Proton-Conductive Metal-Organic Frameworks

Explains why direct acidic-ligand synthesis can fail, reviews tag-based postsynthetic modification, and presents protection-complexation-deprotection as a route to acidic sites without unwanted coordination.

Relevance: Core · 3 · 2. Subtractive Approach for Introducing Acidic Functional Sites

Uses functional-group-modified MIL-53 to relate conductivity order, activation energy, water adsorption steps and hydrogen-bond-network density.

Relevance: Core · 4 · 3.1 Proton Conductivity and Its Mechanism of Functional Group-Modified MIL-53 · Figure 3; Figure 4

Frames proton conductors as fuel-cell materials, contrasts Nafion and cesium hydrogen sulfate with the tunability of PCP/MOF pores, and summarises early proton-conductive coordination polymers.

Relevance: Core · 3 · 1. Introduction

Treats mixed-ligand MIL-53 analogues as coordination copolymers, showing that ligand incorporation, water uptake and gate opening control humidity-dependent conductivity.

Relevance: Core · 6 · 3.2 Precise Control of Proton Conductivity by Mixing Ligands · Chart 1; Figures 5-8; Table 1

Condenses the account into design rules: increase hydrophilic/protonic media, recognise that strong acid sites are not always critical, exploit disorder, and develop films, membranes, size effects and mixed catalytic/electronic functions.

Relevance: Core · 8 · 5. Summary and Outlook

Reviews one-dimensional and two-dimensional oxalate frameworks as stable, non-aromatic proton-conductive PCPs where acidic cations, water disorder and Grotthuss-type motion support high conductivity.

Relevance: Core · 8 · 4. Proton Conductivity of Oxalato-Bridged Coordination Polymers

Routes to acidic functional sites in PCPs

The account organises acidic-site synthesis by whether the acidic group participates in coordination, is installed after framework formation, is masked then revealed, or is tolerated by robust high-valence metal frameworks.

Categories: Direct reaction of acidic ligand with metal ions · Postsynthetic tag functionalisation · Protection-complexation-deprotection · Direct functionalisation using high-valence metal nodes

3-4 · 2. Subtractive Approach

Carrier factors in ionic conductivity

The review separates carrier concentration effects from activation-energy effects, using this distinction to argue that hydrogen-bond-network design is especially important for low-temperature conductivity.

Categories: Carrier charge · Carrier density · Carrier mobility · Activation energy

7 · 3.2 Precise Control by Mixing Ligands · Equation 1

Mixed-ligand PCP arrangements

The review imports organic-copolymer language to describe mixed-ligand PCPs, then cautions that higher-dimensional framework connectivity can make the actual linker arrangement more complex.

Categories: Block · Random · Alternating · Phase-separated · More complex 2D or 3D arrangements

5 · 3.2 Precise Control by Mixing Ligands

Rules of thumb for high ionic conductivity

The review closes with three explicit design heuristics: maximise protonic media, avoid assuming strong acidity alone is decisive, and consider disorder as a beneficial feature rather than only a defect.

Categories: Less hydrophobic frameworks and more proton-conducting media · Strong acid sites are not always essential · Disordered sites can promote protonic conductivity

8 · 5. Summary and Outlook

Reference proton-conductor platforms

The review contrasts established polymer and inorganic proton conductors with PCP/MOF systems, arguing that crystalline, designable pores can expose pathway-structure relationships that are hard to control in the older platforms.

Categories: Organic polymer electrolyte such as Nafion · Inorganic solid acid such as cesium hydrogen sulfate · Porous coordination polymer or MOF

2-3 · 1. Introduction

Functional-group control in MIL-53

MIL-53 derivatives are ordered by the review according to proton conductivity and activation energy, with carboxy and hydroxy derivatives outperforming parent and amino derivatives because of stronger protonated water networks.

Categories: Carboxy-functionalised MIL-53(Fe)(COOH)2 · Hydroxy-functionalised MIL-53(Al)OH · Unsubstituted MIL-53(Al) · Amino-functionalised MIL-53(Al)NH2

4 · 3.1 Proton Conductivity · Figure 3

Hydrated M(ox).nH2O coordination polymers used as stable analogues of early proton-conductive dithiooxamidate compounds.

Conduction: Proton conductivity is interpreted as sensitive to metal-site Lewis acidity, coordinated-water proton donation and mis-stacking defects.

Representative materials: Fe(ox).2H2O · Mg(ox).2H2O · Ni(ox).2H2O · Co(ox).2H2O

Nodes / linkers: Fe · Mg · Ni · Co · Oxalate

8 · 4. Proton Conductivity of Oxalato-Bridged Coordination Polymers

Anionic [M2(ox)3] sheets paired with ammonium cations and spacer molecules, exemplified by hydrated zinc oxalate-adipate.

Conduction: High humidity produces hydrated phases with confined, disordered water and ammonium networks that support Grotthuss-type local proton motion.

Representative materials: (NH4)2(adp)[Zn2(ox)3].nH2O

Nodes / linkers: Zn · Oxalate · Adipic acid spacer · Ammonium countercation

8 · 4. Proton Conductivity of Oxalato-Bridged Coordination Polymers

Water-stable frameworks where amino, sulfonate or carboxylate functionality can be introduced without protection by using high-valence metal nodes.

Conduction: Enable functional group introduction and proton-transport studies in frameworks more stable than early PCD targets.

Representative materials: MIL-53 · MIL-101 · UiO-66 · Zr-PCPs

Nodes / linkers: Al · Fe · Zr · Amino terephthalates · Sulfonated ligands · Carboxylated ligands

4 · 2. Subtractive Approach

Water-stable 1D pore frameworks based on trivalent metal ions, terephthalate or substituted terephthalate, and hydroxo ligands.

Conduction: Bulk proton conductivity varies with acidic functional groups, water adsorption and hydrogen-bond-network density.

Representative materials: MIL-53(Fe)(COOH)2 · MIL-53(Al)OH · MIL-53(Al) · MIL-53(Al)NH2

Nodes / linkers: Al · Fe · Terephthalate · Hydroxyterephthalate · Aminoterephthalate · Dicarboxyterephthalate

4 · 3.1 Proton Conductivity

MIL-53-type mixed-ligand frameworks combining hydroxyterephthalate and aminoterephthalate linkers.

Conduction: At high humidity, higher htp fractions increase water uptake and conductivity; at low humidity, all compositions retain low conductivity dominated by a one-water network.

Representative materials: Al(OH)(htp)x(atp)1-x · MIL-53(Al)OH · MIL-53(Al)NH2

Nodes / linkers: Al · Hydroxyterephthalate · Aminoterephthalate

6 · 3.2 Precise Control by Mixing Ligands · Figures 5-8; Table 1

Zn4O terephthalate-type frameworks used to demonstrate whether protected hydroxy groups can survive complexation and later generate acidic functionality.

Conduction: Mainly a synthetic model in this review; water instability prevented successful deprotection/conductivity exploitation.

Representative materials: MOF-5 · dacoMOF-5

Nodes / linkers: Zn4O clusters · Terephthalate · Diacetoxyterephthalate · Dihydroxyterephthalate precursor

3 · 2. Subtractive Approach · Figure 2

Crystalline porous networks built from metal ions and bridging ligands whose pore size, configuration and environment can be designed.

Conduction: Provide a tunable scaffold for positioning acidic sites, water and other proton-conducting media inside crystallographically addressable pores.

Representative materials: MOF-5 · MIL-53 · MIL-101 · UiO-66

Nodes / linkers: Zn · Al · Fe · Zr · Terephthalates · Substituted terephthalates · Carboxylates

3 · 1. Introduction

Anionic oxalate sheets with ammonium and water media

Use anionic metal oxalate frameworks as counterion sheets and introduce ammonium plus water to form dense, degenerate proton-transfer networks.

Claimed effects: Generates high proton conductivity without relying on conventional strong acid groups by increasing protonic media and disorder.

Controlling variables: Metal valence · Ammonium content · Spacer molecule · Hydration state · Water disorder

Representative materials: (NH4)2(adp)[Zn2(ox)3].nH2O

Caveat: The review presents this as a mechanism-rich benchmark rather than a directly device-ready membrane.

8 · 4. Proton Conductivity of Oxalato-Bridged Coordination Polymers

Direct assembly with acidic ligands

React ligands bearing acidic groups directly with metal ions, accepting that those groups may coordinate and redirect framework formation.

Claimed effects: Can introduce proton-donating groups but often forms undesired structures such as MOF-74/CPO-27 rather than the targeted pore architecture.

Controlling variables: Ligand acidity · Metal-binding preference · Competing coordination modes

Representative materials: MOF-74 · CPO-27(M)

Caveat: The route is conceptually simple but unreliable when acidic groups bind metals strongly.

3 · 2. Subtractive Approach

Mixed-ligand coordination copolymer tuning

Combine isostructural ligands with different functional groups to tune the statistical distribution of proton-donating and hydrogen-bonding sites.

Claimed effects: Allows finer control over water uptake and hydrogen-bond networks than a single substituted linker family.

Controlling variables: Feed ratio · Ligand incorporation ratio · Relative coordination reactivity · Humidity and gate opening

Representative materials: Al(OH)(htp)x(atp)1-x · IRMOF mixed-ligand derivatives

Caveat: Incorporated composition may differ strongly from feed composition because ligands have different coordination abilities.

6 · 3.2 Precise Control by Mixing Ligands · Figure 5; Table 1

Protection-complexation-deprotection

Mask acidic groups before metal coordination, form the PCP, then deprotect so the acidic function is exposed inside the pore.

Claimed effects: Prevents acidic functional groups from coordinating during assembly and can enlarge pores after complexation.

Controlling variables: Protecting group size · Framework stability · Lewis-acid assistance · Deprotection conditions

Representative materials: dacoMOF-5 · Pillared-layer dactp/bipyridine PCPs

Caveat: Water instability of early products prevented harsh deprotection and limited practical use.

3 · 2. Subtractive Approach

Photochemical pore expansion or deprotection

Use photoactive groups so that pore expansion or catechol deprotection occurs through chemistry orthogonal to framework coordination.

Claimed effects: Can expand pores or introduce iron catecholate sites after assembly while reducing interference with coordination chemistry.

Controlling variables: Photoactive group · Irradiation conditions · Framework heat and water stability · Orthogonality to coordination bonds

Representative materials: Azide-containing PCP · UMCM-1 catecholate sites

Caveat: The cited examples were relatively heat stable but not necessarily stable against water, a serious issue for proton-conduction studies.

4 · 2. Subtractive Approach

Postsynthetic tag functionalisation

Introduce a non-coordinating tag into the framework and functionalise it after crystallisation using click or condensation chemistry.

Claimed effects: Can add functional groups without disrupting initial assembly, but small pores may block full conversion or collapse porosity.

Controlling variables: Tag identity · Pore size · Diffusion into crystals · Reaction orthogonality

Representative materials: Tagged PCPs · Azide-functionalised PCPs

Caveat: The review warns that conventional PCP pores are often below 1 nm and postsynthetic reactions may remain peripheral.

3 · 2. Subtractive Approach

Device-relevant intergrain proton conductivity, membrane preparation and conductivity perpendicular to thin films remain insufficiently investigated.

Proposed direction: Measure grain-boundary and through-film transport and develop membrane-processing routes for proton-conductive PCPs.

9 · 5. Summary and Outlook

Detailed crystal structures of some hydrated MIL-53 derivatives were not determined, limiting direct assignment of hydrogen-bond networks.

Proposed direction: Pair conductivity and adsorption data with hydrated-state crystallography or complementary dynamics methods.

5 · 3.1 Proton Conductivity

Fuel-cell electrodes require protons, gases and electrons at catalyst structures, but proton-conductive PCPs still need integration with Pt, nanocarbon, electronic conductivity or catalytic sites.

Proposed direction: Develop proton-conductive PCP composites or inherently catalytic/electronically conductive PCPs for thicker fuel-cell electrodes.

9 · 5. Summary and Outlook

Most systems rely on water; the review calls for proton-conducting media other than water, imidazolium or triazole and broader stimuli-responsive conduction.

Proposed direction: Explore non-aqueous proton carriers and stimuli-responsive PCP/MOF conductors.

9 · 5. Summary and Outlook

Organic and inorganic benchmark proton conductors lack precise control over dopant distribution, cluster connectivity or conducting pathways.

Proposed direction: Use crystalline PCP/MOF pores to design and structurally resolve ideal hydrogen-bond distances and pathways.

3 · 1. Introduction

The influence of size effects on proton-conductive PCPs is identified as an underexplored future direction.

Proposed direction: Investigate crystal size, grain boundary and nanoscale morphology effects on proton transport.

9 · 5. Summary and Outlook

Early PCD-derived PCPs were insufficiently stable against water, preventing harsh deprotection and hydrated-conduction studies.

Proposed direction: Develop water-stable frameworks and orthogonal postsynthetic reactions that preserve porosity while exposing acidic sites.

4 · 2. Subtractive Approach