Design of Ordered Mesoporous Sulfonic Acid Functionalized ZrO2/organosilica Bifunctional Catalysts for Direct Catalytic Conversion of Glucose to Ethyl Levulinate

Accepted Article

Title: Design of ordered mesoporous sulfonic acid functionalized ZrO2/

organosilica bifunctional catalysts for direct catalytic conversion

of glucose to ethyl levulinate

Authors: Daiyu Song, Qingqing Zhang, Yingnan Sun, Panpan Zhang,

Yi-Hang Guo, and Jianglei Hu

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To be cited as: ChemCatChem 10.1002/cctc.201801089

Link to VoR: http://dx.doi.org/10.1002/cctc.201801089

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10.1002/cctc.201801089

ChemCatChem

FULL PAPER

Design of ordered mesoporous sulfonic acid functionalized

ZrO₂/organosilica bifunctional catalysts for direct catalytic

conversion of glucose to ethyl levulinate

Daiyu Song,^[a]Qingqing Zhang,^[a]Yingnan Sun,^[a]Panpan Zhang,^[a]Yi-Hang Guo,*^[a]and Jiang-Lei

Hu*^[b]

Dedication ((optional))

Abstract: Ordered mesoporous sulfonic acid functionalized

ZrO₂/organosilica catalysts (SO₄^2–/ZrO₂-PMO-SO₃H) bearing tunable

Brønsted and Lewis acid site distributions were prepared by a P123-

directed sol-gel co-condensation route followed by ClSO₃H

functionalization. As-prepared catalysts were applied in the

conversion of glucose to ethyl levulinate in ethanol medium. The

chemical or biochemical transformation of lignocellulosic

biomass/derivatives, has emerged more recently. Bio-ethanol,

biomass-derived furanic compounds as well as alkyl levulinates

represent a large family of the ‘second generation’ biofuels that

can be blended directly with fossil fuels.^[4]Among them, alkyl

levulinates have been proposed as promising candidates.^[5,6]

Among the various alkyl levulinates, including methyl, ethyl and

butyl levulinate, ethyl levulinate (EL) is the most suitable fuel

blend in terms of miscibility, reduced emissions and shorter

degradation time. EL can be directly used in diesel engines.^[6,7]

EL can be synthesized by acid-catalyzed ethanolysis of various

biomass-derived substrates such as levulinic acid (LA), furfuryl

SO₄^2–/ZrO₂-PMO-SO₃H-catalyzed target reaction followed

a

glucoseethyl glucosideethyl fructoside5-

ethoxymethylfurfuralethyl levulinate pathway dominated by the

synergistic effect of the super strong Brønsted acidity and moderate

Lewis acidity of the catalysts. Additionally, by combining the

advantages of the considerably high Brønsted (696 μeq g^–1) and

Lewis acid site density (703 μeq g^–1), optimal Brønsted/Lewis molar

ratio (0.99) and excellent porosity properties, the SO₄^2–/ZrO₂-PMO-

SO₃H1.0 obtained at an initial Si/Zr molar ratio of 1.0 exhibited the

highest ethyl levulinate yield (42.3%) among the various tested

catalysts. Moreover, the SO₄^2–/ZrO₂-PMO-SO₃H can be reused three

times without obvious changes in activity, morphology and chemical

structure.

alcohol

(FAL),

monosaccharides,

disaccharides

and

polysaccharides.^[7]Although high EL yield is obtained from LA or

FAL, the higher price of LA or FAL due to the multistep biomass

transformation process used for their production leads to an

uneconomical EL production process. For acid-catalyzed direct

conversion of disaccharides or polysaccharides to EL without

separating intermediates such as LA, 5-hydroxymethylfurfural

(5-HMF) or 5-ethoxymethylfurfural (5-EMF) and byproducts from

the reaction mixture, the yield of EL is considerably low (lower

than 50%) due to poor selectivity for EL.^[8,9]Acid-catalyzed direct

synthesis of EL from monosaccharides such as fructose or

glucose in ethanol medium has been the most favorable

approach, and fructose is reported to give a significantly higher

EL yield compared to glucose.^[10-12]Sustainable glucose is more

cheap and abundant than fructose; however, direct conversion

of glucose to EL is more complex and difficult than conversion of

fructose because the process involves isomerization of

glucose/ethyl glucoside to fructose/ethyl fructoside catalyzed

predominantly by Lewis (L) acid sites.^[13-17]Subsequent

successive dehydration of fructose/or ethyl fructoside to EL is

mainly catalyzed by Brønsted (B) acid sites. From an ecological

and economical viewpoint, bifunctional catalyst systems bearing

both B and L acid sites for the direct production of EL from

Introduction

Catalytic conversion of lignocellulosic biomass/derivatives to

alternative and renewable biofuels is currently of great interest to

alleviate the dependency on rapidly decreasing fossil reserves

and to reduce the emissions of greenhouse and noxious gases

such as CO, CO₂, NO_xand SO_x.^[1]The ‘first generation’ biofuels

are typically represented by biodiesels, which are composed of a

mixture of C₁₂–C₂₂fatty acid methyl esters derived from

acid/base-catalyzed transesterification of oily feedstocks with

light alcohols.^[2,3]Owing to enormous market requirements, the

‘second generation’ biofuels, which are obtainable directly via

abundant biomass-derived glucose can provide

a highly

[a]

Dr. D. Song, Dr. Q. Zhang, Dr. Y. Sun, Dr. P. Zhang, Prof. Y.-H.

Guo*

School of Environment

desirable alternative approach that obviates the dependence on

LA or FAL.^[9]For this purpose, SO₄^2/ZrO₂, protonic forms of

aluminosilicate zeolites such as H, HUSY, HZSM-5, HMOR

and HY have been applied in such conversion process.^[9]

However, the catalytic activity and selectivity of the

aforementioned systems are unsatisfactory because of

moderate B and/or L acidity or inferior porosity properties.

Therefore, the design of robust, efficient and recyclable

bifunctional solid acid catalysts for glucose conversion to alkyl

levulinates remains a challenge.

Northeast Normal University

Changchun 130117, P.R. China

Tel./Fax: (+86)431-89165626

E-mail: guoyh@nenu.edu.cn

Dr. J.-L. Hu*

School of Chemical Engineering

Changchun University of Technology

Changchun 130012, P.R. China

[b]

Supporting information for this article is given via a link at the end of

the document.((Please delete this text if not appropriate))

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Scheme 1. Illustrations of the preparation route and wall structure of bifunctional SO₄^2–/ZrO₂-PMO-SO₃H catalysts.

Recently reported combined catalyst systems consisting of

SO₄^2/ZrO₂(strong B acidity and moderate L acidity) and Sn-

Beta zeolites (considerably high L acidity) have been found to

exhibit superior catalytic activity compared to conventional acid

catalysts in the direct conversion of glucose in methanol to

methyl levulinate.^[9]

Accordingly, the morphologies of lyotropic liquid crystal phases

were well-adjusted. Here, the molar composition was controlled

at P123: BTESB: Zr(OnBu)₄: HCl: H₂O = 0.029: 1: (0.5, 1 or 2):

4.4: 288. At this molar composition, P123 micelles can

aggregate linearly to form cylindrically shaped lyotropic liquid

crystal structures, and the interaction between the cylindrical

P123 liquid crystals and the hydrolyzed Si/Zr precursors ensured

Motivated by the above considerations, here, a series of

highly ordered mesoporous sulfonic acid functionalized

ZrO₂/organosilica catalysts with strong B acidity and moderate L

acidity as well as tunable acid site distributions were prepared

by a carefully designed P123-directed sol-gel co-condensation

route followed by ClSO₃H functionalization. The resulting

bifunctional SO₄^2–/ZrO₂-PMO-SO₃H catalysts with superior

porosity properties, including a large BET surface area, high

pore volume and surface hydrophobicity, were successfully

applied in the conversion of glucose to EL in ethanol medium.

Special attention was paid to studying the reaction pathway and

kinetics of the SO₄^2–/ZrO₂-PMO-SO₃H-catalyzed glucose

conversion on the basis of qualitative and quantitative analyses

of the product distribution, to reveal the contributions of the B

and L acid sites of the catalysts to the formation of EL.

the construction of

a ZrOSiC₆H₄Si framework with

ordered mesostructure. Additionally, the hydrolysis rate of

BTESB was slower than that of Zr(OnBu)₄. To ensure a matched

hydrolysis rate of both precursors for adequate fabrication of the

ZrOSiC₆H₄Si framework, prehydrolysis of BTESB at 40

^oC for 1 h was performed at the beginning of the catalyst

preparation. The ZrOSiC₆H₄Si framework was further

fixed after hydrothermal treatment, and then ZrO₂-PMO with a

2D hexagonal mesostructure was obtained after removal of

P123. Subsequent functionalization of the ZrO₂-PMO framework

with ClSO₃H gave rise to bifunctional SO₄^2–/ZrO₂-PMO-SO₃H

catalysts without changing the structural ordering of the ZrO₂-

2–

PMO. However, at the lowest Si/Zr molar ratio of 0.25, SO₄

/ZrO₂-PMO-SO₃H exhibited a disordered 3D interconnected

mesostructure (see Figure 1b), implying that an unsuitable

BTESB/Zr(OnBu)₄ratio could not maintain the cylindrically

shaped lyotropic liquid crystal structures. Additionally, the acid

site distributions or molar ratio of B-to-L acid sites (n_B/n_L) can be

adjusted by changing the initial Si/Zr molar ratio (Table 1).

Results and Discussion

Preparation and characterization of bifunctional SO₄^2–/ZrO₂-

PMO-SO₃H catalysts

Morphological characteristics

As illustrated in Scheme 1, ordered mesoporous bifunctional

SO₄^2–/ZrO₂-PMO-SO₃H catalysts were facilely prepared by a

P123-directed co-hydrolysis and -condensation of the bridging

organosilane BTESB and Zr(OnBu)₄under acidic condition

followed by ClSO₃H functionalization. In the first step, the key

factor for ensuring a ZrO₂-PMO framework with highly ordered

mesostructure is to optimize the molar composition of the

starting materials, which affects the hydrolysis and condensation

rates of BTESB and Zr(OnBu)₄as well as the

hydrophilicity/hydrophobicity of the preparation system.

The TEM images presented in Figure 1a revealed that ZrO₂-free

PMO-SO₃H exhibited a 2D hexagonal mesostructure with long-

range structural ordering, and the estimated pore diameter was

6 nm. The SO₄^2–/ZrO₂-PMO-SO₃H0.25 catalyst obtained at an

initial Si/Zr molar ratio of 0.25 exhibited a disordered 3D

interconnected mesostructure with a pore diameter of ca. 4 nm

(Figure 1b).

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Figure 1. TEM images of PMO-SO₃H (a), SO₄^2–/ZrO₂-PMO-SO₃H0.25 (b), SO₄^2–/ZrO₂-PMO-SO₃H0.5 (c), SO₄^2–/ZrO₂-PMO-SO₃H1.0 (d), SO₄^2–/ZrO₂-PMO-

SO₃H2.0 (e), SO₄^2–/Nb₂O₅-PMO-SO₃H1.0 (f), SO₄^2–/TiO₂-PMO-SO₃H1.0 (g) and PF-SO₄^2–/ZrO₂-PMO-SO₃H1.0 (h).

Table 1 Textural parameters and acidic nature of various mesoporous sulfonated ZrO₂-organosilicas.

a

b

c

d

e

S_BET

(m²g^–1

D_p

V_p

A_B

A_L

f

n_B/n_L

Catalyst

)

(nm)

6.4

(cm³g^–1

)

(μeq g^–1

)

(μeq g^–1

)

PMO-SO₃H

1120

959

920

900

562

637

793

700

314

700

640

1.33

537

n.d.

SO₄^2–/ZrO₂-PMO-SO₃H2.0

SO₄^2–/ZrO₂-PMO-SO₃H1.0

SO₄^2–/ZrO₂-PMO-SO₃H0.5

SO₄^2–/ZrO₂-PMO-SO₃H0.25

PF-SO₄^2–/ZrO₂-PMO-SO₃H1.0

SO₄^2–/Nb₂O₅-PMO-SO₃H1.0

SO₄^2–/TiO₂-PMO-SO₃H1.0

SO₄^2–/ZrO₂-SiO₂1.0

6.4

5.5

3.9

3.5

5.6

6.4

4.2

2.5

0.7

1.31

1.07

0.68

0.73

0.56

1.09

1.00

0.72

n.d.

566

696

986

908

493

682

595

545

647

423

572

703

835

609

498

710

661

458

n.d.

0.99

1.18

1.49

0.99

0.96

0.90

1.19

n.d.

HY zeolite

Hβ zeolite

n.d.

^aThe BET surface area (S_BET) was calculated using Brunauer–Emmett–Teller (BET) equation.

^bThe pore diameter (D_p) was estimated from BJH adsorption determination.

^cThe pore volume (V_p) was estimated from the pore volume determination using the adsorption branch of the N₂isotherm curve at P/P₀= 0.99 single point.

^dA_B: B acid site density.

^eA_L: L acid site density.

^fThe molar ratio of B acid-to-L acid (n_B/n_L) was estimated by the peak area integral ratio at 1593 and 1443 cm^–1(Figure 4c).

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SO²₄^-/ZrO₂-PMO-SO₃H2.0

SO²₄^-/ZrO₂-PMO-SO₃H1.0

SO²₄^-/ZrO₂-PMO-SO₃H0.5

SO²₄^-/ZrO₂-PMO-SO₃H2.0

SO²₄^-/ZrO₂-PMO-SO₃H1.0

SO²₄^-/ZrO₂-PMO-SO₃H0.5

a

b

SO²₄^-/ZrO₂-PMO-SO₃H0.25

PMO-SO₃H

0

5

10

15

0.0

0.2

0.4

0.6

0.8

1.0

D_P(nm)

P/P₀

SO²₄^-/TiO₂-PMO-SO₃H1.0

SO²₄^-/Nb₂O₅-PMO-SO₃H1.0

PF-SO²₄^-/ZrO₂-PMO-SO₃H1.0

SO²₄^-/TiO₂-PMO-SO₃H1.0

SO²₄^-/Nb₂O₅-PMO-SO₃H1.0

PF-SO²₄^-/ZrO₂-PMO-SO₃H1.0

d

c

0.0

0.2

0.4

0.6

0.8

1.0

0

5

10

15

20

25

30

P/P₀

D_P(nm)

Figure 2. Nitrogen gas adsorption–desorption isotherms (a, c) and BJH pore size distribution profiles (b, d) of various SO₃H-based catalysts.

The SO₄^2–/ZrO₂-PMO-SO₃H0.5, SO₄^2–/ZrO₂-PMO-SO₃H1.0 and

SO₄^2–/ZrO₂-PMO-SO₃H2.0 catalysts obtained at initial Si/Zr

molar ratios of 0.5, 1.0 and 2.0 all possessed an ordered

mesostructure (pore diameter of 4‒6 nm) with helically arranged

mesopores (Figure 1c-e).

/ZrO₂(Nb₂O₅

or

TiO₂)-PMO-SO₃H

with

an

ordered

mesostructure; in addition, the suitable molar composition of

P123: BTESB: Zr(OnBu)₄: HCl: H₂O greatly influenced the

structural ordering of the materials.

SAXS measurements supported the TEM observations. As

shown in Figure S1 in the Supporting Information (SI), ZrO₂-free

PMO-SO₃H exhibited three diffraction peaks in the low-angle

range, i.e., 0.71.1 deg (100), 1.51.7 deg (110) and 1.71.8

deg (200). The first peak possessed the highest intensity,

whereas the other two peaks showed very weak intensity. The

results indicated that the PMO-SO₃H possessed highly ordered

mesoporous channels.

To verify the reproducibility of the aforementioned preparation

route, SO₄^2–/Nb₂O₅-PMO-SO₃H1.0 and SO₄^2–/TiO₂-PMO-

SO₃H1.0 were also prepared. As shown in Figure 1f and g, both

2–

samples exhibited ordered mesostructures similar to their SO₄

/ZrO₂-PMO-SO₃H1.0 counterpart.

However, in the absence of P123, the resulting PF-SO₄

2–

/ZrO₂-PMO-SO₃H1.0

exhibited

a

3D

interconnected

mesostructure (Figure 1h). The results suggested that P123

2–

plays a structure-directing function in the formation of the SO₄

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Zr 3d_5/2

Zr 3d_3/2

Nb 3d_5/2

a

b

Nb 3d_3/2

SO²₄^-/ZrO₂-PMO-SO₃H1.0

SO²₄^-/Nb₂O₅-PMO-SO₃H1.0

ZrO₂

190

Nb₂O₅

215

185

180

210

205

Binding energy (eV)

Ti 2p_3/2

S 2p_1/2

S 2p_3/2

c

d

Ti 2p_1/2

SO²₄^-/TiO₂-PMO-SO₃H1.0

SO²₄^-/Nb₂O₅-PMO-SO₃H1.0

SO²₄^-/ZrO₂-PMO-SO₃H1.0

TiO₂

470

465

460

455

175

170

165

Binding energy (eV)

Figure 3. High resolution XPS spectra of SO₄^2–/ZrO₂-PMO-SO₃H1.0 and ZrO₂in the Zr 3d binding energy region (a), SO₄^2–/Nb₂O₅-PMO-SO₃H1.0 and Nb₂O₅in

the Nb 3d binding energy region (b), SO₄^2–/TiO₂-PMO-SO₃H1.0 and TiO₂in the Ti 2p binding energy region (c) and SO₄^2–/ZrO₂(Nb₂O₅/TiO₂)-PMO-SO₃H1.0 in the

S 2p binding energy region (d).

For the sulfonated ZrO₂-PMO catalysts, the (100) diffraction

peak (0.860.95 deg) gradually became weaker as the Si/Zr

molar ratio decreased from 2.0, 1.0 to 0.5; in addition, the pair of

weak diffraction peaks nearly disappeared. These results

indicated that the SO₄^2–/ZrO₂-PMO-SO₃H possessed structural

ordering that decreased with increasing ZrO₂content. In the

case of PF-SO₄^2–/ZrO₂-PMO-SO₃H1.0, the diffraction peaks in

the low angle range were not found, further indicating its

disordered mesostructure.

Therefore, bridging organosilica units (SiC₆H₄Si) and

P123 with suitable content in the initial catalyst preparation

system are important contributors to the structural ordering of

the bifunctional catalysts.

Porosity properties

The porosity properties of the SO₄^2–/ZrO₂-PMO-SO₃H catalysts

were characterized by nitrogen porosimetry measurements. As

shown in Figure 2a, PMO-SO₃H and SO₄^2–/ZrO₂-PMO-SO₃H

with Si/Zr molar ratios of 0.5, 1.0 and 2.0 exhibited a type IV

isotherm with an H1 type hysteresis loop, reflecting their regular

mesoporous structures without interconnecting channels. In

addition, SO₄^2–/ZrO₂-PMO-SO₃H0.25 had a type IV isotherm

with an H2 type hysteresis loop, indicating its disordered

mesostructure. These results are in consistent with those of

obtained by TEM or SAXS.

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ix

viii

vii

ix

viii

vii

vi

v

vi

v

vi

v

iv

iii

ii

i

iv

iii

B- + L-acid

iii

ii

Si-OH

ii

L-acid

B-acid

C=C, O-H S-O

S=O

C-H

ⁱb

i

-

O-H

a

c

Si-O-Si

SO

3

3000

2000

1000

3000

2000

1000

1650 1500 1350

Wavenumber / cm^-1

2–

Figure 4. FTIR spectra (a) and pyridine-adsorbed FT-IR spectra (b, c) of (i) PMO-SO₃H, (ii) SO₄^2–/ZrO₂-PMO-SO₃H0.25, (iii) SO₄^2–/ZrO₂-PMO-SO₃H0.5, (iv) SO₄

/ZrO₂-PMO-SO₃H1.0, (v) SO₄^2–/ZrO₂-PMO-SO₃H2.0, (vi) PF-SO₄^2–/ZrO₂-PMO-SO₃H1.0, (vii) SO₄^2–/Nb₂O₅-PMO-SO₃H1.0, (viii) SO₄^2–/TiO₂-PMO-SO₃H1.0 and (ix)

ZrO₂.

Additionally, the aforementioned samples possessed narrowed

BJH pore size distribution curves, indicating their well-distributed

pore diameters (Figure 2b).

lower pore volume (0.73 cm³g^–1) originated from the synergistic

effect of the structural disordering and high Zr content. The BET

surface areas of SO₄^2–/Nb₂O₅-PMO-SO₃H1.0 (793 m²g^–1) and

SO₄^2–/TiO₂-PMO-SO₃H1.0 (700 m²g^–1) were smaller than that of

SO₄^2–/ZrO₂-PMO-SO₃H1.0. PF-SO₄^2–/ZrO₂-PMO-SO₃H1.0 also

exhibited a much smaller BET surface area (637 m²g^–1) and

pore volume (0.56 cm³g^–1).

As shown in Figure 2c and d, both SO₄^2–/Nb₂O₅-PMO-

SO₃H1.0 and SO₄^2–/TiO₂-PMO-SO₃H1.0 exhibited similar

porosity

properties

to

their

SO₄^2–/ZrO₂-PMO-SO₃H1.0

counterpart, indicating their ordered mesostructures with well-

distributed pore size. However, the PF-SO₄^2–/ZrO₂-PMO-

SO₃H1.0 sample exhibited an ill-defined nitrogen gas sorption

isotherm and pore size distribution curve, indicating a disordered

mesostructure. The results further confirmed that the structural

ordering of the bifunctional SO₄^2–/ZrO₂(Nb₂O₅/TiO₂)-PMO-SO₃H

catalysts was dominantly influenced by the Si/Zr molar ratio and

structure-directing function of P123.

Composition and structural information

The XPS surface probe technique was applied to study the

2–

surface interactions between the SO₄anion or –SO₃H group

with the ZrO₂-POM framework. Figure 3a shows high-resolution

XPS spectra of the ZrO₂and SO₄^2–/ZrO₂-PMO-SO₃H1.0 in the Zr

3d binding energy region. The binding energies of Zr 3d_5/2and

Zr 3d_3/2of ZrO₂were 181.8 and 184.2 eV, respectively,

corresponding to the characteristics of the Zr(IV) oxidation state

in the ZrO₂. The binding energies of Zr 3d_5/2(183.0 eV) and Zr

3d_3/2(185.3 eV) shifted to higher values for SO₄^2–/ZrO₂-PMO-

As summarized in Table 1, PMO-SO₃H exhibited the largest

BET surface area (1120 m²g^–1) and the highest pore volume

(1.33 m³g^–1) among all tested samples. For three ordered

mesoporous bifunctional catalysts, SO₄^2–/ZrO₂-PMO-SO₃H2.0

2–

(959 m²g^–1), SO₄^2–/ZrO₂-PMO-SO₃H1.0 (920 m²g^–1) and SO₄

/ZrO₂-PMO-SO₃H0.5 (900 m²g^–1), their BET surface areas

decreased slightly compared with PMO-SO₃H, but the

differences in their BET surface areas were negligible. However,

the pore volumes of these three samples (1.31, 1.07 and 0.68

m³g^–1) were obviously different, owing to the Si/Zr molar ratio. In

the SO₄^2–/ZrO₂-PMO-SO₃H catalysts, Zr atoms were

incorporated into the benzene-bridged organosilica framework

SO₃H1.0, implying

a

slight perturbation of the zirconium

2–

environment due to the interaction between SO₄and ZrO₂as

well as the introduction of SiC₆H₄Si units into the ZrO₂

framework. Similar effects were also observed in the high-

resolution XPS spectra of Nb₂O₅and SO₄^2–/Nb₂O₅-PMO-

SO₃H1.0 in the Nd 3d binding energy region (Figure 3b) as well

as TiO₂and SO₄^2–/TiO₂-PMO-SO₃H1.0 in the Ti 2p binding

energy region (Figure 3c), further confirming the interactions

2–

through ZrOSiC₆H₄SiO linkages, whereas SO₄anions

2–

and SO₃H groups interacted with the ZrO₂and phenyl groups,

respectively. The incorporation of these two acid sites into a

benzene-bridged organosilica framework blocked the pore

channels to some extent, which in turn led to the decreased BET

surface area and pore volume; moreover, more acid sites were

incorporated, resulting in more significant decreases in the BET

surface area and pore volume. In the case of SO₄^2–/ZrO₂-PMO-

SO₃H0.25, its much smaller BET surface area (562 m²g^–1) and

between SO₄and Nb₂O₅(TiO₂) as well as the introduction of

SiC₆H₄Si units into the Nb₂O₅(TiO₂) framework.

Figure 3d displays the high-resolution XPS spectra of three

bifunctional catalysts in the S 2p binding energy region, which

were deconvoluted into two peaks centered at 168.5 and 170.1

eV, corresponding to S 2p_3/2and S 2p_1/2spin-orbit components,

respectively.^[18-20]

supports were successfully sulfonated by ClSO₃H.

Therefore,

the

ZrO₂(Nb₂O₅/TiO₂)-PMO

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The formation of a benzene-bridged organosilica framework

and its subsequent sulfonation were further demonstrated by

FT-IR analysis. As shown in Figure 4a, various sulfonated

catalysts all exhibited characteristic IR absorption peaks at 1634

as well as 2977 and 2898 cm^1, which were assigned to

stretching vibrations of C=C and CH bonds from the

SiC₆H₄SiO units.^[21]In addition, the peak at 1634 cm^1was

possibly due to the stretching vibration of OH bond from the

SiOH or ZrOH groups on the surface of the catalysts. The

peaks appearing at 3425, 918 as well as 810 and 526 cm^1were

attributed to the vibrations of OH, SiOH and SiOSi bonds,

respectively.^[22]Additionally, the other three IR absorption peaks

appearing at 1384, 1156 and 1021 cm^1originated from

stretching vibrations of SO and S=O bonds from the SO₃H

¹³C CP-MAS NMR analysis conclusively supported the above

result. As shown in Figure 5a, two resonance signals at 132.8

and 142.4 ppm were found in the ¹³C CP-MAS NMR spectrum of

the representative catalyst, SO₄^2–/ZrO₂-PMO-SO₃H1.0. The

former higher-intensity was attributed to carbon species from the

–Si–C₆H₄–Si– units,^[25]whereas the latter with much weaker

intensity was assigned to carbon species from the sulfonated –

Si–C₆H₄–Si– framework.^[26]The other two weak signals at 57.7

and 15.8 ppm were assigned to the carbon species of the ethoxy

groups that formed during the boiling ethanol washing

process.^[27,28]In the ²⁹Si MAS NMR spectrum of SO₄^2–/ZrO₂-

PMO-SO₃H1.0 presented in Figure 5b, the characteristic

resonances at –63.2 and –71.9 ppm were assigned to

organosiloxane species of T²[C–Si(OSi)₂(OH)] and T³[C–

Si(OSi)₃],^[29]whereas the resonance signal at –80.9 ppm

originated from inorganosiloxane species of Q¹[Si(OSi)(OH)₃].^[29]

These results indicated that not only ZrO₂but also benzene-

bridged organosilica unit can be sulfonated by ClSO₃H during

the SO₄^2–/ZrO₂-PMO-SO₃H prepartion process, thereby

providing two types of B acid sites, i.e., SO₄^2–/ZrO₂and PMO-

SO₃H (Scheme 1).

2–

group and/or SO₄anion,^[20,23,24]which suggested that the ZrO₂-

PMO framework was successfully sulfonated by ClSO₃H.

132.8

a

Si

Acid nature

SO₃H

The acid site nature of SO₄^2–/ZrO₂-PMO-SO₃H was

characterized by in situ pyridine-FT-IR transmission

spectroscopy; for comparison, PMO-SO₃H was also tested

(Figure 4b). Compared with the FT-IR spectrum of pyridine-free

PMO-SO₃H, a new peak at 1593 cm⁻¹was observed in the

pyridine-adsorbed FT-IR spectrum of PMO-SO₃H, which was

assigned to pyridinium ions formed due to protonation by the

SO₃H group.^[30]This result indicated the B acid nature of PMO-

57.7

50

15.8

142.4

150

2–

SO₃H. The pyridine-adsorbed FT-IR spectra of various SO₄

200

100

0

/ZrO₂-PMO-SO₃H bifunctional catalysts showed three new peaks

positioned at 1443, 1489 and 1593 cm⁻¹, respectively, compared

with their FT-IR spectra. The peak at 1443 cm⁻¹corresponded to

pyridine interacting with L acid sites, e.g., the unsaturated

surface Zr⁴⁺site of ZrO₂,^[31]whereas the peak at 1489 cm⁻¹

indicated the co-existence of B and L acid sites in the catalysts.

Therefore, the SO₄^2–/ZrO₂-PMO-SO₃H exhibited both B and L

acidity, in which the B acid sites were contributed from both

SO₄^2–/ZrO₂and SO₃H groups confined on the benzene-bridged

units. Moreover, the SO₄^2/ZrO₂possessed super strong B

acidity, and its strong B acidic protons originated from the

polarization of surface OH groups of ZrO₂induced by the S=O

double bonds of the near sulfate group (Scheme 1).^[32-34]

ppm

b

-71.9

-63.2 -80.9

Similarly, three new peaks at 1443, 1489 and 1593 cm⁻¹were

2–

also found in the pyridine-adsorbed FT-IR spectra of SO₄

/Nb₂O₅-PMO-SO₃H1.0

and

SO₄^2–/TiO₂-PMO-SO₃H1.0,

suggesting their both B and L acidity. Their B acid sites

originated from both SO₄^2–/Nb₂O₅(or SO₄^2–/TiO₂) and the SO₃H

50

0

-50

-100

ppm

-150

-200

group, whereas the

L acid sites were derived from the

unsaturated surface Nb⁵⁺(or Ti⁵⁺) site of Nb₂O₅(or TiO₂).

The B acid site density (A_B) of the SO₄^2–/ZrO₂-PMO-SO₃H was

determined by titrating with dilute NaOH solution, whereas the L

acid site density (A_L) was calculated based on the n_B/n_Lvalue.^[35]

As summarized in Table 1, both B and L acid site densities of

Figure 5. ¹³C CP (a) and ²⁹Si (b) MAS NMR spectra of the SO₄^2–/ZrO₂-PMO-

SO₃H1.0.

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three ordered mesoporous SO₄^2–/ZrO₂-PMO-SO₃H catalysts

decreased gradually with increasing Si/Zr molar ratios from 0.5,

1.0 to 2.0, owing to the gradually decreased proportion of the

SO₄^2–/ZrO₂units in the catalysts. For SO₄^2–/ZrO₂-PMO-

SO₃H0.25, its lower B acid site density compared to SO₄^2–/ZrO₂-

PMO-SO₃H0.5 was due to its significantly lower BET surface

area and thereby decrease in exposed surface acid sites.

of SO₄^2–/ZrO₂-PMO-SO₃H1.0, SO₄^2–/Nb₂O₅-PMO-SO₃H1.0 and

SO₄^2–/TiO₂-PMO-SO₃H1.0 were 0.99, 0.96 and 0.90,

respectively, showing that different metal oxides possess

different acid capacities.^[32]For PF-SO₄^2–/ZrO₂-PMO-SO₃H1.0,

its B and L acid site densities (493 and 488 μeq g^–1) were the

lowest because of its worst porosity and thereby the least-

exposed acid sites.

Catalytic performance

50

a

The catalytic performance of the bifunctional SO₄^2–/ZrO₂-PMO-

SO₃H catalysts was evaluated by conversion of glucose to EL in

ethanol medium (Scheme 2); reference catalysts including PMO-

SO₃H, SO₄^2–/ZrO₂, PF-SO₄^2–/ZrO₂-PMO-SO₃H1.0, SO₄^2–/ZrO₂-

SiO₂1.0, SO₄^2–/Nb₂O₅-PMO-SO₃H1.0 and SO₄^2–/TiO₂-PMO-

SO₃H1.0 as well as commericial HY and Hβ zeolites were also

tested under the same conditions.

40

30

20

10

0

Optimization of reaction conditions

The optimization of the reaction temperature was studied by

selecting SO₄^2–/ZrO₂-PMO-SO₃H1.0 as a representative catalyst.

As shown in Figure 6a, the production of EL from conversion of

glucose in ethanol medium was significantly accelerated by

elevating the reaction temperature from 140, 160 to 170 ^oC, and

the corresponding EL yield reached 6.9, 15.2 and 39.2% after

the reaction was performed for 8 h. Therefore, in subsequent

catalytic tests, the reaction temperature was set at 170 ^oC.

Next, the influence of the glucose-to-ethanol molar ratio

140

170

160

Temperature (^oC)

50

40

30

20

10

0

n_Glucose/n_EtOH= 1/186

2–

(n_glucose/n_EtOH) on the catalytic activity of the representative SO₄

b

/ZrO₂-PMO-SO₃H1.0 was explored at 170 ^oC. As shown in

Figure 6b, over a period of 12 h, the yield of EL reached 21.9,

42.3 and 32.1%, respectively, at n_glucose/n_EtOHratios of 1/186,

1/248 and 1/310. In the current catalytic system, ethanol was

used as both reactant and solvent, and therefore suitable

n_glucose/n_EtOHratio (e.g. 1/248) can facilitate the conversion of

glucose to EL; in addition, soluble polymeric humins derived

from self-polymerization of the intermediates can be inhibited to

some extent.^[36]However, at a lower n_glucose/n_EtOHratio, e.g. 1/310,

the substrate may be diluted, thereby decreasing the EL yield

(32.1%); moreover, excessive ethanol may reduce the economic

favorability of the above process. Extremely low EL yield (21.9%)

was found at a higher n_glucose/n_EtOHratio (1/186), originating from

serious self-polymerization of the intermediates and thereby the

decreased selectivity to EL. The above results were consistent

with the color changes of the reaction media from light-yellow

(n_glucose/n_EtOHratio of 1/310 and 1/248) to brown-black

(n_glucose/n_EtOHratio of 1/186); in addition, the formed polymeric

humins were identified by LC-MS as shown in Figure S2ae of

the SI. Based on these results, an optimum n_glucose/n_EtOHratio of

1/248 was chosen for subsequent catalytic tests.

n_Glucose/n_EtOH= 1/248

n_Glucose/n_EtOH= 1/310

0

4

8

12

Time (h)

Figure 6. Influence of reaction temperature (a) and glucose-to-ethanol molar

ratio (n_glucose/n_EtOH) (b) on the catalytic activity of SO₄^2–/ZrO₂-PMO-SO₃H1.0 in

the conversion of glucose in ethanol. Reaction conditions for Figure 6a: n_glucose

/n_EtOH= 1/248, 8 h, 1.5 wt% catalyst; reaction conditions for Figure 6b: 170 ^oC,

1.5 wt% catalyst.

The B and L acid site densities of both SO₄^2–/Nb₂O₅-PMO-

SO₃H1.0 and SO₄^2–/TiO₂-PMO-SO₃H1.0 were similar to their

SO₄^2–/ZrO₂-PMO-SO₃H1.0 counterpart; however, the n_B/n_Lratios

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Reaction pathway and kinetics

100

80

60

40

20

0

As shown in Figure 7, the conversion of glucose in ethanol

medium over the bifunctional SO₄^2–/ZrO₂-PMO-SO₃H1.0 or

monofunctional PMO-SO₃H or SO₄^2–/ZrO₂proceeded rapidly at

170 ^oC, and glucose conversion was greater than 99% over

period of 15 and 30 min, respectively, over the SO₄^2–/ZrO₂-PMO-

SO₃H1.0 (Figure 7a) and PMO-SO₃H (Figure 7b) or SO₄^2–/ZrO₂

(Figure 7c). However, production of the final product EL

occurred slowly.

Glucose

a

Ethyl glucoside

Ethyl lactate

5-EMF

80

60

40

20

0

EL

2–

To reveal the contributions of the B and L acid sites of SO₄

/ZrO₂-PMO-SO₃H to the formation of EL from glucose, the

reaction pathway of the SO₄^2–/ZrO₂-PMO-SO₃H-catalyzed

conversion of glucose in ethanol medium was studied by

identifying the intermediates and byproducts of the above

process via both LC-MS and GC-MS; afterwards, kinetic studies

were performed by both LC and GC.

0

240

480

720

Time (min)

Generally, the first step of the synthesis of EL from glucose

over bifunctional acid catalysts may experience etherification or

isomerization of glucose to ethyl glucoside^[5]or fructose^[9],

depending on the acid nature of the catalyts. In the current

catalytic system, LC-MS analysis showed that at the initial stage

of the reaction (15 min), SO₄^2–/ZrO₂-PMO-SO₃H1.0-catalyzed

glucose was mainly transformed into ethyl glucoside (retention

time = 8.83 min, m/z = 206.98, Figure S3a and c) and ethyl

fructoside (retention time = 8.50 min, m/z = 207.04, Figure S3a

and d). Additionally, trace anhydro monosaccharide byproducts

with retention times of 6.48 and 6.80 min were detected (Figure

S3a); these byproducts were derived from the dehydration of

glucose. After the reaction proceeded for 4 and 12 h, glucose,

ethyl glucoside, ethyl fructoside and anhydro monosaccharide

disappeared, suggesting that they were further transformed to

the intermediates and EL; additionally, the above transformation

process was accompanied by the self-polymerization of the

intermediates, giving rise to various polymeric humins that were

found by LC-MS (Figure S2d and e). GC-MS analysis indicated

that the intermediate 5-EMF (retention time = 9.30 min),

byproduct ethyl lactate (retention time = 6.97 min) and final

product EL (retention time = 10.97 min) were identified at a

reaction time of 8 h (Figure S4). However, fructose was not

detected in this system.

100

80

60

40

20

0

100

80

60

40

20

0

Glucose

Ethyl glucoside

Ethyl lactate

5-EMF

b

EL

0

240

Time (min)

480

720

100

80

60

40

20

0

100

80

60

40

20

0

c

Glucose

Ethyl glucoside

Ethyl lactate

5-EMF

EL

The above results indicate that the production of EL from the

SO₄^2–/ZrO₂-PMO-SO₃H-catalyzed conversion of glucose in

ethanol medium followed the reaction pathway of glucoseethyl

glucosideethyl

fructoside5-EMFEL

via

successive

etherification, isomerization, dehydration and ring-opening steps,

which were dominated by the synergistic effect of the super

strong B acidity and moderate L acidity of the SO₄^2–/ZrO₂-PMO-

SO₃H.^[5,9,37,38]In detail, as illustrated in Scheme 2, the super

strong B acid sites (SO₄^2–/ZrO₂and SO₃H) of SO₄^2–/ZrO₂-PMO-

SO₃H can activate the hemiacetal hydroxy of glucose first, and

the activated glucose is then attacked by ethanol to produce

ethyl glucoside and water via an etherification reaction.

Subsequently, the L acid sites (Zr⁴⁺) of the bifunctional catalysts

facilitate the breakage of the C–O bond of the six-membered

ring structured ethyl glucoside, followed by ring-opening and

isomerization to generate ethyl fructoside. Next, ethyl fructoside

0

240

480

720

Time (min)

Figure 7. Kinetic studies on conversion of glucose in ethanol over SO₄^2–/ZrO₂-

PMO-SO₃H1.0 (a), PMO-SO₃H (b) and SO₄^2–/ZrO₂(c). n_glucose/n_EtOH= 1/248,

1.5 wt% catalyst, 170 ^oC.

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undergoes dehydration and breakage of C–O bonds with the

assistance of the B acid sites to produce 5-EMF. Finally,

protonation of 5-EMF by the B acid sites gives rise to cyclic

oxonium, and EL is produced by a ring-opening reaction of the

cyclic oxonium under attack by water and ethanol. Therefore, in

the bifunctional SO₄^2–/ZrO₂-PMO-SO₃H catalytic system, super

strong B acidity and double B acid sites are responsible for

etherification of glucose to ethyl glucoside in the first step of

glucose conversion, whereas the moderate L acid sites of the

catalyst are active for the isomerization of ethyl glucoside to

ethyl fructoside; subsequent dehydration of ethyl glucoside to 5-

EMF and then ring-opening of the activated 5-EMF to EL are

catalyzed by the B acid sites of the catalysts.

the PMO-SO₃H-catalyzed glucose conversion reaction

proceeded for 30 min. However, in contrast to bifunctional SO₄

2–

/ZrO₂-PMO-SO₃H1.0, when the reaction time was continuously

increased to 720 min, ethyl glucoside remained in the PMO-

SO₃H system with a yield of 12.4%. Additionally, only trace 5-

EMF yielded, and the maximum 5-EMF yield of 2.5% was

obtained after the reaction was performed for 720 min. Ethyl

lactate was not produced in this system. PMO-SO₃H only had B

acid sites, and in the PMO-SO₃H-catalyzed glucose conversion

process, glucose was etherified to ethyl glucoside. However,

owing to the weaker B acid strength of PMO-SO₃H compared

with SO₄^2–/ZrO₂-PMO-SO₃H1.0, the yield of ethyl glucoside was

lower over PMO-SO₃H. In addition, owing to the absence of L

acid sites, it was difficult to further isomerize ethyl glucoside to

ethyl fructoside. Accordingly, ethyl glucoside still remained in the

system with considerably high yield, which significantly limited

further conversion of ethyl fructoside to 5-EMF and EL.

Accordingly, the EL yield (16.7%, 720 min) over PMO-SO₃H was

obviously lower than that over SO₄^2–/ZrO₂-PMO-SO₃H1.0.

Additionally, the pure B acid nature of the PMO-SO₃H prevented

the formation of ethyl lactate.

2–

On the basis of the above results, kinetic studies of SO₄

/ZrO₂-PMO-SO₃H1.0-, PMO-SO₃H- and SO₄^2–/ZrO₂-catalyzed

conversion of glucose to ethyl glucoside, 5-EMF, ethyl lactate

and EL in ethanol medium were performed. As shown in Figure

7a, at the initial stage of the SO₄^2–/ZrO₂-PMO-SO₃H1.0-

catalyzed glucose conversion reaction, the yield of ethyl

glucoside increased gradually, and the highest yield of ethyl

glucoside (27.3%) was obtained after the reaction proceeded for

30 min. As the reaction time was further prolonged, the yield of

ethyl glucoside decreased quickly and disappeared over a

period of 480 min. This result implies that ethyl glucoside was

transformed to ethyl fructoside and other intermediates rapidly, a

prerequisite for ensuring the production of EL with considerably

high yield. The highest 5-EMF yield of 8.2% was obtained at 30

min, and the yield decreased gradually with increasing reaction

time. The total yields of all detected intermediates were less

than 2% after the reaction was performed for 720 min,

suggesting that they were further converted to the final product

EL and polymeric humins. Additionally, only a small amount of

ethyl lactate (2.9%) was obtained in the current catalytic system.

Formation of ethyl lactate was catalyzed by the L acid sites of

the catalysts via retro-aldol reaction followed by isomerization,

which competed with the dehydration of 5-EMF catalyzed by the

B acid sites.^[15]Since SO₄^2–/ZrO₂-PMO-SO₃H exhibited moderate

L acid sites, the production of ethyl lactate in glucose conversion

was obviously inhibited, which was favorable for the production

of EL with high yield. Consequently, SO₄^2–/ZrO₂-PMO-SO₃H1.0

exhibited a retatively high EL yield compared with reported

catalytic systems.^[5,7,37]For example, after the reaction time

proceeded for 720 min, the EL yield reached 42.3%. However,

for the reported SO₄^2–/ZrO₂-catalyzed ethanolysis of glucose, the

EL yield was ca. 30% under the conditions of catalyst dosage of

Although SO₄^2–/ZrO₂exhibited both B and L acid nature, it

catalyzed the glucose conversion reaction with lower EL yield

(20.5%, 720 min) compared with SO₄^2–/ZrO₂-PMO-SO₃H1.0

(Figure 7c). This result is due to its single B acid sites as well as

very small BET surface area (12 m²g^1).^[32]Additionally, the yield

of ethyl glucoside reached 14.0% at 30 min and then decreased

to 1.8% at 720 min with the aid of L acid sites. The highest 5-

EMF yield of 9.2% was obtained at 240 min, and the yield

decreased to 4.9% at 720 min.

2–

Subsequently, the catalytic activities of the bifunctional SO₄

/ZrO₂-PMO-SO₃H catalysts prepared at Si/Zr molar ratios of 0.25,

0.5, 1.0 and 2.0 were compared in conversion of glucose to EL.

As shown in Figure 8a, SO₄^2–/ZrO₂-PMO-SO₃H1.0 was the most

active in the target reaction, whereas the catalytic activities of

SO₄^2–/ZrO₂-PMO-SO₃H0.25

were similar and both the lowest. After the SO₄^2–/ZrO₂-PMO-

SO₃H0.25-,

SO₄^2–/ZrO₂-PMO-SO₃H0.5-, SO₄^2–/ZrO₂-PMO-

and

SO₄^2–/ZrO₂-PMO-SO₃H2.0

SO₃H1.0- and SO₄^2–/ZrO₂-PMO-SO₃H2.0-catalyzed glucose

conversion reactions proceeded for 12 h, the yield of EL was

20.6, 30.5, 42.3 and 21.5%, respectively. The above results can

be explained by the influence of the initial Si/Zr molar ratios on

the porosity properties, the acid site densities of B and L acid

and the B-to-L acid molar ratio. All of these factors were

important influences on the catalytic activity of SO₄^2–/ZrO₂-PMO-

SO₃H. SO₄^2–/ZrO₂-PMO-SO₃H2.0 possessed perfect porosity

properties, including the largest BET surface area (959 m²g^–1)

and pore size (6.4 nm) as well as the highest pore volume (1.31

cm³g^–1), among four bifunctional SO₄^2–/ZrO₂-PMO-SO₃H

catalysts, and its poor catalytic activity in the target reaction was

due to the lowest acid site density of B and L acids. SO₄^2–/ZrO₂-

PMO-SO₃H0.25 had a considerably high acid site density of B

and L acids, and its poor catalytic activity originated from its

o

2.5 wt%, 200 C and 3 h;^[37]in addition, in the reported sulfated

zirconia grafted SBA-15-catalyzed reaction of glucose with

ethanol, the EL yield of ca. 25% was obtained at 140 ^oC and 24

h.^[5]The formation of polymeric humins due to self-

polymerization of the intermediates during the glucose

conversion process accounted for significant carbon loss of the

feed, causing limited EL yield in the present and reported

catalytic systems (Figure S2f-h).^[39]

In order to further evaluate the catalytic activity of SO₄^2–/ZrO₂-

PMO-SO₃H, as-prepared PMO-SO₃H and SO₄^2–/ZrO₂were also

tested under the same conditions. As displayed in Figure 7b, the

yield of ethyl glucoside reached the maximum value of 18.7% as

2–

inferior porosity properties and the highest n_B/n_Lratio. SO₄

/ZrO₂-PMO-SO₃H0.5 had the highest acid site densities of B and

L acids, and its BET surface area remained sufficiently large.

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Scheme 2. Reaction pathway of the SO₄^2–/ZrO₂-PMO-SO₃H-catalyzed conversion of glucose to ethyl levulinate in ethanol medium.

2–

Its lower activity compared to the most active catalyst, SO₄

50

40

30

20

10

0

SO²₄^-/ZrO₂-PMO-SO₃H0.25

SO²₄^-/ZrO₂-PMO-SO₃H0.5

SO²₄^-/ZrO₂-PMO-SO₃H1.0

SO²₄^-/ZrO₂-PMO-SO₃H2.0

/ZrO₂-PMO-SO₃H1.0 was due to its smaller pore diameter, lower

pore volume and higher n_B/n_Lratio. Therefore, based on the

combination of its advantages of considerably high acid site

densities of B and L acids, the lowest n_B/n_Lratio and excellent

porosity properties, SO₄^2–/ZrO₂-PMO-SO₃H1.0 exhibited the

highest catalytic activity in the conversion of glucose to EL. A

high n_L/n_Bratio can accelerate the isomerization of ethyl

glucoside to ethyl fructoside, one of the key steps for the

conversion of glucose to EL. Additionally, well-ordered

mesostructures of the catalyst can facilitate mass transfer and

diffusion of the reactants to the acid sites, whereas a large BET

surface and high pore volume can provide a high population of

acid sites. Importantly, SO₄^2–/ZrO₂-PMO-SO₃H1.0 possessed a

larger pore diameter (5.5 nm), which can accommodate bulky

glucose and its intermediates in reactions in the mesopores. All

of the above factors led to the increased accessibility of the

reactants to the acid sites and thereby enhanced catalytic

activity in the conversion of glucose to EL.

a

0

4

8

12

Time (h)

Figure 8b presents the catalytic activity of various reference

50

40

30

20

10

0

SO²₄^-/Nb₂O₅-PMO-SO₃H1.0

SO²₄^-/TiO₂-PMO-SO₃H1.0

PF-SO²₄^-/ZrO₂-PMO-SO₃H1.0

SO²₄^-/ZrO₂-SiO₂1.0

2–

catalysts in the target reaction. All were less active than SO₄

b

/ZrO₂-PMO-SO₃H1.0. Over a period of 12 h, the yield of EL

reached 39.0, 16.6, 9.9, 9.8 and 5.3% in the HY-, SO₄^2–/ZrO₂-

SiO₂1.0-, PF-SO₄^2–/ZrO₂-PMO-SO₃H1.0-, Hβ- and ZrO₂-

catalyzed target reaction; additionally, in the same reaction time,

the yield of EL was 22.0 and 19.4% for the SO₄^2–/Nb₂O₅-PMO-

SO₃H1.0- and SO₄^2–/TiO₂-PMO-SO₃H1.0-catalyzed target

HY

H

ZrO₂

2–

reactions. The obviously lower catalytic activity of PF-SO₄

/ZrO₂-PMO-SO₃H1.0 compared to its SO₄^2–/ZrO₂-PMO-SO₃H1.0

counterpart is due to its disordered mesostructure, which leaded

to poor porosity properties as well as low acid site densities of B

and L acids. Similarly, the lower catalytic activity of SO₄^2–/Nb₂O₅-

PMO-SO₃H1.0 or SO₄^2–/TiO₂-PMO-SO₃H1.0 than its SO₄^2–/ZrO₂-

PMO-SO₃H1.0 counterpart is also due to its obviously

decreased BET surface area and acid site density of B and L

acids. The lower catalytic activity of HY zeolite compared to

SO₄^2–/ZrO₂-PMO-SO₃H1.0 is mainly due to its smaller BET

surface area (700 m²g^1, Table 1) and pore diameter (2.5 nm),

which can reduce the accessibility of the substrates to acid sites.

0

4

8

12

Time (h)

Figure 8. Catalytic activity comparison of various SO₄^2–/ZrO₂-PMO-SO₃H

catalysts (a) and reference solid acids (b) in the conversion of glucose to ethyl

levulinate. n_glucose/n_EtOH= 1/248, 1.5 wt% catalyst, 170 ^oC.

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FULL PAPER

50

40

30

20

10

0

a

b

40

30

20

10

0

6

12 0

6

12 0

6

12

0

6

12 0

132.8

6

12 0

6

12

Time (h)

d

Si

SO₃H

57.7

15.4

142.4

200

100

0

ppm

Figure 9. Reusability of SO₄^2–/ZrO₂-PMO-SO₃H1.0 (a) and SO₄^2–/ZrO₂(b) in the conversion of glucose to ethyl levulinate. n_glucose/n_EtOH= 1/248; 1.5 wt% catalyst,

170 ^oC, 12 h. TEM image (c) and ¹³C CP MAS NMR (d) of the third time spent SO₄^2–/ZrO₂-PMO-SO₃H1.0 catalyst.

The microporous Hβ zeolite with a pore diameter of 0.7 nm limits

the accessibility of bulky reactant molecules to the acid sites,

thereby severely hindering diffusion and resulting in poor

ethanolysis activity. The lower catalytic activity of SO₄^2–/ZrO₂-

SiO₂1.0 is due to the following two factors. One, owing to the

lack of bridging benzene groups in the silica framework, the

porosity properties of SO₄^2–/ZrO₂-SiO₂1.0 (BET surface area of

314 m²g^1, pore diameter of 4.2 nm and pore volume of 0.72

cm³g^–1, Table 1) were inferior to those of SO₄^2–/ZrO₂-PMO-

SO₃H1.0, which may limit the accessibility of the reactants to the

acid sites. Second, the SO₄^2–/ZrO₂-SiO₂1.0 had single (i.e.,

SO₄^2–/ZrO₂) rather than double B acid sites; accordingly, its B

the conversion of glucose to EL compared with SO₄^2–/ZrO₂-

PMO-SO₃H1.0.

The surface hydrophobicity of the SO₄^2–/ZrO₂-PMO-SO₃H

catalysts due to incorporation of Si(C₆H₄)Si units into ZrO₂

framework also positively influenced their catalytic activity in the

target reaction, thereby effectively reducing the adsorption of

hydrophilic byproducts such as ethyl lactate, anhydro

monosaccharide and polymeric humins on the catalyst

surface.^[40]However, these products may strongly adsorb on the

SO₄^2–/ZrO₂surface because of its surface hydrophilicity, leading

to decreased accessibility of the acid sites.

2–

acid site density (545 μeq g^–1) was lower than that of SO₄

Regeneration and reusability

/ZrO₂-PMO-SO₃H1.0, which may lead to a slower reaction rate in

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Catalytic stability and reusability are significant challenges for

practical applications of bifunctional SO₄^2–/ZrO₂-PMO-SO₃H in

the conversion of glucose to EL. Here, the stability and

reusability of as-prepared catalysts were evaluated by recycling

catalysts, SO₄^2–/ZrO₂-PMO-SO₃H-catalyzed the conversion of

glucose to EL followed by etherification of glucose with ethanol

as the first step. Additionally, the SO₄^2–/ZrO₂-PMO-SO₃H

catalyst prepared at a Si/Zr molar ratio of 1.0 exhibited the

SO₄^2–/ZrO₂-PMO-SO₃H1.0 three times. After each catalytic cycle, highest EL yield (42.3%) among various as-prepared catalysts,

the spent catalyst was recovered by centrifugation, followed by

thorough ethanol washing before the second and third catalytic

cycles. For comparison, the reusability of SO₄^2–/ZrO₂was also

tested under the same conditions. As shown in Figure 9a, after

the SO₄^2–/ZrO₂-PMO-SO₃H1.0-catalyzed glucose conversion

reaction was performed for 12 h, the yield of EL was 42.3 (1st

cycle), 40.1 (2nd cycle) and 38.5% (3rd cycle), respectively. This

result indicated that the SO₄^2–/ZrO₂-PMO-SO₃H can be reused

and its catalytic activity outperformed HY (EL yield of 39.0%)

and H (EL yield of 9.8%) zeolites. The acid site densities of B

and L acids, the molar ratio of B-to-L acid sites, structural

ordering and porosity properties gave the important influence on

the catalytic activity of the bifunctional SO₄^2–/ZrO₂-PMO-SO₃H in

the target reaction. Moreover, the catalysts exhibited excellent

reusability, which was attributed to the strong covalent bonding

between double B acid sites and the ZrO₂-PMO framework; in

addition, the surface hydrophobicity of the catalysts can

effectively avoid catalyst deactivation, owing to byproduct

adsorption. Therefore, the SO₄^2–/ZrO₂-PMO-SO₃H catalysts are

promising alternatives to hazardous liquid acids employed in the

effective conversion of glucose directly to EL.

2–

three times without obvious activity loss. However, for the SO₄

/ZrO₂-catalyzed target reaction, the yield of EL decreased clearly

from 20.5 (1st cycle), 15.7 (2nd cycle) to 7.0% (3rd cycle), after

the reaction performed for 12 h (Figure 9b). SO₄^2–/ZrO₂, which

has plentiful Zr–OH groups, exhibits a hydrophilic surface, that

can strongly adsorb hydrophilic humins, resulting in deactivation

of SO₄^2–/ZrO₂during its recycling runs.

To support the above results, TEM image and ¹³C CP MAS

NMR of the third time spent SO₄^2–/ZrO₂-PMO-SO₃H1.0 catalyst

were measured. As shown in Figure 9c, the spent SO₄^2–/ZrO₂-

Experimental Section

Materials

PMO-SO₃H1.0 still exhibited

a

perfect 2D hexagonal

mesostructure, implying its excellent mechanical stability and

resistance to byproduct adsorption. These properties ensured

the excellent reusability of SO₄^2–/ZrO₂-PMO-SO₃H.

Pluronic P123 (EO₂₀PO₇₀EO₂₀

CH₂(CH₃)CHO–), 1,4-bis-(triethoxysilyl)benzene

,

EO

=

–CH₂CH₂O–, PO

(BTESB,

=

–

96%),

zirconium n-butoxide [Zr(OnBu)₄, 76–80% in n-butanol], NbCl₅(> 99%),

titanium iso-propylate (TTIP, 97%) and 5-ethoxymethylfurfural (5-EMF, >

97%) were purchased from Sigma-Aldrich. Ethyl levulinate (EL, > 98%),

chlorosulfonic acid (ClSO₃H, > 97%) and ethyl laurate (> 99%) were

purchased from TCI. Ethyl lactate (> 99%) was purchased from J&K

Scientific. Ethyl-β-D-glucoside (> 95%) was purchased from Aikon. Y and

beta zeolite powders were purchased from Nankai University Catalyst

Co.. NH₄-form zeolites were obtained by ion-exchange twice with

NH₄NO₃solution (0.5 mol L⁻¹) at 80 °C for 2 h, and protonic form zeolites

were obtained by calcination of the NH₄-zeolites at 550 °C for 5 h.

¹³C CP-MAS NMR of the spent SO₄^2–/ZrO₂-PMO-SO₃H

revealed all characteristic signals for various carbon species in

SO₄^2–/ZrO₂-PMO-SO₃H, the carbon species of the bridging

benzene units (132.8 ppm) and –SO₃H functionalized benzene

groups (142.4 ppm) are shown in Figure 9d. Additionally, signals

still appeared at 57.7 and 15.8 ppm, owing to the carbon species

of the ethoxy group that formed during the boiling ethanol

washing process.^[27,28]This result suggested that the Si-C

framework remains intact after the catalytic cycles; more

importantly, byproducts were hardly found on the surface of the

spent catalyst. The excellent reusability of SO₄^2–/ZrO₂-PMO-

SO₃H is mainly due to the strong covalent bonding between

double B acid sites and the ZrO₂-PMO framework; in addition,

the surface hydrophobicity of the catalysts can effectively avoid

catalyst deactivation, owing to byproduct adsorption. Therefore,

the SO₄^2–/ZrO₂-PMO-SO₃H catalysts are promising alternatives

to hazardous liquid acids employed in effective conversion of

glucose directly to EL.

Catalytic preparation

Typically, P123 (0.275 g) was dissolved in a mixture of HCl (12 mol L⁻¹

,

0.6 mL) and distilled water (8.5 mL) at room temperature under stirring.

The obtained P123 solution was heated to 40 ^oC, and then BTESB (0.68

mL) was added. After prehydrolysis of BTESB for 45 min, Zr(OnBu)₄(3.0,

1.5, 0.75 or 0.375 mL to adjust the Si/Zr molar ratio to 2.0, 1.0, 0.5 or

0.25, respectively) was added to the above mixture. The resultant white

suspension was stirred at 40 ^oC for 24 h. The suspension was transferred

to an autoclave and heated to 120 ^oC at a heating rate of 2 ^oC min⁻¹, and

then it was held at this temperature for another 24 h. The resulting white

solid powder was air-dried at 60, 80 and 100 ^oC, respectively, for 12 h.

P123 in the product was removed by washing three times with boiling

ethanol, and the mesostructured ZrO₂-organosilica support (ZrO₂-PMO)

was obtained after air drying at 100 ^oC overnight.

Conclusions

A series of bifunctional SO₄^2–/ZrO₂-PMO-SO₃H catalysts with

both B and L acid sites were successfully prepared by a

carefully designed sol-gel co-condensation route followed by

ClSO₃H functionalization. The acid site density, structural

ordering and porosity properties of SO₄^2–/ZrO₂-PMO-SO₃H were

optimally tuned by adjusting the initial Si/Zr molar ratios to 0.25,

0.5, 1.0 and 2.5. Owing to the synergistic effect of the super

strong B acidity and moderate L acidity of the bifunctional

Subsequently, ZrO₂-PMO powder (0.5 g) was dispersed into a mixture

of ClSO₃H (5 mL) and dichloromethane (25 mL), and the obtained

suspension was stirred at 0 ^oC for 12 h under argon gas. In the above

process, ClSO₃H was in excess with respect to the total moles of Si and

Zr. Finally, the suspension was washed with copious amounts of water

until the filtrate was neutral. After centrifugation and washing with ethanol,

the powder product was dried at 100 ^oC overnight. The initial molar ratio

of the reactants was P123: BTESB: Zr(OnBu)₄: ClSO₃H: HCl: H₂O =

0.029: 1: (0.25, 0.5, 1 or 2): 45: 4.4: 288. The product was denoted as

10.1002/cctc.201801089

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FULL PAPER

SO₄^2–/ZrO₂-PMO-SO₃Hx, where x represents the initial Si/Zr molar ratio;

here, x = 0.25, 0.5, 1.0 and 2.0.

PMO-SO₃H and PF-SO₄^2–/ZrO₂-PMO-SO₃H1.0 were prepared based

on the above procedure but in the absence of Zr(OnBu)₄or P123,

respectively.

Ordered mesoporous silica functionalized by sulfonic acid and ZrO₂

(SO₄^2–/ZrO₂-SiO₂1.0) was prepared following a route of similar to that for

SO₄^2–/ZrO₂-PMO-SO₃H1.0 except that TEOS was used as the silicon

precursor instead of BTESB.

of 105 ^oC; and a gas flow rate of 2.3 L min^1. The concentrations of EL,

5-EMF and ethyl lactate were determined periodically on a Shimadzu

2014C gas chromatograph fitted with an HP-INNOWAX capillary column

(ﬁlm thickness, 0.5 m; i.d., 0.32 mm; length, 30 m) and flame ionization

detector, and ethyl laurate was applied as an internal standard. The

intermediates and byproducts obtained during the catalytic processes

were identified by both GC-MS (HP6890GC-5973MSD) and LC-MS

(Thermo Scientific LTQ-Orbitrap XL).

SO₄^2–/Nb₂O₅-PMO-SO₃H1.0 and SO₄^2–/TiO₂-PMO-SO₃H1.0 were

prepared based on the above procedure except that NbCl₅and TTIP,

respectively, were used as the precursors instead of Zr(OnBu)₄.

Acknowledgements ((optional))

This work is supported by the Natural Science Fund Council of

China (21573038 and 21703030) and the Fundamental

Research Funds for the Central Universities (130014725).

Catalyst characterization

TEM observations were performed on a JEM-2100F high resolution

transmission electron microscope at an accelerating voltage of 200 kV.

Small angle X-ray scattering (SAXS) patterns were obtained on a D/max-

2200 VPC diffractometer using CuKa radiation. Nitrogen porosimetry

measurements were performed on a Micromeritics ASAP 2020M surface

area and porosity analyzer after the samples were outgassed under

vacuum at 363 K for 1 h and 393 K for 6 h. XPS was performed on a VG-

ADES 400 instrument with a Mg Kα-ADES source at a residual gas

pressure of less than 10⁻⁸Pa. All binding energies were referenced to

the C 1s peak at 284.8 eV of the surface adventitious carbon. FT-IR

spectra in transmission mode were recorded on a Nicolet Magna 560 IR

spectrophotometer, and the catalyst powder samples were first admixed

with KBr and then pressed into the pellets. ¹³C CP-MAS and ²⁹Si MAS

Keywords: biomass • glucose • ethyl levulinate • solid acid •

Brønsted and Lewis acid

[1]

[2]

L. Yan, Q. Yao and Y. Fu, Green Chem. 2017, 19, 5527-5547.

K. Jacobson, R. Gopinath, L. C. Meher and A. K. Dalai, Appl. Catal. B:

Environ. 2008, 85, 86-91.

[3]

[4]

[5]

A. Sivasamy, K. Y. Cheah, P. Fornasiero, F. Kemausuor, S. Zinoviev

and S. Miertus, ChemSusChem 2009, 2, 278-300.

H. Li, Z. Fang, J. Luo and S. Yang, Appl. Catal. B: Environ. 2017, 200,

182-191.

G. Morales, A. Osatiashtiani, B. Hernandez, J. Iglesias, J. A. Melero, M.

Paniagua, D. R. Brown, M. Granollers, A. F. Lee and K. Wilson, Chem.

Commun. 2014, 50, 11742-11745.

NMR spectra were recorded on

a Bruker AVANCE III 400 WB

spectrometer equipped with a 4 mm standard bore CP MAS probe head.

Chemical shifts for ¹³C CP-MAS NMR and ²⁹Si MAS NMR spectra were

referenced to the signals of a C₁₀H₁₆standard (δCH₂= 38.5) and 3-

(trimethylsilyl)-1-propanesulfonic acid sodium salt standard (δ = 0.0),

respectively.

The nature of the acid sites was distinguished by in situ FT-IR

spectroscopy with chemical adsorption of pyridine. Before the

determination, the samples were pretreated at 100 ^oC for 12 h in a

vacuum. The samples were then exposed to pyridine vapor at 60 ^oC for

12 h in a vacuum, followed by pumping out at 150 ^oC for 1 h to remove

the physisorbed pyridine.

[6]

[7]

[8]

[9]

B. C. Windom, T. M. Lovestead, M. Mascal, E. B. Nikitin and T. J.

Bruno, Energ. Fuel. 2011, 25, 1878-1890.

E. Ahmad, M. I. Alam, K. K. Pant and M. A. Haider, Green Chem. 2016,

18, 4804-4823.

S. Saravanamurugan and A. Riisager, ChemCatChem 2013, 5, 1754-

1757.

L. Jiang, L. Zhou, J. Chao, H. Zhao, T. Lu, Y. Su, X. Yang and J. Xu,

Appl. Catal. B: Environ. 2018, 220, 589-596.

[10] S. Saravanamurugan and A. Riisager, Catal. Commun. 2012, 17, 71-75.

[11] C. H. Kuo, A. S. Poyraz, L. Jin, Y. Meng, L. Pahalagedara, S. Y. Chen,

D. A. Kriz, C. Guild, A. Gudz and S. L. Suib, Green Chem. 2014, 16,

785-791.

L and B acid sites were quantified by integrating the area underneath

the bands at 1450 and 1540 cm^1, respectively,^[35]and the corresponding

molar ratio of B-to-L acid was then estimated.

[12] R. Liu, J. Chen, X. Huang, L. Chen, L. Ma and X. Li, Green Chem. 2013,

15, 2895-2903.

The B acid site density was determined by titration. Typically, fresh

catalyst powder (50 mg) was dispersed in a NaCl solution (2 mol L^1, 10

mL) under stirring at 30 ^oC for 24 h, and then the suspension was titrated

with standard NaOH solution (4.6 mmol L⁻¹).^[41]The B acid site density

(µeq g^1) was calculated from the consumed NaOH, whereas the L acid-

site density was calculated based on molar ratio of B-to-L acid (n_B/n_L)

and the B acid site density.

[13] A. Takagaki, M. Ohara, S. Nishimura and K. Ebitani, Chem. Commun.

2009, 0, 6276-6278.

[14] L. Zhou, H. Zhao, L. Cui, Y. Bai, J. Bian, T. Lu, Y. Su and X. Yang,

Catal. Commun. 2015, 71, 74-78.

[15] I. Delidovich and R. Palkovits, ChemSusChem 2016, 9, 547-561.

[16] H. Nguyen, V. Nikolakis and D. G. Vlachos, ACS Catal. 2016, 6, 1497-

1504.

Catalytic tests

[17] J. M. R. Gallo, D. M. Alonso, M. A. Mellmer and J. A. Dumesic, Green

Chem. 2013, 15, 85-90.

Catalyst powders were dried for 2 h at 120 ^oC under vacuum before the

tests. Conversion of glucose to EL was conducted in an autoclave with a

Teﬂon lining at 170 ^oC, a glucose (0.278 mmol) to ethanol (68.95 mmol)

molar ratio of 1:248 and 50 mg or 1.5 wt% catalyst (with respect to total

reactants); stirring was applied throughout the reaction. Conversion of

glucose and the yield of ethyl glucoside were determined on an Agilent

1260HPLC liquid chromatograph fitted with an ACQUITY UPLC Prevail

Carbohydrate column (film thickness, 5 m; i.d., 4.6 mm; length, 250

mm) and an evaporative light-scattering detector (Alltech ELSD 2000ES).

The flow rate of the mobile phase (acetonitrile: water = 85: 15) was 1 mL

min^1. The operation conditions of the detector were a tube temperature

[18] F. Chen, H. Ma and B. Wang, J. Hazard. Mater., 2009, 162, 668-673.

[19] J. Gardy, A. Hassanpour, X. Lai, M. H. Ahmed and M. Rehan, Appl.

Catal. B: Environ. 2017, 207, 297-310.

[20] Z. Li, R. Wnetrzak, W. Kwapinski and J. J. Leahy, ACS appl. Mater.

Inter. 2012, 4, 4499-4505.

[21] E. M. Domingues, N. Bion, F. M. Figueiredo and P. Ferreira, Micropor.

Mesopor. Mat. 2016, 226,386-395.

[22] J. Valimaña-Traverso, S. Morante-Zarcero, D. Pérez-Quintanilla, M. Á.

García, I. Sierra and M. L. Marina, J.Chromatogr. A 2018, 1566, 135-

145.

10.1002/cctc.201801089

ChemCatChem

FULL PAPER

[23] M. G. Goesten, À. Szécsényi, M. F. de Lange, A. V. Bavykina, K. B. S.

S. Gupta, F. Kapteijn and J. Gascon, ChemCatChem 2016, 8, 961-967.

[24] S. Suganuma, K. Nakajima, M. Kitano, D. Yamaguchi, H. Kato and S.

Hayashi, J. Am. Chem. Soc. 2008, 130, 12787-12793.

[34] V. Adeeva, J. W. Dehaan, J. Janchen, G. D. Lei, V. Schunemann, L. J.

M. Vandeven, W. M. H. Sachtler and R. A. Vansanten, J. Catal. 1995,

151, 364-372.

[35] P. Lanzafame, K. Barbera, S. Perathoner, G. Centi, A. Aloise, M.

Migliori, A. Macario, J. B. Nagy and G. Giordano, J. Catal. 2015, 330,

558-568.

[25] F. Su, Q. Wu, D. Song, X. Zhang, M. Wang and Y. Guo, J. Mater.

Chem. A 2013, 1, 13209-13221.

[26] X. Zhang, Y. Zhao and Q. Yang, J. Catal. 2014, 320, 180-188.

[27] D. Song, S. An, Y. Sun and Y. Guo, J. Catal. 2016, 333, 184-199;

[28] J. Liu, S. Y. Bai, H. Zhong, C. Li and Q. H. Yang, J. Phys. Chem. C

2010, 114, 953-961.

[36] D. Song, Y. Sun, Q. Zhang, P. Zhang, Y. Guo and J. Leng, Appl. Catal.

A: Gen. 2017, 546, 36-46.

[37] L. Peng, L. Lin, J. Zhang, J. Shi and S. Liu, Appl. Catal. A: Gen. 2011,

397, 259-265.

[29] Z. Teng, X. Su, Y. Zheng, J. Zhang, Y. Liu, S. Wang, J. Wu, G. Chen, J.

Wang, D. Zhao and G. Lu, J. Am. Chem. Soc. 2015, 137, 7935-7944.

[30] A. M. Alsalme, P. V. Wiper, Y. Z. Khimyak, E. F. Kozhevnikova and I. V.

Kozhevnikov, J. Catal. 2010, 276, 181-189.

[38] G. D. Yadav, N. P. Ajgaonkar and A. Varma, J. Catal., 2012, 292, 99-

110.

[39] Z. Cheng, J.L. Everhart, G. Tsilomelekis, V. Nikolakis, B. Saha, and D.

G. Vlachos, Green Chem. 2018, 20, 997–1006.

[40] F. Liu, K. Huang, A. Zheng, F.-S. Xiao and S. Dai, ACS Catal. 2017, 8,

372-391.

[31] M. G. Kulkarni, R. Gopinath, L. C. Meher and A. K. Dalai, Green Chem.

2006, 8, 1056-1062.

[32] W. Li, F. Ma, F. Su, L. Ma, S. Zhang and Y. Guo, ChemSusChem 2011,

4, 744-756.

[41] J. A. Melero, G. D. Stucky, R. van Grieken and G. Morales, J. Mater.

Chem. 2002, 12, 1664-1670.

[33] K. Arata, Green Chem. 2009, 11, 1719-1728.

10.1002/cctc.201801089

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Entry for the Table of Contents

FULL PAPER

SO₄^2–/ZrO₂-PMO-SO₃H catalysts with

super strong Brønsted acidity and

moderate Lewis acidity exhibit high

yield of ethyl levulinate from glucose

conversion.

Daiyu Song,^[a]Qingqing Zhang,^[a]

Yingnan Sun,^[a]Panpan Zhang,^[a]Yi-

Hang Guo,* ^[a]and Jiang-Lei Hu*^[b]

Page No. – Page No.

Design of ordered mesoporous

sulfonic acid functionalized

ZrO₂/organosilica bifunctional

catalysts for direct catalytic

conversion of glucose to ethyl

levulinate

Article Doi

Design of Ordered Mesoporous Sulfonic Acid Functionalized ZrO2/organosilica Bifunctional Catalysts for Direct Catalytic Conversion of Glucose to Ethyl Levulinate

DOI: 10.1002/cctc.201801089

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Article abstract of DOI:10.1002/cctc.201801089

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