Abstract
We have investigated the mechanism of anisotropic changes in the unit cell dimensions of SAPO-34 during the methanol-to-olefin (MTO) process. Retained hydrocarbon analysis was employed to quantify the amount of coke in the cages of thermally quenched SAPO-34 samples which had been used as the catalyst in the MTO reaction. Molecular mechanics structural optimizations were carried out on structural models with different coke molecules in the pores, and the results were compared to experimentally determined cell dimension changes and coke composition. Our results indicate no correlation between small hydrocarbon intermediates and cell expansion, with all changes in the unit cell parameters being due to bulky coke molecules like phenanthrene and pyrene. Further, only a small fraction of the cavities in the material need to be filled with such large molecules in order to bring about changes of the magnitude observed experimentally.
