On route to one-pot synthesis of delaminated Al-SSZ-70 zeolite via partial substitution of OSDA with CTAOH surfactant

Abstract

Direct one-pot synthesis of delaminated layered zeolitic materials aims to create two-dimensionally confined catalysts for reactions involving sterically bulky reactants, which are too large to benefit from conventional three-dimensional confinement in micropores. As a prototypical system, several efforts have focused on the development of post-synthetic approaches for delamination of silica-rich MWW layered zeolite precursors, but to date this has required either harsh (halide) multi-step conditions, or the need for gemini quaternary ammonium organic surfactants for direct-synthesis routes. The one-pot synthesis of silica-rich delaminated layered zeolites consisting of MWW layers with simple quaternary ammonium surfactants such as those consisting of cetyltrimethyl ammonium (CTA) head groups remains a challenge. In this study, we describe our attempts for one-pot synthesis of delaminated Al-SSZ-70 zeolite via partial replacement of the organic structure-directing agent (OSDA) with a simple surfactant, CTAOH. Leveraging on our prior demonstration of direct one-pot synthesis of delaminated B-SSZ-70, our goal is to use these dual organic additives in Al-SSZ-70 hydrothermal synthesis, to effectively restrain the MWW layer stacking by capping the external surface of the MWW layer with a surfactant. The synthetic approach reported in this work leads to high-silica delaminated MWW-type zeolite Al-SSZ-70, which benefits from postsynthetic high-shear mixing of the layered zeolite precursor for further delamination. The resulting delaminated zeolites exhibit a similar Brønsted/Lewis acid site distributions as characterized by FTIR spectroscopy of adsorbed pyridine at various temperatures, and enhanced acid catalytic properties over a non-delaminated control. However, we observe a minor amorphous-phase impurity when using the high surfactant loading required for optimum delamination, which is observable using TEM imaging and low-angle powder X-ray diffraction. The presence of this amorphous phase highlights the ongoing challenge of synthesizing a fully crystalline MWW-type delaminated material via direct hydrothermal synthesis, when using simple surfactants, which remains a target for future research.