Revelation on the Complex Nature of Mesoporous Hierarchical FAU-Y Zeolites

Abstract

The texture of mesoporous FAU-Y (FAUmes) prepared by surfactant-templating in basic media is a subject of debate. It is proposed that mesoporous FAU-Y consists of: (1) ordered mesoporous zeolite networks formed by a surfactant-assisted zeolite rearrangement process involving local dissolution and reconstruction of the crystalline framework, and (2) ordered mesoporous amorphous phases as Al-MCM-41, which coexist with zeolite nanodomains obtained by a dissolution-reassembly process. By the present systematic study, performed with FAU-Y (Si/Al = 15) in the presence of octadecyltrimethylammonium bromide and 0 < NaOH/Si ratio < 0.25 at 115 °C for 20 h, we demonstrate that mesoporous FAU zeolites consist, in fact, of a complex family of materials with textural features strongly impacted by the experimental conditions. Two main families have been disclosed: (1) for 0.0625 < NaOH/Si < 0.10, FAUmes are ordered mesoporous materials with zeolite walls, which coexist with zeolite nanodomains (100-200 nm) and (2) for 0.125 < NaOH/Si < 0.25, FAUmes are ordered mesoporous materials with amorphous walls as Al-MCM-41, which coexist with zeolite nanodomains (5-100 nm). The zeolite nanodomains decrease in size with the increase of NaOH/Si ratio. Increasing NaOH/Si ratio leads to an increase of mesopore volume, while the total surface area remains constant, and to a decrease of strong acidity in line with the decrease of micropore volume. The ordered mesoporous materials with zeolite walls feature the highest acidity strength. The ordered mesoporous materials with amorphous walls present additional large pores (50-200 nm), which increase in size and amount with the increase of NaOH/Si ratio. This alkaline treatment of FAU-Y represents a way to obtain ordered mesoporous materials with zeolite walls with high mesopore volume for NaOH/Si = 0.10 and a new way to synthesize mesoporous Al-MCM-41 materials containing extralarge pores (50-200 nm) ideal for optimal diffusion (NaOH/Si = 0.25).

Synthesis of nanostructured catalysts by surfactant-templating of large-pore zeolites

“…The intracrystalline character of mesoporosity and long-range ordering have been previously conrmed for faujasite type materials using TEM, XRD and adsorption studies. 32,45,46 Our results extend this work to a number of large-pore zeolites and provide further evidence for the intracrystalline nature of the generated micro-mesoporous materials, rather than the co-existence of a microporous crystalline zeolite and a mesoporous amorphous silica in a composite mixture. This is based on direct information obtained from the TEM images of these materials and the shape of the H4 hysteresis loop observed in nitrogen adsorption-desorption experiments, which is indicative of the interconnected system of micro-and mesopores.…”

Time-Resolved Dynamics of Intracrystalline Mesoporosity Generation in USY Zeolite

“…high pH), a fraction of the zeolite can be dissolved and an amorphous mesoporous phase is formed ( Figure S13), as reported by García-Martínez and coworkers 4 and recently confirmed independently. 32 Therefore, it is critically important to judiciously select the mesostructuring conditions to produce high quality surfactanttemplated zeolites and to avoid the formation of spurious phases. For severely steamed zeolites (e.g.…”

Development of Bifunctional Hydrodeoxygenation Catalyst Rh‐HY for the Generation of Biomass‐Derived High‐Energy‐Density Fuels

“…0.8Rh‐HY‐R exhibited an increase in surface area, total pore volume, and external surface area compared with the HY‐C sample; however, the micropore volume and micropore surface area decreased. Remarkably, the pore diameters of these materials were similar, indicating that there was no collapse of the faujasite structure . Moreover, the increase in surface area for 0.8Rh‐HY‐R suggests that some pore opening occurred, caused by some dissolution of the silicon or aluminum framework during exposure to the acidic ion exchange in solution.…”