Thermal Stability Improvement of Cu-Based Catalyst by Hydrophobic Modification in Methanol Synthesis

Abstract

Water can cause the growth and oxidation of Cu nanoparticles on the surface of Cu-based catalysts, leading to their deactivation. However, during methanol synthesis process from syngas on Cu-based catalysts, water is inevitably produced as a by-product due to the presence of CO₂. Therefore, enhancing the stability of Cu-based catalysts during the reaction, particularly in the presence of water, is crucial. In this study, Cu/ZnO/Al₂O₃ was first subjected to wet etching and then hydrophobically modified using the sol–gel method with methyltrimethoxysilane (MTMS) and the grafting method with 1H,1H,2H,2H-perfluoroalkyltriethoxysilanes (PFOTES) as modifiers. These modifications aimed to mitigate the impact of water on the catalyst and improve its stability. After modification, the catalysts exhibited excellent hydrophobicity and enhanced catalytic activity in the methanol synthesis process. The surface physical properties, composition, and thermal stability of the catalysts before and after hydrophobic modification were characterized by SEM, FT-IR, BET, XRD and TGA. Additionally, molecular dynamics simulations were employed to compare the diffusion behavior of water molecules on the catalyst surfaces before and after hydrophobic modification. The results indicated that the modified catalyst surface formed a micro/nano structure composed of nanosheets and nanosheet clusters, while the hydrophobic modification did not alter the structure of the catalyst. According to the results of simulations, the hydrophobic layers on the modified catalysts were able to expel water quickly from the surfaces and reduce the relative concentration of water molecules at the active sites, thereby improving the stability of the catalyst. Notably, the thermal stability and hydrophobicity of the PFOTES-modified catalyst were superior to those of the MTMS-modified catalyst, resulting in a more significant enhancement in catalyst stability, which aligned with the experimental results.

1. Introduction

Methanol is a type of chemical raw material and solvent that can be utilized to synthesize a variety of chemical products, including formaldehyde, dimethyl ether, and acetic acid [1,2]. Catalysts used for methanol synthesis can be categorized based on the type of active metals they contain, including Cu-based catalysts [3], noble metal-based catalysts [4,5], and other types of catalysts [6]. Among these, Cu-based catalysts are widely employed in industrial methanol synthesis due to their high catalytic activity, excellent methanol selectivity, and cost-effectiveness. The syngas used for industrial methanol synthesis is primarily derived from coal gasification and the steam reforming of methane, consisting mainly of H₂, CO, and a small amount of CO₂. The presence of CO₂ inevitably leads to the occurrence of the reverse water–gas shift (RWGS) reaction within the reaction system, which is accompanied by the formation of water. Water is detrimental to Cu-based catalysts as it can decrease the number of active sites on the catalyst, increase the size of Cu particles, and alter the valence state of Cu species, thereby reducing both catalyst activity and the space–time yield of products [7]. Martin et al. [8] discovered that water increases the number of hydroxyl groups on the surface of the CuZnAl catalyst, which can block the active sites of the catalyst and diminish its catalytic performance. Wang et al. [9] found that the hydroxyl groups adsorbed around Cu particles can lead to the partial oxidation of Cu⁰ to Cu⁺ and Cu²⁺. The catalytic performance of Cu-based catalysts is closely related to the Cu⁺/Cu⁰ ratio [10,11], indicating that water adversely affects catalyst activity. Furthermore, they also observed that water promotes the growth of Cu crystals and reduces the specific surface area of Cu particles, resulting in catalyst deactivation. During the reaction process, Cu nanoparticles with a low Tammann temperature are prone to migration and aggregation, leading to sintering deactivation of the catalyst. The presence of water molecules accelerates this deactivation process [12,13].

It is essential to enhance the stability of Cu-based catalysts in the presence of water. For industrial applications, extensive research has been conducted on methods to maintain high catalytic activity and long-term stability of catalysts under operational conditions. Catalyst deactivation induced by the by-product water can be eliminated through its removal during reactions. Tian et al. [14] developed a Cu/Zn-BTC@LTA catalytic membrane reactor to remove continuously the by-product water steam during the reaction. Consequently, CO₂ conversion and methanol selectivity remained around 40% and 93.5% after 48 h reaction. In addition, doping of metal sites was served as a promising method for resolving the sintering and oxidation of Cu-based catalysts caused by the poisoning effect of water. Wang et al. [15] developed Cu,Zn-codoped ZrO₂ catalyst with outstanding performance, which can remain without deterioration with methanol selectivity for 250 h, compared to Cu-ZrO₂ and Zn-ZrO₂. Chen et al. [16] prepared a ternary Cu/ZnO/MgO catalyst through co-precipitation and found that it can keep its activity for 180 h without deactivation, demonstrating the effects of MgO on the stability of the Cu/ZnO catalyst.

Some studies demonstrated that the hydrophobicity and hydrophilicity of the catalyst surface significantly influence its catalytic activity and product selectivity [17]. Consequently, hydrophobic modification of the catalyst surface can effectively expel water, mitigate the deactivation effects of water molecules, and enhance both the activity and stability of the catalyst. The wettability of the catalyst surface is determined by a combination of surface roughness and chemical composition [18,19]. The construction of rough surfaces can be achieved through various methods, including electrochemical deposition, chemical vapor deposition, self-assembly, plasma surface treatment, and chemical etching. Among these, chemical etching stands out as a simple, efficient, and time-saving technique for creating micro/nano structures by utilizing the dislocation preferential corrosion mechanism within metals. This method is particularly advantageous as it can be easily scaled to an industrial level without the need for specialized equipment [20,21]. However, significant numbers of hydroxyl (-OH) groups are present on the surface of the Cu/Zn/Al₂O₃ catalyst, which impart hydrophilicity. According to the Wenzel model, while chemical etching enhances the roughness of the catalyst surface, the intrinsic water contact angle remains below 90°. Consequently, the apparent water contact angle decreases as the roughness of the catalyst surface increases, resulting in a more hydrophilic surface. Therefore, to achieve superhydrophobicity on the rough surface, it is essential to modify the etched catalyst surface with low surface energy substances. Sol–gel is the most effective method for manufacturing crystalline or amorphous oxide coatings due to its excellent properties, straightforward processing techniques, cost-effectiveness, and the capability to create complex shapes without the need for specialized instruments, as well as the ease of controlling surface characteristics [22]. Methyltrimethoxysilane (MTMS), methyltriethoxysilane (MTES), and vinyltriethoxysilane (VTES) are the most commonly used precursors for hydrophobic coatings prepared by the sol–gel method [23,24]. Wang et al. [25] prepared a hydrophobic catalyst, Cn/Zn-2 mM, with a water contact angle of 163.8° by using stearic acid as a modifier. Compared to the unmodified catalyst, the methanol selectivity and space–time yield of the modified catalyst were enhanced during the process of CO₂ hydrogenation to methanol. Xu et al. [26] investigated the impact of carbon chain length in silane modifiers on the stability of Cu-based catalysts during methanol synthesis. The results indicated that hydrophobic modification significantly enhances catalyst stability, with longer carbon chains yielding better performance.

Perfluorosilane is widely used in superhydrophobic coatings due to the strong electronegativity and low polarizability of fluorine. This results in weak London dispersion forces, cohesion, and adhesion, causing the -CF₃ groups to exhibit lower surface energy than the -CH₃ groups, thereby demonstrating superior hydrophobicity [27]. Additionally, the long carbon chains of perfluorosilanes exhibit a unique zigzag structure that creates a spatial shielding effect, protected by the surrounding fluorine atoms. Consequently, perfluorosilanes demonstrate greater chemical stability and lower surface energy compared to traditional silanes. However, studies have demonstrated that even when a smooth surface is modified with fluorine, the water contact angle does not exceed 120°, preventing the achievement of a superhydrophobic effect [28]. Therefore, the modification of surfaces to create roughness and low surface energy materials is essential for attaining superhydrophobicity. Thin hydrophobic molecular layers can be prepared using silane as a modifier, which alters the chemical structure of the surface and reduces surface energy without compromising the desired micro-scale roughness, thereby ensuring that the modified surface achieves superhydrophobicity. Although perfluorosilane exhibits excellent hydrophobic properties, research on the hydrophobic modification of Cu-based catalysts by fluoride remains limited.

In this study, two superhydrophobic catalysts, M-Cu/ZnO/Al₂O₃(10) and F-C₈-Cu/ZnO/Al₂O₃, were prepared through chemical etching followed by hydrophobic modification using the sol–gel method and grafting method, respectively. The catalysts were characterized by SEM, FT-IR, BET, XRD, and TGA. Additionally, the stabilities of the superhydrophobic coatings on the surfaces of Cu/ZnO/Al₂O₃(10) and F-C₈-Cu/ZnO/Al₂O₃ were evaluated. The effects of reaction temperature, volume space velocity of syngas, and wettability on the performance of the catalyst in the gas–solid methanol synthesis were investigated. Three catalyst interface models were constructed to simulate the dynamics of water molecules, allowing for an examination of how surface wettability influences the diffusion of water molecules.

2. Experimental Section

2.1. Catalyst Preparation

2.1.1. Preparation of M-Cu/ZnO/Al₂O₃(10)

A commercial Cu-based methanol synthesis catalyst (purchased from Dalian Zeer Catalytic Materials Co., Dalian, China) was utilized to prepare superhydrophobic catalysts (The properties, as provided by the manufacturer, are presented in

Table 1. Commercial Cu-based catalyst properties (catalyst cylinders with domed ends, 5 mm × 5 mm).

Property	Value
chemical composition
Cu	>42%
Zn	20 ± 2%
Al	5 ± 1%
axial crush strength	≥200 N
expected filling density	1100–1400 kg m−3

). Prior to surface modification, 10 g of the as-received catalyst was washed three times with anhydrous ethanol (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) to remove oil and dust, followed by five washes with deionized water. The washed catalyst was then dried for 8 h at 60 °C under vacuum, and the sample was designated as Cu/ZnO/Al₂O₃. Subsequently, Cu/ZnO/Al₂O₃ was immersed in 50 mL of a 0.1 M NaOH (Shanghai Boer Chemical Reagent Co., Ltd., Shanghai, China) solution for wet etching for 0–20 min to create micro/nano rough structures on the catalyst surface. The etched catalyst (Etched-Cu/ZnO/Al₂O₃) was rinsed five times with deionized water and dried for 6 h at 60 °C under vacuum. To prevent a decrease in the concentration of the etching agent, which could lead to a decline in the etching rate, an excess of NaOH solution was used.

The catalyst was modified by the sol–gel method. First, 123 mL of methanol (Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China) and 17.5 mL of MTMS (Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China) were added to a beaker under vigorous stirring. Next, 10 mL of 5 M NH₄OH (Shanghai Macklin Biochemical Technology Co., Ltd., Shanghai, China) was added dropwise to the solution and stirred for 15 min to obtain a sol with a molar ratio of MTMS/methanol/water of 1:25:4. In this process, NH₄OH served as the catalyst for the sol condensation reaction. Subsequently, Etched-Cu/ZnO/Al₂O₃ was immersed in the prepared sol for 1 h and then dried at ambient temperature for 30 min. Finally, the modified catalyst underwent heat treatment at 150 °C for 1 h (with a heating rate of 2 °C min⁻¹, natural cooling) to ensure a strong bond between the catalyst and the coating. The modified catalyst was designated as M-Cu/ZnO/Al₂O₃(x), where M stands for methyl and x represents different etching times (x = 0, 5, 10, 15, 20 min). The preparation process is illustrated in Figure 1.

2.1.2. Preparation of F-C₈-Cu/ZnO/Al₂O₃

The superhydrophobic catalyst (F-C₈-Cu/ZnO/Al₂O₃) was prepared by the grafting method (Figure 2). The pretreatment and etching processes for the catalyst were identical to those used for Cu/ZnO/Al₂O₃(10), with the etching duration set to 10 min. Etched-Cu/ZnO/Al₂O₃ was then immersed in a solution of 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES, Shanghai Titan Scientific Co., Ltd., Shanghai, China) in ethanol (0.1 mol L⁻¹) and maintained at 50 °C for 12 h in a water bath to facilitate the surface coupling reaction. Afterwards, the catalyst was rinsed three times with ethanol and deionized water, then dried for 12 h at 100 °C under vacuum.

2.2. Catalyst Characterization

The surface area, total pore volume, and average pore diameter of the Cu-based catalysts were determined by nitrogen adsorption–desorption isotherms at −196 °C with a Micromeritics ASAP 2460 apparatus (Micromeritics Instrument Corp., Norcross, GA, USA). Prior to measurement, the samples were degassed under vacuum at 120 °C for 8 h. The Barrett–Joyner–Halenda (BJH) method was employed to assess the pore size distribution, while the surface area of the samples was calculated by the BET method. Fourier-transform infrared (FT-IR) spectroscopy was conducted using an American Spectrum 100 infrared spectrometer (PerkinElmer Company, Waltham, MA, USA). The samples were prepared by mixing KBr with the catalyst to facilitate the determination of functional groups. The thermal stability of the samples was evaluated through thermogravimetric analysis (TGA) using a Mettler instrument (Mettler-Toledo, Zurich, Switzerland). The samples were heated at a rate of 10 °C min⁻¹ from 30 °C to 800 °C under a dry nitrogen atmosphere. X-ray diffraction (XRD) was tested on a D8 diffractometer (Bruker AXS, Karlsruhe, Germany) with Cu Kα radiation (λ = 0.15416 nm). The sweep range was 10–80° with a scanning rate of 10° min⁻¹. Scanning electron microscopy (SEM) was performed with a Nova NanoSEM 450 (FEI Company, Hillsboro, OR, USA) to examine the surface morphology of the catalysts before and after surface treatment. Each sample was placed in a holder and coated with platinum to obtain clear images. Water contact angle measurements were conducted using the static drop method. A 5 μL drop of water was placed on the surface, and the morphology of the water droplet was recorded continuously with a digital camera. The water contact angle was calculated based on the recorded images, with each sample measured five times at different locations.

2.3. Catalystic Tests

2.3.1. Thermal Stability Evaluation

(1)

Short-term thermal stability evaluation at different temperatures

M-Cu/ZnO/Al₂O₃(10) and F-C₈-Cu/ZnO/Al₂O₃ were heat-treated in a tubular furnace at temperatures ranging from 100 to 350 °C in N₂ atmosphere for 1 h. The N₂ flow rate was set at 40 mL min⁻¹ and the heating rate was maintained at 2.5 °C min⁻¹. N₂ was introduced for 15 min prior to heat treatment to purge the air from the tubular furnace. After the heat treatment, the water contact angle on the surface of the hydrophobic catalyst was measured.

(2)

Long-term thermal stability evaluation at 240 °C

On the other hand, M-Cu/ZnO/Al₂O₃(10) and F-C₈-Cu/ZnO/Al₂O₃ were heat-treated in a tubular furnace at 240 °C for 0–24 h in N₂ atmosphere to investigate the long-term thermal stability of the hydrophobic layer of the catalyst at methanol synthesis temperature. The heating rate was set at 2.5 °C min⁻¹, and the N₂ flow rate was maintained at 40 mL min⁻¹. N₂ was introduced for 15 min prior to heat treatment to purge the tubular furnace of air. After natural cooling, the water contact angle on the surface of the hydrophobic catalyst was measured at various heat treatment durations.

2.3.2. Catalytic Performance Test

The gas–solid methanol synthesis process was conducted in a fixed-bed reactor measuring 1000 mm in length and 20 mm in diameter. An amount of 11.7 g catalyst was loaded into the reactor, with inert alumina beads placed both above and below the catalyst. N₂ was initially introduced to the reactor to purge the air. Subsequently, the catalyst was reduced by pure H₂ at a flow rate of 70 mL min⁻¹ at 0.5 MPa and 220 °C for 7 h. After the reduction, the feed was switched to syngas (H₂/CO/CO₂/N₂ = 63/9/15/13, Air Liquide Compressed Gas Co., Ltd., Shanghai, China) once the reactor had naturally cooled below 160 °C. The reactor was then flushed with syngas at a flow rate of 100 mL min⁻¹, after which the pressure was gradually increased to 4 MPa. The reactor was subsequently heated to the desired temperature at a rate of 2.5 °C min⁻¹. The compositions of the gaseous products were analyzed by GC-900C gas chromatograph (Shanghai Tianpu Analytical Instrument Co., Ltd., Shanghai, China) every 20 min. N₂ was used as the reference for calculations, as it does not participate in the methanol synthesis process. The outlet gas flow ($F_{out}$, Equation (1)) was determined based on the volume fraction of N₂, allowing for the calculation of CO conversion ($x_{CO}$, Equation (2)) and the methanol generation rate ($r_{MeOH}$, Equation (3)):

$$F_{out}={\displaystyle \frac{F_{in}\times y_{in,N_2}}{y_{out,N_2}}}$$

(1)

$$x_{CO}={\displaystyle \frac{F_{in}\times y_{in,CO}-F_{out}\times y_{out,CO}}{F_{in}\times y_{in,CO}}}\times 100\%$$

(2)

$$r_{MeOH}={\displaystyle \frac{F_{in}\times y_{in,CO}\times x_{CO}}{m_{cat}}}$$

(3)

3. Results

3.1. Surface Modification of Cu/ZnO/Al₂O₃

To investigate the influence of varying etching durations and hydrophobic modifications on the surface morphology of the catalyst, SEM characterization was conducted (Figure 3). The differences in the surface morphology of Etched-Cu/ZnO/Al₂O₃, resulting from different etching durations, are attributed to the combined effects of chemical etching and capillarity [29,30,31]. The NaOH solution penetrates the nanopores of Cu/ZnO/Al₂O₃ through capillary action, dissolving the outer surface and thin walls. As the etching duration increases, the surface and thin walls are progressively dissolved, leading to the formation of nanosheet structures [32] (Figure 3a–c). However, excessively prolonged etching durations result in the dissolution of the initially formed nanosheets, thereby reducing their quantity (Figure 3d) and potentially causing cracking of the catalyst surface (Figure 3e). Consequently, the etching duration was optimized at 10 min. In Figure 3c, a substantial number of polyhedral nanosheet structures (thickness: 30–60 nm, size: 300–400 nm, Figure 3f) are uniformly distributed across the catalyst surface, with the nanosheets irregularly piled together to form nanosheet clusters (size: 0.5–1.5 μm, Figure 3g). This micro/nano hierarchical structure, composed of nanosheets and nanosheet clusters, significantly enhances the surface roughness of the catalyst and traps more air within the gaps of the rough surface, which is a crucial prerequisite for the superhydrophobicity of the catalyst [33]. Furthermore, the surface morphology images of M-Cu/ZnO/Al₂O₃ (10) (Figure 3h) and F-C₈-Cu/ZnO/Al₂O₃ (Figure 3i) demonstrate that the micro/nano structure of the etched catalyst surface remains intact following hydrophobic modification.

The FT-IR spectra of Cu/ZnO/Al₂O₃ before and after hydrophobic modification are shown in Figure 4. In all cases, the absorption peaks at 1627 cm⁻¹ and 3441 cm⁻¹ are derived from bending vibration of H-OH bond and stretching vibration of -OH or physically adsorbed water [34]. For both Cu/ZnO/Al₂O₃ and Etched-Cu/ZnO/Al₂O₃, the absorption peak at 507 cm⁻¹ corresponds to the stretching vibration of the M-O bond (M is Cu, Zn, or Al) [34,35]. After hydrophobic modification, M-Cu/ZnO/Al₂O₃(10) shows new absorption peaks at 459 cm⁻¹, 1090 cm⁻¹, 1218 cm⁻¹ and 1265 cm⁻¹, which are associated with bending vibration [36] and antisymmetric stretching vibration of Si-O-Si bond [37,38], as well as deformation vibration of -CH₃ and stretching vibration of Si-CH₃ [37,38,39], respectively. In addition, the adsorption peaks at 2848 cm⁻¹ and 2917 cm⁻¹ are the symmetric and antisymmetric stretching vibrations of -CH₃ [39,40]. Furthermore, new adsorption peaks at 1141 cm⁻¹, 1207 cm⁻¹, and 1240 cm⁻¹, which are related to the stretching vibrations of -CF₂ and -CF₃ [41,42,43], appeared on the F-C₈-Cu/ZnO/Al₂O₃ surface. New adsorption peaks on the surfaces of M-Cu/ZnO/Al₂O₃(10) and F-C₈-Cu/ZnO/Al₂O₃ indicated that the hydrophobic groups had been successfully grafted.

Moreover, the texture properties of Cu/ZnO/Al₂O₃ before and after hydrophobic modification are listed in

Table 2. Texture property parameters of different catalysts.

Catalysts	BET Surface Area/m2·g−1	Pore Volume/cm3·g−1	BJH Diameter/nm
Cu/ZnO/Al2O3	89.16	0.25	8.11
Etched-Cu/ZnO/Al2O3	86.14	0.21	6.98
M-Cu/ZnO/Al2O3(10)	81.48	0.20	6.74
F-C8-Cu/ZnO/Al2O3	85.11	0.21	6.94

. Obviously, the specific surface area, pore volume, and pore diameter of the catalyst decreased after hydrophobic modification compared to Cu/ZnO/Al₂O₃. The hydrophobic layers on the surface of the modified catalysts possess a three-dimensional network structure, which inevitably obstructs some of the pore channels, leading to a decline in the catalyst structural parameters [44,45]. However, this also indicated that the hydrophobic groups had been successfully grafted onto the catalyst surface, which was consistent with the FT-IR results.

Later, XRD characterization was employed to analyze the crystal structure of the catalyst before and after hydrophobic modification, with the results presented in Figure 5. For all catalyst samples, the characteristic peaks of CuO and ZnO were observed. The diffraction peaks of CuO at 2θ = 35.8°, 38.9°, 48.7°, 61.8°, and 66.2° correspond to the typical reflections of the (002), (111), (−202), (−113), and (022) crystal planes of CuO (JCPDS 02-1040). The ZnO diffraction peaks at 2θ = 32.0° and 68.8° are attributed to the (100) and (112) crystal plane reflections of ZnO (JCPDS 75-1526). However, no characteristic peak of the carrier Al₂O₃ was detected in the XRD patterns of the four catalysts, indicating that Al₂O₃ exists in an amorphous or dispersed structure [46]. Additionally, the XRD pattern of all samples exhibited a C diffraction peak at 2θ = 26.7°, which was attributed to residual graphite in the catalyst (JCPDS 26-1080). Importantly, after etching and hydrophobic modification, the diffraction peak intensities of CuO and ZnO remained relatively unchanged, and no diffraction peaks corresponding to organic functional groups were present in the XRD patterns of the hydrophobic catalyst. This indicates that the crystal structures of CuO and ZnO are well preserved during the preparation process of the hydrophobic catalyst. Therefore, etching and hydrophobic modification do not affect the crystal structure of the catalyst components.

In order to further assess the hydrophobicity of the catalyst surface after hydrophobic modification with varying etching durations, the water contact angle was measured (Figure 6). The water contact angle of M-Cu/ZnO/Al₂O₃(x) initially increases and then decreases as the etching duration increases, reaching a maximum value of 153° at 10 min (Figure 6c). This indicates that M-Cu/ZnO/Al₂O₃(10) possesses the roughest surface and the highest hydrophobicity after the same modification treatment, which aligns with the SEM results. Similarly, the F-C₈-Cu/ZnO/Al₂O₃ surface also demonstrated excellent hydrophobicity, exhibiting a water contact angle of 153.8°.

To determine the thermal stability of the hydrophobic layer binding to the catalyst surface, the thermogravimetric analysis before and after the hydrophobic modification of Cu/ZnO/Al₂O₃ is presented in Figure 7. For Cu/ZnO/Al₂O₃, the weight loss peaks at 0–100 °C, 100–200 °C, and 200–600 °C are derived from the desorption of water molecules, the loss of crystal water from the hydrotalcite (hydroxymagnesite) phase in the catalyst precursor [47], and the thermal decomposition of hydroxyl carbonate and hydrotalcite into metal oxides [46,48]. In contrast to the unmodified catalyst, the intensity of the weight loss peak for the hydrophobic catalyst was significantly reduced at 0–100 °C, which can be attributed to the hydrophobic modification that lowers the surface energy of the catalyst, thereby decreasing the adsorption of water molecules. On the other hand, the weight loss peaks of M-Cu/ZnO/Al₂O₃(10) at approximately 333 °C and 758 °C are attributed to the thermal decomposition of the hydrophobic coating [39] and the release of O₂ during the conversion of CuO to Cu₂O [49]. For F-C₈-Cu/ZnO/Al₂O₃, the weight loss peaks near 289 °C and 356 °C originate from the thermal desorption of physically adsorbed PFOTES molecules and the thermal decomposition of the hydrophobic layer. This indicates that the hydrophobic layer is firmly bound to the catalyst, and the thermal stability of F-C₈-Cu/ZnO/Al₂O₃ is greater than that of M-Cu/ZnO/Al₂O₃(10).

3.2. Evaluation of Catalyst Thermal Stability

The water contact angle values on the surfaces of M-Cu/ZnO/Al₂O₃(10) and F-C₈-Cu/ZnO/Al₂O₃ after heat treatment at temperatures ranging from 100 °C to 350 °C for 1 h are shown in Figure 8. The water contact angle of both hydrophobic catalysts decreases as the heat treatment temperature increases. For M-Cu/ZnO/Al₂O₃(10), hydrophobicity is maintained within the temperature range of 100 °C to 250 °C, with a water contact angle of 137.5° at 250 °C. However, at 325 °C, the water contact angle drops to 88.8°, indicating a transition to hydrophilicity, which suggests that the hydrophobic layer on the catalyst surface begins to decompose [50]. At 350 °C, the water contact angle approaches 0°, allowing water to spread across the catalyst surface, indicating the complete disappearance of the hydrophobic layer. In contrast, F-C₈-Cu/ZnO/Al₂O₃ exhibits greater thermal stability than M-Cu/ZnO/Al₂O₃(10) within the measured temperature range, attributed to the high bond energy and inertness of the C-F bond, which is less prone to fracture during heat treatment [51,52]. Furthermore, the decomposition temperature of the PFOTES-modified hydrophobic layer generally exceeds 300 °C [42,53,54,55], and the water contact angle remains as high as 147.3° at 300 °C, consistent with the results of TGA.

To further assess the hydrophobicity of the hydrophobic catalyst during methanol synthesis, we investigated the long-term thermal stability of M-Cu/ZnO/Al₂O₃(10) and F-C₈-Cu/ZnO/Al₂O₃ at 240 °C (Figure 9). The water contact angle of both hydrophobic catalysts decreased with increasing heat treatment duration. After being subjected to heat treatment at 240 °C for 24 h, the water contact angle of M-Cu/ZnO/Al₂O₃(10) significantly decreased from 153.0° to 95.5°. This reduction is attributed to the rupture of chemical bonds in the hydrophobic layer caused by prolonged heat exposure, leading to the dissociation of hydrophobic -CH₃ groups into smaller gas molecules. Consequently, silica with high surface energy is generated [38], which results in a further decrease in the water contact angle. In contrast, after heat treatment at 240 °C for 24 h, the water contact angle of F-C₈-Cu/ZnO/Al₂O₃ only slightly decreased from 153.0° to 146.5°, indicating that the catalyst surface retains good hydrophobicity. Therefore, F-C₈-Cu/ZnO/Al₂O₃ demonstrates superior long-term thermal stability compared to M-Cu/ZnO/Al₂O₃(10) at 240 °C.

3.3. Catalytic Evaluation

First, the effect of the reaction temperature on the catalytic performance of M-Cu/ZnO/Al₂O₃(10) and F-C₈-Cu/ZnO/Al₂O₃ was investigated at 4 MPa, with a volume space velocity (${\omega }_{sp}$) of 500 L(STP) kg⁻¹ h⁻¹, and a reaction temperature range from 220 °C to 260 °C, as illustrated in Figure 10. The performance of the two hydrophobic catalysts initially increased from 220 °C to 240 °C and subsequently decreased when the temperature was raised from 240 °C to 260 °C. Therefore, 240 °C was identified as the optimal temperature, resulting from the opposing effects of kinetics and reaction equilibrium. The maximum values of $x_{CO}$ and $r_{MeOH}$ for M-Cu/ZnO/Al₂O₃(10) are 37.0% and 1.59 mol kg⁻¹ h⁻¹, respectively, while the maximum values for F-C₈-Cu/ZnO/Al₂O₃ are 38.3% and 1.65 mol kg⁻¹ h⁻¹, respectively.

Moreover, the reaction rates of M-Cu/ZnO/Al₂O₃(10) and F-C₈-Cu/ZnO/Al₂O₃ at 240 °C, 4 MPa and the ${\omega }_{sp}$ of 500–1500 L(STP) kg⁻¹ h⁻¹ are given in Figure 11. The $x_{CO}$ was inversely proportional to the syngas volume space velocity. This relationship arises from the fact that an increase in space velocity reduces the residence time of the syngas in the catalyst bed, leading to insufficient contact between the gas reactants and the catalyst, which results in a decrease in $x_{CO}$. As the ${\omega }_{sp}$ of syngas increased from 500 to 1500 L(STP) kg⁻¹ h⁻¹, the $x_{CO}$ values of M-Cu/ZnO/Al₂O₃(10) and F-C₈-Cu/ZnO/Al₂O₃ decreased from 37% and 38.3% to 11% and 13.4%, respectively.

The catalytic performance of hydrophilic and hydrophobic catalysts in the gas–solid methanol synthesis process was compared at 4 MPa, 260 °C, and the ${\omega }_{sp}$ of 500–1500 L(STP) kg⁻¹ h⁻¹, as shown in Figure 12. Under the measured space velocity, the $x_{CO}$ of the hydrophobic catalyst was enhanced compared to that of the hydrophilic catalyst, with the improvement of F-C₈-Cu/ZnO/Al₂O₃ being greater than that of M-Cu/ZnO/Al₂O₃(10). Some studies have indicated the following: on one hand, the water molecules generated during methanol synthesis can adsorb onto the active sites of the Cu-based catalyst, converting copper into copper carbonate; on the other hand, the presence of water molecules can promote the growth of Cu particles, leading to the deactivation of the Cu-based catalyst. Khodakov et al. [56] investigated the effect of water on the activity of the Cu/ZSM-5 catalyst during the direct synthesis of dimethyl ether from syngas, characterizing the catalyst before and after the reaction by in situ XRD and TEM. The results demonstrated that the presence of water facilitated the migration of small Cu nanoparticles and accelerated the sintering of Cu particles, resulting in the formation of larger Cu nanoparticles, although no Cu oxidation was detected during the reaction. Simanungkalit et al. [57] studied the activity of the Cu/ZnO/Al₂O₃ catalyst during the direct synthesis of methanol from biomass pyrolysis syngas and found that the syngas contained a higher concentration of CO₂ (~20%), which promoted the oxidation of Cu⁺ and Cu⁰ on the catalyst surface to Cu²⁺, leading to catalyst deactivation. The hydrophobic modification of the catalyst surface can accelerate the desorption of by-product water molecules from the active sites, reduce the concentration of water molecules at these sites, and maintain the activity of the catalyst. Consequently, M-Cu/ZnO/Al₂O₃(10) and F-C₈-Cu/ZnO/Al₂O₃ exhibited higher reactivity than Cu/ZnO/Al₂O₃.

Materials Studio 2020 (Accelrys Software Inc., Carson, NV, USA) was utilized to construct three catalyst interface models (Figure 13a–c). The Forcite module was employed to simulate the dynamics of water molecules within these catalyst interface models. Molecular dynamics simulations, lasting 500 ps, were conducted at a temperature of 260 °C within the NVT ensemble to investigate the impact of the wettability of the catalyst surface on the diffusion of water molecules. The mean square displacement (MSD) curves of the water molecules in the three catalyst interface models are shown in Figure 14. The diffusion of the water molecules on the surface of the unmodified catalyst is limited, with a diffusion coefficient of D = 1.84 × 10⁻⁸ m² s⁻¹. However, after hydrophobic modification, the diffusion coefficients of the water molecules on the M-Cu/ZnO/Al₂O₃(10) and F-C₈-Cu/ZnO/Al₂O₃ surfaces increase to 2.38 × 10⁻⁸ m² s⁻¹ and 3.06 × 10⁻⁸ m² s⁻¹, respectively, significantly accelerating the diffusion rate of the water molecules. Moreover, the MSD curve for the water molecules on the surface of F-C₈-Cu/ZnO/Al₂O₃ is greater than that for M-Cu/ZnO/Al₂O₃(10). This difference can be attributed to the fact that perfluorosilane, compared to silane, has lower surface energy, reduced interaction with water, and facilitates easier diffusion of water molecules on the catalyst surface [58].

We further investigated the influence of catalyst surface wettability on the diffusion of water molecules and examined the distribution of these molecules before and after simulation in the direction perpendicular to the catalyst surface, as illustrated in Figure 15. In the initial state, the relative concentration of water molecules reaches its maximum at distances of 3.33 Å, 4.24 Å, and 7.25 Å from the surfaces of Cu/ZnO/Al₂O₃ (Figure 15a), M-Cu/ZnO/Al₂O₃(10) (Figure 15b), and F-C₈-Cu/ZnO/Al₂O₃ (Figure 15c), respectively. Additionally, the relative concentration of water molecules on the surface of the unmodified catalyst is high at the surface and low at the center. In contrast, the relative concentration of water molecules on the surface of the hydrophobic modified catalyst shows minimal variation with spatial position after reaching its maximum value. This indicates that hydrophobic modification of the catalyst surface can effectively reduce the adsorption of water molecules and decrease their relative concentration on the surface [20]. After simulation for the unmodified catalyst, nearly all water molecules were concentrated on the catalyst surface, with the relative concentration of water molecules in the center dropping to zero. In the case of the hydrophobic modified catalysts, although water molecules were slightly clustered near the surface, they were still present in the center. Notably, the relative concentration of water molecules at the center of F-C₈-Cu/ZnO/Al₂O₃ was greater than that of M-Cu/ZnO/Al₂O₃(10). This observation suggests that water molecules tend to accumulate differently on various catalyst surfaces after simulation. However, the hydrophobic modification of the catalyst surface effectively reduces the adsorption of water molecules and enhances their diffusion. F-C₈-Cu/ZnO/Al₂O₃ demonstrated a superior hydrophobic effect compared to M-Cu/ZnO/Al₂O₃(10).

4. Conclusions

In this work, the influence of hydrophobic modification on the stability of Cu/ZnO/Al₂O₃ in the methanol synthesis from syngas was investigated. Superhydrophobic catalysts were prepared by etching Cu/ZnO/Al₂O₃, followed by the sol–gel and grafting methods, respectively, and characterized by SEM, FT-IR, BET, XRD, and TGA. The thermal stability and catalytic properties of M-Cu/ZnO/Al₂O₃(10) and F-C₈-Cu/ZnO/Al₂O₃ were tested. Additionally, the difference of diffusion behaviors of water molecules on the catalyst surfaces before and after hydrophobic modification were compared by molecular dynamics simulations. The results showed that a micro/nano structure composed of nanosheets and nanosheet clusters formed on the surface of the etched catalyst, increasing its surface roughness. Hydrophobic modification did not alter the physical structure of the catalyst; both M-Cu/ZnO/Al₂O₃(10) and F-C₈-Cu/ZnO/Al₂O₃ exhibited water contact angles exceeding 150°. According to the molecular dynamics simulation results, the hydrophobic surfaces of the two modified catalysts accelerated the desorption of by-product water molecules from the active sites of the catalysts and reduced the relative concentration of water molecules at these sites, thereby improving the stability of the catalyst. The catalytic activity of the two hydrophobic catalysts surpassed that of the hydrophilic catalyst, with F-C₈-Cu/ZnO/Al₂O₃ demonstrating superior thermal stability and hydrophobicity compared to M-Cu/ZnO/Al₂O₃(10), as well as higher methanol synthesis activity. Compared to Cu/ZnO/Al₂O₃, F-C₈-Cu/ZnO/Al₂O₃ exhibited a 67.8% increase in the diffusion coefficient of water molecules on its surface. Additionally, CO conversion during the gas–solid methanol synthesis process was enhanced by 43.5% at 260 °C, 4 MPa, and 1500 L(STP) kg⁻¹ h⁻¹. This work provides a novel approach to improve the stability of Cu-based catalysts for methanol synthesis from syngas, addressing the issue of deactivation due to water.