Sustainable Synthesis of Diamide Solvents from Succinic Anhydride under Microwave Conditions for the Production of Graphene via Liquid-Phase Exfoliation

Abstract

Liquid-phase exfoliation (LPE) is a viable method for the large-scale production of two-dimensional (2D) graphene; however, toxic solvents must be replaced. The microwave synthesis of sustainable succindiamide solvents, namely N,N′-diethyl-N,N′-dibutylsuccindiamide (EBSA) and N,N,N′,N′-tetrabutylsuccindiamide (TBSA), was investigated and applied to the LPE of graphite to obtain graphene. The yield of EBSA was 99.4% from succinic anhydride with a K60 silica gel catalyst under microwave conditions (160 °C in 30 min), while TBSA presented an 88.8% yield at 180 °C. The green metric demonstrated that this route has a significantly lower Process Mass Intensity (PMI) and higher Reaction Mass Efficiency (RME) values compared to previously published routes. TBSA demonstrated a ∼150% increase in the graphene and graphene oxide exfoliation (thickness <7 nm) compared to N-methylpyrrolidone (NMP). TBSA outperformed Cyrene in the LPE of graphene oxide. Graphene and graphene oxide nanosheets were characterized by transmission electron microscopy (TEM), Raman spectroscopy, X-ray diffraction (XRD), and atomic force microscopy (AFM). The solvent surface tension, density, viscosity, Hansen Solubility Parameters (HSPs), and Kamlet–Abboud–Taft parameters (KATs) were investigated to determine the key parameters for exfoliation. The high dipolarity (π*) and hydrogen bond-accepting ability (β) of TBSA promoted significant graphene exfoliation, while its greater viscosity produced high-quality graphene sheets, with ID/IG ratios of 0.005 ± 0.02 for graphene flakes of <1 μm. Higher-viscosity solvents demonstrated a limited increase in the graphene concentration with longer sonication, but retained high-quality graphene. TBSA is a potentially greener alternative to NMP for LPE, leading to opportunities for sustainable graphene production.