**4. Conclusions**

CaO catalysts derived from three types of waste seashell were first reported as active catalysts in cyclopentanone self-aldol condensation. Among the investigated catalysts, S-shell-750 catalyst exhibited the highest catalytic activity and selectivity (dimer yield: 92.1%; selectivity: up to 100%), which was comparable with commercial CaO (dimer yield: 88.2%). According to the results of multiple characterizations, the excellent catalytic performance of S-shell catalysts can be rationalized by its relatively higher CaO content, low hydrophilicity, and stronger basicity/acidity. In this reaction, commercial CaO can be substituted directly by S-shell-750 catalyst. This work developed a new route for using cost-effective bio-based catalysts for biomass platform chemical conversion.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/9/8/661/s1, Figure S1: Catalyst reusability of commercial CaO and S-shell-950 catalysts. Reaction conditions: 4.0 g cyclopentanone; 1.0 g catalyst; 180 ◦C; 2h. Catalysts were regenerated by calcination at 950 ◦C for 4 h. Figure S2: SEM-EDS data of catalysts before calcination. (a) commercial CaO; (b) S-shell; (c) C-shell; (d) O-shell. Figure S3: SEM-EDS data of catalysts after calcination at 950 ◦C. (a) commercial CaO; (b) S-shell-950; (c) C-shell-950; (d) O-shell-950.

**Author Contributions:** X.S. conceived and designed the experiments; Q.X. and X.W. performed the experiments; N.L., H.J., M.N. and H.S. analyzed the data; J.Z. and Q.P. contributed reagents and analysis tools; X.S. wrote the paper.

**Funding:** This research was funded by National Natural Science Foundation of China (No. 21808023) and Natural Science Foundation of Liaoning (No. 20180550559).

**Acknowledgments:** We acknowledge the Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province.

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