Supercritical Fischer-Tropsch Synthesis - American Chemical Society

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Supercritical Fischer-Tropsch Synthesis: Heavy Aldehyde Production and the Role of Process Conditions Ed Durham,† Charlotte Stewart, David Roe, Rui Xu, Sihe Zhang, and Christopher B. Roberts* Department of Chemical Engineering, Auburn University, Auburn, Alabama 36849, United States S Supporting Information *

ABSTRACT: Supercritical Fischer-Tropsch Synthesis (SC-FTS) using a potassium-promoted iron-based catalyst has been shown to produce large amounts of heavy (C10+) aldehydes and methyl ketones, while traditional gas phase FTS does not produce these compounds in significant amounts under either fixed or slurry bed operation. In order to better understand this behavior, a series of studies was undertaken to determine the effect of process conditions (H2/CO ratio, temperature, pressure, and supercritical hexanes media ratio) on the performance of iron-based SC-FTS generally, and on aldehyde formation specifically. Over the range of process conditions studied, heavy aldehyde selectivity was found to decrease with increasing temperature, while both elevated pressure and increased media ratio favored aldehyde production. Changes in the H2/CO ratio had little influence on syncrude functionality. The role of potassium promotion was also investigated by operating a potassiumfree iron-based catalyst under SC-FTS conditions. In the absence of potassium promotion, no heavy aldehydes were detected.

1. INTRODUCTION

Increasing the reaction temperature in FTS is generally known to increase catalytic activity while decreasing heavy product selectivity and increasing methane selectivity.9,10 For HTFT, increasing the temperature also decreases the alcohol content of the syncrude.11 In contrast to cobalt-based catalysts, increasing the reaction pressure over iron-based LTFT catalysts appears to have little effect on the overall carbon number distribution of the product.11,12 However, increasing the reaction pressure does generally increase oxygenate selectivity11 as well as the production rate at high syngas space velocities.13 Increasing the H2/CO ratio of the syngas is generally shown to decrease heavy product and oxygenate selectivity while increasing methane selectivity.9 Compared to cobalt FT catalysts, iron-based catalysts show higher water-gas shift activity.14 Relative to that of cobalt-based catalysts, the product selectivity of iron-based catalysts is less sensitive to changes in H2/CO ratio.12 While cobalt catalysts require a H2/CO ratio of about 2, iron catalysts can function over a much broader ratio range, with acceptable H2/CO ratios ranging from 0.5 to 2.5.14 Iron catalyst product selectivity can also be rendered more resistant to changes in gas composition through the incorporation of alkali metal promoters.11 Group 1A elements such as potassium are commonly employed to increase the basicity of iron catalysts. These alkali metal promoters have been shown to have a significant effect on the FT product distribution. For iron-based catalysts, potassium promotion has been reported to increase heavy product formation11 as well as the selectivity toward olefins and alcohols.9

Fischer-Tropsch Synthesis (FTS) is a mature process for the conversion of syngas (a mixture of CO and H 2) to hydrocarbons and oxygenates of various chain lengths and functionalities.1,2 FTS hydrocarbon products include nparaffins, n-olefins (mostly terminal), as well as branched paraffins and olefins. FTS oxygenates include aldehydes (mostly linear), alcohols (mostly terminal and linear), methyl ketones, carboxylic acids, and esters.3 FTS has two main modes of operation: high temperature and low temperature. High temperature Fischer-Tropsch Synthesis (HTFT) is commonly carried out on an iron-based catalyst in a fluidized bed reactor primarily for the production of gasoline and light olefins. Low temperature Fischer-Tropsch Synthesis (LTFT) is aimed at wax production and can be carried out on either an iron-based or cobalt-based catalyst in either a fixed bed or slurry bed reactor.1,2 As an alternative to traditional fixed-bed gas phase FTS (GPFTS) and slurry phase FTS (SP-FTS), Kaoru Fujimoto’s group pioneered the use of supercritical fluids as reaction media for LTFT.4 A number of research groups have investigated supercritical FTS (SC-FTS)5 and have generally observed lower methane selectivity, lower CO2 selectivity, and enhanced heavy olefin selectivity for SC-FTS relative to GP-FTS.5−8 Additionally, our group has shown that SC-FTS carried out over a potassium-promoted iron-based catalyst gives a high selectivity toward heavy aldehydes and methyl ketones.6 We define these heavy aldehydes as long-chain aldehydes with a general formula of CH3(CH2)nCHO, where n ≥ 8. A number of process parameters including temperature, pressure, H2/CO ratio in the reactor feed, and catalyst promotion can affect FTS performance.2 Additionally, SCFTS adds new process parameters such as media choice (not investigated in this study) and media ratio (the molar ratio of the supercritical media to syngas in the feed). © XXXX American Chemical Society

Received: March 19, 2014 Revised: May 15, 2014 Accepted: May 16, 2014

A

dx.doi.org/10.1021/ie5011756 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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The effects of changes to process parameters in SC-FTS are qualitatively similar to those observed in GP-FTS, although they are generally less severe.5,15 This work focuses on the impact of process conditions on heavy aldehyde formation in SC-FTS over an iron-based catalyst.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The 1 Fe: 0.01 Cu: 0.025 K by mole catalyst used in this study was synthesized by a procedure similar to the one outlined by Enrique Iglesia’s group,16 as follows. Unless otherwise noted, all chemicals were used as received. A 1 M iron nitrate nonahydrate (Sigma-Aldrich 216828) solution in DIUF (deionized ultra-filtered) water (Fisher W220) was prepared along with a reducing solution consisting of DIUF water (Fisher W2-20) saturated with ammonium carbonate (Alfa Aesar 36229). The iron solution was added at a rate of 2 mL/min via a pump (Eldex B-100-S) to 50 mL of DIUF water maintained at 80 °C. The reducing solution was added manually to maintain pH 7 (measured by Denver Instrument UB-10) with vigorous agitation. After approximately 50 min, the iron solution flow was ceased. The slurry was maintained at temperature with agitation for several minutes and then allowed to cool to room temperature without agitation. This process was repeated a total of five times, with the five slurries then mixed for subsequent processing. The iron slurry mixture was first vacuum filtered. The resulting filter cake was then reslurried in DIUF water and filtered for a total of five filtrations. The filter cake was next slurried in acetone (BDH 1101-19L), filtered, reslurried in acetone, refiltered, and dried overnight at 80 °C. The dried powder was subsequently calcined in flowing air by ramping at 5 °C/min to 400 °C and holding the temperature constant for 240 min. The pore volume of the calcined powder was measured, and the powder was dried overnight at 80 °C. Copper was added to the powder via a solution of copper nitrate hemipentahydrate (Alfa Aesar 12523) in DIUF water by incipient wetness to give a ratio of 1 mol of Cu/100 mol Fe. The incipient wetness technique involved adding only enough copper nitrate solution to fill the pore volume of the catalyst powder. The powder was then dried and calcined as described above. Potassium was added to the powder via a solution of potassium carbonate (EMD PX1390-1) in DIUF water by incipient wetness to give a molar ratio of 2.5 mol of K/100 mol of Fe. The powder was then dried and calcined as previously described. A 0.50 g amount of catalyst was mixed with 10 g of sand (Acros Organics 370942500) that had been acid washed and calcined to serve as an inert diluent. The catalyst/diluent mixture was loaded in the reactor and reduced in situ using 200 SCCM of syngas/g of catalyst (100 SCCM) at 270 °C and atmospheric pressure for 1 h. Reduction with syngas rather than with pure H2, while resulting in a less active catalyst, has been shown to increase the selectivity toward heavy hydrocarbons.11 2.2. Apparatus. The apparatus used in this experiment is shown below in Figure 1. In this setup, carbon monoxide (A), hydrogen (B), and nitrogen (C) are metered through their respective mass flow controllers (Brooks 5850E), (D), (E), and (F). After these gases are mixed, a slipstream of the feed gas passes through a valve (G) and mass flow controller (H) to a gas chromatograph (GC) with a thermal conductivity detector (TCD) for compositional analysis. The remainder of the syngas is mixed with hexanes (a mixture of hexane isomers;

Figure 1. Experimental apparatus used in GP-FTS and SC-FTS studies. (A) CO cylinder; (B) H2 cylinder; (C) N2 cylinder; (D−F) feed mass flow controllers; (G) valve; (H) slipstream mass flow controller; (I) hexanes drum; (J) HPLC pump; (K) fixed bed reactor; (L) hot trap; (M) back-pressure regulator; (N) condenser; (O) cold trap.

approximate critical point, 234.5 °C and 30.2 bar) (I), which are first fed through an high-performance liquid chromatography (HPLC) pump (Beckman 110B) (J), preheated to the reaction temperature as described below, and then passed through a downflow, fixed bed reactor (K). The reactor effluent passes to the reactor hot trap (L), where liquid is allowed to accumulate. The gas from the hot trap passes through a backpressure regulator (Straval BPH0502T-N2403) (M), to a water-cooled condenser (N), and finally to a water-cooled cold trap (O). The cold trap liquids are allowed to accumulate for manual collection, while the cold trap vapor stream is analyzed and vented. The reactor is a 25.4 cm long, 2.5 cm diameter HIP microreactor. The top 15.2 cm of the reactor has been reamed out to an inside diameter of 1.6 cm, and a stainless steel frit (Western Analytical A-343-02) rests on the inside ledge as a catalyst support. To ensure that the catalyst particles remain compacted and immobilized, glass wool is packed below the frit, between the frit and the catalyst bed, and above the catalyst bed. The temperature in the reactor is measured by a six-point profile thermocouple (Omega Engineering PP6-36-K-G-18), with the reaction temperature controlled by one of the junctions in the catalyst bed and with the pre- and post-reactor temperature controllers (Omega CSC32K) adjusted to maintain thermal uniformity in the reactor. The same type of temperature controller is used to preheat the mixture of syngas and hexanes to reaction temperature in the section of stainless steel tubing directly preceding the reactor. All stainless steel tubing between the outlet of the reactor and the outlet of the back-pressure regulator is also maintained at reaction temperature. The hot trap is maintained at approximately 240 °C. Heating wires (Omegalux FGR and FGH series) are used to supply heat to the reactor, pre- and post-reactor heated tubing, and hot trap, all of which are wrapped in glass insulation tape. The condenser is a shell-and-tube design heat exchanger. Cooling water for the condenser was supplied using a refrigerating circulator (Fisher Scientific Isotemp 3016D). B



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All of the gases used in this study were ultra high purity grade purchased from Airgas. The ACS grade hexanes used as the reaction medium were acquired from VWR. Liquid analysis was performed by manual injection (typically 0.5 μL) into a Varian 3300 GC with flame ionization detection (FID). The GC-FID was equipped with a DB5 column (Agilent 125-5032). Column temperature in the GC-FID was ramped from 35 to 250 °C at a maximum ramp rate of 5 °C/min. Vapor analysis was carried out by automatic injection using a Valco six-way valve (50 μL sample loop) to a Varian 3800 GC-TCD with a Hayesep DB column (Grace Davison 2836PC). Column temperature in the GC-TCD was ramped from 40 to 250 °C at a maximum ramp rate of 5 °C/min. The concentrations of vapor phase and liquid phase products were calculated using molar and mass response factors, respectively. Conversion and selectivity in the vapor phase were then determined based on the molar concentration of N2, which served as an internal standard. Proton nuclear magnetic resonance (NMR) testing of the liquid samples from the GP-FTS study was carried out using a Bruker Avance 400 MHz spectrometer. Samples were prepared for NMR analysis by dilution in deuterated chloroform (Acros, 99.8% atomic purity) at a 2:1 volumetric ratio. 2.3. Procedure. The baseline process parameters in this study were T = 240 °C, Psyngas = 17 bar, supercritical media = hexanes, media ratio = 3.5 (mol of media/mol of syngas), N2 molar concentration in feed = 2.1%, and H2/CO ratio = 1.56. For the purposes of this paper, Psyngas refers to the sum of the partial pressures of CO, H2, and N2. When feasible, testing of each parameter was done in an A-B-A-C-A pattern. The baseline conditions (A) were tested first, third, and fifth while one parameter value (B) was studied second and the other (C) fourth. This A-B-A-C-A pattern made it possible to account for any changes in performance with time on stream. After changing a test parameter, at least 12 h elapsed before the first sample was taken. For the tabulated data (e.g, activity and CH4 selectivity) in section 3, the reported values represent the average of at least two samples which differed by less than 5%. Each series of data points plotted in the figures to follow represents a single liquid sample collected at stable operation. Additionally, as our earlier SC-FTS studies showed a strong dependence of aldehyde selectivity on CO conversion level,6 all tests except for the gas phase study were performed at a CO conversion of approximately 38%. A detailed description of the reaction conditions for the H2/ CO ratio, temperature, pressure, and media ratio studies can be found in the Supporting Information.

Table 1. Activity, CO2 Selectivity, and CH4 Selectivity for H2/CO Ratio Studya portion description H2/CO ratio activity −RCO (SCCM of CO consumed/gcatalyst) CO2 selectivity (%) CH4 selectivity (%)

R2

R3

R4

R5

rich 1.89 2.2 34.1

baseline 1.54 2.3 38.0

lean 1.18 2.0 28.9

baseline 1.59 2.2 37.1

17 1.5

21 1.9

19 1.5

27 1.6

15 1.2

a

CO conversion was 38% for all studies and portions, unless indicated otherwise.

where −RCO is the rate of CO consumption including both FT and the water-gas shift reaction, keff is the effective rate constant, and PCO and PH2 are the partial pressures of CO and H2, respectively. It should be noted that the method for determining the apparent reaction orders took catalytic deactivation into account and was not simply based on the statistical regression of the data in Table 1. Rather, the method that was employed to obtain these parameters is presented in the Supporting Information. A common rate equation for the conversion of CO to hydrocarbons in iron-based FTS9 is shown in eq 2: RFT =

kPCOPH2 PCO + aPH2O

(2)

where k and a are the kinetic and adsorption constants, respectively, and PH2O is the partial pressure of water. At less than 60% syngas conversion (negligible PH2O),9 this kinetic equation reduces to

RFT = kPH2

(3)

On the basis of eqs 2 and 3, it would be expected that the kinetics for hydrocarbon formation would be first order with respect to hydrogen as observed. However, the positive order observed for CO is not consistent with the zero order expected from eqs 2 and 3. This discrepancy could be attributed to the water-gas shift reaction, as well as the influence of the supercritical fluid medium on the apparent reaction kinetics. A slight decrease in activity was observed under lean syngas conditions. However, changes in syngas composition (i.e., H2/ CO ratio) do not appear to have a strong influence on activity over the range of H2/CO ratios studied. While the iron catalyst itself may be responsible for the relative stability of activity with changing H2/CO ratio,12 it is also possible that the presence of the supercritical hexanes reaction medium played a role. In GPFTS, decreased H2/CO ratio is associated with increased carbon deposition9 and wax selectivity,2 both of which can lead to a reduction in observed catalytic activity.11 In a related gas phase study on higher alcohol synthesis17 (a similar exothermic and mass-transfer-limited reaction), it was found that the conversion of both reactants (i.e., H2 and CO) decreased with decreasing H2/CO ratio. However, it was found that the CO and H2 conversion remained relatively stable with changes in H2/CO ratio under SC hexanes conditions. As such, it would be expected that the presence of the supercritical fluid (SCF) medium in this FTS study could similarly allow for more stable activity with changing H2/CO ratio. Previous studies by our

3. RESULTS AND DISCUSSION In the results that follow, activity is defined as SCCM of CO consumed/gcatalyst/P[H2+CO], where P[H2+CO] is absolute pressure in bar. Methane selectivity is reported on a CO2-free basis. 3.1. H2/CO Ratio Study. The activity, CO2 selectivity, and methane selectivity for the H2/CO ratio study are shown in Table 1. On the basis of the results presented in Table 1, it was found that the influence of syngas composition on the rate of CO consumption can be described by the following pseudo rate equation: −R CO = keff PH 21.04PCO0.70

R1 baseline 1.55 2.4 39.9

(1) C



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Over the H2/CO ratio range studied, the composition of syngas was shown to have little influence on the reaction selectivity. As mentioned previously, alkali metal promotion tends to reduce the sensitivity of iron catalysts to changes in syngas composition.11 For the C10−C15 range, increasing the H2/CO ratio slightly decreased the olefin content of the syncrude. This suppression of olefin formation with increased hydrogen partial pressure is consistent with previous observations in the literature.9 For the C20−C24 range, paraffin selectivity increased slightly with H2/CO ratio. However, no significant trend was observed for the other product classes. Not shown in the above data is the propagation probability. For all supercritical phase studies, the propagation probability (α-value) remained essentially unchanged, with a value of approximately 86% (α = 0.86). Propagation probabilities for traditional GP-FTS over an iron-based catalyst typically range from 50 to 70%.19 Increased α-values under SC-FTS conditions have been reported previously.5 3.2. Temperature Study. The effect of temperature on the catalytic performance is shown in Table 2. As would be expected, the catalytic activity increased markedly with increasing temperature. In addition, increasing the temperature increased both the methane and CO2 selectivities. Although activation energy is typically considered to be independent of temperature,20 in this work, the apparent activation energy as determined by the Arrhenius equation showed a strong dependence on temperature. If one were to linearly fit all five activity data points in Table 2, an activation energy of 76 kJ/mol would be obtained. This simple linear regression approach, however, does not account for changes in activity with time on stream and puts disproportionate emphasis on the fitting of the three activity data points obtained at baseline conditions. From 230 to 240 °C, the apparent activation energy was determined to be 59 kJ/mol, while from 240 to 250 °C it was determined to be 89 kJ/mol. In the literature review of van der Laan and Beenackers,9 the activation energies for a number of FTS studies were reported with tabulated values between 56 and 105 kJ/mol, with half of the data points between 80 and 89 kJ/mol. It is known that diffusion resistance can lead to suppressed apparent activation energies.5,21 The lower apparent activation energy observed in this work between 230 and 240 °C could be attributed to increased diffusional resistance caused by the solvent, whose transport properties are more liquid-like near the critical temperature (234.5 °C). The syncrude functionality for the temperature study is shown in Figures 4 (C10−C15) and 5 (C20−C24). Increasing the reaction temperature resulted in a marked increase in the olefin selectivity with a corresponding decrease in the aldehyde selectivity, which suggests that, at higher temperatures, aldehydes are more likely to be readsorbed and

research group, among others, have shown a positive correlation between activity and the presence of a supercritical solvent.5,7,18 Additionally, H2-lean syngas appears to elevate the CO2 selectivity while H2-rich syngas appears to elevate the methane selectivity. Elevated methane selectivity at high H2/CO ratios is well established in the literature. 7,9,10,14 Increased CO 2 formation at the lowest H2/CO ratio studied is most likely the result of increased water-gas shift activity.9 The syncrude functionality for the H2/CO ratio study is shown in Figures 2 (C10−C15) and 3 (C20−C24). These

Figure 2. Syncrude functionality for the C10−C15 range for the H2/ CO ratio study.

Figure 3. Syncrude functionality for the C20−C24 range for the H2/ CO ratio study.

carbon number ranges were chosen due to analytical constraints (i.e., analyte peaks were best resolved in these ranges). In the figures and discussion to follow, n-paraffins, terminal n-olefins, and terminal n-alcohols are referred to as “paraffins,” “olefins,” and “alcohols,” respectively, for the sake of simplicity. Minor FT products such as branched olefins and branched paraffins are lumped together in the “other” product class.

Table 2. Activity, CO2 Selectivity, and CH4 Selectivity for the Temperature Study portion description T (°C) activity CO2 selectivity (%) CH4 selectivity (%)

T1

T2

T3

T4

T5

baseline 240 2.2 15 1.2

low temp 230 1.6 13 1.1

baseline 240 2.1 16 1.2

high temp 250 3.2 19 1.6

baseline 240 2.2 10 1.4

D



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PCO when syngas conversion is less than 60% on a catalyst that is highly active toward water-gas shift. In section 3.1, we derived a pseudo rate equation for CO consumption (including FT and water-gas shift) that was of the order of 1.7 with respect to pressure. The pressure effect study in this section gives an apparent reaction order with respect to pressure of 1.8, which is higher than what would be expected from the literature for hydrocarbon formation, but only slightly higher than that obtained from the syngas ratio study in section 3.1, where the syngas partial pressure was held essentially constant. As the syngas ratio study accounted for both water-gas shift activity and the influence of the supercritical fluid on the apparent order of the reaction, other pressure/solvent effects may be responsible for the small remaining difference in the apparent reaction order between the two studies. Activity was found to increase monotonically with increasing system pressure. As Table 3 shows, the partial pressure of syngas also increased with system pressure in order to maintain a constant 3.5:1 molar ratio of supercritical hexanes to syngas feed. For iron catalysts, the conversion rate of CO to hydrocarbon products is more dependent on the magnitude of the reactants’ partial pressures than on their ratio (i.e., H2/ CO ratio).13 Thus, the changing syngas partial pressure in this study could contribute to the observed trend in activity. Solvent effects could also influence the catalyst performance observed in this pressure study. At elevated pressures, the properties of a supercritical reaction medium more closely resemble those of a liquid. The more liquid-like density and solubility of SCFs at pressures above the critical point (e.g., 75, 71, and 62 bar) can facilitate heavy product extraction and improve the accessibility of catalyst active sites,7 which could explain why, at least in part, higher values for activity were observed at 75 bar relative to 42 bar. Phase behavior studies conducted by our group have indicated that the critical point of a simulated FT reaction mixture with a SC hexanes to syngas molar ratio of 3:1 is around 240 °C and 70 bar.22 While 42 bar is above the critical pressure of the solvent (30.2 bar), the fact that this operating pressure is subcritical with respect to that of the reaction mixture could contribute to the markedly lower activity observed at 42 bar. It is likely that, at conditions below the estimated critical point of the FT mixture (i.e., studies carried out below 240 °C and 70 bar), the reaction mixture is not in the supercritical state, which could account for the relative differences in reaction performance compared with the higher-pressure and higher-temperature investigations. Additionally, increasing pressure suppresses both CO2 and methane selectivity, which is reasonable given that low pressure SC-FTS more closely resembles GP-FTS, and GP-FTS yields more methane and CO2 relative to SC-FTS,6 as stated in section 1. The syncrude functionality for the pressure study is shown in Figures 6 (C10−C15) and 7 (C20−C24). Over the pressure range studied, increasing the reaction pressure was shown to markedly increase the aldehyde selectivity while decreasing the olefin selectivity. Additionally, high pressure mildly suppresses the alcohol selectivity from 62 to 75 bar while showing no consistent effect on the selectivity toward paraffins and “other” syncrude functionalities. There are no data for the C20−C24 range at the lowest pressure (42 bar) as we were unable to collect any C20+ product. Previous work in our group indicates that aldehydes are primary products in potassium-promoted iron-based FTS and can be converted to olefins by secondary reactions.6 Under SC-

Figure 4. Syncrude functionality for the C10−C15 range for the temperature study.

Figure 5. Syncrude functionality for the C20−C24 range for the temperature study.

converted into secondary products, such as olefins. An earlier investigation by our group has shown that olefins may be primary and secondary products in iron-based FTS,6 and previous observations of increased olefin selectivity with temperature3 may indicate that olefins are preferentially formed under high temperature conditions. It is also possible that the more liquid-like properties of hexane at lower temperatures promoted the extraction and stabilization of aldehydes. Higher temperatures slightly suppressed methyl ketone formation while having no clear effect on alcohol or paraffin selectivity. 3.3. Pressure Study. The effects of system pressure on the catalytic activity and the selectivity toward CO2 and methane are presented in Table 3. On the basis of the data in Table 3, the CO consumption rate was determined to be proportional to P1.8. As noted by van der Laan and Beenackers,9 the rate of CO conversion to hydrocarbons should be first order in PH2 and independent of Table 3. Activity, CO2 Selectivity, and CH4 Selectivity for the Pressure Study portion P (bar) media ratio Psyngas (bar) activity −RCO (SCCM of CO consumed/gcatalyst) CO2 selectivity (%) CH4 selectivity (%)

P1

P2

P3

P4

75 3.5 16.9 2.2 36.3 16 1.4

71 3.5 15.8 2.1 33.1 17 1.4

62 3.5 13.8 2.0 27.1 18 1.5

42 3.5 9.3 1.3 12.3 25 2.1 E



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Additionally, as the media ratio is decreased and the reaction environment transitions from SC-FTS to GP-FTS, both the CO2 and methane selectivities increased. Again, this is in keeping with SC-FTS giving a lower CO2 and methane selectivity relative to GP-FTS. As mentioned previously, some of the conditions for the GPFTS experiment deviated slightly from those in the SC-FTS baseline case. Specifically, the syngas pressure was 8.2% higher and the H2/CO ratio was 6.4% higher than in the SC-FTS portions. Of particular note, the CO conversion achieved in GP-FTS operation was 27% instead of 38% as in SC-FTS operation. However, earlier work in our group indicates that under GP-FTS conditions the CO conversion has essentially no influence on the various vapor product selectivities and little influence on the selectivities toward the various liquid product classes. Additionally, negligible quantities of aldehydes and methyl ketones are formed in GP-FTS operation regardless of the conversion level.6 The syncrude functionality for the media ratio study is shown in Figures 8 (C10−C15) and 9 (C20−C24).

Figure 6. Syncrude functionality for the C10−C15 range for the pressure study.

Figure 7. Syncrude functionality for the C20−C24 range for the pressure study.

FTS conditions, aldehyde intermediates can be extracted from active sites by the solvent and stabilized prior to further hydrogenation to olefins or paraffins. Thus, the superior extraction capability of the supercritical solvent accounts for the presence of aldehydes in the SC-FTS product despite their absence in GP-FTS. The results of this study suggest that the increased aldehyde selectivity observed at elevated pressures is due to the effect of pressure on the tunable density of the supercritical fluid medium. Near the critical point, even moderate increases in pressure can significantly enhance the density, and consequently the solvent strength, of the supercritical fluid.7 3.4. Media Ratio Study. The results of the media ratio study are displayed in Table 4. For SC-FTS, the observed catalytic activity decreases with decreasing system pressure/media ratio in the range studied.

Figure 8. Syncrude functionality for the C10−C15 range for the media ratio study.

Table 4. Activity, CO2 Selectivity, and CH4 Selectivity for Media Ratio Study

Figure 9. Syncrude functionality for the C20−C24 range for the media ratio study.

portion P (bar) media ratio Psyngas (bar) activity CO2 selectivity (%) CH4 selectivity (%) a

M1

M2

M3

M4

M5a

75 3.5 16.9 2.2 17 1.4

71 3.2 17.2 1.9 18 1.5

62 2.7 16.9 1.6 20 1.7

42 1.5 16.7 0.91 31 2.8

18.4 0 18.4 0.98 32 5.0

As shown in Figures 8 and 9, in decreasing the media ratio from 3.5 to 1.5, the aldehyde selectivity dropped markedly with a corresponding increase in the alcohol and olefin selectivities. Under gas phase conditions (media ratio of 0), aldehydes were detected in the liquid product by proton NMR (results not shown). However, in GC-FID analysis, aldehydes above C14 were undetected and aldehyde peaks below C14 were too small to quantify reliably.

CO conversion was 27% for this gas phase study. F



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investigated the role of process conditions on aldehyde formation and other aspects of SC-FTS process performance. Adjusting the H2/CO ratio was seen to have little effect on the selectivity of the reaction. Increasing the temperature was shown to favor the formation of olefins over aldehydes, while increasing the reaction pressure was shown to favor the formation of aldehydes over olefins. During supercritical operation, decreasing the media ratio at constant syngas partial pressure was shown to markedly decrease the aldehyde selectivity while increasing the olefin selectivity. Under gas phase conditions, the selectivity toward aldehydes was negligible, with paraffins dominating the product distribution at high carbon numbers (C20+). Consequently, low temperature, high pressure, and a high media ratio are concluded to enhance aldehyde production. However, while many of the parameters studied had a marked impact on the aldehyde and olefin selectivity, the selectivity of the reaction to paraffins and the propagation probability proved almost entirely independent of these adjustments under supercritical conditions. The performance of an iron-based catalyst without potassium promotion was also investigated. This catalyst produced no heavy aldehydes and was shown by incorporation studies to be strongly, but reversibly, deactivated by aldehydes.

For the conditions studied, the olefin selectivity in the C10− C15 range is shown to have a local maximum at a media ratio of 1.5. Under supercritical conditions, lowering the media ratio leads to an increase in the conversion of aldehydes to olefins. At a media ratio below 1.5, aldehyde formation is minimal, so other secondary reactions such as alcohol formation and the hydrogenation of olefins to paraffins are enhanced. As a result, the C20−C24 paraffin selectivity in the gas phase was substantially higher than that in the supercritical phase. 3.5. Potassium-Free Iron-Based Catalyst Study. As part of this investigation, a catalyst similar to that used above was made, except without potassium promotion. This catalyst was then run under standard SC-FTS conditions. The data for this run are shown in Table 5. Table 5. Performance of a Potassium-Free Iron-Based Catalyst in SC-FTS (at Baseline Conditions) parameter

K-free catalyst

CO conversion (%) catalytic activity (SCCM of CO/g·bar) CO2 selectivity (%) CH4 selectivity (%) propagation probability (%) C10−C15 paraffin selectivity (%) C10−C15 “other” selectivity (%) C10−C15 1-olefin selectivity (%) C10−C15 2-olefin selectivity (%) C10−C15 alcohol selectivity (%) C10−C15 aldehyde selectivity (%)

45 0.5 4 8 80 45 6 24 17 8 0

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ASSOCIATED CONTENT

S Supporting Information *

Supporting information for this paper contains a detailed description of the process parameters for the various studies as well as the method for determining the apparent reaction orders in section 3.1. This material is available free of charge via the Internet at http://pubs.acs.org.

■

In the absence of potassium, the catalyst gave a higher selectivity toward methane, much lower CO2 selectivity, lower propagation probability, and a more paraffinic syncrude. All of these observations are consistent with the literature.3,11,23 Additionally, without potassium promotion no detectable quantities of aldehydes were produced. This indicated that the presence of potassium is necessary either for the formation of aldehydes or for stabilizing them against secondary reactions (e.g., hydrogenation). The incorporation of the aldehydes into the SC-FTS reaction was also investigated by the addition of the C8 aldehyde to the hexanes using the same procedure we have described previously.6 With potassium promotion, aldehyde incorporation had no effect on the CO conversion rate. A small quantity (likely below 10%) of the aldehyde was consumed, and a portion of the aldehyde was converted to heavier products. With a potassium-free catalyst, aldehyde incorporation reversibly deactivated the catalyst by 30% (constant syngas rate basis) or 50% (constant CO conversion basis). A vast majority of the C8 aldehyde was converted in the reaction, mostly to the C8 alcohol. These observations indicate that the C8 aldehyde is likely a competitive inhibitor. We were unable to detect any C8 aldehyde conversion to heavier products. These two observations lead us to conclude that one of the functions of the potassium promoter is to weaken the interaction between the catalyst and the aldehyde reaction intermediates.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: (334) 844-2303. Present Address †

Ed Durham: Evonik Corp., Theodore, AL 36582, USA.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the U.S. Department of Energy (Grant EE0003115), the U.S. Department of Agriculture (Grant 2011-67009-20077), the National Science Foundation IGERT Program (NSF-IGERT No. 1069004), and the U.S. Department of Agriculture/NIFAAFRI IBSS Consortium (Grant 2011-68005-30410). We would like to thank Dr. Michael Meadows, director of the Auburn University NMR Lab, for his assistance with the operation of the instrument and with the interpretation of results.

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REFERENCES

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4. CONCLUSION Our group previously reported that a series of iron-based catalysts, operated under SC-FTS conditions, was highly selective toward heavy aldehydes. In this study, we have G



Article

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Supercritical Fischer-Tropsch Synthesis - American Chemical Society

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