Thiophene hydrodesulfurization over noble metal modified Co-clay catalysts

https://doi.org/10.1016/j.apcata.2004.05.016Get rights and content

Abstract

This paper presents the effects of noble metal modification of Co-HPS (high porous saponite) on thiophene hydrodesulfurization. The modification was done by two different ways: (I) single noble metal and (II) bi-noble metal. The noble metal loading was varied from 0.1 to 2 wt.% of the catalyst. In catalyst preparation direct impregnation technique was used for the loading of noble metal and an ion-exchange was followed for Co loading on HPS. The catalysts were characterized by elemental analysis and temperature programmed reduction (TPR). Elemental analysis shows that the Co loading was slightly affected by noble metal modification. TPR study displayed that the reducibility of the catalysts was enhanced in presence of noble metal(s). Thiophene hydrodesulfurization activities were measured in a pulse micro reactor. The noble metal modified catalysts exhibited significantly higher activity as compared to the Co-HPS. Rh shows the highest activity among the three noble metals used for Co-HPS modification. The thiophene hydrodesulfurization activity was further enhanced by a bi-noble metal modification. The Pt-Rh combination shows better performance as compared to the Pd-Rh combination.

Introduction

Hydrodesulfurization probably is the most important hydrotreating process in petroleum refining industry to produce the clean fuels. Recent trend of environmental regulation is becoming increasingly stringent with regards to sulfur content in fuels. Therefore, the demand for efficient hydrotreating process/catalysts becomes more evident. Due to the fact, the hydrotreating catalyst development research has been significantly increased for the last decade to obtain more effective and stable catalysts towards the production of sulfur-free fuels. Many of such recent works are devoted to develop new hydrodesulfurization catalysts to obtain more efficient sulfur removal than the conventional CoMo/Al2O3 catalysts [1].

The key objective in developing a successful catalyst is to achieve the targeted sulfur level under milder reaction conditions [2]. In conventional catalysts suitable transition metals from Group VIB (Mo or W) and/or VIII (Co or Ni) have been used as the active species supported on Al2O3 or zeolites [3], but the demand for more efficient catalyst is ahead to meet the further reduction (possibly to zero) of sulfur level in the coming years. Along with the conventional supports and active metals a number of different support materials and active metals are being explored to achieve the goal. Titina is one of the interesting support materials that has shown some unique performances in residual oil hydrotreating [4]. Silica is also found to be an alternative support, when Ni was deposited on it, the catalysts showed excellent activity and stability in thiophene hydrodesulfurization [5]. The clay minerals also are found to be interesting as a support for hydrotreating catalysts. Comparable (to commercial catalysts) hydrodesulfurization activity was found when Co and Ni pillared high surface area saponite clays were exposed to vacuum gas oil feed stock into a batch reactor [6]. Mesoporous smectite clays have been reported to be very promising for hydrodesulfurization catalysts [7]. Co containing smectite clay displayed high activity in hydrodesulfurization of thiophene [8]. Recently, high porous saponite supported (HPS) Co catalyst reported to be very promising for both hydrocracking and hydorodesulfurization of vacuum gas oil [3].

In the present work, we also have explored the possibility of Co-clay catalysts modified with noble metals for the purpose of hydrodesulfurization. It has been reported that noble metal promoted Co-HPS demonstrated a significant increased activity in treatment of vacuum gas oil in a flow reactor [9]. The present authors also tested the noble metal modified Co-HPS catalysts in a batch autoclave reactor using vacuum gas oil (VGO) as feedstock. Both hydorcracking and hydrodesulfurizarion activities of Co-HPS were significantly increased when the catalysts were modified with a trace amount of Pt-Rh [10]. In this communication we will report the effects of modification of Co-HPS on thiophene hydrodesulfurization performances by single noble metals like Pt, Pd and Rh and their combinations such as Pt-Rh and Pd-Rh.

Section snippets

Catalyst preparation

Before catalyst preparation the high porous saponite clay (received from Kunimine Kogyo Co. Ltd., Japan) was calcined at 600 °C for 4 h. For catalyst preparation, at first the support was modified by noble metal(s) by direct impregnation method using noble metal chloride aqueous solutions. Then it was dried at 120 °C followed by calcinations at 600 °C over night. The Co loading was done by ion-exchange technique, as described below.

Co(NO3)2·6H2O solution was prepared and aged at 80 °C for 2 h. The

Elemental analysis

For bifunction catalysts like HDS and hydorcracking catalysts it is very important to balance the acidity of the support and the amount of metal loading on the catalyst in order to obtain the optimum performance. The element analysis was carried out to have a clear knowledge about the composition of different metals in the fresh catalysts; in particular, the effects of the noble metal on the loading of Co, which is the main active component of the catalyst. Table 1 lists the elemental analysis

Conclusion

The addition of noble metal(s) affects the Co loading on high porous saponite. The reducibility of Co-HPS was significantly enhanced with modification of catalysts by noble metals. Pulse reactor evaluation revealed that all the Pt, Pd and Rh modified catalysts showed higher thiophene hydrodesulfurization and Rh showed the best activity among them. The bi-noble metal promoted catalysts also have shown some interesting results, a trace amount of Rh is sufficient to modify activity of Co-HPS HDS

Acknowledgements

The authors wish to acknowledge Center for Refining and Petrochemicals, The research Institute, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia and Petroleum Energy Center, Japan, for supporting this work under the KFUPM-RI project no. 21151.

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