Ammonia IRMS-TPD measurements and DFT calculation on acidic hydroxyl groups in CHA-type zeolites
Ammonia IRMS-TPD measurements and DFT calculation on acidic hydroxyl groups in CHA-type zeolites
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DOI：
10.1039/b711961e
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发表时间：
2007-01-01
影响因子：
3.3
通讯作者：
Niwa, Miki
中科院分区：
文献类型：
--
作者：
Suzuki, Katsuki;Sastre, German;Niwa, Miki
通过氨红外质谱/程序升温脱附（IRMS-TPD）方法和密度泛函计算研究了H-菱沸石（CHA）沸石（Si : Al-2 = 4.2）的布朗斯台德酸度。在3644、3616、3575和3538 cm(-1)处观察到4个红外谱带，它们归因于CHA结构中四个非等价氧位点上的酸性OH基团。 3538 cm(-1)处的吸收带归属于六元环(MR)中的O( 4) H，该OH基的氨吸附能(DU)是4种OH基中最低的。其他 3 个条带分配给 8MR 中的酸性 OH 基团。据观察，8 和 6MR 中的 DU 分别为 131 (+/- 3) 和 101 kJ mol(-1)。另一方面，周期性边界条件内的密度泛函理论 (DFT) 计算得出 8 和 6MR 中这些 OH 基团的吸附能约为 1。分别为130和110 kJ mol(-1)，与实验观察到的值非常吻合。
Bronsted acidity of H-chabazite (CHA) zeolites (Si : Al-2 = 4.2) was investigated by means of ammonia infrared-mass spectrometry/ temperature-programmed desorption (IRMS-TPD) methods and density functional calculations. Four IR bands were observed at 3644, 3616, 3575 and 3538 cm(-1), and they were ascribable to the acidic OH groups on four nonequivalent oxygen sites in the CHA structure. The absorption band at 3538 cm(-1) was attributed to the O( 4) H in the 6-membered ring (MR), and ammonia adsorption energy (DU) of this OH group was the lowest among the 4 kinds of OH groups. The other 3 bands were assigned to the acidic OH groups in 8MR. It was observed that the DU in 8 and 6MR were 131 (+/- 3) and 101 kJ mol(-1), respectively. On the other hand, the density functional theory (DFT) calculations within periodic boundary conditions yielded the adsorption energies on these OH groups in 8 and 6MR to be ca. 130 and 110 kJ mol(-1), respectively, in good agreement with the experimentally-observed values.