N-Heterocyclic Carbene Functionalized Covalent Organic Framework for Transesterification of Glycerol with Dialkyl Carbonates

The development of a heterogeneous catalyst through the combination of novel carrier and powerful catalytic active sites is of particular interest. Herein, the successful integration of an N-Heterocyclic carbene (NHC) moiety into a covalent organic framework (COF) was achieved by coupling 4,4′,4′′,4′′′-(pyrene-1,3,6,8-tetrayl) tetraaniline (PyTTA) and equimolar 4,7-bis(4-formylphenyl)-1-methyl-1H-benzimidazole (IM) and 2′3′5′6′-tetrafluoro-[1,1′:4′,1′′-terphenyl]-4,4′-dicarbaldehyde (4F) followed by ionization with 1-bromobutane (C4H9Br) and then deprotonation upon addition of a base. The resulting material exhibited promising heterogeneous catalytic activity towards transesterification reaction of glycerol with dialkyl carbonate. Moreover, good recyclability granted no substantial loss of activity upon five cycles. Combination of COFs and NHCs might synergize their characteristics, thus providing more possibilities for creating new patterns of catalytic reactivity.

N-Heterocyclic carbenes (NHCs) are another class of powerful catalysts that would be appealing to feature in COFs. In homogeneous catalysis, they are found to be highly effective for a broad range of rearrangement, addition, and group-transfer reactions entailing carbon-carbon, carbon-nitrogen, and carbon-oxygen bond formation or deletion [69][70][71][72][73].
NHC-Py-COF in the presence of NaCO3. In other words, BMIM4F-Py-COF is a pre-catalyst in this research. Each benzimidazole framework can be essentially treated as a discrete molecule to undergo a reaction normally performed in molecular organic chemistry, providing a completely different framework material. The catalytic performance of the resulting NHC-Py-COF was then evaluated by the transesterification reaction of glycerol with dialkyl carbonates and excellent catalytic activity and good recyclability were observed for the NHC-Py-COF. The crystalline nature of all the COFs was confirmed by powder X-ray diffraction (PXRD) measurements. IM4F-Py-COF exhibited strong diffraction peaks at 2.8°, 3.8°, 5.5°, 8.2°, 10.9°, 13.7°, and 23.2°, which were attributed to the (110), (020), (220), (330), (400), (550), and (001) facets, respectively (Figure 1a, red curve). The use of lattice modeling (Figure 1a, magenta curve) and Pawley refinement processes produce an eclipsed AA stacking model that reproduced the PXRD pattern very well with Rwp of 2.9% and Rp of 1.6% (Figure 1a, black curve). The Pawley refinement using the unit cell parameters of a = 49.83 Å, b = 42.60 Å, c = 3.93 Å, and α = β = γ = 90°, confirmed the peak assignment, as evidenced by the negligible difference (Figure 1a, blue curve). By contrast, an alternatively staggered AB model did not match the observed data ( Figure 1a, olive curve). A schematic arrangement structure was shown in Figure 1b. Moreover, identical PXRD patterns were also observed for BMIM4F-Py-COF and 4F-Py-COF, suggesting that these three COFs possess similar crystal structures ( Figure 2). The slight decrease in the diffraction peak intensity of BMIM4F-Py-COF was attributed to the occupation of the pores by the grafted n-butyl groups. The crystalline nature of all the COFs was confirmed by powder X-ray diffraction (PXRD) measurements. IM4F-Py-COF exhibited strong diffraction peaks at 2.8 • , 3.8 • , 5.5 • , 8.2 • , 10.9 • , 13.7 • , and 23.2 • , which were attributed to the (110), (020), (220), (330), (400), (550), and (001) facets, respectively (Figure 1a, red curve). The use of lattice modeling (Figure 1a, magenta curve) and Pawley refinement processes produce an eclipsed AA stacking model that reproduced the PXRD pattern very well with R wp of 2.9% and R p of 1.6% (Figure 1a, black curve). The Pawley refinement using the unit cell parameters of a = 49.83 Å, b = 42.60 Å, c = 3.93 Å, and α = β = γ = 90 • , confirmed the peak assignment, as evidenced by the negligible difference ( Figure 1a, blue curve). By contrast, an alternatively staggered AB model did not match the observed data ( Figure 1a, olive curve). A schematic arrangement structure was shown in Figure 1b. Moreover, identical PXRD patterns were also observed for BMIM4F-Py-COF and 4F-Py-COF, suggesting that these three COFs possess similar crystal structures ( Figure 2). The slight decrease in the diffraction peak intensity of BMIM4F-Py-COF was attributed to the occupation of the pores by the grafted n-butyl groups.  FT-IR spectroscopy was used to characterize the formation and modification of the COFs. As shown in Figure 3, a strong characteristic stretching peak at 1622 cm -1 was observed for all COFs, revealing imine (-C=N-) linkages of these COFs. Besides, a strong peak appeared at 980 cm -1 , which can be ascribed to the -C-F-stretching vibration. In IM4F-Py-COF and BMIM4F-Py-COF, the peak at 1072 cm -1 was due to the -C-Nstretching vibration, which was absent in 4F-Py-COF. A new peak that appeared at 2956 cm -1 in BMIM4F-Py-COF was ascribed to the characteristic stretching of -CH2-, indicating the successful grafting of n-butyl groups onto the channel walls. The porous properties of all the COFs were characterized by nitrogen adsorptiondesorption isotherms measured at 77 K. As shown in Figure 4a, the three COFs exhibited the typical type-IV adsorption isotherms, indicative of mesoporous characteristics. The BET surface areas were measured to be 1930, 1450, and 1326 m 2 g −1 , and the pore widths were calculated to be 3.43, 3.34, and 3.31 nm for 4F-Py-COF, IM4F-Py-COF, and BMIM4F-Py-COF, respectively (Figure 4b-d). Their corresponding total pore volumes were calculated to be 1.82, 1.11, and 0.65 cm 3 g −1 , respectively (Table 1). Based on the FT-IR spectroscopy was used to characterize the formation and modification of the COFs. As shown in Figure 3, a strong characteristic stretching peak at 1622 cm -1 was observed for all COFs, revealing imine (-C=N-) linkages of these COFs. Besides, a strong peak appeared at 980 cm -1 , which can be ascribed to the -C-F-stretching vibration. In IM4F-Py-COF and BMIM4F-Py-COF, the peak at 1072 cm -1 was due to the -C-N-stretching vibration, which was absent in 4F-Py-COF. A new peak that appeared at 2956 cm -1 in BMIM4F-Py-COF was ascribed to the characteristic stretching of -CH 2 -, indicating the successful grafting of n-butyl groups onto the channel walls.  FT-IR spectroscopy was used to characterize the formation and modification of th COFs. As shown in Figure 3, a strong characteristic stretching peak at 1622 cm -1 was ob served for all COFs, revealing imine (-C=N-) linkages of these COFs. Besides, a stron peak appeared at 980 cm -1 , which can be ascribed to the -C-F-stretching vibration. I IM4F-Py-COF and BMIM4F-Py-COF, the peak at 1072 cm -1 was due to the -C-N stretching vibration, which was absent in 4F-Py-COF. A new peak that appeared at 295 cm -1 in BMIM4F-Py-COF was ascribed to the characteristic stretching of -CH2-, indicat ing the successful grafting of n-butyl groups onto the channel walls. The porous properties of all the COFs were characterized by nitrogen adsorption desorption isotherms measured at 77 K. As shown in Figure 4a, the three COFs exhibited the typical type-IV adsorption isotherms, indicative of mesoporous characteristics. Th BET surface areas were measured to be 1930, 1450, and 1326 m 2 g −1 , and the pore width were calculated to be 3.43, 3.34, and 3.31 nm for 4F-Py-COF, IM4F-Py-COF, and BMIM4F-Py-COF, respectively (Figure 4b-d). Their corresponding total pore volume were calculated to be 1.82, 1.11, and 0.65 cm 3 g −1 , respectively (Table 1). Based on th The porous properties of all the COFs were characterized by nitrogen adsorptiondesorption isotherms measured at 77 K. As shown in Figure 4a, the three COFs exhibited the typical type-IV adsorption isotherms, indicative of mesoporous characteristics. The BET surface areas were measured to be 1930, 1450, and 1326 m 2 g −1 , and the pore widths were calculated to be 3.43, 3.34, and 3.31 nm for 4F-Py-COF, IM4F-Py-COF, and BMIM4F-Py-COF, respectively (Figure 4b-d). Their corresponding total pore volumes were calculated to be 1.82, 1.11, and 0.65 cm 3 g −1 , respectively (Table 1). Based on the PXRD and adsorption isotherm results, it is evident that the ordered framework structure remained after modification.
teraction stemming from a fluoro-substituted and non-substituted unit combination leads to high thermal stability. After the ionization modification, the thermal stability slightly decreased, as reflected by the weight loss at ca. 200 °C, which may be attributed to the decomposition of an ionic moiety attached to the channel walls. However, the frameworks still remain at high thermal stability (>450 °C). Therefore, it is practicable for BMIM4F-Py-COF to be used as a pre-catalyst for the transesterification of glycerol with dialkyl carbonates. The use of 1-alkylimidazlium type ionic liquid and CO2 in the presence of a base as a precatalyst for carbene has been widely reported and the formed carbene catalysts exhibited excellent catalytic performance in carboxylation reaction [84], reduction reactions [85], and cycloaddition reaction [86]. Moreover, the NHC-functionalized polymer was also used for reversible fixation-release of CO2 [87]. During these reactions, an intermediate N-heterocyclic carbene (NHC)-CO2 zwitterionic adduct was formed in situ that was  The thermal behavior of the three COFs was detected by thermogravimetric analysis (TGA). It can be seen from Figure 5 that all the materials decomposed at a temperature above 450 • C, indicating their good thermal stability. The decomposition temperature of IM4F-Py-COF is higher than 4F-Py-COF. This result suggests that strong interlayer interaction stemming from a fluoro-substituted and non-substituted unit combination leads to high thermal stability. After the ionization modification, the thermal stability slightly decreased, as reflected by the weight loss at ca. 200 • C, which may be attributed to the decomposition of an ionic moiety attached to the channel walls. However, the frameworks still remain at high thermal stability (>450 • C). Therefore, it is practicable for BMIM4F-Py-COF to be used as a pre-catalyst for the transesterification of glycerol with dialkyl carbonates.
Catalysts 2021, 11, x FOR PEER REVIEW 6 of 13 then subjected to nucleophilic substitution reaction to give final products due to its strong σ-donor character. In our case, although benzimidazolium moiety was integrated into the skeleton of COF, we assume that the deprotonation to form NHC in the heterogeneous system could also happen just like in a molecular solution.

Catalytic Performance of COFs.
The catalytic activity of the COF was tested in the transesterification reaction of glycerol with dialkyl carbonates by the in situ generated NHC-Py-COF catalyst [88,89]. The reactions were performed at 70 °C, and the reaction time was 8 h. In these reactions, a  The use of 1-alkylimidazlium type ionic liquid and CO 2 in the presence of a base as a precatalyst for carbene has been widely reported and the formed carbene catalysts exhibited excellent catalytic performance in carboxylation reaction [84], reduction reactions [85], and cycloaddition reaction [86]. Moreover, the NHC-functionalized polymer was also used for reversible fixation-release of CO 2 [87]. During these reactions, an intermediate Nheterocyclic carbene (NHC)-CO 2 zwitterionic adduct was formed in situ that was then subjected to nucleophilic substitution reaction to give final products due to its strong σ-donor character. In our case, although benzimidazolium moiety was integrated into the skeleton of COF, we assume that the deprotonation to form NHC in the heterogeneous system could also happen just like in a molecular solution.

Catalytic Performance of COFs.
The catalytic activity of the COF was tested in the transesterification reaction of glycerol with dialkyl carbonates by the in situ generated NHC-Py-COF catalyst [88,89]. The reactions were performed at 70 • C, and the reaction time was 8 h. In these reactions, a 5 mol% NHC-Py-COF catalyst, generated through the addition of 2.5 mol% Na 2 CO 3 into BMIM4F-Py-COF, was used. The catalytic performance is summarized in Table 2. The transesterification of glycerol with dimethyl carbonate (DMC) was used as a model reaction. The reaction cannot proceed without a catalyst ( Table 2, entry 1), which is in accordance with previously reported literature [90,91]. On the contrary, high conversion (99%) and yield (96%) were obtained when NHC-Py-COF was used as a catalyst ( Table 2, entry 2). A similar result was also observed for the transesterification reaction of glycerol with diethyl carbonate (96% for both conversion and yield, Table 2, entry 3). Such high catalytic activity is comparable to that of NHC-[BMIM]Br (Table 2, entry 4), suggesting that the heterogeneous NHC-Py-COF catalyst possesses an excellent catalytic ability that can only be achieved by a corresponding homogenous counterpart. In addition, [BMIM]Br, IM4F-Py-COF, and 4F-Py-COF were tried to catalyze the model reaction, no product can be detected, indicating that the reaction was catalyzed by NHC-Py-COF (Table 2, entry 5-7). The separation and recyclability of the catalyst are further investigated for this heterogeneous reaction. After each cycle, the NHC-Py-COF could be simply regenerated by centrifugation. As shown in Table 2, entry 8-12, the catalyst NHC-Py-COF could be reused at least 5 times without significant loss of its catalytic activity.  Based on our experiments and previous literature, we propose the following mechanism for this reaction (Scheme 2). Take DMC as an example, in situ generated carbene on the skeleton of COF attacks the carbon of the carbonyl group of DMC, leading to the cleavage of one methoxy anion (Step 1), that in turn deprotonates the primary alcohol group of glycerol (Step 2). The generated alkoxide then attracts the carbonyl group on the 1-butyl-3-methylimidazole complex. As a result, the other methoxy anion is lost (Step 3). This methoxy anion then deprotonates the secondary alcohol group (Step 4), giving an alkoxide attached to the 1-butyl-3-methylimidazole complex. This alkoxide then attacks the carbonyl carbon of the complex to produce the product and meanwhile regenerates the active carbene species (Step 5).

Scheme 2.
Proposed reaction mechanism of the transesterification of glycerol and DMC.

Synthesis
Synthesis of IM4F-Py-COF. PyTTA (21.67 mg, 0.04 mmol), IM (13.76 mg, 0.04 mmol), and 4F (14.32 mg, 0.04 mmol) were placed in a glass ampule vessel (10 mL), followed by adding 2 mL of 1,2-dichlorobenzene as a solvent. The mixture was sonicated for 10 min and 0.2 mL of 6.0 M acetic acid was rapidly added. The vessel was then flash-frozen in liquid nitrogen and degassed by three freeze-pump-thaw cycles. The internal pressure of the vessel was controlled below 5 Pa. The system was rapidly sealed with a flame, and then heated at 120 • C for 3 days. The resulting precipitate is filtered out, washed thoroughly with anhydrous THF and acetone to give a yellow colored powder, which was dried at 100 • C under vacuum overnight to give the desired product in 78% yield.
Synthesis of BMIM4F-Py-COF. IM4F-Py-COFT measuring 50 mg, 5 mL of 1-bromobutane, and 20 mL of acetonitrile were added to a 50 mL round-bottom flask. The reaction was heated under reflux at 80 • C and stirred for 48 h. After cooling down to room temperature, the precipitate of the product was collected by filtration, and washed thoroughly with anhydrous ethanol and acetone, respectively. The precipitate was then dried at 100 • C overnight under vacuum, giving a dark yellow product in 95% yield.
Synthesis of 4F-Py-COF. PyTTA (21.67 mg, 0.04 mmol), and 4F (28.64 mg, 0.08 mmol) were placed in a glass ampule vessel (10 mL), followed by adding 2 mL of 1,2-dichlorobenzene. The mixture was sonicated for 10 min and 0.2 mL of 6.0 M acetic acid was rapidly added. The vessel was then flash-frozen in liquid nitrogen and degassed by three freeze-pumpthaw cycles. The internal pressure of the vessel was controlled below 5 Pa. The system was rapidly sealed with a flame, and then heated at 120 • C for 3 days. The resulting precipitate was filtered out and washed thoroughly with anhydrous THF and acetone to give a yellow-colored powder, which was dried at 100 • C under vacuum overnight to give the desired product with 81% yield.

General Procedures for Transesterification of Glycerol with Dialkyl Carbonates
In a typical experiment, the reaction was performed using a 5 mol% catalyst (generated in situ by a 5 mol% pre-catalyst and 2.5 mol% Na 2 CO 3 ), 2.5 mmol of glycerol, and 7.5 mmol of dimethyl carbonate (or diethyl carbonate) at 70 • C for 8 h. No additional solvents were used in this reaction. All experiments were carried out in a Schlenk tube attached with a reflux condenser under an N 2 atmosphere. Dimethyl carbonate and diethyl carbonate were distilled before use. The products were analyzed by flame ionization gas chromatography (GC-17A, Shimadzu), using a capillary column (TC-FFAP). After the reaction, 1 mmol of 1-propanol was added to the reaction solution as an internal standard.

Characterization
Fourier transform infrared (FT-IR) spectra were recorded using KBr pellets on a Bruker model TENSOR 27 spectrophotometer. Powder X-ray diffraction (PXRD) measurements were recorded on a PANalytical X'Pert model Pro Multipurpose Diffractometer using Cu K α radiation at 40 kV and 40 mA. The signals were collected from 2θ of 2.5-30 • at a 0.03 • step scan with an exposure time of 10 s per step. Low-pressure (0-110 kPa) N 2 gas sorption isotherms were measured volumetrically at 77 K using a Quantachrome Autosorb-iQ2 analyzer with ultrahigh-purity gases. The fresh samples were activated at 100 • C for 15 h under a high vacuum prior to analysis. The Brunauer-Emmett-Teller (BET) model was used to determine the specific surface areas using desorption branches over P/P 0 of 0.01-0.05. In all isotherm plots, closed circles describe adsorption data points and open circles are used to represent desorption data points. The pore size distribution was evaluated by a nonlocal density function theory (NLDFT) method. Thermogravimetric analysis (TGA, STA449F3, NETZSCH, Germany) was performed from room temperature to 800 • C at a heating rate of 10 • C min −1 and a N 2 flow rate of 20 mL min −1 . Gas chromatography (GC, Agilent 7890A) equipped with a capillary column (HP-5, 30 m × 0.25 mm) using a flame ionization detector. 1 H and 13 C nuclear magnetic resonance (NMR) spectra were recorded by a Bruker Advance III 400 MHz NMR spectrometer (Bruker BioSpin Corporation, Fällanden, Switzerland).

Conclusions
In summary, we developed a post-synthetic modification strategy to anchor N-Heterocyclic carbene (NHC) moiety into a covalent organic framework (COF) that was initially constructed through a three-component condensation reaction. After successful grafting of 1-butyl groups onto the skeletons of the COF, ionization of the framework was achieved, and NHC-functionalized COF, i.e., NHC-Py-COF was then obtained in the presence of a base. The precursors of the NHC-Py-COF were characterized by various technologies. The in situ generated NHC-Py-COF was used as a catalyst for the transesterification reaction of glycerol with dialkyl carbonates. Excellent catalytic activity and good recyclability were observed, suggesting the excellent catalytic performance of the NHC-functionalized COF. Considering the powerful catalytic activity of NHC and unique features of COFs, we can envisage that the combination of NHC and COF would open up a way to create new patterns of catalytic reactivity.