Learning Objectives
- Understand how MOFs are used for gas storage (H₂, CH₄, CO₂)
- Learn about carbon capture and separation applications
- Explore MOF-based catalysis
- Discover biomedical applications: drug delivery and sensing
- Understand water harvesting from air
- Review commercial success stories
4.1 Gas Storage
Hydrogen Storage
Hydrogen is a promising clean fuel, but its low volumetric energy density makes storage challenging. MOFs offer a potential solution through physisorption of H₂ molecules in their pores.
Why MOFs for H₂ Storage?
- High surface area provides many adsorption sites
- Pore sizes can be optimized for H₂ (~3 Å diameter)
- Lighter than metal hydrides
- Faster kinetics than chemical storage
Performance Metrics
| MOF | H₂ Uptake (wt%) | Conditions |
|---|---|---|
| MOF-177 | 7.5 | 77 K, 70 bar |
| MOF-5 | 5.0 | 77 K, 50 bar |
| NOTT-112 | 10.0 | 77 K, 77 bar |
| NU-100 | 14.0 | 77 K, 56 bar |
The Challenge
Current MOFs achieve high H₂ storage only at cryogenic temperatures (77 K). The U.S. DOE target for onboard hydrogen storage is 6.5 wt% at near-ambient conditions, which remains challenging for physisorption-based systems.
Methane Storage
Natural gas (primarily methane) storage in MOFs is closer to commercial viability than hydrogen. MOFs can store methane at much lower pressures than compressed natural gas (CNG) tanks.
BASF-MOF-5 (Basolite Z 500) for Natural Gas Vehicles
BASF demonstrated that MOF-based tanks can store natural gas at 65 bar instead of 250 bar, reducing tank weight and safety concerns while achieving comparable energy density.
Methane Storage Performance
| MOF | CH₄ Uptake (v/v) | Conditions |
|---|---|---|
| HKUST-1 | 267 | 298 K, 65 bar |
| PCN-14 | 230 | 290 K, 35 bar |
| UTSA-76 | 257 | 298 K, 65 bar |
DOE target: 263 v/v (volumetric) at 35-65 bar - several MOFs now meet or exceed this!
4.2 Carbon Capture and Separation
CO₂ Capture from Flue Gas
Power plants emit flue gas containing ~10-15% CO₂. MOFs can selectively capture CO₂ through:
- Physical adsorption: High surface area, optimized pore size
- Chemical interaction: Amine-functionalized linkers, open metal sites
N₂ + CO₂ + H₂O] --> B[MOF Bed] B --> C[Clean Gas
N₂ + H₂O] B --> D[Captured CO₂] D --> E[Storage or
Utilization] style B fill:#27ae60,stroke:#1e8449,color:#fff style D fill:#3498db,stroke:#2980b9,color:#fff
Leading MOFs for CO₂ Capture
| MOF | CO₂ Uptake | CO₂/N₂ Selectivity | Features |
|---|---|---|---|
| Mg-MOF-74 | 35 wt% | >100 | Open metal sites |
| UTSA-16 | 16 wt% | 315 | High selectivity |
| mmen-Mg₂(dobpdc) | 25 wt% | Very high | Amine-functionalized |
| CALF-20 | Commercial | High | Water stable, BASF product |
Commercial Deployment: BASF CALF-20
In 2023, BASF launched CALF-20 (Calgary Framework-20), developed with the University of Calgary, as a commercial MOF for carbon capture. It combines:
- High CO₂ selectivity
- Water stability
- Low regeneration energy
- Scalable synthesis
Direct Air Capture (DAC)
Capturing CO₂ directly from ambient air (~420 ppm) is more challenging than flue gas capture due to the low concentration. Amine-functionalized MOFs show promise.
4.3 Catalysis
MOFs as Heterogeneous Catalysts
MOFs offer unique advantages for catalysis:
- High surface area: More catalytic sites per gram
- Single-site catalysis: Isolated, well-defined active sites
- Confinement effects: Pores can influence selectivity
- Tunability: Active sites can be engineered
Catalytic Site Types
Example Reactions Catalyzed by MOFs
| Reaction Type | MOF Catalyst | Active Site |
|---|---|---|
| Cyanosilylation | HKUST-1 | Cu open metal site |
| Friedel-Crafts alkylation | MIL-100(Fe) | Fe Lewis acid site |
| Oxidation | Fe-MOF-5 | Fe redox center |
| CO₂ conversion | UiO-66-NH₂ | Amine base + Zr acid |
| Hydrogenation | Pd@MIL-101 | Encapsulated Pd NPs |
4.4 Drug Delivery
MOFs for Biomedical Applications
MOFs can serve as drug carriers with advantages over traditional delivery systems:
- High drug loading: Large pore volumes accommodate more drug
- Controlled release: Pore environment affects release kinetics
- Targeting: Surface functionalization for cell-specific delivery
- Imaging: Metal centers can provide contrast
Key Considerations for Bio-MOFs
Biocompatibility Requirements
- Non-toxic metals: Fe, Zn, Mg, Ca preferred over Cu, Cr
- Biodegradable linkers: Endogenous molecules (fumarate, succinate)
- Appropriate size: Nanoparticles for cellular uptake
- Stability: Stable in biological fluids until reaching target
Example: MIL-Series for Drug Delivery
The MIL-100(Fe) and MIL-101(Fe) MOFs have been extensively studied for delivering anticancer drugs like ibuprofen, busulfan, and cidofovir. Drug loadings can exceed 30 wt%.
4.5 Chemical Sensing
MOF-Based Sensors
MOFs can detect target molecules through changes in:
- Luminescence: Guest binding quenches or enhances emission
- Conductivity: Adsorption changes electrical properties
- Mass: Gravimetric sensors (QCM, SAW)
- Color: Visible color change upon binding
Molecule] --> B[MOF
Sensor] B --> C{Signal Change} C --> D[Luminescence] C --> E[Conductivity] C --> F[Mass] C --> G[Color] style B fill:#9b59b6,stroke:#8e44ad,color:#fff
Sensing Applications
| Target | MOF Sensor | Detection Method | Detection Limit |
|---|---|---|---|
| Explosives (TNT) | Zn-MOF with fluorescent linker | Luminescence quenching | ppb level |
| Volatile organic compounds | ZIF-8 thin film | Conductivity | ppm level |
| Humidity | HKUST-1 | Color change | Visual |
| Toxic gases (NH₃) | Cu-BTC | Conductivity | ppm level |
4.6 Water Harvesting
Atmospheric Water Capture
Some MOFs can capture water vapor from air even in arid conditions and release it upon mild heating. This could provide drinking water in water-scarce regions.
MOF-801 Water Harvester
Yaghi's group at UC Berkeley demonstrated a device using MOF-801 that can harvest water from air at relative humidity as low as 20%. A prototype collected ~2.8 liters of water per day per kg of MOF in desert conditions.
Key MOFs for Water Harvesting
| MOF | Water Uptake | RH Range | Features |
|---|---|---|---|
| MOF-801 | 22 wt% | 20-30% | Low regeneration T |
| MOF-303 | 30 wt% | 30-40% | Higher capacity |
| Al-fumarate | 35 wt% | 25-35% | Cheap, scalable |
| Co₂Cl₂(BTDD) | 82 wt% | 30-45% | Very high capacity |
4.7 Commercial Success Stories
BASF Basolite Series
Commercially Available MOFs
| Product | MOF Type | Applications |
|---|---|---|
| Basolite A 100 | MIL-53(Al) | Gas storage, separation |
| Basolite C 300 | HKUST-1 (Cu-BTC) | Gas storage, catalysis |
| Basolite F 300 | Fe-BTC | Catalysis |
| Basolite Z 1200 | ZIF-8 | Gas separation, catalysis |
NuMat Technologies (ION-X)
NuMat Technologies developed MOF-based cylinders (ION-X) for storing hazardous electronic gases (AsH₃, PH₃, BF₃) used in semiconductor manufacturing. Benefits:
- Store gases at sub-atmospheric pressure (safer)
- 3-5x more gas per cylinder
- Reduced transportation costs and risks
ION-X Commercial Success
ION-X cylinders are now used by major semiconductor manufacturers worldwide. NuMat was acquired by Honeywell in 2023, validating the commercial potential of MOF technology.
Svante (formerly Inventys)
Svante uses proprietary MOF-based adsorbents for industrial carbon capture. Their technology is deployed at cement and steel plants.
Market Overview
MOF Market Growth
| Metric | Value |
|---|---|
| 2024 Market Size | $143.8 million |
| 2033 Projected Size | $443.4 million |
| CAGR | 13.7% |
| Leading Regions | North America, Europe, Asia-Pacific |
| Key Players | BASF, NuMat/Honeywell, Svante, MOF Technologies |
Summary
Key Takeaways
- Gas storage: MOFs enable H₂ and CH₄ storage at lower pressures, critical for clean energy
- Carbon capture: MOFs like CALF-20 are being deployed commercially for CO₂ capture
- Catalysis: Single-site and confined catalysis in MOFs offers selectivity advantages
- Drug delivery: Bio-compatible MOFs can achieve high drug loadings with controlled release
- Sensing: Luminescent and conductive MOFs detect trace chemicals
- Water harvesting: MOFs can extract water from desert air
- Commercialization: Companies like BASF, NuMat, and Svante are bringing MOFs to market
- Market growth: The MOF market is projected to triple by 2033
Check Your Understanding
Question 1
Why do most MOFs require cryogenic temperatures (77 K) for high hydrogen storage capacity?
Click for Answer
Answer: Hydrogen adsorption in MOFs occurs through weak van der Waals interactions (physisorption). At room temperature, H₂ molecules have enough thermal energy to overcome these weak attractions and escape from the pores. At 77 K (liquid nitrogen temperature), the thermal energy is low enough for significant H₂ to remain adsorbed.
Question 2
What makes CALF-20 suitable for commercial carbon capture applications?
Click for Answer
Answer: CALF-20 combines several critical properties: (1) high CO₂ selectivity over N₂, (2) stability in the presence of water (unlike many MOFs), (3) low regeneration energy (can release CO₂ with mild heating), and (4) scalable synthesis. This combination addresses the practical requirements of industrial deployment.
Question 3
How does NuMat's ION-X technology improve safety in semiconductor manufacturing?
Click for Answer
Answer: ION-X stores hazardous gases (like arsine, phosphine) at sub-atmospheric pressure by adsorbing them in MOFs. If a cylinder is damaged, gas doesn't rush out under pressure - it's retained in the MOF. This makes storage, transportation, and handling much safer compared to traditional compressed gas cylinders.