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

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 I D/I G 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.