A Highly E ﬃ cient Bifunctional Catalyst CoO x / tri-g-C 3 N 4 for One-Pot Aerobic Oxidation–Knoevenagel Condensation Reaction

: A highly e ﬃ cient bifunctional catalyst of an s-triazine-based carbon-nitride-supported cobalt oxide is developed for the aerobic oxidation–Knoevenagel condensation tandem reaction of benzyl alcohol and malononitrile, whereby 96.4% benzyl alcohol conversion with nearly 100% selectivity towards benzylmalononitrile can be obtained in 6 h at 80 ◦ C. The excellent catalytic performance derives from the high basicity of carbon nitride and strong redox ability of Co species induced by carbon nitride. The catalyst is also quite stable and can be reused without any regeneration treatment, whose product yield is only an 11.5% reduction after four runs. reaction without separation of intermediate benzaldehyde.


Introduction
The development of alternative clean and efficient processes is one of the priorities for modern chemistry, since traditional chemical synthesis and the resulting large consumption of fossil energy have caused a series of environmental pollution problems, which are destroying our ecological environment. The one-pot procedure has raised extensive interest since it can simplify the synthetic routes and minimize the production waste and energy consumption [1][2][3][4][5]. The challenges for one-pot transformation involve the development of heterogeneous catalysts with different catalytically active sites that can promote different reactions simultaneously and independently while not interfering with each other [6][7][8].
Benzylmalononitrile is a key intermediate for the preparation of antimalarial, which has numerous applications in the fields of pharmaceutical, biological, and synthetic chemistry [9]. Benzylmalononitrile is traditionally prepared by Knoevenagel condensation of malononitrile and benzaldehyde, the latter of which is generally obtained from the selective oxidation of benzyl alcohol, since alcohol is readily available as compared with the corresponding aldehydes. Preparation of benzylmalononitrile through tandem aerobic oxidation-Knoevenagel condensation reaction can greatly improve the synthesis efficiency [10][11][12][13][14][15]. One-pot synthesis of the above tandem reaction has been achieved by several researchers, including Qi [16], Miao [17], and Yan [18]. However, developing a simple and efficient catalyst is still a big challenge, since the activation of molecular oxygen under mild conditions is very difficult, especially without a noble metal.
Graphitic carbon nitride is known as a solid base catalyst that is active for the Knoevenagel condensation of benzaldehyde and malononitrile [19][20][21][22]. Meanwhile, it is also an outstanding support for the transition metal oxides, since the redox catalytic ability of the oxides can be improved greatly due to their unique interaction with g-C 3 N 4 [23][24][25][26][27]. Recently, s-triazine-based carbon greatly due to their unique interaction with g-C3N4 [23][24][25][26][27]. Recently, s-triazine-based carbon nitride (tri-g-C3N4) has been found to be a more ideal solid base catalytic material than g-C3N4 because it has more uncondensed amino groups (-NH, -NH2) in the structure as compared with g-C3N4 [27][28][29][30][31]. These uncondensed amino groups can also be employed as anchors for supporting transition metal oxides.
Herein, a bifunctional catalyst with both basic and redox properties has been designed by supporting CoOx on tri-g-C3N4. Due to the basic activity of tri-g-C3N4 and the enhanced redox ability of Co species, this catalyst has excellent catalytic activity in aerobic oxidation-Knoevenagel condensation tandem reaction of benzyl alcohol and malononitrile, without addition of any auxiliary reagents. The reasons for this remarkable catalytic activity of CoOx/tri-g-C3N4 are also elucidated. Figure 1 shows XRD patterns of CoOx/tri-g-C3N4 with different CoOx contents, together with that of tri-g-C3N4. For tri-g-C3N4, in addition to the two diffraction peaks assigned to g-C3N4 with 2θ of 13.2° and 27.7°, peaks at 12.3°, 20.0°, 24.0 o , 26.7°, 29.5°, and 32.2° can also be observed, which are ascribed to tri-g-C3N4, corresponding to its (100), (110), (200), (002), (102), and (210) crystal planes, respectively [32]. The characteristic diffractions of g-C3N4 decrease and later disappear, while the (100) and (002) diffractions of tri-g-C3N4 remain with the increase of CoOx loading, demonstrating the transformation of g-C3N4 to tri-g-C3N4. No obvious characteristic diffraction peak of CoOx can be identified due to the relatively small CoOx loading and its high dispersion. The XRD patterns of (a) tri-g-C3N4, (b) 1CoOx/tri-g-C3N4, (c) 3CoOx/tri-g-C3N4, (d) 5CoOx/trig-C3N4, and (e) 7CoOx/tri-g-C3N4.

Catalyst Characterization
The generation of the tri-g-C3N4 structure was further confirmed by the FT-IR spectra, which is shown in Figure 2. An intense absorption peak at 808 cm −1 corresponding to the breathing modes of the s-triazine units [32] can be observed for all the catalysts. Skeletal stretching vibrations together with bending vibrations of s-triazine or tri-s-trizaine located at 1250 ∼ 1700 cm −1 also appeared in the spectra. The broad absorption band in the range of 3100∼3300 cm −1 belongs to the stretching vibration of uncondensed N-H. A significant shift of the N-H band can be detected, probably caused by the interaction between the N-H group and CoOx species.
The generation of the tri-g-C 3 N 4 structure was further confirmed by the FT-IR spectra, which is shown in Figure 2. An intense absorption peak at 808 cm −1 corresponding to the breathing modes of the s-triazine units [32] can be observed for all the catalysts. Skeletal stretching vibrations together with bending vibrations of s-triazine or tri-s-trizaine located at 1250~1700 cm −1 also appeared in the spectra. The broad absorption band in the range of 3100~3300 cm −1 belongs to the stretching vibration of uncondensed N-H. A significant shift of the N-H band can be detected, probably caused by the interaction between the N-H group and CoO x species.
The SEM images of the obtained catalysts are presented in Figure 3, where tri-g-C 3 N 4 exhibits a block structure made up of irregular layered sheets. No clear differences in morphology were found after CoO x loading, signifying that CoO x did not destroy the tri-g-C 3 N 4 structure. This was confirmed by N 2 adsorption measurements, with similar type III isotherms with H3 hysteresis loops shown for all of the catalysts (illustrated in Figure S1). The hysteresis loops are more evident for the supported catalysts, revealing the increase of interparticle voids after the CoO x support. Table 1 shows the specific Catalysts 2020, 10, 712 3 of 10 surface areas and total pore volumes of the catalysts. Both become larger after Co loading, probably due to the intercalation of Co species into the layered structure, resulting in more interparticle voids. However, when the Co loading is further increased, the pore channel of tri-g-C 3 N 4 may be partially blocked, leading to a reduction in pore volume. FT-IR spectra of (a) tri-g-C3N4, (b) 1CoOx/tri-g-C3N4, (c) 3CoOx/tri-g-C3N4, (d) 5CoOx/tri-g-C3N4, and (e) 7CoOx/tri-g-C3N4.
The SEM images of the obtained catalysts are presented in Figure 3, where tri-g-C3N4 exhibits a block structure made up of irregular layered sheets. No clear differences in morphology were found after CoOx loading, signifying that CoOx did not destroy the tri-g-C3N4 structure. This was confirmed by N2 adsorption measurements, with similar type III isotherms with H3 hysteresis loops shown for all of the catalysts (illustrated in Figure S1). The hysteresis loops are more evident for the supported catalysts, revealing the increase of interparticle voids after the CoOx support. Table 1 shows the specific surface areas and total pore volumes of the catalysts. Both become larger after Co loading, probably due to the intercalation of Co species into the layered structure, resulting in more interparticle voids. However, when the Co loading is further increased, the pore channel of tri-g-C3N4 may be partially blocked, leading to a reduction in pore volume. . SEM images of (a) tri-g-C3N4, (b) 1CoOx/tri-g-C3N4, (c) 3CoOx/tri-g-C3N4, (d) 5CoOx/tri-g-C3N4, and (e) 7CoOx/tri-g-C3N4. FT-IR spectra of (a) tri-g- FT-IR spectra of (a) tri-g-C3N4, (b) 1CoOx/tri-g-C3N4, (c) 3CoOx/tri-g-C3N4, (d) 5CoOx/tri-g-C3N4, and (e) 7CoOx/tri-g-C3N4.
The SEM images of the obtained catalysts are presented in Figure 3, where tri-g-C3N4 exhibits a block structure made up of irregular layered sheets. No clear differences in morphology were found after CoOx loading, signifying that CoOx did not destroy the tri-g-C3N4 structure. This was confirmed by N2 adsorption measurements, with similar type III isotherms with H3 hysteresis loops shown for all of the catalysts (illustrated in Figure S1). The hysteresis loops are more evident for the supported catalysts, revealing the increase of interparticle voids after the CoOx support. Table 1 shows the specific surface areas and total pore volumes of the catalysts. Both become larger after Co loading, probably due to the intercalation of Co species into the layered structure, resulting in more interparticle voids. However, when the Co loading is further increased, the pore channel of tri-g-C3N4 may be partially blocked, leading to a reduction in pore volume. . SEM images of (a) tri-g-C3N4, (b) 1CoOx/tri-g-C3N4, (c) 3CoOx/tri-g-C3N4, (d) 5CoOx/tri-g-C3N4, and (e) 7CoOx/tri-g-C3N4. SEM images of (a) tri-g- The thermogravimetric method was employed to analyze the thermal stability of CoO x /tri-g-C 3 N 4 catalysts in air atmosphere. The results are shown in Figure 4. CoO x /tri-g-C 3 N 4 catalysts are all stable below 400 • C; only a slight weight loss caused by the desorption of H 2 O or CO 2 was observed at the temperature range of 20-200 • C. The thermal decomposition temperature decreased gradually with the CoO x loadings, indicating that their thermal stability was lowered by the Co loadings. The actual content of Co in the catalyst was calculated by the remaining weight after TGA experiments. As shown Catalysts 2020, 10, 712 4 of 10 in Table 1, these were quite close to the contents added. The slightly increased content was caused by the partial decomposition of the tri-g-C 3 N 4 support during the final calcination. Table 1. Textural and basic properties of tri-g-C 3 N 4 and CoO x /tri-g-C 3 N 4 .

Catalysts
S BET (m 2 /g) The thermogravimetric method was employed to analyze the thermal stability of CoOx/tri-g-C3N4 catalysts in air atmosphere. The results are shown in Figure 4. CoOx/tri-g-C3N4 catalysts are all stable below 400 °C; only a slight weight loss caused by the desorption of H2O or CO2 was observed at the temperature range of 20-200 °C. The thermal decomposition temperature decreased gradually with the CoOx loadings, indicating that their thermal stability was lowered by the Co loadings. The actual content of Co in the catalyst was calculated by the remaining weight after TGA experiments. As shown in Table 1, these were quite close to the contents added. The slightly increased content was caused by the partial decomposition of the tri-g-C3N4 support during the final calcination.  . TG thermograms of (a) 1CoOx/tri-g-C3N4, (b) 3CoOx/tri-g-C3N4, (c) 5CoOx/tri-g-C3N4, and (d) 7CoOx/tri-g-C3N4.
The XPS method was used to detect the chemical states of Co and O on the catalyst surface, the results of which are displayed in Figure S2 and Figure S3. Two peaks at ~796 eV and ~780 eV appeared in Figure S2, which are attributed to the Co 2p1/2 and Co 2p3/2 spin orbits, respectively. The ~780 eV peak can be deconvoluted into two peaks, with peak positions of 780.8 eV and 782.2 eV, corresponding to Co 3+ and Co 2+ species, respectively [26,33,34]. The Co 3+ /Co 2+ ratio after fitting is listed in Table 2. It can be clearly seen that the ratio increases initially with the increase of Co loading. After further increasing the loading of Co, the Co 3+ /Co 2+ ratio decreases. When the Co loading reaches 5%, the Co 3+ /Co 2+ ratio is the largest. A high concentration of Co 3+ may lead to an increase in the number of chemically adsorbed oxygen species on the catalyst surface [27]. This is proven by the O1s spectra illustrated in Figure S3, which can be fitted into two peaks, with peak positions of 532.5 eV and 531.5 eV, corresponding to the adsorbed oxygen species and surface lattice oxygen species, respectively [35,36]. The ratio of the adsorbed oxygen species to the surface lattice oxygen ones has the same change tendency as the ratio of Co 3+ to Co 2+ . The surface-adsorbed oxygen (Oads) is usually located in surface defects and is easier to reduce [37][38][39], which is crucial for catalytic oxidation. The XPS method was used to detect the chemical states of Co and O on the catalyst surface, the results of which are displayed in Figures S2 and S3. Two peaks at~796 eV and~780 eV appeared in Figure S2, which are attributed to the Co 2p 1/2 and Co 2p 3/2 spin orbits, respectively. The~780 eV peak can be deconvoluted into two peaks, with peak positions of 780.8 eV and 782.2 eV, corresponding to Co 3+ and Co 2+ species, respectively [26,33,34]. The Co 3+ /Co 2+ ratio after fitting is listed in Table 2. It can be clearly seen that the ratio increases initially with the increase of Co loading. After further increasing the loading of Co, the Co 3+ /Co 2+ ratio decreases. When the Co loading reaches 5%, the Co 3+ /Co 2+ ratio is the largest. A high concentration of Co 3+ may lead to an increase in the number of chemically adsorbed oxygen species on the catalyst surface [27]. This is proven by the O 1s spectra illustrated in Figure S3, which can be fitted into two peaks, with peak positions of 532.5 eV and 531.5 eV, corresponding to the adsorbed oxygen species and surface lattice oxygen species, respectively [35,36]. The ratio of the adsorbed oxygen species to the surface lattice oxygen ones has the same change tendency as the ratio of Co 3+ to Co 2+ . The surface-adsorbed oxygen (O ads ) is usually located in surface defects and is easier to reduce [37][38][39], which is crucial for catalytic oxidation. The surface basicities of the catalysts were measured by acid-base neutralization titration in aqueous solution, since the reactions investigated here are carried out in the liquid phase. The results are shown in Table 1. A large amount of basic sites were detected on tri-g-C 3 N 4 surface despite its small specific surface area. This was because of the swelling effects in the liquid phase, which was also found on sulfonated carbon catalysts [40]. The amount of basic sites decreased with the Co loadings due to the interaction between the N-H group and CoO x species. However, the reduction was not too big in the range of present CoO x content. Basic sites measuring 2.40 mmol/g still remained on the 7CoO x /tri-g-C 3 N 4 catalyst, which was larger than those on pristine g-C 3 N 4 [19].

Aerobic Oxidation of Benzyl Alcohol
Aerobic oxidation of benzyl alcohol was conducted over CoO x /tri-g-C 3 N 4 catalysts, as well as tri-g-C 3 N 4 . The results are listed in Table 3. It can be seen that tri-g-C 3 N 4 is not active for the oxidation, indicating that redox active sites come from the supported CoO x species. The activity increased sharply with the CoO x loadings, reaching the maximum and then decreasing slowly with further increases of the amount of CoO x . 5CoO x /tri-g-C 3 N 4 has the best activity. The excellent redox catalytic ability can be attributed to the high chemisorbed oxygen species on the surface, since they are thought to be the most active oxygen species in the oxidation reaction, which can promote the formation of oxygen vacancies, as well as enhance the exchange of gaseous oxygen molecules and adsorbed oxygen molecules on the catalyst surface, thus promoting the oxidation reaction [41][42][43]. This can be proven by the XPS results in Table 2, which indicate that the change in activity with the Co loadings has the same trend as in the adsorbed oxygen-to-lattice oxygen ratio (O ads /O latt ). Table 3. Catalyst activity of aerobic oxidation and Knoevenagel condensation.

Knoevenagel Condensation of Benzaldehyde and Malononitrile
The catalytic activities of the CoO x /tri-g-C 3 N 4 catalysts for the Knoevenagel condensation of benzaldehyde and malononitrile were also evaluated, together with that of tri-g-C 3 N 4 . All were very active for the condensation; nearly 100% conversion of benzaldehyde was obtained with the same reaction condition as oxidation (80 • C, 3 h). This can be ascribed to the abundant uncondensed amino groups (-NH, -NH 2 ) in the structure of tri-g-C 3 N 4 . Table 3 summarizes their catalytic activities at the lower temperature of 60 • C. It can be seen that the activity decreased with the CoO x loading, which can be ascribed to the reduction of basic sites, since Knoevenagel condensation is a typical base-catalyzed reaction. However, the decrease in activity is not as evident. The possible reason is that the reduction of basic sites was small, resulting in sufficient active sites remaining for catalyzing Knoevenagel condensation.

Oxidation-Condensation Tandem Reaction
The catalytic performances of CoO x /tri-g-C 3 N 4 catalysts towards one-pot aerobic oxidation-Knoevenagel condensation tandem reaction were evaluated, the results of which are summarized in Table 4. All were active for the tandem reaction. It can be seen that the conversion of benzyl alcohol increases with the CoO x loading, reaches the maximum at 5CoO x /tri-g-C 3 N 4 , then Catalysts 2020, 10, 712 6 of 10 decreases with further increases of the CoO x content. The trend for activity is exactly the same as that of the oxidation reaction, owing to the much faster rate of condensation as compared with that of oxidation. No intermediates benzaldehyde were found in any cases, indicating the oxidation is the rate-determined step. The benzyl alcohol conversion also increased with the reaction time, and 96.4% yield of benzylmalononitrile was obtained in 6 h over 5CoO x /tri-g-C 3 N 4 . The excellent catalytic performance could be due to its dual function-the redox ability of supported CoO x species combined with the basic sites retained in the tri-g-C 3 N 4 support, which was confirmed by the above results of individual oxidation and condensation reactions. Table 4. Catalyst activity of tandem aerobic oxidation-Knoevenagel condensation reaction.
Knoevenagel condensation tandem reaction were evaluated, the results of which are summarized in Table 4. All were active for the tandem reaction. It can be seen that the conversion of benzyl alcohol increases with the CoOx loading, reaches the maximum at 5CoOx/tri-g-C3N4, then decreases with further increases of the CoOx content. The trend for activity is exactly the same as that of the oxidation reaction, owing to the much faster rate of condensation as compared with that of oxidation. No intermediates benzaldehyde were found in any cases, indicating the oxidation is the rate-determined step. The benzyl alcohol conversion also increased with the reaction time, and 96.4% yield of benzylmalononitrile was obtained in 6 h over 5CoOx/tri-g-C3N4. The excellent catalytic performance could be due to its dual function-the redox ability of supported CoOx species combined with the basic sites retained in the tri-g-C3N4 support, which was confirmed by the above results of individual oxidation and condensation reactions.
It can also be seen that the conversion in individual oxidation over 5CoOx/tri-g-C3N4 at 3 h (87.6%) is very close to that in the tandem reaction (83.2%), indicating that the two types of active sites in 5CoOx/tri-g-C3N4 would not interfere with each other during the tandem reaction. Table 4. Catalyst activity of tandem aerobic oxidation-Knoevenagel condensation reaction. The reusability of 5CoOx/tri-g-C3N4 in the aerobic oxidation-Knoevenagel condensation tandem reaction was also investigated. After the reaction, the catalyst was filtered out from the reaction liquid and then dried for the repeatability test. The results are shown in Figure 5. The conversion of benzyl alcohol decreased continuously but slowly from 96.4% to 84.9% after three regeneration processes, while the selectivity remained intact, indicating that 5CoOx/tri-g-C3N4 is quite stable in the liquidphase tandem reactions.

Catalysts Time (h) Conversion of A (%) Yield of C (%)
tri-g-C 3 N 4 6 2. It can also be seen that the conversion in individual oxidation over 5CoO x /tri-g-C 3 N 4 at 3 h (87.6%) is very close to that in the tandem reaction (83.2%), indicating that the two types of active sites in 5CoO x /tri-g-C 3 N 4 would not interfere with each other during the tandem reaction.
The reusability of 5CoO x /tri-g-C 3 N 4 in the aerobic oxidation-Knoevenagel condensation tandem reaction was also investigated. After the reaction, the catalyst was filtered out from the reaction liquid and then dried for the repeatability test. The results are shown in Figure 5. The conversion of benzyl alcohol decreased continuously but slowly from 96.4% to 84.9% after three regeneration processes, while the selectivity remained intact, indicating that 5CoO x /tri-g-C 3 N 4 is quite stable in the liquid-phase tandem reactions.  Table 5 summarizes the catalytic behavior over various catalysts for this tandem aerobic oxidation-Knoevenagel condensation reaction, which has been reported in previous literature. The high yield of benzylmalononitrile was obtained over our present catalyst under mild reaction conditions, without the use of expensive noble metals, giving it advantages over most of the catalysts reported before. Since Co3O4/C3N4 composites are active for aerobic oxidation of various aromatic  Table 5 summarizes the catalytic behavior over various catalysts for this tandem aerobic oxidation-Knoevenagel condensation reaction, which has been reported in previous literature. The high yield of benzylmalononitrile was obtained over our present catalyst under mild reaction conditions, without the use of expensive noble metals, giving it advantages over most of the catalysts reported before. Since Co 3 O 4 /C 3 N 4 composites are active for aerobic oxidation of various aromatic alcohols bearing either electron-withdrawing groups or electron-donating groups [27], the present catalyst is believed to be applicable for aerobic oxidation-Knoevenagel condensation of various aromatic alcohols besides benzyl alcohol. Note: a "m + n" means step-by-step reaction without separation of intermediate benzaldehyde.

Catalyst Preparation
The s-triazine-based g-C 3 N 4 was synthesized as follows: 5 g melamine was dispersed in 30 mL ethylene glycol to obtain a saturated solution. Hereafter, 60 mL 0.1 M HNO 3 solution was added and stirred for 1 h at room temperature. The resulting mixture was filtered and washed three times with ethanol to remove the remaining nitric acid and ethylene glycol. Afterward, the mixture was dried at 120 • C and put into a covered ceramic crucible, heated under air flow at 5 • C/min up to 400 • C, and maintained for 2 h. The resulting product was cooled and grounded into powder.
The tri-g-C 3 N 4 supported CoO x catalysts were synthesized as follows: 1 g obtained tri-g-C 3 N 4 was put into an aqueous solution with a calculated content of Co(NO 3 ) 2 6H 2 O. The mixture was stirred for 1 h at room temperature. After drying at 120 • C, the resulting solid was calcined under air flow at 2 • C/min up to 300 • C and maintained for 3 h. The obtained product was designated as yCoO x /tri-g-C 3 N 4 , where y stands for the weight percentage of Co added.

Characterization of Catalyst
X-ray diffraction (XRD) was carried out on a D2 PHASER X-ray diffractometer (Brucker, Madison, WI, USA) using nickel-filtered Cu-K α (λ = 0.15418 nm) at 40 kV and 30 mA in the range of 10 • -80 • . Fourier transform infrared spectra (FT-IR) were measured on a Nicolet iS10 spectrometer (ThermoFisher, Waltham, MA, USA). Scanning electron microscopy (SEM) was recorded on a Phenom Prox and by field-emission scanning electron microscopy (Ultra 55, Zeiss, Germany) with an acceleration voltage of 20.0 kV. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Perkin-Elmer PHI 5000C spectrometer (Perkin-Elmer, Boston, MA, USA) using Mg K α radiation (hν = 1253.6 eV) as the excitation source. The C 1s peak at 284.6 eV was used as the reference of the binding energy. The N 2 adsorption isotherms were measured on a Micromeritics ASAP 2000 instrument (Micromeritics, Atlanta, GA, USA) at −196 • C. Specific surface areas of the catalysts were calculated by BET method and pore volumes were calculated by one-point method. Thermogravimetric (TG) analysis was carried out on a thermal analyzer TGA8000 (Perkin-Elmer, Waltham, MA, USA) under the inflow of air, with a ramp rate of 10 • C/min from 20-900 • C.

Basicity Measurement
The surface density of base sites was evaluated by neutralization titration. Typically, a 20 mg sample was put into 10 mL 0.05 mol/L aqueous hydrochloric acid solution. The mixture was stirred for 30 min under N 2 flow. The solid was then removed by filtration and the remaining solution was immediately titrated with a 0.05 mol/L sodium hydroxide solution using phenolphthalein as the indicator.

Catalyst Evaluation
Knoevenagel condensation. In a round bottom flask, 1 mmol benzaldehyde, 2 mmol malononitrile, 10 mL N,N-dimethylformamide, and 50 mg catalyst were added in this order and stirred at 60 • C for 0.5 h with reflux. The reaction products were analyzed using a GC9560 gas chromatograph equipped with a SE-30 capillary column (30 m × 0.25 mm × 0.3 µm).
Oxidation of benzyl alcohol. 0.1 g catalyst, 0.25 mmol benzyl alcohol, and 20 mL N,Ndimethylformamide were added into a round-bottom flask. The mixture was reacted at 80 • C for 3 h with an O 2 stream of 50 mL/min under magnetic stirring. The products were analyzed using a GC9560 gas chromatograph equipped with a SE-30 capillary column (30 m × 0.25 mm × 0.3 µm).
Oxidation-condensation tandem reaction. The tandem reaction of benzyl alcohol and malononitrile was carried out in a round-bottom flask equipped with a reflux condenser under magnetic stirring. Typically, 0.1 g catalyst was added into the mixture of 0.25 mmol benzyl alcohol, 0.5 mmol malononitrile, and 20 mL N,N-dimethylformamide. The reaction was performed at 80 • C for 3 h and 6 h, with an O 2 flow of 50 mL/min. Products were analyzed using a GC9560 gas chromatograph equipped with a SE-30 capillary column (30 m × 0.25 mm × 0.3 µm).

Conclusions
A bifunctional catalyst was prepared by supporting cobalt oxide on s-triazine-based carbon nitride. The abundant ammonia species (especially -NH and -NH 2 groups) on the support not only act as basic active sites for Knoevenagel condensation but also enhance the catalytic activity of Co species in oxidation reactions. This catalyst exhibits excellent catalytic performance in the aerobic oxidation-Knoevenagel condensation tandem reaction without any auxiliary reagent and can easily be reused by simple filtration, for which the activity reduces by only 11.5% after four circles. The catalyst has fantastic potential for applications in industrial production.

Conflicts of Interest:
The authors declare no conflict of interest.