Effect of Mesopore Structural Parameters in Alumina Supports on Catalytic Hydrodeoxygenation of Guaiacol to Cycloalkanes via Ni-Supported Al₂O₃ Catalysts

Abstract

The elevated oxygen content in lignin-derived oil restricts its direct application as a liquid fuel. Ni-based Al₂O₃ catalysts are commonly employed to enhance the quality of lignin-derived oil via the hydrodeoxygenation (HDO) process. In this study, we successfully synthesized Ni-supported Al₂O₃ catalysts with diverse mesopore structural parameters of the Al₂O₃ support. Subsequently, we investigated the impacts of mesoporous size and volume on the HDO of guaiacol, a representative model compound of lignin-derived oil. The results indicate that optimizing the mesoporous size can enhance catalyst stability and significantly boost selectivity for cyclohexane. Moreover, an increase in the mesoporous volume can further improve the selectivity of cycloalkanes in the products. When the Ni/meso-Al₂O₃-F-100 catalyst was utilized, the cycloalkane selectivity reached 98.8%. During the upgrading of lignin-derived oil, the Ni/meso-Al₂O₃-F-200 catalyst, featuring a mesopore size of 4.07 nm and a mesopore volume of 0.286 cm³/g, exhibited outstanding performance. Notably, its selectivity for alkanes reached 65.9%, significantly higher than that of the commercial Ni/c-Al₂O₃ catalyst, which has a mesopore size of 3.83 nm and a mesopore volume of 0.184 cm³/g. This work offers valuable insights into the design of efficient and stable Ni-based Al₂O₃ catalysts for upgrading lignin-derived oil.

1. Introduction

With the depletion of conventional fossil energy sources and the severe environmental issues caused by greenhouse gas emissions from fossil fuel combustion, renewable and environmentally friendly energy sources have garnered increasing attention [1,2,3]. As one of the most abundant energy resources, biomass is widely available on Earth, with a continuous annual supply through photosynthesis, making it highly cost-effective [4,5]. Lignocellulose, the primary component of biomass, consists of lignin, cellulose, and hemicellulose [6,7]. Among these, lignin possesses a unique aromatic structure, granting it significant potential for producing liquid fuels and high-value six-membered ring chemicals. Lignin is a three-dimensional amorphous polymer with relatively low energy density. Through pyrolysis [8,9], solid lignin can be converted into liquid products. However, the pyrolysis products of lignin are complex and predominantly composed of various phenolic compounds. These compounds exhibit high oxygen content, high viscosity, poor stability, and strong corrosiveness [10], making them unsuitable for direct use as liquid fuels.

Hydrodeoxygenation (HDO) is an effective method for converting lignin-derived phenolic compounds into high-quality liquid fuels [11,12,13]. Bifunctional metal-solid acid catalysts are commonly employed in the hydrodeoxygenation of oxygen-containing fuel precursors derived from fossil fuels and biomass to produce hydrocarbon fuels [14]. This is attributed to the ability of metal active sites to adsorb and activate C–O bonds, while acid sites facilitate dehydration [15], significantly enhancing the efficiency of HDO reactions. Common active metals include noble metals (Ru, Pd, Pt) and non-noble metals (Ni, Mo, Co). Although noble metals exhibit high catalytic activity, their high cost limits their large-scale industrial application [16]. Among non-noble metals, Ni is widely used as the metal component in HDO catalysts for phenolic compounds due to its low cost and excellent hydrogenation capability [17,18]. Solid acids primarily include oxides, sulfides, metal salts, and zeolite molecular sieves. Common oxides include Al₂O₃, ZrO₂, TiO₂, and SiO₂. Among them, Al₂O₃ possesses relatively strong acidity and is frequently used as a catalyst support [19,20]. Ni-based Al₂O₃ catalysts have been widely applied in upgrading lignin-derived oxygenated compounds to cycloalkane fuels. For instance, Tang et al. prepared a Ni/α-Al₂O₃ catalyst via the impregnation method for the HDO of anisole, achieving an anisole conversion rate of 93.25% and a hydrocarbon yield of 90.47% [21]. However, Povov et al. found that, during the HDO of bio-oil, the surface of Al₂O₃ was prone to carbonate accumulation, which subsequently transformed into coke, thereby reducing catalyst activity [22].

To address this issue, researchers have introduced additional components into Al₂O₃ supports to form composite supports, altering their physicochemical properties. Shu et al. doped SiO₂ into the Al₂O₃ support to enhance Ni dispersion, achieving complete conversion of eugenol at 200 °C with a cyclohexane selectivity of 97.8% [23]. After three cycles, the catalyst exhibited only a slight loss of catalytic activity. More recently, Jiang et al. incorporated carbon into the support to dilute the acid concentration, preparing a Ni/Al₂O₃-C-0.25 catalyst that enabled complete conversion of guaiacol at 220 °C, significantly improving the selectivity for cyclohexane [24]. While these studies have enhanced the coke resistance of catalysts by modifying the support structure, the impact of Al₂O₃ support pore properties on the HDO of lignin-derived phenols remains unclear.

In this work, mesoporous Al₂O₃ supports with different pore sizes and volumes were successfully synthesized using a soft-template hydrothermal method by introducing various organic cationic surfactants and adjusting their dosages. Ni-supported Al₂O₃ catalysts were then prepared via the impregnation method. Guaiacol is a typical oxygen-containing aromatic compound in lignin pyrolysis oil, containing a benzene ring, methoxy group and phenolic hydroxyl group. Studying its HDO process can directly simulate the key reactions in the deoxygenation upgrading of bio-oil. Using guaiacol as a model compound, we investigated the effects of mesoporous Al₂O₃ support pore diameter and porosity on the HDO reaction mechanism and catalyst stability. Additionally, the application of Ni-supported Al₂O₃ catalysts in the HDO of other lignin-derived phenolic compounds and raw lignin oil was explored, providing theoretical guidance for the precise design of highly active and stable Ni-based Al₂O₃ catalysts for industrial HDO of phenolic compounds.

2. Materials and Methods

2.1. Materials

Guaiacol (GC, >99.0%), n-decane (98%), F127 (average M_n ~15,000), P123 (average M_n ~5800) and commercial γ-alumina (≥92%) were purchased from Shanghai Macklin Biochemical Co., Ltd., Shanghai, China. Ni(CH₃COO)₂·4H₂O (99.9%), Al(NO₃)₃·9H₂O (AR, 99.0%), and ammonia solution (25–28%) were obtained from Meryer (Shanghai) Chemical Technology Co., Ltd., Shanghai, China. Anisole (99%), o-cresol (99%), eugenol (99%), and vanillin (99%) were supplied by Shanghai Adamas Reagent Co., Ltd., Shanghai, China. The raw lignin-oil was obtained from the rice husk pyrolysis in a tube furnace.

2.2. Catalyst Preparation

2.2.1. Preparation of Meso-Al₂O₃ with Different Mesoporous Sizes

Dissolve 20.0 g of Al(NO₃)₃·9H₂O in 100 mL of deionized water under stirring until completely dissolved. Subsequently, slowly introduce the templating agent (None/F127/P123) into the solution (the molar ratio of templating agent/Al(NO₃)₃·9H₂O is 1/200) and stir at 45 °C for 48 h. Gradually add concentrated ammonia solution dropwise to the above solution until the pH reaches 8, followed by stirring for 5 min. Transfer the resulting solution into a polytetrafluoroethylene (PTFE)-lined autoclave and react at 100 °C for 24 h. After filtration, wash the obtained precipitate three times with deionized water and dry it overnight at 100 °C. Once dried, grind the sample into a fine powder. The sample is then calcined at 500 °C for 5 h with a heating rate of 5 °C/min, yielding the meso-Al₂O₃, meso-Al₂O₃-F-200 and meso-Al₂O₃-P-200 supports.

2.2.2. Preparation of Meso-Al₂O₃-F with Different Mesoporous Volumes

Similarly, dissolve 20.0 g of Al(NO₃)₃·9H₂O in 100 mL of deionized water under stirring until completely dissolved. Subsequently, introduce the F127 templating agent into the solution at different molar ratios of F127/ Al(NO₃)₃·9H₂O (1:25, 1:50, 1:100, 1:200, 1:400) and stir at 45 °C for 48 h. Gradually add concentrated ammonia solution dropwise until the pH reaches 8, followed by stirring for 5 min. Transfer the resulting solution into a PTFE-lined autoclave and react at 100 °C for 24 h. After filtration, wash the obtained precipitate three times with deionized water and dry it overnight at 100 °C. Once dried, grind the sample into a fine powder. The sample is then calcined at 500 °C for 5 h with a heating rate of 5 °C/min. The samples were denoted as meso-Al₂O₃-F-25, meso-Al₂O₃-F-50, meso-Al₂O₃-F-100 and meso-Al₂O₃-F-400, respectively.

2.2.3. Preparation of Ni-Supported Al₂O₃ Catalysts

The Ni-supported Al₂O₃ catalysts were prepared using the wet impregnation method. For example, 7.5 g nickel (II) acetate tetrahydrate was dissolved in 20 mL deionized water to prepare the impregnation solution, which was then introduced onto the 10.0 g commercial Al₂O₃. After stirring for 3 h, the samples were dried overnight in an oven at 60 °C. Subsequently, the samples were calcined at 500 °C for 5 h with a heating rate of 5 °C/min, yielding the Ni/c-Al₂O₃ catalyst. Similarly, the Ni/c-Al₂O₃, Ni/meso-Al₂O₃, Ni/meso-Al₂O₃-P-200, Ni/meso-Al₂O₃-F-25, Ni/meso-Al₂O₃-F-50, Ni/meso-Al₂O₃-F-100, Ni/meso-Al₂O₃-F-200, and Ni/meso-Al₂O₃-F-400 catalysts were obtained in the same way. The actual content of Ni in these catalysts is presented in Table S1. Prior to HDO reactions, those catalyst samples were reduced in a slow-flowing H₂ atmosphere at 550 °C for 4 h.

2.3. Catalyst Characterization

X-ray diffraction (XRD) was employed to characterize and analyze the crystallinity and crystal structure of the samples. XRD measurements were performed using a Rigaku D/Max2400 diffractometer (Tokyo, Japan) with Cu Kα radiation (λ = 1.5418 Å). The samples were scanned over a 2θ range of 10°~80° with a step size of 0.02° and a scanning speed of 10°/min. The Scherrer equation was used to calculate the crystal grain size.

The specific surface area, pore size, and pore volume of the samples were characterized using a Micromeritics ASAP 2020 nitrogen adsorption/desorption analyzer (Norcross, GA, USA). Prior to N₂ adsorption, the catalysts were degassed under vacuum at 250 °C for 22 h.

H₂-TPR characterization was performed using a Micromeritics AutoChem II 2920 (Norcross, GA, USA). A total of 100 mg of the sample was placed in a U-shaped quartz tube and treated with helium at a flow rate of 100 mL/min at 300 °C for 1 h. The atmosphere was then switched to a H₂ + Ar mixture (H₂:Ar = 0.05:0.95). After the baseline stabilized, the quartz tube was heated from 50 °C to 800 °C at a rate of 10 °C/min, while the hydrogen consumption signal was detected by a thermal conductivity detector (TCD).

Scanning electron microscopy (SEM) images of the catalysts were obtained using a Hitachi S-4800 instrument (Tokyo, Japan) at an acceleration voltage of 10 kV. Transmission electron microscopy (TEM) analysis was conducted using a Jeol JEM-F200 instrument (Tokyo, Japan) with an accelerating voltage of 200 kV.

Thermogravimetric analysis (TGA) was conducted using a Q500 thermogravimetric analyzer (New Castle, DE, USA). Approximately 20 mg of the sample was placed in an alumina crucible and heated from 25 °C to 1000 °C at a rate of 10 °C/min under a flowing N₂ atmosphere (100 cm³/min).

2.4. Catalyst Test

The catalytic activity evaluation experiments were conducted in a 100 mL stainless steel high-pressure mechanically stirred autoclave reactor. In each experiment, 0.5 g of guaiacol, 0.1 g of the pre-reduced catalyst, and 30 mL of n-decane were added to the reactor. The reactor was then sealed, and the air inside was replaced with hydrogen five times. Next, hydrogen was introduced to raise the pressure to 4 MPa. The system was then heated to 280 °C, and the reaction was carried out for 4 h. After completion, the reactor was cooled to room temperature, and the liquid products were collected for analysis. Quantitative analysis of the reaction products was performed using gas chromatography (GC) equipped with a flame ionization detector (FID) and an INNOWAX capillary column, applying the external standard method.

The substrate conversion and product selectivity were calculated using the following equations:

$$\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n} (\mathrm{\%})={\displaystyle \frac{{\mathrm{M}\mathrm{o}\mathrm{l}}_{\mathrm{g}\mathrm{u}\mathrm{a}-\mathrm{i}\mathrm{n}}-{\mathrm{M}\mathrm{o}\mathrm{l}}_{\mathrm{g}\mathrm{u}\mathrm{a}-\mathrm{o}\mathrm{u}\mathrm{t}}}{{\mathrm{M}\mathrm{o}\mathrm{l}}_{\mathrm{g}\mathrm{u}\mathrm{a}-\mathrm{i}\mathrm{n}}}}\times 100\mathrm{\%}$$

$$\mathrm{S}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y} \mathrm{o}\mathrm{f} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}-\mathrm{i} (\mathrm{\%})={\displaystyle \frac{{\mathrm{M}\mathrm{o}\mathrm{l}}_{\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}-\mathrm{i}}}{{\mathrm{M}\mathrm{o}\mathrm{l}}_{\mathrm{a}\mathrm{l}\mathrm{l} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{s}}}}\times 100\mathrm{\%}$$

$$\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d} \mathrm{o}\mathrm{f} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}-\mathrm{i} (\mathrm{\%})={\displaystyle \frac{{\mathrm{M}\mathrm{o}\mathrm{l}}_{\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}-\mathrm{i}}}{{\mathrm{M}\mathrm{o}\mathrm{l}}_{\mathrm{g}\mathrm{u}\mathrm{a}-\mathrm{i}\mathrm{n}}-{\mathrm{M}\mathrm{o}\mathrm{l}}_{\mathrm{g}\mathrm{u}\mathrm{a}-\mathrm{o}\mathrm{u}\mathrm{t}}}}\times 100\mathrm{\%}$$

Here, “gua” represents guaiacol.

3. Results and Discussion

3.1. Textural Properties of the Catalysts

The pore structure characteristics of these Al₂O₃ supports were investigated through N₂ physisorption–desorption analysis. As Figure 1a shows, the N₂ adsorption and desorption isotherms of these Al₂O₃ samples prepared by different templating agents can be classified as an IUPAC type IV, characterized by an H1 hysteresis loop [25]. A distinct hysteresis loop was observed in the relative pressure range of p/p₀ = 0.4–0.9. This observation indicates the presence of mesopores in the c-Al₂O₃, meso-Al₂O₃, meso-Al₂O₃-P-200, and meso-Al₂O₃-F-200 samples. The pore size distributions, derived from the desorption branch of the BJH model for these samples, are presented in Figure 1c. The measured pore diameters of c-Al₂O₃, meso-Al₂O₃, meso-Al₂O₃-P-200, and meso-Al₂O₃-F-200 were 3.83 nm, 5.54 nm, 4.64 nm, and 4.07 nm, respectively. These results clearly demonstrate that, by incorporating different templating agents, Al₂O₃ supports with various mesoporous sizes can be synthesized.

Table 1. Specific surface area, pore volume, and mesoporous size of different samples.

Catalyst	Surface Area (m2/g) a	Total Pore Volume (cm3/g) b	Mesoporous Volume (cm3/g) c	Mesoporous Size (nm) d
c-Al2O3	138.8	0.197	0.184	3.83
meso-Al2O3	173.2	0.275	0.268	5.54
meso-Al2O3-P-200	207.9	0.366	0.357	4.64
meso-Al2O3-F-25	324.9	0.958	0.856	4.37
meso-Al2O3-F-50	292.2	0.419	0.348	4.59
meso-Al2O3-F-100	264.4	0.380	0.305	4.05
meso-Al2O3-F-200	240.6	0.345	0.286	4.07
meso-Al2O3-F-400	232.4	0.338	0.273	4.08

^a MultiPoint Brunauer, Emmett and Teller (BET) method; ^b Single point method; ^c Cumulative pore volume with pore size of 2–50 nm; ^d Barrett, Joyner Halenda (BJH) method.

offers detailed information on the BET surface area, total pore volume, and mesoporous volume of different Al₂O₃ supports. When contrasted with the conventional c-Al₂O₃ and meso-Al₂O₃, the BET surface areas of the meso-Al₂O₃-P-200 and meso-Al₂O₃-F-200 samples show remarkable increases, reaching 207.9 m²/g and 233.7 m²/g, respectively. Correspondingly, the total pore volumes of these samples also increase, with values of 0.366 cm³/g for meso-Al₂O₃-P-200 and 0.333 cm³/g for meso-Al₂O₃-F-200. This indicates that the addition of a templating agent not only controls the mesoporous dimensions of the Al₂O₃ supports but also enhances their specific surface area and pore volume, which are crucial parameters influencing the catalytic performance of the supports in various applications.

The influence of different amounts of the templating agent on the Al₂O₃ supports was investigated in more detail. Figure 1b shows the N₂ adsorption and desorption isotherms of the Al₂O₃ samples synthesized with different amounts of the F127 templating agent. According to the IUPAC classification, these isotherms are of type IV. Notably, the isotherm of the Ni/meso-Al₂O₃-F-25 sample showed an H3 hysteresis loop, different from the other samples, which showed an H1 hysteresis loop. This phenomenon might be due to the incorporation of a large amount of the templating agent. This facilitated the formation of many accumulation pores within the Al₂O₃ [26]. The results of the pore size distribution are shown in Figure 1d. All samples, including meso-Al₂O₃-F-25, meso-Al₂O₃-F-50, meso-Al₂O₃-F-100, meso-Al₂O₃-F-200 and meso-Al₂O₃-F-400, have similar mesopore sizes, ranging from approximately 4.05 to 4.59 nm. Moreover, the addition of the templating agent increased the specific surface area, total pore volume, and mesoporous volume. The measured values increased from 232.4 to 324.9 cm²/g, 0.338 to 0.958 cm³/g, and 0.273 to 0.856 cm³/g, respectively. These findings suggest that, by adjusting the amount of the templating agent in the synthesis process, it is possible to obtain Al₂O₃ supports with similar mesopore sizes but different mesopore capacities.

The crystalline structures of these Ni-supported Al₂O₃ catalysts were analyzed using XRD technology. As shown in Figure 2, diffraction peaks at 45.9° and 67.0°are clearly observed in all catalyst samples, which correspond to the (400) and (440) crystal planes of γ-Al₂O₃ (PDF# 00-029-0063) [27,28,29]. This indicates that the incorporation of Ni metal has less effect on the crystal structure of the Al₂O₃ support. Additionally, distinct diffraction peaks were observed at 37.1°, 43.0°, 62.6°, and 75.4°, corresponding to the (111), (200), (220), and (311) crystal planes of NiO (PDF# 03-065-2901), respectively. No additional impurity peaks were detected, suggesting that the Ni metal in all samples belongs to the NiO species. Notably, as Figure 2a shows, the diffraction peaks corresponding to NiO species are broadened over Ni/meso-Al₂O₃, Ni/Al₂O₃-P-200, and Ni/Al₂O₃-F-200 catalysts, implying that enlarging the mesoporous size contributes to a decrease in the size of Ni species and enhances their dispersion on the Al₂O₃ support [30]. Figure 2b shows that, as the mesoporous volume of the Al₂O₃ supports increases significantly from 0.273 (meso-Al₂O₃-F-400) to 0.856 cm³/g (meso-Al₂O₃-F-25), a notable reduction in the intensity of the diffraction peak corresponding to the NiO species is clearly observed. This phenomenon indicates that an increase in the mesoporous volume of the Al₂O₃ supports facilitates the dispersion of Ni species on the Al₂O₃ surface and contributes to the formation of NiO with a smaller particle size. Furthermore, the XRD results of the reduced Ni-based Al₂O₃ catalysts (Figure S1) showed that the size of metallic Ni evolution trend in the reduced catalyst is similar to that in the unreduced fresh catalyst. In conclusion, augmenting the mesoporous size and increasing the mesoporous volume of the Al₂O₃ supports significantly facilitates a more uniform distribution of Ni species in the Ni-supported Al₂O₃ catalyst.

Figure 3 shows the H₂-TPR profiles of different Ni-based Al₂O₃ catalysts. The reduction peak at 200–500 °C is attributed to the reduction of free NiO, while the peak at 500–750 °C corresponds to the reduction of bulk NiO, which interacts strongly with the alumina support [31]. The peak area reflects the content of NiO at each site. The Ni/c-Al₂O₃ catalyst with the smallest mesoporous size exhibits a lower bulk NiO content. As the mesoporous volume increases, the bulk NiO content gradually rises, suggesting that large mesoporous pore volume is beneficial for improving the interaction between Ni species and the Al₂O₃ support.

The morphological structures of the Ni/meso-Al₂O₃, Ni/meso-Al₂O₃-P-200, and Ni/Al₂O₃-F-200 catalysts were characterized using electron microscopy techniques. As shown in Figure 4a, all samples presented a dense structure where small particles were aggregated. However, after pore expansion through the addition of templating agents, the Ni/meso-Al₂O₃-P-200 and Ni/Al₂O₃-F-200 samples exhibited rougher surfaces with uniformly distributed pores, showing more distinct mesoporous characteristics. The Ni species present on these catalysts were further analyzed using high-resolution transmission electron microscopy (HRTEM). The analysis revealed lattice spacings of approximately 0.240 nm and 0.210 nm, which correspond to the (111) and (200) crystal planes of NiO, respectively. The similar structure and morphology lay a foundation for the subsequent investigation into the influence of the mesopore structural parameters in alumina supports on the catalytic hydrodeoxygenation (HDO) of guaiacol to cycloalkanes.

3.2. HDO of Guaiacol over Ni-Based Al₂O₃ Catalysts

3.2.1. Effect of Al₂O₃ Support’s Mesoporous Size

The catalytic efficacy of Ni/c-Al₂O₃, Ni/meso-Al₂O₃, Ni/meso-Al₂O₃-P-200, and Ni/meso-Al₂O₃-F-200 catalysts in the hydrodeoxygenation (HDO) of guaiacol was evaluated, and the results are presented in Figure 5a. The substrate conversion exceeded 99.9% over all catalysts, indicating that the ability to achieve complete guaiacol conversion is not closely related to the mesoporous sizes of the Al₂O₃ support. As the mesoporous size of the Al₂O₃ support increases, the selectivity of cyclohexane undergoes significant changes. Initially, when the mesoporous size increases from 3.83 nm to 4.07 nm, the cyclohexane selectivity surges from 47.6% to 84.8%. This suggests that an increase in the mesoporous size of the Al₂O₃ support can promote the HDO process of guaiacol to cyclohexane. However, after peaking at 4.07 nm, the selectivity starts to decline as the pore size continues to grow, up to 5.54 nm. This means that the increase in mesoporous size of Al₂O₃ support could facilitate the HDO process of guaiacol to cyclohexane. The reason for this situation is that the mesoporous pore size of the catalyst can affect the mass transfer rate of reactants and products to a certain extent. When the mesoporous pore size is small, the mass transfer of reactants and products is restricted, and it is difficult for molecules to quickly reach the active sites inside the catalyst. Increasing the mesoporous pore size can enhance the mass transfer rate, thereby improving the catalytic activity [32,33]. This trend underscores the importance of optimizing mesoporous sizes to attain the best HDO performance with Ni-based Al₂O₃ catalysts.

The product distribution of different samples was further analyzed. As shown in Figure 5b, the main products identified were cyclopentane, cyclohexane, cyclohexanol, and a small amount of cyclic alcohols (e.g., methyl-cyclohexanol, cyclo-pentyl-methanol, etc.). This finding indicates that the hydrogenation of benzene rings is not significantly affected by the mesopore sizes of Ni-supported Al₂O₃ catalysts. Notably, when using the Ni/c-Al₂O₃ catalyst, the product mixture showed a relatively high cyclohexanol selectivity of 37.3%. In contrast, the deoxidation products, namely cyclopentane and cyclohexane, accounted for only 57.8%. This suggests that the Ni/c-Al₂O₃ catalyst exhibits relatively low activity in deoxidation processes. With the mesoporous size of Al₂O₃ supports increasing to 4.64 nm, the proportion of deoxidation products initially rose to 96.8% over the Ni/meso-Al₂O₃-P-200. This demonstrates that increasing the mesoporous size of Al₂O₃ supports promotes the dehydroxylation of the cyclohexanol intermediate to cyclohexane over Ni-based Al₂O₃ catalysts. Nevertheless, as the mesoporous size increased to 5.54 nm, the proportion of deoxidation products decreased to 88.5% over Ni/meso-Al₂O₃, while the concentration of cyclohexanol increased again to 11.5%. This may be due to the fact that, when the mesoporous size of Al₂O₃ supports is too large, cyclohexanol intermediates tend to escape from the catalyst, reducing the deoxidation efficiency. Therefore, an efficient HDO process for converting guaiacol to cyclohexane requires an optimal mesoporous size of the support in Ni-based Al₂O₃ catalysts.

As depicted in Figure 5c, the yields of cyclohexane significantly declined when using the Ni/c-Al₂O₃ catalyst, dropping from 43.7% to 12.9% after five consecutive runs. Similarly, for the Ni/meso-Al₂O₃-F-200 catalyst, the yield decreased from 79.3% to 60.7% and, for the Ni/meso-Al₂O₃ catalyst, a reduction from 69.7% to 54.3% was observed. These results correspond to catalytic activity losses of 70.5%, 23.4%, and 22.1%, respectively. The deactivation of the catalyst can be primarily attributed to coke formation, as revealed by the thermogravimetric analysis (TGA) in Figure S1 [34]. It is evident that increasing the mesoporous size of the Al₂O₃ support can effectively alleviate the accumulation of carbon deposits. In conclusion, strategically increasing the mesoporous size is beneficial not only for promoting the HDO process of guaiacol but also for reducing carbon deposition and extending the lifespan of Ni-based Al₂O₃ catalysts.

3.2.2. Effect of Al₂O₃ Support’s Mesoporous Volume

The substrate conversion and cyclohexane selectivity for Ni/meso-Al₂O₃-F-x (x = 25–400) catalysts are shown in Figure 6a. All catalysts exhibited high guaiacol conversion, indicating that the mesoporous volume of the Al₂O₃ support negligibly affects substrate conversion. When the mesoporous volume is 0.286 cm³/g, cyclohexane selectivity reaches a maximum of 84.8% (Ni/meso-Al₂O₃-F-200). The product distribution is shown in Figure 6b. As the mesoporous volume increases from 0.273 to 0.305 cm³/g, cycloalkane (cyclohexane and cyclopentane) selectivity increases from 92.6% to 98.8%, while the oxygenate proportion decreases from 7.4% to 1.2%. This indicates that a larger mesoporous volume promotes the deoxygenation process by expanding the pore channel space, reducing mass transfer resistance, and enhancing catalytic activity [35]. As the mesoporous volume increases from 0.286 to 0.348 cm³/g, cyclopentane selectivity rises from 11.1% to 18.4%, likely due to enhanced diffusion of cyclohexanol intermediates, which promotes their isomerization to cyclopentane-methanol. This results in a decrease in cyclohexane selectivity from 84.8% (Ni/meso-Al₂O₃-F-200) to 80% (Ni/meso-Al₂O₃-F-50). When the mesoporous volume reaches 0.856 cm³/g, cycloalkane selectivity slightly decreases to 98.3%, possibly attributed to weakened structural integrity of the Al₂O₃ support. These results suggest that increasing the mesoporous volume positively influences the HDO performance of Ni-based Al₂O₃ catalysts.

3.3. Mechanism for HDO of Guaiacol over the Ni-Supported Al₂O₃ Catalyst

In order to investigate the mechanism of how the mesoporous properties of Al₂O₃ affect the catalytic performance of Ni-based Al₂O₃ catalysts in the hydrodeoxygenation (HDO) of guaiacol, the conversion rates of guaiacol and product distributions were compared between Ni/c-Al₂O₃ and Ni/meso-Al₂O₃-F-200 catalysts at different reaction times (Figure 7). At 0.5 h of reaction time, guaiacol was not completely converted, with conversion rates of 79.8% and 75.7% for Ni/c-Al₂O₃ and Ni/meso-Al₂O₃-F-200, respectively. The main products were cyclohexane, cyclohexanol, and 2-methoxycyclohexanol. After 1 h of reaction time, guaiacol conversion reached 100% for both catalysts. After 2 h of reaction time, the cyclohexane selectivity of Ni/meso-Al₂O₃-F-200 reached 82.1%, while that of Ni/c-Al₂O₃ was only 31.6%. For Ni/meso-Al₂O₃-F-200, the products stabilized after 4 h of reaction time, consisting mainly of cyclohexane and cyclopentane, whereas Ni/c-Al₂O₃ still produced significant amounts of cyclohexanol. The reaction with Ni/c-Al₂O₃ reached near saturation at 8 h, with cyclohexane selectivity at 67.8% and cyclohexanol selectivity at 20.2% in the products. After 12 h of reaction time, cyclohexane selectivity in the products of Ni/c-Al₂O₃ and Ni/meso-Al₂O₃-F-200 reached 68.5% and 87.9%, respectively.

As a model compound for lignin-derived phenols, guaiacol contains three C-O bonds. This characteristic makes its reaction pathways relatively complex and variable, depending on the catalyst [36]. Currently, there are two possible reaction pathways for the conversion of guaiacol to cyclohexane. In one pathway, the catalyst adsorbs a large amount of hydrogen to completely hydrogenate the benzene ring, forming 2-methoxycyclohexanol. Subsequently, the methoxy group is removed to form cyclohexanol and, finally, dehydration occurs to yield cyclohexane. The other pathway involves the direct hydrogenolysis of guaiacol. In this process, both the methoxy and phenolic hydroxyl groups are removed to form benzene, which then undergo hydrogenation to form cyclohexane. However, almost no benzene was detected during the reaction. Therefore, it can be concluded that the first reaction pathway is the main route for the conversion of guaiacol into cyclohexane over the Ni-supported Al₂O₃ catalysts.

Figure 8 compares the changes in the proportion of methoxy group and hydroxy group between the Ni/c-Al₂O₃ and Ni/meso-Al₂O₃-F-200 catalysts at different reaction times. It can be observed that the trends in methoxy group removal were similar for both catalysts. After 1 h of reaction time, a significant number of methoxy groups were removed, leaving 22.3% and 17.7% of the initial amounts for Ni/c-Al₂O₃ and Ni/meso-Al₂O₃-F-200, respectively. After 4 h of reaction time, the products contained almost no methoxy groups. However, the trends for hydroxyl group removal differed significantly between the two catalysts. For Ni/meso-Al₂O₃-F-200, hydroxyl groups were almost completely removed after 2 h of reaction time, while Ni/c-Al₂O₃ still retained 61.2%. The dehydroxylation rate of the Ni/meso-Al₂O₃-F-200 catalyst was approximately 2.5 times that of the Ni/c-Al₂O₃ catalyst. After 12 h of reaction time, Ni/c-Al₂O₃ still retained 17.8% of the hydroxyl groups. This finding indicates that incorporating suitable mesoporous sizes, along with an increase in mesoporous volume within the Al₂O₃ support, can significantly promote the dehydroxylation process during the hydrodeoxygenation (HDO) of guaiacol. This enhancement ultimately facilitates the HDO of guaiacol into cycloalkanes. As illustrated in Scheme 1, precisely engineering the mesoporous architecture of the Al₂O₃ support, specifically through pore size expansion and volumetric enhancement, can significantly accelerate the rate-determining dehydroxylation step of cyclohexanol during the conversion of guaiacol to cycloalkanes via HDO [37]. Therefore, this structural optimization improves the catalytic performance in the HDO conversion of guaiacol into cycloalkanes within the Ni-supported Al₂O₃ catalyst system. Furthermore, such adjustments also contribute to the mitigation of carbon deposit formation by facilitating the diffusion of reactive intermediates [38], ultimately enhancing catalyst stability.

3.4. HDO of Other Phenols and Lignin-Derived Bio-Oil over the Ni-Supported Al₂O₃ Catalyst

The hydrodeoxygenation (HDO) performance of Ni/meso-Al₂O₃-F-200 was further evaluated for other lignin-derived phenolic compounds, including o-cresol, anisole, eugenol, and vanillin, under reaction conditions of 280 °C and an initial hydrogen pressure of 4 MPa. After 12 h of reaction, the conversion rates and major product yields are summarized in

Table 2. Catalytic HDO performance of various lignin-derived phenolic compounds over Ni/meso-Al₂O₃-F-200 catalyst.

Substrate	Conversion	Product and Yield
	100%	97.1%
	100%	95.7%
	100%	12.1% 8.8% 62.2%
	100%	14.1% 74.2%

. All lignin-derived phenolic compounds were completely converted. The yield of cycloalkanes in the conversion products of o-cresol and anisole is over 95%. Eugenol and vanillin presented a greater challenge for hydrodeoxygenation because of their relatively complex structures, which contain not only methoxy and phenolic hydroxyl groups but also double bonds and aldehyde groups. Nevertheless, under the catalysis of Ni/meso-Al₂O₃-F-200, the yield of cycloalkanes exceeded 80%. These results demonstrate that the Ni/meso-Al₂O₃-F-200 catalyst has strong HDO activity and can handle various complex molecular structures.

The Ni/meso-Al₂O₃-F-200 catalyst was also used in the HDO upgrading of raw lignin oil. The compositional changes of the lignin oil are presented in

Table 3. The components of the raw and upgraded lignin-oil analyzed by GC–MS.

Raw Lignin Oil Component	Content (%)	Upgraded Lignin Oil Component	Product Distribution (%)
			Ni/meso-Al2O3-F-200	Ni/c-Al2O3
Hydrocarbons	22.0	Hydrocarbons	65.9	40.5
Longifolene	9.7	Cyclopentane	1.3	0
Retene	12.3	Methylcyclopentane	3.2	1.1
Guaiacols	70.3	Cyclohexane	14.4	3.9
o-Cresol	3.0	Methylcyclohexane	8.4	6.0
p-Cresol	4.7	Ethylcyclohexane	4.3	0
Guaiacol	10.1	Propylcyclohexane	3.5	0
3,5-Dimethylphenol	3.7	Longifolene	0	8.0
2-Methoxy-4-methylphenol	19.9	Retene	2.3	7.2
2-Methoxy-4-ethylphenol	14.2	Retene’s derivatives	28.5	14.3
2-Methoxy-5-propylphenol	3.1	Guaiacols	34.1	59.5
3-Allyl-6-methoxyphenol	5.2	Guaiacol	3.9	8.5
trans-Isoeugenol	6.4	2-Methoxy-4-methylphenol	8.9	17.5
Other Oxygenated Compounds	7.7	2-Methoxy-4-ethylphenol	10.7	14.5
Methyl dehydroabietate	7.7	3-Allyl-6-methoxyphenol	10.6	19.0

. The raw lignin oil contained 22.0% hydrocarbons and 78.0% oxygenated compounds. After HDO upgrading, the hydrocarbon content increased from 22.0% to 65.9%. This indicates that most phenolic compounds were converted into hydrocarbons via HDO, significantly enhancing the heating value of the lignin oil. For comparison, the HDO upgrading of raw lignin oil was also carried out using a Ni/c-Al₂O₃ catalyst. The results showed that the hydrocarbon content only increased from 22.0% to 40.5%. The superior performance of the Ni/meso-Al₂O₃-F-200 catalyst compared to the Ni/c-Al₂O₃ catalyst demonstrates its great potential for practical application in the HDO upgrading of lignin oil.

4. Conclusions

The influence of the mesoporous architecture (pore size and pore volume) of the Al₂O₃ support on the HDO conversion of guaiacol has been investigated in the Ni-supported Al₂O₃ catalyst system. The results indicate that optimizing the mesoporous size can enhance the diffusion of cyclohexanol intermediates. This improvement reduces carbon deposits that result from the retention of these intermediates, thereby enhancing catalyst stability and significantly boosting cyclohexane selectivity. Furthermore, by increasing the mesoporous volume of the Al₂O₃ support, the selectivity of cycloalkanes in the products can reach as high as 98.8% when using the Ni/meso-Al₂O₃-F-100 catalyst. During the upgrading of lignin-derived oil, the Ni/meso-Al₂O₃-F-200 catalyst, characterized by a mesoporous size of 4.07 nm and a mesoporous volume of 0.286 cm³/g, exhibits excellent performance. Its alkane selectivity reaches 65.9%, which is significantly higher than that of the commercial Ni/c-Al₂O₃ catalyst, which has a mesoporous size of 3.83 nm and a mesoporous volume of 0.184 cm³/g. In summary, these superior performances can be attributed to the suitable mesoporous size and large mesoporous volume of the Al₂O₃ support. Such structural features effectively facilitate the dehydroxylation process of the cyclohexanol intermediate during the HDO conversion of guaiacol, ultimately promoting the conversion efficiency and improving the quality of the products in the lignin-derived oil upgrading process.