Elsevier

Journal of Catalysis

Volume 374, June 2019, Pages 24-35
Journal of Catalysis

Potassium-promoted magnesium ferrite on 3D porous graphene as highly efficient catalyst for CO hydrogenation to lower olefins

https://doi.org/10.1016/j.jcat.2019.04.024Get rights and content

Highlights

  • Honeycomb-structured graphene (HSG) was used to support the K-MgFe catalysts.

  • Unprecedentedly high activity and productivity of lower olefins were obtained in FTO.

  • The synergetic effect of Mg and K enhanced the adsorption and dissociation of CO.

  • The catalyst also showed better durability than typical FTO catalysts.

  • HSG enabled fast heat dissipation and confined the sintering of the active phase.

Abstract

Three-dimensional (3D) honeycomb-like structured graphene (HSG)-supported ternary K-promoted magnesium ferrite catalysts (K-MgFe/HSG) are prepared and evaluated in Fischer–Tropsch synthesis to the lower olefins (FTO). The catalysts bear interconnected mesoporous-macroporous framework of graphene nanosheets decorated with homogeneously sized magnesium ferrite nanoparticles (NPs). Under typical FTO reaction conditions, the composition-optimized 1K-MgFe/HSG catalyst affords a reduced CO2 selectivity of 40.4%, an excellent weight specific activity to hydrocarbons of 1825 μmolCO gFe−1 s−1, and an appreciable selectivity to the lower olefins (C2single bondC4 olefins) of 57.8%, thus giving rise to a record high productivity of the lower olefins of 1055 μmolCO gFe−1 s−1. The excellent catalytic efficiency is tentatively attributed to the synergetic effect of Mg and K on adsorption and dissociation of CO. The catalyst also exhibits better durability than previously reported iron-based FTO catalysts, without significant changes in the catalytic performance and the size of the iron carbide NPs after 120 h on stream, highlighting the crucial role of the 3D porous HSG support in restricting the agglomeration of the active phase during the challenging high-temperature and exothermic FTO reaction.

Introduction

The lower olefins are fundamental raw materials in chemical industry for the production of various value-added commodities. For example, the world consumption of ethylene was about 114 Mt in 2009. It was predicted that by 2020 the global production of ethylene will amount to 174 Mt [1]. Considering the limited reserves of crude oil and the protection of the environment, strategies for the production of the lower olefins from syngas (a mixture of CO and H2) derived from non-oil carbon sources, especially renewable biomass, are highly desirable [2], [3], [4], [5]. To transform syngas into the lower olefins, there are indirect routes such as methanol-to-olefins (MTO), dimethyl ether-to-olefins (DMTO), and dehydration of lower alcohols, and direct route, FTO. Since the direct route is technically more concise and cost-effective [6], the development of efficient FTO catalyst becomes one of the focal points in heterogeneous catalysis in recent years. Many studies have succeeded in developing highly selective FTO catalysts towards the lower olefins [3], [4], [7]. However, the high activity and stability of the catalyst, which enable energy-efficient, massive, and uninterrupted production of the lower olefins, are equally important. Besides, due to the high FTO temperature to accelerate the desorption of the lower olefins from the catalyst surface to avoid secondary reactions, the selectivity to the undesired CO2 is usually rather high (>45%) [3], [4], [7], [8], [9], as the water–gas shift (WGS) reaction is kinetically enhanced concomitantly [10]. Moreover, the high reaction temperature [2], together with the highly exothermic nature of the FTO reaction, places severe challenge on the stability of the catalysts.

It has been demonstrated that the support and the promoter pronouncedly influence the catalytic performance of the iron-based catalysts in FTO, which affect not only the dispersion, reduction, and carburization of iron [11], [12], [13], but also the reaction pathway [14]. Recently, two-dimensional (2D) graphene nanosheet was found to be an effective support for the iron-based FTO catalysts. The moderate interaction between the carbonaceous support and iron can render high degree of reduction and carburization, and the nanosheet-like structure of graphene facilitates the fast desorption of the reaction intermediate [15], in the present case, the lower olefins. For example, on the N-doped graphene-supported iron catalyst, a selectivity to the lower olefins of around 50% was reported, while it was only 30% on the carbon black-supported one [16]. On the reduced graphene oxide (rGO)-supported K-promoted iron catalyst (FeK1/rGO), a high selectivity to the lower olefins of 62% was obtained, along with a high specific activity of 646 μmolCO gFe−1 s−1 [9]. The activity was boosted to 1338 μmolCO gFe−1 s−1 over the Mg and K dual-promoted Fe/rGO catalyst. Meanwhile, the CO2 selectivity markedly decreased relative to that on the Mg-free catalyst [17]. Moreover, the stability of the catalyst was better than those of the oxide- and carbon nanotube (CNT)-supported iron-based catalysts in FTO [2], [18], [19].

The 3D porous graphene possesses interconnected network, large pores, and weak interlayer interactions, aside from the intrinsic merits of graphene such as excellent thermal/electrical conductivity, making it a promising material in many advanced applications [20], [21], [22], [23], [24], [25], [26]. Cheng and co-workers synthesized a 3D graphene material with a foam-like network. The prefect interconnection in 3D was believed to be essential to its outstanding electrical conductivity [20]. Ruoff and co-workers prepared graphene paper with 3D network of highly curved atom-thick walls, which possessed ultrahigh surface area and high electrical conductivity [21]. Müllen and co-workers synthesized 3D N-doped graphene aerogel (N-GA)-supported Fe3O4 NPs (Fe3O4/N-GAs), which outperformed the Fe3O4 NPs supported on N-doped carbon black or 2D N-doped graphene sheets for oxygen reduction reaction (ORR) in alkaline media. Furthermore, Fe3O4/N-GAs showed better durability than commercial Pt/C [22]. Hu and co-workers synthesized a 3D honeycomb-structured graphene (HSG) by reacting Li2O with CO. As a counter electrode, HSG was comparable to the Pt electrode for dye-sensitized solar cells [23]. They further synthesized cauliflower-fungus-like graphene for supercapacitor [24]. Chen et al. prepared 3D high-quality graphene flakes, on the basis of which the 3D graphene/g-C3N4 composite was fabricated. With the aid of 3D graphene, the catalytic performance of g-C3N4 in cyclohexane oxidation to cyclohexanone was improved substantially [25]. Wang et al. synthesized a 3D graphene-like macroporous Fesingle bondNsingle bondC catalyst, which exhibited excellent intrinsic ORR activity and superior mass transport properties, as well as good capacitive charge storage performance [26].

Enlightened by the advantages of 3D porous graphene, we recently reported the HSG-supported FeK catalysts for CO2 direct hydrogenation to light olefins. High activity and selectivity to the lower olefins were obtained on the FeK1.5/HSG catalyst, which also showed good stability at the reaction temperature of 613 K [27]. Herein, as an extension of our works, we fabricated the 3D porous HSG-supported K-promoted magnesium ferrite catalysts for FTO. It is expected that the 3D porous structure of HSG can geometrically confine the aggregation of the active components loaded on, which is essential for improving its endurance to the high reaction temperature and high reaction heat, while the large pores do not impede either the accessibility of the active sites to the reactant molecules or the escaping of the lower olefins from the catalyst surface. K is a commonly used promoter for iron-based Fischer–Tropsch catalysts, which can improve the activity while suppress the secondary hydrogenation of the lower olefins [28]. Mg is capable of improving the carbon efficiency in FTO by suppressing the generation of CO2 [17]. Some studies showed that the selectivity to the lower olefins was improved over the MgO-supported iron catalysts [29], [30]. Therefore, the combination of 3D graphene with Mg and K may endow the iron catalyst with unprecedentedly excellent catalytic performance in FTO.

Section snippets

Catalyst preparation

Li2O, Fe(NO3)3·9H2O, Mg(NO3)2·6H2O, and K2CO3 were of analytical grade (A.R.) and purchased from Sinopharm. HCl (36.5 wt%) was purchased from Shanghai Reagent. The gases were all purchased from Shanghai Youjiali.

The HSG support was fabricated following the method developed by Hu and co-workers with slight simplification [23]. About 15 g Li2O powders were loaded in a quartz boat and heated at 823 K for 24 h with a ramping rate of 10 K min−1 under flowing CO at ambient pressure. After the

Basic physical properties of the HSG support

N2 physisorption revealed that the Brunauer–Emmett–Teller surface area (SBET) of the as-fabricated HSG was 285 m2 g−1, and the pore volume (Vpore) and pore diameter (dpore) were as large as 1.28 cm3 g−1 and 15.1 nm, respectively. Fig. 1A shows the scanning electron microscopic (SEM) image of HSG. It is observed that the thin carbon sheets were curved and interconnected, forming a 3D free-standing honeycomb structure. Moreover, the cells with the macropore size ranging from 50 to 200 nm were

Discussion

Owing to the high reaction temperature and highly exothermic nature of FTO, high catalytic activity is accompanied by a high inclination of catalyst deactivation due to sintering. However, compared to the iron-based catalysts reported in the literature, 1K-MgFe/HSG is quite unique: it displays not only excellent activity, but also outstanding stability (Fig. 10). Fig. 11 shows the TEM image and PSD histogram of this catalyst after 120 h on stream in FTO. The 3D porous structure of HSG is well

Conclusions

Because of the 3D porous structure of HSG and the synergetic effects of Mg and K, the ternary K-MgFe/HSG catalysts exhibit record high activity and productivity of the lower olefins, outstanding durability, and reduced selectivity to CO2 in FTO. Aside from the excellent thermal conductivity of graphene that can facilitate the dissipation of the reaction heat, the porous structure of HSG is found to be highly effective in constraining the sintering of the iron carbide NPs in this challenging

Acknowledgements

This work was supported by the National Key R&D program of China (2018YFB0604501, 2017YFB0602204), the National Natural Science Foundation of China (21872035), the Science and Technology Commission of Shanghai Municipality (08DZ2270500), the International Joint Laboratory on Resource Chemistry (IJLRC), and the Shanghai Synchrotron Radiation Facility (SSRF).

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