Zeolite-encaged mononuclear copper centers catalyze CO₂ selective hydrogenation to methanol

ABSTRACT

The selective hydrogenation of CO₂ to methanol by renewable hydrogen source represents an attractive route for CO₂ recycling and is carbon neutral. Stable catalysts with high activity and methanol selectivity are being vigorously pursued, and current debates on the active site and reaction pathway need to be clarified. Here, we report a design of faujasite-encaged mononuclear Cu centers, namely Cu@FAU, for this challenging reaction. Stable methanol space-time-yield (STY) of 12.8 mmol g_cat^-1 h^-1 and methanol selectivity of 89.5% are simultaneously achieved at a relatively low reaction temperature of 513 K, making Cu@FAU a potential methanol synthesis catalyst from CO₂ hydrogenation. With zeolite-encaged mononuclear Cu centers as the destined active sites, the unique reaction pathway of stepwise CO₂ hydrogenation over Cu@FAU is illustrated. This work provides a clear example of catalytic reaction with explicit structure-activity relationship and highlights the power of zeolite catalysis in complex chemical transformations.

Zeolite with confined mononuclear Cu ions can efficiently catalyze the selective hydrogenation of CO₂ to methanol by dihydrogen at relatively low reaction temperatures.

INTRODUCTION

The increasing amount of atmospheric CO₂ from anthropogenic emission is becoming a serious concern worldwide, as it can cause significant environmental problems like global warming and increased ocean acidity. Among all technically feasible approaches for reducing and recycling CO₂, the hydrogenation of CO₂ to green methanol (CH₃OH) using preferentially renewable hydrogen sources has drawn great attention [1–5]. The target product CH₃OH can be directly applied in internal combustion engines and fuel cells or be reserved as a versatile chemical feedstock [6–8]. The CO₂-to-CH₃OH transformation is challenging due to the relative chemical inertness of CO₂ and the difficulty in controlling the side reactions to obtain single product CH₃OH. Many catalyst systems have been explored for CO₂ selective hydrogenation, including Cu-based catalysts [9–13], noble metal catalysts [14,15] and metal oxide catalysts [16,17], etc. [18–20]. Hence, Cu-based catalysts with unparalleled advantages of low-cost and high-abundance have been intensively investigated in the past decades. Cu/ZnO/Al₂O₃ is currently recognized as a benchmark catalyst for CH₃OH synthesis from the hydrogenation of CO₂, CO or CO/CO₂ mixture. The inherent instability and complexity of Cu component as well as the further modifications by promoters like ZnO bring about significant debates on the active Cu sites [21–28] and the reaction pathway thereof [4,29]. Cu-based catalysts with diverse Cu sites generally suffer from low CH₃OH selectivity owing to the exacerbated reverse water gas shift (RWGS) reaction to produce CO and the excessive hydrogenation to methane (CH₄) [30,31]. In practical recycling reactors for CO₂ hydrogenation, elevated temperatures are employed to obtain reasonable CO₂ conversion and CH₃OH space-time-yield (STY), resulting in high energy consumption and further decline in CH₃OH selectivity. On the other hand, the reaction of CO₂-to-CH₃OH is limited by thermodynamic equilibrium, and low temperatures are beneficial to attain both high equilibrium CO₂ conversion and high CH₃OH selectivity. The design of highly active Cu-based catalysts for CO₂ selective hydrogenation to CH₃OH at low reaction temperatures is therefore possible and urgently needed.

Zeolites are widely employed industrial catalysts and support materials with unique confinement effect [32,33] and ionic environment [34]. Transition metal ions (TMIs) can be accommodated within a zeolite matrix, balancing the negative charges of [AlO₄]⁻ units, to create more functionalities [35–38]. Interestingly, isolated TMIs can be easily introduced to, and efficiently stabilized by, a zeolite matrix like faujasite via a ligand-protected in situ hydrothermal route [39,40], providing an opportunity to construct TMIs-containing zeolites toward selective catalysis. Herein, we demonstrate the design of uniform Cu ions confined in faujasite, namely Cu@FAU, for the selective hydrogenation of CO₂ to CH₃OH. Cu@FAU catalyst with exclusive mononuclear Cu centers exhibits high CH₃OH selectivity and STY as well as perfect stability in CO₂ reduction at relatively low reaction temperatures, fulfilling the basic requirements for industrial applications. With well-defined structure of Cu@FAU model catalyst, the current debates on active Cu sites can be addressed and a clear roadmap of stepwise CO₂ reduction to CH₃OH is illustrated.

RESULTS AND DISCUSSION

Construction and characterization of Cu(II) confined in zeolite

An in situ hydrothermal route was developed to encapsulate Cu complexes in the matrix of faujasite (See Supplementary Information for details) and Cu ions confined in faujasite could be obtained via the calcination removal of organic ligands. Post-synthesis modulation was performed and trace exchangeable Cu ions were selectively removed through repeated ion exchange with NaNO₃ aqueous solution, leaving stable Cu ions confined in faujasite. X-ray diffraction (XRD) patterns identify the typical FAU topology in pure-phase (Fig. S1) and Ar sorption isotherms reveal the uniform microporous structure (Fig. S2) of Cu@FAU. Microscopy analyses indicate the characteristic octahedral faujasite morphology with crystal size of 1–2 μm and the ultra-dispersion of Cu species with loading of ∼4.5 wt% (Figs S3–S5, Table S1).

A single Cu²⁺→Cu⁺ reduction peak centered at ∼443 K was observed for Cu@FAU (Figs S6 and S7), while complex Cu⁺→Cu⁰ reduction peaks in the temperature region of 523–873 K were observed for Cu-FAU and Cu/FAU samples [41–43]. That is, uniform Cu ions were formed and efficiently stabilized at specific positions in the zeolite matrix in Cu@FAU, in great contrast to the cases of Cu-FAU and Cu/FAU where significant amounts of CuOx species were present as observed in the TEM images (Fig. S8). The valance state of +2 in Cu@FAU was confirmed by Cu K-edge X-ray absorption near-edge structure (XANES) spectrum (Fig. 1a) and a prominent peak at ∼2.0 Å due to the first shell of Cu-O unit with average coordination number of 3.8 was obtained in the Fourier-transformed (FT) k²-weighted extended X-ray absorption fine structure (EXAFS) spectrum (Fig. 1b, Fig. S9 and Table S2) and the wavelet-transformed (WT) EXAFS oscillations (Fig. 1c). EPR spectrum (Fig. S10) shows a signal of a high-field peak centered at ∼3380 G with EPR parameters g_‖ = 2.35 (g_‖ = 2.05) and A = 176 G, assignable to Cu²⁺ species residing in, or close to, the six-membered rings with two aluminum T-sites [44]. The observed signal should be broad and unresolved due to the magnetic dipole interactions from individual Cu ions in close proximity or Cu particles, while the latter possibility could be excluded by microscopic observations (Fig. S5). The fine structure of Cu@FAU and the exact location of Cu sites in faujasite were identified by synchrotron XRD (Fig. S11). According to the results from Rietveld refinement, the Cu sites exclusively situated in the center of the six-membered ring would be shared by the sodalite cage and supercage of zeolite (Fig. 1d). The possible structure of Cu²⁺ in faujasite was also screened by density functional theory (DFT) calculations and the configuration of Cu²⁺ sitting in the six-membered rings containing Al pairs in the para- or meta-positions could be optimized (Fig. S12). The local coordination environment of Cu sites in faujasite from DFT calculations is shown in Fig. 1e, and the bonding parameters are in good consistency with those from synchrotron X-ray absorbance spectroscopy (Table S2). These results demonstrate the successful construction of Cu@FAU containing uniform and well-defined mononuclear Cu sites, which is analogous to a typical coordination compound with Cu²⁺ as the central ion and faujasite framework as the ligand.

Figure 1.

Fine structure of Cu@FAU model catalyst. (a) Cu K-edge XANES spectra of Cu foil, CuO, Cu@FAU and spent Cu@FAU. (b) FT k²-weighted EXAFS spectra of Cu foil, CuO and Cu@FAU. (c) WT EXAFS oscillations of Cu foil, CuO and Cu@FAU. (d) Schematic view of Cu@FAU from synchrotron XRD Rietveld refinement. (e) Local coordination environment of Cu sites in faujasite from DFT calculations with bond length shown in angstroms.

CO₂ selective hydrogenation to methanol over Cu@FAU

Figure 2a shows the results of CO₂ hydrogenation over representative Cu-based catalysts under relatively mild reaction conditions, i.e. at 513 K and in the feed gas of 3.0 MPa CO₂-H₂ (H₂/CO₂ = 3 : 1). All Cu-containing zeolites, namely Cu-FAU, Cu/FAU and Cu@FAU, can catalyze the CO₂-to-CH₃OH transformation, with CO and methane as major byproducts from RWGS and methanation reactions (Fig. S13), respectively. Cu@FAU is the most active catalyst with 11.5% CO₂ conversion and 89.5% CH₃OH selectivity, offering a CH₃OH STY of 12.8 mmol g_cat^-1 h^-1 distinctly higher than that of Cu-FAU (4.7 mmol g_cat^-1 h^-1) and Cu/FAU (6.7 mmol g_cat^-1 h^-1). Due to the absence of Brønsted acid sites (Figs S14 and S15) and the relatively low reaction temperature employed, the formation of dimethyl ether from methanol dehydration can be greatly suppressed, as confirmed by methanol feeding experiment (Fig. S16). The catalytic performance of Cu-containing zeolites seems to be controlled by specific Cu sites and their chemical environment. Notably, similar CO₂ conversion was achieved with Cu@FAU (4.5wt% Cu) and commercial Cu/ZnO/Al₂O₃ (63.0wt% Cu) catalysts despite the huge difference in Cu loading. Cu@FAU exhibits significantly higher CH₃OH selectivity than Cu/ZnO/Al₂O₃ at 473–553 K with comparable CO₂ conversions (Fig. S17). Cu@FAU surpasses commercial Cu/ZnO/Al₂O₃ catalyst in CO₂ selective hydrogenation at low reaction temperatures.

Figure 2.

Catalytic performance of Cu@FAU in CO₂ selective hydrogenation. (a) Representative Cu-based catalysts in CO₂ hydrogenation. Reaction conditions: 0.15 g catalyst, H₂/CO₂ = 3/1, 3 MPa, 513 K, GHSV = 12 000 h⁻¹. (b) Temperature-dependent behaviors of Cu@FAU catalyst in CO₂ hydrogenation. Reaction conditions: 0.15 g catalyst, H₂/CO₂ = 3/1, 3 MPa, GHSV = 12 000 h⁻¹. (c) Literature survey of Cu-based catalysts for CO₂ hydrogenation. CH₃OH selectivity and STY plotted for comparison. (d) Stability test of Cu@FAU catalyst in CO₂ hydrogenation. Reaction conditions: 0.15 g catalyst, H₂/CO₂ = 3/1, 3 MPa, 513 K, GHSV = 12 000 h⁻¹.

The temperature-dependent behaviors of Cu@FAU catalyst in CO₂ hydrogenation are shown in Fig. 2b. The CO₂ conversion increases almost linearly with increasing reaction temperature from 453 to 573 K, and, meanwhile, two-stage declines in CH₃OH selectivity are observed, namely the mild declines from 453 to 513 K and the sharp declines from 513 to 573 K. The reaction temperature of 513 K can be optimized in view of both CO₂ conversion and CH₃OH selectivity. The second stage decline in the CH₃OH selectivity should be related to the reduction of Cu²⁺ to Cu⁺, as revealed by in situ near-ambient pressure X-ray photoelectron spectroscopy (Fig. S18). Higher pressure and gas hourly space velocity (GHSV) are beneficial to the methanol selectivity (Figs S19 and S20), and CH₃OH selectivity can be promoted to 92.5% with optimized reaction parameters. The catalytic performance of Cu@FAU, in terms of CH₃OH selectivity and STY, is superior to all known Cu-based catalysts under comparable reaction conditions (Fig. 2c, Table S3), and, more importantly, the remarkable catalytic performance is achieved with Cu as a single active component free of modifiers like zinc. Cu@FAU catalyst demonstrates good stability and no activity loss or selectivity decline can be observed for over 200-h run of CO₂ hydrogenation (Fig. 2d, carbon balance >95%), in significant contrast to Cu-FAU (Fig. S21) and Cu/FAU (Fig. S22). Stability is a fatal issue for CH₃OH synthesis from CO and/or CO₂ hydrogenation, and Cu-based catalysts generally suffer from rapid deactivation due to metal sintering. For Cu@FAU catalyst, the isolated Cu ions are efficiently stabilized by zeolite matrix and their coordination environment can be well preserved in long-term running, as confirmed by EXAFS analyses (Table S2, Fig. S23). Overall, Cu@FAU appears to be a qualified catalyst for CH₃OH production from CO₂ hydrogenation at low reaction temperatures, offering high CH₃OH selectivity and STY as well as perfect stability.

Mechanistic insights into CO₂ selective hydrogenation to methanol

CO₂ hydrogenation generally requires both CO₂ and H₂ activation, followed by the stabilization of reaction intermediates for controllable hydrogenation. Dihydrogen cannot be activated on Cu@FAU at the reaction temperature of 513 K, as indicated by the absence of HD signal (m/z = 3) in the H₂-D₂ stream (Fig. 4a) [45]. Upon the introduction of CO₂ pulses, the signals of H₂ and D₂ decline while the HD signal appears, accompanied by the formation of CH₃OH and deuterated CH₃OH (control experiment shown in Figs S24 and S25). These observations clearly demonstrate the CO₂-assisted dihydrogen activation on Cu@FAU and the subsequent hydrogenation of CO₂ to CH₃OH. The surface species involved in the hydrogenation process were then monitored by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). A series of organic surface species were observed and most of these species reached dynamic equilibrium at the early stage of reaction (Fig. 3b). Typically, the infrared band at 1250 cm⁻¹ is assigned to the C–O asymmetric stretching vibrations of mono- or bidentate HCOO^* species, and the band at 1335 cm⁻¹ assigned to the C–O asymmetric stretching vibrations of HCOOH^* species with contribution from bending vibrations. The band at 1385 cm⁻¹ is due to the bending vibrations of CH₃O^* species and the paired bands at 1465 and 1495 cm⁻¹ due to the bending vibrations of CH₃OH^* species. The band at 1385 cm⁻¹ is related to the C–O asymmetric stretching vibrations of CH₂O^* with adjacent co-adsorbed H₂O and the contribution from C–H bending vibrations. The band at 2915 cm⁻¹ is explicitly related to the C–H stretching vibrations of H₂COOH^* species [17,19,46,47]. DFT calculations were employed to support the above assignments, and the structure of key organic species and their vibration frequencies are summarized in Fig. 3c and Table S4. In deuterium labeling experiments, the C-D stretching vibrations of DCOO^* (2160 cm⁻¹), DCOOD^* (2095 cm⁻¹) and CD₃O^* (2060 cm⁻¹) species were observed (Fig. 3d) [17,48]. All these observations hint to the stepwise CO₂ reduction by dihydrogen on the Cu@FAU, which appears to be quite different to the conventional CO pathway [24]. Accordingly, Cu@FAU catalyst shows very low activity in the hydrogenation of CO, with ∼2.5% CO conversion at 513 K (Fig. S26).

Figure 4.

Reaction pathway of CO₂ hydrogenation to CH₃OH on mononuclear Cu centers. Elementary reaction steps of CO₂ hydrogenation over Cu@FAU model catalyst and calculated Gibbs free energy profile with ZPE correction at 0 K.

Figure 3.

Characteristics of CO₂ selective hydrogenation over Cu@FAU catalyst. (a) Mass spectrometry responses of CO₂ pulses fed to Cu@FAU in H₂-D₂ stream. Reaction conditions: 0.2 g catalyst, 0.4 MPa, 513 K, 5 mL/min CO₂, 15 mL/min H₂-D₂ (1/1). (b) In situ DRIFT spectra of surface species formed on Cu@FAU in CO₂-H₂ stream. Time-dependent spectra recorded within 30 min, from light to dark curves. Reaction conditions: 0.02 g catalyst, 3.0 MPa, 513 K, 5 mL/min CO₂, 15 mL/min H₂. (c) Calculated structure of key surface intermediates and their vibration frequencies. (d) In situ DRIFT spectra of surface species formed on Cu@FAU in CO₂-D₂ stream. Reaction conditions: 0.02 g catalyst, 3.0 MPa, 513 K, 5 mL/min CO₂, 15 mL/min D₂.

DFT calculations were finally employed to clarify the reaction pathway of CO₂ hydrogenation to CH₃OH over Cu@FAU, with mononuclear Cu centers confined in faujasite as the catalytically active sites. In the first step, gaseous CO₂ adsorbs on the Cu site in a linear configuration (ln-CO₂^*) with adsorption energy of −0.22 eV at 0 K, followed by the adsorption of dihydrogen. The dihydrogen undergoes facile dissociation into a hydride bonded to the Cu site and a proton bonded to adjacent O site via TS1 (E_a = 0.75 eV, E_r = 0.32 eV). Clearly, the dihydrogen is activated by classical Lewis pairs (Cu–O) with the assistance from adsorbed CO₂ species, as confirmed by the pulse-response experiments in Fig. 3a. The hydride transfer to the C atom of the ln-CO₂^* results in the formation of the monodentate formate (mono-HCOO^*) via TS2 (E_a = 0.64 eV, E_r = −0.12 eV). The mono-HCOO^* rapidly transforms into the bidentate formate (bi-HCOO^*, E_r = −0.29 eV), which is further protonated to the HCOOH^*via TS3 (E_a = 0.37 eV, E_r = 0.17 eV). The second dihydrogen molecule then dissociates into a hydride and a proton at the Cu–O pair site (assisted by adsorbed HCOOH) and the HCOOH^* is hydrogenated to the H₂COOH^*via TS4 (E_a = 0.65 eV, E_r = 0.09 eV). Through a simple rotation, the H₂COOH^* reacts with the proton to form the CH₂O^* and the H₂O^*via TS5 (E_a = 0.00 eV, E_r = −0.29 eV). The H₂O^* leaves the active site with the desorption energy of 0.39 eV and enables the third dihydrogen dissociation at the Cu-O pair site (assisted by adsorbed CH₂O). The CH₂O^* is hydrogenated to the CH₃O^*via TS6 (E_a = 0.63 eV, E_r = −0.64 eV) and the CH₃OH^* is formed by the protonation of the CH₃O^*via TS7 (E_a = 0.00 eV, E_r = −0.58 eV). Finally, the CH₃OH^* desorbs from the active site (E_r = 0.84 eV) and the catalytic cycle ends. For a direct view, the complete reaction pathway of CO₂ hydrogenation to CH₃OH over Cu@FAU model catalyst and the calculated Gibbs free energy profile are shown in Fig. 4 (adsorption energy of key intermediates listed in Table S5 and Table S6). Some of the key reaction intermediates like HCOOH^*, H₂COOH^*, CH₃O^* and CH₃OH^* have been successfully captured by in situ DRIFTS, as shown in Fig. 3d.

According to above analyses, all the energy barriers from TS1 to TS7 are less than 0.75 eV and the reaction energies are less than 0.32 eV, indicating that CH₃OH production from CO₂ hydrogenation on zeolite confined mononuclear Cu centers is thermodynamically and kinetically favorable. The first CO₂-assisted dihydrogen dissociation has the highest activation barrier, and it should be the rate-determining step for the overall reaction. At the optimized temperature of 513 K, the energy barriers and reaction energies are also reasonable (Fig. S27). The oxidation state of copper species shows dynamic changes during reaction while the cationic state of copper can be well preserved, as confirmed by the variations of Bader charge (Fig. S28). The high catalytic activity of the Cu@FAU originates from the unique configuration of Cu^δ+ sitting in the six-membered ring, containing classical Lewis pairs of Cu^δ+–O²⁺ units. The C-containing intermediates can effectively adsorb on Cu sites and assist the dihydrogen activation on Cu–O pairs for subsequent hydrogenation via the so-called associative mechanism [45]. We also search for the bent configuration of CO₂ (bt-CO₂^*) by structure optimization, which is found to be extremely unstable and undergoes spontaneous transformation to ln-CO₂^* on the Cu site. Thereupon, the traditional protonation of bt-CO₂^* to COOH^* followed by dissociation to CO^* [29,49] or bt-CO₂^* direct dissociation to CO^* [50] will not occur on Cu@FAU. With well-defined uniform mononuclear Cu cations confined in faujasite, Cu@FAU is completely different to traditional Cu-based catalysts containing a complicated constitution of Cu species and always modified by Zn species [21–28]. For Cu@FAU, the step-wise CO₂ hydrogenation to CH₃OH is achieved on zeolite-confined Cu–O Lewis pairs and dihydrogen activation is assisted by adsorbates on Cu sites like CO₂, HCOOH and CH₂O. Such unique homogeneous-like mechanism derives unprecedented catalytic performance in CO₂-to-CH₃OH transformation at relatively low temperatures (Fig. 2).

CONCLUSIONS

The selective hydrogenation of CO₂ to CH₃OH provides a technically feasible route for CO₂ recycling and is carbon neutral. The reaction process is very complex and requires the well-balanced C–O dissociation and dihydrogen activation. A complicated catalyst system of Cu/ZnO/Al₂O₃ is currently employed to achieve high activity and moderate selectivity to CH₃OH as well as good catalyst stability, which also brings about significant debates on the active site and reaction mechanism. Herein, we demonstrate that zeolite-encaged uniform mononuclear Cu centers, namely Cu@FAU, can efficiently catalyze the stable CO₂-to-CH₃OH transformation at the relatively low reaction temperature of 513 K, offering a high CH₃OH STY of 12.8 mmol g_cat^-1 h^-1 at selectivity of 89.5%. The reaction sequence of CO₂ hydrogenation over well-defined Cu@FAU catalyst and the full catalytic cycle are successfully depicted. It is disclosed that all the reaction steps can take place on Cu^δ+–O²⁺ Lewis pairs confined in zeolites, following the homogeneous-like mechanism. The unique zeolite-confined catalyst system and reaction pathway contribute to the success of CO₂-to-CH₃OH process for carbon neutral, and may trigger some new thoughts for other complex chemical transformations.

MATERIALS AND METHODS

Cu@FAU was synthesized via a ligand-protected in situ hydrothermal route. The composition of the gel with the molar ratio of 7.8 SiO₂ : 1 Al₂O₃ : 2.2 Na₂O: 0.6 Cu-TAPTS: 174 H₂O (TAPTS = 3-[2-(2-aminoethylamino) ethylamino] propyl-trimethoxysilane) was transferred into an autoclave and heated at 373 K for 4 days under static conditions. The solid was collected by centrifugation, washed with water, dried at 353 K overnight and calcined in flowing air at 823 K for 6 h.

XRD patterns of selected zeolite samples were recorded on a Bruker D8 diffractometer. High resolution synchrotron XRD data were collected at Beamline I11 of Diamond Light Source using multi-analysing crystal-detectors and monochromated radiation [λ = 0.826126(2) Å]. TEM images of selected samples were acquired on a FEI Tecnai G2 F20 electron microscope. EPR spectra were collected with a continuous wave X-band Bruker EMX EPR spectrometer. ¹H magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectra were obtained on a Bruker Avance III spectrometer. In situ near ambient pressure XPS were performed on a SPECS NAPXPS spectrometer. XAS spectra were measured at the BL11B, Shanghai Synchrotron Radiation Facility (SSRF).

The catalytic reaction of CO₂ hydrogenation was carried out in a high-pressure fixed-bed continuous-flow reactor. The products were analyzed using an online gas chromatograph (Shimadzu 2010SE) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). A TDX-01 packed column was connected to the TCD and an RT-Q-BOND-PLOT capillary column was connected to the FID. Product selectivity was calculated on a molar carbon basis, and the TCD and FID signals were correlated by the signal of methane.

FUNDING

This work was supported by the National Natural Science Foundation of China (22121005, 22002062 and 22025203), the China National Postdoctoral Program for Innovative Talents (BX20200171) and the Fundamental Research Funds for the Central Universities (Nankai University).

AUTHOR CONTRIBUTIONS

Y.C. conducted the sample synthesis and catalytic study. B.Q. performed the theoretical calculations. B.L. collected and analyzed the X-ray absorption spectroscopy data. W.D., G.W. and N.G. analyzed the data and provided helpful discussions. L.L. directed and supervised the project. Y.C., B.Q. and L.L. prepared the manuscript.

Conflict of interest statement. None declared.