Using small angle x-ray scattering to examine the aggregation mechanism in silica nanoparticle-based ambigels for improved optical clarity.

Silica-based aerogels are a promising low-cost solution for improving the insulation efficiency of single-pane windows and reducing the energy consumption required for space heating and cooling. Two key material properties required are high porosity and small pore sizes, which lead to low thermal conductivity and high optical transparency, respectively. However, porosity and pore size are generally directly linked, where high porosity materials also have large pore sizes. This is unfavorable as large pores scatter light, resulting in reduced transmittance in the visible regime. In this work, we utilized preformed silica colloids to explore methods for reducing pore size while maintaining high porosity. The use of preformed colloids allows us to isolate the effect of solution conditions on porous gel network formation by eliminating simultaneous nanoparticle growth and aggregation found when using typical sol-gel molecular-based silica precursors. Specifically, we used in situ synchrotron-based small-angle x-ray scattering during gel formation to better understand how pH, concentration, and colloid size affect particle aggregation and pore structure. Ex situ characterization of dried gels demonstrates that peak pore widths can be reduced from 15 to 13 nm, accompanied by a narrowing of the overall pore size distribution, while maintaining porosities of 70%-80%. Optical transparency is found to increase with decreasing pore sizes while low thermal conductivities ranging from 95 +/- 13 mW/m K are maintained. Mechanical performance was found to depend primarily on effective density and did not show a significant dependence on solution conditions. Overall, our results provide insights into methods to preserve high porosity in nanoparticle-based aerogels while improving optical transparency.

Understanding Correlation Between CO2 Insertion Mechanism and Chain Length of Diamine in Metal-Organic Framework Adsorbents.

Although CO2 insertion is a predominant phenomenon in diamine-functionalized Mg2 (dobpdc) (dobpdc4- =4,4-dioxidobiphenyl-3,3'-dicarboxylate) adsorbents, a high-performance metal-organic framework for capturing CO2 , the fundamental function of the diamine carbon chain length in the mechanism remains unclear. Here, Mg2 (dobpdc) systems with open metal sites grafted by primary diamines NH2 -(CH2 )n -NH2 were developed, with en (n=2), pn (n=3), bn (n=4), pen (n=5), hn (n=6), and on (n=8). Based on CO2 adsorption and IR results, CO2 insertion is involved in frameworks with n=2 and 3 but not in systems with n≥5. According to NMR data, bn-appended Mg2 (dobpdc) exhibited three different chemical environments of carbamate units, attributed to different relative conformations of carbon chains upon CO2 insertion, as validated by first-principles density functional theory (DFT) calculations. For 1-hn and 1-on, DFT calculations indicated that diamine inter-coordinated open metal sites in adjacent chains bridged by carboxylates and phenoxides of dobpdc4- . Computed CO2 binding enthalpies for CO2 insertion (-27.8 kJ mol-1 for 1-hn and -20.2 kJ mol-1 for 1-on) were comparable to those for CO2 physisorption (-19.3 kJ mol-1 for 1-hn and -20.8 kJ mol-1 for 1-on). This suggests that CO2 insertion is likely to compete with CO2 physisorption on diamines of the framework when n≥5.