0.3 Å Makes the Difference: Dramatic Changes in Methanol-to-Olefin Activities between H-ZSM-12 and H-ZSM-22 Zeolites

Two one-dimensional zeolite catalysts with similar pore structures and 0.3 angstrom difference in pore sizes (5.6 angstrom x 6.0 angstrom for H-ZSM-12 and 4.6 angstrom x 5.7 angstrom for H-ZSM-22) were exploited to gauge the critical sizes of the key intermediate species formed during the methanol conversion to olefins. Both zeolite catalysts are characterized by powder X-ray diffraction, scanning electron microscopy, (27)Al solid-state NMR spectroscopy, NH(3) temperature-programmed desorption, and N(2) adsorption/desorption analysis. In the methanol-to-olefin (MTO) process, the hydrocarbon-pool mechanism worked on the H-ZSM-12 zeolite with the 6.0 angstrom pore size, while the catalytic cycle could not be completed inside the H-ZSM-22 zeolite with the smaller 5.7 angstrom pore size. This finding strongly confirms that the hydrocarbon-pool mechanism is a space-demanding process and suggests that the size of key reactive hydrocarbon-pool intermediates for the MTO process is between 5.7 and 6.0 angstrom. These results will provide a strong experimental base for both theoretical and experimental studies in the MTO process.