Deactivation mechanisms of Nb2O5 catalyst for esterification of dimethyl adipate: crystal transformation and acid site coking

Abstract

The deactivation mechanisms of Nb₂O₅ catalysts in fixed-bed reactors were systematically investigated, revealing that carbon deposition and structural phase transformations synergistically drive catalyst degradation. During adiponitrile synthesis, carbon deposits evolve progressively from monocyclic to polycyclic aromatic hydrocarbons, leading to blockage of critical acid sites and reduced catalytic activity. Simultaneously, the phase transformation of Nb₂O₅ to Nb₁₂O₂₉ generates oxygen vacancies, which form unsaturated metal coordination sites and alter the catalyst's acidity profile. Regeneration partially reverses this phase transformation, restoring Nb₂O₅ content while reducing Nb₁₂O₂₉, thereby improving stability Crucially, maintaining moderate acidity is essential for achieving high nitrile selectivity (>85%) at efficient conversion rates. The study proposes practical strategies to mitigate coke formation and stabilize active sites, including optimized ammonia flow rates and acidic site control. These findings provide a theoretical foundation for enhancing Nb₂O₅ catalyst stability, reducing energy consumption, and extending operational lifespan in industrial nitrile production processes.