Uncalcined TS-1

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Kinetics, Catalysis, and Reaction Engineering

Surface Engineering and Kinetics Behaviors of Au/Uncalcined TS-1 Catalysts for Propylene Epoxidation with H2 and O2 Gang Wang, Yueqiang Cao, Zhihua Zhang, Jialun Xu, Mengke Lu, Gang Qian, Xuezhi Duan, Weikang Yuan, and Xinggui Zhou Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b03708 • Publication Date (Web): 27 Aug 2019 Downloaded from pubs.acs.org on August 27, 2019

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Industrial & Engineering Chemistry Research

Surface Engineering and Kinetics Behaviors of Au/Uncalcined TS-1 Catalysts for Propylene Epoxidation with H2 and O2 Gang Wang, Yueqiang Cao, Zhihua Zhang, Jialun Xu, Mengke Lu, Gang Qian, Xuezhi Duan*, Weikang Yuan, Xinggui Zhou* State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China * Corresponding

authors: [email protected]; [email protected]

Tel.: +86-21-64253509; Fax.: +86-21-64253528

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Uncalcined TS-1 immobilized Au bifunctional catalysts have been demonstrated to be highly active yet stable for the propylene epoxidation with H2 and O2. The objective of this study is to further engineer the surface properties of uncalcined TS-1 toward enhanced bifunctional catalysis. A strategy by increasing the reduction temperature is proposed to remove the residual TPA+ template on the external surfaces, and the resultant Au/TS-1-B-300 catalyst gives rise to simultaneously enhanced activity, PO selectivity and H2 efficiency. These phenomena are explained by more exposed Ti active sites and targeted catalysts electronic properties based on HAADF-STEM, TGA, UV-vis, FT-IR and XPS measurements. Furthermore, kinetics analysis demonstrates a much lower activation energy for the main reaction to form PO, suggesting the existence of an appropriate reaction temperature for the PO yield. The obtained insights could shed new light on rationally designing and optimizing the catalysts by engineering the surface properties. Keywords: surface engineering, kinetics behaviors, propylene epoxidation, uncalcined TS-1 immobilized Au catalysts.


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Introduction Direct epoxidation of propylene with H2 and O2, as a simple, clean and highly selective process, has been viewed as a dream reaction to produce propylene oxide (PO).1-4 Among the tested catalysts, isolated Ti4+-rich titaniumsilicate-1 (i.e., TS-1) immobilized Au nanoparticles have exhibited state-of-the-art PO formation rates.5-9 In our previous studies,10-12 employing uncalcined TS-1 (i.e., TS-1-B) to immobilize Au nanoparticles has been demonstrated to be a simple yet effective strategy to suppress the catalyst deactivation from the micropore blocking by carbonaceous deposits. Notably, for the employment of TS-1-B, it inevitably suffers from the release of TPA+ template adsorbed on the external surfaces and/or filled in the micropores during the thermal treatment or under the reaction conditions.13,

14

An attempt is therefore

necessary to understand the effects of the thermal/reaction temperature on the release of the template and thus the catalytic performance, aiming to develop highly efficient Au-Ti bifunctional catalysts for the reaction. The released TPA+ template, mainly originating from that adsorbed on the external surfaces due to the higher stability of that filled in the micropores of TS-1-B, has shown to be detrimental to the catalytic performance.14,

15

This indicates that it is very

important to pre-remove such detrimentally released TPA+ species before the reaction, where the TPA+ removal in principle gives rise to significantly different catalyst electronic properties because of its electron-rich nature.16, 17 To this end, we can employ a simple yet commonly used thermal treatment method, where it is crucial to select an appropriate temperature for the thermal treatment toward targeted catalyst properties,



such as no obvious aggregation of Au nanoparticles and balanced electronic properties of the bifunctional Au-Ti active sites.9, 11, 18-20 As a consecutive effort, it is also desirable to understand the kinetics behaviors of the pretreated catalyst with specific catalyst electronic properties and without interference of the released TPA+ template, which would be compared to previously reported un-pretreated catalyst.14 The objective of this study is to reveal the underlying nature of thermal treatment effects on the catalyst structural and electronic properties and thus the kinetics behaviors for direct propylene epoxidation with H2 and O2. Deposition-precipitationurea (DPU) method was employed to prepare Au/TS-1-B catalyst due to the higher Au capture efficiency (> 99%).21-23 Thermal behaviors of both the TS-1-B and catalyst were studied in detail for selecting an appropriate temperature to mainly remove the TPA+ template adsorbed on the external surfaces but doesn’t lead to the occurrence of obvious Au particle aggregation. The catalysts after and before the thermal treatment were tested for the reaction, and a plausible relationship between catalyst structure and performance was established by using multiple techniques, such as HAADF-STEM, TGA, UV-vis, FT-IR and XPS measurements. Finally, kinetics behaviors of the pretreated catalyst were studied and compared to the un-pretreated catalyst to understand the contributions of the released TPA+ template and the changed catalyst electronic properties.

2. Experimental 2.1 Preparation of unpretreated and pretreated Au/TS-1-B catalysts


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Uncalcined TS-1 with the Si/Ti molar ratio of 100 and TPA+ template blocked micropores, i.e., TS-1-B, was hydrothermally synthesized according to the previously reported method.24 Specifically, polyoxyethylene 20 sorbitan monolaurate (Tween 20, 99%) was added into deionized water followed by dropwise addition of tetrapropylammonium

hydroxide

(TPAOH,

25

wt%

in

water)

and

tetraethylorthosilicate (TEOS, 98%), where the molar ratio of TEOS and TPAOH was 1:0.15. Subsequently, a mixture of titanium (IV) tetrabutoxide (TBOT, 99%) and isopropanol (99%) was added to the above solution under vigorous stirring for at least 1 h. The resultant mixture was then crystallized at 443 K for 48 h under autogenous pressure. Finally, the solid was obtained by centrifugation, washed with deionized water and dried at 373 K for 12 h. The deposition of gold onto TS-1-B was carried out by deposition-precipitation-urea method (DPU) as described in literature.21 Specifically, a gram of TS-1-B was added into 40 mL deionized water followed by the addition of HAuCl4 solution and urea. The slurry was heated to 90 oC for 6 h in the absence of light, and then the solid was separated by centrifugation and washed with deionized water. The obtained sample was dried at room temperature overnight under vacuum followed by thermal treatment in a quartz reactor under a gas mixture of 40% H2 in N2 with a flow rate of 50 mL min-1 to obtain Au/TS-1-B-200 or Au/TS-1-B-300 catalyst.

2.2 Catalyst evaluation



Catalytic testing of the two catalysts for the direct propylene epoxidation with H2 and O2 was performed in a fixed bed reactor, and an gas chromatography equipped with RT-QS-Bond and TDX-01 columns was used to determine the effluent concentrations. Typically, a weighed catalyst (0.15 g) was loaded in a quartz reactor at the center of tube, where the catalyst pellet size is lower than 110 m to rule out the internal diffusion limitation. The catalysts were reduced under a gas mixture of 40% H2 in N2 with a flow rate of 50 mL min-1 at 200 or 300 oC for 2 h, followed by that a feed mixture consisted of 10:10:10:70 vol% of C3H6, H2, O2 and N2 with a space velocity of 14,000 mL h-1 gcat-1 was fed into the reactor. During the reaction, organic compounds including propylene, PO, acrolein, ethanal, acetone and propanal were analyzed via a flame ionization detector (FID), while O2, H2, and CO2 were analyzed via a thermal conductivity detector (TCD) using N2 as the carrier gas to obtain an accurate H2 conversion. Sensitivity factors of FID and TCD signals on gas chromatograms were calibrated, and the conversion of propylene, selectivity toward products and H2 efficiency were calculated as below: Propylene conversion = moles of (2/3ethanal + 1/3CO2 + C3-oxygenates) in the reactor effluent / mole of propylene in the feed. C3-oxygenate selectivity = moles of C3-oxygenates/moles of (2/3ethanal + 1/3CO2 + C3-oxygenates). Ethanal selectivity = 2/3 × (mole of ethanal)/moles of (2/3ethanal + 1/3CO2 + C3oxygenates).


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CO2 selectivity = 1/3 × (mole of CO2)/moles of (2/3ethanal + 1/3CO2 + C3oxygenates). H2 efficiency = mole of PO/mole of converted H2.

2.3 Catalyst characterization Thermal gravimetric analyses (TGA) of the samples were performed on s PerkinElmer Pyris 1 under N2 with the heating rate of 5 K min-1. The bulk structures of the catalysts were determined by XRD analysis on a Cu Kα radiation equipped Phillips Panalytical X’pert Pro diffractometer. The diffuse reflectance UV-vis spectrum was employed to probe the coordination state of Ti by using a Perkin Elmer spectrometer with the reference of BaSO4. High-angle annular dark-field imaging (HAADF) was performed to characterize the Au nanoparticle size distribution on a Tecnai G2 F20 STWIN equipped with a digitally processed scanning transmission electron microscope imaging (STEM) system. Fourier transform infrared spectra (FT-IR) of the two catalysts were collected on a Perkin Elmer Spectrum 100 FT-IR spectrometer using the traditional transmission technique in KBr pellets. The electronic structures of the catalysts were probed by X-ray photoelectron spectrum (XPS) measurements on a Kratos XSAM-800 spectrometer with an Al Kα anode, and the binding energies of the samples were calibrated based on the C 1s peak at 284.8 eV. The gold loading of the catalyst was determined to be 0.1 wt% with the inductively coupled plasma atomic emission spectrometer (ICP-AES) on a Varian 710-ES (Agilent Technologies). 3. Results and discussion



3.1 Crucial role of the reduction temperature As described in the Introduction, selecting an appropriate thermal treatment temperature to pre-remove the TPA+ template adsorbed on the external surfaces of TS1-B immobilized Au (i.e., Au/TS-1-B) catalyst in Figure 1a is crucial to suppress the negative effects of the TPA+ release on the catalytic performance. To this end, TGA measurement was employed to probe the thermal behaviors of the uncalcined TS-1-B and thus select one appropriate thermal treatment temperature. As shown in Figure 1b, the weight loss in the range of 200-300 oC is most likely due to the release of the TPA+ template adsorbed on the external surfaces,13, 25 in addition to those at the lower (< 200 oC)

or higher (> 300 oC) temperature ranges resultant from the release of the adsorbed

water or the TPA+ template filled in the micropores of uncalcined TS-1.26 Along this line, the temperature of 300 oC was selected for the thermal treatment to pre-remove the TPA+ template on the external surfaces, and the typical reaction temperature of 200 oC

was also selected to perform the thermal treatment as the reference to comparatively

investigate the effect of TPA+ template. To further confirm the decomposition behaviors of TPA+ template on the external surfaces, the weight losses of the uncalcined TS-1-B samples pretreated at different temperatures were monitored by TGA measurement. As shown in Figure 1c, the weight of TS-1-B sample after being heated to 200 oC slowly decreases even after being treated for 7 h. In contrast, the weight of the TS-1-B sample in Figure 1d pretreated at 300 oC for 2 h and then cooled down to 200 oC almost remains stable within 5 h. This observation indicates that the TPA+


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template on the external surfaces of the uncalcined TS-1-B can be completely removed after being reduced at 300 oC for 2 h.

Figure 1. (a) Schematic diagram of Au/TS-1-B catalyst. (b) TG and DTG curve of uncalcined TS-1-B sample. (c) TG analysis of TS-1-B hold for 7 h at 200 oC. (d) TG analysis of TS-1-B pretreated at 300 oC for 2 h and then hold at 200 oC for 5 h.

It is noted that the presence of TPA+ template on TS-1-B can increase its isoelectron point from 3 to 7,10 and this allows the deposition of Au onto such TS-1-B with urea as precipitation agent having a high Au capture efficiency (> 99%).21 In this regard, the thermal treatment to remove TPA+ template on the external surfaces of TS-1-B was carried out after Au deposition onto TS-1-B, and the catalyst pretreated at 200 oC (Au/TS-1-B-200) and that pretreated at 300 oC (Au/TS-1-B-300) were comparatively investigated to understand the underlying nature of TPA+ template on the uncalcined



TS-1-B immobilized Au catalysts. HAADF-STEM was first employed to obtain the Au particle size distribution of the two catalysts, and the representative images of Au particle sizes are shown in Figure 2a and 2b. Based on about 200 random particles, the two catalysts are determined with similar Au average particle sizes, i.e., 3.0 nm for the Au/TS-1-B-200 catalyst and 3.1 nm for the Au/TS-1-B-300 catalyst. XRD patterns shown in Figure S1 reveal that the two catalysts exhibit typical MFI structure, and the absence of peaks at 38.4o and 44.4o corresponding to the Au (111) surface and (200) surface (JCPDS Card No. 89-3697) indicates that the aggregation of Au nanoparticles doesn’t occur.27 UV-vis spectroscopy was used to explore the Ti coordination states of the two catalysts, and the results are shown in Figure 2c. Obviously, the absorption bands observed at around 207 and 522 nm are respectively attributed to the isolated tetrahedrally coordinated Ti (Ti(IV)) in the framework and the localized surface plasmon resonance (LSPR) of Au particles.12, 24, 28 Notably, a shoulder peak at around 290 nm on the Au/TS-1-B-200 catalyst, and a smaller one on the Au/TS-1-B-300 catalyst are observed, which seems to be assigned to the presence of TPA+ template.29 To prove that, TS-1-B dried at 120 oC with blocked micropores and TS-1-O calcined at 550 oC with open micropores were also characterized. It can be seen in Figure 2c that compared to TS-1-O, TS-1-B exhibits an obvious shoulder peak at around 290 nm, which is in agreement with the UV-vis results of the two catalysts.


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Figure 2. The representative Au particle images of (a) Au/TS-1-B-200 and (b) Au/TS1-B-300 catalysts obtained by HAADF-STEM. (c) UV-vis spectra of Au/TS-1-B-200, Au/TS-1-B-300, TS-1-B and TS-1-O. (d) PO formation rate, PO selectivity and H2 efficiency over Au/TS-1-B-200 and Au/TS-1-B-300 catalysts at 200 oC.

Catalytic performances of the two Au/TS-1-B catalysts were evaluated at 200 oC and the results are shown in Figure 2d. Obviously, compared to the Au/TS-1-B-200 catalyst, the Au/TS-1-B-300 catalyst exhibits simultaneously enhanced PO formation rate, PO selectivity and H2 efficiency. Considering the similar gold particle size distributions and Si/Ti molar ratio of the two catalysts, the greatly enhanced catalytic performance may arise from the difference in the surface and/or electronic properties of the two catalysts.

3.2 Plausible relationship between catalyst surface properties and performance



To understand the difference of surface properties between the two catalysts, FT-IR measurement was carried out to detect the changes in their functional groups caused by increasing reduction temperature. It can be clearly seen in Figure 3a that the Au/TS-1B-300 catalyst exhibits a stronger band at 3435 cm-1 attributed to the stretching vibration of hydroxyl groups and weaker bands at 2800-2900 cm-1 attributed to the asymmetrical and symmetrical stretching vibrations of -CH2 and -CH3 groups in TPA+ template.30, 31 This indicates that the existence of TPA+ template on the external surface would lead to the covering of hydroxyl groups, and with its removal from the catalysts surface, more Si-OH and Ti-OH groups get exposed to the reactants. Considering that Ti-OH group could react with peroxide species to produce active Ti-OOH species which can oxidize propylene to PO,3, 32, 33 the increase in the number of Ti active sites would contribute to the enhancement of catalytic activity. The electronic properties of the two catalysts were explored by XPS measurements. It can be clearly observed in Figure 3b, 3c and S2 that the Au/TS-1-B-300 catalyst exhibits higher Si 2p, Ti 2p and lower N 1s intensities of spectra compared to those of the Au/TS-1-B-200 catalyst, which are consistent with the above TGA and FT-IR results. The Ti 2p and Au 4f spectra shown in Figure 3c and 3d were deconvoluted with a combination of the Shirley base line and a shape of Gaussian-Lorentzian using the non-linear least squares algorithm.9, 34 It can be seen in Figure 3c that due to the removal of electron-rich TPA+ template, the binding energy of Ti4+ 2p3/2 over the Au/TS-1-B300 catalyst (459.8 eV) is higher than that over the Au/TS-1-B-200 catalyst (459.6 eV). Considering that the produced PO can adsorb on the catalysts surface through the


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interaction between Ti site and electron-deficient oxygen atom in PO molecule according to the previously studies.33, 35 Such adsorption behavior has been viewed to be responsible for the subsequent ring-opening of PO and thus formation of byproducts.35-37 Therefore, the increased XPS binding energy of Ti 2p corresponding to the lower electron density will be favorable for weakening that interaction and thus the enhancement of PO selectivity.

Figure 3. (a) FT-IR spectra of Au/TS-1-B-200 and Au/TS-1-B-300 catalysts. XPS spectra of (b) Si 2p, (c) Ti 2p and (d) Au 4f over the Au/TS-1-B-200 and the Au/TS1-B-300 catalysts.

Moreover, the two peaks in Au 4f spectrum of Au/TS-1-B-200 catalyst observed in Figure 3d are attributed to the zerovalent Au 4f7/2 and Au 4f5/2, respectively.34 The



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results show that the Au/TS-1-B-300 catalyst exhibits higher Au0 4f7/2 binding energy (i.e., 83.7 eV) than that of the Au/TS-1-B-200 catalyst (i.e., 83.4 eV). Notably, theoretical calculation and D2 kinetic isotope experiment have revealed that the dissociation of H2 on gold nanoparticles to produce the hydroperoxy intermediate is involved in the rate-determining step for propylene epoxidation.38,

39

Therefore, the

higher Au binding energy over Au/TS-1-B-300 catalyst with stronger ability to break H-H bond in H2 would be favorable for the hydroperoxy intermediate productivity and thus subsequent propylene epoxidation.40 These observations are in good agreement with the results of previous studies that the electron transfer from Ti-containing material to Au nanoparticles plays a crucial role in the catalytic performance.41-43 To confirm the binding energy shift caused by the removal of template, the electronic properties of the Au/TS-1-B catalyst pretreated at 350 oC with more electron-rich TPA+ template being removed was also characterized. It can be seen in Figure 3c and 3d that the Au/TS-1-B-350 catalyst exhibits even higher Ti 2p and Au 4f binding energies compared to those of the Au/TS-1-B-300 catalyst. Notably, the higher pretreatment temperature at 350 oC may also lead to a larger Au nanoparticle size distribution to be 3.6 ± 0.9 nm for the Au/TS-1-B-350 catalyst shown in Figure S3. This could be responsible for the lower PO formation rate, PO selectivity and hydrogen efficiency in Figure S4 than those of Au/TS-1-B-300 catalyst according to the previous studies.8, 11 Therefore, selecting the temperature of 300 oC is appropriate to perform surface pretreatment over the Au/TS-1-B catalyst for obtaining the highly active Au-Ti bifunctional catalysts.


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In addition to the PO activity and selectivity, the byproducts distribution is another important issue for direct propylene epoxidation with H2 and O2. In the catalytic evaluation of the two catalysts, five byproducts were detected and their selectivity are respectively shown in Figure 4a and 4b. Clearly, the Au/TS-1-B-300 catalyst shows greatly lower byproducts selectivity than that over the Au/TS-1-B-200 catalyst. According to the previous studies,44, 45 the reactive allylic hydrogen atoms in propylene molecule would easily suffer from the attack from negatively charged oxygen species bound to Au nanoparticles due to their nucleophilic nature, and the partial oxidation of the methyl would contribute to the formation of acrolein. Combined with the XPS results, the higher Au binding energy will make the acrolein formation through (H2assisted) activation of O2 to be more difficult.46, 47 At the same time, the higher Ti binding energy would weaken the interaction of PO with the titanium, which would be favorable for the decrease of the other byproducts selectivity.



Figure 4. Byproducts selectivity over (a) Au/TS-1-B-200 and (b) Au/TS-1-B-300 catalysts. (c) Proposed reaction network for direct propylene epoxidation with H2 and O2. Based on the above discussion, the pretreatment temperature has an important impact on the surface properties of the resultant uncalcined TS-1 immobilized Au catalyst and thus catalytic performance. The reaction pathway could be schematically depicted in Figure 4c that H2 reacts with O2 on gold nanoparticles to form the hydroperoxy intermediate serving as the oxidation agent for the epoxidation of propylene to PO on Ti sites. For the produced PO, its desorption from catalyst surface is very crucial to suppress the occurrence of side reactions, e.g., the resultant ring opening and cracking.48, 49 For the two Au/TS-1-B catalysts with similar gold particle size distributions and Si/Ti

molar ratio, the existence of TPA+ template on the external surfaces of Au/TS-1 catalyst leads to the covering of Ti sites and lower binding energies of Au and Ti sites due to its


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electron-rich nature. More Ti active sites and higher Au and Ti XPS binding energies of Au and Ti sites on the pretreated Au/TS-1-B-300 catalyst contribute to the enhancement in activity, PO selectivity and H2 efficiency.

3.3 Kinetics behaviors of products formation To further understand the kinetics behaviors of the Au/TS-1-B-300 catalyst, the catalytic performance were tested under different temperatures, and the results of PO formation rate, PO selectivity, and byproducts selectivity are shown in Figure 5a, 5b and 5c, respectively. It can be clearly seen that PO formation rate gets greatly enhanced as the reaction temperature increases, while PO selectivity decreases quickly due to all the byproducts selectivity gets greatly enhanced. Among the five detected byproducts, propanal, acetone and CO2 are produced in large quantity especially when the reaction temperature increases, which indicates that their formation may result from the isomerization and deep oxidation of PO.



300

-1

PO formation rate (gPOh kgcat )

(a)

(c)

250

0.09

180 oC

Ethanal Acrolein Propanal Acetone Carbon dioxide

-1

0.06

200

0.03

150 0.00

200 oC o

50 0

180 C o 220 C

0

2

4

6

8

o

200 C o 240 C

10

12

Time (h)

(b)

1.00 0.95

Byproducts selectivity

100

PO selectivity

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0.06 0.03 0.00

220 oC 0.06 0.03

0.90 0.00

240 oC

0.85 0.06

0.80 o

0.75 0.70

180 C o 220 C

0

2

4

6

8

0.03

o

200 C o 240 C

10

12

0.00

0

Time (h)

2

4

6

8

10

12

Time (h)

Figure 5. Catalytic performance of Au/TS-1-B-300 catalyst under different reaction temperatures. (a) PO formation rate, (b) PO selectivity, and (c) byproducts selectivity.

Considering the reaction rate is the function of temperature and concentration, it could be described as Ea

(1)

r = A exp ( - RT)·f(c)

where A, Ea, R, T and f(c) are the pre-exponential factor, activation energy, ideal gas constant, absolute temperature, and the function of concentration, respectively.. Therefore, the logarithm of the reaction rate could be described as ln (r) = ln (A·f(c)) -

Ea

(2)

RT

Based on the catalytic testing of Au/TS-1-B-300 catalyst under different temperatures, the formation rates with the unit of gh-1kgcat-1 for all the products can be


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obtained. As shown in Figure 6a, the logarithm of the as-obtained logarithms of reaction rates of PO and byproducts were linearly correlated with the reciprocal of absolute temperature (1/T) by the Arrhenius equation to yield the corresponding activation energies (Ea) in Figure 6b. It can be seen that the activation energy for PO is determined to be 29.2 ± 2.4 kJ∙mol-1 over the pretreated Au/TS-1-B-300 catalyst, which is lower than the previously reported 35 kJ∙mol-1 over the un-pretreated Au/TS-1-B catalyst in literature.14 This can be attributed to the more exposed Ti active sites and balanced electronic properties. Moreover, the activation energies of the five byproducts increase in the order of ethanal < acrolein < acetone < propanal < carbon dioxide, and the values of them were respectively determined to be 60.5 ± 4.1, 71.1 ± 3.2, 73.7 ± 2.8, 85.6 ± 3.5 and 92.2 ± 6.1 kJ∙mol-1, which are much higher than the activation energy of PO formation. This observation indicates the existence of an appropriate reaction temperature for the PO yield. Notably, it should be recalled that propanal, acetone and carbon dioxide exhibit relatively higher selectivity than ethanal and acrolein in Figure 5c, which seems contradictory to the results of activation energy. Considering that the reaction rate can also depend on the pre-exponential factors (A) and concentration effect (f(c)), the logarithms of pre-exponential factor and concentration effect ln(A·f(c)) of the byproducts were determined based on the intercepts of linear fitting. As shown in Figure 6b, the trend of obtained ln(A·f(c)) is similar to that of activation energy. This observation indicates that the trade-off between the two parameters contributes to the selectivity of propanal, acetone and carbon dioxide. Therefore, it would be more reliable to conduct kinetics analysis by combining the pre-exponential factor, activation



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energy, and concentration effect for understanding the byproducts selectivity distribution.

Figure 6. (a) Corresponding ln (reaction rate) as a function of 1/T for PO and byproducts. (b) Activation energy (Ea) and the logarithm of multiplication of preexponential factor and concentration effect, i.e., ln(A·f(c)), over the Au/TS-1-B-300 catalyst. Moreover, it has been proposed that the formation of byproducts is mainly caused by the ring-opening and isomerization of PO catalyzed by the acidic sites on catalyst surface.4, 36, 50 To weaken the PO adsorption and thus suppress its side reaction, surface modification would be a promising strategy to enhance the catalytic performance. Previous studies has demonstrated that increasing the hydrophobicity of catalysts surface could improve the catalytic activity and selectivity.4,

51-54

However, these

studies are not in the scope of this work, which would be investigated in our future study. 4. Conclusions In summary, we have engineered the surface properties of Au/TS-1-B bifuntional catalyst to boost the direct propylene epoxidation with H2 and O2. By increasing the


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reduction temperature to remove the TPA+ template on the external surfaces of catalysts, the PO formation rate, PO selectivity and H2 efficiency have been significantly enhanced owing to the more exposed Ti active sites and adjustment of electronic properties of Au and Ti. Further kinetics studies show a much lower activation energy of PO than those of byproducts, suggesting the existence of an appropriate reaction temperature for the PO yield. These results could be valuable for the design of highly efficient Au-Ti catalysts in the direct propylene epoxidation with H2 and O2.

Associated content Supporting Information XRD patterns of the Au/TS-1-B-200 and the Au/TS-1-B-300 catalysts, XPS spectra of N 1s over the Au/TS-1-B-200 and the Au/TS-1-B-300 catalysts, the representative Au particle image of Au/TS-1-B-350 catalyst obtained by HAADF-STEM, and PO formation rate, PO selectivity and H2 efficiency over Au/TS-1-B-350 at 200 oC.

Author Information Corresponding authors Email: [email protected]; Email: [email protected] Tel.: +86-21-64250937; Fax.: +86-21-64253528

Notes The authors declare no competing financial interest.



Acknowledgments This work was financially supported by the Natural Science Foundation of China (21922803 and 91434117), the Natural Science Foundation of Shanghai (17ZR1407300), the Shanghai Rising-Star Program (17QA1401200), the Fundamental Research Funds for the Central Universities (222201718003) and the Open Project of State Key Laboratory of Chemical Engineering (SKL-Che-15C03).

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Table of contents


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Uncalcined TS-1

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