Methanol-to-olefins catalysis on ERI-type molecular sieves: towards enhancing ethylene selectivity

ERI-type molecular sieves (SSZ-98, UZM-12, ERI-type zeolite, SAPO-17) are synthesized with varying Si/Al = 5–9 and Si/T-atoms = 0.034–0.12 using several organic structure-directing agents (OSDAs), and evaluated as catalysts for the methanol-to-olefins (MTO) reaction. SAPO-34 (Si/T-atoms = 0.089) and SSZ-13 (Si/Al = 15) are also prepared and tested for comparison. The ERI-type zeolites gave improved ethylene-to-propylene ratios (E/P = 1.1–1.9) over SSZ-13 (E/P = 0.82) and SAPO-34 (E/P = 0.85). The SAPO-17 samples produced an E/P of 0.7–1.1 and a generally high C₄+ fraction. The differences observed in the olefins product distributions between the zeolites with low framework Si/Al (E/P > 1.5) and SAPO-17 with low Si/T-atom < 0.1 (E/P ≤ 1 and high C₄+) are the result of slower maturation of aromatic hydrocarbon-pool (HP) species and the presence of aromatics with bulky alkyl-groups (C₃-C₄) in the SAPO-17 samples. The rapid formation of cyclic intermediates and the shift in their composition towards less-methylated ((CH₃)_(n≤4)) methylbenzenes and methylnaphthalenes are found to be key to enhancing the ethylene selectivity in ERI-type molecular sieves.