Effect of Pore and Cage Size on the Formation of Aromatic Intermediates During the Methanol-to-Olefins Reaction

Six eight-membered-ring (8MR), microporous materials are synthesized and evaluated as catalysts for the methanol-to-olefins (MTO) reaction. The molecular sieves SSZ-13, SAPO-34, SAPO-39, MCM-35, ERS-7 and RUB-37 are investigated since they have 8MR access to the crystal interior but have differences in pore structure and cage size. The polymethylbenzene species that are the proposed reaction intermediates of the MTO reaction should only be able to form in materials with intra-molecular sieve void spaces of sufficient size to accommodate them. Thus, it is hypothesized that 8MR materials without adequately large pores or cages will be inactive for the MTO reaction. SSZ-13 and SAPO-34 (both with CHA framework topology) have interconnected 3-dimensional pore-and-cage systems sufficiently large for formation of the proposed reaction intermediates, while the other 8MR materials have intra-molecular sieve void spaces that are too small to allow formation of these species. The molecular sieves are tested as MTO catalysts at 400 °C, and only the molecular sieves with the CHA topology show MTO activity. Post-reaction analysis of the organic content of each solid material is accomplished by HF acid digestion with subsequent 1H NMR analysis of the extracted hydrocarbon products to confirm the presence of aromatics in the 8MR materials with sufficiently large cages (those with CHA topology), and absence with materials that have smaller void spaces. These data provide further support for the necessity of polymethylbenzene species in the hydrocarbon pool for MTO activity.