Direct Propylene Epoxidation over RuO2–CuO–NaCl–TeO2–MnOx

Article pubs.acs.org/IECR

Direct Propylene Epoxidation over RuO2−CuO−NaCl−TeO2−MnOx/ SiO2 Catalysts: Optimized Operating Conditions and Catalyst Characterization Anusorn Seubsai,*,†,‡,§ Photchanan Phon-in,† Thanaphat Chukeaw,† Chalinee Uppala,† Paweena Prapainainar,† Metta Chareonpanich,†,§,⊥ Bahman Zohour,∥ Daniel Noon,∥ and Selim Senkan∥ †

Department of Chemical Engineering, Faculty of Engineering, ‡Center for Advanced Studies in Industrial Technology and Faculty of Engineering, §NANOTEC Center for Nanoscale Materials Design for Green Nanotechnology, and ⊥Center for Advanced Studies in Nanotechnology and Its Applications in Chemical, Food and Agricultural Industries, Kasetsart University, Bangkok 10900, Thailand ∥ Department of Chemical and Biomolecular Engineering, University of California, Los Angeles, Los Angeles, California 90095, United States S Supporting Information *

ABSTRACT: Multimetallic catalysts of RuO2−CuO−NaCl− TeO2−MnOx supported on SiO2 were investigated for the direct-gas-phase epoxidation of propylene to propylene oxide (PO) using molecular oxygen under atmospheric pressure. Four operating variables (reactor temperature, O2 to propylene feed gas volume ratio, reactants to carrier gas volume ratio, and total feed gas flow rate) were studied using the Box−Behnken design, which enabled the achievement of a relatively high PO formation rate at 1507 gPO h−1 kgcat−1 with PO selectivity of 25.3% and propylene conversion of 5.9% at 1 h of time on stream. The optimized conditions were a reactor temperature of 297 °C, a O2/C3H6 ratio of 11.95, a (O2 + C3H6)/He ratio of 0.24, and a total feed gas flow rate of 36 cm3 min−1. The physical and chemical properties were characterized using various techniques that revealed that the crystallinity of RuO2 and CuO, the close proximity of RuO2 and CuO, and the surface’s acidity are crucial for PO production. NaCl plays a key role in reducing CO2 combustion. TeO2 and MnOx enhanced the active sites that facilitate PO production. In a typical direct PO generation mechanism, O2 first dissociatively adsorbs onto the catalyst. Gas-phase propylene then reacts with adsorbed atomic oxygen to form an oxametallocycle, allyl radical, or other intermediates.7−10 It has been reported that the formation of an oxametallocycle is the key intermediate that leads to the generation of PO.7 Conversely, an allyl radical resulting from the abstraction of allylic hydrogen of propylene by the adsorbed oxygen irreversibly undergoes complete combustion (i.e., producing CO2 and H2O). If the adsorbed oxygen species is a strong nucleophile, the allyl radical route is more favorable. In contrast, a strongly electrophilic surface oxygen would preferentially make an insertion to the π bond of propylene molecule to form the oxametallocycle. Hence, a material tuned to have a moderate surface acidity would be suitable as a propylene epoxidation catalyst under this mechanism, thus

1. INTRODUCTION Propylene oxide (PO) is an intermediate chemical widely used in the preparation of various textiles and plastics such as polyurethanes, polyester resins, cosmetics, food emulsifiers, and additives.1,2 Currently, the global consumption is approximately 7.5 million tons annually,3 and the average demand of PO is growing quickly at 4.2% per year.4 To meet this enormous demand, the hydrogen peroxide to propylene oxide (HPPO) process, which employs titanium silicalite-1 (TS-1) as a catalyst to epoxidize propylene directly with hydrogen peroxide, has been launched commercially in Italy, China, and Thailand.5,6 However, the discovery of heterogeneous catalysts capable of accomplishing the direct-gas-phase epoxidation of propylene with molecular oxygen (C3H6 + 1/2O2 → C3H6O) has been a critical research focus because of its superior economic upside, safety, and environmental advantages relative to established processes (e.g., chlorohydrin, HPPO, and coproduct processes).1 Nevertheless, no catalyst has been created that can sustainably produce PO on an industrial scale, primarily because of the PO selectivity and C3H6 conversion performance limitations.1 © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

September 28, 2016 December 7, 2016 December 8, 2016 December 8, 2016 DOI: 10.1021/acs.iecr.6b03771 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Aesar]. After that, the mixture was stirred for 4 h at 750 rpm and at room temperature using a heater−stirrer (Fisher Scientific Isotemp, Ceramic Top). Subsequently, the heater’s temperature setting was fixed at 165 °C to dry the solution. Next, the dried sample was ground using a stirring rod in the glass shell vial. The obtained fine solid sample was then calcined in a furnace with air atmosphere by heating from room temperature to 480 °C at a heating rate of 10 °C min−1, holding at 480 °C for 8 h, and then naturally cooling to room temperature. The other catalysts were prepared similarly. For any other presented catalysts that did not all consist of the compositions in the optimal catalyst, the weight percentage of each component (unless otherwise specified) was the same value as the weight percentage of that component in the optimal catalyst. 2.2. Catalytic Performance Evaluation. The catalytic performance tests were carried out in a traditional packed-bed reactor. The prepared catalyst (1.5 mg) was packed (as thin as possible to minimize the complete combustion of PO after the generation) and sandwiched between quartz wool plugs in a quartz tube (0.5 cm in diameter) and then loaded into a tube furnace reactor. For the experiments to compare the relative activity of different catalysts, the feed gases consist of 2 vol % C3H6 (Linde, 99.5%), 8 vol % O2 (Praxair, 99.999%), and He (Praxair, 99.999%) as an inert carrier. The total flow rate was 50 cm3 min−1 (GHSV = 152727 h−1) controlled by mass flow controllers (KOFLOC 3810 DSII). The reactor temperature was set at 250 °C under atmospheric pressure. For optimization of the PO formation rate by using a regular Box−Behnken design (all experiments in one block), four operating variables were studied: O2/C3H6 ratio (0.4−20.0), (O2 + C3H6)/He ratio (0.03−0.33), reactor temperature (190−310 °C), and total flow rate (30−70 cm3 min−1). Product analysis was conducted at a pseudosteady condition (between 1 and 2 h after the reactor reached the target temperature) by online GC (Varian CP-4900 Micro gas chromatograph) with a thermal conductivity detector (TCD), Porapak U (10 m) with molecular sieve 5 Å (10 m). The detected products were mainly PO and CO2 along with traces (34%) can be achieved at reactor temperatures below 250 °C, O2/C3H6 ratios of around 5−15, (O2 + C3H6)/He ratios above 0.1, and total feed flow rates above 50 cm3 min−1. These conditions yielding the highest PO selectivities are in stark contrast to the optimal conditions producing the highest PO formation rates. The rise in CO2 production tends to outpace that of PO for increasing reactor temperatures and (O2 + C3H6)/He ratios or for reducing total feed flow rates, resulting in falling PO

to identify the electronic states of the mixing elements. Transmission electron microscopy (TEM) and EDS was also used to observe the element distribution by a FEI CM120 microscope operating at 120 kV. Continuous hydrogentemperature-programmed-reduction (H2-TPR) measurements were carried out in a continuous-flow Inconel tube reactor held at 25−800 °C with a heating rate of 5 °C min−1. The H2/Ar mixture gas (9.6% H2) was introduced into the catalyst bed at a total flow rate of 30 cm3 min−1. The H2 consumption was continuously monitored using a TCD-equipped gas chromatograph (Shimadzu GC-2014). An ammonia-temperature-programmed-desorption (NH3-TPD; TPD/R/O Thermo Finnigan 1100) technique was used to analyze the catalysts’ acidity. Typically, the catalysts were pretreated under a He flow at 400 °C for 1 h and cooled to 40 °C before 10% NH3/He mixed gas was flowed over the catalysts for 30 min to adsorb on the acid sites. The excess ammonia was eradicated by flowing N2 at 40 °C for 20 min. The catalysts were then heated to 800 °C at a heating rate of 20 °C min−1, while a flow of He passed over the catalysts at 20 cm3 min−1. The TPD profiles were detected by a TCD and analyzed with ChemiSof t TPx software.

3. RESULTS AND DISCUSSION 3.1. Investigation of the Operating Conditions for a RuO2−CuO−NaCl−TeO2−MnOx/SiO2 Catalyst. Four operating variables, which could significantly influence the catalytic performance, were studied to further optimize the PO formation rate of the RuO2−CuO−NaCl−TeO2−MnOx/SiO2 catalyst: reactor temperature, O2 to C3H6 feed volume ratio (O2/C3H6 ratio), reactants to carrier gas volume ratio (O2 + C3H6)/He ratio, and total feed gas flow rate. In general, several thousands of experiments would have to be done if several dependent variables had to be studied experimentally. Therefore, the Box−Behnken methodology was employed in this E

DOI: 10.1021/acs.iecr.6b03771 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research selectivities. Hence, performance optimization hinges on balancing the relative importance of the selectivity and production rate. The use of Box−Behnken designs predicted an optimal condition to maximize the PO formation rate and PO selectivity, as listed in Table 1. The experiments were, therefore, employed to ascertain the predicted values. Remarkably, the highest PO formation rate achieved was 1507 gPO h−1 kgcat−1 (25.3% of PO selectivity and 5.9% of propylene conversion), fairly close to the predicted value at 1501 gPO h−1 kgcat−1. The maximized PO selectivity was also predicted at 40.9% at a relatively low reactor temperature of 190 °C and close to the prediction of 40.4%. Nevertheless, the PO formation rate and propylene conversion were relatively small because of the low reactor temperature. The reliability of the experimental values compared to the predicted values was also confirmed by the parity plots shown in Figure 3. As seen in the parity plots of the PO selectivity (Figure 3a) and PO formation rate (Figure 3b), the correlation between the experimental and predicted values is exceptional with R2 values of 0.9878 and 0.9993, respectively. The maximized PO formation rate obtained was plotted in the graph between the PO formation rate and PO selectivity compared to others reported in the literature from 2005 to the present, as illustrated in Figure 4 (also see Figure S2 and Table

Figure 5. Time-on-stream testing of the optimal RuO2−CuO−NaCl− TeO2−MnOx/SiO2 catalyst with testing conditions of a reactor temperature of 297 °C, a O2/C3H6 ratio of 11.95, a (O2 + C3H6)/He ratio of 0.24, and a total feed gas flow rate of 36 cm3 min−1.

deactivation of the catalyst and its prevention is being intensively performed and will be reported in the very near future. 3.2. Characterization of a RuO2−CuO−NaCl−TeO2− MnOx/SiO2 Catalyst. Various characterizations of the optimal multimetallic catalyst using different techniques were followed up to understand the physical and chemical properties relative to its catalytic activity as follows. The SEM image and its element distributions (Ru, Cu, Na, Te, and Mn) are shown in Figure 6. The particle sizes were roughly around 30−50 nm, and each metal (Figure 6b−f) was uniformly dispersed on the SiO2 support. The TEM images of some selected catalysts are presented in Figure 7. A rutile structure with an approximation of 40 × 100 nm of the RuO2/ SiO2 catalyst is observed in Figure 7a. The unimetallic catalyst of CuO/SiO2 was uniformly dispersed on the SiO2 support with an average size of 3−5 nm, as shown in Figure 7b. Interestingly, for the RuO2−CuO−NaCl/SiO2 and RuO2− CuO−NaCl−TeO2−MnOx/SiO2 catalysts, the particle size of RuO2 was reduced to 30 × 10 mn. Also, the individual CuO particles could not be observed, potentially because they were in proximity with larger RuO2 particles;9 therefore, it is difficult to see any clear dark spots from CuO. More details of the disappearance of CuO particles and a reduction of the RuO2 particle size will be explained in the discussion of the H2-TPR and XRD results, respectively. The NaCl particles were crudely cubic (see Figure S3). Distinct MnOx and TeO2 particles could not be observed, potentially because they were added in small amounts and/or they were crumbled and formed into chunks of large particles of mixed composition. However, as seen in the SEM/EDS and TEM mapping using the EDS technique (see Figure S4), all of the elements can be observed. The BET surface area of the optimal RuO2−CuO−NaCl− TeO2−MnOx/SiO2 catalyst was 74.78 m2 g−1, compared to 89.59 m2 g−1 for the unloaded-metal SiO2 support. The average pore volume and pore size of the catalyst were 0.4309 cm3 g−1 and 19.24 nm relative to its pure SiO2 support at 1.129 cm3 g−1 and 60.4 nm, respectively. This indicates that the active metal components were positioned inside the pores of the SiO2 support after impregnation; thus, the overall pore size and pore volume of the catalyst decreased, resulting in a decrease of the overall surface area.

Figure 4. PO selectivity versus PO formation of various catalysts for the epoxidation of propylene to PO reported in the literature from 2005 to the present (gray ●, epoxidation with H2; red ◆, epoxidation without H2; green ▲, photoepoxidation; blue ■, this work).

S1). Notably, the PO formation rate is relatively high compared to the previous reports. However, for economically sustainable PO production, the PO selectivity of the optimal catalyst should be >70% while maintaining this high PO formation rate.1 Strides must still be taken to upgrade this catalytic material to overcome this challenge. A time-on-stream experiment for 10 h using the optimal catalyst was, therefore, carried out using the maximized conditions of the PO formation rate presented in Table 1. The results are presented in Figure 5. The performance (i.e., PO formation rate, PO selectivity, and propylene conversion) quickly increased during the first 0.5 h and reached maxima at about 0.5−2 h of the testing. Thereafter, deactivation ensued, potentially because of the loss of Cl content35 or combined causes (e.g., sintering, coking, etc.). A full detailed study of F

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Figure 6. SEM/EDS images of the optimal RuO2−CuO−NaCl−TeO2−MnOx catalyst at 25 wt % total metal loading. (a) SEM image and images showing element distributions of (b) Ru, (c) Cu, (d) Na, (e) Te, and (f) Mn.

Figure 8. XRD spectra of various SiO2-supported catalysts. The compositions of Ru, Cu, Na, Te, and Mn were 13.06, 6.53, 3.26, 0.32, and 1.83 wt %, respectively, and balanced using SiO2.

MnO2 (2θ = 26.4°), and trace amounts of mixed Mn2O3 (2θ = 33.4°) and Mn3O4 (2θ = 45.8°). These peaks did not appear in the optimal catalysts perhaps because they were overlapped with the larger peaks of RuO2, CuO, and NaCl. It should be noted that the XRD peaks of RuO2 in the optimal catalyst were broader relative to the single RuO2/SiO2 catalyst, indicating that the crystallite size of RuO2 in the optimal catalyst became smaller according to the Scherrer equation. This is in good agreement with the size reduction of RuO2 observed in the TEM images. However, the XRD peaks for CuO and NaCl were unchanged, suggesting that the particle sizes of CuO and NaCl remained unchanged. Additionally, mixed-metal oxides were absent from these XRD spectra. Table 2 shows the relative catalytic activity of selected catalysts from the combination of RuO2, CuO, NaCl, TeO2, and MnOx. Most of the presented catalysts correspond to the XRD spectra presented in Figure 8. In general, the unimetallic catalysts of SiO2-supported NaCl, TeO2, and MnOx catalysts

Figure 7. TEM images of selected catalysts: (a) RuO2/SiO2, (b) CuO/SiO2, (c) RuO2−CuO−NaCl/SiO2, (d) RuO2−CuO−NaCl− TeO2−MnOx.

The XRD measurements of the optimal catalysts compared with their mono-, bi-, and trimetallic catalysts were performed as shown in Figure 8. The spectrum of the optimal catalyst revealed the characteristic diffraction patterns of RuO2 (2θ = 28.0, 35.0, and 54.0°), CuO (2θ = 35.6, 38.7, and 48.9°), and NaCl (2θ = 31.7 and 45.5°), but no diffraction patterns of TeO2 and MnOx were observed. Clearly, the XRD spectrum of the TeO2/SiO2 catalyst did not show the characteristic peaks of TeO2 crystals, suggesting that TeO2 is in either an amorphous phase or a nanocrystalline size but too small to be detected by XRD. The XRD spectrum of the MnOx/SiO2 catalyst primarily consisted of crystalline MnO (2θ = 35.8, 41.5, and 59.9°), G

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Industrial & Engineering Chemistry Research Table 2. Catalytic Performance of Prepared Catalysts Containing Ru, Cu, Na, Te, and/or Mn on SiO2a selectivity (%)

a

catalyst supported on SiO2

PO

AC

AT

CO2

C3H6 conversion (%)

PO yield (%)

PO formation rate (gPO h−1 kgcat−1)

RuO2− CuO−NaCl−TeO2−MnOx RuO2− CuO−NaCl−MnOx RuO2−CuO−NaCl−TeO2 RuO2−CuO−NaCl RuO2−CuO MnOx TeO2 NaCl CuO RuO2

31.4 26.2 28.9 27.3 4.9 0 0 0 16.7 0.4

0.5 0 0 0 0.7 75 64.3 0 25 1

0 0 0 0 0.1 0 0 0 0 0

68.1 73.8 71.1 72.7 94.3 25 35.7 100 58.3 98.6

5.23 5.08 4.79 5.48 13.32 0.1 0.06 0.02 0.08 9.22

1.65 1.33 1.39 1.50 0.66 0 0 0 0.01 0.03

641 517 538 557 276 0 0 0 5 14

Each catalyst was prepared by using Ru, Cu, Na, Te, and/or Mn at 13.06, 6.53, 3.26, 0.32, and 1.83 wt %, respectively, and balanced using SiO2.

Figure 9. XPS spectra of (a) Ru, (b) Cu, (c) Te, and (d) Mn species of the RuO2−CuO−NaCl−TeO2−MnOx/SiO2 catalyst. Note that unidentified peaks are satellite or plasmon peaks.

dissociation of O2 onto the RuO2 surface, the adsorbed oxygen species can migrate to the CuO surface in close proximity and subsequently react with propylene to produce PO. Apparently, NaCl acted as a promoter to partially inhibit CO2 combustion, resulting in increasing PO selectivity. The addition of either TeO2 or MnOx individually to RuO2−CuO−NaCl/SiO2 slightly decreased the PO formation rates. However, the addition of both TeO2 and MnOx together to the RuO2− CuO−NaCl/SiO2 catalyst exhibited the most superior PO formation relative to the others. The means by which the added presence of both TeO2 and MnOx enhances the functioning of

were inactive for PO formation. The CuO/SiO2 catalyst was active for AC formation higher than PO formation with a small propylene conversion, consistent with previous reports.16,31 The RuO2/SiO2 catalyst was highly active for CO2 formation with a trace amount of PO. Accordingly, the dissociative adsorption of O2 occurs more easily on RuO2 compared to CuO, but the adsorbed oxygen species on RuO2 are strongly nucleophilic, lending to the abstraction of allylic hydrogen from propylene.38 Catalysts containing both RuO2 and CuO together improved the PO formation rate significantly because of their previously documented synergy.9,16 It is possible that, after H

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indicating the additional H2 consumption by CuO (see the details in Table S2). It is important to point out that the particles of CuO are most likely to be in close contact with RuO2, confirming the disappearance of CuO particles in the TEM images (Figure 7) of the catalysts consisting of CuO and RuO2 together. Thus, this proximity enables the synergistic catalytic effects responsible for PO synthesis. It should be additionally noted here that the reductions of NaCl and TeO2 were not observed, because either these materials were present in insufficient quantities or the reduction peaks were not in this range of temperatures. Also, the reduction peaks of MnOx were not clearly seen in the H2-TPR spectrum of the RuO2−CuO− NaCl−TeO2−MnOx/SiO2 catalyst, from which they generally appeared between 175 and 330 °C.44 Potentially, this is because the peak of MnOx is convoluted with the RuO2−CuO peak because the integral peak area around 200 °C is the largest relative to the normal RuO2−CuO peaks. The acidities of the RuO2−CuO/SiO2, RuO2−CuO−NaCl/ SiO2, RuO2−CuO−NaCl−TeO2/SiO2, and RuO2−CuO− NaCl−TeO2−MnOx/SiO2 catalysts was tested using NH3TPD, as presented in Figure 11. The relationship between the

the catalyst’s active sites will be elaborated on in the NH3-TPD results later. Parts a−d of Figure 9 present the surface chemistry of the optimal catalyst measured by XPS. The multiple scan spectra were conducted in the Ru, Cu, Te, and Mn ranges. These spectra confirm that the catalyst comprises immiscible metal oxides of RuO2 (3d3/2, 285.4 eV; 3d5/2, 281.3 eV), CuO (2p1/2, 954.0 eV; 2p3/2, 934.1 eV), TeO2 (3d3/2, 585.0 eV; 3d5/2, 576.3 eV), MnO2 (2p1/2, 653.5 eV; 2p3/2, 642.1 eV), and MnO (2p1/2, 652.4 eV; 2p3/2, 640.5 eV).41 The small XRD peaks corresponding to Mn2O3 and Mn3O4 in Figure 8 combined with the complete absence of any such XPS signals in Figure 9d suggest these compounds were present in trace amounts compared to MnO2 and MnO. In addition, the signal-to-noise ratios of Te and Mn were quite large. This was because the amount of these two elements was small, and thus it was difficult to achieve a high intensity peak even when multiple scans were made. This multiple catalyst can, therefore, be correctly named as RuO2−CuO−NaCl−TeO2−MnOx/SiO2, as currently stated. The metal oxide−metal oxide and metal oxide−support interactions were investigated for the SiO2-supported RuO2, CuO, RuO2−CuO, and RuO2−CuO doped with NaCl, NaCl− TeO2, or NaCl−TeO2−MnOx catalysts using H2-TPR treatment, as shown in Figure 10 (see also the peak area in Table

Figure 11. NH3-TPD profiles of RuO2−CuO/SiO2, RuO2−CuO− NaCl/SiO2, RuO2−CuO−NaCl−TeO2/SiO2, and RuO2−CuO− NaCl−TeO2−MnOx/SiO2 catalysts. Each catalyst has weight percentages of Ru, Cu, Na, Te, and Mn on SiO2 = 13.06, 6.53, 3.26, 0.32, and 1.83, respectively. Figure 10. H2-TPR profiles of all combinations of RuO2, CuO, NaCl, TeO2, and MnOx on SiO2. Each catalyst has weight percentages of Ru, Cu, Na, Te, and Mn on SiO2 of 13.06, 6.53, 3.26, 0.32, and 1.83, respectively.

catalytic performance of each catalyst and its acidity strength is plotted in Figure 12. As presented in Figure 11, each catalyst shows a similar profile in which the strong, medium, and weak acidic sites appear around 700−800, 400−600, and 100−200 °C, respectively. However, some of the relative desorption intensities of each acidic strength and the total surface acidity shown in Figure 12 were significantly different. The addition of NaCl to the RuO2−CuO/SiO2 catalyst obviously reduced the overall surface acidity of the RuO2−CuO/SiO2 catalyst, especially the strong and medium sites, but slightly enhanced the weak site. This resulted in a dramatic reduction in the propylene conversion and an increase in the PO selectivity and PO formation rate (see Figure 12), indicating that the combustion route of propylene is largely inhibited. In other words, NaCl primarily occupies the strong and medium acidic sites that create mostly CO2. As a result, CO2 production decreases. Moreover, NaCl may take into account withdrawal of the electrons from the adsorbed oxygen species, thus inducing the adsorbed oxygen to become more electrophilic, resulting in

S2). The RuO2/SiO2 catalyst showed two peaks. The main peak around 200 °C was attributed to the complete reduction of Ru4+ to Ru0, and the lower temperature peak around 170 °C was associated with ruthenium species interacting with the support.42 The CuO/SiO2 catalyst represented a single peak around 310 °C, suggesting the reduction of bulk CuO species.43 All combinations of RuO2 and CuO appeared as a single sharp peak around 170−180 °C, similar to the range of the reduction temperatures of the RuO2/SiO2 catalyst. Surprisingly, the reduction peak of CuO was barely observed around 310 °C. This indicated that most of the CuO species were rapidly reduced by a H2 spillover during the reduction of RuO2 (i.e., hydrogen spillover mechanism).9 Moreover, the H2 consumption spectra of all materials that include at least RuO2 and CuO together were larger relative to the RuO2/SiO2 spectrum, I

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In measurements of the catalyst surface acidity using a NH3TPD technique, the presence of NaCl in the catalyst was confirmed to play a key role in inhibiting the CO2 formation pathway. Moreover, the addition of TeO2 and MnOx together to the catalyst provided new strong acidic sites, substantially enhancing the PO formation rate.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b03771. Contour plots showing propylene conversion from all combinations of four operating variables, a plot showing PO formation versus PO selectivity of various catalysts reported in the literature, a TEM image showing a NaCl particle, TEM/EDS image showing the element distribution of the optimal catalyst, and a table showing peak areas of the H2-TPR results (PDF)

Figure 12. Relationship between the catalytic performance and acidity of (A) RuO2−CuO (B) RuO2−CuO−NaCl/SiO2, (C) RuO2−CuO− NaCl−TeO2/SiO2, and (D) RuO2−CuO−NaCl−TeO2−MnOx/SiO2 catalysts.

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an enhancement of the PO production.9,35,45 The addition of TeO2 to the RuO2−CuO-NaCl/SiO2 catalyst minimally changed all strong, medium, and weak acidic sites. Thus, the catalytic performances (i.e., PO selectivity, PO formation rate, and propylene conversion) all were virtually unchanged. This indicates that TeO2 has no effect on the acidity of the catalyst surface. Interestingly, the addition of MnOx to the RuO2− CuO−NaCl−TeO2/SiO2 catalyst significantly increased the total surface acidity, especially the strong acidic sites, and lowered the medium and weak acidic sites. As a result, the PO formation rate increased from 538 to 641 gPO h−1 kgcat−1, while both the PO selectivity and propylene conversion also increased from 28.9% to 31.4% and from 4.79% to 5.23%, respectively. This is in good agreement with the result in Table 2, in which MnOx in the presence of TeO2 is able to increase the effective number of active sites (i.e., suitable strong acidic sites) for PO formation. In other words, this is potentially because the lattice oxygen created from the TeO2−MnOxcontaining catalyst (i.e., RuO2−CuO−NaCl−TeO2−MnOx/ SiO2) becomes more electrophilic, enhancing insertion of the lattice oxygen to the π bond of propylene and thereby increasing PO production. Nevertheless, the condition having high and strong surface acidity, which can promote PO production, should be limited under catalysts that can exhibit synergistic catalyst effects. For instance, RuO2 or MnOx itself possesses a high surface acidity, but they produce only CO2.12

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+66) 02-561-4621. ORCID

Anusorn Seubsai: 0000-0001-8336-6590 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Kasetsart University Research and Development Institute, the Thailand Research Fund, and the Commission on Higher Education (Grant MRG5980240).

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REFERENCES

(1) Khatib, S. J.; Oyama, S. T. Direct oxidation of propylene to propylene oxide with molecular oxygen. Catal. Rev.: Sci. Eng. 2015, 57, 306−344. (2) 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, 3447−3459. (3) Prieto, A.; Palomino, M.; Díaz, U.; Corma, A. Propylene epoxidation with in situ generated H2O2 in supercritical conditions. Catal. Today 2014, 227, 87−95. (4) Ring, K. L.; deGuzman, M. Chemical Economics Handbook: Propylene Oxide; IHS Chemical: 2016. (5) Russo, V.; Tesser, R.; Santacesaria, E.; Di Serio, M. Chemical and technical aspects of propene oxide production via hydrogen peroxide (HPPO Process). Ind. Eng. Chem. Res. 2013, 52, 1168−1178. (6) Teles, J. H.; Hermans, I.; Franz, G.; Sheldon, R. A. Oxidation. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2015. (7) Torres, D.; Lopez, N.; Illas, F.; Lambert, R. M. Low-basicity oxygen atoms: a key in the search for propylene epoxidation catalysts. Angew. Chem., Int. Ed. 2007, 46, 2055−2058. (8) Chukeaw, T.; Seubsai, A.; Phon-in, P.; Charoen, K.; Witoon, T.; Donphai, W.; Parpainainar, P.; Chareonpanich, M.; Noon, D.; Zohour, B.; Senkan, S. Multimetallic catalysts of RuO2-CuO-Cs2O-TiO2/SiO2 for direct gas-phase epoxidation of propylene to propylene oxide. RSC Adv. 2016, 6, 56116−56126. (9) Seubsai, A.; Zohour, B.; Noon, D.; Senkan, S. Key mechanistic insight into the direct gas-phase epoxidation of propylene by the RuO2−CuO−NaCl/SiO2 catalyst. ChemCatChem 2014, 6, 1215− 1219.

4. CONCLUSIONS Four operating variables, including the reactor temperature, O2 to C3H6 feed volume ratio, reactants to carrier gas volume ratio, and total feed gas flow rate, were investigated for the RuO2− CuO−NaCl−TeO2−MnOx/SiO2 catalyst in direct-gas-phase epoxidation by using the Box−Behnken design to optimize PO formation. At 1−2 h of time on stream, the highest PO formation was achieved at 1507 gPO h−1 kgcat−1 and with 25.3% PO selectivity and 5.9% propylene conversion at a reactor temperature of 297 °C, a O2/C3H6 ratio of 11.95, a (O2 + C3H6)/He ratio of 0.24, and a total feed gas flow rate of 36 cm3 min−1. Investigations using SEM/EDS and TEM/EDS indicated that RuO2 and CuO nanoparticles were uniformly dispersed on the SiO2 support. The H2-TPR and TEM measurements suggested that the RuO2 and CuO crystals were in close proximity, creating a highly active site for PO synthesis. J

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DOI: 10.1021/acs.iecr.6b03771 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX