catalysts Review New Trends in the Conversion of CO2 to Cyclic Carbonates Erivaldo J.C. Lopes, Ana P.C. Ribeiro * and Luísa M.D.R.S. Martins * Centro de Química Estrutural and Departamento de Engenharia Química, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal; [email protected] * Correspondence: [email protected] (A.P.C.R.); [email protected] (L.M.D.R.S.M.); Tel.: +351-21-841-9201 (A.P.C.R.); +351-21-841-9264 (L.M.D.R.S.M.) Received: 1 April 2020; Accepted: 26 April 2020; Published: 27 April 2020 Abstract: This work concerns recent advances (mainly in the last five years) in the challenging conversion of carbon dioxide (CO2) into fine chemicals, in particular to cyclic carbonates, as a meaningful measure to reduce CO2 emissions in the atmosphere and subsequent global warming effects. Thus, efficient catalysts and catalytic processes developed to convert CO2 into different chemicals towards a more sustainable chemical industry are addressed. Cyclic carbonates can be produced by different routes that directly, or indirectly, use carbon dioxide. Thus, recent findings on CO2 cycloaddition to epoxides as well as on its reaction with diols are reviewed. In addition, indirect sources of carbon dioxide, such as urea, considered a sustainable process with high atom economy, are also discussed. Reaction mechanisms for the transformations involved are also presented. Keywords: carbon dioxide; cyclic carbonate; diol; epoxide; catalysis; CO2 conversion 1. Introduction Modern life is sustained by an unremitting stream of energy delivered to final users as fuels, electricity and heat. Currently, over 80% of the world’s primary energy supply is provided by fossil fuels carbon sources (oil, coal or natural gas). Since the industrial revolution, i.e., for the last two centuries, fossil fuels, generated from biomass over millions of years, have been extensively used in anthropic activities, thereby increasing carbon dioxide emission into the atmosphere. Over the last few decades it has become clear that carbon dioxide released in this way is affecting the climate stability of the biosphere (see Figure1) with countless consequences. Scientists predict social, environmental, economic and health issues if the actual trend continues: melting glaciers, droughts, early snowmelt, extreme weather changing, wildfires, water shortages, rising sea level, new pests, infectious diseases and air pollution, just to mention a few [1–4]. In December 2015, the United Nations conference on climate change (the 21st Conference of the Parties, COP21) brought public awareness to the steps required to slow down global warming and set a framework to mitigate climate change by agreeing on 17 sustainable development goals. Moreover, the Intergovernmental Panel on Climate Change (IPCC) set as a goal to keep the warming under 2 ◦C above preindustrial levels and pursuing efforts to limit the temperature increase to 1.5 ◦C above preindustrial levels [3]. Thus, it is urgent to adopt behaviors to reduce anthropogenic emissions of carbon dioxide. Among all the measures, there are four considered more significant: (i) the reduction of energy consumption by increasing its efficiency; (ii) the replacement of fossil fuels with renewable energy sources; (iii) CO2 capture and storage (CCS) and (iv) CO2 capture and usage (CCU). Catalysts 2020, 10, 479; doi:10.3390/catal10050479 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 479 2 of 14 Catalysts 2020, 10, x FOR PEER REVIEW 2 of 15 Figure 1.1. EvolutionEvolution of of Earth’s Earth’s average average temperature temperature in comparison in comparison with thewith atmospheric the atmospheric carbon dioxidecarbon dioxideover the lastover decades. the last Reproduced decades. Reproduced with permission with frompermission [4], copyright from 2020,[4], c NOAAopyright Climate.gov 2020, NOAA. Climate.gov. 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However,Two main often approaches such procedures to convert require carbon high dioxide pressures into and fine harsh chemicals reaction can conditions, be considered thus according limiting theirto their practical energetics. applications One is the in the reductive chemical CO industry.2 conversion that requires a large amount of energy and quiteTo powerful overcome reducing CO2’s agents,kinetic suchstability, as hydrogen, catalysts andand itcatalytic is used toprocesses produce have chemicals been likedeveloped methanol to quicklyor formic achieve acid [8 the,14]. reaction The other equilibrium one is non-reductive allowing the (the reaction+4-oxidation to take stateplace of under carbon mild is maintained), conditions. Thus,either a moderately significant endothermicnumber of successful or exothermic, research andes widely on the used coordination to provide chemistry products of like CO carbonates,2 with the polycarbonates,ultimate aim of discovering urea, polyurethanes, new catalysts carboxylates, for CO2 pyrenes,conversion lactones has been or carbamatesreported [6 []8 (see,10,15 Scheme,16]. 1). In this respect, homogeneous catalysis is important and its potential for CO2 conversion was already discussed in several reviews [7–10]. Added-value chemicals including organic carbonates [11] urethanes [12], carbamates [10], lactones [10], pyrenes [10], formic acid and its derivatives [13] are Catalysts 2020, 10, x FOR PEER REVIEW 3 of 15 well-known to be synthesized by homogeneous catalytic processes. However, only a few energy-efficient processes employing CO2 have been commercialized. Catalysts 2020, 10, x FOR PEER REVIEW 3 of 15 Catalysts 2020, 10, 479