W-SBA based materials as efficient catalysts for the ODS of model and real feeds: Improvement of their lifetime through active phase encapsulation

Highlights

• Incorporated HPW-SBA catalysts are more resistant toward deactivation in oxidative desulfurization of real fuel than impregnated ones.

• Incorporated HPW-SBA catalyst showed enhanced interaction between supported phase and carrier as shown by Tof-SIMS analysis.

• No deactivation was observed for 9 days on incorporated HPW-SBA catalyst in the ODS of a SRGO 2000 ppmS.

• This preparation method appears promising to reduce deactivation in the ODS of real feeds.

Abstract

Incorporated W-SBA based catalysts were prepared by a direct synthesis method involving phosphotungstic acic (HPW) together with mixed structure directing agents, and by incipient wetness impregnation for comparison purpose. In the oxidative desulfurization (ODS) of dibenzothiophene (DBT, 500–1500 ppmS) and of a Straight Run Gas Oil (SRGO, 2000 ppmS) in batch reactor, the impregnated solids showed higher efficiency than the incorporated ones, which was attributed to a better accessibility of the active phase present at the surface of the support in the impregnated catalysts. However, in continuous evaluation using a fixed bed reactor, the incorporated catalyst showed a stable conversion in the ODS of SRGO 2000 ppmS for 9 days while within 24 h the impregnated solid was almost completely deactivated. Higher interaction of the active phase with the support in incorporated catalysts was evidenced by Time-of-Flight Secondary Ions Mass Spectrometry (ToF-SIMS) analysis, which can be at the origin of less leaching of tungsten species during reaction and thus explain their better resistance towards deactivation. The lower efficiency of incorporated catalysts compared to impregnated ones in ODS reaction carried out in a batch reactor is clearly compensated by their resistance towards leaching of the active phase in a continuous test. These results highlight the importance of evaluating not only the catalysts performance but also their lifetime in conditions more representative of industrial ones.

Introduction

Hydrodesulfurization (HDS) is the main industrial process for the removal of undesirable sulfur containing compounds present in petroleum feedstocks. However the continuous tightening of environmental regulations concerning the sulfur content in diesel led to consider and develop complementary and/or alternative solutions, among which oxidative desulfurization (ODS) is arousing an increasing interest [[1], [2], [3]]. In the ODS process, the sulfur containing molecules are oxidized in sulfones, which, due to their polar properties, can be further separated from the reaction mixture by extraction or adsorption [[4], [5], [6]]. One major advantage of this reaction is that it is carried out under mild conditions regarding temperature and pressure and without the use of costly hydrogen [7]. Moreover, refractory compounds in HDS such as 4,6-dimethyldibenzothiophene (4,6-DMDBT) are more readily converted in the ODS process than benzothiophene (BT), which has been related to the higher electron density on the sulfur atom in DBT than in BT [[8], [9], [10], [11]].

Polyoxometalates (POM) have been extensively used as ODS catalysts in homogeneous systems, in particular Keggin-type heteropolyacids (HPA), like phospho(silico)tungstic or molybdic acids [[12], [13], [14], [15]]. Due to the difficulty associated to catalyst recovery, heterogeneous systems are however preferred, with dispersion of the HPA at the surface of a carrier. Mesoporous silica have been reported as suitable supports for ODS catalysts, due to their large surface area and high stability, like HMS [16], MCM-41 [17,18] and SBA [19,20]. However these impregnated catalysts suffer from strong deactivation, which major sources have been identified as sulfones adsorption on the catalysts as well as leaching of the active phase during reaction [15,21]. It is thus of paramount importance to optimize not only the efficiency of a catalyst but also its resistance regarding deactivation. If sulfones retention on the catalyst surface is important in the ODS of model molecules, it is much less present in the ODS of real feedstocks, as sulfones are more soluble in these media [19]. Concerning the active phase leaching, several strategies aiming at the stabilization of the active phase have been developed, either by anchoring the HPA on a modified surface or through encapsulation within the silica framework by direct synthesis. For instance phosphotungstic acid (HPW) has been immobilized on the surface of MCM-41 [22,23] and SBA-15 [24] functionalized by amino groups and in all the cases the obtained catalysts could be reused without significant loss of activity in the ODS of model compounds. However modification of the support requires extra steps in the catalyst synthesis which may be detrimental to the overall cost of the process. Alternatively one-pot synthesis approaches have been developed for the incorporation of polyoxometalates, introduced during the silica support preparation [[25], [26], [27], [28], [29]]. Several authors reported on the use of such one-pot catalysts in ODS reaction [[30], [31], [32], [33], [34], [35]] but only a few addressed the reusability of the catalytic solids. With one exception [32], the majority of the authors reported the preservation of the catalytic efficiency after several runs [33,[31], [32], [33], [34], [35]]. Interaction of HPW with the support was claimed to be responsible for this activity stabilization, but only Du et al. brought experimental evidence by FT-IR of this interaction [33].

In this work we propose to evaluate the efficiency and the lifetime in ODS of incorporated HPW catalysts with various HPW contents, prepared by a direct synthesis method in the presence of poly-(ethylene oxide)-poly(propylene oxide) copolymer (Pluronic P123) and cetyltrimethylammonium bromide (CTAB) as structure directing agents. Impregnated catalysts on SBA-15 with similar HPW contents were also prepared for comparison purpose. The originality of our work lies first in the testing of these solids, whose efficiency is not only evaluated in the ODS of model feed in batch reactor, but also in the ODS of a Straight Run Gas Oil (SRGO) 2000 ppmS using a fixed bed reactor during a 9 days period, when most of the literature deals with model feeds in batch reactors. Moreover Time-of-Flight Secondary Ions Mass Spectrometry (Tof-SIMS) analysis allowed probing the interaction between the HPW and the support depending on the preparation method.