Abstract

Environmental concerns have highlighted the need to transition to renewable energy sources to reduce dependence on fossil fuels. Biodiesel has emerged as a promising alternative to traditional diesel due to its environmental benefits, such as biodegradability and renewability. Nevertheless, its large-scale production still faces some challenges related mainly to the reliance on edible oils and homogeneous catalysts, which lead to costly purification processes, and the food vs. fuel debate. In this context, scientists have begun exploring the use of non-edible and waste cooking oils, in addition to the development of heterogeneous catalysts to make the biodiesel process more sustainable.
Recently, carbon-based materials have gained significant attention as heterogeneous catalysts for biodiesel, owing to the sustainability of their synthesis from inexpensive and widely available sources such as biomass. Furthermore, their high surface area, thermal stability, and tunable surface chemistry endow exceptional physicochemical properties that can translate to excellent catalytic activity through functionalization with different functional groups or elements to introduce specific active basic, acidic, or dual active sites.
Herein, carbon-based catalysts were developed and investigated for biodiesel synthesis using refined and acidic oils. Reaction parameters such as oil-to-methanol molar ratio, catalyst loading, temperature, and time were optimized aiming at the highest conversion. First, a basic catalyst was obtained through the calcination of sodium citrate at moderate temperatures. Thorough characterization efforts were carried out to glean an understanding of the effect of calcination temperature on the physicochemical properties of the materials and their relationship with catalytic activity. The material showed high conversion of canola and waste cooking oils and exhibited remarkable free fatty acid tolerance up to 10 wt.%.
To overcome the FFA resistance faced by the basic catalyst, a sulfonated carbon material derived from sucrose was prepared using a rapid and energy-efficient synthesis approach. An assessment of the effect of synthesis parameters on the physicochemical properties of the materials and their catalytic activity was carried out to better comprehend the underlying interplay. The materials showed excellent catalytic activity towards the esterification of oleic acid, highlighting the potential of sulfonated carbon as an efficient solid acid catalyst.
Lastly, owing to the monofunctional nature of the previous catalytic systems, a bifunctional catalyst was designed. Mesoporous carbons derived from biomass and doped in situ with Zn and Ni were synthesized to enable simultaneous transesterification and esterification reactions. Characterization results confirmed the presence of acidic and basic sites within the material structure. Moreover, the effect of feedstock acidity on the catalytic activity was also explored, employing canola oil acidified at different levels, proving the effectiveness of the material even at high levels of acidity.