Synthesis of phosphorus-modified small-pore zeolites utilizing tetraalkyl phosphonium cations as both structure-directing and phosphorous modification agents

Abstract

A rational strategy for the synthesis of phosphorus-modified small-pore zeolites by utilizing tetraalkyl phosphonium cations as both a structure-directing agent and a phosphorous modification agent was developed. Initially, we investigated the hydrothermal conversion of FAU zeolites as a starting silica/alumina source in the presence of tetraalkyl phosphonium cations with different structures. Various types of zeolites, including small-pore zeolites with 8-MR windows such as LEV, GIS, AEI, and CHA zeolites, were obtained. Next, we applied the dual-template method with a mixture of the N-containing organic structure-directing agent (OSDA) N,N,N-trimethyl-1-adamantammonium cation and the P-containing OSDA tetraethyl phosphonium cation to the synthesis of phosphorus-modified zeolites with small pores, especially CHA zeolites. Using this method, the proportion of P-containing OSDA/N-containing OSDA in as-synthesized CHA zeolites can be easily controlled. After the calcination of the as-synthesized zeolites, CHA zeolites with different degrees of phosphorus modification were easily obtained without significant pore occlusion or toxification of catalytically active sites. The phosphorus-modified CHA zeolite had high thermal stability and retained its structure even after calcination at 1050 °C for 1 h. In addition, in the selective catalytic reduction of NOx with NH3, the Cu-loaded phosphorus-modified CHA zeolite exhibited NO conversion above 90%, even after hydrothermal treatment at 900 °C for 8 h, indicating extremely high hydrothermal stability.