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Article

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

by
Nataliia Nastasiienko
1,*,
Tetiana Kulik
1,*,
Borys Palianytsia
1,
Julia Laskin
2,
Tetiana Cherniavska
1,
Mykola Kartel
1 and
Mats Larsson
3
1
Chuiko Institute of Surface Chemistry, NAS of Ukraine, 17 General Naumov Str., 03164 Kyiv, Ukraine
2
Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA
3
Department of Physics, AlbaNova University Center, Stockholm University, SE-106 91 Stockholm, Sweden
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2021, 11(16), 7205; https://0-doi-org.brum.beds.ac.uk/10.3390/app11167205
Submission received: 1 July 2021 / Revised: 25 July 2021 / Accepted: 28 July 2021 / Published: 5 August 2021
(This article belongs to the Special Issue Achievements and Prospects of Biomass Pyrolysis)
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Abstract

:
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 CeO2 surface to generate hydroxybenzene. The formation of alkylated products accompanies catalytic pyrolysis of acids due to processes of transalkylation on the surface.

1. 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 (wine-distilleries, 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]. СеО2 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 Ce3+. The concentration of defects, both Ce3+ ions and oxygen vacancies, on the oxide surface is higher than in the bulk [41]. Therefore, nanosized cerium oxide has higher concentrations of Ce3+ 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 com-pounds 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], CeO2 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/CeO2 catalysts have also been used to convert 4-propyl-phenol to propyl-cyclohexane. [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 CeO2 and ZrO2 have proven their effectiveness [16,35,38,43]. They have many advantages over the individual oxides [35]. In particular, the combination of CeO2 and ZrO2 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 CeO2/ZrO2 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 CeO2, CeO2-ZrO2, Ni/CeO2, and Ni/CeO2-ZrO2 catalysts for hydro-deoxygenation of phenol at intermediate temperature and pressure (275 °C and 100 bar) in a batch reactor was tested in [43]. Oxide catalysts (CeO2, CeO2/ZrO2) 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/CeO2 and Ni/CeO2-ZrO2 were the most active catalysts for the initial hydrogenation of phenol to cyclohexanol but were insufficiently active in the next stage of deoxygenation [43].
Catalysts CeO2, ZrO2, Al2O3, CeO2-SiO2, and CeO2-ZrO2 were used in the ketonization of valeric acid into 5-nonanone, [44,45,46] and various fatty acids (C2-C10, D6CCOOH, and CH3COOH) into symmetric ketones [46]. The majority of the studied acids [46], especially valeric, acetic, levulinic, furandicarboxylic, and other acids, can be obtained by converting lignocellulosic biomass [47]. Conversely, lignin is a source for a number of natural cinnamic acids [1,48].
In [38], the activity and surface properties based on oxides of CeO2 and ZrO2 coated with nickel (Ni/CeO2-ZrO2) were used in the thermochemical conversion of cellulose. The catalyst Ni/CeO2-ZrO2 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/ZrO2. 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/CeO2-ZrO2 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 com-pounds 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 phenol-containing plant raw materials over the CeO2-based catalytic systems.

2. Materials and Methods

The model compounds used in this work were рyrocatechol (99%, Changzhou Winsun Import & Export Co., Ltd, Changzhou, Jiangsu, China), guaiacol (≥ 98%, Alfa Aesar, Karlsruhe, Germany), vanillic acid (97%, Sigma-Aldrich, Buchs, Switzerland) and ferulic acid (98%, Alfa Aesar, Karlsruhe, Germany). No further purification of these compounds was conducted in this work. Nanosized cerium dioxide (99.5%, SAr = 71 m2/g, Alfa Aesar, Karlsruhe, Germany) was pre-calcined at 500 °C for 2 h to remove organic matter.
A series of samples, P/CeO2, G/CeO2, VA/CeO2, and FA/CeO2, 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 CeO2 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.
Infrared spectra were recorded on a Thermo Nicolet Nexus FT-IR instrument (Thermo Nicolet Corporation, Madison, WI, USA) within the range of 4000–400 cm−1, working in diffuse reflection mode. The resolution was 4 cm−1, scanning speed—0.5 cm/s, and number of scans—50. Before the FTIR studies, pure СеО2 and samples P/CeO2, G/CeO2, VA/CeO2, and FA/CeO2 were mixed with KBr (≥ 99%, Alfarus, Kyiv, Ukraine) (1:10). KBr was pre-calcined for 2 h at 500 °C. Pure phenolic compounds were mixed with KBr in a ratio of 1:100.
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 Tmax, reaction order n, activation energy E≠, pre-exponential factor ν0, 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.

3. Results and Discussion

3.1. FT-IR Spectroscopic Studies

3.1.1. Pyrocatechol

FTIR spectra of CeO2, pure Р, and samples P/CeO2 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.
According to [53], most of the bands of this dihydroxybenzene have a mixed shape. Figure 1 shows significant changes in the P/СеО2 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/СеО2 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 CeO2. 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/CeO2 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/CeO2 samples, these bands are shifted toward lower frequencies, and their relative intensity decreases.
In addition, for P/CeO2 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/CeO2. 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(СОН) [51,53]), and 1281 cm−1 (ν(СО)) [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/CeO2 spectra contained a new band at 1297 cm−1, which may indicate the formation of a bond between pyrocatechol and CeO2. 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 (СС) was registered at 1471, 1514, 1603, and 1620 cm−1 [53], while in the P/CeO2 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/CeO2 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 CeO2 surface. The obtained results indicate the formation of chemisorbed pyrocatechol complexes on the CeO2 surface.
The P/CeO2 spectrum (Figure 1) is similar to the IR spectra of pyrocatechol adsorbed on the nanosized TiO2 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 TiO2, 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/CeO2 spectrum correspond to the bands 1263 and 1483 cm−1 (Figure 1), it is probable that the P complexes on the CeO2 surface also have a bidentate bridge structure (two oxide atoms of phenolic groups of P interact with two metal atoms).

3.1.2. Guaiacol

The FT-IR spectra obtained for pure G and immobilized on the surface of CeO2 are shown in Figure 2. The absence for G/CeO2 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 CeO2.
At the same time, for these samples, the bands of symmetric (νs) and asymmetric (νаs) valence vibrations of the С-О-СН3 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(CH3) 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 CeO2.
The new bands at 1288 and 1323 cm−1 in the spectra of G/CeO2 samples were a sign of the formation of chemisorbed G complexes [62]. The absorptions for ν(CH3) at 2843 cm−1 partially remained in the spectra of G/CeO2.Therefore, the interaction of the COCH3 group with the oxide surface probably occurred through an oxygen atom without cleavage of CH3. 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 CeO2 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/CeO2 samples is shifted to 1498 cm−1. According to [62], such a shift may be a sign of the presence on the CeO2 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.

3.1.3. Vanillic Acid

Figure 3 shows the spectra of pure VA immobilized on CeO2. 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/CeO2. 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 CeO2. In particular, for VA/CeO2 samples, a decrease in the intensity of bands at 1030 cm−1 (νs(COCH3)), 1240 cm−1 (ναs(COCH3)) [15,64] and at 1113, 1188, 1456, and 1473 cm−1 (β(CH3)) [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 СOH groups ((COH)ar) of acid [15,65]. In the spectra of the VA/CeO2 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 CeO2.
According to studies of complexes of ferulic and caffeic acids with the metal ions (Cu2+, Al3+, Na+) CuCl2, AlCl3, 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/CeO2 spectra. In particular, the absorption at 1686 cm−1 (ν(С = О)) and 920 cm−1 (δ(СОН)) [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 (νs(COO−)) and 1539 cm−1 (νas(COO−). The presence in this part of the spectrum of absorption bands ν(СС) (at 1523 cm−1—for pure VA) prevented an accurately identification of the bands at ~1510 cm−1 for VA/CeO2 samples. It may correspond to both νas(COO−) and ν(CC). The difference, ∆ν = νas(COO) − νs(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/CeO2 was 129 cm−1, which corresponded to the bidentate VA complexes with a bridge structure formed on the nanoceria surface.

3.1.4. 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/CeO2 (Figure 4). Bands 1036 cm−1 (νs(C-O-CH3)) and 1205 cm−1 (νas(C-O-CH3)) [72,73] for FA/CeO2 were shifted to 1034 cm−1 and 1211 cm−1, respectively. In addition, bands at 1115 and 1178 cm−1 disappeared in the FA/CeO2 spectra, which was most likely associated with β(CH3) vibrations [72], and a new band appeared at 1124 cm−1. The intensity of the band 1466 cm−1 (β(CH3)) [64,72] significantly decreased (Figure 4). This indicates the participation of the methoxyl group in the interaction with the nanoceria surface.
The absorption ratios at 1115 cm−1 for pure FA in the literature data differ between (β(CH3)—[72]) and (β(CH)—[74]), as well as the absorption ratios of bands at 1113 cm−1 for VA (β(CH3)—[65]) and (β(CH)—[15]). However, these bands all corresponded to (β(CH3)), while the new bands, which appeared at 1124 cm−1 for FA/CeO2 and 1130 cm−1—for VA/CeO2, corresponded to β(CH3) vibrations of methoxyl groups involved in the interaction with the surface.
The interaction of the aromatic OH group of FA with CeO2 can be evidenced by the absence of bands in the spectra of the FA/CeO2 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 CeO2 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/CeO2 samples (Figure 4). The appearance of bands at 1405 cm−1 (CO) and 1608 cm−1 (C = O) may be associated with the formation of monodentate carboxylate complexes (Δν = ν(C = O) − ν(C–O) = 203 cm−1). The band 1608 cm−1 overlapped with the absorption of the aromatic ring vibration (1601 cm−1). The new bands at 1450 cm−1 (νas (COO−)) and 1502 cm−1 (νas(COO−)) corresponded to the bidentate chelate complexes, since Δν = 52 cm−1.
The C = O vibrations at 1666 and 1691 cm−1 as well as β(COH) at 1325 cm−1 and δ(COH) at 949 cm−1 ([64,74] in the spectra of FA/CeO2 (0.1‒0.6 mmol/g) disappeared. The appearance of these and other bands of pure acid in the FA/CeO2 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/CeO2 samples. Such a type of shift was also found in the interaction of FA with metals [74,75].

3.2. Thermal Transformations of Model Lignin Compounds on the Surface of CeO2

3.2.1. Pyrocatechol

The study of thermal transformations of P/CeO2 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/CeO2 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 (C6H11O), m/z 85 (С6Н12), m/z 71 (С5Н11), m/z 57 (С4Н9), and m/z 43 (С3Н7)). 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 CeO2 and CeO2-ZrO2 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 CeO2, 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 C2H4 (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).
Figure 6 and Table 5 show the results of a DTG/DTA/TG study of CeO2 and P/CeO2 samples. The main weight loss of the CeO2 sample is recorded in the temperature range 20–200 °С, which was probably associated with the desorption of water from the oxide surface. The decomposition of the G/CeO2 sample, according to the DTG curve (Figure 6A), proceeded in two stages: 100–150 °С and 150–430 °С. All stages were exothermic. The main weight loss occurred in the second stage.

3.2.2. 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 (CH3OH) 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(СОН)) and the appearance of new bands at 1288 and 1323 cm−1 in the IR spectra of the G/CeO2 samples (Figure 2). The work [35] confirms the possibility of forming such G-complexes on the CeO2 surface. It was found that the hydrodeoxygenation of guaiacol over CeO2-ZrO2 catalysts required two oxygen vacancies [35].
Intense peaks on the TPD curves of ions with m/z 28 (C2H4), m/z 14 (CH2), m/z 12 (C), and m/z 3 (H) at 225 °C were observed during thermal decomposition of the G/CeO2 samples (Figure 7B). The presence of these peaks indicates intense desorption of ethylene (C2H4, 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/CeO2 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/CeO2 sample. The first stage corresponded to Tmax~100 °C, the second to Tmax~180 °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/CeO2 samples (Table 6 and Figure 7 and Figure 8) may indicate that the greatest weight loss occurs due to the decomposition of the G-complexes formed through the methoxyl group.

3.2.3. Vanillic Acid

The TPD MS study results of the VA/CeO2 sample are presented in Figure 9. The decomposition interval of VA on the CeO2 surface was 100–750 °C (Figure 9). According to thermograms and the P/T-curve (Figure 9C,D), the pyrolysis of VA on the CeO2 surface occurred in several main stages: 130, 270, 370, 550, and 650 °C.
The low-intense peak of decarboxylation (m/z 44, Tmax~130 °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 CeO2 [79] (Figure 9C). The formation of such associates in the VA/CeO2 sample was confirmed by the presence in their IR spectra of bands at 1686 cm−1 (ν(C = O)) and 920 cm−1 (δ(СОН)) (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, Tmax ~ 270 °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 СеО2 surface [79] and on the SiO2 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) (Tmax~330 °C, Figure 9D), which can be formed by pyrolysis of VA molecules bound to the oxide through OHar. The possibility of such a bond is indicated by the transformation of the 1284 cm−1 band, which corresponds to the vibrations of the COap 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 OHar. 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 CeO2 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 Tmax ≈ 294 °C are probably related to desorption methanol (CH3OH, M.r. = 32 Da, m/z 32, 31) [76] (Figure 9D).
In our opinion, the release of hydroxybenzene (Tmax~380 °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 CeO2 and SiO2 [79,80].
The ion with m/z 151 was observed in the mass spectra of the VA/CeO2 sample in a wide temperature range (150–450 °C) (Figure 9A). Tmax 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/CeO2 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 (Tmax = 265 °C) (Table 7).

3.2.4. Ferulic Acid

The results of the TPD MS study of the FA/CeO2 sample are presented in Figure 11 and Figure 12. According to Figure 11, the main decomposition products were the 3-methoxy-4-vinyl phenol (MVPh) (M.r. = 150 Da, m/z 150, Tmax≈110 and 220 °C), guaiacol (Mr = 124 Da, m/z 124, Tmax ≈ 261 °C), 4-Vinylmethylguaiacol (Mr = 164 Da, m/z 164, Tmax ≈ 334 °C) and hydroxybenzene (Mr = 94 Da, m/z 94, Tmax ≈ 407 °C). The decomposition of the sample occurred from 55 to 700 °C (Figure 12A,B).
The MVPh was formed as a result of the decarboxylation of FА. 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 Tmax 480 °C [23]. The temperature maximum rate of the MVF formation on the SiO2 surface corresponded to 400 °C [50]. Thus, the interaction of FA with the CeO2 surface led to a significant decrease in the temperature of the MVPh formation. The release of the MVPh at Tmax = 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/CeO2. 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/CeO2 (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/CeO2 by the appearance of bands ν(СОО−) 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/CeO2 (Figure 4). This indicated an increase in the relative amount of these complexes at higher acid concentrations.
The destruction of FА complexes formed on the surface of CeO2 with the participation of aromatic ligands was accompanied by the release of guaiacol (M.r. = 124 Da, m/z 124) (Scheme 6) and hydroxybenzene (M.r. = 94 Da, m/z 94) (Figure 12D). The participation in the interaction with the oxide of both active groups of the aromatic ring was confirmed by the obtained data of FTIR spectroscopy, presented in Figure 4. The thermal transformation of FА molecules bound to the oxide surface through the OCH3 group occurred with the formation of guaiacol (M.r. = 124 Da, m/z 124, Tmax≈ 265 °C).
FA complexes formed due to the simultaneous interaction of OH and OCH3 groups of the aromatic ring with the CeO2 surface being destroyed at about 400 °C (Scheme 7). As a result, hydroxybenzene was formed (M.r. = 94 Da, m/z 94, Tmax≈407 °C) (Figure 12D). In the same way, similar complexes of caffeic acid decomposed on the CeO2 surface [79].
In addition, the presence of the TPD peak for the ions with m/z 164 and 178 may be due to the methylation processes of the aromatic ring, according to the Scheme 7, with the formation of methylated 4-vinylguaiacols (Figure 14, Scheme 8).
At higher temperatures, desorption of aromatic products such as naphthalene (m/z 128, Tmax ≈ 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 SiO2 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/CeO2 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).

3.2.5. Kinetic Parameters of the Catalytic Reactions of Vanillic and Ferulic Acids during Pyrolysis over Nanoceria Catalyst

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.
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).

4. 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 ((−OHar) and (−OCH3)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 CeO2 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 CH3O-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 OH- and CH3O-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 CH3O-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.

Author Contributions

Conceptualization, T.K., M.K. and M.L.; methodology, N.N., B.P. and T.K.; investigation, N.N., B.P., T.C. and T.K.; resources, T.K.; writing—original draft preparation, N.N. and T.K.; writing—review and editing, N.N., T.K., J.L. and M.L.; visualization, N.N., T.K., and B.P.; supervision, M.K., T.K., J.L. and M.L.; project administration, T.K. and J.L.; funding acquisition, T.K. and M.L. All authors have read and agreed to the published version of the manuscript.

Funding

This publication is based on work supported by grant FSA3-20-66700 from the United States Civilian Research & Development Foundation (CRDF Global) with funding from the United States Department of State, by the Swedish Research Council (VR) under contract 348-2014-4250, by STCU (grant P707), and by NAS of Ukraine (program “new functional substances and materials of chemical production”).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The study did not report any data.

Conflicts of Interest

The authors declare no conflict of interest.

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Applsci 11 07205 g001 550
Figure 1. Fourier transform-infrared (FT-IR) spectra of pure СеО2 (a), samples of P/СеО2 with different contents of FA (0.1: b; 0.3: c; 0.6: d; 0.9: e; 1.2 mmol/g: f) and pure P (g).
Figure 1. Fourier transform-infrared (FT-IR) spectra of pure СеО2 (a), samples of P/СеО2 with different contents of FA (0.1: b; 0.3: c; 0.6: d; 0.9: e; 1.2 mmol/g: f) and pure P (g).
Applsci 11 07205 g001
Applsci 11 07205 g002 550
Figure 2. Fourier transform-infrared (FT-IR) spectra of pure СеО2 (a), samples of G/СеО2 with different contents of G (0.1: b; 0.3: c; 0.6: d; 0.9: e; 1.2 mmol/g: f) and pure G (g).
Figure 2. Fourier transform-infrared (FT-IR) spectra of pure СеО2 (a), samples of G/СеО2 with different contents of G (0.1: b; 0.3: c; 0.6: d; 0.9: e; 1.2 mmol/g: f) and pure G (g).
Applsci 11 07205 g002
Applsci 11 07205 g003 550
Figure 3. Fourier transform-infrared (FT-IR) spectra of pure СеО2 (a), samples of VA/СеО2 with different contents of VA (0.1: b; 0.3: c; 0.6: d; 0.9: e; 1.2 mmol/g: f) and pure VA (g).
Figure 3. Fourier transform-infrared (FT-IR) spectra of pure СеО2 (a), samples of VA/СеО2 with different contents of VA (0.1: b; 0.3: c; 0.6: d; 0.9: e; 1.2 mmol/g: f) and pure VA (g).
Applsci 11 07205 g003
Applsci 11 07205 g004 550
Figure 4. Fourier transform-infrared spectra of pure СеО2 (a), samples of FA/СеО2 with different contents of FA (0.1: b; 0.3: c; 0.6: d; 0.9: e; 1.2 mmol/g: f) and pure FA (g).
Figure 4. Fourier transform-infrared spectra of pure СеО2 (a), samples of FA/СеО2 with different contents of FA (0.1: b; 0.3: c; 0.6: d; 0.9: e; 1.2 mmol/g: f) and pure FA (g).
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Applsci 11 07205 g005 550
Figure 5. Thermal decomposition of P/CeO2 (0.6 mmol/g); vapor pressure measured as a function of temperature for the FA/Al2O3 sample (A); mass spectra at 120 °C (B); TPD curves for ions with m/z 18, 28, and 44 (C); for ions with m/z 43, 57, 71, 85, and 99 (D).
Figure 5. Thermal decomposition of P/CeO2 (0.6 mmol/g); vapor pressure measured as a function of temperature for the FA/Al2O3 sample (A); mass spectra at 120 °C (B); TPD curves for ions with m/z 18, 28, and 44 (C); for ions with m/z 43, 57, 71, 85, and 99 (D).
Applsci 11 07205 g005
Applsci 11 07205 g006 550
Figure 6. Differential thermal analysis (DTA), differential thermogravimetric analysis (DTG), and thermogravimetric (TG) curves for СеО2 (A) and P/СеО2 (0.6 mmol/g) (B).
Figure 6. Differential thermal analysis (DTA), differential thermogravimetric analysis (DTG), and thermogravimetric (TG) curves for СеО2 (A) and P/СеО2 (0.6 mmol/g) (B).
Applsci 11 07205 g006
Applsci 11 07205 g007 550
Figure 7. Thermal decomposition of G/CeO2 (0.6 mmol/g); mass spectra at 120 °C and vapor pressure measured as a function of temperature for the G/CeO2 sample (A); TPD curves for ions with m/z 3, 18, 28, 31, and 44 (B); for ions with m/z 43, 57, 71, 85, and 99 (С); for ions with m/z 3, 12, 15, 31, 50, 78, and 94 (D).
Figure 7. Thermal decomposition of G/CeO2 (0.6 mmol/g); mass spectra at 120 °C and vapor pressure measured as a function of temperature for the G/CeO2 sample (A); TPD curves for ions with m/z 3, 18, 28, 31, and 44 (B); for ions with m/z 43, 57, 71, 85, and 99 (С); for ions with m/z 3, 12, 15, 31, 50, 78, and 94 (D).
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Applsci 11 07205 g008 550
Figure 8. Differential thermal analysis (DTA), differential thermogravimetric analysis (DTG), and thermogravimetric (TG) curves for G/СеО2 (0.6 mmol/g).
Figure 8. Differential thermal analysis (DTA), differential thermogravimetric analysis (DTG), and thermogravimetric (TG) curves for G/СеО2 (0.6 mmol/g).
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Applsci 11 07205 g009 550
Figure 9. Thermal decomposition of VA/CeO2 (0.6 mmol/g); mass spectra at 270 °С (A) and 410 °С (B); TPD-curves for ions with m/z 18, 24, 44, and vapor pressure measured as a function of temperature for the VA/CeO2 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).
Figure 9. Thermal decomposition of VA/CeO2 (0.6 mmol/g); mass spectra at 270 °С (A) and 410 °С (B); TPD-curves for ions with m/z 18, 24, 44, and vapor pressure measured as a function of temperature for the VA/CeO2 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).
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Applsci 11 07205 sch001 550
Scheme 1. The decarboxylation of VA molecules bound to the oxide surface through the carboxyl group.
Scheme 1. The decarboxylation of VA molecules bound to the oxide surface through the carboxyl group.
Applsci 11 07205 sch001
Applsci 11 07205 sch002 550
Scheme 2. The decomposition of the phenolic complex of VA.
Scheme 2. The decomposition of the phenolic complex of VA.
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Applsci 11 07205 g010 550
Figure 10. Differential thermal analysis (DTA), differential thermogravimetric analysis (DTG), and thermogravimetric (TG) curves for VA/СеО2.
Figure 10. Differential thermal analysis (DTA), differential thermogravimetric analysis (DTG), and thermogravimetric (TG) curves for VA/СеО2.
Applsci 11 07205 g010
Applsci 11 07205 g011 550
Figure 11. Mass spectra obtained by decomposition of FA/CeO2 at a temperature of 250 °С (А), 330 °С (B), and 400 °С (C).
Figure 11. Mass spectra obtained by decomposition of FA/CeO2 at a temperature of 250 °С (А), 330 °С (B), and 400 °С (C).
Applsci 11 07205 g011
Applsci 11 07205 g012 550
Figure 12. Thermal decomposition of FA/CeO2 (0.6 mmol/g); vapor pressure measured as a function of temperature for the FA/CeO2 sample (A); TPD curves for ions with m/z 18, 24, and 44 (B), for ions with m/z 77, 107, 135, and 150 (C) and for ions with m/z 94, 108, 124, 128, and 164 (D).
Figure 12. Thermal decomposition of FA/CeO2 (0.6 mmol/g); vapor pressure measured as a function of temperature for the FA/CeO2 sample (A); TPD curves for ions with m/z 18, 24, and 44 (B), for ions with m/z 77, 107, 135, and 150 (C) and for ions with m/z 94, 108, 124, 128, and 164 (D).
Applsci 11 07205 g012
Applsci 11 07205 sch003 550
Scheme 3. The decomposition of the monodentate carboxylate complexes of FA.
Scheme 3. The decomposition of the monodentate carboxylate complexes of FA.
Applsci 11 07205 sch003
Applsci 11 07205 sch004 550
Scheme 4. The decomposition of FA associates.
Scheme 4. The decomposition of FA associates.
Applsci 11 07205 sch004
Applsci 11 07205 sch005 550
Scheme 5. The decomposition of the bidentate carboxylate complexes of FA.
Scheme 5. The decomposition of the bidentate carboxylate complexes of FA.
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Applsci 11 07205 g013 550
Figure 13. TPD-curves for the ion with m/z 150 (3-methoxy-4-vinilphenol), obtained by thermal decomposition of samples of FA/CeO2 (а: 0.6 mmol/g, b: 0.9 mmol/g, c: 1.2 mmol/g).
Figure 13. TPD-curves for the ion with m/z 150 (3-methoxy-4-vinilphenol), obtained by thermal decomposition of samples of FA/CeO2 (а: 0.6 mmol/g, b: 0.9 mmol/g, c: 1.2 mmol/g).
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Applsci 11 07205 sch006 550
Scheme 6. The guaiacol formation.
Scheme 6. The guaiacol formation.
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Applsci 11 07205 sch007 550
Scheme 7. The hydroxybenzene formation.
Scheme 7. The hydroxybenzene formation.
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Applsci 11 07205 g014 550
Figure 14. TPD-curves for the ion with m/z 93, 121, 147, 164, and 178, obtained by thermal decomposition of samples of FA/CeO2.
Figure 14. TPD-curves for the ion with m/z 93, 121, 147, 164, and 178, obtained by thermal decomposition of samples of FA/CeO2.
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Applsci 11 07205 sch008 550
Scheme 8. The methylated 4-vinylguaiacols formation.
Scheme 8. The methylated 4-vinylguaiacols formation.
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Applsci 11 07205 g015 550
Figure 15. Differential thermal analysis (DTA), differential thermogravimetric analysis (DTG), and thermogravimetric (TG) curves for FA/СеО2.
Figure 15. Differential thermal analysis (DTA), differential thermogravimetric analysis (DTG), and thermogravimetric (TG) curves for FA/СеО2.
Applsci 11 07205 g015
Table
Table 1. Assignments of infrared bands of pure P and of P/CeO2 (0.6 mmol/g).
Table 1. Assignments of infrared bands of pure P and of P/CeO2 (0.6 mmol/g).
AssignmentsFrequency (cm−1)Ref.
PP/СеО2
β(СН)721–[53]
β(СН)937–[53]
δ(OH) + δ(CH))756–[53]
ν(CC) + ν(CO) + β(CCC))769–[53]
ν(CO) + ν(CC) + β(CCH)1167–[53]
β(CCH) + ν(CC) + β(COH)11881188[51,53]
β(CCH) + β(COH) + ν(CC)1242–[53]
β(CCH) + β(COH) + ν(CC)12551261[51,53]
ν(CC) + ν(CO) + β(CCC)1281–[51,53]
β(CCH) + β(COH) + ν(CC)13631361[53]
β(COH) + ν(CО)–1273 *
ν(CО)-–1297
β(CCH) + ν(СС) + β(COH) + β(CCO)14401446[53]
ν(СС) + β(CH)1471–[53]
ν(СС) + β(CH)–1483
ν(СС) + β(CH)15141514 *[53]
ν(СС) + β(CH)–1576
ν(СС) + β(CH)16031603 *[53]
ν(СС) + β(CH)1620–[53]
* band appears for P concentrations ˃ 0.6 mmol/g.
Table
Table 2. Assignments of characteristic infrared bands of pure G and of G/CeO2 (0.6 mmol/g).
Table 2. Assignments of characteristic infrared bands of pure G and of G/CeO2 (0.6 mmol/g).
AssignmentsFrequency (cm−1)Ref.
GG/СеО2
νs(СОСН3)10241018[63]
νаs(СОСН3)12251217[63]
ν(CОН)12611261[63]
ν(CО)-–1288[63]
ν(CО)-–1323[63]
β(СОН)1363–[63]
β(СН3)1444–[63]
β(СН3)1469–[63]
ν(CC) + β(СН3)14581456[63]
ν(CO)15081498[63]
ν(CC)15971595[63]
ν(CC)1617–[63]
Table
Table 3. Assignments of characteristic infrared bands of pure VA and of VA/CeO2 (0.6 mmol/g).
Table 3. Assignments of characteristic infrared bands of pure VA and of VA/CeO2 (0.6 mmol/g).
AssignmentsFrequency (cm−1)Ref.
VAVA/СеО2
δ(СOН)920- *[15]
νs(СОСН3)10301018[15]
β(СН3)11131113[61]
β(СН3)11881188[15,65]
νаs(СОСН3)12401240[15]
ν(СОар)12841288[15]
ν(CОH) + β(OH)1299-[15]
ν(CО)--1275
β(СН3)14561456[15,65]
β(СН3)14731467[15,65]
νs(СОО−)-1410
νas(СОО−)-1539
ν(C = O)1686- *[15]
* for VA concentrations 0.1–0.3 mmol/g.
Table
Table 4. Assignments of characteristic infrared bands of pure FA and of FA/CeO2 (0.6 mmol/g).
Table 4. Assignments of characteristic infrared bands of pure FA and of FA/CeO2 (0.6 mmol/g).
AssignmentsFrequency (cm−1)Ref.
FAFA/СеО2
δ(СOН)920-[64,74]
νs(СОСН3)10361034[72,73]
β(СН3)11151124[73]
β(ОН)ар1167-[73]
β(ОН)ар1178-[73]
νаs(СОСН3)12051211[72,73]
ν(СО)ар1290-[72,73]
ν(CО)ар−-1296
ν(CО)-1405
νs(СОО−) 1450
β(СН3)14661468[64,72]
νаs(СОО−)-1502
ν(СС)ар16011601[72]
ν(C = О)-1608
ν(С = С)16201637[72]
νs(СОО−)1666-[72]
νs(СОО−)1691-[72]
Table
Table 5. Pyrolysis yields for TGA pyrolysis of P/CeO2 (0.6 mmol/g).
Table 5. Pyrolysis yields for TGA pyrolysis of P/CeO2 (0.6 mmol/g).
SampleStageTmax (°С)Volatiles (%)Char (%)
P/СеО2I858.419.3
II22472.3
Σ(I + II) 80.7
Table
Table 6. Pyrolysis yields for TGA pyrolysis of G/CeO2 (0.6 mmol/g).
Table 6. Pyrolysis yields for TGA pyrolysis of G/CeO2 (0.6 mmol/g).
SampleStageTmax (°С)Volatiles (%)Char (%)
G/СеО2I10020.330.4
II18049.3
Σ(I + II) 69.6
Table
Table 7. Pyrolysis yields for TGA pyrolysis of VA/CeO2 (0.6 mmol/g).
Table 7. Pyrolysis yields for TGA pyrolysis of VA/CeO2 (0.6 mmol/g).
SampleStageTmax (°С)Volatiles (%)Char (%)
VA/СеО2I1154.321.8
VA/СеО2II17510.9
VA/СеО2III26563.0
VA/СеО2Σ(I + II + III) 78.2
Table
Table 8. Pyrolysis yields for TGA pyrolysis of FA/CeO2 (0.6 mmol/g).
Table 8. Pyrolysis yields for TGA pyrolysis of FA/CeO2 (0.6 mmol/g).
SampleStageTmax (°С)Volatiles (%)Char (%)
FA/СеО2I1207.712.6
II15811.5
III21923.0
IV25945.2
Σ(I–IV) 87.4
Table
Table 9. Kinetic parameters (temperature of the maximum desorption rate Tmax, reaction order n, activation energy E≠, pre-exponential factor ν0, and change of activation entropy ∆S≠), temperature range (Trange) of formation and peak intensities (I) of the catalytic reactions of vanillic and ferulic acids during pyrolysis over nanoceria catalyst.
Table 9. Kinetic parameters (temperature of the maximum desorption rate Tmax, reaction order n, activation energy E≠, pre-exponential factor ν0, and change of activation entropy ∆S≠), temperature range (Trange) of formation and peak intensities (I) of the catalytic reactions of vanillic and ferulic acids during pyrolysis over nanoceria catalyst.
Pyrolysis Product or
Its Fragment Ion		Schemem/z1I, a.u.Trange,
C		Tmax, °CnE≠, kJ/molν0,
s −1		∆S≠,
cal/(K×mol)		R2 2
FA
Phenol7940.107250–57040711185.013 × 106−290.949
Guaiacol61240.139150–3752601781.665 × 105−360.957
4-Vinylguaiacol21503.070–200120-----
31502.2100–400220-----
4-Vinylmethylguaiacol81640.209188–42733211091.628 × 107−270.984
4-Vinyldimethylguaiacol81780.047237–43231811144.200 × 107−250.951
Naphthalene-1280.04320–520430-----
VA
Phenol2940.05295–47138211211.371 × 107−270.965
Cresol-1080.023238–37432211373.383 × 109−160.944
Guaiacol11240.067193–3902751911.575× 106−310.938
1090.097190–4112761901.124 × 106−320.905
810.087190–3972741821.743 × 105−350.937
Vanillin-1510.026180–420~271-----
1 m/z: ratio of ion mass to ion charge.2 R2: coefficient of determination.
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Nastasiienko, N.; Kulik, T.; Palianytsia, B.; Laskin, J.; Cherniavska, T.; Kartel, M.; Larsson, M. Catalytic Pyrolysis of Lignin Model Compounds (Pyrocatechol, Guaiacol, Vanillic and Ferulic Acids) over Nanoceria Catalyst for Biomass Conversion. Appl. Sci. 2021, 11, 7205. https://0-doi-org.brum.beds.ac.uk/10.3390/app11167205

AMA Style

Nastasiienko N, Kulik T, Palianytsia B, Laskin J, Cherniavska T, Kartel M, Larsson M. Catalytic Pyrolysis of Lignin Model Compounds (Pyrocatechol, Guaiacol, Vanillic and Ferulic Acids) over Nanoceria Catalyst for Biomass Conversion. Applied Sciences. 2021; 11(16):7205. https://0-doi-org.brum.beds.ac.uk/10.3390/app11167205

Chicago/Turabian Style

Nastasiienko, Nataliia, Tetiana Kulik, Borys Palianytsia, Julia Laskin, Tetiana Cherniavska, Mykola Kartel, and Mats Larsson. 2021. "Catalytic Pyrolysis of Lignin Model Compounds (Pyrocatechol, Guaiacol, Vanillic and Ferulic Acids) over Nanoceria Catalyst for Biomass Conversion" Applied Sciences 11, no. 16: 7205. https://0-doi-org.brum.beds.ac.uk/10.3390/app11167205

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