Ti(IV) doping: An effective strategy to boost Lewis acidic performance of ZnO catalyst in fluorescein dye synthesis

Article abstract of DOI:10.1016/j.catcom.2018.11.008

Zn_1-xTi_xO NPs were efficiently synthesized using a simple solution free mechanochemical method. The synthesized ZnO and Ti(IV)-doped ZnO catalysts are exhibited polycrystallinity, a hexagonal crystal structure, and roughly spherical agglomerates. The surface areas of the Zn_1-xTi_xO catalysts were positively correlated with the doping percentage of Ti(IV), which also enhanced the Lewis acidity of the NPs. The catalysts exhibited excellent activity during the synthesis of fluorescein dyes. This methodology was also extended to sulfone-fluorescein dye synthesis.

Full text of DOI:10.1016/j.catcom.2018.11.008

Accepted Manuscript
Ti(IV) doping: An effective strategy to boost Lewis acidic
performance of ZnO catalyst in fluorescein dye synthesis
Nirajkumar H. Jadhav, Dnyaneshwar R. Shinde, Sachin S. Sakate,
Nishant K. Rasal, Ramdas A. Pawar
PII:
S1566-7367(18)30452-7
DOI:
Reference:
https://doi.org/10.1016/j.catcom.2018.11.008
CATCOM 5552
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
9 September 2018
4 November 2018
15 November 2018
Please cite this article as: Nirajkumar H. Jadhav, Dnyaneshwar R. Shinde, Sachin S.
Sakate, Nishant K. Rasal, Ramdas A. Pawar , Ti(IV) doping: An effective strategy to boost
Lewis acidic performance of ZnO catalyst in fluorescein dye synthesis. Catcom (2018),
https://doi.org/10.1016/j.catcom.2018.11.008
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ACCEPTED MANUSCRIPT
Ti(IV) doping: An effective strategy to boost Lewis acidic performance of ZnO
catalyst in fluorescein dye synthesis
Nirajkumar H. Jadhav,a,d Dnyaneshwar R. Shinde,b Sachin S. Sakate,c Nishant K. Rasala and
Ramdas A. Pawar*a,b
a PDEA’s Baburaoji Gholap College, Sangvi, Pune 411027, India.
b PDEA’s Prof. Ramkrishna More College, Akurdi, Pune 411044, India.
c P. E. Society’s Modern College of Arts, Science and Commerce, Shivajinagar, Pune 411005,
India.
d Dr. D. Y. Patil School of Engineering, Lohegaon, Pune 412105, India.
*Corresponding author:
E-mail: rapawar9@gmail.com (Dr. Ramdas A. Pawar)

ACCEPTED MANUSCRIPT
Abstract
Zn1-xTixO NPs were efficiently synthesized using a simple solution free mechanochemical
method. The synthesized ZnO and Ti(IV)-doped ZnO catalysts are exhibited polycrystallinity,
a hexagonal crystal structure, and roughly spherical agglomerates. The surface areas of the
Zn1-xTixO catalysts ware positively correlated with the doping percentage of Ti(IV), which
also enhanced the Lewis acidity of the NPs. The catalysts exhibited excellent activity during
the synthesis of fluorescein dyes. This methodology was also extended to sulfone-
fluorescein dye synthesis.
Keywords: Doped Metal oxide, Fluorescein, Friedel–Craft’s acylation, Lewis acid, Zn1-xTixO
NPs.

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1. Introduction
Zinc oxide (ZnO) NPs have intrinsic structural defects, such as oxygen vacancies (Vo) and Zn
interstitials (Zni), which cause deviation from the stoichiometry [1]. These deviation
automatically result in the formation of an n-type semiconducting material [2, 3]. Thus a
small deviation from the stoichiometry and doping with appropriate donor atoms facilitates
the successful development of the n-type ZnO. Similarly, ZnO NPs having various
morphologies; they exhibit different structural properties, and act as a heterogeneous
catalyst in various organic transformations such as Friedel–Craft’s acylation [4], and
esterification [5] as well as in the synthesis of N-acyl sulfonamides and sulfonate esters [6],
naphthaoxazinones and xanthenes [7, 8], chalcones [9], β-enaminones [10], β- acetamido
ketones or esters [11] and ureas [12].
Sometimes, the perfect crystal surfaces of NPs of the metal oxides exhibit either minimal or
no catalytic activity owing to the lack of active sites or defects in the crystal structure [13,
14]. Studies have reported that small amounts of guest cations with sizes similar to those of
the host atoms can be incorporated the oxide lattice of host atoms (or surface layers); thus,
doping is a powerful technique for enhancing the catalytic performance of heterogeneous
catalysts. The metal used for doping (dopant) not only alters the surface properties but also
enhances the structural, optical, magnetic and electrical properties of ZnO NPs [15-17].
Among the available transition metal dopants, the ionic sizes of Ti(IV) (0.68 Å) and Zn(II)
(0.74 Å) are similar [18]; hence, Ti(IV) an excellent choice among the other dopants.
Similarly, studies have reported that Ti(IV) has higher charge than Zn(II); therefore the
properties of ZnO NPs can be easily tuned and controlled by doping ZnO with Ti(IV) for
modern applications [19, 20].
Fluorescein derivatives represent a class of xanthene dyes, which exhibit high molar
absorption coefficients, high fluorescence quantum yields, high photo stabilities and
absorption and emission at high wavelengths (400-700 nm) [21-23]. Fluorescein
derivatives are used as surgical adjuvants to enhance the visualization of ocular
tissues [24]. Its derivatives are widely used as fluorescent probes for the detection of
various analytes including metal ions, anions, small molecules and bio-molecules in
living cells [25]. These analytes play crucial roles in biological, chemical and

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environmental processes; therefore a lack or an excess of these analytes results in
physiological and environmental problems. Considering the biological and chemical
application of fluorescein dyes, various homogeneous catalyzed methods such as
those involving Fe(HSO4)3 [26], ZnCl2 [27], H3PO4 [28], NbCl5 and MeSO3H [29] and
microwave irradiation [30] have been reported in the literature. The first reported
successful heterogeneously catalyzed synthesis of fluorescein was performed at 473
K in the channels of the acidic zeolite HY [31]. However, these methods have
associated with drawbacks such as high reaction time, high cost, high energy
requirement, use of harmful acids, requirement of inert conditions and tedious work-
up procedures. In addition reaction that require microwave ovens are generally not
suitable for large-scale synthesis. The development of an efficient and
environmentally benign approach that can overcome all the aforementioned
limitations is necessary.
In present work, we developed Zn1-xTixO NPs by using a simple solution free
mechanochemical method. The Zn1-xTixO NPs exhibited high surface areas and Lewis
acidity measured using Brunauer-Emmett-Teller (BET) method and Pyridine-FTIR (PY-
FTIR). Zn1-xTixO NPs acts as active heterogeneous catalysts and provided an efficient,
environmentally friendly and solvent free fluorescein synthesis.
2. Experimental
2.1 Materials and methods
All required chemicals were purchased from Aldrich, Thomas Baker and Finar and
used as received without any further purification. The wide angle X-ray diffraction
(XRD) patterns of the ZnO and Zn1-xTixO catalysts were analyzed using a Rigaku Mini
Flex 600 difractometer. The IR spectra were recorded using FTIR (Spectrum 2000
PerkinElmer) instrument and KBr pellets. The BET method was used to calculate the
multipoint BET surface areas. Scanning electron micrograph (SEM) was obtained
using (XL-30 FEG SEM, Philips) system, at 20 kV. 1H and 13C NMR spectra were
recorded in the solvent DMSO-d6 on a Bruker Advance-III 500 MHz spectrometer
using TMS as an internal standard.

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2.2 Synthesis of ZnO and Zn1-xTixO NPs
The catalysts were prepared by the solution free mechanochemical method [32]. In a
typical procedure, zinc acetate (50 mmol) and oxalic acid (60 mmol) were mixed and
ground for 1h at room temperature. Next, zinc oxalate dihydrate (Zn (C2O4).2 H2O)
precursor was calcined at 600 OC for 2h in air to obtain ZnO NPs. The Zn1-xTixO
catalysts were similarly obtained by the addition of an appropriate quantity of
titanium isopropoxide in the preparation of precursor. Zn1-xTixO containing 25, 50, 75
and 100 mmol Ti(IV) were synthesized.
2.3 General experimental procedure for fluorescein synthesis
In a typical experimental procedure, phthalic anhydride (1 mmol), resorcinol (2
mmol) and Zn0.950Ti0.050O (10 mol%) catalyst were heated with constant stirring for
1.33h at 160 OC and the progress of the reaction was monitored by TLC. After the
completion of reaction, the reaction mixture was dissolved in 1N NaOH (10 mL) and
the catalyst was separated using centrifugation. The filtrate was acidified using 1N
HCl until the solution was neutralized. During neutralization, the precipitation of the
dye was observed which was further purified by recrystallization with methanol.
2.4 General experimental procedure of sulfone-fluorescein synthesis
The mixture of 2-sulfonebenzoic anhydride (1 mmol), phenol (2 mmol) and
Zn0.950Ti0.050O (10 mol%) catalyst were heated with constant stirring for 2h at 140 OC
and the progress of reaction was monitored by TLC. After the completion of reaction,
the reaction mixture was dissolved in methanol and catalyst was separated using
centrifugation. The filtrate was concentrated and further purified by recrystallization
with methanol.
3. Results and discussion
3.1 Textural properties
The crystalline phase of the ZnO catalyst was identified using the recorded powder XRD
patterns (Fig. 1). The powder XRD pattern of the ZnO catalyst exhibited sharp and intense

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peaks at the 2 values of 31.74O, 34.40O, 36.21O, 47.51O, 56.55O, 62.83O, 66.32O, 67.92O,
69.13O, 72.61O, and 77.02O. From the observed 2 values inter-planar spacing’s (d) was
calculated by Brag’s equation. The d’ values from XRD pattern exhibited good agreement
with JCPDS card No. 36-1451; thus, the ZnO catalyst exhibited the typical characteristics of
the hexagonal crystal structure. The powder XRD of Zn1-xTixO catalysts showed similar
patterns to that of the ZnO NPs. No extra peaks in addition to those of hexagonal phase of
ZnO NPs were observed, thus indicating perfect doping of Ti(IV) in the Zn1-xTixO NPs.
However, a decreases in the peak intensities in the XRD patterns of Zn1-xTixO were observed
with an increase in the Ti(IV) content. This can be attributed to the poorer crystallization of
ZnO due to the incorporation of Ti(IV). Crystallite sizes have been reported to decrease with
an increase in the Ti(IV) content of the Zn1-xTixO NPs.
Fig. 1 Wide angle XRD pattern of ZnO, Zn0.900Ti0.100O, Zn0.975Ti0.025O, Zn0.950Ti0.050O and Zn0.925Ti0.075O.
The presence of acidic sites in the ZnO, Zn0.950Ti0.050O and Zn0.925Ti0.075O catalysts were
identified through PY- FTIR (Figure 2). The band in the region of 1490 cm−1 was common to
both Lewis and Brønsted types of acidic sites whereas band in the 1452 cm-1 region (metal
coordinated bond) was assigned to Lewis acid sites. However, the band at 1611 cm−1 was
assigned to the Brønsted acid sites due to hydrogen bonded pyridine [33, 34].

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Fig. 2 Pyridine IR spectra of ZnO, Zn0.950Ti0.050O and Zn0.925Ti0.075O.
The surface areas of the ZnO and Zn1-xTixO catalysts were determined by using the BET
method. Among the catalysts, ZnO exhibited the lowest surface area (15.74 m2/g);
Zn0.900Ti0.100O exhibited the highest surface area (49.11 m2/g); and Zn0.975Ti0.025O,
Zn0.950Ti0.050O and Zn0.925Ti0.075O exhibited intermediate surface area between these two
(28.22, 42.82 and 44.13 m2/g) respectively. The surface areas were correlated with
crystallite sizes and grain sizes of the catalysts. Ti(IV) doping in ZnO resulted in poor
crystallization of doped ZnO which reflects in terms of smaller crystallite size. Therefore with
increasing Ti(IV) in ZnO surface area of catalyst goes on increasing. The increase in surface
area due to doping ZnO with Ti(IV) augmented the catalytic activity of Zn1-xTixO catalysts.
3.2 Catalytic screening and optimization of reaction conditions
The catalyst activity was tested in the model reaction of fluorescein synthesis using
phthalic anhydride and resorcinol performed under solvent-free conditions (Table 1).
Initially to verify the role of catalyst, the reaction was performed without catalyst for
24h. However the formation of the product was not observed despite the long
reaction time (Table 1, Entry 1). Under similar reaction conditions, different metal
oxides were screened for their activity, ZnO and TiO2 showed highest catalytic activity

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with fluorescein yields of 78% and 68% respectively (Table 1, Entry 2-3). The products
yields
Table 1 Catalytic screening and optimization for fluorescein synthesis
HO
O
OH
O
HO
OH
Catalyst
+
O
O
Reaction conditions
-H2O
O
O
1a
2a
3a
Catalyst amount
(mol%)
Temp. (OC)
Entry
1
Catalyst
-
Time (h)
Yield (%)*
-
160
160
24
1.5
0
2
ZnO
10
10
10
10
10
10
10
10
10
78
68
42
38
21
84
95
91
86
3
TiO2
ZrO2
SiO2
CeO2
1.5
160
160
160
160
160
160
160
160
160
160
25
4
1.5
5
1.5
6
1.5
7
Zn0.975Ti0.025
Zn0.950Ti0.050
Zn0.925Ti0.075
Zn0.900Ti0.100
Zn0.950Ti0.050
Zn0.950Ti0.050
Zn0.950Ti0.050
Zn0.950Ti0.050
Zn0.950Ti0.050
Zn0.950Ti0.050
O
1.33
1.33
1.33
1.33
8
O
O
O
O
O
O
O
O
O
9
10
11
12
13
14
15
16
5
2
1.33
24
3
83
95
0
15
10
10
10
10
140
150
170
41
73
94
3
1.33

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Reaction conditions: phthalic anhydride (1 mmol), resorcinol (2 mmol), catalyst, catalyst amount,
temperature and time specified as given, *isolated yield.
with ZrO2, SiO2 and CeO2 as catalysts were 42%, 38% and 21% respectively (Table 1,
Entry 4-6). Based on these results, we investigated the effects of the Ti(IV) doped ZnO
catalysts on the synthesis of fluorescein under the same reaction conditions. The use
of Zn0.975Ti0.250O did not significantly increase the product yield (84%) (Table 1, Entry
7) because of the very low doping concentration of Ti(IV). Further increase in the
Ti(IV) doping concentration in ZnO increased the yield of fluorescein (95% and 91%)
when Zn0.950Ti0.050O and Zn0.925Ti0.075O respectively (Table 1, Entry 8-9) were used as
catalyst. A slight decrease in yield (86 %) was observed with Zn0.900Ti0.100O (Table 1,
Entry 10). From the experimental results, Zn0.950Ti0.050O exhibited the highest catalytic
activity among the all prepared catalyst. For the optimization of synthesis reaction
conditions for fluorescein, the reaction was carried out using the Zn0.950Ti0.050
O
catalyst. First, to study the effect of catalyst loading under similar reaction
conditions, the catalyst concentration was reduced to 5 mol%; consequently, the
yield reduced to 83 % and reaction required relatively longer time (of approximately
0
2h at 160 C) for completion (Table 1, Entry 11). When the catalyst concentration was
10 mol%, the yield was 95% under similar reaction conditions (Table 1, Entry 8).
However, increasing the catalyst concentration to 15 mol%, did not show any
improvement in the yield (95%) of the product (Table 1, Entry 12). No reaction was
observed at room temperature about 24h
(Table 1, Entry 13). At lower temperature, the yield of the product decreased to 41%
and 73% respectively (Table 1, Entry 14 and 15). However, as temperature increased,
no further improvement was observed in the yield of product (Table 1, Entry 16).
Therefore, the optimized reaction conditions were 10 mol% Zn0.950Ti0.050O NPs at 160
OC under solvent free conditions for 1.33h and fluorescein yield was 95%.
3.3 Substrate scope
After optimization of the reaction conditions, Friedel-Craft’s acylation reaction was
successfully conducted using the Zn0.950Ti0.050O catalyst with different substituted
phenols exerting electronic effects with symmetrical phthalic anhydride derivatives
produce fluorescein dyes in moderate to excellent yields (54-96%) (Scheme 1).

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Initially
resorcinol
(2a)
reacted smoothly with phthalic anhydride
(1a),
tetrachlorophthalic anhydride (1b) and tetrabromophthalic anhydride (1c); excellent
yields (86-95%) of the respective fluorescein dyes were obtained (Scheme 1, Entry
3a-3c). Similarly anhydride
O
O
R
R
R
R
X
O
HO
O
O
OH
OH
HO
O
R
S
O
O
1a-c
O
O
O
10 mmol% Zn0.950Ti0.050
140 OC, solvent free
-H2O
O
HO
OH 10 mmol% Zn0.950Ti0.050
O
S
O
X
160 OC, solvent free
-H2O
O
O
3a-q
4a-b
2a-h
HO
O
OH
HO
O
OH
HO
O
OH
HO
O
OH
Cl
Br
O
O
O
O
Cl
Br
O
O
O
Cl
Br
O
Cl
O
Br
O
MeO
MeO
O
OMe
OMe
HO
O
HO
OH
OH
3d: 1.5h, 94%
3a: 1.33h, 95%
3b: 2h, 89%
3c: 2h, 86%
Cl
Cl
O
O
Cl
Cl
Br
O
O
Cl
Br
O
O
O
O
Cl
Cl
Br
Cl
Br
3e: 1.5h, 88%
3f: 1.5h, 85%
3h: 4h, 68%
3g: 4h, 71%
MeO
Br
O
O
O
O
OMe
Br
Br
Cl
Br
O
O
O
O
Cl
Br
O
O
O
O
Cl
Br
Br
Cl
Br
3i: 4h, 66%
3j: 4h, 64%
3l: 4h, 61%
3k: 4h, 60%
HO
Cl
O
OH
O
OH
Cl
HO
O
OH
OH
O
OH
HO
Cl
OH
HO
Cl
Cl
Cl
O
O
O
O
O
Cl
O
O
Cl
O
3m: 1.5h, 88%
3n: 1.5h, 82%
3o: 2h, 79%
3p: 1h, 96%
Cl
O
O
OH
HO
Cl
OH
OH
Cl HO
Cl
O
OH
Cl
Cl
O
O
O
S
O
O
O
S
O
Cl
Cl
O
3q: 5h, 54%
4a: 2h, 61%
4b: 2h, 58%
(Reaction conditions: anhydride (1 mmol), phenol (2 mmol) and Zn0.950Ti0.050O (10 mol%), time and yield specified
as given, isolated yields)

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Scheme 1 Friedel-Craft’s acylation of anhydride derivatives with different phenols.
derivatives (1a-1c) on reaction with 2-methyl resorcinol (2b) produced fluorescein
dyes (Scheme 1, Entry 3d-3f) in excellent yields (85-94%). Fluorescein dyes (Table 4,
Entry 3g-3l) obtained from 3-methoxy phenol (2c) and 3-methyl phenol (2d)
produced relatively low yields (60-71%) and require long reaction time approximately
4h. Furthermore 4-chlororesorcinol (2e) produced satisfactory yields of the products
(Scheme 1, Entry 3m-3n) with phthalic anhydride (1a) and tetrachlorophthalic
anhydride (1b); however, tetrabromophthalic anhydride (1c) did not react with 4-
chlororesorcinol (2e). Orcinol (2f) and pyrogallol (2g) reacted only with the phthalic
anhydride and produced fluorescein dyes about 79 and 96 % respectively (Scheme 1,
Entry 3o-3p), while 3-chloro phenol (2h) reacted only with the tetrachlorophthalic
anhydride (1b) and produced a lower yield of dye. (Scheme 1, Entry 3q). Similarly the
sulfone-fluorescein derivatives were synthesized using 2-sulfonebenzoic anhydride
and phenols under similar reaction conditions (10 mol% Zn0.95Ti0.05O at 140 0C) to
obtain yields of 61 and 58% respectively. (Scheme 1, Entry 4a -4b). In this study, we
conducted a series of experiments as result of these reactions, to create a small
library of fluorescein and sulfone-fluorescein derivatives in moderate to excellent
yield within a short period using Zn0.950Ti0.050O NPs.
3.4 Plausible mechanism for fluorescein synthesis using Zn0.950Ti0.050O NPs.
Scheme 2 Plausiblereaction mechanismfor fluorescein synthesis.

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It is assumed that, the catalyst is act as a Lewis acid and the reaction proceeds
through the Friedel craft’s acylation between phthalic anhydride and resorcinol. The
catalyst activates the carbonyl group of phthalic anhydride and resorcinol attacks the
same site to form monoacylated product (III). Then, the product formed reacts with
second molecule of resorcinol, followed by proton transfer with the loss of water,
thus leading to the formation of fluorescein (Scheme 2).
3.5 Recyclability study
Fig. 3 Catalyst recycling study. (Reaction conditions: phthalic anhydride (1 mmol), resorcinol (2 mmol) and
Zn0.950Ti0.050O (10 mol%); Time - 1.33h and temperature - 160 OC)
Fig. 4 Zn0.950Ti0.050O catalyst filtration test (leaching test)
The recyclability of the Zn0.950Ti0.050O NPs was studied in the reaction between
phthalic anhydride and resorcinol under the optimized reaction conditions up to four
recycles (Fig. 3). After completion of the reaction, the catalyst was separated using

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centrifugation. Then, the catalyst was washed 2 to 3 times with ethanol and dried in
air oven at 100 OC. The dried catalyst was further used in this reaction without any
prior activation or additives. The experimental results showed that the catalytic
activity of Zn0.950Ti0.050O NPs showed only marginal decrease in yield (95 to 86 %)
after four catalytic cycles. The reaction crude did not show any leaching of
Zn0.950Ti0.050O, which was confirmed using hot filtration test (Fig. 4). Thus, the catalyst
was quite stable under these reaction conditions, the decrease in activity could be
due to handling losses of the catalyst after each run, as the initial catalyst charges
was only 10 mol%. The XRD pattern of the recycled Zn0.950Ti0.050O catalyst was studied
after the four cycles, and was found to be indistinguishable from those of the fresh
catalyst (Fig. 1). This observation suggests the high chemical stability of Zn0.950Ti0.050
catalyst.
O
Conclusion
In summary, Zn0.950Ti0.050
O
NPs were synthesized by a simple solution free
mechanochemical method. The textural properties of the NPs such as surface area
and Lewis acidity were characterized using BET surface area and PY-FTIR. An increase
in the concentration of TI(IV) enhanced the Lewis acidity of the catalyst. Considering
the acidic nature of the catalyst Zn0.950Ti0.050O, it was further tested for the Friedel-
Craft’s acylation reaction as a heterogeneous catalyst. Different substituted phenols
were reacted with symmetrical phthalic anhydride or 2-sulfobenzoic anhydride
respectively, for the synthesis of fluorescein and sulfone-fluorescein in good to
excellent yields. The result of recyclability studies showed that, Zn0.950Ti0.050O NPs
were stable up to four recycles.
Acknowledgements
One of the authors is thankful to the Principal Dr. M. G. Chaskar, of PDEA’s B. Gholap
College, Sangavi, Pune for providing the necessary research facilities for the present
work.
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