Efficient synthesis of 3-methylindole using biomass-derived glycerol and aniline over ZnO and CeO2 modified Ag/SBA-15 catalysts

Article abstract of DOI:10.1016/j.mcat.2020.111038

An efficient mesoporous catalyst of Ag/SBA-15 modified with ZnO and CeO₂ was successfully constructed with the purpose of efficiently synthesizing 3-methylindole by biomass-derived glycerol and aniline, which up to 62% yield and 75% selectivity for 3-methylindole were achieved when Ag loading was 1.00 mmol/g^?1, ZnO or CeO₂ content was 1.00 or 0.05 mmol/g^?1, respectively. And only 3% yield decreased when the catalyst was circulated five times. The characterizations researches on N₂ physical adsorption, Fourier transform infrared (FT-IR), scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDX), temperature programmed reduction of hydrogen (H₂-TPR), X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature programmed desorption (TPD) of NH₃ and CO₂, thermogravimetric and differential thermal analysis (TG-DTA) indicated that adding the promoter of ZnO to Ag/SBA-15 increased the dispersion of the silver particles significantly and controlled the aggregation of Ag nanoparticles during the reaction remarkably because doping ZnO greatly increased the polarity of the composite carrier SBA–15–ZnO and brought about the interaction between Ag and carrier enhanced. CeO₂ could promote the reduction of Ag₂O and suppress the formation of carbon deposition effectively. In addition, doping ZnO and CeO₂ increased the number of the weak-acid centers of the SBA-15 supported Ag-based catalyst observably, thereby the catalyst of Ag/SBA-15-ZnO-–ZnO–CeO₂ acquired a good selectivity. Moreover, the reaction pathway for 3-methylindole synthesis by biomass-derived glycerol and aniline was probed in depth and a reasonable route was proposed.

Full text of DOI:10.1016/j.mcat.2020.111038

MolecularCatalysis493(2020)111038
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Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Efficient synthesis of 3-methylindole using biomass-derived glycerol and
aniline over ZnO and CeO2 modified Ag/SBA-15 catalysts
Yi Qu, Yining Gao, Shuyi Lin, Lei Shi*
College of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian, 116029 Liaoning, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
An efficient mesoporous catalyst of Ag/SBA-15 modified with ZnO and CeO2 was successfully constructed with
the purpose of efficiently synthesizing 3-methylindole by biomass-derived glycerol and aniline, which up to 62%
yield and 75% selectivity for 3-methylindole were achieved when Ag loading was 1.00 mmol/g−1, ZnO or CeO2
content was 1.00 or 0.05 mmol/g−1, respectively. And only 3% yield decreased when the catalyst was circulated
five times. The characterizations researches on N2 physical adsorption, Fourier transform infrared (FT-IR),
scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDX), temperature programmed
reduction of hydrogen (H2-TPR), X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature
programmed desorption (TPD) of NH3 and CO2, thermogravimetric and differential thermal analysis (TG-DTA)
indicated that adding the promoter of ZnO to Ag/SBA-15 increased the dispersion of the silver particles sig-
nificantly and controlled the aggregation of Ag nanoparticles during the reaction remarkably because doping
ZnO greatly increased the polarity of the composite carrier SBA–15–ZnO and brought about the interaction
between Ag and carrier enhanced. CeO2 could promote the reduction of Ag2O and suppress the formation of
carbon deposition effectively. In addition, doping ZnO and CeO2 increased the number of the weak-acid centers
of the SBA-15 supported Ag-based catalyst observably, thereby the catalyst of Ag/SBA-15-ZnO-–ZnO–CeO2 ac-
quired a good selectivity. Moreover, the reaction pathway for 3-methylindole synthesis by biomass-derived
glycerol and aniline was probed in depth and a reasonable route was proposed.
3-Methylindole
Ag/SBA-15 catalyst
ZnO and CeO2 promoters
Biomass-derived glycerol
Reaction pathway
1. Introduction
added chemical of 3-methylindole [21]. It’s well known that indole and
its derivatives are important fine chemicals widely used in the pro-
With the increasing shortage of fossil fuels, the efficient use of re-
newable biomass resources to partially replace fossil fuels has received
widespread attention [1–4]. Biodiesel, as one of the typical re-
presentatives of biofuels and a green energy source, has the advantages
of renewable, degradable, and low pollution [5–7]. And in recent years,
the research concerning the production of biodiesel have become of a
great interest [8]. However, the production of biodiesel would emerge a
large amount of by-product glycerol [9,10]. How to effectively convert
biomass-derived glycerol to high value-added chemicals is a topic of
great concern to chemical researchers [11], which can improve the
sustainability and economic viability of biodiesel production. Delight-
fully, many value-added chemicals have been successfully produced
from biomass-derived glycerol by various methods [12–14]. For ex-
ample, hydrogenolysis, hydrodeoxygenation, dehydration, etherifica-
tion, transesterification and polymerization, etc. [15–20].
duction of drug, pesticides, spices and dye [22–26]. 3-Methylindole, a
noted compound of indoles, has an extensive application in many fields
due to its unique structure. For example, it can be used to prepare
antihypertensive drugs, stimulants, analgesics, and anticancer drugs,
etc. [27,28].
At present, there are many methods to synthesize 3-methylindole.
Among them, the most famous method is the Fischer synthesis.
Simoneau et al. [29] reported the Fischer cyclization of 1-nitropropane
and phenylhydrazine catalyzed by sulfuric acid and a 50% yield of
target product was obtained. Kanchupalli et al. [30] proposed the
transannulationin from pyridazine N-oxide and pyrrole to produce 3-
methylindole over Rh2(esp)2 with the yield of 67%. Moreover, Cam-
panati et al. [31] carried out the catalytic reaction with aniline and 1,2-
propanediol as raw materials in a fixed bed at atmospheric pressure
over ZrO2/SiO2, however 3-methylindole yield was merely 12%. Since
there are many disadvantages for the current synthesis of 3-methy-
lindole such as a high cost of reactants or catalysts, a low yield of target
In our recent study, it has been found that biomass-derived glycerol
with aniline could be used as raw materials to synthesize a high value-
⁎ Corresponding author.
E-mail address: shilei515dl@126.com (L. Shi).
https://doi.org/10.1016/j.mcat.2020.111038
Received 23 February 2020; Received in revised form 20 May 2020; Accepted 21 May 2020
2468-8231/©2020ElsevierB.V.Allrightsreserved.

Y. Qu, et al.
MolecularCatalysis493(2020)111038
product, a complicated experimental operation or the use of a large
amount of toxic solvents, etc, it is therefore, essential to develop an
efficient synthesis method of 3-methylindole to meet the requirements
of cost-saving as well as environmentally friendly. Using aniline and
biomass-derived glycerol as raw materials to synthesize 3-methylindole
is in line with the concept of atom economy and is currently the most
promising method because of a low cost and environmental friendli-
ness. From our indepth research on the reaction in recent years, it has
been found that the catalyst carrier should have a large specific surface
area and a large number of weak-acid centers to ensure its high activity
and good selectivity [21,32,33].
The catalyst of Ag/SBA-15-ZnO was synthesized by sequential im-
pregnation. First, a certain amount of SBA-15 carrier was soaked in a
certain concentration of aqueous solution of Zn(NO3)2 for 15.5 h. After
dried at 393 K, the mixture was calcined at 873 K for 4.0 h to obtain
SBA-15–ZnO. The active component was then loaded on the modified
carrier of SBA-15–ZnO and the process was the same as the preparation
of silver catalyst described above.
Ag/SBA-15–ZnO–CeO2 was also prepared by the same method of
sequential impregnation. The process was the same as the preparation
of the catalyst of Ag/SBA-15–ZnO.
Ag loading to all the SBA-15 supported silver-based catalysts was
Mesoporous materials are greatly favored in the field of catalysis
due to their unique structure and properties [34,35]. Among them,
mesoporous silica SBA-15 is a good alternative as carrier of hetero-
geneous catalysts [36–38] because of its large specific surface area,
good hydrothermal stability, adjustable pore size and regular pore
structure, etc [39–42]. And now it has been successfully applied in
many catalytic reactions [43–45]. ZnO and CeO2 additives have ex-
cellent promotion effect in many catalytic reactions [46–49]. Therefore,
in this work, SBA-15 was proposed as the carrier to prepare the Ag-
based catalysts for the catalytic reaction of glycerol and aniline to
synthesize 3-methylindole. To improve the performance of the catalyst,
ZnO and CeO2 promoters were added to Ag/SBA-15 catalyst in se-
quence and the effects of ZnO and CeO2 were investigated thoroughly
by N2 physical adsorption, FT-IR, SEM-EDX, H2-TPR, XRD, TEM, NH3/
CO2-TPD, TG-DTA. The reaction pathway of 3-methylindole synthesis
on Ag/SBA-15–ZnO–CeO2 was further studied meticulously and a rea-
sonable route was proposed.
1.00 mmol/g−1
.
2.3. Evaluation of catalysts
The catalytic activity and selectivity of Ag-based catalyst were
measured in a fixed-bed continuous flow glass reactor. The catalytic
reaction was proceeded at 483 K. The space velocity (SV) of the reac-
tion was 1700 h−1, and the liquid hourly space velocity (LHSV) of
aniline and glycerol was 0.4 h−1. The reactant solution (glycerol/ani-
line molar ratio of 1:3) pre-heated in a buffer bottle to evaporate was
carried into the catalyst bed with H2 (15 mL min−1), N2 (9 mL min-1)
and steam (16 mL min-1).
2.4. Analysis of products
The qualitative analysis of 3-methylindole was proceeded on a 500
superconducting NMR spectrometer of Bruker. The other products were
qualitatively detected by Shimadzu QP2010 GC–MS instrument with
DB-5MS, in which the temperature was changed from 373 to 533 K at a
15 K min−1 temperature increasing rate. The reaction products were
quantitatively determined by gas chromatography with HP-5 capillary
column and N-hexanol was the internal standard.
2. Experimental
2.1. Materials
Silver nitrate (AgNO3, ≥99.0%), zinc nitrate (Zn(NO3)2∙6H2O,
≥99.0%), cerium nitrate (Ce(NO3)3∙6H2O, ≥99.0%), aniline
(C6H5NH2, ≥99.5%), and n-Hexanol (C6H13OH, ≥95.0%) were ob-
tained from Tianjin Damao Chemical Reagent Co., Ltd. (China).
Tetraethyl orthosilicate (TEOS, (C2H5O)4Si, ≥98.0%), hydrochloric
acid (HCl, 36–38%), glycerol (C3H8O3, ≥99.0%), acetol
(C3H6O2, > 99.0%) and 1,2-propanediol (C3H8O2, > 99.0%) were ob-
tained from Tianjin Kemiou Chemical Reagent Co., Ltd. (China).
Pluronic P123 (Mn = 5800, EO20PO70EO20) were obtained from
Beijing Reagent Co., Ltd. (China). The above chemicals were not pur-
ified before use.
Glycerol conversion, 3-methylindole yield or selectivity was calcu-
lated as follows:
no nt
no
Conv. (%) =
× 100%
np
no
Yield(%) =
× 100%
Yield
Conv.
Sel. (%) =
× 100%
Here no or nt was the amount of initial or final glycerol, respectively.
And np was the amount of the target product.
2.2. Preparation of SBA-15 carrier and silver-based catalysts
2.5. Characterization of catalysts
SBA-15 carrier was hydrothermally prepared according to the lit-
erature [50] with a minor modification. A 4 g of P123 (EO20PO70EO20
)
The N2 adsorption-desorption measurement of SBA-15 or its sup-
ported Ag-based catalyst was measured using a physical adsorption
instrument of Micromeritics ASAP 2010 at 10−5 kPa, where the sample
was evacuated and purify at 573 K for 3.0 h. The specific surface area of
sample was calculated with the Brunauer–Emmett–Teller (BET)
method. The relative pressure (P/P0) of 0.05–0.30 was selected on the
adsorption isotherm.
was dispersed in 160 mL (1.5 M) hydrochloric acid solution. After the
solution was stirred thoroughly with the magnetic stirrer until the
surfactant was completely dissolved and uniformly dispersed, a 8.5 g of
TEOS was dissolved in the above solution, and stirred at 313 K for 24.0
h using a magnetic stirrer. Then the miscible liquids underwent an
additional aging at 353 K for 24.0 h, and the suspension was filtered,
washed. The sample was air-dried and calcined at 773 K for 6.0 h in a
muffle furnace. Finally, the sample of SBA-15 was obtained. From Fig. 1
it can be known that the SBA-15 carrier was successfully prepared,
which had a P6mm symmetrical hexagonal structure [51].
The mid-infrared analysis of sample was executed on the Nicolets50
Fourier transform infrared spectrometer. The scanning range was
2000–400 cm−1
.
The compositions of Ag/SBA-15–ZnO–CeO2 were examined using
field-emission scanning electron microscopy of Germany Carl Zeiss
Supra55 with energy-dispersive X-ray spectroscopy (EDS) of UK Oxford
X-MaxN. The acceleration voltage was 15 kV.
The catalyst of Ag/SBA-15 was obtained by equal volume impreg-
nation method. A certain amount of SBA-15 carrier was soaked into a
certain concentration of an aqueous solution of AgNO3 for 15.5 h in air.
After the impregnation, the sample was dried at 393 K, and finally
calcined at 773 K for 4.0 h. The catalyst was reduced at 453 K for 2.0 h
with a mixture flow of H2 (15 mL min−1)–N2 (15 mL min−1) before the
catalytic reaction.
H2-TPR measurement was proceeded in the 6 mm × 350 mm quartz
tubular reactor. 100 mg catalyst precursor was filled and reactor was
placed in the heating furnace connected to SP-6890a gas chromato-
graph with TCD detector. The sample was heated at 573 K in N2 (30 mL
2

Y. Qu, et al.
MolecularCatalysis493(2020)111038
Fig. 1. XRD pattern (A) and TEM image (B) of the synthesized SBA-15 carrier.
min−1) flow and purged at the temperature for 1.0 h to remove surface
moisture and impurities, then lowered to 313 K. Finally the reduction
was carried out in a mixed gas (30 mL min−1) of H2–Ar (VH2/VAr of 1/
9) from 313 K to 873 K.
Table 1
The activity and selectivity of Ag/SBA-15−ZnO catalyst with different ZnO
content.
ZnO content
Glycerol
3-Methylindole
3-Methylindole sel.
(mmol/g−1
)
conv. (%)
yield (%)
(%)
The crystal structures of SBA-15 carrier and Ag-based catalysts were
detected on the Rigaku Smartlab 9 diffractometer using Cu-Kα radia-
tion with small-angle scanning range (2θ) from 0.5° to 5° and wide-
angle scanning range (2θ) from 30° to 80°. Scan rates were 1°/min and
5°/min, respectively. The tube current and tube voltage were 200 mA
and 45 kV, respectively.
0.00
0.60
0.80
1.00
1.20
1.40
52
82
86
89
91
93
13
40
44
47
43
40
25
49
51
53
47
43
TEM study of SBA-15 carrier or SBA-15 supported Ag-based cata-
lysts was proceeded on the JEM-2000EX microscope operated at an
operating voltage of 50 kV. The sample was finely ground and sonicated
for 0.5 h to make it fully dispersed in ethanol and then dripped on a
porous grid for measurement.
Reaction conditions: 210 °C, aniline:glycerol = 3:1 M ratio, SV = 1700 h−1
,
LHSV = 0.4 h−1, H2 = 15 mL/min−1, steam = 16 mL/min−1, N2 = 9 mL/
min. Ag loading: 1.00 mmol/g−1. The first hour results.
The NH3/CO2-TPD experimental device is the same as that of H2-
TPR. Under He (35 mL min−1) gas flow, a 150 mg of catalyst was he-
ated from 293 K to 773 K ramping at 9.6 K min-1, and held at 773 K for
1.0 h to remove impurities. Then the sample was adsorbed by NH3 (or
CO2) at 373 K and purged at this temperature in He (35 mL min−1) flow
for 2.0 h. After that, the sample was heated to 973 K in a He gas flow
(35 mL min−1), and held at 973 K for 0.5 h.
Table 2
The activity and selectivity of Ag/SBA-15–ZnO–CeO2 catalyst with different
CeO2 content.
CeO2 content
Glycerol
3-Methylindole
3-Methylindole sel.
(mmol/g−1
)
conv. (%)
yield (%)
(%)
0.00
0.02
0.03
0.04
0.05
0.06
89
87
86
85
83
82
47
53
57
60
62
59
53
61
66
71
75
72
The carbon deposition analyses for used catalysts were carried out
on
a 6300 Diamond TG-DTA thermogravimetric instrument of
PerkinElmer. 7 mg of used catalyst was purged in a stream of nitrogen
(20 mL min−1) at 573 K, then lowered to 333 K, and maintained the
temperature for 0.25 h. Following, the used catalyst was heated from
333 K to 1073 K with the air flow of 20 mL min-1.
Reaction conditions: 210 °C, aniline:glycerol = 3:1 M ratio, LHSV = 0.4 h−1
,
SV = 1700 h−1, H2 = 15 mL/min−1, steam = 16 mL/min−1, N2 = 9 mL/min.
Ag loading: 1.00 mmol/g−1, ZnO content: 1.00 mmol/g−1. The first hour re-
sults.
2.6. Regeneration of Ag/SBA-15–ZnO–CeO2 catalyst
The deactivated catalyst sample of Ag/SBA-15–ZnO–CeO2 was re-
generated in a mixed gas (60 mL min−1) of O2–N2 (VO2/VN2 of 3/57) at
773 K for 4 h. Before the cycle reaction, the catalyst was reduced in situ
at 453 K in a H2 (15 mL min−1)–N2 (15 mL min−1) flow.
From Table 2 it can be seen that adding the promoter of CeO2 to Ag/
SBA-15–ZnO significantly increased the selectivity of 3-methylindole
further, but glycerol conversion decreased slightly. And with increase of
CeO2 content, 3-methylindole selectivity also showed a tendency of
firstly going up and then going down. When CeO2 content was 0.05
mmol/g−1, the optimal activity and selectivity were obtained with 3-
methylindole yield up to 62%, which was higher than the best result
[32].
3. Results and discussion
3.1. Catalytic performance
Fig. 2 shows the 3-methylindole yield versus time on stream over
SBA-15 supported Ag-base catalysts (Ag loading of 1.00 mmol/g−1, the
content of ZnO or CeO2 of 1.00 or 0.05 mmol/g−1). After adding the
promoter of ZnO to Ag/SBA-15, the initial activity of the catalyst was
improved observably. Continue to add CeO2 promoter to Ag/SBA-15-
ZnO, not only the activity was improved significantly, but also the
deactivation of the catalyst was inhibited effectively, indicating that
both ZnO and CeO2 were the good promoters to improve the perfor-
mance of the catalyst.
The activity and selectivity of Ag/SBA-15 modified with ZnO and
CeO2 for the reaction of glycerol and aniline to synthesize 3-methy-
lindole are shown in Tables 1 and 2. It can be observed from Table 1
that the conversion of glycerol was as low as 52% and the selectivity of
3-methylindole was only 25% over the catalyst of Ag/SBA-15. Both
glycerol conversion and 3-methylindole selectivity increased re-
markably when ZnO promoter was added to Ag/SBA-15. Additionally,
with the content of ZnO increased, the selectivity of target product
presented earlier increase and later decrease trend. When ZnO content
was 1.00 mmol/g−1, 3-methylindole yield reached 47%.
3

Y. Qu, et al.
MolecularCatalysis493(2020)111038
Table 3
Specific surface areas and pore volume of SBA-15 and SBA-15 supported Ag-
based catalystsa.
Sample
SBET (m2/g−1
)
Pore volume (cm3/g−1
)
SBA-15
521
406
379
345
0.543
0.558
0.562
0.568
Ag/SBA-15
Ag/SBA-15−ZnO
Ag/SBA-15–ZnO–CeO2
a
Ag loading: 1.00 mmol/g−1, ZnO content: 1.00 mmol/g−1, CeO2 content:
0.05 mmol/g−1
.
Fig. 2. The 3-methylindole yield versus time on stream over SBA-15 supported
Ag-base catalysts.
Ag loading: 1.00 mmol/g−1, ZnO content: 1.00 mmol/g−1, CeO2 content: 0.05
mmol/g−1
3.2. Characterization of catalysts
3.2.1. N2 physical adsorption
The N2 adsorption-desorption isotherms, average pore width, spe-
cific surface areas and pore volume of SBA-15 and three Ag-based
catalysts (Ag loading of 1.00 mmol/g−1, the content of ZnO or CeO2 of
1.00 or 0.05 mmol/g−1) were shown in Fig. 3 and Table 3. From Fig. 3A
it can be known that both SBA-15 and its supported silver-based cata-
lysts exhibited typical IV isotherms with H1-type hysteresis loops,
which indicates that these four samples existed mainly in mesoporous
form [52]. From Fig. 3B it can be seen that there were two kinds of size
distribution on SBA-15. The number of the mesopore at 3.63 nm was
much more than the number of micropore at 1.36 nm. When Ag, ZnO
and CeO2 were loaded on SBA-15 successively, the number of micro-
pore decreased clearly, while the number of mesopore obviously in-
creased in sequence, and pore size gradually grew.
Fig. 4. Middle FT-IR spectra of SBA-15 (a), Ag/SBA-15 (b), Ag/SBA-15–ZnO (c)
and Ag/SBA-15–ZnO–CeO2 (d).
Ag loading: 1.00 mmol/g−1, ZnO content: 1.00 mmol/g−1, CeO2 content: 0.05
mmol/g−1
1230 cm−1, attached to absorption vibration peaks of typical Si-O-Si
[53,54], meanwhile an absorption peak of 940 cm-1 was observed,
which belonged to winding vibration peak of Si−OH [55]. After SBA-
15 was supported with Ag, ZnO and CeO2, the bending vibration in-
tensity of Si−OH weakened gradually, however the infrared absorption
peaks at 466, 800, 1091, and 1230 cm−1 still existed, indicating that
the SBA-15 skeleton on the silver-based catalysts remained stable.
As it is obvious from Table 3, the specific surface area decreased
with loading of Ag, ZnO and CeO2 consecutively, indicated the presence
of the active component on the SBA-15 framework as well as the pro-
moters of ZnO and CeO2. The pore volume of SBA-15 supported Ag-
based catalysts increased a little after Ag, ZnO and CeO2 were loaded on
SBA-15 successively.
3.2.2. FT-IR
3.2.3. SEM–EDX
Fig. 4 shows the mid-infrared spectroscopic data of carrier and
silver-based catalysts (Ag loading of 1.00 mmol/g−1, the content of
ZnO or CeO2 of 1.00 or 0.05 mmol/g−1). It can be seen that on the
sample of SBA-15, there were absorbance peaks at 466, 800, 1091, and
Fig. 5 presents the SEM–EDX mappings of Ag/SBA-15–ZnO–CeO2
catalyst (Ag loading of 1.00 mmol/g−1, the content of ZnO or CeO2 of
1.00 or 0.05 mmol/g−1) and the result adequately confirmed a pre-
sence of Ag, Zn or Ce in the sample of Ag/SBA-15–ZnO–CeO2.
Fig. 3. N2 adsorption-desorption isotherms (A) and pore size distribution (B) of SBA-15 and SBA-15 supported Ag-based catalysts.
Ag loading: 1.00 mmol/g−1, ZnO content: 1.00 mmol/g−1, CeO2 content: 0.05 mmol/g−1
4

Y. Qu, et al.
MolecularCatalysis493(2020)111038
Fig. 5. SEM-EDX mappings of as-reduced Ag/SBA-15–ZnO–CeO2 catalyst.
(a) dark-field image, (b) the EDX spectrum, (c→f) elemental SEM-EDX mapping images of the measured Si intensity, Ag intensity, Zn intensity and Ce intensity
Ag loading: 1.00 mmol/g−1, ZnO content: 1.00 mmol/g−1, CeO2 content: 0.05 mmol/g−1
Moreover, the EDS mappings showed homogeneous distributions of the
loaded metal ions, i.e., Ag, Zn and Ce.
3.2.5. XRD
Fig. 7 presents XRD patterns of small angle (A) and wide angle (B) of
a series fresh and used catalysts with Ag loading of 1.00 mmol/g−1, the
content of ZnO or CeO2 of 1.00 or 0.05 mmol/g−1, respectively. From
Fig. 7A it can be observed that three diffraction peaks appeared at 2θ of
0.9, 1.6, and 1.9° on Ag/SBA-15 catalyst, assigning to the (100), (110),
(200) reflections of SBA-15. Although the diffraction intensity turned to
weak after the incorporation of ZnO and CeO2 into Ag/SBA-15 catalyst
in sequence no matter on the fresh or the used silver-based catalysts, the
SBA-15 skeleton still remained. The conclusion was consistent with the
FT-IR result in Fig. 4.
3.2.4. H2-TPR
H2-TPR is known as a good method to study the effect of promoter
on the interaction between active component and carrier. Fig. 6 shows
the H2-TPR results of the composite carrier SBA-15–ZnO, SBA-
15–ZnO–CeO2 and the samples of Ag2O/SBA-15, Ag2O/SBA-15–ZnO,
Ag2O/SBA-15–ZnO–CeO2 (Ag2O loading of 1.00 mmol/g−1, the content
of ZnO or CeO2 of 1.00 or 0.05 mmol/g−1). No hydrogen consumption
peak was observed on SBA-15–ZnO or SBA-15–ZnO–CeO2, indicating
that ZnO and CeO2 could not be reduced in the range of 373– 643 K.
When ZnO was added to Ag2O/SBA-15, the reduction temperature of
Ag2O was significantly increased, suggesting that ZnO strengthened the
interaction between the active component and the carrier, which
brought about the reduction of Ag2O difficult. After adding 0.05 mmol/
g−1 content of CeO2 promoter to Ag2O/SBA-15–ZnO, the reduction
peak of Ag2O obviously turned to a lower temperature, revealed that
CeO2 altered the electronic property of Ag2O, which made Ag2O acquire
electrons more easily [56,57].
From Fig. 7B it can be observed that there were two silver crystal
diffraction peaks at 2θ of 38.2° and 44.4° on Ag/SBA-15 catalyst in
30–60°, attributed to (111) and (220) crystal plane of silver [58]. After
adding the promoter of ZnO to Ag/SBA-15, the diffraction peak dis-
tributed to (111) become very wide, while the peak assigned to (220)
completely disappeared, revealed that ZnO promoter significantly im-
proved the dispersion of Ag particles. After CeO2 was added to Ag/SBA-
15–ZnO, the intensity of the diffraction peak of silver increased a little,
demonstrating that CeO2 could not further promote the dispersion of Ag
5

Y. Qu, et al.
MolecularCatalysis493(2020)111038
silver particles only increased to 7.7 nm at the same reaction time, il-
lustrating that the addition of ZnO inhibited the aggregation of silver
particles during the reaction effectively. This should be directly related
to the fact that ZnO can enhance the interaction between active com-
ponent and carrier. On the used Ag/SBA-15–ZnO–CeO2 catalyst, the
size of silver particles increased from 7.7 nm to 10.1 nm, indicating that
the aggregation of silver particles increased slightly after adding the
promoter of CeO2 to Ag/SBA-15–ZnO. From Fig. 6 it can be known that
adding the promoter of CeO2 to Ag2O/SBA-15–ZnO made Ag2O be re-
duced more easily, suggested that CeO2 could weaken the interaction
between Ag2O and the carrier, therefore the aggregation of active
component silver occurred with ease.
3.2.7. NH3/CO2-TPD
Fig. 9 is the NH3/CO2-TPD data of silver-based catalysts with Ag
loading of 1.00 mmol/g−1, ZnO or CeO2 content of 1.00 or 0.05 mmol/
g−1, respectively. It is obvious in Fig. 9A that the number of weak-acid
centers assigned to the desorption peak of NH3 below 473 K was fairly
small on Ag/SBA-15, however the amount of medium-strong acid
centers attributed to the desorption peak of NH3 between 473 ∼ 723 K
was a lot. After doping the promoter of ZnO into Ag/SBA-15, the
number of medium-strong acid centers decreased somewhat, but the
number of weak-acid centers increased a lot, which led to a significant
increase in the selectivity of the catalyst for 3-methylindole [33]. After
CeO2 promoter was added to Ag/SBA-15-ZnO, the number of weak-acid
centers decreased slightly as well as the number of medium-strong acid
centers, meanwhile the acidity of the catalyst was weakened for the
peak of NH3 attributed to medium-strong acid shifted to a lower tem-
perature, which was beneficial to the reaction because carbon deposi-
tion could be suppressed correspondingly.
Fig. 6. H2-TPR profiles of SBA-15–ZnO (a), SBA-15–ZnO–CeO2 (b), Ag2O/SBA-
15 (c), Ag2O/SBA-15–ZnO (d) and Ag2O/SBA-15–ZnO–CeO2 (e).
Ag2O loading: 1.00 mmol/g−1, ZnO content: 1.00 mmol/g−1, CeO2 content:
0.05 mmol/g−1
on the composite carrier of SBA-15–ZnO.
Comparing the XRD patterns on the fresh catalyst of Ag/SBA-15, the
intensity of silver diffraction peak on the used catalyst of Ag/SBA-15
was significantly enhanced, indicating that a severe aggregation hap-
pened during the reaction. The aggregation of catalyst decreased ob-
viously after adding ZnO to Ag/SBA-15, indicating that the promoter of
ZnO could restraint the aggregation of Ag particles during the reaction
effectively. CeO2, however, could not play the function.
Fig. 9B demonstrates CO2-TPD profiles of silver-based catalyst. On
the three SBA-15 supported Ag-based catalysts, there was only one
desorption peak of CO2 belonged to medium-strong base. With the
addition of ZnO promoter to Ag/SBA-15, not only the number of
medium-strong base centers increased, but also the alkalinity enhanced.
It was the addition of ZnO that increased the polarity (i.e. acidity and
alkalinity) of SBA-15-ZnO composite carrier, so the interaction between
silver and the carrier was heightened, as a result, the silver dispersion
and sintering resistance of the catalyst were improved. When doping
the promoter of CeO2 to Ag/SBA-15–ZnO, the desorption peak of CO2
obviously shifted to low temperature, revealed that the alkalinity of the
catalyst weakened significantly.
3.2.6. TEM
Fig. 8 is the TEM images and size distributions of Ag particles on the
fresh and the used Ag-based catalysts with Ag loading of 1.00 mmol/
g−1, ZnO or CeO2 content of 1.00 or 0.05 mmol/g−1, respectively. It
can be observed that the size of silver particles was ca.14.1 nm on the
fresh Ag/SBA-15. After adding 1.00 mmol/g−1 of ZnO promoter to Ag/
SBA-15, the average diameter of silver particles decreased to 4.8 nm,
showing that ZnO improved silver dispersion greatly. When CeO2 with
the content of 0.05 mmol/g−1 was added to the catalyst of Ag/SBA-
15–ZnO, the average size of silver particles increased to 6.5 nm, re-
vealed that doping CeO2 did not improve the dispersion of active
component silver, which was the same as the XRD result obtained in
Fig. 7A.
3.2.8. TG-DTA
Compared with the result in Fig. 8(a), silver particles on the used
Ag/SBA-15 grew a lot with the average size up to 22.1 nm, indicating
that a severe aggregation of active component occurred after 4.0 h of
the reaction. When ZnO was added to Ag/SBA-15, the average size of
It is well known that acidity can bring about the formation of carbon
deposition, which will obstruct the synthesis of 3-methylindole [59]. In
order to know the formation of carbon deposition on catalyst during the
reaction, the used Ag-based catalysts were characterized by TG-DTA
Fig. 7. XRD patterns of small angle (A) and wide angle (B) for the fresh and used Ag/SBA-15 (a, a’), Ag/SBA-15–ZnO (b, b’) and Ag/SBA-15–ZnO–CeO2 (c,c’).
Ag loading: 1.00 mmol/g−1, ZnO content: 1.00 mmol/g−1, CeO2 content: 0.05 mmol/g−1
6

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MolecularCatalysis493(2020)111038
Fig. 8. TEM images and normal distributions of Ag particle size for the fresh and used Ag/SBA-15 (a, a’), Ag/SBA-15−ZnO (b, b’) and Ag/SBA-15–ZnO–CeO2 (c, c’).
Ag loading: 1.00 mmol/g−1, ZnO content: 1.00 mmol/g−1, CeO2 content: 0.05 mmol/g−1
Fig. 9. NH3-TPD (A) and CO2-TPD (B) profiles of Ag/SBA-15 (a), Ag/SBA15−ZnO (b) and Ag/SBA-15–ZnO–CeO2 (c).
Ag loading: 1.00 mmol/g−1, ZnO content: 1.00 mmol/g−1, CeO2 content: 0.05 mmol/g−1
Fig. 10. TG(A)-DTA(B) results of the used Ag/SBA-15 (a), Ag/SBA15−ZnO (b) and Ag/SBA-15–ZnO–CeO2 (c).
Ag loading: 1.00 mmol/g−1, ZnO content: 1.00 mmol/g−1, CeO2 content: 0.05 mmol/g−1
and Fig. 10 revealed the amount of carbon deposition and the degree to
which carbon deposition can be oxidized. In Fig. 10A, an obvious mass
loss existed on the used Ag/SBA-15, indicating the formation of carbon
deposition in the reaction. After adding the promoter of ZnO to Ag/
SBA-15, the amount of carbon deposition increased, indicating that ZnO
facilitated the formation of carbon deposition. It was undoubtedly due
7

Y. Qu, et al.
MolecularCatalysis493(2020)111038
acetol, etc., there was also a small number of 1,2-propanediol. Based on
the reported literatures [60–63], 1,2-propanediol and acetol can be
generated from glycerol, therefore, the catalytic hydrogenolysis of
glycerol on SBA-15 supported silver-based catalyst was carried out. It
was found that the main product was acetol, 1,2-propanediol was only a
little. On the basis of literatures [64,65], 1,2-propanediol can also be
formed from acetol, so the catalytic conversion of acetol was further
proceeded on the catalyst and 1,2-propanediol was indeed found. Since
3-methylindole syntheses using 1,2-propanediol and aniline as raw
materials over SiO2 supported silver-based catalysts were achieved
[66,67], the conversion reaction of 1,2-propanediol and aniline over
Ag/SBA-15–ZnO–CeO2 was carried out as well as the conversion of
acetol with aniline and it was found that both the reactions could
produce the objective product of 3-methylindole.
Based on the above experiments and literatures [33,68], a possible
pathway to synthesize 3-methyindole in vapor-phase by glycerol with
aniline over Ag/SBA-15–ZnO–CeO2 catalyst was proposed as shown in
Scheme 1. 1,2-Propanediol or acetol can be made from the hydro-
genolysis or dehydration of glycerol. Some acetol could converted into
1,2-propanediol by hydrogenation. 1,2-propanediol as well as acetol
can be catalyzed with aniline to generate the target product of 3-me-
thylindole, in which 1,2-propanediol reacted with aniline to form 1-
phenylamino-acetone by dehydration, then produced the target product
of 3-methylindole by dehydration and dehydrogenation [33], while
acetol was firstly transformed to 2-hydroxy-1-propanal (F) via enol-keto
tautomerism, and F reacted with aniline to generate 3-methylindole
with 1-phenylimino-2-propanol as the intermediate [68].
Fig. 11. Recycling activity of Ag/SBA-15–ZnO–CeO2 catalyst.
Ag loading: 1.00 mmol/g−1, ZnO content: 1.00 mmol/g−1, CeO2 content: 0.05
mmol/g−1
to the increased acidity of the catalyst after the addition of ZnO, and
which was consistent with the NH3-TPD result in Fig. 9A. The amount
of carbon deposition decreased after adding the promoter of CeO2 to
Ag/SBA-15–ZnO, which was associated with the weakening of acidity
of the catalyst after the addition of CeO2. The same result can also be
obtained from the DTA profiles in Fig. 10B. In addition, from the DTA
exotherms on the used Ag-based catalysts it can be known that the
carbon deposition was not easily oxidized after adding ZnO to Ag/SBA-
15. The situation was modified a lot when CeO2 was added to Ag/SBA-
15–ZnO, indicating that the types of carbon deposition on the three
used Ag-based catalysts were somewhat different.
4. Conclusion
3-Methylindole was efficiently synthesized in a low-cost and green
way over Ag/SBA-15 modified with ZnO and CeO2 using biomass-de-
rived glycerol and aniline as raw materials. The catalyst exhibited a
superior catalytic activity and selectivity, which a 62% yield and a 75%
selectivity of 3-methylindole were obtained. A series of characteriza-
tions researches showed that the addition of ZnO promoter efficiently
improved the dispersion of silver and inhibited the aggregation of Ag
particles during the reaction because ZnO could increase the interaction
between the silver and composite carrier. Adding the promoter of CeO2
promoter to the Ag/SBA-15–ZnO catalyst could cut down the formation
of carbon deposition in the reaction process. Furthermore, with adding
the promoters of ZnO and CeO2, the quantity of weak-acid centers of
catalysts increased obviously, this contributed to achieve a higher se-
lectivity of the target product. When Ag/SBA-15–ZnO–CeO2 was cir-
culated five times, the yield of 3-methylindole still reached 59%, in-
dicating a good stability was obtained for the catalyst. Through the
research on the reaction pathway, it was concluded that glycerol can be
converted to acetol and 1,2-propanediol on the catalyst of Ag/SBA-
15–ZnO–CeO2, and they can both react with aniline to produce the
important chemical of 3-methylindole.
3.3. Catalyst reusability
The Reusability of Ag/SBA-15–ZnO–CeO2 (Ag loading of 1.00
mmol/g−1, ZnO content of 1.00 mmol/g−1, CeO2 content of 0.05
mmol/g−1) was tested. The catalyst underwent the regeneration after
each run, and was reduced in situ again prior to activity test. From
Fig. 11 it can be seen that only 3% of 3-methylindole yield declined
when Ag/SBA-15–ZnO–CeO2 was circulated five times, indicating that
it had a good practicality.
3.4. Research on the reaction pathway
In recent years, we have conducted a meticulous investigation for
the catalytic pathway of glycerol and aniline to 3-methylindole over
copper-based catalysts [32,33]. Here, the reaction pathway for the re-
action catalyzed by Ag/SBA-15–ZnO–CeO2 catalyst was researched.
From GC–MS analysis it was known that, except the main by-products
of N-methylaniline, p-methylaniline, N-ethylaniline, p-ethylaniline,
Scheme 1. A possible pathway for the vapor-phase synthesis of 3-methylindole from glycerol and aniline over Ag/SBA-15–ZnO–CeO2 catalyst.
8

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MolecularCatalysis493(2020)111038
Declaration of Competing Interest
methylindole: vapor-phase synthesis from aniline and glycerol over Cu-based cat-
alyst, Catal. Commun. 12 (2010) 147–150, https://doi.org/10.1016/j.catcom.2010.
08.011.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
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Acknowledgment
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We are gratefully thankful to the financial supported by the
National Natural Science Foundation of China (Grant No. 21576128).
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