## 3 Results and Discussion
### Thermal Transformations of Model Lignin Compounds on the Surface of CeO\({}_{2}\)
#### 3.2.3 Vanillic Acid  
The TPD MS study results of the VA/CeO\({}_{2}\) sample are presented in Figure 9. The decomposition interval of VA on the CeO\({}_{2}\) surface was 100-750 \({}^{\circ}\)C (Figure 9). According to thermograms and the P/T-curve (Figure 9C,D), the pyrolysis of VA on the CeO\({}_{2}\) surface occurred in several main stages: 130, 270, 370, 550, and 650 \({}^{\circ}\)C. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **Stage** & **T\({}_{\text{max}}\)(\({}^{\circ}\)C)** & **Volatiles (\%)** & **Char (\%)** \\ \hline \multirow{3}{*}{G/CeO\({}_{2}\)} & I & 100 & 20.3 & \multirow{3}{*}{30.4} \\  & II & 180 & 49.3 & \\ \cline{1-1}  & \(\Sigma\)(I + II) & & 69.6 & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Pyrolysis yields for TGA pyrolysis of G/CeO\({}_{2}\) (0.6 mmol/g). Figure 8: Differential thermal analysis (DTA), differential thermogravimetric analysis (DTG), and thermogravimetric (TG) curves for G/CeO\({}_{2}\) (0.6 mmol/g). The low-intense peak of decarboxylation (\(m/z\) 44, T\({}_{\rm max}\)\(\sim\)130 \({}^{\circ}\)C) probably corresponds to the decomposition of associates of VA, which formed on the oxide surface, as was recorded during the pyrolysis of caffeic acid on CeO\({}_{2}\)[79] (Figure 9C). The formation of such associates in the VA/CeO\({}_{2}\) sample was confirmed by the presence in their IR spectra of bands at 1686 cm\({}^{-1}\) (v(C = O)) and 920 cm\({}^{-1}\) (\(\delta\)(COH)) (Figure 3) as well as in the region 2400-2700 cm\({}^{-1}\), which belongs to pure acid. Intense release of guaiacol (M.r. = 124 Da, \(m/z\) 124, T\({}_{\rm max}\)\(\sim\) 270 \({}^{\circ}\)C) can be a consequence of the decarboxylation of VA molecules bound to the oxide surface through the carboxyl group (Scheme 1), similar to the decomposition of the caffeic acid carboxylate complexes on the CeO\({}_{2}\) surface [79] and on the SiO\({}_{2}\) surface [80]. The involvement of the carboxyl group of VA in the interaction with the oxide was confirmed by the appearance of v(COO\({}^{-}\)) bands (1410, 1539 cm\({}^{-1}\)) in the IR spectra (Figure 3). The second low-intensity peak was observed for guaiacol (\(m/z\) 124) (T\({}_{\rm max}\)\(\sim\)330 \({}^{\circ}\)C, Figure 9D), which can be formed by pyrolysis of VA molecules bound to the oxide through OH\({}_{\rm ar}\). The possibility of such a bond is indicated by the transformation of the 1284 cm\({}^{-1}\) band, which corresponds to the vibrations of the CO\({}_{\rm np}\) group (Figure 3). Along with guaiacol at about 330 \({}^{\circ}\)C, the release of a product with \(m/z\) 108 was recorded, which can be identified as anisole. It can also be a conversion product of a VA complex linked through OH\({}_{\rm ar}\). However, since the C-OH bond was strong [14], and the probability of breaking this bond is small, a product with \(m/z\) 108 may correspond to cresol. It is known that phenol can be transalkylated to various cresol isomers [81]. This reaction requires Lewis acid sites, which promote the formation of a methyl cation as an  
Figure 9: Thermal decomposition of VA/CeO\({}_{2}\) (0.6 mmol/g); mass spectra at 270 \({}^{\circ}\)C (**A**) and 410 \({}^{\circ}\)C (**B**); TPD-curves for ions with \(m/z\) 18, 24, 44, and vapor pressure measured as a function of temperature for the VA/CeO\({}_{2}\) sample (**C**); TPD-curves for ions with \(m/z\) 31, 94,109, 124, 151, and 182 and for ions with \(m/z\) 94, 108, and 124 (**D**). intermediate in the alkylation reaction [82]. In addition, it is known [81] that CeO\({}_{2}\) is a good catalyst for ortho-methylation of phenol with methanol. Therefore, it is likely that surface methoxy groups, which are formed by the decomposition of VA complexes associated with the methoxyl group, can react with oxygen vacancies, turning into surface methyl groups involved in the transalkylation reaction [35]. The presence of the methoxyl and methyl groups linked to the surface was confirmed by peaks on the TPD curves of the ions with \(m/z\) 32, 31, and 15.