Supported dual-acidic 1,3-disulfoimidazolium chlorozincate@HZSM-5 as a promising heterogeneous catalyst for synthesis of indole derivatives

Article abstract of DOI:10.1002/aoc.4672

HZSM-5-supported Br?nsted and Lewis acidic ionic solid 1,3-disulfoimidazolium chlorozincate materials ([dsim]₂[ZnCl₄]@HZSM-5) were synthesized with various ratios (3, 6, 9, 17 and 50% w/w). The heterogeneous materials were characterized via a variety of spectroscopic techniques. Dual acidity of these materials was determined using specified techniques. These acidic solids were examined as stable heterogeneous catalysts for the Fischer indole reaction of equimolar amounts of phenylhydrazine hydrochloride and various aliphatic or aromatic ketones at 80–90°C in neat condition to produce substituted indole derivatives. The efficient 17% ionic salt-loaded HZSM-5 composite was easily reused for ten consecutive cycles with a slight loss of its activity. The recycled catalyst was further analysed using powder X-ray diffraction and inductively coupled plasma optical emission spectrometric techniques.

Full text of DOI:10.1002/aoc.4672

Received: 10 July 2018
DOI: 10.1002/aoc.4672
Revised: 28 September 2018
Accepted: 3 October 2018
F U L L P A P E R
Supported dual‐acidic 1,3‐disulfoimidazolium
chlorozincate@HZSM‐5 as a promising heterogeneous
catalyst for synthesis of indole derivatives
Susmita Saikia
| Krishna Puri
| Ruli Borah
Department of Chemical Sciences, Tezpur
University, Napaam, Tezpur 784028, India
HZSM‐5‐supported Brönsted and Lewis acidic ionic solid 1,3‐
disulfoimidazolium chlorozincate materials ([dsim]2[ZnCl4]@HZSM‐5) were
synthesized with various ratios (3, 6, 9, 17 and 50% w/w). The heterogeneous
materials were characterized via a variety of spectroscopic techniques. Dual
acidity of these materials was determined using specified techniques. These
acidic solids were examined as stable heterogeneous catalysts for the Fischer
indole reaction of equimolar amounts of phenylhydrazine hydrochloride and
various aliphatic or aromatic ketones at 80–90°C in neat condition to produce
substituted indole derivatives. The efficient 17% ionic salt‐loaded HZSM‐5 com-
posite was easily reused for ten consecutive cycles with a slight loss of its activ-
ity. The recycled catalyst was further analysed using powder X‐ray diffraction
and inductively coupled plasma optical emission spectrometric techniques.
Correspondence
Ruli Borah, Department of Chemical
Sciences, Tezpur University, Napaam,
Tezpur 784028, India.
Email: ruli@tezu.ernet.in
Funding information
SERB‐DST India, Grant/Award Number:
EMR/2016/002108
KEYWORDS
1,3‐disulfoimidazolium chlorozincate, Brönsted–Lewis acidic, HZSM‐5‐supported ionic salt, indoles,
reusable catalyst
1 | INTRODUCTION
Si/Al ratio of the ZSM‐5 support prevents de‐alumination
of framework Al which is a common problem of many
The growing importance of supports in research fields
has attracted attention towards zeolite materials for their
ubiquitous influence as acidic, porous, crystalline, high‐
surface‐area materials. Combining the characteristics of
zeolite with target‐specific solid or liquid catalysts via
immobilization often results surface‐modified stable reac-
tive heterogeneous catalytic systems with high surface
area. The selection of siliceous ZSM‐5 as support among
various types of zeolites is considered in terms of its
unique shape selectivity, solid acidity, ion exchangeabil-
ity, pore size, thermal stability and structural network.[1]
The presence of medium pore size (5.1–5.6 Å) bearing
three‐dimensional channels defined by 10‐membered
rings and smaller crystallite sizes of ZSM‐5 framework
can work as better support for immobilization of active
catalytic species on its surface.[2,3] Furthermore, high
other zeolites used as supports for immobilization of
acidic materials. Nowadays, immobilization of catalyti-
cally active metals and nanomaterials onto heteroge-
neous supports is very common.[4–7] Large numbers of
zeolite‐supported catalytic systems have been developed
for utilization as efficient heterogeneous catalysts in
organic synthesis.[8,9] Most of these catalysts include
reactive metal, their salts, oxide, metal complexes,
nanomaterials, etc.[10–13]
There are only a few reports of the design and uses of
ionic liquid/solid supported zeolite composites as hetero-
geneous catalysts.[14–17] The concept of an ‘ionic liquid’ is
an area beyond the scope of the present paper.[18] Amidst
the numerous members of the ionic liquid family, some
of them have extended their use from homogeneous room
temperature liquids to heterogeneous solids. Inclusion of
Appl Organometal Chem. 2019;e4672.
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https://doi.org/10.1002/aoc.4672

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SAIKIA ET AL.
complex metal halide anions with organic cations gener-
ates solid or semisolid‐like materials. Transition metal
halides have been utilized as anionic counterparts of an
ionic liquid‐based system which has been defined as
‘chlorometallate’ as one important class of functionalized
ionic solid. Structural variations of cationic and anionic
counterparts of ionic liquids mainly contribute the spe-
cific behavioural differences depending on the functional-
ities present. Most of them are hygroscopic solids
(or semisolids) with low thermal stability which reduces
their lifetime although they have excellent Brönsted–
Lewis acidic natures.[19] The water sensitivity of these
materials can be improved after being loaded on the sur-
face of ZSM‐5 zeolite in various compositions. There is a
possibility for increasing the thermal stability of compos-
ites with modification of catalytic sites of the acidic ionic
material on the surface of a support. This process can be
utilized to combine certain specific features of metal‐
containing ionic solids (e.g. Brönsted–Lewis acidity,
magnetic properties, fluorescence, etc.) with unique prop-
erties of a zeolite support for efficient applications as
solid acidic composites.[20] In this context, we aimed to
immobilize a known[19] Brönsted–Lewis acidic 1,3‐
disulfoimidazolium chlorozincate ([dsim]2[ZnCl4]) ionic
salt on HZSM‐5 support with various ratios (3, 6, 9 17
and 50% w/w) for the formation of stable composites by
a wet impregnation method (Scheme 1).[21]
The prepared composites were characterized using
various analytical techniques to understand their thermal
stability, acidic strength, surface morphology and
Brunauer–Emmett–Teller (BET) surface area and also
the interaction of the ionic salt with the zeolite support.
Finally they were examined as possible reusable sup-
ported acid catalysts for the Fischer indole reaction
(Scheme 2) in solvent‐free medium.
The potential of the Fischer indole reaction has been
observed with stoichiometric amounts of a variety of
Brönsted or Lewis acid catalysts (e.g. H2SO4, HCl,
polyphosphoric acid, ZnCl2, FeCl3, AlCl3, etc.) and also
with other solid acids.[22–26] These catalytic systems
SCHEME 2 Fischer indole reaction
suffer from considerable drawbacks such as low yield,
harsh or delicate reaction condition, by‐products, high
temperatures, prolonged reaction times and tedious
work‐up procedures. Few catalytic systems have been
introduced for the Fischer indole reaction involving ionic
liquid as reaction medium with added Lewis acid catalyst
or use of task‐specific acidic ionic liquids to work as dual
solvent–catalysts in solution or solvent‐free medium at
various temperatures.[27–29] Indole and its derivatives
constitute a major class of N‐heterocycles in various
biochemical processes as they are a part of some essen-
tial amino acids and most proteins. Many of them act
as anticancer, antileukemic, antidepressant, antihyper-
tensive, immune modulating agents, etc.[30,31] Vincristine
is a drug molecule of clinical interest possessing anti‐
tumour activities.[30] The initial Lewis acidic ionic liquid
catalyst 1‐butyl‐pyridinium chloride·3AlCl3 worked as
dual solvent–catalyst for the Fischer indole reaction
with excellent catalytic activity.[32] Ionic liquid choline
chloride·2ZnCl2 was also successfully used as solvent–
catalyst for this reaction.[33] In spite of the robust cata-
lytic activities of such Lewis acidic ionic liquids, they
were not recycled because of their air‐ and moisture‐
sensitive nature. Catalytic recyclable Brönsted acidic
imidazolium ionic liquids functionalized with two alkyl
sulfonic acid groups were designed by Xu et al. for the
Fischer indole reaction in water at 80–100°C for
0.5–6 h reaction time.[34] Neuhaus and co‐workers also
conducted the Fischer indole synthesis of wide variety
of hydrazine and ketones in the presence of the acidic
ionic liquid tetramethylguanidinium propanesulfonic
acid trifluoromethyl acetate in water at 90°C.[27] In
2012, Li et al. designed a series of ―SO3H‐functionalized
imidazolium ionic liquids and employed them as reus-
able catalysts for one‐pot Fischer indole synthesis under
microwave irradiation in water.[35] An ultrasound‐
assisted route for the same synthesis was introduced by
Tao et al. in water in the presence of imidazolium ionic
liquid bearing two alkyl sulfonic acid groups at 100°C
SCHEME 1 Synthesis of [dsim]2[ZnCl4]@HZSM‐5

SAIKIA ET AL.
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within a short reaction time.[36] The literature also
reveals one example of a silica‐supported ―SO3H‐bear-
ing acidic ionic liquid as a heterogeneous catalyst for
the reaction of phenylhydrazines with ketones or
aldehydes at room temperature.[37]
2.3 | Catalytic investigation
A typical reaction of phenylhydrazine hydrochloride
(1 mmol) with ketone (or aldehyde) (1 mmol) using a
catalytic amount (10 mg) of [dsim]2[ZnCl4]@HZSM‐5
was performed at 80–100°C under neat condition for a
specific reaction time to afford the respective indole
derivatives. Product formation was monitored by TLC
using 1:5 EtOAc–hexane as mobile phase. After that
the reaction mixture was cooled immediately to room
temperature and the organic mixture extracted with
dichloromethane (DCM). The catalyst was insoluble in
DCM. Thus decantation of DCM with further washing
several times using the same solvent gave the recycled
catalyst. Then the organic phase was dried over Na2SO4
and the solvent was removed in a rotary evaporator.
Recrystallization from ethanol and water gave the
desired solid product in satisfactory yield. This process
was repeated two to three times to obtain analytically
pure product.
2 | EXPERIMENTAL
2.1 | Catalyst characterization
A Nicolet Impact‐410 spectrophotometer was used for
recording Fourier transform infrared (FT‐IR) spectra.
Thermogravimetric analysis (TGA) was done with a
Shimadzu TGA‐50. The Brönsted acidities of the sup-
ported composites were obtained with a UV 2550 spectro-
photometer using 4‐nitroaniline as basic indicator.
Scanning electron microscopy (SEM) images were
obtained with a JEOL JSM‐6390LVSEM, equipped with
energy‐dispersive X‐ray (EDX) analyser. Raman spectra
of the composites were obtained using a Renishaw spec-
trophotometer equipped with a He–Ne laser of excitation
wavelength 514 nm. BET isotherms were recorded with a
Quantachrome NOVA 1000E surface pore size analyser.
Powder X‐ray diffraction (PXRD) patterns were recorded
with a Rigaku Multiflex instrument using a nickel‐filtered
Cu K (0.15418 nm) radiation source and scintillation
counter detector. UV–visible diffuse reflectance spectra
were obtained using a Shimadzu UV 2450spectrophotom-
eter. Melting points were determined with a Buchi‐560
melting point apparatus. The 1H NMR and 13C NMR
spectra of the indole derivatives were obtained with a
JEOL 400 MHz spectrophotometer with CDCl3 as solvent.
Inductively coupled plasma optical emission spectrome-
try (ICP‐OES) was conducted with a PerkinElmer Optima
2100DV instrument.
3 | RESULTS AND DISCUSSION
At first, the synthesis of five [dsim]2[ZnCl4]@HZSM‐5
composite materials was conducted after impregnation
of [dsim]2[ZnCl4] ionic salt with ratios of 3, 6, 9 17
and 50% (w/w) on HZSM‐5 support according to
Scheme 1. Then these five samples were subjected to
various analytical techniques which included FT‐IR
spectroscopy, TGA, PXRD, Raman spectroscopy, SEM,
EDX, Hammett acidity and Lewis acidity determination,
electronic spectroscopy and BET analysis. These analyses
gave valuable information about the physicochemical
nature of the composites such as thermal stability,
hygroscopic properties, Brönsted–Lewis acidities, surface
area and also crystallinity of the HZSM‐5 support. In the
next step, we studied the possible uses of these compos-
ites as supported solid acid catalysts for the Fischer
indole reactions of phenylhydrazine with a variety of
carbonyl compounds in solvent‐free medium at various
temperatures.
2.2 | Preparation of [dsim]2[ZnCl4]
@HZSM‐5
The ammoniated ZSM‐5 powders (Si/Al = 80) and all
other chemicals used in this study were purchased from
Acros Chemicals. The ionic chlorometallate was synthe-
sized according to a reported method.[18] Then ZSM‐5
powder was calcined in a muffle furnace at 550–600°C
to afford the final degassed sample as HZSM‐5. After cal-
cining, HZSM‐5 powder was treated with pre‐synthesized
1,3‐disulfoimidazolium chlorozincate (1) with various
ratios (3, 6, 9, 17 and 50% w/w) in methanol at 80°C for
12 h (Scheme 1). The resultant supported materials were
collected after evaporation of methanol in a rotary evapo-
rator and were dried again in a vacuum oven at 100°C.
3.1 | FT‐IR study of 1@HZSM‐5
Various structural constituents and crystallinity related to
their characteristic occupancy in the zeolite framework
were primarily distinguished from FT‐IR stretching
and bending vibrations for the support HZSM‐5 and
hybrid samples of Zn2+ (2a–2e) (Figure 1; Table 1).
Evidence for the existence of crystalline phases in all
the modified materials was determined using the two
absorptions at 543 and 450 cm−1 [38,39]
The degree of
.

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SAIKIA ET AL.
crystallinity estimated from the ratio of the intensities
of these two peaks (Table S1; Figure 2) was found to be
in the range 0.78–0.82 which was in accordance with
the reported value of 0.8 for pure HZSM‐5.[40,41] The crys-
tallinity analysis along with the respective FT‐IR spectra
made a clear point about loss of some degree of crystallin-
ity in the case of the 50% loaded material (Table S2;
intensity ratio > 0.85) (2e) which was later confirmed
by PXRD analysis. Also the FT‐IR spectrum indicates
the destruction of the framework by displaying
additional peak at 1592 cm−1 as well as some discrepancy
around 797–801 cm−1. We analysed the O―H stretching
vibration of hybrid samples within the frequency
range 3000–3800 cm−1 (Figure 3). The reason behind this
behaviour shown by the highest loaded composite can be
accounted for by framework discontinuity due to possible
insertion and reinsertion of Al atoms associated with the
zeolite network. Reinsertion of EFAl (extraframework Al)
species into the zeolite as framework Al is evidenced
from the reduction of peak intensity at 3662 cm−1 with
increasing loading up to 17% of ionic salt in composite
materials. The absorption maximum of hydroxyl peak at
3452 cm−1 shifts towards 3610 cm−1 as a broad band
which can be reasoned as the presence of framework Al
with increasing loading of the ionic salt.[42] This type of
acidic salt‐mediated insertion of Al is supported by the
work done by Fan et al.[43] Alkali‐mediated desilication
of highly siliceous HZSM‐5 structure with 50% loaded
hybrid samples can be indicated by absorption in the
range 2800–3200 cm−1 which again may occur by in situ
formation of metal hydroxide of Zn2+ from water‐
sensitive chlorometallates during the preparation of 50%
loaded zeolite. Therefore, we have got comparable
thermal stabilities of the 50% loaded sample (2e) with
the bare ionic salt (1) in TGA (Figure 4).
FIGURE 1 FT‐IR spectra of 1@HZSM‐5 (2a–2e) and HZSM‐5
TABLE 1 FT‐IR peak assignments of 1@HZSM‐5
Peak (cm−1
)
Assignment
450
T–O bending vibrations of SiO4 and AlO4
internal tetrahedron
543–553
797–801
1075
Double five‐ring lattice vibration of
external linkages
External symmetric vibrations of
T–O–T linkage (T = Si or Al)
Asymmetric stretching vibration of –SO3H
group overlapped with asymmetric
T–O band
1106–1230
Internal asymmetric stretching vibrations
of T–O–T linkage
1630
Bending vibration due to adsorbed water
on HZSM‐5 framework merged with C¼N
stretching vibration of imidazolium cation
FIGURE 2 FT‐IR absorption spectra of 1@HZSM‐5 (2a–2e)
FIGURE 3 OH stretching vibration bands of 1@HZSM‐5 (2a–2e)

SAIKIA ET AL.
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decrease in peak intensity at 2θ = 7.78°, 23.07° and
45.1°.[44,45] PXRD patterns strongly established the
retention of crystallinity in synthesized materials 2a–2d
without displaying any characteristic peak of the bare
ionic salt. Figure 6 shows the degree of crystallinity of
the loaded materials for the four intense peaks in the
region 2θ = 8–45° with respect to HZSM‐5. The values
calculated from the graph indicate the gradual loss of
crystallinity of the 1@HZSM‐5 materials up to 17%
loading. Sample 2e has lost maximum crystallinity
due to its hydrophilic nature and thus destabilization
results from the dissolution of the HZSM‐5 framework
evidenced from Figure 3 (O―H stretching) and Figure 4
(TGA profile).
FIGURE 4 TGA curves of 1@HZSM‐5 (2a–2e), HZSM‐5 and 1a
3.4 | Raman spectra
3.2 | Thermogravimetric analysis
Owing to the ambiguous behaviour of the highest loaded
hybrid 2e, we chose to consider only 1@HZSM‐5 mate-
rials up to 17% loading. Therefore, Raman spectra
(Figure 7) were recorded for only the lowest (3%, 2a)
and highest loaded materials (17%, 2d) with respect to
the parent HZSM‐5. Characteristic peaks showed evi-
dence of respective hybrids with intactness of the MFI
topology of HZSM‐5. The symmetric stretching modes of
Si―O―Si bonds of five‐membered silicate frameworks
of MFI unit structure in addition to other characteristics
The TGA profiles displayed 3–8% weight loss below
100°C which may be attributed to the loss of absorbed
moisture. Beyond this, the curves express high thermal
stability in the case of 2a–2d (3–17%) up to 600°C, similar
to that of parent HZSM‐5. But 2e (50% loaded material)
behaved more like the original ionic salt which is very
hygroscopic as well.
bands appeared at 290, 380 and 460 cm−1 [46]
.
The basic
3.3 | PXRD analysis
peak of HZSM‐5 at 298 cm−1 slightly changed its position
to 300 and 295 cm−1 due to the presence of Zn2+
.
Figure 5 reveals identical patterns of PXRD peaks in
the case of chlorometallate‐loaded HZSM‐5 hybrids
1@HZSM‐5 (2a–2d) to that of the parent zeolite, except
for 2e. The highest loaded material (50%, 2e) showed a
The other important peak at 460 cm−1 is also shifted to
some extent.
FIGURE 6 Crystallinity of 1@HZSM‐5 (2a–2e) with respect to
parent HZSM‐5
FIGURE 5 PXRD patterns of 1@HZSM‐5 (2a–2e), HZSM‐5 and 1

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SAIKIA ET AL.
FIGURE 7 Raman spectra of 1@HZSM‐5 (2a and 2d) with
respect to parent HZSM‐5
3.5 | SEM and EDX analyses
Surface morphological changes and discrepancies
resulting from loading of ionic salt 1 onto HZSM‐5 can be
assessed from SEM analyses of 2a and 2d along with
HZSM‐5 (Figure 8). Composite 2a displayed a little
aggregation on the surface with negligible change in
crystalline phase while 2d showed clogging of pores with
some cluster formation due to expected hydrogen bonding
occurring between Brönsted acidic sites of the ionic salt
with framework Al generated from re‐alumination of EFAl
to the parent HZSM‐5 as described in the study of O―H
stretching vibrations (Figure 3). Likewise, the EDX
analysis of 2d confirmed the presence of Zn2+ along with
Al, Si, Cl, N, O and S of the 1@HZSM‐5 material.
3.6 | Hammett acidity determination
using UV–visible spectrophotometry
We determined the trend in Brönsted acidity in loaded
zeolite materials 1@HZSM‐5 (2a–2d) with Hammett
plots using a UV–visible spectrophotometer (Figure 9).
p‐Nitroaniline was used as basic indicator [I] with
pKa = 0.99 showing maximum absorbance at 380 nm
in blank run which lost its absorption intensity with
increasing loading of ionic salts on HZSM‐5 (Figure 9),
and this change was reflected in Hammett acidity values
(Table S3). Hammett acidity function H0 was determined
using the following equation:[47]
H0 ¼ pKðIÞaq þ log½Iꢀ=½IHþꢀ
(1)
The order of acidity for all the hybrid samples can be
arranged with reference to the parent ionic salt as:
FIGURE 8 SEM images of 2a, 2d and HZSM‐5, and EDX
analysis of 2d

SAIKIA ET AL.
7 of 12
FIGURE 9 Hammett plots of 1@HZSM‐5 in ethanol
2d > 2c > 2b > 2a > 1a. This increasing trend in
Brönsted acidity towards the highest loaded sample may
be reasoned in terms of acidic ionic salt‐mediated
re‐alumination of EFAl in HZSM‐5 which increased the
acidic sites in composites, which was also previously
explained in the case of ―OH band in absorption
spectra[48] (Figure 3).
3.7 | Lewis acidity determination
The presence of Lewis acidic sites in the synthesized
materials (2a–2e) was determined using pyridine as probe
molecule. FT‐IR analysis was conducted, mixing pyridine
with 1@ HZSM‐5 (1:3) for all samples. Neat pyridine
displays a characteristic band at 1437 cm−1 which was
shifted to higher wavenumbers in the case of Lewis acidic
materials (1440 cm−1) along with shoulder broadening at
1450–1455 cm−1 (Figure 10). This can be reasoned as the
Lewis acidity of materials arising from the presence of
2−
ZnCl4 anion in the ionic salt. Also, the new absorption
peak at 1538 cm−1 may be explained in terms of the inter-
action between chlorometallate and pyridine to form
pyridinium ion. However, for 2a and 2b this peak did
not appear. This may be due to the minimal loading of
ionic salt on HZSM‐5 which was not sufficient to express
the formation of pyridinium ion in FT‐IR spectra.[49]
It was obvious that the highest loaded sample showed
significant Lewis acidity.
FIGURE 10 FT‐IR analysis of Lewis acidity of 1@HZSM‐5 using
pyridine
3.8 | UV–visible diffuse reflectance
(Figure 11). All the spectra clearly indicate the existence
of characteristic constituents of chlorometallate moiety
in HZSM‐5. Weak absorptions at 260 and 319 nm can
be correlated with the intra‐ligand charge transfer transi-
tions of the loaded ionic salt 1 at 222 and 340 nm.[19]
spectra
The UV–visible diffuse reflectance spectra of the loaded
materials with respect to HZSM‐5 were recorded to
examine the effect of the loading amount of ionic salt

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SAIKIA ET AL.
FIGURE 11 UV–visible diffuse reflectance spectra of 1@HZSM‐5
(2a–2e) with respect to HZSM
3.9 | BET analysis
According to the IUPAC classification, Figure 12a shows
characteristics of combined type I and type IV isotherms
with type H4 hysteresis loops which indicate the forma-
tion of a hierarchical porous system containing both
micropores and mesopores. Capillary condensation in
inter‐/intra‐crystalline mesopore void spaces present in
the initially de‐aluminated HZSM‐5 and hybrid materials
can be assumed in the region 0.4 < P/P0 < 0.95.[50–52]
Figure 12b portrays the Barrett–Joyner–Halenda (BJH)
pore size distribution plot of the parent HZSM‐5 and 2d
(17% loading) using only the adsorption branch of the
isotherm. The mean size of the micropores in HZSM‐5
and the highest loaded composites was estimated to be
about 17.34 Å (1.7 nm).
FIGURE 12 (a) Nitrogen isotherms of HZSM‐5 and 2d. (b) BJH
curves of HZSM‐5 and 2d
The t‐plots of HZSM‐5 and 2d were obtained using the
equation given by De Boer:
3.10 | Catalytic activity
À Á
0:5
t Å ¼ ½13:99=ð logðpo=pÞ þ 0:034Þꢀ
(2)
The catalytic activity was investigated for all the synthe-
sized materials (2a–2d) in the Fischer indole reaction
to synthesize indole derivatives using phenylhydrazine
hydrochloride and various ketones or aldehydes (ali-
phatic/aromatic). The reaction was optimized by using
various amounts of acidic catalysts, at various tempera-
tures and also for various solvents. To optimize the tion
The positive intercept of the straight line to the
y‐axis gives the pore volume (VMP = I × 0.001547 cm3).
From the slopes of the straight lines derived from t‐plots
of respective samples in Figure 13, one can determine
the external surface areas and mesoporous areas
(A (m2 g−1) = slope × 15.47).[53]
given by reaction conditions,
a
model reaction
(Scheme 3) was adopted involving phenylhydrazine
hydrochloride (1 mmol) and cyclohexanone (1 mmol)
to yield 1,2,3,4‐tetrahydrocarbazole (3a) in the presence
of various amounts (5, 10 and 15 mg) of catalysts 2a–
2d in solvent‐free medium at 100°C (Table 3, entries 1–
4). The best result was observed for 10 mg of catalyst 2d
(Table 3, entry 4). The reaction temperature was
All the results obtained from nitrogen adsorption–
desorption isotherms, BJH curves and t‐plots are summa-
rized in Table 2. We noticed that a significant decrease in
BET surface areas occurred for the highest loaded mate-
rial as compared to the parent zeolite, which can be
justified in terms of clogging or filling of pores by the
loaded ionic salt.

SAIKIA ET AL.
9 of 12
TABLE 3 Optimization of catalyst amount for synthesis of 3a
Time (h)
Entry Catalyst
5 mg 10 mg 15 mg Yield (%)a,b
1
2
3
4
5
6
1@HZSM‐5 = 3%
(2a)
2.75 2.5
1.75
1.75
1.5
75 80 82
80 82 82
85 88 88
88 95 95
−/75/94/95c
90/82/90d
1@HZSM‐5 = 6%
(2b)
2.75 2.5
1@HZSM‐5 = 9%
(2c)
2
1.5
1@HZSM‐5 = 17% 1.5
(2d)
1@HZSM‐5 = 17% 24/1/0.75/0.75c
(2d)c
0.75
0.5
1@HZSM‐5 = 17% 0.83/1/1.16
(2d)d
aIsolated yield.
bReactions of entries 1–4 were conducted at 100°C in neat condition.
cReactions were conducted at room temperature, 60°C, 80°C and 100°C
respectively using 10 mg of 2d in neat condition.
dReactions were performed in CH2Cl2, EtOH and H2O respectively at tem-
peratures below their boiling points using 10 mg of 2d as catalyst.
TABLE 4 Comparison of catalytic activity of 2d and previously
reported catalysts
Reaction
conditions
Time Yield
(h) of 3a (%)
Catalyst
1@HZSM‐5 = 17% (2d) 10 mg, 100°C
10 mg, 100°C
p‐Toluenesulfonic acid 10 mg, 100°C
0.75
94
75
FIGURE 13 t‐plots of HZSM‐5 and 2d
TABLE 2 Summery of BET analysis
1
1.5
8
68
ZnCl2
10 mg, 100°C
6
70
H‐Y
1 g, MeOH, 60°C
70°C in ionic liquid
18
1
69.3[54]
92[55]
92[56]
95[57]
[bmim][HSO4]
SBET
SEXT
Vmp
Smp
Material
(m2 g−1 a
)
(m2 g−1 b
)
(cm3 g−1 b
)
(m2 g−1 c
)
50 mol% [cmmim][BF4] 140°C
2
20 mol% [bmim][BF4]
Reflux in MeOH
7
HZSM‐5
428.77
134.94
53.34
33.97
0.178
0.064
375.43
100.97
10 mol% [C10MIM][Br] MW, 300 W
3 min 58.05[58]
3 min 94.25[58]
2d
10 mol% [C10MIM]
[Br] + ZnCl2
MW, 300 W
aSurface area calculated form multipoint BET.
bExternal surface areas and micropore volumes measured by t‐plot method.
cSmp = SBET − SEXT
.
optimized at 80°C for the same catalyst amount (Table 3,
entry 5). The reaction was also conducted in each of
CH2Cl2, EtOH and water at temperatures below their
boiling points and good results were obtained (Table 3,
entry 6).
Based on these optimization results, we extended our
investigation with a variety of cyclic, acyclic and aro-
matic ketone and aliphatic aldehyde compounds with
10 mg of catalyst 2d at 80–90°C to synthesize substituted
SCHEME 3 Synthesis of 1,2,3,4‐tetrahydrocarbazole

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SAIKIA ET AL.
TABLE 5 Substrate scope of indole derivatives using 1@HZSM‐5 = 17% (2d) as catalyst
Entry
Ketone
Time (h)
Product
Yield of product (%)a
1
0.75
94
2
3
1
90
90
0.83
4
5
0.83
0.75
90
92
6
7
1
1
82
80
8
9
0.75
0.75
88
85
10
11
2
2
—
—
Complex mixture
Complex mixture
aIsolated yield.
indole derivatives (Table 4). The reaction involving
ketones gave excellent performance in terms of product
yield, easy isolation and minimal side product. But in
the case of aliphatic aldehydes, namely propionaldehyde
and isobutyraldehyde, we obtained complex mixture of
products which can be accounted for by acid‐catalysed
self‐condensation of aldehydes containing acidic α‐
hydrogen with respect to electron‐deficient aldehyde
group (Table 4, entries 10 and 11). A few data are pre-
sented in Table 5 for comparison of the percentage
yields of model product 3a for the present catalyst and
previously reported ones[54–58]
.
FIGURE 14 Recyclability profile of 2d

SAIKIA ET AL.
11 of 12
these catalysts lies in the possession of all the properties
of HZSM‐5 similar to heterogeneous catalysts which
bring selectivity to products without formation of many
unwanted side products followed by simple isolation
and easy recyclability. Also, the dual‐acidic character
3.11 | Reaction mechanism
A plausible reaction mechanism may be expected from
simultaneous interactions of both the Brönsted–Lewis
acidic sites of the supported catalyst with C¼O and also
with ―NHNH2 groups through hydrogen bonding as
depicted in Scheme S1 (supporting information).
−
imparted by Lewis acid ZnCl4 and Brönsted acid group
―SO3H contributes to the good catalytic performance.
Thus, this work provides an easy and beneficial route
towards heterogeneous catalysis.
3.12 | Recyclability study
Catalyst recyclability was investigated using the reaction
shown in Scheme 3. The model reaction was carried out
with 5 mmol of substrates using 50 mg of 2d as a solid
acidic catalyst under standard reaction conditions.
After completion of the reaction as monitored by TLC,
the reaction mixture was dissolved in DCM whereas the
catalyst was insoluble in the same. Decantation of the
organic extract separated the solid catalyst from the
reaction mixture. Reactivation of the spent catalyst was
done after washing two to three times with absolute
ethanol and then dried in a vacuum oven at 100°C for
1 h. The dried catalyst was reused and recycled for nine
more cycles (Figure 14). ICP‐OES and PXRD analyses
were performed with the reused catalyst (Table S4 and
Figure S1 of supporting information). The ICP‐OES
results showed slight leaching of metal content in every
consecutive cycle which may be accounted for by in situ
formation of base from the ionic salts and thus causing
desilication of the HZSM‐5 support after repeated ethanol
washing and drying at 100°C during work‐up. This type
of deactivation process limits the reactivation of spent
catalyst using additional ionic salts.
ACKNOWLEDGEMENTS
The authors are grateful to Sophisticated Analytical
Instrumentation Centre, Tezpur University, for analyses
of various samples and SERB‐DST, India for funding
(research grant no. EMR/2016/002108) to R.B.
ORCID
Susmita Saikia http://orcid.org/0000-0003-1276-7025
Krishna Puri http://orcid.org/0000-0003-1824-556X
Ruli Borah https://orcid.org/0000-0001-5362-398X
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