at 0.14 M and NH4OH for the regulation of the pH value at pH 7. The suspension was kept under stirring for 24 h and then was filtered using membrane filters (Millipore, 0.22 mm). The resulting solid was dried at 110 8C for 24 h and then it was calcined at 300 8C for 1 h. The Mn content in the prepared samples was determined using atomic absorption spectroscopy (Shimadzu AA 6501). Following WI, an amount of g-Al2O3 equal to about 5 g, accurately weighted, was placed in a 500 mL spherical flask of a rotary evaporator and then 50 mL of impregnating solution were added. The impregnating solution was prepared by dissolving in water a calculated amount of (CH3COO)2Mn4H2O in order the final samples to have the same Mn loading with the corresponding samples prepared by EDF. The suspension was left under rotation for 1 h and then the water was evaporated within 4 h, at temperature 40 8C and
2.2.2. Diffuse reflectance spectroscopy (DRS) The diffuse reflectance spectra of the catalysts were recorded in the range 200–800 nm at room temperature (SBW: 2 nm), using a UV–vis spectrophotometer (Varian Cary 3) equipped with an integration sphere. The Al2O3 carrier, treated with the same way as the corresponding catalyst except of the Mn addition, was used for base line correction and as a reference sample. The samples were mounted in a quartz cell, which provided a sample thickness greater than 3 mm. 2.2.3. X-ray photoelectron spectroscopy (XPS) The XPS analysis was performed at room temperature in UHV chamber (base pressure 8 1010 mbar), which consists of a fast specimen entry assembly, a preparation and an analysis chamber. The residual pressure in the analyzer chamber was below 108 mbar. The latter was equipped with a hemispherical electron energy analyzer (SPECS, LH 10) and a twin-anode Xray gun for XPS. The unmonochromatized Mg Ka line at
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1253.6 eV and a constant pass energy mode for the analyzer were used in the experiments. Pass energies of 36 and 97 eV resulted in a full width at half maximum (FWHM) of 0.9 and 1.6 eV, respectively, for the Ag 3d5/2 peak of a reference foil. The binding energies were calculated with respect to the C 1s peak (C–C, C–H) set at 284.6 eV. 2.3. Catalytic tests The catalytic behavior of the prepared samples was tested in a 10 mL pyrex batch reactor. Six milliliters of the reaction mixture constituted by benzaldehyde/ethanol with a molar ratio 1/20 were placed in the reactor and the temperature was regulated at 78 8C. Then a weighted amount of catalyst was added to obtain a solid to liquid ratio equal to 0.05 g mL1. Reaction mixture aliquots of 1 mL were pooled out and analyzed in a Gas-Chromatograph (Shimadzu GC-8A) equipped with a wide bore fused silica column (25 m 0.53 mm, CP-Sil 5 CB DF = 1.0 mm supplied by Chrompack) and a flame ionization detector. 3. Results and discussion 3.1. Physicochemical characterization As may be seen in Table 1, the loading of the prepared catalysts ranges up to 1.2% (w/w) Mn. Such low loading is expected to result in very small nano-particles of MnOx on the g-Al2O3 surface. The values of the SSAs of the samples, illustrated in the same Table, decrease with the Mn loading. This shows that the deposition of even small amount of MnOx on the g-Al2O3 surface provokes textural changes, which probably are combined with structural ones. However, the values of the Mn/(Mn + Al + O) surface atomic ratios (see Table 1) determined by XPS on the surfaces of the samples with the highest Mn loading show that the Mn surface concentration and thus the dispersion of manganese oxide phase is higher when it is deposited by EDF. The UV–vis/DR spectra of the prepared catalysts are illustrated in Fig. 1 and show a main absorption band centred at
Fig. 2. Diffuse reflectance spectrum of a MnOx/g-Al2O3 mechanical mixture containing 1.2% (w/w) Mn.
ca. 250 nm and a wide band of lower intensity centred at ca. 450 nm which covers almost all the visible range of the spectrum. According to the literature, the first absorption is associated with O2 ! Mn2+ charge transfer transition whereas the latter is attributed to badly resolved absorbance bands (d–d transitions) originated from Mn(III)- and Mn(IV)oxo species [25–28]. Fig. 2 shows that the intensity of the band at ca. 250 nm is negligible in the spectrum of a MnOx/g-Al2O3 mechanical mixture containing 1.2% (w/w) Mn. Thus, we have attributed the enhanced intensity of this band observed in the supported catalysts to the absorption of Mn species interacting with the gAl2O3 surface. Such species are created upon impregnation as it can be concluded from the appearance of the above mentioned absorption band already in the spectra of the impregnated samples after drying (See Fig. 3). The comparison of the corresponding spectra illustrated in Figs. 1 and 3 shows that the intensity of this band increases remarkably after calcination, when the interactions exerted between the supported Mn species and the alumina surface are expected to be strengthened. This finding is in accordance with the literature [25]. Considering the intensity of the band at ca. 250 nm as a measure of the surface concentration of the Mn species
Fig. 1. Diffuse reflectance spectra of the (a) WI and (b) EDF prepared catalysts recorded after calcination.
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than that for the WI ones (mean value 3). This shows that the strong interaction exerted between the MnOx phase and the gAl2O3 surface in the first case stabilizes higher amounts of Mn2+ species on the catalyst surfaces after calcination. 3.2. Deposition mechanism
interacting with the g-Al2O3 surface we can conclude that the concentration of these species increases with the Mn loading. The use of DRS for quantitative analysis of transition metal ions in solid samples is well established [25,29–31]. The validity of this analysis is ensured under certain conditions, described by Weckhuysen and Schoonheydt [29], and adopted in the present study. Fig. 4 shows that in the case of the WI samples this increase is linear. The same linear increase is observed in the case of EDF samples up to 1% (w/w) Mn loading, where a plateau is reached. The appearance of the plateau in the case of EDF samples shows that the Mn-acceptor sites on the g-Al2O3 surface are fully covered when the Mn loading reaches 1% (w/w) Mn and the EDF method is used for its deposition. Such a plateau cannot be attributed to the Kubelka–Munk theory limitations related to the darkness of the samples. Indeed, the colour of our samples was pale brown. Moreover, according to the literature the intensity of the adsorption band of Mn increases linearly with the loading up to 4.5% (w/w) Mn [25]. In contrast to the EDF samples, Mn acceptor sites are not consumed at least up to 1.2% (w/w) Mn loading when the WI method is used. This means that the dispersion of MnOx phase formed in the latter case is lower in agreement to our XPS results (Table 1). This view is also corroborated by the fact that the intensity ratios of the DRS absorbance at ca. 250 nm to that at ca. 450 nm are higher for the EDF samples (mean value 4)
In order to explain the above findings it seems to us reasonable to investigate the mechanism by which EDF increases considerably the Mn dispersion of the final catalysts with respect to that achieved at the WI samples. Let us first study the interfacial region developed between the surface of the alumina particles and the impregnating solution. The Mn2+ ion is rather stable and not easily oxidized in neutral or acid solutions. In a basic solution Mn2+ precipitates in the form of Mn(OH)2. This is the reason for keeping constant the pH of our impregnating solutions at 7 h, during the preparation of the EDF samples. However, in the solution not only Mn2+ but quite a few aqueous species are present, whose relative concentration depends on the total Mn concentration. Taking into account that manganese acetate ((CH3COO)2Mn4H2O) and ammonium nitrate (NH4NO3) were used for the preparation of the impregnating solution, and using the computer program Visual MINTEQ [32], we calculated, at pH 7, the distribution of the total Mn among the various species found in the impregnating solution. The results are presented in Fig. 5, where the total Mn concentration ranges between 102 M and 101 M, in order to include the values of the initial Mn concentration used for the preparation of the EDF samples (see Table 1). According to Fig. 5 the Mn2+ and [Mn–Ac]+ ions (where Ac stands for the acetate) are the predominant species at this pH value. The Mn2+ ion is hydrated in the form [Mn(H2O)6]2+ having an octahedral symmetry [33] whereas in [Mn–Ac]+ a water molecule has been substituted by an acetate ion (CH3COO). Following EDF manganese is deposited on the surface via adsorption of the above mentioned species. However, the surface of the g-Al2O3 used is slightly positively charged at pH 7 (point of zero charge = 7.5) [34]. This means that only a small portion of the surface sites of the alumina is
Fig. 4. Correlation of the intensity of the DRS peak at ca. 250 nm with the % (w/w) Mn content, for the two series of the prepared catalysts after calcination.
Fig. 5. Variation of the concentration of the Mn(II) aqueous species found in the impregnating solution with the total Mn(II) concentration at pH 7.
Fig. 3. Diffuse reflectance spectra of the WI-1.2 and EDF-1.2 catalysts recorded after drying.
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negatively charged (acidic sites) and thus available to the adsorption of the positively charged Mn species. Moreover, the presence of the ligand CH3COO in the complex [Mn–Ac]+ increases its size and consequently each adsorbed [Mn–Ac]+ complex covers a relatively large number of surface sites. The above explain the saturation of the available acceptor sites on the g-Al2O3 surface at the relatively low Mn loading of 1% (w/ w) Mn, when the EDF method is used for its deposition. However, the fact that the deposition of Mn proceeds via adsorption at the EDF sample has a serious advantage. Due to the adsorption mechanism described above the adsorbed manganese species cover uniformly the alumina surface and result to a very good dispersion of the Mn amount. This dispersion remains intact even after drying and calcination because the adsorbed manganese complexes are strongly bounded on the alumina surface in accordance to our DRS results and relevant literature [35,36]. Moreover, the adsorbed [Mn–Ac]+ complexes possibly do not favour Mn polymerization or precipitation which could lead to a lower dispersion of the manganese phase. On the contrary, concerning the samples prepared with the wet impregnation technique, only a small portion of the total manganese is deposited via adsorption. The major amount is deposited on the alumina surface in the step of slow evaporation of the impregnating solution by precipitation. Thus, less strong interactions are exerted between the precipitated phase and the alumina surface in agreement with our DRS findings. This results to lower dispersion of the manganese phase on the final catalysts, in agreement with the XPS results. 3.3. Catalytic behavior The chromatographic analysis of the samples taken from the reactor revealed that, as the concentrations of the two reactants were reduced, the reaction mixture was continuously enriched in acetaldehyde, benzyl alcohol, and cinnamaldehyde. This observation shows that besides the main reaction (1), the only side reaction which takes place is the aldol condensation between the benzaldehyde and the produced acetaldehyde (2). No ethyl ester of benzoic acid was detected indicating that the Tischenko reaction (3) is not favoured over the catalysts studied: CH3 CH2 OH þ C6 H5 CHO ! CH3 CHO þ C6 H5 CH2 OH (1) CH3 CHO þ C6 H5 CHO ! C6 H5 CH ¼ CHCHO þ H2 O
(2)
CH3 CHO þ C6 H5 CHO ! C6 H5 CO2 C2 H5
(3)
However, the relative yield towards benzyl alcohol was found to be greater than 92% in all cases. The activity of the catalysts studied (expressed as benzaldehyde turn over numbers (TONs) measured at reaction time 20 h) is plotted against the Mn loading in Fig. 6. TON is defined as the moles of benzaldehyde converted per square meter of catalytic surface being in the reactor. An inspection of this figure shows that the EDF catalysts were more active than the WI ones, while the EDF catalyst containing 1%
Fig. 6. The activity of the catalysts studied expressed as benzaldehyde turn over numbers (TONs) in correlation with their Mn loading.
(w/w) Mn was the most active one. Moreover, it should be noticed that the deposition of Mn species on the g-Al2O3 surface by WI does not improve the catalytic activity of the support, even at the optimum Mn content (1%, w/w Mn). 3.4. Acid–base properties and activity As stated in Section 1 the MPVO reactions on the solid catalyst surfaces proceed via a hydrogen transfer mechanism from an alcohol to carbonyl compound adsorbed on pairs of adjacent acid–base sites [14]. This mechanism explains the catalytic activity of the support, taking into account that the alumina used is amphoteric or slightly basic (pHpzc 7.5) [34]. Thus, one can expect that it exposes a relatively high number of surface acid–base pairs. The deposition of Mn species by WI, where a relatively low dispersion is achieved, probably destroys a portion of such pairs by covering them with relatively large MnOx particles exhibiting only acidic sites. Indeed, MnOx surface particles expose such acidic sites as it can be concluded taking into account that manganese oxides are relatively acidic (pHpzc 2.3–3.3) [37]. However, on the boundaries of the MnOx particles new acid–base pairs of higher activity can be created. The above described opposite effects could explain the no improvement of the support activity by depositing the Mn species following the WI method. On the contrary, when the EDF method is followed for the preparation of the MnOx/Al2O3 catalysts, the higher dispersion achieved ensures the creation of small MnOx nano-particles on the support surface, which do not destroy the initial acid–base pairs of the support. Indeed, according to the deposition mechanism described above, in the EDF preparation Mn species are deposited on the acidic surface sites of the support replacing thus them by the more acidic sites of MnOx nanoparticles. Fig. 7 illustrates a very interesting correlation found between the activity of the studied manganese oxide containing samples and their absorbance at 250 nm (F(R250)). An inspection of this figure shows that the activity of the samples increases linearly with the intensity of the aforementioned peak independently of the method followed for their preparation (unsupported manganese oxide, WI and EDF samples). Considering this
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Fig. 7. Correlation of the activity of the studied manganese oxide containing samples with the intensity of their DRS peak at ca. 250 nm.
intensity proportional both to the dispersion of MnOx phase and the Mn loading of the samples, as stated above, we can conclude that the activity of the MnOx/Al2O3 catalysts for the reduction of benzaldehyde with ethanol is inversely proportional to the size of MnOx nano-particles formed on the alumina surface and proportional to Mn content. This conclusion is in good agreement with the view that the MPVO reactions on the solid catalyst surfaces proceed via a hydrogen transfer mechanism from an alcohol to carbonyl compound adsorbed on pairs of adjacent acid–base sites [14]. 4. Conclusions The following conclusions can be drawn from the present work: (i) Following EDF methodology the deposition of MnOx species on the alumina takes place via adsorption of [Mn(H2O)6]2+ and [Mn–Ac]+ species on the negatively charged surface sites. On the contrary, following wet impregnation the deposition takes place mainly via precipitation in the step of the solvent evaporation. (ii) The extent of the interactions exerted between the support and supported phase is higher in the EDF samples. These interactions created during the impregnation step were detected by DRS after drying and calcination. (iii) Higher dispersion of the MnOx phase is achieved when it is deposited on the g-Al2O3 surface by the EDF than the WI method. (iv) As the size of the MnOx nano-particles decreases the catalytic activity of the MPVO reaction of benzaldehyde with ethanol increases. References [1] T. Ooi, H. Otsuka, T. Miura, H. Ichikawa, K. Maruoka, Org. Lett. 4 (2002) 2669.
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Influence of composition and preparation method on the activity of MnOx/Al2O3 catalysts for the reduction of benzaldehyde with ethanol N. Stamatis a,b, K. Goundani b, J. Vakros b, K. Bourikas b, Ch. Kordulis b,c,* a
Department of Aquaculture and Fisheries, Faculty of Agricultural Technology, Technological Education Institute of Messolonghi, Messolonghi, Greece b Department of Chemistry, University of Patras, GR-265 00 Patras, Greece c Institute of Chemical Engineering & High Temperature Chemical Processes (FORTH/ICE-HT), P.O. Box 1414, GR-265 00 Patras, Greece Received 25 July 2006; accepted 7 February 2007 Available online 3 March 2007
Abstract Two series of MnOx/Al2O3 catalysts with varying Mn loading (0–1.2%, w/w Mn) prepared by equilibrium deposition filtration (EDF) and wet impregnation (WI) methodology were used for studying the influence of the composition and the preparation method on their activity for the reduction of benzaldehyde with ethanol. The prepared catalysts were characterized by BET measurements, X-ray photoelectron spectroscopy and diffuse reflectance UV–vis spectroscopy. It was found that following EDF methodology, the deposition of MnOx species on the alumina takes place via adsorption of [Mn(H2O)6]2+ and [Mn–Ac]+ species on the negatively charged surface sites. On the contrary, following wet impregnation the deposition takes place mainly via precipitation in the step of the solvent evaporation. The extent of the interactions exerted between the support and supported phase is higher in the EDF samples. These interactions created during the impregnation step were detected by DRS after drying and calcination. Higher dispersion of the MnOx phase is achieved when it is deposited on the g-Al2O3 surface by the EDF than the WI method. The catalysts of the first series exhibited higher activity related well with the dispersion of MnOx supported nano-particles. # 2007 Elsevier B.V. All rights reserved. Keywords: MnOx/Al2O3 catalysts; Supported manganese oxide catalysts; Equilibrium deposition filtration; MPVO reaction; Active sites
1. Introduction The oxidation of alcohols into the corresponding carbonyl compounds is undoubtedly one of the indispensable transformations in organic synthesis, and numerous studies have been devoted to the development of a more efficient and milder oxidation method [1]. Oppenauer (OPP) oxidation, the reverse process of the Meerwein–Ponndorf–Verley (MPV) reduction is a classical, yet useful, method and is widely employed for the synthesis of steroids, terpenoids and other important intermediates in the pharmaceutical, fragrance, flavor, and agrochemical industry. The Meerwein–Ponndorf–Verley–Oppenauer (MPVO) process permits the reduction of carbonylic group by alcohols via a hydrogen transfer reaction, usually catalyzed by metal alkoxides such as Al(O-iPR)3 in the homogeneous phase [2].
* Corresponding author at: Department of Chemistry, University of Patras, GR-265 00 Patras, Greece. Fax: +30 2610 994 796. E-mail address: [email protected] (Ch. Kordulis). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.02.044
In spite of the simplicity of the above reaction the alkoxides needed as catalysts must be used in stoichiometric amounts and cannot be regenerated. This leads to large waste streams (corrosive streams in many cases), and causes undesirable side reactions, i.e., dehydration of alcohols and aldol condensations. The use of a solid catalyst can overcome these disadvantages [3]. These catalysts can be separated from the reaction mixture and reused. Moreover, these ‘‘green’’ materials are not harmful to the environment, because they are not corrosive and do not produce corrosive side-products. Recently, various heterogeneous catalysts have been demonstrated to efficiently catalyze MPVO reactions under mild conditions. Acid solids, such as silica [4] and zeolites [5,6], and basic solids such as magnesium oxides [7–9], calcium oxides [10], and double-layered hydroxides [11–13] have been used as catalysts for these reactions. The MPVO reactions on the solid catalyst surfaces seems to proceed via a hydrogen transfer mechanism from an alcohol to carbonyl compound adsorbed in the vicinity of the alcohol. If the hydrogen transfer is assumed to be the reaction
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rate-determining step, then pairs of adjacent acid–base sites will be required to adsorb both reactants [14]. Taking into account the above we studied, in the present work, the catalytic performance for the MPVO reaction of solid catalysts consisted by two oxides, one exposing weak basic surface sites and another one exposing weak acidic sites. Gamma alumina has been selected as a weak base and manganese oxide as a weak acid. Alumina is known as high surface area catalytic support and the deposition of manganese oxide on its surface is expected to result in an increase of population of acid sides decorated by basic sites. Impregnation is the simplest way to deposit a catalytically active phase on a high surface area support. In the last decades, our group has studied and developed an impregnation methodology called ‘‘equilibrium-deposition-filtration’’ (EDF), alternatively called equilibrium adsorption, for depositing such phases on oxidic supports [15–24]. This methodology favors monolayer structures of the deposited phases. In the frame of this work, we prepared two series of manganese oxide supported on alumina catalysts with varying Mn loading using the above-mentioned methodology (EDF) and the conventional wet impregnation (WI) one. Thus, we studied the influence of the preparation method and the composition of the prepared catalysts on their catalytic activity in the MPVO reaction of benzaldehyde with ethanol trying to clarify the role of basic and acidic centers in the activity and the selectivity of these catalysts.
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Table 1 The catalysts prepared, the initial concentration of the impregnating solution used to prepare the EDF samples, the specific surface area (SSA) of the samples and the Mn/(Mn + Al + O) surface atomic ratio determined by XPS Catalyst EDF-0.5 EDF-1.0 EDF-1.2 WI-0.5 WI-1.0 WI-1.2 g-Al2O3 MnOx
C0 (mol dm3) 2
1.75 10 3.5 102 5.5 102 1 102 2 102 2.5 102 – –
SSA (m2 g1)
Mn/(Mn + Al + O)
252 233 213 220 232 219 260 24
– – 1.52 – – 0.94 – –
pressure 40–50 mbar. The resulting solid was then dried and calcined as described above. A sample of unsupported manganese oxide was also prepared by precipitation from an aqueous solution, which had been prepared by mixing two aqueous solutions containing (CH3COO)2Mn and NH4OH at stoichiometric amounts. The precipitate was aged for 24 h in contact with the mother liquids. Then it was filtered and underwent the same thermal treatment with the supported samples. The supported catalysts prepared are symbolized by the abbreviation of the preparation method used (EDF or WI) followed by a number indicating the Mn loading (%, w/w Mn). All the prepared samples are illustrated in Table 1.
2. Experimental
2.2. Characterization of the catalysts
2.1. Preparation of catalysts
2.2.1. Specific surface area measurements Specific surface area (SSA) measurements of the prepared samples have been carried out in a laboratory constructed apparatus by the three-point dynamic BET method. Pure nitrogen and helium (Air Liquide) were used as adsorption and carrier gas, respectively. A thermal conductivity detector of a gas chromatograph (Shimadzu GC 8A) was used to detect the adsorbed amount of nitrogen at a given partial pressure.
Six samples of manganese oxide supported on g-Al2O3 catalysts were prepared using the EDF and the WI method. Following EDF, 5 g of g-Al2O3 (AKZO, Alumina Extrudates, HDS-000-1.5 mm E) crushed and sieved into particles of 90– 150 mm were placed in a 10 L spherical flask immersed in a thermostated bath at 25 1 8C. 4.5 L of dilute manganese acetate ((CH3COO)2Mn4H2O, Alfa, ref. 87336) impregnating solution of suitable concentration was added. NH4NO3 was used for the regulation of ionic strength of this solution at 0.14 M and NH4OH for the regulation of the pH value at pH 7. The suspension was kept under stirring for 24 h and then was filtered using membrane filters (Millipore, 0.22 mm). The resulting solid was dried at 110 8C for 24 h and then it was calcined at 300 8C for 1 h. The Mn content in the prepared samples was determined using atomic absorption spectroscopy (Shimadzu AA 6501). Following WI, an amount of g-Al2O3 equal to about 5 g, accurately weighted, was placed in a 500 mL spherical flask of a rotary evaporator and then 50 mL of impregnating solution were added. The impregnating solution was prepared by dissolving in water a calculated amount of (CH3COO)2Mn4H2O in order the final samples to have the same Mn loading with the corresponding samples prepared by EDF. The suspension was left under rotation for 1 h and then the water was evaporated within 4 h, at temperature 40 8C and
2.2.2. Diffuse reflectance spectroscopy (DRS) The diffuse reflectance spectra of the catalysts were recorded in the range 200–800 nm at room temperature (SBW: 2 nm), using a UV–vis spectrophotometer (Varian Cary 3) equipped with an integration sphere. The Al2O3 carrier, treated with the same way as the corresponding catalyst except of the Mn addition, was used for base line correction and as a reference sample. The samples were mounted in a quartz cell, which provided a sample thickness greater than 3 mm. 2.2.3. X-ray photoelectron spectroscopy (XPS) The XPS analysis was performed at room temperature in UHV chamber (base pressure 8 1010 mbar), which consists of a fast specimen entry assembly, a preparation and an analysis chamber. The residual pressure in the analyzer chamber was below 108 mbar. The latter was equipped with a hemispherical electron energy analyzer (SPECS, LH 10) and a twin-anode Xray gun for XPS. The unmonochromatized Mg Ka line at
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1253.6 eV and a constant pass energy mode for the analyzer were used in the experiments. Pass energies of 36 and 97 eV resulted in a full width at half maximum (FWHM) of 0.9 and 1.6 eV, respectively, for the Ag 3d5/2 peak of a reference foil. The binding energies were calculated with respect to the C 1s peak (C–C, C–H) set at 284.6 eV. 2.3. Catalytic tests The catalytic behavior of the prepared samples was tested in a 10 mL pyrex batch reactor. Six milliliters of the reaction mixture constituted by benzaldehyde/ethanol with a molar ratio 1/20 were placed in the reactor and the temperature was regulated at 78 8C. Then a weighted amount of catalyst was added to obtain a solid to liquid ratio equal to 0.05 g mL1. Reaction mixture aliquots of 1 mL were pooled out and analyzed in a Gas-Chromatograph (Shimadzu GC-8A) equipped with a wide bore fused silica column (25 m 0.53 mm, CP-Sil 5 CB DF = 1.0 mm supplied by Chrompack) and a flame ionization detector. 3. Results and discussion 3.1. Physicochemical characterization As may be seen in Table 1, the loading of the prepared catalysts ranges up to 1.2% (w/w) Mn. Such low loading is expected to result in very small nano-particles of MnOx on the g-Al2O3 surface. The values of the SSAs of the samples, illustrated in the same Table, decrease with the Mn loading. This shows that the deposition of even small amount of MnOx on the g-Al2O3 surface provokes textural changes, which probably are combined with structural ones. However, the values of the Mn/(Mn + Al + O) surface atomic ratios (see Table 1) determined by XPS on the surfaces of the samples with the highest Mn loading show that the Mn surface concentration and thus the dispersion of manganese oxide phase is higher when it is deposited by EDF. The UV–vis/DR spectra of the prepared catalysts are illustrated in Fig. 1 and show a main absorption band centred at
Fig. 2. Diffuse reflectance spectrum of a MnOx/g-Al2O3 mechanical mixture containing 1.2% (w/w) Mn.
ca. 250 nm and a wide band of lower intensity centred at ca. 450 nm which covers almost all the visible range of the spectrum. According to the literature, the first absorption is associated with O2 ! Mn2+ charge transfer transition whereas the latter is attributed to badly resolved absorbance bands (d–d transitions) originated from Mn(III)- and Mn(IV)oxo species [25–28]. Fig. 2 shows that the intensity of the band at ca. 250 nm is negligible in the spectrum of a MnOx/g-Al2O3 mechanical mixture containing 1.2% (w/w) Mn. Thus, we have attributed the enhanced intensity of this band observed in the supported catalysts to the absorption of Mn species interacting with the gAl2O3 surface. Such species are created upon impregnation as it can be concluded from the appearance of the above mentioned absorption band already in the spectra of the impregnated samples after drying (See Fig. 3). The comparison of the corresponding spectra illustrated in Figs. 1 and 3 shows that the intensity of this band increases remarkably after calcination, when the interactions exerted between the supported Mn species and the alumina surface are expected to be strengthened. This finding is in accordance with the literature [25]. Considering the intensity of the band at ca. 250 nm as a measure of the surface concentration of the Mn species
Fig. 1. Diffuse reflectance spectra of the (a) WI and (b) EDF prepared catalysts recorded after calcination.
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than that for the WI ones (mean value 3). This shows that the strong interaction exerted between the MnOx phase and the gAl2O3 surface in the first case stabilizes higher amounts of Mn2+ species on the catalyst surfaces after calcination. 3.2. Deposition mechanism
interacting with the g-Al2O3 surface we can conclude that the concentration of these species increases with the Mn loading. The use of DRS for quantitative analysis of transition metal ions in solid samples is well established [25,29–31]. The validity of this analysis is ensured under certain conditions, described by Weckhuysen and Schoonheydt [29], and adopted in the present study. Fig. 4 shows that in the case of the WI samples this increase is linear. The same linear increase is observed in the case of EDF samples up to 1% (w/w) Mn loading, where a plateau is reached. The appearance of the plateau in the case of EDF samples shows that the Mn-acceptor sites on the g-Al2O3 surface are fully covered when the Mn loading reaches 1% (w/w) Mn and the EDF method is used for its deposition. Such a plateau cannot be attributed to the Kubelka–Munk theory limitations related to the darkness of the samples. Indeed, the colour of our samples was pale brown. Moreover, according to the literature the intensity of the adsorption band of Mn increases linearly with the loading up to 4.5% (w/w) Mn [25]. In contrast to the EDF samples, Mn acceptor sites are not consumed at least up to 1.2% (w/w) Mn loading when the WI method is used. This means that the dispersion of MnOx phase formed in the latter case is lower in agreement to our XPS results (Table 1). This view is also corroborated by the fact that the intensity ratios of the DRS absorbance at ca. 250 nm to that at ca. 450 nm are higher for the EDF samples (mean value 4)
In order to explain the above findings it seems to us reasonable to investigate the mechanism by which EDF increases considerably the Mn dispersion of the final catalysts with respect to that achieved at the WI samples. Let us first study the interfacial region developed between the surface of the alumina particles and the impregnating solution. The Mn2+ ion is rather stable and not easily oxidized in neutral or acid solutions. In a basic solution Mn2+ precipitates in the form of Mn(OH)2. This is the reason for keeping constant the pH of our impregnating solutions at 7 h, during the preparation of the EDF samples. However, in the solution not only Mn2+ but quite a few aqueous species are present, whose relative concentration depends on the total Mn concentration. Taking into account that manganese acetate ((CH3COO)2Mn4H2O) and ammonium nitrate (NH4NO3) were used for the preparation of the impregnating solution, and using the computer program Visual MINTEQ [32], we calculated, at pH 7, the distribution of the total Mn among the various species found in the impregnating solution. The results are presented in Fig. 5, where the total Mn concentration ranges between 102 M and 101 M, in order to include the values of the initial Mn concentration used for the preparation of the EDF samples (see Table 1). According to Fig. 5 the Mn2+ and [Mn–Ac]+ ions (where Ac stands for the acetate) are the predominant species at this pH value. The Mn2+ ion is hydrated in the form [Mn(H2O)6]2+ having an octahedral symmetry [33] whereas in [Mn–Ac]+ a water molecule has been substituted by an acetate ion (CH3COO). Following EDF manganese is deposited on the surface via adsorption of the above mentioned species. However, the surface of the g-Al2O3 used is slightly positively charged at pH 7 (point of zero charge = 7.5) [34]. This means that only a small portion of the surface sites of the alumina is
Fig. 4. Correlation of the intensity of the DRS peak at ca. 250 nm with the % (w/w) Mn content, for the two series of the prepared catalysts after calcination.
Fig. 5. Variation of the concentration of the Mn(II) aqueous species found in the impregnating solution with the total Mn(II) concentration at pH 7.
Fig. 3. Diffuse reflectance spectra of the WI-1.2 and EDF-1.2 catalysts recorded after drying.
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negatively charged (acidic sites) and thus available to the adsorption of the positively charged Mn species. Moreover, the presence of the ligand CH3COO in the complex [Mn–Ac]+ increases its size and consequently each adsorbed [Mn–Ac]+ complex covers a relatively large number of surface sites. The above explain the saturation of the available acceptor sites on the g-Al2O3 surface at the relatively low Mn loading of 1% (w/ w) Mn, when the EDF method is used for its deposition. However, the fact that the deposition of Mn proceeds via adsorption at the EDF sample has a serious advantage. Due to the adsorption mechanism described above the adsorbed manganese species cover uniformly the alumina surface and result to a very good dispersion of the Mn amount. This dispersion remains intact even after drying and calcination because the adsorbed manganese complexes are strongly bounded on the alumina surface in accordance to our DRS results and relevant literature [35,36]. Moreover, the adsorbed [Mn–Ac]+ complexes possibly do not favour Mn polymerization or precipitation which could lead to a lower dispersion of the manganese phase. On the contrary, concerning the samples prepared with the wet impregnation technique, only a small portion of the total manganese is deposited via adsorption. The major amount is deposited on the alumina surface in the step of slow evaporation of the impregnating solution by precipitation. Thus, less strong interactions are exerted between the precipitated phase and the alumina surface in agreement with our DRS findings. This results to lower dispersion of the manganese phase on the final catalysts, in agreement with the XPS results. 3.3. Catalytic behavior The chromatographic analysis of the samples taken from the reactor revealed that, as the concentrations of the two reactants were reduced, the reaction mixture was continuously enriched in acetaldehyde, benzyl alcohol, and cinnamaldehyde. This observation shows that besides the main reaction (1), the only side reaction which takes place is the aldol condensation between the benzaldehyde and the produced acetaldehyde (2). No ethyl ester of benzoic acid was detected indicating that the Tischenko reaction (3) is not favoured over the catalysts studied: CH3 CH2 OH þ C6 H5 CHO ! CH3 CHO þ C6 H5 CH2 OH (1) CH3 CHO þ C6 H5 CHO ! C6 H5 CH ¼ CHCHO þ H2 O
(2)
CH3 CHO þ C6 H5 CHO ! C6 H5 CO2 C2 H5
(3)
However, the relative yield towards benzyl alcohol was found to be greater than 92% in all cases. The activity of the catalysts studied (expressed as benzaldehyde turn over numbers (TONs) measured at reaction time 20 h) is plotted against the Mn loading in Fig. 6. TON is defined as the moles of benzaldehyde converted per square meter of catalytic surface being in the reactor. An inspection of this figure shows that the EDF catalysts were more active than the WI ones, while the EDF catalyst containing 1%
Fig. 6. The activity of the catalysts studied expressed as benzaldehyde turn over numbers (TONs) in correlation with their Mn loading.
(w/w) Mn was the most active one. Moreover, it should be noticed that the deposition of Mn species on the g-Al2O3 surface by WI does not improve the catalytic activity of the support, even at the optimum Mn content (1%, w/w Mn). 3.4. Acid–base properties and activity As stated in Section 1 the MPVO reactions on the solid catalyst surfaces proceed via a hydrogen transfer mechanism from an alcohol to carbonyl compound adsorbed on pairs of adjacent acid–base sites [14]. This mechanism explains the catalytic activity of the support, taking into account that the alumina used is amphoteric or slightly basic (pHpzc 7.5) [34]. Thus, one can expect that it exposes a relatively high number of surface acid–base pairs. The deposition of Mn species by WI, where a relatively low dispersion is achieved, probably destroys a portion of such pairs by covering them with relatively large MnOx particles exhibiting only acidic sites. Indeed, MnOx surface particles expose such acidic sites as it can be concluded taking into account that manganese oxides are relatively acidic (pHpzc 2.3–3.3) [37]. However, on the boundaries of the MnOx particles new acid–base pairs of higher activity can be created. The above described opposite effects could explain the no improvement of the support activity by depositing the Mn species following the WI method. On the contrary, when the EDF method is followed for the preparation of the MnOx/Al2O3 catalysts, the higher dispersion achieved ensures the creation of small MnOx nano-particles on the support surface, which do not destroy the initial acid–base pairs of the support. Indeed, according to the deposition mechanism described above, in the EDF preparation Mn species are deposited on the acidic surface sites of the support replacing thus them by the more acidic sites of MnOx nanoparticles. Fig. 7 illustrates a very interesting correlation found between the activity of the studied manganese oxide containing samples and their absorbance at 250 nm (F(R250)). An inspection of this figure shows that the activity of the samples increases linearly with the intensity of the aforementioned peak independently of the method followed for their preparation (unsupported manganese oxide, WI and EDF samples). Considering this
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Fig. 7. Correlation of the activity of the studied manganese oxide containing samples with the intensity of their DRS peak at ca. 250 nm.
intensity proportional both to the dispersion of MnOx phase and the Mn loading of the samples, as stated above, we can conclude that the activity of the MnOx/Al2O3 catalysts for the reduction of benzaldehyde with ethanol is inversely proportional to the size of MnOx nano-particles formed on the alumina surface and proportional to Mn content. This conclusion is in good agreement with the view that the MPVO reactions on the solid catalyst surfaces proceed via a hydrogen transfer mechanism from an alcohol to carbonyl compound adsorbed on pairs of adjacent acid–base sites [14]. 4. Conclusions The following conclusions can be drawn from the present work: (i) Following EDF methodology the deposition of MnOx species on the alumina takes place via adsorption of [Mn(H2O)6]2+ and [Mn–Ac]+ species on the negatively charged surface sites. On the contrary, following wet impregnation the deposition takes place mainly via precipitation in the step of the solvent evaporation. (ii) The extent of the interactions exerted between the support and supported phase is higher in the EDF samples. These interactions created during the impregnation step were detected by DRS after drying and calcination. (iii) Higher dispersion of the MnOx phase is achieved when it is deposited on the g-Al2O3 surface by the EDF than the WI method. (iv) As the size of the MnOx nano-particles decreases the catalytic activity of the MPVO reaction of benzaldehyde with ethanol increases. References [1] T. Ooi, H. Otsuka, T. Miura, H. Ichikawa, K. Maruoka, Org. Lett. 4 (2002) 2669.
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