Elsevier

Applied Catalysis A: General

Volume 539, 5 June 2017, Pages 48-58
Applied Catalysis A: General

Feature Article
δ-Alumina supported cobalt catalysts promoted by ruthenium for Fischer-Tropsch synthesis

https://doi.org/10.1016/j.apcata.2017.04.003Get rights and content

Highlights

  • Reduction of cobalt cations by hydrogen occurs at significantly lower temperatures for the ruthenium-promoted Сo-δAl2O3 catalysts.

  • FTS was carried out after activation catalyst under the conditions ensuring the comparable extent of cobalt reduction.

  • Selectivity of Ru-promoted Сo-δAl2O3 catalysts for olefins and high-molecular hydrocarbons is higher in comparison with unpromoted catalysts.

  • At 0.2 wt.% Ru loading, the major part of ruthenium resides in metallic cobalt particles.

  • When 0.5–1.0 wt.% of Ru is introduced to catalyst, a part of Ru forms individual ultradisperse particles, which are not active in FTS.

Abstract

The paper presents the low-temperature nitrogen adsorption, TG, XRD, IR spectroscopy, XPS, TEM and SEM data for ruthenium promoted (0.2–1 wt.%) Сo-δAl2O3 catalysts and characteristics of the catalysts in Fischer-Tropsch synthesis after their activation under the conditions ensuring the reduction of comparable fractions of metallic cobalt. It was shown that cobalt in oxide precursors is a component of the spinel-like Со3-xAlxO4 phase containing the impurity anions СО32−, NO3, ОН, and NO in promoted samples, which belong to the thermolysis product of the Ru precursor. Average sizes of Со3-xAlxO4 crystallites are in a range of 5–10 nm. As ruthenium content in the catalyst increases upon reduction, the temperature of metallic phase formation decreases substantially (by more than 150 °C). After the reduction, selectivity of promoted catalysts for α-olefins and high-molecular hydrocarbons was higher in comparison with unpromoted catalysts, without a noticeable decrease in catalytic activity. Therewith, in 0.5–1.0 wt.% catalysts, a part of ruthenium forms individual ultradispersed metallic particles ca. 1 nm in size that are located on the surface of oxide support and are not active in Fischer-Tropsch synthesis. The oxide layer decorating the surface of metallic cobalt particles is also strongly enriched with ruthenium. In the 0.2 wt.% catalyst, the major part of ruthenium resides in metallic cobalt particles. Although the ruthenium-cobalt alloy segregates with enrichment of the surface with cobalt, the presence of ruthenium in the metallic particles and probably in the decorating oxide layer exerts a considerable effect on selectivity of the catalysts.

Introduction

Gas-to-liquid technology (GTL) is one of the known processes for the production of liquid synthetic hydrocarbons from natural gas. This technology includes several steps: syngas production from natural gas, syngas conversion to hydrocarbon products via the catalytic Fischer-Tropsch synthesis (FTS), and upgrading of hydrocarbons (hydrocracking and hydroisomerization). The composition of Fischer-Tropsch synthesis products depends on the catalyst features and reaction conditions. It is known that cobalt-containing catalysts are most efficient for the production of linear hydrocarbons with the C5+ chain length at low temperatures and pressures of the synthesis, with low selectivity for the formation of methane (10% and lower), and low activity in WGSR. Supports for such catalysts are commonly represented by inorganic systems with the high specific surface area and sufficient mechanical strength (SiO2, Al2O3, TiO2 and ZrO2). Active catalytic sites of supported cobalt catalysts are nanostructured metallic particles with optimal size from 5 to 10 nm [1], [2]. The particles are formed by thermal reduction in a hydrogen-containing mixture from the oxide precursor. Metallic particles can be obtained also by direct decomposition of organic salts (cobalt citrate) or other organic precursors in an inert atmosphere (IR pyrolysis of a common solution of polymer and cobalt salt) [3], [4], [5].

It is known that thermal reduction of both the bulk Со3О4 and the oxide precursor of any supported cobalt catalyst consists of two steps: Со3О4*  СоО* and СоО*  Со0 [6], [7], [8], [9], [10], where asterisks indicate the possible presence of anions of the precursor compound, cations of the oxide support or promoting metals in the oxide. Temperature intervals and features of the reduction steps depend on the size of oxide particles as well as on the nature and amount of impurity anions and cations, and are caused by the nature of precursor compounds, support, methods and conditions of the synthesis, the presence of promoters, and conditions of thermal treatment. The effect of support nature is clearly demonstrated in the case of strong interaction of alumina with cobalt compounds during thermal decomposition, which leads to the formation of relatively small particles of the active component precursor, on the one hand, and spinel-like compounds with the composition Со3-xAlxO4 (0  x  2), on the other hand [11], [12]. As a result, the formation of metallic particles via the reduction of cobalt ions in a hydrogen-containing mixture of the indicated compounds proceeds at higher temperatures as compared to the reduction of bulk Со3О4 structure, which is completely reduced at 350–400 °C. According to [6], [7], [13], the appearance of metallic phase upon non-isothermal reduction at a partial hydrogen pressure of 0.1–1 atm is observed at a temperature not lower than 400–500 °C.

A possible way to decrease the activation temperature of cobalt-containing catalysts is the introduction of noble metals, for example Pt, Pd, Ru or Re. Such metals promote cobalt reduction to the metallic state; however, platinum and palladium adversely affect its selectivity owing to their intrinsic activity in CO hydrogenation to methane and light hydrocarbons [14], [15]. On the other hand, the introduction of up to 5 wt.% ruthenium either slightly decreases or even increases the selectivity of cobalt catalysts for high-molecular hydrocarbons. Several research groups showed earlier [14], [15], [16], [17], [18], [19], [20] that the promotion of Co-Al2O3 catalysts with small amounts of ruthenium decreases the characteristic temperature of cobalt reduction by 100–150 °C (according to TPR data) and increases initial activity of the catalyst with the retained selectivity for C5+ products. Ruthenium affects also the dispersion of active component particles: the introduction of promoter produces a twofold or even greater increase in the dispersion (according to data of hydrogen chemisorption followed by oxygen titration). In [21], hydrogen chemisorption data indicated that promotion of the cobalt catalyst did not increase the dispersion; however, transmission electron microscopy showed that the fraction of ultradispersed particles (below 6 nm) decreased from 83 to 49%, whereas the fraction of particles with the optimal for FTS sizes in the range of 6–12 nm increased from 17 to 51%.

In all the listed works, the catalytic properties of ruthenium promoted and unpromoted Со-Al catalysts were compared under similar conditions of reductive activation before catalytic testing in FTS: the temperature (350 °C [14], [15], [17], [18], 400 °C [16], [19], [21], 420 °C [20]) and duration of isothermal mode (10–16 h) were the same. Since for unpromoted Co-Al catalysts the temperatures of metallic cobalt particles formation are above 450 °C and ruthenium promotion substantially decreases the cobalt reduction temperature, in all the cited works ruthenium promoted catalysts expectedly showed a higher reduction degree and a greater amount of active metal sites. For example, after 10 h of reductive activation at 350 °C the fraction of cobalt reduced to metallic state in the 15%Со-Al2О3 catalyst only reached 30% [14]. Therewith, the characteristic temperature of metallic phase formation, according to TPR data, was ca. 600 °C. The introduction of 0.5 wt.% ruthenium decreased the characteristic temperature by 100 °C, thus increasing to 50% the fraction of cobalt reduced to metallic state. In the study of 20% Со-Al2О3 catalyst [18], the fraction of cobalt that was reduced to metallic state by promotion with 0.5 wt.% ruthenium increased from 60 to 85% (the characteristic temperature of the reduction process, according to TPR data, decreased from 600 to 450 °C). The catalytic activity of such systems in Fischer-Tropsch synthesis is determined by the amount of metallic cobalt; so, the activity was expectedly higher for the promoted catalysts. A comparison of the catalytic properties of Ru promoted and unpromoted Co-Al catalysts that were reduced at optimal (and different) temperatures could reveal the role of ruthenium directly in the catalytic process and separate it from the plausible effect of ruthenium on the reductive activation of Co-Al catalysts. However, such studies were not found in the literature.

The study was carried out with the cobalt-alumina FTS catalysts promoted with fac-Ru(NO)(NH3)2(NO3)3 ruthenium complex or unpromoted. The goal of the work was to compare the catalytic properties of ruthenium promoted and unpromoted Co-Al catalysts in Fischer-Tropsch synthesis after their activation under the conditions ensuring the comparable extent of cobalt reduction, and to elucidate the role of ruthenium directly in the catalytic Fischer-Tropsch synthesis.

Section snippets

Catalysts preparation

Cobalt-alumina catalysts were synthesized using deposition by precipitation with urea (DPU) [22]. The synthesis was carried out with granulated δ-Al2O3 cylinders (diameter 2.5 mm, length 4–5 mm) that were obtained by thermal treatment of pseudoboehmite in air at 900 °C for 3 h (A64, JSC Angarsk Catalysts and Organic Synthesis Plant), Сo(NO3)2·6H2O salt (pure, GOST 4528-78), and urea (analytically pure, GOST 6691-77). 20 g of the support suspended in 93 ml of an aqueous mixture of cobalt nitrate and

Texture of catalyst granules and spatial uniformity of Co and Ru distribution

Data of elemental analysis of the dried samples are listed in Table 1. Cobalt content in all the dried samples varies from 9.14 to 9.53 wt.%. Ru content in the modified samples is lower than the specified value by 6–10%. Different concentrations of Со and Ru are related to the presence of impurity anions and water in the catalysts. According to SEM and TEM data for the catalysts used in the process, cobalt and ruthenium are distributed nonuniformly in the granule. A more detailed discussion is

Conclusion

The promotion of cobalt-containing catalysts supported on δ-Al2O3 with ruthenium from the fac-Ru(NO)(NH3)2(NO3)3 complex provides a substantial decrease in their reduction temperature. Reductive activation under the conditions ensuring the formation of a comparable fraction of metallic cobalt increased selectivity of the promoted catalysts for α-olefins and high-molecular hydrocarbons without neither considerable loss, nor an improvement in the catalytic activity. Therefore, the observed in

Acknowledgements

This work was conducted within the framework of budget project No. 0303-2016-0013 for Boreskov Institute of Catalysis. The authors are grateful to Prof. V.A. Emelyanov (Novosibirsk State University) who afforded fac-Ru(NO)(NH3)2(NO3)3 complex for the study and Dr. N.A. Rudina for SEM and EDAX studies.

References (63)

  • Ø. Borg et al.

    J. Catal.

    (2008)
  • L. Shi et al.

    Catal. Today

    (2014)
  • G. Jacobs et al.

    Appl. Catal. A Gen.

    (2007)
  • P. Arnoldy

    J. Catal.

    (1985)
  • N. Tsubaki et al.

    J. Catal.

    (2001)
  • W. Ma et al.

    Appl. Catal. A Gen.

    (2012)
  • G. Jacobs et al.

    Appl. Catal. A Gen.

    (2002)
  • S.H. Song et al.

    Catal. Commun.

    (2008)
  • A. Kogelbauer et al.

    J. Catal.

    (1996)
  • J.Y. Park et al.

    J. Mol. Catal. A Chem.

    (2011)
  • M.J. Parnian et al.

    Appl. Surf. Sci.

    (2014)
  • K.M. Cook et al.

    Appl. Catal. A Gen.

    (2012)
  • J.H. Scofield

    J. Electron Spectros. Relat. Phenomena

    (1976)
  • J.W. Evans et al.

    Appl. Catal.

    (1983)
  • W.L. Roth

    J. Phys. Chem. Solids

    (1964)
  • S. Kurajica et al.

    Mater. Chem. Phys.

    (2012)
  • A. Boumaza et al.

    J. Solid State Chem.

    (2009)
  • J. Preudhomme et al.

    Spectrochim. Acta Part A Mol. Spectrosc.

    (1971)
  • A.A. Khassin et al.

    J. Mol. Catal. A Chem.

    (2001)
  • N. Kosova et al.

    Solid State Ionics

    (2008)
  • J. Mendialdua et al.

    J. Mol. Catal. A Chem.

    (2005)
  • A.A. Khassin et al.

    J. Mol. Catal. A Chem.

    (2001)
  • M.C. Biesinger et al.

    Appl. Surf. Sci.

    (2011)
  • H. Xiong et al.

    Fuel Process. Technol.

    (2009)
  • M. Hu et al.

    J. Catal.

    (2004)
  • M.H. Kim et al.

    Cat. Comm.

    (2007)
  • M. Reinikainen et al.

    Appl. Catal. A: Gen.

    (1998)
  • K.S. Kim et al.

    J. Catal.

    (1974)
  • V. Mazzieri et al.

    Appl. Surf. Sci.

    (2003)
  • L.A. Pedersen et al.

    J. Catal.

    (1980)
  • Ş. Sayan et al.

    J. Mol. Struct.

    (1997)
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