The concurrent propagation of the aromatics-based and olefins-based catalytic cycles at early stages of the methanol-to-olefins reaction over HSAPO-34 and the resulting consequences on light olefins selectivities are demonstrated with 13C3-propylene/12C2-dimethyl ether isotopic tracing studies at 623 K and sub-complete dimethyl ether conversions. Transients in effluent product selectivities were rationalized by the maturation of the entrained hydrocarbon pool where catalyst turnover number is introduced as a compendious descriptor of hydrocarbon pool maturity and reaction progress. The distinct 13C-content of ethylene from other effluent products and its agreement with the 13C-content of entrained polymethylbenzenes indicate that ethylene is a product of aromatics-based dealkylation events while the match between methylation-predicted and experimentally observed 13C-contents for C5+ olefins establishes that they are products of olefins-based methylation events. Methanol-to-olefins conversion proceeds through a dual cycle mechanism proposed earlier for methanol conversion over other solid acid catalysts where the topology of HSAPO-34 specifically engenders the prevalence of the aromatics-based cycle at >∼200 molcmol-1H+ catalyst turnovers.
Bibliographical noteFunding Information:
The authors thank Dr. Yu Liu and Dr. Dean M. Millar (The Dow Chemical Company) for synthesis of the SAPO-34 sample. We gratefully acknowledge the financial support for this research from The Dow Chemical Company and the United States National Science Foundation under Grant No. CBET-1055846 . Powder X-ray diffraction and scanning electron microscopy were performed at the University of Minnesota Characterization Facility which receives partial support from NSF through the MRSEC program.
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- Dual cycle
- Hydrocarbon pool