Metrics for green syntheses: two case studies in biorefinery

Green metrics is a relatively new concept in Green Chemistry. The application of rigorous green metrics must go along with the experimental validation of synthetic procedures; this is necessary to give precise guidelines so to define a green synthetic approach and avoid misunderstanding and pretentious claims originating from subjective rather than objective evaluations. 5-(hydroxymethyl)furfural (HMF) has been labelled as the “sleeping giant” of the bio-based platform-chemicals realm, due to its versatility and being the starting point for endless chemical transformations into novel monomers for producing bio-based polymers. Among HMF derivatives, 2,5-Furandicarboxylic acid (FDCA) has been extensively studied as monomer for the production of polyesters such as polyethylene furanoate (PEF), considered as one of the most valuable bio-based substitute of the petroleum-derived polyethylene terephthalate (PET). However, the sustainability of the synthetic procedures leading to HMF and its derivatives represent a key aspect that must be addressed in order to foster their entrance into the bio-based plastic market. In this scenario, green metrics such as the environmental factor (E-factor) and the process mass intensity (PMI) represent an useful tool towards this goal.1 From these premises, the present study reports an alternative synthetic procedure for the production of 2,5-furandicarboxylic acid dimethyl ester (FDME) starting from galactaric acid via dimethyl carbonate (DMC) chemistry. Both sulfonic resins and an iron-based Lewis acid showed to promote the one-pot formation of FDME. The pure product was retrieved as a white crystalline solid with an isolated yield of up to 70%. Based on the different intermediates identified, a possible reaction mechanism was proposed, which highlights the essential contribute of DMC in the product formation.2 This presentation is also focussing on some key points inherent to the IUPAC project 2017-030-2-041: Metrics for Green synthesis. Thus, the greenness of the herein discussed synthetic procedure was evaluated using the most common green metrics and compared with other available synthetic pathways.