(141c) HDS and HDA Characteristics of Pt-Encapsulated Zeolite Catalysts

Noble metal catalysts (e.g. Pt, Pd) are commonly used in hydrogenation processes. They have higher hydrogenation capability than base catalysts (e.g. Co, Mo, Ni), but can be easily poisoned by sulphur compounds and lose their activity. This paper presents a specially prepared noble metal catalyst which uses a Pt encapsulated zeolite material to prevent the Pt particles from sulphur poisoning. A Mo-Ni catalyst was also prepared using the same procedure for comparison. A set of pilot plant tests was conducted with naphthalene as a hydrodearomatization/hydrogenation model compound and dibenzothiophene as a sulphur model compound. Two-dimensional GCxGC analysis was performed to identify and quantify the model compounds and their hydrotreated products.

The prepared Pt-encapsulated catalyst exhibited naphthalene conversion comparable to that obtained using the Mo-Ni catalyst. However, the Pt catalyst yielded significant amounts of the deep naphthalene hydrogenation product decalin in the low-sulphur poisoning environment. In this case, the relative amount of decalin increased concomitantly with reaction temperature. In the high-level sulphur poisoning environment, the Pt catalyst gave much more intermediate hydrogenation product tetralin than decalin. These observations indicate that sulphur poisoning has an impact on the hydrogenation selectivity. At the same time, the Mo-Ni catalyst gave similar amounts of tetralin and decalin regardless of the presence of sulphur poisoning. Based on the analysis of HDS products of dibenzothiophene, it was observed that the HDS of dibenzothiophene over the Pt- encapsulated catalyst proceeded mainly via the direct desulphurization pathway and yielded biphenyl as the main product. This observation is contrary to the commonly accepted understanding that the hydrogenation pathway is preferred for noble metal catalysts. The Mo-Ni catalyst, on the other hand, yielded cyclohexylbenzene as the main product. The implication is that use of the Pt catalyst leads to lower hydrogen consumption than the Mo-Ni catalyst while retaining comparable HDS conversion.