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Synthesis of nitrogen-containing ordered mesoporous carbon materials with tunable nitrogen distributions and their application for metal-free catalytic synthesis of dimethyl carbonates
Molecular Catalysis 485 (2020) 110848
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Molecular Catalysis journal homepage: www.elsevier.com/locate/mcat
Synthesis of nitrogen-containing ordered mesoporous carbon materials with tunable nitrogen distributions and their application for metal-free catalytic synthesis of dimethyl carbonates
T
Jie Xu*, Lin-Zhi Wen, Yu-Lin Gan, Bing Xue* Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Gehu Middle Road 21, Changzhou, Jiangsu, 213164, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Mesoporous carbon Metal-free catalysis Base catalyst Transesterification Dimethyl carbonate
Dicyandiamide (DCDA) was utilized as a facile nitrogen source for the fabrication of nitrogen-containing ordered mesoporous carbon (NOMC) samples via a one-pot soft-templating approach under aqueous phase. X-ray diffraction, N2 adsorption–desorption, Transmission electron microscopy, Scanning electron microscopy, Raman and X-ray photoelectron spectroscopy have been applied to analyze the physicochemical properties of the synthesized NOMC materials. The characterization results showed that the textural parameters (545–589 m2 g−1), graphitic crystallinity and distribution of various nitrogen species of the synthesized NOMC materials were largely dependent on the adding mass of DCDA. Besides DCDA, NOMC materials have been also successfully fabricated by employing urea and melamine as nitrogen sources. As metal-free heterogeneous catalysts, the NOMC materials showed good catalytic activity and selectivity in the transesterification of ethylene carbonate to dimethyl carbonate, affording a maximum yield of dimethyl carbonate up to 76 % at 3 h under 120 °C.
1. Introduction The past decades have witnessed the remarkable advances in the synthesis and application of ordered mesoporous carbon (OMC) materials, because of their multiple physicochemical properties including large surface areas, adjustable mesopores, mechanical stability and excellent conductivity [1–3]. To date, OMC materials have been regarded as promising candidates to traditional carbon materials (such as graphite) in various research fields including thermal catalysis, supercapacitors, gas adsorption, sensors, and energy storage and conversion [4–6]. Unfortunately, most OMC materials have a limited number of catalytically active sites, and meanwhile the surface thereof is highly hydrophobic, therefore restraining their extensive application, especially in the hydrophilic liquid phase. Alternatively, the incorporation of heteroatoms (e.g. boron, phosphorous, and nitrogen) into the carbon skeleton of OMC materials could significantly improve the semiconducting, field-emission, and surface polarity as well as electron-donor properties of the original OMC materials [1,7–11]. Among the various heteroatoms induced into the carbon matrix of OMC materials, nitrogen-containing ordered mesoporous carbon (NOMC) materials have attracted particular interest
⁎
[8,12–14], as such NOMC materials featuring basicity demonstrate enhanced performance in CO2 capture [13,15–17], catalysis [2,7,18,19], supercapacitors [1,11,20], etc. The hard-templating procedure is a conventional approach to synthesize NOMC materials [2,8,21]. The method usually employs nitrogen-containing molecules (or polymers) as carbon and nitrogen sources [1], and presynthesized ordered mesoporous siliceous templates (SBA-15, FDU-12, KIT-6, etc.) as the sacrificial scaffolds [16]. Although the hard-templating method is well developed [7], it is actually time-consuming, and expensive to carry out at large scale [1,2]. In particular, the detemplating procedure thereof involves hazardous and very corrosive hydrogen fluoride solution [22–24]. In sharp contrast, the soft-templating strategy based on co-assembly of high-molecular-weight co-polymer (structure-directing agent) and nitrogen-containing carbon precursors [2,7], provides a sustainable and facile technique for the synthesis of NOMC and OMC-related materials. Recently, Wang et al. reported the one-pot synthesis of NOMC materials via a soft-templating method under aqueous phase [4,7]. Compared with other evaporation-induced self-assembly (EISA) processes [1,14], such a synthetic route adopts no volatile organic solvent and thus is much more convenient. Nevertheless, the adjustment of nitrogen
Corresponding authors. E-mail addresses: [email protected] (J. Xu), [email protected] (B. Xue).
https://doi.org/10.1016/j.mcat.2020.110848 Received 23 December 2019; Received in revised form 12 February 2020; Accepted 16 February 2020 Available online 24 February 2020 2468-8231/ © 2020 Elsevier B.V. All rights reserved.
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2.3. Catalytic evaluation
content, as well as the distribution of various nitrogen species, mainly relied on the carbonization temperature. Moreover, although the NOMC materials own multiple nitrogen-containing species and are regarded as typical solid bases, the basic strength of the reported NOMC materials is relatively weak [14,18,25]. Up to now, the application examples of NOMC materials as heterogeneous basic catalysts only include transesterification of β-esters [18] and Knoevenagel condensation [25]; however, the wide catalytic application is rarely reported. In this work, we developed a new one-pot approach to synthesize NOMC materials by adopting resol and dicyandiamide (DCDA) as carbon and nitrogen precursors based on the previous liquid-phase softtemplating strategy. In this synthetic route, DCDA molecules and resol interact with the F127 template, thereby inducing nitrogen species into the resin framework assembled on the F127 micelle during the condensation procedure. Upon adjusting the amount of DCDA, the final nitrogen contents along with the distribution of nitrogen species can be easily tuned. Furthermore, based on the synthetic strategy, other nitrogen molecules such as urea and melamine could also be employed for the production of NOMC materials. As heterogeneous base catalysts, the synthesized NOMC materials demonstrated good catalytic activities in transesterification reactions between ethylene carbonate and CH3OH.
Transesterification of EC with CH3OH was performed in a stainless steel autoclave (25 mL). 200 mmol of CH3OH, 25 mmol of EC, and 0.1 g of the pre-dried catalyst were added into the autoclave and mixed well. After being filled with nitrogen gas up to 0.6 MPa, the reactor was heated at 120 °C for 3 h under stirring. After the completion of the reaction, the autoclave was cooled down in ice water and then exhausted slowly. The liquid-phase mixture was separated by centrifugation and analyzed by a gas chromatographymass spectrometer (GC–MS, GCMS-QP2010, Shimadzu) to qualitatively confirm the products. The quantitative analysis of the reaction mixture was determined by a gas chromatograph (GC, SP-6890, Shandong Lunan Ruihong Chemical Instrument Co.) equipped with a SE-54 capillary column and FID. Owing to the higher stoichiometric of EC than CH3OH adopted in the reaction, the catalytic conversion was calculated based on EC. The detailed EC conversion and selectivity to the target molecule (DMC) were calculated based on an area-normalization method of GC, and the corresponding calculation equations are as follows:
Conv . =
ADMC × fDMC + AHEMC × fHEMC AEC × fEC + ADMC × fDMC + AHEMC × fHEMC =
2. Experimental section
ADMC × fDMC ADMC × fDMC + AHEMC × fHEMC
Sel.
,
where A, and f are the peak areas of GC, and response factor for each component. HEMC stood for the 2-hydroxyethyl methyl carbonate as a byproduct. For the recycling experiment, the reaction mixture was filtrated, and the filtrated catalyst was rinsed by methanol for three times. After drying in a vacuum (100 °C for 2 h), the recycled catalyst was used for the next catalytic run without the addition of the fresh catalyst.
2.1. Synthesis NOMC materials were synthesized by a one-pot method, using resorcinol and hexamethylenetetramine (HMTA) as the precursors, pluronic F127 as the structure-directing agent, and DCDA as the nitrogen source. In brief, 2 g of F127 was fully dispersed in 50 ml of H2O, followed by the addition of DCDA, and 0.56 ml of 1,3,5-trimethyl benzene (TMB). Next, 1.10 g of resorcinol, 0.7 g of HMTA, and 1.3 ml of NH3 (28 wt%, a.q.) were added into the above liquid successively under stirring. The liquid was then heated at 80 °C and stirred for another 24 h. After that, the mixture was centrifuged and dried, and the subsequent solid was heated from room temperature to 600 °C (1 °C min−1) and kept at this temperature for 2 h, in a tubular furnace under nitrogen (10 ml min−1). The resultant black solid was grounded and labeled as NOMC-xD, where x represents the mass (g) of DCDA. Likewise, urea or melamine was also used to substitute DCDA, and the corresponding products were labeled as NOMC-xU, and NOMC-xM, respectively. The material synthesized without adding DCDA/urea/ melamine was labeled as OMC.
3. Results & discussion 3.1. Material characterization The specific surface areas and porous properties of the materials were analyzed by N2 adsorption. The OMC synthesized without using nitrogen-containing precursor, presents type-IV isothermal curves featuring a hysteretic loop in the relative pressure (p/p0) ranging from 0.45 to 0.65 (Fig. 1A), indicating the sample has mesopores. Like OMC, NOMC materials show apparent mesostructure along with narrow pore distributions (Fig. 1B). However, the adsorption quantity of NOMC materials rises continuously in high relative pressure (p/p0 > 0.95). This means that, in addition to mesopores, NOMC materials still possess a certain amount of macropores, which might be associated with the packing pores of NOMC materials. The surface areas and pore sizes and volumes are summarized in Table 1. The BET surface area of the pure OMC is 565 m2 g−1 and the pore volume thereof is 0.39 cm3 g−1. As for NOMC materials, their surface areas are very similar to that of OMC, whereas the pore sizes become much smaller for NOMC. It is found that the textural parameters of NOMC materials are related to the feeding mass of DCDA. With the increase of the mass of DCDA, the surface areas, as well as pore volumes, decrease monotonously (Table 1). More importantly, it is detected that the product mass of NOMC is very sensitive to the mass of DCDA. In fact, when the mass of DCDA is above 2.4 g, the product yield of NOMC is quite low. One probable reason is that, during the subsequent carbonization procedure, the addition of more DCDA would bring out more volatile intermediates, which therefore promote the decomposition of nitrogen-containing frameworks of the as-synthesized NOMC materials (discussed below). Besides DCDA, we have also prepared other NOMC materials by applying urea and melamine as nitrogen-containing sources. The N2
2.2. Characterization X-ray diffraction (XRD) patterns of the materials were recorded with a D/max 2500 PC X-ray diffractometer (Rigaku) with a graphite monochromator (40 kV, 40 mA) using Ni-filtered Cu-Kα radiation. The surface areas and porous information were measured using an ASAP 2020 analyzer (Micromeritics). Before the analysis, the samples were degassed at 150 °C for at least 4 h. The specific surface areas were calculated based on the Brunauer–Emmet–Teller (BET) method, and pore size distributions were determined by the Barret–Joyner–Halenda method. Scanning electron microscopy (SEM) images were obtained on a Philips XL 30 microscope. Transmission electron microscopy (TEM) images were recorded on a JEOL 2010 electron microscope (200 kV). Xray photoelectron spectra (XPS) were recorded on an ESCALAB 250XI spectrometer (Thermo) using an Al anode as an X-ray source (12.5 kV). Raman spectra were collected on a Lab Ram HR evolution spectrometer (Jobin Yvon) using a 532 nm line as the excitation source. Elementary analysis (EA) was performed with Elementar Vario EL III instrument to calculate the carbon, nitrogen and hydrogen contents of the samples. 2
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Fig. 1. N2 adsorption–desorption isotherms (A) and the pore size distributions (B) of OMC and NOMC-xD materials. Table 1 Physical and chemical properties of OMC and NOMC-xD materials. Sample
SBET (m2 g−1)
Dpore (nm)a
Vpore (cm3 g−1)
a0 (nm)c
t (nm)d
ID/IG
Product mass (g)
Chemical compositione
OMC NOMC-0.8D NOMC-1.6D NOMC-2.4D NOMC-1.6Db
565 589 570 545 524
4.3 3.3 3.4 4.0 3.4
0.39 0.45 0.43 0.40 0.38
10.15 10.29 10.31 10.75 10.35
5.83 6.97 6.87 6.77 6.91
0.89 0.78 0.87 0.88 0.86
0.77 0.70 0.61 0.48 –
C60.7NH20.4 C39.5NH18.1 C32.8NH17.4 C21.5NH13.6 C34.5NH18.3
a b
Determined by the adsorption branches. Spent catalyst after five consecutive runs. 1 2 dhkl
4 h2 + hk + k2 3 a02
c
The lattice parameter (a0) is calculated by
d
The thickness of the mesoporous carbon wall (t) is calculated by t = a0 − Dpore. Determined by CHN elemental analysis.
e
=
based on (110) reflection.
Scheme 1. A possible synthetic route of NOMC via a one-pot co-assembly method using DCDA as a nitrogen source.
analyzed by EA. As listed in Table 1, the molar ratio of carbon to nitrogen in OMC is 60.7:1, indicating that the material contains a large number of carbon species. By comparison, NOMC materials possess higher contents of nitrogen than OMC. Moreover, with the increase of the amount of DCDA, the molar ratio of carbon to nitrogen of NOMC is decreased, suggesting that the nitrogen contents of NOMC materials can be easily controlled by adjustment of the amount of DCDA. According to the soft-templating synthetic approaches of NOMC
adsorption–desorption isotherms (Fig. S1) indicate that the materials have mesostructures along with narrow pore size distributions like NOMC-xD materials. Accordingly, the surface areas of NOMC-0.8U and NOMC-0.8 M are 553 and 538 m2 g−1 (Table S1), respectively. Likewise, the NOMC-M material adopting more melamine results in a slight decline in both surface area and pore volume, which is in accordance with the above NOMC-xD materials. The bulk chemical compositions of OMC and NOMC materials were 3
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further transforms into ordered mesostructure (Step 3). During the pyrolysis under nitrogen gas, F127 decomposes by itself (generally occurring at ca. 400 °C [7]), and the mesostructured NOMC material is finally obtained under higher carbonization temperature (Step 4). It should be mentioned that DCDA would also partially decompose below 400 °C and release a large amount of gas, thus inducing the decomposition or even partial collapse of ordered carbon framework of NOMC. Therefore, adding the mass of DCDA results in lower productivity of NOMC (Table 1). The small-angle XRD (SAXRD) was applied to further study the mesostructures of NOMC materials. The SAXRD pattern of OMC shows two pronounced diffraction peaks at 2θ = 0.8 and 1.44° (Fig. 2). The ratio of the corresponding d-spacing values of the peaks is close to 3 1 . It can be inferred that the two peaks are indexed as (100) and (110) reflections of two-dimensional hexagonal symmetry (p6mm), respectively [5,7]. The similar patterns are also observed in the SAXRD patterns of NOMC materials, which prove the existence of ordered mesostructure in NOMC. Moreover, the results of SAXRD patterns indicate that the incorporation of a small amount of DCDA into the original synthetic procedure has not altered the regular mesostructures of OMC. However, as the mass of DCDA is increased, the position of diffraction peak (110) is shifted towards lower 2θ values, corresponding to larger lattice parameters (a0) of NOMC (Table 1). This means that the lattice parameters of the p6mm symmetry of NOMC materials undergo expansion. Accordingly, the thickness of the carbon wall of mesostructures becomes thinner from NOMC-0.8D to NOMC-2.4D. Furthermore, the SAXRD patterns of NOMC-0.8U and NOMC-0.8 M (Fig. S2) manifest that the materials fabricated using urea and melamine as nitrogen source also possess ordered mesoporous structures. The micro-morphology of NOMC-0.8D and NOMC-1.6D samples are observed by SEM and TEM techniques. The SEM images of NOMC (Fig. 3) reveal rod-like domains with a mean width of ca. 100 nm. The aggregated particles of NOMC are like that of siliceous SBA-15 [26]. The existence of mesopores in NOMC materials detected by N2 adsorption–desorption analysis should originate from the interparticle voids. TEM images of NOMC materials exhibit well regular shutter-like strips (Fig. 4), further confirming the ordered mesoporous structures of
Fig. 2. Small-angle XRD patterns of OMC (a) and NOMC-xD (x = 0.8 (b), 1.6 (c), and 2.4 (d)) materials.
previously reported in the literature [1,4,5,7], a possible synthetic route of NOMC using DCDA, urea, and melamine as nitrogen sources has been proposed in this study. As illustrated in Scheme 1, the present synthetic method for NOMC is indeed based on the co-assembly of nitrogencontaining sources and structure-directing agent (tri-block PEO-PPOPEO F127 polymer). First, F127 molecules transform into stripe-like micelle in the aqueous solution and TMB prefers to locate inside and swell the hydrophobic domain (PPO) of F127 micelle (Step 1) [5]. Simultaneously, DCDA molecules are adsorbed on the hydrophilic section (PEO) of F127 via hydrogen-bonding interaction. On the other hand, HMTA hydrolyzes under 80 °C, thus generating formaldehyde. The liquid phase with basic condition originating from ammonia induces the condensation of resorcinol and formaldehyde, thereby gradually forming resorcinol-formaldehyde resin. Afterwards, the resin interacts with the hydrophilic section of F127 micelle by a hydrogen bond (Step 2). Namely, both resin and DCDA are located at the exterior of the F127 strip-type micelle. The co-assembly of DCDA-resol/F127 micelles
Fig. 3. SEM images of NOMC-0.8D (A&B) and NOMC-1.6D (C&D) materials.. 4
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Fig. 4. TEM images of NOMC-0.8D (A&B) and NOMC-1.6D (C&D) materials.
Fig. 7. XPS survey of OMC and NOMC-xD materials.
NOMC materials. Directly judged from the images, the pore sizes of NOMC are ca. 4 nm, close to the results gained from N2 sorption analysis in Table 1. Fig. 5 depicts the wide-angle XRD (WAXRD) patterns of OMC and NOMC. The two diffraction peaks appearing at 2θ = ca. 22.5 and 43.1°, which are ascribed to (002) and (100) reflections of discorded graphite [13,15,27] in NOMC. For NOMC-0.8D, the intensity of the diffraction peak is comparable to that of OMC. Whereas, further adding the mass of DCDA, the eventual intensity of peak is weaken. Raman spectroscopy was used to further investigate the graphitic character of the samples. The resultant spectra of both OMC and NOMC (Fig. 6) reveal two primary signals at 1356 and 1586 cm−1, which are assigned to the disordered (D) and graphitic (G) modes of graphiteanalogous material [15,22,27]. The intensity ratio of the two peaks (ID/ IG) is commonly applied to appraise the graphitic crystallinity. The calculated ID/IG values of NOMC materials (Table 1) are 0.78–0.89, illustrating that the synthesized NOMC materials possess comparatively highly sp2-hybridized carbon species, The results of Raman spectra agree with the WAXRD pattern described above. The chemical compositions of OMC and NOMC were probed by XPS technique. The spectrum of OMC (Fig. 7) shows carbon and oxygen elemental signals detected at binding energies of 285 and 533 eV. The occurrence of the oxygen element of OMC is possibly owing to the
Fig. 5. Wide-angle XRD patterns of OMC (a) and NOMC-xD (x = 0.8 (b), 1.6 (c), and 2.4 (d)) materials.
Fig. 6. Raman spectra of OMC (a) and NOMC-xD (x = 0.8 (b), 1.6 (c), and 2.4 (d)) materials.
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Fig. 8. N 1s spectra of NOMC-xD materials.
Scheme 2. Chemical structure of NOMC with various nitrogen functionalities.
Table 2 Distributions of various nitrogen species in NOMC materials. Sample
NOMC-0.8D NOMC-1.6D NOMC-2.4D NOMC-0.8 M NOMC-0.8U
Fig. 9. Correlation between the catalytic conversion of EC and the proportion of graphitic nitrogen species of various NOMC materials.
Nitrogen component (mol%) pyridinic
pyrrolic
graphitic
pyridinic oxide
29.1 27.1 22.2 28.9 27.0
39.6 41.0 54.4 41.5 42.1
14.3 20.7 14.1 16.4 16.3
17.0 11.2 9.3 13.2 14.6
residual water molecules adsorbed on the surface of OMC. In addition to the two elements, a weak peak at 400 eV is observed in each spectrum of NOMC. Calculated based on the peak areas integrated, the surface atomic ratios of carbon to nitrogen of NOMC materials are in the range of 20.7:1–37.8:1, depending on the adding mass of DCDA in the synthetic procedure of NOMC. The XPS survey spectra of NOMC0.8 M and NOMC-0.8U are depicted in Fig. S3. The carbon, nitrogen,
Table 3 Catalytic performances of OMC and NOMC materials in transesterification reactions of EC with CH3OHa.
Catalyst
Conv. (%)
Sel. (%)
Yield (%)
– OMC NOMC-0.8D NOMC-1.6D NOMC-2.4D NOMC-0.8 M NOMC-0.8U
41 53 76 86 71 83 82
40 61 69 89 69 80 82
16 32 52 76 49 66 67
a
200 mmol of CH3OH, 25 mmol of EC, T =120 °C, Wcatal. = 0.1 g, and t =3 h. 6
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3.2. Catalyst evaluation Dialkyl carbonates represented by dimethyl carbonate (DMC) and diethyl carbonate (DEC) have been extensively applied for numerous applications in the contemporary chemical industry [28]. Due to its high octane number and excellent biodegradability [29–31], DMC is a sustainable additive to gasoline. Furthermore, DMC is considered as a potential candidate to replace phosgene and dimethyl sulfate in carbonylation, and methylation processes [32–34], respectively. Among various synthetic processes, including phosgenation and oxidative carbonylation of methanol (CH3OH), transesterification of cyclic carbonates (such as ethylene carbonate and propylene carbonate) with CH3OH has been proposed as the most convenient and eco-benign approach for the manufacture of DMC [32,35,36]. The transesterification is an acid/base-catalyzed reaction. Up to now, the most efficient catalysts suggested for the transesterification of EC to DMC or DEC are mainly acidic and basic ionic liquids [37,38]. Despite their high catalytic activities, such homogeneous catalysts suffer from difficulty in product separation and catalyst recycling. In sharp contrast, heterogeneous catalysts are more convenient to be recycled, and more importantly, nanostructured and mesoporous heterogeneous catalysts have been well developed in recent years [39–45]. However, it should be pointed out, most of the heterogeneous catalysts including metal oxides [46] and their composites [33,47–49], and dawsonites [36,50], proposed for the transesterification reactions, contain metal cations. The potential metal contamination is undoubtedly unfavorable for the further application of DMC as the electrolyte [51]. Regarding these viewpoints, it is highly desired to develop a metal-free solid catalyst for the clean production of DMC via transesterification. Herein, as the present NOMC materials contain rich nitrogen species and more importantly, no metallic component has been involved in the overall synthetic procedure, the NOMC materials are applied as catalysts for the metal-free catalytic production of DMC. Initially, the reaction was performed without any catalyst (Table 3), and the blank result revealed that the conversion of EC was low (41 %). In the case of catalytic selectivity, the main product was mainly 2-hydroxyethyl methyl carbonate (HEMC) instead of the desired DMC. The monoester byproduct was probably because of the incomplete transesterification of EC [32,52]. After the addition of 0.1 g of OMC, the catalytic activity is obviously improved. The conversion of EC is 53 %, and the selectivity to DMC is increased to 61 %, respectively. Unfortunately, the yield of DMC (32 %) is still not satisfactory. Using NOMC materials instead, the catalytic conversion is remarkably enhanced. Among the three NOMCxD materials, NOMC-1.6D demonstrates the highest catalytic conversion and selectivity. In addition to NOMC-xD materials, the catalytic performances of other NOMC materials prepared using urea, and melamine as nitrogen precursors were tested under identical reaction conditions. The corresponding catalytic results (Table 3) show that the transesterification reactions could also be smoothly promoted in the presence of these NOMC materials. The EC conversion and DMC selectivity for each case are ca. 80 %. The catalytic results evidence that using other typical nitrogen-containing compounds, the synthesized NOMC samples could also act as efficient catalysts to promote the transesterification reactions between EC and CH3OH. The above XPS analysis (Fig. 8) has revealed that there are four kinds of nitrogen species existing in NOMC, and on the other hand, the transesterification of EC with CH3OH in this work undoubtedly belongs to base-mediated catalysis. To find out possible catalytic sites for the reactions promoted by NOMC, the correlation between the catalytic activity of various NOMC materials with their corresponding nitrogen species is tentatively elucidated. As described in Fig. 9, the catalytic activity has a good relationship with the proportion of graphitic nitrogen species of NOMC materials, suggesting that these graphitic nitrogen species have a positive effect for NOMC in the transesterification
Fig. 10. Effects of reaction temperature (A) and time (B) on the transesterification on catalytic performances Reaction conditions: 200 mmol of CH3OH, 25 mmol of EC, 0.1 g of NOMC-1.6D, T =120 °C (A), and t =3 h (B).
Fig. 11. Recycling catalytic performance of NOMC-1.6D in transesterification of EC with CH3OH. Reaction condition: 200 mmol of CH3OH, 25 mmol of EC, Wcatal. = 0.1 g (the first run), T =120 °C, and t =3 h.
and oxygen elements are observed in the two spectra, and the atomic ratios of carbon to nitrogen of NOMC-0.8 M and NOMC-0.8U are 29.6:1 and 30.5:1, respectively. For most of the nitrogen-containing carbon materials, there exist various nitrogen species in the forms of pyridine, pyrrole [8,9,15], etc. In order to elucidate the detailed chemical environment of nitrogen species, the N 1s spectra with high resolution of NOMC samples are deconvoluted. As described in Fig. 8, the N 1s spectra can be separated into four independent peaks. The intensive peak appearing at 398.4 eV is attributed to pyridinic nitrogen atoms [6,13], and the signal at 400.4 eV is attributed to pyrrolic nitrogen species (Scheme 2). The two nitrogen species account for the major proportion of the whole nitrogen species. The broad peak centered at 401.2 eV is assigned to graphitic nitrogen whereas the signal of 403.0 eV is ascribed to pyridinic oxide on the surface of NOMC [6,7]. Additionally, the deconvoluted N 1s spectra of NOMC-0.8U and NOMC-0.8M are presented in Fig. S4 and the corresponding molar proportions of various nitrogen components are listed in Table 2. From the full-range XPS spectra and deconvoluted N 1s spectra, it can be concluded that NOMC materials own multiple nitrogen-containing species, which thus dictate the basic character of the materials, which thus could be applied for the base-mediated organocatalysis.
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Table 4 Catalytic performances of NOMC-1.6D for transesterification reactions with various substratesa.
Cyclic carbonate
a b
Alcohol
T (°C)
t (h)
Conv. (%)
Sel. (%)b
CH3OH
120
3
86
89
C2H5OH
140
3
63
41
C2H5OH
140
6
76
78
CH3OH
140
6
37
84
200 mmol of alcohol, 25 mmol of cyclic carbonate, and Wcatal. = 0.1 g. Selectivities to the corresponding dialkyl carbonates.
EC and C2H5OH, even the temperature is elevated up to 140 °C, the selectivity to the target molecular (diethyl carbonate) is only 41 %. The dissociation of the OeH bond in C2H5OH is more difficult than that in CH3OH [48,52], which is responsible for the lower selectivity in the cases of C2H5OH than CH3OH. Despite this, upon prolonging the reaction to 6 h, higher selectivity along with enhanced conversion is obtained. Additionally, propylene carbonate could also be moderately converted into EC.
reactions. The attribution of such graphitic nitrogen to catalytically active sites agrees well with the relevant literature involving basemediated reactions (e.g. Knoevenagel condensation [53–55] and cycloaddition of CO2 with epoxides [56]) catalyzed by mesostructure carbon nitride materials. Since NOMC-1.6D exhibits superior catalytic activity to other NOMC materials under the same reaction conditions, the influences of reaction conditions on the eventual catalytic results have been evaluated adopting NOMC-1.6D as a representative catalyst. After the first hour, the catalytic EC conversion is only 50 % while the DMC selectivity is 53 % (Fig. 10A). Prolonging reaction, both the conversion and selectivity are increased progressively. After 3 h of reaction time, the EC conversion shows no further incensement. The catalytic reaction is also found to be sensitive to the reaction temperature (Fig. 10B). In the case of product distribution, the major product is HEMC rather than the target DMC as the temperature is below 100 °C. This means that, under low temperatures, the second step of transesterification between HEMC and CH3OH is difficult to proceed. Elevating the temperature could effectively promote the transfer of EC and HEMC into the DMC. Notwithstanding, the catalytic activity and selectivity level off as the temperature is above 120 °C. The catalytic results using various amounts of NOMC-1.6D catalyst are given in Fig. S5. Upon increasing the feeding amount of the catalyst, the EC conversion is gradually enhanced. It is apparent that more catalyst together with higher temperature is beneficial to convert more EC and/or HEMC to DMC. Fig. 11 shows the catalytic performances of the recycled NOMC-1.D material during five consecutive runs. The mass of the recycled catalyst for each run was almost 95 mg. For the second run, the catalytic conversion decreases to 72 %. This might result from the leaching of some nitrogen-containing fragments unsteadily adsorbed on the surface of NOMC into the liquid phase. A similar phenomenon has been also found in our previous work involving the transesterification of β-esters [18] and Knoevenagel condensation [25] catalyzed by NOMC materials. Nevertheless, the catalyst still demonstrates stable conversion and high selectivity within the subsequent catalytic runs. As mentioned above, catalytic transesterification of cyclic carbonates with CH3OH is a more sustainable approach for the production of dialkyl carbonates. Herein, in order to examine the catalytic versatility of NOMC, other cyclic carbonate and alcohol have been used in the transesterification reactions. As summarized in Table 4, in the case of
4. Conclusion In summary, adopting DCDA as a nitrogen source, a series of NOMC materials have been synthesized by a one-pot co-assembly approach under the liquid phase. The NOMC materials possess ordered mesostructures (two-dimensional hexagonal symmetry), with surface areas of 545–589 m2 g−1. Besides DCDA, urea and melamine could be also successfully applied as nitrogen sources for the fabrication of NOMC. The introduction of nitrogen into carbon materials is probably based on the interaction between nitrogen sources and F127 micelle via hydrogen bonding. The textural properties, graphitic crystallinity, and distribution of nitrogen species are related to the adding amount of DCDA. In the transesterification of EC, the synthesized NOMC materials as solid bases showed high catalytic activity and selectivity. The catalytic active sites might originate from the graphitic nitrogen atoms, the proportion of which correlated with the final catalytic activity. The work offers a convenient method to prepare NOMC materials, also enlarges the catalytic application of NOMC for the metal-free synthesis of dialkyl carbonates.
CRediT authorship contribution statement Jie Xu: Conceptualization, Writing - review & editing. Lin-Zhi Wen: Formal analysis, Writing - original draft. Yu-Lin Gan: Validation. Bing Xue: Supervision.
Declaration of Competing Interest The authors declare that there are no conflicts of interest. 8
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Acknowledgments
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Molecular Catalysis journal homepage: www.elsevier.com/locate/mcat
Synthesis of nitrogen-containing ordered mesoporous carbon materials with tunable nitrogen distributions and their application for metal-free catalytic synthesis of dimethyl carbonates
T
Jie Xu*, Lin-Zhi Wen, Yu-Lin Gan, Bing Xue* Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Gehu Middle Road 21, Changzhou, Jiangsu, 213164, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Mesoporous carbon Metal-free catalysis Base catalyst Transesterification Dimethyl carbonate
Dicyandiamide (DCDA) was utilized as a facile nitrogen source for the fabrication of nitrogen-containing ordered mesoporous carbon (NOMC) samples via a one-pot soft-templating approach under aqueous phase. X-ray diffraction, N2 adsorption–desorption, Transmission electron microscopy, Scanning electron microscopy, Raman and X-ray photoelectron spectroscopy have been applied to analyze the physicochemical properties of the synthesized NOMC materials. The characterization results showed that the textural parameters (545–589 m2 g−1), graphitic crystallinity and distribution of various nitrogen species of the synthesized NOMC materials were largely dependent on the adding mass of DCDA. Besides DCDA, NOMC materials have been also successfully fabricated by employing urea and melamine as nitrogen sources. As metal-free heterogeneous catalysts, the NOMC materials showed good catalytic activity and selectivity in the transesterification of ethylene carbonate to dimethyl carbonate, affording a maximum yield of dimethyl carbonate up to 76 % at 3 h under 120 °C.
1. Introduction The past decades have witnessed the remarkable advances in the synthesis and application of ordered mesoporous carbon (OMC) materials, because of their multiple physicochemical properties including large surface areas, adjustable mesopores, mechanical stability and excellent conductivity [1–3]. To date, OMC materials have been regarded as promising candidates to traditional carbon materials (such as graphite) in various research fields including thermal catalysis, supercapacitors, gas adsorption, sensors, and energy storage and conversion [4–6]. Unfortunately, most OMC materials have a limited number of catalytically active sites, and meanwhile the surface thereof is highly hydrophobic, therefore restraining their extensive application, especially in the hydrophilic liquid phase. Alternatively, the incorporation of heteroatoms (e.g. boron, phosphorous, and nitrogen) into the carbon skeleton of OMC materials could significantly improve the semiconducting, field-emission, and surface polarity as well as electron-donor properties of the original OMC materials [1,7–11]. Among the various heteroatoms induced into the carbon matrix of OMC materials, nitrogen-containing ordered mesoporous carbon (NOMC) materials have attracted particular interest
⁎
[8,12–14], as such NOMC materials featuring basicity demonstrate enhanced performance in CO2 capture [13,15–17], catalysis [2,7,18,19], supercapacitors [1,11,20], etc. The hard-templating procedure is a conventional approach to synthesize NOMC materials [2,8,21]. The method usually employs nitrogen-containing molecules (or polymers) as carbon and nitrogen sources [1], and presynthesized ordered mesoporous siliceous templates (SBA-15, FDU-12, KIT-6, etc.) as the sacrificial scaffolds [16]. Although the hard-templating method is well developed [7], it is actually time-consuming, and expensive to carry out at large scale [1,2]. In particular, the detemplating procedure thereof involves hazardous and very corrosive hydrogen fluoride solution [22–24]. In sharp contrast, the soft-templating strategy based on co-assembly of high-molecular-weight co-polymer (structure-directing agent) and nitrogen-containing carbon precursors [2,7], provides a sustainable and facile technique for the synthesis of NOMC and OMC-related materials. Recently, Wang et al. reported the one-pot synthesis of NOMC materials via a soft-templating method under aqueous phase [4,7]. Compared with other evaporation-induced self-assembly (EISA) processes [1,14], such a synthetic route adopts no volatile organic solvent and thus is much more convenient. Nevertheless, the adjustment of nitrogen
Corresponding authors. E-mail addresses: [email protected] (J. Xu), [email protected] (B. Xue).
https://doi.org/10.1016/j.mcat.2020.110848 Received 23 December 2019; Received in revised form 12 February 2020; Accepted 16 February 2020 Available online 24 February 2020 2468-8231/ © 2020 Elsevier B.V. All rights reserved.
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2.3. Catalytic evaluation
content, as well as the distribution of various nitrogen species, mainly relied on the carbonization temperature. Moreover, although the NOMC materials own multiple nitrogen-containing species and are regarded as typical solid bases, the basic strength of the reported NOMC materials is relatively weak [14,18,25]. Up to now, the application examples of NOMC materials as heterogeneous basic catalysts only include transesterification of β-esters [18] and Knoevenagel condensation [25]; however, the wide catalytic application is rarely reported. In this work, we developed a new one-pot approach to synthesize NOMC materials by adopting resol and dicyandiamide (DCDA) as carbon and nitrogen precursors based on the previous liquid-phase softtemplating strategy. In this synthetic route, DCDA molecules and resol interact with the F127 template, thereby inducing nitrogen species into the resin framework assembled on the F127 micelle during the condensation procedure. Upon adjusting the amount of DCDA, the final nitrogen contents along with the distribution of nitrogen species can be easily tuned. Furthermore, based on the synthetic strategy, other nitrogen molecules such as urea and melamine could also be employed for the production of NOMC materials. As heterogeneous base catalysts, the synthesized NOMC materials demonstrated good catalytic activities in transesterification reactions between ethylene carbonate and CH3OH.
Transesterification of EC with CH3OH was performed in a stainless steel autoclave (25 mL). 200 mmol of CH3OH, 25 mmol of EC, and 0.1 g of the pre-dried catalyst were added into the autoclave and mixed well. After being filled with nitrogen gas up to 0.6 MPa, the reactor was heated at 120 °C for 3 h under stirring. After the completion of the reaction, the autoclave was cooled down in ice water and then exhausted slowly. The liquid-phase mixture was separated by centrifugation and analyzed by a gas chromatographymass spectrometer (GC–MS, GCMS-QP2010, Shimadzu) to qualitatively confirm the products. The quantitative analysis of the reaction mixture was determined by a gas chromatograph (GC, SP-6890, Shandong Lunan Ruihong Chemical Instrument Co.) equipped with a SE-54 capillary column and FID. Owing to the higher stoichiometric of EC than CH3OH adopted in the reaction, the catalytic conversion was calculated based on EC. The detailed EC conversion and selectivity to the target molecule (DMC) were calculated based on an area-normalization method of GC, and the corresponding calculation equations are as follows:
Conv . =
ADMC × fDMC + AHEMC × fHEMC AEC × fEC + ADMC × fDMC + AHEMC × fHEMC =
2. Experimental section
ADMC × fDMC ADMC × fDMC + AHEMC × fHEMC
Sel.
,
where A, and f are the peak areas of GC, and response factor for each component. HEMC stood for the 2-hydroxyethyl methyl carbonate as a byproduct. For the recycling experiment, the reaction mixture was filtrated, and the filtrated catalyst was rinsed by methanol for three times. After drying in a vacuum (100 °C for 2 h), the recycled catalyst was used for the next catalytic run without the addition of the fresh catalyst.
2.1. Synthesis NOMC materials were synthesized by a one-pot method, using resorcinol and hexamethylenetetramine (HMTA) as the precursors, pluronic F127 as the structure-directing agent, and DCDA as the nitrogen source. In brief, 2 g of F127 was fully dispersed in 50 ml of H2O, followed by the addition of DCDA, and 0.56 ml of 1,3,5-trimethyl benzene (TMB). Next, 1.10 g of resorcinol, 0.7 g of HMTA, and 1.3 ml of NH3 (28 wt%, a.q.) were added into the above liquid successively under stirring. The liquid was then heated at 80 °C and stirred for another 24 h. After that, the mixture was centrifuged and dried, and the subsequent solid was heated from room temperature to 600 °C (1 °C min−1) and kept at this temperature for 2 h, in a tubular furnace under nitrogen (10 ml min−1). The resultant black solid was grounded and labeled as NOMC-xD, where x represents the mass (g) of DCDA. Likewise, urea or melamine was also used to substitute DCDA, and the corresponding products were labeled as NOMC-xU, and NOMC-xM, respectively. The material synthesized without adding DCDA/urea/ melamine was labeled as OMC.
3. Results & discussion 3.1. Material characterization The specific surface areas and porous properties of the materials were analyzed by N2 adsorption. The OMC synthesized without using nitrogen-containing precursor, presents type-IV isothermal curves featuring a hysteretic loop in the relative pressure (p/p0) ranging from 0.45 to 0.65 (Fig. 1A), indicating the sample has mesopores. Like OMC, NOMC materials show apparent mesostructure along with narrow pore distributions (Fig. 1B). However, the adsorption quantity of NOMC materials rises continuously in high relative pressure (p/p0 > 0.95). This means that, in addition to mesopores, NOMC materials still possess a certain amount of macropores, which might be associated with the packing pores of NOMC materials. The surface areas and pore sizes and volumes are summarized in Table 1. The BET surface area of the pure OMC is 565 m2 g−1 and the pore volume thereof is 0.39 cm3 g−1. As for NOMC materials, their surface areas are very similar to that of OMC, whereas the pore sizes become much smaller for NOMC. It is found that the textural parameters of NOMC materials are related to the feeding mass of DCDA. With the increase of the mass of DCDA, the surface areas, as well as pore volumes, decrease monotonously (Table 1). More importantly, it is detected that the product mass of NOMC is very sensitive to the mass of DCDA. In fact, when the mass of DCDA is above 2.4 g, the product yield of NOMC is quite low. One probable reason is that, during the subsequent carbonization procedure, the addition of more DCDA would bring out more volatile intermediates, which therefore promote the decomposition of nitrogen-containing frameworks of the as-synthesized NOMC materials (discussed below). Besides DCDA, we have also prepared other NOMC materials by applying urea and melamine as nitrogen-containing sources. The N2
2.2. Characterization X-ray diffraction (XRD) patterns of the materials were recorded with a D/max 2500 PC X-ray diffractometer (Rigaku) with a graphite monochromator (40 kV, 40 mA) using Ni-filtered Cu-Kα radiation. The surface areas and porous information were measured using an ASAP 2020 analyzer (Micromeritics). Before the analysis, the samples were degassed at 150 °C for at least 4 h. The specific surface areas were calculated based on the Brunauer–Emmet–Teller (BET) method, and pore size distributions were determined by the Barret–Joyner–Halenda method. Scanning electron microscopy (SEM) images were obtained on a Philips XL 30 microscope. Transmission electron microscopy (TEM) images were recorded on a JEOL 2010 electron microscope (200 kV). Xray photoelectron spectra (XPS) were recorded on an ESCALAB 250XI spectrometer (Thermo) using an Al anode as an X-ray source (12.5 kV). Raman spectra were collected on a Lab Ram HR evolution spectrometer (Jobin Yvon) using a 532 nm line as the excitation source. Elementary analysis (EA) was performed with Elementar Vario EL III instrument to calculate the carbon, nitrogen and hydrogen contents of the samples. 2
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Fig. 1. N2 adsorption–desorption isotherms (A) and the pore size distributions (B) of OMC and NOMC-xD materials. Table 1 Physical and chemical properties of OMC and NOMC-xD materials. Sample
SBET (m2 g−1)
Dpore (nm)a
Vpore (cm3 g−1)
a0 (nm)c
t (nm)d
ID/IG
Product mass (g)
Chemical compositione
OMC NOMC-0.8D NOMC-1.6D NOMC-2.4D NOMC-1.6Db
565 589 570 545 524
4.3 3.3 3.4 4.0 3.4
0.39 0.45 0.43 0.40 0.38
10.15 10.29 10.31 10.75 10.35
5.83 6.97 6.87 6.77 6.91
0.89 0.78 0.87 0.88 0.86
0.77 0.70 0.61 0.48 –
C60.7NH20.4 C39.5NH18.1 C32.8NH17.4 C21.5NH13.6 C34.5NH18.3
a b
Determined by the adsorption branches. Spent catalyst after five consecutive runs. 1 2 dhkl
4 h2 + hk + k2 3 a02
c
The lattice parameter (a0) is calculated by
d
The thickness of the mesoporous carbon wall (t) is calculated by t = a0 − Dpore. Determined by CHN elemental analysis.
e
=
based on (110) reflection.
Scheme 1. A possible synthetic route of NOMC via a one-pot co-assembly method using DCDA as a nitrogen source.
analyzed by EA. As listed in Table 1, the molar ratio of carbon to nitrogen in OMC is 60.7:1, indicating that the material contains a large number of carbon species. By comparison, NOMC materials possess higher contents of nitrogen than OMC. Moreover, with the increase of the amount of DCDA, the molar ratio of carbon to nitrogen of NOMC is decreased, suggesting that the nitrogen contents of NOMC materials can be easily controlled by adjustment of the amount of DCDA. According to the soft-templating synthetic approaches of NOMC
adsorption–desorption isotherms (Fig. S1) indicate that the materials have mesostructures along with narrow pore size distributions like NOMC-xD materials. Accordingly, the surface areas of NOMC-0.8U and NOMC-0.8 M are 553 and 538 m2 g−1 (Table S1), respectively. Likewise, the NOMC-M material adopting more melamine results in a slight decline in both surface area and pore volume, which is in accordance with the above NOMC-xD materials. The bulk chemical compositions of OMC and NOMC materials were 3
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further transforms into ordered mesostructure (Step 3). During the pyrolysis under nitrogen gas, F127 decomposes by itself (generally occurring at ca. 400 °C [7]), and the mesostructured NOMC material is finally obtained under higher carbonization temperature (Step 4). It should be mentioned that DCDA would also partially decompose below 400 °C and release a large amount of gas, thus inducing the decomposition or even partial collapse of ordered carbon framework of NOMC. Therefore, adding the mass of DCDA results in lower productivity of NOMC (Table 1). The small-angle XRD (SAXRD) was applied to further study the mesostructures of NOMC materials. The SAXRD pattern of OMC shows two pronounced diffraction peaks at 2θ = 0.8 and 1.44° (Fig. 2). The ratio of the corresponding d-spacing values of the peaks is close to 3 1 . It can be inferred that the two peaks are indexed as (100) and (110) reflections of two-dimensional hexagonal symmetry (p6mm), respectively [5,7]. The similar patterns are also observed in the SAXRD patterns of NOMC materials, which prove the existence of ordered mesostructure in NOMC. Moreover, the results of SAXRD patterns indicate that the incorporation of a small amount of DCDA into the original synthetic procedure has not altered the regular mesostructures of OMC. However, as the mass of DCDA is increased, the position of diffraction peak (110) is shifted towards lower 2θ values, corresponding to larger lattice parameters (a0) of NOMC (Table 1). This means that the lattice parameters of the p6mm symmetry of NOMC materials undergo expansion. Accordingly, the thickness of the carbon wall of mesostructures becomes thinner from NOMC-0.8D to NOMC-2.4D. Furthermore, the SAXRD patterns of NOMC-0.8U and NOMC-0.8 M (Fig. S2) manifest that the materials fabricated using urea and melamine as nitrogen source also possess ordered mesoporous structures. The micro-morphology of NOMC-0.8D and NOMC-1.6D samples are observed by SEM and TEM techniques. The SEM images of NOMC (Fig. 3) reveal rod-like domains with a mean width of ca. 100 nm. The aggregated particles of NOMC are like that of siliceous SBA-15 [26]. The existence of mesopores in NOMC materials detected by N2 adsorption–desorption analysis should originate from the interparticle voids. TEM images of NOMC materials exhibit well regular shutter-like strips (Fig. 4), further confirming the ordered mesoporous structures of
Fig. 2. Small-angle XRD patterns of OMC (a) and NOMC-xD (x = 0.8 (b), 1.6 (c), and 2.4 (d)) materials.
previously reported in the literature [1,4,5,7], a possible synthetic route of NOMC using DCDA, urea, and melamine as nitrogen sources has been proposed in this study. As illustrated in Scheme 1, the present synthetic method for NOMC is indeed based on the co-assembly of nitrogencontaining sources and structure-directing agent (tri-block PEO-PPOPEO F127 polymer). First, F127 molecules transform into stripe-like micelle in the aqueous solution and TMB prefers to locate inside and swell the hydrophobic domain (PPO) of F127 micelle (Step 1) [5]. Simultaneously, DCDA molecules are adsorbed on the hydrophilic section (PEO) of F127 via hydrogen-bonding interaction. On the other hand, HMTA hydrolyzes under 80 °C, thus generating formaldehyde. The liquid phase with basic condition originating from ammonia induces the condensation of resorcinol and formaldehyde, thereby gradually forming resorcinol-formaldehyde resin. Afterwards, the resin interacts with the hydrophilic section of F127 micelle by a hydrogen bond (Step 2). Namely, both resin and DCDA are located at the exterior of the F127 strip-type micelle. The co-assembly of DCDA-resol/F127 micelles
Fig. 3. SEM images of NOMC-0.8D (A&B) and NOMC-1.6D (C&D) materials.. 4
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Fig. 4. TEM images of NOMC-0.8D (A&B) and NOMC-1.6D (C&D) materials.
Fig. 7. XPS survey of OMC and NOMC-xD materials.
NOMC materials. Directly judged from the images, the pore sizes of NOMC are ca. 4 nm, close to the results gained from N2 sorption analysis in Table 1. Fig. 5 depicts the wide-angle XRD (WAXRD) patterns of OMC and NOMC. The two diffraction peaks appearing at 2θ = ca. 22.5 and 43.1°, which are ascribed to (002) and (100) reflections of discorded graphite [13,15,27] in NOMC. For NOMC-0.8D, the intensity of the diffraction peak is comparable to that of OMC. Whereas, further adding the mass of DCDA, the eventual intensity of peak is weaken. Raman spectroscopy was used to further investigate the graphitic character of the samples. The resultant spectra of both OMC and NOMC (Fig. 6) reveal two primary signals at 1356 and 1586 cm−1, which are assigned to the disordered (D) and graphitic (G) modes of graphiteanalogous material [15,22,27]. The intensity ratio of the two peaks (ID/ IG) is commonly applied to appraise the graphitic crystallinity. The calculated ID/IG values of NOMC materials (Table 1) are 0.78–0.89, illustrating that the synthesized NOMC materials possess comparatively highly sp2-hybridized carbon species, The results of Raman spectra agree with the WAXRD pattern described above. The chemical compositions of OMC and NOMC were probed by XPS technique. The spectrum of OMC (Fig. 7) shows carbon and oxygen elemental signals detected at binding energies of 285 and 533 eV. The occurrence of the oxygen element of OMC is possibly owing to the
Fig. 5. Wide-angle XRD patterns of OMC (a) and NOMC-xD (x = 0.8 (b), 1.6 (c), and 2.4 (d)) materials.
Fig. 6. Raman spectra of OMC (a) and NOMC-xD (x = 0.8 (b), 1.6 (c), and 2.4 (d)) materials.
5
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Fig. 8. N 1s spectra of NOMC-xD materials.
Scheme 2. Chemical structure of NOMC with various nitrogen functionalities.
Table 2 Distributions of various nitrogen species in NOMC materials. Sample
NOMC-0.8D NOMC-1.6D NOMC-2.4D NOMC-0.8 M NOMC-0.8U
Fig. 9. Correlation between the catalytic conversion of EC and the proportion of graphitic nitrogen species of various NOMC materials.
Nitrogen component (mol%) pyridinic
pyrrolic
graphitic
pyridinic oxide
29.1 27.1 22.2 28.9 27.0
39.6 41.0 54.4 41.5 42.1
14.3 20.7 14.1 16.4 16.3
17.0 11.2 9.3 13.2 14.6
residual water molecules adsorbed on the surface of OMC. In addition to the two elements, a weak peak at 400 eV is observed in each spectrum of NOMC. Calculated based on the peak areas integrated, the surface atomic ratios of carbon to nitrogen of NOMC materials are in the range of 20.7:1–37.8:1, depending on the adding mass of DCDA in the synthetic procedure of NOMC. The XPS survey spectra of NOMC0.8 M and NOMC-0.8U are depicted in Fig. S3. The carbon, nitrogen,
Table 3 Catalytic performances of OMC and NOMC materials in transesterification reactions of EC with CH3OHa.
Catalyst
Conv. (%)
Sel. (%)
Yield (%)
– OMC NOMC-0.8D NOMC-1.6D NOMC-2.4D NOMC-0.8 M NOMC-0.8U
41 53 76 86 71 83 82
40 61 69 89 69 80 82
16 32 52 76 49 66 67
a
200 mmol of CH3OH, 25 mmol of EC, T =120 °C, Wcatal. = 0.1 g, and t =3 h. 6
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3.2. Catalyst evaluation Dialkyl carbonates represented by dimethyl carbonate (DMC) and diethyl carbonate (DEC) have been extensively applied for numerous applications in the contemporary chemical industry [28]. Due to its high octane number and excellent biodegradability [29–31], DMC is a sustainable additive to gasoline. Furthermore, DMC is considered as a potential candidate to replace phosgene and dimethyl sulfate in carbonylation, and methylation processes [32–34], respectively. Among various synthetic processes, including phosgenation and oxidative carbonylation of methanol (CH3OH), transesterification of cyclic carbonates (such as ethylene carbonate and propylene carbonate) with CH3OH has been proposed as the most convenient and eco-benign approach for the manufacture of DMC [32,35,36]. The transesterification is an acid/base-catalyzed reaction. Up to now, the most efficient catalysts suggested for the transesterification of EC to DMC or DEC are mainly acidic and basic ionic liquids [37,38]. Despite their high catalytic activities, such homogeneous catalysts suffer from difficulty in product separation and catalyst recycling. In sharp contrast, heterogeneous catalysts are more convenient to be recycled, and more importantly, nanostructured and mesoporous heterogeneous catalysts have been well developed in recent years [39–45]. However, it should be pointed out, most of the heterogeneous catalysts including metal oxides [46] and their composites [33,47–49], and dawsonites [36,50], proposed for the transesterification reactions, contain metal cations. The potential metal contamination is undoubtedly unfavorable for the further application of DMC as the electrolyte [51]. Regarding these viewpoints, it is highly desired to develop a metal-free solid catalyst for the clean production of DMC via transesterification. Herein, as the present NOMC materials contain rich nitrogen species and more importantly, no metallic component has been involved in the overall synthetic procedure, the NOMC materials are applied as catalysts for the metal-free catalytic production of DMC. Initially, the reaction was performed without any catalyst (Table 3), and the blank result revealed that the conversion of EC was low (41 %). In the case of catalytic selectivity, the main product was mainly 2-hydroxyethyl methyl carbonate (HEMC) instead of the desired DMC. The monoester byproduct was probably because of the incomplete transesterification of EC [32,52]. After the addition of 0.1 g of OMC, the catalytic activity is obviously improved. The conversion of EC is 53 %, and the selectivity to DMC is increased to 61 %, respectively. Unfortunately, the yield of DMC (32 %) is still not satisfactory. Using NOMC materials instead, the catalytic conversion is remarkably enhanced. Among the three NOMCxD materials, NOMC-1.6D demonstrates the highest catalytic conversion and selectivity. In addition to NOMC-xD materials, the catalytic performances of other NOMC materials prepared using urea, and melamine as nitrogen precursors were tested under identical reaction conditions. The corresponding catalytic results (Table 3) show that the transesterification reactions could also be smoothly promoted in the presence of these NOMC materials. The EC conversion and DMC selectivity for each case are ca. 80 %. The catalytic results evidence that using other typical nitrogen-containing compounds, the synthesized NOMC samples could also act as efficient catalysts to promote the transesterification reactions between EC and CH3OH. The above XPS analysis (Fig. 8) has revealed that there are four kinds of nitrogen species existing in NOMC, and on the other hand, the transesterification of EC with CH3OH in this work undoubtedly belongs to base-mediated catalysis. To find out possible catalytic sites for the reactions promoted by NOMC, the correlation between the catalytic activity of various NOMC materials with their corresponding nitrogen species is tentatively elucidated. As described in Fig. 9, the catalytic activity has a good relationship with the proportion of graphitic nitrogen species of NOMC materials, suggesting that these graphitic nitrogen species have a positive effect for NOMC in the transesterification
Fig. 10. Effects of reaction temperature (A) and time (B) on the transesterification on catalytic performances Reaction conditions: 200 mmol of CH3OH, 25 mmol of EC, 0.1 g of NOMC-1.6D, T =120 °C (A), and t =3 h (B).
Fig. 11. Recycling catalytic performance of NOMC-1.6D in transesterification of EC with CH3OH. Reaction condition: 200 mmol of CH3OH, 25 mmol of EC, Wcatal. = 0.1 g (the first run), T =120 °C, and t =3 h.
and oxygen elements are observed in the two spectra, and the atomic ratios of carbon to nitrogen of NOMC-0.8 M and NOMC-0.8U are 29.6:1 and 30.5:1, respectively. For most of the nitrogen-containing carbon materials, there exist various nitrogen species in the forms of pyridine, pyrrole [8,9,15], etc. In order to elucidate the detailed chemical environment of nitrogen species, the N 1s spectra with high resolution of NOMC samples are deconvoluted. As described in Fig. 8, the N 1s spectra can be separated into four independent peaks. The intensive peak appearing at 398.4 eV is attributed to pyridinic nitrogen atoms [6,13], and the signal at 400.4 eV is attributed to pyrrolic nitrogen species (Scheme 2). The two nitrogen species account for the major proportion of the whole nitrogen species. The broad peak centered at 401.2 eV is assigned to graphitic nitrogen whereas the signal of 403.0 eV is ascribed to pyridinic oxide on the surface of NOMC [6,7]. Additionally, the deconvoluted N 1s spectra of NOMC-0.8U and NOMC-0.8M are presented in Fig. S4 and the corresponding molar proportions of various nitrogen components are listed in Table 2. From the full-range XPS spectra and deconvoluted N 1s spectra, it can be concluded that NOMC materials own multiple nitrogen-containing species, which thus dictate the basic character of the materials, which thus could be applied for the base-mediated organocatalysis.
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Table 4 Catalytic performances of NOMC-1.6D for transesterification reactions with various substratesa.
Cyclic carbonate
a b
Alcohol
T (°C)
t (h)
Conv. (%)
Sel. (%)b
CH3OH
120
3
86
89
C2H5OH
140
3
63
41
C2H5OH
140
6
76
78
CH3OH
140
6
37
84
200 mmol of alcohol, 25 mmol of cyclic carbonate, and Wcatal. = 0.1 g. Selectivities to the corresponding dialkyl carbonates.
EC and C2H5OH, even the temperature is elevated up to 140 °C, the selectivity to the target molecular (diethyl carbonate) is only 41 %. The dissociation of the OeH bond in C2H5OH is more difficult than that in CH3OH [48,52], which is responsible for the lower selectivity in the cases of C2H5OH than CH3OH. Despite this, upon prolonging the reaction to 6 h, higher selectivity along with enhanced conversion is obtained. Additionally, propylene carbonate could also be moderately converted into EC.
reactions. The attribution of such graphitic nitrogen to catalytically active sites agrees well with the relevant literature involving basemediated reactions (e.g. Knoevenagel condensation [53–55] and cycloaddition of CO2 with epoxides [56]) catalyzed by mesostructure carbon nitride materials. Since NOMC-1.6D exhibits superior catalytic activity to other NOMC materials under the same reaction conditions, the influences of reaction conditions on the eventual catalytic results have been evaluated adopting NOMC-1.6D as a representative catalyst. After the first hour, the catalytic EC conversion is only 50 % while the DMC selectivity is 53 % (Fig. 10A). Prolonging reaction, both the conversion and selectivity are increased progressively. After 3 h of reaction time, the EC conversion shows no further incensement. The catalytic reaction is also found to be sensitive to the reaction temperature (Fig. 10B). In the case of product distribution, the major product is HEMC rather than the target DMC as the temperature is below 100 °C. This means that, under low temperatures, the second step of transesterification between HEMC and CH3OH is difficult to proceed. Elevating the temperature could effectively promote the transfer of EC and HEMC into the DMC. Notwithstanding, the catalytic activity and selectivity level off as the temperature is above 120 °C. The catalytic results using various amounts of NOMC-1.6D catalyst are given in Fig. S5. Upon increasing the feeding amount of the catalyst, the EC conversion is gradually enhanced. It is apparent that more catalyst together with higher temperature is beneficial to convert more EC and/or HEMC to DMC. Fig. 11 shows the catalytic performances of the recycled NOMC-1.D material during five consecutive runs. The mass of the recycled catalyst for each run was almost 95 mg. For the second run, the catalytic conversion decreases to 72 %. This might result from the leaching of some nitrogen-containing fragments unsteadily adsorbed on the surface of NOMC into the liquid phase. A similar phenomenon has been also found in our previous work involving the transesterification of β-esters [18] and Knoevenagel condensation [25] catalyzed by NOMC materials. Nevertheless, the catalyst still demonstrates stable conversion and high selectivity within the subsequent catalytic runs. As mentioned above, catalytic transesterification of cyclic carbonates with CH3OH is a more sustainable approach for the production of dialkyl carbonates. Herein, in order to examine the catalytic versatility of NOMC, other cyclic carbonate and alcohol have been used in the transesterification reactions. As summarized in Table 4, in the case of
4. Conclusion In summary, adopting DCDA as a nitrogen source, a series of NOMC materials have been synthesized by a one-pot co-assembly approach under the liquid phase. The NOMC materials possess ordered mesostructures (two-dimensional hexagonal symmetry), with surface areas of 545–589 m2 g−1. Besides DCDA, urea and melamine could be also successfully applied as nitrogen sources for the fabrication of NOMC. The introduction of nitrogen into carbon materials is probably based on the interaction between nitrogen sources and F127 micelle via hydrogen bonding. The textural properties, graphitic crystallinity, and distribution of nitrogen species are related to the adding amount of DCDA. In the transesterification of EC, the synthesized NOMC materials as solid bases showed high catalytic activity and selectivity. The catalytic active sites might originate from the graphitic nitrogen atoms, the proportion of which correlated with the final catalytic activity. The work offers a convenient method to prepare NOMC materials, also enlarges the catalytic application of NOMC for the metal-free synthesis of dialkyl carbonates.
CRediT authorship contribution statement Jie Xu: Conceptualization, Writing - review & editing. Lin-Zhi Wen: Formal analysis, Writing - original draft. Yu-Lin Gan: Validation. Bing Xue: Supervision.
Declaration of Competing Interest The authors declare that there are no conflicts of interest. 8
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Acknowledgments
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