Mechanisms of thiophene conversion over the modified Y zeolites under catalytic cracking conditions

The mechanisms of thiophene conversion over various Y zeolites, viz., HY, Ni-modified NiY and rare earths metal-modified REY, were investigated under catalytic cracking conditions in a fix-bed reactor. The final products of thiophene conversion were analyzed by using a gas chromatography with flame ionization detector (GC-FID), a gas chromatography with sulfur chemiluminescence detector (GC-SCD) and an in situ infrared spectrometer. The results were then correlated with the acidic properties of the zeolites, which indicated that the mechanisms of thiophene interacting with these Y zeolites are dependent on the state and occurrence mode of the modified metallic cation and/or the extraframework aluminum species, which can modulate the Brnsted and Lewis acidities in the zeolites. For the NiY zeolites, the thiophene molecules are mainly adsorbed on the Lewis acidic sites associated with the NiOH⁺ species; the species of Ni₄AlO₄³⁺ and those generated from the Ni²⁺ and the extraframework aluminum can weaken the Brnsted acidity and then reduce the cracking activity of the zeolite. Polymerization and cracking reactions are observed for the conversion of thiophene over the HY and REY zeolites. For the HY zeolites, 2,2',5',2''-terthiophene can be found, which is ascribed to the Brnsted acid sites adjacent to extraframework aluminum species (AlO⁺ or so on); some subsequent reactions of hydrogen transfer and cracking can also be detected. Over the REY zeolite, hydrogen transfer and cracking reactions of thiophene oligomer species derived on the Brnsted acid sites adjacent to extraframework aluminum species (Al(OH)₂⁺, Al(OH)²⁺ and so on) can be promoted by the Lewis acid center related to the RE species.