A facile and acid-free approach towards the preparation of sulphonated multi-walled carbon nanotubes as a strong protonic acid catalyst for biodiesel production

The process of grafting multi-walled carbon nanotubes (MWCNTs) with SO3H groups is known as sulphonation. The most common method to sulphonate MWCNTs is via thermal treatment with concentrated sulphuric acid. However, the major drawbacks of this sulphonation process are that it is time consuming, energy intensive and requires the use of concentrated acid. In contrast, in this study, sulphonated MWCNTs (s-MWCNTs) were prepared using a facile and acid-free approach via the thermal decomposition of ammonium sulphate ((NH4)2SO4) with the aid of ultrasonication. The effects of the concentration of the (NH4)2SO4 solution and the ultrasonication period of MWCNTs in the (NH4)2SO4 solution were studied and optimised to enhance the catalytic activity of s-MWCNTs. These results showed that s-MWCNTs with the best performance could be obtained by ultrasonicating the purified MWCNTs in a 10 wt.% (NH4)2SO4 solution for 10 min followed by heating at 235 °C for 30 min. The resulting s-MWCNTs can produce a biodiesel yield of 84.9%. The high solubility of palm fatty acid distillate (PFAD) and the good dispersion behaviour of s-MWCNTs in methanol caused a reduction in reaction phases and thus minimised the mass transfer limitation encountered by conventional heterogeneous catalysts used for biodiesel production.