Rational design on hierarchically porous Cu/ZSM-5 zeolite for promoting low-temperature NH3-SCR performance based on protectively alkali-etching strategy

Abstract

In this study, organic structure-directing agents (OSDA) of TPA+ are used as templating agents to synthesize zeolites with different topologies, and are also served as protectants for alkali treatment to inhibit the severe silicon extraction from zeolite skeleton. The TPA+ cations could be bound to the negatively charged Al sites in the framework, so we prepared a series of Cu-based ZSM-5 catalysts with mesoporous/microporous composite structure by using TPA+ cation as a protectant for alkali treatment and evaluated the pore structures by using the hierarchy factor (HF). The results show the hierarchically mesoporous/microporous structure of the alkali-etched catalyst with the maximal HF (0.115) effectively improved the low-temperature activity on selective catalytic reduction (SCR) of NO by ammonia, achieving a complete NOx to N2 conversion at 225–400 ºC. It was shown that the TPA+ cation played an important role on protecting the crystallinity, long-range order, and pore connectivity of zeolites, and substantially widened the mesopore volume with the loss of a small portion of micropores. In addition, the alkali treatment strategy involving protectant was systematically studied, including the corresponding effect on the pore structure, redox ability, acidity, and copper ion dispersion. The N2 adsorption-desorption experiments revealed that the maximization of the HF can be obtained by adjusting the ratio of protectant to etchant (TPA+/OH-). Among them, the Cu/ZSM-5 catalyst associated with the ratio of TPA+/OH-=0.4 and the best activity has a maximum surface area of 515 m2/g, which enhances the mesopore volume (0.325 cm3/g) by approximately three times simultaneously with only a 16.9% loss of the micropore volume (0.154 cm3/g). The hierarchically porous zeolites not only enhance the diffusion of mobile hydrated copper ions and ammonia-solvated copper complexes, but also facilitate the dispersion of copper ions. This work provides a feasible strategy to rapidly enhance the low-temperature deNOx efficiency on the zeolite catalysts.