Development of deep hydrodesulfurization catalysts: II. NiW, Pt and Pd catalysts tested with (substituted) dibenzothiophene

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Abstract

The applicability of NiW- and Pt-based catalysts for the deep hydrodesulfurization (HDS) of diesel fuel was evaluated from gas phase experiments with the model reactants DBT (dibenzothiophene), 4MDBT (4-methyl dibenzothiophene), and 4E6MDBT (4-ethyl, 6-methyl dibenzothiophene). NiW/Al2O3 is less active than NiMo/Al2O3 in DBT HDS, but more active in 4E6MDBT HDS because of its greater hydrogenation activity. Replacing Al2O3 by ASA (amorphous silica–alumina) causes a general lowering of the activity under H2S-free conditions, but induces a dramatic increase in the H2S tolerance of NiW. Thus, the activity of NiW/ASA was much higher than that of NiMo/ASA or NiMo/Al2O3 during 4E6MDBT HDS in the presence of 0.2% H2S. The higher activity over the acidic supports could not be attributed to dealkylation or isomerization; however, it was associated with C–C bond scission in the thiophenic ring, either before or after the desulfurization step. Pt/ASA has DBT and 4MDBT HDS activities which are comparable with those of the NiW catalysts; in 4E6MDBT HDS Pt/ASA is much more active because of its superior hydrogenation activity. Its sulfur tolerance, on the other hand, is much lower than that of NiW catalysts, rendering Pt/ASA less active than NiW/ASA during 4E6MDBT HDS in the presence of 0.2% H2S.

Introduction

In the previous study [1], the performance of CoMo and NiMo catalysts was studied in the desulfurization of (un)substituted dibenzothiophenes (DBT). It was found that the main reaction path was direct hydrogenolysis, a route which is impaired by the presence of substituents, especially when both at the 4- and 6-position. Therefore, the activity of the catalysts for 4MDBT (4-methyl dibenzothiophene) and especially 4E6MDBT HDS (4-ethyl, 6-methyl dibenzothiophene hydrodesulfurization) was clearly lower than for DBT HDS. It was pointed out that appreciable hydrogenation, dealkylation or C–C bond scission activity may lead to more facile pathways for 4E6MDBT HDS. With CoMo and NiMo catalysts, only a small increase in hydrogenation activity could be realized by the use of amorphous silica–alumina (ASA) as a support. In the present study, we will test NiW and noble metal catalysts, which are known for their HDS activity as well as high(er) hydrogenation activity 2, 3. Supports with varying acidity will be used to influence the activity for dealkylation or C–C bond scission. Special attention will be paid to the sensitivity of these catalysts to the presence of H2S in the feed gas. By means of feed flow variation and hydrogenation experiments, information will be obtained about the 4E6MDBT HDS reaction network.

Section snippets

Catalyst preparation

The following supports were used in catalyst preparation (0.25–0.50 mm particles).

Activated carbon (Norit RX3 Extra: pore volume (P.V.) 1.0 ml/g, surface area (S.A.) 1200 m2/g), γ-Al2O3 (Ketjen 001–1.5E: (P.V.) 0.60 ml/g, (S.A.) 276 m2/g), ASA (ex Shell: Si/Al ratio 0.70 at/at, (P.V.) 0.72 ml/g, (S.A.) 389 m2/g), as well as a more acidic variation of the latter ASA, referred to as ASA+.

The NiW/Al2O3, NiW/ASA and NiW/ASA+ catalysts were all obtained from Delft University [4]. They were prepared

DBT HDS

Fig. 1 shows the DBT HDS activities, calculated from experiments at 573 K. For the NiW catalysts, the order in activity is NiW/ASA<NiW/ASA+<NiW/Al2O3; all three are less active than the NiMo/Al2O3 catalysts tested previously [1]. The activity of Pt/ASA is comparable with that of the NiW catalysts; Pt/Al2O3 is clearly less active.

The three NiW catalysts show approximately the same H2S sensitivity: at a level of 0.2% H2S, their activities are all reduced by a factor of six to seven. The Pt

NiW catalysts

The performance of the NiW catalysts is clearly dependent on the support. In 4E6MDBT, 4MDBT and DBT HDS at 573 K (no H2S added), the differences in activity are fairly modest and the order is usually Al2O3>ASA>ASA+. However, the amount of C7–C8 or C6–C7 products increases considerably in the reverse order. From feed flow variation experiments, it appeared that the latter products are formed from (substituted) BP and/or CHB's in a consecutive reaction, possibly on the acidic support. A different

Conclusions

The HDS activity rankings for the catalysts studied appeared quite dependent on the type of reactant. The degree of steric hindrance in the desulfurization reaction increases in the order DBT<4-MDBT<4E6MDBT; therefore, activity for 4MDBT and especially 4E6MDBT HDS is favoured by a high hydrogenation activity, providing an easier reaction pathway. The activity trends, measured without additional H2S (CoMo, NiMo: 4E6MDBT≪4MDBT<DBT; NiW: 4E6MDBT<4MDBT≈DBT; Pt: 4E6MDBT≈4MDBT≈DBT) are therefore

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