Perlite impregnated with HPA/α-Fe2O3: Magnetically separable catalyst for the synthesis of 3,3,6,6-tetramethyl-9-substituted-3,4,6,7-tetrahydro-2H-xanthene-1,8-(5H,9H)-diones

Article abstract of DOI:10.1016/j.jpcs.2021.110192

Phosphotungstic acid (HPA)/α-Fe₂O₃-loaded Perlite has been prepared as a recyclable heterogeneous catalyst for the synthesis of xanthenes. The prepared catalyst has been characterized by Fourier-transform infrared (FTIR) spectroscopy

Full text of DOI:10.1016/j.jpcs.2021.110192

Journal of Physics and Chemistry of Solids 157 (2021) 110192
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
Perlite impregnated with HPA/ -Fe O : Magnetically separable catalyst for
α
2 3
the synthesis of 3,3,6,6-tetramethyl-9-substituted-3,4,6,7-tetrahydro-2H-
xanthene-1,8-(5H,9H)-diones
a
a
,
*
b
Komalavalli Lakshminarayanan , Amutha Parasuraman , Manawwer Alam ,
b
c
d
,
**
d,***
Naushad Ahmad , Balu Krishnakumar , Annamalai Raja
, Misook Kang
a
Department of Chemistry, PSGR Krishnammal College for Women, Coimbatore, 641004, India
b
Department of Chemistry, College of Science, King Saud University, Riyadh, 11451, Saudi Arabia
c
Department of Chemistry, Yonsei University, Seoul, 03722, Republic of Korea
d
Department of Chemistry, College of Natural Sciences, Yeungnam University, 280 Daehak-Ro, Gyeongsan, Gyeongbuk, 38541, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords:
α
2 3
Phosphotungstic acid (HPA)/ -Fe O -loaded Perlite has been prepared as a recyclable heterogeneous catalyst for
Perlite
the synthesis of xanthenes. The prepared catalyst has been characterized by Fourier-transform infrared (FTIR)
spectroscopy, X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) with energy-dispersive X-ray
Phosphotungstic acid
α
2 3
-Fe O
(
EDX) spectroscopy, transmission electron microscopy (TEM) with selected-area electron diffraction (SAED)
Magnetically separable
Heterogeneous catalyst
Reusable
pattern analysis, thermogravimetric analysis (TGA), and Brunauer–Emmett–Teller (BET) measurements. XRD
◦
◦
◦
◦
◦
(
peaks at 2θ = 9.08 and 26.73 (HPA) and 36.03 , 39.72 , and 62.6
-Fe on the Perlite surface. TEM images of the prepared HPA/
loading of -Fe
α
-Fe
2
O
3
) showed efficient loading of HPA/
α
2
O
3
α-Fe
2
O
3
/Perlite corroborated the successful
α
2
O
3
on the Perlite. Advantages of this new protocol include facile handling, low cost, easy work-up
procedure, and remarkable reusability over several cycles without loss of the catalytic activity.
1
. Introduction
Superior alternatives to homogeneous catalysts are solid supports
incorporating immobilized magnetic nanoparticles, such as Fe
2 3
O [20,
Currently used heterogeneous catalysts have proven to be superior to
21], Fe , Fe , or Fe [22], which are known for their ease of
2
O
5
2
O
4
3 4
O
their homogeneous predecessors. They have enabled synthetic routes to
novel compounds in a cost-effective and benign manner. They are also
well known for their ease of handling, low toxicity, and high-yielding
transformations of organic compounds. Impregnated solid supports
can act as facile and improved heterogeneous catalysts for synthetic
organic transformations. They can retain their characteristics and pro-
vide excellent reaction enhancement [1–3]. Nanomaterials show
outstanding properties, opening new possibilities for various applica-
tions [4–8]. Nano-sized and suitably blended solid-acid catalysts have
proven to be suitable scavengers for biologically recalcitrant ions such as
Cu(II), Cr(VI), and Cr(II), as well as noxious organic waste, as a result of
their inherent characteristics of high porosity and large surface area. The
most attractive feature of heterogeneous solid-acid catalysts is their safe
and facile removal and handling [9–19].
separation using a simple permanent magnet without any tedious steps,
extended reactivity, and increased number of active sites. Among the
2 3
various iron oxides, Fe O is highly versatile, of low toxicity, easily
recoverable, and has been widely applied in various fields, such as
nucleophilic substitution reactions [23], heterocycle synthesis [24],
coupling reactions [25], catalysis [26], and sensors [27]. Owing to its
conductivity, it is notably used in supercapacitors [28] and in
conjunction with surfactants [29]. Solid supports incorporating
magnetically separable materials are used in synthesis, as well as
biomedical fields, such as bio-imaging, drug delivery, and so on [30,31].
α
2 3
Of these, those incorporating -Fe O nanoparticles can be obtained by a
range of techniques, such as sol–gel, microemulsion, dispersion, etc. The
co-precipitation methodology has proved to be favorable for the prep-
aration of stable composites that can be magnetically separated [32,33].
*
Corresponding author. Department of Chemistry, PSGR Krishnammal College for Women, Coimbatore, 641004, India.
*
*
* Corresponding author.
** Corresponding author.
E-mail addresses: amuchem@gmail.com (A. Parasuraman), rajaannamalai88@gmail.com (A. Raja), mskang@ynu.ac.kr (M. Kang).
https://doi.org/10.1016/j.jpcs.2021.110192
Received 30 November 2020; Received in revised form 18 May 2021; Accepted 19 May 2021
Available online 26 May 2021
0
022-3697/© 2021 Elsevier Ltd. All rights reserved.

K. Lakshminarayanan et al.
Journal of Physics and Chemistry of Solids 157 (2021) 110192
Heteropoly acids (HPA) have been reported as better alternatives to
conventional catalysts, owing to their protonating ability with superior
active sites, possess advantages of high stability, non-toxicity, and
non-corrosivity, and as such are exploited as benign systems for organic
processes [34–36]. HPA-catalyzed transformations are usually free from
side reactions and provide good yields. Developing a solid-acid catalyst
with Keggin-type structured HPA supported on an inert material has
proven to be a good choice for obtaining various heterocycles, due to the
increased catalytic activity of such systems. A catalyst with a higher
surface area will clearly show enhanced activity, and thus impregnating
a solid inert silica-like material with HPA ensures excellent surface area
and available active sites [37–39].
Catalyst systems based on the naturally occurring mineral Perlite
have acquired significance lately due to characteristic features of ease of
access, economy, reaction-enhancing ability at minimal loading,
reduced time consumption, and high yield. The synthesis of heterocyclic
compounds using Perlite-supported solid-acid catalysts has been widely
investigated. Researchers have paid more attention to heterocycles due
to their unique properties that underpin their numerous biological and
pharmaceutical applications. Among these heterocycles, the synthesis of
xanthenes is of substantial interest owing to their pharmacological ac-
tivities, such as anti-oxidant, anti-microbial, anti-cancer, analgesic, and
anti-inflammatory effects [40–43]. Hence, in continuation of our work
on heterogeneous catalysis [44], we set out to prepare a solid-acid
methods. The corresponding 1H and 13C NMR spectra are given as
Figs. S1–S18 (see the Supporting Information).
3,3,6,6-Tetramethyl-9-phenyl-3,4,6,7-tetrahydro-2H-xanthene-1,8-
ꢀ
1
(5H,9H)-dione (1a): White solid; IR (KBr, cm ):
1626.75, 1463.57, 1357.89, 1134.14, 1002.98, 694.37; H NMR (500
MHz, CDCl
, ppm): δ 7.30–7.08 (m, 5H, Ar–H), 4.75 (s, 1H, H-9), 2.46 (s,
4H, H-2, H-7), 2.25–2.22 (d, 2H, H-4), 2.18–2.15 (d, 2H, H-5), 1.10 (s,
ν
2954.95, 1658.78,
1
3
1
3
6H, (CH
196.39 (carbonyl C
144.08–126.38 (Ar–C), 50.75 (C-2, C-7), 40.89 (C-9), 32.21 (C-4, C-5),
31.84 (C-3, C-6), 29.27 (C–CH ), 27.34 (C–CH ).
9-(2-Chlorophenyl)-3,3,6,6-tetramethyl-3,4,6,7-tetrahydro-2H-xan-
3
)
2
), 0.99 (s, 6H, (CH
3
)
2
); C NMR (125 MHz, CDCl
3
, ppm): δ
–
–
O), 162.24 (C-12, C-13), 115.69 (C-11, C-14),
3
3
ꢀ 1
thene-1,8-(5H,9H)-dione (1b): Yellow solid; IR (KBr, cm ):
ν
2926.05,
1639.38, 1342.73, 1169.27, 738.74, 450.27; H NMR (500 MHz, CDCl
ppm): δ 7.45–7.06 (m, 4H, Ar–H), 5.02 (s, 1H, H-9), 2.47 (s, 4H, H-2, H-
7), 2.26–2.22 (d, 2H, H-4), 2.19–2.16 (d, 2H, H-5), 1.12 (s, 6H, (CH ),
, ppm): δ 196.46
O), 162.95 (C-12, C-13), 139.82–126.28 (Ar–C), 113.64
(C-11, C-14), 50.65 (C-2, C-7), 40.76 (C-9), 31.97 (C-4, C-5), 31.82 (C-3,
C-6), 29.23 (C–CH ), 27.32 (C–CH ).
9-(4-Chlorophenyl)-3,3,6,6-tetramethyl-3,4,6,7-tetrahydro-2H-xan-
1
3
,
3 2
)
1
3
3 2 3
1.03 (s, 6H, (CH ) ); C NMR (125 MHz, CDCl
–
(carbonyl C
–
3
3
ꢀ
1
thene-1,8-(5H,9H)-dione (1c): Yellow solid; IR (KBr, cm ):
1631.78, 1520.91, 1327.03, 1168.86, 742.59; 1H NMR (500 MHz,
CDCl
, ppm): δ 7.27–7.17 (m, 4H, Ar–H), 4.71 (s, 1H, H-9), 2.46 (s, 4H,
H-2, H-7), 2.25–2.22 (d, 2H, H-4), 2.18–2.15 (d, 2H, H-5), 1.10 (s, 6H,
ν
2924.09,
3
heterogeneous catalyst of phosphotungstic acid (HPA)/
α
-Fe
2 3
O /Perlite
1
3
by a co-precipitation method. The activity of HPA/
α-Fe
2
O
3
/Perlite has
(CH
196.38 (carbonyl C
15.27 (C-11, C-14), 50.69 (C-2, C-7), 40.85 (C-9), 32.21 (C-4, C-5),
31.47 (C-3, C-6), 29.28 (C–CH ), 27.29 (C–CH ).
-(4-Bromophenyl)-3,3,6,6-tetramethyl-3,4,6,7-tetrahydro-2H-xan-
3
)
2
), 0.99 (s, 6H, (CH
3
)
2
); C NMR (125 MHz, CDCl
3
, ppm): δ
–
–
been investigated in the synthesis of xanthenes.
O), 162.45 (C-12, C-13), 142.70–128.22 (Ar–C),
1
2
. Experimental
3
3
9
ꢀ 1
2
.1. Synthesis of
α
-ferric oxide [45,46]
thene-1,8-(5H,9H)-dione (1d): Light-yellow solid; IR (KBr, cm ):
ν
2
926.01, 1654.92, 1363.67, 1157.29, 1145.39, 1001.02, 846.25,
1
α
-Fe
0.6 g) and FeCl
mL). This mixture was then added dropwise with stirring to 25% NH
2
O
3
was prepared by a co-precipitation method. FeCl
⋅6H O (1.5 g) were dissolved in 2 M HCl solution (30
OH
2
⋅4H
2
O
697.53; H NMR (500 MHz, CDCl
3
, ppm): δ 7.34–7.16 (m, 4H, Ar–H),
(
3
2
4.70 (s, 1H, H-9), 2.46 (s, 4H, H-2, H-7), 2.25–2.22 (d, 2H, H-4),
1
3
4
2.18–2.15 (d, 2H, H-5), 1.10 (s, 6H, (CH
NMR (125 MHz, CDCl , ppm): δ 196.37 (carbonyl C
C-13), 143.23–120.24 (Ar–C), 115.19 (C-11, C-14), 50.69 (C-2, C-7),
40.84 (C-9), 32.21 (C-4, C-5), 31.56 (C-3, C-6), 29.28 (C–CH ), 27.30
(C–CH ).
9-(4-Fluorophenyl)-3,3,6,6-tetramethyl-3,4,6,7-tetrahydro-2H-xan-
3
)
2
), 0.99 (s, 6H, (CH
3
)
2
);
C
–
–
solution (75 mL), resulting in a solution of pH 11. Further 2 M HCl was
added dropwise until pH 2 was attained, and the resulting mixture was
stirred for 2 h. It was then centrifuged, and the collected solid was
3
O), 162.47 (C-12,
3
washed with water to remove excess NH
4
OH. The obtained product was
3
◦
dried at 80 C for 8 h.
ꢀ 1
thene-1,8-(5H,9H)-dione (1e): White solid; IR (KBr, cm ):
1654.46, 1602.92, 1503.63, 1364.10, 1196.92, 1145.39, 1005.86,
14.80; 1H NMR (500 MHz, CDCl
, ppm): δ 7.27–6.88 (m, 4H, Ar–H),
4.72 (s, 1H, H-9), 2.47 (s, 4H, H-2, H-7), 2.25–2.22 (d, 2H, H-4),
ν
2951.67,
2
α
2 3
.2. Synthesis of -Fe O -loaded perlite
8
3
Perlite and ferric oxide in a 2:1 ratio were suspended in water/
ethanol (1:1; 25 mL) and the mixture was stirred for 8 h. The solvents
1
3
2.19–2.15 (d, 2H, H-5), 1.10 (s, 6H, (CH
NMR (125 MHz, CDCl , ppm): δ 196.53 (carbonyl C
C-13), 139.95–129.81 (Ar–C), 115.50 (C-11, C-14), 50.70 (C-2, C-7),
40.84 (C-9), 32.21 (C-4, C-5), 31.20 (C-3, C-6), 29.26 (C–CH ), 27.28
C–CH ).
3,3,6,6-Tetramethyl-9-(3,4,5-trimethoxyphenyl)-3,4,6,7-tetrahydro-
3
)
2
), 0.99 (s, 6H, (CH
3
)
2
);
–
–
O), 162.41 (C-12,
C
◦
were then evaporated and the residue was dried at 80 C for 8 h.
3
2
.3. Synthesis of HPA/
α
-Fe
2
O
3
/Perlite
3
(
3
α
-Ferric oxide/Perlite and phosphotungstic acid (H
3
[P(W
3
O
10
)
4
]⋅
ꢀ
1
xH
2
O) (1:1.2) were suspended in acetonitrile, and the mixture was
2H-xanthene-1,8-(5H,9H)-dione (1f): Yellow solid; IR (KBr, cm ):
ν
◦
heated under reflux at 60 C for 24 h. The solvent was then evaporated,
2943.92, 1661.01, 1590.36, 1454.01, 1185.03, 1120.58, 1006.54,
◦
◦
1
and the residue was dried at 80 C for 8 h and calcined at 300 C for 3 h.
830.53, 671.87; H NMR (500 MHz, CDCl
.71 (s, 1H, H-9), 3.79 (s, 6H, Ar-CH ), 3.77 (s, 6H, m-OCH
3H, o-OCH
), 2.48 (s, 2H, H-2), 2.47 (s, 2H, H-7), 2.24–2.23 (m, 4H, H-4,
H-5), 1.12 (s, 6H, (CH
CDCl ppm): 196.51 (carbonyl
152.80–136.57 (Ar–C), 115.50 (C-11, C-14), 60.71–56.10 (Ar-CH
50.75 (C-2, C-7), 40.91 (C-9), 32.19 (C-4, C-5), 31.81 (C-3, C-6), 29.38
(C–CH ), 27.20 (C–CH ).
3,3,6,6-Tetramethyl-9-(3,4-dimethylphenyl)-3,4,6,7-tetrahydro-2H-
3
, ppm): δ 6.51 (s, 2H, Ar–H),
4
3
3
), 3.69 (s,
2
.4. General procedure for the synthesis of 3,3,6,6-tetramethyl-9-
3
1
3
substituted-tetrahydro-2H-xanthene-1,8-(5H,9H)-diones (1a–i)
3
)
2
), 1.03 (s, 6H, (CH
3
)
2
); C NMR (125 MHz,
O), 162.35 (C-12, C-13),
),
–
3
,
δ
C
–
A mixture of a diketone (2 mmol) and the requisite aromatic alde-
hyde (1 mmol) in acetonitrile was added to a dispersion of catalyst (0.1
g) in acetonitrile, and the resulting mixture was heated under reflux at
3
3
3
◦
8
0 C for the stipulated duration. Progress of the reaction was monitored
ꢀ 1
by thin-layer chromatography (TLC), and on completion, the mixture
was cooled and the catalyst was separated by means of an external
magnet. The solvent was evaporated and the resulting crude product
was recrystallized from ethanol. The structures of the xanthenes [47]
obtained were confirmed by physical data and IR and NMR spectral
xanthene-1,8-(5H,9H)-dione (1g): Light-yellow solid; IR (KBr, cm ):
ν
2958.79, 1752.73, 1656.07, 1604.74, 1456.87, 1196.57, 1134.25,
1
987.60, 892.28, 818.95; H NMR (500 MHz, CDCl
Ar–H), 6.95 (d, 2H, Ar–H), 4.68 (s, 1H, H-9), 2.46 (s, 4H, H-2, H-7),
2.24–2.14 (m, 10H, H-4, H-5, Ar-(CH ), 1.09 (s, 6H, (CH ), 1.00 (s,
3
, ppm): δ 7.08 (s, 1H,
3
)
2
3 2
)
2

K. Lakshminarayanan et al.
Journal of Physics and Chemistry of Solids 157 (2021) 110192
Fig. 1. FTIR spectra of a) pure Perlite, b)
α
2 3
-Fe O , c) HPA, and d) HPA/
α
2 3
-Fe O /Perlite.
Fig. 2. XRD patterns of a) pure Perlite, b)
-Fe /Perlite.
α
2 3
-Fe O , c) HPA, and d) HPA/
6
H, (CH
3
)
2
); 13C NMR (125 MHz, CDCl
–
–
O), 162.08 (C-12, C-13), 141.56–125.50 (Ar–C), 115.84 (C-11, C-
3
, ppm): δ 196.51 (carbonyl
α
2 3
O
C
1
2
4), 50.79 (C-2, C-7), 40.90 (C-9), 32.23 (C-4, C-5), 31.35 (C-3, C-6),
9.24 (C-(CH ), 27.43 (C-(CH ).
3
. Results and discussion
3
)
2
3
)
2
), 19.86–19.39 (Ar-CH
3
3
,3,6,6-Tetramethyl-9-(4-methylphenyl)-3,4,6,7-tetrahydro-2H-
3
.1. Characterization of HPA/ -Fe O /Perlite
α
2 3
xanthene-1,8-(5H,9H)-dione (1h): White solid; IR (KBr, cm 1):
ꢀ
ν
2
6
4
941.14, 1659.44, 1448.63, 1173.98, 1133.03, 1001.72, 830.67,
87.32; 1H NMR (500 MHz, CDCl
, ppm): δ 7.18–7.01 (m, 4H, Ar–H),
.71 (s, 1H, H-9), 2.45 (s, 3H, Ar-CH
), 2.24–2.14 (m, 7H, H-4, H-5, Ar-
), 1.09 (s, 6H, (CH ), 0.99 (s, 6H, (CH
CDCl ppm): 196.45 (carbonyl
The immobilization of the solid-acid catalyst by loading HPA and
-Fe on the surface of Perlite was studied by FTIR measurements.
3
α
2 3
O
3
The IR spectrum of pure Perlite is depicted in Fig. 1a. The vibrations at
1
3
CH
3
3
)
2
3
)
2
); C NMR (125 MHz,
ꢀ 1
6
77.01, 985.62, 1066.64, 1527.62, and 3578.60 cm correspond to
–
C
–
3
,
δ
O), 162.11 (C-12, C-13),
Si–O–Si bending, Si–O–Al bending, Si–O stretching, Si–OH stretching,
and O–H stretching, respectively [48].
1
9
2
41.20–128.25 (Ar–C), 115.77 (C-11, C-14), 50.77 (C-2, C-7), 40.88 (C-
), 32.20 (C-4, C-5), 31.44 (C-3, C-6), 29.27 (C–CH
1.06 (Ar-CH ).
3
), 27.38 (C–CH
3
),
The IR spectra of
α
-Fe
2
O
3
α
2 3
, HPA, and HPA/ -Fe O /Perlite are dis-
3
played in Fig. 1b, c, and 1d, respectively. In Fig. 1b, the vibrations at
9
-(4-Isopropylphenyl)-3,3,6,6-tetramethyl-3,4,6,7-tetrahydro-2H-
ꢀ
1
1
041.56, 810.10, 594.08, and 439.77 cm can be ascribed to Fe–O
stretchings and bendings, and are characteristic of -Fe [49]. The
stretching frequencies at 1074.35, 964.41, and 900.76 cm in Fig. 1c
xanthene-1,8-(5H,9H)-dione (1i): Light-yellow solid; IR (KBr, cm 1):
ꢀ
ν
α
2 3
O
2
8
958.79, 1658.53, 1608.95, 1460.21, 1357.33, 1191.23, 1046.21,
ꢀ
1
1
28.05, 711.53; H NMR (500 MHz, CDCl
3
, ppm): δ 7.18–7.04 (m, 4H,
–
correspond to P–O, W
indicates characteristic vibrations at 3348.42, 1656.02, 1527.89, and
–
O, and W–O–W vibrations, respectively. Fig. 1d
Ar–H), 4.73 (s, 1H, H-9), 2.99–2.96 (m, 1H, methine H), 2.46 (s, 4H, H-
2
6
, H-7), 2.25–2.22 (d, 2H, H-4), 2.20–2.16 (d, 2H, H-5), 1.29–1.27 (d,
H, isopropyl CH
ꢀ 1
1
066.20/1049.28 cm , corresponding to O–H stretching, Si–OH
13
3
), 1.09 (s, 6H, (CH ); C NMR
–
–
, ppm): δ 196.84 (carbonyl C O), 177.41–155.28 (C-
3
)
2
), 1.00 (s, 6H, (CH
3
)
2
stretching, Si–O–Si stretching, and Si–O–Al bending vibrations, respec-
(
125 MHz, CDCl
3
ꢀ 1
tively. Further peaks at 810.10, 601.79, 547.78, and 439.77 cm can be
ascribed to Fe–O stretchings of -Fe . The vibrations at 897.42 and
82.36 cm can be attributed to W–O–W and W
Keggin-type HPA in HPA/ -Fe /Perlite [50].
1
4
2
2, C-13), 130.38–126.14 (Ar–C), 115.84 (C-11, C-14), 50.73 (C-2, C-7),
α
2 3
O
4.88, 40.88 (C-4, C-5), 32.21 (C-9), 34.48–33.60 (aliphatic C),
ꢀ 1
–
–
O, and correspond to
9
9.70–27.46 (C–CH
3
), 23.91–23.63 (aliphatic CH
3
).
α
2 3
O
3

K. Lakshminarayanan et al.
Journal of Physics and Chemistry of Solids 157 (2021) 110192
Fig. 3. SEM images of a)
α
-Fe
2
O
3
α
2 3
and b) HPA/ -Fe O /Perlite.
4

K. Lakshminarayanan et al.
Journal of Physics and Chemistry of Solids 157 (2021) 110192
Fig. 4. EDX of a)
α
-Fe
2
O
3
α
2 3
and b) HPA/ -Fe O /Perlite.
The synthesized catalysts were analyzed by the powder XRD tech-
nique. The XRD patterns of pure Perlite, -Fe , HPA, and HPA/
-Fe /Perlite are shown in Fig. 2. The XRD patterns corroborate the
crystalline nature of the product and the effective loading of -Fe and
compound is consistent with the nano- and polycrystalline nature of
-Fe . TEM images of the prepared HPA/ -Fe /Perlite indicate the
successful loading of -Fe (see arrow marks) (Fig. 5d and e). The
layered structure of Perlite can clearly be seen, and the spherical par-
ticles of -Fe are embedded on the surface of layered Perlite sheets
(Fig. 5d and e). The diffraction pattern of HPA/ -Fe /Perlite (Fig. 5f)
α
2
O
3
α
2
O
3
α
2 3
O
α
2
O
3
α
2 3
O
α
2 3
O
HPA on Perlite. The XRD pattern of Perlite shows a strong band at
α
2 3
O
◦
2
4.74 , evidencing its amorphous nature (Fig. 2a). The 2θ and hkl
α
2 3
O
indices in Fig. 2b conform well with JCPDS no. 87–1164, confirming the
with irregular spots showed the catalyst to be semicrystalline and
polycrystalline in nature. The particle size distribution curve from the
◦
◦
formation of
α-Fe
2
O
3
[49]. In Fig. 2c, the peaks at 2θ = 10.38 , 20.58 ,
◦
◦
◦
◦
2
6.24 , 28.51 , 31.86 , and 38.18 can be attributed to Keggin-type
TEM results showed that the prepared
20–30 nm particles, with an average particle size of 24.65 nm (Fig. 6).
The thermal stability of the HPA/ -Fe /Perlite catalyst was
2 3
α-Fe O consisted of 70% of
HPA. The XRD pattern of HPA/
It features the peaks of both HPA and
loading of HPA/ -Fe on the Perlite surface.
The surface morphology of the prepared catalyst was examined by
SEM. The SEM image (Fig. 3a) shows the prepared -Fe to be
α
-Fe
2
O
3
/Perlite is displayed in Fig. 2d.
α
-Fe , consistent with efficient
2
O
3
α
2 3
O
α
2
O
3
examined by TGA. Fig. 7 shows a gradual thermal decomposition up to
◦
180 C, corresponding to the loss of water. Thereafter, the decrease from
◦
α
2
O
3
200 to 400 C indicates a 4% loss of Keggin-type HPA, and this is fol-
◦
spherical in nature, and the clusters formed are due to magnetic dipole
interactions, evidencing the magnetic nature of the prepared compound
lowed by a sudden loss at around 500 C, corresponding to complete loss
◦
of HPA. A steady loss after 600 C reflects the removal of hydroxyl
[
49]. Fig. 3b depicts the SEM image of HPA/
α
-Fe
2
O
3
/Perlite, which
groups from the surface of Perlite [51,52].
reveals the effective and uniform loading of -Fe
α
2
O
3
on the Perlite (see
α
2 3
Surface area analysis of the Perlite and HPA/ -Fe O /Perlite was
arrow marks) as well as the layer structure of the latter [44].
Fig. 4 depicts the percentage compositions of the elements in
performed by the BET method. The pore volumes, pore diameters, and
surface areas are tabulated in Table 2, from which it can be seen that
there was an increase in surface area and pore volume of Perlite after
2 3
α-Fe O
and HPA/
peak for W (23%) and the increase in Fe content corroborate the loading
of HPA and -Fe on Perlite.
TEM images and SAED patterns of the prepared materials (
and HPA/ -Fe /Perlite), along with particle size distributions of
-Fe , are given in Figs. 5 and 6. From the TEM images, the
morphology of -Fe was seen to be spherical and of nanometer di-
mensions (Fig. 5a and b). The diffraction pattern (Fig. 5c) of the
2 s
α-Fe O /Perlite. As is evident from Table 1, the additional
HPA/
3.2. Catalytic activity of HPA/
2 3
In order to test the effectiveness of the HPA/ -Fe O /Perlite catalyst,
2 3
α-Fe O loading.
α
2 3
O
α-Fe
2
O
3
α
2 3
-Fe O /Perlite
α
2 3
O
α
2
O
3
α
α
2
O
3
9-substituted xanthenes were prepared by reactions of a diketone with
aromatic aldehydes in its presence (Scheme 1).
5

K. Lakshminarayanan et al.
Journal of Physics and Chemistry of Solids 157 (2021) 110192
Table 1
Composition of the prepared HPA/
α-Fe
2
O
3
/Perlite catalyst.
Fe
Sample
O
Na
Al
Si
K
Ca
W
Perlite
20.28
30.69
1.04
4.12
3.09
1.20
11.05
3.91
60.24
33.76
4.33
1.95
0.82
0.01
–
HPA/
α
-Fe
O
2 3
/Perlite
23.77
The reaction conditions were initially optimized for the reaction of
dimedone with benzaldehyde to afford 3,3,6,6-tetramethyl-9-phenyl-
in each case (Table 5). Of the solvents used, acetonitrile showed good
reaction progress, giving a 95% yield within 1 h (Table 5). The rapid
attainment of high yield made acetonitrile the preferred solvent for
further reactions. Hence, the preparation of xanthene in the presence of
3
,4,6,7-tetrahydro-2H-xanthene-1,8-(5H,9H)-dione. The synthesis of
xanthenes was first evaluated using bare Perlite and Perlite/Fe . Only
a trace amount of product was obtained with Perlite, whereas utilization
of 0.1 g of Perlite/Fe produced an appreciable yield of 89%.
Accordingly, the efficiency of HPA/ -Fe /Perlite was evaluated
2 3
O
◦
0.1 g of catalyst at 80 C using acetonitrile as solvent was identified as
2 3
O
α
2 3
O
further. The amount of catalyst was varied from 0.01 to 0.20 g (Table 3).
The zero yield from the uncatalyzed reaction proved the need for a
catalyst. The reaction with 0.1 g of catalyst was found to proceed with
significant yield. An increase in the amount of the catalyst raised the
yield from 89 to 95% (Table 3), but a further increment in the catalyst
loading beyond 0.1 g decreased the yield. Thus, the catalyst amount was
optimized at 0.1 g for this reaction.
Temperature and solvent effects were also examined to optimize the
protocol. The reaction was carried out at various temperatures
◦
◦
(
60–110 C) with the optimized catalyst loading. At 80 C, the yield of
◦
the product was satisfactory. Increasing the temperature beyond 80 C
did not produce a beneficial effect on the yield (Table 4). Thus, further
◦
reactions were controlled at 80 C.
Furthermore, the reaction was attempted in different solvents,
◦
namely ethanol, water, and acetonitrile, at 80 C using 0.1 g of catalyst
α
2 3
Fig. 6. Particle size distribution of -Fe O .
Fig. 5. TEM images and SAED patterns of a–c)
α
-Fe
2
O
3
and d–f) HPA/
α
2 3
-Fe O /Perlite.
6

K. Lakshminarayanan et al.
Journal of Physics and Chemistry of Solids 157 (2021) 110192
Table 3
Effect of catalyst loading in the synthesis of 3,3,6,6-tetramethyl-9-phenyl-
3
,4,6,7-tetrahydro-2H-xanthene-1,8-(5H,9H)-dione.
a
b
Catalyst
Catalyst amount (g)
Yield (%)
–
–
none
trace
89
Perlite
0.1
0.1
0.01
0.05
0.1
0.15
0.2
α
2 3
-Fe O /Perlite
HPA/
HPA/
HPA/
HPA/
HPA/
α
α
α
α
α
-Fe
-Fe
-Fe
-Fe
-Fe
2
2
2
2
2
O
O
O
O
O
3
3
3
3
3
/Perlite
/Perlite
/Perlite
/Perlite
/Perlite
84
89
95
93
90
a
Reaction conditions: benzaldehyde (1 mmol), dimedone (2 mmol), solvent
=
acetonitrile, time = 60 min at 80 ◦C.
b
Isolated yield.
α
2 3
Fig. 7. TGA for HPA/ -Fe O /Perlite.
Table 4
Effect of temperature in the synthesis of 3,3,6,6-tetra-
methyl-9-phenyl-3,4,6,7-tetrahydro-2H-xanthene-1,8-
5H,9H)-dione.
(
Table 2
◦
a
b
Temperature ( C)
Yield (%)
Pore analysis of Perlite and HPA/
α
2 3
-Fe O /Perlite.
6
0
72
86
95
95
95
95
2
Catalyst
Surface area (m /
g)
Pore volume (cc/
g)
Pore diameter
(Å)
70
80
ꢀ
ꢀ
3
3
90
Perlite
1.874
2.084
2.899 × 10
3.661 × 10
3.910
3.485
100
HPA/
α
2 3
-Fe O /
1
10
Perlite
a
Reaction conditions: benzaldehyde (1 mmol), dime-
done (2 mmol), solvent = acetonitrile, time = 60 min.
b
Isolated yield.
Table 5
Effect of solvent in the synthesis of 3,3,6,6-tetramethyl-9-phenyl-3,4,6,7-tetra-
hydro-2H-xanthene-1,8-(5H,9H)-dione.
a
b
Solvent
Yield (%)
Time (h)
water
87
91
95
12
8
ethanol
3
CH CN
1
a
Reaction conditions: benzaldehyde (1 mmol), dimedone (2 mmol).
Isolated yield.
b
optimal.
Having identified the optimal conditions, this protocol was extended
to various substituted xanthenes. Besides the appreciable results ob-
tained, application of the reaction was studied for several aromatic al-
dehydes (Table 6). As is evident from Table 6, the proposed protocol
proved to be applicable to a variety of substituted aldehydes. Neither
electron-withdrawing nor electron-releasing groups had any appreciable
influence on the reaction outcome. To study the effectiveness of this
protocol, the results were correlated with previously published results
(
Table 7). Although previous reports of the use of heteropoly acids for
the synthesis of xanthenes quoted comparable yields, reaction times
were longer and reusability of the catalyst was difficult. The use of iron
oxide and composites thereof has produced satisfactory yields, but the
reaction temperature was relatively high. Hence, to reconcile these
2 3
differences, we set out to combine the properties of HPA and Fe O by
preparing a composite on Perlite. The strong adsorbability, chemical and
thermal stability, low toxicity, and natural occurrence of Perlite, mainly
comprising silica and alumina, make it a good support for the assembly
of heterogeneous catalysts. These remarkable properties prompted us to
Scheme 1. Synthesis of xanthenes.
α
2 3
prepare HPA/ -Fe O /Perlite. The proposed strategy is promisingly
superior to preceding methods.
The reusability of the catalyst for the synthesis of xanthene from
7

K. Lakshminarayanan et al.
Journal of Physics and Chemistry of Solids 157 (2021) 110192
Table 6
α
2 3
Preparation of 3,3,6,6-tetramethyl-9-substituted-3,4,6,7-tetrahydro-2H-xanthene-1,8-(5H,9H)-diones using HPA/ -Fe O /Perlite.
a
b
◦
◦
Product
R
Product structure
Yield (%)
95
Time (min)
m.p. ( C)
Ref. m.p. ( C)
1a
1b
1c
H
60
45
48
203–205
203–204 [53]
228–230 [54]
230–231 [53]
2-Cl
4-Cl
91
93
229–231
228–229
1
1
1
1
1
1
d
e
f
4-Br
94
92
91
89
85
80
50
55
50
55
70
75
231–232
222–223
184–185
220–221
211–212
213–214
230–231 [55]
224–225 [54]
186–188 [56]
–
4-F
3 3
3,4,5-(OCH )
g
h
i
3 2
3,4-(CH )
4-(CH
3
)
211–214 [57]
4-isopropyl
–
a
b
Reaction conditions: aldehyde (1 mmol), dimedone (2 mmol), catalyst (0.1 g) under reflux in CH
Isolated yield.
3
CN.
benzaldehyde was evaluated. The catalyst used in the reaction could be
easily removed by means of a permanent magnet (Fig. 8).
4. Conclusion
The heterogeneous solid-acid catalyst HPA/
The catalyst was separated from the product after the reaction by
simple filtration. It was washed with hot ethanol, dried, and reused for
further reaction. The results revealed that the prepared catalyst could be
reused up to five times without incurring a significant loss of activity
α-Fe
2
O
3
/Perlite has been
prepared and used for the synthesis of substituted xanthenes. After
optimizing the protocol, the optimal conditions were applied to the
synthesis of various substituted xanthenes. The favorable immobiliza-
(
Table 8).
tion of the HPA/
α
-Fe
2
O
3
on the Perlite was evident from FTIR, XRD,
2 3
-Fe O /
SEM-EDX, TEM, TGA, and BET studies. The efficiency of HPA/
α
8

K. Lakshminarayanan et al.
Journal of Physics and Chemistry of Solids 157 (2021) 110192
Table 7
Effectiveness of HPA/
α
2 3
-Fe O /Perlite compared with other described catalysts in the synthesis of 9-substituted xanthenes.
◦
Catalyst
Catalyst amount
Temp. ( C)
Time
Yield (%)
Ref.
a
SBSSA
0.03 g
0.05 g
0.05 g
200 mg
0.25 g
0.03 g
0.4 mol%
1 mol%
0.02 g
15 mg
20 mol%
0.1 g
RT
10 h
98
84
95
92
87
89
90
90
89
95
90
95
[53]
[47]
[55]
[58]
[59]
[60]
[61]
[62]
[63]
[64]
[65]
our work
MCM-41-SO
b
3
H
60
60 min
2 h
SBSAN
50
Amberlyst-15
reflux
100
reflux
120
90
5 h
HPWA/MCM-41
5 h
c
SBNPSA
2–3 h
H
14NaP
PMo11TiO40
NPs
γ-Fe2O3@HAp-Ag NPs
Fe
5
W
30
O
110
1.5–1.9 h
20–45 min
30–40 min
30 min
2 h
H
5
Fe
3
O
4
100
60
2
O
3
RT
HPA/
α-Fe
2
O
3
/Perlite
80
60 min
a
SBSSA: silica-bonded S-sulfonic acid.
b
c
SBSAN: silica boron–sulfuric acid nanoparticles.
SBNPSA: silica-bonded N-propyl sulfamic acid.
support from King Saud University, Deanship of Scientific Research and
College of Science-Research Center.
Table 8
Reusability of HPA/
α
-Fe
1
2
O
3
/Perlite.
Run
2
3
4
5
Appendix A. Supplementary data
Yield (%)
95
95
93
91
91
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.jpcs.2021.110192.
Authors statement
Komalavalli Lakshminarayanan is involved in the preparation of the
materials and specifically writing the initial draft. Amutha Parasuraman
contributed in the supervision and leadership responsibility for the
research activity planning and execution, including mentorship external
to the core team. Manawwer Alam and Naushad Ahmad are involved in
materials characterization. Balu Krishnakumar is involved in material
characterization’s discussion and correcting the manuscript. Annamalai
Raja and Misook Kang both are involved in the validation of the
manuscript and overall replication/reproducibility of results/experi-
ments and other research outputs.
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