SO3H-Functionalized Mesoporous Carbon-silica Composite With a Spherical Morphology and Its Excellent Catalytic Performance for Biodiesel Production

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SO3H-functionalized mesoporous carbon/silica compositewith a spherical morphology and its excellent catalytic

performance for biodiesel production

Binbin Chang • Yanlong Tian • Weiwei Shi •

Jiyang Liu • Fengna Xi • Xiaoping Dong

Springer Science+Business Media New York 2013

Abstract Mesoporous carbon/silica composites func-

tionalized with –SO3H groups were prepared via poly-merization and carbonization of glucose into mesoporous

silica SBA-15 and a followed sulfonation by sulphuric

acid. These composites were characterized by powder

X-ray diffraction, N2 adsorption–desorption and transmis-

sion electron microscopy, which suggested the preservation

of ordered mesoporous structure, as well as a novel

spherical morphology. The result of fourier transform

infrared spectroscopy indicated the successful modification

of –SO3H groups and the acidity of catalysts was deter-

mined by an indirect titration method. The composite with

40 % carbon loading possessing the highest acidity in

synthesized catalysts and the ordered mesoporous structure

without pore blocking exhibited a remarkable catalytic

activity for biodiesel production. Experimental parameters

including the carbon loading, molar ratio of reactants,

reaction time and reaction temperature were optimized. In

addition, a superior recycling property was exhibited after

five consecutive cycles.

Keywords Mesoporous carbon/silica composite Porous

solid acid Catalysis

1 Introduction

Biodiesel, an eco-friendly fuel production, has exhibited

great potential as substitute for traditional fossil energy

resources [1–5]. During the past decade, biodiesel has

been derived from either the transesterification of tri-glycerides or the esterification of free fatty acids with

short-chain alcohols. In these reactions, catalysts play an

important role in accelerating the transesterification and

esterification process. Traditional liquid acids (such as

HCl and H2SO4) are widely used in chemical industry,

however, the difficulty to separate from products makes

them lose the recyclability. Furthermore, the consumption

of liquid acids generally produces considerable toxic

waste and results in serious environmental problems.

Thus, out of the concept of environmentally friendly,

the developments of efficient solid acid catalysts to

instead of traditional liquid acids have became an inevi-

table trend.

In recent years, acid functionalized materials as solid

acid such as inorganic-carbon composite [6], organic-

carbon composite [7], mesoporous silicas [8, 9] and so on

[10–12], have been paid much attention by researchers.

Mesoporous silica based on self-assembly principles are

novel materials with high surface area, large pore volume,

uniform channels, tailored and ordered structures, which

make them become potential catalysts or catalytic supports.

Currently, the preparation of acid functionalized meso-

porous silica mainly concentrates on the doping of metal

ions in silica framework and the modification of acidic

groups on pore surface [13–15]. Though metal dopants

bring strong acid sites in the inert silica wall, the doped

amount is limited because the mesoporosity of silica

decreases with the increase of metal amount [16]. The

surface acidic groups, mainly –SO3H groups, are usually

obtained from the oxidation of –SH groups by a post-

grafted method [17]. Unfortunately, the acidity is also

constrained by the low concentration of Si–OH groups on

pore surface in post-grafted strategies.

B. Chang Y. Tian W. Shi J. Liu F. Xi X. Dong (&)

Department of Chemistry, School of Sciences, Zhejiang

Sci-Tech University, 928 Second Avenue, Xiasha Higher

Education Zone, Hangzhou 310018, China

e-mail: [email protected]

1 3

J Porous Mater

DOI 10.1007/s10934-013-9728-9



Another important kind of acid functionalized solid

catalysts used in catalysis of biodiesel products is carbon-

based materials bearing –SO3H groups. These acid groups

can be loaded on the surface of carbon by a sulfonation

treatment of incompletely carbonized sugar in concentrated

sulfuric acid. It has been confirmed that amorphous carbons

bearing –SO3H groups possess high catalytic activity for

the production of biodiesel [18, 19]. However, these car-bon-base acid catalysts prepared from sugars possess low

surface area (\5 cm2 g-1), poor porosity and relatively

low acid density ranging from 0.37 to 1.34 mmol H? g-1

[18, 19]. Mesoporous carbon (MC) presents high chemical

stability, high surface area, and simplification of func-

tionalization with active groups [20, 21]. Consequently, it

has been widely used as an admirable catalyst or catalyst

support in a variety of industrial and environmental

applications [22–24]. SO3H–modified MC materials with

excellent catalytic performance have been successfully

synthesized by different routes [25–28]. These materials

retain mesoporous structure with high surface area andregular pore size, whereas, the acidity is much lower than

those of incompletely carbonized catalysts. The modifica-

tion of –SO3H groups on carbon surface involves the

reaction of –C–OH and –COOH with H2SO4 [29]. In other

words, the acidity is mainly determined by the concentra-

tion of surface oxygen-containing groups. However, car-

bonizations at high temperature are necessary for MC

synthesis, which would result in the lack of oxygen-con-

taining groups on pore surface. Therefore, the increase of

surface acid sites meantime keeping of the mesoporosity is

vital for developing efficient MC solid acid catalysts.

Herein, we reported the preparation of SO3H–function-

alized mesoporous carbon/silica composite (C/Si–SO3H)

with a novel spherical morphology. The hydrothermal con-

dition for the carbonization of precursors is favorable for

remaining abundant oxygen-containing groups, accordingly,

a high acidity produced after the sulfonation by concentrated

sulphuric acid. These mesoporous carbon/silica composites

exhibited relative high specific surface area, connected pore

structure,high acidity andexcellent catalytic activity, as well

as superb recycled performance for the esterification of oleic

acid (OA) with methanol.

2 Experimental

2.1 Preparation of C/Si–SO3H composite materials

Mesoporous silica SBA-15 was synthesized using an amphi-

philic triblock copolymer, pluronic P123 (EO20PO70EO20)

as a structure directing agent [30]. Carbon / silica composites

were prepared by a simple hydrothermal method. In a typical

experiment, 1.0 g SBA-15 was dispersed in a specific

concentration of glucose solution and stirred for 2 h. Then,

the glucose / SBA-15 solution was placed in a Teflon–sealed

autoclave and maintained at 453 K for 15 h. The black

products were obtained by filtration, washed repeatedly with

distilled water, oven–dried at 353 K, anddesignated as C/Si–

x, where x refers to the mass of loaded carbon and was

determined by a thermogravimetry method. A series of

samples with different carbon loading were obtained byvarying the mass of impregnated glucose in the range of

1.0–3.0 g. Subsequently, these C/Si– x materials were sul-

fonated using concentrated sulfuric acid at 423 K for 10 h in

a Teflon–sealed autoclave. Then, these sulfonated C/Si– x

catalysts were washed repeatedly with hot distilled water

([353 K) until the sulfateionswereno longer detected in the

wash water (BaCl2 precipitation test) and were dried at

373 K for 6 h. The resulting materials were denoted as C/Si–

x–SO3H.

For comparison, SBA-15 was sulfonated using concen-

trated sulfuric acid at 423 K for 10 h in a Teflon–sealed

autoclave, designated as SBA-15–SO3H.

2.2 Characterizations

The X–ray diffraction (XRD) patterns of powder samples

were taken by a Bruker D8 Advance diffractometer using

Cu K a radiation (k = 0.15418 nm) as an X–ray source.

Nitrogen adsorption–desorption isotherms were carried out

at 77 K using a Micromeritics ASAP 2020 analyzer.

Before adsorption, the samples were out-gassed at 373 K

for 4 h. The specific surface area (S BET ) was evaluated

using the Brunauer–Emmett–Teller (BET) method, while

the pore volume and pore size were calculated according to

the Barrett–Joyner–Halenda (BJH) formula applied to the

adsorption branch. Fourier transform infrared spectroscopy

(FTIR) spectra of a sample in KBr pellet were recorded on

a Nicolet Avatar 370 spectrometer. The morphology and

pore structure were observed from a scanning electron

microscope (SEM, Hitachi S-4800) and a JEOL JEM–2100

transmission electron microscope (TEM) with an acceler-

ating voltage of 200 kV. The thermogravimetric (TG)

analysis curves were obtained on a Mettler analyzer (TGA/

SDTA851e/5FL1100).

The number of acid sites was estimated by using an

indirect titration method [31, 32], which involves an

aqueous ion-exchange step of the catalyst H? ions with

base of NaHCO3, followed by titration of the resulting

solution with HCl aqueous solution (0.1 M). In a typical

experiment, 30 mg of the catalyst was dispersed in 50 mL

of 5 9 10-3 mol L-1 NaHCO3 solution, which was stirred

for 24 h and separated by filteration. Then 5 mL of filtrate

was taken out for titration with 0.1 M of HCl aqueous

solution. Titration was performed three times and the

average number was reported. The amount of acid groups

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in the solid acid catalysts was estimated by the NaHCO3

consumed.

2.3 Catalytic testing

Catalytic esterification of OA with methanol was per-

formed in a 100 mL three necked round bottomed flask

equipped with a reflux condenser, magnetic stirrer and awater bath maintained at a specified temperature. In a

typical experiment, 0.05 mol of OA was mixed in anhy-

drous methanol (MeOH) in the round bottom flask and the

required quantity of solid acid catalyst (0.1 g) was added.

At regular time intervals, 4 mL of samples were extracted

to check the progress of esterification reaction. The sam-

ples drawn from the reaction mixture were centrifuged to

separate the catalyst powder, then, the methanol and water

were evaporated out of the samples, and the product ana-

lyzed for acid value (AV) by titration [33]. The conversion

of OA was calculated using the following formula:

Conversion % ¼ ð1 AV x=AV0Þ 100%

where AV x is the instant AV of samples drawn from the

reaction mixture, AV0 is the initial AV of oleic acid.

3 Results and discussion

3.1 Characteristics of catalysts

The carbon loading in these carbon/silica composites were

determined by the thermogravimetry method, and Fig. 1

shows the TG curves measured under an air atmosphere forC/Si composites. In the temperature range from 500 to

800 K, the C/Si composites lost respectively *20, *40

and *60 %, which indicates the relative amounts of car-

bon in the composites are *20, *40 and *60 %, as 1, 2

and 3 g of carbon source were used. Figure 2 shows the

low–angle XRD patterns of SBA-15, C/Si– x and C/Si– x–

SO3H composites. The characteristic peaks assigned to

(100) (110), and (200) diffractions of 2D hexagonal mes-

oporous structure can be clearly observed in all samples,

which indicate that C/Si– x and C/Si– x–SO3H samples still

retain the ordered mesoporous structure of SBA-15. In

addition, the (100) diffraction peak of carbon / silica com-posites shifts to a lower angle compared to SBA-15,

resulting in their larger d 100 values (Table 1). It is well

known that the structural contraction of mesoporous silica

occurs, due to the further condensation of Si–OH groups,

when organic templates are removed by calcination at high

temperature [34, 35]. The structural expansion of carbon /

silica composites could be ascribed to the partial hydrolysis

of the Si–O–Si framework to form Si–OH groups when

calcined SBA-15 is re-hydrothermal treated [36].

Figure 3 a shows N2 adsorption–desorption isotherms of

SBA-15, C/Si– x and C/Si– x–SO3H composites. All these

samples show typical type IV curves with a clear hysteresis

loop at relative pressure from 0.65 to 0.85, which indicate

that the uniform cylindrical mesoporous channels are

retained. The adsorption volume of nitrogen decreasing

with the increase of carbon loading suggests the reducing

of surface area. These decreases should be caused by

Fig. 1 The TG curves of C/Si- x composites materials

Fig. 2 a Low-angle XRD patterns of SBA-15, C / Si– x and b C / Si– x–

SO3H composite materials

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carbon species deposited on the internal surface of SBA-

15. The pore structure parameters are listed in Table 1. The

specific surface area and the pore volume continue to

reduce after sulfonation treating with concentrated sulfuric

acid, which may be attributed to the presence of a largenumber of -SO3H groups. The pore diameter distributions

of SBA-15, C/Si– x and C/Si– x–SO3H composites are

shown on Fig. 3b. The decrease in pore diameter and the

enhancement of pore wall thickness demonstrate that car-

bon species have successfully loaded on the pore channels

of SBA-15. In addition, the hysteresis loop of C/Si–60 %–

SO3H exhibits an obvious delay at relative pressure range

of 0.55–0.65, which implies that carbon particles accu-

mulate to block the channels [37, 38].

To reveal the morphology and structure of materials,

SEM and TEM images of the C/Si–40 % and C/Si–60 %

particles before and after the sulfonated modification are

shown in Figs. 4 and 5. A spherical shape nanoparticle is

observed in mesoporous carbon/silica composites fromFig. 4. The transformation of morphology may be related

to the hydrothermal treatment [39]. After sulfonation, the

spherical morphology is still maintained (Fig. 5a, b),

indicating that the process of sulfonation does not destroy

the structure of materials. Figure 5c and d show the TEM

images of single carbon/silica composite spheres of C/Si–

40 %–SO3H and C/Si–60 %–SO3H. A highly ordered

mesoporous structure is directly observed in C/Si–40 %–

SO3H sample, whereas, an indiscernible pore structure is

Table 1 Textural parameters and chemical properties of catalysts

Sample S BET a (m2g-1) V P

b (cm3g-1) D pc (nm) d (100)

d (nm) a0e (nm) D pw

f (nm) Acidityg

(mmol H?

g-1)

SBA-15 746 1.03 8.38 8.5 9.82 1.44 –

SBA-15–SO3H 507 0.88 7.76 8.6 9.93 2.17 0.35

C/Si–20 % 358 0.75 7.55 8.9 10.28 2.73 –

C/Si–40 % 260 0.55 7.42 9.2 10.62 3.20 –C/Si–60 % 166 0.37 7.27 9.4 10.85 3.58 –

C/Si–20 %–SO3H 298 0.66 7.47 9.0 10.39 2.92 0.98

C/Si–40 %–SO3H 209 0.44 7.41 9.2 10.62 3.21 1.68

C/Si–60 %–SO3H 158 0.41 7.14 9.4 10.85 3.71 1.40

a Specific surface area estimated using BET methodb Pore volume estimated from BJH formulac Pore diameter of peak value in Fig. 3bd d-spacing of 100 diffractione Cell parameterf Pore wall thicknessg Measured by acid–base titration

Fig. 3 a N2 adsorption–desorption isotherms and b pore diameter distributions of SBA-15, C / Si– x and C / Si– x–SO3H samples

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shown in C/Si–60 %–SO3H sample, which should be due

to the excess carbon particles accumulate to block the pore.

This result is in accordance with that of N2 adsorption–

desorption isotherm.

Fourier transform infrared spectroscopy spectra (Fig. 6)

are used to characterize the functional groups. In SBA-15,

the bands centered at 1,086, 800 and 470 cm-1 are attrib-

uted to asymmetric stretching, symmetric stretching and

Fig. 4 SEM and TEM images of a, c C/Si–40 % and b, d C/Si–60 % samples samples

Fig. 5 TEM images of a C/Si–

40 %–SO3H and b C/Si–60 %–

SO3H samples. TEM images of

single carbon/silica composite

spheres of c C/Si–40 %–SO3H

and d C/Si–60 %–SO3H

samples

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bending modes of Si–O–Si respectively and the band at

965 cm-1 is related to the characteristic stretching vibra-

tion of non-bridged Si–OH groups [40, 41]. These

absorptions of Si–O–Si weaken in carbon/silica composite,

and the absorption of Si–OH even becomes illegible, which

may be because that the Si–OH group reacted with C–OH

group of carbon in the hydrothermal condition. Moreover,

the relative intensity of the absorption band at*3,450 cm-1 corresponding to the stretching vibration of

–OH group is significantly enhanced, indicating the plen-

tiful oxygen-containing groups, which is favorable for the

modification of –SO3H groups. In comparison with C/Si

sample, an additional peak at 1,020 cm-1 is found in the

samples with sulfonation treating. This absorption can be

ascribed to the S=O symmetric stretching vibrations [42,

43], and demonstrates that –SO3H groups have been suc-

cessfully modified onto the MC/silica composites. Addi-

tionally, the peak at 1,635 cm-1 is raised from the C=C

stretching vibration, suggesting the presence of polycyclic

aromatic rings, which are considered as the product of thecarbonization of organic substances [44, 45].

3.2 Catalytic performances of C/Si–x–SO3H

composites

The solid acid catalyzed esterification is an equilibrium

limited reaction. To investigate the catalytic performances

of C/Si– x–SO3H composite catalysts, esterification of OA

with methanol was carried out. Scheme 1 shows the

mechanistic steps during the esterification reaction [46].

The solid acid catalyst activates the esterification reaction

by affording a proton to the carboxylic acid molecule to

form a carbonium ion, which is readily attacked by an

alcohol (CH3OH) to give the corresponding ester after

losing a H2O and a proton.

3.2.1 Effect of carbon loading

Figure 7 depicts the catalytic performance of various cat-

alysts in esterification of OA with methanol. It clearly

displayed that the catalytic activity enhanced with the

increase of carbon loading from 20 to 40 %, which was

owed to the increase of acidity of catalysts (Table 1).

Nevertheless, with higher carbon loading, the catalyticactivity declined from C/Si–40 %–SO3H to C/Si–60 %–

SO3H and the OA conversion dropped from *80 to

*65 %. This behavior could be explained that the higher

carbon loading block the pore of SBA-15 to result in the

decrease of acidity and the reduced accessibility to acid

sites. For comparison, the catalytic performance of SBA-

15–SO3H was also tested. The observed OA conversion on

SBA-15–SO3H was only *20 % and much lower than that

of C/Si–40 %–SO3H, and this result may be ascribed to its

low acidity.

To further estimate the catalytic activity of C/Si–40 %–

SO3H, the influences of reaction parameters, includingreaction time, reaction temperature and molar ratio of

MeOH/OA, were investigated as following.

3.2.2 Effect of reaction time

The reaction time is an important parameter for evaluate

catalytic activity of solid acid catalyst. Figure 8 shows the

OA conversion dependent on reaction time from 0 to 3 h

with a MeOH/OA molar ratio of 10:1 at 343 K. This cat-

alyst exhibits a highly catalytic activity and reaction rate.

The OA conversion rapidly mounted to *70 % in the

initial 0.5 h, and reached an equilibrium value of *80 %

after 1.5 h. The connected mesoporous structure and large

surface area bearing abundant –SO3H groups should be

responsible for this excellent performance, which allow

reactants to promptly transfer in the catalyst system to

reach active sites, and in the meantime products formed on

catalyst surface can quickly diffuse out of catalyst to the

solution.

R C+H+

O

OH R C+

OH

OH+O

CH3

H

R C

OH

OH

O+

H

CH3

R C

OH

O+H2

_H2O

R+C

OH

H3CO_H+

OCH3C

O

R

R= C17H33

Oleic acid Methanol

Solid acid catalyst

Methyl oleate

OCH3

Scheme 1 Mechanism of solid acid catalyzed esterification of OA

with methanolFig. 6 FTIR spectra of SBA-15, C/Si–x and C/Si–x–SO3H samples

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3.2.3 Effect of reaction temperature

The effect of reaction temperature on esterification was

investigated at different temperatures (323, 333, 343 and

353 K) for 3 h. Figure 9 obviously demonstrates that the

reaction rate is dependent on the reaction temperature.

Under a lower reaction temperature of 323 K, the esteri-

fication reaction got to an equilibrium state after *2 h.With the gradual enhancement of temperature to 333 and

343 K, the times to equilibrium were shortened to *1.5

and *1 h, which was ascribed to the much faster diffusion

rate of reactants under a higher reaction temperature. As

the reaction temperature was raised up to 353 K, however,

the catalytic performance was similar to that under 343 K.

The reason may be that this temperature has greatly

exceeded the boiling temperature of methanol, and a sig-

nificant portion of methanol was under refluxing to reduce

its concentration in reaction solution. Furthermore, because

the esterification reaction is a reversible endothermic pro-

cess, appropriately increasing temperature is favorable for

the forward reaction. Consequently, the OA conversion

gradually increases from *68 % (323 K) to *71 %

(333 K) to *80 % (343 and 353 K).

3.2.4 Effect of MeOH/OA molar ratio

The molar ratio of methanol to OA is another most

important variable to affect the OA conversion. Becauseesterification is an equilibrium reaction, the amount of

methanol is usually excessive to force the reaction towards

the formation of ester [47]. Figure 10 depicts the OA

conversion versus MeOH/OA molar ratio from 1:1 to 30:1

at 343 K for 2 h. An extremely low OA conversion of

*35 % was obtained with a MeOH/OA molar ratio of 1:1.

With the increase of the molar ratio, the OA conversion

dramatically increased. As using a 30:1 molar ratio, the OA

conversion reached above 90 %. This behavior demon-

strates that the increased MeOH/OA molar ratio play an

important role in driving the equilibrium to the product

side. When the MeOH/OA molar ratio was at a low value, a

Fig. 8 Dependence of the OA conversion on reaction time over

C / Si–40 %–SO3H (MeOH/OA = 10:1; 343 K; 0.1 g catalyst)

Fig. 9 Effect of reaction temperature on the conversion of OA over

C / Si–40 %–SO3H (MeOH/OA = 10:1; 3 h; 0.1 g catalyst)

Fig. 10 Effect of MeOH/OA molar ratio on the conversion of OA

over C / Si–40 %–SO3H (2 h; 343 K; 0.1 g catalyst)

Fig. 7 Comparison of catalytic activities for conversion of OA with

various catalysts (MeOH/OA = 10:1; 2 h; 343 K; 0.1 g catalyst)

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large number of methanol molecules were vaporized into

reflux condenser. As a result, the methanol concentration in

reaction solution was decreased in a great degree, which

induced the low OA conversion. On the other hand, as

enhancement of the methanol concentration, the rate of

forward reaction was sharply raised, and in the meantime,

the rate of reverse reaction was restricted. Consequently,

the enhancement of MeOH/OA molar ratio contributes todrive the equilibrium to the product side and bring a higher

OA conversion.

3.3 The reusability of catalyst

The catalyst recycling is extremely important to estimate

the efficiency of solid acid catalysts, which contributes to

reduce the cost of practical applications process. The

deactivation of solid acid catalyst occurred due to the

leaching of sulfate groups caused by the water presented in

the reaction medium [48, 49]. In order to prove the reus-

ability of catalyst, after each catalytic reaction the solidacid catalyst was separated by centrifugation and washed

repeatedly with ethanol and distilled water. The washed

catalyst was dried at 373 K and used for the next experi-

ment. Figure 11 presents the reusability of C/Si–40 %–

SO3H catalyst through five consecutive cycles with a

MeOH/OA molar ratio of 30:1 at 343 K for 2 h. It was

noteworthy to mention that the catalyst was reusable

without any appreciable loss in activity. Meanwhile, for

checking the stability of –SO3H groups, the resulting

mixture after catalytic reaction was washed with water to

extract sulfate ions possibly leached from catalyst. The

BaCl2 precipitation test demonstrates no sulfate ions exist

in the washed water, suggesting the stability of –SO3H

groups on the surface of catalyst.

4 Conclusion

In conclusion, hydrothermal treatment strategy in the pres-

ence of glucose and SBA-15 was applied to prepare a

spherical mesoporous carbon/silica composite. The material

exhibits a novel spherical morphology, uniform and con-

nected pore structure, well dispersion of acid sites and a

striking catalytic performance on esterification. Importantly,leaching of functional groups from the hybrid catalyst in the

reaction system was inhibited and the catalyst can maintain

its initial activity after five consecutive catalytic cycles. In

addition, such a synthetic methodproposes a newstrategy for

controlling and preparing the different morphology of hybrid

mesoporous materials with high activities.

Acknowledgments The authors gratefully acknowledge the finan-

cial support from the National Natural Science Foundation of China

(21001093), the Qianjiang talent project of Zhejiang Province of

China (2011R10048), the Science Foundation of Zhejiang Sci-Tech

University (0913848-Y) and the project-sponsored by SRF for ROCS,

SEM.

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Fig. 11 Recyclability of C / Si–40 %–SO3H catalyst for the esterifi-

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SO3H-Functionalized Mesoporous Carbon-silica Composite With a Spherical Morphology and Its Excellent Catalytic Performance for Biodiesel Production