Esterification of Acrylic Acid with Different Alcohols Catalyzed by

Apr 28, 2012 - Res. , 2012, 51 (19), pp 6666–6671 .... temperature was reached, alcohol was added into the reactor and this was taken as zero time f...
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Esterification of Acrylic Acid with Different Alcohols Catalyzed by Zirconia Supported Tungstophosphoric Acid Emine Sert* and Ferhan Sami Atalay Chemical Engineering Department, Ege University, Bornova, Izmir, Turkey ABSTRACT: The liquid phase esterification of acrylic acid with different alcohols (butanol, iso-butanol, or hexanol) was investigated in a batch reactor with zirconia supported tungstophosphoric acid (TPA) as heterogeneous catalyst. The prepared catalysts with different TPA loadings (20, 25, and 30 wt %) and calcination temperatures (550, 650, and 750 °C) were characterized by nitrogen adsorption studies, X-ray diffraction (XRD), and thermogravimetric analysis (TGA) techniques. In addition, acidity measurements were performed by potentiometric titration with n-butylamine. The activitiy of catalysts strongly dependent on the acidic characteristic of the catalysts. The most active catalyst, 25 wt % TPA, calcined at 650 °C, gave more than 33%, 31%, and 27% conversions of acrylic acid for butyl, iso-butyl, and hexyl acrylate synthesis, respectively.

1. INTRODUCTION Acrylic esters are versatile monomers and widely used for the production of coatings, adhesives, textiles, and plastics.1 Their excellent clarity, toughness, color retention, UV stability and chemical inertness make acrylic ester emulsion polymers prime paint binders. Acrylics are widely used in all types of paint formulations. Large volumes of acrylic emulsion polymers are used as binders for fiberfill and nonwoven fabrics, textile bonding or laminating, flocking, back coating, and pigment printing binders. These are also used for leather finishing, textile and fiberfill bonding, and as adhesives.2 Particularly, acrylates are used as a comonomer in the manufacture of polymers. Polymers made with butyl acrylate or iso-butyl acrylate are primarily used in surface coatings, in films and pressure sensitive adhesives, in dispersions, and in construction materials. Esterification of acrylic acid with alcohol has commercially been performed by using liquid catalysts such as sulfuric acid, hydrofluoric acid, and para-toluene sulfonic acid, but these are toxic, corrosive, and often hard to remove from the reaction solution. Thus, it is keenly desirable to develop new types of solid acid catalysts to replace them, because the solid acids are less toxic and facilitate the recovery and recycling of the catalysts.3 Heteropolyacids, particularly tungstophosphoric acid (H3− PW12O40·xH2O, TPA) have been proven to be an alternative to traditional acid catalysts because of their strong acidity and appropriate redox properties.4 Heteropolyacids are typical strong Bro̷ nsted acids and catalyze a wide variety of reactions in homogeneous phase offering strong option for efficient and cleaner processing. The major disadvantages of heteropolyacids as catalysts lie in their low thermal stability, low surface area (1−10 m2/g), and separation problems from reaction mixture. Heteropolyacids can be made an ecofriendly insoluble solid acid with high thermal stability and high surface area by supporting them onto suitable supports.5 Various supports such as silica,6,7 titania,7 zirconia,8,9 alumina,10 and MCM-4111 have been used for supporting heteropolyacids. © 2012 American Chemical Society

A number of studies related to the catalytic esterification reactions of acrylic acid with some other alcohols have been reported. Dupont et al.12 have reported the esterification of acrylic acid with 1-butanol over H3PW12O40 supported on active carbon. They claimed that the specific activity of the supported heteropolyacid was comparable to the values for the parent H3PW12O40 and H2SO4. Chen et.al.3 studied the esterification of acrylic acid with 1-butanol over various solid acids including Cs2.5H0.5PW12O40. In the solid−liquid reaction system, Cs2.5H0.5PW12O40 exhibited the highest catalytic activity in the unit of catalyst weight among the solid oxide catalysts, while the activity of Cs2.5H0.5PW12O40 was less than those of organic resins such as Nafion and Amberlyst 15. Although there are a number of such studies in the literature, no study has been reported for the esterification of acrylic acid with iso-butanol and hexanol. In the present study, the heterogeneous catalyzed esterification of acrylic acid with different alcohols (butanol, iso-butanol, hexanol) was carried out. We describe the preparation and the textural and structural properties of zirconia supported TPA as catalyst. Also, the effects of calcination temperature and TPA loading on the catalytic activity were investigated; as well, the effects of the length of alcohol’s carbon chain and the OH group location were studied for the esterification of acrylic acid.

2. EXPERIMENTAL STUDY 2.1. Preparation of Zirconia Supported TPA. Zirconia supported TPA catalyst was synthesized as reported in literature.13 Support Preparation. Zirconium hydroxide was prepared by hydrolysis of 0.5 M zirconyl chloride solution by the dropwise addition of aqueous NH3 (25%) to a final pH of 10. The precipitate was filtered and washed with water until it was determined to be free from chloride ions by the silver nitrate Received: Revised: Accepted: Published: 6666

November 16, 2011 April 27, 2012 April 28, 2012 April 28, 2012 dx.doi.org/10.1021/ie202609f | Ind. Eng. Chem. Res. 2012, 51, 6666−6671

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test. The hydrous zirconia thus obtained was dried at 120 °C for 12 h, powdered well, and dried for another 12 h. Catalyst Preparation. 4 mL of methanol per 1 g of solid support was used, and the mixture was stirred. After stirring, the excess methanol was removed at 60 °C. The resulting solid materials were dried at 120 °C for 24 h and ground well. TPA was loaded to zirconia at 20, 25, and 30% to understand the role of TPA loading on the catalyst activity. The dried samples were calcined at 650 °C for catalysts prepared with different TPA loadings. Also, samples with 25% TPA loading were calcined at 550 and 750 °C to investigate the effect of calcination temperature. 2.2. Characterization of Zirconia Supported TPA. Zirconia supported TPA catalysts were characterized by different instrumental techniques. Thermal analysis of zirconia supported TPA catalysts that are prepared with different TPA loadings and calcination temperatures was carried out by using Perkin-Elmer Diamond TG/DTA. The samples under examination are heated in N2 stream (20 mL/min) at a heating rate of 10 °C/min. The specific surface area of the calcined sample was determined from N2 adsorption conducted at −196 °C. Measurements of the surface area and pore volume of catalyst were performed in an automatic gas adsorption system (Micromeritics Gemini V) by N2 adsorption. The phase composition of five samples was determined by means of X-ray diffraction (XRD). XRD studies were accomplished using Philips X’Pert Pro diffractometer using Cu Kα and α scanning range 2θ of 4−80°. The nature of the acid sites (Brønsted and Lewis) of the catalyst samples was characterized by FTIR (Perkin-Elmer Spectrum 100) spectroscopy with adsorbed pyridine. The acidity of prepared catalysts was measured by potentiometric titration with n-butylamine in acetonitrile.14 A small quantity of 0.1 N n-butylamine in acetonitrile was added to a known mass of solid and agitated for 3 h. Later, the suspension was titrated with the same base at 0.05 mL/min. The electrode potential variation was measured with a pH meter. 2.3. Experimental Set-up and Procedure. Butanol, isobutanol, or hexanol (Merck) and acrylic acid (Merck) were used as the reactants. Initial volume of the reaction mixture was approximately 300 mL. The experimental assembly was consisted of a reactor fitted with a condenser to prevent any loss of products. The electrical heater was used to heat the reaction mixture, and the batch content is stirred magnetically using a magnetic stirrer. Temperature of the reaction mixture was controlled using a temperature controller. In a typical run, acrylic acid, catalyst, and phenothiazine as inhibitor were charged into the reaction vessel in a predetermined ratio. The temperature of the reactor was set to the desired value. After the desired temperature was reached, alcohol was added into the reactor and this was taken as zero time for a run. About 1 mL of liquid sample was withdrawn from the reactor at regular intervals for gas chromatographic analysis.

Table 1. Standard Enthalpies and Gibbs Energies of Formation component

ΔHf (kJ/mol)

ΔGf (kJ/mol)

butanol hexanol iso-butanol acrylic acid butyl acrylate iso-butyl acrylate hexyl acrylate water

−274.60 −316.54 −283.20 −336.45 −385.00 −394.00 −441.09 −242.00

−150.30 −134.47 −167.43 −286.25 −221.12 −229.34 −201.45 −228.77

and hexyl acrylate synthesis, respectively. The equilibrium constants were weakly dependent on temperature because of the low value of heat of reaction. Equilibrium constants were calculated from thermodynamic data at a reference temperature from the standard Gibbs energies of reaction: ln K (T0) = −

ΔG0 RT0

Equilibrium constants at different temperatures were obtained from the integrated form of Van’t Hoff equation; ln K (T ) = ln K (T0) −

ΔH0 ⎛ 1 1⎞ ⎜ − ⎟ R ⎝T T0 ⎠

For the esterification of butanol/iso-butanol/hexanol with acrylic acid, using the thermodynamic data, the equilibrium constants were calculated at different temperatures. Constant enthalpy of reaction is assumed because of the small temperature interval investigated. The variations of chemical equilibrium constant with temperature for each reaction were illustrated in Figure 1. It can also be seen that the reaction is exothermic, since the equilibrium constant decreased with increasing temperature.

Figure 1. Equilibrium constant versus temperature for the synthesis of butyl acrylate, iso-butyl acrylate, and hexyl acrylate.

3. RESULTS AND DISCUSSION 3.1. Thermodynamic Analysis. The heat of reaction and equilibrium constant are very important for equilibrium-limited reactions such as esterification, transesterification, isomerization, etc. The heat of reaction can be calculated from the standard enthalpy of formation. The standard enthalpies of formation for the reactants and products in the liquid phase are listed in Table 1. The heat of reactions were found to be −15.2, −15.6, and −30.1 kJ/mol for butyl acrylate, isobutyl acrylate,

3.2. Characterization of Zirconia Supported TPA Catalysts. The catalyst prepared with 20 wt % TPA loading and at calcination temperature of 650 °C showed a surface area of 95.4 m2/g. The addition of TPA (25 wt %) to the support increases the surface area from 95.4 m2/g to 98.2 m2/g where it reached a maximum. This can be explained by the fact that the added TPA forms a surface over layer that reduces the surface diffusion of zirconia and exhibits sintering.13 Further addition of TPA loading up to 30 wt % resulted in loss of the catalyst 6667

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surface area, and the surface area decreased to 88.6 m2/g. The effect of calcination temperature was investigated by changing the calcination temperature in the range 550−750 °C at a TPA loading of 25 wt %. An increase in the calcination temperature from 550 to 750 °C lowers the surface area from 157.9 to 69.8 m2/g as a result of the micropore area created by calcination. In Figure 2, effect TPA loading on XRD patterns is shown. Figure 2 shows that the prepared catalysts show crystalline

observed, indicating that TPA is highly dispersed on the support. When the calcination temperature reaches 750 °C, new peaks appear in the region of 23−25°, which is characteristic of WO3.13 To check the stability, the prepared catalysts were studied for thermogravimetric analysis. The TGA of catalysts (Figure 4)

Figure 4. TGA curves of zirconia supported TPA catalysts. Figure 2. Effect of TPA loading on X-ray diffraction patterns.

shows initial weight loss up to 200 °C is due to loss of adsorbed water and water of crystallization. The weight loss between 200 to 500 °C may be due to the decomposition of TPA. All prepared catalysts show the weight losses between 2 and 6% until 500 °C. In the zirconia supported TPA catalyst, no appreciable change in weight between 500 and 800 °C indicates an increase in the stability of TPA after being supported onto zirconia. This may result from the formation of intermolecular hydrogen bonds between the support and TPA and indicates the presence of chemical interaction between zirconia and TPA. As shown in Figure 4, as calcination temperature was increased, the thermal stability of catalyst increased. The acidity measurements of the catalysts by means of potentiometric titration with n-butylamine were carried out. This method enables the determination of the total number of acid sites and their distribution. As a criterion to interpret the obtained results, it was suggested that the initial electrode potential (E) indicates the maximum acid strength of the sites. The value of meq amine/g solid, where the plateau is reached, indicates the total number of acid sites.16 The initial electrode potential indicates the maximum acid strength of the surface sites, and the values where the plateau is reached indicate the total number of acid sites. The acid strength of surface sites can be assigned according to the following ranges: very strong site, E > 100 mV; strong site, 0 mV < E < 100 mV; weak site, −100 mV < E < 0 mV; and very weak site, E < −100 mV. The titration curves obtained for the prepared catalysts are shown in Figure 5 and the initial electrode potential values are listed in Table 2. Addition of TPA from 20 wt % to 25 wt % increases both surface acidity and acid strength, followed by a decrease at 30 wt % TPA loading. These results show that 25% TPA loading causes the strongest and the highest number of acid sites, and this is confirmed by the surface area values of prepared catalysts. The increases in the surface area enhance the dispersion of acidic protons of TPA up to monolayer coverage at 25 wt % TPA loading. The further increase in TPA content above monolayer leads to the aggregaration of TPA that will lead to a decrease in surface acidity.4 The increase in the calcination temperature from 650 to 750 °C lowers the acid

structure and three peaks around 2θ values of 30.15, 50.20, and 60.19, which are attributed to ZrO2.14 Zirconia may be amorphous, tetragonal, cubic, or monoclinic. Upon thermal treatment, first transformation of amorphous results in a mixture of tetragonal or monoclinic zirconia. The higher temperature leads to higher monoclinic fraction.8 In Figure 2, zirconia was found to be purely tetragonal. Pure zirconia is mainly monoclinic, with only a small amount of the tetragonal phase.13 XRD patterns do not show indication of any crystalline phases related to TPA. The most intense peak of pure solid TPA is expected at about 2θ values of 10°. The reason for this result may be that TPA particles are too small and/or too well dispersed and therefore undetectable by XRD.15 In Figure 3,

Figure 3. Effect of calcination temperature on X-ray diffraction patterns.

the effect of calcination temperature on the XRD patterns at constant TPA loading of 25 wt % is shown. At 550 °C, zirconia supported TPA shows amorphous structure. As calcination temperature was increased, crystalline structure occurred. For catalysts calcined at 550 and 650 °C, no XRD peaks that could be attributed to the polyacid or its decomposition products are 6668

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Figure 5. Potentiometric titration curves of zirconia supported TPA catalysts.

Table 2. Surface Acidites of Prepared Catalysts TPA loading (wt %)

calcination temp. (°C)

E (mV)

no. acid sites

B/L

20 25 30 25 25 25

650 650 650 550 650 750

630 710 600 645 710 520

0.58 0.74 0.68 0.61 0.74 0.45

0.532 0.998 0.621 0.574 0.998 0.480

Figure 6. FTIR spectra of pyridine adsorbed catalysts: (a) effect of TPA loading at a calcination temperature of 650 °C, (b) effect of calcination temperature with 20% TPA loading.

strength and the surface acidity because of the decomposition of heterolpolyacid. Adsorption of pyridine is one of the most frequently applied methods for the characterization of surface acidity. The use of IR spectroscopy to detect adsorbed pyridine enables us to distinguish among different acid sites. FTIR pyridine adsorption spectra of catalysts with different TPA loading and calcination temperatures are shown in Figure 6. The absorption band at ∼1450 cm−1 was assigned to pyridine adsorbed on Lewis acid sites, and the absorption band at ∼1550 cm−1 was assigned to pyridine adsorbed on Bro̷ nsted acid sites. The B/L ratios calculated from the IR absorbance intensities are given in Table 2. The B/L ratio shows that the relative Bronsted acidity increases with TPA loading up to 25 wt % and decreases with further loading. For 25 wt % catalyst, the Bronsted acidity increases up to 650 °C, and above calcination temperature of 650 °C, an increase in Lewis acidity is observed. So, the catalyst with a TPA loading of 25 wt % and calcination temperature of 650 °C shows the maximum Bronsted acidity. 3.3. Performance of Zirconia Supported TPA Catalysts in the Esterification of Acrylic Acid with Different Alcohols. The esterification of alcohol (butanol, iso-butanol, or hexanol) with acrylic acid catalyzed by zirconia supported TPA catalysts was investigated at a temperature of 358 K and a molar ratio of 1. The catalytic data was collected in 5 h, and when the conversion values remain constant, corresponding values were compared to understand the effect of TPA loading and calcination temperature on acrylate synthesis. Effect of TPA Loading. Figure 7 shows the effect of TPA loading on the catalytic performance of zirconia supported TPA catalysts. To show the effect of supporting on the catalytic activities of TPA, the reaction of alcohol (butanol/iso-butanol or hexanol) and acrylic acid in the presence of the same amounts of nonsupported TPA was carried out under the same reaction conditions. Zirconia gave the minimum conversion

values for all three reactions. Figure 7 showed that the catalytic activity of TPA has been increased by supporting it on zirconia up to 25 wt % TPA loading. For iso-butyl acrylate synthesis, conversion of acrylic acid with 20 wt % TPA loading is 31%, while with 25 wt % and 30 wt % TPA loadings the corresponding values are 33% and 29%, respectively. It is seen that conversion increases with TPA loading and reaches the maximum at 25 wt % TPA loading, but conversion of acrylic acid decreased with further TPA loading. These results show that the presence of TPA plays a role in making the material more porous up to the amount required to form monolayer. However, when the TPA content increases beyond 30 wt %, pore blocking occurs due to the presence of an excess amount of TPA crystallites.4 In the same manner, for butyl acrylate and hexyl acrylate production, maximum conversion was obtained at 25 wt % TPA loading. Also, the decrease in the conversion of acrylic acid by the increase of the TPA content to 30 wt % might be attributed to both the decrease in the specific surface area and the surface acidity. Effect of Calcination Temperature. Figure 8 shows the effect of calcination temperature on the catalytic performance of zirconia supported TPA catalysts. For the effect of calcination temperature catalyst with 25 wt % TPA loading calcined at temperatures of 550, 650, and 750 °C. The catalyst gives the maximum conversion at calcination temperature of 650 °C, and conversion of acrylic acid decreased at the calcination temperature of 750 °C for the esterification of acrylic acid with butyl/iso-butyl/hexyl alcohol. This decrease may come from decrease in the specific surface area and the surface acidity. 6669

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Figure 7. Effect of TPA loading on the conversion of acrylic acid in the esterification of acrylic acid with (a) butanol, (b) iso-butanol, (c) hexanol. Figure 8. Effect of calcination temperature on the conversion of acrylic acid in the esterification of acrylic acid with (a) butanol, (b) isobutanol, (c) hexanol.

It is well-known that the strong acidity of HPW originates from its characteristic Keggin structure; destruction of such Keggin structure would result in a significant loss of the acidity and would therefore decrease its activity for acid-catalyzed reactions.17−19 Effect of the Length of the Alcohol’s Carbon Chain. The catalyst with optimum TPA loading (25 wt %) and calcination temperature (650 °C) was taken to study the role of the length of the alcohol’s carbon chain. The esterification of acrylic acid with different alcohols (butanol and hexanol) using zirconia supported TPA catalyst was performed. Experiments were carried out in a batch reactor at a temperature of 358 K and an acid to alcohol mol ratio of 1, and results are given in Figure 9. Butanol had a higher final conversion and initial reaction rate than hexanol for the esterification of acrylic acid. Effect of the OH Group Location. The catalyst with optimum TPA loading (25 wt %) and calcination temperature (650 °C) was taken to study the role of the OH group location.

To evaluate the significance of the location of the alcoholic group, 1-butanol and iso-butanol were used in the esterification of acrylic acid. In Figure 10, an increase in the final conversion of 1-butanol could be seen, as compared with iso-butanol. This effect might be due to a steric effect on the secondary alcohol. Reusability of Catalyst. The stability of the active species has been of concern for solid acidic catalysts, especially for supported materials. Since 25 wt % TPA loading and calcination temperature of 650 °C showed the best results, reusability of this catalyst was investigated in the esterification of acrylic acid with different alcohols by this catalyst. It showed a loss of activity after three successive runs, the conversion of acrylic acid was decreased by 3−7% after third run for the esterification of acrylic acid with butanol/isobutanol/hexanol. 6670

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REFERENCES

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Figure 9. Effect of length of alcohol’s carbon chain on the conversion of acrylic acid.

Figure 10. Effect of OH location group on the conversion of acrylic acid.

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CONCLUSION This study indicates that zirconia supported TPA catalysts can be used as a catalyst in the esterification of acrylic acid with high catalytic activity and thermal stability. The activity of catalyst was found to depend on TPA loading and calcination temperature. The results showed that 25 wt % TPA loading and calcination temperature of 650 °C are the most efficient catalyst preparation conditions since the maximum conversion values of acrylic acid were achieved for all alcohols. This catalyst has the advantages of an easy catalyst separation from the reaction medium and no problems of corrosion. Also, in this study, effects of length of alcohol’s carbon chain and the OH group location were investigated. Both have a significant effect on the conversion of acrylic acid achieved.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 90 232 311 1493, 90 232 388 7776. E-mail: emine. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by Ege University Scientific Research Project (09-MUH-054). The kind help of Simge Karakuş and Aslı Deniz Buluklu during laboratory studies is also acknowledged. 6671

dx.doi.org/10.1021/ie202609f | Ind. Eng. Chem. Res. 2012, 51, 6666−6671