**4. Conclusions**

We developed a microwave-assisted protocol to convert furfural and acetone very quickly into their aldol reaction (C8-OH) and condensation products (C8 and C13). The catalysts used are easy to prepare and to separate from the reaction mixture, they are not corrosive, and only contain earth-abundant elements. Our neat process is selective for the preparation of C8, which is achieved with satisfactory C8:C13 selectivity. This selectivity can be increased even further if the furfural concentration is decreased, whereas shifting the selectivity towards C13 requires the use of a solvent (or of a different catalyst). MX is by far the most active catalyst of the ones tested, followed by MMO and HT. Although this reactivity trend has been observed in many other reports, HT activity has not. HTs are usually reported to be inactive, or they are not even considered for catalytic testing. The formation of hotspots in the HT structure caused by MW irradiation may account for the atypical activity of this catalyst. The reaction also works well at temperatures lower than 100 ◦C but, of course, a much longer reaction time is required.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4352/10/9/833/s1: Figure S1: Parameter profiles in a typical microwave experiment; Figure S2: Mass spectra of the various chemical species detected in the final reaction mixtures by GC-MS.

**Author Contributions:** Investigation: A.T. and M.L.; writing—original draft preparation: A.T.; writing—review and editing: F.M. and M.C.; supervision: F.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** A.T. thanks AGAUR (Generalitat de Catalunya) and ESF (European Union) for his postgraduate scholarship (2018 FI\_B 01124). F.M. and M.C. thank the Ministerio de Economía y Competitividad for financial support (RTI2018-098310-B-I00).

**Acknowledgments:** S. Dominguez and the technicians of the Servei de Recursos Científics i Tècnics of URV are gratefully acknowledged for assistance in the characterizations.

**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.
