Organic-Free Synthesis of CHA-Type Zeolite Catalysts for the Methanol-to-Olefins Reaction
2015-07-02T00:00:00Z (GMT) by
Chabazite (CHA)-type zeolites are prepared from the hydrothermal conversion of faujasite (FAU)-type zeolites, dealuminated by high-temperature steam treatments (500–700 °C), and evaluated as catalysts for the methanol-to-olefins (MTO) reaction. The effects of temperature and partial pressure of water vapor during steaming are investigated. Powder X-ray diffraction (XRD) and Ar physisorption data show that the steam treatments cause partial structural collapse of the zeolite with the extent of degradation increasing with steaming temperature. <sup>27</sup>Al MAS NMR spectra of the steamed materials reveal the presence of tetrahedral, pentacoordinate, and octahedral aluminum. NH<sub>3</sub> and <i>i</i>-propylamine temperature-programmed desorption (TPD) demonstrate that steaming removes Brønsted acid sites, while simultaneously introducing larger pores into the CHA materials that make the remaining acid sites more accessible. Acid washing the steamed CHA-type zeolites removes a significant portion of the extra-framework aluminum, producing an increase in the bulk Si/Al ratio as well as the adsorption volume. The proton form of the as-synthesized CHA (Si/Al = 2.4) rapidly deactivates when tested for MTO at a reaction temperature of 400 °C and atmospheric pressure. CHA samples steamed at 600 °C performed the best among the samples tested, showing increased olefin selectivities as well as catalyst lifetime compared to the unsteamed CHA. Both lifetime and C<sub>2</sub>–C<sub>3</sub> olefin selectivities are found to increase with increasing reaction temperature. At 450 °C, CHA steamed at 600 °C reached a combined C<sub>2</sub>–C<sub>3</sub> olefin selectivity of 74.2% at 100% methanol conversion, with conversion remaining above 80% for more than 130 min of time-on-stream (TOS) before deactivating. More stable time-on-stream behavior is observed for 600 °C-steamed CHA that underwent acid washing: conversion above 90% for more than 200 min of TOS at 450 °C with a maximum total C<sub>2</sub>–C<sub>3</sub> olefin selectivity of 71.4% at 100% conversion.