Sustainable processes for the catalytic synthesis of safer chemical substitutes of N-methyl-2-pyrrolidone

N-methyl-2-pyrrolidone (NMP) is a chemical widely used as solvent in several industrial applications. However, many studies and regulations, including REACH, have indicated NMP as a potential toxic agent. Therefore, functional substitutes with high marketability are actively sought. This paper reviews the methods reported in the literature for the sustainable synthesis of the 5-methyl-homologues of NMP. Focus is on liquid phase processes using heterogeneous catalysts and bio-derived feedstock, i.e. levulinates, since N-substituted-5-methyl-2-pyrrolidones (5 M Ps) are readily obtainable from them through one-pot reductive amination. The contributions appeared up to July 2018 are covered and discussed in terms of reaction mechanism, catalysts' properties and applicability.