Synthesis of binderless FAU-X (13X) monoliths with hierarchical porosity

Abstract

Self-standing binderless FAU-X monoliths with hierarchical trimodal porosity have been synthesized for the first time by a double pseudomorphic transformation. Parent silica monoliths obtained by the combination of spinodal decomposition and sol-gel process have first been synthesized. The silica monoliths have been then transformed into silica-alumina monoliths (0.25 < 0.40) in low NaOH concentration ([NaOH] = 0.24 mol/L) at 40 °C for 24 h. Silica-alumina monoliths have been then transformed into FAU-X monoliths featuring nanocrystals in the struts at high NaOH concentration ([NaOH] = 2.2 mol/L) with an aging step at 40 °C for 4 days and a crystallization step at 100 °C for 24 h. The FAU-X monoliths feature macropores with diameters adjustable from 3 to 20 μm (similar to the parent silica monolith). The skeleton of the FAU-X monoliths is formed by an aggregation of two populations of FAU-X nanocrystals (100–200 nm/400–500 nm) generating a secondary porosity between the nanocrystals of 30–1000 nm in diameter, centered at 300 nm. The FAU-X monoliths present three levels of porosity with a macropore volume of ca. 1.0 mL/g, a secondary pore volume of ca. 0.40 mL/g and a micropore volume of 0.30 mL/g. These new FAU-X monoliths with hierarchical porous structure fulfill the requirements of high performance adsorbents for continuous flow process intensification.