The Relation of Ni/ZnO Nano Structures With Properties of Reactive Adsorption Desulfurization

Introduction

The desulfurization is one of the most important processes in the petroleum refining industry. The conventional technology is hydrodesulfurization (HDS) where molybdenum or tungsten supported on alumina support promoted with cobalt or nickel is used as a catalyst (Babich and Moulijn 2003). Recently, many governments make more and more stringent regulations controlling the sulfur content in transport fuels. For example, in the Europe V standard, the sulfur content in gasoline and diesel is limited to below 10 ppm. The HDS process faces a serious challenge to achieving this objective. Thus other desulfurization technologies have been vividly investigated. Among them, reactive adsorption desulfurization (RADS) emerges as a perspective field, where a sorbent is used to remove the sulfur impurity from oil stream and keeps the sulfur in the sorbent by chemical reaction (Ge et al. 2016). The great challenge for RADS technology is to develop sorbent with high desulfurization activity and high sulfur capacity. Nano Ni/ZnO sorbent is found as an ideal candidate for this objective.

Ni/ZnO was considered one of the most excellent sorbents for the RADS. The metallic Ni can strongly adsorb sulfur, meantime, the sulfur is difficult to be removed. In the Ni/ZnO system, the Ni captures sulfur from the sulfur-containing compound in feed forming NiS_x, then the sulfur is transferred to the ZnO component in the presence of hydrogen, and the Ni active center is regenerated, while the hydrocarbon portion of the sulfur-containing compound is released back into the process stream (Tawara et al. 2000, 2001a). Due to the dual identities of Ni/ZnO, namely, both as a catalyst and as sorbent, it has been called different names in references, such as “catalyst”, “sorbent”, or “adsorbent” etc.. In this chapter, for the consistency, we adopt the term “sorbent”. To RADS sorbent, there are three key specifications, one is the activity of desulfurization, another is the sulfur capacity, namely possible adsorbed sulfur quantity when sulfur content is kept below some content, and the third is the regeneration ability. Many methods have been used to improve the reaction properties of RADS sorbents. By controlling the nano morphology and size of Ni/ZnO sorbent, the activity and sulfur capacity can be remarkably improved (Zhang et al. 2012a). Addition of Mn forms the new phase of ZnMnO₃, which enhances activity and renewability (Zhang et al. 2013). Substitution ZnO component with MnO increases the sulfur capacity (Tang et al. 2015). Introduction of a second metal, such as Co, Fe, Cu, Silver, can inhibit the hydrogenation to olefins (Khare 1999, Gyanesh 2001).

Ryzhikov et al. (2008) observed that NiO/ZnO can be reduced directly in situ and shows the better RADS of thiophene than the pre-reduced counterpart. They suggested that the H₂ pretreatment results in the formation of Ni-Zn alloy and agglomeration of nano particles, leading the decrease of activity. However, the pre-reduction to NiO/ZnO-SiO₂-Al₂O₃ sorbent can improve the desulfurization capability (Fan et al. 2010), which is attributed to the additives of alumina and silica stabilizing the particles (Wen et al. 2012, Meng et al. 2013). Decreasing the size of ZnO nano particles leads to efficient contact between Ni and ZnO particles and enhances the desulfurization ability and sulfur adsorption capacity (Zhang et al. 2012a). But small ZnO particles are not stable under the calcination or reaction conditions, the sinter of particles lead a noticeable drop of activity (Bezverkhyy et al. 2008). Thus structure additives, such as diatomite, perlite, attapulgite, silica sol, pseudo-boehmite or their mixtures are added to reinforce the texture of nano sorbent (Shangguan et al. 2013, Zhou et al. 2013).