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Mg2AlO hydrotalcite catalysts
Applied Catalysis A: General 381 (2010) 209–215
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Selective hydrogenation of crotonaldehyde in liquid-phase over Au/Mg2 AlO hydrotalcite catalysts Hsing-Yu Chen a , Ching-Tu Chang b , Shu-Jen Chiang a , Biing-Jye Liaw c,∗ , Yin-Zu Chen a,∗ a b c
Department of Chemical and Materials Engineering, National Central University, Jhongli 32001, Taiwan, ROC Institute of Nuclear Energy Research Atomic Energy Council, Longtan 32546, Taiwan, ROC Department of Chemical and Materials Engineering, Graduate School of Materials Applied Technology, Nanya Institute of Technology, Jhongli 32091, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 23 September 2009 Received in revised form 26 March 2010 Accepted 7 April 2010 Available online 14 April 2010 Keywords: Gold catalysts Hydrotalcite Promoters Selective hydrogenation Crotonaldehyde
a b s t r a c t The liquid-phase selective hydrogenation of crotonaldehyde to crotyl alcohol over Au supported on a solid base of Mg2 AlO-hydrotalcite (Mg/Al = 2) was investigated. The 2% Au/Mg2 AlO catalysts were prepared using a modified deposition precipitation method without adjusting the pH of the initial HAuCl4 solution. The influence of the calcination temperatures of the Mg2 AlO support and of the 2% Au/Mg2 AlO catalyst, and the effects of the reaction medium (solvent), temperature, pressure and concentration were studied. The promoter, Fe, Mo or W, was co-deposited with Au on Mg2 AlO to maximize the yield of crotyl alcohol. 2% Au/Mg2 AlO was compared with Au supported on FeOOH, Fe2 O3 , CeO2 , TiO2 and Al2 O3 . The correlation between the gold states (Au3+ /Au0 ) and the activity and selectivity of crotyl alcohol was extensively examined. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The selective hydrogenation of ␣,-unsaturated aldehydes with a conjugated C C/C O group to the corresponding unsaturated alcohols is not only of commercial interest in the intermediate production of fine chemicals [1] but also of special scientific interest. Thermodynamics and kinetics favor the hydrogenation of C C over the C O bond on conventional hydrogenation catalysts that are based on Pt, Ru, Pd or Ni [2,3]. Nevertheless, designing catalysts for the preferred hydrogenation of the conjugated C O group is challenging. Researchers have used platinum doped with promoters (such as Fe and Sn) [3–6] or supported on easily reducible oxides (SMSI effect) [7–9], or bimetallic catalysts [10], to enhance the hydrogenation of the conjugated C O bond. Several groups have recently reported that gold-supported catalysts exhibit remarkable selectivity toward the hydrogenation of the conjugated C O bond in ␣,-unsaturated aldehydes and ketones [1,11–13]. In their studies, iron oxide (FeOOH, ␥-Fe2 O3 and ␣-Fe2 O3 ), TiO2 , Al2 O3, SiO2 , ZnO and ZrO2 were used as supports for the gold catalysts. Such a support affects the reactivity and selectivity of gold. Milone et al. [12] demonstrated Au supported on FeOOH is the most active and selective catalyst to the hydrogenation of the conjugated C O bond. Bus et al. [1] compared Au/Al2 O3 with Pt/Al2 O3 , and showed that
∗ Corresponding authors. Tel.: +886 3 4227151; fax: +886 3 4252296. E-mail addresses: [email protected] (B.-J. Liaw), [email protected] (Y.-Z. Chen). 0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.04.009
Au/Al2 O3 is much more active and selective than Pt/Al2 O3 in the selective hydrogenation of C O bond in cinnamaldehyde, whereas Au/Al2 O3 is less active by two orders of magnitude than Pt/Al2 O3 in the hydrogenation of the C C bond in cyclohexene. Gold has attracted much attention in the recent decade as a catalyst, since Haruta et al. reported that gold particles with a nanosize of less than 5 nm exhibit very high activity in the oxidation of CO at room temperature [13]. Most attention has been focused on oxidation reactions [14–16], and only a few work on selective hydrogenation on gold catalysts have been published [17,18]. The deposition precipitation (DP) method, developed by Haruta et al. [13], has been widely adopted to prepare gold catalysts, and the salt HAuCl4 is most often used as a precursor [19]. The equilibrium species of chloro-hydroxy complexes ([AuClx (OH)4−x ]− , x = 1–4) depend on the pH of the gold solution [20,21]. In the DP method, the pH of the initial HAuCl4 solution is adjusted to 7, and the complex [AuCl(OH)3 ]− is the major species, which is predominantly deposited on the support to yield an active gold catalyst [22]. In this method, the iso-electric point (IEP) of the support is critical to the deposition of gold species. Oxides with an IEP of approximately 7, including CeO2 (IEP = 6.75) [23], Fe2 O3 (IEP = 6.5–6.9) [13], TiO2 (IEP = 6), and ZrO2 (IEP = 6.7) [24], have been extensively used as supports for obtaining an active catalyst; acidic supports such as SiO2 (IEP = 1–2) [25] or basic supports such as MgAlO hydrotalcite (IEP = 10) [26] and MgO (IEP = 12) [27] are not typically used. MgO is well known for the stabilization of nano-sized particles of gold on it [28]. The Mgx AlO hydrotalcites, consisting of positively charged brucite-like Mg(OH)2 layers separated by interlayer
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anions and water molecules [29], are generally used as catalysts or support precursors [30,31]. No suitable method for obtaining an active gold catalyst supported on MgO or Mgx AlO hydrotalcite [26,32] had been proposed before our previous studies [33–35]. Chang et al. [33] used a modified DP method, in which Mgx AlO suspended in water was added to the gold solution without preadjusting the pH of the initial solution, to obtain an active catalyst of Au/Mg2 AlO for the oxidation of CO and the selective hydrogenation of ␣,-unsaturated aldehydes [34]. You et al. [35] found that high yields of unsaturated alcohol (85% from cinnamaldehyde and 95% from citral) were obtained over Au/Mg2 AlO. They also found that the activity of cinnamaldehyde hydrogenation and its selectivity to unsaturated alcohol increased with the Au3+ /Au0 ratio of the catalyst. Among many examples of selective hydrogenation of ␣,unsaturated aldehydes, the hydrogenation of crotonaldehyde to crotyl alcohol is considered to be particularly difficult because the hindrance of the substituent at the C C bond is weak. In the hydrogenation of crotonaldehyde, according to Okumura et al. [36], the selectivity toward crotyl alcohol was in the range 10–25% for Au/TiO2 , Au/Al2 O3 and Au/SiO2 . Claus and co-workers [37–39], working with Au/SiO2, Au/TiO2, Au/ZrO2 , Au/ZnO, and Au-In/ZnO, concluded that the support determines the morphology of the gold particles and suggested that the edges of gold crystallites are active sites for C O hydrogenation. Campo et al. [40,41] reported that the Au/CeO2 catalyst is highly selective toward crotyl alcohol (78% selectivity) in the gas-phase hydrogenation of crotonaldehyde, but unselective in the liquid-phase hydrogenation (< 10% selectivity). Of the gold catalysts of the hydrogenation of ␣,-unsaturated aldehydes that have been identified in the literature, Au/Mg2 AlO seems to be the most active. In this study, 2% Au/Mg2 AlO catalysts for the selective hydrogenation of crotonaldehyde in liquid phase were investigated. The effects of the calcination temperatures of the support and of the catalyst were examined. The effects of the reaction parameters, including the solvent used as the reaction medium, temperature, pressure and concentration were studied. Promoters of the catalyst were studied to maximize the yield of crotyl alcohol. 2% Au/Mg2 AlO was compared with gold supported on FeOOH, Fe2 O3 , CeO2 , TiO2 and Al2 O3 for the hydrogenation of crotonaldehyde. The correlation between gold states (Au3+ /Au0 ) and the activity/selectivity of the gold catalysts to crotyl alcohol were examined extensively.
dispersed well in 100 ml of water, and then poured into the gold solution slowly at a rate of about 3 ml/min for deposition precipitation at 70 ◦ C. The obtained catalyst precursor was aged at 70 ◦ C for 3 h, and then washed and dried overnight at 100 ◦ C. The catalysts were calcined at various temperatures (100–300 ◦ C) for 4 h. Moreover, an extra metal additive (Fe, Mo, W) was introduced by dissolving its precursor salt in the solution in which Mg2 AlO was suspended. Then, the solution was well stirred for 3 h before mixing with the gold solution. 2.2. Characterization of catalysts The specific surface areas (SBET ) of samples were determined by nitrogen adsorption with a Micromeritics ASAP-2020 apparatus at −196 ◦ C following degassing at 100 ◦ C. The compositions of samples were analyzed by inductively coupled plasma analysis (ICP) using a JOBIN JY-24 device. X-ray diffraction (XRD) patterns were obtained using a Siemens-500 diffractometer with Cu K␣ radiation ( = 0.1542 nm). Transmission electron microscopy (TEM) photographs were taken using a JEOL JEM-2999FMII apparatus. Samples for TEM analysis were prepared by gently grinding a catalyst into powder in a mortar; the synthesized powder was ultrasonically dispersed in ethanol. The solution was deposited onto a carbon-coated Cu mesh grid using a pipette; the solvent was then allowed to evaporate naturally. Mean particle diameters were determined by counting ∼150 particles in enlarged photographs. X-ray photoelectron spectroscopy (XPS) measurements were made using a Thermo VG Scientific Sigma Prob spectrophotometer with Al K␣ radiation (1486.6 eV). The nanoparticles were firstly pressed into a 10-mm diameter disk; these disks were fixed onto the sample holders, and immediately transferred to the pretreatment chamber. In the chamber, each sample was degassed overnight at 1 × 10−6 Torr to remove the volatile contaminants and was then transferred to the analyzing chamber for XPS analysis. The spectra were obtained using an analyzer pass energy of 25.5 eV and an electron take-off angle of 45◦ . The vacuum in the test chamber was maintained at below 1.33 × 10−8 Torr during the collection. Binding energies were corrected for surface charging by referencing them to the energy of the C 1s peak of the contaminant carbon at 284.6 eV. 2.3. Selective hydrogenation
2. Experimental 2.1. Preparation of support and catalyst Hydrotalcite supports of Mg2 AlO (Mg/Al molar ratio = 2) were prepared by co-precipitation of an aqueous solution of magnesium and aluminum salts with a highly basic carbonate solution. The salt solution (1 M) contained Mg(NO3 )2 ·6H2 O and Al(NO3 )3 ·9H2 O dissolved in deionized water at various Mg/Al molar ratios (x = 1–4). The basic solution contained equal volumes of KOH and K2 CO3 , with molar ratios of CO3 2− /(Al + Mg) = 0.67 and OH− /(Al + Mg) = 2.25. These two solutions were mixed at a rate of 60 ml/h, maintained at a constant pH of about 10, and then aged overnight with stirring. The white precipitate was washed, dried at 100 ◦ C, and then calcined in air at various temperatures (100–300 ◦ C). The catalysts except Au/Mg2 AlO were prepared using the DP method that was developed by Haruta et al. [13]. The Au/Mg2 AlO catalysts were prepared using a modified DP method [33]. In the conventional DP method, the pH of the initial HAuCl4 solution (pHi 4–9) was firstly adjusted by adding a 0.5 M NaCO3 solution. In the modified DP method, the pH of about 2 of the initial solution (pHi 2) was not adjusted. The hydrotalcite support of Mg2 AlO was firstly
A fresh, as-prepared catalyst was used for each reaction run. In some cases, the as-prepared catalyst was calcined before the reaction. The hydrogenation reactions were performed in a magnetically stirred autoclave (160 ml, Parr-4842) under a constant total pressure of 929 kPa (120 psig); in the autoclave pure hydrogen was continuously supplied to compensate for the consumed hydrogen to maintain the total pressure. Notably, the amount of catalyst was sufficiently small and the stirring rate was sufficiently high to minimize the mass transfer resistance. The crotonaldehyde was hydrogenated in ethanol or other solvents with a 2% Au/Mg2 AlO hydrotalcite catalyst with reactant/ethanol/catalyst = 2 ml/78 ml/0.5 g. The hydrogenated samples were analyzed using a gas chromatograph with a SUPELCOWAXTM 10 capillary column of 30 m × 0.25 mm × 0.25 m film thickness attached to a flame ionization detector. 3. Results and discussion 3.1. Effect of calcination temperature of Mg2 AlO and catalyst Fig. 1 displays the XRD spectra of the Mg2 AlO calcined at 100, 200 and 300 ◦ C. The Mg2 AlO exhibited the characteristic diffraction
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Table 1 Effect of calcination temperature of support on the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO. Ta (◦ C)
SBET b (m2 g−1 )
Au loadingb (wt%)
dAu b (nm)
Au3+ /Au0
100 200 300
71.6 70.1 236.4
1.20 1.05 1.07
3.6 3.7 3.8
1.20 1.08 0.75
b
Activityc (mmol gcat −1 min−1 )
Conversiond (%)
0.096 0.091 0.086
23.6 22.4 21.3
Selectivityd (%) UOL
SAL
SOL
62.1 56.4 48.9
35.2 36.9 44.2
2.7 6.7 6.9
a
Calcination temperature of Mg2 AlO. Data from Ref. [35]. c The average rate for 2 h of reaction time. d Reaction conditions: T = 120 ◦ C; P = 929 kPa; crotonaldehyde/cyclohexane/catalyst = 2 ml/78 ml/0.5 g; reaction time = 2 h; UOL, unsaturated alcohol of crotyl alcohol; SAL, saturated aldehyde of butyraldehyde; SOL, saturated alcohol of 1-butanol. b
Table 2 Effect of calcination temperature of catalyst on the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO. Ta (◦ C)
SBET (m2 g−1 )
dAu (nm)
Au3+ /Au0
100 200 300
84.6 83.6 97.1
3.6 3.8 3.9
1.20 0.89 0.00
b
Activityc (mmol gcat −1 min−1 )
Conversiond (%)
0.096 0.052 0.015
23.6 12.9 3.8
Selectivityd (%) UOL
SAL
SOL
62.1 44.3 37.0
35.2 51.5 61.2
2.7 4.2 1.8
a
Calcination temperature of Au/Mg2 AlO. Data of XPS results from Ref. [35]. c The average rate for 2 h of reaction time. d Reaction conditions: T = 120 ◦ C; P = 929 kPa; crotonaldehyde/cyclohexane/catalyst = 2 ml/78 ml/0.5 g; reaction time = 2 h; UOL, crotyl alcohol; SAL, butyraldehyde; SOL, 1-butanol. b
pattern of the laminated hydrotalcite at 2 = 11.02◦ , 23.55◦ , 34.84◦ , 39.03◦ , 46.29◦ , 60.96◦ , 62.26◦ and 66.29◦ . The Mg2 AlO hydrotalcite maintained its layered structure even when it was calcined up to 300 ◦ C. In the previous study on 2% Au/Mg2 AlO [35], You et al. found that the calcination temperatures of the support and the catalyst determine the activity and selectivity in the liquid-phase hydrogenation of cinnamaldehyde. These effects of the calcination temperatures were examined again in the hydrogenation of crotonaldehyde, and the results are shown in Tables 1 and 2. All 2% Au/Mg2 AlO catalysts in Table 1 were calcined at 100 ◦ C. The catalyst with Mg2 AlO calcined at 100 ◦ C was the most active one and a high calcination temperature of Mg2 AlO was associated with a low average rate. The basicity of Mg2 AlO increased with the calcination temperature, affecting the pH of the gold solution during DP and the state of gold on the catalysts [33]. The surface areas of Mg2 AlO hydrotalcites increased with the calcination temperature from about 70 m2 /g (100 and 200 ◦ C) to
236 m2 /g (300 ◦ C). The actual loadings and particle sizes of gold for 2% Au/Mg2 AlO catalysts with Mg2 AlO calcined at 100, 200 and 300 ◦ C were approximately the same; thus, the Au3+ /Au0 ratio dominated the performance of the catalysts. Both the activity and the selectivity of crotyl alcohol (UOL) decreased as the calcination temperature increased. Therefore, the activity and selectivity of crotyl alcohol increased as the Au3+ /Au0 ratio increased. This correlation between activity/selectivity and Au3+ /Au0 ratio over 2% Au/Mg2 AlO was suggested in the general liquid-phase hydrogenation of ␣,unsaturated aldehydes. This correlation has not yet been proposed over other gold catalysts. The as-prepared 2% Au/Mg2 AlO (with Mg2 AlO calcined at 100 ◦ C) catalysts were calcined at 100, 200 and 300 ◦ C prior to reactions. The surface areas of the corresponding catalysts (85, 84 and 97 m2 /g) did not increase obviously as Mg2 AlO support was calcined at various temperatures and the TEM analysis revealed no significant agglomeration of gold particles (Table 2). The Au3+ /Au0 ratio was about 1.2 for the 2% Au/Mg2 Al catalyst calcined at 100 ◦ C, and decreased to 0.9 for the catalyst calcined at 200 ◦ C. When the calcination temperature reached 300 ◦ C, Au3+ was completely reduced to Au0 . As presented in Table 2, the activity of 2% Au/Mg2 AlO declined sharply as the calcination temperature increased above 200 ◦ C, such a result was consistent with the disappearance of the oxidized state of gold (Au3+ ) as the calcination temperature exceeded 200 ◦ C. The selectivity of crotyl alcohol also decreased as the calcination temperature increased; such relationship was associated with a decrease in the Au3+ /Au0 ratio. To clarify the correlation between activity/selectivity and the Au3+ /Au0 ratio for general gold catalysts, the final section discusses the effect of the support on the hydrogenation of crotonaldehyde.
3.2. Effect of reaction solvent
Fig. 1. XRD spectra of Mg2 AlO calcined at 100, 200 and 300 ◦ C.
Catalytic hydrogenation is often carried out in a solvent for various reasons, such as to dissolve solid reactants and products, to absorb the exothermic heat of the reaction, and to keep the surface of the catalyst free from impurities and carbonaceous deposits. The nature of the solvent has a considerable effect on the rate and/or
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Table 3 Effect of solvent on the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO. Solvent
Activitya (mmol gcat −1 min−1 )
Conversionb (%)
1-Propanol (C3 -OH) 1-Pentanol (C5 -OH) 2-Propanol (i-C3 -OH) n-Hexane (n-C6 ) n-Heptane (n-C7 ) Cyclohexane (cyclo-C6 )
0.027 0.047 0.067 0.098 0.086 0.096
6.6 11.7 16.4 24.2 21.3 23.6
Selectivityb (%) UOL
SAL
SOL
CP
38.4 52.3 65.7 52.9 63.0 62.1
58.9 43.4 26.3 40.1 34.4 35.2
2.7 4.3 3.7 7.0 2.6 2.7
0.0 0.0 4.3 0.0 0.0 0.0
a
The average rate for 2 h of reaction time. Reaction conditions: T = 120 ◦ C; P = 929 kPa; crotonaldehyde/solvent/catalyst = 2 ml/78 ml/0.5 g; reaction time = 2 h; UOL, crotyl alcohol; SAL, butyraldehyde; SOL, 1butanol; CP, condensation product. b
the selectivity. The general solvents methanol and ethanol are liable to aldocondensate with crotonaldehyde over a solid base such as Mg2 AlO, so they were not used as reaction media. In this work (Table 3), the polar alcohols (C3 -OH, C5 -OH and i-C3 -OH) and the non-polar alkanes (n-C6 , n-C7 and cyclo-C6 ) were used as the reaction media. The effects of the solvent include solubility of hydrogen in it as the reaction medium, competitive adsorption between the solvent and the reactant, and an intermolecular interaction with the reactant molecules. The non-polar solvents were preferable to the polar ones for the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO, as presented in Table 3. The solubility of hydrogen in the non-polar solvent of cyclohexane is about twice that in the polar solvent of ethanol [42]. Undoubtedly, the solubility of hydrogen in the solvent was its major effect on the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO. The opposite was true over Group VIII metals [43]: the polar solvents were preferable to the non-polar ones for the hydrogenation reaction. Generally, hydrogen is dissociatively adsorbed on Group VIII metals. This opposite effect of solvent on 2% Au/Mg2 AlO may be due to the specific state of hydrogen non-dissociatively adsorbed on 2% Au/Mg2 AlO. The effect of hydrogen pressure will be discussed in the following section. Cyclohexane was used as the reaction medium to examine the course of the hydrogenation of crotonaldehyde (UAL) over 2% Au/Mg2 AlO, as shown in Fig. 2(a). The amount of crotyl alcohol (UOL) increased with the reaction time, and the amount of butyraldehyde (SAL) increased to a maximum before decreasing. The completely reduced product of 1-butanol (SOL) occurred at around 100 min and increased gradually. The conversion values of UAL and the selectivities of products are shown in Fig. 2(b). The selectivity of UOL was maintained at about 60% before the complete conversion of UAL. The selectivity of SAL decreased and that of SOL increased with the conversion, but the total selectivity of SAL and SOL remained constant at about 40%. Accordingly, UOL is difficult to reduce to SOL and most of the SOL was obtained from the successive reduction of SAL, as demonstrated in Scheme 1. The chemisorption of reactants on catalysts is typically a necessary step in the catalytic reaction over Group VIII metals. The physical-like adsorption through dipole–dipole, dipole-induced dipole and dispersion forces has been suggested to occur when reactants are adsorbed on 2% Au/Mg2 AlO [35]. The activity and selectivity of 2% Au/Mg2 AlO are reportedly to be associated with the gold states of Au3+ and Au0 [35]. Butyraldehyde with a single C O bond and crotyl alcohol with a single C C bond were hydrogenated under the same conditions as those for the hydrogenation of crotonaldehyde. The conversion of butyraldehyde was 4.4 times great that of crotyl alcohol; such a result was associated with the stronger adsorption of butyraldehyde with the polar C O bond than of crotyl alcohol with non-polar C C bond. The conjugated C C/C O was delocalized as Cı+ –Cı− –Cı+ –Oı− , and adsorbed on the catalyst through electrostatic interaction with Au3+ . Most of the Cı+ –Oı− bonds were reduced to yield crotyl alcohol; besides these, some of the delocalized Cı+ –Cı− bonds were also reduced to
Fig. 2. Courses of the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO (a) and selectivity of products (b).
form butyraldehyde in the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO. The polar C O bond in butyraldehyde was adsorbed much more strongly than the non-polar C C bond in crotyl alcohol, so butyraldehyde was preferentially reduced to 1-butanol. 3.3. Effects of temperature, pressure and concentration The selectivity to crotyl alcohol was not obviously influenced as hydrogen pressure, crotonaldehyde concentration and temperature (<150 ◦ C) increased. The average rates of hydrogenation increased with the hydrogen pressure from 5.7 to 19.3 mmol gcat −1 min−1 with an apparent order of dependence of
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Scheme 1. Hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO.
Fig. 4. Effect of crotonaldehyde concentration on the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO.
(100 ◦ C) of the catalysts. The ratio of Au3+ /Au0 decreased from 1.2 to 0.85 − 0.89 at reaction temperatures of 120–150 ◦ C, and decreased to 0.5 at 160 ◦ C. The marked decrease of the Au3+ /Au0 ratio might result in the sharp decrease of the conversion and selectivity for reaction at 160 ◦ C. Besides, the competitive adsorption of hydrogen was less favorable than that of crotonaldehyde as the temperature exceeded 150 ◦ C. The apparent activation energy, estimated from the plot of ln R versus 1/T in the range of 120–150 ◦ C, was 38 kJ/mol. 3.4. Effect of promoter
Fig. 3. Effect of hydrogen pressure on the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO.
1.84 (Fig. 3), which was three times as greater than the order (0–0.5) in a simplified power law based on the Langmuir–Hingshellwood (L–H) model. The L–H model for the catalytic reaction of chemisorbed reactants clearly cannot be fitted to the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO. The rate of crotonaldehyde hydrogenation was also much more sensitive to the hydrogen pressure over 2% Au/Mg2 AlO than over Group VIII metals. That result is consistent with the aforementioned discussion of the dominant factor (solubility of hydrogen) in the solvent effect. In contrast, the average rates decreased as the concentration of crotonaldehyde increased with an apparent order of −0.55 (Fig. 4), revealing that the adsorption strength of crotonaldehyde substantially exceeded that of hydrogen. As presented in Fig. 5, the conversion increased and the selectivity changed slightly at about 62%, as the temperature increased to 150 ◦ C, beyond which both decreased. The catalysts after reaction were analyzed by XPS and the deconvolution results of Au 4f spectra are listed in Table 4. The states of Au3+ and Au0 changed as the reaction temperature exceeded the calcination temperature
Some promoters that co-deposited with Au on Mg2 AlO were investigated to improve the activity/selectivity for the hydrogenation of crotonaldehyde. As listed in Table 5, Fe, Mo and W substantially increased the activity of 2% Au/Mg2 AlO and slightly improved the selectivity of crotyl alcohol. Of these promoters, Fe was the most effective one and the optimal ratio of Fe/Au was 0.5. The activity of 2% Au-Fe0.5 /Mg2 AlO was approximately 2.2 times that of 2% Au/Mg2 AlO, and the selectivity of crotyl alcohol increased from 62% to 66%. Fe was deposited in an ionic state, and its affinity to
Table 4 Au3+ /Au0 ratio on 2% Au/Mg2 AlO after reaction (XPS). Reaction temperature (◦ C)
Au3+
Au0
Au3+ /Au0
120 150 160
0.47 0.46 0.33
0.53 0.54 0.67
0.89 0.85 0.50
Fig. 5. Effect of temperature on the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO.
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Table 5 Effect of promoter on the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO. Mx a
Activityb (mmol gcat −1 min−1 )
Conversionc (%)
– Fe0.3 Fe0.5 Fe0.7 Mo0.3 Mo0.5 Mo0.7 W0.3 W0.5 W0.7
0.096 0.145 0.209 0.168 0.129 0.123 0.086 0.106 0.123 0.086
23.6 35.7 51.5 41.5 31.7 30.4 21.2 26.1 30.3 21.2
Selectivityc (%) UOL
SAL
SOL
62.1 62.9 65.8 64.1 62.6 63.1 63.4 61.3 61.4 60.0
35.2 29.3 22.0 26.4 29.4 29.3 31.7 32.5 31.3 34.0
2.7 7.8 12.2 9.5 8.0 7.6 4.9 6.2 7.3 6.0
a
x = the molar ratio of M/Au. The average rate for 2 h of reaction time. c Reaction conditions: T = 120 ◦ C; P = 929 kPa; crotonaldehyde/cyclohexane/catalyst = 2 ml/78 ml/0.5 g; reaction time = 2 h; UOL, crotyl alcohol; SAL, butyraldehyde; SOL, 1-butanol. b
Table 6 Effect of support on the hydrogenation of crotonaldehyde over the gold catalyst. Catalyst
dAu (nm)
Au3+ /Au0
Activity a (mmol gcat −1 min−1 )
Conversionb (%)
2% Au/Mg2 AlO 2% Au/FeOOH 2% Au/Fe2 O3 2% Au/CeO2 2% Au/TiO2 2% Au/Al2 O3
3.6 4.3 3.1 3.5 3.0 –
1.20 0.06 0.00 0.05 0.11 0.00
0.096 0.004 0.002 0.009 0.011 0.004
23.6 1.0 0.4 2.1 2.7 1.1
Selectivityb (%) UOL
SAL
SOL
62.1 66.8 50.4 59.0 61.2 46.6
35.2 33.2 49.6 41.0 38.8 53.4
2.7 0.0 0.0 0.0 0.0 0.0
a
The average activity for 2 h of reaction time. Reaction conditions: T = 120 ◦ C; P = 929 kPa; crotonaldehyde/cyclohexane/catalyst = 2 ml/78 ml/0.5 g; reaction time = 2 h; UOL, crotyl alcohol; SAL, butyraldehyde; SOL, 1-butanol. b
the electronegative dipole of crotonaldehyde enhanced the adsorption and activation of crotonaldehyde on 2% Au-Fe0.5 /Mg2 AlO. The promoters enhanced the activation of not only the C O bond but also the C C bond in the conjugated C O/C C group that was delocalized as a resonance state, Cı+ –Cı− –Cı+ –Oı− . 3.5. Effect of support Gold was supported on FeOOH, Fe2 O3 , CeO2 , TiO2 , and Al2 O3 with the typical DP method, and each catalyst was then calcined at 100 ◦ C. The as-prepared catalysts were used in the hydrogenation of crotonaldehyde, and results were compared with those for 2% Au/Mg2 AlO (Table 6). The average sizes of Au were 4.3 nm (Au/FeOOH), 3.1 nm (Au/Fe2 O3 ), 3.5 nm (Au/CeO2 ) and 3 nm (Au/TiO2 ). Clearly, the size effect was not the major effect on the activity and selectivity in this study. Table 6 also shows the Au3+ /Au0 ratio determined from XPS analyses. 2% Au/Mg2 AlO was the most active catalyst and it had the largest Au3+ /Au0 ratio. The Au3+ /Au0 ratio on Au/Fe2 O3 and Au/Al2 O3 was zero; this value was associated with their low activities. The correlation between activity and Au3+ /Au0 ratio could be found among the gold catalysts in this work. This viewpoint needs more support work in the further studies.
alysts supported on FeOOH, Fe2 O3 , CeO2 , TiO2 and Al2 O3 . In the liquid-phase hydrogenation over 2% Au/Mg2 AlO, high selectivity of crotyl alcohol, about 62%, was obtained near complete conversion and the crotyl alcohol was not successively reduced to 1-butanol. Fe, co-deposited on 2% Au-Fe0.5 /Mg2 AlO, effectively doubled the activity of the catalyst and improved the selectivity of crotyl alcohol. Non-polar solvents as reaction mediums favored the hydrogenation reaction over Au/Mg2 AlO. The hydrogenation rates were much more sensitive to the hydrogen pressure, with an apparent order of 1.8, over 2% Au/Mg2 AlO than over Group VIII metals. These different performances over 2% Au/Mg2 AlO catalyst were ascribed to the different interactions of reactants with gold. Reactants were suggested to be adsorbed on gold catalysts by physical-like adsorption through dipole–dipole, dipole-induced dipole and dispersion forces. Acknowledgements The authors would like to thank the Ministry of Economic Affairs of the Republic of China, Taiwan, for financially supporting this research under Contract No. MOEA 97-EC-17-A-09-S1-022. References
4. Conclusion Gold was dispersed and stabilized on the solid base of Mg2 AlO hydrotalcite using a modified DP method for use in the hydrogenation of crotonaldehyde to crotyl alcohol. The calcination temperatures of the Mg2 AlO support and the catalyst dominated the ratio of gold states (Au3+ /Au0 ) on the catalyst. Increasing the Au3+ /Au0 ratio on the catalysts increased the activity and selectivity of crotonaldehyde to crotyl alcohol. This correlation between Au3+ /Au0 and activity/selectivity was also observed in gold cat-
[1] E. Bus, R. Prins, J.A. van Bokhoven, Catal. Commun. 8 (2007) 1397–1402. [2] M.A. Aramendia, V. Borau, C. Jimenez, J.M. Marinas, A. Porras, F.J. Urbano, J. Catal. 172 (1997) 46–54. [3] M. English, V. Ranade, J. Lercher, J. Mol. Catal. 121 (1997) 69–80. [4] V. Ponec, Appl. Catal. A: Gen. 149 (1997) 27–48. [5] J.L. Margitfalvi, A. Tompos, I. Kolosova, J. Valyon, J. Catal. 174 (1998) 246–249. [6] N. Mahata, F. Goncalves, M. Fernando, R. Pereira, J.L. Figueiredo, Appl. Catal. A: Gen. 339 (2008) 159–168. [7] M.A. Vannice, B. Sen, J. Catal. 115 (1989) 65–78. [8] H. Yoshitake, Y. Iwasawa, J. Chem. Soc. Faraday Trans. 88 (1992) 503–510. [9] A. Sepulveda-Escribano, F. Coloma, F. Rodriguez-Reinoso, J. Catal. 178 (1998) 649–657.
H.-Y. Chen et al. / Applied Catalysis A: General 381 (2010) 209–215 [10] D. Goupil, P. Fouilloux, R. Maurel, React. Kinet. Catal. Lett. 35 (1987) 185–193. [11] R. Zanella, C. Louis, S. Giorgio, R. Touroude, J. Catal. 223 (2004) 328–339. [12] C. Milone, C. Crisafulli, R. Ingoglia, L. Schipilliti, S. Galvagno, Catal. Today 122 (2007) 341–351. [13] M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, J. Catal. 115 (1989) 301–309. [14] N.A. Hodge, C.J. Kiely, R. Whyman, M.R.H. Siddiqui, G.H. Hutchings, Q.A. Pankhurst, P.E. Wanger, R.R. Rajaram, S.E. Golunski, Catal. Today 72 (2002) 133–144. [15] E.D. Park, J.S. Lee, J. Catal. 186 (1999) 1–11. [16] S. Scire, S. Minico, C. Crisafulli, C. Satriano, A. Pistone, Appl. Catal. B: Environ. 40 (2003) 43–49. [17] C. Milone, R. Ingoglia, S. Galvagno, Gold Bull. 39 (2006) 54–65. [18] P. Claus, Appl. Catal. A: Gen. 291 (2005) 222–229. [19] G.C. Bond, C. Louis, D.T. Thompson, Catalysis by Gold, Imperial College Press, London, 2006. [20] F. Moreau, G.C. Bond, A.O. Taylor, J. Catal. 231 (2005) 105–114. [21] S. Ivanova, V. Pitchon, C. Petit, H. Herschbach, A.V. Dorsselaer, E. Leize, Appl. Catal. A: Gen. 298 (2006) 203–210. [22] R. Zanella, L. Delannoy, C. Louis, Appl. Catal. A: Gen. 291 (2005) 62–72. [23] D. Andreeva, V. Idakiev, T. Tabakova, L. Ilieva, P. Falaras, A. Bourlinos, A. Travlos, Catal. Today 72 (2002) 51–57. [24] A. Knell, P. Barnickel, A. Baiker, A. Wokaun, J. Catal. 137 (1992) 306–321. [25] A. Wolf, F. Schuth, Appl. Catal. A: Gen. 226 (2002) 1–13. [26] I. Dobrosz, K. Jiratova, V. Pitchon, J.M. Rynkowski, J. Mol. Catal. A: Chem. 234 (2005) 187–197.
215
[27] J.L. Margitfalvi, A. Fasi, M. Hegeds, F. Lonyi, S. Gbolos, N. Bogdanchikova, Catal. Today 72 (2002) 157–169. [28] L.M. Molina, B. Hammer, Phys. Rev. Lett. 90 (2003) 206102-1–206102-4. [29] K. Jiratova, P. Cuba, F. Kovanda, L. Hilaire, V. Pitchon, Catal. Today 76 (2002) 43–49. [30] S. Narayanan, K. Krishna, Appl. Catal. A: Gen. 174 (1998) 221–229. [31] F. Kovanda, K. Jiratova, J. Rymes, D. Kolousek, Appl. Clay Sci. 18 (2001) 71–80. [32] S. Ivanova, C. Petit, V. Pitchon, Appl. Catal. A: Gen. 267 (2004) 191–201. [33] C.T. Chang, B.J. Liaw, C.T. Huang, Y.Z. Chen, Appl. Catal. A: Gen. 332 (2007) 216–224. [34] C.T. Chang, B.J. Liaw, Y.P. Chen, Y.Z. Chen, J. Mol. Catal. A: Chem. 300 (2009) 80–88. [35] K.J. You, C.T. Chang, B.J. Liaw, C.T. Huang, Y.Z. Chen, Appl. Catal. A: Gen. 361 (2009) 65–71. [36] M. Okumura, T. Akita, M. Haruta, Catal. Today 74 (2002) 265–269. [37] S. Schimpf, M. Lucas, C. Mohr, U. Rodemerck, A. Bruckner, J. Radnik, H. Hofmeister, P. Claus, Catal. Today 72 (2002) 63–78. [38] C. Mohr, H. Hofmeister, P. Claus, J. Catal. 213 (2003) 86–94. [39] C. Mohr, H. Hofmeister, J. Radnik, P. Claus, J. Am. Chem. Soc. 125 (2003) 1905–1911. [40] B. Campo, M. Volpe, S. Ivanova, R. Touroude, J. Catal. 242 (2006) 162–171. [41] B. Campo, C. Petit, M.A. Volpe, J. Catal. 254 (2008) 71–78. [42] R.A. Rajadhyaksha, S.L. Karwa, Chem. Eng. Sci. 41 (1986) 1765–1770. [43] Y.Z. Chen, B.J. Liaw, S.J. Chiang, Appl. Catal. A: Gen. 284 (2005) 97–104.
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Selective hydrogenation of crotonaldehyde in liquid-phase over Au/Mg2 AlO hydrotalcite catalysts Hsing-Yu Chen a , Ching-Tu Chang b , Shu-Jen Chiang a , Biing-Jye Liaw c,∗ , Yin-Zu Chen a,∗ a b c
Department of Chemical and Materials Engineering, National Central University, Jhongli 32001, Taiwan, ROC Institute of Nuclear Energy Research Atomic Energy Council, Longtan 32546, Taiwan, ROC Department of Chemical and Materials Engineering, Graduate School of Materials Applied Technology, Nanya Institute of Technology, Jhongli 32091, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 23 September 2009 Received in revised form 26 March 2010 Accepted 7 April 2010 Available online 14 April 2010 Keywords: Gold catalysts Hydrotalcite Promoters Selective hydrogenation Crotonaldehyde
a b s t r a c t The liquid-phase selective hydrogenation of crotonaldehyde to crotyl alcohol over Au supported on a solid base of Mg2 AlO-hydrotalcite (Mg/Al = 2) was investigated. The 2% Au/Mg2 AlO catalysts were prepared using a modified deposition precipitation method without adjusting the pH of the initial HAuCl4 solution. The influence of the calcination temperatures of the Mg2 AlO support and of the 2% Au/Mg2 AlO catalyst, and the effects of the reaction medium (solvent), temperature, pressure and concentration were studied. The promoter, Fe, Mo or W, was co-deposited with Au on Mg2 AlO to maximize the yield of crotyl alcohol. 2% Au/Mg2 AlO was compared with Au supported on FeOOH, Fe2 O3 , CeO2 , TiO2 and Al2 O3 . The correlation between the gold states (Au3+ /Au0 ) and the activity and selectivity of crotyl alcohol was extensively examined. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The selective hydrogenation of ␣,-unsaturated aldehydes with a conjugated C C/C O group to the corresponding unsaturated alcohols is not only of commercial interest in the intermediate production of fine chemicals [1] but also of special scientific interest. Thermodynamics and kinetics favor the hydrogenation of C C over the C O bond on conventional hydrogenation catalysts that are based on Pt, Ru, Pd or Ni [2,3]. Nevertheless, designing catalysts for the preferred hydrogenation of the conjugated C O group is challenging. Researchers have used platinum doped with promoters (such as Fe and Sn) [3–6] or supported on easily reducible oxides (SMSI effect) [7–9], or bimetallic catalysts [10], to enhance the hydrogenation of the conjugated C O bond. Several groups have recently reported that gold-supported catalysts exhibit remarkable selectivity toward the hydrogenation of the conjugated C O bond in ␣,-unsaturated aldehydes and ketones [1,11–13]. In their studies, iron oxide (FeOOH, ␥-Fe2 O3 and ␣-Fe2 O3 ), TiO2 , Al2 O3, SiO2 , ZnO and ZrO2 were used as supports for the gold catalysts. Such a support affects the reactivity and selectivity of gold. Milone et al. [12] demonstrated Au supported on FeOOH is the most active and selective catalyst to the hydrogenation of the conjugated C O bond. Bus et al. [1] compared Au/Al2 O3 with Pt/Al2 O3 , and showed that
∗ Corresponding authors. Tel.: +886 3 4227151; fax: +886 3 4252296. E-mail addresses: [email protected] (B.-J. Liaw), [email protected] (Y.-Z. Chen). 0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.04.009
Au/Al2 O3 is much more active and selective than Pt/Al2 O3 in the selective hydrogenation of C O bond in cinnamaldehyde, whereas Au/Al2 O3 is less active by two orders of magnitude than Pt/Al2 O3 in the hydrogenation of the C C bond in cyclohexene. Gold has attracted much attention in the recent decade as a catalyst, since Haruta et al. reported that gold particles with a nanosize of less than 5 nm exhibit very high activity in the oxidation of CO at room temperature [13]. Most attention has been focused on oxidation reactions [14–16], and only a few work on selective hydrogenation on gold catalysts have been published [17,18]. The deposition precipitation (DP) method, developed by Haruta et al. [13], has been widely adopted to prepare gold catalysts, and the salt HAuCl4 is most often used as a precursor [19]. The equilibrium species of chloro-hydroxy complexes ([AuClx (OH)4−x ]− , x = 1–4) depend on the pH of the gold solution [20,21]. In the DP method, the pH of the initial HAuCl4 solution is adjusted to 7, and the complex [AuCl(OH)3 ]− is the major species, which is predominantly deposited on the support to yield an active gold catalyst [22]. In this method, the iso-electric point (IEP) of the support is critical to the deposition of gold species. Oxides with an IEP of approximately 7, including CeO2 (IEP = 6.75) [23], Fe2 O3 (IEP = 6.5–6.9) [13], TiO2 (IEP = 6), and ZrO2 (IEP = 6.7) [24], have been extensively used as supports for obtaining an active catalyst; acidic supports such as SiO2 (IEP = 1–2) [25] or basic supports such as MgAlO hydrotalcite (IEP = 10) [26] and MgO (IEP = 12) [27] are not typically used. MgO is well known for the stabilization of nano-sized particles of gold on it [28]. The Mgx AlO hydrotalcites, consisting of positively charged brucite-like Mg(OH)2 layers separated by interlayer
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anions and water molecules [29], are generally used as catalysts or support precursors [30,31]. No suitable method for obtaining an active gold catalyst supported on MgO or Mgx AlO hydrotalcite [26,32] had been proposed before our previous studies [33–35]. Chang et al. [33] used a modified DP method, in which Mgx AlO suspended in water was added to the gold solution without preadjusting the pH of the initial solution, to obtain an active catalyst of Au/Mg2 AlO for the oxidation of CO and the selective hydrogenation of ␣,-unsaturated aldehydes [34]. You et al. [35] found that high yields of unsaturated alcohol (85% from cinnamaldehyde and 95% from citral) were obtained over Au/Mg2 AlO. They also found that the activity of cinnamaldehyde hydrogenation and its selectivity to unsaturated alcohol increased with the Au3+ /Au0 ratio of the catalyst. Among many examples of selective hydrogenation of ␣,unsaturated aldehydes, the hydrogenation of crotonaldehyde to crotyl alcohol is considered to be particularly difficult because the hindrance of the substituent at the C C bond is weak. In the hydrogenation of crotonaldehyde, according to Okumura et al. [36], the selectivity toward crotyl alcohol was in the range 10–25% for Au/TiO2 , Au/Al2 O3 and Au/SiO2 . Claus and co-workers [37–39], working with Au/SiO2, Au/TiO2, Au/ZrO2 , Au/ZnO, and Au-In/ZnO, concluded that the support determines the morphology of the gold particles and suggested that the edges of gold crystallites are active sites for C O hydrogenation. Campo et al. [40,41] reported that the Au/CeO2 catalyst is highly selective toward crotyl alcohol (78% selectivity) in the gas-phase hydrogenation of crotonaldehyde, but unselective in the liquid-phase hydrogenation (< 10% selectivity). Of the gold catalysts of the hydrogenation of ␣,-unsaturated aldehydes that have been identified in the literature, Au/Mg2 AlO seems to be the most active. In this study, 2% Au/Mg2 AlO catalysts for the selective hydrogenation of crotonaldehyde in liquid phase were investigated. The effects of the calcination temperatures of the support and of the catalyst were examined. The effects of the reaction parameters, including the solvent used as the reaction medium, temperature, pressure and concentration were studied. Promoters of the catalyst were studied to maximize the yield of crotyl alcohol. 2% Au/Mg2 AlO was compared with gold supported on FeOOH, Fe2 O3 , CeO2 , TiO2 and Al2 O3 for the hydrogenation of crotonaldehyde. The correlation between gold states (Au3+ /Au0 ) and the activity/selectivity of the gold catalysts to crotyl alcohol were examined extensively.
dispersed well in 100 ml of water, and then poured into the gold solution slowly at a rate of about 3 ml/min for deposition precipitation at 70 ◦ C. The obtained catalyst precursor was aged at 70 ◦ C for 3 h, and then washed and dried overnight at 100 ◦ C. The catalysts were calcined at various temperatures (100–300 ◦ C) for 4 h. Moreover, an extra metal additive (Fe, Mo, W) was introduced by dissolving its precursor salt in the solution in which Mg2 AlO was suspended. Then, the solution was well stirred for 3 h before mixing with the gold solution. 2.2. Characterization of catalysts The specific surface areas (SBET ) of samples were determined by nitrogen adsorption with a Micromeritics ASAP-2020 apparatus at −196 ◦ C following degassing at 100 ◦ C. The compositions of samples were analyzed by inductively coupled plasma analysis (ICP) using a JOBIN JY-24 device. X-ray diffraction (XRD) patterns were obtained using a Siemens-500 diffractometer with Cu K␣ radiation ( = 0.1542 nm). Transmission electron microscopy (TEM) photographs were taken using a JEOL JEM-2999FMII apparatus. Samples for TEM analysis were prepared by gently grinding a catalyst into powder in a mortar; the synthesized powder was ultrasonically dispersed in ethanol. The solution was deposited onto a carbon-coated Cu mesh grid using a pipette; the solvent was then allowed to evaporate naturally. Mean particle diameters were determined by counting ∼150 particles in enlarged photographs. X-ray photoelectron spectroscopy (XPS) measurements were made using a Thermo VG Scientific Sigma Prob spectrophotometer with Al K␣ radiation (1486.6 eV). The nanoparticles were firstly pressed into a 10-mm diameter disk; these disks were fixed onto the sample holders, and immediately transferred to the pretreatment chamber. In the chamber, each sample was degassed overnight at 1 × 10−6 Torr to remove the volatile contaminants and was then transferred to the analyzing chamber for XPS analysis. The spectra were obtained using an analyzer pass energy of 25.5 eV and an electron take-off angle of 45◦ . The vacuum in the test chamber was maintained at below 1.33 × 10−8 Torr during the collection. Binding energies were corrected for surface charging by referencing them to the energy of the C 1s peak of the contaminant carbon at 284.6 eV. 2.3. Selective hydrogenation
2. Experimental 2.1. Preparation of support and catalyst Hydrotalcite supports of Mg2 AlO (Mg/Al molar ratio = 2) were prepared by co-precipitation of an aqueous solution of magnesium and aluminum salts with a highly basic carbonate solution. The salt solution (1 M) contained Mg(NO3 )2 ·6H2 O and Al(NO3 )3 ·9H2 O dissolved in deionized water at various Mg/Al molar ratios (x = 1–4). The basic solution contained equal volumes of KOH and K2 CO3 , with molar ratios of CO3 2− /(Al + Mg) = 0.67 and OH− /(Al + Mg) = 2.25. These two solutions were mixed at a rate of 60 ml/h, maintained at a constant pH of about 10, and then aged overnight with stirring. The white precipitate was washed, dried at 100 ◦ C, and then calcined in air at various temperatures (100–300 ◦ C). The catalysts except Au/Mg2 AlO were prepared using the DP method that was developed by Haruta et al. [13]. The Au/Mg2 AlO catalysts were prepared using a modified DP method [33]. In the conventional DP method, the pH of the initial HAuCl4 solution (pHi 4–9) was firstly adjusted by adding a 0.5 M NaCO3 solution. In the modified DP method, the pH of about 2 of the initial solution (pHi 2) was not adjusted. The hydrotalcite support of Mg2 AlO was firstly
A fresh, as-prepared catalyst was used for each reaction run. In some cases, the as-prepared catalyst was calcined before the reaction. The hydrogenation reactions were performed in a magnetically stirred autoclave (160 ml, Parr-4842) under a constant total pressure of 929 kPa (120 psig); in the autoclave pure hydrogen was continuously supplied to compensate for the consumed hydrogen to maintain the total pressure. Notably, the amount of catalyst was sufficiently small and the stirring rate was sufficiently high to minimize the mass transfer resistance. The crotonaldehyde was hydrogenated in ethanol or other solvents with a 2% Au/Mg2 AlO hydrotalcite catalyst with reactant/ethanol/catalyst = 2 ml/78 ml/0.5 g. The hydrogenated samples were analyzed using a gas chromatograph with a SUPELCOWAXTM 10 capillary column of 30 m × 0.25 mm × 0.25 m film thickness attached to a flame ionization detector. 3. Results and discussion 3.1. Effect of calcination temperature of Mg2 AlO and catalyst Fig. 1 displays the XRD spectra of the Mg2 AlO calcined at 100, 200 and 300 ◦ C. The Mg2 AlO exhibited the characteristic diffraction
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Table 1 Effect of calcination temperature of support on the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO. Ta (◦ C)
SBET b (m2 g−1 )
Au loadingb (wt%)
dAu b (nm)
Au3+ /Au0
100 200 300
71.6 70.1 236.4
1.20 1.05 1.07
3.6 3.7 3.8
1.20 1.08 0.75
b
Activityc (mmol gcat −1 min−1 )
Conversiond (%)
0.096 0.091 0.086
23.6 22.4 21.3
Selectivityd (%) UOL
SAL
SOL
62.1 56.4 48.9
35.2 36.9 44.2
2.7 6.7 6.9
a
Calcination temperature of Mg2 AlO. Data from Ref. [35]. c The average rate for 2 h of reaction time. d Reaction conditions: T = 120 ◦ C; P = 929 kPa; crotonaldehyde/cyclohexane/catalyst = 2 ml/78 ml/0.5 g; reaction time = 2 h; UOL, unsaturated alcohol of crotyl alcohol; SAL, saturated aldehyde of butyraldehyde; SOL, saturated alcohol of 1-butanol. b
Table 2 Effect of calcination temperature of catalyst on the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO. Ta (◦ C)
SBET (m2 g−1 )
dAu (nm)
Au3+ /Au0
100 200 300
84.6 83.6 97.1
3.6 3.8 3.9
1.20 0.89 0.00
b
Activityc (mmol gcat −1 min−1 )
Conversiond (%)
0.096 0.052 0.015
23.6 12.9 3.8
Selectivityd (%) UOL
SAL
SOL
62.1 44.3 37.0
35.2 51.5 61.2
2.7 4.2 1.8
a
Calcination temperature of Au/Mg2 AlO. Data of XPS results from Ref. [35]. c The average rate for 2 h of reaction time. d Reaction conditions: T = 120 ◦ C; P = 929 kPa; crotonaldehyde/cyclohexane/catalyst = 2 ml/78 ml/0.5 g; reaction time = 2 h; UOL, crotyl alcohol; SAL, butyraldehyde; SOL, 1-butanol. b
pattern of the laminated hydrotalcite at 2 = 11.02◦ , 23.55◦ , 34.84◦ , 39.03◦ , 46.29◦ , 60.96◦ , 62.26◦ and 66.29◦ . The Mg2 AlO hydrotalcite maintained its layered structure even when it was calcined up to 300 ◦ C. In the previous study on 2% Au/Mg2 AlO [35], You et al. found that the calcination temperatures of the support and the catalyst determine the activity and selectivity in the liquid-phase hydrogenation of cinnamaldehyde. These effects of the calcination temperatures were examined again in the hydrogenation of crotonaldehyde, and the results are shown in Tables 1 and 2. All 2% Au/Mg2 AlO catalysts in Table 1 were calcined at 100 ◦ C. The catalyst with Mg2 AlO calcined at 100 ◦ C was the most active one and a high calcination temperature of Mg2 AlO was associated with a low average rate. The basicity of Mg2 AlO increased with the calcination temperature, affecting the pH of the gold solution during DP and the state of gold on the catalysts [33]. The surface areas of Mg2 AlO hydrotalcites increased with the calcination temperature from about 70 m2 /g (100 and 200 ◦ C) to
236 m2 /g (300 ◦ C). The actual loadings and particle sizes of gold for 2% Au/Mg2 AlO catalysts with Mg2 AlO calcined at 100, 200 and 300 ◦ C were approximately the same; thus, the Au3+ /Au0 ratio dominated the performance of the catalysts. Both the activity and the selectivity of crotyl alcohol (UOL) decreased as the calcination temperature increased. Therefore, the activity and selectivity of crotyl alcohol increased as the Au3+ /Au0 ratio increased. This correlation between activity/selectivity and Au3+ /Au0 ratio over 2% Au/Mg2 AlO was suggested in the general liquid-phase hydrogenation of ␣,unsaturated aldehydes. This correlation has not yet been proposed over other gold catalysts. The as-prepared 2% Au/Mg2 AlO (with Mg2 AlO calcined at 100 ◦ C) catalysts were calcined at 100, 200 and 300 ◦ C prior to reactions. The surface areas of the corresponding catalysts (85, 84 and 97 m2 /g) did not increase obviously as Mg2 AlO support was calcined at various temperatures and the TEM analysis revealed no significant agglomeration of gold particles (Table 2). The Au3+ /Au0 ratio was about 1.2 for the 2% Au/Mg2 Al catalyst calcined at 100 ◦ C, and decreased to 0.9 for the catalyst calcined at 200 ◦ C. When the calcination temperature reached 300 ◦ C, Au3+ was completely reduced to Au0 . As presented in Table 2, the activity of 2% Au/Mg2 AlO declined sharply as the calcination temperature increased above 200 ◦ C, such a result was consistent with the disappearance of the oxidized state of gold (Au3+ ) as the calcination temperature exceeded 200 ◦ C. The selectivity of crotyl alcohol also decreased as the calcination temperature increased; such relationship was associated with a decrease in the Au3+ /Au0 ratio. To clarify the correlation between activity/selectivity and the Au3+ /Au0 ratio for general gold catalysts, the final section discusses the effect of the support on the hydrogenation of crotonaldehyde.
3.2. Effect of reaction solvent
Fig. 1. XRD spectra of Mg2 AlO calcined at 100, 200 and 300 ◦ C.
Catalytic hydrogenation is often carried out in a solvent for various reasons, such as to dissolve solid reactants and products, to absorb the exothermic heat of the reaction, and to keep the surface of the catalyst free from impurities and carbonaceous deposits. The nature of the solvent has a considerable effect on the rate and/or
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Table 3 Effect of solvent on the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO. Solvent
Activitya (mmol gcat −1 min−1 )
Conversionb (%)
1-Propanol (C3 -OH) 1-Pentanol (C5 -OH) 2-Propanol (i-C3 -OH) n-Hexane (n-C6 ) n-Heptane (n-C7 ) Cyclohexane (cyclo-C6 )
0.027 0.047 0.067 0.098 0.086 0.096
6.6 11.7 16.4 24.2 21.3 23.6
Selectivityb (%) UOL
SAL
SOL
CP
38.4 52.3 65.7 52.9 63.0 62.1
58.9 43.4 26.3 40.1 34.4 35.2
2.7 4.3 3.7 7.0 2.6 2.7
0.0 0.0 4.3 0.0 0.0 0.0
a
The average rate for 2 h of reaction time. Reaction conditions: T = 120 ◦ C; P = 929 kPa; crotonaldehyde/solvent/catalyst = 2 ml/78 ml/0.5 g; reaction time = 2 h; UOL, crotyl alcohol; SAL, butyraldehyde; SOL, 1butanol; CP, condensation product. b
the selectivity. The general solvents methanol and ethanol are liable to aldocondensate with crotonaldehyde over a solid base such as Mg2 AlO, so they were not used as reaction media. In this work (Table 3), the polar alcohols (C3 -OH, C5 -OH and i-C3 -OH) and the non-polar alkanes (n-C6 , n-C7 and cyclo-C6 ) were used as the reaction media. The effects of the solvent include solubility of hydrogen in it as the reaction medium, competitive adsorption between the solvent and the reactant, and an intermolecular interaction with the reactant molecules. The non-polar solvents were preferable to the polar ones for the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO, as presented in Table 3. The solubility of hydrogen in the non-polar solvent of cyclohexane is about twice that in the polar solvent of ethanol [42]. Undoubtedly, the solubility of hydrogen in the solvent was its major effect on the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO. The opposite was true over Group VIII metals [43]: the polar solvents were preferable to the non-polar ones for the hydrogenation reaction. Generally, hydrogen is dissociatively adsorbed on Group VIII metals. This opposite effect of solvent on 2% Au/Mg2 AlO may be due to the specific state of hydrogen non-dissociatively adsorbed on 2% Au/Mg2 AlO. The effect of hydrogen pressure will be discussed in the following section. Cyclohexane was used as the reaction medium to examine the course of the hydrogenation of crotonaldehyde (UAL) over 2% Au/Mg2 AlO, as shown in Fig. 2(a). The amount of crotyl alcohol (UOL) increased with the reaction time, and the amount of butyraldehyde (SAL) increased to a maximum before decreasing. The completely reduced product of 1-butanol (SOL) occurred at around 100 min and increased gradually. The conversion values of UAL and the selectivities of products are shown in Fig. 2(b). The selectivity of UOL was maintained at about 60% before the complete conversion of UAL. The selectivity of SAL decreased and that of SOL increased with the conversion, but the total selectivity of SAL and SOL remained constant at about 40%. Accordingly, UOL is difficult to reduce to SOL and most of the SOL was obtained from the successive reduction of SAL, as demonstrated in Scheme 1. The chemisorption of reactants on catalysts is typically a necessary step in the catalytic reaction over Group VIII metals. The physical-like adsorption through dipole–dipole, dipole-induced dipole and dispersion forces has been suggested to occur when reactants are adsorbed on 2% Au/Mg2 AlO [35]. The activity and selectivity of 2% Au/Mg2 AlO are reportedly to be associated with the gold states of Au3+ and Au0 [35]. Butyraldehyde with a single C O bond and crotyl alcohol with a single C C bond were hydrogenated under the same conditions as those for the hydrogenation of crotonaldehyde. The conversion of butyraldehyde was 4.4 times great that of crotyl alcohol; such a result was associated with the stronger adsorption of butyraldehyde with the polar C O bond than of crotyl alcohol with non-polar C C bond. The conjugated C C/C O was delocalized as Cı+ –Cı− –Cı+ –Oı− , and adsorbed on the catalyst through electrostatic interaction with Au3+ . Most of the Cı+ –Oı− bonds were reduced to yield crotyl alcohol; besides these, some of the delocalized Cı+ –Cı− bonds were also reduced to
Fig. 2. Courses of the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO (a) and selectivity of products (b).
form butyraldehyde in the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO. The polar C O bond in butyraldehyde was adsorbed much more strongly than the non-polar C C bond in crotyl alcohol, so butyraldehyde was preferentially reduced to 1-butanol. 3.3. Effects of temperature, pressure and concentration The selectivity to crotyl alcohol was not obviously influenced as hydrogen pressure, crotonaldehyde concentration and temperature (<150 ◦ C) increased. The average rates of hydrogenation increased with the hydrogen pressure from 5.7 to 19.3 mmol gcat −1 min−1 with an apparent order of dependence of
H.-Y. Chen et al. / Applied Catalysis A: General 381 (2010) 209–215
213
Scheme 1. Hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO.
Fig. 4. Effect of crotonaldehyde concentration on the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO.
(100 ◦ C) of the catalysts. The ratio of Au3+ /Au0 decreased from 1.2 to 0.85 − 0.89 at reaction temperatures of 120–150 ◦ C, and decreased to 0.5 at 160 ◦ C. The marked decrease of the Au3+ /Au0 ratio might result in the sharp decrease of the conversion and selectivity for reaction at 160 ◦ C. Besides, the competitive adsorption of hydrogen was less favorable than that of crotonaldehyde as the temperature exceeded 150 ◦ C. The apparent activation energy, estimated from the plot of ln R versus 1/T in the range of 120–150 ◦ C, was 38 kJ/mol. 3.4. Effect of promoter
Fig. 3. Effect of hydrogen pressure on the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO.
1.84 (Fig. 3), which was three times as greater than the order (0–0.5) in a simplified power law based on the Langmuir–Hingshellwood (L–H) model. The L–H model for the catalytic reaction of chemisorbed reactants clearly cannot be fitted to the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO. The rate of crotonaldehyde hydrogenation was also much more sensitive to the hydrogen pressure over 2% Au/Mg2 AlO than over Group VIII metals. That result is consistent with the aforementioned discussion of the dominant factor (solubility of hydrogen) in the solvent effect. In contrast, the average rates decreased as the concentration of crotonaldehyde increased with an apparent order of −0.55 (Fig. 4), revealing that the adsorption strength of crotonaldehyde substantially exceeded that of hydrogen. As presented in Fig. 5, the conversion increased and the selectivity changed slightly at about 62%, as the temperature increased to 150 ◦ C, beyond which both decreased. The catalysts after reaction were analyzed by XPS and the deconvolution results of Au 4f spectra are listed in Table 4. The states of Au3+ and Au0 changed as the reaction temperature exceeded the calcination temperature
Some promoters that co-deposited with Au on Mg2 AlO were investigated to improve the activity/selectivity for the hydrogenation of crotonaldehyde. As listed in Table 5, Fe, Mo and W substantially increased the activity of 2% Au/Mg2 AlO and slightly improved the selectivity of crotyl alcohol. Of these promoters, Fe was the most effective one and the optimal ratio of Fe/Au was 0.5. The activity of 2% Au-Fe0.5 /Mg2 AlO was approximately 2.2 times that of 2% Au/Mg2 AlO, and the selectivity of crotyl alcohol increased from 62% to 66%. Fe was deposited in an ionic state, and its affinity to
Table 4 Au3+ /Au0 ratio on 2% Au/Mg2 AlO after reaction (XPS). Reaction temperature (◦ C)
Au3+
Au0
Au3+ /Au0
120 150 160
0.47 0.46 0.33
0.53 0.54 0.67
0.89 0.85 0.50
Fig. 5. Effect of temperature on the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO.
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H.-Y. Chen et al. / Applied Catalysis A: General 381 (2010) 209–215
Table 5 Effect of promoter on the hydrogenation of crotonaldehyde over 2% Au/Mg2 AlO. Mx a
Activityb (mmol gcat −1 min−1 )
Conversionc (%)
– Fe0.3 Fe0.5 Fe0.7 Mo0.3 Mo0.5 Mo0.7 W0.3 W0.5 W0.7
0.096 0.145 0.209 0.168 0.129 0.123 0.086 0.106 0.123 0.086
23.6 35.7 51.5 41.5 31.7 30.4 21.2 26.1 30.3 21.2
Selectivityc (%) UOL
SAL
SOL
62.1 62.9 65.8 64.1 62.6 63.1 63.4 61.3 61.4 60.0
35.2 29.3 22.0 26.4 29.4 29.3 31.7 32.5 31.3 34.0
2.7 7.8 12.2 9.5 8.0 7.6 4.9 6.2 7.3 6.0
a
x = the molar ratio of M/Au. The average rate for 2 h of reaction time. c Reaction conditions: T = 120 ◦ C; P = 929 kPa; crotonaldehyde/cyclohexane/catalyst = 2 ml/78 ml/0.5 g; reaction time = 2 h; UOL, crotyl alcohol; SAL, butyraldehyde; SOL, 1-butanol. b
Table 6 Effect of support on the hydrogenation of crotonaldehyde over the gold catalyst. Catalyst
dAu (nm)
Au3+ /Au0
Activity a (mmol gcat −1 min−1 )
Conversionb (%)
2% Au/Mg2 AlO 2% Au/FeOOH 2% Au/Fe2 O3 2% Au/CeO2 2% Au/TiO2 2% Au/Al2 O3
3.6 4.3 3.1 3.5 3.0 –
1.20 0.06 0.00 0.05 0.11 0.00
0.096 0.004 0.002 0.009 0.011 0.004
23.6 1.0 0.4 2.1 2.7 1.1
Selectivityb (%) UOL
SAL
SOL
62.1 66.8 50.4 59.0 61.2 46.6
35.2 33.2 49.6 41.0 38.8 53.4
2.7 0.0 0.0 0.0 0.0 0.0
a
The average activity for 2 h of reaction time. Reaction conditions: T = 120 ◦ C; P = 929 kPa; crotonaldehyde/cyclohexane/catalyst = 2 ml/78 ml/0.5 g; reaction time = 2 h; UOL, crotyl alcohol; SAL, butyraldehyde; SOL, 1-butanol. b
the electronegative dipole of crotonaldehyde enhanced the adsorption and activation of crotonaldehyde on 2% Au-Fe0.5 /Mg2 AlO. The promoters enhanced the activation of not only the C O bond but also the C C bond in the conjugated C O/C C group that was delocalized as a resonance state, Cı+ –Cı− –Cı+ –Oı− . 3.5. Effect of support Gold was supported on FeOOH, Fe2 O3 , CeO2 , TiO2 , and Al2 O3 with the typical DP method, and each catalyst was then calcined at 100 ◦ C. The as-prepared catalysts were used in the hydrogenation of crotonaldehyde, and results were compared with those for 2% Au/Mg2 AlO (Table 6). The average sizes of Au were 4.3 nm (Au/FeOOH), 3.1 nm (Au/Fe2 O3 ), 3.5 nm (Au/CeO2 ) and 3 nm (Au/TiO2 ). Clearly, the size effect was not the major effect on the activity and selectivity in this study. Table 6 also shows the Au3+ /Au0 ratio determined from XPS analyses. 2% Au/Mg2 AlO was the most active catalyst and it had the largest Au3+ /Au0 ratio. The Au3+ /Au0 ratio on Au/Fe2 O3 and Au/Al2 O3 was zero; this value was associated with their low activities. The correlation between activity and Au3+ /Au0 ratio could be found among the gold catalysts in this work. This viewpoint needs more support work in the further studies.
alysts supported on FeOOH, Fe2 O3 , CeO2 , TiO2 and Al2 O3 . In the liquid-phase hydrogenation over 2% Au/Mg2 AlO, high selectivity of crotyl alcohol, about 62%, was obtained near complete conversion and the crotyl alcohol was not successively reduced to 1-butanol. Fe, co-deposited on 2% Au-Fe0.5 /Mg2 AlO, effectively doubled the activity of the catalyst and improved the selectivity of crotyl alcohol. Non-polar solvents as reaction mediums favored the hydrogenation reaction over Au/Mg2 AlO. The hydrogenation rates were much more sensitive to the hydrogen pressure, with an apparent order of 1.8, over 2% Au/Mg2 AlO than over Group VIII metals. These different performances over 2% Au/Mg2 AlO catalyst were ascribed to the different interactions of reactants with gold. Reactants were suggested to be adsorbed on gold catalysts by physical-like adsorption through dipole–dipole, dipole-induced dipole and dispersion forces. Acknowledgements The authors would like to thank the Ministry of Economic Affairs of the Republic of China, Taiwan, for financially supporting this research under Contract No. MOEA 97-EC-17-A-09-S1-022. References
4. Conclusion Gold was dispersed and stabilized on the solid base of Mg2 AlO hydrotalcite using a modified DP method for use in the hydrogenation of crotonaldehyde to crotyl alcohol. The calcination temperatures of the Mg2 AlO support and the catalyst dominated the ratio of gold states (Au3+ /Au0 ) on the catalyst. Increasing the Au3+ /Au0 ratio on the catalysts increased the activity and selectivity of crotonaldehyde to crotyl alcohol. This correlation between Au3+ /Au0 and activity/selectivity was also observed in gold cat-
[1] E. Bus, R. Prins, J.A. van Bokhoven, Catal. Commun. 8 (2007) 1397–1402. [2] M.A. Aramendia, V. Borau, C. Jimenez, J.M. Marinas, A. Porras, F.J. Urbano, J. Catal. 172 (1997) 46–54. [3] M. English, V. Ranade, J. Lercher, J. Mol. Catal. 121 (1997) 69–80. [4] V. Ponec, Appl. Catal. A: Gen. 149 (1997) 27–48. [5] J.L. Margitfalvi, A. Tompos, I. Kolosova, J. Valyon, J. Catal. 174 (1998) 246–249. [6] N. Mahata, F. Goncalves, M. Fernando, R. Pereira, J.L. Figueiredo, Appl. Catal. A: Gen. 339 (2008) 159–168. [7] M.A. Vannice, B. Sen, J. Catal. 115 (1989) 65–78. [8] H. Yoshitake, Y. Iwasawa, J. Chem. Soc. Faraday Trans. 88 (1992) 503–510. [9] A. Sepulveda-Escribano, F. Coloma, F. Rodriguez-Reinoso, J. Catal. 178 (1998) 649–657.
H.-Y. Chen et al. / Applied Catalysis A: General 381 (2010) 209–215 [10] D. Goupil, P. Fouilloux, R. Maurel, React. Kinet. Catal. Lett. 35 (1987) 185–193. [11] R. Zanella, C. Louis, S. Giorgio, R. Touroude, J. Catal. 223 (2004) 328–339. [12] C. Milone, C. Crisafulli, R. Ingoglia, L. Schipilliti, S. Galvagno, Catal. Today 122 (2007) 341–351. [13] M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, J. Catal. 115 (1989) 301–309. [14] N.A. Hodge, C.J. Kiely, R. Whyman, M.R.H. Siddiqui, G.H. Hutchings, Q.A. Pankhurst, P.E. Wanger, R.R. Rajaram, S.E. Golunski, Catal. Today 72 (2002) 133–144. [15] E.D. Park, J.S. Lee, J. Catal. 186 (1999) 1–11. [16] S. Scire, S. Minico, C. Crisafulli, C. Satriano, A. Pistone, Appl. Catal. B: Environ. 40 (2003) 43–49. [17] C. Milone, R. Ingoglia, S. Galvagno, Gold Bull. 39 (2006) 54–65. [18] P. Claus, Appl. Catal. A: Gen. 291 (2005) 222–229. [19] G.C. Bond, C. Louis, D.T. Thompson, Catalysis by Gold, Imperial College Press, London, 2006. [20] F. Moreau, G.C. Bond, A.O. Taylor, J. Catal. 231 (2005) 105–114. [21] S. Ivanova, V. Pitchon, C. Petit, H. Herschbach, A.V. Dorsselaer, E. Leize, Appl. Catal. A: Gen. 298 (2006) 203–210. [22] R. Zanella, L. Delannoy, C. Louis, Appl. Catal. A: Gen. 291 (2005) 62–72. [23] D. Andreeva, V. Idakiev, T. Tabakova, L. Ilieva, P. Falaras, A. Bourlinos, A. Travlos, Catal. Today 72 (2002) 51–57. [24] A. Knell, P. Barnickel, A. Baiker, A. Wokaun, J. Catal. 137 (1992) 306–321. [25] A. Wolf, F. Schuth, Appl. Catal. A: Gen. 226 (2002) 1–13. [26] I. Dobrosz, K. Jiratova, V. Pitchon, J.M. Rynkowski, J. Mol. Catal. A: Chem. 234 (2005) 187–197.
215
[27] J.L. Margitfalvi, A. Fasi, M. Hegeds, F. Lonyi, S. Gbolos, N. Bogdanchikova, Catal. Today 72 (2002) 157–169. [28] L.M. Molina, B. Hammer, Phys. Rev. Lett. 90 (2003) 206102-1–206102-4. [29] K. Jiratova, P. Cuba, F. Kovanda, L. Hilaire, V. Pitchon, Catal. Today 76 (2002) 43–49. [30] S. Narayanan, K. Krishna, Appl. Catal. A: Gen. 174 (1998) 221–229. [31] F. Kovanda, K. Jiratova, J. Rymes, D. Kolousek, Appl. Clay Sci. 18 (2001) 71–80. [32] S. Ivanova, C. Petit, V. Pitchon, Appl. Catal. A: Gen. 267 (2004) 191–201. [33] C.T. Chang, B.J. Liaw, C.T. Huang, Y.Z. Chen, Appl. Catal. A: Gen. 332 (2007) 216–224. [34] C.T. Chang, B.J. Liaw, Y.P. Chen, Y.Z. Chen, J. Mol. Catal. A: Chem. 300 (2009) 80–88. [35] K.J. You, C.T. Chang, B.J. Liaw, C.T. Huang, Y.Z. Chen, Appl. Catal. A: Gen. 361 (2009) 65–71. [36] M. Okumura, T. Akita, M. Haruta, Catal. Today 74 (2002) 265–269. [37] S. Schimpf, M. Lucas, C. Mohr, U. Rodemerck, A. Bruckner, J. Radnik, H. Hofmeister, P. Claus, Catal. Today 72 (2002) 63–78. [38] C. Mohr, H. Hofmeister, P. Claus, J. Catal. 213 (2003) 86–94. [39] C. Mohr, H. Hofmeister, J. Radnik, P. Claus, J. Am. Chem. Soc. 125 (2003) 1905–1911. [40] B. Campo, M. Volpe, S. Ivanova, R. Touroude, J. Catal. 242 (2006) 162–171. [41] B. Campo, C. Petit, M.A. Volpe, J. Catal. 254 (2008) 71–78. [42] R.A. Rajadhyaksha, S.L. Karwa, Chem. Eng. Sci. 41 (1986) 1765–1770. [43] Y.Z. Chen, B.J. Liaw, S.J. Chiang, Appl. Catal. A: Gen. 284 (2005) 97–104.