The roles of formic acid and levulinic acid on the formation and growth of carbonaceous spheres by hydrothermal carbonization

Abstract

The catalytic conversion of biomass into 5-hydroxymethylfurfural (HMF) and acid has been investigated. However, until now there has been little mechanism research on the roles of acids produced during the process of hydrothermal carbonization of biomass on the formation and growth of carbonaceous spheres. In this work, we specifically demonstrate how the formic acid and levulinic acid molecules participate in the formation and growth of carbonaceous spheres by analyzing the size variations of solid products obtained from the four solution systems. The formic acid molecules play a critical role in catalytic conversion of fructose into HMF, which promotes the growth of carbonaceous spheres considerably. The primary function of levulinic acid molecules is taking part in the growth as building units and slowing the growth by reducing the surface density of hydroxyl groups of carbonaceous spheres. In addition, the levulinic acids molecules existed in solutions also promote the conversion from fructose to HMF to a certain extent.

---

Introduction

In the last few decades, carbon-based materials including carbon nanotubes, carbon fibers and more recently graphene trigger widespread attention due to their superior performances in solar cell, energy storage, supercapacitors.^1–3 Nowadays, functional carbon spheres with controllable size, surface morphology, porosity, and chemical composition are a fascinating topic in the carbon community for their promising applications in various fields, such as catalysts or catalyst supports, electrode materials, CO₂ capture, microwave adsorption and sacrifice templates.^3–12 Moreover, ultrafine carbon nanospheres and hollow nanospheres with sizes below 200 nm could be easily internalized into cells by intracellular endocytosis and thus have been extended to biomedical and pharmaceutical applications such as drug delivery, imaging agents.^13–16 To date, there have been several successful routes for synthesis of carbon spheres: chemical vapor deposition (CVD),¹⁷ arc-discharge process,¹⁸ hydrothermal carbonization (HTC) of biomass,³ modified Stöber synthesis,¹⁹ organic–organic self-assembly,^20,21 and so on. Among them, hydrothermal carbonization of biomass is widely considered as a versatile method for producing functionalized carbonaceous spheres in large scale because of low temperature (e.g., <300 °C), self-generated pressure (e.g., ∼1.0 MPa), available raw materials (such as polymers,²² carbohydrates,²³ microalgae²⁴ and so on).

For carbonaceous spheres via hydrothermal carbonization of carbohydrates, the mechanism of their formation and growth is still a complex process even though tremendous efforts have been made in this regard.^25,26 Combined with previous researches,^23,27 we recently reported a new three-step mechanism using sucrose as a precursor material.²⁸ However, there are still some confusions on the roles of acids produced during hydrothermal process, especially for formic acid, which is regarded as an important chemical to accelerate conversion from fructose to HMF.^29,30 Although experiments involving formic acid and levulinic acid have been conducted,^31,32 the results of these experiments mainly focus on conversion of reactants into 5-hydroxymethylfurfural (HMF) in a catalytic perspective without available description or analysis of the formation of carbonaceous spheres. Moreover, some additives are also involved in the process of hydrothermal carbonization of carbohydrates to change functional groups on the surface of carbonaceous spheres and improve monodispersity,^33,34 but these additives such as sodium polyacrylate and acrylic acid couldn't be produced during the hydrothermal process and even disturb the understanding of the mechanism for formation and growth of carbonaceous spheres to some degree.

Based on our previous experimental results,²⁸ several by-products including formic acid, levulinic acid and acetic acid have been found in residual solutions after hydrothermal reaction by solution ¹³C nuclear magnetic resonance (NMR) characterization. As reported in previous literature, formic acid and levulinic acid are directly generated by rehydration of HMF,³⁵ and acetic acid comes from fragmentation of fructose.³⁶ And these acids lower the pH value of solutions, facilitate the conversion of fructose to HMF subsequently.³⁷ As for acetic acid, it was initially detected in 0.100 M sucrose solution after 3.75 h, besides, the peak intensity of acetic acid in ¹³C solution NMR spectra was very weak.²⁸ So the effect of acetic acid on the formation and growth of carbonaceous spheres could be assumed to be trivial due to its trace amount and late appearance. In this paper, we focus intensively on revealing the roles of formic acid and levulinic acid in the formation and growth of carbonaceous spheres, and illuminating the critical point to obtain monodispersed carbonaceous spheres, which are also effective complementing to the whole mechanism for hydrothermal carbonization of biomass.

Experimental

All the chemicals were of analytical grade purchased from Sinopharm Chemical Reagent Co., Ltd. and were used without further purification.

A typical synthesis of carbonaceous spheres with formic acid as additive was shown in following procedure. Firstly, 1.711 g sucrose was dispersed in 50 ml water to form 0.1 M sucrose solution by vigorous stirring. And 0.3 ml formic acid was added into 50 ml 0.1 M sucrose solution, after stirring, the mixed solution with a volume of 30 ml was transferred to a 45 ml Teflon-lined stainless steel autoclave. Then the reactor vessel was directly transferred into muffle furnace maintaining 180 °C and maintained for a period of time. After this, the reactor vessel was immediately immersed in water to quench the reaction. Finally, the precipitates were collected by centrifugation (4000 rpm, 30 min), and rinsed with distilled water and anhydrous ethanol more than three times, then dried at 80 °C in a vacuum drying oven for 12 h.

Herein, different additives including formic acid and levulinic acid were selected for hydrothermal experiments based on our previous work. There were four solution systems under hydrothermal carbonization including the 0.100 M sucrose solution without any additive, the 0.100 M sucrose and 0.159 M formic acid mixed solution, the 0.100 M sucrose and 0.100 M levulinic acid mixed solution, the 0.100 M sucrose, 0.100 M levulinic acid and 0.050 M formic acid mixed solution in this paper, which could be indicated as the original system, the adding formic acid system, the adding levulinic acid system and the adding mixed acids system correspondingly.

The morphologies and sizes of carbonaceous spheres were characterized by a field emission scanning electron microscope (FESEM, ZEISS & ULTRA PLUS). The diameters of carbonaceous spheres were measured by image processing software from SEM images of each sample, and statistical analysis was performed subsequently. The sampling number of each sample was more than 1000. The X-Ray Photoelectron Spectroscopy (XPS) analysis was carried out with a Thermo ESCALAB 250 instrument using monochromatic Al Kα radiation (1486.6 eV). Binding energies for the high-resolution spectra were calibrated by setting C 1s at 284.6 eV. The detailed statistic results of carbonaceous spheres in this work are listed in Tables S1–S4 (ESI†).

Results and discussion

The functional groups present on the outer layer of carbonaceous spheres were characterized by X-ray photoelectron spectroscopy (XPS). Fig. 1 shows the C 1s and O 1s XPS spectra for carbonaceous spheres reacted for 3.75 h in the original system. The C 1s spectrum (Fig. 1a) could be deconvoluted into four individual component peaks at 284.6, 285.7, 287.2, 289.0 eV. These peaks are attributed to carbon groups (C=C, CH_x, C–C), hydroxyl groups or ethers (–C–OR), carbonyl or quinone groups (>C=O), and carboxylic groups, esters, or lactones (–COOR) respectively.^25,38 For the O 1s spectrum (Fig. 1b), two peaks were identified at 531.7 and 533.1 eV. The former peak corresponds to carbonyl groups (C=O), while the peak at 533.1 eV is attributed to hydroxyl groups or ethers (C–OH/C–O–C).^25,38 Combined with Fourier transform infrared spectroscopy (FT-IR) results in our recently published paper,²⁸ it is concluded that the outer layer of carbonaceous spheres contains lots of reactive groups (i.e. hydroxyl, carbonyl), and that these oxygen-containing functional groups make outer layer hydrophilic and provide numerous active sites for further growth of carbonaceous spheres.

Fig. 1 The typical X-ray photoelectron spectra of C 1s (a) and O 1s (b) of carbonaceous spheres reacted for 3.75 h in the original system.

The 0.100 M sucrose solution without any additive was initially selected as controlled group, which was indicated as the original system. The SEM images and diameter histograms of carbonaceous spheres obtained from the original system are shown in Fig. 2. These data are directly taken from our paper recently published,²⁸ except for the cases for 3.25 h and 6 h. Interestingly, a notable phenomenon that a sudden drop of mean diameter of carbonaceous spheres takes place with the extension of time when adopting the original system as reaction medium could be observed. And this phenomenon is related with a complicated mechanism for formation and growth of carbonaceous spheres, which has been expounded in our paper.²⁸

Fig. 2 SEM images and diameter histograms of carbonaceous spheres under various experimental conditions. 0.100 M sucrose solutions hydrothermally treated at 180 °C for (a) 3 h, (b) 3.25 h, (c) 3.5 h, (d) 3.75 h, (e) 4 h, (f) 4.5 h, (g) 5 h, (h) 5.5 h, (i) 6 h in rapid heating route.

For the adding formic acid system (0.100 M sucrose and 0.159 M formic acid mixed solutions), the diameters of solid products obtained were two size greater than those of the original system entirely, as shown in Fig. 2–4. Despite the increasing trend of diameters of carbonaceous spheres for these two systems, the larger size difference between original system and adding formic acid system has been amplified with the reaction time prolonging. Almost all the sizes of carbonaceous spheres in the original system were less than 300 nm, while the sizes in adding formic acid system exceeded 500 nm apart from the case reacted for 3.75 h.

Fig. 3 SEM images and diameter histograms of carbonaceous spheres under various experimental conditions. 0.100 M sucrose and 0.159 M formic acid mixed solutions hydrothermally treated at 180 °C for (a) 3 h, (b) 3.25 h, (c) 3.5 h, (d) 3.75 h, (e) 4 h, (f) 4.5 h, (g) 5 h, (h) 5.5 h, (i) 6 h in rapid heating route.

Fig. 4 The variations of diameters of carbonaceous spheres obtained from the original system and the adding formic acid system. The bound of error bar represents 90% range of statistical samples, and the intermediate point reflects the mean diameter of carbonaceous spheres among 50% range in each group.

As shown in Fig. 4, the variation trend of sizes of carbonaceous spheres in adding formic acid system is extremely obvious. The mean diameter of products increases linearly with the reaction time before 3.5 h, while the growth rate of carbonaceous spheres is on the decline after 4 h. In addition, a sudden drop of mean diameter is observed clearly at 3.75 h for adding formic acid system. Although a similar phenomenon has also been confirmed in adding formic acid system, its mechanism is distinctly different from that of original system. For the original system, large amounts of newly formed HMF molecules coming from convention of fructose catalyzed by yielded acids result in the appearance of large amounts of carbonaceous spheres, which accounts for the sudden drop in mean diameter.²⁸ As regards the adding formic acid system, formic acid has been added in 0.100 M sucrose solution before hydrothermal carbonization, so it is reasonable to assume there may be other factors that needed to be taken into consideration. At this time, this could be attributed to the influence of levulinic acid. As mentioned above, formic acid and levulinic acid are directly generated simultaneously by rehydration of HMF in original system under hydrothermal condition.³⁵ The existence of a large amount of formic acid suppresses the rehydration of HMF for adding formic acid system, which leads to extremely low concentration of levulinic acid in initial stage of hydrothermal carbonization. However, the concentration of HMF increases rapidly due to convention of fructose catalyzed by formic acid with the prolongation of reaction time. Subsequently, levulinic acid molecules in large quantity are obtained and they may take a role in the formation and growth of carbonaceous spheres as “Capping Agents” to some extent. The main component of carbonaceous spheres obtained from hydrothermal carbonization is HMF molecule which has been proved by solid state ¹³C nuclear magnetic resonance (NMR) spectroscopy,²⁷ and combined with the XPS spectroscopy above, it is easy to get a conclusion that there are many reactive groups including hydroxyl, aldehyde and carboxyl groups existed in the surface of carbonaceous spheres. And these reactive groups, hydroxyl groups in particular, are likely to react with carboxyl groups of levulinic acid molecules, which makes levulinic acid participate in the growth of carbonaceous spheres. This view could be supported by two-dimensional ¹³C DQ-SQ MAS NMR correlation spectrum by Baccile et al.,²³ they have confirmed the existence of levulinic acid in carbonaceous spheres and developed a visualized structural model in which levulinic acid accounts for around 15 wt% carbon mass. And the variation of pH values of solutions from the adding formic acid system under different reaction time also provides evidence of this view, as shown in Fig. S1 (ESI†). Before hydrothermal carbonization, the pH value is 2.25 due to 0.3 ml formic acid adding in sucrose solution. The pH value decreases generally with the process of reaction, meaning that more and more formic acid and levulinic acid are produced by the rehydration of HMF molecules. The adding formic acid system gets low pH values at 3.5 and 3.75 h, corresponding with the drop of mean diameter at 3.75 h. To put it simply, the surface density of reactive groups (especially for hydroxyl group) for carbonaceous spheres decreases rapidly due to high concentration of levulinic acid, which makes it difficult for carbonaceous spheres pre-existing to grow and prompts the emergence of a new batch of carbonaceous spheres. That is, levulinic acid molecules in large quantity result in the abnormal change of mean diameter in adding formic acid system.

In order to demonstrate the roles of formic acid and levulinic acid, two groups of contrast experiments including the adding levulinic acid system and the adding mixed acids system have also been conducted systematically. Generally speaking, the sizes of carbonaceous spheres in the adding levulinic acid system are slightly larger than those in the original system, as shown in Fig. 2, 5 and 6. And the trend of mean diameter change for the adding levulinic acid system is mostly consistent with that for the original system. As reported in our previous paper, the sudden drop of mean diameter at 3.75 h for the original system is closely related to the high concentration of HMF, and the emergence of the second batch of carbonaceous spheres in abundant accounts for the unique phenomenon.²⁸ However, there are some distinguished details for the integral changing tendency of mean diameter for adding levulinic acid system, the mean diameters of carbonaceous spheres decrease moderately in the period from 3.75 to 4.5 h, as shown in Fig. 6. Actually, the postponed decent point of mean diameter and the extended generation period of the second batch of carbonaceous spheres are noteworthy for analyzing the role of levulinic acid in this system. Similar to the adding formic acid system, levulinic acid molecules in large quantity suppress the rehydration of HMF, which results in a tiny amount of formic acid molecules in the initial stage. At this time, HMF molecules in small quantity couldn't provide powerful support for the formation of carbonaceous spheres, and conventional growth pattern is presented. But after a period of time, more and more formic acid molecules are produced with the gradually increasing concentration of HMF, which further accelerates conversion from fructose to HMF. Understandably, the surface density of reactive groups (especially for hydroxyl group) of carbonaceous spheres declines with their sizes increasing. Due to the existence of levulinic acid in large quantity, the growth of carbonaceous spheres is almost inhibited completely, and HMF molecules mainly contribute to the formation of new batch of carbonaceous spheres. This is the reason for the drop of mean diameter of carbonaceous spheres in the period from 3.75 h to 4.5 h for adding levulinic acid system. So it is reasonable that levulinic acid molecules are involved in the formation and growth of carbonaceous spheres as indispensable building units, they decrease the growth rate of carbonaceous spheres to some degree by preventing many hydroxyl groups in the surface of carbonaceous spheres from reactions with HMF molecules.

Fig. 5 SEM images and diameter histograms of carbonaceous spheres under various experimental conditions. 0.100 M sucrose and 0.100 M levulinic acid mixed solutions hydrothermally treated at 180 °C for (a) 3 h, (b) 3.25 h, (c) 3.5 h, (d) 3.75 h, (e) 4 h, (f) 4.5 h, (g) 5 h, (h) 5.5 h, (i) 6 h in rapid heating route.

Fig. 6 The variations of diameters of carbonaceous spheres obtained from the original system and the adding levulinic acid system. The bound of error bar represents 90% range of statistical samples, and the intermediate point reflects the mean diameter of carbonaceous spheres among 50% range in each group.

Moreover, the adding mixed acids system is convincing proof to illustrate the roles of formic acid and levulinic acid. The SEM images and diameter histograms are shown in Fig. 7, and the variations of diameters of carbonaceous spheres obtained from all four systems are presented in Fig. 8. On the whole, the sizes of carbonaceous spheres in the adding mixed acids system are between those of the adding levulinic acid system and adding formic acid system. Besides that, the variation trends of mean diameters of carbonaceous spheres are roughly the same between the adding mixed acids system and the adding formic acid system. These are interesting results due to the comprehensive influences of formic acid and levulinic acid. It is worthy to note that the mean diameters of carbonaceous spheres decrease twice at 3.5 h and 4 h for the adding mixed acids system. And the declines could both be attributed to many newly-formed carbonaceous spheres with smaller sizes, which could be reasonably interpreted by the diameter histograms of products. The carbonaceous spheres with diameter above 400 nm obtained from 3.25 h group (Fig. 7b) account for more than 90% of the total, while those with diameter above 400 nm in 3.5 h group (Fig. 7c) only occupy about 35%. Simultaneously, carbonaceous spheres with diameter above 450 nm reacted for 3.75 h in adding mixed acids system (Fig. 7d) take over 60% of the total number, and those with size larger than 450 nm reacted for 4 h (Fig. 7e) make up around 45% of sample number. These convincing evidences clearly manifest that the newly-formed carbonaceous spheres with smaller size dilute the percentage of those with larger size.

Fig. 7 SEM images and diameter histograms of carbonaceous spheres under various experimental conditions. 0.100 M sucrose, 0.050 M formic acid and 0.100 M levulinic acid mixed solutions hydrothermally treated at 180 °C for (a) 3 h, (b) 3.25 h, (c) 3.5 h, (d) 3.75 h, (e) 4 h, (f) 4.5 h, (g) 5 h, (h) 5.5 h, (i) 6 h in rapid heating route.

Fig. 8 The variations of diameters of carbonaceous spheres obtained from four solution systems. The bound of error bar represents 90% range of statistical samples, and the intermediate point reflects the mean diameter of carbonaceous spheres among 50% range in each group.

As mentioned above, the surface density of hydroxyl group for carbonaceous spheres declines with their sizes increasing, consequently, the role of levulinic acid as “Capping Agent” would be gradually revealed. A certain amount of HMF molecules could be obtained before 3.25 h for the adding mixed acids system, due to adding formic acid at the beginning. The carbonaceous spheres grow rapidly to around 450 nm at 3.25 h. Subsequently, a large quantity of newly-formed carbonaceous spheres appear in the period from 3.25 h to 4 h. Obviously, the consistent production of HMF molecules catalyzed by formic acid leads to this. However, the mean diameters of carbonaceous spheres tend to hover around 450 nm in the period. And the slow growth of carbonaceous spheres from 4 h to 4.5 h is clearly shown in Fig. 8. The levulinic acid molecules are definitely involved in this phenomenon. As a matter of fact, the definite concentration of levulinic acid is very sensitive to the surface density of hydroxyl group of carbonaceous spheres. Without enough HMF molecules, levulinic acid molecules would restrict the size of carbonaceous spheres to a limit. Once the amount of carbonaceous spheres with limited size reach the extreme, the growth of carbonaceous spheres restart again. And the continuous consumption of levulinic acid molecules takes place in the period from 4 h to 4.5 h.

Discussion

Based on experimental results above, the growth of carbonaceous spheres in the adding formic acid system are promoted due to numerous HMF molecules and a relatively small amount of levulinic acid molecules. Conventionally, HMF could be prepared via dehydration of fructose or fructose precursors in the presence of an acid catalyst, which mainly involves homogeneous catalysts (such as HCl, H₂SO₄, formic acid, oxalic acid, etc.).^30,39 Moreover, it has been reported that formic acid (pK_a 3.75) with stronger acid strength shows a higher catalytic efficiency than levulinic acid (pK_a 4.95) in a methyl isobutyl ketone/water biphasic system.³¹ Therefore, formic acid molecules in large quantity essentially lead to adequate growth of carbonaceous spheres for the adding formic acid system compared with the original system and adding levulinic acid system. Besides, no solid-state ¹³C NMR signal of formic acid has been detected for carbonaceous spheres obtained from hydrothermal carbonization in the previous literature. As a conclusion, formic acid molecules don't participate in growth of carbonaceous spheres as building agents, they play a critical role in catalytic conversion of fructose into HMF, which promotes the growth of carbonaceous spheres considerably.

Intensive analysis of variations of diameters of carbonaceous spheres obtained from four solution systems combined with detection of solid-state ¹³C NMR signal of levulinic acid indicates that levulinic acid molecules participate in the growth of carbonaceous spheres as building units. Although levulinic acid molecules constitute an important part of the carbonaceous spheres, they sharply reduce the growth rate of carbonaceous spheres by replacing HMF molecules to react with hydroxyl groups on the surface of carbonaceous spheres. And the role of levulinic acid on the growth of carbonaceous spheres is illustrated schematically in Fig. 9. For the adding levulinic acid system, the sizes of carbonaceous spheres are slightly larger than those from the original system, which can be attributed to higher yield of HMF catalyzed by numerous levulinic acid molecules. Generally, the primary function of levulinic acid molecules is involved in the growth of carbonaceous spheres as building units and slowing the growth down by reducing the surface density of hydroxyl groups of carbonaceous spheres. In addition, levulinic acid molecules existed in solutions also promote the conversion from fructose to HMF to a certain extent.

Fig. 9 Schematic illustration of the role of levulinic acid on the growth of carbonaceous spheres.

For the synthesis of hollow structures for photocatalysts and sensors, carbonaceous spheres have been widely adopted as sacrificial templates. As is well known, the chemical and physical properties of nanomaterials are tightly related to the sizes and morphologies of their precursors. Especially for hollow inorganic metal oxide spheres obtained from removing the carbonaceous core via calcinations, carbonaceous spheres with uniform sizes are the optimal choice to avoid the size influence on experimental results. However, in order to effectively control the size of carbonaceous spheres obtained by hydrothermal carbonization, changing reaction time is extensively accepted as the traditional method. But there are still several deficiencies in the traditional method needed to be improved for further researches and applications, including the agglomeration and size inhomogeneity of carbonaceous spheres. From the discussion above about the roles of formic acid and levulinic acid, it is reasonable that massive amounts of monodisperse carbonaceous spheres with specific sizes could be obtained by adding appropriate amounts of these two acids into sucrose solution. A moderate amounts of formic acid molecules results in emerging of building units (i.e., HMF molecules) in multitude, which ensures a high carbon conversion ratio from sucrose to carbonaceous spheres. Simultaneously, the proper amount of levulinic acid molecules slow down the growth rate of carbonaceous spheres, which leads to the appearance of monodisperse carbonaceous spheres with relatively small sizes under sufficient HMF molecules condition. As a consequence, the method involving adding formic acid and levulinic acid provides a sensible approach to control the sizes of carbonaceous spheres for hydrothermal carbonization.

Conclusions

In this study it has been shown how the formic acid and levulinic acid molecules take part in the formation and growth of carbonaceous spheres obtained by hydrothermal carbonization of sucrose. The four different solution systems including original system, adding formic acid system, adding levulinic acid system and adding mixed acids system, were chose for investigation. By analyzing the size variations of solid products obtained from the four solution systems. We conclude that the formic acid molecules play a critical role in catalytic conversion of fructose into HMF and they promote the growth of carbonaceous spheres considerably. The primary function of levulinic acid molecules is taking part in the growth as building units and slowing the growth by reducing the surface density of hydroxyl groups of carbonaceous spheres. Simultaneously, the levulinic acids molecules existed in solutions also promote the conversion from fructose to HMF to a certain extent.

Overall, the present work provides clear cognition on the roles of formic acid and levulinic acid on the formation and growth of carbonaceous spheres, which are also effective complementing to the whole mechanism for hydrothermal carbonization of biomass. Besides, it is a sustainable and facile route to control the size of carbonaceous spheres and improve product yield by adding appropriate amounts of formic acid and levulinic acid molecules, compared with the traditional additive-free hydrothermal process.

Acknowledgements

The authors wish to acknowledge financial support from the National Natural Science Foundation of China (grant No. 51172040) and the Fundamental Research Funds for the Central Universities (grant No. N130105001).

Notes and references

1. L. Zhao, L. Z. Fan, M. Q. Zhou, H. Guan, S. Qiao, M. Antonietti and M. M. Titirici, Adv. Mater., 2010, 22, 5202.
2. C. Falco, J. M. Sieben, N. Brun, M. Sevilla, T. V. D. Mauelen, E. Morallón, D. Cazorla-Amorós and M. M. Titirici, ChemSusChem, 2013, 6, 374.
3. B. Hu, K. Wang, L. Wu, S. H. Yu, M. Antonietti and M. M. Titirici, Adv. Mater., 2010, 22, 813.
4. N. Wang, W. Fan, A. M. Xie, X. Dai, M. Sun, Y. Qiu, Y. Wang, X. Lv and M. Wang, RSC Adv., 2015, 5, 40531.
5. P. Zhang, Z. A. Qiao and S. Dai, Chem. Commun., 2015, 51, 9246.
6. X. Lai, J. E. Halpert and D. Wang, Energy Environ. Sci., 2012, 5, 5604.
7. A. D. Roberts, X. Li and H. Zhang, Chem. Soc. Rev., 2014, 43, 4341.
8. S. Dutta, A. Bhaumik and K. C. W. Wu, Energy Environ. Sci., 2014, 7, 3574.
9. S. Feng, W. Li, Q. Shi, Y. Li, J. Chen, Y. Ling, A. M. Asiri and D. Zhao, Chem. Commun., 2014, 50, 329.
10. T. Liu, R. Kavian, Z. Chen, S. Cruz, S. Noda and S. W. Lee, Nanoscale, 2016, 8, 3671.
11. C. Zhou, S. Geng, X. Xu, T. Wang, L. Zhang, X. Tian, F. Yang, H. Yang and Y. Li, Carbon, 2016, 108, 234.
12. R. Zhang, T. Zhou, L. Wang, Z. Lou, J. Deng and T. Wang, New J. Chem., 2016, 40, 6796.
13. Y. Fang, S. Guo, D. Li, C. Zhu, W. Ren, S. Dong and E. Wang, ACS Nano, 2012, 6, 400.
14. J. Gu, S. Su, Y. Li, Q. He and J. Shi, Chem. Commun., 2011, 47, 2101.
15. Y. Fang, G. Zheng, J. Yang, H. Tang, Y. Zhang, K. Biao, Y. Lv, G. Xu, A. M. Asiri, J. Zi, F. Zhang and D. Zhao, Angew. Chem., Int. Ed., 2014, 126, 5407.
16. L. Wang, Q. Sun, X. Wang, T. Wen, J. J. Yin, P. Wang, R. Bai, X. Q. Zhang, L. H. Zhang, A. H. Lu and C. Chun, J. Am. Chem. Soc., 2015, 137, 1947.
17. A. Ma, X. Wang, T. Li, X. Liu and B. Xu, Mater. Sci. Eng., A, 2007, 443, 54.
18. S. Kim, E. Shibata, R. Sergiienko and T. Nakamura, Carbon, 2008, 46, 1523.
19. J. Liu, S. Z. Qiao, H. Liu, J. Chen, A. Orpe, D. Zhao and G. Q. Lu, Angew. Chem., Int. Ed., 2011, 50, 5947.
20. Z. Sun, Y. Liu, B. Li, J. Wei, M. Wang, Y. Qin, Y. Deng, S. Kaliaguine and D. Zhao, ACS Nano, 2013, 7, 8706.
21. A. H. Lu, T. Sun, W. C. Li, Q. Sun, F. Han, D. H. Liu and Y. Guo, Angew. Chem., Int. Ed., 2011, 50, 11765.
22. D. Wang, M. Chen, C. Wang, J. Bai and J. Zheng, Mater. Lett., 2011, 65, 1069.
23. N. Baccile, G. Laurent, F. Babonneau, F. Fayon, M. M. Titirici and M. Antonietti, J. Phys. Chem. C, 2009, 113, 9644.
24. M. Sevilla, C. Falco, M. M. Titirici and A. B. Fuertes, RSC Adv., 2012, 2, 12792.
25. M. Sevilla and A. B. Fuertes, Chem.–Eur. J., 2009, 15, 4195.
26. M. Sevilla and A. B. Fuertes, Carbon, 2009, 47, 2281.
27. M. Zhang, H. Yang, Y. Liu, X. Sun, D. Zhang and D. Xue, Carbon, 2012, 50, 2155.
28. Y. Qi, M. Zhang, L. Qi and Y. Qi, RSC Adv., 2016, 6, 20814.
29. F. S. Asghari and H. Yoshida, Ind. Eng. Chem. Res., 2006, 45, 2163.
30. R. J. V. Putten, J. V. D. Waal, E. D. Jong, C. B. Rasrendra, H. J. Heeres and J. G. D. Vries, Chem. Rev., 2013, 113, 1499.
31. H. Ma, F. Wang, Y. Yu, L. Wang and X. Li, Ind. Eng. Chem. Res., 2015, 54, 2657.
32. A. Ranoux, K. Djanasvili, I. W. C. E. Arends and U. Hanefeld, ACS Catal., 2013, 3, 760.
33. Y. Gong, L. Xie, H. Li and Y. Wang, Chem. Commun., 2014, 50, 12633.
34. R. Demir-Cakan, N. Baccile, M. Antonietti and M. M. Titirici, Chem. Mater., 2009, 21, 484.
35. T. M. Aida, Y. Sato, M. Watanabe, K. Tajima, T. Nonaka, H. Hattori and K. Arai, J. Supercrit. Fluids, 2007, 40, 381.
36. H. R. Holgate, J. C. Meyer and J. W. Tester, AIChE J., 1995, 41, 637.
37. F. S. Asghari and H. Yoshida, Ind. Eng. Chem. Res., 2006, 45, 2163.
38. T. I. T. Okpalugo, P. Papakonstantinou, H. Murphy, J. McLaughlin and N. M. D. Brown, Carbon, 2005, 43, 153.
39. A. A. Rosatella, S. P. Simeonov, R. F. M. Frade and C. A. M. Afonso, Green Chem., 2011, 13, 754.

---

Footnote

† Electronic supplementary information (ESI) available: Four tables showing synthesis conditions and statistic results, one figure supplying evidence for conclusion. See DOI: 10.1039/c6ra21312j

---