450
For example, the temperature-programmed studies (TPR) shown in Fig. 15a
indicate that reduction temperature of cobalt is ~800 °C, forming very stable cobalt
silicates and this is consistent with the results from XPS studies [45].
The studies with MCM-41 support resulted in very low selectivity toward higher
hydrocarbons despite showing the highest stability over a longer time on stream
(TOS). This prompted us to investigate different types of mesoporous silica supports to gain an in-depth understanding of the influence of the structure of mesoporous silica material and its role in the reaction kinetics of FT synthesis.
The kinetic studies for FT synthesis were conducted with three different types of
mesoporous silica (MCM-41, SBA-15, and KIT-6) supports having 10% Co and 5%
Ru in the support framework [24]. A one-pot hydrothermal process was used to
synthesize CoRu-MCM-41, CoRu-KIT-6, and CoRu-SBA-15, in order to study the
effect of these long-range ordered mesoporous catalysts on the kinetics of FT reaction. The catalysts were coated in the microchannels of microreactor with 4% PVA
catalyst slurry and the kinetic performances were evaluated at three different temperatures based on our previous studies with MCM-41-supported catalysts. The
reactions were performed at 210, 240, and 270 °C at different weight hourly space
velocities (WHSV = 2.15–25.2 kgcat h/kmol) with 2:1 (H 2 :CO) constant syngas
ratio. The theoretical kinetic models were proposed based on the LangmuirHinshelwood and the Eley-Rideal mechanisms. Table 8 shows the proposed six FT
mechanisms, FT 1–6 with elementary reactions, and kinetic eqs. FT-1, FT-2, FT-4,
and FT-5 are based on the Eley-Rideal-type mechanism and FT-3 and FT-6 are
based on the Langmuir-Hinshelwood mechanism.
All the six FT mechanistic models were fitted against the experimental data to
get the best-fitted model for each catalyst. The type of silica support has a significant effect on the kinetic studies. FT-3 was the best kinetic model to fit the data for
the CoRu-MCM-41 catalyst, whereas, for CoRu-SBA-15 and CoRu-KIT-6, FT-2
and FT-6 were found to be statistically relevant to the experimental kinetic data.
Deactivation studies show that the CoRu-KIT-6 is more stable and active than that
of the other two catalysts in terms of CO conversion and activation energy, respectively. The type of mesoporous silica material clearly shows the difference in the
kinetic performance of the catalyst which is consistent with the differences shown
in extensive characterization of the materials. For example, the TPR studies indicate
that the metals Co and Ru on these three supports have different hydrogen consumption temperatures to get active metal into the support framework. Figure 15b shows
the TPR profiles of CoRu-MCM-41, CoRu-KIT-6, and CORu-SBA-15. The reduction temperature of cobalt in MCM-41 is around 780 °C, whereas, in Co- KIT-6 and
Co-SBA-15, the total hydrogen consumption is observed at much lower
temperature.
Sun et al. studied the kinetics of FT synthesis in a reactor having channels of
1 × 1 × 40 mm on 8 × 8 cm stainless steel metal plate with Co-Ni bimetallic catalyst
on SiO 2 support. They investigated FT kinetics over a wide range of operating conditions: T = 280–320 °C, 10–50 bar pressure varying syngas (H 2 :CO) ratio from 1
to 3 [49]. The comprehensive theoretical kinetic model based on the LangmuirHinshelwood- Hougen-Watson carbide mechanism was derived and compared with
N. Mohammad et al.
For example, the temperature-programmed studies (TPR) shown in Fig. 15a
indicate that reduction temperature of cobalt is ~800 °C, forming very stable cobalt
silicates and this is consistent with the results from XPS studies [45].
The studies with MCM-41 support resulted in very low selectivity toward higher
hydrocarbons despite showing the highest stability over a longer time on stream
(TOS). This prompted us to investigate different types of mesoporous silica supports to gain an in-depth understanding of the influence of the structure of mesoporous silica material and its role in the reaction kinetics of FT synthesis.
The kinetic studies for FT synthesis were conducted with three different types of
mesoporous silica (MCM-41, SBA-15, and KIT-6) supports having 10% Co and 5%
Ru in the support framework [24]. A one-pot hydrothermal process was used to
synthesize CoRu-MCM-41, CoRu-KIT-6, and CoRu-SBA-15, in order to study the
effect of these long-range ordered mesoporous catalysts on the kinetics of FT reaction. The catalysts were coated in the microchannels of microreactor with 4% PVA
catalyst slurry and the kinetic performances were evaluated at three different temperatures based on our previous studies with MCM-41-supported catalysts. The
reactions were performed at 210, 240, and 270 °C at different weight hourly space
velocities (WHSV = 2.15–25.2 kgcat h/kmol) with 2:1 (H 2 :CO) constant syngas
ratio. The theoretical kinetic models were proposed based on the LangmuirHinshelwood and the Eley-Rideal mechanisms. Table 8 shows the proposed six FT
mechanisms, FT 1–6 with elementary reactions, and kinetic eqs. FT-1, FT-2, FT-4,
and FT-5 are based on the Eley-Rideal-type mechanism and FT-3 and FT-6 are
based on the Langmuir-Hinshelwood mechanism.
All the six FT mechanistic models were fitted against the experimental data to
get the best-fitted model for each catalyst. The type of silica support has a significant effect on the kinetic studies. FT-3 was the best kinetic model to fit the data for
the CoRu-MCM-41 catalyst, whereas, for CoRu-SBA-15 and CoRu-KIT-6, FT-2
and FT-6 were found to be statistically relevant to the experimental kinetic data.
Deactivation studies show that the CoRu-KIT-6 is more stable and active than that
of the other two catalysts in terms of CO conversion and activation energy, respectively. The type of mesoporous silica material clearly shows the difference in the
kinetic performance of the catalyst which is consistent with the differences shown
in extensive characterization of the materials. For example, the TPR studies indicate
that the metals Co and Ru on these three supports have different hydrogen consumption temperatures to get active metal into the support framework. Figure 15b shows
the TPR profiles of CoRu-MCM-41, CoRu-KIT-6, and CORu-SBA-15. The reduction temperature of cobalt in MCM-41 is around 780 °C, whereas, in Co- KIT-6 and
Co-SBA-15, the total hydrogen consumption is observed at much lower
temperature.
Sun et al. studied the kinetics of FT synthesis in a reactor having channels of
1 × 1 × 40 mm on 8 × 8 cm stainless steel metal plate with Co-Ni bimetallic catalyst
on SiO 2 support. They investigated FT kinetics over a wide range of operating conditions: T = 280–320 °C, 10–50 bar pressure varying syngas (H 2 :CO) ratio from 1
to 3 [49]. The comprehensive theoretical kinetic model based on the LangmuirHinshelwood- Hougen-Watson carbide mechanism was derived and compared with
N. Mohammad et al.