Deactivation of HDT Catalysts by Formation of Silica Gels from Silicone Oil. Characterization of Spent Catalysts from HDT of Coker Naphtha Using 29Si and 13C CP/MAS NMR

Abstract

Catalysts used for hydrotreating of coker naphtha are found to deactivate very quickly because of deposition of Si-containing species on their surface. These species originate from the silicone oil (polydimethylsiloxane) added to the coker unit in order to suppress foaming during the coking process. Six samples of a spent catalyst from a HDT reactor have been analyzed by 29Si MAS. 29Si CP/MAS, and 13C CP/MAS NMR spectroscopy. From these studies it is concluded that the silicone oil in the naphtha feed is transformed (oxidized) to modified silica gels, i.e., silica with a partly methylated surface, under the operating conditions of the catalyst. Physi- or chemisorption of the silica gels on the catalyst explains the fast irreversible loss of activity during HDT of coker naphtha.