O2 over Bioreduction Au

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Vapor-Phase Propylene Epoxidation with H2/O2 over Bioreduction Au/TS-1 Catalysts: Synthesis, Characterization, and Optimization Guowu Zhan, Mingming Du, Daohua Sun, Jiale Huang,* Xin Yang, Yao Ma, Abdul-Rauf Ibrahim, and Qingbiao Li* Department of Chemical and Biochemical Engineering, and National Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, and Key Lab for Chemical Biology of Fujian Province, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, P. R. China ABSTRACT: Au/TS-1 catalysts could be prepared by immobilizing the biosynthesized Au sol on TS-1 supports. A variety of techniques, such as N2 physisorption, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectorscopy, UV visible diffuse reflectance, and thermogravimetric analysis, were employed to characterize both the supports and the bioreduction catalysts. The influence of various parameters (Si/Ti molar ratio, Au loading, immobilization pH, reaction temperature, and space velocity) on the catalytic performance in vapor phase propylene epoxidation with H2/O2 was systematically analyzed. Parameter optimization results manifested that the optimum catalytic activity and stability of bioreduction Au/TS-1 catalysts were obtained under optimum operation conditions of Si/Ti molar ratio of 35, Au loading of 1 wt %, immobilization pH of 2, reaction temperature of 573 K, and space velocity of 4000 8000 mL gcat 1 h 1. Furthermore, efforts were also made to clarify the plausible reaction routes over the bioreduction catalysts.

1. INTRODUCTION Propylene oxide (PO), a versatile chemical intermediate, is one of the bulk products in the petrochemical industry. Commercially, PO production is limited to liquid phase methods, namely, chlorohydrin and hydroperoxidation processes.1 However, both processes suffer from serious disadvantages; the former should meet two disposal problems (brine and chlorinated byproducts), while the latter produces equal amounts of coproducts requiring heavy capital investment. Recently, direct vaporphase propylene epoxidation using H2/O2, pioneered by Haruta in 1998,2 has received intense research due to its higher atom efficiency. For this purpose, Au was found to exhibit higher catalytic activity than other metals under extensive researches. In the past decade, series of different support materials have been used as carriers of Au nanoparticles for vapor phase propylene epoxidation, including TiO2, Ti-SiO2, Ti-TUD, TS-1, Ti-MCM-41, TiMCM-48, Ti-HMS, 3D mesoporous titanosilicates, and so forth.2 9 In particular, TS-1 as a support is our preferred choice due to its remarkable catalytic performance, as noted by Delgass et al.8,10 A survey of published literature on commercial standards7 shows that there is still no economically viable process in vapor-phase propylene epoxidation. This clearly indicates that new catalysts are needed to overcome the problems of low propylene conversion, poor stability, low H2 efficiency and/or low PO selectivity. On the other hand, the past decade has witnessed intensive research in the biosynthesis of Au nanoparticles with biological organisms (plants or microorganisms), a more economical and environmentally friendly manner as compared to conventional physical and chemical methods,11,12 with surprising results.13 However, few reports have actually contributed to the catalytic application of biosynthesized Au nanoparticles to date. The only attempt, perhaps, was the recent demonstration by Zepeda and co-workers,14 who obtained Au(AgAu)/SiO2 Al2O3 catalysts for oxidation and hydrogenation of CO, using a bioreduction wetness impregnation r 2011 American Chemical Society

method. Since biological approach has been demonstrated as a wellestablished method for synthesizing Au nanoparticles, it is therefore imperative to prepare active Au catalysts based on such an approach, hence, our proposal for the preparation of Au catalysts employing biosynthesized Au nanoparticles. In this present work, Au catalysts were prepared by solimmobilization (SI) technique through the immobilization of biosynthesized Au sol onto TS-1 supports, where the SI technique was somewhat similar to those in literature.15 17 The Au sol was obtained by the reduction of chloroauric acid with Cacumen Platycladi (CP) extract (bioreduction). This bioreduction method is advantageous because the size of the Au particles can be controlled. Another advantage is that the Au is already reduced (by the extract) before its combination with the support. Both the Au catalysts and the supports were characterized using different techniques, such as N2 physisorption, transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectorscopy (FTIR), UV visible diffuse reflectance (DRUV-vis), and thermogravimetric (TG) analysis. Optimization of the catalytic performance of the bioreduction Au/TS-1 catalysts in vapor phase propylene epoxidation with H2/O2 was done, including both the catalyst preparation factors (Si/Ti molar ratio, Au loading, immobilization pH) and the reaction operating conditions (reaction temperature, space velocity). This work aimed at developing the catalytic application of the biosynthesized Au nanoparticles and determining the optimal technological parameters for vapor phase propylene epoxidation with H2/O2 over bioreduction Au/TS-1 catalysts. Received: January 16, 2011 Accepted: June 21, 2011 Revised: June 12, 2011 Published: June 21, 2011 9019

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Table 1. Overview of Bioreduction Au/TS-1 Catalyst Samples samplea

support

Au loading (wt %)

DTS-1 (nm)b

SBET (m2/g)

VP (cm3/g)c

DP (nm)d

DAu (nm)e

Au/TS-135/0.25%

TS-135

0.25

162 ( 19

400

0.35

6.3

2.2 ( 0.4

Au/TS-135/0.5%

TS-135

0.5

162 ( 19

383

0.33

6.3

3.0 ( 0.6

Au/TS-135/1.0%

TS-135

1.0

162 ( 19

376

0.32

6.2

4.2 ( 0.8

Au/TS-135/1.5%

TS-135

1.5

162 ( 19

368

0.29

6.2

4.6 ( 1.1

Au/TS-135/2.0%

TS-135

2.0

162 ( 19

346

0.27

6.2

6.6 ( 2.2

Au/TS-135/2.5%

TS-135

2.5

162 ( 19

337

0.27

6.4

8.6 ( 3.2

Au/TS-120/1.0%

TS-120

1.0

193 ( 25

368

0.32

6.4

4.6 ( 0.6

Au/TS-150/1.0% Au/TS-175/1.0%

TS-150 TS-175

1.0 1.0

154 ( 20 130 ( 16

379 395

0.29 0.29

5.7 5.4

4.6 ( 0.5 4.7 ( 0.9

Au/TS-1100/1.0%

TS-1100

1.0

126 ( 20

392

0.28

7.1

4.4 ( 0.9

a

All catalysts were prepared under immobilization pH of 2. b Diameter of TS-1 support. c Pore volume. d Pore diameter. e Diameter of Au nanoparticles on catalyst.

2. EXPERIMENTAL SECTION 2.1. Preparation of CP Extract. The CP (purchased from Xiamen Jiuding Drugstore, China) was first milled (with a Philip blender) and then sieved (20 mesh). A 1.0 g portion of the sieved power was dispersed in 100 mL deionized water in a water bath shaker (at 303 K, 150 rpm) for 4 h. The mixture was then filtrated to remove residual biomass and the filtrate (i.e., CP extract) was used for the experiments. 2.2. Preparation of TS-1 Supports. All chemical reagents were purchased from Sinopharm Chemical Reagent Co. Ltd., China, and were used without treatments. TS-1 supports were prepared according to the established procedure.18 Tween 20 (2 g) was dissolved in deionized water (24 mL), followed by the addition of tetrapropylammonium hydroxide (TPAOH, 40 wt %, 27 mL) while stirring. Tetraethyl silicate (TEOS, 38.6 mL) was slowly added to the above solution under vigorous stirring for 1 h. Afterward, a dropwise of tetrabutyl titanate (TBOT, 1.8 mL) dissolved in isopropyl alcohol (IPA, 20 mL) was added (still under vigorous stirring) for another 1 h. The resulting solution was then crystallized (at 443 K, 18 h) under autogenous pressure. Finally, the solid recovered from centrifugation was washed with distilled water, dried in vacuum oven (at 323 K, 12 h) and calcined (at 823 K, 5 h) in air. The as-synthesized support was named TS-135, where the subscripts denote the Si/Ti molar ratio. Based on the experiments conditions (discussed below), TS-1 supports (with other Si/Ti molar ratio) were also prepared similarly, except for the dosage of TBOT. 2.3. Preparation of Au/TS-1 Catalysts. Au sol was synthesized by the reduction of the Au precursor (chloroauric acid) with CP extract. Addition of CP extract (30 mL) to the aqueous chloroauric acid solution (30 mL, 0.5 mM) led to a sharp color change (pale yellow to brownish red), indicating the formation of Au sol. After 30 min of complete sol generation, the Au sol was immobilized by adding TS-1 support under vigorous stirring condition. After 90 min the suspension was filtered, washed thoroughly with distilled water, and dried in a vacuum oven (at 323 K, 6 h). The catalysts were activated by calcination (at 623 K, 2 h) in air. To examine the influence of the catalyst parameters on the reaction performance, a series of catalysts with a Au loading range of 0.25 2.5 wt % and Si/Ti molar ratio range of 20 100 were prepared using the SI technique with immobilization pH range of 0 10. The catalysts were labeled by their Au loading and the Si/Ti molar ratio. For example, Au/TS-135/1.0% corresponds to the catalyst with Si/Ti molar ratio of 35 and Au loading of 1.0 wt %.

2.4. Catalyst Characterization. N2 physisorption experiments were measured at 77 K on a Micromeritics TriStar 3000 porosimetry analyzer, using static adsorption procedures. The surface area was calculated with Brunauer Emmett Teller (BET) method. The total pore volume was obtained by the single point desorption at P/P0 = 0.99. The average pore diameter was determined using the Barret Joyner Halenda (BJH) method applied to the adsorption branch of the isotherms. TEM observations were performed on a Phillips Analytical FEI Tecnai 30 electron microscope (300 kV). XRD analysis was conducted on a Phillips Panalytical X’pert Pro diffractometer equipped with Cu KR radiation (40 kV, 30 mA). FTIR spectra were recorded on a Nicolet Avatar 660, where the samples were ground with KBr and pressed into thin wafer. DRUV vis spectra were collected on a Varian Cary 5000 spectrometer equipped with a diffuse-reflectance accessory, using dehydrated BaSO4 as a reference. TG studies were carried out on a Netzsch TG209F1 thermobalance under flowing air atmosphere at a heating rate of 10 K min 1 from 295 to 1073 K. 2.5. Catalyst Evaluation. The reaction was carried out in a vertical flow reactor by using 150 mg of the catalyst at 1 atm. The compositions of the feed gas (C3H6/O2/H2/N2) were adjusted to 10/10/10/70 (vol %) with a specific space velocity during a test, and the temperature was measured with a thermocouple located at the center of the catalyst bed. The concentrations of the reactant and product concentrations were measured online by two gas chromatographs (GC), equipped with a TCD (Porapak Q packed column) detector and a FID (Porapak T packed column) detector, respectively. The substances, detected by GC analysis, were propylene, PO, acetone, ethanal, acrolein, CO2, H2, N2, O2, and H2O. Unless otherwise mentioned, the data were measured after 1 h of steady-state activity. The experimental results allowed calculations of the values of propylene conversion, PO selectivity, H2 efficiency, and PO formation rate.

3. RESULTS AND DISCUSSION 3.1. Characterization of TS-1 and Au/TS-1. 3.1.1. N2 Physisorption and TEM Observations. Fresh catalyst samples were

analyzed by N2 physisorption and TEM to determine their different characteristics, including BET surface, pore size, pore volume, Au size, and TS-1 support size. The results (in Table 1) show that both the BET surface and pore volume decrease with increasing Au loading, indicating that the Au nanoparticles were immobilized within the channels of TS-1 support which thereby 9020

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Figure 1. XRD spectra of TS-1 supports varying Si/Ti molar ratios: (a) 100, (b) 75, (c) 50, (d) 35, and (e) 20. Au/TS-135 catalysts varying Au loading: (f) 0.25%, (g) 0.5%, (h) 1.0%, (i) 1.5%, (j) 2.0%, (k) 2.5%.

Figure 3. DRUV vis spectra of TS-1 supports varying Si/Ti molar ratio: (a) 100, (b) 75, (c) 50, (d) 35, (e) 20. Au/TS-135 catalysts varying Au loading: (f) 0.25%, (g) 0.5%, (h) 1.0%, (i) 1.5%, (j) 2.0%, (k) 2.5%.

Figure 2. FTIR spectra of Au/TS-1/1% catalysts varying Si/Ti molar ratio: (a) 20, (b) 35, (c) 50, (d) 75, (e) 100, and the TS-135 support (f).

Figure 4. Comparison of catalytic performance of bioreduction Au/TS1/1.0% varying Si/Ti molar ratio. Reaction condition: space velocity, 4000 mL gcat 1 h 1; temperature, 573 K; feed pressure, 1 atm (all data taken at 1 h of time-on-stream).

led to the reductions of the BET surface and pore volume. Analysis of the average pore size indicates that the role of the Au nanoparticles is of interest in the extension as they cause slight increase of the catalyst pore size. As expected the size of TS-1 decreased monotonously with increasing Si/Ti molar ratio.19 Furthermore, the average diameters of the Au particles decreased with decreasing Au loading; consistent with previous report.2 3.1.2. XRD Analysis. The XRD patterns of the TS-1 support and Au/TS-1 catalysts are shown in Figure 1. All patterns were similar to the well-defined crystalline TS-1 (MFI structure); evidenced by the disappearance of splitting peaks at 2θ = 24.5° and 2θ = 29.5°.20 XRD analysis of the Au catalysts show that there were no obvious characteristic diffraction peaks of Au specie with less than 1 wt % Au loading, because they were highly dispersed on the catalysts. Accordingly, the intensity of the metallic Au (111) peaks at 38.3° increased with increasing Au loading (indicated by the dash frame). 3.1.3. FTIR Analysis. The absorption FTIR band at 960 cm 1 (assigned to the stretching vibration of Si O Ti bonds through tetrahedral coordination) is considered as the fingerprint of the TS-1 material.21 As shown in Figure 2, the intensity of the 960 cm 1 FTIR band was considerably attenuated with the increase of Si/Ti molar ratio, indicating lesser Ti contents into the framework of SiO4 tetrahedra. Furthermore, the FTIR spectrum of Au/TS-135 catalyst (curve b) was almost identical with that of the pure support (TS-135, curve f), revealing no change in the crystallinity of TS-1 after Au immobilization. 3.1.4. DRUV vis Analysis. The quality of the TS-1 samples was further determined by DRUV vis spectroscopy shown in Figure 3

(curves a e). All the spectra were similar, with strong absorptions at 210 nm associated with the isolated environment of Ti4+ in TS-1 structure.22 However, the absorption shoulder in the region 280 340 nm, representing the presence of extra framework of TiO2, was only present in the sample with the highest Ti content (TS-120). The spectra of the Au/TS-1 catalysts in Figure 3 (curves f k) show one characteristic absorption band around 540 nm, at higher Au loadings, corresponding to the surface plasmon resonance of Au nanoparticles.6 3.2. Epoxidation of Propylene over Bioreduction Au/TS-1 Catalysts. 3.2.1. Influence of Si/Ti Molar Ratio. Five TS-1 samples varying Si/Ti molar ratio were used. Figure 4 shows the effect of Si/Ti molar ratio on the catalytic performance of Au/TS-1 catalysts. As shown, propylene conversion was almost proportional to the content of Ti. The catalyst with Si/Ti molar ratio of 35 exhibited the highest propylene conversion and PO selectivity, hence the maximum PO formation rate (78 gPO kgcat 1 h 1). The catalyst with the highest Si/Ti ratio, that is, the lowest Ti content (Au/TS-1100) gave the lowest propylene conversion, PO selectivity, and H2 efficiency. These results suggest that Ti is a necessity for the catalytic reaction as it catalyzes the formation of intermediate species for PO production.23,24 However, the catalytic performance of Au/TS-120 catalyst was inferior to that of Au/TS-135 probably due to the presence of extra framework Ti species, as evidenced by DRUV vis spectra (Figure 3). The H2 efficiency increased monotonously with the increase of Ti content because of the sufficient and available sites of the Lewis acids in the higher Ti content.6 The acidic species inhibit the 9021



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decomposition of H2O2 and limit the combustion of H2 to H2O.25 Therefore, catalysts with higher Ti content facilitate H2 utilization for PO production resulting enhanced H2 efficiency. Since the TS-135 support was the best candidate for the catalytic reaction here, the influence of other parameters on the catalytic performance was studied with TS-135 as the support. 3.2.2. Influence of Au Loading. Gold loading is shown to be particularly important for propylene epoxidation.8,10 From Figure 5, the Au loading of the catalysts significantly affects the catalytic performance. Both propylene conversion and PO formation rate initially rose and then fell with the increase of Au loading (0.25 to 2.5 wt %), whereas PO selectivity was slightly changed. The inferior performance with the low Au loading might be attributed to the inadequate presence of active gold sites, and thus lower activity of propylene epoxidation. Continuously increasing the Au loading to 2.5 wt %, however led to the decrease in propylene conversion which was closely associated with the presence of larger Au particles (Figure 6) less active for PO formation.19 Therefore, an optimum Au loading of sufficient active sites with small Au sizes is required for optimum performance. The catalysts with 1.0 wt % Au showed the maximum propylene conversion (12%) and PO formation rate (76 gPO kgcat 1 h 1). As far as the H2 efficiency was concerned, it decreased significantly with increasing Au loading, indicating the size-dependent activity of the Au nanoparticles. Smaller Au nanoparticles are capable of producing H2O2 (from H2 and O2) via a reaction pathway with lower activation barrier26 and

consequently higher H2 utilization efficiency. From the results here, the Au loading of 1.0 wt % was more desirable. 3.2.3. Influence of Immobilization pH. The effect of different immobilization pH (0 10) is shown in Figure 7; and reveals that immobilization pH is a dominated parameter that affects the ultimate catalyst performance. This is essentially due to the fact that the actual amount of Au to be immobilized depends on the immobilization pH. In our work, the isoelectric point (IEP) of the TS-1 support was about 2.5. Since the Au sol is often negatively charged, the electrostatic interaction between the support and the Au sol proceeds quickly as the surface of the support is positively charged with a pH value below that of the IEP. Hence, a pH value below that of the IEP benefited the solimmobilization process. However, the very low immobilization pH (pH value of 0) resulted in inferior catalytic activity due to the low stability of the Au sol.27 On the contrary, when the pH value was above that of the IEP, both the support and the Au sol were negative charged; and thus, spontaneous immobilization of Au on the TS-1 surface did not occur resulting in incomplete Au loading.15,16 The explanation was confirmed by investigating the sol-immobilization filtrate through UV vis analysis (Figure 8) to determine the complete or otherwise loading of Au on the support.15 The results confirmed that immobilizing Au under basic conditions (above the IEP) led to the incomplete loading of Au (curves f and g). Likewise, the variation of H2 efficiency in Figure 7 indicated that catalysts prepared under acidic conditions possessed high H2 efficiency, probably, owning to the formation of H2O2 promoted by the acidity of the catalyst. At the

Figure 5. Comparison of catalytic performance of bioreduction Au/TS-1 varying Au loadings. Reaction condition: space velocity, 4000 mL gcat 1 h 1; temperature, 573 K; feed pressure, 1 atm (all data taken at 1 h of time-onstream).

Figure 7. Comparison of catalytic performance of bioreduction Au/TS1 varying immobilization pH. Reaction condition: space velocity, 4000 mL gcat 1 h 1; temperature, 573 K; feed pressure, 1 atm (all data taken at 1 h of time-on-stream).

Figure 6. Representative TEM images and Au size distributions of bioreduction catalysts. (a) Au/TS-135/0.25%; (b) Au/TS-135/1.0%; (c) Au/TS-135/2.5%. 9022



Figure 8. UV vis spectra of the sol-immobilization filtrate under different immobilization pH values: (a) 0, (b) 1, (c) 2, (d) 4, (e) 6, (f) 8, and (g) 10. The inset is UV vis spectra of Au sol and CP extract.

Figure 9. Catalytic performance of bioreduction catalyst (Au/TS-135/ 1.0%) as a function of reaction temperature. Reaction condition: space velocity, 4000 mL gcat 1 h 1; feed pressure, 1 atm (all data taken at 1 h of time-on-stream).

immobilization pH of 2, no trace of Au species was detected in the filtrate (by both atomic absorption spectrophotometer (AAS) and UV vis analysis) indicating the total immobilization of Au.17 Again, the propylene conversion was the highest at that pH. Therefore, the immobilization pH of 2 was the best for preparation of the bioreduction catalysts for propylene epoxidation. 3.2.4. Influence of Reaction Temperature. It is a well-known fact that reaction temperature plays an important role in the formation of PO. Figure 9 shows the effect of reaction temperature (473 613 K) on the catalytic performance. As shown, the propylene conversion increased with increasing reaction temperature while the PO selectivity decreased monotonously. When the reaction temperature was as low as 473 K, the PO selectivity (81%) was high, whereas the propylene conversion (3%) was very low. However, the elevated temperature led to the reaction shifting to more favored pathways (toward CO2 and ethanal), thereby decreasing the PO selectivity (as depicted in Figure 10). When the reaction temperature was raised to 613 K, PO selectivity decreased to less than 50%. In contrast, little ethanal was formed below 473 K, but its selectivity grew significantly to 20% at 613 K. Furthermore, H2 efficiency was very sensitive to the reaction temperature. As the reaction temperature was raised to 613 K, the H2 efficiency decreased severely to 11%, indicating most of the H2 converted to water instead of PO. The reaction temperature of 573 K was the best for the catalytic reaction toward about 12% propylene conversion, 68% PO selectivity, 77 gPO kgcat 1 h 1 PO formation rate, and 27% H2 efficiency. It is about 100 K higher than that required for nonbioreduction Au catalysts. Haruta and Qi reported that silylated Au catalysts

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Figure 10. Effect of reaction temperature on products selectivity in propylene epoxidation over bioreduction catalyst (Au/TS-135/1.0%): Ac, acetone; An, acrolein; EA, ethanal; PO, propylene oxide. Reaction condition: space velocity, 4000 mL gcat 1 h 1; feed pressure, 1 atm (all data taken at 1 h of time-on-stream).

Figure 11. Catalytic performance of bioreduction catalyst (Au/TS-135/ 1.0%) as a function of space velocity. Reaction condition: temperature, 573 K; feed pressure, 1 atm (all data taken at 1 h of time-on-stream).

required higher temperature; about 100 K higher than the optimum temperature for nonsilylated Au catalyst.28 This led us to believe that the residual plant biomass on the catalyst (validating by the following TG analysis) might modify bioreduction catalysts like silylation, which needs further investigations. Moreover, based on the reaction temperature investigations, the Arrhenius plots for the formation of PO, CO2, acetone, ethanal, and acrolein gave apparent activation energies of 29, 64, 26, 58, and 35 kJ mol 1, respectively. These activation energy values are close to those reported in literature,10,19 suggesting the reactions occur via similar mechanisms. 3.2.5. Influence of Space Velocity. The influence of the pseudocontact time, expressed as space velocity (volume of feed flow per catalyst weight per time), on the propylene conversion and products selectivity was investigated. The catalytic performance and the product selectivity versus space velocity were illustrated in Figure 11 and Figure 12, respectively. As shown, with increasing space velocity the propylene conversion dropped rapidly, whereas the PO selectivity and H2 efficiency consistently went up. This indicates that at very high space velocity (24000 mL gcat 1 h 1), there is insufficient contact time for the reactants leading to insufficient reaction. Similarly, at very low space velocity, poor performance also appeared due to the further combustion of PO during the long contact time. Not all of the oxidation of H2 to H2O is retarded by the high space velocity (short contact time) resulting in higher H2 efficiency. It is important to note that PO formation rate is defined as the weight of PO formed per catalyst weight per time (h); therefore, higher 9023


Industrial & Engineering Chemistry Research space velocities will result in higher rates of PO formation despite the poor conversion of propylene. Therefore, it will be confusing to compare the catalytic performance based on PO formation rate both here (Figure 11) and in the works of literature; unless, of course it is for those with the same space velocities. As a consequence, the preferred space velocities are the same with those widely used in literature (between 4000 and 8000 mL gcat 1 h 1).7,29 31 3.2.6. Plausible Reaction Routes over the Catalysts. From the analysis (particularly the influence of temperature and space velocity) and based on our experimental results, we propose a reaction scheme for propylene epoxidation over bioreduction catalysts in Scheme 1. The selectivity of acrolein was almost independent of the contact time (Figure 12), indicating that acrolein was one of the major primary products at the initial stage of the reaction. Hence, the reaction scheme was based on the assumption that all partial oxidation products other than acrolein went through PO first, which was similarly demonstrated in literature.19,32 As shown in Scheme 1, there are other competing (side) reactions besides the objective reaction (epoxidation of propylene to PO): two parallel reactions of propylene oxidation; two consecutive reactions of PO; and one combustion reaction of H2. The ultimate PO selectivity depended, of course, on the relative rates of the first five reactions (Scheme 1), which in turn depended on a number of factors (discussed above). Our results

Figure 12. Effect of space velocity on products selectivity in propylene epoxidation over bioreduction catalyst (Au/TS-135/1.0%): Ac, acetone; An, acrolein; EA, ethanal; PO, propylene oxide. Reaction condition: temperature, 573 K; feed pressure, 1 atm (all data taken at 1 h of time-onstream).

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showed that the apparent activation energies of CO2 (64 kJ mol 1) and ethanal (58 kJ mol 1) were similar (to one significant figure), indicating they mainly formed in the same reaction (reaction 5). The assumption was supported by the kinetic experiments of Oyama3 who showed that CO2 was formed primarily from the further oxidation of PO (reaction 5) rather than the oxidation of propylene (reaction 3). Further oxidation of PO should be considered, especially when the space velocity was very low or the reaction temperature was very high, as PO would further crack to ethanal and CO2 (reaction 5) or isomerize to acetone (reaction 4). 3.2.7. Stability of Au/TS-1 Catalysts. In general, Au catalysts suffer from rapid deactivation.6,31 However, the bioreduction Au/TS-1 catalysts do not have this problem. Figure 13 illustrates the catalytic performance over operating time and reveals that there was no significant decrease in both the activity and the selectivity during the long time test indicating remarkable stability. The catalyst sample was quenched at 48 h of time-onstream for TEM analysis, revealing that the Au remained much the same particle-size (4.5 nm) without agglomeration. To probe the contributors to catalyst stability, the residual plant biomass on the bioreduction catalysts were analyzed by TG and differential TG (DTG) analysis (Figure 14). According to the TG DTG curves, four weight loss peaks above 673 K presented in the catalyst sample, corresponding to the desorption of residual plant biomass which accounted for 0.4 wt % of the total catalyst weight. Since the plant biomass was demonstrated to be

Figure 13. Catalytic performance of bioreduction catalyst (Au/TS-135/ 1.0%) as a function of time-on-stream. Reaction condition: space velocity, 4000 mL gcat 1 h 1; temperature, 573 K; feed pressure, 1 atm.

Scheme 1. Proposed Reaction Scheme for Propylene Epoxidation with H2/O2 over Bioreduction Catalyst

9024



Figure 14. Simultaneous TG and DTG analysis profiles: (a) TS-135 support, (b) Au/TS-135/1% catalyst.

responsible for stabilization of the nanoparticles,11 it is reasonable to assume that the residual plant biomass benefited the stability of the bioreduction Au/TS-1 catalyst, due to the prevention of Au from agglomeration. In addition, plant biomass might also contribute to the catalyst stability by inhibiting the formation of deactivating compounds (propoxy species) via changing the catalyst surface property.33 Works toward the detailed mechanism of the stability of bioreduction catalysts are underway in our group.

4. CONCLUSIONS In conclusion, we have designed the new bioreduction Au/TS1 catalysts for vapor phase propylene epoxidation with H2/O2, which exhibited both good activity and stability. The optimum operation conditions, Si/Ti molar ratio of 35, Au loading of 1.0 wt %, immobilization pH of 2, reaction temperature of 573 K, and space velocity of 4000 8000 mL gcat 1 h 1, were obtained. In addition, for the first time this work proposed the plausible reaction routes of propylene epoxidation over bioreduction catalysts. Although it would be premature to use the bioreduction method as the general strategy for the catalysts preparation, the present study provides an optimal process for propylene epoxidation over bioreduction catalysts with high PO yields and remarkable stability. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (Q. Li) and [email protected] (J. Huang). Tel.: (+86) 592-2189595. Fax: (+86)592-2184822.

’ ACKNOWLEDGMENT This work was supported by key program (No.21036004) and general program (Nos. 20776120 and 20976146) of the National Natural Science Foundation of China, and the Natural Science Foundation of Fujian Province of China (Nos. 2010J05032, 2010J01052, and 2008J0169). ’ REFERENCES (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.

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