(714f) Structural Changes and Catalytic Performance of SAPO-34 after Prolonged Hydrothermal Treatments

Silicoaluminophosphate SAPO-34 is used commercially as a solid acid catalyst in the Methanol-to-Olefins (MTO) process due to high selectivity to ethylene and propylene and deactivation resistance. However, the catalyst is continuously exposed to water at high temperatures inside the fluidized catalytic reactor and during oxidative regeneration, eventually leading to loss in performance. In this work we have studied transformations taking place in SAPO-34 after hydrothermal treatment under industrially relevant temperatures and timeframes. Samples were treated at atmospheric pressure and 95% steam inside a horizontal quartz tube furnace for up to 10 days at temperatures 600 – 750 ^oC. Detailed characterisation before and after steaming was obtained using the following techniques: thermal analysis, diffraction, elemental composition, electron micrographs, acid site titration with ammonia, N₂ adsorption, microcatalytic testing, solid state NMR and synchrotron IR microspectroscopy.

Three samples of SAPO-34 in the protonic form are compared. Depending on the deactivation conditions and the Si content of the parent sample we observed different rates of Brønsted acid site loss, a redistribution of Si bonds, and/or creation of mesopores. Steaming samples at higher temperatures (above 700 ^oC) caused severe structural changes, loss of micropore volume, and an optically dark core inside individual SAPO-34 crystals. Compositionally, the core was found to be a SAPO phase, but a catastrophic loss of structure was visible in SEM micrographs. Infrared concentration maps of the Brønsted acid sites from individual crystals revealed that the crystal core had no hydroxyls left after steaming. From these data a two-stage mechanism is outlined of the deactivation processes that occur on the individual crystal level.