**4. Discussions**

A catalyst characterization study using various analytical techniques including XRD, BET, TGA, H2-TPR, NH3-TPD, and ICP-OES along with topography analysis of the samples using the FESEM method revealed that the zinc metal is a promising active site to be applied for hydrodeoxygenation of oxygenated compounds such as phenol. Zinc is an abundant element in the earth's crust, which makes it a promising candidate as a cost-effective active site. Thermogravimetry analysis revealed that the silica-supported zinc catalysts are highly stable at high temperatures up to 800 ◦C, which is a crucial factor for a catalyst to be applied for HDO reactions. High surface area and large porosities of the samples, analyzed by N2-adsorption analysis, make the surface reactions possible during the HDO process. Significant H2 consumption of the samples in the range of 400–550 ◦C explored by H2-TPR proved that the catalysts have decent reducibility due to the zinc metal. Furthermore, as it can be perceived from Figure 3, all the samples have one broad peak, which is representative of the one-step reduction of the active metal. Based on the literature [50], the surface acidity of a catalyst plays an important role in the reactivity and selectivity of the HDO reactions. In other words, higher acidity results in a higher conversion efficiency and selectivity of the products. NH3-TPD analysis proved that the total acidities of the samples were in the range of 0.108–0.481 mmol/g. Elevating the active site doping resulted in a higher value for the total acidity, and the sample with 3% of Zn represented the highest surface acidity of 0.408 mmol/g. The topography analysis micrographs (Figure 4) of the selected samples showed no sign of agglomeration of the zinc metals on the silica surface. Agglomeration is an imperative reason for a reduction in conversion efficiency [51].

Along with the characterization analysis, reactivity analysis revealed that the highest conversion efficiency could be achieved using the sample with 3% metal loading (80%), which was predictable. The selectivity of the HDO reaction products was analyzed using the GC-FID method and revealed that the products of the reactions were cyclohexane, cyclohexene, and benzene. According to the literature [52–54], hydrodeoxygenation of phenol progresses through two dissimilar reaction mechanisms. The first one is breaking the C-O bonds of phenol through direct hydrogenolysis. The final main product of this mechanism is cyclohexane. The second mechanism proceeds through hydrogenation of the aromatic ring, which results in different products in comparison to the first mechanism such as cyclohexanone, cyclohexanol, cyclohexene, and cyclohexane. Accordingly, regarding the selectivity of the products, the second mechanism is the main route in this survey.
