Quasi in Situ Sequential Sulfidation of CoMo/Al₂O₃ Studied Using High-Resolution Electron Microscopy

Abstract

Using a specially developed, quasi in situ vacuum-sample-transfer system, the stepwise sequential sulfidation of CoMo/γ-Al2O3 was studied with transmission electron microscopy without exposing the catalyst to air. In a first step, the sulfidation was performed at 400 °C at either 1 or 10 bar pressure. In a second step, the sulfidation was performed at 600 °C and 1 bar. As the second sulfidation step was performed on the sample on the transmission electron microscopy grid, the same areas of the samples that had been studied for the first sulfidation step could be studied once more, providing detailed data on the changes in the sulfided phase. The slab length and degree of stacking have been studied in detail from the TEM/HREM images. Sulfidation at higher pressure or higher temperature results in a higher average slab size. Detailed study of the same areas of the catalysts before and after the second, more severe, sulfidation step shows that although the dispersion of the sulfided phase decreases, the total amount visible in the TEM images remains unchanged. This indicates that practically all the sulfide present is actually imaged on the micrographs. Additionally, detailed HRTEM study shows increased crystallinity of the slabs after the second sulfidation step. XPS analysis shows that after sulfidation at 400 °C and 1 bar, the degree of sulfidation is 95%, whereas after sulfidation at 600 °C and 1 bar, sulfidation is complete. The catalytic performance for thiophene hydrodesulfurization is much higher after sulfidation at 10 bar and 400 °C as compared to 1 bar and 400 °C, and decreases after the second sulfidation step (1 bar 600 °C). The latter effect is more pronounced for the sample sulfided at 10 bar in the first step, indicating that a distorted yet fully sulfided structure is the most active phase for thiophene HDS.

Effect of High Pressure Sulfidation on the Structure of Sulfide Sites of Hydrotreatment Catalysts

“…The used sulfidation procedures can strongly vary with respect to the sulfiding agent, temperature and pressure. In industrial conditions, sulfidation is generally performed under high pressure (3)(4)(5), high temperature (573-723 K) and using a mixture of petroleum feed and organo-sulfide molecules as dimethyl(di)sulfide (DMDS, DMS). Conversely in academic studies, sulfidation is mostly carried out with H 2 S/H 2 at atmospheric pressure.…”

“…Conversely in academic studies, sulfidation is mostly carried out with H 2 S/H 2 at atmospheric pressure. H 2 S is generally used as sulfiding agent not only because it is easier to handle but also because organo-sulfide molecules decompose into H 2 S and CH 4 . However, to get more insight into the sulfidation step, the effect of pressure has to be considered.…”

“…To obtain more relevant structure-activity relationships, the sulfidation stage of academic studies should be performed closer to industrial conditions. However, only few papers relate structural changes of the sulfide phase to the activity of catalysts sulfided under more realistic conditions i.e., using organo-sulfide molecules [1][2][3] or high pressure [2,[4][5][6][7].…”

“…As far as the effect of sulfidation pressure increase is concerned, literature generally reports an increase of the HDS activity [4,5,8]. However, some discrepancies appear in the exact origin of this beneficial effect.…”

“…However, some discrepancies appear in the exact origin of this beneficial effect. Kooyman et al [4] compared the morphology of a CoMo/Al 2 O 3 catalyst (C444-Criterion) after sulfidation with H 2 S/H 2 at 673 K under 0.1 or 1.0 MPa, and observed an increase of the slab length (from 3.1 to 3.8 nm). By contrast, Dugulan et al [5] analyzed the effect of pressure on a CoMo/Al 2 O 3 catalyst sulfided at 673 K, but did not report any difference of the sulfide slab size after sulfidation under 0.1 or 4.0 MPa of H 2 S/H 2 (L = 2.7 nm).…”