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Fischer-Tropsch Synthesis Performance of Supported Nano-Iron Catalysts Synthesized By a Gas-Expanded Liquid Deposition Technique Rui Xu, Pranav Shekhar Vengsarkar, David P. Roe, and Christopher Brian Roberts Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02971 • Publication Date (Web): 28 Feb 2017 Downloaded from http://pubs.acs.org on March 3, 2017

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Fischer-Tropsch Synthesis Performance of Supported NanoIron Catalysts Synthesized By a Gas-Expanded Liquid Deposition Technique Rui Xu #, Pranav S. Vengsarkar # , David Roe and Christopher B. Roberts* Department of Chemical Engineering, Auburn University, Auburn, AL, USA 36849 *Corresponding author: [email protected]; Phone: (+1)334 844-2303; Fax: (+1) 334 844-2063 #

Denotes Equal Contributions

Abstract A gas-expanded-liquid technique (GXL) was applied to the preparation of supported iron catalysts for Fischer-Tropsch synthesis (FTS). A mixture of pre-synthesized iron oxide nanoparticles with an average size of 12 nm was deposited onto a SiO2 support in a manner such that nanoparticles smaller than 5 nm were excluded from the final catalyst product. A series of SiO2-supported iron oxide nanoparticle catalysts were prepared with iron loadings of 11.4, 18.0, 24.0, and 28.8 wt%. Catalysts were characterized by nitrogen adsorption, ICP-OES, H2-TPR, XRD, TEM, XPS, and CO-TPD. TEM analysis showed that iron oxide nanoparticles were welldistributed over the surface of the SiO2 support, with an increase in the iron loading resulting in the formation of multilayers and three-dimensional islands of iron oxide nanoparticles. H2-TPR indicated that the reducibility of the iron oxide nanoparticles increased monotonically with iron loading. According to XRD results, the increase in the iron loading resulted in an increase in the iron oxide crystallite size, and iron carbides were present in the used catalysts after solvent treatment. FTS was carried out in a fixed-bed reactor at reaction conditions of 230 °C, 2 MPa, H2/CO=1.70, and GHSV=3000 L/kgcat/h. For the catalysts studied, the highest syngas conversion 1 ACS Paragon Plus Environment

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and iron time yield were observed at the intermediate (18-24 wt%) iron loadings. The C5+ selectivity and carbon chain growth probability factor were about 68% and 0.76, respectively, for each catalyst. A 16 wt% iron on SiO2 catalyst prepared by the same GXL technique was promoted with 0.7 wt% potassium using the incipient wetness method. As expected, K promotion resulted in a slight decrease in FTS activity and increased selectivity toward CO2 and C5+ selectivity.

Keywords: iron oxide, nanoparticle, supported iron catalyst, gas-expanded liquids, FischerTropsch synthesis

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1

Introduction Iron catalysts have been commonly used in the commercial Fischer-Tropsch synthesis

(FTS) process, which produces various hydrocarbon fuels and chemicals from synthesis gas (a mixture of CO and H2) via surface catalyzed polymerization reactions.1,2 Iron based catalysts are well suited for applications involving a relatively CO-rich syngas feedstock in FTS since they also catalyze the water-gas shift reaction (WGS), which can generate H2 from CO and H2O. This WGS activity enables the utilization of coal derived synthesis gas without an adjustment of the H2/CO ratio.3 Relative to other FTS catalysts, iron catalysts also give higher selectivity towards olefins and alcohols. Unsupported iron catalysts are commonly prepared by co-precipitation methods and doped with promoters such as K and Cu. Due to their poor mechanical stability, precipitated iron catalysts can be unsuitable for high temperature applications or use in stirred tank slurry reactors. As an alternative to the use of unsupported iron catalyst, iron can be dispersed onto the surface of a support material such as alumina or silica in order to provide additional physical stability. Supported iron catalysts provide a few advantages including better mechanical strength and increased attrition resistance relative to unsupported iron4, enhanced dispersion of the active phase over a higher surface area5, and resistance to sintering6. The development of a commercially viable supported iron catalyst has been met with only limited success. Previous investigations of supported iron catalysts for FTS have shown that the catalytic performance of supported iron catalysts was inferior to that of precipitated iron catalysts. The inadequate activity and selectivity towards desired hydrocarbon products have been attributed to a strong support-metal interaction4, the reduction of potassium promotion


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effectiveness7, and ineffective preparation methods8. For example, Galvis et al. indicated that a strong metal-support interaction inhibited the formation of catalytically active iron carbides, resulting in a low activity.4 Utilization of an inert support material was suggested as a way to impart mechanical stability to the iron nanoparticles without inhibiting their activation for high temperature FTS light olefin production. Bukur and Sivaraj7 evaluated and compared the activity and selectivity of several supported iron FTS catalysts in a slurry reactor. They found that silica supported catalysts had higher initial activity, lighter products, and lower olefin content compared to their precipitated counterparts. They stated that the interaction between potassium and the support materials resulted in a reduction in the effectiveness of potassium promotion. SiO2 was found to be superior to Al2O3 in terms of providing a relatively inert surface, and therefore, the reduction behavior of iron oxide was more similar to that of the precipitated iron catalysts.7 Xu et al.8 prepared 10% Fe/SiO2 and FePt/SiO2 catalysts using a nonaqueous evaporative deposition technique and previously dehydroxylated silica supports. They stated that this preparation method led to moderate dispersion and relatively high extent of reduction due to minimization of surface hydroxyl groups, which can react with the iron precursor to form iron oxide-support complexes.8 In our group’s previous research, a technique was developed that uses a gas expanded liquid (GXL) to controllably deposit pre-synthesized iron oxide nanoparticles from an organic liquid dispersion onto an oxidic support material, such as alumina and silica. This GXL deposition technique can be performed in such a manner that nanoparticles smaller than a certain size (e.g. 5 nm) can be excluded from the final product (i.e. not deposited on the surface) while all larger particles can be conformally mapped onto the support surface. Vengsarkar et al.9 4 ACS Paragon Plus Environment

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prepared a 10 wt% Fe/SiO2 catalyst using this GXL method and compared it to a 10 wt% Fe/SiO2 prepared by a conventional incipient wetness method. The GXL-prepared catalyst demonstrated a higher extent of reduction, indicating a weaker interaction between iron oxide nanoparticles and the support materials. Higher CO conversion, higher C5+ selectivity and lower CH4 selectivity were observed over the GXL catalysts compared to their incipient wetness counterparts. The objective of this work was to explore the benefits of using the GXL technique to prepare SiO2-supported catalysts with moderate iron loadings for Fischer Tropsch synthesis applications. The catalytic performance of GXL prepared Fe/SiO2 catalysts with different iron loadings were evaluated in terms of CO conversion, CH4 selectivity, C5+ selectivity and C5+ productivity. The use of SiO2 as the support material allows for i) improvement in the thermal stability of the Fe catalysts, ii) better maintenance of high surface areas upon high temperature treatment compared to the unsupported catalysts, and iii) inhibition of the sintering of the iron particles6. In addition to the effect of iron loading, we have also investigated the effect of reaction conditions, such as reaction temperature, on the catalytic performance of our GXL catalysts in order to understand the influence of operating conditions on the selectivity and productivity of desired hydrocarbon products.

2 2.1

Experimental Section Materials: Iron (III) chloride hexahydrate (FeCl3·6H2O, 99.9%), iron (II) chloride tetrahydrate

(FeCl2·4H2O, 99.9%), and n-hexane (HPLC grade, 95%), were obtained from Alfa Aesar. Oleic acid (99%), was obtained from Sigma Aldrich, and ammonium hydroxide (5M) was obtained 5 ACS Paragon Plus Environment

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from BDH Chemicals. Deionized ultra-filtered (DIUF) water was obtained from Fisher Scientific. Silicon oxide pellets (0.125 inch pellets, 250 m2/g surface area) were obtained from Alfa Aesar. Carbon dioxide (SFC/SFE grade) was obtained from Airgas. All chemicals were used as received without further purification. 2.2

Catalyst preparation: The procedure for synthesizing iron oxide nanoparticles has been reported previously10.

Briefly, two solutions of iron chloride (Fe2+ and Fe3+) were synthesized with concentrations formulated to achieve the desired amount (wt. %) of iron to later be deposited onto the SiO2 support. These solutions were then mixed in a three-necked flask. Under constant magnetic stirring and an N2 atmosphere, a solution of ammonium hydroxide was added to precipitate iron oxide nanoparticles from solution. The flask was then heated from ambient to 80 °C. During the heating process, a predetermined amount of oleic acid was added to the mixture. The elevated temperature was maintained for 30 min to evaporate the ammonia from the solution. After cooling, the suspended oleic acid-coated Fe nanoparticles in the flask were magnetically immobilized and washed thoroughly with DIUF water to remove any excess reagents and impurities. The particles were then dried under flowing nitrogen and then dispersed in n-hexane via sonication. Four batches of catalysts were prepared by depositing pre-synthesized iron oxide nanoparticles onto the SiO2 support using a gas-expanded technique with targeted iron loadings of 10, 20, 25 and 30 wt. %.9 Actual iron loadings differed slightly from the intended values as will be described below. Herein, these four catalysts will be referred to as G12Si, G18Si, G24Si and G29Si, respectively. The procedure to deposit iron oxide nanoparticles onto the support has been described previously.9 Briefly, 8 g of SiO2 support (212-355 µm range) was first added to a 1L Parr reactor 6 ACS Paragon Plus Environment

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along with a dispersion of oleic acid-coated iron oxide nanoparticles in n-hexane and this mixture was agitated using a stainless steel impeller. The reactor system was then pressurized to 4.83 MPa (700 psi) by introducing CO2 from a syringe pump (ISCO 260D). As CO2 dissolves in the catalyst precursor suspension, it acts as an antisolvent and causes larger nanoparticles to destabilize and precipitate from the solution to the support materials.

The reactor was

maintained at 4.83 MPa under stirring for a period of 20 hours to ensure that the n-hexane and CO2 mixture reached equilibrium. After 20 hours, the impeller was stopped and the particles were allowed to settle for 30 minutes. The reactor was equipped with an internal siphon tube that allowed for withdrawal of the solvent and smallest nanoparticles—still dispersed—from the vessel while leaving the SiO2 support and the deposited nanoparticles in the vessel. The gasexpanded hexane was displaced from the reactor via the siphon tube, while the system pressure was maintained at 4.83 MPa. The hexane removal continued until only CO2 gas was emitted from the outlet of the siphon tube and no liquid effluent was observed. The system was then slowly depressurized to atmospheric pressure and the catalyst precursor was dried by flowing CO2 through the system for 15 mins. The catalyst precursor was further dried overnight in air at 100 °C then calcined under air flow. During calcination the temperature was increased by 5 °C/min until reaching 375 °C, then held constant for 4 hours before cooling. 2.3

Catalyst characterization BET surface areas of the catalysts were determined by N2 adsorption at -196 °C using a

Micromeritics Tristar II surface area analyzer. The pore-size distributions were calculated from the BJH desorption branch of the isotherm. Before BET analysis, all samples were degassed at 350 °C for 4 hours under a flow of helium.


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The actual iron loading of the calcined catalysts was determined by ICP-OES using a Spectro Ciros ICP (SPECTRO Analytical Instruments, Kleve, Deutschland). Samples were weighed to 0.1 g and digested using EPA Method 3051A, which is a microwave digestion procedure to extract total elements. Microwave digestion was performed on a Mars Xpress system (CEM Corporation, NC) with 10 mL of concentrated nitric acid. Digested samples were filtered and the volume made up to 100 mL with deionized water. X-ray diffractometer (XRD) data was obtained through the use of a Bruker D8 diffractometer using Cu Kα radiation operated at 40 kV and 40 mA. The sample was prepared by placing a small portion of the powder on a glass slide. The diffraction patterns were collected using a step size of 0.01° and 0.2 s/step count time from 5° ≤ 2θ ≤ 90°. The obtained peaks were compared to available spectra for known compounds using the International Center for Diffraction Data (ICDD) database to determine the species present. The used FTS catalyst samples were toluene treated for one hour using Foss 245 Soxhlet extraction equipment in order to remove the waxy product residues. TEM images were obtained on a Zeiss EM 10 transmission electron microscope and sized using the ImageJ software package. High resolution TEM image was taken on a FEI 200 kV Tecnai F-20 TEM equipped with an EDX detector. Samples were prepared by dropcasting an ethanol solution of the catalyst onto carbon-coated copper grids. The surface characterization of the samples by X-ray Photoelectron Spectroscopy (XPS) was done with a Kratos Ultra Axis DLD (delay lines detector) spectrometer under high vacuum using a monochromatic Al source at 10 kV and 15 mA. The data was analyzed using the software of XPSPEAK41. The calibration of the binding energy was referenced to the peak of Si 2p at 103.3 eV.11


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H2-TPR experiments were performed in a Micromeritics Chemisorb 2750 system equipped with ChemiSoft TPx. An approximately 0.15g catalyst sample was placed in a quartz tube in a temperature-controlled furnace and connected to a thermal conductivity detector (TCD). The sample was first purged under 50 ml/min of N2 flow at 100 °C for 1 hour and then cooled to room temperature. The carrier gas was switched to a reducing gas mixture (10% H2 in Ar) with a flow rate of 25 mL/min and the temperature was increased to 900 °C at a constant rate of 5 °C/min. The iron dispersion of the used catalysts was analyzed via temperature programmed desorption (TPD) using the same system described for TPR analysis. Prior to TPD analysis, heavy wax products were removed from the used catalyst surface and pores via a one hour Soxhlet extraction in a Foss 245 Soxtec with toluene. An approximately 0.30 g sample of treated catalyst was then reduced for 20 hours at 400 °C in a 50 ml/min flow of 5%H2/N2. The temperature was raised from room temperature to 400 °C at a rate of 5 °C/min. The reduced catalyst was allowed to cool to 40 °C in the flow of H2/N2, followed by a one hour purge with 50 ml/min of argon. For H2-TPD, hydrogen desorption was monitored and recorded as temperature ramped from 40 °C to 600 °C at a rate of 10 °C/min in a flow of 40 ml/min argon. The catalyst was again allowed to cool, and 10% CO in helium was pulsed through the sample bed at 40 °C until saturation coverage was achieved. Subsequently, CO-TPD was performed by ramping the furnace temperature from 40 °C to 700 °C at 10 °C/min. Temperature was held at 700 °C until the TCD response returned to its baseline (about 30 minutes). The values of % CO active sites were calculated from the CO-TPD results, assuming a Fe:CO stoichiometry of 2:1.


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2.4

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FTS Catalytic Studies: The catalytic FTS experiments were carried out in a fixed bed reactor using a feed gas

mixture of CO (36.49%), H2 (62.02%) and N2 (1.49%). The reaction system used in this study is described in our previous work

9,12

. One gram of catalyst (45-70 mesh) was mixed with 2 gram

of glass beads (40-60 mesh) and was placed into the reactor bed. The catalyst was then reduced in situ for 12 hours at 370 °C under a stream of H2 (5% in N2, 100 ml/min). The temperature of the reactor bed was then cooled to 300 °C, and the atmosphere was switched from H2/N2 to syngas with a flow rate of 50 ml/min for 24 hours. The temperature of the reactor was then reduced to 230 °C under a flow of helium at 50 ml/min. The system pressure was then raised to 2 MPa. While the reactor was maintained at 230 °C and 2MPa, syngas was introduced at a flow rate of 3000 L/kgcat/h. The reaction conditions in this study are consistent with the conditions employed by our group in our previous research involving low temperature Fischer-Tropsch synthesis over an iron-based catalyst.9,13,14 The liquid products resulting from the FTS catalytic process were analyzed with a Bruker 430 Gas Chromatograph equipped with a DB-5 column (DB-5, 30m × 0.53mm) and a FID detector. The gas phase products were analyzed online on a Varian CP-3380 Gas Chromatograph equipped with a Haysep-DB column and a TCD detector. The CO conversion was calculated as:

X 1

In which, the and is the gas composition measured by GC-TCD for CO and N2 in the feed gas, respectively. The selectivity toward CO2 in the effluents is defined as 10 ACS Paragon Plus Environment

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100 ∗

∗

In addition, the selectivity towards CH4 was reported in a CO2 free basis, and defined as:

100 ∗

∗ ∗

Activity is also reported as iron-time yield (FTY, mol CO/gFe/s), and was calculated as the moles of CO converted to hydrocarbons per gram of iron per second.

3 3.1

Results and discussion Catalyst Characterization The BET surface area of the SiO2 support used in this study was 230 m2/g, and the total

pore volume and average pore width was 0.91 cm3/g and 6.6 nm, respectively. The deposition of iron oxide nanoparticles on the SiO2 support slightly affected the BET surface area. The total pore volume was found to decrease with an increase in iron loading, while the average pore width was largely unaffected, as shown in Table 1. XRD spectra of the calcined catalysts are shown in Figure 1. As discussed in previous findings9, the XRD spectra of the catalysts synthesized using the GXL technique show characteristic peaks of γ-Fe2O3 (maghemite, JCPDS 80-2186) at 30°, 35°, 43°, 57°, and 63°. In this series of samples, the peaks become sharper and higher at elevated iron loading, indicating an increase in the mean γ-Fe2O3 crystallite size. A small amount of α-Fe2O3 (hematite, JCPDS 71-5088) were also detected via XRD, and was found slightly affected by the increase in iron loadings.


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The crystallite sizes of the iron oxide on the SiO2 support were determined using the Scherrer equation and are listed in Table 1. The mean crystallite diameter of the maghemite in G12Si and G18Si were similar, approximately 11 nm, while the crystallite sizes for G24Si and G29Si were approximately 18 nm and 16 nm, respectively. The XRD results for the used catalysts are shown in Figure 2. The most notable changes between the fresh and used catalysts are in the 30° to 65° 2θ range. Firstly, the used catalysts demonstrated characteristic peaks of Fe3O4, which is consistent with previous observations with various iron based FTS catalysts that Fe3O4 was present in FTS used catalysts.15,16 It also should be noted that there is a highly similarly between the XRD spectra for Fe3O4 (01-87-0245) and γFe2O3 (maghemite, JCPDS 80-2186), as shown in Figure 2. Although Fe3O4 has been widely accepted in literature for the used FTS catalyst, other characterization method might be needed in order to assist the discrimination between these two components in the used GXL catalysts. Another remarkable change in the used catalysts is the existence of the characteristic peaks of iron carbide, i.e. Fe5C2, as shown in Figure 2. As the iron loading increased from G12Si to G24Si, it is observed that the intensity of peaks in the 39° to 44° 2θ range increased significantly. Figure 3 shows TEM images for each of the four catalyst samples. The left column (Fig. 3a-d) shows that the iron oxide nanoparticles were well-dispersed on the SiO2 support, while the right column (Fig. 3a’-d’) shows the high-resolution images which illustrate crystallite lattice fringes. In a previous study9, our laboratory demonstrated the ability to separate an identical, polydisperse solution of iron oxide nanoparticles in hexane into two distinct size fractions using the same GXL size fractionation technique described in this paper. Specifically, a hexane solution of dispersed iron oxide nanoparticles with an average size of 12.1 nm ± 4.5 nm were processed using the GXL deposition method such that the nanoparticles larger than 5 nm were 12 ACS Paragon Plus Environment

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selectively deposited onto a catalyst support structure via controlled CO2 pressurization (thereby inducing precipitation of the larger particles from solution), while the smaller particles (with an average size of 2.7 nm ± 1.0 nm) remained in the hexane solution and were excluded from the catalyst support. Fig. 3a shows that iron oxide nanoparticles ranging from 5 to 13 nm are distributed over the surface of the SiO2 catalyst support with some agglomerates as well as some vacancies on the support surface. At elevated iron loadings, the frequency of vacancies on the surface decreases and three-dimensional islands of iron oxide nanoparticles are formed, as shown in Fig. 3b-d. In addition, agglomerates of iron oxide nanoparticles were observed over the surface of G24Si and G29Si catalysts samples as well. It can also be observed from the TEM images that as the iron loading increases multilayers of iron oxide nanoparticles are present on the SiO2 surface. The reduction behavior of the catalysts was investigated by temperature programmed reduction (TPR) experiments with hydrogen. The H2-TPR profiles of the four supported Fe catalysts prepared by the GXL deposition method are shown in Figure 4. The catalysts’ TPR profiles have been normalized on the basis of iron mass to facilitate comparison. The degree of reduction is calculated as the ratio of H2 consumed during reduction process to the theoretical amount of H2 required for complete reduction of Fe2O3. The degree of reduction increased from 52% to 84% as the iron loading was increased from 11.6% to 28.8%. This increase in the reducibility with the increase in iron loading corresponds well with previous findings in the literature.17 Each of the four TPR profiles shows two distinct groups of reduction peaks which correspond to the two-step reduction process of Fe2O3. 6,7,18 The first group of peaks—appearing in the range of 200 to 375 °C—is a product of the reduction of Fe2O3, which typically forms 13 ACS Paragon Plus Environment

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Fe3O4. Within this group of peaks there is a primary peak which appears at a lower temperature and a secondary peak that appears at a higher temperature and increases in intensity at elevated iron loadings. As indicated by the XRD results, the iron oxide is present on the SiO2 as a mixture of maghemite (γ-Fe2O3) with trace amounts of hematite (α-Fe2O3). Considering both the XRD results and information about H2-TPR profiles of maghemite and hematite in the literature,19,20 these results indicate that the first, larger of the peaks in the first reduction step is the result of γFe2O3, while the second, smaller peak is due to the reduction of α-Fe2O3. The second peak starts ca. 350 ºC for each catalyst and is attributed to the reduction of Fe3O4 to metallic Fe. This reduction step is commonly reported as occurring slowly as temperature rises, as indicated by the large peak width observed in the H2-TPR profile.7 As shown in Figure 4, the Fe3O4 reduction peak for the G12Si and G18Si catalysts occurs at lower temperatures—approximately 470-480 ºC—while higher iron loadings delayed the reduction peaks for G24Si and G29Si to 530 ºC and 565 ºC, respectively. This shift to higher temperature at elevated iron loadings can be simply attributed to the increased amount of H2 required for reducing Fe3O4. It is also observed that a small shoulder peak appears at a temperature higher than 650 °C, which can be attributed to some Fe3O4 being reduced to metallic iron via an intermediate, stable wüstite phase.18,20 Additional information about TPR peak areas, peak ratios, and the extent of reduction can be found in Table S1 in the supporting information. The peak area ratio is defined as the area under the curve resulting from the second reduction step divided by the area resulting from the first reduction step. Assuming the complete reduction of Fe2O3 to Fe3O4 in the first step and of Fe3O4 to α-Fe in the subsequent step, the peak area ratio should theoretically be 8.0. The observed peak area ratios range from 4.6 to 6.3. These relatively low peak area ratios most likely 14 ACS Paragon Plus Environment

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indicate either that the first reduction step proceeds beyond Fe3O4 or that the second reduction stage is incomplete.21 In order to differentiate between these two possibilities, the degree of reduction for the first step was also calculated and listed in Table S1. For the first reduction step alone, the G12Si catalyst was the only one to consume less (7% degree of reduction) than the theoretical amount of H2 (11.1% degree of reduction) indicating that it is the only one of the catalysts to have inhibited reduction of Fe2O3 to Fe3O4. The remaining catalysts each consumed slightly more than the theoretical amount of H2 in this step. We therefore conclude that, in the absence of the reduction of Fe2O3 beyond Fe3O4 in the first reduction step, the deviations from the theoretical peak area ratio observed for each catalyst are primarily due to inhibited reduction of Fe3O4 to metallic iron. The reduction behavior shown in H2-TPR profiles is impacted by the extent of metalsupport interactions. For co-precipitated bulk iron catalysts, the reduction of Fe2O3 typically proceeds via the formation of Fe3O4. In contrast, for supported iron catalysts, other reduction intermediates such as Fe2SiO4 and FexO have been reported17,18,22. When the reduction of Fe2O3 proceeds via these intermediates, the first reduction peak in the TPR profile is broad and shifted to elevated temperature. Yuen et al.22 reported that the reduction of a 1 wt% Fe/SiO2 catalyst resulted in the formation of ferrous ions which strongly interacted with the silica support, generating an iron (II) silicate. Zhang et al. demonstrated that the reduction of SiO2-supported Fe2O3 can form FexO as well as Fe2SiO4, which accounts for the incomplete reduction observed in that TPR study.18 The formation of iron (II) silicate was attributed to the intimate contact between the support and iron precursor, forming a Fe-O-Si structure.23 The reduction behavior of the four GXL catalysts in this study more closely resembles a reduction via Fe3O4 rather than other intermediates. In a previous paper9, our lab demonstrated that this G12Si catalyst exhibited 15 ACS Paragon Plus Environment

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better reducibility than its counterpart prepared using the incipient wetness method at the same metal loading. While this G12Si catalyst exhibited better reducibility than its incipient wetness counterpart, it exhibited the lowest reducibility among the four catalysts examined in the current paper, where reducibility increased monotonically with increasing metal loading. These observations indicate that reducibility, and therefore relative strength of the metal-support interaction, can be affected by the metal loading in highly dispersed catalysts produced using the GXL process. One explanation for the relationship between Fe loading and reducibility is that the SiO2 support influences the electronic structure of Fe atoms and leads to an electron-deficient state of all iron species, resulting in strengthened Fe-O covalent bonds.23 As the iron loading is increased however, the effect of the SiO2 support on the Fe-O covalent bonds becomes less significant as additional Fe nanoparticles serve to dilute the effect and multilayers of iron oxide form with less-intimate contact with the support. To investigate the electronic state of Fe on the surface of the SiO2 support, XPS spectra were collected for these four catalysts after calcination. Figure 5 displays the Fe 2p XPS spectra as a function of Fe loading. The peaks at ca. 710.5 eV, 719.0 eV, and 724.3 eV were assigned to the 2p3/2, the satellite, and the 2p1/2, respectively, for the Fe3+ species. The observance of these peaks can be attributed to the presence of γ-Fe2O3.9,11 As shown in Figure 5, as Fe loading increases from 12% to 24% for the G12Si, G18Si, and G24Si catalysts, peak intensity increases correspondingly. The theoretical Fe/Si bulk ratio as well as the experimental atomic ratio of Fe/Si, which was determined from the XPS spectra, is tabulated in Table 1. According to the XPS data, the surface atomic ratio of Fe/Si ranged from 0.18 to 0.22 for this set of catalysts, but the lower value of 0.18 corresponds to the highest loading.


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CO-TPD was used to investigate the effect of iron loading on the dispersion of iron oxide nanoparticles on the SiO2 support. TPD profiles normalized by the mass of iron in the sample are shown in Figure 6. For each catalyst, there are two peaks visible in the CO-TPD profile. The earlier, lower temperature peak represents more weakly adsorbed CO, while the later, higher temperature peak represents more strongly adsorbed CO.24,25 It is observed that the second, more intense peak appears at the same temperature for each of iron loadings, while the desorption peak of the weakly adsorbed CO (the initial, less intense peak), was much smaller and shifted to a significantly lower temperature at each of the two highest iron loadings. The CO desorbed (µmol/g·cat) and % CO active sites were calculated for each catalyst and the values are tabulated in Table 1 with the assumptions that the signal in the CO-TPD profiles was entirely due to the emission of CO, and that the Fe/CO stoichiometry ratio is 2:1. Note that the method of calculating iron oxide dispersion differs significantly between various literature sources.26,27 The current method aims to provide an approximate estimate of the relative quantity of iron FTS active sites for the four catalysts used in this study. Of the catalysts studied, the G12Si catalyst has the lowest % CO active sites (15%), while the G18Si catalyst possesses the highest (36%). The relatively low value of % CO active sites in G12Si can be attributed to the intimate contact of iron with the SiO2 support, which is also reflected in the lower degree of reduction observed in the TPR study. The decrease in the % CO active sites with the increase in iron loading from G18Si to G29Si can be attributed to the formation of nanoparticle aggregates as well as three-dimensional islands of iron oxide which corresponds well with TEM observations and the crystallite sizes determined from XRD results.


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The performance of catalysts in FTS reaction The catalytic activity of the supported iron catalysts prepared by the GXL method were

tested in a fixed-bed FTS reactor at 230 ºC, 2.0 MPa, and a GHSV of 3000 L/kgcat/h. The catalytic performance results of these four catalysts are presented in Table 2. It should be noted that the conversion and selectivity data shown in Table 2 are the average values of at least 20 data points obtained once the reaction performance was stable, as determined by CO conversion.

3.2.1

The effect of iron loading amount on the catalytic performance The CO conversion as a function of time on stream for each of the four catalysts studied

is shown in Figure 7. From this data, it appears that these catalysts reached steady state after no more than 20 hours of time on stream. At 230 °C and 2.0 MPa, these four catalysts demonstrated stable performance with minimal deactivation with time on stream up to 100 hours. CO conversion, H2/CO usage ratio, and CO2 selectivities are presented in Table 2. A H2/CO usage ratio of 1.7 was achieved over the G12Si catalyst, and was found to be approximately the same as the feed ratio of H2/CO. This relatively low H2/CO usage ratio, i.e. less than 2.0, corresponds well with previous findings observed over an iron based catalyst operating in a fixed bed reactor at similar reaction temperature, where a H2/CO consumption ratio of 1.65 was reported at 225 °C.28 For each incremental increase in iron loading, the G12Si, G18Si, G24Si, and G29Si catalysts exhibited a significant change in their observed catalytic behavior. As might be expected, the first incremental increase in iron loading—from G12Si to G18Si—simply resulted in increased CO and H2 conversion. This increased conversion was accompanied by only a slight change in CO2 selectivity or H2/CO usage ratio which could be 18 ACS Paragon Plus Environment

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attributed to water availability for WGS. The second incremental increase—from G18Si to G24Si—also resulted in increased CO conversion, but this increase in CO consumption was accompanied by a shift in H2/CO usage and was almost entirely due to increased production of CO2. With the final increase in iron loading from G24Si to G29Si, CO conversion decreased and was accompanied by a modest reduction in the CO2 selectivity. The disproportionately high CO2 selectivity for the two catalysts with the highest iron loading is the subject for later discussion, as it is beyond what might be expected based on their increased CO conversion. The CH4, C2-C4, and C5+ selectivities for each of the four catalysts are presented on a CO2-free basis in Table 2. For each catalyst studied, methane selectivity was approximately 10% and the selectivities toward C2-C4 and C5+ hydrocarbons were approximately 22% and 68%, respectively. The distribution of the hydrocarbon products in terms of wt% and productivity are shown in Figure 8. Apart from a slightly higher selectivity towards diesel range hydrocarbons and a lower selectivity towards gasoline range hydrocarbons for the G12Si catalyst, iron loading had a negligible impact on the hydrocarbon selectivity in this study. The discrepancy in productivity is a product of the differences in CO conversion and CO2 selectivity between catalysts. The hydrocarbon product distribution for each catalyst is well represented by an ASF plot, with the carbon chain growth probability factor (α) in the range of 0.75-0.77. The α value observed over the G12Si catalyst is slightly higher, which corresponds to the higher selectivity towards the diesel range hydrocarbons observed for that catalyst. Changes in iron loading, conversion, and CO2 selectivity left the hydrocarbon distribution relatively unaffected, and this leads us to conclude that the iron loading did not significantly impact the FTS propagation over the GXL catalysts in this study. One possible explanation for the increased CO conversion and 19 ACS Paragon Plus Environment

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CO2 selectivity of G24Si relative to the other catalysts is related to crystallite size. At low iron loading, iron oxide particles are widely dispersed over the SiO2 surface. Beyond a certain loading however, iron oxide particles would be forced to attach to existing iron oxide particles to form bigger particles or multiple layers, which could then agglomerate to form larger crystallites under calcination or reaction temperatures. The XRD results for the four catalysts after calcination reveal that the size of the γ-Fe2O3 crystallites in G24Si are the largest among the four catalysts, i.e. 17.7 nm. In comparison, the size of the γ-Fe2O3 crystallites in G12Si, G18Si, and G29Si are 11.6 nm, 11.0 nm, and 15.6 nm, respectively. This variation in crystallite size, more so than iron loading alone, correlates well with, and might serve to explain, both the CO conversion and CO2 selectivity observed in this study. CO2 selectivity observed over both G18Si and G29Si catalysts are shown as function of CO conversion in Figure 9. These data were collected by varying the gas space velocity of the syngas while the reaction temperature and H2/CO ratio of the feed were maintained at 230 °C and 1.70, respectively. For both catalysts, an increase in the CO conversion (decrease in the syngas space velocity) resulted in elevated selectivity towards CO2. The increased CO2 selectivity was attributed to the reaction of water with CO-derived intermediates in secondary reactions, foremost among which is the water-gas-shift reaction29. At comparable levels of CO conversion, the CO2 selectivity observed over G29Si was higher than that obtained over G18Si. The extent of water-gas-shift reaction relative to CO conversion (i.e. the slope of the CO2 selectivity versus CO conversion) can provide some indication as to the tendency of the catalysts to promote water-gas-shift reaction29. As such, the larger slope for G29Si in Figure 9 suggests that this catalyst yielded a higher rate of the WGS reaction relative to the G18Si.


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The iron time yield (FTY), which represents the FTS reaction rate per gram of iron, is listed in Table 2 for each catalyst. The FTY obtained at both low (G12Si) and high (G29Si) iron loadings were lower than those observed at intermediate iron loadings. As indicated by H2-TPR, XRD, and CO-TPD results, at low iron loading, the SiO2 support inhibits the reduction of Fe2O3 and thereby reduces the number of iron carbide active sites. Conversely, at higher iron loadings, we have observed a tendency for the iron nanoparticles to aggregate and form three-dimensional islands which are lower in FTS activity due in part to lower surface area and proportionally fewer FTS active sites. This tendency of the nanoparticles to aggregate could be the result of the iron oxide nanoparticle deposition process occurring via a Stranski - Krastanov type of behavior.30 Stranski - Krastanov film growth is characterized by the sequential formation of multiple monolayers at lower concentrations followed by the formation of islands and aggregates at higher concentrations. Hurst et al. adapted this film growth mechanism for nanoparticles deposition on to silica substrates where Stranski – Krastanov type of behavior could be obtained.31 The inferior FTY observed at both very high and very low iron nanoparticle loadings indicate that optimal FTS performance could be achieved over a catalyst with an intermediate iron nanoparticle loading, where the quantity of iron nanoparticles would be high enough to dilute the deactivation effect seen at low loadings due to metal-support interactions, but not so high as to form less-active iron agglomerates and islands. Potassium is a commonly used promoter for iron-based FTS catalysts that can enhance catalytic activity as well the formation of olefins.32–36 In general, potassium increases the strength of CO adsorption on the catalyst surface and weakens the H2 adsorption strength.35 Potassium loading titrates residual acid and hydrogenation sites, thereby increasing the olefin selectivity and the average molecular weight of final hydrocarbon products.34 Over co21 ACS Paragon Plus Environment

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precipitated catalysts, K promotion has been found to increase water-gas shift activity, but its influence on FTS activity is more complicated, and is highly dependent on the ratio of K to Fe as well as the catalyst preparation technique.32 The overall FTS rate can be positively or even negatively affected by K promotion depending on the operating conditions, conversion and the subsequent influence of the extra H2 production via the WGS reaction.37 Kinetic studies performed by Davis’ group showed that K had a negative effect on the FTS reaction rate constant, while it enhanced the adsorption parameter—the relative ratio of water and CO adsorption equilibrium constants.37 Since decreases in the reaction rate constant and adsorption parameter affect the FTS rate in opposite directions, for a given set of catalyst and conditions, the degree of potassium loading must be tuned to maximize the FTS activity. Relative to co-precipitated iron catalysts, supported iron catalysts typically require higher degrees of potassium promotion to obtain similar results due to higher surface areas and partitioning of K into the support.32 In order to investigate the interaction between of potassium promotion, temperature, and the GXL catalyst preparation method, a K-promoted catalyst was synthesized and tested. Potassium was added to a supported iron catalyst prepared by the GXL technique via the incipient wetness method using potassium carbonate. As determined via ICP, the final catalyst has Fe and K loadings of 16.0 % and 0.7 wt%, respectively, and is denoted as KG16Si. The catalytic performance of KG16Si is shown in Table 2. At the same reaction conditions, the K promoted KG16Si catalyst demonstrated a markedly lower CO and H2 conversion relative to G18Si catalyst, while yielding a lower CH4 selectivity and considerably higher CO2 selectivity. The iron time yield (FTY), which represents the FTS reaction rate, obtained over KG16Si was significantly lower than the one observed over G18Si, i.e., 1.31 versus 2.06. This decrease in the FTY with K promotion indicates a negative impact of potassium on the FTS reaction rate for 22 ACS Paragon Plus Environment

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these conditions. K promotion decreased C2-C4 selectivity, while it increased the C5+ selectivity to 75.5%. The ratio of olefins to paraffins in the light (C2-C4 range) products increased when the GXL-prepared catalyst was promoted with K, as shown in Table 2. These observations— specifically the depressed selectivity towards CH4 and increased selectivity towards CO2 as well as C5+ hydrocarbons—confirm that the catalytic performance of this incipient wetness promoted KG16Si catalyst was affected by the K promotion in a manner consistent with the literature.2,33,35,38 The KG16Si catalyst was also evaluated at various elevated reaction temperatures with results shown in Table 3. The lower temperatures that were investigated corresponded to reaction temperatures typical for low temperature Fischer-Tropsch synthesis, which aims to produce long chain hydrocarbons (waxy products).39 The highest temperature studied is consistent with the temperature suggested for the Fischer-Tropsch to Olefins (FTO) synthesis process, which aims to produce C2-C4 olefins.4 As the temperature was varied between 230 °C and 340 °C, CO conversion increased significantly while the H2/CO usage ratio was depressed. CO2 selectivity approached 40% at the highest temperature. CH4 selectivity increased from 7.5% to 27.3%, while the C2-C4 selectivity increased to around 28% at elevated temperatures. The ratio of olefins to paraffins in C2-C4 range products declined significantly with increasing reaction temperature. This decrease in olefinicity can be attributed to the enhanced hydrogenation occurred at the high CO conversion levels, since an increase in GHSV at 340 °C enhanced this olefin-to-paraffin ratio significantly (data not shown). For comparison, at a similar CO conversion level (81%), Galvis et al. reported a 50% selectivity towards C2 through C4 olefins along with a CH4 selectivity of 15% for a 25 wt% Fe/α-Al2O3 catalyst.4 The superior selectivity towards light C2-C4 olefins was attributed to the inert support as well as trace amounts 23 ACS Paragon Plus Environment

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of Na and S. It should be noted that the C5+ selectivity for the KG16Si catalyst is considerably higher than those observed by Galvis et al. Therefore, to adapt this catalyst for FTO, further improvement in the catalyst composition, such as using a more inert support material, optimizing the K content and adding more promoters, is suggested.

4

Conclusion This study illustrated that SiO2-supported iron oxide nanoparticle catalysts for Fischer-

Tropsch synthesis can be effectively prepared via the gas-expanded-liquid (GXL) technique. Presynthesized iron oxide nanoparticles were successfully deposited on SiO2 from a polydisperse hexane dispersion. Particles smaller than 5 nm were selectively excluded from the deposition process. Using this GXL deposition system, four catalysts with iron loadings ranging from 12 wt% up to 29 wt% were prepared and compared in terms of reducibility, FTS activity, and product selectivity. A catalyst with a moderate iron loading ca. 18 wt% demonstrated good reducibility and a correspondingly high number of FTS active sites as well as high FTS activity with relatively low selectivity toward CO2. For the four catalysts studied, variations in iron loading had little to no effect of on FTS hydrocarbon selectivity and α remained consistent ca. 0.76. As expected, promotion of the GXL catalyst with potassium resulted in a slight decrease in FTS activity, increase in CO2 selectivity, and elevated carbon chain growth probability. The effect of temperature on the hydrocarbon selectivity was also evaluated over the K promoted catalyst. At 340 °C, the K16Si catalyst demonstrated a C5+ selectivity of 45.4%, indicating that improvements would be needed in order for this GXL catalyst to be applicable in high temperature Fischer-Tropsch synthesis for the production of light olefins.


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Further improvement in the GXL deposition system could likely be obtained by using a nanoparticle solution with lower concentration while employing a larger volume of solution or multiple deposition cycles in order to achieve better deposition of iron oxide nanoparticles for catalysts with high iron loadings.

Supporting information Additional information about H2-TPR analysis results, including peak areas, peak ratios, and the extent of reduction for the first stage and total reduction process

Acknowledgements The authors would like to acknowledge financial support from the National Science Foundation IGERT Program (NSF-IGERT No.1069004) and the U.S. Department of Agriculture/NIFA-AFRI IBSS Consortium (grant No. 2011-68005-30410). The authors would like to thank Johnny Goodwin at the Central Analytical Facility at the University of Alabama at Tuscaloosa for his excellent assistance in the HRTEM and Steven Moore in the Materials Engineering Center at Auburn University for his help with particle characterization.


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Table 1 ICP, BET and XRD results of the G12Si, G18Si, G24Si, and G29Si catalysts BET surface area (m2/g)

Total pore volume (cm3/g)

Average pore width (nm)

Crystallite size (nm)

XPS Fe/Si ratio

Fe/Si bulk ratio

CO desorbed (µmol/g·cat)

CO active sites%

NA

230

0.91

6.6

NA

NA

NA

NA

NA

G12Si

11.4±0.5%

240

0.91

6.2

11.6

0.19±0.02

0.15

151

15

G18Si

18.0±1.0%

230

0.85

6.0

11.0

0.21±0.01

0.26

576

36

G24Si

24.0±0.5%

217

0.80

6.1

17.7

0.22±0.02

0.39

557

26

G29Si

28.8±0.5%

211

0.76

6.6

15.6

0.18±0.02

0.53

405

16

Sample Raw SiO2

ICP results

NA: not applicable CO active sites% 1234567 89 :6;8782 ;>=6; ⁄=8=?@ 234567 89 A6 46=?@ ?=84; C ∗ 100

Table 2 CO conversion and product selectivity of the iron catalysts prepared by GXL (2.0 MPa, 230 °C, H2/CO = 1.70, GHSV = 3000 L/kgcat/h) Hydrocarbon selectivity (%) CO conversion (%)

H2 conversion (%)

H2/CO usage ratio

CO2 selectivity (%)

G12Si

18.0

17.7

1.73

5.4

G18Si

34.1

33.7

1.74

G24Si

46.1

33.9

G29Si

35.9

KG16Si

20.4

Sample

FTY (10-5 molCO/gFe.s)*

α value**

CH4

C2-C4

C5+

(C2-C4)=/(C2-C4)-

1.73

9.2

21.5

69.4

1.48

0.77

8.4

2.06

10.1

22.6

67.4

1.16

0.75

1.31

22.3

1.89

10.4

21.3

68.4

1.16

0.75

29.3

1.45

17.0

1.29

10.1

21.7

68.2

1.24

0.76

15.4

1.34

13.6

1.31

7.5

16.8

75.7

1.67

0.81

* FTY was calculated as the moles of CO converted to hydrocarbons per gram of Fe per second ** α value was obtained from the plot of ln1wn/nC as a function of carbon number n, 1 ≤ n ≤ 27

(C2-C4)=/(C2-C4)- was obtained as the olefin/paraffin ratio for C2-C4 products


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Table 3 CO conversion and product selectivity of the KG16Si catalyst prepared by GXL (2.0 MPa, H2/CO = 1.70, GHSV = 3000 L/kgcat/h) Hydrocarbon selectivity (%) CO

H2

H2/CO

CO2

conversion

conversion

usage

selectivity

(%)

(%)

ratio

(%)

230

20.4

15.4

1.34

260

40.1

27.5

280

50.7

320 340

Temperature

α

FTY (10-5 molCO/gFe.s)

CH4

C2-C4

C5+

(C2-C4)=/(C2-C4)-

value**

13.6

1.31

7.5

16.8

75.7

1.67

0.81

1.22

23.8

2.28

10.1

19.4

70.5

1.44

0.80

31.4

1.10

29.5

2.66

13.5

19.2

67.2

1.32

0.77

78.1

41.5

0.94

37.6

3.64

23.9

28.1

47.9

0.84

0.54*

84.8

45.4

0.95

38.2

3.91

27.3

27.4

45.4

0.69

0.51*

(°C)

* α value at these two temperature was obtained from the plot of ln1wn/nC as a function of carbon number n, 1 ≤ n ≤ 7 in order to keep a strict linear relationship

Figure 1 XRD spectra for catalysts with different iron loading


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Figure 2 XRD spectra for FTS used catalysts with different iron loadings


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Figure 3 TEM images of iron oxide nanoparticles dispersed on SiO2: G12Si (a, a’), G18Si (b, b’), G24Si (c, c’), and G29Si (d, d’).


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Figure 4 H2-TPR profiles for the four catalysts with different amount of iron loadings


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Figure 5 Fe 2p XPS spectra of calcined iron catalysts supported on SiO2 prepared by gasexpanded liquid technique


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Figure 6 CO-TPD profiles of the FTS used catalysts

Figure 7 CO conversion as a function of time on stream of the catalysts with different iron loadings. Reaction conditions: 230 °C, 2.0 MPa, H2/CO = 1.70, GHSV = 3000 L/kgcat/h.


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Figure 8 Product selectivity (in wt%) and productivity distribution by carbon number for the four catalysts G12Si, G18Si, G24Si, and G29Si. Syngas conversions for each catalyst were listed in Table 2.

Figure 9 CO2 selectivity as a function of CO conversion on G18Si and G29Si catalyst at 230 °C, 2.0 MPa, H2/CO = 1.70


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