(606b) Cage-Defining Ring: A New Molecular Sieve Structural Indicator for Olefin Product Distribution from the Methanol-to-Olefins Reaction
The methanol-to-olefins (MTO) process is a way to produce high value-added light olefins from non-petroleum sources. Acidic zeotypes containing cages bounded by 8-ring (small pore) windows can effectively catalyze the MTO reaction, since their cages can accommodate the necessary aromatic intermediates while letting only the light olefin products escape. There have been numerous attempts to elucidate relationships between the framework architectures and the resultant olefin product distributions. Although the mechanism of the MTO reaction has been investigated thoroughly in recent years, the structure-property relationships have yet to be elucidated. In this work, we report MTO reaction results from various small pore, cage-containing SAPO/MAPOs and zeolites under the same reaction conditions. The MTO behavior of microporous materials having the following topologies are summarized: LEV, ERI, SAT, CHA, AFX, SFW, AEI, DDR, ITE, SAV, LTA and RHO. Based on the experimentally observed product distributions, the behavior of tested zeolites is classified into four structural categories (SAPO and MAPO materials also conform to these classifications). The concept of the cage-defining ring sizes is introduced here as an intuitive but reliable gauge to estimate the product distributions of light olefins. The cage-defining ring is defined as the minimum number of tetrahedral atoms of the ring encircling the center of cages. We show that the cage-defining ring size correlates with light olefin product selectivity. This new structure-property relationship may aid in the design of zeotypes with specified light olefin product distributions.