Green synthesis of sulfonated organosilane functionalized multiwalled carbon nanotubes and its catalytic activity for one-pot conversion of high free fatty acid seed oil to biodiesel

Abstract

An environmentally friendly surface modification method was applied to synthesize solid acid catalyst suitable for biodiesel production. The catalyst was prepared from multiwalled carbon nanotube (MWCNT) and 3-mercaptopropyltrimethoxysilane (3-MPTMS) oxidized in hydrogen peroxide under supercritical carbon dioxide (scCO2). The carbon dioxide under supercritical condition with ethanol as cosolvent allows swift transportation and promotes uniform distribution of organosilane groups on randomly entangled and layered orientation of MWCNT. The catalyst was characterized by using Field emission scanning electron microscopy–energy dispersive x-ray (FESEM-EDX), Thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, X-ray powder diffraction (XRD), Brunauer–Emmett-Teller (BET) analysis and Time-of-Flight secondary ion mass spectrometry (TOF-SIMS). The catalytic activity of the catalyst was tested using a high free fatty acid (FFA)-containing Hibiscus cannabinus (kenaf) oil, and the fatty acid methyl esters (FAME) products from simultaneous esterification and transesterification reactions were quantified. To compare with scCO2 method, an acid catalyst was also prepared using liquid chemical deposition that resulted in only 45.11% biodiesel conversion. On the other hand, the optimum conversion of 93.10% was obtained using a scCO2 synthesized catalyst at the following transesterification conditions: temperature = 63 °C, methanol:oil ratio = 14:1, 10 wt % catalyst and time = 240 min.