Formation of the V Center on H-Type Zeolites with Thermal Treatment

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• 作者：Zhenping Qu ; Mojie Cheng ; Chuan Shi ; Weixin Huang ; Weiping Zhang ; and Xinhe Bao
• 刊名：Langmuir
• 出版年：2002
• 出版时间：February 19, 2002
• 年：2002
• 卷：18
• 期：4
• 页码：1200 - 1205
• 全文大小：76K
• 年卷期：v.18,no.4(February 19, 2002)
• ISSN：1520-5827

文摘

The effect of thermal treatment on H-MCM-22 and H-ZSM-5 zeolites was investigated using theelectron spin resonance technique. A six-line signal (denoted as A, g = 2.048, A = 22.15 G) was detectedon H-MCM-22 after He purging at high temperatures, whose intensities increased with the treatingtemperature. The same signal was also found on H-ZSM-5 zeolites with different crystal sizes. Theparamagnetic center was identified as a V center, namely, a hole of an electron trapped on an oxygen atombonding to a nearby aluminum atom. These signals appeared only on a dealuminated sample or a sampleconcomitantly with dealumination. The formation of the hole might involve an electron transferring fromthe lattice oxygen to a nonframework aluminum species, and the hyperfine splitting is caused by theinteraction between the electron hole locating on the p orbit of oxygen and the framework aluminumbonding with the oxygen. The signal disappeared after the sample was exposed to air or oxygen at roomtemperature. However, the process was reversible. A new set of signals (denoted as B, g₁ = 2.008, g₂ =2.003, g₃ = 1.9985) was observed after oxygen adsorption on the H-MCM-22 pretreated with He at 973K or He purging at 973 K on the H-MCM-22 pretreated with oxygen at 813 K, which was attributed tothe O^- species.