Adsorption And Transformation Mechanisms Of Thiophene And Methylthiophenes On The Active Sites Of Zeolites Y

Thiophene and methylthiophenes account for the main proportion in thiophenic sulfur compounds of fluid catalytic cracking?FCC?gasoline before and after treating the FCC gasoline by using different depth desulphurizationtechnologies.Theremovalofthiopheneand methylthiophenes has became an inevitable challenge in reducing the sulfur content in FCC gasoline deeply.So far,the transformation mechanisms of these sulfur compounds has been stayed B acid and L acid in general with a macroscopic level,and ignored the studies that the correlation between active center of zeolites Y and thiophene and methylthiophenes.Therefore,the analysis of the structures,location and natures of the active center in the modified zeolites Y in-depth,which play a role in the process of adsorption and transformation thiophene and methylthiophenes.All the studies can provide the theory basis for reducing the sulfur content of FCC gasoline deeply and the design and development of the catalyst with an important guiding significance in the future.In this paper,textural properties of NaY,HY,NiY,and REUSY zeolites were characterized by using N₂ adsorption,X-ray powder diffraction?XRD?and inductively coupled plasma emission spectrometer?ICP-AES?.Their acid properties were measured by pyridine adsorption followed by Fourier transform infrared?Py-FTIR?spectroscopy and ammonia-temperature programmed desorption?NH₃-TPD?.The results show that the NaY only exists the Na⁺active sites.The adjacent non-framework aluminum species?Al O⁺?active sites in the HY enhances the acidity of the silicon aluminum bridge hydroxyl?Si-OH-Al?active sites.In the NiY,apart from the NiOH⁺active sites,the structure of the Ni₄Al O₄³⁺species being from the Ni²⁺combined with the non-framework aluminum reduces the acidity of Si-OH-Al active sites.Two kinds of active sites,respectively,assigned to the non-framework aluminum species[Al?OH?²⁺]and the rare earth ion species[RE?OH?²⁺]in the REUSY significantly strengthened the acidity of the Si-OH-Al,and more stronger than those in the HY.Adsorption and transformation mechanisms of thiophene over the active sites in the modified Y zeolites of a HY,a NiY and a REUSY were studied under catalytic cracking conditions by employing a fix-bed reactor and an in situ FTIR technique.The results indicate that,for the Ni Y zeolite,the thiophene molecules are mainly adsorbed on the the NiOH⁺active sites.The species of Ni₄AlO₄³⁺and those generated from the Ni²⁺and the extra framework aluminum can weaken the acidity of the Si-OH-Al,resulting in the reduction of the cracking performance of the zeolite.Polymerization and cracking reactions can be mainly observed for the conversion of thiophene over the HY and REUSY zeolites.For the HY zeolite,2,2',5',2''-terthiophene can be found,which can be ascribed to the effect of the Si-OH-Al active sites,and some subsequent reactions of hydrogen transfer and cracking can be detected.While for the REUSY zeolite,hydrogen transfer and cracking reactions of thiophene oligomer species derived from the effect of the Si-OH-Al active sites can be promoted by the active centers related to the RE species.Correlation between the adsorption-transformation behaviors of2-methylthiophene and 3-methylthiophene and the active sites of NaY,HY and rare earth ion-exchanged ultra-stable Y?REUSY?zeolites have been studied by using an in situ FTIR technique and an intelligent gravimetric analyser-mass spectrometer?IGA-MS?coupling technique,and combining with the GC-SCD and GC-MSD detected techniques.The results show that the bridging hydroxyls?OH?of Si and Al atoms located in the supercage of zeolites Y are the preferred active sites,leading to the protonation and cracking reactions of the methylthiophenes.The AlO⁺active sites in the HY,and the Al?OH?²⁺and RE?OH?²⁺active sites in the REUSY both can make the oligomerization reaction occur at 303 and 373 K,and especially,the RE?OH?²⁺active sites significantly improve the oligomerization abilities.Besides,the activities and selectivities of the protonated 2-methylthiophene and 3-methylthiophene play a decisive role in the oligomerization pathways.As system temperatures of>473 K,the oligomerization reaction is restricted,while the cracking reaction is improved.Thereby,modulation of the key active sites in adsorbents and system temperatures will be a good prospect for ultra-deep desulfurization in the future.