Further synthetic investigation of YNU-5 zeolite and its application as a solid acid catalyst for the MTO reaction

The synthesis conditions for YNU-5 zeolite were further optimized using dimethyldipropylammonium hydroxide as the organic structure-directing agent (OSDA). The use of FAU-type zeolite as the Si and Al source was essential for obtaining a high-quality YNU-5 samples, although the formation of the YFI framework was still possible without using the FAU-type zeolite. After removing the OSDA, the pure YNU-5 sample (denoted as YNU-5_cal), was converted to its proton form via ammonium ion exchange followed by calcination. YNU-5_cal was then treated with various concentrations of nitric acid under reflux conditions to obtain its dealuminated form with different Si/Al ratios while maintaining framework stability. During the investigation of the dealumination process, distinct dealumination tendencies were observed depending on the type of zeolite framework, specifically Beta, MSE, YFI, and CHA. The obtained samples were evaluated as catalysts for the methanol-to-olefin (MTO) reaction. In this reaction, YNU-5 catalysts with Si/Al ratios of 9–100 exhibited rapid deactivation due to heavy coking. In contrast, when the Si/Al ratio was as high as 200, deactivation was significantly suppressed. Notably, the regeneration of the deactivated catalyst was achievable when deactivation resulted from coking. The approximate location of acid sites within the framework was inferred based on dealumination behavior, NMR data, and the catalytic performance in the MTO reaction.