Nanosized Al-rich MFI zeolite synthesized by zeolite hydrothermal conversion in the presence of zeolite nanosheet seeds

Abstract

The development of nanosized zeolites with enhanced catalytic performance presents a significant challenge due to the need for cost-effective and sustainable synthesis methods suitable for large-scale production and practical applications. This study investigates the synthesis of nanosized Al-rich MFI zeolite without using an inorganic structure-directing agent. We performed the hydrothermal conversion of FAU to MFI zeolite in the presence of zeolite self-pillared pentasil (SPP) seeds, designated as SPP(FAU). For comparison, nanosized MFI zeolites were conventionally synthesized using an amorphous silica-alumina precursor along with zeolite seeds prepared with TPAOH, denoted as TPA(Amor.). Structural characterization confirmed the successful formation of highly crystalline MFI zeolites in both synthesis routes, wherein elongated plate-like or rod-like crystals with thicknesses ranging from approximately 10 to 35 nm were organized into secondary aggregated particles. Catalytic performance assessment using the bulky molecule 1,3,5-triisopropylbenzene demonstrated superior stability and activity for SPP(FAU) in comparison to TPA(Amor.) and commercial MFI materials, which can be attributed to the presence of large mesopores and a high external surface area. The crystallization behaviors of the SPP(FAU) and TPA(Amor.) during hydrothermal synthesis at various heating durations were also investigated. The SPP(FAU) system exhibited a rapid enhancement in crystallinity after 40 hours and maintained a relatively high solid yield throughout the hydrothermal treatment. Consequently, the specific intermediate state provided by SPP(FAU) plays a crucial role in achieving an effective mesopore distribution for the cracking of bulky molecules. The insights obtained in this study are beneficial for designing zeolites for industrial catalytic applications.