Structure Controlling And Catalytic Performance Of HZSM-5 Zeolites

HZSM-5 zeolites are widely used in the reaction processes, such as esterification, etherification, alkane cracking and methanol aromatization, due to its strong acidity, good shape selectivity and excellent stability. However, the selectivity of target products decreases and the lifetime is shortened due to the small pore size and uneven acid distribution on the HZSM-5 zeolites. Therefore, it is very important to investigate the controlling of structure and catalytic performance of the HZSM-5 zeolites.In this work, a series of modified HZSM-5 zeolite nanoparticles, micro-mesoporous composites of HZSM-5 zeolites and monolithic zeolites were prepared in order to adjust the structure of the HZSM-5 zeolites to obtain a proper pore structure and acidity, and were applied to methanol dehydration to dimethyl ether and catalytic cracking of n-decane. The structure, acidity and catalytic performance of the modified HZSM-5 zeolites were characterized by scanning electron microscopy(SEM), high-resolution transmission electron microscopy(HRTEM), X-ray diffraction(XRD), Fourier transform infrared spectroscopy(FTIR), energy dispersive spectroscopy(EDS), N2 adsorption-desorption, in-situ FTIR of adsorbed pyridine(Py-FTIR), NH3 temperature-programmed desorption(NH3-TPD) and activity evaluation.Firstly, the HZSM-5 zeolites with different particle sizes were prepared by changing the hydrothermal synthesis conditions. The diffusion performance and acidity of the HZSM-5 zeolites could be effectively adjusted in order to obtain a good catalytic performance by changing their particle sizes. The nano-HZSM-5 zeolite had a higher total specific surface area, an external specific surface area and a larger total pore volume. As a result, the nano-HZSM-5 zeolite showed higher catalytic activity, target product selectivity and longer lifetime than the micron-HZSM-5 zeolite.Secondly, the nano-HZSM-5 zeolites with the different SiO2/Al2O3 ratios and the diameter of spherical particles about 200-500 nm were prepared by the hydrothermal technique. The acidity of the HZSM-5 zeolites could be effectively controlled in order to obtain a good catalytic performance by varing the SiO2/Al2O3 ratio of the HZSM-5 zeolites. The amount of Br?nsted acid sites and the ratio of the amount of Br?nsted acid sites to the amount of Lewis acid sites showed a decreasing tendency with increasing the SiO2/Al2O3 ratio of the nano-HZSM-5 zeolites. The nano-HZSM-5 zeolite with the SiO2/Al2O3 ratio of 100 showed the highest catalytic activity, a high selectivity of target products and a long lifetime.Thirdly, the HZSM-5/MCM-41 composite molecular sieves were prepared by employing the HZSM-5 zeolites with the different particle sizes and SiO2/Al2O3 ratios as raw materials. The structure and property of the composite molecular sieves depended on the parent HZSM-5 zeolites. The composite molecular sieve prepared with the nano-HZSM-5 zeolite as raw materials consisted of spherical particles and irregular particles, and had a higher total specific surface area, an external specific surface area and a larger total pore volume. Reducing the crystal size of the HZSM-5 zeolites could effectively improve their diffusibility resulting in a good catalytic performance. A proper SiO2/Al2O3 ratio of the HZSM-5 zeolites benefited their acidity distribution, and then enhanced good catalytic performance of the composite molecular sieves. The composite molecular sieve with the nano-HZSM-5 zeolite with the SiO2/Al2O3 ratio of 150 as raw materials had the highest catalytic activity, a high selectivity of target products and a long lifetime.Finally, the monolithic HZSM-5 zeolites were prepared by the impregnation method with the nano-HZSM-5 zeolite or the corresponding HZSM-5/MCM-41 composite molecular sieve as active components and the Al2O3/cordierite as carrier. The treated cordierite carrier with acid could increase the specific surface area, adsorption and roughness of the carrier, which could improve the loading and stability of the binder and active component, and the introduction of the Al2O3 into the surface of the cordierite carrier to prepare the composite carrier could also raise the specific surface area and adhesion strength of the carrier, which could improve the loading and stability of the active component. In addition, the catalytic activity of the monolithic zeolites was similar to the corresponding particle zeolites, and the selectivity of light olefins over the monolithic zeolites increased and the selectivity of aromatics decreased as compared to the corresponding particle zeolites during the catalytic cracking of n-decane. Moreover, the catalytic activity of the monolithic zeolite with the HZSM-5 zeolite as the active component was higher than the monolithic zeolite with the corresponding HZSM-5/MCM-41 composite molecular sieve as the active component. However, the catalytic activity of the monolithic zeolite with the HZSM-5 zeolite as active component rapidly decreased with ongoing the stream.