Synthesis and Evaluation of Cu-SAPO-34 Catalysts for Ammonia Selective Catalytic Reduction. 1. Aqueous Solution Ion Exchange

Abstract

SAPO-34 molecular sieves are synthesized using various structure directing agents (SDAs). Cu-SAPO-34 catalysts are prepared via aqueous solution ion exchange (IE). Catalysts are characterized with surface area/pore volume measurements, temperature programmed reduction (TPR), electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) spectroscopies. Catalytic properties are examined using standard ammonia selective catalytic reduction (NH3–SCR) and ammonia oxidation reactions. During solution IE, different SAPO-34 samples undergo different extent of structural damage via irreversible hydrolysis. Si content within the samples (i.e., Al–O–Si bond density) and framework stress are key factors that affect irreversible hydrolysis. Even using very dilute Cu acetate solutions, it is not possible to generate Cu-SAPO-34 samples with only isolated Cu2+ ions. Small amounts of CuOx species always coexist with isolated Cu2+ ions. Highly active and selective Cu-SAPO-34 catalysts for NH3-SCR are readily generated using this synthesis protocol, even for SAPO-34 samples that degrade substantially during solution IE. High-temperature aging is found to improve the catalytic performance. This is likely due to reduction of intracrystalline mass-transfer limitations via formation of additional porosity in the highly defective SAPO-34 particles formed after IE.