Ind. Eng. Chem. Res. 2006, 45, 7365-7373
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APPLIED CHEMISTRY Technological Parameter Optimization for Epoxidation of Methallyl Alcohol by Hydrogen Peroxide over TS-1 Catalyst Agnieszka Wro´ blewska,* Ewelina Ławro, and Eugeniusz Milchert Institute of Organic Chemical Technology, Technical UniVersity of Szczecin, Pulaskiego 10, Pl 70-322 Szczecin, Poland
Optimization of technological parameters for epoxidation of methallyl alcohol (MAA) by 30 wt % hydrogen peroxide was presented. The reaction was performed in a glass reactor at atmospheric pressure and in methanol (protic solvent). The titanium silicalite TS-1 catalyst was used to activate the hydrogen peroxide. The influence of the following parameters was investigated: temperature (20-60 °C), MAA/H2O2 molar ratio (1:1-5:1 mol/mol), methanol concentration (5-90 wt %), TS-1 concentration in the reaction mixture (0.1-5.0 wt %), and reaction time (0.5-5 h). The optimal conditions were established by a mathematical method of design of experiments (rotatable-uniform design). The obtained results were described using the following mathematical functions: the selectivity of transformation to 2-methylglycidol in relation to methallyl alcohol consumed, conversion of methallyl alcohol, and the selectivity of transformation to organic compounds in relation to hydrogen peroxide consumed. The courses of these functions were plotted in the system of two variables with the other parameters determining the maximum of the function. 1. Introduction Epoxide compounds are valuable intermediates and final products of the organic chemical industry. Currently, new methods for their preparation are being developed while previous ones have been improved. Considerable effort is directed toward the use of catalysts and reactants that are not harmful to the environment. The titanium silicalite TS-1 is highly important heterogeneous catalyst of the zeolite type. In low-tonnage processes the following catalysts are also used: TS-2, Ti-beta, Ti-MCM-41, and Ti-MCM-48. By the application of these catalysts, hydrogen peroxide can be activated and used for epoxidation of unsaturated compounds. Simultaneously, by the choice of optimal technological parameters, a high selectivity of transformation to epoxide at a relatively high conversion of substrates can be achieved. The performance of epoxidation under mild conditions (low temperature, atmospheric pressure) ensures better security and allows the reduction of operating costs. A heterogeneous catalyst can be easily separated from the postreactive mixture and recycled to the process. However, its activity becomes lower due to the leaching of titanium from the catalyst structure. This phenomenon can be limited by the choice of appropriate technological parameters to minimize formation of polyhydric alcohols (polyols).1 2-Methylglycidol finds various applications, among others; it is utilized for the preparation of oxazoline used in the production of anticonvulsant drugs. It is also used in the preparation of various natural products and their analogues such as vitamin D3 and choline.2 2-Methylglycidol is a raw material in the preparation of cyanodiols and R-methylcysteine, which are intermediates in the synthesis of compounds exhibiting antitumor and anti-HIV activities.3,4 This compound also finds an applica* To whom correspondence should be addressed. Tel.: +48 914494875. Fax: + 48 914494365. E-mail: Agnieszka.Wroblewska@ ps.pl.
tion in the manufacture of immunomodulator, which improves not only the effectiveness of antitumor adriamycins and mitomycins C, used in the treatment of people who have leukemia, but also reduces the toxicity of these compounds.5 2-Methylglycidol is employed in the production of quick-drying lacquers, inks, and printing glues.2,6 This compound was also detected in AK toxins, which are responsible for the necrosis of leaves and fruit of the Japanese pear. It enables the production of synthetic AK toxins and the prevention of necrosis. Similarly to glycidol, it is utilized in the production of surface-active agents, cosmetics, pharmaceuticals, and plant protective compounds.7 The aim of this work was to determine optimal technological parameters for epoxidation of methallyl alcohol to 2-methylglycidol by 30 wt % hydrogen peroxide in the presence of the titanium silicalite TS-1 catalyst. The influence of the following parameters was investigated: temperature, MAA/H2O2 molar ratio, concentration of methanol (solvent), concentration of TS-1 catalyst, and reaction time. The course of the process was evaluated on the basis of the following functions: the selectivity of transformation to 2-methylglycidol in relation to methallyl alcohol consumed, conversion of methallyl alcohol, and the selectivity of transformation to organic compounds in relation to hydrogen peroxide consumed. Each function was presented in the form of a mathematical equation for which a maximum value and the optimal technological parameters were established. Moreover, the courses of the functions were plotted for the changes of two parameters in the established range of their variability. The other parameters were held constant at a level corresponding to a maximum of the function. 2. Experimental Section 2.1. Preparation of TS-1 Catalyst and Its Characteristics. TS-1 catalyst was prepared by a method of Thangaraj et al.8 Tetraethyl orthosilicalite (Fluka) and tetrabutyl orthotitanate (Fluka) were used as raw materials, whereas tetrapropylammo-
10.1021/ie0514556 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/22/2006
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Figure 1. X-ray diffraction pattern of TS-1 catalyst.
nium hydroxide, TPAOH (Fluka), was employed as the template agent. The gel crystallization was carried out under static conditions at 175 °C for 8 days. After separation of the crystals from the parent liquid, they were dried in a furnace at 120 °C for 12 h and subsequently dried at 550 °C for 24 h. A calcined catalyst was activated by washing with a 10% aqueous solution of ammonium acetate at 80 °C and recalcined at 550 °C for 24 h. Characterization of the catalyst was performed using conventional techniques. The chemical composition of the catalyst was established by means of X-ray fluorescence spectroscopy (XRF) on a VRA 30 spectrometer. The catalyst contained 1.7 wt % Ti. The crystalline structure of a sample was confirmed by X-ray diffraction (XRD) using an XPERT PRO diffractometer with Co KR radiation at a wavelength of 0.179 nm. The XRD pattern was recorded between 6° and 60° 2Θ (Figure 1). It was found that the XRD pattern was similar to those reported by Perego et al. and Thangaraj et al.9,10 The Fouier transform infrared (FT-IR) spectrum of a TS-1 sample was recorded on a JASCO FT/IR-430 instrument using the KBr pellet techniques. The catalyst concentration in the pellet amounted to 1.4 wt %. The ∼960 cm-1 absorption band was present in this spectrum. The UV-vis spectrum was recorded on a SPECORD M40 instrument. The UV-vis spectrum shows a characteristic band at 220 nm, and confirms the incorporation of titanium into the crystalline structure of silica. There were no additional bands showing the presence of extraframework titanium (Figure 2a). The morphology of the crystals was determined on the basis of the micrographs taken on a JEOL JSM-6100 scanning electron microscope (SEM). They were uniform crystallites with a size of 0.7 µm (Figure 2b).
2.2. Epoxidation Procedure, Apparatus, and Analytical Methods. Epoxidation of methallyl alcohol was carried out using the following reagents: methallyl alcohol (MAA) (98 wt %, Fluka), hydrogen peroxide (30 wt % aqueous solution, POCh, Gliwice), methanol (analytical grade, POCh, Gliwice), and TS-1 catalyst. Hydrogen peroxide consumption was determined iodometrically.11 A quantitative analysis of 2-methylglycidol (MG) was performed chromatographically on a Chrom 5 apparatus equipped with a flame ionization detector (FID) with the use of a column packed with Chromosorb 101 (60/80 mesh). The column temperature was programmed as follows: isothermally at 170 °C for 6 min, followed by an increase at the rate of 15 °C/min, isothermally at 200 °C for 10 min, then an increase at the rate of 20 °C/min, and isothermally at 225 °C for 14 min. These analyses were performed by the external standard method. Epoxidation was carried out in a glass reactor with a capacity of 50 cm3, which was equipped with a reflux condenser, thermometer, mechanical stirrer, and dropping funnel. The reagents MAA, solvent (methanol), and the catalyst were consecutively placed in the reactor. When the process temperature was reached, hydrogen peroxide was added dropwise under vigorous stirring. After the reaction was completed, the mass balance was performed. The results of experiments allowed calculations of the values of the following functions: the selectivity of transformation to 2-methylglycidol in relation to MAA consumed (SMG/MAA), the conversion of MAA (XMAA), and the selectivity of transformation to organic compounds in relation to H2O2 (Sorg/H2O2) consumed. These functions were calculated from the following equations:
SMG/MAA )
amt 2-methylglycidol × 100% amt methallyl alcohol consumed
XMAA )
amt MAA consumed × 100% init amt MAA
Sorg/H2O2 )
amt obtained compds × 100% amt H2O2 consumed
3. Results and Discussion The major product of methallyl alcohol epoxidation by 30% hydrogen peroxide in methanol medium over the TS-1 catalyst is 2-methylglycidol (MG). Depending on the technological parameters, the following byproducts were obtained in various amounts: 2-methylglycerol, 2-methylacrolein, methallyl ether,
Figure 2. UV-vis spectrum and scanning electron micrograph of TS-1 catalyst.
Ind. Eng. Chem. Res., Vol. 45, No. 22, 2006 7367 Scheme 1
methallyl methylglycidyl ether, 2-methoxy-2-methylpropane1,3-diol, 3-methoxy-2-methylpropane-1,2-diol, and polymers. Reactions proceeding during the epoxidation are presented in Scheme 1. The influence of technological parameters (independent factors, input variables) on the course of epoxidation process was examined by the mathematical method of experimental design.12-15 A rotatable-uniform design was used to achieve this goal. The studies were performed over the following ranges of changes of technological parameters: temperature 20-60 °C, MAA/H2O2 molar ratio 1:1-5:1, solvent concentration (methanol) 5-90 wt %, TS-1 catalyst concentration 0.1-5.0 wt %, and reaction time 0.5-5.0 h. The experimental design and calculations were performed by computer using the software Cadex: Esdet 2.2.16 The plan was realized for five technological parameters x1-x5, where x1 is temperature, x2 is MAA/H2O2 molar ratio, x3 is methanol concentration, x4 is TS-1 concentration, and x5 is reaction time. The total number of experiments was 32: in the nucleus of the experimental design 16 experiments, in the star points 10 experiments, and in the center 6 experiments. The real values of the input variables x1-x5 were recalculated into the normalized values (dimensionless) according to the equation
Xi ) [2R(xk - xk,min)/(xk,max - xk,min)] - R where Xi ∈ (-R;R); Xi is the normalized input variable, i ) 1, ..., 5; R is star arm (R ) 2); xk is the real input variable, k ) 1, ..., 5; xk,max is the maximum value of the real input variable, k
) 1, ..., 5; and xk,min is the minimum value of the real input variable, k ) 1, ..., 5. The real and normalized (coded) input variables at the levels resulting from the experimental design are shown in Table 1. A matrix of the experimental design and the experimentally determined values of the response functions are shown in Table 2: z1 is selectivity of transformation to 2-methylglycidol in relation to methallyl alcohol consumed (SMG/MAA), z2 is conversion of methallyl alcohol (XMAA), and z3 is selectivity of transformation to organic compounds in relation to hydrogen peroxide consumed (Sorg/H2O2). The influence of normalized independent factors X1-X5 (technological parameters) of methallyl alcohol epoxidation on the values of the response functions (SMG/MAA, XMAA, and Sorg/H2O2) is presented in the form of a second-order polynomial (regression equation), containing linear components, square, and dual products: k
Zi ) bo +
∑ i)1
k
biXi +
∑ i1 despite the same maximum conversion as at MAA/H2O2 ) 1:1 is not reasonable with regard to the necessity of recirculation of a considerable quantity of MAA. The interaction of the parameters temperature-methanol concentration (Figure 4b) indicates that the highest conversion of MAA can be achieved over the whole range of temperatures at the methanol concentration of at least 60 wt %. This concentration can be lowered with elevation of temperature. At the process temperature of 20 °C the methanol concentration is 60 wt %; however, at 60 °C this concentration can be reduced to 50 wt % to achieve MAA conversion of 90 mol %. From the point of view of conversion a useful range of methanol concentration is 60-90 wt %. This range is within the methanol concentrations determining the most beneficial technological parameters of the z1 function (the selectivity of transformation to 2-methylglycidol in relation to methallyl alcohol consumed). After completion of the synthesis, methanol can be easy recovered by distillation and returned to the process. A similar tendency was found in the case of the dependence of temperature-TS-1 concentration (Figure 4c), where the z2 function reaches its maximum value over the whole range of
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Figure 4. Influence of technological parameters on conversion of MAA: (a) temperature and MAA/H2O2 molar ratio; (b) temperature and methanol concentration; (c) temperature and TS-1 concentration; (d) temperature and reaction time. Constant parameters: MAA/H2O2 molar ratio 2:1, methanol concentration 68 wt %, TS-1 concentration 4 wt %, and reaction time 5 h.
examined temperatures and catalyst concentration of at least 3 wt %. This concentration allows achievement of the maximum of z1 function. The reaction time required to achieve the highest conversion of MAA can be shortened along with elevation of the process temperature (Figure 4d). Thus the reaction time amounts to 4.5 h at 20 °C and about 3.0 h at 60 °C. However, it is more advantageous to run the epoxidation at lower temperatures for a longer time. This is associated with a limitation of the occurrence of side reactions such as hydrolysis, etherification, and polymerization as well as with a lower consumption of energy. The course of the MAA conversion (z2 function) allows extension of the range of the technological parameters to a region in which this function reaches its maximum value. These parameters are as follows: temperature 20 °C, MAA/H2O2 molar ratio 1:1, methanol concentration 60-90 wt %, catalyst TS-1 concentration 3-5 wt % and reaction time 4.5-5.0 h. 3.3. Influence of Technological Parameters on the Selectivity of Transformation to Organic Compounds in Relation to Hydrogen Peroxide Consumed (z3). Hydrogen peroxide undergoes both effective and ineffective transformation during the epoxidation process. Effective transformation relies on the formation of the active intermediate with the titanium atom that allows transfer of the oxygen atom to the double bond of methallyl alcohol. Ineffective transformation is associated with the decomposition of hydrogen peroxide to oxygen and water. Oxygen originated from the decomposition does not participate in the epoxidation. Both high temperatures and the presence of
the catalysts in the form of titanium compound enhance the rate of ineffective decomposition of H2O2. Therefore, it is reasonable to investigate the influence of the technological parameters on the course of function of the selectivity of transformation to organic compounds in relation to hydrogen peroxide consumed. This function determines the amount of the reacted H2O2 that is consumed to form the organic compounds (z3). With regard to the separation of MG by the distillation method, hydrogen peroxide should be first removed from the postreactive solution to prevent explosion. Hydrogen peroxide can be decomposed by cautious heating; however, this is associated with losses of the epoxide due to the polymerization and other reactions such as hydrolysis, solvolysis, and etherification. For this reason the temperature is lowered and the decomposition of hydrogen peroxide is carried out in the presence of active carbon, pumice, or some metals such as ground silver, gold, and platinum. The chemical decomposition by solutions of iron salts and sodium bisulfate(IV) has significance on a small scale due to the contamination by the introduced salts. The calculations of extremes of the z3 function (Table 4) indicate that this function achieves its maximum value at 77 wt % for the following technological parameters: temperature 37 °C, MAA/H2O2 molar ratio 5:1, methanol concentration 78 wt %, TS-1 catalyst concentration 4 wt %, and reaction time 2.5 h. The influence of changes of two selected technological parameters on the course of the z3 function is shown in Figure 5. The remaining parameters had constant values and allowed achievement of the maximum of the function.
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Figure 5. Influence of technological parameters on selectivity of transformation to organic compounds in relation to hydrogen peroxide consumed: (a) temperature and MAA/H2O2 molar ratio; (b) temperature and methanol concentration; (c) temperature and TS-1 concentration; (d) temperature and reaction time. Constant parameters: MAA/H2O2 molar ratio 5:1, methanol concentration 78 wt %, TS-1 concentration 4 wt %, and reaction time 150 min.
Figure 5a shows that the z3 function decreases along with the elevation of temperature at each molar ratio of MAA/H2O2. At the temperatures up to 30 °C and at the equimolecular ratio of the substrates, or at a temperature of 40 °C after elevation of the MAA/H2O2 molar ratio to 5:1, the highest selectivity of transformation to organic compounds in relation to hydrogen peroxide consumed can be achieved. The selectivity is always lower above the temperature of 40 °C. A decrease of the selectivity at higher temperatures is associated with increased conversion of hydrogen peroxide. This conversion proceeds at a slower rate after an increase of the MAA/H2O2 molar ratio. An analysis of the interaction of temperature and the MAA/ H2O2 molar ratio reveals that the process also proceeds with the highest selectivity at a temperature of 20 °C and molar ratio of reagents 1:1. The interaction of the parameters temperature-methanol concentration (Figure 5b) results that the highest selectivities are achieved in the upper range of variations of methanol concentration as well as in the bottom range. In case of the process operation at higher temperature, the methanol concentration should be above 40 wt % to maintain the selectivity (function z3) at a high level. A dilution of the reaction medium by methanol prevents the ineffective decomposition of hydrogen peroxide as well as the polymerization of methallyl alcohol and 2-methylglycidol. Hence, the methanol concentration should amount to 70-90 wt % when the process is operated at a temperature of 20 °C.
The interaction of the parameters temperature and TS-1 catalyst concentration (Figure 5c) also indicates the necessity of increasing the catalyst concentration during the elevation of temperature. When the process is run at a temperature of 20 °C and methanol concentration of 70-90 wt %, the TS-1 catalyst loading can be established within the range of 1-5 wt %. The course of the functions temperature-reaction time (Figure 5d) indicates a significant decrease in the selectivity after exceeding the temperature of 50 °C. However, the highest selectivities of transformation to organic compounds (function z3) cannot be achieved under these conditions. Simultaneously, the time of process operation can be varied over a relatively wide range of 1.5-5 h at a temperature of 20 °C. 4. Conclusions Taking into consideration the courses of the functions representing the interaction among the technological parameters as well as the possibility of obtaining 2-methylglycidol with the highest selectivity in relation to methallyl alcohol consumed (function SMG/MAA), it was established that the epoxidation process should be run at the following technological parameters: temperature 20 °C, MAA/H2O2 molar ratio 1:1, methanol concentration 70 wt %, TS-1 concentration about 3 wt %, and reaction time 3 h. These conditions allow reduction of the quantity of side products formed in the process, particularly 2-methylglycerol, ethers (methallyl and methallyl methylglycidol ether), and
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polymers. Moreover, the established parameters allow maintaining the catalyst performance over a long period of time because leaching of titanium from the catalyst structure and pore blocking by polymers are restricted. These parameters are relatively mild. The solvent (methanol) and unreacted MAA can be distilled off and recycled to the process. Moreover, an incomplete conversion of the hydrogen peroxide was found in this process. Hydrogen peroxide should be removed from the postreaction mixture because 2-methylglycerol is separated by the distillation method. Decomposition of hydrogen peroxide may be performed either by cautious heating of the postreaction mixture or by the use of reduction agents such as sodium bisulfate(IV) and iron salts(II). Literature Cited (1) Davies, L.; McMorn, P.; Bethell, D.; Bulman Page, P. C.; King, F.; Hancock, F. E.; Hutchings, G. J. Byproduct formation causes leaching of Ti from the redox molecular sieve TS-1. Chem. Commun. 2000, 1807. (2) Milchert, E.; Wro´blewska, A. Otrzymywanie glicydolu. Przem. Chem. 1996, 75, 367. (3) Hatakeyama, S.; Fukuyama, H.; Mukugi, Y.; Irie, H. Total Synthesis of (+)-Conagenin. Tetrahedron Lett. 1996, 37 (23), 4047. (4) Lakner, F. J.; Hager, L. P. Chloroperoxidase-mediated asymmetric epoxidation. Synthesis of (R)-dimethyl 2-methylaziridine-1,2-dicarboxylates a potential R-methylamino acid synthon. Tetrahedron: Asymmetry 1997, 8 (21), 3547. (5) Avenoza, A.; Cativiela, C.; Peregrina, J. M.; Sucunza, D.; Zurbano, M. M. An alternative approach to (S)- and (R)-2-methylglycidol O-benzyl ether derivatives. Tetrahedron: Asymmetry 2001, 12, 1383. (6) Hutchings, G. J.; Lee., D. F. Epoxidation of allyl alcohol to glycidol using titanium silicalite TS-1: effect of the method of preparation. Catal. Lett. 1995, 33, 369. (7) Klunder, J. M.; Ko, S. Y.; Sharpless, K. B. Asymmetric epoxidation of allyl alcohol: efficient routes to homochiral beta-adrenergic blocking agents. J. Org. Chem. 1986, 51, 3710. (8) Thangaraj, A.; Kumar, R.; Ratnasamy, P. Direct catalytic hydroxylation of benzene with hydrogen peroxide over titanium-silicate zeolites. Appl. Catal. 1990, 57, L1-L3.
(9) Perego, G.; Bellussi, G.; Corno, C.; Taramasso, M.; Buonomo, F.; Esposito, A. Titanium-silicalite: a novel derivative in the pentasil family. Stud. Surf. Sci. Catal. 1986, 28, 129. (10) Thangaraj, A.; Kumar, R.; Mirajkar, S. P.; Ratnasamy, P. Catalytic properties of crystalline titanium silicalites. Synthesis and characterization of titanium-rich zeolites with MFI structure. J. Catal. 1991, 130, 1. (11) Brill, W. F. The origin of epoxides in the liquid-phase oxidation of olefins with molecular oxygen. J. Am. Chem. Soc. 1963, 85, 141. (12) Polan´ski, Z. Experiments Planning in Technique; Science Publisher: Warsaw, 1984. (13) Achnazarowa, S. Ł.; Kafarow, W. W. Optimization of Experiments in Chemistry and Chemical Technology; Scientific-Technical Publishers: Warsaw, 1982. (14) Nalimow, W. W., Czernowa, N. A. Statistical Methods of Planning Experiments; Scientific-Technical Publishers: Warsaw, 1982. (15) Montgomery, D. C. Design and analysis of experiments, 6th ed.; John Wiley & Sons: New York, 2005. (16) Polan´ski, Z.; Go´recka-Polan´ska, R. Cadex: Esdet 2.2, Program, Planning and Statistical Analysis of Experimental InVestigations Applying Determined Statistical Analysis; Department of Technical Development and Implementation: Cracow, Poland, 1992. (17) Clerici, M. G.; Ingallina, P. Epoxidation of lower olefins with hydrogen peroxide and titanium silicalite. J. Catal. 1993, 140, 71. (18) Adam, W.; Corma, A.; Martinez, A.; Mitchell, C. M.; Redy, T. I.; Renz, M.; Smerz, A. K. Diastereoselective epoxidation of allylic alcohols with hydrogen peroxide catalyzed by titanium-containing zeolites or methyltrioxorhenium versus stoichiometric oxidation with dimethyldioxirane: clues on the active species in the zeolites lattice. J. Mol. Catal. A: Chem. 1997, 117, 357. (19) Adam, W.; Corma, A.; Redy, T. I.; Renz, M. Diastereoselective catalytic epoxidation of chiral allylic alcohols by the TS-1 and Ti-β zeolites: evidence for a hydrogen-bonded, peroxy-type loaded complex as oxidizing species. J. Org. Chem. 1997, 62, 3631.
ReceiVed for reView December 29, 2005 ReVised manuscript receiVed June 20, 2006 Accepted August 23, 2006 IE0514556
