Magnetic Resonance Studies of the Diffusion Dynamics of Molecular Systems Relevant to Fischer-Tropsch Catalysis

The Fischer-Tropsch (FT) reaction is a heterogeneous catalytic reaction that produces synthetic linear hydrocarbon fuels from syngas. Recently there has been renewed academic and industrial interest in the reaction due to its potential to be used in the production of carbon-neutral fuels. The reaction typically occurs inside mesoporous catalyst particles. It is well known that at steady state, the catalyst mesopores are filled with heavy liquid hydrocarbon product. The diffusion of reactants and products through this liquid has a significant influence on reaction selectivity and activity, however this is currently poorly understood in the literature. This thesis presents the use of magnetic resonance techniques to understand this diffusion, and in particular how it depends on the composition of the heavy liquid product. The Cohen-Turnbull-Bueche (CTB) free volume model of diffusion is shown to be an accurate description of the diffusion of liquid hydrocarbon products. As such, a novel optimisation method is developed that is able to calibrate the model when applied to hydrocarbon mixtures confined in mesopores. This enables the reconstruction of carbon number distributions of confined mixtures using Pulsed-Field Gradient (PFG) Nuclear Magnetic Resonance (NMR) techniques. An advancement on this calibration technique is used to show that the diffusion mechanism of these liquids, in addition to the activation energy and molecular free volume, changes significantly when they are confined in mesopores compared to their bulk liquid counterpart. Further, temperature variations at FT conditions are shown to influence the diffusion mechanism. It is also found that a simple Arrhenius activation energy description of mixture diffusion fails to yield the correct enthalpic activation energy; a more careful treatment based on the CTB model yields the correct result. The CTB model is parametrised for FT reaction conditions at 220 ºC. Using spatially resolved PFG NMR, the carbon number distributions of liquid products as they form in situ inside catalyst mesopores during an operando FT reaction have been computed. Resolution of pores within a single catalyst pellet was achieved. It is shown that the average carbon number of liquid products inside pores is smaller than liquid products accumulating between pellets. Finally, an analysis of the diffusion of CO and H2 (syngas) dissolved in model FT hydrocarbon liquid confined in mesopores is presented. Using the results, a simplified model based on existing literature that depends only on the solute and the temperature is shown to describe well the temperature dependence of diffusion.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2020

Author dc:creator

• Williams, Jack

Advisor dc:contributor.advisor

• Gladden, Lynn F

• Fischer-Tropsch Catalysis
• diffusion
• porous media
• NMR
• MRI

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/f1bde392-d03f-4434-a42d-fb6e227eae69/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.71766

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/324311