CO2 fixation using recoverable chromium salen catalysts: Use of ionic liquids as cosolvent or high-surface-area silicates as supports

Two series of recoverable and reusable chromium salen catalysts for the CO₂ addition to styrene epoxide to form cyclic carbonate have been tested under supercritical conditions (100 bar and 80 °C). In one of the cases, tetra-tert-butylsalen chromium (substrate-to-catalyst ratio 875) was dissolved in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF₆), and after the products were extracted with diethyl ether the ionic liquid containing the complex was reused up to four consecutive runs. However, only modest conversions (> 50 %) and product selectivities (> 79 %) were achieved under these conditions, although decreasing the substrate-to-catalyst ratio showed some enhancement in the epoxide conversion. In the other system, a chromium salen catalyst was adsorbed on aminopropylsilyl-modified silica and ITQ-2. The solids were tested as heterogeneous catalysts but complete desorption of the complexes was observed after the first use. The best recoverable catalyst of the series was obtained when chromium salen was covalently anchored to silica support through a linker resulting from nucleophilic substitution between aminopropylsilyl-modified silica and chloromethyl-substituted complex. In this case, the solid was still active after the fourth reuse and exhibited 100% carbonate selectivity at about 70% epoxide conversion.