**5. Conclusions**

The replacement of Cr3+ in MIL-101 by more benign metal cations, Fe3<sup>+</sup> and Sc3+, leads to an instability of the structure and the formation of the polymorphic MOF MIL-88B structure. Although MIL-88B(Fe,Sc) materials are unstable under aqueous hydrothermal conditions, they are stable in DMSO at 140 ◦C and e fficiently transform glucose to 5-HMF with high conversion and selectivity. While DMSO is less desirable as a solvent than water, the simplicity of the synthesis of the MIL-88B catalyst materials in a single-step is a significant advantage over the more conventional solid–acid catalysts that have so far been proposed for this reaction.

**Supplementary Materials:** Additional characterisation data. The following are available online at http://www. mdpi.com/2073-4344/9/5/43/s1. Figure S1: XPS of MIL-88B(Fe), Table S1: Assignment of key bands in the IR spectra, Figure S2: Thermogravimetric analysis of MIL-88B catalysts, Figure S3: Powder on samples immersed in DMSO, Figure S4: Powder XRD of MIL-88(Fe,Sc) as-made, after catalysis, and after ammonia TPD.

**Author Contributions:** Conceptualization, V.D., Y.K.K., and R.I.W.; Formal Analysis, R.P., R.O., D.L.B, T.W.C, N.C., M.W., R.J.K.; Investigation, R.P., R.O., D.L.B, T.W.C, N.C., M.W., R.J.K.; Writing—Original Draft Preparation, R.P, and R.I.W.; Writing—Review and Editing, R.P., R.O., D.L.B., Y.K.K, V.D.; Supervision, V.D., Y.K.K., and R.I.W.; Project Administration, V.D., Y.K.K., and R.I.W.; Funding Acquisition, V.D., Y.K.K., and R.I.W.

**Acknowledgments:** This work was supported by the Royal Society [Challenge Grant CH160099], the UK Engineering and Physical Sciences Research Council (EPSRC) [grant EP/P511432/1 Global Challenge Research Fund (GCRF) Institutional Award for the University of Warwick], and a Research Grants for Higher Education 2016 from DGHE, Indonesia [No.1136/UN2.R12/HKP/05.00/2016]. We thank Baiwen Zhao for his assistance with measuring surface area. The data supporting this article are available at https://wrap.warwick.ac.uk/116888.

**Conflicts of Interest:** The authors declare no conflict of interest. The sponsors had no role in the design, execution, interpretation, or writing of the study.
