Defect hydroxides in UiO-66 shuttle protons
Simulations of missing-linker UiO-66 put charge-balancing OH on under-coordinated Zr and show those sites swap between hydroxide and water by rapid proton transfer with extra-framework water.
Source
Dynamic acidity in defective UiO-66
Study at a glance
- Design
- Computational / modelling — PBE0+D3 and AIMD of defective UiO-66 proton transfer and NH3 binding
- N
- Periodic DFT/AIMD study of MOF defects — no sample N
- Population
- Defective UiO-66 zirconium MOF models
- Outcome
- Dynamic Brønsted acidity and proton-transfer barriers at defect sites
Structured fields used in claim comparison tables when every cited study has a complete layer.
Inspect source passages
Defect hydroxides in UiO-66 shuttle protons
These passages occur verbatim in the stored paper text. They support the points listed below; they do not verify every statement in the explanation.
Mean AIMD Zr⋯O distance was 2.23 Å, matching 2.24(3) Å experiment at 300 K.
Averaging the two Zr⋯O distances over the trajectory of the whole AIMD simulation, we obtain a mean Zr⋯O distance of 2.23 Å, which is in excellent agreement with the experimental Zr⋯O distance of 2.24(3) Å at the same temperature of 300 K by Trickett et al. and 2.22 Å by Øien et al. at 100 K.
Direct proton transfer barrier is 6.5 kJ mol−1, 21.0 kJ mol−1 below the O3-mediated path.
We estimate the transition barrier of this single proton transfer process in the same manner as the O3 mediated case, which is found to be only 6.5 kJ mol −1 per defect centre (at the PBE0 + D3 level of theory), 21.0 kJ mol −1 per defect centre lower than the two-step O3 mediated case.
What they did
Authors ran PBE0+D3 static optimisations and AIMD at 100–700 K on defective UiO-66, compared Zr–O distances to diffraction, estimated proton-transfer barriers, and used NH3 binding as a Brønsted-acidity probe.
What they found
Average Zr⋯O = 2.23 Å matches 300 K experiment. Direct O0–O1 proton transfer barrier is only 6.5 kJ mol−1 versus 27.5 for an O3-mediated path. NH3 binds 110.1 vs 75.8 kJ mol−1 at defect vs perfect regions. Water can leave, exposing frustrated Lewis pair-like Zr sites.
The limits
What it doesn't show
The dynamics are computed; they are invisible to time-averaged XRD and are not a measured catalytic turnover of a substrate.
Key terms
- UiO-66
- Zr6-oxo MOF with 12 BDC linkers per cluster in the ideal structure.
- Missing-linker defect
- A removed BDC2− compensated here by bound hydroxide/water.
- AIMD
- Ab initio molecular dynamics following nuclei on a DFT potential.
- μ3-OH
- Hydroxide bridging three Zr atoms on the intact cluster face.
- Frustrated Lewis pair
- Under highly activated conditions, a bare Zr and hydroxide that could cooperate.
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Quiz yourself
Missing BDC2− charge is balanced by:
Common questions
What balances the missing BDC2− charge?
Hydroxide bound to under-coordinated Zr.
How fast is proton transfer at 300 K?
About one double transfer per 15 ps with water at O3.
Direct proton-transfer barrier?
6.5 kJ mol−1 per defect.
NH3 binding at the defect vs bulk?
110.1 vs 75.8 kJ mol−1.
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