1 Introduction

In the twenty-first century, there is a growing concern about the excessive emission of carbon dioxide into the atmosphere [1,2,3]. This is related to the rapidly increasing number of inhabitants, human activities, and consumption of fossil fuels. Global warming and climate change are two serious problems that result from atmospheric CO2 emissions [4, 5]. One of the important disadvantages of CO2 for the global environment is the adsorption of CO2 by oceans and seas, which causes a rise in seawater acidity [6, 7]. On the other hand, CO2 is an essential requirement for plants that use it in photosynthesis to form organic molecules and oxygen [8, 9]. In the past years, numerous attention was drawn to the transformation of CO2 into valuable chemical products such as carbon monoxide (CO) [10], formic acid [11, 12], methane [13, 14], methanol [15,16,17], and ethanol [18, 19]. In industry, CO2 plays a vital role in producing many compounds, including drugs, fragrances, and fuels [20,21,22,23,24]. In particular, CO is used as fuel for heat, light, and manufacturing of organic chemicals [25, 26]. As a result, considerable efforts have been dedicated to physically absorbing CO2 for storage and chemically converting CO2 to other chemicals [27,28,29]. Among the utilized methods, photocatalytic reduction by primarily using sunlight energy and CO2 insertion into the epoxide are ideal approaches for transforming CO2 into fine chemicals [30,31,32]. One of the promising candidates for CO2 transformation is metal–organic frameworks (MOFs) [33,34,35]. MOFs are a class of crystalline porous coordination polymers built-up of metal cluster nodes interconnected with multi-dentate organic linkers [36]. Benefiting from their outstanding chemical and physical properties, such as high surface area and pore volume, and tunable structure, MOFs have emerged as a mediate for various applications, including drug delivery [37], catalyst [38], sensing [39, 40], separation [41], adsorption [42], etc. Among the MOF-based porous materials, UiO-based MOFs are a class of materials with Zr cluster and phenyl-dicarboxylate ligands [43]. Zr-UiO-67-Bipydc (ZBU) can be utilized as a platform for the post-synthetic method (PSM) to incorporate secondary metals such as Co, Mn, Ru, Rh, etc. [44]. PSM of ZBU with metals provides a versatile tool for improving catalytic conversion, like CO2 fixation [45, 46]. Also, there are a few reports on ZBU@metal (Co, Re, Ru, Rh, Ni, Mn, Pt, and Cu) with high conversion in photocatalytic CO2 reduction.[47,48,49,50,51,52,53,54,55]. In this report, we synthesized a Co-modified ZBU (ZBU-Co) by PSM method under a solvothermal condition (Fig. 1). The rich nature of catalytic activation modes in the afforded MOF prompted us to employ it in CO2 transformations. First, we envisioned that multiple Lewis acid/Lewis base sites in the as-synthesized ZBU-Co can conduct CO2 insertion into the epoxides. Fortunately, ZBU-Co showed promising results in the production of cyclic carbonate adducts. Next, we hypothesized that Co moiety in ZBU-Co can also play the role of charge transfer medium in photocatalytic reaction. To our delight, further investigation revealed that ZBU-Co offers a practical photocatalytic approach for the reduction of CO2 to CO. Therefore, ZBU-Co represents a potential candidate for CO2 transformations featuring bi-functional catalytic manner, which is unprecedented in MOFs catalysis CO2 transformations [44]. According to our results, in both of the CO2 transformations, ZBU-Co exhibited superior catalytic performance compared to the pristine ZBU and homogeneous analogs. Spectral and instrumental analysis including XRD, TGA, SEM, BET, ICP, XPS, fluorescence, and 1H NMR were employed to get insight into the catalytic activity of the MOF.

Fig. 1
figure 1

Synthetic procedure of ZBU-Co via PSM and schematic illustration of ZBU-Co catalytic functions

Full size image

2 Results and Discussion

2.1 Characterization of ZBU-Co

First, powder X-ray diffraction (PXRD) pattern of ZBU and ZBU-Co demonstrated the isoreticular and crystalline nature of the MOFs (Fig. 2a). Additionally, the crystallinity was well retained after post-metallation. To the finding the permanent porosity and calculate the surface area of the MOFs, N2 adsorption–desorption isotherms at 77 K were performed. As shown in Fig. 2b, the isotherms of both MOFs exhibited type I, which were identified as microporous materials. At relative low pressures (P/P0 <0.1) and high relative pressure (P/P0 > 0.99), the N2 adsorption amount of ZBU was 455 and 800, respectively. Also, 234 and 390 cm3 g−1 were obtained for N2 adsorption of ZBU-Co ((P/P0 <0.1) and (P/P0 > 0.99), respectively). Moreover, the Brunauer−Emmett−Teller (BET) and Langmuir surface areas were calculated 1098 and 1800 cm3 g−1 for ZBU and 603, 972 cm3 g−1 for ZBU-Co, respectively. Thermal stability evaluation of MOFs was investigated by thermal gravimetric analysis (TGA) in air atmosphere at the range of 30–800 °C (Fig. 2c). TGA curves of ZBU and ZBU-Co exhibited 20% decomposition in the range of 30–500 °C for ZBU and 30–400 °C for ZBU-Co, which can be assigned to the solvent and residual molecules. The second weight loss occurred in the range of 500–600 °C for ZBU and 400–500 °C for ZBU-Co, which can be attributed to the complete decomposition of the frameworks. These data illustrate the remarkable thermal stability of the MOFs. SEM images of the MOFs exhibited cubic nanoparticles before and after metalation (Fig. 2d). The elemental analysis of ICP-OES (inductively coupled plasma optical emission spectroscopy) and XPS (X-ray photoelectron spectroscopy) has proven the successful loading of Co into the ZBU. The atomic ratio of cobalt in ZBU-Co exhibited that for each Zr atom in the cluster, there is one Co atom, so the atomic ratio of Zr/Co was obtained 1:0.3 wt%. Furthermore, XPS analysis demonstrated the presence of elements in the composite. As illustrated in Fig. 3, the XPS spectrum of the ZBU-Co exhibited the presence of Co, Zr, C, Cl, O, and N. The main peaks at 795 eV and 780 eV are shown attributed to Co2p, corresponding to 2P1/2, and 2P3/2 of Co2+, respectively. [56, 57] The production rate of CO was detected using a gas chromatograph (GC) by injection of 5 mL gas into the reactor at 1 h interval.

Fig. 2
figure 2

a PXRD pattern, b adsorption and desorption isotherms of N2 at 77 K, c TGA profiles at air condition, and d SEM images of ZBU and ZBU-Co

Full size image
Fig. 3
figure 3

High-resolution XPS spectrum of ZBU-Co

Full size image

2.2 Catalytic Studies

2.2.1 CO2 Insertion Into Epoxides

As depicted in Fig. 1, ZBU-Co takes advantage of various activation modes to improve CO2 capture and fixation.

Zr-clusters and incorporated Co open-metal sites are Lewis acid centers capable of activation of epoxides. Moreover, N-donors sites in bi-pyridine linkers are considered as Lewis base sites for CO2 activation [58]. To evaluate the dual activation mode of ZBU-Co (Zr and Co centers) in the conversion of CO2 to cyclic carbonates, epichlorohydrin (ECH) was selected as a model substrate. The conversion efficiency of the catalysts was analyzed by 1H NMR (See Supporting Information File) First, the effect of the molar ratio of ZBU-Co and TBAB (co-catalyst) on the conversion of ECH to the desired adduct 1,3-dioxolane-2-one was investigated (Table 1). Accordingly, as the molar ratio of TBAB increases from 1 to 3 mol%, the catalytic conversion increases significantly from 10 to 99% (Table 1, entries 1–3). Then, by lowering the amount of catalyst from 0.5 to 0.1 mol%, the catalytic conversion decreased from 99 to 43% (Table 1, entries 3–5). No catalytic conversion was observed upon removal of the co-catalyst from the reaction mixture (Table 1, entry 6). Also, removal of ZBU-Co from the reaction drastically reduced the conversion from 99 to 38% (Table 1, entry 7). Finally, trying the reaction with less than 2 mmol ECH showed a decrease in efficiency (Table 1, entry 8). Consequently, performing the reaction under CO2 (1 bar), tetrabutylammonium bromide (TBAB) (3 mol%), ECH (2 mmol), and 0.5 mol% ZBU-Co at room temperature for 6 h was established as an optimized condition.

Table 1 Investigation the optimal conditions for the catalytic synthesis of 1,3-dioxolane-2-one
Full size table

With optimal conditions in hand, we compared the catalytic performance of ZBU-Co with different homogeneous and heterogeneous catalysts under the considered condition. Based on our experiments, a dramatic decrease in conversion was observed as a result of conducting the reaction by various homogeneous catalysts, including ZrCl4 and Co(OAc)2 as Lewis acid, and 2,2′-Bipy-5,5′-dicarboxylic acid as Lewis base (Table 2, entries 3–5). Additionally, the pristine ZBU exhibited a catalytic conversion of 74%, while ZBU-Co showed superior activity toward CO2 insertion with complete conversion (Table 2, entries 1–2).

Table 2 Evaluation of homogeneous and heterogeneous catalysts in cycloaddition of CO2 with ECH
Full size table

Next, to investigate the efficiency of the catalytic system in diversity-oriented epoxides, a range of epoxides were tested in the CO2 insertion reaction under the optimized conditions (Table 3). As demonstrated, bulkier epoxides exhibited lower efficiency compared to ECH (Table 3, Figure S20-S25). This can be attributed to the steric constraints that affect the diffusion rate of the substrates in the MOF pores [59]. Therefore, it can be concluded that the catalytic reaction occurs mostly in the MOF pores. To enhance the catalytic efficiency of the bulky epoxides, the temperature was increased from 25 to 40 °C. Successfully, all of the epoxides exhibited complete conversion (Table 3).

Table 3 Cycloaddition reaction of CO2 with various epoxides
Full size table

The versatility of ZBU-Co in this reaction was further proved by leaching test. Accordingly, no catalytic activity was observed by removing the catalyst after 2 h. Thus, it can be rationalized that no leaching occurred at the active catalyst sites (Fig. 4a). In addition, to investigate the chemical stability of ZBU-Co, a recyclability test was performed (Fig. 4b). As seen in Fig. 4c, the remaining PXRD patterns of the recovered catalyst showed remarkable stability after five catalytic cycles (Figure S26-30).

Fig. 4
figure 4

a Leaching test of ZBU-Co, b recycle experiments of ZBU-Co for cycloaddition of CO2 with ECH under solvent free, 6 h, 1 bar and room temperature condition. Conversion in each cycles: run 1, 100%; run 2, 99%; run 3, 98%; run 4, 97%; run 5, 95%; and c PXRD patterns of ZBU-Co after each catalytic cycles

Full size image

2.2.2 Photocatalytic Conversion of CO2 to CO

To evaluate the efficiency of photocatalytic CO2 reduction in the MOFs, CO2 adsorption experiments were performed on ZBU and ZBU-Co at 298 K. During the CO2 reduction process, CO2 molecules were absorbed by the catalytic centers (Co). The CO2 reduction efficiency can be attributed to the amount of CO2 adsorbed. Therefore, MOFs with higher CO2 capacity would exhibit higher catalytic conversion. As shown in Fig. 5, ZBU-Co had a CO2 adsorption capacity of 36.7 cm3 g−1, which was higher than the CO2 adsorption capacity of ZBU (22.2 cm3 g−1) (Fig. 5).

Fig. 5
figure 5

CO2 adsorption capability of ZBU and ZBU-Co at 298 K

Full size image

Next, the ability of light absorption was evaluated using UV–Vis diffuse reflectance spectroscopy (DRS). As shown in Fig. 6, both complexes showed photo-absorption from UV light to visible light. The Co-modified catalyst had higher light absorption intensity than the pristine MOF. Generally, enhanced light absorption is correlated with better catalytic activity [60].

Fig. 6
figure 6

UV–Vis diffuse reflectance spectra of Zr-Bipy-UiO-67-Co(OAc)2 and Zr-Bipy-UiO-67

Full size image

Then, photocatalytic CO evolution experiments were performed under visible light illumination. When the photocatalytic CO2 reduction was run with ZBU-Co and Ru(Bipy)3Cl2 (photosensitizer) for 4 h, 15.09 μmol CO was produced with a formation rate of 3452 μmol h−1 g−1 (Fig. 7a). In contrast, only a trace amount of CO was produced when ZBU was used, implying the versatility of the modified MOF. Further details regarding optimized conditions can be found in Table S1.

Fig. 7
figure 7

a Monitoring CO evolution, b production yields of CO over ZBU-Co catalyst in four repeated cycles

Full size image

The chemical stability of the MOF was tested by a recyclability experiment (Fig. 7b). In good agreement with the PXRD patterns (Figure S1), the conversion ability of the ZBU-Co catalyst remained unchanged after four cycles. To gain insight into the electron transfer process of Co sites during CO2 reduction, a photoluminescence (PL) experiment was carried out. Based on our findings, the emission intensity of the photosensitizer/ZBU-Co catalyst was significantly decreased compared to the photosensitizer/ZBU (Fig. 8). This can be ascribed to the fast electron transfer from the photosensitizer to the ZBU-Co, which further corroborates the combination of the photosensitizer/ZBU-Co in a photocatalytic system (Tables 2, 3).

Fig. 8
figure 8

PL spectra of photosensitizer before and after addition of ZBU and ZBU-Co

Full size image

Furthermore, the electron transfer in CO2 reduction was further investigated by electrochemical testing (Fig. 9). As seen in Fig. 9, − 0.89 V (vs. Ag/AgCl) was recorded as the initial point for the increase in current density of ZBU-Co under CO2 atmosphere compared to the current density of ZBU-Co under N2 atmosphere. Therefore, − 0.89 V was recognized as the initial potential in the CO2 reduction. Consequently, as − 1.31 V (vs. Ag/AgCl) has been recorded for singlet state of Es([Ru(bpy)3]2+*/[Ru(bpy)3]3+ [61], it was found that the CO2 reduction is thermodynamically favorable (ΔG = − 1.31 V–(− 0.89 V) =  − 0.42 V, i.e. < 0) [62].

Fig. 9
figure 9

CV of Zr-Bipy-UiO-67–Co(OAc)2 under CO2 and N2

Full size image

Moreover, the XPS analysis of ZBU-Co after photo-reduction was not significantly altered, demonstrating the stability of the catalytic system (Figure S3). The mechanism of photocatalytic reduction is depicted in Fig. 10. In the first step, the photosensitizer is excited to produce [Ru(bpy)3]2+*. Then, electrons are transferred from the excited species [Ru(bpy)3]2+* to ZBU-Co. As a result, ZBU-Co is reduced to [ZBU-Co]¯ and ([Ru(bpy)3]3+) is produced. Next, CO2 molecules are reduced to CO by [ZBU-Co]¯. Finally, after the reaction of TEOA (triethanolamine) with ([Ru(bpy)3]3+ to form TEOA+/[Ru(bpy)3]3 + , photocatalytic CO2 reduction is completed and H2 released as a side product [51] (Figure S3). Despite the feasibility of other side products such as formaldehyde, methanol, and hydrocarbons, the high selectivity of ZBU-Co towards CO production is remarkable. In MOFs, pores are the sites where catalysis and product formation take place. Therefore, product selectivity is controlled by pore size. In the case of ZBU-Co, it is believed that the pore size of ZBU-Co is ideally suited for CO synthesis with high selectivity [44].

Fig. 10
figure 10

The photocatalytic mechanism of the ZBU-Co for the conversion of CO2 to CO

Full size image

3 Conclusion

In summary, it is the first record of MOFs-catalyzed CO2 transformation reactions with bifunctional catalysts. ZBU-Co can catalyze CO2 insertion into epoxides under mild, green, solvent-free, and temperature-free conditions, achieving complete conversion. The CO2 uptake capacity of the MOF was recorded at 36.7 cm3 g−1, which is higher than the parent ZBU MOF (22.2 cm3 g−1). Additionally, ZBU-Co was used for the photocatalytic reduction of CO2 to CO under Xe lamp irradiation. The superior photocatalytic performance of ZBU-Co compared to the pristine ZBU is due to its higher charge transfer ability and CO2 adsorption capacity. Moreover, the chemical stability and catalytic performance of the ZBU-Co remained unchanged after five cycles. Taken together, our findings open a new horizon for the rational design of efficient MOF catalysts, especially in the bi-functional mode for CO2 transformation reactions.