(360c) A Mechanistic Investigation into Basic Metal Oxide Co-Catalyst Functionality in Methanol Conversion Reactions

Small pore silicoaluminophosphate molecular sieves, for example SAPO-34, are well known to be effective for the industrially important conversion of methanol to olefins (MTO), predominately ethylene and propylene. Small pore molecular sieves tend to trap aromatic molecules in their cages, resulting in high coke selectivities and shorter catalyst lifetimes. Ground breaking research by ExxonMobil in the early 2000s led to the discovery that the use of basic metal oxide co-catalysts significantly reduced the coke selectivity in methanol conversion reactions such as MTO. These basic metal oxides may be Group 3 oxides, e.g. Y₂O₃ [US 6,995,111], Group 4 Oxides, e.g. ZrO₂ [US 6,844,291], Group 2 Oxides, e.g. MgO [US 7,208,442], or hydrotalcite [US 6,951,830]. The presence of these basic oxides in a physical mixture with a molecular sieve catalyst leads to a significant increase in catalyst lifetime, defined as the amount of methanol converted per gram of molecular sieve before methanol conversion declines to 10%. This surprising discovery raised a number of questions. First, how does the basic metal oxide reduce coke selectivity and extend catalyst lifetime? Second, how can the metal oxide co-catalyst work at a distance from the cage where the MTO chemistry is occurring? Third, why do only basic metal oxides extend catalyst life? Fourth, why is one metal oxide better than another in reducing coke selectivity? This presentation will address these questions and show how fundamental research can impact the development of industrial processes.