Preparation and characterization of basic graphene-based catalysts and their application in biodiesel synthesis

Abstract

A series of N-doped graphene-based materials was prepared from thermally reduced graphene oxide (TRGO) at elevated temperature (850 or 950 °C) under ammonia flow, and by impregnation of TRGO with melamine followed by subsequent thermal treatment. The performed analyses revealed efficient doping of TRGO with N functionalities of different types, the content of which was found to be between 3.4 wt% and 12.3 wt%, depending on the modification route and conditions. The functionalization of TRGO with NH3 was considerably less effective than that with melamine, however, the NH3-treated carbons were significantly more basic in nature (basicity up to 0.77 mmol g−1) due to the presence of quaternary N (NQ). It was also found that NQ was preferentially formed under more severe conditions, i.e., higher temperature and longer reaction time. The catalytic performance of the prepared samples was investigated in the transesterification of rapeseed oil with methanol under elevated pressure at 130 °C. The activity of carbons was strongly related to the content of NQ, and the highest yield of fatty acid methyl esters (of 65%) was obtained in the process performed with TRGO functionalized with ammonia at 950 °C for 8 h, which showed the highest number of quaternary nitrogen.