Mingdeng
Wei
*a,
Kiyomi
Okabe
a,
Hironori
Arakawa
a and
Yasutake
Teraoka
b
aNational Institute of Advanced Industrial Science and Technology, Tsukuba Central #5, Ibaraki 305-8565, Japan. E-mail: wei-mingdeng@aist.go.jp; Fax: +81 298 61 4487
bDepartment of Applied Chemistry, Faculty of Engineering, Nagasaki University, Nagasaki, 852-8521, Japan
First published on 9th January 2002
Mesoporous alumino-silicate (MPAS) was prepared at ambient temperature by utilizing hexadecylpyridinium chloride as a surfactant, and used as the support of cobalt catalysts for Fischer–Tropsch synthesis in a slurry phase. The results of XRD and N2 adsorption measurements indicate that cations such as Co2+ and Ir4+ were effectively loaded on the surface of the mesopore walls, retaining mesoporous characteristics. The Co–Ir catalyst supported on MPAS showed excellent thermal stability, higher selectivity for C10–20 hydrocarbons and lower selectivity for CH4. These properties are affected by Al incorporated into the silica framework.
XRD patterns of mesoporous alumino-silicates (MPAS), as well as mesoporous silicate (MPS) synthesized by the same method,7 are depicted in Fig. 1, curves a and b. They show three peaks in the region 2θ=
2–8°, corresponding to the (100), (110) and (200) reflections of a typical hexagonal lattice, indicating that MPAS and MPS are structurally analogous to MCM-41.8 It is important to mention that the position of the main peak varies significantly depending on the synthesis conditions, and on the type of Si and Al sources. When aluminum is incorporated into the framework, the spacing of the (100) reflection shifts from the 3.2 nm found in purely siliceous MPS to the 3.3 nm of MPAS with a Si/Al ratio of 29. The interplanar spacing of the MPAS material is higher than that of the pure silicate MPS material, and increases with the amount of aluminum incorporated. This is consistent with the incorporation of increasing amounts of tetrahedral Al into the MPS framework, due in part to the longer Al–O bond length (1.75 Å) compared to the Si–O bond (1.60 Å). On the other hand, with the increase in aluminum content, the (100) diffraction peak becomes broader and less intense. The effect of the incorporation of aluminum into the framework is clearly indicated by the changes in the thickness of the pore walls (calculated by subtracting the pore diameter from the lattice parameter). N2 adsorption–desorption isotherms of MPAS and MPS exhibit a isotherm of type IV with a sharp step at the relative pressure 0.2
<
P/P0
<
0.3, which is due to capillary condensation inside the mesopores. The transmission electron micrographs (TEM) of MPAS and MPS show that the (100) direction still retains a regular hexagonal array of uniform channels, characteristic of mesoporous MCM-41 (see Fig. 2).
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Fig. 1 Powder XRD patterns of (a) MPS, (b) MPAS with a Si/Al ratio of 29, (c) 1 wt% Ir–5 wt% Co/MPAS, (d) 1 wt% Ir–10 wt% Co/MPAS, (e) 1 wt% Ir–15 wt% Co/MPAS, (f) used 1 wt% Ir–10 wt% Co/MPAS catalyst. |
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Fig. 2 TEM images of (a) MPAS with a Si/Al ratio of 19 and (b) MPS. |
A series of Co–Ir catalysts supported on MPAS with a Si/Al ratio of 29 (1 wt% Ir–x wt% Co/MPAS) was prepared by the impregnation or evaporation-to-dryness methods, in which x is 5, 10 or 15. Their crystal structure was checked by powder X-ray diffraction (XRD) using Cu-Kα irradiation, as shown in Fig. 1, curves c–e. The (100) peak was observed for all samples, though the intensity decreased remarkably with Co content, indicating a considerable lowering in structural regularity. The results of N2 adsorption measurements showed that all of these catalysts have a well-defined mesoporous distribution, although their peak height decreased slightly. The 1 wt% Ir–10 wt% Co/MPAS catalyst employed was calcined at 400°C for 5 h in order to remove residual wax, and then was checked by XRD, as shown in Fig. 1, curve f. The intensity of the (100) peak was almost unchanged, indicating that the mesoporous structure can be retained after F–T synthesis. Table 1 lists the mesopore parameters of the samples mentioned above. The interplanar spacing (d100) was almost unchanged after cation loading, but the surface area decreased from 1202 to 678 m2 g−1 upon loading up to 15 wt% Co. The pore size gradually decreased with Co content, resulting in an obvious increment of pore wall thickness. In view of the results mentioned above, we suggest that the metal ions, Co2+ and Ir4+, are effectively loaded on the surface of the mesopore walls.
Samplea | d 100/nm | Unit cell parameterb/nm | S BET/m2 g−1 | Pore volume/cm3 g−1 | Pore diameter/nm | Wall thickness/nm |
---|---|---|---|---|---|---|
a
Si/Al![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
||||||
MPAS | 3.4 | 3.9 | 1202 | 1.10 | 3.66 | 0.24 |
1 wt% Ir–5 wt% Co/MPAS | 3.4 | 3.9 | 825 | 0.76 | 3.66 | 0.24 |
1 wt% Ir–10 wt% Co/MPAS | 3.4 | 3.9 | 778 | 0.65 | 3.32 | 0.52 |
1 wt% Ir–15 wt% Co/MPAS | 3.3 | 3.8 | 678 | 0.52 | 3.08 | 0.72 |
The F–T synthesis was carried out in a slurry phase over Co–Ir catalysts supported on MPAS, and the reaction results are shown in Fig. 3. It indicates that CO conversion increases drastically when noble metal Ir is added to the Co catalysts, due to the increased reducibility of surface Co on the catalysts.9,10 On the other hand, the catalytic activity over Ir–Co/MPAS catalyst was more stable than that over Ir–Co/MPS catalyst, and insignificant deactivation was observed, even after a reaction time of more than 44 h.
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Fig. 3 Time course of the Fisch–Tropsch synthesis in slurry phase. (O) 20 wt% Co/MPS, (□) 0.5 wt% Ir–20 wt% Co/MPS, (△) 0.5 wt% Ir–20 wt% Co/MPAS (Si/Al![]() ![]() ![]() ![]() ![]() ![]() |
The effects of Al in the MPAS framework are shown in Fig. 4. It is obvious that the catalytic activity is enhanced by Al, resulting in increased CO conversion as well as chain growth probability α, while CH4 selectivity was restrained. The influence of Al on the F–T product distribution is shown in Table 2. The selectivity for C10–20 hydrocarbons increased by almost twofold, even when only a small amount of Al was incorporated into the framework. When the Si/Al ratio was 19, the selectivity for C10–20 hydrocarbons was 38.5%, while CH4 selectivity was only 8.2%.
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Fig. 4 Effect of Al in the silica framework on the product distribution in F–T synthesis: (♦) CO conversion, (●) CH4 selectivity, (△) chain growth probability α. |
Selectivity for | ||||||
---|---|---|---|---|---|---|
Catalyst (Si/Al ratio) | CO conv. | CH4 | C2–4 | C5–9 | C10–20 | α a |
a The chain growth probability α was calculated from a Schultz–Flory distribution. | ||||||
20 wt% Co/MPS | 16.5 | 18.5 | 14.4 | 22.0 | 12.2 | 0.76 |
0.5 wt% Ir–20 wt% Co/MPS | 50.8 | 14.2 | 25.2 | 27.7 | 13.0 | 0.76 |
20 wt% Co/MPAS (19) | 49.2 | 8.0 | 10.4 | 16.9 | 30.6 | 0.88 |
0.5 wt% Ir–20 wt% Co/MPAS (49) | 32.7 | 12.7 | 10.7 | 17.0 | 26.0 | 0.84 |
1 wt% Ir–20 wt% Co/MPAS (49) | 29.9 | 15.9 | 14.5 | 21.1 | 23.7 | 0.81 |
1 wt% Ir–20 wt% Co/MPAS (39) | 39.1 | 12.2 | 11.8 | 18.9 | 29.3 | 0.85 |
1 wt% Ir–20 wt% Co/MPAS (29) | 44.7 | 11.2 | 10.4 | 17.2 | 31.1 | 0.88 |
1 wt% Ir–20 wt% Co/MPAS (19) | 56.6 | 8.2 | 9.2 | 16.4 | 38.5 | 0.91 |
In order to elucidate the effect of the Al state in the catalysts on F–T synthesis, four types of Al-containing supports were used: (A) MPAS (Si/Al=
29), (B) MPS impregnated with Al (Si/Al
=
29), (C) SiO2 impregnated with Al (Si/Al
=
29), and (D) commercial SiO2–Al2O3
(Si : Al
=
1 : 9). The synthesis results are summarized in Table 3. Catalyst A showed higher CO conversion, C10–20 selectivity and chain growth probability α, and lower CH4 selectivity than the others. In addition, the reaction was more stable over catalyst A for a long time, while serious deactivation occurred over catalysts B, C and D. The difference in the state of Al between catalysts A and B/C is that Al in the former is incorporated into the silica framework, forming a tetrahedral coordination with oxygen, while in the latter two it is in the form of Al2O3. As for catalyst D, there might be too much Al in the support SiO2–Al2O3, which caused deactivation as well as lower selectivity for C10–20 hydrocarbons.
Selectivity for | ||||||
---|---|---|---|---|---|---|
Catalyst (Si/Al ratio) | CO conv. | CH4 | C2–4 | C5–9 | C10–20 | α a |
a The chain growth probability α was calculated from a Schultz–Flory distribution. | ||||||
A: 1 wt% Ir–15 wt% Co/MPAS (29) | 45.4 | 10.8 | 13.5 | 21.9 | 29.9 | 0.85 |
B: 1 wt% Ir–15 wt% Co–1.3 wt% Al/MPS (29) | 39.1 | 13.9 | 20.7 | 28.0 | 20.8 | 0.78 |
C: 1 wt% Ir–5 wt% Co–1.2 wt% Al/SiO2 (29) | 40.0 | 11.7 | 13.2 | 24.2 | 26.6 | 0.81 |
D: 1 wt% Ir–20 wt% Co/SiO2–Al2O3 (1/9) | 39.1 | 17.4 | 18.0 | 29.9 | 21.3 | 0.76 |
In summary, mesoporous alumino-silicate (MPAS) as a catalyst support has been used for F–T synthesis in the slurry phase. The characterization results show that the cations Co2+ and Ir4+ are effectively loaded on the surface of the mesopore walls and that the materials still retain a mesoporous structure. Compared with mesoporous silicate (MPS) and SiO2 supports, the thermal stabilization of MPAS was obviously enhanced; this might be attributed to Al incorporated into the silica framework. F–T synthesis over Co–Ir catalysts supported on MPAS showed that CO conversion and selectivity for C10–20 hydrocarbon products increased and selectivity for CH4 was effectively restrained.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2002 |