J. Phys. Chem. C 2007, 111, 17427-17436
17427
Kinetic Study of Propylene Epoxidation with H2 and O2 over a Gold/Mesoporous Titanosilicate Catalyst Juan J. Bravo-Sua´ rez,† Jiqing Lu,†,‡ Carlos Gregorio Dallos,£ Tadahiro Fujitani,† and S. Ted Oyama*,†,§ Research Institute for InnoVation in Sustainable Chemistry, National Institute of AdVanced Industrial Science and Technology, AIST Tsukuba West, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan, Zhejiang Key Laboratory for ReactiVe Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal UniVersity, Jinhua 321004, China, Escuela de Ingenierı´a Quı´mica, UniVersidad Industrial de Santander, A.A. 678, Bucaramanga, Colombia, and EnVironmental Catalysis and Nanomaterials Laboratory, Department of Chemical Engineering (0211), Virginia Polytechnic Institute and State UniVersity, Blacksburg, Virginia 24061 ReceiVed: June 30, 2007; In Final Form: August 26, 2007
The kinetics of propylene oxidation to propylene oxide (PO) with H2/O2 mixtures on gold supported on the mesoporous titanium silicate, Ti-TUD, was investigated using Langmuir-Hinshelwood (L-H) models and power-rate law (PRL) models. The catalyst gave stable activity and was appropriate for the kinetic studies, giving high selectivity to PO (>95%) at low conversions of propylene ( propylene > H2. Similar traces were obtained for samples of Si-TUD, Au/Si-TUD, and Ti-TUD but are not shown. Figure 3 shows van’t Hoff-type plots of the adsorption amounts of H2, O2, and propylene for the various
17430 J. Phys. Chem. C, Vol. 111, No. 46, 2007
Figure 1. Approach to steady state of the Au-Ba/Ti-TUD (9) catalyst. Reaction conditions: C3H6 ) H2 ) O2 ) 10.1 kPa, Ar ) balance, SV ) 7000 cm3 h-1 gcat-1, T ) 423 K, P ) 0.1 MPa.
Figure 2. Temperature-programmed desorption traces of H2, O2, and propylene from Au-Ba/Ti-TUD (9) adsorption at 323 (top), 373 (middle), and 423 K (bottom). Relative scale bars are shown.
Figure 3. Adsorption data in van’t Hoff form.
samples. Table 1 presents a summary of the adsorption data for each sample, gas, and temperature. Figure 4 shows the results of in situ FTIR measurements at reaction conditions on the working Au-Ba/Ti-TUD (9) catalyst. Bands arising at 2980, 2939, and 2880 cm-1 are assigned to the C-H stretching vibrations of bidentate propoxy species, which can result from PO decomposition on acidic Ti sites.41
Bravo-Sua´rez et al.
Figure 4. In situ Fourier transform infrared spectroscopy for propylene epoxidation on Au-Ba/Ti-TUD (9) (a) before reaction in He at 423 K; (b) under C3H6 ) H2 ) O2 ) 10.1 kPa, Ar ) balance, SV ) 7000 cm3 h-1 gcat-1, T ) 423 K, P ) 0.1 MPa at the beginning of the reaction; (c) under reaction conditions after 5 h; (d) the difference spectra after the steady-state, (c) - (d); and (e) after switching to He for 0.25 h at 423 K.
Table 2 presents the dependence of the formation rates of PO, CO2, and H2O on the partial pressures of C3H6, H2, and O2 at 423 K and 101.3 kPa of total pressure. The data were obtained at propylene conversions lower than 6% and PO selectivities ranging from 85 to 100%. The rates are expressed in units of mmol kgcat-1 h-1 and can be converted into turnover frequencies (TOF, s-1) based on gold surface atoms by the conversion factor (1/S)(1/3600), where S is the surface gold content (5.28 µmol gcat-1). Gold is the appropriate basis (rather than Ti) because it is present in much smaller quantities than Ti (about 1/100), and the H2O2 formation rate on Au is known to be slower than the epoxidation rate on Ti. The formation rates are labeled with four shading gradations from light to dark, indicative of the rates from a low to high level. The partial pressure levels are denoted as low (L ) 2.9 kPa), medium (M ) 10.1 kPa), and high (H ) 20.2 kPa). It can be clearly seen that the formation rates increased with the partial pressure of each reactant. For example, when the partial pressures of O2 and C3H6 were both high and the H2 partial pressure was increased from low to high, the PO formation rate rose from 290 to 925 mmol kgcat-1 h-1. Similarly, when the partial pressures of H2 and O2 were both medium and the propylene partial pressures were increased from low to medium to high, the PO formation rates rose from 237 to 388 and 454 mmol kgcat-1 h-1, respectively. Also, when the H2 and propylene partial pressures were both medium and the O2 partial pressures were increased from low to medium to high, the PO formation rates rose from 307 to 388 and 468 mmol kgcat-1 h-1, respectively. Similar trends were also found for the CO2 and H2O formation rates. The highest PO formation rate (925 mmol kgcat-1 h-1) was obtained when partial pressures of all three reactants were high. This corresponds to a PO formation yield of 54 g kgcat-1 h-1. One experiment was carried out to determine the effect of varying the water vapor partial pressure at steady-state. It was found at 423 K and 101.3 kPa with the H2, O2, and propylene partial pressures being 10.1 kPa (10% each) that the propylene conversion was unchanged at 1.1% and the PO selectivity was invariant at 98.5% when water vapor was introduced at a level of 3.2 kPa (3.2%) in the feed. Table 3 summarizes the results of the fitting of the kinetic data in Table 2 to various rate expressions. The table shows the rate expressions in the second column, an explanation of the origin of the expression in the third column, the rate
Propylene Epoxidation with H2 and O2
J. Phys. Chem. C, Vol. 111, No. 46, 2007 17431
TABLE 1: Summary of Adsorption Amounts on Different Samples H2 adsorption (µmol g-1)
O2 adsorption (µmol g-1)
C3H6 adsorption (µmol g-1)
-heat of adsorption (kJ mol-1)
sample
323 K
373 K
423 K
323 K
373 K
423 K
323 K
373 K
423 K
H2
O2
C3H6
Si-TUD Au/Si-TUD Ti-TUD Au-Ba/Ti-TUD (9)
0.57 1.43 11.3 43.5
0.28 0.44 3.88 15.0
0.14 0.19 1.44 4.88
2.89 10.0 45.6 159.0
1.32 1.67 14.3 57.8
0.82 0.57 5.26 18.8
1.56 5.39 38.4 96.0
0.55 1.96 11.0 34.3
0.32 0.95 4.96 12.7
16.2 23.0 23.3 24.7
14.4 32.7 24.5 24.1
18.3 19.7 23.3 22.9
TABLE 2: Formation Rate of PO, CO2, and H2O as a Function of the Partial Pressures of H2, O2, and Propylene
parameters in the fourth column, the calculated F value and regression coefficient (for comparison) in the fifth column, and the relative model probability of being the correct model in the last column. A detailed explanation of each rate expression will be presented in the next section. 4. Discussion 4.1. General Aspects. The catalyst chosen for this investigation, Au-Ba/Ti-TUD (9) was selected because of its stability. As in the case of the Au/titanosilicate catalysts studied previously by the group of Moulijn17 and the Au/TS-1 catalysts studied by the group of Delgass,18-20 the Ti is highly dispersed and in the form of isolated tetrahedral units, as indicated by a UV-vis band at 230 nm and absence of intensity above 300 nm.23 Poisoning by adsorption of PO is believed to occur on adjacent Ti-O-Ti sites to form a bidentate intermediate.41 The barium component is in the form of an inert carbonate and was added to induce higher precipitation of the gold component.23 The catalyst activity as a function of time (Figure 1) shows an initial drop in activity but stabilization after 6 h. The
attainment of stable activity permitted the kinetic measurements described in this study. The epoxidation of propylene with hydrogen-oxygen mixtures on gold-based catalysts has been studied extensively, and considerable information is available on the mechanism. In the original paper by Haruta and co-workers on Au/TiO2,10 several important points were made, which would later be confirmed by other researchers. First, the combination of Au and TiO2 was indispensable for the reaction as neither Au nor TiO2 alone exhibited any notable catalytic activity or selectivity between 303 and 573 K. Second, Au in ultrafine particle form was unique in its properties, as Pt or Pd gave the hydrogenation product propane, Cu gave CO2, and Ag was inactive. Third, Au on a TiO2/SiO2 support was especially active for PO formation. Finally, the PO formation rate increased with increasing concentration of H2 and O2 and suggested that an active oxygen species was formed from their reaction. Not mentioned in this first work was the low catalyst stability, a characteristic of gold catalysts with high titania contents.42
17432 J. Phys. Chem. C, Vol. 111, No. 46, 2007
Bravo-Sua´rez et al.
TABLE 3: Summary of Kinetic Fits model
comments
rPO ) k(H2)l(O2)m(C3H6)n
1
rPO ) k
2
(H2)(O2)(C3H6) [1 + R(H2) + β(O2) + γ(C3H6)]3
(H2)(O2)(C3H6)
3
rPO ) k [1 + R(H2) + β(O2)]2[1 + γ(C3H6)]
4
rPO ) k
[
5
rPO ) k
[
6
rPO ) k ln[1 + R(H2)]ln[1 + β(O2)]ln[1 + γ(C3H6)]
7
rPO ) k
8 rPO )
a
][
β(O2)
γ(C3H6)
1 + R(H2) 1 + β(O2) 1 + γ(C3H6) R(H2)1/2
][
β(O2)
][
]
]
γ(C3H6)
[
R(H2)1/2(O2)
] [
1 + R(H2)1/2(O2)
(H2)1/2
γ(C3H6)
k ) 24.7 l ) 0.596
m ) 0.248 n ) 0.359
F ) 2.11 R2 ) 0.986
24.6
generic model Langmuir-Hinshelwood competitive site (single site)
k ) 8.75 R ) 0.0243
β ) 0.0790 γ ) 0.0586
F ) 24.0 R2 ) 0.882
1.9
generic model Langmuir-Hinshelwood independent sites (two sites)
k ) 10.6 R ) 0.0307
β ) 0.0760 γ ) 0.442
F ) 10.1 R2 ) 0.944
2.0
F ) 6.77 R2 ) 0.959
5.4
F ) 5.83 R2 ) 0.955
6.3
generic model Frumkin-Temkin independent sites
k ) 31.2 R ) 0.212
β ) 7.21 γ ) 1.52
F ) 3.90 R2 ) 0.975
10.3
coupled cycles Langmuir-Hinshelwood
k ) 257 R ) 0.113
γ ) 0.225
F ) 4.50 R2 ) 0.968
5.2
independent cycles Langmuir-Hinshelwood
R ) 2.78 × 1011 F ) 5.34 R2 ) 0.969 β ) 0.0189 δ ) 9.52 × 109 γ ) 0.109 kD /κ ) 4.44 (δ(H2)1/2 . 1 R/δ ) 29.3)
11.8
independent cycles hybrid model
k ) 82.4 m ) 0.250
32.6
1 + γ(C3H6)
R(O2)(H2)(H2)1/2
(C3H6)
[1 + β(O2) + γ(O2)(H2) ][1 + δ(H2) ] [kD/κ + (C3H6)] 1/2
(C3H6) [kD/κ + (C3H6)]
β ) 0.440 γ ) 0.223
generic model k ) 1.62 × 106 β ) 0.443 Langmuir-Hinshelwood R ) 1.57 × 10-4 γ ) 0.224 independent sites (three sites) dissociative H2 adsorption
]
calculated F value, Pm,N+1 regression coefficient (%)b
generic model power-rate law model
generic model k ) 2345 Langmuir-Hinshelwood R ) 0.0511 independent sites (three sites)
1 + R(H2)1/2 1 + β(O2) 1 + γ(C3H6)
rPO ) k(H2)l(O2)m
9
][
R(H2)
calculated parametersa
1/2
l ) 0.611 kD/κ ) 4.36
F ) 1.76 R2 ) 0.988
The parameters have units so that the rate is given in mmol kgcat-1 h-1 when pressures are in kPa. b Model relative probability.
4.2. Adsorption of Propylene, Oxygen, and Hydrogen. The first study directed to examining possible surface intermediates in propylene epoxidation on gold catalysts was carried out by the group of Moulijn using infrared spectroscopy.41 This work was conducted on deactivating Au/TiO2 catalysts as well as on stable PO catalysts consisting of highly dispersed gold on silicatitania and TS-1.17 It was found that the adsorption of propylene was reversible on these catalysts between 300 and 473 K and that no irreversible adsorbed species were formed. It was therefore concluded that deactivation on Au/TiO2 was not due to irreversible nonoxidative interactions of propylene with the catalyst. On the adsorption of propylene, Haruta and co-workers had earlier reported that the rate of PO formation was independent of C3H6 concentration and proposed that it was strongly adsorbed on the catalyst surface to near saturation.10 However, these kinetics were obtained on a deactivating catalyst and are probably not general.22 The adsorption of hydrogen and oxygen on gold catalysts is well documented through studies of the hydrogen oxidation reaction. On silica-supported gold nanoparticles, Naito and Tanimoto43 observed that large gold particles (0.1-5 wt % Au/ SiO2) largely catalyzed H2O formation while small gold particles ( Ti-TUD > Au/Si-TUD > Si-TUD (Table 1). For Au-Ba/Ti-TUD (9), the adsorption amounts at the reaction temperature of 423 K were 4.88 µmol g-1 for H2, 18.8 µmol g-1 for O2, and 12.7 µmol g-1 for propylene. It is useful to compare these quantities with the amounts of the active elements
Propylene Epoxidation with H2 and O2 in the sample, which were 5.28 µmol g-1 for surface Au and 476 µmol g-1 for the dispersed Ti component. It can be deduced that, at least for O2 and propylene, some of the adsorption probably occurred on the Ti component because the adsorbed quantities exceed the surface gold atoms available. However, adsorption at 423 K on the Ti-TUD support alone gave substantially lower adsorption amounts of 1.44 µmol g-1 for H2, 5.26 µmol g-1 for O2, and 4.96 µmol g-1 for propylene. This strongly indicates that the presence of gold is somehow enhancing adsorption on the titanium sites, probably by a spillover mechanism. The plain support does not contain barium, but this is likely to be inconsequential, as the barium is in the form of carbonate and is not expected to interact strongly with the gaseous species. Gold by itself does not seem to adsorb much, as the quantity adsorbed at 423 K on Au/Si-TUD is very low and similar to that on the plain Si-TUD support itself. Yet, the total titanium sites, comprising 476 µmol g-1, are much more numerous than the adsorption quantities, suggesting that the enhanced adsorption is occurring on Ti sites, and specifically on limited Ti sites, perhaps in a localized area close to the gold particles or their periphery. The amounts of H2, O2, and propylene adsorbed at lower temperatures on the materials based on Ti-TUD are much higher than those based on the Si-TUD samples but still less than the total amount of Ti in the materials, suggesting that these molecules are weakly held in association with Ti sites. Indeed, prior work on the adsorption of species like H2O or NH3 on titanosilicate indicates strong interactions between these molecules and Ti centers.45,46 Since H2, O2, and propylene are weaker Lewis bases, their interactions are expected to be weaker but still higher than those with the silicious materials. It should be noted that our measurements were carried out in a flow system at different temperatures and required a purge ( 5.94). Similar observations are noted when comparing the models’ relative probabilities (Pm,N+1). The PRL has the highest model relative probability (26.8%) of being the correct model, in line with the F test, while the other models have probabilities lower than about 10%. A lot is known about the mechanism of the reaction, which can be used to derive more meaningful rate expressions. Considerable evidence exists that the role of gold is to oxidize hydrogen to form hydrogen peroxide or an intermediate oxidizing agent51-54 and that the role of titanium is to carry out the epoxidation step.17,55,56 The group of Delgass22 has carried out a kinetic evaluation of the reaction on a stable Au/TS-1 catalyst and proposed a mechanism based on the above evidence. Key assumptions were that hydrogen peroxide is produced on gold sites to form one intermediate (HOOH-S1), that propylene adsorbs on titanium sites to form another intermediate (C3H6S2), and that these react in a single rate-determining step to form the product propylene oxide. The resulting rate expression is reproduced as model 7 in Table 3. The model is also statistically meaningful with a calculated F value of 4.50 that ranks behind the top two generic models considered earlier. These authors
(8)
where A is a species and x is the exponent. In this manner, they arrived at the power-rate law form presented as model 1, which describes their data well. Their fit is probably good, as it is in our case, because in the range of partial pressures considered, the coverages are not changing drastically, and the (A)x approximation can be used accurately. The mechanism of the group of Delgass assumes that the hydrogen peroxide species on the gold site reacts with the propylene species associated with the titanium sites in a ratedetermining step. This means that the catalytic cycles on the gold and titanium sites are necessarily coupled and turning at the same rate. This is illustrated in Figure 5b. The mechanism would also require that the two sites be in close proximity. Our adsorption data discussed earlier do not rule this out. However, the synchronization of the cycles appears to be an unnecessary requirement. This is particularly the case because it is known that the epoxidation step with hydrogen peroxide is much more rapid than the synthesis of hydrogen peroxide. On TS-1 and other titanosilicates, the epoxidation occurs readily at close to room temperature,55,57,58 more than 100 K below the temperature of the H2/O2 plus propylene reaction. Although the former are measured in the liquid phase, there is no reason to believe the situation would change in the gas phase. The lack of a compelling reason for concluding that the cycles were operating in a coupled manner motivated us to explore the possibility that the two cycles operate in an independent fashion. A means of decoupling the cycles occurring on the Au and Ti sites is to recognize that the hydrogen peroxide produced on the Au could migrate to the Ti sites to carry out the epoxidation reaction but could also separately decompose. This is illustrated in Figure 5c. Separate expressions may be derived for the rate of hydrogen peroxide synthesis, the rate of propylene oxide production, and the rate of hydrogen peroxide reaction. This can be done by assuming (a) the steps of hydrogen peroxide synthesis proposed by Barton and Podkolzin,44 (b) separate adsorption steps of hydrogen peroxide and propylene on Ti with a rate-determining reaction between the adsorbed entities, and (c) a first-order decomposition of hydrogen peroxide. Three rate expressions need to be solved simultaneously for a solution; however, they can be simplified to get model 8.21 Because its calculated F value (5.34) is lower than the tabulated F at a 95% confidence level and (10,4) degrees of freedom (5.96),34 the model is deemed to describe the data appropriately. Examination of the fitting parameters reveals that the adsorption constant for hydrogen in the last term in the denominator is very large, δ ) 9.52 × 109, so that 1 + δ(H2)1/2 may be approximated as δ(H2)1/2. This results in an identical equation as that in model 7, the model derived from the assumption of coupled cycles, which leads to the interesting conclusion that different models and assumptions can lead to the same expression. This actually is not unexpected, as it is a well-known maxim in kinetics that a mechanism cannot be proved or disproved from just the validity of a rate expression. However, model 8 can be refined slightly by assuming that the left side can be expressed as a PRL expression. This gives rise to the expression shown in model 9, which represents a hybrid model since it combines both L-H and PRL forms. The reason this is presented is that the resulting expression gives the best fit with the lowest calculated F value (1.76) and the highest model relative probability of being the correct model (32.6%). It is interesting
Propylene Epoxidation with H2 and O2 that this expression can be derived from both the assumption of coupled cycles or independent cycles. 4.5. Assessment of Models using Adsorption Data. Beyond the indications from the F test and the model relative probabilities, the models can be evaluated by their concordance with adsorption data. Recalling that the adsorption amounts at the reaction temperature of 423 K were 4.88 µmol g-1 for H2, 18.8 µmol g-1 for O2, and 12.7 µmol g-1 for propylene gives an order of O2 > C3H6 > H2 for the amounts. In model 1, the PRL equation is rPO ) k(H2)0.60(O2)0.25 (C3H6)0.36, where the order of exponents is H2 > C3H6 > O2, which is the reverse of the order of adsorption amounts. However, from eq 8, the expression for adsorption (A)/[1 + K(A)] ≈ (A)x gives, in the case of strong adsorption (K(A) . 1), x f zero, and in the case of weak adsorption (K(A) , 1), x f 1; therefore, the exponent x is inversely related to the adsorption amount. Thus, the exponents in the PRL equation agree with the adsorption data. No quantitation of this trend will be made with this or other models because the quantities involved are obtained in different experiments. Specifically, the rate parameters were obtained at reaction conditions, while the adsorbed amounts were determined for the individual gases at reaction temperature. Although the agreement may seem fortuitous, if correct, the results would indicate that the various species are adsorbing together at reaction conditions (but not necessarily on the same sites) and that there are no strong tendencies for displacement of one species for another. In models 2 and 3, which consist of one- and two-site L-H models, respectively, the equilibrium adsorption constants R, β, and γ are in the order C3H6 > O2 > H2, which conflict with the adsorption data. This indicates that one- and two-site models are not consistent with the adsorption data. Note also that these models had F values that were too high and were thus not statistically meaningful. The same held true for model 4, which assumed an unlikely associative H2 adsorption. In model 5, which consists of a L-H equation with three independent sites and dissociative H2 adsorption, examination of the expression shows that the equilibrium adsorption constants R, β, and γ are in the order O2 > C3H6 > H2, which follows the order of adsorption. Thus, this model is in agreement with the adsorption data. In model 6, which consists of a nonuniform surface model with independent sites, the equilibrium constants R, β, and γ are in the order O2 > C3H6 > H2, which follows the order of adsorption. This model is also consistent with the adsorption data. In model 7, which assumes coupled cycles on Au and Ti, the exponent R combines two equilibrium constants for the formation of hydrogen peroxide from H2 and O2. Since these reactants are on opposite ends of the sequence for adsorption, O2 > C3H6 > H2, the combination of the constants does not permit comparison to the equilibrium constant for propylene, and the adsorption data cannot be used to evaluate this model. In model 8, which assumes independent cycles on Au and Ti, the same problem arises as that in model 7, and the adsorption data do not provide independent verification. Model 8 can be simplified to give the same rate expression as model 7, with different interpretation of the rate parameters. In hybrid model 9, the adsorption of propylene is taken to occur by a L-H process, while the dependencies on H2 and O2 are described with a PRL expression. The exponents l and m are consistent with the observed adsorption order O2 > H2, but although the dependency on C3H6 cannot be directly assessed in the same manner, a transformed expression for the C3H6 term
J. Phys. Chem. C, Vol. 111, No. 46, 2007 17435 (from eq 4, x ≈ 1 - ln[1 + K(A)]/ln(A)), in the studied partial pressure interval of 2.9-20.2 kPa, gives equivalent orders in the 0.42-0.52 range, which lie between those of O2 and H2. Therefore, it can be concluded that, overall, the expression is supported by the adsorption data. In summary, the generic models power-rate law, L-H model with three independent adsorption sites (dissociative H2 adsorption), and the model for a nonuniform surface (models 1, 5, and 6) were supported by the adsorption results. The model with the coupled cycles (model 7) was indeterminate, as was the model with sequential cycles (model 8). The semiempirical hybrid equation (model 9) was consistent with the adsorption experiments. In terms of the calculated F values, the statistically meaningful models fell in the following order of decreasing fit, model 9 > model 1 > model 6 > model 7 > model 8 > model 5. 5. Conclusions A kinetic study of the propylene epoxidation reaction with H2/O2 mixtures at 423 K was carried out on a stable gold catalyst supported on the mesoporous titanosilicate, Ti-TUD. The data were obtained using a factorial design and were fit using various rate expressions that had terms for the adsorption of the H2, O2, and C3H6 reactants. Independent adsorption measurements of the reactants were carried out at the same temperature of reaction, and the amounts adsorbed were found to decrease in the order O2 > C3H6 > H2. Six generic models were evaluated, which included power-rate law (PRL), Langmuir-Hinshelwood (L-H), and Frumkin-Temkin nonuniform surface equations. The models incorporating three independent sites (dissociative H2 adsorption) and nonuniform surface sites were consistent with the adsorption experiments. L-H models with competitive sites (one adsorption site), two adsorption sites, and three adsorption sites (associative H2 adsorption) did not describe the data appropriately as evaluated from an F test. Two different models based on the existence of coupled cycles or independent sequential cycles gave the same rate expression. The expression gave a moderate fit to the reaction data but could not be evaluated with the adsorption data. The best rate expression was obtained by a variation of this last expression rPO ) k(H2)l (O2)m (C3H6)/[kD/κ + (C3H6)], which incorporated elements of PRL and L-H expressions and was supported by the adsorption data. Acknowledgment. The authors are grateful for financial support from the Ministry of Economy, Trade and Industry (METI, Minimum energy chemistry project) and the National Science Foundation. J.J.B.-S. and S.T.O. acknowledge support from the Japan Society for the Promotion of Science (JSPS) through the Postdoctoral Fellowship for Foreign Researcher Program (No. P05627) and the Invited Fellow Program. References and Notes (1) Kirschner, M. Chem. Mark. Rep. 2004, 266, 31. (2) Buijink, J. K. F.; van Vlaanderen, J. J. M.; Crocker, M.; Niele, F. G. M. Catal. Today 2004, 93-95, 199. (3) Trent, D. L. Propylene oxide. In Kirk Othmer Encyclopedia of Chemical Technology, on-line edition; John Wiley & Sons: New York, 2001. (4) Mimoun, H.; Mignard, M.; Brechot, P.; Saussine, L. J. Am. Chem. Soc. 1986, 108, 3711. (5) Monnier, J. R. Appl. Catal., A 2001, 221, 73. (6) Oyama, S. T.; Murata, K.; Haruta, M. Shokubai (Catalysts and Catalysis) 2004, 46, 13. (7) Lu, J. Q.; Bravo-Sua´rez, J. J.; Takahashi, A.; Haruta, M.; Oyama, S. T. J. Catal. 2005, 232, 85. (8) Lu, J. Q.; Bravo-Sua´rez, J. J.; Haruta, M.; Oyama, S. T. Appl. Catal., A 2006, 302, 283.
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