Efficient and selective adsorption and separation of methylene blue (MB) from mixture of dyes in aqueous environment employing a Cu(II) based metal organic framework

Abstract

In view of the increasing demand of the better adsorbents for separation of organic aromatic pollutants from waste water, we have made an attempt to design a stable polymeric material for the purpose. In this report, a copper based metal organic framework (MOF), [Cu2(Hbtc)2(H2O)2(µ2-H2O)]H2O (Cu-btc-1), where H3btc = benzene-1,2,4-tricarboxylic acid is synthesized employing one pot solvothermal method. The crystalline material, Cu-btc-1 is characterized by single crystal X-ray, XRPD, TGA and various spectroscopic techniques. X-ray data confirm that Cu(II) is present in square pyramidal environment. Topological features reveal the hxl topology of the MOF. The present MOF (Cu-btc-1) is water stable and recyclable and therefore has been employed in adsorption and separation of methylene blue with excellent efficiency from the mixture of three organic dyes, methylene blue (MB), methyl orange (MO) and rhodamine (Rh-B) in aqueous phase. MB was observed to show highest adsorption of nearly 98% at neutral pH and optimum temperature (25 °C). At equilibrium, the removal efficiency of Cu-btc-1 towards MB from the mixture of dyes was found to be 98.67%. The efficiency and highest adsorption of MB could be attributed to the cationic nature of the dye and nucleophilic aromatic moieties of carboxylates of Cu-btc-1 framework where strong electrostatic interactions take place for adoption of the MB+ molecules. Not only the cationic nature but also the linearity of the MB helps enhance its adsorption by the adsorbent by easily approaching to the sites available on MOF. H-bonding and π–π interactions are also responsible for the adsorption of the dye molecules here. Thus, mechanism of adsorption is the result of interplay of the electrostatic attraction/repulsion, π–π interactions, H-bonding and weak Van der Waals interactions. The present MOF thus represents an excellent class of adsorbent material and establishes a fine tuning between the structure-efficiency relationships for separation of the aromatic organic dyes in aqueous medium for future endeavours.