摘要
A reconstruction strategy has been developed to synthesize Cu-SAPO-34 with a wide crystallization phase region,high solid yield,and tunable Si and Cu contents.Cu-rich SAPO-34 was prepared from a Cu-amine complex,which acted as a precursor and Cu source for the reconstruction synthesis.The role of the Cu-amine complex as a template was restricted,which allowed easier control over the Cu and Si contents than in the previously reported"one-pot"synthesis method.Characterization of the material revealed that the Si(4Al)coordination environment dominates the synthesized Cu-SAPO-34 catalysts.High-temperature hydrothermal treatment increased the isolated Cu2+content slightly,and the acid sites in the low-silica catalyst are more resistant to hydrothermal treatment than those of the existing catalysts.The obtained materials,especially the low-silica Cu-SAPO-34 sample,exhibit excellent catalytic activity and hydrothermal stability for the selective catalytic reduction of NOx by NH3(NH3-SCR).In addition,the influence of the catalyst acidity on the NH3-SCR reaction was also investigated and is discussed.The high synthetic efficiency and outstanding catalytic performance make Cu-SAPO-34 synthesized by the reconstruction method a promising catalyst for the NH3-SCR process.
氨选择催化还原NOx(NH3-SCR)是重要的柴油车尾气脱硝技术.发展高效且稳定的催化剂是提升该技术指标、应对严苛排放标准的关键.近年来,以Cu-SSZ-13和Cu-SAPO-34为代表的金属离子负载分子筛催化剂材料因其较宽的活性温度窗口和高水热稳定性受到研究者的广泛关注.Cu-SSZ-13分子筛催化剂已被BASF公司商业应用.相较Cu-SSZ-13,Cu-SAPO-34具有更优的低温活性与高温水热稳定性,且合成成本低廉.但Cu-SAPO-34的低温耐水性差,当采用传统的金属离子负载过程进行反复多次离子交换时,存在分子筛结晶度下降(部分骨架结构塌陷)的风险.使用铜四乙烯五胺络合物(Cu-TEPA)作为模板剂和铜源一步合成Cu-SAPO-34可以避免反复的离子交换过程,但由于Cu-TEPA的模板导向能力过强,Cu负载量难以控制,而降低Cu-TEPA的投料量则会损失产品收率.同时,该方法合成的Cu-SAPO-34中含有大量硅岛,分子筛催化剂的水热稳定性显著降低.针对这些问题,本文提出利用一步合成的高铜含量Cu-SAPO-34作为前驱体与铜源合成Cu-SAPO-34的方法(命名为重构法).该方法不仅产品收率高,而且Cu和Si的含量/分布可控,合成样品的NH3-SCR催化性能和水热稳定性也明显提升.EDX结果显示,焙烧后的Cu-SAPO-34中铜分布均匀,说明前驱体充分参与了Cu-SAPO-34的重构.13C NMR和元素分析结果显示,相较一步法合成的Cu-SAPO-34,重构的Cu-SAPO-34中的四乙烯五胺在模板剂中的比例明显下降.Cu-TEPA限域在前驱体的CHA笼中,有效抑制其结构导向剂的作用.Cu引入量由前驱体加入量决定.29Si NMR结果显示,Cu-SAPO-34中的硅主要呈Si(4Al)分布,这有利于Cu-SAPO-34保持良好的高温水热稳定性.NH3-SCR反应结果显示,相较一步法合成的Cu-SAPO-34,重构的Cu-SAPO-34表现出良好的NH3-SCR反应活性和更优异的高温水热稳定性.不同硅含量的重构Cu-SAPO-34的NH3-SCR反应结果显示:在低温段,低硅含量的Cu-SAPO-34具有更高的反应活性;在高温段,高硅含量的Cu-SAPO-34具有更高的反应活性.通过NH3-TPD和原位红外漫反射分析发现,低硅含量的Cu-SAPO-34具有相对较弱的酸性,使得吸附的氨具有较高的反应性,催化剂的低温活性较高.
基金
国家自然科学基金(21476228,21606221,21676262,21991091)
中国科学院前沿科学重点研究计划(QYZDB-SSW-JSC040).
