Developing Performance Indicators for MOFs in Atmospheric Water Harvesting

Original Authors: Muhammad Anique Azam, Mubashir Ali Siddiqui, Haider Ali

Original paper is accessible at: https://doi.org/10.1016/j.seppur.2024.129660

Introduction

Metal–organic frameworks (MOFs) have emerged as promising materials for atmospheric water harvesting (AWH) due to their high surface area, tunable pore sizes, and selective adsorption capabilities. However, the performance of MOF-based atmospheric water generators (AWGs) has often been reported without a unified benchmark, making it difficult to compare across systems. This study proposes a new set of performance indicators that link MOF material properties with AWG system-level behavior, offering a standardized framework for evaluation.

Methodology

The authors developed four performance indicators based on uptake capacity, thermodynamics, surface/structural characteristics, and adsorption–desorption kinetics.

• Adsorption Equation of State: Using modified virial equations, the team derived water adsorption isotherms for MOFs and calculated isosteric heat of adsorption.

• Kinetics Modeling: A linear driving force model was applied to predict adsorption/desorption rates and time required to reach 99% saturation.

• System Definition: A simplified AWG model assumed uniform thin-coating of MOFs on substrates with continuous airflow.

• Performance Factor: The overall indicator (xpf) combines uptake (xu), thermodynamics (xt), surface factor (xs), and kinetics (xk), yielding a unified performance score.

Case Study: MOF-801

The framework was applied to MOF-801, one of the most studied MOFs for AWH. Results showed:

• Equilibrium adsorption capacity of ~18.8 mol/kg at 60% RH and 15 °C.

• Desorption released ~2.1 mol/kg water under 50 °C heating conditions, even without assisted heating.

• Uptake and thermodynamic indicators revealed the sensitivity of MOF-801 performance to adsorption/desorption temperatures and cycle times.

Validation with Literature Data

Three independent experimental studies of MOF-801 were re-analyzed using the new indicators:

1. Tao et al. – Arid desert conditions (40 °C, 30% RH) showed good recovery with spherical MOF-801 beads.

2. Hashjin et al. – High humidity experiments (80% RH, 25 °C) confirmed strong uptake but slower desorption.

3. Fathieh et al. – Solar-assisted setups achieved complete recovery but lower uptake due to substrate limitations.

The analysis showed that system configuration and substrate design strongly influence the overall indicator values, even when using the same.

Results and Implications

• Standardization: The proposed indicators allow fair comparison between different MOFs and AWG setups.

• Design Guidance: Uptake and thermodynamic multipliers highlight optimal adsorption–desorption conditions.

• Scalability: Structural and kinetic factors emphasize the role of substrate design and cycle time in large-scale deployment.

Conclusion

This research establishes the first comprehensive performance benchmark for MOF-based AWGs, bridging the gap between laboratory adsorption data and real-world device performance. By integrating uptake, thermodynamics, surface, and kinetics indicators, the study provides a robust framework for evaluating and ranking MOF–AWG systems, ultimately guiding the design of efficient, scalable solutions for water harvesting in arid and water-stressed environments.