High Proton Conduction at above 100 °C Mediated by Hydrogen Bonding in a Lanthanide Metal–Organic Framework

A lanthanide
metal–organic framework (MOF) compound of the
formulation [Eu2(CO3)­(ox)2(H2O)2]·4H2O (1, ox =
oxalate) was prepared by hydrothermal synthesis with its structure
determined crystallographically. Temperature-dependent but humidity-independent
high proton conduction was observed with a maximum of 2.08 ×
10–3 S cm–1 achieved at 150 °C,
well above the normal boiling point of water. Results from detailed
structural analyses, comparative measurements of conductivities using
regular and deuterated samples, anisotropic conductivity measurements
using a single-crystal sample, and variable-temperature photoluminescence
studies collectively establish that the protons furnished by the Eu­(III)-bound
and activated aqua ligands are the charge carriers and that the transport
of proton is mediated along the crystallographic a-axis by ordered hydrogen-bonded arrays involving both aqua ligands
and adjacent oxalate groups in the channels of the open framework.
Proton conduction was enhanced with the increase of temperature from
room temperature to about 150 °C, which can be rationalized in
terms of thermal activation of the aqua ligands and the facilitated
transport between aqua and adjacent oxalate ligands. A complete thermal
loss of the aqua ligands occurred at about 160 °C, resulting
in the disintegration of the hydrogen-bonded pathway for proton transport
and a precipitous drop in conductivity. However, the structural integrity
of the MOF was maintained up to 350 °C, and upon rehydration,
the original structure with the hydrogen-bonded arrays was restored,
and so was its high proton-conduction ability.