Room temperature synthesis of biodiesel using sulfonated graphitic carbon nitride

Sulfonation of graphitic carbon nitride (g-CN) affords a polar and strongly acidic catalyst, Sg-CN, which displays unprecedented reactivity and selectivity in biodiesel synthesis and esterification reactions at room temperature.

The Sg-CN catalyst was characterized using scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and solid-state nuclear magnetic resonance ( 13 C-NMR). The SEM analysis clearly indicates the incorporation of sulfonic group in the graphitic carbon nitride framework; change in morphology of g-CN after sulfonation was also discerned (Fig. 2).
The examination of Sg-CN and starting material graphitic carbon nitride (g-CN) reveals the crystalline nature of Sg-CN which is reaffirmed by comparing the XRD pattern of Sg-CN and g-CN (Fig. 3a). The stability of the catalyst and temperature tolerance have been studied using thermogravimetric analysis (TGA), which confirms that the synthesized sulfonated graphitic carbon nitride is stable up 250 °C (Fig. 3b). The change in functional group and electronic behavior has been studied using FTIR and solid state 13 Fig. S8) also confirmed the immobilization of sulfonic group. There is sharp decline in the surface area after the immobilization which is may be due to the creation of ionic character on the nitrogenous framework of g-CN culminating in the better interlayer attraction in graphitic carbon nitride.  The concentration of sulfur was determined using elemental analysis which corresponds to the 5.47 mmol/g of the catalyst, which is equivalent to the acid concentration in Sg-CN. The acid strength is expected to be higher due anticipated positive charge developed on the nitrogenous framework after the attachment of -SO 3 H.

Results and Discussion
The application of Sg-CN was explored in the synthesis of biodiesel via the esterification of fatty acids. Oleic acid was used as a model substrate to optimize of reaction conditions (Fig. 4). Initially, one gram of oleic acid was treated with 100 mg of Sg-CN in methanol at room temperature ( Table 1, entry 1) and the reaction was monitored using GC-MS at regular time intervals of time. Complete conversion of oleic acid to the corresponding methyl ester occurred within 4 hours. The most enthusing observation was the product purity attained after a simple decantation and distillation of reaction mixture. Experiments were conducted then to determine the optimum catalyst charge required for this efficient fatty acid esterification. Accordingly, oleic acid was treated with 75 mg, 50 mg, 25 mg, 10 mg and 5 mg of Sg-CN (Table 1, entries 2-5). Complete conversion of oleic acid to methyl oleate was discerned again (Table 1, entries 2-4) whereas the reaction with 10 mg of Sg-CN required overnight stirring for the completion of esterification at room temperature. However, further reduction in Sg-CN quantity does not allow the reaction to be completed even after 24 hours of stirring (Table 1, entry 6). A control experiment with pure support g-CN has been performed under similar conditions; even a trace of the product formation after 24 hours was not discernible. After finding optimum catalyst loading of Sg-CN, it was imperative to compare our results with the reported acid catalyst. We were pleasantly surprised to see that Sg-CN completed the reaction within 4 h at room temperature which was not precedence in earlier reports (Table 2) 13,[36][37][38][39] After establishing the optimum catalyst charge required for efficient esterification, a broader scope of the reaction was explored deploying a wide range of fatty acids and their analogues for the esterification and biodiesel   production ( Table 3). Most of the long chain fatty acids were efficiently converted into corresponding esters. The presence of unsaturation in the backbone does not affect the reaction outcome as all the acids were converted into corresponding esters almost in quantitative yield. Treatment of a bi-functional dicarboxylic acid with Sg-CN under similar conditions afforded the corresponding diester (Table 3, entry 5) although a relatively longer reaction time was required. The transesterification reactions were also performed using ethyl benzoate and ethyl cinnamate using methanol as a solvent and Sg-CN as a catalyst (see Supplementary, Table S1). GCMS (see Supplementary) confirmed that the equilibrium shift completed towards the corresponding methyl esters. The solidification of the reaction towards the right may be due to higher concentration of methanol which is used as a reaction media in transesterification.
Recycling and reusability of the Sg-CN. The stabilty and recyclability aspects of the catalyst were studied thereafter using oleic acid and Sg-CN. Upon reaction completion, the catalyst was seperated, washed with methanol, dried under vacuum and reused for the next set of reactants. The outcome of the recycling experiments authenticate that the catalyst can be reused up to 5 cycles without any loss in activity (see Supplementary, Fig. S2).

Conclusion
A sulfonated graphitic carbon nitride (Sg-CN) has been synthesized via simple sulfonation and its application has been demonstrated in the efficient synthesis of biodiesel. The unique attribute of Sg-CN is its unprecedented reactivity which enables the esterification at room temperature, affording product that does not require any purification. The salient features of this catalyst include its relatively benign nature, easy accessibility, low cost and stability over several reaction cycles.  Table 3. Sg-CN catalyzed esterification of fatty acids a . a Reaction Condition: Fatty acid (1.0 g), methanol (5.0 ml), Sg-CN (25 mg), room temperature, 4 h; b Reaction was stirred for 8 h at room temperature.