Influence of the Si/Al ratio and crystal size on the acidity and activity of HBEA zeolites

Three HBEA samples were prepared through ion exchange followed by calcination under dry air flow at 550 °C from parent samples synthesized with different total Si/Al ratios or crystal sizes and characterized by various techniques. The denomination, Si/Al ratio and crystal size were the following—Cal: 12.5, ∼20 nm; LC: 14.5, 3000–10,000 nm; RAl: 7.0, ∼50 nm. Pyridine adsorption followed by FTIR shows very different values of the Lewis/Brønsted site concentration ratio: 1.2 with Cal, 0.2 with LC and 0.6 with RAl, which indicates a predominant positive effect of the crystal size on the framework stability. Methylcyclohexane transformation at 450 °C was found to occur through both protolysis and the carbenium ion chain mechanism. Protolysis plays a significant role on Cal only. This very difficult reaction which requires very strong protonic sites, most likely sites resulting from interaction of bridging OH groups with neighbouring Lewis sites was 3 and 12 times faster on Cal than on RAl and LC samples, respectively. Furthermore, from the low turnover frequency value of the protonic sites of LC, severe diffusion limitations were demonstrated to occur on this large crystal size sample.

Graphical abstractThree HBEA samples were prepared with different values of Si/Al ratio and crystal size (from 7 to 14.5 and 20 to 10,000 nm, respectively). Textural, structural and acid properties were examined. Catalytic characterization was performed by methylcyclohexane transformation; product distribution was investigated and the reaction rates of both protolytic and carbenium ion chain mechanism were estimated. Figure optionsDownload full-size imageDownload as PowerPoint slide