Carbon nanotube supported aluminum porphyrin-imidazolium bromide crosslinked copolymer: A synergistic bifunctional catalyst for CO2 conversion

The increased awareness of the catastrophic consequences caused by the accumulation of greenhouse gases into the atmosphere has generated a large mobilization aimed at CO2 mitigation. Herein, in the spirit of the transformation of a waste as CO2 into value added products, we propose an efficient preparation of two different hybrid systems based on aluminum chloride tetrastyrylporphyrin (TSP-Al-Cl) and 1,4-butanediyl-3,3′-bis-1-vinylimidazolium dibromide copolymerized in the presence (MWCNT-TSP-AlCl-imi) and in absence (TSP-AlCl-imi) of multi-walled carbon nanotubes (MWCNTs) for the CO2 utilization in the synthesis of cyclic carbonates. The so-prepared materials have been thoroughly characterized by means of several spectroscopic and analytical techniques. The MWCNT-TSP-AlCl-imi heterogenous catalyst enabled the highly efficient chemical transformation of CO2 and epoxides into cyclic carbonates with high turnover number (TON) and frequency (TOF) values at low temperature down to 30 °C in solvent-free conditions. MWCNT-TSP-AlCl-imi proved to be a very stable and reusable heterogeneous catalyst in consecutive cycles without the need of any reactivation procedure and no leaching phenomena. Furthermore, the optimal morphology of MWCNT-TSP-AlCl-imi, with the crosslinked polymer uniformly distributed onto MWCNTs backbone, resulted in a more active catalyst with a TON double than the unsupported one. The enhanced activity of MWCNT-TSP-AlCl-imi can be ascribed to its higher surface area that permits fully accessible catalytic sites. Interestingly, MWCNT-TSP-AlCl-imi also showed a catalytic activity comparable to a reference homogeneous catalytic system, proving that synergism occurred between the metal centers and the nucleophilic sites due to their close proximity.