Solid Solutions Derived from Schiff Base Bridged UiO-66 Composites

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Functional Nanostructured Materials (including low-D carbon)

MxOy-ZrO2 (M = Zn, Co, Cu) Solid Solutions Derived from Schiff Base Bridged UiO-66 Composites as High-Performance Catalysts for CO2 Hydrogenation Wen Li, Kuncan Wang, Junjie Huang, Xiao Liu, Dun Fu, Jiale Huang, Qingbiao Li, and Guowu Zhan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11547 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 22, 2019

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ACS Applied Materials & Interfaces

MxOy-ZrO2 (M = Zn, Co, Cu) Solid Solutions Derived from Schiff Base Bridged UiO-66 Composites as High-Performance Catalysts for CO2 Hydrogenation Wen Li,a Kuncan Wang,b Junjie Huang,b Xiao Liu,b Dun Fu,a Jiale Huang,b,* Qingbiao Li,b,d,* and Guowu Zhanc,* Department of Ecological Engineering for Environmental Sustainability, College of the Environment and Ecology, Xiamen University, Xiamen 361102, P. R. China b Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, National Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, Key Lab for Chemical Biology of Fujian Province, Xiamen University, Xiamen 361005, P. R. China c College of Chemical Engineering, Huaqiao University, Xiamen 361021, P. R. China d College of Food and Biology Engineering, Jimei University, Xiamen, Fujian 361021, China a

ABSTRACT: Metal-organic frameworks (MOFs) have been exploited as excellent solid precursors and templates for the

preparation integrated nanocatalysts with multicomponent and hierarchical structures. Herein, a novel synthetic protocol has been developed to fabricate versatile Zr-based solid solutions (such as ZnO-ZrO2, Co3O4-ZrO2, and CuO-ZrO2) via pyrolysis of Schiff base modified UiO-66 octahedrons (size < 100 nm), which were then utilized as efficient catalysts for CO2 hydrogenation. Schiff base serves as an effective bridge to doping secondary metal ions into UiO-66 frameworks with a controlled amount (0.3 wt% to 8.8 wt%) which are initially hard to achieve. Interestingly, by simply changing the loading metal ions, the selectivity of C1 hydrogenation products can be facilely tuned. For instance, the maximum CO2 conversion of ZnO-ZrO2, Co3O4-ZrO2, and CuO-ZrO2 solid solutions were 5.8%, 11.4%, and 22.5%, with main product selectivity of 70% CH3OH, 92.5% CH4, and 86.7% CO, respectively. Moreover, in-situ DRIFTS characterizations reveal that the significant difference in C1 product selectivity is mainly determined by the balance of *HCOO, *CH3O, and *CO intermediate species over the Zr-based solid solutions.

KEYWORDS: MOFs, in-situ DRIFTS, Schiff base, solid solutions, CO2 hydrogenation

 INTRODUCTION Metal-organic frameworks (MOFs) are a new kind of porous materials which have been widely studied for gas storage,1 membrane separation,2 catalysis,3-5 drug delivery,6 etc., due to the high flexibilities in organic linkers. Until now, there are more than 20,000 MOFs reported in the literature.7 However, the low intrinsic catalytic activity of both building motifs still hinders MOFs’ application in catalysis. To this end, an effective way is to incorporate secondary metal ions into the frameworks, that is, to prepare bimetallic MOFs.8 For instance, bimetallic Au/ZnMOF nanocages derived from ZIF-8 could be used to catalyze the cycloaddition of CO2 and epoxide under mild reaction conditions with excellent yields (95-99%).9 Thus, a great deal of efforts has been devoted to functionalize MOFs to prepare heterometallic catalysts,10 such as Co/ZnMOF-5,11 Co/Zn-ZIF-8,12 and Au/Zn-MOF,9 etc. Nevertheless, it is still not easy to synthesize various bimetallic MOFs by a general method due to that the coordination environments in MOFs are unfavorable to efficiently incorporate new metal ions and the

incorporation of the secondary metal ions often results in fragile frameworks.13, 14 The Zr-based MOFs have attracted tremendous research attention because of their excellent thermal, acid and water resistance, as compared to other conventional MOFs.15 Among them, UiO-66 is a highly stable MOF composed of Zr-metal nodes and H2BDC (1,4-dicarboxybenzene) as organic linkers. Currently, incorporation of secondary metal ions into UiO-66 frameworks has remained a challenge. Previous studies have shown that doping metal ions to UiO-66 cannot be achieved with flexible and tunable metal ratios by the traditional one-pot method or metallic exchange method.8, 16, 17 It is worth noting that the diversity and flexibility of organic ligands in MOFs offer a myriad of opportunities to the post-modifications of MOFs. For instance, Schiff base is one of the most widely used molecular linkers in chelating heavy metal ions,18 which could be employed as a colorimetric fluorescent probe for detecting metal ions.19, 20 Great success has been achieved in functionalizing MOFs with salicylaldehyde, which then forms Schiff base to introduce the secondary

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metal ions into the frameworks.21-24 Recently, UiO-66-NH2 has been functionalized with salicylaldehyde which could be further loaded with a series of metal ions like Co,25 Mo,23 Ir,24 Pd,26 Fe,27 to form the Schiff base complex. CO2 conversion into value-added products like chemicals and fuels is an attractive way to mitigate CO2 emission. Cu/ZnO based catalysts are most widely studied for CO2 utilization.28, 29 Recently, In2O3 with oxygen vacancies that can activate CO2 molecule has also emerged as a promising catalyst for CO2 hydrogenation.30, 31 On the other hand, the solid solution presents a group of heterogeneous catalysts owning a wide range of catalytic properties. Recently, a newly discovered ZnO-ZrO2 solid solution showed super stability and high selectivity for methanol synthesis from CO2 hydrogenation, with a high selectivity of 86%.32 CexZr1-xO2 solid solution has been prepared for directly synthesis dimethyl carbonate from CO2 and methanol.33, 34 To the best of our knowledge, both physicochemical and catalytic properties of solid solutions are strongly affected by the surface property and the crystal structure, which are highly depended on the preparation methods.34 Accordingly, various studies have been carried out to develop new methods to synthesize solid solutions with controlled physicochemical properties. For instance, Zhang et al have prepared a series of CexZr1-xO2 solid solutions with bimodal pore structure by sol-gel method.35 Avgouropoulos et al have reported that CuO-CeO2 solid solution can be prepared by urea-nitrates combustion method which showed excellent CO oxidation activity with a reaction rate of 1.8 μmol/g/s at 70 ℃ and low CO2 inhibition effect.36 In addition, a high-energy milling ball method has also been used for V2O5-Yb2O3 solid solution synthesis.37

Scheme 1. Schematic illustrations of the preparation of Schiff base bridged UiO-66, wherein, UiO-66-NH2-SB is Schiff base modified UiO-66, UiO-66-NH2-SB-Mδ+ (Mδ+ = Zn2+, Co2+, or Cu2+) is bimetallic UiO-66 prepared via Schiff base modification.

As mentioned above, there are only limited reports for the use of MOFs as solid precursors for the synthesis of solid solutions. In this work, Schiff base modified bimetallic UiO-66 frameworks have been applied to prepare MxOy-ZrO2 (M = Zn, Co, or Cu) solid solutions. The Schiff base successfully bridges different metal ions (Zn2+, Co2+, or Cu2+) into the robust UiO-66 frameworks wherein the doping amount can be well-controlled. As illustrated

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in Scheme 1, the approach includes four consecutive steps: i) the synthesis of UiO-66-NH2 octahedral nanoparticles, ii) post-modification by salicylaldehyde to form Schiff base, iii) chelating the secondary metal ions (such as Zn2+, Co2+, or Cu2+) by the Schiff base bridge, and iv) calcination of the bimetallic MOFs template to obtain Zr-based solid solutions (viz., ZnO-ZrO2, Co3O4-ZrO2, and CuO-ZrO2). Then, the catalytic performance of the three prepared solid solutions was evaluated in CO2 hydrogenation reaction at 3 MPa and 320 °C. In addition, the surface intermediates during the reaction were examined by in-situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) to explore the CO2 hydrogenation mechanism.

 EXPERIMENTAL SECTION Materials. The following chemicals were used without further purification. Zirconium nitrate (analytical reagent), zinc nitrate (99.0%), 2-aminoterephthalic acid (H2BDCNH2, 99.0%), acetic acid (99.5%), N, Ndimethylformamide (99.5%), sodium hydroxide (98.0%), cobalt nitrate (98.5%), copper nitrate (99%), salicylaldehyde (98%), ethanol (99.7%), hydrofluoric acid (40%), and DMSO-d6 (99.9%), were purchased from Aladdin Chemicals Co. Ltd. And all the chemicals were used as received. Deionized water was used for all experiments. Preparation of MxOy-ZrO2 by Schiff base modification method. Preparation of UiO-66-NH2. UiO66-NH2 octahedral nanoparticles were prepared by using a modified method.38 In details, 0.15 mmol of Zr(NO3)4 and 0.15 mmol of H2BDC-NH2 were added to a mixed solvent of DMF (15 mL) and acetic acid (1 mL). Then the solution was stirred and transferred into a 100 mL Teflon-lined autoclave for heating at 120 °C for 12 h. The obtained product was collected by centrifugation, washed with excess ethanol for three times, and dried at 80 °C overnight. Preparation of UiO-66-NH2-SB. 1 mmol of UiO-66-NH2 was dispersed in 30 mL of ethanol under ultrasonication and stirred vigorously. The solution was vigorously stirred when 400 μL of salicylaldehyde dissolved in 15 mL of ethanol was added with an addition rate of 0.5 mL/min. After that, the solution was continuously stirred for 6 h. The product was recovered by centrifugation and washed with excess ethanol for three times, and dried at 80 °C to obtain Schiff base modified UiO-66-NH2 (i.e., UiO-66NH2-SB). Preparation of Zr-based solid solutions. 1 mmol of UiO66-NH2-SB was dispersed in 30 mL of ethanol via ultrasonication and vigorously stirring. Then, to the solution, precursor solution (various metal nitrates dissolved in 15 mL of ethanol) was added with an addition rate of 0.5 mL/min. The reaction was carried out at room temperature for 6 h. Finally, the product (viz., UiO-66NH2-SB-Mδ+) was collected by centrifugation and washed with ethanol for three times. The UiO-66-NH2-SB-Mδ+ was


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obtained with different theoretical molar ratios of Mδ+ to Zr: 0.3, 0.5, 1.0, and 3.0. Then the UiO-66-NH2-SB-Mδ+ was calcined in a muffle furnace in air at 500 °C for 5 h with a heating rate of 2 °C/min to obtain Zr-based solid solutions, which were denoted as MxOy-ZrO2 (Mδ+ = Zn2+, Co2+, or Cu2+). Preparation of solid solution by metallic exchange method. Typically, 1 mmol of UiO-66-NH2 was dispersed in 30 mL of ethanol and ultrasonic dispersed. The solution was stirred for 5 min, then n mmol of Zn(NO3)2·6H2O was added into the above solution and stirred for 8 h (n = 1.0, 5.0, 10, 20). The product was collected by centrifugation and washed with ethanol for three times, and dried at 80 °C overnight. Then, the obtained product was calcined in a muffle furnace in air at 500 °C for 5 h with a heating rate of 2 °C/min to obtain Zr-based solid solution. Preparation of solid solution by the one-pot method. Briefly, 0.15 mmol of Zr(NO3)4·5H2O, n mmol of Zn(NO3)2·6H2O and 0.15 mmol of H2BDC-NH2 were added to the mixed solvent of 15 mL DMF and 1 mL acetic acid (n = 1.0, 5.0, 10, 20). Then the solution was stirred and transferred into a 100 mL Teflon-lined autoclave for heating at 120 °C for 12 h. The obtained solid was collected by centrifugation, washed with excess ethanol for three times, and dried at 80 °C overnight. Then the product was calcined in a muffle furnace in air at 500 °C for 5 h with a heating rate of 2 °C/min to obtain Zr-based solid solution. Catalytic performance evaluation. The CO2 hydrogenation was carried in a quartz fixed-bed continuous flow reactor loading catalysts (containing 100 mg of the prepared catalyst diluted with 400 mg quartz sand). The reaction was carried out under condition of 3 MPa, 320 °C, V(CO2)/V(H2)/V(N2) = 24%/72%/4%, and GHSV = 18,000 mL/g/h. The gaseous products were measured online with a gas chromatographic system equipped with both TCD and FID detectors. Characterization methods. The morphologies of the MOFs and MxOy-ZrO2 solid solutions were observed using SEM system (ZEISS SIGMA, Germany). Transmission electron microscopy (TEM) images of the samples were obtained by a Tecnai F30 microscope (TECNAI F30, USA). Organic functional groups of the samples were analyzed with FTIR spectroscopy (Nicolet 6700, USA). The crystal information was analyzed by XRD (Rigaku Ultima IV, Japan) with Cu Kα radiation. Temperature-programmed reduction (H2-TPR) was conducted using a Micromeritics AutoChem II 2920 instrument with a thermal conductivity detector (TCD). Temperature-programmed desorption of CO2 (CO2-TPD) was used to assess the CO2 adsorption ability and desorption amount. Metal contents in the solid samples were determined by ICP-OES (PerkinElmer ICP 2100) or ICP-MS (PerkinElmer NexION 300X). The amount of adsorbed CO2 was monitored by Micromeritics AutoChem II ASAP 2920 apparatus with a mass spectrometer (MS) detector. The sample was pretreated at 500 °C under He flow at a rate of 30 mL/min and then cooled down to 50 °C. The desorption was performed from

50 to 800 °C with a heating rate of 10 °C/min. X-ray photoelectron spectroscopy (XPS) was detected on Thermo Fisher Scientific diffractometer employing Al Kα (1486.6 eV) radiation as an X-ray source in which the binding energy was calibrated with C 1s 284.8 eV. Inductively coupled mass spectrometry (ICP-MS, Agilent 7500CE) combined with a concentric nebulizer and was used to determine the concentration of metal. 1H-NMR spectra of the digested samples were collected on a 500 MHz Bruker AVANCE III spectrometer. For UiO-66-NH2 and UiO-66-NH2-SB, 20 mg of the solid sample, 120 μL of 5% HF and 0.5 mL of DMSO-d6 were added into NMR tubes, and the mixture was sonicated for 10 min until a clear solution was obtained. The specific BET surface areas of the UiO-66-NH2, UiO-66-NH2-SB, and UiO-66-NH2-SB-Zn samples were measured on Tristar II3020, USA. Before the N2 physisorption measurement, all samples were degassed at 200 °C for 3 h. In-situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) were performed using FTIR spectrometer (Thermo, Nicolet iS50). Before measurements, each catalyst was pre-treated at 300 °C in a 30 mL/min H2 flow for 30 min and N2 at 350 °C for 30 min. After cooling to 30 °C, the background spectra were recorded. At 30 °C, CO2 flow was pulsed over the samples (30 mL/min) for 18 min, then changed to N2 flow (30 mL/min) for 40 min until a stable signal was observed. Following the N2 treatment, the intermediate background spectra were collected in flowing N2 at 280 °C and automatically subtracted from the sample spectra. Then the samples were treated with a CO2/H2 mixture (20 mL/min CO2 and 60 mL/min H2) for 100 min.39

 RESULTS AND DISCUSSION Characterizations of UiO-66-NH2-Mδ+. In the beginning, the light yellow UiO-66-NH2 with octahedral morphologies were synthesized by coordination reactions between Zr(NO3)4 and 2-aminoterephthalic acid (H2BDCNH2), as shown in Figure 1a,b. The sharp XRD diffraction peaks pattern of the synthesized sample (as shown in Figure 1g) match well with the simulated UiO-66-NH2,40 which indicated the successful formation of UiO-66 frameworks. The textural properties of samples were characterized by N2 physisorption measurements as shown in Figure S1. The BET surface areas were 1074, 936, and 846 m2/g, and pore volumes were 0.362, 0.319, and 0.284 cm3/g for the as-prepared UiO-66-NH2, UiO-66-NH2-SB, and UiO-66-SB-Zn samples, respectively. The decrease in surface areas and pore volumes after surface modification and metal ion loading was attributed to the aldehyde (or metal ion) moieties grafted on the surface and occupied the larger pore space.41 Anyway, the specific BET surface areas and pore sizes of UiO-66-NH2-SB, and UiO-66-SB-Zn kept essentially unchanged as compared with those of pristine UiO-66-NH2, revealing the similar framework in UiO-66-NH2 after Schiff base modification. Representative SEM images of the samples also show the unchanged morphology after Schiff base post-modification and



metalation (as shown in Figure 1c-f). Specifically, the amine groups reacted with salicylaldehyde to produce (R-N=CC6H4OH) moiety as π-conjugated molecules sites which could be expected to serve as metal ions binding sites (see Scheme 2). Such a structure transformation could also be easily observed from the color change turning from light yellow to bright yellow. Afterward, the UiO-66-NH2-SBMδ+ were prepared by introducing different M(NO3)δ alcohol solution at room temperature. The products were characterized by using XRD and SEM. As revealed in Figure 1e,f,g and Figure S2, all the synthesized products are of monodispersed with uniform size and well-defined octahedral nanocrystal shapes.

Figure 1. Representative SEM images of (a, b) UiO-66NH2, (c, d) UiO-66-NH2-SB, (e, f) UiO-66-NH2-SB-Mδ+ (M δ+ = Zn2+), and (g) XRD patterns of the different samples. Insets show the structural models and the photos of the ethanolic suspensions illustrating the transformations from UiO-66-NH2 to UiO-66-NH2-SB and UiO-66-NH2SB-Mδ+. O NH2

UiO-66-NH2

H OH

OH N

UiO-66-NH2-SB

O M(NO3)δ in ethanol

N

M

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introduction of metal ions is caused by the electron density reduction of N atoms after coordination with metal ions.44 The band corresponding to (C-N) shifted from 1160 to 1145 cm-1 also shows the electron density decrease of N atoms in UiO-66-NH2-SB-Zn2+.45 This feature clearly indicates the interaction between metal ions and Schiff base during the metal complexation process. However, the transmission bands of (Zn-N) and (Zn-O) cannot be observed due to that they are overlapped by the strong vibration of (ZrO).46 The successful coordination of Mδ+ (Mδ+ = Zn2+, Co2+, or Cu2+) to UiO-66-NH2-SB was further confirmed by XPS (Figure 2c,d, and Figure S3). As compared to UiO-66-NH2SB, N 1s peak in UiO-66-NH2-SB-Mδ+ clearly shifts from 399.6 to 399.7, 399.8, and 399.9 eV, indicating the coordination of Mδ+ with N atoms.45, 47 The slight increase in the value of binding energy is due to the decreased electron density of nitrogen after the formation of N-Mδ+ bond.48 While the binding energy of O 1s peak decreases from 531.6 to 531.3, 531.5, and 531.5 eV after Mδ+ coordination (as shown in Figure 2d).47 The shift of O 1s peak was caused by electron density change of O after Mδ+ coordinated with O atoms forming O-Mδ+ coordination band. These results confirmed the implementation of the Schiff base in the coordination with Mδ+ ions. The TEM images and the corresponding EDX elemental mapping of UiO-66-NH2-M are shown in Figure S4. Clearly, EDX elemental maps revealed that Mδ+ (M= Zn, Co, Cu) ions were homogeneously distributed throughout the whole UiO-66-NH2 crystals rather than the external surface.

NO3 NO3

UiO-66-NH2-SB-Mδ+

Scheme 2. The scheme of the preparation of UiO-66-NH2SB-Mδ+. FTIR characterization was conducted to prove the successful modification of UiO-66-NH2 by salicylaldehyde. Figure 2a shows the FTIR spectra of UiO-66-NH2, UiO-66NH2-SB, and UiO-66-NH2-SB-Zn2+. The vibration of -NH2 group in UiO-66-NH2 appears in the broad intense band of 3500-3300 cm-1 and 1600-1500 cm-1.42, 43 The strong band at 1580 cm-1 can be ascribed to the vibration of N-H group. The band of 1500 cm-1 represents the typical C=C vibration of a benzene ring, and the characteristic bands at 767, 656, and 478 cm-1 are all associated with O-H, C-H groups in BDC2-, respectively.42 After Schiff base modification, a new band appeared at 1640 cm-1 which can be ascribed to C=N stretch vibration due to the formation of Schiff base. A redshift of (C=N) band from 1640 to 1624 cm-1 with the

Figure 2. (a) FTIR spectra of UiO-66-NH2, UiO-66-NH2SB, UiO-66-NH2-SB-Zn2+, insets show the structural models of the corresponding samples, (b) 1H NMR spectra of the digested UiO-66-NH2 and UiO-66-NH2-SB samples, (c) N 1s XPS spectra, and (d) O 1s XPS spectra for (1) UiO66-NH2-SB, (2) UiO-66-NH2-SB-Zn2+, (3) UiO-66-NH2-SBCo2+, and (4) UiO-66-NH2-SB-Cu2+.


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The degree of UiO-66-NH2 modified by Schiff base was further analyzed by 1H NMR via digesting the solid samples. As shown in Figure 2b, the 1H NMR spectra of the digested UiO-66-NH2-SB displayed several new proton peaks at 7.65 ppm, 7.53-7.48 ppm, and 7.00-6.93 ppm, which can be attributed to a Schiff base ligand. The 1H NMR results further confirmed the successful modification of UiO-66-NH2. In addition, the degree of Schiff base conversion was estimated by comparison of the 1H NMR peak at 7.36 ppm corresponding to NH2-bdc and those integration peaks at 7.53 ppm corresponding to the aldehyde. Accordingly, the degree of conversion was calculated as 22%. The data is similar to those reported for MOF incorporation with Schiff base (see also Table S1).27, 41 In addition, the zinc loading amounts in the solid solutions prepared with different theoretical Zn/Zr ratios and solution pH was measured by ICP-OES (see Figure 4b). The Zn loading amount varied from 0.3 wt% to 8.8 wt% as increasing the theoretical molar ratio of Zn/Zr from 0.5 to 20. The Schiff base modification method enables the maximum Zn doping amount of 8.8 wt%. The solution pH of 7 favors the high zinc loading which is due to that protons could occupy some of the adsorption sites on MOF surface at lower solution pH.25 Combining the above NMR and ICP data, the maximum Zn loading in the UiO-66-NH2-SB-Zn solid corresponds to an N/Zn molar ratio of ca. 0.9, indicating that nearly one of the Schiff base moiety was able to coordinate with one Zn ion. Although a very high Ir loading of 30 wt% on UiO66-NH2-SB was reported by Kuntal Manna,26 it is believed that the adsorption capacity of metal ions by Schiff base varied with the different metal species.25 Characterizations of MxOy-ZrO2 solid solutions. To compare the structural advantages of UiO-66-NH2-SB for anchoring metal ions, one-pot method and metallic exchange method which are commonly used for preparing bimetallic MOFs were also adopted to load Zn2+ on UiO66-NH2. As shown in Figure 3a, the XRD pattern of pristine ZrO2 is mainly tetragonal phase mixing with some monoclinic phase. The three characteristic reflection peaks at 2θ = 30.5, 50.1, and 60° are assigned to tetragonal ZrO2(JCPDS No. 50-1089),49 and the three reflection peaks with low intensity at 2θ = 24.0, 28.1, and 31.5° can be indexed as monoclinic ZrO2(JCPDS No. 37-1484). It was found that XRD patterns of ZnO-ZrO2 solid solutions prepared by different methods show the same phase of tetragonal ZrO2, indicating all the samples are in solid solution structure.50 After calcination, the BET surface areas and pore volumes of ZnO-ZrO2 solid solution decreased to 62 m2/g and 0.016 cm3/g which is due to the framework collapse (as shown in Figure S1). Furthermore, by careful comparison (insert in the top right corner of Figure 3a), the XRD diffraction peak at 50.2° of pristine ZrO2 show slightly shift to higher 50.4° for samples prepared by the one-pot method and metallic exchange method and 50.7° for ZnO-ZrO2 fabricated by our newly

designed method, which is probably due to the different loading amount of Zn2+. For comparison, Zn concentrations in ZnO-ZrO2 prepared by different methods were detected by ICP-MS (as shown in Figure 3b). It was found that traditional methods were inferior to introduce the secondary metal ions into UiO-66 frameworks, as compared to our designed Schiff base modification method. Zn concentrations in the samples remained very low even the ratio of Zn/Zr was raised up to 20, which further confirms that the obtained UiO-66-NH2SB facilitated Zn ions complexation.

Figure 3. (a) XRD patterns of ZnO-ZrO2 solid solutions prepared by different methods, and (b) the comparison of Zn concentrations in the solid solutions prepared by different synthetic methods ((1) the pristine ZrO2, (2) metallic exchange method, (3) one-pot method, and (4) Schiff base modification method).

Figure 4. (a, c, and d) XRD patterns of UiO-66 derived ZnO-ZrO2, Co3O4-ZrO2, and CuO-ZrO2 solid solution, (b) the comparison of Zn concentrations in ZnO-ZrO2 solid solution prepared under different conditions. To demonstrate the general application of Schiff base serving as metal loading bridge, other metal ions like Co2+ and Cu2+ with different metal loading amounts were also introduced into UiO-66 frameworks in the same manner. The XRD patterns (Figure 4a,c, and d) reveal that pure solid solution phases of MxOy-ZrO2 are formed for various M to Zr molar ratios from 0.5 to 3.0 (M = Zn, Co, or Cu).51



Moreover, combined with the ICP-MS results (Figure 4b), the metal complexation efficiency could be regulated by changing the pH value of the solution. For instance, a high Zn amount (0.29%) was achieved when the molar ratio of Zn: Zr was 0.5 as adjusting the pH of the solution to 7. These results highlight the significant metal doping ability of Schiff base on the modified UiO-66 with flexible doping amount. It is anticipated that the general synthetic strategy may also be applicable to modified Zr-based MOF with other metal ions.

Figure 5. Representative TEM images of (a-c) ZnO-ZrO2, (d-f) Co3O4-ZrO2, and (g-i) CuO-ZrO2.

Figure 6. EDX elemental maps of (a) ZnO-ZrO2, (b) Co3O4ZrO2, and (c) CuO-ZrO2 solid solutions. TEM images of the obtained MxOy-ZrO2 (M = Zn, Co, or Cu) nanoparticles are shown in Figure 5. It is clear that ZnO-ZrO2 (Figure 5a-c), Co3O4-ZrO2 (Figure 5d-f) and

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CuO-ZrO2 (Figure 5g-i) nanoparticles are all well monodispersed, with uniform octahedral shapes (see also SEM images in Figure S5). As illustrated in Figure 5, the mean edge lengths of ZnO-ZrO2, Co3O4-ZrO2, and CuOZrO2 particles are around 100 nm with stacked nanoparticles, which the average crystallite sizes estimated from Scherrer’s equation are 7.8, 4.7, and 5.2 nm, respectively. The pristine surface of UiO-66 nanoparticles became rougher along with the calcination, but the overall morphology is still maintained. EDX elemental maps of ZnO-ZrO2 (as shown in Figure 6a) show that Zr, O, Zn are evenly distributed on the ZnO-ZrO2 solid solution structures. Similarly, the Co (Figure 6b) and Cu (Figure 6c) elements also distribute uniformly throughout the whole octahedral nanoparticles MxOy-ZrO2 (M = Co, or Cu). Therefore, the Schiff base modification method benefits the formation of uniform solid-solution particles. CO2 hydrogenation over MxOy-ZrO2 solid solutions. We next examined the CO2 hydrogenation performance of the octahedral MxOy-ZrO2 solid solutions. First, for comparison, a blank experiment was carried out by using pristine ZrO2 derived from pure UiO-66 as a catalyst. As presented in Figure 7a, the CO2 conversion was negligible, that is, 0.7% at 320 ℃ and 3 MPa. Whereas, much higher activities were obtained for the ZnO-ZrO2 solid solution under the same reaction conditions. In addition, the sample of ZnO-ZrO2 fabricated by Schiff base modification method exhibited much higher CO2 conversion (5.7%) than solid solutions prepared by the one-pot method (1.9%) and metallic exchange method (1.7%) (see the results in Figure 7a, and Figure S6). The catalytic activity varies greatly with different Zn concentrations (as shown in Figure 7a,b), which is consistent with the previous reports.39, 51 It appears that with increasing the amount of Zn, CO2 conversion drastically increases, and the CH3OH selectivity becomes more prominent. The maximum CH3OH selectivity (70%) and CO2 conversion (5.7%) were achieved at the molar ratio of Zn: Zr of 1: 1, but further increasing molar ratio of Zn: Zr ratio (Zn: Zr = 3: 1) did not lead to any further changes of product selectivity and CO2 conversion. All the results again indicate that the Schiff base modification method can lead to successfully introducing the secondary metals into UiO-66 efficiently, thus dramatically enhance the catalytic activity of ZnOZrO2 solid solution. Other Zr-based solid solutions (e.g., Co3O4-ZrO2 and CuO-ZrO2) were also synthesized by Schiff base modification method. As shown in Figure 7c,d, Co3O4ZrO2, and CuO-ZrO2 also showed high catalytic activity for CO2 hydrogenation with different main products. The Co3O4-ZrO2 solid solution exhibited excellent activity for CO2 hydrogenation with the main product of CH4. The maximum of CO2 conversion and CH4 selectivity were 11.4% and 92.5%, respectively, when the molar ratio of Co: Zr was 0.5: 1. The CO2 conversion increased quickly with the increase of Co amount, but further increased the molar ratios of Co: Zr from 0.5 to 3.0, the CO2 conversion


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decreased dramatically to only 1.9%. Compared to pristine ZrO2, the selectivity of CH4 for Co3O4-ZrO2 increased from 0.8% to 92.5%, and the CO2 conversion was increased about 16 times. Differently, the CuO-ZrO2 solid solution was more active in CO formation from RWGS reaction as shown in Figure 7d. The catalytic activity was significantly improved with CO selectivity of 86.7% and CO2 conversion of 22.5% when the molar ratio of Cu: Zr was 0.5:1. It is worth noting that the CO2 conversion of CuO-ZrO2 was 32 times higher than those of pristine ZrO2. All the above results indicate that there is a strong synergetic effect between the introduced secondary M metal (M = Zn, Co, or Cu) and Zr in the catalytic activity for CO2 hydrogenation reaction. In addition, Figure 7e shows there is no activity loss of ZnOZrO2 in CO2 hydrogenation reaction for 80 h on stream. As shown in Figure 7f, the morphology of the spent ZnO-ZrO2 particles remains monodisperse. No obvious crystalline structure change was found from XRD characterization (Figure S7), suggesting the high stability of ZnO-ZrO2 catalyst.

Figure 7. CO2 hydrogenation performance of the prepared catalysts. (a) CO2 conversion over (1) pristine ZrO2, (2) ZnZr solid solution prepared by metallic exchange method, (3) Zn-Zr solid solution prepared by one-pot method, (4) Zn-Zr solid solution prepared by Schiff base modification method, and (b-d) MxOy-ZrO2 with different M: Zr molar ratios (M = Zn, Co, or Cu), (e) catalyst stability test of ZnOZrO2 during 80 h, and (f) TEM image of the spent ZnOZrO2 catalyst after being subjected to 80 h activity evaluation.

Figure 8. (a) CO2-TPD, (b) H2-TPR characterizations of pristine ZrO2, ZnO-ZrO2, Co3O4-ZrO2, and CuO-ZrO2 solid solutions. CO2-TPD was performed to assess CO2 adsorption ability of the catalyst samples. As shown in Figure 8a, there are two desorption peaks being found: one at low temperature from 50 to 300 °C, and the other at high temperature from 300 to 500 °C. Obviously, the total CO2 adsorption amount of MxOy-ZrO2 solid solutions at low temperature was much higher than pristine ZrO2, which might facilitate the CO2 hydrogenation performance.52 H2-TPR was performed to investigate the reducibility of the catalyst samples as illustrated in Figure 8b. There are two H2 consumption peaks for all MxOy-ZrO2 solid solutions. The peak at 100500 °C can be attributed to the reduction of highly dispersed MxOy species, while the peak at 500-600 °C is the reduction peak for ZrO2. Specifically, for ZnO-ZrO2 sample, the peak at 400-500 °C assigned to the reduction of ZnO.53 For CuO-ZrO2 sample, two broad peaks in the range of 100200 °C and 200-300 °C were observed, which corresponding to the reduction of copper oxide to metallic copper.54 Additionally, for Co3O4-ZrO2 sample, a small peak at 300-400 °C is caused by the reduction of Co3+ to Co2+, while the broad peak at high temperature 500-600 °C is related to the overlap two reduction peaks of Co2+ to Co0 and ZrO2.55, 56 Compared with the pristine ZrO2 sample, the reduction temperature of MxOy-ZrO2 solid solutions all shift to lower temperature, confirming that MxOy-ZrO2 solid solutions are more easily to be reduced than pristine ZrO2 (> 600 °C). Therefore, on the basis of H2-TPR and CO2-TPD results together with CO2 hydrogenation performance, it can be derived that the synergistic effect between M and Zr in MxOy-ZrO2 solid solutions would enhance the activation of CO2 and H2 leading to higher catalytic performance in CO2 hydrogenation.39 DRIFTS study over MxOy-ZrO2 solid solutions catalysts. The superior catalytic activity of MxOy-ZrO2 (M = Zn, Co, or Cu) solid solutions is assumed to arise from the uniform dispersion of M dopants in its structures. Based on the above results, it can be concluded that there is a synergetic effect between M and Zr, which obviously promoted the activation of CO2 and H2, and consequently resulted in the high catalytic performance for CO2 hydrogenation. To understand the roles of Zr-based solid solution catalysts in determining the different product selectivity (viz., CH3OH, CH4, or CO), the surface species evolved in the CO2 hydrogenation reaction were monitored by in-situ



DRIFTS (as shown in Figure 9). Firstly, a series of IR spectra were obtained after introducing CO2 over the sample of ZnO-ZrO2 (Figure 9a), Co3O4-ZrO2 (Figure 9b), and CuOZrO2 (Figure 9c). A strong peak at 2357 cm-1 can be detected and reached steady after 1.5 min, which is assigned for CO2 in reactor and physisorption on the surface of catalysts. Meanwhile, the adsorption of CO2 also results in the formation of peaks at 1652, 1497 and 1296 cm-1, all of which are attributed to different symmetrical stretching and asymmetric stretching vibration of chemisorption carbonates species.57, 58 Afterward, CO2 was cut off, and the samples were flushed by N2. The introduction of N2 resulted in the disappearance of physisorption peak of CO2 at 2357 cm-1, while the chemisorption peaks of CO2 assigned to carbonates species remained unchanged. According to the obtained DRIFTS, we propose that CO2 could be efficiently adsorbed on the surface of ZnO-ZrO2, Co3O4-ZrO2, and CuO-ZrO2 solid solutions, which is consistent with CO2-TPD results. Since the observed differences in CO2 hydrogenation catalytic activity can be related to different reaction intermediates over the catalyst surface, in-situ DRIFTS of ZnO-ZrO2 (Figure 9d), Co3O4-ZrO2 (Figure 9e), and CuOZrO2 (Figure 9f) solid solutions in the reaction gases were employed to shed more light on the catalytic mechanism. Figure 9d shows the intermediate species during CO2 hydrogenation reaction over ZnO-ZrO2 solid solution. With reaction gas (CO2/H2) onward, the peaks appeared at 1592 and 1372 cm-1 are assigned to asymmetric and symmetric OCO stretching vibrations respectively, which belong to the adsorbed *HCOO species.39 And the peaks at 2932, 2827, and 1044 cm-1 are attributed to *CH3O species. The bands at 2880 and 1386 cm-1 are assigned to the stretching vibration ν(CH) and bending vibration δ(CH).59 Therefore, over the ZnO-ZrO2 solid solution catalyst, the *HCOO and *CH O species were the main intermediate 3 species with intense adsorption.

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Figure 9. In-situ DRIFTS of (a, d) ZnO-ZrO2, (b, e) Co3O4ZrO2, (c, f) CuO-ZrO2 catalysts after exposing to CO2 (30 °C, 30 mL/min CO2) and in the CO2 hydrogenation reaction (280 °C, 20 mL/min CO2 + 60 mL/min H2). The different reaction intermediates over Co3O4-ZrO2 catalyst during the methanation reaction are presented in Figure 9e. After introducing reaction mixture (CO2/H2) for 0.6 min, new peaks centered at 2870 and 1425 cm-1 are the indication of δ(CH3) and νs(CH3), suggesting the presence of adsorbed *CH3O species. Accordingly, the surface *CH3O intermediate is the key likely precursor species for methane formation.60, 61 Furthermore, for CuO-ZrO2 solid solution (as shown in Figure 9f), the bands at 1567 and 1370 cm-1 suggest the presence of *HCOO surface intermediates,39, 62 while the band at 2124 cm-1 is ascribed to surface-adsorbed *CO species.63, 64 Hence, the absorbed CO2 is mainly transformed into *HCOO and *CO species, which are the main intermediates for CO formation via RWGS reaction.62, 65 Overall, all results show that different reaction intermediate species lead to different CH3OH, CH4 or CO selectivity. Particularly, the balance of *HCOO and *CH3O governs the selectivity to CH3OH, CH4 and CO. This could be tuned to achieve the desired product selectivity by optimizing the loading metal amount (Zn, Co, or Cu).

 CONCLUSIONS In summary, we have successfully prepared a series of MxOy-ZrO2 (M = Zn, Co, or Cu) solid solutions with an


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excellent catalytic performance for CO2 hydrogenation by pyrolysis of Schiff base modified bimetallic UiO-66. It was found that the Schiff base facilitated doping of the secondary metal ions into the rigid metal-organic frameworks with adjustable loading amount. Among the three obtained catalysts, Co3O4-ZrO2 showed the highest CH4 selectivity of 92.5% with a CO2 conversion of 11.4%, CuO-ZrO2 exhibited 22.5% conversion of CO2 with 86.7% CO selectivity, and ZnO-ZrO2 owned the highest methanol selectivity of 70% with 5.7% CO2 conversion. According to the in-situ DRIFTS of the obtained catalysts, both *HCOO and *CH3O species were observed as the intermediates over ZnO-ZrO2, and *CH3O species were observed over Co3O4ZrO2, whereas *HCOO and *CO species were observed over CuO-ZrO2. Therefore, the C1 product selectivity in CO2 hydrogenation can be facilely tuned by changing the secondary metal ions during the preparation of solid solutions. For future studies, more effort should be focused on the further promoting the CO2 conversion and product selectivity, such as adjusting the size of UiO-66 to introduce more active sites, integrating other active sites on solid solutions to construct multifunctional catalyst.

 ASSOCIATED CONTENT Supporting Information Available: Additional experimental results of the studied samples by using TEM, SEM, EDX, BET, XPS, and XRD, including Figures S1 to S7 and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

 AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (G. Zhan), [email protected] (J. Huang), [email protected] (Q. Li). Notes The authors declare no competing financial interest. ORCID Guowu Zhan: 0000-0002-6337-3758

 ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21536010 and 41673088). Dr. Mingzhi Wang in Xiamen University and Qinghua Chen’s group in Fujian Normal University are gratefully acknowledged for aiding with some characterizations.

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Solid Solutions Derived from Schiff Base Bridged UiO-66 Composites

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