Learning Objectives
- Understand Secondary Building Units (SBUs) and their role in MOF structure
- Learn about different types of organic linkers and their properties
- Grasp the concept of network topology in MOFs
- Understand key MOF properties: surface area, porosity, and tunability
- Recognize iconic MOF structures and their characteristics
2.1 Secondary Building Units (SBUs)
What are SBUs?
Secondary Building Units (SBUs) are the fundamental structural units that serve as the vertices (corners) in MOF architecture. They consist of metal ions or clusters coordinated by organic groups, creating geometrically defined connection points.
Why "Secondary"?
The term "secondary" distinguishes these units from the primary building blocks (individual atoms or ions). SBUs are pre-formed or formed in situ during synthesis and maintain their geometry throughout the framework.
Common SBU Geometries
| SBU Type | Metal Center | Geometry | Connectivity | Example MOF |
|---|---|---|---|---|
| Zn₄O(CO₂)₆ | Zn²⁺ | Octahedral | 6-connected | MOF-5, IRMOF series |
| Cu₂(CO₂)₄ | Cu²⁺ | Paddlewheel | 4-connected | HKUST-1 |
| Zr₆O₄(OH)₄(CO₂)₁₂ | Zr⁴⁺ | Octahedral cluster | 12-connected | UiO-66 |
| Zn(Im)₄ | Zn²⁺ | Tetrahedral | 4-connected | ZIF-8 |
| Al(OH)(CO₂)₂ | Al³⁺ | Chain (rod) | 4-connected | MIL-53 |
SBU Examples in Detail
Zn₄O Cluster (MOF-5)
The Zn₄O cluster consists of four zinc atoms tetrahedrally arranged around a central oxygen atom. Six carboxylate groups from organic linkers coordinate to this cluster, creating an octahedral connection geometry.
Cu Paddlewheel (HKUST-1)
The copper paddlewheel features two copper atoms bridged by four carboxylate groups in a square arrangement, resembling a paddlewheel or lantern. This creates a 4-connected node with square geometry.
Zr₆ Cluster (UiO-66)
The Zr₆ cluster is exceptionally stable due to strong Zr-O bonds. Six zirconium atoms form an octahedron, with bridging oxygen and hydroxyl groups, and can coordinate up to 12 carboxylate linkers.
2.2 Organic Linkers
Types of Linkers
Organic linkers connect the SBUs to form extended frameworks. They are characterized by:
- Coordinating groups: Functional groups that bind to metals (carboxylate, imidazolate, pyridyl, etc.)
- Length: Distance between coordinating groups affects pore size
- Geometry: Linear, triangular, tetrahedral, etc.
- Functionality: Additional groups for specific properties
Common Linkers
| Linker | Full Name | Geometry | Features |
|---|---|---|---|
| BDC | 1,4-Benzenedicarboxylic acid (Terephthalic acid) | Linear | Most common linker, rigid, ~7 Å length |
| BTC | 1,3,5-Benzenetricarboxylic acid (Trimesic acid) | Triangular | 3-connected, forms different topologies |
| NDC | 2,6-Naphthalenedicarboxylic acid | Linear | Longer than BDC (~11 Å), larger pores |
| BPDC | 4,4'-Biphenyldicarboxylic acid | Linear | Extended linker (~11 Å), isoreticular MOFs |
| MeIM | 2-Methylimidazole | Angular | Forms ZIF structures, zeolite-like topology |
Linker Functionalization
Linkers can be modified with functional groups to introduce specific properties:
Common Functional Groups
- -NH₂ (amino): Enhanced CO₂ capture, basic sites for catalysis
- -NO₂ (nitro): Electron-withdrawing, affects pore polarity
- -OH (hydroxyl): Hydrogen bonding sites
- -Br, -Cl (halogens): Tuning pore size and hydrophobicity
- -SO₃H (sulfonic acid): Strong acid sites for catalysis
2.3 Network Topology
What is Topology?
Network topology describes the connectivity pattern of nodes and linkers in a MOF, independent of specific chemical composition. Different combinations of SBUs and linkers can create the same topology, a principle called isoreticular chemistry.
Common MOF Topologies
| Topology Symbol | Name | Description | Example MOF |
|---|---|---|---|
| pcu | Primitive cubic | 6-connected nodes in cubic arrangement | MOF-5 |
| tbo | Twisted boracite | 3,4-connected net | HKUST-1 |
| fcu | Face-centered cubic | 12-connected nodes | UiO-66 |
| sod | Sodalite | 4-connected tetrahedral | ZIF-8 |
| dia | Diamond | 4-connected tetrahedral | Various ZIFs |
Isoreticular Chemistry
The IRMOF series (Isoreticular MOFs) demonstrates how the same topology can be achieved with different linkers:
BDC linker
Pore: 12 Å] IRMOF8[IRMOF-8
NDC linker
Pore: 18 Å] IRMOF16[IRMOF-16
TPDC linker
Pore: 29 Å] end MOF5 --> IRMOF8 IRMOF8 --> IRMOF16 style MOF5 fill:#3498db,stroke:#2980b9,color:#fff style IRMOF8 fill:#9b59b6,stroke:#8e44ad,color:#fff style IRMOF16 fill:#e74c3c,stroke:#c0392b,color:#fff
By using longer linkers while maintaining the same SBU and topology, pore size can be systematically increased.
2.4 Key Properties of MOFs
Surface Area
Surface area is one of the most remarkable properties of MOFs, measured by gas adsorption techniques (typically N₂ at 77 K) and reported as BET surface area.
| Material | BET Surface Area (m²/g) |
|---|---|
| Activated Carbon | 500-3,000 |
| Zeolites | 200-800 |
| MOF-5 | ~3,800 |
| MOF-177 | ~4,500 |
| NU-110 | 7,140 |
| DUT-60 | 7,839 |
Surface Area Perspective
The record holder DUT-60 has a surface area of 7,839 m²/g. This means:
- 1 gram has more surface area than an Olympic swimming pool (~2,500 m²)
- A 10 mg sample has the internal surface area of a typical classroom
Porosity
Porosity refers to the void space within the MOF structure. Key parameters include:
- Pore volume: Total empty space per unit mass (cm³/g)
- Void fraction: Percentage of volume that is empty (up to 90% in some MOFs)
- Pore size: Diameter of pore openings and cavities
- Pore size distribution: Range of pore sizes present
Pore Size Classification
| Classification | Pore Diameter | Examples |
|---|---|---|
| Microporous | < 2 nm (20 Å) | ZIF-8, UiO-66 |
| Mesoporous | 2-50 nm | MIL-101, MOF-177 |
| Macroporous | > 50 nm | Hierarchical MOFs |
Tunability
MOF properties can be precisely controlled through:
- Metal selection: Different metals provide different coordination geometries, stability, and functionality
- Linker design: Length, geometry, and functional groups affect pore properties
- Topology choice: Different networks have different pore shapes and connectivity
- Post-synthetic modification: Adding functional groups after MOF formation
- Defect engineering: Controlled introduction of defects for enhanced properties
Stability
MOF stability is crucial for practical applications:
| Stability Type | Description | Stable MOF Examples |
|---|---|---|
| Thermal | Resistance to decomposition at high temperature | UiO-66 (up to 500°C), ZIF-8 (up to 450°C) |
| Chemical | Resistance to acids, bases, solvents | UiO-66 (acid stable), MIL-101 (water stable) |
| Mechanical | Resistance to pressure and stress | ZIF-8, UiO-66 |
Stability Considerations
Many early MOFs (like MOF-5) are sensitive to moisture, which limits their practical applications. Research has focused on developing water-stable MOFs using high-valent metals (Zr⁴⁺, Al³⁺, Cr³⁺, Ti⁴⁺) or hydrophobic linkers.
2.5 Iconic MOF Structures
MOF-5 (IRMOF-1)
The Prototype MOF
- Composition: Zn₄O(BDC)₃
- SBU: Zn₄O cluster (6-connected)
- Linker: BDC (terephthalate)
- Topology: pcu (primitive cubic)
- Surface Area: ~3,800 m²/g
- Pore Size: 12 Å (windows), 18 Å (cavities)
- Significance: First highly porous, stable MOF; demonstrated reticular synthesis
HKUST-1 (Cu-BTC)
The Paddlewheel MOF
- Composition: Cu₃(BTC)₂
- SBU: Cu₂ paddlewheel (4-connected)
- Linker: BTC (trimesate, 3-connected)
- Topology: tbo
- Surface Area: ~1,500-2,000 m²/g
- Features: Open metal sites (after activation), commercially available (Basolite C 300)
- Applications: Gas storage, catalysis, sensing
ZIF-8
The Zeolitic MOF
- Composition: Zn(MeIM)₂
- SBU: Zn²⁺ (tetrahedral coordination)
- Linker: 2-Methylimidazolate
- Topology: sod (sodalite, zeolite-like)
- Surface Area: ~1,600 m²/g
- Pore Window: 3.4 Å (but flexible, admits larger molecules)
- Significance: Exceptional chemical and thermal stability, zeolite-like properties
UiO-66
The Benchmark Stable MOF
- Composition: Zr₆O₄(OH)₄(BDC)₆
- SBU: Zr₆ cluster (12-connected)
- Linker: BDC (terephthalate)
- Topology: fcu
- Surface Area: ~1,200-1,500 m²/g
- Features: Exceptional stability (thermal up to 500°C, acid/base stable)
- Applications: Catalysis, drug delivery, gas separation
Comparison of Iconic MOFs
| Property | MOF-5 | HKUST-1 | ZIF-8 | UiO-66 |
|---|---|---|---|---|
| Metal | Zn | Cu | Zn | Zr |
| Surface Area | 3,800 m²/g | 1,800 m²/g | 1,600 m²/g | 1,200 m²/g |
| Water Stability | Poor | Moderate | Good | Excellent |
| Thermal Stability | 300°C | 280°C | 450°C | 500°C |
| Pore Size | Large | Medium | Small | Medium |
| Open Metal Sites | No | Yes | No | No |
Summary
Key Takeaways
- SBUs are the metal-based vertices that define MOF geometry and connectivity
- Organic linkers bridge SBUs and can be modified to tune properties
- Topology describes the connectivity pattern, enabling rational design
- Surface areas can exceed 7,000 m²/g, far surpassing other porous materials
- Tunability comes from choosing metals, linkers, topology, and post-synthetic modifications
- Stability (thermal, chemical, mechanical) is crucial for applications
- Iconic MOFs like MOF-5, HKUST-1, ZIF-8, and UiO-66 serve as benchmarks and platforms for further development
Check Your Understanding
Question 1
What makes the Zr₆ cluster in UiO-66 particularly stable?
Click for Answer
Answer: The strong Zr-O bonds (high-valent Zr⁴⁺ with hard oxo/hydroxyl ligands) and the high connectivity (12 linkers per cluster) provide exceptional thermal and chemical stability. The cluster can tolerate missing linkers (defects) while maintaining structural integrity.
Question 2
How does linker length affect MOF properties in an isoreticular series?
Click for Answer
Answer: Longer linkers increase pore size and total pore volume while maintaining the same topology. However, very long linkers may lead to framework interpenetration (multiple frameworks grown through each other) or reduced stability.
Question 3
Why does HKUST-1 have "open metal sites" while MOF-5 does not?
Click for Answer
Answer: In HKUST-1, the Cu paddlewheel SBU has axial positions that are occupied by solvent molecules during synthesis. When activated (heated under vacuum), these solvent molecules are removed, exposing coordinatively unsaturated Cu²⁺ sites. In MOF-5, all coordination sites of the Zn₄O cluster are satisfied by carboxylate linkers.