The enhancement of direct amide synthesis reaction rate over TiO2@SiO2@NiFe2O4 magnetic catalysts in the continuous flow under radiofrequency heating

Article abstract of DOI:10.1016/j.jcat.2017.09.010

A series of TiO₂@SiO₂@NiFe₂O₄ composite magnetic catalyst with a core-double shell structure was synthesized by a sol-gel method. The morphology of the catalysts was studied by XRD, SEM, N₂ physisorption and their magnetic properties were examined with magnetometry, and specific absorption rate measurements. The catalytic activity was determined in a direct amide synthesis reaction between aniline and phenylbutyric acid at 150 °C in a fixed bed flow reactor under radiofrequency heating. The intermediate silica layer of the catalyst increased the porosity of the outer titania layer and the specific absorbance rate of the catalyst. The initial reaction rate increased by 61% as compared to a similar core-shell TiO₂@NiFe₂O₄ catalyst showing the detrimental effect of nickel ferrite on titania. The reaction rate was further increased by a factor of 3.5 after a sulfation treatment due to an optimum Lewis acid site strength. The highest specific reaction rate over TiO₂@SiO₂@NiFe₂O₄ was observed at a 7.5 wt% sulfate loading which was 2.6 times higher as compared to a mechanical mixture of the same composition. The initial reaction rate decreased by 36% after a period of 55 h on stream. The catalyst activity was restored after a treatment with a H₂O₂ solution.

Full text of DOI:10.1016/j.jcat.2017.09.010

Journal of Catalysis 355 (2017) 120–130
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
The enhancement of direct amide synthesis reaction rate over
TiO2@SiO2@NiFe2O4 magnetic catalysts in the continuous flow under
radiofrequency heating
Yawen Liu a, Nikolay Cherkasov a, Pengzhao Gao b, Javier Fernández a, Martin R. Lees c,
Evgeny V. Rebrov a,d,
⇑
a School of Engineering, University of Warwick, Coventry CV4 7AL, UK
b College of Materials Science and Engineering, Hunan University, Changsha 410082, China
c Department of Physics, University of Warwick, Coventry CV4 7AL, UK
d Department of Biotechnology and Chemistry, Tver State Technical University, 170026, Nab. A. Nikitina 22, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of TiO2@SiO2@NiFe2O4 composite magnetic catalyst with a core-double shell structure was syn-
thesized by a sol-gel method. The morphology of the catalysts was studied by XRD, SEM, N2 physisorption
and their magnetic properties were examined with magnetometry, and specific absorption rate measure-
ments. The catalytic activity was determined in a direct amide synthesis reaction between aniline and
phenylbutyric acid at 150 °C in a fixed bed flow reactor under radiofrequency heating. The intermediate
silica layer of the catalyst increased the porosity of the outer titania layer and the specific absorbance rate
Received 9 June 2017
Revised 5 September 2017
Accepted 12 September 2017
Keywords:
Sol-gel method
of the catalyst. The initial reaction rate increased by 61% as compared to
a similar core-shell
TiO2@NiFe2O4 catalyst showing the detrimental effect of nickel ferrite on titania. The reaction rate was
further increased by a factor of 3.5 after a sulfation treatment due to an optimum Lewis acid site strength.
The highest specific reaction rate over TiO2@SiO2@NiFe2O4 was observed at a 7.5 wt% sulfate loading
which was 2.6 times higher as compared to a mechanical mixture of the same composition. The initial
reaction rate decreased by 36% after a period of 55 h on stream. The catalyst activity was restored after
a treatment with a H2O2 solution.
Core-shell
Composite magnetic catalysts
Sulfated titania
Amide synthesis
Radiofrequency heating
Induction heating
Ó 2017 Elsevier Inc. All rights reserved.
1. Introduction
molecular friction caused by molecules alignment in the quickly
changing field and (ii) ionic conduction for conductive substances
Electromagnetic heating is an efficient method for direct heat-
ing of a reaction medium providing high product yields and energy
efficiency [1]. The short distance between the heating source and
the substrate provides an excellent temperature uniformity, fast
heating rates and decreased heat losses which are the main prob-
lems in conventional heating. There are two main methods for
electromagnetic heating: (i) microwave heating operating in the
GHz frequency range and (ii) radiofrequency (RF) heating operat-
ing in the kHz range [2].
Microwave heating, due to low costs and availability of domes-
tic units, is increasingly used in chemical processes such as the
synthesis of organic chemicals and nanomaterials, drug discovery
and polymer synthesis [3,4]. The main heating principles are (i)
[5]. Microwave heating, however, has several major drawbacks
such as very limited penetration depth in polar solvents (in the
order of a few cm), difficulties in control of the heating rates [6],
limited scalability of the microwave cavities [7,8], and field non-
uniformity resulting in hot spots and accelerated catalyst deactiva-
tion [9].
Radiofrequency (RF) heating does not suffer from these draw-
backs. It provides a much higher penetration depth and utilizes
easier scalable hardware such as magnetic materials and induction
coils connected to an RF generator [2]. During RF heating, the heat
is generated by eddy currents, relaxation loss on the conductive
material, as well as by the rotating magnetic domains in the ferro-
magnetic nanoparticles [10,11]. RF heating is traditionally applied
on a large scale to the areas that need high temperatures such as
metal tempering and welding, alloy synthesis [12], the formation
of carbon [13] and silicon carbide nanocomposites [14]. These
applications demonstrate scalability of the method [15]. In the last
⇑
Corresponding author at: School of Engineering, University of Warwick,
Coventry CV4 7AL, UK.
E-mail address: E.Rebrov@warwick.ac.uk (E.V. Rebrov).
https://doi.org/10.1016/j.jcat.2017.09.010
0021-9517/Ó 2017 Elsevier Inc. All rights reserved.

Y. Liu et al. / Journal of Catalysis 355 (2017) 120–130
121
decade, RF heating was applied in catalytic organic synthesis.
Kirschning et al. studied several organic reactions in an RF-
heated continuous reactor operated at 25 kHz. They reported
higher reaction rates as compared to a conventionally-heated reac-
tor due to the direct heat generation in the material [16]. A high
product yield of 92% was reported in the condensation reaction
to form thiazole over a Pd/silica-coated magnetite catalyst. A reac-
tion rate of up to 4 mmol Lꢀ1 hꢀ1 was reported in the continuous
synthesis of neurolepticum olanzapine over a MagSilica300 com-
mercial magnetic catalyst [17]. A gold-functionalized MagSilica300
catalyst was studied in the oxidation of allylic and benzylic alco-
hols in a flow reactor and showed a close to 100% yield [18].
Under RF heating, the composite magnetic catalyst should pro-
vide both high heating and reaction rates. A core-shell structure is
often used because the shell layer often acts as a catalyst and pro-
tects the magnetic core from chemical erosion and aggregation
[19,20]. However, the charge transfer between the core and shell
elements is often detrimental for catalytic properties [20,21].
Beydoun et al. [21] observed the photodissolution of the catalytic
titania shell at the boundary with the magnetic core which changed
the magnetic properties and reduced the catalytic activity. To avoid
this detrimental effect, an intermediate silica layer was introduced
to prevent the solid state reaction at the interface [22,23].
(ii) limited reaction rate. A long-term irreversible deactivation
was also observed, caused by the interaction of titania with the
magnetic core at high temperature required to carry out the
reaction.
The limited reaction rates over titania catalysts can be
improved by a sulfation treatment as demonstrated by Nagarajan
et al. [28] for a range of aromatic acids and amines. The sulfated
TiO2 is an efficient catalyst in the amidation of various acid and
amine combinations at 115 °C with the amides yield usually
exceeding 95% after a reaction time of 3 h [29]. Chaudhari et al.
[30] studied the amide synthesis over acids and sulfated tung-
states, and identified that catalytic activity significantly depends
on the acid type. Arena et al. [31] showed the following reaction
rate sequence is observed TiO2 (P25) > ZrO2 ꢁ CeO2 >
c-Al2O3
caused by the chemical and surface properties of the catalysts. To
the best of our knowledge however, there is very little systematic
studies about the effect of sulfation treatment on the direct amide
synthesis reaction rates to allow for efficient catalyst development.
In this study, we investigated the effect of the acid site density
and strength over a range of sulfated titania and composite cata-
lysts. In addition, an intermediate silica layer has been introduced
in an attempt to suppress the interactions between titania and
nickel ferrite to enhance the catalytic activity.
Following this idea, we developed a series of core-double shell
composites with an intermediate silica layer for direct amide syn-
thesis, which is one of the most important reactions in pharmaceu-
tical industry. About 25% of all drugs on the market contain amide
bonds, and the amide synthesis covers 65% of drug candidates [24].
Detailed methodologies for the amide formation are presented in
several recent reviews [24–27]. In summary, the traditional syn-
thesis involves the activation of carboxylic acids followed by
ammonolysis of the activated compound to obtain amide as pre-
sented in Fig. 1. The activation step is performed with coupling
agents, and the final product is either separated from the reaction
mixture or obtained in situ. Typical activating agents involve thio-
nyl chloride (SOCl2), phosphorous oxychloride (POCl3), oxalyl chlo-
ride (COCl)2 or a two-step approach of forming a hydrazine
derivative followed by nitrous acid treatment [27]. All the com-
pounds are hazardous, result in reactions with very poor atom effi-
ciency (meaning toxic waste) and complicate the synthesis
procedure. The direct amide synthesis from carboxylic acids and
amines without the activation stage is also possible, however, it
is more challenging because it requires high temperature due to
low reaction rates and a stable acidic catalyst.
2. Experimental
2.1. Synthesis of composite magnetic catalysts
A nickel ferrite sol was prepared by redispersion of 3 g of nickel
ferrite nanoparticles obtained by a sol-gel method [32] in 285 mL
of a 2 M ammonia in a water-ethanol (1:3 v/v) solution under con-
tinuous sonication for 2 h followed by the addition of 0.3 g of cetri-
monium bromide (CTAB) and aging overnight under continuous
stirring. A titania sol (0.48 mol L-1 TiO2) was prepared by dissolving
titanium(IV) butoxide (85 mL), diethanolamine (25 mL) in ethanol
(285 mL). Then 10 mL of a water-ethanol (1:10 v/v) solution was
added dropwise and the mixture obtained was stirred for 48 h.
In the next step, the desired amount of the titania sol was added
dropwise to the nickel ferrite sol under stirring and the mixture
was aged for 48 h. Then the temperature of the mixture was
increased to 40 °C under reflux for 24 h and finally to 60 °C without
reflux to evaporate the solvent. The solids obtained were dried at
80 °C to produce as-synthesised TiO2@NiFe2O4 (shell@core) cata-
lysts which will be referred to as T2A(n) hereafter, where n repre-
sents the titania loading in wt%. Some of these T2 A samples were
calcined at a desired temperature for 1 h with a heating rate of
1 °C min-1. The calcined catalyst will be referred to as T2(n) here-
after. A scheme of the catalyst synthesis and the corresponding
sample codes are shown in Fig. 2. The synthesis methods are
described in the Supplementary Material, SM1, in more details.
The synthesis of TiO2@SiO2@NiFe2O4 composite magnetic cata-
lysts started with the preparation of SiO2@NiFe2O4 samples. They
were prepared by dropwise addition of 12.5 mL the silica sol
(0.49 mol Lꢀ1; obtained from tetraethyl orthosilicate) to the previ-
ously prepared suspension containing NiFe2O4 nanoparticles. The
temperature of the resulting suspension was increased to 40 °C
under reflux for 48 h and finally to 60 °C without reflux to fully
evaporate the solvent. The powder obtained was dried at 80 °C
and then calcined in air at 700 °C for 1 h at a heating rate of
2 °C min-1. The sample will be referred to as Si2 hereafter. In the
final step of preparation, the Si2 powder was redispersed in a
water-ethanol solution and the desired amount of titania sol was
added following the same aging and calcination procedures as
described above for the T2 samples. The as-synthesized and cal-
cined samples will be referred to as T3A and T3 respectively.
This study follows our previous work [20,28,31] where we
developed different titania-containing composite catalysts for this
reaction. The main problems with these catalysts were (i) the rel-
atively fast catalyst deactivation due to product inhibition, as a
result of a strong product adsorption on the titania surface and
Fig. 1. The general scheme of amide synthesis, adapted from Refs. [24,27].

122
Y. Liu et al. / Journal of Catalysis 355 (2017) 120–130
The magnetic properties of the catalysts were measured by a
vibrating sample magnetometer (VSM, Oxford Instruments) in a
magnetic field ranging from ꢀ5 to 5 kG. The saturation magnetiza-
tion and coercive field were determined from the hysteresis loop.
The specific absorption rate (SAR) was determined by recording
the temperature change of a mixture containing catalyst (10 mg
in water) placed in the center of a 5-turn coil connected to a
radiofrequency generator (Easyheat, 300 kHz, 200 A) [33].
The specific surface area and the total pore volume were deter-
mined by nitrogen physisorption at 77 K using a Micromeritics
ASAP 2020 porosimeter. Prior to the measurements, the samples
were degassed until a residual pressure of 10ꢀ4 mbar at 250 °C
for 1 h was reached.
The identification of the nature of acid sites was performed
using pyridine adsorption performed inside an in-situ IR flow cell
for a catalyst wafer maintained at a desired temperature. Prior to
the experiments, the catalyst was degassed by heating the cell to
150 °C for 30 min under argon flow. The sample was then cooled
down to 25 °C and the Ar flow was replaced with a flow of
2.0 vol% pyridine in Ar. After 30 min, the temperature was
increased to the desired temperature (40, 100, 150, or 200 °C)
and the pyridine mixture cell was replaced with an Ar flow. Then
the IR spectrum was recorded with an FTIR spectrometer at a res-
olution of 2 cmꢀ1 (Bruker Vertex 70).
The concentration profiles of CO, CO2 and H2O desorbed from
the spent catalysts during temperature programmed oxidation
(TPO) were recorded using a quadrupole mass spectrometer. In
these experiments, the sample (50 mg) was placed in a quartz tube
inserted into an oven. A mixture of 20 vol% O2 and 5 vol% Ar (inter-
nal standard) in He was fed over the sample and the temperature
Fig. 2. Scheme of the catalyst synthesis and the corresponding sample codes.
2.2. Sulfation of the catalysts
was increased at a rate of 5 °C minꢀ1
.
The T1 (commercial P25 from Sigma Aldrich), T2 or T3 catalysts
(1 g) were suspended in 40 mL of a sulfuric acid solution and
mixed for 4 h. The extent of sulfation was varied by changing the
concentration of the sulfuric acid in the solution. The slurry was
then centrifuged, dried and calcined at 500 °C. The sulfated sam-
ples are labelled with the index Sx in the sample code, and the
number x refers to the total number of calcination steps. This num-
Ammonia chemisorption profiles were recorded in the same
setup. The catalyst sample (100 mg) was placed in a quartz tube,
heated to 400 °C for 30 min under nitrogen flow (20 mL minꢀ1
STP prior to the adsorption). The sample was cooled down to
150 °C and adsorption was performed from a mixture containing
1.9 vol% NH3 and 2.4 vol% Ar (internal standard) in N2. A blank
experiment with the same volume of SiC pellets was performed
to subtract the background signal due to ammonia chemisorption
onto the gas lines maintained at 110 °C [34].
ber is
1 when the sulfated catalysts were prepared from
as-synthesized T2A or T3A samples and the number is equal to 2
when the T2 and T3 samples were used. The nominal loading of
sulfate species (in wt%) is shown in brackets in the sample code.
Two reference catalysts have also been prepared. A physical
mixture of the T1S and NiFe2O4 pellets was prepared in a shaker
(referred as Mech hereafter). The other reference catalyst was
obtained by ball-milling of a mixture of the T1 S catalyst with
NiFe2O4 nanoparticles in an agate jar for 30 min (referred as Ball
hereafter).
2.4. Catalytic activity measurements
Two reactor types were used in this study. A continuously stir-
red tank reactor was used to optimize the catalyst composition. In
these experiments, a degassed solution of 4-phenylbutyric acid
(50 mM) and aniline (50 mM) in p-xylene and the catalyst pellets
(fraction size: 10–40 lm) were loaded into a stainless steel Parr
reactor. The temperature was increased and the pressure was set
at 10 bar using nitrogen. The absence of external mass-transfer
limitations was demonstrated by the constant reaction rate at
the stirring rate above 650 rpm. The absence of internal mass
transfer limitations was confirmed as the highest Weisz-Prater
number in this study was 0.14 ꢃ 3 [35].
2.3. Catalyst characterization
Powder X-ray diffraction (XRD) patterns were recorded using a
PANalytical Empyrean diffractometer with a Cu K
a radiation at a
scanning rate of 2° 2h minꢀ1. The morphology of the samples was
studied with a field emission scanning electron microscope (SEM,
Carl Zeiss Sigma). The elemental composition was determined with
an energy dispersive spectrometer (EDS) calibrated using the sam-
ples with known concentrations of the elements. Prior to analysis,
the catalyst samples were not coated with a conductive layer to
ensure accurate measurements. The surface chemical composition
and electronic state of the elements were analyzed using an X-ray
photoelectron spectrometer (XPS, ESCALab250) with a monochro-
A glass reactor (Omnifit HiT, i.d 6.6 mm, 250 cm long) was used
in flow experiments (Fig. 3). Two HPLC pumps (Shimadzu
LC-20ADXR) were used to feed the N2-degassed solutions of
4-phenylbutyric acid (100 mM) and aniline (100 mM) in p-xylene
to the reactor. The initial concentrations were selected such that
after the mixing, they would be the same in the batch reactor.
The flow reactor was packed with the catalyst pellets (fraction size:
125–250 mm) and it was placed in the center of an 8-turn RF coil
operated at 300 kHz. A 5-mm preheating zone was packed with
Fe3O4 pellets of the same size. A fiber optic temperature sensor
(Opsens) was attached to the outer surface of catalytic zone to con-
trol the temperature within 0.1 °C of the set point using a PID con-
matic 150 W Al K
a radiation. The pass energy for the high-
resolution scan was 30 eV and the base pressure of 6.5 ꢂ 10ꢀ10
mbar. The binding energies were referenced to the C1s line at
284.8 eV.

Y. Liu et al. / Journal of Catalysis 355 (2017) 120–130
123
Fig. 3. Scheme of the RF-heated flow reactor used for direct amide synthesis.
troller connected to the RF generator. The reactor was insulated
with a 1 cm thick layer of ceramic wool. A total pressure of 7 bar
(gauge) was maintained in the reactor with a back pressure con-
troller to keep the reaction mixture in the liquid phase.
The samples were collected at regular time intervals and ana-
lyzed by a Shimadzu GC-2010 gas chromatograph (GC) equipped
with a FID detector and a 30 m Stabilwax capillary column.
Tetradecane was used as internal standard, carbon balance for all
the reactions was 99 2%.
The spent catalysts (typically after 2 days on stream) were
regenerated in the same flow set-up at 90 °C using a 35 wt%
H2O2 solution which was fed at a flow rate of 100 mL minꢀ1 for
22 h. Before and after the regeneration, the catalysts were flushed
with a flow of isopropanol (10 ml total).
the 390 °C peak indicates oxidation of the coke formed by dehydra-
tion of the organic compounds. The highest temperature peak had
the highest C/H ratio of 2.9 demonstrating a graphitic nature of the
coke remaining inside the pores. The presence of graphitic coke is
also indirectly confirmed by the high oxidation temperature of
485 °C [36–38]. Importantly, no formation of H2O and CO2
occurred above 500 °C, showing that this temperature is sufficient
for the full removal of the surfactant from the composite catalysts.
To study the effect of calcination time, the as-synthesized T3 A
catalyst was calcined at 500 °C for different time intervals. The
most dramatic change was observed between 1 and 2 h. The cata-
lyst calcined for 1 h had a surface area of 49 m2 g-1 (Table 1) similar
to that of the reference T1 catalyst. However, the surface area
decreased to 11 m2 gꢀ1 after calcination for 2 h which was likely
caused by the sintering of the porous TiO2 shell. Therefore for fur-
ther study, all the samples were calcined for 1 h.
The actual elemental composition of T3(17) (Table 1) was in
good agreement with its nominal composition of 11 wt% SiO2
and 14 wt% TiO2. These data suggest that the thickness of both tita-
nia and silica shells are similar considering their close apparent
density. It can also be concluded that the catalyst composition
can be controlled by adjusting the initial composition of the corre-
sponding precursors.
The morphology of the T3(17) catalyst was studied with TEM
(Fig. 5). The mean size of NiFe2O4 core was 70 20 nm, which
was the same as that in the initial NiFe2O4 nanoparticles. This indi-
cates the absence of the solid-state reaction between the NiFe2O4
core and the adjacent silica layer during the calcination steps.
The total thickness of the shell layers was 30 10 nm.
3. Results and discussion
3.1. Characterization of composite catalysts
Temperature-programmed oxidation experiments over the T3A
catalysts were performed to select the appropriate calcination
temperature for the removal of surfactants (Fig. 4). Below 150 °C,
only physically adsorbed water is desorbed as indicated by the
absence of CO2 evolution. The main peaks of H2O and CO2 are cen-
tred at 250 °C. A very high H2O peak and the C/H ratio of only about
0.02 also suggest a significant removal of strongly adsorbed water
from the catalyst pores or dehydration of the adsorbed organic
species. The presence of the CO2 peak indicates oxidation or
decomposition of organic species, likely surfactant molecules from
the as-synthesised catalyst. At a higher temperature, two smaller
CO2 peaks at 390 and 485 °C observed show a stepwise decompo-
sition of the organic compounds. The increased C/H ratio of 2.0 for
Fig. 6 shows XRD patterns of the T2(17) and T3(17) catalysts.
The peaks at 25.4 and 48.2° 2h correspond to the (1 0 1), (2 0 0)
crystal planes of anatase (JCPDS card No. 78-2486). The peaks at
30.7, 36.0, 37.6, 43.8, 54.8, and 64.4° 2h are assigned to the
NiFe2O4 structure (card 74-2081). Minor amounts of a-Fe2O3 (card
33-0664) are confirmed by the peaks at 31.4 and 41.3° 2h. The
Table 1
Physical, magnetic, and catalytic properties of composite catalysts.
Catalyst
T2(17)
T3(17)
NiFe2O4 content (wt%)
SiO2 content (wt%)
83
0
70
13
TiO2 content (wt%)
17
25
51
0.27
25.0
0.43
1.1
17
49
51
0.27
23.5
0.51
1.5
BET surface area (m2 gꢀ1
)
Mean core particle sizea (nm)
Coercivity (kG)
Magnetic saturation (emu gꢀ1
)
Hysteresis area (J gꢀ1
)
SARb (W gꢀ1
)
a
Estimated by the Scherrer equation.
Specific absorption rate of RF radiation.
Fig. 4. CO2 and H2O concentration profiles (left y-axis) and temperature profile
(right y-axis) in a TPO experiment over the T3A catalyst.
b

124
Y. Liu et al. / Journal of Catalysis 355 (2017) 120–130
Fig. 7. Magnetisation versus the applied field curves for the T2(17) and T3(17)
catalysts.
results which characterize the heating power released per gram of
the material being placed under RF field. A similar effect of
increased magnetization of silica-encapsulated CoFe2O4 magnetic
nanoparticles was observed by Hutlova et al. [41] caused by an
enhanced anisotropy, which was also likely the case for the
silica-layered T3 catalyst.
Fig. 5. TEM image of the T3(17) catalyst.
Likely, due to the presence of a chelating ligand (diethanola-
mine), mesopores with a mean size of 3.0 nm were formed in both
catalysts as shown in Fig. 8 [44–46]. Interestingly, both catalysts
have a very similar pore size distribution in the mesoporous area.
The formation of a mesoporous layer is greatly beneficial to
enhance the mass transfer towards the catalytic sites [35].
The T3(17) catalyst has a twofold higher specific surface area
and the total pore volume as compared to T2(17) (Table 1). We
can assume that the total surface area of the composite catalyst
is a linear combination of the areas of the core and the shell
(Eq. (1)).
Atotal ¼ xshellAshell þ xcoreAcore
ð1Þ
where xshell and xcore are the weight fractions of the shell and the
core, Ashell and Acore are the corresponding specific surface areas,
respectively. The assumption is valid if the shell provides complete
access for the nitrogen molecules to the core, which seems reason-
able in our case considering the low thickness of the shell (Fig. 5)
and its high porosity (Fig. 8). The NiFe2O4 core has a very low area
of 2.0 m2 gꢀ1. From Eq. (1), the specific surface area of the titania
shell in the T2(17) and T3(17) catalysts is estimated to be 125
and 136 m2 gꢀ1, respectively. This result illustrates that the specific
surface area of titania shell increases when deposited onto a silica
layer rather than directly onto the magnetic core. This is another
advantage of the double shell catalysts in addition to the elimina-
Fig. 6. Powder XRD patterns of the (a) T2(17) and (b) T3(17) catalysts.
a
-Fe2O3 phase is an impurity usually observed in the low-
temperature nickel ferrite synthesis [39]. No SiO2 peaks are
detected in the XRD pattern of T3(17) suggesting its amorphous
structure. The mean crystal size of the NiFe2O4 nanoparticles was
found to be the same (51 nm) in both T2(17) and T3(17) catalysts.
This size is slightly below the mean size obtained from the TEM
images. It appears some agglomeration of nickel ferrite nanoparti-
cles occurs prior to their enclosure by the silica layer. This observa-
tion also confirms that the deposition of an outer shell and
subsequent chemical and thermal treatments do not change the
size of the magnetic core.
Magnetic hysteresis loops of the T2(17) and T3(17) catalysts are
shown in Fig. 7. The coercivity was the same for both catalysts as it
is mainly determined by the mean particle size of the magnetic
core [40], which in turn, was the same in both samples (Table 1).
The remnant magnetization was, however, higher for T3(17) com-
pared to T2(17), which is likely explained by the different defect
density in the magnetic materials [41–43]. A lower saturation
magnetization for T3(17) compared to T2(17) was due to an addi-
tional shell present in this sample which increased the total weight
of non-magnetic layers (Table 1).
The hysteresis area of T3(17) was higher than that of T2(17)
despite the lower saturation magnetization (Table 1). This area is
proportional to the energy released per cycle of magnetic field
reversal. These data are consistent with the specific absorption rate
Fig. 8. Pore size distribution for the catalysts studied.

Y. Liu et al. / Journal of Catalysis 355 (2017) 120–130
125
Fig. 9. The scheme of the 4,N-diphenylbutynamide synthesis.
Fig. 10. Direct amide synthesis reaction rates as a function of time on stream over
the magnetic composite catalysts at the flow rate of 40 mL minꢀ1 (50 mM 4-
phenylbutyric acid, 50 mM aniline), 320 mg of the catalyst, 150 °C, 7 bar(g) in the
RF-heated reactor.
tion of detrimental core-shell electronic interaction by an interme-
diate layer [20,21], and its higher SAR values.
The catalytic activity was compared in the reaction between 4-
phenylbutyric acid and aniline (Fig. 9). In all the experiments, no
products other than the amide were identified and the carbon bal-
ance was 99 2%. The reaction kinetics is the first order in respect
to both amine and acid and it is controlled by strong product inhi-
bition under the given experimental conditions at conversion
levels above 50%. Hence, for proper comparison of the catalysts
studied, the conversion was kept below 40% and the results are dis-
cussed in terms of the initial reaction rates (see SM2 in the Supple-
mentary Material for details).
The reaction rate over the T3(17) catalyst was 52% higher com-
pared to T2(17) (see Fig. 10). The reference Si2 catalyst showed a
negligible activity confirming that the silica layer behaves as an
inert material in this reaction. The difference between the catalysts
becomes even larger when we compare the rates normalized per
available surface area of titania in these catalysts. The reaction rate
normalized by surface area over the T3(17) catalyst was larger by
61% compared to T2(17) at the same reaction conditions. Hence,
the addition of the SiO2 layer provides a significant increase in
the intrinsic reaction rate over the catalytic sites located onto the
titania surface. This is likely caused by the elimination of charge
transfer between the nickel ferrite and titania [20,21]. The deacti-
vation rate of T3(17) was also found to be slower than that of
T2(17).
3.2. Effect of the sulfation treatment
The electron-withdrawing Lewis acid sites play the main role in
the direct amide synthesis over titania catalysts. It is well accepted
that the final step of the reaction mechanism includes an attack of
the nucleophilic nitrogen atom of the amine towards the carbon
atom of the acid forming the amide bond. The electron density
on the carbon atom is decreased via an interaction with the car-
boxylate oxygen atoms [47]. The contribution of the Brønsted acid
sites is considerably lower [48].
Fig. 11. X-ray photoelectron spectra of the T1S catalysts with various sulfate
loading in brackets, (a) Ti 2p3/2, (b) O1s, (c) S 2p.
of SO24- loading had not been studied in details to the best of our
knowledge. Therefore, we prepared a range of sulfated titania sam-
ples to obtain the structure-activity relationship between the
The direct amide synthesis reaction rate over titania can be
increased using a sulfation treatment [28,49]. However, the effect

126
Y. Liu et al. / Journal of Catalysis 355 (2017) 120–130
amount of SO24- species and the catalytic activity. To simplify the
analysis and quantification of the data, the T3 catalysts were
replaced with the corresponding T1 catalysts with the same titania
morphology. The phase composition study showed no change on
sulfation as presented in Fig. SM3 of the Supplementary Material.
The Ti 2p3/2 XPS line in the T1 catalyst is observed at 458.0 eV
(Fig. 11a). After sulfation, the peak shifts to higher binding energies
due to the electron-withdrawal effect of sulfate groups [50] and
associated surface species [51]. The presence of sulfate species also
results in the formation of a shoulder in the spectrum of the O 1 s
at 531.4 eV next to the main line centered at 529.9 eV (Fig. 11b). Its
intensity increases with the concentration of sulfate species. The
formation of the shoulder agrees with the XPS studies over sulfated
titania catalysts and is attributed to the presence of O-S bonds [52].
The main oxygen line also shifts to higher energy indicating the
presence of S-O-Ti bridges [53–55]. The S 2p line is present in all
XPS spectra of sulfated samples (Fig. 11c). The binding energy of
sulfur in the range from 168.4 to 168.7 eV is characteristic for a
hexavalent oxidation state in the SO24ꢀ species.
The surface sulfur loading determined by XPS versus nominal
loading is shown in Fig. 12. It can be seen that the surface sulfur
loading is very close to the nominal composition up to a loading
of 7.5 wt% suggesting that all sulfur is present near the outer sur-
face of the nanoparticles. This behavior suggests that all the exter-
nal sites are covered with SO24ꢀ at a loading of 7.5 wt%. At higher
loadings, the sulfur incorporation into the catalysts decreases
likely due to a change in the coordination from bridge to chelate
[51,56].
The nature of the acid sites on the sulfated catalysts was inves-
tigated by IR study of adsorbed pyridine (Fig. 13a). A characteristic
Brønsted acid site band at 1540 cmꢀ1 [57–59] was not observed in
all spectra showing that the sulfated catalysts have no Brønsted
acidity. The presence of Lewis acid sites is confirmed by the bands
at 1445, 1574, 1604 cmꢀ1 and 1490 cmꢀ1 [60,61].
The relative strength of the acid sites was studied in the tem-
perature programmed desorption of pyridine (Fig.13a). As the tem-
perature increases, the intensity of all bands decreases due to
desorption of pyridine. This method allows to estimate the relative
acid strength of different sites while it remains rather qualitative
because the band intensity changes with temperature [62,63].
Therefore, the intensity of all bands was normalized to the inten-
sity of the strongest band at 1445 cmꢀ1 measured at 40 °C to study
the relative acid strength. In the sample with the highest sulfur
loading of 28 wt%, the relative intensity was as high as 80% even
at 200 °C indicating very strong adsorption of pyridine. Interest-
ingly, the slope of the lines in Fig. 13b decreases as the sulfur load-
ing increases. It shows that both the desorption activation energy
Fig. 13. (a) FTIR spectra of pyridine over sulfated titania catalysts and (b)
temperature dependence of the normalized intensity of the 1445 cmꢀ1 band at
40 °C.
of pyridine and the strength of Lewis acid sites monotonously
increase with sulfur loading.
The total amount of acid sites was studied using ammonia
chemisorption from the NH3 capacity in the adsorption mode at
150 °C. This temperature was chosen to minimize the amount of
physically adsorbed ammonia [64]. It should be mentioned that
the desorption mode (TPD) could not provide quantitative data
because the decomposition of sulfur sites occurs below the tem-
perature of complete ammonia desorption (>450 °C) [58–60]. The
results show that the extent of sulfation does not affect the number
of acid sites on the surface (Fig. 12). While Wang et al. [65]
reported a different dependence, their catalysts had both Brønsted
and Lewis acid sites. Therefore, no direct comparison is possible,
because the presence of Brønsted acid sites may have altered the
adsorption mode of sulfate species.
Fig. 14. Reaction rate between 4-phenylbutyric acid and aniline as a function of the
SO24ꢀ loading in
a stirred tank reactor. Reaction conditions: 100 mL p-xylene
Fig. 12. SO24ꢀ surface loading (left y-axis) and total NH3 chemisorption capacity
solution of 4-phenylbutyric acid (50 mM) and aniline (50 mM), 320 mg sulfated
(right y-axis) as a function of nominal composition in the T1S catalysts.
catalyst, 6 bar(g), 200 °C.

Y. Liu et al. / Journal of Catalysis 355 (2017) 120–130
127
From the pyridine adsorption and ammonia chemisorption
experiments, it can be concluded that the sulfation treatment does
not change the total number of the Lewis acid sites, but signifi-
cantly increases their strength. As a result, the reaction rate
demonstrates a classical volcano curve of catalytic activity with a
maximum at a 7.5 wt% sulfur loading (Fig. 14). It is likely that at
the lower loadings, the reaction rate is reduced due to a decreased
catalyst coverage with the reactants, while at the higher loadings
the reaction rate is reduced due to strong product adsorption (pro-
duct inhibition). The optimum titania loading was chosen for the
follow-up study with the T2 and T3 composite magnetic catalysts.
3.3. Sulfation of the composite catalysts
Two sulfation methods have been compared. In the first
method, the as-synthesized T2A and T3A catalysts were sulfated
following the approach already optimized for the T1 catalysts as
discussed in the previous section. In the second method, the corre-
sponding calcined catalysts (T2 and T3) were sulfated. The elemen-
tal analysis data (Table 2) show that both methods result in a slight
decrease of titania loading likely due to its partial redispersion in
the acidic solution.
The first method increases the surface area of sulfated catalysts
by 13 and 46%, respectively, as compared to the corresponding T2
and T3 catalysts. On the contrary, the surface area of the T2S2 and
T3S2 catalysts, prepared by the second method, decreased by 17
and 30%, respectively. This is caused by sintering of porous titania
layer after the second calcination step (Table 2).
The SAR values of the catalysts obtained by the first method
were lower as compared to the corresponding unsulfated catalysts
(Table 2). This can be explained by a partial dissolution of nickel
ferrite core in a sulfuric acid solution at a pH below 2 [66]. In con-
trast, the catalysts prepared by the second method showed higher
SAR values as compared to the corresponding unsulfated catalysts.
The SAR value increases due to higher coercivity of nickel ferrite
nanoparticles with a mean size close to the transition region
between single domain and multidomain structures [32]. It can
be concluded that the intermediate silica shell protects the mag-
netic core from dissolution as the SAR value of T3S2 is considerably
higher than that of T2S2 catalyst.
The rates for the reaction between 4-pheylbutyric acid and ani-
line over the sulfated composite catalysts are listed in Table 2. In
line with the data obtained over T1 catalysts (Fig. 14), the reaction
rates increased over the T3S2 and T3S1 sulfated catalysts by 30%
and 80%, respectively, as compared to the corresponding unsul-
fated catalysts.. While the sulfation increased the reaction rate over
the T1S catalyst by a factor of 3.5 (compared to T1), the observed
enhancement over the composite catalysts was lower due to a
lower titania loading. It should be noted that the effect over the
T2 sulfated catalyst is less pronounced. This can be explained by
the negative influence of the magnetic core on the titania shell as
discussed above. The activity increases by 1.5 times after the sulfa-
tion treatment. The second calcination step then reduces the rate
enhancement factor to 1.2 in the T2S2 catalyst.
Fig. 15. Specific reaction rate as a function of time on stream over (a) core-shell and
(b) core double shell catalysts as well as mechanical and ball-milled mixtures of the
catalyst components. Reaction conditions: flow rate of 40 mL minꢀ1 (50 mM 4-
phenylbutyric acid, 50 mM aniline), 320 mg of the catalyst, 150 °C, 7 bar(g) in the
RF-heated reactor.
The comparison of catalyst performance is presented in Fig. 15.
No deactivation for 400 min on stream was observed over both
T2S1 and T3S1 catalysts. However, the rate of deactivation over
T2S2 and T3S2 catalysts was rather high and similar to that over
the corresponding non-sulfated catalysts. This effect can be associ-
ated with the reduced rate of coke formation because the T2S1 and
T3S1 catalysts have no Brønsted acid sites. A partial collapse of
porous structure, as indicated by the reduced surface area (Table 2),
always results in the formation of new Brønsted acid sites in the
porous framework [47]. Likely, these sites are responsible for cok-
ing and therefore much higher deactivation rates are observed over
these samples.
The comparison of reaction rates between the T3S1 composite
catalyst, ball milled catalyst and the mechanical mixture of the
same composition shows a notable difference (Fig. 15b). Over the
ball-milled catalyst, the initial reaction rate was 2.5 times lower
as compared to T3S1. In addition, a much faster deactivation kinet-
ics was observed over the ball-milled catalyst. Over the mechanical
mixture, the initial rate decreased by 1.7 times, without major
changes in the deactivation kinetics. The decreased reaction rates
in mixtures indicate that the close contact between the magnetic
core and the shell in the ball-milled catalysts results in decreased
reaction rates and faster deactivation. This effect is reduced in
the mechanical mixture and completely eliminated in the core-
double shell catalysts.
Table 2
Specific surface area (SBET), specific absorption rate (SAR), elemental analysis and
catalytic activity data for the catalysts studied.
SBET
SAR
TiO2
(wt%)
SiO2
(wt%)
Reaction rate
ꢀ1
(m2 gꢀ1
)
(W gꢀ1
)
(mmol g
sꢀ1
)
TiO2
T2S2
T2S1
T2
T3S2
T3S1
T3
42.9
58.7
49.0
33.8
71.7
52.0
0.6
0.8
0.9
0.6
1.1
1.5
26
25
32
16
14
17
0
0
0
12
9
0.18
0.22
0.15
0.28
0.37
0.21
Because the T3S1 catalyst showed the highest catalytic activity
with no deactivation for 6 h on stream, a further deactivation study
was performed, Fig. 16. The reaction rate gradually decreased by
43% after 55 h on stream. The spent catalyst was flushed with a
35 wt% H2O2 solution at 90 °C adapting the method reported for
13

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Y. Liu et al. / Journal of Catalysis 355 (2017) 120–130
acidity of the catalysts for direct amide synthesis should be opti-
mized to obtain the highest reaction rates.
Sulfation of the core-shell catalysts showed a further perfor-
mance enhancement. The reaction rate over the double-shell
TiO2@SiO2@NiFe2O4 catalyst was 1.7 times higher compared to
the single-core shell TiO2@NiFe2O4 catalyst and 2.5 times higher
compared to mechanical mixtures of the catalyst constituents.
Moreover, the deactivation rates were considerably lower over
the sulfated core-double shell catalysts compared to single-shell
and non-sulfated catalysts. The core-double shell catalysts also
showed a high deactivation resistance working for more than
150 h with an overall activity decline of only 28% after two H2O2
regeneration cycles.
Acknowledgements
Fig. 16. Reaction rate as a function of time on stream catalyzed by the T3S1 catalyst
over 2 regeneration cycles with H2O2. Reaction conditions: flow rate of 40 mL minꢀ1
(50 mM 4-phenylbutyric acid, 50 mM aniline), 320 mg of the catalyst, 150 °C, 7 bar
(g) in the RF-heated reactor.
Financial support provided by the European Research Council
(ERC) project 279867 and the Russian Science Foundation project
15-13-20015 is gratefully acknowledged.
batch recovery. The catalyst recovered showed the same initial
activity as the fresh catalyst likely due to the complete coke
removal [20]. However, after 9 h on stream, the regenerated cata-
lyst demonstrated a faster deactivation rate compared to the fresh
catalyst. As a result, the activity at the end of the second run was
14% lower compared to that observed after the first run. The cata-
lyst activity after the second regeneration cycle was restored to
93% compared to the fresh catalyst and reached 32% of the initial
activity for the fresh catalyst after the third run. Increasing deacti-
vation rates suggest that H2O2 treatment removed light coke from
the active sites, but that some coke remained resulting in the faster
blockage of pores and a faster deactivation. Another possible rea-
son is the negative effect of the hydrothermal regeneration treat-
ment which could result in a partial collapse of the mesoporous
network. However, the low amount of deactivated catalyst did
not allow to perform a more detailed study.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2017.09.010.
References
[1] C.O. Kappe, Controlled microwave heating in modern organic synthesis,
Angew. Chem. Int. Ed. Engl. 43 (2004) 6250–6284, https://doi.org/10.1002/
anie.200400655.
[2] P. Piyasena, C. Dussault, T. Koutchma, H.S. Ramaswamy, G.B. Awuah, Radio
frequency heating of foods: principles, applications and related properties–a
review, Crit. Rev. Food Sci. Nutr. 43 (2003) 587–606, https://doi.org/10.1080/
10408690390251129.
[3] P. Lidström, J. Tierney, B. Wathey, J. Westman, Microwave assisted organic
synthesis—a review, Tetrahedron 57 (2001) 9225–9283 (accessed May 9,
2013) http://www.sciencedirect.com/science/article/pii/S0040402001009061.
[4] F. Wiesbrock, R. Hoogenboom, U.S. Schubert, Microwave-assisted polymer
synthesis: State-of-the-art and future perspectives, Macromol. Rapid Commun.
25 (2004) 1739–1764, https://doi.org/10.1002/marc.200400313.
[5] C. Oliver Kappe, Microwave dielectric heating in synthetic organic chemistry,
Chem. Soc. Rev. 37 (2008) 1127–1139, https://doi.org/10.1039/b803001b.
[6] T.N. Glasnov, C.O. Kappe, Microwave-assisted synthesis under continuous-flow
conditions, Macromol. Rapid Commun. 28 (2007) 395–410, https://doi.org/
10.1002/marc.200600665.
The regeneration data show that the H2O2 treatment restores
activity significantly. The integrated reaction rate (that is the total
product output) for the second regeneration was only 28% lower
compared to the fresh catalyst demonstrating good reusability
and a long catalyst lifetime.
[7] J.D. Moseley, P. Lenden, M. Lockwood, K. Ruda, J.P. Sherlock, A.D. Thomson, J.P.
Gilday, A comparison of commercial microwave reactors for scale-up within
process chemistry, Org. Process Res. Dev. 12 (2008) 30–40, https://doi.org/
10.1021/op700186z.
4. Conclusion
[8] A. Díaz-Ortiz, A. de la Hoz, J. Alcázar, J.R. Carrillo, M. Antonia Herrero, A.
Fontana, J. de Mata Muñoz, Reproducibility and scalability of solvent-free
microwave-assisted reactions: from domestic ovens to controllable parallel
applications, Comb. Chem. High Throughput Screen. 10 (2007) 163–169,
https://doi.org/10.2174/138620707780126679.
[9] F. Benaskar, N.G.G. Patil, V. Engels, E.V.V. Rebrov, J. Meuldijk, L. a. A. Hulshof, V.
Hessel, A.E.H.E.H. Wheatley, J.C.C. Schouten, Microwave-assisted Cu-catalyzed
Ullmann ether synthesis in a continuous-flow milli-plant, Chem. Eng. J. 207–
208 (2012) 426–439, https://doi.org/10.1016/j.cej.2012.06.147.
[10] I. Sharifi, H. Shokrollahi, S. Amiri, Ferrite-based magnetic nanofluids used in
hyperthermia applications, J. Magn. Magn. Mater. 324 (2012) 903–915,
https://doi.org/10.1016/j.jmmm.2011.10.017.
[11] A.E. Deatsch, B.A. Evans, Heating efficiency in magnetic nanoparticle
hyperthermia, J. Magn. Magn. Mater. 354 (2014) 163–172, https://doi.org/
10.1016/j.jmmm.2013.11.006.
[12] T.W.B. Riyadi, T. Zhang, D. Marchant, X. Zhu, Synthesis and fabrication of NiAl
coatings with Ti underlayer using induction heating, Surf. Coatings Technol.
258 (2014) 154–159, https://doi.org/10.1016/j.surfcoat.2014.09.037.
[13] Z. Li, B. Zhao, P. Liu, B. Zhao, D. Chen, Y. Zhang, Synthesis of high-quality single-
walled carbon nanotubes by high-frequency-induction heating, Phys. E Low-
Dimensional Syst. Nanostructures. 40 (2008) 452–456, https://doi.org/
10.1016/j.physe.2007.06.062.
The performance of the core-shell (TiO2@NiFe2O4) and core-
double shell (TiO2@SiO2@NiFe2O4) composite magnetic catalysts
was compared in the direct amide synthesis reaction between ani-
line and phenylbutyric acid in a flow reactor under radiofrequency
heating at 300 kHz. Despite the similar magnetic core content, the
double shell catalyst showed a 36% increase in specific heating rate
compared to the core-shell catalyst. Due to the presence of an
intermediate silica layer, the reaction rate normalized by weight
of titania increased by 52% and the rate normalized by titania sur-
face area increased by 61% compared to the core-shell catalyst. The
data show that middle silica layer prevents the negative effect of
interaction between the magnetic core and the titania shell and
also increases the specific surface area of the catalyst layer.
The sulfation was optimised for titania catalysts containing only
Lewis acid sites. The pyridine IR and ammonia chemisorption stud-
ies revealed that an increasing sulfate loading resulted in a higher
acidity with the number of active sites being constant. The direct
amide synthesis reaction rates, however, changed non-
monotonously with the maximum at the sulfate loading of
7.5 wt% and the reaction rate 3.5 times higher compared to
non-sulfated titania. The effect of sulfation demonstrates that the
[14] L. Wang, Y. Peng, X. Hu, X. Xu, Combustion synthesis of high purity SiC powder
by radio-frequency heating, Ceram. Int. 39 (2013) 6867–6875, https://doi.org/
10.1016/j.ceramint.2013.02.021.
[15] V. Rudnev, D. Loveless, R.L. Cook, M. Black, Handbook of Induction Heating,
Marcel Dekker, New York. (2003) 796. http://www.asminternational.org/web/
hts/home/-/journal_content/56/10192/75024G/PUBLICATION.

Y. Liu et al. / Journal of Catalysis 355 (2017) 120–130
129
[16] S. Ceylan, C. Friese, C. Lammel, K. Mazac, A. Kirschning, Inductive heating for
organic synthesis by using functionalized magnetic nanoparticles inside
microreactors, Angew. Chemie - Int. Ed. 47 (2008) 8950–8953, https://doi.
org/10.1002/anie.200801474.
[17] J. Hartwig, S. Ceylan, L. Kupracz, L. Coutable, A. Kirschning, Heating under high-
frequency inductive conditions: Application to the continuous synthesis of the
neurolepticum olanzapine (Zyprexa), Angew. Chemie - Int. Ed. 52 (2013)
9813–9817, https://doi.org/10.1002/anie.201302239.
[18] S.R. Chaudhuri, J. Hartwig, L. Kupracz, T. Kodanek, J. Wegner, A. Kirschning,
Oxidations of allylic and benzylic alcohols under inductively-heated flow
conditions with gold-doped superparamagnetic nanostructured particles as
catalyst and oxygen as oxidant, Adv. Synth. Catal. 356 (2014) 3530–3538,
https://doi.org/10.1002/adsc.201400261.
[40] J.M. Wesselinowa, Size and anisotropy effects on magnetic properties of
antiferromagnetic nanoparticles, J. Magn. Magn. Mater. 322 (2010) 234–237,
https://doi.org/10.1016/j.jmmm.2009.08.045.
[41] A. Hutlova, D. Niznansky, J.L. Rehspringer, C. Estournès, M. Kurmoo, High
Coercive Field for Nanoparticles of CoFe2O4 in Amorphous Silica Sol-Gel, Adv.
Mater. 15 (2003) 1622–1625, https://doi.org/10.1002/adma.200305305.
[42] V.K. Sharma, A. Baiker, Superparamagnetic effects in the ferromagnetic
resonance of silica supported nickel particles, J. Chem. Phys. 75 (1981)
5596–5601, https://doi.org/10.1063/1.441997.
[43] C. De Julián Fernández, G. Mattei, C. Maurizio, E. Cattaruzza, S. Padovani, G.
Battaglin, F. Gonella, F. D’Acapito, P. Mazzoldi, Magnetic properties of Co-Cu
nanoparticles dispersed in silica matrix, J. Magn. Magn. Mater. 290 (2005)
187–190, https://doi.org/10.1016/j.jmmm.2004.11.178.
[19] M. Zhu, C. Wang, D. Meng, G. Diao, In situ synthesis of silver nanostructures on
magnetic Fe3O4@C core-shell nanocomposites and their application in
catalytic reduction reactions, J. Mater. Chem. A. 1 (2013) 2118–2125, https://
doi.org/10.1039/C2TA00669C.
[20] Y. Liu, P. Gao, N. Cherkasov, E.V. Rebrov, Direct amide synthesis over core–shell
TiO 2 @NiFe 2 O 4 catalysts in a continuous flow radiofrequency-heated
[44] J. Zhou, G. Zhao, X. Ren, B. Song, G. Han, Effects of diethanolamine on the
evolution of silver/titanium dioxide sol–gel process, J. Sol-Gel Sci. Technol. 58
(2011) 148–155, https://doi.org/10.1007/s10971-010-2369-4.
[45] D. Fattakhova-Rohlfing, A. Zaleska, T. Bein, Three-dimensional titanium
dioxide nanomaterials, Chem. Rev. 114 (2014) 9487–9558, https://doi.org/
10.1021/cr500201c.
reactor, RSC Adv.
C6RA22659K.
6
(2016) 100997–101007, https://doi.org/10.1039/
[46] L.M. Nikolic´, L. Radonjic´, V.V. Srdic´, Effect of substrate type on nanostructured
titania sol-gel coatings for sensors applications, Ceram. Int. 31 (2005) 261–
266, https://doi.org/10.1016/j.ceramint.2004.05.012.
[47] H. Lundberg, F. Tinnis, N. Selander, H. Adolfsson, Catalytic amide formation
from non-activated carboxylic acids and amines, Chem. Soc. Rev. 43 (2014)
2714, https://doi.org/10.1039/c3cs60345h.
[21] D. Beydoun, R. Amal, G. Low, S. McEvoy, Occurrence and prevention of
photodissolution at the phase junction of magnetite and titanium dioxide, J.
Mol. Catal. A Chem. 180 (2002) 193–200, https://doi.org/10.1016/S1381-1169
(01)00429-0.
[22] B. Chang, Y. Tian, W. Shi, J. Liu, F. Xi, X. Dong, Magnetically separable porous
carbon nanospheres as solid acid catalysts, RSC Adv. 3 (2013) 20999–21006,
https://doi.org/10.1039/c3ra43208d.
[23] A.L. Costa, B. Ballarin, A. Spegni, F. Casoli, D. Gardini, Synthesis of
nanostructured magnetic photocatalyst by colloidal approach and spray-
drying technique, J. Colloid Interface Sci. 388 (2012) 31–39, https://doi.org/
10.1016/j.jcis.2012.07.077.
[48] C. Deiana, Y. Sakhno, M. Fabbiani, M. Pazzi, M. Vincenti, G. Martra, Direct
synthesis of amides from carboxylic acids and amines by using heterogeneous
catalysts: evidence of surface carboxylates as activated electrophilic species,
ChemCatChem. 5 (2013) 2832–2834, https://doi.org/10.1002/cctc.201300164.
[49] M. Hosseini-Sarvari, E. Sodagar, M.M. Doroodmand, Nano sulfated titania as
solid acid catalyst in direct synthesis of fatty acid amides, J. Org. Chem. 76
(2011) 2853–2859, https://doi.org/10.1021/jo2002769.
[24] R.M. De Figueiredo, J.S. Suppo, J.M. Campagne, Nonclassical routes for amide
bond formation, Chem. Rev. 116 (2016) 12029–12122, https://doi.org/
10.1021/acs.chemrev.6b00237.
[50] E. Barraud, F. Bosc, D. Edwards, N. Keller, V. Keller, Gas phase photocatalytic
removal of toluene effluents on sulfated titania, J. Catal. 235 (2005) 318–326,
https://doi.org/10.1016/j.jcat.2005.08.017.
[25] A. El-Faham, F. Albericio, Peptide coupling reagents, more than
a
letter
[51] Y.A. Chesalov, G.B. Chernobay, T.V. Andrushkevich, FTIR study of the surface
complexes of b-picoline, 3-pyridine-carbaldehyde and nicotinic acid on
sulfated TiO2 (anatase), J. Mol. Catal. A Chem. 373 (2013) 96–107, https://
doi.org/10.1016/j.molcata.2013.03.007.
soup, Chem. Rev. 111 (2011) 6557–6602, https://doi.org/10.1021/
cr100048w.
[26] M.M. Joullié, K.M. Lassen, Evolution of amide bond formation, Ark. Online J.
Org.
ark.5550190.0011.816.
[27] C.A.G.N. Montalbetti, V. Falque, Amide bond formation and peptide coupling,
Chem.
2010
(2010)
189–250,
https://doi.org/10.3998/
[52] H. Zhao, P. Jiang, Y. Dong, M. Huang, B. Liu, A high-surface-area mesoporous
sulfated nano-titania solid superacid catalyst with exposed (101) facets for
esterification: facile preparation and catalytic performance, New J. Chem. 38
(2014) 4541–4548, https://doi.org/10.1039/C4NJ00494A.
Tetrahedron
61
(2005)
10827–10852,
https://doi.org/10.1016/j.
tet.2005.08.031.
[53] L.G. Devi, R. Kavitha, Enhanced photocatalytic activity of sulfur doped TiO2 for
[28] S. Nagarajan, P. Ran, P. Shanmugavelan, M. Sathishkumar, A. Ponnuswamy, K.
Suk, Nahm, G. Gnana kumar, The catalytic activity of titania nanostructures in
the synthesis of amides under solvent-free conditions, New J. Chem. 36 (2012)
1312–1319, https://doi.org/10.1039/c2nj40119c.
[29] T.K. Houlding, K. Tchabanenko, M.T. Rahman, E.V. Rebrov, Direct amide
formation using radiofrequency heating, Org. Biomol. Chem. 11 (2013) 4171–
4177, https://doi.org/10.1039/c2ob26930a.
the decomposition of phenol: a new insight into the bulk and surface
modification, Mater. Chem. Phys. 143 (2014) 1300–1308, https://doi.org/
10.1016/j.matchemphys.2013.11.038.
[54] P. Goswami, J.N. Ganguli, C. Sao’an, Z. Jianqing, C. Chunan, J. Zhao, R. Xu, A
novel synthetic approach for the preparation of sulfated titania with enhanced
photocatalytic activity, RSC Adv.
c3ra23451g.
3 (2013) 8878, https://doi.org/10.1039/
[30] P.S. Chaudhari, S.D. Salim, R.V. Sawant, K.G. Akamanchi, Sulfated tungstate: a
new solid heterogeneous catalyst for amide synthesis, Green Chem. 12 (2010)
1707–1710, https://doi.org/10.1039/c0gc00053a.
[31] F. Arena, C. Deiana, A.F. Lombardo, P. Ivanchenko, Y. Sakhno, G. Trunfio, G.
Martra, Activity patterns of metal oxide catalysts in the synthesis of N-
phenylpropionamide from propanoic acid and aniline, Catal. Sci. Technol. 5
(2015) 1911–1918, https://doi.org/10.1039/c4cy01504e.
[32] P. Gao, X. Hua, V. Degirmenci, D. Rooney, M. Khraisheh, R. Pollard, R.M.
Bowman, E.V. Rebrov, Structural and magnetic properties of Ni1ꢀxZnxFe2O4
(x=0, 0.5 and 1) nanopowders prepared by sol–gel method, J. Magn. Magn.
Mater. 348 (2013) 44–50, https://doi.org/10.1016/j.jmmm.2013.07.060.
[33] T.K. Houlding, P. Gao, V. Degirmenci, K. Tchabanenko, E.V. Rebrov,
Mechanochemical synthesis of TiO2/NiFe2O4 magnetic catalysts for
operation under RF field, Mater. Sci. Eng. B Solid-State Mater. Adv. Technol.
193 (2015) 175–180, https://doi.org/10.1016/j.mseb.2014.12.011.
[34] O. Vaittinen, M. Metsälä, S. Persijn, M. Vainio, L. Halonen, Adsorption of
ammonia on treated stainless steel and polymer surfaces, Appl. Phys. B. 115
(2013) 185–196, https://doi.org/10.1007/s00340-013-5590-3.
[35] M.A. Vannice, Kinetics of Catalytic Reactions, Springer Science+Business
Media, New York (2005), https://doi.org/10.1007/b136380.
[36] N.B. Cherkasov, S.V. Savilov, A.S. Ivanov, V.V. Lunin, Bomb calorimetry as a bulk
characterization tool for carbon nanostructures, Carbon NY 63 (2013) 324–
329, https://doi.org/10.1016/j.carbon.2013.06.085.
[37] N. Cherkasov, S.V. Savilov, A.S. Ivanov, A.V. Egorov, V.V. Lunin, A.O. Ibhadon,
Template synthesis and characterization of carbon nanomaterials from
ferrocene crystals, Appl. Surf. Sci. 308 (2014) 388–395, https://doi.org/
10.1016/j.apsusc.2014.04.181.
[55] K.J.A. Raj, B. Viswanathan, Single-step synthesis and structural study of
mesoporous sulfated titania nanopowder by a controlled hydrolysis process,
ACS Appl. Mater. Interfaces. 1 (2009) 2462–2469, https://doi.org/10.1021/
am900437u.
[56] J. Zhong, J. Li, J. Zeng, S. Huang, W. Hu, J. Chen, M. Li, J. Wang, S. Zhang,
Enhanced photo catalytic activity of sulfated silica-titania composites
prepared by impregnation using using ammonium persulfate solution,
Mater. Sci. Semicond. Process. 26 (2014) 62–68, https://doi.org/10.1016/j.
mssp.2014.04.008.
[57] R.M. de Almeida, L.K. Noda, N.S. Gonçalves, S.M.P. Meneghetti, M.R.
Meneghetti, Transesterification reaction of vegetable oils, using superacid
sulfated TiO2-base catalysts, Appl. Catal. A Gen. 347 (2008) 100–105, https://
doi.org/10.1016/j.apcata.2008.06.006.
[58] S. Yamazaki, K. Ichikawa, A. Saeki, T. Tanimura, K. Adachi, Photocatalytic
degradation of chlorinated ethanes in the gas phase on the porous TiO2
pellets: effect of surface acidity, J. Phys. Chem. A. 114 (2010) 5092–5098,
https://doi.org/10.1021/jp911842t.
[59] J.L. Ropero-Vega, A. Aldana-Pérez, R. Gómez, M.E. Niño-Gómez, Sulfated titania
[TiO2/SO42ꢀ]: a very active solid acid catalyst for the esterification of free
fatty acids with ethanol, Appl. Catal. A Gen. 379 (2010) 24–29, https://doi.org/
10.1016/j.apcata.2010.02.020.
[60] Y. Wang, Y. Gan, R. Whiting, G. Lu, Synthesis of sulfated titania supported on
mesoporous silica using direct impregnation and its application in
esterification of acetic acid and n-butanol, J. Solid State Chem. 182 (2009)
2530–2534, https://doi.org/10.1016/j.jssc.2009.07.003.
[61] J. Sun, K. Zhu, F. Gao, C. Wang, J. Liu, C.H.F. Peden, Y. Wang, Direct conversion of
bio-ethanol to isobutene on nanosized Zn xZryOz mixed oxides with balanced
acid – base sites, J. Am. Chem. Soc. 133 (2011) 11096–11099, https://doi.org/
10.1021/ja204235v.
[62] M. Niwa, K. Suzuki, N. Katada, T. Kanougi, T. Atoguchi, Ammonia IRMS-TPD
study on the distribution of acid sites in mordenite, J. Phys. Chem. B. 109
(2005) 18749–18757, https://doi.org/10.1021/jp051304g.
[38] S. Savilov, N. Cherkasov, M. Kirikova, A. Ivanov, V. Lunin, Multiwalled carbon
nanotubes and nanofibers: similarities and differences from structural,
electronic and chemical concepts; chemical modification for new materials
design, Funct. Mater. Lett.
S1793604710001421.
3 (2010) 289–294, https://doi.org/10.1142/
[39] A.E. Paladino, Phase equilibria in the ferrite region of the system Fe-Ni-O, J.
Am. Ceram. Soc. 42 (1959) 168–175, https://doi.org/10.1111/j.1151-
2916.1959.tb12941.x.
[63] N. Cherkasov, T. Vazhnova, D.B. Lukyanov, Quantitative infra-red studies of
Brønsted acid sites in zeolites: case study of the zeolite mordenite, Vib.
Spectrosc. 83 (2016) 170–179, https://doi.org/10.1016/j.vibspec.2016.02.002.

130
Y. Liu et al. / Journal of Catalysis 355 (2017) 120–130
[64] C.U. Ingemar Odenbrand, J.G.M. Brandin, G. Busca, Surface acidity of silica-
titania mixed oxides, J. Catal. 135 (1992) 505–517, https://doi.org/10.1016/
0021-9517(92)90050-R.
[65] X. Wang, J.C. Yu, P. Liu, X. Wang, W. Su, X. Fu, Probing of photocatalytic surface
sites on SO42ꢀ/TiO2 solid acids by in situ FT-IR spectroscopy and pyridine
adsorption, J. Photochem. Photobiol. A Chem. 179 (2006) 339–347, https://doi.
org/10.1016/j.jphotochem.2005.09.007.
[66] K. Shih, T. White, J.O. Leckie, Nickel stabilization efficiency of aluminate and
ferrite spinels and their leaching behavior, Environ. Sci. Technol. 40 (2006)
5520–5526, https://doi.org/10.1021/es0601033.