## 3 Results
### Catalytic Activity  
In preliminary tests conducted without using diethyl ether as a cosolvent, the immiscibility between methanol and oil created significant challenges in the reactor. At high flow rates, the reactants did not remain in contact with the catalyst long enough, resulting in low biodiesel yields. Conversely, at lower flow rates, phase separation occurred within the reactor, further hindering reactant interaction. The addition of DEE overcame these limitations by homogenizing the phases, improving fluid dynamics, and ensuring effective contact between reactants and the catalyst. The catalytic performance of the LiSA catalyst was evaluated over a reaction period of 508 min, with reaction products collected at the reactor outlet. A total of 28 samples, each of 25 mL, were taken approximately every 18 min. The %FAME content of these biodiesel samples ranged from 94.7% to 99.6%, with an average of 97.1%, as shown in Table 2 and plotted over time in Figure 5. These results confirm the reliable long-term performance of the catalyst, demonstrating its efficiency in sustaining biodiesel production that consistently \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & **S\({}_{\text{BET}}\) (m\({}^{2}\)/g)** & **D (nm)** & **mg Li/g** \\ \hline SAS & 0.62 & 80.3 & 0.0 \\ LiSA & 0.18 & 102.3 & 15.2 \\ Used LiSA & 0.16 & 158.7 & 33.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Surface area, average pore diameter, and lithium content of the studied materials. meets commercial standards, exceeding the minimum requirement of 96.5% established by the UNE-EN 14214:2020 standard for biodiesel commercialization [21]. \begin{table}
\begin{tabular}{c c c} \hline
**Sample Number** & **t (min)** & **FAME (\%)** \\ \hline  
\hline \end{tabular}
\end{table} 
Table 2: %FAME content for each sample collected at the outlet of the fixed-bed reactor with LiSA catalyst. Figure 5: Evolution of %FAME content over time during transesterification reaction using the LiSA as a catalyst. Additionally, Figure 6 displays chromatograms for two randomly selected samples: sample 12 (at 220 min), with a %FAME content of 96.2%, and sample 28 (at 508 min), with 97.1%. These chromatograms confirm the stability and reproducibility of the process over time, with only minor variations in the FAME content observed among the samples. This consistent performance highlights the high catalytic activity of the LiSA catalyst for sustained biodiesel production. Furthermore, for the two samples, a photograph of the reaction products is shown once the MeOH and DEE have evaporated. The upper phase, containing the FAME (biodiesel phase), and the lower phase, consisting of glycerol, can be observed. Although no studies have currently been found on the development of molds for the synthesis of catalysts for biodiesel production in a fixed-bed reactor, there are some studies that, although not using 3D synthesis methods or fixed-bed reactors, do use catalysts impregnated with lithium for biodiesel production in batch processes. Among these studies, it is worth mentioning that [PERSON] et al. [22] used Kalsilite as a heterogeneous catalyst for  
Figure 6: Chromatograms of (**a**) sample 12 at 220 min (%FAME 96.2%) and (**b**) sample 28 at 508 min (%FAME 97.1%). biodiesel production from soybean oil and methanol. Kalsilite exhibited low catalytic activity, but by introducing 2.3% by weight of Li (using lithium nitrate as a precursor) via the impregnation method, the activity was significantly enhanced. Specifically, a 100% FAME yield was achieved at a reaction temperature of 120 \({}^{\circ}\)C. On the other hand, [PERSON] et al. [23] used lithium-zeolite as a catalyst for biodiesel production from non-edible castor oil. A maximum FAME yield of 92% was reached at 60 \({}^{\circ}\)C, 1 h, with a 6:1 MeOH/oil molar ratio and a catalyst load of 1.5%. In contrast to studies that operate at elevated temperatures, the present study works at room temperature, which offers a significant advantage in terms of energy efficiency.