Abstract
Mesoporous Y zeolites were prepared by the sequential chemical dealumination (using chelating agents such as ethylenediaminetetraacetic acid, H4EDTA, and citric acid aqueous solutions) and alkaline desilication (using sodium hydroxide, NaOH, aqueous solutions) treatments. Specifically, the ultrasound-assisted alkaline treatment (i.e., ultrasonic treatment) was proposed as the alternative to conventional alkaline treatments which are performed under hydrothermal conditions. In comparison with the hydrothermal alkaline treatment, the ultrasonic treatment showed the comparatively enhanced efficiency (with the reduced treatment time, i.e., 5 min vs. 30 min, all with 0.2 mol·L−1 NaOH at 65°C) in treating the deal-uminated Y zeolites for creating mesoporosity. For example, after the treatment of a dealuminated zeolite Y (using 0.1 mol·L−1 H4EDTA at 100°C for 6 h), the ultrasonic treatment produced the mesoporous zeolite Y with the specific external surface area (Sexternal) of 160 m2·g·−1and mesopore volume (Vmeso) of 0.22 cm3·g−1, being slightly higher than that by the conventional method (i.e., Sextemal= 128 m2·g−1 and Vmeso = 0.19 cm3·g−1). The acidic property and catalytic activity (in catalytic cracking of n-octane) of mesoporous Y zeolites obtained by the two methods were comparable. The ultrasonic desilication treatment was found to be generic, also being effective to treat the dealuminated Y zeolites by citric acid. Additionally, the first step of chemical dealumination treatment was crucial to enable the effective creation of mesopores in the parent Y zeolite (with a silicon-to-aluminium ratio, Si/Al = 2.6) regardless of the subsequent alkaline desilication treatment (i.e., ultrasonic or hydrothermal). Therefore, appropriate selection of the condition of the chemical dealumination treatment based on the property of parent zeolites, such as Si/Al ratio and crystallinity, is important for making mesoporous zeolites effectively.
Highlights
Zeolites as a class of crystalline porous materials with intrinsic micropores (i.e., < 1 nm) are being used considerably by industry as adsorbents and catalysts due to their high specific surface areas, excellent hydrothermal stability, inclusion of acidity and ion exchange ability
We explored the possibility of using ultrasound irradiation to intensify the alkaline treatment step in the sequential post-synthesis treatment of pristine zeolite Y, aiming at preparing hierarchical Y zeolites more efficiently than the conventional sequential method under hydrothermal conditions
The findings of this study suggest that dealumination of the parent zeolite is necessary before the ultrasound-assisted alkaline treatment for preparing mesoporous zeolites via the post-synthesis modification of zeolites with low Si/Al ratios, exemplified by the Y zeolite
Summary
Zeolites as a class of crystalline porous materials with intrinsic micropores (i.e., < 1 nm) are being used considerably by industry as adsorbents and catalysts due to their high specific surface areas, excellent hydrothermal stability, inclusion of acidity and ion exchange ability. The microporous structure of zeolite Y imposes the accessibility and diffusion limitation on their applications, which is one of the primary contributing factors to the catalyst deactivation [2], due to the coke deposition covering acidic sites and/or clogging the. To improve the catalytic performance of zeolites, especially FAU zeolite Y for FCC, it is necessary to make zeolites with hierarchical meso-micro-pores (or mesoporous zeolites) to mitigate the accessibility and diffusion limitation. The bottom-up method can remarkably enhance pore accessibility by soft [6,7] or hard templating [8], the associated cost and environmental issues (of using templates) can be problematic for largescale applications in industry [9]. The top-down method, which creates the intracrystalline mesopores by the postsynthesis dealumination and/or desilication (via steaming, chemical treatment, acid and base washing) [9,10,11,12,13], is simple and effective, as well as being practical for industrial adoption
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