Modified Spent FCC Catalysts with Accessible and Balanced Distribution of Acid Sites for the Stereospecific Synthesis of Turpentine into High-Quality Borneol and Associated Kinetics

Abstract

The spent fluid catalytic cracking catalyst (SFCC), as both hazardous solid wastes and valuable resources, requires environmentally safe and resource-efficient treatment to support the sustainable development of the petrochemical industry. In this study, a ″top-down″ approach was employed to modify SFCC, aiming to address the poor selectivity of existing catalysts for endobornyl ester in the esterification of α-pinene to borneol. The BET surface area of the modified catalyst (HS-SFCC) was 143.09 m² g^–1, with a hierarchical structure consisting of mesopores of 1.3 nm and macropores ranging from 9 to 25 nm. Advanced infrared techniques with different probe molecules precisely revealed a significant number of Lewis acid sites exposed on the outer surface of HS-SFCC, along with a fraction of Brønsted acid sites at the micropore openings. The balanced distribution of accessible acid sites contributed to the remarkable catalytic performance of HS-SFCC in esterification reactions. After a 6-h esterification reaction of turpentine with HS-SFCC followed by a 1-h saponification reaction, high-quality borneol was synthesized with an endoborneol content of up to 80.5%, indicating excellent catalytic activity and high selectivity of HS-SFCC. Furthermore, HS-SFCC exhibited satisfactory stability and reusability after 12 cycles. A complex reaction network for the esterification of turpentine with oxalic acid catalyzed by the HS-SFCC catalyst was constructed, and the intrinsic kinetics of turpentine esterification were first investigated using a lumped kinetic model, with activation energies and pre-exponential factors calculated from the Arrhenius relationship. The activation energies for the main esterification ranged from 99.46 to 115.63 kJ mol^–1.