Abstract 

1. The article( Adv. Mater. 2023, 2211302) introduces a novel strategy leveraging polymorphism in aluminum-based metal-organic frameworks (MOFs) to adjust their hydrophilicity, which is crucial for water vapor adsorption technologies.
2. The researchers synthesized MOFs with either trans- or cis- μ-OH-connected AlO4(OH)2 octahedra chains, highlighting the case of MIP-211 [Product Page], which has a 3D network with sinusoidal channels formed by trans, trans-muconate linkers and cis-μ-OH-connected octahedra.
3. A minor structural change in MIP-211 resulted in a significant shift in the water adsorption isotherm step position compared to MIL-53-muc, enhancing its hydrophilic behavior.
4. Solid-state NMR and Grand Canonical Monte Carlo simulations revealed the initial adsorption mechanism between hydroxyl groups, favored by MIP-211's cis-positioning.
5. Theoretical evaluations demonstrated MIP-211's potential for achieving a high coefficient of performance for cooling (COPc) at an ultralow driving temperature of 60°C, outperforming existing sorbents, especially for small temperature lifts.

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Research Background

1. Industry Problems: Traditional cooling technologies face challenges of high energy consumption and greenhouse gas emissions. Sustainable alternatives rely on efficient water vapor adsorbents.
2. Existing Solutions: Previous research has focused on developing MOFs with tunable water sorption profiles and high porosity for applications in adsorption-driven heat transformation (AHT) and atmospheric water harvesting (AWH).
3. Innovative Approach: This study introduces polymorphism in MOFs as a new approach to modulate water sorption properties, with a focus on aluminum muconate MOFs, demonstrating a significant shift in hydrophilicity and water adsorption capacity.

Experimental Details

1. Synthesis of MIP-211: Involved a reaction between t,t-muconic acid and aluminum sulfate octadecahydrate in a water/DMF mixture under reflux, leading to the formation of MIP-211 with a unique structure.
2. Synthesis of MIL-53-muc: Obtained through a similar process using aluminum nitrate hexahydrate, resulting in a structurally distinct phase with poor crystallinity.
3. Characterization Techniques: Utilized high-resolution PXRD, solid-state NMR, and DFT calculations to confirm the structural differences and polymorphic relationship between MIP-211 and MIL-53-muc.

Analysis and Testing

1. Nitrogen Adsorption Isotherms: MIP-211 showed a type I isotherm with a BET surface area of 1450 m² g⁻¹ and pore volume of 0.60 cm³ g⁻¹, closely matching MIL-53-muc's values.
2. Thermal Stability: MIP-211 was found to be thermally stable up to 250°C, with decomposition occurring at 300°C, as determined by TGA and VT-PXRD.
3. Water Sorption Profile: MIP-211 exhibited a distinct S-shaped isotherm with a step at P/P0 ≈ 0.3, indicating higher hydrophilicity compared to MIL-53-muc.

Conclusion 

1. The study successfully demonstrated the tunability of hydrophilicity in Al-MOFs through polymorphism, with MIP-211 showing superior water sorption characteristics.
2. MIP-211's unique structure and water sorption profile make it an excellent candidate for adsorption-driven cooling systems and atmospheric water harvesting.
3. The findings open up new avenues for the design of MOFs with tailored properties for specific applications, with the potential for green synthesis and large-scale production.

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Prospective

1. Unaddressed Issues: Further research is needed on the long-term stability and performance of MIP-211 under continuous cycling conditions.
2. Shortcomings: The study could benefit from a more comprehensive analysis of the mechanical properties of MIP-211 and its resistance to degradation in various environmental conditions.
3. Future Research: It would be valuable to explore the synthesis of MIP-211 using alternative, more sustainable methods and to investigate its performance in real-world applications.