Applied Catalysis A: General 287 (2005) 227–235 www.elsevier.com/locate/apcata
‘‘Clean’’ limonene epoxidation using Ti-MCM-41 catalyst M.V. Cagnoli a, S.G. Casuscelli b, A.M. Alvarez a, J.F. Bengoa a, N.G. Gallegos a, N.M. Samaniego a, M.E. Crivello b, G.E. Ghione b, C.F. Pe´rez b, E.R. Herrero b, S.G. Marchetti a,* a
CINDECA, Fac. Cs. Exactas, Fac. Ingenierı´a, U.N.L.P., CIC, CONICET Calle 47 No. 257 (1900) La Plata, Argentina b CITeQ, Facultad Regional Co´rdoba, U.T.N., CC.36 (5016) Co´rdoba, Argentina Received 17 November 2004; received in revised form 1 April 2005; accepted 2 April 2005 Available online 11 May 2005
Abstract The limonene oxidation with H2O2 (‘‘clean’’ oxidation) using a Ti-MCM-41 catalyst was studied. This catalyst was synthesized by a sol– gel method and characterized by XRD, DRS, ICP and BET. The operative reaction conditions, under which no Ti leaching was detected, were determined. In these conditions the catalyst has shown a high activity, and very good selectivity towards epoxides. The kinetic parameter measurements were carried assuming a heterogeneous reaction and empirical power rate law. Using the initial rate method, a first order with respect to the concentrations of catalyst, limonene and hydrogen peroxide were determined. An apparent activation energy value of 16.4 kJ/ mol and a pre-exponential Arrhenius´s factor of 7.1 l2/g mol h were obtained. # 2005 Elsevier B.V. All rights reserved. Keywords: Catalytic limonene epoxidation; Ti-MCM-41 catalysts; Ti leaching; Limonene epoxidation kinetic
1. Introduction The selective oxidation of organic compounds has been widely studied [1–3]. In order to replace the stoichiometric oxidation reactions by processes involving catalytic oxygen transference (using innocuous oxidants for the environment requirements), it is necessary to look for heterogeneous catalysts, like supported heteropolyacids, hydrotalcites and materials containing titanium ions [4–9]. In this way, the TS1 zeolite, which incorporates Ti in the silicalite framework, is effective in the oxidation of a variety of small substrates when H2O2 is used as oxidant [10–12]. Although, zeolites exhibit remarkable shape selectivity, the diffusion of bulky reactants and products through the pore structure is often hindered. A new family of mesoporous molecular sieves was discovered by the researchers at Mobil Co., at the beginning of the 1990s [13,14]. This fact led to extend the application * Corresponding author. Tel.: +54 221 4210711; fax: +54 221 4211353. E-mail addresses:
[email protected] (S.G. Casuscelli),
[email protected] (S.G. Marchetti). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.04.001
of redox molecular sieves to the oxidation of organic molecules with large critical size. One member of this mesoporous family is the MCM-41 with hexagonal arrangement and narrow pore size distribution varying between 3 and 10 nm, according to the surfactant used in the synthesis, high specific surface area (ffi1000 m2/g) and structure thermal stability, but with low hydrothermal stability. The Ti incorporation into the MCM-41 framework was first successfully obtained in 1994 [15,16]. The results obtained indicate that this solid was active when dealing with large molecules such as a-terpineol and nor-bornene, at 343 K using H2O2 or tert-butyl hydroperoxide as oxidants. Several authors found that the Ti ions are extracted out of the structure when the hydrogen peroxide is used as oxidant (titanium leaching) in different reaction systems [17–22]. However, Schuchardt et al. [23] working with ciclohexane/ H2O2/acetone system did not detect Ti leaching. Catalyst deactivation and/or homogeneous catalysis might occur when Ti is leached out of the solid [18,21,22]. In order to study some aspects of these solids behavior, we have prepared and characterized a Ti-MCM-41 following the Koyano and Tatsumi protocol [24], to be used in the
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limonene oxidation. This reaction shows a special interest since the products (e.g. 1,2-epoxylimonene) are used in the flavors and fragrances industry [3]. We have verified the Ti4+ ion location into the framework of the mesoporous solid, and the heterogeneity of the catalytic process in the limonene oxidation with H2O2. Finally, we have determined the activity and selectivity of the catalyst and the kinetic parameters of the reaction.
2. Experimental The Ti-MCM-41 catalyst, with a Ti content of 1.27% Ti (w/w) determined by inductively coupled plasma (ICP) with optical emission detector, was prepared using tetra-ethylortho-silicate (TEOS), titanium-tetra-butoxide (TBOT) and cetyl trimethyl-ammonium-bromide as surfactant (CTMABr). An alcoholic solution of CTMABr was added to a mixture of TEOS and TBOT, following the methodology proposed by Koyano and Tatsumi [24]. The solid was characterized by X-ray diffraction (XRD) in a low angles range using Cu Ka radiation, diffuse reflectance spectroscopy (DRS) in the UV–vis range, pore radii distribution and specific surface area by nitrogen adsorption (BET). The activity and selectivity measurements in the limonene oxidation reaction with H2O2 have been carried out in a batch reactor at 343 K equipped with refrigerant, thermometer and magnetic stirrer. For a standard reaction, 40 mg of catalyst were added to a reaction mixture of 4.32 mmol of limonene (L), 2.70 g of acetonitrile (AcN), used as solvent, and 1.17 mmol of H2O2 35% w/w. Samples at different reaction times were obtained through a lateral tube of the reactor and were analyzed by GC using a capillary column (cross-linked methyl–silicone gum, 30 m 0.53 mm 2.65 mm film thickness). Reaction products were identified by GC–MS. The H2O2 conversion was measured by iodometric titration. The conversion was defined as the ratio of converted species to initial concentration. Also it was calculated the ratio of limonene conversion to theoretically possible conversion (mol% of max.), that is the maximum amount of the oxygenated products that could be obtained if all H2O2 was consumed. The limonene conversion thus defined was named XL.
3. Results and discussion 3.1. Catalyst characterization The textural properties of the Ti-MCM-41 are: 2.8 nm pore diameter, 1105 m2/g of specific surface area and 1.52 cm3/g of specific pore volume. The N2 adsorption isotherm (not shown) allowed us to confirm the existence of a unimodal and narrow pore size distribution that verify a model of a solid with cylindrical, equal and straight pores.
Fig. 1. Fresh and used Ti-MCM-41 XRD patterns.
Combining the XRD and BET results according to Beck et al. [14], a 2.1 nm of wall thickness was obtained. The X-ray diffraction patterns of the fresh and used TiMCM-41 are shown in Fig. 1. The fresh sample displays an intense peak at 2u = 2.38 and one additional peak, less intense and wider, at 2u = 4.48 characteristics of a hexagonal arrangement [14,24]. However, the presence of only one wide peak between 3.58 and 58 implies a decrease of the spatial regularity compared with the typical structure of MCM-41. The spectrum of the used catalysts (washed with AcN and dried) shows the maintenance of the solid structure. Fig. 2 displays the DRS spectra of the fresh and used TiMCM-41 after calcination in air at 773 K during 20 h. There is a strong transition around 205 nm and a shoulder at about 258 nm in both samples. The spectra were fitted using two Lorentzian functions. The parameters (position, full linewidth at half-height and area) were not held fixed in fitting. These bands were assigned to charge transfer transition (CT) from O ion to a Ti4+ ion (O2 ! Ti4+) located in the MCM-41 structure like in TS-1 system. This comparison is possible, since in both solids some of the Si4+ ions of silica framework have been substituted by Ti4+ ions. According with Lamberti et al. [25] we have demonstrated [26] the existence of two different framework Ti4+ sites in the TS-1: perfect ‘‘closed’’ and defective ‘‘open’’ sites. The ‘‘closed’’ sites structure can be expressed by Ti(OSi)4, while the ‘‘open’’ sites are generated by the rupture of a Si–O–Ti bridge of a ‘‘closed’’ site with the appearance of two OH groups, implying the insertion of a fifth oxygen atom in the first coordination sphere of Ti. Rhee and Lee [27] also have demonstrated that Ti replaces Si in the MCM-41 framework and its coordination environment was similar to that of TS-1. In this way, in the present work, the 205 nm band was assigned to ‘‘closed’’ sites and the 258 nm band to ‘‘open’’ sites. It is important to remark the anatase absence in the
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Fig. 2. Fresh and used Ti-MCM-41 UV–vis DRS spectra. Solid lines: experimental spectra. Dash lines: fit curves spectra.
solid since there is no band at 330 nm [28]. Its presence would produce the H2O2 decomposition decreasing the oxidant amount in the reaction environment. No significant changes were observed between the DRS spectra of fresh and used catalysts, indicating the maintenance of the structure of the Ti4+ sites after the reaction. 3.2. Activity and selectivity measurements Fig. 3a shows the activity and selectivity results of the limonene epoxidation using H2O2 as oxidant. A blank experiment was carried out mixing the reactants in absence of catalyst. A limonene conversion of 1% was obtained. This value was subtracted in all experiments, in order to evaluate only the catalyst behavior. The limonene conversion reached at seven hours of reaction time was high (52% of max.) in comparison with the conversion (7.4% of max.) obtained previously [9] using a catalyst prepared by a different method, with a similar titanium content (2.1 wt.% TiO2). The TON value (total number of product molecules formed per active site) calculated at 7 h of reaction time was 57 molecules/site. This number was calculated assuming that all isolated Ti4+ are located on the surface. The catalyst showed a good selectivity towards 1,2 and 8,9-epoxylimonene (60%) along the reaction time. Other products obtained were carveol, carvone, di-epoxylimonene and glycols. In Fig. 3b are shown the H2O2 conversion and its efficiency (percentage of this reactive converted to oxidized
Fig. 3. (a) Activity and selectivity of limonene oxidation on Ti-MCM-41 catalyst. (b) Limonene and H2O2 conversions and H2O2 efficiency vs. reaction time. All line curves are intended only as a visual guide. Reaction conditions: T = 343 K, 40 mg of catalyst, 4.32 mmol of L, 2.70 g of AcN, 1.17 mmol of H2O2.
products). The efficiency is lower than 100% due to the partial decomposition of H2O2 in our reaction conditions. We have not observed Ti leaching after 7 h, in our reaction conditions. We have measured the Ti loading in the solid and liquid phases, before and after reaction by ICP and colorimetry. The solid was separated from the liquid phase by filtration at the reaction temperature. The catalyst was attacked with H2SO4 and HF up to SO3 white smoke appearance. H2SO4 and H2O2 (3% w/w) were added to the resulting liquid and the final volume was adjusted. The liquid phase of the reaction medium was evaporated, calcined and then was treated in the same way as the solid. The solutions thus obtained were used for the colorimetric and ICP determinations. The Ti content remained constant in
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comparison with that reached when the catalyst is present in the reaction medium (Fig. 3a). It can be seen in Fig. 5 that the conversion increase, in the experiment without catalyst, is much lower than that corresponding to the reaction when the catalyst is present in the overall range time (Fig. 3a) 0 0 (XL420 XL25 ffi 12 and 35%, respectively). In order to explained why the reaction continues after the catalyst removal considering the Ti leaching absence, we must take into account that the oxidation of an organic substrate may occur by three different mechanisms [19]: (a) Free radical chain auto-oxidation (there is at least one of these reactions present, even in the absence of the catalyst). (b) Catalytic oxygen transfer with variable valence metals. (c) Catalytic oxygen transfer without change in the metal oxidation state (Mars-van Krevelen mechanism). Fig. 4. Limonene oxidation on used Ti-MCM-41. Reaction conditions: T = 343 K, 40 mg of catalyst, 4.32 mmol of L, 2.70 g of AcN, 1.17 mmol of H2O2.
the catalyst, while it was not detected Ti in the liquid phase considering the detection limits of the used techniques (0.5 ppb by ICP). These results were corroborated since the catalyst activity remained constant when it was re-used in several reactions. The catalyst was separated, after being used in the reaction, by filtration at the reaction temperature and placed it in an other reactor with fresh reactants. A catalyst used in two previous reactions was re-used without loss of activity and selectivity (Fig. 4). Notwithstanding the above results, the oxidation reaction continues when the catalyst is removed from the reaction medium. Thus, if the catalyst is removed by filtration at the reaction temperature after 15 min, the reaction continued although with a low limonene conversion (Fig. 5) in
Fig. 5. Limonene oxidation with the extraction of the Ti-MCM-41 from the reaction medium after 15 min of reaction. Reaction conditions: T = 343 K, 40 mg of catalyst, 4.32 mmol of L, 2.70 g of AcN, 1.17 mmol of H2O2.
From the viewpoint of selectivity, it would be desirable to avoid mechanism (a) because it produces a high number of oxidation products. Mechanism (b) is also undesirable, since variable valence metals can also be expected to catalyze some steps of the free radical chain auto-oxidation (a) [19]. Therefore, mechanism (c) seems to be the most adequate one to obtain a good selectivity towards to epoxides. A new experiment was carried in order to verify if the mechanism (a) occurs. The radical scavenger, 2,6-di-tertbutyl-4-methylphenol (BHT) was added to the reaction medium at 15 min of reaction time, after removing the catalyst by filtration (Fig. 6). It can be seen that the reaction is stopped. Fig. 7 shows the different limonene conversion behavior:
Fig. 6. Limonene oxidation with the extraction of the Ti-MCM-41 from the reaction medium after 15 min of reaction and BHT adding. Reaction conditions: T = 343 K, 40 mg of catalyst, 4.32 mmol of L, 2.70 g of AcN, 1.17 mmol of H2O2.
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Fig. 7. Limonene conversions comparison: (i) with catalyst during all reaction time, (ii) extracting the catalyst after 15 min of reaction time, and (iii) extracting the catalyst after 15 min of reaction time and adding the radical scavenger. Reaction conditions: T = 343 K, 40 mg of catalyst, 4.32 mmol of L, 2.70 g of AcN, 1.17 mmol of H2O2.
(i) when the catalyst is present all reaction time; (ii) when the catalyst is extracted from the reactor after 15 min; (iii) when the catalyst is extracted after 15 min and the BHT is added.
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Fig. 8. Limonene oxidation with Ti-MCM-41 with the addition of a radical scavenger at the beginning of the reaction. Reaction conditions: T = 343 K, 40 mg of catalyst, 4.32 mmol of L, 2.70 g of AcN, 1.17 mmol of H2O2.
Sheldon [19,30] and considering that Ti4+ catalyzes oxygen transfer by peroxo-metal pathway, we suggest a sequence for the free-radical auto-oxidation. During the first 15 min, the catalyst promotes the formation of hydroxi-radicals as it is shown in Scheme 1: When the catalyst is removed, the OH and the O2 of the system in the liquid phase, react with limonene as it is shown in Scheme 2: This hypothesis justifies the experimental results:
The figure reveals that: the reaction was inhibited when the radical scavenger was added; the reaction continues by a radical mechanism when the catalyst is removed without BHT presence [29]. Other authors [18,21] have attributed this behavior to the existence of simultaneous homogeneous catalysis due to the Ti4+ presence in the reaction medium after verifying the Ti leaching. When the catalyst and the radical scavenger were added at the beginning of the reaction, the results obtained (Fig. 8) were similar to those found when the reaction was carried out in catalyst presence and without BHT (Fig. 3a). It has been found that the carveol and carvone are formed by both a free radical mechanism and a concerted process via catalytic oxygen transfer [29]. In this case, since the BHT inhibits the radical mechanism, the presence of approximately 20% of carveol and/or carvone, would indicate that these products are formed from the epoxide. When the catalyst is removed (after 15 min of reaction time) the reaction, initiated by the catalyst, continues by radical mechanism. According to the ideas proposed by
(a) an increase of the selectivity to carvone and carveol from 20 to 40% when the catalyst is removed (Figs. 3a and 5); (b) the reaction suppression when a radical scavenger is added once the catalyst is removed from the reaction medium (Fig. 7).
Scheme 1.
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Scheme 2.
According to these results we conclude that in catalyst presence there are two competitive mechanisms: the catalytic oxygen transfer and free radical auto-oxidation, being the former faster than the last. The dissolved oxygen and/or the impurities of the solvent do not have influence in the reaction without catalyst since the OH initiators are absent. It was verified through the blank experiment where only 1% of conversion was obtained, after 7 h. On the other hand, when the catalyst is present during all reaction time, the oxygen presence have not also influence since the catalytic oxygen transfer is faster than the free radical auto-oxidation. Considering all the above description, we have demonstrated that the limonene oxidation, in our reaction conditions, appears to be heterogeneously catalyzed by Ti-MCM-41.
As it was mentioned in Section 1, several authors had found that the titanium ions are extracted from the solid when the catalyst is contacted with the reaction medium [17–22]. Therefore, our results seem to go against actual knowledge about the Ti leaching. With the purpose to study this topic, we carried out the reaction changing the limonene/H2O2 molar ratio from 3.7 to 0.7. Titanium leaching was observed only with the molar ratio L/ H2O2 < 1. When the molar ratio was 0.7 (H2O2 in excess) we detected, by ICP, a titanium loss of 13.2 and 29.4% after 7 and 70 h of reaction time, respectively. Bearing in mind that Ti4+ ions are located in stressed positions into the MCM-41 framework, the hydroperoxide could easily reach the Ti4+ sites, leaching them. However, it has been demonstrated that the sylanol distribution on the inner surface of MCM-41 is in-homogeneous, producing
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localized hydrophobic zones contained in a solid essentially hydrophilic [31]. Therefore, the Ti4+ ions seem to be protected by adsorption of organic molecules onto the surface when there is an important excess of organic substrate and non-protic solvent in the reaction medium [22]. Therefore, we conclude that the Ti leaching depends, not only on the structural solid properties, but also on the reaction operative variables. 3.3. Kinetic measurements The results obtained allowed us to determine the kinetic parameters under the following assumptions: The reaction is heterogeneous. The Ti leaching is avoided maintaining the limonene concentration in excess respect to the H2O2 concentration in all experiments. It is worth-mentioning that even if Ti traces were present in solution, they will not affect the heterogeneity of the reaction since they show little activity in homogeneous reaction [19,30]. Taking into account the aim of this work we will not use the Langmuir–Hinshelwood or Hougen–Watson models to obtain the reaction rate equation. Instead, we will find the functionality between reaction rate and the reactant concentrations through an empirical power law. This is a methodology also useful since the reaction rate equation obtained from the mechanistic models can be compared to empirical power law considering that the kinetic constant and the reaction orders will be apparent [32]. The initial rate method [33] was used to measure the kinetic parameters of the reaction in absence of external and internal diffusional effects. A third order polynomial has been used to fit the conversion–time curves. The dependence of the reaction rate with the oxidant concentration was determined fixing the limonene and solvent concentration (1.00 mol/l and 634 g/l, respectively), the catalyst mass (9 g/ l) and the reaction temperature (343 K). The limonene conversion versus the reaction time for a H2O2 concentration range of 0.15–0.76 mol/l was measured. To obtain the initial reaction rate, each experiment was extrapolated to the initial condition and plotted versus H2O2 concentration initially added (Fig. 9). A first order respect to the peroxide concentration was obtained. The limonene reaction order was determined varying its concentration between 0.50 and 1.26 mol/l, maintaining constant all the other variables (0.25 mol/l of H2O2, 9 g/l of catalyst, 634 g/l of AcN and 343 K of temperature). Fig. 10 displays a linear relation between the initial reaction rate calculated for each experiment and the initial limonene concentration, indicating also a first order respect to substrate concentration. The effect of the catalyst mass on the reaction rate was studied varying it between 6 and 12 g/l and maintaining constant all of the other variables (0.25 mol/l of H2O2,
Fig. 9. Effect of the H2O2 concentration on the initial reaction rate. [L] = 1.00 mol/l; [catalyst] = 9 g/l; [AcN] = 634 g/l.
Fig. 10. Effect of the limonene concentration on the initial reaction rate. [H2O2] = 0.25 mol/l; [catalyst] = 9g/l; [AcN] = 634 g/l.
1.00 mol/l of L, 634 g/l of AcN and 343 K of temperature). Fig. 11 shows the initial reaction rate versus the catalyst concentration obtaining a first order with respect to the catalyst concentration.
Fig. 11. Effect of the catalyst concentration on the initial reaction rate. [L] = 1.00 mol/l; [H2O2] = 0.25 mol/l; [AcN] = 634 g/l.
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Fig. 12. Limonene conversion vs. reaction time at different temperatures. [L] = 1.00 mol/l [H2O2] = 0.25 mol/l; [catalyst] = 9 g/l [AcN] = 634 g/l.
Fig. 13. Specific reaction rate vs. inverse of temperature (Arrhenius’s graph) for limonene oxidation.
The reaction temperature was varied between 313 and 343 K to obtain the specific reaction rate at different temperatures and the apparent activation energy. The limonene conversion (XL) and the reaction time (t) can be related applying the integral method [33] through the following expression:
4. Conclusions
ln
M XL ¼ k½cat ½L 0 ðM 1Þt Mð1 XL Þ
being M ¼
½H2 O2 0 ½L 0 , 0
(1)
= [initial].
Fig. 12 displays the graph obtained plotting expression (1) versus t for each temperature. Table 1 shows the fitting parameters and the correlation coefficients. The k values were obtained from these fittings. The linearity of the data at 323, 333 and 343 K corroborates the reaction orders obtained above. However, at 313 K, the data are not linear. Fig. 13 shows the typical Arrhenius´s plot. It can be seen that at 313 K the linearity is broken. This behavior suggests a probable change in the reaction mechanism at temperatures lower than 323 K. An apparent activation energy (Ea) of 16.4 kJ/mol and a frequency factor (k0) of 7.1 l2/g mol h were obtained for the limonene oxidation, in the 323–343 K temperature range, using the Arrhenius´s equation (Fig. 13). Finally, the kinetic expression obtained for the limonene oxidation with H2O2 between 323 and 343 K is r ¼ k½cat ½L ½H2 O2
Ti-MCM-41 synthesized by Koyano and Tatsumi [24] protocol has presented a high performance in the limonene epoxidation with hydroperoxide as oxidant agent: a good level of limonene conversion (>50%) and an interesting selectivity towards epoxide products (50– 60%). These results have been obtained in the past using other oxidant agents, different than H2O2 [9]. The catalyst maintained its activity and selectivity even after several reactions. Using a molar ratio of organic substrate/H2O2 > 1, titanium leaching has not been observed. When the catalyst is separated from the reaction mixture, the reaction continues through a radical mechanism with activity and selectivity loss. Therefore, the Ti-MCM-41 favors the catalytic process of oxygen transference avoiding the radical mechanism that produces a loss of selectivity towards epoxides. In the temperature range studied and in reaction conditions that inhibits the Ti leaching, a first reaction order was found respect to limonene, hydrogen peroxide and catalyst concentrations. The apparent reaction activation energy (Ea) and frequency factor (k0) were found to be of 16.4 kJ/mol and 7.1 l2/g mol h.
(2) Acknowledgments
Table 1 Fitting parameters of the experimental points in Fig. 12 T (K)
a
b
R
323 333 343
0 0 0
0.10921 0.12701 0.15013
0.99211 0.9923 0.99862
Fitting function: y = a + bx.
The authors acknowledge the support for this work to the Consejo Nacional de Investigaciones Cientı´ficas y Te´ cnicas, Comisio´ n de Investigaciones Cientı´ficas de la Provincia de Buenos Aires, Universidad Nacional de La Plata and Universidad Tecnolo´ gica Nacional, Facultad Regional Co´ rdoba and Agencia Nacional de Promocio´ n Cientı´fica y Tecnolo´ gica (PICT No. 14-08456).
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