Generating silicon-rich AFX zeolite using rigid diquaternary alkylammonium structure directing agent towards stable selective catalytic reduction of NOx by NH3

Abstract

New synthetic protocols to manipulate Si/Al ratios of small pore zeolites hold promise to find better selective NOx reduction catalyst using NH3 as reducing agent (NH3-SCR). AFX zeolite as NH3-SCR catalyst is currently impeded by the insufficient hydrothermal stability owing to the low Si/Al ratio. In this contribution, we report the synthesis of AFX zeolite using N,N-(1,4-phenylenebis(methylene))bis(N,N,N-triethylammonium) as organic structure directing agent (OSDA) to increase the framework Si/Al ratio to 7.8, giving rise to a marked enhancement of hydrothermal stability to withstand hydrothermal aging at 850 °C for up to 10 h. The increase in Si/Al ratio for AFX zeolite minimizes catalytic cycle driven by Brønsted acid site and broadens high temperature operation window (150–515 °C) for Cu-exchanged catalyst. Moreover, the hydrothermally aged Cu-AFX catalyst exhibits comparable catalytic activity to that of the fresh catalyst owing to Cu predominant distribution close to the 6-MR.