Structural characterization of coke on spent hydroprocessing catalysts used for processing of vacuum gas oils

The molecular structure and composition of carbonaceous deposits commonly known as coke deposited inside the pores and on the surface of aged hydroprocessing (hydrotreating and hydrocracking) catalysts in value upgradation of vacuum gas oils were investigated by various analytical methods, such as NMR, HPLC and TGA. The soft (dichloromethane extractable) and hard (solvent insoluble) coke deposits from two spent hydrotreating catalysts (CoMo/g-Al2O3 and NiMo/g-Al2O3) and one spent commercial hydrocracking catalyst with carbon contents ranging from 5 to 10% were characterized. HPLC and high resolution liquid state 1H and 13C NMR methods were used for the characterization of the soft coke whereas hard insoluble coke deposits were characterized by high resolution solid-state 13C NMR and TGA. The results indicate that the composition of soft coke is mostly alkylated mono-, di-aromatics and fewer amounts of polyaromatics. The composition of the hard coke in spent hydrotreating catalysts is relatively high in aliphatic content with high H/C ratio obtained in the present study unlike the highly aromatic coke obtained with residue hydrotreating as reported in the literature. However, the insoluble coke deposits in spent hydrocracking catalysts are highly aromatic in nature (aromaticity, fa > 0.95). Keywords: Coke; Deactivation; Hydroprocessing; Catalyst; NM

Structural characterization of coke deposits on industrial spent paraffin dehydrogenation catalysts

Coke deposition on spent noble metal catalysts used in petroleum/petrochemical industries is of serious concern on account of its impact on the catalyst deactivation and consequent loss in the production yield. In order to counteract the effects of coke deposition, it is vital to know the location as well as the nature and composition coke deposited on the spent catalysts. In the present study, spent Pt-Sn/Al2O3 catalysts used in the industrial reactors for selective dehydrogenation of C10–C13 n-paraffins to mono-olefins at different coke loading (approximately 7–9%, w/w) were characterized. The characterization of coke deposits were analyzed by the combination of analytical techniques including HPLC, solid-state 13CCP/MASNMR,and TGA.Average structural information has been obtained from the quantitative analysis of NMR data. The results indicate that the nature of coke present in the soluble coke extracts of spent catalysts is rich in alkylated mono- and diaromatics with low percentage of polyaromatics whereas the nature of insoluble coke is highly polyaromatic (aromaticity, fa > 0.95). In addition, temperature programmed oxidation studies by TGA reveals that the coke is deposited on the dispersed metal as well as on the support. © 2002 Elsevier Science B.V. All rights reserved