Suppr超能文献

沸石中的悬浮效应:正戊烷异构体在沸石 NaY 中的准弹性中子散射和分子动力学研究。

Levitation effect in zeolites: Quasielastic neutron scattering and molecular dynamics study of pentane isomers in zeolite NaY.

机构信息

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India.

出版信息

J Chem Phys. 2010 Apr 14;132(14):144507. doi: 10.1063/1.3367894.

Abstract

We report the quasielastic neutron scattering (QENS) and molecular dynamics (MD) investigations into diffusion of pentane isomers in zeolite NaY. The molecular cross section perpendicular to the long molecular axis varies for the three isomers while the mass and the isomer-zeolite interaction remains essentially unchanged. Both QENS and MD results show that the branched isomers neopentane and isopentane have higher self-diffusivities as compared with n-pentane at 300 K in NaY zeolite. This result provides direct experimental evidence for the existence of nonmonotonic, anomalous dependence of self-diffusivity on molecular diameter known as the levitation effect. The energetic barrier at the bottleneck derived from MD simulations exists for n-pentane which lies in the linear regime while no such barrier is seen for neopentane which is located clearly in the anomalous regime. Activation energy is in the order E(a)(n-pentane)>E(a)(isopentane)>E(a)(neopentane) consistent with the predictions of the levitation effect. In the liquid phase, it is seen that D(n-pentane)>D(isopentane)>D(neopentane) and E(a)(n-pentane)<E(a)(isopentane)<E(a)(neopentane). Intermediate scattering function for small wavenumbers obtained from MD follows a single exponential decay for neopentane and isopentane. For n-pentane, a single exponential fit provides a poor fit especially at short times. Cage residence time is largest for n-pentane and lowest for neopentane. For neopentane, the width of the self-part of the dynamic structure factor shows a near monotonic decrease with wavenumber. For n-pentane a minimum is seen near k=0.5 A(-1) suggesting a slowing down of motion around the 12-ring window, the bottleneck for diffusion. Finally, the result that the branched isomer has a higher diffusivity as compared with the linear analog is at variation from what is normally seen.

摘要

我们报告了在 NaY 沸石中戊烷异构体扩散的准弹性中子散射(QENS)和分子动力学(MD)研究。对于这三种异构体,垂直于长分子轴的分子截面发生变化,而质量和异构体-沸石相互作用基本保持不变。QENS 和 MD 结果均表明,在 300 K 下,支链异构体新戊烷和异戊烷在 NaY 沸石中的自扩散系数高于正戊烷。这一结果为自扩散系数与分子直径的非单调、异常依赖性(即悬浮效应)的存在提供了直接的实验证据。从 MD 模拟中得出的瓶颈处的能垒存在于正戊烷中,正戊烷处于线性区域,而在新戊烷中则不存在这种能垒,新戊烷显然处于异常区域。活化能的顺序为 E(a)(正戊烷)>E(a)(异戊烷)>E(a)(新戊烷),与悬浮效应的预测一致。在液相中,D(正戊烷)>D(异戊烷)>D(新戊烷),E(a)(正戊烷)<E(a)(异戊烷)<E(a)(新戊烷)。从 MD 得到的小波数的中间散射函数对于新戊烷和异戊烷遵循单一指数衰减。对于正戊烷,单指数拟合在短时间内提供了较差的拟合。笼驻留时间对于正戊烷最大,对于新戊烷最小。对于新戊烷,动态结构因子的自部分的宽度随波数呈近单调减小。对于正戊烷,在 k=0.5 A(-1)附近观察到最小值,表明在扩散的瓶颈,即 12 元环窗口周围的运动速度减慢。最后,支链异构体的扩散系数高于线性类似物的结果与通常的观察结果不同。

文献AI研究员

20分钟写一篇综述,助力文献阅读效率提升50倍。

立即体验

用中文搜PubMed

大模型驱动的PubMed中文搜索引擎

马上搜索

文档翻译

学术文献翻译模型,支持多种主流文档格式。

立即体验