*3.3. Catalyst Stability and Reusability*

The catalyst with the highest reactivity, 3% Zn/SiO2, was selected for the stability studies of phenol HDO. To identify the activity alteration of the catalyst with the time on stream, the conversion of the phenol was assessed using the extracted experimental data at various TOS (Figure 7). As can be illustrated from the Figure, the catalyst was highly active up to 240 min with a minor decrease in inactivity. Following that, the activity of the catalyst steadily diminished with elevating TOS. After around 340 min of TOS, the alteration in the catalyst activity became insignificant and was around 43%. Additionally, the variation in the product selectivity was likewise insignificant after 420 min of the TOS. In terms of the selectivity of the products, after a TOS of 420 min, the selectivity of the cyclohexane slightly decreased with a minor increase in the selectivity on cyclohexene. Henceforth, from the stability results, it could be concluded that the silica-supported zinc with 3% loading is highly active until 340 min and, after that, must be replaced or regenerated.

**Figure 7.** Alteration of the products selectivity and conversion efficiency of phenol with time on stream (TOS). Catalyst 3% Zn/SiO2; temperature 500 ◦C; pressure 1 atm; weight hourly space velocity (WHSV) (h<sup>−</sup>1) 0.32; feed flow rate (mL/min) 0.5; and hydrogen flow rate (mL/min) 150.

The applied catalyst for the stability test has been used for the reusability study. The spent catalyst was regenerated by calcination at 500 ◦C for one hour under the air atmosphere after each cycle. The freshly calcinated sample was then examined by the HDO of phenol. This procedure was continued until four cycles of regeneration. Figure 8 represents the results of the regeneration study. As can be perceived from the Figure, the catalyst recovered fully and had almost the same results as the fresh catalyst in its first and second regeneration steps. In the third and fourth cycles of regeneration, the conversion efficiency of the catalyst was eliminated. The main reason for eliminating the phenol conversion efficiency after the third regeneration cycle might be because of coking and occupying the pores of the support.

**Number of cycles**

**Figure 8.** Catalyst reusability survey displaying the conversion and yield of products for hydrodeoxygenation of phenol over 3% Zn/SiO2. Temperature 500 ◦C; pressure 1 atm; WHSV (h<sup>−</sup>1) 0.32; feed flow rate (mL/min) 0.5; and H2 volumetric flow rate (mL/min) 150.
