A Facile Synthesis of Pd–C3N4@Titanate Nanotube Catalyst: Highly Efficient in Mizoroki–Heck, Suzuki–Miyaura C–C Couplings

Article abstract of DOI:10.1007/s10562-019-02955-9

Abstract: A Pd–C₃N₄@titanate nanotube (Pd–C₃N₄@TNT) catalyst workable in water medium, robust against leaching and agglomeration was prepared in a facile synthetic procedure using quite common chemicals such as TiO₂ powder, urea and palladium acetate. The Pd–C₃N₄@TNT catalyst has been characterized by BET surface area and pore size distribution, X-ray diffraction, solid-state ¹³C NMR spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. The Pd–C₃N₄@TNT is a green catalyst for the Miziroki–Heck and Suzuki–Miyaura C–C coupling reactions in water medium with high efficiency (??99% product yields) due to atomic level immobilization of Pd in C₃N₄ networked titanate nanotubes. Pd–C₃N₄@TNT is robust against leaching and agglomeration due to stable and furthermore it is recyclable and usable at least for five repeated cycles. The use of water as solvent, absence of leaching and agglomeration, recyclability and reusability ascertains the greenness of Pd–C₃N₄@TNT) catalyst and process. Graphic Abstract: Novel Pd–C₃N₄@titanate nanotube catalyst prepared from bulk TiO₂ and urea by simple hydrothermal and thermal pyrolysis followed by immobilization of Pd is active and selective for Mizoroki–Heck, Suzuki–Miyaura C–C couplings in water medium.[Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02955-9

Catalysis Letters
https://doi.org/10.1007/s10562-019-02955-9
A Facile Synthesis of Pd–C3N4@Titanate Nanotube Catalyst: Highly
Efcient in Mizoroki–Heck, Suzuki–Miyaura C–C Couplings
Venkata Ramana Kumar Velpula1,2 · Thirupathaiah Ketike1 · Gidyonu Paleti1,2 · Seetha Rama Rao Kamaraju1,2
David Raju Burri1,2
·
Received: 19 June 2019 / Accepted: 4 September 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
A Pd–C3N4@titanate nanotube (Pd–C3N4@TNT) catalyst workable in water medium, robust against leaching and agglomera-
tion was prepared in a facile synthetic procedure using quite common chemicals such as TiO2 powder, urea and palladium
acetate. The Pd–C3N4@TNT catalyst has been characterized by BET surface area and pore size distribution, X-ray difraction,
solid-state 13C NMR spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. The Pd–C3N4@
TNT is a green catalyst for the Miziroki–Heck and Suzuki–Miyaura C–C coupling reactions in water medium with high
efciency (˃ 99% product yields) due to atomic level immobilization of Pd in C3N4 networked titanate nanotubes. Pd–C3N4@
TNT is robust against leaching and agglomeration due to stable and furthermore it is recyclable and usable at least for fve
repeated cycles. The use of water as solvent, absence of leaching and agglomeration, recyclability and reusability ascertains
the greenness of Pd–C3N4@TNT) catalyst and process.
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-019-02955-9) contains
supplementary material, which is available to authorized users.
* David Raju Burri
david.iict@gov.in
1
Catalysis & Fine Chemicals Department, CSIR-Indian
Institute of Chemical Technology, Hyderabad 500007, India
2
CSIR-Academy of Scientifc and Innovative Research
(CSIR-AcSIR), New Delhi, India
Vol.:(0123456789)
1 3

V. R. K. Velpula et al.
Graphic Abstract
Novel Pd–C3N4@titanate nanotube catalyst prepared from bulk TiO2 and urea by simple hydrothermal and thermal pyroly-
sis followed by immobilization of Pd is active and selective for Mizoroki–Heck, Suzuki–Miyaura C–C couplings in water
medium.
Keywords Titanate nanotube · C–C coupling · Pd · C3N4
Abbreviations
for the cross-coupling reactions of Mizoroki–Heck and
Suzuki–Miyaura reactions, which are the most promising
synthetic routes in pharmaceutical industries. Most of these
industries rely on homogeneous Pd catalysts, but the involve-
ment of difculties in catalyst separation and its reusability
are unavoidable. Palladium contamination in the product is
also one of the notable issues. To overcome these difculties
the research directed towards the development of hetero-
geneous catalysts. Unambiguously, the problem of catalyst
separation and recyclability was solved with heterogeneous
catalysts, but the leaching and agglomeration of palladium
[4–7] are the most unwanted issues still to be resolved. Phos-
phine ligands are being used to stabilize Pd species, preven-
tion of Pd agglomeration followed by inactive palladium
TNT
Titanate nanotubes
H-TNT Hydrogenated titanate nanotubes
GVL
DCM
GC
Gamma-Valerolactone
Dichloromethane
Gas chromatography
GC-MS Chromatography–mass spectrometry
1 Introduction
Noble metals are capable of catalyzing most of the organic
transformations [1–3], wherein, palladium is one of the key
transition metals, generally engaged as an efcient catalyst
1 3

A Facile Synthesis of Pd–C3N4@Titanate Nanotube Catalyst: Highly Efcient in Mizoroki–He…
black formation [8, 9]. Product contamination with ligand
residues is one of the notable issues in pharmaceutical
industries. The permissible limit of contamination is gener-
ally below 10 ppm [10], hence developments of alternative
eco-friendly heterogeneous catalysts are highly desirable.
To circumvent the problem of palladium leaching, diferent
porous materials having peculiar porous network were used
as supports, which are capable of preventing the leaching
by accommodate palladium particle in the pore texture. But,
still complete prevention of palladium leaching is not yet
been achieved. In this context, immobilization of palladium
with certain functional groups that are anchored to supports
like silica, carbon, polymer, Pd@CDNS–g–C3N4, ordered
mesoporous SBA-15/PrSO3 and metal–organic framework
[11–18] is one of the important options.
non-toxic bio-mass derived renewable solvent like
γ-Valerolactone [24]. But less attention is being paid to use
water as a solvent even though it is cheap, abundant and
clean.
The present work deals with the design and synthesis
of novel heterogeneous palladium catalyst (Pd–C3N4@
TNT) for effective utilization in Mizoroki–Heck and
Suzuki–Miyaura carbon–carbon bond forming reactions
under eco-friendly water medium. To the best of our knowl-
edge synthesis of melon (C3N4) anchored titanium nanotubes
(TNT), immobilization of palladium in melon network to
attain atomic level dispersion of palladium, maintain the
leaching-free, agglomeration-free environment, applica-
tion as catalyst for Mizoroki–Heck and Suzuki–Miyaura
carbon–carbon bond forming reactions under eco-friendly
water medium is the frst report. Herein, details of catalyst
preparation, characterization and catalytic applications are
delineated.
Supports having high surface area, suitable porosity, held
with organic functional groups capable of frm anchoring
of palladium are highly desirable to prevent the leaching.
Hydrogen titanate nanotubes are intensively studied mate-
rials, attracting much attention in the past decade due to
unique textural and physicochemical properties including
versatile applications, particularly in catalysis and photo
catalysis. Titanate nanotubes that are synthesized from
TiO2 nanoparticles (particle size<25 nm) by hydrothermal
method possess high specifc surface area (≈ 200–400 m2/g),
which are widely used because of their fexibility towards
desired modifcations. Albeit, the preparation of titanate
nanotubes from TiO2 nanoparticles well established, pre-
paring from commercial TiO2 powder are limited.
2 Experimental Section
2.1 Synthesis of Pd–C3N4@TNT Catalyst
and Catalytic Activity
Synthesis of catalyst involves (i) the transformation of bulk
TiO2 powder into titanate nanotubes (ii) initial formation of
sodium form of titanate nanotubes, (iii) transformation of
sodium form of titanate nanotubes (Na-TNT) into H form of
titanate nanotubes (H-TNT), (iv) anchoring of C3N4 (melon)
to H-TNT, and (v) immobilization of palladium in the C3N4
network. The detailed catalyst synthesis is as follows.
Amine and imine functional groups are the most com-
mon chelating agents which are anchored to the surface of
the catalyst supports. Polydopamine (PDA) deposition on
the silicone nanoflament is one of the best known example
[19].The suppression ofPd leaching by nitrogen and oxygen
containing polymer is exploited in nitrogen rich triazine
functionalized mesoporous covalent organic polymer [20].
According to earlier reports TiO2 and SiO2 were modifed
with urea and termed as N-doping or N-modifcation, but
the actual N-species has not been identifed. The subsequent
studies revealed that the N-doped species is C3N4 on the sur-
face of TiO2 powder using the surface –OH of bulk TiO2. It
can be ascertain that it is not only a simple nitrogen insertion
but also surface modifcation with triazine ring derivatives
(malon) [21]. Pyrolysis of urea beyond 400°C gives graphitic
carbon nitride [22], similarly, pyrolysis of urea in presence
of titania and silica at 350–400°C produces a unique materi-
als of (C3N4 triazine) derivatives covalently attached to the
semiconductor which is also called as covalently coupled
inorganic–organic semiconductor [23].
2.2 Synthesis of H‑TNT
Typically, 2.5 g of commercially available TiO2
(Sigma–Aldrich) powder was dispersed in 10 M NaOH
(200 ml) aqueous solution and subjected to hydrothermal
treatment at 130 °C in a Tefon-lined autoclave (250 ml
capacity) for 48 h under autogeneous pressure in static con-
dition. After cooling to room temperature, precipitate was
separated by fltration and washed thoroughly with distilled
water. The resultant white precipitate is the sodium form of
titanate nanotubes (Na-TNT), which was treated with 0.1 M
HCl (1000 ml) at room temperature under constant stirring
for 12 h, fltered, washed, dried at 100 °C for 12 h, which
results the formation of H form titanate nanotubes, hereafter
this material is used as H-TNT [25].
The commonly used solvents for Miziroki–Heck and
Suzuki–Miyaura C–C coupling are dimethylformamide,
dimethylsulfoxide, N-methylpyrrolidin-2-one and N,N-
dimethylacetamide. Of late, attention is diverted towards
2.3 Anchoring of C3N4 to H‑TNT
Anchoring of C3N4 to H-TNT was achieved by pyroly-
sis of urea. In a typical procedure, 1 g of HTNT was
1 3

V. R. K. Velpula et al.
dispersed in 50 ml of aqueous urea solution (5 g urea in
50 ml of water) stirred at RT for 1 h and evaporated the
excess water on a water bath, dried at 60°C for 6 h fol-
lowed by hot air oven at 100 °C for 12 h. The resultant
material was taken in a glass reactor in between the two
layers of glass wool. On the top layer of glass wool 5 g of
urea was placed and increased the temperature to 400 °C
in the flow of N2 gas and calcined at 400°C for 2 h in N2
gas environment (inert atmosphere). The obtained yellow
materials was washed with water (200 ml) and ethanol
(50 ml) and dried at 100°C in a hot air oven for 12 h,
yielded the graphitic-C3N4 anchored to H-TNT, which is
hereafter presented as C3N4@TNT.
2.4 Immobilization of Palladium
0.975 g of C3N4@TNT was dispersed in 30 ml of DCM and
sonicated for 30 min. 0.053 g of palladium acetate dissolved
in 50 ml of DCM was slowly added to the sonicated C3N4@
TNT under content stirring at room temperature for 3 h, cen-
trifuged and the obtained solid material was kept in oven for
drying at 100°C for 12 h, followed by annealing at 250 °C
for 1 h under N2 fow. The resultant palladium immobilized
C3N4@TNT is coded as Pd–C3N4@TNT, where the palla-
dium loading is around 2 wt% determined by SEM–EDX
analysis Fig. 1.
The proposed mechanism of C3N4 formation and its
anchoring on TNT was displayed in Scheme 1. As per
previously reported in literature [21, 23] the thermal
pyrolysis of urea to melamine which give to polymer-
ized to C3N4 lead in four steps (1) urea decomposition to
isocyanic acid (2) the reacts with isocyanic acid forms
Ti–NH2 and CO2 (3) some of isocyanic acid convert to its
tautomer cyanic acid (4) cyanic acid reacts with Ti–NH2
gives cyanamide. It is necessary to presence of OH groups
containing a catalyst to convert cyanamide, which can tri-
merize to melamine, in the absence of the heterogeneous
catalyst, melamine is formed only under high-pressure
conditions. Calcination environment is a crucial step to
get C3N4@TNT. Unless N2 atmosphere is maintained the
resultant product is N-doped TiO2 nanotubes (N-TNT)
which is confirmed by 13C NMR.
3 Results and Discussion
The elemental mapping of Pd–C3N4@TNT was made and
the images are presented in Fig. 2, which shows that the
spacial arrangement of C, N, Pd, Ti, and O respectively and
are homogeneously distributed.
Surface area and pore volume of H-TNT, C3N4@TNT
and Pd–C3N4@TNT are listed in Table 1, gradual decrease
in surface area and pore volume from H-TNT to C3N4@TNT
confrming the anchoring of C3N4 onto H-TNT. Since sur-
face area and pore volume of C3N4@TNT and Pd–C3N4@
TNT are more or less equal, immobilization of palladium
within C3N4 networked titanate nanotubes can be ascer-
tained. The amount of Pd in Pd–C3N4@TNT was measured
by ICP-OES and it is 1.7 wt%.
TEM images were shown in Fig. 3, convey the titanate
nanotubes produced from titanium powder retained their
Scheme 1 Schematic representation of C3N4@TNT synthesis
1 3

A Facile Synthesis of Pd–C3N4@Titanate Nanotube Catalyst: Highly Efcient in Mizoroki–He…
Fig. 1 SEM-EDX of Pd–
C3N4@TNT
Fig. 2 a SEM image of Pd–C3N4@TNT the resultant color-coded Single-element distribution maps of Pd–C3N4@TNT containing, b C, c N, d
Pd, e Ti, f O
Table 1 BET surface area of TNT catalysts
tubular structure with internal diameter 4.834 nm and inter-
layer distance of 0.35 nm even after the anchoring of C3N4
and the palladium immobilization From Fig. 4, the C3N4
formation has been confrmed using 13C NMR spectroscopic
analysis. Solid state 13C NMR chemical shifts values for
Pd–TNT@C3N4 obtained in the region of 163.86 ppm and
S. no
Sample
BET (m2/g)
1
2
3
H-TNT
C2N4@TNT
Pd–C2N4@TNT
181
114
105
1 3

V. R. K. Velpula et al.
Fig. 3 TEM images a titanate
nanotubes, b Pd–C3N4@TNT
Fig. 4 Solid state 13C NMR of
C3N4@ TNT and N-TNT
155.54 ppm. The 13C NMR spectral signals at 165 ppm
and 155.1 ppm represent the poly(tri-s-triazine) structure
which is the characteristic of carbon nitride [26–29].Close
observation of 13C spectral signals at 163.86 ppm and was
assigned to C atoms that were not associated with sp3 N,
whereas a second peak at 155.54 ppm was suggested to rep-
resent C atoms close to the bridging –N groups rather than
at 165 ppm and 155.1 revealing the interaction of C3N4 with
titanate nanotubes as well as immobilized palladium spe-
cies. It is one of the evidences for the formation of malon
structure of graphitic carbon nitride (C3N4) and its interac-
tion with both titanate nanotube and immobilized palladium.
Similar observation was made by Gao et al. [30] when potas-
sium was interacted with C3N4.
Fig. 5 XRD patterns of commercial TiO2 powder, H-TNT, C3N4@
The XRD pattern of commercially purchased TiO2 pow-
der shown in Fig. 5 is in the anatase form in accordance with
the difraction peak positions at 2θ=25.4, 37.9, 48.0, 54.0,
55.1,62.7, 68.8,70.4 and 75.1° and the corresponding (101),
(004), (200), (105), (211), (204), (116), (200) and (215)
crystal planes (JCPDS: 01-07-2486C). The XRD pattern
TNT and Pd–C3N4@TNT
of H-TNT is poorly resolved confrming the transformation
of highly crystalline anatase TiO2 into nanoscopic mate-
rial (H-TNT). Albeit, the XRD pattern of H-TNT is poorly
1 3

A Facile Synthesis of Pd–C3N4@Titanate Nanotube Catalyst: Highly Efcient in Mizoroki–He…
crystalline, four difraction peaks were observed at 2θ=9.33,
24.1, 28.3, and 48.4°, which are typical of monoclinic lay-
ered hydrogen titanate (JCPDS-ICDD Card No. 47-0561).
The difractograms of C3N4@TNT and Pd–C3N4@TNT are
similar to that of the difractogram of H-TNT, except the
appearance of XRD refection at 2θ = 12.4° implying the
distance between fragments of tri-s-triazine (C3N7) in (100)
plane of graphitic–C3N4, (C6N7)–N–(C6N7), the strong-
est peak at 27.6° (002) plane is a characteristic interplanar
stacking peak of 2 aromatic systems.
H-TNT and C3N4@TNT were found at 459.29 and 464.6 eV
of TiO2 correspond to the Ti 2p3/2 and 2p1/2 respectively
[34, 35].
In FT-IR spectra (Fig. 7) several bands are observed in the
range of 1200–1650 cm−1 for C3N4@TNT, is characteristic
of the usual stretching modes of C–N heterocycles. Another
band found at 808 cm−1 is contributing from the ring sex-
tant out-of-plane bending vibrations of triazine rings. The
absence of a band at 2500 cm−1 indicating the absence of
urea moiety, the broad bands at appears 3330 cm−1 and
3160 cm−1 are indicative of stretching vibration modes for
OH and NH. These results further confrming that g-C3N4
structure in TNT prepared by the supramolecular aggrega-
tion from urea.
According to X-ray photoelectron spectra (Fig. 6) the
binding energies of carbon at 288.24 eV and N at 398.78 eV
are responsible for sp2 C=N bond in the g-C3N4 triazine
ring and the peak for N at 400.99 assigned to (C)3–N or
(C)2–N–H. The peaks at 288.24 eV and 284.6 eV in the C
zone are attributed to electrons originating from a sp2 car-
bon (C) atom attached to an NH2 group and to an aromatic
carbon atom [31, 32]. Whereas, the weak peak at 286.4 eV
is ascribed to C–O from adsorbed CO2 and absence of a peak
at 290.7 eV indicating the complete consumption of urea
[33]. The binding energies observed for both Pd–C3N4@
TNT annealed catalyst (at 250°C) and used catalyst are at
343.0 eV, 337.8 eV and 341.0 eV, 335.8 eV respectively cor-
responding to Pd 3d3/2and Pd 3d5/2 indicating that metallic
Pd and Pd2+ in both the catalysts Fig. 6d and e, confrming
that both the catalysts reduced some extent during annealing
and reaction, this phenomenon has been reported elsewhere.
In Fig. 6c and f the shift in XPS spectra of titania 2p for
Fig. 7 FT-IR spectra of H-TNT, C3N4@TNT and Pd–C3N4@TNT
Fig. 6 XPS spectra of Pd–C3N4@TNT a N region, b C region, c Ti in H-TNT, d Pd in fresh catalyst, e Pd in used catalyst, f Ti in C3N4@TNT
1 3

V. R. K. Velpula et al.
Table 3 Scope of Mizoroki–Heck coupling over Pd–C3N4@TNT
3.1 Mizoroki–Heck Reaction
catalyst
The catalytic activity of Pd–C3N4@TNT was determined
in the liquid phase. In a typical procedure, 1 mmol acrylate
1 mmol iodobenzenes, 5 mol percent of TBAI, 10 mg of the
Pd–C3N4@TNT catalyst and 3 ml of water were added to
a RB fask and stirred magnetically at 75–80 °C in atmos-
pheric conditions 45 min, the products were extracted with
ethyl acetate and DCM mixture 3:1 the catalyst was sepa-
rated by centrifugation. The catalytic activity of Pd–C3N4@
TNT catalyst for the Mizoraki–Heck reaction was deter-
mined and data obtained at optimized conditions has been
displayed in Table 2 and the scope of the catalyst perfor-
mance has also been determined and the data displayed in
Table 3. The TON and TOF of the Pd–C3N4@TNT catalyst
for this reaction is 625 and 833 h−1 respectively.
Entry
R1
R2
Yield (%)
1
2
3
4
5
6
7
8
H
n-Butyl
n-Butyl
n-Butyl
n-Butyl
n-Butyl
n-Butyl
n-Butyl
Isobutyl
Isobutyl
Isobutyl
Isobutyl
Isobutyl
Isobutyl
Isobutyl
≈ 99
93
89
≈ 99
71
91
83
97
96
65
70
98
84
65
4-Methyl
2-Methoxy
4-Methoxy
4-Iodo
2,4-Dimethoxy
4-Amino
H
4-amino
4-iodo
2-methoxy
4-methyl
2,4-dimethoxy
4-nitro
9
10
11
12
13
14
3.2 Suzuki–Miyaura Coupling Reaction
Reaction conditions: butyl acrylate (1 mmol), aryl halide (1 mmol),
and catalyst (10 mg), base (2 mmol), TBAI (tet-rabutyl ammonium
iodide, 0.5 mmol) and solvent (2 ml)
The reaction was performed by taking 1.2 mmol of phenyl
boronic acid, 1 mmol of iodobenzene, 1.5 mmol of Na2CO3
and 3 ml of water–ethanol (2:1 v/v) solvent at 80 °C for
1 h. The product mixture was extracted using ethyl acetate-
dichloromethane (3:1, v/v) and analyzed by GC, prior to
Table 2 Optimization of reaction parameters over Pd–C3N4@TNT catalyst for Mizoroki–Heck coupling
Entry
Solvent
Base
Temp. (°C)
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
DMF
DMSO
GVL
Water
ACN
Water
Water
Water
Water
Water
Water
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
50
50
50
50
50
50
65
75
75
75
75
60
60
60
60
60
60
60
45
45
45
45
9
Negligible
42.93
45.6
19.5
45
94
≈ 100
44
9
10
11
K2CO3
aDBU
bNEDA
53
92
Reaction conditions: n-butyl acrylate (1 mmol), aryl halide (1 mmol), and catalyst (10 mg), base (2 mmol), TBAI (tetrabutyl ammonium iodide,
0.5 mmol) and solvent (2 ml), aDBU: 8-Diazabicyclo [5.4.0] undec-7-ene, bNEDA: N-ethyldiisopropylamine
1 3

A Facile Synthesis of Pd–C3N4@Titanate Nanotube Catalyst: Highly Efcient in Mizoroki–He…
Table 4 Scope of Suzuki–Miyaura coupling over Pd–C3N4@TNT catalyst
Entry
R1
R2
Yield (%)
1
2
3
4
5
6
7
8
9
H
H
H
H
H
≈ 100
≈ 100
≈ 100
70
≈ 100
≈ 100
≈ 100
≈ 100
≈ 100
≈ 100
4-Me
4-NO2
4-NH2
4-OMe
H
4-Me
4-NO2
4-NH2
4-OMe
H
4-OMe
4-OMe
4-OMe
4-OMe
4-OMe
10
Reaction conditions: 1.2 mmol of Aryl boronic acids, 1 mmol of aryl halide, 1.5 mmol of Na2CO3 and 3 ml of water–ethanol (2:1 v/v) solvent at
80 °C for 1 h
regular analysis the products were identifed by GC–MS
observed, revealing the robustness against agglomeration
of active component, i.e., palladium species. Palladium
leaching test was conducted by stopping the reaction at
about half of the reaction time (after 30 min) and sepa-
rated the catalyst from product mixture by centrifugation
and continued the reaction for 3 h, but no change either
in the conversion or in the product yield, confrming the
absence of leached Pd in the product mixture. The product
mixture was also analyzed by ICP-AES, no Pd species
were observed. The spent Pd–C3N4@TNT catalyst was
(Table 4).
3.3 Catalyst Reusability and Leaching Test
At the end of each cycle the catalyst was separated by
centrifugation, washed with ethyl acetate-DCM (3:1,
v/v) mixture and reused. Within 5 repeated cycles no sig-
nifcant loss in activity of Pd–C3N4@TNT catalyst was
Table 5 Comparison of catalytic performance of Pd–C3N4@TNT catalyst with reported catalytic systems
Catalyst
Reaction conditions
Yield (%) Ref.
SBA–Pd° catalyst (10 mg, 0.157 mol%)
poly (AAm–coHEMA)@TAP–POSS5@Pd–NPs H2O (2 ml), 2 mmol K2CO3, 100 °C, 5 h
ZrO2@AEPH2–PPh2–Pd (0) (0.2 mol%)
Pd(0)–Arg–boehmite (2.7 mol%)
Pd–pyridyl complexes flms
(GA-FSNP@Pd) (0.47 mol% Pd)
IRMOF-3–Pd
PEG (2 ml), and 3 mmol Na2CO3, at 100 °C, 20 min
91
96
95
98
98
90
100
[36]
[37]
[38]
[39]
[40]
[41]
[42]
PEG 600 (3 ml), 1.5 mmol K2CO3, at 100 °C, 20 min
DMF, K2CO3, 120 °C, 50 min
DMF (6 ml), 2 mmol Na2CO3, 140 °C, 24 h
DMF:H2O (2:1 ml), 110 °C, 30 min
DMA (0.5 ml), iodobenzene (0.2 mmol), methyl acrylate
(0.3 mmol), base (0.3 mmol), Hexamethylenediamine, 140 °C,
15 min
SMNPs-DF–Pd
Pd(OAc)2@SBA–15/PrEn
Pd–C3N4@TNT
Solvent free, DABCO (1.5 mmol), 140 °C, 2 h
DMF:H2O (2 ml),Et3N 2 mmol, 80 °C, 1 h
92
97
~ 99
[43]
[44]
Present work
Base (2 mmol), TBAI (tetrabutyl ammonium iodide, 0.5 mmol)
and solvent (2 ml), 45 min
1 3

V. R. K. Velpula et al.
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is no signifcant structural change was observed. These
results reveal that the Pd–C3N4@TNT is heterogeneous
in nature. The high stability (neither leach nor agglomer-
ate) of Pd–C3N4@TNT catalyst is due to reasons (i) Pd
immobilization in the C3N4 network (ii) frm anchoring of
C3N4 to TNT through elimination of water molecule from
Ti–OH and –NH2 group of C3N4 via Ti–NH bonding. As
it is a solid catalyst ease of separation. High efciency
of Pd–C3N4@TNT is mainly due to isolated Pd species
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A novel palladium immobilized titanate nanotube
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forming reaction is on par with the reported superior cata-
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Acknowledgements The author VVRK thanks Council of Scientifc
Industrial Research (CSIR), New Delhi, India for award of Fellowship.
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