*2.3. Characterization of the Catalysts*

The prepared calcinated catalysts and the bare support were analyzed by XRD (PANalytical X'Pert with Cu-Kα radiation). Scherrer's formula was applied to estimate the crystallite size of silica-supported zinc catalysts. In Equation (1), λ represents the X-ray wavelength, *dxrd* denotes the mean size of the crystallite, β*hkl* stands for the full-width at half-maximum (FWHM), *k* is the shape factor, and θ denotes the Bragg angle. Data were collected at ambient conditions at a scanning range from 5◦ to 80◦ with a step size of 0.0260◦. The slit and the time step were set to 0.1 mm and 0.1 s, respectively.

$$d\_{\rm xrd} = \frac{k\lambda}{\beta\_{\rm hkl}\cos\theta} \tag{1}$$

N2-adsorption analysis was performed using a Micromeritics TrisStar II 3020 V1.04 to detect the BET surface area, pore size, and pore volume of the calcinated catalysts in this study. The measurement temperature was set at −196 ◦C and the degassing of the catalysts were performed for one hour at 90 ◦C and, subsequently, 4 h at 200 ◦C, prior to the test and under vacuum atmosphere.

Thermogravimetric (TGA) analysis was employed for the bare SiO2 and freshly calcinated catalysts to examine the extent of water and organic compounds on the samples. A TGA Q500 V20.10 (TA instruments) instrument was applied in this survey. In a typical run, 80 mg of a sample was placed in the sample holder and was heated with a heating rate of 20 ◦C/min up to 800 ◦C.

H2 temperature-programmed reduction (H2-TPR) was conducted by a Micromeritics Chemisorb 2720 apparatus. Prior to hydrogen adsorption, using the pure nitrogen at 500 ◦C, the samples were outgassed for 30 min. Subsequently, the catalysts were reduced, using 5% hydrogen diluted in argon gas, for one hour and 500 ◦C. In the following, the catalysts were cooled and, to remove the physically adsorbed molecules, flushed with N2 gas at a flow rate of 20 mL/min. Finally, the catalysts were heated up to 900 ◦C in the nitrogen atmosphere. A thermal conductivity detector (TCD) monitored and recorded the H2 consumption during the reduction process.

A QUANTA 450 FEG instrument was applied for the FESEM analysis. The morphology of two calcinated catalysts (1% Zn/SiO2 and 3% Zn/SiO2) and the bare silica was examined.

Inductively coupled plasma optical emission spectrometry (ICP–OES) was applied to determine the metal content of the prepared catalysts. In this analysis, 100 mg of each catalyst was digested with HNO3 (10 mL). Zinc metal solutions for the calibration standard preparation were made using a certain quantity of Zn metal to 7 mL of nitric acid and 14 mL of distilled water, for each standard sample.

The total acidity of the prepared catalysts was tested using the NH3-TPD method. Prior to outgassing the catalyst (50 mg) in the quartz U-tube, it was heated from room temperature up to 300 ◦C. Then, under a flow rate of 20 mL/min of helium, the sample was degassed for 30 min at 300 ◦C. Subsequently, the catalyst was cooled down to room temperature and was then ready for the ammonia chemisorption step. A 5% NH3/He gas mixture was selected for this step and the ammonia chemisorbed on the sample for 30 min. To remove the physisorbed molecules, the sample was purged using the helium gas at 100 ◦C (30 min). Finally, applying the heating rate of 40 ◦C/min, the sample was heated up to 900 ◦C in the helium atmosphere (20 mL/min). For this analysis, a Micromeritics apparatus was used (Model: Chemisorb 2720).
