Catalytic Pyrolysis of Lignin Model Compounds (Pyrocatechol, Guaiacol, Vanillic and Ferulic Acids) over Nanoceria Catalyst for Biomass Conversion

: Understanding the mechanisms of thermal transformations of model lignin compounds (MLC) over nanoscale catalysts is important for improving the technologic processes occurring in the pyrolytic conversion of lignocellulose biomass into biofuels and value-added chemicals. Herein, we investigate catalytic pyrolysis of MLC (pyrocatechol (P), guaiacol (G), ferulic (FA), and vanillic acids (VA)) over nanoceria using FT-IR spectroscopy, temperature-programmed desorption mass spectrometry (TPD MS), and thermogravimetric analysis (DTG/DTA/TG). FT-IR spectroscopic studies indicate that the active groups of aromatic rings of P, G, VA, and FA as well as carboxylate groups of VA and FA are involved in the interaction with nanoceria surface. We explore the general transformation mechanisms of different surface complexes and identify their decomposition products. We demonstrate that decomposition of carboxylate acid complexes occurs by decarboxylation. When FA is used as a precursor, this reaction generates 4-vinylguaiacol. Complexes of VA and FA formed through both active groups of the aromatic ring and decompose on the CeO 2 surface to generate hydroxybenzene. The formation of alkylated products accompanies catalytic pyrolysis of acids due to processes of transalkylation on the surface.


Introduction
Lignocellulose feedstock is a major potential renewable source of bio-oils and a large number of valuable chemicals. A total of 10-25% of lignocellulosic raw material is lignin [1]. The exact structure of lignin is still unknown; however, it is believed that lignin is formed through biosynthesis of p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol [2]. Lignin determines the strength of trunks and stems of plants [3]. Due to its rigid structure, this natural polymer is currently used mainly for heat and energy production [4][5][6][7]. However, the growing amount of research on the production of bio-oil and various chemicals from lignocellulosic raw materials motivates the search for effective ways to process lignin [4,5,7].
Due to its unique structure, lignin can be a source of a large number of aromatic compounds, both monomers and polymers [1,8,9]. Since the structure of this natural polymer is extremely complex, model compounds are often used to study its conversion to high value-added chemicals [8,10]. Ferulic and vanillic acids, as well as pyrocatechol and guaiacol are common products of lignin processing [1,8,[11][12][13], and therefore are often used as model lignin compounds [4,8,11,[14][15][16][17]. These compounds are lignin structural units of various sizes and contain almost the entire list of functional groups present in the lignin macromolecule. Among these products, guaiacol is an ideal model compound for assessing the performance of various catalysts, as it is a typical component of pyrolysis oil and one of the most complex methoxyphenols for deoxygenation [18,19]. Pyrocatechol is often an intermediate in converting other model lignin compounds, for example, guaiacol [8,20]. Ferulic acid may be used for the production of valuable aromatic chemicals, in particular 4-vinylguaiacol [20][21][22][23][24], vanillin, and vanillic acid [24][25][26].
In addition to lignin, these compounds are found in large quantities in other polymers of vegetable raw materials and in the waste generated during their processing (winedistilleries, olive oil processing, table olive industries, pulp paper processing, etc.) [26][27][28][29]. Due to the high toxicity of phenol-containing compounds to microorganisms [29][30][31], special attention is directed to their utilization [24,32].
Pyrolysis is commonly used for lignin processing due to its ability to effectively separate this polymer's strong structure [8,11,33,34]. However, high-temperature conversion of these raw materials generates a large number of final pyrolysis products. Various catalytic systems are used to overcome these imperfections [34]. CeO 2 is often used as a catalyst in the catalytic conversions of lignocellulosic raw materials [35][36][37][38][39][40]. Its catalytic properties are attributed to the ease with which it alternates between the 3+ and 4+ oxidation states, depending on the environmental conditions. [35]. The corresponding number of oxygen vacancies compensates for the decrease in the positive charge of Ce 3+ . The concentration of defects, both Ce 3+ ions and oxygen vacancies, on the oxide surface is higher than in the bulk [41]. Therefore, nanosized cerium oxide has higher concentrations of Ce 3+ ions and, accordingly, redox activity compared to large particles, since the surface-to-volume ratio increases [42]).
In the conversion and valorization of lignocellulosic raw materials and model compounds of lignin, cerium oxide is combined with other catalysts, particularly metals [38,39] and zirconium oxide [16,35,43]. In the work of Deng and co-authors [32], CeO 2 oxide with platinum deposited on its surface was used for the oxidative conversion of lignin and 2-phenoxy-1-phenyl-ethanol, which contains β-O-4 bonds and Cα-hydroxyl groups, in monomeric aromatic compounds (4-methoxy-phenol, acetophenone, methyl benzoate (82%, 38%, and 40%). Pt/CeO 2 catalysts have also been used to convert 4-propyl-phenol to propylcyclohexane. [37]. The yield of propyl-cyclohexane was 83%, and propyl-cyclohexanol was < 1%. In this case, the catalyst worked effectively both in the presence of water and in anhydrous conditions [37].
Catalytic systems composed of both CeO 2 and ZrO 2 have proven their effectiveness [16,35,38,43]. They have many advantages over the individual oxides [35]. In particular, the combination of CeO 2 and ZrO 2 promotes a decrease in the required surface temperature and reduction volume as well as an increase in the number of oxygen vacancies. Such catalytic systems dissociate hydrogen and create oxygen vacancies under mild conditions. The inclusion of zirconium ions in the cerium lattice leads to structural compression and promotes the formation of oxygen vacancies. In addition, the presence of zirconium partially suppresses crystallization during the synthesis of catalysts and leads to the formation of small and active crystallites [35].
Using a CeO 2 /ZrO 2 catalyst for the hydrodeoxygenation of guaiacol [35], valuable products such as phenol, catechol, and benzene were obtained. In addition, the use of this catalytic system increased the conversion efficiency of guaiacol and eliminated the formation of the undesirable oligomeric products containing hydrogenated rings. Moreover, these catalysts showed no signs of deactivation after 72 h of flow [35].
The efficiency of CeO 2 , CeO 2 -ZrO 2 , Ni/CeO 2 , and Ni/CeO 2 -ZrO 2 catalysts for hydrodeoxygenation of phenol at intermediate temperature and pressure (275 • C and 100 bar) in a batch reactor was tested in [43]. Oxide catalysts (CeO 2 , CeO 2 /ZrO 2 ) showed low activity on the hydrodeoxygenation of phenol under these conditions due to their inability to hydrogenate the phenolic ring [43]. Reduced metal catalysts for both noble and base metals were significantly more active. Hydrodeoxygenation of phenol proceeded through initial hydrogenation to cyclohexanone, which rapidly hydrogenates to cyclohexanol [43]. Ni/CeO 2 and Ni/CeO 2 -ZrO 2 were the most active catalysts for the initial hydrogenation of phenol to cyclohexanol but were insufficiently active in the next stage of deoxygenation [43].
In [38], the activity and surface properties based on oxides of CeO 2 and ZrO 2 coated with nickel (Ni/CeO 2 -ZrO 2 ) were used in the thermochemical conversion of cellulose. The catalyst Ni/CeO 2 -ZrO 2 was effective in producing hydrogen during the conversion of cellulose raw materials. The presence of cerium oxide in such catalytic systems contributed to a slower decrease in their activity versus Ni/ZrO 2 . It has been shown [38] that the tested catalysts allow the efficient formation of a gaseous fraction [38]. At the same time, in studies [16], Ni/CeO 2 -ZrO 2 catalytic systems were used for lignin conversion. The main phenolic compounds in the obtained lignin oil were: guaiacol, methylguaiacol, ethylguaiacol, vanillin, and homovanillic acid [16]. However, the exact mechanisms of the transformations that occurred remain unclear.
In this work, we examined thermal transformations of several aromatic model compounds of lignin (guaiacol, pyrocatechol, vanillic acid, and ferulic acid) over the nanoceria catalyst. The results reported in this study are important for establishing the catalytic transformation mechanisms of both lignin and its processing products and other phenolcontaining plant raw materials over the CeO 2 -based catalytic systems.
A series of samples, P/CeO 2 , G/CeO 2 , VA/CeO 2 , and FA/CeO 2 , with concentrations of phenolic compounds of 0.1, 0.3, 0.6, 0.9, and 1.2 mmol/g were prepared. The concentration range of 0.1-1.2 mmol/g was selected based on previous studies [49]. According to [49], the maximum adsorption values for cinnamic acid and its derivatives, including ferulic acid, are almost equal and amount to ≈ 2.9 × 10 -4 mol/g irrespective of the differences in the reaction sites of their molecules. The samples were prepared by impregnation of 100 mg of CeO 2 with 2 mL of P, G, VA, and FA solutions in ethanol. The suspensions were stirred for several minutes and then dried at room temperature in the air.
The TPD MS-experiment was performed on an MX-7304 monopole mass spectrometer (Electron, Sumy, Ukraine) with electron ionization, re-equipped for thermal desorption measurements [45,46,50]. At the beginning of the experiment, a sample weighing 10-20 mg was placed in a quartz-molybdenum ampoule and pumped out at room temperature to a pressure of~5 × 10 -5 Pa. The rate of programmed linear heating was 0.17 • C/s. Heating was increased from room temperature to 750 • C. Volatile products of thermolysis entered the ionization chamber of the mass spectrometer and were ionized and fragmented under the action of electrons. The range of the investigated masses was 1-210 a.m.u. The total number of mass spectra recorded during the experiment reached~240. The slow heating of the sample and the high pumping rate of volatile thermolysis products made it possible to neglect diffusion effects. Under such conditions, the intensity of the ion current was proportional to the rate of desorption.
Kinetic parameters of the chemical reactions and processes of the lignin model compound on the nanoceria surface (temperature of the maximum desorption rate T max , reaction order n, activation energy E = , pre-exponential factor ν 0 , and change of activation entropy ∆S = ) were calculated from the TPD-MS data by an in-house computer program using the linear form of the Arrhenius equation [45,46,50]. Thermogravimetric analysis, differential thermogravimetric analysis (DTG), and differential thermal analysis (DTA) were performed using a TGA/DTA analyzer (Q-1500D, Hungary). Samples weighing 100 mg were heated from room temperature to 1000 • C. The heating rate was 10 • C/min in an air atmosphere.

Results and Discussion
3.1. FT-IR Spectroscopic Studies 3.1.1. Pyrocatechol FTIR spectra of CeO 2 , pure Р, and samples P/CeO 2 are presented in Figure 1. Interpretation of the obtained results, performed on the basis of experimental data [51,52] and the results of quantum chemical calculations of the frequencies of normal vibrations of pyrocatechol in the crystalline and gaseous states [53], are presented in Table 1. The symbol ν is used to denote valence vibrations, β for out-of-plane deformation vibrations, and δ for non-planar deformation vibrations.  Table 1. Assignments of infrared bands of pure P and of P/CeO 2 (0.6 mmol/g).
According to [53], most of the bands of this dihydroxybenzene have a mixed shape. Figure 1 shows significant changes in the P/CeO 2 spectra compared with pure P (Figure 1, Table 1), which is the result of the interaction of P molecules with oxide. The absorption bands at 721 and 937 cm −1 , which correspond to C-H stretching vibrations [53] of pure pyrocatechol molecules in the crystalline state, are a sign of association due to intermolecular hydrogen bonds [53]. The disappearance of these absorptions for P/CeO 2 indicates the destruction of this association structure of P due to interaction with the oxide surface.
The obtained spectra of the studied samples contain several other features indicating the interaction of OH groups of pyrocatechol with CeO 2 . In particular, one of these signs is a significant decrease in the intensity of the bands 756 (δ(OH) + δ(CH)) and 769 cm −1 (ν(CC) + ν(CO) + β(CCC)) [53].
There are bands 849 and 859 cm -1 ( Figure 1) for pyrocatechol in the region of 800-900 cm −1 , which according to [48,53], are mixed and associated with non-planar deformation vibrations of CH, deformation vibrations of CC, as well as vibrations of COH groups. For P/CeO 2 samples, changes in this region of the spectrum are observed due to the interaction of OH groups of P with the oxide surface. Absorption at 1041 cm −1 for pyrocatechol is mainly due to the stretching vibrations of the CC, and 1095 cm −1 is associated with vibrations of CC, CH, and COH groups. For P/CeO 2 samples, these bands are shifted toward lower frequencies, and their relative intensity decreases.
In addition, for P/CeO 2 samples, changes were found in the range 1150-1400 cm −1 . This region's absorption mainly corresponds to stretching and bending vibrations of COH [51,53].
Analysis of the IR spectra of pyrocatechol in the crystalline state and the gas phase [48] shows that the ultrathin structure of the bands in this region is associated with the formation of intermolecular and intramolecular bonds. From Figure 1, it is seen that the maximum of 1167 cm −1 , which is due mainly to ν(CO) vibration [53], essentially disappears for all concentrations of P/CeO 2 . At the same time, the intensity of the bands at 1188 and 1365 cm −1 , which partially corresponds to the β(COH) vibrations [51,53], decreases significantly. Instead of a wide band with three peaks at 1242, 1255 (β(CОН) [51,53]), and 1281 cm −1 (ν(CО)) [51,53], for lower concentrations, one maximum was detected at 1261 cm −1 , and only when the concentration of P increases to 0.6 mmol/g, a peak appears at 1273 cm −1 .
At the same time, all P/CeO 2 spectra contained a new band at 1297 cm −1 , which may indicate the formation of a bond between pyrocatechol and CeO 2 . It is known that the new bands found in this area by the interaction of phenol and phenolic compounds with metal surfaces, as well as surfaces of oxides and hydroxides of metals [54][55][56][57][58], were signs of the formation of new bonds.
The binding of P to the oxide surface significantly affected the absorption of its aromatic ring, which manifested in the displacement of the absorption bands of P and changes in their intensity. In particular, for pure P, the vibration band ν (CC) was registered at 1471, 1514, 1603, and 1620 cm −1 [53], while in the P/CeO 2 spectrum, the corresponding absorptions were registered at 1446, 1483, and 1576 cm −1 . Bands characteristic of pure P (1471, 1514, 1603, and 1620 cm −1 ) also appeared in the spectra of P/CeO 2 samples with a P concentration above 0.6 mmol/g ( Figure 1). This is probably because the amount of P exceeds the number of active centers of the CeO 2 surface. The obtained results indicate the formation of chemisorbed pyrocatechol complexes on the CeO 2 surface.
The P/CeO 2 spectrum ( Figure 1) is similar to the IR spectra of pyrocatechol adsorbed on the nanosized TiO 2 surface [51,[59][60][61]. According to the results of quantum chemical calculations [61] and experimental data [51,56,60,61], this type of spectra of P adsorbed on TiO 2 , in which the bands 1250 cm −1 and 1475 cm −1 are the most intense, is more characteristic of complexes with a bidentate bridge structure. Since the most intense absorptions in the P/CeO 2 spectrum correspond to the bands 1263 and 1483 cm −1 (Figure 1), it is probable that the P complexes on the CeO 2 surface also have a bidentate bridge structure (two oxide atoms of phenolic groups of P interact with two metal atoms).

Guaiacol
The FT-IR spectra obtained for pure G and immobilized on the surface of CeO 2 are shown in Figure 2. The absence for G/CeO 2 of the absorption band at 1364 cm −1 , which corresponds to β(OH) [62] (Table 2), as well as the decrease in the intensity of absorption at 1261 cm −1 (ν(CO) [62]), indicates the participation of the OH group of G in the interaction with CeO 2 .  Table 2. Assignments of characteristic infrared bands of pure G and of G/CeO 2 (0.6 mmol/g).

Assignments
Frequency 1617 - [63] At the same time, for these samples, the bands of symmetric (ν s ) and asymmetric (ν as ) valence vibrations of the C-О-CН 3 groups, which were registered for guaiacol at 1024 and 1225 cm −1 [63], shifted to 1018 and 1217 cm −1 , respectively. In addition, absorption bands in the range 1444-1469 cm −1 , corresponding to the β(CH 3 ) vibrations [62,63], were transformed. In particular, the peaks at 1444 and 1469 cm −1 disappeared, and the maximum at 1458 cm −1 shifted to 1456 cm −1 . These changes indicate the interaction of the methoxyl groups of G with CeO 2 .
The new bands at 1288 and 1323 cm −1 in the spectra of G/CeO 2 samples were a sign of the formation of chemisorbed G complexes [62]. The absorptions for ν(CH 3 ) at 2843 cm −1 partially remained in the spectra of G/CeO 2 .Therefore, the interaction of the COCH 3 group with the oxide surface probably occurred through an oxygen atom without cleavage of CH 3 . At the same time, it is possible that several methoxyl groups may have been freed.
Thus, the obtained data ( Figure 2) indicate the formation of G complexes on the nanoceria CeO 2 bound to the oxide surface through the phenolic and methoxyl groups simultaneously, as well as separately through each of these groups.
In addition, the shift of the band of C = C vibrations may also indicate the interaction of the aromatic ring G with the oxide surface [62]. Figure 2 shows that the C = C band, which for pure G is about 1502 cm −1 , for the G/CeO 2 samples is shifted to 1498 cm −1 . According to [62], such a shift may be a sign of the presence on the CeO 2 surface of weakly bound G complexes, which occur due to the simultaneous formation of hydrogen bonds between the CH-group of the aromatic ring and the surface hydroxyl as well as OH group of G and the surface cerium atom. Figure 3 shows the spectra of pure VA immobilized on CeO 2 . The presence of different COH groups in the VA molecule complicates the interpretation of the obtained spectra. Literature data [15,59,64,65] and our FT-IR spectroscopic studies of P, G, and carboxylic acids [22,45,50] were used to analyze the IR spectra of pure VA and VA/CeO 2 . From the obtained spectra ( Figure 3, Table 3), it was found that both the carboxyl group and the active groups of the aromatic ring of VA are involved in the interaction with CeO 2 . In particular, for VA/CeO 2 samples, a decrease in the intensity of bands at 1030 cm −1 (ν s (COCH 3 )), 1240 cm −1 (ν αs (COCH 3 )) [15,64] and at 1113, 1188, 1456, and 1473 cm −1 (β(CH 3 )) [15,65] was observed. This indicates the participation of the methoxyl group in the interaction with the oxide surface.  A wide intense band, which in the spectra of pure acid has two maxima at 1284 cm −1 and 1299 cm −1 , occurs mainly as a result of vibrations of carboxyl and aromatic COH groups ((COH) ar ) of acid [15,65]. In the spectra of the VA/CeO 2 samples, it undergoes significant transformations; instead of two maxima, a peak appears at 1288 cm -1 and a shoulder in the 1275 cm −1 region. From this, we can conclude that both the carboxyl and phenolic groups can be involved in binding to the surface of CeO 2 .

Vanillic Acid
According to studies of complexes of ferulic and caffeic acids with the metal ions (Cu 2+ , Al 3+ , Na + ) CuCl 2 , AlCl 3 , and Na [66], new absorption bands associated with the formation of bonds between aromatic ligands of these acids and metal ions can appear in the FT-IR spectra in the region from 1090 to 1300 cm −1 . Their position depends on the type of metal and the reagent ratio [66]. The high reactivity of phenolic OH groups of a number of carboxylic aromatic acids (vanillic, gallic, and caffeic) in interaction with cerium oxide was also recorded in [67] by UV-Vis spectroscopy.
A number of signs indicating the formation of carboxylate complexes were detected in the VA/CeO 2 spectra. In particular, the absorption at 1686 cm −1 (ν(C = О)) and 920 cm −1 (δ(CОН)) [15]) disappeared for concentrations of 0.1-0.3 mmol/g, and for higher concentrations the intensity of these bands were smaller compared to pure VA. At the same time, the broad bands appeared at~1410 cm −1 (ν s (COO − )) and 1539 cm −1 (ν as (COO − ). The presence in this part of the spectrum of absorption bands ν(CC) (at 1523 cm −1 -for pure VA) prevented an accurately identification of the bands at~1510 cm −1 for VA/CeO 2 samples. It may correspond to both ν as (COO − ) and ν(CC). The difference, ∆ν = ν as (COO) − ν s (COO − ), was used as a criterion to establish the coordination of the COO − group to the metal [68] and oxide surfaces [22,45,59,[69][70][71]. The value of ∆ν for VA/CeO 2 was 129 cm −1 , which corresponded to the bidentate VA complexes with a bridge structure formed on the nanoceria surface.

Ferulic Acid
FA has a more complex structure compared to VA because it has a group C = C in the aliphatic part of the acid. This is manifested in its vibrational spectrum (Figure 4, Table 4). The interaction of FA with the oxide surface caused a number of changes in the spectra of FA/CeO 2 (Figure 4). Bands 1036 cm −1 (ν s (C-O-CH 3 )) and 1205 cm −1 (ν as (C-O-CH 3 )) [72,73] for FA/CeO 2 were shifted to 1034 cm −1 and 1211 cm −1 , respectively. In addition, bands at 1115 and 1178 cm −1 disappeared in the FA/CeO 2 spectra, which was most likely associated with β(CH 3 ) vibrations [72], and a new band appeared at 1124 cm −1 . The intensity of the band 1466 cm −1 (β(CH 3 )) [64,72] significantly decreased ( Figure 4). This indicates the participation of the methoxyl group in the interaction with the nanoceria surface.   ) 1691 - [72] The absorption ratios at 1115 cm −1 for pure FA in the literature data differ between (β(CH 3 )- [72]) and (β(CH)- [74]), as well as the absorption ratios of bands at 1113 cm −1 for VA (β(CH 3 )- [65]) and (β(CH)- [15]). However, these bands all corresponded to (β(CH 3 )), while the new bands, which appeared at 1124 cm −1 for FA/CeO 2 and 1130 cm −1 -for VA/CeO 2 , corresponded to β(CH 3 ) vibrations of methoxyl groups involved in the interaction with the surface.
The interaction of the aromatic OH group of FA with CeO 2 can be evidenced by the absence of bands in the spectra of the FA/CeO 2 samples at 1167 cm −1 β(OH) ar [72] and 1290 cm −1 (ν(CO) ar ) [72], and the appearance of absorption at 1296 cm −1 (0.1-0.6 mmol/g). Thus, we can discuss the interaction of FA with the surface of CeO 2 through both methoxyl and phenolic groups.
The formation of carboxylate complexes of FA on the cerium oxide surface was also detected by the FT-IR spectra of FA/CeO 2 samples (Figure 4). The appearance of bands at 1405 cm −1 (CO) and 1608 cm  64,74] in the spectra of FA/CeO 2 (0.1-0.6 mmol/g) disappeared. The appearance of these and other bands of pure acid in the FA/CeO 2 spectra (0.9-1.2 mmol/g) was due to the formation of intermolecular acid associates. This was confirmed by the presence of absorptions in the region of 2400 cm −1 , which correspond to ν(OH) of dimers [64]. FA associates can form on the surface when the number of FA molecules exceeds the available active centers of the oxide surface.
A significant shift up to 1637 cm −1 was observed for the absorption band ν(C = C) for the FA/CeO 2 samples. Such a type of shift was also found in the interaction of FA with metals [74,75].

Thermal Transformations of Model Lignin Compounds on the Surface of CeO 2 3.2.1. Pyrocatechol
The study of thermal transformations of P/CeO 2 samples by the TPD MS method is presented in Figure 5. According to thermograms and the P/T-curve ( Figure 5), the thermal decomposition of P/CeO 2 occurred in several stages at the range of 50-750 • C. At the same time, no desorption of pyrocatechol in molecular form (M.r. = 110 Da, m/z 110) was observed over the entire temperature range studied. This indicates that pyrocatechol binds to the surface of nanoceria, and its pyrolysis occurs due to the transformation of surface complexes. This is confirmed by the data of IR spectroscopy, according to which changes in the absorption of COH groups and the appearance of a new band at 1297 cm −1 indicate chemisorption of pyrocatechol.
In this case, one feature was observed: the peaks at a temperature of about 120 • C on the TPD curves for aliphatic series ions m/z 99, 85, 71, 57, and 43. According to [76], such a set of ions is characteristic of aliphatic compounds, namely, alkyl derivatives, alicyclic alcohols (m/z 99 (C 6 H 11 O), m/z 85 (C 6 Н 12 ), m/z 71 (C 5 Н 11 ), m/z 57 (C 4 Н 9 ), and m/z 43 (C 3 Н 7 )). The formation of aliphatic products can result from the decomposition of phenolate complexes of pyrocatechol. It is known that the Ar-OH bond is one of the strongest types of C-O bonds, for example, for guaiacol and other phenol derivatives [35]. It is believed that, before breaking, this bond must first be weakened by hydrogenating the aromatic ring [43,77,78]. According to [43], deoxygenation in the presence of the catalysts CeO 2 and CeO 2 -ZrO 2 occurs more easily from the saturated cycle than from the unsaturated cycle. This is due to the dissociation energy of the CO bond in alcohols, which decreases in the following order: aromatic alcohol (469 kJ/mol), secondary alcohol (385 kJ/mol), primary alcohol (383 kJ/mol), tertiary alcohol (379 kJ/mol) [43]. We do not rule out the possibility of such a process on the surface of CeO 2 , because during the study, we recorded an intense signal of hydrogen evolution (m/z 3) ( Figure 5B). The formation of aliphatic products was likely due to complex redox processes on the surface of nanoceria. However, the intensity of these processes was low. The main processes were of deep destruction of pyrocatechol. This conclusion can be made by comparing the intensities of TPD peaks of the aliphatic series with TPD peaks, which characterize the processes of dehydration (m/z 18), decarboxylation (m/z 44), and desorption of CO and C 2 H 4 (m/z 28) ( Figure 5C,D). The strong influence of the interactions with catalyst surface on the structure of the pyrocatechol molecule was also indicated by the obtained IR spectra. Figure 1 shows that significant changes occurred not only in the absorption of COH groups, through which the interaction occurs, but also in the absorption of the aromatic ring. In particular, the absorption at 1471 cm −1 shifted by 12 cm −1 to the high-frequency region, and the absorption at 1603 and 1620 cm −1 was almost invisible for small concentrations (0.1-0.3 mmol/g).   Table 5 show the results of a DTG/DTA/TG study of CeO 2 and P/CeO 2 samples. The main weight loss of the CeO 2 sample is recorded in the temperature range 20-200 • C, which was probably associated with the desorption of water from the oxide surface. The decomposition of the G/CeO 2 sample, according to the DTG curve ( Figure 6A), proceeded in two stages: 100-150 • C and 150-430 • C. All stages were exothermic. The main weight loss occurred in the second stage.

Guaiacol
Pyrolysis of guaiacol on the surface of nanoceria proceeded similarly to pyrolysis of pyrocatechol (Figure 7). Namely, there was no desorption of guaiacol in molecular form (M.r. = 124 Da, m/z 124); the formation of aliphatic products (m/z 99, 85,71,57,43) occurred in the same temperature range. Consequently, their formation was probably the result of transformations of similar surface complexes P and G on the nanoceria surface. It could also be phenolate complexes, which were confirmed above using IR spectroscopy. The presence of peaks on the TPD curves for ions with m/z 31, 32 (CH 3 OH) and m/z 94 (PhOH) indicates the presence of an additional stage of pyrolysis, probably as a result of thermal transformations of G-complexes bound to the surface through the methoxyl group, the formation of which was evidenced by a number of changes recorded in the absorption of the methoxyl group during the study of these samples by the FT-IR spectroscopy ( Figure 2 and Table 2).
A peak at~358 • C on the TPD curve for the ion with m/z 78 (benzene) was recorded as well as a minor release of the product with m/z 94 (hydroxybenzene). The intensity of the latter was low. These peaks could be associated with the decomposition of G-complexes, which were formed due to the interaction of both active groups of the aromatic ring with two active centers of the cerium oxide surface. The existence of bidentate structures on the oxide surface was evidenced by the disappearance of absorption at 1363 cm −1 (β(CОН)) and the appearance of new bands at 1288 and 1323 cm −1 in the IR spectra of the G/CeO 2 samples (Figure 2). The work [35] confirms the possibility of forming such G-complexes on the CeO 2 surface. It was found that the hydrodeoxygenation of guaiacol over CeO 2 -ZrO 2 catalysts required two oxygen vacancies [35].
Intense peaks on the TPD curves of ions with m/z 28 (C 2 H 4 ), m/z 14 (CH 2 ), m/z 12 (C), and m/z 3 (H) at 225 • C were observed during thermal decomposition of the G/CeO 2 samples ( Figure 7B). The presence of these peaks indicates intense desorption of ethylene (C 2 H 4 , M.r. = 28, m/z 28, 14,12), that can serve as confirmation of the alkylation processes of the nanoceria surface, which occurs during the pyrolysis of G. The data presented in [35] confirm the possibility of alkylation of the oxide surface during pyrolysis of G.
The DTG/DTA/TG study results of the G/CeO 2 pyrolysis are presented in Figure 8. The thermal decomposition of the sample occurred up to 400 • C. On the DTG curve, we can distinguish two stages of weight loss for the G/CeO 2 sample. The first stage corresponded to T max~1 00 • C, the second to T max~1 80 • C. All stages were exothermic. The maximum weight loss occurred in the second stage at~180 • C (Table 6).  Analysis of TPD MS and DTG data of the G/CeO 2 samples (Table 6 and Figures 7 and 8) may indicate that the greatest weight loss occurs due to the decomposition of the Gcomplexes formed through the methoxyl group.

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 • 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 • C. The low-intense peak of decarboxylation (m/z 44, T max~1 30 • 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 (ν(C = O)) and 920 cm −1 (δ(CОН)) ( 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 max~2 70 • 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 ν(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 max~3 30 • C, Figure 9D), which can be formed by pyrolysis of VA molecules bound to the oxide through OH 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 ap group (Figure 3). Along with guaiacol at about 330 • 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 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 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. These peaks at T max ≈294 • C are probably related to desorption methanol (CH 3 OH, M.r. = 32 Da, m/z 32, 31) [76] (Figure 9D).
In our opinion, the release of hydroxybenzene (T max~3 80 • C, Figure 9D) can also result from the transformation of the phenolic complex linked through both active groups of the aromatic ring (Scheme 2). Such a decomposition mechanism was observed for similar caffeic acid complexes on the surfaces of CeO 2 and SiO 2 [79,80]. The ion with m/z 151 was observed in the mass spectra of the VA/CeO 2 sample in a wide temperature range (150-450 • C) ( Figure 9A). T max of TPD peak for this ion was located at~380 • C ( Figure 9D). The presence of this peak could be related to the formation of the vanillin (M.r. = 152 Da, m/z 151 (100%), m/z 152 (93%), m/z 81 (32%), m/z 109 (25%), and m/z 123 (18%) [83]). Vanillin can form due to the VA reduction processes on the nanoceria surface.
The DTG/DTA/TG data obtained during the pyrolysis of the VA/CeO 2 sample are shown in Figure 10. Thermal decomposition of the sample proceeded in three main stages and continued in the range from 100 to 500 • C. All stages were exothermic. Analysis of the results of the TG/DTG and TPD MS studies shows that the greatest weight loss occurred during the decomposition of the carboxylate complexes with the release of guaiacol (T max = 265 • C) ( Table 7).

Ferulic Acid
The results of the TPD MS study of the FA/CeO 2 sample are presented in Figures 11 and 12. According to Figure 11   The MVPh was formed as a result of the decarboxylation of FA. The TPD curve of the MVPh molecular ion had two peaks at 110 and 220 • C ( Figure 12C). During the decomposition of FA in the pristine state in the air atmosphere, the release of MVPh was registered at 280 • C [22]. Conversely, during the TPD MS study, its formation was observed at T max 480 • C [23]. The temperature maximum rate of the MVF formation on the SiO 2 surface corresponded to 400 • C [50]. Thus, the interaction of FA with the CeO 2 surface led to a significant decrease in the temperature of the MVPh formation. The release of the MVPh at T max = 110 • C may be associated with the decomposition of monodentate carboxylate complexes (Scheme 3) and the destruction of FA associates (Scheme 4), the presence of which was evidenced by the FT-IR spectra of FA/CeO 2 . The presence of associates was indicated by bands at 1666, 1691 (C = O), 1325 β(COH), and at 949 cm −1 δ(COH), which were revealed in the IR-spectra of the samples FA/CeO 2 (0.6-1.2 mmol/g) ( Figure 4).  The second peak (~220 • C) was formed due to the transformation of the bidentate carboxylate complexes (Scheme 5). The second peak (~220 • C) was formed due to the transformation of the bidentate carboxylate complexes (Scheme 5), which were detected in the IR spectroscopic study of FA/CeO 2 by the appearance of bands ν(CОО − ) at 1405, 1450, and 1502 cm −1 . Increasing the amount of FA on the oxide surface, the first TPD peak of the MVPh desorption increased compared to the second (Figure 13). In this case, the relative intensity of the absorption band (1608 cm −1 ) corresponding to C = O vibrations of VA molecules, which form monodentate carboxylate complexes, also increased in the IR spectra of FA/CeO 2 ( Figure 4). This indicated an increase in the relative amount of these complexes at higher acid concentrations. FA complexes formed due to the simultaneous interaction of OH and OCH 3 groups of the aromatic ring with the CeO 2 surface being destroyed at about 400 • C (Scheme 7). As a result, hydroxybenzene was formed (M.r. = 94 Da, m/z 94, T max ≈407 • C) ( Figure 12D). In the same way, similar complexes of caffeic acid decomposed on the CeO 2 surface [79].  At higher temperatures, desorption of aromatic products such as naphthalene (m/z 128, T max ≈ 430 • C) was detected ( Figure 12D). However, the intensity of their release was low. The formation of polyaromatic products was also detected during the pyrolysis of caffeic and ferulic acids and a number of coumarins on the surfaces of nanoscale oxides [50,80,84]. However, the decomposition of cinnamic acid on the SiO 2 surface did not reveal such products [85]. Therefore, their formation was seemingly due to the transformation of complexes bound to the oxide through the active groups of the aromatic ring.
In accordance with DTG/DTA/TG data obtained by pyrolysis of the FA/CeO 2 sample (Figure 15), the decomposition of FA occurred in the temperature range of 100 to 500 • C in four stages. All stages were exothermic. The maximum weight loss corresponded to the third stage (~259 • C) ( Table 8).  The kinetic parameters of the formation of the main products during VA and FA catalytic pyrolysis were calculated in this study (Table 9). Based on calculated negative values of activation entropy, the formation processes of phenol, guaiacol, cresol, and methylated 4-vinylguaiacols run through highly ordered cyclic transition states on the nanoceria surface. Table 9. Kinetic parameters (temperature of the maximum desorption rate T max , reaction order n, activation energy E = , pre-exponential factor ν 0 , and change of activation entropy ∆S = ), temperature range (T range ) of formation and peak intensities (I) of the catalytic reactions of vanillic and ferulic acids during pyrolysis over nanoceria catalyst. The formation of phenol was characterized by similar kinetic parameters for both vanillic and ferulic acids. The value of the activation energy was~120 kJ mol −1 , and the change in the entropy of activation is~28-29 (cal K −1 mol −1 ). Close kinetic parameters were observed for the formation of guaiacol for both vanillic and ferulic acids. This indicates that FA and VA have common pyrolysis pathways, probably due to thermal transformations of the same types of surface complexes. The processes of the formation of methylated products such as cresol in the case of VA and methylated 4-vinylguaiacols in the case of FA are also characterized by close temperatures of the maximum desorption rate of~318-332 • C (Table 8).

Conclusions
The interactions of model compounds of lignin (P, G, VA, and FA) with the nanoceria surface were investigated by FT-IR spectroscopy. It was found that active groups of the aromatic ring ((−OH ar ) and (−OCH 3 ) ar ) as well as carboxylate groups, in the case of VA and FA, were involved in the interaction with the oxide. According to the FT-IR spectra, VA formed carboxylate complexes with a bidentate structure on the CeO 2 surface. In contrast, for FA, in addition to bidentate complexes, the existence of monodentate complexes was confirmed.
Thermal decomposition of P and G bound to the nanoceria surface through the OH group was probably accompanied by hydrogenation of the aromatic ring and its opening. The intensity of these processes was low. As a result of the thermal destruction of G complexes formed through the methoxyl group, hydroxybenzene was released. The thermal decomposition of P and G revealed signs of alkylation of the oxide surface. Catalytic pyrolysis of guaiacol and pyrocatechol led to the deep destruction of these compounds. The decomposition of carboxylic acids was accompanied by active processes of dehydration, decarbonylation, and decarboxylation. The main pyrolysis products of VA on the nanoceria surface were guaiacol and hydroxybenzene. Guaiacol can be formed due to the destruction of carboxylate complexes and the complexes formed through OH-and CH 3 O-groups of the aromatic ring. Destruction of FA carboxylate complexes led to the formation of 3-methoxy-4-vinylphenol. As a result of the transformation of the complexes formed through OHand CH 3 O-groups of the aromatic ring, guaiacol, and hydroxybenzene were formed. The decomposition of carboxylate complexes occurred at lower temperatures than complexes formed through OH-and CH 3 O-groups. Thermolysis of both acids was accompanied by alkylation of the oxide surface. Polycyclic aromatic hydrocarbons (naphthalene) were also registered during the FA catalytic pyrolysis.
The kinetic parameters of the formation of the main products' catalytic pyrolysis (phenol, guaiacol, cresol, and methylated 4-vinylguaiacols) were calculated. The catalytic pyrolysis processes of VA and FA occurred through highly ordered cyclic transition states on the nanoceria surface.  Data Availability Statement: The study did not report any data.