Surfactant-Assisted Direct Crystallization of CON-Type Zeolites with Particle Size and Acid-Site Location Controlled

CON-type boroaluminosilicate zeolites with varying Si/Al ratios ([A, B]-CON-x-C; x: Si/Al ratio; x = 200-600) were synthesized via a direct crystallization method from a synthetic gel containing cetyltrimethylammonium bromide (CTAB). We confirmed that the [Al, B]-CON-x-C zeolites obtained from the CTAB-containing gel exhibited smaller particles and almost twice the external surface area compared to those of the [Al, B]-CON-x zeolites crystalized from gels that did not contain CTAB, suggesting a considerable influence of CTAB on the crystallization process. In the methanol-to-olefin (MTO) reaction, [Al, B]-CON-x-C zeolites showed a longer catalyst lifetime compared to the corresponding [Al, B]-CON-x zeolites, together with a characteristic high propylene selectivity (∼50%). Systematic investigations using 27Al, 29Si, and 11B magic-angle spinning (MAS) NMR as well as 27Al 3QMQMAS NMR spectroscopy, together with Fourier transform infrared (FT-IR) spectroscopy with CO as the probe molecule, confirmed the negligible impact of CTAB on the local structure of [Al, B]-CON zeolites. On the other hand, a lower amount of Al and related Brønsted-acid sites on the external surface of [Al, B]-CON-x-C zeolites relative to the corresponding [Al, B]-CON-x zeolites was confirmed by FT-IR spectroscopy using quinoline (Qu) as the probe molecule. A systematic investigation of the catalytic MTO reaction, together with the structural, physicochemical, and spectroscopic analyses unveiled a dual role for CTAB during the crystallization process, i.e., control over the particle size and over the location of Al sites within the particles, which contributes to prolonging the MTO catalytic lifetime without changing the acidic properties and undermining the propylene selectivity.