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Propane Dehydrogenation over Alumina-supported Iron/ Phosphorous Catalysts: Structural Evolution of Iron Species Leading to High Activity and Propylene Selectivity Shuai Tan, Bo Hu, Wun-gwi Kim, Simon H. Pang, Jason S. Moore, Yujun Liu, Ravindra S. Dixit, John G. Pendergast, David S. Sholl, Sankar Nair, and Christopher W Jones ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01286 • Publication Date (Web): 20 Jul 2016 Downloaded from http://pubs.acs.org on July 26, 2016

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Propane Dehydrogenation over Aluminasupported Iron/Phosphorous Catalysts: Structural Evolution of Iron Species Leading to High Activity and Propylene Selectivity Shuai Tan,1 Bo Hu,1 Wun-Gwi Kim,1 Simon H. Pang,1 Jason S. Moore, 2 Yujun Liu, 2 Ravindra S. Dixit,2 John G. Pendergast,2 David S. Sholl,1 Sankar Nair*1 and Christopher W. Jones*1

1

School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332

2

Engineering & Process Sciences, The Dow Chemical Company, Freeport, TX 77541

*To whom correspondence should be addressed: Christopher W. Jones Sankar Nair School of Chemical & Biomolecular Engineering Georgia Institute of Technology Atlanta, GA, 30332 Email: [email protected] Tel: 404-385-1683 (C. W. Jones) Email: [email protected] Tel: 404-894-4826 (S. Nair)

1 ACS Paragon Plus Environment

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Abstract A series of alumina-supported Fe-based catalysts is prepared via a dry impregnation method in the presence of a phosphorous source (phosphate salt) and then used for the catalytic dehydrogenation of propane. Specifically, supported catalysts with Fe:P molar ratio of 1:1, 2:1, 3:1 are prepared and their chemical composition, textural properties and redox properties are characterized with an array of techniques. In the non-oxidative dehydrogenation (PDH) of propane at 600 °C and atmospheric pressure, the most active catalyst (Fe:P ratio of 3:1) exhibits 15% propane conversion and >80% C3H6 selectivity. The calculated activity is 9.9 mmol/h/gFe (mass basis) or 13 µmol/h/m2 (surface area basis), with a corresponding TOF of 19 h-1. During the initial stages of reaction under PDH conditions, the precatalyst is reduced and Fe(0) species are generated, eventually giving way to iron carbide species. During this induction period, significant carbon is incorporated into the catalyst and propylene selectivity is low. Only after the iron carbide phase appears does the reactivity and selectivity achieve steady state conditions with high propylene selectivity and good activity. The addition of the phosphorous source in the pre-catalyst is found to be important in obtaining a catalyst with superior performance.

Keywords iron carbide; propane dehydrogenation; propylene; phosphide; phosphate;


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1. Introduction Propylene is a significant feedstock for a variety of industrial applications, such as polymer and petrochemical synthesis.1-2 The global propylene market is anticipated to exhibit considerable growth in coming years and is estimated to reach 165 million tons by 2030.3 With petrochemical feedstocks continuing to shift away from naphtha and C3 volumes diminishing, there is growing interest in alternative feedstocks for propylene production to capitalize on the supply gap.4-6 Among the well-known approaches for propylene production, catalytic propane dehydrogenation (PDH) is of particular interest. The most well-studied catalysts

are

Cr-based7-9

and

Pt-based,10-11

which

represent

the

two

commercialized catalysts to date. Despite the success of these catalysts, due to the scale of propylene production whereby even minor improvements in productivity, selectivity or catalyst stability can lead to substantial cost savings, many efforts have been made in further optimizing these catalysts. Examples include inclusion of additional metals with Pt,12-17 or surface modification of Cr.1821

In parallel, investigations of other types of catalysts continue to appear, with the aim of developing alternative materials that are cheap and environmentally benign, such as binary/ternary Group IIIA metal oxides,22-23 acidic zeolites,24-26 and isolated transition metal atoms on oxide supports.27-29 Iron based catalysts have been widely used due to their earth abundance and (generally) low toxicity.30-31 From this perspective, iron based materials are promising candidates, although most studies to date using iron based catalysts have focused on


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oxidative PDH. For example, Fe-ZSM-5 zeolite catalysts were found to be active in presence of O2 and N2O, with propane conversion of 51.7% and propylene selectivity of 39.7%.32-33 In another example, Michorczyk and co-workers studied supported Fe2O3 catalysts in a C3H8/CO2 co-feed, with conversions ranging from 2.3-29.7%, and propylene selectivities ranging from 71.5-92.5%.34-35 More recently, Lobo et al. investigated Fe-substituted ZSM-5 for non-oxidative PDH, achieving 78-85 % propylene selectivity; however, the conversion was very modest (ca. 3.6 %).36 Meanwhile, Li’s group reported non-oxidative PDH over Fe2O3 supported on sulfated Al2O3 and proposed that the addition of sulfur species was responsible for enhanced catalytic performance.37-38 In this contribution, we describe the synthesis an array of alumina-supported iron-based catalysts in the presence of a phosphorous source (phosphate salt) for non-oxidative propane dehydrogenation. The resulting materials were characterized by an array of techniques, including X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), N2 physisorption, H2 temperature-programmed reduction (H2-TPR), elemental analysis and Raman spectroscopy. These ironbased catalysts show promising catalytic performance in terms of selectivity, stability and activity. In addition, the presence of phosphorous species plays a key role in maintaining the catalyst stability.

2. Experimental 2.1 Catalyst preparation


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The following chemicals were used as received with at least 99 % purity from Sigma Aldrich: Aluminum oxide (120 m2/g, gamma-phase), iron nitrate nonanhydrate (Fe(NO3)39H2O), Citric acid (C6H8O7), and ammonium phosphate dibasic ((NH4)2HPO4). The iron-based catalyst precursors were synthesized via a dry impregnation method. In a typical synthesis (mole ratio of Fe:P=3:1 as example), 5.76 g citric acid (0.03 mol), 0.66 g (NH4)2HPO4 (0.005 mol) and 6.06 g Fe(NO3)39H2O (0.015 mol) were mixed in a centrifugation vial. Citric acid was used for assisting dissolving the Fe salt due to its ability to ligate the metal and prevent precipitation. The molar ratio of citric acid:Fe was 2:1. Then, ca. 15 mL DI H2O was added in the vial and the mixture was left sonicating for 20 min to ensure all salts were dissolved. At this stage, the mixture turned a maroon color. The mixture was then added dropwise to 4 g of Al2O3 powder with a pipette using the dry impregnation method. After adding the solution, the Al2O3 powder was put in oven at 70 °C for 90 min until most of the H2O was evaporated. Such a cycle was repeated several times until all the solution was added. After final drying step, the Al2O3 powder with deposited Fe/P salts became a dark red color. Then the material was transferred into a crucible and loaded in the furnace. The calcination step was as follows: a ramp rate of 5 °C/min with a 6 h hold at 600 °C, then cooling to room temperature within 8 h. This resulting precursor powder contained FePO4 (and Fe2O3 depending on the Fe:P ratio in the recipe). The precursor was then loaded into the reactor and in-situ reduced under an H2 flow right before the propane dehydrogenation tests.


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The resulting Fe-based catalyst precursors are labelled as (xFe:P)/Al2O3, whereas x denotes the molar ratio of Fe/P in the recipe. For example, (2Fe:P)/Al2O3 refers to sample with Fe:P in a ratio of 2:1.

2.2 Catalyst characterization The X-ray diffraction (XRD) patterns were measured with an X’pert Pro X-ray diffractometer from PANalytical with nickel-filtered Cu Kα radiation (λ=1.5418 Å), with generator settings of 45 kV and 40 mA. A scanning region of 20-60° with a scanning step size of 0.008° was used. XPS analysis was carried out with a Thermo K-Alpha spectrometer equipped with a monochromatic Al Kα X-ray source. Vacuum (pressure below 3×10-7 mbar) was maintained in the analytical chamber during the surface analysis. The C1s peak at ca. 284.8 eV with an uncertainty of ±0.2 eV was used as reference for calibration of the binding energies (B.E.) of all elements. Textural properties were determined by N2 adsorption-desorption isotherms carrying out with a Tristar II 3020 from Micromeritics. About 0.1 g of sample was degassed under vacuum and preheated at 120 °C for 12 h on a Schlenk line before the measurement. Elemental analysis for each element within the catalysts was performed by using inductively coupled plasma-optical emission spectroscopy (ICP-OES). This service was provided by ALS Environmental. CO chemisorption was performed with an Autochem II Chemisorption Analyzer from Micromeritics to probe the dispersion of the Fe-based catalysts.


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About 30 mg of sample was preheated to 600 °C with a ramp of 5 °C/min under a 30 mL H2/Ar flow and held under those conditions for 1 h. Then the sample was cooled to 35 °C in a 30 mL He flow, with a ramp of 15 °C/min. After further purging for 30 min, the CO pulse chemisorption was started. The CO consumption was followed with a thermal conductivity detector (TCD) and recorded. The dispersion (D%) and particle size (dp) were calculated with the empirical formula:39-40 % =

1.117 ∗

=

96 %

where X is the CO uptake (µmol/g), W is the Fe wt% within the catalyst, and dp (nm) is the average particle size if assuming a spherical shape for the metal domain. High Resolution TEM (HR-TEM) was performed on a FEI Tecnai G2 F30 Transmission Electron Microscope operating at 300 kV. The well dispersed catalyst suspension in H2O was dropped onto a Lacey carbon-coated copper grid and dried in ambient air for sample preparation. Coke analysis based on Raman spectra obtained with a Thermo Nicolet Dispersive Raman Spectrometer equipped with a CCD detector and confocal optics was performed. The signal was collected in backscattering mode. The beam is 488 nm wavelength with a power of 10 mW. The acquisition time for background is 20 s, followed by 15 s sample scanning.


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X-ray absorption spectroscopy was carried out on the bending magnet beam line of the Materials Research Collaborative Access Team (MRCAT, 10 BM) at the Advanced Photon Source, Argonne National Laboratory. Ionization chambers were optimized at the midpoint of the Fe spectrum for the maximum current with linear response (ca. 1010 photons detected per second) using 35 vol% He in N2 (ca. 15 % absorption) in the incident X-ray detector and a mixture of 17 % Ar in N2 (ca. 70 % absorption) in the transmission X-ray detector. A third detector simultaneously collected the Fe foil reference spectrum with each measurement for energy calibration. A water cooled double-crystal Si (111) monochromator was used and detuned to 50 % to minimize the presence of harmonics. The Xray beam size was 1×1 mm2. The catalyst was diluted by mixing with Al2O3 powder and was pressed into a 4 mm diameter wafer and loaded in a stainless steel 6-shooter sample holder. The reactor was made of a straight quartz tube with 1 inch diameter and 10 inches length. Both ends of the reactor were equipped with shut-off valves with an Ultra-Torr fitting. The tube reactor was sealed by Kapton windows and O-rings. The reactor has an inserted thermocouple placed near the samples that controls the clam shell furnace. The catalysts were pretreated at 600 °C in 50 sccm H2 flow for 1 h followed by purging in a 100 sccm Ar flow for 30 min. For in-situ measurements, 50 sccm 3 vol% C3H8/He was flowed over pretreated samples at 600 °C for different times, followed by measurement at room temperature without exposure to air.

2.3 Catalyst evaluation


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The PDH reaction was carried out in a U-shape quartz reactor of ca. 1/8-inch diameter and ca. 80 cm length. In a typical test, 0.2 g catalyst precursor (150-212 µm pellets, with reaction conditions previously suggested to avoid significant mass/heat transfer limitations22-23) was loaded on a quartz wool bed. The reactor was then heated to the desired temperature (i.e., 600 °C) in a Techne FB-08 Al2O3 fluidized bath under 30 sccm H2 flow (in-situ reduction). After the temperature reached 600 °C, the sample was purged with 20 sccm N2 for 10 min. Then, the feed gas was switched to a flow of 10 sccm of 5 vol % of C3H8 with balancing N2. The hydrocarbon products as well as H2 were analyzed on line by a Shimadzu GC-2014 gas chromatograph equipped with a RESTEK Column (RtAlumina BOND/Na2SO4, 30 m × 0.25 mm × 4 µm) and a TCD, respectively. The data were collected after flowing the feed gas for 10 min. The carbon mass balance was calculated and had an average deviation from closure of 5% or less. After reaction, the catalyst was purged with N2, cooled to room temperature under its protection, removed from the reactor and saved in a glass vial for additional characterization. The conversion and atomic selectivity were calculated by using the following equations: % =

%&'( )*(&&+ % =

, − , ,

!"

, , !" × 3 , − ,


× 100%

× 100% !"

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Whereas Xi,out refers to concentration of products i (i.e., CH4, C2H6, C2H4, C3H6), and Yi is the number of carbon atoms in the product i molecules.

3. Results and Discussion 3.1 Catalyst Structural Properties Figure 1a shows the XRD patterns of the various supported catalyst precursors at different Fe:P mole ratios. It can be seen that for the (1Fe:P)/Al2O3 sample, peaks were apparent at 20.3°, 25.8°, 35.5°, 36.6°, 38.1°, 41.4° and 48.6°. They are assigned as FePO4 (JCPDS: 29-0715).

In contrast, for the

(3Fe:P)/Al2O3 sample, the FePO4 phase was not observable. Instead, a completely different pattern appeared, with peaks at 24.1°, 33.2°, 35.7°, 40.9°, 49.4° and 50.1°. These peaks are due to the diffraction of Fe2O3 (JCPDS: 850987). This might be expected based on stoichiometry, since the excess Fe atoms were oxidized under air at high temperature to form large domains of iron oxide. It should be mentioned that for the (2Fe:P)/Al2O3 sample, there were no XRD peaks observable. This is possibly due to possible formation of dispersed domains of Fe2O3 and FePO4, with all domains being too small to be detected. To support this hypothesis, a series of unsupported precursors was synthesized via the same approach and the XRD patterns are shown in the supporting information (Figure S1). In short, the unsupported 2Fe:P sample contained a mixture of Fe2O3 and Fe2P2O7 (JCPDS: 76-1762) domains. The unsupported


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1Fe:P and 3Fe:P samples showed FePO4 and Fe2O3 as dominant phases, respectively, which is consistent with the case of the supported samples. Figure 1b shows the XRD patterns of the precursors after H2-pretreatment (at 600 °C, right before PDH reaction) as well as after 6 h of reaction. It can be seen that for the (1Fe:P)/Al2O3 sample, Fe2P2O7 (JCPDS: 76-1762) was formed after reduction, due to the partial reduction of FePO4.41-42 Meanwhile, Fe3(PO4)2 (JCPDS: 83-0801) was also found, which is generally associated with the reaction between ferrous iron and phosphate. In contrast, a featured peak due to Fe(0) (JCPDS: 89-7194) appeared for the (2Fe:P)/Al2O3 and (3Fe:P)/Al2O3 samples at ca. 44.7°, indicating reduction of the Fe2O3 phase to metallic iron. For the samples after PDH reaction, the (1Fe:P)/Al2O3 showed almost identical patterns, indicating it contained a relatively stable mixture of Fe2P2O7 and Fe3(PO4)2. However, for (3Fe:P)/Al2O3, an abundant amount of the Fe(0) was transformed into Fe3C (JCPDS: 89-2722). In all the catalysts before and after reaction, no indication of the formation of iron phosphide domains was observed. To further understand when phosphide domains might form, the series of supported Fe/P precursors was also reduced in H2 at different temperatures to examine the stability of the Fe-P system, and the XRD patterns of the resulting solids are shown in the supporting information (Figure S2). The patterns suggest that the reduction temperature played a key role in determining the final chemical structure of the Fe-based catalysts. To be specific, when the samples were reduced at 800 °C, a strong Fe(0) peak remained, indicating predominant metallic domains, with a small amount of iron


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phosphide species. However, when the reduction temperature was increased to 900 °C, strong peaks associated with phosphide species appeared. These data suggest that under the normal reaction conditions employed in this study (H2 reduction at 600 °C), the likelihood of formation of iron phosphide species was quite small. (a)

[102]

(1Fe:P)/Al2O3 precursor

100

(2Fe:P)/Al2O3 precursor

Intensity (a.u.)

(3Fe:P)/Al2O3 precursor [100]

(): Fe2O3, JCPDS: 85-0987

[104] [110]

[]: FePO4, JCPDS: 29-0715

[112] [200]

[203]

20

(1-10)

(211)

(110)

30

(220)

(210)

40

(321)

50

2θ (ο)

(b)

60

(1Fe:P)/Al2O3 after H2 reduction

150

(1Fe:P)/Al2O3 after reaction (2Fe:P)/Al2O3 after H2 reduction

Fe2P2O7 and Fe3(PO4)2

(2Fe:P)/Al2O3 after reaction (3Fe:P)/Al2O3 after H2 reduction (3Fe:P)/Al2O3 after reaction

Intensity (a.u.)

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

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Fe (×0.1) Fe3C

20

30

40

2θ (ο)

50

60

Figure 1. XRD patterns of Fe-based precatalysts and catalysts: (a) after calcination and (b) the samples after (600 °C) H2 reduction and PDH reactions.


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XPS was utilized to characterize the speciation of phosphorus, as shown in the supporting information (Figure S3). No significant binding energy (B.E.) shifts were observed within the error of the measurements. It was observed that there was a broad peak centered at ca. 134.9 eV representing P2p1/2 and P2p3/2 binding energies, which is supported by prior studies characterizing iron(III) phosphate and iron(II) phosphate.43-44 The current results are also in accord with the XRD observation that the phosphorous element existed in either phosphate (PO43-) or pyrophosphate (P2O74-) forms during the reaction. Both species contain phosphorous in the same oxidation state and should not exhibit significant B.E. shifts. The textural properties of the supported iron-based catalysts both before (pre) and after (post) the PDH reactions, as determined from N2-physisorption isotherms (Figure 2), are listed in Table 1. It can be seen that the bare support, Al2O3, exhibited the largest surface area, while the samples with Fe/P deposition had decreased surface areas as well as pore volumes.

Table 1. Textural properties of the Fe-based catalysts before/after PDH reactions. Catalyst

Stotala (m2/g)

Vtotalb (cm3/g)

Pore sizeb (nm)

(1Fe:P)/Al2O3 pre

89

0.13

5.3

(1Fe:P)/Al2O3 post

83

0.13

5.4

(2Fe:P)/Al2O3 pre

83

0.10

5.3

(2Fe:P)/Al2O3 post

75

0.08

5.3


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(3Fe:P)/Al2O3 pre

103

0.15

5.4

(3Fe:P)/Al2O3 post

103

0.16

5.5

Al2O3

120

0.18

5

a

Based on BET method.

b

Based on BJH method.

As depicted in Figure 2, the isotherms of all the samples were similar and can be identified as type IV, which is characteristic of mesoporous materials. In addition, type H2 hysteresis loops, according to IUPAC, were detected in all catalysts. This type of loop is characteristic of bottlenecked pores and of solids composed of small spherical particles. 140 (1Fe:P)/Al2O3 pre (1Fe:P)/Al2O3 post

120

Quantity Adsorbed (cm3/g STP)

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

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(2Fe:P)/Al2O3 pre (2Fe:P)/Al2O3 post (3Fe:P)/Al2O3 pre

100

(3Fe:P)/Al2O3 post Al2O3

80

60

40

20

0 0.0

0.2

0.4

P/P0

0.6

0.8

1.0

Figure 2. N2-physisorption isotherms of the supported Fe-based catalysts before (pre) and after (post) PDH reactions.

The chemical composition of the catalysts was measured by ICP-OES, and the results are summarized in Table 2. It can be seen that the iron and


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phosphorous content remained constant during the reaction, and the corresponding atomic ratios roughly followed those of the preparation recipe. In addition, varied amounts of coke were found in the catalysts after 6 h of reaction.

Table 2. Elemental analysis of the Fe-based catalysts before and after PDH reactions. Element Composition (wt%)

Fe:P mole ratio

Catalyst C

Fe

P

(1Fe:P)/Al2O3 pre

-

11.7

6.4

1:1

(1Fe:P)/Al2O3 post

0.3

11.1

6

1:1

(2Fe:P)/Al2O3 pre

-

14.4

3.9

2.1:1

(2Fe:P)/Al2O3 post

2.7

11.4

3

2.1:1

(3Fe:P)/Al2O3 pre

-

13.2

2.1

3.4:1

(3Fe:P)/Al2O3 post

3.5

14.2

2.3

3.4:1

To determine the reducibility of the materials, H2-TPR experiments were carried out and the corresponding plots are given in Figure 3. Quartz wool, Al2O3 as well as a sample without the Fe precursor salt were examined as control experiments. These were found to show negligible reduction signals. To better elucidate the redox properties of the samples, a supported Fe2O3 sample prepared by same method was also examined. It is well accepted that reduction


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of Fe2O3 goes through the following steps: Fe2O3→Fe3O4→FeO→Fe.35,

45

The

multistep reduction generally causes broad reduction envelopes. For example, two peaks appeared in the supported Fe2O3 sample with centers at ca. 350 and 547 ºC, respectively. In contrast, samples with phosphorous showed broader peaks with a shift to higher temperature (ca. 377 - 395 ºC), indicating that the presence of phosphate strengthened the interactions between Fe2O3 and Al2O3, leading to a stabilization of the Fe2O3 domains. The broader peak around 460 507 ºC was due to the reduction of FePO4 to Fe2P2O7.46 The signal at T > 650 ºC has been proposed to be due to further reduction of Fe2P2O7 to iron phosphide.42, 47

(1Fe:P)/Al2O3 (2Fe:P)/Al2O3 (3Fe:P)/Al2O3

TCD signal (a.u.)

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Fe2O3/Al2O3 Quartz wool Al2O3 P/Al2O3 395 377

460 507 504 500

377

547

350

100

200

300

400

500

600

Temperature (oC)

700

800

900

Figure 3. H2-TPR profiles of the Fe containing pre-catalysts and control samples.

3.2 Catalytic reactions Prior to the catalytic tests, control experiments were performed in an empty tube, over the Al2O3 support, and over Al2O3 impregnated with a phosphorous


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source (labeled as P/Al2O3) under the same reaction conditions. All experiments had ca. 5 s residence times. It was found that the background propane conversion was negligible and addition of the phosphorous source in the absence of Fe did not significantly enhance the conversion or selectivity under these conditions. A direct comparison of the catalytic performance for the Febased samples with different Fe:P ratios is illustrated in Figure 4. It was found that as the Fe:P ratio increased, the propane conversion significantly increased. To be specific, the 1Fe:P, 2Fe:P, and 3Fe:P samples exhibited a steady level of conversion of around 6, 11 and 17 %, respectively, at a common set of conditions. Since the Fe content in each catalyst only varied in a narrow range based on ICP-OES analysis (11.7 ~ 14.4 wt%), the different catalytic activities could be primarily ascribed to different active Fe-containing species within each catalyst. As mentioned in the discussion of the XRD data, the precursor of Fe:P=1:1 sample was mainly FePO4, which transformed to Fe2P2O7 and Fe3(PO4)2 after H2 pretreatment. Hence, Fe2P2O7 and Fe3(PO4)2 are proposed to be least active of the crystalline species contained within the various catalyst samples. With an increase in the Fe:P ratio, an extra phase, Fe2O3, formed in the precursors, as observed by XRD (see Figure 1a). The iron oxide phase was found to be reducible during the H2-pretreatment step to form Fe(0). Hence, by correlating the increased conversion for the phosphorous lean samples with the presence of Fe(0) and Fe3C within the samples before and after reaction, it can be suggested that these two species are more active phases. This is explored further, below.


ACS Catalysis

It should be noted that all three Fe-based samples showed low C3H6 selectivity at the data first point (10 min time on stream). Subsequently, the propylene selectivity rapidly increased to 75 - 90 % within the next 30 - 60 min. Meanwhile, a relatively large amount of H2 was observed during the initial period of reaction. The high propane conversion, together with a poorly closed carbon balance, indicated that during the induction period, the C3H8 feed gas was primarily converted to carbon-containing species that remained in the solid state. It is well known that iron carbide can be formed from metallic Fe(0) in the presence of hydrocarbon/H2 co-feeds.48-50 Hereby, we propose that during the induction period, a key reaction occurring is the in-situ formation of active and selective iron carbide phase(s). 30

(a)

(1Fe:P)/Al2O3 (2Fe:P)/Al2O3

25

Conversion (%)

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

Page 18 of 42

(3Fe:P)/Al2O3

20

15

10

5

0 0

50

100

150

200

Time (min)

250


300

350

Page 19 of 42

100

10

(b)

80

60

6

40

4

20

2

0 0

1.05

50

100

150

200

Time (min)

250

300

H2 vol(%)

C3H6 selectivity (%)

8

0 350

(c)

1.00

Carbon balance

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

ACS Catalysis

0.95

0.90

(1Fe:P)/Al2O3

0.85


0.80 0

50

100

150

200

Time (min)

250

300

350

Figure 4. (a) Propane conversion, (b) propylene selectivity (closed symbols) and H2 volume percentage (open symbols), and (c) carbon balance closure over Fe-based catalysts with different Fe:P ratios. Reaction conditions: 0.2 g catalyst, 600 °C, 10 sccm 5 vol% C3H8/N2.

To probe this hypothesis, the following experiment was carried out with aim of examining the catalyst at different reaction times by ex-situ XRD measurements. For these experiments, 1 g of (3Fe:P)/Al2O3 catalyst was initially loaded in the reactor. After the H2 reduction at 600 °C, the PDH reaction was started. Then,


ACS Catalysis

after different times on stream, the reaction was paused by purging with N2 and cooling down to room temperature. At each point (0, 30, 75, 120, 360 min), a small amount of catalyst was taken out for later XRD and ICP-OES analysis. The total flowrate was fixed, while the C3H8 percentage was adjusted to keep constant the residence time and catalyst contact time. It can be seen from Figure 5 that during the induction period, all the gaseous hydrocarbon products were present at low selectivity, while H2 was formed in high concentrations (ca. 8 % by vol.).

100

10

90 80

8

60

Conversion CH4

50

C2H6 C2H4

40

4

C3H6 H2 Vol.

30

6

20

H2 vol. (%)

70

%

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

Page 20 of 42

2

10 0

0 0

50

100

150

200

250

300

350

Time(min)

Figure 5. Catalytic experiment using the (3Fe:P)/Al2O3 catalyst. Reaction conditions: 0.2 - 1 g catalyst, 600 °C, 10 sccm of 2 - 5 vol% C3H8/N2. Arrows: different reaction times where catalyst was removed (0, 30, 75, 120, 360 min).

The corresponding series of catalysts in Figure 5 was also examined by ICPOES and the results are summarized in Table 3. It was found that the Fe:P mole ratio was constant within error throughout the experiment, and was close to the


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ACS Catalysis

recipe value of 3:1, indicating that the phosphorous species were stable under the reaction conditions. Meanwhile, according to the phase diagram of the Fe-C system, when the carbon content is lower than 6.67 wt%, the binary system should be a mixture of α-Ferrite (solution of C in BCC structure of Fe) and Fe3C (Cementite).51 After 30 minutes of reaction, the carbon content of the catalyst (relative to the total composition of Fe and C) was only 1.3 % by wt., which is not enough to transform all the Fe to the carbide phase (Table 3). However, after 75 minutes on stream, there was sufficient carbon present to fully form the carbide phase. Interestingly, it is around this stage, between 75 and 150 minutes on stream, where the catalysts start to display steady reactivity and good propylene selectivity. Thus, these data imply that Fe carbide phases may be important to achieving good activity and selectivity with this system.

Table 3. Elemental analysis of the (3Fe:P)/Al2O3 catalyst after different reaction times. Element Composition Fe:P mole ratio

C/(C+Fe) wt%

(wt%)

Reaction Time (min) C

Fe

P

0

-

13.2

2.1

3.4:1

-

30

0.3

13.3

2.2

3.4:1

1.9

75

1.6

13.3

2.2

3.4:1

10.7

120

1.9

13.6

2.2

3.4:1

12.1

360

2.7

14.2

2.3

3.4:1

16.2


ACS Catalysis

The catalyst samples removed at different reaction times (arrows in Figure 5) were characterized by XRD and the results are shown in Figure 6. It can be seen that after H2 reduction at 600 °C (0 min), only one sharp peak at 44.67° due to metallic Fe (JCPDS: 89-7194) was present, which is consistent with the data in Figure 1b. After 75 min, several other peaks appeared at 37.77°, 42.90°, 43.75°, 44.59°, 45.01°, 45.88°, 48.62°, and 49.14°. The new peaks are assigned as Fe3C (JCPDS: 89-2722). The gradually appearing Fe3C peaks in Figure 6 are concomitant with the development of C3H6 selectivity, as observed in Figure 5. In addition to the elemental analysis, these observations suggest that Fe3C could be responsible for the combined high catalytic activity and selectivity to propylene. [022]

[]: Fe3C, JCPDS: 89-2722

[113] [122]

360 min

(): Fe, JCPDS: 89-7194 [103] [211]

[021]

[121] [210]

250

120 min 75 min

30 min (110)

Intensity (a.u.)

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

Page 22 of 42

0 min (after H2 reduction) Precursor

20

30

40

2θ (ο)

50

60

Figure 6. Ex-situ XRD patterns of the (3Fe:P)/Al2O3 catalyst after different reaction times.

To assess the activity of the catalyst, a second series of experiments was carried out. The goal was to obtain conversions at a similar level in a low range


Page 23 of 42

(i.e., ~10 %) by only adjusting the amount of catalyst while fixing the other reaction conditions. The corresponding results are shown in Figure 7. Taking the catalyst

amount

into

account,

the

activity

followed

the

order

of

(3Fe:P)/Al2O3 >(2Fe:P)/Al2O3 > (1Fe:P)/Al2O3. In addition, it was found that all samples only suffered mild deactivation during the initial 30 min on stream. As for the propylene selectivity, all three samples showed similar steady state selectivity around 75 - 90 %, with a similar induction period for ca. 30 min. 30

(a)


25

(3Fe:P)/Al2O3

Conversion (%)

20

15

10

5

0 0

50

100

150

200

Time (min)

250

300

350

100

(b)

80

C3H6 selectivity (%)

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

ACS Catalysis

60

40

20

(1Fe:P)/Al2O3 (2Fe:P)/Al2O3 (3Fe:P)/Al2O3

0 0

50

100

150

200

250

300

350

Time (min)

Figure 7. (a) Propane conversion, (b) propylene selectivity over the Fe-based catalysts with different Fe:P ratios. Reaction conditions: 600 °C, 10 sccm of 5 vol%


ACS Catalysis

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

C3H8/N2. Catalyst amount: (1Fe:P)/Al2O3 0.4 g, (2Fe:P)/Al2O3 0.2 g, (3Fe:P)/Al2O3 0.14 g.

The calculated activities over the series of Fe-based catalysts are summarized in Table 4. The reaction rates are presented on both a surface area basis and mass of iron basis. Two types of activities (rates) are reported. The instantaneous activity is calculated by using the 1st data point at 10 min on stream, while the intrinsic activity was estimated by extrapolating the linear region of conversion back to time zero. The results suggest that the activity increased as the ratio of Fe:P increased. With that, the highest activities among the Fe-based catalysts were for (3Fe:P)/Al2O3 at 9.9 mmol/h/gFe, and 13 µmol/h/m2. The corresponding propane conversion was ca. 14 %, with a steady level of C3H6 selectivity at 82.4 %. If only considering the (potentially) active metal species, Fe, the calculated activity (9.9 mmol/h/gFe) was 2 - 5 fold higher than the binary oxide system (1.9 - 4.9 mmol/h/gIn-Ga)22 and comparable to the advanced ternary oxide system (5.7 - 13.8 mmol/h/gIn/Ga)23 that our group reported previously. As an additional comparison, Zhuang’s work on In2O3-Al2O3 mixed oxides exhibited an activity of 2.1 mmol/h/gIn.52 Based on the data collected thus far, the nature of the active species is not unambiguously known, though the circumstantial data provided above may suggest that Fe carbide phases may be important. As an estimate for the size of such reduced iron domains, CO chemisorption experiments were carried out to allow estimation of the dispersion of such domains. The CO uptake for


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ACS Catalysis

(3Fe:P)/Al2O3, (2Fe:P)/Al2O3 and (1Fe:P)/Al2O3 precursors after H2 reduction at 600 °C were 53.8, 37.4, and 7.1 µmol/g, respectively. As indicated in the above discussion, the phosphate species (i.e., Fe2P2O7 and Fe3(PO4)2) appear to be quite less active than iron or iron carbide. From the above characterization results, an increase of the Fe:P ratio led to increased formation of Fe(0) domains after reduction. Accordingly, it is assumed the difference in the CO uptake from the Fe:P sample to the (2Fe:P)/Al2O3 and (3Fe:P)/Al2O3 samples is solely due to metallic Fe surface adsorption, the calculated dispersion was ca. 3.6 %, with a particle size of ca. 26 nm for the most active (3Fe:P)/Al2O3 catalyst. It should be mentioned that the CO uptake for (1Fe:P)/Al2O3 precursor without H2 reduction was ca. 8.6 µmol/g. The minor difference between the reduced and unreduced catalysts (7.1 vs. 8.6 µmol/g) may be within experimental error. Hence, these CO titration experiments suggest the possibility of formation of small Fe(0) domains in the Fe:P/Al2O3 catalyst, which might not be detected by XRD, was unlikely. The average size of the Fe3C domains was also estimated from the XRD patterns of (3Fe:P)/Al2O3 sample using the Scherrer equation. The [022] peak of Fe3C at 44.8° in Figure 6 suggests the carbide species have particles that are ca. 33 nm in size, on average, with an estimated dispersion of ca. 2.9 %. Thus, the estimates of dispersion via CO chemisorption and XRD examination gave consistent results, within the limit of instrument error. These features can be further probed by TEM (see supporting information, Figure S4). For the (3Fe:P)/Al2O3 sample after 6 h reaction, domains of roughly spherical shape were observed, with the presence of interplanar distances of ca. 2.07 Å, which


ACS Catalysis

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

Page 26 of 42

corresponds to (210) crystalline plane of orthorhombic phase of iron carbide. The particle size was ca. 30 nm, which is consistent with the XRD and chemisorption results described above. It is interesting to note that these iron carbide domains are surrounded by coke layers on the catalyst surface. Based on the hypothesis that Fe3C domains contain the active sites, the corresponding TOF for most active catalyst ((3Fe:P)/Al2O3) was calculated to be ca. 19 h-1. This estimated value is about half of that reported for metallic Fe nanoparticles deposited on SiO2 (43 h-1), but with 6-fold improvement in C3H6 selectivity (80 % vs. 14 %). Moreover, this TOF is almost 20-fold higher than the value reported for single site Fe catalysts (1.1 h-1) at even higher reaction temperatures (650 °C). 28

Table 4. Catalytic performance of the Fe-based catalysts reported in this work. Conversiona

Catalyst

Atomic selectivityb (%)

Activityc (%) mmol/h/gFe µmol/h/m2

(%)

CH4

C2H6

C2H4

C3H6

(1Fe:P)/Al2O3

15.3 (13.5)

2.7 (4.2)

1.8 (2.3)

0 (8.1)

45 (82.3)

4.6 (4.1)

6.2 (5.4)

(2Fe:P)/Al2O3

16.8 (12.9)

2 (4.2)

1.2 (1.9)

1.3 (6.5)

20 (79.8)

9.4 (7.6)

12.5 (9.6)

(3Fe:P)/Al2O3

14 (12)

2.4 (3.1)

1.9 (2)

2.1 (5.8)

26 (82.4)

9.9 (8.1)

13 (11.1)

Al2O3

3.4

7

4.8

14.9

64.2

P/Al2O3

3.4

10.7

5.9

24.7

52.6

Blank reactor

2

11.7

8.7

28.3

49.4

a

Initial conversions are data obtained at 10 min. Numbers in parenthesis are

data obtained at 6 h on stream.


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ACS Catalysis

b

Initial selectivities are data obtained at 10 min. Numbers in parenthesis are the

average value at steady state. c

Instantaneous activities are calculated by using the first data point. Intrinsic

activities (in parenthesis) are estimated by extrapolating the linear region of conversion to time zero.

This study suggests that a higher Fe:P ratio leads to enhanced catalytic performance. To further probe this finding, a control catalyst (Fe2O3/Al2O3) without phosphorous (probing the extreme condition of Fe:P = ∞) was synthesized using the same method with the same Fe loading. Under the standard reaction conditions, it was found that the resulting material coked severely within 2 h, to the extent that the reactor was clogged and gas flow was substantially impeded. A significant amount of H2 and CH4 was observed with low C3H6 selectivity. This control experiment suggests that the presence of phosphorous (or perhaps related, see below) species is crucial for creation of active sites that allow for combined high activity and high C3H6 selectivity under these conditions. It was recently reported that Fe2O3 deposited on sulfated Al2O3 exhibited interesting activity and selectivity for propane dehydrogenation.38, 53 In contrast, our above results suggest that pre-reducing the iron-containing catalysts results in promising performance. To this end, it is worth examining the effect of H2 pretreatment as a way of comparing and distinguishing if Fe2O3 or Fe(0) is a better precursor for formation of an active and selective iron phase. Figure 8


ACS Catalysis

shows reactivity data for the (3Fe:P)/Al2O3 catalyst with and without pretreatment using H2. As can be seen in Figure 8a, the catalyst with H2 pretreatment showed conversion that was 2-fold higher (17 % vs. 9 %) than the same catalyst without pretreatment, with the steady-state level of selectivity being slightly lower (Figure 8b). The higher content of C1 and C2 species over the unreduced Fe2O3 sample further suggest that the oxide species could also promote unwanted cracking side reactions. However, over the Fe0 surface formed by hydrogen pretreatment, the cracking reactions were suppressed. In addition, it should be mentioned that with H2 pretreatment, the catalyst again showed an induction period (low C3H6 selectivity and high H2 content), whereby Fe3C was formed, while for Fe2O3 catalyst (without H2 reduction), no induction period was apparent. According to these results, it is inferred that Fe2O3 and Fe(0) pre-catalysts may form different catalytic species, leading to different activities and selectivities during propane dehydrogenation. 5

30 (a)

Conversion, without H2 pretreatment Conversion, with H2 pretreatment

25

H2 vol%, without H2 pretreatment

4

H2 vol%, with H2 pretreatment 20 3 15 2 10 1

5

0 0

50

100

150

200

Time (min)


0 250

H2 vol (%)

Conversion (%)

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

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Page 29 of 42

100

(b)

90 80

Atomic selectivity (%)

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

ACS Catalysis

70

C3H6

60

CH4

50

C2H6 C2H4

40

C3H6

30

CH4 C2H6

20

C2H4

10 0 0

50

100

150

200

250

Time (min)

Figure 8. (a) Propane conversion and H2 volume percentage, (b) atomic selectivity to C1-C3 products over the (3Fe:P)/Al2O3 catalyst with (open symbol) and without (closed symbol) H2 pretreatment. Reaction conditions: 0.2 g catalyst, 600 °C, 10 sccm of 5 vol% C3H8/N2.

For further insights into the possible active Fe site(s), the (2Fe:P)/Al2O3 precursor with H2 reduction pretreatments at different temperatures, creating Fe(0) and Fe phosphide domains, was studied in the PDH reaction. As can be seen in Figure 9, the catalyst that was reduced at 600 °C showed almost a 2-fold higher steady-state conversion compared to the material pre-reduced at 900 °C (ca. 13 % vs. ca. 7 %). Both cases exhibit a similar level of propylene selectivity (ca. 80 %). Taking the XRD measurements into account, the collected data indicate that under the lower reduction temperature (600 °C), Fe(0) was likely to be that starting material, and the formed carbide phase during the induction period created the primary active sites. In contrast, with a higher reduction temperature (900 °C), the resulting phosphide phase appears to be less active.


ACS Catalysis

The incomplete removal of the induction period could be due to the presence of small Fe(0) domains (not observable with XRD). It should be mentioned that the post-reaction iron phosphide sample was also examined by XRD, and was found to show identical iron phosphide patterns as the fresh sample, with no indication of the formation of iron carbide domains.

3.0

100

2.5

80

1.5 40

H2 vol. (%)

2.0 60

%

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

Page 30 of 42

1.0 20

0.5

0 0

50

100

150

Time (min)

200

250

0.0 300

Figure 9. Propane conversion (squares), propylene selectivity (circles) and H2 volume percentage (triangles) over the (2Fe:P)/Al2O3 precursors reduced at 900 °C (open symbols) and 600 °C (closed symbols). Reaction conditions: 0.2 g catalyst, 600 °C, 10 sccm of 5 vol% C3H8/N2.

The best catalyst, (3Fe:P)/Al2O3, was chosen for a longer duration experiment to examine the stability and the results are shown in Figure 10. It could be seen


Page 31 of 42

that over 24 h of reaction, the catalyst still exhibited stable activity, with the conversion remaining at ca. 20 % throughout the experiment. The C3H6 selectivity started at ca. 75 % while slowly dropping to ca. 65 % over the course of the experiment. The results suggest that the catalyst offers a moderate degree of stability under the conditions employed. 100 90 80 70 60

%

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

ACS Catalysis

50

Conversion CH4

40

C2H6 C2H4

30

C3H6

20 10 0 0

5

10

Time (h)

15

20

25

Figure 10. Stability test over the (3Fe:P)/Al2O3 precursors reduced at 600 °C. Reaction conditions: 0.4 g catalyst, 600 °C, 20 sccm of 5 vol% C3H8/N2.

Because some authors operate this reaction at lower temperatures when using highly active catalysts, such as those used commercially (Pt and Cr based systems), the best catalyst, (3Fe:P)/Al2O3, was also studied at 570 °C (Figure S5a-b). Operation at this temperature led to a dramatic drop of conversion (from ca. 20 % to ca. 6 %). However, the steady state selectivity increased from 75 % to 90 %. As expected, lower temperatures (and lower conversions) suppress side reactions such as cracking and coke formation. This can be confirmed with the higher degree of carbon balance closure, as shown in Figure S5b.


ACS Catalysis

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

In summary, the above tests as well as ex-situ XRD measurements suggest that Fe3C, formed from reduced Fe(0), is more active than Fe2O3 or iron phosphide in propane dehydrogenation, while also allowing for high propylene selectivity. To readily form the carbide phase, H2 pre-reduction is helpful and the presence of phosphorous species plays a key role in maintaining the stability of the catalyst.

3.3 In-situ XAS measurements In-situ XAS measurements at the Fe K-edge (ca. 7112 eV) were carried out to gain insight into the changes of the Fe species during the reaction. The XANES data can provide useful information regarding the electronic properties (i.e., oxidation state) of the metal centers. As can be seen in Figure 11a, for the (3Fe:P)/Al2O3 catalyst prior to H2-reduction (precursor), the pre-edge energy of the 3d orbital transition was at ca. 7114.5 eV, indicating the presence of Fe(III) species.

28

After H2 reduction, the pre-edge peak disappeared. Moreover, as the

PDH reaction time increased, the signal intensity at ca. 7115 eV kept decreasing until a steady state value was reached. Walton’s group studied the chlorination of iron carbide by XAS and the final spectrum in Figure 11a is similar to the spectrum of Fe3C in their work,

54

which suggests that iron carbide-like species

are formed during PDH, consistent with the ex-situ XRD analysis discussed above. The R-space of the EXAFS spectra are plotted in Figure 11b. Before the reduction, the strongest signal was at ca. 1.4 Å, associated with the Fe-O bond. After H2 pretreatment, a strong peak appeared at ca. 2.2 Å, together with a


Page 32 of 42

Page 33 of 42

weakening of the peak at 1.4 Å, indicating the formation of metallic Fe domains (see Fe foil standard) and fewer iron oxide species. As the PDH reaction started, the Fe-Fe bond intensity decreased along with an increase of a peak at ca. 1.5 Å. The new peak is suggested to be associated with Fe-C bonds in Fe3C. Such observations are consistent with the transformation of Fe(0) to Fe3C species, as also described above. 1.4

(a)

1.2

Normalized µ(E)

1.0

0.8

Foil H2 reduction 1 h

0.6

reaction 0.5 h reaction 1 h reaction 2 h reaction 3 h precursor

0.4

0.2

0.0 7110

4.0

7120

7130

7140

Energy (eV)

(b)

3.5

reaction 0.5 h reaction 1 h reaction 2 h reaction 3 h precursor

3.0 2.5

3

2.0

χ(

()

Å | R |

7150

Foil H2 reduction 1 h

1.5 1.0 0.5 0.0 0

1

2

3

Å

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

ACS Catalysis

4

Radical distance ( )

5

6

Figure 11. (a) X-ray absorption near-edge spectroscopy (XANES) and (b) the magnitude of the Fourier transform of the EXAFS spectra from in-situ PDH over the (3Fe:P)/Al2O3 catalyst at 600 °C.


ACS Catalysis

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Page 34 of 42

In summary, the in-situ XAS measurements are consistent with the reactivity tests and ex-situ XRD measurements, suggesting a change of the Fe species in the samples proceeding down the following pathway under reaction conditions: Fe2O3 Fe Fe3C. The final state of the iron species, Fe3C, is proposed to likely be the main catalytically active phase.

Nonetheless, based on the

compiled data in this first report, some activity associated with iron oxide domains that are unconverted to metallic iron or iron carbide cannot be excluded in the catalysts. Control experiments suggest that iron phosphide phases are also catalytically active if formed, though under the conditions employed (600 °C reduction) in this work, such phases are not suggested to exist.

3.4 Coke analysis Coke generation is always a concern for PDH catalysis, as it may cause loss of activity and necessitate frequent catalyst regeneration. Hence, the properties of the carbon deposited on the catalysts have been investigated with Raman Spectroscopy by several groups. The corresponding D and G band ratio (D/G) from Raman spectra has been used as a factor that reflects the properties of the deposited coke. For example, Weckhuysen et al. reported coke analysis associated with Pt based catalysts.55-56 They found that Pt/Al2O3 gave values in the range of 0.54 - 0.81, while addition of Sn and Ga provided coke in the range of 0.67 - 0.78 and 0.7 - 0.97, respectively. Meanwhile, we previously examined mixed oxides containing Group IIIA elements for PDH. The reported D/G values


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for these catalysts varied from 0.7 - 0.88 for the binary system (In2O3-Ga2O3), and 0.45 - 0.6 for the ternary system (In2O3-Ga2O3-Al2O3).22-23 All together, these studies suggest that coke with more graphitic properties correlate to catalysts with higher C3H6 selectivity. In this work, we examined the spent Fe-based catalysts using Raman spectroscopy. The Raman spectra analysis was carried out in same way as in our previous reports.22-23, 57 The deconvolution results are given in the supporting information (Figure S6). The results are summarized in Table 5. As can be seen from the data, all three samples had quite similar D1/G ratios (0.6 - 0.7) within the limit of instrument error. Meanwhile, the steady levels of C3H6 were also quite similar (79.8 % - 82.4 %). It thus seems that the empirical trend correlating coke properties with propylene selectivity holds for the current series of Fe-based catalysts. Others have also studied the carbon species present in iron carbide phases by Raman spectroscopy.

Li et al. synthesized Fe3C deposited on N-doped

graphene and found the D/G band ratios were ca. 1:1 regardless of the synthesis temperature (700 - 900 °C).58 In another study, Weckhuysen and co-workers synthesized a series of ε-θ-χ iron carbides via the Fischer-Tropsch reaction. However, due to the co-existence of multiple phases of carbide, a quantitative analysis of the Raman spectra was difficult.59 Howe et al. investigated the reduction and carburization of Fe2O3 under harsher conditions (750 and 835 °C) and found the resultant Fe3C was covered by a thin layer of amorphous carbon.60


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Unfortunately, in the current study, we were unable to distinguish the carbon species in the iron carbide and in deposited coke. The D/G bands for both carbide and coke appear at ca. 1360/1590 cm-1.61 To this end, the possibility of simultaneously forming both carbide and coke under the reaction conditions cannot be excluded, although we propose that during the induction period, carbide species may be primarily formed.

Table 5. D1/G ratios of the catalysts after reaction. Catalyst

(D1/G) Intensity ratio

C3H6 selectivitya

Coke amountb (wt%)

(1Fe:P)/Al2O3

0.63

82.3 %

0.3

(2Fe:P)/Al2O3

0.7

79.8 %

2.7

(3Fe:P)/Al2O3

0.6

82.4 %

3.5

a

: Estimated steady value from Figure 7.

b

: Obtained from samples used in Figure 4, by ICP-OES.

4. Conclusions This work describes a new class of Fe-based catalysts prepared in the presence of a phosphorous source for propane dehydrogenation. The evolution of the nature of the iron species is tracked in the initial stages of the catalytic reaction, and the results suggest that the pre-reduction to Fe(0) by H2 lead to enhanced reactivity. The formation of iron carbide species during the induction period is responsible for the high activity and selectivity of the resulting catalyst. The presence of phosphorous element plays key role in maintaining the stability


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of the resulting catalysts. Of the series of tested materials, the (3Fe:P)/Al2O3 composition showed superior performance, with ca. 14 % conversion and > 80 % C3H6 selectivity under a standard set of reaction conditions. The corresponding activity was 9.9 mmol/h/gFe, and 13 µmol/h/m2, correlating to a TOF of 19 h-1. The current series of Fe-based oxide pre-catalysts, after a reductive pretreatment, display an induction period (ca. 30 - 60 min) under PDH reaction conditions. During this period, the gaseous hydrocarbon selectivities were low, and substantial carbon was incorporated into the catalyst. XRD measurements, XAS measurements and elemental analysis suggest that during this induction period, the Fe species were first reduced to Fe(0) and then evidence for formation of Fe3C appeared. Once the iron carbide phase appeared, the catalyst entered a period of steady operation, where propylene selectivity was high and carbon deposition was low.

These observations suggest that iron carbide is likely

responsible for the PDH activity. Control experiments suggest that addition of the phosphorous source (phosphate) was helpful in creating catalysts that displayed good activity (with little deactivation) and high C3H6 selectivity, while effectively suppressing cracking reactions.

Supporting Information XRD patterns of unsupported catalysts, XRD patterns of supported catalysts treated at different reduction temperatures, XPS spectra, TEM images, and reactivity data for the (3Fe:P)/Al2O3 catalyst over an extended reaction period.


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Author Information Corresponding Authors *

[email protected]

*

[email protected]

Acknowledgements This work was financially supported by the Dow Chemical Company. Use of the Advanced Photon Source was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DEAC02-06CH11357. MRCAT operations are supported by the Department of Energy and the MRCAT member institutions. The authors also would like to especially thank Seung-Won Choi from Georgia Tech, and Dr. Guanghui Zhang (Argonne National Laboratory) for fruitful discussions.

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Graphical Abstract


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Propane Dehydrogenation over Alumina ... - ACS Publications

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