Unravelling the role of Ag[sbnd]Cr interfacial synergistic effect in Ag/Cr2O3 nanostructured catalyst for the ammoxidation of toluene via low temperature activation of Csp3-H bond

Article abstract of DOI:10.1016/j.catcom.2021.106290

An environmentally benign and cost-effective synthesis of benzonitrile is reported by direct oxidative cyanation of aromatic Csp³-H bonds of toluene over Ag(I)/Cr₂O₃ nanostructured catalyst. The catalyst was prepared by a

Full text of DOI:10.1016/j.catcom.2021.106290

Catalysis Communications 152 (2021) 106290
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Short communication
Unravelling the role of Ag Cr interfacial synergistic effect in Ag/Cr O
nanostructured catalyst for the ammoxidation of toluene via low
–
2 3
3
sp
temperature activation of C -H bond
1
1
*
Shankha Shubhra Acharyya , Shilpi Ghosh , Rubina Khatun, Rajaram Bal
Nanocatalysis Area, Light Stock Processing Division, CSIR-Indian Institute of Petroleum, Dehradun 248005, India
A R T I C L E I N F O
A B S T R A C T
An environmentally benign and cost-effective synthesis of benzonitrile is reported by direct oxidative cyanation
Keywords:
Ag (I) oxide
Cr2O3
3
2 3
of aromatic Csp -H bonds of toluene over Ag(I)/Cr O nanostructured catalyst. The catalyst was prepared by a
hydrothermal route using cationic surfactant cetyltrimethylammonium bromide and was characterized by
powder XRD, XPS, SEM, TEM, TGA, TPD, TPR, UV-visible, FTIR, XANES, and ICP-AES methods. Conjugal effect
Nanoparticles
Toluene
of Lewis plus Brønsted acidity coupled with Ag
–
Cr interfacial synergistic interaction appears as detrimental for
its high catalytic activity and selectivity (98.4%) for benzonitrile. In situ DRIFTS analysis suggested that the
Benzonitrile
Ammoxidation
Cyanation
catalytic system proceeded via a reaction pathway explained by Mars-van Krevelen mechanism.
1
. Introduction
Direct functionalization of C s3 p-H hybridized inactive hydrocarbons
latter reaction causes a partial reduction of the oxide surface, particu-
larly at high temperatures [20]. Therefore, control of the rate of unse-
lective oxidation of NH
selectivity of the nitrile product in the ammoxidation reaction [20],
because this side reaction limits the availability of surface adsorbed NH
(N-insertion) species that are necessary for nitrile formation. It is known
that the rate of oxidation of NH is faster than that of the ammoxidation
reaction; excess NH modifies the surface oxidation characteristics of a
3 2 x
to N /NO is a crucial factor in determining the
to form oxygenated products under mild conditions is a major challenge
from an industrial point of view [1–8]. Ammoxidation of methylar-
omatics, an avenue to produce organic nitriles, these have been
commercially used as common building blocks that are integral parts for
producing pharmaceuticals, agrochemicals and fine chemicals [9–11].
Generally, organic nitriles are synthesized by cyanation of aldehydes
using HCN or metalcyanides, which are hypertoxic and cause environ-
3
3
3
catalyst and decreases the ammoxidation activity [20].
Nevertheless, it is also very difficult to completely avoid NH
3
mental disasters [11]. Industrially, gas-phase ammoxidation of C
–
H
oxidation but it can only be minimized to the maximum possible extent.
Besides, coordinatively unsaturated sites act as Lewis acidic sites for the
bond is performed over V
–
Cr oxides, however, the use of high reaction
◦
temperature (>350 C) and toxic metals refrained from the claims of
3
chemisorption of methyl aromatics and NH . Also, the acidity of the
environmental benignity [12].
catalyst plays a crucial role in the ammoxidation reaction [17,20]. It is
obvious that enhanced acidity is favourable for the better performance
The vapor phase ammoxidation of toluene to benzonitrile has been
extensively studied as a model reaction and typically with metal and
metal oxide catalysts of V, Mo and others [12–21]. Very recently, Iwa-
of catalysts, as it is known that NH
3
can adsorb on the catalyst surface in
+
the form of either NH
NH
always leads to the formation of undesired products, such as de-
methylated products and CO due to the strong adsorption of methyl
4
on Bronsted acid sites or coordinatively adsorbed
+
sawa et al. [22] reported the synthesis of single Cs /Y catalyst which
3
on Lewis acid sites. Furthermore, the strong acidity of the catalyst
furnished benzonitrile from toluene with very high yield. But so far re-
searchers always wade through several problems embroiled with ac-
tivity and stability of the catalyst, which further become constraints to
its industrialization. Moreover, there is always a competition between
x
aromatic compounds [20]. Therefore, it is indispensable to maintain a
good balance between the redox and acidic properties of the solid sur-
face to attain high ammoxidation activity and selectivity.
3
ammoxidation and NH oxidation on the catalyst surface, where the
*
Corresponding author.
E-mail address: raja@iip.res.in (R. Bal).
1
These authors contributed equally
https://doi.org/10.1016/j.catcom.2021.106290
Received 13 November 2020; Received in revised form 3 February 2021; Accepted 5 February 2021
Available online 14 February 2021
1
566-7367/© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

S.S. Acharyya et al.
Catalysis Communications 152 (2021) 106290
Since the last decade, Bal and his group has explored the develop-
internal standard after considering the fact that methane does not react
◦
ment of Ag-WO
investigated their activities in
5,6,23–29]. However, after thorough screening with WO
members of group VI metal oxides as supports in ammoxidation of
toluene, Cr was chosen to be the best one owing to its activity and
3
nanoarchitectures with several morphologies and
activation-based reactions
and other
over the present catalyst up to 500 C. The individual gas flows were
–
C H
adjusted by mass flow controllers (Brooks). The reaction stream was
analyzed by an online GC (Agilent), which was equipped with a flame
ionization detector (FID), and connected to an HP5 capillary column (30
[
3
2
O
3
m length, 0.28 mm i.d., 0.25
(CH ) and benzonitrile analysis, and a thermal conductivity detector
(TCD, column Porapak Q column) for O , N , CO, CO , NH , CH and
O analysis.
μm film thickness) for toluene, methane
long-term stability in the reaction stream. Thus, we came out of this
issue for the sustainable production of benzonitrile by means of
4
2
2
2
3
4
ammoxidation of toluene using Ag/Cr
2
O
3
nanostructured catalyst,
H
2
which showed high efficiency of NH utilization in the ammoxidation of
3
toluene with very high selectivity (98.4%), a benzonitrile yield of 72%,
and sustained activity for 18 h. To the best of our knowledge, this is the
highest activity ever shown by a catalyst in terms of activity and stability
3. Results and discussion
3.1. Catalyst characterization
2
in the direct benzonitrile production from toluene with O in a single-
step gas-phase reaction at comparatively lower reaction temperature
The Ag–Cr-based catalyst was characterized thoroughly by powder
XRD, XPS, SEM, TEM, FTIR, TG-DTG, XANES and ICP-AES. The crystal
structure and phase purity of the catalyst were analyzed based on the X-
ray diffractogram recorded (Fig. 1). The latter showed diffraction peaks
at 2θ values of 24.44, 33.6, 36.2, 39.8, 41.5, 44.2, 50.2, 54.85, 58.4,
◦
(
ca. 280 C).
2
. Experimental
◦
2
.1. Materials and reagents
63.5, 65.1, 73.2, 76.9 and 79.1 that can be indexed to the rhombohe-
dral phase of crystalline Cr
2
O
3
(JCPDS 82–1484), thus indicating the
Cr(NO
polyvinylpyrrolidone (PVP 10), HPLC grade (~ 9.9%) were purchased
from Sigma-Aldrich Co. NH , O and helium gas cylinders were also
3
)
3
.9H
2
O, AgNO
3
, cetyltrimethylammonium bromide (CTAB),
presence of trivalent Cr in the sample. From the FWHM of the line
broadening corresponding to the diffraction angles and applying Lor-
entzian peak-fit, we determined (obeying the Scherrer’s equation) that
the mean crystallite size of the fresh and spent catalyst was 32.3 and
3
2
commercially provided. All the chemicals were used without further
purification. Methane was used as an internal standard because it never
reacts with the examined catalysts in the present system. Double
distilled water was used during the preparation of the catalyst.
36.1 nm, respectively (Table 1). Similarly, the mean crystallite size of
IMP
the catalyst prepared under impregnation process (Ag(I)/Cr
2
O
3
) was
determined to be 52.2 nm, indicating somewhat larger particle size than
the former catalyst. Moreover, an intense diffraction peak at 2θ value of
◦
2
.2. Synthesis of catalyst
32.2 , followed by some small peaks at 2θ values of 26.5, 37.9, 54.3 and
◦
6
4.6 corresponding to Ag
2
O crystal faces of (111,110,200), (220,311),
CB
Ag(I)/Cr
2
O
3
nanostructured catalyst was prepared according to the
respectively, which coincide well with literature values (JCPDS No.
CB
protocol described elsewhere [23]. In a typical synthesis procedure, an
aqueous solution of 7.9 g of Cr(NO .9H O (from Sigma-Aldrich) was
76–1393); thus, denoted as Ag(I)/Cr
2 3
O . Powder XRD of the spent
3
)
3
2
catalyst also showed that there was no structural deformation or change
of oxidation state of the duo species after catalysis, demonstrating the
high structural stability of the catalyst.
dissolved in 27 g of deionized water to give a clear dark blue homoge-
neous solution. The pH of the solution was kept at 8–9 by gradual
addition of a few drops of ammonia solution, and dark green precipi-
tation was finally observed. An ethanolic solution (10%) of 2.7 g of
The presence of other mixed-valent Cr or Ag species in the catalyst
was ruled out completely, when the sample was examined in XPS. The
wide-scan XPS spectrum (survey spectrum, Fig. 2) of the catalyst depicts
◦
CTAB (from Sigma-Aldrich) was made and allowed to stir at 50 C for 15
min. The two solutions were mixed maintaining the molar ratio Cr:
CTAB: H
Ag wrt Cr
2
O = 1: 1: 200. Eventually, a hot solution of 0.094 g AgNO
3
(4%
2 3
O
by weight) and 0.001 g polyvinylpyrrolidone (PVP 10) was
added dropwise to the former solution with a dropper. The mixture was
allowed to stir for additional 15 min and was subjected to hydrothermal
◦
treatment at 180 C for 24 h in a Teflon-lined autoclave vessel under
autogenous pressure. The autoclave was then cooled until it reached
room temperature. Then, the green fluffy mixture was taken out as such.
The whole mixture was then washed several times with ethanol and
◦
◦
dried at 110 C for 10 h, followed by calcination in air at 750 C for 6 h
◦
(
ramped at 1 C/min) to get eventually green powder of Ag
–
Cr oxide
composite.
.3. Reaction setup
Vapor phase ammoxidation of toluene was carried out at atmo-
2
spheric pressure using a fixed-bed flow reactor system (Pyrex glass tube)
with an inner diameter of 6 mm and externally heated by electric re-
sistances. The reaction temperature was measured inside the catalyst
bed by a thermocouple, whose tip was inside the upper side of the
catalyst bed. Catalyst powder were pressed to pellets, crushed, and
sieved (typically, 0.2 g, sieved into ~500 m particles) and were packed
μ
between quartz wools. Toluene was fed continuously into the gas stream
by using a syringe pump. The reactor was fed with a gas mixture of
toluene/NH
3
/O
2
/He in the molar (or volume) ratio of 0.2/2.8/1.0/8
CB
2 3
Fig. 1. Powder XRD diffractograms of (a) fresh and (b) spent Ag(I)/Cr O
with a total volume flow rate (F) of 12.0 mL/min. The conversion and
selectivity in the current system were calculated by using methane as an
nanostructured catalysts and that of commercial (c) Ag(I) oxide, (d) metallic Ag
(0), (e) metallic Cr(0), (f) Cr(VI) oxide and (g) Cr(III) oxide.
2

S.S. Acharyya et al.
Catalysis Communications 152 (2021) 106290
Table 1
2 3
A comparative physicochemical study of Ag(I)/Cr O catalysts.
Catalyst
Oxidation state of
Particle size (nm)
Total acidity
NH Uptake
2
O Uptake
[
a]
ꢀ 1 [f]
Ag
(
μmolg )
Crystallite size from
Average particle size from
[c]
3
Pyridine Uptake
[
b]
ꢀ 1 [d]
ꢀ 1 [e]
XRD (nm)
TEM (nm)
(mmolg
)
(mmolg
)
CB
3
Ag(I)/Cr
2
2
O
+ 1
+ 1
+ 1
32.3
26.6 ± 0.89
30.0 ± 0.9
500–800
3.87
0.016
1.09
–
[
f]
CB
3
Ag(I)/Cr
O
36.1
52.2
–
–
[
g]
Ag(I)/
0.15
0.003
0.92
IMP[h]
2 3
Cr O
Determined using [a] XPS and Ag LIII-edge XANES spectra; [b] Scherrer’s equation; [c] TEM histogram; [d] NH
3 2 2 3
-TPD; [e] O -TPD; [f] fresh and [g] spent Ag(I)/Cr O
catalyst prepared by hydrothermally mediated by cationic surfactant CTAB method, and [h] Ag(I)/Cr catalyst prepared by impregnation method.
2 3
O
Fig. 2. (a) Wide-scan XPS spectrum (survey spectrum), and narrow scan XPS spectrum (core level spectra) of (b) Ag3d, (c) Cr2p and that of (d) O1s in the Ag(I)/
CB
2 3
Cr O nanostructured catalyst.
the presence of Ag, Cr and O. From the narrow-high resolution scan XPS
spectrum of silver (Ag3d, Fig. 2b) the two symmetric peaks located at
the corresponding Ag LIII-edge XANES spectrum (Fig. S1, Electronic
Supporting Information, ESI). The nature of edge energies for the fresh
+
3
68.4 and 374.4 eV are assigned to the binding energies of Ag 3d5/2 and
and spent catalysts resembled that of Ag valence in AgNO
3
(and not to
+
Ag 3d3/2, respectively, indicating only the presence of Ag in the pre-
pared composite [27], which supports the results of powder XRD. The
that with metallic silver foil), that further supports XPS results.
CB
2 3
The Ag(I)/Cr O catalyst possesses a regular texture with small and
peak at 576.8 and 586.4 eV correspond to Cr 2p3/2 and Cr 2p1/2
,
almost homogeneously distributed uniform particles (25–55 nm in size)
as imaged by SEM (Fig. S2a, ESI) and TEM (Fig. 2a-c and Fig. S3a, ESI)
that was entirely different from that prepared by the impregnation
process, which is composed of larger and non-uniform particles (as
evident from its TEM image, Fig. S6, ESI). Fig. S3b (ESI) shows broad
size distribution (Gauss) of silver particles possessing diameters in the
range of 2–5 nm. The mean particles sizes were determined (by Image-J
respectively of the characteristic Cr (III) in the sample (Fig. 2c) [30].
Notably, no XPS peak around 575.8 eV was observed, which certainly
thrives out the possibility of the presence of Cr4 species on the surface of
the catalyst [30]. The O1s signal of the catalyst (Fig. 2d) shows two
almost overlapping peaks at 532.2 and 530.2 eV, assigned to lattice
+
oxygen ions in different chemical environments (e.g., Ag
2
O and Cr
2
O
3
+
respectively). Furthermore, the presence of monovalent Ag species in
the catalyst did not change before and after catalysis, as confirmed from
software and Gaussian function) to be 26.6 ± 0.89 nm and 30 ± 0.9 nm,
CB
2 3
for the fresh and spent Ag(I)/Cr O catalysts, respectively (Table 1 and
3

S.S. Acharyya et al.
Catalysis Communications 152 (2021) 106290
◦
Fig. S4a-d, ESI). Thus, the structure-directing effect of CTAB is clearly
evident from the SEM analysis. From the EDAX image (Fig. S2b, ESI) it
can be seen that the sample contains only Ag, Cr, and O and from the
STEM image (Fig. 3d) and the corresponding elemental mapping
2–6) totally failed to show any catalytic activity, even at 280 C. How-
ever, the catalyst prepared under the impregnation process (Ag(I)/
IMP
3
Cr
2
O
2 3
) taking commercial Cr O as support, showed very poor activity
(Table 2, Entry 7); although, similarly prepared catalyst (Ag(0)/
IMP
3
(
Fig. 3e-g) the homogeneous distribution of Ag, Cr and O in the catalyst
Cr
2
O
) with metallic Ag(0) as active site failed to do so (Table 2, Entry
is demonstrated. The HRTEM image (Fig. 3c) clearly displays the
interplanar spacing 0.33 nm, which can be indexed to the (012) plane of
8), demonstrating the importance of cationic silver in the reaction.
CB
2 3
Then the prepared Ag(I)/Cr O catalyst was tested under the same
Cr
indexed to (111) plane of Ag
peaks assignment was further confirmed by TEM. The generation of
uniformly Ag(I)/Cr nanoparticles and the thorough dispersion of
2
O
3
(JCPDS 82–1484) and was easily discriminated from 0.27 nm
reaction conditions. Although this catalyst exhibited same elemental
2
O (JCPDS 76–1393). Moreover, the XRD
composition and corresponding metal oxidation states, and similar
IMP
2 3
loading of silver, it was proved much superior to the Ag(I)/Cr O
2
O
3
catalyst, furnishing 73.1% conversion of toluene with PhCN selectivity
of 98.4% (yield = 72%, Table 2, Entry 10). Interestingly, the catalyst
provided stable activity for 18 h without any significant activity loss.
After 18 h, the catalytic activity started decreasing. This experimental
finding can be attributed to two major facts. After 18 h, continuous
decrease of PhCN selectivity was observed due to the increased forma-
supported silver nanoparticles are attributable to the use of the cationic
surfactant of CTAB. Therefore, the important role of CTAB in controlling
CB
2 3
the formation of nano architectures of Ag(I)/Cr O , under the so called
“
Template Effect”, cannot be avoided. The embedding of CTAB mole-
cules on the pre-calcined catalyst surface was further confirmed from
TGA analysis (Fig. S7, ESI) and ex-situ Fourier Transform Infrared (FTIR)
analysis (Fig. S8, ESI).
tion of benzamide, apart from CO
of excess water that facilitated nucleophilic substitution mechanism
12]. Perhaps, the formed benzamide blocked the access of reactants to
2
; this fact demonstrates the formation
[
active metal i.e. Ag(I) surface sites, and interrupted the involved redox
catalysis. Moreover, accumulation of coke was also formed that prob-
ably encapsulated Ag(I) surface sites and thereby completely deacti-
vated the catalytic system. TGA analysis (Fig. S7c, ESI) of the spent
catalyst (after 18 h) revealed a mass loss ~8% and the major loss was
3
.2. Catalytic performance
Spurred by the earlier reports, we started screening the activities of
◦
several Cr-based catalysts from 200 C (Table 2) in the ammoxidation of
◦
◦
toluene with (O
2
+ NH
3
) reaction in the fixed bed reactor. From 200 to
noticed in the 180 -450 C range, indicating a considerable amount of
coke deposition that accounted for catalyst deactivation. The catalyst
showed 67.0% toluene conversion with PhCN selectivity of 90.2%
(Table 2, Entry 11).
◦
2
50 C, no catalyst showed any activity. Even neat reaction (Table 2,
Entry 1) was performed to check the necessity of the catalyst, but neither
benzonitrile (PhCN) nor any oxygenate (e.g. benzaldehyde/ benzoic
acid) was detected. Commercial catalysts, like Cr(III) oxide, Cr(VI)
oxide, metallic chromium, Ag(I) oxide, metallic silver (Table 2, Entry
It was also noticed that when the loading of Ag was increased to
CB
Fig. 3. TEM images (a-c) in ascending resolution, and (d) STEM image and STEM-EDS elemental mapping of (e) Ag, (f) Cr and that of (g) O in the Ag(I)/Cr
nanostructured catalyst.
2 3
O
4

S.S. Acharyya et al.
Catalysis Communications 152 (2021) 106290
Table 2
catalyst is preserved during catalysis [32].
3
The Lewis acid sites in the catalyst coordinatively adsorb NH and
Catalytic performances in selective ammoxidation of toluene to benzonitrile
a
reaction over various Ag-Cr-based catalysts.
toluene as well [16] and compel them to get reacted with the active site
+
Entry
1
Catalyst
No
C
T
S
BN
Y
BN
TOF
NH
3
Reacted/
present (e.g. Ag ). We also conducted toluene TPD over these two cat-
b
c
d
ꢀ 1
e
(
%)
(%)
(%)
(h
)
PhCN Formed
alysts and noticed that the quantitative amount of desorbed toluene for
ꢀ
1
ꢀ 1
these catalysts were 0.058 mmol g and 0.021 mmol g . That means
CB
Catalyst
COM
2 3
the amount of toluene adsorbed is ~3 times greater on the Ag(I)/Cr O
IMP
3
2
3
4
5
6
7
Cr
2
O
3
COM
3
–
–
–
–
–
–
–
–
–
–
–
–
–
than Ag(I)/Cr
it was detected that the Ag-species were reduced at higher temperatures
compared to the commercial Ag O, demonstrating high interaction of
2
O
2
catalyst. Furthermore, from H -TPR (Fig. S13, ESI),
CrO
–
–
–
COM
COM
COM
Cr(0)
Ag
–
–
–
2
2
O
–
–
–
+
CB
Ag(0)
–
–
–
Ag and Cr O in the Ag(I)/Cr O catalyst; therefore, one can attribute
2 3 2 3
Ag(I)/
4.1
16.4
0.67
1.8
253
the catalytic discrepancies in terms of the interface between the support
IMP
3
Cr
2
O
+
and active metal (here Ag NP), that plays pivotal roles in the
8
Ag(0)/
–
–
–
–
–
IMP
ammoxidation reaction.
Cr
2 3
O
f
9
Ag(I)/
63.2
73.1
67.0
45.7
–
88.9
98.4
90.2
83.8
–
56.2
71.9
60.4
38.3
–
27.5
31.9
29.2
19.9
–
2.9
3.2
5.2
4.5
–
CB
3
Cr
2
O
3.3. Reaction mechanism
1
1
1
1
1
0
1
2
3
4
Ag(I)/
CB
Cr
2 3
O
It is generally agreed that activation of the methyl group is the RDS
during toluene ammoxidation [13]. The nature of this activated species,
however, is still under debate. Meanwhile, introducing p-nitrotoluene as
g
h
i
Ag(I)/
CB
Cr
2 3
O
Ag(I)/
CB
3
Cr
2
O
2
substrate (Table 2, Entry 13), furnished only CO , which strongly pro-
Ag(I)/
voked the involvement of heterolytic C H rupture producing a carbo-
–
CB
Cr
2 3
O
j
cationic intermediate, and this hypothesis got rendered by spectroscopic
Ag(I)/
88.0
95.5
84.0
38.4
0.8
CB
evidence (in situ DRIFTS, Fig. S14, ESI). Even after seizing toluene flow,
Cr
2 3
O
we speculated the formation of benzaldehyde, which certainly indicates
a
Reaction Conditions: toluene/O
2
/NH
3
/He = 0.2: 1.0: 2.8: 8.0 mL/min;
+
the fact that the catalyst (Ag species) is capable of activating the C
–
H
◦
catalyst weight = 0.2 g; loading of Ag = 3.8 wt%; reaction temp = 280 C.
b
3
bond of toluene without the intervene of NH . These experimental
C
T
= conversion of toluene based upon the FID-GC (GC equipped with a
findings also compel us to hypothesis that the surface lattice oxygen
induces toluene oxidation reaction, and gaseous oxygen species accel-
flame ionization detector), and is equal to 100 × (mol of toluene reacted)/
(
initial mol of toluene used).
c
S
BN = is the selectivity of PhCN and is equal to 100 × (total mol of PhCN)/
total mol of toluene consumed).
erate the replenishment and migration of reactive oxygen species
IMP
(
through an oxygen vacancy. In contrast, Ag(I)/Cr
the direct dissociation of O
oxidation and produces large concentration of CO
2
O
3
catalyst needs
d
Y
BN is the yield of PhCN and is equal to (C
TOF is defined as reacted toluene(mol)/total metal (mol)/h.
toluene/O /NH
/He = 0.4: 1.0: 2.8: 8.0 mL/min.
After 18 h.
T
× SBN)/ 100.
on the surface of the catalyst for toluene
as a consequence.
2
e
f
2
2
3
Also, the detection of benzaldehyde (benzyl species) as previously
g
h
i
observed by Azimov et al. [33] demonstrates the fact that probably it
–
served as intermediate that reacts with NH to produce PhCN, and C H
3
Loading of Ag = 9.8 wt%.
p-Nitrotoluene.
j
activation in toluene is the RDS. This was evidenced by when benzal-
dehyde itself, which gave a very similar IR spectrum evidence (Fig. S15,
ESI). Since the formation rate of PhCN by ammoxidation of benzalde-
hyde was much higher than that by toluene (Table 2, Entry 14), this
suggests that the proposed mechanism, including the partial oxidation of
toluene to benzaldehyde, is the RDS. Had benzyl species been interme-
diate, a degree of surface reduction of the catalyst is expected [33],
Benzaldehyde was used as feedstock, respectively.
~
10%, the catalyst was able to furnish PhCN with 38.3% yield (Table 2,
Entry 12). The drastic increment of Ag(I) oxide particle size (confirmed
from TEM images, Fig. S5, ESI) with higher loading, probably accounts
for this experimental finding. Moreover, when benzaldehyde was used
as feedstock under ammoxidation conditions, it led to the formation of
PhCN with high yields and much higher rates than toluene itself
again, which is contradicting the H
2
-TPR results (Fig. S13, ESI)., because
+
Ag species present in the catalyst is not easily reducible compared to
(
Table 2, Entry 13).
+
the commercial Ag
2
O; therefore, partial reduction of Ag species is
CB
2 3
To get insight of the superiority of Ag(I)/Cr O catalyst over Ag(I)/
highly expected as observed in Mars–van Krevelen mechanism [12–21].
CB
IMP
3
Cr
2
O
, we integrated several characterization results (Fig. S3-S12, ESI)
Therefore, we can assume that acidic sites in the Ag(I)/Cr
catalyst coordinatively adsorb -cloud density of toluene and to some
extent few NH molecules (chemisorbed) [14] to generate NH
2 3
O
of these two catalysts. Thorough dispersion of Ag nanoparticles in Ag(I)/
π
CB
3
Cr
2
O
catalyst, may be one of the key reasons of its superiority, because
+
4
ions that
CB
3
smaller crystallites in Ag(I)/Cr
2 3
O catalyst offered a higher density of
are able to intervene in the ammoxidation mechanism, by increasing the
nucleophilic character of oxygen surface sites, and interacting with the
coordinatively unsaturated sites. We also investigated their comparative
◦
acidic character by means of NH
3
-TPD (Fig. S12a, ESI) from 80 C-
–
Me groups, making them prone towards activation [34]. Adsorbed
◦
IMP
3
6
50 C. The TPD peaks for NH
3
on Ag(I)/Cr
2
O
were observed be-
ꢀ
toluene produces a carbocationic intermediate and an H ion on the Ag
◦
◦
◦
tween 205 C -450 C with three maxima at 205, 327 and 450 C;. On the
(
I)- active site via heterolytic C
it (Ag(I)- active site in silver cluster) [35] partially; however, although
Cr does not take part in this redox-system directly, its
–
H bond rupture, and the latter reduces
CB
2 3 3
other hand, for the Ag(I)/Cr O catalyst, NH -TPD curve was observed
◦
◦
within a broad range, ca. 250 C-530 C, with maxima at 487 and
2 3
O
◦
5
27 C, demonstrating the fact that the overall (Lewis + Brønsted)
dehydrogenative-character assists the C
–
H heterolytic cleavage and
CB
3
surface acidity of Ag(I)/Cr
2
O
catalyst was greater than that of Ag(I)/
catalyst. From their respective calibration curves, it was
calculated that the amount of desorbed NH over these catalysts was
.868 and 0.155 mmol gcat respectively, demonstrating that ~25 times
greater concentration of acidic sites is present in the former catalyst (Ag
ꢀ
stabilizes the H ion. The lattice oxygen takes part in the oxidation
process, resulting in the formation of an alkoxide type intermediate,
IMP
2 3
Cr O
3
ꢀ
which then undergoes H abstraction by silver cluster to give a benzyl-
ꢀ 1
3
+
type intermediate that easily gets cyanation by NH
4
ions and thus the
entire mechanistic process might proceed through the Mars–van Kre-
CB
3
(
I)/Cr
Cr
vents sintering of the Ag(I) oxide and, hence, the surface area of the
2
O
); and this contribution majorly comes from the surface of
+
CB
velen mechanism. Importantly, Ag (present in Ag(I)/Cr
Ag Cr interfacial synergistic interaction (suspected to have generated
from H ) oxidation,
-TPR ~ 200–280 ◦C) that refrain from its (NH
2 3
O ) or strong
2 3
O [31], which also believed to act as a textural promoter that pre-
–
2
3
5

S.S. Acharyya et al.
Catalysis Communications 152 (2021) 106290
because if NH
(or NO ) would have been detected. The existence of a strong syn-
ergistic effect at the interface sites between Cr and Ag(I)-
nanoparticles probably plays the key factor for making it highly
3
could easily reduce Ag(I) center, massive production of
of Tokyo) for conducting XAFS measurements which were performed at
KEK-IMSS-PF with the approval of the Photon Factory Advisory
Committee.
N
2
x
2 3
O
CB
active, selective, and a robust Ag(I)/Cr
2
O
3
hetero-nanostructured cat-
Appendix A. Supplementary data
alytic system. However, more studies concerned with the role of surface
lattice oxygen are highly required in the future to reveal the reaction
mechanism by integrating several in situ techniques and DFT, followed
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.catcom.2021.106290.
by further optimization of the catalytic performance of the Ag(I)/Cr
2 3
O
catalyst for real industrial applications.
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