Sequential steps of ammoniation of the microporous silicoaluminophosphates H-SAPO-34 and H-SAPO-37 investigated by in situ CF MAS NMR spectroscopy
Sequential steps of ammoniation of the microporous silicoaluminophosphates H-SAPO-34 and H-SAPO-37 investigated by in situ CF MAS NMR spectroscopy
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DOI：
10.1021/jp030249d
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发表时间：
2004-03-11
影响因子：
3.3
通讯作者：
Hunger, M
中科院分区：
文献类型：
--
作者：
Buchholz, A;Wang, W;Hunger, M
通过原位H-1和Al-27 MAS NMR研究硅铝磷酸盐型沸石H-SAPO-34和H-SAPO-37在连续流动（CF）条件下的氨化反应，确定了两步吸附过程。第一氨化步骤由氨仅在布朗斯台德酸性桥接OH基团（SiOHA 1）处的吸附组成，导致铵离子（NH 4-形式）的形成。第二个氨化步骤发生在较高的氨覆盖率下，由氨分子与框架Al原子的配位组成，相当于P-O-Alequivalent到桥。该第二吸附步骤导致铝配位从四面体配位变为八面体配位。在413 K吹扫时，氨与Al原子的配位是可逆的。NH 4-型硅铝磷酸盐（氨化桥接OH基团）的水合作用导致水分子仅与Al原子配位，相当于P-O-与桥的配位，导致骨架水解。因此，如果桥接OH基团（SiOHA 1）以及铝磷酸盐骨架（相当于P-O-Alevalent）被氨覆盖，则硅铝磷酸盐骨架的水解受阻。后者可以支持预加载的氨对H-型硅铝磷酸盐朝向水合和弱水热处理的稳定作用，如最近对H-SAPO-34观察到的。
By studying the ammoniation of the silicoaluminophosphate-type zeolites H-SAPO-34 and H-SAPO-37 by in situ H-1 and Al-27 MAS NMR under continuous-flow (CF) conditions, a two-step adsorption process was determined. The first ammoniation step consists of an adsorption of ammonia exclusively at Bronsted acidic bridging OH groups (SiOHAl), leading to the formation of ammonium ions (NH4-form). The second ammoniation step, which occurs at a higher ammonia coverage, consists of a coordination of ammonia molecules to framework Al atoms in equivalent toP-O-Alequivalent to bridges. This second adsorption step causes a change of the aluminum coordination from a tetrahedral coordination to an octahedral coordination. The ammonia coordination to Al atoms is reversible when the material is purged at 413 K. The hydration of NH4-form silicoaluminophosphates (ammoniated bridging OH groups) causes a coordination of water molecules exclusively to Al atoms in equivalent toP-O-Alequivalent to bridges, leading to a hydrolysis of the framework. Therefore, a hydrolysis of the silicoaluminophosphate framework is hindered if the bridging OH groups (SiOHAl), as well as the aluminophosphate framework (equivalent toP-O-Alequivalent to), is covered by ammonia. The latter may support the stabilizing effect of preloaded ammonia on H-form silicoaluminophosphates toward hydration and weak hydrothermal treatments, as recently observed for H-SAPO-34.