Coke deactivation of Pd/H-mordenite catalysts used for C₅/C₆ hydroisomerization

Coke formation in C₅/C₆ alkane isomerization on a Pd/H-Mordenite catalyst discharged from a pilot reactor was studied. Experimental techniques like XRD, EPR, MAS-NMR, FTIR, TGA and TPO, were applied for the characterization of coke and coked catalysts. It was shown that coke content, chemical composition, its structure and nature depended upon time-on-stream and section of the catalyst bed. The coke content increased from 3.0 in the top section, to 4.1 wt.% in the central section and 6.2 wt.% in the bottom of the catalyst bed and C/H ratio of the corresponding coke increased from 1:1.12, 1:0.55 to 1:0.35, which significantly affected the catalytic behavior and regeneration of the catalysts. The selectivity to 2,2-dimethyl butane during n-hexane isomerization, respectively, decreased from 19% for the fresh catalysts, to 10% and 8.3% for the coked catalysts in the central and outlet sections. As time-on-stream increased, coke initially formed on palladium metals and then moved to acidic sites in the support where polyaromatic or pseudographite-like structures were formed through further acid catalyzed reactions. Two kinds of coke with different nature could be combusted below 300 and 600°C, respectively. These results are very useful for choosing operation parameters to restore initial activity of the coked catalysts in the regeneration units in our pilot plant.