Q. Zhang et al.
Applied Catalysis A, General 611 (2021) 117973
reaction is facilitated correspondingly. Similar to transesterification
reaction, esterification of Gly with LA also starts from the protonation of
provide a low degree of pore opening and strong diffusion and mass-
transport limitation [72], leading to the inaccessibility of the acid sites
to bulky reactant molecules and thereby poor transesterification and
esterification activity.
3
carbonyl groups of LA molecules by the Ar/PrSO H sites, followed by
nucleophilic addition of oxygen atoms of glycerol molecules with oxo-
nium ions in LA [71].
Finally, the incorporation of bridging ethyl groups within the silica/
carbon framework can increase the surface hydrophobicity of the Ar/
On the basis of the above reaction mechanism, the trans-
esterification/esterification activity of the Ar/PrSO
3
H–Si(Et)Si nano-
PrSO
3
H–Si(Et)Si nanotubes, which also influence their trans-
tubes strongly depends on their Brønsted acid nature; additionally, the
morphological characteristics, porosity properties and surface hydro-
phobicity influence the accessibility of the acid sites to bulky reactant
molecules and thus the activity in some extent.
esterification and esterification activity positively. For TP trans-
esterification reaction, the hydrophobic environment in the tube
channels is in favor of enrichment of hydrophobic reactant, while the
hydrophilic byproduct is expelled. As a consequence, the formation of
MP is accelerated. Additionally, after mixing hydrophobic TP and hy-
Firstly, the intrinsic strong Brønsted acid nature of the Ar/PrSO
Et)Si nanotubes plays the key role to boost the target reactions. With
the assistance of the protons from the Ar/PrSO
3
H–Si
(
drophilic MeOH with hydrophobic Ar/PrSO
lyst powder under stirring, we observe that a homogeneous Pickering
emulsion is formed, in which TP and Ar/PrSO
H–Si(Et)Si nanotube
catalyst show good miscibility. Accordingly, the improved accessibility
of TP to the acid sites of Ar/PrSO
H–Si(Et)Si nanotube catalyst can be
obtained, leading to little mass transfer limitation of the Ar/PrSO
H–Si
3
H–Si(Et)Si nanotube cata-
3
H–Si(Et)Si nanotubes,
superstrong Brønsted acidity with high acid site density can activate
sufficiently the bulky reactant molecules and further assure the trans-
esterification/esterification reaction proceeds rapidly. Lower trans-
3
3
esterification/esterification activity of the PrSO
than the corresponding ArSO
H–Si(Et)Si counterparts is due to their
weaker Brønsted acid strength.
Secondly, the morphological characteristics and porosity properties
of the Ar/PrSO
H–Si(Et)Si nanotubes influence their transesterification/
esterification activity obviously. One-dimensional tubular Ar/
PrSO
H–Si(Et)Si nanocatalysts possess outstanding advantages such as
3
H–Si(Et)Si nanotubes
3
3
(Et)Si catalyst system [73–75]. As for the esterification of Gly with LA,
hydrophobic surface of the nanotubes is benefit to adsorb hydrophobic
LA molecules; meanwhile, byproduct water escapes from the catalyst
surface. Accordingly, the formation of glycerol laurate esters is pro-
moted; meanwhile, hydrolysis of glycerol laurate esters by water can be
inhibited obviously.
3
3
open and flexible structure, thin wall, tunable inner diameter and
length, high dispersity, high pore volume as well as large BET surface
area, and the most of the acid sites situate at the interior of the nano-
tubes. These acid site-confined nanotubes with large inner diameter can
serve as the nanoreactors to allow free diffusion and fast mass-transport
of the reactants and products, particularly for bulky reactants (TP and
Gly) and products (MP, MLG and DLG). In addition, large BET surface
3.3.2. Catalytic stability and reusability
The issue of acid catalyst reusability and stability has been a central
investigative theme for catalyst research in the area of biomass con-
version. The acid catalysts containing sulfonic acid active sites, such as
polymeric resins and sulfonated carbon materials, are susceptible to
breakdown during biomass conversion because the process generally
carries out under harsh conditions. Under these conditions, sulfonic acid
groups are readily hydrolyzed, leading to leaching to the reaction media.
The other reason of the deterioration of the catalyst is strong adsorption
of carbonaceous products and byproducts on the catalyst surface. To
evaluate the catalytic reusability and stability of as-prepared nano-
3
area of the nanotubes can help the Ar/PrSO H groups homogeneously
dispersed throughout the silica/carbon framework, giving rise to high
population of the exposed acid sites confined in the surface of the
nanotubes and thus of high accessibility of the acid sites to the reactants.
These advantages positively influence the catalytic activity of the Ar/
PrSO
tubes show the smallest inner diameter of the tubes (5 nm) among ten as-
prepared Ar/PrSO H-Si(Et)Si nanotubes, their inner diameter and pore
3
H–Si(Et)Si nanotubes. Although the Ar/PrSO
3
H-Si(Et)Si-1 nano-
catalysts, ArSO
3
H–Si(Et)Si-1 is selected, and it catalyzes TP trans-
esterification and Gly esterification reactions being repeated for five
times. After each catalytic cycle, the recovered catalyst is washed by
3
◦
diameter are still large enough for free diffusion and fast mass-transport
of bulky reactants (TP and Gly) and products (MP, MLG and DLG) within
the nanotubes. The highest transesterification and esterification activity
boiling ethanol and then dried at 80 C. As shown in Fig. 8a, after the
ArSO
3
H–Si(Et)Si-1-catalyzed TP transesterification reaction proceeds
for 12 h, the yield of MP is 97.9 (1 st run), 94.7 (2nd run), 94.1 (3rd run),
92.6 (4th run) and 95.0 % (5th run), respectively. In the case of the
of the Ar/PrSO
to their largest BET surface area. In the cases of other four pairs of the
Ar/PrSO
H–Si(Et)Si nanotubes, their BET surface areas are similar, and
3
H–Si(Et)Si-1 nanotube among their counterparts is due
ArSO
3
H–Si(Et)Si-1-catalyzed Gly esterification reaction, over period of
3
60 min, the total yields of MLG and DLG reach 93.7 (1 st run), 95.4 (2nd
run), 90.1 (3rd run), 93.3 (4th run) and 93.1 % (5th run), respectively
their transesterification and esterification activity are closely related to
the inner diameter and length of the nanotubes. Namely, larger inner
diameter and shorter tube length can accelerate the diffusion and mass
transport of bulky reactants and products significantly, originating from
the enlarged interior channel of the nanotubes and shortened mass
(Fig. 8b). The results suggest that ArSO
3
H–Si(Et)Si-1 nanotubes exhibit
excellent catalytic reusability in both target reactions, and the activity
loss is hardly observed after five consecutive cycles.
To account for excellent catalytic reusability of the Ar/PrSO
3
H–Si
transport distance. Accordingly, the Ar/PrSO
3
H–Si(Et)Si-5 nanotubes
(Et)Si nanotubes, leaching of sulfonic acid groups from the catalyst to
reaction media is firstly tested. After digesting the separated clear re-
with the largest inner diameter and the shortest tube length exhibit
, the concentration of SO2
–
comparable transesterification and esterification activity to the Ar/
action solution by HNO -HClO ion in the
3 4 4
PrSO
3
H–Si(Et)Si-1 counterparts, which compensate their disadvantage
reaction solution is determined by ion chromatography. It shows only
0.85 % of sulfonic acid groups are found to drop to the reaction media
during the transesterification process, while the loss of sulfonic acid
groups is hardly detected during the esterification process. The result
provides one of the important evidence to confirm the excellent catalytic
of smaller BET surface areas. The similar explanation can also be found
in Gong’s and Liu’s work, and they pointed out that short nanotubes are
of interest in catalysis [57,58].
On the basis of the above discussion it is also inferred that higher BET
surface area and larger inner diameter of the ArSO
than their PrSO
3
H–Si(Et)Si nanotubes
stability of the Ar/PrSO
3
H–Si(Et)Si nanotubes. Additionally, the struc-
3
H–Si(Et)Si counterparts are the other reasons that
ture of the fifth time spent ArSO
3
H–Si(Et)Si-1 nanotubes is characterized
1
3
13
dominate their higher transesterification and esterification activity than
the latter. Additionally, the inferior transesterification and esterification
by C CP-MAS NMR method. As displayed in Fig. 8c and d, C CP-MAS
NMR spectra of the spent catalyst exhibits all characteristic signals
2
activity of Amberlyst-15 is attributed to its low BET surface area (50 m
concerning about various carbon species existed in the fresh ArSO
(Et)Si-1 nanotubes, indicating that the chemical structure of the
ArSO
H–Si(Et)Si-1 nanotubes remains intact after five consecutive cat-
alytic cycles. Additionally, for the fifth time spent ArSO
H–Si(Et)Si-1
3
H–Si
ꢀ
1
g
) although it has strong Brønsted acid nature, which results in it low
exposing degree of the acid sites to the reactants. As for acidic ZSM-5, its
3
3
ꢀ 1
micropore size (0.5 × 0.5 nm) and low pore volume (0.15 cm g ) only
3
1
1