Hydrodesulfurization (HDS) represents a crucial component of current petroleum refining operations in terms of both environmental and economic considerations. At the same time that crude oils are becoming heavier with increasing amounts of sulfur, product regulations are becoming increasingly more stringent. Therefore, new support materials and new active phases for high performance HDS catalysts are needed. Because of the large size of the organic molecules in the heavy oil fractions, HDS catalyst supports contain primarily mesopores and macropores. At present, a focus of intensive effort has been the development of new support materials with pore diameters optimized for desulfurization of heavy oil molecules.

Two types of synthetic mesoporous materials were investigated as a size selective supports for HDS catalysts. In the first, mesoporous synthetic clays (MSCs) were synthesized in the presence of a neutral polymer, polyvinylpyrrolidone (PVP), to systematically vary the pore size (in the mesoporous range of 40-100 Å). The second material, M41S is a new family of mesostructured molecular sieves patented by Mobil scientists in early 90's. M41S has a uniform, one dimensional and hexagonal pore structure in the range of 20-100 Å. Both materials have very uniform pore size distributions and show great promise as a catalyst support for HDS purposes.

The pore diameter of the MSC catalysts was found to have a strong effect on both the HDS activity and selectivity. An increasing pore diameter led to a nearly linear increase of activity and a boost in selectivity. The activity profile may indicate that the internal diffusion of large molecules (such as dibenzothiophene) still plays a critical role in the HDS reaction. The higher selectivity was achieved in larger pores because larger pores make it easier for primary products to diffuse out of the pores preventing further hydrogenation and hydrocracking.

The Si/Al ratio of the M41S catalysts has an effect on HDS performance. The HDS activity at 400 °C increased with increasing Al content (to a maximum at Si/Al = 60). This suggests that increasing amounts of Al may create more sites for anchoring the Co-Mo-S within the M41S framework. Increasing the Si/Al ratio, however, results in a decrease in the HDS selectivity. The selectivity loss is probably due to an increase in acidity which shifts the reaction network from HDS to hydrocracking. Titration of the acid sites on the surface minimized this problem.