Selective catalytic reduction of NOxwith ammonia (NH3-SCR)has been regarded as the most efficient technology to facilitate the NOxreduction in oxygen-rich exhaust released from diesel engines.Metal-exchanged conventional aluminosilicatezeolites or silicoaluminophosphates (SAPO) molecular sieves (MSs) have been extensively investigated for catalysing this reaction. Although Cu-SSZ-13 catalyst has been commercialized, its thermal durability after the high-temperature(HT)hydrothermal aging, arising fromthe regenerationprocessof diesel particulate filter (DPF),can be further improved.Moreover,Cu-SAPO-34 catalyst has some disputes on the low-temperature (LT) hydrothermal stability. Therefore, it stimulates the interest of researchers to explore other novel zeolite frameworks with small pore openings. Recently, Honget al.have reported a Cu exchangedLTA-type high-silica (3D 8×8×8-ring channel system) zeolite which has the better activity and durability than Cu-SSZ-13 after HT hydrothermal aging. However, the expensive organic structure-directing agents (OSDAs) and the presence of the toxic fluoride used initsrecipe might inhibit its further utilization as an NH3-SCR catalyst. It is of interest to note that SAPO-42 with theLTAtopology might beapotential catalyst for the NH3-SCR reaction.
Recently, a research team led by Prof. Zhongmin Liu, Prof. Peng tian and Associate Professor Peng Guo from the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences developed a targeted synthesis strategy based on understanding the host (SAPO framework) - guest (OSDA) interaction unravelled by Rietveld refinement, denoted asRSS.TheRSSmethod includes three steps: 1)Refine known samples against experimental diffraction data and then identify the locations of OSDA and host - guest interactions, 2)Summarize the structural features of OSDA, and 3)Search for suitable OSDA candidates. By utilizing theRSSapproach, the synthesis of DNL-6 (RHOtopology)have been extended with a series of OSDAs predicted (J. Mater. Chem. A, 2018,6, 24186-24193). In DNL-6s, the protonated OSDA form the classical hydrogen bonding interactions with O atoms in the 8-rings, which connectltacage andd8r. Meanwhile, the shorter alkyl groups of OSDAs stretch intod8r, while the larger one can stabilize theltacage. Since SAPO-42 (LTAtopology) is constructed byltacages connected by the face-to-face mode through single 8-rings, it is highly possible that the symmetric DPA and “asymmetric” OSDAswith appropriate groups can direct the synthesis of SAPO-42. Finally, a series of cheap and commercialized OSDAs are successfully applied for the targeted synthesis of SAPO-42 in the fluoride-free system. Before this research, most SAPO-42 can only be synthesized by using the complex and expensive OSDAs in the toxic F-medium.Furthermore, Cu-SAPO-42 catalysts have been utilized for NH3-SCR. Among these catalysts, Cu-SAPO-42 prepared with 2-(Butylamino)ethanol (BAEA) as OSDA demonstrates the excellent activity even after hydrothermal aging at 800 °C for 16 h, which shows much better hydrothermal stability than the commercialized Cu-SAPO-34 catalyst with comparable Si and Cu contents. EPR (electron paramagnetic resonance) spectroscopy andRietveld refinement were performed to identify the locations of active Cu2+ions.It turns out that the active Cu2+ions are distributed near the center of single 6-rings of theltacage.
This new synthetic methodology might open an avenue for the targeted synthesis of specific SAPO MSs with tailored catalytic performance in the near future. This work published inSmall (DOI: 10.1002/smll.202000902)，special research assistant DrNana Yan andPhD. Chao Macontributed equally to this work.
Rational Design of a Novel Catalyst Cu-SAPO-42 for NH3-SCR Reaction. Nana Yan, Chao Ma, Yi Cao, Xiaona Liu, Lei Cao, Peng Guo*, Peng Tian*, Zhongmin Liu*, SMALL, 16(33):2000902, 2020.
