**1. Introduction**

Due to global warming, many countries have moved from fossil fuel-based energy toward renewable energy. There are several forms of renewable energy that have been effectively executed out there. Many countries have successfully implemented renewable energy generated from solar [1,2]. Where some others have successfully utilized hydro, wave, geothermal, and wind as a secondary energy source [1,3], Malaysia and Indonesia have successfully utilized biofuel as one of the alternative energy sources [4–6]. Biofuel has been proven to reduce greenhouse gas emissions significantly

in these countries [7]. Therefore, among all alternative sources of energy to replace the restricted natural fossil fuel reservoirs, biofuel represents the sole available sustainable energy source that could properly replace petroleum [8,9]. Nevertheless, due to its component's complexity, which results in its instability, corrosiveness, and low heating value, the pyrolyzed biomass, known as bio-oil, must be upgraded before utilization [10–12]. Hydrodeoxygenation is a catalytic reaction that applies hydrogen to eliminate oxygen from the oxygenated compounds. These reactions are an efficient alternative method to attain bio-oils from biomass-derived oxygenated compounds [13]. Nevertheless, several issues must first be addressed before the hydrodeoxygenation (HDO) process can be entirely commercialized. The development of efficient, robust, cost-effective, and selective catalysts enable the advancement of this method to partly prevail these disadvantages [14].

Various kinds of active sites have been tested in the last decade for the HDO of the oxygenated compounds, which have had promising results including transition metal oxides such as MoO3 [15,16], Mo2C [17], Pt/(Al2O3, SiO2, H-Beta zeolie, activated carbon) [18–20], Fe/(SiO2, activated carbon) [21,22], Ni/(Al2O3, SiO2, HZSM-5 zeolite) [23], and Ga/(HBETA, SiO2, ZSM-5) [24]; precious metals such as Ru/TiO2 [25], and W/carbon [26]; phosphides such as Ni2P/SiO2, Fe2P/SiO2, MoP/SiO2, Co2P/SiO2, and WP/SiO2 [27]; and bifunctional such as NiMo/Al2O3 [28] and Pd-FeOX/SiO2 [29].

Zinc has been reported as an active site for the hydroprocessing of bio-oil oxygenated compounds, resulting in aromatic compounds and/or hydrocarbons [30–38]. The interdependent effect of nickel and zinc metals on Al2O3 support has been investigated by Cheng et al. [39]. They found that the pine sawdust bio-oil could efficiently be converted by 15%Ni.5%Zn/Al2O3 at 44.64 wt%. According to their report, the highest hydrocarbon content of 50.12% could be achieved by applying this bifunctional catalyst. Bifunctional catalysts possess two types of active sites and catalyze two dissimilar types of reactions. In another study by Parsell et al. [40], Zn/Pd was used for the HDO of lignin. Based on the author's claims, 80%–90% of conversion yield could be achieved using Pd/Zn as the HDO catalyst. Besides, they reported that the bifunctional Pd/Zn/C catalyst was more effective in the HDO of lignin molecules (β-O-4) and aromatic compounds, in comparison to the Pd/C sample. Additionally, they expressed that the bifunctional Pd/Zn/C sample is recyclable and robust, and no further addition of the zinc metal is required after each reaction cycle. The efficiency upsurge of the Kapuk seed oil hydrocracking process using the zinc metal has been reported recently by Mirzayanti et al. [41]. They stated that using Zn-Mo/HZSM-5 with a loading of 2.99 wt% for Zn and 7.55 wt% for Mo, the highest performance of the catalyst for the hydrocracking process could be reached.

Due to the complexity of the bio-oils causing complicated reactions during the HDO process, model compounds such as phenol, lignin, and others have been applied. Using the model compounds, one can attain enough data regarding the mechanism and reaction networks of the process. Phenol and its derivatives are known as the most refractory compound in the bio-oil and, hence, it has been selected as the model compound in this investigation [42].

The current research aims to study the physicochemical characteristics of the Zn/SiO2 catalyst as a potential and promising catalyst for the HDO of bio-oil oxygenated compounds using various analytical equipment. Furthermore, using the phenol as a model compound for the oxygenated bio-oil compound, the reactivity, performance, stability, and reusability of the selected catalyst have been examined in a continuous fixed-bed reactor.
