Direct Oxidative Esterification of Aldehyde with Alcohol to Ester over

Article pubs.acs.org/IECR

Direct Oxidative Esterification of Aldehyde with Alcohol to Ester over Pd/Styrene-Divinyl Benzene Copolymer Catalyst Baohe Wang,* Weili Ran, Wenjuan Sun, and Kun Wang 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: The direct oxidative esterification of methacrolein (MAL) with methanol in the presence of molecular O2 at low temperature and ambient pressure to methyl methacrylate (MMA) was carried out over a novel hydrophobic styrene-divinyl benzene copolymer (SDB) supported palladium catalyst, and the results were compared with conventional hydrophilic catalysts over SiO2 and γ-Al2O3. Among the catalysts studied, SDB supported palladium catalyst showed higher activity than the conventional catalysts. The Pd/SDB catalyst was characterized by X-ray powder diffraction (XRD), environmental scanning electron microscope (ESEM), and thermal gravimetric analysis (TGA). A systematic investigation was carried out for the oxidative esterification of MAL with methanol over SDB supported catalyst. The effects of catalyst loading and alcohol/ aldehyde molar ratio were studied. The reaction kinetics was studied, and the result showed that the fitting results agreed well with the experimental data. On the basis of the Langmuir−Hinshelwood adsorption model, the probable mechanism was suggested.

1. INTRODUCTION Methyl methacrylate (MMA) is a commercially important chemical, which is normally used for producing acrylic plastics, polymer dispersions for paints, and coatings.1 The production capacity of MMA is approximately 3 millions of tons worldwide in 2010.2 As of now, it is mainly produced by the traditional method of the acetone cyanohydrin process, though there are many routes available.3 However, there are severe difficulties in the procurement of the raw material hydrogen cyanide and the high cost for the waste treatment of by-produced ammonium sulfide. Recently, extensive and intensive studies have been made on the newly developed method for producing MMA, which comprises subjecting methacrolein (MAL) and methanol to an oxidative esterification reaction in the presence of molecular oxygen and in the presence of a catalyst, to thereby produce MMA by one step directly from MAL.4 For the catalyzed reaction, from a viewpoint of green chemistry, this transition-metal-catalyzed oxidative esterification is expected to be a versatile procedure directly giving esters from aldehydes and alcohols. Palladium is the most active catalyst in the process among transition metal catalysts and thus has been widely used in many organic syntheses. In the environmental context of today, one of the challenging issues for chemists is to develop cost-effective, green, mild, and efficient catalytic routes that minimize hazardous waste.4 Because the production of MMA has been commercialized for several decades, improving the catalysts with respect to rate and MMA selectivity is a challenge, particularly given that the nature of the active site and mechanism for MMA synthesis Pdbased catalysts have not been established. Although there have been a lot of research about the MMA production from MAL and methanol in the presence of molecular O2 over supported Pd catalyst,5−8 it is based on hydrophilic supports; there are very few reports concerning the synthesis of MMA with catalyst © 2012 American Chemical Society

over hydrophobic support. One problem, which is seldom discussed, is the deactivation of these hydrophilic catalysts by water due to capillary condensation and slow water desorption. At low reaction temperatures, condensation of product water on the catalyst surface is an unavoidable problem. However, the problem may be overcome by the use of a hydrophobic catalyst. With respect to the application of hydrophobic styrene-divinyl benzene copolymer (SDB) used in the reaction involving water, there are many reports in the open literature.9−18 However, it is mainly used in the selective oxidation of alcohols and aldehydes or olefins to desired compounds. There is an opportunity for new environmentally friendly processes for the production of esters. In the present study, direct oxidative esterification of MAL with methanol to MMA in the presence of molecular O2 with water generated in the reaction was carried out over a hydrophobic SDB supported palladium catalyst (Scheme 1).The effects Scheme 1. Oxidative Esterification of MALwith Methanol to MMA over Pd/SDB

of molar ratio and catalyst loading on reactivity were studied for the formation of MMA over SDB supported palladium catalysts. Moreover, the investigation on reaction kinetics provides a useful tool for large-scale direct production of MMA with higher performances over the hydrophobic catalysts. Received: Revised: Accepted: Published: 3932

November 22, 2011 February 13, 2012 February 21, 2012 February 21, 2012 dx.doi.org/10.1021/ie202701k | Ind. Eng. Chem. Res. 2012, 51, 3932−3938

Industrial & Engineering Chemistry Research

Article

2. EXPERIMENTAL SECTION 2.1. Material and Catalyst Preparation. All chemicals were purchased from Tianjin Reagent Company (China). The metallic precursors were PdCl2, 99.99% metal basis. The percent loading of the catalyst was based strictly on the initial weight of the chemical complexes. The SDB catalyst support, in the form of 30−40 mesh granules, was prepared by polymerizing divinylbenzene in ethylvinylbenzene (DVB). 2-Methyl-1-pentanol and 2,2-azobis(2-methylpropionitrile) (AIBN) were used as solvent and initiator, respectively.19 SDB supported palladium catalyst with palladium containing 3 wt % (denoted as Pd3/SDB) was prepared using a conventional impregnation method. DMF (99.5+% purity) was used as a solvent for Pd solution instead of water due to the hydrophobicity of SDB support. The obtained catalyst was rotary evaporated at 353 K under a slight vacuum to remove the DMF and then dried under infrared light for 6 h and reduced with 50 mL (NTP)/min H2 at 473 K until the pH of the furnace outlet became neutral. Catalysts containing 3 wt % of palladium supported on SiO2 and γ-Al2O3 (denoted as Pd3/ SiO2 and Pd3/γ-Al2O3) were prepared from a water solution of PdCl2 by aqueous impregnation. The water was evaporated at an increased temperature (353 K) under vacuum. The catalysts were dried in air at 383 K for 6 h, calcined at 773 K for 4 h in oxygen atmosphere, and then reduced in hydrogen atmosphere for 2 h at 473 K until the pH of the furnace outlet became neutral. 2.2. Catalytic Performance. The catalytic performance tests were performed in a thermostatted glass reactor with an inside diameter of 5 cm and volume of 100 mL with a stirrer, a O2 supply system. The gas flow rates were controlled by mass flow meters. The reaction was conducted over the range of 303−333.15 K, pH = 10. The mixture was stirred at 500 rpm, and oxygen was bubbled through at 6 mL min−1. It was observed that a further increase in the speed of stirring did not cause further changes in activity significantly. The choice of the stirring speed and O2 flow rate served to minimize transport limitations. Once the desired reaction temperature was attained, the catalyst was charged to the reactor, and this time was considered the zero reaction time. Samples of the reaction medium trapped by a condenser at 253 K were taken every 30 min, filtered, and analyzed using a gas chromatograph (GC) (Aglient gas chromatograph model GDX-103; TCD detector, equipped with a DB-WAX capillary column, 6 m × Φ3; and a SP4270 data processor). The retention times of all of the individual compounds were verified using authentic samples. The reproducibility of the results was checked, and the error in all experimental measurements was less than 2%. Catalytic results were expressed as conversion rate (X, %), selectivity (S, %), and yield (Y, %) which were determined by GC. Those parameters are defined as ⎛ ⎛ C ⎞⎞ X = ⎜⎜1 − ⎜⎜ MAL ⎟⎟⎟⎟ × 100% ⎝ C0,MAL ⎠⎠ ⎝

(1)

⎛ ⎞ CMMA ⎟⎟ × 100% S = ⎜⎜ ⎝ C0,MAL − CMAL ⎠

(2)

Y = XS

(3)

molar concentration of aldehyde after time t, and CMMA is the molar concentration of the corresponding ester after time t. 2.3. Catalyst Characterization. The chemical composition of the samples was determined using ICP (IRIS Intrepid II XSP, Thermo). Analysis results of ICP from Table 1 showed Table 1. Surface Area and Pore Structure of Catalysts theoretical composition

practical atomic ratio Pd

surface area (m2/g)

average pore size (nm)

pore volume (mL/g)

SDB Pd3/Al2O3 Pd3/SiO2 Pd3/SDB

2.9 3 3

483 480 473 468

16.8 12.4 13.3 15.9

1.15 0.76 0.77 1.11

that the actual chemical compositions were close to theoretical compositions. The surface areas and porosities of supports were determined in a Micromeritics TriStar 3000 (American Micromeritics) automated adsorption system apparatus by physical adsorption of nitrogen (99.99% purity). Samples were previously outgassed to remove impurities at 523 K in a helium stream during 1 h. Then, a mixture of 30% N2/He was allowed to flow through the sample previously immersed in a liquid nitrogen bath at 77.36 K. Both adsorbed and desorbed N2 were recorded. The amount of desorbed nitrogen was used to calculate total surface area. Experiments were repeated twice or until surface areas were reproducible within ±5%. X-ray powder diffraction (XRD) spectra were collected on powdered a.p. samples with a Philips powder/thin film diffractometer PW1140 equipped with a PW3020 goniometer using a Cu Kα1 (λ = 1.54) radiation. The samples were scanned at a rate of 0.02°/step over the range of 30° ≤ 2θ ≤ 80° (scan time = 2 s/step). The particles morphology was recorded by the use of an environmental scanning electron microscope (ESEM) (XL30 ESEM-FEG, FE I, USA) operating at 30 kV. Before being transferred into the ESEM chamber, a sample ultrasounddispersed with ethanol was settled on a holder and then moved into the vacuum evaporator in which a thin gold film was deposited after sample drying. The thermostability of SDB support was indicated by the thermogravimetric analysis. The thermal analysis experiments were performed with a SHIMADZU model TGA-50 with a working station (TA50WSI) for collecting data in a nitrogen flow of 30 mL min−1. The temperature was calibrated with a high-purity indium standard. The runs were carried out at a heating rate of 10 °C/ min. Each sample was placed in an open silica crucible and heated from room temperature to 773 K. The thermogravimetric weight loss curve (TG, mg) was recorded as a function of time and temperature.

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization Results. Figure 1 shows the XRD analysis of Pd3/Al2O3, Pd3/SiO2, and Pd3/SDB catalysts. The diffraction features at Bragg angles of nearby 40°, 46°, and 68° were detected corresponding to the ⟨111⟩, ⟨200⟩, and ⟨220⟩ Pd planes, indicating that the catalyst particles were palladium black. PdCl2 was reduced to metal Pd completely after the hydrogen reduction for all the catalysts due to no PdO peaks observed. Figure 2 shows the ESEM images of SDB and Pd3/SDB samples. It is observed that the SDB possesses a typical sphere architecture which is 200−600 μm in diameter (Figure 2a).The existence of abundant pores in the inner of the

wherein C0,MAL is the molar concentration of aldehyde at the beginning of the direct transformation process, CMAL is the 3933

dx.doi.org/10.1021/ie202701k | Ind. Eng. Chem. Res. 2012, 51, 3932−3938


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Figure 1. XRD patterns of Pd3/SiO2 (a); Pd3/Al2O3 (b); Pd3/SDB (c).(▲), (▼), and (■) for ⟨111⟩, ⟨200⟩, and ⟨220⟩ Pd plane, respectively. ⧫ (SiO2), ● (γ-Al2O3).

catalyst support offered a suitable environment for active component. The shine spot was active components of Pd particle. Particle size was small, and its distribution was uniform (Figure 2b). The results of thermal gravimetric analysis (TGA) for the bare SDB and the fresh Pd3/SDB are shown in Figure 3. The pattern of weight loss of the palladium on SDB was the same as that of bare SDB in the same temperature range, and the weight loss was negligible at temperatures below 300 °C. However, the weight loss of Pd3/SDB was a little larger than that of SDB above 300 °C, which might be attributed to the slight structural change of SDB after the loading of Pd; thus, the thermostability was reduced. Consequently, it is concluded that palladium does not affect the thermal stability of SDB in this study. 3.2. Absence of Mass Transfer Limitations. Preliminary experiments were conducted varying the stirring speed and particle size, to quantify the influence of external resistances to heat and mass transfer. These experiments showed that there was very little effect of speed of agitation in the range of 300− 800 rpm and particle size in the range of 20−60 mesh on the overall rate of the reaction. Hence, all further experiments were conducted at a stirrer speed of 500 rpm and particle size of 30−40 mesh, ensuring that there were no external mass transfer resistances. Moreover, the influence of oxygen partial pressure was studied. The effect of O2 partial pressure is mainly to improve the MAL conversion, accelerating the total oxidation reactions, but it has little influence on the MAA formation, which is in agreement with that reported in ref 5. It was also confirmed that no attrition of the resin particle took place under the experimental conditions employed for this work. 3.3. Oxidative Esterification over Various Supports. Catalytic performance of oxidative esterification of MAL with

Figure 3. TGA analysis of (a) bare SDB; (b) fresh Pd3/SDB. Heating rate: 10 °C/min; carrier gas: nitrogen; final temperature: 500 °C.

methanol with 3 wt % Pd catalysts over various supports was carried out, and the results are shown in Figure 4. As can be

Figure 4. Conversion and selectivity versus time for the oxidative esterification over various supports. ■, ⧫, ●: conversion catalyzed by Pd3/SiO2, Pd3/γ-Al2O3, and Pd3/SDB, respectively; □, ◊, ○: selectivity catalyzed by Pd3/SiO2, Pd3/γ-Al2O3, and Pd3/SDB, respectively. Reaction condition: T = 323 K; pH = 10; O2 flow rate = 6 mL min−1; alcohol/aldehyde molar ratio = 40/1. The catalyst additive amount was 3% (wt) in all experiments.

seen, SDB supported Pd catalyst shows the highest catalytic activity among all the catalysts investigated. The MAL conversion and MMA selectivity catalyzed by Pd3/SDB catalysts

Figure 2. ESEM photography of SDB (a) and Pd3/SDB (b). 3934



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increased when Pd loading was between 0.5% and 3% but presented a downtrend with increasing Pd loading over 3%.The reason may be that increasing palladium loading may result in the formation of larger Pd particles and consequently cause some side effects for the catalytic performance, resulting in the deep oxidization of MAL to CO2, i.e., the decarbonylation of MAL. It can be concluded that 3% Pd loading is the optimum in the reaction condition. 3.4.2. Molar Ratio. The effects of molar ratio (methanol/ MAL) on MAL conversion and MMA selectivity were studied for the oxidative esterification of MAL with methanol catalyzed by Pd3/SDB, and the results obtained are given in Figure 6.

are higher than that catalyzed by the Pd3/γ-Al2O3 and Pd3/SiO2 catalysts. After 5 h on stream, the conversion of the Pd3/SDB catalysts was up to 44%. In the case of a hydrophilic support, γ-Al2O3 and SiO2, the activities of catalysts were lower than those of the Pd catalyst on hydrophobic support, SDB. Experimental results show that the contact angles (θ) between water and γ-Al2O3 and SiO2 are 4.6° and 3.5°, respectively, while it is 107.6° between water and SDB. Hence, the low activity of the Pd3/γ-Al2O3 and Pd3/SiO2 catalysts was thought to be caused by the diffusion hindrance apart from their physical structures. When conventional hydrophilic catalysts used as carriers are exposed to an aqueous solution, capillary condensation takes place until it reaches thermodynamic equilibrium dictated by the Kelvin equation,20 ln(P/P0) = 2 Vμ 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. 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. For hydrophilic material-supported Pd catalysts, the contact angle with an aqueous solution would be close to zero. In the process studied involving water as byproduct, water film was formed and covered the surface of catalyst, consequently inhibited catalytic activity, and could generate side reactions.11 On the other hand, if a material with contact angle greater than 90° is selected as a catalyst support, its pore will remain dry and accessible to gaseous reactants and thus independent of water. It well proves that the final catalytic properties depend on the combination of the types of metal and support. Particularly, the support can exhibit a direct influence on the catalytic reaction, as its surface is often active toward reactants and reaction products, but an indirect influence also occurs in that the physical−chemical properties of the support influence the metal dispersion, its resistance to sintering, and the accessibility of active sites to reactants.21 Similar results have been observed for the oxidation of water-containing organic compounds.9−11,14,15 3.4.1. Optimization of Reaction Conditions. Catalyst Loading. Effects of catalyst loading on the MAL conversion and MMA selectivity over SDB have been studied as shown in Figure 5, keeping all other parameters constant. It has been

Figure 6. Effect of molar ratio (methanol/MAL) on MAL conversion and MMA selectivity (at 300 min) catalyzed by Pd3/SDB. ■, MAL conversion; □, MMA selectivity. Reaction condition: T = 323 K; pH = 10; O2 flow rate = 6 mL min−1.

Oxidative esterification of MAL with methanol is an equilibrium-limited chemical reaction, and because the position of equilibrium controls the amount of ester and water formed, the use of an excess of methanol increases the conversion of MAL based on chemical equilibrium theory. The equilibrium conversion of MAL increased from about 15% at a feed mole ratio (methanol/MAL) of 10:1 to about 50% at a feed mole ratio (methanol/MAL) of 50:1. However, it is also indicated that it is not economical when the molar ratio (methanol/MAL) is in excess of 40 in view of both MAL conversion and MMA selectivity and the subsequent separation and purification. What’s more, the MAL conversion and MMA selectivity were independent of a little water in reactant due to the hydrophobicity of SDB. This further proves the utilization of hydrophobic SDB has a good advantage in the reaction involving water. 3.5. Recycling of the Catalyst. Recycling of the catalyst is an important aspect of any industrial process.22 The reaction catalyzed by Pd3/SDB was finished after 5 h, and the resulting reaction medium was transferred and dried. The resulting solid was then reused as catalyst for a new reaction under the same conditions, and the results are shown in Table 2. After five Table 2. Catalyst Recycle

Figure 5. Effect of Pd loading on MAL conversion and MMA selectivity (at 300 min). ■, MAL conversion; □, MMA selectivity. Reaction condition: T = 323K; pH = 10; O2 flow rate = 6 mL min−1; alcohol/aldehyde molar ratio = 40/1.

reused times

observed that catalytic activity improved with the increase of Pd loading. The reason was that, the more catalysts loaded, the more active sites were available for reaction. MMA selectivity

catalytic performance (%)

1

2

3

4

5

X S

43.2 54.3

42.8 54.1

42.5 55.8

42.2 53.4

40.8 53.2

cycles, the selectivity of MMA had slightly decreased from 54.3% to 53.2%, indicating that these catalysts supported on 3935



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with temperature is shown in Figure 8. According to Arrhenius equation defined as

SDB can be easily recycled for the reaction of MAL with methanol in the presence of molecular O2 to MMA. A small decrease could be due to some loss of the catalyst during filtration and due to no addition of any fresh catalyst to keep the identical amount of catalyst used. That is, reproducible conversion and selectivity were obtained during the course of several experiments and the catalyst was not poisoned by the feed components or the reaction products.

ln k = ln k 0 −

Ea RT

(4)

The energy of activation for this reaction was found to be 51.24 kJ mol−1. This indirectly confirms the observation of the absence of diffusional resistances. Estimated orders of the oxidative esterification reaction are shown in Table 3, which has indicated that the reaction order is not integral in the condition investigated. To verify the reliability of the study on kinetics, the experiment was carried out at 318.15 K, keeping all the other parameters constant, and the result is shown in Figure 9. The predicted values are in fair agreement with the experimental values (R-squared = 0.99753), indicating that the reaction kinetics was suitable in the range of temperature investigated.

4. KINETICS STUDY In the present study, the influence of methanol on reaction rate can be neglected due to their excess existing in the system studied in all experiments. As aforementioned, based on ref 5, MMA formation reaction does not depend on the low oxygen partial pressure. The temperature-dependent rates in the range of 303.15− 333.15 K are shown in Figure 7. Because of the change of catalyst activity and selectivity during the reaction, determining time-varying mass-transfer resistance is very difficult; thus, rigorous kinetic analysis was not attempted.17 The data were correlated via unlinear regression analysis. The variation of k

5. POTENTIAL MECHANISM Diao et al.6 reported that, for direct transformation of aldehyde with alcohol to corresponding ester over Pb, Mg-doped Al2O3supported Pd catalysts in the reaction atmosphere of O2 or He,

Figure 7. Temperature dependence of the reaction rate catalyzed by Pd3/SDB. Reaction condition: pH = 10; O2 flow rate = 6 mL min−1; alcohol/ aldehyde molar ratio = 40/1. (a) T at 303.15 K; (b) T at 313.15 K; (c) T at 323.15 K; (d) T at 333.15 K. 3936



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Figure 10. The proposed mechanism for the MMA production.

species to decompose to the methoxy group and hydride, and (III) the coupling of acyl group and methoxy group to form MMA. The remaining Had is oxidized to Pd metal with oxygen to generated water. Nevertheless, the mechanism of MMA synthesis may very well be more complex than suggested here. Studies are currently under way to detect intermediate species in order to fully understand overall reaction mechanism.

Figure 8. Temperature dependence of the reaction rate constant.

Table 3. Estimated Orders of the Oxidative Esterification Reaction T (K) 303.15 313.15 323.15 333.15

k 0.1938 0.3086 0.7201 1.1214

± ± ± ±

6. CONCLUSIONS In the present study, the direct oxidative esterification of MAL with methanol in the presence of molecular O2 to MMA at atmospheric pressure was carried out over a hydrophobic SDB supported palladium catalysts, and the results were compared with conventional hydrophilic catalysts over SiO2 and γ-Al2O3. Among the catalysts studied, SDB supported palladium catalysts showed higher activities than the conventional catalysts supported on SiO2 and γ-Al2O3.This can be mainly ascribed to the hydrophobicity of SDB. The effects of catalyst loading, alcohol/aldehyde molar ratio, and temperature were systematic investigated for the esterification of MAL with methanol over SDB supported palladium catalyst. The activity and selectivity suggest that high hydrophobicity of the catalyst is necessary for maintaining its high activity and selectivity to MMA, especially at low temperatures. Although the detailed reaction mechanism of the synthesis of MMA from MAL and methanol over Pd/SDB is not very clear at this stage, the preliminary experimental results show that SDB as a novel environmentally friendly catalyst support is not only possible but also quite satisfactory. Further detailed study is still ongoing.

n 0.02 0.02 0.02 0.02

2.1848 1.9159 2.1954 2.0941

± ± ± ±

0.02 0.02 0.02 0.02

■

Figure 9. Comparison of experimental results and fitting results catalyzed by Pd3/SDB. Reaction condition: T = 318.15 K; pH = 10; O2 flow rate = 6 mL min−1.

AUTHOR INFORMATION

Corresponding Author

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

the intermediate might be hemiacetal and proposed a tentative mechanism for the direct transformation of aldehydes with alcohols to corresponding esters.

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).

It is known that the path of the reaction network is strongly dependent on the nature of the material.23 In our study, there was no hemiacetal and methacrylic acid observed, maybe due to the use of hydrophobic catalyst. In the catalytic system catalyzed by Pd3/SDB, the presumptive mechanism based on the Langmuir−Hinshelwood adsorption model for the MMA production is shown in Figure 10 and the sign ∗ represents adsorption active site. The suggested reaction route contains (I) dissociative chemisorptions of aldehyde on the active Pd species to decompose to the acyl group and hydride, (II) dissociative chemisorptions of methanol on the active Pd

■

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Direct Oxidative Esterification of Aldehyde with Alcohol to Ester over

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