Scalable and super-stable exfoliation of graphitic carbon nitride in biomass-derived γ-valerolactone: enhanced catalytic activity for the alcoholysis and cycloaddition of epoxides with CO2

Zhimin Xue *a, Feijie Liu b, Jingyun Jiang b, Jinfang Wang b and Tiancheng Mu *b
aBeijing Key Laboratory of Lignocellulosic Chemistry, College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, China. E-mail: zmxue@bjfu.edu.cn
bDepartment of Chemistry, Renmin University of China, Beijing 100872, China. E-mail: tcmu@ruc.edu.cn

Received 23rd August 2017 , Accepted 18th September 2017

First published on 21st September 2017


Biomass-derived γ-valerolactone (GVL) could exfoliate bulk g-C3N4 to form a super-stable dispersion of few-layer g-C3N4 nanosheets with a high concentration of up to 0.8 mg mL−1 due to the polarity and the appropriate surface energy of GVL. The exfoliation process can be easily extended to a 200 ml scale and should be extended further. The formed g-C3N4 nanosheets showed enhanced activity for the alcoholysis of epoxides and the cycloaddition of epoxides with CO2 owing to their higher specific surface areas and more exposed active centers than the bulk g-C3N4. This affords a green, facile and scalable method to form few-layer g-C3N4 nanosheets and further expand the application of g-C3N4 materials to the field of non-photocatalysis.


Two-dimensional (2D) layered materials have attracted tremendous attention due to their unique properties and diverse applications.1–5 In the context of 2D materials, metal-free and nontoxic graphitic carbon nitride (g-C3N4) with high thermal and chemical stability can be easily synthesized using low-cost and abundant nitrogen-rich raw materials through a simple calcination process,6 and has emerged as a very attractive 2D layered material in solar energy conversion, electrocatalysis, and especially in photocatalysis.7–12 However, the applications of g-C3N4 in non-photocatalysis are very limited.13 The exploration of new non-photocatalytic applications of g-C3N4 has attracted more attention, which can significantly expand the application field of g-C3N4 to make full use of this unique metal-free material.

Generally, ultrathin g-C3N4 with few layers possesses better performances in the application of catalysis due to the higher specific surface area and the easy contact of active centers compared with the bulk one. Therefore, great efforts have been devoted in the synthesis of few-layer g-C3N4 nanosheets. Inspired by the strategies for other 2D layered materials (e.g., graphene, MoS2, and black phosphorus),14–16 exfoliation has great potential for the effective preparation of few-layer g-C3N4 nanosheets from the bulk g-C3N4.17 Several methods have been developed for the exfoliation of bulk g-C3N4, including chemical,18,19 micromechanical,20,21 thermal,22 and liquid-phase23–25 methods. Among them,26 liquid-phase exfoliation has gained more attention owing to the ease of obtaining few-layer g-C3N4 nanosheets in large quantities under relatively mild conditions. To date, several organic solvents (e.g., isopropanol,23 1,3-butanediol,25 and N-methyl-pyrrolidone23) and some mixed solvents24,27 have been applied in the exfoliation of bulk g-C3N4. However, these solvents are usually not so environmentally benign. Meanwhile, the obtained suspensions of few-layer g-C3N4 nanosheets are still at relatively low concentrations, and are not stable with precipitation or aggregation generated in a relatively short time. Therefore, further exploration of environmentally friendly solvents for the large-scale exfoliation of bulk g-C3N4 to form super-stable suspensions is still highly desirable in order to improve the exfoliation efficiency and fulfill the versatile applications of few-layer g-C3N4 nanosheets.

Nowadays, the utilization of biomass-derived compounds as solvents has become a hot field.28 In this regard, increasing interest has been paid to γ-valerolactone (GVL), which can be produced in a large scale from cellulose. GVL has been considered as an alternative dipolar aprotic solvent for common organic solvents due to its properties of biocompatibility and biodegradability.29–31 As is well known, solvents possessing suitable polarity, surface tension, and electron donation ability, etc., are the most efficient solvents for the exfoliation of bulk g-C3N4.17,25 Therefore, considering its polarity and surface tension, GVL as a biomass-derived solvent has great potential for the formation of few-layer g-C3N4 nanosheets by liquid-phase exfoliation.

Herein, we conducted the first study on the utilization of biomass-derived GVL to exfoliate bulk g-C3N4. This green process had high efficiency. A dispersion of few-layer g-C3N4 nanosheets with a high concentration of 0.8 mg mL−1 could be generated in GVL, which was stable for twelve months at ambient temperature without obvious precipitation or aggregation. Surprisingly, the formed few-layer g-C3N4 nanosheets showed enhanced activity for the alcoholysis of epoxides (a new reaction catalyzed over g-C3N4 nanosheets) and the cycloaddition of epoxides with CO2, compared with the bulk g-C3N4 (Fig. 1).


image file: c7gc02583a-f1.tif
Fig. 1 Schematic illustration of the GVL-exfoliation process from the bulk g-C3N4 to g-C3N4 nanosheets.

Similar to some reported exfoliation processes for other 2D layered materials, as shown in Fig. 1, bulk g-C3N4 was initially suspended in GVL at a concentration of 3 mg mL−1, and then sonicated (100 W) at a frequency of 40 kHz at room temperature for the gradual exfoliation of the bulk g-C3N4 to form few-layer g-C3N4 nanosheets. After a sonication time of 24 h, the dispersion was centrifuged at 4000 rpm for 30 min to remove aggregations, obtaining a homogeneous dispersion of few-layer g-C3N4 nanosheets with a concentration of up to 0.8 mg mL−1, which was calculated according to the Lambert–Beer law (Fig. S1),32 and was consistent with the results obtained by weighing. The obtained dispersion showed an obvious Tyndall effect (Fig. 1),25 and was stable after being stored for twelve months with no precipitation or aggregation (Fig. S2). Meanwhile, the concentration of few-layer g-C3N4 nanosheets in GVL after 12-month storage was still about 0.8 mg mL−1 according to the Lambert–Beer law (Fig. S1), further indicating the super stability of the g-C3N4 nanosheet dispersion in GVL, and the FT-IR spectra of the few-layer g-C3N4 nanosheets after being stored in GVL for twelve months were similar to those of the bulk one (Fig. S3), suggesting no change of the chemical structure after being stored for a long period (12 months). Additionally, the color of the exfoliated few-layer g-C3N4 nanosheets became lighter than that of the bulk g-C3N4 (Fig. S4). Furthermore, this method was still effective when the scale was expanded to 200 mL (Fig. S5). There were two main reasons for the high efficiency of GVL to exfoliate bulk g-C3N4. Firstly, as a dipolar aprotic solvent, GVL could form hydrogen bonds with the dangling hydrogens on the layers of bulk g-C3N4,24,25 and thus bulk g-C3N4 was expanded and exfoliated in GVL assisted by weak sonication. Secondly, GVL has appropriate surface energy (∼60 mJ m−2),33 which was matched with van der Waals bonded surfaces (∼70 mJ m−2);23,34 therefore, GVL showed remarkable performance for the exfoliation of bulk g-C3N4 to form few-layer g-C3N4 nanosheets.

The morphology of the obtained few-layer g-C3N4 nanosheets was determined by transmission electron microscopy (TEM) and high resolution TEM (HR-TEM). As shown in Fig. 2a, few-layer g-C3N4 nanosheets were indeed generated through the GVL-exfoliation process, which illustrated an obvious comparison with the bulk g-C3N4 (Fig. S6). The HR-TEM images (Fig. 2b) showed that the exfoliated g-C3N4 was very thin and transparent, indicating that the obtained g-C3N4 was made of monolayers or a few atomic layers. The structure of the as-prepared few-layer g-C3N4 nanosheets was also characterized by using the atomic force microscopy (AFM) method. The results of AFM in Fig. 2c and d showed that the average thickness of the few-layer g-C3N4 nanosheets was about 2 nm, suggesting that the achieved g-C3N4 consisted of about 6 layers.24,35


image file: c7gc02583a-f2.tif
Fig. 2 TEM image (a), HR-TEM image (b), AFM image (c), and the corresponding thickness analysis (d) taken around the blue line in (c) for the obtained few-layer g-C3N4 nanosheets.

The crystal structure of the few-layer g-C3N4 nanosheets was examined through the X-ray diffraction (XRD) method (Fig. 3a). A strong peak for the (002) plane at 27.7° was found, which was the characteristic interlayer stacking reflection of conjugated aromatic systems.25 However, the intensity of this peak for the few-layer g-C3N4 nanosheets significantly decreased compared with that for the bulk g-C3N4, indicating that the bulk g-C3N4 had been successfully exfoliated as we desired, which was consistent with the results obtained from TEM and AFM (Fig. 2). Furthermore, the X-ray photoelectron spectroscopy (XPS) technique was conducted to determine the chemical states of the prepared few-layer g-C3N4 nanosheets and the bulk one (Fig. 3b). Meanwhile, in the high resolution XPS spectra of C 1s (Fig. 3c), the peak appearing at 288.4 eV originated from the sp2-bonded carbon (N–C–N) in the aromatic ring, while the peak at 284.8 with low intensity was related to carbon contamination.23 Additionally, from high resolution XPS of N 1s (Fig. 3d), the spectrum was divided into three peaks indicating three types of N-bonding in the g-C3N4. The three peaks of N 1s at 398.4, 399.5 and 400.8 eV were attributed to sp2 N atoms in triazine rings, tertiary nitrogen (N–(C)3) and terminal amino functions (C–N–H),24 respectively. These results of XPS examinations showed that there was no obvious change for the chemical states of both carbon and nitrogen in both bulk g-C3N4 and the few-layer g-C3N4 nanosheets obtained in GVL (Fig. 3b, c, and S7).


image file: c7gc02583a-f3.tif
Fig. 3 XRD pattern (a), XPS survey spectra (b), high-resolution N 1s XPS spectra (c), high-resolution N 1s XPS spectra (d), FT-IR spectra (e) and UV-visible absorption spectra (f) of the g-C3N4 nanosheets and the bulk g-C3N4.

The chemical structure of the few-layer g-C3N4 nanosheets was further studied by using FT-IR spectra (Fig. 3e). The broad peaks from 3000 to 3600 cm−1 resulted from the N–H stretches, suggesting the existence of dangling hydrogens in the g-C3N4 layers,23,24 which could form the hydrogen bond with GVL, and thus cause the exfoliation of the bulk g-C3N4 as discussed above. The peak at 807 cm−1 was the characteristic breathing mode for the s-triazine ring.23 Other peaks in the FT-IR spectra were due to the stretching vibration of C[double bond, length as m-dash]N and C–N heterocycles.25 Meanwhile, the spectra of few-layered g-C3N4 nanosheets were similar to that of bulk g-C3N4, indicating that the chemical structure was unchanged after exfoliation in GVL, which was consistent with the results obtained from XPS examinations. Meanwhile, UV-vis diffuse reflectance spectroscopy (Fig. 3f) showed that the absorption spectrum of few-layer g-C3N4 nanosheets displayed a slight blue shift in comparison with the bulk g-C3N4, and the band gap of bulk g-C3N4 of ∼2.64 eV increased to 2.78 eV for the obtained few-layer g-C3N4 nanosheets. Additionally, from the Raman spectra, the exfoliated g-C3N4 nanosheets showed nearly identical Raman shifts of bulk g-C3N4 (Fig. S8a), indicating that the obtained g-C3N4 nanosheets retained the same crystal structure as that of the bulk g-C3N4. However, the exfoliated g-C3N4 nanosheets showed a blue shift of about 1.7 cm−1 (Fig. S8b), ascribed to the phonon confinement effect of the as-exfoliated nanosheets and inferring their ultrathin thickness.24

Generally speaking, few-layer g-C3N4 nanosheets possess better performance than the bulk g-C3N4 when it is used as a catalyst because the few-layer g-C3N4 nanosheets have higher specific surface areas and more exposed active centers compared with the bulk one, which makes the reactant come into contact with the catalytic centers more easily. Herein, we attempted to utilize g-C3N4 materials (few-layer nanosheets or the bulk one) as efficient catalysts in non-optical reactions in order to expand the application field of g-C3N4 materials. We conducted the alcoholysis of epoxides (a new reaction catalyzed over g-C3N4 nanosheets) and the cycloaddition of epoxides with CO2 (Scheme S1) to examine the difference of catalytic activity between the obtained few-layer g-C3N4 nanosheets and the bulk g-C3N4 (Tables 1 and 2). Initially, control experiments indicated that 120 °C was a suitable reaction temperature for the alcoholysis of glycidyl phenyl ether over few-layer g-C3N4 nanosheets under our reaction conditions (Fig. S9). Subsequently, kinetic experiments for the alcoholysis of glycidyl phenyl ether (Fig. 4) showed that the reaction over few-layer g-C3N4 nanosheets could be completed in 5 h at 120 °C, showing a higher rate than that over the bulk one, which needed about 8 h to fulfill the reaction. Meanwhile, we observed that the reaction rate over the bulk g-C3N4 in 3 h was lower than that from 3 h to 8 h, which may be caused by the exfoliation effect of methanol in the initial 3 h. This interesting phenomenon to some extent proved the higher activity of few-layer g-C3N4 nanosheets. We also conducted the alcoholysis of other epoxides with different alcohols (Table 1, entries 2–7) over few-layer g-C3N4 nanosheets and the bulk one, and all the experiments provided the same tendency that few-layer g-C3N4 nanosheets possessed higher activity than the bulk one. Additionally, the few-layer g-C3N4 nanosheets could be reused at least four times for the alcoholysis of glycidyl phenyl ether (Fig. S10) without obvious aggregations of the C3N4 layers (Fig. S11) due to the exfoliation effect of the used alcohol to some extent.23 Furthermore, the cycloaddition of epoxides with CO2, an important reaction for CO2 transformation,36–38 was also examined over both few-layer g-C3N4 nanosheets and the bulk one. As shown in Table 2, few-layer g-C3N4 nanosheets still showed a higher activity than the bulk one for the cycloaddition of all the examined epoxides and CO2 at 120 °C with a CO2 pressure of 3 MPa. In addition, the few-layer g-C3N4 nanosheets could also be reused at least four times for the cycloaddition of glycidyl phenyl ether with CO2 (Table S1) without obvious g-C3N4 nanosheet aggregations (Fig. S12) due to the exfoliation effect of the in situ formed cyclic carbonates. These above results proved the superiority of the exfoliated few-layer g-C3N4 nanosheets as efficient catalysts compared with the bulk one.


image file: c7gc02583a-f4.tif
Fig. 4 Kinetic experiments for the alcoholysis of glycidyl phenyl ether over g-C3N4 nanosheets and bulk g-C3N4. Reaction conditions: Glycidyl phenyl ether, 1 mmol; methanol, 4 g; catalyst, 0.1 g; reaction temperature, 120 °C.
Table 1 Catalytic activity of g-C3N4 nanosheets and bulk g-C3N4 for the alcoholysis of epoxidesa
Entry Epoxide Alcohol Product Time (h) Yieldb (%)
a Reaction conditions: Epoxide, 1 mmol; alcohol, 4 g; temperature, 120 °C; g-C3N4, 0.1 g. b The yields were determined by GC using anisole as the internal standard. The values in parentheses are for the reactions conducted over bulk g-C3N4.
1 image file: c7gc02583a-u1.tif Methanol image file: c7gc02583a-u2.tif 5 98.7 (89.6)
2 image file: c7gc02583a-u3.tif Methanol image file: c7gc02583a-u4.tif 6 97.9 (82.3)
3 image file: c7gc02583a-u5.tif Methanol image file: c7gc02583a-u6.tif 5 99.4 (86.5)
4 image file: c7gc02583a-u7.tif Methanol image file: c7gc02583a-u8.tif 8 96.8 (82.3)
5 image file: c7gc02583a-u9.tif Methanol image file: c7gc02583a-u10.tif 6 97.1 (85.6)
6 image file: c7gc02583a-u11.tif Ethanol image file: c7gc02583a-u12.tif 8 94.9 (79.3)
7 image file: c7gc02583a-u13.tif 1-Propanol image file: c7gc02583a-u14.tif 10 87.7 (78.3)


Table 2 Catalytic activity of g-C3N4 nanosheets and bulk g-C3N4 for the cycloaddition of epoxides with CO2[thin space (1/6-em)]a
Entry Epoxide Product Pressure (MPa) Time (h) Yieldb (%)
a Reaction conditions: Epoxide, 10 mmol; g-C3N4, 50 mg; CO2, 3 MPa; temperature, 120 °C. b The yields were determined by GC using n-butanol as the internal standard. The values in parentheses are for the reactions conducted over bulk g-C3N4. c The reaction was conducted at 110 °C.
1 image file: c7gc02583a-u15.tif image file: c7gc02583a-u16.tif 0.5 4 53.6 (36.1)
2 image file: c7gc02583a-u17.tif image file: c7gc02583a-u18.tif 1 4 82.7 (50.2)
3 image file: c7gc02583a-u19.tif image file: c7gc02583a-u20.tif 2 4 93.2 (68.4)
4 image file: c7gc02583a-u21.tif image file: c7gc02583a-u22.tif 3 4 99.2 (73.3)
5c image file: c7gc02583a-u23.tif image file: c7gc02583a-u24.tif 3 4 88.3 (59.1)
6 image file: c7gc02583a-u25.tif image file: c7gc02583a-u26.tif 3 4 98.9 (81.9)
7 image file: c7gc02583a-u27.tif image file: c7gc02583a-u28.tif 3 8 96.3 (77.6)
8 image file: c7gc02583a-u29.tif image file: c7gc02583a-u30.tif 3 20 97.2 (87.4)


Conclusions

In summary, biomass-derived GVL had been successfully used as an efficient solvent to exfoliate bulk g-C3N4 to generate few-layer g-C3N4 nanosheets. A stable dispersion of few-layer g-C3N4 nanosheets with a high concentration of up to 0.8 mg mL−1 could be obtained due to the polarity and the appropriate surface energy of GVL, and no obvious precipitation or aggregation was found after being stored for twelve months at ambient temperature. More importantly, the formed few-layer g-C3N4 nanosheets showed enhanced activity for the alcoholysis of epoxides and the cycloaddition of epoxides with CO2 compared with the bulk g-C3N4, which was caused by the higher specific surface areas and more exposed active centers of the few-layer g-C3N4 nanosheets. This work expands the applications of few-layer g-C3N4 nanosheets except for their utilization in photocatalysis and provides a helpful example that few-layer g-C3N4 nanosheets have catalytic activity compared to bulk g-C3N4. We believe that GVL has great potential to exfoliate various 2D materials efficiently for various applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Fundamental Research Funds for the Central Universities (2017ZY40) and the National Natural Science Foundation of China (21503016, 21773307).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7gc02583a

This journal is © The Royal Society of Chemistry 2017