**5. Conclusions**

In summary, the catalyst characterization study revealed that the physicochemical properties of zinc-supported catalysts depended on their active site content. The incorporation of zinc as an active site over SiO2 mostly involved the filling of pores with zinc, which occupied up to 6.58% of the surface area. The accumulation of zinc on silica also caused an upsurge in acid site concentration due to the regeneration of new proton sites on the catalyst. The effect of metal loading amount on the catalyst performance was also studied in a continuous fixed-bed reactor. It was found that 3%Zn/SiO2 represents the highest phenol conversion efficiency among other samples. However, the results indicate that the selectivity of the products including cyclohexane, cyclohexene, and phenol remained almost steady by varying the doping amount of the active site. The time-on-stream investigation showed that the silica-supported zinc was highly active up to 240 min of phenol HDO, with a conversion efficiency up to 80%, and after 420 min of TOS, the activity decreased to around a conversion rate of 43%. Deposition of the impurities and coke on the surface of the catalysts is responsible for the deactivation of the catalysts. Reusability tests revealed that the catalyst displayed outstanding reusability and could be regenerated fully after several reusing rounds. Further investigation on catalyst design, reaction mechanism, and regeneration should facilitate the progress of bio-oil oxygenated compounds using silica-supported zinc catalysts.

**Author Contributions:** Conceptualization, H.P., W.M.A.W.D. and F.A.; Methodology, H.P. and F.A.; Software, H.P. and M.K.A.; Validation, H.P., F.A. and W.M.A.W.D.; Formal analysis, H.P. and F.A.; investigation, H.P.; resources, F.A., W.M.A.W.D., M.K.A. and T.M.I.M.; Data curation, H.P. and F.A.; Writing—original draft preparation, H.P.; Writing—review and editing, F.A. and T.M.I.M.; Visualization, H.P.; Supervision, W.M.A.W.D., M.K.A. and F.A.; Project administration, W.M.A.W.D. and F.A.; Funding acquisition, W.M.A.W.D., F.A. and T.M.I.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors gratefully acknowledge the financial support from the University of Malaya, Malaysia, and the Deanship of Scientific Research (DSR) at King Abdulaziz University, Jeddah.

**Acknowledgments:** The authors gratefully acknowledge the financial support from the University of Malaya, Malaysia, and the Deanship of Scientific Research (DSR) at King Abdulaziz University, Jeddah.

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