Desulfurization in the FCC riser is achieved by modifying zeolite catalysts with La³⁺ ions, promoting H₂S formation from thiophenic compounds while suppressing secondary reactions. This approach enhances gasoline quality by increasing selectivity for H₂S and enabling efficient condensation and alkylation, supporting cleaner fuel production and compliance with stringent environmental regulations.

Abstract

This study evaluates the incorporation of lanthanum into zeolites Y, Beta, and ZSM-5 to promote the desulfurization of a gasoline stream directly in the riser of a fluid catalytic cracking (FCC) process. The catalysts were prepared using wet impregnation and characterized by X-ray diffraction (XRD) (crystallinity and phase integrity), XRF (La content and Si/Al ratio), X-ray photoelectron spectroscopy (XPS), solid state nuclear magnetic resonance (NMR) of ²⁹Si and ²⁷Al, N₂ adsorption, high-resolution transmission electron microscopy (HRTEM), temperature-programmed desorption of ammonia (TPD-NH₃), Fourier-transform infrared spectroscopy (FTIR), and pyridine chemisorbed FTIR (FTIR-Py). Catalytic tests were conducted in a tubular reactor, using a mixture of cyclohexane and thiophene (20000 ppm) at 773 K and 1.3 atm. Catalytic activity was assessed in a fixed-bed reactor using a cyclohexane/thiophene feed, and product selectivities were quantified by gas chromatography with flame ionization and sulfur chemiluminescence detectors. The addition of lanthanum altered the acidity of the catalysts, reducing the total density of acid sites while increasing the strength of Lewis acid sites. The protonated zeolites, particularly HY, facilitated sulfur removal, achieving a 91% selectivity for H₂S through secondary hydrogen transfer reactions. In contrast, the lanthanum-modified zeolites enhanced thiophene alkylation and condensation reactions, as evidenced by the selectivity observed in La/BEA and La/ZSM-5, which reached 15%. The results indicated that incorporating lanthanum changes the desulfurization profile, making the catalysts more selective for forming higher molecular weight sulfur compounds, thereby shifting the products toward the FCC diesel range. The modulation of acidity and structural accessibility of the zeolites proves crucial in converting sulfur compounds into H₂S or heavy sulfur compounds in the FCC riser, leading to a reduction in the sulfur content of the gasoline fraction, which is economically advantageous.