The Mordenite-type zeolite (MOR) is one of the most popular commercial zeolite structures because of its large surface area and silica content, excellent separation performance for gas molecules, and sustainable synthesis. However, MOR zeolite shows a lower activity in the liquid-phase alkylation of benzene due to the low pore accessibility of acid sites for pyridine (its similar size to benzyl alcohol and benzene) [1, 2]. In order to improve this catalytic property, MOR is often functionalized with diazonium derivatives or other organic molecules to increase the accessibility of acid sites through the modification of pore sizes.
Among all these strategies, the reduction in the diffusion length of molecules between the liquid phase and MOR pore channels has received the most attention. To achieve this, MOR zeolite is often treated by top-down methods such as dealumination and desilication or through bottom-up processes such as silica addition to the reaction mixture and subsequent calcination. The MOR zeolite pore system is formed by five rings connected to each other, and the one-dimensional channel system extends through the centers of these rings. The pore diameters of the rings vary depending on their position in the structure (e.g., the eight-ring MOR zeolite has a much larger pore diameter than the 12-ring MOR zeolite) [3, 4].
To increase the surface area of MOR zeolite and enhance the accessibility of acid sites for pyridine and benzyl alcohol, the authors used a novel method to prepare nanocrystalline MOR zeolite by hydrothermal synthesis with CTAOH as the sole organic structure-directing agent, in which the aluminum source was varied from Al(NO3)3 to AlCl3 and Al(OH)3. The catalysts were characterized for their performance in the alkylation reaction of benzene with benzyl alcohol and bifunctional hydroconversion of n-hexadecane.
The structural characteristics of the synthesized MOR zeolite were analyzed by powder X-ray diffraction (XRD) using a MPD X'Pert Pro diffractometer operated with Cu Ka radiation and an Angular Range 2nd detector at 22 degC. The crystallinity of the zeolite was determined by measuring the relative intensity of the diffraction peaks at 282, 354, and 480
The results showed that the synthesis of MOR zeolite with small crystallization temperature and large mesoporous volume with NMP-assisted strategy was successful. The synergistic effect of Na + in the synthesis solution effectively promoted the nucleation of MOR and suppressed the TEAOH decomposition during the synthesis, which is beneficial for the reuse of TEAOH. The NMP-assisted synthesis process also resulted in a highly crystalline MOR zeolite with high silica content and a large mesoporous volume. The adsorption capacity of CO2, CH4, and N2 at 298, 333, and 298 K were determined for the MOR zeolite with different treatment periods of the dealumination process, which confirmed its enhanced separation performance. This study provides a simple and effective method to prepare MOR zeolite with desirable properties for gas separations, such as improved adsorption capacity and high separation efficiency.
Tags:hy zeolite
