Article pubs.acs.org/IECR
Highly Selective SnCl2‑Catalyzed Solketal Synthesis at Room Temperature Fernanda D. L. Menezes, Matheus D. O. Guimaraes, and Márcio J. da Silva* Chemistry Department, Federal University of Viçosa, Viçosa, Minas Gerais 36590-000, Brazil ABSTRACT: The selective transformation of glycerol into value-added products remains a challenging task due to its polyfunctional nature. Conversion of glycerol into 2,2-dimethyl-1,3-dioxolane-4-methanol (i.e., solketal) was efficiently catalyzed by SnCl2 at room temperature and in solvent-free conditions. Solketal is an useful additive for the formulation of gasoline, diesel, and biodiesel. Tin chloride, an inexpensive, water-tolerant, and minimally corrosive Lewis acid catalyst, has demonstrated excellent catalytic behavior in the acetalisation of glycerol with acetone to yield solketal with a higher efficiency than traditional Brønsted acids (i.e., p-toluenesulfonic acid or sulfuric acid). The effects of various parameters, such as catalyst loading, acetone/ glycerol molar ratio, and temperature on the selectivity and conversion of glycerol was investigated in detail. Although used in the homogeneous phase, the SnCl2 catalyst was easily recovered and reused without any reactivation treatment up to six times, keeping constant its activity. K.13 Transition metal-based catalysts are also an option and have been used in either the homogeneous or heterogeneous phase to produce diois acetals.14 Glycerol cyclic acetals with furfural were obtained using Lewis acid catalysts at 373−393 K.15 Those authors found that ZnCl2 and MCM-41 were the more effective catalysts in homogeneous and heterogeneous catalysts, respectively. On the other hand, solketal was successfully obtained in reactions performed at 393 K using mesoporous substituted silicates as catalysts.15 In the present work, we wish to demonstrate an optimized and environmentally friendly synthesis of solketal, a high valueadded product, from SnCl2-catalyzed glycerol ketalization reactions performed at room temperature and in solvent-free conditions. To the best of our knowledge, this is the first report of solketal synthesis in presence of SnCl2, a homogeneous Lewis acid catalyst that was easily recovered and reused without loss activity. The results clearly show that the catalyst is effective for selective solketal synthesis and uses a selective and environmentally benign process.
1. INTRODUCTION Driven by the depletion of fossil fuels and environmental concerns, biofuels have attracted considerable interest as an alternative fuel with remarkably lower exhaust emissions of particulate matter and greenhouse gases.1 In fact, biodiesel generation has increased from almost nothing to 20 billion liters per year in just over a decade.2 However, glycerol is an unavoidable coproduct that is forecast to increase globally, with a net global production of around 1.2 million tons by 2012.3 Nowadays, the surplus of glycerol caused by increased biodiesel production threatens to become an economic disadvantage for the development of biodiesel itself.4 Glycerol is produced by both the bioethanol and biodiesel industries; it will probably continue to be available at a low cost for the foreseeable future, which encourages the search for new innovative applications.2 Due to its low cost and availability, the broadest-based opportunity for glycerol consumption makes it a primary renewable building block for biorefinery, analogous to those of the petrochemical industry.5 Although glycerol is being widely used, traditional markets are unable to absorb its excessive surpluses. Consequently, several processes for converting glycerol into high value-added products are currently being developed.6,7 Glycerol derivatives (i.e., ethers, esters, diols, and acetals) find applications in the fuel, plastic, and fine chemical industries. In particular, glycerol acetals may find applications as fuel additives, surfactants, disinfectants, antifreeze additives, and flavors.8−11 Solketal (i.e., 2,2-dimethyl-1,3-dioxolane-4methanol), is a product of glycerol condensation with acetone and is an useful additive for the formulation of gasoline, diesel, and biodiesel. Frequently, solketal synthesis is accomplished by p-toluenesulfonic acid-catalyzed reactions performed at 373 K for 12 h.12 However, homogeneous Brønsted acid catalysts have several drawbacks that have reduced their utility, such as difficulty in separation, reactor corrosion, and incapability for reuse. Alternatively, glycerol acetals from different aldehydes were successfully obtained using Amberlyst-15 acid solid resin as a catalyst and DMSO as solvent after a 2 h reaction at 393 © 2013 American Chemical Society
2. EXPERIMENTAL PROCEDURES 2.1. Materials and Physical Methods. Unless otherwise noted, all chemicals were purchased from commercial sources and were used without further purification. Sulfuric acid (98.5%, Vetec) and p-toluenesulfonic acid monohydrate (PTSA; Sigma, 98.5%) were acquired and used as received. The tin salts (SnBr2, SnCl2·2H2O, SnF2, and Sn(OAc)2), as well as the solvents, were acquired from Sigma-Aldrich (98.5%). Glycerol (99%) and acetone (99%) were also purchased from Sigma-Aldrich. 2.2. Monitoring and Identification of the Reaction Products. Aliquots at regular time intervals were collected and Received: Revised: Accepted: Published: 16709
July 15, 2013 September 18, 2013 October 31, 2013 October 31, 2013 dx.doi.org/10.1021/ie402240j | Ind. Eng. Chem. Res. 2013, 52, 16709−16713
Industrial & Engineering Chemistry Research
Article
analyzed via gas chromatography (GC Varian 450, FID, fitted with Carbowax capillary column). The temperature profile was as follows: 150 °C/1 min, 10 °C/min up to 240 °C/5 min; injector temperature, 250 °C; detector temperature, 280 °C. The conversions were calculated by matching the areas of GC peaks with the corresponding calibration curves. Reaction products were identified by analyses carried out on a Shimadzu GC17A gas chromatograph coupled with a MS-QP 5050A mass spectrometer (Tokyo, Japan). He was the carrier gas at 2 mL/min, and the MS detector was operated in the EI mode at 70 eV, with a scanning range of m/z 50−400. 2.3. Brønsted or Lewis Acid-Catalyzed Glycerol Ketalization: Effect of Catalyst Nature. The reactions were performed in a 50 mL three-necked glass flask equipped with a sampling system, a reflux condenser, and a thermostatic bath with a magnetic stirrer. The reaction’s temperature was 298 or 333 K. Typically, acetone (4.79 mmol) and glycerol (4.79 mmol) were introduced in the reactor containing CH3CN (15.0 mL solution), and magnetically stirred and heated (if required); then, an adequate catalyst (i.e., SnX2, X = F−, Cl−, Br−, or −OAc or PTSA; 1 mol %) was added and the reactions were started. 2.4. Effect of the Reactants’ Molar Ratio on the SnCl2Catalyzed Glycerol Ketalization. The acetone/glycerol molar ratio varied from 1:1 to 1:4 (i.e., keeping constant acetone, 4.79 mmol), whereas the glycerol/acetone ratio varied from only 1:1 to 1:6 (i.e., keeping constant glycerol, 4.79 mmol). In a typical run, acetone and glycerol in adequate proportions were introduced in the reactor containing CH3CN and magnetically stirred and heated to 333 K; then, the SnCl2 catalyst (1 mol %) was added and the reactions were started. 2.5. Effect of the SnCl2 Concentration on the Glycerol Ketalization. To assess the effect of catalyst concentration on the reaction rate, the kinetic measurements should be made under conditions in which the reaction rate dependence with regards to the reactants’ concentration is of a pseudozero order. For this reason, a large excess of acetone (i.e., ketone/glycerol molar ratio equal to 7:1) was used, and kinetic measurements were obtained during the first hour of reaction. The catalyst load ranged from 1.0 to 4.0 mol %, and the reaction temperature was 298 K. 2.6. Effect of Temperature on the SnCl2-Catalyzed Glycerol Ketalization. The kinetics of the SnCl2-catalyzed reactions was studied by varying the reaction temperature in the range 298 to 333 K. Typically, glycerol (4.79 mmol) and acetone (19.16 mmol) were solved in acetonitrile, stirred, and heated at reaction temperature; after the addition of the SnCl2 catalyst, the reactions were started. 2.7. Recovery/Reutilization of the SnCl2 Catalyst. A simple procedure was used for recovery of the homogeneous SnCl2 catalyst. The reactions were performed at room temperature with ketone/glycerol at 1:6 proportion and SnCl2 concentration equal to 10 mol % related to glycerol. At the end of the reaction, the solution was concentrated under reduced pressure and the acetone excess recovered. Thus, after cooling the solution at room temperature, the solid SnCl2 catalyst was recovered by filtration, washing with hexane/ethyl acetate (10:1). Then, the catalyst was dried at 80 °C and reused six times. The tin content recovered from the reaction medium was gravimetrically determined. In addition, the tin content in the recovered solid from the reaction was determined by elemental analysis of atomic absorption spectroscopy as described in the literature.19
3. RESULTS AND DISCUSSION 3.1. General Aspects. In a patent published by Bruchmann et al., solketal synthesis was performed in PTSA-catalyzed glycerol ketalization reactions with acetone (ca. 1:3 acetone/ glycerol molar ratio, PTSA 1 mol %) for 16 h under reflux.16 Herein, this same reaction was made using SnCl2 catalyst, which allows a noticeable reduction in the time and reaction temperature without lowering conversion or selectivity (Figure 1).
Figure 1. SnCl2-catalyzed solketal synthesis.
The effect of reactants’ molar ratio was first assessed under a temperature of 373 K using acetonitrile to keep the total volume of reaction constant (Figure 2).
Figure 2. Kinetic curves of SnCl2-catalyzed solketal synthesis: effect of glycerol/acetone molar ratio. Reaction conditions: glycerol (4.79 mmol); SnCl2 (1 mol %); CH3CN; 333 K. Conversion determined via GC analysis calculated on the basis of the limiting reactant (glycerol).
3.2. Effect of Reactants Concentration. It is noteworthy that we did not observe the significant formation of any byproduct in the glycerol ketalization with acetone when SnCl2 was the catalyst. Reactions in which the ketone concentration was kept constant and the amount of glycerol was increased were also performed (Figure 3). When glycerol (4.79 mmol) reacted with acetone in excess, the highest conversion was obtained using a ketone/glycerol molar ratio equal to 1:4. An amount of ketone higher than 1:4 produced a decrease in the conversion (Figure 2). Similarly, when ketone (4.79 mmol) reacted with glycerol in excess, the highest conversion was also reached using a glycerol/acetone molar ratio equal to 1:4 (Figure 3). Experiments with a higher amount of glycerol were not performed due to high viscosity, 16710
dx.doi.org/10.1021/ie402240j | Ind. Eng. Chem. Res. 2013, 52, 16709−16713
Industrial & Engineering Chemistry Research
Article
In general, solketal was always the major product in all reactions catalyzed by Brønsted or Lewis acids; however, in the absence of a catalyst, it was not formed. Remarkably, in the presence of tin catalysts, a lowering of the temperature did not produce a decrease in the conversion as it did in the case of Brønsted acid catalysts (Table 1). Actually, at room temperature, a significant improvement in the conversion was achieved, making SnCl2 the best catalyst. The mass balance showed that in certain reactions, a decrease in the area of glycerol GC peak occurred, but no product was detected (nd products, runs 1, 3 and 7, Table 1). Possibly, these products are glycerol oligomers, which are not detectable via GC analysis. Some tin catalysts were not soluble in the reaction medium (i.e., SnF2 and Sn(OAc)2). However, different from the Sn(OAc)2, which was the less-active catalyst, neither the conversion nor the selectivity of the SnF2-catalyzed ketalization reaction were hindered. Conversely, the results showed that both the SnCl2 and SnBr2 catalysts were as active as ptoluenesulfonic acid, which is the most used catalyst in the homogeneous phase of solketal synthesis.16 The kinetic curves displayed in Figure 4 reveal that tincatalyzed glycerol ketalization reactions achieved high con-
Figure 3. Kinetic curves of SnCl2-catalyzed solketal synthesis: effect of acetone/glycerol molar ratio. Reaction conditions: ketone (4.79 mmol); SnCl2 (1 mol %); CH3CN; 333 K. Conversion determined via GC analysis calculated on the basis of the limiting reactant (acetone).
which would hamper the homogenization of the reaction medium. 3.2. Glycerol Ketalization in the Presence of Tin Salts: Effect of Catalyst Nature at Different Temperatures. In order to investigate the effect of catalyst nature on the conversion and selectivity of glycerol ketalization, reactions at temperatures of 298 and 333 K were performed in the presence of Brønsted (i.e., PTSA and H2SO4) or Lewis acid catalysts (i.e., SnX2 with X = F−, Cl−, Br−, or −OAc). In this study, with the aim of making the effect of the catalyst more noticeable, the reactants were employed in stoichiometric proportions (i.e., 1:1) and also at 1:4 glycerol/acetone molar ratio in CH3CN as solvent. Because an equal behavior in relation the activity of catalysts was observed, only the results using 1:4 glycerol/ acetone molar ratio were reported (Table 1).
Figure 4. Kinetic curves of glycerol ketalization in the presence of Lewis or Brønsted catalysts.
Table 1. Glycerol Ketalization in the Presence of Lewis or Brønsted Catalysts Using a Glycerol/Ketone Molar Ratio Equal to 1:4a
versions within the first hour of reaction. Conversely, the reactions in the presence of Brønsted acid catalysts have a different behavior; the reaction conversions have a noticeable increasing with the increase of reaction time when PTSA or H2SO4 were the catalysts. The best performance of SnCl2 when compared to Brønsted acids can be also attributted to its higher water tolerance. Assessing this behavior, we have found that SnCl2 was much more water tolerant than PTSA in FFA esterification reactions performed in presence of additional water amount.17 3.3. SnCl2-Catalyzed Glycerol Ketalization Reactions: Effect of Catalyst Concentration. The kinetic curves obtained by varying the catalyst concentration in the range of 1.0−4.0 mol % (ca. 0.0479−0.192 mmol) are displayed in Figure 5. Because the reactions using a glycerol/acetone molar ratio equal to 1:1 were those that reached the lower conversion, it was selected to assess the effect of the SnCl2 catalyst concentration. Although the initial reaction rate demonstrated a slight improvement, the final conversions achieved in all reactions were very close to each other (Figure 5). Consequently, it was not possible to determine the order in relation to SnCl2 catalyst
products selectivityb (%) run
catalyst
conversion (%)
ndb
solketal
othersc
temperature (K)
298
333
298
333
298
333
298
333
1 2 3 4d 5 6 7d
15 65 59 75 81 76 23
15 78 70 74 77 76 29
0 98 97 95 98 98 85
0 98 85 98 98 99 0
95 0 0 0 0 0 10
95 0 10 0 0 0 95
5 2 3 5 2 2 5
5 2 5 2 2 1 5
PTSA H2SO4 SnF2 SnCl2 SnBr2 Sn(OAc)2
a
Reaction conditions: acetone (19.16 mmol); glycerol (4.79 mmol); catalyst (1 mol %); CH3CN solution (15 mL); 1.5 h reaction. b Determined by GC analyses. In some reactions, a decrease in the area of glycerol GC peak occurred, but no product was detected (runs 1 and 7). The mass balance was checked in all reactions. This difference was attributed to formation of nondetectable products (i.e., “nd” in Table 1). cComplex mixture of nonidentified products. dCatalysts were almost insoluble in the reaction medium. 16711
dx.doi.org/10.1021/ie402240j | Ind. Eng. Chem. Res. 2013, 52, 16709−16713
Industrial & Engineering Chemistry Research
Article
(i.e., Lewis acid) after metal coordination. Both processes can favor a nucleophilic attack by the hydroxyl group of glycerol, resulting in the formation of the intermediate in solution that, after a water elimination step, results in solketal. 3.6. Recovery/Reutilization of the SnCl2 Catalyst. As described in Section 2.7, SnCl2 catalyst was easily recovered and reused six times without any reactivation procedure, keeping constant its activity. A high recovery rate was obtained in all of the recycle steps (omitted in the Figure 7 by simplification; ca. 98−99%),
Figure 5. Effect of the SnCl2 catalyst concentration on the glycerol ketalization with the reactants’ molar ratio equal to 1:1. Reaction conditions: glycerol (4.79 mmol), acetone (4.79 mmol), CH3CN solution (15 mL), 298 K.
concentration. Actually, in the concentration range of 1−4 mol %, an increase in catalyst concentration did not significantly favor the conversion of glycerol into solketal. On the other hand, additional experiments showed that a decrease on catalyst concentration resulted in a significant decrease in the conversion reaction. On the other hand, if we take into account only the initial rate reaction for each run, we can note that an increase in catalyst concentration resulted in a significance increase on the conversion reached through the first 30 min of reaction. 3.5. Insights on the SnCl2-Catalyzed Glycerol Ketalization. The literature describes that in reactions of glycerol ketalization using a high temperature (ca. 438 K), two main products were preferentially formed as expected: a higher portion of the kinetically favored product (dioxolane) and a lower portion of the thermodynamically favored product (dioxane).15 Herein, even when using temperatures equal to room temperature or 333 K, only the dioxolane product (solketal) was always the majority formed. The literature and the results obtained suggest a simplified mechanism of SnCl2catalyzed ketalization (Figure 6).18 The solubility problems of SnF2 and Sn(OAc)2 hampers an adequate evaluation of ligand anion effect on the activity of tin catalysts (i.e., only SnCl2 and SnBr2 were completely soluble). In general, the role of the Lewis acid or Brønsted acid catalysts in ketalization reactions is to activate the ketone carbonyl group through a protonation step (i.e., Brønsted acids) or polarization
Figure 7. Conversions of glycerol into solketal obtained after successive recycles of SnCl2 catalyst. Reaction conditions: glycerol/ ketone molar ratio (1:6); SnCl2 catalyst (10 mol %); 298 K.
indicating the efficiency of procedure employed. These recovery percentages were also assessed by elemental analysis performed as described in literature.19 Three solid samples were recovered and, after acid treatment, were analyzed by AAS. The results showed that a real recovery of SnCl2 ranged from 93 to 97%. The difference in relation to the gravimetrically determined percentage can be attributed to possible contamination of the solid by glycerol or product. The catalyst recycling was also performed at conditions of low conversion (Figure 8). In this case, reactions were performed with 1 mol % catalyst and equal proportions of acetone and glycerol. The SnCl2 catalyst remained active after 4 successive cycles of reuse.
Figure 6. Proposed mechanism based on literature for SnCl2-catalyzed solketal synthesis at room temperature where Sn(II) acts as Lewis acid.18
Figure 8. Conversions of glycerol into solketal obtained after successive recycles of SnCl2 catalyst. Reaction conditions: glycerol/ acetone molar ratio (1:1); SnCl2 catalyst (1 mol %); 298 K. 16712
dx.doi.org/10.1021/ie402240j | Ind. Eng. Chem. Res. 2013, 52, 16709−16713
Industrial & Engineering Chemistry Research
Article
(9) Climent, M. J.; Corma, A.; Veltry, A. Synthesis of Hyacinth, Vanilla, and Blossom Orange Fragrances: the Benefit of Using Zeolites and Delaminated Zeolites as Catalysts. Appl. Catal., A 2004, 263, 155. (10) Sari, P.; Razzak, M.; Tucker, I. G. Isotropic Systems of MediumChain Mono and Diglycerides for Solubilization of Lipophilic and Hydrophilic Drugs. Pharm. Dev. Technol. 2004, 9, 97. (11) Delfort, B.; Durand, I.; Jaecker, A.; Lacome, T.; Montagne, X.; Paille, F. Diesel Fuel Compositions with Reduced Particulate Emission, Containing Glycerol Acetal Derivatives. WO Patent 2003163949A1, 2004. (12) Suriyaprapadiloka, N.; Kitiyanana, B. Synthesis of Solketal from Glycerol and Its Reaction with Benzyl Alcohol. Energy Procedia 2011, 9, 63. (13) Silva, P. H. R.; Gonçalves, V. L. C.; Mota, C. J. A. Glycerol and Acetals as Antifreezing Additives for Biodiesel. Bioresour. Technol. 2010, 101, 6225. (14) Krompieca, S.; Penkala, M.; Szczubiałka, K.; Kowalska, E. Transition Metal Compounds and Complexes as Catalysts in Synthesis of Acetals and Orthoesters: Theoretical, Mechanistic and Practical Aspects. Coord. Chem. Rev. 2012, 256, 2057. (15) Wegenhart, B. L.; Liu, S.; Thom, M.; Stanley, D.; Abu-Omar, M. M. Solvent-Free Methods for Making Acetals Derived from Glycerol and Furfural and Their Use as Biodiesel Fuel Component. ACS Catal. 2012, 2, 2524. (16) Bruchmann, B.; Haberle, K.; Gruner, H.; Hirn, M. U. S. Patent 5,917,059, 1999. (17) da Silva, M. J.; Figueiredo, A. P.; Cardoso, A. L.; Natalino, R. J. Am. Oil Chem. Soc. 2011, 88, 1431. (18) Li, L.; Korányi, T. I.; Sels, B. F.; Pescarmona, P. P. HighlyEfficient Conversion of Glycerol to Solketal over Heterogeneous Lewis Acid Catalysts. Green Chem. 2012, 14, 1611. (19) da Silva, M. J.; Cardoso, A. L.; Natalino, R. Int. J. Chem. React. Eng. 2010, 8, 1.
4. CONCLUSION A highly selective solketal synthesis at room temperature in the presence of tin catalysts was developed. The SnCl2 and SnBr2 catalysts were as active as p-toluenesulfonic acid, which is the most used catalyst in the solketal synthesis in a homogeneous phase. We found that, among the tin catalysts studied, SnF2 and Sn(OAc)2 have problems in regards to solubility. Nevertheless, it is worth noting that SnF2 showed high conversion and selectivity in the reactions performed at room temperature. Among the other homogeneous Lewis acid catalysts assessed, SnBr2 and SnCl2 stood out as potential candidates for further study; these should be assessed in reactions with other carbonylic substrates. At room temperature, for SnCl2-catalyzed glycerol ketalization, by increasing the acetone-to-glycerol molar ratio (i.e., 4:1), conversion and selectivity toward solketal increased to a maximum level (i.e., 81% and 98%, respectively). A procedure for recovery/reuse of the catalyst was proposed on the basis of vaporization of ketone excess followed by washing of SnCl2 solid catalyst with ethyl acetate. SnCl2 catalyst activity remains unaltered after six cycles of recovery and reuse. This deserves highlighting that product distillation is a necessary step to isolate solketal from acetone excess in either glycerol ketalization process under homogeneous as well as heterogeneous catalysis conditions.
■
AUTHOR INFORMATION
Corresponding Author
*M. J. da Silva. E-mail:
[email protected] or silvamj2003@ yahoo.com.br. Fax: (+55)-31-3899-3065. Phone: (+55)-313899-3071 or (+55)-31-3899-3210. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors are grateful for the financial support from CAPES, CNPq, FAPEMIG, and FUNARBE (Brazil). They also wish to thank Professor Luis Claudio Barbosa for the GC-MS analyses.
■
REFERENCES
(1) Sharma, Y. C.; Singh, B.; Upadhyay, S. N. Advancements in Development and Characterization of Biodiesel: A review. Fuel 2008, 87, 2355. (2) Bauer, F.; Hulteberg, C. Is There a Future in Glycerol as a Feedstock in the Production of Biofuels and Biochemicals? Biofuels, Bioprod. Bioref. 2013, 7, 43. (3) Zhou, C. H.; Beltramini, J. N.; Fan, Y. X.; Lu, G. Q. Chemoselective Catalytic Conversion of Glycerol as a Biorenewable Source to Valuable Commodity Chemicals. Chem. Soc. Rev. 2008, 37, 527. (4) De Torres, M.; Jiménez-osés, G.; Mayoral, J. A.; Pires, E.; de los Santos, M. Glycerol Ketals: Synthesis and Profits in Biodiesel Blends. Fuel 2012, 94, 614. (5) Zheng, Y.; Chen, X.; Shen, Y. Commodity Chemicals Derived from Glycerol, an Important Biorefinery Feedstock. Chem. Rev. 2008, 108, 5253. (6) Pagliaro, M.; Ciriminna, R.; Kimura, H.; Rossi, M.; Pina, C. D. From Glycerol to Value-Added Products. Angew. Chem., Int. Ed. 2007, 46, 4434. (7) Jérôme, F.; Pouilloux, Y.; Barrault, J. Rational Design of Solid Catalysts for the Selective Use of Glycerol as a Natural Organic Building Block. ChemSusChem 2008, 1, 586. (8) Garcia, E.; Laca, M.; Pérez, E.; Garrido, A.; Peinado, J. New Class of Acetal Derived from Glycerin as a Biofuel Component. Energy Fuel 2008, 22, 4274. 16713
dx.doi.org/10.1021/ie402240j | Ind. Eng. Chem. Res. 2013, 52, 16709−16713
