The strong SDA/framework interactions and acidity study of high-silica LTA-type zeolites

Abstract

The zeolite community spent 50 years in pursuing the synthesis of high-silica LTA-type zeolites, and ultimately succeeded with the pure-silica polymorph by applying specific structure-directing agents (SDA). The high-silica zeolite A has exhibited unique properties and strong potential to be applied as an adsorbent and catalyst in several novel processes. In the present study, the high-silica polymorphs of Si/Al ratios from 15 to ∞ were synthesized and subjected to thorough characterizations using multiple physicochemical methods. It has been revealed that there exists strong interactions between the organic SDAs and the framework. Therefore, removing the SDAs by calcination in air is difficult, and can only be done above 700 °C with a certain degree of framework dealumination. The retained framework structure shows connectivity defects and a unique combination of Brønsted and Lewis acidity. Using methanol-to-olefins (MTO) as a probe reaction, the catalytic performance of high-silica LTA-type zeolites has been evaluated. Compared to the well-studied CHA and MFI-type catalysts, high-silica zeolite A exhibited a shorter life-time and higher selectivity towards C4 olefins, while methane selectivity increased with the deactivation.