Synthesis of Nano Cu-SAPO-34 Zeolite and Its Catalytic Performance of NH₃-SCR AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY25010028

Chinese Library Classification Number:O643

Citation Information:

NO_x contained in the exhaust gas emitted by motor vehicles not only causes serious harm to the environment but also poses a threat to human health. Given that the current National Ⅵ B emission standards impose increasingly strict limits on NO_x, there is an urgent need to develop more efficient selective catalytic reduction of nitrogen oxides with ammonia (NH₃-SCR) catalysts. The small crystal grain size of nano-molecular sieves not only endows them with a high specific surface area and rich pore structure, providing more adsorption sites for gases, but also effectively reduces the influence of internal diffusion resistance on the mass transfer rate, thereby enhancing catalytic performance. Among the myriads of molecular sieve catalysts, Cu-SAPO-34 stands out due to its versatility in employing various templating agents and its exceptional high-temperature hydrothermal stability, coupled with superior NH₃-SCR catalytic performance. Consequently, it holds promising potential for commercial applications. Meanwhile, compared to other chemical products, the natural mineral attapulgite boasts abundant reserves, low cost, and convenient transportation, making it a viable silicon source for synthesizing molecular sieves. Currently, there is limited research on synthesizing SAPO-34 molecular sieves using attapulgite as the silicon source, which presents a novel direction for exploration. In this work, nano SAPO-34 molecular sieves were successfully synthesized via a dynamic hydrothermal method, employing natural attapulgite as the silicon source, pseudoboehmite as the aluminum source, and phosphoric acid as the phosphorus source. Additionally, triethylamine and tetraethylammonium hydroxide were selected as co-templating agents. Subsequently, a series of Cu-SAPO-34 molecular sieve catalysts was prepared using the impregnation method. In order to delve deeply into the impact of the silicon-to-aluminum ratio (Si/Al) on catalytic performance, we conducted a systematic series of experimental studies. Comprehensive characterization of the catalyst's structure and physicochemical properties was carried out using various techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) analysis for specific surface area and pore size distribution, ammonia temperature-programmed desorption (NH₃-TPD), hydrogen temperature-programmed reduction (H₂-TPR), ultraviolet-visible spectroscopy (UV-Vis), and electron paramagnetic resonance (EPR). The experimental results showed that when Si/Al ratio was between 0.13 and 1, the crystal size of all samples was smaller than 1 μm, and with the increase of Si/Al ratio, the crystallinity of SAPO-34 gradually decreased, accompanied by a reduction in crystal size. Nevertheless, all samples maintained a high specific surface area, with a surface area of more than 600 m²·g^-1 and a pore size of about 0.49 nm. The results of NH₃-TPD showed that Si/Al ratio had a significant effect on the acid amount of Cu-SAPO-34 zeolite catalyst. Specifically, when Si/Al ratio was either too low or too high, the amount of medium and strong acids decreased；when Si/Al ratio increased from 0.4 to 0.6, the amount of medium and strong acids increased with the increase of Si/Al ratio. When Si/Al ratio exceeded 0.6, the acidic sites decreased due to the increase in the number of silicon islands, and the acid content of medium and strong acids also decreased. Therefore, when Si/Al ratio was 0.6, the medium and strong acid content of Cu-SAPO-34 catalyst reached the maximum value. Further UV-Vis and H₂-TPR analyses showed that copper species in Cu-SAPO-34 molecular sieve catalysts were mainly in the form of CuO, Cu⁺ and isolated Cu²⁺. Variations in Si/Al ratio did not exert an influence on the types of copper species. EPR test results showed that at normal temperature, the isolated Cu²⁺ existed in the six-membered ring of CHA structure only in a set-off way. At the same time, Si/Al ratio also affected the amount of isolated Cu²⁺ as the active site in Cu-SAPO-34 catalyst. H₂-TPR and EPR quantitative analyses showed that the content of isolated Cu²⁺ was lower in both Si/Al ratios greater than or less than 0.6 than in Si/Al ratios of 0.6, with a trend consistent with the quantity of medium strong acids, which was because isolated Cu²⁺ was generated by an ion exchange between Cu²⁺ and the H⁺ occurrence of the acidic site of Cu-SAPO-34 catalyst, and a higher amount of acid means that more isolated Cu²⁺ active sites could be formed. In summary, the sample with Si/Al ratio of0.6 exhibited the highest acid content and the most isolated Cu²⁺, and the catalyst exhibited the best activity in the NH₃-SCR reaction with 99.29% NO_x conversion and the widest temperature window of 250~550 ℃. This research result provided a new idea for the development of an efficient NH₃-SCR catalyst.