Studies on MCM-48 supported cobalt catalyst for Fischer–Tropsch synthesis

Abstract

MCM-48 molecule sieve was used as a support of cobalt catalyst for Fischer–Tropsch synthesis (FTS). Co/MCM-48 catalysts were prepared using incipient witness impregnation method with cobalt loadings of 5, 10 and 15 wt.%, respectively. The catalysts were characterized by N₂ physisorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), hydrogen temperature programmed desorption (H₂-TPD) followed by pulse oxygen titration and transmission electron microscopy (TEM). The catalytic properties for FTS were tested in a fixed bed reactor. TPR and oxygen titration indicated different extent of overall reduction of Co cobalt species in different cobalt content catalysts. With increasing Co loading, CO conversion and C5+ selectivity first increased remarkably and then remained nearly invariable. Lower reducibility of smaller cobalt particles on 5Co/MCM-48 is likely to be one of the reasons responsible for the lower CO conversion and highest methane selectivity. Co loading exceeding 10 wt.% had no significant effect on FTS properties of the catalysts.

Introduction

Fischer–Tropsch (FT) synthesis is a process for hydrocarbon production from carbon monoxide and hydrogen. The synthetic fuels produced via FT synthesis have high cetane numbers, low contents of sulfur and aromatics. This makes them suitable for diesel engines and friendly for environment [1]. Supported Co-based catalysts have been widely used to achieve high yields of paraffinic hydrocarbons in FT synthesis [2], [3], [4], [5], [6]. Cobalt is usually supported on a high surface area support such as SiO₂ [7], [8], [9], Al₂O₃ [7], zeolites [10] and TiO₂ [7] with microporous and mesoporous structures in order to obtain a high metal dispersion. These conventional mesoporous supports are irregularly spaced and their pore sizes are broadly distributed. Thus, it is rather hard to study the effect of support and its porosity on FT reaction rate and hydrocarbon selectivity [11]. Highly ordered mesoporous silica, such as MCM-41 [12], FSM-16 [13], SBA-15 [14] and HMS [15], have been recently used as a support for metal catalysts, which were found to have well-defined periodic mesopores, consequently provide very narrow pore size distributions, and possess large pore volumes of 1–2 cm³/g and high surface areas reaching 1000 m²/g. The pore diameters can be controlled in the range of 2–30 nm by using various surfactants, additives, and different synthetic conditions [16]. The narrow pore size distributions are suitable for evaluation of pore size and texture effects on cobalt dispersion and also it may make it possible to design new catalysts with higher productivity for long chain hydrocarbons. Thus, the use of periodic mesoporous silicas as supports for preparing Co-based FTS catalysts has been recently explored [17], [18], [19], [20], [21], [22].

Catalytic behavior of silica supported cobalt catalysts in FT synthesis was found to depend on the nature of cobalt species, cobalt particle size and catalyst mesoporous structure. The properties of the cobalt particles were greatly affected by the pore size of the mesoporous support. Previous reports showed that reducibility of cobalt particles supported by silica [20], [23], [24], titania [25] and alumina [26] depended mostly on their sizes. Smaller cobalt particles are usually more difficult to reduce than larger ones. For example, with alumina supported cobalt catalysts, even though the true metal cluster size is smaller at lower loadings, the poor extent of reduction leads to a low cobalt surface metal active site density under FT conditions. Heavier loadings are often used to increase the cluster size and thereby break the cobalt oxide–support interaction, resulting in increased extent of reduction and leading to improved cobalt surface active site densities. It is often considered [7], [15] that the presence of larger cobalt particles leads to higher selectivity to heavier hydrocarbons.

Considering the discussion above, larger cobalt particles sizes formed in wider pore mesoporous supports, such as SBA-15, and were more reducible leading to catalysts with higher catalytic activity and lower methane selectivity than smaller particles in narrower pore materials, such as MCM-41. Lower FT activity and higher methane selectivity observed on the narrow pore cobalt catalysts are principally attributed to the lower reducibility of small cobalt particles [18], [20].

It is usually suggested that the size of supported metal and oxide particles and thus their catalytic behavior are principally affected by overall metal content in supported catalysts. It is generally assumed that an increase in metal loading would almost automatically result in lowering of metal dispersion [27]. Co/SBA-15 catalysts cobalt loadings of 10–40 wt.% were prepared and a maximum CO conversion was obtained for the catalyst loaded with ca. 30 wt.% Co presenting the highest density of surface Co⁰ sites. This finding highlights the importance of further considering the impact of the cluster size on the reducibility of cobalt clusters. That is, at low cobalt loadings, the cobalt oxide–support interaction is strengthened, making it difficult to achieve reduction of the cobalt clusters and therefore, generate surface active sites. Product selectivities were also influenced by Co loading. The product distribution shifted toward the formation of lighter hydrocarbons (methane, C2–C4) for the less reducible low-loaded sample [28].

The synthesis of the silica-based M41S was first reported in 1992 [29]. The specific properties of these materials are large surface areas and a narrow pore-size distribution. MCM-48, as a member of M41S, has a cubic structure indexed in the space group Ia3d [30], recently modelled as a gyroid minimal surface with its high specific surface area up to 1600 cm² g⁻¹ specific pore volume up to 1.2 cm³ g⁻¹ and high thermal stability. This is because of its interwoven and branched pore structure, which provides more favourable mass transfer kinetics in catalytic and separation applications than MCM-41 with its unidirectional pore system [31]. Various ordered mesoporous silica for cobalt catalysts have been used, but an application of MCM-48 as metal support to cobalt-based FT catalysts has not previously been reported. In this work, MCM-48 was utilized to investigate how MCM-48, its pore size and Co loading affect surface reaction parameters during CO hydrogenation. Co/MCM-48 catalysts with different Co content were prepared, characterized by N₂ physisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature programmed reduction (TPR), hydrogen temperature programmed desorption (H₂-TPD) with pulse oxygen titration, and tested in a fixed-bed reactor for the Fischer–Tropsch synthesis reaction.