Juan Baia,
Ziwei Song*a,
Lijuan Liua,
Xu Zhua,
Faming Gaoa and
Raghunath V. Chaudhari*b
aHebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China
bCenter for Environmentally Beneficial Catalysis, Department of Chemical & Petroleum Engineering, University of Kansas, 1530 W15th Street, Lawrence, Kansas 66045, USA
First published on 15th September 2022
Metal–organic frameworks (MOF) have been studied extensively for the adsorption and catalytic conversion of CO2. However, previous studies mainly focused on the adsorption capabilities of partially or totally Ce substituted UiO-66, there are few studies focusing on transformation of the structure and catalytic activity of these materials. In this work, a series of Zr/Ce-based MOFs with UiO-66 architecture catalysts were prepared for the conversion of CO2 into value-added dimethyl carbonate (DMC). Owing to the different addition order of the two metals, significantly varied shapes and sizes were observed. Accordingly, the catalytic activity is greatly varied by adding a second metal. The different catalytic activities may arise from the different acid–base properties after Ce doping as well as the morphology and shape changes. Besides, the formation of terminal methoxy (t-OCH3) was found to be the rate limiting step. Finally, the reaction mechanism of CO2 transformation in the presence of a dehydrating agent was proposed.
Scheme 1 Reactions involved in the reaction of CO2 and MeOH with 2-cyanopyridine as dehydrating agent. |
The direct catalytic conversion of CO2 to DMC is simple, clean and with high atom efficiency. But because of the extreme thermodynamic limitation, achieving high DMC yield is still challenging.15 Besides, the H2O formed during the reaction leads to the easily deactivation of the catalysts. To overcome these challenges, various catalytic systems have been developed, such as ionic liquids,16,17 alkali carbonates,18,19 metal oxides,20,21 supported metal oxides,22 heteropoly acids,23 metal–organic frameworks,24,25 etc. Among these catalysts, metal–organic frameworks (MOFs) show a promising prospect.
MOFs are porous materials formed by metal ions or metal clusters with organic bridging linkers, showing a diverse range of potential applications, such as used for gas adsorption, catalysis, ionic conduction, etc.26–28 Previous studies showed that MOFs offer excellent capacities of CO2 uptake due to its ultra-high surface areas.29 MOFs can also be used as catalyst for CO2 conversion.27 The extreme stable UiO-66 was reported to be active for the conversion of CO2 with 100% selectivity of DMC without dehydrating agent, but with a low DMC yield of around 0.015%.30 Studies demonstrated that the catalytic activity of MOFs could be tuned by introducing/substituting the functional group in an organic linker.31,32 Xuan K. and co-workers modified UiO-66 (Zr) with trifluoroacetic acid (TFA), a higher DMC yield (∼0.084%) was observed, which could be attributed to the increased number of active sites and the enlargement of porosity originated from TFA modulation.30 In addition, the exchange of the metal ions in UiO-66 by Hf, Ti and Ce could also be used to adjust the catalytic activity of MOFs. Vasudeva and co-workers found that after changing Zr to Hf in the nods of UiO-66, the TOF of the material for solketal synthesis increased from 153 h−1 to 13886 h−1, increased by 90 times.33
Owing to its high oxygen mobility, easy and reversible transition between Ce4+ and Ce3+, introducing Ce into MOF materials has also been implemented.34,35 Nouar et al. partially substituted Zr in UiO-66 by Ce, leading to easy catalytic decomposition of the methanol, which can be attributed to structural defects and redox activity generated from the introduction of Ce.35 Besides, Stawowy's research found that missing linker molecules in UiO-66(Ce) results in enhanced CO2 adsorption.32 All these properties obtained through Ce doping can benefit the direct conversion of CO2. However, to the best of our knowledge, previous studies mainly focused on the adsorption capabilities of partially or totally Ce substituted UiO-66, there are few studies focusing on transformation of the structure and catalytic activity of these materials. In this study, a series of UiO-66 MOFs were prepared and evaluated for the CO2 conversion to address the transformation of morphologies, structures, texture properties, and catalytic activity that caused by Ce substitution. Furthermore, the cause of the activity difference and the reaction mechanism of CO2 conversion on MOF were proposed based on experimental and characterization results.
For Ce doped Zr-MOF and Zr doped Ce-MOF catalysts, the preparation procedure is as follows: 2.79 g (NH4)2Ce(NO3)6/1.16 g ZrCl4 was dissolved in 50 ml DMF and stirred for 30 min, denoted as A solution. 1.16 g ZrCl4/2.79 g (NH4)2Ce(NO3)6 and 0.84 g BDC were dissolved in 125 ml DMF and stirred for 30 min, denoted as B solution. Then A solution was dropwise added to B solution at room temperature under vigorous stirring. After this, the mixture was vigorously stirred for 30 min at room temperature. Then the same procedure as that of Zr-MOF was applied.
The textural properties of the synthesized catalyst were determined by N2-adsorption–desorption isotherms using Autosorb iQ-Chemisorption & Physisorption Gas Sorption Analyzer. The specific surface area was evaluated by the Brunauer–Emmett–Teller (BET) method and pore size distribution of the samples was determined by the Horvath–Kawazoe (HK) method.
XPS spectra were recorded with X-ray photo electron spectrometer Escalab 250Xi apparatus with an Al-Kα X-ray source (hv = 1486.6 eV) and a monochromator. The XPS measurement was carried out in the electron binding energy ranges corresponding to carbon 1s, oxygen 1s, zirconium 3d and cerium 3d core excitations. Spectra were deconvoluted by using XPSPEAK 4.0 software. The peak was fittedusing Lorentzian–Gaussian function with an asymmetric parameter of 0.
Elemental analysis was performed over an Agilent-7700 inductively coupled plasma mass spectrometry (ICP-MS). 50 mg sample was dissolved and diluted for the analysis.
Fourier transform infrared (FT-IR) spectra for the catalyst samples were recorded in the range of 500–4000 cm−1 on a FT-IR Nicolet iS10 spectrometer. The acidity of the samples was also detected using FT-IR with pyridine as probe molecule. The catalyst samples were immersed in pyridine stream for 24 hours and then dried under vacuum at 60 °C for 8 hours. After cooling to room temperature, the FT-IR spectra of the samples were collected. Diffuse-reflectance UV-Vis absorption spectra were acquired using a UV-2550 spectrometer in the range of 200–600 nm at room temperature at a scanning wavelength interval of 0.5 nm.
Acid–base properties of the samples were studied by temperature-programmed desorption of ammonia (NH3-TPD) and temperature-programmed desorption of carbon dioxide (CO2-TPD) on a BELCAT-B instrument. Prior to adsorption, the sample was pretreated by argon (40 ml min−1) at 150 °C for 1 h. After cooling to room temperature, the argon was switched to NH3 or CO2 (40 ml min−1) for 1 h to perform the adsorptive process. The physisorbed NH3 or CO2 was then flushed by argon at room temperature for 1 h. The desorption process was conducted in argon (40 ml min−1) from room temperature to 450 °C with a heating rate of 10 °C min−1.
All the liquid samples were analyzed using an Agilent gas chromatography (GC-7890A) equipped with a WAX capillary column and a flame ionization detector (FID). MeOH conversion and the turn over frequency (TOF) as follows:
Fig. 1 (a) XRD patterns of Zr/Ce-based MOF materials, (b) N2-adsorption–desorption isotherm, TEM images of (c) Ce-MOF; (d) Zr–Ce-MOF; (e) Ce–Zr-MOF; (f) Zr-MOF. |
Interestingly, with doping of Ce in the material, obvious structural distortion (blue shift in the XRD patterns) was detected, as shown in Fig. 1(a). It can be clearly seen that, with the increasing content of Ce in the materials, the distortion is more obvious. For example, the reflection angle of the (111) facet for Zr-MOF is at 7.62°, while for other three materials, the angle has shifted to 7.5° for Zr–Ce-MOF and Ce–Zr-MOF, and to 7.24° for Ce-MOF. Similar shift has also been observed for other planes, corroborating that the second metal have been successfully incorporated into the framework lattice. Specifically, lattice compression may due to the different radii of Ce4+ (0.97 Å) and Zr4+ (0.84 Å).32,39 Besides, different valence state of Ce (Ce3+) can also lead to deformation of the crystal structure. Furthermore, apparent morphology change has been observed by TEM after incorporation of Ce or Zr into the MOF materials, as shown in Fig. 1(c)–(f). Evolution from tetrahedron to spherical was detected after doping of the second metal. In addition, much smaller particle size for Zr–Ce-MOF than the other three materials was observed. The decreasing particle size indicates that the addition order of the metals plays a significantly role in the formation of the solid composite.
N2 adsorption–desorption analysis was carried out to characterize the textural properties of the samples, as shown in Fig. 1(b). The adsorption isotherms of all these catalysts exhibited type I behavior, suggesting microporous structure for all material.31,40,41 However, for the material Zr–Ce-MOF, H3 type of hysteresis loop was detected, indicating possible existing of mesopores in this material.40,42 The corresponding textural properties (BET surface area, pore volume) are listed in Table 1. It is noted that Zr-MOF showed the highest surface area (1159.00 m2 g−1) than the other three materials. However, with the introduction of Ce, the surface area began to decrease, from 1159.00 m2 g−1 to 960.40 m2 g−1 (Ce–Zr-MOF) to 757.91 m2 g−1 (Zr–Ce-MOF). Interestingly, for all the three materials, the pore size is in the same range of 0.5–1.0 nm [as shown in Fig. 1(b)], confirming again that the materials are microporous material.43 Nevertheless, there is no obvious trend for the amount of micropores. For the Ce-MOF, extremely small surface area was detected (as shown in Table 1). Ar atmosphere isothermal adsorption and desorption analysis was also performed, the results of which is shown in Table S1 (ESI†). Similar small surface area and small pore size were observed, which may result from the non-porous structure of this material, or smaller pore size of this material [smaller than the kinetic diameter of N2 (0.364 nm), N2 cannot enter into the pores, only adsorption on the surface occurred] or that the pore structure was collapsed during the N2-adsorption–desorption process. Furthermore, no obvious pore structure was observed on the surface from the SEM images (Fig. S4†), indicating super-microporous or non-porous structure of Ce-MOF. Additionally, the Ce-MOF have been prepared for three times, with nearly the same textural properties, which are also included in Table S1.†
Overall, the above results confirmed that both the single metal MOFs and the doped MOFs are successfully synthesized, all of which are crystalline microporous materials with high surface area (above 758 m2 g−1). Besides, when trying to incorporate Zr into the Ce-MOF frameworks, a tetragonal particles shape (more like Zr-MOF) was obtained. Similar phenomenon was observed for the Ce–Zr-MOF preparation, which possessed the shape of the Ce-MOF.
Fig. 2 (a) Catalytic evaluation and (b) product distribution of Zr/Ce-based MOF materials. Reaction conditions: 0.5 mol MeOH, 0.25 mol 2-cyanopyridine, T = 150 °C, P = 2.6 MPa, t = 6 h. |
Samples | Conc. (wt%) of elementsa | Conc. (wt%) of elementsb | Conc.a of Ce3+ (wt%) | Conc.a of Ov (wt%) | ||||
---|---|---|---|---|---|---|---|---|
Ce | Zr | C | O | Ce | Zr | |||
a Concentration of Ce3+ and OV is estimated from XPS data.b Determined by ICP-MS. | ||||||||
Ce-MOF | 34.02 | 0 | 41.72 | 24.26 | 20.97 | 0 | 62.87 | 18.67 |
Zr–Ce-MOF | 3.83 | 17.23 | 51.31 | 27.63 | 1.16 | 23.85 | 52.37 | 28.92 |
Ce–Zr-MOF | 1.72 | 24.44 | 45.35 | 28.49 | 1.23 | 19.62 | 19.88 | 21.89 |
Zr-MOF | 0 | 26.25 | 44.16 | 29.59 | 0 | 17.61 | — | 19.74 |
As shown in Scheme 1, except for the hydration of 2-CY, several other reactions are also involved in the reaction system. The evolution of the formation of the chemicals with different MOF composites are displayed in Fig. 2(b). It is noted that after doping Ce, the activity for the hydration of 2-CY (blue) and reaction of 2-PA with methanol (red) was significantly decreased (total formation of 2-PA and P-2 decreased from 51.11 mmol to 17.83–20.42 mmol). In particular, when Ce-MOF was used, the reaction of 2-PA with methanol (red) diminished to nearly none, only 1.3 mmol of P-2 was formed. However, Zr-MOF has the smallest activity for the reaction of 2-CY with methanol (green), indicating there is competition for the synthesis of the side-products.
Next, the reaction temperature of the Zr–Ce-MOF catalyst was optimized from 140 °C to 170 °C [Fig. 3(a)]. When the temperature was increased, the catalytic activity first increased and then decreased, reaching an optimum DMC value at 160 °C. On the other hand, yields of by-products such as 2-PA and P-2 were also followed the same trend. But yield of P-C decreased with increasing temperature, indicating over-reactions and side reactions take place more easily at lower temperatures. As these by-products is not favorable for DMC synthesis, the optimal reaction temperature is determined to be 160 °C.
The effect of CO2 pressure was investigated in the range of 1.6 to 4.6 MPa, and the results are shown in Fig. 3(b). DMC formation increased with increasing CO2 pressure. Moderate amount (1.57 mmol) of DMC was obtained in the presence of 1.6 MPa of CO2, and higher yield (4.62 mmol) was achieved using 4.6 MPa CO2. In contrast, the yields of these by-products were increased with increasing CO2 pressure till 2.6 MPa, then decreased with further increasing CO2 pressure. As these by-products are produced through reaction with methanol as shown in Scheme 1, and alcohol is known to be activated on acid–base pair sites.46–49 Considering that CO2 can also be strongly adsorbed on the acid–base pair sites, high pressure CO2 will suppress the formation of these by-products by covering the active sites.47,48
As for the effect of molar ratio of methanol to 2-CY, the DMC formation amount was found to be increased with increasing molar ratio. When the solar ratio of methanol to 2-CY was changed from 1:4 to 4:1, the DMC formation increased to 4.95 mmol from 0.51 mmol, while the P-C formation first increased then decreased to varying degrees. These results suggested that high MeOH concentration enhances the formation of DMC and inhibits the formation of by-products. Previous studies have also shown that the molar ratio of methanol to 2-CY plays a crucial role in the reaction.14
Finally, the reaction time effect in DMC synthesis from CO2 and methanol was studied. Interestingly, in 1 h only 1.16 mmol of DMC was formed, but large amount of P-C was synthesized (∼66.30 mmol), indicating faster reaction rate of 2-CY with methanol than CO2 with methanol. With increasing reaction time, DMC amount increased significantly, when the reaction time reached 9 h, DMC amount reached 5.02 mmol. Meanwhile, the amount of by-products was decreased with increasing reaction time. Especially, the amount of P-C dropped significantly with reaction time, which is attributed to the reversible reaction (e) in Scheme 1. These results show that an obvious competition is existing between the DMC formation and P-C formation.
In the high-resolution Zr 3d spectra [Fig. 4(c)], two peaks at around 182.9 eV and 185.3 eV were observed, which represents Zr 3d5/2 of the zirconium atoms in Zr6 clusters and Zr 3d3/2 of the zirconium atoms in missing-linker defects, respectively.30,50 Besides, with the introduction of Ce, the binding energies of Zr 3d shifted to lower energies [Fig. 4(c)], indicating the successful introduction of Ce into the material, which is consistent with the results from XRD. Furthermore, with the increasing content of Ce in the material (as shown in Table 2), more obvious shift was detected, which could be attributed to the varies electron withdrawing capability of Ce and Zr.
The Ce 3d spectrum could be deconvoluted into four pairs of spin-orbital doublet peaks (3d3/2 and 3d5/2) as shown in Fig. 4(d): 881.7 eV/900.2 eV, 885.5 eV/903.9 eV, 888.9 eV/907.1 eV and 898.5 eV/917.0 eV. The peaks at 881.7 eV/900.2 eV, 888.9 eV/907.1 eV and 898.5 eV/917.0 eV are assigned to Ce(IV) species, while the peaks at 885.5 eV/903.9 eV are attributed to Ce(III) species.55,56 These results reveal co-existence of Ce(III) species and Ce(IV) species on the surface of Ce-MOF, Zr–Ce-MOF and Ce–Zr-MOF, which leads to more defects in the material. Defects in UiO-66 is known to enhance Lewis acidic sites, hence, more Lewis acidic sites should be found in Zr–Ce-MOF and Ce–Zr-MOF. To verify this, the pyridine-IR analysis of the samples was investigated, and the result is shown in Fig. 6(c). Zr–Ce-MOF and Ce–Zr-MOF presented much higher peak than pure Zr-MOF and Ce-MOF at 1580 cm−1.
The authentic composition of these four MOF materials was also investigated by ICP-MS, and the metal loading was summarized in Table 2. Notably, the total metal mass fraction increased from 17.61 wt% to 20.85 wt%, wherein Zr content improved from 17.61 wt% to 19.62 wt% when Ce was doped into the Zr-MOF, indicating that Ce doping is conducive to generation of active sites. On the other hand, if Zr was added later, the bulk Ce decreased from 20.97 wt% to 1.16 wt%, while the content of Zr was as high as 23.85 wt%, suggesting a stronger coordination of Zr metal with the organic linker than Ce. Furthermore, higher surface Ce content on Zr–Ce-MOF than on Ce–Zr-MOF was detected, demonstrating the crucial role Ce played for the CO2 transformation reaction, which is consistent with previous reports.46,74
The preceding discussed XRD and XPS results gave a strong indication that the incorporation of Ce had been achieved. In order to ascertain if this had any effect on other properties of MOF materials, UV-Vis DRS analyses were conducted, as shown in Fig. 5(a). After incorporation of Ce into the Zr-MOF, the absorption edge of the composite displayed an obvious red-shift, from 341 nm in the UV region to 409 nm and 419 nm in the visible light region, indicating the composite begins to absorb visible light after introduction of Ce, which could be attributed to both the structure effect and the electronic effect.57,58 Especially, for the Ce-MOF, the absorption edge was approximately 450 nm [Fig. 5(a)]. In addition, the optical band gaps of the as-prepared materials were calculated using Kubelka–Munk (KM) method, and the results are demonstrated in Fig. 5(b).46,59,60 It can be clearly seen that with the increasing content of Ce in the MOF (Zr-MOF, Ce–Zr-MOF, Zr–Ce-MOF and Ce-MOF), the band gaps decrease (3.53 eV > 2.88 eV > 2.87 eV > 2.63 eV). Previous reports have shown that band gap changes indicates obvious changes in the amount of defects in the materials, especially the oxygen vacancy change.46,61 Thus, the results indicate that the substitution of Ce with Zr species leads to the formation of oxygen vacancy in the material through distorting the lattice structure.
Fig. 5 (a) UV-Vis diffuse absorbance spectra, (b) Tauc plots, (c) FT-IR spectra of fresh Zr/Ce-based MOF materials and (d) FT-IR spectra of used Zr/Ce-based MOF materials. |
To further investigate the chemical structure of the MOF materials, FT-IR spectra were obtained, as shown in Fig. 5(c). The characteristic peak of the CO stretching vibration from BDC in the frameworks at 1650 cm−1 are detected in all the four materials.39 However, the one for Ce-MOF is wider compared to that of the other three MOFs, indicating that the Ce ions in the material have been successfully coordinated with the CO of the BDC ligand.62 All materials show intense bands at 1580 cm−1 and 1390 cm−1, corresponding to the asymmetric and symmetric stretching vibrations of the –COOH from BDC ligand.63 The bands at 1500 cm−1 and 748 cm−1 could be assigned to the typical vibration of CC and the bending vibration of C–H on the benzene ring, respectively.52,54 For Zr-MOF, Zr–Ce-MOF and Ce–Zr-MOF, characteristic peaks for stretching vibration of Zr–Oμ3-O and Zr–Oμ3-OH bonds of the Zr6 cluster at 658 cm−1 and 484 cm−1 were detected, verifying again that Zr-MOF structures can be formed more easily when Ce and Zr co-existing during the preparation procedure.63 An obvious shift for the two peaks was observed for Ce-MOF, suggesting similar structure of Ce-MOF to that of Zr-MOF. In the low-frequency region, the band near 548 cm−1 is ascribed to Zr–O–C or Ce–O–C asymmetric stretching vibration.63
It is widely reported that acid–base properties are crucial to the CO2 conversion.64 As basic sites are required to activate the CO2 molecule, while acid–base pair sites are often reported to be mandatory for methanol activation (CH3O− and CH3+ formation). Acidity and basicity properties of the four samples were characterized by NH3-and CO2-TPD, as presented in Fig. 6(a) and (b), respectively. NH3-TPD profiles, as illustrated in Fig. 6(a), presented two broad peaks centered at about 120 °C and 270 °C for all the samples. However, the amount of acid sites (desorption peak area) was significantly affected by the introduction of Ce into Zr-MOF lattice. The amount of acid sites reached maximum for Zr–Ce-MOF. Moreover, strong acid sites with the desorption temperature over 400 °C were observed in Zr–Ce-MOF, demonstrating the influence of dopant Ce on the acidity of Zr-based MOF material.
Fig. 6 Temperature programmed desorption (TPD) of (a) NH3, (b) CO2 on the prepared MOF materials and (c) pyridine-IR results of MOF materials. |
Meanwhile, the CO2-TPD profiles are also shown in Fig. 6(b). CO2 desorption peaks in the regions of 100–350 °C were observed and could be ascribed to the desorption of different carbonate species (linear species, bidentate carbonate and monodentate carbonate). A distinguished sharp desorption peak at 180 °C was observed in the CO2-TPD of Zr–Ce-MOF, which can be assigned to the removal of bidentate carbonate species on the surface.46,65 The absence of this particular peak in the other three samples could explain the smaller catalytic activity of the three samples.
In addition, Py-IR experiments are performed to distinguish Lewis acidic sites (LAS) and Brønsted acidic sites (BAS) on Zr-based MOFs. As shown in Fig. 6(c), the bands at 1610, 1580 and 1440 cm−1 are ascribed to LAS with the label P-L, while the bands at 1655 and 1540 cm−1 are assigned to BAS with the label P-B.47,66 It is also observed that a combined additional band at 1504 cm−1 is attributed to the vibration of pyridine on both LAS and BAS acid sites with the label P-L+B presents on catalysts.66,67 The Py-IR results showed small bands of LAS at 1580 cm−1 on both Zr-MOF and Ce-MOF, but doping of Ce into the Zr-MOF significantly increased the intensity of the band. The LAS was reported to be important in the adsorption of CO2.46,68 Therefore, by doping Ce, varied catalytic activity can be derived for Zr-based MOFs.69,70
Overall, the catalytic activities of the MOF materials were not linearly dependent on the special surface areas, suggesting that other properties of the catalysts should be considered for the better activities of Zr–Ce-MOF. As reported previously, the catalytic activity of catalysts for CO2 conversion related to the surface area, crystallite size, acid–base properties as well as the surface ratio of Ce4+/Ce3+. However, based on the N2 adsorption–desorption, XRD and XPS results in the present study, there is no direct correlation between the crystallite size, surface area and surface proportion of Ce4+/Ce3+ and the activity of the MOF materials.
The acid–base properties of the MOF catalysts were also examined in this study, but the amounts of acidic and basic sites were not in obvious linear relationship with the yield of DMC. The amount of basic sites, which were in favor of CO2 adsorption and activation, has no direct relationship with the doping of Ce. However, the weak basic site is a key factor for the catalytic activity, which can form bidentate carbonate species on the surface with CO2. By inspection of Fig. 6(a), it could be seen that the more acid sites catalysts possessed, the better catalytic performance they displayed [Fig. 2(a)]. As shown by Fig. 6(c), the Lewis acid sites responsible for the adsorption and activation of OH group of alcohols is another key factor for the catalytic activity. Therefore, the improved acid–base properties might take major part of the responsibility for the better catalytic performance of Zr–Ce-MOF material.
Integrated analysis of the IR result and the catalytic evaluation results showed that, the activation of methanol is crucial during the reaction, especially the generation of t-OCH3 species from LAS activation of methanol, which could be the limiting step of the reaction. Though more m-CO3 species was detected from Ce-MOF and Zr-based MOF, smaller TOF illustrates that the activation of CO2 is not the limiting step.
Based on the above analysis, we propose a possible reaction mechanism, as shown in Scheme 2. During the DMC formation process from methanol and CO2, methanol gets activated by Lewis acid sites to form b-OCH3 and t-OCH3, CO2 is activated on basic sites and reacted with the CH3O− species to generate m-CH3OCOO−. DMC is formed through the reaction of CH3OCOO− with another activated methanol, and the H atom from methanol activation then reacts with the surface OH group to form H2O.30,73,74
DMC | Dimethyl carbonate |
2-CY | 2-Cyanopyridine |
2-PA | 2-Picolinamide |
P-2 | Methyl pyridine-2-carboximidate |
P-C | Methyl picolinimidate |
Footnote |
† Electronic supplementary information (ESI) available. See https://doi.org/10.1039/d2ra02680e |
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