Abstract

Energy demands are continuously rising, and currently, society remains dependent on fossil fuels as the primary energy source. The overconsumption of fossil fuels offers many drawbacks, including rising greenhouse gas emissions. Therefore, the search for more sustainable and renewable energy sources, such as biodiesel, has garnered significant interest. Commercially, biodiesel is produced using a homogeneous strong base catalyst (e.g. NaOH) through the transesterification of vegetable oils. While efficient, this process presents several limitations, such as the inability to recover and reuse the catalyst coupled with costly purification requirements.
Carbon-based nanomaterials have already been reported as efficient heterogeneous catalysts for biodiesel production. However, despite promising results, these materials usually require high temperatures and pressures to drive the reaction, increasing the energy requirements for the transesterification reaction and rendering large-scale use economically unfeasible. Polymeric carbon nitride-based structures (PCNs) and carbon dots (CDs) have emerged as promising candidates to overcome these limitations. PCNs and CDs are well-known for their low-cost synthesis and interesting optical properties, yet their role in driving chemically-catalyzed reactions remains relatively unexplored.
Herein, PCNs structures with different morphologies (nanosheets, fibers and dots) and amine-passivated CDs were synthesized and investigated as heterogeneous catalysts in the transesterification reaction of canola oil to biodiesel. In addition, the surface chemistry, morphology and physico-chemical properties of all materials were thoroughly characterized.
Bulk PCNs were synthesized and post-modified through acid and thermal treatments, whiled fibers were produced through a molten salt method. These different strategies give rise to materials with different morphologies, surface area and functional groups, allowing for understanding the effect of these parameters in their catalytic activity. It is demonstrated that surface chemistry is the most important aspect in designing more efficient PCN-based catalysts, as oxygenated functional groups are crucial to achieving high biodiesel yields.
Amine-passivated CDs (N-CDs) were chosen as a model system further to extend our knowledge of the role of surface chemistry in heterogeneous catalyzed transesterification reactions. By varying the type and concentration of the amine passivating agents, it was possible to control the degree of carboxylic acid to amine and amide functionalization. These results shed light on a plausible governing mechanism for the N-CD-catalyzed transesterification of canola oil to biodiesel, suggesting that both amines and carboxylic acids are active catalytic sites.
The transesterification reaction parameters were optimized, achieving >96% biodiesel conversion for both PCNs and N-CDs systems. The optimal conditions were achieved using 1 wt% catalyst loading for 3h in a solvothermal reactor at 150 °C and 100 °C for PCNs and N-CDs, respectively. In addition, the N-CDs were reused for at least five reaction cycles and demonstrated the system's ability to maintain high catalyst performance over several reaction cycles.
Lastly, we aimed to explore the knowledge acquired from the previous systems and reduce the energy requirements for biodiesel synthesis. We synthesized and thoroughly characterized polymeric carbon nitrides nanosheets (PCNNs) and dots (PCNDs) through a solid phase, low-temperature reaction. Their morphology and surface chemistry were tuned by varying the precursors’ ratio, and the materials were investigated as catalysts in the transesterification reaction of canola oil to biodiesel. Biodiesel conversion yields > 98% were achieved using a 5 wt% catalyst loading, oil to methanol ratio of 1:36 at 90 °C for 4 h, with the performance maintained over at least five reuse cycles. The reactions were carried out in a non-pressurized vial and at lower temperatures when compared to PCNs and N-CDs, reducing the energy requirements. In addition, kinetics studies were carried out to glean a deeper understanding of the catalytic mechanism, suggesting that the reaction mechanisms occur through a pseudo-first order mechanism.