Ind. Eng. Chem. Res. 2004, 43, 6409-6415
6409
MATERIALS AND INTERFACES M-Mn-Al Hydrotalcite-like Compounds as Precursors for Methyl Benzoate Hydrogenation Catalysts Aimin Chen, Hualong Xu, Yinghong Yue, Wei Shen, Weiming Hua,* and Zi Gao Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, People’s Republic of China
Mn-Al and M-Mn-Al (M ) Mg, Zn, Ni, Pb, Cr) hydrotalcite-like compounds (HTlcs) were synthesized by a coprecipitation method. The hydrogenation of methyl benzoate to benzaldehyde over the calcined HTlcs was investigated. Mn2Al1 catalyst calcined in N2 gives a better performance than a conventional 10% Mn/γ-Al2O3 catalyst. Incorporation of Mg2+, Zn2+, Pb2+, and Cr3+ into the Mn-Al hydrotalcite-like precursor may further enhance the catalytic activity of the calcined catalysts. The conversion of methyl benzoate and selectivity to benzaldehyde on Pb0.2Mn1.8Al1 catalyst at 370 °C are 94.4 and 84.9%, respectively. X-ray diffraction, temperatureprogrammed reduction, and CO2 temperature-programmed desorption studies show that the redox and basic properties of the catalysts play an important role in the reaction. Introduction Aromatic aldehydes are important intermediates in fine chemical industries, manufacturing pharmaceuticals, agrochemicals, perfumes, and flavors.1 The production of aromatic aldehydes has been increasing over the years. Currently, aromatic aldehydes are mainly produced by partial oxidation of alkyl aromatics,2 halogenation of alkyl aromatics followed by hydrolysis,3 and halogenation of aromatic acids followed by hydrolysis, the so-called Rosenmund reduction.4 Nevertheless, all of these processes have their own drawbacks, such as low selectivity, low yield, and large amounts of harmful wastes polluting the environment. Therefore, much attention has been paid to explore a new ecologically benign synthetic route. The direct hydrogenation of carboxylic acids or carboxylic acid esters to corresponding aldehydes is an example of “green” technology and an alternative to the current aldehyde synthesis with high yield and low amounts of undesirable byproducts.5 γ-Al2O36 and copper chromite7 were reported as catalysts for hydrogenation of methyl benzoate (MB) or benzoic acid to benzaldehyde in the 1970s. Since then, various kinds of metal oxides, including alkali-earth oxides, transition-metal oxides, and rare-earth oxides, have been suggested for use as the hydrogenation catalysts.5,8-18 In 1988, Mitsubishi Chemicals established a commercial process using chromium-modified zirconia as the catalyst to manufacture benzaldehyde from benzoic acid with high yield.19 Among these metal oxide catalysts, manganese oxide catalysts have received growing attention because of their good activity and selectivity to benzaldehyde at rather low temperature.12-17 In the patent literature,12 the use of activated alumina as a support enhances the * To whom correspondence should be addressed. Tel.: (+86)21-65642409. Fax: (+86)-21-65641740. E-mail: wmhua@ fudan.edu.cn.
catalytic activity of manganese oxide. In our previous work,20 hydrogenation of MB to benzaldehyde over various supported manganese oxide catalysts has been studied. It was found that the strong interaction between manganese oxide and the γ-Al2O3 support plays a positive role in the hydrogenation reaction. Hydrotalcite-like compounds (HTlcs) have the general formula [MII1-xMIIIx(OH)2]x+(An-x/n)‚mH2O, where MII is a divalent cation, MIII is a trivalent cation, and An- is an anion. These types of layered compounds have attracted increasing interest in recent years because of their potential applications as catalysts, catalyst supports, ion exchangers, adsorbents, and ionic conductors.21 The catalysts obtained by thermal decomposition of hydrotalcite-like precursors often display good activity because of their smaller crystal size, higher surface area, basic surface properties, and formation of a homogeneous mixture of oxides. In the present work, a series of manganese oxide catalysts were prepared from M-Mn-Al (M ) Mg, Zn, Ni, Pb, Cr) HTlcs. The catalytic behaviors of the catalysts in the dehydrogenation of MB to benzaldehyde were investigated. The activity and selectivity of these new catalysts are correlated with the results of surface basicity measurements and temperature-programmed reduction (TPR) tests. Experimental Section Preparation of Catalysts. A series of Mn-Al HTlcs with different Mn-Al molar ratios were synthesized by a coprecipitation method.22 Two aqueous solutions, one containing a mixture of Mn(NO3)2 and Al(NO3)3 and the other KOH (1 mol/L), were added dropwise into a flask at room temperature under a nitrogen atmosphere while the pH of the solution was maintained between 9 and 10. The resulting slurry was aged at 60 °C for 24 h under a nitrogen atmosphere. The precipitate was washed with distilled water until the pH of the filtrate was around 7 and dried at 60 °C for 24 h. The ternary M-Mn-Al (M ) Mg, Zn, Ni, Pb, Cr) HTlcs with a
10.1021/ie049753a CCC: $27.50 © 2004 American Chemical Society Published on Web 08/31/2004
6410
Ind. Eng. Chem. Res., Vol. 43, No. 20, 2004
M-Mn-Al molar ratio of 0.2:1.8:1 were synthesized in the same way. The hydrotalcite precursors were calcined at 500 °C in a nitrogen atmosphere for 3 h, unless otherwise stated. The potassium-modified catalysts were prepared by impregnating the hydrotalcite precursors with an aqueous solution of KNO3, followed by drying at 60 °C for 24 h and calcining in a nitrogen atmosphere at 500 °C for 3 h. The content of potassium in the catalysts was 3 or 5 wt %. Characterization. X-ray diffraction (XRD) patterns were recorded on a Siemens D8 Advance X-ray diffractometer (Cu KR radiation, 40 kV, 40 mA) with a scan speed of 8°/min. The Brunauer-Emmett-Teller surface areas of the catalysts were measured by N2 adsorption at -196 °C on a Micromeritics ASAP 2000 instrument. Elemental analyses were carried out on a Thermo Elemental inductively coupled plasma atomic emission spectrometer after dissolution of the solid sample in a HCl solution. TPR experiments were carried out using a Micromeritics temperature-programmed desorption (TPD)/TPR 2900 instrument. A total of 25 mg of catalyst was pretreated in N2 at 300 °C for 3 h. A reduction run was then performed under a gas flow (40 mL/min) of hydrogen (10 vol %) and argon (90 vol %). The temperature was increased from 50 °C at a heating rate of 10 °C/min. A thermal conductivity detector was used to monitor the hydrogen consumed during the course of TPR. CuO was used as a standard sample for the calibration of hydrogen consumption. CO2-TPD was performed in a flow-type fixed-bed reactor at atmospheric pressure. The samples were pretreated at 500 °C for 3 h and then cooled to 80 °C in a He flow. Pure CO2 was injected until adsorption saturation, followed by purging with He for 1 h. The temperature was then raised from 80 to 500 °C at a rate of 10 °C/min to desorb CO2. Activity Measurement. The catalytic test was carried out in a fixed-bed reactor with 800 mg of catalyst in the form of 20-40-mesh particles. The reaction was performed at 350-430 °C and ambient pressure. MB was pumped to a vaporizer to mix with hydrogen (gas hourly space velocity ) 700 h-1), and the mixture (H2:MB ) 20:1 vol %) was preheated to 210 °C before entering the reactor. Prior to the reaction, the catalyst was reduced at 260 °C for 2 h, at 360 °C for 3 h, and finally at 420 °C for 2 h in a H2 flow (30 mL/min). The reaction products were condensed and analyzed by means of a gas chromatograph (GC)-mass spectrometer (Finnigen Voyager) or a GC (HP 5890) equipped with a 30-m SE-54 capillary column and a flame ionization detector. For all of the catalysts, the reaction reached steady state after running for 3-4 h. The reaction data reported in this work were obtained under steady-state conditions, and they were calculated in the following way:
MB conversion (%) )
MBf - MBp × 100 MBf
BA selectivity (%) )
BAp × 100 MBf - MBp
BA yield (%) )
BAp × 100 MBf
where MBf, MBp, and BAp represented MB in the feed,
Figure 1. XRD patterns of Mn-Al hydrotalcite-like precursors: (a) Mn:Al ) 3:1; (b) Mn:Al ) 2:1; (c) Mn:Al ) 1:1; (d) Mn:Al ) 1:2; (e) Mn:Al ) 1:4; (b) HTlcs; (1) Mn3O4; (9) Al(OH)3; ([) Al2O3‚ H2O.
Figure 2. XRD patterns of M-Mn-Al hydrotalcite-like precursors: (a) Mg0.2Mn1.8Al1; (b) Zn0.2Mn1.8Al1; (c) Ni0.2Mn1.8Al1; (d) Pb0.2Mn1.8Al1; (e) Cr0.2Mn1.8Al1; (b) HTlcs; (9) MnO2; (2) Mn2O3; (1) Mn3O4.
MB in the product, and benzaldehyde in the product, respectively. Results Hydrotalcite Precursors. The binary Mn-Al HTlcs were synthesized at Mn-Al molar ratios ranging from 1:4 to 3:1 using a coprecipitation method under a nitrogen atmosphere. Figure 1 shows the XRD patterns of the samples. The diffraction peaks at 11.5, 21.9, 34.3, 38.0, 46.6, 60.5, and 61.8° are characteristic of HTlcs. It can be seen in Figure 1 that the hydrotalcite-like structure is formed when the Mn-Al ratio is 3:1 or 2:1. However, a small amount of Mn3O4 appears in the MnAl HTlc precursor because of the oxidation of the sample during preparation. For Mn-Al molar ratios smaller than 2:1, no hydrotalcite phase but an aluminum hydroxide or oxide phase is observed. The above result indicates that the Mn-Al ratio of 2:1 to 3:1 is probably an appropriate composition range for the formation of Mn-Al HTlcs. MII ions having ionic radii not too different from that of Mn2+ can also form HTlcs.23 The ternary M-Mn-Al (M:Mn:Al ) 0.2:1.8:1, M ) Mg, Zn, Ni, Pb, Cr) HTlcs were prepared by the same method. Their XRD patterns are presented in Figure 2. Obviously, all of the samples still keep the hydrotalcite-like structure, albeit there is a small amount of Mn3O4 in the Zn-Mn-Al and NiMn-Al HTlc precursors. Several different manganese oxide phases and an aluminum oxide phase are found in the Pb-Mn-Al HTlc precursor.
Ind. Eng. Chem. Res., Vol. 43, No. 20, 2004 6411 Table 1. Chemical Composition of Hydrotalcite Precursors chemical analysis (wt %)a sample
M
Mn
Al
K
molar ratio for M-Mn-Al
Mn3Al1 Mn2Al1 Mg0.2Mn1.8Al1 Zn0.2Mn1.8Al1 Pb0.2Mn1.8Al1 Ni0.2Mn1.8Al1 Cr0.2Mn1.8Al1
0 0 1.51 2.78 8.74 2.95 2.76
41.4 36.2 25.3 22.4 19.8 22.6 22.5
6.9 9.4 6.98 6.08 5.43 6.17 6.24
0.40 0.20 0 0.15 0.17 0.29 0.31
0:2.95:1 0:1.89:1 0.24:1.78:1 0.19:1.81:1 0.21:1.79:1 0.22:1.80:1 0.23:1.77:1
a
The rest is oxygen and water.
Figure 3. XRD patterns of Mn-Al catalysts calcined at 500 °C in air (a and b) and N2 (c and d): (a and c) Mn3Al1; (b and d) Mn2Al1; (9) MnO2; (1) Mn3O4.
The chemical compositions of the binary and ternary HTlc precursors are summarized in Table 1. The M-Mn-Al molar ratios of the precursors are close to those of the reactant mixtures, suggesting that the precipitation of the metal cations is almost complete. KOH was employed as the precipitant in the synthesis instead of NaOH to avoid the detrimental effect of Na+ on the activity of the final catalysts. The amount of K+ left in the HTlc precursors after washing is less than 0.4 wt %.
Hydrogenation on Calcined Mn-Al HTlcs. The Mn-Al HTlc precursors were calcined at 500 °C in air and N2, and their XRD patterns are compared in Figure 3. The layered structure of the HTlc precursor is completely destroyed, and small crystallites of Mn3O4 and MnO2 are formed after calcination at 500 °C in both air and N2. The catalysts calcined in N2 display weaker and broader diffraction peaks than those calcined in air, indicating that the manganese oxides in the former catalysts have smaller crystallite size. The surface areas of the catalysts calcined in N2 are also much higher than those of the catalysts calcined in air, as shown in Table 2, which is consistent with the XRD result. Therefore, calcination in N2 is probably more preferable for the synthesis of these types of catalysts. The activity and selectivity of the calcined catalysts for the hydrogenation of MB at 350-430 °C are listed in Table 2. The catalytic activity is increased with the reaction temperature, but the selectivity to benzaldehyde is diminished because of the increased concentration of toluene in the product. The Mn2Al1 catalyst calcined in N2 is the most active one among all of the calcined HTlcs because of its high surface area. The activity and selectivity of this catalyst as a function of the reaction temperature are depicted in Figure 4. At 390 °C, the conversion of MB and yield of benzaldehyde on this catalyst are 93.6 and 73.2%, respectively. In comparison with the result on the 10% Mn/γ-Al2O3 catalyst, the reaction temperature for the same yield level is reduced by 20 °C. It is clear in Table 2 that catalysts calcined in N2 have higher surface areas and are more active than the same catalysts calcined in air. Hence, in the following experiments, the ternary M-MnAl HTlc precursors were all calcined at 500 °C in a N2 atmosphere. Hydrogenation on Calcined M-Mn-Al HTlcs. Hydrogenation of MB was performed over M-Mn-Al (M ) Mg, Zn, Ni, Pb, Cr) catalysts prepared by calcining the respective HTlc precursors. The conversion of MB and the product distribution after reaction in the
Table 2. Hydrogenation of MB over Calcined Mn-Al Catalysts selectivity (%)a catalyst b
Mn3Al1
Mn2Al1b
Mn3Al1c
Mn2Al1c
10% Mn/γ-Al2O3
a
T (°C)
convn (%)
BA
B
T
BOH
ME
others
BA yield (%)
350 370 390 410 430 350 370 390 410 430 350 370 390 410 430 350 370 390 410 430 350 370 390 410 430
7.7 15.5 40.8 63.8 94.5 20.8 52.3 73.2 90.9 92.9 21.7 44.3 80.2 95.8 98.3 28.1 61.4 93.6 96.5 98.4 17.5 30.3 59.6 92.7 93.5
99.0 98.0 92.0 87.3 74.5 96.5 92.1 86.0 78.2 73.9 94.2 91.5 85.5 70.6 51.3 93.5 91.7 78.2 64.2 47.1 86.9 89.2 84.2 78.2 69.5
0 0 0.4 0.5 0.7 0 0 0.2 0.3 0.5 0.2 0.2 0.4 0.9 2.3 0.2 0.2 0.3 1.0 2.6 0 0.9 0.9 0.6 0.6
0 0 1.1 4.1 15.0 0.5 1.4 5.9 8.0 12.3 0.8 1.6 5.3 16.5 30.3 1.8 2.1 9.0 21.9 27.9 6.7 2.4 3.5 6.4 13.3
0 0 0.9 3.0 3.5 0 0 5.4 6.6 5.5 0.8 2.0 4.0 4.4 1.8 0.9 4.0 7.3 3.5 1.8 1.9 2.9 4.9 4.7 4.4
0 0 0 0 0 0 0 1.9 2.2 2.1 0 0 0 0 0 0 0 0 0 0 0.9 1.5 1.7 2.6 4.1
1.0 2.0 5.6 5.1 6.3 3.0 6.5 0.6 4.7 5.7 4.0 4.7 4.8 7.6 14.3 3.6 2.0 5.2 9.4 20.6 3.6 3.1 4.8 7.5 8.1
7.6 15.4 37.5 55.8 70.2 20.1 47.9 63.1 71.1 68.7 20.4 40.5 68.5 67.7 50.4 26.3 56.3 73.2 62.0 46.4 15.2 27.0 50.2 72.5 65.0
surface area (m2/g)
13.3
88.3
87.2
121.1
73.9
BA ) benzaldehyde; B ) benzene; T ) toluene; BOH ) benzyl alcohol; ME ) methyl benzyl ether. b Calcined in air. c Calcined in N2.
6412
Ind. Eng. Chem. Res., Vol. 43, No. 20, 2004
Figure 4. Hydrogenation of MB over Mn2Al1 calcined at 500 °C in N2: (9) MB conversion; (b) benzaldehyde selectivity; (2) benzene selectivity; (1) toluene selectivity; (O) benzyl alcohol selectivity; (4) others selectivity.
temperature range of 350-430 °C are summarized in Table 3. The activity of the Mn2Al1 catalyst is promoted by the addition of a small amount of Mg, Zn, Pb, or Cr; however, after incorporation of Ni, the activity is diminished. The conversion of MB on M-Mn-Al catalysts varies as Zn0.2Mn1.8Al1 > Pb0.2Mn1.8Al1 > Cr0.2Mn1.8Al1 > Mg0.2Mn1.8Al1 > Mn2Al1 > Ni0.2Mn1.8Al1, whereas the maximum yield of benzaldehyde follows the order of Pb0.2Mn1.8Al1 > Cr0.2Mn1.8Al1 ∼ Mg0.2Mn1.8Al1 ∼ Mn2Al1 > Zn0.2Mn1.8Al1 . Ni0.2Mn1.8Al1. The conversion of MB and selectivity to benzaldehyde on the Pb0.2Mn1.8Al1 catalyst at 370 °C are 94.4 and 84.9%, respectively. In comparison with the conversion and selectivity of 92.7 and 78.2% at 410 °C on a 10% Mn/ γ-Al2O3 supported catalyst, the reaction temperature for the same yield level is reduced by at least 40 °C. The activity and selectivity of the Pb0.2Mn1.8Al1 catalyst as a function of the temperature are depicted in Figure 5. As compared with calcined Mn-Al HTlcs, calcined M-Mn-Al HTlcs exhibit a similar trend of variation in catalytic activity and selectivity with reaction temperature. The XRD patterns of the M-Mn-Al catalysts calcined at 500 °C in N2 are presented in Figure 6. The oxidation
Figure 5. Product distribution in the hydrogenation of MB over the Pb0.2Mn1.8Al1 catalyst: (9) MB conversion; (b) benzaldehyde selectivity; (2) benzene selectivity; (1) toluene selectivity; (O) benzyl alcohol selectivity; (4) others selectivity.
Figure 6. XRD patterns of M-Mn-Al and K-modified catalysts calcined at 500 °C in N2: (a) Mg0.2Mn1.8Al1; (b) Zn0.2Mn1.8Al1; (c) Ni0.2Mn1.8Al1; (d) Pb0.2Mn1.8Al1; (e) Cr0.2Mn1.8A1; (f) 3% K/Pb0.2Mn1.8Al1; (g) 3% K/Mg0.2Mn1.8Al1; (9) MnO2; (2) Mn2O3; (1) Mn3O4.
state of manganese oxide in these catalysts is different. Only Mn3O4 is observed in the patterns of the M-MnAl (M ) Mg, Zn, Ni, Cr) catalysts, but the diffraction peaks of the MnO2, Mn2O3, and Mn3O4 phases are found
Table 3. Hydrogenation of MB over Calcined M-Mn-Al Catalysts selectivity (%) catalyst
T (°C)
convn (%)
BA
B
T
BOH
ME
others
BA yield (%)
surface area (m2/g)
Mg0.2Mn1.8Al1
350 370 390 410 430 350 370 390 410 430 350 370 390 410 430 350 370 390 410 430 350 370 390 410 430
33.1 69.4 95.2 96.8 98.0 64.6 94.7 98.3 99.5 100 50.3 94.4 97.0 97.9 99.4 28.3 54.3 63.9 96.4 98.5 47.2 93.3 97.5 98.0 99.5
92.6 91.4 77.2 57.0 35.9 81.5 73.8 68.2 50.2 43.5 90.0 84.9 72.3 53.3 32.7 50.0 42.3 28.8 23.8 18.5 92.0 80.0 73.5 57.4 37.5
0.2 0.2 0.3 1.2 3.0 0.5 0.6 0.7 2.0 3.1 0.2 0.2 0.3 1.0 2.7 18.9 22.5 27.2 29.2 31.8 0 0.2 0.4 1.7 3.8
1.2 3.1 10.9 25.8 39.0 2.7 8.6 18.2 23.8 27.6 2.6 9.5 20.3 33.5 40.7 16.7 9.6 13.3 14.8 17.6 1.0 5.9 13.3 22.4 31.7
2.5 3.9 5.9 3.8 1.2 3.8 10.9 4.1 2.7 1.5 2.7 5.2 3.0 1.8 1.0 2.5 3.9 3.8 5.9 4.2 0.4 8.6 5.2 2.3 1.7
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2.7 1.8 1.2 0.2
3.5 1.4 5.7 12.2 20.9 11.5 6.1 8.8 21.3 24.3 4.5 0.2 4.1 10.4 22.9 11.9 21.7 26.9 26.3 27.9 6.6 2.6 5.8 15.0 25.1
30.7 63.4 73.6 55.2 38.3 52.7 69.9 67.1 50.0 43.5 45.2 80.1 70.1 52.2 32.5 14.2 23.0 18.4 22.9 18.2 43.4 74.6 71.7 56.3 37.4
136.2
Zn0.2Mn1.8Al1
Pb0.2Mn1.8Al1
Ni0.2Mn1.8Al1
Cr0.2Mn1.8Al1
128.4
108.2
133.5
132.6
Ind. Eng. Chem. Res., Vol. 43, No. 20, 2004 6413 Table 4. Hydrogenation of MB over K/Mg0.2Mn1.8Al1 and K/Pb0.2Mn1.8Al1 Catalysts selectivity (%) catalyst
T (°C)
convn (%)
BA
B
T
BOH
ME
others
BA yield (%)
surface area (m2/g)
3% K/Mg0.2Mn1.8Al1
350 370 390 410 430 350 370 390 410 430 350 370 390 410 430 350 370 390 410 430
24.0 58.0 90.0 94.6 96.5 40.5 86.9 90.1 97.2 98.5 19.6 50.3 84.0 94.8 98.2 35.6 69.7 84.6 95.4 96.3
96.5 93.1 88.0 78.0 68.7 94.0 90.8 80.8 69.3 60.2 95.8 92.8 90.2 78.9 67.6 95.1 91.7 83.7 73.3 60.4
0.5 0.7 0.7 1.6 5.7 2.5 2.3 2.0 4.3 7.2 0.7 0.8 0.8 3.3 5.5 0.7 0.8 3.3 4.5 7.7
0.5 0.2 0.5 1.6 3.3 1.6 1.1 2.3 5.4 10.7 1.4 0.7 0.4 2.4 2.6 0.6 0.7 1.3 2.5 9.6
1.1 3.3 6.4 6.3 5.6 1.3 3.7 4.0 4.2 3.7 1.0 2.6 4.0 7.1 4.5 1.8 2.8 3.2 4.5 3.9
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1.4 2.7 4.4 12.5 16.7 0.6 2.1 10.9 16.8 18.2 1.1 3.1 4.6 8.3 19.8 1.8 4.0 8.5 15.2 18.4
23.2 54.0 79.2 73.8 66.3 38.1 78.9 72.8 62.4 59.3 18.4 46.7 79.1 74.8 66.4 34.7 66.7 70.8 69.9 61.1
79.3
3% K/Pb0.2Mn1.8Al1
5% K/Mg0.2Mn1.8Al1
5% K/Pb0.2Mn1.8Al1
71.3
61.3
56.4
Table 5. TPR Data of M-Mn-Al Catalysts
in the pattern of the Pb-Mn-Al catalyst. The specific surface areas of the M-Mn-Al catalysts are slightly higher than that of the Mn2Al1 catalyst, except the PbMn-Al catalyst (see Table 3). Effect of a Potassium Additive. The XRD patterns of the 3% K/Mg0.2Mn1.8Al1 and 3% K/Pb0.2Mn1.8Al1 catalysts calcined at 500 °C in N2 are also shown in Figure 6. The patterns of the K-modified catalysts are similar to those of the unmodified catalysts, but the intensities of the peaks are increased, showing that the crystallite sizes of the manganese oxides in the catalysts are probably increased. The specific surface areas of the catalysts are reduced after incorporation of the alkali, as shown in Table 4, which is consistent with the XRD results. The conversion of MB and the product distribution on the K-modified catalysts are listed in Table 4. The catalysts are less active after the addition of a small amount of K, but the selectivity to benzaldehyde is increased substantially because of the decreased concentration of toluene in the product. Moreover, the selectivity to benzaldehyde decreases less abruptly as the reaction temperature is increased. The MB conversion and benzaldehyde selectivity on the 3% K/ Pb0.2Mn1.8Al1 catalyst at 370 °C are 86.9 and 90.8%, respectively. TPR and CO2-TPD Studies. The TPR profiles of the M-Mn-Al catalysts calcined at 500 °C in N2 are illustrated in Figure 7. All of the catalysts show a twostep reduction except the Ni0.2Mn1.8Al1 catalyst. The first two peaks in the profiles correspond to subsequent
reduction of MnO2 to Mn2O3/Mn3O4 and MnO,24 demonstrating that there are quite a number of very small and dispersed MnO2 crystallites in the catalysts that are undetectable by XRD (see Figure 6). It is notable that the third peak (654 °C) of the Ni0.2Mn1.8Al1 catalyst corresponds to the reduction of Ni2+ to Ni.25 The reduction peak temperatures and the H2 consumptions are summarized in Table 5. The temperature of the reduction to MnO for the catalysts increases in the order of Pb0.2Mn1.8Al1 < Zn0.2Mn1.8Al1 < Mg0.2Mn1.8Al1 ∼ Cr0.2Mn1.8Al1 ∼ Ni0.2Mn1.8Al1 < Mn2Al1. The TPR profiles of the K/Mg0.2Mn1.8Al1 and K/Pb0.2Mn1.8Al1 catalysts calcined at 500 °C in N2 are shown in Figure 8, and the results are summarized in Table 6. It seems that the catalysts become more difficult to reduce after the addition of a small amount of K. The XRD patterns of the catalysts in this work after reaction are illustrated in Figure 9. Manganese oxide exists in the form of MnO in all of the catalysts after reaction.
Figure 7. TPR profiles of M-Mn-Al catalysts calcined at 500 °C in N2: (a) Mn2Al1; (b) Mg0.2Mn1.8Al1; (c) Zn0.2Mn1.8Al1; (d) Ni0.2Mn1.8Al1; (e) Pb0.2Mn1.8Al1; (f) Cr0.2Mn1.8Al1.
Figure 8. TPR profiles of K/Mg0.2Mn1.8Al1 and K/Pb0.2Mn1.8Al1 catalysts: (a) 3% K/Pb0.2Mn1.8Al1; (b) 3% K/Mg0.2Mn1.8Al1; (c) 5% K/Pb0.2Mn1.8Al1; (d) 5% K/Mg0.2Mn1.8Al1.
H2 consumption (mmol/g)
peak (°C) catalyst
I
II
Mn2Al1 Mg0.2Mn1.8Al1 Zn0.2Mn1.8Al1 Ni0.2Mn1.8Al1 Pb0.2Mn1.8Al1 Cr0.2Mn1.8Al1
353 333 298 376 324 324
478 460 451 467 402 463
III
654
I
II
0.047 0.046 0.035 0.038 0.049 0.048
0.042 0.039 0.036 0.031 0.041 0.029
III
0.013
total 0.089 0.085 0.071 0.082 0.090 0.077
6414
Ind. Eng. Chem. Res., Vol. 43, No. 20, 2004
Table 6. TPR Data of K/Mg0.2Mn1.8Al1 and K/Pb0.2Mn1.8Al1 peak (°C)
H2 consumption (mmol/g)
catalyst
I
II
I
II
total
3% K/Mg0.2Mn1.8Al1 3% K/Pb0.2Mn1.8Al1 5% K/Mg0.2Mn1.8Al1 5% K/Pb0.2Mn1.8Al1
335 332 357 352
507 440 481 498
0.043 0.059 0.056 0.052
0.030 0.040 0.017 0.048
0.077 0.099 0.073 0.100
Table 7. CO2-TPD Data of Some Representative Catalysts temp (°C) catalyst
I
Mg0.2Mn1.8Al1 Pb0.2Mn1.8Al1 3% K/Mg0.2Mn1.8Al1 3% K/Pb0.2Mn1.8Al1 5% K/Mg0.2Mn1.8Al1 5% K/Pb0.2Mn1.8Al1
229 187 208 240 210 201
II 242 257 328
CO2 desorbed (mmol/g) I 0.035 0.017 0.061 0.060 0.128 0.122
II 0.020 0.055 0.056
total 0.035 0.037 0.116 0.060 0.184 0.122
The CO2-TPD profiles of some representative catalysts are shown in Figure 10. The desorption temperatures and the CO2 uptakes are listed in Table 7. The CO2-TPD profiles of the catalysts feature a main broad peak centered around 190-240 °C, convoluted with a shoulder peak at higher temperatures. The addition of potassium to the catalysts increases the total amount of CO2 uptake, showing that the number of the basic sites on the catalysts is increased.
Figure 9. XRD patterns of M-Mn-Al and K-modified catalysts after reaction: (a) Mn2Al1; (b) Mg0.2Mn1.8Al1; (c) Zn0.2Mn1.8Al1; (d) Ni0.2Mn1.8Al1; (e) Pb0.2Mn1.8Al1; (f) Cr0.2Mn1.8Al1; (g) 3% K/Mg0.2Mn1.8Al1; (h) 3% K/Pb0.2Mn1.8Al1; ([) MnO.
Discussion Manganese oxide may exist in different oxidation states on the catalysts, such as MnO2, Mn2O3, Mn3O4, and MnO. In our previous work,20 it has been observed that under the reaction conditions the manganese oxides in the catalysts are reduced to MnO, and we have proposed that MnO is probably the active component for the reaction. Here in this work, MnO is again identified in all of the catalysts after reaction (see Figure 9). These results show that the redox properties of the manganese oxides and the oxygen vacancies on the catalyst surface created during reduction could play an important role in the hydrogenation reaction. It has been suggested by various authors5,17,18,26 that the hydrogenation of carboxylic acids or carboxylates proceeds via a reverse Mars and van Krevelen mechanism consisting of two steps. The first step is the dissociative adsorption of benzoate on the oxygen vacancies of MnO to form ArCOO- species. The second step is the reaction between the ArCOO- species and the activated hydrogen on the catalyst surface followed by benzaldehyde desorption from the catalyst. According to this mechanism, the existence of an appropriate amount of oxygen vacancies on the catalyst facilitates the hydrogenation reaction. The oxides of Pb and Zn are themselves active for the hydrogenation of benzoic acid, whereas the oxides of Cr and Mg are less active for the reaction.17 Hence, the promoting effect of Pb, Zn, Cr, and Mg could be related to both the increased reducibility to MnO (see Figure 7 and Table 5) and the increased number of active sites. The main byproduct of the hydrogenation reaction is toluene. At elevated temperature, a small amount of benzene and benzyl alcohol are also formed in the reaction. Toluene is formed either by the direct reduction of benzoate or as a secondary product from benzaldehyde.27 A high concentration of oxygen vacancies formed by overreduction of the oxide could lead to the direct reduction of benzoate to toluene at adjacent oxygen vacancy sites and reduce the selectivity to benzaldehyde. This may account for the decrease in selectivity with increasing reaction temperature. On the other hand, toluene is formed indirectly via desorption and readsorption of benzaldehyde. In this case, benzoate and benzaldehyde compete for the oxygen vacancies. The selectivity to benzaldehyde is high when benzoate adsorbs more strongly on the catalyst than benzaldehyde. The basicity of the catalyst enhances the dissociative adsorption of benzoate and inhibits the readsorption and consecutive hydrogenation of benzaldehyde. This gives an explanation for the improvement in selectivity for the K-modified catalysts and the drop in selectivity at high benzoate conversion. Conclusion
Figure 10. CO2-TPD profiles of some represented catalysts: (a) Pb0.2Mn1.8Al1; (b) Mg0.2Mn1.8Al1; (c) 3% K/Pb0.2Mn1.8Al1; (d) 3% K/Mg0.2Mn1.8Al1; (e) 5% K/Pb0.2Mn1.8Al1; (f) 5% K/Mg0.2Mn1.8Al1.
A series of Mn-containing HTlcs were prepared by a coprecipitation method. The Mn2Al1 catalyst prepared by calcination of the hydrotalcite-like precursor in N2 is more active than a conventional 10% Mn/γ-Al2O3 catalyst for the hydrogenation of MB to benzaldehyde. The activity of the Mn2Al1 catalyst is enhanced by the addition of a small amount of Mg, Zn, Pb, or Cr, whereas after incorporation of Ni, the activity is diminished. The promoting effect of the third component is associated with not only their activity toward the reaction but also the increased reducibility to MnO. A selectivity to
Ind. Eng. Chem. Res., Vol. 43, No. 20, 2004 6415
benzaldehyde of 84.9% at a conversion of 94.4% is achieved on the Pb0.2Mn1.8Al1 catalyst at 370 °C, whereas the conversion and selectivity on the 10% Mn/γ-Al2O3 supported catalyst at 410 °C are 92.7 and 78.2%, respectively. The lower conversion of MB and the higher selectivity to benzaldehyde of the catalysts added with potassium could be related to a decreased reducibility to MnO and an increase in basicity, respectively. Acknowledgment This work was financially supported by the Chinese Major State Basic Research Development Program (2000077507), the Shanghai Major Basic Research Program (03DJ14004), and the National Natural Science Foundation of China (20303004). Literature Cited (1) Ho¨lderich, W. F. New reactions in various fields and production of specialty chemicals. Stud. Surf. Sci. Catal. 1993, 75, 127. (2) Saito, N.; Nakamura, I.; Ueshima, M.; Takatsu, K.; Nagi, I. Method of producing substituted benzaldehydes. WO Patent 8606715, 1986. (3) Maki, T.; Yokoyama, T. Recent progress in manufacturing technology for aromatic aldehydes. Org. Synth. Chem. 1991, 49, 195. (4) Rosenmund, K. W. New method for the preparation of aldehydes. Berichte 1918, 51, 585. (5) Yokoyama, T.; Yamagata, N. Hydrogenation of carboxylic acids to the corresponding aldehydes. Appl. Catal. A 2001, 221, 227. (6) Feinstein, A.; Fields, E. K. Vapor phase conversion of aromatic esters to aromatic aldehydes. U.S. Patent 3,935,265, 1976. (7) Wada, A.; Nakajima, C.; Hironaka, T. Benzyl alcohol. Japan Kokai Patent 7558022, 1975. (8) King, S. T.; Strojny, E. J. An in situ study of methyl benzoate and benzoic acid reduction on yttrium oxide by infrared spectroscopic flow reactor. J. Catal. 1982, 76, 274. (9) Ho¨lderich, W. F.; Tjoe, J. Direct hydrogenation of aromatic carboxylic acids to their corresponding aldehydes with zinc oxide catalysts. Appl. Catal. A 1999, 184, 257. (10) Kondo, J.; Ding, N.; Maruya, K.; Domen, K.; Yokoyama, T.; Fujita, N.; Maki, T. Infrared study of hydrogenation of benzoic acid to benzaldehyde on ZrO2 catalysts. Bull. Chem. Soc. Jpn. 1993, 66, 3085. (11) Sakata, Y.; Ponec, V. Reduction of benzoic acid on CeO2 and the effect of additives. Appl. Catal. A 1998, 166, 173. (12) Gelbein, A. P.; Hansen, R. Hydrogenation of carboxylic acid compounds to aldehydes using MnO2 on gamma alumina as catalyst. U.S. Patent 4,585,899, 1986.
(13) van Geem, P. C.; Janssen, L. H. W. Process for the preparation of benzaldehyde in the gas phase. European Patent 290096, 1988. (14) Xu, H.; Shen, W.; Pan, X.; Xiang, Y.; Liu, J. Vapour-phase hydrogenation of benzoic acid to benzaldehyde over MnO/γ-Al2O3 catalysts. Chin. J. Catal. 1997, 18, 384. (15) Yang, Y.; Xu, H.; Shen, W.; Chen, J.; Zhou, Y.; Xiang Y. Study on catalytic hydrogenation of methyl benzoate to benzaldehyde. Chin. J. Catal. 1999, 20, 530. (16) Xu, H.; Shen, W.; Yang, Y. Characterization and activity of MnO/γ-Al2O3 for hydrogenation of methyl benzoate to benzaldehyde. Chin. J. Chem. 2001, 19, 647. (17) Sakata, Y.; van Tol-Koutstaal, C. A.; Ponec, V. Selectivity problems in the catalytic deoxygenation of benzoic acid. J. Catal. 1997, 169, 13. (18) de Lange, M. W.; van Ommen, J. G.; Lefferts, L. Deoxygenation of benzoic acid on metal oxides 1. The selective pathway to benzaldehyde. Appl. Catal. A 2001, 220, 41. (19) Yokoyama, T.; Setoyama, T.; Fujita, N.; Nakajima, M.; Maki, T. Appl. Catal. A 1992, 88, 149. (20) Chen, A.; Xu, H.; Yue, Y.; Hua, W.; Shen, W.; Gao, Z. Support effect in hydrogenation of methyl benzoate over supported manganese oxide catalysts. J. Mol. Catal. A 2003, 203, 299. (21) Vaccari, A. Preparation and catalytic properties of cationic and anionic clays. Catal. Today 1998, 41, 53. (22) Velu, S.; Ramaswamy, V.; Ramani, A.; Chanda, B. M.; Sivasanker, S. New hydrotalcite-like anionic clays containing Zr4+ in the layers. Chem. Commun. 1997, 2107. (23) Cavani, F.; Trifiro, F.; Vaccori, A. Hydrotalcite-type anionic clays: preparation, properties and applications. Catal. Today 1991, 11, 173. (24) Kapteijn, F.; Singoredjo, L.; Andreini, A. Activity and selectivity of pure manganese oxides in the selective catalytic reduction of nitric oxide with ammonia. Appl. Catal. B 1994, 3, 173. (25) Wang, J.; Dong, L.; Hu, Y.; Zheng, G.; Hu, Z.; Chen, Y. Dispersion of NiO supported on γ-Al2O3 and TiO2/Al2O3 supports. J. Solid State Chem. 2001, 157, 274. (26) Pestman, R.; Koster, R. M.; Boellaard, E.; van der Kraan, A. M.; Ponec, V. Identification of the active sites in the selective hydrogenation of acetic acid to acetaldehyde on iron oxide catalysts. J. Catal. 1998, 174, 142. (27) de Lange, M. W.; van Ommen, J. G.; Lefferts, L. Deoxygenation of benzoic acid on metal oxides. 2. Formation of byproducts. Appl. Catal. A 2002, 231, 17.
Received for review March 28, 2004 Revised manuscript received June 29, 2004 Accepted July 21, 2004 IE049753A
