Synthesis of two-dimensional mesoporous carbon nitride under different carbonization temperatures and investigation of its catalytic properties in Knoevenagel condensations

Abstract

A two-dimensional mesoporous carbon nitride (MCN-1) material with tunable surface area, pore volume and nitrogen content has been synthesized by using carbon tetrachloride and ethylenediamine as precursors and SBA-15 as a template. The effect of the carbonization temperature on the textural properties, N content and the types of nitrogen species of the MCN-1 samples was also investigated by several characterization techniques. The results reveal that the higher temperature favors the formation of porous structure during the thermal condensation process. In addition, the nitrogen content and the surface N/C ratio in the samples monotonically decrease with the increase of carbonization temperature. The catalytic activity of MCN-1-T is not only related to its surface nitrogen content but also dependent on the structural diversities of the surface nitrogen species. The MCN-1 sample synthesized at 400 °C shows the higher catalytic activity and stability for Knoevenagel condensations.

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1. Introduction

Carbon nitride (CN) is a well-known and promising material that has drawn considerable attention due to its unique combination of various physicochemical properties, which endows it with versatile applications in many fields such as heterogeneous catalysis, photocatalysis, fuel cells and hydrogen-storage.^1–11 Incorporation of nitrogen into the carbon matrix can offer different types of nitrogen species, e.g. amine, imine and pyridinic nitrogen. This gives rise to the material with intrinsically different basic functions, and thus, the capability of acting as efficient solid base catalyst for various catalytic reactions, such as transesterification of β-keto esters, Friedel–Crafts reactions and Knoevenagel condensations.^1,12,13 Bitter and co-workers reported that the catalytic activity of basic nitrogen-containing carbon nanotubes for the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate was related to their amount of pyridinic nitrogen species.¹⁴ This shows that the catalytic performance of CN materials depends on the type and content of nitrogen species in the carbon matrix.

Compared with bulk CN material, mesostructured carbon nitride (MCN) material possesses more active sites on the surface and higher mass diffusion rate because of its large surface area and nanosized pores.^15–17 Vinu et al. prepared a series of mesoporous carbon nitrides (MCN-1) with tunable pore diameters by using SBA-15 as template and ethylenediamine (EDA) and carbon tetrachloride (CTC) as precursors. They found that MCN-1 can be used as a metal-free catalyst for the Friedel–Crafts reaction of benzene and showed high catalytic activity.^18,19 Zhao et al. synthesized hierarchical MCN spheres using spherical mesostructured cellular silica foams as a hard template and EDA and CTC as precursors. It was found that these MCN spheres showed excellent CO₂ capture performance and recycling stability.¹⁶ It should be noted that these MCN materials mostly have low nitrogen content, i.e. the N/C ratio is less than 0.20, being far lower than the theoretical value of C₃N₄. Therefore, it is necessary to increase the nitrogen content in the MCN. The nitrogen atoms in the wall structure of MCN exist in the form of amine, imine and/or pyridinic nitrogen species, which determine the basic character, and thus, the basic catalytic performance of the materials. Many researchers tried to improve the nitrogen content of the MCN materials by using a single-molecule precursor with high nitrogen content and a template with small nanoparticles size,^15,20 and found that the higher carbonization temperature led to a lower nitrogen content due to the low thermal stability of nitrogen species.²⁰ However, few reports studied the effects of carbonization temperature on the textural properties, structural diversities and amounts of nitrogen species, and basic catalytic properties of MCN-1 materials.

Knoevenagel condensation is one of the well-known reactions for forming carbon–carbon bond between aldehydes/ketones and active methylene-containing compounds.²¹ This reaction is a practical way to attain fine chemical intermediates and therapeutic and pharmacological products.^22–24 It is generally catalyzed by homogenous bases such as ammonia, ammonium salts, pyridine, and primary and secondary amines.^25,26 Although these catalysts show high catalytic activity, the thorny issues associated with the separation and recycling of the catalysts cannot meet the increasing demand on green and sustainable chemical industry.²⁵ Therefore, development of heterogeneous catalysts has been paid much attention. These materials may have some advantages in achieving high selectivity to desired product, facilitating the separation and recovery of the catalyst from the reaction mixture and lowering the operation cost.²⁷ Thus, a number of solid base catalysts such as silica supported amines,²⁸ nitrided zeolites²⁹ and metal–organic frameworks³⁰ have been prepared. However, these base catalysts may still suffer from certain deficiencies, e.g. partial blockage of the pores by the basic guests and the possible leaching of the basic components during the reaction.³¹ Hence, it is highly desirable to develop an effective heterogeneous catalyst for Knoevenagel condensations. Recently, MCN materials have been extensively used as solid base catalysts for Knoevenagel condensations, since they can somehow overcome these drawbacks and show good catalytic performance resulting from their high surface area and basicity of nitrogen-containing groups.³²

The aim of this study is to reveal the effect of carbonization temperatures of MCN-1 materials on their textural properties, structural diversities and amounts of nitrogen species, and catalytic activity for Knoevenagel condensations. The catalytic activity of MCN-1 materials for the Knoevenagel condensation of benzaldehyde or acetone and malononitrile was also discussed in details. This study may shed a new light on the understanding of the effects of textural properties, and structural diversities and amounts of nitrogen species of MCN-1 materials on its catalytic properties in Knoevenagel condensation reactions.

2. Experimental

2.1 Materials

Pluronic P123 was purchased from Sigma-Aldrich. Decane was obtained from Alfa Aesar China (Tianjin) Co., Ltd. Absolute ethanol, tetraethyl silicate (TEOS), hydrochloric acid (HCl), ethylenediamine, carbon tetrachloride, benzaldehyde, malononitrile, acetone and sodium hydroxide (NaOH) were all obtained from Sinopharm Chemical Reagent Co., Ltd. All reagents were used as received without further purification.

2.2 Synthesis of mesoporous silica SBA-15 and ordered mesoporous carbon nitride materials

SBA-15 was synthesized following the previous procedures reported by Zhao et al.³³ In a typical batch, specific amounts of Pluronic P123 were mixed with water and 2 M HCl solution at 38 °C under the stirring condition. Then, TEOS was dropwise added to the above solution. The mixture was further stirred for 24 h at 38 °C. After that, the mixture was hydrothermally treated at 100 °C for 24 h in an oven. Finally, the solid product was filtered, washed and dried at 80 °C in the oven. The template was removed by calcination at 550 °C for 10 h.

The ordered mesoporous carbon nitride materials were prepared according to the method reported by Vinu et al.¹⁹ Typically, 1 g of the calcined SBA-15 was first added into the previously well-mixed solution containing 6.0 g of CTC and 2.7 g of EDA. Then, the mixture was refluxed and stirred at 90 °C for 6 h, followed by drying at 80 °C overnight. The obtained dark-brown solid was ground into fine powder and transferred into a crucible, and heated from room temperature to target temperature and kept for 5 h under nitrogen atmosphere. Afterwards, the as-synthesized dark powder was treated with dilute sodium hydroxide solution at 120 °C for 24 h, followed by filtration and washing with water and ethanol several times. Finally, the dark sample was dried in oven at 100 °C for 12 h. The resultant mesoporous CN material was designated as MCN-1-T (T representing the carbonization temperatures ranging from 300 °C to 800 °C). Attempt to prepare MCN-1-200 by the same method failed maybe due to the low carbonization temperature.

2.3 Characterization

Wide-angle X-ray powder diffraction (XRD) patterns were collected on a Rigaku MiniFlex II desktop X-ray diffractometer with CuKα radiation (λ = 0.154 nm, 30 kV, 15 mA). Small-angle XRD patterns (SXRD) were recorded on a Bruker D8 ADVANCE X-ray diffractometer with Ni-filtered CuKα radiation (λ = 0.154 nm, 40 kV, 40 mA). Thermogravimetric analysis (TGA) of MCN-1-T was performed on a Setaram Setsys Evolution 16/18 analyzer (France) at a heating rate of 10 °C min⁻¹ in an argon flow to test their thermal stability. Nitrogen sorption isotherms were measured at −196 °C on a TriStar II 3020 volumetric adsorption analyzer manufactured by Micromeritics Instrument Corporation. Before the measurement, the samples were outgassed at 150 °C for 8 h in order to remove and/or desorb impurities and moisture on their surfaces. The specific surface areas of the samples were calculated using the Brunauer–Emmett–Teller (BET) method within the relative pressure range of 0.05–0.30. The pore diameter, pore volume and pore size distribution were determined from the adsorption branch of the nitrogen isotherms using the Barrett–Joyner–Halenda (BJH) method. The morphology of the solid was identified on a JEOL JSM-7001F field emission scanning electron microscope (SEM). High resolution transmission electron microscopy (HRTEM) images were measured on a JEOL JEM-2100F transmission electron microscope operating at 200 kV. Fourier transform infrared (FT-IR) spectra of the samples were collected in transmission mode at room temperature using the KBr self-supported pellet technique on a Bruker Tensor 27 spectrometer in the region of 400–4000 cm⁻¹. The mass ratio of the sample to KBr was 1 to 300. Raman spectra were recorded on a laser Raman spectrometer (Renishaw inVia Reflex, U.K.) at room temperature using the 532 nm line of an argon ion laser as the excitation source. X-ray photoelectron spectra (XPS) were measured on an AXIS ULTRA DLD spectrometer equipped with a monochromatic AlKα radiation source. This analysis was used to identify the chemical states and estimate the surface contents of nitrogen-containing species in the catalysts. The carbonaceous C 1s line (284.8 eV) was used as the reference to calibrate the binding energies. Elemental analyses were conducted on an Elementar Vario EL CUBE elemental analyzer.

2.4 Catalytic tests

The catalytic properties of the prepared MCN-1-T materials were tested for the Knoevenagel condensation of benzaldehyde or acetone and malononitrile. The reaction was conducted in a Teflon-lined stainless steel autoclave without adding any additional solvent. The reaction conditions were as follows: 1 mmol of benzaldehyde or acetone, 3 mmol of malononitrile, 20 mg of catalyst, 40 °C. The products were analyzed by a Shimadzu GC-2014C gas chromatograph equipped with a 60 m DB-1 capillary column and a flame ionization detector. After each reaction run, the used catalyst was washed with water and ethanol, and dried overnight at 100 °C for next running.

3. Results and discussion

3.1 Characterization results

The pore structure orderings of MCN-1-T materials and their parent silica templates were investigated by powder XRD technique. Fig. 1A shows the SXRD patterns of SBA-15 and MCN-1-T materials. The parent SBA-15 template exhibits three well-resolved peaks that can be indexed as (100), (110), and (200) facets of p6mm symmetry.³³ All the MCN-1-T samples show a diffraction peak at 2θ of 1.10–1.20°, which is similar to that of SBA-15 mesoporous silica template, and indexed as (100) plane. This indicates that the MCN-1 materials possibly possess a two-dimensional mesoporous structure replicated from the SBA-15 template. It is clear that increase of the carbonization temperature increases the intensity of this peak, showing that the structural order of the MCN-1 material increases with the carbonization temperature. The wide-angle XRD patterns of MCN-1-T samples (Fig. 1B) exhibit a single broad diffraction peak near 25.3°, which is attributed to the (002) plane, and characteristic of interplanar stacking peaks of aromatic systems in graphitic materials.^19,34 The d spacing calculated from the (002) plane of MCN-1-T samples is around 0.35 nm, being similar to the value obtained from nonporous carbon nitrides materials. This indicates that the wall structures of these samples are composed of carbon and nitrogen that are arranged in a turbostratic form.

Fig. 1 Small-angle XRD patterns (A) of SBA-15 (a) and MCN-1-T (T = 300 (b), 400 (c), 500 (d), 600 (e), 700 (f), and 800 (g)) samples, and wide-angle XRD patterns (B) of MCN-1-T samples.

To investigate the thermal stability of MCN-1-T samples, TGA profiles were measured (Fig. 2). The weight loss below 200 °C is attributed to the desorption of water molecules in the pore and/or on the surfaces of the MCN-1 material. The strong weight loss takes place when the temperature is higher than the carbonization temperature for all the samples, and the lost weight significantly decreased with increasing carbonization temperature. This shows that the thermal stability of MCN-1-T materials highly depends on their carbonization temperature, and it is stable below the carbonization temperature. The weight loss occurred at the temperature higher than carbonization temperature may be caused by the thermal decomposition of the samples into ammonia and nitrogen,¹⁶ and the evaporation of carbon species.^35–37

Fig. 2 TGA profiles of MCN-1-T samples.

The nitrogen sorption isotherms and BJH pore-size distributions of the samples are displayed in Fig. 3. SBA-15 shows a type IV curve with a steep H1 hysteresis loop at P/P₀ = 0.65–0.80, indicative of a typical mesoporous structure with large pore size and narrow pore size distribution (PSD). Table 1 shows that the surface area and total pore volume of SBA-15 reach 1015.5 m² g⁻¹ and 1.07 cm³ g⁻¹ respectively. All the MCN-1 materials, like SBA-15, also display type IV sorption isotherms with H1 hysteresis loops, implying the presence of mesopores. The MCN-1-T samples carbonized at different temperatures show significant differences in the mesostructures. The MCN-1-300 sample has a very low nitrogen adsorption capacity, while MCN-1-400 exhibits a broader capillary condensation step, indicative of a slight disordered mesoporous structure. In addition, the sorption hysteresis loop of MCN-1-400 located at P/P₀ of 0.4–0.85, suggesting that the pore size of MCN-1-400 should be smaller than that of SBA-15. In contrast, the other four MCN-1-T samples exhibit two capillary condensation steps in the sorption isotherms. One is at a low relative pressure and the other is at a high relative pressure region. This indicates that MCN-1 materials prepared at high carbonization temperatures possess ordered mesoporous structures with bimodal pores, as confirmed by the BJH pore-size distribution (Fig. 3b). Two peaks were observed in the PSD curves of the samples carbonized at temperature higher than 500 °C. The first peak near the 4 nm may come from the small mesopores formed after dissolution of the silica template, whereas the other peak should be attributed to the partial collapse of MCN-1 mesostructures and interparticle voids. It is worth noting that the pore diameters of MCN-1 samples increase with the carbonization temperature.

Fig. 3 Nitrogen sorption isotherms (a) and BJH pore-size distributions (b) of MCN-1-T samples.

Table 1 Textural properties and chemical compositions of MCN-1-T and SBA-15 samples

Sample	SBET^a (m^2 g^−1)	Vpore^b (cm^3 g^−1)	Pore diameter^c (nm)		Composition (wt%)
			Primary mesopore diameter	Large mesopore diameter	C	N	H	Other possible elements (Si, O, Cl and Na)
a BET surface area.b Total pore volume obtained at P/P0 = 0.99.c Most probable pore size.
MCN-1-300	19.4	0.016	—	—	56.91	24.15	3.80	15.14
MCN-1-400	306.4	0.22	—	6.27	57.29	23.52	3.25	15.94
MCN-1-500	491.4	0.42	3.36	7.30	61.83	21.69	2.94	13.54
MCN-1-600	618.4	0.67	3.56	8.08	65.76	15.75	2.55	15.94
MCN-1-700	790.3	0.95	3.71	9.27	68.54	12.97	2.22	16.27
MCN-1-800	711.4	0.93	3.92	9.41	75.67	8.69	1.45	14.19
SBA-15	1015.5	1.07	8.06	—	—	—	—	—

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The textural parameters, including the specific surface area, pore volume and pore diameter of MCN-1-T samples are given in Table 1. MCN-1-300 shows a low surface area (19.4 m² g⁻¹) and pore volume (0.016 cm³ g⁻¹). This could be attributed to the pore collapse during the removal of silica template as a result of low polymerization degree of the formed mesoporous silica–carbon nitride composites. Nevertheless, an increase in the carbonization temperature led to an increase in the surface areas and pore volumes. When the carbonization temperature was increased to 700 °C, the surface area and pore volume reached the highest values of 790.3 m² g⁻¹ and 0.95 cm³ g⁻¹ respectively. Thus, it can be drawn a conclusion that high temperature favors the formation of porous structure during the thermal condensation process.

The SEM images of MCN-1-T samples are shown in Fig. 4. All the samples exhibit a rod-like morphology with uniform particles, the diameters of which are around 200 nm, being similar to those of the parent SBA-15 templates. This shows that the replication process of MCN-1 from SBA-5 template is successful and the morphology of MCN-1 samples is highly similar to that of the SBA-15 even after SBA-15 was removed. The HRTEM images of MCN-1-400, MCN-1-600 and MCN-1-800 are shown in Fig. 5. Hexagonally arrayed mesostructure was clearly observed for all the samples, and the structural order increases with the carbonization temperature. This is consistent with the results obtained with powder XRD patterns. The pore diameters obtained from the HRTEM images are similar to those calculated from nitrogen sorption isotherm.

Fig. 4 SEM images of MCN-1-300 (a), MCN-1-400 (b), MCN-1-500 (c), MCN-1-600 (d), MCN-1-700 (e), and MCN-1-800 (f).

Fig. 5 HRTEM images of MCN-1-400 (a), MCN-1-600 (b) and MCN-1-800 (c).

Fig. 6 shows the FT-IR spectra of MCN-1-T samples. All the samples exhibit three intense bands around 1260 cm⁻¹, 1610 cm⁻¹ and 3420 cm⁻¹, which are corresponding to the aromatic C–N stretching vibration, aromatic ring modes, and the stretching mode of the N–H groups in the aromatic ring and/or the O–H stretching vibration of residual water molecules.^13,19 This suggests that the MCN-1-T samples are mainly composed of pyridine and benzene rings interconnected by nitrogen atoms.

Fig. 6 FTIR spectra of MCN-1-T samples.

The relative intensity ratio of D-band to G-band (I_D/I_G) in the Raman spectrum is a measurement of the ordering of nitrogen-containing carbon solid.³⁸ Fig. 7 shows, as an example, the Raman spectrum of MCN-1-400 since all the MCN-1 samples exhibit similar spectra. Two intense bands were observed at 1371 cm⁻¹ and 1575 cm⁻¹, which are ascribed to the D (disordered) and G (graphitic) bands of the sp²-based carbon. The presence of the D band is characteristic of some disordered graphitic carbons in the walls of material.^22,39 Fig. 7 shows that the intensity of the G band is higher than that of the D band (I_D/I_G = 0.83), evidencing a high graphitization degree in the MCM-1-400 structure. This reveals that many sp²-hybridized C species are present in the pore walls, being consistent with the XRD results.

Fig. 7 Raman spectrum of MCN-1-400.

In order to further investigate the chemical and electronic states of nitrogen and carbon atoms in the MCN-1-T samples, XPS analysis (Fig. 8) was carried out. It should be mentioned that all the MCN-1-T samples show similar C 1s, N 1s and O 1s spectra, indicative of similar surface compositions. For simplicity, only the C 1s and O 1s spectra of MCN-1-300 are presented in Fig. 8b and c. The XPS survey spectrum of the MCN-1-300 sample confirms that its surface is mainly composed of C, N and O atoms (Fig. 8a). The O species may come from the moisture, ethanol and atmospheric O₂ or CO₂ adsorbed on the surface,¹⁹ and partial oxidation of the precursors.¹⁶ The C 1s spectrum (Fig. 8b) of MCN-1-300 was divided into three peaks at 288.6, 285.6 and 284.4 eV. The intense peak at 284.4 eV results from the pure graphitic species in the amorphous CN matrix,^12,40,41 while the peak located at 285.6 eV can be assigned to the sp² C atoms bonded to N inside the aromatic structures.^42,43 As for the peak with the binding energy at 288.6 eV, it could be ascribed to the presence of sp² C atoms bonded to aliphatic amine (–NH₂ or –NH–) in the aromatic rings.^12,44

Fig. 8 XPS survey spectrum (a) and deconvolved C 1s (b), and O 1s (c) of MCN-1-300 and N 1s spectra of MCN-1-T samples (d–i) (T = 300 (d), 400 (e), 500 (f), 600 (g), 700 (h), and 800 (i)).

The N 1s spectra (Fig. 8d–i) indicate that several types of nitrogen species with different electronic states exist in the sample. Two peaks were observed at 397.8 and 399.5 eV in the N 1s spectrum of MCN-1-300 sample (Fig. 8d). The one at 399.5 eV is ascribed to the N atoms trigonally bonded to all sp² carbons or bonded with the sp² carbon atoms and hydrogen atoms,^13,19 viz. tricoordinated nitrogen species and/or amino groups. The other one at 397.8 eV arises from the sp² N atoms bonded to C atoms in aromatic rings, i.e. pyridine nitrogen.^16,19 The surface N/C ratio, the relative concentration of the functional groups in the C 1s, O 1s and N 1s, and the relative fractions of different types of nitrogen species were estimated by deconvolving the XPS spectra, and the results are summarized in Table 2. The surface nitrogen content gradually decreases with the carbonization temperature due to release of more nitrogenous volatiles at higher carbonization temperature. MCN-1-300 shows the highest surface N/C ratio of 0.34. The peak analyses of N 1s show that the MCN-1-T samples all contain several types of nitrogen species, but with different amounts. The fraction of pyridine nitrogen decreases but the proportion of the tricoordinated nitrogen and/or the amino groups increases with increasing carbonization temperature. This suggests that the pyridine nitrogen should be chemically transformed into nitrogen species with higher binding energies through the condensation reactions during the carbonization treatment. As a result, the surface pyridine nitrogen content gradually decreases with the increase in the carbonization temperature, but the content of the surface tricoordinated nitrogen and/or amino groups increases until the carbonization temperature reached 400 °C, despite that a further increase in the carbonization temperature led to a significant decrease.

Table 2 Relative surface concentrations of C, O and N of MCN-1-T samples obtained from XPS data

Sample	Relative surface concentrations (at.%) and relative fraction of nitrogen species^a (%)					Surface N/C ratio
	C 1s	N 1s	O 1s	Pyridine nitrogen	Tricoordinated nitrogen and/or amino groups
a Values in parentheses are relative fractions of different types of nitrogen species.
MCN-1-300	67.3	22.6	10.1	8.6 (37.9)	14.0 (62.1)	0.34
MCN-1-400	69.1	22.6	8.4	8.3 (36.9)	14.3 (63.1)	0.33
MCN-1-500	71.5	21.0	7.5	7.6 (36.2)	13.4 (63.8)	0.29
MCN-1-600	73.7	16.7	9.6	6.0 (35.8)	10.7 (64.2)	0.23
MCN-1-700	80.2	11.9	7.9	4.1 (34.8)	7.8 (65.2)	0.15
MCN-1-800	82.8	8.3	8.9	2.6 (31.7)	5.7 (68.3)	0.10

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It should be noted that the depletion rates of nitrogen and carbon atoms are different during the high temperature carbonization process, according to the C, H, and N elemental analyses (Table 1). The nitrogen content continuously decreases with the carbonization temperature, and all the samples have lower nitrogen content than the theoretical carbon nitride. This can be accounted for by the low thermodynamic stability of nitrogen species in the carbon matrix and the preferable state of nitrogen species at higher temperature being nitrogen molecules.¹³ Trace amounts of H comes from either the moisture or ethanol adsorbed on the surface, or the amino groups on the MCN-1. The hydrogen contents correspondingly decrease with increasing carbonization temperature because most of hydrogen atoms are bound to the edges and defects of the layers. It can be seen from Table 1 that other elements such as Si, Na, O or Cl are also present in the MCN-1-T after removal of silica template. The XPS analysis results reveal the presence of only a small amounts of silica (ca. 1 wt%), Na and Cl but a large amount of O in the obtained carbon nitride matrix (Fig. 8a).

3.2 Catalytic activity of MCN-1-T in Knoevenagel condensation

The catalytic properties of MCN-1-T were investigated by using Knoevenagel condensations of benzaldehyde or acetone and malononitrile as model reactions, and the obtained results are shown in Table 3. It should be pointed out that the water was not removed during the reaction. The blank test (without catalyst) gave a quite low benzaldehyde or acetone conversion (entries 1 and 8). In contrast, all the MCN-1-T samples show high catalytic activity and selectivity. Irrespective of this, the catalytic activity of MCN-1-T is highly dependent on its carbonization temperature. With respect to the Knoevenagel condensation of benzaldehyde and malononitrile, MCN-1-300 showed a benzaldehyde conversion of 95.9%, MCN-1-400 gave a benzaldehyde conversion of 99.3% (entries 2 and 3), but a further increase in the carbonization temperature resulted in a decrease of the benzaldehyde conversion. Nonetheless, the MCN-1-800 sample carbonized at 800 °C still exhibited high catalytic activity, giving a benzaldehyde conversion of 91.5%. This shows that all the MCN-1-T samples can effectively catalyze this condensation reaction. When acetone, instead of benzaldehyde, was used as the starting material, a similar effect of the carbonization temperature on the catalytic activity of MCN-1-T was observed (entries 9–14 in Table 3), although they show a lower catalytic activity for the condensation of acetone with malononitrile due to the lower reactivity of ketones than aldehydes (entries 10–14).⁴⁵ Nevertheless, even for such a tough reaction, MCN-1-400 still gave an acetone conversion of 92.1%. Table 3 shows that the product (benzylidene malononitrile and isopropylidenemalononitrile) selectivities in both the condensations are close to 100% for all the MCN-1-T samples.

Table 3 Catalytic results of MCN-1-T for different Knoevenagel condensation reactions

Entry	Catalyst	X^a (%)	S^b (%)
a Benzaldehyde or acetone conversion.b Selectivity to benzylidene malononitrile (I) or isopropylidenemalononitrile (II). Reactions conditions: 1 mmol benzaldehyde or acetone, 3 mmol malononitrile, 20 mg catalyst, 40 °C, 2 h.
I. Benzaldehyde + malononitrile
1	None	9.8	89.5
2	MCN-1-300	95.9	100
3	MCN-1-400	99.3	100
4	MCN-1-500	94.9	100
5	MCN-1-600	94.1	100
6	MCN-1-700	92.0	100
7	MCN-1-800	91.5	100
II. Acetone + malononitrile
8	None	0.17	100
9	MCN-1-300	88.6	100
10	MCN-1-400	92.1	100
11	MCN-1-500	83.5	100
12	MCN-1-600	30.8	100
13	MCN-1-700	25.2	100
14	MCN-1-800	14.0	100

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As mentioned above, Knoevenagel condensation reactions can be catalyzed by the basic catalysts, and the catalytic activity is related to the amounts and types of active sites and textural properties of the catalysts. By comparing Table 1 with Table 3, it can be found that MCN-1-400 shows higher activity than MCN-1-300 although its nitrogen content is slightly lower than that of MCN-1-300. This is certainly related to its high surface area or developed porosity. However, the catalytic activity of the MCN-1-T samples monotonically decreases with increasing carbonization temperatures from 400 °C to 800 °C (entries 3–7 and entries 10–14), although their surface areas are all much higher than that of MCN-1-400. This implies that the textural properties of the catalysts should be not the dominant factor determining the catalytic activity for the Knoevenagel condensations, as confirmed by the fact that MCM-1-300 shows significantly higher catalytic activity than the samples carbonized at ≥ 600 °C, but its surface area is far lower. This suggests that the catalytic activity of MCN-1-T should be also related to the amount of nitrogen species. In addition, the XPS analyses show that several types of nitrogen species exist on the surface of MCN-1-T materials. Xu et al. reported that tertiary N atoms in the three-dimensional mesostructured carbon nitride have a positive effect on the catalytic activity for the reaction of benzaldehyde and malononitrile.² Indeed, it was found that MCN-1-400 has a higher content of tricoordinated nitrogen and/or amino groups than MCN-1-300 (Table 2). Thus, the higher catalytic activity of MCN-1-400 than MCN-1-300 may be also attributed to its higher content of the tricoordinated nitrogen and/or amino groups on the surface. In contrast, when the carbonization temperature is higher than 400 °C, both the bulk and surface nitrogen contents decrease with the carbonization temperature. This is consistent with the decrease of the catalytic activity. This shows that the catalytic activity of MCN-1-T is not only dependent on the amount of available basic sites on the surface, but also related to the types of nitrogen species. The tricoordinated nitrogen and amino groups might show higher catalytic activity than the other type of nitrogen species.

To further explore the active sites, the benzaldehyde and acetone conversions were plotted against the surface concentrations of tricoordinated nitrogen and/or amino groups, and total nitrogen species (Fig. 9). It is clear that the catalytic activities of MCN-1-T samples are in a good agreement with the contents of the tricoordinated nitrogen and/or amino groups but slightly different from those of total surface nitrogen species, further confirming that tricoordinated nitrogen and amino groups are more active than other type of nitrogen species.

Fig. 9 Plots for the benzaldehyde and acetone conversions obtained in the Knoevenagel condensations of benzaldehyde or acetone and malononitrile against the surface concentrations of tricoordinated nitrogen and/or amino groups (a) and total nitrogen species (b).

To investigate the catalytic stability of MCN-1-T, MCN-1-400 as an example was recycled to catalyze the reaction between benzaldehyde and malononitrile. It was found within five repeated runs with regeneration that the obtained benzaldehyde conversions are all higher than 90% with benzylidene malononitrile selectivity of about 100%, showing that MCN-1-T materials are stable heterogeneous catalysts and can be reused for Knoevenagel condensation reactions.

4. Conclusions

A series of two-dimensional mesoporous carbon nitride (MCN-1) materials have been prepared by using carbon tetrachloride and ethylenediamine as precursors and SBA-15 as a template. By altering the carbonization temperature between 300 °C and 800 °C, the MCN-1 samples were obtained with tunable surface area, pore volume, and structural diversities and amounts of nitrogen species. Among the prepared MCN-1-T samples, MCN-1-400 shows the highest catalytic activity for Knoevenagel condensations, giving benzaldehyde and acetone conversions of 99.3% and 92.1% respectively in the reactions of benzaldehyde or acetone and malononitrile. The catalytic activity of MCN-1-T is not only related to its surface nitrogen content but also dependent on the structural diversities of the surface nitrogen species.

Acknowledgements

This work is financially supported by the National Basic Research Program of China (2011CB201400), the National Natural Science Foundation of China (21273263, 21273264), the Natural Science Foundation of Shanxi Province (2012011005-2), and the Research Project Supported by Shanxi Scholarship Council of China (2014-102).

Notes and references

1. F. Z. Su, M. Antonietti and X. C. Wang, Catal. Sci. Technol., 2012, 2, 1005–1009.
2. J. Xu, K. Shen, B. Xue, Y. X. Li and Y. Cao, Catal. Lett., 2013, 143, 600–609.
3. Y. Wang, H. R. Li, J. Yao, X. C. Wang and M. Antonietti, Chem. Sci., 2011, 2, 446–450.
4. J. S. Zhang, M. W. Zhang, G. G. Zhang and X. C. Wang, ACS Catal., 2012, 2, 940–948.
5. X. C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen and M. Antonietti, Nat. Mater., 2009, 8, 76–80.
6. S. Kumar, T. Surendar, B. Kumar, A. Baruah and V. Shanker, RSC Adv., 2014, 4, 8132–8137.
7. F. Z. Su, S. C. Mathew, G. Lipner, X. Z. Fu, M. Antonietti, S. Blechert and X. C. Wang, J. Am. Chem. Soc., 2010, 132, 16299–16301.
8. P. F. Zhang, Y. Wang, J. Yao, C. M. Wang, C. Yan, M. Antonietti and H. R. Li, Adv. Synth. Catal., 2011, 353, 1447–1451.
9. P. F. Zhang, Y. Wang, H. R. Li and M. Antonietti, Green Chem., 2012, 14, 1904–1908.
10. J. Liang, Y. Zheng, J. Chen, J. Liu, D. Hulicova-Jurcakova, M. Jaroniec and S. Z. Qiao, Angew. Chem., Int. Ed., 2012, 51, 3892–3896.
11. S. Giraudet, Z. H. Zhu, X. D. Yao and G. Q. Lu, J. Phys. Chem. C, 2010, 114, 8639–8645.
12. J. Xu, T. Chen, X. Wang, B. Xue and Y. X. Li, Catal. Sci. Technol., 2014, 4, 2126–2133.
13. S. N. Talapaneni, S. Anandan, G. P. Mane, C. Anand, D. S. Dhawale, S. Varghese, A. Mano, T. Mori and A. Vinu, J. Mater. Chem., 2012, 22, 9831–9840.
14. S. van Dommele, K. P. de Jong and J. H. Bitter, Top. Catal., 2009, 52, 1575–1583.
15. X. Jin, V. V. Balasubramanian, S. T. Selvan, D. P. Sawant, M. A. Chari, G. Q. Lu and A. Vinu, Angew. Chem., Int. Ed., 2009, 48, 7884–7887.
16. Q. Li, J. P. Yang, D. Feng, Z. X. Wu, Q. L. Wu, S. S. Park, C. S. Ha and D. Y. Zhao, Nano Res., 2010, 3, 632–642.
17. S. S. Park, S. W. Chu, C. F. Xue, D. Y. Zhao and C. S. Ha, J. Mater. Chem., 2011, 21, 10801–10807.
18. A. Vinu, K. Ariga, T. Mori, T. Nakanishi, S. Hishita, D. Golberg and Y. Bando, Adv. Mater., 2005, 17, 1648–1652.
19. A. Vinu, Adv. Funct. Mater., 2008, 18, 816–827.
20. S. N. Talapaneni, G. P. Mane, A. Mano, C. Anand, D. S. Dhawale, T. Mori and A. Vinu, ChemSusChem, 2012, 5, 700–708.
21. F. Freeman, Chem. Rev., 1980, 80, 329–350.
22. R. M. Freire, A. H. de Morais Batista, A. G. de Souza Filho, J. M. Filho, G. D. Saraiva and A. C. Oliveira, Catal. Lett., 2009, 131, 135–145.
23. G. W. Li, J. Xiao and W. Q. Zhang, Green Chem., 2011, 13, 1828–1836.
24. X. F. Zhang, E. S. M. Lai, R. Martin-Aranda and K. L. Yeung, Appl. Catal., A, 2004, 261, 109–118.
25. Y. Kubota, Y. Nishizaki and Y. Sugi, Chem. Lett., 2000, 29, 998–999.
26. S. Balalaie, M. Sheikh-Ahmadi and M. Bararjanian, Catal. Commun., 2007, 8, 1724–1728.
27. J. Weitkamp, M. Hunger and U. Rymsa, Microporous Mesoporous Mater., 2001, 48, 255–270.
28. K. M. Parida and D. Rath, J. Mol. Catal. A: Chem., 2009, 310, 93–100.
29. H. K. Min, S. H. Cha and S. B. Hong, Chem. Commun., 2013, 49, 1115–1117.
30. A. R. Burgoyne and R. Meijboom, Catal. Lett., 2013, 143, 563–571.
31. D. T. On, D. Desplantier-Giscard, C. Danumah and S. Kaliaguine, Appl. Catal., A, 2001, 222, 299–357.
32. M. B. Ansari, H. L. Jin, M. N. Parvin and S. E. Park, Catal. Today, 2012, 185, 211–216.
33. D. Y. Zhao, Q. S. Huo, J. L. Feng, B. F. Chmelka and G. D. Stucky, J. Am. Chem. Soc., 1998, 120, 6024–6036.
34. Y. Qiu and L. Gao, Chem. Commun., 2003, 2378–2379.
35. H. Montigaud, B. Tanguy, G. Demazeau, I. Alves, M. Birot and J. Dunogues, Diamond Relat. Mater., 1999, 8, 1707–1710.
36. E. G. Gillan, Chem. Mater., 2000, 12, 3906–3912.
37. D. R. Miller, J. J. Wang and E. G. Gillan, J. Mater. Chem., 2002, 12, 2463–2469.
38. M. S. Dresselhaus, F. Villalpando-Paez, G. G. Samsonidze, S. G. Chou, G. Dresselhaus, J. Jiang, R. Saito, A. G. Souza Filho, A. Jorio, M. Endo and Y. A. Kim, Phys. E, 2007, 37, 81–87.
39. S. Jun, S. H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna and O. Terasaki, J. Am. Chem. Soc., 2000, 122, 10712–10713.
40. D. Marton, K. J. Boyd, A. H. Albayati, S. S. Todorov and J. W. Rabalais, Phys. Rev. Lett., 1994, 73, 118–121.
41. Y. H. Cheng, B. K. Tay, S. P. Lau, X. Shi, X. L. Qiao, J. G. Chen, Y. P. Wu, Z. H. Sun and C. S. Xie, J. Mater. Res., 2002, 17, 521–524.
42. P. Srinivasu, A. Vinu, S. Hishita, T. Sasaki, K. Ariga and T. Mori, Microporous Mesoporous Mater., 2008, 108, 340–344.
43. A. Vinu, P. Srinivasu, D. P. Sawant, T. Mori, K. Ariga, J. S. Chang, S. H. Jhung, V. V. Balasubramanian and Y. K. Hwang, Chem. Mater., 2007, 19, 4367–4372.
44. J. Xu, K. Z. Long, T. Chen, B. Xue, Y. X. Li and Y. Cao, Catal. Sci. Technol., 2013, 3, 3192–3199.
45. B. M. Choudary, M. L. Kantam, P. Sreekanth, T. Bandopadhyay, F. Figueras and A. Tuel, J. Mol. Catal. A: Chem., 1999, 142, 361–365.

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