Catalytic Pyrolysis of Lignin Model Compound (Ferulic Acid) over Alumina: Surface Complexes, Kinetics, and Mechanisms

Abstract

Studies of the thermochemical properties of the important model compound of lignin-ferulic acid (FA) and its surface complexes are substantial for developing technologies for catalytic pyrolysis of renewable biomass into biofuels and lignin-derived chemicals as well as for bio-oil upgrading. In this work, the catalytic pyrolysis of ferulic acid over alumina was studied by temperature-programmed desorption mass spectrometry (TPD MS), in situ FT-IR spectroscopy, thermogravimetric analysis, and DFT calculations. We established that both the carboxyl group and the active groups (HO and CH₃O) of the aromatic ring interact with the alumina surface. We calculated the kinetic parameters of formation of the main products of catalytic pyrolysis: 4-vinylguaiacol, guaiacol, hydroxybenzene, benzene, toluene, cresol, naphthalene, and PACs. Possible methods of their forming from the related surface complexes of FA are suggested.

1. Introduction

Concerns for climate change, high oil prices, depletion of fossil fuels, energy security, and technological diversification of energy sources are driving increasing demand on renewable energy sources. Bio-based fuels, chemicals, and materials are expected to be an essential part of the future bioeconomy. Rising demands for these renewable commodities during post-COVID recovery confirms these expectations.

Catalytic pyrolysis is suitable for second-generation non-food biomass (2G biomass) conversion in an economically feasible way and has the inherent advantage of the ability to convert virtually any type of biomass into a mixture of liquid pyrolysis bio-oil, gaseous, and solid products [1,2,3,4,5,6,7]. At the same time, the cost of 2G biomass is low, as was shown by Wit and Faaij [8], where costs for agricultural waste are 1–7 € GJ⁻¹ and for forest residues are 2–4 € GJ⁻¹. The main components of pyrolysis oils [1,2,3,4,5,6,7,9,10,11] are carboxylic acids, phenols, aldehydes, ketones, alcohols, furans, phenols, alkanes, etc.

The current literature analysis shows that the only feasible technology for producing biofuels at an industrial scale is catalytic pyrolysis [12,13,14,15,16,17]. Pyrolysis is a process of thermal transformation of organics in the absence of oxygen. Under these conditions, the three main products are produced: bio-oil, pyrolysis gases/syngas, and charcoal. Future efforts should be made to improve the pyrolysis process towards higher yield and quality of liquid bio-oils. These tasks can be solved only by the use of fundamental knowledge about reaction mechanisms of biomass and bio-oil components during catalytic pyrolysis [12].

Carbohydrate components (polysaccharides) make up about 75% of the total biomass and lignin about 20% [18,19]. The annual increase in lignin is about 20 billion tonnes, and its total amount on the Earth exceeds 300 billion tonnes. [18,20]. Lignin is the primary source of sustainable aromatic compounds (benzene, toluene, cresol, etc.) [20]. Lignin-derived aromatic compounds could be upgraded through catalytic hydrogenation to liquid alkanes and used as biofuels. Lignin-derived products have different prices; however, lignin waste prices are low and stable, making this aromatic biopolymer a better feedstock for further conversion into biofuels than fossil fuels, as fossil fuel prices are volatile and constantly increasing [20].

Lignin is composed of phenylpropane blocks: p-coumaryl-, sinapyl-, and coniferyl-units. [18]. In addition, plant biomass includes substantial amounts of hydroxycinnamates cross-linked via different bonds with macromolecule blocks of lignocellulose, which is considered the most attractive 2D biomass for conversion into biofuels and value-added chemicals [21,22,23].

Coniferyl-units are the most common in the structure of lignin, which is especially characteristic of coniferous wood. Therefore, ferulic acid is one of the most important lignin model compounds since it contains the main functional groups and structural elements inherent in this macromolecule. Thus, the elucidation of the structure of the surface complexes of ferulic acid and the mechanisms of thermal decomposition of these complexes on the catalyst surface can help to establish the fundamental mechanisms of pyrolysis processes of lignocellulosic biomass at the molecular level.

Among the compounds that can be obtained from this biomass are phenolic and cinnamic acids, including ferulic acid [21,24,25,26]. They can be obtained by the conversion of lignin. In plant raw materials, phenolic acids are mainly in the conjugated form [24]. FA has been found in alkaline oxidation products of lignosulfonates, alkaline hardwood extracts [27], and various grains [28]. The value of this acid is largely associated with its pronounced antioxidant properties, which are due to the presence in the aromatic nucleus of a hydroxyl group in the para position and a methoxy group in the ortho position to the OH group [29,30,31].

Due to this, FA today has significant potential for cosmetology [32,33] medicine [34], and food technology [35,36]. In addition, in our previous studies of the thermal transformations of phenolic acids both in pure form [27,37] and on the surface of oxide materials (SiO₂, CeO₂) [38,39,40,41], it was shown that they are a potential source of useful chemicals. In particular, the thermal decomposition of FA leads to the formation of 4-vinylguacol [27,37] whose value is much higher than FA [42], which can also be a source of other valuable compounds.

For the extraction of FA and other phenolic acids from biomass has used alkaline and enzymatic hydrolysis combined with various methods of identification and quantitative analysis [43,44,45,46,47,48,49]. These are complex techniques that are not always highly selective and involve harmful substances, which pose a threat to the environment. Therefore, today, an active search continues for environmentally friendly, selective, and low-cost methods to isolate FA and other valuable chemical products from plant materials. One of such methods may be direct or catalytic pyrolysis of biomass [50,51,52,53]. The use of selective catalysts can increase its effectiveness. This will allow us to vary the products of the transformation of biomass components, their relative content, and the temperature of the transformations [52,53].

According to the literature, alumina and alumina-based materials, due to their acid–base properties, are effective catalysts for biomass conversion and upgrading bio-oil [54,55,56,57,58,59]. Therefore, in this work, catalytic pyrolysis of FA on the alumina surface was studied using the TPD MS [38,40,41,57,60,61], FT-IR, thermogravimetric analysis, and quantum chemical methods [62,63,64,65,66,67,68]. Our previous works [38,40,57,61] indicated that combining these methods allows us to identify the formed surface complexes more accurately and to track the pathways, kinetics, and products of their thermal transformations.

As a result of the study, we found that the interaction of various active groups of the acid with the alumina surface led to the formation of different carboxylate complexes with a bidentate chelate and a monodentate structure as well as phenolate complexes. Their pyrolysis led to the formation of 4-vinylguaiacol, guaiacol, hydroxybenzene, benzene, toluene, cresol, naphtalene, PACs, etc. Lignin-derived aromatic compounds and PACs can be upgraded through catalytic hydrogenation to liquid alkanes and used as biofuels. Therefore, an understanding of the mechanisms of ferulic acid pyrolysis can help to obtain more selective catalysts and carry out the catalytic conversion of biomass into biofuel more efficiently.

2. Results and Discussion

2.1. FT-IR Spectroscopy

The FA molecule has a rich vibration spectrum (Figure 1, Table 1), which is caused by the presence of an aromatic ring and several functional groups (COOH, OH, and CH₃O) in the FA structure. The analysis and attribution of absorption bands in the IR spectra of FA in pristine and adsorbed states were performed based on literature data [28,29,30,31,32,33,34,35,36,37,38,39,40,41,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85]. The symbol ν is used to denote stretching vibrations, β—in-plane deformation vibrations, and δ–deformation vibrations.

Figure 1 shows the spectra of samples with different FA content (0.1–1.2 mmol/g). The obtained spectra reveal that both the carboxyl group and the aromatic ring are involved in the interaction of FA with the oxide surface.

In particular, for FA immobilized on the oxide surface, in the region 1150–1500 cm⁻¹ one can see a number of signs of the interaction of the methoxyl group of acid with Al₂O₃. The bands 1036 and 1205 cm⁻¹ correspond to ν_s(C–O–CH₃) and ν_as(C–O–CH₃) vibrations [69,70,71,72]. These bands for FA/Al₂O₃ were shifted to 1030 and 1211 cm⁻¹, respectively. In addition, the absorption of δ(CH₃) at 1115 cm⁻¹ [69] and maximum 1178 cm⁻¹(δ(CH₃)) [69] disappear for FA/Al₂O₃, but a new band appears at 1124 cm⁻¹. A significant decrease in the band intensity is observed at 1466 cm⁻¹ (δ(CH₃)) [69,71].

A band of 1325 cm⁻¹ was not detected for the FA/Al₂O₃ sample. It can correspond to both vibrations of the β(C–O–H) carboxyl group of FA [69] and β(C–O–H)_ar, the absorption of which, as is known [73,74,75,76,77,78,79] can appear for phenol-containing compounds in the region of 1300–1400 cm⁻¹.

The formation of phenolate complexes of FA on Al₂O₃ is indicated by the disappearance in the spectra of FA/Al₂O₃ absorption at 1167 cm⁻¹ (β(O–H)_ar [69] and the band at ~1290 cm⁻¹ (ν(C–O)_ar) [69,70,71,72], (Figure 1, Table 1). At the same time, a new band appears around 1296 cm⁻¹. It is known that new bands that appeared in the phenol spectra as a result of interaction with cerium oxide [76], as well as phenol-containing compounds with transition metals [78], hydroxides [78], and metal oxides [79,80,81] indicated the formation of chemical bonds between them.

The data obtained allow us to confirm the formation of complexes of FA on the Al₂O₃ surface that bonded through the active groups of the aromatic ligand of acid. They can be formed due to the interaction with the oxide of both groups simultaneously and each group separately.

The results of FT-IR study (Figure 1, Table 1) also indicated the presence of carboxylate complexes of FA on the surface of alumina. In particular, for FA/CeO₂, the absorption for ν(C=O) at 1670 and 1691 cm⁻¹ disappear [70,72]. However, instead, there appear new carboxylate bands: at 1396 ν(CO), 1450 cm⁻¹ ν_s(COO⁻) and at 1549, ν_as(COO⁻) 1608 cm⁻¹ ν_as(COO⁻) [70,72].

It should be noted that, in the 1400–1500 cm⁻¹ region, in addition to the vibrations ν_s(COO^–), absorption of the δ(CH₃) vibrations can also occur [69], which significantly complicates the identification of the band at 1450 cm⁻¹ in the FA/Al₂O₃ spectra. However, when studying the interaction of myristic and succinic acids, as well as deuterated succinic acid-d₄ with alumina, two new bands in the 1400–1500 cm⁻¹ region were also found in their IR-spectra, which were attributed to symmetric carboxylate stretching vibrations [56]. These data confirm that the absorption at 1450 cm⁻¹ for FA/Al₂O₃ samples really attributed to the vibrations ν_s(COO⁻), (Figure 1).

The difference ∆ν = ν_as(COO⁻) − ν_s(COO⁻) or ∆ν = ν(C=O) − ν(CO) is often used to establish the coordination of COOH groups when interacting with metal ions [57,83], oxide, and hydroxide surfaces [41,57,84,85,86,87,88]. Since, ∆ν = ν_as(COO⁻) − ν_s(COO⁻) for FA/Al₂O₃ takes values 99 cm⁻¹, this indicates the presence on the surface of aluminum oxide of carboxylate complexes with a bidentate chelate structure.

We also suppose the existence of monodentate complexes on the Al₂O₃ surface, the respective ν(C=O) of which appear at 1608 cm⁻¹ [89] (∆ν = ν(C=O) − ν(CO) = 212 cm⁻¹), but the presence of intense absorption (ν(C–C/C=C)) for the aromatic ring in this region (1597 cm⁻¹) prevents good visualization of these bands. The maximum at 1684 cm⁻¹ in the spectra of this sample is a sign of the formation of weakly bound hydrogen complexes of FA [82].

In the FA/Al₂O₃ spectra, absorption at 1620 cm⁻¹ (ν(C=C) for pure FA) [69,70,72] is absent. It is likely that the vibrations ν(C=C) of the acid molecules bound to the oxide surface (0.1…0.6 mmol/g) correspond to a broad band with a maximum at 1639 cm⁻¹. This can be confirmed by studies of FA complexes with Mn, Cu, Zn, Cd, and Ca oxides [72], as well as complexes of o-coumaric [90] and p-coumaric acid [91] with alkali metals. A new band in this region, found in these complexes FT-IR spectra, was also assigned to ν(C=C) [88,90,91]. Such a shift indicates an increase in the electron density around the C=C bond of acid molecules located on the oxide surface.

With an increase in acid concentration in FA/Al₂O₃ samples (0.6–1.2 mmol/g), absorption bands characteristic of pure acid appear. This indicates a decrease in the interaction of FA with the surface and its increase between acid molecules, which leads to the formation of associates. Their presence is confirmed by the appearance of corresponding bands in the 2400–2700 cm⁻¹ region.

2.2. In-Situ FTIR Spectroscopic Study of FA Thermal Transformations over an Alumina Surface

In Figure 2, one can trace the changes in the FT-IR spectrum of the FA/Al₂O₃ sample (0.6 mmol/g) when it is heated from 20 to 450 °C. From the obtained data, it is seen that with increasing temperature the intensity of all bands gradually decreases. At 350 °C, the absorption bands of hydrogen-bound complexes of acid (1670 and 1684 cm⁻¹) and associates of FA (1690 cm⁻¹) disappear. Absorption of carboxylate complexes at 1396 cm⁻¹ (ν_s(COO⁻)) is not traced at temperature 380 °C, while bands 1450 cm⁻¹ (ν_s(COO⁻)) and in the 1541–1560 cm⁻¹ region (ν_as(COO⁻)) remain noticeable at 450 °C.

This can be explained by the fact that the complexes corresponding to these bands have a bidentate structure and are stronger than monodentate complexes. At the temperature 450 °C, we can still see bands at 1032, 1124, 1209, and 1296 cm⁻¹, which correspond to the active groups of the aromatic ring involved in bonds with the Al₂O₃ surface. This indicates the strength of the interaction of these complexes with the surface of aluminum oxide. Absorption bands that are fully or partially caused by vibrations of the aromatic ring (1278, 1433, 1516, and 1596 cm⁻¹ (ν(C–C/C=C_ar)), and vibration ν(C=C) (1634 cm⁻¹) at 450 °C have a sufficiently high intensity, which can be evidence of the formation of condensation structures on the surface of the oxide, Figure 2.

2.3. Quantum Chemical Calculation

The optimized structures along with geometrical parameters of the considered cluster are presented in Figure 3. This cluster serves as a simple model of the alumina surface. We believe that the difference in the structure of these clusters is primarily due to the different coordination numbers of aluminum atoms.

Structure 1A is characterized by the coordination numbers of aluminum equal to five and four, and, in the case of Figure 3(1B), it is equal to four only. Calculations of the Gibbs free energy indicate that the structure 1B is thermodynamically more stable than the structure drawn in Figure 3(1A). Nevertheless, we believe that both of these structures may represent specific features of a surface of alumina. Finally, the structure presented in Figure 3(1B) was used as a starting structure to study the interaction with ferulic acid.

First, we considered the interaction of a neutral molecule of ferulic acid with Al₂O₆H₆ structures. The primary subject of our interest is feasibility of the formation of intermolecular complexes between Al₂O₆H₆ that interact by the OH groups and FA that interacts by the oxygen of the carbonyl or phenolic group. We found these complexes. However, they also form a coordination bond with one of the aluminum atoms. In addition, it is also important to assess the energy parameters of the dissociation process of the carboxyl or phenolic group. This will allow us to estimate the probability of a dissociation of carboxylic and phenolic groups.

We were unable to localize any structure where FA interacts with Al₂O₆H₆ only by its OH groups. Figure 3(2A,3A) demonstrates the structures of intermolecular complexes for this system, where FA interacts with Al₂O₆H₆ through the OH group, but an aluminum atom necessarily participates in this process. The peculiarity of structure 2A is that the interaction involves the carboxyl group of FA.

A similar interaction that involves a phenolic group is presented in Figure 3(3A). Analysis of the Gibbs free energy values allow us to find the most favorable process to form complexes where the aluminum atoms are directly involved. The calculated value of the interaction energy for the thermodynamically most advantageous complex is −11.51 kcal/mol (complex 2A) and 3.99 kcal/mol (complex 3A). These are the structures that we have chosen as pre-reaction complexes for dissociation processes.

Figure 3 shows transition state (TS) structures for the dissociation of carboxyl Figure 3(2B) and phenolic Figure 3(3B) groups of FA. In addition, Figure 3(2C,3C) shows the products of this reaction. In both cases, water molecules, formed in the course of the reaction, continue to be held in the coordination sphere of the aluminum atom. This is demonstrated by the values of interatomic distances (1882 Å for 2C and 2029 for 3C) and by the bond orders between aluminum and water oxygen (0.4249 for 2C and 0.5789 for 3C).

The values of the activation energies (see Table 2) indicate a high probability of the process of dissociation of carboxyl and phenolic groups in this system. In addition, it should be noted that, in both considered cases, these processes are exothermic. This is another argument to expect that the considered mechanism of adsorption is realized in experimental conditions.

2.4. Thermogravimetric Analysis

Thermogravimetric analysis showed that the thermal decomposition of this sample occurs in three main steps, Figure 4. The first stage lasts from room temperature to 120 °C, the second for 120–270 °C, and the third (T_max~375 °C) for 270–550 °C. The maximum rate of weight loss occurs in the third stage at 370 °C (DTG-curve). Analysis of the thermogravimetric data (Table 3) showed that 85% of the FA was converted into volatile products and 15% was converted into char on the surface. According to the DTA-curve, all three stages of decomposition are exothermic.

2.5. TPD MS Study of FA Catalytic Pyrolysis

The results of the TPD MS study of the decomposition of FA on the CeO2 surface, in particular, the curve of pressure versus temperature (p = f(T)), thermograms of the main decomposition products of the FA/Al₂O₃ sample and pure oxide, as well as the corresponding mass spectra shown in Figure 5 and Figure 6. Analysis of the p = f(T) curve (Figure 5A), mass spectra, and TPD-curves of the pyrolysis products (Figure 5B and Figure 6) shows that the releasing of volatile products of FA decomposition occurs in several stages: 60, 155, 240, and 408 °C.

The main gaseous products produced during pyrolysis of FA are CO (m/z 28), CO₂ (m/z 44), CH₃OH (m/z 32, 31) and H₂O (m/z 18) (Figure 5 and Figure 6 and Table 4). Releasing these compounds reduces the oxygen content and, thus, increases the final product’s caloric content. The processes of dehydration, decarboxylation, decarbonylation, and demethoxylation are essential for biomass catalytic conversion technologies.

Interestingly, the FA molecular ion peak with m/z 194 and the peaks of its most intense fragment ions (m/z 179, 133, and 195) are absent in the mass spectra of the gaseous products of the catalytic pyrolysis of ferulic acid in the temperature range 20–700 °C, [92]. According to the NIST database [92] in the electron ionization spectrum of ferulic acid, the following ions are present: m/z 194 (100%), m/z 179 (~20%), m/z 133 (~18%), m/z 77 (~13%), m/z 195 (~11%), m/z 51 (~11%), m/z 105 (~8%), m/z 107 (~7%), m/z 151 (~6%), m/z 134 (~6%), m/z 161 (~4%), m/z 150 (~3%), m/z 109 (~3%), m/z 135 (~2%), m/z 94 (~2%), m/z 44 (~2%), and m/z 162 (~1%).

This fact suggests that ferulic acid is not desorbed from the catalyst surface in molecular form but undergoes a number of catalytic transformations with the formation of several chemical products, as will be shown in the course of further discussion. In the mass spectra of the products of direct pyrolysis of FA without a catalyst, the molecular ion and its characteristic fragment ions are absent. As previously established in a study of the direct pyrolysis of ferulic acid [37], on the first pyrolysis stage (150 °C), the formation of some oligomeric structures occurred, which was accompanied by partial decarboxylation (m/z 44).

The decomposition and decarboxylation of oligomeric structures with the formation of 4-vinylguaiacol (m/z 150) and CO₂ (m/z 44) were observed only during the second stage (350–700 °C) [37]. The most intense ions are molecular ions of decarboxylation products with m/z 150 (4-viniguaiacol) and m/z 44 (CO₂.). On the other hand, in the FA electron impact mass spectrum, the intensity of ions with m/z 150 and 44, which correspond to the decarboxylation channel of the FA molecular ion, is only 4–3% [92]. The most intense channel of FA ion-molecular reactions is demethylation. The intensity of the ion with m/z 179, formed via the release of the methyl group, is about ~20% [92].

In addition to decarboxylation, the catalytic action of alumina triggers other reaction channels leading to the formation of other valuable products. The FA decomposition on the Al₂O₃ surface is more similar to the decomposition of caffeic (CA) and ferulic acids on the nanoceria surface [41,93]. The decarboxylation of the FA begins almost at 70 °C. In this case, the formation of 4-vinylguaiacol (M.r. = 150 Da, m/z 150) occurs in a wide temperature range of 70–300 °C, (Figure 6A,B).

According to the NIST database [92] in the electron ionization spectrum of 4-vinylguaiacol, the following fragment pattern is present: m/z 135 (100%), 150 (~98%), 107 (~68%), and 77 (~70%). The TPD profiles of the peaks corresponding to 4-vinylguaiacol generation (m/z 150 and 135) display the same shapes and T_max, (Figure 6A). In addition, we observed a very similar ratio for the 4-vinylguaiacol fragment ion intensities: m/z 135 (100%), 150 (~95%), 107 (~70%), and 77 (~80%) (see Figure 6B). Likely, the TPD-peak at ~160 °C for the molecular ion of 4-vinyl guaiacol is the result of decarboxylation of two types of FA complexes in a very close temperature range (Scheme 1 and Scheme 2).

Deconvolution of the TPD curve for the molecular ion of 4-vinylguaiacol with m/z 150 into separate Gaussians (Figure 7) made it possible to determine the temperatures of the maximum decomposition rate of these complexes (T_max = 148 and T_max = 197 °C) with the formation of 4-vinylguaiacol. The presence of monodentate bound complexes and bidentate chelate complexes was confirmed by the FT–IR data (Figure 1 and Figure 2, Table 1) and analysis of the differences ∆ν = ν_as(COO⁻) − ν_s(COO⁻) = 99 cm⁻¹ and ∆ν = ν(C=O) − ν(CO) = 212 cm⁻¹ as well as by quantum chemical calculations (Figure 3).

Likely, the monodentate complexes decompose at a lower temperature (T_max ≈ 148 °C) according to Scheme 1 because they are the less strongly bound complexes. In contrast, the more stable bidentate chelate complexes decompose at a higher temperature (T_max ≈ 197 °C), according to Scheme 2. The formation of 4-vinylguaiacol is similar to the formation of 3,4-dihydroxyphenylethylene during the decomposition of various types of carboxylate complexes of caffeic acid on the surfaces of SiO₂ and CeO₂ oxides [40,41]. It should be noted that the formation of 4-vinylguaiacol on the surface of alumina occurs at significantly lower temperatures (T_max ≈ 148 and 197 °C) than on the silica surface (T_max ≈ 408 °C) [38], as well as during the decomposition of FA in pristine state (>350 °C) [37].

Interestingly, parallel to the formation of 4-vinylguaiacol, the formation of product with m/z 164 is observed, Figure 8. We identified it as 4-vinylmethylguaiacol (M.r. = 164 Da, m/z 164, 149, 138, and 121) based on the presence of peaks on the TPD curves for its fragment ions with m/z 149, 138, and 121. These peaks have the same shape and localization as the molecular ion peak. We presented the possible pathways for the fragmentation of the molecular ion of methylvinylguaiacol in Scheme 3.

The calculated kinetic parameters for these TPD peaks showed close values, Table 4. Thus, the pre-exponential factor has one order of 10⁶ (s⁻¹), which indicates that this process proceeds through a highly ordered transition state. The formation of this product is likely due to the methylation of some part of the carboxylate bidentate chelate complexes on the catalyst surface, Scheme 3.

In contrast to the thermal transformations of the FA in the condensed state and on the SiO₂ surface, products, such as guaiacol (M.r. = 124 Da, m/z 124, T_max ≈ 234 °C) and hydroxybenzene (M.r. = 94 Da, m/z 94, T_max ≈ 409 °C), were recorded during the FA pyrolysis on the alumina surface (Figure 6C,D). In our opinion, their release is due to the transformation of FA complexes, which are formed as a result of the interaction of HO- and CH₃O- groups of the aromatic ring with an alumina surface.

The presence of such complexes on the surface is confirmed by corresponding absorption bands in the FT-IR spectra, Figure 1 and Figure 2, Table 1. Moreover, quantum-chemical calculations have shown that the formation of complexes with the participation of HO and CH₃O groups of the aromatic ring is exothermic, an energetically favorable process, Table 3, Figure 3(3A–C). Moreover, both the phenolic hydroxyl group and the methoxy group can be involved in binding to the surface, Figure 3(3C). Likely, guaiacol is a decomposition product of the complexes formed by the interaction of one functional group of an aromatic ring with an oxide (Scheme 4).

The mechanism shown in Scheme 4 can be confirmed by our previous results of caffeic acid pyrolysis on the nanoceria surface [41]. According to [41], pyrocatechol (m/z 110) was released in the same temperature range (T_max~240 °C) during caffeic acid pyrolysis and, like guaiacol, was the result of decomposition of caffeic acid complexes formed due to the interaction of the OH_ar group with oxide. The close formation temperatures of guaiacol and pyrocatechol are explained by the similar structure of the complexes.

The thermal transformation of the surface complexes formed on alumina with the participation of two functional groups of aromatic rings (Scheme 5) leads to the release of hydroxybenzene (M.r. = 94 Da, m/z 94, T_max ≈ 409 °C), (Figure 6C,D, Table 4). This process occurs at temperatures close to the pyrolysis temperature of similar caffeic acid complexes on the nanoceria surface (~390 °C) [41].

At the same time, the decomposition of FA on the alumina surface, as well as the caffeic acid on the nanoceria surface [41], is likely accompanied by the alkylation of these surfaces. This further leads to cresol formation (M.r. = 108 Da, m/z 108, 107, T_max ≈ 375 °C) (Figure 6C,D). The formation of cresol is likely due to the alkylation of some part of the phenolate complexes of ferulic acid, Scheme 5, Table 4.

In addition, desorption of aromatic and polycyclic aromatic hydrocarbons (PAHs) was detected during catalytic pyrolysis, in particular, naphthalene (m/z 128, T_max ≈ 432 °C), methylnaphthalene (C₁₁H₉, m/z 141, T_max ≈ 409 °C), toluene (C₇H₈, m/z 92, 91, T_max ≈ 430 °C), benzene (C₆H₆, m/z 78, T_max ≈ 450 °C), and indene (C₉H₇, m/z 115, T_max ≈ 422 °C) (Figure 9). Likely, PHCs are formed as a result of the transformation of the complexes bound through the phenolic groups of the aromatic ring because their formation was observed during catalytic pyrolysis only for cinnamic acids [39,40,41] and coumarins [94] with the phenolic groups in the benzene ring.

The formation of aromatic and PAHs proceed with negative values of the change in the entropy of activation, Table 4. This indicates that these processes proceed through highly ordered transition states. This is likely due to the destruction of the formed polyaromatic coating on the catalyst surface. The existence of the polyaromatic coating is also evidenced by vibrations of the aromatic ring (1278, 1433, 1516, and 1596 cm⁻¹ (ν(C–C/C=C_ar)), and vibration ν(C=C) (1634 cm⁻¹) in the FT-IR spectra obtained at 430–450 °C (Figure 2).

The poisoning of the catalysts during the production of bio-oil and during its catalytic upgrading into biofuel is likely due to the formation of phenolate complexes. Since their subsequent thermal transformation leads to the formation of a polyaromatic coating on the surface, as a result, this leads to blocking the catalyst’s active sites and a drop in its catalytic activity. The alumina sample turned black after pyrolysis, likely due to a carbon coating formed on its surface.

We performed the regeneration procedure of the alumina sample by classical calcination in air at 500 °C for two hours. This procedure made it possible to restore the original snow-white color of the sample, and this fact suggests the possibility of fairly easy regeneration of the catalyst. In addition, based on the results obtained by Sharanda et al. [54], it can be assumed that the regeneration temperature of about 500 °C can be optimal since the expected number of base active sites should be about ~3.0 × 10¹⁸ m⁻², and the expected number of acid active sites should be about ~3.5 × 10¹⁸ m⁻² as at the same temperature of pretreatment 500 °C [54].

We carried out a TPD MS study of FA pyrolysis on the surface of a regenerated alumina sample, Figure 10. Analysis of mass spectra and TPD curves showed that the formation of the same products (vinyl guaiacol, guaiacol, and aromatic products) is observed during pyrolysis over the regenerated alumina, as in pyrolysis using the initial sample, Figure 10B,C.

However, establishing the optimal conditions for regeneration and their effect on the catalytic activity of the regenerated catalyst requires deep future research. Such studies are of great practical interest since they make it possible to establish the possibility of reusing the catalysts [95,96,97]. Regeneration and reusing of the catalysts could obtain a great economic effect.

3. Materials and Methods

3.1. Materials

Ferulic acid (≥98%) was purchased from Alfa Aesar, Karlsruhe, Germany. Nonporous fumed alumina γ-Al₂O₃ (S_a = 111 m²g⁻¹, d_a~40 nm) was supplied by Degussa AG (Essen, Germany). This pyrogenic oxide was synthesized by hydrolysis of its chloride in a hydrogen–oxygen flame. Its acid-base properties were studied by Sharanda et al. [54] to examine changes in the number of acid and base sites depending on the pre-treatment temperature. It was obtained that the number of acid and base sites are 2.74 and 1.56 × 10¹⁸ m⁻², respectively, for the non-treated sample.

Moreover, the number of active centers changes nonlinearly with an increase in the pre-treatment temperature. The number of base sites increases to ~3.6 × 10¹⁸ m⁻² at a pre-treatment temperature of 900 °C. The maximum number of acid sites ~3.5 × 10¹⁸ m⁻² is observed at a treatment temperature of about 500 °C. The presence of acidic and basic sites is confirmed by the activity of this alumina sample in ketonization and ketenization reactions of valeric acid [57]. It is known that ketenization is catalyzed by acid sites and ketonization by basic sites [57,98].

In this work, nanoscale alumina was calcined at 450 °C for two hours to remove organic impurities. According to the data of [54], such temperature treatment should have led to an increase in the number of active sites on the surface to ~3.4 × 10¹⁸ m⁻² for acid sites and up to ~3.0 × 10¹⁸ m⁻² for basic sites. This calcined sample was used for FA loading. After the pyrolysis of FA from 20 before 500 °C under TPD MS condition, the alumina sample was regenerated at 500 °C for two hours in the air for the oxidation of surface coke. This regenerated sample was used for loading FA (0.6 mmol/g) and studying its catalytic activity by TPD MS.

3.2. Loading of FA on the Alumina Surface

A series of samples FA/Al₂O₃ with concentrations of FA of 0.1, 0.3, 0.6, 0.9, and 1.2 mmol/g were prepared by mixing 100 mg of Al₂O₃ with 2 mL of FA solutions in ethanol. The suspensions were stirred for several minutes and then dried at room temperature in the air.

3.3. FT-IR Spectroscopic Studies

In situ FT-IR spectroscopy was recorded on a Thermo Nicolet Nexus Fourier-transform IR spectrometer (Thermo Nicolet Corporation, Madison, WI, USA), using a “Nexus Smart Collector” in diffuse reflection mode. The number of scans was set at 50 with a resolution of ±4 cm⁻¹, over the range of 4000–400 cm⁻¹. The scan velocity was 0.5 cm/s. The samples Al₂O₃ and FA/Al₂O₃ were mixed with KBr in the ratio 1:10. Pure FA was mixed with KBr at a ratio of 1:100. KBr was pre-calcined at 500 °C for 2 h. The IR spectra of all FA/Al₂O₃ samples (0.1–1.2 mmol/g) were obtained and presented in this work. This series of concentrations makes it possible to detect bands of carboxylate complexes, which, for higher FA concentrations (0.9–1.2 mmol/g), are lost behind intense absorption bands that are characteristic of pure acid.

3.4. TPD MS Study

TPD MS measurements were performed on a mass spectrometer MX-7304A (Electron, Sumy, Ukraine) with electron ionization, adapted to study catalytic and direct pyrolysis kinetics [57,60,61,94]. The procedure of obtaining the kinetic parameters (temperature of the maximum desorption rate T_max, reaction order n, activation energy E^≠, pre-exponential factor ν₀, and change of activation entropy ∆S^≠) from TPD-MS by using the classic Arrhenius plot method was described previously in our work [57,94].

The test sample, weighing ~20 mg, was placed in a quartz molybdenum ampoule and pumped at room temperature to ~5 × 10⁻⁵ Pa pressure. The programmable linear heating of the sample was performed from 20 to ~750 °C with a heating rate of 0.17 °C/s. Volatile pyrolysis products entered the mass spectrometer chamber through a high-vacuum valve, where they were ionized and fragmented under the influence of electron impact. The ionic current of the products of desorption and pyrolysis was recorded with a VEU-6 secondary electron multiplier.

Mass spectra were recorded and analyzed using an in-house developed computerized data acquisition and processing unit. The range of the studied masses was 1–210 Da. The number of mass spectra recorded during the TD MS experiment reached 240. The slow heating of the sample and the high rate of removal of volatile pyrolysis products made it possible to neglect the effects of diffusion. Therefore, we propose that the intensity of the ion current was proportional to the rate of desorption.

3.5. Thermogravimetric Analysis

Thermogravimetric analyses were performed using a TGA/DTA analyzer (Q-1500D, Budapest, Hungary). Samples weighing 100 mg were heated from room temperature to 1000 °C. The heating rate was 10 °C/min in an air atmosphere.

3.6. Density Functional Theory (DFT) Calculations

Density Functional Theory (DFT) calculations were performed using the wB97XD functional having Grimme’s D2 dispersion corrections [62], which was conjugated with a 6-311++G(d, p) basis set. A Gaussian 09 set of software was used [63] Full optimization of geometry was performed for the studied molecular systems.

The total charge of the system was equal to zero. Transition-state points were localized using the Berny algorithm and GEDIIS procedure [64] in redundant internal coordinates [65]. A Hessian matrix was obtained for each optimized structure. It was confirmed that the Hessian matrix of any local minima contained only positive eigenvalues, whereas the transition states were identified by the presence of one negative eigenvalue.

In addition, the structure of the transition-state vector was analyzed. This structure confirms whether the given TS belongs to the studied reaction channel. The results of the calculations (geometrical structures and harmonic frequencies) were visualized using the molecular graphics program MaSK v. 1.3.0 (Jackson, MS, USA) [66]. Natural Bond Orbital analysis (NBO) (NBO version 3.1 [67] was performed to obtain bond order values.

All considered processes of interaction or dissociation ferulic acid (FA) include just one chemical transformation step. The activation Gibbs free energies are calculated as a difference between the Gibbs free energies of pre-reaction complexes and corresponding TS at 298.15 K.

The cluster having the composition Al₂O₆H₆ was used as the simplest model of aluminum oxide. The basis set superposition error (BSSE) was computed using the counterpoise method [68].

4. Conclusions

Based on quantum chemical calculations, in situ FT-IR spectroscopic, and TPD MS studies, it has been established that both the carboxyl group and the active groups (OH and OCH₃) of the aromatic ring are involved in the interaction of FA with alumina surface. Moreover, quantum chemical calculations showed that the formations of both carboxylate and phenolate complexes are exothermic processes.

The structures of surface complexes of ferulic acid, temperature ranges, and products of their decomposition were established. The kinetic parameters were calculated for some product formations (guaiacol, phenol, cresol, naphthalene, methylnaphthalene, toluene, benzene, etc.), and their formation’s probable mechanisms were proposed. It turned out that the catalytic pyrolysis processes of ferulic acid are characterized by negative changes in the entropy of activation. This indicates that thermal transformations proceed through highly ordered transition states on the catalyst surface.

Establishing the structure of surface complexes responsible for the catalytic conversion of bio-oil components into desired products with high added value and biofuels is extremely important for developing technologies for catalytic conversion of biomass.

It was found that the main product of the decomposition of carboxylate complexes is 4-vinyl guaiacol, which is a promising monomer for the preparation of new types of polymer materials. The process of catalytic decarboxylation of ferulic acid with the formation of 4-vinyl guaiacol proceeds at a much lower temperature (T_max = 148 and 197 °C) than in the direct pyrolysis of ferulic acid (>350 °C) [37].

Catalytic transformations of phenolate complexes occur with the formation of hydroxybenzene and guaiacol. In addition, we assume that phenolate complexes are responsible for forming a polyaromatic coating on the catalyst surface. Decomposition of this coating leads to the formation and desorption of aromatic compounds. These aromatic compounds could be upgraded through catalytic hydrogenation to liquid alkanes and used as biofuels.

During the catalytic pyrolysis of ferulic acid, trans-methylation processes were observed. Trans-methylation is likely associated with the processes of demethoxylation of the benzene ring of ferulic acid. This process is most intense at ~300 °C in the range 120–360 °C with the formation of methanol. A certain part of carboxylate and phenolate complexes undergo methylation on the surface of aluminum oxide. After that, they are transformed with the formation of methylated products (methylated 4-vinylguaiacol and cresol). Their pyrolysis occurs in temperature ranges that are close to those for the corresponding un-methylated products.

Author Contributions

Conceptualization, T.K. and M.L.; methodology, T.K., N.N., M.I. and B.P.; investigation, N.N., B.P., M.I. and T.K.; resources, T.K.; writing—original draft preparation, T.K. and N.N.; writing—review and editing, T.K., N.N. and M.L.; visualization, N.N., T.K. and B.P.; supervision, T.K. and M.L.; project administration, T.K.; 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 the grant FSA3-20-66700 from the U.S. 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”).

Acknowledgments

This work was performed using computational facilities of the joint computer cluster of SSI “Institute for Single Crystals” of the National Academy of Sciences of Ukraine and Institute for Scintillation Materials of the National Academy of Sciences of Ukraine incorporated into the Ukrainian National Grid.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures, Schemes and Tables

Figure 1. Fourier transform-infrared (FT-IR) spectra of pure Al2O3 (a), pure FA (b) and samples of FA/Al2O3 with different contents of FA (0.1—c; 0.3—d; 0.6—e; 0.9—f; and 1.2 mmol/g—g).

Figure 2. FT-IR spectra of pure Al2O3, pure FA and sample of FA/Al2O3 (0.6 mmol/g) at different temperatures (from 20 to 450 °C).

Figure 3. Geometrical structures of Al2O6H6 cluster (1A,1B). Structure of intermolecular complex (2A), transition state (2B), and reaction product (2C) for the interaction of ferulic acid with Al2O6H6 through the carboxyl group. Structure of intermolecular complex (3A), transition state (3B), and reaction product (3C) for the interaction of ferulic acid with Al2O6H6 through the phenolic group. The interatomic distances are given in Angstroms.

Figure 4. Differential thermal analysis (DTA), differential thermogravimetric analysis (DTG), and thermogravimetric (TG) curves for FA/Al2O3.

Figure 5. (A) Vapor pressure of pyrolysis products measured as a function of temperature. (B) TPD-curve for the ions with m/z 44 (CO2), 32 (MeOH), 31 (MeOH), 28 (CO), and 18 (H2O) for the FA/Al2O3 sample. (C) TPD-curve for molecular and fragment ions the ions with m/z 44 (CO2), 28 (CO, CO2), and 17 (H2O) for the Al2O3.

Figure 6. TPD curves for ions with m/z 164, 150, and 135 (A); mass spectra at 221 °C (B); TPD curves for ions with m/z 94, 108, 124, 128, and 164 (C); and mass spectra at 410 °C (D).

Scheme 1. The possible way of 4-vinylguaiacol formation during decomposition of the monodentate complexes over an alumina surface.

Scheme 2. The possible way of 4-vinylguaiacol formation during decomposition of the bidentate chelate carboxylate over an alumina surface.

Figure 7. Deconvolution of the TPD-curve for the molecular ion of 4-vinylguaiacol with m/z 150 into separate Gaussians.

Figure 8. TPD-curves of molecular ion of methylated 4-vinylguaiacol and its fragment ions with m/z 164, 149, 138, and 121.

Scheme 3. The possible way of methylated 4-vinylguaiacol formation.

Scheme 4. The possible way of guaiacol formation over an alumina surface.

Scheme 5. The possible way of hydroxybenzene and cresol formation over an alumina surface.

Figure 9. TPD-curves for the molecular and/or fragment ions of aromatics with m/z 78 (benzene), 91 (tropylium ion, C7H7+), 115 (indene), 128 (naphtalene), and 141 (methylnaphtalene).

Figure 10. (A) Vapor pressure of pyrolysis products measured as a function of temperature. (B) TPD-curve for the ions with m/z 44 (CO2), 32 (MeOH), 31 (MeOH), 28 (CO), and 18 (H2O) for the FA/regenerated-Al2O3 sample. (C) Mass spectra at 220 °C.

Table 1

Assignments of some characteristic infrared bands of pure FA and of FA/Al₂O₃ (0.6 mmol/g).

Assignments	Frequency (cm−1)		Ref.
	FA	FA/Al2O3
δ (CH3)	1115	–	[69]
δ (CH3)	–	1124
δ (CH3)	1178	–	[69]
δ (CH3)	1379	1379	[69]
δ(CH3)	1466	1464	[69]
νs(COCH3)	1036	1030	[69,70,71,72]
νas(COCH3)	1205	1211	[69,70]
β(OHap)	1167	–	[69]
β(CHap)	1155	1159	[69]
ν(COap)	1290	–	[69,70]
ν(CO–)	–	1296	-
ν(CCap)	1518	1518	[69,72]
	1601	1597	[69,72]
ν(C=C)	1620	1639	[69,72]
ν(C=O)	1666	–	[69,70,71,72]
ν(C=O)	1691	1691 *	[69,70,71,72]
ν(C=O)	–	1670	[69,70,71,72]
ν(C=O)	–	1684 **	[69,70,71,72]
ν(CO)	–	1396	[69]
νs(COO–)	–	1450	[56]
νas(COO–)	–	1549	[70,71,72]
ν(C=O)	–	1608	[89]

* FA/Al₂O₃ (1.2–0.9 mmol/g), ** for FA/Al₂O₃ (0.3–0.6 mmol/g).

Table 2

The values of enthalpy (H₂₉₈) and Gibbs free energy (G₂₉₈) of the process of FA dissociation (see Figure 3).

Structure	H298, a.u.	G298, a.u.	∆G, kcal/mol
2A	−1627.939549	−1628.024516	0.0
2B	−1627.940484	−1628.023856	0.41 *
2C	−1627.937973	−1628.022237	0.99 **
3A	−1627.911185	−1628.000011	0.0
3B	−1627.903850	−1627.988411	7.28 *
3C	−1627.931414	−1628.014775	−9.26 **

* Values of activation energy; ** Values of thermal effect of reaction.

Table 3

Pyrolysis yields obtained during thermogravimetric analysis of the sample FA/Al₂O₃ (0.6 mmol/g).

Stage	Tmax, °C	Volatiles (%)	Char (%)
I	80	6.8
II	210	20.6
III	360	57.6
Σ(I + II + III)		85	15

Table 4

Kinetic parameters (temperature of the maximum desorption rate T_max, reaction order n, activation energy E^≠, pre-exponential factor ν₀, and activation entropy ∆S^≠), temperature range (T_range) of formation and peak intensities (I) of catalytic processes/reactions during FA pyrolysis over an alumina catalyst.

Product or Its Fragment Ion	Scheme	m/z	I, a.u.	Trange, °C	Tmax, °C	n	E≠, kJ/mol	ν0, s−1	∆S≠, cal/(K × mol)	R2 *
Decarboxylation
CO2	-	44	3.1	~70–250	155	-	-	-	-	-
	-	44	4.2	~100–400	240	-	-	-	-	-
Demethoxylation
MeOH	-	31	0.739	~220–390	~300	1	88	3.64 × 106	−29	0.936
	-	32	0.583	~220–390	~300	1	85	1.75 × 106	−31	0.952
Decarbonylation
CO	-	28	1.4	~100–220	160	-	-	-	-	-
	-	28	1.5	~180–300	235	-	-	-	-	-
	-	28	1.0	~300–500	400	-	-	-	-	-
Dehydration
H2O	-	18	9.3	~20–200	100	-	-	-	-	-
	-	18	4.2	~180–350	265	-	-	-	-	-
	-	18	4.1	~300–600	410	-	-	-	-	-
Decomposition of carboxylate complexes
4-Vinylguaiacol	1	150	~1.0	~70–270	~148	-	-	-	-	-
4-Vinylguaiacol	2	150	~0.7	~100–300	~197	-	-	-	-	-
Decomposition of carboxylate complexes with desorption of methylated products
4-Vinyl-methylguaiacol	3	164	0.077	164–324	240	1	84	1.12 × 106	−32	0.943
		138	0.040	183–292	234	1	85	2.11 × 106	−31	0.932
		121	0.054	180–292	250	1	90	5.91 × 106	−29	0.963
Decomposition of phenolate complexes
Guaiacol	4	124	0.057	160–295	234	1	106	3.48 × 108	−21	0.961
Cresol	5	108	0.057	317–453	375	1	140	6.41 × 108	−20	0.959
Phenol	5	94	0.082	301–491	409	1	135	8.14 × 107	−24	0.961
Desorption of aromatic/polycyclic hydrocarbons
Naphthalene	-	128	0.060	333–530	432	1	125	4.77 × 106	−30	0.94
Methylnaphthalene	-	142	0.038	326–486	409	1	99	1.10 × 105	−37	0.97
Indene	-	115	0.084	349–487	422	1	177	9.22 × 1010	−10	0.932
Benzene	-	78	0.268	353–536	447	1	129	5.32 × 106	−29	0.943
Toluene	-	92	0.053	350–490	428	1	151	7.65 × 108	−19	0.959
Tropylium ion, C7H7+	-	91	0.130	100–570	~170~230~430	-	-	-	-	-
			0.125
			0.07

* R²—Coefficient of determination.

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