Heteropolyacid generated on the surface of iron phosphate nanotubes: Structure and catalytic activity studies

Article abstract of DOI:10.1039/c5ra11175g

Structural and catalytic properties of a Mo based heteropolyacid generated after impregnation of MoO_x on the surface of porous iron phosphate nanotubes (FeP) were studied. Nano sized MoO_x-FeP composites with different Mo molar loadin

Full text of DOI:10.1039/c5ra11175g

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PAPER
Heteropolyacid generated on the surface of iron
phosphate nanotubes: structure and catalytic
activity studies†
Cite this: RSC Adv., 2015, 5, 63917
*
*
Ebtesam Al-Mutairi, Katabathini Narasimharao and Mohamed Mokhtar
Structural and catalytic properties of a Mo based heteropolyacid generated after impregnation of MoOx on
the surface of porous iron phosphate nanotubes (FeP) were studied. Nano sized MoOx–FeP composites
with different Mo molar loadings (1–5%) have been prepared under acidic conditions. Synthesized
composites were characterised by elemental analysis, X-ray diffraction, transmission electron
microscopy, Raman spectroscopy, UV-vis spectroscopy, X-ray photoelectron spectroscopy, acidity
measurements using FTIR, N2-physisorption and H2-temperature programmed reduction methods.
Spectroscopic characterization results suggest that Keggin-type FeMoP species were formed on the
surface of the iron phosphate nanotubes in the case of catalysts with higher (4 and 5 mol%) Mo loadings.
Pure iron phosphate nanotubes, Fe2O3, MoO3 and bulk FeMoP HPA samples are less active than MoOx–
FeP composite samples for the benzylation of benzene with benzyl chloride, particularly the 5 mol% Mo
loaded catalyst showed high activity. The enhanced catalytic activity of this catalyst is attributed to the
presence of easily reducible, acidic and porous FeMoP heteropolyacid species. These materials can be
readily separated from the reaction system for reuse. They are resistant to leaching of the active
heteropolyacid species.
Received 11th June 2015
Accepted 21st July 2015
DOI: 10.1039/c5ra11175g
www.rsc.org/advances
toxicity. The development of reusable heterogeneous catalysts
has gained practical importance in overcoming the problems
associated with homogenous catalysts.4
1 Introduction
The ne chemical industry has experienced tremendous growth
over the past few years due to the high demand for products like
pharmaceuticals, pesticides, fragrances, avourings and food
additives.1 The stoichiometric organic synthesis that largely fol-
lowed so far leaves huge quantities of inorganic salts as by-
products, the disposal of which is a serious problem due to
keen environmental awareness and tightened regulations.2
Increased competition in the industry has pushed the research
and development activity on ne chemicals toward nding more
cost-effective catalytic routes. Diphenylmethane, also known as
benzyl benzene, is a valuable intermediate in the chemical
industry due to its use in the dye and perfume manufacturing
processes.3 Friedel–Cra type benzylation of benzene using
benzyl chloride to afford diphenylmethane is a well-known
method of synthesising diphenylmethane. The commonly used
homogeneous catalysts, such as AlCl3, BF3, H3PO4 and H2SO4,
suffer from several disadvantages, including difficulty in sepa-
ration, recovery, disposal of used catalyst, corrosion and high
The utilization of solid heteropolyacids (HPAs) as alternate
heterogamous catalysts for conventionally used reagents, such
as HF, H2SO4 is well known for many years.5 HPAs are oxo-
clusters of transition metals, such as W and Mo. It is well
known that HPAs are insoluble in non-polar solvents, but highly
soluble in polar ones without structure change. Due to their
unique combination of acid–base and redox properties, these
kinds of solids have been used successfully as solid catalysts in
their proton or salt form for acid and redox catalysed reactions.6
Thermally stable and high surface area heteropoly acids and
salts of heteropoly acids supported on solid metal oxides have
been used as catalysts for alkylation and acylation reactions.7
Kamalakar et al.8 dispersed HPAs on mesoporous silica, such as
MCM-41, FSM-16 and SBA-15, by utilising the impregnation
method and testing the supported HPA catalysts in the benzy-
lation of benzene and substituted aromatics with benzyl
alcohol. Tipnis et al.9 used bulk and supported Fe(III) and Al(III)
salts of tungstophosphoric acid catalysts for the benzylation
reaction. The authors observed that clay-supported Fe salt of
tungstophosphoric acid showed the highest activity for the
reaction, resulting in the complete conversion of benzyl chlo-
ride in 60 minutes with a selectivity of 98.7% towards diphe-
nylmethane, however, leaching of active heteropolyacid catalyst
is a problem.
Department of Chemistry, Faculty of Science, King Abdulaziz University, P. O. Box
80203, Jeddah 21589, Saudi Arabia. E-mail: nkatabathini@kau.edu.sa;
katabathini@yahoo.com; mmokhtar2000@yahoo.com; Fax: +966-26952292; Tel:
+966-538638994, +966-500558045
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra11175g
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Literature reports revealed that due to ionic interactions thermally treated at 300 ꢀC for 4 h, and were stored in air prior
between the support surface and HPA clusters are responsible to analysis and reaction testing. The following nomenclature
in generation of the active species, however, due to weak nature was applied for the sample codes: xMo–FeP, wherein ‘x’ repre-
of interaction, it is possible for HPA clusters to leach out from sents the amount of Mo loading and FeP represents ‘iron
the support during reactions in solvents.10 Previously, we have phosphate’. FePMo12O40 heteropolyacid salt (FePMo) was
synthesised the ammonium salt of molybdophosphoric acid on synthesized using the methodology reported in ref. 9 to its
the surface of niobium phosphate by using a novel in situ activity against that Mo–FeP catalysts.
method of preparation.11 In this method, the phosphate ions on
the support were made to react in situ with the ammonium 2.2 Characterization of synthesised materials
heptamolybdate in the solution under acidic conditions.
Subsequently, the heteropoly Keggin ions were grown at specic
sites on the support.
In the present work, we prepared a series of porous nano-
composite of MoOx–FeP catalysts with different Mo loadings to
generate HPA on the surface of iron phosphate nanotubes. The
The elemental composition of the materials was determined
using inductively coupled plasma mass spectrometry (ICP-MS),
Optima 7300DV, Perkin Elmer Corporation, USA. X-ray powder
diffraction (XRD) studies were performed for all of the prepared
solid samples using a Bruker diffractometer (Bruker D8 advance
target). The patterns were run with copper Ka and a mono-
synthesised materials were used as catalysts for the benzylation
of benzene using benzyl chloride. A systematic characterisation
was performed to study the physico-chemical properties of the
catalysts. An effort was made to correlate the benzylation
activity of the catalysts with their physico-chemical properties.
˚
chromator (l ¼ 1.5405 A) at 40 kV and 40 mA. The particle size
of the catalyst was calculated using Scherrer's equation:
D ¼ Bl/b1/2 cos q
(1)
where D is the average crystallite size of the phase under
investigation, B is the Scherer constant (0.89), l is wavelength of
2 Experimental
2.1 Preparation of materials
˚
the X-ray beam used (1.54056 A), b1/2 is the full width at half
maximum (FWHM) of the diffraction peak and q is the
2.1.1 Porous iron phosphate nanotubes. Porous iron diffraction angle. Identication of the different crystalline
phosphate nanotubes were prepared by following the method phases in the samples was performed by comparing the data
reported.12 In a typical method, 4.0 g of ferric nitrate was dis- with the Joint Committee for Powder Diffraction Standards
solved in 20 mL of distilled water and 9.0 g mono-hydrogen (JCPDS) les.
sodium phosphate was dissolved in 60 mL of distilled water.
The Raman spectra of samples were measured with a Bruker
The two solutions were mixed under vigorous stirring to obtain Equinox 55FT-IR spectrometer equipped with a FRA106/SFT-
a precipitate. The obtained precipitate was recovered by Raman module and a liquid N2 cooled Ge detector, using the
centrifugation and was suspended in 10 mL of aqueous solution 1064 nm line of an Nd:YAG laser with an output laser power of
contained 2.88 g sodium dodecyl sulfate. Then, 1.8 mL hydro- 400 mW. A 200 kV Philips CM200FEG microscope equipped
uoric acid (40 wt%) was dropped into the suspension with with a eld emission gun was used for transmission electron
vigorous stirring. The resulting transparent solution was microscopy (TEM) analysis. The coefficient of the spherical
continuously ꢀstirred at room temperature for 24 h and then aberration was Cs ¼ 1.35 mm. The information limit was better
soaked at 60 C for 12 h. Aer cooling to room temperature, a than 0.18 nm. High resolution images with a pixel size of 0.044
light yellow precipitate was observed in the solution and nm were taken with a charge-coupled device (CCD) camera. The
precipitate was recovered by centrifugation. The obtained textural properties of the prepared samples were determined
precipitate was washed with water for four times and then from nitrogen adsorption/desorption isotherm measurements
acetone and drying at 100 ꢀC. The dried solid was suspended in at ꢁ196 ꢀC using the model Autosorb-1 surface analyser,
absolute ethanol and subjected to hydrothermal treatment in a Quantachrome. Prior to measurement, each sample was
ꢀ
Teon-lined autoclave at 150 ꢀC for 24 h. Then, the resultant degassed for 6 h at 150 C. The specic surface area, SBET, was
precipitate was recovered by centrifugation, followed by calculated by applying the Brunauer–Emmett–Teller (BET)
washing with ethanol and dried 100 ꢀC. The dodecyl sulfate equation. The average pore radius was estimated from the
surfactant was removed by thermal treatment at 500 ꢀC for 4 h relation 2Vp/SBET, where Vp is the total pore volume (at P/P0 ¼
in air and the resultant sample was denoted as FeP.
0.975). Pore size distribution over the mesopore range was
2.1.2 Synthesis of MoOx–FeP nanocomposites. A series of generated through the Barrett–Joyner–Halenda (BJH) analysis of
catalysts were prepared with different Mo loadings over the the desorption branches. Then, the values for the average pore
range of 1 to 5 mol%. Simple impregnation method was size were calculated.
employed and it involved the dissolution of required amount of
FTIR spectra of calcined catalysts obtained at room
ammonium molybdate (Aldrich 99%) in 20 mL distilled water in temperature using Perkin-Elmer Spectrum 100 FTIR spectrom-
a round-bottomed ask, followed by the addition of the porous eter. Then, the samples were subjected to pyridine adsorption
FeP support. The pH of the contents was maintained at 2 by analysis. The analysis was carried out over a catalyst disk which
adding the diluted HCl solution. The resulting mixture was le was treated under vacuum for 5 h. Later, the samples were
ꢀ
overnight followed by evaporation to a dry, free owing powder treated with pyridine vapor and nally heated at 100 C under
at 60 ꢀC under vacuum was obtained. Finally, the catalysts were vacuum for 30 min. The amount of Bronsted and Lewis acid
¨
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sites was calculated via integration of the area of the absorption and Mo–FeP samples are shown in Fig. 1. Pure FeP sample
bands showing the maximum values of intensity at 1446 cmꢁ1 showed diffraction peaks of FePO4 indexed from the standard
and 1536 cmꢁ1, respectively. Integrated absorbance of each diffraction peaks [JCPDF le no. 29-0715], indicating that the
band was obtained using the appropriate soware by applying crystal structure is a hexagonal system with space group P321.
the corresponding extinction coefficient and normalized by the Diffraction peaks related to phase impurities were not observed
weight of the samples. in the XRD pattern, conrming the high purity of the FePO4
The XPS measurements were carried out using a SPECS nanotubes.
GmbH X-ray photoelectron spectrometer. Prior to analysis, the
The observed broad diffraction pattern of the FeP sample
samples were degassed under vacuum inside the load lock for revealed the nanosize nature of the solid material. The XRD
16 h. The binding energy of the adventitious carbon (C 1s) line patterns of the MoOx deposited FeP solids also showed only
at 284.6 eV was used for calibration and the positions of other pure FePO4 phase with hexagonal structure. There were no
peaks were corrected according to the position of the C 1s diffraction peaks observed due to MoO3 or any species due to
signal. For the measurements of the high resolution spectra, the interaction between FePO4 and MoOx. This is possibly due to
analyser was set to the large area lens mode with energy steps of either ne dispersion of supported Mo species on the surface of
25 meV and in Fixed Analyser Transmission (FAT) mode with FeP or the phase formation might be under 5%, which is the
pass energies of 34 eV and dwell times of 100 ms. The photo- minimum X-ray detection limit. However, it is interesting to
electron spectra of the four samples were recorded with the note that the diffraction peaks of MoOx deposited FeP samples
acceptance area and angle of 5 mm in diameter and up to 5ꢀ, are more intense and narrower than that of the pure FeP
respectively. The base pressure during all measurements was 5 sample, indicating that the MoOx deposited samples are more
ꢂ 10ꢁ9 mbar. A standard dual anode excitation source with Mg crystalline than pure FeP.
Ka (1253.6 eV) radiation was used at 13 kV and 100 W. H2-TPR
The intensity of the diffraction peaks of FePO4 changed with
patterns of the samples were recorded using CHEMBET-3000 change of the Mo mol%, conrming the role of MoOx in the
(Quantachrome, USA) instrument. TPR experiments were enhancement of the transformation of the partially crystalline
carried out in a quartz reactor; 100 mg of catalyst sample was FeP nanotubes. To understand the role of MoOx role in
loaded into reactor, thermally treated at 120 ꢀC for 2 hours and enhancement of crystallinity of Mo–FeP samples, a FeP sample
cooled to 30 ꢀC under the ow of helium gas and then the was prepared by subjecting it to all the post treatments similar
ꢁ1
ꢀ
ꢀ
sample is heated from 30 to 800 C at the rate of 5 C min
to preparation of Mo–FeP, but without addition of Mo. The XRD
under the ow of 90% helium–10% hydrogen (by volume) gas at pattern of this sample was presented in the ESI.† The XRD
ow rate of 60 mL minꢁ1. The effluent hydrogen concentration pattern of this sample showed a small improvement in the
was detected by the thermal conductivity detector (TCD).
crystallinity of the sample, when it compared to crystallinity of
pure FeP, but not as signicant as observed in the Mo–FeP
samples. This observation indicating that both Mo addition and
the post treatment conditions are affecting the crystallinity of
the Mo–FeP samples. A possible dispersion or incorporation of
MoOx species on the surface or bulk of FeP was studied using
the XPS technique and the results are discussed in the later part
of this section.
2.3 Benzylation of benzene using benzyl chloride
The liquid phase benzylation of benzene with benzyl chloride
(BC) was carried out in a three necked round-bottomed ask
equipped with a reux condenser and electrically heated in a
precisely controlled oil bath under atmospheric pressure. In a
typical run, 13 mL of benzene was added to 50 mg catalyst
(which had been activated overnight at 100 ꢀC). The reaction
mixture was maintained for 30 min at the required reaction
temperature and then 1 mL of benzyl chloride was added. The
moment was regarded as initial reaction time. Liquid samples
were withdrawn at regular intervals and analysed using gas
chromatography (HP-6890) equipped with a ame ionisation
detector (FID) and HP-5 capillary column. The products were
also identied by gas chromatography-mass spectrometry (GC-
MS, HP-5975C) analysis. Since benzene was in excess, conver-
sion was calculated based on the benzylating reagent (i.e., BC).
Selectivity to the product diphenylmethane (DPM) was
expressed as the amount of the particular product divided by
the amount of the total products and multiplied by 100.
The average crystallite sizes of the FeP and 5 mol% Mo
deposited sample are 25 nm and 40 nm (Table 1), respectively,
3 Results and discussion
3.1 Powder X-ray diffraction
Powder X-ray diffraction (XRD) analysis was used to determine
the phase purity of the samples. The XRD patterns of pure FeP Fig. 1 Powder XRD patterns of FeP and Mo–FeP samples.
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and are estimated from the (212) reection based on the
Scherrer eqn (1). The crystallite sizes of the MoOx deposited
samples determined using XRD analysis are bigger than
observed in TEM images. This might be due to the formation of
another phase (amorphous or highly dispersed or under the
detection of XRD limit) during the impregnation of the MoOx in
acidic solution.
Fig. 2 shows TEM images of the pure FeP and representative
MoOx impregnated FeP catalysts. The pure FeP sample
possesses mainly tubular structures with diameters of 25–30
nm and lengths of several microns. The walls of the nanotubes
range from 4 to 6 nm in thickness. The samples, which con-
tained 1 mol% and 4 mol% of Mo, showed particulates apart
from the nanotubes as shown in Fig. 2(b) and (c). The increase
of Mo molar loading to 5% resulted in the change of the tubular
morphology into wormlike and spherical Keggin structures with
meso and macro size voids [Fig. 2(d)]. TEM analysis clearly
showed that there are different morphologies for Mo–FeP
samples, which is leading to different intensities and reection
broadenings in XRD patterns.
Raman spectra of pure FeP and MoOx impregnated FeP
catalysts are shown in Fig. 3(A). The pure FeP sample exhibited
two major Raman vibrations at 1010 and 1053 cmꢁ1 along with
minor vibrations at 408, 440, 595, 665 and 1166 cmꢁ1. It was
reported that the stretching and bending vibrations of phos-
phate groups appear in the range of 1000–1200 and 400–700
cmꢁ1, respectively.13 The major Raman vibrations at 1010 and
1053 cmꢁ1 can be attributed to alternatively connected tetra-
hedral FeO4 and PO4 groups, respectively.14
Fig. 2 TEM images of (a) FeP (b) 1Mo–FeP (c) 4Mo–FeP (d) 5Mo–FeP.
transitions, respectively.14–16 For the MoO3 impregnated FeP
samples, a new broad absorbance band at higher wavelength
(around 370 nm) was observed along with peaks due to FeP
indicating that the local structure of the MoOx impregnated FeP
samples differed from that of the FeP to some extent. The
samples with Mo content from 1 to 4 mol% showed very similar
absorption spectrum with a very broad ultraviolet absorption
extending from 200 to 400 nm. The strong band cantered
around 370 nm showing the presence of octahedrally coordi-
nated Mo species in Mo–FeP samples.17 Further increase of Mo
content to 5 mol%, the UV-vis spectrum showed a sharp
absorption peak at 310 nm, which could be due to formation of
bulk HPA species. The material with different Mo composition
is expected to favour different oxidation states for molybdenum
species.18 From these results, it appears that the FeP support is
required between 4 and 5% Mo loading to form bulk type het-
eropolyacid species. In our previous publication,19 we studied
the variation in surface W content as a function of bulk tung-
sten loading in case of WO3–ZrP composite. The surface W
coverage rose continuously as a function of bulk content, with a
slight plateau obtained around 5 wt% W, followed by a rapid
rise for bulk loadings in excess of 10 wt% W. This variation is
suggestive of in-pore adsorption of WOx species for low load-
ings, with the onset of multilayer/crystalline growth occurring at
higher loadings. A very similar phenomenon can be expected in
case of MoOx–FeP.
The vibrations at 1167 and 665 cmꢁ1 can be ascribed to the
presence of metal phosphate groups. The Mo–FeP catalysts
showed new vibrations at 988, 876 and 608 cmꢁ1, which corre-
sponds to ns(Mo]Od), nas(M–Ob–Mo) and ns(Mo–Oc–Mo) due to
the characteristic of the Keggin anion.15 It is clear from the
Raman data that Keggin type HPA formation is taking place
with stoichiometric phosphorous presented in the FeP support.
Fig. 3(B) shows the UV-vis absorption spectra extending from
200 to 800 nm of the FeP and MoOx impregnated FeP samples.
The pure FeP sample showed a sharp absorbance bands at
around 230 nm and a broad band at around 280 nm, which
could be attributed to the P–O and Fe–O charge transfer
Table 1 Crystallite size of catalysts determined from XRD and TEM
measurements
X-ray photoelectron spectroscopy analysis of the samples
was performed to investigate the nature of the surface species.
The Fe 2p XPS core spectra for all the samples are shown in
Fig. 4(A) and deconvoluted Fe 2p XPS spectrum for 5Mo–FeP
sample was also depicted at the bottom of the gure. Fe 2p3/2
binding energies in the range of 711.1–711.5 eV were observed
for FeP and MoOx deposited FeP samples. Alptekin et al.20
observed a binding energy of 712.5 eV for pure FeP and Wang
et al.21 reported a value of 713.2 eV for the same material. The
Crystallite size (nm)
S. no.
Catalyst
XRD
TEM
1
2
3
4
5
6
FeP
25
29
33
35
38
40
25a
26a
28a
29a
30a
32b
1Mo–FeP
2Mo–FeP
3Mo–FeP
4Mo–FeP
5Mo–FeP
a
b
Diameter of tubes or worm shape particles. Diameter of spherical binding energies of the Fe 2p1/2 and Fe 2p3/2 peaks attributed to
Fe3+ in pure FeP are 724.4 eV and 711.5 eV, respectively. A small
particles.
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Fig. 3 (A) Raman (B) UV-vis absorption spectra of FeP and Mo–FeP samples.
shi in the binding energies of the Fe 2p peaks in the MoOx the absence of a characteristic O 1s peak of iron oxide (Fe–O)
deposited FeP samples was observed. The deconvoluted Fe 2p3/2 bond at 530 eV,25 indicating that the Fe cations are bound as
spectra for 5Mo–FeP sample showed two peaks centred at 711 phosphates and not as iron oxides. Deconvoluted spectrum of
eV and 712.8 eV due to Fe3+ species. It is interesting to note that 5Mo–FeP sample clearly showed a shoulder peak at 531.5 eV.
a new small peak appeared at 714 eV in the XPS spectrum of This peak can be attributed to the bridging of the oxygen P–O–P
5Mo–FeP sample. Appearance of a new peak with 2.5 eV binding contribution.
energy difference for the 5Mo–FeP sample relative to bulk FeP is
The binding energy value observed in all the samples for P
high enough to attribute this peak to the surface oxidation state 2p3/2 XPS peak [Fig. 4(D)] is around 132 eV and this peak can be
of iron between three and two, which is possibly due to FeMoP assigned to P with an oxidation state of 5+.26 The XPS analysis of
HPA species.
the FePO4 showed a peak at 133.8 eV for P 2p3/2. The binding
The Mo 3d XPS core spectra for all the samples are shown in energies for the mixture of FePO4 and MoO3 provided very
Fig. 4(B). The FeP, 1Mo–FeP and 2Mo–FeP samples did not similar value for P as the FePO4 material. The surface atomic
show any dened Mo 3d contributions. Specically, 1Mo–FeP Mo/P and Fe/P ratios were calculated and at lower loadings, the
and 2Mo–FeP samples did not show any contributions of Fe/P ratio is identical to that observed for pure FePO4.20 A low
surface Mo species, probably due to the fact that the actual surface enrichment of Mo becomes evident as the Mo loading
concentration is very small and the deposited Mo atoms entered level goes up (Table 2). It appears that Mo is interacting with
into the framework of FeP. In contrast, the 3Mo–FeP and 4Mo– specic sites on the FeP surface and that the Mo atoms are
FeP samples showed two broad peaks centred at 231.1 eV. sorbed or bonded on top of the FeP. As noted above, procedures
Meanwhile, 232 eV correspond to Mo6+ (3d3/2) and Mo5+ (3d5/2), similar to those employed here signicantly overestimate the
respectively.22 However, in the case of the 5Mo–FeP catalyst, the absolute value of the surface P to the metal ratio.27 The trend of
broadness of the peaks disappeared and a doublet Mo 3d5/2 and the surface Mo enrichment, with increasing Mo loading was
3d3/2 peaks at 231.3 eV and 235.5 eV was observed. These two clearly observed.
species can be attributed to typical Mo6+ species.23 Presence of
The bulk elemental composition of the catalysts was per-
reduced Mo ions was not observed in case of 4Mo–FeP and formed using ICP-MS analysis. The calculated percentages of
5Mo–FeP samples. The binding energy values for all low Mo the elements Fe, Mo and P are given in Table S1 (ESI†). The
loading samples are approximately the same, probably due to results indicate that P/Fe mole ratio of the calcined FeP sample
the presence of Mo with the low oxidation state (5+) in these is very close to the stoichiometry of FePO4, while the P/Fe mole
samples. These results provide evidence that the Keggin type ratio of MoOx deposited samples changed as expected.
species were formed in case of the 4Mo–FeP and 5Mo–FeP Considering the P/Fe and P/Mo mole ratios of samples and the
samples, although the Mo ions were placed in a distorted difference among them, we can notice that a reaction took place
octahedral environment of oxide ions.24 The O 1s peak for pure between MoOx and FePO4 during the impregnation in the acidic
FeP at 531 eV [Fig. 4(C)] is attributed to the Fe–O–P bond and solution.
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Fig. 4 XPS core spectra for all the samples (A) Fe 2p (B) Mo 3d (C) O 1s (D) P 2p and the deconvoluted spectra for 5Mo–FeP sample.
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Table 2 Textural properties of catalysts from N2-physisorption and chemical composition of catalysts from XPS analysis
XPS chemical analysis (molar percent)
Catalyst
SBET (m2 gꢁ1
)
Vp (cm3 gꢁ1
)
Av. pore diameter (nm)
Mo
Fe
P
O
FeP
123
107
91
76
58
0.294
0.254
0.219
0.188
0.171
0.155
20.0
16.8
13.0
12.3
10.8
7.5
0.0
0.1
0.2
1.4
2.5
3.6
36.5
36.4
36.2
36.1
36.1
35.8
20.0
20.1
20.1
20.2
20.1
20.0
43.5
43.4
43.5
42.3
41.3
40.6
1Mo–FeP
2Mo–FeP
3Mo–FeP
4Mo–FeP
5Mo–FeP
39
Fig. 5(A) and (B) shows the typical N2 adsorption–desoprtion
Temperature programmed reduction (TPR) proles were
isotherms (inset; corresponding pore size distribution) of FeP recorded for all the samples to determine the reduction
and 5Mo–FeP samples respectively. The samples are possessed temperatures for all the samples [Fig. 6]. Pure FeP exhibited one
type-IV isotherm with narrow hysteresis loop which indicates broad asymmetric reduction peak centred at 700 ꢀC. In
the presence of mesopores related to inter-particle voids as comparison to the pure FeP sample, peak position was not
dened by the International Union of Pure and Applied changed aer the deposition of 1% Mo. However, a low, intense
Chemistry (IUPAC).28 Accordingly, the pore size distribution broad peak at 550 C appeared due to the reduction of the Mo
ꢀ
pattern displays a unimodal shape pores with maximum pore species. Approximately 2Mo–FeP and 3Mo–FeP samples
˚
˚
radius at 200 A for FeP and 75 A for the 5Mo–FeP samples, underwent reduction by hydrogen at higher temperatures,
respectively. showing modied valence stability of the Fe in the FeP nano-
The mesoporous nature observed from the results of the N2- material. It was reported that the reduction temperature were
physisorption is consistent with the pores visualized in the TEM 690 ꢀC for FePO4, 900 ꢀC for MoPO4 and 640 ꢀC, 690 ꢀC, 730 ꢀC
analysis, indicating that the mesopores were raised from the and 790 ꢀC for the mixture of FePO4 and MoO3.29 The difference
morphology of the FeP nanotubes or Mo–FeP particles. Quan- in the reduction temperature in the MoOx deposited FeP
titative calculation shows that pure FeP possesses Brunauer, samples and the broadening of the peaks suggest a chemical
Emmet and teller (BET) surface area of 123 m2 gꢁ1 and a pore interaction between the FePO4 and MoOx in the MoOx deposited
volume of 0.294 cm3 gꢁ1. A pronounced decrease of surface FeP samples during the reduction. It is interesting that the
area, pore volume and pore diameter were observed with the Mo reduction peak remarkably shied to lower temperatures for
loading over FeP. The sample with 5 mol% Mo loading the 4% and 5% Mo loaded samples. As the loading amount
ꢀ
possessed surface area, pore volume and pore diameter of 39 reached 5 Mo mol%, the main peak shied to 675 C and the
m2 gꢁ1, 0.155 cm3 gꢁ1 and 7.5 nm, respectively (Table 2). The shoulder peak appeared distinctly. The major peak at 675 ꢀC
decrease in the values of the textural properties with the and 700 ꢀC probably arose from the reduction of the crystalline
increase in Mo loading is not only due to the formation of FePO4. These results strongly suggest that the Keggin type Mo
Keggin type FePMo heteropolyacid species, but also due to species encapsulated within the mesopore channels of FeP can
complete transformation of the initial FeP particle morphology. be reduced at lower temperatures as compared with the crys-
talline FeP.
Fig. 5 N2 adsorption–desorption isotherms of (A) FeP (B) 5-MoFeP (inset pore size distribution).
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As previously mentioned, the goal of this research is to
synthesize the heteropolyacids by interaction of Mo with iron
phosphate nanotubes and it is important to study the acidity of
the synthesized Mo–FeP catalysts. FTIR spectra of samples aer
pyridine (Py) adsorption at 100 ꢀC followed by evacuation at the
same temperature was shown in Fig. 7. It is known that pyridine
adsorbed samples exhibit peaks at 1446, 1486 and 1536 cmꢁ1
The peaks at 1446 and 1536 cmꢁ1 are characteristic of Lewis-
.
¨
coordinated pyridine (L) and Bronsted coordinated pyridine
(B) respectively. The band at 1486 cmꢁ1 is due to Lewis and
30
¨
Bronsted coordinated pyridine (L + B). The pure FeP sample
showed a small peak at 1486 cmꢁ1, which is due to minor
¨
contribution of Lewis and Bronsted acid sites. However, a major
difference was noticed in FTIR spectra MoOx impregnated FeP
samples, particularly in 5Mo–FeP and 4Mo–FeP samples. The
formation of new Lewis sites was clearly observed as revealed by
the intense band at 1446 cmꢁ1 along with the peak due to Lewis
¨
and Bronsted acid sites.
The Lewis acid sites density for all the catalysts was deter-
mined. Pure FeP sample possessed total acid sites density of 0.9,
however aer impregnation of MoOx resulted increase of
density of acid sites. A gradual increase of acidity was observed
from 1 to 3 mol% (1Mo–FeP ¼ 4.5, 2Mo–FeP ¼ 6.2, 3Mo–FeP ¼
8.9) and further increasing of Mo loading did not change the
density of acid sites (4Mo–FeP ¼ 15.2, 5Mo–FeP ¼ 16.1). The
surface species responsible for acidity in these samples are
coordinated unsaturated metal ions (Fe3+ and Mo5+ ions). These
cations possess an uncompensated positive charge and coor-
dinate molecules with a free electron pair and act as Lewis acid
sites.31
Fig. 7 FTIR spectra of samples after pyridine adsorption.
accompanied by di- and poly-alkylations. The benzylation can
also produce dibenzylation products (4, 5 and 6) as side prod-
ucts.1 Thus, dibenzylbenzene, tribenzylbenzene and other side
products can be formed in the reaction, with dibenzylbenzene
being the most prominent side product.
The conversion of benzyl chloride with reaction time over
ꢀ
FeP and MoOx impregnated FeP catalysts at 80 C is shown in
Fig. 8(A). As observed, pure FeP offered very low catalytic
activity, with a benzyl chloride conversion of 4% aer a reaction
time of 240 min an increase in the conversion of benzyl chloride
(50%) was observed upon the addition of 1 mol% Mo on FeP.
A signicant increase of activity was observed aer loading 2
mol% Mo on FeP, where 24% of the conversion of benzyl
3.2 Benzylation of benzene
Benzylation of benzene was used as a model reaction to check
the capability of the synthesized Mo–FeP materials as redox
catalysts. Benzylation of benzene (1), with benzyl chloride (2) as
an alkylating agent, produces mainly diphenylmethane (3)
(Scheme 1). It is known that Friedel–Cras reaction is always
Scheme 1 Reaction pathway of benzylation of benzene with benzyl
Fig. 6 H2-TPR patterns for FeP and Mo–FeP samples.
63924 | RSC Adv., 2015, 5, 63917–63929
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Fig. 8 (A) The conversion of benzyl chloride with the reaction time over FeP, Mo–FeP and FeMoP catalysts, reaction temperature: 80 ꢀC (B) plot
of ln[1/1 ꢁ x] as a function of the reaction time over FeP and MoOx impregnated FeP catalysts (C) the Arrhenius plots of benzylation reaction for all
the catalysts, benzyl chloride : benzene ¼ 1 : 15, wt of catalyst: 50 mg, the selectivity to DPM was varied between 96–98%.
chloride was observed aer 15 min. Conversion reached to to the time required for reaching equilibrium temperature. A plot
100% aer a reaction time of 120 min. An increase of Mo of ln[1/1 ꢁ x] as a function of time was shown in Fig. 8(B). It is
loading to 3, 4 and 5 mol% led to further increase in conversion shown in Fig. 8(B) and the data presented in Table S2 (ESI†) that
of benzyl chloride. In particular, 5Mo–FeP catalyst offered 100% the reaction rate constant ‘ka’ for the 5Mo–FeP catalyst is higher
conversion just aer 45 minutes of reaction. The conversion of than that of pure FeP and twice that of 2Mo–FeP. The turnover
benzyl chloride prole for bulk FeMoP heteropolyacid salt was frequency (TOF) was estimated based on the mole number of
included to compare its activity with Mo–FeP catalysts. It is clear benzyl chloride converted per mole of Mo and Fe species per
that FeMoP showed better conversion than pure FeP and 1Mo– second. Based on the comparison of the TOF data, it can be seen
FeP catalysts, but lower than other Mo–FeP composite catalysts. that the catalytic performance is greater for MoOx impregnated
It is also observed that Mo–FeP composite catalysts with higher FeP catalysts than the FeP, MoO3 and Fe2O3 materials under the
Mo loading, offer much higher conversion of benzyl chloride same reaction conditions. For instance, the TOF of 5Mo–FeP is 10
aer one hour of reaction time.
times higher than that of FeP and 30 times than that of MoO3.
The kinetic data for the benzylation of benzene in benzene The dependency of TOF on the MoOx loading apparently suggests
[stoichiometric ratio (benzene/benzyl chloride) ¼ 15] over all the the formation of several MoOx–FeP interactive species having
catalysts could be tted well to a pseudo-rst-order rate law: ln[1/ different intrinsic activities for the benzylation. In addition, the
1 ꢁ x] ¼ ka [t ꢁ to], where ka is the apparent rst-order rate TOF of bulk FeMoP catalyst was calculated and it is lower than
constant, x is the fractional conversion of benzyl chloride, ‘t’ is Mo–FeP catalysts except 1Mo–FeP. The selectivity to DPM is very
the reaction time and ‘to’ is the induction period corresponding similar for all the catalysts. The obtained products comprised
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mainly of DPM (above 96%), with a small amount of DBB (below
A noticeably high difference in activity can be observed in the
4%). Polyaromatic condensation compounds 5 and 6 were not MoOx deposited catalysts with reaction temperature and
detected in the product analysis. different compositions; thus, it can be assumed that the cata-
Friedel–Cras alkylation is an aromatic electrophilic lytic activity of these catalysts in the benzylation of benzene was
substitution reaction in which the carbocation is formed by the affected by their content and physico-chemical properties.
complexation of alkyl halide with the catalyst. The carbocation
Fig. 9(A) illustrates the conversion of benzyl chloride at
ꢀ
attacks the aromatic species for alkylation; hence, the forma- different reaction temperatures of 60, 70, 80 and 90 C on the
tion of the carbocation is an important step in the reaction most active catalyst, 5Mo–FeP. It is clear that the 5Mo–FeP
mechanism. The activity has signicantly increased with the sample exhibits signicant different catalytic performances in
deposition of 1 mol% of Mo loading and the activity increased the benzylation of benzene. It can be seen that the conversion
linearly with the increase of loading from 1 to 5 mol% at 70 ꢀC. levels increased with the increase in reaction temperature. The
However, the conversion of benzyl chloride has reached conversion levels are very low at 60 ꢀC; the maximum conversion
maximum level (100%) for 2 mol% Mo at 80 ꢀC. To obtain a is 7% even aer 240 minutes. Increasing the reaction temper-
better insight of the catalysts behaviour, we tested the catalysts ature to 70 ꢀC resulted in minimal improvement in the
with high Mo loadings (3, 4 and 5 mol%) for the benzylation of conversion levels (11% maximum). However at 80 ꢀC, the
benzene. These samples also offered 100% conversion with conversion was increased to 100% just aer 45 min. A similar
short reaction times at 80 ꢀC. Fig. 8(C) shows the Arrhenius conversion pattern was observed ꢀwith a further increase in the
plots of the benzylation for all the catalysts. The observed Ea reaction temperature of up to 90 C.
values are tabulated in Table S2.† The activation energy for the
To investigate the effects of the benzyl chloride-to-benzene
most active catalysts, 5Mo–FeP and 4Mo–FeP, is 50.5 kJ molꢁ1 molar ratio to the most active catalyst 5Mo–FeP, the benzyla-
and 59.8 kJ molꢁ1, respectively. As expected, pure FeP showed a tion experiments were conducted by changing the molar ratio
higher activation energy of 78.6 kJ molꢁ1
.
from 1 : 1, 1 : 6 to 1 : 15, while at the same time keeping the
The conversion of benzyl chloride and the rate of reaction reaction temperature and the catalyst amount constant at 80 ꢀC
per unit surface area of FeP and MoOx deposited FeP catalysts at and 50 mg, respectively. Fig. 9(B) demonstrates the effect of the
different reaction temperatures are presented in Table 3. It is molar ratio of benzyl chloride to benzene on benzyl chloride
clear that when pure FeP was used as a catalyst, only 22% conversion for the 5Mo–FeP sample. The reaction developed
conversion was observed aer 240 min at 70 ꢀC, clearly indi- slowly and the 100% conversion was observed in 240 min when
cating that the presence of active species is essential to obtain a molar ratio is 1 : 1. Depending on the different molar ratios of
high conversion in this reaction. The benzyl chloride conver- benzyl chloride to benzene, benzylation reached a state of
sion increased with the increase of the reaction temperature for equilibrium at different reaction times. The conversion of
all the catalysts. In terms of the rates of the benzyl chloride benzyl chloride reached 100% at 180 minutes for 1 : 6. It took
conversion per unit surface area of catalyst, the 5Mo–FeP cata- only 45 minutes to get 100% conversion with a 1 : 15 molar
lyst showed the highest benzylation reaction rate. The rates per ratio. A similar pattern was observed in the case of other
unit surface area were consistent among all catalysts. The samples; however, the other catalyst requires longer reaction
greatest difference was observed at high reaction temperature times to convert benzyl chloride. It is known that benzene
ꢀ
(80 C), where the 5Mo–FeP catalyst was more active than FeP serves as a solvent as well. An excess of benzene can shi the
and other MoOx impregnated catalysts. It is interesting to note equilibrium to the right side, improving the conversion of
that bulk FeMoP heteropolyacid salt offered less rates per unit benzyl chloride. The difference in the catalytic activity among
surface area than all of the Mo–FeP catalysts even though its the MoOx impregnated FeP catalysts is possibly due to the
showed more conversion than 1Mo–FeP catalyst. This is due to difference in the nature of the interactive species formed
very low surface area (5 m2 gꢁ1) of the bulk FeMoP catalyst.
between the MoOx and FeP in these catalysts.
Table 3 Conversion of benzyl chloride and reaction rate per unit surface area of FeP and MoOx impregnated FeP catalysts at different reaction
temperaturesa
60 ꢀ
C
70 ꢀ
C
80 ꢀ
C
ꢁ1
ꢁ1
k (min mꢁ2), 10ꢁ5
Conversion (%)
k (minꢁ1 mꢁ2), 10ꢁ5
ꢀ
Conversion (%)
k (min mꢁ2), 10ꢁ5
ꢀ
ꢀ
Catalyst
Conversion (%)
FeP
16 (96)
24 (96)
35 (97)
42 (97)
48 (98)
52 (98)
31 (97)
0.40
0.49
0.72
1.31
1.53
2.78
0.42
22 (96)
49 (96)
56 (96)
68 (96)
75 (97)
89 (97)
50 (97)
0.47
0.79
0.90
1.34
2.42
3.20
0.51
23 (96)
63 (96)
99 (96)
99 (96)
99 (96)
100 (97)
78 (97)
0.47
2.16
3.28
4.33
5.02
5.52
1.58
1Mo–FeP
2Mo–FeP
3Mo–FeP
4Mo–FeP
5Mo–FeP
FeMoP
a
Benzyl chloride : benzene stoichiometric ratio ¼ 1 : 15 and 50 mg of catalyst; the values in parenthesis represents selectivity to DPM.
63926 | RSC Adv., 2015, 5, 63917–63929
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Fig. 9 (A) Conversion of benzyl chloride at different reaction temperatures over 5Mo–FeP (B) effect of molar ratio of benzyl chloride to benzene
on benzyl chloride conversion for the 5Mo–FeP sample, reaction temperature: 80 ꢀC, wt of catalyst: 50 mg, the selectivity to DPM was varied
between 95–98%.
The obtained characterization results suggest that
Comparison of porous nanosized MoOx–FePO4 catalysts with
increasing the amounts of Mo atoms up to 4.4 mol% of Mo, other reported catalysts was tabulated and is presented in Table
gradually introduced into the FeP framework, have an effect on S3 (ESI†). In comparison with previous reports on mesoporous
the Raman, UV-vis spectra, reducibility and acidity of the cata- iron based catalysts in the benzylation of benzene with benzyl
lysts. The observed changes unequivocally suggest that Mo chloride, 5Mo–FeP catalyst is equally efficient catalyst. It was
atoms in the synthesised materials were introduced into the FeP reported that Fe-MCM-41 (ref. 35) exhibited 90% conversion of
framework by the breaking of oxygen bridges (i.e. P–O–P and benzyl chloride with 95% selectivity towards diphenylmethane;
Fe–O–P) and the following formation of bonds, such as P–O–Mo however, this catalyst has a major disadvantage that almost
and/or Fe–O–Mo. Modications occurring in the XPS spectra 33% of the iron atoms have been removed from the framework
mainly involved the Mo 3d peak, which is not so much assigned of MCM-41 during reaction cycles. Sun et al.36 reported that 10
for the value of the binding energy (which remained approxi- wt% iron oxide loaded SBA-15 catalyst showed 100% conversion
mately constant for relatively high loading samples), but rather of benzyl chloride aer a reaction time of 30 min and 15 wt%
in the form of the peak, which is highly sensitive to the atom iron oxide loaded SBA-15 gave 100% conversion aer are action
environment.32 On the other hand, it is not possible to easily time of 45 min. Leng et al.37 used iron-exchanged mordenite
distinguish between Fe3+ in the tetrahedral coordination and zeolite catalyst for benzylation of benzene by benzyl chloride.
Fe3+ in the octahedral coordination by analysis of the Fe 2p The authors observed 100% conversion in just 30 min. However,
peak. Fe3+ and Fe2+ in the high-spin state give rise to a multiple there are serious questions about the heterogeneous nature and
effect, which is responsible for peak broadening. However, the reusability of iron exchanged porous zeolite and SBA-15 cata-
Mo 3d peak position provides evidence that the Keggin type lysts. Shinde and Sawant38 used various ferrites such as
species were formed in the case of the 4Mo–FeP and 5Mo–FeP CuFe2O4, NiFe2O4, CoFe2O4, ZnFe2O4, and MgFe2O4 for benzy-
samples.
lation of benzene with benzyl chloride. Among the catalysts,
There are also several reports indicating a decrease in the ZnFe2O4 is highly active, which offered 100% conversion of
reducibility of the total catalyst with the increase of active benzyl chloride in 10 min. The authors did not presented any
component loading, as well as on the linking of the catalytic data on the selectivity of diphenylmethane, and also the ferrite
activity and selectivity to the reducibility of the catalyst.33 As the samples are micropores in nature with low surface area and
FeMoP heteropolyacid species become more bulk-like; that is, there is a clear possibility for the diffusional problems. Koyande
as the particle size increases with an increasing loading, they et al.39 observed maximum 80% of diphenylmethane yield over
become more difficult to reduce due to bulk diffusion limita- sulfate-promoted ZrO2–Fe2O3 catalyst. It was observed that the
tions.34 This is the reason for the selection of 1 to 5 molar% of activity of the catalyst decreased drastically in the second run
Mo. It is clear from our previous study that higher loadings (5 to itself and suffered gradual loss in activity. Although in the above
25 wt%) of Mo and W produce bulk-like HPA species.11,19 The works the amounts of catalyst and benzyl chloride are different
H2-TPR results clearly indicated that the 5Mo–FeP catalyst can from those used in this work, it can be estimated that 5Mo–FeP
be easily reduced compared to the parent FeP. This may indi- is still a very active catalyst in this reaction aer the effect factor
cate partial reduction of the iron or an increase in the ease of of amounts of catalyst, reaction time and benzyl chloride is
reduction of the MoOx deposited FeP.
deducted. The main advantage of using porous nanosized
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MoO3–FePO4 catalyst in this reaction is its higher hydrothermal
and water stability as compared to other mesoporous catalysts;
in addition, in situ generated FeMoP Keggin species are very
difficult to remove from the crystalline framework of iron
phosphate.11
Reusability is one of the main advantages of heterogeneous
catalysts. To test the reusability of Mo deposited FeP catalysts,
the benzylation of benzene with benzyl chloride reaction was
carried out using 5Mo–FeP catalyst at 80 ꢀC. The conversion
levels of the benzyl chloride of this catalyst for ve consecutive
cycles were determined. The fresh catalyst offered 100%
conversion at a reaction time of 45 min correspondingly; the
rst and second reused catalysts presented exactly the same
activity. A slight decrease of conversion (96%) was observed in
the third, fourth and h cycles. This could be due to the loss of
catalyst amount during the reaction recycling. These results
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The authors gratefully thank King Abdulaziz City for Science 20 G. O. Alptekin, A. M. Herring, T. R. Ohno, D. L. Williamson
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