Effects of basic sites and defects of MgAl-LDHs on direct carboxylation of fluorene with CO<sub>2</sub>

Converting CO₂ into high value-added chemicals is essential for mitigating environmental pollution caused by CO₂ and reducing dependence on fossil fuels. One significant approach to resource utilization is the direct synthesis of carboxylic acids using CO₂ as a carboxyl source. Utilizing fluorene as the raw material along with CO₂ for the synthesis of 9-fluorenecarboxylic acid (9-FCA) provides a readily available feedstock while eliminating the need for toxic phosgene or carbon monoxide. Due to the thermodynamic stability and kinetic inertness of CO₂, an appropriate catalytic system is crucial for ensuring that the reaction proceeds efficiently. Layered double hydroxides (LDHs) represent an important class of solid bases that facilitate the control of alkalinity by adjusting the composition and ratio of M²⁺ and M³⁺ elements within the LDH layers. Their catalytic performance is influenced by both the basic strength and the quantity of the active phase. Layered double oxides (LDOs), which are derived from LDHs after calcination, exhibit developed pore structures and abundant tunable surface basic sites, which are beneficial for the adsorption and activation of CO₂. Exploring and understanding the effects of basic sites and defects on the direct carboxylation of fluorene with CO₂ catalyzed by MgAL-LDHs is beneficial to the non-dissociation activation of CO₂ and has certain reference significance for improving the yield of 9-FCA under mild conditions. To achieve efficient CO₂ conversion under mild conditions, this study investigates the effects of solid bases such as anhydrous K₂CO₃, K₂CO₃/LDHs and K₂CO₃ + LDHs/LDO on the direct carboxylation of CO₂ to synthesize 9-FCA. The variations in catalytic activity were analyzed using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), CO₂ temperature-programmed desorption (CO₂-TPD), and Fourier-transform infrared spectroscopy (FT-IR). Results indicate that during the recycling of K₂CO₃, weak alkaline sites gradually transform into medium-strength alkaline sites; however, the alkali content decreases, leading to reduced reaction activity. When the regeneration temperature exceeds 250 °C, the crystal structure transitions from high-activity γ-K₂CO₃ to low-activity β-K₂CO₃. After three cycles, the yield of 9-FCA declined from an initial 64.61% to 29.10%. Factors such as the preparation method, crystallinity, number of basic sites, and basic centers in MgAl-LDHs significantly affect catalytic performance, as evidenced by differences in 9-FCA yields. Notably, MgAl-LDHs synthesized via coprecipitation exhibit lower crystallinity but demonstrate superior catalytic effects compared to those produced through hydrothermal methods. Lower crystallinity corresponds to a higher number of medium-strength alkaline active centers, thereby enhancing catalytic reactivity. The cycle stability of the MgAl-LDHs catalyst is commendable, with no phase change observed. The presence of K₂CO₃ influences the number and strength of alkaline centers in LDHs, yielding 66.15% for Mg₂Al-LDH-C-40% and 44.45% for Mg₃Al-LDH-H-40%, respectively. The spinel phase (MgAl₂O₄) in Mg₂Al-LDO-500 increases, while hydroxyl groups (−OH) and crystallinity decrease, resulting in an increase in intermediate strength alkaline sites and oxygen vacancies (Ov). Both crystallinity and pore volume of LDO decrease, while the strength and number of medium-strong alkaline sites increase. The presence of more medium-strong alkaline sites enhances the non-dissociative activation of CO₂, significantly increasing the yield of 9-FCA. When 7% Mg₂Al-LDO-500 is added, the yield of 9-FCA rises to 78.36%.