Organic-Free Synthesis of a Highly Siliceous Faujasite Zeolite with Spatially Biased Q4 (nAl) Si Speciation

Abstract

We report the most siliceous FAU-type zeolite, HOU-3, prepared via a one-step organic-free synthesis route. Computational studies indicate that it is thermodynamically feasible to synthesize FAU with SAR=2-7, though kinetic factors seemingly impose a more restricted upper limit for HOU-3 (SAR≈3). Our findings suggest that a slow rate of crystallization and/or low concentration of Na⁺ ions in HOU-3 growth mixtures facilitate Si incorporation into the framework. Interestingly, Q⁴ (nAl) Si speciation measured by solid-state NMR can only be modeled with a few combinations of Al positioning at tetrahedral sites in the crystal unit cell, indicating the distribution of Si(-O-Si)_4-n (-O-Al)_n species is spatially biased as opposed to being random. Achieving higher SAR is desirable for improved zeolite (hydro)thermal stability and enhanced catalytic performance, which we demonstrate in benchmark tests that show HOU-3 is superior to commercial zeolite Y.

Keywords: acidity; faujasite; heterogeneous catalysis; thermal stability; zeolite.