Natural Nanofibrous Cellulose-Derived Solid Acid Catalysts

Natural Nanofibrous Cellulose-Derived Solid Acid Catalysts

PDF

Zhen-Yu Wu¹, Peng Yin¹, Huan-Xin Ju², Zhi-Qin Chen¹, Chao Li¹, Si-Cheng Li¹, Hai-Wei Liang¹, Jun-Fa Zhu², Shu-Hong Yu¹

Research. Vol 2019 Article ID 6262719

Less

Research. Vol 2019 Article ID 6262719

• Research Article •

Natural Nanofibrous Cellulose-Derived Solid Acid Catalysts

Full

Zhen-Yu Wu¹, Peng Yin¹, Huan-Xin Ju², Zhi-Qin Chen¹, Chao Li¹, Si-Cheng Li¹, Hai-Wei Liang¹, Jun-Fa Zhu², Shu-Hong Yu¹

Affiliations

¹Division of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at the Microscal, CAS Center for Excellence in Nanoscience, Hefei Science Center of CAS, Collaborative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China

²National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026, China

About Author:

Authors’ Contributions

Zhen-Yu Wu and Peng Yin contributed equally to this work. Shu-Hong Yu, Hai-Wei Liang, Zhen-Yu Wu, and Peng Yin conceived and designed the experiments. Shu-Hong Yu and Hai-Wei Liang supervised the research. Peng Yin, Zhi-Qin Chen, Chao Li, and Si-Cheng Li conducted the synthesis of the catalysts. Zhen-Yu Wu and Peng Yin performed the catalyst characterizations and catalytic experiments. Huan-Xin Ju and Jun-Fa Zhu assisted with XAS measurement and data analysis. Shu-Hong Yu, Hai-Wei Liang, Zhen-Yu Wu, and Peng Yin cowrote the paper. All authors discussed the results and commented on the manuscript.

Published: 2019-04-20 doi: 10.34133/2019/6262719

Outline

Abstract

Less

Solid acid catalysts (SACs) have attracted continuous research interest in past years as they play a pivotal role in establishing environmentally friendly and sustainable catalytic processes for various chemical industries. Development of low-cost and efficient SACs applicable to different catalysis processes are of immense significance but still very challenging so far. Here, we report a new kind of SACs consisting of sulfonated carbon nanofibers that are prepared via incomplete carbonization of low-cost natural nanofibrous cellulose followed by sulphonation with sulfuric acid. The prepared SACs feature nanofibrous network structures, high specific surface area, and abundant sulfonate as well as hydroxyl and carboxyl groups. Remarkably, the nanofibrous SACs exhibit superior performance to the state-of-the-art SACs for a wide range of acid-catalyzed reactions, including dimerization of α-methylstyrene, esterification of oleic acid, and pinacol rearrangement. The present approach holds great promise for developing new families of economic but efficient SACs based on natural precursors via scalable and sustainable protocols in the future.

Cite this Article

Zhen-Yu Wu, Peng Yin, Huan-Xin Ju, Zhi-Qin Chen, Chao Li, Si-Cheng Li, Hai-Wei Liang, Jun-Fa Zhu, Shu-Hong Yu. Natural Nanofibrous Cellulose-Derived Solid Acid Catalysts[J]. Research, 2019 , 2019 (4) : 6262719 - . DOI: 10.34133/2019/6262719

Full Text

Less

1. Introduction

Less

Acid catalysis plays a crucial role in many industrial chemical processes, such as fine-chemical synthesis, oil refining, and biomass conversions [1–7]. Traditional liquid acid catalysts, e.g., H₂SO₄, HCl, H₃PO₄, and HF, are highly efficient and widely used in these processes [2, 5, 7, 8]. Nevertheless, liquid acid catalysts suffer from problems of safety threat, corrosivity to equipments, catalyst waste, and difficulty of separation and recovery, which significantly limit their practical applications [2–4, 8]. The principles of green and sustainable chemistry stimulate the replacement of the homogeneous liquid acid catalysts with heterogeneous solid acid catalysts (SACs) with inherent advantages of being safe, environmentally friendly with respect to corrosiveness, less waste, and ease of separation and recovery [2, 5, 8–10]. Yet the catalytic performance of the state-of-the-art SACs, such as heteropolyacids, metal oxides (e.g., niobic acid), phosphates, zeolite (e.g., H-Mordenite), and sulfonated resins (e.g., Amberlyst-15, nafion) [1, 8], needs to be significantly improved owing to their low densities of acid sites, poor stability, and high cost [7, 8].

To this end, some novel SACs, such as phenylene-sulfonic acid functionalized ethenylene-silica [11], sulfonated polymers [12], carbon-silica composites [13, 14], and carbon-based materials [15–22], have been developed recently and showed promising performance for various acid-catalyzed reactions. A particularly important contribution to this field is the development of carbonaceous SACs by the sulfonation of incompletely carbonized D-glucose and sucrose by Hara et al. [16]. The incompletely carbonized carbohydrates with flexible polycyclic carbon frameworks and large amount of oxygen-based groups are beneficial to the followed sulfonation process [4, 16, 18, 20, 23]. As a result, the prepared SACs contain abundant -SO₃H acid sites as well as high density of hydrophilic functional groups and thus show good catalytic performance in some hydrophilic reactions, such as hydration, hydrolysis, and esterification reactions [4, 16, 18, 20, 23]. However, the low specific surface area and poor porosity of carbohydrate-derived carbonaceous SACs hinder the diffusion of hydrophobic reactants to active acid sites and therefore result in poor even no catalytic activity of these catalysts for hydrophobic reactions [13, 24–26]. Developing carbon-based SACs with high -SO₃H density, porous nanostructure, and large specific surface area is therefore highly desirable but challenging to promise these catalysts applicable to both hydrophilic and hydrophobic reactions as well as other important reactions [24–26].

Herein, we report a new type of highly porous carbon-based SACs that are prepared via a two-step process of carbonization and sulfonation of natural nanofibrous cellulose. Owing to the low-cost natural cellulose precursor and simple preparation process, our method is environmentally friendly, inexpensive, and easy to scale up. Importantly, the prepared SACs well inherit the three-dimensional (3D) nanofiber network structure of natural cellulose precursor and thus exhibit high specific surface areas (up to 837 m² g⁻¹) and large pore volumes (up to 0.92 cm³ g⁻¹); the efficient sulfonation process endows the nanofibrous SACs with abundant Brönsted acid sites including -SO₃H groups (up to 2.42 mmol g⁻¹) as well as hydroxyl (-OH) and carboxyl (-COOH) groups (total acid density of up to 3.88 mmol g⁻¹). As a result, in a wide range of important acid-catalyzed reactions, including dimerization of α-methylstyrene (AMS, a hydrophobic acid-catalyzed reaction), esterification of oleic acid (a hydrophilic acid-catalyzed reaction), and pinacol rearrangement (an acid strength-dependent reaction), our nanofibrous SACs exhibit much better performance than the state-of-the-art SACs, e.g., Amberlyst-15, H-Mordenite and niobic acid, and in some cases even better than H₂SO₄ under similar reaction conditions.

2. Results

Less

2.1. Preparation and Structural Characterizations of Nanofibrous SACs

We recently demonstrated two kinds of natural nanofibrous celluloses, i.e., bacterial cellulose (BC) and wood-based nanofibrillated cellulose (NFC), as ideal precursors for preparation of carbonaceous nanofiber aerogels [27–29]. BC, a low-cost biomass consisting of 3D nanofibrous cellulose network that can be now produced on an industrial scale via a microbial fermentation process [27], was selected as a typical precursor in this work for preparing nanofibrous SACs. The preparation of nanofibrous SACs involves a two-step process of (1) carbonization of BC aerogels at 400-800°C under a N₂ atmosphere to generate carbon-based nanofibers (BC-CNFs) aerogels and (2) subsequent sulfonation of the BC-CNFs with concentrated H₂SO₄ or fuming H₂SO₄ (Figure 1(a)). The obtained catalysts are denoted as BC-CNFs-x-SO₃H or BC-CNFs-x-SO₃H

∗

, where

x

is the pyrolysis temperature and asterisk indicates the SACs prepared with fuming H₂SO₄. The commercial availability of low-cost BC precursors and simple preparation processes allow BC-CNFs-SO₃H catalysts to be easily scaled up (Figures 1(b) and 1(c)). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images clearly indicate that the nanofibrous network structures of original BC precursors are kept well in all of the BC-CNFs-

x

-SO₃H and BC-CNFs-

x

-SO₃H

∗

catalysts even after carbonization and harsh sulfonation processes (Figures 1(d) and 1(e) and Figures S1-S4). The effective sulfonation is roughly confirmed by energy dispersive X-ray spectroscopy (EDS) spectra (Figure S5). The corresponding elemental mapping images show that S and O species are homogeneously distributed throughout the individual nanofiber as well as the whole nanofibrous network structure (the insets of Figure 1(e) and Figures S6-S8), implying that -SO₃H groups have been successfully grafted onto BC-CNFs and were uniformly distributed within the whole nanofibrous network. The XRD patterns of BC-CNFs-

x

-SO₃H show broad but weak diffraction peaks (2θ = 10-30°, Figure S9), indicating the amorphous carbon structure composed of aromatic carbon sheets in a considerably random fashion, which is consistent with Raman analysis results (Figure S10) [18].

N₂ sorption measurements were performed to investigate the textural properties of BC-CNFs-SO₃H SACs and the results were summarized in Table 1 and Figures S11 and S12. All of BC-CNFs-x-SO₃H SACs show high specific surface area ranging from 496 to 837 m² g⁻¹ and large pore volume ranging from 0.41 to 0.92 cm³ g⁻¹. These values are much higher than those of previously reported biomass-derived SACs, such as D-glucose [23], sucrose [16], and microcrystalline cellulose derived SACs [18]. Note that the sulfonation with concentrated H₂SO₄ did not damage the porous structure of BC-CNFs, while the fuming H₂SO₄ sulfonation led to a distinct decrease of specific surface area; the specific surface area considerably dropped from 512 m² g⁻¹ for BC-CNFs-400 to 192 m² g⁻¹ for BC-CNFs-400-SO₃H

∗

. Additionally, the pore size distribution analysis manifests the presence of a large number of mesopores for BC-CNFs-SO₃H SACs (Figures S11 and S12). As far as we know, this work represents the first case of preparing SACs with high specific surface area and large pore volume from biomass precursors without templating or activation processes.

2.2. Surface Chemistry Characterization of Nanofibrous SACs

We then carried out the Fourier transform infrared spectroscopy (FT-IR), ¹³C magic angle spinning nuclear magnetic resonance (¹³C MAS NMR), acid-base back titration, X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) analyses to reveal the detailed surface chemistry of the nanofibrous SACs.

Figure 2(a) shows the FT-IR spectra of BC-CNFs-400 before and after sulfonation. For BC-CNFs-400, the peaks at 3000-3700 cm⁻¹, 2880-2990 cm⁻¹ and 1350-1400 cm⁻¹ are assigned to O-H, C-H and aromatic C-O stretching vibrations, respectively [30]; The peaks at 1700 cm⁻¹ and 1500-1670 cm⁻¹ are attributed to C=O and C=C stretching vibration, respectively [30]. The rich oxygen-containing groups in BC-CNFs-400 are beneficial to followed sulfonation processes. After sulfonation, three new peaks at 1338 cm⁻¹, 1065 cm⁻¹ and 658 cm⁻¹ attributed to -SO₃H groups emerge in the FT-IR spectrum of BC-CNFs-400-SO₃H [18, 30]. The surface functional groups on the nanofibers were further analyzed by ¹³C MAS NMR (Figure 2(b) and Figure S13). The peaks at 128, 155, and 182 ppm assignable to polycyclic aromatic carbon atoms, phenolic OH, and COOH groups are found in the both ¹³C MAS NMR spectra of BC-CNFs-x and BC-CNFs-x-SO₃H as well as BC-CNFs-400-SO₃H

∗

[13, 18, 23]. Meanwhile, all of BC-CNFs-SO₃H SACs exhibit a shoulder peak at 140 ppm, which belongs to the aromatic carbon with SO₃H groups (Ar-SO₃H, ca. 140 ppm) [23, 24]. The abundant oxygen-containing groups make these nanofibrous SACs have an excellent hydrophilic property (Figure S14).

Further, the total acid density (referring to total H⁺ number per gram of Brönsted acids, i.e., the total density of SO₃H, COOH, and OH groups) of the nanofibrous SACs was estimated by acid-base back titration method; the SO₃H density was determined by elemental analyses based on the fact that sulfur element exclusively came from -SO₃H group in the nanofibrous SACs. The results of the nanofibrous SACs as well as H₂SO₄ and several reference SACs were summarized in Table 1. The BC-CNFs-400-SO₃H possesses a high total acid density of 1.84 mmol g⁻¹ and SO₃H density of 0.85 mmol g⁻¹, comparable to that of Nafion R-1100 (1.13 mmol g⁻¹ and 1.02 mmol g⁻¹, respectively). With the increase of carbonization temperature, the total acid density and SO₃H density drop dramatically for BC-CNFs-600-SO₃H and BC-CNFs-800-SO₃H because of the increased difficulty in sulfonation for the carbons with higher carbonization degree [24]. Of note, BC-CNFs-400-SO₃H∗ prepared with fuming H₂SO₄ sulfonation exhibits a much higher total acid density of 3.88 mmol g⁻¹ and SO₃H density of 2.42 mmol g⁻¹; the SO₃H density surpasses most of reported carbon-based SACs [13, 16–19, 23–26] and even can compete with Amberlyst-15 with a high SO₃H density of 4.36 mmol g⁻¹.

The surface composition and chemical state of the constituent elements of BC-CNFs-x and corresponding SACs were additionally characterized by XPS (Figures 2(c)–2(f) and Figure S15). The C1s spectrum of BC-CNFs-400 can be deconvoluted into four peaks at 284.8 eV, 285.5 eV, 286.7 eV, and 288.8 eV corresponding to C-C and/or C=C, C-O, C=O, and COOH, respectively (Figure 2(d)) [31, 32]. Interestingly, the intensity of C=O and COOH peaks increases obviously in BC-CNFs-400-SO₃H, probably because of the oxidization of carbon during the sulfonation process. In the high-resolution O1s spectra, the peaks at 531.5 eV and 533.4 eV in BC-CNFs-400 are assigned to oxygen atoms in C=O and C-O, respectively (Figure 2(e)) [30]. A new O1s peak centered at 532.6 eV appears for BC-CNFs-400-SO₃H, which can be attributed to oxygen atoms in -SO₃H groups [30]. The high-resolution S2p spectrum of BC-CNFs-400-SO₃H can be deconvoluted into two peaks at 167.6 eV and 168.7 eV (Figure 2(f)), corresponding to S2p_3/2 and S2p_1/2 in -SO₃H groups, respectively [30]. No other sulfur-containing configurations are observed by XPS analyses. The XPS of other nanofibrous SACs were also analyzed and the results were presented in Figure S15.

XAS can probe unoccupied electronic states and is particularly sensitive to the local electronic configuration. XAS measurements of C, O, and S K-edge were therefore performed to further investigate the electronic structure of the nanofibrous SACs. The C K-edge XAS spectra of BC-CNFs-400 and BC-CNFs-400-SO₃H clearly show the presence of unoccupied π∗ (1s→π∗) and σ∗ (1s→σ∗) states at around 284.9 and 292.1 eV, respectively, indicating the aromaticity of both samples (Figure 2(g)) [33]. However, no recognizable structures can be identified between 293 and 310 eV, probably because the amorphous nature of these two samples induces a spread of the σ∗ resonances [34]. The peaks at photon energies of 285.7, 286.3, and 288.7 eV are associated with various functional oxygen-containing groups in BC-CNFs-400 and BC-CNFs-400-SO₃H [33, 34]. The peaks of a1, a2, and a3 correspond to the π∗ resonances of carbons being in a chemical bond with oxygen in hydroxyl groups (C-OH), carbons in epoxy groups (C-O-C), and carbons in the double bonds (C=O) in carbonyl and carboxyl (C=O, COOH), respectively [33–35]. Remarkably, the peak of a3 of BC-CNFs-400-SO₃H is much sharper and stronger than that of BC-CNFs-400, indicating much increased contents of carbonyl and carboxyl groups after sulfonation, agreeing with the XPS results well. The O K-edge XAS spectra further reveal the changes of surface chemistry during sulfonation process. BC-CNFs-400 and BC-CNFs-400-SO₃H show very similar peaks at 531.8 (b1), 536.0 (b3) and 540.5 eV (b5), which can be assigned to the π∗ resonance of C=O/COOH bonds, σ∗ resonances of C-OH bonds and C=O/COOH bonds, respectively (Figure 2(h)) [35, 36]. Two new peaks at photon energies of 532.9 and 537.5 eV emerge at O K-edge XAS spectrum of BC-CNFs-400-SO₃H, which is associated with the π∗ and σ∗ resonances of -SO₃H groups [37]. Finally, the S K-edge XAS spectra directly reveal the successful sulfonation: BC-CNFs-400-SO₃H displays a peak at 2481.2 eV that is assigned to -SO₃H groups with S in the +6 state [38], while BC-CNFs-400 has no peak at 2470-2495 eV (Figure 2(i)). The C, O, and S K-edge of other nanofibrous SACs were also tested and carefully analyzed; all of them exhibit similar results with BC-CNFs-400-SO₃H (Figure S16).

Overall, the above FT-IR, ¹³C MAS NMR, acid density test, and XPS and XAS analyses indicate unambiguously that sulfonation can efficiently endow the incompletely carbonized natural nanofibrous cellulose with abundant -SO₃H groups and other oxygen-containing groups, meanwhile maintaining the nanofibrous structures and high specific surface areas.

2.3. The Performance of Acid-Catalyzed Reactions

Three different but highly important acid-catalyzed liquid-phase reactions were selected for evaluating the catalytic performance of our nanofibrous SACs, including a hydrophobic acid-catalyzed reaction (dimerization of AMS), a hydrophilic acid-catalyzed reaction (esterification of oleic acid with methanol), and an acid strength-dependent reaction (pinacol rearrangement). H₂SO₄ and several state-of-the-art SACs, i.e., Amberlyst-15, Nafion R-1100, H-ZSM-5, H-mordenite, and niobic acid, were also tested under the same conditions for comparison.

2.4. Catalytic Dimerization of AMS

The dimerization of AMS produces valuable unsaturated dimers (2,4-diphenyl-4-methyl-1-pentene and 2,4-diphenyl-4-methyl-2-pentene) accompanied with an undesirable saturated dimer (1,1,3-trimethyl-3-phenylindan) [13, 25]. The unsaturated dimers are industrially important chemicals in the syntheses of styrene-butadiene-rubber and acrylonitrile-butadiene-styrene resin; thus it is necessary to selectively synthesize the unsaturated dimers by inhibiting the formation of the saturated dimer [13, 25]. H₂SO₄ gave a nearly 100% conversion but a very poor selectivity; 95.7% AMS was converted into the undesirable saturated dimer (Table 2). The time courses of conversion and yield tests show that most of AMS was rapidly converted into unsaturated and saturated dimers during 0.5 h over H₂SO₄. Unfortunately, the unsaturated dimers were gradually converted into saturated dimers at following reaction process; finally the yield of unsaturated dimers was decreased to 4.3% after 20 h (Figure S17a), indicating that the intramolecular Friedel-Crafts alkylation proceeded continuously over H₂SO₄. As a conventional resin-based strong solid acid, Nafion R-1100 with high density of SO₃H groups could not efficiently catalyze the AMS dimerization, which is associated with its low specific surface area [13], while Amberlyst-15 with relatively large surface area exhibited a superior catalytic conversion of 98.8% but a low selectivity of 56.3% for unsaturated dimers (Table 2). The whole reaction process of the dimerization of AMS for Amberlyst-15 was very similar to that of H₂SO₄ (Figure S17b), but its reaction speed was much slow than that of H₂SO₄, probably because of its low SO₃H density. In addition, H-ZSM-5, H-Mordenite and niobic acid with acidic OH groups also displayed negligible catalytic activity for the dimerization of AMS (lower that 10% conversion).

Remarkably, when the BC-CNFs-x-SO₃H catalysts were tested, they showed remarkable catalytic performance for the AMS dimerization. Particularly, the conversion of AMS and selectivity for unsaturated dimers could reach 96.3% and 96.8% for BC-CNFs-400-SO₃H and 92.9% and >99% for BC-CNFs-600-SO₃H after 20 h (Table 2). The much higher selectivity of BC-CNFs-x-SO₃H to the unsaturated dimers over H₂SO₄ and Amberlyst-15 is probably due to the blocking of intramolecular Friedel-Crafts alkylation on BC-CNFs-x-SO₃H enriched with phenolic OH groups (Figure S17c) [25]. Indeed, the incorporation of p-cresol in the H₂SO₄-catalyzed AMS dimerization could distinctly improve the selectivity to unsaturated dimers (Figure S18). Of note, when using microcrystalline cellulose and glucose derived SACs (i.e., cellulose-600-SO₃H and glucose-600-SO₃H) with low specific surface area of <1 m² g⁻¹ for catalyzing the dimerization of AMS, very low activity (lower than 5% conversion) was observed despite their high total acid density and SO₃H density (Figures S17d and S19). These results clearly manifest the vital role of the high porosity resulting from the nanofibrous structure of BC-CNFs-x-SO₃H in hydrophobic acid-catalyzed reaction.

The reusability of BC-CNFs-600-SO₃H for AMS dimerization was evaluated by a simple decantation method (Figure S20). No obvious decrease in activity and selectivity was observed after three reuses of the same sample, though the activity decreased in the fourth cycle. We found that the nanofibrous microstructure of BC-CNFs-600-SO₃H was well retained (Figure S21), but the total acid density dropped to 0.51 mmol g⁻¹, which should be an important reason for the catalytic performance degradation. Besides, the blocking of active SO₃H groups by adsorbed reactants and/or products and the unavoidable catalyst loss during the recycle process should be considered as other reasons for the decrease of catalytic performance [25, 39].

2.5. Catalytic Esterification of Oleic Acid with Methanol

The esterification of fatty acids with methanol was selected to further demonstrate the wide applicability of BC-CNFs-

x

-SO₃H for various acid-catalyzed reactions (Table 3). The esterification of fatty acids with alcohols to produce fatty acid esters is an important step in the production of biodiesel, which is considered a promising alternative fuel to replace petroleum-based fuels because of its biodegradability, nontoxicity, and favorable combustion emission profile [4, 16, 22]. Methyl oleate was only product detected by gas chromatography in this acid-catalyzed liquid-phase reaction. When three conventional SACs including H-ZSM-5, H-Mordenite, and niobic acid which are free of -SO₃H were used for esterification of oleic acid with methanol, the conversion of oleic acid was very low (<15%) after 6 h reaction and the corresponding turnover frequencies (TOFs) were less than 1 h⁻¹. Under the same conditions, Nafion R-1100 with high SO₃H density also showed a poor activity, while Amberlyst-15 with higher SO₃H density and larger specific surface area gave a moderate activity with a conversion of 42.4% and a TOF of 8.86 h⁻¹. Remarkably, BC-CNFs-400-SO₃H and BC-CNFs-600-SO₃H with relatively low SO₃H density even exhibited much better catalytic performance; ca. 90% oleic acid was converted on them within 12 h and their TOFs are more than 15.0 h⁻¹. The higher catalytic activity of BC-CNFs-SO₃H over Nafion R-1100 and Amberlyst-15 is attributed to not only their high specific surface areas but also the flexibility carbon structures with large amounts of hydrophilic functional groups that provide access for the reactant molecules to the active SO₃H groups inside the carbon matrix [18, 23]. As expected, H₂SO₄, as a homogeneous acid catalyst, had the highest activity with a conversion of 98.3% and a TOF of 41.6 h⁻¹. When BC-CNFs-400-SO₃H∗ with higher SO₃H density (nearly triple of that in BC-CNFs-400-SO₃H) which was prepared by fuming H₂SO₄ sulfonation was used for this esterification reaction, it showed nearly the same conversion and comparable TOF (29.3 h⁻¹) as the high-performance H₂SO₄ catalyst. Furthermore, BC-CNFs-400-SO₃H could be well recycled for four times without obvious performance degradation (Figure S22).

2.6. Catalytic Pinacol Rearrangement

Pinacol rearrangement is a valuable process for synthesizing aldehydes or ketones through the skeletal rearrangement of 1,2-diols and elimination of water [40]. The reaction involves the evolution of a cationic intermediate, which is stabilized under highly acidic conditions [11]. We therefore selected this reaction to show the association of acid strength of the BC-CNFs-SO₃H catalysts with their catalytic performance (Table 4). H-ZSM-5, H-Mordenite, niobic acid, and Nafion R-1100 showed very poor conversion (lower that 10%), as results of their low acid strength. In contrast, all of BC-CNFs-SO₃H catalysts exhibited high performance for pinacol rearrangement with >85% conversion and >70% selectivity for pinacolone. The BC-CNFs-400-SO₃H catalyst could be recycled for three times with a slight performance degradation (Figure S23). Particularly, BC-CNFs-400-SO₃H∗ showed a conversion of 99.4% and a selectivity for pinacolone of 81.9%, which is very close to H₂SO₄ and exceeds Amberlyst-15. The high acid strength of BC-CNFs-400-SO₃H∗ was verified by ³¹P MAS NMR tests (Figure S24).

2.7. Catalytic Performance for Other Reactions

Besides being used for the above acid-catalyzed reactions, the BC-CNFs-SO₃H catalysts were also explored for other liquid-phase reactions, including the synthesis of β-enamino ketones/esters and hydrogenation of nitrobenzene. Enamination of β-dicarbonyl compounds, an important and widely used transformation, can form β-enamino ketones and esters [41, 42]. The obtained compounds are a highly versatile class of intermediates for the synthesis of heterocycles and biologically active compounds [41]. Various amines (aromatic, cyclic, and aliphatic) and β-diketones or β-ketoesters were investigated for synthesizing β-enamino ketones/esters in the presence of BC-CNFs-400-SO₃H (Table S1). Most yields of β-enamino ketones/esters exceeded 90%, which was close comparable with the results of reported metal-based catalysts [41]. The relatively low yield (52.5%) of the reaction between acetylacetone and o-chloroaniline probably results from the stereo-hindrance effect. The high catalytic performance of BC-CNFs-400-SO₃H for synthesis of β-enamino ketones/esters is believed to be associated with high solid acid groups and active defect sites [42]. In addition, BC-CNFs-400-SO₃H also exhibited promising activity for catalyzing the hydrogenation of nitrobenzene. Using N₂H₄·H₂O as the reductant, nitrobenzene could completely converted into aniline over BC-CNFs-400-SO₃H with a superior yield of 99.6%, which suppresses most of the reported carbon-based catalysts (Table S2). The BC-CNFs-400-SO₃H catalyst could be well recycled; only a slight activity decrease (ca. 8%) was observed after 5 runs of catalytic reactions (Figure S25), demonstrating good reusability as a heterogeneous carbon-based catalyst.

These catalysis tests clearly confirm the distinct advantages of BC-CNFs-SO₃H SACs over H₂SO₄ and the state-of-the-art SACs in a wide range of liquid-phase acid-catalyzed reactions. The high performance of BC-CNFs-SO₃H SACs is associated with their unique nanofibrous network structure, abundant functional groups, high -SO₃H density, and acid strength. Another superiority of BC-CNFs-SO₃H catalysts is easy to scale up due to the commercially available low-cost biomass precursors and simple preparation processes. Particularly, the more sustainable and cheaper wood-based NFC could also be employed as precursor to prepare efficient nanofibrous SACs (Figure S26). Moreover, our developed method can also be extended to other nanofibrous SACs, such as phosphorylated BC-CNFs (Figure S27).

3. Discussion

Less

In summary, we report a facile, low-cost, and scalable method to prepare a new type of nanofibrous SACs by incomplete carbonization and sulfonation of natural nanofibrous celluloses. The prepared nanofibrous SACs showed distinctly better catalytic performance than the state-of-the-art SACs in various important acid-catalyzed liquid-phase reactions, including both hydrophilic and hydrophobic acid-catalyzed reactions. The superior performance of the nanofibrous SACs is associated to their unique structural feature and surface chemistry, i.e., high specific surface areas, large pore volumes, and high SO₃H density and total acid density. The concept of converting nanofibrous cellulose to SACs demonstrated in this work sheds new light on the further development of highly efficient catalysts based on nanostructured biomass for green and sustainable chemistry.

4. Materials and Methods

Less

4.1. Materials and Chemicals

Raw materials of purified BC pellicles with fiber content of ~1% (vol/vol) were kindly provided by Ms C. Y. Zhong (Hainan Yeguo Foods Co. Ltd, Hainan, China). The fuming sulfuric acid (50 wt% SO₃) was purchased from Nanjing Chemical Reagent Co. Ltd (Nanjing, China). Amberlyst-15, Nafion R-1100, and niobic acid were obtained from Puzhen Biological Technology Co. Ltd., Alfa Aesar (China) Chemicals Co. Ltd., and Macklin Biochemical Co. Ltd. (Shanghai, China), respectively. H-ZSM-5 and H-Mordenite were purchased from Shengtan Environmental New Materials Co. Ltd (Shanghai, China). Other chemicals were of analytical grade, commercially available from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China), and used as received without further purification.

4.2. Preparation of BC-CNFs-S

O 3

H SACs

The as-received purified BC pellicles with a thickness of 1 cm were first soaked with deionized water for 3 days and cut into small pieces. Then, the small BC pieces were immersed in 10 mM NH₄H₂PO₄ solution for 3 days and freeze-dried to obtain BC aerogels. The BC aerogels were subsequently heated to 400-800°C under a N₂ atmosphere with at a heating rate of 5°C min⁻¹ speed and kept for 2 h to generate BC-CNFs aerogels. The BC-CNFs aerogels were then sulfonated using concentrated H₂SO₄ at 150°C or fuming H₂SO₄ (50 wt% SO₃) at 100°C for 12 h, respectively, before cooling to room temperature and washing with hot distilled water (>80°C) until impurities, such as sulfate ions, were no longer detected in the wash water. Finally, the sulfonated products were freeze-dried to obtain BC-CNFs-SO₃H SACs. The cellulose-600-SO₃H and glucose-600-SO₃H as reference samples were prepared by the same processes using concentrated H₂SO₄ for sulfonation. Of note, the precursor of glucose-600-SO₃H, i.e., glucose, has not been impregnated with deionized water and 10 mM NH₄H₂PO₄ solution before use, due to the high solubility in water.

4.3. Characterization

SEM images were taken with a Zeiss Supra 40 scanning electron microscope at an acceleration voltage of 5 kV. TEM images were obtained using a Hitachi H7650 or Hitachi H7700 transmission electron microscope with a CCD imaging system and an accelerating voltage of 100 kV. XRD data was collected on a Philips X’Pert PRO SUPER X-ray diffractometer equipped with graphite monochromatic Cu Kα radiation (λ = 1.54056 Å). EDS spectrum and corresponding elemental mapping were acquired using a Talos F200X transmission electron microscope at an accelerating voltage of 200 kV equipped with an energy dispersive detector. FT-IR spectroscopy was measured on a Bruker Vector 22 Fourier transform infrared spectrometer. ¹³C MAS NMR spectra were recorded on a Bruker AVANCE 400WB NMR spectrometer (400 MHz). N₂ sorption analysis was carried out using an ASAP 2020 accelerated surface area and porosimetry instrument (Micromeritics), equipped with automated surface area, at 77 K using Barrett-Emmett-Teller (BET) calculations for the surface area. The pore size distribution plot was analyzed from the adsorption branch of the isotherm based on the Barrett-Joyner-Halenda (BJH). Raman scattering spectra were recorded with a Renishaw System 2000 spectrometer using the 514.5 nm line of Ar+ for excitation. XPS was performed on an X-ray photoelectron spectrometer (ESCALab MKII) with an excitation source of Mg Kα radiation (1253.6 eV). The XAS tests of C K-edge and O K-edge were carried out at the BL11U beamline of National Synchrotron Radiation Laboratory (NSRL, Hefei, China). The S K-edge was performed at the beamline 4B7A beamline of Beijing Synchrotron Radiaition Facility (BSRF, Beijing, China). The XAS spectra in the figures have been normalized to the background before and after the main features. The GC instrument was equipped with a Restek-5 capillary column (5% diphenyl and 95% dimethylsiloxane, 0.32 mm diameter, 60 m length) and a flame ionization detector (FID). Gas chromatograph (GC, Shimadzu GC-2014) was used to calculate the conversion and selectivity of the catalytic products.

4.4. Catalytic Reactions

Dimerization of AMS was performed in a 5 mL glass reactor sealed with a Teflon lid. AMS (1 ml) was reacted over the catalyst (5 mg) at 100°C for 20 h unless otherwise specified. After cooling to room temperature,

n

-heptadecane was added as an internal standard and then ethanol (10 ml) as a diluent was added. Samples were withdrawn at intervals from the reaction mixtures and analyzed by gas chromatography using a capillary column.

Esterification of higher fatty acids was performed in a mixture containing 0.010 mol methanol, 0.001 mol oleic acid, 1 ml n-octane mixture, and 5 mg catalyst at 80°C for 6 h. After cooling to room temperature, n-heptadecane was added as an internal standard and then n-octane (10 ml) as a diluent was added. The liquid phase was separated from the reaction mixtures and analyzed by gas chromatography using a capillary column.

Pinacol-pinacolone rearrangement was carried out in the liquid phase as follows. 5 mg catalyst, pinacol (1.0575 mol), and 1 ml water were added in glass reactor. Then they were heated to 130°C and kept for 12 h. After cooling to room temperature, n-heptadecane was added as an internal standard and toluene (10 ml) as a diluent was also added. At last, the liquid phase was analyzed using gas chromatography with capillary columns.

Catalytic synthesis of β-enamino ketones/esters was carried out in a round bottom flask. Typically, 10 mg catalyst was added into a reactant mixture of diketone (5 mmol) and amine (5.2 mmol). The reaction was performed in air atmosphere with magnetically stirring at 60°C for 0.1-3 h. The reactants and products were analyzed by gas chromatography, and the yield of all reactions refers to the GC yield.

Hydrogenation of nitrobenzene was performed in the presence of 0.3 g nitrobenzene, 0.85 mL hydrazine monohydrate, and 5 mg catalyst at 100°C for 4 h. After cooling to room temperature, 10 ml methanol (diluent) and n-dodecane (internal standard) were added, and the liquid phase was analyzed using gas chromatography with capillary columns.

Funding

Less

National Natural Science Foundation of China(21431006; 21761132008)
Foundation for Innovative Research Groups of the National Natural Science Foundation of China(21521001)
Key Research Program of Frontier Sciences, CAS(QYZDJ-SSW-SLH036)
Users with Excellence and Scientific Research Grant of Hefei Science Center of CAS(2015HSC-UE007)
Recruitment Program of Global Experts
National Natural Science Foundation of China(21671184)
Fundamental Research Funds for the Central Universities(WK2340000076)
National Postdoctoral Program for Innovative Talents(BX201600142)
China Postdoctoral Science Foundation(2017M610383)
Fundamental Research Funds for the Central Universities(WK2060190077)
National Natural Science Foundation of China(21703229)

References

Less

Corma

, “Inorganic solid acids and their use in acid-catalyzed hydrocarbon reactions,” Chemical Reviews, vol. 95, no. 3, pp. 559–614, 1995

Corma

, “Solid acid catalysts,” Current Opinion in Solid State & Materials Science, vol. 2, no. 1, pp. 63–75, 1997

Okuhara

, “Water-tolerant solid acid catalysts,” Chemical Reviews, vol. 102, no. 10, pp. 3641–3666, 2002

Hara

, “Biomass conversion by a solid acid catalyst,” Energy & Environmental Science, vol. 3, no. 5, pp. 601–607, 2010

Takagaki

, Tagusagawa

, Hayashi

, Hara

, and Domen

, “Nanosheets as highly active solid acid catalysts for green chemical syntheses,” Energy & Environmental Science, vol. 3, no. 1, pp. 82–93, 2010

Shimizu

K.-I.

, and Satsuma

, “Toward a rational control of solid acid catalysis for green synthesis and biomass conversion,” Energy & Environmental Science, vol. 4, no. 9, pp. 3140–3153, 2011

Gupta

, and Paul

, “Solid acids: green alternatives for acid catalysis,” Catalysis Today, vol. 236, pp. 153–170, 2014

Liu

, Huang

, Zheng

, Xiao

F.-S.

, and Dai

, “Hydrophobic solid acids and their catalytic applications in green and sustainable chemistry,” ACS Catalysis, vol. 8, no. 1, pp. 372–391, 2018

Zhang

, Zhao

, Xu

, Yang

, Liu

, Wei

, and Yang

, “Polystyrene sulphonic acid resins with enhanced acid strength via macromolecular self-assembly within confined nanospace,” Nature Communications, vol. 5, p. 3170, 2014

Kitano

, Nakajima

, Kondo

J. N.

, Hayashi

, and Hara

, “Protonated titanate nanotubes as solid acid catalyst,” Journal of the American Chemical Society, vol. 132, no. 19, pp. 6622–6623, 2010

Nakajima

, Tomita

, Hara

, Hayashi

, Domen

, and Kondo

J. N.

, “A stable and highly active hybrid mesoporous solid acid catalyst,” Advanced Materials, vol. 17, no. 15, pp. 1839–1842, 2005

Xing

, Liu

, Wu

, Jiang

, Chen

, He

, and Wu

, “Novel solid acid catalysts: Sulfonic acid group-functionalized mesostructured polymers,” Advanced Functional Materials, vol. 17, no. 14, pp. 2455–2461, 2007

Nakajima

, Okamura

, Kondo

J. N.

, Domen

, Tatsumi

, Hayashi

, and Hara

, “Amorphous carbon bearing sulfonic acid groups in mesoporous silica as a selective catalyst,” Chemistry of Materials, vol. 21, no. 1, pp. 186–193, 2009

Yue

, Wang

, Wei

, Deng

, Liu

, Che

, Tu

, and Zhao

, “A template carbonization strategy to synthesize ordered mesoporous silica microspheres with trapped sulfonated carbon nanoparticles for efficient catalysis,” Angewandte Chemie International Edition, vol. 51, no. 41, pp. 10368–10372, 2012

Hara

, Yoshida

, Takagaki

, Takata

, Kondo

J. N.

, Hayashi

, and Domen

, “A carbon material as a strong protonic acid,” Angewandte Chemie International Edition, vol. 43, no. 22, pp. 2955–2958, 2004

Toda

, Takagaki

, Okamura

, Kondo

J. N.

, Hayashi

, Domen

, and Hara

, “Green chemistry: biodiesel made with sugar catalyst,” Nature, vol. 438, no. 7065, p. 178, 2005

Wang

, Liu

, Waje

M. M.

, Chen

, Yan

, Bozhilov

K. N.

, and Feng

, “Sulfonated ordered mesoporous carbon as a stable and highly active protonic acid catalyst,” Chemistry of Materials, vol. 19, no. 10, pp. 2395–2397, 2007

Suganuma

, Nakajima

, Kitano

, Yamaguchi

, Kato

, Hayashi

, and Hara

, “Hydrolysis of cellulose by amorphous carbon bearing SO₃H, COOH, and OH groups,” Journal of the American Chemical Society, vol. 130, no. 38, pp. 12787–12793, 2008

, Zhang

, Chen

, Wang

, Zhang

, and Fan

, “Sulfonated graphene as water-tolerant solid acid catalyst,” Chemical Science, vol. 2, no. 3, pp. 484–487, 2011

Nakajima

, and Hara

, “Amorphous carbon with SO₃H groups as a solid brønsted acid catalyst,” ACS Catalysis, vol. 2, no. 7, pp. 1296–1304, 2012

Peng

, Zhang

, Wang

, Lv

, and Yu

, “Sulfonated carbon nanotubes as a strong protonic acid catalyst,” Carbon, vol. 43, no. 11, pp. 2405–2408, 2005

Poonjarernsilp

, Sano

, and Tomon

, “Hydrothermally sulfonated single-walled carbon nanohorns for use as solid catalysts in biodiesel production by esterification of palmitic acid,” Applied Catalysis B: Environmental, vol. 147, pp. 726–732, 2014

Okamura

, Takagaki

, Toda

, Kondo

J. N.

, Domen

, Tatsumi

, Hara

, and Hayashi

, “Acid-catalyzed reactions on flexible polycyclic aromatic carbon in amorphous carbon,” Chemistry of Materials, vol. 18, no. 13, pp. 3039–3045, 2006

Kitano

, Arai

, Kodama

, Kousaka

, Nakajima

, Hayashi

, and Hara

, “Preparation of a sulfonated porous carbon catalyst with high specific surface area,” Catalysis Letters, vol. 131, no. 1-2, pp. 242–249, 2009

Suganuma

, Nakajima

, Kitano

, Kato

, Tamura

, Kondo

, Yanagawa

, Hayashi

, and Hara

, “SO₃H-bearing mesoporous carbon with highly selective catalysis,” Microporous and Mesoporous Materials, vol. 143, no. 2-3, pp. 443–450, 2011

Fukuhara

, Nakajima

, Kitano

, Hayashi

, and Hara

, “Synthesis and acid catalysis of zeolite-templated microporous carbons with SO₃H groups,” Physical Chemistry Chemical Physics, vol. 15, no. 23, pp. 9343–9350, 2013

Z.-Y.

, Liang

H.-W.

, Chen

L.-F.

, Hu

B.-C.

, and Yu

S.-H.

, “Bacterial cellulose: a robust platform for design of three dimensional carbon-based functional nanomaterials,” Accounts of Chemical Research, vol. 49, no. 1, pp. 96–105, 2016

Z.-Y.

, Li

, Liang

H.-W.

, Chen

J.-F.

, and Yu

S.-H.

, “Ultralight, flexible, and fire-resistant carbon nanofiber aerogels from bacterial cellulose,” Angewandte Chemie International Edition, vol. 52, no. 10, pp. 2925–2929, 2013

, Hu

, Ding

, Liang

, Li

, Yu

, Wu

, Chen

, and Yu

, “Wood-derived ultrathin carbon nanofiber aerogels,” Angewandte Chemie International Edition, vol. 57, no. 24, pp. 7085–7090, 2018

Zhao

, Liu

, Li

, Hu

, Yuan

, Yan

, Guo

, and Luo

, “Fabrication of −SO₃H functionalized aromatic carbon microspheres directly from waste Camellia oleifera shells and their application on heterogeneous acid catalysis,” Molecular Catalysis, vol. 433, pp. 193–201, 2017

Gong

, Wang

, Wei

, Xie

, and Wang

, “An efficient way to introduce hierarchical structure into biomass-based hydrothermal carbonaceous materials,” ACS Sustainable Chemistry & Engineering, vol. 2, no. 10, pp. 2435–2441, 2014

Zhang

, Wu

, Liu

, Wang

, Lv

, and Jia

, “Graphene oxide mediated cellulose-derived carbon as a highly selective catalyst for the hydrolysis of cellulose to glucose,” Applied Catalysis A: General, vol. 543, pp. 218–224, 2017

Ganguly

, Sharma

, Papakonstantinou

, and Hamilton

, “Probing the thermal deoxygenation of graphene oxide using high-resolution in situ X-ray-based spectroscopies,” The Journal of Physical Chemistry C, vol. 115, no. 34, pp. 17009–17019, 2011

Lenardi

, Piseri

, Briois

, Bottani

C. E.

, Li Bassi

, and Milani

, “Near-edge X-ray absorption fine structure and Raman characterization of amorphous and nanostructured carbon films,” Journal of Applied Physics, vol. 85, no. 10, pp. 7159–7167, 1999

Kuznetsova

, Popova

, Yates J. T. Jr., Bronikowski

M. J.

, Huffman

C. B.

, Liu

, Smalley

R. E.

, Hwu

H. H.

, and Chen

J. G.

, “Oxygen-containing functional groups on single-wall carbon nanotubes: NEXAFS and vibrational spectroscopic studies,” Journal of the American Chemical Society, vol. 123, no. 43, pp. 10699–10704, 2001

Wolcott

, Schiros

, Trusheim

M. E.

, Chen

E. H.

, Nordlund

, Diaz

R. E.

, Gaathon

, Englund

, and Owen

J. S.

, “Surface structure of aerobically oxidized diamond nanocrystals,” The Journal of Physical Chemistry C, vol. 118, no. 46, pp. 26695–26702, 2014

Todd

E. C.

, Sherman

D. M.

, and Purton

J. A.

, “Surface oxidation of chalcopyrite (CuFeS₂) under ambient atmospheric and aqueous (pH 2-10) conditions: Cu, Fe L- and O K-edge X-ray spectroscopy,” Geochimica et Cosmochimica Acta, vol. 67, no. 12, pp. 2137–2146, 2003

Sarret

, Connan

, Kasrai

, Bancroft

G. M.

, Charrié-Duhaut

, Lemoine

, Adam

, Albrecht

, and Eybert-Bérard

, “Chemical forms of sulfur in geological and archeological asphaltenes from Middle East, France, and Spain determined by sulfur K- and L-edge X-ray absorption near-edge structure spectroscopy,” Geochimica et Cosmochimica Acta, vol. 63, no. 22, pp. 3767–3779, 1999

López

D. E.

, Goodwin J. G. Jr., Bruce

D. A.

, and Lotero

, “Transesterification of triacetin with methanol on solid acid and base catalysts,” Applied Catalysis A: General, vol. 295, no. 2, pp. 97–105, 2005

Chen

S.-Y.

, Lee

J.-F.

, and Cheng

, “Pinacol-type rearrangement catalyzed by Zr-incorporated SBA-15,” Journal of Catalysis, vol. 270, no. 1, pp. 196–205, 2010

Sun

, Dong

, Li

, Zhang

, Wei

, Shi

, and Li

, “Ag nanoparticles in hollow magnetic mesoporous spheres: A highly efficient and magnetically separable catalyst for synthesis of β-enaminones,” Materials Chemistry and Physics, vol. 140, no. 1, pp. 1–6, 2013

Deng

, Xiao

, Chung

I.-M.

, Kim

I. S.

, and Gopiraman

, “Industrial-quality graphene oxide switched highly efficient metaland solvent-free synthesis of β-ketoenamines under feasible conditions,” ACS Sustainable Chemistry & Engineering, vol. 5, no. 2, pp. 1253–1259, 2017

Fukuhara

, Nakajima

, Kitano

, Kato

, Hayashi

, and Hara

, “Structure and catalysis of cellulose-derived amorphous carbon bearing SO₃H groups,” ChemSusChem, vol. 4, no. 6, pp. 778–784, 2011

Suganuma

, Nakajima

, Kitano

, Hayashi

, and Hara

, “Sp³-Linked amorphous carbon with sulfonic acid groups as a heterogeneous acid catalyst,” ChemSusChem, vol. 5, no. 9, pp. 1841–1846, 2012

Appendix

Less

Year 2019 volume 2019 Issue 4

PDF

Cite this Article

BibTeX

Article Info

doi: 10.34133/2019/6262719

Receive Date：2018-11-30
Online Date：2026-10-27
Published：2019-04-20

Article Data

Affiliations

History

Received：2018-11-30
Accepted：2019-01-27

Funding

National Natural Science Foundation of China(21431006; 21761132008)

Foundation for Innovative Research Groups of the National Natural Science Foundation of China(21521001)

Key Research Program of Frontier Sciences, CAS(QYZDJ-SSW-SLH036)

Users with Excellence and Scientific Research Grant of Hefei Science Center of CAS(2015HSC-UE007)

Recruitment Program of Global Experts

National Natural Science Foundation of China(21671184)

Fundamental Research Funds for the Central Universities(WK2340000076)

National Postdoctoral Program for Innovative Talents(BX201600142)

China Postdoctoral Science Foundation(2017M610383)

Fundamental Research Funds for the Central Universities(WK2060190077)

National Natural Science Foundation of China(21703229)

Affiliations

²National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026, China

References

Share

https://castjournals.cast.org.cn/joweb/research/EN/10.34133/2019/6262719

Share to

Scan QR to access full text

Cite this article

BibTeX

Citations

表12种不同金属材料的力学参数

科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
多孔菌科 Polyporaceae	8	14	6.70	蜡蘑属 Laccaria	5	2.39
红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
				光柄菇属 Pluteus	5	2.39
				红菇属 Russula	17	8.13
				栓菌属 Trametes	5	2.39

关闭全屏

BibTeX
EndNote
RefWorks
TxT

Table 1 The structure and surface properties of BC-CNFs-SO₃H SACs and reference catalysts.

Catalyst	Specific surface area (m² g⁻¹)	Pore volume(cm³ g⁻¹)	Total acid density(mmol g⁻¹ $) a$	SO₃H density(mmol g⁻¹ $) b$
BC-CNFs-400-SO₃H	496	0.41	1.84	0.85
BC-CNFs-600-SO₃H	686	0.76	1.26	0.43
BC-CNFs-800-SO₃H	837	0.92	1.08	0.32
BC-CNFs-400-SO₃H∗	192	0.42	3.88	2.42
BC-CNFs-400	512	0.38	0.58	/
H₂SO₄	/	/	20.4	/
Amberlyst-15	50	0.2-0.4	4.50	4.36
Nafion R-1100	<1	/	1.13	1.02
H-ZSM-5	462	~0.21	0.62	/
H-Mordenite	387	~0.20	0.44	/
Niobic acid	124	0.02	0.41	/

a

Total acid density is determined by acid-base back titration;

b

SO₃H densities are estimated from the S content in sample compositions determined by elemental analysis.

Table 2 Catalytic performance of the catalysts tested for dimerization of α-methylstyrene.


Entry	Catalyst	Conversion (%)	Selectivity (%)
Entry	Catalyst	Conversion (%)	Unsaturated dimers $a$	Saturated dimers $b$
1	BC-CNFs-400-SO₃H	96.3	96.8	3.20
2	BC-CNFs-600-SO₃H	92.9	>99%	Trace
3	BC-CNFs-800-SO₃H	20.7	>95%	Trace
4	BC-CNFs-400-SO₃H∗	97.7	83.6%	16.4%
5	BC-CNFs-600	Trace	Trace	Trace
6	H₂SO₄	ca. 100	4.30	95.7
7	Amberlyst-15	98.8	56.3	43.7
8	Nafion R-1100	Trace	Trace	Trace
9	H-ZSM-5	8.92	>99%	Trace
10	H-Mordenite	6.35	>99%	Trace
11	Niobic acid	Trace	Trace	Trace

2 a

,4-diphenyl-4methyl-1-pentene and 2,4-diphenyl-4-methyl-2-pentene;

1 b

,1,3-trimethyl-3-phenylindan.

Table 3 Catalytic performance of the catalysts tested for esterification of oleic acid with methanol.


Entry	Catalyst	Conversion (%)	TOF (h⁻¹) $a$
1	BC-CNFs-400-SO₃H	90.4	18.3
2	BC-CNFs-600-SO₃H	87.9	15.6
3	BC-CNFs-800-SO₃H	80.3	9.5
4	BC-CNFs-400-SO₃H∗	97.2	29.3
5	BC-CNFs-400	19.1	1.23
6	H₂SO₄	98.3	41.6
7	Amberlyst-15	42.4	8.86
8	Nafion R-1100	13.2	0.48
9	H-ZSM-5	10.9	0.88
10	H-Mordenite	12.2	0.53
11	Niobic acid	13.3	0.21

T h e a

TOF is calculated by the conversion of oleic acid per acid site per hour with conversion less than 40%; the active sites are defined as H⁺ measured by acid-base titration method.

Table 4 Catalytic performance of the catalysts tested for pinacol rearrangement.


Entry	Catalyst	Conversion (%)	Selectivity (%)
Entry	Catalyst	Conversion (%)	Pinacolone	2,3-Dimethyl-1,3-butadiene
1	BC-CNFs-400-SO₃H	96.3	78.9	21.1
2	BC-CNFs-600-SO₃H	92.9	76.7	23.3
3	BC-CNFs-800-SO₃H	85.6	72.2	27.8
4	BC-CNFs-400-SO₃H∗	99.4	81.9	18.1
5	BC-CNFs-400	10.2	97.2	2.80
6	H₂SO₄	ca. 100	86.5	13.5
7	Amberlyst-15	97.3	65.3	34.7
8	Nafion R-1100	3.20	37.5	62.5
9	H-ZSM-5	8.92	94.6	5.40
10	H-Mordenite	6.35	63.1	36.9
11	Niobic acid	Trace	Trace	Trace

Figure 1 Preparation and structural characterizations of nanofibrous SACs. (a) Schematic illustration for the preparation processes of BC-CNFs-SO₃H SACs. Digital images of large-scale preparation of (b) BC and (c) BC-CNFs aerogels. (d) SEM and (e) TEM images of BC-CNFs-400-SO₃H. The insets in (e) are elemental mapping images for an individual nanofiber.

Figure 2 Surface chemistry characterization of nanofibrous SACs. (a) FT-IR spectra, (b) ¹³C MAS NMR spectra and (c) XPS survey spectra of BC-CNFs-400 and BC-CNFs-400-SO₃H. (d) C 1s, (e) O 1s and (f) S 2p high resolution XPS spectra of BC-CNFs-400 and BC-CNFs-400-SO₃H. (g) C K-edge, (h) O K-edge, and (i) S K-edge XAS spectra of BC-CNFs-400 and BC-CNFs-400-SO₃H.

Articles: Latest Articles; Most Read; Collections

Updates: Events; News; Multimedia

About: About Us

Contact

No. 86 Xueyuan South Road, Haidian District, Beijing

100081

010-62662699(Operation and Technical Support)

qkjq@cast.org.cn

Copyright © 2025 China Association for Science and Technology. All rights reserved. For all open access content, the relevant licensing terms apply.
Sponsored by the Office of the Leading Group for Cybersecurity and Informatization of CAST, and supported by Science and Technology Review Publishing House