Design of Br?nsted acidic ionic liquid functionalized mesoporous organosilica nanospheres for efficient synthesis of ethyl levulinate and levulinic acid from 5-hydroxymethylfurfural

Catalysis

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Design of Brønsted acidic ionic liquid

functionalized mesoporous organosilica

nanospheres for efficient synthesis of ethyl

levulinate and levulinic acid from

5-hydroxymethylfurfural†

Cite this: Catal. Sci. Technol., 2021,

11, 1827

*

Daiyu Song, Jingyu Liu, Chaoyue Zhang and Yihang Guo

Brønsted acidic ionic liquids (BAILs) have brought new vitality in catalytic transformation of biomass to fuels

and chemicals, but practical applications of BAILs suffer from drawbacks of slow diffusion and difficulty in

separation. Chemical immobilization of BAILs is an effective way to circumvent these problems. Here, we

demonstrate a series of monodispersed mesoporous organosilica nanosphere-immobilized BAIL catalysts,

[C₃PrIm][OTf]-MONSs (C₃= PrSO₃H, OTf = SO₃CF₃), by a quaternary ammonium surfactant micelle-

directed liquid-interface assembly strategy followed by successive chemical modifications, and the particle

size (180–360 nm) and pore morphology (periodic centrally radialized and 3D interconnected mesopores)

of the catalysts are well-adjusted by changing the cations and/or anions of the surfactants as well as the

preparation conditions. As-prepared [C₃PrIm][OTf]-MONSs serve as nanoreactors to transform an

important biomass-derived platform molecule, 5-hydroxymethylfurfural, in ethanol and water media to

valuable chemicals, ethyl levulinate (EL) and levulinic acid (LA). By the combination of their superstrong

Brønsted acidic nature, outstanding open mesoporous spherical nanostructures and excellent textural

properties, the [C₃PrIm][OTf]-MONSs exhibit high ethanolysis and hydrolysis activity and selectivity. The

particle size and pore morphology of the catalysts significantly influence the selectivity and thereby the

yield of the products. The cetyltrimethylammonium tosylate-directed [C₃PrIm][OTf]-MONSs catalyst with

the smallest particle size (210 nm) and a wormhole-like interconnected mesostructure shows the highest

yield of EL (93.6%) and LA (72.8%) under the optimum reaction conditions. The catalyst also displays good

reusability in ethanolysis reaction, originating from chemical bonding [C₃PrIm][OTf] within the hydrophobic

silica framework.

Received 5th October 2020,

Accepted 24th December 2020

DOI: 10.1039/d0cy01941k

rsc.li/catalysis

ring, in its structure.⁷HMF can be converted to a variety of

targeted high value-added chemicals, including highly

1. Introduction

Catalytic transformation of lignocellulosic biomass and

related platform molecules into valuable chemicals and fuel

products represents an efficient strategy to decrease the

current dependence on exhaustible resources.^1–4As one of

such promising C₆sugar-derived platform molecules,

5-hydroxymethylfurfural (HMF) has received greatly interest in

green chemistry and sustainable chemical industry.^4–6HMF

exhibits enormous synthetic potential and high reactivity,

which are provided by the presence of three functional

centers, the aldehyde group, hydroxymethyl group and furan

promising new biofuel candidates and important platform

compounds. Among these, Brønsted acid catalyzed ring-

opening reactions of HMF in ethanol and water media can

produce ethyl levulinate (EL) and levulinic acid (LA) in an

atom economic approach.^8,9EL has been found to be

extensively applied in the perfume and flavor industries and

as a blending agent for diesel fuel formulation,^10,11while LA

is one of the most attractive and important intermediates for

the synthesis of a broad range of chemicals (e.g., diphenolic

acid and δ-aminolevulinic acid), fuel additives (e.g., alkyl

levulinates and γ-valerolactone) and liquid hydrocarbon

fuels.^12–19Although liquid Brønsted acids (e.g., H₂SO₄and

p-toluenesulfonic acid) show high efficiency for the target

reactions, they suffer from severe problems of corrosiveness

to equipment, pollution to the environment and difficulty in

separation and recovery.^20–22As for generally applied solid

School of Environment, Northeast Normal University, 2555 Jingyue Street,

Changchun 130117, P.R. China. E-mail: guoyh@nenu.edu.cn

† Electronic supplementary information (ESI) available. See DOI: 10.1039/

d0cy01941k

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acids (e.g., sulfated zirconia, sulfonic acid functionalized

silica/carbon, acidic resins and zeolites), they bear the

drawbacks of poor accessibility of the acid sites for the

substrates and irreversible deactivation by the formation of

by-products, humins, due to the oligomerization and

polymerization of HMF on their surface.^8,23,24Therefore, the

crucial challenge for efficient and selective synthesis of EL

and LA from HMF is to develop recoverable and

environmentally-benign Brønsted acid catalysts.

hydrophobicity

are

better

support

candidates

in

nanocatalysis.³⁸In the current work, we design a quaternary

ammonium surfactant micelle-directed liquid-interface

assembly strategy to prepare monodispersed chloropropyl

functionalized mesoporous organosilica nanosphere supports

(PrCl-MONSs), and the particle size and pore morphology of

the supports are well-adjusted by changing the cations and/

or anions of the surfactants as well as the preparation

conditions. After successive chemical modification of the

Here, we develop a novel solid acid, Brønsted acidic ionic

liquid functionalized mesoporous organosilica nanospheres

([C₃PrIm][OTf]-MONSs, C₃= PrSO₃H, Im = imidazole, OTf =

SO₃CF₃), for high yield synthesis of EL and LA from

ethanolysis and hydrolysis of HMF. Ionic liquids (ILs) have

attracted considerable attention due to their distinguishing

properties such as nonvolatility, strong dissolution and

structural designability, and they are preferable innovative

supports

with

imidazole,

1,3-propanesultone

and

trifluoromethane sulfonic acid (HOTf), monodispersed [C₃-

PrIm][OTf]-MONSs catalysts with different particle sizes and

pore morphologies are successfully achieved. As-prepared [C₃-

PrIm][OTf]-MONSs serve as nanoreactors to accommodate

efficient and selective conversion of HMF to EL and LA in

ethanol and water media, respectively, and the particle size-

and pore morphology-dependent catalytic activity and

selectivity are studied at an appropriate reaction temperature

and ethanol/water-to-HMF molar ratio. It is found that the

high reactivity and selectivity of the [C₃PrIm][OTf]-MONSs

catalysts in both target reactions are dominated by the

combination of superstrong Brønsted acidity, spherical

nanostructures with abundant mesopores and surface

hydrophobicity. Finally, the catalytic reusability and stability

of the [C₃PrIm][OTf]-MONSs are evaluated.

green

solvents

in

chemical

reactions;

moreover,

functionalized ILs have exhibited outstanding catalytic

performances, thus bringing new vitality in numerous

biomass transformation-associated processes.^25–29In this

context, Brønsted acidic ILs (BAILs) have emerged. For

example, C_nMIm-based BAILs show excellent catalytic activity

and selectivity in the synthesis of 100% bio-based

poly(ethylene 2,5-furandicarboxylate) from the esterification

of ethylene glycol with biomass-derived 2,5-furandicarboxylic

acid;³⁰sulfonic acid-functionalized ILs display high selectivity 2. Experimental section

and reactivity in conversion of chitin to LA (ref. 27) or

2.1. Catalyst preparation

synthesis of furylmethane derivatives from the condensation

of furan with furfural.³¹However, the practical applications

Four quaternary ammonium surfactants with the same

hydrophobic chain length (hexadecyl groups) but different

counterions including cetyltrimethylammonium bromide

of BAILs remain

a challenge because they show high

solubility in most of solvents, and thus tedious separations

of BAILs from the reaction media for recycling are needed;

meanwhile, the catalytic reactions are limited by diffusion

processes owing to the high viscosity and low diffusion

coefficients of BAILs. Chemical immobilization of BAILs on

suitable supports is an effective way to circumvent these

problems; meanwhile, the cost of BAILs can be reduced

owing to a significantly decreased dosage. For examples,

BAIL-functionalized organosilica hollow nanospheres,³²

polysiloxane networks³³and dendritic fibrous nitrogen-doped

carbon nanospheres³⁴as well as imine-linked COF (covalent

organic framework)-confined BAILs³⁵have been reported to

exhibit excellent catalytic activity and high reusability in

biomass conversion-associated processes.

Among various catalyst supports, mesoporous inorgano/

organosilicas, prepared by sol–gel chemistry, are one of the

best choices due to their facilely-controlled morphologies

and well-defined mesoporosities and thus excellent textural

properties, which endow silica supported ILs distinguishing

catalytic performances in comparison to bulk ILs through

facilitating mass transfer and increasing active site

populations.^36,37Moreover, in comparison to traditional

mesoporous silicas, mesoporous organosilicas with a flexible

silica/carbon framework and high hydrothermal stability as

well as a tunable morphology and surface hydrophilicity/

(CTAB),

cetyltrimethylammonium

chloride

(CTAC),

cetylpyridinium bromide (CPB) and cetyltrimethylammonium

tosylate (CTATOS) are selected to prepare four PrCl-MONSs

supports including CTAB PrCl-MONSs, CTAC PrCl-MONSs,

CPB PrCl-MONSs and CTATOS PrCl-MONSs. Subsequent

functionalization of the supports with BAILs gave rise to four

[C₃PrIm][OTf]-MONSsx catalysts, where x = 6, 12 and 24,

referring to the hydrolysis time of organosilica precursors of

6, 12 and 24 h, as described in the ESI.†

2.2. Characterization of the [C₃PrIm][OTf]-MONSs catalysts

Transmission electron microscopy (TEM) images were

obtained by using a JEM-2100F high resolution transmission

electron microscope at an accelerating voltage of 200 kV.

Field emission scanning electron microscopy (FESEM) images

were collected using an XL-30 ESEMFEG field emission

scanning electron microscope. Nitrogen gas porosimetry

measurements were performed on a Micromeritics ASAP

2020M surface area and porosity analyzer after the samples

were calcined at 393 K in a vacuum for 2 h, and outgassed

under vacuum at 393 K for another 12 h. The surface areas

were calculated using the Brunauer–Emmett–Teller (BET)

equation, while pore diameter distribution curves were

calculated using the Barrett–Joyner–Halenda (BJH) desorption

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Paper

branch of the isotherms, and the pore volume was

(HP6890GC-5973MSD) and LC-MS (Agilent 1260HPLC-6490

LC/QQQ/Thermo Scientific LTQ-Orbitrap XL), while the

concentration of formic acid (by-product of the HMF

hydrolysis reaction) was determined using a Metrohm Eco

ion chromatograph (IC).

accumulated up to P/P₀

=

0.99. X-ray photoelectron

spectroscopy (XPS) was carried out on an Axis Ultra DLD

instrument with a monochromated Al-Kα source at a residual

gas pressure below 10⁻⁸Pa, and all the binding energies were

referenced to the

C 1s peak at 284.8 eV of surface

adventitious carbon. ¹³C cross polarization-magic angle 3. Results and discussion

spinning (CP-MAS) NMR and ²⁹Si MAS NMR spectra were

3.1. Fabrication and characterization of the [C₃PrIm][OTf]-

recorded on a Bruker AVANCE III 400 WB spectrometer

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

The dried and finely powdered samples were packed in a

ZrO₂rotor closed with a Ke–F cap and were spun at a 12 KHz

rate.

MONSs catalysts with tunable particle sizes and pore

morphologies

Fabrication of the monodispersed [C₃PrIm][OTf]-MONSs

catalysts includes a liquid-interface assembly strategy to

prepare chloropropyl functionalized mesoporous organosilica

nanosphere supports (PrCl-MONSs) followed by chemical

modification with imidazole, 1,3-propanesultone and HOTf

successively. At first, the PrCl-MONSs supports are prepared by

a quaternary ammonium surfactant micelle-directed sol–gel

chemistry process. In order to adjust the particle sizes and pore

morphologies of the supports, four quaternary ammonium

surfactants (serving as a structure-directing agent, SDA) such

as CTAB, CTAC, CPB and CTATOS are selected, leading to CTAB

PrCl-MONSs, CTAC PrCl-MONSs, CPB PrCl-MONSs and CTATOS

PrCl-MONSs, respectively. As illustrated in Scheme 1, in the

initial stage of preparation of the support, SDA self-assembles

into monodispersed spherical micelles with an orderly

arrangement of hydrophobic chains inside the EtOH–water–

The Brønsted acid strength of the as-prepared [C₃PrIm]

[OTf]-MONSs catalysts was measured using a WDDY-2008J

microcomputer

automatic

potentiometric

titration

instrument, and the procedure followed has been previously

reported.³⁹50 mg of sample was suspended in acetonitrile

and stirred for 3 h, and then the suspension was titrated with

a 0.1 mol L⁻¹solution of n-butylamine in acetonitrile. The

initial electrode potential (E_i, mV) indicated the maximum

acid strength of the surface sites. The Brønsted acid site

densities (A, μeq g⁻¹) of the catalysts were determined by

acid–base titration with a NaOH solution.⁴⁰Typically, the

sample (50 mg) was added into a NaCl solution (10 mL, 2

mol L⁻¹) and continually stirred at room temperature for 24

h. Then, the acid amount in the solution was determined by

titration with a NaOH (0.0044 mol L⁻¹) standard solution,

and therefore the Brønsted acid site density of the tested

sample can be calculated.

NH₄OH

environment.

After

slowly

dropping

an

organoalkoxysilane precursor 1,2-bis(trimethoxysilyl)ethane

(BTMSE) and 3-chloropropyltrimethoxysilane (CPTMS)-

containing cyclohexane solution to the above surfactant-

containing colloidal solution, co-hydrolysis and co-

condensation of BTMSE and CPTMS in the as-formed weakly

alkali O/W microemulsion system occur, which creates strongly

2.3. Catalytic tests

2.3.1. The synthesis of EL from the ethanolysis of HMF.

The ethanolysis reaction was carried out in an autoclave

under the conditions: 120°C, an ethanol (EtOH, 69 mmol)-to-

HMF (0.278 mmol) molar ratio of 248 and HMF-to-catalyst

weight ratio of

1 wt/wt. The concentrations of HMF,

5-ethoxymethylfurfural (EMF) and EL were monitored using a

Shimadzu 2014C gas chromatograph (GC) equipped with an

HP-INNOWAX capillary column and

a flame ionization

detector, and ethyl laurate was applied as an internal

standard.

2.3.2. The synthesis of LA from the hydrolysis of HMF.

The hydrolysis reaction was conducted in an autoclave under

the conditions: 140 °C, a water (166.7 mmol)-to-HMF (0.4

mmol) molar ratio of 417 and 1% catalyst (with respect to the

total reactants). The concentrations of HMF and LA were

simultaneously analyzed using an Agilent Technologies

1260HPLC equipped with a Prevail™ Organic Acid column and

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

The catalytic performance was evaluated quantitatively by

conversion (Conv, %), yield (Y, %) and selectivity (Sel, %),

respectively.

The generated intermediates and by-products during the

above two reaction processes were identified by both GC-MS

Scheme 1 Morphological evolution and wall structure of the [C₃PrIm]

[OTf]-MONSs catalysts.

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negatively charged polysilicates at the O/W liquid interface.

Co-assembly of these polysilicates with the spherical SDA

cation micelles via electrostatic interactions leads to

polysilicate–SDA supramolecule aggregates around the

outside of the spherical SDA micelles. Continuous stirring of

the microemulsion followed by hydrothermal treatment

results in slow extension of the supramolecule aggregates

along the direction of the hydrophobic chains, which

the PrCl-MONSs are modified with imidazole to yield

imidazole modified supports (PrIm-MONSs), accompanied

with the release of HCl molecules. Through further

quaternary ammonization of PrIm-MONSs with 1,3-propane

sultone and then protonation with HOTf, the [C₃PrIm][OTf]-

MONSs are ultimately obtained.

The TEM images shown in Fig. 1a and b clearly reveal that

the representative support, CTAB PrCl-MONSs12, exhibits

monodispersed spherical nanostructures with an average

particle size of 270 nm (Table 1), in which periodic centrally

radialized mesopores are full of nanospheres. After

functionalization of the CTAB PrCl-MONSs12 support with [C₃-

PrIm][OTf] groups, the resulting CTAB [C₃PrIm][OTf]-MONSs12

catalyst can well retain the mesoporous spherical

nanostructures of the support; moreover, the catalyst possesses

the same particle size as that of the support (Fig. 1c and d). As

for the CTAC [C₃PrIm][OTf]-MONSs12 (Fig. 1e and f) and CPB

[C₃PrIm][OTf]-MONSs12 (Fig. 1g and h) catalysts, they display

the same monodispersed spherical nanostructures and pore

morphologies as those of the CTAB [C₃PrIm][OTf]-MONSs12;

however, their particle sizes are different (350 and 260 nm for

CTAC [C₃PrIm][OTf]-MONSs12 and CPB [C₃PrIm][OTf]-

provides

a prerequisite for the generation of centrally

radialized mesoporous structure within the spherical

organosilica support; meanwhile, as-formed Si–C–C–Si–O–Si–

PrCl framework is further strengthened. After extraction of

SDA from spherical supramolecule aggregates with boiling

ethanol, monodispersed mesoporous PrCl-MONSs supports

are subsequently fabricated. In the above preparation

process, co-hydrolysis and co-condensation of BTMSE and

CPTMS happen in a weakly alkali O/W microemulsion system

at a suitable cyclohexane-to-EtOH molar ratio, which can

control the co-hydrolysis and co-condensation rates of

BTMSE and CPTMS in a favorable manner. Accordingly,

individual

chloropropyl

functionalized

mesoporous

organosilica nanospheres are facilely constructed. Afterwards,

Fig. 1 TEM images of CTAB PrCl-MONSs12 (a and b), CTAB [C₃PrIm][OTf]-MONSs12 (c and d), CTAC [C₃PrIm][OTf]-MONSs12 (e and f), CPB

[C₃PrIm][OTf]-MONSs12 (g and h), CTATOS [C₃PrIm][OTf]-MONSs12 (i and j), CTAB [C₃PrIm][OTf]-MONSs6 (k and l), CTAB [C₃PrIm][OTf]-MONSs24

(m and n), and CTAB PrSO₃H-MONSs12 (o and p).

1830 | Catal. Sci. Technol., 2021, 11, 1827–1842

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Table 1 Brønsted acid site density (A), initial electrode potential (E_i), average particle size (d), pore diameter (D_p), BET surface area (S_BET) and pore

volume (V_p) of the CTAB PrCl-MONSs support, various [C₃PrIm][OTf]-MONSs catalysts and reference acid catalyst

Catalyst

d (nm)

D_p(nm)

S_BET(m²g⁻¹

)

V_p(cm³g⁻¹

)

A (μeq g⁻¹

)

E_i(mV)

CTAB PrCl-MONSs12

270

180

270

360

350

260

210

270

3.3

3.2

3.0

2.9

3.3

3.8

3.2

915

553

538

433

258

537

581

551

564

0.90

0.64

0.58

0.45

0.24

0.54

1.13

0.94

0.50

n.d.

873

865

792

675

865

880

873

704

n.d.

CTAB [C₃PrIm][OTf]-MONSs6

CTAB [C₃PrIm][OTf]-MONSs12

CTAB [C₃PrIm][OTf]-MONSs24

CTAC [C₃PrIm][OTf]-MONSs12

CPB [C₃PrIm][OTf]-MONSs12

CTATOS [C₃PrIm][OTf]-MONSs12

CTATOS [C₃PrIm][OTf]-MONSs12 3rd^a

CTAB PrSO₃H-MONSs12

456.1

424.9

412.8

412.6

510.4

545.5

n.d.

340.3

a

The catalyst was used for the third time.

MONSs12, Table 1). Interestingly, CTATOS [C₃PrIm][OTf]-

MONSs12 exhibits monodispersed spherical nanostructures but

with a wormhole-like interconnected mesoporous structure;

additionally, its particle size (210 nm) is the smallest among

those of the as-prepared four [C₃PrIm][OTf]-MONSs catalysts

(Fig. 1i and j). The results suggest that the anion size and

structures of SDAs influence not only the particle size but also

the pore morphology of the [C₃PrIm][OTf]-MONSs catalysts.

CTATOS and CTAC with the largest and smallest anion

facilitate the growth of the polysilicate–SDA supramolecule

aggregates, thereby enlarging the diameter of the PrCl-MONSs

support, which finally gives rise to a larger particle size of the

[C₃PrIm][OTf]-MONSs catalyst.

CTAB PrSO₃H-MONSs12, prepared by a similar route to

that of CTAB [C₃PrIm][OTf]-MONSs12, exhibits the same

spherical nanostructure and pore morphology as well as

particle size with the latter, further confirming the

reproducibility of the preparation method (Fig. 1o and p).

FESEM results of the support (Fig. 2a), [C₃PrIm][OTf]-

MONSs catalysts (Fig. 2b–g) and CTAB PrSO₃H-MONSs12

(Fig. 2h) coincide exactly with those from TEM

observations, further confirming their monodispersed

(MePhSO₃and Cl⁻) size can lead to CTATOS [C₃PrIm][OTf]-

−

MONSs12 and CTAC [C₃PrIm][OTf]-MONSs12 with the smallest

and largest particle sizes; in the cases of CPB [C₃PrIm][OTf]-

MONSs12 and CTAB [C₃PrIm][OTf]-MONSs12 prepared with

SDAs with the same anion (Br⁻), their particle sizes are very

close. This is due to the fact that the larger anion size of the

SDA may limit the growth of polysilicate–SDA supramolecules,

leading to a smaller particle size of the [C₃PrIm][OTf]-MONSs.

Additionally, during the process of the synthesis of CTATOS

PrCl-MONSs, the hydrophobic chains of CTATOS twist together

spherical

nanostructures

with

plentiful

mesopores.

Moreover, the surface of CTATOS [C₃PrIm][OTf]-MONSs12

(Fig. 2g) is more open in comparison to those of the other

three catalysts. Additionally, the particle size distributions

of the four [C₃PrIm][OTf]-MONSs12 catalysts (Fig. S1a′–d′†)

are also provided according to FESEM observations (Fig.

S1a–d†). For CTAB [C₃PrIm][OTf]-MONSs12, CTAC [C₃PrIm]

[OTf]-MONSs12, CPB [C₃PrIm][OTf]-MONSs12 and CTATOS

[C₃PrIm][OTf]-MONSs12, their particle sizes mainly center at

ca. 270, 350, 260 and 210 nm, respectively. The result

confirms the uniformity of the as-prepared [C₃PrIm][OTf]-

MONSs catalysts.

−

with large-sized MePhSO₃ions inside the micelles, which

causes a twisted and short rod-like arrangement of hydrophobic

groups of CTATOS and thus finally results in CTATOS [C₃PrIm]

[OTf]-MONSs12 with

a

wormhole-like interconnected

mesostructure. Different from CTATOS PrCl-MONSs, during the

process of the synthesis of CTAB-, CTAC- and CPB-directed

MONSs, SDA micelles can offer an ordered and stretched

arrangement of hydrophobic groups, which ultimately leads to

CTAB-, CTAC- and CPB-directed [C₃PrIm][OTf]-MONSs with a

periodic centrally radialized mesostructure.

To study the influence of co-hydrolysis and co-condensation

time of organoalkoxysilane precursors on the pore morphology

and particle size of the [C₃PrIm][OTf]-MONSs catalysts, CTAB

[C₃PrIm][OTf]-MONSs6 and CTAB [C₃PrIm][OTf]-MONSs24 are

prepared. As shown in Fig. 1k–n, both catalysts present the

same monodispersed spherical nanostructures and pore

morphologies as those of the CTAB [C₃PrIm][OTf]-MONSs12 but

different particle sizes (180 and 360 nm for CTAB [C₃PrIm][OTf]-

MONSs6 and CTAB [C₃PrIm][OTf]-MONSs24, Table 1). The result

indicates that the co-hydrolysis and co-condensation time of

organoalkoxysilane precursors has little influence on the

morphology of the [C₃PrIm][OTf]-MONSs catalysts; however,

prolonging the co-hydrolysis and co-condensation time can

Fig. 2 FESEM images of CTAB PrCl-MONSs12 (a), CTAB [C₃PrIm][OTf]-

MONSs6 (b), CTAB [C₃PrIm][OTf]-MONSs12 (c), CTAB [C₃PrIm][OTf]-

MONSs24 (d), CTAC [C₃PrIm][OTf]-MONSs12 (e), CPB [C₃PrIm][OTf]-

MONSs12 (f), CTATOS [C₃PrIm][OTf]-MONSs12 (g) and CTAB PrSO₃H-

MONSs12 (h).

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The interesting monodispersed mesoporous spherical

nanostructures of the [C₃PrIm][OTf]-MONSs catalysts give

them excellent textural properties. As shown in Fig. S2a and

c,† the CTAB PrCl-MONSs12 support and various [C₃PrIm]

Catalysis Science & Technology

spectra of the four different SDA-directed [C₃PrIm][OTf]-

MONSs catalysts are deconvolved into four individual peaks

positioned at 284.6, 285.7, 286.7 and 292.4 eV, respectively,

corresponding to C–C bonds from bridging ethyl units and

propyl groups, C–N, C–O and C–F bonds within the [C₃PrIm]

[OTf] groups, respectively.^41,42The immobilization of [C₃-

PrIm][OTf] groups within the PrCl-MONSs support is further

evidenced by the appearance of two characteristic peaks

centered at 168.5 and 169.5 eV in the high resolution S 2p

XPS spectra (Fig. 3b), attributed to the S 2p_3/2and S 2p_1/2

states of the PrSO₃H group and OTf anion.³⁴Additionally, on

the basis of the XPS whole spectra of the CTAB-, CTAC-, CPB-

and CTATOS-directed [C₃PrIm][OTf]-MONSs12 catalysts (Fig.

S3†), the content (At%) of the nitrogen, sulphur and fluorine

elements and their corresponding atomic ratios in the

catalysts are estimated (Table S1†). As expected, the

determined atomic ratios of N : S : F in the four [C₃PrIm][OTf]-

MONSs12 catalysts are very close to the stoichiometric ratio

(2 : 2 : 3) of the parent ionic liquid (Scheme 1). Therefore, the

[C₃PrIm][OTf] groups are successfully incorporated into the

silica framework without inducing structural changes.

[OTf]-MONSs catalysts all exhibit

a type IV isotherm,

reflecting that they possess mesoporous structures; however,

their pore-condensation steps appear in different relative

pressure (P/P₀) ranges, which is related to their pore

morphologies. For CTAB PrCl-MONSs12, CTAB [C₃PrIm][OTf]-

MONSs12, CTAC [C₃PrIm][OTf]-MONSs12, CPB [C₃PrIm][OTf]-

MONSs12 and CTAB PrSO₃H-MONSs12 with a periodic pore

morphology, their pore-condensation step happens around

the range of P/P₀= 0.4–0.6, while the pore-condensation step

occurs at P/P₀= 0.4–0.8 for CTATOS [C₃PrIm][OTf]-MONSs12

with an interconnected pore morphology. The results indicate

that CTATOS [C₃PrIm][OTf]-MONSs12 has

a larger pore

diameter than the other catalysts. The BJH pore size

distribution curves displayed in Fig. S2b and d† suggest that

all tested samples show a sharp pore size distribution,

suggesting that the mesopores within the nanospheres are

uniform.

From the determined textural parameters based on

nitrogen sorption isotherms, it is found that the CTAB PrCl-

MONSs12 support possesses a large BET surface area (915

m²g⁻¹) and high pore volume (0.90 cm³g⁻¹), contributed by

MAS NMR analysis can further support the above results.

As shown in Fig. 3c, three characteristic resonance signals at

−61.6, −66.7 and −73.7 ppm are detected in the ²⁹Si NMR

MAS spectrum of CTAB [C₃PrIm][OTf]-MONSs12, and they are

assigned to organosilica species including T¹[C–Si(OSi)

(OH)₂], T²[C–Si(OSi)₂(OH)] and T³[C–Si(OSi)₃]. These

organosilica species come from co-hydrolysis and co-

condensation of organoalkoxysilane precursors (BTMSE and

CPTMS), and they construct the silica/carbon framework of

the [C₃PrIm][OTf]-MONSs catalysts.^43–45More abundant

structural information concerning the formation of

chloropropyl functionalized silica/carbon frameworks (Si–C–

C–Si–O–Si–PrCl) and the immobilization of the [C₃PrIm][OTf]

groups within the silica/carbon frameworks are provided by

the ¹³C CP-MAS NMR spectra of CTAB PrCl-MONSs12, CTAB

PrIm-MONSs12 and CTAB [C₃PrIm][OTf]-MONSs12. As shown

in Fig. 3d, four carbon species (C¹–C⁴) are identified in the

CTAB PrCl-MONSs12 support. Specifically, the strongest peak

positioned at 5.2 ppm is assigned to carbon species (C¹) of

bridging ethyl units of the silica/carbon framework, while the

other three peaks centered at 10.1 (C²), 26.3 (C³) and 45.7

ppm (C⁴) are attributed to carbon species from the

chloropropyl group.^32,46–48In the case of CTAB PrIm-

MONSs12, the aforementioned C¹–C⁴species still can be

found, accompanied with the appearance of two new and

weak peaks positioned at 120.1 (C⁵) and 135.1 ppm (C⁶). The

C⁵and C⁶species originate from the imidazole unit, and

therefore, the imidazole group is successfully incorporated

into the silica/carbon framework to form –Si–C–C–Si–O–Si–

PrIm linkage.^32,48As for CTAB [C₃PrIm][OTf]-MONSs12, the

C¹–C⁶species originated from the imidazole-functionalized

silica/carbon framework remaining intact; meanwhile, two

weak peaks emerged at 51.9 (C⁷) and 122.4 ppm (C⁸), and

they are derived from the –PrSO₃H group covalently bonded

to an imidazole cation and the introduced OTf anion,

its

well-defined

mesostructures

(Table

1).

After

functionalization, the resulting CTAB [C₃PrIm][OTf]-MONSs12

shows a decreased BET surface area (538 m²g⁻¹) and pore

volume (0.58 cm³g⁻¹); additionally, the pore diameter of

CTAB [C₃PrIm][OTf]-MONSs12 (3.2 nm) decreases slightly in

comparison to that of CTAB PrCl-MONSs12 (3.3 nm). The

above results confirm that most of the [C₃PrIm][OTf] groups

are immobilized on the internal surface of the nanospheres,

which leads to the decreased BET surface area and pore

volume as well as somewhat blocked pore channels. For the

three CTAB [C₃PrIm][OTf]-MONSs catalysts, their BET surface

areas and pore volumes decrease gradually with increasing

hydrolysis time from 6 to 12 and 24 h (Table 1), originating

from the gradually enlarged particle sizes. Finally, the four

different surfactant-directed catalysts follow the pore

diameter, BET surface area and pore volume order CTATOS

[C₃PrIm][OTf]-MONSs12 (3.8 nm, 581 m²g⁻¹, 1.13 cm³g⁻¹) >

CPB [C₃PrIm][OTf]-MONSs12 (3.3 nm, 537 m²g⁻¹, 0.54

cm³g⁻¹) ≈ CTAB [C₃PrIm][OTf]-MONSs12 (3.2 nm, 538

m²g⁻¹, 0.58 cm³g⁻¹) > CTAC [C₃PrIm][OTf]-MONSs12 (2.9

nm, 285 m²g⁻¹, 0.24 cm³g⁻¹). In the case of CTAB PrSO₃H-

MONSs12, it has similar textural parameters to those of its

CTAB [C₃PrIm][OTf]-MONSs12 counterpart. The above results

suggest that the textural properties of the [C₃PrIm][OTf]-

MONSs catalysts are dominated by their pore morphologies

and particle sizes.

XPS surface probe and MAS NMR techniques are applied

to confirm the formation of inorganic–organic silica/carbon

framework in the [C₃PrIm][OTf]-MONSs catalysts and the

immobilization of the [C₃PrIm][OTf] groups within the

support. As presented in Fig. 3a, the high resolution C 1s XPS

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Fig. 3 High resolution XPS spectra of the different SDA-directed [C₃PrIm][OTf]-MONSs12 catalysts in the C 1s (a) and S 2p (b) binding energy

region. (c) ²⁹Si MAS NMR spectrum of CTAB [C₃PrIm][OTf]-MONSs12 and (d) ¹³C CP-MAS NMR spectra of CTAB PrCl-MONSs12, CTAB PrIm-

MONSs12 and CTAB [C₃PrIm][OTf]-MONSs12.

respectively.⁴⁹Therefore, it is evidenced that the [C₃][OTf]

group is successfully bonded to the imidazole-functionalized

silica/carbon framework to create the [C₃PrIm][OTf]-MONSs

catalysts. It should be noted that due to the different

chemical environments of C³and C⁴species in CTAB PrCl-

MONSs12, CTAB PrIm-MONSs12 and CTAB [C₃PrIm][OTf]-

MONSs12, their chemical shifts have slight differences.

From the combination of XPS and MAS NMR analysis

results, we firmly speculate that [C₃PrIm][OTf] functionalized

ethyl-bridged organosilica is successfully constructed by

using the currently designed liquid-interface assembly

strategy followed by further chemical modifications, and they

constructed the wall structure of the catalyst, as illustrated in

Scheme 1.

The strength of Brønsted acid sites of the [C₃PrIm][OTf]-

MONSs catalysts is measured by nonaqueous potentiometric

titration with n-butylamine in acetonitrile, and the obtained

initial electrode potential (E_i) value indicates the maximum

acid strength of the surface sites. Generally, a material with

an E_ivalue higher than 100 mV is defined as a very strong

solid acid.⁵⁰As presented in Table 1, the measured E_ivalues

of as-prepared [C₃PrIm][OTf]-MONSs are in the range of 456.1

to 545.5 mV, indicating their superstrong Brønsted acidic

nature. The Brønsted acid site density of the [C₃PrIm][OTf]-

MONSs are determined by acid–base titration with a dilute

NaOH solution. As listed in Table 1, for the three CTAB [C₃-

PrIm][OTf]-MONSs catalysts with different particle sizes, i.e.,

CTAB [C₃PrIm][OTf]-MONSs6, CTAB [C₃PrIm][OTf]-MONSs12

and CTAB [C₃PrIm][OTf]-MONSs24, their Brønsted acid site

density (873, 865 and 792 μeq g⁻¹) gradually decreases with

continuously increasing particle size (180, 270 and 360 nm).

In the cases of the four [C₃PrIm][OTf]-MONSs catalysts

prepared under the same conditions but using different

SDAs, i.e., CTATOS [C₃PrIm][OTf]-MONSs12, CPB [C₃PrIm]

[OTf]-MONSs12, CTAB [C₃PrIm][OTf]-MONSs12 and CTAC [C₃-

PrIm][OTf]-MONSs12, the difference among their Brønsted

acid site density is also related to the particle size.

Specifically, CTATOS [C₃PrIm][OTf]-MONSs12 and CTAC [C₃-

PrIm][OTf]-MONSs12 with the smallest and the largest

particle size (210 vs. 350 nm) show the highest and the lowest

Brønsted acid site density (880 vs. 675 μeq g⁻¹), while CPB

[C₃PrIm][OTf]-MONSs12 and CTAB [C₃PrIm][OTf]-MONSs12

with very close particle sizes (260 and 270 nm) display the

same acid site density (865 μeq g⁻¹). This is due to the fact

that the increase of the particle size of the catalyst leads to

somewhat a decrease of its BET surface area and pore

volume, which results in less exposed surface Brønsted acid

sites and thereby a reduced Brønsted acid site density.

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3.2. Evaluation of the catalytic performance of the [C₃PrIm]

[OTf]-MONSs

3.2.1. Catalytic activity and selectivity. The acid catalytic

activity and selectivity of the as-prepared [C₃PrIm][OTf]-

MONSs catalysts are evaluated by transformation of HMF to

EL and LA in green solvents like EtOH and H₂O media.

The synthesis of EL from the ethanolysis of HMF. As

illustrated in Scheme S1a,† the acid-catalyzed transformation of

HMF to EL by using EtOH as both the reactant and solvent

starts from the protonation of the hydroxymethyl group of

HMF.

Subsequently,

the

main

intermediate,

ethoxymethylfurfural (EMF), is rapidly formed via etherification

reaction under the nucleophilic attack of EtOH molecules,

accompanied with the release of water molecules (identified by

GC-MS, retention time of 9.88 min in Fig. S4†). The final

formation of EL from EMF is the rate-limiting step, which

undergoes multiple steps including nucleophilic addition,

dehydration and ring opening.⁵¹In the search for optimum

reaction conditions for the synthesis EL with high yield, the

influence of the important experimental parameters such as

reaction temperature and EtOH-to-HMF molar ratio on the

catalytic activity and selectivity is studied by using CTAB [C₃-

PrIm][OTf]-MONSs12 as the representative catalyst. Afterwards,

the particle size- and pore morphology-dependent activity and

selectivity of the [C₃PrIm][OTf]-MONSs in the target reaction are

revealed under the optimum reaction conditions.

Fig. 4 Influence of reaction temperature (a) and n_EtOH: n_HMFratio (b)

on the catalytic activity and selectivity of CTAB [C₃PrIm][OTf]-

MONSs12 in ethanolysis of HMF. (a) 0.278 mmol HMF, 69 mmol EtOH,

n_EtOH: n_HMF= 248, HMF-to-catalyst weight ratio of 1 wt/wt, 120 min;

(b) 0.278 mmol HMF, 120 °C, HMF-to-catalyst weight ratio of 1 wt/wt,

120 min.

As presented in Fig. 4a, in the CTAB [C₃PrIm][OTf]-

MONSs12-catalyzed ethanolysis of HMF reaction system, the

conversion of HMF increases rapidly from 82.4 to 96.8% (120

min) with elevating reaction temperature from 80 to 100 °C,

and by further increasing the reaction temperature to 120

and 140 °C, the conversion of HMF reaches 99.9%.

Simultaneously, the selectivity of the catalyst to EMF reaches

the maximum value (45.6%) at 100 °C, and then it

continuously reduces to 23.9% with elevating reaction

temperature to 140 °C, whereas the selectivity to EL increases

remarkably with the temperature, e.g., from 3.0 to 16.4 and

65.6% at 80, 100 and 120 °C, respectively. By further elevating

the reaction temperature to 140 °C, the selectivity to EL has a

slight decrease (62.2%). The above results indicate that the

transformation of HMF to EL extremely depends on the

reaction temperature. Lower reaction temperature (e.g., 80

°C) not only limits the transformation of HMF to EL but also

to EMF, while higher reaction temperature (e.g., 120 °C)

boosts further transformation of EMF to EL, thus reducing

the selectivity to EMF. At a reaction temperature of 140 °C,

the slightly decreased selectivity to EL is due to the formation

of by-products, e.g., 2,5-bis(furan-2-ylmethyl)furan (identified

by GC-MS, retention time of 12.58 min in Fig. S4a†) by self-

polymerization of HMF. Therefore, much higher reaction

temperature (e.g., 140 °C) can not only promote the

conversion of HMF but also facilitate the self-polymerization

of HMF to oligomeric by-products, thus decreasing the

selectivity to EL. Consequently, the reaction temperature of

120 °C is set for following tests.

On the basis of the results displayed in Fig. 4a, the

Arrhenius model (ln k = ln k₀− E_a/RT) is applied by fitting

the formation rate of EL at 80, 100, 120 and 140 °C. As

shown in Fig. S5,† the determined activation energy (E_a) is

68.1 kJ mol⁻¹for the CTAB [C₃PrIm][OTf]-MONSs12-

catalyzed HMF ethanolysis reaction. According to the

literature, if the E_avalue is higher than 41.8 kJ mol⁻¹, the

HMF ring-opening reaction follows

a reaction-limited

mechanism.⁵²Therefore, it is more consistent with

reaction-limited kinetics rather than diffusion-limited

kinetics for the [C₃PrIm][OTf]-MONSs-catalyzed-ethanolysis

of HMF reaction, in which the ring-opening step is involved.

At the reaction temperature of 120 °C and reaction time

of 120 min, the influence of EtOH-to-HMF molar ratio

(n_EtOH: n_HMF) on the conversion of HMF and selectivity to

EL is studied subsequently. From the result shown in

Fig. 4b, it is found that the transformation of HMF is

finished at various n_EtOH: n_HMFratios. Additionally, the

selectivity of CTAB [C₃PrIm][OTf]-MONSs12 to EL increases

apparently from 17.9 to 63.9 and 65.5% with increasing

n_EtOH: n_HMFratio from 124 to 186 and 248; by further

increasing the ratio to 310, the selectivity to EL decreases to

58.8%. The selectivity of CTAB [C₃PrIm][OTf]-MONSs12 to

EMF is relatively low (lower than 25.4%) at n_EtOH: n_HMF

ratios of 124, 186, 248 and 310. The above results are

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explained below. At a lower n_EtOH: n_HMFratio (e.g., 124), the

reactant HMF has a high concentration, thus creating a

favourable condition for the self-polymerization of HMF to

oligomeric by-products, e.g., 2,5-bis(furan-2-ylmethyl)furan,

with high concentration under the acid catalysis over CTAB

[C₃PrIm][OTf]-MONSs12, as confirmed by GC-MS (retention

time of 12.58 min, Fig. S4b†). As a consequence, the total

selectivity of CTAB [C₃PrIm][OTf]-MONSs12 to both EL and

EMF is relatively low. Otherwise, at a higher n_EtOH: n_HMF

ratio (e.g., 310), HMF is diluted significantly, which may

decelerate the formation of EMF and EL to some extent.

Accordingly, a suitable n_EtOH: n_HMFratio (e.g., 248) can

avoid equilibrium limitation and facilitate the conversion of

HMF to EL. In the following tests, the n_EtOH: n_HMFratio is

set at 248.

Under the conditions: 120 °C and n_EtOH: n_HMFratio of 248,

the particle size- and pore morphology-dependent activity

and selectivity of [C₃PrIm][OTf]-MONSs in ethanolysis of

HMF to EL are studied. At first, the three CTAB [C₃PrIm]

[OTf]-MONSs catalysts with different particle sizes (180, 270

and 360 nm) are tested. As shown in Fig. 5a, conversion of

HMF occurs rapidly over the three tested CTAB [C₃PrIm]

[OTf]-MONSs catalysts, regardless of their particle sizes.

After the reaction proceeds for 20 min, the conversion of

HMF reaches 94.6, 94.3 and 92.4% in CTAB [C₃PrIm][OTf]-

MONSs6-, CTAB [C₃PrIm][OTf]-MONSs12- and CTAB [C₃PrIm]

[OTf]-MONSs24-catalyzed ethanolysis of HMF, respectively.

By prolonging the reaction time to 60 min, conversion of

HMF is completed. However, the selectivity of CTAB [C₃-

PrIm][OTf]-MONSs to EL is obviously influenced by the

Fig. 5 Particle size- and morphology-dependent activity (a and c) and selectivity (b and d) of [C₃PrIm][OTf]-MONSs in ethanolysis of HMF. Activity

(e) and selectivity (f) of the reference solid acids in the target reaction. n_EtOH: n_HMF= 248, HMF-to-catalyst weight ratio 1 wt/wt, 120 °C.

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reaction time and particle size of the catalysts (Fig. 5b).

Specifically, the selectivity of the three CTAB [C₃PrIm][OTf]-

MONSs catalysts to EL gradually increases with prolonged

reaction time; meanwhile, the selectivity of the catalysts to

EMF increases at the initial stage of the reaction, and then

it gradually decreases with the reaction time after reaching

the peak. The result reflects that HMF can be transformed

to EMF quickly over the CTAB [C₃PrIm][OTf]-MONSs;

however, further transformation of EMF to EL occurs slowly

because it suffers from multiple steps. Additionally, CTAB

[C₃PrIm][OTf]-MONSs6 and CTAB [C₃PrIm][OTf]MONSs12

with a smaller particle size (180 and 270 nm) show much

higher selectivity to EL rather than EMF in comparison to

CTAB [C₃PrIm][OTf]-MONSs24 with a larger particle size

(360 nm). For example, the selectivity to EL reaches 92.3

(CTAB [C₃PrIm][OTf]-MONSs6), 90.6 (CTAB [C₃PrIm][OTf]-

MONSs12) and 51.0% (CTAB [C₃PrIm][OTf]-MONSs24),

respectively, after the ethanolysis reaction proceeds for 240

min; simultaneously, the corresponding selectivity to EMF

is 1.0, 1.4 and 18.0%, respectively.

Subsequently, the four different SDA-directed catalysts are

compared. As shown in Fig. 5c, the tested catalysts show fast

transformation of HMF, and the transformation is finished

within 60 min. In the case of the selectivity of the four

catalysts to EMF, it increases in the beginning of the

ethanolysis reaction, and then it starts to decrease as the

reaction time is prolonged (Fig. 5d). Among the four

catalysts, CTAC [C₃PrIm][OTf]-MONSs12 exhibits the highest

selectivity to EMF (68.0%, 60 min); additionally, over a period

of 240 min, most of EMF has been transformed to EL under

the catalysis of CTAB [C₃PrIm][OTf]-MONSs12, CPB [C₃PrIm]

[OTf]-MONSs12 or CTATOS [C₃PrIm][OTf]-MONSs12. However,

a considerable amount of EMF still exists in the CTAC [C₃-

PrIm][OTf]-MONSs12-catalyzed reaction system. For example,

the selectivity of CTATOS [C₃PrIm][OTf]-MONSs12, CPB [C₃-

PrIm][OTf]-MONSs12, CTAB [C₃PrIm][OTf]-MONSs12 and

CTAC [C₃PrIm][OTf]-MONSs12 to EL is 93.7, 91.4, 90.6 and

48.7%, respectively, at the reaction time of 240 min;

simultaneously, the corresponding selectivity to EMF is 1.0,

1.0, 1.4 and 15.0%, respectively.

Additionally, the three catalysts display continuously

increasing selectivity to EL with the reaction time; however,

remarkably increased selectivity to EL is only found for

Amberlyst-15 resin (76.7%, 240 min), while CTAB PrSO₃H-

MONSs12 (30.2%) and HY zeolite (27.6%) exhibit poor

selectivity to EL at the reaction time of 240 min. Therefore,

except for CTAC [C₃PrIm][OTf]-MONSs12 and CTAB [C₃PrIm]

[OTf]-MONSs24 with a large particle size, the other four as-

prepared [C₃PrIm][OTf]-MONSs all show obviously higher

ethanolysis activity and selectivity to EL, outperforming

Amberlyst-15 resin and HY zeolite.

On the basis of the above results, we conclude that the

various [C₃PrIm][OTf]-MONSs catalysts, regardless of their

particle sizes and pore morphologies, all can transform HMF

in EtOH media rapidly and completely. The high conversion

of HMF over the [C₃PrIm][OTf]-MONSs is mainly dominated

by the inherent superstrong Brønsted acidic nature of the

catalysts, which can boost the fast transformation of HMF;

additionally, the unique spherical morphology of the [C₃-

PrIm][OTf]-MONSs with an open surface and 2D periodic or

3D interconnected mesopore channels is readily accessible,

and they serve as nanoreactors that are favourable for

diffusion and mass transfer of the reactants and products,

which leads to the positive influence on the fast

transformation of HMF. However, the selectivity of the [C₃-

PrIm][OTf]-MONSs to EL is remarkably influenced by the

particle sizes and pore morphologies of the catalysts, which

finally results in different yields of EL. For example, CTAB

[C₃PrIm][OTf]-MONSs6 and CTATOS [C₃PrIm][OTf]-MONSs12

with the smallest particle size (180 and 210 nm) show the

highest (92.2 and 93.6%, 240 min) yield of EL, while CTAC

[C₃PrIm][OTf]-MONSs12 and CTAB [C₃PrIm][OTf]-MONSs24

with the largest particle size (350 and 360 nm) show the

lowest yield of EL (48.7 and 50.9%, 240 min). As for the other

two catalysts, CPB [C₃PrIm][OTf]-MONSs12 and CTAB [C₃-

PrIm][OTf]-MONSs12, their particle size (260 and 270 nm)

and pore morphology are almost the same, and they show

the same yield of EL (91.3 and 90.5%, 240 min). The smaller

particle size of the mesoporous [C₃PrIm][OTf]-MONSs

catalysts can shorten the diffusion distance of reactants and

products, which favors the reaction kinetics and thereby fast

transformation of HMF to the final product EL; meanwhile,

the smaller [C₃PrIm][OTf]-MONSs catalyst possesses a larger

BET surface area and higher pore volume and thus more

exposed acid sites, which can improve the accessibility of the

acid sites. This is also one of the important reasons for the

acceleration of the transformation of HMF to EL, giving rise

to higher selectivity to EL and thus high yield of EL. For the

largest CTAB [C₃PrIm][OTf]-MONSs24 and CTAC [C₃PrIm]

[OTf]-MONSs12, the increased diffusion distance of reactants

and products, and inferior textural properties accompanied

with less exposed acid sites obviously slow down the

transformation of HMF to EL; as a consequence, they show

relatively high selectivity to EMF (or lower selectivity to EL)

but low yield of EL. It should be noted that CTATOS [C₃PrIm]

[OTf]-MONSs12 possesses a unique open surface and 3D

Under identical experimental conditions, reference solid

acid catalysts including CTAB PrSO₃H-MONSs12, Amberlyst-

15 resin and HY zeolite are also tested for comparison. As

shown in Fig. 5e, after the CTAB PrSO₃H-MONSs12-,

Amberlyst-15 resin- and HY zeolite-catalyzed ethanolysis

reaction proceeds for 20 min, the conversion of HMF is 85.9,

86.2 and 63.7%, respectively. By prolonging the reaction time

to 60 min, the transformation of HMF over the reference

catalysts is completed. The result implies that the ethanolysis

activity of the three reference solid acids is lower than that of

as-prepared [C₃PrIm][OTf]-MONSs to different extents. From

the result shown in Fig. 5f, it is found that three catalysts

show the highest selectivity to EMF at the reaction time of 40

min, which is 59.7 (CTAB PrSO₃H-MONSs12), 57.5 (Amberlyst-

15 resin) and 50.5% (HY zeolite), respectively, and then the

selectivity to EMF gradually decreases with the reaction time.

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wormhole-like interconnected mesostructure, which gives it

excellent porosity properties including the largest pore

diameter, the largest BET surface area and the highest pore

volume among the as-prepared six [C₃PrIm][OTf]-MONSs

catalysts. All of these advantages give rise to the high

exposure degree of the acid sites of CTATOS [C₃PrIm][OTf]-

MONSs12 to the reactants; accordingly, it shows the highest

ethanolysis activity and selectivity to EL. Amberlyst-15 resin is

a superstrong Brønsted acid (E_ivalue of 617.0 mV)⁵³with an

extremely high Brønsted acid-site density (4800 μeq g⁻¹),^11,54

which leads to the considerably fast transformation of HMF

to EL. However, the small BET surface area of Amberlyst-15

resin (50 m²g⁻¹)^11,54results in a low exposure degree of the

acid sites to the reactants. Accordingly, Amberlyst-15 resin

shows a somewhat slow transformation rate for EL, and at a

reaction time of 240 min, a considerable amount of EMF still

has not been transformed (selectivity to EMF is 15.1%).

Although the particle size and pore morphology of CTAB

PrSO₃H-MONSs12 are similar to those of its CTAB [C₃PrIm]

[OTf]-MONSs12 counterpart, the relatively weak Brønsted acid

strength and lower acid site density make CTAB PrSO₃H-

MONSs12 have very low selectivity to EL, accompanied with

the formation of more by-products such as 5-methylfuran-2-

carbaldehyde and 2,5-bis(furan-2-ylmethyl)furan (retention

time of 5.67 and 12.61 min, detected by GC-MS, Fig. S4c†)

from the dehydration and self-polymerization of HMF. In the

case of HY zeolite, its microporosity with a pore diameter of

0.7 nm (ref. 53) and low Brønsted acid site density (608 μeq

g⁻¹)⁵³make it have poor conversion ability for EL, and more

by-products are produced in this system (retention time of

5.64 and 12.61 min, detected by GC-MS, Fig. S4d†). The above

results confirm that the particle size- and morphology-

controlled preparation of the supported BAILs is an effective

strategy to improve their catalytic activity and selectivity in

transformation of HMF to valuable chemicals such as EL.

Finally, the surface hydrophobicity of the [C₃PrIm][OTf]-

MONSs originating from bridging ethyl units within the

silica/carbon framework also contributes to the high catalytic

activity and selectivity to EL to some extent by preventing the

adsorption of hydrophilic by-products such as oligomers and

water on the catalyst surface, boosting the transformation of

HMF at a fast rate.

transformation process of reactant-to-transition product-to-

final product. In order to efficiently and selectively synthesise

LA from the hydrolysis of HMF, the influence of important

experimental parameters such as reaction temperature, H₂O-

to-HMF molar ratio and organic solvent on the hydrolysis

activity and selectivity to LA is firstly studied by using the

most active CTATOS [C₃PrIm][OTf]-MONSs12 as the

representative catalyst. Subsequently, the influence of particle

size and pore morphology on the hydrolysis activity and

selectivity to LA is revealed under the optimum reaction

conditions.

As presented in Fig. 6a, with increasing reaction

temperature, the conversion of HMF increases remarkably. For

example, after the CTATOS [C₃PrIm][OTf]-MONSs12-catalyzed

The synthesis of LA from the hydrolysis of HMF. As

illustrated in Scheme S1b,† the Brønsted acid-catalyzed

transformation of HMF to LA in water medium begins from

the protonation of the hydroxymethyl group of HMF. Under

the attack of water molecules, the protonated HMF

undergoes successive dehydration, ring opening, release of

formic acid and then formation of the final product (LA).

Different from the transformation of HMF in EtOH medium,

the transformation of HMF in water media suffers from

harsher reaction conditions such as higher temperature,

which may lead to more oligomer by-products and thereby

low selectivity to LA. Moreover, leaching of BAIL sites may

occur due to the solvent effect of water, which provides a

stronger polarity and hydrogen bonding, limiting the

Fig. 6 Influence of reaction temperature (a), n_{H O}: n_HMFratio (b) and

2

solvent (c) on the catalytic activity and selectivity of CTATOS [C₃PrIm]

[OTf]-MONSs12 in hydrolysis of HMF. (a) 0.4 mmol HMF, 166.7 mmol

H₂O, 120 min; (b) 0.4 mmol HMF, 140 °C, 120 min; (c) 0.4 mmol HMF,

2 mL organic solvents and 1 mL H₂O mixture, 140 °C, 30 mg catalyst,

120 min.

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HMF hydrolysis reaction proceeds for 120 min, the conversion

of HMF reaches 32.7 (100 °C), 35.7 (120 °C), 74.1 (140 °C) and

99.9% (160 °C), respectively; additionally, the highest selectivity

of the catalyst to LA (82.2%) is achieved at 140 °C, and by

further increasing the temperature to 160 °C, the selectivity to

LA (73.1%) decreases. The decreased selectivity to LA at 160 °C

is due to the formation of more oligomer by-products such as

5,5′-oxybis(methylene)difuran-2-carbaldehyde (m/z = 235) via

etherification reaction of two molecules of HMF and

(5-formylfuran-2-yl)methyl 4-oxopentanoate (m/z = 225) via

esterification reaction of HMF with LA (identified by LC-MS

analysis, Fig. S6a and b†). Therefore, in order to ensure a

considerably rapid HMF hydrolysis rate and avoid the formation

of a high concentration of oligomer by-products simultaneously,

the optimum hydrolysis temperature of 140 °C is set in the

subsequent catalytic tests.

compared. As displayed in Fig. 7a, the conversion of HMF

obviously increases with prolonged reaction time;

additionally, similar to their activity trend for the HMF

ethanolysis reaction, CTATOS [C₃PrIm][OTf]-MONSs12 and

CTAC [C₃PrIm][OTf]-MONSs12 also exhibits the highest and

lowest activity in the HMF hydrolysis reaction among the four

tested [C₃PrIm][OTf]-MONSs catalysts, while CTAB [C₃PrIm]

[OTf]-MONSs12 and CPB [C₃PrIm][OTf]-MONSs12 show

similar hydrolysis activity. After the HMF hydrolysis reaction

proceeds for 240 min, the conversion of HMF reaches 99.9

(CTATOS [C₃PrIm][OTf]-MONSs12), 92.4 (CPB [C₃PrIm][OTf]-

MONSs12), 87.2 (CTAB [C₃PrIm][OTf]-MONSs12) and 81.8%

(CTAC [C₃PrIm][OTf]-MONSs12), respectively. Under the same

conditions, the hydrolysis activity of Amberlyst-15 resin

(conversion of HMF is 99.9%) is comparable to that of the

most active CTATOS [C₃PrIm][OTf]-MONSs12; however, HY

zeolite shows the lowest conversion of HMF (63.9%).

Next, at 140 °C and 120 min, the influence of the H₂O-to-

HMF molar ratio (n_{H O}: n_HMF) on the HMF hydrolysis activity

and selectivity of CTATOS [C₃PrIm][OTf]-MONSs12 is studied.

As displayed in Fig. 6b, the conversion of HMF approaches

As displayed in Fig. 7b, the selectivity of [C₃PrIm][OTf]-

MONSs12 and the reference catalysts to LA reaches the

highest value when the HMF hydrolysis reaction is performed

for 120 min, and the corresponding selectivity is 82.2

(CTATOS [C₃PrIm][OTf]-MONSs12), 73.1 (CPB [C₃PrIm][OTf]-

MONSs12), 71.5 (CTAB [C₃PrIm][OTf]-MONSs12), 66.7 (CTAC

[C₃PrIm][OTf]-MONSs12), 56.1 (Amberlyst-15 resin) and

82.6% (HY zeolite), respectively. By extending the reaction

time to 240 min, the selectivity decreases to some extent,

2

100% at a lower n_{H O}: n_HMFratio (139 and 278), and by

2

increasing the ratio to 417 and 556, the conversion decreases

to 74.4 and 57.9%, respectively. This is due to the fact that a

lower n_{H O}: n_HMFratio or higher HMF concentration can

2

significantly facilitate the transformation of HMF at a fast

rate. As for the selectivity to LA, it continuously increases

from 39.5 to 80.9% with increasing n_{H O}: n_HMFratio from

2

139 to 556. The reason for this is that a higher n_{H O}: n_HMF

2

ratio or lower HMF concentration can inhibit the self-

polymerization of HMF to oligomer by-products. Therefore,

considering the fast and selective transformation of HMF to

LA, in subsequent catalytic tests, the n_{H O}: n_HMFratio is set

2

at 417.

Solvent is one of the key parameters that influences HMF

hydrolysis activity and selectivity to LA. Here, at an organic

solvent (THF, DMSO or acetone)-to-H₂O volume ratio of 2,

140 °C and 120 min, the conversion of HMF and selectivity to

LA in the CTATOS [C₃PrIm][OTf]-MONSs12 system is studied.

As displayed in Fig. 6c, the highest hydrolysis activity and

selectivity to LA occur in a pure water system, and in the

presence of an organic solvent, both the conversion of HMF

and selectivity to LA decrease remarkably. For example, the

conversion of HMF and selectivity to LA are 74.1% and 82.2%

(water), 53.3% and 36.2% (acetone–H₂O), 43.0% and 62.1%

(DMSO–H₂O) and 29.4% and 25.9% (THF–H₂O). Therefore,

the selectivity to LA is related to the polarity of the solvent,

and HMF and H₂O with the weakest and strongest polarity

show the lowest and highest selectivity to LA. This is due to

the fact that the strong polarity of H₂O can promote the

release of H⁺from the [C₃PrIm][OTf] group, leading to rapid

transformation of HMF to LA. Additionally, LA is highly

soluble in water. Therefore, H₂O as both reactant and solvent

is chosen for the HMF hydrolysis reaction.

Fig.

7 Catalytic activity (a) and selectivity (b) comparison of the

Under the conditions: 140 °C, an n_{H O}: n_HMFmolar ratio

2

various SDA-directed [C₃PrIm][OTf]-MONSs catalysts and the reference

of 417 and water medium, the HMF hydrolysis activity and

selectivity of the four different SDA-directed catalysts are also

solid acids in HMF hydrolysis reactions. n_{H O}: n_HMF= 417, 30 mg

catalyst, 140 °C.

2

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which is due to the formation of more oligomeric by-

products i.e., (5-formylfuran-2-yl)methyl 4-oxopentanoate (m/z

= 225, detected by LC-MS analysis, Fig. S6a and c†).

[C₃PrIm][OTf]-MONSs12-catalyzed hydrolysis of HMF under

the given reaction conditions. Therefore, the synthesis of EL/

LA over the [C₃PrIm][OTf]-MONSs with high selectivity and

high yield can be achieved by complete conversion of HMF,

and thus humin formation is a minor pathway.

Similar to the ethanolysis testing results, the above results

further confirm that the superstrong Brønsted acidic nature

of the [C₃PrIm][OTf]-MONSs catalysts can accelerate the

transformation of HMF in water media to LA, leading to high

conversion of HMF; additionally, the particle sizes and pore

morphologies of the catalysts influence the hydrolysis activity

and selectivity remarkably. CTATOS [C₃PrIm][OTf]-MONSs12

with the smallest particle size, largest pore diameter, largest

BET surface area, highest pore volume and 3D wormhole-like

interconnected mesostructure shows high accessibility of the

acid sites to the reactants; accordingly, it also shows the

highest hydrolysis activity and selectivity to LA and thus the

highest LA yield among the four tested [C₃PrIm][OTf]-

MONSs12 catalysts. For example, the yield of LA reaches

72.8% after the CTATOS [C₃PrIm][OTf]-MONSs12-catalyzed

HMF hydrolysis reaction proceeds for 240 min under the

reaction conditions listed in Fig. 7. In the case of Amberlyst-

15 resin, its superstrong Brønsted acidic nature and high

acid site density lead to high hydrolysis activity; however, its

low exposure degree of the acid sites to the reactants may

limit the transformation of HMF to the final product (LA).

HY zeolite catalyzing the HMF hydrolysis reaction possesses

relatively low catalytic activity but high selectivity to LA. This

is due to the microporous structure of HY zeolite possibly

retarding the esterification reaction between HMF molecules

and LA because the produced bulky oligomeric by-products

are hardly formed within the micropores (confirmed by LC-

MS analysis, Fig. S6d†). Similar to ethanolysis of HMF, the

HY zeolite-catalyzed hydrolysis of HMF also produces some

oligomeric by-products from self-polymerization of HMF on

the surface of the catalyst.

The turnover frequency (TOF) accounts for differences in

acid site density by normalizing the EL and LA formation rate

by the number of Brønsted acid sites on each catalyst. As

displayed in Fig. 8, in the HMF ethanolysis reaction, the

tested catalysts follow the TOF value order CTATOS [C₃PrIm]

[OTf]-MONSs12 (67.5 × 10⁻³min⁻¹) > CPB [C₃PrIm][OTf]-

MONSs12 (57.7 × 10⁻³min⁻¹) > CTAB [C₃PrIm][OTf]-MONSs12

(54.5 × 10⁻³min⁻¹) > CTAC [C₃PrIm][OTf]-MONSs12 (35.6 ×

10⁻³min⁻¹) > CTAB PrSO₃H-MONSs12 (25.4 × 10⁻³min⁻¹) >

HY zeolite (15.7 × 10⁻³min⁻¹) > Amberlyst-15 resin (6.1 × 10⁻³

min⁻¹); as for HMF hydrolysis reaction, they follow the TOF

value order CTATOS [C₃PrIm][OTf]-MONSs12 (75.5 × 10⁻³

min⁻¹) > CPB [C₃PrIm][OTf]-MONSs12 (64.2 × 10⁻³min⁻¹) >

CTAC [C₃PrIm][OTf]-MONSs12 (62.6 × 10⁻³min⁻¹) > HY

zeolite (61.8 × 10⁻³min⁻¹) > CTAB [C₃PrIm][OTf]-MONSs12

(58.6 × 10⁻³min⁻¹) > Amberlyst-15 resin (7.0 × 10⁻³min⁻¹).

Therefore, CTATOS [C₃PrIm][OTf]-MONSs12 still performs the

best in the synthesis of EL and LA from ethanolysis and

hydrolysis of HMF as revealed by the TOF values.

To further evaluate the catalytic performance of the [C₃-

PrIm][OTf]-MONS catalysts, sugars as the substrates are also

The carbon balances in CTATOS [C₃PrIm][OTf]-MONSs12-

catalyzed ethanolysis and hydrolysis of HMF have been

provided by taking into account the products, intermediates

and by-products under the optimal conditions. As shown in

Scheme S2a,† for the CTATOS [C₃PrIm][OTf]-MONSs12-

catalyzed HMF ethanolysis reaction performed under the

conditions: n_EtOH: n_HMFmolar ratio of 248, 120 °C and 240

min, HMF (Conv = 99.9%) is transformed to product EL (Y_EL

= 93.6%), intermediate EMF (Y_EMF= 1%) and by-products

including 2,5-bis(furan-2-ylmethyl)furan (identified by GC-

MS, Fig. S4†) as well as a small amount of humins. The

carbon balance is therefore 94.6% in CTATOS [C₃PrIm][OTf]-

MONSs12-catalyzed ethanolysis of HMF under the given

reaction conditions. As shown in Scheme S2b,† in the

CTATOS [C₃PrIm][OTf]-MONSs12-catalyzed hydrolysis of HMF

performed at an n_{H O}: n_HMFmolar ratio of 417, 140 °C and

2

240 min, HMF (Conv = 99.9%) is transformed to product LA

(Y_LA= 72.8%), by-products including formic acid (FA, Y_FA

8.6%), oligomers like 5,5′-oxybis(methylene)difuran-2-

carbaldehyde and (5-formylfuran-2-yl)methyl 4-oxopentanoate

(identified by LC-MS, Fig. S6c†) and a small amount of

humins. The carbon balance is therefore 72.8% in CTATOS

=

Fig. 8 Catalytic activity comparison of various [C₃PrIm][OTf]-MONSs

and reference solid acids in ethanolysis (a) and hydrolysis (b) of HMF

revealed by TOF_ELand TOF_LAvalues (min⁻¹), respectively. (a) n_EtOH

:

n_HMF= 248, HMF-to-catalyst weight ratio of 1 wt/wt, 120 °C, 40 min;

(b) n_{H O}: n_HMF= 417, 30 mg catalyst, 140 °C, 60 min.

2

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applied in the synthesis of EL/LA over CTATOS [C₃PrIm][OTf]-

MONS12. As shown in Fig. S7a,† in the ethanolysis of sugar

reactions, the conversion of sugar over CTATOS [C₃PrIm]

[OTf]-MONS12 reaches 98.1 (fructose), 94.7 (glucose) and

96.3% (sucrose), respectively, over a period of 240 min;

meanwhile, the selectivity of the catalyst to EL is 74.6

(fructose), 21.5 (glucose) and 39.3% (sucrose), respectively. In

the hydrolysis of sugar reactions, the conversion of sugar to

LA in water media over CTATOS [C₃PrIm][OTf]-MONS12

reaches 98.0 (fructose), 94.2 (glucose) and 96.8% (sucrose),

respectively, over a period of 240 min; meanwhile, the

selectivity of the catalyst to LA is 47.8 (fructose), 18.8

(glucose) and 31.6% (sucrose), respectively (Fig. S7b†). The

result indicates that CTATOS [C₃PrIm][OTf]-MONS12 can

transform not only HMF but also sugars in EtOH/H₂O media

rapidly and completely. The lower selectivity of the catalyst to

EL/LA (or lower EL/LA yield) by using sugars as the reactants

in comparison to that using HMF is due to the complicated

structure of sugars, and therefore the transformation of

sugars to EL/LA in EtOH/H₂O media suffers from a more

complex process, accompanied with the production of more

by-products. Additionally, for the transformation of glucose

and sucrose in EtOH/H₂O media, the poor EL/LA yield over

CTATOS [C₃PrIm][OTf]-MONS12 is due to the lack of Lewis

acid sites, which limits the transformation of glucose or

sucrose to fructose.

3.2.2. Catalytic stability and reusability. The issue of the

reusability and stability of immobilized BAIL catalysts has

been an essential investigative theme for catalyst research in

the field of biomass transformation. Under the harsh

reaction conditions for catalytic transformation of biomass or

biomass-derived platform molecules, immobilized BAILs may

suffer from leaching into the reaction media because of the

high solubility of BAILs in most of solvents; on the other

hand, strong adsorption of hydrophilic by-products on the

catalyst surface may also deteriorate the catalytic reusability.

In the current [C₃PrIm][OTf]-MONSs catalyzed HMF

ethanolysis reaction system, the color of the catalyst changes

from white to light yellow after the reaction. However, the

catalyst becomes dark brown after it catalyzes the HMF

hydrolysis reaction, which is due to the formation of more

humins during the complex HMF hydrolysis process. These

humins strongly adsorb on the surface of [C₃PrIm][OTf]-

MONSs, which results in difficulty in catalyst recycling.

Therefore, the reusability and stability of the [C₃PrIm][OTf]-

MONSs catalysts are evaluated in the HMF ethanolysis

reaction system by using CTATOS [C₃PrIm][OTf]-MONSs12 as

the representative catalyst. After each catalytic cycle, the

spent catalyst is recovered by centrifugation and then drying

at 100 °C for the next cycle. As shown in Fig. 9a, the

conversion of HMF reaches ca. 100% for each catalytic run

after the CTATOS [C₃PrIm][OTf]-MONSs12-catalyzed HMF

ethanolysis reaction proceeds at 120 °C for 40 min; at the

same time, the selectivity to EL slightly decreases from 42.7

(first run) to 42.4 (second run) and 38.5% (third run), while

the selectivity to EMF increases somewhat from 48.9 (first

Fig. 9 Catalytic reusability of the representative CTATOS [C₃PrIm]

[OTf]-MONSs12 in ethanolysis of HMF (a). Nitrogen gas adsorption–

desorption isotherm and pore size distribution curve (b) as well as the

TEM image (c) of the three times spent CTATOS [C₃PrIm][OTf]-

MONSs12. 0.278 mmol HMF, n_EtOH: n_HMF

weight ratio of 1 wt/wt, 120 °C, 40 min.

= 248, HMF-to-catalyst

run) to 49.8 (second run) and 51.5% (third run). Accordingly,

the total selectivity of the catalyst to EMF and EL remains

unchangeable (ca. 91.3%). Therefore, CTATOS [C₃PrIm][OTf]-

MONSs12 exhibits good stability in HMF ethanolysis

reaction, and the slightly decreased selectivity to EL is due to

the loss of a small amount of catalyst powder, which may

slightly decelerate the formation rate of EL. To account for

the good catalytic reusability of CTATOS [C₃PrIm][OTf]-

MONSs12, the textural properties and morphology of the

three times spent CTATOS [C₃PrIm][OTf]-MONSs12 are tested.

As shown in Fig. 9b, the three times spent CTATOS [C₃PrIm]

[OTf]-MONSs12 still displays well-defined sorption isotherms

with the pore-condensation step occuring at P/P₀= 0.4–0.8;

simultaneously, its mesopore size distribution is still

centered at 3.8 nm, consistent with that of its original state;

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however, its BET surface area and pore volume have

somewhat decreased as compared with those of the fresh

catalyst (551 m²g⁻¹and 0.94 cm³g⁻¹vs. 581 m²g⁻¹and 1.13

cm³g⁻¹, Table 1). From the TEM observation displayed in

Fig. 9c, it is found that the three times spent CTATOS [C₃-

PrIm][OTf]-MONSs12 still maintains perfect spherical

nanostructures with open interconnected mesopores, and the

adsorption of by-products is hardly observed on the surface

of the spent catalyst. The retention of excellent textural

properties and perfect morphology of the spent catalyst

ensures the good catalytic reusability; additionally, the

hydrophobic microenvironment of the silica/carbon

framework may avoid acid site deactivation due to strong

adsorption of hydrophilic humin by-products.

acidic nature. Among the various [C₃PrIm][OTf]-MONSs

catalysts, CTATOS [C₃PrIm][OTf]-MONSs12 with the smallest

particle size, superior textural properties and 3D wormhole-

like interconnected mesostructure shows the fastest HMF

transformation rate and significantly improved accessibility

for the reactants in both target reactions, which gives it the

highest conversion of HMF and selectivity to LA and EL. The

work suggests that the particle size- and pore morphology-

controlled preparation of immobilized BAILs is an effective

strategy to improve the catalytic activity and selectivity in

transformation of HMF to valuable chemicals such as EL and

LA.

Conflicts of interest

Finally, the catalytic stability of CTATOS [C₃PrIm][OTf]-

MONSs12 is tested by monitoring the leaching of the [C₃PrIm]

[OTf] group during the catalytic process. It shows that the

Brønsted acid site density (873 μeq g⁻¹) of the three times

spent CTATOS [C₃PrIm][OTf]-MONSs12 is almost the same as

that of the fresh catalyst. Additionally, the sulfur element

coming from the [C₃PrIm][OTf] group is not found by ICP-

OES determination in the reaction medium after three cycles

of ethanolysis reaction. Both results indicate the excellent

catalytic stability of CTATOS [C₃PrIm][OTf]-MONSs12. This is

mainly attributed to the chemical interaction between the [C₃-

PrIm][OTf] group and silica/carbon framework; additionally,

[C₃PrIm][OTf] groups are confined within the abundant

mesopores of organosilica nanospheres, which effectively

avoids their leaching. Therefore, as-prepared [C₃PrIm][OTf]-

MONSs can work as efficient and recyclable solid acid

catalysts in biomass transformation.

There are no conflicts to declare.

Acknowledgements

This work is financially supported by the National Natural

Science Foundation of China (22072016 and 21703030).

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Article Doi

Design of Br?nsted acidic ionic liquid functionalized mesoporous organosilica nanospheres for efficient synthesis of ethyl levulinate and levulinic acid from 5-hydroxymethylfurfural

DOI: 10.1039/d0cy01941k

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Article abstract of DOI:10.1039/d0cy01941k

Full text of DOI:10.1039/d0cy01941k

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