3.1. Material Characterization
All prepared materials were characterized using various techniques. The methods were chosen to confirm the materials’ composition, and to find the properties that may influence their catalytic performance. The composition was determined by EA (
Table 2).
Raw montmorillonite K10 did not contain any carbon (as expected), which was the most considerable difference compared to all modified materials. In all acid-treated materials, a small amount of combustible carbon was present (0.14–0.27). No traces of sulfur and nitrogen were detected in any case, meaning acids (H
2SO
4, HNO
3) were successfully removed from the materials, A small amount (218 resp. 225 ppm) of chlorine was observed in the case of treating by acids containing chlorine. The overall small content of analyzed elements (C, Cl) and no content of S and N confirmed the successful washing out of acids used to treat montmorillonite K10. X-ray fluorescence (
Table S1) results and X-ray diffraction (XRD) results were presented in our previous work [
20,
31]. XRD analysis revealed preserved montmorillonite structure after modification (
Figure S1).
The treatment by acids was distinctively positive from the point of view of material acidity. Modifying montmorillonite with different acids led to an insignificant increase of this parameter, which is especially important in the catalytic properties (
Table 3).
Raw montmorillonite K10 contained only one temperature maximum (ca 270 °C) assigned to weak acid sites. Acid-treated montmorillonites also had this band at ca 270 °C. However, they also possessed a significant band with a maximum at 540 °C, which corresponded to strong acid sites (
Figure S2). Origination of a new band correlates with the incorporation of new H
+ ions into the material structure. Our observation follows previous statements that the surface composition of clay base materials is pH-dependent (e.g., [
32]). The main groups responsible for montmorillonite acidity are ≡M-OH and ≡M-OH
2+ (where M represents Si or Al). The treatment by sulfuric and nitric acids (MMT-H
2SO
4, MMT-HNO
3) showed a significant decrease in amount of weak sites and the formation of strong sites. The treatment by chlorinated acids showed a low reduction of weak acid site amount and the formation of strong acid sites. Different behavior may be explained by the traces of chlorine in the materials, which might influence the overall acidity.
UV-Vis spectroscopy using adsorbed pyridine showed (
Figure S3) the dominant presence of hydroxyl groups that are responsible for the catalytic properties. The most significant was the band at ca 39,900 cm
−1 assignable to π-π* excitation of pyridinium ion [
30]. The bands of pure (without adsorbed pyridine) materials hid the bands potentially allocated to Lewis acid sites. However, we suppose that mainly Brønsted acid sites were present in all materials due to the preparation and drying methods. The differences between the materials were almost negligible.
Laser light scattering was used to monitor the particle size distribution of raw and acid-treated montmorillonite (
Table 4,
Figure 2). Curves of material volume density did not differ significantly. However, raw montmorillonite had a little higher Dv(50) value (17.4 µm) than acid-treated materials (11.3–13.7 µm), meaning that it contained slightly bigger particles. The stirring of the materials during the treatment was probably responsible for the change. Dv(10) values, i.e., the boundary value for 10% of the smallest particles, were very similar for all materials (3.07–3.69 µm). Raw and acid-treated materials, except for MMT-CH
3COOH, also contained a small amount of bigger particles with the size around 700 µm. Curves of raw [
33,
34] and acid-treated [
20] materials are similar to already published results.
Nitrogen physisorption was used to compare material textural properties (
Table 5,
Figure 3). All samples of catalysts showed adsorption isotherms of type IVa (IUPAC classification). Samples consisted of only mesopores with classical type hysteresis loop H3 (IUPAC classification), which corresponded to non-rigid aggregates of plate-like particles (e.g., clays).
Table 5 summarizes the results of the textural analyses of the catalysts. Total pore volume was 0.36 cm
3/g in all cases.
Figure 3 compares the pore size distribution from N
2 adsorption for samples using the BJH method. Based on the results of the mesopores volume distribution (BJH method), all samples contained narrow mesopores (about 7 nm). Similar to the case of XRD, textural characteristics of raw and acid-treated montmorillonite were similar.
Temperature programmed oxidation (TPO) was used to determine the amount of carbonaceous residues on catalysts after reuse (
Figure 4). Two types of recycling were performed—the first of them involved washing of catalyst with toluene and drying at 80 °C overnight (MMT-H
2SO
4-RE80); the second recycling process was similar—the only difference was calcination (300 °C) overnight (MMT-H
2SO
4-RE300). The assumption was that if any carbonaceous deposits were present on the material, carbon dioxide would originate during the TPO. The original amount of carbonaceous residues on material could be calculated from the amount of evolved carbon dioxide. In fresh MMT-H
2SO
4, a small amount of carbonaceous deposits was found on the material surface—2 mg
carbon/g
mat (which was also confirmed using EA—2.7 mg
carbon/g
mat). Carbonaceous deposits were present in a higher amount in recycled materials—4 mg
carbon/g
mat for MMT-H
2SO
4-RE300 and 8 mg
carbon/g
mat for MMT-H
2SO
4-RE80. The organic compounds remaining in the structure were obviously the compounds from the reaction mixture. The recycling treatment should remove all of them from the material to enable the recovery of the catalyst activity. Higher calcination temperature during catalyst recycling decreased carbonaceous residues compared to simple catalyst drying at 80 °C. All organic compounds should be removed from the material above 300 °C as carvone, carvacrol, and the used solvent have boiling points lower than this temperature. However, any used recycling process did not remove all carbonaceous compounds.