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Selective esterification of glycerol to bioadditives over heteropoly tungstate supported on Cs-containing zirconia catalysts
Applied Catalysis A: General 386 (2010) 166–170
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Selective esterification of glycerol to bioadditives over heteropoly tungstate supported on Cs-containing zirconia catalysts K. Jagadeeswaraiah, M. Balaraju, P.S. Sai Prasad, N. Lingaiah ∗ Catalysis Laboratory, I&PC Division, Indian Institute of Chemical Technology, Hyderabad 500607, India
a r t i c l e
i n f o
Article history: Received 23 February 2010 Received in revised form 14 July 2010 Accepted 26 July 2010 Available online 3 August 2010 Keywords: Esterification Glycerol Acetic acid Tungstophosphoric acid Cesium Zirconia
a b s t r a c t Esterification of glycerol with acetic acid was carried out over tungstophosphoric acid (TPA) supported on Cs-containing zirconia. The catalysts were prepared by impregnation method and characterized by FT-infrared spectroscopy, X-ray diffraction and temperature program desorption of NH3 . The catalysts exhibited more than 90% conversion within a short reaction time. The catalytic activity depends on the amount of exchangeable Cs with TPA on zirconia, which is in tern related to the acidity of the catalysts. The acidity of the catalysts varied with the presence of residual protons of TPA. The effects of various parameters, such as reaction temperature, catalyst concentration and molar ratio of glycerol to acetic acid, were studied and optimized reaction conditions are established. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Glycerol is the main by-product in biodiesel production by transesterification of oil with methanol or ethanol. The increasing use and production of biodiesel has resulted in substantial amount of glycerol accumulation. This has allowed researchers to look for new usages of glycerol [1,2]. In future, glycerol will be a costeffective raw material for the preparation of valuable chemicals and fuel additives. Different methods for the conversion of glycerol to value-added chemicals such as hydrogenolysis to propane diols and for the preparation of different oxidation products have been explored [3–12]. The uses of glycerol-based additives to improve properties of biodiesel are also being explored [13]. The esterification of glycerol with acetic acid can be a good application for glycerol utilization. The products of glycerol esterification are monoacetin, diacetin and triacetin, which have great industrial applications. The products like triacetin have different applications, going from cosmetics to fuel additives. The mono and diacetylated esters are also have applications, particularly in cryogenics and syntheses of biodegradable polyesters [14]. Acetylation of glycerol has been carried using solid acid catalysts such as zeolites. The main aspect of the catalyst is its selectivity for this reaction. Heteropoly acids (HPAs) are typical strong Bronsted acids and catalyze a wide variety of reactions in both homogeneous and
∗ Corresponding author. Tel.: +91 40 2719 3163; fax: +91 40 2716 0921. E-mail address: [email protected] (N. Lingaiah). 0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.07.046
heterogeneous phases [15]. The major disadvantages of HPAs as catalysts lie in their low thermal stability, low surface area (1–10 m2 /g) and solubility in polar media. HPAs can become eco friendly insoluble solid acid catalysts with high thermal stability and high surface area by exchanging their protons with metal/alkali metal ion and supporting them on suitable supports. The support not only increases the surface area but also its stability. Supported HPAs were also studied for simple esterification reactions and were found to be highly active [16]. The support plays an important role in the dispersion of HPA and in the acidic nature of the final catalyst. The activity of the HPAs can be exchanged by supporting them on acidic supports and by exchanging the proton partially with alkali metal ions like Cs+ . It is known that the Cs salts of heteropoly acids are more acidic than parent HPAs [17]. Partial substitution of H+ by Cs+ changes the number of available surface acid sites and the resulting materials exhibit significantly higher activities than the parent acid in acid-catalyzed reactions [18]. Cs-containing phospho tungstates are well-known as a water-insoluble strong Brønsted acid and a versatile solid acid catalyst possessing high thermal stability (≥500 ◦ C) and water tolerance [19–22]. Even though the Cs salts possess more surface area compared to HPAs, their overall surface area and stability is minimum. It is not possible to prepare directly Cs salts of HPA dispersed on support. The usual way to prepare is firstly dispersing Cs on support and then supporting HPA on it by anticipating the partial exchange of Cs present on support with HPA. There are few studies where the support is doped with Cs and heteropoly acid is supported on it. Yang et al. reported the heteropoly tungstate supported on Cs modified
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mesoporous silica catalysts and their catalytic activity towards nbutane isomerization [23]. In another study cesium salts of TPA supported on dealuminated ultra-stable Y zeolite catalysts are reported. The Cs salt of TPA was generated by first dispersing Cs on Y zeolite, followed by impregnation of TPA [24]. Yadav and George reported the novelty of Cs substituted HPA supported on clay for acid-catalyzed reactions [25]. In the present study, tungstophosphoric acid supported on Cscontaining zirconia catalysts are prepared and evaluated for the acetylation of glycerol with acetic acid. This reaction is tested under different reaction parameters to yield the desired product. The catalyst performance is discussed with the observed physico-chemical properties derived from different characterization methods. It is interesting to know the activity of the catalysts when Cs present on zirconia is partially exchanged with TPA.
2. Experimental 2.1. Preparation of the catalysts Initially Cs is doped on zirconia and this material is used as support to disperse TPA. The chemicals CsNO3 and H3 PW12 O40 are purchased from Aldrich Chemicals. The support hydrous zirconia was prepared by hydrolyzing the aqueous solution of ZrOCl2 ·8H2 O with ammonium hydroxide at a pH of 10. The precipitate was filtered off and thoroughly washed with deionized water several times until the chloride content was negligible. The precipitate is dried at 120 ◦ C for 36 h. The hydrous zirconia after thorough drying was used as support. Firstly, the required amount of CsNO3 was dissolved in aqueous solution and added to the hydrous zirconia. The solution was allowed to sit for 3 h and then the excess water was evaporated on a water bath. The dried catalyst masses were kept for further drying in an air oven and calcined at 500 ◦ C for 2 h. Later TPA supported on Cs-containing zirconia was prepared in the same fashion by taking calculated amounts of TPA in aqueous solution. The final catalysts were calcined at 350 ◦ C for 4 h in air. The quantity of Cs is varied such that Cs can be exchangeable with 1, 2 and 3 protons of TPA. A catalyst without Cs also prepared in the similar way by dispersing TPA on zirconia. The active component TPA is kept constant at 20 wt% for all the catalysts. These catalysts are denoted as 20%TPA/Csx -ZrO2 where x = 0, 1, 2 and 3.
Fig. 1. XRD profiles of TPA supported on Cs-containing zirconia catalysts. (o) TPA, (#) Monoclinic phase of ZrO2 , (*) Tetragonal phase of ZrO2 .
3. Results and discussion 3.1. Catalyst characterization The XRD patterns of the catalysts are shown in Fig. 1. The catalysts showed main patterns related to the support zirconia. Both tetragonal and monoclinic phases of ZrO2 are present, monoclinic being predominant. The characteristic diffraction peaks related to Keggin ions of TPA are observed at 2Â of 10.3◦ and 24.4◦ [26]. XRD analysis suggests the presence of intact Keggin ion structure of TPA on Cs-ZrO2 . FT-IR spectra give an informative fingerprint about the presence of Keggin structure of heteropoly tungstates. The FT-IR spectra of the catalysts are presented in Fig. 2. The catalysts mainly exhibited bands at 1081, 990, 887 and 798 cm−1 related to the asymmetric vibrations of (P–O), (W Ot ), (W–Oc–W) and (W–Ob –W) modes, respectively [27]. These results endorse the existence of Keggin structure of TPA on support. The FT-IR results support the observations made from XRD analysis.
2.2. Characterization of the catalysts X-ray diffraction (XRD) patterns of the catalysts were recorded on a Rigaku Miniflex diffractometer using CuK␣ radiation ˚ at 40 kV and 30 mA. The measurements were obtained (1.5406 A) in steps of 0.045◦ with a account time of 0.5 s and in the 2Â range of 10–80◦ . FT-IR spectra were recorded on Biorad Excalibur series using KBr disc method. Temperature programmed desorption of ammonia (TPA) was carried out on a laboratory-built apparatus equipped with a gas chromatograph using a TCD detector. In a typical experiment about 0.05 g of the oven dried sample was taken in a quartz tube. Prior to TPD studies, the catalyst sample was treated at 300 ◦ C for 1 h by passing pure He gas (99.9%, 50 ml/min). After pretreatment, the sample was saturated with anhydrous ammonia (10% NH3 ) at 100 ◦ C at a flow rate of 50 ml/min for 1 h and was subsequently flushed with He at the same temperature to remove physisorbed ammonia. The process was continued until a stabilized base line was obtained in the gas chromatograph. Then the TPD analysis was carried out from ambient temperature to 700 ◦ C at a heating rate of 10 ◦ C/min. The amount of NH3 evolved was calculated from the peak area of the already calibrated TCD signal.
Fig. 2. FT-IR of TPA supported on Cs-containing ZrO2 catalysts.
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Fig. 4. Esterification of glycerol over TPA supported on Cs-containing ZrO2 catalysts.
Fig. 3. TPD of NH3 pattern of the catalysts.
Ammonia adsorption–desorption technique permits the determination of the strength of acid sites present on the catalyst surface, together with total acidity. The TPD profiles of the catalysts are shown in Fig. 3. The TPD profiles of the catalysts without Cs (TPA/ZrO2 catalyst) are also included for the sake of comparison. All samples showed broad TPD profiles, revealing that the surface acid strength is widely distributed. Three desorption peaks centered at 240, 430 and 570 ◦ C, are observed; these are related to the presence of weak, moderate and strong acid sites. The catalyst TPA/Cs2 -ZrO2 showed the highest amount of total acidity with more strong acidic sites than other catalysts. The variation in number of acidic sites depended on the amount of the cesium on zirconia. The cesium present on zirconia is exchangeable with the protons of TPA during the impregnation. The partial exchange of TPA protons with Cs results in enhanced surface acidic sites due to the presence of residual protons [17,18]. Generally these residual protons are mobile and lead to generation of strong acidic sites. The catalyst without Cs showed the presence of more weak to moderate acidic sites compared to the Cs-containing catalysts.
maximum conversion within 2 h. Initially, at the start of the reaction, the selectivity towards monoacetin was high; as the reaction prolonged, the selectivity towards di and triacetin increased at the expense of that toward monoactin. The increase in selectivity for di and triacetin with time is mainly due to the further acetylation of monoacetin as the glycerol conversion is nearly completed with in 2 h of reaction time. The high activity of the catalysts mainly related to the presence of intact Keggin ions on the support. During the preparation, there was a possibility for the formation of Cs exchanged TPA on ZrO2 . The highest activity showed by TPA/Cs2 ZrO2 catalyst might be due to the partial exchange of Cs, which results in the presence of residual protons. The TPD results of the catalysts suggest the presence of a maximum amount of acidity for this catalyst. The catalyst with Cs amount equal to three protons of TPA showed the lowest activity as expected due to the absence of residual protons. The conversion and selectivity during glycerol acetylation not only depends on the nature of the catalyst but also on the reaction parameters. The influence of different reaction parameters such as reaction temperature, catalyst weight and glycerol to acetic acid molar ratio are studied.
3.2. Glycerol acetylation The acetylation of glycerol over TPA/Csx -ZrO2 catalysts leads to the formation of monoacetin, diacetin and triacetin. Fig. 4 compares the activity of the catalysts for the acetylation of glycerol with acetic acid. The TPA/ZrO2 catalyst reaction profile is also included for comparison. The catalytic activity varied with the exchangeable amount of Cs on ZrO2 . The catalyst without Cs showed relatively less activity compared to all other catalysts. The catalyst with Cs content equal to 2 moles exchangeable with two protons of TPA showed the highest activity. The glycerol conversion is increased with reaction time for all the catalysts and a attained maximum conversion within 3 h of reaction time. As the catalyst TPA/Cs2 -ZrO2 showed excellent esterification activity, the reaction profile over this catalyst on varying the reaction time was studied and the results are presented in Fig. 5. The conversion of glycerol was very high even at 30 min and attained
Fig. 5. Reaction profile of TPA/Cs2 -ZrO2 catalysts during glycerol esterification.
K. Jagadeeswaraiah et al. / Applied Catalysis A: General 386 (2010) 166–170
Fig. 6. Effect of reaction temperature on glycerol acetylation with acetic acid.
3.3. Influence of reaction temperature
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Fig. 8. Effect of catalyst weight on glycerol acetylation with acetic acid.
The influence of reaction temperature on the esterification of glycerol over TPA/Cs2 -ZrO2 was evaluated by varying the reaction temperature from 60 to 120 ◦ C; results are shown in Fig. 6. As expected, the conversion of glycerol increased with temperature. At the same time, the selectivity also varied with temperature. The selectivity to di and tri acetin increased with increase in temperature from 60 to 120 ◦ C. The present catalyst showed considerable activity even at low reaction temperature.
increased with increase in the amount of acetic acid and maximum conversion was attained at a mole ratio of 1:5. A marginal increase is noticed as the results are taken after 4 h of reaction time. The conversion did not varied above the molar ratio of 1:5. However, the selectivity varied with glycerol to acetic acid molar ratio. More monoacetin formed at lower glycerol to acetic acid ratio and its selectivity decreased with increase in acetic acid amount. At higher acetic acid concentrations, one expects to get more di and triacetin due to the availability of more acetylating agent.
3.4. Effect of glycerol to acetic acid molar ratio
3.5. Effect of catalyst weight
Glycerol conversion and selectivity during acetylation of glycerol also depends on glycerol to acetic acid molar ratio. The reaction was carried out for the glycerol to acetic acid molar ratios from 1:4 to 1:7; the results are shown in Fig. 7. The conversion of glycerol does not vary drastically with change in molar ratio. The conversion
Fig. 8 shows the effect of catalyst weight on glycerol conversion. The increase in catalyst loading from 0.1 to 0.4 g results in a marginal variation in glycerol conversion. When the catalyst amount increased from 0.1 to 0.2 g, only a moderate increase in glycerol conversion was observed. Thus, it is clear that the increase in catalyst loading above 0.2 g did not help to improve the initial rate of the reaction. The results suggest that a small amount catalyst is sufficient to attain maximum conversion. The selectivity of catalyst did not vary much with the change in catalyst weight. 4. Conclusions Heteropoly tungstate supported on Cs-containing ZrO2 catalysts was prepared with retention of Keggin ion structure. The characterization results suggest the presence of strong acidic sites, where acid strength depends on the exchangeable Cs content on zirconia. The esterification activity is related to the acidity of the catalysts. The Cs-containing catalysts possess stronger acidic sites compared to the catalyst without Cs. The presence of Cs substantially enhanced the activity. The glycerol esterification activity and selectivity not only depended on the nature of the catalyst but also on some reaction parameters such as temperature, time and mole ratio of glycerol to acetic acid. Acknowledgement
Fig. 7. Influences of glycerol to acetic acid molar ratio on glycerol acetylation with acetic acid.
The authors KJ and MB are thankful to CSIR for the awards of junior and senior research fellowships, respectively.
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Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Selective esterification of glycerol to bioadditives over heteropoly tungstate supported on Cs-containing zirconia catalysts K. Jagadeeswaraiah, M. Balaraju, P.S. Sai Prasad, N. Lingaiah ∗ Catalysis Laboratory, I&PC Division, Indian Institute of Chemical Technology, Hyderabad 500607, India
a r t i c l e
i n f o
Article history: Received 23 February 2010 Received in revised form 14 July 2010 Accepted 26 July 2010 Available online 3 August 2010 Keywords: Esterification Glycerol Acetic acid Tungstophosphoric acid Cesium Zirconia
a b s t r a c t Esterification of glycerol with acetic acid was carried out over tungstophosphoric acid (TPA) supported on Cs-containing zirconia. The catalysts were prepared by impregnation method and characterized by FT-infrared spectroscopy, X-ray diffraction and temperature program desorption of NH3 . The catalysts exhibited more than 90% conversion within a short reaction time. The catalytic activity depends on the amount of exchangeable Cs with TPA on zirconia, which is in tern related to the acidity of the catalysts. The acidity of the catalysts varied with the presence of residual protons of TPA. The effects of various parameters, such as reaction temperature, catalyst concentration and molar ratio of glycerol to acetic acid, were studied and optimized reaction conditions are established. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Glycerol is the main by-product in biodiesel production by transesterification of oil with methanol or ethanol. The increasing use and production of biodiesel has resulted in substantial amount of glycerol accumulation. This has allowed researchers to look for new usages of glycerol [1,2]. In future, glycerol will be a costeffective raw material for the preparation of valuable chemicals and fuel additives. Different methods for the conversion of glycerol to value-added chemicals such as hydrogenolysis to propane diols and for the preparation of different oxidation products have been explored [3–12]. The uses of glycerol-based additives to improve properties of biodiesel are also being explored [13]. The esterification of glycerol with acetic acid can be a good application for glycerol utilization. The products of glycerol esterification are monoacetin, diacetin and triacetin, which have great industrial applications. The products like triacetin have different applications, going from cosmetics to fuel additives. The mono and diacetylated esters are also have applications, particularly in cryogenics and syntheses of biodegradable polyesters [14]. Acetylation of glycerol has been carried using solid acid catalysts such as zeolites. The main aspect of the catalyst is its selectivity for this reaction. Heteropoly acids (HPAs) are typical strong Bronsted acids and catalyze a wide variety of reactions in both homogeneous and
∗ Corresponding author. Tel.: +91 40 2719 3163; fax: +91 40 2716 0921. E-mail address: [email protected] (N. Lingaiah). 0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.07.046
heterogeneous phases [15]. The major disadvantages of HPAs as catalysts lie in their low thermal stability, low surface area (1–10 m2 /g) and solubility in polar media. HPAs can become eco friendly insoluble solid acid catalysts with high thermal stability and high surface area by exchanging their protons with metal/alkali metal ion and supporting them on suitable supports. The support not only increases the surface area but also its stability. Supported HPAs were also studied for simple esterification reactions and were found to be highly active [16]. The support plays an important role in the dispersion of HPA and in the acidic nature of the final catalyst. The activity of the HPAs can be exchanged by supporting them on acidic supports and by exchanging the proton partially with alkali metal ions like Cs+ . It is known that the Cs salts of heteropoly acids are more acidic than parent HPAs [17]. Partial substitution of H+ by Cs+ changes the number of available surface acid sites and the resulting materials exhibit significantly higher activities than the parent acid in acid-catalyzed reactions [18]. Cs-containing phospho tungstates are well-known as a water-insoluble strong Brønsted acid and a versatile solid acid catalyst possessing high thermal stability (≥500 ◦ C) and water tolerance [19–22]. Even though the Cs salts possess more surface area compared to HPAs, their overall surface area and stability is minimum. It is not possible to prepare directly Cs salts of HPA dispersed on support. The usual way to prepare is firstly dispersing Cs on support and then supporting HPA on it by anticipating the partial exchange of Cs present on support with HPA. There are few studies where the support is doped with Cs and heteropoly acid is supported on it. Yang et al. reported the heteropoly tungstate supported on Cs modified
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mesoporous silica catalysts and their catalytic activity towards nbutane isomerization [23]. In another study cesium salts of TPA supported on dealuminated ultra-stable Y zeolite catalysts are reported. The Cs salt of TPA was generated by first dispersing Cs on Y zeolite, followed by impregnation of TPA [24]. Yadav and George reported the novelty of Cs substituted HPA supported on clay for acid-catalyzed reactions [25]. In the present study, tungstophosphoric acid supported on Cscontaining zirconia catalysts are prepared and evaluated for the acetylation of glycerol with acetic acid. This reaction is tested under different reaction parameters to yield the desired product. The catalyst performance is discussed with the observed physico-chemical properties derived from different characterization methods. It is interesting to know the activity of the catalysts when Cs present on zirconia is partially exchanged with TPA.
2. Experimental 2.1. Preparation of the catalysts Initially Cs is doped on zirconia and this material is used as support to disperse TPA. The chemicals CsNO3 and H3 PW12 O40 are purchased from Aldrich Chemicals. The support hydrous zirconia was prepared by hydrolyzing the aqueous solution of ZrOCl2 ·8H2 O with ammonium hydroxide at a pH of 10. The precipitate was filtered off and thoroughly washed with deionized water several times until the chloride content was negligible. The precipitate is dried at 120 ◦ C for 36 h. The hydrous zirconia after thorough drying was used as support. Firstly, the required amount of CsNO3 was dissolved in aqueous solution and added to the hydrous zirconia. The solution was allowed to sit for 3 h and then the excess water was evaporated on a water bath. The dried catalyst masses were kept for further drying in an air oven and calcined at 500 ◦ C for 2 h. Later TPA supported on Cs-containing zirconia was prepared in the same fashion by taking calculated amounts of TPA in aqueous solution. The final catalysts were calcined at 350 ◦ C for 4 h in air. The quantity of Cs is varied such that Cs can be exchangeable with 1, 2 and 3 protons of TPA. A catalyst without Cs also prepared in the similar way by dispersing TPA on zirconia. The active component TPA is kept constant at 20 wt% for all the catalysts. These catalysts are denoted as 20%TPA/Csx -ZrO2 where x = 0, 1, 2 and 3.
Fig. 1. XRD profiles of TPA supported on Cs-containing zirconia catalysts. (o) TPA, (#) Monoclinic phase of ZrO2 , (*) Tetragonal phase of ZrO2 .
3. Results and discussion 3.1. Catalyst characterization The XRD patterns of the catalysts are shown in Fig. 1. The catalysts showed main patterns related to the support zirconia. Both tetragonal and monoclinic phases of ZrO2 are present, monoclinic being predominant. The characteristic diffraction peaks related to Keggin ions of TPA are observed at 2Â of 10.3◦ and 24.4◦ [26]. XRD analysis suggests the presence of intact Keggin ion structure of TPA on Cs-ZrO2 . FT-IR spectra give an informative fingerprint about the presence of Keggin structure of heteropoly tungstates. The FT-IR spectra of the catalysts are presented in Fig. 2. The catalysts mainly exhibited bands at 1081, 990, 887 and 798 cm−1 related to the asymmetric vibrations of (P–O), (W Ot ), (W–Oc–W) and (W–Ob –W) modes, respectively [27]. These results endorse the existence of Keggin structure of TPA on support. The FT-IR results support the observations made from XRD analysis.
2.2. Characterization of the catalysts X-ray diffraction (XRD) patterns of the catalysts were recorded on a Rigaku Miniflex diffractometer using CuK␣ radiation ˚ at 40 kV and 30 mA. The measurements were obtained (1.5406 A) in steps of 0.045◦ with a account time of 0.5 s and in the 2Â range of 10–80◦ . FT-IR spectra were recorded on Biorad Excalibur series using KBr disc method. Temperature programmed desorption of ammonia (TPA) was carried out on a laboratory-built apparatus equipped with a gas chromatograph using a TCD detector. In a typical experiment about 0.05 g of the oven dried sample was taken in a quartz tube. Prior to TPD studies, the catalyst sample was treated at 300 ◦ C for 1 h by passing pure He gas (99.9%, 50 ml/min). After pretreatment, the sample was saturated with anhydrous ammonia (10% NH3 ) at 100 ◦ C at a flow rate of 50 ml/min for 1 h and was subsequently flushed with He at the same temperature to remove physisorbed ammonia. The process was continued until a stabilized base line was obtained in the gas chromatograph. Then the TPD analysis was carried out from ambient temperature to 700 ◦ C at a heating rate of 10 ◦ C/min. The amount of NH3 evolved was calculated from the peak area of the already calibrated TCD signal.
Fig. 2. FT-IR of TPA supported on Cs-containing ZrO2 catalysts.
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Fig. 4. Esterification of glycerol over TPA supported on Cs-containing ZrO2 catalysts.
Fig. 3. TPD of NH3 pattern of the catalysts.
Ammonia adsorption–desorption technique permits the determination of the strength of acid sites present on the catalyst surface, together with total acidity. The TPD profiles of the catalysts are shown in Fig. 3. The TPD profiles of the catalysts without Cs (TPA/ZrO2 catalyst) are also included for the sake of comparison. All samples showed broad TPD profiles, revealing that the surface acid strength is widely distributed. Three desorption peaks centered at 240, 430 and 570 ◦ C, are observed; these are related to the presence of weak, moderate and strong acid sites. The catalyst TPA/Cs2 -ZrO2 showed the highest amount of total acidity with more strong acidic sites than other catalysts. The variation in number of acidic sites depended on the amount of the cesium on zirconia. The cesium present on zirconia is exchangeable with the protons of TPA during the impregnation. The partial exchange of TPA protons with Cs results in enhanced surface acidic sites due to the presence of residual protons [17,18]. Generally these residual protons are mobile and lead to generation of strong acidic sites. The catalyst without Cs showed the presence of more weak to moderate acidic sites compared to the Cs-containing catalysts.
maximum conversion within 2 h. Initially, at the start of the reaction, the selectivity towards monoacetin was high; as the reaction prolonged, the selectivity towards di and triacetin increased at the expense of that toward monoactin. The increase in selectivity for di and triacetin with time is mainly due to the further acetylation of monoacetin as the glycerol conversion is nearly completed with in 2 h of reaction time. The high activity of the catalysts mainly related to the presence of intact Keggin ions on the support. During the preparation, there was a possibility for the formation of Cs exchanged TPA on ZrO2 . The highest activity showed by TPA/Cs2 ZrO2 catalyst might be due to the partial exchange of Cs, which results in the presence of residual protons. The TPD results of the catalysts suggest the presence of a maximum amount of acidity for this catalyst. The catalyst with Cs amount equal to three protons of TPA showed the lowest activity as expected due to the absence of residual protons. The conversion and selectivity during glycerol acetylation not only depends on the nature of the catalyst but also on the reaction parameters. The influence of different reaction parameters such as reaction temperature, catalyst weight and glycerol to acetic acid molar ratio are studied.
3.2. Glycerol acetylation The acetylation of glycerol over TPA/Csx -ZrO2 catalysts leads to the formation of monoacetin, diacetin and triacetin. Fig. 4 compares the activity of the catalysts for the acetylation of glycerol with acetic acid. The TPA/ZrO2 catalyst reaction profile is also included for comparison. The catalytic activity varied with the exchangeable amount of Cs on ZrO2 . The catalyst without Cs showed relatively less activity compared to all other catalysts. The catalyst with Cs content equal to 2 moles exchangeable with two protons of TPA showed the highest activity. The glycerol conversion is increased with reaction time for all the catalysts and a attained maximum conversion within 3 h of reaction time. As the catalyst TPA/Cs2 -ZrO2 showed excellent esterification activity, the reaction profile over this catalyst on varying the reaction time was studied and the results are presented in Fig. 5. The conversion of glycerol was very high even at 30 min and attained
Fig. 5. Reaction profile of TPA/Cs2 -ZrO2 catalysts during glycerol esterification.
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Fig. 6. Effect of reaction temperature on glycerol acetylation with acetic acid.
3.3. Influence of reaction temperature
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Fig. 8. Effect of catalyst weight on glycerol acetylation with acetic acid.
The influence of reaction temperature on the esterification of glycerol over TPA/Cs2 -ZrO2 was evaluated by varying the reaction temperature from 60 to 120 ◦ C; results are shown in Fig. 6. As expected, the conversion of glycerol increased with temperature. At the same time, the selectivity also varied with temperature. The selectivity to di and tri acetin increased with increase in temperature from 60 to 120 ◦ C. The present catalyst showed considerable activity even at low reaction temperature.
increased with increase in the amount of acetic acid and maximum conversion was attained at a mole ratio of 1:5. A marginal increase is noticed as the results are taken after 4 h of reaction time. The conversion did not varied above the molar ratio of 1:5. However, the selectivity varied with glycerol to acetic acid molar ratio. More monoacetin formed at lower glycerol to acetic acid ratio and its selectivity decreased with increase in acetic acid amount. At higher acetic acid concentrations, one expects to get more di and triacetin due to the availability of more acetylating agent.
3.4. Effect of glycerol to acetic acid molar ratio
3.5. Effect of catalyst weight
Glycerol conversion and selectivity during acetylation of glycerol also depends on glycerol to acetic acid molar ratio. The reaction was carried out for the glycerol to acetic acid molar ratios from 1:4 to 1:7; the results are shown in Fig. 7. The conversion of glycerol does not vary drastically with change in molar ratio. The conversion
Fig. 8 shows the effect of catalyst weight on glycerol conversion. The increase in catalyst loading from 0.1 to 0.4 g results in a marginal variation in glycerol conversion. When the catalyst amount increased from 0.1 to 0.2 g, only a moderate increase in glycerol conversion was observed. Thus, it is clear that the increase in catalyst loading above 0.2 g did not help to improve the initial rate of the reaction. The results suggest that a small amount catalyst is sufficient to attain maximum conversion. The selectivity of catalyst did not vary much with the change in catalyst weight. 4. Conclusions Heteropoly tungstate supported on Cs-containing ZrO2 catalysts was prepared with retention of Keggin ion structure. The characterization results suggest the presence of strong acidic sites, where acid strength depends on the exchangeable Cs content on zirconia. The esterification activity is related to the acidity of the catalysts. The Cs-containing catalysts possess stronger acidic sites compared to the catalyst without Cs. The presence of Cs substantially enhanced the activity. The glycerol esterification activity and selectivity not only depended on the nature of the catalyst but also on some reaction parameters such as temperature, time and mole ratio of glycerol to acetic acid. Acknowledgement
Fig. 7. Influences of glycerol to acetic acid molar ratio on glycerol acetylation with acetic acid.
The authors KJ and MB are thankful to CSIR for the awards of junior and senior research fellowships, respectively.
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