Evolution of carbonaceous deposits on H-mordenite and Pt-doped H-mordenite during n-butane conversion

The formation and evolution of carbonaceous deposits on H-mordenite and platinum-doped H-mordenite during conversion of n-butane in a flow reactor were investigated using in situ UV–vis–NIR spectroscopy. When n-butane (10%) was mixed with H2 (90%), both catalysts exhibited stable performance at a reaction temperature of 623 K and a total pressure of 1 atm, and no unsaturated compounds accumulated on their surfaces. When the H2 in the feed was replaced with helium, rates jumped to a higher level, but then declined rapidly while unsaturated surface species formed on the zeolite surface. Spectra of both catalysts showed defined bands at 286, 335, 395, and 460 nm; bands in these ranges have been previously assigned to cationic species. Butenes, which were detected in increased concentrations in the absence of H2 in the feed, are demonstrated to be the precursors of these surface species. Deactivation was more severe for the platinum-doped sample and affected isomerization, disproportionation, and hydrogenolysis functions but not dehydrogenation and H2 activation. Return to a H2-containing feed reduced the concentration of unsaturated surface species on the Pt-containing catalyst, with a very slow recovery of isomerization activity.