Ruthenium Modification on Mn and Zr-Modified Co/SiO2 Catalysts for Slurry-Phase Fischer-Tropsch Synthesis
Biomass Refinery Research Center, Advanced Industrial Science and Technology (AIST) 3-11-32 Kagamiyama, Higashihiroshima, Hiroshima 737-0046, Japan
*
Author to whom correspondence should be addressed.
Catalysts 2015, 5(1), 26-37; https://doi.org/10.3390/catal5010026
Submission received: 10 October 2014
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Accepted: 4 January 2015
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Published: 12 January 2015
Abstract
:The addition of Ru to Mn and Zr-modified Co/SiO2 catalysts, while applying different preparation orders and loading amounts, was investigated as a means of enhancing the Fischer-Tropsch synthesis reaction. The coimpregnation of Zr/SiO2 with Co, Mn and Ru gave the most attractive catalytic properties. This can be attributed to the higher dispersion of Co metal resulting from the coimpregnation of Co and Mn as well as enhanced reducibility due to the presence of Ru. The addition of a moderate amount of Ru together with the appropriate order of addition affected both the Co reducibility and the catalytic activity, primarily because of increased reducibility. The addition of even 0.1 wt.% Ru resulted in an obvious enhancement of Fischer-Tropsch synthesis activity.
1. Introduction
Fischer-Tropsch (FT) synthesis is a useful means of producing liquid fuels and other chemicals from various carbon sources (such as coal, natural gas and biomass) via syngas [1,2]. Co-based catalysts are the most widely applied for this reaction since they exhibit higher activity and selectivity and thus are more economical [3,4]. It is generally accepted that the activity of Co catalysts for FT synthesis depends on the presence of surface-exposed Co metal atoms [5,6], and therefore Co species should be both highly dispersed and reduced on the catalyst surface. For this reason, Co is generally deposited on high surface area supports such as SiO2 [7,8,9,10,11], Al2O3 [12,13] and TiO2 [14,15], in order to increase the quantity of active Co metal species. However, strong interactions between Co atoms and the support material can decrease both the reducibility and the activity of a Co catalyst [7,8].
The support materials used for Co catalysts can significantly affect the state of Co on the catalyst surface, including factors such as particle size and reducibility. Our group is focused on the use of SiO2 as a catalyst support and has investigated the modification of this support using both metals and oxide additives. It is well known that the addition of Zr to a Co/SiO2 catalyst (Zr/Co/SiO2) increases the FT synthesis activity of the catalyst [9,10,11]. Furthermore, the addition of Mn to Zr/Co/SiO2 enhances the catalyst stability during the FT synthesis reaction [16]. In a previous study, we found that the addition of moderate amounts of Mn and Zr affected both the reducibility of Co and the degree of Co dispersion. Various characterization results suggested that the enhanced FT synthesis activity observed following coimpregnation to produce a Mn+Zr+Co/SiO2 catalyst can be attributed to the formation of highly dispersed Co metal particles interacting with Mn species [17].
However, on such modified catalysts, the degree of reduction of the Co component remains below 50%. Therefore, the goal of the present study was to increase the Co reducibility and the catalytic activity of Mn and Zr-modified Co/SiO2 catalysts for the FT synthesis reaction. It has been demonstrated that the addition of noble metals to supported cobalt catalysts increases the Co reducibility [10,18,19,20,21,22] and so this project investigated the promotional effect of Ru on a Mn+Zr+Co/SiO2 catalyst, in addition to the effects of reduction conditions.
2. Results and Discussion
2.1. Effect of Noble Metal Addition and Impregnation Order
In a previous study [17], we prepared a Mn and Zr-modified Co/SiO2 catalyst and found that the coimpregnation method was preferable in terms of improving the catalytic activity of Co/SiO2 by Mn and Zr addition, and that the optimum loading amounts of Zr and Mn were 8 and 1 wt.%, respectively. The present study investigated the effect of adding a small amount of a noble metal to this Mn and Zr-modified Co/SiO2 catalyst. Table 1 summarizes the effect of noble metal addition and impregnation order on Mn and Zr-modified Co/SiO2 when applied to FT synthesis. Here STY indicates the space time yield of C5+ liquid products.
It is evident that the simple addition of a noble metal to the Co+Mn+Zr/SiO2 catalyst following calcination was not effective, in the case of both Ru and Pt, since the CO conversion and C5+ yield over the Ru and Pt-modified catalysts were lower than that over the Co+Mn+Zr/SiO2 catalyst. It is believed that surface Co metal active sites on the Ru and Pt-treated catalysts were covered during the additional impregnation step thus decreasing their activity. In contrast, when Ru was added simultaneously with Co, the FT synthesis activity was enhanced in all cases. The effect of Ru addition was also found to be much more pronounced than that of Pt. Pt modification of the Co+Mn+Zr/SiO2 catalyst was ineffective, whether accomplished by sequential or simultaneous impregnation. In addition, although details are not provided here, it was determined that Ru+Mn+Zr/SiO2 and Ru/SiO2 catalysts without Co exhibited only very low activity during FT synthesis, with less than 5% CO conversion.
| Catalyst | Conversion/% | Selectivity/% | C5+ yield/% | STY/g/kg·h | ||||
|---|---|---|---|---|---|---|---|---|
| CO | H2 | CO2 | CH4 | C2–C4 | C5+ | |||
| Co/SiO2 | 30.6 | 36.8 | 2.4 | 12.8 | 9.5 | 75.3 | 23.1 | 281.5 |
| Co+Ru/SiO2 | 31.9 | 38.0 | 1.1 | 9.9 | 3.8 | 85.2 | 27.2 | 310.7 |
| Co+Mn+Zr/SiO2 | 37.4 | 44.7 | 1.9 | 11.1 | 3.4 | 83.5 | 31.2 | 380.4 |
| Pt/Co+Mn+Zr/SiO2 | 34.2 | 42.8 | 2.4 | 17.3 | 6.3 | 74.1 | 25.3 | 308.9 |
| Pt+Co+Mn+Zr/SiO2 | 38.5 | 46.5 | 2.0 | 11.8 | 5.0 | 81.2 | 31.2 | 381.0 |
| Ru/Co+Mn+Zr/SiO2 | 36.9 | 45.3 | 1.7 | 14.3 | 5.0 | 79.1 | 29.2 | 355.6 |
| Ru+Co+Mn+Zr/SiO2 | 44.2 | 53.3 | 1.9 | 11.9 | 3.5 | 82.7 | 36.6 | 445.7 |
| Ru+Co/Mn+Zr/SiO2 | 45.0 | 54.0 | 2.1 | 12.1 | 3.5 | 82.3 | 37.1 | 452.0 |
| Ru+Co+Mn/Zr/SiO2 | 46.2 | 55.2 | 2.1 | 11.0 | 3.0 | 83.9 | 38.8 | 473.0 |
Catalyst: 1.0 g, W/F: 4 g·h/mol, T=503 K, P=1.0 MPa; Co loading: 20 wt%, Mn loading 1 wt%, Zr loading 8 wt%, Ru or Pt loading 0.5 wt%
Figure 1 presents the H2-TPR profiles of Ru-Co-Mn-Zr/SiO2 catalysts prepared using various orders of addition. As a reference, the profiles of Co+Mn+Zr/SiO2 and Pt/Co+Mn+Zr/SiO2 are also shown.
Figure 1.
H2-TPR profiles of various catalysts: (a) Co+Mn+Zr/SiO2, (b) Pt/Co+Mn+Zr/SiO2; (c) Ru/Co+Mn+Zr/SiO2; (d) Ru+Co+Mn+Zr/SiO2; (e) Ru+Co+Mn/Zr/SiO2; (f) Ru+Co/Mn+Zr/SiO2.
Figure 1.
H2-TPR profiles of various catalysts: (a) Co+Mn+Zr/SiO2, (b) Pt/Co+Mn+Zr/SiO2; (c) Ru/Co+Mn+Zr/SiO2; (d) Ru+Co+Mn+Zr/SiO2; (e) Ru+Co+Mn/Zr/SiO2; (f) Ru+Co/Mn+Zr/SiO2.
It is evident that the reduction of Co species without noble metals has its onset at approximately 550 K and shows a maximum at around 640 K on the Co+Mn+Zr/SiO2 catalyst. Reduction over the Co+Mn+Zr/SiO2 catalyst during the 8 h hold proceeded slowly although a gradual reduction occurred and the extent of reduction eventually achieved is twice that calculated from the reduction below 673 K. In the case of the noble metal-modified catalysts, however, the reduction peaks are shifted to lower temperatures and the reduction of Co species took place during the 8 h hold at 673 K. The Pt/Co+Mn+Zr/SiO2 catalyst exhibited a startup temperature for the lower reduction peak in the vicinity of 470 K with a noticeably sharper and higher peak compared to the other Ru-modified catalysts. Similar behavior was observed for a Pt-modified Co/SiO2 catalyst by Pirola et al. [23]. Based on these results, it appears that the addition of 0.5 wt.% Pt generates a cubic structure in the Pt-containing phase. In contrast, the same level of Ru addition produces a Ru−Co solid solution with a hexagonal close packed structure. This difference in the metal structures can be related to the differences in the TPR profiles. The sharp reduction peaks obtained from the Pt/Co+Mn+Zr/SiO2 catalyst can be explained by the H2 spillover effect from the cubic structure of Pt. Among the Ru-modified Ru+Co+Mn+Zr/SiO2, Ru+Co+Mn/Zr/SiO2 and Ru+Co/Mn+Zr/SiO2 catalysts, broad reduction peaks are seen at lower temperatures of approximately 440 K, although the peak heights are very low, while the main reduction peaks appear around 510 to 540 K. The peaks at lower temperatures are assigned to the reduction of Co3O4 to CoO, and the high temperature peaks are attributed to the reduction of CoO to Co metal. The Ru/Co+Mn+Zr/SiO2 catalyst, compared with the other Ru-modified catalysts, has both its lower peak and its main peak at higher temperatures; the lower peak is situated at approximately 500 K and the main peak at 560 K. Table 2 summarizes the physical properties and CO conversions during FT synthesis reactions over these catalysts.
Table 2.
Physical properties and CO conversions during Fischer-Tropsch (FT) synthesis reaction over various catalysts.
| Catalyst | CO conv./% | H2 consumption/mmol/g-cat | Reduction degree/% (at 673 K) | Reduction degree/% (after hold) | Dispersion/% |
|---|---|---|---|---|---|
| Co/SiO2 | 30.6 | 12.1 | 32 | 94 | 0.66 |
| Co+Ru/SiO2 | 31.9 | 13.1 | 41 | 96 | 0.70 |
| Co+Mn+Zr/SiO2 | 37.4 | 14.7 | 20 | 48 | 1.58 |
| Pt/Co+Mn+Zr/SiO2 | 34.2 | 19.7 | 33 | 68 | 1.49 |
| Pt+Co+Mn+Zr/SiO2 | 38.5 | 22.5 | 34 | 71 | 1.62 |
| Ru/Co+Mn+Zr/SiO2 | 36.9 | 20.4 | 33 | 76 | 1.38 |
| Ru+Co+Mn+Zr/SiO2 | 44.2 | 25.8 | 36 | 79 | 1.67 |
| Ru+Co/Mn+Zr/SiO2 | 45.0 | 24.9 | 37 | 74 | 1.73 |
| Ru+Co+Mn/Zr/SiO2 | 46.2 | 25.9 | 34 | 71 | 1.87 |
It is clear that the degree of reduction of Co was significantly enhanced by noble metal addition in all cases. However, the activities during FT synthesis of both Ru/Co+Mn+Zr/SiO2 and Pt/Co+Mn+Zr/SiO2 were decreased compared to that of Co+Mn+Zr/SiO2. From the H2 chemisorption results, the dispersion of Co metal was decreased during the Ru or Pt modification by simple sequential impregnation of Co+Mn+Zr/SiO2. This likely resulted in spite of the enhanced reducibility because coverage of the Co metal particles by the sequential impregnation decreases the Co dispersion and hence the FT synthesis activity. In contrast, the catalyst that had been simultaneously impregnated with Ru and Co exhibited higher Co reducibility and FT synthesis activity compared to the Co+Mn+Zr/SiO2. From these results, it can be expected that Ru-Co solid solutions were formed during the coimpregnation process, leading to a greater degree of Co reducibility. The H2-TPR profiles and catalytic properties are similar among these Ru and Co coimpregnation catalysts, and the differences in their FT synthesis activities are minor, although Ru+Co+Mn/Zr/SiO2 exhibited the highest activity. Ru+Co+Mn/Zr/SiO2 also showed the highest level of metal dispersion and this can be related to its superior catalytic activity. In addition, the catalytic activity normalized to the level of dispersion was almost constant among all the Ru-modified catalysts. This result indicates that the site-time yields over the Co catalysts were independent of the metal dispersion, and that the nature of the active sites on the catalysts is likely unchanged, which is also consistent with the findings of Iglesia et al. [6]. Our previous work [17] suggested that, during the coimpregnation catalyst preparation process, the interaction between Co and Mn species enhances Co dispersion and therefore FT synthesis activity. Hence, it is thought that the higher dispersion of Co metal results from coimpregnation of the Co and Mn while the higher reducibility is due to the presence of Ru, and these two effects combine such that Ru+Co+Mn/Zr/SiO2 shows the highest activity among the Ru-modified catalysts.
2.2. Effects of Ru Quantity on Ru+Co+Mn/Zr/SiO2
In the previous section, it was shown that the addition of Ru clearly enhances the catalytic activity of Ru+Co+Mn/Zr/SiO2. However, Ru is a less than ideal additive due to its high cost and limited availability. Therefore, the variations in the catalytic activity of the Ru+Co+Mn/Zr/SiO2 catalyst with Ru content were evaluated. Figure 2 shows the H2-TPR profiles of Ru+Co+Mn+Zr/SiO2 catalysts with different Ru amounts. The preparation and reaction procedures applied in these trials were the same as those described above. (wt.%).
Figure 2.
H2-TPR profiles of Ru+Co+Mn/Zr/SiO2 catalysts with different Ru amounts. Numbers in brackets indicate the Ru loading amounts. (a) Without Ru; (b) 0.1 wt%; (c) 0.3wt%; (d) 0.5wt%; (e) 1.0wt%
Figure 2.
H2-TPR profiles of Ru+Co+Mn/Zr/SiO2 catalysts with different Ru amounts. Numbers in brackets indicate the Ru loading amounts. (a) Without Ru; (b) 0.1 wt%; (c) 0.3wt%; (d) 0.5wt%; (e) 1.0wt%
It is evident that higher concentrations of Ru allow the Co to be reduced more readily. Without Ru addition, the reduction of Co species starts around 550 K and the reduction peak is centered at approximately 640 K. Even 0.1 wt.% Ru decreases the peak temperature to 590 K and 0.3 wt.% lowers the peak temperature still further, to 550 K. At more than 0.3 wt.% Ru, however, the enhancement effect appears to plateau: the difference in the peak temperatures between 0.3 and 1.0 wt.% is only 20 K. Figure 3 summarizes the effect of Ru concentration on the FT synthesis activity.
Figure 3.
Ru amount dependence of the FT synthesis activity over the Ru+Co+Mn/Zr/SiO2 catalyst. Catalyst: 1.0 g, W/F: 4 g·h/mol, Co loading: 20 wt%, Pressure: 1.0–4.0 MPa, Temperature: 503 K.
Figure 3.
Ru amount dependence of the FT synthesis activity over the Ru+Co+Mn/Zr/SiO2 catalyst. Catalyst: 1.0 g, W/F: 4 g·h/mol, Co loading: 20 wt%, Pressure: 1.0–4.0 MPa, Temperature: 503 K.
Both the CO conversion and STY were increased by addition of 0.1 wt.% Ru, although the selectivity was not as dependent on the Ru amount. In addition, the effects of increasing the Ru concentration on the CO conversion and STY were also not overly significant above 0.1 wt.%. Table 3 summarizes the physical properties and CO conversions during the FT synthesis reaction over these catalysts.
Table 3.
Physical properties and CO conversions during the FT synthesis reaction over Ru-modified catalysts.
| Ru amount/wt% | CO conv./% | H2 consumption/mmol/g-cat | Reduction degree/% (at 673 K) | Reduction degree/% (after hold) | Dispersion/% |
|---|---|---|---|---|---|
| 0.0 | 38.0 | 14.6 | 20 | 47 | 1.60 |
| 0.1 | 44.3 | 21.1 | 24 | 61 | 1.77 |
| 0.3 | 45.0 | 25.2 | 42 | 71 | 1.84 |
| 0.5 | 46.2 | 25.9 | 34 | 71 | 1.87 |
| 1.0 | 45.4 | 25.7 | 41 | 73 | 1.81 |
These data clearly show that the degree of reduction of Co after H2-TPR is increased by the Ru addition and that this increase was almost saturated at 0.3 wt.% Ru addition. In addition, in accordance with increasing reduction degree, the H2 consumption also increased. Therefore, the increased quantity of active Co metal sites on the Ru-modified Ru+Co+Mn/Zr/SiO2 may be attributed to the enhanced reducibility of the Co resulting from its interaction with the Ru. However, an overly high Ru concentration actually decreases both the metal dispersion and catalytic activity. As an example, the material with 0.5 wt.% Ru exhibited the highest FT synthesis activity, due to the segregation of Ru [21]. From these results, it can be said that moderate amounts of Ru addition to the Ru+Co+Mn/Zr/SiO2 catalysts enhance the FT synthesis activity primarily due to the increasing reducibility. Even 0.1 wt.% Ru addition generated an obvious enhancement of the catalytic activity and reducibility.
2.3. Effect of H2 Reduction Conditions on the Ru+Co+Mn/Zr/SiO2
As discussed in the previous section, Ru addition improved the catalyst reducibility under 1.0 MPa pressure. To confirm this improvement, the FT activity of Ru+Co+Mn/Zr/SiO2 under various H2 reduction conditions was evaluated. The H2 reduction temperature was varied from 573 to 773 K and the pressure was varied between ambient and 1.0 MPa. In each trial, the same temperature increase rate (10 K/min) and reduction time (8 h hold at the desired temperature) were applied. Table 4 summarizes the variations in the FT reaction with changing reduction conditions.
| Reduction Condition | Conversion/% | Selectivity/% | C5+ yield/% | STY/g/kg·h | ||||
|---|---|---|---|---|---|---|---|---|
| Temperature, Pressure | CO | H2 | CO2 | CH4 | C2–C4 | C5+ | ||
| 673 K, 1.0 MPa | 41.4 | 48.7 | 1.9 | 11.1 | 3.4 | 83.5 | 34.5 | 421.1 |
| 573 K, 0.1 MPa | 46.1 | 54.9 | 2.2 | 11.8 | 3.4 | 82.6 | 38.1 | 464.3 |
| 673 K, 0.1 MPa | 46.2 | 55.2 | 2.1 | 11.0 | 3.0 | 83.9 | 38.8 | 473.0 |
| 773 K, 0.1 MPa | 43.4 | 51.8 | 1.5 | 12.8 | 4.5 | 81.2 | 35.2 | 429.7 |
Catalyst: 1.0 g, W/F: 4 g h/mol, T=503 K, P=1.0 MPa; Co loading: 20 wt%, Mn loading 1 wt%, Zr loading 8 wt%, Ru 0.5 wt%
It can be seen that reduction with this catalyst at 1.0 MPa generates less activity than reduction at ambient pressure (0.1 MPa), thus H2 reduction at 1.0 MPa is not suitable for this catalyst. It is thought that reduction at higher pressures decreases both the metal dispersion and the catalytic activity. In addition, H2 reduction at 773 K also lowers the FT activity compared to the reaction at lower temperatures, although the difference between the catalytic activities at 573 and 673 K was minimal. As discussed in Section 2.1, the addition of Ru to the catalyst enables sufficient Co reduction at 573 K and generates almost the same level of FT synthesis activity. It can be expected that the energy input required for H2 reduction would be decreased by the lowering of the reduction temperature from 673 K to 573 K.
2.4. FT Synthesis Reaction under High Pressure Conditions
As noted above, Ru addition improved the FT synthesis activity of the Mn and Zr-modified Co/SiO2 catalysts under a pressure of 1.0 MPa. This reaction pressure is sufficiently low so as to avoid any restrictions associated with the Japanese High-Pressure Gas Safety Law. However, on an industrial scale, the FT synthesis reaction is typically performed under higher pressure conditions, such as 2.0–5.0 MPa [7]. Hence the catalytic FT synthesis reactivity at 4.0 MPa was also investigated and compared with the results obtained at 1.0 MPa. Table 5 presents the effect of reaction pressure on the Co/SiO2, Co+Mn+Zr/SiO2 and Ru+Co+Mn/Zr/SiO2 catalysts during FT synthesis. The activity order of these catalysts was: Ru+Co+Mn/Zr/SiO2 > Co+Mn+Zr/SiO2 > Co/SiO2 at both pressures. The Ru/Co+Mn/Zr/SiO2 produced greater CO and H2 conversions, and exhibited higher C5+ selectivity and chain growth probability (α), compared to Co+Mn+Zr/SiO2 and Co/SiO2. The results also indicated that, at higher reaction pressures, higher conversions, C5+ selectivity and α values were achieved in the case of all catalysts. In addition, CH4 selectivity was drastically reduced at higher pressures, leading to greater C5+ selectivity, particularly with the Ru/Co+Mn/Zr/SiO2 catalyst. In this research, the raw material synthesis gas included 5.0 vol% CH4 and so CH4 selectivity was calculated by subtracting the CH4 from the raw synthesis gas, possibly generating artificially low values for the CH4 selectivity. Despite this potential source of error in the selectivity estimations, we are confident that the general selectivity trends identified here are correct. Although both CO conversion and STY were seen to increase with pressure, these parameters increased at rates lower than the pressure elevation rates; when the reaction pressure was quadrupled from 1 to 4 MPa, the CO conversions and STY increased by only approximately 50%. It was determined that the Ru/Co+Mn/Zr/SiO2 catalyst exhibited the highest FT synthesis activity compared to the Co/SiO2 and Co+Mn+Zr/SiO2.
| Catalyst | Pressure/MPa | Conversion/% | Selectivities/% | C5+ yield/% | α | STY/g/kg·h | Dispersion degree b/% | Reduction degree c/% | ||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| CO | H2 | CO2 | CH4 | C2–C4 | C5+ | |||||||
| Co/SiO2 | 1.0 | 30.6 | 36.8 | 2.4 | 12.8 | 9.5 | 75.3 | 23.1 | 0.86 | 281 | 0.66 | 94 |
| 4.0 | 44.4 | 53.4 | 0.7 | 2.7 | 8.0 | 88.5 | 39.3 | 0.87 | 427 | |||
| Co+Mn+Zr/SiO2 | 1.0 | 37.4 | 44.7 | 1.9 | 11.1 | 3.4 | 83.5 | 31.2 | 0.86 | 380 | 1.57 | 48 |
| 4.0 | 53.6 | 58.7 | 0.5 | 1.6 | 5.9 | 92.0 | 49.3 | 0.90 | 601 | |||
| Ru+Co+Mn/Zr/SiO2 | 1.0 | 46.2 | 55.2 | 2.1 | 11.0 | 3.0 | 83.9 | 38.8 | 0.86 | 473 | 1.87 | 71 |
| 4.0 | 70.5 | 77.8 | 1.4 | 1.3 | 3.4 | 93.9 | 66.2 | 0.90 | 758 | |||
a Catalyst: 1.0 g, W/F: 4.0 g h/mol, Pressure: 1.0-4.0 MPa, T=503 K; b Determined by H2 chemisorption at 373 K; c Reduction percentage of Co3O4 to Co0, mesured by TPR at 673 K.
3. Experimental Section
3.1. Catalyst Preparation
SiO2 Q-10 used as support material for the Co catalysts (average pore size: 10 nm, surface area: 300 m2/g) was purchased from the Fuji Silysia Chemical Co., Ltd. (Kasugai, Japan). The Co loading was 20 wt.% on each catalyst and supports were impregnated using an aqueous solution of Co(NO3)2·6H2O. Attempts were made to improve the catalytic activity, stability and reducibility by modification with Mn, Zr and Ru or Pt. The loading amounts of Mn and Zr was 1 and 8 wt.%, respectively, while Ru was added in the concentration range of 0.1 to 1.0 wt.%. Mn(NO3)2·6H2O (Nacalai Tesque, Inc., Kyoto, Japan), ZrO(NO3)2·2H2O (Wako Pure Chemical Industries, Ltd., Osaka, Japan) and RuCl3·nH2O (SOEKAWA chemical Co., Ltd., Tokyo, Japan assay: 41%) or H2PtCl6·2H2O (Wako Pure Chemical Industries, Ltd., Osaka, Japan) were used as precursors. The purities of these chemical reagents were 99%. SiO2 supports were impregnated with Co, Mn, Zr and Ru or Pt using aqueous solutions of the above precursors, followed by drying and calcination. In this impregnation process, some catalysts were impregnated twice. A typical procedure, used to fabricate Ru/Co+Mn+Zr/SiO2, was as follows. The SiO2 support was first impregnated with Co, Mn and Zr using aqueous solutions of the above precursors, followed by drying and calcination. Subsequently, the support was impregnated with Ru using an aqueous solution of RuCl3·nH2O and again subjected to drying and calcination. In the catalyst nomenclature applied herein, the “/” symbol indicates that the material to the right of the slash was impregnated with the components shown on the left. To evaluate the effect of Ru addition and the addition order, Co+Mn+Zr/SiO2, Ru/Co+Mn+Zr/SiO2, Ru+Co+Mn+Zr/SiO2, Ru+Co+Mn/Zr/SiO2 and Ru+Co/Mn+Zr/SiO2 were prepared. In addition, Co/SiO2, Co+Ru/SiO2, Pt/Co+Mn+Zr/SiO2 and Pt+Co+Mn+Zr/SiO2 were fabricated as reference catalysts. In all cases, drying conditions were 383 K for 12 h, and calcination condition conditions were 673 K for 3 h (temperature increase rate: 10 K/min).
3.2. Characterization
Hydrogen temperature-programmed reduction (H2-TPR) and hydrogen chemisorption were carried out using a BELCAT-B automatic instrument equipped with a thermal conductivity detector (TCD) and a mass spectrometer detector. During the H2-TPR measurements, each calcined sample (ca. 0.10 g) was flushed with Ar at room temperature while held in a quartz tubular reactor, after which a 5% H2/Ar mixture was introduced at a flow rate of 30 mL/min. The temperature was increased at a rate of 10 K/min from ambient to 673 K and the sample was held at the final temperature for 3 h. In hydrogen chemisorption trials, prior to the adsorption of H2, the catalysts were treated in a 5% H2/Ar flow at 673 K for 8 h, followed by Ar purging during cool down. Hydrogen chemisorption was carried out at 373 K. The Co metal surface area and hydrogen consumption were calculated according to a method reported in the literature [24,25,26]. In this work, catalysts loaded with both Co and a noble metal (Ru or Pt) were characterized and so it was difficult to differentiate the hydrogen consumption resulting from Co compared to that resulting from Ru or Pt. However, the amounts of noble metals added were much smaller than the quantity of Co present (a ratio of approximately 1:40 by weight), and so the contributions of the noble metals to the total chemisorption amount can be regarded as minimal. The dispersion values in this paper were calculated by subtracting the chemisorption of the noble metals assuming full reduction.
3.3. Syngas Preparation
The syngas used during FT synthesis trials in this research was produced and stored using a bench-scale BTL plant installed at the Biomass Refinery Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Japan. In this apparatus, syngas produced from a gasifier was purified by passing through a scrubber and desulfization towers [27]. The final compressed gas composition employed for FT synthesis was (in vol%): H2 = 59.9, CO = 29.8, CH4 = 5.0, N2 = 4.9, CO2 = 0.4.
3.4. FT Reaction
FT synthesis was carried out using a continuous stirred tank reactor system equipped with a 0.13 L volume reactor. Each catalyst (100–200 mesh) was set in the reactor and reduced in situ under an atmospheric pressure H2 flow at 673 K for 8 h before reaction. After cooling down the reactor, 50 mL of n-hexadecane was added into the reactor as a solvent by pumping through a valve without introducing any of the external atmosphere. The FT synthesis reaction was typically carried out at 503 K and 1.0 MPa for 8 h, during which time the syngas feed rate was controlled at a W/F ratio of 4.0 g·h/mol. During the reaction, the gaseous products were continuously analyzed by two on-line gas chromatographs (GCs) in order to calculate CO conversion. Inorganic gases (N2, H2, CO and CO2) were analyzed using a Shimadzu GC-2014 equipped with a TCD and a Porapak-Q column. Light hydrocarbons (C1–4) were analyzed with a Shimadzu GC-2014 equipped with a flame ionization detector (FID) and an RT-QPLOT capillary column. No obvious deactivation was observed over the course of an 8 h reaction with almost all of the catalysts. After each reaction, the gasoline-range hydrocarbons (C5–10) collected in the trap and the heavy hydrocarbons (C11+) dissolved in the solvent were analyzed and quantified using an Agilent 6890 N GC equipped with an FID and a UA-DX30 capillary column.
4. Conclusions
The effects of small amounts of noble metal addition on the performance of a Mn-Zr-Co/SiO2 catalyst were investigated. Simultaneously impregnation of Ru with Co gave much higher reactivity than simple sequential addition and this effect was much more remarkable when adding Ru compared to Pt. In H2-TPR measurements, reduction peaks were shifted to lower temperatures and the extent of reduction of Co was shown to be significantly enhanced by the Ru addition. The combination of higher dispersion of Co metal obtained via the coimpregnation of Co and Mn and higher reducibility due to the presence of Ru generated the highest activity in the case of the Ru+Co+Mn/Zr/SiO2 among the Ru-modified catalysts.
The effect of Ru concentration on the Ru+Co+Mn/Zr/SiO2 catalyst showing the highest FT synthesis activity was also examined. Ru addition clearly enhanced the Co reducibility and FT synthesis activity. However, more than 0.3 wt.% Ru gave no further enhancement of the reducibility. A moderate amount of Ru addition enhances the catalytic activity mainly due to the increasing reducibility, and as little as 0.1 wt.% Ru generated an obvious enhancement of catalytic activity and reducibility.
The Ru+Co+Mn/Zr/SiO2 catalyst exhibited superior catalytic properties under various FT synthesis conditions. These results demonstrate that the addition of a small amount of Ru using coimpregnation is an effective means of improving the performance of Mn-Zr-Co/SiO2 catalysts.
Author Contributions
T.M. carried out the catalyst preparation and hydrocracking behavior studies and drafted the manuscript. T.H., K.S. and S.H. participated in the design of the study and assisted in drafting the manuscript. All authors read and approved the final manuscript.
Conflicts of Interest
The authors declare no conflict of interest.
References
- Souza-Aguiar, E.F.; Noronhac, F.B.; Faro, A., Jr. The main catalytic challenges in GTL (gas-to-liquids) processes. Catal. Sci. Technol. 2011, 1, 698–713. [Google Scholar] [CrossRef]
- Luque, R.; de la Osa, A.R.; Campelo, J.M.; Romero, A.A.; Valverde, J.L.; Sanchez, P. Design and development of catalysts for Biomass-to-Liquid-Fischer-Tropsch (BTL-FT) processes for biofuels production. Stud. Surf. Sci. Catal. 2012, 5, 5186–5202. [Google Scholar]
- Schulz, H. Short history and present trends of Fischer-Tropsch synthesis. Appl. Catal. A 1999, 186, 3–12. [Google Scholar] [CrossRef]
- Reuel, R.C.; Bartholomew, C.H. Effects of support and dispersion on the CO hydrogenation activity/selectivity properties of cobalt. J. Catal. 1984, 85, 78–88. [Google Scholar] [CrossRef]
- Schulz, H.; van Steen, E.; Claeys, M. Selectivity and mechanism of Fischer-Tropsch synthesis with iron and cobalt catalysts. Stud. Surf. Sci. Catal. 1994, 81, 455–460. [Google Scholar]
- 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, 212–224. [Google Scholar] [CrossRef]
- Iglesia, E. Design, synthesis, and use of cobalt-based Fischer-Tropsch synthesis catalysts. Appl. Catal. A 1997, 161, 59–78. [Google Scholar] [CrossRef]
- Khodakov, A.Y.; Chu, W.; Fongarland, P. Advances in the Development of Novel Cobalt Fischer-Tropsch Catalysts for Synthesis of Long-Chain Hydrocarbons and Clean Fuels. Chem. Rev. 2007, 107, 1692–1744. [Google Scholar] [CrossRef] [PubMed]
- All, S.; Chen, B.; Goodwin, J.G. Zr Promotion of Co/SiO2 for Fischer-Tropsch Synthesis. J. Catal. 1995, 157, 35–41. [Google Scholar] [CrossRef]
- Bae, J.W.; Park, S.J.; Woo, M.H.; Cheon, J.Y.; Ha, K.S.; Jun, K.W.; Lee, D.H.; Jung, H.M. Enhanced Catalytic Performance by Zirconium Phosphate-Modified SiO2-Supported Ru-Co Catalyst for Fischer-Tropsch Synthesis. ChemCatChem 2011, 3, 1342–1347. [Google Scholar] [CrossRef]
- Moradi, G.R.; Basir, M.M.; Taeb, A.; Kiennemann, A. Promotion of Co/SiO2 Fischer-Tropsch catalysts with zirconium. Catal. Commun. 2003, 4, 27–32. [Google Scholar] [CrossRef]
- Zhang, J.; Chen, J.; Ren, J.; Li, Y.; Sun, Y. Support effect of Co/Al2O3 catalysts for Fischer-Tropsch synthesis. Fuel 2003, 82, 581–586. [Google Scholar] [CrossRef]
- Xiong, H.; Zhang, Y.; Wang, S.; Li, J. Fischer-Tropsch synthesis: The effect of Al2O3 porosity on the performance of Co/Al2O3 catalyst. Catal. Commun. 2005, 6, 512–516. [Google Scholar] [CrossRef]
- Morales, F.; de Smit, E.; de Groot, F.M.F.; Visser, T.; Weckhuysen, B.M. Effects of manganese oxide promoter on the CO and H2 adsorption properties of titania-supported cobalt Fischer-Tropsch catalysts. J. Catal. 2007, 246, 91–99. [Google Scholar] [CrossRef]
- Li, J.; Coville, N.J. The effect of boron on the catalyst reducibility and activity of Co/TiO2 Fischer-Tropsch catalysts. Appl. Catal. A 1999, 181, 201–208. [Google Scholar] [CrossRef]
- Liu, Y.; Hanaoka, T.; Miyazawa, T.; Murata, K.; Okabe, K.; Sakanishi, K. Fischer-Tropsch synthesis in slurry-phase reactors over Mn- and Zr-modified Co/SiO2 catalysts. Fuel Process. Technol. 2009, 90, 901–908. [Google Scholar] [CrossRef]
- Miyazawa, T.; Hanaoka, T.; Shimura, K.; Hirata, S. Mn and Zr modified Co/SiO2 catalysts development in slurry-phase Fischer-Tropsch synthesis. Appl. Catal. A 2013, 467, 47–54. [Google Scholar] [CrossRef]
- Kogelbauer, A.; Goodwin, J.G., Jr.; Oukaci, R. Ruthenium Promotion of Co/Al2O3 Fischer-Tropsch Catalysts. J. Catal. 1996, 160, 125–133. [Google Scholar] [CrossRef]
- Tsubaki, N.; Sun, S.; Fujimoto, K. Different Functions of the Noble Metals Added to Cobalt Catalysts for Fischer-Tropsch Synthesis. J. Catal. 2001, 199, 236–246. [Google Scholar] [CrossRef]
- Jacobs, G.; Das, T.K.; Zhang, Y.; Li, J.; Racoillet, G.; Davis, B.H. Fischer-Tropsch synthesis: Support, loading, and promoter effects on the reducibility of cobalt catalysts. Appl. Catal. A 2002, 233, 263–281. [Google Scholar] [CrossRef]
- Song, S.H.; Lee, S.B.; Bae, J.W.; Sai Prasad, P.S.; Jun, K.W. Influence of Ru segregation on the activity of Ru-Co/γ-Al2O3 during FT synthesis: A comparison with that of Ru-Co/SiO2 catalysts. Catal. Commun. 2008, 9, 2282–2286. [Google Scholar] [CrossRef]
- Ma, W.; Jacobs, G.; Keogh, R.A.; Bukur, D.B.; Davis, B.H. Fischer-Tropsch synthesis: Effect of Pd, Pt, Re, and Ru noble metal promoters on the activity and selectivity of a 25%Co/Al2O3 catalyst. Appl. Catal. A 2012, 437–438, 1–9. [Google Scholar] [CrossRef]
- Pirola, C.; Scavini, M.; Galli, F.; Vitali, S.; Comazzi, A.; Manenti, F.; Ghigna, P. Fischer-Tropsch synthesis: EXAFS study of Ru and Pt bimetallic Co based catalysts. Fuel 2014, 132, 62–70. [Google Scholar] [CrossRef]
- Khodakov, A.Y.; Griboval-Constant, A.; Bechara, R.; Zholobenko, V.L. Pore Size Effects in Fischer Tropsch Synthesis over Cobalt-Supported Mesoporous Silicas. J. Catal. 2002, 206, 230–241. [Google Scholar] [CrossRef]
- Zhang, Y.; Nagamori, S.; Hinchiranan, S.; Vitidsant, T.; Tsubaki, N. Promotional Effects of Al2O3 Addition to Co/SiO2 Catalysts for Fischer-Tropsch Synthesis. Energy Fuels 2006, 20, 417–421. [Google Scholar] [CrossRef]
- Koizumi, N.; Suzuki, S.; Niiyama, S.; Shindo, T.; Yamada, M. Preparation of Co/SiO2 Using Several Glycols for Enhanced Fischer-Tropsch Synthesis Activity and Dispersion of Co0 Nanoparticles with Unique Co0 Particle Size Effect. Catal. Lett. 2011, 141, 931–938. [Google Scholar] [CrossRef]
- Hanaoka, T.; Miyazawa, T.; Nurunnabi, M.; Hirata, S.; Sakanishi, K. Liquid Fuel Production from Woody Biomass via Oxygen-enriched Air/CO2 Gasification on a Bench Scale. J. Jpn. Inst. Energy 2011, 90, 1072–1080. [Google Scholar] [CrossRef]
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Miyazawa, T.; Hanaoka, T.; Shimura, K.; Hirata, S. Ruthenium Modification on Mn and Zr-Modified Co/SiO2 Catalysts for Slurry-Phase Fischer-Tropsch Synthesis. Catalysts 2015, 5, 26-37. https://doi.org/10.3390/catal5010026
AMA Style
Miyazawa T, Hanaoka T, Shimura K, Hirata S. Ruthenium Modification on Mn and Zr-Modified Co/SiO2 Catalysts for Slurry-Phase Fischer-Tropsch Synthesis. Catalysts. 2015; 5(1):26-37. https://doi.org/10.3390/catal5010026
Chicago/Turabian StyleMiyazawa, Tomohisa, Toshiaki Hanaoka, Katsuya Shimura, and Satoshi Hirata. 2015. "Ruthenium Modification on Mn and Zr-Modified Co/SiO2 Catalysts for Slurry-Phase Fischer-Tropsch Synthesis" Catalysts 5, no. 1: 26-37. https://doi.org/10.3390/catal5010026
