3.1.1. XRD

Figure 1 represents the XRD diffractograms of the silica-supported zinc catalysts at various loadings and the bare SiO2. The XRD patterns of the samples confirmed the existence of Zn metal in all zinc-doped catalysts (b, c, d, and e). According to the literature, the observed peaks at certain 2θ values (32◦, 34◦, 36◦, 47◦, 56◦, 63◦, and 68◦) represent hexagonal zinc [43]. Table 1 shows the calculated average size of zinc (calculated from 2θ = 32◦, 34◦, and 36◦) crystallite, which was elevated with the increment in Zn metal doping. This phenomenon occurs due to the agglomeration of the zinc particles outside the pores [44]. The diffraction peaks of some planes including (112), (200), and (110) have not been found, due to the good dispersion of the small crystallites formed during the impregnation or forming in negligible quantity. Comparable degrees of crystallinity and slight peak broadening of all XRD diffractograms indicate that the addition of 0.5–3 wt% Zn metal insignificantly transformed the crystallinity of the samples. The peaks of Zn metal crystallites (Figure 1) were not distinguished, probably due to the decent diffusion of Zn particles in the surface of the support (SiO2) or the low quantity of the zinc.

**Table 1.** Physicochemical characteristics results of the bare SiO2 and Zn-doped catalysts.


**Figure 1.** XRD patterns of bare SiO2 and doped catalysts applied in this study [bare SiO2 (a), 0.5% Zn/SiO2 (b), 1% Zn/SiO2 (c), 2% Zn/SiO2 (d), and 3% Zn/SiO2 (e)].
