Preparation and Characterization of γ-Al₂O₃ with Rich Brønsted Acid Sites and Its Application in the Fluid Catalytic Cracking Process

Abstract

The objective of this work is to investigate the surface acidity of γ-Al2O3 after modification and its application in reducing coke formation in the fluid catalytic cracking (FCC) process. γ-Al2O3 with rich Brönsted acid sites and reduced Lewis acid sites was prepared by the sol–gel method using NH4BF4 as a modifier to develop a new functional material to adjust surface acidity. N2 sorption, powder X-ray diffraction (XRD), 27Al magic angle spinning nuclear magnetic resonance (27Al MAS NMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FT-IR) spectroscopy were used to characterize the structure and surface properties of the prepared γ-Al2O3. The results showed that partial fluorination of the surface of γ-Al2O3 generated small quantities of a pyrochlore-type phase which was formed mainly by substitution of the OH group on six-coordinated aluminum with fluorine. In addition, boron insertion in the structure of γ-Al2O3 reduced the Lewis acid concentration and increased the surface area of γ-Al2O3. After aging in an alkaline solution with a F/Al mole ratio of 0.45, the modified γ-Al2O3 demonstrated high surface area (276 m2/g) with gamma alumina structure due to the synergistic effect of boron and fluorine doping by using NH4BF4. However, most importantly the modified γ-Al2O3 exhibited the high Brönsted/Lewis acid ratio of 0.75. In the bench-scale tests with heavy oil, the modified γ-Al2O3 showed good catalytic performance by increasing conversion of heavy oil to light oil while reducing coke formation.