Elisa HERNÁNDEZ1, Alejandro BELINCHÓN1, Pablo NAVARRO1, Rubén SANTIAGO1, Cristian MOYA1, Daniel HOSPITAL-BENITO1, Jesús LEMUS1, Rola EL-BIJOU1, José PALOMAR1
1Autonomous University of Madrid, Madrid, Spain
Global warming solutions mainly consist of reducing the current alarming CO2 levels in the atmosphere. Two main strategies are implemented to achieve said purpose: Carbon Capture and Storage (CCS), which aims to concentrate CO2 and store it, and Carbon Capture and Utilization (CCU), which uses CO2 as feedstock of new processes to produce value-added compounds.
In particular, CO2 valorization with epoxides to obtain cyclic carbonates using ionic liquids (ILs) as catalysts is being studied as reaction path due to the high yields obtained. The challenge to design a CO2 conversion process based on ILs is to solve the product/catalyst separation efficiently and sustainably, especially dealing with a homogeneous catalytic step. The common strategy proposed in the literature consists of a distillation step in which the IL is obtained as pure compound in the residue and the carbonate is recovered as distillate. Nevertheless, propylene carbonate (PC) presents a high boiling point, which requires high reboiler temperatures, incompatible with ionic liquids thermal stability. This distillation can be achieved by displaying extreme vacuum conditions, but it implies high electricity expenses. Recently, IL/PC separation by liquid-liquid extraction (LLE) was successfully employed in different systems containing ionic liquids of different nature. However, literature evaluating the sustainability of these kind of processes is scarce, which is a very important shortcoming given the importance of ensuring sustainability and an adequate CO2 balance.
In this work, the environmental impacts associated to the distillation-based reference and two proposed LLE-based approaches to produce PC are evaluated by means of process modelling and life cycle impact assessment tools. Simulations prove that tuning operating variables in terms of environmental benefits also improves the cost-effectiveness of the processes. In addition, sustainability of the processes is nearly not altered by the IL selection when an effective recovery is designed. The emissions associated to the proposed approaches vary between 0.12 and 0.22 kg CO2 equiv. per kgPC, while the operational costs range from 3 to 8 $ per tPC. The water-mediated LLE-based approach imposes the best environmental, capital expenditures, and operating expenses performance.