**4. Conclusions**

Hydrodeoxygenation of guaiacol over Al-MCM-41 supported Pd–Co and Pd–Fe catalysts were studied at 400 ◦C and ambient atmosphere. The Fe catalyst gave a higher HDO yield and lower gas-phase yield compared with the Co catalyst in HDO of guaiacol. The bimetallic Pd–Co and Pd–Fe achieved a higher conversion and HDO yield than the monometallic Co and Fe. Interestingly, the addition of Pd significantly improved the stability of the catalysts, since it could suppress the coke deposition on the catalysts. Furthermore, the Pd–Fe catalyst presented a higher stability and regeneration ability than the Pd–Co catalyst. The coke deposits were mostly removed by the treatment at 500 ◦C in air, which was confirmed by TGA and XPS results. The regenerated Pd–Co catalyst showed a decrease in HDO yield and stability, while the Pd–Fe catalyst presented consistent activity during three HDO cycles. This can be explained by the lower thermal stability coke deposit and the formation of lower binding energy Si–O–Fe bonds of the used Pd–Fe catalyst.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/2227-9 717/9/3/430/s1, Figure S1: De-convoluted Si 2p, Al 2p and Pd 3d XPS spectra of fresh, used, and regenerated Pd–Co/Al-MCM-41catalysts; Figure S2: De-convoluted Si 2p, Al 2p and Pd 3d XPS spectra of fresh, used, and regenerated Pd–Fe/Al-MCM-41catalysts.

**Author Contributions:** Conceptualization, N.T. and Y.U.; methodology, N.T.; software, T.T.; formal analysis, N.T.; investigation, N.T.; writing—original draft preparation, N.T.; writing—review and editing, Y.U., A.R., and T.T.; supervision, Y.U. and A.R.; project administration, Y.U.; funding acquisition, Y.U. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Mitsubishi Corporation Educational Trust Fund.

**Data Availability Statement:** Data sharing is not applicable.

**Acknowledgments:** We wish to thank Universiti Teknologi PETRONAS for providing a congenial work environment and state-of-the-art research facilities. We also thank Masaharu Komiyama for his valuable support, comments, and suggestions. We acknowledge the support, both in terms of time and financially, from Ho Chi Minh City University of Technology (HCMUT).

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