Ind. Eng. Chem. Res. 2008, 47, 8011–8015
8011
New Two-Step Process for Propene Oxide Production (HPPO) Based on the Direct Synthesis of Hydrogen Peroxide Gema Blanco-Brieva,† M. Carmen Capel-Sanchez,† M. Pilar de Frutos,‡ Ana Padilla-Polo,‡ Jose M. Campos-Martin,*,† and Jose L. G. Fierro*,† Instituto de Cata´lisis y Petroleoquı´mica, CSIC, Marie Curie, 2, Cantoblanco, E-28049 Madrid, Spain, and Centro de Tecnologı´a Repsol YPF, A-5, Km. 18, E-28930 Mo´stoles, Madrid, Spain
In previous papers, we showed that (i) neutral solutions of hydrogen peroxide can be safely obtained by the direct reaction of H2 and O2 gas mixtures in the presence of Pd-loaded sulfonic acid resins and (ii) low molecular weight olefins can be successfully epoxidized using aqueous solutions of H2O2 in the presence of amorphous Ti/SiO2 catalysts. Against this background, this paper seeks to go one step further in our on-site H2O2 strategy by combining the direct synthesis of nonacidic H2O2 solutions with the catalyzed epoxidation of alkenes with hydrogen peroxide. In a first step, we optimized the reaction conditions for the direct synthesis of H2O2 working in a semibatch reactor. Aqueous solutions of 9 wt % H2O2 were then used in the epoxidation of oct-1-ene on a Ti-loaded amorphous silica catalyst, and reaction conditions were optimized. Finally, the propene epoxidation reaction was conducted in a continuous mode under the optimum reaction conditions selected (343 K, H2O2/catalyst ratio ) 1:4, propene/catalyst ratio ) 25, residence time 45 min). At steady-state, the conversion level of H2O2 reached 96% with a selectivity of hydrogen peroxide to propene oxide of 95%. After 135 h of reaction time, a slight decrease in the selectivity of H2O2 to epoxide was observed, with a decrease of H2O2 conversion from 96 to 80%. This catalyst deactivation is reversible, as original activity is fully recovered upon regeneration in air at 873 K. Introduction Propene oxide (PO) is one of the major starting materials in the chemical industry. It is widely used for making polyurethane, unsaturated resins, surfactants, and other products. PO is currently produced by two different types of processes involving chlorohydrin and hydroperoxide. In the first process, Cl2 reacts with propene to yield chlorohydrin, whereupon it is dehydrochlorinated with aqueous solution of alkalis.1,2 This method yields an equimolar quantity of waste aqueous solution of alkali metal chlorides, as well as considerable amounts of waste chlororganic derivatives, which are the products of chlorine addition to the double bond (usually excreting more than 40 tons of CaCl2-containing wastewater per ton of PO). A safer environmental approach is the oxidation of olefins with hydroperoxides in the presence of catalysts. The economy of the process depends on the market price of the coproduct obtained with PO (these processes produce more than 2 tons of styrene or 1.5 tons of tert-butanol per ton of PO).1 Current global PO production is approximately 6 million metric tons per year, and both technologies are employed: hydroperoxides (practiced in various forms by Lyondell, Shell, Sumitomo, and Repsol) and chlorohydrins (Dow).2–4 Today, there is a pressing need for new PO-synthesizing processes because of growing environmental and economic concerns. It is well-known that PO can be produced by the epoxidation of propene with dilute hydrogen peroxide (H2O2) over titanium silicalite (TS-1) in methanol solvent. This epoxidation process produces PO with very high selectivity (95% or more) and, theoretically, excretes only H2O as a byproduct. It has been extensively studied in recent decades as an ideal * Corresponding author. E-mail:
[email protected] (J.L.G.F.),
[email protected] (J.M.C.-M.). Fax: +34 915854760. Home page: http://www.icp.csic.es/eac/index.htm. † CSIC. ‡ Centro de Tecnologı´a Repsol YPF.
alternative for commercial PO processes.1 However, its commercialization has been hindered largely by the supply of H2O2. H2O2 is now almost exclusively produced by the anthraquinone process; production costs, investment in equipment, and the expense of H2O2 transportation are too high for the production of bulk chemicals like PO. The strategies that have so far been proposed to address the problems of H2O2 include (i) integration of the anthraquinone process for H2O2 synthesis with the propene epoxidation process catalyzed by TS-1.5 In such a combined process, a suitable solvent system has to be developed to facilitate not only the epoxidation of propene but also the sequential hydrogenation and oxidation of alkyl-anthraquinone and (ii) direct synthesis of H2O2 from H2/O2 mixtures with supported palladium (or palladium alloy) catalysts in acidified solvents (water, methanol).6,7 The second option is the more interesting option. The direct reaction of H2 + O2 f H2O2 is clearly the simplest method to form hydrogen peroxide, thereby reducing capital investment and operating costs. The catalysts described in the literature are based on noble metals or combinations thereof supported on a wide variety of substrates such as alumina, silica, and carbon.7 Besides the heterogeneous catalyst, acids are often incorporated into the reaction medium to delay or prevent the decomposition of hydrogen peroxide, and the addition of halides is necessary to obtain hydrogen peroxide.7 However, operating with highly concentrated acid solutions requires special equipment to avoid corrosion. Furthermore, the presence of acid solutions and halogen ions favors the solution of the active metal7 (platinum group), whose primary consequence is catalyst deactivation. In the case of palladium catalyst, colloidal Pd is believed to be involved in a catalytic cycle that may, in part, be homogeneous. Although such a system is of great interest, the management of a colloid would be difficult in a commercial process, as its recovery is nonviable at the very low dissolved metal concentration used. In addition, the use of high concentrations of promoters in the reaction medium, for example, acid promoters,
10.1021/ie800245r CCC: $40.75 2008 American Chemical Society Published on Web 06/11/2008
8012 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008
Figure 1. Flowchart of the novel integrated process for synthesizing PO based on direct H2O2 synthesis in neutral solutions and the catalyzed epoxidation of propene.
halogenated products, and/or other additives, requires the addition of stabilizers, involving the laborious processing of the H2O2 solution prior to its use in oxidation reactions. Acidic supports allow for working with neutral solutions and are therefore often used to reduce the impact of the promoters. Excellent yields of hydrogen peroxide have recently been reported using neutral solutions and heterogeneous catalysts consisting of functionalized carbons with sulfonic acid groups,8 or sulfonic acid functionalized polystyrene resins (PS-SO3H).9,10 Our results on the epoxidation of linear alkenes with aqueous hydrogen peroxide using titanium-silica catalysts on silica revealed high efficiency.11–14 These catalysts are simple to prepare and have good mechanical strength and high thermal stability, especially in comparison with the usual catalysts employed in the epoxidation of alkenes with hydrogen peroxide, such as those with Ti incorporated into the framework of zeolites. In view of this, we decided to go one step further in our on-site H2O2 strategy by combining the direct synthesis of nonacidic solutions of H2O2 with the catalyzed epoxidation of alkenes (Figure 1). We call this propylene oxide production based on the direct on-site synthesis of hydrogen peroxide, abbreviating it to HPPO. Experimental Section Direct Hydrogen Peroxide Synthesis. Catalyst was prepared using mesoporous ion-exchange resins functionalized with sulfonic groups (K2641, Lewatit Bayer AG). In a first step, the resin was washed three times with acetone, using equal volumes of solvent and resin. A resin suspension (4 g) was then stirred with 50 mL of methanol. A Pd(II) acetate solution (86 mg) in acetone (20 mL) was added to this suspension dropwise. The suspension was concentrated to half of the solvent in a rotary evaporator; the temperature of the bath was 318 K. The remaining solution was filtered off, and the solid obtained was washed and air-dried at 333 K for 2 h. This procedure leads to the synthesis of very active and selective catalysts, as we have shown in a previous paper. 9,10 This catalyst was tested in the direct synthesis of hydrogen peroxide. A total of 1.6 g of the catalyst was placed in an
autoclave with 150 g of a methanol:water mixture (96:4) and 24 ppm of HBr, and the mixture was heated to 313 K. The system was then pressurized with a H2:O2:N2 mixture with a total flow of 2500 mL N min-1 at 9.5 MPa without stirring, after which stirring was starting (1500 rpm) to initiate the reaction. Hydrogen consumption was determined by GC-TCD using a Varian CP-4900 microGC device. Hydrogen peroxide and water concentrations were measured by iodometric and Karl Fischer standard titrations, respectively. Alkene Epoxidation. The catalysts were prepared as follows. Titanium isopropoxide (Aldrich, reagent grade) (0.75 g) was dispersed in cyclohexanol (150 mL). The solution was heated to 423 K under stirring conditions, and silica (5.0 g; Grace Davison G-952; specific area, 310 m2 g-1; pore volume, 1.5 mL g-1) was then added to the solution, maintaining the mixture under vigorous stirring at 423 K for 2 h. The solid thus obtained was filtered off and washed twice with 150 mL of hot solvent. The solid was dried at 383 K and finally calcined at 773 K for 5 h. This procedure also leads to the synthesis of very active and selective catalysts, as previously noted.12–14 We tested the use of nonacidic solutions of hydrogen peroxide synthesized by two different epoxidation procedures (Figure 1). Epoxidation of Oct-1-ene at Atmospheric Pressure. These epoxidation reactions were carried out batch-wise in a mechanically stirred 250 mL thermostatic glass reactor equipped with thermometer, reflux condenser, and a septum for withdrawing samples. In a typical run, oct-1-ene (0.2 mol), 11 g of solvent, and 1 g of catalyst were mixed in the reactor, and the suspension was heated at 333 K. Subsequently, 4 g of a 5 wt % organic solution of H2O2 was added dropwise to the reactor suspension over 0.5 h. This procedure was adopted to simulate a continuousstirred tank reactor (CSTR). These activity data can then be extrapolated to an industrial process. We decided to select oct1-ene because it is well-known that primary aliphatic alkenes are the most difficult to epoxidize, and hence it is a useful model to describe the reactivity in the epoxidation of propene to PO in the industrial process of PO production without coproduct. We employed an excess of alkene, because hydrogen peroxide is the most costly reactant in the PO process.
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Epoxidation of Propene. The hydrogen peroxide solution was diluted with tert-butanol. A continuous stirred tank reactor, equipped with a filter to maintain the catalysts inside the reactor, was loaded with 13.88 g of powdered catalyst. The reactor was heated to reaction temperature (323-363 K), and then 319.5 g/h of propene and 222.2 g/h hydrogen peroxide solution diluted with tert-butanol were fed continuously into it. The organic compounds were analyzed on a GC-FID HewlettPackard 6890-plus device equipped with a HP-WAX capillary column. Hydrogen peroxide consumption was evaluated by standard iodometric titration. Epoxide selectivity was related to the hydrogen peroxide converted according to the equation: S(%) ) 100 × [Epoxide] ⁄ ([H2O2]0 - [H2O2])
(1)
where the subscript 0 stands for initial values and all concentrations are expressed on a molar basis. Results and Discussion Direct Hydrogen Peroxide Synthesis. In a first step, a study was made of the variation of several reaction conditions operating under semibatch conditions with a continuous gas flow but static liquid solutions. We studied the effect of hydrogen and oxygen concentration, as well as initial water concentration in the liquid. The hydrogen concentration in the gas feed was varied between 1 to 3.6 molar %, keeping the concentration of oxygen constant at 46.5% O2, by balancing with N2. The total flow was 2500 mL N min-1, and the overall pressure was fixed at 9.5 MPa. We did not study higher concentrations of hydrogen because the flammability concentration limit for hydrogen is 4%. Hydrogen is the limiting reactant because the stoichiometric ratio regarding oxygen is 1:1 and the solubility of oxygen in methanol is higher than hydrogen.15 Hence H2 concentration has a decisive influence on reaction rate. Under these reaction conditions, a high conversion of hydrogen is reached in all concentrations studied. As a general rule, the higher concentration of H2 employed results in an increase in reaction rate, although it is worth considering that Pd2+ and/or PdO species can be reduced to metallic palladium in an excess of H2. If so, the selectivity toward hydrogen peroxide drops because metallic palladium favors secondary reactions, such as the decomposition and hydrogenation of formed hydrogen peroxide.7 The optimum hydrogen concentration reached, outside flammability limits, is 3.6:46.4:50 H2:O2:N2. For comparative purposes, we have plotted the hydrogen peroxide concentration profile of these optimum conditions in an experiment using 2% hydrogen, with these conditions being published previously.8 The concentration of hydrogen peroxide clearly increases linearly in both experiments (Figure 2), but H2O2 concentration is higher operating at 3.6% of hydrogen during all experiments. The final hydrogen peroxide concentration reached is 9.6% versus 6.3%, and without a notable loss of selectivity, 77 versus 79%. In both cases, hydrogen conversion is higher than 90%. Once the hydrogen concentration had been selected, the oxygen concentration was varied between 20 and 60%, while keeping the concentration of hydrogen constant (3.6%). The reduction in oxygen concentration in the gas reactor feed leads to a decrease in hydrogen peroxide concentration (Figure 3). This effect is much more pronounced for O2 concentration below 30%. In addition, the highest oxygen concentration selected (60%) also causes a drop in hydrogen peroxide production. These data indicate that the optimum oxygen concentration in the feed falls within the range 45-46%. A possible interpretation of these results involves considering two opposite effects. In the region of low oxygen concentrations,
Figure 2. Dependence between H2O2 concentration and reaction time for catalyst EG-2 for two different H2 concentrations at the reactor inlet.
Figure 3. Dependence between H2O2 concentration and reaction time for catalyst EG-2 for four different O2 concentrations at the reactor inlet.
an excess of reducing species (hydrogen and methanol) exists on the surface of catalyst particles. Under these conditions, reduced palladium species are favored, and this reduced Pd surface catalyzes the secondary reactions of water formation.7 By contrast, when oxygen concentration is high, that is, 60% O2 in the feed, the catalyst surface becomes saturated by adsorbed oxygenated species, which limits the access of hydrogen to metal sites. Thus, an oxygen concentration of around 45-46% in the feed can be expected to yield a moderate coverage of Pd particles while keeping the other fraction of the surface available to adsorb hydrogen. As the reaction of H2O2 formation takes place on the palladium surface between adsorbed species, probably in the atomic state, its rate is expected to be strongly dependent on the coverage. The high sensitivity of the H2O2 formation rate on the O2 partial pressure or, in other words, on the coverage of palladium particles by adsorbed oxygen, emphasizes the need to optimize reaction conditions and thus reach the highest yield of H2O2. In this process, methanol is used as a carrier/solvent for the hydrogen peroxide produced in the first step of the epoxidation unit; methanol is therefore recycled continuously (Figure 1). The correct design of the process requires knowing the concentration of water in methanol that can be fed into the reactor employed in the direct synthesis of H2O2, as the
8014 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 Table 1. Effect of the Variation of Propene/Catalyst Ratio in the Epoxidation of Propene with Hydrogen Peroxide at 323 K Reaction Temperature propene/catalyst weight ratio
% H2O2 conversion
% selectivity of H2O2 to epoxide
100 50 25
25 50 75
52 50 55
Table 2. Effect of the Variation of Reaction Temperature in the Epoxidation of Propene with Hydrogen Peroxide Using a Propene/ Catalyst Ratio ) 25
Figure 4. Operation stability of the direct synthesis of hydrogen peroxide.
methanol/water mixtures will be separated by distillation and the capital investment and operating costs of this purification stage are largely influenced by the water concentration allowed in the methanol recycle stream. Moreover, as oxygen and hydrogen solubility in water (reaction product) is quite different from that in methanol, it is important to know how water affects the yield of the reaction toward hydrogen peroxide formation. Accordingly, several experiments were conducted varying the initial concentration of water (0-10%) in the reaction mixture. No significant differences in hydrogen peroxide yield were observed in the whole range of water concentration explored; only a slight decrease in activity is observed for the higher water concentration examined, and this decrease can be linked to the lower solubility of both H2 and O2 in water with respect to methanol. Even considering that the effect of water concentration on the H2O2 yield is negligible, it should be noted that large amounts of water are detrimental to the epoxidation reaction because water favors the oxirane ring-opening reaction with subsequent loss of epoxide yield. Thus, we have decided to keep around 4% of water in the methanol stream as the optimum concentration for this two-step HPPO process. Once these variables had been determined, the synthesis reaction was operated for a period of 43 days in on-stream continuous operation. Optimal conditions were recorded when working under semibatch conditions. The reactor was fed with a methanol:water ) 96:4 (by weight) liquid mixture with a residence time of 2 h, H2:O2:N2 ) 4.6:46.4:50 gas mixture with a total flow of 2500 mL N min-1, overall pressure of 9.5 MPa, and a reaction temperature of 313 K. As can be seen in Figure 4, activity data indicate that the reaction is very stable along the time on-stream investigated. A minor change in the conversion of hydrogen (ca. 95%) and selectivity of hydrogen to hydrogen peroxide (ca. 78%) was observed. Moreover, the concentration of hydrogen peroxide was almost unchanged (within (0.1 wt %). Epoxidation of Alkenes. To establish the suitability of using hydrogen peroxide solutions prepared in direct synthesis with no pretreatment, we used solutions of reactor effluent with 9% hydrogen peroxide in methanol/water for the epoxidation of alkenes with amorphous Ti/SiO2 catalysts.12–14 For operating
reaction temperature (K)
% H2O2 conversion
% selectivity of H2O2 to epoxide
323 343 363
75 95 96
55 75 60
simplicity, the first screening was performed with oct-1-ene, as its epoxidation can be performed at atmospheric pressure. For this reaction, we used reaction conditions previously tested in the literature12–14 (see Experimental Section). The H2O2 conversion was 93%, and efficiency reached 95% at 1 h of reaction taken from the moment hydrogen peroxide was added. Epoxide was the only organic product detected from oct-1-ene. Several methods for direct hydrogen peroxide synthesis employ acidic solutions, usually 0.1-1.0 M H2SO4 or H3PO4.7 For comparative purposes, we simulated these acidic solutions of hydrogen peroxide by mixing with a 1 wt % H2SO4 in water and then tested this solution without pretreatment in the epoxidation of oct-1-ene. A 60% selectivity of H2O2 to products derived from oct-1-ene was observed, and no epoxide formation was detected. From results of converted oct-1-ene, the selectivity to methyl-ethers (2-methoxy-octan-1-ol and 1-methoxy-octan2-ol) was 98% and 2% to 1,2-octanediol. Once the suitability of hydrogen peroxide solutions prepared by direct synthesis was proven in the epoxidation of oct-1-ene, we carried out the same experiments in propene epoxidation, which is the alkene of industrial interest. For the reaction test, we started with the reaction conditions used with oct-1-ene, although the epoxidation rate of propene is higher than that of oct-1-ene. Accordingly, certain reaction parameters have to be optimized, including reaction temperature, molar ratio of catalyst/propene, propene/H2O2, residence time, and temperature. For these studies, we started with a reaction temperature of 323 K and selected 45 min as the optimum residence time. This residence time was fixed for all subsequent studies. The propene/ catalyst ratio was varied between 100 and 10. Reaction data showed that an increase in the catalyst amount results in an increase in the conversion of hydrogen peroxide, but the selectivity of hydrogen peroxide was similar for the entire range examined (Table 1). The lower propene/catalyst ratio was discarded because it is difficult to work with a high concentration of solids in a slurry reactor; for this reason, the optimum ratio selected was 25. The next step in this research was the study of reaction temperature variation (323 to 363 K). A rise in reaction temperature from 323 to 343 K produces a sharp increase in hydrogen peroxide conversion (Table 2), but a further increase in temperature to 363 K does not lead to an increase in H2O2 conversion. Nevertheless, hydrogen peroxide selectivity to epoxide follows a different trend: it initially increases when the temperature is raised from 323 to 343 K and then decreases from 343 to 363 K (Table 2). These data indicate that hydrogen peroxide conversion increases with temperature but decomposes at the highest temperatures on the walls of the reactor, thereby
Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8015 Table 3. Effect of the Variation of H2O2/Catalyst Ratio in the Epoxidation of Propene with a Propene/Catalyst Ratio ) 25 and 343 K Reaction Temperature H2O2/catalyst weight ratio
% H2O2 conversion
% selectivity of H2O2 to epoxide
1:1 1:2 1:4
95 95 95
75 80 96
decreasing epoxidation yield. Indeed, reaction temperature must be carefully tuned to maintain a high H2O2 conversion while recording the maximum epoxide yield. In addition, the H2O2/ catalyst ratio was varied from 1:1 to 1:4 (Table 3). The results show that H2O2 conversion was unchanged and very high (95%) for all the H2O2/catalyst ratios studied; however, hydrogen peroxide selectivity to epoxide was found to increase upon decreasing the H2O2/catalyst ratio. Thus, a 96% selectivity is achieved when using a H2O2/catalyst ratio ) 1:4 (Table 3). Finally, the epoxidation reaction was conducted in continuous mode under the optimum reaction conditions selected (343 K, H2O2/catalyst ratio ) 1:4, propene/catalyst ratio ) 25, residence time 45 min). When steady state was reached, the conversion level of H2O2 attained 96% with hydrogen peroxide selectivity to PO of 95%. Upon running the reaction for 135 h, a slight decrease in hydrogen peroxide selectivity to epoxide was observed, with H2O2 conversion decreasing from 96 to 80%. These data indicate that the catalyst undergoes slight deactivation, but this deactivation is reversible as original activity is fully recovered by subjecting the used catalyst to air atmosphere at 873 K. Conclusions PO synthesis by a combined two-step process based on the direct synthesis of hydrogen peroxide and propene epoxidation (HPPO) with a heterogeneous catalyst based on highly disperse titanium supported on silica has been successfully recorded. This new process is based on the direct synthesis of nonacidic solutions of H2O2, which are then fed, without pretreatment, to an epoxidation unit. Once the operation variables had been optimized (methanol:water ) 96:4 (by weight), residence time of 2 h, H2:O2:N2 ) 4.6:46.4:50 gas mixture with a total flow of 2500 mL N min-1, overall pressure of 9.5 MPa, and a reaction temperature of 313 K), this HPPO process operated for a period of 43 days of on-stream continuous operation. Experimental data confirmed that activity appeared very stable along the time onstream investigated. Only a very small change in the conversion of hydrogen (ca. 95%) and selectivity of hydrogen to hydrogen peroxide (ca. 78%), with hydrogen peroxide concentration remaining within ( 0.1 wt %, was observed.
Acknowledgment The authors acknowledge financial support from Repsol-YPF (Spain) and the Spanish Ministry of Science and Education in the projects PSE-310200-2006-2 and FIT-320100-2006-88. G.B.-B. and M.C.C.-S. gratefully acknowledge fellowships granted by Repsol-YPF. Literature Cited (1) Nijhuis, T. A.; Makkee, M.; Moulijn, J. A.; Weckhuysen, B. M. The production of propene oxide: Catalytic processes and recent developments. Ind. Eng. Chem. Res. 2006, 45, 3447. (2) Kobe J. M.; Evans, W. E.; June, R. L.; Lemanski, M. F. EpoxidationIndustrial. In Encyclopedia of Catalysis; (Horva´th, I. T. Ed.; Wiley-VCH, Weinhein (Germany), 2003; Vol. 3, 246. (3) Merlau, M. L.; Borg-Breen, C. C.; Nguyen, S. B. T. EpoxidationHomogeneous. In Encyclopedia of Catalysis; (Horva´th, I. T. Ed.; WileyVCH, Weinhein (Germany), 2003; Vol. 3, p 155. (4) Buijink, J. K. F.; van Vlaanderen, J. J. M.; Crocker, M.; Niele, F. G. M. Catal. Today 2004, 199, 93–95. (5) Wang, C.; Wang, B.; Meng, X.; Mi, Z. Study on process integration of the production of propylene oxide and hydrogen peroxide. Catal. Today 2002, 74, 15. (6) Zhao, J.; Zhou, J.; Su, J.; Guo, H.; Wang, X.; Gong, W. Propene Epoxidation with In-Site H2O2 Produced by H2/O2 Non-Equilibrium Plasma. AIChE J. 2007, 53 (12), 3204. (7) Campos-Martin, J. M.; Blanco-Brieva, G.; Fierro, J. L. G. Hydrogen peroxide synthesis: An outlook beyond the anthraquinone process. Angew. Chem., Int. Ed. 2006, 45, 6962. (8) Paparatto, G.; d’AloisioR., de AlbertiG.; Furlan, P.; Arca, V.; Buzo´n, R. (Enichem S. p. a.) New catalyst, process for the production of hydrogen peroxide and its use in oxidation processes. EP 0978316, 2000 (Chem. Abstr. 2000, 132, 142643). (9) de Frutos, M. P.; Campos-Martin, J. M.; Fierro, J. L. G.; CanoSerrano, E.; Blanco-Brieva, G. (REPSOL QUIMICA S.A.) Catalyst and process to obtain hydrogen peroxide, EP1344747, 2003. Chem. Abs. 2003, 139, 247564. (10) Blanco-Brieva, G.; Cano-Serrano, E.; Campos Martin, J. M.; Fierro, J. L. G. Direct synthesis of hydrogen peroxide solution with palladiumloaded sulfonic acid polystyrene resins. Chem. Commun. 2004, 1184. (11) Campos-Martin, J. M.; de Frutos, M. P. (REPSOL QUIMICA S.A.) Process for epoxidation of olefinic compounds with hydrogen peroxide. WO9948884, 1999 (Chem. Abstr. 2000, 131, 257434. (12) Capel-Sanchez, M. C.; Campos-Martin, J. M.; Fierro, J. L. G.; de Frutos, M. P.; Padilla Polo, A. Effective alkene epoxidation with dilute hydrogen peroxide on amorphous silica-supported titanium catalysts. Chem. Commun. 2000, 855. (13) Capel-Sanchez, M. C.; Campos-Martin, J. M.; Fierro, J. L. G. Influence of solvent in the synthesis steps of titanium-supported amorphous silica epoxidation catalysts. J. Catal. 2003, 217, 195. (14) Capel-Sanchez, M. C.; Campos-Martin, J. M.; Fierro, J. L. G. Influence of the textural properties of supports on the behaviour of titaniumsupported amorphous silica epoxidation catalysts. J. Catal. 2005, 234, 488. (15) Graaf, O. H.; Smit, H. J.; Stamhuls, E. J.; Beenackers, A. A. C. M. Gas-liquid solubilities of the methanol synthesis components in various solvents. J. Chem. Eng. Data 1992, 37, 146.
ReceiVed for reView February 11, 2008 ReVised manuscript receiVed March 28, 2008 Accepted April 4, 2008 IE800245R
