Synthesis of 2-Methylbenzimidazole in Continuous Flow: Mechanism of Cu-Pd/(K)-Al₂O₃-Catalyzed Deactivation and Regeneration

Benzimidazole compounds are a pivotal structure within various pharmaceuticals and possess significant medical value. Utilizing Cu-Pd/gamma-Al2O3 as the catalyst, benzimidazole can be synthesized directly from 2-nitroaniline and ethanol, offering advantages such as readily available starting materials, high efficiency, and a straightforward process. Modification of the Cu-Pd/gamma-Al2O3 catalyst with potassium (K) can significantly enhance its catalytic activity. The introduction of K into the Cu-Pd/gamma-Al2O3 catalyst effectively sustains and promotes the formation of active sites within the CuPd alloy. Furthermore, K facilitates the dehydrogenation of alcohols, thereby expediting the overall coupling reaction rate. The impact of various factors such as space velocity, reaction temperature, reaction pressure, and solvent on the catalyst's performance was investigated during the continuous flow synthesis of benzimidazole from ethanol and ortho-nitroaniline. Additionally, characterization techniques such as Nitrogen gas adsorption/desorption, TEM, TPD, TPO, XRD, and TG were employed to explore the catalyst deactivation mechanism. The research outcomes demonstrate that the 5 wt %Cu-5 wt %Pd/gamma-Al2O3 catalyst modified with K exhibited excellent catalytic performance, achieving complete conversion of ortho-nitroaniline and impressive 98.2% selectivity toward 2-methylbenzimidazole under the conditions of a reaction temperature of 433 K, a pressure of 5 MPa, a mass space velocity of 0.28 h(-1), and a water-to-ethanol volume ratio of 1:3. Nevertheless, catalyst deactivation became evident after 42 h of continuous operation. Analysis revealed that catalyst deactivation was attributed to changes in the carrier's crystalline phase, catalyst coking, and trace amounts of CO poisoning. The deactivated catalyst could be effectively regained through high-temperature calcination and reduction. The findings of this study provide a straightforward method for regenerating efficient catalysts within the "alcohol dehydrogenation-hydrogen transfer-hydrogenation" coupled reaction system.