Synthesis of CHA from MFI by three interzeolite transformation strategies and its application in NH3-SCR reaction

Abstract

CHA Zeolites are currently considered as the most effective catalysts to meet the increasingly stringent emission requirements of diesel vehicles. Herein, the synthesis of SSZ-13 zeolites(CHA topology) using ZSM-5 (MFI topology) with various SiO₂/Al₂O₃ ratios as parent zeolites were investigated in the presence of N,N,N, trimethtyl-1-adamantammonium hydroxide (TMAdaOH). The crystallization processes of three different strategies, that is, high silica ZSM-5 with additional Al source(HSZ + Al), completely zeolite to zeolite(CZTZ) transformation and low silica ZSM-5 with additional Si source(LSZ + Si) were compared. The results show that pure SSZ-13 zeolites with high crystallinity can be synthesized at 160 °C for only 6 h by CZTZ strategy. While for the HSZ + Al and LSZ + Si synthesis systems, the complete transformation from MFI to CHA can even be shortened to 4.5 h at 160 °C, suggesting the promoting effect of additional Al- or Si- source for MFI-CHA transformation. The rapid MFI-CHA transformation may be related to fast disintegration of parent ZSM-5 under the promotion of TMAda⁺ template. Meanwhile, the five-membered rings predominating in the MFI framework rapidly disassembled and rearranged into favorable double six-membered ring and CHA cage composite building units, thus facilitate the rapid formation the CHA framework. Additionally, the resultant samples, after Cu²⁺ exchange, showed superior catalytic activity and hydrothermal stability for the selective catalytic reduction of NOx with NH₃. The operation temperature window (NOx conversion > 90%) of HSZ-4.5 h, CZTZ-6.0 h and LSZ-4.5 h samples were all about 200 ∼ 500 °C. Among three samples, the HSZ-4.5 h presents best low-temperature catalytic activity, while CZTZ-6.0 h and LSZ-4.5 h samples show more superior hydrothermal stability.