Hydrogels have attracted attention due to their interesting mechanical properties which makes them suitable for wide range of applications. Metallo-supramolecular hydrogels are particularly important due to their tenability. In this study, we report a simple method for synthesizing copper-containing polymer hydrogels made from nontoxic poly(methyl vinyl ether-alt-maleic anhydride) (PVM-MA) in the absence or presence of added carboxylate ligands: dicarboxylates, such as adipate and terephthalate, tricarboxylates, such as nitrilotriacetate (NTA) and citrate or tetracarboxylate such as ethylenediaminetetraacetic acid. Our copper hydrogels are wet precursors to a new family of amorphous porous materials, consisting of a metal-polycarboxylate backbone and carboxylate spacer ligands between polymer strands engineered via non-covalent interactions. Rheological measurements revealed that the mechanical stability of the hydrogels was enhanced by the addition of supplementary dicarboxylate ligands. We determined that the optimal ratio of polymer to dicarboxylate to Cu2+ was 10:4:2.5. Our scanning electron microscope (SEM) and cryo-SEM imaging and physical adsorption measurements revealed the formation of pores.
The Brunauer–Emmett–Teller (BET) surface area of the dried hydrogels was tunable with the addition of supplementary dicarboxylate ligands. The BET surface area increased from 177.96 m2 g−1 in a dried hydrogel without added dicarboxylate to 646.9 and 536.4 m2 g−1 with the addition of adipate and terephthalate, respectively. Moreover, addition of dicarboxylate ligands increased the pore volume and CO2 gas adsorption capacity. Separation of CO2 from post-combustion flue gases is important for environmental and economic sustainability. Our copper-based hydrogel with dicarboxylate spacer ligands offers the possibility of a new material for post-combustion CO2. At commercial conditions (298 K and 1 bar), the hydrogel samples with adipate as a spacer ligand, showed notable promising CO2/N2 selectivity of 78.46 and a high CO2/CH4 selectivity reaching 26.09 at 1 bar.
Furthermore, we investigated in detail the effect of metal ion on the rigidity and structure of hydrogels. A series of transition metal ions such as iron nitrate (Fe(NO3)3, aluminum nitrate (Al(NO3)3, zinc nitrate Zn(NO3)2, nickel nitrate Ni(NO3)2 and cobalt nitrate Co(NO3)2 were investigated for hydrogel formation. Scaling up of the synthesis of hydrogels was achieved using the filling machine which was used to fill silver hydrogels with antibacterial properties.