HZSM-5-Catalyzed Vapor-Phase Conversion of Dimethyl Adipate to Adiponitrile: The Influence of the SiO₂/Al₂O₃ Ratio on Reactivity

The vapor-phase nitrilation of dimethyl adipate (DMA) with ammonia (NH₃) is an environmentally benign route leading to adiponitrile (ADN) with great application value. In this study, the performance of HZSM-5 zeolites, widely used in industry as the solid acid catalysts, was investigated. It exhibits a strong correlation between the SiO₂/Al₂O₃ ratio (SAR) in HZSM-5 zeolites and the reactivity (both activity and selectivity). The influence of Al density closely related to the value of SAR on the reactivity of HZSM-5 was thus elucidated by mechanistic experiments. As a result, the introduction of Al element to the framework is beneficial to the activation of ester functionality, thus improving the activity, especially at a high liquid hourly space velocity (LHSV). However, a high Al density will lead to poor selectivity and deactivation due to too strong adsorption of esters and intermediates (e. g., amides). Herein, the balanced adsorption/desorption capability was reached on HZSM-5 with SAR as 631 (i. e., HZSM-5-631), which provides the best selectivity to nitriles (S_ADN+MCP, 92.5 %) at a high single-pass conversion of DMA (C_DMA, 98.0 %).