H-SAPO-5 as methanol-to-olefins (MTO) model catalyst: Towards elucidating the effects of acid strength

The methanol-to-hydrocarbons (MTH) reaction was studied over a moderately acidic zeotype material, H-SAPO-5, at 350–450 °C and with WHSV = 0.3–5 h−1. C3–C5 alkenes were the main products of reaction, followed by C6+ aliphatics. Conversion-selectivity plots from experiments conducted at various contact times revealed that coking did not influence product selectivity significantly. Steady-state isotope transient experiments (12CH3OH//13CH3OH) were performed at 450 °C. 13C incorporation was more rapid in the alkene products than in the polymethylated benzene molecules that were retained inside the catalyst after testing, suggesting that polymethylbenzenes contribute only to a minor extent to alkene formation in H-SAPO-5. Co-feed studies of methanol and benzene at 350 °C revealed that benzene shifts the product selectivity towards ethene and aromatic products. Co-feeding 13CH3OH and benzene at 250 °C, giving <2% conversion of both reactants, indicated that polymethylbenzenes, when present in excessive amounts, may contribute to ethene and propene, but not to C4+ alkene, formation. Furthermore, the isotopic labelling pattern of ethene provided the first direct experimental evidence for ethene formation by a paring-type reaction from polymethylated benzene intermediates. Overall, the results obtained in this study suggest that a lower acid strength promotes an alkene-mediated MTH reaction mechanism, and that acid strength is therefore an important design parameter for selectivity optimisation in zeotype catalysis.

Isotopic labelling studies of the methanol-to-hydrocarbons reaction revealed different origins of light and higher alkenes, respectively, over H-SAPO-5. The study suggests a shift in the dominating reaction cycle with a change in catalyst acid strength.Figure optionsDownload high-quality image (103 K)Download as PowerPoint slideHighlights
► Ethene formation from polymethylbenzenes proceeds by the paring reaction mechanism.
► Isobutene is predominantly formed from alkene cracking in H-SAPO-5.
► Evidence of a shift in mechanism towards aliphatic intermediates in SAPO-5 compared to previously studied large-pore zeolites such as H-Beta.