Controlling the Crystallisation and Hydration State of Crystalline Porous Organic Salts

Abstract

Crystalline porous organic salts (CPOS) are a subclass of molecular crystals. The low solubility of CPOS and their building blocks limits the choice of crystallisation solvents to water or polar alcohols, hindering the isolation, scale‐up, and scope of the porous material. In this work, high throughput screening was used to expand the solvent scope, resulting in the identification of a new porous salt, CPOS‐7, formed from tetrakis(4‐sulfophenyl)methane (TSPM) and tetrakis(4‐aminophenyl)methane (TAPM). CPOS‐7 does not form with standard solvents for CPOS, rather a hydrated phase (Hydrate2920) previously reported is isolated. Initial attempts to translate the crystallisation to batch led to challenges with loss of crystallinity and Hydrate2920 forming favorably in the presence of excess water. Using acetic acid as a dehydrating agent hindered formation of Hydrate2920 and furthermore allowed for direct conversion to CPOS‐7. To allow for direct formation of CPOS‐7 in high crystallinity flow chemistry was used for the first time to circumvent the issues found in batch. CPOS‐7 and Hydrate2920 were shown to have promise for water and CO2 capture, with CPOS‐7 having a CO2 uptake of 4.3 mmol/g at 195 K, making it one of the most porous CPOS reported to date.

Crystalline porous organic salts

“…However, this choice of solvents always brings strong interactions between the often-polar crystallization solvents and the highly charged framework. 35 Organic salt networks formed by sulfonic acids and guanidine, and carboxylic acids and amidines also exhibit significant Δp K a values. Nevertheless, many of these systems have yet to be verified to possess permanent porosity.…”

“…Very recently, Cooper et al employed a high-throughput screening (HTS) approach to meticulously modify the solvent composition and introduce a dehydration agent, effectively preventing the formation of CPOS hydrated. 35 This innovative methodology led to the discovery of a novel CPOS, namely CPOS-7, consisting of TSPM and tetrakis(4-amino-phenyl)methane (TAPM), boasting remarkable stability and exhibiting an exceptional maximum CO 2 adsorption capacity of 96 cm 3 g −1 at 195 K. The exceptional gas sorption performance of CPOS-7 establishes it as one of the most porous CPOSs hitherto reported.…”

“…23–25 In contrast, ionic bonds facilitate the formation of porous organic materials termed CPOSs. 26–38 Depending on their long-range structural order, these materials can further be categorized as amorphous or crystalline porous organic materials.…”

Porous isoreticular non-metal organic frameworks

“…By contrast, although porous salts show promise for some applications 5 , 20 , 21 , they lack many of the basic design principles that apply to isoreticular frameworks. For example, porous molecular salts can be subject to polymorphism 22 because the interactions between net charges in organic salts are less directional than is the case for coordinate covalent bonding in MOFs.…”