SilicaâCarbon Nanocomposite Acid Catalyst with Large Mesopore

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Silica-Carbon Nanocomposite Acid Catalyst with Large Mesopore Interconnectivity by Vapor-Phase Assisted Hydrothermal Treatment RuYi Zhong, Yuhe Liao, Li Peng, Remus Ion Iacobescu, Yiannis Pontikes, Riyang Shu, Longlong Ma, and Bert F. Sels ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01003 • Publication Date (Web): 23 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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ACS Sustainable Chemistry & Engineering

Silica-Carbon Nanocomposite Acid Catalyst with Large Mesopore Interconnectivity by Vapor-Phase Assisted Hydrothermal Treatment Ruyi Zhong,a, Yuhe Liao,a, Li Peng,b Remus Ion Iacobescu,c Yiannis Pontikes,c Riyang Shu,d Longlong Ma,d and Bert F. Selsa,* ‡

a

‡

Centre for Surface Science and Catalysis, KU Leuven, Corelab, Celestijnenlaan 200F, B3001 Heverlee, Belgium.

b

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, 5 Xinmofan Road, Nanjing 210009, China. c

Department of Materials Engineering, KU Leuven, Kasteelpark Arenberg 44, B3001 Heverlee, Belgium.

d

Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, 2 Energy Road, Guangzhou 510640, China.

‡ These authors contributed equally. * E-mail: [email protected]

KEYWORDS: Vapor-phase assisted hydrothermal treatment, Mesoporous silica-carbon nanocomposite, Solid acid, Sylvan condensation

ABSTRACT: Mesostructured silica-carbon nanocomposites with large mesopore interconnectivity are created from sucrose as sustainable carbon source using a mild vapor-phase assisted hydrothermal treatment procedure. The resultant mesostructured silica-carbon nanocomposite can be readily sulfonated to provide a strong acid catalyst with high sulfonic acid density, or the carbon phases of the nanocomposite can be removed by calcination to produce a silica material with ultrahigh porosity (Vpore = 1.25 to 1.34 cm3 g-1). A superior catalytic activity is demonstrated for the solvent-less condensation of 2-methylfuran with furfural; both product yield and conversion rate surpass that of reference catalysts such as their counterparts from dry pyrolysis and the commercial strong acid resins. The enhanced catalytic activity is attributed to the higher SO3H acid density (0.64 to 1.08 mmol g-1), the larger and better communicating mesopores (Vmeso = 0.38 to 0.82 cm3 g-1) and the abundant presence of surface oxygen-containing functional groups on the vapor-phase assisted hydrothermally treated samples. The origin of the well-developed large interconnecting mesopores is investigated and discussed. The mild hydrothermal treatment causes local etching of the original mesopores in the precursor material, creating unexpected interconnectivity between the pores, while the original micropores are basically eliminated during the treatment. Therefore, the here specified hydrothermal treatment provides a promising method to conventional pyrolysis for the efficient and eco-friendly synthesis of highly mesoporous silica-carbon nanocomposites and modification of their physicochemical properties.

INTRODUCTION Mesoporous silica-carbon nanocomposites are an interesting class of materials, comprising of inorganic silica and organic carbon in its mesostructure. As mesostructures, these nanocomposites have high surface area and large pore volume. The silica and carbon moieties in the structure provide advantages of both the inorganic, viz. the hydrophilicity and thermal/mechanical stability, and

the organic component, viz. the hydrophobicity and capability of functionalization.1-3 These composite materials have shown potentials in e.g. catalysis, adsorption, energy storage, drug delivery, and sensors.1-3 Evaporation-induced self-assembly (EISA) of silica and carbon precursors in the presence of copolymer soft template has been proven an efficient one-pot strategy for the fabrication of mesoporous silica-carbon nanocompo-

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sites.4-7 Zhao et al. and Lu et al. reported the EISA processes of TEOS, resol, and F127.6,7 Our previous work successfully replaced resol with the renewable and inexpensive carbohydrate as the carbon source for the formation of mesoporous silica-carbon nanocomposites from the triconstituent EISA.4,5 Also, the time-consuming classic EISA method can be readily replaced by a more practical rotavap method without compromising the materials’ properties.8 Pyrolysis in an inert atmosphere is often required for the opening of the mesopores and carbonization of the carbon precursor to obtain the final mesoporous silicacarbon nanocomposites.1,9,10 The mesoporous silica-carbon nanocomposites can be functionalized with SO3H to act as strong solid acid catalysts, which are easier to separate from the products, cause less corrosion to equipment, and offer better recyclability than the homogeneous mineral acid catalysts. Sulfonation can be carried out with concentrated H2SO4.4,5,11-14 The too-high degree of carbonization (graphitization) brought by a too-high pyrolysis temperature is unfavorable for the introduction of the surface SO3H moieties.11,15 On the contrary, a too-low carbonization degree, or even direct treatment of the carbon precursor with concentrated H2SO4, results in the aggregation of carbon species on the silica surface, i.e., macro-phase separation of the nanocomposite.16 Besides, the soft aggregation of polycyclic aromatic carbon is prone to leaching and degrades rapidly during catalytic reactions.13 Therefore, the final pore structure and SO3H density in the sulfonated mesoporous silica-carbon nanocomposites are highly dependent on the carbonization conditions. However, previous papers often overlooked the important impact of the degree of carbonization on sulfonation, otherwise mostly limited to the dry pyrolysis temperature optimization.11,12,14,16,17 Dry gel conversion (DGC) - treating gel powder with the vapor of water or other volatile solvents - has been applied in the preparation of zeolites, hierarchical titanosilicate zeolite, mesoporous silica, and MOFs.18-23 Very recently, we have found that the setup for dry gel conversion of physical separation of the carbon precursor and the liquid water can be used for direct hydrothermal carbonization of sucrose.24 The resultant carbon material shows a surprisingly high surface area, with a similar degree of carbonization and high oxygen-containing surface functional groups relative to the carbon microspheres obtained from conventional hydrothermal treatment. The vapor-phase assisted hydrothermal carbonization was applied in this research for the first time to the assynthesized mesostructured silica-carbon nanocomposites, followed by sulfonation in concentrated H2SO4, aiming at sulfonated mesoporous silica-carbon nanocomposites with promoted sulfonation efficacy. The catalytic reaction investigated was the acidcatalyzed sylvan (2-methylfuran) condensation with furfural (Table 2).25-29 2-methylfuran and furfural are furanic compounds derived from hemi-cellulose forming a large

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nm-sized product, 2,2’-(2-furylmethylene)bis(5methylfuran) (FMBM), which is an important biodiesel precursor.25-29 The reactions were conducted to examine the influence of the SO3H density, pore structure, and other surface properties of the catalysts on the catalytic performance. The comparison between sulfonated mesoporous silica-carbon nanocomposites from vapor-phase assisted hydrothermal treatment and dry pyrolysis process shows a superior catalytic performance of the former. EXPERIMENTAL Synthesis of parental mesostructured silica-carbon nanocomposites Mesostructured silica-carbon composites were synthesized using the classic evaporation-induced triconstituent co-assembly (EISA) method, wherein sucrose is used as carbon precursor, pre-hydrolyzed tetraethylorthosilicate (TEOS) as silica precursor, and Pluronic F127 (EO106PO70EO106, Mw = 12600) as template.4,5 First, 6.4 g of F127 was added to 32 g of ethanol with 0.3 g of concentrated HCl (37 %), which was sonicated to achieve a homogeneous solution. 2.8 g of sucrose was dissolved in 10 mL H2O and added to the template solution together with 8.32 g of TEOS. The mixture was homogenized for 1 h through sonication, then transferred into dishes to form thin films and evaporate the ethanol at 40 oC for 20 h. The dishes were then kept in an oven of 160 oC for 24 h for curing. The as-made product, named Si66C33, was scraped from the dishes. Si50C50 and Si33C66 were synthesized by a similar procedure, but increasing the starting sucrose amount to 5.6 and 11.2 g, respectively. Vapor-phase assisted hydrothermal treatment The scraped pieces of the parental mesostructured silica-carbon nanocomposite (3.0 g) were placed in a 10 mL glass vial in a Teflon liner (volume 34 ± 1 mL) in a stainless steel autoclave, which was sealed to allow the sample to contact with the vapor from 3 mL H2O at 200 oC for 24 h. The schematic illustration of vapor-phase assisted hydrothermal treatment is shown in Scheme 1. The hydrothermally-treated sample, denoted as SimCnHT (m and n represent the initial weight percentages of silica and carbon components of the parental composite), was extensively washed with H2O to remove weakly-adsorbed soluble species. The sample was dried at 100 oC in air overnight. The product yield was about 70 wt%.


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Scheme 1. Schematic illustration of vapor-phase assisted hydrothermal treatment of mesostructured silica-carbon nanocomposites.

Derived pure silica or pure carbon structure SimCnHT was pyrolyzed in N2 at 800 oC for 2 h for the removal of F127 surfactant and carbonization to produce SimCnHT-800. For comparison, SimCn-800 was prepared by directly pyrolyzing the as-synthesized SimCn in N2 at 800 o C for 2 h. Pure silica and pure carbon structures were derived from SimCnHT-800 by calcination in air at 550 oC and washing by HF solution, respectively.5 The denotations are SimCnHT-Si and SimCnHT-C, accordingly. Sulfonation 1 g of the brown powder (SimCnHT) was mixed with 9 mL of concentrated H2SO4 in a 10 mL glass vial. This vial was immediately moved into a Teflon-lined stainless steel autoclave and heated at 150 oC for 15 h. The solid was washed extensively with hot distilled water (> 80 oC) until no sulfate ions were detected with Ba(NO3)2. On the one hand, only very strong sulfonation reagent, such as ClSO3H, could sulfonate Si-OH.47 On the other hand, SiO-SO3H is not as stable as C-SO3H and can be easily hydrolyzed to Si-OH and H2SO4.48 The as-obtained composite with only the carbon part sulfonated was dried in air overnight and named SimCnHT-SO3H. Control samples SimCn-400-SO3H were made according to our previous work.5 The as-synthesized SimCn was pyrolyzed in N2 at 400 oC for 15 h and subjected to the same sulfonation procedure. Characterization The thermogravimetric analysis (TGA) curves were obtained on Q500 instrument (TA Instruments, Brussels, Belgium) by heating the samples from room temperature to 800 oC at 5 oC min-1 under O2. Small angle X-ray scattering (SAXS) patterns were recorded using a SAXSess mc2 instrument (Anton Paar, Graz, Austria) with line-collimated CuKα radiation and a 2D imaging plate detector. Nitrogen sorption isotherms were measured with a Micromeritics Tristar 3000 sorptometer at -196 oC after degassing of the samples at 120 oC in a N2 flow overnight. The surface area was calculated with the BrunauerEmmett-Teller (BET) equation, micropore volumes with the t-plot method, and pore sizes with the BrunauerJoyner-Halenda (BJH) method based on the adsorption branch of the isotherms. The total pore volume was estimated from a single point on the adsorption branch at p/p0 of 0.97. Scanning Electron Microscopy (SEM) images in backscattering electron mode (BSE) were acquired using a Philips XL30 FEG microscope, and spatial elemental distribution maps for Si, C, S, and O were recorded on a Jeol Hyperprobe JXA-8530F FEG electron probe microanalyzer (EPMA) equipped with five wavelength dispersive spec-

trometers (WDS). Prior to investigation, the samples were embedded in resin and allowed to harden. The resin surface was polished and coated with Pt-Pd layer of 1 nm thickness. The microprobe was operated at 7 kV, 15 nA, 33 nm step size (in x and y) and a counting time of 40 ms per step. The signals of Si, C, S, and O were detected using a TAP, LED2H, PETH, and LDE1 crystal, respectively. The contents of Si, C, and S were quantified using Tugtupite, Carbon, and Galena standards, respectively. Transmission electron microscopy (TEM) images were acquired on a JEM-2100 electron microscope operating at 200 kV. The samples for TEM were prepared by depositing an ethanolic suspension of silica-carbon nanocomposite onto holey carbon copper grids and drying. Raman spectra were collected on a LabRam HR 800 Raman Spectrometer using 633 nm Laser. Each sample was placed across a glass slide. For each spectrum, 10 scans were recorded. Peak fitting was performed with the OriginLab software. 13

C cross-polarization (CP) magic angle spinning (MAS) NMR spectra were recorded on a Bruker Avance III spectrometer. The sample was loaded into a 4 mm ZrO2 rotor and spun at a frequency of 6.5 kHz. The recycle delay was 2.5 s and the CP contact time was 2 ms. 29Si single-pulse MAS NMR spectra were recorded using a Bruker AMX300 spectrometer with a frequency of 59.63 MHz and a recycling delay of 60 s. The sample was packed in a 4 mm rotor and the spinning frequency was 5 kHz. Both 13C and 29 Si NMR spectra were referenced to tetramethylsilane. The X-ray photoelectron spectroscopy (XPS) measurements were performed using an ESCALAB 250 xi system with Al Kα1 radiation (1486.7 eV). Binding energies for the high-resolution C 1s, O 1s, Si 2p, and S 2p spectra were calibrated by setting C 1s at 284.6 eV. Peak fitting was performed with the XPSpeak41 software. The sulfonic acid group content was determined by acid-base titration. For this, 0.05 g of previously-dried solid was stirred in 10 mL of 2 M NaCl at room temperature for 24 h. Then the filtrate was titrated potentiometrically with 1 mM NaOH solution. Catalytic reaction The condensation of 2-methylfuran with furfural was performed in a 10 mL glass vial. 2-methylfuran (13.2 mmol) and furfural (6 mmol) were added to the reactor, which was pre-charged with 50 mg of catalyst that was dried overnight at 100 oC in air and 20 mg of naphthalene as internal standard. The reactor was put in a heating copper block at 50 oC and vigorous stirring at 700 rpm was initiated. After a fixed reaction time, the reaction was stopped by cooling in icy H2O and a sample of reaction mixture was taken. After filtration of the solid catalyst, the reaction solution was analyzed by GC (Agilent 6890 series, using a HP-1 column and a flame ionization detector). Two blank tests were performed under the same reaction conditions, without adding any catalysts or with the addition of the SO3H-free Si66C33HT.



The filtrate after 24 h (e.g. with Amberlyst 15) was evaporated with a rotary evaporator under 35 mbar at 65 oC in order to remove the excess 2-methylfuran and formed H2O. After dissolution in deuterium chloroform (CDCl3), 1 H NMR spectra were recorded with a Bruker spectrometer at a frequency of 400 MHz and 13C NMR spectra at a frequency of 75 Hz, and the product yield was compared with the GC analysis. For the recyclability test, after a first catalytic run of 6 h, the catalyst was separated and simply flushed by acetone to become the spent catalyst after drying at room temperature in air. The spent catalyst was subjected to the sulfonation procedure (in concentrated H2SO4 at 150 oC for 15 h) to produce the re-sulfonated sample. Meanwhile, as a control, the spent catalyst was protonated by ionexchanging with 0.2 M H2SO4 solution (at a ratio of 100 mL g-1) at 30 oC under stirring for 4 h. The sample was filtered, washed thoroughly until neutral pH and dried at 100 oC for the catalytic reaction.

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structure of the composite can be considered as superposition of the structures of the derived pure SiO2 and C materials. Hence, SimCnHT-Si and SimCnHT-C were prepared from SimCnHT-800 by removal of C and SiO2 component, respectively, for structural insights. Characterization of SimCnHT-SO3H The TGA/DTG curves of SimCnHT, SimCnHT-800, SimCnHT-SO3H, and SimCn-400-SO3H samples obtained under oxygen atmosphere are shown in Figure 1. The carbon contents, as measured as weight loss in the curves below 600°C, are listed in Tables 1 and 2. The sharp weight loss at ca. 200 oC for the SimCnHT samples is due to the removal of F127-derived alkoxy compounds,30 which is absent in the pyrolyzed and sulfonated samples. This weight loss decreases from 27 wt% to 10 wt% with an increase of the carbon content in the composite, in accordance with the dosing of F127 during the EISA synthesis.

RESULTS AND DISCUSSION Synthesis of the different materials Scheme 2 presents all the materials synthesized in this work. Mesostructured SiO2-sucrose-F127 was obtained from the one-pot evaporation-induced self-assembly of pre-hydrolyzed silica oligomers and sucrose in the presence of F127 tri-block copolymer.5 The rapid evaporation of solvent (here ethanol) drives the formation and alignment of F127 tri-block copolymer micelles. Simultaneously, owing to the hydrogen-bonding between the hydrophilic PEO part of the copolymer and the hydroxyl groups from silicate oligomers and sucrose, the silica oligomers and sucrose aggregate around the EO domains of F127 micelles. The resultant SiO2-sucrose-F127 mesophase was then subjected to curing at 160 oC, to yield the assynthesized SimCn, with partial polymerized sucrose (with m and n being the initial weight percentages of silica and carbon components). The further carbonization of SimCn was conducted here, to our knowledge for the first time, via vapor-phase assisted hydrothermal treatment to generate SimCnHT, while pyrolysis in N2 is applied to form SimCn-400 as reference. The carbonized silica-carbon nanocomposites were decorated with SO3H through sulfonation in concentrated H2SO4 for solid acid catalytic applications. SimCnHT was also pyrolyzed at 800 oC to check the structural stability of the highly porous structure. Since formation of Si-C species is insignificant at 800 oC,5,6 the

Figure 1. TGA and DTG curves of (A) SimCnHT, (B) SimCnHT800, (C) SimCnHT-SO3H, and (D) SimCn-400-SO3H, black, red and blue corresponding to Si33C66, Si50C50 and Si66C33, respectively.

The sharp shape of the DTG peak at ca. 200 oC for SimCnHT indicates the relative independent state of the F127-decomposed residues. It has been reported that the Pluronic copolymer partially decomposes at 200 oC.30,31 Both pyrolysis and sulfonation can remove these residues. For instance, a temperature of 350 oC has been reported for removal of F127 during pyrolysis in N2,6 and extraction with sulfuric acid solution (48 wt%) under reflux for complete elimination of F127.32 Aside from the weight loss below 100 oC, which is caused by H2O desorption, the single peak at high temperature range arises from the burnoff of carbon in the composite. This DTG peak for SimCnHT-800 locates at ca. 500 oC, for SimCnHT-SO3H and SimCn-400-SO3H at ca. 440 oC, indicating the highest carbonization degree for SimCnHT-800 and similar degrees for SimCnHT-SO3H and SimCn-400-SO3H. The carbon weight losses fairly well fit with the initial sugar carbon content in the preparation.


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Scheme 2. Schematic illustration of the different materials synthesized from vapor-phase assisted hydrothermal treatment or pyrolysis in N2.

Table 1. Compositional and textural properties of vapor-phase assisted hydrothermally treated silicacarbon nanocomposites and their corresponding SiO2 and C frameworks obtained after removal of carbon and silica, respectively. Catalyst

Ca ab Vpored Vmicroe SBETc Dporef -1 -1 -1 (%) (nm) (m² g ) (cm³ g ) (cm³ g ) (nm)

Si33C66HT

78

14.1

37

0.16

0.00

15.3

Si50C50HT

71

13.7

33

0.12

0.00

13.2

Si66C33HT

67

13.9

23

0.09

0.00

15.0

Si33C66HT-800

60

14.1

279

0.57

0.08

17.0

Si50C50HT-800

50

12.9

306

0.72

0.07

16.3

Si66C33HT-800

16

14.3

316

0.89

0.05

14.6

Si33C66-800

49

12.4

155

0.06

0.05

5.2

Si50C50-800

32

11.3

177

0.20

0.04

6.3

Si66C33-800

19

11.5

156

0.22

0.02

6.4

Si33C66HT-Si

0

14.8

289

1.34

0.02

19.4

Si50C50HT-Si

0

17.9

315

1.25

0.03

16.3

Si66C33HT-Si

0

15.7

301

1.26

0.03

16.6

Si33C66HT-C

100

-

n.d.

n.d.

n.d.

n.d.

Si50C50HT-C

100

-

n.d.

n.d.

n.d.

n.d.

Si66C33HT-C

100

-

394

0.24

0.15

-

a

Carbon content was determined as weight percentage of total dry sample weight by TGA. The carbon content covers the percentage of F127 residues in SimCnHT. b The average adjacent pore distance (a) was determined from the SAXS analysis. c Surface area (SBET) was calculated by the Brunauer–Emmett–Teller (BET) method. d Total pore volume (Vpore) was calculated from the saturation plateau at high relative pressures. e Micropore volume (Vmicro) was calculated by t-plot method. f

Pore size (Dpore) was calculated from the adsorption branch of the isotherms by the Barrett-Joyner-Halenda (BJH) method.

Table 2. Profiles and catalytic performances of SimCnHT-SO3H and SimCn-400-SO3H in sylvan condensationa

En- Catalyst try

1

Si33C66HTSO3H

[SO3H] Carbon (mmol con-1 c g ) tent (%) b

1.08

75

e

f

h

j

Vmicro Vmeso Dpore a Yield of Rate of FFA Rate of TOF of TOF of SBET Vtot g -1 -1 k l -1 m (cm³ (cm³ (cm³ (nm) FMBM (mmol g h ) FMBM FFA (h ) FMBM -1 -1 -1 i -1 -1 -1 g ) after 6 h g ) (nm (mmol g h ) (h ) (m² g ) -1 ) g ) (%) d

94

0.39

0.01

0.38 16.9 14.8

86 (90)


291

196

270

182


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2

Si50C50HTSO3H

0.92

61

138

0.62

0.02 0.60 16.8 13.2

85 (90)

333

242

362

263

3

Si66C33HTSO3H

0.64

45

201 0.84

0.02 0.82 16.7 15.7

84 (90)

211

153

330

239

4

Si66C33HTSO3H_ n spent

n.d.

55

n.d. n.d.

n.d.

n.d. n.d. n.d.

22 (79)

n.d.

n.d.

n.d.

n.d.

5

Si66C33HTSO3H_ 0.2MH2SO4

0.26

55

150

0.59

0.02

0.57 17.4 n.d.

48 (85)

n.d.

n.d.

n.d.

n.d.

6

Si66C33HT0.65 SO3H_ rep sulfonated

50

186

0.73

0.02

0.71 17.4 n.d.

79 (92)

204

162

314

248

7

Si33C66-400SO3H

0.57

62

471

0.20

0.18

0.02

13.6

46 (91)

38

37

66

65

8

Si50C50-400SO3H

0.40

49

424 0.25

0.12

0.13 7.6

12.7

50 (89)

84

82

210

205

9

Si66C33-400SO3H

0.31

30

493 0.44

0.11

0.33 6.9

12.7

77 (88)

204

154

659

496

10

Amberlyst 15, dry

4.70

-

37

0.20

0

0.20

-

-

85 (95)

281

253

60

54

11

Nafion NR50

0.80

-

0

0

0

0

-

-

52 (82)

74

48

93

60

12

H2SO4 (conc.)

10.00

-

-

-

-

-

-

-

94 (94)

812

392

81

39

o

-

a

Reaction conditions of sylvan condensation: 2-methylfuran (13.2 mmol), furfural (6 mmol), catalyst (50 mg), naphthalene (20 mg) as internal standard, 50 oC. Density of SO3H groups was determined by titration. c Carbon content was determined as weight percentage of total dry sample weight by TGA. d Surface area (SBET) was calculated by the Brunauer-Emmett-Teller (BET) method. e Total pore volume (Vtot) was calculated from the saturation plateau at high relative pressures (P/P0). f Micropore volume (Vmicro) was calculated by the t-plot method. g Dpore was obtained from the N2 adsorption branch at maxima of pore size distributions according to the Barrett-Joyner-Halenda (BJH) model. h Lattice parameter (a) was calculated from d100 spacing in SAXS patterns. i FMBM is the main condensation product, 2,2’-(2-furylmethylene)bis(5-methylfuran), from 2-methylfuran condensation with furfural. The value in the brackets denotes the selectivity of FMBM in relative to the conversion of furfural. j The initial conversion rate of furfural (FFA) was calculated from the conversion of furfural within 5 min of reaction. k The initial formation rate of FMBM was calculated from the yield of FMBM within 5 min of reaction. m TOF of FFA was calculated based on the initial conversion rate of furfural in 5 min and the SO3H group density in the catalyst. n TOF of FMBM was calculated based on the initial formation rate of FMBM in 5 min and the SO3H group density in the catalyst. n Si66C33HT-SO3H_spent was collected from the reaction mixture after a first run of 6 h by filtration, washing, and drying. o Si66C33HT-SO3H_0.2MH2SO4 was obtained from Si66C33HT-SO3H_spent after treatment with 0.2 M H2SO4 solution (stirring at room temperature for 4 h, washing and drying). p Si66C33HT-SO3H_resulfonated was obtained from Si66C33HT-SO3H_spent after another sulfonation process with concentrated H2SO4 at 150 oC for 15 h. b

Note that the carbon contents for Si33C66HT-SO3H, Si50C50HT-SO3H, and Si66C33HT-SO3H are 75, 61, and 45 wt%, respectively, and thus significantly higher than the corresponding SimCn-400-SO3H sample. As shown in Figure S1 and Table 1, the carbon contents for Si33C66HT-800, Si50C50HT-800 and Si66C33HT-800 are also higher than the corresponding SimCn-800 sample. These results indicate that the carbon nature is modified by vapor-phase assisted hydrothermal treatment and that probably more oxygen-containing groups have been attached to the carbon phase in the nanocomposite.

The morphology and microstructure of SimCnHT-SO3H and Si66C33-400-SO3H were characterized with SEM, as shown in Figure S2. The vapor-phase assisted hydrothermal treatment did not significantly influence the microscale morphology; like Si66C33-400-SO3H, Si66C33HTSO3H also show holes of several micrometers resulting from fast evaporation of solvent.


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Figure 2. Elemental distribution maps of C, Si, S and O in Si66C33HT-SO3H (top row) and Si66C33-400-SO3H (bottom row). The colored scale bars show the level of elemental concentration. EPMA mapping conditions: 7 kV, 15 nA, 33 nm step size (in x and y) and a counting time of 40 ms per step. Each map is 30 μm × 22.5 μm. The sample was embedded in resin, which was polished and coated with Pt-Pd layer after hardening.

The compositional difference, for instance between Si66C33HT-SO3H and Si66C33-400-SO3H, is shown by electron probe microanalysis (EPMA) in Figure 2. For both samples, the elemental distribution is highly homogeneous throughout the mapping area. The color difference indicates the decrease of Si and O contents and the increase of C content in hydrothermally treated Si66C33HTSO3H relative to the classically pyrolyzed Si66C33-400SO3H sample. The vapor-phase assisted hydrothermal treatment thus clearly helps to retain more carbon content in the nanocomposite than the dry pyrolysis, proving similarity to the conventional hydrothermal treatment in the case of pure carbon.24 TEM images of SimCnHT-SO3H and Si66C33-400-SO3H are shown in Figure 3. The carbon phase in the nanocomposite is not expected to contribute significantly to the TEM contrast, for instance as observed from the TEM images of SimCn-400-SO3H (Figure S3). The TEM images of Si33C66HT-SO3H, Si50C50HT-SO3H, and Si66C33HT-SO3H are therefore quite similar. However, when compared to the highly-ordered hexagonally-arranged mesoporous channels in Si66C33-400-SO3H, the ordered regions in SimCnHT-SO3H samples are much smaller, with some blurring at their edges. Meanwhile, there are interconnected channels within the pore walls, in contrast to the smooth pore walls in Si66C33-400-SO3H. The width of the channels in SimCnHT-SO3H samples are only slightly larger than those in Si66C33-400-SO3H. Similar hierarchical structures, albeit in silica materials, were observed with SBA-15 synthesized at 200 oC with additional cations (K+, Mg2+, Al3+), where the cations created tunnels between 33 P123 micelle-templated mesopores. Hence, the hybrid pore walls in SimCnHT-SO3H are likely to be penetrated by H2O vapor during the vapor-phase assisted hydrothermal treatment, resulting in interconnecting mesopores with a relatively poor mesostructured ordering. Figure 4 shows TEM images of Si50C50HT-800 and its derived Si50C50HT-Si and Si50C50HT-C materials. The roughness and interstices of the pore walls after vaporphase assisted hydrothermal treatment is clearly observed

in the magnified image in Figure 4B. The representative hexagonal arrangement of the channel is denoted by the blue box, while the red and yellow lines represent the pore diameter and the distance between adjacent pores, respectively. The all-silica Si50C50HT-Si shows very much the same structure, as indicated by comparison of the TEM images. The short-range ordering of the mesochannels disappears in the Si50C50HT-C sample, possibly due to the soft structure of the carbon phase in the nanocomposite in contrast to the rigid silica framework.5

Figure 3. TEM images of (A) Si33C66HT-SO3H, (B) Si50C50HTSO3H, (C) Si66C33HT-SO3H and (D) Si33C66-400-SO3H.

Figure 4. TEM image of (A) Si50C50HT-800, (B) magnified region (yellow box in A) of Si50C50HT-800, (C) Si50C50HT-Si, and (D) Si50C50HT-C.

The SAXS patterns of SimCnHT, SimCnHT-800, SimCnHTSi, SimCnHT-C and SimCnHT-SO3H samples are shown in Figure 5. Tables 1 and 2 display the corresponding average distance between neighboring pores, i.e., lattice parameter a calculated as 4π/(√3q*), where q* is the scattering vector of the first peak. Aside from the main scattering peak of q*, two more peaks can be observed in some pat-



terns at √3q* and 2q*, as denoted by the vertical bars. The peaks demonstrate a 2D hexagonal mesostructure with p6mm symmetry of the silica-carbon nanocomposite or pure silica, whereas the structure of pure carbon is basically non-ordered. The a values for all vapor-phase assisted hydrothermal treated samples, even SimCnHT-800, are larger than those of SimCn-800 and SimCn-400-SO3H samples, indicating a significant modification of the silica framework during the vapor-phase assisted hydrothermal treatment. The lattice parameters for Si33C66HT-SO3H, Si50C50HT-SO3H and Si66C33HT-SO3H are 14.8, 13.2 and 15.7 nm respectively, only slightly larger than the corresponding SimCnHT-800. The a value for Si50C50HT-800 determined from SAXS is close to the adjacent mesochannel distances read from the TEM image (Figure 4B). The consistency of SAXS measurement results with TEM images is also observed with SimCnHT-SO3H samples.

Figure 5. SAXS patterns of: (A) vapor-phase assisted hydrothermally treated silica-carbon nanocomposites a) Si33C66HT, b) Si50C50HT, c) Si66C33HT, and after pyrolysis d) Si33C66HT800, e) Si50C50HT-800, f) Si66C33HT-800; (B) silica or (C) carbon samples after removal of carbon or silica from the pyrolyzed composites; and (D) after sulfonation, i), ii) and iii) corresponding to their mother composites of Si33C66HT, Si50C50HT, and Si66C33HT, respectively.

The N2 sorption curves, which indicate a mesoporous structure of SimCnHT-800, SimCnHT-Si, and SimCnHTSO3H, are shown in Figure 6. The textural properties are listed in Tables 1 and 2. All vapor-phase assisted hydrothermally treated samples exhibit the typical Type IV isotherms with steep and parallel H1 type hysteresis loop, indicating the high pore size uniformity. The sharp increase in uptake of N2 appears at about P/P0 = 0.8, indicative of the presence of relatively large mesopores. Note that the pore sizes calculated using the BJH model from the adsorption branch are between 15 to 17 nm, comparable to, or even slightly larger than, the average pore distance values. They are much larger than the read-out channel diameter from the TEM images, such as the red lines in Figure 4B. These results therefore strongly suggest that the F127-derived mesopores are intersected, in agreement with the visible interstices in the pore walls in the TEM images (with an ideal representation of the pore size enlargement in Scheme 1). Therefore, under the interaction with chemically active H2O vapor during the vapor-phase assisted hydrothermal treatment, the silicacarbon intertangled pore walls are likely etched and rearranged.

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Compared to SimCn-400-SO3H (Figure S4 and Table 2), the hydrothermally treated SimCnHT-SO3H possesses a pore size (12 to 16 vs. 7 to 8 nm) and total specific pore volume (0.4 to 0.8 vs. 0.2 to 0.4 cm2 g-1) that is two times larger, at the sacrifice of losing micropores (2 to 3 vs. 25 to 90% of the total pore volume) and BET surface area (100 to 200 vs. 400 to 500 m2 g-1). The close to zero density of micropores in the mesopore walls after high-temperature treatment (200 oC) has also been observed elsewhere, which is likely due to the partial decomposition of F127 and the separation of hydrophilic PEO segments from the pore walls.30,31,34 However, high mesopore volume is achieved by vapor-phase assisted hydrothermal treatment. Compared to SimCnHT-SO3H, the mesopore volume grows even larger for SimCnHT-800, leading to the larger surface area than SimCn-800, wherein micropores are largely eliminated by pyrolysis at 800 oC. These results indicate that the mesopores in the vapor-phase assisted hydrothermally treated samples, are not merely derived from F127 templates, but also from the pore wall cut-through. After removal of the carbon phase, the total pore volume for SimCnHT-Si, is 1.25 to 1.34 cm3 g-1 (Table 1), which is much higher than the conventionally synthesized mesoporous silica from F127 templating.34,46 The pure silica materials barely have microporosity, like their parental SimCnHT-800. The pore volume is even larger than the mesostructures synthesized with a micelle expander like xylene,34 or dodecane46. Such large mesoporous silica networks with high porosity can be promising not only in adsorption and separation of biomolecules,33 but also as insulators in low dielectric (k) materials.35,36

Figure 6. (Top) N2 adsorption (closed symbols)/desorption (open symbols) isotherms and (Bottom) pore size distribution of (A) SimCnHT-800, (B) SimCnHT-Si, and (C) SimCnHTSO3H; i), ii), and iii) corresponding to their mother composites of Si33C66HT, Si50C50HT, and Si66C33HT, respectively. For clarity, the offset between the isotherms along the y-axis is 3 3 200 cm /g in A/C and 500 cm /g in B.

The solid state 13C and 29Si MAS-NMR spectra of Si66C33HT-SO3H, Si66C33-400-SO3H, and the as-synthesized Si66C33 are depicted in Figure 7. The 13C NMR spectrum of the as-synthesized Si66C33 from the EISA process indicates


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the primarily polymerized sucrose, which closely entangles with the silica framework, and the F127 polymer, which is relatively independent.5 For Si66C33HT-SO3H and Si66C33-400-SO3H, the peak for aliphatic carbon (ca. 20 ppm) and the peaks for C-O groups (90 - 55 nm) disappear. They both show resonance signals at 170, 152, and 128 ppm, which can be assigned to COOH, phenolic OH, and aromatic carbon atoms, respectively.5,12 The main signal of Si66C33HT-SO3H is attributed to the aromatic carbon, similar to that of Si66C33-400-SO3H. The relative content of oxygen-containing groups are thus clearly much higher for Si66C33HT-SO3H than for Si66C33-400SO3H, showing the advantageous characteristics of hydrothermally-treating carbons,37,38 though here using the original strategy of vapor-phase assisted hydrothermal treatment. The 29Si MAS NMR spectra show the presence of Q2 (92 ppm), Q3 (-101 ppm), and Q4 (-110 ppm) resonances for the Si atoms whose next neighbored atoms are two, three, and four Si atoms, respectively. The decrease of geminal silanol Q2 sites and the increase of four siloxane-bonded Q4 sites, in the order of Si66C33, Si66C33HT-SO3H, and Si66C33-400-SO3H, indicate the increased condensation degree of silica phase in the nanocomposite. The vaporphase assisted hydrothermal treatment leads to an obvious consolidation of the silica framework compared to the as-synthesized composite.

13

29

Figure 7. Solid-state C and Si NMR of (black) Si66C33HTSO3H, (blue) Si66C33, and (red) Si66C33-400-SO3H.

applied to assess the degree of graphitization of the carbon phase in the nanocomposite. The ID1/(IG + ID1 + ID2) values for SimCnHT-SO3H are ca. 0.49 (for all samples), while the values for SimCn-400-SO3H are ca. 0.61. These results indicate that the variation of silica-carbon ratio does not influence the graphitization degree, and that the vapor-phase assisted hydrothermal treatment results in a less organized carbon structure, compared to the pyrolyzed material.

Figure 8. Deconvoluted Raman spectra of Si33C66HT-SO3H, Si50C50HT-SO3H, Si66C33HT-SO3H, and Si66C33-400-SO3H.

The C 1s and S 2p XPS spectra were recorded to reveal the presence of functionalities on the carbon surface in the nanocomposite, as shown in Figure 9. The C exists in a variety of moieties according to the C 1s spectra. Four individual component peaks were identified after deconvolution, representing carbon group (C=C, CHx, C-C, Peak 1 at 284.6 eV), hydroxyl groups or ethers (C-OH, CO-C, Peak 2 at 285.9 eV), carbonyl groups (C=O, Peak 3 at 287.2 eV), and carboxylic groups or esters (COO, Peak 4 at 288.8 eV).41 SimCnHT-SO3H samples show higher surface content of oxygen-containing groups (lower area percentage of Peak 1 and higher area percentage of Peak 4) compared to Si66C33-400-SO3H, in agreement with the 13C MAS NMR data. They all give a similar S 2p signal at 169.0 eV, close to that of Amberlyst 15 (168.9 eV), suggesting that they have similarly strong acidic SO3H groups.42

The Raman spectra of SimCnHT-SO3H and SimCn-400SO3H are shown in Figures 8 and S5. There are two main bands, D band at about 1370 cm-1, which is associated with the disorder or defect in the organization of the carbon atoms, and G band at about 1590 cm-1, which shows the sp2 in-plane vibration of the carbon atoms.39 The D and G bands can be deconvoluted into five components, which are assigned to polyenes (D4, 1208 cm-1), graphene edges (D1, 1352 cm-1), amorphous carbon (D3, 1529 cm-1), graphitic carbon (G, 1572 cm-1), and graphene sheets (D2, 1598 cm-1), respectively.40 ID1/(IG + ID1 + ID2) values were



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Catalytic activity of SimCnHT-SO3H

Figure 9. C 1s and S 2p XPS spectra of Si33C66HT-SO3H, Si50C50HT-SO3H, Si66C33HT-SO3H, and Si66C33-400-SO3H.

The SO3H density was determined by acid-base titration, and the data are listed in Table 2. The SO3H density of Si33C66HT-SO3H, Si50C50HT-SO3H, and Si66C33HT-SO3H is 1.08, 0.92, and 0.64 mmol g-1, respectively, which is about two times that of the corresponding SimCn-400SO3H pyrolyzed sample. These SO3H densities are among the highest values ever measured on carbon, even compared with pure sulfonated carbonaceous materials.43 The correlation of SO3H density with carbon content is found to be linear for the SimCnHT-SO3H and SimCn-400-SO3H samples, as shown in Figure S6. The silica phase is not observed to hinder the access of H2SO4 to the carbon surface during sulfonation, probably due to the presence of F127-templated mesostucture as well as the cut-through in the mesopore walls in the case of SimCnHT-SO3H. Sulfonation of carbon materials depends on the content of polycyclic aromatic carbon, i.e., the aromaticity, which is available for functionalization.11,15 The higher amount of oxygen-containing groups on the vapor-phase assisted hydrothermally treated nanocomposites leads to bonding with a higher density of SO3H groups. Meanwhile, the treatment brings in carbonaceous networks rather than weakly connected hydrocarbon polymers containing aromatic compounds that are generated by pyrolysis at temperatures below 250 oC.11,38 Thus, the vapor-phase treatment has the specific advantage of generating carbons, functionalized with a high density of SO3H groups. The above characterization results of vapor-phase assisted hydrothermally treated samples shows that SimCnHT-SO3H is purely mesoporous (see scheme 1 for an idealized representation) compared to the micro/mesoporous SimCn-400-SO3H. The mesopores originate not only from F127 templating, but also from their interconnection formed under the vapor-phase assisted hydrothermal circumstances, which creates much larger mesopores. The mesoporous structure is still supported by silica framework, but with mesoscopic order limited to smaller length scale. The carbon phase is still entangled with the silica phase, and is of polyaromatic nature with plenty of functionalities (phenolic OH, COOH, and SO3H), which can be beneficial for the acid catalysis.

The catalytic reactions of solvent-less condensation of 2-methylfuran (MF) with furfural (FFA), as presented in Table 2 and Scheme S1, were conducted in a well-stirred closed glass vial under reaction conditions, which are close to those typically used,28,29 in order to be able to compare the data. The main reaction pathways involve a hydroxyalkylation/alkylation (HAA) step for the formation of the target product, 2,2’-(2-furylmethylene)bis(5methylfuran) (FMBM), as shown in Scheme S1. Control experiments were conducted under the same reaction conditions, but without solid catalyst added. Near zero conversion and selectivity was observed without the catalyst, thus indicating that MF and FFA do not undergo auto-condensation reactions. The selectivity for FMBM is also roughly zero with the use of SO3H-free Si66C33HT, indicating that strong acidic sites such as SO3H groups are entailed. In addition, the presence of other oxygenated functionalities, such as phenolic OH and COOH, could also facilitate the adsorption and activation of the furanic substrates, and thus the proceeding of the reaction.25 Two commercial resins, Amberlyst 15 in its dry form and Nafion NR50, were also tested for comparison. The 1H and 13C NMR spectra of the evaporated product from the reaction with Amberlyst 15 for 24 h were shown in Figure S7. High purity of the desired product FMBM (> 95%) was demonstrated by the NMR results, in good agreement with the GC analysis results (92 % of FMBM molar yield, corresponding to 92 wt.% of FMBM in the evaporated product), showing the credibility of product analysis by GC using naphthalene as internal standard. The time evolution of the FMBM yield and the selectivity for FMBM versus the conversion on FFA for the different catalysts are shown in Figure 10. The FMBM selectivity is always above 80 % for FFA conversions larger than 60 %, indicating similarity of the functioning active sites (here, the SO3H groups). The formation of hydroxyalkylation or other intermediates is not observed, or only in insignificant amounts. The catalytic results for homogeneous H2SO4 are shown in Figure S8. The yield of FMBM reached a maximum of 96% at reaction time of 2 h and decreased afterwards, indicating the occurrence of side reactions.

Figure 10. Product yields obtained in the condensation of 2methylfuran (MF) with furfural (FFA) (left) and selectivity for FMBM against conversion of FFA (right). The yield of FMBM is based on FFA. Reaction conditions: MF (13.2 mmol),


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FFA (6 mmol), catalyst (50 mg), naphthalene (20 mg) as ino ternal standard, 50 C.

The yield and selectivity for FMBM after 6 h reaction, the initial conversion rate of FFA, the initial formation rate of FMBM, and turnover frequencies (TOFs; measured after 5 minutes of reaction) based on the conversion of FFA and the formation of FMBM respectively, are shown in Table 2. Since molecular size of FMBM is about 0.9 nm as shown in Figure S9 and the diffusion issues need to be taken into account, the formation rate and TOF of FMBM are paid more attention to. Besides, the variation of the conversion rate and TOF of FFA between different catalysts are roughly the same as that of the formation rate and TOF of FMBM. Internal diffusion limitations of FMBM can be expected to occur within catalysts consisting mainly of micropores.26,44 Therefore, for SimCn-400SO3H samples, where the carbonization degree and oxygen functionalities are similar, with the increase of silicacarbon ratio and decrease of micropore volume, the catalytic activity is much improved in terms of FMBM yield after 6 hours and its formation rate at the beginning of the reaction. The TOF of FMBM, which is based on each measured SO3H site, increases more significantly with the increase of silica-carbon ratio, highlighting the importance of acid site accessibility. Meanwhile, the external diffusion limitation is excluded, as all the tested solid acids show higher TOF of FMBM values than H2SO4. The SimCnHT-SO3H samples possess larger pore size, larger mesopore volume, higher SO3H density, and higher surface oxygen functionality than SimCn-400-SO3H. As expected, therefore, all SimCnHT-SO3H show high catalytic activity, outperforming SimCn-400-SO3H. The improvement is most pronounced upon comparison of the data of Si33C66-400-SO3H and Si33C66HT-SO3H. The two SimCn catalysts were prepared in the same way, except for a carbonization stage of vapor-phase assisted hydrothermal treatment or dry pyrolysis. Since the carbonization degree, oxygen functionality, and SO3H density are mainly determined by the carbonization conditions, the ratio of TOF of FMBM values, viz. 2.8, 1.3 and 0.5 for Si33C66, Si50C50, and Si66C33 respectively, is a strong indication of the structural modification, i.e., the accessibility of SO3H site between SimCnHT-SO3H and SimCn-400-SO3H. Therefore, the lower TOF of FMBM of Si66C33HT-SO3H compared to that of Si33C66-400-SO3H could be accounted for by the reduced mesostructural ordering. However, for Si33C66HT-SO3H and Si50C50HT-SO3H, the opening and interconnecting of mesopores, along with the higher SO3H density and oxygen functionality, outweigh this adverse effect, leading to a higher TOF of FMBM value than the corresponding SimCn-400-SO3H. The catalytic behavior of Amberlyst 15 and Nafion NR50 can also be understood from the combination effect of porosity, SO3H density, and oxygen-containing groups. The benefit of their high SO3H density is counterbalanced by the poor porosity and the non-existence of oxygenated

functionality. The catalytic performance for FMBM production at 6 h of SimCnHT-SO3H (up to 86 % product FMBM yield) slightly exceeds Amberlyst 15 (85 %), and largely exceeds Nafion NR50 (77 %). The activity of SimCnHT-SO3H is even more apparent by comparing the TOF of FMBM vaues: 182 to 263 h-1 for SimCnHT-SO3H versus 54 h-1 for Amberlyst 15 and 60 h-1 for Nafion NR50. Catalytic reusability The catalytic reusability was investigated by taking Si66C33HT-SO3H as an example. The results of recycling for condensation of 2-MF and FFA are shown in Figure 11 and Table 2. After the first 6 h run, the solid was separated from the liquid by filtration, washed with acetone, and dried in air. The recovered catalyst was reused in the second cycle, showing significant deactivation. The splitting test shows unchanged FMBM yield for the filtrate, suggesting that most of the effective catalysis occurred heterogeneously, which is also verified by the splitting test result of Si66C33HT-SO3H_resulfonated after 5 min. The deactivation of the spent catalyst might therefore be connected with the inefficient removal of adsorbed byproducts, which foul the catalyst surface and its active sites, leading to active site passivation and decreasing the amount of accessible SO3H groups.

Figure 11. (A) TGA/DTG curves, and (B) N2 sorption isotherms/pore size distributions of the fresh, spent, 0.2 M H2SO4 solution-treated and re-sulfonated Si66C33HT-SO3H. (C) Reusability of Si66C33HT-SO3H in the condensation of 2methylfuran with furfural after regeneration by resulfonation. The filled and open symbols denote the yields obtained from the fresh and re-sulfonated catalysts respectively. The arrow Reuse points to the reaction results of the spent catalyst (symbols with a horizontal bar) collected by filtration after a first 6 h run. The arrow Splitting test points to the reaction results of the filtrate (symbols with a vertical bar) under the same stirring and temperature conditions for



another 18 h. The splitting test was also conducted with Si66C33HT-SO3H_resulfonated after 5 min of reaction. The arrow 0.2MH2SO4 points to the reaction results of the spent catalyst after treatment with 0.2 M H2SO4 (symbols with cross bars). Reaction conditions: MF (13.2 mmol), FFA (6 mmol), catalyst (50 mg), naphthalene (20 mg) as internal o standard, 50 C.

A first regeneration was attempted with treatment in 0.2 M H2SO4 aqueous solution at 30 oC for 4 h in accordance with a literature procedure.45 Si66C33HTSO3H_0.2MH2SO4 shows a higher yield of FMBM at 6 h than the spent catalyst, but still lower than the fresh catalyst. The spent catalyst was also regenerated by conducting a second sulfonation (in concentrated H2SO4 at 150 oC for 15 h). The re-sulfonated catalyst shows 78.8% yield of FMBM at 6 h: 94% of the yield gained from the fresh catalyst. The initial rate of the re-sulfonated catalyst is slightly higher than that of the fresh catalyst. Therefore, regeneration by re-sulfonation could help regain the majority of the catalytic activity. The acid titration of the regenerated samples reveals that the SO3H density is only 0.26 mmol/g in Si66C33HTSO3H_0.2MH2SO4, but it becomes 0.65 mmol/g in Si66C33HT-SO3H_resulfonated. The DTG curves of Si66C33HT-SO3H_spent and Si66C33HT-SO3H_0.2MH2SO4 show similarly an additional bump in the temperature range of 200-350 oC, in comparison with the fresh Si66C33HT-SO3H and Si66C33HT-SO3H_resulfonated sample (Figure 11A). The carbon content of Si66C33HTSO3H_resulfonated is 50 wt%, in between the fresh Si66C33HT-SO3H (45 wt%) and Si66C33HT-SO3H_spent (55 wt%). The main DTG peak of Si66C33HTSO3H_resulfonated locates in the temperature range of 300-450 oC, which is similar to Si66C33HT-SO3H. Meanwhile, Si66C33HT-SO3H_0.2MH2SO4 exhibits lower SBET and Vtot than the fresh sample, and Si66C33HTSO3H_resulfonated has relatively more opened mesopores, as ascertained by the N2 sorption measurement (Figure 11B). These results suggest that treatment with 0.2 M H2SO4 is not sufficient to remove the accumulated organics during the reaction, while re-sulfonation could transform such entrapped organics into C-SO3H, bringing in more SO3H sites. Furthermore, with the robust silica phase in the nanocomposite, the open and wellconnected mesopores in Si66C33HT-SO3H were largely preserved.5,14 Thus, re-sulfonation could serve as an efficient regeneration method for the vapor-phase assisted hydrothermally treated SimCnHT-SO3H catalysts.

CONCLUSIONS In this study, vapor-phase assisted hydrothermal treated and sulfonated silica-carbon nanocomposites were synthesized for the first time, and characterized by SEM, TEM, TGA, N2 sorption, SAXS, NMR, Raman, XPS, etc.. They exhibited superior catalytic performance in the 2-

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methylfuran condensation with furfural, relative to their counterparts from dry pyrolysis. The better performance is due to a higher carbon content, higher SO3H density, and most importantly, the presence of more open and interconnected mesopores in the silica-carbon nanocomposite and the higher oxygen functionality as a direct consequence of the specified mild vapor-phase assisted hydrothermal treatment. Catalyst fouling occurs, but the activity can be regenerated by conducting re-sulfonation cycles of the spent catalyst.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Scheme S1 and Figure S1-S8; reaction pathways, TGA, SEM, TEM, N2 sorption, Raman spectra, correlation of SO3H density with carbon content, NMR analysis of the reaction product from sylvan condensation, catalytic results with H2SO4 and molecular size of FMBM.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No.51536009). R.Z. and Y.L. acknowledge the Chinese Scholarship Council (No. 201206210307 and No. 201404910467 respectively) for financial support. L.P. acknowledges the State Key Laboratory of Materials-Oriented Chemical Engineering (No. ZK201719) for financial support.

ABBREVIATIONS EISA, evaporation-induced self-assembly; DGC, dry-gel conversion; TEOS, tetraethylorthosilicate; HAA, hydroxyalkylation/alkylation; MF, 2-methylfuran; FFA, furfural; FMBM, 2,2’-(2-furylmethylene)bis(5-methylfuran).

REFERENCES (1) de Clippel, F.; Dusselier, M.; Van de Vyver, S.; Peng, L.; Jacobs, P. A.; Sels, B. F. Tailoring nanohybrids and nanocomposites for catalytic applications. Green Chem. 2013, 15, 1398-1430. (2) Nicole, L.; Laberty-Robert, C.; Rozes, L.; Sanchez, C. Hybrid materials science: a promised land for the integrative design of multifunctional materials. Nanoscale 2014, 6, 62676292. (3) Díaz, U.; Brunel, D.; Corma, A. Catalysis using multifunctional organosiliceous hybrid materials. Chem. Soc. Rev. 2013, 42, 4083-4097. (4) Van de Vyver, S.; Peng, L.; Geboers, J.; Schepers, H.; de Clippel, F.; Gommes, C. J.; Goderis, B.; Jacobs, P. A.; Sels, B. F. Sulfonated silica/carbon nanocomposites as novel catalysts for hydrolysis of cellulose to glucose. Green Chem. 2010, 12, 15601563. (5) Zhong, R.; Peng, L.; de Clippel, F.; Gommes, C.; Goderis, B.; Ke, X.; Van Tendeloo, G.; Jacobs, P. A.; Sels, B. F. An eco-friendly soft template synthesis of mesostructured silica-


Page 13 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


carbon nanocomposites for acid catalysis. ChemCatChem 2015, 7, 3047-3058.

(20) Lu, N.; Zhou, F.; Jia, H.; Wang, H.; Fan, B.; Li, R. Drygel conversion synthesis of Zr-based metal–organic frameworks. Ind. Eng. Chem. Res. 2017, 56, 14155-14163.

(6) Liu, R.; Shi, Y.; Wan, Y.; Meng, Y.; Zhang, F.; Gu, D.; Chen, Z.; Tu, B.; Zhao, D. Triconstituent co-assembly to ordered mesostructured polymer-silica and carbon-silica nanocomposites and large-pore mesoporous carbons with high surface areas. J. Am. Chem. Soc. 2006, 128, 11652-11662.

(21) Jiao, Y.; Fan, X.; Perdjon, M.; Yang, Z.; Zhang, J. Vaporphase transport (VPT) modification of ZSM-5/SiC foam catalyst using TPAOH vapor to improve the methanol-to-propylene (MTP) reaction. Appl. Catal. A 2017, 545, 104-112.

(7) Hu, Q.; Kou, R.; Pang, J.; Ward, T. L.; Cai, M.; Yang, Z.; Lu, Y.; Tang, J. Mesoporous carbon/silica nanocomposite through multi-component assembly. Chem. Commun. 2007, 601603.

(22) Koekkoek, A. J. J.; Degirmenci, V.; Hensen, E. J. M. Dry gel conversion of organosilane templated mesoporous silica: from amorphous to crystalline catalysts for benzene oxidation. J. Mater. Chem. 2011, 21, 9279-9289.

(8) Zhong, R.; Peng, L.; Iacobescu, R. I.; Pontikes, Y.; Shu, R.; Ma, L.; Sels, B. F. Scalable synthesis of acidic mesostructured silica-carbon nanocomposite catalysts by rotary evaporation. ChemCatChem 2017, 9, 65-69.

(23) Yue, M. B.; Sun, M. N.; Xie, F.; Ren, D. D. Dry-gel synthesis of hierarchical TS-1 zeolite by using P123 and polyurethane foam as template. Micropor. Mesopor. Mater. 2014, 183, 177-184.

(9) Enterría, M.; Figueiredo, J. L. Nanostructured mesoporous carbons: tuning texture and surface chemistry. Carbon 2016, 108, 79-102.

(24) Zhong, R.; Liao, Y.; Shu, R.; Ma, L.; Sels, B. F. Vaporphase assisted hydrothermal carbon from sucrose and its application in acid catalysis. Green Chem. 2018, 20, 1345-1353.

(10) Antonietti, M.; Fechler, N.; Fellinger, T. Carbon aerogels and monoliths: control of porosity and nanoarchitecture via sol-gel routes. Chem. Mater. 2014, 26, 196-210.

(25) Dutta, S.; Bohre, A.; Zheng, W.; Jenness, G. R.; Nunez, M.; Saha, B.; Vlachos, D. G. Solventless C-C coupling of low carbon furanics to high carbon fuel precursors using an improved graphene oxide carbocatalyst. ACS Catal. 2017, 7, 39053915.

(11) Nakajima, K.; Hara, M. Amorphous carbon with SO3H groups as a solid Brønsted acid catalyst. ACS Catal. 2012, 2, 12961304. (12) Nakajima, K.; Okamura, M.; Kondo, J. N.; Domen, K.; Tatsumi, T.; Hayashi, S.; Hara, M. Amorphous carbon bearing sulfonic acid groups in mesoporous silica as a selective catalyst. Chem. Mater. 2009, 21, 186-193. (13) Takagaki, A.; Toda, M.; Okamura, M.; Kondo, J. N.; Hayashi, S.; Domen, K.; Hara, M. Esterification of higher fatty acids by a novel strong solid acid. Catal. Today 2006, 116, 157-161. (14) Janaun, J.; Ellis, N. Role of silica template in the preparation of sulfonated mesoporous carbon catalysts. Appl. Catal. A 2011, 394, 25-31. (15) Xing, R.; Liu, Y.; Wang, Y.; Chen, L.; Wu, H.; Jiang, Y.; He, M.; Wu, P. Active solid acid catalysts prepared by sulfonation of carbonization-controlled mesoporous carbon materials. Micropor. Mesopor. Mater. 2007, 105, 41-48.

(26) Corma, A.; de la Torre, O.; Renz, M. Production of high quality diesel from cellulose and hemicellulose by the sylvan process: catalysts and process variables. Energy Environ. Sci. 2012, 5, 6328-6344. (27) Corma, A.; de la Torre, O.; Renz, M.; Villandier, N. Production of high-quality diesel from biomass waste products. Angew. Chem. Int. Ed. 2011, 50, 2375-2378. (28) Li, S.; Li, N.; Li, G.; Li, L.; Wang, A.; Cong, Y.; Wang, X.; Zhang, T. Lignosulfonate-based acidic resin for the synthesis of renewable diesel and jet fuel range alkanes with 2-methylfuran and furfural. Green Chem. 2015, 17, 3644-3652. (29) Li, G.; Li, N.; Wang, Z.; Li, C.; Wang, A.; Wang, X.; Cong, Y.; Zhang, T. Synthesis of high-quality diesel with furfural and 2-methylfuran from hemicellulose. ChemSusChem 2012, 5, 1958-1966.

(16) Du, B.; Zhang, X.; Lou, L.; Dong, Y.; Liu, G.; Liu, S. Synthesis of acid-functionalized composite via surface deposition of acid-containing amorphous carbon. Appl. Surf. Sci. 2012, 258, 7166-7173.

(30) Han, Y.; Li, D.; Zhao, L.; Song, J.; Yang, X.; Li, N.; Di, Y.; Li, C.; Wu, S.; Xu, X.; Meng, X.; Lin, K.; Xiao, F. Hightemperature generalized synthesis of stable ordered mesoporous silica-based materials by using fluorocarbon-hydrocarbon surfactant mixtures. Angew. Chem. Int. Ed. 2003, 42, 3633-3637.

(17) Fang, L.; Zhang, K.; Li, X. H.; Wu, H. H.; Wu, P. Preparation of a carbon-silica mesoporous composite functionalized with sulfonic acid groups and its application to the production of biodiesel. Chin. J. Catal. 2012, 33, 114-122.

(31) Liu, F.; Li, C.; Ren, L.; Meng, X.; Zhang, H.; Xiao, F. High-temperature synthesis of stable and ordered mesoporous polymer monoliths with low dielectric constants. J. Mater. Chem. 2009, 19, 7921-7928.

(18) Matsukata, M.; Ogura, M.; Osaki, T.; Hari Prasad Rao, P. R.; Nomura, M.; Kikuchi, E. Conversion of dry gel to microporous crystals in gas phase. Top. Catal. 1999, 9, 77-92.

(32) Zhuang, X.; Qian, X.; Lv, J.; Wan, Y. ：An alternative method to remove PEO-PPO-PEO template in organic–inorganic mesoporous nanocomposites by sulfuric acid extraction. Appl. Surf. Sci. 2010, 256, 5343-5348.

(19) Xu, W.; Dong, J.; Li, J.; Li, J.; Wu, F. A novel method for the preparation of zeolite ZSM-5. Chem. Commun. 1990, 755-756.



(33) Zhang, P.; Wu, Z.; Liu, T.; Li, Z. High-Temperature Synthesis of ordered hexagonal mesoporous silica materials (SBA-15) with adjustable large mesopores for selective adsorption of biomolecules. Eur. J. Inorg. Chem. 2014, 5577-5584. (34) Huang, L.; Yan, X.; Kruk, M. Synthesis of ultralargepore FDU-12 silica with face-centered cubic structure. Langmuir 2010, 26, 14871-14878. (35) Zhao, D.; Yang, P.; Melosh, N.; Feng, J.; Chmelka, B. F.; Stucky, G. D. Continuous mesoporous silica films with highly ordered large pore structures. Adv. Mater. 1998, 10, 1380-1385. (36) Eslava, S.; Kirschhock, C. E. A.; Aldea, S.; Baklanov, M. R.; Iacopi, F.; Maex, K.; Martens, J. A. Characterization of spin-on zeolite films prepared from Silicalite-1 nanoparticle suspensions. Micropor. Mesopor. Mater. 2009, 118, 458-466.

Page 14 of 15

(46) Roucher, A.; Bentaleb, A.; Laurichesse, E.; Dourges, M.; Emo, M.; Schmitt, V.; Blin, J.; Backov, R. First macromesocellular silica SBA-15-Si(HIPE) Monoliths: conditions for obtaining self-standing materials. Chem. Mater. 2018, 30, 864873. (47) Balakrishnan, M.; Sacia, E. R.; Bell, A. T. Etherification and reductive etherification of 5-(hydroxymethyl)furfural: 5(alkoxymethyl)furfurals and 2,5-bis(alkoxymethyl)furans as potential bio-diesel candidates. Green Chem. 2012, 14, 1626-1634. (48) Gallo, J. M. R.; Alamillo, R.; Dumesic, J. A. Acidfunctionalized mesoporous carbons for the continuous production of 5-hydroxymethylfurfural. J. Molecular Catal. A 2016, 422, 13-17.

(37) Libra, J. A.; Ro, K. S.; Kammann, C.; Funke, A.; Berge, N. D.; Neubauer, Y.; Titirici, M.; Fühner, C.; Bens, O.; Kern, J.; Emmerich, K. Biofuels 2010, 2, 71-106. (38) Titirici, M.; Thomas, A.; Antonietti, M. Hydrothermal carbonization of biomass residuals: a comparative review of the chemistry, processes and applications of wet and dry pyrolysis. J. Mater. Chem. 2007, 17, 3412-3418. (39) de Clippel, F.; Harkiolakis, A.; Vosch, T.; Ke, X.; Giebeler, L.; Oswald, S.; Houthoofd, K.; Jammaer, J.; Van Tendeloo, G.; Martens, J. A.; Jacobs, P. A.; Baron, G. V.; Sels, B. F.; Denayer, J. F. M. Graphitic nanocrystals inside the pores of mesoporous silica: synthesis, characterization and an adsorption study. Micropor. Mesopor. Mater. 2011, 144, 120-133. (40) Sadezky, A.; Muckenhuber, H.; Grothe, H.; Niessner, R.; Poschl, U. Raman microspectroscopy of soot and related carbonaceous materials: spectral analysis and structural information. Carbon 2005, 43, 1731-1742. (41) Sevilla, M.; Fuertes, A. B. Chemical and structural properties of carbonaceous products obtained by hydrothermal carbonization of saccharides. Chem. Eur. J. 2009, 15, 4195-4203. (42) Wang, L.; Zhang, J.; Yang, S.; Sun, Q.; Zhu, L.; Wu, Q.; Zhang, H.; Meng, X.; Xiao, F. Sulfonated hollow sphere carbon as an efficient catalyst for acetalisation of glycerol. J. Mater. Chem. A 2013, 1, 9422-9426. (43) Yu, L.; Brun, N.; Sakaushi, K.; Eckert, J.; Titirici, M. M. Hydrothermal nanocasting: synthesis of hierarchically porous carbon monoliths and their application in lithium-sulfur batteries. Carbon 2013, 61, 245-253. (44) Locus, R.; Verboekend, D.; Zhong, R.; Houthoofd, K.; Jaumann, T.; Oswald, S.; Giebeler, L.; Baron, G.; Sels, B. F. Enhanced acidity and accessibility in Al-MCM-41 through aluminum activation. Chem. Mater. 2016, 28, 7731-7743. (45) Russo, P. A.; Antunes, M. M.; Neves, P.; Wiper, P. V.; Fazio, E.; Neri, F.; Barreca, F.; Mafra, L.; Pillinger, M.; Pinna, N.; Valente, A. A. Solid acids with SO3H groups and tunable surface properties: versatile catalysts for biomass conversion. J. Mater. Chem. A 2014, 2, 11813-11824.


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For Table of Contents Use Only

A silica-carbon nanocomposite with large mesopore interconnectivity is synthesized by a specified vapor-phase assisted hydrothermal treatment and shows great potential towards the synthesis of excellent solid acid catalyst via classic sulfonation, as illustrated for the 2-methylfuran condensation reaction with furfural.


SilicaâCarbon Nanocomposite Acid Catalyst with Large Mesopore

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SilicaâCarbon Nanocomposite Acid Catalyst with Large Mesopore