Formation of active species for propane dehydrogenation with hydrogen sulfide co-feeding over transition metal catalyst

Applied Catalysis A, General 587 (2019) 117238

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Formation of active species for propane dehydrogenation with hydrogen sulfide co-feeding over transition metal catalyst Ryo Watanabea, Nozomu Hirataa, Kazuya Miurab, Yuta Yodaa, Yuya Fushimia, Choji Fukuharaa,

T ⁎

a Applied Chemistry and Biochemical Engineering Course, Department of Engineering, Graduate School of Integrated Science and Technology, Shizuoka University, 3-5-1, Johoku, Naka-ku, Hamamatsu, Shizuoka, Japan b Advanced Automotive Energy Engineering, Faculty of Engineering, Shizuoka University, 3-5-1, Johoku, Naka-ku, Hamamatsu, Shizuoka, Japan

A R T I C LE I N FO

A B S T R A C T

Keywords: Propane dehydrogenation Hydrogen sulfide Metal sulfide

This study focuses on a novel route for propylene production using a transition metal (Fe, Co, Ni, Mn, Cu) catalyst via propane dehydrogenation (PDH) with co-feeding hydrogen sulfide (H2S). The SiO2-supported Fe (Fe/ SiO2) catalyst was found to be highly active and selective for PDH with co-feeding H2S. The Fe/SiO2 catalyst displayed higher stability than the commercial CrOx/γ-Al2O3 catalyst. The durability of the Fe/SiO2 catalyst was tuned by changing the ratio of H2S to propane (H2S/C3) from 0.2 to 2.6: the most stable performance was obtained over the Fe/SiO2 catalyst with a H2S/C3 ratio of below 0.4 for 50 h. The structure of the active site on the Fe/SiO2 catalyst was investigated using XRD, XPS, and XAFS analyses. The co-supplying of H2S promoted formation of an iron sulfide species during the reaction, which was the active site for PDH. The thermodynamic calculations indicate that the iron sulfide active phase was non-stoichiometric, e.g., Fe1–xS. This structure was maintained by co-supplying H2S. DFT calculations of the dehydrogenation path suggest that PDH occurs on an iron sulfide active site via an intermediate of the propyl group combining with the surface sulfide ion.

1. Introduction Propylene is an important intermediate for production of polypropylene, propylene oxide, and acrylonitrile [1,2]. The main routes for producing propylene are thermal cracking of naphtha and fluid catalytic cracking (FCC) [3–5]. In these processes, the main products are ethylene and gasoline; therefore, propylene is produced as a by-product. The global propylene production capacity reached approximately 100 million tons in 2015 [6]. The propylene demand will increase from about 100 million to 120 million tons by 2020; steam cracking is estimated to produce 60 million tons of propylene by 2020, i.e., only half of the demand [7]. Therefore, propylene demand is growing more rapidly than production by steam cracking. The FCC units are optimized for the production of gasoline, which impedes sufficient production of propylene to meet the demand. Therefore, a novel route for propylene production is required to fulfill the increased demand [8,9]. Recently, significant attention has been focused on simple propane dehydrogenation (PDH; C3H8 → C3H6 + H2) to produce propylene [10–13]. CrOx/γ-Al2O3 is one of the commercialized catalysts for PDH, which was discovered in pioneering work by Frey and Huppke [14]. The CrOx/γ-Al2O3 catalyst shows a high activity for PDH and excellent selectivity for the production of propylene: the propane conversion was

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about 50% with a 90% selectivity for propylene [15–20]. Numerous papers have been published regarding Cr-based catalysts such as CrOx/ SiO2, CrOx/γ-Al2O3, and CrOx/ZrO2 [21–24]. However, severe coke deposition occurs during PDH, which quickly deactivates dehydrogenation on Cr-based catalysts. Normally, steam can be used to suppress coke deposition; however, in this case, steam oxidizes and deactivates the Cr-based catalyst. Therefore, a regeneration process to combust the coke using diluted air is required after very short reaction periods [25]. Recently, many promising catalytic systems have been proposed, basing on vanadium, gallium, iron, tungsten, indium oxides or platinum component [26–35]. In our previous study, a novel dehydrogenating catalyst was explored for the PDH process [36]. We found that a transition metal (Fe, Co, Ni) catalyst loaded onto aluminum oxide (γ-Al2O3) with a sulfate ion (SO42–) was selective for PDH. Sun et al. also observed such promotional effect in PDH process [37]. Using this catalyst system, propylene selectivity improved significantly, and coke deposition was decreased dramatically by the sulfidation treatment, which comprised the addition of ammonium sulfate to the γ-Al2O3 precursor sol. The loaded sulfate species (SO42–) were reduced to sulfide ions (S2–) in the reaction atmosphere and then functioned as the active site for PDH. Other researchers also reported the effectivity of metal sulfide catalysts for the

Corresponding author. E-mail address: [email protected] (C. Fukuhara).

https://doi.org/10.1016/j.apcata.2019.117238 Received 18 May 2019; Received in revised form 28 July 2019; Accepted 4 September 2019 Available online 05 September 2019 0926-860X/ © 2019 Elsevier B.V. All rights reserved.

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gaseous reactants and products in the effluent gases (i.e., C3H8, CO, CH4, C2H4, C2H6, and C3H6) were collected by micro-syringe and subsequently injected into the off-line thermal conductivity detection gas chromatograph and flame ionization detection gas chromatograph (GC8A; Shimadzu Inc., Japan). Other products except for the above products were not confirmed in the effluent gas. In addition, carbon balance was almost 100%. Propane conversion (PC) and the selectivity for the products (SP) were calculated using the following equations:

dehydrogenation of alkanes: Wang et al. investigated the catalytic performance of metal sulfides for dehydrogenation of isobutane to isobutene [38,39]. The metal sulfide catalyst showed superior selectivity in dehydrogenation to metal oxide catalysts. It was proposed that the sulfide catalyst activates the CeH bond without breaking the CCe bond thereby suppressing the side reaction of isobutane cracking. Resasco et al. developed a nickel sulfide catalyst for dehydrogenation of isobutane to isobutene [40]. They proposed that the active site constituted nickel anchored to three sulfur species. The mechanism involves isobutane dissociatively adsorbing as an isobutyl cation onto the unsaturated Ni center, leaving H bonded to the sulfur atom. Via β-H elimination from the isobutyl cation, the β-H combines with the H on the sulfur atom, which is followed by the formation and desorption of H2. The catalytic cycle is completed by desorption of isobutylene from the unsaturated Ni–S species. Metal sulfide catalysts are considered as one of the promising candidates for dehydrogenation of alkanes. However, there is a crucial problem regarding the loss of a sulfide ion (S2–) from the metal sulfide via a reaction with H2, which leads to decreased dehydrogenation activity [41]. Therefore, suppression of the release of S species from the metal sulfide is necessary to maintain high dehydrogenation activity. In petrochemical industries, hydrodesulfurization is performed to remove sulfur species from sulfur-containing compounds. A large quantity of hydrogen sulfide (H2S) gas is exhausted during this process. The exhausted H2S is generally removed using the well-known Claus process (2H2S + O2 → 2S + 2H2O). However, global sulfur supplies could exceed demand by about several million tons, which suggests the need for a new application of H2S [42]. H2S is known to decompose to produce H2 and S2– on various metal components [43]. Therefore, the effect of the addition of a co-supplying H2S with propane on the PDH performance of transition metal catalysts (Fe, Co, Ni, Mn, Cu) was investigated. We expected that co-supplying H2S would improve the stability of the performance of the catalyst through continuous regeneration of S2− during the reaction. In addition, the structure of the active site of the high-performance catalyst was investigated using Xray diffraction (XRD), X-ray absorption fine structure (XAFS), and X-ray photoelectron spectroscopy (XPS) analyses. Based on these results, we considered the active site for PDH with co-feeding H2S and the reaction mechanism by density functional theory (DFT) calculation.

PC =

FCO + FCH4 + FCO2 + 2 × FC2H4 + 3 × FC3H6 × 100 3 × FC3H8

(1)

SP=

N ×FP × 100 FCO + FCH4 + FCO2 + 2 × FC2H4 + +3 × FC3H6

(2)

Where, F denotes the flow rate of the reactant and products, P indicates a specific product, and N indicates the number of carbon atoms in the product, respectively. 2.3. Characterization of catalysts The amount of deposited carbon on the catalyst after the reaction was measured using an element analyzer Flash EA 1112 (Thermo Fisher Electron Inc., USA). During the measurement, 15 mg of the catalyst was placed in an aluminum foil cup. The deposited carbon on the catalyst was oxidized by increasing the temperature from room temperature to 900 °C under air atmosphere. Based on the produced COx detected by GC-FID-methanizer, the amount of coke deposition was calculated. The crystalline structure of the prepared catalyst was determined by XRD analysis using CuKα radiation (λ = 1.54 Å, Ultima IV, Rigaku Inc., Japan). Transmission electron microscopy (TEM; JEM-2100F, JEOL Japan)) operating at accelerating voltages of 200 kV. The components of the products were evaluated by energy-dispersive X-ray spectroscopy (EDX). The Fe K-edge XAFS measurement was performed at the hard Xray XAFS beamlines of BL5S1 in the Aichi Synchrotron Radiation Center (Aichi SR, Japan); this center has an electron storage ring with a circumference of 72 m that is operated at an electron energy of 1.2 GeV with a current of 300 mA. The XANES spectra were normalized with respect to the edge jump energy. EXAFS oscillation χ(k) was weighted by k3, in order to compensate for the diminishing amplitude in the high k range due to the decay of the photoelectron wave. The filtered k3weighted χ(k) was then Fourier transformed into R space (k range: 2.0–14.0 Å–1). In the curve-fitting step, the possible backscattering amplitude and phase shift were calculated by using the FEFF 6 code. To determine the atomic concentrations of iron and sulfur species in the catalysts, XPS (Axis Ultra DLD; Shimadzu Inc., Japan) analyses were performed using monochromatic AlKα radiation. The binding energy was referenced to C1s = 284.7 eV.

2. Experimental 2.1. Catalyst preparation The SiO2 (JRC-SIO-4) support was supplied by the Catalysis Society of Japan. The transition metal oxide (Fe, Co, Ni, Mn, Cu) and sulfate ions (SO42–) were loaded using an impregnation method: The corresponding metal nitrate (i.e., Fe(NO3)3, Co(NO3)2, Ni(NO3)2, Mn(NO3)2, Cu(NO3)2) and ammonium sulfate ((NH4)2SO4) were impregnated on the support and then evaporated until dry. After calcination of the catalyst at 700 °C for 1 h, the sulfated catalyst, containing 5 wt% SO42– and 20 wt% transition metal, was obtained. For comparison, a CrOx/γAl2O3 catalyst was prepared using a conventional impregnation method with Cr(NO3)3·9H2O (Kanto Kagaku Inc., Japan) and γ-Al2O3 followed by sequential calcination at 650 °C in air. The loadings of Cr components were 10, 15, and 20 wt%, respectively.

2.4. DFT calculation Propane dehydrogenation was computed at the level of DFT by using the CASTEP. The exchange correlation energy was described with GGA-PBEsol. Ionic cores were described by the ultrasoft pseudopotential and the Kohn-Sham one-electron states were expanded in a plane wave basis set up to 410 eV. A Fermi smearing of 0.1 eV was utilized. Brillouin zone integration was approximated by a sum over special kpoints chosen using the Monkhorst-Pack scheme (3 3 1). Without counting the adsorbates, the vacuum between the slabs was set to span the range of 20 Å for the slabs without significant interaction. Geometry optimizations were carried out by BFGS method. Transition states (TS) were located by the LST/QST method and MEP method. Hubbard repulsion parameter (U = 2.5 eV) was introduced for 3d electrons of Fe atoms. The convergence criteria for structure optimization and energy calculation were 1.0 × 104 eV·Cell–1 for SCF, 1.0 × 103 eV·Cell–1 for energy difference, 100 eV·Å–1 for maximum force and 100 Å for maximum displacement.

2.2. Activity tests The PDH performances of the prepared catalysts, including the catalytic activity, selectivity for the product, and stability, were examined in a conventional fixed-bed reactor. The reaction was performed at 600 °C under atmospheric pressure. Propane was supplied to the catalyst bed with co-feeding H2S, as balanced by He. The molar ratio of H2S to propane ranged from 0.2 to 2.6. The propane feed rate was 2.5 mL·min−1 (SATP), and the catalyst weight was 0.25 g. The 2

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Fig. 2. Effect of the addition of H2S on the C3H6 yield of the Fe/SiO2 and Cr/γAl2O3 catalysts. Catalytic performances were evaluated for PDH (a) without and (b) with H2S. The reaction conditions were as follows: (without H2S) reaction temperature was 600 °C, the catalyst weight was 250 mg, and the ratio of C3H8/ He flow rates was 2.5:23.5 mL·min−1; (with co-feeding H2S) reaction temperature was 600 °C, catalyst weight was 250 mg, and the ratio of H2S/C3H8/He flow rates was 3.5:2.5:20 mL·min−1.

this high selectivity was almost maintained throughout the dehydrogenation, as shown in the Supporting information (Fig. S1). The C3H6 selectivities of the other catalysts were as follows: 88.2%, 67.5%, 58.3%, and 52.8% over the Co/SiO2, Ni/SiO2, Mn/SiO2, and Cu/SiO2 catalysts, respectively. In terms of the catalytic performance, the Fe/ SiO2 catalyst is a suitable material for PDH with co-feeding H2S. The dehydrogenation performance of the Fe/SiO2 catalyst was compared to that of the CrOx/γ-Al2O3 catalyst, which is known to be highly active catalyst for PDH. Fig. 2(a) and (b) show the yield of C3H6 with and without co-feeding H2S, respectively. Here, CrOx loadings of 10, 15, and 20 wt% were investigated. The CrOx/γ-Al2O3 catalysts showed high initial activities; however, their activities rapidly decreased over time, as shown in Fig. 2(a). In addition, the activity of the CrOx/γ-Al2O3 catalysts decreased even in the presence of H2S in the dehydrogenation atmosphere, as shown in Fig. 2(b). Although the Fe/ SiO2 catalyst had almost no activity in the absence of H2S, the addition of H2S significantly improved the performance. The stability of the Fe/ SiO2 catalyst was superior to that of the CrOx/γ-Al2O3 catalyst. The reason for the high activity of the Fe/SiO2 catalyst with co-feeding H2S is discussed in terms of the bulk structure of the catalyst in this paper. Table 1 shows the amount of coke deposited after PDH with co-feeding H2S. The amount of coke was a little on the transition metal catalyst after the PDH with co-feeding H2S; 1.2 wt% coke was deposited on the highly-active and stable Fe/SiO2 catalyst. While, the amounts of coke deposited on the CrOx/γ-Al2O3 catalysts with loadings of 10, 15, and 20 wt% were 7.6, 7.2, and 6.0 wt%, respectively; these values are higher than that for the transition metal catalyst. Co-feeding H2S significantly improved the performance of the Fe/SiO2 catalyst.

Fig. 1. (a) Propane conversion and (b) product selectivity over Metal/SiO2 catalysts (Metal: Fe, Co, Ni, Mn, and Cu) with co-feeding H2S.

3. Results and discussion 3.1. Propane dehydrogenation with co-feeding H2S The catalytic performances were investigated in the PDH with cofeeding H2S over SiO2-supported various transition metal (Fe, Co, Ni, Mn, Cu) catalysts (described as Metal/SiO2). Fig. 1(a) shows the propane conversion over the Metal/SiO2 catalysts. The Fe/SiO2, Co/SiO2, Ni/SiO2 catalysts showed a high propane conversion at initial stage of the reaction. In addition, the Fe/SiO2 catalyst displayed better stability during PDH with co-feeding H2S than the other transition metal catalysts. The Mn/SiO2 and Cu/SiO2 catalysts showed a low propane conversion. The stability of these catalysts was largely dependent on the type of active metals. The product selectivity at 200 min is shown in Fig. 1(b). Propylene (C3H6), methane (CH4), and ethylene (C2H4) were confirmed as the products over all the catalysts. CH4 and C2H4 byproducts were produced by propane cracking (C3H8 → CH4 + C2H4). Conley et al. have reported the formation mechanism of by-product over the Cr/SiO2 catalyst [44]. Cracking is associated with the cleavage of a CeC bond in propyl intermediate (Cr–C3H7). Then, β-alkyl transfer proceeds, and forms the Cr-(ethylene)(CH3) species. This Cr-species releases ethylene to form methyl intermediate. Subsequent proton transfer from Si−(μ−OH)−Cr to Cr–CH3 forms the methane. Such reaction mechanism proceeded on the catalysts, which produced CH4 and C2H4. Comparison of the C3H6 selectivity over each catalyst revealed that the Fe/SiO2 catalyst had the highest selectivity of 92.8%; 3

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sufficient dehydrogenation performance over the Fe/SiO2 catalyst. Upon supplying H2S (H2S/C3 = 0.2 and 0.4), the Fe/SiO2 catalyst showed a stable performance with high selectivity for propylene. Furthermore, the activity was enhanced by increasing the H2S/C3 from 1.4 to 2.6; however, the stability and selectivity for propylene was slightly decreased. Cummins et al. investigated the mechanism for sulfidation of hematite to iron sulfide by feeding H2S [45]. Upon exposing hematite to H2S, iron sulfide formed on the hematite surface. They proposed the following mechanism: Above the hematite surface, H2S firstly dissociates into H2 and S. These compounds diffuse onto the hematite, while the iron cation (Fe3+) and lattice oxygen (O2–) diffuse toward the surface. The H2 and O2– react to produce H2O accompanied by the reduction of Fe3+ to Fe2+. The produced Fe2+ then reacts with the sulfur atom to form iron sulfide on the surface. During dehydrogenation with H2S over the Fe/SiO2 catalyst, the conversion increased over time, as shown in Fig. 3. This result suggests that there is an induction period, which might involve reduction of the Fe cation and subsequent sulfidation. This induction period was shortened by increasing the H2S/C3. A high H2S/C3 might promote the formation of iron sulfide on the catalyst, thereby improving the catalytic performance. Fig. 4 shows the durability of the Fe/SiO2 catalyst during a 50-h reaction period using H2S/C3 values of 0.2, 0.4, and 1.4. The initial propylene yield was high at a H2S/C3 of 1.4. However, the catalyst was deactivated as the reaction proceeded: After 35 min, the conversion was 49.5% with 94.4% propylene selectivity; the activity was decreased to a conversion of 15.0% with 78.2% propylene selectivity after 50 h. This result indicated that the catalyst was degraded significantly during the reaction. At H2S/C3 values of 0.2 and 0.4, the initial conversion was lower than that at a H2S/C3 value of 1.4. However, there was only a slight decrease in the conversion and selectivity. Additionally, the catalyst only underwent slight deactivation over the 50 h reaction under H2S/C3 values of 0.2 and 0.4; for example, relatively high conversions and 90% selectivity were obtained at a H2S/C3 of 0.4.

Table 1 Amount of coke deposited on various catalysts. Catalyst

a)

Fe/SiO2 Co/SiO2 Ni/SiO2 Mn/SiO2 Cu/SiO2 10 wt%-CrOx/γ-Al2O3 15 wt%-CrOx/γ-Al2O3 20 wt%-CrOx/γ-Al2O3

Amount of coke / wt% 1.2 0.4 0.7 0.1 0.5 7.6 7.2 6.0

a) after 200 min with co-feeding H2S.

3.3. Bulk structure of Fe/SiO2 catalyst Based on the results of the performance evaluation, the Fe/SiO2 catalyst was high performance for PDH with co-feeding H2S. To investigate the reason for this performance, the composition of the catalyst after PDH with co-feeding H2S (i.e., the used catalyst) was analyzed by STEM-EDX mapping. Fig. 5 shows a STEM image and element maps of the silicon (Si), iron (Fe), and sulfide (S) components of the used Fe/SiO2 catalyst. From the element maps, it is evident that Fe and S components were detected on the catalyst, as shown in Fig. 5(c) and (d). The element map of the S species suggests dispersion of the Fe component on the SiO2 support. This result provides evidence of Fe–S bond formation during the reaction. The generated Fe–S species might play an important role during PDH in the presence of H2S. The XRD pattern of the catalyst after PDH with co-feeding H2S was examined to determine the structure of the Fe–S species in the catalyst.

Fig. 3. Effect of the H2S/C3 in the feed gas on the (a) C3H6 yield and (b) product selectivity after 65 min of reaction over the Fe/SiO2 catalyst.

3.2. Effect of H2S ratio on dehydrogenation performance of Fe/SiO2 catalyst Since co-feeding H2S has a profound effect on the dehydrogenation properties of the catalyst, the effect of the ratio of H2S to propane (abbreviated as H2S/C3, where C3 indicates propane) on the dehydrogenation performance was investigated over the Fe/SiO2 catalyst. Fig. 3 shows the C3H6 yield and product selectivity under various H2S/ C3 values. Here, the partial pressure of propane was fixed at 9.7 kPa (equilibrium conversion: 70.9%) and H2S/C3 was varied from 0 to 2.6. When H2S was not supplied in the feed (i.e., H2S/C3 = 0), the propylene yield was very low. Therefore, it is essential to supply H2S for

Fig. 4. Effect of H2S/C3 in feed gas on C3H8 conversion and C3H6 selectivity over the Fe/SiO2 catalyst for 50 h reaction. 4

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Fig. 5. (a) STEM image and element maps of the (b) silicon (Si), (c) iron (Fe), and (d) sulfide (S) components in the used Fe/SiO2 catalyst.

Fig. 6. XRD pattern of the Fe/SiO2 catalyst before and after the dehydrogenation reaction.

Fig. 7. Schematic of the troilite structure, as reported by Skinner.46 The Fe and S sites are represented by green and yellow spheres, respectively. The Fe–Fe distance is 2.919 Å for L1, 2.947 Å for L2, and 2.984 Å for L3 (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

Fig. 6 shows the XRD pattern of the Fe/SiO2 catalyst before and after the dehydrogenation reaction. Before the reaction, the iron species of the catalyst had a hematite (α-Fe2O3) structure. After PDH with cofeeding H2S, the α-Fe2O3 structure changed to a troilite (FeS) or 5

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spectra of the Fe/SiO2 catalysts before and after PDH with co-feeding H2S. XANES spectra of the reference Fe species, i.e., hematite (αFe2O3), magnetite (Fe3O4), wüstite (FeO), and cation-deficient iron sulfide (Fe1-xS), are also shown in Fig. 9. Note that α-Fe2O3, Fe3O4, and FeO were obtained from Wako Pure Chemical Industries (Osaka, Japan). The Fe1-xS was prepared by supplying H2 and H2S to Fe2O3 at 700 °C for 2 h. The XANES spectra of Fe1-xS corresponding to the reported pyrrhotite phase [50]. As shown in Fig. 9(a), the XANES spectrum of the Fe/SiO2 catalyst before the reaction corresponded with αFe2O3. The XANES spectrum of the Fe/SiO2 catalyst was different after the reaction. The XANES spectrum of the Fe/SiO2 catalyst was completely identical to that of Fe1-xS. The Fe phase on the SiO2 support transforms into the Fe1-xS species during the reduction and subsequent sulfidation that occurs in the reaction. Fig. 9(b) shows the Fourier transforms of filtered EXAFS oscillations k3χ(k) of the Fe K-edge in the Fe/SiO2 before and after reaction into R space in the k range of 2–14 Å–1. The filtered k3χ(k) plots and the curve fitting line in k space calculated by the FEFF 6 code was shown in Fig. 9(c). The Fe/SiO2 catalyst before the reaction showed a well-defined first Fe-O coordination shell and a second coordination Fe-Fe shell, which is in good accordance with Fe2O3. While, the Fe/SiO2 catalyst after the reaction exhibited a first Fe-S coordination shell located at a distance of 2.40 Å, suggesting the pyrrhotite of iron sulfide phase on SiO2 support. Based on this result, it is evident that the iron sulfide phase is important for high dehydrogenation activity. To fully elucidate the role of H2S on the catalyst, catalytic performance tests and XPS analysis were performed on the Fe/SiO2 catalyst. The catalytic performance test involved the following method: firstly, PDH with co-feeding H2S was conducted, followed by purging of the surface adsorbate using flowing He; secondly, PDH was carried out without co-feeding H2S, followed by purging and desorption of the surface adsorbate (mainly physically adsorbed H2S on the catalyst); finally, PDH with co-feeding H2S was conducted again. Fig. 10 shows propane conversion over the Fe/SiO2 catalyst with reaction time. The conversion reached as high as 50%; however, it significantly decreased when the co-feed of H2S was stopped. When H2S was co-supplied again to the PDH, the catalytic activity was recovered to 33.1%. Under the cofeeding H2S conditions, the catalyst surface might be covered by a surface lattice sulfide atom (S2–), which imparted the high activity. In contrast, during PDH without H2S, H2 reduction probably occurred at the expense of a surface lattice sulfide atom (S2–). Therefore, a decrease in the dehydrogenation activity might occur as the sulfide atoms were consumed. However, by supplying H2S again, the surface lattice sulfide atoms (S2–) were regenerated, resulting in a recovery of the dehydrogenation activity. This hypothesis was supported by the XPS analyses results. The surface atom concentrations for sulfur (S) and iron (Fe) atoms and the atomic ratio for S/Fe were obtained. Table 2 shows the atomic concentrations calculated from the areas of the Fe2p and S2p XPS spectra. Here, the three catalyst samples, which are indicated by red circles in Fig. 10, were prepared by performing PDH with and without co-feeding H2S. Each XPS spectra in provided in the Supporting information (Fig. S2). Under co-feeding H2S conditions, the atomic ratio of S/Fe was 1.71. Such a non-stoichiometric composition indicates that the catalyst surface contains excess sulfur. In contrast, removing the H2S feed from the PDH reaction decreased the S/Fe ratio of 1.05, which is very close to stoichiometric. This result implies that the catalyst surface contained the iron cation, while the surface sulfide ion (S2–) was probably consumed by oxidative PDH or H2 reduction in the following reactions:

Fig. 8. Predominance diagram for the Fe–H–S system at 600 °C.

pyrrhotite (Fe1-xS, 0 ≤ x ≤ 0.125) structure. Fig. 7 describes the structural images of troilite that were reported by Skinner [46]. The structure of troilite is similar to that of the NiAs-type hexagonal closepacked structure with each iron atom octahedrally coordinated to six sulfur atoms [47]. In contrast, for pyrrhotite, the Fe atoms are coordinated to either five or six sulfur atoms because of metal vacancies [48]. Pyrrhotite has a slightly distorted NiAs-type structure with a nonstoichiometric composition including Fe7S8, Fe8S9, Fe9S10, and Fe10S11 [49]. The FeS stoichiometric compound and Fe1-xS (Fe8S9, Fe9S10, and Fe10S11) intermediates have orthorhombic and hexagonal structures, respectively. The iron-deficient Fe7S8 compound has a monoclinic symmetry. These Fe–S species might be formed by the reduction and subsequent sulfidation of α-Fe2O3 during PDH with co-feeding H2S. To verify the stable phase of the Fe/SiO2 catalyst during PDH with co-feeding H2S, thermodynamic calculations on these reaction conditions were performed using HSC Chemistry® ver. 6.0 to generate phase diagrams of the ternary component systems of Fe–H–S. Fig. 8 shows a phase diagram of the iron species at the reaction temperature of 600 °C. The dehydrogenation atmosphere in this calculation includes 13.6 kPa H2S and H2 as the dehydrogenation product at 3.0 kPa, which was the partial pressure calculated based on the propylene yield. The point that corresponds to these conditions is indicated in Fig. 8. The calculation result predicts the existence of Fe0.877S rather than the stoichiometric compound FeS, which is not stable under these reaction conditions. In the region of a high partial pressure of H2S, the iron disulfide (FeS2) phase and Fe metal are stable phases when there is a high partial pressure of H2. α-Fe2O3 on the SiO2 support was transformed to a deficient iron sulfide during reduction and subsequent sulfidation. Since the iron sulfide phase was only observed in the catalyst, this iron sulfide might be the active phase for dehydrogenation. According to Wang et al., sulfide catalysts have been reported as being selective for dehydrogenation of isobutane through suppression of the sequential reaction due to the decreased adsorption of isobutene [41]. Modification of the metal catalyst by sulfur increases the electron density of the surface atom, which facilitates a repulsive interaction between isobutene and the catalyst surface. Similarly, the excellent dehydrogenation performance of the Fe/SiO2 catalyst might be caused by the effect of sulfidation on the electronic state of the Fe cation. The high durability of the catalyst might be attributed to maintenance of the thermodynamically stable iron sulfide phase by co-feeding H2S in the feed. 3.4. Identification of the iron phase

C3H8 + S2– → C3H6 + H2S

(3)

In the presence of co-feeding H2S, iron sulfide was formed over the Fe/SiO2 catalyst in the reaction atmosphere. To elucidate the origins of the high performance of this catalyst for PDH with co-feeding H2S, the active species of the Fe-based catalysts after the reaction were investigated using XANES analysis. Fig. 9(a) shows Fe K-edge XANES

H2 + S2– → H2S

(4)

After resuming the co-feed of H2S, the S/Fe ratio recovered to 1.58. The surface sulfide ion (S2–) was regenerated through the reverse of the reaction shown in Eq. (4). One possible reason why the S/Fe did not 6

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Fig. 9. (a) Fe K-edge XANES spectra of the Fe/ SiO2 catalyst before and after PDH with cofeeding H2S, (b) Fourier transforms of filtered EXAFS oscillations k3χ(k) into R space and (c) filtered k3χ(k) of Fe K edge in the k rage of 2–14 Å–1.

feeding H2S. The low activity of Mn/SiO2 was possibly caused by the electronic structure of metal sulfide catalyst. Gómez-Balderas performed DFT study of the 3d transition metal sulfides [51]. For the Co, Ni and Fe sulfides, calculation results showed that the electron density of states (DOS) was existed in the occupied states region below the Fermi level. In contrast, the DOS of MnS is increased in the unoccupied states region above the Fermi level. They suggested that the possibility of a strong interaction with charge donating sulfur adsorbate atoms poisoned the active surface of MnS. In this work, H2S, which was charge donating sulfur molecule, was co-fed with propane. The charge donating sulfur molecule of H2S might strongly adsorb on the catalyst, which would produce a low dehydrogenation performance of the MnS catalyst. Higher H2S/C3 leads to a quicker deactivation rate than the cases with lower H2S/C3, as shown in Fig. 3. Under high H2S/C3 condition, the rate of consumption of S2− is accelerated by the progress of the reactions of (3) and (4). Under such condition, the rate for the S2– regeneration (5) did not catch up with the rate for the consumption of S2-, which might cause micro-structural changes in the iron sulfide and deactivate the catalyst.

Fig. 10. Propylene yield over the Fe/SiO2 catalyst for PDH with and without cofeeding H2S. Table 2 Atomic concentration over the Fe/SiO2 catalyst after PDH with co-feeding H2S. Timea) / min

50 125 200

Area / a.u.

S2p

Fe2p3/2

1624 927 2554

3513 3257 5998

Area ratio (S2p/ Fe2p3/ 2) / –

0.4622 0.2846 0.4258

Atom concentration / mol% S

Fe

4.26 2.16 4.24

2.49 2.05 2.69

H2S + Vs → H2 + S2–

Atomic ratio of S/Fe /–

(5)

Another possible reason was the accumulation of coke. Table 3 shows the coke deposition of the Fe/SiO2 catalyst under various H2S/C3 values. Higher H2S/C3 value caused a higher amount of coke deposition on the catalyst, because of the increase of the selectivity to by-products of CH4 and C2H4 as shown in Fig. 3(b). Such accumulation of coke on the catalyst might accelerate the quicker deactivation. An important question to address is how the reaction mechanism proceeds on the sulfur-rich iron sulfide. Based on the structure of the Fe/SiO2 catalyst, DFT calculations were performed to identify potential reaction pathways and reaction intermediates on iron sulfide. Fig. 11 schematically shows the energy diagram for PDH on the low-index (001) facet of the iron-sulfide phase. Note that the (001) facet of the iron sulfide was reported to be the most stable surface [52,53]. As a

1.71 1.05 1.58

a) The value corresponds to the circle symbol in Fig. 12.

reach its original value of 1.71 upon the re-supply of H2S might be micro-structural changes in the iron sulfide due to redox cycling. Based on the atomic concentration in Table 2, the high activity might be derived from the presence of the sulfur-rich iron sulfide phase. Therefore, one of the key roles of the H2S is the formation of an iron-sulfide phase with a sulfur-rich composition. Based on the performance in Fig. 1 the Fe/SiO2, Co/SiO2, and Ni/ SiO2 catalysts showed a high dehydrogenation performance, however, the Mn/SiO2 and Cu/SiO2 catalysts had much lower activity. There are two main reasons for the low activity over the Cu/SiO2 and Mn/SiO2 catalysts. Fig. S3 in supporting information shows XRD pattern of used catalysts of the M/SiO2 (M: Mn, Fe, Co, Ni, Cu). The metal sulfide was formed as a single phase on these catalysts, except for the Cu/SiO2 catalyst. While, the catalyst of Cu/SiO2 had the multiple phases with different valences of monovalent and divalent. The formation of such multiple phases was considered to cause a low activity for PDH with co-

Table 3 Amount of deposited coke on the Fe/SiO2 catalyst after PDH with various H2S/C3.

7

H2S/C3

Amount of coke / wt%

2.6 1.4 0.4 0.2 0

1.3 1.2 0.6 0.4 0

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Fig. 11. Schematic view of the hydrogenated surface of iron sulfide (001).

Fig. 12. Free energy profiles of PDH on FeS(001). The transition states of the first and second dehydrogenation steps are indicated as 2-TS and 4-TS, respectively. The corresponding transition state geometries are shown in the insets.

hydrogenated structure as a starting point for PDH, the CHe bond in the second carbon of propane dissociates on the sulfide ion to produce an alkyl species and H atom. The produced H atom reacts with the adsorbed H atom to produce a H2 molecule, as shown in Fig. 12. The alkyl species then bonds with the sulfide ion, forming the alkyl sulfide species as an intermediate species on the catalyst. Subsequently, the H atoms were removed from the alkyl sulfide intermediate species. The

preliminary experiment, we calculated the most stable surface state of the (001) facet in the dehydrogenation atmosphere by adding a hydrogen atom to the surface. In addition, the London dispersion force was corrected using the method reported by Tkatchenko and Scheffler [54]. Fig. 11 indicates the stabilization energies of various hydrogenadded surfaces. The hydrogen atom was present on the top site of the sulfide atom and hollow site of the iron atom. On the basis of this 8

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catalyst cycle was completed by desorption the propylene product from the catalyst surface. Yun et al. investigated the reaction mechanism for PDH over H-[Fe]-ZSM-5 [55]. They proposed a redox mechanism involving the formation of a propane radical cation as a reaction intermediate over H-[Fe]-ZSM-5. The reaction of PDH over the Fe/SiO2 catalyst must have proceeded via a similar route due to the nature of H2S. H2S could be decomposed radical species such as SH and H species at high temperature [56]. Propane was initially activated on the iron sulfide catalyst and/or radical species, to form a propane radical cation and H atom. The removed H atom reacted with the originally adsorbed H atom to produce a H2 molecule. At the same time, the Fen+ (n: 2, 3) on the iron sulfide was reduced to Fe(n−1)+ through the transfer of an electron from the propane radical. This generated the propyl cationic species, which interacted with the surface sulfide, as shown in Fig. 12 (step 3). Such an alkyl sulfide species formed as the reaction intermediate. The H atom was subsequently removed from the reaction intermediate. The catalytic cycle was completed by the production of propylene accompanied by oxidation of Fe(n−1)+ to Fen+. Although future work is required to verify the proposed radical mechanism, the present study introduced a novel dehydrogenation system, in which the lattice-S–mediated reaction path results in highly selective and continuous production of propylene by co-feeding H2S.

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4. Conclusions The catalytic properties of transition metal (Fe, Co, Ni, Mn, Cu) catalyst for PDH with co-feeding H2S were evaluated. The Fe/SiO2 catalyst displayed a high activity and selectivity for the production of C3H6; its overall performance was superior to that of the CrOx/γ-Al2O3 catalyst. By tuning the ratio of H2S to propane (H2S/C3) in the feed to 0.4, the catalyst proved to be highly stable for about 50 h of reaction. Based on the structural characterization results of XRD, XPS, and XANES analyses, iron sulfide formed on the Fe/SiO2 catalyst. During the reaction, the co-fed H2S promoted the formation of iron sulfide, which was the active site for PDH. Based on the DFT calculations, PDH occurs on the formed iron sulfide site via the combination of the intermediate propyl group with the surface sulfide ion. Acknowledgment This study was financially supported by Japan Petroleum Energy Center (JPEC) for Creation of Technological Seeds of Innovative Refining. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcata.2019.117238. References [1] A. Corma, F.V. Melo, L. Sauvanaud, F. Ortega, Catal. Today 107 (2005) 699–706. [2] J.S. Plotkin, Catal. Today 106 (2005) 10–14. [3] A. Corma, F. Melo, L. Sauvanaud, F.J. Ortega, Appl. Catal. A Gen. 265 (2) (2004) 195–206. [4] B. Basu, D. Kunzru, Ind. Eng. Chem. Res. 31 (1) (1992) 146–155. [5] J. Verstraete, V. Coupard, C. Thomazeau, P. Etienne, Catal. Today 106 (1–4) (2005) 62–71. [6] C.C.E. Christopher, A. Dutta, S. Farooq, I.A. Karimi, Ind. Eng. Chem. Res. 56 (49) (2017) 14557–14564.

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