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Amberlyst-15 is the most active among all of the tested acid
catalysts. For the Amberlyst-15-, 7.2ArSO3H-Si(Et)Si-Ph-NTs-,
7.3ArSO3H-Si(Et)Si-NTs-, 6.8PrSO3H-Si(Et)Si-NTs-, and 6.7PrSO3H-
SBA-15-catalyzed ethanolysis reactions, the yield of ethyl levuli-
nate was 87.4, 85.0, 83.5, 78.1, and 75.2%, respectively, after
120 min. During the initial stage of the ethanolysis reaction,
the yield of ethyl levulinate increased rapidly in the homoge-
neous H2SO4-catalyzed reaction system. For example, the yield
of ethyl levulinate reached 63.5% after 30 min. However, with
a further increase of the reaction time to 120 min, the ethanol-
ysis activity remained unchanged. This may be because many
oligomeric condensation products of furfuryl alcohol can be
formed in the H2SO4-catalyzed ethanolysis reaction because of
the extremely high Brønsted acid strength and strong surface
hydrophilicity of H2SO4.[6] The highest yield of ethyl levulinate
obtained for the Amberlyst-15-catalyzed ethanolysis reaction is
because of its much higher Brønsted acid site density
[4800 meq(H+)gÀ1] and low surface hydrophilicity, which can
provide many more acid sites for the target reaction and de-
crease the absorption of oligomeric condensation byproducts
as well.
cohol is considered to better understand the excellent acid
catalytic activity of the multifunctional ArSO3H-Si(Et)Si-Ph-NTs
hybrid nanocatalysts.
In the ArSO3H-Si(Et)Si-Ph-NTs-catalyzed esterification reaction
(SchemeS1a), the strong Brønsted acidity of the catalyst can
protonate the carbonyl groups of levulinic acid molecules in
the first step to give oxonium ions that are readily attacked by
ethanol molecules through an exchange reaction to produce
ethyl levulinate molecules after the loss of one hydrogen
atom.[28,39]
The mechanism of the ArSO3H-Si(Et)Si-Ph-NTs-catalyzed etha-
nolysis reaction is also put forward based on the literature and
our previous work[40,41] as well as the identified intermediates
(Scheme S1b). Similar to the first step in the esterification reac-
tion, the furfuryl alcohol molecules are firstly activated by the
Brønsted acid sites, and the activated furfuryl alcohol mole-
cules are attacked by ethanol molecules to form 2-(ethoxyme-
thyl)furan and water. Next, intermediate A is formed by the
ring-opening reaction caused by the attack of water molecules
on the cyclic oxonium obtained by the protonation of epoxy
groups in the presence of Brønsted acid sites. Subsequently,
the formation of ethyl levulinate may proceed through the fol-
lowing possible pathways: intermediate A is isomerized with
the release of one hydrogen atom to form ethyl levulinate
(Path I). Intermediate A is isomerized and the produced com-
pound is activated by the Brønsted acid sites. After attack by
ethanol molecules, 4,5-diethoxy-5-hydroxypentan-2-one is
formed. One ethanol molecule is released from 4,5-diethoxy-5-
hydroxypentan-2-one followed by isomerization to form ethyl
levulinate; meanwhile, 4,5-diethoxy-5-hydroxypentan-2-one ac-
tivated by the Brønsted acid sites is further attacked by etha-
nol molecules, and 4,5,5-triethoxypentan-2-one is formed with
the release of water molecules (Path II). Intermediate A is iso-
merized and the produced compound is activated by the
Brønsted acid sites. After attack by ethanol molecules, 4,5,5-
triethoxypentan-2-one is formed. After the release of one di-
ethyl ether molecule from 4,5,5-triethoxypentan-2-one fol-
lowed by isomerization, ethyl levulinate is formed (Path III). In
the above reaction processes, the intermediates and byprod-
ucts (identified by GC–MS, Table S1 and Figure S3) are pro-
duced inevitably. The produced intermediates are 2-(ethoxyme-
thyl)furan (main), 4,5-diethoxy-5-hydroxypentan-2-one (minor),
and 4,5,5-triethoxypentan-2-one (minor), whereas the byprod-
ucts found are diethyl ether (nonproductive consumption of
ethanol) and some oligomeric products of furfuryl alcohol such
as difuran-2-ylmethane, 2-[(furan-2-ylmethoxy)methyl]furan,
and 2,5-bis(furan-2-ylmethyl)furan.
GC–MS analysis confirms that the intermediate 2-(ethoxyme-
thyl)furan was formed during the conversion of furfuryl alcohol
to ethyl levulinate. The yield of 2-(ethoxymethyl)furan de-
creased gradually with the increase of the reaction time from
30 to 120 min (Figure 8b), which implies that 2-(ethoxyme-
thyl)furan continued to be converted into ethyl levulinate. The
formation rate for 2-(ethoxymethyl)furan over various SO3H-
based catalysts is Amberlyst-15>7.2ArSO3H-Si(Et)Si-Ph-NTs>
7.3ArSO3H-Si(Et)Si-NTs>6.8PrSO3H-Si(Et)Si-NTs>6.7PrSO3H-SBA-
15. However, for the homogeneous H2SO4-catalyzed ethanoly-
sis reaction, 2-(ethoxymethyl)furan was hardly formed, which
suggests that ethyl levulinate and the oligomeric condensation
byproducts were formed rapidly in the H2SO4-catalyzed etha-
nolysis process.
If we consider the different acid site densities of the various
acid catalysts tested, the ethanolysis activity was compared in
terms of the yield of ethyl levulinate per acid site of each cata-
lyst (TOF) after 30 min. The tested catalysts follow the TOF
order 7.2ArSO3H-Si(Et)Si-Ph-NTs (18.3 hÀ1)>7.3ArSO3H-Si(Et)Si-
NTs (14.9 hÀ1)>6.8PrSO3H-Si(Et)Si-NTs (14.4 hÀ1)>6.7PrSO3H-
SBA-15 (13.4 hÀ1)>Amberlyst-15 (5.5 hÀ1)> H2SO4 (4.0 hÀ1
)
(Figure 8c). The lower TOF value and higher yield of ethyl levu-
linate for the H2SO4- and Amberlyst-15-catalyzed ethanolysis
reaction is because of the higher Brønsted acid site density.
Therefore, 7.2ArSO3H-Si(Et)Si-Ph-NTs is still catalytically active in
the ethanolysis reaction.
The strong Brønsted acidity of the SO3H-based silica nanohy-
brids, which includes the Brønsted acid site density and acid
strength, plays a key role to ensure that both of the target re-
actions proceed at a considerably fast rate. For the 4.8ArSO3H-
Si(Et)Si-Ph-NTs- and 7.2ArSO3H-Si(Et)Si-Ph-NTs-catalyzed esterifi-
cation and ethanolysis reactions, the lower catalytic activity of
4.8ArSO3H-Si(Et)Si-Ph-NTs than 7.2ArSO3H-Si(Et)Si-Ph-NTs is
mainly because of the lower Brønsted acid site density of the
former. Phenyl-free 7.3ArSO3H-Si(Et)Si-NTs and 6.8PrSO3H-
Si(Et)Si-NTs both possess a 1D hollow tubular nanostructure
Discussion
The excellent esterification and ethanolysis activity of the
ArSO3H-Si(Et)Si-Ph-NTs organic–inorganic hybrid nanocatalysts
can be explained by the combination of the strong Brønsted
acidity, unique hollow nanotube morphology, excellent porosi-
ty properties, and interesting hydrophobic surface.
Firstly, a possible mechanism of the Brønsted acid catalyzed
esterification of levulinic acid and the ethanolysis of furfuryl al-
ChemCatChem 2016, 8, 2037 – 2048
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