Metal-organic frameworks (MOFs) as innovative adsorbents for indoor environment control: preparation, shaping, characterization, and novel applications

Growing concerns about global warming and the energy crisis have heightened the urgency of conserving and managing energy consumption. The building sector accounts for over one-third of global energy consumption and emissions. Quick changes in building practices are necessary. Since space cooling (including dehumidification) significantly contributes to increasing energy demand in buildings, extensive innovation has been conducted on materials and technologies to reduce the latent load, resulting from dehumidification, of HVAC systems. Traditional hygroscopic building materials (e.g., wood panel, gypsum plaster, etc.) and conventional desiccants (e.g., silica gel, and zeolite, etc.), exhibit relatively poor performance for indoor moisture control due to either low water adsorption capacity or high regeneration temperature. Therefore, developing novel advanced materials is essential.

Metal-organic frameworks (MOFs) are one of the upcoming and promising candidates for indoor moisture control. MOFs are a group of nano or mesoporous materials consisting of metal iron nodes and organic linkers. MOFs have ultrahigh porosity and specific surface area, good adsorption capacity, and mild regeneration conditions. In addition, MOFs have tuneable structure and allow customized design based on the specific application. These features create MOFs unique advantages for indoor moisture control.

Though numerous different MOFs have been reported, only a few of them have been explored for indoor humidity control because applying MOFs in built environment requires good water stability, high moisture adsorption capacity, and non-toxicity etc. Synthesizing and discovering more potential MOFs is one of the primary works that should be done. Therefore, the first part of this PhD research synthesized and characterized some new MOFs, Al-pda and Al-MILmuc. Their properties are compared with reported MOFs, MIL-100(Fe) and MOF-303, and conventional sorbent zeolite 13X. All the selected MOFs have S-shaped isotherms and mild regeneration conditions. However, since different MOFs have different trigger points, water vapor capacity, and hysteresis, they can be chosen based on the specific application scenario and method.

MOFs are primarily synthesized in powdered form, which is unsuitable for direct industrial applications. Therefore, the second part of the PhD research investigated two methods to shape powdered MOFs into engineered materials. The MIL-100(Fe) electrospinning nanofiber membrane (MIL-100(Fe) NFM) was prepared with a 60% MOF loading rate. MIL-100(Fe) NFM has good water adsorption performance and is further improved by adding a small amount of LiCl in MOF’s pores. Building energy simulation results indicate that MOF@LiCl NFM is a promising material for reducing indoor moisture fluctuation and building energy consumption in dry and moderate climate areas. In addition to electrospinning, a simple and sustainable paper-making method was also investigated. MIL-100(Fe) cellulose paper sheet, with a high MOF loading rate of up to 75%, has good water adsorption and desorption performance. The simulation results show it can remove about 45%-55% of the latent load in most European areas.

When evaluating novel materials performance in indoor moisture control, traditional Moisture Buffer Value (MBV) theory assumes materials are semi-infinite. Existing commercial building energy simulation model and programs greatly simplified materials isotherms, which is not suitable for materials with special isotherm shape and hysteresis, for example, MOFs. Therefore, the third part of the PhD research developed a numerical model to calculate a material’s MBV with different thicknesses, thus finding its optimal thickness and maximum MBV. Based on the proposed numerical model, a building energy simulation model was also developed. The advantage of these models is that the material’s actual water adsorption isotherm and hysteresis are integrated into the calculation.