Enhanced Hydrolytic Stability of Self-Assembling Alkylated Two-Dimensional Covalent Organic Frameworks

Abstract

The stability and bulk properties of two-dimensional boronate ester-linked covalent organic frameworks (COFs) were investigated upon exposure to aqueous environments. Enhanced stability was observed for frameworks with alkylation in the pores of the COF compared to nonalkylated, bare-pore frameworks. COF-18Å and COF-5 were analyzed as "bare-pore" COFs, while COF-16Å (methyl), COF-14Å (ethyl), and COF-11Å (propyl) were evaluated as "alkylated-pore" materials. Upon submersion in aqueous media, the porosity of alkylated COFs decreased ∼25%, while the nonalkylated COFs were almost completely hydrolyzed, virtually losing all porosity. Similar trends were observed for the degree of crystallinity for these materials, with ∼40% decrease for alkylated COFs and 95% decrease for nonalkylated COFs. SEM was used to probe the particle size and morphology for these hydrolyzed materials. Stability tests, using absorbance spectroscopy and (1)H NMR, monitored the release of monomers as the COF degraded. While nonalkylated COFs were stable in organic solvent, hydrolysis was rapid in aqueous environments, more so in basic compared to neutral or acidic aqueous media (minutes to hours, respectively). Notably, alkylation in the pores of COFs slows hydrolysis, exhibiting up to a 50-fold enhancement in stability for COF-11Å over COF-18Å.