Metal Organic Frameworks Synthesis-Ultrasonic and Solvothermal Treatment with Organic/Inorganic Solvents Combinations

Abstract

Metal organic frameworks (MOF) are a class of materials which are formed by a combination of metal with organic linkers. The properties of organic ligands such as bond angles, length, bulkiness and chirality plays an important part in determination of resultant framework. The metal ions can also adopt various geometries and affect the structure of MOFs. These solids have stable and ordered structures with high surface areas. Metal organic frameworks (MOFs) have different names such as porous coordination networks (PCNs) or porous coordination polymers (PCPs), yet all of these refer to same kind of materials. These materials have attracted a huge attention over the last three decades because of their applications in gas storage, gas separation, catalysis, luminescent and fluorescent materials, and drug storage and drug delivery. By convention, metal organic frameworks have been synthesized by solvo thermal process which involves a reaction of organic ligands with metal salts at relatively low temperature (below 300°C). Additionally, other methodologies such as electrochemical, mechanochemical, microwave assisted heating and sonochemical routes have also been employed successfully with appreciable product yields. Copper and Nickel based MOFs were synthesized from ultrasonic and solvo thermal techniques and their properties were investigated. Cu-BTC has been synthesized by ultrasonic treatment while using different combinations of organic and inorganic solvents. The solvent combination Dimethylformamide (DMF)/De-ionized water (H2O) produced a MOF product of highest surface area (1434m2/g) in one of these reactions. The product yield tend to vary with sonication time and applied power of sonication. Particle size and morphology vary with the type of solvent combination being employed. The ultrasonic technique was also used to synthesize Ni-BTC by reacting Nickel metal precursor (NiNO3.6H2O) with benzenetricaroxylic acid at various probe temperatures and power levels. The probe temperatures and intermediate power level are the key operating principles for obtaining meaningful product yield. The Ni-BTC has also been synthesized by solvo thermal reactions. Seven different solvent combinations were used at identical operating conditions. The product yields were consistent but surface areas and other physico-chemical properties of MOFs tend to differ with each solvo thermal reaction mixture. An intermediate/low temperature hydrothermal process approach was adopted for synthesis of Fe-BTC. This low temperature and short time reaction approach made the synthesis economically feasible and less time consuming compared to conventional processes. High temperature stability of synthesized MOF was investigated by comparing the XRD patterns of as prepared and heat treated samples of Fe-BTC.