Coupling of carbon dioxide with epoxides to form cyclic carbonates using manganese based-polyoxometalate metal organic frameworks

Abstract

This study reports manganese based polyoxometalate metal organic frameworks (POMOF A and POMOF B) as catalytic systems for the cycloaddition of carbon dioxide with epoxides to form cyclic carbonates. The POMOF catalysts were synthesized under hydrothermal conditions at moderately acidic conditions (pH ≈ 5). POMOF A comprised of arsenic as the hetero atom, manganese as the active metal and glutaric acid as the organic ligand. While POMOF B comprised of selenium as the hetero atom, manganese as the active metal and terephthalic acid as the organic ligand. The POMOF catalysts were characterized using Fourier transform infra-red spectroscopy (FTIR), powder X-ray diffraction (PXRD), inductively coupled plasma – mass spectroscopy (ICP -MS), transmission electron microscopy (TEM), scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM-EDX) and ultra violet – visible spectroscopy (UV-Vis). The catalysts were employed in the coupling of CO2 and epoxides under varying reaction conditions using propylene oxide as the model oxide. The reaction was carried out in a pressurized autoclave reactor at specific set conditions of temperature (100-160 oC), CO2 pressure (10-35 bars), catalyst load (0-0.1 g) and reaction time (2-9 h), in presence of a tetrabutylammonium bromide as co-catalyst and acetonitrile as a solvent. Determination of the optimal reaction conditions such as temperature, reaction time, pressure of CO2 and catalyst load were done using response surface methodology (RSM). The identification and quantification of the reaction products was achieved using GC-MS and GC-FID respectively. Under optimal reaction conditions (140 oC, 6 hrs, 0.075 g, 25 bars), the POMOF catalysts exhibited good performance in coupling of CO2 and propylene oxide producing the desired propylene carbonate in high yields (≥ 87%). For both catalyst systems, the conversion of propylene oxide was ≥ 89% and the selectivity for propylene carbonate was ≥ 99%. The catalytic activity of the POMOFs was also tested in the coupling of CO2 and other epoxides such as styrene oxide, cyclohexene oxide and butylene oxide. In all cases, the corresponding cyclic carbonates were produced in excellent yields (83-95%) with ≥ 99% selectivity for the desired cyclic carbonates. Interestingly the POMOF catalysts could be recovered after the reactions and reused without significant loss in activity.