Hydrodesulfurization of hindered dibenzothiophenes: an overview

doi:10.1016/S0920-5861(00)00308-4

Catalysis Today

Volume 59, Issues 3–4, 25 June 2000, Pages 423-442

https://doi.org/10.1016/S0920-5861(00)00308-4 Get rights and content

Abstract

Hydrodesulfurization is a well-documented process which has been commonly used in the refining of crude oil for over 60 years. It is a process for which interest is frequently renewed due to the requirement to use new feedstocks and the application of more severe environmental legislation, for example, the need to reduce sulfur levels in fuels. Of particular importance in achieving low sulfur levels in fuels is the problem posed by a particular class of compounds, namely hindered dibenzothiophenes, e.g. dibenzothiophene, 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene. Dibenzothiophenes demonstrate resilience to hydrodesulfurization using current catalyst formulations. This overview addresses the key area of hydrodesulfurization chemistry concerning the desulfurization of highly hindered sulfur containing molecules.

Introduction

The industrial practice of oil refining is continually changing. These changes can be either due to quality of the feeds, or due to the standards to which the products are manufactured. Both cause the refining industry to react, either by improving old technologies or by inventing new ones. For example, in the USA the average crude oil utilized has changed to a poorer quality with respect to sulfur content since sweeter USA crudes (average S=0.87 wt.%) have been largely replaced by sourer South American crudes (average S=2.20 wt.%). In addition, the overall sulfur content of crudes is also expected to rise in the USA due to increasing imports. In Western Europe, crude oil quality has remained fairly constant over the last 10 years (average S=1.02 wt.%). However, it can be anticipated that the same trend towards higher sulfur crude oils will occur due to the decreasing availability of North Sea crude (average S=0.23 wt.%).

As well as the variation in the quality of oil feedstocks, the refining industry is also under constant pressure to meet more stringent standards with respect to product specification. These specification changes are driven by environmental concerns and are summarized in Table 1 [1].

Since the amount of sulfur in crudes encountered by the refiner is expected to increase in Western Europe as well as in the USA, it can be clearly deduced that oil refining technologies, in particular desulfurization, will have to react to these changes. There is, therefore, a considerable interest in investigating the already well-documented process of hydrodesulfurization (HDS), particularly the HDS of hindered, less reactive S containing molecules. This is because highly hindered sulfur containing molecules, such as dibenzothiophenes, are not particularly reactive under typical HDS reaction conditions. Industrial research is, therefore, aimed at improving catalyst activity with respect to the hydrogenation, and successive desulfurization of relatively unreactive sulfur containing molecules. This article aims to overview aspects of the literature concerning the HDS of hindered, less reactive sulfur containing molecules, since it is these molecules that must be desulfurized if future fuel specifications are to be met.

Section snippets

The HDS reaction

The HDS reaction for thiophene is shown below:

HDS is the process by which sulfur is removed from sulfur containing compounds by reaction with hydrogen, thereby forming H₂S. The process is widely employed throughout the world and has been used for over 60 years. The reaction has recently been extensively reviewed by Whitehurst et al. [2]. It is a catalyzed reaction usually involving a metal sulfide catalyst, in particular sulfided Co/Mo/Al₂O₃ or sulfided Ni/Mo/Al₂O₃. The resultant H₂S that is

Reaction pathways for thiophenes

The majority of thiophene HDS studies have been performed at atmospheric pressure. It has been suggested that thiophene HDS proceeds via two parallel pathways [8]. One proposition is that hydrogenolysis of the C–S bond precedes hydrogenation of the aromatic ring [9]. Alternatively, Kolboe [10] proposed an intramolecular dehydrosulfurization pathway, whereby the hydrogen in the H₂S product originates from positions β to the sulfur atom of the thiophene. Studies by Lipsch and Schuit [11] proposed

Co/Mo/Al₂O₃ and Ni/Mo/Al₂O₃ catalysts

Kilanowski et al. [27] carried out a study of HDS of methyl substituted BT and DBT with methyl substitution in different positions. The different compounds were 3-methylbenzothiophene, 3,7-dimethylbenzothiophene, 2-methylbenzothiophene, 7-methylbenzothiophene, 2,3-dihydro-benzothiophene, 4-methyldibenzothiophene, 2,8-DMDBT and 4,6-DMDBT. The catalytic experiments were carried out in a pulse microreactor at temperatures between 350 and 450°C and the results are given in Table 6 for a Co/Mo/Al₂O₃

Conclusions

The general findings for the studies of the HDS reaction of hindered dibenzothiophenes over Co/Mo/Al₂O₃ and Ni/Mo/Al₂O₃ catalysts presented in this paper are considered to be very consistent. A range of experimental conditions has been examined in the broad range of studies and these are summarized in Table 13. When compared to the reactivity of unsubstituted dibenzothiophene and other substituted dibenzothiophenes, the order of reactivity is now acknowledged as follows: $2,8- DMDBT > DBT >4- MDBT >4,6-$

Acknowledgements

The authors wish to thank Dr. Keith Holder and Dr. Reg Gregory (BP Amoco, Sunbury) for useful discussions, advice and support in the preparation of this paper.

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Hydrodesulfurization of hindered dibenzothiophenes: an overview

Abstract

Introduction

Section snippets

The HDS reaction

Reaction pathways for thiophenes

Co/Mo/Al2O3 and Ni/Mo/Al2O3 catalysts

Conclusions

Acknowledgements

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Co/Mo/Al₂O₃ and Ni/Mo/Al₂O₃ catalysts