Pt, Ir and Pd promoted Co/MSU catalysts for hydrotreating of tetralin: A thiotolerance study

https://doi.org/10.1016/j.apcatb.2006.11.018Get rights and content

Abstract

Cobalt based catalysts doped with noble metals (Pt, Ir or Pd) supported on Zr-MSU type materials were studied in the hydrogenation and hydrogenolysis/hydrocracking of tetralin at different temperatures. Sulphur tolerance was studied in order to evaluate the possible application of these catalytic systems in second stage processes. The catalytic test was carried out in a high-pressure fixed-bed continuous-flow stainless steel catalytic reactor operating at a pressure of 6.0 MPa. Textural, structural, acidic and metallic properties were studied by XRD, XPS, H2-TPR, NH3-TPD, O2 chemisorption measurements, TEM and elemental chemical analysis. Three catalysts were prepared with 10 wt% of cobalt and 0.5 wt% of noble metal (Pt, Pd or Ir). Moreover, other three monometallic noble based catalysts (0.5 wt% of noble metal) were prepared to demonstrate the improvement found in the bimetallic ones. The results indicate that the presence of a noble metal in cobalt catalysts, especially Pt, improves their catalytic properties in hydrogenation reactions with or without dibenzothiophene (DBT) in the feed by operating not only at low contact times, but also at low H2/THN molar ratios. The conversion values and the yield of hydrogenation products were higher for noble metal doped catalysts at all temperatures studied; meanwhile, the yield of hydrocracking and hydroisomerization compounds is appreciable at higher temperatures, especially for platinum and iridium catalysts, despite the formation of considerable amounts of undesirable naphthalene. The platinum promoted catalyst displays very good properties when DBT is added to the feed. After 10 h on stream, with 600 ppm of DBT in the feed and at 315 °C, it was found to maintain its high conversion of tetralin (90.5%) and a yield of hydrogenation products of 82.2%.

Introduction

The worldwide environmental policy, both current and future, is imposing a reduction of diesel fuel exhaust emissions on the industrial sector. This is specifically aimed at NOx, hydrocarbons, CO, sulphur and particulate matter. Diesel exhaust particles are the major constituent of urban carbonaceous pollution, considered as causing a wide range of adverse environmental and health effects. Health risks are of particular concern as the particles are small and prone to be inhaled, provoking respiratory and cardiac diseases. These particles possess a carbonaceous centre, coated in a soluble organic fraction, being mainly formed from heavy and condensed hydrocarbons, and water-soluble ions in the form of sulphates.

It is important to note that allergy cases are on the rise, in spite of relatively constant historical pollen concentrations, as shown by the data accumulated over the last 25 years. Further, it is known that the incidence of allergies recorded in urban areas is now considerably higher than those recorded in rural ones [1], [2], [3]. Since a diesel engine produces six times more microscopic particles than a gasoline engine, and that diesel vehicle sales have recently increased notably due to being more economical, durable and efficient, we are led to suppose that the environmental pollution caused by these vehicles is responsible for this problem. Pollen can accumulate coats of diesel exhaust particles, aggravating the damage that pollen alone causes. Diesel particles also have a negative effect on local air quality and the global climate, due to the involvement of carbonaceous particles in several atmospheric chemical processes. Their role in radiation absorption results in a rise in atmospheric temperature.

The amount of this particulate matter in the environment is strongly dependant on fuel quality, in the view of its effects, considerable improvements need to be achieved in terms of diesel emissions. This should mainly be tackled by fuel reformulation to reduce aromatic and sulphur content [4].

The two-step process has been the preferred choice for upgrading diesel fuel quality in hydrotreating technology. In the first step of this process, a catalyst reduces the sulphur and nitrogen content, and in the second step a catalyst capable of hydrogenating aromatic molecules is applied to the sulphur and nitrogen free-fuel. The second step catalysts use noble metals as an active phase. However, the main drawback is their low sulphur tolerance, which is present in the fuel, which results from the first stage. Therefore, second step catalysts should not only display good hydrogenation properties, but also be capable of resisting the poisoning effect of small amounts of sulphur coming from the first step. A great variety of studies have centred on this problem by trying to modify the physical–chemical properties of the metallic ions via the addition of a second metal, or by employing an acidic support, which provokes an electronic deficiency in the metallic particles, thus reducing the bond strength with sulphur compounds [5], [6], [7]. In this way, different supports have been tested, not only with different acidic properties, but also with different pore sizes. Yashuda et al. [8] have reported the higher thiotolerance of Pt and Pd catalysts when a zeolitic support is used. Besides this, MCM-41, Al-MCM-41, amorphous silica-alumina, USY zeolite, silica and γ-alumina have also been tested in naphthalene hydrogenation [9], the most acidic support being shown to be the most thiotolerant. Improved hydrogenating properties, due to higher platinum dispersion, were found with the mesoporous materials. Further, the addition of a second metal has proved a subject of great interest since the second metal could modify the catalytic properties of the former as a result of electronic and structural effects, together with the particle size and dispersion. The addition of metals, such as Co, Mo, Ni, Re, Ag and Pd to a γ-Al2O3 supported Pt catalyst has revealed that the highest thiotolerance was that displayed by the PdPt catalyst [10]. Indeed, Hu et al. [11], after adding Cr, W, La, Mn, Mo and Ag to Pd/HY-Al2O3, observed that the Pd performance was modified in all cases, with the Cr-Pd catalyst giving the best thiotolerant properties. Conversely, the Pd-Ag catalyst suffered a rapid deactivation in the presence of sulphur as the Ag increased the Pd electronic density, therefore making the Pd more vulnerable to sulphur poisoning. On the other hand, Reinhoudt [12] have indicated that the prevention of active phase agglomeration is the main factor in the addition of Pd to Pt catalysts. The good performance of bimetallic PdPt catalysts has also been reported by Rodríguez-Castellón et al. [13] and Jacquin et al. [14] in the hydrogenation of tetralin with the presence of DBT as a poisoning agent. Other noble active phases with good results in this kind of reaction have been Ru-Os [15].

Although noble metals have been the main active phases studied, given their good hydrogenating properties [16], new alternatives have been studied in order to reduce the cost of catalysts. For this reason, molybdenum and tungsten doped nickel catalysts [17], [18], [19] have been reported as giving interesting results in the hydrogenation and ring opening of naphthenes in the presence of sulphur compounds, with nickel having become an interesting alternative to noble metal catalysts.

Recently, Co based catalysts [20], [21] supported on an acidic and mesoporous support (Zr-MSU type material) have been tested in this type of reaction. This Zr-MSU material was prepared by using a non-ionic surfactant as template, polyethyleneoxide, these surfactants are biodegradable, cheaper than ionic surfactants and therefore they are promising templating agents for the synthesis of new porous materials. The Zr-MSU materials have demonstrated to be stable at high temperatures, and possess good stability in aqueous media, mechanical resistance and under steaming conditions. Their high BET surface areas and high acidities make them excellent candidates to be used in heterogeneous catalysis as both acid catalysts and supports. These cobalt based catalysts have displayed good catalytic properties in the hydrogenation of tetralin, especially molybdenum doped cobalt catalysts, where the cobalt loading in the catalyst is seen to be important for achieving a good catalytic performance, with or without sulphur compounds in the feed. Given the encouraging results previously obtained with cobalt catalysts and the hydrogenation properties of noble metals, which are fully documented in the literature, bimetallic cobalt-noble metal catalysts supported on an acidic support have been prepared in this study with the principle aim of obtaining good thiotolerance properties when exposing them to different concentrations of DBT.

Section snippets

Preparation of catalysts

Zirconium doped mesoporous silica (Zr-MSU) with a molar ratio Si/Zr = 7 was used as the support, and prepared as described elsewhere [22]. Bimetallic catalysts with cobalt and noble metals (Pt, Ir or Pd) were prepared by the incipient wetness method. The support was pelletized (0.85–1.00 mm) before being impregnated with an aqueous solution of both precursor salts, i.e. an aqueous solution containing both the cobalt salt, Co(NO3)2·6H2O, and the noble metal salt, [Pt(NH3)4](NO3)2, IrCl3·xH2O or

Catalyst characterization

A characterization of the catalysts was carried out for a better understanding of the catalytic results. The results obtained from X-ray diffraction measurements of the precursor samples at high angles give evidence, in all cases, of the formation of cobalt spinels, as indicated by the diffraction lines at 31.3°, 36.8°, 44.8°, 59.4° and 65.2°. Apart for the lines corresponding to cobalt spinels, only the Pd-containing precursor shows a small peak at 2θ = 33.8°. This is due to the presence of

Conclusions

A new series of catalysts based on noble metals have been prepared with the main aim of obtaining thiotolerant catalysts to be used in second stage processes of mild-hydrotreating. A family of cobalt catalysts doped with platinum, palladium and iridium was tested in the hydrogenation and hydrogenolysis/hydrocracking of tetralin, in the presence of DBT acting as a sulphur poison.

After a profound characterization, the catalytic results reveal the good performance of these bimetallic catalysts,

Acknowledgements

We gratefully acknowledge the Ministerio de Ciencia y Tecnología (Spain) (Project MAT2003-02986 and MAT2006-02465) and EU Commission's GROWTH Program (Contract G5RD-CT-2001-0537) for funding this work. A.I.M. also thanks the Ministerio de Ciencia y Tecnología (Spain) for a fellowship.

References (48)

  • L.J. Simon et al.

    J. Catal.

    (2001)
  • J. Wang et al.

    Appl. Catal. A

    (1998)
  • A. Corma et al.

    J. Catal.

    (1997)
  • L. Hu et al.

    J. Mol. Catal.

    (2001)
  • E. Rodríguez-Castellón et al.

    Appl. Catal. A

    (2004)
  • M. Jacquin et al.

    J. Catal.

    (2004)
  • D. Eliche-Quesada et al.

    Appl. Catal. A

    (2005)
  • U. Nylén et al.

    Appl. Catal. A

    (2006)
  • R. Hernández-Huesca et al.

    J. Catal.

    (2001)
  • D. Eliche-Quesada et al.

    Appl. Catal. A

    (2004)
  • A. Infantes-Molina et al.

    Appl. Catal. A

    (2005)
  • A. Infantes-Molina et al.

    J. Catal.

    (2006)
  • A. Infantes-Molina et al.

    Micropor. Mesopor. Mater.

    (2004)
  • C.H. Bartholomew et al.

    J. Catal.

    (1976)
  • F.B. Noronha et al.

    J. Catal.

    (1999)
  • H. Tian et al.

    Appl. Catal. A

    (2001)
  • A. Boix et al.

    J. Catal.

    (2003)
  • M. Meng et al.

    Appl. Catal. A

    (2006)
  • D. Pietrogiacomi et al.

    Appl. Catal. B

    (2000)
  • N. Li et al.

    J. Catal.

    (2004)
  • R. Moreno-Tost et al.

    Appl. Catal. B

    (2004)
  • Y. Yazawa et al.

    J. Catal.

    (1999)
  • B. Pawelec et al.

    Appl. Surf. Sci.

    (2005)
  • P. Reyes et al.

    J. Mol. Catal.

    (2000)
  • Cited by (29)

    • Bimetallic platinum/iridium modified mesoporous catalysts applied in the hydrogenation of HMF

      2021, Catalysis Today
      Citation Excerpt :

      In the case of PtIr bimetallic catalysts, the synergistic effect between Pt and Ir are crucial factors to influence the catalytic performance of catalysts. The characteristic of PtIr bimetallic nanoparticles could promote the catalyst to adsorb CO bond and, thereby, improve the selectivity of 2,5-DMF [43]. The rapid passage of BHMF to 2,5-DMF through MFA requires a site that promotes hydrogenolysis and this feature is provided in this case by Ir in the bimetallic alloy, as an example of the strong tendency of metallic iridium to catalyze hydrogenolysis reactions [43].

    View all citing articles on Scopus
    View full text