*2.2. Catalyst Characterization*

To ge<sup>t</sup> a deeper insight of activity difference of catalysts derived from different sources, multiple characterizations of the catalysts were performed. According to the results of X-ray diffraction (XRD) in Figure 3, the only CaO diffraction peaks were detected after all types of waste seashell calcined at 950 ◦C for 4 h. This indicates that the CaCO3 ingredient in waste seashell completely converted to CaO at pretreatment conditions (calcined at 950 ◦C for 4 h).

The Brunauer-Emmet-Teller (BET) specific surface areas of the investigated catalysts were characterized by physical adsorption. As the results shown in Table 1, *SBET* values of four types of catalysts showed no significant differences, which were around 10 m<sup>2</sup> g<sup>−</sup>1.

However, the CaO content of these catalysts showed obvious variations (as shown in Table 1). There is an evident correspondence between the activity and the CaO content of catalysts. Among the investigated waste seashell catalysts, S-shell-950 exhibited the highest purity, which is comparable with commercial CaO (97.78% vs. 98.00%). The CaO content sequence for the investigated catalysts was commercial CaO > S-shell-950 > C-shell-950 > O-shell-950, which was consistent with the activity sequence of catalysts mentioned in Figure 1. This result was also confirmed by SEM-EDS (as shown in Figures S2 and S3 in the supporting information). Compared with S-shell and C-shell, O-shell has more impurity elements (such as Na, Mg, Si, S, Cl), even if calcined at 950 ◦C.

**Table 1.** Specific Brunauer-Emmet-Teller surface areas (*SBET*), CaO content, and contact angle of the calcined shell catalysts.


1: measured by physical adsorption, 2: measured by ICP, 3: contact angle-measured by optical contact angle measurement.

Moreover, the contact angle (CA) of water on the catalyst surface was also measured to evaluate the hydrophilicity of the catalysts (as shown in Table 1 and Figure 4). The CA sequence of catalysts was commercial CaO > S-shell-950 > C-shell-950 > O-shell-950. This sequence was also consistent with the activity sequence of catalysts mentioned in Figure 1. It can be assumed from this result that in this reaction, the activities of CaO catalysts are also related to its hydrophilicity. The more hydrophilic a catalyst is, the lower the activity it obtains. This assumption can be supported by following the literature [20]: CaO catalysts are unavoidably poisoned by atmospheric H2O or produced H2O during the reaction. Therefore, if a CaO catalyst is hydrophilic, H2O can be absorbed by catalyst more easily, which leads to bad catalytic activity. In conclusion, the excellent performance of S-shell-950 catalysts can be explained by two reasons: the high content of CaO and the relatively low hydrophilicity.

Another question to be revealed is the relationship between the calcination temperature and the catalytic activity of S-shell catalysts. As the XRD results in Figure 5a show, when the calcination temperature was below 750 ◦C, CaCO3 residue was found in S-shell catalysts. When the calcination temperature was raised above 750 ◦C, no CaCO3 di ffraction peak was observed, which indicated CaCO3 was completely transformed to CaO. Figure 5b shows the visible e ffect of the S-shell with di fferent calcination temperatures. The outside surface of the S-shell transformed from black color to grey color as the calcination temperature increased, which was owing to the decomposition of organism residues. When the calcination temperature was above 750 ◦C, the calcined shell became fragile and easy to mill into powder. The abovementioned results indicate that CaCO3 in S-shell was completely transformed into CaO after calcination at 750 ◦C or above.

**Figure 3.** X-ray diffraction (XRD) pattern of commercial CaO and waste seashell after calcined at 950 ◦C for 4 h.

**Figure 4.** Contact angle of water on catalysts. (**a**) commercial CaO; (**b**) S-shell-950; (**c**) C-shell-950; (**d**) O-shell-950.

**Figure 5.** (**a**) XRD pattern of S-shell catalysts. (**b**) The original appearance of S-shell and visible alterations of appearance after calcination at 550–950 ◦C for 4 h.

In addition, CO2 temperature-programmed desorption (CO2-TPD) and NH3-TPD of S-shell catalysts were also carried out. From the results shown in Figure 6 and Table 2, we can see the calcination temperature has a strong influence on the base sites amount and base strength of the catalysts. When the calcination temperature increased to 750 ◦C, the amount of base sites and base strength both increased significantly. Likewise, the amount of acid sites was also increased with the increment of the calcination temperature, but the amount was relatively low, which was only ~10 μmol g<sup>−</sup>1. Among the investigated S-shell catalysts, S-shell-750, S-shell-850, and S-shell-950 catalysts have relatively stronger basicity and acidity. Therefore, we can attribute the excellent performance of S-shell-750/850/950 to the higher CaO content, relatively low hydrophilicity, and stronger acidity and basicity of these catalysts.

**Figure 6.** (**a**) CO2 temperature-programmed desorption (CO2-TPD) profiles of S-shell calcined at different temperatures. (**b**) NH3-TPD profiles of S-shell calcined at different temperatures.


**Table 2.** The amounts of acid sites or base sites of S-shell calcined at different temperatures.

### **3. Materials and Methods**

### *3.1. Materials and Catalyst Preparation*

Waste seashells were collected from home food residue with subsequent washing and drying. Before activity testing, the seashells were calcined in a muffle furnace at the proper temperature for 4 h and milled into powders. Commercial CaO and cyclopentanone were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
