An anionic metal–organic framework and its r-graphene oxide composite for selective adsorption and separation of methyl blue from aqueous solution†

Abstract

A mono-metallic MOF (Mn-FM) and its r-GO composite (rGO@Mn-FM) were synthesized with a simple and cost-effective method. The MOF contains a Mn²⁺ metal centre with an in situ generated ligand from the decomposition of DMF used as a reaction medium under solvothermal conditions. The unit cell also contains an in situ generated dimethyl ammonium (DMA) cation entrapped inside the pores of the framework. Due to the presence of the DMA cation, the MOF exhibits selective adsorption of the anionic dye (namely, methyl blue, methyl orange) through ionic interactions. Again, upon removal of the DMA cation, adsorption was significantly enhanced owing to the formation of higher charge density species in the framework. The water stability of the MOF was significantly improved through composite formation with r-graphene oxide (namely rGO@Mn-FM) which in turn improved the adsorption of methyl blue as well. The adsorption efficiency could be achieved up to 84% within 40 minutes. Moreover, the composite was found to be highly sensitive and capable of adsorbing trace amounts of the MeB dye (up to 1 ppm) from the aqueous solution. The composite follows pseudo-second order kinetics and the Langmuir isotherm model signifying monolayer chemisorption with homogeneous sites. The release/desorption of adsorbed methyl blue from the rGO@Mn-FM surface establishes the reusability of the dye and subsequently the stability and potential of the adsorbent to reuse for multiple cycles.