One-Pot Method Synthesis of Bimetallic MgCu-MOF-74 and Its CO2 Adsorption under Visible Light

A magnesium-based metal–organic framework (Mg-MOF-74) exhibits excellent CO2 adsorption under ambient conditions. However, the photostability of Mg-MOF-74 for CO2 adsorption is poor. In this study, MgxCu1–x-MOF-74 was synthesized by using a facile “one-pot” method. Furthermore, the effects of synthesis conditions on the CO2 adsorption capacity were investigated comprehensively. X-ray diffraction, Fourier transform infrared, scanning electron microscopy, thermo gravimetric analysis, inductively coupled plasma atomic emission spectroscopy, ultraviolet–visible spectroscopy and photoluminescence spectroscopy, and CO2 static adsorption–desorption techniques were used to characterize the structures, morphology, and physicochemical properties of MgxCu1–x-MOF-74. CO2 uptake of MgxCu1–x-MOF-74 under visible light illumination was measured by the CO2 static adsorption test combined with the Xe lamp. The results revealed that MgxCu1–x-MOF-74 exhibited excellent photocatalytic activity. Furthermore, the CO2 adsorption capacity of MgxCu1–x-MOF-74 was excellent at a synthesis temperature and time of 398 K and 24 h in dimethylformamide (DMF)-EtOH-MeOH mixing solvents, respectively. MgxCu1–x-MOF-74 retained a crystal structure similar to that of the corresponding monometallic MOF-74, and its CO2 uptake under visible light was superior to that of the corresponding monometallic MOF-74. Particularly, the CO2 uptake of Mg0.4Cu0.6-MOF-74 under Xe lamp illumination for 24 h was the highest, up to 3.52 mmol·g–1, which was 1.18 and 2.09 times higher than that of Mg- and Cu-MOF-74, respectively. The yield of the photocatalytic reduction of CO2 to CO was 49.44 μmol·gcat–1 over Mg0.4Cu0.6-MOF-74 under visible light for 8 h. Mg2+ and Cu2+ functioned as open alkali metal that could adsorb and activate CO2. The synergistic effect between Mg and Cu metal strengthened MgxCu1–x-MOF-74 photostability for CO2 adsorption and broadened the scope of its photocatalytic application. The “bimetallic” strategy exhibits considerable potential for use in MOF-based semiconductor composites and provides a feasible method for catalyst design with remarkable CO2 adsorption capacity and photocatalytic activity.


■ INTRODUCTION
Metal−organic framework (MOF) materials, featuring highly effective structures (i.e., ultrahigh surface areas, extraordinary porosity, a highly ordered porous structure, and homogeneous active sites), exhibit considerable potential for use in carbon capture 1−3 and sequestration 4−6 (CCS) and heterogeneous catalysis, 7−9 especially in photocatalysis. 10−12 Among the numerous MOFs, Mg-MOF-74 (or CPO-27) exhibits the best CO 2 uptake in low pressures because of its open metal sites. 13,14 Yazaydin et al. analyzed the CO 2 adsorption capacities of M-MOF-74 (M = Ni, Zn, Co, and Mg) with simulation and experimental methods at 1 bar and 298 K and proved that the CO 2 uptake of Mg-MOF-74 was highest, up to 8 mmol·g −1 . 15 Bao et al. proved that the CO 2 uptake of Mg-MOF-74 was considerably higher than that of zeolite 13X, up to 8.61 mmol·g −1 . Mg-MOF-74 exhibits open metal sites. Compared with Na + in zeolite 13X, unsaturated Mg 2+ has a smaller ionic radius and a larger ionic valence, which results in stronger adsorbate−metal interactions. 16 MOFs with excellent CO 2 adsorption capacity have been used as co-catalysts in semiconductor composites for CO 2 photocatalytic reduction. 17, 18 Zhao et al. reported a novel strategy for incorporating Mg-MOF-74 as a co-catalyst to prepare Zn 2 GeO 4 /Mg-MOF-74 and revealed that the CO 2 photocatalytic reduction of the semiconductor composites was considerably higher than that of single semiconductor materials. The productive rate of CO (12.94 μ mol·g cat −1 ) was 7 times higher than that of Zn 2 GeO 4 (1.8 μmol·g cat −1 ). 19 The high concentration of open alkaline metal (Mg 2+ ) of Mg-MOF-74 was beneficial to the CO 2 adsorption and activation, which improved electron transfer from Zn 2 GeO 4 to Mg-MOF-74 and effectively inhibited the excited electron−hole recombination of composites. Wang et al. revealed that Mg-MOF-74 (CPO-27)/TiO 2 exhibited enhanced photocatalytic reduction of CO 2 to CO and CH 4 . Furthermore, Mg-MOF-74 exhibited excellent CO 2 adsorption capacity and open alkaline metal sites, which enhanced the photocatalytic performance of the composites. 20 Furthermore, although the CO 2 uptake of the Cu-based MOF was lower than that of Mg-MOF-74, the semiconductor composite of the Cu-based MOF exhibited excellent CO 2 photocatalytic reduction and physicochemical stability. Li et al. prepared a Cu 3 (BTC) 2 @TiO 2 semiconductor composite, and the yield of photocatalytic reduction of CO 2 to CH 4 was up to 2.64 μmol· g cat −1 ·h −1 . Cu 3 (BTC) 2 functioned as a co-catalyst for Cu 3 (BTC) 2 @TiO 2 that exhibited excellent stability of the morphology and compositions after CO 2 photocatalytic reduction. 21 A strategy using Mg-MOFs as a co-catalyst and with excellent CO 2 adsorption capacity is required to enhance the photocatalytic performance of composite semiconductors. 22 However, the CO 2 uptake of Mg-MOF-74 is poor under ambient conditions. The existing studies of Mg-MOF-74 showed improved CO 2 uptake under humid conditions (i.e., water stability of CO 2 ). 23,24 The CO 2 uptake of Mg-MOF-74 under long-time illumination (i.e., photostability of CO 2 ) is yet to be discussed.
In bimetallic MOFs, two inorganic metal nodes are used to integrate two monometallic MOFs. Bimetallic MOFs considerably outperform their corresponding monometallic MOFs. 25−27 Although bimetallic Mg-MOF-74 is widely used in thermocatalysis 28,29 and adsorption, 30,31 its use in CO 2 photocatalytic reduction is limited. 32 Guo et al. prepared bimetallic NiMg-MOF-74 catalysts by using a solvothermal method and revealed that CO 2 can be photocatalytically reduced into formate over NiMg-MOF-74 under extremely harsh flue gas conditions. Furthermore, the excellent stability of NiMg-MOF-74 in the photocatalytic reaction was demonstrated through five cyclic photocatalytic experiments and X-ray diffraction (XRD) patterns after the photocatalytic reaction. 32 Consequently, based on the good CO 2 uptake of Mg-MOF-74 and the photostability of the Cu-based MOF, a bimetallic strategy was proposed to strengthen Mg-MOF-74 photostability for CO 2 adsorption.
We used the one-pot method to prepare a series of bimetallic Mg x Cu 1−x -MOF-74 (x = 0.4, 0.2, and 0.17). The effects of the preparation conditions (i.e., temperatures, time, and solvents) and Mg/Cu molar ratio on CO 2 adsorption were discussed. The relationship between the structure and CO 2 adsorption under visible light illumination was investigated through inductively coupled plasma atomic emission spectroscopy (ICP-AES), XRD, Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), thermo gravimetric analysis (TGA), and density functional theory (DFT). The photocatalytic performance of the MgCu-MOF was verified. In this study, an executable and competitive method was proposed for design of MOF-based semiconductor composites for excellent CO 2 adsorption capacity and considerable photocatalytic activity.

■ MATERIALS AND EXPERIMENT
Materials. All organic and inorganic chemicals in this study were of commercially available analytical grade and were used without further purification. Magnesium  Characterization. The crystal structure was identified using an X-ray diffractometer (7000S/L, Shimadzu) with Cu Kα radiation. The morphology was observed using a scanning electron microscope (S-4800, Hitachi) with a magnification of 2000−20,000 times and an acceleration voltage of 5 kV. The chemical structure and functional groups were determined using a FT-IR spectroscope (Tensor 27, Bruker); each spectrum was obtained from the acquisition of 32 scans from 4000 to 400 cm −1 with 2 cm −1 . Thermogravimetry and derivative thermogravimetry (Mettler Toledo) were performed using a TGA analyzer. Elemental quantitative analysis was performed using an inductively coupled high-frequency plasma emission spectrometer (715-ES, Agilent). Before the test, the sample was dissolved completely with concentrated nitric acid and subsequently diluted to 250 mL with ultrapure water. The band gap width was measured using an ultraviolet−visible spectrophotometer (UV-2600, Shimadzu). The photoluminescence (PL) spectra were recorded on a fluorescence spectrophotometer (PL, F-4600, Hitachi). The excitation wavelength was set to 370−580 nm. The pore structures were determined by N 2 adsorption−desorption isotherms at 77 K using the Micromeritics ASAP 2460 adsorption apparatus. All samples were as-treated by heating at 473 K for 10 h in a dynamic vacuum before measurement. The surface area and micropore diameter were calculated through DFT. Micropore volume and average micropore diameter were calculated using the Horvath−Kawazoe (H−K) method. Furthermore, Mg 0.5 Cu 0.5 -MOF-74 was synthesized at the three temperatures (398, 408, and 418 K), at three times (12, 24, and 36 h), and in three solvents (A, B, and C). A is DMF-MeOH-EtOH, B is DMF-EtOH-IPA, and C is DMF-MeOH-IPA.
CO 2 Adsorption Measurement. The CO 2 static adsorption−desorption isotherms of activated samples were obtained using the Micromeritics ASAP 2460 adsorption apparatus, which were measured at 298 K and gas pressure up to 760 mm Hg. The temperature control system was achieved using a Dewar bottle with a circulating sleeve connected to a thermostatic bath.
Photostability of Mg x Cu 1−x -MOF-74 for CO 2 Adsorption Measurement. The activated samples were treated using a Xe lamp (CEL-HXL300, China Education Au-Light Co., Ltd., Beijing) at three times (12,24, and 36 h), and the CO 2 uptake was evaluated. The test method was the same.
Photocatalytic Activity of Mg x Cu 1−x -MOF-74 for CO 2 Measurement. A 300 W Xe lamp was used as the visible light. The samples (20 mg) and deionized water (2 mL) were both placed in a closed reactor (CEL-HPR100T) with a sapphire window and temperature control system. Before the reaction, the reactor was blown with pure CO 2 with a flow rate of 20 mL·min −1 for 15 min to ensure elimination of all air. The pressure and temperature of the sealed reaction system were increased to 1 MPa and 150°C, respectively, and stirred in the dark for 30 min to ensure that the catalysts reached the adsorption−desorption equilibrium. After a certain time, the gases produced were analyzed and quantified through BFRL SP-3510 gas chromatography (GC) and quantified with a TCD and FID.
■ RESULTS AND DISCUSSION Metal Composition Measurement of As-Prepared Samples. To confirm the metal composition of samples, ICP-AES analyses were conducted (Table S1). According to the ICP-AES results, Mg and Cu coexist in Mg x Cu 1−x -MOF-74. With the increase in the Cu content, the Mg/Cu molar ratio decreased. The results are consistent with the experimental system and proved that Mg x Cu 1−x -MOF-74 with various Mg/Cu metal ratios can be synthesized. However, the Cu content in the Mg x Cu 1−x -MOF-74 structure was considerably higher than the Mg content. This phenomenon can be attributed to the higher stability of Cu-MOF-74 than that of Mg-MOF-74. Cu is more easily coordinated with the H 4 dhtp to form MOF-74 than Mg, and the synthesis temperature of Cu-MOF-74 (353 K) is lower than that of Mg-MOF-74 (398 K). 33,34 Therefore, Cu-MOF-74 is preferentially formed in the synthesis process of Mg x Cu 1−x -MOF-74, and Mg 2+ replaces part of Cu 2+ into the lattice. Because the Cu−O bond length is longer than that of the Mg−O bond, the coordination of the Mg x Cu 1−x -MOF is distorted and deformed because of the Jahn−Teller effect. 35 The coordination distortion affects the synthesis of Mg x Cu 1−x -MOF-74, which results in a lower Mg content than Cu. The result is consistent with TG analysis mentioned previously.
According to ICP-AES analysis, the Mg/Cu metal ratio was adjusted to Mg 0. 4   Notably, the functional groups of Mg 0.2 Cu 0.8 -MOF-74 were primarily affected by the synthesis temperature and less affected by synthesis time and solvents. The characteristic peaks at 889 and 823 cm −1 of Mg 0.2 Cu 0.8 -MOF-74 nearly disappeared at a synthesis temperature of 145°C, which indicated that the synthesis temperature affected the coordination reaction between metal ions and organic ligands ( Figure S1d). 38 The Cu metal ratio of Mg x Cu 1−x -MOF-74 exceeded 50%, and the characteristic peaks of 1589 and 1191 cm −1 in the spectrum of Mg-MOF-74 transferred to 1552 and 1191 cm −1 , respectively (Figure 1b). This phenomenon is attributed to the conjugated effect of Mg x Cu 1−x -MOF-74. The CO group conjugates with the CC bond, and delocalization of the π-electrons occurred between two unsaturated bands. The double bond characteristics of CO were reduced, which resulted in the shift of the absorption frequency toward a lower wave number. 36 Therefore, strongly coordinated olefin molecules existed in the metal centers of Mg x Cu 1−x -MOF-74.
The morphology and structures of samples at three synthesis temperatures (398, 408, and 418 K) were observed through SEM ( Figure S2a). The morphology structure of Mg x Cu 1−x -MOF-74 was affected by synthesis temperature. Furthermore, Mg 0.2 Cu 0.8 -MOF-74 at 398 K exhibited a spherical crystal structure formed by the reaggregation of needle-like crystals in diameters less than approximately 1 μm (Figure S2a,b). 36 The morphology of Mg 0.2 Cu 0.8 -MOF-74 at 418 K exhibited flake crystal branches ( Figure S2d). Mg 0.2 Cu 0.8 -MOF-74 at 408 K formed flake crystal branches ( Figure S2c), which are slightly larger than the needle crystal branches of Mg 0.2 Cu 0.8 -MOF-74 at 398 K. The temperature can promote crystal growth; however, excessively high temperature leads to grain agglomeration. 43 With the increase in temperature, the dispersion degree of the attachment energy of each modified crystal surface increased, which resulted in various growth rates between the crystal surfaces. 44,45 The morphology of Mg 0.2 Cu 0.8 -MOF-74 at 418°C was nonspherical. The morphology structure of Mg x Cu 1−x -MOF-74 was also affected by the Mg/Cu molar ratio ( Figure 2). Mg-MOF-74 exhibited a chrysanthemum-like morphology formed by polyhedral prism crystal branches (Figure 2a,b). The spherical structures of Cu-MOF-74 consisted of needle-like crystal branches, and the average crystal is less than 5 μm (from Figure 2c−f). With the increase in the Cu content, the petal structure of Mg x Cu 1−x -MOF-74 changed from prismatic petals to needle-like petals. Therefore, the average particle size of Mg 0.17 Cu 0.83 -MOF-74 with the lowest Mg/Cu molar ratio (n Mg /n Cu = 0.20) was close to 5 μm. Mg x Cu 1−x -MOF-74 with a hydrangea-like morphology exhibited a structure denser than that of the corresponding monometallic MOF-74, which results in its specific surface area becoming larger than that of Cu-MOF-74 and its thermal stability being superior to that of Mg-MOF-74.
Influence of the Mg/Cu Molar Ratio on Thermal Stability. Two main mass loss stages exist in the TG curves of Mg x Cu 1−x -MOF-74 in a N 2 atmosphere, which revealed the weight loss temperature of each sample (Figure 3a). The first mass loss stage started with heating and lasts until the start of the second stage at various temperatures. For Cu-MOF-74, the starting and finishing temperatures reduced to 308 and 475°C, respectively. For Mg-MOF-74, the starting and finishing temperatures reduced to 267 and 609°C, respectively. The mass loss temperatures of Mg x Cu 1−x -MOF-74 in the second stage were between those of Mg-and Cu-MOF-74. In the first stage, the mass loss was attributed to the removal of adsorbed water, gas molecules, and residual solvents, such as methanol, ethanol, or DMF. 27,46 The weightlessness in the second stage corresponds to the collapse of the framework structure, which disintegrated organic ligands and formed metal oxides. 35,47 The TG curve of Mg-MOF-74 reveals that the thermal stability of Mg-MOF-74 was not satisfied. However, the thermal stability of Mg x Cu 1−x -MOF-74 was enhanced considerably. The considerable difference in the thermostability of Mg x Cu 1−x -MOF-74 could be attributed to the synergy between Mg and Cu. 48 To avoid the structural collapse of Mg x Cu 1−x -MOF-74, its thermal treatments were strictly performed at less than 200°C .  (Figure 3b,c). Mg x Cu 1−x -MOF-74 exhibited a typical type I isotherm and type H 3 hysteresis loops, which indicated that both microporous and mesoporous structures were the same. The micropores of Mg x Cu 1−x -MOF-74 were predominantly between 0.84 and 1.70 nm, which is consistent with the previous report ( Figure  3c). 24,36 The surface area of Mg 0.4 Cu 0.6 -MOF-74 calculated according to the DFT method was 1561.96 m 2 ·g −1 , which was   inhibit the recombination of electron−holes and enhance photocatalytic activity because the lower intensity of PL indicates higher separation efficiency of electron−hole pairs, that is, the superior photocatalytic activity. 12,49,50 Influence of Synthesis Conditions on CO 2 Adsorption Capacity. To confirm the effect of synthesis conditions on the CO 2 uptake of Mg x Cu 1−x -MOF-74, the CO 2 uptake of S at 298 K and 1 bar under three synthesis conditions (time, temperatures, and solvents) was tested (Table S2). The CO 2 uptake of S-398-24-A (i.e., 398 is temperature, 24 is time, and A is the solvent) was the highest, up to 3.24 mmol·g −1 . Thus, the best synthesis condition was 398 K, 24 h, and in A.
To verify that Mg x Cu 1−x -MOF-74 was successfully synthesized, we obtained the mechanical combination of two corresponding monometallic MOF-74 in a molar ratio of 2:1 and tested CO 2 uptake (Figure 4a). The CO 2 uptake of Mg 0.4 Cu 0.6 -MOF-74 outperformed that of the mechanically mixed MOF-74(M), which proved that Mg x Cu 1−x -MOF-74 was synthesized successfully. The synergistic effects originating from two adjacent Mg and Cu metals considerably improved the CO 2 adsorption capacity. 28 Evaluation of the Photostability of Mg x Cu 1−x -MOF-74 for CO 2 Adsorption. To investigate the effect of the Mg/Cu molar ratio on the photostability of Mg x Cu 1−x -MOF-74 for CO 2 adsorption, the adsorption−desorption isotherms at 298 K and 1 bar were compared to those obtained by the static volumetric CO 2 adsorption method (Figure 4b). The CO 2 uptake of Mg x Cu 1−x -MOF-74 was higher than that of Cu-MOF-74 but lower than that of Mg-MOF-74. Specifically, the CO 2 uptake of Mg 0.4 Cu 0.6 -MOF-74 was the highest, up to 4.58 mmol·g −1 , and that of Mg 0.17 Cu 0.83 -MOF-74 was the lowest, up to 1.52 mmol·g −1 . This phenomenon could be attributed to the surface area and the pore volume of Mg 0.4 Cu 0.6 -MOF-74, which is consistent with the pore structure analysis. Moreover, the CO 2 adsorption capacity of Mg x Cu 1−x -MOF-74 was influenced by the Mg content. With the decrease in the Mg content, the CO 2 uptake of Mg x Cu 1−x -MOF decreased, which was attributed to the CO 2 adsorption heat of Mg ions being larger than that of Cu ions in a low-pressure region. 35 To investigate the effect of the Mg/Cu molar ratio on the chemical stability and photostability of Mg x Cu 1−x -MOF-74, the CO 2 uptake of the Mg x Cu 1−x -MOF-74 treated using a Xe lamp was measured by using the static volumetric CO 2 adsorption method (Figure 4c). The CO 2 uptake of Mg x Cu 1−x -MOF-74 decreased with the increase in the illumination time, and the CO 2 uptake of those treated using an X-lamp for more than 24 h was the lowest and stable. Furthermore, the CO 2 uptake values of Mg-, Mg 0.4 Cu 0.6 -, Mg 0.2 Cu 0.8 -Mg 0.17 Cu 0.83 -, and Cu-MOF-74 treated by using the X-lamp for 24 h were 2.98, 3.52, 2, 1.69, and 1.68 mmol· g −1 , respectively. To analyze the effect of the Mg/Cu molar ratio on the photostability of Mg x Cu 1−x -MOF-74 for CO 2 adsorption, the decrease rate in the CO 2 uptake of those treated by the X-lamp for 12, 24, 36, and 48 h was quantitatively discussed (Figure 4c). The CO 2 uptake decrease rate (100%) of Mg x Cu 1−x -MOF-74 under long-term illumination is defined as follows Here, Y indicates the CO 2 decrease rate; C n indicates the CO 2 uptake of Mg  (Figure 4d). The CO 2 uptake of Mg-MOF-74 was the highest and reached 7.18 mmol·g −1 . However, its photostability was the weakest (Figure 4c,d). The photostability of Cu-MOF-74 for CO 2 uptake was the best. The CO 2 uptake of Cu-MOF-74 was the lowest, only up to 2.09 mmol·g −1 . Mg x Cu 1−x -MOF-74 photostability of CO 2 adsorption was superior to that of two corresponding monometallic MOF-74 ( Figure 4c). Specifically, the photostability of Mg 0.4 Cu 0.6 -MOF-74 treated using the X-lamp for 24 h was the best, and its CO 2 uptake and decrease rate were 3.52 mmol·g −1 and 23.13%, respectively. A strong relationship exists between photostability and metal composition of Mg x Cu 1−x -MOF-74. Because of the synergistic effect between Cu and Mg, the photostability of Mg x Cu 1−x -MOF-74 was enhanced considerably. 32 Furthermore, the stability of Mg-MOF-74 was highly affected by the M−O bond on the top of the metal−organic frame (M represents the metal species) and the metal center atoms (Mg). 31 The distortion and deformation of the coordination environment of Cu 2+ (i.e., Jahn−Teller effect) lead to the contraction of M−O bonds and improved photostability of Mg x Cu 1−x -MOF-74. 35 Evaluation of the CO 2 Photocatalytic Activity of Mg 0.4 Cu 0.6 -MOF-74. To confirm the potential application of Mg 0.4 Cu 0.6 -MOF-74 as a co-catalyst for MOF-based semiconductor composites, its CO 2 photoreduction activity was investigated without any sacrificial agent ( Table 2). The main gaseous product of CO 2 photoreduction in an aqueous system was CO. The yield of CO was 49.