Hierarchical Silica Composites for Enhanced Water Adsorption at Low Humidity

To combat water scarcity in remote areas around the world, adsorption-based atmospheric water harvesting (AWH) has been proposed as a technology that can be used alongside existing water production capabilities. However, commonly used adsorbents either have low water adsorption loadings or are difficult to regenerate. In this work, we developed two novel hierarchical silica-salt composites that both exhibit high water adsorption loadings under dry and humid conditions. The total water vapor loading, kinetics, and heats of water adsorption for both silica-salt composites were investigated. As hierarchical silicas have tunable pores and large pore volumes, these materials serve as effective host matrixes for the hygroscopic salt LiCl. Our results suggest that hierarchical pores play a significant role in water adsorption: micropores and some smaller mesopores act as “storage” sites for hygroscopic salt, whereas larger mesopores and macropores increase the accessibility of water vapor into the silica. Using this mix of pores, we achieved greater than 0.4 g H2O/g composite at 10% RH and 27 °C. Additionally, we found that the salt-impregnated silica and bare silica had the same heat of adsorption: 80–90 kJ/mol. The results suggest that the H-bond interactions are similar for both systems and that the primary mechanism at play here is water cluster adsorption/desorption. Despite the similar energies, the LiCl-containing materials exhibited considerably slower kinetics than bare silica materials. Of equal importance to the adsorption capacity and kinetics of these composites is their mechanical stability. To assess their mechanical stability, high-energy ball milling of silica was conducted to create more uniform particle sizes. However, reduced particle sizes came at a cost—the BET surface areas and pore volumes were drastically decreased after more than 1 h of ball milling. Findings from this study suggest that short-term ball milling may be a viable large-scale option to reduce particle size in silica materials without sacrificing significant performance.


Table of Contents
Table S1: LiCl impregnation of HS-PEG and HS-PEG-2xCTAB characterization

Figure S1 :
Figure S1: Non-Local Density Functional Theory Pore Size Distributions Figure S2: Mercury intrusion plot of HS-PEG and HS-PEG-2xCTAB Figure S3: Nitrogen adsorption isotherms of ball milled HS-PEG Figure S4: SEM of HS-PEG ball milled for 30 min Figure S5: SEM of HS-PEG ball milled for 1 h Figure S6: SEM of HS-PEG ball milled for 2 h Figure S7: SEM of HS-PEG ball milled for 3 h Figure S8: SEM of HS-PEG ball milled for 4 h Figure S9: SEM of HS-PEG ball milled for 5 h Figure S10: SEM of HS-PEG ball milled for 12 h Adsorption enthalpy calculation procedure Table S2: Adsorption enthalpies for bare/impregnated HS-PEG and HS-PEG-2xCTAB Figure S11: TGA/DSC plots for HS-PEG Figure S12: TGA/DSC plots for HS-PEG-2xCTAB Figure S13: TGA/DSC plots for 30wt% LiCl in MeOH HS-PEG-2xCTAB Figure S14: TGA/DSC plots for 20wt% LiCl in H 2 O/MeOH HS-PEG-2xCTAB Figure S15: TGA/DSC plots for 25wt% LiCl in MeOH HS-PEG

Figure S1 .Figure S2 .
Figure S1.Pore size distributions calculated using the Non-Local Density Functional Theory (NLDFT) method for a) HS-PEG and LiCl impregnated HS-PEG and b) HS-PEG synthesized with 2x the amount of C 16 TAB (HS-PEG-2xCTAB) and LiCl impregnated HS-PEG-2xCTAB.The solid lines are cumulative pore volumes of the materials Figure S3.a) Nitrogen adsorption isotherms at 77K for 0-12h ballmilled HS-PEG, and b) Pore size distributions for 0h, 30min, 1h ball-milled HS-PEG calculated using non-local density functional theory (NLDFT).Solid lines are cumulative pore volume.

Figure S4 .
Figure S4.Measured particle sizes for 30 minutes ball-milled HS-PEG using SEM.The approximate particle radius for this sample was estimated by halving the diameter of the largest particle in the image (~45 µm).

Figure S5 .
Figure S5.Measured particle sizes for 1h ball-milled HS-PEG using SEM.The approximate particle radius for this sample was estimated by halving the diameter of the largest particle in the image (~38 µm).

Figure S6 .
Figure S6.Measured particle sizes for 2h ball-milled HS-PEG using SEM.The approximate particle radius for this sample was estimated by halving the diameter of the largest particle in the image (~25 µm).

Figure S7 .
Figure S7.Measured particle sizes for 3h ball-milled HS-PEG using SEM.The approximate particle radius for this sample was estimated by halving the diameter of the largest particle in the image (~30 µm).

Figure S8 .
Figure S8.Measured particle sizes for 4h ball-milled HS-PEG using SEM.The approximate particle radius for this sample was estimated by halving the diameter of the largest particle in the image (~20 µm).

Figure S9 .
Figure S9.SEM for 5h ball-milled HS-PEG with zoomed-in view on the right.The approximate particle radius for this sample was estimated by halving the diameter of the largest particle in the image (~20 µm).

Figure S10 .
Figure S10.Measured particle sizes for 12h ball-milled HS-PEG using SEM.The approximate particle radius for this sample was estimated by halving the diameter of the largest particle in the image (~20 µm).

Figure S11 :
Figure S11: TGA/DSC data on the water adsorption of HS-PEG in a 200 ml/min flow of nitrogen at a constant 27 °C.The humidity of the gas flow was changed from 0 to 10 %RH 30 min into the adsorption step.Enthalpy of adsorption values were derived from the integration of the heat flow (red) and the quantity of water adsorbed (black).The enthalpy of adsorption was calculated to be 85.7 kJ/mol.

Figure S12 :
Figure S12: TGA/DSC data on the water adsorption of HS-PEG 2xCTAB in a 200 ml/min flow of nitrogen at a constant 27 °C.The humidity of the gas flow was changed from 0 to 10 %RH 30 min into the adsorption step.Enthalpy of adsorption values were derived from the integration of the heat flow (red) and the quantity of water adsorbed (black).The enthalpy of adsorption was calculated to be 84.7 kJ/mol.

Figure S13 :
Figure S13: TGA/DSC data on the water adsorption of LiCl@HS-PEG 2xCTAB, 30 wt% in MeOH in a 200 ml/min flow of nitrogen at a constant 27 °C.The humidity of the gas flow was changed from 0 to 10 %RH 30 min into the adsorption step.Enthalpy of adsorption values were derived from the integration of the heat flow (red) and the quantity of water adsorbed (black).The enthalpy of adsorption was calculated to be 84.5 kJ/mol.

Figure S14 :
Figure S14: TGA/DSC data on the water adsorption of LiCl@HS-PEG 2xCTAB, 20 wt% in MeOH/H 2 O in a 200 ml/min flow of nitrogen at a constant 27 °C.The humidity of the gas flow was changed from 0 to 10 %RH 30 min into the adsorption step.Enthalpy of adsorption values were derived from the integration of the heat flow (red) and the quantity of water adsorbed (black).The enthalpy of adsorption was calculated to be 81.6 kJ/mol.

Figure S15 :
Figure S15: TGA/DSC data on the water adsorption of LiCl@HS-PEG (1x CTAB), 25 wt% in MeOH in a 200 ml/min flow of nitrogen at a constant 27 °C.The humidity of the gas flow was changed from 0 to 10 %RH 30 min into the adsorption step.Enthalpy of adsorption values were derived from the integration of the heat flow (red) and the quantity of water adsorbed (black).The enthalpy of adsorption was calculated to be 80.6 kJ/mol.