**4. Conclusions**

Active carbon-based acid catalysts were prepared by hydrothermal treatment of glucose and cellulose and their subsequent sulfonation. It was found that the starting material (glucose or cellulose) and the synthesis parameters strongly influenced the nature of the carbon catalyst. All the sulfonated

carbons showed a high activity in the synthesis of solketal at room temperature in a batch reactor, this activity was comparable to the one by Dowex 50Wx2 resin and higher than the ones obtained with Amberlyst 15, Deloxan or composites such as Nafion-silica-SAC-13. Di fferences in initial TOFs observed for the sulfonated carbons were di fficult to relate to any structural parameter (acid content, surface area, pore volume, acid density, acidity both total and sulfonic, and hydrothermal index). Nevertheless, the solids obtained at longer hydrothermal treatment times were generally less active, probably due to a poor access of the reactants to catalytic sites. The use of felts covered with sulfonated carbons derived from both glucose and cellulose allowed the design of a flow system for continuous solketal production. The use of ethanol in the feeding mixture was crucial to avoid catalyst deactivation and to maintain solketal productivity at least at short times on stream (one hour).

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/9/10/804/s1, Hydrothermal index definition, SEM images of HTC and SHTC, Plots of CO2 adsorption isotherms and pore distribution of HTC and SHTC, Relationship between H.I. and micropore volume, Productivity vs time plots for the synthesis of Solketal catalyzed by sulfonated hydrothermal carbons from cellulose prepared at 195 ◦C, Productivity vs time plots for the synthesis of Solketal catalyzed by sulfonated hydrothermal carbons from cellulose prepared at 215 ◦C, Productivity vs time plots for the synthesis of Solketal catalyzed by commercial resins and Glu-195-20 h-S, Initial TOF in solketal synthesis vs sulfonic acidity of reused catalyst plot with second-used SHTC as catalyst.

**Author Contributions:** E.G.-B. was responsible for hydrothermal carbon synthesis, J.M.F. carried out catalyst characterizations and E.P. and P.F. performed catalytic tests. J.M.F., E.G.-B. and E.P. are responsible for conceptualization and discussion of the results and contributed equally to the writing and design of the manuscript. J.M.F. and E.G.-B. secured funding for the research work.

**Funding:** Financial support from Ministerio de Ciencia, Innovación y Universidades (project RTI2018-093431-B-I00 and ENE2016-79282-C5-1-R) and the Gobierno de Aragón (Group E37\_17R) co-funded by FEDER 2014-2020 "Construyendo Europa desde Aragón" is acknowledged.

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