(91c) Pressure: The Neglected Variable in High Pressure Processing

High pressures are required for many processes in the chemical, oil and gas, pharmaceutical and other industries. Examples can be found in many heterogeneous gas reactions, in the production of pharmaceuticals, and in the synthesis of high pressure solid phases of materials with desired properties, such as semiconducting or superconducting behaviour. In some cases pressures of thousands or tens of thousands of bar are necessary. For some applications even higher pressures may be needed, approaching one million bar or more; such pressures can be achieved only in diamond anvil cells for very small samples, and are not practical at present on an industrial scale. Even when possible, achieving such high pressures is very energy-intensive, expensive, and harmful to the environment. In this talk an alternative approach to the production of high pressures will be considered, based on the very strong attractive force fields exerted by some solid substrates. Such an approach is not energy-intensive, environmentally damaging or expensive, and so could lead to a promising new technology. It is mainly limited by our current lack of fundamental understanding of the atomic level processes involved.

There is an abundance of anecdotal evidence that nano-phases adsorbed on solid substrates or within nano-porous materials can exhibit high pressures as a result of the confinement^1,2. For example, phase changes and chemical reactions that only occur at high pressures in the bulk phase occur in the adsorbed film or confined phase at bulk phase pressures that are orders of magnitude lower. The pressure in the film is different in different directions; for simple surface geometries there is a pressure normal to the surface of the substrate, and one parallel to the walls components (tangential pressure).

For simple fluids in pores that are up to a few nanometers in width, molecular simulations show that both the normal and tangential pressures can be locally very high (thousands or tens of thousands of bars, for example) in the film, even though the bulk phase in equilibrium with the pore is at a pressure of one bar or less. The cause of these high in-film pressures will be discussed, and where possible comparison with experimental results will be made³.

When the molecules in the confined nano-phase react with each other chemically, or with the pore walls, it may be possible to achieve even higher tangential pressures, in the megabar range. Evidence for this is provided by recent experiments on sulfur (an insulator at ambient conditions) in narrow single-walled carbon nanotubes, carried out by Kaneko and coworkers⁴. They find that the sulfur atoms within the pore covalently bond to form a one-dimensional phase that is metallic. In the bulk phase sulfur forms a metallic phase only at pressures above 95 GPa. In our recent molecular dynamics simulations of this system⁵ we find that the sulfur atoms are covalently bonded in the pore and that they experience tangential pressures in excess of 100 GPa as a result of the strong confinement. In a second example, the nitric oxide dimer reaction, 2NO = (NO)₂, the dimer molecules interact strongly (‘chemically’) with the pore wall, leading to a 100% yield of the dimer in the pores, in contrast to a yield of less than 1% in a gas phase at the same thermodynamic conditions. Again the tangential pressures in the reacting phase near the pore walls is found to be in the megabar range.⁶

¹ Yun Long, Jeremy C. Palmer, Benoit Coasne, MaÅ‚gorzata Åšliwinska-Bartkowiak and Keith E. Gubbins, “Pressure enhancement in carbon nanopores: A major confinement effect”, Physical Chemistry Chemical Physics, 13, 17163-17170 (2011).

² Yun Long, Jeremy C. Palmer, Benoit Coasne, MaÅ‚gorzata Åšliwinska-Bartkowiak, George Jackson, Erich A. Müller and Keith E. Gubbins, “On the Molecular Origin of High Pressure Effects in Nanoconfinement: Effects of Surface Chemistry and Roughness”, Journal of Chemical Physics, 139, 144701 (2013)

³ M. Åšliwinska-Bartkowiak, H. Drozdowski, M. Kempinski, M. Jazdzewska, Y. Long, J.C. Palmer and K.E. Gubbins, „Structural Analysis of the Behavior of Water Adsorbed in Activated Carbon Fibers”, Physical Chemistry Chemical Physics, 14, 7145-7153 (2012).

⁴ Y. Fujimori, A. Morelos-Gómez, Z. Zhu, et al., “Conducting Linear Chains of Sulphur Inside Carbon Nanotubes”, Nature Comm., 4, 3162 (2013).

⁵ Cody K. Addington, J. Matthew Mansell and Keith E. Gubbins, “Computer Simulation of Conductive Linear Sulfur Chains Confined in Carbon Nanotubes”, Molecular Simulation, 43, 519-525 (2016).

⁶ Deepti Srivastava, Erik E. Santiso and Keith E. Gubbins, “Dimerization of Nitric Oxide: Effect of Confinement in Carbon Nanopores”, to be published (2017).