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Article

Effect of NaOH Concentration on Rapidly Quenched Cu–Al Alloy-Derived Cu Catalyst for CO2 Hydrogenation to CH3OH

by
Xuancheng Liu
1,
Dong Sun
1,
Yushan Ji
1,
Sijie Zu
1,
Yan Pei
1,
Shirun Yan
1,
Minghua Qiao
1,*,
Xiaoxin Zhang
2 and
Baoning Zong
2,*
1
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University, Shanghai 200438, China
2
State Key Laboratory of Catalytic Materials and Reaction Engineering, Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(6), 391; https://0-doi-org.brum.beds.ac.uk/10.3390/catal14060391
Submission received: 10 May 2024 / Revised: 12 June 2024 / Accepted: 17 June 2024 / Published: 19 June 2024
(This article belongs to the Special Issue Exclusive Papers in Green Photocatalysis from China)
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Abstract

:
By utilizing greenhouse gas CO2 and renewable energy-sourced H2 to produce methanol, the “methanol economy” can replace fossil fuels and H2 as the energy storage medium, which not only reduces CO2 emissions, but also mitigates the energy shortage issue. However, the traditional Cu-based catalysts for CO2-to-methanol conversion suffer from low activity at low temperature and high vulnerability to sintering and deactivation. In this contribution, rapidly quenched skeletal Cu catalysts (RQ Cu) are prepared by leaching the RQ Cu–Al alloy with NaOH aqueous solutions of different concentrations. It is found that high NaOH concentration of 10 wt% favors the preparation of the RQ Cu-10 catalyst with higher porosity, lower residual Al content, and larger active Cu surface area (SCu) than the RQ Cu-3 catalyst leached with 3 wt% of NaOH solution. However, in aqueous-phase CO2 hydrogenation at 473 K and 4.0 MPa, the CO2 conversion over the RQ Cu-3 catalyst is more than two times greater than that over the RQ Cu-10 catalyst, and the selectivity and productivity of methanol are 1.20 and 2.69 times of the corresponding values over the RQ Cu-10 catalyst. At 5.0 MPa, the selectivity and productivity of methanol are further boosted to 97.9% and 1.329 mmol gCu–1 h–1 on the RQ Cu-3 catalyst. It is identified that the SCu of the RQ Cu-3 catalyst is well preserved after reaction, while dramatic growth of the Cu crystallites occurs for the RQ Cu-10 catalyst. The better catalytic performance and stability of the RQ Cu-3 catalyst are tentatively attributed to the presence of more residual Al species by using NaOH solution with lower concentration for Al leaching, which acts as the dispersant for the Cu crystallites during the reaction.

1. Introduction

Cu-based catalysts have been widely used in the industrial production of methanol from syngas. Recently, concern about the greenhouse effect has aroused interest in the CO2 economy, making CO2 hydrogenation to methanol a hot topic [1]. The Cu-based catalyst is the most widely studied catalyst for CO2 hydrogenation to methanol because of its good selectivity to methanol. However, the Cu-catalyzed CO2 hydrogenation usually occurs at a high temperature (573 K) and high pressure (>5 MPa) due to the chemical inertness of CO2, and produces a lot of CO byproduct. When the classical Cu/ZnO/Al2O3 catalyst is used for CO2 hydrogenation, the methanol selectivity is about 50–60%, and the water generated during the reaction will accelerate the sintering of the Cu active sites, thus reducing the activity and selectivity [2]. Note that CO2 hydrogenation to methanol is an exothermic reaction, so lowering the reaction temperature is conducive to improving the equilibrium conversion and methanol selectivity. At the reaction temperature of 573 K, the theoretical maximum selectivity to methanol is no higher than 30%. When the reaction temperature is lowered to 473 K, the theoretical maximum selectivity to methanol can increase to more than 90% [3]. Hence, it is of great practical significance to develop highly active and stable Cu-based catalysts at low reaction temperatures.
The skeletal Cu catalyst is prepared by leaching Al from the Cu–Al alloy with alkali solution. The resulting skeletal Cu catalyst has a porous structure, thus giving it a large active surface area, and it can be used as a catalyst for CO2 or CO hydrogenation to methanol [4,5], ester hydrolysis [6,7,8], glycerol hydrogenation to propylene glycol [9], methanol steam reforming, water–gas shift reaction, and so on [10]. For instance, Kong et al. [6] leached Al in the Cu–Al alloy with NaOH solution to prepare the Raney Cu catalyst. In the gas-phase hydrogenation of dimethyl oxalate, the selectivity to methylglycolate was 95.0%, and the catalysts were stable for 100 h. Tanielyan et al. [9] leached the Al in the Cu–Al, Cu–Al–Cr, and Cu–Al–Ni–Cr alloys with NaOH solution to prepare Raney Cu and Raney copper alloy catalysts. In glycerol hydrogenation on a fixed-bed reactor, the selectivity to ethylene glycol on the Raney Cu catalyst was significantly suppressed compared to the Raney Ni catalyst. Meanwhile, the selectivity to 1,3-PDO on the Raney Cu catalyst was higher than those on the Raney copper alloy catalysts doped with Cr or Ni. Hesselmann et al. [5] leached the Al in the Cu–Al alloy with a mixed NaOH and Na2Zn(OH)4 solution to form the Zn(OH)2 precipitate simultaneously to prepare the Zn-doped Raney Cu catalyst. In CO hydrogenation, methanol and dimethyl ether were the main products, and the selectivity to dimethyl ether increased with the increase in the reaction temperature.
The conventional Raney Cu catalyst is leached from the Cu–Al alloy prepared by solidifying the Cu–Al melt at the rate of 102–10–2 K s–1. The Cu–Al alloy thus prepared has the shortcomings of large grain size and uneven structure. For comparison, the rapidly quenched alloy is solidified by cooling the melt at a rate as fast as 106–107 K s–1. Compared to the conventional alloy, the rapidly quenched alloy has the advantages of small grain size, uniform composition, and high density of the active sites [11]. Currently, the study on the rapidly quenched catalyst mainly focuses on the skeletal Ni catalyst, which can be used for CO hydrogenation, unsaturated functional group hydrogenation, and other reactions [12,13,14]. In the present work, a rapidly quenched Cu–Al alloy (RQ Cu50Al50, w/w) is used as the precursor, and the rapidly quenched skeletal Cu catalyst (RQ Cu) is prepared by leaching with NaOH aqueous solution. The catalyst is evaluated in low-temperature aqueous-phase CO2 hydrogenation to methanol, with special emphasis on the effects of the concentration of NaOH solution (10 wt% and 3 wt%) for Al leaching on the physicochemical properties and catalytic performances, as the concentration of NaOH solution has been reported to impose a profound effect on the catalytic performance of the Raney Ni catalyst [15,16]. It was demonstrated that the catalysts prepared in a way similar to that of Raney Ni but using much less NaOH for Al leaching not only produced much fewer waste salts for disposal, but also showed the same or higher catalytic activity and stability than Raney Ni in liquid phase hydrogenation and ethylene glycol reforming, which was attributed to the stabilizing effect of the residual hydrated alumina on Ni crystallites [15,16].

2. Results and Discussion

2.1. Characterization of the As-Leached RQ Cu Catalysts

Firstly, the pristine RQ Cu–Al alloy is characterized by X-ray diffraction (XRD, Bruker D2 PHASER, Billerica, MA, USA). As presented in Figure 1, distinct peaks at 2θ of 20.7°, 29.4°, 37.9°, 42.6°, 47.4°, and 47.8° are indexable to the (110), (200), (211), (112), (310), and (202) reflections of the CuAl2 phase (JCPDS 65-2695). Additional peaks at 2θ of 38.5° and 44.7° are assignable to the (111) and (200) reflections of the α-Al phase (JCPDS 04-0787). The presence of the α-Al phase in addition to the CuAl2 phase is compatible with the bulk composition of the RQ Cu50Al50 alloy.
Figure 2 shows the XRD patterns of the as-leached RQ Cu catalysts. The peaks at 2θ of 43.2°, 50.3°, and 74.1° are indexable to the (111), (200), and (220) reflections of the face-centered cubic (fcc) Cu (JCPDS 04-0836). For RQ Cu-10 leached with 10 wt% of NaOH, there is an additional peak at 2θ of 36.4° assignable to the (111) reflection of Cu2O (JCPDS 05-0667). It can be seen that the diffraction peaks of RQ Cu-10 are much stronger than those of RQ Cu-3 leached with 3 wt% of NaOH, which is consistent with their difference in the content of Cu determined by the inductively coupled plasma-atomic emission spectroscopy (ICP–AES, Thermo Elemental IRIS Intrepid, Waltham, MA, USA) summarized in Table 1 and confirms that in RQ Cu-10 most of the Al in the RQ Cu–Al alloy has been leached out, leaving mainly metallic Cu. Although the ICP–AES result shows that there is still a small amount of Al in RQ Cu-10, no relevant diffraction peak is discerned, suggesting the high dispersion of the residual Al species. For RQ Cu-3, there are additional diffraction peaks due to CuAl2 and gibbsite (Al(OH)3, JCPDS 33-0018), while the diffraction peaks for α-Al disappear, signifying that the Al in the CuAl2 phase is more resistant to NaOH leaching than the α-Al phase. According to the Scherrer equation, the crystallite size of Cu decreases from 18.6 nm for RQ Cu-10 to 17.5 nm for RQ Cu-3, suggesting that the remaining Al species can confine the growth of the Cu crystallites.
Table 1 summarizes the physicochemical properties of the RQ Cu catalysts. The ICP–AES results show that most of the Al has reacted with NaOH. Moreover, the content of Al in the RQ Cu catalyst increases from 2.2% in RQ Cu-10 to 28.8% in RQ Cu-3 with the decrease in the concentration of NaOH. N2 physisorption (TriStar 3000, Micromeritics Instrument Corp., Norcross, GA, USA) shows that with the decrease in the concentration of NaOH, the specific surface area (SBET) increases from 15 m2 g–1 to 17 m2 g–1, while the average pore size (dpore) decreases from 28.1 nm to 9.7 nm, along with the decrease in the pore volume (Vpore) from 0.15 cm3 g–1 to 0.045 cm3 g–1, which signifies that more Al has been removed for RQ Cu-10. With the decrease in the concentration of NaOH, the active copper surface area (SCu) determined by N2O titration (Chemisorb 2750, Micromeritics Instrument Corp., Norcross, GA, USA) decreases from 9.8 m2 g–1 to 5.2 m2 g–1, and the proportion of the SCu in the SBET also decreases from 65% for RQ Cu-10 to 31% for RQ Cu-3, indicating that metallic Cu contributes more to the SBET of RQ Cu-10, while the Al-containing phases such as gibbsite and CuAl2 contribute more to the SBET of RQ Cu-3.
For heterogeneous catalysts, the surface composition and chemical state are closely related to the catalytic performance. Hence, X-ray photoelectron spectroscopy (XPS, Perkin-Elmer PHI 5000C, Waltham, MA, USA) is used to characterize the RQ Cu catalysts. Due to the relatively low content of Al and the overlapping of the Al 2p peak with the Cu 3p peak, the analysis of the Al 2p spectra is not performed. Figure 3 shows the Cu 2p spectra of RQ Cu-10 and RQ Cu-3. In the figure, the peaks with the binding energies (BEs) of 952.8 and 932.8 eV are ascribed to the Cu 2p3/2 and Cu 2p1/2 levels, respectively [17]. These peaks are much stronger for RQ Cu-10, which is in line with its higher bulk Cu content and larger SCu (Table 1). In addition, there is a broad feature at around 944 eV for RQ Cu-10, which is assigned to the shake-up peak of the Cu2+ species possibly due to the oxidation of a small fraction of metallic Cu to CuO during the washing and transfer of the sample.
Since the Cu 2p3/2 BE difference between Cu+ and Cu0 is only about 0.1 eV, in order to further distinguish the chemical state of Cu on the RQ Cu catalysts, the Cu LMM spectra are acquired by XAES and deconvoluted; the results are illustrated in Figure 4. For RQ Cu-10, three XAES peaks are identified at 918.3, 915.4, and 916.8 eV, which are assigned to the Cu0, Cu+, and Cu2+ species, respectively [18,19]. While RQ Cu-3 also possesses the Cu+ species, the Cu2+ species is unavailable, which is consistent with the XPS result. As summarized in Table 2, the surface Cu2+ content among all the Cu species is about 2.6% on RQ Cu-10. The surface Cu+ content on RQ Cu-3 is 26.7%, which is comparable to that on RQ Cu-10. Both the Cu0 and Cu+ species have been reported to be active in CO2 hydrogenation to methanol [20,21].
The morphologies of the RQ Cu catalysts are observed by scanning electron microscopy (SEM, FEI Nova NanoSem 450, Lausanne, Switzerland); the images are shown in Figure 5. After NaOH leaching, the particle sizes of the as-leached RQ Cu catalysts are somewhat smaller than that of the RQ Cu50Al50 alloy (100–200 meshes, i.e., 75–150 μm). As expected, the particle size of RQ Cu-10 with more Al being leached is smaller than that of RQ Cu-3. Moreover, RQ Cu-10 displays a rough and porous morphology as a result of the higher degree of Al leaching (Figure 5a), while the surface of RQ Cu-3 is relatively smooth (Figure 5b). The EDX mapping images of RQ Cu-3 as a representative shown in Figure 6 confirm the homogeneous distribution of the Cu, Al, and O elements on the surface.
Figure 7a shows the H2-TPD profiles of the RQ Cu-3 and RQ Cu-10 catalysts conducted on a Micromeritics AutoChem (Repentigny, QC, Canada) II 2920 instrument coupled to an MKS Cirrus mass spectrometer. Both catalysts give one H2 desorption peak at ca. 500 K, which is assignable to the adsorption of atomic hydrogen on metallic Cu [22]. However, the desorption peak of the RQ Cu-3 catalyst is much stronger than that of the RQ Cu-10 catalyst, which indicates a higher H2 adsorption capacity of the former. This discrepancy suggests that smaller Cu crystals or the presence of more residual Al are beneficial for the dissociative adsorption of H2. Figure 7b shows the CO2-TPD profiles of the RQ Cu-3 and RQ Cu-10 catalysts. The CO2 desorption peak at ca. 590 K may be attributed to the CO2 adsorbed in the form of HCO3– or HCO2– [23]. Similarly, the adsorption capacity of CO2 on the RQ Cu-3 catalyst is much larger than that on the RQ Cu-10 catalyst.

2.2. Catalytic Performance in CO2 Hydrogenation

The catalytic performances of the as-leached RQ u catalysts in CO2 hydrogenation are evaluated in the aqueous phase and at the low temperature of 473 K if unspecified. After reaction, the reaction liquid is analyzed by GC-MS first for qualification purposes. An obvious signal at m/z = 31 is identified, evidencing the formation of methanol. GC analysis shows that the major by-product in the gas phase is CO. However, since the CO signal is very weak, the content of CO is not quantified. In addition, small peaks of by-products are also identified in the aqueous phase by GC analysis. However, because no chromatographic signals for these by-products are observed upon GC-MS analysis, their quantitative analyses are also not performed. Thus, the selectivity to methanol over the RQ Cu catalysts is calculated on the basis of CO2 conversion and the content of methanol in the aqueous phase.
Table 3 summarizes the catalytic results of the RQ Cu catalysts leached with NaOH of different concentrations in CO2 hydrogenation. As can be seen from the table, with the decrease in the concentration of NaOH, the CO2 conversion increases dramatically from 6.1% on RQ Cu-10 to 13.7% on RQ Cu-3, which is compatible with the higher adsorption capacities for H2 and CO2 on RQ Cu-3 than on RQ Cu-10. The selectivity to methanol on RQ Cu-10 is 78.8%, which markedly increases to 94.4% on RQ Cu-3. Because of the high CO2 conversion on RQ Cu-3, the space–time yield of methanol (STYMeOH) amounts to 0.772 mmol gCu–1 h–1. As compared to the literature on Cu-based catalysts for CO2 hydrogenation to methanol (Table 4), the RQ Cu-3 catalyst is unique in exhibiting both high CO2 conversion and high methanol selectivity at low reaction temperatures.
For the RQ Cu-3 catalyst, the effects of the reaction temperature and pressure on the catalytic performance are further investigated. The catalytic results are summarized in Table 5 and Table 6. According to Table 5, the CO2 conversion increases first, maximizes at 473 K, and then decreases at higher temperatures. Considering the chemical inertness of the CO2 molecule, kinetically the hydrogenation rate of CO2 and hence the CO2 conversion increases with the reaction temperature. However, since CO2 hydrogenation to methanol is an exothermic reaction, thermodynamically the equilibrium CO2 conversion decreases with the increase in the reaction temperature [3,33,34,35,36,37]. As a result, the CO2 conversion evolves in volcanic shape with respect to the reaction temperature. On the other hand, the selectivity to methanol decreases monotonically with the increase in the reaction temperature. At 453 K, the selectivity to methanol reaches 95.9%. As a result, the STYMeOH increases first, maximizes at 473 K, and decreases at higher temperatures. As shown in Table 6, both the CO2 conversion and the selectivity to methanol increase steadily with the reaction pressure. At 5.0 MPa, the CO2 conversion is improved to 18.2%, and the selectivity to methanol is elevated to as high as 97.9%, thus further boosting the STYMeOH to 1.329 mmol gCu–1 h–1.

2.3. Characterization of the RQ Cu Catalysts after Reaction

The physicochemical properties of the RQ Cu catalysts after reaction are listed in Table 7. For the RQ Cu-10 catalyst, the SBET and Vpore dramatically drop to 5 m2 g–1 and 0.02 cm3 g–1 after reaction, respectively. The N2 physisorption isotherms of the RQ Cu-10 catalyst in Figure 8 directly demonstrate that its pores virtually disappear after reaction, indicating that the skeletal structure of the RQ Cu-10 catalyst has essentially collapsed. In addition, the color of the RQ Cu-10 catalyst powders changes from black before reaction to purplish red with metal luster after reaction, substantiating the occurrence of severe sintering of the Cu crystallites. The SCu of the RQ Cu-10 catalyst after reaction is close to its SBET, inferring that Cu0 dominates on the catalyst surface. In contrast, the SBET and Vpore of the RQ Cu-3 catalyst are well preserved or even slightly increase after reaction, and its SCu also increases. The SCu/SBET ratio of the RQ Cu-3 catalyst also increases markedly from 31% before reaction to 42% after reaction.
Figure 9 compares the XRD patterns of the RQ Cu catalysts after reaction. The diffraction peaks at 2θ of 43.2°, 50.3°, and 74.1° attributable to the (111), (200), and (220) reflections of fcc Cu are narrowed significantly for both catalysts. According to the Scherrer equation, after reaction the Cu crystallite size of the RQ Cu-10 catalyst increases to 40.6 nm, and that of the RQ Cu-3 catalyst increases to 32.3 nm (Table 7), leading to the difference in the crystallite size of 26% after reaction as compared to only 6% before reaction. For the RQ Cu-3 catalyst, the (111) peak of Cu2O becomes visible after reaction, while this peak is invisible before reaction, indicating that the dispersion of the Cu+ species decreases during the reaction.
In addition, the characteristic diffraction peaks assignable to boehmite (AlOOH, JCPDS 21-1307) can be found on both RQ Cu catalysts after reaction, which are more intensive for the RQ Cu-3 catalyst. Meanwhile, the diffraction peaks of gibbsite and CuAl2 previously observed on the RQ Cu-3 catalyst are diminished after reaction. It was reported that gibbsite can be transformed into boehmite in the aqueous phase at a similar reaction temperature [16]. In addition, the metallic Al in the residual CuAl2 phase or dispersed in the skeleton of the RQ Cu catalyst may also be transformed into boehmite during the reaction conducted in the aqueous phase. Because the content of Al in the RQ Cu-3 catalyst is much higher than that in the RQ Cu-10 catalyst, the diffraction peaks of boehmite are much stronger after reaction.
Figure 10 shows that the morphologies of both RQ Cu catalysts after reaction are greatly changed as compared to their pristine morphologies. In particular, the particles with regular shapes are observed on both catalysts, which can be attributed to the growth of the Cu crystallites. Moreover, the particle size of Cu on the RQ Cu-10 catalyst is much larger than that on the RQ Cu-3 catalyst, which explains its much smaller SCu.

2.4. Structure–Activity Relationship of the RQ Cu Catalyst

According to the N2 physisorption and SEM results, the as-leached RQ Cu-10 catalyst is porous and possesses a higher specific surface area and active surface area than the as-leached RQ Cu-3 catalyst. However, due to the large cohesive energy of Cu, Cu particles tend to sinter during the reaction, thus ripening into large regular Cu crystallites as observed by SEM and losing the high porosity characteristic of the skeletal catalyst as confirmed by N2 physisorption. It is generally acknowledged that the growth of the Cu crystallite is accompanied by the deactivation of the catalyst due to the lowering of the active surface area. Moreover, theoretical calculations revealed that the flat surface of the Cu crystals is inactive in the hydrogenation reaction [38], which can additionally account for the lower activity of the RQ Cu-10 catalyst with larger crystallite size in CO2 hydrogenation to methanol.
Alumina is commonly used as the structural promoter in the Cu-based catalysts to improve the dispersion and stability of metallic Cu and enhance the mechanical strength of the catalyst [36]. For the RQ Cu catalyst prepared by leaching with 3 wt% of NaOH solution, XRD characterization reveals the existence of gibbsite, which may play the role of structural promoter in the RQ Cu-3 catalyst. Moreover, the Al in the residual CuAl2 phase can be transformed to boehmite during the reaction, which may also confine the growth of the Cu crystallites by physically segregating individual Cu crystallites. As a result, after reaction the crystallite size of Cu in the RQ Cu-3 catalyst is much smaller than that in the RQ Cu-10 catalyst, and the active surface area of the RQ Cu-3 catalyst is well retained and even surpasses that of the RQ Cu-10 catalyst, thus giving rise to much better activity and selectivity on the RQ Cu-3 catalyst in low-temperature aqueous-phase CO2 hydrogenation to methanol.

3. Materials and Methods

The rapidly quenched Cu–Al alloy (RQ Cu50Al50) is prepared by the single roller melt-spinning method in an Ar atmosphere. First, equal weight of metallic Cu and Al powders are melted at 1573 K in a quartz vacuum induction furnace for a sufficiently long time to ensure the homogeneity of the melt. Then, the RQ Cu50Al50 alloy is obtained by spraying the melt onto a high-speed rotation copper wheel with a cooling rate of 106 K s–1. Finally, the brittle ribbons with a width of ca. 5 mm and thickness of ca. 20 μm are ground and sieved, and the powders of 100–200 meshes in size are collected for NaOH leaching.
To prepare the RQ Cu catalyst, 0.2 g of the RQ Cu50Al50 alloy powders are immersed in 4 mL deionized water at 323 K first. While stirring, 4 mL of NaOH aqueous solution with concentrations of 5 M or 1.5 M are added slowly. After addition, the resulting NaOH concentration is ca. 10 wt% or 3 wt%. After stirring at 323 K for 1 h, the black powders are centrifuged and washed with deionized water five times to remove the residual NaOH and/or soluble Al species such as NaAlO2. The as-leached RQ Cu-10 and RQ Cu-3 are stored in deionized water for characterization and reaction purposes.
Low-temperature aqueous-phase CO2 hydrogenation is carried out in a 25 mL-capacity Parr Hastelloy high-pressure batchwise autoclave. After the loading of the catalyst and deionized water, the air in the autoclave is expelled by purging with the reaction gas (H2/CO2/N2 = 72/24/4, volume ratio) six times. The autoclave is pressurized with the reaction gas and heated to the reaction temperature. After 5 h of reaction at the stirring speed of 500 rpm, the autoclave is cooled down to room temperature. The gas-phase products are collected with a gas bag. The aqueous-phase products are centrifuged to remove the catalyst prior to analysis.
The gas- and aqueous-phase products are qualified first on an Agilent (Santa Clara, CA, USA) 8860 GC System fixed with an HP-PLOT/Q capillary column (30 m × 0.32 mm × 20 μm) and an Agilent 5977B GC/MSD detector. The gas-phase products are quantitatively analyzed on a GC-9560 gas chromatograph fixed with a 2 m-long TDX-01 column and a thermal conductivity detector (TCD) and on a GC-9160 gas chromatograph fixed with a PONA capillary column (50 m × 0.2 mm × 0.5 μm) and a flame ionization detector (FID). The aqueous-phase products are also analyzed on the GC-9560 gas chromatograph fixed with an HP-INNO Wax column (30 m × 0.32 mm × 0.50 µm) and an FID detector using n-butanol as the internal standard.

4. Conclusions

The concentration of the NaOH aqueous solution for the leaching of Al from the RQ Cu50Al50 alloy markedly influences the physicochemical properties and catalytic performances of the resulting RQ Cu catalysts in low-temperature aqueous-phase CO2 hydrogenation to methanol. Whilst higher NaOH concentration favors the preparation of the RQ Cu-10 catalyst with higher porosity and larger SCu, excessive leaching of Al greatly deteriorates its stability in CO2 hydrogenation. In contrast, for the RQ Cu-3 catalyst leached with NaOH of lower concentration, the SCu is nicely preserved by the residual Al species during the reaction, thus giving rise to high selectivity and STY of methanol. This work demonstrates the important role of the concentration of NaOH in the preparation of high-performance skeletal Cu catalyst for the challenging low-temperature aqueous-phase CO2 hydrogenation reaction. Further research is needed to fully understand the structure–activity relationships and optimize the catalyst design for practical CO2 hydrogenation applications of the RQ Cu catalyst. Moreover, the exploration of the potential of the RQ Cu catalysts for other catalytic applications would be an interesting area for future research.

Author Contributions

X.L.: Experimental work, Data curation, Writing—original draft preparation. D.S.: Experimental work, Data curation. Y.J.: Experimental work. S.Z.: Experimental work. Y.P.: Characterization of the catalysts. S.Y.: Supervision. M.Q.: Writing—review and editing, Supervision. X.Z.: Characterization of the catalysts. B.Z.: Writing—review and editing, Supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Natural Science Foundation of China (No. 22272030), State Key Laboratory of Catalytic Materials and Reaction Engineering (RIPP, SINOPEC), and Science and Technology Commission of Shanghai Municipality (No. 19DZ2270100).

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare that this study received funding from SINOPEC. The funder was not involved in the study design, collection, analysis, interpretation of data, the writing of this article or the decision to submit it for publication.

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Catalysts 14 00391 g001
Figure 1. XRD pattern of the RQ Cu50Al50 alloy.
Figure 1. XRD pattern of the RQ Cu50Al50 alloy.
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Catalysts 14 00391 g002
Figure 2. XRD patterns of the RQ Cu catalysts leached with different concentrations of NaOH.
Figure 2. XRD patterns of the RQ Cu catalysts leached with different concentrations of NaOH.
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Catalysts 14 00391 g003
Figure 3. Cu 2p spectra of the RQ Cu catalysts.
Figure 3. Cu 2p spectra of the RQ Cu catalysts.
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Catalysts 14 00391 g004
Figure 4. Cu LMM XAES spectra of the RQ Cu catalysts.
Figure 4. Cu LMM XAES spectra of the RQ Cu catalysts.
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Catalysts 14 00391 g005
Figure 5. SEM images of the RQ Cu catalysts leached with NaOH of different concentrations. (a) RQ Cu-10 and (b) RQ Cu-3.
Figure 5. SEM images of the RQ Cu catalysts leached with NaOH of different concentrations. (a) RQ Cu-10 and (b) RQ Cu-3.
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Catalysts 14 00391 g006
Figure 6. EDX mapping images of RQ Cu-3.
Figure 6. EDX mapping images of RQ Cu-3.
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Catalysts 14 00391 g007
Figure 7. (a) H2-TPD and (b) CO2-TPD profiles of the RQ Cu catalysts.
Figure 7. (a) H2-TPD and (b) CO2-TPD profiles of the RQ Cu catalysts.
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Catalysts 14 00391 g008
Figure 8. N2 physisorption isotherms at 77 K for the RQ Cu catalysts before and after reaction.
Figure 8. N2 physisorption isotherms at 77 K for the RQ Cu catalysts before and after reaction.
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Catalysts 14 00391 g009
Figure 9. XRD patterns of the RQ Cu catalysts after reaction. ◆ Cu (JCPDS 04-0836), ▽ Cu2O (JCPDS 05-0667), and ★ Boehmite (JCPDS 21-1307).
Figure 9. XRD patterns of the RQ Cu catalysts after reaction. ◆ Cu (JCPDS 04-0836), ▽ Cu2O (JCPDS 05-0667), and ★ Boehmite (JCPDS 21-1307).
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Catalysts 14 00391 g010
Figure 10. SEM images of the (a) RQ Cu-10 and (b) RQ Cu-3 catalysts after reaction.
Figure 10. SEM images of the (a) RQ Cu-10 and (b) RQ Cu-3 catalysts after reaction.
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Table 1. Basic physicochemical properties of the RQ Cu catalysts.
Table 1. Basic physicochemical properties of the RQ Cu catalysts.
CatalystBulk Comp. a
(wt%)		SBET b
(m2 g–1)		Vpore b
(cm3 g–1)		dpore b
(nm)		SCu c
(m2 g–1)		SCu/SBET
(%)		dcryst d
(nm)
RQ Cu-10Cu97.8Al2.2150.1528.19.86518.6
RQ Cu-3Cu71.2Al28.8170.0459.75.23117.5
a Determined by ICP–AES. b Determined by N2 physisorption. c Determined by N2O chemisorption. d Determined by XRD.
Table 2. Cu LMM XAES fitting results of the RQ Cu catalysts.
Table 2. Cu LMM XAES fitting results of the RQ Cu catalysts.
CatalystKinetic Energy (eV)Cu+/Cu
(%)		Cu0/Cu
(%)		Cu2+/Cu
(%)
Cu0Cu+Cu2+
RQ Cu-10918.3915.4916.826.471.02.6
RQ Cu-3918.2915.3-26.773.3-
Table 3. The catalytic results of the RQ Cu catalysts in CO2 hydrogenation to methanol a.
Table 3. The catalytic results of the RQ Cu catalysts in CO2 hydrogenation to methanol a.
CatalystConv.
(%)		Sel.MeOH
(%)		STYMeOH
(mmol gCu–1 h–1)
RQ Cu-106.178.80.287
RQ Cu-313.794.40.772
a Reaction conditions: 0.1 g of Cu, T = 473 K, P = 4.0 MPa, H2/CO2/N2 = 72/24/4, 10 mL of water, stirring rate of 500 rpm, and reaction time of 5 h.
Table 4. Comparison of the RQ Cu-3 with the literature on Cu-based catalysts for CO2 hydrogenation to methanol.
Table 4. Comparison of the RQ Cu-3 with the literature on Cu-based catalysts for CO2 hydrogenation to methanol.
CatalystTemperature
(K)		Pressure
(MPa)		H2/CO2 RatioConv.
(%)		Sel.MeOH
(%)		Ref.
RQ Cu-34734.0313.794.4This work
Cu/ZnO/Al2O35434.52.210.972.7[24]
Cu/ZnO/SiO24933.0313.557.2[25]
Cu/Ga/ZnO5432.036.088.8[26]
Cu@ZnOx5233.032.3100[27]
Cu/ZrO25132.036.348.8[28]
Cu/Zn/ZrO25133.0317.056.2[29]
La-Cu/ZrO24933.035.872.0[30]
Cu/AlCeO-75534.0322.035.0[31]
CuZn@UiO-bpy5234.033.3100[32]
Table 5. The catalytic results of the RQ Cu-3 catalyst in CO2 hydrogenation to methanol at different temperatures a.
Table 5. The catalytic results of the RQ Cu-3 catalyst in CO2 hydrogenation to methanol at different temperatures a.
Temperature
(K)		Conv.
(%)		Sel.MeOH
(%)		STYMeOH
(mmol gCu–1 h–1)
4537.695.90.556
47313.794.40.772
4938.481.20.407
5134.943.80.128
a Reaction conditions: 0.1 g of Cu, P = 4.0 MPa, H2/CO2/N2 = 72/24/4, 10 mL of water, stirring rate of 500 rpm, and reaction time of 5 h.
Table 6. The catalytic results of the RQ Cu-3 catalyst in CO2 hydrogenation to methanol at different pressures a.
Table 6. The catalytic results of the RQ Cu-3 catalyst in CO2 hydrogenation to methanol at different pressures a.
Pressure
(MPa)		Conv.
(%)		Sel.MeOH
(%)		STYMeOH
(mmol gCu–1 h–1)
3.08.090.50.324
4.013.794.40.772
5.018.297.91.329
a Reaction conditions: 0.1 g of Cu, T = 473 K, H2/CO2/N2 = 72/24/4, 10 mL of water, stirring rate of 500 rpm, and reaction time of 5 h.
Table 7. Basic physicochemical properties of the RQ Cu catalysts after reaction.
Table 7. Basic physicochemical properties of the RQ Cu catalysts after reaction.
CatalystSBET a
(m2 g–1)		Vpore a
(cm3 g–1)		dpore a
(nm)		SCu b
(m2 gCu–1)		SCu/SBET
(%)		dcryst c (nm)
RQ Cu-1050.02023.04.99840.6
RQ Cu-3210.0618.86.14232.3
a Determined by N2 physisorption. b Determined by N2O chemisorption. c Determined by XRD.
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Liu, X.; Sun, D.; Ji, Y.; Zu, S.; Pei, Y.; Yan, S.; Qiao, M.; Zhang, X.; Zong, B. Effect of NaOH Concentration on Rapidly Quenched Cu–Al Alloy-Derived Cu Catalyst for CO2 Hydrogenation to CH3OH. Catalysts 2024, 14, 391. https://0-doi-org.brum.beds.ac.uk/10.3390/catal14060391

AMA Style

Liu X, Sun D, Ji Y, Zu S, Pei Y, Yan S, Qiao M, Zhang X, Zong B. Effect of NaOH Concentration on Rapidly Quenched Cu–Al Alloy-Derived Cu Catalyst for CO2 Hydrogenation to CH3OH. Catalysts. 2024; 14(6):391. https://0-doi-org.brum.beds.ac.uk/10.3390/catal14060391

Chicago/Turabian Style

Liu, Xuancheng, Dong Sun, Yushan Ji, Sijie Zu, Yan Pei, Shirun Yan, Minghua Qiao, Xiaoxin Zhang, and Baoning Zong. 2024. "Effect of NaOH Concentration on Rapidly Quenched Cu–Al Alloy-Derived Cu Catalyst for CO2 Hydrogenation to CH3OH" Catalysts 14, no. 6: 391. https://0-doi-org.brum.beds.ac.uk/10.3390/catal14060391

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