Synthesis and evaluation of Cu/SAPO-34 catalysts for NH3-SCR 2: Solid-state ion exchange and one-pot synthesis

Abstract

Cu/SAPO-34 catalysts are synthesized using two methods: solid-state ion exchange (SSIE) and one-pot synthesis. SSIE is conducted by calcining SAPO-34/CuO mixtures at elevated temperatures. For the onepot synthesis method, Cu-containing chemicals (CuO and CuSO 4) are added during gel preparation. A high-temperature calcination step is also needed for this latter method. Catalysts are characterized with surface area/pore volume measurements, temperature programmed reduction (TPR), electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectroscopies, and scanning electron microscopy (SEM). Catalytic properties are examined using standard ammonia selective catalytic reduction (NH 3-SCR) and ammonia oxidation reactions. For Cu/SAPO-34 samples prepared by SSIE, Cu presents both as isolated Cu 2+ ions and unreacted CuO. The former are highly active and selective in NH 3-SCR, while the latter catalyzes a side reaction; notably, the non-selective oxidation of NH 3 above 350 C. Using the one-pot method followed by a high-temperature aging treatment, it is possible to form Cu/SAPO-34 samples with predominately isolated Cu 2+ ions at low Cu loadings. However at much higher Cu loadings, isolated Cu 2+ ions that bind weakly with the CHA framework and CuO clusters also form. These Cu moieties are very active in catalyzing non-selective NH 3 oxidation above 350 C. At very low reaction temperature temperatures (< 155 C), standard NH 3-SCR over Cu/SAPO-34 catalysts appears to be kinetically limited. However at higher temperatures, multiple rate limiting factors are possible.

Impact of Copper Loading on NH3-Selective Catalytic Reduction, Oxidation Reactions and N2O Formation over Cu/SAPO-34

“…At lower Cu contents, they are positioned at sites in the 6-membered rings and at higher contents, also within the large cages [19]. There are indications that Cu ions may be sited similarly in Cu/SAPO-34 [11]. In addition to its importance for SCR activity, copper loading may also yield useful information on the reaction selectivities and pathways, as we have previously shown for Cu/Beta [16].…”

“…Thus, a number of studies have been published on Cu/SAPO-34 synthesised via one-pot synthesis or solid state ion exchange and on commercially-produced catalysts [11,12]. However, very few details are available as to the procedure of aqueous ion exchange for Cu/SAPO-34.…”

“…At lower Cu contents, they are positioned at sites in the 6-membered rings and at higher contents, also within the large cages [19]. There are indications that Cu ions may be sited similarly in Cu/SAPO-34 [11].…”

High hydrothermal stability of Cu–SAPO-34 catalysts for the NH3-SCR of NOx

“…These results suggested clearly that appropriate calcination temperature was important for the catalytic activity of Cu-SAPO-34. Gao et al [13] studied solid-state reaction between SAPO-34 and CuO under different calcination temperatures and times and found that a portion of CuO ($67%) converted to isolated Cu 2+ ions after the SSIE treatment at 800°C for 16 h. It suggested that the copper species in Cu-SAPO-34 can be redistributed to yield an active catalyst upon high temperature treatment. However, copper species distribution of Cu 5 -SAPO-34-T samples did not change distinctly ( Fig.…”

“…The aqueous solution ion exchange method involves a two-step exchange process, filtering, washing and calcining, greatly limiting its application [11,12]. The solid-state ion exchange (SSIE) method is carried out under rather harsh conditions (calcining at 800°C), leading to decomposition of the framework [13,14]. Recently, one-pot hydrothermal synthesis method used low-cost copper-tetraethylenepentamine (Cu-TEPA) as Cu source and template to prepare Cu-CHA directly which was much simpler to perform, and might be applicable at industrial scale [15].…”