Effect of Boron Substitution in Chabazite Framework: IR Studies on the Acidity Properties and Reactivity Towards Methanol

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Abstract

The importance of methanol to light olefins (MTO) process in today's economy, using H-SAPO-34 as a catalyst, has stimulated the investigation of new zeolitic material with the same CHA topology but characterized by a different acidic strength. In this work, we report a detailed IR investigation on the symmetry, accessibility, acidic strength, and reactivity of B sites in a CHA framework. As the prepared material exhibits [B(OSi)4] units in a Td-like geometry, upon template burning, the break of a B−O−Si bond results in [B(OSi)3] units in a D3h-like geometry, testified by the appearance of the IR fingerprint at 1390 cm-1. Interaction with a strong base as H2O allows the Td-like geometry to be reversibly restored. Weak bases, such as H2 and CO, are unable to displace B species from the D3h-like geometry but testify the weak Brønsted acidity of internal OH species. Interaction with CH3OH gives rise to a much more complex spectroscopy, reflecting a rich reactivity of methanol with both [B(OSi)3] units and adjacent SiOH species. This reactivity, discovered in the present IR investigation, justifies on a molecular level the unsuitability of the B-CHA system for the MTO process.

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