Kinetics of Fischer–Tropsch Synthesis in a 3-D Printed Stainless Steel Microreactor Using Different Mesoporous Silica Supported Co-Ru Catalysts

Abstract

Fischer–Tropsch (FT) synthesis was carried out in a 3D printed stainless steel (SS) microchannel microreactor using bimetallic Co-Ru catalysts on three different mesoporous silica supports. CoRu-MCM-41, CoRu-SBA-15, and CoRu-KIT-6 were synthesized using a one-pot hydrothermal method and characterized by Brunner–Emmett–Teller (BET), temperature programmed reduction (TPR), SEM-EDX, TEM, and X-ray photoelectron spectroscopy (XPS) techniques. The mesoporous catalysts show the long-range ordered structure as supported by BET and low-angle XRD studies. The TPR profiles of metal oxides with H₂ varied significantly depending on the support. These catalysts were coated inside the microchannels using polyvinyl alcohol and kinetic performance was evaluated at three different temperatures, in the low-temperature FT regime (210–270 °C), at different Weight Hourly Space Velocity (WHSV) in the range of 3.15–25.2 kgcat.h/kmol using a syngas ratio of H₂/CO = 2. The mesoporous supports have a significant effect on the FT kinetics and stability of the catalyst. The kinetic models (FT-3, FT-6), based on the Langmuir–Hinshelwood mechanism, were found to be statistically and physically relevant for FT synthesis using CoRu-MCM-41 and CoRu-KIT-6. The kinetic model equation (FT-2), derived using Eley–Rideal mechanism, is found to be relevant for CoRu-SBA-15 in the SS microchannel microreactor. CoRu-KIT-6 was found to be 2.5 times more active than Co-Ru-MCM-41 and slightly more active than CoRu-SBA-15, based on activation energy calculations. CoRu-KIT-6 was ~3 and ~1.5 times more stable than CoRu-SBA-15 and CoRu-MCM-41, respectively, based on CO conversion in the deactivation studies.

1. Introduction

Although Fischer–Tropsch (FT) synthesis was discovered by Franz Fischer and Hans Tropsch in the 1920s in Germany [1], it has gained immense attention in last few years due to depletion of non-renewable energy sources. FT synthesis is an environmental friendly route for alternative fuels and can produce liquid fuels from carbon sources by coal-to-liquid (CTL), natural gas-to-liquid (GTL) and biomass-to-liquid (BTL) [2] processes. Three types of reactors have been utilized commercially for FT synthesis: Fixed bed, fluidized bed, and slurry bubble column bed by leading GTL companies like Shell, Sasol, Exxon Mobil, and Energy Int. [3]. There is a minimum scale limit of this FT process to be economical; for example, the Pearl GTL, a collaboration between Shell and Qatar petroleum, producing 140,000 bpd (barrels per day) is considered as a profitable economic scale for the FT GTL process [4]. The limitation of the scale-up considerations to commercialize small scale plants to be more profitable has driven industries and researchers to pursue an interest in alternative technologies. Since FT synthesis is highly exothermic in nature, there is a need for much process intensification technologies. The microreactor platform, which contains microstructured units called microreactors, uses a large number of small, parallel channels with different channel designs. This technology provides an alternative platform for controlling highly exothermic reactions like FT synthesis with enhanced mass and heat transfer. It has gained much attention in process intensification of FT synthesis [5,6] as isothermal operating conditions are well maintained in a microreactor with good control over process parameters which favors quick screening of catalyst for different chemical reactions. The reaction zone for these microreactors are several parallel microchannels with small geometry. The specific surface area of the reaction zone is greatly enhanced by the design of microchannels resulting in an efficient FT synthesis. In addition to efficient heat and mass transfer with good heat dissipation, microreactors also have advantages such as high reaction throughput, easy scale-up, good portability, and lower cost over conventional reactors [6,7,8,9,10]. This has been demonstrated commercially and in R&D by Velocys and Micrometrics Corporations [11,12,13,14].

Iron, cobalt, and ruthenium catalysts have been extensively used for FT synthesis [15]. To increase the performance of catalysts, different supports have been used; some of the previous studies examined the role of Al₂O₃ [16,17,18,19,20], TiO₂ [21,22,23,24,25,26,27,28], SiO₂ [29,30,31,32,33], and CNTs [34,35,36] as supporting materials for the formation of higher alkanes. These supports tend to enhance the FT process by increasing the active number of catalytic sites and good metal dispersion with the high surface area. Therefore, the selection of support and study of its interaction with the incorporated metal ion plays an important role in catalysis. In our previous studies, sol-gel encapsulated catalysts were used in silicon microreactors for FT synthesis [37,38,39]. While Al₂O₃ and SiO₂ sol-gel supports show similar behavior in formation of higher alkanes such as ethane, propane, and butane for the reactions at 1 atm, TiO₂ has a profound effect on FT synthesis [40] and the stability of the catalysts are observed in reverse order from that observed with SiO₂ and Al₂O₃. However, in all these studies, sol-gel coated catalysts in silicon microreactors tend to have challenges such as low surface area, clogging of microchannels and difficulty in reducing the metal oxides to expose active sites. In addition, the Si-microreactors are fragile and they break easily and require a large infrastructure for fabrication. Further, it’s more difficult to increase pressure for FT studies using Si-microreactors. Thus, we have turned our attention to 3D printed stainless steel (SS) microreactors which are easy to fabricate by direct metal laser sintering layer-by-layer additive manufacturing technique. Recently, 3D printed microreactors have been used as flow devices in many chemical reactions such as fast difluoromethylation [41], a customizable Lab-On-Chip device for optimization of carvone semicarbazon [42], a micro fuel cell [43,44], and wide range of organic and inorganic reactions [45,46,47]. Further, these metal printed microreactors have been used for high pressure and temperature chemical reactions providing a new fast developing reactor technology in process development to industrial scale [47], which makes them suitable for reactions like FT synthesis due to its good mechanical and thermal properties. Although the specific surface area of stainless steel microreactor is less when compared to silicon microreactors used in our previous studies [40], the use of stainless steel material increases heat transfer, its chemical and mechanical resistances play a major role in process intensification of chemical processes. In order to increase specific surface area of the reaction zone in microreactors, we synthesized catalysts with surface area greater than 1000 m²/g using mesoporous MCM-41 support. The use of high surface area MCM-41 for FT catalysis stems from our previous studies, which can be prepared easily by one-pot hydrothermal procedure and are extremely stable, for steam reforming of methanol to produce hydrogen [48,49,50,51]. Bimetallic catalysts containing Co and one other metal—Fe, Ru, or Ni—were prepared to investigate the synergistic effect of bi-metallic species on the FT performance (manuscript submitted), The results show that CoRu-MCM-41 is more active than other bimetallic catalysts in producing longer-chain hydrocarbons at one atmosphere.

In this manuscript, we have focused on the kinetics of FT synthesis in a 3-D printed Stainless Steel(SS) microreactor using CoRu bimetallic catalysts supported by MCM-41, and two other mesoporous silica supports: SBA-15 and KIT-6. In order to translate new advancements in the laboratory as well as industry on both catalysis and microreactors for FT synthesis, chemical kinetics is a key issue in developing mathematical models for the reactors. However, to our knowledge, the kinetics of FT synthesis using mesoporous materials in a microreactor is relatively unknown in the literature. So, in order to understand more about the interaction between silica mesoporous materials and the metal, and especially kinetics, three different types of silica mesoporous materials (MCM-41, SBA-15, and KIT-6) containing cobalt and ruthenium metals were synthesized by one-pot hydrothermal method. To address the thermodynamic stability of the catalysts, CO-conversion using these three catalysts in the 3-D printed SS microchannel microreactor was also investigated.

2. Results and Discussion

2.1. Catalyst Characterization

2.1.1. Textural Evaluation of Catalysts

The textural properties of the catalysts were evaluated using nitrogen Brunner–Emmett–Teller (BET) physisorption analysis.

Table 1. Brunner–Emmett–Teller (BET) surface area, pore size, pore volume and EDX metal loadings of synthesized catalysts.

Mesoporous Silica Supported Catalyst with Intended Metal Loadings	Surface Area a (m2/g)	Pore Volume b (cm3/g)	Pore Size c (nm)	Metal Loadings Obtained from SEM-EDX (wt %)
10% Co5%Ru-MCM-41	1025	0.77	3.2	9%Co3.9%Ru-MCM-41
10% Co5%Ru-SBA-15	691	0.73	4.2	8.4%Co4.5%Ru-KIT-6
10%Co5%Ru-KIT-6	690	0.92	5.3	11.1%Co5.6%Ru-SBA-15

^a = Variation range ±2%, ^b = Variation range ±3%, ^c = Variation range ±5%.

shows the BET surface area, pore volume and the average pore diameter of all three catalysts. The surface areas of the catalysts are different depending upon the type of silica support. While the surface area of CoRu-MCM-41 was 1025 m²/g, that of CoRu-SBA-15 and CoRu-KIT-6 was around 691 m²/g and 690 m²/g, respectively. The general trend is consistent with that reported in the literature [48,52,53]. The pore diameter in the range of 3.2–5.3 nm was obtained from BJH desorption plot. The pore volume was in the range of 0.77–0.92 cm³/g. Figure 1a shows nitrogen adsorption–desorption isotherms for all the catalysts with pore size distribution of mesoporous materials. These isotherms represent the category of Type IV isotherms which is typical for mesoporous materials as mentioned in IUPAC classification [54]. These isotherms are classified into three different types of regions. The initial part of isotherm is a linear increment of nitrogen uptake at lower relative pressures (P/P₀ = 0–0.2) called Type II isotherm. This is due to the adsorption of N₂ on monolayer and multilayer within the pore walls of the catalyst. For relative pressure in the range of P/P₀ = 0.2–0.4, there is an exponential increment in the isotherms which indicates the ordered mesoporous structure of the catalysts. Especially, the steepness of CoRu-MCM-41 is sharp when compared to the other two samples which indicate that MCM-41 support is more ordered in the nature of all the catalysts. Finally, the third region, in the relative pressure range of P/P₀ = 0.4–0.95, has a long plateau for all the catalysts and it corresponds to the multilayer adsorption on the outer surface of the catalyst. The hysteresis loop for the samples is associated with condensation of N₂ uptake in the interstitial voids of mesopores of the support [55]. Figure 1b shows pore size distribution obtained from BJH desorption plots. A sharp single peak for pore size with narrow distribution is observed for all the catalysts covering uniformly the pores with sizes in the range of 3.2 to 5.3 nm as shown in Table 1. The pore sizes of KIT-6 support appear to be larger and wider when compared to that of MCM-41 and SBA-15 supports. Furthermore, the pore distribution of MCM-41 is bi-modal, having major pores distributed in the range of 2 to 3 nm and minimal pore distribution between 3–4 nm.

2.1.2. SEM-EDX Analysis

The metal loadings in the catalysts (wt%) and the surface morphology were obtained by SEM-EDX analysis. Figure S1 shows the SEM-EDX images of a typical MCM-41 catalyst showing uniform metal distribution with porous morphology. The actual and intended metal loadings are quite similar (Table 1) and suggest that the one-pot hydrothermal synthesis is one of the best routes to prepare mesoporous materials with uniform metal distribution. This uniformity plays a key role in the activity of FT catalysts; it not only decreases sintering but also increases the thermal stability of the catalysts for long-term studies.

2.1.3. Transmission Electron Microscopic (TEM) Imaging

The size of the metal particles and the structure of the mesoporous support in all catalysts were obtained from TEM studies. The high magnification images in Figure 2 show the uniform ordered hexagonal pores present in the support. It is also worth noting that MCM-41 support has well defined hexagonal pores when compared to KIT-6 and SBA-15 and this is consistent with the BET surface area and the low angle XRD studies (discussed below). A uniform metal distribution with black dots, as shown in Figure S2, having almost circular in shape is clearly evident in the mesoporous silica matrix.

2.1.4. Powder X-Ray Diffraction (XRD) Studies of Calcined Catalysts

In order to obtain information about the structural phases of the catalysts, XRD studies were carried out. Figure 3 shows the small angle XRD diffraction patterns for different mesoporous silica supported catalysts. The variations of peaks are probably due to the presence of metal nanoparticles present in the catalyst. For CoRu-MCM-41 catalyst, a sharp intense peak between 2-theta values 2–3° and two broad peaks between 2-theta values 4–5.5° corresponds to (100), (110), and (200) reflections of hexagonal mesoporous structure. This confirms that these catalysts are highly ordered mesoporous in nature with no deformation of hexagonal framework even after the addition of metals and this is consistent with the observed TEM images. For CoRu-SBA-15 catalyst, the peak between 2-theta value 1–2° indicates the mesoporous structure with 2D hexagonal symmetry with p6mm space group and long range ordered mesoporous structure [56]. For CoRu-KIT-6 catalyst, the peak at 2-theta value 0.94° corresponds to (211) plane and two low intensity peaks between 1.5–2° ascribes to (420) and (332) diffraction planes. These planes confirmed the characteristic three-dimensional nature of mesoporous KIT-6 reported in the literature [57].

The wide-angle XRD (WAXRD) analysis was carried out to determine the crystallinity of metal oxides in different mesoporous supports. Figure 4 shows the WAXRD patterns of these samples. The observed 2θ angles are compared with the JCPDS (Joint Committee on Powder Diffraction Standards) database. For all catalysts, the peaks at 18.90° (111), 31.09° (220), 36.74° (311), 38.36° (222), 44.72° (400), 59.25° (511), and 65.26° (440) correspond to the cubic structure of Co₃O₄ (JCPDS-42-1467) [58,59]. The orthorhombic structure of RuO₂ (JCPDS-88-0323) is consistent with the observed peaks at 28.18° (110), 35.27° (101), and 54.56° (211) in all the catalysts.

2.1.5. X-Ray Photoelectron Spectroscopy (XPS)

To determine the oxidation states of Co and Ru in MCM-41, KIT-6, and SBA-15, XPS studies were performed. Figure S3 shows the XPS spectra of Si 2p and O 1s containing a single spectrum which is centered at 104 eV and 532 eV, respectively, and confirms the presence of silicates in the sample. Figure 5a shows the Co 2p spectra for all the samples; the Co 2p_3/2, and Co 2p_1/2 peaks are clearly observed to indicate the presence of cobalt in two oxidation states in the silica matrix [60,61,62]. The peaks centered at 780.5 eV and 796.2 eV are associated with Co 2p_3/2 and Co 2p_1/2, respectively in the MCM-41 matrix. Whereas, in the case of KIT-6, the peaks for Co 2p_3/2 and Co 2p_1/2 are observed at 779.6 eV and 795.8 eV, respectively. For SBA-15, the similar peaks are noticed at 779.7 eV and 794.8 eV, respectively. It is clear from these data that the binding energy for cobalt in the MCM-41 matrix is distinctly higher when compared to that of cobalt in KIT-6 and SBA-15. This suggests that cobalt in two oxidation states in MCM-41 is in a different environment from that of SBA-15 and KIT-6. This is also consistent with temperature programmed reduction (TPR) profile showing much higher reduction temperatures for CoRu-MCM-41 catalyst as discussed below. Similar XPS spectra for Co 2p were observed and analyzed by Bhoware et al., [63]. Figure 5b shows the XPS spectra for the ruthenium metal in the catalyst. The presence of Ru in the sample is confirmed by the Ru 3d spectra which is centered almost at 284.8 eV and it is associated with the Ru 3d_3/2 oxidation state [64]. However, in contrast to cobalt, there is no significant difference in the binding energy of the Ru metal ions in different mesoporous silica supports.

2.1.6. H₂ Temperature Programmed Reduction (H₂ TPR)

Temperature programmed reduction (TPR) is an ideal technique to analyze the reduction behavior of metal oxides in mesoporous silica. It helps to investigate the interaction between metal and the support by providing information on physiochemical properties of the material. All the calcined catalysts are treated with 10%H₂ to record TPR profiles for Co and Ru metal oxides shown in Figure 6. All the samples contain well defined peaks for ruthenium at low reduction temperatures and cobalt at much higher reduction temperatures. The TPR profiles of all the catalysts show that the ruthenium oxide is reduced with H₂ at a relatively lower temperature between 100 °C and 250 °C with main hydrogen consumption peaks for Ru³⁺ to Ru⁰ which is also reported by Panpranot et al. [65]. However, the reduction behavior of Co₃O₄ (Co₃O₄ → CoO → Co⁰) [66] in three samples to Co⁰ is remarkably different depending on the type of support. For MCM-41, as reported by Lim et al., the small peak centered around 310 °C ascribes the reduction of cobalt to CoO, while the second main peak corresponds to the reduction of CoO to metal ions Co²⁺ into the silica network [67]. The last hydrogen uptake has a peak centered almost 780 °C which suggests that the cobalt and the MCM-41 support have strong interaction which is also confirmed by the binding energy obtained from XPS in Figure 5a [48,68]. This could be due to the formation of a spinel structure as cobalt silicates [69] and consistent with the XPS and XRD data. Unlike MCM-41, the reduction temperatures of Co inSBA-15 and KIT-6 were quite low around 365 °C and 375 °C, respectively, confirming weaker metal interactions with SBA-15 and KIT-6 supports [53]. However, no separate three peaks were observed for the reduction of cobalt, this may be due to the absence of silicates in SBA-15 and KIT-6 samples. The shift in the reduction peaks to the lower temperatures can also be due to the incorporation of Ru metal in the support [70]. Although the 5% weight of Ru is maintained in the catalyst sample, there might be a slight difference in the actual loadings of the Ru metal as shown in EDX

Table 2. Elementary reaction steps for Fischer–Tropsch synthesis.

Model	No	Elementary Reaction
FT-1	1	$CO+*\underset{k_{-1}}{\overset{k_1}{\rightleftharpoons }}CO*K_1=\frac{k_1}{k_{-1}}$
	2	$CO*+H_2\stackrel{k}{\to }C+D+*$
FT-2	1	$H_2+*\underset{k_{-1}}{\overset{k_1}{\rightleftharpoons }}{H}_2*K_1=\frac{k_1}{k_{-1}}$
	2	$H_2*+CO\stackrel{k}{\to }C+D+*$
FT-3	1	$CO+*\underset{k_{-1}}{\overset{k_1}{\rightleftharpoons }}CO*K_1=\frac{k_1}{k_{-1}}$
	2	$H_2+*\underset{k_{-2}}{\overset{k_2}{\rightleftharpoons }}{H}_2*K_2=\frac{k_2}{k_{-2}}$
	3	$CO*+H_2*\stackrel{k}{\to }C+D+2*$
FT-4	1	$CO+*\underset{k_{-1}}{\overset{k_1}{\rightleftharpoons }}CO*K_1=\frac{k_1}{k_{-1}}$
	2	$CO*+H_2\underset{k_{-2}}{\overset{k_2}{\rightleftharpoons }}CO{H}_2*K_2=\frac{k_2}{k_{-2}}$
	3	$CO{H}_2*\stackrel{k}{\to }C+D+*$
FT-5	1	$H_2+*\underset{k_{-1}}{\overset{k_1}{\rightleftharpoons }}{H}_2*K_1=\frac{k_1}{k_{-1}}$
	2	$H_2*+CO\underset{k_{-2}}{\overset{k_2}{\rightleftharpoons }}CO{H}_2*K_2=\frac{k_2}{k_{-2}}$
	3	$CO{H}_2*\stackrel{k}{\to }C+D+*$
FT-6	1	$CO+*\underset{k_{-1}}{\overset{k_1}{\rightleftharpoons }}CO*K_1=\frac{k_1}{k_{-1}}$
	2	$H_2+*\underset{k_{-2}}{\overset{k_2}{\rightleftharpoons }}{H}_2*K_2=\frac{k_2}{k_{-2}}$
	3	$CO*+H_2*\underset{k_{-3}}{\overset{k_3}{\rightleftharpoons }}CO{H}_2*+*K_3=\frac{k_3}{k_{-3}}$
	4	$CO{H}_2*\stackrel{k}{\to }C+D+*$

. Qin et al., have studied the effect of the Ru metal on Co-SBA-15 catalyst at different loading and found a remarkable effect on the activity of catalyst during the FT synthesis [70]. Thus, the overall interactions of metal–metal and metal–support have a strong influence on the reducibility and reactivity of the catalysts for FT synthesis. Since the operating temperature zone of the FT synthesis is less than 350 °C, the activity of the catalyst is more dependent on the ease of reducibility of the metal oxides to pure metals (active sites) in the support.

The amount of hydrogen consumed by CoRu-MCM-41, CoRu-SBA-15, and CoRu-KIT-6 catalysts was calculated and quantified to be 0.108, 0.05, and 0.032 mmol H₂/gm, respectively in the temperature range of 25 to 1000 °C. The amount of hydrogen consumed by CoRu-MCM-41 was found almost two times more than that of CoRu-SBA-15 and 3 times more than by CoRu-KIT-6. Higher hydrogen uptake by CoRu-MCM-41 is most likely due to the reduction of Co-silicates ~750 °C.

Although MCM-41 exhibits higher hydrogen consumption than other catalysts, the amount of hydrogen adsorbed by CoRu-KIT-6 in the temperature range of 25–311 °C is higher when compared to that by CoRu-MCM-41 and CoRu-SBA-15 in the same range of temperature.

2.2. FT Reaction Mechanism

In order to have a better understanding of the effect of metal and support interaction on catalysts, kinetic studies were carried out in the SS microchannel microreactors. The main difficulty to describe the FT kinetics is the complexity of its mechanism and a larger number of possible chemical species involved. Kinetic models of FT synthesis using Co based catalysts are less abundant than Fe based catalysts in literature [71]. Most of the existing models are mainly based on power-law models where Langmuir–Hinselwood (LH) type equations have been used by different researchers [72,73,74,75,76]. Although the simple power-law expression is widely recognized in the field of catalysis, it was recognized to have limited application in FT synthesis due to the narrow range of reaction conditions [77,78]. However, LH type equations are widely used for prediction of rates over a wide range of reaction conditions. As an example, Yates and Satterfield [72] worked on Co-catalyst and fitted the rate data obtained at 220–240 °C. They found that the rate data were best fitted with simple LH expression. Rautavuoma and van dar Baan [79] reported the rate of reaction at 1 atm pressure and 250 °C. They observed that reaction proceeds through CO dissociation and formation of -CH₂- surface intermediate.

2.2.1. Reaction Mechanism

In the microchannels of the microreactor, the flow of reactants is basically laminar. The complexity of the microchannel microreactor increases due to the parabolic type velocity profile; so, an average velocity profile is approximated during the development of the model for the microreactor system [80]. The outlet concentrations of the limiting reactant (CO), which was related to the rate of reaction, were calculated by an in-line GCMS. The following differential equation was used for a reactor model defined as Equation (1):

$$\frac{W_{cat}}{F_{in,CO}}={\displaystyle {\int }_{X_{CO,in}}^{X_{CO,out}}\frac{d{X}_{CO}}{-r_{CO}}}$$

(1)

• $W_{cat}$ = Wt. of the catalyst
• $F_{in,CO}$ = Molar feed rate of CO
• $X_{CO}$ = Conversion of CO
• $-r_{CO}$ = Disappearance rate of CO

Equation (2), below, is used to calculate the disappearance rate of CO

$$-r_{CO}=\frac{X_{CO}{F}_{in,CO}}{W_{cat}}$$

(2)

The following boundary conditions (BC) were used:

• W = 0; F_i =F_i(inlet)
• W = W_cat; F_i = F_i(exit)

The partial pressure of the compound was calculated using the following equations:

$$p_i=\frac{m_i}{{\displaystyle {\sum }_{i=1}^{N_c}{m}_i}}{P}_T$$

(3)

where p_i is the partial pressure of the component, P_T is the total pressure of the reactor at the inlet (1 atm) and N_c is the total number of components. m_i is the number of moles of component i.

2.2.2. Mechanism and Kinetics

In order to determine the most suitable kinetic model for a particular catalyst, all possible combinations of FT reactions were considered and rate equations were developed based on CO conversion. A number of Langmuir–Hinshelwood and Eley–Rideal models have been developed for kinetics of CO hydrogenation to hydrocarbons in different types of reactors over the past few years [80,81,82]. In this study, it was assumed that the FT reactions occur only at active sites and proposed six possible mechanisms for FT reactions to develop kinetic models in the microchannel microreactor as shown in Table 2. In order to derive an appropriate model that describes a suitable FT equation, we considered six cases with different elementary reaction steps for each case as shown in Table 2.

In FT-1, CO is adsorbed on active site (*) of catalyst to form a CO* intermediate. Then, CO* reacts with H₂ to give the products C (hydrocarbons) and D (H₂O). Similarly, H₂ can be adsorbed on the catalyst site (*) to form H₂* intermediate and this intermediate subsequently reacted with CO to yield products in the FT-2 mechanism. There are two steps of adsorption in the FT-3 mechanism. In 1^st and 2^nd steps, CO and H₂ both are adsorbed on catalyst active site (*) to form two intermediates (CO* and H₂*). In the last step (surface reaction), these two intermediates react with each other to give products C and D. The FT-4 model consisted of three different steps. In the 1^st step (adsorption), CO is adsorbed on catalyst site (*) to form CO*. In 2^nd step (surface reaction), the intermediate (CO*) reacts with H₂ to form another intermediate (COH₂*). In the last step (desorption), the final intermediate gave products (C and D). Like FT-4, FT-5 also consists of three different steps—adsorption, surface reaction, and desorption. In the 1^st step (adsorption), H₂ is adsorbed on catalyst site (*) to form the H₂* intermediate. This intermediate reacts with CO to form another intermediate (COH₂*) in the 2^nd step (surface reaction). In 3^rd step, the products (C and D) are formed from the intermediate (COH₂*). In contrast to other models, FT-6 consists of four different steps. The 1^st and 2^nd steps are like that of the FT-3 mechanism. The 3^rd step is the surface reaction where two intermediates (CO* and H₂*) react with each other to give another intermediate COH₂* and released one active site (*). In last step (desorption), the intermediate (COH₂*) yields products (C and D).

Using the six models described above, six rate equations can be deduced for FT reactions by considering surface reaction and rate-limiting desorption as shown in

Table 3. Proposed kinetic equations for Fischer–Tropsch synthesis.

Model	Rate Controlling Step (RCS)	Kinetic Equation
FT-1	(2)	$-r_{CO}=\frac{k{K}_1{p}_{CO}{p}_{H_2}}{(1+K_1{p}_{CO})}$
FT-2	(2)	$-r_{CO}=\frac{k{K}_1{p}_{CO}{p}_{H_2}}{(1+K_1{p}_{H_2})}$
FT-3	(3)	$-r_{CO}=\frac{k{K}_1{K}_2{p}_{CO}{p}_{H_2}}{{(1+K_1{p}_{CO}+K_2{p}_{H_2})}^2}$
FT-4	(3)	$-r_{CO}=\frac{k{K}_1{K}_2{p}_{CO}{p}_{H_2}}{(1+K_1{K}_2{p}_{CO}{p}_{H_2}+K_1{p}_{CO})}$
FT-5	(3)	$-r_{CO}=\frac{k{K}_1{K}_2{p}_{CO}{p}_{H_2}}{(1+K_1{K}_2{p}_{CO}{p}_{H_2}+K_1{p}_{H_2})}$
FT-6	(4)	$-r_{CO}=\frac{k{K}_1{K}_2{K}_3{p}_{CO}{p}_{H_2}}{(1+K_1{K}_2{K}_3{p}_{CO}{p}_{H_2}+K_1{p}_{CO}+K_2{p}_{H_2})}$

. (See Appendix A for the rate equation derived using the FT-3 kinetic model).

All the models presented in Table 3 were verified against experimental data to obtain the best suitable mechanism with the best fit. The models FT-1, FT-2, FT-4, and FT-5 are based on Eley–Rideal-type mechanism. In this case, one reactant gets adsorbed and another reactant reacts directly from the gas phase to form intermediates that yield products. Other models FT-3 and FT-6 are based on the Langmuir–Hinshelwood mechanism, which means all reactants are adsorbed on the catalyst surface before the products are formed. The kinetic parameter (k) and equilibrium constants (K₁, K₂, K₃) at each temperature were evaluated by non-linear regression analysis based on Levenberg–Marquart algorithm in POLYMATH software by minimizing the sum of squared residuals of reaction rates [83,84]. The objective function is defined as:

$$F={\displaystyle \sum _{i=1}^N{(r_{ca{l}_i}-r_{{\exp }_i})}^2}$$

(4)

where, $N$ is the number of total observations, $r_{ca{l}_i}$ and $r_{{\exp }_i}$ are calculated from the model equation and experimental rates at the different reaction temperatures.

The rate constants and the equilibrium constants can be related to The Arrhenius equation and van’t Hoff laws as shown below:

$$k_i(T)=A\exp (-\frac{E_{ai}}{RT})$$

(5)

$$K_i(T)=K\exp (-\frac{\Delta H_i}{RT})$$

(6)

where k_i and K_i are reaction and equilibrium constants, respectively. E_ai and ΔH_i are the apparent activation energy and standard enthalpy change of i species.

2.3. Effect of Space Velocity on CO Conversion

The influence of the weight hourly space velocity (WHSV) on the CO conversion for three different catalysts at 1 atm and H₂/CO molar ratio 2 with error bar is shown in Figure 7a–c. The reactions were carried out at three different temperatures (210 °C, 240 °C, and 270 °C). CO conversion increases with the increase of space velocity and temperature. While CO conversion increases quickly with the increase of space velocity at the beginning, it remains almost constant at higher space velocity as the reaction reaches the equilibrium state. The variation of CO conversion was within 10% as observed during these reactions.

2.4. FT Kinetic Model

The kinetic models derived from Langmuir–Hinshelwood and Eley–Rideal mechanisms consider elementary reactions consuming CO and H₂ to produce hydrocarbons and water. Recently, the mechanistic aspects of FT synthesis were well investigated by computational catalysis studies using DFT-based quantum chemical models [85,86,87]. However, the use of Langmuir–Hinshelwood and Eley–Rideal models facilitates understanding of the FT mechanism more easily. In this work, all the kinetic models derived based on these mechanisms are investigated and fitted with experimental data to check the feasibility of the proposed mechanism. The objective function (F) in Equation (4) was utilized to measure the goodness of the model to select the best-fitted mechanism for FT synthesis.

Table 4. Kinetic experimental data for all the catalysts.

Temperature	W/F (Kgcat.h/kmol)	CoRu-MCM-41		CoRu-SBA-15		CoRu-KIT-6
		Reaction Rate (Kmol/Kg.h)	Conversion (%)	Reaction Rate (Kmol/Kg.h)	Conversion (%)	Reaction Rate (Kmol/Kg.h)	Conversion (%)
210 °C	25.2	0.035	89.53	0.035	88.35	0.033	84.12
	12.6	0.070	88.43	0.068	86.08	0.065	83.03
	8.39	0.100	84.29	0.1	84.29	0.098	82.41
	6.30	0.130	81.97	0.13	81.97	0.13	81.97
	5.04	0.156	78.64	0.162	81.52	0.158	79.34
	4.20	0.177	74.45	0.188	79.11	0.186	78.15
	3.60	0.181	65.46	0.208	74.74	0.21	75.45
	3.15	0.196	61.98	0.231	72.79	0.224	70.43
240 °C	25.2	0.036	93.09	0.037	94.43	0.034	87.39
	12.6	0.071	90.09	0.071	90.36	0.067	85.27
	8.39	0.105	88.84	0.01	85.18	0.101	84.79
	6.30	0.127	80.14	0.013	82.07	0.126	79.63
	5.04	0.153	77.34	0.157	79.24	0.154	77.48
	4.20	0.160	67.23	0.182	76.32	0.176	74.02
	3.60	0.176	63.42	0.209	75.25	0.201	72.35
	3.15	0.187	59.02	0.218	68.76	0.224	70.44
270 °C	25.2	0.037	94.95	0.038	95.98	0.036	92.40
	12.6	0.070	88.39	0.072	91.55	0.069	87.39
	8.39	0.100	83.96	0.107	89.90	0.101	85.01
	6.30	0.115	72.70	0.135	85.22	0.130	81.80
	5.04	0.133	67.48	0.162	81.42	0.153	77.08
	4.20	0.158	66.53	0.182	76.32	0.177	74.18
	3.60	0.175	63.20	0.206	74.19	0.2	71.93
	3.15	0.194	61.34	0.222	69.90	0.222	70.00

shows the experimental data obtained for all catalysts at 210 °C, 240 °C, and 270 °C.

The kinetic parameters obtained for all the mechanisms for CoRu-MCM-41 are shown in

Table 5. Kinetic parameters obtained from proposed mechanisms for CoRu-MCM-41

Model	Temperature
	210 °C	R2	240 °C	R2	270 °C	R2
FT-1	k = 0.872 ± 0.0037 K1 = 21.19 ± 1.79	0.88	k = 0.23 ± 0.027 K1 = 16.49 ± 7.66	0.98	k = 0.38 ± 0.118 K1 = 3.92 ± 2.46	0.98
FT-2	k = 0.87 ± 0.0037 K1 = 21.19 ± 1.79	0.88	k = 0.74 ± 0.0014 K1 = 21.19 ± 0.84	0.72	k = 0.69 ± 0.0067 K1 = 6.05 ± 0.34	0.93
FT-3	k = 1.32 ± 0.000029 K1 = 5.04 ± 0.000194 K2 = 2.877 ± 0.000468	0.97	k = 2.21 ± 0.017 K1 = 4.02 ± 0.103 K2 = 0.51 ± 0.0064	0.96	k = 3.22 ± 0.068 K1 = 1.41 ± 0.05 K2 = 0.5 ± 0.019	0.97
FT-4	k = 0.443 ± 1.69 K1 = 1.88 ± 19.53 K2 = 2.49 ± 34.08	0.98	k = 0.85 ± 0.015 K1 = 11.09 ± 0.686 K2 = 0.365 ± 0.0077	0.98	k = 2.01 ± 0.037 K1 = 3.13 ± 0.11 K2 = 0.228 ± 0.0045	0.98
FT-5	k = 0.536 ± 0.000355 K1 = 21.19 ± 0.403 K2 = 2.42 ± 0.0023	0.97	k = 0.48 ± 0.000205 K1 = 21.19 ± 0.265 K2 = 2.33 ± 0.0015	0.90	k = 0.41 ± 0.009 K1 = 0.33 ± 0.014 K2 = 11.09 ± 0.396	0.97
FT-6	k = 0.704 ± 0.0022 K1 = 0.931 ± 0.0039 K2 = 31.29 ± 3.21 K3 = 1.77 ± 0.0074	0.96	k = 0.67 ± 0.0012 K1 = 0.897 ± 0.0021 K2 = 31.29 ± 1.83 K3 = 1.69 ± 0.0041	0.87	k = 0.243 ± 0.00018 K1 = 3.14 ± 0.0052 K2 = 31.29 ± 0.76 K3 = 1.67 ± 0.0027	0.83

. It can be inferred from data that the value of the rate constant (k) increases with increasing temperature with only one of the 6 mechanisms which is FT-3. Therefore, for CoRu-MCM-41, the model FT-3 is best fitted with the kinetic data.

In order to determine the activation energy and the frequency factor from the Arrhenius equation, the logarithm of the rate constant was plotted against the inverse of reaction temperature as shown in Figure 8. The activation energy was determined to be 32.21 kJ/mol and the frequency factor was 4099 kmol/KgCat.hr. (atm)² for CoRu-MCM-41 catalyst.

Table 6. Kinetic parameters obtained from proposed mechanisms for CoRu-SBA-15.

Model	Temperature
	210 °C	R2	240 °C	R2	270 °C	R2
FT-1	k = 3.02 ± 0.092 K1 = 0.46 ± 0.015	0.94	k = 2.01 ± 0.059 K1 = 0.655 ± 0.021	0.94	k = 2.01 ± 0.026 K1 = 0.727 ± 0.01	0.98
FT-2	k = 9.08 ± 0.266 K1 = 0.166 ± 0.005	0.94	k = 11.09 ± 0.281 K1 = 0.118 ± 0.0033	0.95	k = 13.12 ± 0.005 K1 = 0.109 ± 0.000046	0.99
FT-3	k = 11.09 ± 0.698 K1 = 2.23 ± 0.145 K2 = 20.62 ± 1.49	0.82	k = 2.01 ± 0.043 K1 = 10.79 ± 0.285 K2 = 17.51 ± 0.536	0.79	k = 11.09 ± 0.337 K1 = 2.01 ± 0.063 K2 = 18.10 ± 0.645	0.95
FT-4	k = 11.09 ± 0.379 K1 = 0.161 ± 0.0057 K2 = 0.761 ± 0.026	0.94	k = 2.01 ± 0.0026 K1 = 0.283 ± 0.00042 K2 = 2.31 ± 0.0032	0.93	k = 2.01 ± 0.037 K1 = 3.13 ± 0.11 K2 = 0.228 ± 0.0045	0.98
FT-5	k = 2.01 ± 0.026 K1 = 0.189 ± 0.0031 K2 = 4.41 ± 0.062	0.92	k = 2.01 ± 0.0237 K1 = 0.193 ± 0.0029 K2 = 3.92 ± 0.050	0.94	k = 2.01 ± 0.032 K1 = 0.205 ± 0.0036 K2 = 3.62 ± 0.063	0.98
FT-6	k = 2.01 ± 0.037 K1 = 0.988 ± 0.021 K2 = 0.894 ± 0.031 K3 = 1.57 ± 0.032	0.90	k = 6.05 ± 0.253 K1 = 0.273 ± 0.012 K2 = 0.266 ± 0.014 K3 = 3.56 ± 0.153	0.94	k = 2.01 ± 0.018 K1 = 0.937 ± 0.0094 K2 = 2.18 ± 0.057 K3 = 1.09 ± 0.011	0.97

shows that the rate constant increases with the increase of reaction temperature. However, the two adsorption equilibrium constants (K₁ and K₂) decrease with the increase of reaction temperature. Since adsorption is an exothermic process, the adsorption equilibrium constant decreases with rise in temperature.

Table 4 shows the experimental data of the kinetic runs for CoRu-SBA-15 at 210 °C, 240°C, and 270 °C. When the data are fit against all the kinetic models, FT-2 was the best-fitted model obtained for CoRu-SBA-15. The kinetic parameters for all of the proposed mechanisms are shown in Table 6.

The activation energy and frequency factor of this catalyst were evaluated from the Arrhenius equation by plotting the logarithm of the rate constant to the inverse of reaction temperature as shown in Figure 9. The activation energy was determined to be 13.39 kJ/mol and the frequency factor was 254 kmol/KgCat.hr. atm. Table 6 shows that for FT-3 mechanism, the reaction rate constant increases with reaction temperature and adsorption equilibrium constant (K₁) decreases with the increase of reaction temperature.

Table 4 shows the experimental data for CoRu-KIT-6 catalyst. All the models were fitted with this experimental kinetic data and FT-6 was found to be the best fitted model for CoRu-KIT-6. The kinetic parameters for all the proposed mechanisms are shown in

Table 7. Kinetic parameters obtained from all the proposed mechanisms for CoRu-KIT-6.

Model	Temperature
	210 °C	R2	240 °C	R2	270 °C	R2
FT-1	k = 2.18 ± 8.89 K1 = 0.73 ± 3.28	0.97	k = 0.79 ± 0.543 K1 = 2.13 ± 1.91	0.98	k = 1.38 ± 1.42 K1 = 1.19 ± 1.44	0.99
FT-2	k = 1.42 ± 0.00044 K1 = 11.09±0.038	0.95	k = 1.29±0.022 K1 = 11.09±2.09	0.98	k = 2.02±3.30 K1 = 1.94±8.71	0.98
FT-3	k = 11.09±0.316 K1 = 0.892±0.031 K2 = 0.279±0.013	0.96	k = 10.09±0.166 K1 = 0.882±0.018 K2 = 0.279±0.0073	0.98	k = 11.09±0.088 K1 = 1.94±0.016 K2 = 15.64±0.15	0.97
FT-4	k = 2.01±0.052 K1 = 0.122±0.0038 K2 = 7.27±0.203	0.97	k = 2.01±0.088 K1 = 1.195±0.067 K2 = 0.694±0.033	0.98	k = 2.01±0.072 K1 = 0.355±0.015 K2 = 2.29±0.089	0.99
FT-5	k = 2.01±0.051 K1 = 0.121±0.0038 K2 = 7.27±0.203	0.97	k = 1.46±0.00069 K1 = 10.08±0.055 K2 = 0.91±0.00049	0.92	k = 2.01±0.028 K1 = 0.842±0.022 K2 = 1.62±0.024	0.99
FT-6	k = 1.71±0.02 K1 = 1.39±0.02 K2 = 0.828±0.018 K3 = 1.61±0.021	0.95	k = 2.01±0.039 K1 = 1.11±0.027 K2 = 0.454±0.013 K3 = 2.21±0.048	0.98	k = 2.41±0.040 K1 = 0.91±0.017 K2 = 0.315±0.0071 K3 = 3.12±0.056	0.99

and the activation energy from Figure 10 was determined to be 12.59 kJ/mol and the frequency factor was 39 kmol/KgCat.hr. atm.

Based on our experimental and kinetic model data, it can be concluded that only one of the six mechanisms for each catalyst is statistically relevant for fitting the model. The rate constant (k), for some of the other five mechanisms, does not show an increasing trend with the increase in temperature or remains constant, while the equilibrium constants, K₁ and K₂, did not show the decreasing trend with the increase in temperature. Thus, FT-3, FT-2, and FT-6 mechanisms were considered as kinetically relevant model equations for CoRu-MCM-41, CoRu-SBA-15, and CoRu-KIT-6, respectively.

The results from our studies in a microreactor are similar to those reported in literature. Mansouri et al., developed a similar mechanism to estimate kinetic parameters for FT synthesis using cobalt-based catalyst with silica support and found that the experimental data were best fitted with surface reaction mechanism proposed based on Langmuir-Hineshelwood model and the optimal activation for the proposed kinetic model was found to be 31.57 kJ/mol [88]. Very recently, Sonal et al., detailed mechanistic approach for FT synthesis based on Langmuir–Hinshelwood–Hougen–Watson (LHHW) and Eley–Rideal using Fe–Co based catalyst. They claimed that a mechanism based on the adsorption and desorption have a satisfactory fit to the experimental data with the activation energies for the formation of methane, paraffin and olefin to be around 70 kJ/mol, 113 kJ/mol, and 91 kJ/mol, respectively [89]. In order to improve the efficiency of FT synthesis significantly, detailed kinetic rate expressions were derived which is very similar to our work reported in literature for both fixed bed reactor as well as microreactor using iron or cobalt-based catalyst [80,81,90,91]. The elementary steps in the above studies were used to develop kinetic mechanisms considering FT reactions with and without water gas shift (WHS) reactions occurring on the surface of the catalysts forming intermediates with active sites. A similar approach was considered in this present study where CO*, COH₂* are assumed to form as intermediates, where * is an active site of the catalyst. From the activation energy calculations, shown in Figure 8, Figure 9 and Figure 10, the FT activation energy is observed in the order, CoRu-MCM-41 > CoRu-SBA-15 > CoRu-KIT-6. Almost 20 kJ/mol less activation energy was obtained for SBA-15 supported catalyst than that of MCM-41 catalyst. The activation energy of KIT-6 supported catalyst is a bit less than that of SBA-15 supported catalyst. This reflects that activation energy depends on metal–support interactions in different mesoporous catalysts. The variation of activity in different mesoporous catalysts might arise due to experimental uncertainties and operating conditions [92]. In addition, the FT activation energy is sensitive to the reactor system. Sun et al., [93] reported that the activation energy in a microchannel microreactor is smaller than that observed in a fixed bed reactor (FBR).

Figure 11 shows the variation of the model predicted rate with the experimental rate for all catalysts. The best fitted models i.e., FT-3, FT-2, and FT-6 for CoRu-MCM-41, CoRu-SBA-15, CoRu-KIT-6, respectively, were chosen to plot the graph for predicted and experimental rates. The correlation coefficient (R²) for all cases was above or equal to 0.95 with an error band of ±15% to ±30%. This indicates that the error between experimental and predicted values lies within the statistical permissible limits at all reaction temperatures for all the catalysts and consistent with the mechanistic models proposed in the literature. Moazami et al., conducted kinetic studies for FT synthesis in a fixed bed reactor with cobalt-based catalyst over silica support and found that 60% of the results were predicted with a relative error of less than 15%, while the rest of the proposed kinetic models has error less than 32% with confidence interval of 0.99 [94]. They also proposed a pseudo-homogenous one-dimensional model to evaluate the kinetic performance of the catalyst and achieved less than 8% error with the predicted data for kinetic experiments [95]. More recently, Marchese et al., performed kinetic studies with Co-Pt/γ-Al₂O₃ catalyst in a lab-scale tubular reactor and reported an error band around ± 25% with a confidence level of 0.95 stating it lies in the suitable acceptable limits with many mechanistic models proposed in the literature [89,96,97,98].

2.5. Deactivation Studies

In order to further understand how the interaction of Co and Ru metals with different mesoporous silica supports affects the stability of the FT catalysts, deactivation studies were performed. Figure 12 shows the deactivation rates of the catalysts tested continuously for 60 h at 240 °C, 1 atm, and H₂:CO ratio of 2:1. All the catalysts maintained fairly consistent CO conversion with very little fluctuation during the first 10 h. More specifically, the catalysts maintained 65%–79% CO conversion in the first 10 h of the reaction with CoRu-KIT-6 exhibiting the highest conversion and CoRu-MCM-41, the lowest. The activity of all the catalysts dropped by 20% after 24 h and then started to decline further. At the end of 60 h, the activity of the CoRu-MCM-41 dropped by 70% whereas, in the case of CoRu-KIT-6 and CoRu-SBA-15, the CO conversion decreased by 64% and 84%, respectively. Our results suggest that in terms of stability, the support has a significant impact on FT performance. More significantly, MCM-41 and KIT-6 supports are more stable when compared to the FT stability studies with SBA-15.

Many deactivation mechanisms have been proposed for FT studies that include catalysts poisoning, sintering, oxidation, the effect of water, carbon deposition and surface reconstruction [99]. The syngas used in our studies is a mixture of ultrahigh pure 5.0 CO and H₂ gases; therefore, there is very little or no chance of catalyst deactivation due to poisoning by the gas feed at the inlet to the reactor. It was also observed in our previous studies that the support (SiO₂ vs TiO₂) can enhance the stability of the catalyst to resist deactivation [38,40]. Iglesia et al., noticed that silica supported materials are less stable when compared to the other supports like Al₂O₃ [100] in their FT studies with Co-based catalyst. They also reported that CoRu-TiO₂ and CoRu-SiO₂ were found to have almost the same activation energy upon the addition of Ru to the Co catalyst; however, there were strong differences in the deactivation rates of catalysts depending upon the support [101]. Based on our CO- conversion studies, the ability of catalysts to withstand or retard the FT deactivation rate was in the order of CoRu-KIT-6 > CoRu-MCM-41 > CoRu-SBA-15.

3. Materials and Methods

3.1. Materials

The reagents used for catalysis synthesis were of analytical grade with no further purification. Tetramethyl orthosilicate, 99% (TMOS) and ammonium hydroxide, Tetraethyl orthosilicate reagent grade, 98% (TEOS), Pluronic acid (P123), Hydrochloric acid (HCl), Cetyltrimethylammoniumbromide (CTAB), Co(NO₃)_2.6H₂O, RuCl₃·xH₂O were purchased from Sigma Aldrich. Ethanol (anhydrous), Butanol and acetone, ACS grade, were obtained from Fischer Scientific, Branchburg, New Jersey, USA.

3.2. Fabrication of Microreactor

The microchannel microreactor and the respective cover channel were fabricated using 3D printing technology. Typically, the microreactor and its cover channel are designed using AutoCAD software which is schematically shown in Figure 13. The design is based on the split and recombination principle which has 11 microchannels of 500 μm × 500 μm × 2.4 cm as reaction zone in between them. This stainless-steel 3D printed microreactor is assembled in a custom-built heating block with an inlet and outlet system which facilitates the flow of syngas through the channels.

3.3. Catalyst Synthesis and Loading

Three types of Co-Ru based nanocatalysts supported by different mesoporous silica supports i.e., MCM-41, SBA-15 and KIT-6 supports are used in this study. A constant metal loading of 10%Co and ~5% Ru in weight was maintained in all preparations and this metal loading was also determined using the amount of the precursor. Three catalysts using different mesoporous support—10%Co5%Ru-MCM-41, 10%Co5%Ru-SBA-15 and 10%Co5%Ru-KIT-6—were synthesized using the one-pot hydrothermal procedure ( as shown below) [48]. The catalysts were labeled as CoRu-MCM-41, CoRu-SBA-15, and CoRu-KIT-6 in this manuscript.

For the synthesis of CoRu-MCM-41, TMOS, CTAB, DI-water, and ethanol were used in a molar ratio of 1:0.13:130:20 as described elsewhere [48]. In short, CTAB was dissolved in DI-water at 30 °C to produce a clear solution. The metal precursors were dissolved in ethanol in a separate beaker. The precursor, TMOS, which is a limiting agent for this synthesis, was added dropwise to the mixture of the two solutions prepared previously. Ammonium hydroxide was added dropwise to precipitate metal hydroxides in the solution, till the final pH was ~10. The precipitate was stirred for 3 h, followed by 18 h of aging at 65 °C. The precipitate was then washed with DI water till the filtrate reached a pH of 7, finally washed with ethanol and filtered. The filtered material is air-dried for a day and then oven-dried at 110 °C for 24 h. The dried catalyst is calcined at 550 °C for 16 h with a ramp rate of 2 °C/min to remove the CTAB template.

For the synthesis of CoRu-SBA-15: TEOS, CTAB, water, ethanol, pluronic acid, and hydrochloric acid were mixed in molar ratios of 1:0.081:41:7.5:0.0168:5.981. In a typical synthesis procedure, P123 was dissolved in 2M HCl at 35 °C till a clear solution was obtained. Another solution was prepared by dissolving CTAB in DI water at 35 °C until a homogenous mixture was produced. These two solutions were mixed and stirred for 35 min. Ethanol containing metal precursors were added dropwise into the solution and stirred for 30 min. Afterwards, TEOS which was limiting reagent in this procedure was also added dropwise and stirred for 20 h at 35 °C. The aqueous solution was aged for 48 h at 98 °C followed by air drying for 24 h. The material was then oven-dried at 110 °C for 24 h. Finally, the dried material is then calcined in stepwise fashion with heating rate of 1 °C/min at 350 °C, 450 °C, and 550 °C for 8 h each, respectively to remove CTAB and pluronic acid.

In the case of CoRu-KIT-6, TEOS, P123, HCl, DI water, and butanol were mixed in a molar ratio of 1:0.017:1.83:195:1.31 [102]. For a typical procedure, P123 was added to HCl at 35 °C till a clear solution was obtained. A separate solution was prepared with butanol containing metal precursors and poured to the previous solution and stirred until a homogeneous solution was obtained. To this mixture, TEOS, which was the limiting reagent, was added dropwise and stirred at 500 rpm for 24 h. The final solution was aged for 24 h at 100 °C, followed by air drying for 24 h under the fume hood. The material is oven-dried at 110 °C for 24 h and then calcined at 550 °C for 4 h, to remove P123, the structure directing agent, SDA, with heating and cooling rates of 1 °C/min.

The catalyst is loaded into the microchannels of the microreactor using a PVA suspension containing the catalyst, DI water, binder PVA (polyvinyl alcohol 98%–99% hydrolyzed MW: 31000) and acetic acid of weight ratio 1:5:0.25:0.05. This suspension with well-dispersed catalyst was dip-coated and dried in air and then calcined in presence of air at 400 °C for 2 h with heating and cooling rates of 5 °C/min. Figure 13d shows the SEM image of the catalyst coated microreactor prior to the in-situ reduction.

3.4. Catalyst Characterization

Specific surface area, pore size, pore volume and TPR studies of the catalyst were carried out using Micromeritics, 3-Flex instrument. The Brunner–Emmett–Teller (BET) method was used to calculate the surface area of the catalyst where an equation was obtained from adsorption isotherm in the relative pressure range of 0.07–0.03. The surface area was calculated from adsorption isotherm in the relative pressure range of P/P₀ = 0.07–0.3 using the Brunner–Emmett–Teller (BET) equation. The total volume per gram of catalyst was determined from the amount of N₂ adsorbed at P/P₀ = 1. The N₂ desorption from the catalyst surface provides information about the pore size distribution using BJH (Barret–Joyner–Halenda) plots [103]. The H₂ temperature programmed reduction (TPR) analysis was also done with the same instrument which has a TCD detector to monitor the reduction signals of the catalyst. Around 50 mg of the catalyst was loaded into the quartz sample tube in which a stream of 10% H₂/Ar at flowrate 110 mL/min was passed through and the temperature is increased to 1000 °C with 10 °C/min ramp rate. The small and wide-angle powder x-ray diffraction (XRD) were recorded using D8 Discover X-ray and Rigaku SmartLab X-ray diffractometers, respectively, with Cu K-alpha radiation (wavelength = 0.15418 nm) radiation generated at 40 mA and 40 kV. The step size and time per step used in these measurements are 0.05° and 3 secs/step, respectively. The crystal sizes of the metal oxides were determined using the Scherrer equation. In the Scherrer equation below, τ stands for the crystal size, λ is the wavelength of the Cu Kα radiation, β is the full width half maximum and θ is the Bragg diffraction angle.

$$\mathsf{\tau }=\frac{0.9\mathsf{\lambda }}{\mathsf{\beta }*\mathrm{Cos}\mathsf{\theta }}$$

(7)

The morphology and the size of the catalysts were analyzed using transmission electron (TEM Carl Zeiss Libra 120) at 120 KeV and scanning electron microscopy (Zeiss Auriga FIB/FESEM). The sample for TEM was prepared by dispersing a small quantity of catalyst in 3 mL of ethanol followed by vortex dispersion and sonication for a few minutes. Then the suspension was drop coated on a carbon-coated copper grid of 300 µm mesh size, followed by drying in an oven at 100 °C for 12 h.

The elemental composition and oxidation states of the metals were analyzed using Energy Dispersive X-ray spectrometry (Zeiss Auriga FIB/FESEM obtained from Carl Zeiss, Oberkochen, Germany) and oxidation states by X-ray photoelectron spectroscopy (XPS-Escalab Xi+-Thermo Scientific obtained from Thermo Scientific, West Sussex, UK), respectively.

3.5. Fischer–Tropsch Synthesis in Microreactor and Kinetic Data Collection

An in-house LabVIEW automated experimental setup was built to carry out the FT experiments for precise control over the operating conditions. The experimental setup is shown in Figure 14. The flowrates of the syngas mixture which is a mixture of hydrogen and carbon monoxide were controlled by precalibrated mass flow controllers obtained from cole parmer with flow rates ranging from 0–1 sccm. Nitrogen was used as a carrier gas into the system and was controlled by Aalborg mass flow controller with a maximum flow rate of 10 sccm. The upstream and downstream pressures were continuously monitored by pressure gauges obtained from Cole–Parmer and the data are fed to Aalborg solenoid valve from which the reaction pressure is controlled and kept constant throughout the reaction. All these controllers are operated by LabVIEW 2018 program. The product stream is directly fed to the GC-MS (Agilent Technologies 7890B GC and 5977 MSD). Prior to the start of the kinetic experiments, the microreactors were reduced ex-situ in Carbolite Gero tubular furnace with 10% H₂Ar. To compensate the losses while transferring the microreactor to the heating block the microreactor containing the catalyst was reduced again in-situ for 6 h at 350 °C before the start of FT reaction. The kinetic studies were performed by varying the weight hourly space velocity (WHSV = Wcat/F_CO,in, where W_Cat = weight of the catalyst and F_CO,in = molar flow rate of CO in feed) in the range of ~25.2–3.15 kgcat.h/kmol. The reactions were performed with syngas having a feed molar ratio (H₂/CO) of 2:1 at 210 °C, 240 °C, and 270 °C while the reaction pressure was maintained at 1 atm. Based on our previous FT studies using this setup and preliminary runs, all reactions reached a steady state after an hour at each setpoint of WHSV. Deactivation studies were also performed for all three catalysts at 240 °C using syngas feed molar ration of 2:1. CO conversion was calculated based on following equation:

$$X_{CO}\%=\frac{F_{CO,in}-F_{CO,out}}{F_{CO,in}}\times 100$$

(8)

4. Conclusions

Three different types of mesoporous silica supported Co-Ru based catalysts were synthesized using the one-pot hydrothermal method and performance for FT was evaluated. These catalysts resulted in high surface area with hexagonal ordered mesoporous structure as supported by BET, low angle XRD and TEM studies. The interaction between the metal and different types of support has a significant effect on the kinetic and stability studies of FT synthesis. Six mechanistic models were developed based on the Langmuir–Hinshelwood and Eley-Radiel mechanisms. The best fitted model for all catalysts was obtained on the basis of non-linear regression by comparing with the objective function which is equation-7 in this paper. The proposed model FT-3 was best fitted with the kinetic data for CoRu-MCM-41 catalyst. Whereas, FT-2 and FT-6 were well fitted with the kinetic data for CoRu-SBA-15 and CoRu-KIT-6, respectively. CoRu-KIT-6 was found to be more active than other catalysts with a low activation energy of 12.59 kJ/mol, whereas for CoRu-SBA-15 and CoRu-MCM-41 the activation energies are 13.39 and 32.21 kJ/mol, respectively. An average error of 5.65%, 1.76%, and 3.70% was obtained for catalysts CoRu-MCM-41, CoRu-SBA-15, CoRu-KIT-6, respectively considering the best fitted model explained above for FT synthesis. The predicted data provided by kinetic models were satisfactory with the experimental data. These results highlight the potential of the mechanistic FT models as well as reaction mechanisms to further improve the performance of FT synthesis. In addition, this information can help to design more active and selective catalysts for the optimized FT process. Furthermore, all catalysts exhibited significant resistance to the deactivation rate following the order CoRu-KIT-6> CoRu-MCM-41>CoRu-SBA-15. This study suggests that even if the support is of same type, the structure of the support plays a vital role in catalyst performance for FT synthesis.