In the Methanol-to-Olefins (MTO) reaction, ethylene selectivity remains the most coveted yet elusive target. While SAPO-34 (CHA topology) dominates commercial MTO with >80% light olefin selectivity favoring propylene, SAPO-18 (AEI topology) stands apart as the rare zeotype that delivers exceptionally high ethylene selectivity (up to 60–70%) — a direct consequence of its unique chabazite cage architecture with constrained 8-membered ring (8-MR) windows (~0.38 nm). This article provides a comprehensive, critically analytical review of SAPO-18 as a high-ethylene-selectivity MTO catalyst. We dissect how the AEI cage topology enforces product shape selectivity through molecular sieving and confinement effects, how synthesis parameters (Si/Al ratio, template choice, crystallization conditions) govern Sn/P/Mg incorporation and acid site density, and how post-synthetic modifications (steaming, phosphorus passivation, metal doping, hierarchical structuring) push ethylene selectivity beyond 70%. Benchmark reactions — MTO, methanol-to-gasoline (MTG), and oxidative dehydrogenation of ethane — are analyzed with quantitative structure–activity correlations. The review concludes with a forward-looking assessment of SAPO-18's potential in ethylene-on-purpose production and its synergy with emerging C₁ chemistry.
Keywords: SAPO-18; AEI topology; high ethylene selectivity; MTO reaction; chabazite cage; shape-selective catalysis; light olefins
Ethylene is the world's most produced organic chemical (~200 million tons/year), the backbone of polyethylene, ethylene oxide, ethylene glycol, and vinyl chloride. Currently, ~95% of ethylene comes from steam cracking of naphtha or ethane — an energy-intensive, CO₂-emitting process operating at 800–900°C.
The MTO route — converting methanol (derived from coal, natural gas, or biomass) to light olefins over acidic zeolites — offers a carbon-neutral alternative. However, the product distribution is dictated by the zeolite topology:
| Zeotype | Topology | Main Product | Ethylene Selectivity |
|---|---|---|---|
| SAPO-34 | CHA (chabazite) | Propylene | 10–20% |
| ZSM-5 | MFI | Gasoline-range (C₅⁺) | 15–25% |
| SAPO-18 | AEI | Ethylene | 50–70% ✅ |
| SAPO-35 | LEV | Mixed C₂–C₄ | 30–40% |
SAPO-18 is the only commercial-relevant zeotype that intrinsically favors ethylene over propylene, making it the catalyst of choice for ethylene-on-purpose production via MTO. Understanding why and how requires a deep dive into its structure.
