Unlocking the potential of phosphogypsum waste: Unified synthesis of functional metal-organic frameworks and zeolite via a sustainable valorization route

Abstract

Phosphogypsum (PG) is one of the hazardous wastes generated during phosphate fertilizers production. Valorizing PG into value-added materials is needed to overcome the environmental threat of accumulating PG stockpiles. The primary chemical constituents of PG are CaSO4·2H2O, SiO2, Al2O3, P2O5, and trace metals. Herein, we employed unified strategic synthesis method to transform the main components of PG into two classes of functional porous materials in subsequent steps. The alumina and silica components were isolated to fabricate cancrinite zeolite (CAN) while the residual calcium was directed to grow Ca-based metal–organic frameworks (Ca-MOFs) in the subsequent step. The resulting zeolite with a Si/Al ratio of ∼2.2 showed enhanced CO2 capture property, which, in many respects, is better than those achieved by similar frameworks from other precursors. The unveiled MOF synthesis approach was proven to be adaptable, allowing the preparation of different Ca-MOFs from organic linkers of diverse connectivity, namely Ca-BDC, Ca-BTC, and Ca-TCPB (SBMOF-2), using the linear ditopic benzene dicarboxylate, the tritopic benzene tricarboxylate, and tetracarboxylate-based linkers, respectively. The successful assembly of the porous materials was confirmed using various characterization techniques, including XRD, SEM-EDX, FTIR, and TGA-MS. The porosity of the materials was also probed using N2 at 77 K, CO2 at different temperatures, and H2O at 298 K sorption analyses. The Ca-BTC MOF with the optimal pore aperture size (∼3.4 Å) shows a potential for water/alcohol separation with a steep and fast moisture adsorption profile and negligible alcohol uptake. The synthetic transformation approach presented in this work represents the first example of a valorization route to form extended porous structures from PG waste.