Design of organosulfonic acid functionalized organosilica hollow nanospheres for efficient conversion of furfural alcohol to ethyl levulinate

Article abstract of DOI:10.1039/c4gc02161d

A series of propylsulfonic or arenesulfonic acid functionalized ethane-or benzene-bridged organosilica hollow nanospheres (Pr/ArSO₃H-Et/Ph-HNS) were demonstrated by a one-step condensation method combined with in situ H₂O₂ oxidation. The key preparation route included using nonionic surfactant F127 as the soft template, 1,3,5-trimethylbenzene as the micelle expanding agent and adjusting suitable molar ratios of total Si to F127 in the initial gel mixture. The inner diameter of the Pr/ArSO₃H-Et/Ph-HNS materials was ca. 10 nm, and their shell thickness was ca. 3-5 nm. As the novel organic-inorganic hybrid solid acid catalysts, the catalytic activity of the Pr/ArSO₃H-Et/Ph-HNS was evaluated by ethanolysis of biomass-derived furfural alcohol to ethyl levulinate. The obtained acid catalytic activity that was superior to that of their bulk mesoporous counterparts was explained in terms of their strong Bronsted acidity and unique hollow nanospherical morphology. Finally, the recyclability of the hybrid catalysts was studied through four consecutive catalytic runs. This journal is

Full text of DOI:10.1039/c4gc02161d

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PAPER
Design of organosulfonic acid functionalized
organosilica hollow nanospheres for efficient
conversion of furfural alcohol to ethyl levulinate†
Cite this: DOI: 10.1039/c4gc02161d
a
a
a
b
c
c
Bo Lu, Sai An, Daiyu Song, Fang Su, Xia Yang and Yihang Guo*
A series of propylsulfonic or arenesulfonic acid functionalized ethane- or benzene-bridged organosilica
hollow nanospheres (Pr/ArSO H-Et/Ph-HNS) were demonstrated by a one-step condensation method
combined with in situ H oxidation. The key preparation route included using nonionic surfactant F127
as the soft template, 1,3,5-trimethylbenzene as the micelle expanding agent and adjusting suitable molar
ratios of total Si to F127 in the initial gel mixture. The inner diameter of the Pr/ArSO H-Et/Ph-HNS
materials was ca. 10 nm, and their shell thickness was ca. 3–5 nm. As the novel organic–inorganic hybrid
solid acid catalysts, the catalytic activity of the Pr/ArSO H-Et/Ph-HNS was evaluated by ethanolysis of
3
2 2
O
3
3
Received 6th November 2014,
Accepted 9th January 2015
biomass-derived furfural alcohol to ethyl levulinate. The obtained acid catalytic activity that was superior
to that of their bulk mesoporous counterparts was explained in terms of their strong Brønsted acidity and
unique hollow nanospherical morphology. Finally, the recyclability of the hybrid catalysts was studied
through four consecutive catalytic runs.
DOI: 10.1039/c4gc02161d
www.rsc.org/greenchem
Porous organosulfonic acid functionalized silica catalysts
1
. Introduction
generally possess large surface areas, uniform pore sizes and
1
Organosulfonic acid functionalized inorganic mesoporous high pore volumes, leading to their excellent catalytic activity.
silica or periodic mesoporous organosilica (PMO) materials However, their long pore channels may lead to the lengthened
represent an attractive alternative to the hazardous homo- diffusion distance of reactants and products, which can limit
geneous acids owing to their ease of separation, strong their catalytic activity to some extent because of insufficient
Brønsted acid properties and plentiful acid sites, and they accessibility of the acid sites. Moreover, most of biomass con-
have been widely applied in a wide variety of acid-catalyzed version-related processes perform under hydrothermal (or sol-
reactions including alkylation and acylation of hydrocarbons, vothermal) conditions. The harsh reaction conditions such as
condensation of methylfuran, hydration of alkenes, alcohols high temperature and high pressure often lead to the anchored
1
–4
coupling and polymerization of THF.
More recently, the sulfonic acid groups becoming unstable and leaching into the
threat of an oil shortage has been stimulating the search for reaction media; meanwhile, accumulation of carbonaceous
alternative feedstocks to fuels and fine chemicals, and there- materials on the catalyst surface always occurs. Both disadvan-
fore, efficient conversion of renewable biomass to fuels and tages lead to severe catalyst deactivation. Accordingly, develop-
valuable chemicals is one of the most popular topics in green ment of more efficient and stable organosulfonic acid
and sustainable chemistry. Organosulfonic acid functionalized functionalized silica materials for biomass conversion-related
silica materials are also of interest for these biomass conver- processes is still a challenge.
sion-related processes such as esterification, transesterifica-
For the purpose of further improvement of the hetero-
tion, hydrolysis and dehydration, showing better catalytic geneous acid catalytic activity and stability of –SO H-based
3
performance than conventional homogeneous and other catalysts, in the present work, a series of propylsulfonic or are-
5
–11
heterogeneous catalytic systems.
nesulfonic acid functionalized ethane- or benzene-bridged
organosilica hollow nanospheres (Pr/ArSO H-Et/Ph-HNS) are
3
prepared by a co-condensation method using nonionic surfac-
a
School of Chemistry, Northeast Normal University, Changchun 130024, P.R. China
Experiment center, Liaoning Normal University, Dalian 116029, P.R. China
tant F127 (EO106PO EO106) as the soft template and 1,3,5-tri-
70
b
methylbenzene (TMB) as the micelle expanding agent. Hollow
nanosphere materials with outstanding properties including
hollow interiors, permeable and thin shells, shortened
diffusion distance, low density, thermal and mechanical
c
School of Environment, Northeast Normal University, Changchun 130117,
P.R. China. E-mail: guoyh@nenu.edu.cn
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4gc02161d
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stability as well as excellent porosity have attracted particular acid with the corresponding alkyl alcohols in the presence of
2
4
interest, and they can serve as novel nanoreactors for various acid catalysts. However, levulinic acid has a high cost as raw
1
2–15
chemical transformations.
Generally, various active material for this purpose; additionally, yielded water may
species can be incorporated into the hollow interior of silica, decelerate the esterification reaction process. Accordingly, one
organosilica or carbon hollow nanospheres, however, com- obvious disadvantage is that a large amount of energy is
pared with the preparation of pure silica, organosilica or needed for water evaporation. The development of inexpensive
carbon hollow nanospheres, fabrication of silica-, organosilica- feedstock and a facile catalyst system to produce alkyl levuli-
or carbon-based hollow nanosphere hybrids functionalized by nates is therefore strongly demanded. An atom-economical
various catalytic active sites via a one-step co-condensation and convenient method for the synthesis of alkyl levulinates
route rather than a postsynthesis grafting method is more can be envisioned by alcoholysis of furfural alcohol in the
difficult, and there are only a limited number of research presence of acid catalysts. Conventional catalysts for the target
1
6–19
papers concerning this topic.
The preparation route reaction are inorganic liquid acids. They are efficient in the
should be designed carefully to ensure the structural integrity above process, but suffer from serious contamination and cor-
of the functionalities and the perfect hollow spherical struc- rosion problems that make essential the implementation of
ture of the nanohybrids. General approaches to the fabrication good separation and purification steps. A “green” approach to
of hollow nanosphere hybrids include soft/hard templates and synthesize alkyl levulinates stimulates the application of sus-
self-templating methods depending on the nature of the tem- tainable solid acids as replacements for such liquid acids so
1
9
plate. The hard template approach leads to hollow nano- that the use of harmful substances and generation of toxic
spheres with larger size (several hundreds-of nanometers) and wastes are avoided; meanwhile, the ease of catalyst separation
thicker walls (several tens-of nanometers), which may decrease after the reactions can be realized. Some conventional solid
3
2
the accessibility of active sites to the substrates and increase acids such as mesoporous aluminosilicates (e.g. Al-TUD-1),
the mass-transport of reactants and products. Otherwise, the macroporous sulfonic acid-functionalized polystyrene resins
3
3
soft template approach (or so-called single micelle templating) (e.g. Amberlyst-15 or -70),
microporous zeolites (e.g.
3
4
allows one to obtain hollow and spherical particles on the HZSM-5),
sulfonic acid-grafted/functionalized silica and
3
4
4 2
tens-of nanometers to several tens-of nanometers length scale carbon and SO super acid have been applied in the
2−/ZrO
23
and extremely thinner wall, thereby significantly facilitating above reactions. However, moderate yields of alkyl levulinates
the mass-transport. For example, mesoporous hollow silica were obtained; additionally, catalyst deactivation was found
and phosphosilicate nanospheres with uniform particle sizes during recycling of the solid acids. By the combination of
of several tens of nanometers have been prepared by a co- advantages including interesting morphologies, strong
condensation route in the presence of triblock copolymer Brønsted acid properties and hydrothermal stabilities, as-
3
micelles, and they have been successfully applied in drug prepared Pr/ArSO H-Et/Ph-HNS nanohybrids are expected to
2
0
21
storage and delivery.
Herein, F127-single micelle templated Pr/ArSO
exhibit unique heterogeneous acid catalytic performance in
H-Et/Ph- biomass conversion-related reactions.
3
HNS possesses an extremely thin shell and excellent textural
properties (e.g. very large BET surface area and hierarchical
porous structure). To evaluate the heterogeneous acid catalytic
3
activity and stability of the Pr/ArSO H-Et/Ph-HNS nanohybrids,
ethanolysis of biomass-derived furfural alcohol to produce
ethyl levulinate is selected as the model reaction. The catalytic
2
. Experimental
2.1. Materials
activity of the Pr/ArSO
pylsulfonic acid functionalized ethane-bridged organosilica silyl)benzene
materials with 3D interconnected (PrSO H- Et-3Dint) and cubic (EO106PO70EO106, where EO = –CH
PrSO H-Et-3Dcub) mesostructures. Therefore, the influence of CHO–, M = 12 600) are purchased from Sigma-Aldrich. 3-Mer-
Brønsted acid-site density and morphological and textural pro- captopropyltrimethoxysilane (MPTMS, 97%) is obtained from
perties on the heterogeneous acid catalytic activity of –SO H- J&KCHEMICA (China). 2-(4-Chlorosulfonylphenyl)ethyl tri-
3
H-Et/Ph-HNS is also compared with pro- 1,2-Bis(trimethoxysilyl)ethane (BTMSE, 97%), 1,4-bis(triethoxy-
(BTESB, 96%) and Pluronic F127
CH O–, PO = –CH (CH )-
3
2
2
2
3
(
3
W
3
based nanohybrids can be found out; meanwhile, commer- methoxysilane (CSPTMS, 50% in dichloromethane) is obtained
cially available sulfonic ion-exchange resin (Amberlyst-15) and from Gelest Inc. 1,3,5-Trimethylbenzene (TMB) is received
H-form zeolite (H-ZSM-5) are also tested under the same con- from Tianjin Guangfu fine chemical research institute (China).
ditions for comparison.
Furfural alcohol is purchased from Sinopharm Chemical
Ethyl levulinate is a member of the alkyl levulinates family Reagent Co. Ltd (China). Commercially available Amberlyst-15
that is interesting for different sectors of the chemical industry (Alfa Aesar) is used as a reference catalyst. H-ZSM-5 is prepared
2
2–28
(
e.g. polymers, flavour, fragrances and fuels).
Alkyl levuli- from calcination of ZSM-5 (ammonium) that was supplied by
nates are also versatile building blocks that can be used for the J&KCHEMICA (China) at 550 °C for 6 h. The above reagents are
synthesis of different chemicals and drugs via a chemical reac- used as purchased without further purification. All other
2
9–31
tion at the ester and keto groups.
In industry, alkyl levuli- chemicals are of analytical grade, and they are purchased from
nates are mainly obtained by direct esterification of levulinic Beijing Fine Chemical Co. (China).
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2
.2. Catalyst preparation
.2.1. Organosulfonic acid functionalized alkyl-bridged
2.2.2. Propylsulfonic acid functionalized ethane-bridged
organosilica with 3D sponge-like and cubic mesostructure
2
PrSO H-Et-3Dint. The preparation process was the same as
3
organosilica hollow nanospheres
3
that of PrSO H-Et-HNS but with a change of the molar ratio of
total silicon to F127 from 432 to 605; additionally, the starting
amount of BTMSE, MPTMS and TMB was 5.4, 1.2 and
PrSO H-Et-HNS. Typically, F127 (0.028 mmol) was dissolved
3
in HCl (2 mol L−1, 30 mL). After stirring the solution at 40 °C
for 1 h, TMB (2.9 mmol) was added dropwise, followed by con-
tinuous stirring for another 2 h. Subsequently, a selected
amount of BTMSE (5.7, 5.4, 4.8, 4.5 and 4.2 mmol, respect-
ively) was added. The resulting mixture was stirred for 45 min,
and then MPTMS (0.6, 1.2, 2.4, 3.0, and 3.6 mmol, respectively)
and 30 wt% H O (5.4, 10.8, 21.6, 27.0 and 32.4 mmol, respect-
2 2
ively) were added dropwise, successively. After homogenizing
the mixture at 40 °C for 24 h, the suspension was transferred
2.1 mmol, respectively. The determined –SO
3
H loading in the
PrSO H-Et-3D was 5.8%.
3
int
PrSO H-Et-3Dcub. The preparation process was the same as
3
3
that of PrSO H-Et-HNS but with a change of molar ratio of
total silicon to F127 from 432 to 756; additionally, the starting
amounts of BTMSE, MPTMS and TMB were 5.4, 1.2 and
1.7 mmol, respectively. The determined –SO
3
H loading in the
PrSO H-Et-3Dcub was 5.3%.
3
to an autoclave and heated at 100 °C with a heating rate of
2
tered, washed with deionized water and ethanol, and then air-
°C min− for an additional 24 h. Finally, the product was fil-
1
2.3. Catalyst characterization
dried at 60 °C in a vacuum oven overnight. F127 was removed Nitrogen gas adsorption/desorption isotherms were measured
from the product by washing with boiling ethanol under reflux on a Micromeritics ASAP 2020M surface area and porosity ana-
for 12 h. The process was repeated 3 times to ensure removal lyzer after the samples were outgassed under vacuum at 363 K
of most of the F127. The final product after being dried at for 1 h and 373 K for another 12 h. TEM analyses were per-
1
00 °C for 12 h was denoted as PrSO
3
H-Et-HNSx, where x rep- formed using a JEM-2100F high resolution transmission elec-
resented loading of the–SO H group.
tron microscope at an accelerating voltage of 200 kV. Before
3
In the above preparation, the total molar numbers of testing, the samples were suspended in ethanol using ultra-
silicon (2nBTMSE + nMPTMS) in the starting preparation system sound, after which a droplet of the suspension was dried on a
1
3
was unchangeable (12.0 mmol), and the molar ratio of H O to carbon film on a Cu grid. C cross polarization-magic angle
2
2
2
9
MPTMS was constant (9); changing the amount of BTMSE, spinning (CP-MAS) NMR and Si MAS NMR spectra were
MPTMS and H
was done to adjust the various loadings of recorded on a Bruker AVANCE III 400 WB spectrometer
the –SO H group in as-prepared hollow nanosphere hybrid equipped with a 4 mm standard bore CP MAS probe head. The
2 2
O
3
materials. Herein, x = 5.4, 9.0, 12.5, 15.8 and 17.0%, respect- dried and finely powdered samples were packed in a ZrO
ively, determined by ICP-OES.
rotor closed with a Ke–F cap and spun at a 12 kHz rate. Chemi-
2
1
3
29
ArSO H-Et-HNS. The preparation process was the same as cal shifts for C CP-MAS NMR and Si MAS NMR spectra
3
that described above but with replacement of MPTMS by were referenced to the signal of C H standard (δCH = 38.5)
1
0
16
2
CSPTMS (1.2 mmol) to provide ArSO
3
H groups directly, and and 3-(trimethylsilyl)-1-propanesulfonic acid sodium salt stan-
the starting amount of BTMSE was 5.4 mmol. The determined dard (δ = 0.0), respectively. Sulfur content or –SO H loading in
3
–
SO H loading in the ArSO H-Et-HNS was 7.4%.
as-prepared hybrid catalysts was determined by an ICAP6300-
3
3
PrSO
that of PrSO
3
H-Ph-HNS. The preparation process was the same as Thermoscientific ICP-OES, and therefore, the Brønsted acid-
+
−1
3
H-Et-HNS but using BTESB (5.4 mmol) instead of site density (AICP-OES, μeq(H ) g ) was calculated (see Table 1).
−
1
BTMSE as the organosilica precursor, and the starting amount Before determination, the samples were dissolved by 2 mol L
of MPTMS was 1.2 mmol. The determined –SO
the PrSO H-Ph-HNS was 5.9%.
3
H loading in NaOH solution and then diluted with deioned water to suit-
able volumes.
3
Table 1 Textural parameters and acid capacity of various SO
3
H-based hybrid catalysts
a
2
–1
b
c
3
–1
titration [μeq(H ) g−1]
d
+
A
ICP-OES [μeq(H ) g−1]
e
+
Catalysts
Et-HNS
S
BET [m g
]
D
p
[nm]
V
p
[cm g
]
A
615
596
568
529
486
487
537
648
569
451
7.5/47.1
7.6/83.4
7.5/159.5
6.4/98.2
6.4/81.7
6.4/45.0
6.4
4.9
6.4
6.4
0.51
0.55
0.79
0.83
0.94
0.91
0.64
0.72
0.36
0.26
—
692
1037
1438
1509
1643
920
699
691
681
—
665
1110
1545
1951
2100
911
734
716
654
PrSO
PrSO
PrSO
PrSO
PrSO
3
3
3
3
3
H-Et-HNS5.4
H-Et-HNS9.0
H-Et-HNS12.5
H-Et-HNS15.8
H-Et-HNS17.0
H-Et-HNS7.4
H-Ph-HNS5.9
H-Et-3Dint5.8
H-Et-3Dcub5.3
ArSO
3
PrSO
PrSO
PrSO
3
3
3
a
b
Surface area was calculated using the Brunauer–Emmett–Teller (BET) equation. Pore diameter (D
p
) was estimated from BJH adsorption
c
determination. Pore volume (V
P/P = 0.99 single point. From titration with NaOH solution (0.0092 mol L ). From ICP-OES determination.
0
p
) was estimated from the pore volume determination using the adsorption branch of the N
2
isotherm curve at
d
−1
e
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The Brønsted acid-site density of as-prepared hybrid cata-
lysts was also determined by acid–base titration (Atitration,
+
−1
−1
μeq(H ) g ) after ion exchange with 2 mol L of NaCl solu-
tion (10 g). Fresh catalyst powder (0.0500 g) was in contact
+
with the NaCl solution at 30 °C for 24 h. The evaluation of H
concentration was then performed after the obtained suspen-
sion cooling down to room temperature by titration with
.0092 mol L− of NaOH standard solution that had been
1
0
titrated with a standard potassium hydrogen phthalate solu-
tion (0.0100 mol L− ). Atitration could be calculated from the
consumed NaOH amount, and it was expressed by the number
1
+
of equivalents of H .
2
.4. Catalytic tests
The catalysts were dried for 2 h at 120 °C in a vacuum before
the catalytic tests. The ethanolysis reaction was performed at
1
20 °C in a Teflon-lined autoclave reactor with vigorous stir-
ring and held isothermally in an oil batch. The molar ratio of
furfural alcohol (0.1 mL) to ethanol (4 mL) was 1 : 60, and the
catalyst amount was 1.5 wt% with respect to the total amount
of reactants.
Scheme 1 Illustration of one-step preparation of propylsulfonic acid
functionalized ethane-bridged organosilica nanohybrids with hollow
sphere-like, 3D interconnected and 3D cubic morphologies.
After the reaction, all suspension in the autoclave was de-
canted into a 10 mL centrifuge tube, and the liquid products
were separated by centrifugation. The liquid products diluted
by acetone were analyzed by a Shimadzu 2014C gas chromato-
graph (GC) to obtain the concentration of the yielded ethyl levu-
linate. The GC was equipped with a HP-INNOWAX capillary
column (film thickness, 0.5 μm;i.d., 0.32 mm; length, 30 m)
and a flame ionization detector. The injection port temperature
was 250 °C, the oven temperature was maintained at 60 °C for
single micelle template and TMB as the micelle expanding
agent. After formation of a Pr/ArSO H-functionalized silica/
3
carbon (or alkyl-bridged organosilica) framework, it suffers
from hydrothermal treatment to further fasten the linkage of
silica/carbon framework with organosulfonic acid groups, and
hence cleavage of the Si–C bond in subsequent catalytic
process is expected to be avoided.
Co-condensation is a direct synthesis method, and it can
result in a more uniform distribution and a higher loading of
the functional groups compared to the post grafting procedure;
moreover, the bonding strength of functional groups to the
silica/carbon framework is high enough through covalently
attaching to prevent sulfur leaching. Additionally, this approach
5
8
min and then raised to 120 °C for 30 min at a heating rate of
°C min− , the flow rate of nitrogen gas was 1.0 mL min , and
1
−1
ethyl laurate was applied as an internal standard.
The catalytic activity of the catalysts was evaluated quanti-
tatively by the yields of ethyl levulinate (Y, %) and turnover fre-
quency (TOF, h− ), respectively. Herein, Y (%) = (M /M ) × 100,
1
D
T
2 2
allows in situ oxidation of thiol groups by H O with greater oxi-
where M
D T
and M are the number of moles of ethyl levulinate
1
−
1
dation efficiency compared with postsynthetic treatments. By
produced and expected, respectively; TOF (h ) = [M
D
/(Atitration
m)] × t− , where m (g) is the mass of the catalyst used in the
1
the combination of excellent textural properties and greater oxi-
dation efficiency, the directly prepared RSO H-functionalized
3
×
reaction and t (h) is the reaction time.
organosilica materials are expected to possess much higher acid
capacity than those achieved with postoxidative methods.
In general, the surfactants interacting with silica or organo-
silica sources may form bulk porous structures under the
aforementioned similar conditions, however, in the present
3
. Results and discussion
3
.1 Preparation and characterization of organosulfonic acid
functionalized alkyl-bridged organosilica hollow nanospheres
As illustrated in Scheme 1, organosulfonic acid functionalized through co-assembly between the hydrolyzed organosilica
alkyl-bridged organosilica hollow nanospheres, Pr/ArSO
H-Et/ species and F127 single micelle template by adjusting suitable
3
work, Pr/ArSO H-Et/Ph hollow nanospheres are fabricated
3
Ph-HNS, are prepared by co-condensation of both organic molar ratio of total silicon (2nBTMSE + nMPTMS) with respect to
building blocks, i.e., bis-silylated organic unit BTMSE or F127 micelles.
BTESB (main organic building block to construct silica/carbon
framework of the hollow nanosphere hybrids) with MPTMS acid loading on the morphology of organosulfonic acid func-
secondary organic building block to provide PrSO H sites tionalized alkyl-bridged organosilica hybrid materials is visibly
The influence of the Si/F127 molar ratio and organosulfonic
(
3
after in situ oxidation by 30% H
organic building block to provide ArSO
acidic conditions by using nonionic surfactant F127 as the prepared hybrid materials are composed of uniform spheres
2
O
2
) or CSPTMS (secondary revealed by TEM observations. From the result shown in Fig. 1
H sites directly) under it is found that at the total Si/F127 molar ratio of 432, all the
3
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Fig. 1 TEM images of various SO
3
H-based nanohybrid. (a) PrSO
3
H-Et-HNS5.4; (b) PrSO
3
H-Et-HNS9.0; (c) PrSO
3
H-Et-HNS12.5; (d) PrSO
H-Et-3Dcub5.3.
3
H-Et-
HNS15.8; (e) PrSO
3
H-Et-HNS17.0; (f) ArSO
3
H-Et-HNS7.4; (g) PrSO H-Ph-HNS5.9; (h) PrSO
3
3
H-Et-3Dint5.8 and (i) PrSO
3
with hollow interiors, regardless of the structure of organic
building blocks and loading of organosulfonic acid groups F127 single micelle-templated sulfonic acid-modified organo-
Fig. 1a–g). For propylsulfonic acid functionalized ethane- silica hollow nanospheres is explained in terms of relatively
3
Taking PrSO H-Et-HNS as an example, in situ fabrication of
(
bridged organosilica hollow nanospheres, the estimated slow hydrolysis and condensation rate of bridging organo-
average particle size, inner diameter and shell thickness are silanes and a low Si/F127 molar ratio, and it can extend to the
1
6, 10 and 3 nm, respectively (Fig. 1a–e); moreover, the particle nanohybrids with different kinds of organicsilica units in the
size and shell thickness are hardly affected by the loading of silicon/carbon framework or the incorporated organosulfonic
SO H group. As for the ArSO H-Et-HNS7.4 and PrSO H-Ph- acid groups (e.g. PrSO H-Ph-HNS and ArSO H-Et-HNS). At the
–
3
3
3
3
3
HNS5.9 obtained by the same route but different organic pre- beginning of preparation of sulfonic acid-modified organo-
cursors, they also possess uniform hollow and spherical mor- silica nanohybrids, spherical or cylindrical (depending on the
phology, however, their inner diameter (ca.
ArSO H-Et-HNS7.4 and 7 nm for PrSO
H-Ph-HNS5.9) is slightly dilute HCl solution (2 mol L− ) by copolymer assembly, and
smaller and shell is somewhat thicker (ca. 5 nm for ArSO H-Et- the micelles can further aggregate forming lyotropic liquid
9 nm for molar ratio of Si/F127) F127 single micelles are formed in a
1
3
3
3
HNS7.4 and ca. 4 nm for PrSO H-Ph-HNS5.9). Therefore, pro- crystal structures, such as ordered arrays of spheres or cylin-
3
pylsulfonic acid functionalized ethane-bridged organosilica ders. In the liquid crystal structures, hydrophobic PPO blocks
hollow nanospheres exhibit more perfect spherical structure form a core of the micelles, while hydrophilic PEO blocks form
compared with arenesulfonic acid functionalized ethane- a hydrated corona around the core. Subsequently, co-hydrolysis
bridged or propylsulfonic acid functionalized benzene-bridged and -condensation of BTMSE with MPTMS together with
organosilica hollow nanospheres. From Fig. 1 it is also in situ oxidation by H , HSO
observed that an increasing Si/F127 molar ratio (decreasing CH CH –Si(OSi) (OH) (n = 1–3) framework is formed.
3−n
2
O
2
3 2 2 2 n
CH CH CH –Si(OSi) (OH)3−n–
2
2
n
3
proportions of F127) from 432 to 605 or 756, and transition Assembly of this hydrolyzed PrSO H-modified organosilica
from an individual hollow nanospherical structure to a bulk species around the corona of individual F127 micelles via
mesoporous structure occurs for propylsulfonic acid functiona- supramolecular interactions results in the formation of supra-
lized ethane-bridged organosilica materials, and they exhibit molecular complexes
D sponge-like disordered (Si/F127 molar ratio of 605, Fig. 1h) CH CH –Si(OSi) (OH)3−n/EO106PO70EO106. After hydrothermal
and 3D cubic ordered (Si/F127 molar ratio of 756, Fig. 1i) treatment and subsequent boiling ethanol extraction (for
3 2 2 2 n
HSO CH CH CH –Si(OSi) (OH)3−n–
3
2
2
n
mesostructure, respectively.
3 2 2 2 3 2 2 3
F127 removing), HSO CH CH CH –Si(OSi) –CH CH –Si(OSi)
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(
abbreviated as PrSO
3
H-Et) is formed and constructs the shell nitrogen gas sorption isotherms featuring two capillary con-
framework of hybrid materials. For the prepared densation steps at a relative pressure P/P = 0.45–0.75 and P/P
0
0
HSO
3
CH
2
CH
2
CH
2
–Si(OSi)
3
–CH
2
CH
2
–Si(OSi)
3
material with = 0.90–0.99 (close to the saturation vapour pressure), respect-
H-Et-HNS
sufficiently low Si to F127 micelles molar ratio (i.e. 432), it ively. The result indicates that as-prepared PrSO
3
favors the formation of a hollow spherical nanostructure. At a nanohybrids possess a bimodal pore structure, contributing
lower Si/F127 molar ratio, the concentration of BTMSE or its from both the hollow interior and void space between the
partially hydrolyzed products on the surface of F127 micelles loosely arranged spherical particles.
is likely to be low, and therefore the cross-linking between the
organosilica/micelle nanoparticles is hindered, thus preventing reflected on pore size distribution curves shown in Fig. 2b.
the precursor-filled micelles from consolidating into periodic The smaller (primary) pore of the PrSO H-Et-HNS (7.6–6.4 nm
mesostructures. Increasing the Si/F127 molar ratio to 605 and depending on –SO H loading, Table 1) is well-distributed, fea-
The aforementioned interesting porosity properties are
3
3
7
56, affording a sufficiently high concentration of BTMSE or turing narrower curves, corresponding to the uniform hollow
its partial hydrolyzed products induces cross-linking between interior of the nanohybrids. The larger (secondary) pore
the organosilica/micelle nanoparticles, creating PrSO H-Et (159.5–45.0 nm, Table 1) is unevenly distributed, featuring
3
3
5
materials with interconnected 3D and a cubic mesostructure.
wider curves, corresponding to the void space between the
The textural properties of as-prepared sulfonic acid functio- loosely packed spherical particles. Similar porosity properties
nalized organosilica hollow nanospheres are evaluated by (Fig. 2c and d) are also found for the propylsulfonic acid func-
nitrogen gas porosimetry measurement, and for comparison, tionalized benzene-bridged organosilica hollow nanospheres
as-prepared PrSO
3 3 3
H-Et-3Dint5.8 and PrSO H-Et-3Dcub5.3 are (PrSO H-Ph-HNS5.9) and arenesulfonic acid functionalized
also tested. Fig. 2a and b display nitrogen gas adsorption/ ethane-bridged organosilica hollow nanospheres (ArSO H-Et-
3
3
desorption isotherms and BJH pore size distribution curves of HNS7.4). It should be noted that the primary pore of PrSO H-
propylsulfonic acid functionalized ethane-bridged organosilica modified benzene-bridged organosilica hollow nanospheres is
hollow nanospheres (PrSO H-Et-HNS) with various –SO H the smallest (4.9 nm, Table 1) among seven tested –SO H-
3
3
3
loadings. From Fig. 2a it can be seen that five tested based organosilica hollow nanospheres, which is consistent
PrSO H-Et-HNS (regardless of –SO H loading) show similar with TEM observations.
3
3
3
Fig. 2 Nitrogen gas adsorption–desorption isotherms (a, c) and BJH pore size distribution profiles (b, d) of various SO H-based nanohybrids.
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3 3
In the cases of PrSO H-Et-3Dint5.8 and PrSO H-Et-
3
Dcub5.3 hybrid materials, they both exhibit type IV isotherms
with a steep H2 hysteresis loop at the relative pressure P/P
0
=
0
3
.45–0.70 (Fig. 2c), characteristic of mesoporous materials with
D cage-like pore structure. From the BJH pore size distri-
bution curves shown in Fig. 2d it is found that both of the
porous PrSO H-Et hybrid materials possess uniform pore dia-
3
meter centered at 6.4 nm (Table 1).
The calculated textual parameters based on nitrogen gas
sorption isotherms are summarized in Table 1. Compared to
–
SO H-free organosilica hollow nanospheres, the BET surface
3
area and primary pore diameter of the functionalized organo-
silica hollow nanospheres decrease; moreover, for PrSO H-Et-
HNS nanohybrids, their BET surface area and pore diameter
decrease gradually on increasing –SO H loading from 5.4 to
5.8%, and on further increasing –SO H loading to 17.0%, the
BET surface area and pore diameter stop decreasing. As for the
ArSO H-Et-HNS7.4 and PrSO H-Ph-HNS5.9, they possess
textual parameter values similar to those of PrSO H-Et-HNS.
3
3
1
3
3
3
3
The above results indicate that most of Ar/PrSO H groups are
3
confined inside the nanospheres because spherical Pr/
ArSO
voids in the interior. Therefore, incorporation of a large
amount of Ar/PrSO H groups can block their interior to some
extent, which in turn leads to the decreased BET surface area
and pore diameter. However, the pore volume of the CH
PrSO H-Et-HNS nanohybrids increases with –SO
H loading condensation of BTMSE under acidic conditions followed by
increasing from 5.4 to 15.8%, probably due to their bimodal hydrothermal treatment; meanwhile, cleavage of Si–C bonds in
pore structure. As for the mesoporous PrSO H-Et-3Dint5.8 and the silica/carbon framework of PrSO H-Et-HNS12.5 is avoided.
In the C CP-MAS NMR spectrum of PrSO H-Et-HNS12.5, a
3
H-Et/Ph-HNS nanohybrids possess a large fraction of
29
13
Fig. 3 (a) Si MAS NMR and (b) C CP-MAS NMR spectra of the rep-
resentative SO H-based nanohybrid, PrSO H-Et-HNS12.5.
3
3
3
2
2
–CH –Si–O1.5–) is successfully created after hydrolysis and
3
3
3
3
1
3
PrSO
(
3
H-Et-3Dcub5.3, they possess a larger BET surface area
3
2
−1
569 and 451 m g ) and pore diameter (6.4 nm) but low pore remarkable peak at 5.1 ppm originates from the carbon
3
−1
1
volume (0.36 and 0.26 cm g ).
Through analyzing the above results, it is found that (–Si– CH
RSO
possess excellent textural properties including a very large BET CH
species
( C)
of
ethane-bridged
–Si–), further evidencing the formation of an
H-functionalized organosilica hollow nanospheres ethane-bridged organosilica framework (–Si–O1.5–Si–CH
–Si–O1.5–). The other resonance signals detected at 54.3,
), high pore volume 18.7 and 12.1 ppm belong to three types of carbon species (i.e.
organosilica
units
1
1
2
– CH
2
–
3
2
2
2
−1
surface area (486–648
(
m
g
3
−1
2
3
4
2
3
4
0.55–0.94 cm g ) and bimodal pore structure that are favor-
able to the catalytic reactions. Additionally, the Ar/PrSO H-site- SO
C, C and C) of propylsulfonic acid groups (– CH
2
CH
H), implying that propylsulfonic acid groups are success-
confined hollow interiors of the spherical nanohybrids can fully bonded on the ethane-bridged organosilica framework
provide a nanoreactor for the reactions, ensuring efficient reac- via –Si–O1.5–Si–CH –CH –Si–O1.5–CH CH CH –SO H covalent
tant diffusion and acid site accessibility during the reactions.
linkage, and this organic–inorganic hybrid framework con-
Formation of a silica/carbon (i.e. alkyl-bridged organosilica) structs shell structure of PrSO H-Et-HNS materials. In
framework and successful incorporation of organosulfonic addition, from the C CP-MAS NMR spectrum of PrSO H-Et-
CH –
2 2
3
6
3
3
2
2
2
2
2
3
3
1
3
3
acid groups in as-prepared sulfonic acid functionalized orga- HNS12.5 the resonance signals from F127 are not found, indi-
nosilica hollow nanosphere hybrids by current in situ prepa- cating that F127 is removed completely after boiling ethanol
2
9
ration route are confirmed by Si MAS NMR (Fig. 3a) and extraction.
1
3
C CP-MAS NMR (Fig. 3b), and PrSO H-Et-HNS12.5 is selected
The Brønsted acidity of as-prepared hybrid catalysts is
3
as the representative sample.
characterized by the Brønsted acid-site density that is deter-
In the Si MAS NMR spectrum of PrSO
3
H-Et-HNS12.5, the mined by titration with dilute NaOH solution (0.0092 mol L )
2
9
−1
predominant signals centered at −60.3 and −66.7 ppm can be and ICP-OES, and the results are summarized in Table 1. From
2
attributed to siloxane species of T [–CH
2
CH
2
–Si(OSi)
2
(OH)] the data of Atitration it is found that: (i) for propylsulfonic acid
3
2 2 3
and T [–CH CH –Si(OSi)
] within the ethane-bridged organo- functionalized ethane-bridged organosilica hollow nano-
silica framework. Additionally, there is no resonance signal spheres, their Brønsted acid-site density (692–1643 μeq(H )
species such as Q [Si(OSi)
, −120 ppm]. The result confirms to 17.0%; and (ii) at similar –SO
that the ethane-bridged organosilica framework (–Si–O1.5–Si– fonic acid functionalized benzene-bridged organosilica hollow
+
3
−1
assignable to inorganic SiO
4
3
(OH),
g
) increases gradually on increasing –SO
3
H loading from 5.4
4
−90 ppm] and Q [Si(OSi)
4
3
H loading (ca. 5%), propylsul-
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nanosphere PrSO
H-Ph-HNS5.9 (699 μeq(H ) g−1) has almost 3.2 Heterogeneous acid catalytic performance of
+
3
the same Brønsted acid-site density to PrSO H-Et-HNS5.4 organosulfonic acid functionalized alkyl-bridged organosilica
3
+
−1
(
692 μeq(H ) g ). Similar results can also be obtained for two hollow nanospheres
mesostructured PrSO H-Et-3Dcub5.3 (681 μeq(H ) g ) and
PrSO H-Et-3D 5.8 (691 μeq(H ) g ). Similar results can also
+
−1
3
3
.2.1 Heterogeneous acid catalytic activity. Pr/ArSO H-Et/
3
+
−1
3
int
Ph-HNS organic–inorganic hybrid materials in virtue of strong
Brønsted acidity, hollow nanospherical morphology and excellent
textural properties are expected to exhibit unique heterogeneous
acid catalytic performance. Herein, the heterogeneous acid cata-
be obtained from AICP-OES data. The results indicate that the
Brønsted acidity of as-prepared organosulfonic acid functiona-
lized alkyl-bridged organosilica nanohybrids is dominated by
loading of organosulfonic acid groups, and all tested spherical
and bulk mesoporous hybrid materials possess large BET
surface areas, and therefore, their Brønsted acid-site density
has little to do with their morphological properties.
The thermal stability of as-prepared organosulfonic acid
functionalized organosilica hollow nanosphere hybrids is
studied by TGA in the range of 25 to 600 °C, and the TGA plots
3
lytic performance of Pr/ArSO H-Et/Ph-HNS is evaluated by etha-
nolysis of furfural alcohol to produce ethyl levulinate (Scheme 2).
The synthesis of ethyl levulinate from ethanolysis of fur-
fural alcohol is conducted under the conditions of the molar
ratio of furfural alcohol to ethanol of 1 : 60, 1.5 wt% catalyst
and 120 °C, and the results are shown in Fig. 5. Fig. 5a
3
of the representative samples (PrSO H-Et-HNS5.4 and
PrSO H-Ph-HNS5.9) are depicted in Fig. 4. The TGA profile
3
shows two weight loss steps: the first weight loss (ca. 6%),
occurring below 100 °C is related to the loss of physisorbed
water at the hybrid material surface. The second weight loss
(
3 3
8.9% for PrSO H-Et-HNS5.4 and 8.6% for PrSO H-Ph-HNS5.9)
Scheme 2 Conversion of furfural alcohol to ethyl levulinate catalyzed
by the RSO H-R’-HNS materials. R = Pr/Ar; R’ = Et/Ph.
3
occurring at temperatures higher than 420 °C is attributed to
the decomposition of organic groups in the hybrid materials
3
including bridging ethane or benzene groups and PrSO H
groups. Therefore, as-prepared hybrid materials are thermally
stable below 420 °C.
Fig. 5 Catalytic activity of (a) PrSO
3
H-Et-HNS hollow nanospheres with
H-based nanohybrids
various –SO H group loadings and (b) various SO
3
3
with different morphologies towards ethanolysis of furfural alcohol to
produce ethyl levulinate. Conditions: molar ratio of furfural alcohol to
ethanol = 1 : 60, 1.5 wt% catalyst and 120 °C.
3 3
Fig. 4 TGA plots of (a) PrSO H-Et-HNS5.4 and (b) PrSO H-Ph-HNS5.9.
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Fig. 6 (a) Catalytic activity comparison of as-prepared SO
3
H-based hybrid catalysts in terms of per acid site (TOF). For comparison, Amberlyst-15
3 3
and H-ZSM-5 were also tested. Reaction time 60 min; (b) Recyclability of the representative SO H-based nanohybrid, PrSO H-Et-HNS9.0, towards
ethanolysis of furfural alcohol to produce ethyl levulinate. Conditions: molar ratio of furfural alcohol to ethanol = 1 : 60, 1.5 wt% catalyst, 120 min
and 120 °C.
presents the catalytic activity of PrSO H-Et-HNS hybrids with HNS5.9-, PrSO H-Et-3Dint5.8- and PrSO H-Et-3Dcub5.3-cata-
3
3
3
various –SO
7.0%) in the ethanolysis of furfural alcohol. It shows that the 69.0, 52.8, 46.0 and 39.1%, respectively, over a period of
yield of ethyl levulinate increases gradually for the PrSO H-Et- 60 min.
3
H group loadings (e.g. 5.4, 9.0, 12.5, 15.8 and lyzed ethanolysis reaction, the yield of ethyl levulinate reaches
1
3
HNS5.4-, PrSO
lyzed ethanolysis reaction; as for the PrSO
3
H-Et-HNS9.0- and PrSO
3
H-Et-HNS12.5-cata-
In order to evaluate the influence of morphologies on the
H-based hybrid
3
H-Et-HNS15.8- and catalytic activity of as-prepared various –SO
3
PrSO H-Et-HNS17.0-catalyzed ethanolysis reaction, the yield of materials more accurately, yields of ethyl levulinate per acid
3
H-Et- site (i.e. TOF, h− ) of the representative materials including
1
ethyl levulinate decreases slightly as compared to PrSO
HNS12.5. For example, after the reaction proceeds for 60 min, PrSO
the yield of ethyl levulinate reaches 52.5 (PrSO H-Et-HNS5.4), PrSO H-Et-3D 5.8 and PrSO H-Et-3Dcub5.3 are presented in
3
3
H-Et-HNS5.4, PrSO H-Ph-HNS5.9, ArSO H-Et-HNS7.4,
3
3
3
3
int
3
6
0.3 (PrSO
3
H-Et-HNS9.0), 74.3 (PrSO
3
H-Et-HNS12.5), 73.3 Fig. 6a. For comparison, commercially available Amberlyst-15
H-Et-HNS17.0), (a kind of excellent macroreticular solid acid catalyst with very
(PrSO H-Et-HNS15.8) and 72.0% (PrSO
3
3
respectively. Further increasing the reaction time to 120 min, high Brønsted acid-site density) and H-ZSM-5 are also given
all tested hybrid catalysts show similar yields of ethyl levuli- under the same conditions. It shows that three tested hollow
nate, i.e., 79.9, 79.1, 84.9, 82.4 and 81.4%, respectively, for spherical
nanohybrids
including
3
PrSO H-Et-HNS5.4,
PrSO H-Et-HNS5.4, PrSO H-Et-HNS9.0, PrSO H-Et-HNS12.5, PrSO H-Ph-HNS5.9 and ArSO H-Et-HNS7.4 exhibit almost the
3
3
3
3
3
PrSO
3
H-Et-HNS15.8 and PrSO
3
H-Et-HNS17.0.
3
same TOF values, regardless of the structure of –SO H groups
Influence of –SO
3
H group loading on the catalytic activity of and alkyl-bridged organosilica units; moreover, they are the
PrSO H-Et-HNS is explained in terms of the Brønsted acid-site most active among all tested catalysts towards the target reac-
3
density and porosity properties. For the PrSO
3
H-Et-HNS5.4, tion. As for two mesoporous counterparts, PrSO
3
H-Et-3Dint5.8
PrSO H-Et-HNS9.0 and PrSO H-Et-HNS12.5, they have similar and PrSO
3
3
3
H-Et-3Dcub5.3, they show similar activity, but their
BET surface area, and their activity difference is mainly deter- activity is obviously lower than their hollow spherical counter-
mined by their Brønsted acid-site density. However, for parts. In the case of Amberlyst-15, it has the smallest TOF
PrSO H-Et-HNS15.8 and PrSO H-Et-HNS17.0, the slightly value, although the yield of ethyl levulinate is the highest
3 3
decreased activity with respect to PrSO H-Et-HNS12.5 is attri- (92.4%) for Amberlyst-15-catalyzed ethanolysis reaction. This is
3
+
buted to their decreased BET surface area and pore diameter, attributed to its high Brønsted acid-site density (4800 μeq(H )
although they have much higher Brønsted acid-site density.
−
1 37
g ). The ethanolysis activity of H-ZSM-5 is also lower than
Therefore, it is concluded that the catalytic activity of that of Pr/ArSO H-Et/Ph-HNS.
3
PrSO
3
H-Et-HNS is closely related to both Brønsted acid-site
Combination of the results of Fig. 5 and 6a confirms that
Brønsted acid properties play a key role in the catalytic activity
density and porosity properties.
Fig. 5b presents the catalytic activity of two other hollow of as-prepared organosulfonic acid functionalized alkyl-
nanospherical hybrids (i.e. ArSO H-Et-HNS7.4 and PrSO H-Ph- bridged organosilica hybrid catalysts, which can ensure that
HNS5.9) and two mesostructured hybrids (i.e. PrSO H-Et- the target reaction proceeds at a fast rate; additionally, the
Dint5.8 and PrSO H-Et-3Dcub5.3). At similar –SO H loading, catalytic activity is also dominated to a great extent by their
3
3
3
3
3
3
3
hollow nanospherical hybrids show higher activity with respect morphological properties. For Ar/PrSO H-Et/Ph-HNS materials,
to their mesostructured counterparts towards the target reac- their unique hollow nanospherical morphology with shell
tion. For example, for the ArSO H-Et-HNS7.4-, PrSO H-Ph- thickness of several nanometers can serve as the nanoreactors
3
3
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where furfural alcohol ethanolysis reaction takes place. Accord- important because leaching of active sites under the harsh
ingly, the diffusion path of the reactants and products is shor- conditions and adsorption of carbonaceous byproducts on the
tened; moreover, the accessibility of acid sites to the reactants catalyst surface severely deteriorate catalytic activity. Herein,
can be increased. Both of the factors can speed up the mass PrSO
transport of the reactants and/or products, which results in study the regeneration and reusability of as-prepared hollow
the enhancement of the ethanolysis activity. nanospherical hybrid solid acid catalysts. After the first cataly-
3
H-Et-HNS9.0 is selected as the representative catalyst to
Additionally, the unique hollow nanospherical structure of tic run, the catalyst is recovered by centrifugation and then
Ar/PrSO H-Et/Ph-HNS leads to their excellent textural pro- washed thoroughly with ethanol at room temperature four
3
perties including bimodal porous structure, large BET surface times followed by drying in an oven at 100 °C in air for sub-
2
−1
3
area (486–596 m g ) and high pore volume (0.55–0.94 cm
g
sequent catalytic cycles. As shown in Fig. 6b, PrSO
3
H-Et-
−
1
), which can provide better dispersion of the acid sites HNS9.0 exhibits excellent stability and maintains a similar
throughout the hybrid catalysts and increase the population of level of activity after three reaction cycles (the yields of ethyl
the acid sites. Accordingly, the accessibility of furfural alcohol levulinate are 79.1, 76.5 and 71.4%, respectively, for the first,
molecules to the Ar/PrSO H sites also can be increased, having second and third cycles). After the fourth cycle, loss of the
3
a positive influence on the acid catalytic activity. However, for catalytic activity is obvious (the yield of ethyl levulinate is
the mesostructured PrSO
3
H-Et-3Dint5.8 and PrSO
3
H-Et- 60.4%). In order to find out the reason for activity loss, a series
3
4
Dcub5.3, although they have a large BET surface area (569 and of experiments and result analysis are performed. Firstly, the
51 m g ), their lengthened diffusion pathway (pore chan- reaction rate constants are extracted from the data of yield vs.
2
−1
nels of bulk mesoporous material are longer) and low pore time (Fig. 6b), and they are 0.5034, 0.5153, 0.5046 and
3
−1
−1
volume (0.36 and 0.26 cm
g
) limit their catalytic activity, 0.5086 h for the first, second, third and fourth cycles. It indi-
H-Et-HNS9.0 catalyst used for the fourth
leading to their lower acid-catalytic activity compared with cates that the PrSO
3
their hollow nanospherical counterparts.
time still maintains an initial rate similar to the catalyst used
2
6,38
According to the literature
and the intermediates for the first time. Secondly, the composition, chemical struc-
3
identified, the possible mechanism of synthesis of ethyl levuli- ture, and morphology of the PrSO H-Et-HNS9.0 catalyst used
1
3
nate from ethanolysis of furfural alcohol catalyzed over the Pr/ for the fourth time are characterized by ICP-OES, C CP-MAS
ArSO H-Et/Ph-HNS is presented (Scheme 3). In the first step, NMR and TEM. It shows that the –SO H loading in the used
3
3
hydroxyl groups of furfural alcohol molecules are protonated PrSO
3
H-Et-HNS9.0 is 8.9%, which is close to its initial value
3 3
by –SO H sites, which leads to oxonium ions (species A). The (9.0%); and leaching of –SO H group into the reaction
formed oxonium ions are readily attacked by ethanol mole- medium is trace (lower than 0.01%). Additionally, all charac-
cules to form ethoxymethylfuran (EMF, identified by GC-MS, teristic resonance signals concerning carbon species
see Fig. S1 of ESI†) and water; moreover, it is found that EMF is
a main intermediate produced during the conversion of furfural
alcohol to ethyl levulinate. Subsequently, EMF undergoes 1,4-
addition with ethanol to produce species B. After one ethanol
molecule is withdrawn from species B, species C is formed. Pro-
tonation of this diene yields the cyclic oxonium (species D).
Afterwards, the exocyclic oxonium (species E) is formed through
electron-pair transfer of species D. Finally, in the presence of
water molecules, species F is formed via ring-open reaction, and
then species F is isomerized to form ethyl levulinate.
3
.2.2. Regeneration and reusability. The stability of
anchored active sites and the reusability of the heterogeneous
catalysts in the liquid reaction system have been of concern for
the hybrid catalysts. Under hydrothermal or solvothermal con-
ditions, evaluation of the reusability of hybrid catalysts is more
13
Scheme 3 Proposed mechanism of synthesis of ethyl levulinate from
Fig. 7 (a) C CP-MAS NMR and (b) TEM image of the fourth time used
PrSO H-Et-HNS9.0 catalyst.
ethanolysis of furfural alcohol catalyzed over the Pr/ArSO
3
H-Et/Ph-HNS.
3
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1
(
(
C, 5.5 ppm) of ethane-bridged organosilica units Century Excellent Talents in University of Ministry of Edu-
1
1
2
3
4
–Si– CH – CH –Si–) and carbon species ( C, C and C, 54.3, cation of China (NCET-13-0723).
2 2
1
8.2 and 12.7 ppm) of propylsulfonic acid groups
2
3
4
(
– CH
2 2 2 3 3
CH CH –SO H) are found in the PrSO H-Et-HNS9.0
catalyst used for the fourth time (Fig. 7a). Finally, the hollow
nanospherical morphology of the PrSO H-Et-HNS9.0 catalyst
3
Notes and references
used for the fourth time remains intact (Fig. 7b). The above
results suggest that the physicochemical properties of the
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PrSO
3
H-Et-HNS9.0 catalyst used for the fourth time are
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retained, which can ensure its excellent catalytic stability, and
the activity drop after the fourth catalytic cycle is due to the
loss of the catalyst powder. The excellent reusability of as-
prepared organosulfonic acid functionalized organosilica
hollow nanospheres is due to strong chemical interactions
between organosulfonic acid groups and the silica/carbon
framework; meanwhile, the hydrophobic character of alkyl-
containing silica can reduce the acid site deactivation associ-
ated with adsorption of polar byproducts. Therefore, these
hybrid materials work effectively as recyclable solid acid cata-
lysts in the hydrolysis of furfural alcohol to yield ethyl
levulinate.
7 J. A. Melero, L. F. Bautista, G. Morales, J. Iglesias and
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