**4. Conclusions**

The MoO3-Al2O3 catalytic system has provided positive results consisting of high triglycerides conversions into biodiesel under conditions of high FFAs content that would have made the conventional transesterification reaction impossible in a single step with basic catalysts. Moreover, the FFAs are efficiently converted into biodiesel as well. Compared to bulk MoO3, alumina-supported MoO3 leads to a more efficient utilization of the active phase and enhanced stability towards molybdenum leaching by the reaction medium. These effects likely arise from the interaction established between MoO3 and alumina that affect the textural and acidic properties of the catalysts. However, much effort is still required to develop a sufficiently stable catalyst based on molybdenum oxide for transforming triglycerides with high FFAs content into biodiesel.

**Author Contributions:** All the authors participated actively in the writing and editing of the manuscript. Furthermore: conceptualization, A.N. and L.M.G.; methodology, A.N.; catalysts preparation, textural and structural characterization, and catalytic testing, A.N. and I.R.; acid properties characterization, E.J.-B. and F.R.-S.; XPS characterization J.L.; results interpretation and discussion, and literature review, all the authors; project administration and funding acquisition, L.M.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** Financial support from Spanish Ministerio de Ciencia, Innovación y Universidades, and the European Regional Development Fund (ERDF/FEDER) (grant RTI2018-096294-B-C31) is gratefully acknowledged. L.M.G. thanks Banco de Santander and Universidad Pública de Navarra for their financial support under "Programa de Intensificación de la Investigación 2018" initiative. JL is a Serra Húnter Fellow and is grateful to ICREA Academia program and 2017 SGR 128.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
