3.1.7. NH3-TPD

Figure 5 represents the NH3-TPD profile of the catalysts. It can be observed that all samples absorb ammonia in a broad range of temperatures (100 ◦C–800 ◦C). From Figure 5 and Table 1, it can be observed that after the 0.5%–3% zinc metal was doped on SiO2, the total acidity of the impregnated samples evidently raised and an elevation in the area of the ammonia desorption peak could be perceived. Furthermore, considering the desorption peak temperature of the samples, the position of the NH3 desorption peak temperatures increased by elevating the zinc doping to the samples. It could be concluded that doping the zinc metal onto the silica surface results in the interaction and exchange of proton sites on the support surface. This might be due to the interaction and exchange of proton spots on the silica surface with the loading of zinc metal, initiating the renewal of new proton spots on the samples. As Table 1 indicates, the total acidity of the catalysts was expressively raised from 0.108 to 0.481 mmol/g within the upsurge in zinc doping from 0% to 3%. According to Kernajanakon et al. [50], loading the optimum amount of active sites on the support is crucial and ranges from 1% to 2.5%. By increasing the loading amount such as 5% and 10%, the acidity of the catalysts decreases due to the concealing of acid spots by the produced ZnO cluster. Furthermore, the authors implied that the dispersion efficiency of the metal on the surface of the support might be influenced by the amount of the loaded metal.

**Figure 5.** NH3-TPD profile of the impregnated catalysts and the bare SiO2.

#### *3.2. Reactivity*

Products selectivity and phenol conversion over *x*Zn/SiO2 catalysts in the hydrodeoxygenation of phenol at various loadings of the zinc metal are shown in Figure 6. From Figure 6, it can be perceived that all impregnated samples are active and able to convert the phenol. Table 2 details the data attained by GC-FID analysis. Phenol conversion efficiency using various loads of Zn metal, 0.5–3%, varied from 15% to 80%, respectively. By elevating the zinc metal loading of the catalysts, the total conversion increased, and the highest conversion efficiency was achieved with 3% doping of the active site. In a separate study [42], we observed that a loading of 4% of the active metal (Zn) on the support surface (SiO2) resulted in a slight reduction in the HDO conversion efficiency. The main reason was found to be the occupation of the support surface pores by zinc metal. However, the zinc metal loading (0.5–3%) had a slight effect (up to 13%) on the selectivity of the products including cyclohexane, cyclohexene, and phenol. The selectivity of the cyclohexane represents the highest value (71.62%) using the 2% Zn/SiO2. However, around a 3.14% decrease has been observed in its selectivity, by elevating the loading of the active site from 2% to 3%. This minor reduction in selectivity might be referred to the occupying of the surface porosities by the zinc metal nanoparticles.

**Figure 6.** Conversion efficiency and selectivity of the products at various dopings of Zn metal.


**Table 2.** Results of selectivity and performance of the catalysts.

Note: The experimental conditions for all samples include temperature 500 ◦C; pressure 1 atm; WHSV (h−1) 0.32; feed flow rate (mL/min) 0.5; and hydrogen flow rate (mL/min) 150.
