Cobalt-based Fischer-Tropsch synthesis: Effect of the catalyst granule thermal conductivity on the catalytic performance

doi:10.1016/j.mcat.2021.111395

Molecular Catalysis

Volume 502, February 2021, 111395

https://doi.org/10.1016/j.mcat.2021.111395 Get rights and content

Highlights

•
Model FTS catalyst granules with different thermal conductivity were prepared.
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Overheating effect was exist within catalyst granule with low thermal conductivity.
•
Thermal conductivity of FTS catalyst granules could influent the catalytic performance.
•
Thermal conductivity could affect syngas adsorption, hydrogenation and chain growth.

Abstract

A series of model Fischer-Tropsch synthesis (FTS) catalyst granules with different thermal conductivity were prepared to study the influence of catalyst granule thermal conductivity on FTS performance. A clear correlation was found between catalyst granule thermal conductivity and FTS activity, CH₄ selectivity, C₅₊ selectivity, mole ratio of olefin to paraffin and branched hydrocarbon selectivity at high CO conversion. This could be due to that the overheating effect occurred within catalyst granule with low thermal conductivity. On the basis of the widely accepted ‘alkyl’ mechanism for FTS, the overheating effect was able to affect the adsorption of H₂ and CO molecules, the hydrogenation of carbide species as well as chain growth. In addition, the overheating effect could lead to the catalyst structure reconstruction, sintering and deactivation.

Graphical abstract

Introduction

The demand for oil keeps growing in recent years due to the rapid growth of world population and fast development of industry. Fischer-Tropsch synthesis (FTS) process, which converting synthesis gas (the gas mixture of CO and H₂) to liquid hydrocarbons in the presence of catalysts (Fe, Co or Ru), is considered as one ideal alternative route for fuel production [[1], [2], [3]]. The FTS catalysts are usually prepared by loading active metal phase on porous carriers. Especially, a rising attention has been attracted for cobalt-based catalyst in a fixed-bed reactor due to the high activity, good resistance to deactivation and fine operating flexibility [4]. Improving heavy hydrocarbons selectivity and suppressing the formation of light hydrocarbons are the focus of FTS investigation.

Generally, several factors including the chemical nature of supports, the promoter, the cobalt particle size and the diffusional restrictions could affect the cobalt-based FTS activity and products selectivity according to previous study [[5], [6], [7]]. First, the effect of support chemical nature on FTS performance. With the aim of studying the relationship between the support chemical nature and FTS performance, Bartholomew et al. prepared a series of cobalt-based catalysts by loading cobalt on SiO₂, TiO₂, Al₂O₃ and MgO, respectively. The FTS activities decreased as follows: Co/TiO₂ > Co/SiO₂ > Co/Al₂O₃ > Co/MgO [8]. Rane et al. also reported a certain support effect on the product selectivity over cobalt nanoparticles deposited on Al₂O₃ with different crystal structures [9]. Next, the effect of promoter effect on FTS performance. Transition metal oxides, rare earth metal oxides as well as noble metals are the typical promoters used for the preparation of cobalt-based FTS catalyst. Alexis T. Bell’s research showed that the addition of Mn element can lead to an increase of C₅₊ selectivity, which could be attributed to the improvement of CO adsorption on the catalysts surface [10]. With regard to the cobalt particle size effect, de jong et al. have observed a clear decrease of CO hydrogenation turnover frequency (TOF) and C₅₊ selectivity when the cobalt particle size was smaller than 8 nm at 35 bar, which could be due to the structure sensitivity [11]. At last, the effect of diffusional restrictions on FTS performance [12]. In general, relative large size (1−3 mm) of catalyst granules are needed in an industrial fixed-bed reactor to reduce the pressure drop. However, this may result in serious intraparticle diffusion limitation, which could significantly influence the FTS activity and hydrocarbon selectivity [13]. Iglesia et al. systematically investigated the influence of catalyst granule size on the activity and products distribution for FTS, in which a structural parameter χ (Eq. (1)) was used to determine whether the intraparticle diffusion limitations occurred. An optimal activity and C₅₊ selectivity were obtained with the χ values in the range of 200–2000 (m⁻¹ $\times$ 10^-16), which suggested that the intrinsic reaction behavior was maintained while the structural parameters were in that range [14].χ = L²Φθ/r_pwhere L was the granule diameter (m), Φ was the granule porosity (%), θ was the cobalt site density (Co atoms/m²), r_p was the average pore diameter of the granule (m).

Recently, considerable investigations have been focused on the preparation of high thermal conductive Co/SiC FTS catalyst on account of the high C₅₊ selectivity and low CH₄ selectivity [15,16]. Researchers have pointed out that the ‘hot spots’ were easily formed on the surface of low thermal conductive FTS catalyst (such as Co/Al₂O₃ and Co/SiO₂), which lead to the generation of light hydrocarbon products. As far as the high thermal conductive Co/SiC, the SiC substrate could transfer the reaction heat effectively during reaction, which prevented the overheating effect on catalyst surface [17]. That is to say, some researchers believed that the thermal conductive property of the catalyst support was also an important factor affecting the FTS products distribution. However, this conclusion may be in some speculative, since no work has been carried out directly to study the influence of the thermal conductive property of catalyst on FTS products selectivity in the case of eliminating the interference of other factors. Actually, besides the difference of thermal conductivity, the surface chemical nature of support was also different between Co/SiC and those low thermal conductive FTS catalysts (such as Co/Al₂O₃ and Co/SiO₂). Our previous work has convinced that the chemical nature of SiC also plays a crucial role in the FTS performance over Co/SiC [18,19]. This suggests that the effect of surface chemical nature might mask the effect of thermal conductivity on FTS performance. In fact, it may be insignificant to study the influence of catalyst support thermal conductivity on FTS performance. Generally, FTS catalyst was shaped into small granules and then loaded into the fixed-bed reactor. During reaction, the temperature distribution of each catalyst granule is not uniform on account of the strong exothermic property of FTS. Derevich et al. also reported that the temperature of granule center was much higher than that of granule surface by using simulation method [20]. The variation of catalyst granule thermal conductivity was thus may affect the temperature distribution within catalyst granule, ultimately leading to the variation of activity and products selectivity. Therefore, this points to the need of an intensive research on the effect of catalyst granule thermal conductivity on FT performance.

In this research, we try to study the effect of catalyst granule thermal conductive property on FTS performance. In order to obtain the model catalyst granules with different thermal conductivity, Co/SiO₂ catalyst was first mixed with high thermal conductive aluminum powders in different proportion. The mixtures were then pressed into tablets and finally broke into small granules (Scheme S1). The dependence of FTS activity and products selectivity on the catalyst thermal conductivity were carefully studied. The reason of the dependence was also analyzed on the basis of the widely accepted ‘alkyl’ mechanism.

Section snippets

Catalyst preparation

Co/SiO₂ FTS catalyst was prepared by an incipient wetness impregnation method using SiO₂ (Aladdin, 30 nm) as support and Co(NO₃)₂·6H₂O (Aladdin, Analytical Grade) as precursor in deionized water (prepared in our laboratory). Then the sample was dried overnight in an oven and calcined in air at 673 K for 6 h to obtain the Co/SiO₂ catalyst (20 wt% cobalt loading). In order to change the thermal conductivity of catalyst granule, Co/SiO₂ was mixed with high thermal conductive aluminum powders in

Catalyst properties

XRD pattern of the Co/SiO₂ catalyst is displayed in Fig. S1. Co₃O₄ was found to be the only crystalline cobalt species in fresh catalyst. Detailed textural properties of the model CAT-n (n = 0, 1, 2, 3) catalysts are shown in Table S1. The porosity decreased whereas mean pore size increased with the increase of the proportion of metallic Al powders for CAT-n (n = 0, 1, 2, 3), probably be due to the non-porous property of Al powders. H₂-TPR result (Fig. S2) showed three reduction peaks for the

Conclusion

A comprehensive study of the FTS performances for CAT-n (n = 0, 1, 2, 3) were carried out to study the effect of the thermal conductivity of catalyst granule on Fischer-Tropsch synthesis. The following conclusions could be drawn from this study:

(1)
Fischer-Tropsch synthesis is a strong exothermic reaction and ‘hot spots’ are locating inside the catalyst granules with low thermal conductivity during reaction. These ‘hot spots’ could lead to the overheating effect within catalyst granules. A definite

CRediT authorship contribution statement

Da Wang: Conceptualization, Funding acquisition, Writing - original draft. Lei Chen: Data curation. Guangci Li: Investigation. Zhong Wang: Data curation, Investigation. Xuebing Li: Writing - review & editing. Bo Hou: Supervision, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (21808235 and 21761132006).

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Cobalt-based Fischer-Tropsch synthesis: Effect of the catalyst granule thermal conductivity on the catalytic performance

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Catalyst preparation

Catalyst properties

Conclusion

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgement

Appl. Catal. A-Gen.

Mol. Catal.

Mol. Catal.

Mol. Catal.

Mol. Catal.

Appl. Catal. A-Gen.

J. Catal.

Mol. Catal.

J. Catal.

Fuel

Appl. Catal. A-Gen.

Fuel

Int. J. Heat Mass Transf.

J. Catal.

Catal. Today

Chem. Eng. J.

Appl. Catal. A-Gen.

Appl. Catal. A-Gen.

Catal. Today