Poisoning of a Silica-Supported Cobalt Catalyst due to Presence of

Article pubs.acs.org/IECR

Poisoning of a Silica-Supported Cobalt Catalyst due to Presence of Sulfur Impurities in Syngas during Fischer−Tropsch Synthesis: Effects of Chelating Agent Ashish S. Bambal,†,‡,§ Vidya S. Guggilla,†,‡,⊥ Edwin L. Kugler,†,‡ Todd H. Gardner,‡ and Dady B. Dadyburjor*,†,‡ †

Department of Chemical Engineering, West Virginia University, Morgantown, West Virginia 26506, United States National Energy Technology Laboratory, U.S. Department of Energy, Morgantown, West Virginia 26505, United States

‡

ABSTRACT: The effects of sulfur impurities on the performance of cobalt-based Fischer−Tropsch catalysts are evaluated under industrially relevant operating conditions of temperature, pressure, and impurity levels. Chelating agents (CAs) were used to modify the SiO2 support, and the performances of the CA-modified catalysts are compared with conventional Co/SiO2 catalysts. For both the Co/SiO2 and CA-modified catalysts, the presence of sulfur in the inlet syngas results in a notable drop in the CO conversion, an undesired shift in the hydrocarbon selectivity toward short-chain hydrocarbons, more olefins in the products, and lower product yields. In the post-poisoning stage, i.e., after termination of sulfur introduction in the inlet syngas, the CA-modified catalysts recover activity and selectivity (to some extent at least), whereas such trends are not observed for the base-case, i.e., unmodified Co/SiO2 catalyst. The improved performance of the CA-modified catalysts in the presence of sulfur is attributed to higher densities of active sites.

1. INTRODUCTION Fischer−Tropsch synthesis (FTS) is a well-known and, more importantly, eco-friendly route for conversion of feedstock such as natural gas, biomass, shale gas, or coal into liquid fuels (gasoline, diesel, jet fuel, etc.) via syngas production.1 The development of FT catalysts with improved activity, selectivity, and stability has been the main interest in the recent years. Cobalt-based catalysts are commercially favored for FTS due to an excellent performance in terms of high syngas conversion, high paraffin selectivity, and low water gas shift (WGS) activity. However, being expensive in comparison to Fe-based catalysts, Co-based catalysts have to display a better dispersion and stability to offset the higher cost. A fundamental understanding of catalyst deactivation is important in the development of improved Co-based catalysts. Most earlier2,3 studies identified oxidation as being the major deactivation mechanism for Co-based catalysts. However, strong evidence4,5 contradicts this, especially in the presence of reducing environments. Sintering, carbon deposition, and surface reconstruction remain the main causes for degradation of catalyst performance. These processes can actually be reversed by carefully tailoring regeneration steps that include dewaxing, oxidation, and reduction. In fact, this approach has been reported4 to restore the activity of spent catalyst to that of the fresh catalyst during large-scale operations. In addition to these routes for catalyst deactivation, the impurities present in the syngas, such as sulfur- and nitrogenbased compounds and heavy tars, are detrimental to catalyst life.6 Gas purification, e.g., sulfur removal by adsorption, tar reforming, etc., can substantially reduce concentration of these impurities but do not eliminate them completely, due to process economics. Therefore, concentrations of these impurities at the ppm or ppb level can reach the downstream catalytic bed and these effects are © 2014 American Chemical Society

often sufficient to affect the life of the catalyst. The scope of the present study is limited to investigate the effect of ppm levels of sulfur impurities on cobalt-based FT catalyst. Sulfur-based impurities, mainly H2S and organic sulfur, are formed by the conversion of sulfur present in coal, biomass, or natural gas.7,8 The sulfur content in coal is relatively high compared to that in biomass or natural gas. Additionally, the range of sulfur present depends on the source and type of coal. The concentration of sulfur in the syngas will largely depend on the source used. The sensitivity of many industrially used catalysts to even very low concentrations of impurities is due to the strong adsorption of sulfur species on most of the metal catalyst, and this can lead to undesired modifications in the adsorption of reactants and a significant loss in catalytic activity.9 Therefore, poisoning studies are very relevant to evaluating the tolerance of the catalyst to impurities, and to the development of robust catalysts. At an early stage of development of FT, the upper limit of sulfur allowed in the syngas was 1−2 mg/m3, but advances in the fundamental knowledge of the deactivation process applied to catalyst manufacture have reduced these values by about 2 orders of magnitude.6 Some of these advances in knowledge of the deactivation process are described below. Many previous studies7,8,10−12 have elucidated the effect of sulfur on commercially used Fe-based8,10,13−15 and Cobased11,16,17 catalysts for Fischer−Tropsch synthesis (FTS). Madon and Shaw2 have presented a detailed review of earlier work on the effect of sulfur on FT catalysts. The majority of Received: Revised: Accepted: Published: 5846

January 24, 2014 March 12, 2014 March 13, 2014 March 25, 2014 dx.doi.org/10.1021/ie500243h | Ind. Eng. Chem. Res. 2014, 53, 5846−5857

Industrial & Engineering Chemistry Research

Article

Figure 1. Fixed-bed reactor setup to perform Fischer−Tropsch synthesis.

nitrilotriacetic acid (NTA), alters the physical characteristics and FT activity and selectivity of the catalyst. The modification with CA results in a more-dispersed Co-containing phase, i.e., a moreuniform repartitioning of the Co. The CA-modified catalysts display higher CO conversions, product yields, reaction rates and rate constants. The improved FT performance of CA-modified catalysts is attributed to the formation of stable complexes with Co. The EDTA-modified catalysts show an even better performance than the NTA-modified catalysts, consistent with the increased values of the equilibrium constant for complex formation for the EDTA, i.e., the higher affinity of the former for complex formation with Co ions. Specifically, in this study, we evaluate the effect of sulfur compounds in the feed on FTS activity and selectivity of Co/ SiO2 catalysts. A cobalt loading of 20 wt % is used, consistent with commercial loadings. As mentioned earlier, the silica support is modified with either EDTA or NTA before impregnation of the cobalt salt. The FT performances of the catalysts are compared in various stages of poisoning to determine the outcome of CA modification of the catalyst support. An alkanethiol is used as a surrogate molecule for sulfur introduction. The thiols constituent the organic sulfur class (which includes thiols, sulfides, disulfides, and thiophenes) commonly found in coal and hence is believed to represent sulfur impurities in syngas used in coal-toliquid (CTL) process.

research confirms that sulfur has an undesired, and most likely a permanent, effect on activity and product selectivity of FT catalysts. In general, the reasons for sulfur poisoning of metal catalysts include strong binding of sulfur with active sites, formation of stable but undesired metal sulfides on the surface, and limiting the adsorption of reactants on the active sites.18 However, some researchers15,16 have reported a beneficial effect of sulfur on the performance of the catalysts. Clearly, the role of sulfur varies with the type of catalyst and the operating conditions used. Tsakoumis et al.19 have summarized in their review that there is a mismatch between the effect of sulfur on activity and selectivity of FT catalyst between ex situ and in situ studies. Sparks et al.20 studied the effect of in situ H2S addition during Fischer−Tropsch synthesis on alumina-supported Co−Pt catalysts in a slurry-bed reactor. After discounting normal catalyst aging, they report that sulfur levels up to 500 ppb do not impact the catalyst activity; however, above this sulfur level, there is a linear relationship between the sulfur levels in the syngas and the rate of loss of catalyst activity. Pansare and Allison21 report that syngas containing up to 50 ppbv sulfur are acceptable for FTS. The use of chelating agents (CAs) is becoming widespread in catalyst synthesis, and CAs have shown improved performance for processes such as hydrotreatment,22,23 hydrodesulfurization,22,24 and hydrodenitrogenation.23 CA addition has already shown25 promising results in improving the activity of FT catalysts. The performance improvements due to CA modification are discussed in quantitative detail in our earlier work.26 In brief, the modification of the SiO2 support with one of two chelating agents, ethylenediaminetetraacetic acid (EDTA) and

2. MATERIALS AND METHODS 2.1. Catalyst Preparation. Commercial technical-grade SiO2 support (SS61138) was received from Saint-Gobain 5847

dx.doi.org/10.1021/ie500243h | Ind. Eng. Chem. Res. 2014, 53, 5846−5857


Article

quantification issues associated with the use of n-octane were resolved by performing additional calibration runs, as explained below. The mixture of butanethiol and n-octane was prepared inside a fume hood using hand gloves, a laboratory coat, an ear-loop face mask, and safety glasses as necessary personal protective equipment. The precautions are necessary to avoid direct exposure of the hazardous butanethiol to the operator and other laboratory users. The mixture of butanethiol and n-octane was subsequently loaded into the syringe pump of the liquiddelivery system (in Figure 1). Two solutions of butanethiol and n-octane were prepared. These corresponded to concentrations of sulfur in syngas of 10 and 50 ppm when flow rates of liquid and syngas are 0.01 cc/min (liquid) and 225 sccm (syngas), and the poisoning occurs for 15 h. 2.4. Experimental Procedure. Prior to FTS, approximately 1000 mg of the catalyst was reduced in situ by H2 at 400 °C, 1 atm, and 200 cc/min for 12 h. At the end of the reduction step, the H2 was replaced by inert He to flush the lines while cooling to 170 °C. At this stage, the system was pressurized under syngas up to 20 bar. Thereafter, the temperature of the bed was slowly increased at 1 °C/min to the desired temperature. The nominal operating conditions for FTS used in the study were: H2/CO = 2 (mol ratio) at 230 °C, 20 bar, and GHSV values of 9000 or 13 500 sccm/h/gcat for 72 h. The performances of the three catalysts are compared in the absence26 and presence of sulfur (this work) by accounting for changes in conversions at the two different space velocities. The total time on stream (TOS) of 72 h is divided into three stages of 24 h durations. For the first 24 h, the pre-poisoning stage, the syngas alone is in the feed. During the next 24 h period, termed the poisoning stage, n-octane or the n-octane− butanethiol mixture is introduced in the inlet syngas through the liquid-delivery system for the first 15 h. As mentioned earlier, the flow rate of the liquid is kept constant at 0.01 cc/min. After 15 h, the flow of liquid is stopped and only syngas is fed to the reactor. In the last 24 h period, termed the post-poisoning stage, again only pure syngas flows through the reactor. The following expressions were used to calculate the values of the performance parameters of the catalysts used:

Norpro. This support was used to prepare the catalysts by the incipient-wetness method with a Co loading of 20 wt %. If a chelating agent (CA) was to be used, the SiO2 support was first impregnated with an aqueous solution of the appropriate CA, either nitrilotriacetic acid (NTA) or ethylenediaminetetraacetic acid (EDTA). The amount of CA added was such that the molar ratio of the chelating agent to Co would be unity. The pH of the aqueous solution was maintained at 5.5 by using aqueous NH4OH. The sample was subsequently dried at 110 °C for 12 h. Next, the dry sample was impregnated by an aqueous solution of cobalt nitrate in two steps, each step equivalent to 10 wt % of cobalt loading, with a drying step (at the same conditions as above) between the two impregnation steps and another drying step afterward. Finally, the dried catalyst sample was calcined at 350 °C for 4 h. The catalyst samples in this study were denoted as Co/X/SiO2, representing modification of Co/SiO2 by the chelating agent X (either NTA or EDTA). 2.2. Reaction Equipment. The FT synthesis was carried in a BenchCAT unit built by Altamira Instruments to our specifications. The block diagram of the fixed-bed reactor system is shown in Figure 1, and various components of the unit have been described earlier.26 In brief, the reactor unit contains four inlet gas lines for feeding CO, H2, N2, and He/Ar. The system uses Brooks 5850S mass-flow controllers (MFCs) to feed gases to the process manifold. The setup also includes a liquid-delivery system, consisting of a syringe pump followed by an evaporator and a volume expander. Heat tapes or trace heaters wrap the plumbing from the reactor to the mixing point of the feeds. The system incorporates three stages of liquid separation after the reactor exit: a wax separator, a hot separator, and a cold separator. Typically, during FT runs, the wax separator was operated at 190 °C, the hot separator at 40 °C, and the cold separator at 4 °C. Gases leaving the third condenser include He, H2, CO2, Ar, CH4, CO, and lower hydrocarbons. These are analyzed at 30 min intervals by an online Perkin-Elmer GC unit (Claurus 500), using a HaySep packed column with a dual thermal-conductivity detector (TCD) and a capillary column with a flame-ionization detector (FID). Finally, the condensed products that remain in the three separator stages, liquid and wax phases, which contain higher hydrocarbons and alcohols, are collected every 24 h during the entire run. These products contain an oil phase and an aqueous phase, separated by letting them settle for 24 h. Analysis of these layers is by an off-line HP 3400 GC. The organic layer of the liquid samples is analyzed with a capillary column and FID, whereas the aqueous phase is analyzed by a Propak-Q packed column and FID. 2.3. Selection of Organic Sulfur Poisoning Agent and Solvent. For poisoning experiments, the liquid-delivery system on the reactor setup was used to introduce the sulfur impurities into inlet syngas. The experimental plan involves a sulfur compound dissolved in a solvent entering an evaporator followed by a volume expander before mixing with inlet syngas, as shown in Figure 1. The selection of solvent and carrier was primarily influenced by the following: (i) the maximum allowable temperature attainable by using the heating tapes and (ii) minimal interference of the solvent GC peak with FT product peaks. Accordingly, an alkanethiol, butanethiol (C4H10S), was selected as the sulfur carrier and n-octane (C8H18) was selected as a solvent. Both the evaporator and expander were operated at ∼140 °C, well above the boiling points of both butanethiol and noctane (98 and 126 °C, respectively). Of course, n-octane interferes with the C8 peaks of FT product analysis. However,

turnover frequency (TOF, s−1) = (mol rate of CO consumption)/(mol of surface Co 0 atoms)

(1a)

total TOF (TT, s−1) = (mol rate of CO consumption)/(mol of total Co) (1b)

hydrocarbon {or alcohol} yield (g C/kgcat/h) = (hydrocarbon {or alcohol} production rate, g C/h) /(catalyst mass, kgcat) 5848

(2)



Article

where the term (t − 24) represents the time under sulfur poisoning, which starts 24 h after the start of the run (at t = 0). Accordingly, the design equation for a fixed-bed reactor during deactivation (24 h < t < 48 h) can be written as

selectivity of hydrocarbon {or alcohol} with carbon number of n (wt%) = (rate of hydrocarbon {or alcohol} produced with carbon number of n)

υ0CCO,0

/(rate of total hydrocarbons {or alcohols} produced)

2.5. Characterization Equipment. Surface areas and pore volumes of the support and calcined catalysts were measured on a Micromeritics ASAP 2020 instrument. The X-ray diffraction (XRD) patterns of the support and the prepared catalysts were obtained using a PANalytical diffractometer (XPert PRO) with Cu (Kα) radiation (1.5418 Å) operated at 40 kV and 25 mA. The average particle sizes of the Co3O4 phases were calculated from the Scherrer equation for the most intense peak (311) located at θ = 36.8°. The H2-chemisorption experiments were performed with a Micromeritics ASAP 2020 unit. To calculate the metal dispersion from chemisorption studies, it was assumed that two cobalt sites were covered by one hydrogen molecule.27 Energy dispersive spectroscopy (EDS) yields qualitative information on elemental composition. The EDS characterization of the catalyst particles was performed using a Hitachi S-4700 scanning electron microscope with an EDAX energy dispersive spectrometer. Detailed procedures for all these characterizations have been described earlier.26 2.6. Kinetic Modeling. Zennaro et al.28 have proposed a kinetic model for a FT catalyst with a Co loading of 11.7 wt % from their work with a differential fixed-bed reactor over a wide range of operating conditions, including the conditions used in the present work. Hence, their power-law rate expression rCO = kpH

0.74 2

pCO−0.24

rCO = 20.74kCCO,0 0.5

υ0CCO,0

(9)

(10)

for carbon numbers (n) from 1 to 40. For a negative value of ε, eq 9 can be integrated to obtain exit conversions XCO,out for a fixedbed reactor: ⎛ ln⎜1 − ⎜ ⎝ +

(4)

(1 − XCO,out)(1 + εXCO,out) ⎡ (1 + ε) ⎢ (1 + εXCO , out ) + −ε(1 − XCO,out) ln ⎢ −ε 1 + −ε ⎣

⎛ 20.74kW ⎞ cat ⎟ = ln⎜⎜ − kd(t − 24) 0.5 ⎟ ⎝ υ0CCO,0 ⎠

⎤⎞ ⎥⎟ ⎥⎟ ⎦⎠

(11)

The value of kd can be determined by plotting the left-hand side of eq 11, termed f(XCO), vs (t − 24). In addition, the half-life of the catalyst, t1/2, can be defined as

(5a)

t1/2 = −ln(0.5)/kd

(5b)

(12)

3. RESULTS 3.1. Characterization of Support and Fresh Catalyst. The BET surface area, pore volume, and average pore diameter measured by N2-physisorption for the SiO2 support, Co/SiO2, Co/NTA/SiO2, and Co/EDTA/SiO2 catalysts are presented in Table 1. On average, the three catalysts show a 22% drop in surface area and a 26% drop in pore volume after impregnation. It has been confirmed previously26 that structural characteristics are retained even after the chelation step. The diffraction patterns of catalysts calcined at 350 °C confirm the presence of Co3O4 as a dominant phase.26 Using the full width at half-maximum (fwhm) of the diffraction peak and Scherrer’s equation, the average Co3O4 crystallite sizes were determined and are shown in Table 1. In general, the crystallite sizes are noted to be smaller for the CA-modified catalysts compared to the base catalyst. The values of the metal dispersion of the reduced catalyst samples measured by H2-chemisorption are also presented in Table 1. Recall that each catalyst contains 20 wt % cobalt. These

(6a)

where t is the time (h) under deactivation, kd is the poisoning rate constant (h−1), and a is the activity function accounting for the deactivation due to sulfur expressed as r a = CO0 rCO (6b) where rCO0 is the rate (mol/h/gcat) of the reaction before the initiation of sulfur poisoning. Integration of eq 6a yields a = exp[−kd(t − 24)] for 24 h < t < 48 h

dXCO (1 − XCO) = 20.74kCCO,0 0.5 ×a dWcat (1 + εXCO)

ε = −0.67

was used. A simple model of deactivation of the cobalt catalyst by sulfur poisoning has the deactivation rate proportional to the unpoisoned fraction of active sites on the catalyst: da = −kda dt

(8)

The value of ε was determined by considering the paraffin formation reaction as a representative one, especially for cobalt catalysts. The average value of ε was calculated to be

is used in the present study. Further, the reaction stoichiometry (mol of H 2 reacted) = 2 × (mol of CO reacted)

(1 − XCO) (1 + εXCO)

where the expansion factor, ε, accounts for the change in total molar volume due to reaction. Substituting eq 8 in eq 7

was adopted in this study. Here, rCO is the rate of CO consumption (mol/h/gcat), pH2 and pCO are the partial pressures of reactants (Pa), and k is the rate constant (mol CO/h/Pa0.5/ gcat). The inlet molar ratio of H2 to CO M = 2.0

(7)

where XCO is the conversion of CO given in terms of the inlet concentration CCO,0, υ0 is the volumetric flow rate (m3/h), and Wcat is the weight of the catalyst (gcat). From eq 4

(3)

× 100

dXCO = rCO × a dWcat

(6c) 5849



Article

base-case catalyst was used, and the n-octane was added during what would be the poisoning stage (TOS 24−48 h). The CO conversion remains steady at ca. 18% during the entire 72 h operation, even after the n-octane flow is stopped. Therefore, noctane, as a solvent, shows no measurable effect toward the base catalyst, syngas, and other components of the reactor unit, and therefore can be used in the poisoning studies. It is reasonable to assume that n-octane would display the same inactivity toward the CA-modified catalysts under the same conditions. 3.2.2. Modified Procedure for Product Analysis. The only difference between the two runs represented in Figure 2 is the addition of n-octane during the operation. For the run where noctane is added, the oil product analyses are expected to have anomalies in the paraffin distribution, because n-octane is also one of the paraffin products of FT reaction. Therefore, it is necessary to account for the externally added n-octane from the FT products. In the modified approach, the amount of actual noctane present in the FT product is obtained by the difference between the amount of n-octane collected between 24 and 48 TOS for the run with n-octane addition and the normal FT run. No changes are needed for other products of the FT reaction. Note that the n-octane is added to the syngas only after the catalyst reaches some pseudo-steady-state conversion. In our opinion, the approach adopted here could be considered to account for the changes in conversion more realistically compared to a situation where impurities would be introduced in the feed from the beginning of the run and without attaining the pseudo-steady-state conversions. 3.3. CO Conversions. As mentioned earlier, the base case Co/SiO2 catalyst was subjected to two inlet sulfur concentrations, 10 and 50 ppm. The effects of these additions on the CO conversion are shown in Figure 3. In the pre-poisoning stage, the

Table 1. Physical Characteristics of a SiO2 Support and SiO2 Supported Co Catalysts BET analysisa

SiO2 supportb Co/SiO2 Co/ EDTA/ SiO2c Co/NTA/ SiO2c

surface area (m2/g)

pore volume (cm3/g)

pore diameter (nm)

crystallites sizes by XRD (nm)a

dispersion (D) by H2chemisorption %a

224

0.93

12.4

NA

NA

163 170

0.67 0.65

12.5 11.7

17.7 11.0

1.1 4.7

187

0.73

11.6

16.0

2.4

a Catalysts calcined at 350 °C; CA/Co = 1.0 if CA-modified. bStandard pretreated support before impregnation. cCA/Co = 1.0.

results confirm that both the CA-modified catalysts display higher metal dispersions, and hence higher densities of active sites, compared to the base-case catalyst. It was shown earlier26 that the modification of silica support by a CA affects the structure of the cobalt catalysts: CA modifications result in smaller crystallites and better-dispersed active phases. 3.2. Catalytic Activity. 3.2.1. Effect of n-Octane under FT Conditions. As mentioned in section 2.3, the poisoning studies required addition of sulfur impurities to the inlet syngas, and this was carried out by evaporating a liquid mixture into the feed syngas before introduction to the reactor. Therefore, it was necessary to establish the activity of the selected solvent alone, i.e., n-octane, under the relevant FT conditions. These preliminary runs were performed to determine the products formed from n-octane with the following: (i) the cobalt catalyst in the presence of inert He and no syngas, (ii) the catalyst-free reactor containing quartz chips in the presence of syngas, and (iii) the cobalt catalyst in the presence of syngas. It was confirmed29 that there are no measurable products formed from n-octane in cases (i) and (ii) under reaction conditions. The results from case (iii) are discussed below. The CO conversions for the runs with only syngas and with syngas plus n-octane are compared in Figure 2. In both cases, the

Figure 3. Comparison of CO conversions for the base-case Co/SiO2 catalyst in the presence of 10 and 50 ppm sulfur poison (experimental conditions: T = 230 °C; P = 20 bar; GHSV = 13 500 sccm/h/gcat).

average CO conversions are identical within experimental error. With the addition of 10 ppm sulfur in the poisoning stage, conversion drops only marginally in the first 8 h, whereas it drops noticeably from 19% to 9% by the end of the poisoning stage. The decrease in CO conversion is related to the poisoning of active sites due to sulfur. In the post-poisoning stage, in the presence of pure syngas, the average CO conversion is only ca. 5%, and tends to reach a steady value toward the end of 72 h. Finally, addition of 50 ppm sulfur during the poisoning stage results in more significant changes in the performance of the catalyst. At this higher sulfur level, the CO conversion drops

Figure 2. Comparison of CO conversions for the base-case Co/SiO2 catalyst during a “normal” run containing only syngas, and a run containing a mixture of syngas and n-octane (experimental conditions: T = 230 °C; P = 20 bar; GHSV = 13 500 sccm/h/gcat). 5850



Article

with the CA-modified catalysts were carried out with 10 ppm sulfur only. All the other operating conditions were kept similar to the ones mentioned earlier. The trends of CO conversions vs TOS are plotted for the EDTA- and NTA-modified catalysts in Figure 5. In the pre-

rapidly from the initial value of 18% to 6% toward the end of the poisoning stage. In the post-poisoning stage, the CO conversion continues to drop; the catalyst shows virtually no activity at the end of 72 h. (As discussed later, there are no measurable products of online GC analysis at this point in the run.) In Figure 3, there is a time lag to fill up the relatively large volumes before the butanethiol reaches the catalyst bed. The time for butanethiol to reach the catalyst bed from the start of feed injection is estimated by back-of-the-envelope calculations as about 9 h. Likewise, we expect about the same time lag before alkanethiol is entirely flushed out from the system after termination of the butanethiol feed. It is observed from both the 10 and 50 ppm sulfur poisoning runs that the catalyst does not regain the lost activity for at least 24 h after the removal of sulfur poison in the feed. Therefore, the presence of even 10 ppm sulfur in the inlet can lead to at least a partial loss of activity of FT catalyst, and this observed activity loss seems likely to be permanent. In addition, with 50 ppm sulfur, the complete loss in the activity also suggests that the extent of severity is higher. These observations are qualitatively consistent with the reported ones for the cobalt-based catalyst.21,30,31 Average CO conversions for each of the three stages for runs with different inlet feed compositions are summarized in Figure 4

Figure 5. CO conversion data for Co/EDTA/SiO2 and Co/NTA/SiO2 catalyst in presence of 10 ppm sulfur (experimental conditions: T = 230 °C; P = 20 bar; GHSV = 13 500 sccm/h/gcat).

poisoning stage, the CO conversions for both the modified catalysts are higher than that for the base-case catalyst (Figure 3) and the highest conversion is noted for the EDTA-modified catalyst, which is consistent with our earlier work.26 Also, the drops in CO conversions, i.e., change in absolute conversion values, during the poisoning stage for both the CA-modified catalysts are similar to that for the base-case catalyst. For the EDTA-modified catalyst, at the end of the poisoning stage, the conversion drops to 37%, and reaches 31% by the end of the post-poisoning stage. However, for the NTA-modified catalyst, the conversion at the end of the poisoning stage is 17% and at the end of post-poisoning stage steadies around 13%. Although the post-poisoning stage conversion decreases more for the NTAmodified catalyst than for the EDTA-modified catalyst, it should be noted that absolute conversion values are higher for both the modified catalysts than that for the base-case catalyst under the same sulfur concentrations. Figure 6b,d summarizes the average conversions for the EDTA- and NTA-modified catalysts, respectively, in the presence and absence of sulfur. Figure 6a,c shows the corresponding average values for the two CA-modified catalysts but now at lower GHSV values and in the absence of sulfur. The lower GHSV values correspond to those used in our earlier paper26 and show that the experiments are reproducible between that paper and the present work. The differences in the conversion during the first 24 h between Figure 6a,b, and between Figure 6c,d, are accounted for by the change in the space velocity. The comparison at two different space velocities is shown merely to understand the conversion trend for these catalysts in the absence and presence of sulfur. Similar to Figure 5, Figure 6 also shows that the drop in conversions in the presence of the sulfur is clearly higher for both the CA-modified catalysts (Figure 6b,d) compared to those in the absence of the sulfur (Figure 6a,c); however, all these conversion levels are measurably higher than the corresponding ones for the base catalyst. The post-poisoning stage conversions for all the catalysts are clearly lower than those in the pre-poisoning stage and in the

Figure 4. Average CO conversion for the base-case Co/SiO2 catalyst with different inlets: (a) “normal” run with syngas feed, (b) run with syngas and n-octane feed, (c) run with 10 ppmv S equivalent in syngas feed, and (d) run with 50 ppmv S equivalent in syngas feed (experimental conditions: T = 230 °C; P = 20 bar; GHSV = 13 500 sccm/h/gcat).

for the base-case Co/SiO2 catalyst. The operating conditions during the pre-poisoning stage are essentially similar for all the four experimental runs in Figure 4a−d, and the conversions are expected to be comparable. The small deviations with the measured data can be attributed to experimental error. As discussed earlier, the n-octane addition is found to have no effect on the CO conversions. Therefore, in both the normal FT run (in Figure 4a), and the one with pure n-octane (in Figure 4b), the average CO conversions for all the three stages are nearly the same. On the other hand, the addition of 10 ppm sulfur results in a drop in the CO conversion during both the poisoning stage and the post-poisoning stage. The addition of 50 ppm of sulfur shows an even more detrimental effect and, as noted earlier, results in virtually a complete loss in activity at the end of 72 h. In the next step, the CA-modified catalysts were subjected to sulfur poisoning during FT operation. Because the use of 50 ppm sulfur resulted in complete deactivation of the base catalyst, runs 5851



Article

worst, less than 0.2% of the available sites are directly affected (“killed”) by sulfur and less than 2% of the available sites are nearest neighbors to “killed” sites (“influenced by sulfur”) in the syngas. In absolute numbers, the reaction rate, TOF, and TT values for the EDTA-modified catalyst are about more than 2 times of the NTA-modified catalyst after introduction of 10 ppm S. These numbers are consistent with the activity losses noted in the previous subsection. 3.4. Product Selectivity. The overall hydrocarbon selectivities for the base and modified catalysts are listed in Table 3 using lumps C1, C2−C5, C5+, and C10−C20 (FT diesel, a subset of C5+). The results are compared based on the changes in selectivity among the pre-poisoning, poisoning, and post-poisoning stages. For the base catalyst with 10 ppm sulfur in the inlet, selectivities of C1 and C2−C5 in the poisoning stage are higher compared to those in the pre-poisoning stage. This increase in selectivities for the short-chain hydrocarbons is compensated for by an equivalent drop in C5+ selectivities from the pre-poisoning to the poisoning stage. Interestingly, in the post-poisoning stage, selectivities for the short-chain hydrocarbons continue to increase, and the selectivity continues to drop for C5+. For 50 ppm sulfur in the inlet, the trends in selectivity change for the base catalyst are mostly similar to the case of 10 ppm sulfur. In particular, the C10−C20 selectivity is severely affected at the higher sulfur concentration, and this product lump completely disappears in the post-poisoning stage. In addition, even after removal of inlet sulfur in the post-poisoning stage, the product selectivity does not show signs of approaching the selectivity observed during the pre-poisoning stage. This is similar to trends observed for conversions. In summary, for the base-case catalyst, for both the poisoning and the post-poisoning stages, product selectivities shift toward the short-chain hydrocarbons, with an equivalent drop in C5+ selectivities. Therefore, the sulfur poisoning causes the undesired shifts in product selectivity. As the catalyst fails to recover from its lost selectivities, sulfur poisoning results in the permanent shift in product selectivity. For the EDTA- and NTA-modified catalysts in the presence of 10 ppm sulfur, the trends are similar to the one observed for the base catalyst. However, the post-poisoning stage selectivities do not show any further drop from those in the poisoning stage, unlike the case of the base catalyst. For both the CA-modified catalysts, selectivities for C5+ as well as C10−C20 appear to improve in the post-poisoning stage from those in the poisoning stage, though still lower than those in the pre-poisoning stage. The C5+ selectivities for the NTA-modified catalyst are higher compared to the EDTA-modified catalyst in all the three poisoning stages. This is counter to the relative improvements in conversion.

Figure 6. Comparison of the average CO conversions for CA-modified catalyst: (a) Co/EDTA/SiO2 at GHSV = 9000 sccm/h/gcat and no S, (b) Co/EDTA/SiO2 at GHSV = 13 500 sccm/h/gcat with 10 ppmv S equivalent, (c) Co/NTA/SiO2 at GHSV = 9000 sccm/h/gcat and no S, and (d) Co/NTA/SiO2 at GHSV = 13 500 sccm/h/gcat with 10 ppmv S equivalent (experimental conditions: T = 230 °C; P = 20 bar).

poisoning stage. As a result, the poisoning would seem to be permanent. However, it has been reported by Sparks et al.20 that after termination of introduction of low levels of sulfur impurities (∼276−611 ppb), the activity of cobalt can be restored, at least in part, in a few days (∼200 h TOS). In this context, the postpoisoning period here is relatively short (∼24 h TOS) and no such recovery in the terms of activity is observed for all the catalysts that were poisoned in the poisoning stage. The reactors used are also different, with a slurry column used by Sparks et al.20 3.3.1. Reaction Rates, TOF, TT, and Percent Sites Lost. The average reaction rates (mol/h/gcat), TOF (s−1), and TT (s−1) for the pre-poisoning, poisoning, and post-poisoning stages are compared in Table 2. The sites lost due to sulfur for the poisoning and the post-poisoning stages are shown using the TOF values in the pre-poisoning stage, although essentially the same values are obtained using the TT values. The higher reaction rates for the CA-modified catalyst compared to the base catalyst in the pre-poisoning stage confirm the higher active-site densities for the modified catalyst. With the onset of poisoning, the rates drop progressively in the poisoning and post-poisoning stage. For the base catalysts, the 10 ppm sulfur impurity results in the loss of 77% of the Co sites by the end of the post-poisoning stage, whether on a TT or TOF basis. The corresponding sites lost with 50 ppm sulfur are even higher, at 95%. The corresponding numbers for CA-modified catalysts with 10 ppm S are only ca. 39%, almost 50% lower compared to the base catalyst. In this context, based on the data for 2 years of commercial operation at Sasol, it has been reported14 that, at

Table 2. Reaction Rates, TT, TOF, and Percent Sites Lost by Poisoning of Cobalt Catalysts reaction rate (mol/h/gcat), × 10−2

a

TTa (s−1), × 10−2

% sites lost by poisoningb

TOFa (s−1)

TOS (h)

0−24

24−48

48−72

0−24

24−48

48−72

0−24

24−48

48−72

24−48

48−72

Co/SiO2; normal run Co/SiO2; n-octane run Co/SiO2; 10 ppm S Co/SiO2; 50 ppm S Co/EDTA/SiO2; 10 ppm S Co/NTA/SiO2; 10 ppm S

3.9 3.5 3.7 3.3 9.8 4.3

3.5 3.3 2.8 2.6 8.7 3.6

3.3 3.3 0.9 0.2 5.9 2.5

1.06 1.02 1.10 0.98 1.30 0.59

1.02 0.98 0.83 0.78 1.16 0.49

0.99 0.97 0.25 0.05 0.78 0.34

1.1 1.0 1.1 1.0 0.5 0.3

1.0 1.0 0.9 0.8 0.4 0.3

1.0 1.0 0.3 0.05 0.3 0.2

24 20 11 16

77 95 39 41

TT calculations are on total Co basis; TOF on metal Co basis. bBased on TOF values. 5852



Article

Table 3. Hydrocarbon Selectivity (wt %) of the Base and Modified Catalysts in the Presence on Sulfur Impurities Co/SiO2 (10 ppm S)

Co/SiO2 (50 ppm S)

TOS (h)

0−24

24−48

48−72

0−24

C1 C2−C5 C5+ C10−C20

13.9 11.8 74.3 59.4

21.0 12.3 66.7 46.1

31.4 13.7 54.9 34.1

11.8 11.1 77.1 53.3

24−48

Co/EDTA/SiO2 (10 ppm S)

48−72

selectivity (wt %) 18.5 25.6 8.0 41.0 73.5 33.4 52.9 0.0

Co/NTA/SiO2 (10 ppm S)

0−24

24−48

48−72

0−24

24−48

48−72

15.8 13.2 71.0 44.1

21.3 18.4 60.2 31.3

20.7 13.0 66.3 43.5

4.2 3.8 92.0 68.1

7.7 5.2 87.1 59.1

7.8 4.8 87.4 66.0

Table 4. Product Yields and Product Distributions in the Presence of Sulfur Impurities Co/SiO2 (10 ppm S) TOS (h)

0−24

24−48

Co/SiO2 (50 ppm S)

48−72

0−24

paraffin olefin alcohol total

247.4 27.8 8.8 284.0

137.2 23.5 19.2 179.9

21.6 8.4 6.1 36.1

234.7 23.0 12.0 269.7

paraffin olefin alcohol

87.1 9.8 3.1

76.3 13.1 10.7

59.9 23.3 16.8

87.0 8.5 4.4

24−48

Co/EDTA/SiO2 (10 ppm S)

48−72

0−24

product yield (g C/kgcat/h) 158.6 0.2 735.7 18.1 0.0 79.4 14.8 0.0 36.2 191.5 0.2 851.3 % product distribution 82.8 88.1 86.4 9.5 11.9 9.3 7.7 0.0 4.3

Co/NTA/SiO2 (10 ppm S)

24−48

48−72

0−24

24−48

48−72

567.8 139.5 55.3 762.6

379.9 140.4 18.9 539.3

365.7 26.1 5.6 397.4

255.6 21.9 5.7 283.2

195.4 8.5 4.6 208.5

74.5 18.3 7.2

70.5 26.0 3.5

92.0 6.6 1.4

90.3 7.7 2.0

93.7 4.1 2.2

Figure 7. Plot of f(XCO) vs (t − 24) for: (a) Co/SiO2 catalyst, 10 ppm S; (b) Co/SiO2 catalyst, 50 ppm S; (c) Co/EDTA/SiO2 catalyst, 10 ppm S; (d) Co/NTA/SiO2 catalyst, 10 ppm S.

3.5. Product Yield and Distribution. Product yields and distributions for the catalysts are compared in Table 4 for the different stages. The total yield is the sum of all hydrocarbon and oxygenate yields, on a gC/kgcat/h basis. The product distribution

percent is based on the contribution of hydrocarbon and oxygenate yields. For the base catalyst in the presence of 10 ppm sulfur, the paraffin and olefin yields are suppressed, whereas the alcohol 5853



Article

The half-life values of the CA-modified catalysts are higher than that of the base catalyst. This is consistent with the higher conversions noted for CA-modified catalysts than that for the base catalyst in the post-poisoning stage. The half-lives of the base catalyst in the presence of 10 and 50 ppm sulfur in Table 5 confirm the higher severity at the higher sulfur concentration. 3.7. Characterization of Spent Catalyst. The fresh and spent catalysts were characterized by EDS, as shown in Figure 8. The fresh Co/SiO2 catalyst in Figure 8a shows peaks of Co, Si, O, and C, as expected from the catalyst composition. The appearance of the Ag peak is due to the use of a Ag coating material during the sample preparation stage. The appearance of the sulfur on the EDS spectra in Figure 8b−d confirms the sulfur uptake by the spent catalysts during the poisoning stage. The S peak increases in the order: base-case catalyst (Figure 8b), EDTA-modified catalyst (Figure 8c), and NTA-modified catalyst (Figure 8d), but not much can be deduced from this. It is possible that all the catalyst particles in the bed may not receive the same sulfur dosing during the poisoning stage. Because the process of sulfur adsorption is generally rapid and irreversible, that could be why the concentration of sulfur on the surface of a catalyst particle and through the bed length is nonuniform.9 Therefore, the concentration of sulfur may vary for particles taken from the reactor, as seen in Figure 8b−d. A similar observation has been reported,33 where sulfur analysis on the spent catalyst shows a much higher sulfur deposition on material taken from the top third of a fixed-bed reactor than on particles from the remaining bed.

yield increases (all in absolute values), in comparison to the values in the pre-poisoning stage. The trends for absolute values of the olefin and paraffin yields are also observed in the postpoisoning stage, but now accompanied by a drop in alcohol yields. Correspondingly, the total yield shows a progressive drop with sulfur poisoning. The trends are similar in the presence of 50 ppm sulfur. In that case, however, there are no measurable products in the post-poisoning stage, consistent with the total loss of conversion as shown in Figure 3. The trends are consistent for both the modified catalysts as well: overall yields drop with sulfur addition. However, yields are higher for the modified catalysts in all the three stages than those for the base catalyst. The EDTA-modified catalyst displays the highest total product yields in all the stages. This is due to the relative improvement in conversion for that catalyst, in spite of the decrease in the selectivity. The product distribution percent provides further insights in the effects of poisoning on the FT products. For the base catalyst, in the poisoning as well as the post-poisoning stage, the olefin and alcohol contents in the products increase, with a corresponding drop in the paraffin percentages. Similar trends are noted for the modified catalysts in the poisoning stage. However, these catalysts show signs of recovery in the postpoisoning stage, as the product distribution becomes somewhat similar to that in the pre-poisoning stage. The sulfur poisoning suppresses the total product yield of the FT catalysts, but increases the olefins and the alcohols in the products. Similar observations have been reported32 for Co-based FT catalysts, for which sulfided catalysts show more yield of low-molecularweight products and much smaller amounts of wax, i.e., heavy products, in comparison to nonsulfided catalysts. Similar to the observation of increased olefin content due to sulfur poisoning, the alcohol content also increases in the products due to the poisoning effect. However, the alcohol contribution to overall FT products is less than 7% in most cases. Therefore, it would not be possible to comment on the change in alcohol distribution due to sulfur poisoning. 3.6. Determination of Deactivation Constant and HalfLife. The values of the deactivation rate constant kd for catalysts during poisoning were determined by plotting the left-hand side of eq 11, f(XCO), vs (t − 24). The plots are presented in Figure 7a−d. The calculated values of kd and the half-life t1/2 (from eq 12) are presented in Table 5.

4. DISCUSSION On the basis of the observed drop in TOF and TT and the loss of sites during the poisoning runs, sulfur impurities are confirmed to decrease the total adsorption of one or more reactants. Moreover, the catalyst shows no signs of recovering its lost activity in the post-poisoning stage, and this strongly suggests that the sulfur adsorbs irreversibly on the active sites present on the catalyst, leading to a permanent activity loss. (It must be noted that the post-poisoning stage in the present study lasts for 24 h (48−72 h TOS) and the term “permanent activity loss” is based on that period: with longer TOS, the catalyst activity could be restored.) Both the CA-modified catalysts show an improved performance over the base catalysts in the presence of sulfur, and in particular, the superior performance of EDTA-modified over NTA-modified catalyst correlates directionally with the active site densities. The observed drop in activity due to sulfur is consistent with some other studies;11,16,32 however, the observed trend disagrees with trends reported by others.14−16 For example, Borg et al.34 report a slight positive step increase in the catalyst activity after removal of sulfur from the inlet. It is important to note that the effect of sulfur can be dependent on its concentration in syngas. For example, Sparks et al.30 have indicated that, at a H2S range of 500−1000 ppb, the poisoning effect can be reversed after termination of H2S addition. Pansare and Allison21 suggest that 50 ppb of sulfur may be the limiting concentration in syngas, with sulfur having a detrimental effect above this value. In the present work, the concentrations of sulfur used are equal to or greater than 10 ppm, which is more than 2 orders of magnitude higher. The drop in activity is because of the sulfur poisoning of active sites. The interaction of sulfur with FT catalysts (Co and also Fe) poisons the catalyst due to the formation of either irreversible surface sulfides at low concentration (0.6−1.8 ppm H2S), or a multilayer metal sulfide at higher concentrations (6 ppm H2S and

Table 5. Deactivation Rate Constant and Half-Life of FT Catalysts under Sulfur Poisona

a

catalyst

kd (h−1)

t1/2 (h)

Co/SiO2; 10 ppm S Co/SiO2; 50 ppm S Co/EDTA/SiO2; 10 ppm S Co/NTA/SiO2; 10 ppm S

0.045 0.270 0.014 0.013

15.4 2.6 49.5 53.3

FT runs at 230 °C, TOS of 24−72 h.

As expected, the value of kd is higher for 50 ppm than for 10 ppm sulfur in the feed. In addition, for the CA-modified catalysts with more active sites and better FT activity in the presence of sulfur, the values of kd are expectedly smaller. The reported21 values of kd for a similar cobalt catalyst in the presence of 0.6 and 1.1 ppm sulfur are 0.017 and 0.023 h−1, respectively. The latter value is about 50% of the value from the present work (base-case catalyst) when the present work contains approximately an order of magnitude higher percentage of S in the feed. 5854



Article

Figure 8. EDS analyses of Co/SiO2 catalysts: (a) fresh sample, base-case catalyst; (b) spent catalyst with 10 ppm sulfur in the feed, base-case catalyst; (c) spent catalyst with 10 ppm sulfur in the feed, Co/EDTA/SiO2 catalyst; (d) spent catalyst with 10 ppm sulfur in the feed, Co/NTA/SiO2 catalyst.

above).21,35 Bartholomew et al. have reported35 that the sulfur molecules (in the form of H2S) adsorb strongly on the active metal sites with stoichiometry values (ratio of S to metal) from 0.5 to as high as 3. More importantly, the stoichiometry in their work was established exclusive of interference from the support, by confirming that the silica support had no role in influencing sulfur adsorption on the catalyst. In another case, Curtis et al.16 reported that sulfur present on pre-sulfided catalysts inhibits CO adsorption onto the surface of Co catalysts for sulfur concentrations between 100 and 200 ppm. This may be because of site blockage and/or inhibited reduction of metal. In an exception to this, DFT simulations indicate4 that, at low sulfur concentrations (∼100−400 ppb), sulfur has no clear preference for a particular cobalt site but, due to the size and electronic effect of the sulfur, it poisons adjacent sites. The interaction of sulfur with catalytic sites results in a loss of activity for those specific sites. However, due to higher densities of active sites on the CAmodified catalysts in this study, the extent of sulfur-induced poisoning of active sites is reduced by this modification. The post-poisoning selectivity toward long-chain hydrocarbons for CA-modified catalysts is clearly superior in comparison to that of the base catalyst. Even so, sulfur poisoning results in a permanent drop in selectivity toward higher hydrocarbons and causes an increase in selectivities for lower hydrocarbons, in comparison to the pre-poisoning stage for all the catalysts. It is therefore reasonable to say that sulfur

impurities affect the ability of the catalyst to form C−C bonds, and thereby to reduce formation of the long-chain hydrocarbons; this observation is in accordance to other studies on Co-based catalysts.11,21,31 Liu et al.36 have reported that in situ sulfur poisoning (in the form of COS) over an iron catalyst similarly increases the fraction of C1 and C2−C4 hydrocarbons in the product, while decreasing the C5+ fraction. Such shifts in the hydrocarbon selectivities may be due to physical blocking of the catalytic sites by sulfur. In fact, a sulfur atom adsorbed on an alumina-supported cobalt catalyst may poison more than two cobalt atoms.37 Further, the in situ characterization work of Curtis et al.16 shows that sulfur not only reduces the amount of CO adsorbed on the catalyst surface, but also weakens the COcobalt bond. Therefore, a small concentration of sulfur can potentially mask a substantial numbers of sites on the surface of the catalyst, leading to steric effects. The surface areas and pore volumes of the fresh base-case and CA-modified catalysts are comparable, as shown in Table 1, and therefore these steric limitations are likely due to sulfur adsorbed on the surface of catalyst. Such limitations decrease the readsorption probabilities of the reaction intermediates, and eventually cause shifts in the product selectivities, leading to higher selectivities for short-chain hydrocarbons. Likewise, in the present work of in situ poisoning, physical blockage by sulfur results in reduced CO adsorption on the Co catalysts. It is also evident from the drop in conversion and the shifts in selectivities that both the sites responsible for 5855



Article

level: below 100 ppm, S affects only CO conversion, whereas above 100 ppm, S affects conversions as well as product distribution. In this work (in situ poisoning), we report that, even at 10 ppm, S in the inlet syngas gas has undesired effects on conversion and selectivities. This agrees with the reported11 trend of a greater activity loss during in situ poisoning than presulfiding, at the same concentration of S.

CO adsorptions and the sites for chain growth processes are poisoned in the in situ poisoning experiment. It must be emphasized that correlating the effect of sulfur with product selectivity is not as straightforward as it is for conversion. The partial pressure of water formed during FTS, as one of the coproducts, along with hydrocarbons and oxygenates, has a strong influence on product selectivities. Moreover, the formation of water is dependent on CO conversion, and therefore the drop in C5+ selectivities in the presence of sulfur could also be supposed to arise from the drop in conversion due to sulfur poisoning.34 Another reason for the shift in the selectivity may be due to the increased surface acidity caused by sulfur adsorption on the catalyst surface. Such changes can potentially lead to higher selectivity for the short-chain hydrocarbons, as reported32 for pre-sulfided cobalt-based catalysts. The higher activity of the CA-modified catalysts can be attributed to a greater availability of accessible sites, more of which seem to remain largely unpoisoned even in the presence of sulfur. The higher readsorption probability of reaction intermediates on these highly dispersed catalysts can lead to formation of long-chain hydrocarbons. The higher C 5+ selectivities of the CA-modified catalysts in the post-poisoning stage also suggest that the sites responsible for the C−C formation are still present in sufficient numbers even after the sulfur poisoning. Further, under FT conditions, there exist interactions of CO not only with metal, i.e., Co−CO, but also with other CO molecules, e.g., bridging of CO−CO species.16 This type of interaction requires larger surface sites. The presence of sulfur on the metal surface causes a steric effect and, by blocking surface sites, reduces CO−CO bridging. In support of this hypothesis, the greater availability of surface sites on the CA-modified catalysts is perceived to be beneficial in sustaining activity even in the presence of the sulfur poison. In general, different active sites on the same catalyst may catalyze different reactions. The sulfur species present in syngas can preferentially adsorb on certain catalytic sites and can thus inhibit the reactions that normally occur on those sites. Therefore, the selective blocking of sulfur can alter distributions of FT product types. It is generally believed that the olefin species appears in FT products due to desorption of reaction intermediates and incomplete hydrogenation. The increase in the olefin content due to sulfur poisoning can be attributed to a drop in the hydrogenation ability of the catalysts.38 In this context, Bartholomew et al.39 have suggested that selective sulfur blocking can effectively reduce the adsorption probability of H2, and hence the products may desorb before the hydrogenation step. Visconti et al.31 have also postulated that sulfur adsorption on a FT catalyst can lead to the poisoning of the sites responsible for CO adsorption and hydrogenation. In the propylene-topropane hydrogenation reaction, Visconti et al. claim that the hydrogenation capability of a pre-poisoned Co catalyst significantly decreases with increasing sulfur loading. Additionally, in the commercial operation at Sasol, there is strong evidence14 of sulfur promoting selectivity shifts toward more olefinic C2 products over Fe-based catalysts, and this has been considered as a potential handle in maximizing C2 olefinicity. The changes observed in the product distribution for all the catalysts in this study, showing higher olefin content in the products during the poisoning and the post-poisoning stage, are consistent with these reported findings. Visconti et al.31 further suggest that the poisoning effect of sulfur on pre-poisoned Co is determined by the concentration

5. CONCLUSIONS The addition of sulfur into the inlet syngas causes a prompt and significant loss in activity for the base catalyst, but a relatively lower loss for catalysts modified by chelating agents (CAs) under similar conditions. Two CAs are used, nitrilotriacetic acid (NTA) and ethylenediaminetetraacetic acid (EDTA). For all the catalysts, shifts in selectivity are observed to favor shorter-chain hydrocarbons and olefinic products. These shifts are consistent with those in other reports. However, no promotional (positive) effects of sulfur are observed on the catalyst activity. The increase in the selectivity of short-chain hydrocarbons comes at the expense of a drop in the selectivity for desirable long-chain hydrocarbons and, more importantly, a significant drop in product yields. In the poisoning stage as well as the postpoisoning stage, the CA-modified catalysts with higher active-site densities display a superior performance relative to the base (unmodified) catalyst. Additionally, the CA-modified catalysts, with their higher active-site densities, benefit from a partial recovery of product selectivity and yields in the post-poisoning stage, and this recovery is not observed for the base catalyst. Therefore, catalysts with larger active-sites densities, e.g., CAmodified catalysts in this study, not only can be beneficial for improved product yields in the presence of pure syngas26 but also have an advantage of an acceptable performance, to an extent, in the presence of syngas with sulfur impurities. In particular, the EDTA-modified catalyst shows higher CO conversion, total product yields, reaction rates, TOF, and TT, compared to NTAmodified catalyst, in all the stages. These desired attributes of the EDTA-modified catalyst are due to smaller crystallites and higher metal dispersion, leading to better active-site densities on this catalyst. In summary, the use of CA appears to be promising in the design of improved cobalt catalysts for FTS.

■

AUTHOR INFORMATION

Corresponding Author

*D. B. Dadyburjor. Phone: (304) 293-9337. Fax: (304) 2934139. E-mail: [email protected]. Present Addresses §

A. S. Bambal. UOP LLC, United States V. S. Guggilla. Reliance Industries Limited, Mumbai, India

⊥

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was performed with partial support obtained under the National Energy Technology Laboratory, RDS Contract DEAC26-04NT41817.

■

REFERENCES

(1) Anderson, R. B.; Seligman, B.; Shultz, J. F.; Kelly, R.; Elliott, M. A. Fischer−Tropsch Synthesis. Some Important Variables of the Synthesis on Iron Catalysts. Ind. Eng. Chem. 1952, 44, 391.

5856



Article

(25) (a) Koizumi, N.; Ibi, Y.; Hongo, D.; Hamabe, Y.; Suzuki, S.; Hayasaka, Y.; Shindo, T.; Yamada, M. Mechanistic Aspects of the Role of Chelating Agents in Enhancing Fischer−Tropsch Synthesis Activity of Co/SiO2 Catalyst: Importance of Specific Interaction of Co with Chelate Complex during Calcination. J. Catal. 2012, 289, 151. (b) Koizumi, N.; Mochizuki, T.; Yamada, M. Preparation of Highly Active Catalysts for Ultra-Clean Fuels. Catal. Today 2009, 141, 34. (26) Bambal, A. S.; Kugler, E. L.; Gardner, T. H.; Dadyburjor, D. B. Effect of Surface Modification by Chelating Agents on Fischer−Tropsch Performance of Co/SiO2 Catalysts. Ind. Eng. Chem. Res. 2013, 52, 16675. (27) Iglesia, E.; Soled, S. L.; Fiato, R. A. Fischer−Tropsch Synthesis on Cobalt and Ruthenium - Metal Dispersion and Support Effects on Reaction-Rate and Selectivity. J. Catal. 1992, 137, 212. (28) Zennaro, R.; Tagliabue, M.; Bartholomew, C. H. Kinetics of Fischer−Tropsch Synthesis on Titania-Supported Cobalt. Catal. Today 2000, 58, 309. (29) Bambal, A. Study of the Effect of Surface Modification and Sulfur Impurities in Syngas on the Fischer−Tropsch Performance of Cobalt Catalysts. Ph.D. Dissertation, West Virginia University, Morgantown, WV, 2012. (30) Sparks, D. E.; Jacobs, G.; Gnanamani, M. K.; Pendyala, V. R. R.; Ma, W.; Kang, J.; Shafer, W. D.; Keogh, R. A.; Graham, U. M.; Gao, P.; Davis, B. H. Poisoning of Cobalt Catalyst used for Fischer−Tropsch Ssynthesis. Catal. Today 2013, 215, 67. (31) Visconti, C. G.; Lietti, L.; Forzatti, P.; Zennaro, R. Fischer− Tropsch Synthesis on Sulphur Poisoned Co/Al2O3 Catalyst. Appl. Catal., A 2007, 330, 49. (32) Madikizela-Mnqanqeni, N. N.; Coville, N. J. The Effect of Sulfur Addition during the Preparation of Co/Zn/TiO2 Fischer−Tropsch Catalysts. Appl. Catal., A 2008, 340, 7. (33) Duvenhage, D. J.; Coville, N. J. Deactivation of a Precipitated Iron Fischer−Tropsch CatalystA Pilot Plant Study. Appl. Catal., A 2006, 298, 211. (34) Borg, Ø.; Hammer, N.; Enger, B. C.; Myrstad, R.; Lindvåg, O. A.; Eri, S.; Skagseth, T. H.; Rytter, E. Effect of Biomass-Derived Synthesis Gas Impurity Elements on Cobalt Fischer−Tropsch Catalyst Performance Including in situ Sulphur and Nitrogen Addition. J. Catal. 2011, 279, 163. (35) Bartholomew, C. H.; Agrawal, P. K.; Katzer, J. R. Sulfur Poisoning of Metals. In Adv. Catal., Eley, D., Pines, H., Weisz, P., Eds.; Academic Press: New York, 1982, 31, 135. (36) Liu, Z.-T.; Zhou, J.-L.; Zhang, B.-J. Poisoning of Iron Catalyst by COS in Syngas for Fischer–Tropsch Synthesis. J. Mol. Catal. 1994, 94, 255. (37) Agrawal, P. K.; Katzer, J. R.; Manogue, W. H. Methanation over Transition Metal Catalysts: III. CoAl2O3 in Sulfur-Poisoning Studies. J. Catal. 1981, 69, 327. (38) Hanlon, R. T.; Satterfield, C. N. Reactions of Selected 1-Olefins and Ethanol Added during the Fischer−Tropsch Synthesis. Energy Fuels 1988, 2, 196. (39) Bartholomew, C. H.; Weatherbee, G. D.; Jarvi, G. A. Sulfur Poisoning of Nickel Methanation Catalysts: I. In situ Deactivation by H2S of Nickel and Nickel Bimetallics. J. Catal. 1979, 60, 257.

(2) Madon, R. J.; Shaw, H. Effect of Sulfur on the Fischer−Tropsch Synthesis. Cat. Rev. − Sci. Eng. 1977, 15, 69. (3) Dry, M. E. The Fischer−Tropsch process: 1950−2000. Catal. Today 2002, 71, 227. (4) Saib, A. M.; Moodley, D. J.; Ciobîcă, I. M.; Hauman, M. M.; Sigwebela, B. H.; Weststrate, C. J.; Niemantsverdriet, J. W.; van de Loosdrecht, J. Fundamental understanding of deactivation and regeneration of cobalt Fischer−Tropsch synthesis catalysts. Catal. Today 2010, 154, 271. (5) van de Loosdrecht, J.; Balzhinimaev, B.; Dalmon, J. A.; Niemantsverdriet, J. W.; Tsybulya, S. V.; Saib, A. M.; van Berge, P. J.; Visagie, J. L. Cobalt Fischer−Tropsch synthesis: Deactivation by oxidation? Catal. Today 2007, 123, 293. (6) Tsakoumis, N. E.; Rønning, M.; Borg, Ø.; Rytter, E.; Holmen, A. Deactivation of cobalt based Fischer−Tropsch catalysts: A review. Catal. Today 2010, 154, 162. (7) Chaffee, A. L.; Campbell, I.; Valentine, N. Sulfur Poisoning of Fischer−Tropsch Synthesis Catalysts in a Fixed-Bed Reactor. Appl. Catal. 1989, 47, 253. (8) Anderson, R. B.; Karn, F. S.; Shultz, J. F. Factors in Sulfur Poisoning of Iron Catalysts in Fischer−Tropsch Synthesis. J. Catal. 1965, 4, 56. (9) Bartholomew, C. H. Mechanisms of Catalyst Deactivation. Appl. Catal., A 2001, 212, 17. (10) Bayer, J.; Stein, K. C.; Hofer, L. J. E.; Anderson, R. B. Effect of Preadsorbed Sulfur Compounds on Chemisorption of CO and CO2 on Iron Catalysts. J. Catal. 1964, 3, 145. (11) Bartholomew, C. H.; Bowman, R. M. Sulfur Poisoning of Cobalt and Iron Fischer−Tropsch Catalysts. Appl. Catal. 1985, 15, 59. (12) Madon, R. J.; Taylor, W. F. Effect of Sulfur on the Fischer− Tropsch Synthesis. In Hydrocarbon Synthesis from Carbon Monoxide and Hydrogen; Kugler, E. L., Steffgen, F. W., Eds.; American Chemical Society: Washington, DC, 1979, 178, p 93. (13) Bromfield, T. C.; Coville, N. J. The Effect of Sulfide Ions on a Precipitated Iron Fischer−Tropsch Catalyst. Appl. Catal., A 1999, 186, 297. (14) Kritzinger, J. A. The Role of Sulfur in Commercial Iron-Based Fischer−Tropsch Catalysis with Focus on C2-Product Selectivity and Yield. Catal. Today 2002, 71, 307. (15) Wu, B.; Bai, L.; Xiang, H.; Li, Y.-W.; Zhang, Z.; Zhong, B. An Active Iron Catalyst Containing Sulfur for Fischer−Tropsch Synthesis. Fuel 2004, 83, 205. (16) Curtis, V.; Nicolaides, C. P.; Coville, N. J.; Hildebrandt, D.; Glasser, D. The Effect of Sulfur on Supported Cobalt Fischer−Tropsch Catalysts. Catal. Today 1999, 49, 33. (17) Li, J.; Coville, N. J. Effect of Boron on the Sulfur Poisoning of Co/ TiO2 Fischer−Tropsch Catalysts. Appl. Catal., A 2001, 208, 177. (18) Wise, H.; Mechanisms of Catalyst Poisoning by Sulfur Species. In Stud. Surf. Sci. Catal., Bartholomew, C., Butt, J., Eds.; Catalyst Deactivation 1991, Proceeding of 5th International Symposium, Evanston, IL; Elsevier: Amsterdam; 1991, 68, p 497. (19) Tsakoumis, N. E.; Rønning, M.; Borg, Ø.; Rytter, E.; Holmen, A. Deactivation of Cobalt based Fischer−Tropsch Catalysts: A Review. Catal. Today 2010, 154, 162. (20) Sparks, D. E.; Jacobs, G.; Gnanamani, M. K.; Pendyala, V. R. R.; Ma, W.; Kang, J.; Shafer, W. D.; Keogh, R. A.; Graham, U. M.; Gao, P.; Davis, B. H. Poisoning of Cobalt Catalyst used for Fischer−Tropsch Synthesis. Catal. Today 2013, 215, 67. (21) Pansare, S. S.; Allison, J. D. An Investigation of the Effect of UltraLow Concentrations of Sulfur on a Co/[gamma]-Al2O3 Fischer− Tropsch Synthesis Catalyst. Appl. Catal., A 2010, 387, 224. (22) Shimizu, T.; Hiroshima, K.; Honma, T.; Mochizuki, T.; Yamada, M. Highly Active Hydrotreatment Catalysts Prepared with Chelating Agents. Catal. Today 1998, 45, 271. (23) Sun, M.; Nicosia, D.; Prins, R. The Effects of Fluorine, Phosphate and Chelating Agents on Hydrotreating Catalysts and Catalysis. Catal. Today 2003, 86, 173. (24) Kaluž, L.; Zdražil, M.; Vít, Z.; Gulková, D. CoMo/ZrO2 Hydrodesulfurization Catalysts Prepared by Chelating Agent Assisted Spreading. Procedia Eng. 2012, 42, 261. 5857


Poisoning of a Silica-Supported Cobalt Catalyst due to Presence of

Recommend Documents