Strongly Confined, Dense N2 and O2 at High Pressures

Confining dense simple systems at high pres-sures, at the angstrom scale, produces novel and exotic sub-nanophases of these systems [1,2]. I will first pre-sent our recent results on this type of confinement for molecular N2 and O2 in the 1D channels (about 5.5 Å diameters) of a purely SiO2 zeolite, TON, at several GPa. Structural refinements based on synchrotron X-ray dif-fraction (XRD) data, and guided by classical Monte Carlo computer simulations show that the guest molecules fill the channels of the zeolite in a very dense/packed way, with up to 4 molecules per zeolite unit cell having intermolecular distances comparable to those of the same bulk molecular systems. Raman spec-troscopy results are compatible with the structural information. The evolution of this kind of packing at higher pressures, i. e. at the tens of GPa is then very intriguing. In this respect, I will present on a recently obtained dense and highly disordered form of N2, sub-nanoconfined in a pure SiO2 zeolite, silicalite-1, with 3D pore system, under pressure, up to 50 GPa [3]. In this form, N2-N2 interactions and distances are found to be very close to those of bulk N2, as shown by Raman spec-troscopy. This result, together with those on confined molecules in TON, provides a rationale for the polymer-ization mechanism of simple C-bearing molecules in the channels of non-catalytic zeolites under pressure [4-7]. Importantly, we may also foresee the formation of strongly confined polymeric, non-molecular N forms in zeolites above 1 Mbar. Finally, I will present Raman and IR spectroscopy results on a freshly discovered sub-nano phase of dense O2, confined in TON above 10 GPa [8]. This exotic phase is made of 1D stacked clusters of O2 molecules driven by the 1D host channels of TON, and it occurs in the same pressure range where, in bulk solid O2, the O8 tetramer was observed several years ago [9,10]. Investigations were performed using diamond anvil cells, optical spectroscopy, and XRD. [1] M. Santoro, et al., PNAS U. S. A. 108, 7689 (2011) [2] J. Haines, et al., JPC C, 122, DOI: 10.1021/acs.jpcc.8b01827 (2018) [3] M. Santoro, et al., JPCL 8, 2406 (2017) [4] M. Santoro, et al., Nature Commun. 4, 1557 (2013) [5] D. Scelta, et al., Chem. Mater. 26, 2249 (2014) [6] M. Santoro, et al., Chem. Mater. 27, 6486 (2015) [7] M. Santoro, et al., Chem. Mater. 28, (2016) [8] M. Santoro, et al., in preparation [9] Lundegaard et al., Nature 443, 201 (2006) [10] Fujihisa et al., PRL 97, 085503 (2006)