Achieving a Superlong Lifetime in the Zeolite-Catalyzed MTO Reaction under High Pressure: Synergistic Effect of Hydrogen and Water

Zeolites are usually considered to be acid catalysts, which are prone to deactivation due to the coke deposition in the hydrocarbon conversions such as methanol-to-olefins (MTO) reaction. Herein, a high-pressure MTO process with cofeeding H2 and H2O is reported, which can effectively prolong the catalytic lifetime of SAPO-34. The corresponding methanol handling capacity is about 200 times larger than that under the normal-pressure condition. Investigation reveals that the ultralong lifetime originates from the hydrogenation ability of the acid sites on SAPO-34 for aromatic species, which can hydrogenate the heavy aromatic deposits (especially the phenanthrene composed of three benene rings) to active aromatic intermediates (methylbenzenes and methylnaphthalenes) and thus slow down the evolution of coke species. A positive synergistic effect between H2 and H2O on prolonging the catalyst lifetime is observed at higher H2O partial pressure, likely resulting from the reduced barriers of hydrogenation reactions in the presence of H2O. Furthermore, the evolution pathways of coke species are markedly affected by reaction temperature, and fast deactivation may occur below 400 °C due to the formation of large molecule diadamantanes.