31P MAS NMR固体核磁共振研究稀土改性Y分子筛的酸性

摘要

Detailed qualitative and quantitative information on the effects of rare-earth (RE) cations on the types (Brönsted and Lewis), strengths, and distributions of acid sites on Y zeolite was studied by solid-state 31P magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, using adsorbed trimethylphosphine oxide (TMPO) and tributylphosphine oxide (TBPO) as probe mole-cules. A total of seven 31P resonance peaks, with 31P NMR/TMPO chemical shifts atδ=78, 70, 65, 62, 58, 55, and 53, corresponding to sites with different acid strengths, were identified. The peaks atδ=78 and 70 arose from external and internal acid sites, the peaks atδ=65, 62, 58, and 53 were from internal Brönsted acid sites, and the peak atδ=55 was from internal Lewis acid sites. With increas-ing RE content, the number of medium strength Brönsted acid sites (δ=62 and 58) increased sig-nificantly, whereas those of strong Brönsted acid sites (δ=65) and weak Lewis acid sites (δ=55) decreased. These experimental results were explained in terms of the influence of framework Al, extra-framework Al, and RE cations on the Y zeolite acidity.%以三甲基磷氧(TMPO)和三丁基磷氧(TBPO)为探针分子,用31P魔角旋转核磁共振(31P MAS NMR)法对稀土改性Y型分子筛的酸性进行了定性和定量分析。结果表明,以TMPO为探针分子的31P MAS NMR谱分别在δ=78,70,65,62,58,55和53处存在与酸中心相关的吸收峰,其中δ=78和70处吸收峰与分子筛内部和外部酸性有关,δ=65,62,58和53处吸收峰归属于TMPO在分子筛内部Brönsted酸中心上的贡献,δ=55处吸收峰归属于TMPO在分子筛内部Lewis酸中心上的贡献。随着稀土含量的增加,中等强度Brönsted酸中心(δ=62和58)数量显著增加,而强Brönsted酸中心(δ=65)和较弱Lewis酸中心(δ=55)数量显著降低。结合分子筛骨架铝和非骨架铝对分子筛酸性的影响进一步探讨了稀土改型Y分子筛的酸性。