SDB Bimetallic Catalysts for The Direct Oxidative Esterification

Oct 29, 2012 - Pd–Pb/SDB catalyst prepared by depositing lead and palladium on hydrophobic styrene-divinylbenzene (SDB) copolymer constitutes a new ...
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Pd−Pb/SDB Bimetallic Catalysts for The Direct Oxidative Esterification of Methacrolein to Methyl Methacrylate Baohe Wang,* Wenjuan Sun, Jing Zhu, Weili Ran, and Shuang Chen Key Laboratory for Green Chemical Technology of Ministry of Education, Research and Development Center of Petrochemical Technology, Tianjin University, Tianjin 300072, People’s Republic of China ABSTRACT: Pd−Pb/SDB catalyst prepared by depositing lead and palladium on hydrophobic styrene-divinylbenzene (SDB) copolymer constitutes a new and highly efficient catalyst for the oxidative esterification of methacrolein with methanol to methyl methacrylate under mild conditions. Compared to the monometallic Pd/SDB catalyst and conventional hydrophilic catalysts supported on γ-Al2O3, the Pd−Pb/SDB bimetallic catalyst shows higher reaction rate and product selectivity. The Pd−Pb/SDB catalyst was prepared by impregnating method and characterized by N2 adsorption−desorption isotherm, X-ray diffraction (XRD), environmental scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results showed that Pd−Pb atoms formed intermetallic Pd3Pb1 crystals and the active component had a better dispersion and a smaller particle size than Pd/SDB. The SDB-supported catalyst could be recycled easily and reused, even up to seven cycles, without any appreciable loss efficiency. Based on the experimental results, the potential reaction mechanism was also proposed.

1. INTRODUCTION Methyl methacrylate (MMA) is an important monomer that is mainly used for the production of polymethyl methacrylate and other polymer dispersions for paintings and coats.1 It has been widely spread recently for the optical usage in electro-optical materials such as optical fibers or a light guide plates.2 MMA can be manufactured by numerous routes based on C2−C4 hydrocarbon feedstocks.3−5 Currently, the production of MMA is mainly produced by the acetone cyanohydrin process, but there are problems of dealing with the ammonium bisulfate waste and handling of toxic hydrogen cyanide.3 Recently, an environmentally benign one-pot procedure2,6 for the oxidative esterification of methacrolein (MAL) to MMA has been developed. This process uses molecular oxygen as the oxidant in the presence of methanol and a catalyst. Liu et al. developed the Pd-catalzyed direct aerobic oxidative esterification of benzyl alcohols with methanol and various long-chain aliphatic alcohols to corresponding esters.7 We have recently reported the oxidative reaction over monometallic palladium (Pd),8 but the MMA yield was only moderate (44%). This may be due to the oxidation of the metal, which occasionally caused a severe leaching of the metal into the reaction medium.9,10 The development of the catalyst is the key technology of the process. Many bimetallic catalysts have been studied and Pdlead (Pb)10−13 is one of the best-known metallic catalysts used in the oxidative esterification of alcohol with aldehyde to corresponding esters. Miyake et al. found that when the molar ratio of Pd to Pb is equal 3, the Pd−Pb intermetallic compound has a face-centered cubic structure and only a small amount of Pb could improve the selectivity drastically.12 Catalyst supports play an important role, especially for reactions involving water as a reactant or product. Hydrophilic supports such as ZnO,14 metallosilicate,15 Al2O3,16 and combinations17 of different metal oxides have been studied in detail to different catalysts systems for the reaction of aldehyde with alcohol to esters. Zhao et al. compared the effects of different supports and found that γ-Al2O3 was better than MgO, © 2012 American Chemical Society

CaCO3, ZnO2, and SiO2 supports in MMA selectivity and MAL conversion.16 However, the oxidative esterification reaction is a reversible process, and the generated water not only impedes the forward chemical reaction but also forms a water film on the surface of the catalysts, which retards the diffusion of reactants to active sites,18 resulting in the deactivation of the catalyst. To overcome this problem, hydrophobic materials have been employed as carriers. It is noted that styrene-divinylbenzene copolymer (SDB) support is indeed hydrophobic beads19 with an inert high surface area that is suitable for supporting noble metal catalysts.20 Catalysts over the hydrophobic SDB support have been used for many reactions21−27 but not the oxidative esterification reaction. Yen et al. investigated the effect of support on the oxygen adsorption behavior and found that oxygen might be absorbed by Pd and then diffused into the surface of SDB, but no oxygen adsorption was found on γAl2O3 support.28 The present work aims to study the effect of Pb on Pd-based catalysts and the effect of supports (SDB and γ-Al2O3) for the one-pot oxidative esterification of MAL to MMA in the presence of molecular oxygen. The influence of the active components and their contents on the reaction was studied. Preliminary experiments8 were conducted by varying the amount of Pd from 0.5 to 5 wt % on the SDB support, and it was found that 3 wt % Pd loading is the optimum for the reaction. To study the interactions between Pd and Pb, we chose the Pd3/SDB, Pd3Pb0.6/SDB, and Pd3Pb1/SDB catalysts as objects of our study, here the number 3, 0.6, and 1 represent the contents of Pd and Pb compared with the support by weight, respectively. The X-ray diffraction (XRD) results Received: Revised: Accepted: Published: 15004

June 25, 2012 September 27, 2012 October 29, 2012 October 29, 2012 dx.doi.org/10.1021/ie301674r | Ind. Eng. Chem. Res. 2012, 51, 15004−15010

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combined with flash camera equipment (GBX, France) at room temperature. The measured contact angles were an average of five measurements. 2.3. Catalytic Performance. A double-walled glass reactor with an inside diameter of 5 cm and volume of 100 mL with water circulation for thermostatic control, a stirrer, and an oxygen supply system was used to perform the oxidative esterification of MAL to MMA. The reaction was carried out at atmospheric pressure, 323 K, pH = 10.5, and a molar ratio methanol/MAL of 40. The mixture was stirred at 500 rpm, and oxygen was bubbled through at a constant flow rate of 6 mL/ min. Once the temperature was stabilized, the catalyst was added to the solution and the oxidation reaction started by introducing oxygen. The reaction samples trapped by a condenser at 253 K were taken every 30 min, filtered and analyzed by gas chromatography (GC). After the reaction, the isolated catalyst was washed with ethanol several times, dried in vacuum at 353 K for one day, and this could be reused when new amounts of substrates were added. The reaction samples were analyzed using a gas chromatograph (Aglient gas chromatograph model GDX-103; thermal conductivity detector (TCD), equipped with a DB-WAX capillary column, 6 m × Φ3; and a SP4270 data processor). Catalytic results were expressed as conversion rate (X, %), selectivity (S, %), and yield (Y, %), and they were defined as

proved a strong interaction between Pd and Pb with the formation of binary alloys of Pd3Pb. Three γ-Al2O3 supported catalysts with the same composition as the SDB-supported catalysts were also prepared to study the role of the supports. The reaction mechanism was also proposed and the π-ally intermediate was supposed to react with methanol directly to form the corresponding ester.

2. EXPERIMENTAL SECTION 2.1. Material and Catalyst Preparation. The SDB catalyst support was prepared by polymerizing divinylbenzene in ethylvinylbenzene. 2-Methyl-1-pentanol and 2,2-azobis(2methylpropinotrile) were used as solvent and initiator, respectively. At first, the SDB support was washed with ethanol to remove any water and insoluble impurities that might be absorbed in the resin support, and then, it was dried at 373 K in the air. The support of 10 g γ-Al2O3 was calined at 773 K for 4 h prior to impregnation. The SDB-supported catalysts were prepared by the incipient wetness impregnation method using PdCl2 (99.99% metal basis) and Pb(NO3)2 (99.99% metal basis) as the metallic precursors for Pd and Pb, respectively. The Pd leaching in the water solution is slightly higher than in the N,N-dimethyl formamide (DMF) solution,29 and so, we chose DMF (99.5+% purity) as a solvent for metal solution. The SDB-supported catalysts were prepared by the impregnation of 10 g SDB with the metal solution containing the required contents of Pd(OAC)2 and Pb(NO3)2 overnight, followed by vacuum drying at 353 K for 8 h, and then infrared drying for 6 h. The catalyst samples were reduced until the pH of the furnace outlet became neutral. The catalysts supported on γ-Al2O3 were prepared by aqueous impregnation in water solution of Pd(OAC)2 and Pb(NO3)2. At first, the catalysts were dried at 353 K for 8 h under a slight vacuum to remove the water. The catalysts were further dried, calcined, and then reduced with 50 mL (NTP) min−1 of hydrogen at 473 K until the pH of the furnace outlet became neutral. 2.2. Catalyst Characterization. The polymer, SDB, and catalysts were characterized by different analytical techniques. The catalysts were characterized with respect to Brunauer− Emmett−Teller (BET) surface area and porosities by nitrogen adsorption at 77 K using a Micromeritics TriStar 3000 (American Micromeritics) automated system. The amount of desorbed nitrogen was used to calculate total surface area. The porosities were calculated by Barrett−Joyner−Halenda (BJH) method. The contents of Pd and Pb in each catalyst sample were determined using inductively coupled plasma (ICP) spectroscopy (IRIS Intrepid II XSP, Thermo). X-ray diffraction (XRD) spectra were obtained ex situ using a Pilips power/thin film diffractometer PW1140 with a Ni-filtered Cu Kα radiation (λ = 0.1541 nm), at 40 kV. The examples were scanned at a rate of 0.02°/step with 2θ varying from 30° to 90°. The morphology of the examples was characterized by environmental scanning electron microscopy (ESEM) (XL30 ESEMFEG, FE I, U.S.A.) operating at 30 kV. Transmission electron microscopy (TEM) analysis of the metal particles was performed using a Tecnai G2 F20 microscope. Particle size measurements were performed particle by particle, using ITEM software on digitized micrographs. The average metal particle sizes, d, were determined from the measurement of at least 100 metal particles, and d was calculated using the following formula: d =Σnidi/∑ni, where ni is the number of particles of diameter di. The static contact angles of water drops on the surfaces were measured with an automatic contact angle meter

X (%) =

S (%) =

C0,MAL − Ct ,MAL C0,MAL

× 100

CMMA × 100 C0,MAL − Ct ,MAL

Y = X ·S

wherein C0,MAL is the molar concentration of MAL at the beginning of the reaction process, Ct,MAL the molar concentration of MAL after time t (min), and CMMA is the molar concentration of MMA after time t.

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. The XRD patterns of the catalysts were measured and are shown in Figure 1. The main

Figure 1. XRD patterns of catalysts: (a) Pd3/SDB; (b) Pd3Pb0.6/SDB; (c) Pd3Pb1/SDB; (d) Pd3 /γ-Al2O3; (e) Pd3Pb0.6/γ-Al2O3; (f) Pd3Pb1/ γ-Al2O3. 15005

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Figure 2. ESEM images of (a) SDB support and (b) Pd3Pb0.6/SDB catalyst at higher magnification.

Figure 3. TEM images of (a) Pd3/SDB catalyst and (b) Pd3Pb0.6/SDB catalyst.

Table 1. Surface Area and Pore Structure of Catalysts targeted composition (wt %) catalyst SDB Pd/SDB Pd−Pb/SDB Pd−Pb/SDB γ-Al2O3 Pd/γ-Al2O3 Pd−Pb/γ-Al2O3 Pd−Pb/γ-Al2O3

Pd 3 3 3

Pb

Pd

Pb

0.6 1.0

3 2.9 2.9

0.6 0.9

0.6 1.0

2.9 2.8 2.8

0.6 0.9

3 3

measured composition (wt %) BET surface area (m2/g)

avg. pore size (nm)

total pore vol. (mL/g)

particle size (nm)

483 473 469 464 493 480 465 460

16.8 15.9 15.4 15.2 13.9 12.4 12.2 12.0

1.15 1.11 1.11 1.11 0.89 0.76 0.70 0.69

6.8 5.9 5.7 8.3 7.4 6.7 6.8

incorporation of Pb, which confirmed that adding the second metal to the SDB-supported catalysts improved the metal dispersion on the SDB support. For the Pd3/γ-Al2O3 catalyst, the peak at about 40.1° (Figure 1d) was corresponding to d111 reflection of monometallic Pd crystal. The new peaks of catalysts Pd3Pb0.6/γ-Al2O3 and Pd3Pb1/γ-Al2O3 at about 38.7° (Figure 1e and f) were corresponding to d111 reflection of intermetallic Pd3Pb1 crystals, which indicated that alloying had occurred. ESEM analysis indicated that the SDB support was a sphere, with neat particle shape and uniform particle size distribution (400−750 μm), which was favorable to reduce the catalyst

Pd peaks at 40.0°, 46.5°, 68.1°, and 81.9° were obvious for the SDB-supported monometallic Pd catalyst (Figure 1a). While for the catalysts of Pb deposited on SDB-supported palladium, XRD patterns did not match well with that of monometallic Pd catalysts, the peaks shifted to lower angles from Pd metal at 2θ values of 39.6°, 46.2°, 67.3°, and 81.3° (Figure 1b) for the Pd3Pb0.6/SDB catalyst and 39.5°, 46.0°, 67.1°, and 81.2° (Figure 1c) for the Pb3Pb1/SDB catalyst. Additionally, no Pb peaks can be seen in both bimetallic catalysts’ XRD patterns. This indicated that the Pb was either in an amorphous state or alloyed with the Pd.30 The diffraction intensity of the monometallic Pd catalysts’ peaks was also reduced with the 15006

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Figure 4. (a) MAL conservation (%) versus time and (b) MMA selectivity (%) versus time (weight of the catalyst = 1.8 g, T = 50 °C, P = 0.1 MPa, O2 flow rate = 6 mL/min, and methanol/MAL molar ratio = 40): (a) 3 wt % Pd/γ-Al2O3; (b) 3 wt % Pd-1 wt % Pb/γ-Al2O3; (c) 3 wt % Pd/SDB; (d) 3 wt % Pd-1 wt % Pb/SDB; (e) 3 wt % Pd-0.6 wt % Pb/γ-Al2O3; (f) 3 wt % Pd-0.6 wt % Pb/SDB.

one of the reasons of the increase in catalytic activity by incorporation of Pb into Pd. Hence, it is expected that the SDB-supported 3 wt % Pd catalysts promoted with 0.6 wt % Pb may perform better than the others. 3.2. Catalyst Performance. The catalytic performance tests were carried out under similar conditions at atmospheric pressure, 323 K, pH = 10.5, and a molar ratio methanol/MAL of 40. Figure 4a gives the results of the conversion of MAL versus time during the oxidative esterification over the catalysts with 3.0 wt % Pd, as well as the bimetallic catalysts promoted with different loadings of Pb on SDB and γ-Al2O3. Previous work showed that MMA selectivity was a function of Pd metal loading and 3 wt % Pd loading was the optimum. It is possible that, by increasing metal loading, larger Pd clusters (particles) are formed that could be more active toward side reactions. As Figure 4a shows, MAL conversion rate on SDB-supported monometallic Pd catalysts reached 44% after 6 h. The SDBsupported bimetallic catalysts with 0.6 wt % Pb showed the highest MAL conservation of 79% among others. The Pd catalyst with 1 wt % Pb has a MAL conversion of 48%, which confirms that the amount of Pb has an optimal value on the SDB-supported catalysts. A 28% MAL conversion is observed on the γ-Al2O3-supported monometallic Pd catalyst. The γAl2O3-sopported catalysts promoted with 0.6 and 1 wt % Pb showed MAL conservation of 59% and 42%, respectively. Figure 4b shows the results of the selectivity of the MMA versus time during the oxidative esterification over the catalysts with 3.0 wt % Pd, as well as the bimetallic catalysts promoted with different loadings of Pb supported on SDB and γ-Al2O3, respectively. It can be seen that, over the SDB-supported 3 wt % Pd catalysts, the MMA selectivity was 54%. When the metal Pb was added on the SDB-supported catalyst, the selectivity of MMA kept all most the same value of 86%. The selectivity of MMA on the γ-Al2O3-supported monometallic Pd catalyst was 31%. Increasing the amount of Pb on the γ-Al2O3-supported bimetallic catalysts from 0.6 to 1 wt %, compared to the MAL conversion rate, the selectivity of MMA decreased from 80% to

colliding breakage (Figure 2a). The abundant pores of the catalyst support offered a suitable environment for the active component to well disperse in the internal structure, which was the basis of catalyst performance. In addition, the neat distribution of active components without blocking pores assured the high activity of Pd/SDB catalyst (Figure 2b). The particle size and distribution for the Pd3/SDB and Pd3Pb0.6/SDB catalysts were investigated by TEM and are shown in Figure 3. The TEM images revealed that catalyst particles are well dispersed on the SDB support. The average particle size of 3 wt % Pd/SDB was round 6 nm (Figure 3a) and when 0.6 wt % Pb was added, however, it decreased to about 5 nm (Figure 3b). These results are in line with the crystal size estimated by XRD. The Pd and Pb contents of the samples were measured by ICP and are given in Table 1, along with the targeted compositions. Analysis results showed that the measured contents are slightly lower than the targeted contents, which may be due to the hygroscopic nature of the metal precursors. For γ-Al2O3-supported catalysts, the deviation is greater, which indicates that metal particles are not uniformly dispersed on the support. The results of surface area, average pore size and total pore volume of each example are shown in Table 1. The SDBsupported monometallic Pd catalyst showed a BET surface area of 473 m2 g−1 and an average pore size of 15.9 nm. Increasing the amount of Pb from 0.6 to 1 wt % decreased the BET surface of the SDB-supported bimetallic catalysts from 469 to 464 m2 g−1 and the average pore size from 15.4 to 15.2 nm. After impregnating with metal species, the γ-Al2O3-supported catalysts had a slight fall in surface area and average pore size. The particle sizes calculated by the Scherrer equation31 are listed in Table 1. Analysis results showed that the crystallite sizes of the two bimetallic catalysts were close to each other, while the average particle size of the monometallic Pd/SDB catalyst was a little larger. These results mean that Pb is able to penetrate into the crystal lattice of Pd and cocrystallize together with the Pb crystals to form a fine dispersion of Pd,32 which is 15007

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3.4. Proposed Mechanism. The mechanism for the oxidation esterification of MAL to MMA has been seldom discussed. Based on the relevant literature7,24,34−41 about the oxidative esterification of similar aldehydes to their corresponding esters, we studied the mechanism further. Gowrisankar et al. reported that the oxidative esterification of benzyl alcohol using methanol in Pd catalyst systems reacted to corresponding esters.39 Yamamoto et al. performed a competitive experiment using methanol with alcohol and aldehyde, and they both formed the ester.40 Agrawal et al. studied the oxidative esterification of benzaldehyde with methanol and obtained the desired product of methyl benzoate.41 Table 3 shows yields of products and byproducts in different reaction conditions. Small amounts of byproducts methyl formate, propylene, and carbon dioxide were formed in the reaction with the bimetallic catalyst (entry 4). Besides the oxidation esterification over the bimetallic catalysts, the byproduct methyl isobutyrate was also formed. The principal reaction shown in eq 1 and the side reactions listed in eqs 2−4 were proposed to explain the byproducts listed in Table 3.

74%. These results confirmed that the MAL selectivity decreases with increased Pb content from 0.6 to 1 wt %, because of the formation of CO2, which is produced from the decarbonylation of MAL. From Figure 4a and b it is concluded that the MAL conversion rate increased distinctly with the reaction time, but the MMA selectivity did not change as a function of reaction time. The activity and selectivity of the SDB-supported catalyst are comparatively higher than those of the catalysts supported on γAl2O3-supported catalysts. This can be explained from the hydrophobic properties of the catalysts. On the basis of the Kelvin equation,33 ln(P/P0)) = 2Vμ cos θ/(rRT), where r is the radius of the capillary, V is the molar volume of the liquid, and μ is the surface tension, if hydrophilic materials are exposed to an aqueous solution, capillary condensation takes place until it reaches thermodynamic equilibrium. For the contact angle θ less than 90°, liquid condenses in the capillary at a pressure P less than the saturated P0 at temperature T.18 Experimental results showed that the contact angles between water and γAl2O3, SDB are 4.6° and 107.6°, respectively, and thus, the SDB-supported catalysts prevent the capillary condensation. In contrast, for the hydrophilic γ-Al2O3-supported catalysts, because of the capillary condensation, water accumulates in the catalyst pore and impedes the adsorption of reactants and subsequently reduces the activities of the catalysts. 3.3. Recycling of the Catalyst. In general, the copolymer supported catalysts can activate the oxidative of MAL with methanol in the presence of molecular O2 to the corresponding esters to high yield, which is a green approach from environmental as well as industrial points of view. The reaction catalyzed by Pd3Pb0.6/SDB was finished after 6 h, and the resulting reaction medium was transferred and dried. The resulting solid was then reused as the catalyst for a new reaction under the same conditions. The catalyst is stable, as evidenced from the series of reactions up to seven cycles without any appreciable loss efficiency (Table 2), and no dissolved catalyst

reused times 1

2

3

4

5

6

7

X (%) S (%)

78.6 86.2

78.2 86.0

77.7 85.5

77.0 84.9

76.2 84.2

75.3 83.6

74.5 83.0

(1)

CH3OH + O2 → HCOOCH3 + H 2O

(2)

MAL + O2 → CH 2 = CHCH3 + CO2

(3)

On the basis of the selectivity results and discussion, the following reaction mechanism, eqs 5−9, could be proposed here. The reactant of MAL first adsorbs on the catalyst surface; then, the H atom of the carbonyl group is abstracted by Pd, and the π-ally intermediate is formed. This π-ally intermediate reacts with methanol to form the targeted product MMA and leaves empty active sites. Nagai et al. reported that the methacrylic acid was also observed in the BASF’s process and isobutyric acid process over hydrophilic catalysts.5 Diao et al. used the hydrophilic Al2O3 as catalyst support and suggested that the intermediate might be hemiacetal for the reaction.11 In our study, when using the hydrophilic Al2O3 as support, we also found the byproducts of methacrylic acid. While employing our hydrophobic catalyst, no hemiacetal and methacrylic acid was observed, and this maybe due to the use of hydrophobic support. O2 would be adsorbed on the edge and the corner Pd atoms, and the resulting oxide species would react with the intermediate to form propylene and carbon dioxide. The adsorbed MMA intermediate would also react with the hydride species to form the byproduct methyl isobutyrate over the bimetallic catalyst but not the monometallic Pd catalyst. The selectivity of MMA of the reaction over the monometallic Pd

Table 2. Recyclability of the SDB-Supported Bimetallic Catalysts catalytic performance

MAL + CH3OH + O2 → MMA + H 2O

was observed. This indicates that, for these bimetallic catalysts supported on SDB, no catalyst was poisoned by the feed components or the reaction products and they can be easily recycled for the reaction of MAL with methanol in the presence of molecular oxygen to MMA.

Table 3. Yields of Products and Byproducts in Different Reaction Conditions yield of byproducts (%) entry 1 2 3 4 5 a

reactant CH3OH, MAL, O2 CH3OH, O2 MAL, O2 CH3OH, MAL, O2 CH3OH, MAL, O2

catalyst

yield of MMA (%) a

Pd3/SDB Pd3/SDB Pd3/SDB Pd3Pb0.6/SDB

n.r. n.d.b n.d. 24.8% 67.8%

HCOOCH3

CH2CHCH3

CO2

(CH3)2CHCOOCH3

n.r. 2.8% n.d. 1.0% 0.5%

n.r. n.d. 0.8% 0.5% 0.1%

n.r. n.d. 0.4% 0.2% 0.1%

n.r. n.d. n.d. n.d. 0.1%

n.r. = no reaction. bn.d. = not determined. 15008

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support. On the other hand, with the incorporation of Pb, Pd− Pb atoms formed intermetallic Pd3Pb1 crystals and Pb atom plays a promoting role as well as an inhibiting one. The reaction mechanism was also proposed, and the π-ally intermediate was supposed to react with methanol to form the corresponding ester.

catalyst is lower than that of the Pd−Pb bimetallic catalyst for the production of CO2. We can conclude that the atom Pd has relatively strong adsorption with O2 and results the oxide species, as shown in eqs 5 and 7. From the above phenomena, it is conclude that the influence of the promoter, Pb, for the oxidation esterification, has changed the absorptivity of Pd and weaken the interaction of Pd and oxygen atom. Furthermore, the collaborative effect of Pb is mainly manifested in two aspects: on the one hand, with the decreasing production of CO2, the selectivity of MMA increased; on the other hand, it makes the amount of the hydride species relatively higher resulting in the production of the byproduct methyl isobutyrate. It is obvious that the promoter of Pb has an optimal amount, and it affects the oxide species concentration directly, which plays the critical role on the selectivity of the catalyst. O2 + Pd → Pd[O]

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AUTHOR INFORMATION

Corresponding Author

*Tel.: + 86 22 27406959. Fax: +86 22 27406591. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of National Natural Science Foundation of China (No. 21076153).

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REFERENCES

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4. CONCLUSIONS In this work, we have described the synthesis and characterization of the mesoporous copolymer, SDB, and γ-Al2O3 supported Pd monometallic and Pd−Pb bimetallic catalysts. The SDB-supported bimetallic catalysts proved to be active and reusable catalysts for the direct oxidative esterification of MAL with methanol to MMA under mild reaction conditions. Compared to the monometallic catalysts and conventional hydrophilic catalysts supported on γ-Al2O3, the SDB-supported bimetallic catalysts show higher reaction rate and product selectivity. These results suggest that the hydrophobic property of catalysts is necessary and the copolymer SDB is a appropriate 15009

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dx.doi.org/10.1021/ie301674r | Ind. Eng. Chem. Res. 2012, 51, 15004−15010