Large‐Scale, Homogeneous Solution Crystallization of 2D Covalent Organic Frameworks with Integrated Crystallinity and Solubility for Ultrafast Noble Metal Recovery

Abstract

ABSTRACT Solution‐processable covalent organic frameworks (COFs) are crucial for advancing practical applications, yet traditional top‐down exfoliation into solvophilic nanosheets suffers from low efficiency and limited scalability. This study presents a large‐scale homogeneous crystallization strategy to constitute a family of soluble 2D COFs with high crystallinity and exceptional solubility via a dissolution‐recrystallization route. By employing transformation from 3D precursors into 2D frameworks, a highly symmetric D 3h building block, hexachlorocyclotriphosphazene, is adopted to construct a three‐connected 2D framework via a [3+3] reaction pathway, simultaneously retaining the three unreactive P‐Cl sites on each D 3h center. A monoamine‐substituted short‐chain modulator is thus covalently incorporated to enhance framework solubilization, enabling a homogeneous crystallization via dissolution‐recrystallization. This process accelerates crystallization kinetics, achieves ultrahigh yields (89.7%) and concentrations (138.8 mg mL −1 ), and allows gram‐scale synthesis. Given their superior solution properties, such materials exhibit ultrafast gold recycling (k 2 = 0.141 g mg −1 min −1 ) through redox adsorption, significantly outperforming bulk analogues. Mechanistic studies reveal that phosphazene ring protonation lowers binding energy with AuCl 4 − via electrostatic and coordination interactions, followed by reduction to Au(0)/Au(I) and accompanied by water oxidation. This work explores a homogeneous crystallization system for 2D COFs, advancing their solution processability for scalable applications.