Evolution of active intermediates in MTO reaction on a Chabazite catalyst

Abstract

The activity of the chabazite zeolite in the Methanol to Olefins (MTO) reaction is tightly connected to the organic intermediates confined in its pores. The intermediates and the pores of near dimensions construct a reciprocal reaction environment for the MTO, known as the hydrocarbon pool (HP) mechanism. The chabazite structure (CHA) stands out in MTO due to the three-dimensional system of large pores connected by small openings, allowing the confinement of the intermediates, allowing small olefins to diffuse in and out. Herein, the organic intermediates in the HP arose as soon as the CHA catalyst was subjected to reaction temperature under a methanol flow. HP was identified to be composed of bridged-ring alkanes (at very low temperatures) and polyaromatics of up to five rings, which grew throughout the reaction, leading to the deactivation of the catalyst. The most active species are the least polymeric. The systematic increase in reaction temperature from 190 to 450 °C revealed a combined change of the zeolite's crystalline structure, which elongates into the c-direction, while olefins are formed. DFT computational simulations confirmed confinement is a spontaneous process forming a hybrid organic-inorganic environment active in the MTO reaction. Other DFT results expanded the understanding of the molecular confinement related to interaction energies with and without system deformation in obtaining geometric parameters beyond the experimental resolution.