Experimental and theoretical study of deactivated HDT catalysts by Si species deposited on their surfaces: Models proposition, structural and thermodynamic analysis

It is known that silicon species originate from the thermal degradation of the polydimethylsiloxane, adsorb on the catalysts surface used for hydrotreatment in the oil refining industry. This severely affects the performance of these catalysts and considerably reduce their lifetime. Combining Si-29 and C-13 CP/MAS NMR experiments and DFT calculations, we investigated the poisoning process of these catalysts by silicon species in order to obtain a better understanding of this process. Two structural models (mono and bilayer) for amorphous silica deposited on the gamma-Al2O3 surface and some intermediates that can be formed during this process were proposed. From the calculated Si-29 chemical shifts, it was possible to identify chemical shifts variations due to different structural elements for each species, thereby, it was possible to make a more appropriate assignment of the resonances found in Si-29 CP/MAS NMR spectra in the literature. Besides that, through of the good agreements obtained between the experimental and simulated Si-29 CP/MAS NMR spectra, we identified that probably the D-4, T-1, T-3, T-4, Q(1) and Q(2) species are present in the initial stage of the contamination and that the amorphous silica bilayer model deposited on the gamma-Al2O3 surface best characterizes the final stage of the contamination. Moreover, the thermodynamic analysis revealed that the chemisorption reaction of the Hexamethylcyclotrisiloxane (D-3) compound is much more stable than the Octamethylcyclotetrasiloxane (D-4) compound and of a fragment (corresponding to 1/3 or 1/4 of the D-3 and D(4)compounds, respectively), and the chemisorption reaction of the fragment presented the highest values of Delta G degrees. This leads us to deduce that the D3 and D4 compounds chemisorb preferentially without breaking. Besides, it also showed that the amorphous silica formation process on the gamma-Al2O3 surface is spontaneous and the bilayer formation reaction is more stable than the monolayer. (C) 2020 Elsevier Inc. All rights reserved.