- Email: [email protected]
Catalytic acetylation of glycerol with acetic anhydride
Catalysis Communications 11 (2010) 1036–1039
Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Catalytic acetylation of glycerol with acetic anhydride Leonardo N. Silva, Valter L.C. Gonçalves, Claudio J.A. Mota ⁎ Universidade Federal do Rio de Janeiro, Instituto de Química. Av Athos da Silveira Ramos 149, CT Bloco A, 21941-909, Rio de Janeiro, Brazil INCT de Energia e Ambiente, UFRJ, 21941-909, RJ, Brazil
a r t i c l e
i n f o
Article history: Received 16 March 2010 Received in revised form 4 May 2010 Accepted 7 May 2010 Available online 16 May 2010 Keywords: Glycerol Triacetin Zeolites Acid catalysis
a b s t r a c t We studied the acetylation of glycerol with acetic anhydride using different solid acid catalysts. The results indicated that at 60 °C, zeolite Beta and K-10 Montmorillonite showed 100% selectivity to triacetin within 20 min, with a molar ratio of 4:1. Amberlyst-15 acid resin yielded 100% triacetin after 80 min, whereas niobium phosphate gave diacetin and triacetin in 53% and 47% selectivity, respectively. All catalysts were more selective to triacetin than the uncatalyzed reaction. By contrast, zeolite Beta gave poor yield of triacetin when acetic acid was used as acetylating agent. The different behavior was explained in terms of the stabilization of the acylium ion intermediate. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Glycerol is a byproduct of transesterification of vegetable oil to produce biodiesel [1]. It is normally produced in 10 wt.% mass balance from the transesterification reaction, and its global production is estimated to reach 1.2 million tons by 2012 [2]. This enormous amount of glycerol must find an economical destination. The chemical transformation [3–5] of glycerol into more valuable compounds is one of the most promising applications. Glycerol acetates, namely mono, di and triacetin, have major applications in cryogenics [6], plastics, [7] and fuel additives [8]. However, the efficient production of triacetin (glycerol triacetate) requires large excess of the acetylation reactant, normally acetic acid, as well as long reaction times or expeditious water removal to shift equilibrium. We wish to report in this communication, the fast and easy production of triacetin using acetic anhydride and a solid acid catalyst. Biodiesel is presently one of the major biofuels used worldwide. It is normally produced from the transesterification of triglycerides with methanol, under base catalysis conditions. This reaction affords the fatty acid methyl esters, the biodiesel themselves, and glycerol. Today, one of the most challenging aspects of the biodiesel technology is related with the economical utilization of the glycerol, or glycerin, formed. The major uses of this compound are in personal care products, cosmetics and soaps, but these sectors will not be able to drain the tons of glycerol coming from biodiesel production.
⁎ Corresponding author. Universidade Federal do Rio de Janeiro, Instituto de Química. Av Athos da Silveira Ramos 149, CT Bloco A, 21941-909, Rio de Janeiro, Brazil. Fax: +55 21 25627106. E-mail address: [email protected] (C.J.A. Mota). 1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.05.007
Glycerol is a good feedstock for other chemicals. Hydrogenolysis to 1,2 and 1,3-propanediol over metal catalysts [9–11] has been extensively studied. Dehydration to acrolein, a useful intermediate for plastic production, has been reported to occur over solid acid catalysts [12–14]. Anaerobic fermentation of glycerol is a potential route for the production of butanol and ethanol [15], which can be used as biofuels. Glycerol reforming to synthesis gas, a mixture of carbon monoxide and hydrogen, has been studied over noble metal catalysts [16], and could be a possible pathway for the production of hydrocarbons in the diesel and gasoline range. Glycerol acetates are also good candidates to drain part of the glycerol produced by the biodiesel industry. Traditionally, this chemical is used as a plasticizer in cigarette filters, but in recent years it has been tested as a biodiesel additive [17,18], improving the cold flow properties. This application is extremely interesting, because the glycerol produced from transesterification could be used in the biodiesel chain, as an additive to improve the properties of this biofuel. Triacetin can be produced in the acid-catalyzed reaction of glycerol with acetic acid. There are many studies [19–22] using different solid acid catalysts and operational conditions, but the selectivity to triacetin is normally limited. The presence of water, shifting equilibrium and weakening the catalyst acid strength, as well as the sequential acetylation of the hydroxyl groups contributes for this result. Recently, triacetin was produced in 100% selectivity from glycerol with the use of a two-step process [23]. Initially, glycerol is reacted with nine fold molar excess of acetic acid in the presence of Amberlyst-35 acid resin. After 4 h at 105 °C, acetic anhydride is introduced in the reaction medium to complete the acetylation, yielding 100% triacetin. Calculations suggest [24] that acetylation with acetic acid is an endothermic process, requiring high energy demand for the introduction of the third acetyl group to form triacetin. In
L.N. Silva et al. / Catalysis Communications 11 (2010) 1036–1039
1037
Scheme 1. Acetylation of glycerol with acetic anhidride.
comparison, acetylation with acetic anhydride is exothermic, favoring formation of triacetin. These studies prompted us to show our results of glycerol acetylation with acetic anhydride in the presence of solid acid catalysts (Scheme 1).
Reactions with acetic acid as acylating reagent were also carried out. The procedure was similar as the one reported previously [20], with a molar ratio acid/glycerol of 4. 3. Results and discussion
2. Experimental The reactions were carried out in batch conditions, mixing 2.0 g of glycerol, 8.2 mL of acetic anhydride (4:1 molar ratio anhydride to glycerol) and a mass of catalyst corresponding to 2.0 mmol of acid sites. The acidity was obtained from n-butylamine termodesorption experiments, and the procedure was reported elsewhere [25]. In some experiments, a molar ratio of 3:1 of anhydride to glycerol was used. Reactions were carried out at 60 °C and, in some cases, at 120 °C. Blank experiments, without adding catalysts, were also carried out for comparison purposes. The product distribution was determined, withdrawing samples from the reaction mixture at specific time intervals and analyzing them by gas chromatography coupled with mass spectrometry. In all cases, 1,4-dioxane was used as external standard.
In all reactions with acetic anhydride, no glycerol was detected among the products, indicating 100% conversion. However, the formation of di and triacetin involves consecutive acetylations of the remaining hydroxyl groups and although we express the results in terms of selectivity, they are indirectly related with catalyst activity. Table 1 shows the results of glycerol acetylation with acetic anhydride. One can see that 100% selectivity to triacetin is achieved within 20 min with the use of zeolite H-Beta and K-10 Montmorillonite as catalyst, at 60 °C and 4:1 molar ratio of anhydride to glycerol, respectively. Reduction of the molar ratio to the stoichiometric value decreases the selectivity to triacetin, even after 2 h of reaction time, but triacetin is still the major product formed. Amberlyst-15 acid resin
Table 1 Selectivity to glycerol acetates in reactions of glycerol with acetic anhydride at different conditions.a Catalyst
H-Betab H-Beta K-10c K-10 Amberlyst-15d Amberlyst-15 Niobium phosphatee Niobium phosphate Blank Blank
Molar ratio anhydride: glycerol
Temperature (°C)
4:1 3:1 4:1 3:1 4:1 4:1 4:1 4:1 4:1 4:1
60 60 60 60 60 60 60 120 60 120
Time (min)
20 120 20 120 80 20 120 80 120 120
Selectivity to acetins % Mono
Di
Tri
– – – – – – – – 10 –
– 38 – 22 – 10 53 – 56 6
100 62 100 78 100 90 47 100 34 94
Fig. 1. Glycerol conversion ( ) and selectivity to monoacetin ( ), diacetin ( ), triacetin ( ) and acetol ( ) in the reaction with acetic acid at 120 °C, catalyzed by zeolite Beta.
a Reaction carried out with 2.0 g of glycerol. In all cases, glycerol conversion was 100%. b Si/Al = 16, area = 633 m2/g, acidity = 1.6 mmol/g. Pre-treatment at 500 °C/30 min. c Si/Al= 6.6, area= 240 m2/g, acidity= 0.53 mmol/g. Pre-treatment at 150 °C/30 min. d Area = 50 m2/g, acidity = 4.7 mmol/g. Pre-treatment at 120 °C/30 min. e Area = 187 m2/g, acidity = 0.4 mmol/g. No pre-treatment temperature.
Table 2 Selectivity for glycerol acetates in reactions of glycerol and acetic acid at 120 °C.a Catalystb
Conversion (%)
Selectivity (%) Monoacetin
Diacetin
Triacetin
Acetol
H-Beta K-10 Niobium phosphate Amberlyst-15 Blank
94 100 100 100 91
48 36 38 18 50
39 52 49 55 40
4 6 7 24 4
9 6 5 3 7
a b
Molar ratio acetic acid to glycerol of 4:1. Results after 120 min of reaction. Mass of catalyst used corresponds to 2.0 mmol of acid sites.
Fig. 2. Glycerol conversion ( ) and selectivity to monoacetin ( ), diacetin ( ), triacetin ( ) and acetol ( ) in the reaction with acetic acid at 120 °C, catalyzed by Amberlyst-15.
1038
L.N. Silva et al. / Catalysis Communications 11 (2010) 1036–1039
Scheme 2. AAC2 mechanism with formation of a tetrahedral intermediate.
is slightly less active, yielding 100% selectivity to triacetin only after 80 min of reaction and a molar ratio of 4:1. Within the first 20 min, the selectivity to triacetin with this catalyst is 90%, with the remaining 10% as diacetin. Niobium phosphate is considerably less active at 60 °C. After 120 min, the selectivity to triacetin was only 47%. At 120 °C, this catalyst showed 100% selectivity to triacetin within 80 min of reaction time. By contrast, the uncatalyzed (blank reaction) showed a considerably lower selectivity at both temperatures. At 60 °C, the selectivity to triacetin was 34% after 120 min, whereas at 120 °C the selectivity was about 94% within the same period. These results show that triacetin can be obtained in excellent selectivity at 60 °C with the use of zeolite H-Beta or K-10 Montmorillonite as catalysts and a molar ratio of anhydride to glycerol of 4 to 1. Table 2 shows the results of glycerol acetylation with acetic acid at 120 °C, using the same solid acid catalysts. One can see that in all cases, the selectivity to triacetin is significantly lower when compared with reactions with acetic anhydride. The best selectivity to triacetin (24%) was achieved with use of Amberlyst-15. The other catalysts showed selectivity in the range of 4 to 7%, similar to the uncatalyzed reaction. Zeolite Beta showed the worse performance among the catalysts tested, presenting glycerol conversion lower than 100% and the highest selectivity to monoacetin (Fig. 1). These results are similar to what was found [20] in the acetylation of glycerol with acetic acid catalyzed by HUSY and HZSM-5 zeolites, which did not show good conversion and selectivity to triacetin either. Exclude by contrast, Amberlyst-15 was the best catalysts tested (Fig. 2). The acid resin is the strongest acid catalysts tested [25] and this might partly explain the results. However, neither K-10, nor niobium phosphate have higher acid strength than zeolites [25] and their performance might be associated with other factors. A possible explanation is that their structure would be capable of adsorbing the water formed, probably shifting the equilibrium or preventing catalyst deactivation. It is interesting to note that K-10 Montmorillonite also showed a good performance in glycerol etherification with benzyl alcohol [26], a reaction that also releases water as byproducts. Acetol (hydroxiacetone), arisen from glycerol dehydration, was observed over all catalysts tested. This compound was not significantly observed in the
reactions with acetic anhydride (only traces were observed in some reactions, especially at higher temperatures). It is not completely clear why the acetylation with acetic anhydride performs well on zeolite Beta, whereas it does not with acetic acid. We have shown that zeolite Beta is a good catalyst for the acetalyzation of glycerol with formaldehyde [27], which is sensitive to the presence of water. The explanation was that, due to the high Si/Al ratio of this zeolite, its pore environment is hydrophobic [28] and does not favor the adsorption of the water released during the reaction on the active sites. It might also prevent or minimize the reverse reaction, because the water tends to diffuse out of the pores. Thus, we cannot explain the low selectivity to triacetin in the glycerol acetylation with acetic acid on zeolite Beta to the effect of water, by shifting equilibrium or weakening the acid sites. One can envisage two possible mechanisms [29] for acetylation in strong acidic medium: the first one is the normal AAC2 mechanism, involving protonation of the carbonyl oxygen atom and nucleophilic attack in the carbonyl to form a tetrahedral intermediate, presenting a quaternary carbon atom (Scheme 2); the second mechanism is the AAC1, where protonation takes place in the oxygen atom attached to the carbonyl group, followed by formation of an acylium ion (Scheme 3). The first pathway is normally less energetic, because of the higher stability of the intermediate formed upon protonation in the carbonyl oxygen atom. On the other hand, formation of the tetrahedral intermediate is space demanding, and the second mechanism, involving the acylium ion, prevails in situations of steric constraints [30]. Zeolites are known for their shape selectivity properties [31], especially concerning the formation of bulk transition states. It is possible that in acetylation with acetic anhydride, formation of the tetrahedral intermediate inside the zeolite pores might be prevented, due to transition state shape selectivity. This might be especially difficult for the third acetylation to form triacetin. Therefore, acylation takes place through the AAC1 mechanism. By contrast, it seems that the same mechanism is difficult to operate in the case of acetylation with acetic acid, even at higher temperature. This point is not completely clear and cannot be answered with the data of this study. We propose an explanation based on the interaction of the protonated acetic acid
Scheme 3. AAC1 mechanism with formation of an acylium ion.
L.N. Silva et al. / Catalysis Communications 11 (2010) 1036–1039
1039
compound from acetic acid, structural arrangements do not permit assistance from the framework of oxygen atoms of the zeolite. Nevertheless, formation of the acylium ion from the anhydride allows the participation of the zeolite framework, stabilizing the formation of the intermediate. Acknowledgements Scheme 4. Proposed structure of the protonated acetic acid on the zeolite surface and its dehydration to the acylium ion (no zeolite assistance).
Authors thank financial support from FINEP, CNPq and FAPERJ. References
Scheme 5. Proposed structure of the protonated acetic anhydride and its transformation to the acylium ion (zeolite-assisted).
and protonated acetic anhydride with the zeolite surface, prior to the acyl cation formation. In the case of acetic acid, the two hydrogen atoms are interacting with the zeolite structure through hydrogen bonds (Scheme 4). This type of structure has already been proposed in other zeolite-catalyzed reactions involving protonation of hydroxyl groups [32–34], and might impair the formation of the acylium ion, because it would be away from the zeolite surface and would not be well stabilized. On the other hand, in the case of acetic anhydride, formation of the acylium ion might be assisted by the zeolite surface (Scheme 5), favoring its formation. It is well recognized that the zeolite surface assists the formation of cationic species [35], as well as stabilizes them through covalent bonding. The use of acetic anhydride in the preparation of triacetin from glycerol may become feasible with zeolite Beta, K-10 Montmorillonite or Amberlyst acid resin as catalysts. The reaction is exothermic and can be carried out at mild temperatures and short reaction times with use of these catalysts. This permits a better control of the system for scaling up purposes. 4. Conclusions The use of zeolite Beta or K-10 Montmorillonite as catalysts in the acetylation of glycerol with acetic anhydride produces triacetin in 100% selectivity at 60 °C within 20 min of reaction. Amberlyst-15 acid resin requires longer reaction times, whereas niobium phosphate gives 100% selectivity only at higher temperatures. By contrast, zeolite Beta showed the lowest selectivity to triacetin when acetic acid is used as catalyst. This result was explained in terms of the stability of the acylium ion intermediate. Upon formation of this
[1] F.R. Ma, M.A. Hanna, Bioresour. Tech. 70 (1999) 1–15. [2] C.H. Zhou, J.N. Beltramini, Y.X. Fan, G.Q. Lu, Chem. Soc. Rev. 37 (2008) 527–549. [3] A. Behr, J. Eilting, K. Irawadi, J. Leschinski, F. Lindner, Green Chem. 10 (2008) 13–30. [4] M. Pagliaro, R. Ciriminna, H. Kimura, M. Rossi, C.D. Pina, Angew. Chem. Int. Ed. 46 (2007) 4434–4440. [5] F. Jérôme, Y. Pouilloux, J. Barrault, ChemSusChem 1 (2008) 586–613. [6] A.V. Nikolenko, A.M. Kompaniets, V.I. Lougovoy, Probl. Kriobiologii 4 (1995) 36–41. [7] [7] Y. Taguchi, A. Oishi, Y. Ikeda, K. Fujita, T. Masuda, JP patent 298099, 2000. [8] R. Wessendorf, Erdoel & Kohle, Erdgas Petrochemie 48 (1995) 138–142. [9] M.A. Dasari, P.P. Kiatsimkul, W.R. Sutterlin, G.J. Suppes, Appl. Catal. A 281 (2005) 225–231. [10] Y. Kusunoki, T. Miyazawa, K. Kunimori, K. Tomishige, Catal. Commun. 6 (2005) 645–649. [11] I. Furikado, T. Miyazawa, S. Koso, A. Shimao, K. Kunimori, K. Tomishige, Green Chem. 9 (2007) 582–588. [12] S.H. Chai, H.P. Hang, Y. Liang, B.Q. Xu, Green Chem. 10 (2008) 1087–1093. [13] H. Atia, U. Armbruster, A. Martin, J. Catal. 258 (2008) 71–82. [14] E. Tsukuda, S. Sato, R. Takahashi, T. Sodesawa, Catal. Comm. 8 (2007) 1349–1353. [15] S.S. Yazdani, R. Gonzalez, Curr. Opin. Biotech. 18 (2007) 213–219. [16] R.R. Soares, D.A. Simonetti, J.A. Dumesic, Angew. Chem. Int. Ed. 45 (2006) 3982–3985. [17] E. Garcia, M. Laca, E. Pérez, A. Garrido, J. Peinado, Energy & Fuel 22 (2008) 4274–4280. [18] J. Delgado, Es Pat. 2201894 (2002). [19] D. Gelosa, M. Ramaioli, G. Valente, M. Morbidelli, Ind. Eng. Chem. Res. 42 (2003) 6536–6544. [20] V.L.C. Gonçalves, B.P. Pinto, J.C. Silva, C.J.A. Mota, Catal. Today 133–135 (2008) 673–677. [21] J.A. Melero, R. van Grieken, G. Morales, M. Paniagua, Energy & Fuel 21 (2007) 1782–1791. [22] P. Ferreira, I.M. Fonseca, A.M. Ramos, J. Vital, J.E. Castanheiro, Appl. Catal. B 91 (2009) 416–422. [23] X. Liao, Y. Zhu, S.G. Wang, Y. Li, Fuel Proc. Tech. 90 (2009) 988–993. [24] X. Liao, Y. Zhu, S.G. Wang, H. Chen, Y. Li, Appl. Catal. B 94 (2010) 64–70. [25] V.L.C. Gonçalves, R.C. Rodrigues, R. Lorençatto, C.J.A. Mota, J. Catal. 248 (2007) 158–164. [26] C.R.B. da Silva, V.L.C. Gonçalves, E.R. Lachter, C.J.A. Mota, J. Braz. Chem. Soc. 20 (2009) 201–204. [27] C.X.A. da Silva, V.L.C. Gonçalves, C.J.A. Mota, Green Chem. 11 (2009) 38–41. [28] T. Okuhara, Chem. Rev. 102 (2002) 3641–3666. [29] T.H. Lowry, K.S. Richardson, Mechanism and Theory in Organic Chemistry, 2nd Ed. Harper & Row, New York, 1981 pp. 652–658. [30] M.L. Bender, H. Ladenheim, M.C. Chen, J. Am. Chem. Soc. 83 (1961) 123–127. [31] B. Smit, T.L.M. Maessen, Nature 451 (2008) 671–678. [32] A. Thursfield, M.W. Anderson, J. Phys. Chem. 100 (1996) 6698–6707. [33] S.R. Blaszkowski, R.A. van Santen, J. Am. Chem. Soc. 118 (1996) 5152–5153. [34] T. Maihom, B. Boekfa, J. Sirijaraensre, T. Nanok, M. Probst, J. Limtrakul, J. Phys, Chem. C 113 (2009) 6654–6662. [35] J.F. Haw, J.B. Nicholas, T. Xu, L.W. Beck, D.B. Ferguson, Acc. Chem. Res. 29 (1996) 259–267.
Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Catalytic acetylation of glycerol with acetic anhydride Leonardo N. Silva, Valter L.C. Gonçalves, Claudio J.A. Mota ⁎ Universidade Federal do Rio de Janeiro, Instituto de Química. Av Athos da Silveira Ramos 149, CT Bloco A, 21941-909, Rio de Janeiro, Brazil INCT de Energia e Ambiente, UFRJ, 21941-909, RJ, Brazil
a r t i c l e
i n f o
Article history: Received 16 March 2010 Received in revised form 4 May 2010 Accepted 7 May 2010 Available online 16 May 2010 Keywords: Glycerol Triacetin Zeolites Acid catalysis
a b s t r a c t We studied the acetylation of glycerol with acetic anhydride using different solid acid catalysts. The results indicated that at 60 °C, zeolite Beta and K-10 Montmorillonite showed 100% selectivity to triacetin within 20 min, with a molar ratio of 4:1. Amberlyst-15 acid resin yielded 100% triacetin after 80 min, whereas niobium phosphate gave diacetin and triacetin in 53% and 47% selectivity, respectively. All catalysts were more selective to triacetin than the uncatalyzed reaction. By contrast, zeolite Beta gave poor yield of triacetin when acetic acid was used as acetylating agent. The different behavior was explained in terms of the stabilization of the acylium ion intermediate. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Glycerol is a byproduct of transesterification of vegetable oil to produce biodiesel [1]. It is normally produced in 10 wt.% mass balance from the transesterification reaction, and its global production is estimated to reach 1.2 million tons by 2012 [2]. This enormous amount of glycerol must find an economical destination. The chemical transformation [3–5] of glycerol into more valuable compounds is one of the most promising applications. Glycerol acetates, namely mono, di and triacetin, have major applications in cryogenics [6], plastics, [7] and fuel additives [8]. However, the efficient production of triacetin (glycerol triacetate) requires large excess of the acetylation reactant, normally acetic acid, as well as long reaction times or expeditious water removal to shift equilibrium. We wish to report in this communication, the fast and easy production of triacetin using acetic anhydride and a solid acid catalyst. Biodiesel is presently one of the major biofuels used worldwide. It is normally produced from the transesterification of triglycerides with methanol, under base catalysis conditions. This reaction affords the fatty acid methyl esters, the biodiesel themselves, and glycerol. Today, one of the most challenging aspects of the biodiesel technology is related with the economical utilization of the glycerol, or glycerin, formed. The major uses of this compound are in personal care products, cosmetics and soaps, but these sectors will not be able to drain the tons of glycerol coming from biodiesel production.
⁎ Corresponding author. Universidade Federal do Rio de Janeiro, Instituto de Química. Av Athos da Silveira Ramos 149, CT Bloco A, 21941-909, Rio de Janeiro, Brazil. Fax: +55 21 25627106. E-mail address: [email protected] (C.J.A. Mota). 1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.05.007
Glycerol is a good feedstock for other chemicals. Hydrogenolysis to 1,2 and 1,3-propanediol over metal catalysts [9–11] has been extensively studied. Dehydration to acrolein, a useful intermediate for plastic production, has been reported to occur over solid acid catalysts [12–14]. Anaerobic fermentation of glycerol is a potential route for the production of butanol and ethanol [15], which can be used as biofuels. Glycerol reforming to synthesis gas, a mixture of carbon monoxide and hydrogen, has been studied over noble metal catalysts [16], and could be a possible pathway for the production of hydrocarbons in the diesel and gasoline range. Glycerol acetates are also good candidates to drain part of the glycerol produced by the biodiesel industry. Traditionally, this chemical is used as a plasticizer in cigarette filters, but in recent years it has been tested as a biodiesel additive [17,18], improving the cold flow properties. This application is extremely interesting, because the glycerol produced from transesterification could be used in the biodiesel chain, as an additive to improve the properties of this biofuel. Triacetin can be produced in the acid-catalyzed reaction of glycerol with acetic acid. There are many studies [19–22] using different solid acid catalysts and operational conditions, but the selectivity to triacetin is normally limited. The presence of water, shifting equilibrium and weakening the catalyst acid strength, as well as the sequential acetylation of the hydroxyl groups contributes for this result. Recently, triacetin was produced in 100% selectivity from glycerol with the use of a two-step process [23]. Initially, glycerol is reacted with nine fold molar excess of acetic acid in the presence of Amberlyst-35 acid resin. After 4 h at 105 °C, acetic anhydride is introduced in the reaction medium to complete the acetylation, yielding 100% triacetin. Calculations suggest [24] that acetylation with acetic acid is an endothermic process, requiring high energy demand for the introduction of the third acetyl group to form triacetin. In
L.N. Silva et al. / Catalysis Communications 11 (2010) 1036–1039
1037
Scheme 1. Acetylation of glycerol with acetic anhidride.
comparison, acetylation with acetic anhydride is exothermic, favoring formation of triacetin. These studies prompted us to show our results of glycerol acetylation with acetic anhydride in the presence of solid acid catalysts (Scheme 1).
Reactions with acetic acid as acylating reagent were also carried out. The procedure was similar as the one reported previously [20], with a molar ratio acid/glycerol of 4. 3. Results and discussion
2. Experimental The reactions were carried out in batch conditions, mixing 2.0 g of glycerol, 8.2 mL of acetic anhydride (4:1 molar ratio anhydride to glycerol) and a mass of catalyst corresponding to 2.0 mmol of acid sites. The acidity was obtained from n-butylamine termodesorption experiments, and the procedure was reported elsewhere [25]. In some experiments, a molar ratio of 3:1 of anhydride to glycerol was used. Reactions were carried out at 60 °C and, in some cases, at 120 °C. Blank experiments, without adding catalysts, were also carried out for comparison purposes. The product distribution was determined, withdrawing samples from the reaction mixture at specific time intervals and analyzing them by gas chromatography coupled with mass spectrometry. In all cases, 1,4-dioxane was used as external standard.
In all reactions with acetic anhydride, no glycerol was detected among the products, indicating 100% conversion. However, the formation of di and triacetin involves consecutive acetylations of the remaining hydroxyl groups and although we express the results in terms of selectivity, they are indirectly related with catalyst activity. Table 1 shows the results of glycerol acetylation with acetic anhydride. One can see that 100% selectivity to triacetin is achieved within 20 min with the use of zeolite H-Beta and K-10 Montmorillonite as catalyst, at 60 °C and 4:1 molar ratio of anhydride to glycerol, respectively. Reduction of the molar ratio to the stoichiometric value decreases the selectivity to triacetin, even after 2 h of reaction time, but triacetin is still the major product formed. Amberlyst-15 acid resin
Table 1 Selectivity to glycerol acetates in reactions of glycerol with acetic anhydride at different conditions.a Catalyst
H-Betab H-Beta K-10c K-10 Amberlyst-15d Amberlyst-15 Niobium phosphatee Niobium phosphate Blank Blank
Molar ratio anhydride: glycerol
Temperature (°C)
4:1 3:1 4:1 3:1 4:1 4:1 4:1 4:1 4:1 4:1
60 60 60 60 60 60 60 120 60 120
Time (min)
20 120 20 120 80 20 120 80 120 120
Selectivity to acetins % Mono
Di
Tri
– – – – – – – – 10 –
– 38 – 22 – 10 53 – 56 6
100 62 100 78 100 90 47 100 34 94
Fig. 1. Glycerol conversion ( ) and selectivity to monoacetin ( ), diacetin ( ), triacetin ( ) and acetol ( ) in the reaction with acetic acid at 120 °C, catalyzed by zeolite Beta.
a Reaction carried out with 2.0 g of glycerol. In all cases, glycerol conversion was 100%. b Si/Al = 16, area = 633 m2/g, acidity = 1.6 mmol/g. Pre-treatment at 500 °C/30 min. c Si/Al= 6.6, area= 240 m2/g, acidity= 0.53 mmol/g. Pre-treatment at 150 °C/30 min. d Area = 50 m2/g, acidity = 4.7 mmol/g. Pre-treatment at 120 °C/30 min. e Area = 187 m2/g, acidity = 0.4 mmol/g. No pre-treatment temperature.
Table 2 Selectivity for glycerol acetates in reactions of glycerol and acetic acid at 120 °C.a Catalystb
Conversion (%)
Selectivity (%) Monoacetin
Diacetin
Triacetin
Acetol
H-Beta K-10 Niobium phosphate Amberlyst-15 Blank
94 100 100 100 91
48 36 38 18 50
39 52 49 55 40
4 6 7 24 4
9 6 5 3 7
a b
Molar ratio acetic acid to glycerol of 4:1. Results after 120 min of reaction. Mass of catalyst used corresponds to 2.0 mmol of acid sites.
Fig. 2. Glycerol conversion ( ) and selectivity to monoacetin ( ), diacetin ( ), triacetin ( ) and acetol ( ) in the reaction with acetic acid at 120 °C, catalyzed by Amberlyst-15.
1038
L.N. Silva et al. / Catalysis Communications 11 (2010) 1036–1039
Scheme 2. AAC2 mechanism with formation of a tetrahedral intermediate.
is slightly less active, yielding 100% selectivity to triacetin only after 80 min of reaction and a molar ratio of 4:1. Within the first 20 min, the selectivity to triacetin with this catalyst is 90%, with the remaining 10% as diacetin. Niobium phosphate is considerably less active at 60 °C. After 120 min, the selectivity to triacetin was only 47%. At 120 °C, this catalyst showed 100% selectivity to triacetin within 80 min of reaction time. By contrast, the uncatalyzed (blank reaction) showed a considerably lower selectivity at both temperatures. At 60 °C, the selectivity to triacetin was 34% after 120 min, whereas at 120 °C the selectivity was about 94% within the same period. These results show that triacetin can be obtained in excellent selectivity at 60 °C with the use of zeolite H-Beta or K-10 Montmorillonite as catalysts and a molar ratio of anhydride to glycerol of 4 to 1. Table 2 shows the results of glycerol acetylation with acetic acid at 120 °C, using the same solid acid catalysts. One can see that in all cases, the selectivity to triacetin is significantly lower when compared with reactions with acetic anhydride. The best selectivity to triacetin (24%) was achieved with use of Amberlyst-15. The other catalysts showed selectivity in the range of 4 to 7%, similar to the uncatalyzed reaction. Zeolite Beta showed the worse performance among the catalysts tested, presenting glycerol conversion lower than 100% and the highest selectivity to monoacetin (Fig. 1). These results are similar to what was found [20] in the acetylation of glycerol with acetic acid catalyzed by HUSY and HZSM-5 zeolites, which did not show good conversion and selectivity to triacetin either. Exclude by contrast, Amberlyst-15 was the best catalysts tested (Fig. 2). The acid resin is the strongest acid catalysts tested [25] and this might partly explain the results. However, neither K-10, nor niobium phosphate have higher acid strength than zeolites [25] and their performance might be associated with other factors. A possible explanation is that their structure would be capable of adsorbing the water formed, probably shifting the equilibrium or preventing catalyst deactivation. It is interesting to note that K-10 Montmorillonite also showed a good performance in glycerol etherification with benzyl alcohol [26], a reaction that also releases water as byproducts. Acetol (hydroxiacetone), arisen from glycerol dehydration, was observed over all catalysts tested. This compound was not significantly observed in the
reactions with acetic anhydride (only traces were observed in some reactions, especially at higher temperatures). It is not completely clear why the acetylation with acetic anhydride performs well on zeolite Beta, whereas it does not with acetic acid. We have shown that zeolite Beta is a good catalyst for the acetalyzation of glycerol with formaldehyde [27], which is sensitive to the presence of water. The explanation was that, due to the high Si/Al ratio of this zeolite, its pore environment is hydrophobic [28] and does not favor the adsorption of the water released during the reaction on the active sites. It might also prevent or minimize the reverse reaction, because the water tends to diffuse out of the pores. Thus, we cannot explain the low selectivity to triacetin in the glycerol acetylation with acetic acid on zeolite Beta to the effect of water, by shifting equilibrium or weakening the acid sites. One can envisage two possible mechanisms [29] for acetylation in strong acidic medium: the first one is the normal AAC2 mechanism, involving protonation of the carbonyl oxygen atom and nucleophilic attack in the carbonyl to form a tetrahedral intermediate, presenting a quaternary carbon atom (Scheme 2); the second mechanism is the AAC1, where protonation takes place in the oxygen atom attached to the carbonyl group, followed by formation of an acylium ion (Scheme 3). The first pathway is normally less energetic, because of the higher stability of the intermediate formed upon protonation in the carbonyl oxygen atom. On the other hand, formation of the tetrahedral intermediate is space demanding, and the second mechanism, involving the acylium ion, prevails in situations of steric constraints [30]. Zeolites are known for their shape selectivity properties [31], especially concerning the formation of bulk transition states. It is possible that in acetylation with acetic anhydride, formation of the tetrahedral intermediate inside the zeolite pores might be prevented, due to transition state shape selectivity. This might be especially difficult for the third acetylation to form triacetin. Therefore, acylation takes place through the AAC1 mechanism. By contrast, it seems that the same mechanism is difficult to operate in the case of acetylation with acetic acid, even at higher temperature. This point is not completely clear and cannot be answered with the data of this study. We propose an explanation based on the interaction of the protonated acetic acid
Scheme 3. AAC1 mechanism with formation of an acylium ion.
L.N. Silva et al. / Catalysis Communications 11 (2010) 1036–1039
1039
compound from acetic acid, structural arrangements do not permit assistance from the framework of oxygen atoms of the zeolite. Nevertheless, formation of the acylium ion from the anhydride allows the participation of the zeolite framework, stabilizing the formation of the intermediate. Acknowledgements Scheme 4. Proposed structure of the protonated acetic acid on the zeolite surface and its dehydration to the acylium ion (no zeolite assistance).
Authors thank financial support from FINEP, CNPq and FAPERJ. References
Scheme 5. Proposed structure of the protonated acetic anhydride and its transformation to the acylium ion (zeolite-assisted).
and protonated acetic anhydride with the zeolite surface, prior to the acyl cation formation. In the case of acetic acid, the two hydrogen atoms are interacting with the zeolite structure through hydrogen bonds (Scheme 4). This type of structure has already been proposed in other zeolite-catalyzed reactions involving protonation of hydroxyl groups [32–34], and might impair the formation of the acylium ion, because it would be away from the zeolite surface and would not be well stabilized. On the other hand, in the case of acetic anhydride, formation of the acylium ion might be assisted by the zeolite surface (Scheme 5), favoring its formation. It is well recognized that the zeolite surface assists the formation of cationic species [35], as well as stabilizes them through covalent bonding. The use of acetic anhydride in the preparation of triacetin from glycerol may become feasible with zeolite Beta, K-10 Montmorillonite or Amberlyst acid resin as catalysts. The reaction is exothermic and can be carried out at mild temperatures and short reaction times with use of these catalysts. This permits a better control of the system for scaling up purposes. 4. Conclusions The use of zeolite Beta or K-10 Montmorillonite as catalysts in the acetylation of glycerol with acetic anhydride produces triacetin in 100% selectivity at 60 °C within 20 min of reaction. Amberlyst-15 acid resin requires longer reaction times, whereas niobium phosphate gives 100% selectivity only at higher temperatures. By contrast, zeolite Beta showed the lowest selectivity to triacetin when acetic acid is used as catalyst. This result was explained in terms of the stability of the acylium ion intermediate. Upon formation of this
[1] F.R. Ma, M.A. Hanna, Bioresour. Tech. 70 (1999) 1–15. [2] C.H. Zhou, J.N. Beltramini, Y.X. Fan, G.Q. Lu, Chem. Soc. Rev. 37 (2008) 527–549. [3] A. Behr, J. Eilting, K. Irawadi, J. Leschinski, F. Lindner, Green Chem. 10 (2008) 13–30. [4] M. Pagliaro, R. Ciriminna, H. Kimura, M. Rossi, C.D. Pina, Angew. Chem. Int. Ed. 46 (2007) 4434–4440. [5] F. Jérôme, Y. Pouilloux, J. Barrault, ChemSusChem 1 (2008) 586–613. [6] A.V. Nikolenko, A.M. Kompaniets, V.I. Lougovoy, Probl. Kriobiologii 4 (1995) 36–41. [7] [7] Y. Taguchi, A. Oishi, Y. Ikeda, K. Fujita, T. Masuda, JP patent 298099, 2000. [8] R. Wessendorf, Erdoel & Kohle, Erdgas Petrochemie 48 (1995) 138–142. [9] M.A. Dasari, P.P. Kiatsimkul, W.R. Sutterlin, G.J. Suppes, Appl. Catal. A 281 (2005) 225–231. [10] Y. Kusunoki, T. Miyazawa, K. Kunimori, K. Tomishige, Catal. Commun. 6 (2005) 645–649. [11] I. Furikado, T. Miyazawa, S. Koso, A. Shimao, K. Kunimori, K. Tomishige, Green Chem. 9 (2007) 582–588. [12] S.H. Chai, H.P. Hang, Y. Liang, B.Q. Xu, Green Chem. 10 (2008) 1087–1093. [13] H. Atia, U. Armbruster, A. Martin, J. Catal. 258 (2008) 71–82. [14] E. Tsukuda, S. Sato, R. Takahashi, T. Sodesawa, Catal. Comm. 8 (2007) 1349–1353. [15] S.S. Yazdani, R. Gonzalez, Curr. Opin. Biotech. 18 (2007) 213–219. [16] R.R. Soares, D.A. Simonetti, J.A. Dumesic, Angew. Chem. Int. Ed. 45 (2006) 3982–3985. [17] E. Garcia, M. Laca, E. Pérez, A. Garrido, J. Peinado, Energy & Fuel 22 (2008) 4274–4280. [18] J. Delgado, Es Pat. 2201894 (2002). [19] D. Gelosa, M. Ramaioli, G. Valente, M. Morbidelli, Ind. Eng. Chem. Res. 42 (2003) 6536–6544. [20] V.L.C. Gonçalves, B.P. Pinto, J.C. Silva, C.J.A. Mota, Catal. Today 133–135 (2008) 673–677. [21] J.A. Melero, R. van Grieken, G. Morales, M. Paniagua, Energy & Fuel 21 (2007) 1782–1791. [22] P. Ferreira, I.M. Fonseca, A.M. Ramos, J. Vital, J.E. Castanheiro, Appl. Catal. B 91 (2009) 416–422. [23] X. Liao, Y. Zhu, S.G. Wang, Y. Li, Fuel Proc. Tech. 90 (2009) 988–993. [24] X. Liao, Y. Zhu, S.G. Wang, H. Chen, Y. Li, Appl. Catal. B 94 (2010) 64–70. [25] V.L.C. Gonçalves, R.C. Rodrigues, R. Lorençatto, C.J.A. Mota, J. Catal. 248 (2007) 158–164. [26] C.R.B. da Silva, V.L.C. Gonçalves, E.R. Lachter, C.J.A. Mota, J. Braz. Chem. Soc. 20 (2009) 201–204. [27] C.X.A. da Silva, V.L.C. Gonçalves, C.J.A. Mota, Green Chem. 11 (2009) 38–41. [28] T. Okuhara, Chem. Rev. 102 (2002) 3641–3666. [29] T.H. Lowry, K.S. Richardson, Mechanism and Theory in Organic Chemistry, 2nd Ed. Harper & Row, New York, 1981 pp. 652–658. [30] M.L. Bender, H. Ladenheim, M.C. Chen, J. Am. Chem. Soc. 83 (1961) 123–127. [31] B. Smit, T.L.M. Maessen, Nature 451 (2008) 671–678. [32] A. Thursfield, M.W. Anderson, J. Phys. Chem. 100 (1996) 6698–6707. [33] S.R. Blaszkowski, R.A. van Santen, J. Am. Chem. Soc. 118 (1996) 5152–5153. [34] T. Maihom, B. Boekfa, J. Sirijaraensre, T. Nanok, M. Probst, J. Limtrakul, J. Phys, Chem. C 113 (2009) 6654–6662. [35] J.F. Haw, J.B. Nicholas, T. Xu, L.W. Beck, D.B. Ferguson, Acc. Chem. Res. 29 (1996) 259–267.