Catalysts based on Co/zirconium doped mesoporous silica MSU for the hydrogenation and hydrogenolysis/hydrocracking of tetralin

Abstract

Zirconium doped mesoporous silica (MSU) has been used as a support of Co catalysts with different Co-loadings. These catalysts have been characterised by several techniques such as XRD, XPS, textural properties, H₂-TPR, NH₃-TPD, chemisorption of H₂, DRIFTS and FT-IR of adsorbed NO. The performance in the hydrogenation and hydrogenolysis/hydrocracking of tetralin at different temperatures has been examined in a high-pressure fixed-bed continuous-flow stainless steel catalytic reactor operating at 6.0 MPa of pressure. The incorporation of cobalt by using the incipient wetness method with an aqueous solution of cobalt nitrate is effective for preparing catalysts with 5–20 wt.% Co. The behaviour of these catalysts demonstrates that the higher the metallic loading, the contact time between catalyst and reactants, and the H₂/tetralin molar ratio, the better the results with conversions in the steady-state near to 100% and better the yields of hydrogenation and hydrogenolysis/hydrocracking products. Thus, the catalyst with 15 wt.% of Co, operating at 315 °C, with a contact time of 3.6 s and an H₂/tetralin molar ratio of 15, gives rise to a conversion of tetralin of nearly 100%, yields of hydrogenation of 71%, and of hydrogenolysis/hydrocracking of 28% after 6 h on-stream. In addition, the thiotolerance of these catalysts has been tested with a feed of 425 and 850 ppm of dibenzothiophene (DBT). With 425 ppm of DBT, the performance of the best catalyst is only reduced by 17% after 5 h on-stream. However, with 850 ppm of DBT, a dramatic decrease in the activity is observed, although this activity can be recovered after new thermal and reduction treatments.

Introduction

The materials designed as mesoporous molecular sieves have given new perspectives to heterogeneous catalysis. This is due to the fact that in any catalytic process a high specific surface area or high active phase dispersion, together with a fast mass transfer of the reactants and products to and from the catalytic sites, is required. Indeed, mesoporous materials have a very high specific surface area, uniform pore diameters and a stereoregular arrangement of their channels. Due to these properties, since this family of materials was first reported by Mobil's researchers [1], [2], the literature devoted to their catalytic applications increased sharply. An account of these developments can be found in a recent review by Trong-On et al. [3].

The hydrotreatment of petroleum fractions is an example of the application of these materials in catalysis. This is due to the need to treat feeds with increasingly large molecules, higher sulfur and nitrogen content, and the hydrogenation of aromatics to obtain distillates, which must be adapted to comply with the more and more stringent regulations which, in many countries, will further lower the contents of sulfur and aromatics for the period 2004–2007 [4]. Sulfur content in diesel fuel is an environmental concern due to the SO_x formed upon combustion, which contributes to acid rain and poisons the TWC converters for exhaust emission treatment. At the same time, a high aromatic content in distillate fuels lowers the fuel quality and contributes significantly to the formation of environmentally harmful emissions [5], [6]. Aromatic saturation by catalytic hydrotreating can increase the cetane number significantly and improve combustion properties [7]. Recently, Song and Ma [8] have reviewed deep desulfurization and deep dearomatization of diesel fuels concluding that for developing affordable ultra-clean fuels many aspects of the catalytic process are important to the formulation of new catalysts and to the study of processing and engineering.

There are many papers describing the use of mesoporous molecular sieves such as MCM-41, FSM-16 and SBA-15 as a support and the role played by their porosity in the degree of metal dispersion and metal reduction and the corresponding effect on catalytic behaviour [9], [10], [11], [12]. For economic and technological reasons, the impregnation method is one of the most widely adopted in the industry to prepare a catalyst supported on a high surface area material to improve the accessibility of the active phase [13]. The impregnation of a mesostructured support with an active phase increases the number of exposed atoms and hence the activity.

On the other hand, the use of a more acidic zirconium-doped mesoporous silica as a support has shown to improve the activity in many catalytic processes such as the hydroconversion of tetralin in catalysts based on Ni [14], [15], Pt, Pd, Rh and Pd–Pt [16]. However, very recently a new synthetic approach was reported [17], whereby a low cost zirconium doped mesoporous silica (SiZr7) with high surface area and acidity was prepared from precursors different from those of alcoxides and a non-ionic surfactant template.

As for the use of new catalysts, hydrotreating is typically carried out in a two-stage process where bimetallic (CoMo, NiMo, NiW) supported catalysts, with high activity in HDS reactions, are used in the first stage, and monometallic ones (Ni, Pd, Pt), with a high hydrogenation activity, in the second stage, and where the feeds have a much lower S-content [18]. Cobalt-based catalysts are known to be effective in reactions involving H-transfer. Thus, the best application known of cobalt-based catalysts is the Fischer-Tropsch reaction [19], [20], [21]. These catalysts have also been studied in aromatic hydrogenation [22], [23], [24], but essentially with the emphasis on known catalyst structures. A rapid decay of the activity to zero was observed when the reduction temperature were increased from 400 to 600 °C, which could be due to the encapsulation of Co crystallites migrating on the support at reduction temperatures higher than 500 °C.

In the present study, zirconium-doped mesoporous MSU silica is used as a cobalt-based catalyst support to perform the second stage of the hydrotreating process using tetralin as a molecule probe. The influence of the Co-content and experimental conditions on the hydrogenation and hydrogenolysis/hydrocracking of tetralin is studied, as well as the influence on catalytic activity of high amounts of DBT acting as a sulfur pollutant source.