Local and nanoscale methanol mobility in different H-FER catalysts
Local and nanoscale methanol mobility in different H-FER catalysts
复制标题
不同 H-FER 催化剂中的局部和纳米级甲醇迁移率
DOI：
10.1039/d1cy02001c
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发表时间：
2022
影响因子：
5
通讯作者：
Porter A
中科院分区：
文献类型：
--
作者：
Porter A
用准弹性中子散射（QENS）和经典分子动力学（MD）模拟方法研究了甲醇在H-FER分子筛中的动力学行为，考察了不同Si/Al比对甲醇在不同Brønsted酸性FER催化剂中动力学行为的影响。QENS探测了甲醇饱和下的商业FER样品（Si/Al = 10）中在273-333 K下的甲醇迁移率，以及由天然来源的加纳高岭土（FER-GHA，Si/Al = 35-48）合成的FER样品中也在饱和下的甲醇迁移率。在两种样品中均观察到有限的流动性，并且各向同性旋转模型可以拟合所观察到的甲醇运动，商业样品中的平均移动的分数为20%，FER-GHA中为15%，测量的旋转扩散系数范围为0.82-2.01 × 1011 s−1。采用互补分子动力学模拟来研究在相同温度范围内，在全硅H-FER系统和Si/Al = 35的H-FER系统中，在负载量为0.6wt%（接近实验饱和）的情况下，H-FER中的甲醇迁移率，以了解布朗斯台德酸位点的存在对局部和纳米级迁移率的影响。模拟结果表明，甲醇的扩散系数显着降低后，引入的布朗斯特酸网站到系统中的高达1/3，在300 K，由于与这些网站的强烈相互作用，停留时间的顺序为2-3 ps。MD计算的平移扩散率发生在所用QENS光谱仪可观察范围之外的时间尺度上，范围为0.34-3.06 × 10−11 m2 s−1。QENS观测值从模拟中重现，给出了相同的各向同性旋转运动，旋转扩散系数落在与实验观察到的相似的范围内，在300 K和400 K之间从2.92-6.62 × 1011 s−1。
The dynamical behaviour of methanol confined in zeolite H-FER has been studied using quasielastic neutron scattering (QENS) and classical molecular dynamics (MD) simulations to investigate the effects of the Si/Al ratio on methanol dynamics in different Brønsted acidic FER catalysts. QENS probed methanol mobility at 273–333 K in a commercial FER sample (Si/Al = 10) at methanol saturation, and in a FER sample synthesised from naturally sourced Ghanaian kaolin (FER-GHA, Si/Al = 35–48), also at saturation. Limited mobility was observed in both samples and an isotropic rotation model could be fit to the observed methanol motions, with average mobile fractions of ∼20% in the commercial sample and ∼15% in the FER-GHA, with rotational diffusion coefficients measured in the range of 0.82–2.01 × 1011 s−1. Complementary molecular dynamics simulations were employed to investigate methanol mobility in H-FER over the same temperature range, at a loading of ∼6 wt% (close to experimental saturation) in both a fully siliceous H-FER system and one with Si/Al = 35 to understand the effect of the presence of Brønsted acid sites on local and nanoscale mobility. The simulations showed that methanol diffusivity was significantly reduced upon introduction of Brønsted acid sites into the system by up to a factor of ∼3 at 300 K, due to strong interactions with these sites, with residence times on the order of 2–3 ps. The MD-calculated translational diffusivities took place over a timescale outside the observable range of the employed QENS spectrometer, varying from 0.34–3.06 × 10−11 m2 s−1. QENS observables were reproduced from the simulations to give the same isotropic rotational motions with rotational diffusion coefficients falling in a similar range to those observed via experiment, ranging from 2.92–6.62 × 1011 s−1 between 300 and 400 K.