## 4 Conclusions  
In this study, the performance of two commercial lipase-based biocatalysts, the novel Novozym(r) 40086 and the well-known Novozym(r) 435, has been evaluated in the solvent-free synthesis of NPGDL, a common skin-conditioning and emulsifying agent. For this purpose, the enzyme concentration and temperature have been optimized, proving the great impact of this latter parameter in the performance of _Rhizomuocr miehei_ lipase, since a decrease in its catalytic activity has been observed when operating above 60 \({}^{\circ}\)C. Results under the best reaction conditions have proven the great performance of Novozym(r) 435, which is not only more thermostable but also appears to have a higher specificity for the substrates and the intermediate product involved in this solvent-free esterification process.  
However, when further progress is made in the comparative study of both biocatalysts, with the aim of establishing the most advantageous industrial process for the production of neopentyl glycol dilaurate, the best choice is a different one. Once the conditions in which both enzymatic derivatives can be reused have been established, and the direct production costs of the NPGDL have been calculated, Novozym(r) 40086 becomes the most cost-effective alternative. The direct costs, i.e., the sum of raw materials, biocatalyst, and energy consumption, is 25.28 EUR/kg if the enzyme used is Novozym(r) 40086, and 32.39 EUR/kg if Novozym(r) 435 is chosen.  
From a sustainability point of view, these processes that use enzymes as catalysts do not use any solvents in the reactor, and achieve high conversions that are environmentally friendly. The calculated sustainability indicators are evidence of this statement. In any case, the choice of one or the other would also be in favor of Novozym(r) 40086, which has a lower optimum working temperature.  
\begin{table}
\begin{tabular}{c c c} \hline  & **Novozym® 40086** & **Novozym® 435** \\ \hline Atom economy (AE) 1 (\%) & 92.87 & 92.87 \\ Simple E-factor 2 & 0.017 & 0.018 \\ Complete E-factor 3 & 0.095 & 0.093 \\ Carbon mass efficiency (CME) 4 (\%) & 91.35 & 91.46 \\ Process mass intensity (PMI) 5 & 1.05 & 1.15 \\ \hline \end{tabular}  
* \(\mathrm{AE}=\mathrm{molecular weight}\) of all sandwicular \(\times\) 100. 2 \(\mathrm{E}-\mathrm{factor}=\mathrm{kg of\ waste}\) of several product. 3 Complete E \(-\mathrm{factor}=\mathrm{kg of\ waste}\) (including water). 4 CME \(=\mathrm{kg of\ carbonated\ residual}\times 100\). 5 \(\mathrm{PMI}=\mathrm{kg of\ of\ material\ material}\) kg of desired product.  
\end{table}
Table 2: Green metrics for the biocatalytic synthesis of NPGDL using Novozym® 40086 (7.5% \(w/w\), 60 \({}^{\circ}\)C) and Novozym® 435 (3.75% \(w/w\), 80 \({}^{\circ}\)C).  
**Author Contributions:** Conceptualization, C.M., F.M. and J.B.; methodology, M.S.-A. and S.O.-R.; validation, M.S.-A., S.G.-M. and S.O.-R.; formal analysis, M.S.-A., S.G.-M. and S.O.-R.; investigation, S.G.-M., M.S.-A. and S.O.-R.; writing--original draft preparation, M.S.-A., S.G.-M. and F.M.; writing--review and editing, F.M., C.M. and J.B.; supervision, C.M., F.M. and J.B.; project administration, C.M., F.M. and J.B.; funding acquisition, F.M. and J.B. All authors have read and agreed to the published version of the manuscript.  
This work was funded with support from RTI 2018-094908-B-I00 MCIN/AEI/10.13039/501100011033/ and ERDF \"A way of making Europe\".  
Not applicable.  
**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.  
The authors wish to acknowledge [PERSON] (Novozymes Espana S.A.) who kindly provided the biocatalysts.  
The authors declare no conflict of interest.