Synergistic zeolite synthesis via a fluoride-deficient mixed approach

Abstract

Here, we present a new fluoride-deficient synthesis approach to beta nanozeolites that results in synergistic effects of mineralizers on the crystallization kinetics, attained at intermediate fluoride and hydroxide concentrations (controlled by the ratio of hydrofluoric acid (HF) to organic structure-directing agent (OSDA) in hydroxide form). This approach represents an improved methodology as compared to the traditional hydroxide and fluoride routes, and it is likely transferable to other zeolites with different framework topologies. The method affords not only improved kinetics but also a control of the crystal size, reaching the nanoscale regime (≤100 nm) and framework defect concentration (≤3.5 per unit cell). Upon loading with Pd species, the resulting Pd/beta catalysts are active for wet methane combustion and their performance is dependent on the concentration of zeolite defects, crystal size, and zeolite Si/Al ratio. The 3.0 wt% Pd catalyst supported on Na⁺-post-exchanged 500 °C-calcined beta zeolite with a Si/Al ratio of 45 and a crystal size of ca. 75 nm, obtained at a synergistic mixture HF/OSDA ratio of 0.50, is the best among those studied: it shows a light-off temperature of 300 °C, together with excellent catalyst stability (over 90% methane conversion at 350 °C even after 100 h on stream), in the presence of 10% water vapor. This study provides an example where the performance of zeolite-supported metal catalysts can be significantly improved by controlling both the zeolite crystal size and defect concentration.