Uniaxial rotational diffusion in the smectic B phase of TBBA observed by quasi-elastic neutron scattering

Dynamical behaviour of n- octane and 2,5-dimethylhexane in H-ZSM-5 zeolite catalysts of differing Si/Al ratios (15 and 140) was probed using quasi-elastic neutron scattering, to understand molecular shape and Brønsted acid site density effects on the behaviour of common species in the fluid catalytic cracking (FCC) process, where H-ZSM-5 is an additive catalyst. Between 300 and 400 K, n -octane displayed uniaxial rotation around its long axis. However, the population of mobile molecules was larger in H-ZSM-5(140), suggesting that the lower acid site concentration allows for more molecules to undergo rotation. The rotational diffusion coefficients were higher in H-ZSM-5(140), reflecting this increase in freedom. 2,5-dimethylhexane showed qualitative differences in behaviour to n -octane, with no full molecule rotation, probably due to steric hindrance in the constrictive channels. However, methyl group rotation in the static 2,5-dimethylhexane molecules was observed, with lower mobile fractions in H-ZSM-5(15), suggesting that this rotation is less hindered when fewer Brønsted sites are present. This was further illustrated by the lower activation barrier calculated for methyl rotation in H-ZSM-5(140). We highlight the significant immobilizing effect of isomeric branching in this important industrial catalyst and show how compositional changes of the zeolite can affect a range of dynamical behaviours of common FCC species upon adsorption. This article is part of a discussion meeting issue ‘Science to enable the circular economy’.

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A quasi elastic neutron scattering and molecular dynamics study of the rotational diffusion of n- and iso-alkanes in the cages of zeolite MCM-22

From Zeolites to Porous MOF Materials - The 40th Anniversary of International Zeolite Conference, Proceedings of the 15th International Zeolite Conference - Studies in Surface Science and Catalysis ◽

10.1016/s0167-2991(07)80936-0 ◽

2007 ◽

pp. 882-892

Author(s):

Shiping Huang ◽

Vincent Finsy ◽

Mark T.F. Telling ◽

Gino V. Baron ◽

Joeri F.M. Denayer

Keyword(s):

Molecular Dynamics ◽

Neutron Scattering ◽

Rotational Diffusion ◽

Elastic Neutron ◽

Elastic Neutron Scattering

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Methanol dynamics in H-ZSM-5 with Si/Al ratio of 25: a quasi-elastic neutron scattering (QENS) study

Topics in Catalysis ◽

10.1007/s11244-021-01450-z ◽

2021 ◽

Author(s):

Santhosh K. Matam ◽

C. Richard A. Catlow ◽

Ian P. Silverwood ◽

Alexander J. O’Malley

Keyword(s):

Neutron Scattering ◽

Rotational Diffusion ◽

Jump Diffusion ◽

Translational Diffusion ◽

Elastic Neutron ◽

Measurement Temperature ◽

Diffusion Dynamics ◽

Mobile Fraction ◽

Jump Diffusion Model ◽

Elastic Neutron Scattering

AbstractMethanol dynamics in zeolite H-ZSM-5 (Si/Al of 25) with a methanol loading of ~ 30 molecules per unit cell has been studied at 298, 323, 348 and 373 K by incoherent quasi-elastic neutron scattering (QENS). The elastic incoherent structure factor (EISF) reveals that the majority of methanol is immobile, in the range between 70 and 80%, depending on the measurement temperature. At 298 K, ≈ 20% methanol is mobile on the instrumental timescale, exhibiting isotropic rotational dynamics with a rotational diffusion coefficient (DR) of 4.75 × 1010 s−1. Upon increasing the measurement temperature from 298 to 323 K, the nature of the methanol dynamics changes from rotational to translational diffusion dynamics. Similar translational diffusion rates are measured at 348 and 373 K, though with a larger mobile fraction as temperature increases. The translational diffusion is characterised as jump diffusion confined to a sphere with a radius close to that of a ZSM-5 channel. The diffusion coefficients may be calculated using either the Volino–Dianoux (VD) model of diffusion confined to a sphere, or the Chudley–Elliot (CE) jump diffusion model. The VD model gives rise to a self-diffusion co-efficient (Ds) of methanol in the range of 7.8–8.4 × 10–10 m2 s−1. The CE model gives a Ds of around 1.2 (± 0.1) × 10–9 m2 s−1 with a jump distance of 2.8 (either + 0.15 or − 0.1) Å and a residence time (τ) of ~ 10.8 (either + 0.1 or − 0.2) ps. A correlation between the present and earlier studies that report methanol dynamics in H-ZSM-5 with Si/Al of 36 is made, suggesting that with increasing Si/Al ratio, the mobile fraction of methanol increases while DR decreases.

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Membranes studied using neutron scattering and NMR

Canadian Journal of Physics ◽

10.1139/p95-102 ◽

1995 ◽

Vol 73 (11-12) ◽

pp. 687-696 ◽

Cited By ~ 16

Author(s):

Myer Bloom ◽

Thomas M. Bayerl

Keyword(s):

Nuclear Magnetic Resonance ◽

Magnetic Resonance ◽

Neutron Scattering ◽

Spectral Characteristics ◽

Nmr Relaxation ◽

Elastic Neutron ◽

Fluid Membranes ◽

Dynamical Properties ◽

Elastic Neutron Scattering ◽

Complementary Techniques

After reviewing some of the basic measurements that characterize the study of physical properties of matter using neutron scattering and nuclear magnetic resonance (NMR), connections between information obtained in current research on fluid membranes using these two complementary techniques are explored in two major chapters. In the first, the type of information on the structure of fluid membranes obtained from coherent elastic neutron scattering is compared with that from NMR spectral characteristics. Then, the type of information obtained on dynamical properties from NMR relaxation (T1 and T2) measurements is compared with that from quasi-elastic neutron scattering. Examples of such connections are given with an emphasis on relationships between the time and distance scales intrinsic to neutron scattering and NMR.

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