Aminoguanidine Derived N‐Rich Mesoporous Carbon Nitrides with Tunable Nitrogen Contents for Knoevenagel Condensation

Nitrogen‐rich carbon nitrides are desired materials for base‐catalysed transformations; however, their synthesis is challenging due to the volatile nature of N at high temperatures. Herein, we report on the temperature‐controlled synthesis of ordered N‐rich mesoporous carbon nitrides (MCNs) via pyrolysis of aminoguanidine by using SBA‐15 as a hard template. The properties and the nitrogen content of the materials were tuned by varying the carbonization temperature in the range of 350–500 °C. At 350 and 400 °C, higher amounts of N could be retained in the MCN framework with the predominant formation of C3N6 having a six‐membered aromatic ring with diamino‐s‐tetrazine moiety, whereas C3N5 with 1‐amino/imino‐1,2,4‐triazole moieties was produced at 450 and 500 °C. The base catalytic activity of MCNs in Knoevenagel condensation of benzaldehyde with malononitrile revealed that the MCN‐400 exhibited the highest catalytic performance by displaying a 96.4 % product yield with toluene as a solvent. The superior catalytic activity of MCN‐400 is attributed to high N content (62.6 wt%), high surface area (235 m2 g−1), and large pore volume (0.74 cm3 g−1). The optimum temperature for obtaining the highest yield of the products is 80 °C, and the catalyst showed good cycling stability for 5 consecutive cycles.


Introduction
Knoevenagel condensation is one of the key approaches uniquely suited for C=C bond formation in synthetic organic chemistry. [1] This reaction has successfully been implemented to obtain α,β-unsaturated acids (commonly known as cinnamic acids), which are key intermediates used in the synthesis of numerous therapeutic drugs and various cosmetics as well as perfumes. [2,3] Homogeneous catalysts in the category of alkali metal alkoxides and organic amines such as pyridine and piperidine have widely been employed for these processes because of their high catalytic activities. However, cumbersome separation and purification protocols need to be followed for such catalysts. Heterogeneous catalysts based on metal oxides, such as alkaline earth metal oxides, metal-doped mesoporous materials, zeolites, and metal oxides supported on solid materials have been widely proposed to overcome the demerits of homogeneous ones. [4][5][6][7][8][9][10] However, these metal oxide-based catalysts are not environmentally friendly and suffer from drawbacks, such as contamination and low catalytic activities. [11,12] Therefore, novel metal-free solid base catalysts are urgent subjects of development for Knoevenagel condensation to synthesize various types of key organic compounds. These novel catalysts must be thermodynamically stable, should excel in catalytic performance, and in addition should also exhibit recyclability. [13] Carbon nitrides (C 3 N 4 ), containing atomic sheets composed of C and N atoms, are non-metallic materials with properties and features suited for various applications, including organic catalysis. [14][15][16][17][18] However, C 3 N 4 lacks sufficient active centres with local electron density because of its synthesis at high carbonization temperatures. Retaining more nitrogen through manipulation of the synthesis temperature is one of the feasible strategies that can improve the catalytic performance of carbon nitrides. [19] Therefore, it is urgent to develop novel nitrogen-rich carbon nitrides to resolve the technological and scientific gaps in their structures with atomic-scale pores surrounded by a large number of edge N atoms, their thermal stability, tunable pore diameter, large surface area, and strong basicity. [20][21][22][23][24] They are expected to be useful for a broad range of applications for metal-free catalysts in environmentally benign organocatalysis, such as Knoevenagel and Claisen-Schmidt condensations. [25][26][27] Along with suitable nitrogen content, porosity is another attractive feature of carbon nitrides that can improve their catalytic performance. A combination of both these features and a highly uniform ordered pore system has recently been realised through the discovery of N-rich ordered mesoporous carbon nitrides (MCNsÀ C 3 N 4 , C 3 N 5, and C 3 N 6 ) synthesized by the use of KIT-6, SBA-15, and MCM-48 with 2D and 3D structures as the hard mesoporous templates. [26,[28][29][30][31] Thermodynamically the most stable C 3 N 4 structure does not have enough edged nitrogen because of high-temperature carbonization; however, recently, syntheses of high nitrogen content MCN species C 3 N 5 (termed as  and C 3 N 6 (termed as MCN-4) have been reported from amino tetrazole and aminoguanidine hydrochloride respectively using KIT-6 or SBA-15 as a hard template. [31][32][33] In this article, we report our synthesis of MCN series by the carbonization of aminoguanidine hydrochloride by using SBA-15 as the template at the temperatures: 350-500°C to know the influences of the carbonization on the properties of the resultant materials. We also investigated the influence of the carbonization temperature on the structure and the nitrogen contents of MCN materials and their influence on Knoevenagel condensation. From the results, it was found that MCN-400 has a high nitrogen content (62.6 wt.%) on the surface and a high surface area (235 m 2 g À 1 ), large pore volume (0.74 cm 3 g À 1 ), and a mesopore size of 7.1 nm, which accounted for its superior catalytic activity for Knoevenagel condensation of benzaldehyde with malononitrile as compared to other MCN materials. The use of such materials could be extended beyond catalysis into other application areas as well.

Results and Discussion
The chemical composition of synthesized MCN materials obtained by CHNS analysis is summarized in Table 1. A decrease in N/C atomic ratio was observed with an increase in carbonization temperature. MCN-350 and 400 had N/C ratio of 1.86 and 1.79, respectively. However, it decreased to 1.59 when the carbonization temperature was increased to 500°C. Depending on the carbonization temperature, the molecular formulae of the materials based on these N/C ratios could possibly be considered as C 3 N 6 , and C 3 N 5 , as shown in Scheme 1. The molecular formula of C 3 N 6 for materials MCN-350 and 400 was also confirmed in the previous reports, [34] which has a sixmembered aromatic ring linked with nitrogen atoms trigonally and containing six nitrogen atoms and a highly stable polymer composed of diamino-s-tetrazine. However, the materials MCN-450 and 500 show decreased N/C ratio of 1.59 which occurs due to the release of unstable nitrogen moieties, resulting in the formulae of C 3 N 5 . The crystallinity of the synthesized MCN materials was determined using powder XRD patterns ( Figure 1a). In low angle XRD (Figure 1a inset), the materials MCN-350, 400, and 450 show well-ordered peaks which can be indexed as (100), (110), and (200) Bragg reflection of a hexagonal mesostructured with the p6mm symmetry, that indicates the highly ordered mesoporous system with linear arrays of hexagonal pores. When observed in detail, there is a reduction in diffraction angle with the increase in carbonization temperature (see the inset in Figure 1a), which indicates the loss of structural order when the carbonization temperature was increased above 450°C. These changes also show the increase in the pore diameter with increasing temperature by the loss of mesostructured order. Overall, the materials MCN-350 and 400°C exhibit the highest order among all materials. Higher angle XRD patterns of MCN materials are shown in Figure 1b. A single broad diffraction peak near 2θ = 26.7°corresponds to (002) plane with a d-spacing~0.33 nm. [21,31] This peak shows the highest intensity for MCN-350, which indicates its highly crystalline nature and a gradual reduction in intensity with increasing carbonization temperature is also observed. With increasing carbonization temperature, the peak shows broadening which could be due to the reduction in the size of the crystallites. These results indicate that a lower carbonization temperature facilitates the formation of a crystalline carbon nitride with high nitrogen content.
The porosity of all materials was investigated using the N 2 sorption technique. Figure 1c shows the N 2 adsorption-desorption isotherms of all the materials and the resulting textural parameters are listed in Table 2. All materials show type IV adsorption isotherm according to the IUPAC classification, and the capillary condensation of small hysteresis in the highpressure region indicates well-ordered mesoporous structure and uniform pore size distribution in all the material. The capillary condensation step is shifted to the higher relative pressure region with an increase in carbonization temperature which is evidence of a larger pore size. This can be correlated with the BJH pore size data given in Table 2. The pore size shows an increasing trend with increasing carbonization temperature for the first three materials, i. e., (MCN-350 (5.7 nm) < MCN-400 (7.1 nm) < MCN-450 (7.2 nm), as shown in Figure 1d and is in accordance with the capillary condensation point and the hysteresis observation in N 2 sorption isotherms. This is possible due to the decomposition of a large amount of aminoguanidine precursor, and hence, the creation of large size mesopores in the structure. However, MCN-500 shows a decrease in pore size to 5.4 nm which could be due to the collapse of the large-size mesopores into a smaller size. Carbonization temperature also plays a key role in controlling the surface area and pore volume of the materials. The material MCN-350 shows a surface area of 175 m 2 g À 1 and a pore volume of 0.36 cm 3 g À 1 , increasing to 235 m 2 g À 1 and 0.74 cm 3 g À 1 for MCN-400. Thereafter, these values get lowered for the other two materials, indicating that higher temperatures may disrupt the porous structure. Among the materials prepared, MCN-400 registered the highest specific surface area and the largest pore volume, revealing a large number of basic active sites in this material.
TGA profiles were measured to identify the thermal stability of MCN materials in a nitrogen atmosphere and the obtained curves are shown in Figure S4. The thermogravimetric data shows two weight loss steps from 50°C to 400°C. The weight loss of less than 1-4 % around 100°C is due to the absorbed moisture present over the surface of the materials. The weight loss at around 400°C arises due to the rapid decomposition step, where the polymerized aminoguanidine materials starts to decompose while increasing the temperature. All materials are stable up to 400°C; however, the MCN-350 and 400 showed slightly higher weight loss than other materials due to higher decomposition of the unreacted constituents or the incomplete polymerized network in the Carbon Nitride framework at a temperature higher than 400°C. Finally, all the materials almost decomposed at around 500-600°C, revealing that the prepared materials are not stable after 500°C. After the complete decomposition of the MCNs, we found that 2-6 % of silica remained in all materials which were silica from the SBA-15 template after HF treatment. The SEM-EDS also showed that 2-6 % of silica was present in the samples, which supports the results of the TGA. The TGA changes in MCN-400 were further confirmed with DTG curve (Figure 2a). The surface functional groups were probed using FT-IR spectroscopy and the obtained spectra of all the MCN materials are shown in Figure 2b. All the materials exhibit a broad vibration peak at 1626 cm À 1 , which is attributed to CÀ N and the peaks at 1327 and 1429 cm À 1 are attributed to C=N and N=N stretching modes in tetrazine moieties. [35] The materials MCN-350 and MCN-400 show a peak at 1324 cm À 1, corresponding to the C=N stretching vibration. The bands at 774-786 cm À 1 , attributed to the N=N tetrazine ring, are more visible for C 3 N 6 than the C 3 N 5 material. [23,36,37] Due to the aromatic ring modes, one more band appearing at 1574 cm À 1 primarily represents a graphite-like CÀ N band mainly due the aromatic ring mode of diamino-s-tetrazine molecules. Further, two peaks were observed at 3174 and 3341 cm À 1 assigned to NÀ H stretching of amino-and imino-(À NH 2 and >  NH) groups. These two peaks are more prominent in MCN-350 and 400, and become weaker with the increase in carbonization temperature. These results suggest the amounts of amino/ imino groups decreased with rising carbonization temperatures. These observations of rich amino/imino groups suggest the high catalytic performances of MCN-350 and 400, particularly MCN-400, in base-catalysed Knoevenagel condensation, because the catalysis starts by the H + elimination from nucleophiles. Carbon nitride materials are rich in nitrogen-containing species and are typically solid bases; therefore, CO 2 -TPD measurement was carried out to explore the basic property of all the MCNs material prepared at different carbonization temperatures. Figure 3 shows the CO 2 TPD profiles of all materials at temperatures < 400°C. All the materials exhibit a peak around 100-250°C. These peaks are due to physical adsorption of CO 2 on basic sites, and the amounts of basic sites can be estimated from peak size. [38,39] The peaks are prominent for the samples carbonized at 350 and 400°C, however, they are small for the samples carbonized at 450 and 500°C. From these results, we can suggest the amounts of basic sites of MCN-350 and 400 are rich among all materials, resulting in their high catalytic performance in Knoevenagel condensation. The surface chemical bonding features of carbon and nitrogen atoms in the materials were investigated using XPS. The obtained XPS survey spectrum (Figure S1) of MCNs material shows the presence of mainly carbon and nitrogen on the surface. A negligible amount of oxygen is also noticed which could be attributed to the atmospheric moisture or CO 2 adsorbed on the surface. The deconvoluted high-resolution C 1s and N 1s spectra are shown in Figures 4a and 4b. The C1s spectra of the materials show the two main peaks at the binding energy position of 289.1 and 288.4 eV and a small third peak at 285.5 eV. The prominent peak at 288.4 eV is identified as the C�N bond of the trigonal CÀ N network, and the peak at 285.5 eV corresponds to the sp 2 carbon atoms. The peak at the highest binding energy position of 289.1 eV can be assigned to the sp 2 -hybridized carbon in the aromatic ring attached to the terminal uncondensed NH 2 group. The N 1s spectra of the prepared materials were de-convoluted into three peaks at 398.5, 399.4, and 400.7 eV, revealing the presence of three types of nitrogen bonding. The lowest energy contribution at 398.5 eV corresponds to the nitrogen atoms bonding to sp 2 carbon atoms in the diamino-s-tetrazine-based CN network. The  highest contribution at 399.4 and 400.7 eV are attributed to the nitrogen trigonally bonded to the carbon atoms and to the N=N, respectively. The N=N bonding is originated from the tetrazine-like ring nitrogen atoms arrangement in the wall of the structure in all materials, even though the quantity of this nitrogen is different in different materials prepared. [23,40,41] It should be noted that the area of the peak at 398.5 eV and 399.4 eV is slightly higher in the materials prepared at 350°C and 400°C as compared to that of the materials prepared at 450°C and 500°C, revealing a large number of basic sites in the former materials. The XPS quantification of elements, carbon and nitrogen (Table S1), showed a very similar trend to the CHN analysis observations, confirming that the two elements are uniformly distributed in the bulk and surface of the materials.
Carbon nitrides have basic properties and are active for the Knoevenagel condensation of aldehydes with nucleophiles such as active methylene compounds. We examined the condensation of benzaldehyde with malononitrile, ethyl cyanoacetate, and dimethyl malonate to know the catalytic properties of materials, MCN from aminoguanidine with different carbonization temperatures. Figure 5a shows Knoevenagel condensation using benzaldehyde (I) and malononitrile (II) over MCN at 80°C in toluene as solvent. MCN-400, carbonized at 400°C, shows superior catalytic activity, and the product yield of benzylidenemalononitrile (III) lies~96.4 %. These high performances could be attributed to the highly moderate basicity of this material as well the highest surface area among all materials. Both of these factors are crucial in providing a greater number of catalytic active sites, leading to good product yield. For comparison purposes, the catalytic activity of MCN-400 was also compared with the bulk CN and MCN without HF washing synthesized at 400°C ( Figure S3). The product yield for these two materials was far lower than MCN-400, probably due to their non-porous structure. From these studies, MCN-400 was chosen as the best material for the base catalysed condensations. Further, the reaction of I and II using MCN-400 as a catalyst was performed in various solvents and it was found that toluene is the best reaction solvent that provides the highest product yield (96.4 %). Methanol, ethanol, dichloromethane, ethyl acetate, and acetonitrile provided lower product yields of 89.3 %, 85.4 %, 64.2 %, 46.1 %, and 17.6 %, respectively ( Figure 5b). The dosage of the catalyst is an important parameter that can influence the reaction. MCN-400 was used in different weight amounts to gauge the optimum amount for obtaining high product yield. It was found that 50 mg of MCN-400 is the suitable amount that yields the highest amount of the product (Figure 5c). Further, the influence of the reaction temperature and time was also carried out, and it was found that 80°C for 4 hrs is the optimum temperature for maximizing the product yield of III ( Figure 5d).
Knoevenagel condensation is highly influenced by the presence of substituents on the substrate, benzaldehydes and therefore, four different p-substituted benzaldehydes including p-NO 2 À , p-ClÀ , p-Me, and p-MeOÀ were studied with malononitrile. The benzaldehydes with p-NO 2 À and p-ClÀ gave higher yields than other benzaldehydes ( Figure 6a). Conversely, the aldehydes with p-MeOÀ , and p-ClÀ gave lower yields than benzaldehyde. These results illustrate that the electron density on carbonyl moieties influences the catalytic performances of Knoevenagel condensation. Probably, low electron density on CO moieties created by electron-withdrawing groups such as Cl À and NO 2 accelerates the catalysis, and the condensation is retarded by rich electron density on CO moieties created by Me or OMe groups. The structure and activity of active methylene compounds are largely influenced by the electronegativities of methylene moieties. To confirm this, three methylene compounds, malononitrile, ethyl cyanoacetate, and diethyl malonate, were tested for Knoevenagal condensation with benzalde-hyde using MCN-400 as the catalyst (Figure 6b). The cyano moieties enhance the catalytic performances in the order of malononitrile > ethyl cyanoacetate > dimethyl malonate, which is the order of electron densities on the methylene moieties. The recovery and reuse of a catalyst are important from a commercial point of view. MCN-400 was subjected to five continuous cycles for Knoevenagel condensation and it was found that the activity dropped by 65.3 % after the fourth cycle and stayed near the same after the fifth cycle ( Figure 6c). This shows that a product yield of~60 % is possible after five cycles. The gradual decrease in the yield could be attributed to the incomplete recovery of the catalyst from the reaction mixture. We expect the decrease could be uplifted by bringing more sophistication into the experiments. Figure 7a shows the removal of the catalyst during the reaction. The normal reaction smoothly finished after 6 h. We removed the catalyst after 1 h of starting the reaction, and continued the reaction using the resultant reaction mixtures; however, there is no further increase in the yield of condensation product even after 6 h. The results mean the condensation occurred by the solid catalyst, MCN400, and no significant catalysis occurred by leached species in separated reaction mixtures. We also examined the TGA of the catalyst after the use for the condensation. The catalyst without the washing shows a dip around 200°C ascribed to volatile organic compounds; however, the decrease disappeared by washing with EtOH and drying at 200°C under vacuum. These results indicate that there is no significant amount of non-volatile deposits during the reaction. FT-IR spectra of the catalyst after the reaction are also shown in Figure S2 which also indicateno significant absorption after EtOH washing. These results prove thatMCN-400 is active for condensation without leaching to the reaction solution and there is no significant deactivation of volatile compounds on the catalyst surface.
The Knovenagel condensation is a typical base-catalyzed condensation. We discussed the mechanistic aspects of condensation in our recent review. [42] Basic sites trigger to accelerate the extraction of a proton from nucleophiles, resulting in anion attacks positively charged carbon of aldehyde to give the adducts. The condensation will finish by dehydration to yield the products. It is believed that the catalytic performance of carbon nitrides prepared from aminoguanidine originates from their basicity, which is generated from the CN structure and the free amino and imino moieties: the structures were varied by the carbonization temperature, as discussed in previous sections. The low-temperature materials carbonized at 350 and 450°C have the formula C 3 N 6 intermediate; however, materials carbonized at 450 and 500°C have the formula of C 3 N 5 .
The catalytic performances of MCN expressed by condensate yield were influenced by carbonization temperature between 350-500°C shown in Figure 5a. The sample calcined at 400°C had the highest performance, and the materials prepared at a higher carbonization temperature offered low catalytic performance. These tendencies of the arrangements can be due to changes in basicities and the specific surface area originating from the change in carbonization temperatures. Among these nitrides, the sample carbonized at 450 and 500°C, which have mainly composed of s-triazine nuclei with Lewis basic characters, showed only low catalytic activities; compared to these two materials, their nitrides prepared at less than have higher catalytic activities, in which material has amino and imino moieties. These results suggest that the Lewis base in the carbon nitrides is not the principal catalytic species, whereas the remaining amino and imino moieties should play significant roles in the base-catalyzed Knoenenagel condensation. These catalytic processes occurred by the electrophilic condensation of active methylene compounds against the carbonyl groups, where electronic withdrawing moieties with both electrophiles and carbonyls can enhance the condensation as observed for psubstituted benzaldehyde with active methylenes, as shown in Figure 6a. Table 3 shows a comparison of the Knovenagel condensation for the materials synthesised at 400 and 500°C and reported literature. [32,43] The higher catalytic performances were observed in the samples carbonized at 400°C because of rich amino and imino moieties.   [43] 5-Amino-1H-Tetrazole KIT-6 Toluene 60 4 82 20 [32] Carbon tetracloride and Ethylenediamine SBA- 15  -40  2  96  88 Recently, carbon nitrides derived from numerous precursors by varying the carbonization conditions are reported, including C 3 N 4 , C 3 N 5 , C 3 N 6 , C 5 N 2 , and C 3 N 2 . [16][17][18][19]23,24,37,42,44] Among them, high catalytic performances are expected from the carbon nitrides with high nitrogen content which imparts high basicity to them. We expect the possibility of new carbon nitrides with varied content of nitrogen for base catalysis in future research.

Conclusions
MCNs materials were synthesized at various carbonization temperatures using aminoguanidine hydrochloride as a precursor and SBA-15 as a template. The obtained MCN materials exhibited highly ordered mesoporous structures with high nitrogen content, active basic sites, and tunable pore diameters. The structure of MCN materials changed with a change in carbonization temperature. The C 3 N 6 predominantly forms at lower carbonization temperatures of 350-400°C whereas the nitrogen loss leads to the formation of C 3 N 4 or C 3 N 5 at higher temperatures (450-500°C). Among the materials studied, MCN prepared at 400°C (MCN-400) registered the highest specific surface area and the largest specific pore volume.
The materials were applied for Knoevenagel condensation between benzaldehyde and malononitrile and maximum catalyst performance was obtained with MCN-400. A decrease in performance was observed with materials synthesized at higher carbonization temperatures. The performance of catalysts was correlated with their surface area and basicity with MCN-400 showing the best performance. These catalysts are highly promising and could be further modified to enhance their basicity for increasing their efficiency for Knoevenegal condensation.

SBA-15 Silica synthesis:
Synthesis of SBA-15 was attained using Pluronic P123 triblock copolymer (EO 20 PO 70 EO 20 ) poly (ethylene glycol)-block-poly (propylene glycol)-block-poly (ethylene glycol) (Sigma-Aldrich) as a template [45]. 4 g of Pluronic P123 was dissolved in 30 ml of water and 120 ml of 2 M HCl solution, followed by stirring for 5 h. Then, 9 g of tetraethyl orthosilicate (TEOS) was added to the homogenous solution under stirring. The resulting solution was then stirred at 40°C for 24 h. Afterwards, the solution was transferred into an autoclave and aged at 150°C for 48 h. The white precipitate was then filtered and dried overnight at 100°C. The resultant solid was calcined at 540°C to decompose the triblock copolymer under the ambient condition.

Synthesis of MCNs:
The carbon nitrides were synthesized by previously reported procedures from aminoguanidine hydrochloride using SBA-15 as a hard template. [27] The materials were prepared by using 1 g of calcined SBA-15, and 4 g of aminoguanidine hydrochloride dissolved in 3 g of water. The resultant gel was mixed well and heated at 100°C for 6 h, and then, at 160°C for 6 h. Afterwards, the precursors were heated for 5 h under nitrogen flow at different carbonization temperatures from 350 to 500°C at the ramp of 3°C min À 1 . The MCNs were obtained after removing the SBA-15 silica template in 5 wt.% hydrofluoric acid, washed with ethanol several times, and dried at 100°C overnight. The materials carbonized at 350-500°C were named MCN-T, where MCN stands for mesoporous carbon nitride, and T is the carbonisation temperature. Characterization of MCNs: N 2 adsorption and desorption isotherms were obtained from Micrometrics ASAP 2420. Brunauer-Emmett-Teller (BET) method was used for surface area calculation and Barrett-Joyner-Halenda (BJH) method was utilized for the pore-size distribution using the adsorption branch of sorption isotherms. Before the measurement, the materials were outgassed at 250°C under a vacuum for 12 h. The pore volume was evaluated from the amount adsorbed at a relative pressure P/P 0 = 0.98. X-ray powder diffraction (XRD) patterns were recorded in PANalytical instruments operating with CuKα 1 and Kα 2 radiations and the wavelength of λ = 1.5405 Å and 1.5444 Å with a generator voltage of 40 kV and current 40 mA that produce copper radiation. TG measurements were carried out on the STA-8000 model Perkin Elmer instrument, where the sample was heated from 10°C to 600°C at a ramping rate of 20°C min À 1 in α-Al 2 O 3 crucibles under a nitrogen atmosphere. FT-IR spectra were recorded by Perkin Elmer instrument, USA, in the range of 4000 to 400 cm À 1 , measuring in transmission mode using the KBr supported pellet technique. Temperatureprogrammed desorption (TPD) of CO 2 was performed on a BELCAT II, MicrotracBEL instrument, Japan. The materials were pre-treated for 2 hours at 200°C in a helium environment and cooled to room temperature. The sample was exposed to 1 % CO 2 in helium at that temperature for one hour. After purging, the materials were heated to 400°C at a ramp rate of 3°C/min. Moreover, the thermal conductivity detector (TCD) monitored the CO 2 amount desorbed. X-ray photoelectron spectra (XPS) were obtained on an Ultraviolet Photoelectron Spectroscopy (UPS) by Thermo Scientific, UK to identify the chemical state, estimate the surface nitrogen-containing species in the materials, and analyse the Al Kα radiation source.

Catalysis reaction:
To identify the catalytic properties of MCN 350-500°C materials were tested for Knoevenagel condensation. A mixture of benzaldehyde (0.5 mmol) with malononitrile (0.6 mmol) and 50 mg MCN in toluene (5 ml) was set in a 20 ml of glass vial. The glass vials were kept at 40-80°C under a nitrogen atmosphere. After the reaction, the reaction mixtures were centrifuged to remove the catalyst, and the liquid product was analysed by Clarus 580 Gas Chromatograph, Perkin Elmer with a capillary column of TC-17 (60 m × 0.25 mm × 0.25 μm film thickness, GL Science, Japan). The conversion and selectivity for the products are calculated based on starting amounts. The recycling experiments were carried out using the catalysts recovered by centrifugation and washing with ethanol.