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 gcat-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.


Hydrothermal synthesis of Cu@FAU
Cu@FAU was synthesized via a ligand-protected in situ hydrothermal route.In a typical experiment, 4.13 g Cu(NO 3 ) 2 was added into 98 mL deionized water containing 11.71 g TAPTS and stirred for 30 minutes to obtain the Cu-TAPTS solution.Then, 5.80 g NaAlO 2 and 6.05 g NaOH were added into the solution in turn.After stirring for 1 h, 34 g silica sol (50 wt.% SiO 2 ) was dropwise added into the above mixture under vigorous stirring to form the synthesis gel.Finally, the gel with the molar ratio of 7.8 SiO 2 : 1 Al 2 O 3 : 2.2 Na 2 O: 0.6 Cu-TAPTS: 174 H 2 O was transferred into an autoclave and heated at 373 K for 4 days under static conditions.The solid was collected by centrifuging, washed with water, dried at 353 K overnight and calcined in flowing air at 823 K for 6 h.
The calcined solid samples were subsequently ion-exchanged with 1 M NaNO 3 solution for three times to selectively remove the Cu ions at the exchangeable sites, dried at 353 K overnight, and calcined in flowing air at 823 K for 6 h to derive Cu@FAU sample for catalysis.

Preparation of Cu-FAU and Cu/FAU
Commercial Na-FAU zeolite (Si/Al = 3.5) was employed as zeolite host and Cu species were introduced to the zeolite by ion-exchanged with 1.0 M Cu(NO 3 ) 2 aqueous solution for three times at the constant temperature of 353 K.After each ion-exchange process, the slurry was filtered and washed with distilled water.The final solid product was dried at 353 K overnight and calcined in flowing air at 823 K for 6 h to derive Cu-FAU.
Cu species were also introduced into Na-FAU zeolite (Si/Al = 3.5) by wet impregnation, followed by similar drying and calcination steps.The final product was denoted as Cu/FAU.

Sample characterization
The chemical compositions of samples were analyzed on an IRIS Advantage inductively coupled plasma atomic emission spectrometer (ICP-AES).
The X-ray diffraction (XRD) patterns of selected zeolite samples were recorded on a Bruker D8 diffractometer using Cu-Kα radiation (λ= 0.1541 nm) in the region of 2θ = 5-50 o at a scanning rate of 6 o /min.High resolution synchrotron X-ray powder diffraction data of selected samples were collected at Beamline I11 of Diamond Light Source using multi-analysing crystal-detectors and The surface areas of samples were determined by Ar adsorption/desorption isotherms at 87 K collected on a Quantachrome iQ-MP gas adsorption analyser.The total surface area was calculated via the Brunauer Emmett Teller (BET) equation and the micropore volume was determined using the t-plot method.Prior to Ar adsorption, the sample of ~0.1 g was desolvated under dynamic vacuum at 473 K for 12 h.
Transmission electron microscopy (TEM) images of selected samples were acquired on a FEI Tecnai G2 F20 electron microscope.
The experiments of temperature-programmed reduction by hydrogen (H 2 -TPR) were performed on a Quantachrome ChemBET 3000 chemisorption analyzer.In a typical experiment, the sample of ~0.1 g was calcined in dry air at 823 K for 1 h and cooled to 323 K in flowing Ar.H 2 -TPR profile was recorded in flowing 5%H 2 /Ar at a heating rate of 10 K /min from 323 to 1123 K.
Electron paramagnetic resonance (EPR) spectra were collected with a continuous wave X-band Bruker EMX EPR spectrometer with the ER 4102ST cavity with a gunn diode microwave source in the field interval 220-400 mT.
The experiments of temperature-programmed desorption of ammonia (NH 3 -TPD) were performed on a Quantachrome ChemBet 3000 chemisorption analyzer.In a typical experiment, the sample was saturated with 5% NH 3 /He at 323K and then purged with He at the same temperature for 1 h to eliminate the physical absorbed ammonia.The NH 3 -TPD profile was recorded in flowing He at a heating rate of 10 K/min from 323 to 873 K.
The solid-state magic-angle-spinning nuclear magnetic resonance (MAS NMR) measurements were performed on a Bruker Avance III spectrometer at resonance frequencies of 400.1 MHz for 1 H nuclei. 1 H MAS NMR spectra were obtained upon a single-pulse excitation of π/2 with pulse duration of 2.6 μs and a repetition time of 20 s, respectively.The ammonia loading of the dehydrated samples was done on a vacuum line by adsorption of 100 mbar ammonia (Griesinger) at physisorbed ammonia.
In situ near ambient pressure X-ray photoelectron spectra (XPS) of Cu-containing zeolites were performed on a SPECS NAPXPS spectrometer with monochromatic Al Ka X-ray (hν= 1486.6 eV) as the excitation source.The binding energies (± 0.1 eV) were determined with respect to the position of Si 2p peak of SiO 2 at 103.3 eV.
The X-ray absorption spectra (XAS) were measured at the BL11B, Shanghai Synchrotron Radiation Facility (SSRF) (S1), including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra at the Cu K-edge.A Si (111) double-crystal monochromator was used for the energy selection.The energy was calibrated by Cu foil as a reference and all samples were measured in the transmission mode.The Athena software package was used to analyze the data.

Catalytic reaction of CO 2 hydrogenation
The catalytic reaction of CO 2 hydrogenation was carried out in a high-pressure fixed-bed continuous-flow reactor.Typically, catalyst sample of 0.2 g was placed in the quartz reactor, pretreated in Ar at 673 K for 1 h, and cooled down to designated reaction temperature.Afterwards, the reaction was conducted under the reaction conditions of 1.0-4.0MPa, 453-573 K, V H2 /V CO2 /V Ar of 72/24/4, and gas hourly space velocity (GHSV) of 8000-20,000 /h.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.

In situ diffuse reflectance infrared Fourier transform spectroscopy
The reaction of CO 2 hydrogenation to methanol was in situ monitored by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS).The experiments were performed on a Bruker Tensor 27 spectrometer equipped with an in situ reaction chamber and a liquid N 2 cooled high sensitivity mercury cadmium telluride detector.Typically, ~20 mg of finely-ground catalyst powders were placed in the reaction chamber and pretreated in Ar at 673 K for 1 h.After cooling down to the designated temperature, the reactant gas mixture containing H 2 /CO 2 (3/1) or D 2 /CO 2 (3/1) was fed into the chamber at GHSV of 12000 h -1 , and time-resolved spectra were recorded at a resolution of 4 cm -1 and with an accumulation of 128 scans against blank background.

Computational methods and modeling
The spin-polarized DFT calculations were performed using the Vienna ab initio simulation package (VASP) (S2,S3).The Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional (S4) and the projector-augmented wave (PAW) potentials (S5) were used to describe the electron-ion interaction.The Bayesian error estimation functional with van der Waals correlation (BEEF-vdW) (S6) and an energy cut-off of 400 eV were employed in this study.All structures were optimized using Г point.The electronic energy of the supercell was converged to 10-4 eV, and the force on all unconstrained atoms were converged to 0.01 eV Å.
The structure of the Cu@FAU zeolite was built according to the characterization results.All atoms in the structure were allowed to relax.The zero-point energies (ZPE), enthalpies, entropies, and Gibbs free energies were calculated from harmonic frequencies, identical to our previous work (S7).
Transition states were obtained using the climbing image nudged elastic band (CI-NEB) method

Figures & Tables
Figure S1 XRD pattern of Cu@FAU zeolite Relative pressure (P/P 0 ) Figure S3 SEM images of Cu@FAU sample

Figure S4 Figure S5
Figure S4 TEM image of Cu@FAU sample with selected area energy dispersive X-ray spectrum

Table S1
Physico-chemical properties of Cu containing zeolites under study

Table S3
Methanol STY of the Cu@FAU compared to other Cu-based catalysts reported in the

Table S4
Stretching bands of key intermediates involved in CO 2 hydrogenation from DFT calculations

Table S5
All intermediates in the energy profile in Figure4

Table S6
Adsorption energy of key intermediates involved in CO 2 hydrogenation