Sulfonated graphene as highly efficient and reusable acid carbocatalyst for the synthesis of ester plasticizers

Plasticizers are well known for their effectiveness in producing flexible plastics. The automotive, plastic and pharmaceutical industries, essential to a healthy economy, rely heavily on plasticizers to produce everything from construction materials to medical devices, cosmetics, children toys, food wraps, adhesives, paints, and 'wonder drugs'. Although HSO is commonly used as commodity catalyst for plasticizer synthesis it is energy-inefficient, non-recyclable, and requires tedious separation from the homogeneous reaction mixture resulting in abundant non-recyclable acid waste. In this study, for the first time, we report an efficient synthesis of ester plasticizers (>90% yields) using sulfonated graphene (GSOH) as an energy-efficient, water tolerant, reusable and highly active solid acid carbocatalyst. The hydrothermal sulfonation of reduced graphene oxide with fuming HSO at 120°C for 3 days afforded GSOH with remarkable acid activity as demonstrated by P magic-angle spinning (MAS) NMR spectroscopy. The superior catalytic performance of GSOH over traditional homogeneous acids, Amberlyst™-15, and acidic ionic liquids has been attributed to the presence of highly acidic and stable sulfonic acid groups within the two dimensional graphene domain, which synergistically work for high mass transfer in the reaction. Furthermore, the preliminary experimental results indicate that GSOH is quite effective as a catalyst in the esterification of oleic and salicylic acid and thus may pave the way for its broad industrial applications in the near future.