Silicalite-1 Zeolite-Supported Cu Nanoparticles for Ethanol Dehydrogenation: Influence of Silanols

Abstract

The selective dehydrogenation of ethanol to acetaldehyde is an efficient alternative to biomass valorization. Herein, a series of Cu catalysts supported on Silicalite-1 zeolites with tunable contents of surface silanols and the same Cu loading of 3 wt% were synthesized by an impregnation method. The parent Silicalite-1 supports and as-synthesized Cu/S-1 catalysts were characterized by N₂ adsorption, XRD, SEM, TEM, TGA, DRIFT, ²⁹Si MAS NMR, XPS, and TPR. The Cu dispersion and Cu species distribution of Cu/S-1 catalysts can be modulated by engineering the amount of silanol groups on the support. More silanols present on the surfaces of parent Silicalite-1 supports can promote the Cu dispersion, and lead to a higher Cu⁺/Cu⁰ molar ratio arising from strong interfacial interaction between Cu species and silanols on the Silicalite-1 support via the formation of Si-O-Cu bonds. Thus, higher catalytic activity is achieved.

1. Introduction

Currently, the attention to environmental issues is rising, with increasing in-depth research focused on renewable energy [1]. Among these research initiatives, renewable biomass energy has become a research hotspot due to its unique advantages [2]. As a product of biomass with high productivity and versatility [3,4,5], ethanol is primarily used as a renewable fuel additive and serves as a key platform chemical for upgrading into various high-value chemicals and fuels, including acetaldehyde, propene, n-butanol, butadiene, and ethyl acetate [6,7,8,9,10,11]. The direct dehydrogenation of ethanol is of great significance, as acetic acid, trichloroacetaldehyde, and other important chemicals can be produced by using acetaldehyde. Thus, selective ethanol dehydrogenation to acetaldehyde has attracted increasing interest due to its fundamental and technological importance [6,12,13,14,15,16,17,18,19,20].

In the ethanol dehydrogenation reaction, the cleavage of O-H and C-H bonds represents the rate-determining step [6,12]. Previous studies have shown that copper catalysts exhibit distinctive activity and selectivity in this process owing to their superior capability of O-H and C-H cleavages [21,22,23]. Because of this ability combined with their low price, Cu-based catalysts have received significant attention. Various kinds of supports were developed to disperse Cu nanoparticles (NPs), such as mixed metal oxides possessing both acidic and basic sites [24,25,26], silica-based materials with relatively weak acid sites including amorphous silica, mesoporous silica, and Silicalite-1 zeolite [27,28,29,30,31,32], carbon materials exhibiting inert surface properties [22,33], and basic oxides [20,34]. The Cu⁺ species are considered to be the most active sites for ethanol dehydrogenation [28,31,35]. Thus, maintaining a high proportion of Cu⁺ species in the catalyst can enhance the activity. Pang et al. [28] found that Silicalite-1 zeolite-supported copper catalysts displayed high catalytic stability for ethanol dehydrogenation with high activity. The formation of Cu-O-Si bonds between Cu species and silanol groups of the Silicalite-1 support stabilizes the Cu⁺ species, which plays a pivotal role in their superior performance.

However, to our knowledge, there has been no report about a detailed investigation of the impact of the number of silanols on Silicalite-1 zeolites on the catalytic activity of a supported Cu catalyst for the dehydrogenation of ethanol. In this work, Silicalite-1 zeolites with tunable amounts of surface silanols were prepared via a hydrothermal method, and used as supports to prepare copper catalysts by an impregnation method. The influences of silanols of Silicalite-1 supports on Cu species and thereby catalytic activity for ethanol dehydrogenation were studied.

2. Results and Discussion

2.1. Characterization of Catalysts

The XRD patterns of parent Silicatlite-1 supports presented in Figure S1 show typical diffraction peaks of MFI topological structure (PDF #44-0003) [36,37]. As depicted in Figure 1, in addition to the typical diffraction peaks of MFI crystals, two peaks at 2θ = 43.3° and 50.4° can be observed for Cu/S-1(2), Cu/S-1(3), and Cu/S-1(4) catalysts, which are indexed to the (111) and (200) planes of Cu crystallites, respectively (PDF #04-0836). No reflections assigned to metallic Cu can be discernible for the Cu/S-1(1) catalyst. The intensity of the Cu (111) peak increases slightly in the order of Cu/S-1(2) < Cu/S-1(3) < Cu/S-1(4). The XRD observation indicates that the Cu nanoparticle size follows the order of Cu/S-1(1) < Cu/S-1(2) < Cu/S-1(3) < Cu/S-1(4).

As shown in Figure S2, the SEM images of parent Silicatlite-1 supports show that both S-1(1) and S-1(2) samples exhibit a predominantly uniform and spherical morphology, with grain sizes of approximately 70 and 260 nm, respectively. The S-1(3) and S-1(4) samples possess both submicron-sized and micron-sized grains. For the synthesis of Silicalite-1 zeolites, increasing the concentration of OH⁻ (i.e., higher TPAOH/SiO₂ ratio and lower H₂O/SiO₂ ratio) can accelerate the dissolution of Si, thus leading to the generation of Silicalite-1 with smaller crystallite size. The N₂ sorption isotherms and BJH adsorption pore size distributions of the parent Silicatlite-1 supports are illustrated in Figure S3. The N₂ adsorption isotherms are of type I as classified by IUPAC recommendations, indicating their microporous character. For the S-1 support, a significant hysteresis at p/p₀ between 0.9 and 1.0 was observed, suggesting the existence of additional mesopores contributing from intercrystalline voids in this sample. The mesopore size is centered at 30.3 nm, as shown in Figure S3b. The BET surface area follows the order of S-1(1) > S-1(2) > S-1(3) ≈ S-1(4), while the micropore surface area and volume are similar for differently prepared Silicalite-1 zeolites (

Table 1. Textural properties of parent Silicalite-1 supports.

Sample	SBET	Sext	Smicro	Vmicro	n(OH)	Q3/Q4
	(m2/g)	(m2/g) a	(m2/g) a	(cm3/g) a	(mmol/g) b	Ratio c
S-1(1)	491	148	343	0.143	1.09	0.108
S-1(2)	437	93	344	0.148	1.02	0.100
S-1(3)	405	79	326	0.143	0.77	0.073
S-1(4)	400	75	325	0.142	0.63	0.050

^a External surface area (S_ext), micropore surface area (S_micro), and micropore volume (V_micro), as determined by the t-plot method; ^b Amount of silanols as determined by TGA; ^c Peak area ratio of Q³ to Q⁴ as determined by ²⁹Si MAS NMR.

). The external surface area decreases in the order of S-1(1) > S-1(2) > S-1(3) > S-1(4), indicating that a Silicalite-1 zeolite with smaller crystallite size possesses a higher external surface area.

The HAADF-STEM image analysis of Cu/S-1(1) and Cu/S-1(2) as representative catalysts reveals that well-dispersed Cu nanoparticles with average sizes of ~1.5 and ~3.2 nm, respectively, are formed (Figure 2a,c), which are consistent with the average particle sizes estimated by N₂O titration (

Table 2. Physicochemical properties of the Cu/S-1 catalysts.

Catalyst	Cu Loading	SBET	Sext	Smicro	Vmicro	DCu	dCu	Cu+/Cu0	SCu0	SCu+
	(wt%) a	(m2/g)	(m2/g) b	(m2/g) b	(cm3/g) b	(%) c	(nm) c	Ratio d	(m2/g) c	(m2/g) e
Cu/S-1(1)	3.03	408	72	336	0.140	65	1.5	1.17	12.2	14.2
Cu/S-1(2)	3.02	409	72	337	0.145	32	3.1	1.08	6.6	7.1
Cu/S-1(3)	3.03	366	47	319	0.141	22	4.5	0.96	4.4	4.2
Cu/S-1(4)	3.02	382	61	321	0.141	12	8.3	0.75	2.3	1.8

^a Determined by ICP-AES; ^b External surface area (S_ext), micropore surface area (S_micro), and micropore volume (V_micro), as determined by the t-plot method; ^c Cu dispersion (D_Cu), Cu average volume-surface diameter (d_Cu), and surface area of Cu⁰ (S_Cu⁰), as determined by N₂O titration; ^d Molar ratio of Cu⁺ to Cu⁰ as determined by Cu LMM XAES; ^e Surface area of Cu⁺ (S_Cu⁺) as calculated by Cu⁺/Cu⁰ × S_Cu⁰.

). The corresponding EDX mapping analysis clearly shows that Cu is uniformly distributed on the S-1(1) and S-1(2) supports (Figure 2b,d). An even distribution of Cu on S-1(3) and S-1(4) was also observed (Figure S4).

In comparison with parent Silicatlite-1 supports, the corresponding Cu/S-1 catalysts show a major loss of the external surface area and yet a limited decrease in the micropore surface area and volume after Cu addition (Table 1 and Table 2). The above observation indicates that most supported Cu NPs are located on the external surfaces of differently prepared Silicalite-1 zeolites [31]. The elemental analysis of ICP-AES indicates that the actual Cu loadings of Cu/S-1 catalysts are equivalent to the theoretical Cu content (3 wt%), as shown in Table 2.

The amount of surface silanols on parent Silicatlite-1 supports was estimated by thermogravimetry in a N₂ flow. Figure S5 illustrates the TGA and DTG curves. According to the result from a previous study [38], the steep weight loss below 100 °C and slow weight loss between 100 and 200 °C are attributed, respectively, to the removal of physically adsorbed and chemically adsorbed water. The weight loss between 200 and 850 °C is attributed to the dehydroxylation by condensation of silanols. Based on the weight loss, the estimated silanol numbers for S-1(1), S-1(2), S-1(3), and S-1(4) are 1.09, 1.02, 0.77, and 0.63 mmol/g, respectively, as shown in Table 1.

²⁹Si MAS NMR can offer information about the local environment of Si atoms in the MFI-type zeolite framework. Figure 3 illustrates the ²⁹Si MAS NMR spectra of differently prepared Silicalite-1 zeolites. The peaks at −113 and −103 ppm are attributed to the Q⁴ (Si-[(OSi)₄]) and Q³ (HO-Si-[(OSi)₃]) structures in Silicalite-1, respectively [39,40]. The additional peak at −110 ppm observed for S-1(3) and S-1(4) is also assigned to the Q⁴ species [40]. The presence of a Q³ signal indicates the presence of silanol defects, while the low-resolution Q⁴ signal is also a typical manifestation of silanol defects in the Silicalite-1 zeolite framework [41]. As listed in Table 1, the peak area ratio of Q³ to Q⁴ shows a progressive decrease from samples S-1(1) to S-1(4), which indicates that the amount of silanol groups gradually decreases. This result is in accordance with the quantitative data of surface silanols determined by TGA (Table 1). For the synthesis of Silicalite-1 zeolites, the positive charge of the structure-directing agent (TPA⁺) is balanced by siloxy defects (≡Si–O⁻). Thus, an increased concentration of hydroxide anions (i.e., higher TPAOH/SiO₂ ratio and lower H₂O/SiO₂ ratio), along with an increased concentration of cations (TPA⁺), contributes to the generation of a higher number of framework defects as more silanols get deprotonated to compensate the charge of cations [42].

To gain a deep insight into the nature of surface silanols on parent Silicatlite-1 supports and their interaction with Cu species, additional investigation was performed using DRIFT spectra. Figure 4 reveals the presence of three types of silanol groups (Si-OH). The intense and sharp peak at 3730 cm⁻¹ is assigned to isolated silanols on the external surface of the Silicalite-1 zeolite, while the small peak observed at 3686 cm⁻¹ is associated with vicinal silanols within the micropores [43,44,45,46,47,48]. The nest silanols are interacted by multiple silanol groups through extended hydrogen bonds, corresponding to the intense and wide peak at 3490 cm⁻¹ [43,44,46,47,49]. The peak intensities of surface silanols follow the order of S-1(1) > S-1(2) > S-1(3) > S-1(4), suggesting that the amount of silanol groups decreases in this order, which is in agreement with the results of TGA and ²⁹Si MAS NMR. After loading of Cu NPs, an obvious decrease in the peak intensities of three types of silanols were observed when comparing with the bare Silicalite-1 supports, affording a decrease by 51%, 47%, 32%, and 19%, respectively, for Cu/S-1(1), Cu/S-1(2), Cu/S-1(3), and Cu/S-1(4). This result indicates that a portion of surface Si-OH groups were occupied by Cu species via the formation of Si-O-Cu bonds [28,50], revealing the strong interaction between Cu species and silanols on the surfaces of Silicalite-1 supports. The formation of Si-O-Cu bonds stabilizes the Cu⁺ species.

To investigate the reducibility of CuO/S-1 precursors, H₂-TPR analysis was performed, and the results are illustrated in Figure 5. For CuO/S-1(1) and CuO/S-1(2), an intense peak at ca. 200 °C coupled with a small reduction peak at relatively high temperature can be found. The low-temperature peak and high-temperature one can be attributed to the reduction of dispersed CuO with small size and bulk CuO with large size, respectively [28,51]. Differently, over CuO/S-1(3) and CuO/S-1(4), two reduction peaks are overlapped. The large proportion of high-temperature peaks observed for CuO/S-1(3) and CuO/S-1(4) suggests the larger CuO particles. Thus, CuO particles supported on Silicalite-1 zeolite with less silanols have larger size. The overlap of two reduction peaks could be associated with the larger CuO particles on CuO/S-1(3) and CuO/S-1(4). To gauge the Cu dispersion and exposed surface area, an N₂O titration experiment was conducted. As shown in Table 2, the copper dispersion of Cu/S-1(1), Cu/S-1(2), Cu/S-1(3), and Cu/S-1(4) decreases gradually from 65% to 12%. Accordingly, the surface area of Cu⁰ follows the same order. Combining the amount of silanols on parent Silicalite-1 supports, it can be concluded that more silanols present on the surface of a parent Silicalite-1 zeolite favor the dispersion of copper on the support.

The Cu/S-1 catalysts were further investigated by XPS to reveal the chemical state of Cu species on differently prepared Silicalite-1 zeolites. The XPS survey spectra and C 1s and Si 2p XPS spectra are illustrated in Figures S6 and S7, respectively. Figure 6a illustrates the Cu 2p XPS spectra of the catalysts. Two peaks can be observed at 932.7 eV and 952.7 eV, which are attributed to Cu⁰ and/or Cu⁺ species of Cu 2p_3/2 and Cu 2p_1/2, respectively [27,28,29,52,53,54]. To further distinguish and quantify Cu⁰ and Cu⁺ species which have similar binding energy, the Cu LMM X-ray-induced AES (XAES) analysis was performed, and the results are illustrated in Figure 6b. The Auger signal was deconvoluted into two symmetrical peaks. The peak at 914.2 eV signifies Cu⁺ species, while the one at 917.4 eV denotes Cu⁰ species [27,28,55,56]. The Cu⁺/Cu⁰ molar ratio, calculated according to the corresponding areas of deconvoluted peaks, follows the order of Cu/S-1(1) (1.17) > Cu/S-1(2) (1.08) > Cu/S-1(3) (0.96) > Cu/S-1(4) (0.75). Based on the Cu⁰ surface area and Cu⁺/Cu⁰ molar ratio, the surface area of Cu⁺ was calculated [57,58]. It was found that the Cu⁺ surface area decreases in the order of Cu/S-1(1) > Cu/S-1(2) > Cu/S-1(3) > Cu/S-1(4). More silanols of the Silicalite-1 support are conducive to anchoring Cu⁺ species. Hence, the Cu species distribution of Silicalite-1-supported Cu catalysts can be tailored by engineering the amount of silanols on the support. The above findings reveal the co-existence of Cu⁰ and Cu⁺ species in the Cu/S-1 catalysts. Cu⁺ species may adhere to Cu particles [29,53]. The appearance of Cu⁺ species confirms strong interfacial interaction between Cu species and the Silicalite-1 support through the formation of Si-O-Cu bonds [28,29,50,59,60].

By combining TGA, N₂O titration, and Cu LMM XAES results, it can be found that the Cu⁺/Cu⁰ ratios and total surface areas of Cu⁰ and Cu⁺ for the Cu/S-1 catalysts increase with the amount of silanols present on parent Silicalite-1 supports (Figure 7). To sum up, more silanols present on the surfaces of parent Silicalite-1 supports can promote Cu dispersion, and lead to higher Cu⁺/Cu⁰ ratios and surface areas of Cu⁰ and Cu⁺. Thus, higher catalytic activity can be expected.

2.2. Catalytic Performance

The catalytic dehydrogenation of ethanol to acetaldehyde over Silicalite-1 zeolite-supported Cu catalysts was investigated at 250 °C, and the results are shown in Figure 8. The ethanol conversion decreases in the order of Cu/S-1(1) (77.6%) > Cu/S-1(2) (65.8%) > Cu/S-1(3) (52.1%) > Cu/S-1(4) (25.5%). Combined with the amount of silanols on parent Silicalite-1 supports, it can be found that the more silanols bare Silicalite-1 supports have, the more active supported Cu catalysts are. A positive linear correlation between the reaction rate and the number of silanols on the support was established, as illustrated in Figure 9. The acetaldehyde selectivities of Cu/S-1(1), Cu/S-1(2), Cu/S-1(3) and Cu/S-1(4) increase gradually from 92.0% to 96.8%. Besides the main product acetaldehyde, small amounts of byproducts were observed, including acetone, butyraldehyde, ethyl acetate, ethylene, and butanol.

We have correlated the reaction rates of Cu/S-1 catalysts with their Cu⁺/Cu⁰ ratios and the total surface areas of Cu⁰ and Cu⁺. As illustrated in Figure 10, the reaction rates are positively correlated with the Cu⁺/Cu⁰ ratios and total surface areas of Cu⁰ and Cu⁺ for the Cu/S-1 catalysts. On the basis of DFT calculations, Pang et al. have proposed that both Cu⁺ and Cu⁰ species are active for the ethanol dehydrogenation [28]. Cu⁺ species served as active sites for C-H bond cleavage which is the rate-controlling step in ethanol dehydrogenation. Simultaneously, the generated hydrogen atoms were transferred to Cu⁰ sites for combination to release H₂. Obviously, our results (Figure 10) are in good agreement with the reaction mechanism proposed by Pang et al. [28].

3. Materials and Methods

3.1. Reagent and Materials

The tetraethyl orthosilicate (TEOS), tetrapropylammonium hydroxide (TPAOH), and ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). The Cu(NO₃)₂·3H₂O was purchased from Macklin Biochemical Co. Ltd. (Shanghai, China). All the chemicals were of analytical grade and used as received.

3.2. Catalyst Preparation

Four kinds of Silicalite-1 zeolites with different contents of OH groups designated as S-1(1), S-1(2), S-1(3), and S-1(4) were synthesized via a hydrothermal method. They were synthesized with the same procedure upon varying the TPAOH/SiO₂ ratio, H₂O/SiO₂ ratio, and crystallization time.

Synthesis of S-1(1): TEOS and TPAOH (40 wt%) were added to deionized water, resulting in a molar composition of 1.0SiO₂: 0.25TPAOH: 8.0H₂O. The solution was stirred at 80 °C for 24 h to form a gel, which was then transferred to a 100 mL Teflon-lined stainless-steel autoclave, and heated at 170 °C for 24 h. Subsequently, the obtained solid was washed with deionized water, dried at 100 °C overnight, and finally calcined in air at 550 °C for 6 h.

Synthesis of S-1(2): TEOS and TPAOH (25 wt%) were added to deionized water, resulting in a molar composition of 1.0SiO₂: 0.18TPAOH: 15.4H₂O. The solution was stirred at room temperature for 6 h to form a gel, which was then transferred to a 100 mL Teflon-lined stainless-steel autoclave, and heated at 170 °C for 48 h, followed by the same treatment as above.

Synthesis of S-1(3): The procedure was the same as that of S-1(2), except that the molar composition was changed to 1.0SiO₂: 0.12TPAOH: 19.2H₂O, and the crystallization time was changed to 72 h.

Synthesis of S-1(4): The procedure was the same as that of S-1(2), except that the molar composition was changed to 1.0SiO₂: 0.15TPAOH: 44.2H₂O.

The Silicalite-1 zeolite-supported Cu catalysts (labelled as Cu/S-1) were synthesized by an impregnation method. A certain amount of Cu(NO₃)₂·3H₂O was dissolved in deionized water. Subsequently, a certain amount of Silicalite-1 zeolites were added to the solution, which were dried under an infrared lamp. The resulting solids were dried at 100 °C overnight, and then calcined at 500 °C for 4 h in air to yield the CuO/S-1 precursors. The CuO/S-1 precursors were reduced at 300 °C for 1 h in a flow of 10 vol% H₂/Ar to yield the Cu/S-1 catalysts with 3 wt% nominal Cu mass loading, which was calculated according to the formula:

$$\mathrm{C}\mathrm{u} \mathrm{l}\mathrm{o}\mathrm{a}\mathrm{d}\mathrm{i}\mathrm{n}\mathrm{g} \left(\mathrm{w}\mathrm{t}\mathrm{\%}\right)= {\displaystyle \frac{m_{\mathrm{C}\mathrm{u}}}{m_{\mathrm{C}\mathrm{u}}+m_{\mathrm{s}\mathrm{u}\mathrm{p}\mathrm{p}\mathrm{o}\mathrm{r}\mathrm{t}}}} \times 100\mathrm{\%}$$

3.3. Catalyst Characterization

X-ray diffraction (XRD) patterns were measured on a D2 PHASER diffractometer (Bruker, Billercia, MA, USA) with Cu Kα radiation at 30 kV and 10 mA. Scanning electron microscopy (SEM) images were acquired on a Nova NanoSem 450 microscope (FEI, Hillsboro, OR, USA). The high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images and elemental mapping were obtained on a Tecnai G² F20 S-TWIN microscope (FEI, Hillsboro, OR, USA). The BET surface areas and micropore volumes were measured by nitrogen adsorption at −196 °C using a Tristar 3020 apparatus (Micromeritics, Atlanta, GA, USA). The samples were dehydrated under a vacuum at 300 °C for 5 h before the measurement. Thermogravimetric analysis (TGA) was conducted in a N₂ flow from room temperature to 900 °C at a ramp rate of 10 °C/min using an SDT Q600 instrument (TA, New Castle, DE, USA). ²⁹Si magic angle spinning nuclear magnetic resonance (MAS NMR) measurements were conducted on an Avance III 400 WB spectrometer (Bruker, Billercia, MA, USA) operating at a resonance frequency of 400 MHz. Diffuse reflectance infrared Fourier transform (DRIFT) spectra were collected using an iS50 spectrometer (Thermo Fisher, Waltham, MA, USA), which was equipped with a heating accessory. The samples were pretreated for 1 h in a He flow at 450 °C, followed by cooling to 300 °C under flowing He. Then, DRIFT spectra were measured at 300 °C. The background spectrum was collected at 300 °C using KBr as the reference material. The X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) measurements were conducted using an AXIS Kratos Supra⁺ spectrometer (Kratos, Trafford, GM, UK), excited by an Al Kα radiation source. The charge increase was balanced using a built-in electron flood gun during measurements. The binding energy values were calibrated against the adventitious C 1s core level at 284.6 eV. The actual Cu loadings of the catalysts were determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) using an Optima 8000 instrument (Perkin-Elmer, Waltham, MA, USA).

The H₂ temperature-programed reduction (H₂-TPR) analysis was conducted using an AutoChem II instrument (Micromeritics, Atlanta, GA, USA). For this process, 0.1 g of CuO/S-1 (40–60 mesh) was pretreated at 300 °C under a He flow for 1 h, followed by cooling to 60 °C in flowing He. The gas was changed to 10 vol% H₂/Ar (30 mL/min). The temperature was then ramped up to 500 °C at a rate of 10 °C/min. The Cu dispersion was determined by N₂O oxidation and then by H₂ titration on the same apparatus [61]. Next, 0.1 g of CuO/S-1 (40–60 mesh) were initially reduced at 300 °C for 1 h in a 10 vol% H₂/Ar flow (30 mL/min), followed by cooling to 60 °C. Then, the gas was switched to 20 vol% N₂O/Ar (30 mL/min) and kept at this temperature for 1 h to oxidize surface Cu atoms to Cu₂O, followed by sweeping with He (30 mL/min) for 1 h. Finally, the same H₂-TPR experiment was conducted as above. A₁ and A₂ denote the hydrogen consumption amount in the first and second H₂-TPR experiments, respectively. The copper dispersion (D_Cu), the area of surface Cu⁰ (S_Cu⁰), and average volume-surface diameter (d_Cu) were calculated according to the equations:

$$D_{\mathrm{C}\mathrm{u}} = {\displaystyle \frac{2\times A_2}{A_1}} \times 100\%$$

$$S_{{\mathrm{C}\mathrm{u}}^0} = {\displaystyle \frac{2\times A_2\times N_{\mathrm{A}}}{A_1\times M_{\mathrm{C}\mathrm{u}}\times 1.4\times {10}^{19}}} ={\displaystyle \frac{1353\times A_2}{A_1}}\left({\mathrm{m}}^2/{\mathrm{g}}_{\mathrm{C}\mathrm{u}}\right)$$

$$d_{\mathrm{C}\mathrm{u}} = {\displaystyle \frac{6000}{S_{{\mathrm{C}\mathrm{u}}^0}\times {\rho }_{\mathrm{C}\mathrm{u}}}} \left(\mathrm{n}\mathrm{m}\right)$$

where N_A is Avogadro’s constant, M_Cu is Cu molar mass (63.46 g/mol), 1.4 × 10¹⁹ is the number of Cu atoms/m², and ρ_Cu is the copper density (8.92 g/cm³).

3.4. Catalytic Tests

The dehydrogenation of ethanol was conducted in a flow-type fixed-bed reactor under ambient pressure, with a quartz tube internal diameter of 6 mm. For this process, 0.17 g of CuO/S-1 (40–60 mesh) was in situ reduced at 300 °C for 1 h under a flow of 10 vol% H₂/Ar, followed by cooling to a reaction temperature of 250 °C. Then, a gas mixture (5 vol% ethanol and 95 vol% N₂) with a total flow rate of 120 mL/min was passed through the reactor. Ethanol was introduced into the reactor through a bubbler, with a weight hourly space velocity (WHSV) of 4 h⁻¹. The analysis of the products was conducted online using a gas chromatograph equipped with a flame ionization detector and an FFAP capillary column (30 m × 0.32 mm × 0.25 µm). The calculation formulas of ethanol conversion and acetaldehyde selectivity are as follows:

$$\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n} \left(\mathrm{\%}\right)= {\displaystyle \frac{{\left[\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{o}\mathrm{l}\right]}_{\mathrm{i}\mathrm{n}}-{\left[\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{o}\mathrm{l}\right]}_{\mathrm{o}\mathrm{u}\mathrm{t}}}{{\left[\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{o}\mathrm{l}\right]}_{\mathrm{i}\mathrm{n}}}} \times 100\mathrm{\%}$$

$$\mathrm{S}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y} \left(\mathrm{\%}\right)= {\displaystyle \frac{{\left[\mathrm{a}\mathrm{c}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{d}\mathrm{e}\mathrm{h}\mathrm{y}\mathrm{d}\mathrm{e}\right]}_{\mathrm{o}\mathrm{u}\mathrm{t}}}{{\left[\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{o}\mathrm{l}\right]}_{\mathrm{i}\mathrm{n}}-{\left[\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{o}\mathrm{l}\right]}_{\mathrm{o}\mathrm{u}\mathrm{t}}}} \times 100\mathrm{\%}$$

4. Conclusions

In this contribution, a series of Cu/S-1 catalysts with 3 wt% Cu loading were prepared by an impregnation method using differently synthesized Silicalite-1 zeolites with tunable amounts of surface silanols as supports. The Cu dispersion and Cu⁺/Cu⁰ molar ratio of Cu/S-1 catalysts can be tailored by engineering the amount of silanols on the support. Cu⁰ and Cu⁺ species co-exist in the Cu/S-1 catalysts. The occurrence of Cu⁺ species originates from strong interfacial interaction between Cu species and silanols on the Silicalite-1 support through the formation of Si-O-Cu bonds. The amount of surface silanols on the parent Silicalite-1 was found to correlate positively with Cu dispersion and Cu⁺/Cu⁰ ratio. Thus, the Cu/S-1 catalyst with more silanols on the parent Silicalite-1 support exhibits a higher activity for ethanol dehydrogenation to acetaldehyde.