Fully Copper‐Exchanged High‐Silica LTA Zeolites as Unrivaled Hydrothermally Stable NH₃‐SCR Catalysts

Abstract

Tighter CO2 emission standards from mobile sources are being legislated globally in order to address the concerns regarding anthropogenic climate change. Hence, automotive manufacturers have developed a variety of new propulsion systems, including battery electric, plug-in hybrid, and even fuel cell electric vehicles.[1] However, there is a general consensus that internal combustion engines will continue to dominate the market for the foreseeable future. [1,2] As such, fuel-efficient combustion technologies like diesel engines offer superior green-house gas reduction potential. One technical obstacle to broader diesel implementation is the required lean NOx aftertreatment system, especially to meet upcoming strict emission regulations. NOx is extremely difficult to reduce under an oxygen-rich environment.[3] Although its selective catalytic reduction by urea (urea-SCR) has recently been commercialized, the operation window of this technology is severely limited by the decomposition temperature (~ 200 °C) of urea into NH3 and by SCR catalyst deactivation at temperatures higher than 750 °C.[4]This prohibits closer placement of the catalyst to the engine, requiring an aggressive warm-up with extra fuel burning during the cold-start. Furthermore, when integrating a mandatory particulate filter in the modern diesel aftertreatment system to mitigate soot and ash, the frequent regeneration of diesel particulate filters is required before a certain accumulation of soot, resulting in large temperature spikes. Improvement of the thermal durability of the SCR catalysts would, therefore, be the key to maximizing the fuel efficiency, as well as to producing clean emissions from diesel engines. Metal-exchanged zeolites have drawn much attention as diesel vehicle SCR catalysts, and with copper-exchanged ZSM-5 and SSZ-13, which are medium-and small-pore zeolites with MFI and CHA topologies, respectively, [5] have been most widely studied for this reaction.[4] Cu-SSZ-13 has recently been implemented as the current standard catalyst in the mobile SCR technology because of its superior thermal durability compared to already known catalysts. When aged at 850 °C, however, even this catalyst, whose fresh form achieves greater than 90% NOx conversion at 250 -400 °C in steady state, loses its CHA structure and forms copper oxide (CuOx) species, leading to severe activity loss. [6] Although zeolite A (framework type LTA) is the first synthetic zeolite to be prepared, [7] its catalytic applications have long been severely restricted due to its poor thermal stability originating from the high framework Al content (Si/Al = 1.0). However, a recent success in the benzylimidazolium-mediated synthesis of its unprecedented high-silica (Si/Al > 8) form provides a key advantage in terms of structural stability with tunable loading of catalytically-active metal centers.[8] Here we report that when the copper ion exchange level increases to 100% (Cu/Al = 0.50), the high-silica (Si/Al = 16-23) Cu-LTA catalysts hydrothermally aged at 900 °C, i.e., t...

Applications of Zeolites in Sustainable Chemistry

“…Recently, Cu-exchanged high-silica LTA zeolites were reported to be a promising alternative to commercialized Cu-SSZ-13. 59,60 Cu-LTA exhibited excellent NH 3 -SCR activity under simulated vehicle conditions even after hydrothermal aging at 900 C, a critical temperature that current commercial Cu-SSZ-13 catalyst could not overcome. Such superior performance was attributed to the Cu 2+ ions bound at the center of 6-rings in Cu-LTA, which acted not only as a catalytically active center but also as a dealumination suppressor.…”

Transient modes of zeolite surface growth from 3D gel-like islands to 2D single layers

“…In this study, we focus on LTA (or zeolite A), which is a three-dimensional small-pore (0.42-nm) zeolite. Conventional synthesis of LTA results in a high-aluminum content (Si/Al = 1.0) that is ideal for adsorption and ion exchange 16 , whereas high-silica LTA (Si/Al > 10) has garnered interest as a catalyst owing to recent findings 17 that Cu-exchanged LTA possesses unique properties for the selective catalytic reduction of NOx. Hypotheses of LTA crystallization derived from spectroscopy 18 , 19 , microscopy 20 , 21 , and molecular simulations 22 , 23 postulate diverse pathways that include gel transformations 24 , precursor agglomeration and densification 25 , and the attachment of composite building units (CBUs) 26 , 27 .…”

SO2 promoted in situ recovery of thermally deactivated Fe2(SO4)3/TiO2 NH3-SCR catalysts: From experimental work to theoretical study

“…The selective catalytic reduction of NOx with NH3 (NH3-SCR) is an eff ective approach to controlling the emission of NOx from the com-bustion of fossil fuels [1][2][3][4]. This approach has also been shown to be highly successful for NOx abatement in the exhausts of diesel engines [5][6][7]. Current commercial SCR catalysts are typically vanadium-based materials which are due to their high efficiency for NOx removal and excellent tolerance of SO2.…”