Mild-Temperature Supercritical\nWater Confined in Hydrophobic\nMetal–Organic Frameworks

Abstract

Fluids under extreme confinement show characteristics\nsignificantly\ndifferent from those of their bulk counterpart. This work focuses\non water confined within the complex cavities of highly hydrophobic\nmetal–organic frameworks (MOFs) at high pressures. A combination\nof high-pressure intrusion–extrusion experiments with molecular\ndynamic simulations and synchrotron data reveals that supercritical\ntransition for MOF-confined water takes place at a much lower temperature\nthan in bulk water, ∼250 K below the reference values. This\nlarge shifting of the critical temperature (<i>T</i>_c) is attributed to the very large density of confined water\nvapor in the peculiar geometry and chemistry of the cavities of Cu₂tebpz (tebpz = 3,3′,5,5′-tetraethyl-4,4′-bipyrazolate)\nhydrophobic MOF. This is the first time the shift of <i>T</i>_c is investigated for water confined within highly hydrophobic\nnanoporous materials, which explains why such a large reduction of\nthe critical temperature was never reported before, neither experimentally\nnor computationally.