**4. Conclusions**

In recent decades, the increase in the production of biodiesel has generated a huge amount of crude glycerol, whose valorization would have a grea<sup>t</sup> influence on the biodiesel process, making it more a ffordable from an economical point of view. In this context, the research of cheaper and more sustainable routes to transform glycerol into value-added products in the presence of heterogeneous catalysts are being widely developed.

Among all the routes being studied, the production of oxygenated additives for fuels has been raised as one of the most promising options. Specifically, the di- and tri-*tert*-Butyl ethers (h-GTBE) obtained by the reaction of glycerol, either with isobutene or *tert*-Butyl alcohol, improve the e fficiency of the diesel combustion and reduce the particulate matter and the soot emissions with outstanding results. Furthermore, some studies foresee that these additives could be produced on industrial scale, so the search for suitable heterogeneous acid catalysts for the etherification reaction has been of grea<sup>t</sup> interest in recent years.

In general, a good catalyst for this reaction, implying a high yield to h-GTBE, should exhibit not only a high amount of strong acid sites but also textural properties that allow the formation of the products of the reaction, which are molecules with a large volume. In addition, an adequate hydrophobic–hydrophilic character and water-resistance are characteristics also required to avoid the deactivation of the acid sites, mainly when TBA is employed as a reactant. The study of di fferent catalysts has been combined with the study of several engineering designs, in order to improve the production of the desired products. Thus, di fferent alternatives, such as the use of water permselective membranes or any other sorbents of water, as well as di fferent flow reactors have been studied.

Despite the fact that there are catalysts that have shown remarkable results in the etherification with *tert*-Butyl alcohol, the selectivity to h-GTBE can be improved in order to attain similar results than with isobutene. For this purpose, the shift of the reaction equilibrium to the formation of poly-substituted ethers is still a challenge. Thus, researches contemplating catalysts with ideal properties to perform the reaction, as well as new forms to remove the water formed in the reaction, could be a possible way to go forward.

Nevertheless, further research e fforts are necessary in order to overcome some of the remaining challenges regarding these reactions. For example, the use of crude glycerol directly obtained from biodiesel production, as the majority of these studies employed pure glycerol as a reactant. Furthermore, the oxygenated fuel additives must be tested in di fferent fuel blends to ensure their behavior, according to specification standards, etc. Likewise, the production of these additives on a larger scale in a feasible way needs to be achieved.

**Author Contributions:** R.E. conceived, reviewed, and wrote the manuscript. L.A.-D. helped in writing the manuscript. D.L. and F.M.B. supervised the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** This research is supported by the MEIC funds (Project ENE 2016-81013-R), that cover the costs to publish in open access.

**Conflicts of Interest:** The authors declare no conflicts of interest.
